Crystallographic studies of fac- and mer-isomers of Cu[1, 4, 7, 10, 13, 16-hexaazacyclooctadecane]2+. Influence of counterion and crystallization solvent on crystal structure and dynamic Jahn-Teller effects. Computational studies of structures, energetics, and mechanism of mer-fac isomerization. (1st October 2022)

• Record Type: Journal Article
• Authors: Hughes, Russell P.; Rheingold, Arnold L.; Wong, Edward H.; Bulkowski, John E.
• Is Part Of: Polyhedron. Volume 225(2022)
• Publication Date: 2022-10-01
• Subjects: DFT Jahn-Teller isomerization macrocycle copper(II); Chemistry, Inorganic -- Periodicals; Chimie inorganique -- Périodiques; Organometaalverbindingen; Anorganische chemie; 546.05
• DOI: 10.1016/j.poly.2022.116072
• Languages: English
• ISSNs: 0277-5387

Abstract

Graphical abstract: Crystallographic structures of Cu(II) complexes of the 18N6 macrocyclic ligand show tetragonal distortions in the mer- conformers, but not their fac -counterparts, due to a dynamic Jahn-Teller effect in the latter; DFT calculations corroborate this interpretation and provide a plausible mechanism for facile mer-fac isomerization. Abstract: The syntheses and crystal structures of mer - and fac -isomers of [Cu(18N6)] 2+ (X − )2 are reported (18 N6 = 1, 4, 7, 10, 13, 16-hexaazacyclooctadecane; X − = BF4 −, ClO4 −, NO3 − ) and compared to a previously reported example (X − = Cl − ). Solution structures of the [Cu(18N6)] 2+ dication in MeNO2 or MeOH are shown by FT-IR to be almost exclusively mer, and simple evaporation of these solutions or crystallization using Et2 O co-solvent also afford the solid mer -salts. However, using C6 H6 or CCl4 as crystallization co-solvents afforded the corresponding fac -isomers of the BF4 − and ClO4 − salts, in which the anions and the symmetrical solvent occupy locations on the crystallographic 3-fold axis. Similar treatment of the less symmetrical NO3 − salts affords the mer- isomer without inclusion of C6 H6 . The solid-state structures of the mer -isomers each exhibit the anticipated tetragonal Jahn-Teller distortions, with two longer Cu-N bonds. In contrast the fac -isomers all show internally identical Cu-N distances, due to a dynamic exchange between Jahn-Teller distorted structures in the crystal. Density Functional Theory (DFT) is used to provide rationalizations of these observations and to elucidate an energetically feasible mechanistic pathway for the facile interconversion of mer and fac isomers in solution.