Structural Changes in the Carbon Sphere of a Dirhodium Complex Induced by Redox or Deprotonation Reactions

Abstract

A carbon-rich molecule is synthesized, which mainly contains conjugated sp² and sp hybridized carbon centers. Alkenyl and alkynyl binding sites are arranged such that this compound serves as ligand to a binuclear metal unit with a Rh^I─Rh^I bond. Furthermore, CH units are placed in proximity to the metal centers. The dicationic complex [Rh₂(bipy)₂{Ph₂Ptrop^(C≡CCy)2}]²⁺(OTf^-)₂ allows to study possible responses of the carbon-framework to redox reactions as well as deprotonation reactions. All products are, whenever possible, characterized by X-ray diffraction (XRD) methods, NMR and EPR spectroscopy as well as electrochemical methods. It is shown that the carbon skeleton of the ligand framework undergoes C─C bond rearrangement reactions of remarkable diversity. In combination with DFT (density functional theory) studies, these results allow to gain insight into the electronic structure changes caused by metal sites in a carbon-rich environment, which may be of relevance for the properties of metal particles on carbon support materials when they are exposed to hydrogen, electrons, or protons.

Keywords: alkynyl complexes; dinuclear complexes; low‐valent metals; metal‐metal bonds; rearrangements; redox chemistry; rhodium.

Figure 1

a) DFT studies of H₂ splitting on a heterogenous small rhodium cluster and carbon support. b) H₂ activation by dinuclear, molecular dirhodium complexes. c) Compound investigated in this work.

Figure 2

a) Synthetic routes to obtain complex [4] ²⁺(OTf⁻)₂. b) Synthesis of complex [6] ⁺(OTf⁻). Conditions: [a] [Rh₂(µ₂‐Cl)₂(COE)₄], benzene, 18 h; [b] [Rh₂ (COE)₄(µ₂‐OTf)₂], THF, 2 h; [c] KOTf, bipy, THF, 4 h; [d] bipy, THF, 1 h; [e] AgOTf, DCM, 4 h; [f] [Rh₂(µ₂‐Cl)₂(H₂C = CH₂)₄], benzene, 18 h.

Figure 3

a) Reduction reaction of [6] ⁺ via [6] ^• to [6] ₂ with CoCp*₂ as reductant in THF. b) Redox wave at E^½ = −1.63 V (vs Fc/Fc⁺) of [6] ⁺ at different scan rates. Conditions: 1 mm analyte, 100 mm [nBu₄N]PF₆ electrolyte, 1,2‐Dimethoxyethane (DME), working electrode (WE): glassy carbon (GC), counter electrode (CE): Pt on TiO_x, reference electrode (RE): Ag/Ag⁺. c) EPR spectrum of [6] ⁺ (generated in situ from [6] ⁺ with one equivalent of CoCp*₂ at room temperature in toluene/acetonitrile). d) Reduction of [4] ²⁺ via [4] ^•+ and [4] to [7] with CoCp*₂ as reductant in THF. e) Redox waves at E^½ = −1.35 and E^½ = −1.61 V (vs Fc/Fc⁺) of [4] ²⁺ at different scan rates. Conditions: 1 mm analyte, 100 mm [nBu₄N]PF₆ electrolyte, THF, WE: Pt, CE: Pt on TiO_x, RE: Ag/Ag⁺. f) EPR spectrum of [4] ^•+ in frozen solution in toluene/acetonitrile.

Figure 4

Solid state structures of a) [4] ²⁺, b) [6] ⁺, c) [7], and d) [6] ₂. Hydrogen atoms, counter ions, and solvent molecules omitted for clarity.

Figure 5

a) Minimum energy path for the interconversion between the native form of double reduced complex [4] to the rearranged complex [7] via the transition state TS. Calculated structures, bipy ligands and H atoms omitted for clarity. b) Redox waves at E^½  = ‐1.37 and E^½ = ‐1.01 V (vs Fc/Fc⁺) of [7] in THF, OCP and scan direction are shown by an arrow. Conditions: 1 mm analyte, 100 mm [nBu₄N]PF₆ electrolyte, WE: Pt, CE: Pt on TiO_x, RE: Ag/Ag⁺, 100 mV s⁻¹ scan rate.

Figure 6

a) Synthesis of deprotonated complexes [8] ⁺ and [9] with KOtBu in THF. b) Solid state structure of, [8] ⁺ hydrogen atoms, counter ion and solvent molecules are omitted for clarity. c) Redox waves of [8] ⁺ in THF, the open circuit potential (OCP) and scan direction are shown by an arrow. Conditions: 1 mm analyte, 100 mm [nBu₄N]PF₆ electrolyte, WE: Pt, CE: Pt on TiO_x, RE: Ag/Ag⁺, 100 mV s⁻¹ scan rate.