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

摘要

The fragmentation of the 1,1,2-tris-(diphenylphosphino)ethane ligand in [RuCp~*-((Ph_2P)_2CHCH_2PPh_2)][PF_6] (1) was explored through treatment with base under aprotic conditions. The neutral phosphido complex RuCp~*(PPh_2CH=CHPPh_2)(PPh_2) (2) with a (Z)-1,2-bis(diphenylphosphino)ethene (dppen) ligand was generated through a base-facilitated dehydrophosphina-tion 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~*(Ph_2PCHCHP-(p-tol)_2)(PPh_2) (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 pK_a~(THF) of 28 for the acid/base pair [RuCp~*(Ph_2P(o-C_6H_4)PPh_2)(P(p-tolyl)_2H)]~+/RuCp~*(Ph_2P(o-C_6H_4)PPh_2)(P(p-tolyl)_2) versus 25 for the acid/base pair [RuCp~*(Ph_2P(o-C_6H_4)PPh_2)(PPh_2H)]~+/RuCp~*(Ph_2P(o-C_6H_4)PPh_2)(PPh_2) (7). For comparison, the approximate pK_a~(THF) values for free P(p-tolyl)_2H/[K(crypt)]P(p-tolyl)_2 and free PPh_2H/[K(crypt)]PPh_2 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 PPh_2H to acrylonitrile, but they deactivate fairly rapidly. The pK_a~(THF) 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(C_5(CH_3)_4(CH_2C_6H_5))(Ph_2PCH_2CH_2PPh(o-C_6H_4)PPh) (9) in high yields, demonstrating the unpredictable reactivity of phosphido ligands.