Phosphorus-phosphorus coupling constants in mixed-phosphine tricarbonyl iron complexes, Fe(CO)3LL′. Crystal structure of trans-Fe(CO)3(PEt3) (PPh3)

Article abstract of DOI:10.1021/om9609224

Mixed-ligand complexes, trans-Fe(CO)₃LL' (L = PPh₂Me, L' = PPh₂Me, PPhMe₂, PMe₃, PPh₂-Et, PEt₃, PPh₂CH=CH₂, PPh₂H, AsPh₃, P(OPh)₃; L = PMe₃, L' = PEt₃, PPh₂Et, PCy₃, PPh₂Me, PPhMe; L = PEt₃, L' = PPh₂Me; L = PPh₂H, L' = PPh₂CH=CH₂, PPh₂Et; L = AsPh₃, L' = PPhMe₂, P(OPh)₃, P(OMe)3, P(OEt)₃), have been obtained from the stepwise reaction of phosphines with Fe(CO)₃(BDA) (BDA = benzylideneacetone) or Fe(CO)₃(AsPh₃)₂ and from the reaction of phosphine with Fe(CO)₄PPh₃ in the presence of base. A strong negative correlation exists between ²Jpp coupling constant values and the sum of the phosphine pK_a values. By application of quantitative analysis of ligand effects, it has been shown that ²Jpp for the mixed-ligand complexes correlates strongly with both γ and E_ar, but not with θ. Although a near perfect fit is obtained from the three-parameter equation, a statistical analysis suggests that for this small data set there are no predictive advantages over the one-parameter pK_a model. It is possible to calculate reliable ²Jpp values for transFe(CO)₃L₂ complexes with either model. An X-ray structure of solid-state frans-Fe(CO)₃-(PEt₃)(PPh₃) shows equal Fe-PEt₃ and Fe-PPh₃ bond distances, implying that bond strength equalization may occur when two rather different phosphines occupy trans coordination sites.