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 (31)P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding sigma P-P orbital. Clearly, a one-electron bond (2.763 A) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 A 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) A) 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) A) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon-sodium-phosphorus bridges.

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.

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 P2S2 Rh (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‘ P2S2 Rh (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 (ρRh =0.35 for [L‘ P2S2 Rh (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.

Paramagnetic complexes M(CO) 5 P(C 6 H 5 ) 2 , with M ) Cr, Mo, W, have been trapped in irradiated crystals of M(CO) 5 P(C 6 H 5 ) 3 (M ) Cr, Mo, W) and M(CO) 5 PH(C 6 H 5 ) 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) 5 P(C 6 H 5 ) 2 , is consistent with both the number of EPR sites and the crystal structures. The g and 31 P hyperfine tensors measured for M(CO) 5 P-(C 6 H 5 ) 2 present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph 2 P • , the 31 P 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) 5 P(C 6 H 5 ) 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.

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 pi 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*PdC(NMe 2 ) 2 (1) and Mes*PdC(CH 3 )NMe 2 (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 [Cp 2 Fe]PF 6 leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental 31 P and 13 C 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(NMe 2 ) 2 ) + . Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe 2 ) 2 moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P-• C(NMe 2 ) 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 •+ PF 6 -. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphoruscarbon bond caused by the presence of the amino groups.

The radical cation of the redox active ligand 3,4-dimethyl-3 0 ,4 0 -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 S 2 CQCS 2 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 1 , 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 31 P 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 1 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.

Paramagnetic resonance spectra of the spin-label 2,2,6,6-tetramethylpiperidinyl-l-oxy have been used to study phase separations in binary mixtures of dimyristoyl-phosphatidylcholine and cardiolipin. Two different samples of cardiolipin were used: (i) One sample contained calcium ions at a mole ratio of calcium:cardiolipin = 1:2; the experimental data support the view that cardiolipin is present in the bilayer membrane as calcium ion linked dimers, (CL)2 Ca2+. (ii) A calcium-free sodium cardiolipin sample yielded remarkable spin-label partition data that were quite different from those obtained in the presence of Ca2+. In both cases the spin-label data provide evidence for compound formation and for fluid-fluid immiscibility in the bilayer membrane.