Disilver(I) rectangular-shaped metallacycles: X-ray crystal structure and dynamic behavior in solution

Reaction of the ditopic semirigid ligand 1,2-bis(imidazolylmethyl)benzene (1,2-blmb) or the flexible ligand 1,4-bis-(2-benzimidazolyl)butane (C4Blm) with AgX (X = CIO4-, BF4-, CF3CO2-) afforded five new complexes, namely, [Ag-2(1,2-blmb)(2)](CIO4)(2) (1), [Ag2(1,2-blmb)(2)](BF4)(2) (2), [Ag-2(1,2-blmb)(2)](CF3CO2)(2)center dot 2CH(3)OH (3 center dot 2CH(3)OH), [Ag-2(C4Blm)(2)](CIO4)(2)center dot 2DMF (4 center dot 2DMF), and [Ag-2(C4Blm)(2)](CF3CO2)(2)center dot 2H(2)O (5 center dot 2H(2)O), all of which contain a centrosymmetric, rectangular-shaped cationic disilver(l) metallacycle [Ag-2(L)(2)](2+). In 1-3, a pair of 1,2-bimb ligands takes on the syn conformation to connect two Ag(I) ions to give a compressed rectangle with a transannular Ag center dot center dot center dot Ag separation of 3.27-3.36 angstrom, whereas in 4 and 5, the pair of planar C4Blm ligands acts in the cis conformation to connect two Ag(I) ions to yield a normal rectangle with a transannular Ag center dot center dot center dot Ag separation of 7.67-7.91 angstrom. The anions form Ag center dot center dot center dot O or Ag center dot center dot center dot F weak interactions in 1-3 and O-H center dot center dot center dot O or N-H center dot center dot center dot O hydrogen bonds in 4 and 5 in crystal packing but exhibit no significant influence on the formation of the disilver(I) macrocycles. The solution structure and dynamic behaviour of the complexes studied by electrospray ionization mass spectrometry, H-1 NMR, and variable-temperature NMR indicated that the dynamic equilibrium between the [Ag-2(L)(2)](2+) cation and the open-ring oligomers or other potential species occurs via solvent-assisted dissociative exchange. The metal-ligand exchange barrier was estimated to be 54.5 U mol(-1).