Three-Way Cooperativity in d super(8) Metal Complexes with Ligands Displaying Chemical and Redox Non-Innocence

• Published in: Chemistry : a European journal 2014-11, Vol.20 (46), p.15178-15187
• Main Authors: Deibel, Naina, Hohloch, Stephan, Schweinfurth, David, Weisser, Fritz, Grupp, Anita, Sarkar, Biprajit
• Format: Article
• Language: English
• Subjects: Activation; Bonding; Coordination compounds; Dipping; Ligands; Protonation; Reservoirs; Transformations
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201403276

Reversible proton- and electron-transfer steps are crucial for various chemical transformations. The electron-reservoir behavior of redox non-innocent ligands and the proton-reservoir behavior of chemically non-innocent ligands can be cooperatively utilized for substrate bond activation. Although site-decoupled proton- and electron-transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod sub(-H))M( mu -L super(2-))M(cod sub(-H))] (M=Pt super(II), [1] or Pd super(II), [2], cod=1,5-cyclooctadiene, H sub(2)L=2,5-di-[2,6-(diisopropyl)anilino]-1,4-be nzoquinone), in which cod acts as a proton reservoir, and L super(2-) as an electron reservoir. Protonation of [2] leads to an unusual tetranuclear complex. However, [1] can be stepwise reversibly protonated with up to two protons on the cod sub(-H) ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L super(2-) ligand. The doubly protonated form of [1] is also shown to react with OMe super(-) leading to an activation of the cod ligands. The site-decoupled proton and electron reservoir sources work in tandem in a three-way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton- and electron-transfer reactions by using metal complexes of non-innocent ligands. Spectroelectrochemistry: Site-decoupled proton- and electron-transfer steps have been characterized by using an arsenal of crystallographic, spectroscopic, and spectroelectrochemical methods. The protons and electrons were shown to participate in cooperative bond-activation reactions (see scheme; Dipp=diisopropylphenyl).