Steric-influenced fluxionality in monomeric two and three coordinate copper(I) complexes with a trans-chelating bis-pyrazole ligand

Two new trans-chelating bispyrazolyl ligands, L ^H, where L ^H = 1,3-bis-(3,5-dimethyl-pyrazol-1-ylmethyl)-2,3-dihydro-1H- perimidine, and LCHPh ₂, where LCHPh ₂=2-benzhydryl-1,3-bis- (3,5-dimethyl-pyrazol-1-ylmethyl)-2,3-dihydro-1H-perimidine, and their two and three coordinate copper(I) derivatives [Cu(L ^H)](ClO ₄) (3·ClO ₄), [Cu(LCHPh ₂)](ClO ₄) (4·ClO ₄), and [Cu(L ^H)I] (5) were prepared and structurally characterized by X-ray crystallography. The variable-temperature ¹H NMR spectra showed that complex 4·ClO ₄ is rigid, whereas complexes 3·ClO ₄ and 5 are nonrigid in CH ₂Cl ₂ solution. This suggests that the steric bulk of the 2-substituents in the dihydroperimidine moiety could impart some steric effects upon the fluxional behavior of individual two and three coordinate copper(I)-pyrazole complexes, in comparison with the rigid three coordinate copper(I) complex [Cu(L ^Ph)I] (2) and dynamic two coordinate copper(I) complex [Cu(L ^Ph)](ClO ₄) (1·ClO ₄) found previously [17]. To the best of our knowledge, complex 5 represents the first example of a dynamic monomeric three coordinate copper(I) complex.