Improving the Solubility of Hexanuclear Heterometallic Extended Metal Atom Chain Compounds in Nonpolar Solvents by Introducing Alkyl Amine Moieties

The highest occupied
molecular orbital–lowest unoccupied
molecular orbital (HOMO–LUMO) interaction at the dz2 orbital between two kinds of metal complex
is useful for obtaining heterometallic one-dimensional (1D) chains
as well as heterometallic metal string compounds (HMSCs). Platinum
dinuclear complexes, [Pt2(piam)2(NH2R)4]­X2 (piam = pivalamidate, R = CH3, C2H5, C3H7, or C4H9, X = anion), comprising σ* as HOMO were
mixed with [Rh2(O2CCH3)4] comprising σ* as LUMO in solvents to afford single crystals
of [{Rh2(O2CCH3)4}­{Pt2(piam)2(NH2R)4}2]­X4 (2–5). Single-crystal
X-ray analyses revealed that 2–5 are
hexanuclear complexes that are one-dimensionally aligned as Pt–Pt–Rh–Rh–Pt–Pt
with metal–metal bonds, where the alkyl moieties at end Pt
atoms obstruct further 1D extension. Complexes 2–5 appear as if they are cut off from an infinite chain [{Rh2(O2CCH3)4}­{Pt2(piam)2(NH3)4}2]n(PF6)4n·6nH2O (1) aligned
as −{Pt–Pt–Rh–Rh–Pt–Pt}n–. The diffuse reflectance spectrum
of 1 depicts broad shoulder bands, which are not present
in the spectra of 2–5, proving that
the infinite chain 1 forms a band structure. Compounds 4 and 5 with propyl or butyl moieties at amine
ligands, respectively, are soluble in nonpolar solvents, such as CH2Cl2, without the dissociation of their hexanuclear
structures. Taking advantage of their solubility, measurement of cyclic
voltammetry in CH2Cl2 become possible, which
shows the quasi-reversible oxidation and reduction waves at 4: Eox = 0.86 V and Ered = 0.69 V and 5: Eox = 0.87 V and Ered = 0.53 V.