Regiospecific Acylation of Cycloplatinated Complexes: Scope, Limitations, and Mechanistic Implications

A series of platinum complexes based on the tridentate cyclometalating ligand derivatives 6-arylamino-2,2′-bipyridine, 6-phenoxy-2,2′-bipyridine, 6-phenylthio-2,2′-bipyridine, 6-benzyl-2,2′-bipyridine, and 6-benzoyl-2,2′-bipyridine were synthesized, and their acylation reactions were studied. Acylation of platinum complexes based on 6-(4-R-phenylamino)-2,2′-bipyridine derivatives (R = CH₃O, CH₃, Cl, COOEt) tolerates both electron-donating and electron-withdrawing substituents on the aryl ring that are para to the amino group. However, platinum complexes based on 6-(3-R′-phenylamino)-2,2′-bipyridine (R′ = CH₃, Cl, Br) did not undergo the acylation reaction under the same conditions. Interestingly, the acylation of the platinum complexes based on 6-(3-fluorophenylamino)-2,2′-bipyridine proceeded smoothly, and the results indicate that the acylation is regiospecific and occurs at the metalated carbon. Complexes based on 6-phenoxy-2,2′-bipyridine, 6-phenylthio-2,2′-bipyridine, and 6-benzyl-2,2′-bipyridine are also regioselectively acylated. A cyclometalated platinum complex based on 6-benzoyl-2,2′-bipyridine, where the benzene is more electron deficient than those in other cyclometalated platinum complexes, failed to undergo the acylation reaction. The acylation can be carried out in acetic acid, 1,2-dichloroethane, benzonitrile, and acetonitrile. Other acyl halides such as benzoyl chloride and crotonyl chloride are also effective acylating reagents. On the basis of the fact that the reaction is discouraged by the electron deficiency of the phenyl ring and contrasting results of the acylation of platinum complexes based on 6-(3-R′-phenylamino)-2,2′-bipyridine (R′ = CH₃, F, Cl, Br), an unprecedented electrophilic addition–platinum migration–rearomatization cascade mechanism is proposed for the regiospecific acylation reaction

Computational and Experimental Study on Selective sp²/sp³ or Vinylic/Aryl Carbon–Hydrogen Bond Activation by Platinum(II): Geometries and Relative Stability of Isomeric Cycloplatinated Compounds

Cyclometalating ligands 6-(1-phenylethyl)-2,2′-bipyridine (<b>L4</b>), 6-(1-phenylvinyl)-2,2′-bipyridine (<b>L5</b>), and 6-(prop-1-en-2-yl)-2,2′-bipyridine (<b>L6</b>) were synthesized by the Negishi coupling of 6-bromo-2,2′-bipyridine with the corresponding organozinc reagents. The reaction of <b>L4</b> with K₂PtCl₄ produced only the cycloplatinated compound <b>4a</b> via sp² C–H bond activation. The reactions of <b>L5</b> and <b>L6</b> produced exclusively the cycloplatinated compounds <b>5b</b> and <b>6a</b>, respectively, via vinylic C–H bond activation. DFT calculations were performed on 12 possible cycloplatination products from the reaction of <i>N</i>-alkyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine (alkyl = methyl (<b>L1</b>), ethyl (<b>L2</b>), and isopropyl (<b>L3</b>)) and <b>L4</b>–<b>L6</b>. The results show that compounds <b>1b</b>–<b>3b</b> resulting from the sp³ C–H bond activation of <b>L1</b>–<b>L3</b> are thermodynamic products, and their relative stability is attributed to the planar geometry that allows for a better conjugation. Similar reasoning also applies to the stability of products from vinylic C–H bond activation of <b>L5</b> and <b>L6</b>. The relative stability of isomeric cycloplatinated compounds <b>4a</b> and <b>4b</b> may be due to the different strengths of C–Pt bonds. The steric interaction is the major cause of severe distortion from a planar coordination geometry in the cycloplatinated compounds, which leads to instability of the corresponding cyclometalated products and a higher kinetic barrier for C–H bond activation

Computational and Experimental Study on Selective sp²/sp³ or Vinylic/Aryl Carbon–Hydrogen Bond Activation by Platinum(II): Geometries and Relative Stability of Isomeric Cycloplatinated Compounds

Cyclometalating ligands 6-(1-phenylethyl)-2,2′-bipyridine (<b>L4</b>), 6-(1-phenylvinyl)-2,2′-bipyridine (<b>L5</b>), and 6-(prop-1-en-2-yl)-2,2′-bipyridine (<b>L6</b>) were synthesized by the Negishi coupling of 6-bromo-2,2′-bipyridine with the corresponding organozinc reagents. The reaction of <b>L4</b> with K₂PtCl₄ produced only the cycloplatinated compound <b>4a</b> via sp² C–H bond activation. The reactions of <b>L5</b> and <b>L6</b> produced exclusively the cycloplatinated compounds <b>5b</b> and <b>6a</b>, respectively, via vinylic C–H bond activation. DFT calculations were performed on 12 possible cycloplatination products from the reaction of <i>N</i>-alkyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine (alkyl = methyl (<b>L1</b>), ethyl (<b>L2</b>), and isopropyl (<b>L3</b>)) and <b>L4</b>–<b>L6</b>. The results show that compounds <b>1b</b>–<b>3b</b> resulting from the sp³ C–H bond activation of <b>L1</b>–<b>L3</b> are thermodynamic products, and their relative stability is attributed to the planar geometry that allows for a better conjugation. Similar reasoning also applies to the stability of products from vinylic C–H bond activation of <b>L5</b> and <b>L6</b>. The relative stability of isomeric cycloplatinated compounds <b>4a</b> and <b>4b</b> may be due to the different strengths of C–Pt bonds. The steric interaction is the major cause of severe distortion from a planar coordination geometry in the cycloplatinated compounds, which leads to instability of the corresponding cyclometalated products and a higher kinetic barrier for C–H bond activation

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

Reaction of <i>N</i>‑Isopropyl‑<i>N</i>‑phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄: Selective C–H Bond Activation, C–N Bond Cleavage, and Selective Acylation

The selective C–H bond activation of <i>N</i>-isopropyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine promoted by Pt­(II) was complicated by the low selectivity of sp² C–H bond activation in acetonitrile and low yield of sp³ C–H activation in acetic acid. The use of a base was found to effectively suppress the competing sp³ C–H bond activation in acetonitrile, improving the selectivity of sp² C–H bond activation from 70% to 99%. In the reaction in acetic acid, the low yield was due to the competing C–N bond cleavage. The use of a base reduced the C–N bond cleavage, but not completely. The reaction of <i>N</i>-<i>tert</i>-butyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄ in acetic acid produced the cyclometalated complex with complete C–N bond cleavage and its acylated derivative. These results indicated that the C–N bond cleavage might proceed via heterolytic C–N bond dissociation. The acylation following the C–N cleavage in the reaction in acetic acid is regioselective. Further experiments showed that the reaction of <i>N</i>-phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄ in acetic acid produced the cyclometalated complex, while the reaction in a mixture of acetic anhydride and acetic acid produced the acylated cyclometalated complex. An X-ray crystal structure study revealed strong intramolecular H bonding in the acylated complexes. The regioselectivity was explained in terms of H bonding and the electron distribution predicted by the DFT calculations