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

Abstract

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.