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Preparation and Properties of Dinitrogen-Molybdenum Complexes Masanobu Hidai," Kenichi Tominari, and Yasuzo Uchida Contribution from the Department of Industrial Chemistry, The University of Tokyo, Hongo, Tokyo, Japan. Received June 4, 1971
Abstract: Dinitrogen-molybdenum complexes of the types Mo(N2)(PPh&.C6H6CH3(1) and trans-Mo(N&(Ph2P(CH2),PPh2)2(n = 1, 2, and 3) were prepared from molybdenum(II1) acetylacetonate, organoaluminum compounds, phosphine compounds, and molecular nitrogen. The bis(dinitrogen) complex trans-Mo(Nz)o(DPE)z (2) (DPE, 1,2-bis(diphenyIphosphino)ethane) reacts reversibly with hydrogen gas to give the molybdenum hydride complex trans-MoHz(DPE)z (3) or [trans-MoHz(DPE)]2-pDPE(4) depending upon the solvent used. The former hydride complex was obtained in benzene or tetrahydrofuran, while the latter hydride complex was formed in toluene. The hydride complexes are interconvertible. The reaction of the bis(dinitrogen) complex 2 with trihydridotris(triphenyIphosphine)cobalt(III) was carried out as an inorganic model of nitrogen reductase. The dinitrogen ligand transferred from the molybdenum atom to the cobalt atom and the hydride complex 3 or 4 was formed depending upon the solvent in addition to hydridodinitrogentris(triphenylphosphine)cobalt(I). Reactions of dinitrogen-molybdenum complexes with carbon monoxide were also studied. ince the discovery of the first N, complex of a transipresence of a tenfold excess of triphenylphosphine in tion metal,' many Nz complexes of group VI11 toluene under nitrogen, an orange complex 1 was obtransition metals have been prepared by several methods Mo(acac)3 + PPh3 + AIR, + NZ4 in toluene and their properties have been extensively studied in MO(NZ)(PPh,)z. CsHsCH3 relationship to nitrogen fixation in biological systems.2 1 Some nitrogen-fixing enzymes in biological systems contain molybdenum, nonheme iron, and labile sulfide. R = Et or i-Bu Reactions of molecular nitrogen with iron complexes3 tained, which was empirically formulated as Moor sulfur-containing organic compounds have been (NZ)(PPh&.C8H5CH3. The Nz complex is moderreported. In view of the key role of molybdenum in ately air stable. The yield of the complex decreased to both chemical and biological nitrogen fixation, we have one-fifth when triisobutylaluminum was used in place of attempted to prepare dinitrogen-molybdenum comtriethylaluminum. On the other hand, the complex was plexes. Previously we reported briefly the preparation not obtained by using trimethylaluminum. Molar of dinitrogen-molybdenum complexes of two types, ratios of triphenylphosphine to molybdenum(II1) acetylMo(N2)(PPh3),.C,H,CH, (1) and t r ~ n s - M o ( N ~ ) ~ ( P h ~ acetonate remarkably affect the yield of the complex. PCH2CH2PPh2),(2), which were obtained by the reThe yield decreased with the decrease in the molar ratio duction of molybdenum(II1) acetylacetonate with triof PPh3/Mo(acac)3. At the ratio of PPh,/Mo(acac), = alkylaluminum in the presence of triphenylphos3 the complex was scarcely obtained. phine or 1,2-bis(diphenyIphosphino)ethane in an atThe infrared spectrum of the complex shows a very mosphere of n i t r ~ g e n . ~Recently preparations of strong absorption at 2005 cm-1 assignable to the cothe latter dinitrogen-molybdenum complex by different ordinated N-N stretching frequency in addition to bands routes have been briefly reportedn6 This paper is concharacteristic of triphenylphosphine. The complex decerned with the syntheses and reactivities of the dinitrocomposed in U ~ C U Oabove ca. 90" evolving a gas congen-molybdenum complexes. taining 0.9 mol of nitrogen molecule, 0.2 mol of toluene, and 0.2 mol of benzene per mole of molybdenum in adResults and Discussion dition to a trace of hydrogen gas. The complex may Preparation and Properties of Dinitrogen-Molybbe polynuclear since it has slight solubility in organic denum Complexes. When molybdenum(II1) acetylsolvents such as benzene, ether, acetone, and tetrahyacetonate was reduced with triethylaluminum in the drofuran. Using benzene or mesitylene as the solvent instead of (1) A. D . Allen and C. V. Senoff, Chem. Commun., 621 (1966). toluene, an analogous orange complex with the N-N (2) For recent reviews, see (a) R. Murray and D. C. Smith, Coord. stretching frequency at 2005 cm-1 was obtained. The Chem. Reu., 3, 429 (1968); (b) G. H. Olive and S. Olive, Angew. Chem., 81, 679 (1969); (c) Yu. G . Borod'ko and A. E. Shilov, Usp. Khim., 38, elemental analysis indicates that the complex does not 761 (1969). contain the solvent and is formulated as Mo(N2)(3) (a) C. H . Campbell, A. R. Dias, M . L. H . Green, T. Saito, and (PPh&. The complex shows the same chemical propM . G. Sandwick, J . Organometal. Chem., 14, 349 (1968); (b) A. Sacco and M. Aresta, Chem. Commun., 1223 (1968); (c) G. M. Bancroft, erties as the above NZcomplex 1. M. J. Mays, and B. E. Prater, ibid., 585 (1969); J . Chem. SOC.A , 2146 No N2 complex was obtained by using other mono( 1970). (4) D. C. Owsley and G. K. Helmkamp, J . Amer. Chem. SOC.,89, dentate phosphines such as tri-n-butylphosphine, tri-p4558 (1967); J. Ellermann, F.Poersch, R. Kunstmann, and R. Kramotolylphosphine, and ethyldiphenylphosphine. HOWlowsky, Angew. Chem., 81, 183 (1969). ever, a new series of dinitrogen-molybdenum coni( 5 ) M. Hidai, I
