Reactivity of Ruthenium Phosphido Species Generated through the Deprotonation of a Tripodal Phosphine Ligand and Implications for Hydrophosphination

The fragmentation
of the 1,1,2-tris­(diphenylphosphino)­ethane ligand
in [RuCp*­((Ph2P)2CHCH2PPh2)]­[PF6] (1) was explored through treatment
with base under aprotic conditions. The neutral phosphido complex
RuCp*­(PPh2CHCHPPh2)­(PPh2)
(2) with a (Z)-1,2-bis­(diphenylphosphino)­ethene
(dppen) ligand was generated through a base-facilitated dehydrophosphination
reaction. Installation of a bis­(p-tolyl)­phosphido
ligand was attempted by combining bis­(p-tolyl)­phosphine
with RuCp*­(dppen)Cl in the presence of KOtBu, but
surprisingly, the unsymmetrical diphenylphosphido compound RuCp*­(Ph2PCHCHP­(p-tol)2)­(PPh2) (5) was generated instead. The ligand rearrangement
reaction was driven by the greater electron density on the bis­(p-tolyl)­phosphido moiety. Density functional theory calculations
showed that fragmentation to the 1,2-disubstituted ligand was thermodynamically
favored over the 1,1-disubstituted ligand and that intramolecular
phosphido exchange was kinetically accessible at room temperature.
The greater basicity of the bis­(p-tolyl)­phosphido
ligand was experimentally verified by the measured pKaTHF of 28
for the acid/base pair [RuCp*­(Ph2P­(o-C6H4)­PPh2)­(P­(p-tolyl)2H)]+/RuCp*­(Ph2P­(o-C6H4)­PPh2)­(P­(p-tolyl)2) versus 25 for the acid/base pair [RuCp*­(Ph2P­(o-C6H4)­PPh2)­(PPh2H)]+/RuCp*­(Ph2P­(o-C6H4)­PPh2)­(PPh2) (7).
For comparison, the approximate pKaTHF values for free P­(p-tolyl)2H/[K­(crypt)]­P­(p-tolyl)2 and free PPh2H/[K­(crypt)]­PPh2 are 43 and 38,
respectively. This is the first quantitative measurement of the large
effect that coordination to a metal center, in this case ruthenium­(II),
has on the acidity of secondary phosphines. This is useful information
for designing and understanding hydrophosphination catalysts. Complexes 2 and 7 are catalysts for the addition of PPh2H to acrylonitrile, but they deactivate fairly rapidly. The
pKaTHF measurements are consistent with a catalytic cycle involving
a Michael addition step. Complex 2 in solution underwent
a slow, unprecedented rearrangement of P–C, C–C, and
C–H bonds to give crystalline Ru­(C5(CH3)4(CH2C6H5))­(Ph2PCH2CH2PPh­(o-C6H4)­PPh) (9) in high yields, demonstrating
the unpredictable reactivity of phosphido ligands.