FLUXIONALITY IN HEXACOORDINATED COPPER(II) COMPLEXES WITH 2,2'-6',2''-TERPYRIDINE (TERPY) AND RELATED LIGANDS - STRUCTURAL AND SPECTROSCOPIC INVESTIGATIONS

Variable-temperature EPR spectra of Cu(terpy)2(PF6)2 (terpy = 2, 2′:6′, 2″-terpyridine) revealed that the low-temperature structure contains elongated CuN6 octahedra with an orthorhombic component as a consequence of the Jahn-Teller effect and rigid ligand strain. The high-temperature phase is characterized by compressed octahedra in the time average (planar dynamics). Structural relations between the various low-temperature phases of compounds Cu(terpy)2X2•nH2O (X = NO3, ClO4, PF6, Br) with respect to the cooperative Jahn-Teller order patterns are given, and the structure for the complex with X = Br and n = 3 is reported (298 K). It is triclinic, space group [formula omitted], with a = 19.763 (4) Å, b = 9.563 (2) Å, c = 8.537 (1) Å, a = 95.96 (1)°, β = 93.48 (1)°, γ = 94.44 (1)°, and Z = 2. The least-squares refinement of the structure leads to an R factor of 0.068. Complexes Cu-(tpt)2X2•nH2O [tpt = 2, 4, 6-tris(2-pyridyl)-1, 3, 5-triazine; × = NO3, BF4, C104, n = 1; × = PF6, n = 3] and the compound Cu(bpca)2•H2O [bpca = N-(2-pyridylcarbonyl)-2-pyridinecarboximidate] were characterized by variable-temperature EPR spectroscopy. As for terpy as the ligand, the molecular CuN6 geometry is determined by the vibronic Jahn-Teller coupling and a compression due to the rigid ligand. This model yields a structure of the ground-state potential surface that readily explains the dynamic features with increasing temperature. Finally, bonding parameters have been extracted from the copper hyperfine structures in the EPR signals, which are nicely resolved in some of the pure compounds and in the Cu2+-doped Zn2+ complexes, which have also been prepared. © 1990, American Chemical Society. All rights reserved.