Nineteen-Electron Radicals of the Type (η6-C6Me6)Co(η4-diolefin): Electrochemistry and ESR Results

Access to the Bis-benzene Cobalt(I) Sandwich Cation and its Derivatives: Synthons for a "Naked" Cobalt(I) Source?

The synthesis and structural characterization of the hitherto unknown parent Co(bz)₂⁺ (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO₅⁺ salt, or directly from Co₂ (CO)₈ and a Ag⁺ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(OR^F )₄ ]^- and [F{Al(OR^F )₃ }₂ ]^- {R^F =C(CF₃ )₃ } and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz)₂⁺ were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb)₂⁺ salts are of particular interest for the preparation of further Co^I salts, for example, the structurally characterized low-coordinate 12 valence electron Co(P^t Bu₃ )₂⁺ and Co(NHC)₂⁺ salts.

Evidence from ESR studies for [Co(eta-C2H4)3] produced at 77 K in a rotating cryostat

Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo=7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1=2.284, g2=2.0027, g3=2.1527; A1<-25 MHz, A2=-109 MHz, A3=-198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn-Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(eta-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2-y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene pi/pi* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, theta. At theta=0.00a2 is 0.23 and b2 is 0.78, at theta=0.25a2 is 0.25 and b2 is 0.52 and at theta=0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(eta-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to approximately 220 K indicating a barrier to decomposition of approximately 50 kJ Mol-1

A Combined Electron Paramagnetic Resonance and Fourier Transform Infrared Study of the Co(C6H6)1,2 Complexes Isolated in Neat Benzene or in Cryogenic Matrixes

The products obtained in the reaction of cobalt atoms in neat benzene or in a benzene/argon mixture at low temperature have been reinvestigated. At least three cobalt-containing species were detected by IR, namely, Co(C(6)H(6)), Co(C(6)H(6))(2), and Co(x)(C(6)H(6)), x>1. The IR bands were assigned to these complexes by monitoring their behavior as a function of (a) Co and C(6)H(6) concentration, (b) isotopic substitution, and (c) photoirradiation. We were able to analyze the sample in neat benzene by both electron paramagnetic resonance (EPR) and IR spectroscopy and to determine the magnetic parameters (g tensor and Co hyperfine interaction) for the Co(C(6)H(6))(2) sandwich compound. The large number of fundamental bands observed in the IR spectrum of Co(C(6)H(6))(2), the absorption pattern observed in the Co-ring stretching region of the IR spectrum of the mixed complex, Co(C(6)H(6))(C(6)D(6)) and the orthorhombic g-values extracted from the EPR spectrum are most consistent with nonequivalent benzene ligands in Co(C(6)H(6))(2), i.e., C(s) symmetry. A bonding scheme consistent with both the EPR and IR data for Co(C(6)H(6))(2) is discussed.

Air-Tight Three-Electrode Design of Coaxial Electrochemical-EPR Cell for Redox Studies at Low Temperatures

The weak point of the original Allendoerfer electrochemical-EPR cell has been the reference electrode, placed outside the space-limited electrolysis cavity or not used at all in experiments at low temperatures. We present here an elegant solution to this problem, based on a modified air-tight design of an Allendoerfer cell equipped with a silver-wire pseudoreference electrode. The cell performance is demonstrated on one-electron electrochemical oxidation of heterocyclic 3,6-diphenyl-1,2-dithiine and one-electron reduction of 6-methyl-6-phenylfulvene and the pseudo-octahedral complex fac-[Re(benzyl)(CO)3(dmb)] (dmb = 4,4'-dimethyl-2,2'-bipyridine). In the latter case, the EPR spectrum of the radical anion [Re(benzyl)(CO)3(dmb)]•- points to predominant localization of the unpaired electron on the dmb ligand, in agreement with UV-VIS and IR spectroelectrochemical data.