A series of rhodium(I) complexes of formula Rh{κ2C,X-(IDippR)}(cod) and Rh{κ2C,X-(IDippR)}(CO)2 {IDippR = 1-(R)-3-(2,6-diisopropylphenyl)-imidazolin-2-carbene; X = N, O; R = (6-yl-2-pyridone), (CH2CH2NC(O)Ph), (CH2CH2COO); (cod = 1,5-cyclooctadiene)} containing anionic-tethered NHC-functionalized ligands, including pyridonato, amidato or carboxylato groups at the N-wingtip, has been prepared and their application to alkyne homocoupling has been investigated. The chelate coordination of the NHC-functionalized ligand generates 5, 6 and 7-membered metallacycles for pyridonato, amidato and carboxylato substituents, respectively. Preparation of the mixed bis-NHC derivative Rh{κ2C,O-(IDippCarbx)}(CO)(IPr) {IPr = 1,3-bis-(2,6-diisopropylphenyl)-imidazolin-2-carbene} is possible due to lability of one of the CO ligands in the bis-CO precursor. Reversible protonation is observed for pyridonato and carboxylato moieties but not for amidato counterpart. Alkyne dimerization catalytic activity is reported for N-based anionic groups whereas carboxylato counterparts are inefficient. A proton transfer mechanism is invoked for the gem-selectivity observed for cod-based catalysts whereas alkyne insertion is the regioselectivity-determining step for CO-containing analogues.

The iridium(I) complexes [IrBr(cod)(κC-tBuImCH2PyCH2NRR')] (NRR' = NEt2, NHtBu) have been prepared by reaction of the corresponding functionalized imidazolium salt with the appropriate dinuclear compound [Ir(µ-OR)(cod)]2 (R = OMe, OEt). These compounds react with H2(g) (5 bar) to afford the pincer iridium(III) dihydrido complexes [IrBrH2(κ3C,N,N'-tBuImCH2PyCH2NRR')] in good yields. The complexes [IrBr(cod)(κC-tBuImCH2PyCH2NRR')] efficiently catalyzed the β-alkylation of a series of secondary alcohols and the N-alkylation of a range of aniline derivatives with primary alcohols, with good selectivities for the β-alkylated alcohol and monoalkylated secondary amine products, respectively at low catalyst loading typically 0.1 mol% and sub-stoichiometric amount of base in toluene at 383 K. The pincer iridium(III) dihydrido complexes show a catalytic performance similar to that of the iridium(I) complexes in model alkylation reactions. Mechanistic studies on the activation of the catalytic precursors have shown that both types of complexes have the ability to activate benzyl alcohol through the dearomatization of the pyridine ring by selective deprotonation of the methylene linker between the pyridine and the imidazole-2-ylidene fragment. DFT calculations suggest that activation of both catalytic precursors could lead to the common pincer iridium(I) hydrido species [IrH(κ3C,N,N-tBuImCH2PyCH2NEt2)], which may be key to the borrowing hydrogen reaction mechanism.

The synthesis of pentafulvenes with varied substituents has been efficiently achieved using novel rhodium-based catalysts via (2 + 2 + 1) alkyne cyclotrimerization. A rational design of the catalyst structure, including pyridonato, NHC, and CO ligands, ensures the alkyne chemoselectivity and prevents the formation of robust rhodium-fulvene species. Furthermore, the judicious choice of acidity and steric properties of different alkynes enables the preparation of cross-coupled fulvene derivatives. Stoichiometric and deuteration experiments, as well as DFT calculations, shed light on the reaction mechanism, showing that it includes an initial alkyne deprotonation, two successive alkyne insertions, cyclization, and protonolysis, the first insertion being the rate-determining step.

The compound [Rh(nbd){κ 2 P , N -Ph 2 P(CH 2 ) 3 NMe 2 }][BF 4 ] efficiently catalyzes the polymerization of ring-substituted phenylacetylenes to give highly stereoregular megadalton and ultra-high molecular weight polymers with moderate dispersity.

Graphene oxide has been prepared from an industrial coke-like waste. The morphology of this material (GO-CW) is similar to that of standard graphene oxide (GO-G), although its surface has more defects. Both materials were used to prepare hybrid NHC-Ir(I)/graphene materials, consisting of molecular Ir(NHC) complexes covalently anchored to the graphene surface through the NHC moiety, following two different synthetic routes: (a) direct graphene electrografting of the previously synthesized aniline-functionalized imidazole-2-ylidene-Ir(I) complex, and (b) a two-step sequence comprising initial electrografting of aniline-imidazolium salts and subsequent chemical reaction with Ir(I) precursor anchoring imidazole-2-ylidene-Ir(I) molecular complexes. The synthesized NHC-Ir/graphene hybrid catalysts are active in the oxygen evolution reaction (OER) resulting in current densities in a similar range to those of other NHC-iridium(I) catalysts supported on GO. The highest activity corresponds to hybrid catalysts prepared by the two-step route, with even higher activity and stability when graphene oxide from industrial waste is used. EXAFS spectra of the materials prepared from both synthesis routes before oxidation catalysis reveal the local Ir coordination shell and a structural interaction between Ir and graphene. Both XANES and EXAFS spectra after electrocatalysis point to more oxidized species in which the molecular nature of the iridium catalysts is preserved.

A series of mononuclear square-planar Rh{kappa 2 N,O-BHetA}(eta 2-coe)(NHC) (BHetA = Bis-Heteroatomic Acidato) complexes have been prepared. Modifications of the pyridonato BHetA-type ligand architecture include 4-Me, 5-Me, 6-Me, 3-Br, 4-Br, 4-OMe, and 5-NO2 substitutions as well as pyrimidonato, succinimidato, and 2-piperidonato catalysts. Two structural isomers have been observed for the complexes, depending on the stereoelectronic properties of the ligand. The structure-activity relationship has been studied for gem-specific alkyne dimerization via a cooperative ligand-assisted proton shuttle mechanism. Density functional theory calculations have revealed a mechanistic pathway involving the hemilabile coordination of the BHetA ligand, CMD deprotonation, pi-alkyne protonation, and reductive elimination. The increase in oxygen basicity imparted by the substituent in the pyridonato ligand is key, the 4-methyl derivative being the most active catalyst. However, a favored iminol-amide tautomerization precludes an increase in catalytic activity for the more basic saturated piperidonato catalyst.

Alkynyl rhodium( i ) complexes with functionalized phosphine ligands efficiently catalyze the polymerization of phenylacetylene to give a highly stereoregular linear poly(phenylacetylene) with a fraction of high molecular weight branched polymer.

The [Rh(mu-Cl)(IPr)(eta 2-coe)]2/pyridine system efficiently catalyzes the polyhydrothiolation of a series of dialkynes with dithiols, producing sulfur-rich poly(vinylidene sulfide)s with a typical Mw in the range 20.000-124.000 and vinylidene content of 75-87%. A combination of flexible aliphatic dithiols, including 1,6-hexanedithiol and 2,2 '- (ethylendioxy)diethanethiol, and the rigid aromatic dithiol 4,4 '-thiobisbenzenethiol, with rigid aromatic dia-lkynes, 1,3-diethynylbenzene and 1,4-diethynylbenzene, and flexible dialkynes, including propargyl ether and 1,7-octadiyne, have been used to prepare poly(vinylidene sulfide)s. The copolymerization of flexible dithiols with rigid aromatic dialkynes or vice versa results in high molecular weight polymers, Mw up to 259.000, with low polydispersities. However, polyhidrothiolation of flexible dialkynes with flexible dithiols is much less effi-cient and usually results in the formation of oligomers. The interplay of the IPr and pyridine ligands on the RhCl (IPr)(py)(eta 2-coe) catalyst, which controls the regioselectivity of the alkyne insertion step towards the branched vinyl sulfide, is key in the preparation of these poly(vinylidene sulfide)s.

The iridium(I)-aminophosphane complex [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}(2); SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}(2)). The reaction of [IrCl(cod)(SiNP)] or [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(eta(3)-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(eta(3)-C3H5)((CNBu)-Bu-t)(SiNP)]Cl via the observed intermediate [IrCl2(eta(1)-C3H5)((CNBu)-Bu-t)(SiNP)]. On the other hand, the reaction of [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I-2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2](+) (1 : 2) in acetonitrile affording [Ir{kappa C-3,P,P'-(SiNP-H)}(CH3CN)(3)](2+) isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] with I-2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(kappa P-2,P'-SiNP)(eta(2),eta(3)-C8H11)]I, containing the eta(2),eta(3)-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir-2{kappa C-3,P,P'-(SiNP-H)}(2)(mu-I)(3)]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(kappa P-2,P'-SiNP)(eta(2),eta(3)-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{kappa C-3,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{kappa C-3,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the eta(2),eta(3)-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the β-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent β-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The β-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

A series of rhodium and iridium complexes derived from lutidine-based ligands (lutidine, 2,6-dimethylpyridine) with NHC and amino side-donor functions have been prepared and characterized. Deprotonation of the functionalized imidazolium salts, [(t)BuHImCH(2)PyCH(2)NR(2)]Br, by the bridging methoxo ligands of the dinuclear complexes [M(mu-OMe)(cod)](2) affords [MBr(cod)(kappa C-(t)BuImCH(2)PyCH(2)NR(2))] (M = Rh and Ir) complexes from which a series of Rh(I) and Ir(I) complexes including [M(cod)(kappa C-3,N,N '-(t)BuImCH(2)PyCH(2)NR(2))](+), [Rh(CO)(kappa C-3,N,N '-(t)BuImCH(2)PyCH(2)NR(2))](+), and [IrBr(CO)(2)(kappa C-(t)BuImCH(2)PyCH(2)NR(2))] are readily accessible by halide abstraction and/or carbonylation reactions. In contrast, direct metalation of imidazolium salts with the dinuclear compounds [M(mu-Cl)(cod)](2) (M = Rh and Ir) in the presence of potassium acetate and potassium iodide, a well-established synthetic route to M(III) species, provides access to unusual di-iodido M(III) cyclometalated compounds [MI2{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}] in low yield. Experimental studies combined with DFT calculations suggest that cyclometalated M(III) hydrido [MH(CH3CN){kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}](+) compounds and square-planar cyclometalated M(I) [M{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}] species resulting from their deprotonation by acetate could be intermediates involved in the formation of these compounds. Based on the observed formation of elemental rhodium, disproportionation of square-planar cyclometalated M(I) complexes to afford M(0) and the cationic M(II) species [M{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}](+) is proposed. Reaction of the latter with iodide (I-) would regenerate the M(I) intermediate to give an iodo radical (I center dot) that in turn could dimerize to form diiodine I-2. In this regard, DFT calculations have shown that the oxidative addition of diiodine to the cyclometalated M(I) intermediates leading to the di-iodido M(III) cyclometalated compounds is a highly exergonic process.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A combined ALD/MLD (where ALD and MLD stand for atomic and molecular layer deposition, respectively) deposition strategy using TiCl4, H2O and HQ (hydroquinone) as precursors has been applied for the preparation of inorganic–organic thin films on soda-lime glasses. The alternate deposition of TiO2 layers, by pulsing TiCl4/H2O (ALD), and hybrid layers, using TiCl4/HQ (MLD), results in the formation of thin films that are precursors for porous TiO2-coatings after removal of the HQ template by annealing. The coated-glassed show good photocatalytic activity in the degradation of NO with up to 15% reduction of NO concentration in three successive photocatalytic cycles of 5 h each. Surface Scanning Electron Microscopy (SEM) images show that the TiO2-coating is composed of large grains that are made up of finer subgrains resulting in a porous structure with an average pore size of 3–4 nm. Transmission Electron Microscopy (TEM) images show two regions, a porous columnar structure on top and a denser region over the glass substrate. Energy Dispersive X-Ray (EDX) analysis, nanocrystal electron diffraction and Raman spectroscopy confirm the presence of the anatase phase, which, together with the porosity of the material, accounts for the observed photocatalytic activity.

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η2-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(μ-Cl)(η2-olefin)2]2 with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η2-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η2,η2-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.

This frontiers article highlights recent developments on the application of transition metal-based zwitterionic complexes in catalysis. Recent applications of selected zwitterionic catalysts in polymerization reactions, including the carbonylative polymerization of cyclic ethers, carbon-carbon coupling reactions, the asymmetric hydrogenation of unfunctionalized olefins, and the hydrofunctionalization of alkenes are reviewed. In addition, advances in the field of hydrogenation/dehydrogenation reactions related to energy applications, including the hydrogenation of CO2 and the dehydrogenation of formic acid and N-heterocycles, the functionalization of CO2 with amines and hydrosilanes, and the valorization of polyfunctional bio-based feedstocks, such as the dehygrogenation of glycerol to lactic acid or the reduction of levulinic acid into γ-valerolactone, are also described.

Copper(I) [Cu2(μ-Br)2(tBuImCH2pyCH2L)] n (L = OMe, NEt2, NHtBu) compounds supported by flexible functionalized NHC-based polydentate ligands have been prepared in a one-pot procedure by reacting the corresponding imidazolium salt with an excess of copper powder and Ag2O. An X-ray diffraction analysis has revealed that [Cu2(μ-Br)2(tBuImCH2pyCH2NEt2)] n is a linear coordination polymer formed by bimetallic [Cu(μ-Br)]2 units linked by the lutidine-based NHC-py-NEt2 ligand, which acts as a heteroditopic ligand with a 1κC-2κ2 N,N' coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear [Cu2(μ-Br)2(tBuImCH2pyCH2L)]2 compounds with a coordination mode identical to the functionalized NHC ligands. These compounds have been found to exhibit high catalytic activity in the Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. In particular, [Cu2(μ-Br)2(tBuImCH2pyCH2NEt2)]2 efficiently catalyzes the click reaction of a range of azides and alkynes, under an inert atmosphere at room temperature in neat conditions at a very low catalyst loading, to quantitatively afford the corresponding 1,4-disubstituted 1,2,3-triazole derivatives in a few minutes. The cycloaddition reaction of benzyl azide to phenylacetylene can be performed at 25-50 ppm catalyst loading by increasing the reaction time and/or temperature. Reactivity studies have shown that the activation of the polynuclear catalyst precursor involves the alkyne deprotonation by the NHC moiety of the polydentate ligand to afford a copper(I)-alkynyl species bearing a functionalized imidazolium ligand. DFT calculations support the participation of the dinuclear species [(CuBr)2(μ-tBuImCH2pyCH2NEt2)], resulting from the fragmentation of the tetranuclear compound, as the catalytically active species. The proposed reaction pathway proceeds through zwitterionic dinuclear intermediates and entails the active participation of both copper atoms, as well as the NHC moiety as an internal base, which activates the reacting alkyne via deprotonation.

The iridium(III) hydride compound [IrH{κ3C,P,P'-(SiNP-H)}(CNtBu)2][PF6] (1PF6) was obtained by reaction of [Ir(SiNP)(cod)][PF6] with CNtBu as the result of the intramolecular oxidative addition of the SiCH2-H bond to iridium(I) [SiNP = Si(CH3)2{N(4-tolyl)PPh2}2, SiNP-H = CH2Si(CH3){N(4-tolyl)PPh2}2]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CNtBu)][PF6] (2PF6) forms in the first place and that further reacts with CNtBu, affording the square planar intermediate [Ir(SiNP)(CNtBu)2][PF6] (3PF6) that finally undergoes the intramolecular oxidative addition of the SiCH2-H bond. The reactivity of 1PF6 was investigated. On one hand, the reaction of 1PF6 with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ3C,P,P'-(SiNP-H)}(CNtBu)2][PF6] (X = Cl, 4PF6; Br, 5PF6), and the reaction of 5PF6 with AgPF6 in the presence of acetonitrile affords the solvato species [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)(CNtBu)2]2+ (62+) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF6 with HBF4 gives the iridium(III) compound [IrH(CH2SiF2CH3)(HNP)2(CNtBu)2][BF4] (7BF4) as the result of the formal addition of hydrogen fluoride to the Si-N bonds of 1+ [HNP = HN(4-tolyl)PPh2]. A similar outcome was observed in the reaction of 1PF6 with CF3COOH rendering 7PO2F2. In this case the intermediate [IrH{κ2C,P-CH2SiMeFN(4-tolyl)PPh2}(HNP)(CNtBu)2]+ (8+) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si-N-P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF6, 4PF6 and 7BF4 have been determined by X-ray diffraction measurements.

Rhodium complexes with functionalized NHC ligands catalyze the polymerization of phenylacetylene to give very high molar mass stereoregular polymers. Surprisingly, some chloro-complexes are much more active than related cationic precursors also.

The magnetron sputtering technique has been successfully employed for the preparation of porous TiO2 thin films on soda-lime glasses by means of an oblique angle deposition strategy. The morphology of the thin layers is affected by the deposition parameters, such as, the angle with respect to the target, applied power, total pressure, oxygen pressure and deposition time. It has been shown that 60-65 degrees angles, lead to a compromise between the porosity, the level of oxidation and the thickness of the film. High total pressures of the deposition process result in less dense coatings of greater porosity. Moreover, the oxygen flow during the deposition process must be carefully adjusted for each set of deposition conditions, in order to achieve an optimum degree of oxidation. The evaluation of coated-glasses in the in-flow photocatalytic oxidation of nitrogen oxide has shown that the presence of a porous film is essential to achieve photocatalytic activity. The best performing coated-glass was able to reduce the NO concentration ca. 20% for 5 h. SEM and TEM images of this film show a microstructure composed of nanometric grains and a tilted columnar structure. Nanocrystal electron diffraction, XRD and Raman spec- troscopy have confirmed the deposition of TiO2 anatase.