SYNTHESIS AND CHARACTERIZATION OF AN IRON(II) CIS-HYDRIDE(ETA-2-DIHYDROGEN) COMPLEX STABILIZED BY THE TETRAPHOSPHINE P(CH2CH2PPH2)3

The cis-(H)(H2) complex [(PP3)Fe(H)(H2)]BPh4, 1, [PP3 = P(CH2CH2PPh2)3] has been synthesized by several reactions, including a new one for dihydrogen metal complexes. This involves the one-pot reaction between iron(II) salts, PP3 and HCOOH. Complex 1 is fairly stable in refluxing tetrahydrofuran under argon and does not undergo H/D exchange when treated for 3 h with D2 or D2O. The nonclassical structure of 1 has been established by variable-temperature H-1 and P-31 NMR spectroscopy, T1 measurements and J(HD) values. In the solid state and in solution at low temperature, [(PP3)Fe(H)(H2)]+ is octahedral and the hydride and dihydrogen ligands occupy mutually cis positions. At ambient temperature, 1 is highly fluxional. The dynamic behaviour of the complex in solution has been studied by P-31{H-1} NMR spectroscopy. A satisfactory simulation of the variable-temperature spectra has been obtained assuming exchange between the three configurations P(A)P(M)P(Q)P(Q) if and only if P(A)P(Q)P(Q)P(M)(P(A) = bridgehead phosphorus atom; P(M), P(Q) terminal phosphorus atoms). From the plot of the rate constants against 1/T the activation parameters DELTA-G], DELTA-H] and DELTA-S double-ended dagger for the fluxional process have been calculated. These may suggest the existence in solution of a trihydrogen complex either as an intermediate or a stable species. Complex 1 is able to catalytically decompose formic acid to H-2 and CO2. This potential source of reducing power was used to selectively convert phenylacetylene to styrene.