title: Photothermally induced Bergman cyclization of metalloenediynes via near-infrared ligand-to-metal charge-transfer excitation.
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.CH(3)OH (2) with 1 and subsequent air oxidation yields (4,5-bis(phenylethynyl)-1,2-dihydroxyphenyl)[4-(2-(salicylideneamino)ethyl)imidazolyl]oxovanadium(V).CH(3)OH [VO(SALIMH)CatED], (3), in 85%. The thermal Bergman cyclization temperature for 3 is very high (246 degrees 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 (epsilon = 6063 M(-)(1) cm(-)(1)) and 1028 nm (epsilon = 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 lambda = 785 vs lambda = 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 lambda = 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 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 long-wavelength excitation, as well as the generation of polymeric products.
authors.affiliation: Department of Chemistry and Biochemistry, Indiana University, Bloomington 47405, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Kraft
authors.forename: Nicole L
authors.initials: NL
authors.lastname: Coalter
authors.forename: Mahendra
authors.initials: M
authors.lastname: Nath
authors.forename: Aurora E
authors.initials: AE
authors.lastname: Clark
authors.forename: Allen R
authors.initials: AR
authors.lastname: Siedle
authors.forename: John C
authors.initials: JC
authors.lastname: Huffman
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Zaleski
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D004563:Electrochemistry; D008024:Ligands; D008670:Metals / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D010084:Oxidation-Reduction; D010777:Photochemistry; D013816:Thermodynamics
chemical_list: D000480:Alkynes; D008024:Ligands; D008670:Metals; D009942:Organometallic Compounds