New Donor‐Functionalized Cp Ligands: Synthesis and Complexation Behaviour of Quinoxalyl and Benzothiadiazolyl Systems

Sodium cyclopentadienide reacts as nucleophile with 4,7‐dibromo‐2,1,3‐benzothiadiazole (BTZ) and leads to the new donor‐functionalized ligand Cp^BTZ. Related quinoxalyl Cp systems have been prepared using Pd‐catalyzed coupling with zincated Cp‐metal complexes. The new ligands comprise two N‐donor atoms; one of them is located in a distal position relative to the metal centre so that it cannotcoordinate in a chelating manner. With Cp^BTZ ligand derivatives severalmetal complexes have been synthesized. The new chromium(III) complex Cp^BTZCrCl₂ ( 12 ) becomes upon activation an active catalyst for the polymerization of ethylene. Relying on DFT calculations and analysis of spin‐density distribution combined with paramagnetic NMR data a chelating coordination of the Cp^BTZ ligand is feasible in 12 .     

Quantum chemical interpretation of redox properties of ruthenium complexes with vinyl and TCNX type non-innocent ligands

This review provides an overview of density functional theory (DFT) calculations in a consequence with spectroelectrochemical measurements on mononuclear and symmetrically or unsymmetrically bridged di- and tetranuclear ruthenium complexes of vinyl and TCNX ligands. The DFT approach is used for the calculations of molecular structures, vibrational frequencies, electronic and electron paramagnetic resonance (EPR) spectral data. DFT calculations enable us to identity the primary redox site and the electron and spin-density distribution between the individual components for the individual redox congeners. The DFT technique reproduces the spectral properties of the presented complexes and their radical ions. The generally close correspondence between experimental and quantum chemical results demonstrate that modern DFT is a powerful tool to address issues like ligand non-innocence and electron and spin delocalization in systems containing both redox-active metal ions and redox-active ligands.     

Synthesis,spectral, DFT calculations and antibacterial studies of Fe(III) complexes of new fluorescent Schiff bases derived from imidazo[4',5':3,4]benzo[1,2‐c]isoxazole

New fluorescent heterocyclic ligands were synthesized by the reaction of 8‐(4‐chlorophenyl)‐3‐alkyl‐3H‐imidazo[4',5':3,4]benzo [1,2‐c]isoxazol‐5‐amine with p‐hydroxybenzaldehyde and p‐chlorobenzaldehyde in good yields. The coordination ability of the ligands with Fe³⁺ ion was examined in an aqueous metanolic solution. Schiff base ligands and their metal complexes were characterized by elemental analyses, IR, UV–vis, mass, and NMR spectra. The optical properties of the compounds were investigated and the results showed that the fluorescence of all compounds is intense and their obtained emission quantum yields are around 0.15 – 0.53. Optimized geometries and assignment of the IR bands and NMR chemical shifts of the new complexes were also computed by using density functional theory (DFT) methods. The DFT‐calculated vibrational wavenumbers and NMR chemical shifts are in good agreement with the experimental values, confirming suitability of the optimized geometries for Fe(III) complexes. Also, the 3D‐distribution map for HOMO and LUMO of the compounds were obtained. The new compounds showed potent antibacterial activity and their antibacterial activity (MIC) against Gram‐positive and Gram‐negative bacterial species were also determined. Results of antibacterial test revealed that coordination of ligands to Fe(III) leads to improvement in the antibacterial activity.     

Quantum Chemical Study on the Structure and Energetics of Binary Ionic Porphyrin Complexes

We study the structural and energetic properties of binary ionic porphyrin molecular complexes [H₄TPPS₄]²⁻∙∙∙SnTP using quantum chemical techniques. As the axial ligands and the protonation of pyridine sites highly influence the structure and coordination of metal‐containing porphyrin, various structures of SnTP in the presence and absence of axial ligands and pyridine protons were considered. The constructed porphyrins were then made to interact face to face, and the formed complexes were optimized at the HF/STO‐3G level of theory. The stability and stack‐like interaction of the complexes were analyzed through interplanar spacing, planar angle, and edge‐to‐edge distance. The structural parameters emphasize the importance of axial ligands for the formation of stack‐like structures. The complex that contains axial ligands with pyridine protons, namely [H₄TPPS₄]²⁻∙∙∙[X'SnXTPH]⁴⁺, shows a perfectly stacked layer with a reasonable interplanar distance, which is confirmed from the calculated counterpoise interaction and deformation energies. The energetic parameters were found to correlate well with the obtained geometries. The molecular electrostatic potential (MEP) maps were obtained to infer the presence of nonbonded interaction between the binary ionic porphyrins.     

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid-State NMR Spectroscopy,Polarized Neutron Diffraction,and First-Principle Calculations

The local magnetic structure in the [Fe^III(Tp)(CN)₃]⁻ building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions.     

Design, synthesis, and properties of conformationally fixed semiquinone monoradical species

The design of novel, functionalized semiquinone (SQ) ligands which combine structural rigidity and electron-withdrawing, electron-donating, and electroneutral substituents enables investigation of multiple structure-property relationships and building blocks for new materials, including components of sensors, switches, and molecular spintronics. Along these lines, we report the synthesis of several new SQ ligands containing fused heterocyclic ring systems. Using both electron paramagnetic resonance spectroscopy and quantum chemical calculations, we show how spin density is affected by the fused ring system substituents.     

95Mo NMR: a useful tool for structural studies in solution

Oxomolybdenum(VI) complexes containing diverse ligands from an electronic and topological point of view have been analysed by means of ⁹⁵Mo NMR in solution with the purpose of using this technique as a tool to study their coordination chemistry and reactivity. The relationship between the electronic density on the metal tuned by the electron‐donor ability of the coordinated ligands and the ⁹⁵Mo chemical shift has been proved for mono‐ and bimetallic complexes showing a hexa‐ or hepta‐coordination around the metal centre. The different origins of the signal broadening (associated either to the symmetry of the metallic polyhedron or to the presence of isomers or to the ligand de‐coordination) have been also considered to rationalise the obtained data. Copyright © 2009 John Wiley & Sons, Ltd.     

Gas‐phase doubly charged complexes of cyclic peptides with copper in +1, +2 and +3 formal oxidation states: formation,structures and electron capture dissociation

Copper complexes with a cyclic D‐His‐β‐Ala‐L‐His‐L‐Lys and all‐L‐His‐β‐Ala‐His‐Lys peptides were generated by electrospray which were doubly charged ions that had different formal oxidation states of Cu(I), Cu(II) and Cu(III) and different protonation states of the peptide ligands. Electron capture dissociation showed no substantial differences between the D‐His and L‐His complexes. All complexes underwent peptide cross‐ring cleavages upon electron capture. The modes of ring cleavage depended on the formal oxidation state of the Cu ion and peptide protonation. Density functional theory (DFT) calculations, using the B3LYP with an effective core potential at Cu and M06‐2X functionals, identified several precursor ion structures in which the Cu ion was threecoordinated to pentacoordinated by the His and Lys side‐chain groups and the peptide amide or enolimine groups. The electronic structure of the formally Cu(III) complexes pointed to an effective Cu(I) oxidation state with the other charge residing in the peptide ligand. The relative energies of isomeric complexes of the [Cu(c‐HAHK + H)]²⁺ and [Cu(c‐HAHK ? H)]²⁺ type with closed electronic shells followed similar orders when treated by the B3LYP and M06‐2X functionals. Large differences between relative energies calculated by these methods were obtained for open‐shell complexes of the [Cu(c‐HAHK)]²⁺ type. Charge reduction resulted in lowering the coordination numbers for some Cu complexes that depended on the singlet or triplet spin state being formed. For [Cu(c‐HAHK ? H)]²⁺ complexes, solution H/D exchange involved only the N–H protons, resulting in the exchange of up to seven protons, as established by ultra‐high mass resolution measurements. Contrasting the experiments, DFT calculations found the lowest energy structures for the gas‐phase ions that were deprotonated at the peptide C_α positions. Copyright © 2012 John Wiley & Sons, Ltd.     

Combining NMR of dynamic and paramagnetic molecules: fluxional high-spin nickel(II) complexes bearing bisguanidine ligands

A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.     

Complexes of Uranyl Nitrate with 2,6‐Pyridinedicarboxamides: Synthesis,Crystal Structure,and DFT Study

Two complexes of uranyl nitrate with N,N,N′,N′‐tetrabutyl‐2,6‐pyridinedicarboxamide (TBuDPA) and N,N′‐diethyl‐N,N′‐diphenyl‐2,6‐pyridinedicarboxamide (EtPhDPA) were synthesized and studied. The complex of tetraalkyl‐2,6‐pyridinedicarboxamide with metal nitrate was synthesized for the first time. XRD analysis revealed the different type of complexation: a 1:1 metal:ligand complex for EtPhDPA and complex with polymeric structure for TBuDPA. The quantum chemical calculations (DFT) confirm that both ligands form the most stable complexes that match the minimal values pre‐organization energy of the ligands.     

Electric-field gradients and magnetic hyperfine parameters of square-pyramidal [M(CN)5]3− (M = Co,Rh, and Ir) complexes

Density functional self-consistent spin-polarized calculations with the discrete variational method were performed to obtain the electronic structure of the paramagnetic complexes [Co(CN)₅]^3?, [Rh(CN)₅]^3?, and [Ir(CN)₅]^3? of square-pyramidal geometry. All electrons were kept in the variational space. Electric-field gradients and magnetic hyperfine parameters at the metal site were computed with the molecular charge and spin densities obtained and compared with experimental values derived by electron paramagnetic resonance. It was found that the Fermi interaction is critically dependent on the angle between the axial and equatorial CN ligands. © 1995 John Wiley & Sons, Inc.     

Coordination Chemistry of Organoruthenium Compounds with Benzoylthiourea Ligands and their Biological Properties

Benzoylthiourea derivatives feature several donor atoms capable of coordinating to metal centers. We report here a series of Ru(η⁶‐p‐cymene) complexes employing benzoylthiourea derivatives as ligands. Such ligands often coordinate to metal centers through their S and O donor atoms. We isolated complexes where the ligands were mono‐ or bidentately coordinated to Ru involving the S donor atom and surprisingly in bidentate coordination mode a deprotonated thiourea nitrogen resulting in a 4‐membered ring structure around the metal center. DFT calculations were used to explain the differences in coordination behavior. These were complemented by stability studies and biological investigations of the compounds as anticancer agents. Several of the synthesized derivatives exhibited significant cell growth inhibitory activity, with the complexes featuring bidentate ligands being more potent than their monodentate counterparts. This can be explained by the higher stability of the former under the conditions employed in cell culture assays.     

Protonation of metal‐bound α‐hydroxycarboxylate ligand and implication for the role of homocitrate in nitrogenase: Computational study of the oxy‐bidentate chelate ring opening

Protonation of the metal‐bound oxy‐bidentate ligand in the model complexes of [(HS)₃(NH₃)M(OCH₂COO)]^q (M = Mo, Fe, V, Co; q = ?2, ?1) in the gas phase and in solutions of water and acetonitrile has been explored by the density functional approach. Calculations show that protonation of the carboxyl oxygen can open the α‐hydroxycarboxylate chelate ring ligated to a transition‐metal center under specific oxidation and spin states. The feasibility of the chelate ring opening by protonation depends on the electronic nature of the metal site in tune with conversion of a six‐coordinate with a five‐coordinate metal atom. Such selective dissociation of the metal‐bound chelate ligand manipulates the availability of an empty site at the metal center and significantly affects reactivity of the metal‐mediated chemical processes. Protonation changes the stability of species with different spin multiplicities and impels spin transition at the metal center in dissociation of the oxy‐bidentate ligand. Solvent environments of water and acetonitrile play an important role in stabilizing the negatively charged species. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Cysteine and Methionine Derivatives of Aminophenol Ligands: Synthesis,Iron Complexation,Magnetic, Electronic and Redox Investigation

Two new Cysteine and Methionine derivatives of aminophenol ligands (HL^Cys and HL^Met) were synthesized by a convenient procedure. The ligands were characterized by ¹H NMR, ¹³C NMR and IR spectroscopies, ESI‐MS, elemental analysis and optical activity measurements. Iron(III) complexes (FeL^Cys and FeL^Met) of these ligands were synthesized and characterized by spectroscopic, magnetic susceptibility studies and cyclic voltammetry techniques. The molecular structure of FeL^Cys and FeL^Met determined by ESI‐MS consist of two ligands coordinated to Fe(III) centers. The magnetic susceptibility measurement indicates the monomeric complexes with paramagnetic properties. Both complexes undergo metal‐centred reduction, and ligand‐centred oxidation.     

Ruthenium complexes with lumazine derivatives: structural,electrochemical, computational and radical scavenging studies

In this research study, the formation and characterization of new ruthenium(II) and (III) complexes encompassing multidentate ligands derived from 6-acetyl-1,3,7-trimethyllumazine (almz) are reported. The 1:1 molar coordination reactions of trans-[RuCl₂(PPh₃)₃] with N-1-[1,3,7-trimethyllumazine]benzohydride (bzlmz) and 6-(N-methyloxime)-1,3,7-trimethyllumazine (ohlmz) formed a diamagnetic ruthenium(II) complex, cis-[RuCl₂(bzlmz)(PPh₃)] (1), and paramagnetic complex, cis-[Ru^IIICl₂(olmz)(PPh₃)] (2) [Holmz = 6-(N-hydroxy-N′-methylamino)-1,3,7-trimethyllumazine], respectively. These ruthenium complexes were characterized via physico-chemical and spectroscopic methods. Structural elucidations of the metal complexes were confirmed using single crystal X-ray analysis. The redox properties of the metal complexes were investigated via cyclic voltammetry. Electron spin resonance spectroscopy confirmed the presence of a paramagnetic metal centre in 2. The radical scavenging activities of the metal complexes were explored towards the DPPH and NO radicals. Quantum calculations at the density functional theory level provided insight into the interpretation of the IR and UV–Vis experimental spectra of 1.     

Solid‐State 17O NMR Spectroscopy of Paramagnetic Coordination Compounds

High‐quality solid‐state ¹⁷O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V^III (S=1), Cu^II (S=1/2), and Mn^III (S=2) metal centers, the ¹⁷O isotropic paramagnetic shifts were found to span a range of more than 10 000 ppm. In several cases, high‐resolution ¹⁷O NMR spectra were recorded under very fast magic‐angle spinning (MAS) conditions at 21.1 T. Quantum‐chemical computations using density functional theory (DFT) qualitatively reproduced the experimental ¹⁷O hyperfine shift tensors.     

Neutral bis(alpha-iminopyridine)metal complexes of the first-row transition ions (Cr, Mn, Fe, Co, Ni, Zn) and their monocationic analogues: mixed valency involving a redox noninnocent ligand system

A series of bis(alpha-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic pi radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural characterizations, the neutral complexes are best described as possessing a divalent metal center and two monoanionic pi radicals of the alpha-iminopyridine. The neutral M(L*)2 compounds undergo ligand-centered, one-electron oxidations generating a second series, [(L(x))2M(THF)][B(ArF)4] [where L(x) represents either the neutral alpha-iminopyridine (L)0 and/or its reduced pi radical anion (L*)-]. The cationic series comprise mostly mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L*)-, and the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe and Co complexes exhibit Robin-Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr, and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV-vis spectroscopy. The delocalization versus localization of the ligand radical is determined only by the nature of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a monovalent metal center or [(L)2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between high spin and intermediate spin (S(Fe) = 2 <--> S(Fe) = 1), is described for the complex [(L*)(L)Fe(THF)][B(ArF)4], which consequently is characterized by the overall spin equilibrium (S(tot) = 3/2 <--> S(tot) = 1/2). The two different spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR spectroscopy, zero-field and applied-field M?ssbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.     

Computational modeling of spin crossover phenomenon in adducts of iron <Emphasis Type="Italic">bis</Emphasis>-chelates with <Emphasis Type="Italic">o</Emphasis>-diiminobenzoquinones

A density functional B3LYP*/6-311++G(d,p) quantum chemical study of the interaction of Fe^II complexes with o-diiminobenzoquinones showed that adduct formation is accompanied by oxidation of the metal ion and conversion of the redox-active ligand to the semiquinonate form. Variation of substituents at nitrogen atoms of the bis-chelate and diimine made it possible to reveal the spin-crossover complexes. The nature and strength of the exchange interactions between the unpaired electrons of paramagnetic centers of the adducts studied depend on the spin state of their isomers and on the type of the iron complex.