A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)₃(η⁶-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η⁶-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state.

This contribution investigates Ln^III complexes formed with a small ditopic ligand, L1, and their structural, thermodynamic and photophysical properties. The spectrophotometric and NMR titrations evidence the triangular assemblies [Ln₃(L1-H)₃]⁶⁺ at stoichiometric conditions and their properties are discussed in relation to L2-containing analogues. In addition, the dinuclear species, [Ln₂(L1-H)]⁵⁺, is observed with an excess of metal.

Two tridentate and one bidentate binding strands have been anchored on a carbon atom to provide a new unsymmetrical tripodal ligand L for Ln(III) coordination. The ligand itself adopts a single conformation in solution stabilized by intramolecular hydrogen bonds evidenced in the solid state. The reaction of L with trivalent lanthanides provides different coordination complexes depending on the metal/ligand ratio. The speciation studies with selected lanthanides were performed in solution by means of NMR, ESMS, and spectrophotometric titrations. Differences in coordination properties along the lanthanide series were evidenced and may be associated with the changes in the ionic size. However, thermodynamic stability constants for the species of the same stoichiometry do not significantly vary. In addition, the structure of the dinuclear complex [Eu₂L₂]⁶⁺ has been elucidated in the solid state, where the complex crystallizes predominantly as an M-isomer. The crystal structure shows the coordination of two different ligands to each europium cation through tridentate strands, and the europium nine-coordinate sphere is completed with three solvent molecules. Finally, the results of photophysical investigations of [Eu₂L₂]⁶⁺ are in close agreement with the structural parameters determined by crystallography.

Triangular luminescent box: Self-assembly of a new multidentate receptor with europium cations results in the formation of trinuclear discrete complexes. X-ray crystallography shows that nine-coordinate cations are linked by ligands to provide a triangular complex in the solid state and in solution. Despite the coordinated solvent molecules, this topologically unusual complex exhibits remarkable luminescent properties.

The mononuclear Os^II complex [Os( L1)₃](PF₆)₂ ( L1 = 5-methyl(1-methylbenzimidazol-2-yl)pyridine) is an obvious candidate for the design of an inert d-block-based tripodal receptor capable of binding and photosensitizing trivalent lanthanides (Ln^III). It has thus been prepared and its two enantiomeric meridional (Δ-mer and Λ-mer) and facial (rac-fac) isomers have been separated by ion-exchange chromatography. The optical isomers have been characterized by CD spectroscopy and assignments of absolute configuration confirmed by an X-ray crystallographic study of Λ-mer-[Os( L1)₃](PF₆)₂·1.5MeCN (monoclinic, P2₁, Z = 4). Comparison of the latter structure with that of racemic fac-[Os( L1)₃](PF₆)₂ (monoclinic, C2/c, Z = 8) and [Os(bipy)₃](PF₆)₂ (where bipy = 2,2' -bipyridine) shows minimal structural variations, but differences are observed in the photophysical and electrochemical properties of the respective compounds. Luminescence emissions from Os^II complexes of L1 are typically lower in energy, with shorter lifetimes and lower quantum yields than their bipy analogues, whilst metal-centred oxidation processes are more facile due to the enhanced Ï€-donor ability of L1. The key relationships between these parameters are discussed. Finally, though challenged by (i) the low reactivity of many osmium precursors and (ii) the irreversible formation of competing side products, the synthesis and purification of the heterobimetallic triple-stranded helicate HHH-[OsLu( L2)₃](CF₃SO₃)₅ has been realised, in which L2 is a segmental ligand containing the same bidentate unit as that found in L1 further connected to a tridentate binding site adapted for complexing Ln^III. Its solid-state structure has been established by X-ray crystallography (triclinic, P1^-, Z = 2).

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 incorporation of enantiopure 1-amino-2,3-propanediol as a subcomponent into a dicopper double helicate resulted in perfect chiral induction of the helicate's twist. DFT calculations allowed the determination of the helicity of the complex in solution. The same helical induction, in which S amines induced a Λ helical twist, was observed in the solid state by X-ray crystallography. Electronic structure calculations also revealed that the unusual deep green color of this class of complexes was due to a metal-to-ligand charge transfer excitation, in which the excited state possesses a valence delocalized Cu₂³⁺ core. The use of a racemic amine subcomponent resulted in the formation of a dynamic library of six diastereomeric pairs of enantiomers. Surprisingly, this library converted into a single pair of enantiomers during crystallization. We were able to observe this process reverse upon redissolution, as initial ligand exchange was followed by covalent imine metathesis.

The opposite orientation of the ester spacers in the rodlike ligands L 4^C12 (benzimidazole-OOC-phenyl) and L 5^C12 (benzimidazole-COO-phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, Ln^III, and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4^C12)(NO₃)₃] (Ln=La-Lu) vanishes in [Ln(L 5^C12)(NO₃)₃], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time-dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid-crystalline phases. As a rule of thumb, an alternation of electron-poor and electron-rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4^C12, L 5^C12, [Ln(L 4^C12)(NO₃)₃], but not for [Ln(L 5^C12)(NO₃)₃]. The intercalation of an additional electron-rich diphenol ring on going from [Ln(L 5^C12)(NO₃)₃] to [Ln(L 6^C12)(NO₃)₃] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model.

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.

Inert and optically active pseudo-octahedral Cr^IIIN₆ and Ru^IIN₆ chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL₃]⁶⁺ and HHH-[RuLnL₃]⁵⁺. The crystal structures of [CrLnL₃](CF₃SO₃)₆ (Ln=Nd, Eu, Yb, Lu) and [RuLnL₃](CF₃SO₃)₅ (Ln=Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic M^…Ln distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd→4f communication. Visible irradiation of the Cr^IIIN₆ and Ru^IIN₆ chromophores in HHH-[MLnL₃]^5/6+ (Ln=Nd, Yb, Er; M=Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed.

The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, Ln^III, to give the nine-co-ordinate triple-helical complexes [Ln(Li)₃]³⁺ (i=1–4) existing as mixtures of C₃-symmetrical facial and C₁-symmetrical meridional isomers. Although the β₁₃ formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D₃-symmetrical analogues [Ln(Li)₃]³⁺ (i=5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)₃]³⁺ is significantly larger. The fac-[Ln(Li)₃]³⁺↔mer-[Ln(Li)₃]³⁺ (i =1–4) isomerisation process in acetonitrile is slow enough for Ln=Lu^III to be quantified by ¹H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand -stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 13 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₃H₅N)₂(H₂O) ( 6, C₄₅H₅₄EuF₉N₁₀O₁₃S₃, monoclinic, P2₁/c, Z=4) and [Eu(L4)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₄H₄O)_1.5 ( 7, C₅₁H₆₆EuF₉N₈O_15.5S₃, triclinic, P, Z=2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.

Highly symmetric spirobi[dibenzazepinium] cation 3 reacts with P₄-t-Bu to form exclusively a ring-expanded tertiary amine; this unusual reactivity can be traced back to the geometry of the ylide.

Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)₄Cl₂] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)₃]²⁺ and mer-[Ru(Li)₃]²⁺ (i=2, 3; ΔGΘ_{isomerisation}=-2.7 kJ mol^-1). In more polar solvents (acetonitrile, methanol), the free energy of the facial ↔ meridional isomerisation process favours mer-[Ru(Li)₃]²⁺, which is the only isomer observed in solution at the equilibrium (ΔGΘ_{isomerisation}≤-11.4 kJ mol^-1). Since the latter process takes several days for [Ru(L2)₃]²⁺, fac-[Ru(L2)₃]²⁺ and mer-[Ru(L2)₃]²⁺ have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)₃]²⁺ provides only mer-[Ru(L3)₃](ClO₄)₂ after chromatography (RuC₆₀H₅₁N₉O₈Cl₂, monoclinic, P2₁/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial↔meridional isomerisation is reversed (ΔGΘ_{isomerisation}≥11.4 kJ mol^-1), and the Ru — N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)₃]⁵⁺ within hours ([RuLu(L1)₃](CF₃SO₃)_4.5Cl_0.5(CH₃OH)_2.5: RuLuC₁₀₆H₁₀₉Cl_0.5N₂₁O₁₉S_4.5F_13.5, triclinic, P, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar Ï€-acceptor properties to, but stronger Ï€-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand π→π^* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)₃]²⁺ (i=2, 3) chromophores. The concomitant short lifetime of the ³MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)₃]⁵⁺.

The C₂-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)₂H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF₆)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF₆] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF₆] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein C_α-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature ¹H, ³¹P, and ¹⁷O NMR spectroscopy, variable-pressure ³¹P and¹⁷O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol^-1 (12.2 kcal mol^-1) for the inversion of the pseudo-chirality at the ruthenium center.

Two new, “user-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 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.

A new synthetic approach, reacting alkaline earth metal iodides with butyllithium, lithium hydroxide, and/or lithium butoxide under salt elimination, is presented, giving access to some interesting clusters of calcium, strontium, and barium, partially in combination with lithium. The so far largest calcium cluster Li[{Ca₇(μ₃-OH)₈I₆(thf)₁₂}₂(μ₂-I)]·3THF, 4, and the new strontium compound [Sr₃I₃(OH)₂(thf)₉]I, 5, are shown to feature common building blocks of OH-capped M₃ triangles. On the basis of mainly electrostatic interactions, these clusters are not volatile. By introducing LiO^tBu, the two clusters [IM(O^tBu)₄{Li(thf)}₄(OH)] (6, M = Sr; 7, M = Ba) are prepared, 7 exhibiting volatility as an important physical property, which makes it a potential precursor for chemical vapor deposition. The structural relationship between 4, 5, 6, and 7 and their respective starting materials is shown, and possible reaction mechanisms are proposed. Exhibiting surprising and new structural motifs, the bonding modes of these clusters are investigated by the electron localization function as well as by ab initio calculations.

The hydrolysis of terminal ^tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 − 3H]^3- reacts with Ln(ClO₄)₃ to give the poorly soluble and stable podates [Ln(L13 − 3H)] (log(β₁₁₀) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H₂O)₇ (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in [Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] ( Φ^tot_Eu = 1.8 × 10^-3) and [Tb(L13 − 3H)] ( Φ^tot_Eu = 8.9 × 10^-3) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (Φ^Eu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet (¹Ï€Ï€*) and triplet (³Ï€Ï€*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the “carboxylate effectâ€� in lanthanide complexes are evaluated for programming predetermined properties in functional devices.

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 Ï€ 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.

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.

Lipophilic linear semirigid side arms containing two or three successive phenyl rings separated by carboxylate spacers have been connected to the 5 or 6 positions of bent aromatic terdentate 2,6-bis(benzimidazol-2-yl)pyridine binding units to give extended V-shaped (L11) and I-shaped receptors (L12, L12b, and L13). The carboxylate spacers limit the flexibility of the side arms and provide crossed arrangements of the successive aromatic rings in the crystal structure of L12b (C₆₃H₆₁N₅O₁₀; triclinic, P_↑, Z = 2) in agreement with semiempirical calculations performed on optimized gas-phase geometries. Moreover, the carboxylate spacers in L11−L13 prevent efficient electronic delocalization between the connected aromatic rings and act as weak Ï€ acceptors producing a slight increase of the energy of the ¹Ï€Ï€* and³Ï€Ï€* levels centered on the terdentate binding unit. Intermolecular Ï€-stacking interactions observed in the crystal of L12b are invoked to rationalize (i) the peculiar excimer emission ofL11 in the solid state and (ii) the rich and varied calamitic (I-shaped L12, L12b, and L13) and columnar (V-shaped L11) mesomorphism observed at high temperature. The Col_R mesophase detected for L11 demonstrates that V-shaped bent terdentate binding units are compatible with liquid-crystalline behavior. Complexation of L11 with lanthanide(III) produces I-shaped complexes [Ln(L11)(NO₃)₃] (Ln = La, Eu, Gd, Tb, and Lu) possessing a large axial anisometry as found in the crystal structure of [Lu(L11)(CF₃CO₂)₃(H₂O)] (LuC₈₁H₈₇N₅O₁₇F₉; triclinic, P_↑,Z = 2), which exists in the solid state as H-bonded dimers. No mesomorphism is detected for the complexes as a result of the large perpendicular expansion brought by the metallic coordination site, but the high energy of the ligand-centered ³Ï€Ï€* prevents Eu(⁵D₀) → L11back transfer in the Eu(III) complex, which thus exhibits sizable red luminescence at room temperature, a crucial point for the design of luminescent materials.

The cobalt(II) complexes prepared with a series of enantiopure ligands (1-3) containing the bis(oxazolinyl)pyridine unit have been studied. The ligands form high spin octahedral complexes as shown by the X-ray crystal structure of the homochiral complex [Co(R,R-1)₂](ClO₄)₂(CH₃CN)₃. The diastereoselectivity of complex formation has been studied: equimolar mixtures of RR and SS ligands show mixtures of homochiral and heterochiral complexes for 2 and 3, but the phenyl-substituted ligand 1 shows exclusive formation of the heterochiral species. This selectivity is correlated with structural and electronic properties of the complexes.

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.

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.

Remote site deprotonation of a coordinated imidazole ligand switches the reduction potential of coordinated iron over a narrow pH range from +0.920 to –0.460 V.

A new synthetic strategy has been developed to introduce bent and rigid tridentate 2,6-bis(benzimidazol-2‘-yl)pyridine cores into rodlike ligands L11-17. The crystal structure of the nonmesogenic ligand L13 (C39H37N5O4, triclinic, P, Z = 2) shows the expected trans−trans conformation of the tridentate binding unit, which provides a linear arrangement of the semirigid aromatic sidearms. The crystal structure of the related mesogenic ligand L16 (C61H81N5O4, triclinic, P, Z = 2) demonstrates the fully extended conformation adopted by the lipophilic side chains, leading to a slightly helically twisted I-shaped molecule. A rich and varied mesomorphism results which can be combined with the simultaneous tuning of electronic and photophysical properties via a judicious choice of the spacers between the rigid central core and the semirigid lipophilic sidearms. Ligands L13,14 react with Ln(NO3)3·xH2O to give quantitatively and selectively the neutral 1:1 complexes [Ln(Li)(NO3)3] (Ln = La to Lu), which are stable in the solid state at room temperature but partially dissociate in acetonitrile to give the cationic species [Ln(Li)(NO3)2]+. The crystal structure of [Lu(L13)(NO3)3]·3CH3CN (30, LuC45H46N11O13, monoclinic, C2/c, Z = 8) reveals a U-shaped arrangement of the ligand strand arising from the cis−cis conformation of the coordinated tridentate binding unit. This drastic geometric change strongly affects the thermal behavior and the photophysical and electronic properties of the lipophilic complexes [Ln(L14)(NO3)3]. Particular attention has been focused on structure−properties relationships, which can be modulated by the size of the lanthanide metal ions.

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.

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.

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.

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.

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.

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%).

The main geometric elements of three 3,6-substituted 1,2,4-trioxan-5-ones have been calculated by using molecular mechanics (MM2), and semiempirical (AM1, PM3) methods. The results are compared with those obtained by X-ray analysis.

A series of [Cr(benzene)(CO)2L] complexes with L = PPh3, P(OMe)3, PPh2 ((−)-menthyl), P(OPh)2(O-(−)-menthyl), P(O-(−)-menthyl)3 were subjected to a nueleophile addition/acylation sequence to give trans-5,6-disubstituted cyelohexadienes. Low-to-moderate asymmetric induction was observed with the chira] ligands. Experimental and theoretical evidence for an alkylation at the metal center trans to the P ligand is presented, and a crystal structure determination of a [Cr(η5-cyclohexadienyl)(P(OMe)3)(CO)2SnPh3] complex is included.