3d Transition Metal Complexes Research Articles

Multiplet splitting for the XPS of heavy elements: Dependence on oxidation state

Multiplet splittings in X-ray Photo-electron Spectroscopy, XPS, are a means of distinguishing different open shell occupations, or different oxidation states, in a material being studied. Indeed, especially for 3d transition metal complexes, they have provided fingerprints of the metal oxidation state. The present work provides theoretical and experimental evidence that it may also be possible to use multiplets to characterize the oxidation state of heavy metal, lanthanide and actinide, cations in complexes. However, it is important to make a proper choice of the XPS region to study in order to obtain large multiplet splittings. We identify a low binding energy, BE, peak that had been observed for Ce(III) in CeOx as a high spin coupled multiplet. Furthermore, we show that a low BE feature with reasonable intensity is characteristic of other XPS regions and of other metals. This feature arises from a high spin multiplet and serves as a fingerprint to distinguish closed shell from open shell cations. Evidence is presented that it may also be possible to distinguish different open shell occupations.

Photochemical hydrogen production from 3d transition-metal complexes bearing o-phenylenediamine ligands

Abstract A series of 3d transition-metal complexes bearing o -phenylenediamine (opda) ligands [M(opda) 3 ] 2+ (M = Mn 2+ , Co 2+ , Ni 2+ , and Zn 2+ ) were systematically synthesized as perchlorate salts and their structures were determined. All the opda complexes exhibited hydrogen-evolving activities under photoirradiation conditions as in the case of previously reported [Fe(opda) 3 ] 2+ , whereas replacement of the opda ligands with other aromatic amine ligands caused a dramatic decrease of hydrogen-evolving activities. These results support a ligand-based reaction mechanism for photochemical hydrogen evolution from [M(opda) 3 ] 2+ , which involves 1 ππ* excitation of the opda ligand followed by homolytic N H bond cleavage. In contrast to the non-catalytic reactions, an obvious metal-ion dependency was observed for the catalytic systems including hydroquinone as a proton/electron donor. The results of this work demonstrate a strategy for the design of a molecular photochemical hydrogen-production system under ambient temperature and pressure.

Preference for sulfoxide S- or O-bonding to 3d transition metals – DFT insights

Complexes of 3d transition metals with sulfoxide ligands bound by sulfur were searched on the Cambridge Structural Database. DFT calculations reproduced the experimental geometries and the lowest energy isomer (S or O bound) matched the experimentally observed one. Distances, Wiberg indices and charges (Mulliken, Atomic Polar Tensor and Natural Population Analysis) were compared, showing an elongation and weakening of the S–O bond with O-coordination. Metal charges became more positive with O-coordination, accompanied by a more negative sulfur charge. Examples of O-bound sulfoxides were studied for comparison. An Energy Decomposition Analysis was performed in a series of more relevant complexes, in order to compare the two isomers and identify the reason for the structural preference. In complexes with one S-bound sulfoxide, the attractive interaction terms (electrostatic and orbital) compensate the large Pauli repulsion by a larger amount in the S- than in the O-bound isomer. In the complex with O-bound sulfoxide this tendency was reversed by a very large reorganization energy (to bring fragments from their preferred geometry to the geometry they have in the complex). In the complexes with two ligands, S,S or O,O binding was preferred, and in one example they coexist. The mixed S,O coordination type led to a large orbital stabilization, but required large reorganization energies.

Low-melting cationic 3d-transition metal complexes of azole-based ligands

Ligands N-methylimidazole (MIm) and N-ethylimidazole (EIm) were used for the preparation of [FeII(L)6](NTf2)2·0.5EtOH·0.5H2O. With Tfus=83°C, the EIm complex was characterised as paramagnetic MeIL. Using bidentate imidazole ligands 2-(2-pyridyl)-1-ethylimidazole (L1) and 2-(2-pyridyl)-1-(2-methoxyethyl)imidazole (L2) 3:1 type complexes [MII(Li)3]X2 (M=Fe; Li=L1; X=ClO4, BF4, NTf2; M=Fe; Li=L2, X=ClO4, BF4; M=Co; X=ClO4; Li=L1, L2; M=Ni; X=ClO4; Li=L1, L2) were prepared. All complexes of L2 and complex [FeII(L1)3](NTf2)2 were low-melting with Tfus<200°C. Complexes of L2 additionally had glass transition temperatures below 75°C upon reheating.

CASPT2 study of inverse sandwich-type dinuclear 3d transition metal complexes of ethylene and dinitrogen molecules: similarities and differences in geometry, electronic structure, and spin multiplicity.

The spin multiplicities and coordination structures of inverse sandwich-type complexes (ISTCs) of ethylene and dinitrogen molecules with 3d transition metal elements (Sc to Ni), (μ-C2H4)[M(AIP)]2 and (μ-N2)[M(AIP)]2 (AIPH = (Z)-1-amino-3-iminoprop-1-ene; M = Sc to Ni) were investigated by the CASPT2 method. In both ethylene and dinitrogen ISTCs of the early 3d transition metals (Sc to Cr), sandwiched ethylene and dinitrogen ligands coordinate with two metal atoms in an η(2)-side-on form and their ground states have an open-shell singlet spin multiplicity. The η(1)-end-on coordination structure of dinitrogen ISTCs is considerably less stable than the η(2)-side-on form in these metals. For the late 3d transition metals (Mn to Ni), ethylene and dinitrogen ISTCs exhibit interesting similarities and differences in spin multiplicity and structure as follows: in ethylene ISTCs of Mn to Ni, the ground state has an open-shell singlet spin multiplicity like those of the ISTCs of early transition metals. However, the ethylene ligand is considerably distorted, in which the ethylene carbon atoms have a tetrahedral-like structure similar to sp(3) carbon and each of them coordinates with one metal in a μ-η(1):η(1) structure. These geometrical features are completely different from those of ISTCs of the early transition metals. In dinitrogen ISTCs of Mn to Ni, on the other hand, the ground state has a high spin multiplicity from nonet (Mn) to triplet (Ni). The η(2)-side-on coordination structure of the dinitrogen ligand is as stable as the η(1)-end-on form in the Mn complex but the η(1)-end-on structure is more stable than the η(2)-side-on form in the Fe to Ni complexes. All these interesting similarities and differences between ethylene and dinitrogen ISTCs and between the early and late transition metal elements arise from the occupation of several important molecular orbitals.

Cobalt(II) complexes of a new imidazolium-tagged thiadiazole ligand with bis(trifluormethylsulfonyl)imide or tetraisothiocyanato cobaltate as counterion

The ligand pentanoic acid [5-(2-pyridyl)-1,3,4-thiadiazol-2-yl]-amide-5-(3-ethyl)-3H-1-imidazolium bis(trifluormethylsulfonyl)imide (LNTf2) has been prepared and used for the complexation with “Co(NCS)2”. Compounds [CoII(L)2(NCS)2][Co(NCS)4] and [CoII(L)2(NCS)2](NTf2)2 of L + have been structurally characterised by single-crystal X-ray diffraction analyses and are rare examples of Werner-type 3d transition metal complexes with chromophores of neutral charge but cationic coordination entities, due to imidazolium tags.

Theoretical modelling of biomolecular systems I. Large-scale QM/MM calculations of hydrogen-bonding networks of the oxygen evolving complex of photosystem II

Quantum mechanical (QM)/molecular mechanics (MM) calculations by the use of a large-scale QM model (QM Model V) have been performed to elucidate hydrogen-bonding networks and proton wires for proton release pathways (PRP) of water oxidation reaction in the oxygen evolving complex (OEC) of photosystem II (PSII). Full geometry optimisations of PRP by the QM/MM model have been carried out starting from the geometry of heavy atoms determined by the recent high-resolution X-ray diffraction (XRD) experiment of PSII refined to 1.9 Å resolution. Computational results by the QM/MM calculations have elucidated the hydrogen-bonding O···O(N) and O···H distances and O(N)–H···O angles in PRP, together with the Cl–O(N) and Cl···H distances and O(N)–H···Cl angles for chloride anions. The optimised hydrogen-bonding networks are well consistent with the XRD results and available experiments such as extended X-ray absorption fine structure, showing the reliability of channel structures of OEC of PSII revealed by the XRD experiment. The QM/MM computations have elucidated possible roles of chloride anions in the OEC of PSII. The QM/MM computational results have provided useful information for understanding and explanation of accumulated mutation experiments of key amino acid residues in the OEC of PSII. Implications of the present results are discussed in relation to three steps for theoretical modelling of water oxidation in the OEC of PSII and bio-inspired working hypotheses for developments of artificial water oxidation systems by use of 3d transition-metal complexes.

English

The effect of a schiff base 2-(2-hydroxybenzylideneamino)phenol (HBAP) and its corresponding three 3d-transition metal complexes viz. Mn-HBAP, Co-HBAP and Ni-HBAP on the corrosion inhibiting behaviour against chrome steel bearing balls in 0.1 N HNO3 solution were investigated by means of conventional weight loss method, potentiodynamic polarization and scanning electron microscopy (SEM) studies. The adsorption nature of all inhibitors over chrome steel surface were spontaneous and were best fitted by Langmuir's adsorption isotherm. Stability of complexes in acid media were supported from spectrophotometry measurements. The mixed mode of inhibition was confirmed from polarization and inhibition efficiency (% I.E.) could be increased with concomitant increase in inhibitor concentration in bulk experimental solution. Correlation between the inhibitive effect and molecular structure of inhibitor's was further supported by Density Functional Theory (DFT) like quantum chemical calculation. An effective quick prediction about each and individual inhibitor on inhibitor/metal interface about simultaneous effect of varying concentration and reaction time was successfully exploited by Design of experiments (DOE).

Tracking excited-state charge and spin dynamics in iron coordination complexes.

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

New supramolecular synthons based on 3d transition metal complexes with bidentate bispidines: synthesis and structural, spectroscopic, and electrochemical studies

A new supramolecular synthon, a 2L: 1M complex of bidentate bispidine with transition metal, was suggested. A number of complex compounds of 1,5-dimethylbispidin-9-one with Cu ii , Ni ii , and Co ii salts (perchlorates, chlorides, bromides, nitrates, trifluoroacetates) were synthesized. The structure and composition of complexes obtained were estimated based on the combination of physicochemical methods of analysis (elemental analysis, X-ray diffraction analysis, NMR, IR, and Raman spectroscopy, electron absorption spectroscopy, ESI mass spectrometry, CVA). The composition of most complexes in the solid state corresponds to the formula ML2X2, and in a number of cases the fragment [ML2]2+ is retained in solution, which was shown by NMR spectroscopy for nickel complexes and by mass spectrometry for copper complexes. In the case of copper chloride, the composition of the acetonitrile solutions in the presence of the ligand depends on the ratio of starting compounds and their concentration: an increase in proportion of CuCl2 and concentration shifts the equilibrium to the side of complexes with higher nuclearity. A 2L: 1M complex of bispidine with metal was for the first time structurally characterized using copper(ii) nitrate derivative as an example. The CVA method showed existence of reversible electrochemical reduction of the metal for a number of complexes (the products of the reaction of 1,5-dimethylbispidin-9-one with nickel(ii) and cobalt(ii) chlorides, as well as with nickel(ii) bromide). According to the IR and Raman spectra, chelation of metals with 1,5-dimethylbispidin-9-one leads to the shift of the absorption band of the ligand carbonyl group from 1700 to 1721–1744 cm−1. Nickel(ii) and copper(ii) perchlorates and nitrates were concluded to be the most promising complexation agents for the binding of two NH,NH-bispidine ligands in a coordination polymer.

Terpyridine complexes of first row transition metals and electrochemical reduction of CO2 to CO

Homoleptic terpyridine complexes of first row transition metals are evaluated as catalysts for the electrocatalytic reduction of CO2. Ni and Co-based catalytic systems are shown to reduce CO2 to CO under the conditions tested. The Ni complex was found to exhibit selectivity for CO2 over proton reduction while the Co-based system generates mixtures of CO and H2 with CO : H2 ratios being tuneable through variation of the applied potential.

Synthesis, Characterization and Antimicrobial Screening of Some 4-Acylpyrazoloimines and Some 3d Transition Metal(II) Complexes

Synthesis, Characterization and Antimicrobial Screening of Some 4-Acylpyrazoloimines and Some 3d Transition Metal(II) Complexes

Ab Initio Calculations of X-ray Spectra: Atomic Multiplet and Molecular Orbital Effects in a Multiconfigurational SCF Approach to the L-Edge Spectra of Transition Metal Complexes.

A new ab initio approach to the calculation of X-ray spectra is demonstrated. It combines a high-level quantum chemical description of the chemical interactions and local atomic multiplet effects. We show here calculated L-edge X-ray absorption (XA) and resonant inelastic X-ray scattering spectra for aqueous Ni(2+) and XA spectra for a polypyridyl iron complex. Our quantum chemical calculations on a high level of accuracy in a post-Hartree-Fock framework give excellent agreement with experiment. This opens the door to reliable and detailed information on chemical interactions and the valence electronic structure in 3d transition-metal complexes also in transient excited electronic states. As we combine a molecular-orbital description with a proper treatment of local atomic electron correlation effects, our calculations uniquely allow, in particular, identifying the influence of interatomic chemical interactions versus intra-atomic correlations in the L-edge X-ray spectra.

Photophysical Properties of MM Quadruply Bonded Complexes Supported by Carboxylate Ligands, MM = Mo2, MoW, or W2

While chemists have extensively studied the photophysical properties of d(6), d(8), and d(10) transition metal complexes, their early transition metal counterparts have received less attention. Quadruply bonded complexes of molybdenum and tungsten supported by carboxylate ligands have intense metal-to-ligand charge transfer (MLCT) absorptions that arise from the electronic coupling of the metal-metal (MM) δ orbital with the CO(2) π-system. This coupling may in turn be linked to an extended π-conjugated organic functional group. The major interaction is akin to the so-called back-bonding in metal carbonyl complexes. By the appropriate selection of MM, its attendant ligands, and the organic group, this absorption can be tuned to span the visible and near IR range, from 400 to 1000 nm. Consequently, these complexes offer potential as photon harvesters for photovoltaic devices and photocatalysis. In this Account, we describe recent studies of dinuclear M(II) containing complexes, where M = Mo or W, and show that there are both parallels and disparities to the monomeric transition metal complexes. These early transition metal complexes have relatively long lived excited state singlets when compared to other transition metal complexes. They also often show unusual dual emission (fluorescence and phosphorescence), with singlet (S(1)) lifetimes that range from 1 to 20 ps, and triplet (T(1)) lifetimes from 3 ns to 200 μs. The fluorescent S(1) states are typically (1)MLCT for both M = Mo and W. These extended singlet lifetimes are uncommon for mononuclear transition metal complexes, which typically have very short lived (1)MLCT states due to rapid femto-second intersystem crossing rates. However, the T(1) states differ. This phosphorescence is MLCT in nature when M = W, while this emission comes from the δδ* state for M = Mo. Through time-resolved femtosecond infrared spectroscopy, we can detect the asymmetric stretch of the CO(2) ligand in both the singlet and triplet δδ* states. Through these analytical methods, we can study how the charge distribution in the singlet and triplet excited states changes over time. In addition, we can detect delocalized or localized examples of MLCT states, which represent class III and I excited state mixed valence in the Robin and Day scheme.

Structural and Electronic Comparison of 1st Row Transition Metal Complexes of a Tripodal Iminopyridine Ligand

We report the preparation and characterization of a series of divalent 3d transition metal complexes (Cr to Zn, 1-7), featuring the multidentate, tripodal iminopyridine Schiff-base ligand trimethyl 6,6',6″-((1E,1'E,1″E)-((nitrilotris(ethane-2,1-diyl))tris(azanylylidene))tris-(methanylylidene))trinicotinate (L(5-OOMe)). X-ray structural studies carried out on 1-5 and 7 reveal complex geometries ranging from local octahedral coordination to significant distortion toward trigonal prismatic geometry to heptacoordinate environments. Regardless of coordination mode, magnetic and spectroscopic studies show the ligand to provide moderately strong ligand fields: the Fe complex is low-spin, while the Co and Mn complexes are high-spin at all temperatures probed. Cyclic voltammograms exhibit multiple reversible ligand-based reductions, which are relatively consistent throughout the series; however, the electrochemical behavior of the Cr complex 1 is fundamentally different from those of the other complexes. Time-dependent (TD) density functional theory (DFT) and natural transition orbital (NTO) computational analyses are presented for the ligand, its anion, and complexes 1-7: the computed spectra reproduce the major differential features of the observed visible absorption spectra, and NTOs provide viable interpretations for the observed features. The combined studies indicate that all complexes contain neutral ligands bound to M(II) ions, except for the Cr complex 1, which is best described as a Cr(III) species bound to a radical anionic ligand.

XPS study of the electronic structure of binuclear 3d transition metal pivalate complexes

Binuclear pivalate complexes of 3d transition metals (manganese, iron, cobalt, and nickel) with the same ligand environment and a lantern structure have been studied by X-ray photoelectron spectroscopy. The M2p, M3s, C1s, O1s, and N1s X-ray photoelectron spectra have been examined. A redistribution of electron density in the OCO group has been revealed. It has been shown that the theory fits the experimental data on the energy separation between the high- and low-spin components in the M3s spectra and between the spin doublet components in the M2p spectra. It has been demonstrated that the iron, cobalt, and nickel complexes are paramagnetic at room temperature, whereas the manganese complex exhibits antiferromagnetic properties. There is a correlation between the size of the 3d subshell of the transition metal atom and the M-O and M-N bond lengths.

Theoretical Study of Inverted Sandwich Type Complexes of 4d Transition Metal Elements: Interesting Similarities to and Differences from 3d Transition Metal Complexes

Inverted sandwich type complexes (ISTCs) of 4d metals, (μ-η(6):η(6)-C(6)H(6))[M(DDP)](2) (DDPH = 2-{(2,6-diisopropylphenyl)amino}-4-{(2,6-diisopropylphenyl)imino}pent-2-ene; M = Y, Zr, Nb, Mo, and Tc), were investigated with density functional theory (DFT) and MRMP2 methods, where a model ligand AIP (AIPH = (Z)-1-amino-3-imino-prop-1-ene) was mainly employed. When going to Nb (group V) from Y (group III) in the periodic table, the spin multiplicity of the ground state increases in the order singlet, triplet, and quintet for M = Y, Zr, and Nb, respectively, like 3d ISTCs reported recently. This is interpreted with orbital diagram and number of d electrons. However, the spin multiplicity decreases to either singlet or triplet in ISTC of Mo (group VI) and to triplet in ISTC of Tc (group VII), where MRMP2 method is employed because the DFT method is not useful here. These spin multiplicities are much lower than the septet of ISTC of Cr and the nonet of that of Mn. When going from 3d to 4d, the position providing the maximum spin multiplicity shifts to group V from group VII. These differences arise from the size of the 4d orbital. Because of the larger size of the 4d orbital, the energy splitting between two d(δ) orbitals of M(AIP) and that between the d(δ) and d(π) orbitals are larger in the 4d complex than in the 3d complex. Thus, when occupation on the d(δ) orbital starts, the low spin state becomes ground state, which occurs at group VI. Hence, the ISTC of Nb (group V) exhibits the maximum spin multiplicity.

Complexes with a Single Metal-Metal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals.

The electronic structure of the vanadium dimer complex [V(C5H5)]2Pn with a single metal-metal bond was characterized, and the energies of higher spin states were evaluated. To simplify evaluation of orbital contributions to bonding between atoms and fragments, occupancy-perturbed bond orders were introduced. The structure and experimentally determined singlet-triplet gap in this complex can be used to test the quality of modern exchange-correlation functionals. Most generalized gradient approximation (GGA) functionals were determined to be quite suitable to reproduce the metal-metal distance and the single-triplet energy gap in [V(C5H5)]2Pn. Further accuracy improvement can be achieved by using empirical dispersion corrections. Hybrid exchange-correlation functionals, including the B3LYP functional, performed poorly for both structural and energy predictions. The hybrid functionals significantly overestimate the stability of the singlet state with the antiferromagnetically coupled high-spin metal ions relative to the lowest-energy triplet state and the singlet state with stronger metal-metal interactions. Thus, these XC functionals are not quite suitable for computational studies of multinuclear 3d transition metal complexes with weak-to-intermediate metal-metal bonding.

DNA-binding, oxidative DNA cleavage, and coordination mode of later 3d transition metal complexes of a Schiff base derived from isatin as antimicrobial agents

A Schiff base, obtained by the condensation of isatin monohydrazone with 2,3,5-trichlorobenzaldehyde, and its Co(II), Ni(II), Cu(II), and Zn(II) complexes have been synthesized and characterized. The interaction of these complexes with DNA is investigated using viscosity, absorption titration, and electrochemical techniques. The results indicate that the complexes bind to Calf thymus DNA through intercalation. Oxidative cleavage activities of the complexes are studied using supercoiled pBR322 DNA by gel electrophoresis. Antimicrobial study reveals that copper and zinc complexes are better antimicrobial agents than the Schiff base and its other complexes.

Understanding the Electronic Structure of 4d Metal Complexes: From Molecular Spinors to L-Edge Spectra of a di-Ru Catalyst

L(2,3)-edge X-ray absorption spectroscopy (XAS) has demonstrated unique capabilities for the analysis of the electronic structure of di-Ru complexes such as the blue dimer cis,cis-[Ru(III)(2)O(H(2)O)(2)(bpy)(4)](4+) water oxidation catalyst. Spectra of the blue dimer and the monomeric [Ru(NH(3))(6)](3+) model complex show considerably different splitting of the Ru L(2,3) absorption edge, which reflects changes in the relative energies of the Ru 4d orbitals caused by hybridization with a bridging ligand and spin-orbit coupling effects. To aid the interpretation of spectroscopic data, we developed a new approach, which computes L(2,3)-edges XAS spectra as dipole transitions between molecular spinors of 4d transition metal complexes. This allows for careful inclusion of the spin-orbit coupling effects and the hybridization of the Ru 4d and ligand orbitals. The obtained theoretical Ru L(2,3)-edge spectra are in close agreement with experiment. Critically, existing single-electron methods (FEFF, FDMNES) broadly used to simulate XAS could not reproduce the experimental Ru L-edge spectra for the [Ru(NH(3))(6)](3+) model complex nor for the blue dimer, while charge transfer multiplet (CTM) calculations were not applicable due to the complexity and low symmetry of the blue dimer water oxidation catalyst. We demonstrated that L-edge spectroscopy is informative for analysis of bridging metal complexes. The developed computational approach enhances L-edge spectroscopy as a tool for analysis of the electronic structures of complexes, materials, catalysts, and reactive intermediates with 4d transition metals.