Wolfgang Kaim

The transition metal coordination chemistry of anion radicals

A. Scope and introduction B. Formation of anion radicals C. Ligand properties of anion radicals D. Formation of anion radical complexes (i) Redox reactions of complexes (ii) Coordination of “free” anion radicals (iii) Photochemical generation (iv) Thermal electron transfer between closed-shell species E. Survey of anion radical ligands and of their complexes . . , (i) Carbon ligands (ii) Nitrogen ligands (iii) Oxygen ligands (iv) Phosphorus ligands (v) Sulfur ligands F. Spectroscopy and structure (i) Magnetic resonance and magnetism (ii) Electrochemistry (iii) Electronic spectroscopy (UV/VIS/NIR) ......... (iv) Structural studies and vibrational spectroscopy ..... G. Reactivity of anion radical complexes Acknowledgements References

Radical ions : XXVII. Radical ions of tetrakis(trimethylsilyl)butatriene

Abstract One-electron oxidation and one-electron reduction of the electron-rich acetylene derivative, hexakis(trimethylsilyl)-2-butyne [H 3 C 3 ) 3 Si] 3 CCCC[Si(CH 3 ) 3 ] 3 , unexpectedly produce the persistent radical cation and radical anion of the hitherto unknown neutral compound, tetrakis(trimethylsilyl)butatriene (R 3 Si) 2 CCCC(SiR 3 ) 2 . The radical anion can also be generated from the corresponding diacetylene, bis(trimethylsilyl)-1,3-butadiyne R 3 SiCCCCSiR 3 and potassium metal, obviously via disproportionation. Photoelectron and electron spin resonance spectroscopic data permit the detection and characterization of the individual species. The stability of both the radical anion and the radical cation of the same molecule can be rationalized by the unique combination of the twofold butatriene π-system with 4 R 3 Si substituents, which can act either as electron donors or electron acceptors and thus stabilize the ground state of either the cation or the anion.

Radical ions : XXVIII. Tris(trimethylsilylmethyl)aminium, N(CH2Si(CH3)3)3: A stable fluxional aminium radical cation

Abstract The first ionization potentials of 11 silylmethyl-substituted amines R 3− n N(CH 2 Si(CH 3 ) 3 ) n as determined photoelectron spectroscopy range from 9.07 eV to 7.66 eV. The most easily ionized molecule, N(CH 2 Si(CH 3 ) 3 ) 3 , can also be oxidized with AlCl 3 in H 2 CCl 2 solution to its aminium radical cation. The ESR spectra recorded between 180 K and 310 K display a strong temperature dependence due to rotations around the >NCH 2 bonds.

Binuclear radical complexes of molybdenum and tungsten tetracarbonyls with bridging N-heterocycles and group VA ligands. ESR evidence for a cis configuration

Abstract The reactions of Mo(CO) 6 or W(CO) 6 and of bridging N-heterocyclic ligands L such as pyrazine or 4,4′-bipyridine with potassium have been studied by ESR spectroscopy in the presence of excess phosphane, phosphite or arsane ligands. In many cases, double substitution at the metals could be observed, yielding binuclear paramagnetic complexes [(XR 3 )(CO) 4 M-L-M(CO) 4 (XR 3 )] ⨪ (X = P, As; R = alkyl, aryl, alkoxy; M = Mo, W). ESR investigation of the complexes in THF solution reveals extensive coupling of the unpaired electron with the nuclei 1 H, 13 C(CO), 14 N, 31 P, 75 As, 95,97 Mo and 183 W. Evidence for a cis configuration at the metals is presented, and a π ★ /σ MX ★ hyperconjugation model is used to account for the facile substitution of CO and for the spin transfer to the peripheral nuclei.

Metallaviolenes. An ESR study

Abstract The preparation and ESR-spectroscopic characterization of radical complexes with L = 4,4′-bipyridine is described, where MR n = BePh, MgPh, ZnPh, − BEt 3 , GaMe 2 , InMe 2 and SiMe 3 . While the bis(trimethylsilyl) derivative could be obtained via one-electron oxidation of the corresponding 4,4′-(1H, 1′H)-bi-pyridinylidene, the other complexes were prepared by the reaction of 4,4′-bipyridine with potassium and the organometal MR n +1 or the halide XMR n in THF. Use of organo-cadmium or -thallium compounds in such a procedure leads to deposition of the metals. The Grignard reagents MgPh 2 and BrMgPh reduce 4,4′-bipyridine in an electron transfer process to yield an unsymmetrical complex. The new organometallic 4,4′-bipyridinium (“violence”) radical complexes can be incorporated into a series of related radicals which exhibits the sensitivity of the unpaired electron distribution towards metal coordination.

First observation of resolved ESR hyperfine structure for a reduced ruthenium(II) polyazine complex

Abstract A resolved ESR spectrum of a ruthenium(II) polyazine radical complex has been observed for the first time in the binuclear species [(bpy)4Ru2(dptz)]3+ (dptz: 3,6-di-2-pyridyl-1,2,4,5-tetrazine). Electron spin resonance in conjunction with HMO McLachlan calculations reveal the localization of the unpaired electron in the tetrazine π system and a strongly polarizing effect of the [Ru(bpy)2]2+ fragment.

Zur strukturellen flexibilität der “antiaromatischen” 1,4- dihydropyrazine. Kristall- und molekülstrukturen metallorganischer derivate

Abstract Molecular and crystal structures of the cyclic conjugated 8-π-electron systems 1,4-bis(trimethylgermyl)- and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (I and II) and of 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (III) have been determined by X-ray crystallography at low temperatures. Whereas the first two species exhibit virtually planar six-membered ring conformations and very little pyramidalization at the nitrogen centers, the sterically more crowded tetramethyl derivative III has a pronounced boat conformation with a dihedral angle of about 140° between the ring halves. These structural results are in excellent agreement with theoretical calculations as well as spectroscopic data and illustrate the exceptional structural flexibility of the “antiaromatic” 1,4-dihydropyrazine system.

Radical ions : XI. One-electron oxidation of alkylsilyl benzenes in the gas phase and in solution

Compounds which exhibit first ionization potentials below ~8 eV in their photoelectron spectra can in general be oxidized to their corresponding radical cations. This new criterion for predicting the existence of the less frequent M+. species has been successfully applied to poly(trimethylsilylalkyl)-substituted benzene derivatives. The key to the generation of their cation radicals is a selective one-electron oxidation, involving solid A1C13 in H2CC12. The PE and ESR spectra yield information not only on the charge delocalization but also on the conformations of the radical cations generated.

The stable radical cation of tetrakis(dimethylamino)-p-benzoquinone

Abstract The electron-rich tetrakis(dimethylamino)-p-benzoquinone exhibits a low first ionization energy of only 7.1 eV and can be oxidized electrochemically at +0.25 V vs. SCE to its purple radical cation, whose spin population differs considerably from that of the corresponding semiquinone radical anion.

The molecular geometry of the elusive 1,4-dihydropyrazine. An MNDO study

Abstract Potential surfaces for the as yet unknown molecule 1,4-dihydropyrazine 1 have been calculated by the MNDO method. The configurations at the nitrogen atoms and the folding angle of the six-membered ring in its boat conformation were chosen as critical geometrical variables. Nitrogen and ring inversion barriers were found to be rather small, and geometry optimization yielded a shallow energy minimum at a flat boat conformation with diequatorial N -hydrogens. The MNDO geometry optimization was also applied to the radical cation 1 + and to the dication 1 2+ , both of which were calculated to be planar. The results are discussed with respect to effects in related cyclic systems.