Molecular hydrogen complexes of the transition metals. 4. Preparation and characterization of M(CO)3(PR3)2(.eta.2-H2) (M = molybdenum, tungsten) and evidence for equilibrium dissociation of the H-H bond to give MH2(CO)3(PR3)2

Abstract

30. Spectrophotometry indicated that Ph,PO had no effect on the reaction. The results thus far require that MoO(sap)(DMF) quantitatively reduce Mo02(L-NS2). This point was spectrophotometrically established with use of an equimolar D M F reaction system at ambient temperature.The foregoing results are summarized in Figure 8, which depicts spontaneous intermetal oxo-transfer reactions. In the set of complexes, MoO(sap)(DMF) (17) is the strongest reductant and M o O~( S~C N E~~)~ (19) is the strongest oxidant. This thermodynamic series is the same as kinetic series 19, thereby showing that the activation barrier to oxo transfer is largely set by those factors which stabilize/destabilize Mo(IV) and Mo(V1). This in turn reemphasizes the beneficial effect of anionic sulfur ligands in stabilizing Mo(IV). Lastly, the reactions 17 + ]/,02 -13 and 18 + 1/202 -15 cannot be placed precisely in the oxidative enthalpy series of Table 111. However, it is clear that the first of these reactions lies below the second and that the AH values of both are more negative than that for the oxidation of MoO-(L-NS2)(DMF). The lack of reaction between 17 and 10 equiv of Ph3P0 in DMF for 6 h at ambient temperature suggests that AH k -67 kcal/mol, but slow reaction kinetics cannot be ruled out. In any case, 17 and 18, as MoO(L-NS2)(DMF) and MoO-(S,CNEt2),, should reduce MezSO to Me#. This has been confirmed for the stronger reductant 17, which is quantitatively oxidized to 13 in a system initially containing 2 equiv of Me2S0. All MoIVO complexes in Figure 8 are now recognized to be thermodynamically competent to reduce Me2S0,75 the most re-ductively resistant enzyme substrate for which thermodynamic data are available. The stoichiometric reduction of substrate XO by a MoIVO complex is, therefore, a highly necessary but not a sufficient thermodynamic criterion for a functional oxo-transferase site model. What is required for sufficiency under the oxo atom transfer hypothesis are those factors which permit at least one such atom transfer to or from substrate followed by regeneration of the original Mo'"0 or MoV'02 species either by electron or oxo transfer, such that catalysis is sustained. The results presented here show that anionic sulfur ligation is a critical modulator of these factors and, as already mentioned, appears to place real or effective Mo redox potentials in a range accessible to physiological reactants.Ongoing research on biologically related oxo-transfer reactions includes development of catalytic systems for substrate oxidation and reduction, examination of reactions in aqueous solution, and the possible role of pterins in enzymic electron transfer. Acknowledgment. (75) In the only other related case that has been reported, [MoO(dttd)CIJ1reduces Me2SO: Kaul, B. B.; Enemark, J. H.; Merbs, S. L.; Spence, J. T. J . Am. Chem. SOC. 1985, 107, 2885. dttd = 2,3:8,9-dibenzo-1,4,7,lO-tetrathiadecane( 2-). Abstract: The syntheses, properties, and spectral characterization of the first examples of molecular hydrogen complexes, M(CO),(...

Transition Metal Complexation of σ Bonds

“…In the complex the I3C,H coupling constant 'J(I3C,H) is reduced by up to 50% relative to the uncomplexed ligand (free, ca. 125 Hz, bound 60-120 Hz), just as 'J(H,D) is reduced by about the same amount in unstretched H, complexes (free, 43 Hz, bound [22][23][24][25][26][27][28][29][30][31][32][33]. The signal of the agostic H atom normally appears upfield from tetramethylsilane, and in this respect resembles a metal-bound rather than a C-bound H atom.…”

Molybdenum Nitrosyl Complexes and Their Application in Catalytic Imine Hydrogenation Reactions

“…[4] Furthermore, in related systems, Kubas and co-workers demonstrated the crucial influence of phosphane substitution on hydrogen activation through their electron-donating capability. [21][22][23][24] For example, the complexes [Mo(CO)(iBu 2 PCH 2 CH 2 PiBu 2 ) 2 ] bearing strong electron-donating ligands can split the H 2 molecule at ambient temperature to form a dihydride Mo(H) 2 species, whereas the structurally similar [Mo(CO)-(Ph 2 PCH 2 CH 2 PPh 2 ) 2 ] complex coordinates the hydrogen molecule with an elongated H-H bond. In isoelectronic cationic bis(diphosphane)nitrosylmolybdenum and -tungsten complexes the H 2 ligand adds oxidatively to form dihydride complexes.…”

Chemical bonding of hydrogen molecules to transition metal complexes

“…The infrared spectrum of our tungsten-dihydrogen complex displayed the six vibrational modes that we calculated to exist for a bound H2 (only two exist for a hydride, v(M-H) and 8(M-H)) [1]. The H-H stretch was located as a very broad, weak band near 2700 cm-1 for a solid sample.…”

“…However, our unexpected discovery of stable bonding of neutral dihydrogen (H2) molecules to molybdenum and tungsten complexes in 1983 has led to a new field of inorganic chemistry and has altered the way chemists now perceive bonding and reactions on metal coordination compounds [1,2]. The discovery has been universally characterized to be one of the most significant of the past decade in inorganic chemistry.…”

“…So far three neutron diffraction structures of H2 complexes have been carried out [1,4,12] and they ali show distances of 0.82 A (.-0.79 A for a preliminary refinement of the neutron structure of the above Mo complex). This is probably fortuitous because there is increasing evidence for elongated distances near or even greater than 1.0 A.…”