Complexes of the tripodal phosphine ligands PhSi(XPPh 2 ) 3 (X = CH 2 , O): synthesis, structure and catalytic activity in the hydroboration of CO 2

The coordination chemistry of Fe 2+ , Co 2+ and Cu + ions was explored with the triphosphine and triphosphinite ligands PhSi{CH 2 PPh 2 } 3 ( 1 ) and PhSi{OPPh 2 } 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(κ 3 -PhSi{CH 2 PPh 2 } 3 )(MeCN) 3 ][OTf] 2 ( 3 ) (OTf = O 3 SCF 3 ), [Fe(κ 3 -PhSi{OPPh 2 } 3 )(MeCN) 3 ][OTf] 2 ( 3′ ), [Co(κ 2 -PhSi{CH 2 PPh 2 } 3 )Cl 2 ] ( 4 ), [Co(κ 3 -PhSi{OPPh 2 } 3 )Cl 2 ] ( 4′ ), [Cu(κ 3 -PhSi{CH 2 PPh 2 } 3 )Br] ( 5 ) and [Cu(κ 3 -PhSi{OPPh 2 } 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 CO 2 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 H 2 C(OBBN) 2 with a high molar ratio of H 2 C(OBBN) 2 : CH 3 OBBN up to 92 : 8.