Combining Pyrazolate and N-Heterocyclic Carbene Coordination Motifs: Synthesis and Characterization of a Double-Crowned Silver Complex

Abstract

Reaction of Ag 2 O with the N-heterocyclic carbene (NHC) precursors 3,5-bis [3-(2,4,6-trimethylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1a) and 3,5-bis[3-(2,6-diisopropylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1b) yielded silver(I) complexes 2 and 3, respectively. X-ray crystallography revealed a tetranuclear dimeric solid state structure in the case of 2, whereas an unprecedented octanuclear tetrameric "double-crown"-like structure was found for 3, presumably due to the slightly bulkier ligand. Unlike 2, complex 3 is not stable in solution but in equilibrium with tetranuclear and hexanuclear complexes 4 and 5, respectively. The identity of those species could be elucidated by detailed NMR spectroscopic studies ( 1 H, 13 C, 15 N, 109 Ag, and DOSY experiments), which also provide valuable benchmark data for correlating 1 J AgN couplings and 109 Ag chemical shifts with structural information.

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

“…represent the common structural motif that has been observed for silver complexes with related ligands combining NHC and pyrazole moieties [9,10]. However, not more than a handful of complexes of this type have been structurally characterized to date, and the present work confirms that these tetrametallic arrays are formed also in the case of bulky imidazole substituents such as tert-butyl or 1-adamantyl.…”

“…For [(L 1/3/4 ) 2 Ag 4 ] 2+ such a closed shell interaction might be rather weak if present at all, since the Ag ··· Ag distances are well above 3.1Å [16]. Despite the steric bulk of the peripheral tert-butyl or 1-adamantyl groups, Ag ··· Ag separations are in the same range as in the sterically less demanding silver(I) complex of 3,5-bis[N-methylimidazolium-1-ylmethyl]-1H-pyrazole [9,10] 8 at r. t. afforded colorless crystals that allowed the elucidation of its structure by X-ray diffraction (Fig. 5).…”

“…The hygroscopic bis(imidazolium) chlorides 3 L 4 ]Cl 3 were prepared from 3,5-bis-(chloromethyl)-1-(tetrahydropyranyl-2-yl)-4-phenyl-1H-pyrazole and the respective N-substituted imidazole in close analogy to the procedure reported previously (Scheme 1) [8,10]. Subsequent treatment of aqueous solutions of the chloride salts with an…”

“…The scope is further extended by introducing functional donor substituents to the NHC or combining several NHC moieties in multidentate chelate scaffolds [6]. Some groups including ours have recently reported pyrazolebridged bis(NHC) ligands [7 -9] that provide two proximate binding compartments and give rise to unique multinuclear Ag(I)-NHC complexes [9,10]. Compounds [LAg 2 ] n with two very different structures have been observed: a planar Ag 4 core sandwiched between two pyrazolate/NHC ligands (A; n = 2) [9,10] or an unprecendented Ag 8 double metallocrown (B; n = 4) [10].…”

“…This raises interesting questions concerning the relation and possible competition between these two binding motifs. It was speculated that the formation of different structures A and B despite similar ligand scaffolds and identical metal : ligand ratio (2 : 1 or 4 : 2) in the Ag(I)-pyrazolate/NHC systems might be caused by the different steric bulkiness of the NHC substituents [5,10]. In the present contribution we thus describe the synthesis of several new compartmental pyrazolate/NHC ligands with very bulky imidazolium substituents (tert-butyl, 1-adamantyl), we introduce different substituents at the pyrazolate-C 4 backbone (H, Ph), and we explore the Ag(I) coordination chemistry of the new set of ligand scaffolds.…”