Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3

Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism

The synthesis and structures of nitrile complexes of V(N[^tBu]Ar)₃, 2 (Ar = 3,5-Me₂C₆H₃), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for 2 than for the related complex Mo(N[^tBu]Ar)₃, 1. Kinetic studies reveal similar rate constants for nitrile binding to 2, but the activation parameters depend critically on the nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol^-1, and activation entropies from -9 to -28 cal·mol^-1·K^-1 in an opposing manner. Density functional theory (DFT) calculations provide a plausible explanation supporting the formation of a π-stacking interaction between a pendant arene of the metal anilide of 2 and the arene substituent on the incoming nitrile in favorable cases. Data for ligand binding to 1 do not exhibit this range of activation parameters and are clustered in a small area centered at ΔH^‡ = 5.0 kcal·mol^-1 and ΔS^‡ = -26 cal·mol^-1·K^-1. Computational studies are in agreement with the experimental data and indicate a stronger dependence on electronic factors associated with the change in spin state upon ligand binding to 1.

Reduction of Benzonitriles via Osmium-Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers

The reduction of the N≡C bond of benzonitriles promoted by OsH₆(PⁱPr₃)₂ (1) has been studied. Complex 1 releases a H₂ molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH₄{κ¹- N-(N≡CC₆H₃Me₂)}(PⁱPr₃)₂ (2), which is thermally stable toward the insertion of the nitrile into one of the Os-H bonds. In contrast to 2,6-dimethylbenzonitrile, benzonitrile and 2-methylbenzonitrile undergo insertion, via Os(η²-N≡CR) intermediates, to give the azavinylidene derivatives OsH₃(═N═CC₆H₄R)(PⁱPr₃)₂ [R = H (3) or Me (4)]. The analysis by means of computational tools (EDA-NOCV) of the bonding situation in these compounds suggests that the donor-acceptor nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor-acceptor and pure electron-sharing bonding modes. The N atom is strongly nucleophilic, whereas one of the hydrides is electrophilic. In spite of the different nature of these centers, the migration of the latter to the N atom is kinetically prevented. However, the use of water as a proton shuttle allows hydride migration, as a consequence of a significant decrease in the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH₃{κ²- N, C-(NH═CHC₆H₃R)}(PⁱPr₃)₂ [R = H (5) or Me (6)]. The presence of electrophilic and nucleophilic centers in 3 confers upon it the ability to activate σ-bonds, including H₂ and pinacolborane (HBpin). The reaction with the latter gives OsH₃{κ²- N, C-[N(Bpin)═CHC₆H₄]}(PⁱPr₃)₂ (7).

Copper-catalyzed cascade addition/cyclization: highly efficient access to phosphonylated quinoline-2,4(1H,3H)-diones

A novel Cu-catalyzed addition/cyclization cascade of o-cyanoarylacrylamide has been developed for the synthesis of various phosphonylated quinoline-2,4(1H,3H)-diones.

NO2 bond cleavage by MoL3 complexes

The cleavage of one N-O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N((t)Bu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η(2)-N,O complex (7 kJ mol(-1)). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol(-1)). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol(-1)via the triplet η(1)-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η(1)-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.

Experimental and computational studies on the formation of cyanate from early metal terminal nitrido ligands and carbon monoxide

DFT and experimental studies are used to elucidate key aspects in the design of a transition metal complex that mediates the reduction of dinitrogen by carbon monoxide and an electron source through a terminal metal nitride complex.

Nitrile-nitrile C-C coupling at group 4 metallocenes to form 1-metalla-2,5-diaza-cyclopenta-2,4-dienes: synthesis and reactivity

The CC coupling of aryl nitriles at Group 4 metallocenes leads to unusual ring-strained 1-metalla-2,5-diaza-cyclopenta-2,4-dienes. The structural, energetic, and chemical properties of these complexes are described. The reactions of these compounds towards CH3 CN, H2 , CO2 , and HCl usually lead to the release of one nitrile and its replacement by different co-substrates.

The importance of precursor and successor complex formation in a bimolecular proton-electron transfer reaction

The transfer of a proton and an electron from the hydroxylamine 1-hydroxyl-2,2,6,6-tetramethylpiperidine (TEMPOH) to [Co(III)(Hbim)(H(2)bim)(2)](2+) (H(2)bim = 2,2'-biimidazoline) has an overall driving force of DeltaG degrees = -3.0 +/- 0.4 kcal mol(-1) and an activation barrier of DeltaG(degrees) = 21.9 +/- 0.2 kcal mol(-1). Kinetic studies implicate a hydrogen-bonded "precursor complex" at high [TEMPOH], prior to proton-electron (hydrogen-atom) transfer. In the reverse direction, [Co(II)(H(2)bim)(3)](2+) + TEMPO, a similar "successor complex" was not observed, but upper and lower limits on its formation have been estimated. The energetics of formation of these encounter complexes are the dominant contributors to the overall energetics in this system: DeltaG degrees ' for the proton-electron transfer step is only -0.3 +/- 0.9 kcal mol(-1). Thus, formation of the precursor and successor complexes can be a significant component of the thermochemistry for intermolecular proton-electron transfer, particularly in the low-driving-force regime, and should be included in quantitative analyses.