Coordination of the anion of 2,4-dicyano glutaconic acid diethyl ester to tripodIron(II)-fragments. Formation of helical structures

The anion of 2,4-dicyano glutaconic acid diethyl ester, [NCC(COOEt)CHC(COOE t)CN](1(-)), as an alpha, omega -dinitrile with a three atom spacer is a hi ghly variable ligand capable of binding in a monodentate eta (1)-fashion, i n a bidentate chelate fashion, and as a mu (2)-bridging entity. TripodFe(1) (2), 2, [tripod = CH3C(CH2PPh2)(3)] contains one chelating ligand 1(-) and one terminally coordinated ligand 1(-.) Variable temperature NMR spectrosco py shows, that, while the structure is static at 193 K, dynamic exchange of the donor functions of chelating and terminally bonded ligands occurs at h igher temperature. TripodFe(1)(2), 2, is obtained from a 1:1:2 mixture of t ripod, Fe(II)(aq) (BF4)(2), and Na1 . 2H(2)O. With a stoichiometry 1:1:1 of these ingredients and an additional half equivalent of sodium cyanide, the dinuclear compound [tripodFe{mu -NCC(COOEt)CHC(COOEt)CN}(2){mu -CN}Fetripo d]BF4, 3BF(4), is obtained. The structure of 3(+) shows a helical arrangeme nt of the ligands 1(-) around the Fe . . . Fe axis which is a consequence o f the incomensurability of the bridging ligands 1(-) and mu -CN. The two ir on centers in 3BF(4) differ by their coordination to the carbon or to the n itrogen terminus of the bridging cyano group. This difference is reflected by the NMR spectra, Mossbauer spectra and cyclovoltammograms.