1579211-24-7Relevant academic research and scientific papers
Reactivity of ruthenium phosphido species generated through the deprotonation of a tripodal phosphine ligand and implications for hydrophosphination
Sues, Peter E.,Lough, Alan J.,Morris, Robert H.
, p. 4746 - 4760 (2014)
The fragmentation of the 1,1,2-tris(diphenylphosphino)ethane ligand in [RuCp((Ph2P)2CHCH2PPh2)][PF 6] (1) was explored through treatment with base under aprotic conditions. The neutral phosphido complex RuCp(PPh2CH - CHPPh 2)(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(Ph 2PCHCHP(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-C6H 4)PPh2)(P(p-tolyl)2) versus 25 for the acid/base pair [RuCp(Ph2P(o-C6H4)PPh 2)(PPh2H)]+/RuCp(Ph2P(o-C 6H4)PPh2)(PPh2) (7). For comparison, the approximate pKa THF 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 pKa 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(C5(CH3)4(CH2C6H 5))(Ph2PCH2CH2PPh(o-C 6H4)PPh) (9) in high yields, demonstrating the unpredictable reactivity of phosphido ligands.
