Synthesis, crystal structure and properties of [Cu₂(O₂CC₆H₄Me-p)₃(Me₂NCH₂CH₂NMe₂)₂]PF₆: an unprecedented [Cu₂(μ-O₂CR)₄] into [Cu₂(μ-O₂CR)₃]⁺ core conversion

Abstract

Tris(μ-p-methylbenzoato-O,O')bis(N,N,N',N'-tetramethyl-1,2-diaminoethane)dicopper(II) hexafluorophosphate, [Cu₂(O₂CC₆H₄Me-p)₃(tmen)₂]PF₆1, was prepared from a reaction of [Cu₂(O₂CC₆H₄Me-p)₄(H₂O)₂] with tmen in ethanol at 0°C and characterized by spectral, electrochemical, magnetic and X-ray crystallographic studies. The cationic complex consists of a dicopper(II) unit having three bridging carboxylates and two bidentate chelating tmen ligands. The co-ordination geometry of the copper centres is distorted square pyramidal (CuO₃N₂). The Cu···Cu distance in the core is unusually long, 3.419(2) Å. The Cu-O-C angles at two copper centres are drastically different. Magnetic studies in the temperature range 20-296 K show the presence of an antiferromagnetically coupled dicopper(II) unit giving -2J = 81.4 cm^-1 and g = 2.081. Cyclic voltammetric studies have shown a cathodic peak at -0.92 V with an anodic response at 0.10 V vs. SCE at 100 mV s^-1. The visible electronic spectrum of the complex shows a d-d band at 665 nm (ε=263 M^-1 cm^-1) in MeCN. The magnetostructural data among the syn, syn-carboxylato bridged dicopper(II) systems with a [Cu₂(μ-O₂CR)_m]ⁿ⁺ (m=4−1; n=0−3) core show that while 1 structurally has a tricarboxylato bridged dicopper(II) unit, magnetically it behaves like a dibridged one.