Reversible Displacement of Polyagostic Interactions in 16e [Mn(CO)(R₂PC₂H₄PR₂)₂]⁺ by H₂, N₂, and SO₂. Binding and Activation of η²-H₂ trans to CO Is Nearly Invariant to Changes in Charge and cis Ligands

Abstract

Electrophilic 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) complexes (R = Et, Ph) are synthesized by metathesis of MnBr(CO)(R(2)PC(2)H(4)PR(2))(2) with Na or Li salts of low-coordinating boron or gallium anions (e.g., [B{C(6)H(3)(3,5-CF(3))(2)}(4)](-) or [Ga(C(6)F(5))(4)] (-)). They contain weak polyagostic interactions that are reversibly displaced by H(2), N(2), and SO(2) (which is a surprisingly weak ligand here). The agostic and H(2) complexes, as well as the gallium anions including the new species [{Ga(C(6)F(5))(3)}(2)(&mgr;-Cl)](-), have been characterized by NMR, IR, and X-ray crystallography. The agostic Mn-H distances (e.g., 2.9 Å) are much longer than those found for the single agostic interactions in Mo(CO)(diphosphine)(2) and [Mn(CO)(3)(PCy(3))(2)](+). The H-H and also the Mn-H distances have been determined in the H(2) complex by T(1) measurements for both the H(2) and HD isotopomers. IR data and C-O and M-C bond lengths are used to gauge the pi-acceptor strengths of ligands trans to the CO. The agostic C-H bonds are the weakest ligands and also the weakest acceptors, but the H(2) ligand is an excellent acceptor as strong as N(2) and ethylene. The variation of nu(CO) on increasing the basicity of the cis-phosphine (dppe versus depe) in trans-M(CO)(diphosphine)(2)(L) is less than expected and far less than that on increasing the charge on the complex (M = Mn(+) versus Mo). The H-H bond lengths (0.87-0.90 Å) and J(HD) NMR couplings (32-34 Hz) in [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)(H(2))](+) and other cationic H(2) complexes with trans-CO are strikingly similar to their neutral analogues and nearly invariant. Activation of H-H in the more electrophilic cationic systems occurs primarily via increased sigma donation from H(2) as compared to the more electron-rich neutral analogues where back-bonding dominates. The nature of the ligand trans to H(2) (the strong acceptor CO here) controls the H-H distance more so than all of the cis ligands combined, especially for cationic complexes.

Evaluation of the Thermodynamic Properties of H2 Binding in Solid State Dihydrogen Complexes [M(η2-H2)(CO)dppe2][BArF24] (M = Mn, Tc, Re): An Experimental and First Principles Study

“…Calculations for absorption onto the agostic geometry validate the observation by King et al 28 that direct hydrogenation of the agostic manganese complex in the solid state fails under one atmosphere of hydrogen pressure. Evaluation of Equation 6 for the agostic complex at one atmosphere hydrogen pressure and 313K gives a fractional coverage of 1.443 × 10 −5 , indicating that molecules in the agostic configuration do not contribute to hydrogen binding in the experimental results.…”

“…19, 28 The standard Gibbs free energy for the transformation from the open configuration to the agnostic configuration was also calculated from the vibrational analysis, and was found to be Δ G ∘ = −39.6 kJ/mol, similar to values found for other agostic interactions. 12 …”

“…For the manganese complex, these modes correlated well with literature values obtained from inelastic neutron scattering. 28 In all three cases only the rotational mode, with frequencies of 196.79 cm −1 , 269.2 cm −1 , and 556.7 cm −1 for Mn, Tc, and Re, respectively, was found to be significantly populated at the temperatures of interest. The symmetry of the rotational mode was determined from analysis of the components of the eigenvector associated with the atoms of the hydrogen molecule, and the mode was defined as symmetric if the x,y, and z components were opposite in sign and had differences in their magnitude of less than 0.1 millidyne/angstrom.…”

“…This rapid absorption is likely due to the loose packing of the bulky organic ligands in the solid, which allows the small hydrogen molecule to diffuse rapidly through the structure. 28 …”

A manganese hydride molecular sieve for practical hydrogen storage under ambient conditions

“…The difference peak can be interpreted as showing rotational tunneling transitions for coordinated hydrogen at approximately AE0.4 meV in accordance with previous results on a large number of metal dihydrogen complexes. 30 More specifically, INS results on the only stable Mn(II) dihydrogen complex Mn(I)CO(Z 2 -H 2 )(dppe) 2 31 show rotational tunneling transitions for H 2 at AE0.5 meV. A barrier to rotation can be derived from the observed tunneling frequency if we assume that the reorientation takes place in a plane, and that the rotational potential is one of a double-minimum V 2 .…”