Synthesis and Investigation of Macrocyclic CrIII Bis(alkynyl) Complexes: Structural and Spectroscopic Properties

Abstract

The synthesis and characterization of four new CrIII–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 CrIII 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 µs, 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 C2–Cr–C2 bridge.