Investigation of the [Cp*Mo(PMe₃)₃H]ⁿ⁺ (n = 0, 1) Redox Pair: Dynamic Processes on Very Different Time Scales

Abstract

The compound [Cp*Mo(PMe3)3H] (1) is reversibly oxidized at E1/2 = -1.40 V vs ferrocene in MeCN. Its oxidation with Cp2FePF6 yields thermally stable [Cp*Mo(PMe3)3H]PF6 (2), which has been isolated and characterized by IR and EPR spectroscopy and by single-crystal X-ray diffraction. The 1H and 31P NMR spectra of 1 show two types of PMe3 ligands in a 1:2 ratio at low temperature, but only one average signal at room temperature, with activation parameters of DeltaH++ = 11.7(3) kcal mol-1 and DeltaS++ = -3(1) eu for the exchange process. Although only one species is evidenced by NMR for 1 and by EPR for 2, the solution IR spectra of each complex show two bands in the v(Mo-H) region (1, major at 1794 cm-1 and minor at ca. 1730 cm-1; 2, ca. 1800 and 1770 cm-1 with approximately equal intensity), the position and relative intensity being little dependent on the solvent. A thorough DFT investigation suggests that these are different rotamers involving different relative orientations of the Cp* ring and the PMe3 ligands in these complexes. This ring rotation process is very rapid on the NMR and EPR time scale but slow on the IR time scale. The X-ray data and the theoretical calculations suggest the presence of weak Mo-H...F interactions in compound 2. The possibility of PMe3 dissociation, as well as other intramolecular rearrangements, for 1 and 2 is excluded by experimental and computational studies. Protonation of 1 yields [Cp*Mo(PMe3)3H2]+ (3), which also reveals a dynamic process interconverting the two inequivalent H ligands and the three PMe3 ligands (two sets in a 1:2 ratio in the frozen structure) on the NMR time scale (activation parameters of DeltaH++ = 9.3(1) kcal/mol and DeltaS++ = -4.1(4) eu). A DFT study suggests that this exchange process occurs via a low-energy symmetric dihydride intermediate and not through a dihydrogen complex.

Solvent‐Dependent Dihydrogen/Dihydride Stability for [Mo(CO)(Cp*)H2(PMe3)2]+[BF4]− Determined by Multiple Solvent⋅⋅⋅Anion⋅⋅⋅Cation Non‐Covalent Interactions

“…[28,29] Analogous structures were shown by Xray crystallography for the isoelectronic compounds [27,28,30] and was also proposed for [Mo(CO)(Cp)A C H T U N G T R E N N U N G (dppe)(H) 2 ] + , which, like our complex 3, only shows one hydride signal at low temperatures. [31] Therefore, we presume that structure I (L = CO; R = Me) is also adopted for compound 3.…”

Anion control of tautomeric equilibria: Fe–H vs. N–H influenced by NH⋯F hydrogen bonding

“…Compared to the sum of the van der Waals radii of H and F (about 2.67 Å), these structures indicate that any Fe–H···F interactions are, at most, weak. The shortest H···F distance in the cationic molybdenum hydride, [Cp*MoH(PMe 3 ) 3 ] + PF 6 – , is 3.33 Å 65. While that distance is longer than would normally be considered a hydrogen bond, a computational study provided evidence for a weak interaction.…”

Dihydrogen Bonding, Proton Transfer and Beyond: What We Can Learn from Kinetics and Thermodynamics

“…When the proton donor is too weak and the ion pair local minimum does not exist, all these events can occur in one concerted step (long‐dashed line in Figure 6). 53 Indeed, the intermolecular hydride‐proton or isotope exchange is observed in the NMR spectra even when the dihydrogen bond is not strong enough to allow the proton transfer, e.g., for [Cp*MoH(PMe 3 ) 3 ] or [WH(CO) 2 (NO)(PR 3 ) 2 ] in the presence of CH 3 OD 42,54,55…”