η6-Cycloparaphenylene Transition Metal Complexes: Synthesis, Structure, Photophysical Properties, and Application to the Selective Monofunctionalization of Cycloparaphenylenes

The synthesis, structure, photophysical properties, and reactivity of cycloparaphenylenes (CPPs) coordinated to group 6 transition metal fragments are described. The eta(6)-coordination of [9]CPP or [12]CPP with M(CO)6 (M = Cr, Mo, W) afforded the corresponding [n]CPP-M(CO)(3) complexes (n = 9, 12; M = Cr, Mo, W). In the H-1 NMR spectra of these complexes, characteristic upfield-shifted singlet signals corresponding to the four hydrogen atoms attached to the coordinated C6H4 ring of the CPPs were observed at 5.4-5.9 ppm. The complex [9]CPP-Cr(CO)(3) could be successfully isolated in spite of its instability. X-ray crystallographic analysis and computational studies of [9]CPP-Cr(CO)(3) revealed that chromium-CPP coordination occurs at the convex surface of [9]CPP both in the solid state and in solution. TD-DFT calculations suggested that the emerging high-wavenumber absorption peak upon coordination of [9]CPP to Cr(CO)(3) should be assigned to a weak HOMO-LUMO transition. Moreover, by using the complex [9]CPP-Cr(CO)(3), a rapid and highly monoselective CPP functionalization has been achieved. The established one-pot method, consisting of complexation, deprotonation, nucleophilic substitution, and decomplexation steps, yielded silyl-, boryl-, and methoxycarbonyl-substituted CPPs in up to 93% yield relative to reacted starting material.