Structural Variations in Tetrasilver(I) Complexes of Pyrazolate-bridged Compartmental N-Heterocyclic Carbene Ligands

A set of pyrazole-bridged bis(imidazolium) compounds [H(3)L(1)]X(2) - [H(3)L(4)]X(2) (L(1) = 3,5-bis[1-(tert-butyl)imidazolium-1-ylmethyl]-1H-pyrazole; L(2) = 3,5-bis[1-(tert-butyl)imidazolium-1-ylmethyl]-4-phenyl-1H-pyrazole; L(3) = 3,5-bis[1-(1-adamantyl)imidazolium-1-ylmethyl]-1H-pyrazole; L(4) = 3,5-bis[1-(1-adamantyl)imidazolium-1-ylmethyl]-4-phenyl-1H-pyrazole; X = Cl(-). BF(4)(-) or PF(6)(-)) has been prepared, and three compounds have been characterized by X-ray crystallography. The unique [H(3)L(4)][H(2)L(4)](PF(6))(3) features a dimeric face-to-face arrangement of two molecules due to the involvement of both the pyrazole-NH and the imidazolium C(2)H in hydrogen bonding. [H(3)L(1)]X(2) - [H(3)L(4)]X(2) serve as precursors for silver(I) complexes with compartmental pyrazolate-bridged bis(NHC) ligands. The complexes have been readily prepared by the Ag(2)O route and feature either the known [(L(1-4))(2)Ag(4)](2+) or the new [(H(2)L(1))(4)Ag(4)](8+) motif, depending on the solvent for the reaction (MeCN or acetone). [(H(2)L(1))(4)Ag(4)](PF(6))(8) contains a central (pzAg)(4) ring with pendant imidazolium side arms. Upon further reaction with Ag(2)O in MeCN it was found to undergo transformation to the corresponding [(L(1))(2)Ag(4)](PF(6))(2). All complexes have been thoroughly studied by NMR spectroscopy in solution, and preliminary luminescence data of [(H(2)L(1))(4)Ag(4)](PF(6))(8) have been recorded.