Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

Quantifying the Interdependence of Metal–Ligand Covalency and Bond Distance Using Ligand K‐edge XAS

Bond distance is a common structural metric used to assess changes in metal–ligand bonds, but it is not clear how sensitive changes in bond distances are with respect to changes in metal–ligand covalency. Here we report ligand K‐edge XAS studies on Ni and Pd complexes containing different phosphorus(III) ligands. Despite the large number of electronic and structural permutations, P K‐edge pre‐edge peak intensities reveal a remarkable correlation that spectroscopically quantifies the linear interdependence of covalent M?P σ bonding and bond distance. Cl K‐edge studies conducted on many of the same Ni and Pd compounds revealed a poor correlation between M?Cl bond distance and covalency, but a strong correlation was established by analyzing Cl K‐edge data for Ti complexes with a wider range of Ti?Cl bond distances. Together these results establish a quantitative framework to begin making more accurate assessments of metal–ligand covalency using bond distances from readily‐available crystallographic data.     

Reversing Conventional Reactivity of Mixed Oxo/Alkyl Rare‐Earth Complexes: Non‐Redox Oxygen Atom Transfer

The preferential substitution of oxo ligands over alkyl ones of rare‐earth complexes is commonly considered as “impossible” due to the high oxophilicity of metal centers. Now, it has been shown that simply assembling mixed methyl/oxo rare‐earth complexes to a rigid trinuclear cluster framework cannot only enhance the activity of the Ln‐oxo bond, but also protect the highly reactive Ln‐alkyl bond, thus providing a previously unrecognized opportunity to selectively manipulate the oxo ligand in the presence of numerous reactive functionalities. Such trimetallic cluster has proved to be a suitable platform for developing the unprecedented non‐redox rare‐earth‐mediated oxygen atom transfer from ketones to CS₂ and PhNCS. Controlled experiments and computational studies shed light on the driving force for these reactions, emphasizing the importance of the sterical accessibility and multimetallic effect of the cluster framework in promoting reversal of reactivity of rare‐earth oxo complexes.     

N-heterocyclic phosphenium ligands as sterically and electronically-tunable isolobal analogues of nitrosyls

The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt(0) and Pd(0) precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO(+)) and bent (NO(-)) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.     

Sensitization of NO‐Releasing Ruthenium Complexes to Visible Light

We report a combined spectroscopical–theoretical investigation on the photosensitization of transition metal nitrosyl complexes. For this purpose, ruthenium nitrosyl complexes based on tetradentate biscarboxamide ligands were synthesized. A crystal structure analysis of a lithium‐based ligand intermediate is described. The Ru complexes have been characterized regarding their photophysical and nitric oxide (NO) releasing properties. Quantum chemical calculations have been performed to unravel the influence of the biscarboxamide ligand frame with respect to the molecular electronic properties of the NO‐releasing pathway. A quantitative measure for the ligand design within photosensitized Ru complexes is introduced and evaluated spectroscopically and theoretically by using time‐dependent density functional theory.     

Synthesis, spectroscopic characterization, and 3D molecular modeling of lead(II) complexes of unsymmetrical tetradentate Schiff-base ligands

Solid complexes of Pb(II) with unsymmetrical Schiff-base ligands (H₂L) derived from 2-aminobenzophenone, thiosemicarbazide, semicarbazide, salicylaldehyde, 2-hydroxynaphthaldehyde, and o-hydroxyacetophenone have been synthesized and characterized by elemental analysis, conductance measurements, molecular weight measurement, and UV–Vis, FTIR, ¹H NMR, and ¹³C NMR spectroscopy. The spectral studies suggest the ligands behave as dibasic tetradentate ligands with ONNO/ONNS donor atom sequences toward the central metal ion. From the microanalytical data, the stoichiometry of the complexes was found to be 1:1 (metal:ligand). The physicochemical data suggest a tetracoordinated environment around the metal ion. Three-dimensional molecular modeling and analysis of bond lengths and bond angles have also been conducted for a representative compound, [PbL¹], to substantiate the proposed structures.     

A Series of Diamagnetic Pyridine Monoimine Rhenium Complexes with Different Degrees of Metal‐to‐Ligand Charge Transfer: Correlating 13C NMR Chemical Shifts with Bond Lengths in Redox‐Active Ligands

A set of pyridine monoimine (PMI) rhenium(I) tricarbonyl chlorido complexes with substituents of different steric and electronic properties was synthesized and fully characterized. Spectroscopic (NMR and IR) and single‐crystal X‐ray diffraction analyses of these complexes showed that the redox‐active PMI ligands are neutral and that the overall electronic structure is little affected by the choices of the substituent at the ligand backbone. One‐ and two‐electron reduction products were prepared from selected starting compounds and could also be characterized by multiple spectroscopic methods and X‐ray diffraction. The final product of a one‐electron reduction in THF is a diamagnetic metal–metal‐bonded dimer after loss of the chlorido ligand. Bond lengths in and NMR chemical shifts of the PMI ligand backbone indicate partial electron transfer to the ligand. Two‐electron reduction in THF also leads to the loss of the chlorido ligand and a pentacoordinate complex is obtained. The comparison with reported bond lengths and ¹³C NMR chemical shifts of doubly reduced free pyridine monoaldimine ligands indicates that both redox equivalents in the doubly reduced rhenium complex investigated here are located in the PMI ligand. With diamagnetic complexes varying over three formal reduction stages at the PMI ligand we were, for the first time, able to establish correlations of the ¹³C NMR chemical shifts with the relevant bond lengths in redox‐active ligands over a full redox series.     

Intramolecular Hydrogen Bonding Restricts Gd–Aqua‐Ligand Dynamics

Aqua ligands can undergo rapid internal rotation about the M−O bond. For magnetic resonance contrast agents, this rotation results in diminished relaxivity. Herein, we show that an intramolecular hydrogen bond to the aqua ligand can reduce this internal rotation and increase relaxivity. Molecular modeling was used to design a series of four Gd complexes capable of forming an intramolecular H‐bond to the coordinated water ligand, and these complexes had anomalously high relaxivities compared to similar complexes lacking a H‐bond acceptor. Molecular dynamics simulations supported the formation of a stable intramolecular H‐bond, while alternative hypotheses that could explain the higher relaxivity were systematically ruled out. Intramolecular H‐bonding represents a useful strategy to limit internal water rotational motion and increase relaxivity of Gd complexes.     

锕酰冠醚配合物电子结构和化学成键理论研究进展

本文对锕系化合物的结构和性质的理论研究进行了规律性总结,并结合我们的研究成果,重点介绍了锕酰冠醚配合物的配位化学、电子结构和化学成键的基本特征。尽管近年来出现越来越多的光谱实验和晶体学数据报道,但是对锕系配合物的电子结构和化学成键的理论研究还不够系统。本文对锕酰冠醚配合物的配位结构、稳定能和光谱性质的计算结果进行了综述。大环配体(硫代)冠醚的腔体大小决定了配合物的结构特征。通过理论研究,在锕酰冠醚配合物中存在具有典型的An≡Oactinyl共价键和An-Oligand和An-Sligand离子键。对于离子键An-Oligand和An-Sligand,An和供电子配体之间通过价原子轨道的径向分布重叠形成微弱的共价相互作用。从U到Cm,配体向金属的电荷转移(LMCT)逐渐显著,导致Am和Cm的氧化态降低,金属离子与配体的作用变弱。这一成键规律和金属氧化态的变化规律,为实验上筛选合理且高效的镧锕分离配体提供重要理论指导。     

The Trigonal Prism in Coordination Chemistry

Herein we analyze the accessibility of the trigonal‐prismatic geometry to metal complexes with different electron configurations, as well as the ability of several hexadentate ligands to favor that coordination polyhedron. Our study combines i) a structural database analysis of the occurrence of the prismatic geometry throughout the transition‐metal series, ii) a qualitative molecular orbital analysis of the distortions expected for a trigonal‐prismatic geometry, and iii) a computational study of complexes of several transition‐metal ions with different hexadentate ligands. Also the tendency of specific electron configurations to present a cis bond‐stretch Jahn–Teller distortion is analyzed.     

Intramolecular CH/OH Bond Cleavage with Water and Alcohol Using a Phosphine‐Free Ruthenium Carbene NCN Pincer Complex

Transition metal complexes that exhibit metal–ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine‐free ruthenium NCN pincer complexes with a central N‐heterocyclic carbene donor and methylpyridyl N‐donors. Reaction with base generates a neutral, dearomatized alkoxo–amido complex, which has been structurally and spectroscopically characterized. The tert‐butoxide ligand facilitates regioselective, intramolecular proton transfer through a C?H/O?H bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo–amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine‐free metal–ligand cooperative water splitting.     

Bonding Analysis of N‐Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition‐Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal–ligand bonding in transition‐metal complexes that contain imidazole (IMID), imidazol‐2‐ylidene (nNHC), or imidazol‐4‐ylidene (aNHC). The calculated complexes are [Cl₄TM(L)] (TM=Ti, Zr, Hf), [(CO)₅TM(L)] (TM=Cr, Mo, W), [(CO)₄TM(L)] (TM=Fe, Ru, Os), and [ClTM(L)] (TM=Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID<nNHC<aNHC. The energy levels of the carbon σ lone‐pair orbitals suggest the trend aNHC>nNHC>IMID for the donor strength, which is in agreement with the progression of the metal–ligand bond‐dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand–metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)₅TM(L)] (L=nNHC, aNHC, IMID) with phosphine complexes (L=PMe₃ and PCl₃) shows that the phosphine ligands are weaker σ donors and better π acceptors than the NHC tautomers nNHC, aNHC, and IMID.     

Stable Actinide π Complexes of a Neutral 1,4‐Diborabenzene

The π coordination of arene and anionic heteroarene ligands is a ubiquitous bonding motif in the organometallic chemistry of d‐block and f‐block elements. By contrast, related π interactions of neutral heteroarenes including neutral bora‐π‐aromatics are less prevalent particularly for the f‐block, due to less effective metal‐to‐ligand backbonding. In fact, π complexes with neutral heteroarene ligands are essentially unknown for the actinides. We have now overcome these limitations by exploiting the exceptionally strong π donor capabilities of a neutral 1,4‐diborabenzene. A series of remarkably robust, π‐coordinated thorium(IV) and uranium(IV) half‐sandwich complexes were synthesized by simply combining the bora‐π‐aromatic with ThCl₄(dme)₂ or UCl₄, representing the first examples of actinide complexes with a neutral boracycle as sandwich‐type ligand. Experimental and computational studies showed that the strong actinide–heteroarene interactions are predominately electrostatic in nature with distinct ligand‐to‐metal π donation and without significant π/δ backbonding contributions.     

Comparative analysis of the performance of commonly available density functionals in the determination of geometrical parameters for zinc complexes

A set of 44 Zinc‐ligand bond‐lengths and of 60 ligand‐metal‐ligand bond angles from 10 diverse transition‐metal complexes, representative of the coordination spheres of typical biological Zn systems, were used to evaluate the performance of a total of 18 commonly available density functionals in geometry determination. Five different basis sets were considered for each density functional, namely two all‐electron basis sets (a double‐zeta and triple‐zeta formulation) and three basis sets including popular types of effective‐core potentials: Los Alamos, Steven‐Basch‐Krauss, and Stuttgart‐Dresden. The results show that there are presently several better alternatives to the popular B3LYP density functional for the determination of Zn‐ligand bond‐lengths and angles. BB1K, MPWB1K, MPW1K, B97‐2 and TPSS are suggested as the strongest alternatives for this effect presently available in most computational chemistry software packages. In addition, the results show that the use of effective‐core potentials (in particular Stuttgart‐Dresden) has a very limited impact, in terms of accuracy, in the determination of metal‐ligand bond‐lengths and angles in Zinc‐complexes, and is a good and safe alternative to the use of an all‐electron basis set such as 6‐31G(d) or 6‐311G(d,p). © 2009 Wiley Periodicals, Inc. J Comput Chem 2009