Organic polyanionic high-spin molecular clusters: topological-symmetry controlled models for organic ferromagnetic metals

Abstract

Trianionic spin-quartet and tetraanionic spin-quintet molecular clusters derived from m-dibenzoylbenzene in solution were identified by CW-ESR/pulse-ESR based two-dimensional electron spin transient nutation spectroscopy, and their spin and clustering structures in the ground state were determined in terms of a D-tensor based phenomenological approach and DFT calculations. The molecular structures obtained semiempirically are supported by DFT-based quantum chemical calculations. The DFT calculations have been tested for a sodium ion bridged fluorenone-based cluster, [fluorenone(-)˙ {Na(+)(dme)(2)}](2), whose crystal structure was reported in the literature [H. Bock, H.-F. Herrmann, D. Fenske and H. Goesmann, Angew. Chem., Int. Ed. Engl., 1988, 27, 1067], reproducing the experimentally determined moelcular structure of the dimer cluster. It is suggested that both the quartet and quintet clusters in the 2-MTHF glass and solution form the cross-typed structures with the two m-dibenzoylbenzene moieties in cis-configuration. A dianionic spin-triplet m-dibenzoylbenzene derivative was detected for the first time and its charge and spin densities were studied by the quantum chemical calculations. The high-spin states of the open-shell entities under study were confirmed by X-band pulse-ESR based electron spin nutation spectroscopy in organic frozen glasses. The D values and other spin Hamiltonian parameters of all the polyanionic high-spin species were determined by the hybrid eigenfield spectral simulation for fine-structure ESR spectra. m-Dibenzoylbenzene provides pseudo-degenerate π-LUMOs arising from its topological symmetry of the π-electron network and its dianion in the triplet ground state is a prototypical model for topologically-controlled genuinely organic ferromagnetic metals.

Pulsed electron spin nutation spectroscopy of weakly exchange-coupled biradicals: a general theoretical approach and determination of the spin dipolar interaction

“…The crystal was positioned at the centre of the microwave resonant cavity. This usage of the double detection schemes gives no interference between the 2D-ESN signals due to chemical entities of different spin multiplicities, 32,34,[39][40][41][42][43][44][45][46][47] and thus gives accurate nutation frequencies. The accuracy is more important in the ESN spectroscopy for biradicals than in conventional one for the identification of high spin multiplicities.…”

“…ESN spectroscopy applied to systems with good spin quantum numbers S has been well established. 32,34,[39][40][41][42][43][44][45][46][47] In the weak extreme limit of the irradiation power, the nutation frequency o n for the |M S i -|M S + 1i allowed transition of the isolated spin state S is described by a rather simple expression in first order, 39,44,47…”

“…where o 1 denotes the irradiation strength B 1 , in units of angular frequency, of the microwave field (see eqn (2) below for B 1 ) and G gives the anisotropy of the g-factor. 46,47 The third term of eqn (1) denotes the contribution of nuclear spin transitions involved during the ESR transition. This term becomes significant when the direction of the nuclear quantised axis changes during the ESR transition.…”

“…This term becomes significant when the direction of the nuclear quantised axis changes during the ESR transition. 39,44,47 In the weak extreme limit where the zerofield splitting parameters are much larger than the strength of the microwave irradiation field o 1 , the nutation frequency o n is independent of the magnitude of the zero-field splitting parameters, underlying the usefulness of the 2D-ESN spectroscopy for mixtures of molecular spins in non-oriented media. 46,47 On the other hand, weakly exchange-coupled cases, such as the biradicals 1/2 under study, where the four M S -manifolds are composed of the singlet state and the triplet states with relatively small dipolar interaction, are beyond the scope of eqn (1).…”

“…[35][36][37][38] In addition, twodimensional (2D) pulse-based electron spin transient nutation (2D-ESN or 2D-ESTN) spectroscopy can differentiate different spin multiplicities in a mixture straightforwardly, to yield zerofield splitting parameters by reliable spectral simulations. 32,34,[39][40][41][42][43][44][45][46][47] The nutation experiments in non-oriented media can be carried out with conventional X-band pulsed ESR spectrometers equipped with 1 kW TWT microwave amplifiers. 32,[40][41][42][43] Thus, in high spin chemistry, even if exchange interactions are as small as ca.…”