Pentacyanocobaltate(II): Homogeneous catalysis and complex formation

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PENTACYANOCOBALTATE(II)

HOMOGENEOUS CATALYSIS AND

COMPLEX FORMATION

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT,

OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. R. VAN NAUTA LEMKE, HOOGLERAAR IN DE AFDELING DER ELEKTROTECHNIEK,

VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN TE VERDEDIGEN OP WOENSDAG 29 NOVEMBER 1972 TE 16.00 UUR

DOOR

JOHN BASTERS

scheikundig ingenieur geboren te Katwijk aan Zee

/S>c^c^ 3 ; ^ ^ ^

1972

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DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOREN PROF. DR. IR. L. L. VAN REIJEN EN PROF. DR. IR. H. VAN BEKKUM

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Aan mijn ouders Aan Judy

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Drawings and cover design: A. J. Dekker

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1 Introduction

Historically the first applications of homogeneous catalysis originate in organic chemistry. Homogeneously catalysed reactions involving transition-metal complexes still attract considerable attention. At present research in this field is expanding rapidly. It has been stimulated by the discovery of many new co-ordination com-pounds, possessing unusual catalytic properties or bearing a significant relation to intermediates in heterogeneous catalysis and biochemistry. On the other hand the development in homogeneous catalysis is connected with the important advances in the theoretical understanding of the bonding and electronic structures of co-ordina-tion compounds.

Many investigators in catalysis have recognised the probable relevance of the chemistry of catalytically active organo-metallic complexes with that of metal sur-faces. Organo-metallic complexes are formed as labile intermediates in certain homogeneously catalysed reactions. Similar intermediates are also said to occur at the surface of heterogeneous catalysts during reactions. On this basis, the mechanism of catalytic reactions may be better interpreted and consequently more efficient catalysts designed. Wender and Sternberg for instance regarded carbonyl formation at the surface of a transition metal as an extraction and subsequent stabilization of surface metal atoms by carbon monoxide'. Reactions catalysed by metal carbonyls were considered as counterparts of heterogeneous catalytic reactions. The similarity in behaviour between transition metals and their soluble salts and complexes is ascribed to participation in bonding of d orbitals in chemisorption complexes as well as in co-ordination compounds of transition metals with e.g. hydrogen, carbonyl and unsaturated hydrocarbons.

Supported transition metals or their compounds are used in large-scale industrial processes as heterogeneous catalysts. In the heterogeneous systems, the catalytic process must necessarily take place at the surface of the metal crystallites since only this is exposed to the reactants. Homogeneous catalysts are, however, soluble in the same medium as the reactants and, consequently, all the molecules of the catalyst are available for interaction. Thus homogeneous processes are potentially more efficient, in terms of atoms transition-metal catalyst, than heterogeneous ones. The overall chemical equations and the stoichiometry for both homogeneous and heterogeneous processes are similar. Generally, however, the chemistry of homogeneously catalysed processes is better defined and more easily controlled than that of heterogeneous processes. Thus homogeneous processes may show higher selectivity. On the other hand there is the problem of separating catalysts from substrate phase in

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homoge-neous catalysis, which does not exist in heterogehomoge-neous systems. Recently the use of transition-metal complexes adsorbed at an inert support has been introduced. Other-wise the usefulness of homogeneous catalysts is limited because they are often not stable at higher temperatures.

In the last decade homogeneous catalysis has been developed to such an extent that it has entered industrial application. The first example is the so-called oxo or hydroformylation reaction^'^. This "Oxo-process", an important industrial process for the production of aldehydes and higher alcohols at 90-200°C and CO/Hj pres-sures from 100 to 400 atmospheres using a cobalt carbonyl as catalyst, was already in use in 1960. At the moment its industrial uses have been markedly multiplied. Recent studies have shown that certain rhodium complexes which contain co-ordinated phosphines can also be used as hydroformylation catalysts'*. Especially hydridocarbonyltris(triphenylphosphine)rhodium is more effective than cobalt cata-lysts, allowing the reaction to proceed at room temperature and under atmospheric pressure (CO:H2 = 1)'"*. Moreover a better selectivity is shown, the ratio linear/ branched is raised considerably. A similar rhodium catalyst is used in a recently developed, Monsanto process for the production of acetic acid^. This process com-prises the carbonylation of methanol with chlorocarbonylbis(triphenylphosphine)-rhodium at 200 °C and 35 atmospheres CO pressure. The "Wacker-process" is a process for the homogeneous oxidation of olefins using palladium chloride and cupric chloride in a catalytic cycle*. At this moment especially acetaldehyde is produced on a large scale from ethene using this process. Further is known the "Dow phenol-process" ^, for the production of phenol from benzoic acid by oxidation using cupric benzoate as catalyst.

According to Heinemann^^ in 1971 15% of the total value of products made by catalytic reactions is produced in homogeneously catalysed reactions. This figure would be much greater were it not for the enormous volume of heterogeneously catalysed products based on petroleum refining. The part of the homogeneous industrial catalysis in the catalysed reactions will certainly grow further in the near future.

Several hydrido transition-metal complexes function as intermediates in homo-geneous hydrogenation of unsaturated compounds under mild conditions. Although the first homogeneous hydrogenation catalyst was discovered by Calvin in 1938'',

viz. in the reduction of benzoquinone to quinol with cupric ion and hydrogen, no

industrial applications have been commercialized up to the present. The homoge-neous hydrogenation of olefins is of particular interest for practical use in synthetic organic chemistry. Several catalysts have been developed which excel in properties as selectivity and activity. So it is now possible to reduce selectively a carbon-carbon double bond leaving other functional groups unaffected with chlorotris(triphenyl-phosphine)rhodium at atmospheric hydrogen pressure and room temperature, viz. the reduction of diallylsulfide, w-nitrostyrene (to 2-phenylnitroethane) and 1,4-naph-thoquinone (to l,2,3,4-tetrahydro-l,4-dioxonaphthalene)'^''^. The homogeneous

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platinum-tin catalyst is an useful catalyst for the preparation of c/s-alkenes from ace-tylenes, viz. cw-cinnamic acids from phenylethynecarboxylic acids ^ * ' ' ' .

Our wide-spread interest in homogeneous hydrogenation catalysis has led to the investigations presented in this thesis. The investigations comprise the complex pentacyanocobaltate(n), Co(CN)5~, a rather interesting complex in the areas of both catalysis and co-ordination chemistry. The complex is already known since 1868'*, but still the understanding of this complex can be said to be inadequate. Pentacyano-cobaltate(II), a low-spin 3d^ complex, activates molecular hydrogen in aqueous solution and catalyses the hydrogenation of suitably activated olefins at room tempe-rature and atmospheric hydrogen pressure. The activation of hydrogen takes place according to the reaction:

2 C o ( C N ) r + H 2 ? : i 2 C o ( C N ) 5 H ' - (1.1) The hydrido complex, Co(CN)5H^~, which can be isolated as a stable intermediate,

is rather reactive in homogeneous hydrogenation reactions, transferring the hydrogen to various organic substrates. However, carbon-carbon double bonds are only reduced if this double bond is conjugated with another double bond, a carbonyl or a phenyl group. Small differences in olefin structure may further determine whether reduction actually will occur. For example methacrylic acid is quantitatively reduced whereas acrylic acid is not.

In a study related to this system the homogeneous hydrogenation of a,;6-unsatu-rated carbonyl compounds has been investigated. In the first instance our research was directed to conjugated, substituted cyclohexenes of the type shown in Figure 1.

( V - C R= -OH. -OCHj.-CHj R

Fig. 1. Conjugated cyclohexenes.

The interest in the stereochemistry of cyclohexanes was one of the backgrounds of this investigation. Unfortunately both the carboxylic acid and ester could not be reduced with pentacyanocobaltate(n), whereas l-acetylcyclohexene was reduced very slowly. In order to obtain a better understanding of the factors governing the hydrogenation by means of this catalyst system, next a series of substituted trans- (a) and cw-cinnamic acids (b) has been investigated. These systems have the advantages that both electronic effects (substituents at the ring) and steric effects (or?/!0-substituents) on the homo-geneous hydrogenation can be studied. Moreover the "isolated" double bond offers the opportunity to determine the mechanism by means of isotopic labelling. The cinnamic acids are quantitatively converted into the corresponding 3-phenylpropiomc acids (c). In the present work the kinetics of these hydrogenation reactions have been determined and a detailed mechanism has been elaborated.

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(a) (b) (c)

Fig 2 /ra/ij-Cinnamic acids (a), cw-cmnamic acids (b) and 3-phenylpropionic acids (c)

Accidentally in these investigations, namely in trying to hydrogenate nitro-sub-stituted cinnamic acids, a new class of stable radicals has been discovered By means of ESR techmques a systematic study has been made of these radicals prepared by reaction of aromatic nitro and nitroso compounds with pentacyanocobaltate(II) The structure of the complexes, aryl pentacyanocobalt nitroxides, has been determined by experiments with nitro, corresponding nitroso and dinitro compounds and experi-ments with varying cobalt-substrate and cobalt-cyanide ratios The complexes are characterized by a strong spin delocalization over the transition-metal ion and the aromatic ligand In addition to the proton and nitrogen hyperfine structure the ESR spectra are dominated by a hyperfine structure due to the cobalt nucleus (spin 7/2). Assignments of hyperfine coupling constants have been confirmed by deuterium sub-stitution in the aromatic nucleus Finally, M O calculations by the Extended Huckel method have been performed on these complexes to get a better understanding and a theoretical confirmation of the delocalization of the unpaired electron.

The structure of pentacyanocobaltate(II) and its role in the areas of both co-ordination chemistry and catalysis are considered in detail in Chapter 2 In Chapter 3 the investigations into the homogeneous hydrogenation and deuteration of cis- and ?ra/7i'-cinnamic acids are presented The ESR-investigation of the aryl pentacyano-cobalt nitroxides is described in Chapter 4 Extended Huckel calculations of the phenyl complex are given in Chapter 5 In Chapter 6 a general discussion will be given about the specific properties of the pentacyanocobaltate(II) system as compared to other homogeneous hydrogenation catalysts Part of this work has been published else-w h e r e ' ^ ~ ^ ° or is in the press ^ ' ^^.

LITERATUUR

I / Wender and H W Sternbeig, Advan Catal 9,594(1957)

" / Wender, H W Sternberg, and M Oichm, J Amer Chem Soc 75, 3041 (1953) ' M Orchin,L Kirch, and I Goldfarb,} Amer Chem Soc 78,5450(1956)

* D Evans, J A Osborn,andG Wilkinson,] Chem Soc (A) 1968, 3133

^ D Evans, G Yagupsky, and G Wilkinson, J Chem Soc (A) 1968, 2660 « F E Pauhk, Catal Rev 6, 49 (1972)

' J F Roth, J H Craddock, A Hershman, and F E Paidik, Chem Tech 1971, 600

* J Smidt W Hafner, R Jira, J Sedlmeier, R Sieber, R Ruttingei, and H Kojer, Angew Chem. 71, 176(1959)

» H B Richman, Chem Eng 71, 106 (1964) '» H Hememann Chem Tech 1971, 286

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" Af. Ca/vw, Trans. Faraday Soc. 34, 1181 (1938).

" H. van Bekkum, J. van Gogh, and G. van Minnen-Pathuis, J. Catal. 7, 292 (1967). ^' F. van Rantwijk, C. J. Groenenboom, and H. van Bekkum, to be published. " A.Descamps, Compt. Rend. 67, 330 (1868).

" H. van Bekkum, H. M. A. Buurmans, G. van Minnen-Pathuis, and B. M. Wepster, Reel. Trav. Chim. Pays-Bas 88, 779 (1969).

i« J. Basters, H. van Bekkum, and L. L. van Reijen, Reel. Trav. Chim. Pays-Bas 89, 491 (1970). " / . Basters and P. J. J. M van der Put, J. Magn. Resonance 2, 114 (1970).

2" J. Basters, Reel. Trav. Chim .Pays-Bas 91, 50 (1972).

^' J. Basters, H. van Bekkum, C. J. Groenenboom, and L. L. van Reijen, Reel. Trav. Chim. Pays-Bas, in the press.

^^ / . Basters and C. J. Groenenboom, paper presented at the First lUPAC-conference on Physical Organic Chemistry, Crans sur Sierre, Switzerland, 1972.

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2 Pentacyanocobaltate(II), Co(CN)5 ; structure and reactivity

2.1 Introduction

Pentacyanocobaltate(II), a low spin 3d^ cobalt complex, is one of the typical and interesting complexes in the areas of both co-ordination chemistry and catalysis. The complex has been shown to be rather reactive and undergoes a bewildering number of reactions'. In particular the homogeneous activation of hydrogen and the related catalytic hydrogenation of conjugated olefins have been the subject of several studies ^ ~ '^.

The chemistry of cobaltous cyanide solutions dates back to the last century. Already in 1868 Descamps reported that aqueous solutions of cobaltous salts evolved hydrogen slowly in the presence of cyanide'. Not until 1942 Iguchi showed that the hydrogen evolution occurred at a ratio of 4 to 5 of cyanide to cobalt and not at 6 or more. Moreover, he discovered that these cobaltous cyanide solutions absorb molecular hydrogen when exposed to a hydrogen atmosphere®. Increased quantities of hydrogen were absorbed in the presence of sodium cinnamate or isatin'. Since then, a lot of investigations have been made not only dealing with the nature of the active species, but also with the products of its hydrogenation and the catalytic reactions of this complex. De Vries was first to report on the catalytic activity of the complex in the selective hydrogenation of sorbic acid to 2-hexenoic acid^. Several excellent reviews appeared in this field of homogeneous catalysis ^' ^.

2.2 Structure of the pentacyanocobaltate(II) complex

On mixing an aqueous solution of a cobalt(II) salt with potassium cyanide in an inert atmosphere, a brownish precipitate of cobaltous cyanide, Co(CN)2, is formed. This precipitate redissolves on further addition of cyanide. At the point of five cyanides per cobalt it results in a clear, greenish solution which contains penta-cyanocobaltate(II).

Up to mid 1967, when the present investigation was started, the number and the nature of the complexes present in cobaltous cyanide solutions (CN/Co > 5) were still insufficiently known. The main complex undoubtedly contains five cyanide ligands per cobalt. Already in 1949 Hume and A^o/f/;o/f pointed out that the com-pound believed to be K4Co(CN)6 has only five cyanide ligands, with possibly a water molecule in the sixth position'". Adamson had isolated a violet crystalline material from the greenish cobaltous cyanide solutions (CN/Co ^ 5) by addition of alcohol

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and by cooling in a nitrogen atmosphere'^ Although the solid was not quite pure it was shown to have the empirical formula K3Co(CN)5. The solid was diamagnetic, whereas in aqueous solutions, in an inert atmosphere, a magnetic moment of 1.72 BM was found, corresponding to one unpaired electron per cobalt. In consequence the violet crystalline material was considered to be a dimer complex K6Co2(CN)io. Mills and coworkers have found the same magnetic moment for freshly prepared solutions of this complex, prepared by mixing appropriate solutions of cobaltous chloride and potassium cyanide'^. The paramagnetism of the cobalt(II) solutions decreases with time. This reduction in paramagnetism is accompanied by a loss in capacity for hydrogen absorption. The "ageing" process occurs very slowly in dilute solutions, but quite rapidly at concentrations ^ 0.1 M. De Vries explained this ageing by a dis-proportionation according to reaction (2.1)^.

2Co(CN)^- + H20?iCo(CN)5H^"-t-Co(CN)50H^- (2.1)

King and Winfield have confirmed this with UV spectroscopy'^.

Pure binuclear cobalt(II) compounds were isolated by Nasi and coworkers from aqueous methanolic solutions'*. The compounds were characterized by oxidimetric titration with ferricyanide, Fe(CN)6~, and by means of their IR spectra. The IR spectra indicated a metal-metal bonding in the dimeric compounds. The compounds redissolved in water to give solutions of pentacyanocobaltate(II).

Pratt and Williams have investigated the complexes present in freshly prepared

solutions containing cobalt(II) and cyanide ions by means of UV spectrometry'^. The main complex was identified as Co(CN)5H20^~ by comparison of its spectrum with those of the analogous isocyanide complexes. No evidence was found for the hexacyanide Co(CN)6".

2.2.1 ESR investigation

As the configuration of pentacyanocobaltate(II) in solution was still unknown at the start of this investigation, an ESR investigation was undertaken. On the basis of observed g values an indication about the geometry might be found.

ESR spectra of aqueous solutions of pentacyanocobaltate(II), prepared by mixing appropriate solutions of cobaltous chloride and potassium cyanide under a nitrogen

3050S 3250G

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atmosphere, showed a rather broad resonance at g = 2.12 without any hyperfine structure at 298°K (see Figure I). The same solutions frozen at 133°K gave spectra with some hyperfine structure (see Figure 2). In order to obtain good resolution the pentacyanocobaltate(II) monomers must be dilutely dispersed through the medium, otherwise the strong dipolar coupling between the electron spins of the radicals causes

3000G 3200G

JUUU U JZUUO

Fig. 2. ESR spectrum of pentacyanocobaltate(II) in a frozen aqueous solution at 133 "K. a marked line-broadening. Therefore the same spectra were also run in ethylene glycol-water (2:1) frozen at 76 °K. This mixture forms a good glass and the Co(CN)5 ~ does not dimerise under these conditions. In such a rigid solution the radicals are randomly oriented and the spectrum obtained is an envelope of all the possible spectra from a single crystal. Fortunately, only specific features appear which can often be related to the principle values of the g and hyperfine tensors. The spectra now clearly showed a double 8-line resonance (see Figure 3) which reflects a cobalt hyperfine structure (spin 7/2). Analysis of the spectra gave ^-ji = 1.99, g^ = 2.16, |y4||| = 88 G and MJ.1 = 28 G.

Fig. 3. ESR spectrum of pentacyanocobaltate(U) in a frozen ethylene glycol-water (2:1) solution at 76 °K.

For the pentaco-ordination two configurations are most likely, a trigonal pyramidal structure with D^,, symmetry and a square pyramidal structure with C^.^ symmetry.

Griffith has described the behaviour of the components of the g and A tensors for

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cobalt 3d orbitals '*. As a result it is concluded that the ESR data point to a square pyramidal structure for pentacyanocobaltate(II) in solution. The ground state should be (xz,yz)*(xy)^(3z^ —r^) = •^A, with the unpaired electron placed in an orbital with ^3^2-^2 symmetry. The g tensor calculated to first order by Griffith gives for the elec-tron in |0> ( = | 3 z ^ - r ^ » , g,, = 2.0 and g^_ = 2.0 + 6(C/Ei), where Ej refers to the energy separation (|1>, | — 1 » to |0> in the partial d level diagram (see Figure 4). In the case of D^i, symmetry, a trigonal bipyramid, g|| would be > 2 .

— b, (x2-y2)

H- a, (3z2-r2)

-H- bj (xy)

-tf -ff e (xz.yz)

Fig. 4. Pentacyanocobaltate(ll) in a square pyramidal structure with Civ symmetry and related qualitative crystal-field model.

Pending the experiments similar results were independently reported by Alexander and Gray^''. They found g,, = 1.992, gj_ = 2.157, M||| = 87 G and \A^\ = 28 G, in excellent agreement with our results. Moreover the same authors concluded that their combined ESR and optical spectra data require a square pyramidal structure for the cobalt complex.

Caullon confirmed the conclusions with an analysis of the ligand-field spectrum of

pentacyanocobaltate(II)'^. After that time the molecular and electronic structure of pentacyanocobaltate(II) has still been the subject of several paramagnetic resonance and optical spectral studies. Maher reported slightly different g values, the ESR spectra of aqueous and methanolic solutions and glasses taken at X- and Q-band indicated g\^ is slightly > 2 . 0 0 2 3 " . Tsay and coworkers have concluded that the structure of pentacyanocobaltate(II) in solutions and in polycrystalline media is a slightly distorted square pyramid with no solvent bound at the sixth co-ordination site^". According to Kataoka and Kon the absence of isotropic cobalt hyperfine structure in the ESR spectra of various low-spin cobalt(II) complexes in solution can be explained by considerable mixing of the metal 4^ orbital into the orbital of the un-paired electron (in C^„ symmetry they both belong to the totally symmetric represen-tation)^'-^^. The small amount of 5 character causes a relatively large positive con-tribution to y4iso which will override the negative component due to spin polarization. The normal spin polarization mechanism is known to give a coupling of ± — 90 G for an unpaired electron confined to the d levels ^^. The small <^> indicates that A^, and Ai_ are of opposite sign. In an ESR study of the pentacyanocobaltate(II) ion in various host lattices by Symons and Wilkinson it was concluded that ^n is positive and A^ negative^*.

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2.2.2 Theoretical consideration

The background of the pentaco-ordination and the square pyramidal structure, is rather easy to understand. The regular octahedral configuration with three / j ^ orbitals is the most favourable for accomodation of six d electrons. Additional electrons are forced to occupy strongly antibonding Cg orbitals. Generally it results in a distortion of the octahedron (Jahn-Teller distortion). The formation of pentacyanocobaltate(II) from the stable d^ complex, Co(CN)6 ~, when an electron is added, can be understood in this way. By a loss of a cyanide ligand the complex now gains in energy (see Scheme (2.2)).

Co\ut ^ ^ Co\Ht Co°(CNf + CN'

OCTAHEDRAL SQUARE PYRAMIDAL

x2-y2 3z-r2 y2^y2 j^lr^ xy xz yz xy xz yz (2.2) -4- 3z2-r2 •jt xy -ff ^ f ^ ^ 4 f 4^ . ^ \ xz,yz

A similar situation is found in organic chemistry if an electron is added to a satu-rated carbon compound CX4, e.g. CCI4. The stable bonding orbitals are filled, an extra electron is forced to go into a strongly antibonding orbital. The net result is that the co-ordination number 4 is destabilized and a species of lower co-ordination number is generated, a free radical or a carbanion depending on the relative electron affinities of X and CX3.

cx^ - ^ ^ - e x ; > -0X3 * X- (2.3)

It is thus not surprising that the chemical reactivities of pentaco-ordinated d'' com-plexes resemble those of typical organic radicals. Upon reacting a complex with the original stable hexaco-ordination is formed. For pentacyanocobaltate(II) several reactions are observed which fully endorse this free radical character.

2 C o ( C N ) r + H 2 ^ 2 C o ( C N ) 5 H ' - (2.4) H2O2 ^ 2 C o ( C N ) 5 0 H ^ - (2.5) ICN ^ C o ( C N ) 5 l 3 - + C o ( C N ) ^ - (2.6) RX ^Co(CN)5X3-+Co(CN)5R^- (2.7) O2 ^ [(NC)5CoOOCo(CN)5]«- (2.8) H C s C H ^ [(NC)5CoHC = CHCo(CN)5]^- (2.9)

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The reactions are interpreted in terms of free radical mechanisms of the type

C o ( C N ) r + X - Y -^Co(CN)5X^"-l-Y- (2.10) Co(CN)r+Y- ^Co(CN)5Y^- (2.11) 2Co(CN)?~+X-Y^ Co(CN)5X2' + Co(CN)5Y'" (2.12)

For other low-spin cobalt(II) complexes, e.g. pyridinatobis(dimethylglyoximato)-cobalt(II), Co(DMG)2pyr, similar reactions with organic halides are observed •^^. It is clear that the reductive cleavage of covalent single bonds is of particular importance for catalytic applications.

2.3 Reaction of pentacyanocobaltate(II) with hydrogen

As already has been mentioned in the introduction Iguchi was first to report on the absorption of molecular hydrogen by an aqueous solution of a cobalt(II) salt and excess of cyanide ions*. In the presence of sodium cinnamate increased quantities of hydrogen were absorbed; however, no identification of products was carried out'. Ever since this was first described investigators have been engaged on the problem of determining the products of this hydrogenation and the possible activity of these solutions.

The reaction of cobalt(ll) cyanide solutions with hydrogen appears to be one of the fastest reactions between hydrogen and a transition-metal complex known hitherto. Under favourable conditions the aqueous solutions absorb an amount of hydrogen which corresponds closely to an overall reaction between two cobalt com-plexes and one hydrogen molecule. Iguchi obtained an uptake of 0.8 hydrogen atom per atom of cobalt. Mills and coworkers observed values near 0.95 in 1 M NaOH at 0°C and about 0.8 at 25 ° C ' ^

2Co(CN)^~-l-H2^2Co(CN)5H^" (2.13) The hydrogen uptake resulted in a decrease in paramagnetism and led to the

dis-appearance of the ESR signal. The hydrogenated solutions catalysed the D2-H2O exchange. Griffith and Wilkinson have demonstrated by PMR measurements that the complex formed by hydrogenation or reaction with water does contain a hydrogen atom^*. King and Winfield have described a thorough study of the liquid phase hydro-genation of pentacyanocobaltate(II)' ^. The reaction was considered to be a homolytic cleavage of hydrogen by pentacyanocobaltate(II) to produce a monomeric hydride with the cobalt atom formally written as Co(III). This homolytic cleavage agrees with the radical character of pentacyanocobaltate(II), described in paragraph 2.2.2. The same hydrido complex could also be obtained from reduction of

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pentacyano-cobaltate(II) electrolytically or by means of reduction with sodium borohydride. Moreover they have confirmed the observation by De Vries with spectrophotometric analyses that this hydride is formed in the ageing of pentacyanocobaltate(II) in aqueous solutions. This ageing was proved to be a reaction with water to the hydrido and hydroxo complex, as suggested by De Vries (see reaction (2.1))^.

From De Vries' kinetic investigations of the hydrogen absorption it appeared that the rate-determining step is of second order with respect to cobalt^'. For the de-composition of pentacyanocobaltate(II) under the influence of water the same was found. The reaction between hydrogen and pentacyanocobaltate(II) was considered to result in an equilibrium (ratio Co(CN)5H^~/Co(CN)5~ + 6 at 25°C), the position of which is independent of the initial total cobalt concentration. The observed devia-tions from the stoichiometric amount of hydrogen can be partially ascribed to the position of this equilibrium. At high pentacyanocobaltate(II) concentrations (>0.1) the simultaneously occurring decomposition will also take place. But it constitutes only a small fraction of the total amount of pentacyanohydridocobaltate(IIl) present at equilibrium. De Vries suggested that the dimer complex, [Co2(CN)io]*~, is the ion which reacts with hydrogen according to

2Co(CN)r ^Co2(CN)to" (2.14) Co2(CN)to+H2 ^2Co(CN)5H^- (2.15) Co2(CN)*o+H2O ?^Co(CN)5H'--^Co(CN)50H'" (2.16)

In accordance with this view King and Winfield have shown the presence of small amounts of the binuclear complex in methanolic solutions by means of UV spectro-scopy'^. However, these investigators were unable to find any relationship between the concentration of the binuclear complex and the rate of hydrogen absorption.

Burnett and coworkers have found spectroscopic evidence that the product of the

reaction of pentacyanocobaltate(Il) and hydrogen is the monomeric pentacyano-hydridocobaltate(III)^^. Kinetic and equilibrium studies confirmed their results. They obtained PMR spectra of the reduced compound identical with those recorded by

Griffith and Wilkinson^^. The spectra showed a fast exchange of solvent and hydride

protons. The PMR results also confirmed that the exchange in reaction (2.18) is fast as compared with (2.17) and (2.19) and agrees with the results of Schindewolf in the deuterium exchange reactions catalysed by pentacyanocobaltate(II)'^'.

2Co(CN)5"+D2 ?^2Co(CN)5D^" (2.17) C o ( C N ) 5 D ^ " + H 2 0 ^ C o ( C N ) 5 H ^ " + H D O (2.18) 2Co(CN)5H^" ^ 2 C o ( C N ) 5 " + H2 (2.19)

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H20

The rate-determining step of the isotope exchange (D2 -• HD-I-H2), and

ortho-para hydrogen conversion, was found to be the recombination of hydrogen atoms

in reaction (2.19).

Banks and Pratt isolated Cs2NaCo(CN)5H from hydrogenated aqueous solutions

of pentacyanocobaltate(II)^°. Identical UV spectra were reported for the complex obtained by dissolving the solid in water or by reducing pentacyanocobaltate(II) in solution. The same ion was produced by reduction with sodium borohydride or sodium amalgam. IR spectra of the solid showed bands which were attributed to the hydride.

Concerning the identity of the active species in solution there is now complete evidence that by reduction of pentacyanocobaltate(II) with hydrogen pentacyano-hydridocobaltate(III) is formed. Besides pentacyanopentacyano-hydridocobaltate(III) very low concentrations of other active species may be present in solution. On account of polarographic measurements Hanzlik and Vlcek suggest that at pH > 10 Co(CN)5~ is formed according to reaction (2.20)•"•^^.

Co(CN)5H^"-|-OH" ? i H 2 0 - l - C o ( C N ) r (2.20) It has also been suggested that at low cyanide concentrations (CN/Co < 5)

Co(CN)4H2~ is formed, according to the reactions (2.21) and (2.22).

Co(CN)5H^" ^ H " ' + C N " - | - C o ( C N ) r (2.21)

H2 + C o ( C N ) r ? i C o ( C N ) 4 H r (2.22)

King and Winfield considered the cobalt atom in pentacyanohydridocobaltate(lll)

to be formally in the + 3 oxidation state' ^. Since pentacyanocobaltate(II) is a stronger reducing agent than hydrogen^^ it will reduce hydrogen to hydride ion while it itself is being oxidized to Co(III). According to Halpern the magnetic and spectroscopic properties of pentacyanohydridocobaltate(III) and the Co-H bond-dissociation energy of about 58 kcal/mole are clearly in favour of this oxidation state. Moreover, this oxidation state is thought to be preferentially stabilized by the cyanide ligands^*.

2.4 Homogeneous catalytic hydrogenation by pentacyanocobaltate(n)

In 1960 De Vries reported on the catalytic activity of pentacyanocobaltate(II) in the selective hydrogenation of sorbic acid to 2-hexenoic acid*. The reaction between sorbic acid and pentacyanohydridocobaltate(III) was found to be first order in both reactants. Since De Vries' experiments pentacyanocobaltate(II) has been extensively investigated as a homogeneous catalyst. Kwiatek and coworkers investigated the homogeneous hydrogenation of a variety of substrates by pentacyanocobaltate(II) at room temperature and atmospheric hydrogen pressure'^•'^. From this it appeared

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that a carbon-carbon double bond will be reduced only if this double bond is conju-gated with either another double bond, a carbonyl or a phenyl group. Small differen-ces in olefin structure control the rate of reduction. Methacrylic acid is quantitatively hydrogenated to yield isobutyric acid, but methacroleine is not reduced. On the other hand, crotonaldehyde is partially reduced and crotonic acid is not. It should be re-marked that in several cases stable Co(CN)5H^"-olefin adducts are isolated which consequently are not further reduced.

As there are several excellent reviews which deal with the hydrogenation catalysed by aqueous solutions of pentacyanocobaltate(II) in detail only relevant information will be summarized''^'*'^. Kwiatek, et al. studied the hydrogenation of butadiene by pentacyanocobaltate(II) in detail^'*. They proposed the following mechanism:

H2-l-2Co(CN)r C 4 H 6 - F C O ( C N ) 5 H ' " Co(CN)5(C4H7)' - + C O ( C N ) 5 H ^ ~ = 2 C O ( C N ) 5 H ^ " ^ C O ( C N ) 5 ( C 4 H , ) ^ -^2Co(CN)r + C4H8 (2.23) (2.24) (2.25)

The reversibility of reaction (2.24) has been demonstrated by the formation of mono-, di-, and trideuterated 1-butene in the deuteration of butadiene. For the structure of the intermediate complex three types have been proposed, two c7-complexes and one 7r-allyl type.

H^C-CH = C H - C H , I C O { C N L 3- _{HC = C H - C H - C H ,} ' I 3 Co(CN) 5 -I H^C; ;CH "Co(CN) ,CH. (2.26) Tl

Dependent on the cyanide concentrations in solution various products were obtained.

Kwiatek and Seyler considered that at a high CN/Co ratio (>5.5) 1-butene is formed

from the c7i-complex involving a y-attack by the second hydride. At a low CN/Co ratio (< 5.5) fran5-2-butene is formed from the Ti-allyl complex*. The reaction of allyl halides with pentacyanocobaltate(II) provided more direct evidence for a CN-dependent equilibrium between a- and Tt-allyl intermediates. Burnett and coworkers, however, proposed 1,2-addition via a CTj-complex, followed by a-attack for the pro-duction of 1-butene^ ^. Their detailed kinetic study showed that the two u-butenyl complexes and the ;i-allyl complexes are in equilibrium. Recently Funabiki and

7a-rama studied the hydrogenation of butadiene in aqueous and non-aqueous solvents^*.

A clear solvent effect was observed. Moreover they found PMR evidence for the formation of cis- and fra«i-o--but-2-enyl and 5>'n-;i-(l-methylallyl) complexes in this hydrogenation^'.

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acid. Simandi and Nagy suggested a radical mechanism^*'^^. Their suggestion had its origin in Takahashi's observation which showed that solutions of pentacyano-cobaltate(II) saturated with hydrogen initiate radical polymerization'**'. The signi-ficance of this mechanism is that no organocobalt complex is formed as intermediate.

Co(CN)5H'"-l-S ^ C o ( C N ) r + HS- (2.27) Co(CN)5H^ - -F HS- -> Co(CN)? ~ -h H2S (2.28) A deuteration study has shown that the formation of the radical must be reversible.

The question arises whether this mechanism is applicable to the homogeneous hydrogenation of a,jS-unsaturated compounds. Strohmeier and Iglauer reported that styrene is reduced by the same reaction mechanism*'. The reaction was found to be first order in styrene. Jackman and coworkers studied the hydrogenation of a variety of a,j?-unsaturated acids*^. The reactions were followed in situ by PMR. It was found that carboxylate anions lacking an a-substituent form tr-complexes, whereas those with a-substituents were readily reduced. Their results should also lend support to the radical mechanism suggested by Simandi and Nagy according to the scheme (2.29).

I I H C - C H " I I (2.29) " 1 1 3-HC-C-CO(CN:^

In this scheme the olefin may be hydrogenated, or isomerised, or a stable organoco-balt(III) complex may be formed. It was concluded from experiments with deuterium that the complex formation involves a stereospecific c«-addition, whereas the isomeri-zation and reduction are not stereospecific. Halpern and Lai-Yoong Wong studied the kinetics of the addition of pentacyanocobaltate(II) to some a,^-unsaturated com-pounds*^. Each reaction was found to exhibit first-order kinetics in both reactants in accord with the rate-law, - d [ S ] / d t = k- [Co(CN)5H^~] [H2C = C(R)X]. They also suggest a radical mechanism.

Summing up, little is known about the kinetics of the homogeneous hydrogenation of a,j?-unsaturated carbonyl compounds with pentacyanocobaltate(II), the mechanism of the hydrogenation and the influence of electronic effects and steric effects on this hydrogenation. Consequently an investigation into the homogeneous hydrogenation of a,)?-unsaturated carbonyl compounds, viz. cis- and ?ra/j5-cinnamic acids, was started.

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LITERATURE

1 J Kwiatek, Catal Rev 1, 37 (1967)

2 J Kwiatek, I L Madoi, and J K Seyler, Advan Chem Ser 37, 201 (1963) = J Halpern, Advan Chem Ser 70, 1 (1968)

* J Kwiatek and J K Seyler, Advan Chem Ser 70,207(1968) ' A Descamps, Compt Rend 67,330(1868)

« M Iguchi,} Chem Soc Jap 63, 634(1942), C A 41,2975d (1947) ' M Iguchi,} Chem Soc Jap 63, 1752 (1942), C A 41, 3353c (1947) « B de Vries, Kon Ned Akad Wetensch Proc Ser 8 63,443(1960) « E N Frankeland H J Button, Topics in lipid chemistry 1,171 (1969)

^0 P N Hume and / M Kolthoff, J Amer Chem Soc 71, 867 (1949)

" A W Adammn,] Amer Chem Soc 73,5710(1951)

" G ^ Mills, S Weller, and A Wheeler,] Phys Chem 63,403(1959)

^^ N K King and M E Winfield, J Amer Chem Soc 83, 3366 (1961)

" R Nast, H Ruppert-Mesche, and M Helbig-Neuhauer, Z Anorg Allg Chem 312, 314 (1961)

^^ J M Piatt and R J P Williams, J Chem Soc (A) 1967, 1291

'« / 5 Gnjfith Discuss Faraday Soc 26, 81 (1958)

" J J Alexander and H B Gray, J Amer Chem Soc 89, 3356 (1967) ^' K G Caulton Inorg Chem 7, 392(1968)

'» / P Mahei, J. Chem Soc (A) 1968, 2918

^^ Fun-Dow Tsav, H B Giay, and J Danon, J Chem Phys 54, 3760 (1971)

^' H Kon and N Kataoka in „Electron Spin Resonance of Metal Complexes", ed Ten Fu Yen, Hilger, London, 1969, p 59

'^ N Kataoka and H Kon, J Phys Chem 73, 803 (1969) " 5 i? McGarvey, J Phys Chem 71, 51 (1967)

^* M C R Symons and J G Wilkinson, J Chem Soc (A) 1971, 2069

" /> ff Schneider, P F Phelan, and / Halpern, J Amer Chem Soc 91, 77 (1969)

^^ W P Griffith and C Wilkinson, J Chem Soc 1959, 2757

" B de Vries, ] Catal 1, 480 (1962)

^' M G Burnett, P J Connolly, and C Kemhall, J Chem Soc A 1967, 800.

^' U Schindewolf, Ber Bunsenges Phys Chem 67,219(1963)

'" R G S Banks and J M Pratt, J Chem Soc (A) 1968, 854.

' ' J Hanzhk and A A Vlcek, Chem Commun 1969, 47 3^ J Hanzhk and A A Vlcek, Inorg Chem 8, 669 (1969) 5' G Grube,Z Electrochem 32,561 (1926)

'* J Halpern, Chem Eng News 44, 68 (1966)

'^ M G Buinett, P J Connolly, and C Kemball J Chem Soc (A) 1968, 991

"« T Funahiki and K Tarama, Bull Chem Soc Jap 44, 945 (1971) " T Funabiki and K Tarama, Chem Commun 1971, 1174

" L Simandi and F Nagy, Proc Symp Coord Chem , Tihany, Hung , 1964, p 83 " L Simandi and F Nagy, Acta Chim (Budapest) 46, 137 (1965)

" M Takahashi Bull Chem Soc Jap 36,622(1963) " W Strohmeier and A' Iglauer, Z Phys Chem 51, 50 (1966)

*^ L M Jackman, J A Hamilton and J M Lawloi,} Amer Chem Soc 90, 1914 (1968)

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3 Homogeneous hydrogenation of substituted cis- and

trans-cinnamic acids with pentacyanocobaltate(II)

3.1 Introduction

In this Chapter the homogeneous hydrogenation of a series of substituted trans- and m-cinnamic acids {trans- and c/5-3-phenylpropenoic acids) with aqueous solutions of pentacyanocobaltate(II) at 25 °C and under atmospheric hydrogen pressure will be described. The investigation was undertaken to determine the kinetics of the hydro-genation of these a,^-unsaturated carbonyl compounds, to elucidate the mechanism and to study the effects of substituents at the phenyl ring on kinetics and mechanism. The c/i-compounds are rather interesting in that the conjugation of the double bond and the aromatic nucleus is sterically hindered (see Figure 1). An interplanar angle of 46° between the benzene ring and the ethylenic bond in c/.y-cinnainic acids was re-ported on account of UV absorption spectroscopy by Sandris^.

Fig. 1. Van der Waals radii in trans- (A) and ci.y-cinnamic acid (B).

In this work, the kinetics of the hydrogenation reactions have been determined. Electronic effects (substituents at the ring) and steric effects (or//io-substituents) on the homogeneous hydrogenations have been recorded^. Observed isomerizations in the hydrogenations of the c/5-compounds offered the opportunity to elucidate the mechanism in detail by means of isotopic labelling^"*. Consequently several experi-ments have been performed with deuterium.

3.2 Kinetics of the reduction of substituted cis- and frans-cinnamic acids 3.2.1 trans-Cinnamic acids

When trans-cinnamic acids were added to aqueous solutions of pentacyanohydrido-cobaltate(III), one equivalent of hydrogen was absorbed. By PMR analyses of the reaction products it was demonstrated that the trans-cinnamic acids were quantita-tively converted into the corresponding 3-phenylpropionic acids.

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H

From the slope of the plot of the hydrogen uptake (corrected to 760 mm Hg) against time, pseudo first-order reaction constants were calculated (see Figure 2). The amount of hydrogenated cinnamic acid present at any given time can be calculated as the sum of the amount of absorbed hydrogen and the amount of non-hydrogenated penta-cyanocobaltate(II) which had formed back at that moment. In the hydrogenations of the substituted trans-annamic acids the rate of hydrogen uptake was rather small as compared with the rate of hydrogenation of pentacyanocobaltate(n). Therefore, the concentration of pentacyanohydridocobaltate(III) can be put equal to the equilib-rium concentration of the hydride in reaction (3.2) under the same conditions.

2 C o ( C N ) r + H 2 ^ 2 C o ( C N ) 5 H ' - (3.2) Consequently, the amount of hydrogenated cinnamic acid equals the amount of

absorbed hydrogen From experiments with varying concentrations of pentacyano-hydridocobaltate(III) the reaction was found to be first order in pentacyanohydrido-cobaltate(III). As an example the data for trans-cmnamxc acid are given m Table I.

51 i 9 i7 45 60 to 20 " 60 120 180 2iO 300

Fig. 2. The absorbed amounts of hydrogen (ml) and related ln(S)t as function of the time (min ) in the hydrogenation of trans-cmnamtc acid, 6 0 mmoles, with aqueous pentacyanocobaltate-(II), 2 0 ( 0 ) and 4.0 mmoles ( x ) respectively, at 25 °C and atmospheric hydrogen pressure.

In the experiments with rra/;5-cinnamic acid as well as with 2-chloro-rran5-cinnamic acid a zero-order in cyanide was found A changing initial ratio CN/Co from 5:1 to 15:1 did not affect the rate of hydrogenation within experimental error.

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Table I Reactions of trans-cmnamic acid with pentacyanohydridocobaltate(lll) " cinnamic acid (10-»mole 1-1) 13.3 26.7 40 0 53 3 66.7 80 0 93 3 106 7 80 0 80 0 80 0 80 0 80 0 80 0 80 0 80 0 Co(CN)5H'-(io-» 54.3 54.3 54.3 54.3 54.3 54.3 54.3 54.3 13.0 24.7 37.0 48.6 54.3 58.9 69.8 79.6 mole 1 ') K pseudo first-order (10 ' 1.58 1.58 1.60 1.59 1.58 1.52 1.59 1.54 0 39 0.73 1.12 1.37 1.52 1.68 2 01 2 27 imn" _-') k = K/Co(CN)5H»-(10-M mole-i min-») 2 91 2 91 2.94 2 93 2 91 2.81 2.95 2.85 3 0 0 2 94 3.02 2 82 2 81 2 86 2 88 2 85

" Temperature 25 0 C, atmospheric hydrogen pressure; solvent water (1 MKOH); CN/Co = 6; ionic strength was adjusted by adding KCl

The rate law found is in agreement with the findings of De Vries regarding the hydrogenation of sorbic acid*.

d[S]

dt K • [S] = k • [Co(CN)5 H^" ] • [S] (3 3)

Strohmeier and Iglauer found a similar pseudo first-order reaction in the

hydrogena-tion of styrene^.

Values of the second-order rate constants k for various substituted ?ran5-cinnamic acids are presented in Table II. The data show that the hydrogenation is accelerated by electron-withdrawal and retarded by electron-donation by substituents. Ortho substituents accelerate the reaction considerably, apparently apart from an electronic effect a steric effect is involved.

Table II Reaction rate constants (1 mole ^ min-^) for the homogeneous hydrogenation of sub-stituted trans-cmnan\ic acids with pentacyanocobaltate(II) "

substituent 2-chloro 4-chloro 2-methyl 2-methoxy 3-chloro k 10^ 10 1 5.3 4.4 4.2 4 0 substituent 3-methoxy H 3-methyl 4-methyl 4-methoxy k 10^ 2 9 2.9 2 3 2.1 1 6 subsUtuent 4-dimethylamino a-methyl 4-nitro 3-nitro k 10^ 1.4

-Temperature 25 0 °C; atmospheric hydrogen pressure; solvent water (1 MKOH), substrate con-centration. 0 05-0 22 M, initial Co(CN)6H^- concentration 0 03-0 07 Af (CN/Co = 6); ionic strength was adjusted by adding KCl.

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The nitro-substituted ?/-art.5-cinnamic acids and a-methyl-fra«j-cinnamic acid are not reduced by this catalyst system; 2,6-dichloro-?/-a/w-cinnamic acid is reduced rather slowly.

3.2.2 cis-Cinnamic acids

The c/j-cinnamic acids also absorb one equivalent of hydrogen when added to aqueous solutions of pentacyanohydridocobaltate(III). However, in the hydrogenation of the m-cinnamic acids partial isomerization to the corresponding trans-isomers is ob-served. Both the cis- and /ra«.y-isomer are converted into the corresponding 3-phenyl-propionic acid. As an example the course of the hydrogenation of 2-chloro-m-cin-namic acid is given in Figure 3. Samples of the reaction mixture were analysed by PMR and GLC after esterification. The trans-isomer is formed to a maximum con-centration of 11%. Similar results were obtained for c/5-cinnamic acid and the other substituted cw-cinnamic acids.

100 90 80 70 60 50 1,0 30 20 10 60 120 180 2A0 300 360 420

Fig. 3. Relative concentrations (%) as function of the time (min.) in the hydrogenation of 2-chloro-c/i-cinnamic acid with aqueous pentacyanocobaltate(ll) at 25 °C and atmospheric hydrogen pressure. (2-chloro-e/>cinnamic acid, x ; 2-chloro-/ra«.5-cinnamic acid, O; 3-(2-chloro-phenyl)-propionic acid. A).

Due to the observed isomerization it is not possible to describe the kinetics of the hydrogenation of the substituted d5-cinnamic acids in the same way like that of the ?ra/75-cinnamic acids. Plots of log [cis], against time showed a linear dependence. The hydrogenations of the substituted /ra77.s-compounds were already found to be first order in substrate as well as in pentacyanohydridocobaltate(III). So it is not unre-ahstic to use the triangular reaction scheme (3.4) in which the three overall reactions are described as first-order reactions.

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, ^ t a CO(CN)^7H^ Q ^ i

" . - C O O H

(3.4)

-CH-CH^-COOH

According to this triangular reaction scheme the following differential rate equa-tions will apply

d [cis] dt d [trans]

dt

= - ( k i + k3)[Co(CN)3H^-][cis] (3.5)

= kj [Co(CN)5H^-] [cis] - k a [Co(CN)5H^"] [trans] (3.6)

^ ^^P*""^^ = kz [Co(CN)5H^"] [trans]+ ki [Co(CN)5H^"] [cis] (3.7)

In the hydrogenation of the substituted c«-cinnamic acids the rate of hydrogen up-take was small as compared with the rate of hydrogenation of pentacyanocobaltate(II). Therefore also in this case the concentration of pentacyanohydridocobaltate(III) can be put equal to the equilibrium concentration of the hydride in reaction (3.2) under the same conditions. By means of integration and subsequent substitution three equations are obtained for [cis], [trans] and [prod]. Computer calculation of the three rate constants via the integrated equations from the conversion data obtained by PMR and GLC appeared not to lead to significant results. The sum of the squares of the residuals appeared to be rather insensitive to substantial changes in reaction constants. This is mainly caused by inaccuracies in the measurements of [trans]. As separately measured rate constants kj were also available, they were introduced independently. Now reliable values for k^ and k^ were obtained. The results thus obtained are in concord with rate constants evaluated from the maximum in the curve [trans], against time (kj/kj) and from the direction of the curves [cis] „ [trans], and [prod], in the origin (kj H-kj, kj, k,). For the reaction rate constants see Table III.

The data in Table III show that the rate of conversion of 3- and 4-substituted cis-cinnamic acids (k^-l-kj) roughly equals the rate of hydrogenation of the correspon-ding ?ra«5-compounds. Electronic effects of the substituents on the rate of hydrogena-tion are comparable with the effects observed in the trans-senes. However, in ortho-substituted cw-cinnamic acids no acceleration due to a steric effect is observed, in contrast with orf/jo-substituted ?ra«5-cinnamic acids.

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Table III. Reaction rate constants (Imole-^min-^) for the homogeneous hydrogenation of sub-stituted ci-s-cinnamic acids with pentacyanocobaltate(II) "

substituent kjIO- kjlO^ ka-lO^' 2-chloro 3.9 1.7 10.1 3-chloro 2.8 2.1 4.0 4-chloro 3.4 2.0 5.3 2-methyl 1.7 0.4 4.4 3-methyl 2.0 0.6 2.3 4-methyl 1.6 1.1 2.1 3-methoxy 1.8 1.0 2.9 H 1.7 1.2 2.9 •^ Temperature 25.0°C; atmospheric hydrogen pressure; solvent water (1 MKOH); substrate

con-centration: 0.05-0.15 M; initial Co(CN)5H=- concon-centration: 0.03-0.07 Af (CN/Co = 6); ionic strength was adjusted by adding KCl,

* ka from Table II.

initial ratio CN/Co did not affect the rate of hydrogenation within experimental error.

3.3 Experiments with deuterium; isotopic effects

In order to obtain information regarding the mechanism of the hydrogenation of the cis- and /m«.s-cinnamic acids, experiments have been performed with deuterium using deuterium oxide as the solvent. The deuterations were followed in time, samples of the reaction mixtures were analysed by PMR (100 MHz) and GLC. The location of deuterium in the isomerization products in the deuteration of m-cinnamic acid should allow a decision as to the point of initial addition of deuterium to the double bond in cw-cinnamic acid. The following results have been obtained:

a. Mass-analysis of the reaction mixture of an experiment with deuterium and

trans-cinnamic acid in water showed no deuterium at all in the saturated compound.

This establishes that no conversion with molecular deuterium takes place but that both hydrogens are added to the double bond via pentacyanohydridocobaltate-(III). On account of the rapid exchange

C O ( C N ) 5 D 3 - + H 2 0 -^ Co(CN)5H'--hHDO (3.8)

this complex is essential in the hydrogen form. PMR results independently con-firm that reaction (3.8) is fast compared with the uptake of deuterium by penta-cyanocobaltate(II)^.

b. Mass-analyses of 3-phenylpropionic acid obtained by deuteration of trans-cinna-mic acid in deuterium oxide gave the figures 6.6% di, 78.3% dj and 15.1% dj. We have not been able to detect any wo«o-deuterated /ra/?5-cinnamic acids with PMR during the experiments. Although the addition of

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pentacyanohydridocobal-tate(III) to cinnamic acid is reversible to a certain extent, the reverse reaction must be slow as compared to the continued deuteration.

Mass-analysis of a reaction mixture of a deuteration of m-cinnamic acid (98% conv.) showed 8.6% di, 61.7% dj, 27.5% dj, and 2.2% d4 in the reduced com-pound, 3-phenylpropionic acid. For 4-chloro-m-cinnamic acid the figures 1.0%do, 9.2% di, 62.3% d3 and 0.9% d4 were found and for 3-chloro-cw-cinnamic acid the figures 1.7% do, 8.6% d^, 59.9% d^, 26.4% d, and 2.4% d4.

In the deuteration of cw-cinnamic acid isomerization to the /ra«5-isomer (without deuterium) was observed. Traces of wono-deutero-/ra«j-cinnamic acids were initially not detectable. Possible resonances of a-deutero-?ra/75-cinnamic acid could be obscured by side bands of the aromatic resonance of the m-compound. In order to study this problem further several new experiments with deuterium and substituted cw-cinnamic acids were performed. In the PMR analyses now use has been made of the paramagnetic shift reagent tris(dipivalomethanato)europium, Eu(DPM)3, to improve detection of mowo-deuterated trans-isomcrs. The 100 MHz PMR spectra of the reaction mixtures, after esterification and using Eu(DPM)3, now showed the trans-isomer to consist of a-D, j?-H and a-H, jS-H in a ratio 3:4 with a maximum, total concentration of 7%. In comparison with hydrogenation deuteration leads to a lower extent of isomerization, apparently due to an isotopic effect.

BOO HZ 600Hz

Fig. 4. 100 MHz PMR spectra of a reaction mixture of a deuteration of 4-chloro-cu-cinnamic acid after esterification, (a) without Eu(DPM)3 and (6) with 40 mg Eu(DPM)3. HA and HA' cor-respond with a-H-cis and fi-H-cis (J„^ 13.0 Hz) of cw-cinnamic acid respectively; H B and H B ' correspond with a-H-trans and ^-H-trans (J,^ 16.3 Hz) of /rani-cinnamic acid

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e. The hydrogenation of a,;?-dideutero-c/.y-cinnamic acid was found to be accom-panied by a partial isomerization to a-H,j9-D-rra/M-cinnamic acid (maximum concentration 5%). The detection of the a-H,j8-D-/ra«5-compound confirms that the first hydrogen atom is added to the double bond in c/5-cinnamic acid at the a-carbon atom.

f. Upon deuteration and hydrogenation of 4-chloro-, 3-chloro-, 3-methyl- and 2-methyl-m-cinnamic acid similar results were obtained. In Figure 4 100 MHz PMR spectra are shown to demonstrate the effect of Eu(DPM)3. The observed paramagnetic shifts are the direct consequence of bonding in a labile complex between the metal complex and the ester. This labile complex is in rapid equili-brium (on the PMR time scale) with Eu(DPM)3 and unassociated ester molecules. g. For the cis- and trans-cinnamic acids opposed isotopic effects were observed. The rate of deuteration of fra«5-compounds is higher than the rate of hydrogenation, whereas for the c/5-compounds the reverse is found (see Table IV).

Table IV. Deuteration versus hydrogenation of substituted trans- and c«-cinnamic acids with pentacyanocobaltate(ll); reaction rate constants (l-mole"'-min~')"

substrate //•ani-ciimamic acid cw-cinnamic acid 2-methyl-r»-a7ij-cinnamic acid 2-methyI-cu-cinnamic acid 3-methyl-rra«.s-cinnamic acid 3-methyl-cw-cinnamic acid solvent H2O D 2 O HjO D 2 O HjO D 2 O H^O D 2 O H2O D^O HjO D , 0 atmos-phere H , D, H . D . Ha Da Ha Da Ha Da Ha D . k, xlO^ _ -1.7 0.8 -1.7 0.6 -2.0 0.8 ka xlO^ _ -1.2 0.3 -0.4 0.1 -0.6 0.2 ka xlO^ 2.9 3.8 2.9 3.8 4.4 7.4 4.4 7.4 2.3 2.9 2.3 2.9 " Temperature 25,0 °C; solvents water and deuterium oxide (1 A/NaOH and 1 MNaOD

respec-tively); substrate concentration: 0.05-0.15 M; initialCo(CN)5H^~andCo(CN)5D' concentration: 0.03-0.07 A/(CN/Co = 6).

3.4 Substituent effects

An attempt at correlating substituent effects observed in the reduction of the

trans-cinnamic acids in terms of a Hammett-Yukawa relationship^"^" is shown in Figure 5.

Computer calculations showed optimal correlation in the case of a resonance para-meter 0.12 (correlation coefficient 0.94), a rho value of 0.74 is calculated. A similar treatment has been omitted for the m-series as, at least for a Hammett relationship, too few substituted cw-cinnamic acids have been reduced. Nevertheless, a similar trend can be observed in the reduction of the m-compounds, electron-donating groups retard and electron-withdrawing groups accelerate the hydrogenation.

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tate(III) would yield the hydroxo complex Co(CN)50H^~ ^*. Formation of this hydroxo complex implies poisoning of the catalyst.

More serious consideration was given to a m-ligand insertion mechanism". However, it is ruled out on the basis of three observations: a. a zero-order in cyanide of the hydrogenation; b. the non-stereospecificity of the isomerization; c. no cobalt olefin complex formation. The zero-order in cyanide has recently been observed by Jackman, et al. too^*. From experiments with deuterium and m-cinnamic acids in deuterium oxide we observed that part of the trans-isomcrs contains no deuterium. A c/5-ligand insertion mechanism (see Figure 6) would involve an addition of a deuterium atom and elimination of a hydrogen atom on forming a

mono-deutero-trans-compound. In experiments with methacrylic acid, sorbic acid and trans-cinna-mic acid and at a ratio Co:CN:S = 1:4:1 no cobalt olefin complex formation was

observed. In all cases 0.8 pentacyanocobaltate(II), Co(CN)5", is formed which is hydrogenated to the hydride Co(CN)5H^~.

// H \/ j H - - - C j N C — C o * II S = ^ NC — C o . C -H l / C N NC--,Co—CN NC^I CN

+

v/

_c * II c CN CN-|f +CN 1 -CN 3- \ / [Co(CN) H] I CN / - 2 Co(CN) + C C, < NO —Co—C—C CN

Fig. 6. C«-ligand insertion mechanism.

The results presented here leave then the mechanism suggested by Simandi and

Nagy^^'' ^. In Figure 7 we elaborate this further. The reaction starts with a hydrogen

atom transfer from the hydride Co(CN)5H^" to the double bond. An organic free radical arises. This free radical reacts with a second hydride and the reduction is com-pleted. The problem in this radical mechanism is that we know nothing about the stabilization of the intermediate. What kind of a role is pentacyanocobaltate(II) playing in this part of the reaction? Although there may be some bonding between pentacyanocobaltate(II) and the radical, steric reasons prevent the strong stabilization

II + H — C o - C N : p = ^ C H Co —CN , C—C + N C - C o — CN-H» Ph — H C —CH—COOH /\ NC CM i ' NC CN 1 NC CN tHa I L -O ^ C N -O^CN -NC—Co—CN NC—Co—CN NC CN NC CN Fig. 7. Hydrogen atom transfer mechanism.

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It should be noticed that in general it seems to be rather difficult to put together substituent effects in radical systems^ \ Consequently, a conclusion regarding the direction of the initial hydrogen atom transfer is not to be expected on account of the substituent effects. F o r the used cr-constants a n d Acr^"^ see Table V.

OJ 02 01 0.0 -0 -02 - 0 3 h-nogk/k„ -A-N(CH,), / o / i - C I / ' ' ^ / • ^ ^ 0 3 - C I oH^<S3-OCH3 P2-CH-^ yC-zv.-i -05 -04 -03 -02 -01 00 01 0 2 03 04 05

Fig. 5. Log k/ko plotted against ((T"+0.12 A^r^) for the homogeneous hydrogenation of sub-stituted frani-cinnamic acids catalysed by aqueous pentacyanocobaltate(Il); cr" and ACTr"*" derived from references '•".

Table V. a and ACr* values used in the Hammett-Yukawa relationship "

substituent A(7r* 3-chloro 4-chloro 3-methoxy H 3-methyl 4-methyl 4-methoxy 4-dimethylamino 0.373 0.29 0.076 O.O -0.069 -0.10 -0.09 -0.24 0.0 -0.19 0.0 0.0 0.0 -0.22 -0.70 -1.43 '^ According to Wepster, et al,' and Yukawa, et al.'"

3.5 Mechanistic considerations

3.5.1 Mechanism of the hydrogenation

Several mechanisms can be envisaged for the reaction between pentacyanohydrido-cobaltate(III), Co(CN)5H^~, and cinnamic acids. Two of them can be ruled o u t at once. The first is an initial proton transfer from the hydride t o the double bond. This is n o t likely if we take into account that pentacyanocobaltate(I), Co(CN)5~, is a very strong base which extracts protons from water molecules Hydride transfer yielding a carbanion and pentacyanocobaltate(III), Co(CN)5 , can also be ruled out because the carbanion would be protonated by the solvent a n d the

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pentacyanocobal-by a cobalt-carbon bond that would occur in the case of a m-ligand insertion. It is established from experiments with deuterium and trans-cinnamic acid in water, that, in the second step, a hydride indeed does react and direct conversion with molecular hydrogen does not take place. The deuterium content of 3-phenylpropionic acid in the deuteration of trans-cinnamic acid clearly indicates that the initial addition of penta-cyanohydridocobaltate(III), Co(CN)5H^~, to cinnamic acid is a reversible reaction.

In the hydrogenations and deuterations of cw-cinnamic acids partial isomerization to the corresponding ?ra/3.r-isomers was observed. In this hydrogenation scheme iso-merization can be explained by removal of a hydrogen atom from the radical inter-mediate by pentacyanocobaltate(II) after rotation around the carbon-carbon bond (see Figure 8). OOC + Ph D I ^CN OOC N U -NC D CN ~ r o ^1 CN — CN H P I N C -CN 1 CN " C o - C N yi NC ^ O O C - ^ ^ H O O C ^ j . OOC n . \ H / D

fi

-dc D J\ > h . — H ^ P h — H ^ • P h CN .\^ N C ^ C o -NC*^ 1 CN - C N c = c [Co(CN)Ol " ~ C = C ' I i = ^ ( " " ' C — C C ; I i = i " " " ^ C T ^ C r , " 5 _ ^ Ph-CHD-CHO—COO NC—Co—CN ..„ „ ^ „ - - ^ N C ^ I N C ^ I M / ^ P h ^ OOC^ / H CN CN • < ^ z'—'^-D

Fig. 8. Isomerization in the deuteration of cw-cinnamic acid.

3.5.2 Commentary on kinetics

The experimental results for the hydrogenation of the ?ra«.s-cinnamic acids lead to the following simple reaction scheme

ki

Co(CN)5H^" + S ^ C o ( C N ) r - F H S - (3.9) k,

k3

Co(CN)5H^--hHS-^Co(CN)r + H2S (3.10) The hydride is generated from pentacyanocobaltate(II) by reaction (3.2). The

revers-ibility of reaction (3.9) is indicated by the deuterium content in the reduced com-pound, 3-phenylpropionic acid, in the deuteration of fra«5-cinnamic acid. It should be noticed that we have not been able to detect any intermediary radical by means of

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ESR. This may indicate that reaction (3.10) is so fast under experimental conditions that the concentrations of radicals HS- remain below that required for ESR detection. The following rate equations hold

- ^ - = - k , [ S ] [ C o ( C N ) 5 H ^ - ] + k , [ H S - ] [ C o ( C N ) r ] (3.11)

^ J ^ ^ = ki [S] [Co(CN)5H^-]-k, [SH-] [ C o ( C N ) n - k 3 [SH-] [Co(CN)5H^-] (3.12)

^ i ^ = k3[SH-][Co(CN)5H^-] (3.13)

c . . . . d[SH-] ^

Steady-state approximation gives —-r-— = 0, or [ S H . ] = k.[S][Co(CN)3H3-] k^ [ C o ( C N ) r ] + k3 [Co(CN)5H^-] d [SH,] _ d [ S ] _ j^^ | . g ^ ^ [Co(CN)3H^-] dt dt k3k,[S][Co(CN)5H^-] k, [ C o ( C N ) r ] + k3 [Co(CN)5H^-] (3.15)

This means that in the steady-state approximation the reactions between pentacyano-hydridocobaltate(IIl) and fra^i'-cinnamic acid are first-order in substrate. In the hydrogenations the rate of hydrogen uptake is small to such a degree that the con-centration of pentacyanohydridocobaltate(III) can be put equal to the equilibrium concentration of the hydride in reaction (3.2) under the same circumstances. Conse-quently the concentrations pentacyanocobaltate(II) and pentacyanohydridocobal-tate(III)are known.

The hydrogenations of the fra«j-cinnamic acids were found to be first-order in substrate as well as in pentacyanohydridocobaltate(III). According to equation (3.15) this is only possible if

k 2 [ C o ( C N ) r ] « k 3 [ C o ( C N ) 5 H ^ - ] (3.16) In the hydrogenation experiments the ratio Co(CN)5H-'"/Co(CN)5" is about 6.

This datum and the amount of d^ in 3-phenylpropionic acid in the deuteration of ?rfl«5-cinnamic acid really show that the neglect of k2[Co(CN)5~] is not unrealistic.

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3.5.3 Direction of initial hydrogen atom transfer

As mentioned before the determination of the direction of the initial attack of pentacyanohydridocobaltate(III) at the double bonds in cis- and trans-cinnamic acids was also one of the aims of this investigation. It has already been suggested by several workers that initial addition of hydrogen to double bonds in a,jS-unsaturated compounds, viz. RCH = CHX with R = CH3 or H and X = CN, COOH or CgHj, takes place at the ^-carbon atom'^''*'^*. Indirect indications were found via the formation of cr-complexes, RCH2CH(C02~)-Co(CN)5~, from pentacyanohydrido-cobaltate(III) and a,j8-unsaturated carboxylate ions. However, little is known about the initial hydrogen atom transfer to a double bond which is conjugated with acti-vating groups at both ends as in the present systems. A possible indication is found in the formation of the olefin adduct C6H5CH2CH(CN)Co(CN)5~ from trans-cinnamonitrile^^.

The observed isomerization products in the hydrogenations and deuterations of substituted c/5-cinnamic acids clearly demonstrate that the first hydrogen atom enters the double bond of the cw-compounds at the a-carbon atom. Apparently the addition at the end of the "styrene-type" double bond provides the best compromise for this non-planar system in which both conjugated groups are rotated out of the plane of the double bond. As the resulting radical intermediate will be stabilized by the phenyl group by resonance the intermediate will strive for a planar configuration around the /?-carbon atom.

The //-fl/jj-cinnamic acids have a coplanar configuration and consequently it is

a priori not possible to indicate whether the initial hydrogen atom transfer takes

place at the a- or jS-carbon atom. Either radical intermediate is "optimally" stabilized by one of the activating groups. The mass-spectrometric analysis of 3-phenylpropionic acid obtained in the deuteration of /ran5-cinnamic acid showed the absence of d4. In the deuteration of m-cinnamic acid and chloro-substituted c/i-cinnamic acids about 2% of d4 was found in the reduced compounds. This amount of d4 can be explained if we assume that in the fran^-isomers which are formed by isomerization the first deuterium is introduced at the ^-carbon atom. In Figure 9 this is elaborated in the deuteration of cw-cinnamic acid.

The results for or?/jo-substituted cis- and trans-cinnamic acids are in harmony with the proposed ideas. The ortho-suhstitwtcd ?ra/;5-cinnamic acids are no longer coplanar due to steric hindrance''. An orr/io-methyl substituent is thought to cause an inter-planar angle of about 30° between the phenyl ring and the ethylenic bond. Thus the observed acceleration is rather difficult to understand in the case of a-addition as the

ortho substituents will restrict the stabilizing resonance by the phenyl group. If in

?rart.j-cinnamic acids the first hydrogen atom transfer takes place at the ;8-carbon atom, indeed an acceleration is expected for the or/Zzo-substituted trans-compounds, as the steric strain is decreased in the radical intermediate. In or^/io-substituted cw-cinnamic acids no acceleration is expected as the first hydrogen atom is introduced