Mn(III) O

Polyurethanes are synthesized on industrial scale by the reaction of diisocyanates with diols in the presence of catalysts which are commonly based on tin complexes and amines. However, due to the toxicity and volatility of these tin catalysts and amines, there is the need to develop new catalysts that are more environmentally benign. Herein, we report the synthesis of O^N^O pincer-ligated Mn(III) and Fe(III) complexes that serve as suitable catalysts for urethane formation and are stable to hydrolysis as predicted by computations and observed experimentally. The O^N^O pincer scaffold is vital to the activity of these catalysts, simultaneously ensuring increased solubility in the reaction medium as well as providing a stable framework upon dissociation of co-ligands in the catalytic cycle. In silico mechanistic investigations for urethane formation show that the stabilization of active species in square-planar geometries enabled by these O^N^O ligands permit the simultaneous coordination of alcohol and isocyanate in suitable configuration at the metal center.

Sulfur monoxide thermal release from an anthracene-based precursor, spectroscopic identification, and transfer reactivity.

Sulfur monoxide (SO) is a highly reactive molecule and thus, eludes bulk isolation. We report here on synthesis and reactivity of a molecular precursor for SO generation, namely 7-sulfinylamino-7-azadibenzonorbornadiene (1). This compound has been shown to fragment readily driven by dinitrogen expulsion and anthracene formation on heating in the solid state and in solution, releasing SO at mild temperatures (<100 °C). The generated SO was detected in the gas phase by MS and rotational spectroscopy. In solution, 1 allows for SO transfer to organic molecules as well as transition metal complexes.

Reactivity of Gold Complexes towards Elementary Organometallic Reactions.

For a while, the reactivity of gold complexes was largely dominated by their Lewis acid behavior. In contrast to the other transition metals, the elementary steps of organometallic chemistry-oxidative addition, reductive elimination, transmetallation, migratory insertion-have scarcely been studied in the case of gold or even remained unprecedented until recently. However, within the last few years, the ability of gold complexes to undergo these fundamental reactions has been unambiguously demonstrated, and the reactivity of gold complexes was shown to extend well beyond π-activation. In this Review, the main achievements described in this area are presented in a historical context. Particular emphasis is set on mechanistic studies and structure determination of key intermediates. The electronic and structural parameters delineating the reactivity of gold complexes are discussed, as well as the remaining challenges.

Oxidative addition of carbon-carbon bonds to gold.

The oxidative addition of strained CC bonds (biphenylene, benzocyclobutenone) to DPCb (diphosphino-carborane) gold(I) complexes is reported. The resulting cationic organogold(III) complexes have been isolated and fully characterized. Experimental conditions can be adjusted to obtain selectively acyl gold(III) complexes resulting from oxidative addition of either the C(aryl)C(O) or C(alkyl)C(O) bond of benzocyclobutenone. DFT calculations provide mechanistic insight into this unprecedented transformation.

Mechanisms of syn-insertion of alkynes and allenes into gold-silicon bonds: a comprehensive experimental/theoretical study.

A detailed mechanistic study is reported for the syn-insertion of alkynes and allenes in the Au-Si bonds of complexes (R3P)Au-SiR'Ph2 (R = Ph, Me and R' = t-Bu, Ph). Kinetic experiments indicate that (i) the reaction is first-order in alkyne and gold silyl complex and (ii) it requires a rather low enthalpy of activation and a relatively large negative entropy of activation [ΔH(‡) = 13.7 (±1.6) kcal·mol(-1) and ΔS(‡) = -32.0 (±5.0) cal·mol(-1)·K(-1) for the reaction of (Ph3P)Au-Si(t-Bu)Ph2 with methyl propiolate], in line with a bimolecular associative transformation. The different mechanistic pathways have been explored by DFT calculations. Accordingly, the reaction is found to proceed via a two-step inner-sphere mechanism: (i) first, the alkyne coordinates to the gold silyl complex to form a π-complex; (ii) the subsequent migratory insertion step is rate determining and occurs in a concerted manner. Provided dispersion effects are taken into account (B97D functional), the enthalpy of activation estimated theoretically [ΔH(‡) = 11.5 kcal·mol(-1)] is in good agreement with that measured experimentally. The influence of the π substrate (methyl propiolate, dimethyl acetylene dicarboxylate, phenyl acetylene, ethyl 2,3-butadienoate) has been analyzed theoretically, and the regioselectivity of the insertion has been rationalized. In particular, the unexpected selectivity observed experimentally with the allene is shown to result from the insertion of the terminal nonactivated C═C double bond into the Au-Si bond of (Ph3P)Au-SiPh3, followed by an original isomerization (Au/Si exchange process). This study provides unambiguous evidence for coordination-insertion at gold and thereby strongly supports the possible occurrence of inner-sphere mechanisms during the functionalization of alkynes and allenes.

Facile oxidative addition of aryl iodides to gold(I) by ligand design: bending turns on reactivity.

Thanks to rational ligand design, the first gold(I) complexes to undergo oxidative addition of aryl iodides were discovered. The reaction proceeds under mild conditions and is general. The ensuing aryl gold(III) complexes have been characterized by spectroscopic and crystallographic means. DFT calculations indicate that the bending induced by the diphosphine ligand plays a key role in this process.

Enhanced π-backdonation from gold(I): isolation of original carbonyl and carbene complexes.

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π-backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π-bond character.

Direct evidence for intermolecular oxidative addition of σ(Si-Si) bonds to gold.

Oxidative addition plays a major role in transition-metal catalysis, but this elementary step remains very elusive in gold chemistry. It is now revealed that in the presence of GaCl3, phosphine gold chlorides promote the oxidative addition of disilanes at low temperature. The ensuing bis(silyl) gold(III) complexes were characterized by quantitative (31)P and (29)Si NMR spectroscopy. Their structures (distorted Y shape) and the reaction profile of σ(Si-Si) bond activation were analyzed by DFT calculations. These results provide evidence for the intermolecular oxidative addition of σ(Si-Si) bonds to gold and open promising perspectives for the development of new gold-catalyzed redox transformations.

π Complexes of P

Tricoordinate gold(i) π-complexes containing P-based chelating ligands (P^P and P^N) were prepared. The structure of the gold(i) styrene complexes has been analysed in detail based on NMR and XRD data. The P^N complex is a competent catalyst for indole alkylation. The reaction proceeds with complete C3 and Markovnikov selectivity.

Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent.

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H2CN2 A (1, A = C14H10 or anthracene). The synthesis of this hydrazone is reported from Carpino's hydrazine H2N2 A through treatment with paraformaldehyde. Compound 1 has been found to be an easy-to-handle solid that does not exhibit dangerous heat or shock sensitivity. Effective umpolung of the diazomethane unit imbues 1 with electrophilicity at the methylene carbon center. Its reactivity with nucleophiles such as H2CPPh3 and N-heterocyclic carbenes is exploited for C[double bond, length as m-dash]C bond formation with elimination of dinitrogen and anthracene. Similarly, 1 is demonstrated to deliver methylene to a nucleophilic singlet d2 transition metal center, W(ODipp)4 (2), to generate the robust methylidene complex [2[double bond, length as m-dash]CH2]. This behavior is contrasted with that of the Wittig reagent H2CPPh3, a more traditional and Brønsted basic methylene source that upon exposure to 2 contrastingly forms the methylidyne salt [MePPh3][2[triple bond, length as m-dash]CH].

Terminal tungsten pnictide complex formation through pnictaethynolate decarbonylation.

Tungsten(iv) tetrakis(2,6-diisopropylphenoxide) (1) has been demonstrated to be a competent platform for decarbonylative formation of anionic terminal pnictide complexes upon treatment with pnictaethynolate anions: cyanate, 2-phosphaethynolate, and 2-arsaethynolate. These transformations constitute the first examples of terminal phosphide and arsenide complex formation at a transition metal center from OCP- and OCAs-, respectively. The phosphide and arsenide complexes are also the first to be isolated in a tetragonal, all-oxygen ligand environment. The scalar NMR coupling constants between tungsten-183 and nitrogen-15 or phosphorus-31 have been measured and contextualized using natural bond orbital (NBO) methods in terms of s orbital character in the σ bonding orbital and pnictide lone pair.

An exploding N-isocyanide reagent formally composed of anthracene, dinitrogen and a carbon atom.

Targeted as an example of a compound composed of a carbon atom together with two stable neutral leaving groups, 7-isocyano-7-azadibenzonorbornadiene, CN2A (1, A = C14H10 or anthracene) has been synthesized and spectroscopically and structurally characterized. The terminal C atom of 1 can be transferred: mesityl nitrile oxide reacts with 1 to produce carbon monoxide, likely via intermediacy of the N-isocyanate OCN2A. Reaction of 1 with [RuCl2(CO)(PCy3)2] leads to [RuCl2(CO)(1)(PCy3)2] which decomposes unselectively: in the product mixture, the carbide complex [RuCl2(C)(PCy3)2] was detected. Upon heating in the solid state or in solution, 1 decomposes to A, N2 and cyanogen (C2N2) as substantiated using molecular beam mass spectrometry, IR and NMR spectroscopy techniques.

Synthesis of a ruthenium bis(diisopropylamino(isocyano)borane) complex from the activation of an amino(cyano)borane.

The reactivity of the amino(cyano)borane (i)Pr(2)NBH(CN) (2) towards [RuH(2)(η(2)-H(2))(2)(PCy(3))(2)] (3) was investigated. Our study reveals that the formation of the bis(isocyanoborane) ruthenium complex [RuH(CN)(CN-BHN(i)Pr(2))(2)(PCy(3))(2)] (6) occurs via the intermediacy of the bis(σ-B-H) ruthenium complex [RuH(CN)(H(2)BN(i)Pr(2))(PCy(3))(2)] (5). This transformation involves BCN linkage isomerisation in 2. The diisopropylaminoborane ligand in 5 can be displaced to ultimately produce the bis(σ-borane) complex [RuH(2)(η(2):η(2)-H(2)BN(i)Pr(2))(PCy(3))(2)] (4) as a by-product.