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

Tris(mu-p-methylbenzoato-O,O')bis(N,N,N',N' -tetramethyl-1,2-diaminoethane) dicopper(II) hexafluorophosphate, [Cu-2(O2CC6H4Me-p)(3)(tmen)(2)]PF6 1, was prepared from a reaction of [Cu-2(O2CC6H4Me-p)(4)(H2O)(2)] with tmen in et hanol at O OC and characterized by spectral, electrochemical, magnetic and X-ray crystallographic studies. The cationic complex consists of a dicopper (Ir) unit having three bridging carboxylates and two bidentate chelating tm en ligands. The co-ordination geometry of the copper centres is distorted s quare pyramidal (CuO3N2). The Cu Cu distance in the core is unusually long, 3.419(2) Angstrom. The Cu-O-C angles at two copper centres are drastically different. Magnetic studies in the temperature range 20-296 K show the pre sence 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 ( epsilon = 263 M-1 cm(-1)) in MeCN. The magnetostructural data among the syn , syn-carboxylato bridged dicopper(II) systems with a [Cu-2(mu-O2CR)(m)](n) (m = 4-1; n = 0-3) core show that while 1 structurally has a tricarboxyla to bridged dicopper(II) unit, magnetically it behaves like a dibridged one.