Dioxygen activation at nickel complexes is much less studied than for the biologically more relevant iron or copper systems but promises new reactivity patterns because of the distinct coordination chemistry of nickel. Here we report that a pyrazolate-based dinickel(II) dihydride complex [KL(Ni-H)] (1a) smoothly reacts with O via reductive H elimination to give the μ-1,2-peroxo dinickel(II) complex [KLNi(O)] (2a) and, after treatment with dibenzo[18]-crown-6, the separated ion pair [K(DB18C6)][LNi(O)] (2b); these are the first μ-1,2-peroxo dinickel intermediates to be characterized by X-ray diffraction. In 2a, the K is found side-on associated with the peroxo unit, revealing a pronounced weaking of the O-O bond: d(O-O) = 1.482(2) Å in 2a versus 1.465(2) in 2b; ν̃(O-O) = 720 cm in 2a versus 755 cm in 2b. Reaction of 1a (or 2a/2b) with an excess of O cleanly leads to [LNi(O)] (3), which was shown by X-ray crystallography ( d(O-O) = 1.326(2) Å), electron paramagnetic resonance and Raman spectroscopy (ν̃(O-O) = 1007 cm), magnetic measurements, and density functional theory calculations to feature two low-spin d nickel(II) ions and a genuine μ-1,2-superoxo ligand with the unpaired electron in the out-of-plane π* orbital. These μ-1,2-superoxo and μ-1,2-peroxo species, all containing the O-derived unit within the cleft of the dinickel(II) core, can be reversibly interconverted chemically and also electrochemically at very low potential ( E = -1.22 V vs Fc/Fc). Initial reactivity studies indicate that protonation of 2a, or reaction of 3 with TEMPO-H, ultimately gives the μ-hydroxo dinickel(II) complex [LNi(μ-OH)] (4). This work provides an entire new series of closely related and unusually rugged Ni/O intermediates, avoiding the use of unstable nickel(I) precursors but storing the redox equivalents for reductive O-binding in nickel(II) hydride bonds.

A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi(H)], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi(μ-Br)] (1) with MHBEt (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi(H)][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D was investigated by a suite of H andH NMR experiments, revealing an unusual pairwise H/D exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ∼62 kJ/mol, to give H and an antiferromagnetically coupled [LNi] species. This was confirmed by SQUID monitoring during H release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na or K) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)] → H transition derived from 2D H EXSY spectra confirmed first-order dependence of H release on M·2 concentration and a strong effect of the alkali metal cation M. Treating [LNi(D)] with phenylacetylene led to D and dinickel(II) complex 3 with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi(H)] having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H upon substrate binding is reminiscent of the proposed N binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.

Self-assembly is a powerful means to fabricate multifunctional smart nanotheranostics. However, the complicated preparation, toxicity of responsive carriers, and low loading efficiency of drug cargo hinder the outcome. Herein, we developed a responsive carrier-free noncovalent self-assembly strategy of a metallized Au­(III) tetra-(4-pyridyl) porphine (AuTPyP) anticancer drug for the preparation of a heat/acid dual-stimulated nanodrug, and it generated a better photothermal effect than monomers under irradiation. The photothermal effect promoted the protonation of the hydrophobic pyridyl group and the following release into tumorous acidic microenvironments. With cRGD modification, the released drug induced the aggravation of intracellular reactive oxygen species (ROS) via the activity inhibition of thioredoxin reductase (TrxR) for synergistic chemo-photothermal therapy of tumors.

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H 2 upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L Ph ] 3− with bulky m-terphenyl substituents that can host two Ni−H units in close proximity. Complexes [L Ph (Ni II −H) 2 ] − (3) are prone to intramolecular reductive H 2 elimination, and an equilibrium between 3 and orthometalated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal−metal cooperative phenyl group C(sp 2 )-H oxidative addition to the dinickel(I) intermediate [L Ph Ni I 2 ] − (4). While Ni I species have been implicated in catalytic C−H functionalization, discrete activation of C−H bonds at Ni I complexes has rarely been described. The reversible H 2 and C−H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [L Ph Ni I 2 ] − intermediate 4 either via elimination of H 2 from dihydride 3 or via reductive C−H elimination from monohydride 2. The latter does not release any H 2 byproduct and adds a distinct platform for metal−metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.

The preparation of self-assembled porphyrins with orderly stacked nanostructures for emulating natural photosynthesis has stimulated extensive efforts to optimize the energy conversion efficiency. However, the elucidation of how orderly stacked structures promote photocatalysis at the molecular level remains a great challenge. Here, unique porphyrin nanoleaves with designed and ordered structure are synthesized and show a hydrogen evolution rate higher than that of commercial powder. Photodeposition of cocatalysts and Kelvin probe force microscopy measurement suggest selective aggregation of photogenerated electrons and holes at different active sites. Combined with theoretical calculations, we find that the orderly packing changes molecular symmetry and induces a molecular dipole, which increases linearly along the π–π stacking direction and forms a strong built-in electric field. The built-in electric field drives photogenerated electrons and holes for the unique crossed transportation along different directions. These findings reveal how orderly stacked structures promote photocatalysis and provide a novel approach for highly efficient water splitting.