Preparation of Remote NHC Complexes of Rhodium(I) and Gold(I) by Ligand Transfer

Abstract

The rNHC complexes (5-butyl-1,2-dimethylpyridin-4(1H)-ylidene)Cr(CO) 5 (1), as well as the novel compounds (1,2-dimethyl-5-phenylpyridin-4(1H)-ylidene)M(CO) 5 (2, M = Cr; 3, M = W) and (1-methylpyridin-4(1H)-ylidene)Cr(CO) 5 (7) were prepared from Fischer carbene complexes. Reaction of 1 with [Rh 2 (µ-Cl) 2 (CO) 4 ] then afforded the first rNHC Rh I complex, 4. Transfer of the ligand in 2 to Ph 3 PAu + afforded the cationic rNHC gold complex, 5 and reaction of 3 and 7

Normal and Abnormal N-Heterocyclic Carbene Ligands

“…Thus, pioneering work by Bertrand and coworkers disclosed the high stability of a variety of free carbenes with reduced heteroatom stabilization. 19 In addition, new methodologies for the metalation of heterocyclic carbene precursors have been developed that circumvent the formation of free carbenes, including examples of direct metalation via CdH bond activation, 20 transmetalation, oxidative addition of a CdX bond 21 or a CdH bond, 22 or alkylation of a metal-bound anionic azolyl ligand, 23 thus yielding a series of NHC complexes without the need to stabilize or isolate the free carbene. 17 Even carbenes that have no heteroatom adjacent to the carbene center have successfully been stabilized, as, for example, in amino-substituted cyclopropylidenes or pyrazol-derived bent allenes, 18 unveiling a high stability of these free carbenes.…”

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

“…18 We have been active in this field for a decade and have investigated in particular pyridinium-derived carbenes. [19][20][21][22][23][24][25] With now well-established protocols for the functionalization of pyridine, pyridylidenes offer great potential for variation of steric demand and electronic properties. Moreover, they can bind to the metal in a normal (I, III) or abnormal (II) fashion (Scheme 1) with the possibility of both normal and abnormal carbenes arising in a remote position.…”

A simple, general route to 2-pyridylidene transition metal complexes

“…4 Carbene compounds of these types have long been known 5 and have been generated by various synthetic procedures that include: N-functionalization of pyridyl derivatives; 6 oxidative addition of pyridinium halides to metal complexes; 7 C-H bond activation of pyridinium salts; 8 tautomerization processes of pyridines to N-heterocyclic carbenes mediated by transition metals; 9 cycloaddition reactions to Fischer type carbenes; 10 and transmetalation reactions. 11 Although these methods may be employed in particular instances, they lack generality and moreover suffer from important drawbacks: increase in the metal oxidation state; availability of the starting materials, etc. Deprotonation of pyridinium salts has been reported recently, but the scope of this procedure is still limited to the synthesis of some ruthenium clusters.…”