Synthesis and Characterization of [<i>n</i>]CPP (<i>n</i> = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization

Radical cations and dications of [<i>n</i>]­cyclo-<i>p</i>-phenylenes ([<i>n</i>]­CPPs, <i>n</i> = 5, 6, 10, and 12), which are the models of those of linear oligo-<i>p</i>-phenylenes without a terminus, were synthesized as hexafluoro­antimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF₆ or SbF₅. The radical cations, [<i>n</i>]­CPP^•+, and dications, [<i>n</i>]­CPP²⁺, exhibited remarkable batho­chromic shifts in their UV–vis–NIR absorption bands, suggesting that [<i>n</i>]­CPP^•+ and larger [<i>n</i>]­CPP²⁺ exhibit longer polyene character than the shorter analogues. The larger batho­chromic shift was consistent with the narrower HOMO–SOMO and HOMO–LUMO gaps in larger [<i>n</i>]­CPP^•+ and [<i>n</i>]­CPP²⁺, respectively. In [<i>n</i>]­CPP^•+, the spins and charges were equally and fully delocalized over the <i>p</i>-phenylene rings of the CPPs, as noted by ESR. ¹H NMR revealed that the hydrogen of [<i>n</i>]­CPP²⁺ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [<i>n</i>]­CPP²⁺. The single resonances observed in all [<i>n</i>]­CPP²⁺ strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]­CPP by VT-NMR experiment and theoretical calculation

Through-Space Magnetic Interaction of <i>cis</i>-Azobenzene Biradical

Two chiral PROXYL groups bridged with azobenzene show the change of the magnetic properties with the trans-cis photoisomerization of the azobenzene moiety. The isomerization brought the radicals closer together and increased the exchange interactions. The spin state of the cis-isomer was determined from two-dimensional nutation spectra as a mixture of S = 1/2 and S = 1. This is an example of the optical control of through-space intramolecular magnetic interactions

Characterization of a Stable Ruthenium Complex with an Oxyl Radical

The ruthenium oxyl radical complex, [RuII(trpy)(Bu2SQ)O•-] (trpy = 2,2‘:6‘,2‘ ‘-terpyridine, Bu2SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepared for the first time by the double deprotonation of the aqua ligand of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2 is reversibly converted to [RuIII(trpy)(Bu2SQ)(OH-)]+ upon dissociation of the aqua proton (pKa 5.5). Deprotonation of the hydroxo proton gave rise to intramolecular electron transfer from the resultant O2- to Ru−dioxolene. The resultant [RuII(trpy)(Bu2SQ)O•-] showed antiferromagnetic behavior with a RuII−semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [RuII(trpy)(Bu2SQ)O•-] is a long Ru−O bond length (2.042(6) Å) as the first terminal metal−O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepared and we compared the deprotonation behavior of the aqua ligand with that of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. Deprotonation of the aqua ligand of [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 induced intramolecular electron transfer from OH- to the [RuIII(4ClSQ)] moiety affording [RuII(trpy)(4ClSQ)(OH•)]+, which then probably changed to [RuII(trpy)(4ClSQ)O•-]. The antiferromagnetic interactions (J values) between RuII-semiquinone and the oxyl radical for [RuII(trpy)(Bu2SQ)O•-] and for [RuII(trpy)(4ClSQ)O•-] were 2J = −0.67 cm-1 and −1.97 cm-1, respectively

<i>Meta</i>–<i>Para</i>-Linked Octaaza[1₈]cyclophanes and Their Polycationic States

Octaazacyclophanes, octaaza­[1₈]<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>-cyclophane (<b>2</b>) and octaaza­[1₈]<i>m</i>,<i>p</i>,<i>p</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>p</i>,<i>p</i>-cyclophane (<b>3</b>), as ring-size extended congeners of tetraaza­[1₄]<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>-cyclophane were synthesized, and the electronic states of their polycationic species were investigated by quantum chemical calculations, electrochemical measurements (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), UV–vis–NIR spectroelectrochemical measurements, and pulsed electron spin resonance (ESR) spectroscopy. These octaazacyclophanes exhibited multiredox activities depending on different linkage patterns along the macrocyclic molecular skeletons, and both molecules were oxidizable up to their respective octacations. Spectroelectrochemical measurements demonstrated that <i>p</i>-phenylenediamine (PD) moieties in <b>2</b> could be converted from the semiquinoidal structure to the quinoidal sturcture with increasing oxidation number, whereas higher oxidation states of <b>3</b> did not show definite quinoidal deformation of PD moieties. A pulsed ESR spectrum gave evidence about formation of the almost pure spin-triplet state for <b>3</b>²⁺, whereas the high-spin states of <b>2</b>²⁺ and <b>2</b>⁴⁺ are virtually degenerate with the competing low-spin states even at low temperatures, probably due to the fragility of spin-coupling pathway caused by facile conformational changes

Metal−Metal <i>d</i><i>−</i><i>d</i> Interaction through the Discrete Stacking of Mononuclear M(II) Complexes (M = Pt, Pd, and Cu) within an Organic-Pillared Coordination Cage

Two molecules of planar MII(acac)2 complexes (M = Pt, Pd, and, Cu; acac = acetylacetonato) are efficiently stacked within an organic-pillared coordination cage, exhibiting characteristic spectroscopies (for M = Pt and Pd) and electron spin−spin coupling (for M = Cu) attributable to metal−metal interaction

Fullerenols Revisited as Stable Radical Anions

The first exhaustive purification and characterization of the much-studied “fullerenols”, prepared by reaction of C60 in toluene with an oxygenated, aqueous NaOH solution using tetrabutylammonium hydroxide as a phase transfer catalyst, has been performed. The resulting fullerenol is not simply polyhydroxylated C60 but rather is a structurally and electronically complex C60 radical anion with a molecular formula of Na+n[C60Ox(OH)y]n- (where n = 2−3, x = 7−9, and y = 12−15) for three different, but identical, preparations. Surprisingly, Na+-fullerenol is paramagnetic, exhibiting μB values in aqueous solution of 1.9−2.1 B.M. at 0.5 T and 300 K and R1 proton relaxivities of 0.55−0.77 mM-1s-1 at 20 MHz and 40 °C, values both slightly higher than those expected for a pure S = 1/2 spin system. ESR studies (ESE-FS and 2D nutation) of frozen aqueous solutions at 1.5 and 5.0 K establish that Na+-fullerenol is mainly S = 1/2 with a minor, but significant, component of S = 1. Thus, this is the first report to characterize these widely studied, water-soluble fullerenols as stable radical anions. The stability of the S = 1/2 Na+-fullerenol radical is likely due to a highly derivatized C60 surface that protects a cyclopentadienyl radical center on the fullerene

Efficient Incorporation of a Copper Hydroxypyridone Base Pair in DNA

Recently, we reported the first artificial nucleoside for alternative DNA base pairing through metal complexation (J. Org. Chem. 1999, 64, 5002−5003). In this regard, we report here the synthesis of a hydroxypyridone-bearing nucleoside and the incorporation of a neutral Cu2+-mediated base pair of hydroxypyridone nucleobases (H−Cu−H) in a DNA duplex. When the hydroxypyridone bases are incorporated into the middle of a 15 nucleotide duplex, the duplex displays high thermal stabilization in the presence of equimolar Cu2+ ions in comparison with a duplex containing an A−T pair in place of the H−H pair. Monitoring temperature dependence of UV-absorption changes verified that a Cu2+-mediated base pair is stoichiometrically formed inside the duplex and dissociates upon thermal denaturation at elevated temperature. In addition, EPR and CD studies suggested that the radical site of a Cu2+ center is formed within the right-handed double-strand structure of the oligonucleotide. The present strategy could be developed for controlled and periodic spacing of neutral metallobase pairs along the helix axis of DNA

Metal-Dependent Regioselective Oxidative Coupling of 5,10,15-Triarylporphyrins with DDQ-Sc(OTf)₃ and Formation of an Oxo-quinoidal Porphyrin

Regioselectivity of the oxidative coupling of 5,10,15-triarylporphyrin metal complexes with DDQ-Sc(OTf)3 was dependent on the central metal and meso-aryl substituent. Oxo-quinoidal porphyrin was obtained from Ni(II) porphyrin under the same conditions

Spin Crossover by Encapsulation

Spin Crossover by Encapsulatio