Linear Pyrazole-bridged Tetra(N-heterocyclic carbene) Ligands and Their Hexanuclear Silver(I) Complexes

New hybrid ligands are reported that combine two types of popular donor groups within a single linear scaffold, viz., a central pyrazolate bridge and two appended bis(N-heterocyclic carbene) units; the ligand strands thus provide two potentially tridentate {NCC} compartments. The pyrazole/tetraimidazolium proligands, [H5L1](PF6)(4) and [H5L2](PF6)(4), were synthesized via multi-step protocols, and the NH prototropy of [H5L1](PF6)(4) was examined by variable temperature (VT) NMR spectroscopy, giving solvent dependent activation parameters (Delta H-double dagger = 27.6 kJ center dot mol(-1), Delta S-double dagger = -125 J center dot mol(-1)center dot K-1 in [D-3]MeCN; Delta H-double dagger = 40.4 kJ center dot mol(-1), Delta S-double dagger = -86.9 J center dot mol(-1)center dot K-1 in [D-6]DMSO) that are in the range typical for pyrazoles. Reaction of the proligands with Ag2O gave hexametallic complexes [Ag-6(L-1)(2)](PF6)(4) and [Ag-6(L-2)(2)](PF6)(4) that involve all six potential donor atoms of the ligands, viz. the four C-NHC and two N-pz donors, in metal coordination. X-ray crystallography revealed a chair-like central {Ag-6} deck in both complexes but different arrangements of the ligand strands, which goes along with significantly different Ag-I center dot center dot center dot Ag-I distances that indicate more pronounced argentophilic interactions in case of [Ag-6(L-1)(2)](4+).