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 η(6)-coordination of 9CPP or 12CPP with M(CO)6 (M = Cr, Mo, W) afforded the corresponding nCPP-M(CO)3 complexes (n = 9, 12; M = Cr, Mo, W). In the (1)H 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 9CPP-Cr(CO)3 could be successfully isolated in spite of its instability. X-ray crystallographic analysis and computational studies of 9CPP-Cr(CO)3 revealed that chromium-CPP coordination occurs at the convex surface of 9CPP both in the solid state and in solution. TD-DFT calculations suggested that the emerging high-wavenumber absorption peak upon coordination of 9CPP to Cr(CO)3 should be assigned to a weak HOMO-LUMO transition. Moreover, by using the complex 9CPP-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.