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Bis-iminophosphoranes containing various types of linkers between two R3P=N moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3P=N moieties separated by a âC(CN)=C(CN)âgroup was obtained from its crystal structure. |
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Paramagnetic complexes M(CO)5P(C6H5)2, with M = Cr, Mo, W, have been trapped in irradiated crystals of M(CO)5P(C6H5)3 (M = Cr, Mo, W) and M(CO)5PH(C6H5)2 (M = Cr, W) and studied by EPR. The radiolytic scission of a PâC or a PâH bond, responsible for the formation of M(CO)5P(C6H5)2, is consistent with both the number of EPR sites and the crystal structures. The g and 31P hyperfine tensors measured for M(CO)5P(C6H5)2 present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph2Pâ˘, the 31P isotropic coupling is larger, the dipolar coupling is smaller, and for Mo and W compounds, the g-anisotropy is more pronounced. These properties are well predicted by DFT calculations. In the optimized structures of M(CO)5P(C6H5)2 (M = Cr, Mo, W), the unpaired electron is mainly confined in a phosphorus p-orbital, which conjugates with the metal dxz orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical. |
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Two new, âuser-friendlyâ derivatives of triptycene containing AsH2 and SiH3 fragments were synthesized. Both solids are crystalline, air-stable compounds characterized by elevated melting points and resistance toward moisture. The highly reactive AsâH and SiâH bonds are protected by the presence of the surrounding phenylene hydrogen atoms, which ensure a remarkable kinetic stabilization of these primary hydrides. After X-ray irradiation of a single crystal of triptycenesilane, a persistent silyl radical was trapped and characterized. |
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Cyclic voltammetry of Mes*P==C(NMe2)2 (1) and Mes*P==C(CH3)NMe2 (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp2Fe]PF6 leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental 31P and 13C isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*Pâ˘â(C(NMe2)2)+. Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe2)2 moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)Pââ˘C(NMe2)2. No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the PâC double bond (P-âC+), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal Ď electron distribution (P+âC-): no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1â˘+PF6-. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorusâcarbon bond caused by the presence of the amino groups. |
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X-irradiation of single crystals of TpâGeH3 (Tp: triptycene) led to the trapping of the radical TpââGeH2. The angular variations of the resulting EPR spectra were recorded at 300 and 77 K. The drastic temperature dependence of the spectra was caused by both a strong anisotropy of the g-tensor and a rotation of the âGeH2 moiety around the CâGe bond. The determination of the EPR tensors as well as the analysis of this motion required to take the presence of disorder in the crystal into account. In accordance with DFT calculations, TpââGeH2 is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the CâGeH2 bond is blocked at 90 K and is almost free above 110 K. The experimental barrier, obtained after simulation of the EPR spectra as a function of the rotational correlation time, is equal to 1.3 kcal molâ1, which is slightly inferior to the barrier calculated by DFT (3.6 kcal molâ1). Calculations performed on TpâCH3, TpâGeH3 and TpââGeH2 show that the rotation barrier ÎErot around the CâGe bond drastically decreases by passing from the germane precursor to the germanyl radical and that ÎErot increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined. |
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Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The 31P and 13C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]ââ and [HP---CH=Cp]â have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]ââ is readily protonated and that the EPR spectra are due to the phosphaallylic radical. |
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As shown from the crystal structure, the oxygen atom of Ph3P=CH---C(O)CH3 forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyperfine couplings with those obtained from ab initio calculations shows that the radical cation Ph3P+---CH=C(OH)CH2 is formed under radiolysis. The principal directions of the hyperfine tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph3P---C bond. |
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A new phosphine, the diphenyldibenzobarrelenephosphine 2, was designed to study the barrier to rotation of the PâH group around the Cââ˘P bond. After homolytic scission of a PâH bond by radiolysis, the EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of 2, was studied at 77 K and at room temperature. The directions of the 31P hyperfine eigenvectors were compared with the bond orientations of the undamaged compound as determined from its crystal structure. The temperature dependence of the EPR spectrum was analyzed by using the density matrix formalism; this showed that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond is determinant for explaining the potential energy profile. DFT investigations are consistent with these experimental results. |
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The crystal structure of the title compound, [Pd2Br2(C25H34P)2], a new binuclear phospha-alkene compound containing a trivalent P atom shows a centrosymmetric dimeric arrangement. The Pd2Br2 core is planar and adopts an irregular diamond shape. The coordination of the Pd atom is square planar. No stacking interactions were observed in the molecular packing. |
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Two phosphaalkenes containing either a furane or a thiophene ring bound to the carbon atom of the âPdouble bond; length as m-dashC < bond have been synthesized. The crystal structure of the furane derivative has been determined and the electrochemistry of both compounds has been investigated. THF solutions of these compounds react at 255 K with a potassium mirror to yield the corresponding radical anions which have been studied by EPR in both the liquid and solid states. The resulting hyperfine constants are compared with the values predicted by ab initio calculations on radical anions formed from model phosphaalkenes. |
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Several radiation defects have been detected by EPR in a single-crystal of Pt(dmimt)4Cl2.4H2O (dmit = 1,3-dimethyl-imidazoline-2-thione). In order to identify these rediogenic species, the structure of the crystal has been resolved and the angular dependence of the EPR signals has been analysed. The resulting g tensors and 195Pt hyperfine tensors have been determined and the orientations of their principal axes have been compared to those of the bond directions of the precursor. It is shown that both Pt(I) and Pt(III) complexes are trapped, whereas Pd(dmimt)42+ present as an impurity, leads only to the Pd(I) species. The temperature dependende of the EPR spectra gives information about the relative stability of the paramagnetic species and shows that the formation of some species, especially the Pt(III) complexes, requires drastic modifications of the parent Pt(II) cation. |
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Hydroxyurea (HU), a ribonucleotide reductase inhibitor has been used in the treatment of some malignant and viral diseases and seems now to be promising, in association with 2,3dideoxynucleosides, for the management of AIDS. In an attempt to increase the specificity of action of this radical scavenger, or at least, to study the topological aspects of its reactivity, we introduced the N-hydroxyureido group into nucleosides by using Mitsunobu reaction or by reacting a nucleoside nitrogen nucleophile with a carbonyl electrophile. From the currently available antiviral testing results, concerning the nucleoside analogues it appears that the most noticable activity exert against Varicella Zoster virus (VZV). One acyclonucleoside derivative was found to be very active against the virus HIV-1, its therapeutic index is better than 100.000. We prepared peptid-like dinucleotide analogues33,36 also in which the internucleosidic bridge consists of a spacer of approximately the same length as in the natural compounds. These compounds could be tested as inhibitors of nucleotide-protein interactions, we supposed that they are able to disrupt zinc finger parts of nucleocapsid. Antiviral activity of these dinucleotides were tested in vitro against HIV-1, HIV-2, HSV-1, HSV-2, CMV, VZV and EBV but in no case EC50 values inferior to 10 ľM was found. |
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The synthesis and crystal structure of 1,2-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzene, L, are reported as well as the preparation and conformation of the novel seven-membered ring complex [PdLCl2]; this complex reacts with alcohols (MeOH, EtOH) to give a chiral cyclometallated complex [rac(R)P, (R)C; (S)P, (S)C] where the metal is bound to both a phosphaalkene and a phosphite phosphorus atom. |
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To measure the rotation barrier around an R3CâPH bond in the solid state, 9-phosphinotriptycene50 2 has been synthesized and its crystal structure has been determined. It is shown, by EPR, that the radiogenic radical 3, which results from a homolytic scission of a PâH bond, can indeed be trapped in the crystal matrix. Its g-tensor together with its 31P and 1H hyperfine coupling have been measured at 300 and 77 K. These tensors show that free rotation around the CâP bond occurs at room temperature but is blocked at liquid nitrogen temperature. The temperature dependence of the EPR spectra has been analyzed using the density matrix formalism and has led to a rotation barrier of about 2.5 kcalˇmol-1 . This result and the various hyperfine couplings have been compared with the values predicted by ab initio methods for two isolated model radicals: the tert-butylphosphinyl radical 4 and the barrelenophosphinyl radical 5. |
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A series of 2,3-O-cyclopentylidene C-glycoside analogs in which the furanose ring has been replaced with a N-hydroxypyrrolidine have been prepared. A structural study of these tricyclic compounds and the aminoxyl radicals thereof has been carried out using variable temperature 1H NMR, X-ray diffraction, molecular dynamics and EPR spectroscopy. Both types of compounds, N-hydroxypyrrolidines and pyrrolidine N-oxyls, fundamentally prefer - in solutions- N-endo conformations over the alternate, N-exo forms found by X-ray diffraction studies and computed to be more stable by molecular dynamics. |
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Bis(N-methylimidazolidinethi-2-one)copper(I) chloride has been synthesized and its crystal structure determined. X-Irradiation of a single crystal of this compound leads to the formation of a CuII complex which was studied by EPR: it was shown that this species results from the addition of a radiogenic Cl atom on the CuI precursor. The structural changes induced by this reaction are revealed by the g-tensor and by the hyperfine tensors of one copper and two chlorine nuclei. The structure of this S2Cl2Cu type complex was compared with other sulfur- or chlorine-containing CuII complexes. |
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The new diphosphaalkene 1,3-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzenLe,, h as been synthesized. Due to the presence of two P=C bonds three isomers (EE, EZ, ZZ) were observed by 31P NMR, and the crystal structures of two of them could be determined (EE, ZZ). The electrochemical behavior of L has been studied by cyclic voltametry: a quasi-reversible reduction occurs at -1.89 V/SCE and corresponds to the formation of a radical anion which has been studied by ESR at variable temperature. The experimental 31P and 'H hyperfine constants are consistent with free rotation about the P=C and Cphosphaalkene-Cbenzene bonds at room temperature and agree with ab initio predictions. One of the isomers of L forms complexes with palladium(I1) and platinum(I1) ions. The crystal structures show that L is orthometalated and acts as a terdentate ligand by coordinating the metal with each phosphorus atom. These complexes are electrochemically reduced between -0.92 and - 1.29 V, and the resulting paramagnetic species are studied by ESR in liquid and frozen solutions. This reduction process was shown to be a ligand-centered process, an appreciable part of the unpaired electron is localized on each of the phosphaalkene carbons (20%) and phosphorus atoms (5%). |
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