Synthesis, Structure, and Electrochemical Properties of Sterically Protected Molybdenum Trihydride Redox Pairs: A Paramagnetic “Stretched” Dihydrogen Complex?

Abstract

Complexes [MoCp(#)(PMe(3))(2)H(3)] (Cp(#)=1,2,4-C(5)H(2)tBu(3), 2 a; C(5)HiPr(4), 2 b) have been synthesized from the corresponding compounds [MoCp(#)Cl(4)] (1 a, 1 b) and fully characterized, including by X-ray crystallography and by a neutron diffraction study for 2 a. Protonation of 2 a led to complex [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)H(4)](+) (3 a) in THF and to [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)(MeCN)H(2)](+) (4 a) in MeCN. Complex 4 b analogously derives from protonation of 2 b in MeCN, whereas the tetrahydride complex 3 b is unstable. One-electron oxidation of 2 a and 2 b by [FeCp(2)]PF(6) produces the EPR-active 17-electron complexes 2 a(+) and 2 b(+). The former is thermally more stable than the latter and could be crystallographically characterized as the PF(6) (-) salt by X-ray diffraction, providing evidence for the presence of a stretched dihydrogen ligand (H...H=1.36(6) angstroms). Controlled thermal decomposition of 2 a(+) yielded the product of H(2) elimination, the 15-electron monohydride complex [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)H]PF(6) (5 a), which was characterized by X-ray crystallography and by EPR spectroscopy at liquid He temperature. The compound establishes an equilibrium with the solvent adduct in THF. An electrochemical study by cyclic voltammetry provides further evidence for a rapid H(2) elimination process from the 17-electron complexes. In contrast to the previously investigated [MoCp*(dppe)H(3)](+) system (dppe=1,2-bis(diphenylphosphino)ethane; Cp*=pentamethylcyclopentadienyl), the decomposition of 2 a(+) by H(2) substitution with a solvent molecule appears to follow a dissociative pathway in MeCN.

Spectroscopic, electrochemical and X-ray diffraction studies on nickel(II)-complexes of acetophenone thiosemicarbazones substituted six-carbon groups

“…The ONS chelating ligands and their metal complexes generally give electron transfer reactions at negative potentials (higher than À1.50 V) and at positive potentials (at around 1.50 V) [29][30][31][32][33][34][35][36][37][38]. As results, the obtained experimental data are consistent in comparison with data of similar compounds.…”

Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

“…Moreover, theoretical enthalpy values in THF solvent for parallel pathways for both compounds [Cp t Bu 3 , 7.83 kcal/mol for B3LYP/BS2 and 9.49 kcal/mol for ωB97X-D/BS2; Cp i Pr 4 (rate-determining step for H1 atom), 5.53 kcal/mol for B3LYP/BS2 and 6.71 kcal/mol for ωB97X-D/BS2; also see Table S17] are in good agreement with the experimental enthalpy values for the hydridescrambling process (9.0 ± 0.7 and 8.5 ± 0.3 kcal/mol, respectively, for Cp t Bu 3 and Cp i Pr 4 ). 6 The free-energy calculations ( Table 2) predict a more rapid H exchange for Cp i Pr 4 over that for Cp t Bu 3 in acetonitrile by nearly 2 orders of magnitude. Thus, our modeling explains the NMR results of Baya et al 6 that even at low temperatures (193 K) the fast hydrogen exchange in the parallel mechanism of Cp i Pr 4 prevents the final resolution of the NMR signal.…”

“…6 The free-energy calculations ( Table 2) predict a more rapid H exchange for Cp i Pr 4 over that for Cp t Bu 3 in acetonitrile by nearly 2 orders of magnitude. Thus, our modeling explains the NMR results of Baya et al 6 that even at low temperatures (193 K) the fast hydrogen exchange in the parallel mechanism of Cp i Pr 4 prevents the final resolution of the NMR signal. Moreover, because of the rapid parallel exchange in Cp i Pr 4 , its Figure 6.…”

“…However, aspects of their rearrangement mechanisms and the nature of the so-called stretched dihydrogen bond, 4 Figure 1). 5,6 Unfortunately, crystallography was unable to define positions for the three crucial hydrogens in the Cp i Pr 4 complex. However, the NMR spectra 6 appear to show high similarity between the Cp t Bu 3 and Cp i Pr 4 complexes.…”

Pentabenzylcyclopentadienyl molybdenum and tungsten hydrides: Syntheses, structures and electrochemistry of [MHCpBz(CO)2(L)] (L=CO, PMe3, PPh3)

“…The angles between the carbonyl and the hydride are narrow than CO-PMe 3 [30]. The relative positioning of the benzyl substituents of Cp Bz in relation to the basal ligands in 3 differs from the two trimethylphosphine analogues previously discussed as the benzyl group bending towards the molybdenum atom is positioned between the two adjacent carbonyl ligands with torsion angles C(1)-Cp Bz (centroid)-Mo(1)-C(6) and [29,30,[54][55][56][57]. The distances between Mo(1)-C(6), Mo(1)-C(7) and Mo(1)-P are in the ranges usually reported in the literature.…”