Ligand dependence of the indenyl ring slippage in [(η5-Ind)MoL2(CO)2]0,+ complexes:: Experimental and theoretical studies

The reaction of a series of cationic and neutral complexes [(eta 5-Ind)Mo(CO)(2)L-2](0,+) (1, L =NCMe; 2, L = CNMe; 3, L = OPPh3; 4, L = OP(OMe)(3); 5, L = PMe3; 6, L-2 = (PhSCH2)(2); 7, L-2 = (O2CCF3)(-); 8, L-2 = (O2CCH3)(-); 9, L-2 = (O2CPh)(-); 10, L-2 = [(NPh)(2)CH](-); 11, L-2 = [(N-p-tolyl)(2)CH](-); 12, L-2 = (S2CNEt2)(-); 13; L-2 = (S2P(OEt)(2))(-); 14, L-2 = acac(-); 15, L-2 = MeC-(O)CHC(NPh)CMe) with acetonitrile produces the ring-slipped adducts [(eta(3)-Ind)MoL2(NCMe)(CO)(2)](0,+) only in the cases where L = NCMe, OPPh3, OP(OMe)3 or L-2 = (O2CCF3)(-), acac(-) The adducts are labile and have been characterized by means of the distinctive chemical shift of the central pseudo-allylic proton of the eta(3)-indenyl ligand. In the other cases no ring slippage is observed. In NCMe solvent 6 undergoes substitution to give 1, but the macrocycle trithiacyclononane (ttcn) reacts with 1 to give [(eta 3-Ind)Mo(CO)(2)(kappa(3)-ttcn)](+) as the BF(4)(-)salt. PMe3 reacts with 1 to give substitution products in a stepwise fashion. [(eta(5)-Ind)Mo(NCMe)(PMe3)(CO)(2)]BF4 is formed instantaneously at -70 degrees C,but 5 forms slowly at room temperature without any detectable ring-slipped intermediate. The crystal structure of the isonitrile cation 2 is presented. 2 adds neither NCMe nor CNMe but undergoes CO substitution with excess CNMe. Extended Huckel and density function calculations predict the most stable conformation of [(eta 5-Ind)MoL2(CO)(2)](+) (L = NCR, CNR) to be the one with the ring trans to the carbonyls, while for the [(eta 3-Ind)MoL2(NCR)(CO)(2)](+) complexes the ring lies over the carbonyls. According to the dft calculations, acetonitrile addition induces eta(5) --> eta(3) slipping of the ring in the NCR derivatives but not in the CNR analogues, because the reaction is enthalpically driven in the former and not in the latter. The dft calculations of part of the reaction path indicate that the indenyl ring starts to slip when the incoming ligand is still far from the metal (similar to 400 pm), thus supporting earlier kinetically, based proposals.