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Formation of ([HIPTN3N]Mo(III)H)- by Heterolytic Cleavage of H2 as Established by EPR and ENDOR Spectroscopy.

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Title: Formation of ([HIPTN3N]Mo(III)H)- by Heterolytic Cleavage of H2 as Established by EPR and ENDOR Spectroscopy.
Authors: Kinney, R. Adam1, Hetterscheid, Dennis G. H.2, Hanna, Brian S.2, Schrock, Richard R.2, Hoffman, Brian M.1 bmh@northwestern.edu
Source: Inorganic Chemistry. 1/18/2010, Vol. 49 Issue 2, p704-713. 10p.
Subjects: Electron nuclear double resonance, Electron paramagnetic resonance, Hydrogen, Molybdenum, Dihydrogen bonding, Stoichiometry, Inorganic chemistry
Abstract: MoN2 (Mon [(HlPTNCH2CH2)3N]Mo, where HIPT n 3,5-(2,4,6-i-Pr3C6H2)2C6H3) is the first stage in the reduction of N2 to NH3 by Mo. Its reaction with dihydrogen in fluid solution yields "MoH2", a molybdenum-dihydrogen compound. In this report, we describe a comprehensive electron paramagnetic resonance (EPR) and 1/2H/14N electron nuclear double resonance (ENDOR) study of the product of the reaction between MoN2 and H2 that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g anisotropy relative to MoN2. Analysis of the g values reveal that this anion has the Mo(lll), [dxz, dyz]3 orbital configuration, as proposed for the parent MoN2 complex, and that it undergoes a strong pseudo-Jahn-Teller (PJT) distortion. Simulations of the 2D 350Hz 1H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that us the [MoH]- anion. The 35 GHz Mims pulsed 2H ENDOR spectra of 1 prepared with 2H2 show the corresponding 2H- signal, with a substantial deuterium isotope effect in aiso. Radiotytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D 14N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear Nax-MO-H- molecular axis that is parallel to the unique magnetic direction (g1). The END0R pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo-Neq plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three Neq'S. The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to the FeMo cofactor of the nitrogense turnover intermediate, which has accumulated four electrons/protons (E4), are hydrides that bridge two metal ions, not terminal hydrides. [ABSTRACT FROM AUTHOR]
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Abstract:MoN2 (Mon [(HlPTNCH2CH2)3N]Mo, where HIPT n 3,5-(2,4,6-i-Pr3C6H2)2C6H3) is the first stage in the reduction of N2 to NH3 by Mo. Its reaction with dihydrogen in fluid solution yields "MoH2", a molybdenum-dihydrogen compound. In this report, we describe a comprehensive electron paramagnetic resonance (EPR) and 1/2H/14N electron nuclear double resonance (ENDOR) study of the product of the reaction between MoN2 and H2 that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g anisotropy relative to MoN2. Analysis of the g values reveal that this anion has the Mo(lll), [dxz, dyz]3 orbital configuration, as proposed for the parent MoN2 complex, and that it undergoes a strong pseudo-Jahn-Teller (PJT) distortion. Simulations of the 2D 350Hz 1H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that us the [MoH]- anion. The 35 GHz Mims pulsed 2H ENDOR spectra of 1 prepared with 2H2 show the corresponding 2H- signal, with a substantial deuterium isotope effect in aiso. Radiotytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D 14N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear Nax-MO-H- molecular axis that is parallel to the unique magnetic direction (g1). The END0R pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo-Neq plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three Neq'S. The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to the FeMo cofactor of the nitrogense turnover intermediate, which has accumulated four electrons/protons (E4), are hydrides that bridge two metal ions, not terminal hydrides. [ABSTRACT FROM AUTHOR]
ISSN:00201669
DOI:10.1021/ic902006v