Two- and Three-Electron Oxidation of Single-Site Vanadium Centers at Surfaces by Ligand Design

Abstract

Rational, systematic tuning of single-site metal centers on surfaces offers a new approach to increase selectivity in heterogeneous catalysis reactions. Although such metal centers of uniform oxidation states have been achieved, the ability to control their oxidation states through the use of carefully designed ligands had not been shown. To this end, tetrazine ligands functionalized by two pyridinyl or pyrimidinyl substituents were deposited, along with vanadium metal, on the Au(100) surface. The greater oxidizing power of the bis-pyrimidinyltetrazine facilitates the on-surface redox formation of V(3+), compared to V(2+) when paired with the bis-pyridinyltetrazine, as determined by X-ray photoelectron spectroscopy. This demonstrates the ability to control metal oxidation states in surface coordination architectures by altering the redox properties of organic ligands. The metal-ligand complexes take the form of one-dimensional polymeric chains, resolved by scanning tunneling microscopy. The chain structures in the first layer are very uniform and are based on the same quasi-square-planar coordination geometry around single-site V with either ligand. Formation of a different, dimer structure is observed in the early stages of the second layer formation. These systems offer new opportunities in controlling the oxidation state of single-site transition metal atoms at a surface for new advances in heterogeneous catalysts.

Self‐Assembly and Local Manipulation of Au‐Pyridyl Coordination Networks on Metal Surfaces

“…[20][21][22] Regardingt ot hese distinct properties of the coordinationm otif, researchers have made substantial efforts to control metal-organic coordination self-assembly on metal surfaces, such as by alteringt he type and concentration of organic ligandso rm etallic atoms (e.g. transition, alkali and lanthanide metals), [23][24][25][26][27][28][29][30][31][32][33] by selecting substrates and by tuning the ratio of metal/organic reactants. [6,23,30] Nevertheless, because of the intermediate of substrates and the liability of coordination motifs, controlling coordination assembly,e specially at nanoscale, remainsa challenge.…”

Alkene Hydrosilylation on Oxide‐Supported Pt‐Ligand Single‐Site Catalysts

“…Table shows that for all six Pt‐ligand SSCs, the binding energy of Pt 4 f 2/7 XPS peak does not deviate significantly from 72.8 eV, suggesting that most Pt are Pt(II) single‐sites. BMTZ has a stronger electron affinity (oxidizing potential) than DPTZ and leads to the formation of +3 metal centers with V in model systems (on single crystal metal supports in UHV), but similar systems with Pt prefer a mixed redox isomer of Pt(II)/Pt(0) over odd‐electron Pt oxidation states . Here, it is interesting that simultaneous impregnation of Pt and BMTZ onto CeO 2 and MgO powders generates mainly Pt(II), similar to DPTZ, due to the difference in support interaction.…”

“…Al 2 O 3 ‐ or MgO‐supported SSCs were produced in the same way, except that CeO 2 powders were replaced by Al 2 O 3 (ground to 60 mesh, BET surface area≈195 m 2 /g) or MgO (BET surface area≈5.1 m 2 /g) of the same mass. For PDO or BMTZ‐bound SSCs, DPTZ was replaced by PDO (Sigma Aldrich, 98 %) or BMTZ (synthesized by the group of Prof. Kenneth G. Caulton, Indiana University) of the same mole quantity, and H 2 O or acetone (VWR, ACS grade) were used to replace 1‐butanol as the solvent due to ligand solubility concerns. All metal SSCs were yellow or light yellow powders.…”

Redox Isomeric Surface Structures Are Preferred over Odd‐Electron Pt1+

“…The result differs from other metalligand redox reactions we have studied, which allow complete metal oxidation under milder conditions. [21,29] Thea pparent tendency of the system to form the observed structure of Pt 2 BMTZ more readily than 1:1P t II -BMTZ chains could indicate at hermodynamically favorable mixed valence system. The persistence of the mixed metal species( 2:1s toichiometry) after annealing fora sl ong as 15 hours could indicate at hermodynamic preference and that any 1:1p roduct is significantly reactive towards additional Pt 0 .…”