Polynuclear Transition Metal Complexes Research Articles

Spin Symmetry in Polynuclear Exchange-Coupled Clusters

The involvement of spin symmetry in the evaluation of zero-field energy levels in polynuclear transition metal and lanthanide complexes facilitates the division of the large-scale Hamiltonian matrix referring to isotropic exchange. This method is based on the use of an irreducible tensor approach. This allows for the fitting of the experimental data of magnetic susceptibility and magnetization in a reasonable time for relatively large clusters for any coupling path. Several examples represented by catena-[AN} and cyclo-[AN] systems were modeled. Magnetic data for 20 actually existing endohedral clusters were analyzed and interpreted.

Reduced Tiara-like Palladium Complex for Suzuki Cross Coupling Reactions.

The design of highly active and structurally well-defined catalysts has become a crucial issue for heterogeneous catalysed reactions while reducing the amount of catalyst employed. Beside conventional synthetic routes, the employment of polynuclear transition metal complexes as catalysts or catalyst precursors has progressively intercepted a growing interest. These well-defined species promise to deliver catalytic systems where a strict control on the nuclearity allows to improve the catalytic performance while reducing the active phase loading. This study describes the development of a very active and reusable palladium-based catalyst on alumina (Pd8/Al2O3) for Suzuki cross coupling reactions. An octanuclear tiara-like palladium complex was selected as active phase precursor to give isolated Pd-clusters of ca. 1 nm in size on Al2O3. The catalyst was thoroughly characterised by several complementary techniques to assess its structural and chemical nature. The high specific activity of the catalyst has allowed to carry out the cross-coupling reaction in 30 minutes using only 0.12 mol% of Pd loading under very mild and green reaction conditions. Screening of various substrates and selectivity tests, combined with recycling and benchmarking experiments, have been used to highlight the great potentialities of this new Pd8/Al2O3 catalyst.

Validation of the Green's Function Approximation for the Calculation of Magnetic Exchange Couplings.

In this work, we assess the potential of the Green's function approximation to predict isotropic magnetic exchange couplings and to reproduce the standard broken-symmetry energy difference approach for transition metal complexes. To this end, we have selected a variety of heterodinuclear, homodinuclear, and polynuclear systems containing 3d transition metal centers and computed the couplings using both the Green's function and energy difference methods. The Green's function approach is shown to have mixed results for the cases tested. For dinuclear complexes with large strength couplings (≳50 cm-1), the Green's function method is unable to reliably reproduce the energy difference values. However, for weaker dinuclear couplings, the Green's function approach acceptably reproduces broken-symmetry energy difference couplings. In polynuclear cases, the Green's function approximation worked remarkably well, especially for FeIII complexes. On the other hand, for a NiII polynuclear complex, qualitatively wrong couplings are predicted. Overall, the evaluation of exchange couplings from local rigid magnetization rotations offers a powerful alternative to time-consuming energy differences methods for large polynuclear transition metal complexes, but to achieve a quantitative agreement, some improvements to the method are needed.

Excited‐State Dynamics of Phosphorescent Trinuclear Re(I) Complexes

AbstractThe excited‐state dynamics of cyanide‐bridged trinuclear carbonyl rhenium(I) complexes with general formula of [Re]p[Re]c[Re]p {[Re]c=[(−NC)ReI(CO)2(phen)(CN−)]; [Re]p=[−ReI(CO)2(L L)(La)], L L= diimine or diphosphine, La=triphenylphosphine or carbonyl ligand} and the monomeric peripheral control complexes have been elucidated through a study with ultrafast transient absorption spectroscopy. The time constants for the energy transfer between the MLCT excited states of the peripheral and central Re(I) chromophores (ca. 0.56–0.78 ps) are typical for other cyanide‐bridged polynuclear transition metal complexes. The emissive excited states and the absorption transient have been characterized by nanosecond transient Raman spectroscopy.

Expressibility of comb tensor network states (CTNS) for the P-cluster and the FeMo-cofactor of nitrogenase

Polynuclear transition metal complexes such as the P-cluster and the FeMo-cofactor of nitrogenase with eight transition metal centers represent a great challenge for current electronic structure methods. In this work, we initiated the use of comb tensor network states (CTNS), whose underlying topology has a one-dimensional backbone and several one-dimensional branches, as a many-body wavefunction ansatz to tackle these challenging systems. As an important first step, we explored the expressive power of CTNS with different underlying topologies. To this end, we presented an algorithm to express a configuration interaction (CI) wavefunction into CTNS based on the Schmidt decomposition. The algorithm was illustrated for representing approximate CI wavefunctions obtained from selected CI calculations for the P-cluster and the FeMo-cofactor into CTNS with three chemically meaningful comb structures, which successively group orbitals belonging to the same atom into branches. The conventional matrix product states (MPS) representation was obtained as a special case. We also discussed the insights gained from such decompositions, which shed some light on the future developments of efficient numerical tools for polynuclear transition metal complexes.

Ab Initio Evaluation of the Redox Potential of Cytochrome c.

Various biochemical activities of metabolism and biosynthesis are fulfilled by redox processes with explicit electron exchange, which furnish redox enzymes with high chemical reactivity. However, theoretical investigation of a redox process, which simultaneously involves a complex electronic change at a redox metal center and conformational reorganization of the surrounding protein environment coupled to the electronic change, requires computationally conflicting approaches, highly accurate quantum chemical calculations, and long-time molecular dynamics (MD) simulations, limiting the physicochemical understanding of biological redox processes. Here, we theoretically examined a redox process of cytochrome c by means of a hybrid molecular simulation technique, which enables one to consistently treat the redox center at the ab initio quantum chemistry level of theory and the protein reorganization with long-time MD simulations on the microsecond timescale. The calculations successfully evaluated a large absolute redox potential, 4.34 eV, with errors of only 0.03 to 0.34 eV to the experimental ones without any problem-specific empirical parameters. Through the long-time MD sampling, large and nonlinear reorganization of the protein environment was unveiled and the molecular determinants for the redox potential were identified. The present ab initio approach significantly expands the applicability of theoretical investigation to biological redox systems with more electronically complicated redox centers such as polynuclear transition metal complexes.

Molassembler: Molecular Graph Construction, Modification, and Conformer Generation for Inorganic and Organic Molecules.

We present the graph-based molecule software Molassembler for building organic and inorganic molecules. Molassembler provides algorithms for the construction of molecules built from any set of elements from the periodic table. In particular, polynuclear transition-metal complexes and clusters can be considered. Structural information is encoded as a graph. Stereocenter configurations are interpretable from Cartesian coordinates into an abstract index of permutation for an extensible set of polyhedral shapes. Substituents are distinguished through a ranking algorithm. Graph and stereocenter representations are freely modifiable, and the chiral state is propagated where possible through incurred ranking changes. Conformers are generated with full stereoisomer control by four spatial dimension Distance Geometry with a refinement error function including dihedral terms. Molecules are comparable by an extended graph isomorphism, and their representation is canonicalizeable. Molassembler is written in C++ and provides Python bindings.

Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation.

Magnetic recording using femtosecond laser pulses has recently been achieved in some dielectric media, showing potential for ultrafast data storage applications. Single-molecule magnets (SMMs) are metal complexes with two degenerate magnetic ground states and are promising for increasing storage density, but remain unexplored using ultrafast techniques. Here we have explored the dynamics occurring after photoexcitation of a trinuclear µ3-oxo-bridged Mn(III)-based SMM, whose magnetic anisotropy is closely related to the Jahn-Teller distortion. Ultrafast transient absorption spectroscopy in solution reveals oscillations superimposed on the decay traces due to a vibrational wavepacket. Based on complementary measurements and calculations on the monomer Mn(acac)3, we conclude that the wavepacket motion in the trinuclear SMM is constrained along the Jahn-Teller axis due to the µ3-oxo and µ-oxime bridges. Our results provide new possibilities for optical control of the magnetization in SMMs on femtosecond timescales and open up new molecular-design challenges to control the wavepacket motion in the excited state of polynuclear transition-metal complexes.

Altering the nature of coupling by changing the oxidation state in a {Mn6} cage.

Polynuclear transition metal complexes have continuously attracted interest owing to their peculiar electronic and magnetic properties which are influenced by the symmetry and connectivity of the metal centres. Understanding the full electronic picture in such cases often becomes difficult owing to the presence of multiple bridges between metal centres. We have investigated the electronic structure of a {Mn6} cage complex using computational and experimental approaches with the aim to understand the coupling between the manganese centres. The nature of the various coupling pathways has been determined using a novel methodology that involves perturbing the system while retaining the symmetry and analysing the effect on the coupling strength due to the perturbation. Furthermore, we have investigated the magnetic properties of this complex in higher oxidation states which reveals a switch in the nature of coupling from antiferromagnetic to ferromagnetic in addition to stabilisation of intermediate spin states.

Controlled Molecular Magnetism of Bi- and Polynuclear Transition Metal Complexes Based on Hydrazones, Azomethines, and Their Analogs

The possibilities of the magnetochemical method for the description of structures and properties of bi- and tetranuclear metallochelates and supramolecular architectures based on coordination compounds with restricted types of ligand systems, mainly hydrazones, azomethines, and their analogs, are reviewed. The known published data for the bi- and polynuclear complexes, whose paramagnetic centers are bound by both intra- and intermolecular exchange interactions, are systematized. A relationship between specific features of the electronic and geometric structures of the complexes and the character of the exchange effects is considered. Magnetostructural relations in the discussed compounds are systematized. The compounds discussed are important model objects for the development of the strategy for the targeted design of one-, two-, and three-dimensional magnetically ordered structures.

Hydroxylated phosphines as ligands for chalcogenide clusters: self assembly, transformations and stabilization

Abstract This contribution is a documentation of recent advances in the chemistry of chalcogenide polynuclear transition metal complexes coordinated with mono- and di-phosphines functionalized with hydroxo groups. A survey of complexes containing tris(hydroxymethyl)phosphine (THP) is presented. The influence of the alkyl chain in bidentate phosphines, bearing the P–(CH2)x–OH arms, is also analyzed. Finally, isolation and structure elucidation of the complexes with HP(OH)2, P(OH)3, As(OH)3, PhP(OH)2, stabilized by coordination to Ni(0) and Pd(0) centers embedded into chalcogenide clusters, is discussed.

Towards an ab initio theory for metal L-edge soft X-ray spectroscopy of molecular aggregates.

The Frenkel exciton model was adapted to describe X-ray absorption and resonant inelastic scattering spectra of polynuclear transition metal complexes by means of the restricted active space self-consistent field method. The proposed approach allows to substantially decrease the requirements on computational resources if compared to a full supermolecular quantum chemical treatment. This holds true, in particular, in cases where the dipole approximation to the electronic transition charge density can be applied. The computational protocol was applied to the calculation of X-ray spectra of the hemin complex, which forms dimers in aqueous solution. The aggregation effects were found to be comparable to the spectral alterations due to the replacement of the axial ligand by solvent molecules.

ChemInform Abstract: Heterometallic Cube‐Type Molecular Nitrides

AbstractReview: [emphasis on nitrido complexes with cuboidal [MTi3N4] cores; 96 refs.].

Heterometallic Cube‐Type Molecular Nitrides

AbstractPolynuclear transition‐metal nitrido complexes constitute a class of molecular cage compounds with fascinating structures and interesting bonding properties. However, there is a lack of systematic strategies for the rational construction of aggregates with desired structures and compositions. This article provides a brief overview of the structure and bonding modes of polynuclear nitrido complexes, the most common synthetic approaches used to generate such aggregates, and a systematic review of the development of a family of heterometallic nitrido complexes with [MTi3N4] cube‐type cores. The rational entry to those well‐defined systems is based on the incorporation of transition metals, lanthanides, and main‐group metals into the incomplete cube structure of the trinuclear titanium(IV) imido nitrido complex [{TiCp*(µ‐NH)}3(µ3‐N)] (Cp* = η5‐C5Me5). The great versatility of [{TiCp*(µ‐NH)}3(µ3‐N)] as a preorganized metalloligand is also confirmed by the preparation of complexes with heterometals in low (e.g., Mo0, IrI, and Pt0) and high oxidation states (e.g., ZrIV, TaV, and PtIV).

Photocatalytic production of hydrogen peroxide from water and dioxygen using cyano-bridged polynuclear transition metal complexes as water oxidation catalysts

H2O2 was produced from H2O and O2 using cyano-bridged polynuclear transition metal complexes as water oxidation catalysts with a Ru photocatalyst in water under visible light irradiation.

Ferromagnetism in polynuclear systems based on non-linear [MnII2MnIII] building blocks

Design of new polynuclear transition metal complexes showing ferromagnetic interactions to achieve high spin values is an important challenge due to the scarcity of bridging ligands that provide such coupling.

First principles study of electronic structure for cubane-like and ring-shaped structures of M4O4, M4S4 clusters (M = Mn, Fe, Co, Ni, Cu)

Spin-polarized DFT has been used to perform a comparative study of the geometric structures and electronic properties for isolated M4X4 nano clusters between their two stable isomers - a planar rhombus-like 2D structure and a cubane-like 3D structure with M = Mn, Fe, Co, Ni, Cu ; X = O, S. These two structural patterns of the M4X4 clusters are commonly found as building blocks in several poly-nuclear transition metal complexes in inorganic chemistry. The effects of the van der Waals corrections to the physical properties have been considered in the electronic structure calculations employing the empirical Grimme’s correction (DFT+D2). We report here an interesting trend in their relative structural stability - the isolated M4O4 clusters prefer to stabilize more in the planar structure, while the cubane-like 3D structure is more favorable for most of the isolated M4S4 clusters than their planar 2D counterparts. Our study reveals that this contrasting trend in the relative structural stability is expected to be driven by an interesting interplay between the s-d and p-d hybridization effects of the constituents’ valence electrons.

Magneto-Structural Correlations in a Series of Pseudotetrahedral [Co(II)(XR)4](2-) Single Molecule Magnets: An ab Initio Ligand Field Study.

Over the past several decades, tremendous efforts have been invested in finding molecules that display slow relaxation of magnetization and hence act as single-molecule magnets (SMMs). While initial research was strongly focused on polynuclear transition metal complexes, it has become increasingly evident that SMM behavior can also be displayed in relatively simple mononuclear transition metal complexes. One of the first examples of a mononuclear SMM that shows a slow relaxation of the magnetization in the absence of an external magnetic field is the cobalt(II) tetra-thiolate [Co(SPh)4](2-). Fascinatingly, substitution of the donor ligand atom by oxygen or selenium dramatically changes zero-field splitting (ZFS) and relaxation time. Clearly, these large variations call for an in-depth electronic structure investigation in order to develop a qualitative understanding of the observed phenomena. In this work, we present a systematic theoretical study of a whole series of complexes (PPh4)2[Co(XPh)4] (X = O, S, Se) using multireference ab initio methods. To this end, we employ the recently proposed ab initio ligand field theory, which allows us to translate the ab initio results into the framework of ligand field theory. Magneto-structural correlations are then developed that take into account the nature of metal-ligand covalent bonding, ligand spin-orbit coupling, and geometric distortions away from pure tetrahedral symmetry. The absolute value of zero-field splitting increases when the ligand field strength decreases across the series from O to Te. The zero-field splitting of the ground state of the hypothetical [Co(TePh)4](2-) complex is computed to be about twice as large as for the well-known (PPh4)2[Co(SPh)4] compound. It is shown that due to the π-anisotropy of the ligand donor atoms (S, Se) magneto-structural correlations in [Co(OPh)4](2-) complex differ from [Co(S/SePh)4](2-). In the case of almost isotropic OPh ligand, only variations in the first coordination sphere affect magnetic properties, but in the case of S/SePh ligand, variations in the first and second coordination sphere become equally important for magnetic properties.

Synthesis, structures and magnetic properties of dimeric copper and trimeric cobalt complexes supported by bridging cinnamate and chelating phenanthroline

Two polynuclear transition-metal complexes, [Co II 3 ( cinna ) 6 ( phen ) 2 ] ( 1 ), [Cu II 2 ( cinna ) 2 ( phen ) 2 (NO 3 )(H 2 O)]NO 3 ·CH 3 OH ( 2 ), where cinna = trans -cinnamate, PhCH = CHCO 2 − and phen = 1,10-phenanthroline, have been synthesized and characterized by elemental analyses, infrared spectroscopy, single-crystal X-ray diffraction (XRD) and magnetic measurements. 1 contains two independent geometrical isomers of [Co 3 ( cinna ) 6 ( phen ) 2 ], where the cobalt atoms form a linear spine bridged by carboxylate from cinnamate radiating perpendicular to it, the 1,10-phenanthroline chelating the ends. Weak C H···O hydrogen bonds in two directions generate a bidimensional network. 1 behaves as a paramagnet due to the absence of magnetic exchange between the trimers in the absence of chemical bonds between trimers. 2 contains a positively charged dimer of copper ions bridged to two cinnamate in syn–syn mode with two almost parallel phenanthroline molecules chelating the two copper ions, while a water molecule and one NO 3 − coordinate their axial positions. The dimer is described as a pincer molecule. Weak O H···O and C H···O hydrogen bonds and π–π stacking interactions are consistent with a description of a supramolecular assembly of dimers. The exchange coupling within the doubly-bridged copper atoms is antiferromagnetic ( J = −153(1) K). Deux complexes polynucléaires de cuivre et de cobalt au degré d’oxydation +II, de formules [Co II 3 ( cinna ) 6 ( phen ) 2 ] ( 1 ) et [Cu II 2 ( cinna ) 2 ( phen ) 2 (NO 3 )(H 2 O)]NO 3 ·CH 3 OH ( 2 ) où cinna = trans -cinnamate, PhCH = CHCO 2 − et phen = 1,10-phenanthroline, ont été synthétisés et caractérisés par analyse élémentaire, spectroscopie infrarouge, diffraction des rayons X sur cristaux et par mesures magnétiques. Le complexe de cobalt ( 1 ) contient deux isomères géométriquement indépendants de [Co 3 (cinna) 6 (phen) 2 ], dans lesquels les trois atomes de cobalt sont dans une disposition linéaire, pontés par un carboxylate de l’acide cinnamique et chélatés par le 1,10-phénanthroline. Un réseau bidimensionnel est généré par de faibles liaisons hydrogène du type C H···O. Ce complexe de cobalt a un comportement paramagnétique en raison de l’absence de chemin d’échange magnétique entre les trimères. Le complexe ( 2 ) contient un dimère de cuivre chargé positivement où les deux ions métalliques sont pontés par deux groupements cinnamate en mode syn – syn , les deux molécules de phénanthroline quasi parallèles viennent chélater les deux ions de cuivre, alors que la molécule d’eau et l’anion NO 3 – occupent les positions axiales des deux ions cuivreux. Le dimère est décrit comme une molécule en forme de pince. Des liaisons hydrogène faibles du type O H···O et C H···O et des interactions de type π–π stacking assurent l’assemblage supramoléculaire des dimères. Le couplage d’échange entre les atomes de cuivre doublement pontés est antiferromagnétique. La valeur de la constante de couplage est J = −153 (1) K.

A first-principles approach to the calculation of the on-site zero-field splitting in polynuclear transition metal complexes.

The interpretation of electron paramagnetic resonance spectra of polynuclear transition metal complexes in terms of individual contributions from each paramagnetic center can be greatly facilitated by the availability of theoretical methods that enable the reliable prediction of local spectroscopic parameters. In this work we report an approach that enables the application of multireference ab initio methods for the calculation of local zero field splitting tensors, one of the leading terms in the spin Hamiltonian for exchange-coupled systems of high nuclearity. The method referred to as local complete active space configuration interaction (L-CASCI) represents a multireference calculation with an active space composed of local orbitals of the center of interest. By successive permutation of the active space to include the localized orbitals corresponding to a particular center of the complex, all on-site parameters can be easily obtained at a high-level of theory with a corresponding low computational cost. Benchmark calculations on synthetic complexes confirm the validity of the approach. As an example of the applicability of the L-CASCI method to large systems, we determine the local anisotropy of the Mn(III) ion of the tetranuclear manganese cluster of photosystem II in both structural forms of its S2 state.