Ruthenium PNN(O) Complexes: Cooperative Reactivity and Application as Catalysts for Acceptorless Dehydrogenative Coupling Reactions.

The novel tridentate PNNOH pincer ligand LH features a reactive 2-hydroxypyridine functionality as well as a bipyridyl-methylphosphine skeleton for meridional coordination. This proton-responsive ligand coordinates in a straightforward manner to RuCl(CO)(H)(PPh3)3 to generate complex 1. The methoxy-protected analogue LMe was also coordinated to Ru(II) for comparison. Both species have been crystallographically characterized. Site-selective deprotonation of the 2-hydroxypyridine functionality to give 1' was achieved using both mild (DBU) and strong bases (KOtBu and KHMDS), with no sign of involvement of the phosphinomethyl side arm that was previously established as the reactive fragment. Complex 1' is catalytically active in the dehydrogenation of formic acid to generate CO-free hydrogen in three consecutive runs as well as for the dehydrogenative coupling of alcohols, giving high conversions to different esters and outperforming structurally related PNN ligands lacking the NOH fragment. DFT calculations suggest more favorable release of H2 through reversible reactivity of the hydroxypyridine functionality relative to the phosphinomethyl side arm.

Enantioselective synthesis of tunable chiral Clickphine P,N-ligands and their application in Ir-catalyzed asymmetric hydrogenation.

A small library of highly tunable chiral Clickphine P,N-ligands has been prepared in an enantioselective fashion by Cu(I)-catalyzed asymmetric propargylic amination using a single chiral complex and a subsequent in situ cycloaddition click reaction. The scope of the propargylic amination to yield optically active triazolyl amines is described. The amines are transformed in a one-pot procedure to the corresponding Ir-Clickphine complexes, which serve as catalysts for the asymmetric hydrogenation of di-, tri-, and tetrasubstituted unfunctionalized alkenes. Enantioselectivities of up to 90% ee were obtained in these hydrogenations, which are among the best reported in the case of the tetrasubstituted substrate 2-(4'-methoxyphenyl)-3-methylbut-2-ene (9) (87% ee). This is a demonstration of the effective use of the chiral pool, as from one chiral catalyst a library of chiral Ir complexes has been synthesized that can hydrogenate various alkenes with high selectivity.

N-H bond activation by palladium(II) and copper(I) complexes featuring a reactive bidentate PN-ligand.

The first examples of reactivity at the backbone of a bidentate PN-ligand L1H relevant to N-H activation are described, leading to novel Pd(II) and Cu(I) amido complexes. Activation of the PN-ligand backbone led to selective dearomatization of the pyridyl ring structure. In the case of Pd(II), the intermediate could be efficiently stabilized using PMe(3). Selective N-H bond cleavage of e.g. trifluorosulfonylamide resulted in facile formation of mononuclear metal-amido species 2 and 4, which have been crystallographically characterized. Hydrogen-bonding dimerization is observed in these solid state structures. The results obtained with these structurally versatile and reactive scaffolds likely open up new avenues in cooperative catalysis.

Synthesis and characterization of Pd(II)-methyl complexes with N-heterocyclic carbene-amine ligands.

A number of palladium(ii) complexes with a heteroditopic NHC-amine ligand and their precursor silver(i) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.