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.

In this study, three new terphenyl-substituted NPN ligands bearing pyridyl groups, two phosphonites and one diaminophosphine, were synthesized and fully characterized. Their coordination chemistry with copper(I) was investigated using CuBr and [Cu(NCMe)₄]PF₆ as metal precursors, affording six mononuclear Cu(I) complexes, which were characterized using NMR spectroscopy and, in selected cases, single-crystal X-ray diffraction (SCXRD) analysis. The NPN ligands adopt a κ³-coordination mode, stabilizing the copper centers in distorted tetrahedral geometries. The catalytic performance of these complexes in the S-arylation of thiols with aryl iodides was evaluated. Under optimized conditions, complexes 2a and 2b exhibited excellent activity and broad substrate scope, tolerating both electron-donating and electron-withdrawing groups, as well as sterically hindered and heteroaryl substrates. The methodology also proved effective for aliphatic thiols and demonstrated high chemoselectivity in the presence of potentially reactive functional groups. In contrast, aryl bromides and chlorides were poorly reactive under the same conditions. These findings highlight the potential of well-defined Cu(I)–NPN complexes as efficient and versatile precatalysts for C–S bond formation.

Cyclopentadienyl (Cp), a classic ancillary ligand platform, can be chemically non-innocent in electrocatalytic H-H bond formation reactions via protonation of coordinated η5-Cp ligands to form η4-CpH moieties. However, the kinetics of η5-Cp ring protonation, ligand-to-metal (or metal-to-ligand) proton transfer, and the influence of solvent during H2 production electrocatalysis remain underexplored. We report in-depth kinetic details for electrocatalytic H2 production using Fe complexes [CpFe(CO)2(NCMe)]+ with amine-rich cyclopentadienyl ligands (Cp = enCpN; en = ethylenediamine, N = NHiPr, Pyrrolidinyl), which generate H2 with a maximum turnover frequency of 266 s-1 via η4-CpH intermediates. Under reducing conditions, state-of-the-art DFT calculations reveal that coordinated solvent plays a crucial role in mediating stereo- and regioselective proton transfer to generate (endo-CpH)Fe(CO)2(NCMe), followed by rapid solvent release and ligand-to-metal proton transfer to generate CpFeH(CO)2. The isoelectronic model complex (endo-CpH)Fe(CO)3 is prepared and structurally characterized, and the replacement of CO with NCMe dramatically increases the ligand dissociation barrier from ΔG‡ ≅ 5 kcal/mol to ΔG‡ ≅ 34 kcal/mol. The on-cycle intermediate CpFeH(CO)2 was prepared and cleanly reacts to release H2 and regenerate [CpFe(CO)2(NCMe)]+, which is rate-limiting during electrocatalysis. The solvent-free complex CpFe(CO)2 was found to be catalytically inactive and reversibly dimerizes in solution to form the crystallographically characterized dimer [CpFe(CO)2]2, further supporting the important role of coordinated solvent during H2 production electrocatalysis. Collectively, these experimental and computational results underscore the emerging importance of Cp ring activation, inner-sphere solvation, and metal-ligand cooperativity to perform proton-coupled electron transfer catalysis for chemical fuel synthesis.