SYNTHESIS AND P-31 NMR-SPECTROSCOPY OF TRINUCLEAR, PHOSPHIDO-BRIDGED IRIDIUM AND RHODIUM CLUSTERS - CRYSTAL AND MOLECULAR-STRUCTURES OF [M3(MU-PPH2)3(CO)NL2] (M = IR OR RH, N = 3, L2 = BIS(DIPHENYLPHOSPHINO)METHANE - M = IR, N = 5, L = T-BUNC)

Reaction of [Ir2(cyclooctene)4Cl2] with CO, NHEt2, and PHPh2 provides a synthetic route to the trinuclear, phosphido-bridged iridium clusters [Ir3(mu-PPh2)3(CO)nL2] (n = 3, L = CO or PPh3, L2 =bis(diphenylphosphino)methane (dppm); n = 5, L = t-BuNC). The CO andPPh3 complexes are analogues of previously known rhodium derivatives, and rhodium analogues of the dppm and t-BuNC complexes are also reported. [Ir3(mu-PPh2)3(CO)3(dppm)] (I) and [Rh3(mu-PPh2)3(CO)3(dppm)] (II) crystallize in the Pnma space group (Z = 4) with the following respective unit cell dimensions: a = 23.963 (5) angstrom, b = 24.970 (5) angstrom, c = 11.080 (2) angstrom; a = 24.031 (6) angstrom, b = 25.069 (9) angstrom, c = 11.117 (4) angstrom. [Ir3(mu-PPh2)3(CO)5(t-BuNC)2] (III) crystallizes in the P2(1)/n space group (Z = 4) with a = 23.015 (4) angstrom, b = 20.197 (6) angstrom, c = 11.849 (5) angstrom, and beta = 92.19 (4)degrees. The structures of I and II consist of approximately equilateral triangles of metal atoms (average M-M distances 2.78 (Ir) and 2.79 angstrom (Rh)), with the CO and two PPh2 ligands lying approximately in the M3 plane. The third PPh2 bridge is approximately perpendicular to this plane, linking the two basal metal atoms, which are also bridged by the dppm ligand. In contrast, the structure of III has all three PPh2 bridges approximately in the M3 plane with the t-BuNC ligands added approximately perpendicular to this plane at the apical iridium. The Ir-Ir distances are much longer, averaging 3.23 angstrom. Complete analyses of P-31 NMR spectra are reported for I-III and for IV, the Rh analogue of III. The phosphido bridge shifts reflect the changes in metal-metal distances, with I and II strongly deshielded (by 80-240 ppm) relative to III and IV. There is also a general reduction in the one-bond Rh-P coupling constants in the 50-electron cluster, IV relative to the 46-electron cluster, II.