ABSTRACT
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.
ABSTRACT
The synthesis and structural, electrochemical, spectroscopic, and magnetic characterizations of CrIII(HMC) catecholate and semiquinonate complexes are reported herein, where HMC is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. cis-[Cr(HMC)(Cat)]+ complexes (Cat = catecholate, [1]+; tetrachlorocatecholate, [2]+; and 3,5-di-tert-butylcatecholate, [3]+) were prepared from the reaction between appropriate catechol and [CrIII(HMC)Cl2]Cl reduced in situ by zinc. Chemical oxidation of [3]+ by FcPF6 resulted in cis-[Cr(HMC)(SQ)]2+ ([3]2+, SQ = 3,5-di-tert-butylsemiquinonate). Single crystal X-ray diffraction studies revealed the cis-chelation of the Cat/SQ ligand around the Cr metal center and confirmed the Cat/SQ nature of the ligands. Reversible oxidations of Cat to SQ were observed in the cyclic voltammograms of [1]+-[3]+, while the CrIII center remains redox inactive. The absorption spectrum of the SQ complex [3]2+ exhibits an intense spin-forbidden transition in solution. Time-delayed phosphorescence spectra recorded at 77 K revealed that all catecholate complexes emit from the 2E state, while [2]+ also emits from the 2T1 state. Temperature-dependent magnetic susceptibility measurements indicate the Cat complexes exist as S = 3/2 systems, while the SQ complex behaves as an S = 1 system, resulting from strong antiferromagnetic coupling of the S = 3/2 Cr center with the S = 1/2 SQ radical. Density functional theory (DFT) shows the similarities between the SOMOs of [1]+ and [2]+ and differences in their LUMOs in the ground state.
