Monomer, dimer or cyclic helicate? Coordination diversity with hard–soft P,N-donor ligands

Abstract

We report the synthesis of copper(I) complexes of three ligands which contain a potential P,N,N,P-metal binding site. Elemental analysis confirms that the bulk products possess a composition of [CuL][PF₆] where L = 1, 2 or 3. Electrospray mass spectrometry (ESI MS) provides evidence for speciation in MeCN or MeOH solutions and the formation of both [CuL]⁺ and [Cu₂L₂]²⁺; addition of NaCl to the ESI MS samples aids the observation of dinuclear species as [Cu₂L₂Cl]⁺ ions. NMR spectroscopic data for a CD₃CN solution of [Cu(1)][PF₆] were consistent with a mononuclear species, but more complex multinuclear spectra were observed for the same compound dissolved in CD₂Cl₂. In the solid state, dimeric species dominate. Crystals grown from CH₂Cl₂ solutions of [Cu(1)][PF₆] are found to be [Cu₂(1)₂][PF₆]₂·6CH₂Cl₂; each Cu⁺ ion in the centrosymmetric cation is bound in an N,O,P,P-coordination sphere, the N-donor originating from the pyridine ring. In [Cu₂(3)₂][PF₆]₂, each bridging ligand in the centrosymmetric [Cu₂(3)₂]²⁺ ion acts as a P,N-chelate to each Cu⁺ ion. Competing with this dimeric assembly is that of a circular helicate in which each ligand 3 bridges adjacent pairs of copper(I) ions in a chiral, hexameric complex; both the Δ,Δ,Δ,Δ,Δ,Δ- and Λ,Λ,Λ,Λ,Λ,Λ-enantiomers are present in the crystal lattice; in [Cu₆(3)₆]⁶⁺, each ligand coordinates as a bis(P,N-chelate). The solution absorption spectra of [Cu(1)][PF₆], [Cu(2)][PF₆] and [Cu(3)][PF₆] are dominated by ligand-based transitions and none of the copper(I) complexes exhibits emissive behaviour in solution.