Synthesis, Structure, Photophysics, and a DFT Study of Phosphorescent C*N^∧N- and C^∧N^∧N‑Coordinated Platinum Complexes

The reaction of <i>N</i>,<i>N</i>-diphenyl-2,2′-bipyridin-6-amine (<b>L1</b>) and <i>N</i>,<i>N</i>-diphenyl-6-(1<i>H</i>-pyrazol-1-yl)­pyridin-2-amine (<b>L2</b>) with K₂PtCl₄ produced C*N^∧N-coordinated cycloplatinated compounds with a five–six fused metallacycle <b>1a</b> and <b>2a</b>, respectively, which were then converted into their phenylacetylide derivatives <b>1b</b> and <b>2b</b>, respectively. Similar reactions starting from 2-phenyl-6-(1<i>H</i>-pyrazol-1-yl)­pyridine (<b>L3</b>) produced C^∧N^∧N-coordinated platinum complexes <b>3a</b> and <b>3b</b> with a five–five-fused metallacycle. The structures of <b>1a</b>, <b>1b</b>, <b>2b</b>, <b>3a</b>, and <b>3b</b> were determined by X-ray crystallography. The C*N^∧N-coordinated platinum complexes are closer to a square geometry, whereas the C^∧N^∧N-coordinated complexes display a nearly perfect planar geometry. The π···π interactions were revealed in the crystal packing for <b>1a</b>, <b>2b</b>, and <b>3a</b> with a π···π contact of 3.450, 3.422, and 3.414 Å, respectively. Two conformers were revealed in the crystal structure of <b>2b</b>, one with the phenyl ring of the phenylacetylide being approximately parallel with the coordination plane and the other with the phenyl ring being approximately perpendicular to the coordination plane. Both <b>1a</b> and <b>1b</b> are weakly emissive in the red region. Complexes <b>2a</b> and <b>3a</b> are also weakly emissive, but their acetylide derivatives <b>2b</b> and <b>3b</b> emitted strongly green light at room temperature with quantum yields of 43 and 62%, respectively. DFT/TDDFT calculations were performed to elucidate the nature of their electronic transitions. The calculations suggested that lowest singlet and triplet excited states are characteristic of a mixed state involving one or more charge-transfer transitions such as ILCT, MLCT, and LLCT

Synthesis, Structure, Photophysics, and a DFT Study of Phosphorescent C*N^∧N- and C^∧N^∧N‑Coordinated Platinum Complexes

The reaction of <i>N</i>,<i>N</i>-diphenyl-2,2′-bipyridin-6-amine (<b>L1</b>) and <i>N</i>,<i>N</i>-diphenyl-6-(1<i>H</i>-pyrazol-1-yl)­pyridin-2-amine (<b>L2</b>) with K₂PtCl₄ produced C*N^∧N-coordinated cycloplatinated compounds with a five–six fused metallacycle <b>1a</b> and <b>2a</b>, respectively, which were then converted into their phenylacetylide derivatives <b>1b</b> and <b>2b</b>, respectively. Similar reactions starting from 2-phenyl-6-(1<i>H</i>-pyrazol-1-yl)­pyridine (<b>L3</b>) produced C^∧N^∧N-coordinated platinum complexes <b>3a</b> and <b>3b</b> with a five–five-fused metallacycle. The structures of <b>1a</b>, <b>1b</b>, <b>2b</b>, <b>3a</b>, and <b>3b</b> were determined by X-ray crystallography. The C*N^∧N-coordinated platinum complexes are closer to a square geometry, whereas the C^∧N^∧N-coordinated complexes display a nearly perfect planar geometry. The π···π interactions were revealed in the crystal packing for <b>1a</b>, <b>2b</b>, and <b>3a</b> with a π···π contact of 3.450, 3.422, and 3.414 Å, respectively. Two conformers were revealed in the crystal structure of <b>2b</b>, one with the phenyl ring of the phenylacetylide being approximately parallel with the coordination plane and the other with the phenyl ring being approximately perpendicular to the coordination plane. Both <b>1a</b> and <b>1b</b> are weakly emissive in the red region. Complexes <b>2a</b> and <b>3a</b> are also weakly emissive, but their acetylide derivatives <b>2b</b> and <b>3b</b> emitted strongly green light at room temperature with quantum yields of 43 and 62%, respectively. DFT/TDDFT calculations were performed to elucidate the nature of their electronic transitions. The calculations suggested that lowest singlet and triplet excited states are characteristic of a mixed state involving one or more charge-transfer transitions such as ILCT, MLCT, and LLCT

Redox-Active Bis(phenolate) N‑Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States

A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis­(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co­(II) centers bound to closed-shell [OCO]^2– dianions, of the general formula [(OCO)­Co^IIL] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)­Co^IIL] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc⁺/Fc, corresponding to generation of formally Co­(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [(^SOCO)­Co^III(THF)₂]⁺ containing a closed-shell [^SOCO]^2– diphenolate ligand bound to a <i>S</i> = 1 Co­(III) center, or [(^SOCO^•)­Co^II(THF)₂]⁺ with a low-spin Co­(II) ion ferromagnetically coupled to monoanionic [^SOCO^•]⁻ containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [(^SOCO⁰)­Co^II(THF)₃]²⁺, with a single unpaired electron localized on the d⁷ Co­(II) center and a doubly oxidized, charge-neutral, closed-shell ^SOCO⁰ ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed