Synthesis and Characterization of Molybdenum Complexes Containing Diphosphine Ligands of the Type (ArCH2)2PC2H4P(CH2Ar)2 (Ar = C6H4X). Electronic Control of η2-Dihydrogen versus Dihydride Coordination in MoH2(CO){(RCH2)2PC2H4P(CH2R)2}2 (R = Me, Pri, C6H4X) and Implications on the Reaction Coordinate for H2 Cleavage

Xiao Liang Luo, Gregory J. Kubas, Carol J. Burns, Juergen Eckert

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Abstract

The bis(dinitrogen)molybdenum complexes frans-Mo(N2)2{(ArCH 2)2PC2H4P(CH2Ar)2}2(Ar = C6H 5(1a),C 6H4-m-Me (1b), C6H4-o-Me (1c), C6H4-p-Me (1d), C6H4-o-F (1e), C6H4-m-F (1f) C6H4-p-F (1g), C6H4-m-OMe (1h), C6H4-p-OMe (1i)) are prepared by reduction of MoCl5 with magnesium under a dinitrogen atmosphere in the presence of the appropriate diphosphine ligand. Treatment of 1a and 1b with ethyl acetate in refluxing benzene under argon affords the formally 16-electron complexes Mo(CO){(ArCH2)2PC2H4P(CH2Ar)2}2(Ar=C6H5(2a), C6H4-m-Me (2b)), which are shown by 1H NMR spectroscopy to contain an agostic Mo· ·· H—C interaction between the molybdenum and an ortho aryl hydrogen atom. Complexes 2a and 2b react readily with small molecules H2, N2, and SO2 to give the adducts trans-Mo(L)(CO){(ArCH2)2PC2H4P(CH2Ar)2}2(L =η2-H2, Ar = C6H5(3a), C6H4-m-Me (3b); L = N2, Ar = C6H4(4a), C6H4-m-Me (4b); L = SO2, Ar = C6H 4(5a), C6H4-m-Me (5b)). In contrast to the dihydride formulation in MoH2(CO){(RCH 2)2PC2H4P(CH2R)2}2(R = Me, Pri), the η2-H2 coordination in 3a and 3b is unambiguously established by IR, NMR, and neutron scattering spectroscopies and a single-crystal X-ray diffraction analysis of 3b. Thus, electronic control of η2-dihydrogen versus dihydride coordination has been achieved in complexes of the type MoH2(CO){(RCH2)2PC 2H4P(CH2R)2}2, an η2-dihydrogen complex being formed when R is an electron-withdrawing aryl group, whereas a dihydride is formed when R is an electron-donating alkyl group. The spectroscopic data for 3a and 3b indicate that they exist exclusively as η2-dihydrogen complexes with a normal H—H distance both in solution and in the solid state, and so neither an elongated H—H bond nor an η2-dihydrogen/dihydride tautomeric equilibrium has been observed although the π-basicity of the metal center appears to be very close to the point where the H—H bond cleavage is expected to occur. This supports the suggestion that the reaction coordinate for H2 cleavage in the lybdenum—diphosphine system is rather flat and shows relatively little change in the H—H distance until relatively precipitous cleavage of the H—H bond. Crystallographic data for 3b: monoclinic, space group C2/c, a = 26.782(5) Å, b= 9.365(2) Å, c = 25.399(5) Å, β = 111.75(3)°, V = 5921(5) Å3, Z = 4, and R = 0.058. Crystallographic data for 4a: monoclinic, space group C2/c, a = 23.644(5) Å, b = 13.558(3) Å, c = 17.525(4) Å, β = 108.97(3)°, V = 5313 (4) Å3, Z = 4, and R = 0.034.

Original languageEnglish
Pages (from-to)5219-5229
Number of pages11
JournalInorganic Chemistry
Volume33
Issue number23
DOIs
StatePublished - Nov 1 1994

Fingerprint Dive into the research topics of 'Synthesis and Characterization of Molybdenum Complexes Containing Diphosphine Ligands of the Type (ArCH<sub>2</sub>)<sub>2</sub>PC<sub>2</sub>H<sub>4</sub>P(CH<sub>2</sub>Ar)<sub>2</sub> (Ar = C<sub>6</sub>H<sub>4</sub>X). Electronic Control of η<sup>2</sup>-Dihydrogen versus Dihydride Coordination in MoH<sub>2</sub>(CO){(RCH<sub>2</sub>)<sub>2</sub>PC<sub>2</sub>H<sub>4</sub>P(CH<sub>2</sub>R)<sub>2</sub>}<sub>2</sub> (R = Me, Pr<sup>i</sup>, C<sub>6</sub>H<sub>4</sub>X) and Implications on the Reaction Coordinate for H<sub>2</sub> Cleavage'. Together they form a unique fingerprint.

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