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Abstract Palladium-catalyzed dehydrogenation of formic acid under homogeneous conditions is rare. In most cases, decomposition of palladium complexes is observed under the reaction conditions. In this study, bis(phosphinite)-supported palladium(II) complexes, {2,6-(R2PO)2C6H3}PdX [R = iPr, tBu; X = H (1a–b), OCHO (2a–b)], were examined for catalytic formic acid dehydrogenation (FAD). Their derivative complexes, {2,6-(R2PO)2C6H3}PdCH3 (3a–b), were found to be precatalysts. During catalysis, complex degradation was observed; however, by optimizing the reaction conditions (80 °C, ethanol), a homogeneous and additive-free procedure for FAD was achieved. Alcohols and dihydrogen were identified as the possible sources of catalyst decomposition. Mechanistic investigations reveal that the protonation of {2,6-(R2PO)2C6H3}PdH (1a–b) is both kinetically and thermodynamically favored, which drives FAD while suppressing catalyst degradation pathways. It was found that the sterically crowded {2,6-(tBu2PO)2C6H3}PdCH3 (3b) is a more effective catalyst compared to {2,6-(iPr2PO)2C6H3}PdCH3 (3a). Experimental data show that CO2 elimination, the rate-limiting step of the catalytic cycle, occurs more rapidly from {2,6-(tBu2PO)2C6H3}PdOCHO (2b) than from {2,6-(iPr2PO)2C6H3}PdOCHO (2a), thereby accelerating the overall reaction rate. 

A dodecanuclear copper hydride L₄Cu₁₂H₁₂is shown to be a transient species after the cluster expansion from L₂Cu₄H₄to L₃Cu₆H₆and before the degradation to Cu(0), L, and H₂. 

This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N , N ′-bis(2-hydroxybenzyl)- o -phenylenediamine (H 2 salophan) and N , N ′-bis(2-hydroxy-3-methylbenzyl)- o -phenylenediamine (H 2 salophan_Me). The reaction of H 2 salophan with Ni(OAc) 2 ·4H 2 O generates a dinuclear complex Ni 2 (Hsalophan) 2 (OAc) 2 or Na[Ni 2 (salophan) 2 (OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H 2 salophan_Me with Ni(OAc) 2 ·4H 2 O, however, yields a mononuclear complex Ni(Hsalophan_Me) 2 . Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O 2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes.