Merging two evolving areas in synthesis, namely cooperative bimetallics and N-heterocyclic carbenes (NHCs), this study reports the isolation of the first intermediates of alkali-metal-mediated zincation (AMMZn) of a free NHC and a Zn-NHC complex using sodium zincate [(TMEDA)NaZn(TMP)(tBu) 2 ] (1) as a metallating reagent. The structural authentication of (THF) 3 Na[:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCZn(tBu 2)}] (2) and [Na(THF) 6 ] + [tBu 2 Zn:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCZn(tBu 2)}] À (4), resulting from the reactions of 1 with unsaturated free NHC IPr (IPr ¼ 1,3-bis(2,6-di-isopropylphenylimidazole-2-ylidene) and NHC complex ZntBu 2 IPr (3) respectively demonstrates that in both cases, this mixed-metal approach can easily facilitate the selective C4 zincation of the unsaturated backbone of the NHC ligand. Furthermore, the generation of anionic NHC fragments enables dual coordination through their normal (C2) and abnormal (C4) positions to the bimetallic system, stabilising the kinetic AMMZn intermediates which normally go undetected and provides new mechanistic insights in to how these mixed-metal reagents operate. In stark contrast to this bimetallic approach when NHC-complex 3 is reacted with a more conventional single-metal base such as tBuLi, the deprotonation of the coordinated carbene is inhibited, favouring instead, co-complexation to give NHC-stabilised [IPr$LiZntBu 3 ] (5). Showing the potential of 2 to act as a transfer agent of its anionic NHC unit to transition metal complexes, this intermediate reacts with two molar equivalents of [ClAu(PPh 3)] to afford the novel digold species [ClAu:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCAu(PPh 3)}] (6) resulting from an unprecedented double transmetallation reaction which involves the simultaneous exchange of both cationic (Na +) and neutral (ZntBu 2) entities on the NHC framework.

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A novel water-soluble Au(III)-NHC complex has been synthesized and successfully applied in the intramolecular cyclization of γ-alkynoic acids into enol-lactones under biphasic toluene/water conditions, thus representing a rare example of an active and selective catalyst for this transformation in aqueous media. Remarkably, competing alkyne hydration processes were not observed, even during the desymmetrization reaction of challenging 1,6-diyne substrates. In addition, after phase separation, the water-soluble Au(III) catalyst could be recycled 10 times without loss of activity or selectivity.

The catalytic activity of ruthenium(IV) ([Ru(η(3):η(3)-C(10)H(16))Cl(2)L]; C(10)H(16) = 2,7-dimethylocta-2,6-diene-1,8-diyl, L = pyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, 3-methyl-5-phenylpyrazole, 2-(1H-pyrazol-3-yl)phenol or indazole) and ruthenium(II) complexes ([Ru(η(6)-arene)Cl(2)(3,5-dimethylpyrazole)]; arene = C(6)H(6), p-cymene or C(6)Me(6)) in the redox isomerisation of allylic alcohols into carbonyl compounds in water is reported. The former show much higher catalytic activity than ruthenium(II) complexes. In particular, a variety of allylic alcohols have been quantitatively isomerised by using [Ru(η(3):η(3)-C(10)H(16))Cl(2)(pyrazole)] as a catalyst; the reactions proceeded faster in water than in THF, and in the absence of base. The isomerisations of monosubstituted alcohols take place rapidly (10-60 min, turn-over frequency = 750-3000 h(-1)) and, in some cases, at 35 °C in 60 min. The nature of the aqueous species formed in water by this complex has been analysed by ESI-MS. To analyse how an aqueous medium can influence the mechanism of the bifunctional catalytic process, DFT calculations (B3LYP) including one or two explicit water molecules and using the polarisable continuum model have been carried out and provide a valuable insight into the role of water on the activity of the bifunctional catalyst. Several mechanisms have been considered and imply the formation of aqua complexes and their deprotonated species generated from [Ru(η(3):η(3)-C(10)H(16))Cl(2)(pyrazole)]. Different competitive pathways based on outer-sphere mechanisms, which imply hydrogen-transfer processes, have been analysed. The overall isomerisation implies two hydrogen-transfer steps from the substrate to the catalyst and subsequent transfer back to the substrate. In addition to the conventional Noyori outer-sphere mechanism, which involves the pyrazolide ligand, a new mechanism with a hydroxopyrazole complex as the active species can be at work in water. The possibility of formation of an enol, which isomerises easily to the keto form in water, also contributes to the efficiency in water.

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The bis(isocyanide)−ruthenium(II) complexes trans,cis,cis-[RuX2(CNR)2(dppf)] (X = Cl, R = CH2Ph (2a), Cy (2b), tBu (2c), 2,6-C6H3Me2 (2d), (S)-(−)-C(H)MePh (2e); X = Br, R = CH2Ph (3a), Cy (3b), tBu (3c), 2,6-C6H3Me2 (3d), (S)-(−)-C(H)MePh (3e)) have been prepared by reaction of the bis(allyl)−ruthenium(II) derivative [Ru(η3-2-C3H4Me)2(dppf)] (1) with the appropriate isocyanide ligand, in CH2Cl2 at room temperature and in the presence of the corresponding hydrogen halide HX. The structure of the compound trans,cis,cis-[RuCl2(CN-2,6-C6H3Me2)2(dppf)] (2d) has been confirmed by X-ray crystallography. The catalytic activity of complexes 2 a−e and 3a−e in the transfer hydrogenation of acetophenone by propan-2-ol has been studied, and the most active complex, trans,cis,cis-[RuCl2(CNCH2Ph)2(dppf)] (2a), has been also tested as a catalyst in the transfer hydrogenation of a large variety of ketones. The hydride derivatives cis,cis-[RuHCl(CN-2,6-C6H3Me2)2(dppf)] (4) and cis,cis,cis-[RuH2(CN-2,6-C6H3Me2)2(dppf)] (5) have been isolated and characterized. Although both hydride complexes catalyze the transfer hydrogenation of acetophenone in the absence of base, the reactions proceed ca. 5 times faster with 5 than with 4, pointing out that the real active species are dihydride−ruthenium complexes.

A Ru(IV) catalyst able to promote the selective hydration of nitriles to amides in water, at low metal loadings and under mild conditions, is presented.