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Four new CCC-NHC pincer Zr complexes have been synthesised, characterised, and used in mechanistic studies in the hydroamination/cyclisation of unactivated amino-alkenes. These Zr pre-catalysts will cyclise a primary amino-alkene, but no reaction was observed for a secondary amino-alkene even in the presence of a primary amine. The empirical rate law, experimentally determined activation parameters, and kinetic isotope effects (KIEs) are reported. Several possible mechanisms, including amido- versus imido-insertion and concerted-insertion versus [2 + 2] cycloaddition mechanisms, were modelled computationally at the PBEPBE level of theory with double-zeta quality basis sets. The formation of a catalytically relevant imido complex via the monoamido complexes was accompanied by in situ formation of ammonium salts of the substrates. The experimental and computational data are consistent with an imido-[2 + 2] cycloaddition mechanism for the CCC-NHC pincer diamido Zr complexes that follow saturation kinetics under catalytically relevant concentrations.

Romero and Piers recently reported a mechanistic study in which they detailed the intramolecular exchange of adjacent methylene groups of a ruthenacyclobutane and the intermolecular degenerate exchange of free ethylene with a second-generation Grubbs-type catalyst, a 14-electron ruthenacyclobutane, (NHC)Cl2Ru(CH2CH2CH2). These authors measured activation parameters for the intramolecular and intermolecular exchange processes (ΔG⧧ = 12.3 kcal mol-1 and ΔH⧧ = 13.2(5) kcal mol-1 and ΔS⧧ = −15(2) eu, respectively). The current study applies density functional theory calculations to address their proposed exchange mechanisms. For intramolecular exchange, the formation of a structure with coordinated ethylene proceeds from the cyclobutane species via a direct rotational bond-breaking transition state, which produces an ethylene which is essentially perpendicular to the remaining methylene unit; and the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (ΔGCH2Cl2...

A series of studies have been conducted by experimental and theoretical methods on the synthesis, structures, and reactions of CpRh boryl complexes that are likely intermediates in the rhodium-catalyzed regioselective, terminal functionalization of alkanes. The photochemical reaction of CpRh(eta(6)-C(6)Me(6)) with pinacolborane (HBpin) generates the bisboryl complex CpRh(H)(2)(Bpin)(2) (2), which reacts with neat HBpin to generate CpRh(H)(Bpin)(3) (3). X-ray diffraction, density functional theory (DFT) calculations, and NMR spectroscopy suggest a weak, but measurable, B-H bonding interaction. Both 2 and 3 dissociate HBpin and coordinate PEt(3) or P(p-Tol)(3) to generate the conventional rhodium(III) species CpRh(PEt(3))(H)(Bpin) (4) and CpRh[P(p-tol)(3)](Bpin)(2) (5). Compounds 2 and 3 also react with alkanes and arenes to form alkyl- and arylboronate esters at temperatures similar to or below those of the catalytic borylation of alkanes and arenes. Further, these compounds were observed directly in catalytic reactions. The enthalpies and free energies for generation of the 16-electron intermediate and for the C-H bond cleavage and B-C bond formation have been calculated with DFT. These results strongly suggest that the C-H bond cleavage process occurs by a metal-assisted sigma-bond metathesis mechanism to generate a borane complex that isomerizes if necessary to place the alkyl group cis to the boryl group. This complex with cis boryl and alkyl groups then undergoes B-C bond formation by a second sigma-bond metathesis to generate the final functionalized product.

The reaction of Ir4(CO)12 with an excess of Pt(PBu(t)3)2 at room temperature yielded the bis-Pt(PBu(t)3) adduct Ir4(CO)12[Pt(PBu(t)3)]2 (9), which contains two Pt(PBu(t)3) groups bridging opposite edges of a central Ir4 pseudotetrahedron. The same reaction at 110 degrees C yielded two new higher nuclearity complexes, Ir8(CO)12[Pt(PBu(t)3)]4 (10) and Ir6(CO)10[Pt(PBu(t)3)]4 (11). Compound 10 consists of a central Ir4(CO)4 tetrahedron with four edge-bridging Ir(CO)2 groups and four Pt(PBu(t)3) groups that are each bonded to Ir3 triangles of the Ir4 tetrahedron and two of the Ir(CO)2 groups. Compound 11 consists of a central Ir4(CO)4 pseudotetrahedron with two edge-bridging Ir(CO)2 groups and four Pt(PBu(t)3) groups; one Pt(PBu(t)3) group is bonded to five iridium atoms as found in 10; two are bonded to four iridium atoms, and one is bonded to one of the outer Ir2Pt triangles. Compound 11 reacted with hydrogen at 97 degrees C to give the new tetrahydrido complex Ir6(CO)8[Pt(PBu(t)3)]4(mu-H)4 (12). Compound 12 is formed by the loss of the two bridging carbonyl ligands from 11 and the addition of four hydrido ligands. All four new compounds were characterized by both 1H and 31P NMR and by single-crystal X-ray diffraction analyses. The bonding in 9 was studied by Fenske-Hall molecular orbital calculations, which in this case provides a delocalized bonding description for the Ir-Ir and Ir-Pt bonding, where the attachment of the 0 e- fragments of Pt(PR3) use Ir-Ir bonding orbitals of the Ir4(CO)12 cluster to form multicenter Pt-Ir bonds.

Publisher Summary This chapter discusses “Fenske–Hall” molecular orbital (MO) method, an approximate self-consistent-field (SCF) abinitio method that contains no empirical parameters that began almost 40 years ago in the research group of Richard F. Fenske. It demonstrates for transition metal complexes that the nonempirical Fenske–Hall (FH) approach provides qualitative results that are quite similar to the more rigorous treatment given by density functional theory (DFT) and are quite different from Hartree–Fock–Roothaan (HFR) calculations that have no electron correlation. For example, the highest occupied molecular orbital of ferrocene is metal based for both DFT and FH while it is ligand (cyclopentadienyl) based for HFR. In the doublet (S=1/2) cluster, Cp2Ni2(μ-S)2(MnCO)3, the unpaired electron is delocalized over the complex in agreement with the DFT and FH results, but localized on Mn in the HFR calculation. A brief description of the theory of FH calculations is used to rationalize the origin of its similarity to DFT.

The bis-phosphine compounds M(PBu t 3) 2 , M = Pd and Pt, readily eliminate one PBu t 3 ligand and transfer MPBu t 3 groups to the ruthenium-ruthenium bonds in the compounds Ru 3 (CO) 12 , Ru 6 (CO) 17 -(μ 6 -C), and Ru 6 (CO) 14 (η 6 -C 6 H 6 )(μ 6 -C) without displacement of any of the ligands on the ruthenium complexes. The new compounds, Ru 3 (CO) 12 [Pd(PBu t 3)] 3 , 10, and Ru 6 (CO) 17 (μ 6 -C)[Pd(PBu t 3)] 2 , 11, Ru 6 (CO) 17 (μ 6 -C)-[Pt(PBu t 3)] n , n = 1 (12), n = 2 (13), and Ru 6 (CO) 14 (η 6 -C 6 H 6 )(μ 6 -C)[Pd(PBu t 3)] n , n = 1 (15), n = 2 (16), have been prepared and structurally characterized. In most cases the MPBu t 3 groups bridge a pair of mutually bonded ruthenium atoms, and the associated Ru-Ru bond distance increases in length. Fenske-Hall calculations were performed on 10 and 11 to develop an understanding of the electron deficient metal-metal bonding. 10 undergoes a Jahn-Teller distortion to increase bonding interactions between neighboring Ru(CO) 4 and Pd(PBu t 3) fragments. 11 has seven molecular orbitals important to cluster bonding in accord with cluster electron-counting rules.

The reaction of Mn(2)(CO)(7)(mu-S(2)) with [CpNi(CO)](2) yielded the paramagnetic new compound Cp(2)Ni(2)Mn(CO)(3)(mu(3)-S)(2) (1) and a new hexanuclear metal product Cp(2)Ni(2)Mn(4)(CO)(14)(mu(6)-S(2))(mu(3)-S)(2) (2). Structurally, compound 1 contains two triply bridging sulfido ligands on opposite sides of an open Ni(2)Mn triangular cluster. EPR and temperature-dependent magnetic susceptibility measurements of 1 show that it contains one unpaired electron. The electronic structure of 1 was determined by Fenske-Hall molecular orbital calculations which show that the unpaired electron occupies a low lying antibonding orbital delocalized unequally across the three metal atoms. The selenium homologue Cp(2)Ni(2)Mn(CO)(3)(mu(3)-Se)(2) (3) was obtained from the reaction of a mixture of Mn(2)(CO)(10) and [CpNi(CO)](2) with elemental selenium and Me(3)NO.2H(2)O. It also has one unpaired electron. Compound 1 reacted with elemental sulfur to yield the dinickeldimanganese compound, Cp(2)Ni(2)Mn(2)(CO)(6)(mu(4)-S(2))(mu(4)-S(5)), 4, which can also be made from the reaction of Mn(2)(CO)(7)(mu-S(2)) with [CpNi(CO)](2) and sulfur. Compound 4 was converted back to 1 by sulfur abstraction using PPh(3). The reaction of Mn(2)(CO)(10) with [CpNi(CO)](2) in the presence of thiirane yielded the ethanedithiolato compound CpNiMn(CO)(3)(mu-SCH(2)CH(2)S) (5), which was also obtained from the reaction of Mn(4)(CO)(15)(mu(3)-S(2))(mu(4)-S(2)) with [CpNi(CO)](2) in the presence of thiirane. Compound 5 reacted with additional quantities of thiirane to yield the new compound CpNiMn(CO)(3)[mu-S(CH(2)CH(2)S)(2)], 6, which contains a 3-thiapentanedithiolato ligand that bridges the two metal atoms. Compound 6 was also obtained from the reaction of Mn(2)(CO)(10) with [CpNi(CO)](2) and thiirane. The molecular structures of the new compounds 1-6 were established by single-crystal X-ray diffraction analyses.

A recent crystal structure of beta-phosphoglucomutase from Lactococcus lactis is reported to contain a five-coordinate phosphorus with five oxygen ligands that is a high-energy reaction intermediate during the phosphoryl transfer in the isomerization of beta-glucose 1-phosphate to beta-glucose 6-phosphate. Subsequently, it has been suggested that this structure is a transition state analogue with a five-coordinate magnesium with two oxygen and three fluorine ligands. Two layer ONIOM(B3LYP:PM3MM) calculations have been performed to address the nature of this intermediate and the mechanism of the phosphoryl transfer. These calculations provide evidence that (1) the observed crystal structure is consistent with a five-coordinate magnesium (a stable transition state analogue), not a five-coordinate phosphorus (a phosphorane) as a high-energy intermediate, (2) the active site is stabilized by the extensive hydrogen-bonding network, (3) the transfer of the phosphoryl group proceeds through a moderate barrier (14 kcal mol-1) five-coordinate phosphorus without a stable phosphorane or metaphosphate intermediate, (4) this concerted transition state is directly coupled to a proton transfer from the oxygen of glucose to the carboxylic group of aspartate 10, and (5) a stable glucose 1,6-bis-phosphoglucose intermediate is formed.

Acetyl-coenzyme A (CoA) synthase/carbon monoxide dehydrogenase (ACS/CODH) is a bifunctional enzyme that generates CO from carbon dioxide in the C-cluster of the beta subunit and synthesizes acetyl-CoA from carbon monoxide (CO), CoA, and CH3+ at the active site of the A-cluster in the alpha subunit. On the basis of density functional calculations, we predict that methylation of Nip occurs first, and CO then adds to the NipII-CH3 species to form the intermediate, NipII(CO)(CH3), in which Nip deligates one of its SNid bonds. The CO-insertion/CH3-migration occurs on one metal, the proximal Ni, forming the trigonal planar NipII-acetyl intermediate. The thiolate can bind to NipII and reductively eliminate the thioester. Our calculations disfavor the unprecedented bimetallic CO-insertion/CH3-migration. Ni in the proximal site produces a better catalyst than does Cu.

The recently reported metallation/transmetallation route for the synthesis of CCC-bis(NHC) pincer ligand architectures was extended to 1,3-bis(3'-(trimethylsilylmethyl)-benzimidizol-1'-yl)benzene. The precursor was metallated with Zr(NMe2)4 and transmetallated to Pt using [Pt(COD)Cl2]. This Pt complex was found to resist photobleaching under UV irradiation in ambient conditions. Density functional theory (DFT) computations were used to generate the emission spectrum of the complex and reveal that this spectrum is the result of a transition from the triplet excited state (T1) to the ground state (S0). The Pt complex's molecular structure was determined by X-ray crystallography. The UV-vis absorption and emission spectra in solution and the solid-state emission spectra are reported. The solid-state photostability data and the radiative lifetime is also reported.

Photoejection of one CO ligand from isolated CpM(CO)n+1BR2 (n = 1: M = Fe, Ru; n = 2: M = Mo,W; R2 = catecholate or pinacolate) compounds produces a coordinatively unsaturated 16 e- intermediate, a cyclic dioxaboryl transition metal complex, that can efficiently and selectively initiate regioselective C-H bond activation and can be used in the functionalization of alkanes. This chemistry appears distinct from that reported previously for related CpM(CO)n complexes of alkyl and aryl ligands. We show here by a combination of experimental and theoretical studies that the "unoccupied" p orbital of dioxaboryl ligands are intimately involved in the C-H bond activation step and that this hydrogen transfer to boron occurs by a boron-assisted, metal-mediated sigma-bond metathesis. The "unoccupied" p orbital of boron lowers the energy of the transition state and the intermediates by accepting electron density from the metal. The metal-bound borane then rotates, transfers back through a sigma-bond metathesis to capture the alkyl, and leaves the metal hydride.

Density functional theory calculations were used to examine the role of the urease model complex [Ni2(bdptz)(µ-OH)(µ-H2O)(H2O)2](OTs)3 (A) (bdptz = 1,4-bis(2,2′-dipyridylmethyl)-phthalazine; OTs = tosylate) in the degradation of urea. An elimination mechanism that converts urea to ammonium cyanate was investigated in detail. The lowest energy pathway involves urea coordination through the oxygen atom to a Ni center followed by protonation of a urea NH2 group by the bridging water ligand. Subsequent rotation of the protonated urea, followed by deprotonation of the NH2 by a bridging OH ligand generates the bound, disproportionated urea substrate, HNCONH3, from which ammonium cyanate was produced.

Density functional theory has been used to investigate structural and electronic properties of complexes related to the resting form of the active site of vanadium haloperoxidase as a function of environment and protonation state. Results obtained by studying models of varying size and complexity highlight the influence of environment and protonation state on the structure and stability of the metal cofactor. The study shows that, in the trigonal bipyramidal active site, where one axial position is occupied by a key histidine, the trans position cannot contain a terminal oxo group. Further, a highly negatively charged vanadate unit is not stable. Protonation of at least one equatorial oxo ligand appears necessary to stabilize the metal cofactor. The study also indicates that, while at rest within the protein, the vanadate unit is most likely an anion with an axial hydroxide and an equatorial plane containing two oxos and a hydroxide. For the neutral, protonated state of the vanadate unit, there were two minima found. The first structure is characterized by an axial water with two oxo and one hydroxo group in the equatorial plane. The second structure contains an axial hydroxo group and an equatorial plane composed of one oxo and two hydroxo oxygen atoms. These two species are not significantly different in energy, indicating that either form may be important during the catalytic cycle. These data support the initial crystallographic assignment of an axially bound hydroxide, but an axial water is also a possibility. This study also shows that the protonation state of the vanadate ion is most likely greater than previously proposed.