Amine, Amido, and Imido Complexes of Tantalum Supported by a Pyridine-Linked Bis(phenolate) Pincer Ligand: Ta−N π-Bonding Influences Pincer Ligand Geometry

A series of tantalum imido and amido complexes supported by a pyridine-linked bis(phenolate) ligand has been synthesized. Characterization of these complexes via X-ray crystallography reveals both C_s and C_2 binding modes of the bis(phenolate)pyridine ligand, with complexes containing two or fewer strong π-donor interactions from ancillary ligands giving C_s symmetry, whereas three strong π-donor interactions (e.g., three amido ligands or one amido ligand and one imido ligand) give C_2-symmetric binding of the bis(phenolate)pyridine ligand. DFT calculations and molecular orbital analyses of the complexes have revealed that the preference for C_s-symmetric ligand binding is a result of tantalum−phenolate π-bonding, whereas in cases where tantalum−phenolate π-bonding is overridden by stronger Ta−N π-bonding, C_2-symmetric ligand binding is preferred, likely because conformationally this is the lowest-energy arrangement. This electronically driven change in geometry indicates that, unlike analogous metallocene systems, the bis(phenolate)pyridine pincer ligand is not a strong enough π-donor to exert dominant control over the electronic and geometric properties of the complex

Groups 5 and 6 Terminal Hydrazido(2−) Complexes: N_β Substituent Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation States

Brightly colored terminal hydrazido(2−) (dme)MCl_3(NNR_2) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl_4(NNR_2) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR_2) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N_β lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N_β in order to reduce the overlap between the Nb═Nα π bond and the Nβ lone pair. For W complexes, increasing the energy of N_β eventually leads to reduction from formally [W^(VI)≡N–NR_2] with a hydrazido(2−) ligand to [W^(IV)═N═NR_2] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives

Extracting physical chemistry from mechanics: a new approach to investigate DNA interactions with drugs and proteins in single molecule experiments

In this review we focus on the idea of establishing connections between the mechanical properties of DNAligand complexes and the physical chemistry of DNA-ligand interactions. This type of connection is interesting because it opens the possibility of performing a robust characterization of such interactions by using only one experimental technique: single molecule stretching. Furthermore, it also opens new possibilities in comparing results obtained by very different approaches, in special when comparing single molecule techniques to ensemble-averaging techniques. We start the manuscript reviewing important concepts of the DNA mechanics, from the basic mechanical properties to the Worm-Like Chain model. Next we review the basic concepts of the physical chemistry of DNA-ligand interactions, revisiting the most important models used to analyze the binding data and discussing their binding isotherms. Then, we discuss the basic features of the single molecule techniques most used to stretch the DNA-ligand complexes and to obtain force x extension data, from which the mechanical properties of the complexes can be determined. We also discuss the characteristics of the main types of interactions that can occur between DNA and ligands, from covalent binding to simple electrostatic driven interactions. Finally, we present a historical survey on the attempts to connect mechanics to physical chemistry for DNA-ligand systems, emphasizing a recently developed fitting approach useful to connect the persistence length of the DNA-ligand complexes to the physicochemical properties of the interaction. Such approach in principle can be used for any type of ligand, from drugs to proteins, even if multiple binding modes are present

Complexation of lanthanides, actinides and transition metal cations with a 6-(1,2,4-triazin-3-yl)-2,2’:6’,2’’-terpyridine ligand: implications for actinide(III) /lanthanide(III) partitioning

The quadridentate N-heterocyclic ligand 6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2’:6’,2’’-terpyridine (CyMe4-hemi-BTBP) has been synthesized and its interactions with Am(III), U(VI), Ln(III) and some transition metal cations have been evaluated by X-ray crystallographic analysis, Am(III)/Eu(III) solvent extraction experiments, UV absorption spectrophotometry, NMR studies and ESI-MS. Structures of the 1:1 complexes with Eu(III), Ce(III) and the linear uranyl (UO22+) ion were obtained by X-ray crystallographic analysis, and showed similar coordination behavior to related BTBP complexes. In methanol, the stability constants of the Ln(III) complexes are slightly lower than those of the analogous quadridentate bis-triazine BTBP ligands, while the stability constant for the Yb(III) complex is higher. 1H NMR titrations and ESI-MS with lanthanide nitrates showed that the ligand forms only 1:1 complexes with Eu(III), Ce(III) and Yb(III), while both 1:1 and 1:2 complexes were formed with La(III) and Y(III) in acetonitrile. A mixture of isomeric chiral 2:2 helical complexes was formed with Cu(I), with a slight preference (1.4:1) for a single directional isomer. In contrast, a 1:1 complex was observed with the larger Ag(I) ion. The ligand was unable to extract Am(III) or Eu(III) from nitric acid solutions into 1-octanol, except in the presence of a synergist at low acidity. The results show that the presence of two outer 1,2,4-triazine rings is required for the efficient extraction and separation of An(III) from Ln(III) by quadridentate N-donor ligand

Functionalisation of bolaamphiphiles with mononuclear bis(2,2'-bipyridyl)ruthenium(II) complexes for application in self assembled monolayers

A novel ruthenium(II) polypyridyl complex connected competently to a bolaamphiphile, containing amide linkages to provide rigidity via hydrogen bonding in the monolayer, has been prepared. The ruthenium(II) complexes of this ligand and of the intermediates in the synthesis were prepared by modification of the coordinated ligands, demonstrating the synthetic versatility and robustness of this family of complexes. All ruthenium complexes were characterised by electrochemical and spectroscopic techniques and were found to have similar properties to the parent complex [Ru(bipy)[3]][2][+], and remain versatile photosensitisers, with well-defined properties, despite extensive substitution of the bipy ligand

Thermodynamic Studies of [H_(2)Rh(diphosphine)_2]^+ and [HRh(diphosphine)_(2)(CH_(3)CN)]^(2+) Complexes in Acetonitrile

Thermodynamic studies of a series of [H_(2)Rh(PP)_2]^+ and [HRh(PP)_(2)(CH_(3)CN)]^(2+) complexes have been carried out in acetonitrile. Seven different diphosphine (PP) ligands were selected to allow variation of the electronic properties of the ligand substituents, the cone angles, and the natural bite angles (NBAs). Oxidative addition of H_2 to [Rh(PP)_2]^+ complexes is favored by diphosphine ligands with large NBAs, small cone angles, and electron donating substituents, with the NBA being the dominant factor. Large pK_a values for [HRh(PP)_(2)(CH_(3)CN)]^(2+) complexes are favored by small ligand cone angles, small NBAs, and electron donating substituents with the cone angles playing a major role. The hydride donor abilities of [H_(2)Rh(PP)_2]^+ complexes increase as the NBAs decrease, the cone angles decrease, and the electron donor abilities of the substituents increase. These results indicate that if solvent coordination is involved in hydride transfer or proton transfer reactions, the observed trends can be understood in terms of a combination of two different steric effects, NBAs and cone angles, and electron-donor effects of the ligand substituents

Noninnocence in Metal Complexes: A Dithiolene Dawn

Noninnocence in inorganic chemistry traces its roots back half a century to work that was done on metal complexes containing unsaturated dithiolate ligands. In a flurry of activity in the early 1960s by three different research groups, homoleptic bis and tris complexes of these ligands, which came to be known as dithiolenes, were synthesized, and their structural, electrochemical, spectroscopic, and magnetic properties were investigated. The complexes were notable for facile one-electron transfers and intense colors in solution, and conventional oxidation-state descriptions could not account for their electronic structures. The bis complexes were, in general, found to be square-planar, including the first examples of this geometry for paramagnetic complexes and different formal dn configurations. Several of the neutral and monoanionic tris complexes were found to have trigonal-prismatic coordination, the first time that this geometry had been observed in molecular metal complexes. Electronic structural calculations employing extended Hückel and other semiempirical computational methods revealed extensive ligand–metal mixing in the frontier orbitals of these systems, including the observation of structures in which filled metal-based orbitals were more stable than ligand-based orbitals of the same type, suggesting that the one-electron changes upon oxidation or reduction were occurring on the ligand rather than on the metal center. A summary of this early work is followed with a brief section on the current interpretations of these systems based on more advanced spectroscopic and computational methods. The take home message is that the early work did indeed provide a solid foundation for what was to follow in investigations of metal complexes containing redox-active ligands