Modular O- vs. N-coordination of pyridylidene amide ligands to iron determines activity in alcohol oxidation catalysis

Pincer Platinum(II) Hydrides: High Stability Imparted by Donor-Flexible Pyridylidene Amide Ligands and Evidence for Adduct Formation before Protonation

Donor-flexible ligands are an emerging class of noninnocent ligands. Their ability to adapt their donating strength toward a metal center has had numerous catalytic advantages yet has never been utilized to stabilize and isolate intermediate complexes within these processes. We demonstrate through the use of a pincer ligand containing two donor-flexible pyridylidene amide (PYA) arms in coordination with platinum(II) that this ligand adaptability revealed remarkably stable hydride and formate complexes. These are typically fleeting catalytic intermediates within formic acid dehydrogenation and CO₂ hydrogenation catalytic cycles. The PYA platinum hydride complexes are indefinitely stable in air, while formate complexes show no sign of β-hydrogen elimination. This robustness allowed us to investigate hydride protonation as a seemingly simple reaction, though in-depth kinetic analysis reveals a pre-equilibrium step prior to platinum hydride protonation. This initial step has been attributed to adduct formation and is slower than the protonation, and therefore a relevant aspect when designing catalytic cycles for hydrogen release and its microscopic reverse, viz., hydrogen uptake.

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes

Gold-based homogeneous catalysis is dominated by redox neutral Au^I systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between Au^I and Au^III being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This L-type contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal-ligand interactions.