This study describes the synthesis and characterization of a new class of ferrocene-containing carbon monoxide-releasing molecules (CORMs, 1–3). The ferrocenyl group is both a recognized therapeutically viable coligand and a handle for informative infrared spectroelectrochemistry. Deoxymyoglobin CO-release assays and in situ infrared spectroscopy confirm compounds 2 and 3 as photoCORMs and 1 as a thermal CORM, attributed to the increased sensitivity of the Mn–ferrocenyl bond to protonation in 1. Electrochemical and infrared spectroelectrochemical experiments confirm a single reversible redox couple associated with the ferrocenyl moiety with the Mn tetracarbonyl center showing no redox activity up to +590 mV vs Fc/Fc+, though no concomitant CO release was observed in association with the redox activity. The effects of linker length on communication between the Fe and Mn centers suggest that the incorporation of redox-active ligands into CORMs focuses on the first coordination sphere of the CORM. Redox-tagged CORMs could prove to be a useful mechanistic probe; our findings could be developed to use redox changes to trigger CO release.

AbstractThe facile synthesis of a stable and isolable compound with a fluoroalkynyl group, M−C≡CF, is reported. Reaction of [Ru(C≡CH)(η5-C5Me5)(dppe)] with an electrophilic fluorinating agent (NFSI) results in the formation of the fluorovinylidene complex [Ru(=C=CHF)(η5-C5Me5)(dppe)][N(SO2Ph)2]. Subsequent deprotonation with LiN(SiMe3)2 affords the fluoroalkynyl complex [Ru(C≡CF)(η5-C5Me5)(dppe)]. In marked contrast to the rare and highly reactive examples of fluoroalkynes that have been reported previously, this compound can be readily isolated and structurally characterized. This has allowed the structure and bonding in the CCF motif to be explored. Further electrophilic fluorination of this species yields the difluorovinylidene complex [Ru(C=CF2)(η5-C5Me5)(dppe)][N(SO2Ph)2].

The use of cationic gold(I) species in the activation of substrates containing CC bonds has become a valuable tool for synthetic chemists. Despite the seemingly simple label of ‘alkyne activation’, numerous patterns of reactivity and product structure are observed in systems employing related substrates and catalysts. The complications of mechanistic determination are compounded as the number of implicated gold(I) centres involved in catalysis increases and debate about the bonding in proposed intermediates clouds the number and importance of potential reaction pathways. This perspective aims to illustrate some of the principles underpinning gold–alkynyl interactions whilst highlighting some of the contentious areas in the field and offering some insight into other, often ignored, mechanistic possibilities based on recent findings.

Metal vinylidene complexes are widely encountered, or postulated, as intermediates in a range of important metal-mediated transformations of alkynes. However, fluorovinylidene complexes have rarely been described and their reactivity is largely unexplored. By making use of the novel outer-sphere electrophilic fluorination (OSEF) strategy we have developed a rapid, robust and convenient method for the preparation of fluorovinylidene and trifluoromethylvinylidene ruthenium complexes from non-fluorinated alkynes. Spectroscopic investigations (NMR and UV/Vis), coupled with TD-DFT studies, show that fluorine incorporation results in significant changes to the electronic structure of the vinylidene ligand. The reactivity of fluorovinylidene complexes shows many similarities to non-fluorinated analogues, but also some interesting differences, including a propensity to undergo unexpected C–F bond cleavage reactions. Heating fluorovinylidene complex [Ru(η5-C5H5)(PPh3)2(CC{F}R)][BF4] led to C–H activation of a PPh3 ligand to form an orthometallated fluorovinylphosphonium ligand. Reaction with pyridine led to nucleophilic attack at the metal-bound carbon atom of the vinylidene to form a vinyl pyridinium species, which undergoes both C–H and C–F activation to give a novel pyridylidene complex. Addition of water, in the presence of chloride, leads to anti-Markovnikov hydration of a fluorovinylidene complex to form an α-fluoroaldehyde, which slowly rearranges to its acyl fluoride isomer. Therefore, fluorovinylidenes ligands may be viewed as synthetic equivalents of 1-fluoroalkynes providing access to reactivity not possible by other routes.

Organofluorine chemistry plays a key role in materials science, pharmaceuticals, agrochemicals, and medical imaging. However, the formation of new carbon–fluorine bonds with controlled regiochemistry and functional group tolerance is synthetically challenging. The use of metal complexes to promote fluorination reactions is of great current interest, but even state-of-the-art approaches are limited in their substrate scope, often require activated substrates, or do not allow access to desirable functionality, such as alkenyl C(sp2)–F or chiral C(sp3)–F centers. Here, we report the formation of new alkenyl and alkyl C–F bonds in the coordination sphere of ruthenium via an unprecedented outer-sphere electrophilic fluorination mechanism. The organometallic species involved are derived from nonactivated substrates (pyridine and terminal alkynes), and C–F bond formation occurs with full regio- and diastereoselectivity. The fluorinated ligands that are formed are retained at the metal, which allows subsequent metal-mediated reactivity.

The coordination modes of the [Au(PPh3)]+ cation to metal alkynyl complexes have been investigated. On addition to ruthenium, a vinylidene complex, [Ru(η5-C5H5)(PPh3)2(CCPh{AuPPh3})]+, is obtained while addition to a gold(III) compound gives di- and trinuclear gold complexes depending on the conditions employed. In the trinuclear species, a gold(I) cation is sandwiched between two gold(III) alkynyl complexes, suggesting that coordination of multiple C–C triple bonds to gold is facile.

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru(CCH–C6H2-2,5-R2-4-CCH)Cl(dppm)2]+ (R = H, Me). Subsequent reaction with [NnBu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(CC–C6H2-2,5-R2-4-CClCH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

The synthesis and properties of a novel class of platinum complexes containing Schiff bases as O,N-bidentate ligands is described as are the solution and solid state properties of the uncomplexed ligands. The platinum complexes were prepared from [PtBr2(COD)] (COD = 1,5-cyclooctadiene) and N-(2-hydroxy-1-naphthalidene)aniline derivatives in the presence of base (NaOBut). Instead of a substitution reaction to afford cationic species, the addition of the Schiff base ligands results in both the formal loss of two equivalents of bromide and addition of hydroxide to the COD ligand of the complexes. It is proposed that this reaction proceeds through a cationic platinum complex [Pt(N–O)(COD)]Br which then undergoes addition of water and loss of HBr. An example of a dinuclear platinum complex in which two cyclo–octene ligands are bridged by an ether linkage is also reported. The platinum complexes were evaluated as catalysts for the hydrogenative and dehydrogenative silylation of styrene, the resulting behaviour is substituent, time and temperature dependent.

Reactions of cis-RuCl2(dppm)2 with various terminal alkynes, of the type HCCC6H4-4-R (1 equiv.), in the presence of TlBF4 have resulted in the formation of cationic vinylidene complexes trans-[RuCl(CCHC6H4-4-R)(dppm)2]BF4 ([1]BF4). These complexes can be isolated, or treated in situ with a suitable base (Proton Sponge, 1,8-bis(dimethylamino)naphthalene) to yield the mono-alkynyl complexes trans-RuCl(CCC6H4-4-R)(dppm)2 (2). Through similar reactions between cis-RuCl2(dppm)2 with 2 equiv. of alkyne, TlBF4 and base, trans-bis(alkynyl) complexes, trans-Ru(CCC6H4-4-R)2(dppm)2 (3), can be isolated when R is an electron withdrawing substituent (R = NO2, COOMe, CCSiMe3), whereas reactions with alkynes bearing electron donating substituents (R = OMe and Me) form cationic η3-butenynyl complexes [Ru(η3-{HC(C6H4-4-R)CCCC6H4-4-R})(dppm)2]+ ([4]+). This work highlights the importance of the electronic character of the alkyne in influencing product outcome.

The ruthenium naphthalene complex [Ru(η5-C5H5)(η6-C10H8)]+ is a catalyst precursor for the direct C–H alkenylation of pyridine and related nitrogen heterocycles by terminal alkynes. Stoichiometric studies have demonstrated that the naphthalene ligand may be displaced by either pyridine, 4-methylpyridine or dimethylaminopyridine (DMAP) to give species [Ru(η5-C5H5)L3]+ (L = nitrogen-based ligand). Reaction of in situ-generated [Ru(η5-C5H5)(py)3]+ (py = pyridine) with PPh3 results in the formation of [Ru(η5-C5H5)(PPh3)(py)2]+, the active catalyst for direct alkenylation, some [Ru(η5-C5H5)(PPh3)2(py)]+ is also formed in this reaction. A one-pot procedure is reported which has allowed for the nature of the nitrogen heterocycle and phosphine ligand to be evaluated. The sterically demanding phosphine PCy3 inhibits catalysis, and only trace amounts of product are formed when precursors containing a pentamethylcyclopentadienyl group were used. The greatest conversion was observed with PMe3 when used as co-ligand with [Ru(η5-C5H5)(η6-C10H8)]+.

Experimental results have long suggested that catalyst optimization is an inherently multivariate process, requiring the screening of reaction conditions (temperature, pressure, solvents, precursors, etc.), catalyst structure (metal and ligands), and substrate scope. With a view to demonstrating the feasibility and utility of multivariate computational screening of organometallic catalysts, we have investigated the structural and electronic properties of a library of transition-metal-coordinated alkyne and vinylidene tautomers in different coordination environments. By varying the substituents on the organic moiety of 60 alkyne/vinylidene pairs we were able to capture and quantify the key structural and electronic effects on tautomer preference. For a carefully selected subset of substituents, the effects of metal and ancillary ligands were then explored. We have been able to formulate a protocol for assessing the stabilization of vinylidenes in transition-metal complexes, suggesting that the d6 square-based-pyramidal metal fragment [RuCl2(PR23)(═C═CHR1)], combined with electron-withdrawing substituents R1 and electron-rich groups R2, would provide the ideal conditions favoring the vinylidene form thermodynamically.

Reaction of cis-[RuCl2(DMSO-S)3(DMSO-O)] with cis-1,3,5-triaminocyclohexane (tach) results in the formation of [RuCl(tach)(DMSO-S)2]Cl, a valuable precursor for a wide range of other tach-containing Ru complexes. Reaction of [RuCl(tach)(DMSO-S)2]Cl with the chelating nitrogen-based ligands (N–N = bipyridine, phenanthroline, and ethylenediamine) affords [Ru(N–N)(DMSO-S)2(tach)][Cl]2. A similar reaction between [RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based ligands (P–P = dppm, dppe, dppp, dppb, dppv, and dppben) leads to the formation of [RuCl(P–P)(tach)]Cl. The structures of 10 examples of the tach-containing complexes have been determined by single crystal X-ray diffraction. An examination of the structural metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH2 groups in the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.

Reaction of hydroxyvinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(═C═CHC{OH}R1R2)(PPh3)2] (R1 = R2 = Ph; R1 = R2 = Me; R1 = Ph, R2 = Me) with [CPh3]BF4 results in the formation of the cationic carbene species [Ru(κ2-OAc)(OC{Me}OCC{H}═CR1R2)(PPh3)2]BF4. In these complexes, the κ1-acetate ligand has changed its binding mode in order to stabilize the resulting cationic species. The carbene complexes may be deprotonated, although the outcome of the reaction depends markedly on the substituent present. In the case in which R1 = R2 = Ph, the hydrogen on the β-carbon of the organic ligand is removed to afford an allenylidene complex [Ru(κ1-OAc)(κ2-OAc)(═C═C═CPh2)(PPh3)2]. An examination of the structural and spectroscopic parameters for the allenylidene complex indicates that the electronic influence of this ligand is very similar to the corresponding vinylidene and isonitrile analogues. In the cases where R1 = R2 = Me and R1 = Me, R2 = Ph deprotonation occurs at a methyl group to afford vinylvinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(═C═C{H}-CR2═CH2)(PPh3)2] (R2 = Me, Ph). No interconversion between vinylvinylidene and allenylidene complexes was observed. The overall process is analogous to a formal E1-type elimination in which the cationic carbene complex may be viewed as a stabilized carbocation intermediate. A DFT study provided insight into selectivity of the deprotonation step indicating that the greatest relative difference in energy between all the possible isomers of the vinylvinylidene and allenylidene complexes was ca. 20 kJ mol–1. Interconversion between the two forms of the complex by a [1,3]-hydrogen shift appears to be unlikely due to the higher energy of the corresponding transition state; hence the selectivity in the formation of the vinylvinylidene complexes may be due the site of deprotonation being kinetically controlled. An alternative mechanism for this interconversion between vinylvinylidene and allenylidene complexes in cationic half sandwich metal complexes is proposed, which proceeds via a deprotonation/reprotonation pathway.

A combined experimental and theoretical study has demonstrated that [Ru(η5-C5H5)(py)2(PPh3)]+ is a key intermediate, and active catalyst for, the formation of 2-substituted E-styrylpyridines from pyridine and terminal alkynes HC≡CR (R = Ph, C6H4-4-CF3) in a 100% atom efficient manner under mild conditions. A catalyst deactivation pathway involving formation of the pyridylidene-containing complex [Ru(η5-C5H5)(κ3-C3-C5H4NCH═CHR)(PPh3)]+ and subsequently a 1-ruthanaindolizine complex has been identified. Mechanistic studies using 13C- and D-labeling and DFT calculations suggest that a vinylidene-containing intermediate [Ru(η5-C5H5)(py)(═C═CHR)(PPh3)]+ is formed, which can then proceed to the pyridylidene-containing deactivation product or the desired product depending on the reaction conditions. Nucleophilic attack by free pyridine at the α-carbon in this complex subsequently leads to formation of a C–H agostic complex that is the branching point for the productive and unproductive pathways. The formation of the desired products relies on C–H bond cleavage from this agostic complex in the presence of free pyridine to give the pyridyl complex [Ru(η5-C5H5)(C5H4N)(═C═CHR)(PPh3)]. Migration of the pyridyl ligand (or its pyridylidene tautomer) to the α-carbon of the vinylidene, followed by protonation, results in the formation of the 2-styrylpyridine. These studies demonstrate that pyridylidene ligands play an important role in both the productive and nonproductive pathways in this catalyst system.

Robust, bifunctional catalysts comprising Rh(CO)(Xantphos) exchanged phosphotungstic acids of general formulas [Rh(CO)(Xantphos)]+n[H3–nPW12O40]n− have been synthesized over silica supports which exhibit tunable activity and selectivity toward direct vapor phase methanol carbonylation. The optimal Rh:acid ratio = 0.5, with higher rhodium concentrations increasing the selectivity to methyl acetate over dimethyl ether at the expense of lower acidity and poor activity. On-stream deactivation above 200 °C reflects Rh decomplexation and reduction to Rh metal, in conjunction with catalyst dehydration and loss of solid acidity because of undesired methyl acetate hydrolysis, but can be alleviated by water addition and lower temperature operation.Keywords: carbonylation; EXAFS; heteropoly acid; methanol; rhodium;

A new class of photochemically-activated CO-releasing molecule (photo-CO-RM), based on a Mn(CO)4(C^N) system, is reported in this study. Three CO molecules are released per CO-RM molecule. Complex 3 is a fast releaser, thermally stable in the dark and a viable therapeutic agent.

An investigation into the CO-releasing properties of a range of iron tricarbonyl and chromium and molybdenum tetracarbonyl complexes is presented. Iron tricarbonyl complexes containing the 2,5-bicyclo[2.2.1]heptene (norbornadiene) ligand are shown to be effective CO-releasing molecules, in which the rate and extent of CO release may be modulated by modification of the norbornadiene framework. Species containing the parent norbornadiene and those with a substituent at the 7-position of the organic ligand exhibit CO release; those containing ester substituents at the 2- and/or 3-positions do not. A mechanism for CO release in this species is proposed which involves initial norbornadiene dissociation, a suggestion which is supported by the spectroscopic data and the observation that the addition of excess substituted norbornadiene retards the rate of CO release. CO release from the diester-containing norbornadiene complex may be promoted photochemically, and cell viability studies indicate that in the absence of light this complex is nontoxic, making it an excellent candidate for further study as a photo-CO-RM. Both the chromium and molybdenum tetracarbonyl complexes release CO, which in the case of the molybdenum analogue is rapid.

Reaction of [MoCl(η5-C5H5)(CO)3] with propargyl alcohols HC≡CCR1R2OH in the presence of catalytic amounts of CuI and using NEt2H as solvent results in the formation of alkynyl complexes [Mo(C≡CCR1R2OH)(η5-C5H5)(CO)3]. The structure of the complexes where R1 = R2 = Me and R1 = Me, R2 = Ph were confirmed by single-crystal X-ray diffraction. The method could be extended to a range of propargyl ethers and propargyl esters, which allowed for the preparation of molybdenum complexes containing pendant salicylate and β-d-fructopyranose groups. The alkynyl complex [Mo(C≡CCH2OH)(η5-C5H5)(CO)3] undergoes a further reaction with NEt2H to give the substituted allyl complex [Mo(η3-H2CC{NEt2}CHC{O}NEt2)(η5-C5H5)(CO)3]. Similar products could be obtained from the reaction of [MoCl(η5-C5H5)(CO)3] with HC≡CCH2OH in the presence of CuI using pyrrolidine or piperidine as solvent. In the case of the reaction with piperidine a further product could be isolated that has arisen from the coupling of two propargyl alcohol molecules to afford a butadienyl ligand. The CO-releasing properties of a number of these novel complexes have been investigated. In the case of the water-soluble alkynyl complex containing a β-d-fructopyranose group CO release was shown to be promoted by exposure to UV light, revealing a new class of photochemically activated CO-releasing molecules (PhotoCO-RMs).

The ruthenium bis-acetate complex Ru(κ2-OAc)2(PPh3)2 reacts with HCCPh to afford the vinylidene-containing species Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2. An experimental study has demonstrated that this reaction occurs under very mild conditions, with significant conversion being observed at 255 K. At lower temperatures, evidence for a transient metallo-enol ester species Ru(κ1-OAc)(OC{Me}O–CCHPh)(PPh3)2 was obtained. A comprehensive theoretical study to probe the nature of the alkyne/vinylidene tautomerisation has been undertaken using Density Functional Theory. Calculations based on a number of isomers of the model system Ru(κ1-OAc)(κ2-OAc)(CCHMe)(PH3)2 demonstrate that both the η2(CC) alkyne complex Ru(κ1-OAc)(κ2-OAc)(η2-HCCMe)(PH3)2 and the C–H agostic σ-complex Ru(κ1-OAc)(κ2-OAc)(η2{CH}-HCCMe)(PH3)2 are minima on the potential energy surface. The lowest energy pathway for the formation of the vinylidene complex involves the intramolecular deprotonation of the σ-complex by an acetate ligand followed by reprotonation of the subsequently formed alkynyl ligand. This process is thus termed a Ligand-Assisted Proton Shuttle (LAPS). Calculations performed on the full experimental system Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2 reinforce the notion that lowest energy pathway involves the deprotonation/reprotonation of the alkyne by an acetate ligand. Inclusion of the full ligand substituents in the calculations are necessary to reproduce the experimental observation of Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2 as the thermodynamic product.

The ruthenium bis-acetate complex Ru(κ2-OAc)2(PPh3)2 reacts with HCCPh to afford the vinylidene-containing species Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2. An experimental study has demonstrated that this reaction occurs under very mild conditions, with significant conversion being observed at 255 K. At lower temperatures, evidence for a transient metallo-enol ester species Ru(κ1-OAc)(OC{Me}O–CCHPh)(PPh3)2 was obtained. A comprehensive theoretical study to probe the nature of the alkyne/vinylidene tautomerisation has been undertaken using Density Functional Theory. Calculations based on a number of isomers of the model system Ru(κ1-OAc)(κ2-OAc)(CCHMe)(PH3)2 demonstrate that both the η2(CC) alkyne complex Ru(κ1-OAc)(κ2-OAc)(η2-HCCMe)(PH3)2 and the C–H agostic σ-complex Ru(κ1-OAc)(κ2-OAc)(η2{CH}-HCCMe)(PH3)2 are minima on the potential energy surface. The lowest energy pathway for the formation of the vinylidene complex involves the intramolecular deprotonation of the σ-complex by an acetate ligand followed by reprotonation of the subsequently formed alkynyl ligand. This process is thus termed a Ligand-Assisted Proton Shuttle (LAPS). Calculations performed on the full experimental system Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2 reinforce the notion that lowest energy pathway involves the deprotonation/reprotonation of the alkyne by an acetate ligand. Inclusion of the full ligand substituents in the calculations are necessary to reproduce the experimental observation of Ru(κ1-OAc)(κ2-OAc)(CCHPh)(PPh3)2 as the thermodynamic product.

The ruthenium naphthalene complex [Ru(η5-C5H5)(η6-C10H8)]+ is a catalyst precursor for the direct C–H alkenylation of pyridine and related nitrogen heterocycles by terminal alkynes. Stoichiometric studies have demonstrated that the naphthalene ligand may be displaced by either pyridine, 4-methylpyridine or dimethylaminopyridine (DMAP) to give species [Ru(η5-C5H5)L3]+ (L = nitrogen-based ligand). Reaction of in situ-generated [Ru(η5-C5H5)(py)3]+ (py = pyridine) with PPh3 results in the formation of [Ru(η5-C5H5)(PPh3)(py)2]+, the active catalyst for direct alkenylation, some [Ru(η5-C5H5)(PPh3)2(py)]+ is also formed in this reaction. A one-pot procedure is reported which has allowed for the nature of the nitrogen heterocycle and phosphine ligand to be evaluated. The sterically demanding phosphine PCy3 inhibits catalysis, and only trace amounts of product are formed when precursors containing a pentamethylcyclopentadienyl group were used. The greatest conversion was observed with PMe3 when used as co-ligand with [Ru(η5-C5H5)(η6-C10H8)]+.

A new class of photochemically-activated CO-releasing molecule (photo-CO-RM), based on a Mn(CO)4(C^N) system, is reported in this study. Three CO molecules are released per CO-RM molecule. Complex 3 is a fast releaser, thermally stable in the dark and a viable therapeutic agent.

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru(CCH–C6H2-2,5-R2-4-CCH)Cl(dppm)2]+ (R = H, Me). Subsequent reaction with [NnBu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(CC–C6H2-2,5-R2-4-CClCH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

The coordination modes of the [Au(PPh3)]+ cation to metal alkynyl complexes have been investigated. On addition to ruthenium, a vinylidene complex, [Ru(η5-C5H5)(PPh3)2(CCPh{AuPPh3})]+, is obtained while addition to a gold(III) compound gives di- and trinuclear gold complexes depending on the conditions employed. In the trinuclear species, a gold(I) cation is sandwiched between two gold(III) alkynyl complexes, suggesting that coordination of multiple C–C triple bonds to gold is facile.

The synthesis and properties of a novel class of platinum complexes containing Schiff bases as O,N-bidentate ligands is described as are the solution and solid state properties of the uncomplexed ligands. The platinum complexes were prepared from [PtBr2(COD)] (COD = 1,5-cyclooctadiene) and N-(2-hydroxy-1-naphthalidene)aniline derivatives in the presence of base (NaOBut). Instead of a substitution reaction to afford cationic species, the addition of the Schiff base ligands results in both the formal loss of two equivalents of bromide and addition of hydroxide to the COD ligand of the complexes. It is proposed that this reaction proceeds through a cationic platinum complex [Pt(N–O)(COD)]Br which then undergoes addition of water and loss of HBr. An example of a dinuclear platinum complex in which two cyclo–octene ligands are bridged by an ether linkage is also reported. The platinum complexes were evaluated as catalysts for the hydrogenative and dehydrogenative silylation of styrene, the resulting behaviour is substituent, time and temperature dependent.

The use of cationic gold(I) species in the activation of substrates containing CC bonds has become a valuable tool for synthetic chemists. Despite the seemingly simple label of ‘alkyne activation’, numerous patterns of reactivity and product structure are observed in systems employing related substrates and catalysts. The complications of mechanistic determination are compounded as the number of implicated gold(I) centres involved in catalysis increases and debate about the bonding in proposed intermediates clouds the number and importance of potential reaction pathways. This perspective aims to illustrate some of the principles underpinning gold–alkynyl interactions whilst highlighting some of the contentious areas in the field and offering some insight into other, often ignored, mechanistic possibilities based on recent findings.

Reactions of cis-RuCl2(dppm)2 with various terminal alkynes, of the type HCCC6H4-4-R (1 equiv.), in the presence of TlBF4 have resulted in the formation of cationic vinylidene complexes trans-[RuCl(CCHC6H4-4-R)(dppm)2]BF4 ([1]BF4). These complexes can be isolated, or treated in situ with a suitable base (Proton Sponge, 1,8-bis(dimethylamino)naphthalene) to yield the mono-alkynyl complexes trans-RuCl(CCC6H4-4-R)(dppm)2 (2). Through similar reactions between cis-RuCl2(dppm)2 with 2 equiv. of alkyne, TlBF4 and base, trans-bis(alkynyl) complexes, trans-Ru(CCC6H4-4-R)2(dppm)2 (3), can be isolated when R is an electron withdrawing substituent (R = NO2, COOMe, CCSiMe3), whereas reactions with alkynes bearing electron donating substituents (R = OMe and Me) form cationic η3-butenynyl complexes [Ru(η3-{HC(C6H4-4-R)CCCC6H4-4-R})(dppm)2]+ ([4]+). This work highlights the importance of the electronic character of the alkyne in influencing product outcome.

Metal vinylidene complexes are widely encountered, or postulated, as intermediates in a range of important metal-mediated transformations of alkynes. However, fluorovinylidene complexes have rarely been described and their reactivity is largely unexplored. By making use of the novel outer-sphere electrophilic fluorination (OSEF) strategy we have developed a rapid, robust and convenient method for the preparation of fluorovinylidene and trifluoromethylvinylidene ruthenium complexes from non-fluorinated alkynes. Spectroscopic investigations (NMR and UV/Vis), coupled with TD-DFT studies, show that fluorine incorporation results in significant changes to the electronic structure of the vinylidene ligand. The reactivity of fluorovinylidene complexes shows many similarities to non-fluorinated analogues, but also some interesting differences, including a propensity to undergo unexpected C–F bond cleavage reactions. Heating fluorovinylidene complex [Ru(η5-C5H5)(PPh3)2(CC{F}R)][BF4] led to C–H activation of a PPh3 ligand to form an orthometallated fluorovinylphosphonium ligand. Reaction with pyridine led to nucleophilic attack at the metal-bound carbon atom of the vinylidene to form a vinyl pyridinium species, which undergoes both C–H and C–F activation to give a novel pyridylidene complex. Addition of water, in the presence of chloride, leads to anti-Markovnikov hydration of a fluorovinylidene complex to form an α-fluoroaldehyde, which slowly rearranges to its acyl fluoride isomer. Therefore, fluorovinylidenes ligands may be viewed as synthetic equivalents of 1-fluoroalkynes providing access to reactivity not possible by other routes.