New catalysts with unsymmetrical N-heterocyclic carbene ligands

The importance of unsymmetrical N-heterocyclic carbenes (uNHCs) as ligands in metal-catalyzed reactions is undeniable. While uNHCs show similar properties as compared with symmetrical NHCs, dissymmetrization allows for further fine-tuning. The introduction of chelatization, hemilability, bifunctionality, shielding effects, and chirality-transfer influences the catalyst's stability, reactivity, and selectivity, thus offering access to tailor-made systems including mono- and multidentate uNHC ligands. Based on selected examples, the structure-reactivity relationship of uNHCs employed in metal catalysts is presented. The focus is on catalytically active complexes, which either offer access to new applications or lead to significantly improved results in metal-catalyzed reactions.     

Self-assembled metallo-macrocycle based coordination polymers with unsymmetrical amide ligands

A series of metallo-macrocyclic based coordination polymers has been prepared from flexible amide ligands N-6-[(3-pyridylmethylamino)carbonyl]-pyridine-2-carboxylic acid (L1-CH(3)) and N-6-[(4-pyridylmethylamino)carbonyl]-pyridine-2-carboxylic acid (L2-CH(3)). In all but one case, self-assembled dinuclear metallo-macrocyclic units form the basis of the polymeric structures, whereby discrete metal centres, and dinuclear or trinuclear clusters, are linked by the self-assembled macrocycles to give 1D and 2D coordination polymers. In one instance, a 1D coordination polymer is formed in a reaction carried out under ambient conditions; when the same reaction is conducted under solvothermal conditions a 2D structure is formed. In all but two of these structures, the polymeric chains and nets are close-packed within the crystals. In the case of a 6,3-connected 2D coordination polymer {[Cd(3)(L2-CH(3))(3)(NO(3))(L2)(CH(3)OH)](NO(3))(2)·12?H(2)O}(n) (9), small oval channels percolate down the a-axis of the unit cell.     

New olefin metathesis catalysts with fluorinated unsymmetrical imidazole-based ligands

1. Download high-res image (150KB)
2. Download full-size image

     

Distribution of ligands in polynuclear palladium complexes: DFT study of isomerism of Pd4(L)4(RCO2)4 (L = CO,CH2, NO) complexes

Structural isomerism of the Pd₄(L)₄(RCO₂)₄ (L = CO, CH₂, NO; R = CC1₃, CF₃, CH₃) complexes was studied in the framework of the density functional theory (DFT). Among the Pd₄(CO)₄(RCO₂)₄ and Pd₄(CH₂)4(RCO₂)₄ complexes the most stable were the isomers with alternate coordination of pairs of carbonyl and carboxylate ligands on the sides of a planar rectangular metal core. The isomers with the pairwise coordination of NO/RCO₂ on one side of the metal core are the most stable between the Pd₄(NO)₄(RCO₂)₄ complexes. The features of mutual coordination of ligands in polynuclear complexes of palladium are clarified using the obtained results.

     

Multi-functional,Low Symmetry Pd2L4 Nanocage Libraries**

Although many impressive metallo-supramolecular architectures have been reported, they tend towards high symmetry structures and avoid extraneous functionality to ensure high fidelity in the self-assembly process. This minimalist approach, however, limits the range of accessible structures and thus their potential applications. Herein is described the synthesis of a family of ditopic ligands wherein the ligand scaffolds are both low symmetry and incorporate exohedral functional moieties. Key to this design is the use of Cu^I-catalysed azide-alkyne cycloaddition (CuAAC) chemistry, as the triazole is capable of acting as both a coordinating heterocycle and a tether between the ligand framework and functional unit simultaneously. A common precursor was used to generate ligands with various functionalities, allowing control of electronic properties whilst maintaining the core structure of the resultant cis-Pd₂L₄ nanocage assemblies. The isostructural nature of the scaffold frameworks enabled formation of combinatorial libraries from the self-assembly of ligand mixtures, generating a statistical mixture of multi-functional, low symmetry architectures.     

Self-assembly of [M8L4] and [M4L2] fluorescent metallomacrocycles with carbazole-based dipyrazole ligands

Fluorescent carbazole-based dipyrazole ligands (H(2)L(1-4)) were employed to coordinate with dipalladium corners ([(phen)(2)Pd(2)(NO(3))(2)](NO(3))(2), [(dmbpy)(2)Pd(2)(NO(3))(2)](NO(3))(2), or [(15-crown-5-phen)(2)Pd(2)(NO(3))(2)](NO(3))(2), where phen = 1,10-phenanthroline and dmbpy = 4,4'-dimethyl-2,2'-bipyridine, in aqueous solution to afford a series of positively charged [M(8)L(4)](8+) or [M(4)L(2)](4+) multimetallomacrocycles with remarkable water solubility. Their structures were characterized by (1)H NMR spectroscopy, electrospray ionization mass spectrometry, and elemental analysis and in the cases of 1·8BF(4)(-) ([(phen)(8)Pd(8)L(1)(4)](BF(4))(8)), and 3·4BF(4)(-) ([(phen)(4)Pd(4)L(2)(2)](BF(4))(4)) by single-crystal X-ray diffraction analysis. Complexes 3-8 are square-type hybrid metallomacrocycles, while complexes 1 and 2 exhibit folding cyclic structures. Interestingly, in single-crystal structures of 1·8BF(4)(-) and 3·4BF(4)(-), BF(4)(-) anions are trapped in the dipalladium clips through anion-π interaction. The luminescence properties and interaction toward anions of these metallomacrocycles were discussed.     

Conformational energy penalties of protein-bound ligands

The conformational energies required for ligands to adopt their bioactive conformations were calculated for 33 ligand–protein complexes including 28 different ligands. In order to monitor the force field dependence of the results, two force fields, MM3 and AMBER, were employed for the calculations. Conformational analyses were performed in vacuo and in aqueous solution by using the generalized Born/solvent accessible surface (GB/SA) solvation model. The protein-bound conformations were relaxed by using flat-bottomed Cartesian constraints. For about 70% of the ligand–protein complexes studied, the conformational energies of the bioactive conformations were calculated to be 3 kcal/mol. It is demonstrated that the aqueous conformational ensemble for the unbound ligand must be used as a reference state in this type of calculations. The calculations for the ligand–protein complexes with conformational energy penalties of the ligand calculated to be larger than 3 kcal/mol suffer from uncertainties in the interpretation of the experimental data or limitations of the computational methods. For example, in the case of long-chain flexible ligands (e.g. fatty acids), it is demonstrated that several conformations may be found which are very similar to the conformation determined by X-ray crystallography and which display significantly lower conformational energy penalties for binding than obtained by using the experimental conformation. For strongly polar molecules, e.g. amino acids, the results indicate that further developments of the force fields and of the dielectric continuum solvation model are required for reliable calculations on the conformational properties of this type of compounds.     

The syntheses and catalytic applications of unsymmetrical ferrocene ligands

Interest in the chemistry of ferrocene remains intense, largely due to applications within catalysis. New synthetic routes to unsymmetrical ferrocene ligands have provided another facet to this area, as substituents can be designed to be electronically- and/or sterically-distinct in order to affect the environment around the catalytically-active metal centre. This critical review provides a concise summary of the synthetic routes that have been applied to the synthesis of unsymmetrical ferrocene ligands, along with a systematic survey of the applications of these ligands in homogeneous catalysis. The aim is to help the reader select a suitable ferrocenediyl ligand for a particular synthetic application, and in the synthesis of ligands that require particular structural and/or electronic features.     

A synthesis of unsymmetrical chiral salen ligands derived from 2-hydroxynaphthaldehyde and substituted salicylaldehydes

Novel unsymmetrical chiral salen Schiff base ligands have been synthesised via a stepwise approach. A special feature of these ligands is that they possess two different units: a 2-hydroxynaphthaldehyde moiety on one side and a substituted salicylaldehyde on the other.     

A practical one-pot synthesis of enantiopure unsymmetrical salen ligands

A practical, one-pot synthesis of enantiopure unsymmetrical salen ligands is described, using a 1:1:1 molar ratio of a chiral diamine and two different salicylaldehydes. The new synthetic protocol can be readily performed in good yields (60-85%) on a multigram scale with good tolerance toward various functional groups.     

Tuning facial-meridional isomerisation in monometallic nine-co-ordinate lanthanide complexes with unsymmetrical tridentate ligands

The unsymmetrical tridentate benzimidazole-pyridine-carboxamide units in ligands L1-L4 react with trivalent lanthanides, Ln(III), to give the nine-co-ordinate triple-helical complexes [Ln(Li)3]3+ (i = 1-4) existing as mixtures of C3-symmetrical facial and C1-symmetrical meridional isomers. Although the beta13 formation constants are 3-4 orders of magnitude smaller for these complexes than those found for the D3-symmetrical analogues [Ln(Li)3]3+ (i = 5-6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)3]3+ is significantly larger. The fac-[Ln(Li)3]3+ <--> mer-[Ln(Li)3]3+ (i = 1-4) isomerisation process in acetonitrile is slow enough for Ln = Lu(III) to be quantified by 1H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand pi-stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 1:3 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)2(CF3SO3)2(H2O)](CF3SO3)(C3H5N)2(H2O) (6, C45H54EuF9N10O13S3, monoclinic, P2(1)/c, Z = 4) and [Eu(L4)2(CF3SO3)2(H2O)](CF3SO3)(C4H4O)(1.5) (7, C51H66EuF9N8O(15.5)S3, triclinic, P1, Z = 2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.     

High-pressure synthesis, crystal structure, and properties of BiPd2O4 with Pd2+ and Pd4+ ordering and PbPd2O4

BiPd(2)O(4) and PbPd(2)O(4) were synthesized at high pressure of 6 GPa and 1500 K. Crystal structures of BiPd(2)O(4) and PbPd(2)O(4) were studied with synchrotron X-ray powder diffraction. BiPd(2)O(4) is isostructural with PbPt(2)O(4) and crystallizes in a triclinic system (space group P1, a = 5.73632(4) ?, b = 6.02532(5) ?, c = 6.41100(5) ?, α = 114.371(1)°, β = 95.910(1)°, and γ = 111.540(1)° at 293 K). PbPd(2)O(4) is isostructural with LaPd(2)O(4) and BaAu(2)O(4) and crystallizes in a tetragonal system (space group I4(1)/a, a = 5.76232(1) ?, and c = 9.98347(2) ? at 293 K). BiPd(2)O(4) shows ordering of Pd(2+) and Pd(4+) ions, and it is the third example of compounds with ordered arrangements of Pd(2+) and Pd(4+) in addition to Ba(2)Hg(3)Pd(7)O(14) and KPd(2)O(3). In PbPd(2)O(4), the following charge distribution is realized Pb(4+)Pd(2+)(2)O(4). PbPd(2)O(4) shows a structural phase transition from I4(1)/a to I2/a at about 240 K keeping basically the same structural arrangements (space group I2/a, a = 5.77326(1) ?, b = 9.95633(2) ?, c = 5.73264(1) ?, β = 90.2185(2)° at 112 K). BiPd(2)O(4) is nonmagnetic while PbPd(2)O(4) exhibits a significant temperature-dependent paramagnetic moment of 0.46μ(B)/f.u. between 2 and 350 K. PbPd(2)O(4) shows metallic conductivity, and BiPd(2)O(4) is a semiconductor between 2 and 400 K.     

A Water Soluble Pd2L4 Cage for Selective Binding of Neu5Ac

The sialic acid N-acetylneuraminic acid (Neu5Ac) and its derivatives are involved in many biological processes including cell-cell recognition and infection by influenza. Molecules that can recognize Neu5Ac might thus be exploited to intervene in or monitor such events. A key obstacle in this development is the sparse availability of easily prepared molecules that bind to this carbohydrate in its natural solvent; water. Here, we report that the carbohydrate binding pocket of an organic soluble [Pd₂L₄]⁴⁺ cage could be equipped with guanidinium-terminating dendrons to give the water soluble [Pd₂L₄][NO₃]₁₆ cage 7 . It was shown by means of NMR spectroscopy that 7 binds selectively to anionic monosaccharides and strongest to Neu5Ac with K_a=24 M⁻¹. The cage had low to no affinity for the thirteen neutral saccharides studied. Aided by molecular modeling, the selectivity for anionic carbohydrates such as Neu5Ac could be rationalized by the presence of charge assisted hydrogen bonds and/or the presence of a salt bridge with a guanidinium solubilizing arm of 7 . Establishing that a simple coordination cage such as 7 can already selectively bind to Neu5Ac in water paves the way to improve the stability, affinity and/or selectivity properties of M₂L₄ cages for carbohydrates and other small molecules.     

Self-assembly of Pd4L2 Supramolecular Cage and Permanganate Anion Adsorption Behavior in Water

Journal of Cluster Science - A novel metal–organic cage 1·8PF6? was precisely self-assembled using polynitrogen pyrazine-triazine ligand as linker L1 and bpyPd(NO3)2 as...     

Pd2Ag triangle supported by two micro3-amidopyridine ligands

[Pd(tmeda)(Hampy-N1)(H2O)]2+ (tmeda=N,N,N',N'-tetramethylethylenediamine; Hampy=2-aminopyridine) forms in the presence of Ag+ at pH 8-9 a triangular Pd2Ag complex containing two deprotonated ampy- ligands. It has been crystallized and structurally characterized with nitrate anions and a second co-crystallized AgNO3, [{Pd(ampy)(tmeda)}2Ag(micro-NO3)2Ag(NO3)2]. The two amidopyridine ligands are triply bridging, binding to Ag+ in a monodentate fashion viaN1, and to two PdII centres in a micro2-bridging fashion via the monodeprotonated N2 position. The resulting four-membered Pd(ampy)2Pd metallacycle is syn-planar with Pd[dot dot dot]Pd separations of 3.0878(13) A. The Pd...Ag distances are 3.0879(14) A in (isosceles triangle). In solution (D2O), the two ampy- ligand in are non-equivalent as concluded from a detailed 1H NMR spectroscopic study and confirmed by a 13C NMR spectrum. Removal of Ag+ from, as achieved by addition of Cl-, causes cluster degradation and linkage isomerization of PdII(tmeda) from the exocyclic N2 to the endocyclic N1 position.     

Tetrapalladium complex with bridging germylene ligands. Structural change of the planar Pd4Ge3 core

A complex with a planar hexagonal Pd(4)Ge(3) core, [Pd{Pd(dmpe)}(3)(μ(3)-GePh(2))(3)], was synthesized and characterized by X-ray and NMR measurements as well as by DFT calculations. 4-tert-Butylbenzenethiol converted the Pd(4) complex into a hexapalladium complex, [{Pd(3)(μ-GePh(2))(2)(μ-H)(μ(3)-GePh(2)(SC(6)H(4)(t)Bu-4))}(2)(μ-dmpe)], composed of two Pd(3)Ge(3) units bridged by a dmpe ligand. The addition of CuI or AgI to the Pd(4) complex yielded [Pd(μ-MI){Pd(dmpe)}(3)(μ(3)-GePh(2))(3) ] (M = Cu, Ag), in which Cu or Ag bridges a Pd-Pd bond of the Pd(4)Ge(3) core. The CuI adducts in solution undergo a pivot motion of the CuI on the surface of the Pd(4)Ge(3) plane on the NMR time scale.