Synthesis, Redox Chemistry, and Electronic Structure of the Butadiynyl and Hexatriynyl Complexes [Mo{(C≡C)_<i>n</i>C≡CR}(L₂)(η-C₇H₇)]^<i>z</i>+ (<i>n</i> = 1, 2; <i>z</i> = 0, 1; R = SiMe₃, H; L₂ = 2,2′-bipyridine, Ph₂PCH₂CH₂PPh₂)

Abstract

Two series of extended carbon chain butadiynyl and hexatriynyl complexes, [Mo{(C≡C)nC≡CSiMe3}(bpy)(η-C7H7)] (n = 1, 2; bpy = 2,2′-bipyridine) and [Mo{(C≡C)nC≡CR}(dppe)(η-C7H7)] (n = 1, R = H, SiMe3; n = 2, R = SiMe3; dppe = Ph2PCH2CH2PPh2), have been prepared and structurally characterized. The redox chemistry of these complexes has been investigated by cyclic voltammetry, and the 17-electron radical cations resulting from one-electron oxidation have been characterized by spectroelectrochemical IR and UV–visible methods and EPR spectroscopy. DFT calculations on the H-terminated model complexes [Mo{(C≡C)nC≡CH}(L2)(η-C7H7)]z+ (L2 = bpy, dppe) reveal a largely metal-centered HOMO (z = 0) with a modest increase in carbon chain character with increasing chain length. Spin density calculations for the 17-electron radical cations (z = 1) show large coefficients of spin density at the metal center, consistent with the remarkably high stability of the experimental complexes. However, both DFT theoretical and experimental synthetic studies highlight a distinction between the bpy- and dppe-supported systems. The 17-electron complexes [Mo{(C≡C)nC≡CSiMe3}(bpy)(η-C7H7)]PF6 (n = 1, 2) are unique examples of isolable, metal-stabilized butadiynyl and hexatriynyl radicals. In contrast, the dppe radical [Mo(C≡CC≡CSiMe3)(dppe)(η-C7H7)]+ exhibits chain-centered reactivity, consistent with enhanced coefficients of spin density at Cβ and Cδ in the model complex [Mo(C≡CC≡CH)(dppe)(η-C7H7)]+.