Synthesis and investigation of macrocyclic Cr(III) bis‐alkynyl complexes: structural and spectroscopic properties

The synthesis and characterization of four new Cr-III-bis(alkynyl) complexes bearing the macrocyclic tetraaza ligand DMC (DMC = 5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane) are reported. Complexes trans-[Cr(DMC)(C2R)(2)]X (R = Ph ([1]X), Fc ([2]X), X = Cl, ClO4. C2H ([3]X); X = ClO4, BPh4) and cis-[Cr(DMC)(C4TMS)(2)]Cl ([4]Cl) were studied using UV/Vis and FTIR spectroscopy, and their identities were verified with ESI-MS and elemental analysis. The three trans complexes, [1]Cl, [2](ClO4), and [3](BPh4), were structurally characterized using single-crystal X-ray diffraction, which revealed a pseudo-octahedral geometry around the Cr center with the nitrogen atoms occupying the equatorial plane and the alkynyl ligands residing in the apical positions. Spectroscopic analysis of [1]Cl, [3](BPh4) and [4]Cl shows highly structured d-d bands between 320 and 500 nm. All Cr-III complexes reported herein are emissive, and detailed studies were performed for [1]Cl, [3](BPh4), and [4]Cl, yielding phosphorescence lifetimes (77 K) of 380, 358, and 160 mu s, respectively, and room temperature quantum yields of 0.01% for complex [1]Cl and 0.15% for complex [4]Cl. Voltammetric studies of complex [2](ClO4) indicate a weak but discernible coupling between two ferrocenyl groups across the C-2-Cr-C-2 bridge.