A new synthetic route to prepare imidazolium salts with heteroatom-containing functional groups at the backbone has been reported. Accordingly, the first example of a backbone bisthiofunctionalized imidazolium salt (4) was prepared by sequential metal−halogen exchange reaction of 1-methyl-4,5-diiodoimidazole (1) followed by a quaternization reaction with methyl iodide. The metal−carbene complexes 6, 8, and 10 were synthesized conveniently through three different routes, namely, (a) an in situ generated carbene route, (b) a transmetalation method, and (c) direct reaction with a basic metal precursor, and structurally characterized. Subsequently the electronic properties of the newly prepared 1,3dimethyl-4,5-bis(phenylthio)-imidazol-2-ylidene ((SPh) 2 IMe) was studied by measuring the carbonyl stretching frequency of the corresponding [Ir{(SPh) 2 IMe}(CO) 2 (Cl)] complex. In addition, the air-stable palladium−NHC complex 10 was found to be catalytically active in Suzuki−Miyaura coupling reactions of aryl bromides.

A simple method involving metal−halogen exchange reaction(s) to prepare various phosphine-functionalized imidazole/imidazolium salts and their coordination chemistry with different metal precursors has been described. Interestingly, the reaction of 1,3-dimethyl-2-(diphenylphosphino)-4-iodoimidazolium iodide (6) with Pd 2 (dba) 3 in the presence of triphenylphosphine affords a Pd(II)−NHC complex which involves the cleavage of a C−P bond presumably occurring via oxidative addition of Pd(0) to a C−I bond to afford an in situ generated Pd(II) species, which subsequently reacts with another 1 equiv of 6 through the phosphine center to form an adduct followed by a dephosphinylation reaction. ■ INTRODUCTIONPhosphines and N-heterocyclic carbenes (NHCs) have played a pivotal role in the development of organometallic chemistry, and stitching these two classes of neutral, two-electron σ-donor ligands together would further foster their application as ligands for transition metals which may prove valuable in catalysis. In view of this, several approaches to append a phosphine group to the NHC skeleton have been made 1,2 and metalation via both functionalities was successfully achieved. 2 A detailed account of this subject has appeared recently. 3 Phosphines attached to the imidazolium (NCN) carbon center are better described as NHC−phosphenium adducts and act as poor donors toward transition metals. The dative and labile nature of the C−P bond is echoed in their reactivity toward weak nucleophiles such as chloride ion, which results in heterolytic cleavage of the C−P bond in such systems to give chlorodiphenylphosphine and free NHC which, although not isolated, has been successfully trapped by Chauvin and coworkers. 4 A similar reactivity pattern was observed in the coordination sphere of metal complexes (Pd, Rh, Cu, etc.) to afford metal−NHC complexes. 4,5 Also, cleavage of a C−P bond in NHC−phosphenium salts with an electron-rich metal center such as Pt(0) was reported by Baker and co-workers involving a nonoxidative addition pathway. 6 Metal−halogen exchange reactions 7 with diiodoimidazole are particularly handy to achieve functionalization at the backbone position(s) of imidazole and imidazolium salts with mild reagents and conditions. We have recently reported backbonefunctionalized imidazolium salts which act as precursors for synthesizing the metal complexes of both normal and mesoionic carbenes. 8 In this contribution, we report metal− halogen exchange reaction(s) with 1-methyl-4,5-diiodoimidazole (1) and 1,3-dimethyl-4,5-diiodoimidazolium iodide (5) using chlorodiphenylphosphine as the source of the electrophile to afford a phosphine-functionalized imidazole/imidazolium salt at the backbone position for the former case and at the 2-position for the latter. The coordination chemistry of the aforementioned phosphines (3, 4, and 6) with various metal precursors and, in addition, a rare example of C−P bond cleavage in an NHC−phosphenium salt using a Pd(0) precursor to afford a palladium(II)−carbene complex are discusse...

Three different backbone-monofunctionalized imidazolium salts have been synthesized using the metal− halogen exchange procedure, and their corresponding mesoionic carbene complexes with palladium were prepared via oxidative addition without protection of the C2 position. The donor properties were evaluated with 31 P NMR spectroscopy of the respective palladium complexes. The catalytic activity of these complexes toward Suzuki−Miyaura coupling of aryl bromides was also explored. Also, in one case, a comparison of donor properties was made with those of a "normal" carbene with similar steric bulk.

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Reactions of N,N'-bis(2,6-diisopropylphenyl)formamidinium chloride (1) with palladium acetate in acetonitrile or propanenitrile results in the formation of cyclic six-membered triazapentadiene-palladium dichloride complexes (4 and 5). The reaction of formamidinium salt containing non-coordinating anions, such as (BF4)(-) (2) and (PF6)(-) (3) with palladium acetate in acetonitrile or propanenitrile leads to the formation of cationic bis(1,3,5-triazapentadiene)palladium complexes 6, 7 or 8, 9 respectively. However, the reaction with acrylonitrile afforded an unprecedented cyclic and anionic six-membered palladacycle complex (10). In addition a reaction of 1 with Pd(OAc)2 in THF afforded an acyclic palladium complex (11), which is a possible intermediate formed before conversion to the cyclic six-membered palladium complexes (4, 5 and 10) in presence of organonitriles.