Valence Interconversion of Octahedral Nickel(II/III/IV) Centers

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]+ in which the NiIII and CuIII centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]+, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent MIII species, suggesting the locus of oxidation for both [ML]+ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent MIII species. Notably, [ML]+ can also be obtained via O2 oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]+: -1.16 V, NiL/[NiL]+: -1.01 V vs Fc/Fc+ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..

Nickel-Borolide Complexes and Their Complex Electronic Structure

Borolides (BC₄^2-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr₂NBC₄Ph₂)₂ (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni²⁺ species bound to two monoanionic radical (BC₄^•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Heterometallation of Photoluminescent Silver(I) Sulfide Nanoclusters Protected by Octahedral Iridium(III) Thiolates

The recently-increasing interest in coinage metal clusters stems from their photophysical properties, which are controlled via heterometallation. Herein, we report homometallic Ag^I ₄₆ S₁₃ clusters protected by octahedral fac-[Ir(aet)₃ ] (aet=2-aminoethanethiolate) molecules and their conversion to heterometallic Ag^I ₄₃ M^I ₃ S₁₃ (M=Cu, Au) clusters. The reactions of fac-[Ir(aet)₃ ] with Ag⁺ and penicillamine produced [Ag₄₆ S₁₃ {Ir(aet)₃ }₁₄ ]²⁰⁺ ([1]²⁰⁺ ), where a spherical Ag^I ₄₆ S₁₃ cluster is covered by fac-[Ir(aet)₃ ] octahedra through thiolato bridges. [1]²⁰⁺ was converted to [Ag₄₃ M₃ S₁₃ {Ir(aet)₃ }₁₄ ]²⁰⁺ ([1^M ]²⁰⁺ ) with an Ag^I ₄₃ M^I ₃ S₁₃ cluster by treatment with M⁺ , retaining its overall structure. [1]²⁰⁺ was photoluminescent and had an emission band ca. 690 nm that originated from an S-to-Ag charge transfer. While [1^Cu ]²⁰⁺ showed an emission band with a slightly higher energy of ca. 650 nm and a lower quantum yield, the emission band for [1^Au ]²⁰⁺ shifted to a much higher energy of ca. 590 nm with an enhanced quantum yield.

Interconversion between square-planar palladium(ii) and octahedral palladium(iv) centres in a sulfur-bridged trinuclear structure

Coordination of six thiolato groups from two Rh^III metalloligands stabilizes an octahedral Pd^IV centre, which is interconvertible with a square-planar Pd^II centre retaining the RhPdRh trinuclear structure.

A tetranuclear nickel cluster isolated in multiple high-valent states

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni₄]) neutral cluster. Electrochemical analysis of [Ni₄] reveals three reversible sequential oxidations at 0.248 V (1e^-), 0.678 V (1e^-), and 0.991 V (2e^-) vs. Fc/Fc⁺ corresponding to mono-, di-, and tetra-oxidized species, [Ni₄]⁺, [Ni₄]²⁺, [Ni₄]⁴⁺, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni₄]⁴⁺.

New protective ligands for atomically precise silver nanoclusters

Atomically precise silver nanoclusters (NCs) have emerged as a hot topic attracting immense research interest. Protecting ligands are needed for direct capping on cluster surfaces in order to prevent aggregation and to stabilize NCs. It has been demonstrated that protective ligands are critical to determining the sizes, structures and properties of silver NCs. The past decades have witnessed conventionally used organic ligands (thiolates/selenols, phosphines and alkynyls) and inorganic ligands (chalcogens and halogens) being extensively used to passivate NC surfaces. However, only in the most recent years have new-type protecting ligands beyond the conventional ones begun to be introduced in the protecting sphere of new functional silver NCs. The present Frontier article covers the most recent examples of some new protective agents for well-defined silver NCs. We describe four classes of novel silver NCs stabilized by newly-developed surface ligands, namely, nitrogen-donor organic ligands, oxygen-donor inorganic ligands, metalloligands and macrocyclic hosts, paying attention to the synthesis, structures and properties of these silver NCs. This Frontier article will hopefully attract more cluster scientists to explore more freshly ligated atomically precise silver NCs with novel structures and properties in the years ahead. The literature survey in this review is based on publications up to February 2020. Some suggestions for future directions in this field are also given.

Nickel(iii)-mediated oxidative cascades from a thiol-bearing nickel(ii) precursor to the nickel(iv) product

A nickel(ii) complex, Ni(HPS2)₂ (1) that contains two pendant thiols, is rapidly aerobically oxidized in the presence of an amine to produce a diamagnetic nickel(iv) complex, Ni(PS2)₂ (2). This process was investigated spectroscopically at a temperature of -80 °C. Absorption spectra revealed that the deprotonation of one pendant thiol of 1 triggers an oxidative cascade; EPR findings indicate that single-spin species comprised of nickel(iii) intermediates are produced in the reaction solution. Possible reaction routes were examined by DFT calculations, in which an energy profile indicates that (i) a self-driven formation of 2 favors a sequential proton/electron transfer pathway; (ii) kinetically trapped nickel(iii) intermediates may respond to the specificity of the coordination of 2 in a cis-form. The overall findings help one to rationalize how a nickel(ii) precursor can be oxidized by O₂ to a higher oxidation state.

Heterogeneous catalytic water oxidation controlled by coordination geometries of copper(ii) centers with thiolato donors

Heterometallic coordination compounds containing a thiolato dicopper(ii) core increase heterogeneous electrocatalytic activities for water oxidation in proportion to the numbers of vacant dicopper(ii) coordination sites.