Complexes of the tripodal phosphine ligands PhSi(XPPh2)3 (X = CH2, O): synthesis, structure and catalytic activity in the hydroboration of CO2

Abstract

The coordination chemistry of Fe²⁺, Co²⁺ and Cu⁺ ions was explored with the triphosphine and triphosphinite ligands PhSi{CH₂PPh₂}₃ (1) and PhSi{OPPh₂}₃ (2), so as to evaluate the impact of the electronic properties of the tripodal phosphorus ligands on the structure and reactivity of the corresponding complexes. The synthesis and characterization of the complexes [Fe(κ³-PhSi{CH₂PPh₂}₃)(MeCN)₃][OTf]₂ (3) (OTf = O₃SCF₃), [Fe(κ³-PhSi{OPPh₂}₃)(MeCN)₃][OTf]₂ (3′), [Co(κ²-PhSi{CH₂PPh₂}₃)Cl₂] (4), [Co(κ³-PhSi{OPPh₂}₃)Cl₂] (4′), [Cu(κ³-PhSi{CH₂PPh₂}₃)Br] (5) and [Cu(κ³-PhSi{OPPh₂}₃)I] (5′) were carried out. The crystal structures of 3, 3′, 4, 4′, and of the solvates 5·3THF and 5′·THF are reported. Complexes 3–5′ were shown to promote the catalytic hydroboration of CO₂ with (9-BBN)₂ (9-BBN = 9-borabicyclo[3.3.1]nonane). While the iron and cobalt complexes of the triphosphine 1 are more active than the analogous complexes with 2, the opposite trend is observed with the copper catalysts. Overall, the copper catalysts 5 and 5′ are both more reactive and more selective than the Fe and Co catalysts, enabling the formation of the acetal H₂C(OBBN)₂ with a high molar ratio of H₂C(OBBN)₂ : CH₃OBBN up to 92 : 8.