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

Described herein is the synthesis and characterization of macrocyclic CrIII 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)(C2C6H4tBu)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 CrIII metal center and alkynyl ligands.