Synthesis and Structural Features of Arduengo Carbene Complexes of Group 4 Metallocene Cations

Abstract

Treatment of [Cp 2 TiCH 3 (thf) + ] (5, with [BPh 4 -] anion) with 1,3-diisopropylimidazol-2ylidene (4) at ambient temperature resulted in a rapid displacement of the thf ligand by the stable heterocyclic carbene to yield the Arduengo carbene methyltitanocene cation complex 6a (>90% isolated). The X-ray crystal structure analysis of 6a showed that the heteroatomstabilized carbene ligand [d(Ti-C(carbene) ) 2.289(2) Å, d(Ti-CH 3 ) ) 2.178(3) Å] was bonded to titanium in an orientation where the imidazol-2-ylidene ring lies in the major σ-ligand plane of the bent metallocene moiety. A DFT calculation of 6a and a series of related model compounds has revealed that the Arduengo carbene serves as a pure σ-donor ligand to the titanocene moiety. The observed favored "in-plane" orientation of the ligand is steric in origin. Consequently, complex 6a attains an analogous C s -symmetric structure in solution, featuring symmetry-equivalent Cp rings and a pair of diastereotopic isopropyl substituents as well as chemically differentiated imidazol-2-ylidene C 4 HdC 5 H groups. The reaction of the ion pair [(Cp 2 ZrCH 3 ) + (CH 3 B(C 6 F 5 ) 3 -] (7) with 4 gave the analogous Arduengo carbene zirconocene cation complex 6b (>95% isolated, with [CH 3 B(C 6 F 5 ) 3 -] anion).

π-Acidity and π-basicity of N-heterocyclic carbene ligands. A computational assessment

“…5, solid line), clearly indicates that for all of the complexes investigated in the present study, there exists at least a contribution of 10%. Even the NHC ligand in compound ti1 + , which in a previous calculation has been described as a pure r-donor [61], possesses a 17% p-bonding character.…”

Flexible Structural Features of Pentafulvene Titanium Derivatives: Isolation and Characterization of NHC Complexes

“…As imilar behaviour is found also for titanium NHC adducts (e.g.,N HC-TiCl 4 d = 188.8 [9] or [Cp 2 TiCH 3 (NHC)] + d = 178.2 ppm [10] ). For the exocyclic fulvene carbon atoms in 3 Scheme2.Reactionofp entafulvenetitanium complexes 1a-1c with 1,3,4,5-tetramethylimidazole-2-ylidene (2)t oN HC titanium adducts 3a-3c.…”

Metal–organic frameworks containing N-heterocyclic carbenes and their precursors

“…NHC ligands are considered as simple pure -donors when coordinated to metal ions [100], through → d donation as shown in Fig. 6a.…”