301
syntheses.6 We have also found that congestion correlates well with observed stereospecificities in epoxidation and hydroboration of congested olefins20and expect that it will be applicable to other stereoselective reactions and perhaps t o steric hindrance of ionization, 2 1and structure-activity correlations. (20) W. T. Wipke, T. F. Brownscombe, and P. Gund, in preparation. (21) H. C. Brown, ref 2, p 181. (22) C. Hansch, J . Med. Chem., 13,964 (1970). W. Todd Wipke,* Peter Gund Department of Chemistry, Princeton University Princeton, New Jersey 08540 Received May 30, I973
0 BH
Synthesis of Bimetallocarboranes by Thermal Metal Transfer
Sir : In the course of our investigation of the thermal rearrangements of metallocarboranes, l,z we have discovered a new preparative route to bimetallocarboranes of the formula (C6H5)zCozCzB~H~a based upon thermally induced intermolecular metal transfer (eq 1). CsHsCoCsBsHlo --+ (C~H~)ZCOZC~B~HIO(1) A
(six isomers)
More surprisingly, similar products were obtained upon pyrolysis of the cobalticinium salt of the related commo (eq 2). metallocarborane, [(CsH5)zCo]+[Co(C~B~H1~)~]-
Figure 1. The structure of ~ , ~ - ( ~ - C ~ H & ~ , ~ - C O Z - ~ , ~ ~ - C Z B Table I. 60-MHz IH and 80.5-MHz IlB "r Spectra
Com1H C5H.5, 7
1lBb
I
4.78,4.96
I1
4.49,4.92
-15.9 (2), - 6 . 0 (l), - 2 . 6 (l), + 3 . 0 (I), + 5 . 4 (I), + 8 . 1 (11, $20.1 (1) -21.0(2), -15.2(1). -7.2(1), - 3 . 1 (2), +13.9 (2) -1.4(1), +6.4(1) -15.5 (l), - 6 . 9 (l), $ 0 . 6 (I), +5.0 (I), + 5 . 8 (I), + 9 . 9 (11, +17.1 (l), +20.5 (1) -14.0 (l), - 9 . 5 (l), - 4 . 0 (l), - l . O ( l ) , +1.6(1), +2.8(1), $ 1 1 . 5 (l), +15.9 (1) - 4 . 7 (l), 1-4.3 (2), + 1 3 . 3 (1)
pound0
7
[(C,H~)ZCOI+[CO(C;BEH~O)ZI(C5Hs)zCozGBsHio (2) (five isomers)
I11
4.44
chemical shift (re1 intensity)
IV 4.46,4.98 This discovery provides a new, experimentally convenient route to icosahedral bimetallocarboranes. Rearrangement of 1-(q-C5H6)-1-Co-2,4-CzBBHlo3~4 to V 4.48,4.71 1-(~-CsH6)-1-Co-2,3-CzBBHloS occurs at 350" under high vacuum in a hot tube' or at 145" in solution. AtVI 4.08 tempts to further rearrange 1-(q-CsHs)-1-Co-2,3-CzIn acetone-de. Ppm us. BF3.O(GHs)z; all signals doublets BBHlaat 525" in a hot tube containing ceramic saddles with JB-H = 140 =IC 20 Hz. resulted in an orange sublimate containing a mixture of products. Several species with mass spectral cutoffs at mje 370 corresponding to the 11B~12C121H205gCo2L ion three were confirmed by exact mass measurement.' and isotopic distributions consistent with the formula The llB nmr spectra and cyclopentadienyl proton (CsH6)zCozCzBsHlowere isolated. Increased yields resonances are listed in Table I. resulted when the reaction was carried out in solution. The synthesis of I-V was also accomplished by heatfor 7 hr at After heating I-(q-C6H5)-1-Co-2,3-CzBsHlo ing [(V-C~H~)~CO]+[ 1,1 ' - C O - ( ~ , ~ - C ~ B ~sH ~inter~)~]235" in hexadecane, five new red isomers and a green spersed on ceramic saddles at 525" under high vacuum isomer of (C5Hj)ZCo2C2BsH10 were produced in a total or by heating in hexadecane at 270'. Comparable yield of 32 based on starting material consumed. yields were obtained from both neutral and ionic subColumn and preparative thick-layer chromatography strates. Higher molecular weight products including were used to purify the isomers which eluted in the species believed to be alkylated bimetallics (based on order I-VI.6 Three isomers were characterized by mass spectral and electrochemical data) were also elemental analysis, while the formulas of the other produced using the hexadecane method but not examined further. (1) M. K. Kaloustian, R. J. Wiersema, and M. F. Hawthorne, J. Of the six isomers, the structure of I11 is specified Amer. Chem. SOC.,94,6679 (1972). uniquely by the nmr data as 2,9-(q-C5H&2,9-Coz(2) D . F. Dustin, W. J. Evans, C. J. Jones, R. J. Wiersema, H. Gong, S.Chan, and M. F. Hawthorne, manuscript in preparation. 1,12-CzB~H~~ (Figure 1). This is the first example of an (3) C. J. Jones, J. N. Francis, and M. F. Hawthorne, J. Amer. Chem. icosahedral metallocarborane containing metal atoms SOC.,94,8391 (1972). in the "para" positions. (4) Formulas are numbered according to the guidelines outlined in R.M. Adams, Pure Appl. Chem., 30,683 (1972). Compound VI has been previously reported as a Q
(5) W. J. Evans, G. B. Dunks, and M. F. Hawthorne, J. Amer. Chem. SOC.,95,4565 (1973). (6) Melting points (deg): I, 132-135"; 11, 225-227"; 111, >310°; IV, 255-258"; V, 239-240'; VI (green), 275-277".
(7) For example, Calcd for (CZH~)ZCO~CZB~HIO: C, 39.10; H, 5.47; B, 23.46; Co, 31.97. Found: C, 39.40; H, 5.56; B, 22.42; Co, 31.54. Calcd m/e 370.0973. Found 370.0978 i 0.0007.
Communications to the Editor
302
product of the polyhedral expansion r e a c t i ~ n , ~and !~ overwhelming support and has led to two schools of structurally characterizedg as 2,3-(~-C~H~)~-2,3-C0~-1,7thought. Reasoning primarily from product and rate C2B8H10by X-ray diffraction. The metal atoms occupy data, the first preferred the existence of either one of the cyclopropylcarbinyl (I), cyclobutyl (11), or homoallyl adjacent vertices in this isomer. While a large number of possible structures exist for I, 11, IV, and V, we cations or a rapid equilibration among two or more of tentatively propose the following atomic arrangements them. More recently, however, Olah and his collabbased on a number of empirical observations of their orators3 have suggested the inadequacy of these “classpectral and physical properties; the carbon atoms are sical” representations. in 1,7 positions in all isomers, and cobalt positions are: While all indications point to classical structures for I, 6,9; 11, 3,9; IV, 2.5; and V, 3.6.’O methyl and (geminally) dimethyl-substituted cycloThermal polyhedral rearrangements of carboranes propylcarbinyl cations, with no sign of either degenerate and metallocarboranes have been known for a number rearrangement processes or equilibration with other of years, but this is the first example of the preparation forms taking place, the data seem not to be interpret1)-vertex bimetallocarborane by pyrolysis of of an ( n able as such for the parent species. Rather, Olah best an n-vertex monometallocarborane. The synthesis of represents his (‘H and I3C) nmr data in terms of a rapidly equilibrating set of Q delocalized structures. 12-vertex neutral species by pyrolysis of the cobalticinium salt of the commo-1 1-vertex complex is even more surprising. Previous syntheses of (C5H5)2C02C2B8H10 isomers by polyhedral e x p a n ~ i o nor ~ ~contraction12 ~ produced at most two isomers. The thermal intermolecular metal transfer process described here occurs in an energetically It strikes these authors that Olah’s strongest argument rich environment and would be expected to afford a for such an assignment (or rather for deviation from a diversity of products. The predominant production of classical picture) rests not so much on direct structural icosahedral bimetallocarboranes indicates the greater evidence gathered for parent C4H7+,but more on the kinetic or thermodynamic stability of 12-vertex polyobserved lack of consistency between the nmr paramhedra and also expands the range of isomeric (CjHJ2eters for methyl-substituted ions and that for the parent Co2C2BsHlo species available for further investigation. itself. Generalization of this method to other metallocarIn this communication we present evidence, resulting boranes is under investigation and may afford a new from theoretical ab initio molecular orbital calculasource of heretofore unknown polymetallocarboranes tions, for a classical bisected cyclopropylcarbinyl type of diverse structures. structure for all three ions. We suggest, moreover, that while parent C4H7+may readily undergo facile deAcknowledgment. We thank Dr. K. P. Callahan for generate rearrangement interconverting the three equivresults prior to publication and Dr. R. J. Wiersema for alent cyclopropylcarbinyl valence tautomers, the introthe llB nmr spectra. This research was supported by duction of either one or two (geminal) methyl substituthe Army Research Office (Durham). ents greatly enhances the stability of but one cyclo(8) W. J. Evans and M. F. Hawthorne, J . Chem. Soc., Chem. Compropylcarbinyl form effectively blocking the possibility mun., 611 (1972). of further rearrangement. (9) IC.P. Callahan, C. E. Strouse, A. L. Sims, and M. F. Hawthorne, Recently we reported that all forms of the homoallyl manuscript in preparation. (10) Isomers I1 and V are also formed2t11 by thermal rearrangement cation collapsed without activation energy to bisected - ~ , ~similar -CZB rearrangements S H I O ~ ~ of of ~ , ~ - ( ~ - C ~ H ~ ) Z - ~ , ~ - C O Zand cyclopropylcarbinyl (I).8 We have now resolved the the other isomers of (CSHS)ZCOZCZBSHIO are being examined to confirm structure of the puckered cyclobutyl cation (11) on the above assignments. (1 1) B. Stibr, private communication. this same potential surface. We find that like the (12) C. J. Jones and M. F. Hawthorne, Inorg. Chem., 12,608 (1973). homoallyl systems, the puckered cyclobutyl cation is William J. Evans, M. Frederick Hawthorne* unstable with respect to distortion in the direction of biContribution No. 3192 sected cyclopropylcarbinyl, and hence may be thought Department of Chemistry, University of California of as one possible transition state to degenerate cycloLos Angeles, California 90024 propylcarbinyl-cyclopropylcarbinyl rearrangement. Received August 6, 1973 4-31G level calculations indicate an activation of 9.4 kcal/mol (the difference in stabilities between groundstate cyclopropylcarbinyl and the puckered cyclobutyl Interconverting Cyclopropylcarbinyl Cations
+
Sir : One of the longest sought after goals in mechanistic carbonium ion chemistry has been the nature of the intermediate involved in the rapid equilibration of cyclopropylcarbinyl, cyclobutyl, and homoallyl derivatives in protic solvents.’ The suggestion, made more than a quarter of a century ago by Winstein and Adams,2 that a cation might be involved has received (1) For an excellent recent review see: K. B. Wiberg, B. A. Hess, Jr., and A. J. Ashe, 111, in “Carbonium Ions,” G. A. Olah and P. v. R. Schleyer, Ed., Vol. 3, Wiley, New York, N. Y., 1972, p 1295. (2) S. Winstein and R. Adams, J . Amer. Chem. SOC.,70,838 (1948).
(3) G. A. Olah, C. L. Jeuell, D. P. Kelly, and R. D. Porter, J . A m e r . Chem. Soc., 94,146 (1972). (4) The minimal STO-3G basis6 has been used to calculate equilibrium and transition state geometries, Computations at interesting points on the resulting potential surface are then performed using the 4-31G extended basis set6 in order to more accurately access relative molecular energetics. All calculations have been carried out using the GAUSSIAN 70 series of computer programs.’ ( 5 ) W. J. Hehre, R. F. Stewart, and J. A. Pople, J . Chem. Phys., 51, 2657 (1969). (6) R. Ditchfield, W. J. Hehre, and J. A. Pople, J. Chem. Phys., 54,724 (1971). (7) W. J. Hehre, W. A. Lathan, R. Ditchfield, M. D. Newton, and J. A. Pople, Quantum Chemistry Program Exchange, University of Indiana, Bloomington, Ind., program no. 236. (8) W. J. Hehre and P. C. Hiberty, J. Amer. Chem. SOC.,94, 5917 (1972).
Journal of the American Chemical Society 1 96:l / January 9, 1974
