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

Reaction of 1,2-bis(te-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-CH30H (2) with 1 and subsequent air oxidation yields (4,5-bis(phenylethynyl)1,2-dihydroxyphenyl)[4-(2-(salicylideneamino)ethyl)imidazolyl]oxovanadium(V)-CH30H[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 (6 = 6063 M cm) and 1028 nm ((epsilon) 8098 M cm). 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 /l = 785 vs A 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 /L = 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 regions, indicative of V-O (CatED) and aromatic ring units. The C=C 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 longwavelength excitation, as well as the generation of polymeric products.