Dinuclear Cobalt(II) Bis-Dipyrromethane Complexes: Synthesis via Divergent Transmetalation Reactions.

Abstract

A bis-dipyrromethane ligand, 4,4'-bis(1,1-bis(3,5-dimethyl-1H-pyrrol-2-yl)ethyl)-1,1'-biphenyl, [H4-MeBDPM-BPh] (1), is synthesized via acid-catalyzed electrophilic aromatic substitution of 2,4-dimethylpyrrole with 4,4'-diacetylbiphenyl. Subsequent deprotonation of 1 with n-BuLi yields the corresponding Li-salt [Li4-MeBDPM-BPh] (2). Transmetalation involving 2 and either CoCl2 or [Co(HMDS)2] (HMDS = -N(Si(CH3)3)2) results in dinuclear Co(II) complexes. In the case of CoCl2, the formation of the ate-complex 2[Li(THF)4][(MeBDPM-BPh){Co(μ-Cl)2Li(THF)2}2] (3) is observed where each metal center occupies a tetrahedral geometry with noninteracting metal centers. However, when utilizing [Co(HMDS)2], the dinuclear complex 2[Li(THF)4][(MeBDPM-BPh){Co(HMDS)}2] (4) is obtained with the formation of LiHMDS as the product of elimination. The Co metal centers of complex 4 are both three-coordinate and trigonal planar. Displacement of LiHMDS from 4 readily occurs upon treatment with the ylide base, H2CPPh3, yielding the neutral complex [(MeBDPM-BPh){Co(H2CPPh3)2}2] (5). At room temperature, complexes 3-5 have magnetically noninteracting metal centers with S = 3/2 spin states.