N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of vanadium(III) and vanadium(V)

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

The Facile Hydrolysis of Imidazolinium Chlorides (N-Heterocyclic Carbene Precursors) Under Basic Aqueous Conditions

The hydrolysis of imidazolinium chlorides takes place readily in a basic water/dichloromethane biphasic mixture at room temperature. Experimental parameters were optimized to afford full conversions and high yields of γ-aminoformamides starting from twelve symmetrical substrates with alkyl or aryl substituents on their nitrogen atoms, and five unsymmetrical 1-alkyl-3-arylimidazolinium chlorides. NMR and XRD analyses showed that the cleavage of unsymmetrical salts led to γ-alkylamino-N-arylformamides with a high regioselectivity and that bulky alkyl or aryl groups on the formamide moiety led to the isolation of the (E)-isomer in high stereoisomeric purity (>95 %), whereas smaller and more flexible alkyl substituents afforded mixtures of (E)- and (Z)-rotamers. Control experiments showed that the hydrolysis of 1,3-dimesitylimidazolinium chloride (SIMes ⋅ HCl) did not occur readily in pure or acidic water and that the presence of bulky aromatic substituents on the nitrogen atoms of 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride (SIDip ⋅ HCl) efficiently slowed down its hydrolysis under basic aqueous conditions. Most strikingly, this work highlighted the critical influence of the counteranion on the reactivity of imidazolinium cations. Indeed, the chloride salts underwent a facile hydrolysis in the presence of water and Na2 CO3 , whereas various other NHC ⋅ HX derivatives reacted much slower or remained essentially inert under these conditions.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene ligated half-sandwich complexes of chromium(II) and chromium(I)

The synthesis and characterization of a series of Cr(II) N-Heterocyclic Carbene (NHC) complexes of the type [{Cr(NHC)Cl(μ-Cl)}2] and [(Cyp)Cr(NHC)X] (Cyp = η5-C5H5, cyclopentadienyl; η5-C5Me5, pentamethylcyclopentadienyl; X = Cl, η3-C3H5; NHC = IMeMe, IiPrMe, IMes, IDipp) as well as the cyclic (alkyl)(amino)carbene cAACMe ligated complexes [(η5-C5H5)Cr(cAACMe)X] (X = Cl, NPh2), [(η5-C9H7)Cr(cAACMe)Cl] (C9H7 = Ind, indenyl) and [(η5-C13H9)Cr(cAACMe)Cl] (C13H9 = Fl, fluorenyl) are reported. The reduction of [(η5-C5Me5)Cr(IMeMe)Cl] with KC8 in the presence of CO afforded the NHC ligated Cr(I) metallo-radical [(η5-C5Me5)Cr(IMeMe)(CO)2]. Quantum chemical calculations performed on [(η5-C5Me5)Cr(IMeMe)(CO)2] confirm for this complex a predominantly chromium centered radical.