Gavin W. Roffe

Synthesis of unsymmetrical NCN′ and PCN pincer palladacycles and their catalytic evaluation compared with a related SCN pincer palladacycle

1-(3-(Pyridin-2-yl)phenyl)methanamine derivatives have been synthesized and underwent C–H bond activation to afford unsymmetrical NCN′ pincer palladacycles, which were characterised in the solid state. 2-Pyridinyl-phenol and -benzyl alcohols were then used as precursors to unsymmetrical PCN pincer palladacycles. Catalytic applications, where the palladacycle remains in the Pd(II) state, have been carried out and show good activity and selectivity.

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.

Computational study of the coordination of methane to first row transition metal dication complexes

The coordination of methane, the first step in methane activation, to coordinately unsaturated first row transition metal dication complexes has been studied computationally to determine the most stable metal-methane interaction. The geometries and the vibrational frequencies of the encounter complexes [M(pyridine)2(CH4)](2+) have been determined using density functional theory with the ωB97XD hybrid functional and triple-ζ basis sets. The structure is dependent on the metal center; for the early transition metals η(3) coordination is favored, whereas η(2) is more favorable for the later transition metals. The periodic trend in methane binding energies in the [M(pyridine)2(CH4)](2+) complexes follows the trend in electron affinity until the Mn complex but then exhibits decreasing energies from Fe to Zn. This is attributed to increasing Pauli repulsion and ligand-ligand repulsion. For the most stable complex, [Cr(pyridine)2(CH4)](2+), the structures, energies, and spin states of the key intermediates and products in the oxidative addition/reductive elimination pathway have been investigated. It is found that the reaction is thermodynamically favorable and indicates that two-state reactivity may play an important role in lowering the energy of the hydridomethyl intermediate.

Unsymmetrical Pincer Palladacycles Synthesis and Reactivity

Abstract Pincer palladacycles are an interesting class of monomeric organopalladium complexes with a Pd–C bond, intramolecularly stabilized by two heteroatom containing groups bound to the metal center. Often due to synthetic accessibility, such pincer complexes are mainly symmetrical whereby both heteroatom containing coordinating groups are the same. However, recently a whole raft of novel unsymmetrical examples have emerged. Numerous examples of unsymmetrical pincer palladacycles are presented, underlining their structural diversity and their synthesis. Examples of key aspects of pincer chemistry, catalytic applications, and theoretical studies are also presented.

CCDC 1033101: Experimental Crystal Structure Determination

Related Article: Gavin W. Roffe, Sarote Boonseng, Christine B. Baltus, Simon J. Coles, Iain J. Day, Rhiannon N. Jones, Neil J. Press, Mario Ruiz, Graham J. Tizzard, Hazel Cox, John Spencer|2016|RSOS|3|150656|doi:10.1098/rsos.150656

CCDC 1033102: Experimental Crystal Structure Determination

Related Article: Gavin W. Roffe, Sarote Boonseng, Christine B. Baltus, Simon J. Coles, Iain J. Day, Rhiannon N. Jones, Neil J. Press, Mario Ruiz, Graham J. Tizzard, Hazel Cox, John Spencer|2016|RSOS|3|150656|doi:10.1098/rsos.150656

CCDC 1033103: Experimental Crystal Structure Determination

Related Article: Gavin W. Roffe, Sarote Boonseng, Christine B. Baltus, Simon J. Coles, Iain J. Day, Rhiannon N. Jones, Neil J. Press, Mario Ruiz, Graham J. Tizzard, Hazel Cox, John Spencer|2016|RSOS|3|150656|doi:10.1098/rsos.150656