E–H Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with Hydrogen and p‑Block Element Hydrides

Reactions of complexes [MoMCp­(μ-PMes*)­(CO)6] with
H2 and several p-block element (E) hydrides mostly resulted
in the cleavage of E–H bonds under mild conditions [M = Re
(1a) and Mn (1b); Mes* = 2,4,6-C6H2tBu3]. The reaction
with H2 (ca. 4 atm) proceeded even at 295 K to give the
hydrides [MoMCp­(μ-H)­(μ-PHMes*)­(CO)6]. The same
result was obtained in the reactions with H3SiPh and, for 1a, upon reduction with Na­(Hg) followed by protonation of
the resulting anion [MoReCp­(μ-PHMes*)­(CO)6]−. The latter reacted with [AuCl­{P­(p-tol)3}] to yield the related heterotrimetallic cluster [MoReAuCp­(μ-PHMes*)­(CO)6{P­(p-tol)3}]. The reaction of 1a with thiophenol gave the thiolate-bridged complex [MoReCp­(μ-PHMes*)­(μ-SPh)­(CO)6], which evolved readily to the pentacarbonyl derivative [MoReCp­(μ-PHMes*)­(μ-SPh)­(CO)5]. In contrast, no P–H bond cleavage was observed in
reactions of complexes 1a,b with PHCy2, which
just yielded the substituted derivatives [MoMCp­(μ-PMes*)­(CO)5(PHCy2)]. Reactions with HSnPh3 again
resulted in E–H bond cleavage, but now with the stannyl group
terminally bound to M, while 1a reacted with BH3·PPh3 to give the hydride-bridged derivatives [MoReCp­(μ-H)­(μ-PHMes*)­(CO)5(PPh3)] and [MoReCp­(μ-H)­{μ-P­(CH2CMe2)­C6H2tBu2}­(CO)5(PPh3)], which follow
from hydrogenation, C–H cleavage, and CO/PPh3 substitution
steps. Density functional theory calculations on the PPh-bridged analogue
of 1a revealed that hydrogenation likely proceeds through
the addition of H2 to the MoP double bond of the
complex, followed by rearrangement of the Mo fragment to drive the
resulting terminal hydride into a bridging position.