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

Reported herein are new Fe bis-alkynyl complexes [Fe III (L)(C 2 R) 2 ]BPh 4 based on tetra imine macrocycle (L = HMTI = meso -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; 1a-1c ; R = C 6 H 5 ( a ), C 10 H 9 ( b ), SiMe 3 ( c )) and tetra amine macrocycle (L = HMC = meso -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; 2a-2c ). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of 1a and 2a allowed for investigation of the Fe II oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the Fe II to C 2 Ph 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 2a than in 1a . These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals via density functional theory calculations.