Pyrazole‐Bridged NHC Ligands and Their Dimetallic (Allyl)palladium Complexes

Abstract

A set of compartmental pyrazole‐based ligands with appended NHC donors has been synthesized and isolated as [H4L]Cl3 and [H3L](PF6)2 salts. Dinuclear (allyl)palladium complexes of these ligands are conveniently accessible via the in situ prepared silver species. Three complexes [(allyl)2Pd2L]PF6 and one derivative [(methallyl)2Pd2L]PF6 have been characterized crystallographically, which revealed that the metal ions are positioned in close proximity [d(Pd···Pd) = 3.97–4.05 Å], with two possible mutual orientations of the (meth)allyl ligands within the dimetallic pocket. NMR spectroscopy shows slow interconversion of these isomers (k = 0.05–0.4 s–1), where the (meth)allyl ligands are detached trans to the carbene during this η3‐η1‐η3 dynamic process.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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

“…As concluded from the NMR spectra of [(H 2 L 1 ) 4 Ag 4 ](PF 6 ) 8 , the silver ions are solely bound to the pyrazolate-N atoms, while all side arm imidazolium groups are still protonated and dangling. The imidazolium-C 2 H shows weak hydrogen bonding or close contacts to oxygen The molecular structure of [(H 2 L 1 ) 4 Ag 4 ] 8+ bears a striking resemblance to the recently reported doublecrowned Ag 8 complex B, which features the same central (pzAg) 4 core but hosts four additional silver ions in the peripheral bis(NHC) compartments [10,18] 8 displays emissive properties in solution. Its UV/Vis spectrum shows strong absorption below 240 nm with a pronounced shoulder at around 255 nm, and a broad emission band centered at around 310 nm upon excitation at λ ex = 265 nm was observed (Fig.…”

“…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…”

“…An interesting structural feature is that both the pyrazole-NH and the imidazolium C 2 H are strongly involved in hydrogen bonding interactions. Silver(I) complexes of the ligands can be conveniently prepared by the Ag 2 8 were measured with a Jasco V-550 UV/Vis spectrometer and a Jasco FP-6500 fluorescence spectrometer.…”

Non‐Innocence of N‐Heterocyclic Carbene Ligands: Intermolecular CH Activation in Allyl Palladium NHC Complexes

“…[16] Related ligands based on pyridazines with two imidazolium groups tethered to the 3-and 6-positions of the diazine heterocycle have also been obtained in a straightforward procedure [17] and are expected to serve as useful scaffolds for the synthesis of bimetallic N,N'-bridged NHC complexes. [18,19] The corresponding ligand precursors 1 (Figure 1) bearing only one imidazolium moiety and their (allyl)Pd complexes were studied as mononuclear benchmark systems in order to probe the stability of such NHC/pyridazine hybrids and to assess potential cooperative effects in the bimetallic analogues.…”