SYNTHESIS AND REACTIVITY OF BIMETALLIC ALKOXYSILYL COMPLEXES - CRYSTAL-STRUCTURE OF [FE(CO)3(MU-SI(OME)2(OME))(MU-DPPM)RH(CO)] (DPPM = PH2PCH2PPH2) - A COMPLEX WITH A MU-2-ETA-2-SIO BRIDGE BETWEEN IRON AND RHODIUM

Bimetallic Fe-Rh complexes have been prepared from the (alkoxysilyl)iron derivatives [HFe(CO)3{Si(OMe)3}(dppm-P)] or [HFe(CO)3{Si(OMe)3}(dppm-P)]-, the dppm ligand of which helps assembling and stabilizing the dinuclear unit. The crystal structure of [activated Fe(CO)3{mu-Si(OMe)2(OMe)}(mu-dppm)Rh(CO)] (1a) has been determined: monoclinic, space group P2(1)/n, a = 22.717 (5) angstrom, b = 11.348 (2) angstrom, c = 14.202 (2) angstrom, beta = 94.47 (2)-degrees, V = 3650.2 angstrom 3, Z = 4, R = 0.033, R(w) = 0.052 for 4531 reflections with I > 3-sigma(I). Complex 1a contains an unusual mu-2-eta-2-SiO bridge between the Fe and Rh atoms (Fe-Rh = 2.6283 (7), Rh-O7 = 2.167 (3), O7-Si = 1.683 (3), Si-Fe = 2.249 (1) angstrom). Complex [activated Fe(CO)3{mu-Si(OMe)2(OMe)}(mu-dppm)Rh(PPh3)] (2) reacts with P(OPh)3 to give [activated Fe(CO)3{mu-Si(OMe)2(OMe)}(mu-dppm)Rh{P(OPh)3}] (3). Complex 1a reacts reversibly with CO with opening of the mu-2-eta-2-SiO bridge to give [activated Fe(CO)3{Si(OMe)3}(mu-dppm)Rh(CO)2] (4). Protonation of the Fe-Rh bond of 1a with HBF4 yields [activated Fe(CO)2{mu-Si(OMe2(OMe)}(mu-dppm)(mu-H)(mu-CO)Rh(PPh3)][BF4] (5), which is isoelectronic with [activated Fe(CO)2{mu-Si(OMe)2(OMe)](mu-dppm)(mu-H)(mu-CO)RhCl] (6a), prepared by the addition of [HFe(CO)3{Si(OMe)3}(dppm-P)] to a suspension of [Rh2(mu-Cl)2(COD)2] in Et2O. Reaction of K[Fe(CO)3{Si(OMe)3}(dppm-P)] with [RhCl(CS)(PPh3)2] afforded [activated Fe(CO)3{Si(OMe)3}(mu-dppm)Rh(CS)(PPh3)] (8). The stronger Rh-CS interaction compared with the Rh-CO interaction accounts for the strengthening of its bonding to Rh, preventing the transformation of 8 into 2, and also of the Rh <-- PPh3 bond, which precludes formation of the Rh-CS analogue of 1a. In contrast to the rigid situation in 1a, the lability of the O --> Rh bond in 2, 3, 5, and 6 accounts for the dynamic behavior of the alkoxysilyl ligand, evidenced by variable-temperature H-1 NMR spectroscopy, which renders the methoxy protons equivalent. The observed increase in O --> Rh bond lability parallels the increase in electron density at the Rh center. Attempts to isolate related Fe-Co or Fe-Ir complexes were unsuccessful. In the latter case, a redox reaction resulted in the formation of [Fe(CO)3(PPh3)(dppm-P)].