Toward the development of structural and functional models of the oxygen evolving complex (OEC) of photosystem II (PSII), we report the synthesis of site-differentiated tetranuclear manganese complexes featuring three six-coordinate and one five-coordinate Mn centers. To incorporate biologically relevant second coordination sphere interactions, substituents capable of hydrogen bonding are included as pyrazolates with phenylamino groups. Complexes with terminal anionic ligands, OH− or Cl−, bound to the lower coordinate metal center are supported through the hydrogen-bonding network in a fashion reminiscent to the enzymatic active site. The hydroxide complex was found to to be a competent electrocatalyst for O-O bond formation, a key transformation pertinent to the OEC. In an acetonitrile-water mixture, at neutral pH, electrochemical water oxidation to hydrogen peroxide was observed, albeit with low (15%) Faradaic yield, likely due to competing reactions with organics. In agreement, 9,10-dihydroanthracene is electrochemically oxidized in the presence of this cluster both via H-atom abstraction and oxygenation at ~50% combined Faradaic yield.

Hapticity changes of a Fe2(μ-arene) moiety upon one-electron oxidation highlight the use of an arene as a non-innocent ligand.

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni0 and Pd0–quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M­(OAc)2 precursors. O2 does not react with the Pd0–quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd0–hydroquinone complex (4-H) was prepared using a one-pot metalation with PdII followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η2-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a AgI center incapable of O2 binding (7-H) or a cationic Pd–H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H+/e– transfers.

Metal complexes supported by ligands with chemically modifiable pendant groups are of interest for controlling reactivity. We report on the coordination chemistry and reactivity of a multidentate phosphine ligand framework that contains a Lewis acid binding site. 3,5-Bis(2-phosphinophenyl)pyridine coordinates low-oxidation-state metal centers such as Ni 0 and Pd 0 via the phosphine donors and the π system of the heterocycle. Electrophilic reagents such as B(C 6 F 5 ) 3 , Me + , and BCy 2 OTf bind the available pyridine nitrogen, generating the Ni complexes 2Ni-B(C 6 F 5 ) 3 , 2Ni-Me, and 2Ni-BCy 2 OTf, respectively. Analogous compounds were prepared for Pd (2Pd, 2Pd-B(C 6 F 5 ) 3 , and 2Pd-H). The effect of Lewis acid binding was evaluated by single-crystal Xray diffraction studies and spectroscopy. Lewis acid binding to 2Pd leads to a stronger η 1 interaction between the metal and the heterocycle π system. Ni binds in an η 2 fashion, but the Lewis acid free species is not monomeric. Ni coordination results in disruption of pyridine aromaticity, as indicated by localization of double-and single-bond character in the solid state. CO adducts were prepared for Lewis acid free (4Ni) and Lewis acid bound species (H + -, Me + -, and B(C 6 F 5 ) 3 -bound; 4Ni-H, 4Ni-Me, and 4Ni-B(C 6 F 5 ) 3 ) that show a significant shift of the CO stretching frequency from 1930 to 1966−1976 cm −1 , respectively, indicating communication of ligand electronics to the metal center. An NO adduct (5Ni) with negligible metal−pyridine interactions was obtained upon sequential reaction of 2Ni with [OMe 3 ][BF 4 ] and then [NO][BF 4 ]. Treatment of 2Ni with silanes and boranes results in pyridine dearomatization involving heteroatom−H bond activation, with the heteroatom binding to the pyridine nitrogen and the hydrides delivered to the ortho position of pyridine. This reactivity demonstrates that the pendant pyridine is drastically affected by metal binding, enabling unusual ligand-based substrate activation. The described chemistry highlights a strategy for tuning the properties of metal centers by ligand postsynthetic modifications.

A series of isostructural trinuclear complexes of metal composition Pd2Fe, Pd2Co, and Ni2Fe was synthesized in a stepwise fashion. Using dinuclear precursors (PdI 2 and NiI 2) supported by p-terphenyl diphosphine ligands allowed for the selective incorporation of a single additional metal center. The effects of both metal composition and ligand electronics on cluster properties were studied. These results highlight the importance of metal–metal interactions in mixed-metal sites of structurally analogous clusters.