Activation of Molecular Hydrogen over a Binuclear Complex with Rh2S2 Core: DFT Calculations and NMR Mechanistic Studies

Hydrosulfide complexes of the transition elements: diverse roles in bioinorganic, cluster, coordination, and organometallic chemistry

Molecules containing transition metal hydrosulfide linkages are diverse, spanning a variety of elements, coordination environments, and redox states, and carrying out multiple roles across several fields of chemistry.

Terminal alkyne insertion into a thiolate-bridged dirhodium hydride complex derived from heterolytic cleavage of H2

Thiolate-bridged dirhodium and diiridium complexes can facilely realize heterolytic cleavage of H2 across the metal-sulfur bond to generate the corresponding hydride bridged complexes. Furthermore, terminal alkynes can insert the Rh-H-Rh fragment to afford σ:π alkenyl bridged complexes and then convert to the corresponding alkenes in the presence of a reductant and an acid.

Mechanistic Aspects of Redox-Induced Assembly and Disassembly of S-Bridged [2M-2S] Structures

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands.

Cooperative Catalysis at Metal-Sulfur Bonds

Cooperative catalysis has attracted tremendous attention in recent years, emerging as a key strategy for the development of novel atom-economic and environmentally more benign catalytic processes. In particular, Noyori-type complexes with metal-nitrogen bonds have been extensively studied and evolved as privileged catalysts in hydrogenation chemistry. In contrast, catalysts containing metal-sulfur bonds as the reactive site are out of the ordinary, despite their abundance in living systems, where they are assumed to play a key role in biologically relevant processes. For instance, the heterolysis of dihydrogen catalyzed by [NiFe] hydrogenase is likely to proceed through cooperative H-H bond splitting at a polar nickel-sulfur bond. This Account provides an overview of reported metal-sulfur complexes that allow for cooperative E-H bond (E = H, Si, and B) activation and highlights the potential of this motif in catalytic applications. In recent years, our contributions to this research field have led to the development of a broad spectrum of synthetically useful transformations catalyzed by cationic ruthenium(II) thiolate complexes of type [(DmpS)Ru(PR₃)]⁺BAr^F₄^- (DmpS = 2,6-dimesitylphenyl thiolate, Ar^F = 3,5-bis(trifluoromethyl)phenyl). The tethered coordination mode of the bulky 2,6-dimesitylphenyl thiolate ligand is crucial, stabilizing the coordinatively unsaturated ruthenium atom and also preventing formation of binuclear sulfur-bridged complexes. The ruthenium-sulfur bond of these complexes combines Lewis acidity at the metal center and Lewis basicity at the adjacent sulfur atom. This structural motif allows for reversible heterolytic splitting of E-H bonds (E = H, Si, and B) across the polar ruthenium-sulfur bond, generating a metal hydride and a sulfur-stabilized E⁺ cation. Hence, this activation mode provides a new strategy to catalytically generate silicon and boron electrophiles. After transfer of the electrophile to a Lewis-basic substrate, the resulting neutral ruthenium(II) hydride can either act as a hydride donor (reductant) or as a proton acceptor (Brønsted base); the latter scenario is followed by dihydrogen release. On the basis of this concept, the tethered ruthenium(II) thiolate complexes emerged as widely applicable catalysts for various transformations, which can be categorized into (i) dehydrogenative couplings [Si-C(sp²), Si-O, Si-N, and B-C(sp²)], (ii) chemoselective reductions (hydrogenation and hydrosilylation), and (iii) hydrodefluorination reactions. All reactions are promoted by a single catalyst motif through synergistic metal-sulfur interplay. The most prominent examples of these transformations are the first catalytic protocols for the regioselective C-H silylation and borylation of electron-rich heterocycles following a Friedel-Crafts mechanism.

Heterolytic activation of dihydrogen molecule by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex. Theoretical insights

Heterolytic activation of dihydrogen molecule (H2) by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex [Dmp(Dep)Ge(μ-S)(μ-OH)Ru(PPh3)](+) (1) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl) is theoretically investigated with the ONIOM(DFT:MM) method. H2 approaches 1 to afford an intermediate [Dmp(Dep)(HO)Ge(μ-S)Ru(PPh3)](+)-(H2) (2). In 2, the Ru-OH coordinate bond is broken but H2 does not yet coordinate with the Ru center. Then, the H2 further approaches the Ru center through a transition state TS2-3 to afford a dihydrogen σ-complex [Dmp(Dep)(HO)Ge(μ-S)Ru(η(2)-H2)(PPh3)](+) (3). Starting from 3, the H-H σ-bond is cleaved by the Ru and Ge-OH moieties to form [Dmp(Dep)(H2O)Ge(μ-S)Ru(H)(PPh3)](+) (4). In 4, hydride and H2O coordinate with the Ru and Ge centers, respectively. Electron population changes clearly indicate that this H-H σ-bond cleavage occurs in a heterolytic manner like H2 activation by hydrogenase. Finally, the H2O dissociates from the Ge center to afford [Dmp(Dep)Ge(μ-S)Ru(H)(PPh3)](+) (PRD). This step is rate-determining. The activation energy of the backward reaction is moderately smaller than that of the forward reaction, which is consistent with the experimental result that PRD reacts with H2O to form 1 and H2. In the Si analogue [Dmp(Dep)Si(μ-S)(μ-OH)Ru(PPh3)](+) (1Si), the isomerization of 1Si to 2Si easily occurs with a small activation energy, while the dissociation of H2O from the Si center needs a considerably large activation energy. Based on these computational findings, it is emphasized that the reaction of 1 resembles well that of hydrogenase and the use of Ge in 1 is crucial for this heterolytic H-H σ-bond activation.

Facile activation of dihydrogen by a phosphinito-bridged Pt(I)-Pt(I) complex

The phosphinito-bridged Pt(I) complex [(PHCy(2))Pt(mu-PCy(2)){kappa(2)P,O-mu-P(O)Cy(2)}Pt(PHCy(2))](Pt-Pt) (1) reversibly adds H(2) under ambient conditions, giving cis-[(H)(PHCy(2))Pt(1)(mu-PCy(2))(mu-H)Pt(2)(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy(2))(H)Pt(mu-PCy(2))(mu-H)Pt(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (3). DFT calculations indicate that the reaction of 1 with H(2) occurs through an initial heterolytic splitting of the H(2) molecule assisted by the phosphinito oxygen with breaking of the Pt-O bond and hydrogenation of the Pt and O atoms, leading to the formation of the intermediate [(PHCy(2))(H)Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-P(OH)Cy(2)}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H...H-O-P-Pt ring. Compound D is a labile intermediate which easily evolves into the final dihydride complex 2 through a facile (9-15 kcal mol(-1), depending on the solvent) hydrogen shift from the phosphinito oxygen to the Pt-Pt bond. Information obtained by addition of para-H(2) on 1 are in agreement with the presence of a heterolytic pathway in the 1 --> 2 transformation. NMR experiments and DFT calculations also gave evidence for the nonclassical dihydrogen complex [(PHCy(2))(eta(2)-H(2))Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (4), which is an intermediate in the dehydrogenation of 2 to 1 and is also involved in intramolecular and intermolecular exchange processes. Experimental and DFT studies showed that the isomerization 2 --> 3 occurs via an intramolecular mechanism essentially consisting of the opening of the Pt-Pt bond and of the hydrogen bridge followed by the rotation of the coordination plane of the Pt center with the terminal hydride ligand.

The useful properties of H2O as a ligand of a hydrogenase mimic

This paper investigates the required properties of Ru-coordinated ligands of a Ni-Ru based hydrogenase mimic. A series of ligands, including MeCN, pyridine, H(2)O and OH(-) were coordinated to Ru, with H(2)O being the only ligand to promote H(2)-activation. In addition, a tethered pyridyl moiety was synthesised and found to completely inhibit H(2)-activation. We conclude, therefore, that H(2)O is the ideal ligand for this mimic as a result of both its mild basicity and the availability of two lone pairs for simultaneous binding to Ru and H(2).

Reaction Chemistry of Complexes Containing PtH, PtSH, or PtS Fragments: From Their Apparent Simplicity to the Maze of Reactions Underlying Their Interconversion

The relevance of platinum in the reaction of thiophene and derivatives with homogeneous transition-metal complexes as models for hydrodesulfurization has led us to the study of the reaction chemistry of complexes containing Pt--H, Pt--SH, and Pt--S fragments. Exploration of the reactions triggered by addition of controlled amounts of Na2S or NaSH to [Pt2(H)2(mu-H)(dppp)2]ClO4 (1) has provided evidence of the formation of complexes [Pt2(mu-H)(mu-S)(dppp)2]ClO4 (2), [Pt(H)(SH)(dppp)] (3), [Pt2(mu-S)2(dppp)2] (4), [Pt2(mu-S)(dppp)2] (5) and [Pt(SH)2(dppp)], in which dppp denotes 1,3-bis(diphenylphosphanyl)propane. Consequently, complexes 1, 2, and 5 as well as the already reported 3, 4, and [Pt(SH)2(dppp)] have been obtained and fully characterized spectroscopically. Also the crystal structures of 1 and 2 have been solved. Complexes 1-5 constitute the main framework of the network of reactions that account for the evolution of 1 under various experimental conditions as shown in Scheme 1. Apparently, this network has complexes 2 and 4 as dead-ends. However, their reciprocal interconversion by means of the replacement of one bridging hydride or sulfide ligand in the respective {Pt(mu-H)(mu-S)Pt} and {Pt(mu-S)2Pt} cores enables the closure of the reaction cycle involving complexes 1-5. Theoretical calculations support the existence of the undetected intermediates proposed for conversion from 1 to 2 and from 3 to 2 and also account for the fluxional behavior of 1 in solution. The intermediates proposed are consistent with the experimental results obtained in comparable reactions carried out with labeled reagents, which have provided evidence that complex 1 is the source of the hydride ligands in complexes 2 and 3. Overall, our results show the strong dependence on the experimental conditions for the formation of complexes 1-5 as well as for their further conversion in solution.

A Counterintuitive Structural Effect of Metal–Metal Bond Protonation and Its Electronic Underpinnings

Protonation across the metal-metal bond in the complexes [(CO)(2)M(mu-dppm)(mu-PtBu(2))(mu-H)M(CO)(2)] (M=Fe or Ru, dppm=Ph(2)PCH(2)PPh(2)) induces M-M bond shortening of up to about 0.05 A. DFT calculations on simplified iron models reproduce this trend well. Conversely, the computations show that the M-M distance in the dimer [{Cp*Ir(CO)}(2)] lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the Ir-Ir bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation. An MO analysis indicated similar nature and evolution of the M-M bonding these cases. In particular, the HOMOs of the mono-hydrido cations [Cp(CO)M(mu-H)M(CO)Cp](+) (M=Ir, Co) have evident M-M bent-bond character, and hence subsequent protonation invariably causes a decrease in the bond index. The Fe(2) and Co(2) systems have also been analyzed with the quantum theory of atoms in molecules (QTAIM) method, but in no case was an M-M bond critical point located unless an artificially shorter M-M distance was imposed. However, the trends for the atoms-in-molecules (AIM) bond delocalization indexes delta(M-M) confirm the overall M-M bond weakening on protonation. In conclusion, all the computational results for the iron system indicate that the paradigm of a direct correlation between bond strength and distance is not always applicable. This is attributable to a very flat potential energy surface and various competing effects imposed by the bridging ligands.

Mechanistic Investigations of Imine Hydrogenation Catalyzed by Cationic Iridium Complexes

Complexes [IrH2(eta6-C6H6)(PiPr3)]BF4 (1) and [IrH2(NCMe)3(PiPr3)]BF4 (2) are catalyst precursors for homogeneous hydrogenation of N-benzylideneaniline under mild conditions. Precursor 1 generates the resting state [IrH2{eta5-(C6H5)NHCH2Ph}(PiPr3)]BF4 (3), while 2 gives rise to a mixture of [IrH{PhN=CH(C6H4)-kappaN,C}(NCMe)2(PiPr3)]BF4 (4) and [IrH{PhN=CH(C6H4)-kappaN,C}(NCMe)(NH2Ph)(PiPr3)]BF4 (5), in which the aniline ligand is derived from hydrolysis of the imine. The less hindered benzophenone imine forms the catalytically inactive, doubly cyclometalated compound [Ir{HN=CPh(C6H4)-kappaN,C}2(NH2CHPh2)(PiPr3)]BF4 (6). Hydrogenations with precursor 1 are fast and their reaction profiles are strongly dependent on solvent, concentrations, and temperature. Significant induction periods, minimized by addition of the amine hydrogenation product, are commonly observed. The catalytic rate law (THF) is rate = k[1][PhN=CHPh]p(H2). The results of selected stoichiometric reactions of potential catalytic intermediates exclude participation of the cyclometalated compounds [IrH{PhN=CH(C6H4)-kappaN,C}(S)2(PiPr3)]BF4 [S = acetonitrile (4), [D6]acetone (7), [D4]methanol (8)] in catalysis. Reactions between resting state 3 and D2 reveal a selective sequence of deuterium incorporation into the complex which is accelerated by the amine product. Hydrogen bonding among the components of the catalytic reaction was examined by MP2 calculations on model compounds. The calculations allow formulation of an ionic, outer-sphere, bifunctional hydrogenation mechanism comprising 1) amine-assisted oxidative addition of H2 to 3, the result of which is equivalent to heterolytic splitting of dihydrogen, 2) replacement of a hydrogen-bonded amine by imine, and 3) simultaneous H delta+/H delta- transfer to the imine substrate from the NH moiety of an arene-coordinated amine ligand and the metal, respectively.