The "curly arrow" of Robinson and Ingold is the primary tool for describing and rationalizing reaction mechanisms. Despite this approach's ubiquity and stellar success, its physical basis has never been clarified and a direct connection to quantum chemistry has never been found. Here we report that the bond rearrangements expressed by curly arrows can be directly observed in ab initio computations, as transformations of intrinsic bond orbitals (IBOs) along the reaction coordinate. Our results clarify that curly arrows are rooted in physical reality-a notion which has been challenged before-and show how quantum chemistry can directly establish reaction mechanisms in intuitive terms and unprecedented detail.

No abstract

Our ability to understand and simulate the reactions catalyzed by iron depends strongly on our ability to predict the relative energetics of spin states. In this work, we studied the electronic structures of Fe ion, gaseous FeO and 14 iron complexes using Kohn-Sham density functional theory with particular focus on determining the ground spin state of these species as well as the magnitudes of relevant spin-state energy splittings. The 14 iron complexes investigated in this work have hexacoordinate geometries of which seven are Fe(ii), five are Fe(iii) and two are Fe(iv) complexes. These are calculated using 20 exchange-correlation functionals. In particular, we use a local spin density approximation (LSDA) - GVWN5, four generalized gradient approximations (GGAs) - BLYP, PBE, OPBE and OLYP, two non-separable gradient approximations (NGAs) - GAM and N12, two meta-GGAs - M06-L and M11-L, a meta-NGA - MN15-L, five hybrid GGAs - B3LYP, B3LYP*, PBE0, B97-3 and SOGGA11-X, four hybrid meta-GGAs - M06, PW6B95, MPW1B95 and M08-SO and a hybrid meta-NGA - MN15. The density functional results are compared to reference data, which include experimental results as well as the results of diffusion Monte Carlo (DMC) calculations and ligand field theory estimates from the literature. For the Fe ion, all functionals except M11-L correctly predict the ground spin state to be quintet. However, quantitatively, most of the functionals are not close to the experimentally determined spin-state splitting energies. For FeO all functionals predict quintet to be the ground spin state. For the 14 iron complexes, the hybrid functionals B3LYP, MPW1B95 and MN15 correctly predict the ground spin state of 13 out of 14 complexes and PW6B95 gets all the 14 complexes right. The local functionals, OPBE, OLYP and M06-L, predict the correct ground spin state for 12 out of 14 complexes. Two of the tested functionals are not recommended to be used for this type of study, in particular M08-SO and M11-L, because M08-SO systematically overstabilizes the high spin state, and M11-L systematically overstabilizes the low spin state.

Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through H andC NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV-vis-near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their fac-Re(CO)-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where fac-Re(CO) complexes of triazoles/triazolylidenes are likely to find use.

During the past 10 years iron-catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)3 (NO)](-) , which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)4 ](2-) , which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)3 (NO)](-) cannot be regarded as a Fe(-II) species, but rather is predominantly a Fe(0) species, in which the metal is covalently bonded to NO(-) by two π-bonds. A metal-N σ-bond is not observed.

Proton‐coupled electron transfer (PCET) events play a key role in countless chemical transformations, but they come in many physical variants which are hard to distinguish experimentally. While present theoretical approaches to treat these events are mostly based on physical rate coefficient models of various complexity, it is now argued that it is both feasible and fruitful to directly analyze the electronic N‐electron wavefunctions of these processes along their intrinsic reaction coordinate (IRC). In particular, for model systems of lipoxygenase and the high‐valent oxoiron(IV) intermediate TauD‐J it is shown that by invoking the intrinsic bond orbital (IBO) representation of the wavefunction, the common boundary cases of hydrogen atom transfer (HAT) and concerted PCET (cPCET) can be directly and unambiguously distinguished in a straightforward manner.

This review describes transition-metal-catalysed and -mediated processes for the preparation of oxindoles from anilides through C(3)-C(3a) bond formation. Traditional methods,

The coordination complexes of Ni(II) with the tripodal ligands tpta (tris[(1-phenyl-1H-1,2,3-triazol-4-yl)methyl]amine), tbta ([(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine), and tdta (tris[(1-(2,6-diisopropyl-phenyl)-1H-1,2,3-triazol-4-yl)methyl]amine) and the bidentate ligand pyta (1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole), [Ni(tpta)2](BF4)2 (1), [Ni(tbta)2](BF4)2 (2), [Ni(tdta)2](BF4)2 (3), and [Ni(pyta)3](BF4)2 (4), were synthesized from Ni(BF4)2·6H2O and the corresponding ligands. Complexes 2 and 4 were also characterized structurally using X-ray diffraction and magnetically via susceptibility measurements. Structural characterization of 2 that contains the potentially tetradentate, tripodal tbta ligand revealed that the Ni(II) center in that complex is in a distorted octahedral environment, being surrounded by two of the tripodal ligands. Each of those ligands coordinate to the Ni(II) center through the central amine nitrogen atom and two of the triazole nitrogen donors; the Ni-N(amine) distances being longer than Ni-N(triazole) distances. In case of 4, three of the bidentate ligands pyta bind to the Ni(II) center with the binding of the triazole nitrogen atoms being stronger than those of the pyridine. Temperature dependent susceptibility measurements on 2 and 4 revealed a room temperature χ(M)T value of 1.18 and 1.20 cm(3) K mol(-1), respectively, indicative of S = 1 systems. High-frequency and -field EPR (HFEPR) measurements were performed on all the complexes to accurately determine their g-tensors and the all-important zero-field splitting (zfs) parameters D and E. Interpretation of the optical d-d absorption spectra using ligand field theory revealed the B and Dq values for these complexes. Quantum chemical calculations based on the X-ray and DFT optimized geometries and their ligand field analysis have been used to characterize the metal-ligand bonding and its influence on the magnitude and sign of the zfs parameters. This is the first time that such extensive HFEPR, LFT, and advanced computational studies are being reported on a series of mononuclear, distorted octahedral Ni(II) complexes containing different kinds of nitrogen donating ligands in the same complex.