Revisiting the polytopal rearrangements in penta-coordinate d7-metallocomplexes: modified Berry pseudorotation, octahedral switch, and butterfly isomerization.
ABSTRACT: This paper provides a first-principles theoretical investigation of the polytopal rearrangements and fluxional behavior of five-coordinate d7-transition metal complexes. Our work is primarily based on a potential energy surface analysis of the iron tetracarbonyl hydride radical HFe?(CO)4. We demonstrate the existence of distorted coordination geometries in this prototypical system and, for the first time, introduce three general rearrangement mechanisms, which account for the non-ideal coordination. The first of these mechanisms constitutes a modified version of the Berry pseudorotation via a square-based pyramidal C4v transition state that connects two chemically identical edge-bridged tetrahedral stereoisomers of C2v symmetry. It differs from the classical Berry mechanism, which involves two regular D3h equilibrium structures and a C4v transition state. The second mechanism is related to the famous "tetrahedral jump" hypothesis, postulated by Muetterties for a number of d6 HML4 and H2ML4 complexes. Here, our study suggests two fluxional rearrangement pathways via distinct types of C2v transition states. Both pathways of this mechanism can be described as a single-ligand migration to a vacant position of an "octahedron", thus interchanging (switching) the apical and basal ligands of the initial quasi-square pyramidal isomer, which is considered as an idealized octahedron with a vacancy. Accordingly, we call this mechanism "octahedral switch". The third mechanism follows a butterfly-type isomerization featuring a key-angle deformation, and we thus call it "butterfly isomerization". It connects the quasi-square pyramidal and edge-bridged tetrahedral isomers of HFe?(CO)4 through a distorted edge-bridged tetrahedral transition state of Cs symmetry. Our paper discusses the overall features of the isomers and rearrangement mechanisms as well as their implications. We rationalize the existence of each stationary point through an electronic structure analysis and argue their relevance for isolobal analogues of HFe?(CO)4.
Project description:Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1-xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the "chirality" of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale.
Project description:Six of the seven iron atoms in the iron-molybdenum cofactor of nitrogenase display an unusual geometry, which is distorted from the tetrahedral geometry that is most common in iron-sulfur clusters. This distortion pulls the iron along one C3 axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of the Fe atoms in the iron-molybdenum cofactor with those of the presently and previously reported model complexes, as well as with those in polynuclear iron-sulfur compounds. This understanding of the iron coordination geometry is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.
Project description:The C-ethyl-2-methylresorcinarene (1) forms 1:1 in-cavity complexes with aromatic N,N'-dioxides, only if each of the aromatic rings has an N-O group. The structurally different C-shaped 2,2'-bipyridine N,N'-dioxide (2,2'-BiPyNO) and the linear rod-shaped 4,4'-bipyridine N,N'-dioxide (4,4'-BiPyNO) both form 1:1 in-cavity complexes with the host resorcinarene in C4v crown and C2v conformations, respectively. In the solid state, the host-guest interactions between the 1,3-bis(4-pyridyl)propane N,N'-dioxide (BiPyPNO) and the host 1 stabilize the unfavorable anti-gauche conformation. Contrary to the N,N'-dioxide guests, the mono-N-oxide guest, 4-phenylpyridine N-oxide (4PhPyNO), does not form an in-cavity complex in the solid state. The host-guest complexation and the relative guest affinities were studied through 1H?NMR competition experiments in methanol. Single-crystal X-ray crystallography of the 1:1 complexes supports the proposed solution-state structures, also revealing strong hydrogen bonds between the host and the guests, not observed in solution owing to hydrogen/deuterium (H/D) exchange processes in methanol.
Project description:Bullvalene C<sub>10</sub>H<sub>10</sub> and its analogs semibullvalene C<sub>8</sub>H<sub>8</sub>, barbaralane C<sub>9</sub>H<sub>10</sub>, and 9-Borabarbaralane C<sub>8</sub>BH<sub>9</sub> are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional ?-bonds (2 2c-2e ????2 3c-2e ????2 2c-2e ?) and one fluxional ?-bond (1 2c-2e ????1 4c-2e ????1 2c-2e ?) in their ground states and transition states, unveiling the universal ??+?? double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The <sup>13</sup>C-NMR and <sup>1</sup>H-NMR shielding tensors and chemical shifts of the model compound C<sub>8</sub>BH<sub>9</sub> are computationally predicted to facilitate future experiments.
Project description:Intramolecular Schmidt reactions can be reliably steered toward bridged heterocycles containing orthoamides in high yields. The ketal tether enhances the control of regioselectivity in the migration of the bond distal to the reactive azide nucleophile, thus providing the first examples of the intramolecular Schmidt reaction proceeding with a complete regioselectivity en route to bridged products. The method is broad in scope and allows for systematic study of compounds that are analogous to elusive tetrahedral intermediates of amide addition reactions. Some initial reactivity and structural profiling of these compounds are also reported.
Project description:A number of hydroxyl-substituted azacalixpyridines were synthesized using Pd-catalyzed macrocyclic "2+2" and "3+1" coupling methods and the protection-deprotection strategy of hydroxyl group. While the conformation of the these hydroxyl-substituted azacalixpyridines is fluxional in solution, in the solid state, they adopted shape-persistent 1,3-alternate conformations. Besides, X-ray analysis revealed that the existence of hydroxy groups on the para-position of pyridine facilitated the formation of solvent-bridged intermolecular hydrogen bonding for mono-hydroxyl-substituted while partial tautomerization for four-hydroxyl-substituted macrocycles, respectively. Taking the hydroxyl-substituted azacalixpyridines as molecular platforms, multi-macrocycle-containing architectures and functional building blocks were constructed. The self-assembly behavior of the resulting building blocks was investigated in crystalline state.
Project description:The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N(2)SNi moieties. Square planar [N(2)SNiL](n+) complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N(2)SNi](2) complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N(2)SNiL square plane. Variable temperature (1)H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with density functional theory (DFT)-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization finds Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H(delta+)- - S(delta-) interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N(2)ON(imid)Ni complexes.
Project description:Framework oxides with the capacity to host mobile interstitial oxide anions are of interest as electrolytes in intermediate temperature solid oxide fuel cells (SOFCs). High performance materials of this type are currently limited to the anisotropic oxyapatite and melilite structure types. The langasite structure is based on a corner-shared tetrahedral network similar to that in melilite but is three-dimensionally connected by additional octahedral sites that bridge the layers by corner sharing. Using low-temperature synthesis, we introduce interstitial oxide charge carriers into the La3Ga5-x Ge1+x O14+x/2 langasites, attaining a higher defect content than reported in the lower dimensional oxyapatite and melilite systems in La3Ga3.5Ge2.5O14.75 (x = 1.5). Neutron diffraction and multinuclear solid state 17O and 71Ga NMR, supported by DFT calculations, show that the excess oxygen is accommodated by the formation of a (Ge,Ga)2O8 structural unit, formed from a pair of edge-sharing five-coordinated Ga/Ge square-based pyramidal sites bridged by the interstitial oxide and a strongly displaced framework oxide. This leads to more substantial local deformations of the structure than observed in the interstitial-doped melilite, enabled by the octahedral site whose primary coordination environment is little changed by formation of the pair of square-based pyramids from the originally tetrahedral sites. AC impedance spectroscopy on spark plasma sintered pellets showed that, despite its higher interstitial oxide content, the ionic conductivity of the La3Ga5-x Ge1+x O14+x/2 langasite family is lower than that of the corresponding melilites La1+y Sr1-y Ga3O7+y/2. The cooperative structural relaxation that forms the interstitial-based (Ga,Ge)2O8 units stabilizes higher defect concentrations than the single-site GaO5 trigonal bipyramids found in melilite but effectively traps the charge carriers. This highlights the importance of controlling local structural relaxation in the design of new framework electrolytes and suggests that the propensity of a framework to form extended units around defects will influence its ability to generate high mobility interstitial carriers.
Project description:In the dinuclear centrosymmetric title complex, [Li(2)(C(17)H(30)N(3))(2)(C(4)H(8)O)(2)], the Li(+) cation is coordinated by three N atoms from two guanidinate ligands and an O atom from tetra-hydro-furan (THF) in a strongly distorted tetrahedral environment. In the guanidinate-bridged THF-stabilized dimer the Li?Li separation is short at 2.479?(8)?Å.