SYNTHESES AND PHYSICOCHEMICAL PROPERTIES OF POLYCYANO-SUBSTITUTED BUTA-1,3-DIENYLIDENE-BRIDGED AND ALLENYLIDENE-BRIDGED DIIRON COMPLEXES

Treatment of the (mu-ethenylidene)diiron complexes Cp2Fe2L2(mu-CO)(mu-C=CH2) (L = CO (1a); L2 = dppe (1b), dppm (1c)) with tetracyanoethylene (tcne) results in the formation of the mu-3,4,4-tricyanobuta-1,3-dienylidene complexes CP2Fe2L2(mu-CO)[mu-C=CHC(CN)=(CN)2] (2a-c) and the mu-3,3-dicyanoallenylidene complexes CP2Fe2L2(mu-CO)[mu-C=C=C(CN)2] (3b,c). The tricyanobutadienylidene ligand is shown to be a very good pi-acceptor on the basis of infrared and NMR data. An X-ray diffraction analysis has been carried out on complex 2a (L = CO) (monoclinic, space group P2(1)/c, a = 11.621 (4) angstrom, b = 7.904 (4) angstrom, c = 20.546 (5) angstrom, beta = 94.00 (2)-degrees, Z = 4, R = 0.032, R(w) = 0.031 for 2025 reflections with I > 3-sigma(I)). The bridging tricyanobutadienylidene ligand is planar and exhibits extensive electron delocalization. Complexes 2a (L = CO) and 2b (L2 = dppe) strongly absorb light in the visible region (2a, lambda(max) = 476 nm, epsilon = 25 500 L mol-1 cm-1; 2b, lambda(max) = 539 nm, epsilon = 25 700 L mol-1 cm-1). These absorptions are proposed to be mainly pi --> pi* mu-tricyanobutadienylidene ligand based. More or less restricted rotation about the C(alpha)-C(beta) bond of the mu-tricyanobutadienylidene ligand in 2a,b is observed by dynamic NMR studies. The difference for the energy barriers (2a, DELTA-G(double dagger)298 = 16 +/- 1 kcal mol-1; 2b, DELTA-G(double dagger)211 = 10 +/- 0.2 kcal mol-1) is consistent with the stabilization of a dipolar mesomeric form, as suggested by the X-ray data. The mu-3,3-dicyanoallenylidene complexes (3b,c) have been characterized spectroscopically and by X-ray diffraction analysis (3b-CH2Cl2, monoclinic, space group C2/c, a = 22.222 (8) angstrom, b = 13.677 (5) angstrom, c = 25.746 (9) angstrom, beta = 103.44 (3) angstrom, Z = 8, R = 0.048, R(w) = 0.045 for 3550 reflections with I > 2-sigma(I)). The mu-allenylidene ligand has a short C(alpha)-C(beta) double bond (C(36)-C(37) = 1.285 (5) angstrom) and a normal C(beta)-C(gamma) double bond (C(37)-C(38) = 1.345 (6) angstrom). Cyclic voltammetry data and ESR and H-1 NMR studies on reaction mixtures of 1a,b and tcne allow a reaction mechanism to be proposed. Protonation of the mu-allenylidene complex 3b occurs reversibly at C(beta). The cationic mu-(2,2-dicyanovinyl)carbyne complex Cp2Fe2(mu-CO)(mu-dppe)[mu-CCH=C(CN)2]+BF4- (4b) has been isolated and characterized spectroscopically.