Coordination and Dehydrogenation of Diphosphine–Borane Ph₂PCH₂PPh₂·BH₃ at a Heterometallic MoRe Center to Give an Agostic Boryl-Bridged Derivative

Abstract

The coordination chemistry of the title diphosphine-borane adduct at heterometallic MoRe centers was examined through its reactions with the hydride complex [MoReCp(-H)(-PCy2)(CO)5(NCMe)] (Cp =  5-C5H5). The latter reacted rapidly with stoichiometric amounts of dppm•BH3 (dppm = Ph2PCH2PPh2) in refluxing toluene solution, with displacement of the nitrile ligand, to give [MoReCp(-H)(-PCy2)(CO)5( 1 P-dppm•BH3)], with a P-bound diphosphine-borane ligand arranged trans to the PCy2 group. Decarbonylation of the latter complex was accomplished rapidly upon irradiation with visible-UV light in toluene solution at 263 K, to give the agostic derivative [MoReCp(-H)(-PCy2)(CO)4( 1 P, 2-dppm•BH3)] as major product (MoRe = 3.2075(5) Å), along with small amounts of the diphosphine-bridged complex [MoReCp(-H)(-PCy2)(CO)4(-dppm)]. Extended photolysis of the agostic complex at 288 K promoted an unprecedented dehydrogenation process involving the borane group and the hydride ligand, to give the diphosphine-boryl complex [MoReCp(- 2 : 2 P,B-H2B•dppm)(-PCy2)(CO)4] (MoRe = 3.075(1) Å). The latter displayed a boryl ligand in a novel bridging coordination mode, it being -bound to one of the metal atoms (BRe = 2.38(2) Å) while interacting with the second metal atom via a strong side-on tricentric BHM interaction (BMo = 2.31(1); HMo = 1.9(1) Å). The overall dehydrogenation process was endergonic by 43 kJ/mol, according to Density Functional Theory calculations.

A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation

“…The molecular structure of the resulting complex 3 in the solid state displays a cationic, square planar Pt II complex with a dppm-and a rare, chelating boryl-ligand. 44,45 The assignment of a boryl ligand (X-type) is further supported by the chemical shift in the 11 B{ 1 H} NMR spectrum (d B = 29.9 ppm), while an analysis based on quantum theory of atoms in molecules (QTAIM) and the 1 J PtP constant of 2872.0 Hz suggests the presence of a Pt II complex. 46 As the reactivity of 1 seems to depend on the platinum oxidation state, we investigated possible mechanistic pathways by means of quantum chemical investigations, using density functional theory at the B97D3/def2-TZVP level of theory.…”

“…The molecular structure of the resulting complex 3 in the solid state displays a cationic, square planar Pt II complex with a dppm- and a rare, chelating boryl-ligand. 44,45 The assignment of a boryl ligand (X-type) is further supported by the chemical shift in the 11 B{ 1 H} NMR spectrum ( δ B = 29.9 ppm), while an analysis based on quantum theory of atoms in molecules (QTAIM) and the 1 J PtP constant of 2872.0 Hz suggests the presence of a Pt II complex. 46…”