Platinum thiolate complexes supported by PBP and POCOP pincer ligands as efficient catalysts for the hydrosilylation of carbonyl compounds

Unveiling the Reactivity of Part Per Million Levels of Cobalt-Salen Complexes in Hydrosilylation of Ketones

A series of air-stable cobalt(III)salen complexes Co-1 to Co-4 have been synthesized and employed in the hydrosilylation of ketones. Notably, the most intricately tailored Co-3 pre-catalyst exhibited exceptional catalytic activity under mild reaction conditions. The developed catalytic hydrosilylation protocol proceeded with an unusual ppm level (5 ppm) catalyst loading of Co-3 and achieved a maximum turnover number (TON) of 200,000. A wide variety of aromatic, aliphatic, and heterocyclic ketones encompassing both electron-donating and electron-withdrawing substituents were successfully transformed into the desired silyl ethers or secondary alcohols in moderate to excellent yields.

Notable Catalytic Activity of Transition Metal Thiolate Complexes against Hydrosilylation and Hydroboration of Carbon-Heteroatom Bonds

Chemists tend to use transition metal hydride complexes rather than thiolate complexes to catalyse chemical transformations because the hydride complexes possess diverse catalytic reactivity, although most of them are air/moisture-sensitive and difficult to prepare. By comparing the catalytic performances of pincer ligated group 10 metal thiolate and hydride complexes in catalysing the hydroboration and hydrosilylation of C=O and C=N bonds, we demonstrate in this review that transition metal thiolate complexes are much better catalysts than the corresponding hydride complexes in catalysing this type of reactions. Many hydroboration and hydrosilylation reactions catalysed by pincer ligated group 10 metal hydride complexes can also be catalysed by the corresponding thiolate complexes and the thiolate systems are far more active. Therefore, the applications of transition metal thiolate complexes in the catalytic hydroboration and hydrosilylation of unsaturated carbon-heteroatom bonds deserve special attention in future work.

Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes

The coordination mode of the BH₄^- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH₄^- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(^tBu₂PO)₂C₆H₃]Pt(η¹-HBH₃) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(η²-H₂BH₂) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(^tBu₂PCH₂)₂C₆H₃]Ni(BH₄), [2,6-(^tBu₂PO)₂C₆H₃]M(BH₄) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(BH₄) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH₄^- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH₄^- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH₃ species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH₃ moiety.

Reactions and catalytic applications of a PNCNP pincer palladium hydride complex

A PNCNP-pincer palladium hydride complex possesses strong deprotonating ability and versatile catalytic activity and its pincer backbone exhibits high water stability.