Synthesis and Characterization of Redox-Active C₄-Bridged Rigid-Rod Complexes with Acetylide-Substituted Manganese End Groups

The symmetric d5 trans-bis(alkynyl) complexes [Mn(R‘2PCH2CH2PR‘2)2(C⋮CR)2] (R‘ = Me, R = SiEt3, 3a; R‘ = Me, R = SiiPr3, 3b; R‘ = Me, R = Si(tBu)Me2, 3c; R‘ = Et, R = SiEt3, 4a; R‘ = Et, R = SiiPr3, 4b; R‘ = Et, R = Si(tBu)Me2, 4c) can be prepared by the reaction of [Mn(R‘2PCH2CH2PR‘2)2Br2] (R‘ = Me, 1; R‘ = Et, 2) with 2 equiv of LiC⋮CR (R = SiEt3, SiiPr3, Si(tBu)Me2). The reactions of species 3 and 4 with [Cp2Fe][PF6] yield the corresponding d4 complexes [Mn(dmpe)2(C⋮CR)2][PF6] (R‘ = Me, R = SiEt3, 5a; R‘ = Me, R = SiiPr3, 5b; R‘ = Me, R = Si(tBu)Me2, 5c; R‘ = Et, R = SiEt3, 6a; R‘ = Et, R = SiiPr3, 6b; R‘ = Et, R = Si(tBu)Me2, 6c). The asymmetrically substituted trans-bis(alkynyl) complexes [Mn(dmpe)2(C⋮CR)(C⋮CH)][PF6] (R‘ = Me, R = SiEt3, 10a; R‘ = Me, R = SiiPr3, 10b; R‘ = Me, R = Si(tBu)Me2, 10c) are prepared by the reaction of the vinylidene compounds Mn(dmpe)2(C⋮CR)(CCH2) (R‘ = Me, R = SiEt3, 9a; R‘ = Me, R = SiiPr3, 9b; R‘ = Me, R = Si(tBu)Me2, 9c) with 2 equiv of [Cp2Fe][PF6] and 1 equiv of quinuclidine. The mixed-valent complexes [{Mn(dmpe)2(C⋮CR)}2(μ-C4)][PF6] [11]+ are obtained by the reaction of 10 with 1 equiv of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). The neutral dinuclear Mn(II)/Mn(II) compounds [{Mn(dmpe)2(C⋮CR)}2(μ-C4)] (R = SiEt3, 11a; R = SiiPr3, 11b; R = Si(tBu)Me2, 11c) are synthesized by the reduction of [11]+ with Co(C5Me5)2. Complexes [11]+ can also be oxidized with [Cp2Fe][PF6] to produce the dicationic Mn(III)/Mn(III) species [{Mn(dmpe)2(C⋮CR3)}2(μ-C4)][PF6]2 (R = SiEt3, [11a]2+; R = SiiPr3, [11b]2+; R = Si(tBu)Me2, [11c]2+). Both redox processes are fully reversible. The dinuclear compounds have been characterized by NMR, IR, and Raman spectroscopy, cyclic voltammetry, and elemental analyses. X-ray diffraction studies have also been performed on several complexes.