UNEXPECTED SYNTHESIS OF ORTHO-SUBSTITUTED DISELENOPHENYLENEZIRCONOCENES FROM THE PARASUBSTITUTED DIPHENYLZIRCONOCENES - CHEMICAL AND STRUCTURAL EVIDENCE OF THE PARTICIPATION OF A CYCLOMETALATED INTERMEDIATE THAT BEHAVES AS A BENZYNE ZIRCONOCENE EQUIVALENT

Heating grey selenium with [eta(5)-C5H4(t-Bu)]Zr-2(C6H4(p-OCH3))(2) in boiling octane affords the diselenophenylenezirconocene complex conta ining a methoxy group in the ortho position. A similar reaction, altho ugh less selective, occurs when starting from [eta(5)-C5H4(t-Bu)]Zr-2( C6H4(p-CH3))(2). (C6H4(p-CH3))(2). The structures of the diselenopheny lenezirconocenes obtained were confirmed by NMR spectroscopy and defin itively proved by comparison with the authentic orthosubstituted deriv atives prepared from the corresponding methyl-o-anisyl- and methyl-o-t olylzirconocene. The mechanism proposed to explain this transformation involves the activation of a tert-butyl C-H bond by a transient benzy nezirconocene species. The cyclometalation product, which is obtained by heating the diphenylzirconocene, reacts toward selenium and unsatur ated organic compounds like the benzynezirconocene but the reaction ta kes place at room temperature. The cyclometalation pathway allows a mo re sterically hindered selenium-containing derivative [eta(5)-C5H4(t-B u)]2ZrSe2C6H3(o,o'-(CH3)(2)) to be prepared. The new diselenophenylene zirconocenes described are good precursors to generate benzodiselenage rmoles and related spiro compounds by a zirconium-germanium transmetal ation reaction. All the new products synthesized, zirconocene complexe s and cyclometalated derivatives, benzodiselenagermoles, and spirobisb enzodiselenagermoles, were identified by the multinuclear NMR methods and mass spectrometry. The structure of the cyclometalated product 2,5 -(CH3)(2)C6H3)(eta(1):eta(5)-CH2C(CH3)(2)C5H4)] was determined by X-ra y diffraction analysis. This compound crystallizes in the monoclinic s pace group P2(1)/c with two molecules per asymmetric unit with cell di mensions a = 8.278(3) Angstrom, b = 20.113(9) Angstrom, c = 27.661(9) Angstrom, and beta = 93.13(2)degrees at 295 K. Least-squares refinemen t led to a value for the final R index of 0.0642 for 2701 reflections with I > 2 sigma(I).