Bis-Alkynyl Complexes of Fe(III) Tetraaza Macrocycles─A Tale of Two Rings.

Reported herein are new Fe bis-alkynyl complexes [Fe(L)(CR)]BPh based on tetra macrocycle (L = HMTI = -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; ; R = CH (), CH (), SiMe ()) and tetra macrocycle (L = HMC = -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; ). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of and allowed for investigation of the Fe oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the Fe to CPh metal-to-ligand charge transfer (MLCT) and the ν(C≡C). The ν(C≡C) was further influenced by the oxidation state, though sensitivity to the formal metal oxidation state was much higher in the case of than in . These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals density functional theory calculations.