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]PF₆ 1, 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.