Redox properties of [Cp*Rh] complexes supported by mono-substituted 2,2'-bipyridyl ligands.

The redox properties of half-sandwich rhodium complexes supported by 2,2'-bipyridyl (bpy) ligands can be readily tuned by selection of an appropriately substituted derivative of bpy, but the influences of single substituents on the properties of such complexes are not well documented, as disubstituted bpy variants are much more common. Here, the synthesis, characterization, and redox properties of two new [Cp*Rh] complexes (where Cp* is η-1,2,3,4,5-pentamethylcyclopentadienyl) supported by the uncommon mono-substituted ligands 4-chloro-2,2'-bipyridyl (mcbpy) and 4-nitro-2,2'-bipyridyl (mnbpy) are reported. Single-crystal X-ray diffraction studies and related spectroscopic experiments confirm installation of the single substituents (-Cl and -NO, respectively) on the bipyridyl ligands; the precursor monosubstituted ligands were prepared a divergent route from unsubstituted bpy.

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity.

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2 reduction governed by reduction potential compression/inversion. Treatment of bis(η-naphthalene)chromium(0) with six equiv.

Remote Oxidative Activation of a [Cp*Rh] Monohydride.

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e . Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh process at -0.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes.

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands.

Synthesis, structural studies, and redox chemistry of bimetallic [Mn(CO)] and [Re(CO)] complexes.

Manganese ([Mn(CO)]) and rhenium tricarbonyl ([Re(CO)]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO)] and [Re(CO)] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO)] and [Re(CO)] units supported by 2,2'-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes.

4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes.

4,5-diazafluorene (daf) and 9,9'-dimethyl-4,5-diazafluorene (Medaf) are structurally similar to the important ligand 2,2'-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Medaf as a ligand than those containing bpy. New complexes Mn(CO)Br(daf) (), Mn(CO)Br(Medaf) (), and [Ru(Medaf)](PF) () have been prepared and fully characterized to understand the influence of the Medaf framework on their chemical and electrochemical properties. Structural data for , , and from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Medaf chelate angles in comparison to the analogous Mn(CO)(bpy)Br () and [Ru(bpy)] () complexes.

Ultrafast Spectroscopy of [Mn(CO)] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding.

Manganese tricarbonyl complexes are promising catalysts for CO reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)(bpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (bpy, where R = Bu, H, CF, NO). Transient absorption spectroscopy measurements for Mn(CO)(bpy)Br coordination compounds with R = Bu, H, and CF in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale.

Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand.

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) are attracting renewed interest in coordination chemistry and catalysis, but these useful compounds often undergo net two-electron redox cycling that precludes observation of individual one-electron reduction events. Here, we show that a [Cp*Rh] complex bearing the 4,4'-dinitro-2,2'-bipyridyl ligand (dnbpy) () can access a distinctive manifold of five oxidation states in organic electrolytes, contrasting with prior work that found no accessible reductions in aqueous electrolyte. These states are readily generated from a newly isolated and fully characterized rhodium(III) precursor complex , formulated as [Cp*Rh(dnbpy)Cl]PF₆.

Preparation, Characterization, and Electrochemical Activation of a Model [Cp*Rh] Hydride.

Monomeric half-sandwich rhodium hydride complexes are often proposed as intermediates in catalytic cycles, but relatively few such compounds have been isolated and studied, limiting understanding of their properties. Here, we report preparation and isolation of a monomeric rhodium(III) hydride complex bearing the pentamethylcyclopentadienyl (Cp*) and bis(diphenylphosphino)benzene (dppb) ligands. The hydride complex is formed rapidly upon addition of weak acid to a reduced precursor complex, Cp*Rh(dppb).

Ligand Substituents Govern the Efficiency and Mechanistic Path of Hydrogen Production with [Cp*Rh] Catalysts.

We demonstrate that [Cp*Rh] complexes bearing substituted 2,2'-bipyridyl ligands are effective hydrogen evolution catalysts (Cp*=η -pentamethylcyclopentadienyl). Disubstitution (at the 4 and 4' positions) of the bipyridyl ligand (namely -tBu, -H, and -CF ) modulates the catalytic overpotential, in part due to involvement of the reduced ligand character in formally rhodium(I) intermediates. These reduced species are synthesized and isolated here; protonation results in formation of complexes bearing the unusual η -pentamethylcyclopentadiene ligand, and the properties of these protonated intermediates further govern the catalytic performance.