COMPETITIVE NUCLEOPHILIC-ATTACK BETWEEN CARBON AND METAL ATOMS IN BRIDGING HYDROCARBYLS - X-RAY STRUCTURES OF FE2(CO)5(MU-PPH2)[MU-C=C(PH)PPH2CH2PPH2], A NOVEL ZWITTERIONIC VINYLIDENE COMPLEX, AND FE2(CO)4(MU-PPH2)(MU-2-ETA(2)-CCPR(I))(MU-PH2PCH2CH2PPH2), A DIPHOSPHINE-BRIDGED ACETYLIDE

The sigma-pi acetylide complex Fe2(CO)6(mu-PPh2)(mu(2)-eta(2)-C = CPh) (1A) reacts with the diphosphines bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphino)ethane (dppe) to yield Fe2(CO)5(mu-PPh2)[mu-C = C (Ph)PPh2CH2PPh2] (2A) and Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C = CPh)(mu-dppe) (5A), respectively. The analogue, Fe2(CO)6(mu-PPh2)(mu(2)-eta(2)-C = CPr(i)) (1B) reacts with dppm and dppe to yield Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C = CPr(i))(mu-dppm) (4B) and Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C = CPr(i))(mu-dppe) (5B) while Fe2(CO)6-(mu-PPh2)(mu(2)-eta(2)-C = CBu(t)) (1C) reacts with dppm and dppe to afford Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C = CBu(t))(mu-dppm) (4C) and Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C = CBu(t))(mu-dppe) (5C). The mu-dppm complex Fe2(CO)4(mu-PPh2)(mu(2)-eta(2)-C=CPh)(mu-dppm) (4A) can also be prepared in high yield from Fe2(CO)6(mu-CO)(mu-Ph2PCH2PPh2) and Ph2PC = CPh under UV irradiation and also under prolonged heating of 2A. The reactions were monitored by P-31 NMR spectroscopy, allowing the elucidation of mechanistic details. For Fe2(CO)6(mu-PPh2)(mu(2)-eta(2)-C = CPh) (1A) both dppm and dppe initially attach at the beta-carbon of the acetylide. For Fe2(CO)6(mu-PPh2)(mu(2)-eta(2)-C = CBu(t)) (1C) the initial site is the metal, resulting in displacement of carbon monoxide. For Fe2(CO)6(mu-PPh2)(mu(2)-eta(2)-C = CPr(i)) (1B) both types of intermediate are observed upon reaction with dppe and attack at the beta-carbon of the acetylide predominates for dppm. Complexes 2A and 5B have been characterized crystallographically, confirming that the diphosphine moiety is incorporated into a six-membered ring. Complex 2A has a novel zwitterionic mu-vinylidene ligand formed via nucleophilic attack by one phosphorus atom of dppm at the beta-carbon of the acetylide. The remaining phosphorus atom is bound to a metal center. In 5B the dppe ligand bridges two metal sites. Crystals of 2A are triclinic, space group P1BAR, with unit cell dimensions a = 10.438 (1) angstrom, b = 12.549 (2) angstrom, c = 21.081 (4) angstrom, alpha = 85.07 (2)-degrees, beta = 79.23 (1)-degrees, and gamma = 79.63 (1)-degrees. Crystals of 5 are triclinic, space group P1BAR, with unit cell dimensions a = 10.648 (2) angstrom, b = 12.888 (2) angstrom, c = 23.999 (5) angstrom, alpha = 77.18 (2)-degrees, beta = 88.72 (2)-degrees, and gamma = 69.45 (1)-degrees. The structures were solved and refined to the following R and R(w) values; 2A, R = 0.040 and R(w) = 0.048 on 5093 observed (I > 3-sigma(I)) data; 5B, R = 0.054 and R(w) = 0.064 on 4195 observed (I > 3-sigma(I)) data. In binuclear compounds such as 1A-C, where the hydrocarbyl is activated by sigma-pi-coordination to two metals, nucleophilic attack at carbon and metal is competitive, with the former more favorable as an initial site.