Deciphering the influence of PdII and PdIV oxidation states on non-standard chemical bonds within bis(jN-heterocyclic carbene) complexes: insights from DFT

Bis-N-heterocyclic carbene ligands (bis(NHC)) have introduced a new approach to designing homogeneous and heterogeneous catalysts, demonstrating the versatility of ligand concepts in catalysis. This study presents a computational analysis of palladium (+ii and +iv) complexes containing either a normally (bis(nNHC)) or an abnormally (bis(aNHC)) bound CH2-bridged bis-N-heterocyclic carbene ligand; in addition, ancillary ligands are permuted from chlorides (X = Cl) to bromides (X = Br). Density functional theory at the B3PW91/6-31G(d)/Lanl2DZ level in the gas phase was used to investigate the electronic structure and bonding properties of bis(NHC)PdIIX2 and bis(NHC)PdIVX4 for bis(NHC) palladium(ii) dihalide and palladium(iv) tetrachloride complexes, respectively. Results indicate that all of the palladium complex structures prefer a flexible boat-type conformation with an average C 2v symmetry, according to bond property (Ccarbene-Pd and Pd-Cl[Br]) analysis. The strength of these bonds depends on coordinating halide ions (Cl- and Br-), the type of ligand (bis(nNHC) and bis(aNHC)), and the palladium oxidation state (+ii and +iv). Analysis of thermodynamic parameters (ΔH 0, ΔG 0, and ΔE bind) shows an increase in values from an abnormal to normal chelating mode in tetrahalides, whereas the opposite is observed for dihalide complexes. The lower π-backbonding ability of the metal, which is influenced by the quantity and size of halide ions involved, could be one possible explanation for this deficiency.

Chemoselective Post-Synthesis Modification of Pyridyl-Substituted, Aromatic Phosphorus Heterocycles: Cationic Ligands for Coordination Chemistry

Triazaphospholes are potential polydentate ligands due to the presence of both phosphorus and nitrogen donor atoms within the aromatic 5-membered heterocycle. The incorporation of an additional pyridyl-substituent opens up the possibility of a post-synthesis modification via chemoselective and also stepwise alkylation exclusively of the nitrogen atoms. This can be controlled by the choice and by the stoichiometry of the electrophile and allows the targeted synthesis of a variety of novel mono- and dicationic ligands. Reaction with Cu(I)-halides causes the formation of cuprates of the type [CuXn](n-1)-, which enables coordination of the π-acidic phosphorus donor to the negatively charged metal core, which is favored over the coordination by a strongly σ-donating nitrogen atom. The use of cationic triazaphosphole derivatives can be used as a strategy to enforce the coordination of the ligand to an electron rich metal solely via the phosphorus atom. However, there is a subtle balance between the formation of either coordination polymers or dimeric structures, as the substitution pattern on the heterocycle and the nature of the halide have a large influence on the coordination motifs formed.

The transformations of a methylene-bridged bis-triazolium salt: a mesoionic carbene based metallocage and analogues of TCNE and NacNac

Unusual and unexpected chemical transformations often provide access to completely new types of functional molecules. We report here the synthesis of a methylene-bridged bis-triazolium salt designed as a precursor for a new bis-mesoionic carbene (MIC) ligand. The direct metalation with silver oxide led to the isolation and crystallographic characterization of a cationic tetranuclear octacarbene-silver(i) complex. During metalation the formal bis-MIC precursor undergoes significant structural changes and chemical transformations. A combined synthetic, crystallographic and (spectro-)electrochemical approach is used to elucidate the mechanistic pathway: starting from the methylene-bridged bis-triazolium salt a single deprotonation leads to a NacNac analogue, which is followed by a redox-induced radical dimerization reaction, generating a new tetra-MIC ligand coordinated to silver(i) central atoms. Decomplexation led to the isolation of the corresponding tetratriazoliumethylene, a profoundly electron-poor alkene, which is an analogue of TCNE.

B3LYP, M06 and B3PW91 DFT assignment of nd8 metal-bis-(N-heterocyclic carbene) complexes

This paper is focused on the examination of the bonding properties of a series of [M(NHC)₂X₂] (M = nd⁸ transition metal; X = Cl, Br and I) complexes in normal, abnormal and mixed C^∩C coordination modes. Structures have been optimised in gas phase using B3LYP, M06 and P3BW91 functionals. Two basis sets have been used: the LanL2DZ and a mixed basis set (LanL2DZ for nd⁸ transition metals as well as halogen atoms and 6-311+G(d,p) for other atoms). Results obtained indicate that the B3PW91 bond distances are closer to experimental data. The complexation energies obtained for each binding mode increase in the order: Ni²⁺ < Pd²⁺ < Pt²⁺, independently of the halogen atom adopted. From the Quantum Theory of Atoms in Molecule (QTAIM) approach, the instability has been found to follow this trend: M - X < M - C. The analysis of metal-ligand interactions using the natural bond orbital (NBO) revealed that the strongest metal-ligand interactions are observed in the normal binding mode. The NCH → MX₂ donation terms were found to be interestingly predominant compared with back donation ones in the complexes studied, except in Pt chloride ones. The contribution of electrostatic interaction energy between the above fragments (∆E_elstat term) is in the range 57.48-63.95% traducing the fact that the interactions are mostly electrostatic. Graphical abstract.

Synthesis of water-soluble palladium(ii) complexes with N-heterocyclic carbene chelate ligands and their use in the aerobic oxidation of 1-phenylethanol

Water-soluble palladium(ii) complexes with NHC chelate ligands have been prepared and studied as catalysts in the aerobic oxidation of alcohols in water.

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

Facile base-free in situ generation and palladation of mesoionic and normal N-heterocyclic carbenes at ambient conditions

Synthesis of normal and mesoionic N-heterocyclic carbene Pd(ii) complexes in near quantitative yields from the corresponding NHC precursor salts under base free and ambient conditions is reported.

Non-classical N-Heterocyclic Carbene Complexes

The expansion of N-heterocyclic carbenes as ligands for transition metals to mesoionic ligand systems has led to the discovery of a wide range of non-classical carbene-type ligands. These non-classical carbene-type ligands are characterised by a significantly lower heteroatom stabilisation of the (putative) free carbene, a situation that also affects the ligand donor properties and hence the reactivity of the coordinated metal centre. In consequence, a number of attractive transition metal-catalysed processes have been disclosed in recent years, predominantly in the areas of cross-coupling, hydrogenations, and olefin metathesis reactions.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Rhodium-mediated activation of an alkane-type C-H bond

Abnormal C4-bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp(3))-H bond, whereas no such intramolecular C-H bond breaking is observed when the carbene binds normally through the C2 carbon.