Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

The remarkable influence of N,O-ligands in the assembly of a bis-calix[4]arene-supported [MnIV2MnIII10MnII8] cluster

Bis-calix[4]arene is a highly versatile ligand that is capable of forming a range of polymetallic clusters. Use of an N,O-chelating co-ligand affords a very high nuclearity mixed-valence Mn cluster (shown here) that displays coordination modes relative to each ligand type.

Phosphonate Based High Nuclearity Magnetic Cages

Transition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d-4f) cages with aesthetically pleasing structures and interesting magnetic properties. This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with V(IV/V), Mn(II/III), Fe(III), Co(II), Ni(II), and Cu(II) metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.

Structural variation in cation-assisted assembly of high-nuclearity Mn arsonate and phosphonate wheels

The use of arsonate or phosphonate ligands in related reaction systems afforded the K⁺-assisted assembly of high nuclearity {Mn₈} arsonate and {Mn₂₄} phosphonate complexes.

Synthesis, crystal structures and magnetic properties of a family of manganese phosphonate clusters with diverse structures

Four manganese clusters containing tert-butylphosphonate ligands with diverse structures have been synthesized and characterized.

Synthesis and characterization of nickel(II) phosphonate complexes utilizing pyridonates and carboxylates as co-ligands

The synthesis and structures of five new nickel complexes containing phosphonate ligands are reported. The compounds utilize pivalic acid (HPiv) and 6-chloro-2-pyridonate (Hchp) as co-ligands with the resulting complexes being of formulas [Ni10(chp)4(Hchp)4.5(O3P(t)Bu)3(Piv)5(HPiv)2(OH)6(H2O)4.5](HNEt3)·0.5MeCN·2.5H2O 1, [Ni12(chp)12(Hchp)2(PhPO3)2(Piv)5(HPiv)2(OH)2(H2O)6](F)·4.5MeCN·2H2O 2, [Ni10(chp)6(O3PCH2Ph)2(Piv)8(F)2(MeCN)4] 3, [Ni10(chp)6(O3PMe)2(Piv)8(F)2 (MeCN)4]·5MeCN·2H2O 4, and [Ni10(chp)6(O3PCH2Nap)2(Piv)8(F)2(MeCN)2(H2O)2] 5. The metallic core of compounds 1 and 2 display tetra- and hexa-capped trigonal prismatic arrangements, while the metallic and phosphorus core of 3, 4, and 5 display three face-sharing octahedra. Variable temperature direct current (dc) magnetic susceptibility measurements reveal dominant antiferromagnetic exchange interactions within each cluster, with diamagnetic spin ground states found.

Tetrametallic lanthanide(iii) phosphonate cages: synthetic, structural and magnetic studies

Reaction of lanthanide nitrates with phosphonic acids in the presence of pivalic acid and pyridine lead to a trigonal pyramid of lanthanides with phosphonates capping three of the triangular faces.

Supertetrahedral icosanuclear and ring-like decanuclear mixed valent manganese(II/III) triethanolamine clusters

The synthesis and characterisation of three new mixed-valent manganese clusters [Mn(II)₄Mn(III)₁₆O₁₂(OH)₄(tea)₈(chp)₄]·6MeOH·4H₂O (1), [Mn(II)₆Mn(III)₄(teaH)₄(teaH₂)₂(tpaa)₆(F)₈]·2Et₂O·4MeCN (2) and [Mn(II)₆Mn(III)₄(teaH)₄(teaH₂)₂(2-bpca)₆(F)₈]·4MeCN (3) are reported. They were obtained by the reaction of simple manganese salts with triethanolamine (teaH₃), triethylamine (NEt₃) and the appropriate co-ligand. In the case of 1, 6-chloro-2-hydroxypyridine (Hchp) was used, for 2, triphenylacetic acid (tpaa) and 3, 2-biphenylcarboxylic acid (2-bpca). The core of 1 is a Mn₂₀ supertetrahedron, while the cores of 2 and 3 are identical and have distorted ring-like topologies. Variable-temperature, solid-state DC and AC magnetic studies were performed on 1-3 in the 2-300 K (DC) and 2-18 K (AC) ranges. Cluster 1 has a S = 9 ground state with excited S states, larger in value than 9, close in energy. No SMM features were apparent in 1. In contrast, clusters 2 and 3, with S = 12 or 13 ground states, and with excited S levels of lower value than 12 lying close in energy, do show SMM features, albeit below 2 K in their AC out-of-phase, frequency dependent data.