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, [H 5 L 1 ](PF 6 ) 4 and [H 5 L 2 ](PF 6 ) 4 , were synthesized via multi-step protocols, and the NH prototropy of [H 5 L 1 ](PF 6 ) 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 [D 3 ]MeCN; 605ΔH ‡ = 40.4 kJ·mol -1 , ΔS ‡ = -86.9 J·mol -1 ·K -1 in [D 6 ]DMSO) that are in the range typical for pyrazoles. Reaction of the proligands with Ag 2 O gave hexametallic complexes [Ag 6 (L 1 ) 2 ](PF 6 ) 4 and [Ag 6 (L 2 ) 2 ](PF 6 ) 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 ···Ag I distances that indicate more pronounced argentophilic interactions in case of [Ag 6 (L 1 ) 2 ] 4+ .