A unique series of chromium(iii) mono-alkynyl complexes supported by tetraazamacrocycles

Abstract

Described herein is the synthesis and characterization of macrocyclic Cr^III mono-alkynyl complexes. By using the meso-form of the tetraazamacrocycle HMC (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), trans-[Cr(HMC)(C₂Ph)Cl]OTf (1a), trans-[Cr(HMC)(C₂Np)Cl]OTf (2a), trans-[Cr(HMC)(C₂C₆H₄^tBu)Cl]OTf (3a), and trans-[Cr(HMC)(C₂(3,5-Cl₂C₆H₃))Cl]OTf (4a) complexes have been realized. These complexes were synthesized in high yield through the reaction of trans-[Cr(meso-HMC)(C₂Ar)₂]OTf (1b–4b) with stoichiometric amounts of methanolic HCl. Single crystal X-ray diffraction showed that the trans-stereochemistry and pseudo-octahedral geometry is retained in the desired mono-alkynyl complexes. The absorption spectra of complexes 1a–4a display d–d bands with distinct vibronic progressions that are slightly red shifted from trans-[Cr(HMC)(C₂Ar)₂]⁺ with approximately halved molar extinction coefficients. Time-delayed measurements of the emission spectra for complexes 1a–4a at 77 K revealed phosphorescence with lifetimes ranging between 343 μs (4a) and 397 μs (1a). The phosphorescence spectra of 1a–4a also exhibit more structuring than the bis-alkynyl complexes due to a strengthened vibronic coupling between the Cr^III metal center and alkynyl ligands.