Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

The deaminative hydrogenation of amides is one of the most convenient pathways for the synthesis of amines and alcohols. The ideal source of reducing equivalents for this reaction is molecular hydrogen, though, in practice, this approach requires high pressures and temperatures, with many catalysts achieving only small turnover numbers and frequencies. Nonetheless, during the last ten years, this field has made major advances towards larger turnovers under milder conditions thanks to the development of bifunctional catalysts. These systems promote the heterolytic cleavage of hydrogen into proton and hydride by combining a basic ligand with an acidic metal centre. The present review focuses on the computational study of the reaction mechanism underlying bifunctional catalysis. This review is structured around the fundamental steps of this mechanism, namely the C=O and C–N hydrogenation of the amide, the C–N protonolysis of the hemiaminal, the C=O hydrogenation of the aldehyde, and the competition between hydrogen activation and catalyst deactivation. In line with the complexity of the mechanism, we also provide a perspective on the use of microkinetic models. Both Noyori- and Milstein-type catalysts are discussed and compared.

Precise manipulation of electron transfers in clustered five redox sites

Electron transfers in multinuclear metal complexes are the origin of their unique functionalities both in natural and artificial systems. However, electron transfers in multinuclear metal complexes are generally complicated, and predicting and controlling these electron transfers is extremely difficult. Herein, we report the precise manipulation of the electron transfers in multinuclear metal complexes. The development of a rational synthetic strategy afforded a series of pentanuclear metal complexes composed of metal ions and 3,5-bis(2-pyridyl)pyrazole (Hbpp) as a platform to probe the phenomena. Electrochemical and spectroscopic investigations clarified the overall picture of the electron transfers in the pentanuclear complexes. In addition, unique electron transfer behaviours, in which the reduction of a metal centre occurs during the oxidation of the overall complex (reduction-upon-oxidation process), were discovered. We also elucidated the two dominant factors that determine the manner of the electron transfers. Our results provide comprehensive guidelines for interpreting the complicated electron transfers in multinuclear metal complexes.

Crystal structure of hexakis(N,N-dimethylformamide-κO)iron(III) μ-chlorido-bis(trichloridocadmium)

The title compound, [Fe(C3H7NO)6][Cd2Cl7], crystallizes in the trigonal space group R\overline{3} and is assembled from discrete [Fe(DMF)6]3+ cations (DMF = N,N-dimethylformamide) and [Cd2Cl7]3− anions. In the cation, the iron(III) atom, located on a special position of \overline{3} site symmetry, is coordinated by six oxygen atoms from DMF ligands with all Fe—O distances being equal [2.0072 (16) Å]. A slight distortion of the octahedral environment of the metal comes from the cis O—Fe—O angles deviating from the ideal value of 90° [86.85 (7) and 93.16 (7)°] whilst all the trans angles are strictly 180°. The central Cl atom of the [Cd2Cl7]3− anion is also located on a special position of \overline{3} site symmetry and bridges two corner sharing, tetrahedrally coordinated CdII atoms. The two Cd atoms and the central Cl atom are colinear. The two sets of terminal chloride ligands on either side of the dumbbell-like anion are rotated relative to each other by 30°. In the crystal, the cations and anions, stacked one above the other along the c-axis direction, are held in place principally by electrostatic interactions. There are also C—H...Cl and C—H...O interactions, but these are rather weak. Of the six crystal structures reported to date for ionic salts of [Fe(DMF)6] n+ cations (n = 2, 3), five contain FeII ions. The title compound is the second example of a stable compound containing the [Fe(DMF)6]3+ cation. The existence of both [Fe(DMF)6]2+ and [Fe(DMF)6]3+ cations shows that the DMF ligand coordination sphere can accommodate changes in the charge and spin states of the metal centre.

1st Row Transition Metal Aluminylene Complexes: Preparation, Properties and Bonding Analysis

<i>The synthesis and spectroscopic characterisation of eight new first-row transition metal (M = Cr, Mn, Fe, Co, Cu) aluminylene complexes is reported. DFT and ab<b> </b>initio calculations have been used to provide detailed insight into the metal–metal bond. The σ-donation and π-backdonation properties of the aluminylene ligand are evaluated via NBO and ETS-NOCV calculations. These calculations reveal that these ligands are strong σ-donors but also competent π-acceptors. These properties are not fixed but vary in response to the nature of the transition metal centre, suggesting that aluminylene fragments can modulate their bonding to accommodate both electron-rich and electron-poor transition metals. Ab initio<b> </b>DLPNO-CCSD(T) calculations show that dispersion plays an important role in stabilising these complexes. Both short-range and long-range dispersion interactions are identified. These results will likely inform the design of next-generation catalysts based on aluminium metalloligands. </i>

1st Row Transition Metal Aluminylene Complexes: Preparation, Properties and Bonding Analysis

<i>The synthesis and spectroscopic characterisation of eight new first-row transition metal (M = Cr, Mn, Fe, Co, Cu) aluminylene complexes is reported. DFT and ab<b> </b>initio calculations have been used to provide detailed insight into the metal–metal bond. The σ-donation and π-backdonation properties of the aluminylene ligand are evaluated via NBO and ETS-NOCV calculations. These calculations reveal that these ligands are strong σ-donors but also competent π-acceptors. These properties are not fixed but vary in response to the nature of the transition metal centre, suggesting that aluminylene fragments can modulate their bonding to accommodate both electron-rich and electron-poor transition metals. Ab initio<b> </b>DLPNO-CCSD(T) calculations show that dispersion plays an important role in stabilising these complexes. Both short-range and long-range dispersion interactions are identified. These results will likely inform the design of next-generation catalysts based on aluminium metalloligands. </i>

Manganese(II) complexes derived from acyclic ligands having flexible alcohol arms: structural chracterization and SOD and catalase mimetic studies

In this work, a series of seven MnII complexes of noncyclic flexible ligands derived from 2,6-diformylpyridine and ethanolamine or alkyl-substituted ethanolamines were prepared and characterized, six structurally by single-crystal X-ray diffraction studies. The complexes are dichlorido{2,2′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]diethanol}manganese(II), [MnCl2(C11H15N3O2)] or [MnCl2(L1)], (2), bis{μ-2,2′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]diethanol}bis[dithiocyanatomanganese(II)], [Mn2(NCS)4(C11H15N3O2)2] or [Mn2(NCS)4(L1)2], (3), chlorido{1,1′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]bis(propan-2-ol)}manganese(II) chloride monohydrate, [MnCl(C13H19N3O2)(H2O)]Cl·H2O or [MnCl(L2)(H2O)]Cl·H2O, (4), {1,1′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]bis(propan-2-ol)}dithiocyanatomanganese(II), [Mn(NCS)2(C13H19N3O2)] or [Mn(NCS)2(L2)], (5), aquadichlorido{2,2′-dimethyl-2,2′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]bis(propan-1-ol)}manganese(II) 0.3-hydrate, [MnCl2(C15H23N3O2)(H2O)]·0.3H2O or [MnCl2(L3)(H2O)]·0.3H2O, (6), (dimethylformamide){2,2′-dimethyl-2,2′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]bis(propan-1-ol)}dithiocyanatomanganese(II), [Mn(NCS)2(C15H23N3O2)(C3H7NO)] or [Mn(NCS)2(L3)(DMF)], (7), and (dimethylformamide){2,2′-[(pyridine-2,6-diyl)bis(nitrilomethanylylidene)]bis(butan-1-ol)}dithiocyanatomanganese(II) dimethylformamide monosolvate, [Mn(NCS)2(C15H23N3O2)(C3H7NO)]·C3H7NO or [Mn(NCS)2(L4)(DMF)]·DMF, (8). The crystal structure of ligand L1 is also reported, but that of (5) is not. All four ligands (L1–L4) have five potential donor atoms in an N3O2 donor set, i.e. three N (pyridine/diimine donors) and two alcohol O atoms, to coordinate the MnII centre. The N3O2 donor set coordinates to the metal centre in a pentagonal planar arrangement; seven-coordinated MnII complexes were obtained via coordination of two auxiliary ligands (anions or water molecules) at the axial positions. However, in some cases, the alcohol O-atom donors remain uncoordinated, resulting in five- or six-coordinated MnII complexes. The structurally characterized complexes were tested for their catalytic scavenging of superoxide and peroxide. The results indicated that the complexes with coordinated exogenous water or chloride ligands showed higher SOD activity than those with exogenous thiocyanate ligands.

Structure and NMR properties of the dinuclear complex di-μ-azido-κ4 N 1:N 1-bis[(azido-κN)(pyridine-2-carboxamide-κ2 N 1,O)zinc(II)]

The new diamagnetic complex, [Zn2(N3)4(C6H6N2O)2] or [Zn2(pca)2(μ1,1-N3)2(N3)2] was synthesized using pyridine-2-carboxamide (pca) and azido ligands, and characterized using various techniques: IR spectroscopy and single-crystal X-ray diffraction in the solid state, and nuclear magnetic resonance (NMR) in solution. The molecule is placed on an inversion centre in space group P\overline{1}. The pca ligand chelates the metal centre via the pyridine N atom and the carbonyl O atom. One azido ligand bridges the two symmetry-related Zn2+ cations in the end-on coordination mode, while the other independent azido anion occupies the fifth coordination site, as a terminal ligand. The resulting five-coordinate Zn centres have a coordination geometry intermediate between trigonal bipyramidal and square pyramidal. The behaviour of the title complex in DMSO solution suggests that it is a suitable NMR probe for similar or isostructural complexes including other transition-metal ions. The diamagnetic nature of the complex is reflected in similar 1H and 13C NMR chemical shifts for the free ligand pca as for the Zn complex.

Tuning Extreme Anisotropic Thermal Expansion in 1D Coordination Polymers through Metal Selection and Solid Solutions

The thermal expansion behaviour of a series of 1D coordination polymers has been investigated. Variation of the metal centre allows tuning of the thermal expansion behaviour from colossal positive volumetric...

Structural and electronic studies of substituted m-terphenyl lithium complexes

Spectroscopic and computational investigation of the effects of para-substituted m-terphenyl lithium complexes reveals significant electronic differences at the metal centre.

Synthesis of acetic acid from CO2, CH3I and H2 using a water-soluble electron storage catalyst

A rational design of the electron storage catalyst is demonstrated. This design confirms our long-standing confidence in the advantages of water solubility, metal centre as a point for storing electrons from H2, and electron-withdrawing ligand.