Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-phosphine Ligands

The heteroleptic coordination scenario of silver­(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivatives has been investigated. In addition to the X-ray crystal structural characterization of the resulting mixed ligand Ag­(I) complexes, detailed NMR studies have been performed to disclose the behavior of the prepared silver­(I) complexes in solution. The results obtained with silver­(I) have been also systematically related to the one obtained for copper­(I) with the same combination of PP and NN ligands. Starting from an equimolar mixture of AgBF₄, bis­[(2-diphenylphosphino)­phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag­(POP)­(phen)]⁺ has been obtained as the tetrafluoroborate salt. By following the same experimental procedure starting from bis­(diphenylphosphino)­methane (dppm) or 1,3-bis­(diphenylphosphino)­propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag₂(NN)₂(μ-dppm)₂]²⁺ and [Ag₂(NN)₂(μ-dppp)₂]²⁺ with NN = phen or Bphen (bathophenanthroline), have been isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF₄, phen or Bphen, and 1,2-bis­(diphenyl-phosphino)­ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag₂(NN)₂(μ-dppe)]²⁺ have been obtained as the tetrafluoroborate salts. These compounds have been also prepared in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted us to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag₂(NN)₂(μ-dppm)]­(BF₄)₂ (NN = phen or Bphen) and [Ag₂(NN)₂(μ-dppp)]­(BF₄)₂ (NN = phen or Bphen) have been obtained upon crystallization. When compared to their copper­(I) analogues, the complexation scenario becomes more complex with silver­(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver­(I) shows an adaptive capability leading to various complexes with different coordination geometries and composition. However, as in the case of copper­(I), their solution behavior is highly dependent on the relative thermodynamic stability of the various possible complexes. In most of the cases, a single Ag­(I) complex is observed in solution and the NMR data are in a perfect agreement with their solid state structures. The dppp-containing complexes are the only notable exception; both [Ag₂(NN)₂(μ-dppp)₂]­(BF₄)₂ and [Ag₂(NN)₂(μ-dppp)]­(BF₄)₂ are stable in the solid state but a dynamic mixture is observed as soon as these compounds are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper­(I), it is not the case anymore with silver­(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly suggested by our results