Isolation and characterization of a tetranuclear Pt–Fe⋯Fe–Pt intermediate en route to the trinuclear Pt–Fe–Pt cluster

Abstract

An intermediate compound, [{PtFe(piam)₂(NH₃)₂(OCH₃)}₂(μ-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]·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⋯Fe–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 cm³ mol⁻¹ K per Pt(+2)–Fe(+3)⋯Fe(+3)–Pt(+2) unit at 300 K, which decreases down to 0.04 cm³ 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.