Reduction of monophosphaallenes: An EPR study and ab inition investigations of (HPCCH2)-. and (HPCHCH2). radicals

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

Mono- and Bis(tetrathiafulvalene)-1,3,5-Triazines as Covalently Linked Donor-Acceptor Systems: Structural, Spectroscopic, and Theoretical Investigations

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

Characterization of transient radicals in the reduction product of the -P=C=C=P- system: EPR and theoretical studies

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 initio SCI) 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 to 13C/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

Single-crystal EPR study and DFT structure of the [Mo(CO)5PPh3]+· radical cation

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, similar60% and not, vert, similar4% 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

[CpNi(diselenolene)] Neutral Radical Complexes: Electron Paramagnetic Resonance and Density Functional Theory Investigations

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 gmin 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

EPR and theoretical studies of the reduction product of the fulvenephosphaallene system

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

The structure of diphosphaallenic radical cations as evidenced by EPR experiments and ab initio calculations

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

The diphosphaallene radical anion : EPR an theoretical investigations

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

Role of the aromatic bridge on radical ions formation during reduction of diphosphaalkenes

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