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

Abstract

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