1339 electropositive elements might be expected to have this sign. This is so for ,J(X-C-H) in (CH,),X and related compounds (X = C, Si, Sn, or Pb, allowance being made for the negative magnetogyric ratios of the magnetic Si and Sn isotopes). However, it has been found27 that the negative sign for the geminal H-H coupling applies only if the intervening atom is small (e.g., C), but with other intervening atoms the coupling constant becomes positive and increases with increasing atomic number. A similar result also applies to geminal carbon to hydrogen couplings. Thus the reduced" Pb-Pb-C coupling constant should be positive and larger than the Sn-Sn-C one. This is the case. The 'H and I3C chemical shifts of hexamethyldilead differ only slightly from those found for (CH,),Pb and may well depend on the conditions of measurement. The large high-field shift of the ,07Pb resonance is consistent with the corresponding value of 113 ppm for the '"Sn
+
(27) H. Dreeskamp and C. Schumann, Chem. Phys. Left., 1, 555 (1968). (28) The reduced coupling constant is defined" by KAB= JAB/
fi YAYS.
(29) J. A. Pople and D. P. Santry, Mol. Phys., 8, 1 (1964).
shift in hexamethylditin, the extremely large range3' of ,07Pb chemical shifts being borne in mind. It is generally considered that the local paramagnetic term is the dominant contributor to lead hi el ding,^' and this depends inter alia on the reciprocal of A E (the separation between the ground and excited states) in such a way that small values of A E should correspond to low-field resonances. The cream color of (CH,),Pb, suggests that A E is relatively small ; hence dominance by the paramagnetic term should lead to a '07Pb chemical shift in (CH,),Pb, to the lowjeld of (CH,),Pb. The results for 59C0 and Ig5Pt chemical shifts may be compared here.,,',, We therefore conclude that ,07Pb chemical shift differences between related lead compounds are not necessarily dominated by the paramagnetic term and the neighboring anisotropy effect of the (CH,),Pb group may be more important in hexamethyldilead. (30) C. J. Jameson and H. S . Gutowsky, J . Chem. Phys., 40, 1714 (1960). (31) W. G. Schneider and A. D. Buckingham, Discussions Faraday SOC.,34, 147 (1962). (32) S. S. Dharmatti and C. R. Kanekar, J . Chem. Phys., 31, 1436 (1959). (33) W. McFarlane, Chem. Commun., 393 (1968).
Stereochemically Nonrigid Organometallic Molecules. XX. ' Proton Nuclear Magnetic Resonance Study of the Fluxional Behavior of Some Substituted (1,2,7-Trihaptobenzyl)( pentahaptocyclopentadieny1)dicarbonyl Compounds of Molybdenum and Tungsten's 3a F. A. Cotton and Tobin J. Marks3b Contribution from the Department of Chemistry, Massachusetts tnstitute of Technology, Cambridge, Massachusetts 02139. Received October 3, 1968 Abstract: Several new fluxional molecules containing substituted 1,2,7-trihaptobenzyl groups bound to the (C5Hs)Mo(CO)t and (CsH5)W(C0)2 residues have been prepared and their proton magnetic resonance spectra studied with the objective of elucidating the pathway and evaluating the activation parameters concerned in the flw5onal behavior of this class of molecules. The molecules are less difficult to prepare and more stable than previously reported. The chief qualitative conclusion, derived from the study of the 3,5-diisopropylbenzyI compound, is that the (CsHs)Mo(CO)z residue has access to all four equivalent (including enantiomorphically related) positions of attachment to the benzyl group. A plausible pathway would be via a monohopto- (i.e., 0-) CsHsCH2Mo(CsHs)(CO)z intermediate, in which rotation about the C1-G bond of the benzyl group can occur.
T
he first example of a compound containing a benzyl group bonded to a metal atom in a manner which could be formally considered to involve the C6HsCH, group serving as a 3-electron donor to the metal atom (or (1) Part XIX: F. A. Cotton and C. Reich, J. A m . Chem. SOC.,91, 847 (1969). (2) A more conventional but less precise name for these compounds
would be (rr-benzyl)(x-cyc1opentadienyl)dicarbonylmetal compounds. (3) (a) This work was supported in ;drt by a grant from the Petroleum Research Fund, administered 5j the American Chemical Society, to whom grateful acknowledgment is made. (b) National Science Foundation Predoctoral Fellow, 1966-1969.
the C,H5CH2- anion serving as a 4-electron donor to a cation) was reported in 1966 by King and F r ~ n z a g l i a . ~ The compound in question is (C6H5CH,)(C5H,)Mo(CO), and King and Fronzaglia proposed that the benzyl group is attached to the molybdenum atom through an allylic sequence of three carbon atoms, one of which is the exocyclic methylene carbon atom. In short, the suggested structure was that which would be designated in a recently proposed notation5 as (1,2,7-trihaptobenzyl)(pentahapto(4) R. B. King and A. Fronzaglia, J . Am. Chem. SOC.,88, 709 (1966). (5) F. A. Cotton, ibid., 90, 6230 (1968).
Cotton, Marks 1 Nmr of (1,2,7-Trihaptobenzyl)(pentahaptocyclopentadienyl) dicarbonyl Compounds
1340
cyclopentadienyl)dicarbonylmolybdenum, or (h3-CH,C6H,)(h5-C,H5)(C0)2Mo. A schematic representation is shown as I.
I
111
CH3 IIa, M = Mo b,M=W
IV
Of even greater interest, however, was the observation that the proton magnetic resonance (pmr) spectrum of the benzyl group showed a pronounced temperature dependence, changing from a pattern consistent with I at - 30" to a simpler pattern at t-64". The high-temperature pattern, its relationship to the low-temperature pattern, and the detailed nature of the line-shape changes at intermediate temperatures led to the conclusion that some process (or processes) of intramolecular rearrangement leading to time-average equivalence of the three pairs of protons on opposite edges of the benzyl group was accelerating markedly over the temperature range studied. Several hypotheses as to the rearrangement pathway were described, but no case was pressed for any particular one. It is worth noting that the observations on this compound constitute one of the earliest reported descriptions of a fluxional organometallic molecule' in which the limiting low-temperature spectrum and the spectral changes leading thereto were observed. However, no detailed analysis leading to a more thorough empirical description of the process or to a closer definition of the mechanism was presented. The system seemed to us to merit this kind of study, and the investigation reported here was therefore undertaken. In conjunction with the chemical and pmr studies described here, an X-ray crystallographic study of the 4-methyl analog of I was carried out;' it showed that structures of type I do occur in the C,H,Mo(CO),-benzyl compounds and revealed structural details of importance in discussing rearrangement pathways. The detailed, three-dimensional structure is shown in Figure 1.
Preparation of Compounds The three compounds of main interest in this work are all new ones, having substituents on the benzyl ring. Using the numbering scheme shown in I, they may be designated as the 1,2,7-trihapto-4-methylbenzylderiva(6) CJ F. A. Cotton, Accounrs Chem. Res., 1 , 257 (1968), for a survey of fluxional organometallic molecules and definitions of various terms used here. (7) F. A. Cotton and M. D. LaPrade, J. Am. Chem. Soc., 90, 5418 (1968).
Figure 1. A quasi-perspective view of the (h3-4-CH3C6H4CH2)(h5-C5H5)(C0)2M~ molecule, showing details of the stereochemistry and the more important interatomic distances (data from ref 7).
tives of the (C5H5)M~(C0)2 and (C,H,)W(CO), groups, IIa and IIb, respectively, and the 1,2,7-trihupto-3,5diisopropylbenzyl derivative of (C,H,)Mo(CO),, 111. All were obtained by way of the appropriate monohuptobenzyltricarbonyl intermediates following the general preparative route of King and F r ~ n z a g l i a . ~ For the molybdenum compounds, IIa and 111, we found that conversion of the monohaptobenzyltricarbonyl compounds to the trihaptobenzyldicarbonyl compounds was conveniently accomplished by thermal decarbonylation. Only a few hours heating was required in any preparation, whereas King and Fronzaglia4 indicated as the method of choice ultraviolet irradiation for 5 days to prepare I. For the tungsten compound, IIb, the photochemical method was found to be superior to simple heating. Each of the dicarbonyl compounds we have prepared and studied shows only two sharp CO stretching bands in the infrared spectrum at 25". Thus, unlike the simple allyl complex,8~9C,H5Mo(CO),(C,H,),these compounds appear to be isomerically pure in solution at room temperature.
Nuclear Resonance Studies Plan of Attack. Before reporting the experimental results and considering their interpretation, it is necessary to explain the preliminary analysis upon which the design of the experiments was based. A benzyl group, C6H,CH,, possesses two planes of symmetry, one containing all of the atoms and the other perpendicular to the first and passing through the atoms C,, C4, and C7. As Figure 1 shows, the metal atom of the (C,H,)Mo(CO), group does not lie in either of these planes. In order to discuss its location (and the locations of certain ring substituents) relative to the benzyl skeleton, we shall use the terms face and edge and avoid the equi-
Journal of the American Chemical Society / 9 1 : 6 1 March 12, 1969
(8) R. B. King, Inorg. Chem., 5 , 2242 (1966). (9) A. Davison and W. C. Rode, ibid., 6, 2124 (1967).
1341 shift and have therefore planned and carried out experiments intended to provide such information. One particular distinction we wished to make among the several possible site exchange processes was between the u t, p type, in which Malways remains over the same face, and any other type or combination of types in which M would go from one face to the other. We shall designate these two classes of shift as suprafacial and antarafacial shifts, respectively, It would appear that the published results of King and Fronzaglia can be explained by the simple postulate of suprafacial (t~c, f3) shifts, but the observations are not inconsistent with certain antarafacial shift processes or with complex combinations of both suprafacial and antarafacial shifts. It is clear that in order to determine by pmr studies whether antarafacial shifts occur frequently, the benzyl ring must be provided with one or more proton-bearing substituents capable of sensing differences between the Figure 2. A schematic drawing in which both the CsHs ring and environments over the two faces of the benzyl group. the benzyl group are seen edge-on, showing the four chemically Two additional requirements are (1) that the substituent(s) equivalent locations of the (C,H,)(CO),Mo group relative to the be placed sufficientlyfar from the region of bonding of the benzyl group. The initial position, u, is represented in heavy lines. benzyl group to M that it will not seriously influence the Other positions, 13, 7 , 6 , are shown in lighter lines. nature of the fluxional process and (2) that the equivalence of the two edges of the benzyl group be preserved. These restrictions leave as the only practical possibilities the vocal term side. The two surfaces of the C,H,CH2 group introduction of one substituent in the 4 position or two (ix.,the two sides of the first symmetry plane mentioned identical substituents in the 3 and 5 positions. The use of above) will be called thefaces of the benzyl group. The the 3 and 5 positions was considered preferable since the setsofatoms(cf.I)C7,C,,C,,C3andC7,C,,C,,C5define effect of environmental differences on the two faces what we shall call the edges of the benzyl group. In originating in the region of C7, C,, C,, C6 might be too Figure 1 we see the Mo atom lying in a position defined attenuated for a substituent at the 4 position. (in part) by the specification of one of the two faces and Finally, there remained the question of what substituent one of the two edges, It is also important to note that the to use. Several possibilities involving rigid bodies constructure is further characterized by a particular rotational strained to keep some hydrogen atoms permanently over orientation of the (C,H,)Mo(CO), group relative to the each face of the benzyl group were considered but rejected. benzyl group, taking a line through the Mo atom and Such molecules were necessarily large and elaborate, perpendicular to the mean plane of the benzyl group as the thereby introducing serious synthesis problems and axis of rotation. possibly solubility difficulties. Instead, isopropyl groups It is seen in Figure 2 that there are four equivalent posiwere used in the 3 and 5 positions. It is now well known tions for the (C,H,)Mo(CO), group relative to the benzyl that when a -CXY, group is bound to some dissymmetric group. The two positions in each of the pairs (a, 6) and group the two Y groups cannot achieve equivalence in any (p, y ) are equivalent in the fullest sense, because the rotational conformation or as a result of internal rotation, corresponding molecular configurations are interchangehowever rapid. The simplest alkyl CXY, system would able by rotations of the entire molecule as a rigid body ; be the ethyl group (X = CH,, Y = H), but here the the a and 6 configurations are enantiomorphous to the f3 geminal H-H coupling superimposed on the quartet and y configurations, respectively. It should also be splitting due to the CH, group leads to a complex broad noted that in order to change one of the configurations to resonance," which would be unsuited to our purposes. an enantiomorphous one, not only must the location of The isopropyl group, however, is well suited to our objecthe metal atom be changed, e.g., from c1 to p, but an intertives since CH,-CH3' coupling will be negligible and the nal rotation of the (C5H5)(C0)2M0group relative to the methyl resonances will be split only into doublets by the benzyl group must also occur. unique proton. There are several recent examples of this For brevity in the following discussion, the entire kind of use of isopropyl groups to detect inversion of (C5H5)(C0),Mo or (C5H5)(C0)2Wgroup will be denoted dissymmetric moieties to which they are attached; in M, and the statement that M occupies site c1 (or p, y, 6) will particular, it was the elegant study'' by Whitesides, imply that it has the rotational orientation as well as the supplemented by discussions with Professor Whitesides, positional coordinates appropriate to that site. which led to our adoption of the isopropyl substituents. It was the objective of this investigation to determine If only suprafacial shifts occur in the 3,5-diisopropyl how many of the four sites E, p, y, 6 are accessible to the compound, then, in terms of the labeling scheme shown in M group with sufficient frequency to produce observable Figure 3, there will be only g c,h and g' c,h' exchanges. effects in the pmr spectra. To put it another way, let us Thus, the four separate methyl (doublet) resonances which assume that a molecule starts with M in site c1. There are three distinct types of site exchange possible, viz., u c, p, (10) See, for example, the resonance observed in an early study of this phenomenon by J. S. Waugh and F. A. Cotton, J. Phys. Chem., a- y, and u c,6, and each will have detectably different 65, 562 (1961). effects in suitably designed pmr experiments. We wished (11) G. M. Whitesides and W. M. Bunting, J. Am. Chem. SOC.,89, to know the relative importance of these three types of 6801 (1967). Cotton, Marks 1 Nmr of (1,2,7-Trihaptobenzyl)(pentahapfocyclopentadienyl) dicarbonyl Compounds
1342 Table I. Activation Parameters' for Site Exchanges
Log A
Ea kcal/mol
AH*, kcaljmol
AS*,
Protonsb a, b c, d a, b c, d c, d c, d
16.6k0.2 16.210.3 13.5k0.2 14.91-0.3 14.6k0.3 14.9k0.3
19.2i0.3 18.8k0.4 15.9kO.2 18.7k0.4 1 8 . 3 +O. 5 19.1k0.4
18.6k0.3 18.2k0.4 15.3k0.2 18.1k0.4 17.7i0.5 18.4k0.4
15.6k1.3 14.0+ 1.3 1.3k1.1 7.5k1.3 6 . 2 i 1.3 7 . 6 k 1.3
3
Compound IIa IIa IIb I11 I11 I11
Solvent CDC13 CDC13 CD~CBDS CDC13 CDJCBDS (CD3)zCO
eu
' A H * and AS* were evaluated at the temperatures giving mean residence times of 0.01 sec. *As identified by lower case letters in Figures 3 and 5.
c--- - - ----.Y
