INSERTION OF ISOCYANIDES INTO METAL HYDROGEN-BONDS IN TRIANGULAR HYDRIDO CARBONYL CLUSTERS OF RHENIUM - X-RAY CRYSTAL-STRUCTURES OF [RE3(MU-H)3(MU-ETA-2-CHNR)(CO)10]- (R = PARA-TOLYL) AND OF [RE3(MU-H)3(MU-3-ETA-2-CHNR)(CO)9]- (R = CYCLOHEXYL)

The reaction of the unsaturated triangular cluster anion [Re3(mu-H)4(CO)10]- with isocyanides CNR (R = cyclohexyl or p-tolyl) gives in high yields [Re3(mu-H)3(mu-eta-2-CHNR)(CO)10]-, containing a formimidoyl group originated by the transfer of a hydride ligand to the C atom of the isocyanide. Upon heating, this complex loses one CO ligand, giving with good selectivity [Re3(mu-H)3(mu-3-eta-2-CHNR)(CO)9]-, in a reaction that can be reversed only under high CO pressure (50 atm). The structures of these two formimidoyl species have been elucidated by single-crystal X-ray analyses of their NEt4+ salts: [Re3(mu-H)3(mu-eta-2-CHNC6H4Me)(CO)10]- gives monoclinic crystals, space group P2(1)/c, with a = 16.755 (5) angstrom, b = 12.705 (5) angstrom, c = 16.816 (4) angstrom, beta = 117.5 (2)-degrees, Z = 4, and R = 0.030; [Re3(mu-H)3(mu-3-eta-2-CHNC6H11)(CO)9]- gives monoclinic crystals, space group P2(1)/a, with a = 14.589 (7) angstrom, b = 12.965 (8) angstrom, c = 16.843 (9) angstrom, beta = 93.99 (4)-degrees, Z = 4, and R = 0.039. Variable-temperature H-1 NMR experiments showed that the triple-bridging formimidoyl group undergoes two types of dynamic processes, causing partial or total equalization of the hydrides and consisting presumably in rotations of this ligand on the Re3 face, through the interchange of its three coordination modes (sigma-C, sigma-N, pi) among the vertices of the triangle. Insertion reactions were observed also upon treatment of the unsaturated anion [Re3(mu-H)4(CO)9(CNC6H11)]- with two-electron-donor molecules, as CO (50 atm) or CNC6H11 itself. In the latter case, two isomers of the complex [Re3(mu-H)3(mu-eta-2-CHNC6H11)(CO)9(CNC6H11)]- are formed, likely differing in the coordination of the CNC6H11 ligand (axial syn or anti with respect to the bridging formimidoyl). In all the insertion reactions here reported, the H transfer is supposed to take place in intermediate [Re3(mu-H)3H(CO)9(CNR)L]- adducts, too reactive to be observed. The differences with respect to the analogous reactions of [Os3(mu-H)2(CO)10] are discussed.