Π-coordinated Complex Research Articles

Si–CN Bond Cleavage of Silyl Cyanides by an Iron Catalyst. A New Route of Silyl Cyanide Formation

Abstract The reaction of silyl cyanide (R3SiCN) with hydrosilane (R′3SiH) in the presence of a catalytic amount of [(η5-C5H5)Fe(CO)2Me] formed R′3SiCN and R3SiH. This reaction involves Si–CN bond cleavage and provides a new method for the preparation of silyl cyanides. DFT calculation showed that coordinatively unsaturated [(η5-C5H5)Fe(CO)(SiMe3)] reacts exothermically with Me3SiCN to give the CN π-coordinated complex, [(η5-C5H5)Fe(CO)(η2-Me3SiCN)(SiMe3)], followed by exothermic silyl migration from the iron to the nitrogen atom of the η2-coordinated Me3SiCN with the activation energy of 8.8 kcal mol−1 to give [(η5-C5H5)Fe(CO)(Me3SiC=NSiMe3-κC,κN)]. The complex is one of the intermediates in the catalytic cycle. The related complex [(η5-C5Me5)Fe(CO)(κ-C,N-t-BuMe2SiC=NSiPh3)] was isolated in the reaction of [(η5-C5Me5)Fe(CO)(py)(SiPh3)] with t-BuMe2SiCN and characterized by 1H, 13C, and 29Si NMR spectroscopy. The activation energy of the Si–CN bond cleavage in [(η5-C5H5)Fe(CO)(κ-C,N-Me3SiC=NSiMe3)] was evaluated by DFT calculation to be 33.7 kcal mol−1 which is comparable to that in the reaction of acetonitrile (32.9 kcal mol−1).

DFT Functional Benchmarking on the Energy Splitting of Chromium Spin States and Mechanistic Study of Acetylene Cyclotrimerization over the Phillips Cr(II)/Silica Catalyst

In this work, a two-state reaction mechanism for the acetylene cyclotrimerization over a cluster model for the Phillips Cr(II)/silica catalyst were systematically investigated using density functional theory (DFT). Since spin crossover phenomenon was confirmed in the catalytic cycle, an accurate prediction of the energy gap between low- and high-spin states is crucial for the description of a reaction involving a two-state reactivity. Therefore, a massive DFT functional benchmarking test has been conducted on the cluster model by taking a CASPT2 energy gap as a reference. Consequently, B3PW91* with 28% Hartree-Fock exchange energy was selected for the following mechanistic investigation. Each of the possible potential energy surface including singlet, triplet, and quintet surfaces was explored. On the quintet surface, the reaction begins with a coordination of an acetylene on the chromium center to generate a π-coordinated complex. The following oxidative coupling through further coordination with a second acetylene was predicted to be a two-step reaction to generate a chromacyclopentadiene species. This transformation was found to be energetically prohibitive by the presence of the transition state (5)TS[C-E] (ΔG(‡) = 31.1 kcal/mol). On the triplet surface, however, the coordination of an acetylene generates a chromacyclopropene species without showing any activation barrier. The second acetylene incorporation proceeding via a coordination on the chromium center followed by an insertion into a Cr-C σ-bond of the chromacyclopropene was predicted to be a facile reaction pathway (ΔG(‡) = 10.2 kcal/mol). The third acetylene was captured by the cluster model through the formation of a hydrogen bond. The later transformation on the triplet surface was found to be an intermolecular [4 + 2] cycloaddition to finish the cyclization. The lack of the aromaticity of the benzene ring in (3)L results in an uncompleted reaction pathway on a single triplet surface. Consequently, a two-state reaction pathway that is connected by two low-lying minimum-energy crossing points (MECPs) of the two surfaces is thus described. It is worthy of note that the third acetylene in the tri(acetylene)chromium complex captured by the cluster model only through the formation of a hydrogen bond rules out the [2 + 2 + 2] concerted one-step reaction pathway proposed by Zecchina et al. [Phys. Chem. Chem. Phys.2003, 5, 4414]. The singlet reaction profile is far higher in energy compared with that proceeded on the triplet and quintet surfaces.

Ru(η 6-1,3,5-cyclooctatriene)(η 2-dimethyl fumarate) 2: a novel, versatile zerovalent ruthenium complex with electron-deficient olefinic ligands

The reactivity of a novel zerovalent ruthenium complex, Ru(η 6-cot)(η 2-dmfm) 2 (cot = 1,3,5-cyclooctatriene, dmfm, =dimethyl fumarate), which is readily prepared from Ru(η 4-cod)(η 6-cot) (cod = 1,5-cyclooctadiene) and dmfm was examined. The reaction with monodentate phosphine or amine ligands gave Ru(η 6-cot)(dmfm)(L) (L = ligand) via dissociation of dmfm. Among bidentate phosphines, dppm (dppm = bis(diphenylphosphino)methane) reacted to give Ru(η 4-cot)(dmfm)(dppm) along with releasing a dmfm ligand. In the case of dppe (dppe = 1,2-bis(diphenylphosphino)ethane), two types of complexes were obtained depending on the reaction conditions, Ru(dmfm)(dppe) 2 and an alkyl alkenyl complex; in the formation of the latter complex, sp 2 C–H bond activation of dmfm occurred. Ru(η 4-cot)(dmfm)(N ⌢N) and Ru(dmfm) 2 (N ⌢N ⌢N) were formed by reacting with bidentate and tridentate nitrogen ligands. The reactions with arenes gave π-coordinated complexes, Ru(η 6-arene)(dmfm) 2. p-Quinones and a p-biqunone reacted to give Ru(η 6-cot)( p-quinone) and {Ru(η 6-cot)} 2( p-biquinone), respectively, along with the dissociation of two dmfm ligands. It was found that low-valent ruthenium complexes preferably bear both electron-donating and accepting ligands simultaneously to be thermodynamically stable.

Synthesis of Cyclic Diamino-Substituted Metal Carbene Complexes

A series of heterocyclic metal carbene complexes were prepared by the deoxygenation reaction of M(CO)6 (M = Cr, Mo, W) with RNH(CH2)nNPPh3 (R = H, Et, Ph; n = 2, 3, 4) to form the corresponding isocyanide complex, which subsequently underwent intramolecular cyclization to give (CO)5MCN(R)(CH2)nNH in good yields. None of the carbene complex reacted further with amine−phosphinimine RNH(CH2)nNPPh3 to give the bis(carbene) complex. Deprotonation of the N−H of the carbene complex followed by the reaction of alkyl iodides provided the N-alkylated product. In the case of N-allyl-substituted complexes (CO)5MCNR(CH2)2N(CH2CHCH2) (M = Cr, Mo, W), the carbon−carbon double bond underwent intramolecular ligand displacement of carbonyl ligand to yield the corresponding π-coordinated complex. Unlike the group VI metal carbonyl complexes, treatment of ReBr(CO)5 with 2 molar equiv of Ph3PN(CH2)2NH2 provided a bis(carbene) complex fac-(CO)3BrRe(CNHCH2CH2NH)2. All complexes are characterized by both spectral and elemental analyses. Complexes (CO)5MoCNHCH2CH2CH2NH (18) and fac-(CO)3BrRe(CNHCH2CH2NH)2 (42) were further characterized by X-ray single-crystal analysis.

The activation of the C-H bond in acetylene by second row transition metal atoms

The C-H activation reaction of acetylene by second row transition metal atoms has been studied including electron correlation of all valence electrons. Binding energies have been computed for both π-coordinated complexes and C-H insertion products. It is found that for most atoms the π-coordinated complexes are thermodynamically favoured, just as in the case for the corresponding ethylene reaction. The barrier height for the C-H insertion increases from acetylene to ethylene and to methane. This is in line with the experimental finding that there should be an inverse relation between C-H bond strengths and the difficulty to activate these bonds. To explain the detailed differences between the C-H activation of acetylene and ethylene, the interaction with two, rather than one, π- and π*-orbitals for acetylene is of key importance. The barrier height for the acetylene reaction increases significantly between niobium and molybdenum going to the right in the periodic table, just as for all oxidative addition reactions previously studied. The origin of this increase is that noibium has one empty 4d-orbital but for molybdenum all 4d-orbitals are occupied. Rhodium has the lowest barrier for C-H activation for all systems studied.

A theoretical study of the activation of the carbon-hydrogen bond in ethylene by second-row transition-metal atoms

Calculations including electron correlation of all the valence electrons have been performed for the activation of the C-H bond in ethylene by second row transition metal atoms. The resulting binding energies for the vinyl hydride products are compared to previously calculated binding energies for the π-coordinated ethylene complexes. For the atoms to the right the binding energy difference is 18-28 kcal/mol in favor of the π-coordinated complex. For the atoms to the left the binding energy difference is much smaller, and for zirconium as the only atom, the vinyl hydride product is lower in energy. The product energies and the barrier heights for the ethylene reaction are also compared to previous results for the corresponding methane reaction