ISOMERIZATION AND FLUXIONAL BEHAVIOR OF THE TETRAHEDRO-TRIOSMIUM PLATINUM CLUSTERS OS3PT(MU-H)2(CO)9(PT-PCY3)(OS-PR3) - X-RAY CRYSTAL-STRUCTURE OF OS3PT(MU-H)2(CO)9(PT-PCY3)(OS-PPH3)

The 58-electron clusters Os3Pt(mu-H)2(CO)9(Pt-PCy3)(Os-PR3) (3a; R3 = Me2Ph; 3b, R3 = Me3; 3c, R3 = Ph3; 3d; R3 = (i)Pr3; 3e; R3 = Cy2(i)Pr) have been synthesized by reaction of Os3(mu-H)2(CO)9(PR3) with Pt(C2H4)2(PCy3), and their fluxional behavior has been examined. The structures adopted depend on the steric bulk of the phosphine PR3. One isomer (isomer A) predominates for clusters 3a-c, with some evidence for a very minor component. Crystal data for 3c: monoclinic, space group P2(1)/c, a = 12.448 (2) angstrom, b = 16.319 (3) angstrom, c = 24.936 (5) angstrom, beta = 89.44 (1)-degrees, V = 5065 (2) angstrom 3, Z = 4, and final R (R(w)) values 0.031 (0.042) for 5359 independent observed data, I greater-than-or-equal-to 3-sigma-(I). The structure of 3c shows that isomer A is related to the parent species Os3Pt(mu-H)2(CO)10(Pt-PCy3) (1), with a PPh3 group substituting in a radial site. 3a-c exhibit hydride migration, with DELTA-G double-ended dagger at coalescence being 57.1, 56.5, and 55 kJ mol-1, respectively. A C-13 EXSY spectrum of 3a shows several independent CO exchange pathways, including a concerted migration of a hydride and rotation of the Pt(H)(CO)(PCy3) group resulting in cluster enantiomerization. Clusters 3d,e exist as a approximately 1:1 mixture of two isomers in solution, isomer A and a second isomer B possessing two inequivalent Os(mu-H)Os hydrides. Isomers A and B are in rapid exchange on the NMR time scale at ambient temperatures, and a value for DELTA-G double-ended dagger 233 of 54.9 kJ mol-1 has been estimated for their interconversion in cluster 3d, from analysis of the P-31 EXSY spectrum.