An approach to megalo-boranes.: Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds.: Crystal structure determinations by conventional and synchrotron methods

Several new macropolyhedral metallaboranes have been isolated from thermolytic mixed cluster fusion reactions involving metallaboranes and molten B10H14 as solvent. Co-thermolysis of B10H14 with nine-vertex [(CO)(PMe3)(2)HIrB8H12] (1) engenders 18-vertex [(CO)(PMe3)(2)IrB17H20] (3), via double cluster fusion; this has the 18-vertex configuration of syn-B26H22, but with a metal atom in the 10-position. From the same reaction, triple cluster fusion engenders 28-vertex [(PMe3)(2)IrB26H24Ir(CO)(PMe3)(2)] (4), which structurally is based on an intimate interfusion of closed 10-vertex and 12-vertex subclusters, to generate a tetrahedral tetraboron core that also has a more open commo one-boron linkage to a nido nine-vertex {IrB8} subcluster. Compound 4 exhibits interesting consequences of cluster-crevice formation and introduces the concept of globular megalo-borane structures that have borons-only cores surrounded by boron-hydride sheaths. Examination for incipient megalo-borane globular behaviour in another system, viz. [IrCl(PPh3)(3)] (7) with anti-B18H22, reveals a four-atom core feature in 19-vertex [(PPh3)HIrB18H18(PPh3)] (6), which has a close-type {IrB10} 11-vertex subcluster fused to a nido 10-vertex {B-10} subcluster to generate a four-atom {IrB3} tetrahedron. Examination for mixed cluster fusion in other systems reveals the generation of [(PMe2Ph)(2)Pt-anti-B18H20] (8), from the co-thermolysis of [(PMe2Ph)(2)PtB8H12] (2) and B10H14, and examination for multiple cluster fusion reveals the formation of 30-vertex [(PMe2Ph)(2)(PMe2C6H4)(2)Pt2B28H32] (10), 29-vertex [(PMe2Ph)(2)PtB28H32] (11) and 27-vertex [(PMe2Ph)(2)PtB26H26-(PMe2Ph)] (12) from the same reaction. Structurally, compound 10 is based on a 10-vertex arachno-(6,9-Pt2B8) unit linked, via one B-B two-electron two-centre bond each, to two 10-vertex nido-{B-10} units; it also exhibits molecular condensation in the form of two P-phenylene ortho -cycloboronations. Compound II is based on the 19-vertex [(PMe2Ph)(2)Pt-eta(4)-anti-B18H22] configuration with an additional 10-vertex nido-{B10H13} moiety bound to the non-platinated subcluster via one B-B two-electron two-centre bond. Compound 12 is based on two nido 11-vertex {PtB10} units joined by a single commo Pt vertex, with one of these units conjoined to an arachno eight-boron unit via a two-boron common edge and an open bridging {B-H(exo)-Pt-mu-B-2} link. Thermolysis of [(PMe2Ph)(2)PtB8H12] (2) with the pre-formed double-cluster compound anti-B18H22 generates triple-contiguity 27-vertex [(PMe2Ph)PtB26H26(PMe2Ph)] (13) which, structurally, consists of a nido 11-vertex {PtB10} unit that is fused to a second 11-vertex nido {PtB10} unit with a triangular {PtB2} face in common, and also fused to a 10-vertex nido {B-10} unit with a {B-2} edge in common. The sequence 12 -->11-->10-->13-->4 represents a progression of increasing intimacy of cluster fusion. Small crystals of compounds 3, 11 and 12 necessitated synchrotron X-radiation for sufficient diffraction intensity. (C) 1999 Published by Elsevier Science S.A. All rights reserved.