Dimitar Y. Shopov

Stable Iridium(IV) Complexes of an Oxidation-Resistant Pyridine-Alkoxide Ligand: Highly Divergent Redox Properties Depending on the Isomeric Form Adopted

The preparation of the facial and meridional isomers of [Ir(pyalk)3] (pyalk = 2-(2-pyridyl)isopropanoate), as model complexes for a powerful water oxidation catalyst, is reported. The strongly donating N3O3 ligand set is very oxidation-resistant, yet promotes facile metal-centered oxidation to form stable Ir(IV) compounds. The Ir(III/IV) reduction potentials of the two isomers differ by 340 mV despite the identical ligand set. A ligand field rationalization is advanced and supported by DFT calculations.

A Stable Coordination Complex of Rh(IV) in an N,O-Donor Environment.

We describe facial and meridional isomers of [Rh(III)(pyalk)3], as well as meridional [Rh(IV)(pyalk)3](+) {pyalk =2-(2-pyridyl)-2-propanoate}, the first coordination complex in an N,O-donor environment to show a clean, reversible Rh(III/IV) redox couple and to have a stable Rh(IV) form, which we characterize by EPR and UV-visible spectroscopy as well as X-ray crystallography. The unprecedented stability of the Rh(IV) species is ascribed to the exceptional donor strength of the ligands, their oxidation resistance, and the meridional coordination geometry.

Synthesis and Characterization of Iridium(V) Coordination Complexes With an N,O-Donor Organic Ligand

We have prepared and fully characterized two isomers of [IrIV (dpyp)2 ] (dpyp=meso-2,4-di(2-pyridinyl)-2,4-pentanediolate). These complexes can cleanly oxidize to [IrV (dpyp)2 ]+ , which to our knowledge represent the first mononuclear coordination complexes of IrV in an N,O-donor environment. One isomer has been fully characterized in the IrV state, including by X-ray crystallography, XPS, and DFT calculations, all of which confirm metal-centered oxidation. The unprecedented stability of these IrV complexes is ascribed to the exceptional donor strength of the ligands, their resistance to oxidative degradation, and the presence of four highly donor alkoxide groups in a plane, which breaks the degeneracy of the d-orbitals and favors oxidation.

Synthesis of pyridine-alkoxide ligands for formation of polynuclear complexes

We have prepared and characterized a series of novel polydentate N,O-donor ligands derived from our well-studied ligand 2-(2-pyridinyl)-2-propanol (pyalkH), having the general formula Me{C(OH)(2-py)CH2}nH, where n = 2 or 3. Like pyalkH, these analogues bind via N and O with deprotonation at the latter, thus extending the strongly donor pyridine-alkoxide chelation power of pyalkH to polydentate forms. The greater denticity allows for more effective binding and polynuclear cluster formation with first-row transition metals. Several stable alkoxo-bridged polynuclear clusters of these ligands with Mn, Cu, Co and Ni have been prepared; all reported ligands and complexes have been characterized, including by X-ray crystallography. We report a one-step synthesis of these ligands, alongside pyalkH, on a multi-gram scale from inexpensive starting materials. We have also developed a new scalable procedure for the isolation of pyalkH that avoids the need for chromatography, making large-scale production of this ligand commercially viable.

A Dinuclear Iridium(V,V) Oxo-Bridged Complex Characterized Using a Bulk Electrolysis Technique for Crystallizing Highly Oxidizing Compounds

We report a general method for the preparation and crystallization of highly oxidized metal complexes that are difficult to prepare and handle by more conventional means. This method improves typical bulk electrolysis and crystallization conditions for these reactive species by substituting oxidation-prone organic electrolytes and precipitants with oxidation-resistant compounds. Specifically, we find that CsPF6 is an effective inert electrolyte in acetonitrile, and appears to have general applicability to electrochemical studies in this solvent. Likewise, CCl4 is not only an oxidation-resistant precipitant for crystallization from MeCN but it also enters the lattice. In this way, we synthesized and characterized an Ir(V,V) mono-μ-oxo dimer which only forms at a very high potential (1.9 V vs NHE). This compound, having the highest isolated oxidation state in this redox-active system, cannot be formed chemically. DFT calculations show that the oxidation is centered on the Ir-O-Ir core and facilitated by strong electron-donation from the pyalk (2-(2-pyridinyl)-2-propanolate) ligand. TD-DFT simulations of the UV-visible spectrum reveal that its royal blue color arises from electron excitations with mixed LMCT and Laporte-allowed d-d character. We have also crystallographically characterized a related monomeric Ir(V) complex, similarly prepared by oxidizing a previously reported Ir(IV) compound at 1.7 V, underscoring the general applicability of this method.