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1.
J Chem Phys ; 152(22): 224108, 2020 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-32534543

RESUMO

In this contribution to the special software-centered issue, the ORCA program package is described. We start with a short historical perspective of how the project began and go on to discuss its current feature set. ORCA has grown into a rather comprehensive general-purpose package for theoretical research in all areas of chemistry and many neighboring disciplines such as materials sciences and biochemistry. ORCA features density functional theory, a range of wavefunction based correlation methods, semi-empirical methods, and even force-field methods. A range of solvation and embedding models is featured as well as a complete intrinsic to ORCA quantum mechanics/molecular mechanics engine. A specialty of ORCA always has been a focus on transition metals and spectroscopy as well as a focus on applicability of the implemented methods to "real-life" chemical applications involving systems with a few hundred atoms. In addition to being efficient, user friendly, and, to the largest extent possible, platform independent, ORCA features a number of methods that are either unique to ORCA or have been first implemented in the course of the ORCA development. Next to a range of spectroscopic and magnetic properties, the linear- or low-order single- and multi-reference local correlation methods based on pair natural orbitals (domain based local pair natural orbital methods) should be mentioned here. Consequently, ORCA is a widely used program in various areas of chemistry and spectroscopy with a current user base of over 22 000 registered users in academic research and in industry.

2.
J Comput Chem ; 2020 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-32484577

RESUMO

Segmented all-electron relativistically contracted (SARC) basis sets are presented for the elements 37 Rb-54 Xe, for use with the second-order Douglas-Kroll-Hess approach and the zeroth-order regular approximation. The basis sets have a common set of exponents produced with established heuristic procedures, but have contractions optimized individually for each scalar relativistic Hamiltonian. Their compact size and loose segmented contraction, which is in line with the construction of SARC basis sets for heavier elements, makes them suitable for routine calculations on large systems and when core spectroscopic properties are of interest. The basis sets are of triple-zeta quality and come in singly or doubly polarized versions, which are appropriate for both density functional theory and correlated wave function theory calculations. The quality of the basis sets is assessed against large decontracted reference basis sets for a number of atomic and ionic properties, while their general applicability is demonstrated with selected molecular examples.

3.
J Chem Phys ; 152(16): 164303, 2020 Apr 30.
Artigo em Inglês | MEDLINE | ID: mdl-32357787

RESUMO

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight.

4.
Artigo em Inglês | MEDLINE | ID: mdl-32159921

RESUMO

We describe a highly enantioselective Diels-Alder reaction of cross-conjugated cyclohexadienones with cyclopentadiene, in which five stereocenters are effectively controlled by a strongly acidic and confined imidodiphosphorimidate catalyst. Our approach provides tricyclic products in excellent stereoselectivity. We also report methods to convert the obtained products into useful intermediates and a computational study that aids in gaining deeper insight into the reaction mechanism and origin of stereoselectivity.

5.
J Chem Phys ; 152(11): 114107, 2020 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-32199419

RESUMO

In this work, we present a combined experimental and theoretical study on the V L2,3-edge x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra of VIVO(acac)2 and VIII(acac)3 prototype complexes. The recorded V L2,3-edge XAS and XMCD spectra are richly featured in both V L3 and L2 spectral regions. In an effort to predict and interpret the nature of the experimentally observed spectral features, a first-principles approach for the simultaneous prediction of XAS and XMCD spectra in the framework of wavefunction based ab initio methods is presented. The theory used here has previously been formulated for predicting optical absorption and MCD spectra. In the present context, it is applied to the prediction of the V L2,3-edge XAS and XMCD spectra of the VIVO(acac)2 and VIII(acac)3 complexes. In this approach, the spin-free Hamiltonian is computed on the basis of the complete active space configuration interaction (CASCI) in conjunction with second order N-electron valence state perturbation theory (NEVPT2) as well as the density functional theory (DFT)/restricted open configuration interaction with singles configuration state functions based on a ground state Kohn-Sham determinant (ROCIS/DFT). Quasi-degenerate perturbation theory is then used to treat the spin-orbit coupling (SOC) operator variationally at the many particle level. The XAS and XMCD transitions are computed between the relativistic many particle states, considering their respective Boltzmann populations. These states are obtained from the diagonalization of the SOC operator along with the spin and orbital Zeeman operators. Upon averaging over all possible magnetic field orientations, the XAS and XMCD spectra of randomly oriented samples are obtained. This approach does not rely on the validity of low-order perturbation theory and provides simultaneous access to the calculation of XMCD A, B, and C terms. The ability of the method to predict the XMCD C-term signs and provide access to the XMCD intensity mechanism is demonstrated on the basis of a generalized state coupling mechanism based on the type of the excitations dominating the relativistically corrected states. In the second step, the performance of CASCI, CASCI/NEVPT2, and ROCIS/DFT is evaluated. The very good agreement between theory and experiment has allowed us to unravel the complicated XMCD C-term mechanism on the basis of the SOC interaction between the various multiplets with spin S' = S, S ± 1. In the last step, it is shown that the commonly used spin and orbital sum rules are inadequate in interpreting the intensity mechanism of the XAS and XMCD spectra of the VIVO(acac)2 and VIII(acac)3 complexes as they breakdown when they are employed to predict their magneto-optical properties. This conclusion is expected to hold more generally.

6.
J Chem Theory Comput ; 16(4): 2224-2235, 2020 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-32196337

RESUMO

In this work, a detailed study of spin-state splittings in three spin crossover model compounds with DLPNO-CCSD(T) is presented. The performance in comparison to canonical CCSD(T) is assessed in detail. It was found that spin-state splittings with chemical accuracy, compared to the canonical results, are achieved when the full iterative triples (T1) scheme and TightPNO settings are applied and relativistic effects are taken into account. Having established the level of accuracy that can be reached relative to the canonical results, we have undertaken a detailed basis set study in the second part of the study. The slow convergence of the results of correlated calculations with respect to basis set extension is particularly acute for spin-state splittings for reasons discussed in detail in this Article. In fact, for some of the studied systems, 5Z basis sets are necessary in order to come close to the basis set limit that is estimated here by basis set extrapolation. Finally, the results of the present work are compared to available literature. In general, acceptable agreement with previous CCSD(T) results is found, although notable deviations stemming from differences in methodology and basis sets are noted. It is noted that the published CASPT2 numbers are far away from the extrapolated CCSD(T) numbers. In addition, dynamic quantum Monte Carlo results differ by several tens of kcal/mol from the CCSD(T) numbers. A comparison to DFT results produced with a range of popular density functionals shows the expected scattering of results and showcases the difficulty of applying DFT to spin-state energies.

7.
J Am Chem Soc ; 142(7): 3613-3625, 2020 Feb 19.
Artigo em Inglês | MEDLINE | ID: mdl-31984734

RESUMO

High-level quantum electronic structure calculations are used to provide a deep insight into the mechanism and stereocontrolling factors of two recently developed catalytic asymmetric Diels-Alder (DA) reactions of cinnamate esters with cyclopentadiene. The reactions employ two structurally and electronically very different in situ silylated enantiopure Lewis acid organocatalysts: i.e., binaphthyl-allyl-tetrasulfone (BALT) and imidodiphosphorimidate (IDPi). Each of these catalysts activates only specific substrates in an enantioselective fashion. Emphasis is placed on identifying and quantifying the key noncovalent interactions responsible for the selectivity of these transformations, with the final aim of aiding in the development of designing principles for catalysts with a broader scope. Our results shed light into the mechanism through which the catalyst architecture determines the selectivity of these transformations via a delicate balance of dispersion and steric interactions.

8.
J Phys Chem A ; 124(5): 1025-1037, 2020 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-31977214

RESUMO

Over the last few years, ab initio ligand field theory (AILFT) has evolved into an important tool for the extraction of ligand field models from ab initio calculations. The inclusion of dynamic correlation on top of complete active space self-consistent field (CASSCF) reference functions, which is important for accurate results, was so far realized at the level of second-order N-electron valence state perturbation theory (NEVPT2). In this work, we introduce two alternative methods for the inclusion of dynamic correlation into AILFT calculations, the second-order dynamic correlation dressed complete active space method (DCD-CAS(2)) and the Hermitian quasi-degenerate NEVPT2 (HQD-NEVPT2). These methods belong to the class of multistate perturbation theory approaches, which allow for the mixing of CASSCF states under the effect of dynamic correlation (state-mixing). The two new versions of AILFT were tested for a diverse set of transition-metal complexes. It was found that the multistate methods have, compared to NEVPT2, an AILFT fit with smaller root mean square deviations (rmsds) between ab initio and AILFT energies. A comparison of AILFT excitation energies with the experiment shows that for some systems, the agreement gets better at the multistate level because of the smaller rmsds. However, for some systems, the agreement gets worse, which could be attributed to a cancellation of errors at the NEVPT2 level that is partly removed at the multistate level. An investigation of trends in the extracted ligand field parameters shows that at the multistate level, the ligand field splitting Δ gets larger, whereas the Racah parameters B and C get smaller and larger, respectively. An investigation of the reasons for the observed improvement for octahedral CrIII halide complexes shows that the possibility of state-mixing relaxes constraints that are present at the NEVPT2 level and that keep Δ and B from following their individual preferences.

9.
J Comput Chem ; 41(9): 922-939, 2020 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-31889331

RESUMO

The treatment of the solvation charges using Gaussian functions in the polarizable continuum model results in a smooth potential energy surface. These charges are placed on top of the surface of the solute cavity. In this article, we study the effect of the solute cavity (van der Waals-type or solvent-excluded surface-type) using the Gaussian charge scheme within the framework of the conductor-like polarizable continuum model on (a) the accuracy and computational cost of the self-consistent field (SCF) energy and its gradient and on (b) the calculation of free energies of solvation. For that purpose, we have considered a large set of systems ranging from few atoms to more than 200 atoms in different solvents. Our results at the DFT level using the B3LYP functional and the def2-TZVP basis set show that the choice of the solute cavity does neither affect the accuracy nor the cost of calculations for small systems (< 100 atoms). For larger systems, the use of a vdW-type cavity is recommended, as it prevents small oscillations in the gradient (present when using a SES-type cavity), which affect the convergence of the SCF energy gradient. Regarding the free energies of solvation, we consider a solvent-dependent probe sphere to construct the solvent-accessible surface area required to calculate the nonelectrostatic contribution to the free energy of solvation. For this part, our results for a large set of organic molecules in different solvents agree with available experimental data with an accuracy lower than 1 kcal/mol for both polar and nonpolar solvents.

10.
J Chem Phys ; 152(2): 024116, 2020 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-31941297

RESUMO

The coupled cluster method with single-, double-, and perturbative triple excitations [CCSD(T)] is considered to be one of the most reliable quantum chemistry theories. However, the steep scaling of CCSD(T) has limited its application to small or medium-sized systems for a long time. In our previous work, the linear scaling domain based local pair natural orbital CCSD variant (DLPNO-CCSD) has been developed for closed-shell and open-shell. However, it is known from extensive benchmark studies that triple-excitation contributions are important to reach chemical accuracy. In the present work, two linear scaling (T) approximations for open-shell DLPNO-CCSD are implemented and compared: (a) an algorithm based on the semicanonical approximation, in which off-diagonal Fock matrix elements in the occupied space are neglected [referred to as DLPNO-(T0)]; and (b) an improved algorithm in which the triples amplitudes are computed iteratively [referred to as DLPNO-(T)]. This work is based on the previous open-shell DLPNO-CCSD algorithm [M. Saitow et al., J. Chem. Phys. 146, 164105 (2017)] as well as the iterative (T) correction for closed-shell systems [Y. Guo et al., J. Chem. Phys. 148, 011101 (2018)]. Our results show that the new open-shell perturbative corrections, DLPNO-(T0/T), can predict accurate absolute and relative correlation energies relative to the canonical reference calculations with the same basis set. The absolute energies from DLPNO-(T) are significantly more accurate than those of DLPNO-(T0). The additional computational effort of DLPNO-(T) relative to DLPNO-(T0) is a factor of 4 on average. We report calculations on systems with more than 4000 basis functions.

11.
Inorg Chem ; 59(2): 1556-1565, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31909608

RESUMO

Despite their isoelectronic properties, fluoro and oxo ligands exhibit completely different chemical behavior. Formally speaking, the first is known to exclusively form single bonds, while the latter is generally observed to form double (or even triple) bonds. The biggest difference, however, lies in what is known among inorganic chemists as the Oxo Wall: the fact that six-coordinate tetragonal transition metal oxo complexes are not observed beyond group 7 elements. While the Oxo Wall was explained a few decades ago, some questions regarding the nature of the Oxo Wall remain unanswered. For example, why do group 8 oxo complexes with high oxidation states not violate the Oxo Wall? Moreover, why are transition metal fluoro complexes observed through the whole transition metal series? In order to understand how the small difference between these two isoelectronic ligands can give rise to such different chemical behaviors, we conducted an extensive computational analysis of the geometric and electronic properties of model fluoro and oxo complexes with metals around the Oxo Wall. Among many insights into the details of the Oxo Wall, we mostly learned that the oxygen 2p orbitals are prone to meaningfully interact with transition metal d orbitals, because they match not only spatially but also energetically, while for fluorine the p orbital energies are lower to an extent that interaction with transition metal d orbitals is much reduced. This in turn implies that in those instances where the metal d orbitals principally accessible for interaction are occupied, the oxygen 2p orbitals are too exposed to be stable.

12.
J Chem Phys ; 152(1): 014109, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31914736

RESUMO

Many recent developments in the area of multistate multireference perturbation theories focused on methods that use a state-averaged 0th order Hamiltonian. We recently found that the dynamic correlation dressed complete active space method fails in describing ligand field and charge transfer states in a balanced way precisely because it uses a state-averaged 0th order Hamiltonian [L. Lang and F. Neese, J. Chem. Phys. 150, 104104 (2019)]. The multipartitioning idea allows the use of state-specific 0th order Hamiltonians in a multistate framework and could therefore alleviate the mentioned problem. However, the effective Hamiltonian is non-Hermitian in the traditional formulation of multipartitioning, which can lead to unphysical behavior, especially for nearly degenerate states. In order to achieve a more balanced treatment of states with different physical character and at the same time have a Hermitian effective Hamiltonian, we combine in this work multipartitioning with canonical Van Vleck perturbation theory. At the 2nd order, the result is a Hermitian variant of multipartitioning quasidegenerate N-electron valence state perturbation theory. The effect of model space noninvariance of the method is discussed and the benefit of a Hermitian formulation is highlighted with numerical examples. The method is shown to give good results for the calculation of electronic transitions of the [CuCl4]2 -complex and for the calculation of electron paramagnetic resonance parameters, which are two examples where the balance between ligand field and charge transfer configurations is of utmost importance.

13.
Angew Chem Int Ed Engl ; 59(4): 1382-1383, 2020 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-31917510

RESUMO

On August 23, 2019 Walter Thiel passed away suddenly and unexpectedly. Thiel was a giant in the field of Theoretical Chemistry and has left deep marks as an outstanding scientist and as a wonderful human being. With Walter Thiel, the scientific community has lost a visionary scientific leader and an important voice of reason.

14.
Inorg Chem ; 59(2): 984-995, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31247844

RESUMO

The electronic structure of multinuclear transition metal complexes is a highly challenging problem for quantum chemical methods. The problems to be solved for a successful analysis include the following: (1) many unpaired electrons leading to "highly entangled" wave functions that cannot be calculated by standard electronic structure methods, (2) drastic differences between the one-particle and many-particle spectra and a high density of low-lying states, and (3) the interpretation of such highly complex wave functions in chemical terms. In this work, we continue our research on oligonuclear clusters by presenting an in-depth analysis of the electronic structure of a prototypical iron-sulfur (Fe2S2) dimer. Accurate wave functions are obtained from a variety of advanced wave function based methods. The wave function results are interpreted in terms of an effective Hamiltonian that in turn is parametrized in terms of the angular overlap model (AOM) that provides the chemical insights that we are striving for. A hierarchical analysis allows us to interpret the local electronic structure in terms of the thiolate, sulfide ligands, and metal-metal interaction strengths. The many-particle spectrum is analyzed in terms of configurations involving ligand and metal centers. Finally, we are able to derive simple yet effective interpretations of ligand interaction strengths, the metal-metal interaction strength, and the low-lying many-particle spectrum of the Fe2S2 dimer.

15.
J Am Chem Soc ; 142(3): 1457-1464, 2020 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-31830412

RESUMO

[NiFe] hydrogenases catalyze the reversible oxidation of molecular hydrogen into two protons and two electrons. A key organometallic chemistry feature of the NiFe active site is that the iron atom is co-coordinated by two cyanides (CN-) and one carbon monoxide (CO) ligand. Biosynthesis of the NiFe(CN)2(CO) cofactor requires the activity of at least six maturation proteins, designated HypA-F. An additional maturase, HypX, is required for CO ligand synthesis under aerobic conditions, and preliminary in vivo data indicated that HypX releases CO using N10-formyltetrahydrofolate (N10-formyl-THF) as the substrate. HypX has a bipartite structure composed of an N-terminal module similar to N10-formyl-THF transferases and a C-terminal module homologous to enoyl-CoA hydratases/isomerases. This composition suggested that CO production takes place in two consecutive reactions. Here, we present in vitro evidence that purified HypX first transfers the formyl group of N10-formyl-THF to produce formyl-coenzyme A (formyl-CoA) as a central reaction intermediate. In a second step, formyl-CoA is decarbonylated, resulting in free CoA and carbon monoxide. Purified HypX proved to be metal-free, which makes it a unique catalyst among the group of CO-releasing enzymes.

16.
J Chem Theory Comput ; 16(1): 564-575, 2020 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-31765141

RESUMO

Boron-dipyrromethene (BODIPY) molecules form a class of fluorescent dyes known for their exceptional photoluminescence properties. Today, they are used extensively in various applications from fluorescent imaging to optoelectronics. The ease of altering the BODIPY core has allowed scientists to synthesize dozens of analogues by exploring chemical substitutions of various kinds or by increasing the length of conjugated groups. However, predicting the impact of any chemical change accurately is still a challenge, especially as most computational methods fail to describe correctly the photophysical properties of BODIPY derivatives. In this study, the recently developed coupled cluster method called "domain-based local pair natural orbital similarity transformed equation of motion-coupled cluster singles and doubles" (DLPNO-STEOM-CCSD) is employed to compute the lowest vertical excitation energies of more than 50 BODIPY molecules. The method performs remarkably well yielding an accuracy of about 0.06 eV compared to the experimental absorption maxima. We also provide an estimate to the error made by neglecting vibronic effects in the computed spectra. The dyes selected for investigation here span a large range of molecular sizes and chemical functionalities and are embedded in solvents with different polarities. We have also investigated if the method is able to correctly reproduce the impact of a single chemical modification on the absorption energy. To characterize the method in more specific terms, we have studied four large BODIPY analogues used in real-life applications due to their interesting chemical properties. These examples should illustrate the capacity of the DLPNO-STEOM-CCSD procedure to become a method of choice for the study of photophysical properties of medium to large organic compounds.

17.
J Phys Chem A ; 124(1): 90-100, 2020 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-31841627

RESUMO

In this study we examine the accuracy of domain-based local pair natural orbital coupled cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) on a large benchmark data set. To this end, we use the recently published GMTKN55 superset of molecules that contains 1505 relative energies and 2462 single-point calculations. To our knowledge this is the most comprehensive benchmark evaluation of any highly correlated wave function based ab initio method to date. In the first part of the study, canonical CCSD(T) reference calculations were carried out on the entire test set in order to guarantee that the reference data are of uniform quality. Second, DLPNO-CCSD(T) calculations were carried out under identical conditions. The main finding is that with the exception of two data sets, all data sets have a MAD of 0.4 kcal/mol or less and the majority of sets have a MAD of less than 0.2 kcal/mol. For open shells, the accuracy of the DLPNO calculations was significantly improved through an iterative version of the triples correction.

18.
J Am Chem Soc ; 142(4): 1864-1870, 2020 Jan 29.
Artigo em Inglês | MEDLINE | ID: mdl-31884789

RESUMO

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η2-H, C metal-alkane interactions in the [((t·BuArO)3tacn)UIII(Mecy-C6)]·(Mecy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-OArO and U-Ntacn bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap. For that reason, it is deduced that metal-ligand bonding in the [((t·BuArO)3tacn)UIII] moiety is predominantly ionic. The bonding interaction within the [((t·BuArO)3tacn)UIII] moiety is shown to be dispersive in nature and essentially supported by the upper-rim tBu groups of the (t·BuArO)3tacn3- ligand. Our findings indicate that the axial alkane molecule is held in place by the guest-host effect rather than direct metal-alkane ionic or covalent interactions.

19.
J Am Chem Soc ; 141(43): 17217-17235, 2019 Oct 30.
Artigo em Inglês | MEDLINE | ID: mdl-31566964

RESUMO

Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO}n notation. In this work, we succeeded in characterizing a series of {Fe-NO}6-9 complexes, including a reactive {Fe-NO}10 intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMENMes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENMes)Fe]2+ resulted in formation of the {Fe-NO}6 compound [(TIMENMes)Fe(NO)(CH3CN)](BF4)3 (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO}7,8,9 (2-4). Reduction of {Fe-NO}9 with Cs electride finally yields the highly reduced {Fe-NO}10 intermediate, key to formation of [Cs(crypt-222)][(TIMENMes)Fe(NO)], (5) featuring a metalacyclic [Fe-(NO-NHC)3-] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO}8 complex was found via oxygenation of the nitride complex [(TIMENMes)Fe(N)](BF4). Surprisingly, the resulting {Fe-NO}8 species is electronically and structural similar to the [(TIMENMes)Fe(N)]+ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)Fe(N)]+ as a {Fe-N}8 species.

20.
Nat Chem ; 11(11): 1019-1025, 2019 11.
Artigo em Inglês | MEDLINE | ID: mdl-31611632

RESUMO

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH2 bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.


Assuntos
Materiais Biomiméticos/química , Compostos Ferrosos/química , Proteínas com Ferro-Enxofre/química , Materiais Biomiméticos/metabolismo , Compostos Ferrosos/metabolismo , Proteínas com Ferro-Enxofre/síntese química , Proteínas com Ferro-Enxofre/metabolismo , Modelos Moleculares , Conformação Molecular , Teoria Quântica
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