Photothermally Induced Bergman Cyclization of Metalloenediynes via Near-Infrared Ligand-to-Metal Charge-Transfer Excitation

Publisher Website
Google Scholar
Abstract
Reaction of 1,2-bis(tert-butyldimethylsilyloxy)-4,5-diiodobenzene with 2 equiv of phenylacetylene followed by deprotection with KF/HBr yields the catechol-enediyne ligand 4,5-bis(phenylethynyl)benzene-1,2-diol (CatED, 1). Metathesis of VO(SALIMH)ACAC·CH3OH (2) with 1 and subsequent air oxidation yields (4,5-bis(phenylethynyl)-1,2-dihydroxyphenyl)[4-(2-(salicylideneamino)ethyl)imidazolyl]oxovanadium(V)·CH3OH [VO(SALIMH)CatED], (3), in 85%. The thermal Bergman cyclization temperature for 3 is very high (246 °C), which is expected for a rigid, benzannulated enediyne motif. The electronic spectrum of 3 exhibits two strong ligand-to-metal charge transfer (LMCT) transitions centered at 584 nm (ε = 6063 M-1 cm-1) and 1028 nm (ε = 8098 M-1 cm-1). These transitions derive from CatED-to-V(V) ligand-to-metal charge transfer, the assignment of which is verified by resonance enhancement of several CatED vibrational modes in the Raman spectra obtained with λ = 785 vs λ = 457.9 nm under low power and/or temperature conditions. At elevated temperatures (113−323 K) and powers (2−5 mW), excitation of 3 in the solid state with λ = 785 nm leads to generation of a black, sparingly soluble, fluorescent product that exhibits weak vibrational features in the 580−600, 1200−1350, and 1450−1600 cm-1 regions, indicative of V−O (CatED) and aromatic ring units. The CC ring modes correspond well with the vibrational characteristics of poly(p-phenylene) and derivatives thereof. Additionally, materials generated in both the solid-state thermal and photothermal reactions of 3 demonstrate the formation of high molecular weight species ranging from 5000 to 274 000. On the basis of these data and the literature precedent for formation of poly(p-phenylene) via thermolysis of simple enediynes, the reaction poses a unique approach for photoinitiating Bergman cyclization with long-wavelength excitation, as well as the generation of polymeric products.

This publication has 31 references indexed in Scilit: