RESUMEN
[32](1,4)Naphthalenophanes, bearing carbon-bridge chains (syn- and anti-NPs) and nitrogen-bridge chains (syn- and anti-ANPs), were synthesized, and their X-ray structures and photoreactions were investigated. The intramolecular separation distance between the naphthalene cores for ANPs was shorter than that for NPs, suggesting that intramolecular interactions between the naphthalene rings were more efficient for ANPs compared to NPs. Upon photoirradiation at 300 nm, anti-NP, syn-ANP and anti-ANP produced the corresponding intramolecular [π4s + π4s] cycloadducts, whereas syn-NP gave an unidentified complex product mixture. Quantum yields for the photo-consumption (ΦPC) of NPs and ANPs were evaluated to quantitatively compare their photoreactivity. The ΦPC values of ANPs were approximately two-fold higher than those of ANPs.Noteworthily, the ΦPC value of syn-ANP was estimated to be unity. Based on these results we discuss the effects of the alignments of the naphthalene cores (anti vs. syn) and the bridging elements (C-bridge vs. N-bridge) on the photoreaction efficiencies of [32](1,4)naphthalenophanes.
RESUMEN
Vanadium is accommodated in both the framework (VoutV) and central positions (VinV) in the Keggin-type polyoxometalates (POMs) [VinVVoutVM11O40]4- (M = Mo, W; VinVVoutVM11) and in the central position in [VinVM12O40]3- (VinVM12). The structures of the VinVVoutVM11 class have been determined by X-ray crystallography and compared to those of VinVM12 reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With VinVVoutVM11 and VinVM12 POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of VinVMo12 occurs at the molybdenum framework site to give VinVMoVMo11. In contrast, EPR spectra of one-electron reduced VinVW12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give VinIVW12. In the case of VinVVoutVM11, the initial reduction occurs at the framework VoutV site to give VinVVoutIVM11. The second electron is delocalized over the Mo framework in two-electron reduced VinVVoutIVMoVMo10, whereas it is accommodated on the central V site in VinIVVoutIVW11. The distance between VinIV and VoutIV in VinIVVoutIVW11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in VinVVoutVW11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF3SO3H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [n-Bu4N][PF6]) and allowed acid association constants to be estimated and compared with the literature values available for [XVoutVM11O40]n- (X = S (n = 3), P and As (n= 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51V NMR measurements on the oxidized VV forms of the POMs.