Isolation and characterization of a tetranuclear Pt-Fe•••Fe-Pt intermediate en route to the trinuclear Pt-Fe-Pt cluster

An intermediate compound,[{PtFe(piam)(2)(NH3)(2)(OCH3)}(2)(mu-OCH3)(2)](ClO4)(2) (1, piam = pivalamidate), in the synthetic process to form [Pt2Fe(piam)(4)(NH3)(4)](ClO4)(3) (2) by mixing cis-[Pt(piam)(2)(NH3)(2)]center dot 2H2O and iron sources was successfully isolated and characterized by single-crystal X-ray analysis. In 1, the platinum and iron atoms are bridged by two piam ligands to afford a dinuclear Pt-Fe structure and are further linked to each other by methoxide bridges at the equatorial positions of iron atoms to form a tetranuclear Pt-Fe center dot center dot center dot Fe-Pt complex. The Pt-Fe distances in 1 are 3.0010(16) and 2.9883(17) angstrom, which are significantly longer than those in 2 (2.5566(15) and 2.5718(15) angstrom). X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and magnetic susceptibility measurements revealed that the oxidation states are Pt(+2)-Fe(+3)center dot center dot center dot Fe(+3)-Pt(+2) (1) and Pt(+2)-Fe(+3)-Pt(+2) (2) with high-spin (S = 5/2) configurations in iron atoms. The magnetic susceptibility of 1 has a chi(T) value of 5.83 cm(3) mol(-1) K per Pt(+2)-Fe(+3)center dot center dot center dot Fe(+3)-Pt(+2) unit at 300 K, which decreases down to 0.04 cm(3) mol(-1) K at 7 K due to antiferromagnetic coupling (J = -28 cm(-1)) of the two Fe(+3) centers. Compound 2 maintains its trinuclear structure in MeCN, exhibiting reversible one-electron reduction and oxidation, Pt(+2)-Fe(+2)-Pt(+2) <-> Pt(+2)-Fe(+3)-Pt(+2), at E-1/2 = -0.19 V (vs. Fc/Fc(+)). However, in MeOH, compound 2 is decomposed into a dinuclear structure of Pt-Fe involving an equilibrium between 1 and 2.