The nature of the bonding in symmetrical pincer palladacycles

The Role of Hydrogen Bonds in Interactions between [PdCl4]2- Dianions in Crystal

[PdCl4]2- dianions are oriented within a crystal in such a way that a Cl of one unit approaches the Pd of another from directly above. Quantum calculations find this interaction to be highly repulsive with a large positive interaction energy. The placement of neutral ligands in their vicinity reduces the repulsion, but the interaction remains highly endothermic. When the ligands acquire a unit positive charge, the electrostatic component and the full interaction energy become quite negative, signalling an exothermic association. Raising the charge on these counterions to +2 has little further stabilizing effect, and in fact reduces the electrostatic attraction. The ability of the counterions to promote the interaction is attributed in part to the H-bonds which they form with both dianions, acting as a sort of glue.

Electronic versus steric control in palladium complexes of carboranyl phosphine-iminophosphorane ligands

A new family of carboranyl phosphine-iminophosphorane ligands was prepared and characterized. The new ligands present a carboranyl group directly attached to the iminophosphorane nitrogen atom through a cage carbon atom (C-carboranyl derivatives L1-L3) or through the B3 boron atom (B-carboranyl derivatives L4 and L5), and the phosphine group on a side chain derived from the diphosphine dppm, i.e. with a two-atom spacer between the P and N donor atoms. The non-carboranyl analogue L6, with a biphenyl group on the nitrogen atom, was also synthesized for comparison. These potential (P, N) ligands were used to obtain palladium complexes (Pd1-Pd6) and, thus, study how the different inductive effect of the carboranyl substituents can modify the coordinating ability of the nitrogen atom. The structural analysis of the complexes revealed two different coordination modes for the ligands: the (P, N) chelate coordination and the unexpected P-terminal coordination, which is not observed for non-carboranyl phosphine-iminophosphoranes. These unexpected structural differences led us to perform DFT calculations on the ligands and metal complexes. The calculations show that the final coordination modes depend on the balance between the electronic and steric properties of the particular carboranyl group.

Mechanisms for C(sp2)-Si activation of aryltrimethylsilyl groups in palladium-catalysed couplings

Silyl-substituted aromatic compounds can participate as the electrophilic component in palladium-catalysed cross-couplings, and reactivity is enhanced by a neighbouring silyl-group. Products analogous to those obtained from C-H activation chemistry are accessible by this means with the additional benefit of regiochemistry defined by the site of silyl substitution. DFT studies described here show that the mechanism of C-Si cleavage is distinct from previously recognised mechanisms for C-H cleavage, with a cascade of silyl intermediates en route to a stable product. The amide directing-groups are involved only in the stabilisation of palladacyclic intermediates, and are never disposed to activate silicon directly. 5-Membered and 6-membered palladacycles are known to behave differently in coupling reactions and the calculations reveal underlying reasons in the cationic pathways studied here.

Combining Sanford Arylations on Benzodiazepines with the Nuisance Effect

5-Phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-ones react under palladium- and visible light photoredox catalysis, in refluxing methanol, with aryldiazonium salts to afford the respective 5-(2-arylphenyl) analogues. With 2- or 4-fluorobenzenediazonium derivatives, both fluoroaryl- and methoxyaryl- products were obtained, the latter resulting from a SNAr on the fluorobenzenediazonium salt ("nuisance effect"). A computational DFT analysis of the palladium-catalysed and the palladium/ruthenium-photocalysed mechanism for the functionalization of benzodiazepines indicated that, in the presence of the photocatalyst, the reaction proceeds via a low-energy SET pathway avoiding the high-energy oxidative addition step in the palladium-only catalysed reaction pathway.

A synthetic, catalytic and theoretical investigation of an unsymmetrical SCN pincer palladacycle

The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki-Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C-H activation of 1 with in situ formed Pd(MeCN)4(BF4)2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd-S and Pd-N bonds. It was found that S-coordination to the metal in the rate determining C-H bond activation step leads to better stabilization of the Pd(II) centre (by 13-28 kJ mol(-1)) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd-N bonds are stronger than the Pd-S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.