Cationic Triple‐Decker Complexes with a Bridging 4‐Borataborepine Ligand: Synthesis, Structure, and Bonding

Abstract

AbstractCationic 30 VE triple‐decker complexes [Cp*Ru(μ‐η7:η7‐C5B2Me6H)ML]+ [ML = Co(C4Me4) (3a), RuCp* (4a), Rh(cod) (7a), and Ir(cod) (8a)] with a bridging hexamethyl‐4‐borataborepine ligand were obtained by electrophilic stacking of the sandwich compound Cp*Ru(η7‐C5B2Me6H) (2a) with [ML]+ complex fragments. The reaction of the phenyl‐substituted derivative Cp*Ru(η7‐7‐PhC5B2Me5H) (2b) with [(C4Me4)Co(MeCN)3]+ selectively affords the triple‐decker complex [Cp*Ru(μ‐η7:η7‐7‐PhC5B2Me5H)Co(C4Me4)]+ (3b), whereas a similar reaction with [Cp*RuCl]4/TlBF4 produces a 1:3 mixture of cations, the triple‐decker [Cp*Ru(μ‐η7:η7‐7‐PhC5B2Me5H)RuCp*]+(4b) and the arene‐coordinated [Cp*Ru(μ‐η6:η7‐7‐PhC5B2Me5H)RuCp*]+ (5). Stacking of Cp*Ru(η7‐7‐PhCH2C5B2Me5H) (2c) with [CpRu(MeCN)3]+ selectively gives the triple‐decker complex [Cp*Ru(μ‐η7:η7‐7‐PhCH2C5B2Me5H)RuCp]+ (6c). The dinuclear cations 3–8 were isolated as deep‐colored air‐stable salts with [BF4]– or [PF6]– anions in moderate to high yields. Structures of 3bPF6, 4aBF4, 7aBF4, and 8aBF4 were confirmed by X‐ray diffraction studies. Energy decomposition analysis of complexes CpRu(ring) and [CpRu(ring)RuCp]+ (ring = Cp, C5BH6, C5B2H7) revealed that the insertion of BH units makes the bonding between [ring]– and [RuCp]+ more covalent. According to Mulliken population analysis, weakening of π‐donation and strengthening of δ‐back donation occur in the same order. The electrostatic character of the bond and the contribution of σ‐donation to the covalent bonding are higher in the case of bifacially bonded rings. The boron‐containing triple‐decker complexes are considerably more stable than the cyclopentadienyl analog. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)