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

An intermediate compound, {PtFe(piam)(NH)(OCH)}(μ-OCH) (1, piam = pivalamidate), in the synthetic process to form PtFe(piam)(NH) (2) by mixing cis-[Pt(piam)(NH)]·2HO 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-FeFe-Pt complex. The Pt-Fe distances in 1 are 3.0010(16) and 2.9883(17) Å, which are significantly longer than those in 2 (2.5566(15) and 2.5718(15) Å). X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and magnetic susceptibility measurements revealed that the oxidation states are Pt(+2)-Fe(+3)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 χT value of 5.83 cm mol K per Pt(+2)-Fe(+3)Fe(+3)-Pt(+2) unit at 300 K, which decreases down to 0.04 cm mol K at 7 K due to antiferromagnetic coupling (J = -28 cm) 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 = -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.