688
Organometallics 1983, 2, 688-690 9.
Direct Observation of Metastable Organometallic Cation Radicals from Group 4 8 Alkyls B. W. Waither," F. Wllllams,*'a W. Lau,lb and J. K. Kochi*lb Departments of Chemistry, University of Tennessee Knoxville, Tennessee 37996 and Indiana University Bloomington, Indiana 47405'
Summary: Transient organometallic cations are generated by electron detachment from various tetrahedral group 48 alkyls. Analyses of the ESR spectra of the lead and tin analogs (Me,Pb+, Me,Sn+, and t-BuSnMe,') are in accord with unusual trigonal-pyramidal structures of C ,, symmetry.
The homoleptic organometals, particularly of the group 4B metals (silicon, germanium, tin, and lead), are excellent electron donors.2 For example, the permethyl derivatives are all readily oxidized by outer-sphere electron transfer to various oxidants in solution or at an electrode ~ u r f a c e . ~ Although the kinetics of the chemical and electrochemical oxidations involve a rate-limiting, one-electron process, i.e. Me4M
[-el
Me4M+
b.
(1)
M = Si, Ge, Sn, P b the existence of the cations Me4M+etc. as direct products has merely been inferred, owing to their extremely short lifetimes. Coupled with the relevance to organometallic reaction mechanisms of the ubiquitous charge transfer (CT) interactions extant between such alkylmetals and various ele~trophiles,~ e.g.
-
Me4MBr2
h m
Me4M+Br2-
(2)
we have felt that it is important to establish unambiguously the existence of organometal cations by direct observation and to determine the structure of these transient species as viable intermediates. Indeed, recent developments in the y irradiation of solid solutions have allowed the tetramethylsilane and tetramethylgermane cation radicals to be detected by electron spin resonance (ESR) spectro~copy.~In the course of completing the study with the important lead and tin analogues, we have discovered unusual structural effects which we report herein, since they provide unique insight into these novel organometal cations. The first-derivative ESR spectrum obtained from a dilute solution of Me4Pb in trichlorofluoromethane at 90 K after y irradiation is shown in Figure l a (upper). The ESR spectrum derived from Me4Sn under the same conditions is shown in Figure lb. While these studies were 4B metals (silicon, germanium, tin, and lead), are excellent (1) (a) University of Tennessee. (b) Indiana University. (2) Jonas, A. E.; Schweitzer, G. K.; Grimm, F. A.; Carlson, T. A. J. Electron Spectrosc. Relat. Phenom. 1972,1,29. Evans, S.; Green, J. C.; Joachim. P. J.; Orchard, A. F.; Turner, D. W.; Maier, J. P. J. Chem. Soc., Faraday Trans. 2 1972,68,905. Boschi, R.; Lappert, M. R.; Pedley, M. B.; Schmidt, W.; Wilkins, B. T. J . Organomet. Chem. 1973, 50, 69. (3) Wong, C. L.; Kochi, J. K. J. Am. Chem. SOC.1979, 101, 5593. Klingler, R. J.; Kochi, J. K. Ibid. 1980, 102, 4790. (4) (a) Kochi, J. K. 'Organometallic Mechanisms and Catalysis";Academic Press: New York, 1978 Part 111. (b)Fukuzumi, S.;Mochida, K.; Kochi, J. K. J . Am. Chem. SOC.1979,101,5961; 1980,102,2141; J . Phys. Chem. 1980,84, 2246, 2254. (5) Walther, B. W.; Williams, F. J. Chem. Soc., Chem. Commun. 1982, 270.
*
Figure 1. (a) X-band ESR spectrum of a yirradiated solution of 3 mol % tetramethyllead in trichlorofluoromethane at 85 K shown in first derivative (upper) and second derivative (lower). (b) First derivative ESR spectrum derived from tetramethyltin under similar conditions. The features marked with the asterisk are photobleached by visible light. Table I. ESR Parameters of the Organometal Cations of Group 4B Alkyls' 117,119
cation
proton hfs, G
Sn hfs, G
g value
ref
Al(3 H ) b 14.7 2.111 f A l ( 3 H ) b 13.7 77 (I) 2.044 f 13-14 78 (I) 2.044 g (H,C),CSnMe,+ A l ( 9 H)C 7.6 88 (I) 2.046 f 100 (1) 2.110 (I) h Me,SnSnMe,+ A l ( l 8 H) 3.4 Me,SnGeMe,+ d 115 (I) 2.077 (I) f Me,GeGeMe,+ A l ( ( l 8 H) 5.18 2.0441 (I) i Me,GeSiMe,' A l ( l 8 H)e 5.37 2.0274(1) f Me,SiSiMe,+ A l ( l 8 H ) 5.55 2.0086' i k A (18H) 5.65 H,CPbMe,+ H ,CSnMe,+
Three a In trichlorofluoromethane matrix at MelGe > Me4Sn > Me4Pb as 9.42,9.38,8.85,8.38 order is probably maintained for the inner d subshells of these metals.2 For the Jahn-Teller effect on the photoelectron spectra of the series of Me4M, see ref 2a. (10)For example the heata of formation of methyl, ethyl, isopropyl, and tert-butyl radicals decrease progressively as 34,26, 17.5 and 7 kcal mol-', respectively, at 25 "C. Streitwieser, A., Jr.; Heathcock, C. H. 'Introduction to Organic Chemistry", 2nd ed.; Macmillan: New York, 1981;p 103. For recent measurements, see: Castelhano, A. L.; Marriot, P. R.; Griller, D. J. Am. Chem. SOC. 1981,103, 4262. (11)It is noteworthy that the ESR spectrum of tert-butyl radical is not observed even when the temperature of the matrix is raised to 140 K, and there is a significant decay of the cation. Similarly, the ESR spectrum of the methyl radical is not observed when the matrix temperature of Me4Sn+is raised to 155 K. In this regard, our results differ from those reported by Symons? who managed to trap methyl radicals in his system. Unfortunately, we have not yet been able to repeat this result. (12)May, D.D.;Skell, P. S. J. Am. Chem. SOC.1982,104,4500. (13)For example, steric inhibition to the attainment of planarity in the Me3Sn moiety during fragmentation of t-BuSnMe3+ is conceptually analogous to the explanation put forth by May and Skell12for the relatively slow loss of carbon dioxide from t-BuCO,.. Altematively, a reviewer has suggested an explanation for the apparent persistence of t-BuSnMe3+ compared to the other analogues which is based on polar effects. For example, the relatively low ionization potential of the tert-butyl radical [IP (eV): Me. (9.84),Et. (8.51),i-Pr. (7.69),t-Bu. (6.92)by Houle,.F. A.; Beauchamp, J. L. J.Am. Chem. SOC.1979,101,4067and discussion by Rollick, K. L.; Kochi, J. K. Jbid. 1982, 104, 13191 indicates that the tert-butyl ligand can lead to an enhanced charge stabilization in the cation radical.
690
Organometallics 1983, 2, 690-692
Me3M'MMe3+,which can be generated from their neutral diamagnetic precursors by a similar pr0~edure.l~ Pertinent to the structure of the trimethyltin moiety in 11,the perpendicular component of the tin splitting in the ESR spectrum of the ditin species Me3SnSnMe3+was found to be 100 G, suggesting that the configuration about each tin center is nearly planar as in II.8 Similarly, we found the tin splitting in the heterobimetallic species Me3GeSnMe3+ to be of the same order of magnitude. The ESR parameters listed in Table I thus relate the tetraalkylmetal cations to the family of hexaalkyldimetal cations in a single consistent pattern.I5
Acknowledgment. We thank the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, and the National Science Foundation for financial support of the research carried out at Tennessee and Indiana, respectively, and Dr. K. Mochida for a sample of Me3SnGeMe3. Registry No. H3CPbMe3+,85080-92-8; H3CSnMe3+,8449488-2; (H3C)3CSnMe3+,85005-13-6; Me3SnSnMe3+,81419-26-3; Me3SnGeMe3+, 85005-14-7; Me3GeGeMe3+, 79644-92-1; Me3GeSiMe3+,85005-15-8;Me3SiSiMe3+,77958-47-5. (14)For the donor properties of these dimetallic systems, see: Szepes, L.; Korhyi, T.; N&ray-SzaM, G.; Modem, A.; Distefano, G. J: Organomet. Chem. 1981,217,35. (15)(a) For the related charge-transfer reactions see ref 4. (b) The sizeable configurational changes incurred during cation formation in these donor systems is no doubt related to the large reorganizational energies X observed during electron transfer. See: Klingler, R. J.; Kochi, J. K. J. Am. Chem. SOC. 1981,103,5839. (16)Wang, J. T.; Williams, F. J . Chem. SOC., Chem. Commun. 1981, 666. (17)Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981,79, 179.
Hydrocarbon-Hydrogen Interactlons wlth Metals. A Molecular Orbital Analysis of HFe,( CO),,( v2-CH) Catherine E. Housecrofl and Thomas P. Fehlner' Department of Chemistry, University of Notre Dame Notre Dame, Indiana 46556 Received November 3, 1982
Summary: The electronic structure of HFe,(C0),,(v2-CH) has been examined by using the Fenske-Hall quantum chemical approach with a fragment analysis in terms of the butterfly metal cluster HFe,(CO),,+ and the ligand CH-. The preference for the tilted (v2)orientation of the CHligand over a symmetric vertical orientation can be explained in terms of the unusual properties of the frontier orbitals of the butterfly fragment. The q2 orientation causes the CH bond to be weakened in the complex because of the mixing of an empty CH antibonding orbital with a filled metal cluster orbital.
While characterizing the electronic structure of HFe4(CO)12BH21using the Fenske-Hall quantum chemical approach,* we had cause to examine the isoelectronic compound HFe4(C0)&H, Ib.3 In doing so we observed (1)Wong, K.W.; Scheidt, W. R.; Fehlner, T. P. J . Am. Chem. SOC. 1982,104,1111.Fehlner, T. P.; Housecroft, C. E.; Scheidt, W. R.; Wong, K. S. Organometallics, in press.
0276-7333/83/2302-0690$01.50/0
:FelC013
0.c 0 .H
Figure 1. Generation of HFe4(C0)12(T2-CH) by the capping of an alkylidyne triiron complex, [HFe3(CO)&HI2-,with a Fe(C0):' fragment: X(ca1cd) = 1.82 A, X(measd) = 1.75 A.3 [HFe3(CO),CHI2-geometry was obtained from the known structure of H3Fe3(CO)SCCH3.8
the properties of a tetrametal "butterfly" fragment that facilitate the binding of a CH ligand in a tilted ($) geometry. Thus, not only does the nature of the ligand bonding generated by this multinuclear array of metal atoms provide a mechanism for B-H bond weakening,l but also it suggests one for CH as ell.^,^ Compounds containing transition-metal borane-hydrogen interactions are common;6however, those with metal hydrocarbon-hydrogen interactions are not. Ib has been proposed as a reasonable model for C-H bond activation on a metal surface.' A comparison of the bonding in the observed (tilted) structure with the hypothetical more symmetric (vertical) structure Ia reveals the orbital properties of a tetrametal "butterfly" fragment that permit the v2 binding of CH.
The primary expression of electronic structure is the geometrical relationship between the observed nuclear positions. A fragment analysis of Ib that is very revealing in this regard is shown in Figure 1. The observed geometry of Ib is quantitatively generated by capping a FezC face of a doubly deprotonated (k,-methylidyne)triiron nonacarbonyl complex8with a Fe(C0)32+fragment. The CH axis in the experimental geometry lies close to a C3axis of one metal triangle of the butterfly. Unless fortuitous, (2)Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972,11,768.Hall, M. B. Ph.D. Thesis, University of Wisconsin, Madison, WI, 1971. Fenske, R. F. Pure Appl. Chem. 1971,27,61. (3)The synthesis and structural characterization of the compound HFe4(C0)&H has been reported in detail. Tachikawa, M.; Muetterties, E. L. J. Am. Chem. SOC. 1980,102,4541.Beno, M. A.;Williams, J. M.; Tachikawa, M.; Muetterties, E. L. Ibid. 1980,102,4542. Beno, M. A.; Williams, J. M.; Tachikawa, M.; Muetterties, E. L. Ibid. 1981,103,1485. (4)The C-H bond distance in Ia is significantly longer than the accepted value for hydrocarbons (1.19viz. 1.09 A).3 (5)M-H-C interactions in mononuclear complexes can lead to C-H bond weakening, but apparently to a lesser degree than is accomplished by binding to a multinuclear metal fragment: Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J . Am. Chem. SOC.1980,102,7667. A reported interaction between a 8-CH and a single metal center actually leads to C-H bond shortening: Dawoodi, Z.; Green, M. L. B.;Mtetwa, V. S. B.; Prout, K. J . Chem. SOC.,Chem. Commun. 1982,802. (6)Housecroft, C. E.;Fehlner, T. P. Adu. Organomet. Chem. 1982,21, 57. ( 7 ) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. Reu. 1979,79,91. Gavin, R.M., Jr.; Reutt, J.; Muetterties, E. L. Roc. Natl. Acad. Sci. U.S.A.1981,78, 3981. (8)Wong, K. W.; Haller, K. J.;Dutta, T. K.; Chipman, D. M.; Fehlner, T. P. Inorg. Chem. 1982,21,3197.
0 1983 American Chemical Society
