We report the X-ray diffraction structure and thermal reactivity of the metalloenediyne compound bis(1,2-bis(diphenylphosphinoethynyl)benzene)palladium(0) (Pd(dppeb) 2 , 1). The structure of 1 features a tetrahedral Pd(0) center with four phosphorus atoms from two chelating ligands. The PsPdsP bond angles nearly match the idealized 109.5°geometry expected for a d 10 metal center in a tetrahedral ligand field. The tetrahedral geometry of the metal center forces the alkyne termini separation of the enediyne ligand to a distance of 3.47 Å, which results in a thermally stable compound at room temperature. However, at 115 °C 1 exhibits solvent-dependent reactivity. In o-fluorotoluene, 1 decomposes via ligand dissociation, while in odichlorobenzene, carbon-halide bond activation of solvent occurs leading to the oxidative addition product trans-Pd((2-chlorophenyl)diphenylphosphine) 2 Cl 2 and free (2-chlorophenyl)diphenylphosphine. The thermal reactivity of 1 is markedly more endothermic (44 kcal/mol) than that of the known Pd(dppeb)Cl 2 analogue (12.3 kcal/mol). The diminished reactivity can be attributed to two factors: the increased alkyne termini separation in 1 (3.47 vs 3.3 Å) due to the metal-mandated tetrahedral geometry of the Pd(0) center, and the resistance of the Pd(0) to adopting a planar transition state geometry to promote Bergman cyclization. Overall this study demonstrates that metal binding can impose structural consequences upon the enediyne ligand governed by the oxidation state and corresponding ligand field geometry of the metal center.

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.