FRAMEWORK EXPANSION VERSUS EDGE OPENING IN A 50-E PHOSPHIDO-BRIDGED TRIRUTHENIUM CLUSTER - A CASE-STUDY

The electron rich cluster RU3(mu3-eta2-P(C6H5)(C5H4N))(mu-P(C6H5)2)3(CO)6 (9) is prepared by incorporation of diphenylphosphido groups into the ligand shell of triruthenium complexes that already contain a face-bridging phosphido-pyridyl ligand. The two precursors are (i) the known acyl complex RU3(mu-C(O)(C6H5))(mu3-eta2-P(C6H5)-(C5H4N))(CO)9 (1), which reacts with 3 equiv of diphenylphosphine in refluxing methylcyclohexane to produce 9 in 75% yield, and (ii) the complex RU3(mu3-eta2-P(C6H5)(C5H4N))(mu-P(C6H5)2)(CO)6(mu-CO)2 (4), which also leads to 9 via reaction with 2 equiv of diphenylphosphine (yield 75%). The structure of compound 9 has been determined by X-ray diffraction. Crystal data for 9: monoclinic, C2h5, P2(1)/c, Z = 4, a = 20.246(4) angstrom, b = 13.425(3) angstrom, c = 20.618(4) angstrom, beta = 115.54(2)-degrees (T = -173-degrees-C), final R = 3.7% (R(w) = 4.3%) for 5613 unique reflections (I greater-than-or-equal-to 3 sigma(I)) and 310 variable parameters. The structure consists of a triangular array of ruthenium atoms capped by a phenylpyridylphosphido ligand as referred to the antecedent species. Each Ru-Ru edge is supported by a diphenylphosphido group occupying equatorial coordination sites. The environment of each Ru atom is completed by two terminal carbonyl ligands. Even though this trinuclear species contains 50 cluster valence electrons, the three Ru-Ru bond distances are roughly equivalent within experimental error: Ru(1)-Ru(2) = 3.112(1) angstrom, Ru(1)-Ru(3) = 3.084(1) angstrom, and Ru(2)-Ru(3) = 3.112(1) angstrom. An electrochemical study carried out in CH2Cl2 reveals that the compound undergoes two well-defined reversible one-electron oxidations at E1/2 = 0.16 V and E1/2 = 0.53 V, respectively (vs Ag/AgCl, KCl 0.1 M, H2O). The unusual closed geometry of 9 is rationalized in terms of molecular orbital calculations of extended Huckel type and compared with that of the isostructural 48-e closed complex 4 and the isoelectronic 50-e open cluster Ru2(mu3-eta2-P(C6H5)(C5H4)(mu-P(C6H5)2)(CO)9 (5).