Laura L. Santos

A manganese(I) complex featuring a picoline-derived CNN pincer ligand with two acidic sites ‒the methylene linker and a secondary amino group‒ has been synthesized. In the presence of an alkoxide base, this complex catalyzes the hydrogenation of esters to the corresponding alcohols under relatively mild conditions (1.0 mol% Mn, 10 mol% KOR, 20 bar H2, 110 oC). Stoichiometric reactions of the Mn(I) precursor with an excess of strong base (3 equiv of KHMDS or KOtBu) yield a manganate(I) species (Mn-CNN**) via the deprotonation of both acidic sites of the pincer ligand. X-ray photoelectron spectroscopy (XPS) evinces that the Mn-CNN** complex features a Mn center coordinated by a dianionic CNN** ligand. Finally, detailed DFT calculations were carried out on the two key steps of the catalytic cycle ‒H2 activation and hydride transfer to the ester substrate‒ considering neutral and anionic manganese species. The results indicate that ester hydrogenation proceeds via a Mn(I)-mediated anionic–dianionic manifold, where ligand deprotonation facilitates both H2 cleavage and hydride transfer.

Air-stable ruthenium­(II) complexes based on a picoline-derived CNC pincer ligand, [RuH­(CNC)­(CO)L]X (L = PPh₃, X = Br; L = CO, X = BF₄), were found to catalyze under basic conditions the oxidation with N₂O of a series of alcohols to carboxylates. Both [RuH­(CNC)­(CO)L]X complexes react readily with strong bases (tBuOK or KHMDS), giving rise to a Ru­(II) complex containing a deprotonated CNC* ligand (when L = PPh₃) or a Ru(0)-CNC derivative (for L = CO). Furthermore, the mechanism of the catalytic reaction has been elucidated through density functional theory (DFT) calculations. The catalytic cycle has been shown to proceed through an outer-sphere mechanism comprising four key transformations, which involve Ru­(II) intermediates based on the deprotonated CNC* ligand: (i) alkoxide dehydrogenation to yield a Ru­(II) hydride complex and an aldehyde molecule, (ii) N₂O insertion into the ruthenium–hydride bond to yield a hydroxy ruthenium species and N₂, (iii) nucleophilic attack of the hydroxo ligand in the Ru–OH complex to the intermediate aldehyde, and (iv) dehydrogenation of the formed alcoholate to regenerate the catalytically active Ru­(II) hydride and produce the carboxylate product.

The reaction between the bis(ethylene) complex [TpRh(C₂H₄)₂], 1, (Tp=hydrotris(pyrazolyl)borate), and dimethyl acetylenedicarboxylate (DMAD) has been studied under different experimental conditions. A mixture of products was formed, in which TpRh^I species were prevalent, whereas the presence of trapping agents, like water or acetonitrile, allowed for the stabilization and isolation of octahedral TpRh^III compounds. An excess of DMAD gave rise to a small amount of the [2+2+2] cyclotrimerization product hexamethyl mellitate (6). Although no catalytic application of 1 was achieved, mechanistic insights shed light on the formation of stable rhodium species representing the resting state of the catalytic cycle of rhodium‐mediated [2+2+2] cyclo(co)trimerization reactions. Metallacyclopentene intermediate species, generated from the activation of one alkyne and one ethylene molecule from 1, and metallacyclopentadiene species, formed by oxidative coupling of two alkynes to the rhodium centre, are crucial steps in the pathways leading to the final organometallic and organic products.