Synthesis and properties of novel donor-type metal–dithiolene complexes based on 5,6-dihydro-1,4-dioxine-2,3-dithiol (edo) ligand{

Metal–dithiolene complexes form very attractive categories of molecular metals and superconductors. They can have various oxidation states and can range from electron acceptors to donors. The first observation of metallic behavior in crystalline metal– dithiolene complexes was reported for Li0.75[Pt(mnt)2]?2H2O [mnt~maleonitriledithiolate; Scheme 1] by Underhill et al. in 1981. The conduction pathway in molecular conductors based on metal–dithiolene complexes is via mixed metal(d)–ligand(p) orbitals where the S atoms play an important role. This is in contrast to the case of the partially oxidized conducting platinum complexes, including K2[Pt(CN)4]Br0.3?3H2O (KCP), where a one-dimensional conduction band is formed by the metal d orbital and there is no contribution by the ligand to the conduction band. Another important character of the square-planar metal–dithiolene complex is that the metal d orbitals cannot mix into the HOMO (highest occupied molecular orbital) due to the symmetry while the metal dxz orbital has the appropriate symmetry to mix into the LUMO (lowest unoccupied molecular orbital). This leads to the small energy gap between HOMO and LUMO. Therefore, both the HOMO and LUMO can play an important role in the formation of the conduction band depending on the molecular arrangement. Acceptor-type metal–dithiolene complexes M(dmit)2 [dmit~ 2-thioxo-1,3-dithiole-4,5-dithiolate, M~Ni, Pd, Pt, Au; Scheme 1] have provided various conducting radical anion salts which have been extensively studied. Radical anion salts of Pd(dmit)2 with a monovalent cation Z , Z[Pd(dmit)2]2, are unique two-band systems associated with the HOMO and LUMO bands. In crystals of Z[Pd(dmit)2]2, a strong dimerization of the Pd(dmit)2 units along the stack is observed. In the dimer, both the HOMO and LUMO generate bonding and anti-bonding orbitals separated by an energy gap. When the dimers are assembled into a conduction layer, a ‘‘HOMO– LUMO’’ band inversion occurs because the dimerization gap is large compared with the HOMO–LUMO gap (DE). The conduction band is the HOMO band which lies above the LUMO band. The donor-type metal–dithiolene complexes, where the central CLC bond in the TTF-based organic donor is replaced by a transition metal, are also promising materials. For example, M(dddt)2 [dddt~5,6-dihydro-1,4-dithiine-2,3-dithiolate, M~Ni, Pd, Pt, Au; Scheme 1] complexes which correspond to the organic p-donor BEDT-TTF (ET) [bis(ethylenedithio)tetrathiafulvalene; Scheme 1] are known to provide various conductors. For example, [Ni(dddt)2]3(AuBr2)2 is the first metallic radical cation salt of a metal–dithiolene complex which is stable down to at least 1.3 K. The ‘‘HOMO–LUMO’’ band inversion can also be observed in the donor-type system. The ‘‘HOMO–LUMO’’ band inversion in the donor-type metal– dithiolene system results in the LUMO band as the conduction band. This is the opposite to what is observed in the acceptor system. One drawback with the conventional donor-type metal–dithiolene complexes with the dddt complex is that they are poorly soluble in the usual organic solvents, and hence the preparation of high quality radical cation salts is difficult. In order to improve the solubility and expand the materials chemistry of the donor-type metal–dithiolene complex, we have directed our attention to M(edo)2 [edo~5,6-dihydro-1,4dioxine-2,3-dithiolate; Scheme 1] as an analog of the organic donor BEDO-TTF (BO) [bis(ethylenedioxo)tetrathiafulvalene; Scheme 1]. This donor molecule which is soluble in various organic solvents is known to provide superconducting radical cation salts. We should also emphasize that a large number of