Two molecules containing two phenylphosphaalkene moieties linked by an anthracene (1) or by a naphthalene (2) ring have been synthesized and their crystal structures have been determined. While electrochemical measurements show that these two systems are easily reduced, EPR spectra indicate that, at room temperature, the electronic structures of the two reduction compounds 1˙⁻ and 2˙⁻ are quite different. In 1˙⁻, in good accordance with DFT predictions, the unpaired electron is delocalized on the full molecule while in 2˙⁻ it is confined on a single phosphaalkene moiety. This difference is attributed to the short distance between the two phenylphosphaalkene groups in 2˙⁻ which hinders their reorientation after addition of an electron. The role of this motion is consistent with the fact that two additional paramagnetic species are detected at 145 K: the dianion 2²⁻ characterized by a rather small exchange coupling constant and the radical monoanion 2*˙⁻ resulting from the formation of a one electron P–P bond. These two species are probably reaction intermediates which can lead to the formation of biphosphane.

The ligand 2,4,6-tris(dimethoxyphosphonate)-1,3,5-triazine L has been synthesized and its single crystal X-ray structure determined. The occurrence of P=O···π intermolecular interactions, suggested by the short P=O··· triazine distances of 3.16–3.35 Å, is observed. The electrochemical reduction of the ligand shows its electron acceptor character by the formation of a stable radical anion. The hyperfine structure observed in the EPR spectra, combined with a theoretical DFT study, evidences the full delocalization of the unpaired electron mainly on the triazine core, with some participation of the phosphonate groups. Theoretical calculations are in agreement with the experimental values of the hyperfine coupling constants of 11.81 G for A_iso–³¹P and 1.85 G for A_iso–¹⁴N. Homopolymetallic complexes, formulated as {L[Cu(hfac)₂]₃} (1), ¹_∞{L₂[Co(hfac)₂]₃} (2) and ¹_∞{L₂[Mn(hfac)₂]₃} (3) (hfac = hexafluoroacetylacetonate), have been synthesized and structurally characterized.

A series of bis(TTF) donors containing aromatic linkers between the two TTF units has been synthesized in order to investigate on the electronic structure of the oxidized species from an experimental and theoretical point of view. A mono(TTF)-pyridine compound has been also prepared and characterized by single-crystal X-ray diffraction analysis. Oxidation of a solution of 2,6-bis(TTF)-pyridine (TTF-Pyr-TTF) or of 1,3-bis(TTF)-benzene (TTF-Bz-TTF) in CH₂Cl₂ with less than 0.1 equivalent of [Cp₂Fe][PF₆] gives rise to a seven-line EPR spectrum consistent with the hyperfine structure calculated by DFT for the corresponding radical monocation. Increasing the proportion of oxidant leads to a four-line hyperfine structure, similar to the quartet pattern observed after oxidation of mono(TTF)-pyridine (Pyr-TTF) or mono(TTF)-benzene (Bz-TTF). In good accordance with the very weak value of J calculated by DFT for the dicationic biradicals these four-line spectra are attributed to [2,6-bis(TTF)-pyridine]²⁺ and [1,3-bis(TTF)-benzene]²⁺. Similar experimental results are obtained for 1,4-bis(TTF)-benzene. In this case, however, electrochemical oxidation leads to the monoradical at low potential and to the diradical at higher potential, while only the diradical could be observed by electrochemical oxidation of 2,6-bis(TTF)-pyridine or of 1,3-bis(TTF)-benzene. 

The objective of this paper is to compare the influence of two electron-withdrawing groups (–CN and –NO₂) in the fragmentation of 2-methoxy-4-nitrophenyl benzyl ether and 4-cyanophenyl benzyl ether once these species are reduced. The stability of the corresponding radical anions depends essentially on those substituents. In our design we have chosen two species that are able to attach the electron but which have each a different capacity to retain it. An experimental study is carried out by EPR spectroscopy to analyze the different electronic nature of the generated radical anions. In addition, electronic structure calculations at the DFT level of theory have been performed to study those radical anions and their fragmentation pathways. The thermodynamics and kinetics of the two fragmentations are discussed and compared in detail.

The synthesis and structural characterization of the λ⁵-bis(phosphine sulfide) and the bimetalliccomplexes bis[phosphino-M(CO)₅] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (ortho-DMTTF)-based rigid dimer (PPh)₂(o-DMTTF)₂, containing a central 1,4-dihydro-1,4-diphosphi-nine ring, are described. Single-crystal X-ray analyses have been performed for the trans isomers(PhPX)₂(o-DMTTF)₂ (X = S, Mo(CO)₅, and W(CO)₅) and for the cis isomer [PhPW(CO)₅]_2-(o-DMTTF)₂. Planar or slightly folded boat-type conformations are observed for the central six-membered ring, together with different packings characterized by short intermolecular S · · · Scontacts. The optical signature of the oxidized species in the case of the free ligand (PPh)_2-(o-DMTTF)₂ has been evidenced by UV-vis spectroelectrochemistry measurements. SolutionEPR measurements on the radical cation species of (PPh)₂(o-DMTTF)₂ definitely assess the fulldelocalization of the unpaired electron over both electroactive TTF units, with an associatedcoupling of 0.48 G with 12 equivalent protons. The EPR signal of the dication proves the radicalnature of this species, in favor of a triplet ground state. The radical cation of the cis-[PhPW(CO)₅]_2-(o-DMTTF)₂ isomer was also investigated by EPR, for which the observed hyperfine structuredemonstrates the extended delocalization of the electron, together with a larger coupling constantwith the phosphorus nuclei. DFT calculations for the radical cation of (PPh)₂(o-DMTTF)₂ afford aboat-type conformation for the central ring and a SOMO consistent with a full delocalization of theelectron over both TTF units. Moreover, the calculations indicate that in the case of the dication of(PPh)₂(o-DMTTF)₂ the triplet state is more stable by 11.7 kcal mol^-1 than the singlet state.

A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with ^95/97Mo and ³¹P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)₅PPh₃. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of not, vert, ∼60% and not, vert, ∼4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo^(I) species is verified by calculating the g-tensor and the various ¹⁴N and ¹³C coupling tensors previously reported by Hayes for [Mo(CN)₅NO]^3-. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)₅PH₃, one-electron oxidation of Mo(CO)₅PPh₃ causes an appreciable change in the geometry of the complex. The g-tensor and the ^95/97Mo and ³¹P isotropic and anisotropic coupling constants calculated for [Mo(CO)₅PPh₃]^+· confirm the trapping of this species in the irradiated crystal of Mo(CO)₅PPh₃; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes.

Reaction of 2,4,6-trichloro-1,3,5-triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono- and bis(TTF)-triazines as new covalently linked (multi)donor-acceptor systems. Single-crystal X-ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)-triazine compound, while mixed TTF-triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time-dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)-triazine is paramagnetic, but no spin-spin exchange interaction could be detected.

We describe the preparation of a helicate containing four closely spaced, linearly arrayed copper(I) ions. This product may be prepared either directly by mixing copper(I) with a set of precursor amine and aldehyde subcomponents, or indirectly through the dimerization of a dicopper(I) helicate upon addition of 1,2-phenylenediamine. A notable feature of this helicate is that its length is not limited by the lengths of its precursor subcomponents: each of the two ligands wrapped around the four copper(I) centers contains one diamine, two dialdehyde, and two monoamine residues. This work thus paves the way for the preparation of longer oligo- and polymeric structures. DFT calculations and electrochemical measurements indicate a high degree of electronic delocalization among the metal ions forming the cores of the structures described herein, which may therefore be described as "molecular wires".

⁷⁷Se-enriched CpNi(bds) (bds = 1,2-benzenediselenolate), has been synthesized and its g tensor and ⁷⁷Se hyperfine tensors have been obtained from its frozen solution electron paramagnetic resonance (EPR) spectrum. These parameters are consistent with those calculated by density functional theory (DFT); it is shown that 10% of the spin is localized on each selenium and that the direction associated to the maximum ⁷⁷Se couplings is aligned along the g_min direction, perpendicular to the Ni(bds) plane. EPR measurements and DFT calculations are also carried out on the ⁷⁷Se enriched complex CpNi(dsit) as well on the two dithiolene analogues CpNi(bdt) and CpNi(dmit). The optimized structures of the isolated CpNi(bds) and CpNi(bdt) complexes have been used to generate the idealized dimers (bds)NiCp···CpNi(bds) and (bdt)NiCp···CpNi(bdt) characterized by Cp···Cp overlap. The exchange parameters J calculated at the DFT level for these systems are in reasonable accord with the experimental values. The influence of the geometry of the dimer on its magnetic properties is assessed by calculating the variation of J as a function of the relative orientation of the two Ni(diselenolene) or Ni(dithiolene) planes.

A new type of stable radical ligand featuring a 1,1-bis-phosphinosulfide alkene backbone has been prepared and characterized on the basis of X-ray diffraction, EPR and DFT studies.

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 —C(CN)=C(CN)—group was obtained from its crystal structure.

The synthesis and characterization of two ortho-dimethyltetrathiafulvalene (o-DMTTF)-based rigid dimers containing dimethylsilicon (Me₂Si) or dimethylgermanium (Me₂Ge) linkers are described. Single-crystal X-ray analysis reveals planar geometry for the central 1,4-disilicon or 1,4-digermanium six-membered rings. DFT calculations provide optimized conformations in agreement with the experimental ones, and also emphasize the role of the heteroatomic linkers in the conjugation between the two redox active units. Cyclic voltammetry measurements show sequential oxidation into radical cation, and then dication species. Solution EPR measurements on the radical-cation species indicate full delocalization of the unpaired electron over both electroactive TTF units, with an associated coupling of 0.42 G with twelve equivalent protons. DFT calculations afford fully planar geometry for the radical-cation species and confirm the experimental isotropic coupling constant. Single-crystal X-ray analyses of two charge-transfer compounds obtained upon chemical oxidation, formulated as [(Me₂Si)₂(o-DMTTF)₂]-1/2[TCNQ]·1/2[TCNQF₄] and [(Me₂Ge)₂(o-DMTTF)₂]·[TCNQ], demonstrate the occurrence of genuine mixed-valence radical-cation species, as well as a three-dimensional network of short S···S intermolecular contacts. Temperature-dependent conductivity measurements demonstrate semiconducting behavior for both charge-transfer compounds, with an increase of the absolute value of the conductivity upon applying external pressure. Band structure calculations reveal peculiar pseudo-two-dimensional electronic structures, also confirming electronic interactions through SiMe₂ and GeMe₂ bridges.

[M(CO)₄PPh₃]^•− (M = Mo, W) were trapped at 77 K in X-irradiated single crystals of M(CO)₅PPh₃ and studied by EPR. Structures of [M(CO)₄PPh₃]^•− (M = Cr, Mo, W) were optimized by DFT; predicted g and ³¹P-hyperfine tensors agree with experiments for M = Mo, W. The anions adopt a slightly distorted pyramidal structure with PPh₃ in basal position and the spin mostly delocalized in a metal-d_z² orbital and carbon-p_z orbitals of carbonyls. The EPR tensors are slightly modified by annealing, they suggest that new constraints in the matrix distort the structure of [M(CO)₄PPh₃]^•− (M = Cr, Mo, W).

Neutral, radical [CpNi(dithiolene)] complexes fused with seven-membered rings, formulated as [CpNi{S₂C₂S₂(CH₂)₂X}] (X = CH₂, CF₂, C=CH₂, S), have been synthesized in 30-60 % yields from the reactions of nickelocene with the corresponding neutral, square-planar, (dithiolene)nickel complexes [Ni{S₂C₂S₂(CH₂)₂X}₂]. [CpNi{S₂C₂S₂(CH₂)₂X}] (X = C=O) was prepared from nickelocene and [1,3]dithiolo[4,5-b][1,4]dithiepine-2,6-dione under thermal or photochemical conditions. All complexes exhibit reversible oxidation and reduction waves to the cation and anion form, respectively. The terminal groups (X) in the seven-membered ring shift their redox potentials to anodic potentials in the following order: CF₂ > C=O > S > C=CH₂ > CH₂. The singlet EPR responses of [CpNi{S₂C₂S₂(CH₂)₂(X)}] appear at g ≈ 2.0514-2.0529 in dichloromethane solution at room temperature. An NIR absorption is observed at λ_max ≈ 798-848 nm (ε ≈ 1700-2400 sup>-1 cm^-1) in dichloromethane solution. The X-ray structures of the five complexes show two-legged piano-stool geometries around the central nickel atom (formally Ni^III) and strong distortions from planarity of the seven-membered C₂S₂(CH₂)₂X rings. In the solid state, those radical (S = 1/2) species adopt either one-dimensional alternating chain-like motifs (X = CH₂, C=CH₂, S) or dimeric entities characterized by a singlet-triplet magnetic behavior (X = CF₂, C=O).

One-electron reduction of a diphosphafulvenium dication gives the first stable diphosphafulvenium monoradical cation (see scheme). An X-ray crystal structure analysis, EPR measurements, and DFT calculations clearly show that reduction takes place at the exocyclic double bond and that the excess of electron density is stabilized by the two electron-withdrawing phosphonium groups (see SOMO; P orange, C dark gray, H light gray).

Various preparations of the neutral radical [CpNi(dddt)] complex (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were investigated with CpNi sources, [Cp₂Ni], [Cp₂Ni](BF₄), [CpNi(CO)]₂, and [CpNi(cod)](BF₄), and dithiolene transfer sources, O=C(dddt), the naked dithiolate (dddt^2-), the monoanion of square-planar Ni dithiolene complex (NBu₄)[Ni(dddt)₂], and the neutral complex [Ni(dddt)₂]. The reaction of [CpNi(cod)](BF₄) with (NBu₄)[Ni(dddt)₂] gave the highest yield for the preparation of [CpNi(dddt)] (86%). [CpNi(ddds)] (ddds = 5,6-dihydro-1,4-dithiin-2,3-diselenolate), [CpNi(dsdt)] (dsdt = 5,6-dihydro-1,4-diselenin-2,3-dithiolate), [CpNi(bdt)] (bdt = 1,2-benzenedithiolate), and [CpNi(bds)] (bds = 1,2-benzenediselenolate) were synthesized by the reactions of [Cp₂Ni] with the corresponding neutral Ni dithiolene complexes [Ni(ddds)₂]₂, [Ni(dsdt)₂], [Ni(bdt)₂], and [Ni(bds)₂], respectively. The five, formally Ni^III, radical complexes oxidize and reduce reversibly. They exhibit, in the neutral state, a strong absorption in the NIR region, from 1000 nm in the dddt/ddds/dsdt series to 720 nm in the bdt/bds series with ε values between 2500 and 5000 M^-1 cm^-1. The molecular and solid state structures of the five complexes were determined by X-ray structure analyses. [CpNi(dddt)] and [CpNi(ddds)] are isostructural, while [CpNi(dsdt)] exhibits a closely related structure. Similarly, [CpNi(bdt)] and [CpNi(bds)] are also isostructural. Correlations between structural data and magnetic measurements show the presence of alternated spin chains in [CpNi(dddt)], [CpNi(ddds)], and [CpNi(dsdt)], while a remarkably strong antiferromagnetic interaction in [CpNi(bdt)] and [CpNi(bds)] is attributed to a Cp···Cp face-to-face σ overlap, an original feature in organometallic radical complexes.

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 P−C or a P−H 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 Ph₂P^•, 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 d_xz orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical.

Cyclic voltammetry and EPR spectroscopy show that cationic phospholium groups are good electron acceptors whose reduction leads to a neutral radical where the unpaired electron is mainly delocalized on the carbon atoms of the five-membered ring. DFT calculations together with the crystal structure of phospholiums indicate that the electron addition causes a drastic diminution of the exocyclic CPC angle. The SOMO of reduced phospholium is compared to the SOMO of the phosphole radical anion.

We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)=PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors (³¹P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)₃, PhC(H)=PMes)^·−. The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)₃ to the stabilization of the radical anion.

In many instances, the deduction of spectroscopic parameters from electron paramagnetic resonance spectra depends on spectrum simulation and parameter optimization. We have developed two software packages based on the approximate formulae of Iwasaki for the calculation of line positions and on the Levenberg-Marquardt algorithm for nonlinear least-squares optimization. Our software applies to systems having an anisotropicg-tensor and an arbitrary number of hyperfine interactions with nuclei. They are written in the FORTRAN 77 programming language. At present, neither the nuclear quadrupolar interaction nor the nuclear Zeeman interaction terms are handled. The programs CRISAJU and EPRPOWDERFIT apply to the cases of single crystals and powders, respectively. For use in the latter, thanks to the software ODYSSEE which implements automatic differentiation of algorithms, an ancillary subroutine, which contributes to the performance of the optimization, was created automatically.

Electrochemical oxidation of (C₆H₅)₃P=N(C₆H₅) and (C₆H₅)₃P=N(C₆H₅) leads to EPR spectra which reveal the dimeric structure of the resulting radical cation. In contrast to this behaviour, oxidation of bis-iminophosphorane leads to [(C₆H₅)₃P=N(C₆H₄)N=P(C₆H₅)₃]^•+: In these species the unpaired electron is delocalized on the N(C₆H₄)N moiety and the persistency of the radical depends upon the relative position of the two P=N bonds.

The EPR spectrum of the novel radical Mes*(CH₃)P—PMes* (Mes*=2,4,6-(tBu)₃C₆H₂) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the ³¹P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic ³¹P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P—P bond.

Oxidation of the square planar Rh(I) complex [Rh(SPS^Me)(PPh₃)] (SPS^Me = 1-methyl-1-P-2,6-bis(diphenylphosphinosulfide)-3,5-(bisphenyl)-phosphinine) (1) based on mixed SPS-pincer ligand with hexachloroethane yielded the Rh(III) dichloride complex [Rh(SPS^Me)(PPh₃)Cl₂] (2), which was structurally characterized. The homoleptic Rh(III) complex [Rh(SPS^Me)₂][Cl] (4) was obtained via the stoichiometric reaction of SPS^Me anion (3) with [Rh(tht)₃Cl₃] (tht = tetrahydrothiophene). Complex 4, which was characterized by X-ray diffraction, was also studied by cyclic voltammetry. Complex 4 can be reversibly reduced at E = −1.16 V (vs SCE) to give the neutral 19-electron Rh(II) complex [Rh(SPS^Me)₂] (5). Accordingly, complex 5 could be synthesized via chemical reduction of 4 with zinc dust. EPR spectra of complex 5 were obtained after electrochemical or chemical reduction of 4 in THF or CH₂Cl₂. Hyperfine interaction with two equivalent ³¹P nuclei was observed in liquid solution, while an additional coupling with a spin ¹/₂ nucleus, probably ¹⁰³Rh, was detected in frozen solution. The ³¹P couplings are consistent with DFT calculations that predict a drastic increase in the axial P−S bond lengths when reducing (SPS^Me)₂Rh(III). In the reduced complex, the unpaired electron is mainly localized in a rhodium d_z² orbital, consistent with the g-anisotropy measured at 100 K.

The radical cation of the redox active ligand 3,4-dimethyl-3',4'-bis-(diphenylphosphino)-tetrathiafulvalene ( P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S₂C=CS₂ part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2^•+, one-electron oxidation of [Fe( P2)(CO)₃] gives rise to the metal-centered oxidation species [Fe^(I)( P2)(CO)₃], characterized by a coupling with two ³¹P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe^(I)( P2)(CO)₃] reacts to give the complex [Fe^(I)( P2)₂(CO)] containing two TTF fragments. The two-electron oxidation of [Fe( P2)(CO)₃] leads to decomplexation and to the P2^•+ spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe( P2)(CO)₃] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P — P bond (3.479 Å), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P — P distance in neutral 1 is large (5.8 Å). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P — P single bond (P — P 2.389 Å). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C — Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P — P distance. The ability of the phosphinine radical anion to reversibly form weak P — P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P — P bond.

Two new, “user-friendly” 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 As−H and Si−H 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.

The reduction products of two diphosphaalkenes (1 and 2) and a bis(diphosphene) (3) containing sterically encumbered ligands and corresponding to the general formulas Ar−X==Y−Ar‘−Y==X−Ar, have been investigated by EPR spectroscopy. Due to steric constraints in these molecules, at least one of the dihedral angles between the CXYC plane and either the Ar plane or the Ar‘ plane is largely nonzero and, hence, discourages conformations that are optimal for maximal conjugation of P==X (or P==Y) and aromatic π systems. Comparison of the experimental hyperfine couplings with those calculated by DFT on model systems containing no cumbersome substituents bound to the aromatic rings shows that addition of an electron to the nonplanar neutral systems causes the X==Y−Ar‘−Y==X moiety to become planar. In contrast to 1 and 2, 3 can be reduced to relatively stable dianion. Surprisingly the two-electron reduction product of 3 is paramagnetic. Interpretation of its EPR spectra, in the light of DFT calculations on model dianions, shows that in [3]^2- the plane of the Ar‘ ring is perpendicular to the CXYC planes. Due to interplay between steric and electronic preferences, the Ar−X==Y−Ar‘−Y==X−Ar array for 3 is therefore dependent upon its redox state and acts as a “molecular switch”.

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*P^•−(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)P−^•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 P−C double bond (P^-−C⁺), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal π electron distribution (P⁺−C^-):  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, 1^•+PF₆^-. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorus−carbon bond caused by the presence of the amino groups.

Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (~20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of ~10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4^•-. Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 °C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4^•- is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (π* orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure.

X-irradiation of single crystals of Tp–GeH₃ (Tp: triptycene) led to the trapping of the radical Tp–^√GeH₂. 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 ^√GeH₂ moiety around the C–Ge 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, Tp–^√GeH₂ is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the C–GeH₂ 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 mol⁻¹, which is slightly inferior to the barrier calculated by DFT (3.6 kcal mol⁻¹). Calculations performed on Tp–CH₃, Tp–GeH₃ and Tp–^√GeH₂ show that the rotation barrier ΔE_rot around the C–Ge bond drastically decreases by passing from the germane precursor to the germanyl radical and that ΔE_rot increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined.

Electrochemical and chemical reductions of Rh^(I) complexes of L_P4 (a macrocycle containing four phosphinine rings) and of L_P2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with ³¹P nuclei. An additional coupling (27 MHz) with ¹⁰³Rh is detected, in the liquid state, for the spectrum obtained with [L_P2S2Rh⁽⁰⁾]; moreover, resolved ³¹P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh^(I) complexes of model macrocycles L‘_P4 and L‘_P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L‘_P2S2Rh⁽⁰⁾], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (ρ_Rh =0.35 for [L‘_P2S2Rh⁽⁰⁾]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle.

The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent ³¹P nuclei. Additional couplings with two equivalent ¹³C are observed with Ar–P=¹³C=¹³C=P–Ar. The ³¹P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]^•– and [H–P=CH–CH=P–H]^•–. Although the optimized geometries of these two species are clearly different, neither of them leads to¹³C/³¹P hyperfine tensors in conflict with the experimental results. The absence of any ¹H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]^•– more probable. 

The paper shows the possibilities of the complementary use of the density matrix formalism for the simulation of the anisotropic EPR spectra and the DFT potential energy surface calculations to obtain a detailed picture of the motions of radical molecules. The combined approach is illustrated by a comparative EPR study of three phosphorus derivatives of barrelene. Three compounds were chosen as the model molecules for the observation of different temperature dependent dynamics of radical fragment. Each molecule based on the same barrelene skeleton has a different set of substituents which by influencing the local chemical environment are likely to modify the internal dynamics. The temperature dependent EPR spectra are simulated by means of the density matrix formalism and the geometry of radicals are calculated with DFT. The motion is described in terms of rotational barriers, DFT calculated energy profiles and hypothetical intramolecular distortions. These two approaches lead to a similar microscopic picture of the intramolecular radical motion.

Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1]^•- whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental ³¹P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding σ P−P orbital. Clearly, a one-electron bond (2.763 Å) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Å apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1]^2- which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1]^2-, the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P−P bond whose length (2.305(2) Å) is only slightly longer than a usual P−P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P−P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3]^•- Na⁺; however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)₂]^2-[{Na(THF)₂}₂]²⁺ a P−P bond is formed (2.286(2) Å) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon−sodium−phosphorus bridges.

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]√⁻ and [HP---CH=Cp]√ 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]√⁻ is readily protonated and that the EPR spectra are due to the phosphaallylic radical.

Stable for at least one week below -30°C: crystals of 1, the first highly persistent diphosphanyl radical, have been isolated and characterized. This phosphorus-centered radical exhibits hyperfine coupling whose anisotropy is considerably larger than that for well-established nitrogen radicals (hydrazyls, nitroxides). This feature is of potential interest for studies of fast molecular movements. Mes*=2,4,6-tBu₃C₆H₂.

The radical anion (tmbp)^•-, where tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine, was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus p_π orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine:  [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)₂] led to the 19-electron complex whose g and ³¹P hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)₂] and [Ni(bp)₂]^•-, indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a more planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)₂], metal−ligand back-donation strongly contributes to the metal−ligand bonding.

The g, ³¹P and ¹H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ~= 0.5 kcal mol⁻¹) which correspond to the conformation of the PH₂ moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.

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.

A "CO-like matrix", showing coordination analogous to that of carbonyl groups, is provided by silacalix[4]phosphinine macrocycles. Reaction with Au^I leads to the first gold(I) complexes of macrocycles, which can be reduced with sodium or potassium to the paramagnetic gold(0) complexes (an example is shown), as evidenced by cyclic voltammetry and EPR spectroscopy.

Bicyclonucleosides bearing a 5-deoxy-5-N-hydroxyamino-3,N5-(1,1-ethano)-β-o-furanosyl sugar moiety (15-18) have been prepared by glycosidation of the corresponding bicyclosugars obtained via an intramolecular reverse Cope elimination. The configuration of the asymmetric carbon of the 1,1-ethano bridge is the most important factor directing the conformation of the N-hydroxypyrrolidine ring and its invertomers ratio as shown by variable temperature H NMR experiments.

A new phosphine, the diphenyldibenzobarrelenephosphine 2, was designed to study the barrier to rotation of the P−H group around the C−^•P bond. After homolytic scission of a P−H 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.

Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP•−CHCH2) ↔ (HPCH−•CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.

Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.

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.

Cyclic voltammetry of phosphafulvene and dibenzophosphafulvene shows that in DMF these compounds are reduced at -1.200 and -1.349 V, respectively. The EPR spectra of the corresponding radical anions, formed by electrochemical reduction or by reaction on a potassium mirror, are Recorded between 110 K and room temperature. The g and 31P hyperfine tensors are measured and compared to those previously obtained for a phosphaalkene radical anion. Abinitio investigations on model phosphaalkene and phosphafulvene radical anions show that, in accord with the experimental results, the electronic structure of these two species are quite different: whereas the unpaired electron is delocalized on the whole PC(H)R moiety in the phosphaalkenic anion, it is markedly localized on the phosphorus atom in the phosphafulvene anion. Calculated spin densities and charge distributions for phosphafulvene and azafulvene anions are compared.

Two phosphaalkenes containing either a furane or a thiophene ring bound to the carbon atom of the −Pdouble 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.

The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results.

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.

A bis-cyclometallating ligand bearing two different terdentate coordination sites (N---C---N: dipyridyI-benzene; P---C---P: diphosphaalkenebenzene moieties) has been synthesised. Selective reactions of appropriate metal complex precursors afforded a heterodinuclear ruthenium(II)-palladium(II) complex characterised by 1H, 13P NMR spectroscopy and FAB-MS techniques. We have compared its electrochemical and spectroscopic properties (absorption and emission) with the individual ruthenium(II) and palladium(II) subunits.

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 µM was found.

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.

To measure the rotation barrier around an R3C−•PH 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 P−H 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 C−P 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 kcal·mol-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.

Neoglycolipids bearing a paramagnetic probe in their lipophilic aglycon have been prepared. All belong to the Image -glucose series, both anomers for the glucoside representatives, respectively β and α anomers in the S- and C-glucosyl series. Two different types of radical sites have been used, a relatively short-lived imino N-oxyl group for glucosides and a more stable N-acylamino N-oxyl moiety in the other cases. EPR spectra of these radical species afforded information on the conformation of the lipophilic chain in the vicinity of the paramagnetic probe.

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.

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.

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%).

Two radiogenic radicals trapped in a single crystal of 1-[2,4,6-tri-tert-butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their ³¹P   hyperfine tensors, as being phosphoniumyl radical cations. The spectra modifications caused by ¹³C or ²D enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conformations of the radical. The experimental ³¹P, ¹³C, and ¹H hyperfine tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that these interactions are very sensitive to the geometry of the radical and that their measurement can yield precise structural information.