Heteroleptic Copper(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands

Preparation of [Cu­(NN)­(PP)]+ derivatives has been systematically
investigated starting from two libraries of phenanthroline (NN) derivatives
and bis-phosphine (PP) ligands, namely, (A) 1,10-phenanthroline (phen),
neocuproine (2,9-dimethyl-1,10-phenanthroline, dmp), bathophenanthroline
(4,7-diphenyl-1,10-phenanthroline, Bphen), 2,9-diphenethyl-1,10-phenanthroline
(dpep), and 2,9-diphenyl-1,10-phenanthroline (dpp); (B) bis­(diphenylphosphino)­methane
(dppm), 1,2-bis­(diphenylphosphino)­ethane (dppe), 1,3-bis­(diphenylphosphino)­propane
(dppp), 1,2-bis­(diphenylphosphino)­benzene (dppb), 1,1′-bis­(diphenylphosphino)­ferrocene
(dppFc), and bis­[(2-diphenylphosphino)­phenyl] ether (POP). Whatever
the bis-phosphine ligand, stable heteroleptic [Cu­(NN)­(PP)]+ complexes are obtained from the 2,9-unsubstituted-1,10-phenanthroline
ligands (phen and Bphen). By contrast, heteroleptic complexes obtained
from dmp and dpep are stable in the solid state, but a dynamic ligand
exchange reaction is systematically observed in solution, and the
homoleptic/heteroleptic ratio is highly dependent on the bis-phosphine
ligand. Detailed analysis revealed that the dynamic equilibrium resulting
from ligand exchange reactions is mainly influenced by the relative
thermodynamic stability of the different possible complexes. Finally,
in the case of dpp, only homoleptic complexes were obtained whatever
the bis-phosphine ligand. Obviously, steric effects resulting from
the presence of the bulky phenyl rings on the dpp ligand destabilize
the heteroleptic [Cu­(NN)­(PP)]+ complexes. In addition to
the remarkable thermodynamic stability of [Cu­(dpp)2]­BF4, this negative steric effect drives the dynamic complexation
scenario toward almost exclusive formation of homoleptic [Cu­(NN)2]+ and [Cu­(PP)2]+ complexes.
This work provides the definitive rationalization of the stability
of [Cu­(NN)­(PP)]+ complexes, marking the way for future
developments in this field.