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-bImb) or the flexible ligand 1,4-bis(2-benzimidazolyl)butane (C4BIm) with AgX (X = ClO4-, BF4-, CF3CO2-) afforded five new complexes, namely,
[Ag2(1,2-bImb)2](ClO4)2 (1), [Ag2(1,2-bImb)2](BF4)2 (2), [Ag2(1,2-bImb)2](CF3CO2)2·2CH3OH (3·2CH3OH), [Ag2(C4BIm)2](ClO4)2·2DMF (4·2DMF), and [Ag2(C4BIm)2](CF3CO2)2·2H2O (5·2H2O), all of which contain a centrosymmetric,
rectangular-shaped cationic disilver(I) metallacycle [Ag2(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···Ag separation of
3.27−3.36 Å, whereas in 4 and 5, the pair of planar C4BIm ligands acts in the cis conformation to connect two
Ag(I) ions to yield a normal rectangle with a transannular Ag···Ag separation of 7.67−7.91 Å. The anions form
Ag···O or Ag···F weak interactions in 1−3 and O−H···O or N−H···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 behavior of the complexes studied by electrospray ionization mass spectrometry, 1H NMR, and variable-temperature NMR indicated that the dynamic equilibrium between the [Ag2(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 kJ mol-1.