SYNTHESIS AND CHARACTERIZATION OF MIXED-VALENCE MU-ALKOXO-DIIRON(II,III) COMPLEXES WITH AN UNSYMMETRIC DINUCLEATING LIGAND

A series of dimetal complexes, [(MaMbIII)-M-II(L) (C6H5CO2)(2)](ClO4)( 2), were synthesized (2, L=tpdb, M-a=M-b=Fe; 3, L=Me-4-tpdb, M-a=M-b=F e; 4, L =tpdb, M-a=Fe, M-b =Ga; 5, L=tpdb, M-a=Zn, M-b=Fe; tpdb etraki s(2-pyridylmethyl)-1,4-diaminobutane-2-olate and Me-4-tpdb = N,N,N',N' -tetrakis [2-(6-methylpyridyl) methyl]-1,4-diaminobutane-2-olate). The se dinucleating ligands provide an unsymmetric bridging back born at t wo metal centers, forming five- and six-membered chelate rings in a br idging skeleton. Complex 2 afforded two types of crystal (2a (2.3CH(3) CN) and 2b (2.1.5H(2)O)) suitable for X-ray analyses; 2a crystallizes in the P2(1)2(1)2(1) space group with unit cell parameters a = 18.523( 3), b = 18.871(3), c = 14 838(5) Angstrom, and Z = 4; 2b crystallizes in the P2(1)/c space group with unit cell parameters a = 25.343(5), b = 15.152(2), c = 48.035(7) Angstrom, beta = 91.53(2)degrees, and Z = 1 6. 2a contains only one molecule in an asymmetric unit, whereas 2b con tains four distinct molecules in an asymmetric unit. Crystal structure s showed that all the dinuclear complex cations in 2a and 2b consist o f two distinct iron centers which are triply bridged by the alkoxide o xygen of tpdb and two benzoate groups. The Fem ion tends, preferential ly, to occupy a site which has a five-membered chelate ring in the bri dging skeleton. There are four distinct complex cations in 2b consisti ng of two enantiomeric pairs with respect to an asymmetric carbon atom with a skew boat conformation in the six-membered chelate ring, and w ith a boat conformation in the six-membered chelate ring, respectively . The Mossbauer spectrum of a powdered sample of 2b at 78 K showed thr ee quadrupole doublets; one is due to Fe-III centers and the other two are due to Fe-II centers. The latter two appear to correspond to two types of conformational isomer found for 2b. The Mossbauer spectra of 2b and 3 at room temperature suggested that, in 2b, the fast electron exchange between Fen and Fem begins to occur whereas, in 3, the electr on is localized within the Mossbauer time scale. 2 exhibited three abs orption bands at 18970 cm(-1) (epsilon = 338 mol(-1) dm(3) cm(-1)), 12 900 cm(-1) (epsilon = 284 mol(-1) dm(3) cm(-1)) and 7650 cm(-1) (epsil on = 550 mol(-1) dm(3) cm(-1)). The first and third bands are assignab le to the intervalence charge transfer transition from t(2g) orbitals in the Fe-II center to e(g) and t(2g) orbit als in the Fe-III center, respectively. Two reversible one-electron redox waves corresponding to the (FeFeIII)-Fe-II/(FeFeIII)-Fe-II and (FeFeIII)-Fe-III/(FeFeIII)-Fe -II couples were observed at -110 and 620 mV versus SCE for 2 by cycli c voltammetry, indicating that unsymmetric tpdb stabilizes substantial ly the mixed valence state.