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

Abstract

The synthesis, structure, photophysical properties, and reactivity of cycloparaphenylenes (CPPs) coordinated to group 6 transition metal fragments are described. The η⁶-coordination of [9]­CPP or [12]­CPP with M­(CO)₆ (M = Cr, Mo, W) afforded the corresponding [<i>n</i>]­CPP-M­(CO)₃ complexes (<i>n</i> = 9, 12; M = Cr, Mo, W). In the ¹H 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