Substituent effects in ?-(Tricarbonylchromium) arenes

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Substituent Effects

in

7r-(Tricarbonylchronniunn) arenes

F. van Meurs

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7r-(Tricarbonylchronnium) arenes

81illllili!ii!!f|||.!ll1!iilllll!liJI«"

OD O (jJ - 4 - J O f - 00 O U> CX) o BIBLIOTHEEK TU Delft P 1780 5318 C 526870

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Substituent Effects in

7r-(Tricarbonylchromium) arenes

Proefschrift ter verkrijging van p de graad van doctor in de /y/'cFo ^ d^u

technische wetenschappen

aan de Technische Hogeschool Delft, op gezag van de rector magnificus prof. dr. ir. F.J. Kievits,

voor een commissie aangewezen door het college van dekanen te verdedigen op

donderdag 7 december 1978 te 16.00 uur door

Frank van Meurs, scheikundig ingenieur geboren te 's-Gravenhage

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PROF. DR. IR. H. VAN BEKKUM

The investigation descnbed in this thesis has been supported by the Netherlands Foundation for Chemical Research (SON) w/ith financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO)

Drawings J M Dijksman Typing Miss M A A van Leeuwen

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Contents

V

1 General i n t r o d u c t i o n

I n t r o d u c t i o n 1 Chemical p r o p e r t i e s 2

Molecular s t r u c t u r e and bonding 4

Scope of t h i s t h e s i s 8 R e f e r e n c e s 9 2 ^H NMR Spectroscopy of a l k y l s u b s t i t u t e d I T ( t r i c a r b o n y l -chromium) benzenes I n t r o d u c t i o n 12 E x p e r i m e n t a l 1 3 R e s u l t s 14 D i s c u s s i o n 17 C o n c l u s i o n s 2 1 R e f e r e n c e s 22 3 An IR study of the CO s t r e t c h i n g v i b r a t i o n s in a l k y l s u b s t i t u t e d TT-(tricarbonylchromium)benzenes Introduction 23 Experimental 24 Discussion 25 Conclusions 28 References 29

4 An IR study of the CO stretching vibrations in substituted methyl ii-(tricarbonylchromium)benzoates

Introduction 30 Experimental 31 Results and discussion 33

References 43

5 H NMR Spectroscopy of substituted methyl TT- (tricarbonylchromium) benzoates

Introduction 44 Experimental 45 Discussion 45 References 49

6 Thermodynamic dissociation constants of some substituted IT-(tricarbonylchromium)benzoic acids

Introduction 50 Experimental 51 Results and discussion 53

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7 Dynamic behaviour in

(1' -t-butyl-2', 2' -dimethylpropyl) -n- (tricarbonylchromium)benzene

Introduction 60 Results and discussion 61

References 63 8 The molecular structure of ir-(tricarbonylchromium)toluene

Introduction 64 Experimental 66 Structure determination and refinement 66

Structure description and discussion 67

References 69 9 The molecular structure of

(1'-t-butyl-2',2'-dimethylpropyl)-IT-(tricarbonylchromium)benzene

Structure description and discussion 74

References 77 10 The molecular structure of

4-t-butyl-TT-(tricarbonylchromium)benzoic acid

Results 82 References 85 11 Conformational preference of the tricarbonylchromium group in

alkylsubstituted w-(tricarbonylchromium)benzenes

Introduction 86 Experimental 87 structure description of CgHsRCr(CO)3, R=neoPent, i-Pr and t-Bu 88

Discussion 94 References 97 Summary 98 Samenvatting lOl

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1

GENERAL INTROKJCTION

Introduction

The TT-(tricarbonylchromium)arene chemistry dates from 1957. The first representative of this group of compounds, TT-(tricarbonyl-chromium) benzene, was originally synthesized via a ligand exchange reaction (eqn. 1) between dibenzenechromium and hexacarbonvlchromium

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the difficulty to obtain the required starting sandwich complexes and the rather severe conditions for this reaction (12 h in a sealed tube at 220 °C). In the next years a more general synthetic route was developed. It was found that a large variety of aromatic compounds can react with hexacarbonylchromium when heated either alone or in an inert solvent under anaerobic conditions (eqn. 2) [2-8]. Quite a range of TT-(tricarbonylchromium)arene complexes have been synthesized in

CsHjR + Cr(CO)g » /^''v "•" ^ ^° ^2)

oc v^co

"^ CO

this way. Some complexes (R = CN, CHO, COOH, HC = CH2, C = CH, NO,) cannot be prepared by this reaction, because the decomposition temper-ature of the reaction product is close to, or beneath, the tempertemper-ature required for reaction. In some cases this difficulty could be circum-vented by applying other reactions, such as various types of other ligand exchange reactions [9-22], a photochemical preparation method

[23] and all sorts of side chain reactions in the complexes.

Chemical Properties

^fost TT-(tricarbonylchromium)arenes can be easily prepared, purified and characterized when proper precautions to avoid oxidation are taken. In the solid state they are relatively stable towards air and light. TT-(Tricarbonylchromium)arenes may be transformed chemically by reactions with electrophilic and nucleophilic agents. The chemical reactivity of arenes has been found to be greatly altered upon com-plexation with the tricarbonylchromium moiety. Generally, in comparison with the corresponding arenes the complexes are more susceptible to nucleophiles and less to electrophiles. The mechanisms of these types of reactions are often uncertain. A few examples of both categories

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3

will be recalled.

Halogen substituted TT-(tricarbonylchromium)benzenes undergo rapid

nucleophilic substitution, under the influence of NaOMe or NaNH^ to

yield the methoxy- or amino-substituted analogues [4,24,25].

TT-(Tricarbonylchromium)benzene undergoes base catalyzed H/D exchange

[32-35] under conditions where free arenes will not react. On the

other hand, acid catalyzed H/D exchange [36-37] proceeds much more

slowly than observed for free benzene. A simple electrophilic

substitution mechanism seems rather unlikely for this reaction, since

it is known that in similarly strong acidic media the chromium atom

can be protonated ( H NMR spectroscopy reveals 5(Cr-H) = -4.0 — -5.0

ppm) [37-40]. Acylation of several TT-(tricarbonylchromium) arenes has

shown to be more difficult than that of the free arenes [25-31].

The solvolysis of TT-(tricarbonylchromium)benzyl chlorides and the

hydrolysis of TT-(tricarbonylchromium)benzyl alcohols have been found

to occur 10 times faster than the corresponding free arenes [41-46] .

Apparently, the chromium atom is able to stabilize an a-carbenium ion.

Stereospecific reductions [47-48] and H/D exchange in the side chains

of alkyl-substituted TT-(tricarbonylchromium)arenes also point to the

specific role tJiat a-carbon atoms can play.

The reaction of TT-(tricarbonylchromium)arenes with alkyllithium is

complex and very sensitive to the reaction conditions. In our

laboratory these reactions were studied by Bras [Sl] . Generally,

addition of several sorts of carbanions to the complexes proceeds

under mild conditions. Reaction products are amongst others

carban-ion additcarban-ion products [52-55] and metallatcarban-ion products [55-56].

Other studies have shown that both benzoic acid and phenol are

weaker acids than TT-(tricarbony]chromium)benzoic acid [2,4,57] and

TT-(tricarbonylchromium)phenol [58]. Correspondingly, aniline is shown

to be a stronger base than TT-(tricarbonylchromium)aniline [4,57].

Some of the known properties of TT-(tricarbonylchromium)arenes can be

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group. Others seem to need the assistance of the chromium atom to stabilize an intermediate ion. Thus the electronic and steric effects provide us with the means to effect some chemical transformations which are difficult or not at all possible in free aromatic compounds. Since both bond formation and bond breaking between the arene and the Cr(CO), group are simple processes TT-(tricarbonylchromium)arenes can be useful intermediates in practical organic chemistry.

Molecular Structure and Bonding

The crystal structure of TT-(tricarbonylchromium)benzene was deter-mined by X-ray diffraction in 1959 [59]. Since then several single crystal studies on derivatives of Tr-(tricarbonylchromium)benzene have been carried out. In all studies a "piano stool" arrangement was found for the arene ring and the Cr(CO) group. The results of these studies have been reviewed recently [60] ; some typical distances for TT-(tri-carbonylchromium) benzene [64] are Cr-C(arene) = 2.23 A,

Cr-C(carbonyl) = 1.84 S, C-C = 1.41 S and C-0 = 1.16 R. The Cr-C-0 bonds are linear and their geometry implies an octahedral coordination for the chromium atom.

The results obtained for some mono- and di-substituted Ti-(tricarbonyl-chromium)benzenes [61-63] have led many investigators to the opinion that the conformational preference of the Cr(CO), group with respect to the arene ring is determined by the effects of the substituent(s) on the charge distribution in the aromatic ring. In the solid state two conformations have most frequently been encountered for TT-(tricarbonyl-chromium) arenes , namely a staggered (I) and an eclipsed (II) confor-mation. If the arene ring carries substituents, there are more

I n ni H

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5

possibilities depending on the substitution pattern. For instance, in monosubstituted TT-(tricarbonylchromium)benzenes two conformations in which the Cr(CO), group eclipses the ring carbon atoms are possible, viz. a conformation with the Cr(CO)., group eclipsing the substituent (III) as well and a conformation with staggering of the Cr(CO)_ group with the substituent (IV).

In terms of valence-bond theory, the conformation found in the solid state agrees in many cases with the conformation for which the overlap

2 3

between chromium d sp hybrid orbitals and regions of high electron density on the arene ring is maximum. TT-(Tricarbonylchromium)benzene and some of its derivatives displayed a staggered conformation (I)

[64,60]. It can be argued, however, that for TT-(tricarbonylchromium)-benzene both in valence-bond and ^D formalisms the cylindrical symmetry of the benzene MO's impose no conformational preference for the Cr(CO)T. group. Thus in this case additional factors might have influenced the stereochemistry found.

Attempts to determine the barrier to rotation about the Cr-arene bond in solution by the conventional H NMR method failed, indicating

-1 1

the barrier to be < 8 kcal mol . H NMR line shape analyses provided some insight in the height of this barrier [65,66] ; '^' 2 kcal mol for t-butyl-iT-(tricarbonylchromium)benzene.

The bonding properties of TT-(tricarbonylcliromium)benzene have been the subject of many theoretical studies [64,67-71,75]. The charge distributions calculated in these studies have been compiled in Table

1. For comparison purposes the results of similar calculations for Cr(CO), and (C,H,)2Cr have been included in this Table. The SCCC calculations indicate a drift of charge from the arene ring towards the carbonyl ligands, while CNDO/2 calculations and to a larger extent empirical (from X-ray experiments) and ab initio calculations indicate that the benzene ring has a negative charge. In all calculations the Cr atom is positively charged, althougli the ab initio charges for both C,H Cr(CO), and (C,H,)_Cr seem excessive for non-ionic compounds. In contrast, chemical reactivity data are in favor of a reduced electron density on the arene ring for IT-(tricarbonylchromium)arenes.

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TABLL 1

CltVRGE UISTRIBimON TOR Cjll^Cr(CO),, CrfCO)^ \ND [C^U^) ,CT

The principle of bonding between the Cr atom and the arene ring in TT-(tricarbonylchromium)arenes can be explained by an examination of the ND's involved. The Z-axis is chosen perpendicular to the benzene ring. We take into account only the six valence electrons of Cr and consider the d-level to be split up, yielding d , d 2_ 2, d 2 each formally full and d , d vacant. Using the symmetry of these Cr orbitals and of the benzene MO's the following qualitative bonding scheme can be developed. The a^ benzene orbital can mix with 4s and d 2 because both have a-symmetry with respect to the Z-axis. The e^ benzene orbitals donate electrons into vacant d and d orbitals.

xz yz

This interaction of orbitals of Tr-symmetry with respect to the Z-axis is called donation. However, in most organometallic compounds orbitals of a-symmetry are involved in the donation process. Vacant non-bonding

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7

e. benzene orbitals have 6-symmetry with respect to the Z-axis and are thus suitable to overlap with filled d 2 2 and d orbitals. This kind

^ X -y xy

of interaction, normally of the Tr-symmetry type, is generally known as back-donation and it is supposed to play a dominating role in organo-metallic chemistry. To give an impression of the bond strength in TT-(tricarbonylchromium)benzene, a recent thermochemical investigation revealed the Cr-(benzene) bond enthalpy to be about 42 kcal mol [77].

More elaborate methods, such as extended Hiickel calculations and ab initio calculations [75,7l], confirm the rough scheme given above. An interaction diagram from extended Hiickel calculations for TT-(tri-carbonylchromium)benzene is given in Fig. 1 and it shows basically similar patterns for the stabilization of the d-orbitals in both fragments. ' 2 g _ 4 p _ xy , y z x z , y z 3d x y • x ' - y ' x y , x ' - y ' (CO), Cr Cr Cr CsHs ( C O ) , (CeHs)

Fig. 1. Qualitative MO-diagram for the Cr(CO), and Cr(Cj^llj^) fragments. The fD's of the three CO groups have been rearranged taking into account their geometry [75].

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Scope of this thesis

In 1971 our attention was attracted by the potential application of TT-(tricarbonylchromium)arenes in homogeneous catalysis [78]. We studied the existing literature and noticed that substituent effects in

TT-(tricarbonylchromium)arenes had hardly been investigated. A contro-versy in the literature about conformational effects [65,66,79,80] on

H NMR spectroscopic phenomena as well had our interest. So, for a preliminary investigation we focussed our attention on TT-(tricarbonyl-chromium)ben2oic acids carrying small or large sized alkyl substituents in the aromatic ring. Early in 1972 Ashraf and Jackson [57] published a paper on the same topic. However, in our hands the complexes behaved quite differently [8l]. Unfortunately, we were unable neiter to under-stand the origin of the discrepancies between their results and ours nor to provide a satisfactory explanation for our results. This prompted us to investigate in detail the substituent effects in

TT-(tricarbonylchromium)benzoic acids and related compounds, using

substituents varying in electronic character and in steric requirements. In our opinion the inclusion of large sized substituents in our studies was of vital importance, because conformational effects might show up as part of the substituent effects. The results of these investigations, in which H NMR spectroscopy, IR spectroscopy and equilibrium constant measurements were enployed, are described in Chapters 2-7.

To support the conformational proposals made a number of alkyl-substituted TT-(tricarbonylchromium)arenes were investigated by single crystal X-ray diffraction. The molecular structures of three of these complexes are described in Chapters 8-10. In Chapter 11 the results of some recent additional X-ray studies are reported briefly. In this Chapter the impact of the X-ray data on the interpretation of the results of physical measurements are discussed.

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9

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39. U.N. Kursanov, V.N. Setkina, P.\. Pctrovskii, \ I. Zdanovich, N.K. Baranetskaya .ind I I). Rulun, J. Organometal. Chem., 37 (1972) 339.

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78. For a review see B.R. James, Homogeneous Itydrogenation, John Wilev S Sons, New ^ork, 1')"3, p 50 and p. 354.

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83. F. van Meurs, J.M van der T o o m and H. van Bekkum, J. Organometal. Chem., 113 (1976) 341. 84. F. van Meurs, J.M \ . Baas and H. van Bekkum, J. Organometal. Chem., 113 (1976) 353.

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85. F. van Meurs and H. van Koningsveld, J. Organometal. Cliem., 118 (1976) 295.

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Journal of Organometallic Chemistry, 113 (1976) 341—351

SUBSTITUENT EFFECTS IN 7f-(TRICARBONYLCHROMIUM)ARENES I. 'H NMR SPECTROSCOPY OF ALKYLSUBSTITUTED

7r-(TRICARBONYLCHROMIUM)BENZENES

F. van MEURS , J.M. van der TOORN and H. van BEKKUM

Laboratory of Organic Chemistry, Delft University of Technology, Julianalaan 136, Delft-2208 (The Netherlands)

Introduction

Tlie orientation of the Cr(C0)3 group with respect to the arene ring in 7r-(tricarbonylchromium)benzene and its derivatives has been studied by X-ray and NMR techniques. The first technique showed a staggered conformation I for the benzene and the hexamethylbenzene complex, but eclipsed * conforma-tions II or III for tlie monosubstituted derivatives [ 1 ] . The preference for II or III in the latter compounds seems to be governed by the effect of R on the charge distribution in the benzene ring (preference for II for R = OMe or alkyl, pre-ference for III for R = COOMe). Furthermore, the molecular structure of some disubstituted 7r-(tricarbonylchromium)benzenes [3] is consistent with this view.

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13

R R

( ^ ( ^ ^

I E m

As to the conformation of these compounds in solution, the preference seems far from pronounced. Variable temperature 'H [4,5] and " C [6,7] NMR spectra of monosubstituted 7r-(tricarbonylchromium)benzenes have been analysed in terms of a rapid equilibrium between the conformers II and III. For R = Me, Et, i-Pr or OMe enthalpy differences in favour of II (estimated to be less than 1 kcal mol"' ) are opposed by entropy differences in favour of III [ 4 ] . Here both con-formers are present in solution in substantial amounts at room temperature. With increasing bulkiness of R conformation II is expected t o become destabilised by steric interaction between R and the eclipsing carbonyl ligand. Accordingly, for R = t-Bu ' H NMR spectra [4] indicate that the system prefers conformation III. The effect of steric interactions was further demonstrated by the conforma-tion of 4-t-butyl-7r-(tricarbonylchromium)benzoic acid in the solid phase [ 8 ] .

In order to gain more insight into the directing effects of alkyl groups on the conformation of the 7r-tricarbonylchromium group with respect to the phenyl ring in alkyl-7r-(tricarbonylchromium)benzenes we have measured 'H NMR spec-tra of a series of complexes with R varying in size from Me to CH-t-Bu2. In addition some di- and tri-alkylsubstituted benzene complexes have been included. Moreover, attention was paid to the effects on conformational preference of some other relatively large sized groups, i.e. the trimethylsilyl and the trifluoromethyl group, which differ substantially from the alkyl groups as to their electronic effect.

Experimental

Substituted benzenes were preparations of this laboratory or commercial products; purity was checked by GLC. Commercial hexacarbonylchromium (Strem Chemicals Inc.) was purified by sublimation in vacuo. Solvents were Baker analysed reagents; before use they were freed from oxygen by repeated degassing and saturation with nitrogen. All preparations were carried o u t under nitrogen.

Preparation of the ir-tricarbonylchromium complexes. The complexes were prepared by thermal reaction of equimolar quantities of the benzene derivative and hexacarbonylchromium in boiling dibutyl ether at atmospheric pressure, using the apparatus described by Strohmeier [ 9 ] . The reaction was run until no more hexacarbonylchromium appeared in the cold regions of the system. The time required amounted to 6—15 h. After cooling, the solvent was removed by distillation in vacuo. The residu was extracted with h o t petroleum ether (b.p. 60—80°C). The 7r-tricarbonylchromium complexes were crystallized from this solution. Recrystallization from light petroleum ether usually afforded analytical-ly pure complexes. Anaanalytical-lytical data and melting points are given in Table 1.

Spectral measurements. 'H NMR spectra were obtained on Varian T-60, XL-100-15 and HR-300 spectrometers. Tetramethylsilane was used as internal

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TABLE 1

ANALYTICAL RESULTS OF THE SUBSTITUTED 7 T - ( T R I C A R B O N Y L C H R O M I U M ) B E N Z E N E S

Substituent(s) Me Et neoPent i-Pr t-Bu CEt3 CH-t-Bu2 OMe SiMes none COOMe Chi 1,2 Me2 1,2-Et2 l,2-neoPent2 l,2-i-Pr2 1,2 t-Bu2 l,2-(SiMe3)2 1,3-Me2 l,3-t-Bu2 l,3-(SiMe3)2 l,4-t-BU2 1 4-(SiMe3)2 1,3,5 Me3 1,3 5-Et3 l,3,5-neoPent3 1,3,5-1 Pr3 1 3,5-t-Bu3 l,3-t-BU2-5-Me 1,2.4 t-Bu3 Melting point ("C) 81 0—82 5 48—49 122—123 62—63 79—80 103 5—104 0 139-140 82 5—83 5 72 0—72 5 160—162 97 5—98 5 46 5—47 5 90 0 - 9 0 5 26—27 64 5-65.0 121 0—122 5 119—121 8 1 - 8 2 105—106 136 5—138.0 104 5-106 0 140 5—142 0 125 0 - 1 2 5 5 1 7 2 - 1 7 3 9 6 192 (dec ) 106 5 - 1 0 7 5 1 7 6 - 1 7 7 127—128 119—120 Reference melting point(s) (°C) 8 0 - 8 1 "•'' 48—49 " 63—64'^ 6 1 - 6 2 '^ 83 5 - 8 4 5 *> 79 5—80 5 '^ 8 3 - 8 4 " '= 161 5 - 1 6 3 0 ° 160-162'^ 97 5 - 9 8 5 ** 9 8 - 9 9 ' ' 98 5—99 0 " 1 0 5 - 1 0 6 ' ' 136-138 '^ 1 3 9 - 1 4 1 '^ 1 4 0 - 1 4 1 ' ^ 123—124'' 172—174° 175—177 '^

Analyses found (calcd ) (%)

c 59 3 (59 15) 61 6 ( 6 1 52) 63 6 (63 51) 50 6 (50 34) 42 8 (42 57) 57 6 ( 5 7 77) 64 3 (64 39) 60 2 (60 39) 62 7 (62 56) 50 4 (50 26) 50 3 (50 26) 60 5 (60 39) 67 7 (67 89) 63 7 (63 51) 63 4 ( 6 3 51) 66 1 (65 94) H 5 8 (5 67) 6 7 (6 45) 7 4 (7 11) 5 1 (4 93) 2 2 (1 79) 5 4 (5 18) 7 4 (7 39) 6 2 (6 08) 6 9 (6 80) 6 4 ( 6 19) 6 5 (6 19) 6 3 (6 08) 9 0 (8 55) 7 2 (7 10) 7 4 (7 10) 8 2 (7 91)

" Ref 10 *> Ref 4 '^ Ref 11 ^ Ref 12 ^ Ref 13

reference. The data reported are for approximately 0.5 M solutions in acetone-dft . All peak areas showed the correct relative intensities. The actual chemical shifts of the aromatic protons can be obtained by adding A5 to the chemical shifts of benzene or 7r-(tncarbonylchromium)benzene protons. The A5-values are con-sidered to be better than ±0.02 ppm. Computations for spectrum simulation were performed on an IBM 370/158 computer, using the computer program LAME [ 1 4 ] .

Results

Operating at 100 MHz, 'H NMR spectra of the substituted 7r-(tncarbonyl-chromium)benzenes are easily interpretable. In monoalkylsubstituted benzenes

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TABLE 2

PROTON NMR DATA " OF CgHsR AND THEIR 7r-Cr(CO)3 COMPLEXES

Free hgand Complex

A6(H2,6) A6(H3,s) A6(H4) ^(CHa) «(CH2) 6(CH) A6(H2,6) AS(H3,5) A6(H4) 6(CH3) 6(CH2) 6(CH)

H M e Et n-Pent i-Pr t-Bu CEt3 CH-t-Bu2 O M e SiMe3 COOMe C F 3 0.00 - 0 . 1 6 b —0.14 b - 0 . 2 8 c - 0 . 1 3 b + 0 . 0 2 f" - O l O ' ^ + 0.15'^ —0.28 c —0.45'* - 0 . 1 0 + 0 . 7 2 <* +0.25'^ 0.00 —0.08 *> —0.05 f' —o.is*; —0.08 *> —0.10 I' - 0 . 1 3 c - 0 . 0 6 c —0.12 <: —o.iof* —0.31 + 0 . 1 5 <* + 0 . 1 3 c 0 . 0 0 —0.16 *> —0.17 b - 0 . 2 5 " —0.18 >> —0.24 l> —0.29 <^ —0.15<^ —0.42 <* —0.30 + 0 . 2 6 ' ' + 0 . 2 0 c 2.20 1.17 0 . 9 3 1.21 1.27 0.67 1.04 0 . 2 5 3 . 9 3 2.59 2 . 5 3 1.73 2.88 2.41 0.00 - 0 . 1 9 —0.15 —0.23 —0.06 + 0 . 1 5 + 0 . 1 3 + 0 . 4 5 + 0 . 1 2 —0.19 + 0 . 0 8 + 0 . 6 2 + 0 . 4 0 0 . 0 0 + 0 . 0 3 —0.01 —0 01 —0.06 —0.17 —0.21 —0.25 - 0 . 3 2 + 0 . 2 1 - 0 . 2 0 0 . 0 0 + 0 . 0 2 0 . 0 0 —0.25 —0.19 —0.22 —0.06 - 0 . 0 2 + 0 . 0 6 + 0 . 2 6 —0.51 + 0 . 1 9 +0.27 + 0 . 2 5 2.19 1.21 0.98 1.27 1.32 0.86 0.97 1.33 3.76 0 . 3 2 3.86 2 . 4 6 2.26 1.71 2.68 2.16

" Solvent, acetone-dg, 100 MHz; A6 is the chemical shift increment (m ppm) on substitution relative to CgHj (5 7.35 ppm) for the free ligands and relative to C5H6Cr(CO)3 (6 5.63 ppm) for the complexes; 6-values are given in ppm relative to tetramethylsilane as internal standard. " A6-Values estimated from Bovey et al. [23], 200 MHz; solvent, carbon tetrachloride. '^ From 300 MHz spectra. '* A6-Values taken from ref. 24, 60 MHz; solvent, acetone-dg.

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TABLE 3

PROTON NMR D A T A " OF Dl- A N D TRl-SUBSTITUTED BENZENES A N D THEIR ir-Cr(CO)3 COM-PLEXES Substituents 1,2-Me2 1,2 Me2 1,2-Et2 1,2-Et2 l,2-neoPent2 1,2 neoPcnt2 I,2i-Pr2 l,2-i-Pr2 l,2-t-Bu2 1,2 t BU2 l,2-(SiMe3)2 l,2-(SiMe3)2 1,3-Me2 1,3-Me2 l,3-t-Bu2 l,3-t-Bu2 l,3-(SiMe3)2 l,3-(SiMe3)2 1,4 Me2 ^ 1,4-Me2 « l,4-t-Bu2 l,4-t-Bu2 l,4-(SiMe3)2 l,4-(SiMe3)2 1 3,5-Me3 1,3,5-Me3 1,3,5-Et3 1,3,5-Et3 l,3,5-neoPent3 1,3,5 neoPent3 l,3,5-i-Pr3 1,3 5-1-Pr 3 1,3,5 t B U 3 1,3,5 t B u 3 1,3 t Bu2-5-Me 1,3 t-BU2-5-Me l,2,4-t-Bu3 1 2,4-t-BU3 Type b L 0 L c L c L C L c L c L C L c L c L C L c L c L C L C L c L c L C L c L C AS H 2 — 0 40 — 0 31 +0 02 +0 36 +0 39 +0 12 — 0 30 - 0 07 — 0 03 +0 05 +0 18 — 0 07 - 0 57 - 0 41 — 0 45 - 0 29 - 0 55 — 0 41 - 0 38 — 0 11 - 0 05 +0 47 - 0 08 +0 16 H 3 — 0 29 — 0 05 - 0 24 - 0 07 - 0 22 +0 09 - 0 13 — 0 06 +0 22 +0 18 +0 30 +0 03 — 0 30 - 0 07 — 0 03 +0 05 +0 18 - 0 07 + 0 33 +0 44 H 4 - 0 29 — 0 17 — 0 24 - 0 07 — 0 22 - 0 07 - 0 27 — 0 06 - 0 25 - 0 06 — 0 06 - 0 03 — 0 42 — 0 36 - 0 26 +0 25 +0 19 +0 21 - 0 57 - 0 41 - 0 45 - 0 29 - 0 55 — 0 41 - 0 38 — 0 11 - 0 05 +0 47 — 0 31 +0 16 H 5 — 0 29 — 0 17 - 0 24 - 0 07 - 0 22 - 0 07 — 0 27 - 0 06 - 0 25 - 0 06 — 0 06 - 0 03 — 0 26 +0 08 - 0 24 - 0 26 - 0 02 - 0 30 — 0 30 — 0 07 — 0 03 +0 05 +0 18 - 0 07 - 0 22 +0 24 H6 - 0 29 — 0 05 — 0 24 - 0 07 — 0 22 +0 09 - 0 1 3 — 0 06 +0 22 +0 18 +0 30 +0 03 — 0 42 — 0 36 - 0 26 +0 25 +0 19 +0 21 - 0 30 - 0 07 - 0 03 +0 05 +0 18 — 0 07 - 0 57 — 0 41 - 0 45 - 0 29 - 0 55 — 0 41 - 0 38 - O i l - 0 05 +0 47 — 0 31 +0 16 +0 20 +0 05 6(CH3) 2 19 2 19 1 17 1 23 0 87 1 10 1 21 1 25, 1 27 1 52 1 57 0 37 0 41 2 25 2 21 1 31 1 33 0 29 0 30 1 29 1 32 0 24 0 32 2 22 2 22 1 18 1 23 0 92 1 01 1 23 1 25 1 33 1 30 1 2 8 , 2 30 1 34, 2 21 1 30, 1 53, 1 55 1 30, 1 60 6(CH2) 2 64 2 41 c, 2 64<^ 2 69 2 23'*, 2 37'* 2 58 2 50 2 47 2.31 6 ( C H ) 3 29 3 09 2 88 2 67

" Solvent, acetone-d^ A5 is the chemical shift mcrement (in ppm) on substitution relative to CsHg (6 7 35 p p m ) for the free ligands and relative t o C6H6Cr(CO)3 (& 5 6 3 ppm) for the complexes 5 values are given in ppm relative to tetramethylsilane as internal standard '' Free ligand (L) or TT Cr(CO)3 complex (C) '^ ABX3 system, J ^ g = 1 4 9 Hz, Jy^^ = Jg^ = 7 4 Hz <* AB-system, J ^ g = 7 0 Hz ^ From ref 24

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17 the chemical shifts of the aromatic protons differ only slightly. Accordingly, the interpretation of the aromatic proton region of the spectra of these compounds usually requires high-field magnetic resonance techniques. The assignments of the aromatic protons in neopentylbenzene, (l',l'-diethylpropyl)benzene and (l'-t-butyl-2',2'-dimethylpropyl)benzene, which were not available in the litera-ture, were obtained from 300 MHz ' H NMR spectra.

A problem arises in the assignment of the aromatic protons of the 1,2-di-alkylsubstituted benzenes and the corresponding 7r-tricarbonylchromium com-plexes. Since in all cases a completely symmetric signal was obtained it is not possible to assign e.g. the upfield part of it to either H3,6 or H4_s. For the 1,2-di-methylbenzene complex the assignment was settled by the spectrum of 4-deutero-l,2-dimethyl-7r-(tricarbonylchromium)benzene. Where necessary, the signals have been tentatively assigned on the basis of additivity of the increments obtained for the monosubstituted benzenes, although this is debatable [ 1 5 ] . Furthermore, a conservation of this order is assumed on complex formation.

All assignments have been checked and, if necessary, corrected by matching simulated and experimental spectra. The data obtained are summarized in Tables

2 and 3. Discussion

In substituted benzenes proton chemical shifts are known to reflect the

TT-electron charge densities on the carbon atoms to which the protons are attached [ 1 6 ] . In addition other influences, such as ring current effects and long range effects associated with the substituent, act upon the proton chemical shifts. Ortho protons are influenced rather strongly by inductive substituent effects. Influences on meta and para protons are generally smaller, but less perceptible to other effects [ 1 7 ] . For the Cr(CO)3 complexes 7r-electron densities are not available. Since in monoalkylsubstituted 7r-(tricarbonylchromium)benzenes the electronic and mesomeric effects differ only slightly, it is possible to relate the shift {d{Hx)) of a certain proton on complex formation, defined as ASco^piexCHj.) — A5arene(Hx), to Conformational effects. In Fig. 1 the 0-values for ortho, meta and para protons are given as a function of the bulkiness of the alkyl group used.

Although a theoretical basis is missing, it is generally accepted that in the pre-ferred conformation the protons eclipsed with carbonyl groups are relatively deshielded, i.e. 6 is positive [ 4 , 1 8 ] . Since threefold symmetry is clearly involved, the anisotropy of the carbon monoxide bond might be responsible for the ob-served deshielding. From Fig. 1 we conclude that with increasing bulkiness of R the contribution of conformer III increases at the cost of the electronically favoured conformer II. This is in accordance with the findings of Jackson et al. [ 4 ] . However, it should be emphasized here that for R = CH-t-Bu2 the value of d{meta) — d(para) is distinctly larger than the value * selected by Jackson et al. [ 4 ] .

A high barrier to rotation (approximately 22 kcal mol"' at room temperature) has been reported for the rotation about the C(sp^)—C(sp^) bond in

(I'-t-butyl-* These authors propose B{mcta) — Qipara) = 0.45 ppm, to represent complete preference for conforma-tion i n .

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a n ^poro'"'""' I El I /Pr I CEI3 I M t n e o P e n l ( B u C H - ( B u , 1 El I (Pr I CEI3 I Me n e o P s n l ( B u C H - / B u , Et I i P r 1 C E t , I Me neoPent IBu C H - ( B u ,

Fig. 1. 0-Values for ortho, meta and para protons of the monoalkylsubstituted benzenes.

2',2'-dimethylpropyl)benzene [ 1 9 ] . This is expressed in a significant non-equi-valence of the ortho protons and to a lesser extent of the meta protons.

The preferred conformation of the CH-t-Bu2 group with respect to the arene ring is given in Fig. 2. Moreover, it is not surprising to find the two t-Bu groups t o be non-equivalent in the corresponding Cr(C0)3 complex; the chemical shift of the exo t-Bu group corresponds to the value found in

neopentyl-7r-(tricarbonylchromium)benzene, whereas the endo t-Bu group shows an abnormal low-field resonance. The observed paramagnetic shift may be cooperatively caused by van der Waals repulsion and magnetic anisotropy of the Cr(C0)3 group. For this compound it is evident from molecular models, that in conformation II one of the t-Bu groups would interact unacceptably with the Cr(C0)3 moiety. The contribution of conformation II to the conformational equilibrium is therefore neglected for this compound. Unfavourable interactions are visually minimized in an approximately eclipsed conformation (III). It is supported by IR [ 2 0 ] , that only minor distortion of valence angles are imposed by this geometry.

Starting from a complete conformational preference (III) for R = CH-t-Bu2 (i.e. 0max ~ ~ 1 - 2 1 ppm), it is possible to estimate the conformer population of

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19 T A B L E 4 0 - V A L U E S AND P O P U L A T I O N S O F C O N F O R M A T I O N II ( P j i ) IN C 6 H 5 R C r ( C O ) 3 AT 3 3 ± 5°C R Me Et n e o P e n t i-Pr t-Bu C E t 3 CH-t-BU2 0 " ( p p m ) + 0 . 2 0 + 0 . 0 6 + 0 . 1 4 —0.10 —0.29 - 0 . 4 3 —0.605 Pu (%)'' 67 5 5 6 2 4 2 26 1 5

"& = e(meta) — eipara). * Pji = 1 0 0 X ( 0 . 5 - © / © m a x ) - with ©max = - 1 . 2 1 P P m .

the other monoalkylsubstituted complexes, using the approach of Jackson et al. [ 4 ] . The results are given in Table 4.

Tlie populations obtained are qualitatively in agreement with values reported previously [ 4 ] . The neopentyl group is found t o act like a methyl group, probably due to its positive inductive effect. It should be noted that Roques et al. [ 5 , 6 ] , in their treatment of ' H and '^C NMR chemical shifts of alkylbenzenes and cor-responding Cr(CO)3 complexes, take no account of the incomplete conformational preference reported for t-butyl-7r-(tricarbonylchromium)benzene (AG" ~ 0 . 5 kcal mol"' [ 4 ] ) . Tlie present figures reveal a free energy difference of 0.6 and 1.0 kcal mol"' in favour of conformation III for R = t-Bu and CEt3 respectively.

Although mesomeric and electronic effects of the other substituents investigated differ considerably, the signs ofO(meta) — 0{para) point to conformational pre-ferences which are consistent with results from X-ray and previous 'H NMR investigations. On the present basis further conclusions are without foundation.

A high conformational preference is obtained for trimethylsilyl-7r-(tricarbonyl-chromium)benzene (conformation III ~80%) relatively to t-butyl-7r-(tricarbonyl-chromium)benzene. This is explained by assuming that the steric interaction be-tween R = SiMe3 and the Cr(CO)3 group (though smaller than for R = t-Bu) is hardly opposed by an electronic effect favouring conformation II.

The trifluoromethyl group is known to exert a strong electron-withdrawing effect, whereas its mesomeric effect is considered to be small. In trifluoromethyl-7r-(tricarbonylchromium)benzene, this electronic effect in combination with the bulkiness of the CF3 group, is expected to lead to a strong preference for conforma-tion III. This is, however, n o t reflected in the value of d{meta) — 9(para). Addi-tional work seems required to settle this.

Comparison of the A5-values of the 1,2- and 1,4-dialkylbenzenes with those of the corresponding Cr(C0)3 complexes, does not permit firm conclusions on con-former populations. On close examination of molecular models, taking into

account the small free energy differences for the monoalkylsubstituted complexes, this is not surprising.

In 1,2-diethyl- and l,2-dineopentyl-7r-(tricarbonylchromium)benzene, the

methylene protons are diastereotopic, due to molecular asymmetry. The same holds for the methyl groups in l,2-diisopropyl-7r-(tricarbonylchromium)benzene. For related systems this was shown previously [21 ] . An excellent example of an

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L J L J L J L .

L

2 75 2 50 2 25

Fig. 3 Experimental (top) and calculated (bottom) ' H NMR spectrum of the methylene protons in 1,2-di-ethyl-7r-(tncarbonylchromium)benzene. Me / B u S i M e , + 0 06f-^;¥N+0 09 + 0 5 1 ( ^ 0 ^ + 0 3 4 + 0 0 2 ( ^ Q > J + 0 3 6 ' ^ $ > L . - 0 0 2 v J i J \ - 0 2 8 ^ - V J > J - 0 0 6 TZ Me -0 51 / B u + 0 02 31 -0 27 ' S i M e , rSu tBu

m ::''m

f B u

mi

H046<-^>'' + 0 11 tBu ME

Fig 4 Preferred conformations m some substituted 7T-(tricarbonylchromium)benzenes. (The figures given represent ^-values of the protons concerned.)

0 6 0 4 0 2 0 0 9 (ppm) ° D ° I El I /Pr 1 Me nroPent /Bu

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21

ABX3-system is given in Fig. 3, in which observed and calculated spectra of 1,2-diethyl-7r-(tricarbonylchromium)benzene are shown.

The A5-values obtained for 1,3-dialkylsubstituted 7r-(tricarbonylchromium)-benzenes generally show preference for a single conformation. As suggested pre-viously [4], 1,3-dimethyl- and l,3-di-t-butyl-7r-(tricarbonylchromium)benzene prefer the eclipsed conformations IV and V respectively, as clearly shown by the 0-values obtained here (Fig. 4). The preference found for 1,3-di-trimethylsilyl-7r-(tricarbonylchromium)benzene (VI) is in accordance with the results obtained for the corresponding monosubstituted complex.

Further support for the dominance of steric effects on the conformer equilib-rium is, given by the chemical shift comparison of the symmetric trialkylsubsti-tuted benzenes and the corresponding 7r-Cr(CO)3 complexes. As shown in Fig. 5, |3-branching of the alkyl group had almost no effect, whereas 6 was found to increase progressively upon a-branching. The data obtained for the 1,3-di-t-butyl-5-methyl substituted benzene complex fit in with those of the symmetric tri-alkylsubstituted 7r-(tricarbonylchromium)benzenes. At first sight, 1,2,4-tri-t-butyl-7r-(tricarbonylchromium)benzene would be expected to prefer the staggered conformation VII (Fig. 4). The observed ^(Hs) however, indicates a substantial contribution of the eclipsed conformation VIII (Fig. 4).

Furthermore, it should be noted that irrespective of conformational preference in the complexes, the protons on a-carbon atoms of the alkyl groups are relative-ly shielded compared to the corresponding protons in free arenes. On the other hand a deshielding of protons on /3- and 7-atoms occur. Probably, this should be related to polarisation of the C(sp^)—C(sp^) bond, which increases on complex formation [22,24].

Conclusions

In a 7r-(tricarbonylchromium)benzene complex, an alkyl group, by its elec-tronic effect, tends to orientate the Cr(CO)3 group into conformation II. Ad-verse steric interaction of a bulky alkyl group with a superimposed carbonyl ligand favours conformation III. The bulkiness of the alkyl group determines the conformational equilibrium constant. 'H NMR is shown to be useful as a probe for conformation analysis.

Care should be taken not to overemphasize steric effects; the preparation of Cr(C0)3 complexes from benzene derivates carrying a very bulky group or some t-Bu groups is easily achieved.

Ackno wledgements

The authors wish to thank Mr. A. Sinnema for helpful discussions and Dr. E. Talman of TNO Central Laboratories Delft for recording the 300 MHz spectra. We are also grateful to Messrs. B. van de Graaf, A.J.M. Reuvers and Dr. J.M.A. Baas for providing samples of some substituted benzenes, and to Mr. H.M.A. Buurmans for carrying out tlie elementary analyses.

This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

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References

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4 W.R. J a c k s o n , W.B. J e n n m g s , S.C. Rennison and R. Spratt, J. C h e m . Soc. B, ( 1 9 6 9 ) 1 2 1 4 . 5 C. Segard, B. R o q u e s , C. P o m m i e r and G. G u i o c h o n , Anal. Chem , 4 3 ( 1 9 7 1 ) 1 1 4 6 . 6 B. R o q u e s , C. Segard, S. C o m b r i s s o n and F Wehrh, J. Organometal. Chem., 7 3 ( 1 9 7 4 ) 3 2 7 . 7 W.R. J a c k s o n , C.F. P m c o m b e , I.D. Rae and S. T h a p e b m k a r n , Aust. J Chem., 28 ( 1 9 7 5 ) 1 5 3 5 . 8 F. van Meurs and H. van Konmgsveld, J. Organometal. Chem., 78 ( 1 9 7 4 ) 2 2 9 .

9 W. S t r o h m e i e r , C h e m . Ber., 9 4 ( 1 9 6 1 ) 2 4 9 0 .

10 W.R. J a c k s o n , B. NichoUs and M.C. Whiting, J C h e m . S o c , ( 1 9 6 0 ) 4 6 9 . 11 B. Nicholls and M.C. Whiting, J. Chem S o c , ( 1 9 5 9 ) 5 5 1 .

12 T . F . J u l a and D. Seyferth, Inorg. Chem., 7 ( 1 9 6 8 ) 1 2 4 5 . 1 3 M. Ashraf and W.R. J a c k s o n , J. C h e m . Soc Perkin II, ( 1 9 7 2 ) 1 0 3 .

14 C.W. Haigh, in E . F . Mooney (Ed.), Annual R e p o r t s on NMR S p e c t r o s c o p y , Vol. 4, Academic Press, London—New Y o r k , 1 9 7 1 , p . 3 1 1 .

15 L.M. J a c k m a n and S. Sternhell, Applications of NMR Spectroscopy in Organic C h e m i s t r y , Pergamon Press, L o n d o n , 1 9 6 9 , p . 2 0 3 .

16 T.K. Wu and B.P. DaUey, J. C h e m . Phys., 41 ( 1 9 6 4 ) 2 7 9 6 . 17 H. Spiesecke and W.G. Schneider, J . Chem Phys., 3 5 ( 1 9 6 1 ) 7 3 1 .

1 8 D . E . F . Gracey, W.R. J a c k s o n , W.B. Jennings, S.C. R e n n i s o n and R. S p r a t t , J. C h e m . Soc. B, ( 1 9 6 9 ) 1 2 1 0 . 19 J.M.A. Baas, R e c Trav. Chim. Pays-Bas, 91 ( 1 9 7 2 ) 1 2 8 7 .

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2 1 W.R. J a c k s o n and W.B. J e n n i n g s , J. Chem. Soc. B, ( 1 9 6 9 ) 1 2 2 1 . G. Barbieri and F T a d d e i , Org. Magn. Reson., 3 ( 1 9 7 1 ) 5 0 3 .

22 R.V. E m a n u e l and E.W Randall, J. Chem. Soc. A, ( 1 9 6 9 ) 3 0 0 2 , M.F. Guest, I.H. Hillier, B.R. Higginson and D.R. Lloyd, Mol. Phys., 29 ( 1 9 7 5 ) 1 1 3 .

2 3 F.A. Bovey, F.P. H o o d , E. Pier a n d H.E Weaver, J. Amer. Chem. S o c , 87 ( 1 9 6 5 ) 2 0 6 0 . 2 4 A. Mangini a n d F . T a d d e i , I n o r g . Chim. Acta, 2 ( 1 9 6 8 ) 8.

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23

Journal of Organometallic Chemistry, 113 (1976) 353—359

3

SUBSTITUENT EFFECTS IN 7r-(TRICARBONYLCHROMIUM)ARENES II *. AN IR STUDY OF THE CO STRETCHING VIBRATIONS IN ALKYLSUBSTITUTED 7r-(TRICARBONYLCHROMIUM)BENZENES

F. van MEURS *, J.M.A. BAAS and H. van BEKKUM

Laboratory of Organic Chemistry, Delft University of Technology, Julianalaan 136, Delft-2208 (The Netherlands)

Introduction

Several papers [1—4] report on the carbonyl stretching vibrations of TT-(tricarbonylchromium)benzene and its derivatives, obtained from IR measure-ments on solutions of the complexes. Two well-resolved CO stretching modes are observed, in accordance with the symmetry C3„. The wavenumber of the CO modes is known to reflect the effect of a substituent on the coordinated aromatic ring [1,4—6]. The occasional splitting of the out-of-phase band has been attrib-uted to an active lowering of the symmetry [7—9] by the substituents on the aromatic ring.

So far, no systematic IR study of the CO stretching modes of alkylsubstituted 7r-(tricarbonylchromium)benzenes is available. NMR work [10—13] is indicative

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for a fast equilibrium on the NMR time-scale, between conformers I and II. The question arose whether substituent effects on this conformational equilibrium could be detected by IR spectroscopy. To investigate this, the carbonyl region of several monoalkylsubstituted 7r-(tricarbonylchromium)benzenes was measured

I n

accurately. Furthermore, some other monosubstituted benzenes and a series of di- and trialkylsubstituted 7r-(tricarbonylchromium)-benzenes have been included in this study.

Experimental

Preparation of most of the substituted 7r-(tricarbonylchromium)benzenes has been described in Part I [12]. The complexes of fluorobenzene and biphenyl were prepared by the same method (m.p.'s found are 121—122°C and 84—85°C, respectively).

The IR spectra were measured on a Perkin—Elmer 521 grating spectrometer. The spectral slit-width for the CO region was approximately 2 cm"' . Spectra were recorded with a speed of 7.5 cm"' min"' ; some ill-defined maxima were recorded with a speed of 4.0 cm"' min"' . Isooctane (Uvasol, Merck) was used as the solvent. Concentrations amounted to about 0.001 M. Sodium chloride solution cells with a path length of 0.550 mm were used. The solutions were protected by a germanium filter against the visible part of the radiation, emitted by the IR light-source, and were kept in the dark outside the apparatus to prevent light-induced destruction of the complexes. The wavenumbers of the absorption maxima were calculated from the digital output of the spectrometer, using the computer program of Jones et al. [14]. The wavenumber scale was calibrated using the wavenumbers of water and deuterium chloride absorptions in this region [15]. The wavenumbers are given in Table 1 and are the average of the results from three measurements. S.d.'s in wavenumbers are 0.2 cm"' , unless stated otherwise. The half-band width's have been taken from the recorder-paper, using twentyfold wavenumber scale-expansion. These are accurate within 0.2 cm"' . The experimental error in extinction coefficients is estimated to be ± 10%. The force constants given have been calculated adopting the Cotton— Kraihanzel approximation [16] for cis-(CO)3ML3. In case of asymmetry or splitting of the E-mode the averaged wavenumber value at half band-height was used in this calculation, which seems a reasonable approximation for a small perturbation of C3„ symmetry [ 9 ] . The data obtained are given in Table 1.

The spectra at low temperature were obtained, using a Beckman-RIIC VLT-2 variable temperature cell holder. A solution cell (0.5 mm) with silver chloride windows was used.

Some representative examples of the spectra (only the E-mode) are shown in Fig. 1.

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25

TABLE 1

DATA o ON THE IR CARBONYL STRETCHING BANDS OF 7r-(TRICARBONYLCHROMIUM)BENZENE A N D DERIVATIVES

Substituents A i E K(CO) k, (mdyn (mdyn ^''max ^max '^"112 ^"m^x ^max ^"^U A " ! ) A " ' ) ( c m " ' ) (m^, cm"' (cm ' ) (m^, cm ' m o l " ' ) m o l " ' ) None M e Et neoPent i-Pr t-Bu C E t 3 CH-t-BU2 O M e S i M e 3 Ph F C O O M e CF3 1,2 Me2 1,2-Et2 1,2 neoPent2 l,2-i-Pr2 l,2-t-BU2 l,2-(SiMe3)2 1,3-Me2 l,3t-BU2 l,3-(SiMe3)2 l,4-t-BU2 l,4-(SiMe3)2 l-Me-3,5-t-Bu2 1,2,4 t B U 3 1,3,5-Me3 1 3,5-Et3 l,3,5-neoPent3 l,3,5-i-Pr3 l,3,5-t-BU3 - 4 7 - 5 3 - 5 4 - 6 1 — 7 2 - 8 9 - 8 6 - 5 4 - 6 3 - 3 7 + 7 5 + 8 2 + 14 8 - 1 0 1 — 1 2 0 - 1 3 4 - 1 2 6 - 1 5 4 - 1 1 5 - 9 2 - 1 4 4 - 9 6 — 1 4 6 - 1 1 3 — 1 9 3 - 2 2 2 — 1 3 4 — 1 5 5 - 1 6 1 — 1 8 1 - 2 0 8 850 760 770 1030 790 790 1090 960 900 990 980 800 830 850 830 1040 1170 1090 1160 1070 1040 1110 1220 1090 1110 1070 1110 990 770 930 920 1090 4 0 3 8 4 0 3 6 3 8 3 8 3 6 3 6 4 1 3 7 3 8 4 C 4 1 4 1 4 1 4 1 3 6 3 8 3 9 3 9 3 7 3 6 3 6 3 5 3 6 3 4 3 6 3 7 4 2 3 6 3 9 3 3 — 5 4 - 5 9 - 5 8 _ 7 4 c r — 6 5 6 '^—11 0 b — 1 0 5 - 1 1 O'^ — 7 6 — 5 2 - 1 2 + 10 9'* + I 2 4 C + 21 3^* - 9 5<i - 99<i r-11 5 l> 1^-16 5 b r - 9 6 '^-18 8 r—13 4 - 2 0 3 - 7 l d — 1 0 3 .-14 5'' 1-17 0 b - 8 0 - 1 6 5<* - 1 0 9 - 2 1 5 - 2 5 8 c - 1 4 7 - 1 7 3 - 1 6 2 — 2 0 2 - 2 3 1 840 590 590 830 580 440 1 430 ^ 790 630 840 720 700 600 570 530 460 630 6 4 0 , 6 2 0 ^ 5 7 0 , 6 1 0 ^ 610 1 5 7 0 ^ 600 900 6001 590^ 910 690 1090 910 680 960 690 830 850 970 8 1 9 8 10 4 9 1 11 4 13 1 10 6 11 0 9 0 10 1 10 5 11 8 12 3 12 0 13 6 13 6 13 6 1 5 7 14 9 12 3 9 0 13 0 9 0 10 6 9 1 8 2 11 1 7 2 8 9 7 6 7 6 6 8 15 188 15 108 15 099 15 100 15 080 15 060 15 033 15 039 15 081 15 101 15 156 15 329 15 373 15 473 15 016 15 002 14 973 14 976 14 933 15 033 15 033 14 950 15 055 14 941 15 016 14 865 14 823 14 966 14 928 14 936 14 885 14 841 0 356 0 359 0 358 0 357 0 362 0 363 0 363 0 362 0 366 0 349 0 342 0 346 0 329 0 333 0 361 0 353 0 357 0 362 0 361 0 342 0 360 0 360 0 346 0 363 0 352 0 364 0 362 0 360 0 362 0 354 0 363 0 364

" Solvent isooctane, temperature ~ 3 0 ° C , s d 's in wavenumbers are less than 0 2 c m " ' , see experimental

part ^i^max ~ ''max(^) — ''max(H), whereas for n (tricarbonylchromium)benzene i'max(H) equals 1 9 8 4 3 and 1 9 1 6 5 cm ' for the A j and E m o d e respectively '' S d 0 5 c m " ' '' Asymmetric at high frequency "* Asymmetric at low-frequency

Discussion

The absorptions in the carbonyl stretching region of substituted 7r-(tricarbonyl-chromium )benzenes can be assigned, using local C^^ symmetry, to a non-degenerate symmetric vibration (^i) and a doubly-degenerate asymmetric vibration (E).

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mono-CH-t-Buj 0.3

1920 1900 1M0 1920 laeo 1920 ItaO 1950 W a v e n u m b e r (cm-")

Fig. 1. Out-of-phase carbonyl stretching bands of substituted 7r-(tricarbonylchromium)benzenes, 3 0 ° C , a t - 6 8 ° C .

substituted complex. A splitting of the asymmetric mode can be expected for a small perturbation of C3„ symmetry by the substituent R on the benzene ring. Indeed, such a lifting of the degeneracy of the E-mode is shown by some com-pounds of this type [1,7]. Both electrpnic effects of the substituents and solvent-solute interactions [9] are presumed to play a role.

From 'H and '^C NMR studies [10-13] it is likely that the equihbrium be-tween conformers I and II for small substituents lies not extremely on one side in solution and that the barrier to rotation about the Cr—arene bond in mono-alkylsubstituted 7r-(tricarbonylchromium)benzenes is small (probably less than 2 kcal mol"' for R = t-Bu [17]). However, the time-scale of IR is much shorter than that of NMR. Consequently, the separate conformers with different orienta-tions of the Cr(C0)3 group with respect to the arene ring, may contribute to the observed IR spectrum.

Generally, we observed two bands in the CO region for substituted 7r-(tri-carbonylchromium)benzenes, which were assigned to the adequate symmetry types, in view of intensity ratio and half-band wddth [18].

A, -mode. In all cases only one high-wavenumber band (^i) was observed (apart from the ' ^ C-satellite which gives this mode a characteristic skew appear-ance at the low wavenumber wing). The width's show only small differences. A rather narrow A, -mode is observed for the compounds, in which the rotation of the Cr(C0)3 moiety is considered to be severely restricted.

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2 7

E-mode. The low-wavenumber absorption bands differ considerably in shape, which is illustrated in Fig. 1 and by the tabulated half-band width's. It is evident from Table 1 that this band broadens for the monoalkylsubstituted complexes in the order H < Me < Et < i-Pr < t-Bu. As shown in Fig. 1 two bands with about equal intensity are observed for R = t-Bu. However, the increase in band width is not continued for complexes carrying very bulky alkyl groups like CEts and CH-t-Buj.

The order in experimental band width's is inconsistent with an electronic perturbation of €3^ symmetry, since the electronic effects of the present alkyl groups are approximately equal. Steric effects of the alkyl group, influencing solvent-solute interactions, cannot be solely responsible, because of the discon-tinuity in half-band width observed in the series investigated. Therefore it is tempting to assume that the conformer distribution is involved as well. This indicates a pronounced conformational preference for R = Me and Et (conformer I), for R = CEt3 and CH-t-Bu2 (conformer II) and little or no preference for R = i-Pr and t-Bu. The accordance between this result and the conformer distrib-ution for these compounds, obtained from 'H NMR [12] is satisfactory. Clearly, both conformer distribution and solute—solvent interactions play an important role in the experimental band width's. Furthermore, a direct steric perturbation of C^^ symmetry may be responsible for the asymmetry of the £-band of (l'-t-butyl-2',2'-dimethylpropyl)-7r-(tricarbonylchromium)benzene.

The relatively narrow low-wavenumber band for R = SiMe3 as compared with R = t-Bu, is consistent with the change in conformer population obtained from 'H NMR [12]. The asymmetry of the £'-bands of methyl 7r-(tricarbonylchromium)-benzoate and (trifluoromethyl)-7r-(tricarbonylchromium)benzene, in which rela-tively strong electronic effects are operative, is not surprising.

The 1,2-dialkylsubstituted 7r-(tricarbonylchromium)benzenes show the largest band wddth's of all alkylsubstituted complexes investigated. This is in accordance with an electronic perturbation of the symmetry; the order of dipole moments of the aromatic ligands is ^1,2 > /"i. Mi,3 > Mi,4 , Mi,3 ,s •

For the symmetric trialkylsubstituted complexes C3 symmetry applies to all orientations of the Cr(C0)3 group. 'H NMR spectroscopy [10,12] indicates that conformers III and IV are preferred by the 1,3,5-trimethyl- and the 1,3,5-tri-t-butyl substituted complexes, respectively. For the other 1,3,5-trialkylsub-stituted 7r-(tricarbonylchromium)benzenes, the conformational preference for either III or IV is less pronounced, which might explain the trend in band width's observed.

Y

m

R R

1: m

At low temperatures (—68° C) the A, -band is shifted slightly to lower wave-number. This applies also to the position of the £-band. Furthermore, in a number of cases the asymmetry of the out-of-phase band at ambient temperature leads to a small splitting at low temperature, as is given in Fig. 1 (dotted lines). Although

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0 A 0 3 02 01 0 0 -01 - 0 2 @ A K ( m d y n / A ) c = o COOM« para

®

F Q /

""v

/ ° p h Me / Et 8/ /PrgnDMe / J B U

j

CF3 / j / D 1 / COOMe 0-1 0-1 > 0-1 0-1 0-1 t 0-1 0-1 -0.1 01 03 05 -0.1 01 03 0 5 Fig. 2. Plots of AK(CO) vs. (a) Opgro f^^l •'' ~ 0-566, correlation coefficient = 0.972, s.d. = 0.036 mdyn A"'; (6) a, derived from Omefa- '^'oara'' ^"R '^^^ ""** ^m ^'*^ t*xt), p = 0.584, correlation coefficient = 0.989, s.d. = 0.024 mdyn A"' and r = 0.226.

a small change in conformer population cannot be excluded over this temperature range, increased anisotropy of the solvent at different sites of the molecule is con-sidered to be the main cause of the splitting.

The substituent effects on the frequency of the in- and out-of-phase mode follow a regular electronic order. Consequently, this also applies to the calculated force constants, which are given in Table 1. Two correlations with a-parameters are given (see Fig. 2). The correlation with ap^^a-values [19] is fair (Fig. 2,a), a better correlation is obtained with the equation AX'(CO) = po and a-values: a = / ^ „ a ^ + ( l - / ^ ^ ) ( a 2 + rAa^ )

(Fig. 2,5), in which /^ is the fraction of conformer II. Fractions were taken from our 'H NMR work [12] for the alkyl substituents and were set to 0.5 for the other substituents. At this point it is noteworthy, that the force constant of the methoxybenzene complex and the resonance parameter (r) do not point to a strong electron-donation from this substituent. This is in contrast with the assump-tion made to explain the conformaassump-tional preference in the solid state [ 2 0 ] . For the di- and tri-substituted complexes investigated the substituent effects are approximately additive.

Conclusions

Since the electronic substituent effects play a role in both electronic perturba-tion of C3„ symmetry and conformaperturba-tional preference in the complexes, it is

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29

difficult to distinguish between these in attributing the band shapes observed. In alkylsubstituted 7r-(tricarbonylchromium)benzenes, where steric substituent effects oppose electronic effects in determining the conformational preference, it is possible to observe small effects which cannot be explained on the basis of the static model [ 8 ] . These effects might be due to changes in conformer popula-tion with increasing size of R in the particular series.

Acknowledgements

This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). We are indebted to Mr. A. van Veen and Dr. J. Reedijk for valuable discussions.

References

1 R . D . Fischer, C h e m . Ber., 9 3 ( 1 9 6 0 ) 1 6 5 .

2 D.M. Adams, Metal—Ligand and Related Vibrations, Arnold, New Y o r k , 1 9 6 7 , p . 9 7 and refs. cited; D.M. A d a m s a n d A. Squu-e, J . C h e m . Soc. A, ( 1 9 7 0 ) 8 1 4 and refs. cited

3 D.A. Brown and F.J. Hughes, J. C h e m . Soc. A, ( 1 9 6 8 ) 1 5 1 9 and refs. cited. 4 E.W. Neuse, J O r g a n o m e t a l . Chem., 99 ( 1 9 7 5 ) 2 8 7 .

5 G. K l o p m a n and K. Noack, Inorg. Chem., 7 ( 1 9 6 8 ) 5 7 9 .

6 P.S. Braterman, Metal C a r b o n y l Spectra, Academic Press, L o n d o n , 1 9 7 5 , p . 1. 7 D.A. B r o w n a n d J . R . Raju, J . C h e m . Soc. A, ( 1 9 6 6 ) 1 6 1 7 .

8 L.E. Orgel, Inorg. Chem., 1 ( 1 9 6 2 ) 2 5 .

9 P.S. Braterman, Metal C a r b o n y l Spectra, Academic Press, L o n d o n , 1 9 7 5 , p . 4 6 .

10 W.R. J a c k s o n , W.B. Jennings, S.C. Rennison and R. S p r a t t , J. C h e m . Soc. B, ( 1 9 6 9 ) 1 2 1 4 . 1 1 C. Segard, B. R o q u e s , C. P o m m i e r and G. G u i o c h o n , Anal. C h e m . , 4 3 ( 1 9 7 1 ) 1 1 4 6 .

1 2 F . van Meurs, J.M. van der T o o r n and H. van B e k k u m , J . O r g a n o m e t a l . C h e m . , 1 1 3 ( 1 9 7 6 ) 3 4 1 . 1 3 B.P. R o q u e s , C. Segard, S. Combrisson and F . Wehrli, J . O r g a n o m e t a l . C h e m , 7 3 ( 1 9 7 4 ) 3 2 7 , W.R.

J a c k s o n , C.F. P i n c o m b e , I.D. Rae a n d S. T h a p e b m k a r n , Aust. J. C h e m . , 2 8 ( 1 9 7 5 ) 1 5 3 5 . 1 4 R.N. J o n e s , T.E. Bach, H. F u h r e r , V.B. Kartha, J . Pitha, K.S. S e s h a d n , R. V e n k a t a r a g h a v a n and

R P. Young, C o m p u t e r Programs for Absorption S p e c t r o p h o t o m e t r y , N R C Bull., 11 ( 1 9 6 8 ) 3 9 . 1 5 l U P A C Commission on Molecular S t r u c t u r e and S p e c t r o s c o p y , Tables of Wavenumbers for the

Calibration of Infrared S p e c t r o p h o t o m e t e r s , B u t t e r w o r t h s , L o n d o n , 1 9 6 1 . 16 F.A. C o t t o n a n d C.S. Kraihanzel, J. Amer. C h e m . S o c , 8 4 ( 1 9 6 2 ) 4 4 3 2 . 17 W.R. Jackson, W.B. J e n m n g s and R. Spratt, Chem. C o m r a u n . , ( 1 9 7 0 ) 5 9 3 . 1 8 P.S. Braterman, Metal Carbonyl Spectra, Academic Press, L o n d o n , 1 9 7 5 , p . 4 4 .

19 H. van B e k k u m , P.E. V e r k a d e and B.M. Wepster, R e c . Trav. Chim. Pays-Bas, 78 ( 1 9 5 9 ) 8 1 5 ; A.J. Hoefnagel and B.M. Wepster, J. Amer. Chem. S o c , 9 5 ( 1 9 7 3 ) 5 3 5 7 .

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Journal of Organometallic Chemistry, 129 (1977) 3 4 7 - 3 6 0

SUBSTITUENT EFFECTS IN 7r-(TRICARBONYLCHROMIUM)ARENES III *. AN IR STUDY OF THE CO-STRETCHING VIBRATIONS IN

SUBSTITUTED METHYL 7r-(TRICARBONYLCHROMIUM)BENZOATES

F. van MEURS *, J.M.A. BAAS and H. van BEKKUM

Laboratory of Organic Chemistry, Delft University of Technology, Julianalaan 136, Delft 2208 (The Netherlands)

Introduction

The application of linear free energy relationships in the field of organometal-lic chemistry, and especially to 7r-bonded arene complexes, has received con-siderable attention in the last few years. Recent papers refer to substituent ef-fects in the alkaline hydrolysis of methyl 7r-(tricarbonylchromium)benzoates [1], the acetolysis of TT-tricarbonylchromium complexes of 2-aryl-2-methyl-l-propyl methanesulfonates [2], the pX^-values of 7r-(tricarbonylchromium)phe-nols [3] and of 77-(tricarbonylchromium)benzoic acids [4] and the NMR spec-tral data of 7r-(tricarbonylchromium)anilines [5]. For the most part the correla-tion of rate and equilibrium constants with Hammett a-values was fairly good

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31 though worse than the correlation for the respective arenes.

Several reports on the IR spectra of 7r-(tricarbonylchromium)arenes have been published [6—10]. Klopman and Noack [11] reported the wavenumbers of the carbonyl groups in twelve substituted methyl 7r-(tricarbonylchromium)-benzoates, and found that there was n o direct correlation between the ester CO wavenumbers and the Hammett a-values of the substituents. This induced us to reinvestigate the two types of CO stretching vibrations in an extended series of complexes, with substituents varying in electronic character and in steric re-quirements. In this paper we consider the CO stretching vibrations of some forty sub-stituted methyl benzoates and their 7r-tricarbonylchromium complexed analo-gues.

Experimental Starting materials

The substituted methyl benzoates were prepared by published procedures. Purity was checked by GLC. Hexacarbonylchromium (Strem Chemicals Inc.) was purified by sublimation in vacuo. Solvents were Baker analysed reagents; they were freed from oxygen by repeated degassing and saturating with nitro-gen. All preparations were carried out under nitronitro-gen.

Preparation of the substituted methyl TT-(tricarbonylchromium)benzoates The complexes were prepared by thermal reaction of equimolar quantities of ester and hexacarbonylchromium in boiling dibutyl ether at atmospheric pressure, using the apparatus described by Strohmeier [12]. The reaction was carried on until no more hexacarbonylchromium appeared in the cold regions of the system ( 1 6 - 2 0 h).

After cooling, the solvent was removed by distillation in vacuo. The residue was extracted with petroleum ether (b.p. 40—60°C) and the solution obtained was filtered. Usually, the substituted methyl 7r-(tricarbonylchromium)benzoates could be crystallized from this solution, except for R = 3-Et and 3-CF3. The methyl esters of 3-ethyl- and 3-trifluoromethyl-7r-(tricarbonylchromium)ben-zoic acid were purified by chromatography on a light-protected alumina column, using toluene/petroleum ether as the eluent.

Recrystallization from light petroleum generally yielded 50—80% of the pure complex. Analytical data and melting points are given in Table 1.

Preparation of ir-tricarbonylchromium complexes of methyl 3- and 4-phenyl-benzoates

A solution of 30 mmol hexacarbonylchromium and 20 mmol methyl phenyl-O—Cr(C0)3 K^—Cr(C0)3

O ] K^~Cr(C0)3 COOMe COOMe COOMe