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 (L4), 6-(1-phenylvinyl)-2,2'-bipyridine (LS), and 6-(prop-1-en-2-yl)-2,2'-bipyridine (L6) were synthesized by the Negishi coupling of 6-bromo-2,2'-bipyridine with the corresponding organo-zinc reagents. The reaction of L4 with K2PtCl4 produced only the cycloplatinated compound 4a via sp(2) C-H bond activation. The reactions of L5 and L6 produced exclusively the cycloplatinated compounds 5b and 6a, respectively, via vinylic C-H bond activation. DFT calculations were performed on 12 possible cycloplatination products from the reaction of N-alkyl-N-phenyl-'2,2'-bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and isopropyl (L3)) and L4 L6. The results show that compounds 1b-3b resulting from the sp(3) C-H bond activation of L1-L3 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 L5 and L6. The relative stability of isomeric cycloplatinated compounds 4a and 4b 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.