Dinuclear\nCobalt(II) Bis-Dipyrromethane Complexes:\nSynthesis via Divergent Transmetalation Reactions

A bis-dipyrromethane ligand, 4,4′-bis(1,1-bis(3,5-dimethyl-1H-pyrrol-2-yl)ethyl)-1,1′-biphenyl,\n[H<sub>4</sub>-<sup>Me</sup>BDPM-BPh] (<b>1</b>), is synthesized\nvia acid-catalyzed electrophilic aromatic substitution of 2,4-dimethylpyrrole\nwith 4,4′-diacetylbiphenyl. Subsequent deprotonation of <b>1</b> with <i>n</i>-BuLi yields the corresponding Li-salt\n[Li<sub>4</sub>-<sup>Me</sup>BDPM-BPh] (<b>2</b>). Transmetalation\ninvolving <b>2</b> and either CoCl<sub>2</sub> or [Co(HMDS)<sub>2</sub>] (HMDS = -N(Si(CH<sub>3</sub>)<sub>3</sub>)<sub>2</sub>)\nresults in dinuclear Co(II) complexes. In the case of CoCl<sub>2</sub>, the formation of the ate-complex 2[Li(THF)<sub>4</sub>][(<sup>Me</sup>BDPM-BPh){Co(μ-Cl)<sub>2</sub>Li(THF)<sub>2</sub>}<sub>2</sub>] (<b>3</b>) is observed where each metal center occupies a\ntetrahedral geometry with noninteracting metal centers. However, when\nutilizing [Co(HMDS)<sub>2</sub>], the dinuclear complex 2[Li(THF)<sub>4</sub>][(<sup>Me</sup>BDPM-BPh){Co(HMDS)}<sub>2</sub>] (<b>4</b>) is obtained with the formation of LiHMDS as the product of elimination.\nThe Co metal centers of complex <b>4</b> are both three-coordinate\nand trigonal planar. Displacement of LiHMDS from <b>4</b> readily\noccurs upon treatment with the ylide base, H<sub>2</sub>CPPh<sub>3</sub>, yielding the neutral complex [(<sup>Me</sup>BDPM-BPh){Co(H<sub>2</sub>CPPh<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (<b>5</b>). At room temperature, complexes <b>3</b>–<b>5</b> have magnetically noninteracting metal centers with S = 3/2 spin\nstates.