The chemistry of isomers of icosaborane(26), B20H26: synthesis and nuclear magnetic resonance study of various isomers of platinahenicosaboranes and diplatinadocosaboranes, and the X-ray crystal and molecular structures of 7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane and 4-(2′-nido-decaboranyl)-7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane

Abstract

An improved synthesis of the known compound [(PMe2Ph)2(PtB10H12)] has been developed by deprotonation of B10H14 with NNN′N′-tetramethylnaphthalene-1,8-diamine followed by treatment with cis-[PtCl2(PMe2Ph)2]. This reaction has been applied to the 2,2′, 2,6′, and 1,5′ isomers of (B10H13)2 to prepare various isomeric platinahenicosaborane clusters [(PMe2Ph)2(PtB10H11–B10H13)] which differ either in the position of the conjuncto-linkage or the site of the platinum atom in the cluster. Appropriate modification of the reaction stoicheiometry in the case of 2,2′-(B10H13)2 led to the isolation of cisoid and transoid diplatinadocosaboranes [{(PMe2Ph)2(PtB10H11)}2]. The X-ray crystal structure of [(PMe2Ph)2(PtB10H12)] showed it to contain a platinaundecaborane cluster in which the tetrahapto B10H12 group is twisted by ca. 20° with respect to the PtP2 plane. Similarly, the molecular structure of the isomer of [(PMe2Ph)2Pt(η4-B10H11–B10H13)] obtained from 2,2′-(B10H13)2 is distorted by a twist of ca. 8°. A detailed n.m.r. study of a number of these clusters has been made, using the resonances of 1H, 11B, 31P, and 195Pt. In addition to permitting structural assignments, the data reveal a novel mutual pseudo-rotation of the η4-B10H11X group (X = H or B10H13) and the (PMe2Ph)2 grouping about the central Pt atom. For [(PMe2Ph)2(PtB10H12)] the two sets of 1H-{31P} methyl resonances at 100 MHz coalesce at 71.5 °C with an implied activation energy ΔG‡ of 79 ± 5 kJ mol–1 for the fluxional process. Similar activation energies were deduced for the various isomers of [(PMe2Ph)2(PtB20H24)].