High-resolution EPR spectroscopic investigations of a homologous set of d9-cobalt(0), d9-rhodium(0), and d9-iridium(0) complexes.

The 17-electron complexes [M(tropp(ph))2] (M=Co0, Rh0, Ir0) were prepared and isolated (tropp = tropylidene phosphane). A structural analysis of [Co(tropp(ph))2] revealed this complex to be almost tetrahedral, while the heavier homologues have more planar structures. Partially deuterated tropp complexes [D6][M(tropp(ph))2] were synthesised for M = Rh and Ir in order to enhance the resolution in the EPR spectra. This synthesis involves a four-fold intramolecular C-H activation reaction, whereby alkyl groups are transformed into olefins. Dihydrides were observed as intermediates for M = Ir. The electronic and geometric structures of all complexes [M(tropp(ph))2] (M = Co, Rh, Ir) and [D6][M(tropp(ph))2] (M = Rh, Ir) were investigated by continuous wave (CW) and echo-detected EPR in combination with pulse ENDOR and ESEEM techniques. In accord with their planar structures, cis and trans isomers were detected for [M(tropp(ph))2] (M = Rh0, Ir0) for which a dynamic equilibrium was established. The thermodynamic data show that the cis isomer is slightly preferred by deltaH(o) = -4.7 +/- 0.3 kJ mol(-1) (M = Rh) and delta H(o) = -5.1 +/- 0.5 kJ mol(-1); (M = Ir). The entropies for the process trans-[M(tropp(ph))2] <==> cis-[M(tropp(ph))2] are also negative [deltaS(o) = -5 +/- 1.5 J mol(-1) (M = Rh); deltaS(o) = -17 +/- 3.7 J mol(-1) (M = Rh)], indicating higher steric congestion in the cis isomers. The cobalt(0) and irdium(0) complexes show rather large g anisotropies, while that of the rhodium(0) complex is small (Co: g(parallel) = 2.320, g(perpendicular) = 2.080; cis-Rh: g(parallel) = 2.030, g(perpendicular) = 2.0135; trans-Rh: g(parallel) = 2.050, g(perpendicular) = 2.030; cis-Ir: g(parallel) = 2.030, g(perpendicular) = 2.060; trans-Ir: g(parallel) = 1.980, g(perpendicular) = 2.150). The g matrices of [M(tropp(ph))2] (M = Co, Rh) are axially symmetric with g(parallel) > g(perpendicular), indicating either a distorted square planar structure (SOMO essentially d(x2 - y2) or a compressed tetrahedron (SOMO essentially d(xy)). Interestingly, for [Ir(tropp(ph))2] the inverse ordering, g(perpendicular) > g(parallel) is found; this cannot be explained by simple ligand field arguments and must await a more sophisticated analysis. The hyperfine interactions of the unpaired electron with the metal nuclei, phosphorus nuclei, protons, deuterons and carbon nuclei were determined. By comparison with atomic constants, the spin densities on these centres were estimated and found to be small. However, the good agreement of the distance between the olefinic protons and the metal centres determined from the dipolar coupling parameter indicates that the unpaired electron is primarily located at the metal centre

TROPDAD: A new ligand for the synthesis of water-stable paramagnetic [16+1]-electron rhodium and iridium complexes

The new tetradentate ligand 1,4-bis(5 H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene ((H)tropdad) allows the syntheses of the 16-electron cationic rhodium complexes [M((H)tropdad)](O(3)SCF(3)) (M=Rh, Ir). The structure of the rhodium complex was determined by X-ray analysis and points to a description of these as [M(+1)((H)tropdad)(0)] with short Cd-N bonds (av 1.285 A) and a long C-C bond (1.46 A) in the diazabutadiene (dad) moiety, that is the M-->dad charge-transfer is negligible. Both [Rh((H)tropdad)](+) and [Ir((H)tropdad)](+) are reduced at very low potentials (E(1) (1/2)= -0.56 V and E(1) (1/2)=-0.35 V, respectively) which allowed the quantitative synthesis of the neutral paramagnetic complexes [M((H)tropdad)](0) (M=Rh, Ir) by reacting the cationic precursor complexes simply with zinc powder. The [M((H)tropdad)](0) complexes are stable against protic reagents in organic solvents. Continuous wave and pulse EPR spectroscopy was used to characterize the paramagnetic species and the hyperfine coupling constants were determined: [Rh((H)tropdad)](0): A(iso)((14)N)=11.9 MHz, A(iso)((1)H)=14.3 MHz, A(iso)((103)Rh)= -5.3 MHz; [Ir((H)tropdad)](0): A(iso)((14)N)=11.9 MHz, A(iso)((1)H)=14.3 MHz. In combination with DFT calculations, the experimentally determined g and hyperfine matrices could be orientated within the molecular frame and the dominant spin density contributions were determined. These results clearly show that the complexes [M((H)tropdad)](0) are best described as [M(+1)((H)tropdad)(.-)] with a [16+1] electron configuration

Electron Paramagnetic Resonance spectroscopy

A survey of the activities of the EPR group at ETH is given. The different research areas are discussed briefly and a particular project is highlighted