3078
Journal of the American Chemical Society
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100:10
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M a y 10, 1978
Solution Structure and Dynamics of Binuclear Dinitrogen Complexes of Bis( pent amet hylcyclopen t adieny1)t it anium( I I) and Bis(pent amet hylcyclopen t adieny l)zirconium(11) Juan M. Manriquez, Donald R. McAlister, Edward Rosenberg, Alan M. Shiller, Kenneth L. Williamson, Sunney I. Chan,*' and John E. Bercaw*IJ Contribution No. 5684 from the Arthur Amos Noyes Laborator11 of Chemical Physics, California Institute of Technology, Pasadena, California 91 125. Receiced October 11, I977 Abstract: The solution structure and dynamics of {(17j-CsMej)zZrNz}zN2,((11j-CsMej)zZr(CO)I2N2, {(q5-CsMe5)2Zr(PF3)}2x>,and ((q5-CjMej)2TiN2)2N2have been studied by IH N M R spectrometry. {(q5-CsMej)2ZrN2J2N2is observed to undergo mutual exchange of pentamethylcyclopentadienyl ligands between the two sites of the molecule on the time scale of the order of the ' H N M R experiments at 10 OC. Variable-temperature I5N N M R experiments for {(q5-CjMej)2Zr('5N2)12('5N2) were also carried out, and the results a r e interpreted on the basis of dissociative exchange of the two terminal dinitrogen ligands with free dissolved N2. The observation that nitrogen dissociation is 5-10 times faster than [q5-CsMes] site exchange suggests a mechanism for ring interchange involving stepwise dissociation-association of terminal IV2 ligands with "inversion" at the zirconium centers. Activation parameters calculated from I5N N M R data are E , = 1 1 kca1,mol-' and AS* = t I O eu. The observed temperature dependence of the ' H N M R spectra for the isostructural complex ((qs-CsMej)2TiN2}2Nz suggests that the same mechanism is operative. (($-CsMej)2Zr(C0)}2N2, on the other hand, does not undergo C O dissociation at a rate sufficient to observe [$-CjMes] ring site exchange by ' H N M R spectrometrq even a t 64 "C.
Bis(pentamethylcyclopentadienyl) derivatives of titanium and zirconium have proved to be useful congeners to their bis(cyclopentadieny1) analogues by virtue of enhanced stability, solubility, and crystallizability. Dinitrogen complexes of (q5-C5Me5)2Tiand (v5-C5Me5)2Zr are of particular interest in view of the ready protonation and reduction to hydrazine of their ligated N2.3-8We have recently reported the solid-state structures of {(v5-C5Me5)2Ti)2Nz8 and ((v5-C5Me5)2ZrN2J2N2' as determined by single-crystal x-ray diffraction methods. I n this paper we report the results of an N M R and I R study of the solution structure and dynamics of {($CsMe5)2ZrNz}2Nl,its carbonyl and PF3 derivatives, and the titanium analogue.
Experimental Section Physical Measurements. ' H N M R spectra were recorded on a Varian H R 220 (CW) spectrometer. IsN N M R spectra were obtained at 18.25 M H z on a Bruker W H 1 8 0 (FT) spectrometer. Computersynthesized spectra were obtained using DNMR3, a general N M R line-shape program with symmetry and magnetic equivalence factoring, written by Binsch and Kleier.9 Infrared spectra were obtained on Perkin-Elmer 180. 225, and 457 and Beckman IR-12 spectrophotometers. Materials. All manipulations were performed on a vacuum line, in a glovebox which was evacuated to 3a(FO2).
The success of the polyhedral skeletal electron-count theory2 in rationalizing the geometries of a wide array of cage and cluster compounds and in providing a reliable means of predicting the structures of new species is well d o c ~ m e n t e d .The ~,~ theory is particularly useful in dealing with cluster systems which are not readily amenable to detailed molecular orbital calculations, such as those having low symmetry or containing several types of framework atoms. While full awareness of these ideas among chemists has not yet arrived, inorganic textbooks have begun to incorporate them, and in time the art of skeletal electron counting may be as familiar in chemical education as is, for example, the valence shell electron pair repulsion (VSEPR) model.4 Like the VSEPR and other qualitative approaches to chemical bonding, it is inevitable that limits t o the utility of the skeletal electron-count theory will be found; situations will be discovered in which the theory can be applied only with modification, and others in which it fails altogether. It is obviously important to the further development of cluster chemistry to probe these limiting cases and to try to identify the factors which cause deviation from the predicted structures. It can be hoped that in time this will lead to a more sophisticated version of the theory which will increase its usefulness still further. 0002-7863/78/ 1500-3083$01 .OO/O
In our work we have encountered several situations in which the electron-counting rules are violated, a recent example being the 14-vertex (q5-C5H5)2Fe2(CH3)4C4BgHg isomers which are discussed e l ~ e w h e r eIn . ~ the present article we describe the crystallographic structure determination of a recently prepared6 metallocarborane, (q5-C5H5)Co(CH3)4C4B~H60C2Hj, which is one of five structurally established examples of a 12-vertex, 28-electron cage system. Of these five compounds, we are amazed to find that no fewer than four markedly different polyhedral geometries are represented, and hence a detailed comparison of their structures seems warranted. Experimental Section Red crystals of 12-(C5H 50)-1 ,2,3,7,8-($-C jH j)Co(CH3)4C4B7H,jr prepared6 by the treatment of [(CH&C*B4H4]2FeH* with CoC12 and CsH6 in ethanolic KOH at 70 OC, were grown by the vapor diffusion of pentane into a methylene chloride solution of the compound. One crystal was mounted on a glass fiber in an arbitrary orientation and examined by preliminary precession photographs which indicated acceptable crystal quality. The chosen crystal was a rectangular parallelepiped with one edge truncated, creating a seventh face. Approximate maximum dimensions were 0.21 X 0.23 X 0.48 mm. Crystal data follow: CoOClsB7Hz8; mol wt 359.00; space group
0 1978 American Chemical Society
