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 thermoly tic mixed cluster fusion reactions involving metallaboranes and molten B10H 14 as solvent. Co-thermolysis of B10H14 with nine-vertex [(CO)(PMe3)(2)HIrB 8H12] (1) engenders 18-vertex [(CO)(PMe3)(2)IrB17H20] (3), via double clust er fusion; this has the 18-vertex configuration of syn-B26H22, but with a m etal atom in the 10-position. From the same reaction, triple cluster fusion engenders 28-vertex [(PMe3)(2)IrB26H24Ir(CO)(PMe3)(2)] (4), which structur ally 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. Comp ound 4 exhibits interesting consequences of cluster-crevice formation and i ntroduces 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)HI rB18H18(PPh3)] (6), which has a close-type {IrB10} 11-vertex subcluster fus ed to a nido 10-vertex {B-10} subcluster to generate a four-atom {IrB3} tet rahedron. 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 f usion reveals the formation of 30-vertex [(PMe2Ph)(2)(PMe2C6H4)(2)Pt2B28H32 ] (10), 29-vertex [(PMe2Ph)(2)PtB28H32] (11) and 27-vertex [(PMe2Ph)(2)PtB2 6H26-(PMe2Ph)] (12) from the same reaction. Structurally, compound 10 is ba sed on a 10-vertex arachno-(6,9-Pt2B8) unit linked, via one B-B two-electro n two-centre bond each, to two 10-vertex nido-{B-10} units; it also exhibit s molecular condensation in the form of two P-phenylene ortho -cycloboronat ions. Compound II is based on the 19-vertex [(PMe2Ph)(2)Pt-eta(4)-anti-B18H 22] configuration with an additional 10-vertex nido-{B10H13} moiety bound t o the non-platinated subcluster via one B-B two-electron two-centre bond. C ompound 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-boro n 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 tr iangular {PtB2} face in common, and also fused to a 10-vertex nido {B-10} u nit 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 necess itated synchrotron X-radiation for sufficient diffraction intensity. (C) 19 99 Published by Elsevier Science S.A. All rights reserved.