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.

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).

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.

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.

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.

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.

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 Ã…) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Ã… 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) Ã…) 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) Ã…) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon−sodium−phosphorus bridges.

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.

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.