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

Abstract

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 (ΔH‡ = 27.6 kJ·mol–1, ΔS‡ = –125 J·mol–1·K–1 in [D3]MeCN; ΔH‡ = 40.4 kJ·mol–1, ΔS‡ = –86.9 J·mol–1·K–1 in [D6]DMSO) that are in the range typical for pyrazoles. Reaction of the proligands with Ag2O gave hexametallic complexes [Ag6(L1)2](PF6)4 and [Ag6(L2)2](PF6)4 that involve all six potential donor atoms of the ligands, viz. the four CNHC and two Npz donors, in metal coordination. X‐ray crystallography revealed a chair‐like central {Ag6} deck in both complexes but different arrangements of the ligand strands, which goes along with significantly different AgI···AgI distances that indicate more pronounced argentophilic interactions in case of [Ag6(L1)2]4+.