Hydrochlorination of Ruthenaphosphaalkenyls: Unexpectedly Facile Access to Alkylchlorohydrophosphane Complexes

The novel ruthenaphosphaalkenyls
[Ru­{PC­(H)­SiMe2R}­Cl­(CO)­(PPh3)2] (R = p-C6H4CF3, nBu) have been prepared for the first
time, and studied alongside
precedent analogues (R = Me, Ph, p-tol) for their
reactions with HCl. In contrast to chemistry defined
for the tert-butyl congener [Ru­{PC­(H)tBu}­Cl­(CO)­(PPh3)2], which initially adds a single equivalent of HCl across
the Ru–P linkage, all five silyl derivatives undergo spontaneous
addition of a second equivalent to afford [Ru­{η1-PHCl–CH2­SiMe2R}­Cl­(CO)­(PPh3)2], extremely rare examples of coordinated “PHXR”
type ligands. Where R = SiMe3, a distorted octahedral geometry
with a conformationally restricted “PHXR” ligand is
observed crystallographically; this structure is appreciably retained
in solution, as determined from multinuclear NMR spectroscopic features,
which include a Karplus-like PPPh3–Ru–P–H
spin–spin coupling dependence. Computational data suggest a
silyl-induced increase in negative charge density at the phospha­alkenic
carbon, rather than an intrinsic thermodynamic driver, as the likely
origin of the disparate reactivity.