Organotransition-Metal Metallacarboranes. 59.1 Synthesis and Linkage of Boron-Functionalized Ferracarborane Clusters

Abstract

Negishi cross-coupling has been applied to B−I and B−Br bonds in small closo-ferracarboranes as a means of effecting controlled substitution at boron, with subsequent linkage to form polymetallacarborane systems. Reaction of (η6-C6H6)Fe(2,3-Et2C2B4H3-5-I) (2) with vinyl−, butyl−, (trimethylsilyl)ethynyl−, and phenyl−organozinc reagents (RZnCl) in the presence of catalytic amounts of palladium(0) catalyst produced the corresponding B-functionalized derivatives (3−5 and 8, respectively) in moderate to good yields. Similarly, (η6-C6H6)Fe(Et2C2B4H3-7-C⋮CSiMe3) (12) was prepared from the B(7)−Br (11a) or B(7)−I derivative (11b). A second alkynyl fragment was introduced at B(5) on 12 to yield the B(5,7)-diethynyl-substituted compound 16. Treatment of (η6-C6H6)Fe(2,3-Et2C2B4H3-5-Ph) (8) with Cr(CO)6 in refluxing n-butyl ether gave the heterodimetallic complex (η6-C6H6)Fe[Et2C2B4H3-5-{(η6-C6H5)Cr(CO)3}] (9). A series of diferracarboranes bridged by thiophene (18), dihydrophenanthrene (19), or mono-, di-, or triphenylene (22−24) linking groups was generated via reaction of 2 with bis(chlorozinc) reagents and palladium(0) catalyst, as were B(5)−ferrocenyl derivatives (20, 21). Reaction of the known complex (η6-C8H10)Fe(Et2C2B4H4) with 8 at 180 °C formed (η6-C6H6)Fe(Et2C2B4H3-5-Ph)Fe(Et2C2B4H4) (25) via displacement of cyclooctatriene. Electrophilic B-iodination on the unsubstituted C2B4 cage of 25 afforded a dark red air-stable crystalline B(5)−iodo complex (26). Pd-catalyzed cross-coupling of 26 with PhZnCl, followed by cyclooctatriene displacement from (η6-C8H10)Fe(Et2C2B4H4), produced the triferracarborane-linked oligomer (η6-C6H6)Fe(Et2C2B4H3-5-Ph)Fe(Et2C2B4H3-5‘-Ph)Fe(Et2C2B4H4) (28) in an overall yield from 8 of 40% in this four-step sequence. Deprotection of 5 and 12 with fluoride gave the B(5)− and B(7)−C⋮CH derivatives 6 and 13, respectively. Homocoupling of these terminal alkynes with Pd/Cu catalyst afforded the linked isomers [(η6-C6H6)Fe(Et2C2B4H3-n-C⋮C)]2 (29, n = 5; 31, n = 7). Alternatively, 6 and 13 were treated with 1,3,5-C6H3I3 under similar conditions to form the benzene-centered triferracarborane complexes [(η6-C6H6)Fe(Et2C2B4H3-n-C⋮C)]3C6H3 (30, n = 5; 32, n = 7). X-ray diffraction analyses confirmed the structures of 9, 18, 24, and 26.