Reaction rate constants and mechanistic detail of the Ni+ + butanone reaction

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm?1). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH? or C-C?H? σ-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each σ-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm?1. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C?H?) = (0.92 ± 0.08) × 10? s?1 and k(act)(C-CH?) = (0.37 ± 0.03) × 10? s?1. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C?H? σ-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products.     

Catalytic hydrogenation by triruthenium clusters: a mechanistic study with parahydrogen-induced polarization

The reactivity of the cluster family [Ru(3)(CO)(12-x)(L)(x)] (in which L=PMe(3), PMe(2)Ph, PPh(3) and PCy(3), x=1-3) towards hydrogen is described. When x=2, three isomers of [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] are formed, which differ in the arrangement of their equatorial phosphines. Kinetic studies reveal the presence of intra- and inter-isomer exchange processes with activation parameters and solvent effects indicating the involvement of ruthenium-ruthenium bond heterolysis and CO loss, respectively. When x=3, reaction with H(2) proceeds to form identical products to those found with x=2, while when x=1 a single isomer of [Ru(3)(H)(mu-H)(CO)(10)(L)] is formed. Species [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] have been shown to play a kinetically significant role in the hydrogenation of an alkyne substrate through initial CO loss, with rates of H(2) transfer being explicitly determined for each isomer. A less significant secondary reaction involving loss of L yields a detectable product that contains both a pendant vinyl unit and a bridging hydride ligand. Competing pathways that involve fragmentation to form [Ru(H)(2)(CO)(2)(L)(alkyne)] are also observed and shown to be favoured by nonpolar solvents. Kinetic data reveal that catalysis based on [Ru(3)(CO)(10)(PPh(3))(2)] is the most efficient although [Ru(3)(H)(mu-H)(CO)(9)(PMe(3))(2)] corresponds to the most active of the detected intermediates.     

Interactions in metal clusters : III. Racemization barriers and reversible hydrogen addition of a series of chiral clusters

The barriers to racemization in C₂D₄Cl₂ of [Ir{Fe(P(p-tolyl)₂)(CO)₂(η⁵-C₅H₅)}₂]⁺ BF₄^? and [Ir{Ru(P(p-tolyl)₂)(CO)₂(η⁵-C₅H₅)}₂]⁺ BF₄^? are 17.5 and 17.8 kcal/mol respectively. Both clusters add H₂ reversibly at 1 atmosphere and room temperature. The former also forms an adduct with H₂ and CO when exposed to both gases simultaneously.     

A unifying electron-counting rule for macropolyhedral boranes, metallaboranes, and metallocenes.

A generally applicable electron-counting rule-the mno rule-that integrates macropolyhedral boranes, metallaboranes, and metallocenes and any combination thereof is presented. According to this rule, m + n + o number of electron pairs are necessary for a macropolyhedral system to be stable. Here, m is the number of polyhedra, n is the number of vertices, and o is the number of single-vertex-sharing condensations. For nido and arachno arrangements, one and two additional pairs of electrons are required. Wade's n + 1 rule is a special case of the mno rule, where m = 1 and o = 0. B20H16, for example has m = 2 and n = 20, leading to 22 electron pairs. Ferrocene, with two nido polyhedral fragments, has m = 2, n = 11, and o = 1, making the total 2 + 11 + 1 + 2 = 16. The generality of the mno rule is demonstrated by applying it to a variety of known macropolyhedral boranes and heteroboranes. We also enumerate the various pathways for condensation by taking icosahedral B12 as the model. The origin of the mno rule is explored by using fragment molecular orbitals. This clearly shows that the number of skeletal bonding molecular orbitals of two polyhedral fragments remains unaltered during exohedral interactions. This is true even when a single vertex is shared, provided the common vertex is large enough to avoid nonbonding interactions of adjacent vertices on either side. But the presence of more than one common vertex results in the sharing of surface orbitals thereby, reducing the electronic requirements.     

Novel cyclic tetraselenides of mannose: synthesis and mechanistic studies

In this Letter, we disclose the synthesis of novel cyclic tetraselenides starting from mannose which are very unusual and rare and have been synthesised for the first time. The structures are confirmed by X-ray analysis. The reactivity of the reagent tetraethylammonium tetraselenotungstate, (Et₄N)₂WSe₄ has been compared with the well-known selenium transfer reagents Li₂Se₂ and Na₂Se₂. A tentative reaction mechanism has been proposed.     

Novel metal cluster complexes synthesized by matrix deposition of mass-selected clusters

Metal complexes produced by depositing size selected Fe and Ag cluster cations in N₂ and O₂ matrices respectively are studied by infrared spectroscopy. Unknown species such as Fe(N₂)x, Fe₃ (N₂)x and Ag₃(O₂)x are observed. The IR spectra of Ag⁺, Ag ₂ ⁺ and Ag ₉ ⁺ in excess O₂ indicate that no complexes involving molecular oxygen are formed. However, the strong silver cluster UV-visible absorptions detected in Ar matrices disappear in the oxygen matrices, suggesting that silver-oxygen complexes are formed with dissociated oxygen.     

Metallaborane reaction chemistry. Part 12. Some interactions of acetylenes and isocyanides with selected metallaboranes

Compared to the chemistry associated with the basic syntheses and structures of the metallaboranes, their reaction chemistry is relatively uninvestigated. To illustrate the potential variety of such reaction chemistry, a linked overview of some previously reported and previously unreported reactions of the nido-6-metalladecaboranes [(PPh₃)₂HIrB₉H₁₃], [(PPh₃)(Ph₂PC₆H₄)HIrB₉H₁₂], [(η⁶-C₆Me₆)RuB₉H₁₃], [(η⁶-MeC₆H₄^isoPr)RuB₉H₁₃] and [(η⁵-C₅Me₅)RhB₉H₁₃] with acetylenes and isocyanides is presented, together with some related chemistry derived from the arachno-type 4-metallanonaboranes [(PMe₂Ph)₂PtB₈H₁₂] and [(PMe₃)₂(CO)HIrB₈H₁₂]. Reductions, oligomerisations, and reductive oligomerisations of the unsaturated species are observed, as well as complete or partial incorporation of carbon and nitrogen hetero atoms into the metallaborane clusters.¹     

Novel addition of nitroalkanes to o-quinones

A novel addition reaction of nitroalkanes to o-quinones is found to give 1,3-dioxole derivatives.     

Chiral phosphoramide-catalyzed enantioselective addition of allylic trichlorosilanes to aldehydes. Preparative and mechanistic studies with monodentate phosphorus-based amides

The addition of allylic trichlorosilanes to benzaldehyde promoted by chiral phosphoramides to give the enantioenriched homoallylic alcohol has been investigated. In a survey of Lewis bases as activators for the addition of allyltrichlorosilane to benzaldehyde, phosphorus-based amides have been found to be the most effective promoters. To achieve asymmetric induction, chiral phosphoric triamides derived from chiral diamines have been developed and applied in the allylation reaction albeit with modest enantioselectivities. The addition of 2-butenylsilanes was highly diastereoselective, suggesting a closed, chair-like transition structure. A detailed mechanistic study has been carried out to probe into the origin of activation. From a combination of nonlinear effects and kinetics studies, the reaction was found to likely involve two phosphoramides in both the rate and stereochemistry determining steps. These studies provided the background for the development of highly selective and reactive catalysts.     

Palladium-catalyzed desulfitative conjugate addition of aryl sulfinic acids and direct ESI-MS for mechanistic studies

A new and efficient method for palladium(II) catalytic desulfitative conjugate addition of arylsulfinic acids with α,β-unsaturated carbonyl compound has been developed. The key reacting intermediates including aryl Pd(II) sulfinic intermediate, aryl Pd(II), and C═O-Pd complexes were captured by ESI-MS/MS, which provide new experimental evidence for the understanding of addition mechanism.     

Further mechanistic detail on the fluxional behavior of (η1-cyclopentadienyl)iron complexes

NMR studies on several complexes of type (η⁵-C₅H₅)Fe(CO)L(η¹-C₅H₄R) indicate that the fluxional process in these molecules involves a suprafacial migration with retention of configuration of the migrating group.     

Novel method for geometry optimization of molecular clusters: application to benzene clusters

A heuristic and unbiased method for searching optimal geometries of clusters of nonspherical molecules was constructed from the algorithm recently proposed for Lennard-Jones atomic clusters. In the method, global minima are searched by using three operators, interior, surface, and orientation operators. The first operator gives a perturbation on a cluster configuration by moving molecules near the center of mass of a cluster, and the second one modifies a cluster configuration by moving molecules to the most stable positions on the surface of a cluster. The moved molecules are selected by employing a contribution of the molecules to the potential energy of a cluster. The third operator randomly changes the orientations of all molecules. The proposed method was applied to benzene clusters. It was possible to find new global minima for (C6H6)11, (C6H6)14, and (C6H6)15. Global minima for (C6H6)16 to (C6H6)30 are first reported in this article.     

Novel imidazole derivatives as potential agrochemicals: Synthetical and mechanistic studies

The importance of the imidazole nucleus is briefly outlined and some naturally occurring derivatives listed. The problem associated with direct alkylation on the imino nitrogen is discussed and synthesis of 1-alkyl derivatives by the thermal decarboxylation of 1-alkoxycarbonylimidazoles examined as a possible alternative. Spectroscopic investigation of this mechanism is reported. The fungal threat to plantains in the tropics by black siga-toka and the option of chemical control are discussed. Synthesis of potential fungicides are reported.     

Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study

The kinetics of the reaction between aqueous solutions of Na(2)[Fe(CN)(5)NO].2H(2)O (sodium pentacyanonitrosylferrate(ii), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling ((15)NO), in the pH range 7.1-9.3, I = 1 M (NaCl). The main products were N-methyl-N-nitrosohydroxylamine (MeN(NO)OH) and [Fe(CN)(5)H(2)O](3-), characterized as the [Fe(CN)(5)(pyCONH(2))](3-) complex (pyCONH(2) = isonicotinamide). No reaction occurred with Me(2)NOH (N,N-dimethylhydroxylamine, Me(2)HA) as nucleophile. The rate law was: R = k(exp) [Fe(CN)(5)NO(2-)] x [MeN(H)OH] x [OH(-)], with k(exp) = 1.6 +/- 0.2 x 10(5) M(-2) s(-1), at 25.0 degrees C, and DeltaH(#) = 34 +/- 3 kJ mol(-1), DeltaS(#) = -32 +/- 11 J K(-1) mol(-1), at pH 8.0. The proposed mechanism involves the formation of a precursor associative complex between SNP and MeHA, followed by an OH(-)-assisted reversible formation of a deprotonated adduct, [Fe(CN)(5)(N(O)NMeOH)](3-), and rapid dissociation of MeN(NO)OH. In excess SNP, the precursor complex reacts through a competitive one-electron-transfer path, forming the [Fe(CN)(5)NO](3-) ion with slow production of small quantities of N(2)O. The stoichiometry and mechanism of the main adduct-formation path are similar to those previously reported for hydroxylamine (HA) and related nucleophiles. The nitrosated product, MeN(NO)OH, decomposes thermally at physiological temperatures, slowly yielding NO.     

Deltahedral germanium clusters: insertion of transition-metal atoms and addition of organometallic fragments

Reactions of nine-atom deltahedral clusters of germanium with Ni(COD)2 and/or Ni(PPh3)2(CO)2 in ethylenediamine yielded the Ni-centered heteroatomic 10-atom clusters [Ni@(Ge9Ni-CO)]2- and [Ni@(Ge9Ni-en)]3-, as well as the empty 10-atom heteroatomic cluster [Ge9Ni-CO]3-. A ligand exchange reaction between [Ni@(Ge9Ni-CO)]2- and potassium phenylacetylide produced the organically functionalized species [Ni@(Ge9Ni-CCPh)]3-. The empty cluster [Ge9Ni-CO]3- is a bicapped square antiprism where one of the capping vertexes is the nickel atom. The other three clusters are tricapped trigonal prisms where an additional 10th vertex of monoligated nickel caps a triangular base of the trigonal prism. As a result of this, that base opens up, and the distances within it become nonbonding. This ensures that all atoms of the cluster are equidistant from the central nickel atom, i.e., the cluster is very close to spherical. All species were structurally characterized in crystalline compounds with [K-(2,2,2-crypt)]+ countercations. They were also characterized in solution by mass spectrometry, IR, and 13C NMR.     

Oxidative addition of 2-formyl-furan and related pyrrole and thiophene compounds at triosmium clusters

The bridging acyl complexes [Os₃H(μ-COC₄H₃X)(CO)₁₀] (X = NH, O, or S) have been prepared by oxidative addition of the 2-formyl derivatives of pyrrole, furan, or thiophene (C₄H₃XCHO) at [Os₃(CO)₁₀(MeCN)₂] with cleavage of the aldehydic CH bonds. On heating double decarbonylation of the acyl complexes occurs, to afford high yields of the compounds [Os₃H₂(CO)₉(μ₃-C₄H₂X)], reported previously for X = NH or O. For X = NH, two isomers with this formulation were characterised by ¹H NMR and IR data; the one containing the μ₃-2,3-C₄H₃N ligand isomerises to one containing μ₃-1,2-C₄H₃N. The direct reaction of pyrrole with [Os₃(CO)₁₂] has been re-examined at lower temperatures than before, and observed to give new products, including [Os₃H(CO)₁₀(C₄H₄N)], which contains a bridging non-aromatic tautomeric form of pyrrole. The ability of Os₃ clusters to stabilize non-aromatic tautomers of aromatic ligands is discussed.     

The relevance of boranes and metallaboranes to the structures of p-block element nanoparticles

Recent d-block element metallaborane chemistry, in which metal identity is varied with constant ancillary ligand, demonstrates how the rising energy of the d orbitals as one moves to earlier metals gives rise to non spherical cluster shapes that permit low formal cluster electron counts. In essence, the separation of frontier orbitals from “nonbonding” orbitals required by the isolobal analogy breaks down and the resulting mixing generates additional high-lying empty orbitals concurrently with shape change. A very similar mechanism explains recent p-block cluster chemistry albeit with variation in extent of external cluster ligation as the variable and separation of external lone pair orbitals from cluster bonding as the problem. Sensible, novel explanations of the shape/electron count relationships can be discovered for large group 13 clusters by recognizing the perturbation in cluster orbital energies when stabilization by ligand interactions is removed. These observations are pertinent to an understanding of large p-block clusters with internal atoms often referred to as nanoparticles.     

Computational study of structures and properties of metallaboranes: cobalt bis(dicarbollide)

A density functional study at the BP86/AE1 level is presented for the cobalt bis(dicarbollide) ion [3-Co-(1,2-C2B9H11)2]- (1) and selected isomers and rotamers thereof. Rotation of the two dicarbollide moieties with respect to each other is facile, as judged by the small energetic separation of the three rotamers located (within 11 kJ mol(-1)) and by the low barriers for their interconversion (at most 41 kJ mol(-1)). Among the isomers differing in carbon atom positions that contain two equivalent dicarbollide ligands, the 1,7 ("carbon apart") form [2-Co-(1,7-C2B9H11)2]- is the most stable, 121 kJ mol(-1) below 1. The electronic structure of 1 is characterized in terms of molecular orbitals, population analysis, and excitation energies from time-dependent density functional theory, relevant to UV/Vis spectroscopy. Experimental 11B NMR chemical shifts of 1 are reproduced to better than 5 ppm at the GIAO-B3LYP/II' level, and the computed delta(11B) values are only little affected by rotational averaging or the presence of a polarizable continuum. Larger such effects are found for the as-yet unknown 59Co chemical shift, for which a value in the range between -1800 and -2400 ppm is predicted. Even though the accuracy achieved for the theoretical delta(11B) values is somewhat lower than that for heteroboranes at conventional ab initio levels, the level of density functional employed can afford qualitatively reliable chemical shifts, which can be useful in assignments and structural refinements of heteroboranes containing transition metal.