Synthesis, electrochemistry, magnetic and spectroscopic properties of an exchange-coupled iron(III)-nickel(II)-iron(III) complex. Crystal structure of [L2Fe2(dmg)3Ni](PF6)2.0.5CH3OH (L = 1,4,7-trimethyl-1,4,7-triazacyclononane; dmg = dimethylglyoximate(2-))