Designed synthesis of multi-functional carboranes and organotransition metal—carborane complexes

General synthetic routes to functionalized Cand B-substituted derivatives of nido-C9B4H8, C4B5H12, and C2B3H72 are described together with their use as buiTding-blociZ units in the planned construction of multidecker transition-metal sandwich complexes and related systems. Specifically discussed are (1) synthesis of C2B4 nido-carboranes containing sterically demanding and/or electronically active groups; (2) trinsitionmetal 7-complexation of the C2B4 derivatives at carboranyl and aryl sites; (3) controlled stepwise oxidative fusion of C2Ba-metal complexes to form asymmetrically substituted C4B8 clusters; and (4) current efforts in the author's laboratory directed toward the preparation of electron-delocalized polymers incorporating carboranes or metallacarboranes. INTRODUCTION Boron clusters, particularly the carboranes and their metal-containing derivatives, present intriguing possibilities for the construction of large electron-delocalized multilayer metal sandwich complexes, and, of particular interest, polymers exhibiting low-dimensional electrical conductivity. Following the synthesis of the first ionic (ref. 1) and electrically neutral (ref. 2) triple-decker sandwich complexes, a number of tripleto hexadecker boron-containing sandwiches have been characterized (ref. 3, 4), and an electrically conducting nickel-diborolyl polymer has been reported (ref. 5). As a group, carboranes exhibit high chemical and thermal stability, substantial electron delocalization in the cage framework, ease of complexation with transition metals (for open-cage species), and relatively straightforward functionalization by organic groups at cage carbon locations. These properties have been extensively exploited, in the icosahedral C2B1OH12 carboranes (ref. 6), to prepare uniquely stable high polymers (ref. 7), metallacarborane-based catalysts (ref. 8), and agents for '0B neutron capture therapy of tumors (ref. 9). Until recently, C2B10H12 derivatives were relatively accessible via the base-promoted reaction of alkynes witfi B10H14, but the once-large U.S. Government stockpile of that hydride has virtually disappeared. In contrast, a substantial quantity of B5H9 (ca. l0 kg) remains available for research and hence this borane is a convenient precursor to carboranes and other materials. Particularly useful are the organo derivatives of nido2,3-C2B4H8 (first prepared by Onak, Williams, and Weiss a quarter-century ago (ref. 10)), which can be directly and easily prepared from B5H9 (ref. 11) and which exhibit considerable synthetic versatility. This article describes our group's efforts to utilize nidoC2B4 units and their derivatives in the directed syntheses of multidecker (and eventually polydecker) sandwich complexes and other potentially novel materials. PROPERTIESOF C2B4H62 AND C2B3H72 IONS IN METAL COMPLEXATION The pentagonal pyramidal C2B4H62 ion (1, Fig. 1) and its Cand B-substituted derivatives are ideal ligands for the preparation of stable transition-metal organometallic complexes, especially stacked sandwich compounds. The planar C2B3 basal ring readily undergoes fmetal coordination (ref. 4), particularly with d6 ions such as Fe2 and Co3; like the C5H5 ion and neutral arenes, 1 is a formal 6-electron donor. The dinegative charge on 1 is advantageous in stabilizing high metal oxidation states (which is difficult to do with arene or cyclopentadienyl ligands). Moreover, owing to the lower electronegativity of boron vs. carbon, carborane-transition-metal bonds are more covalent and exhibit generally higher stability than their metal-hydrocarbon counterparts. In C2B4-metal-hydrocarbon complexes, even the metal-hydrocarbon linkage is strengthened by the presence of the carborane ligand, as illustrated by the remarkably stable cyclooctatetraene complexes (178-CRH8)M(Et2C2B4H4) [M = Ti(IV), V(IV)] (ref. 12) which are extremely rare if not unique exampTes of .o-C8H8 -first row transition metal complexes that are unreactive toward atmospheric oxygen. 847