Dimerization of Paramagnetic Trinuclear Complexes by Coordination Geometry Changes Showing Mixed Valency and Significant Antiferromagnetic Coupling through −Pt···Pt– Bonds

Paramagnetic trinuclear complexes, trans-[Pt₂M­(piam)₄(NH₃)₄]­(ClO₄)_x (t-M; piam = pivalamidate, M = Mn, Fe, Co, Ni, and Cu, x = 2 or 3), aligned as Pt–M–Pt were successfully synthesized and characterized. The dihedral angles between the Pt and M coordination planes in t-M are approximately parallel, showing straight metal–metal bonds with distances of approximately 2.6 Å. Except for t-Fe, the trinuclear complexes are dimerized with close contact (approximately 3.9 Å) between the end Pt atoms to form Pt–M–Pt···Pt–M–Pt alignments with high-spin M­(+2) containing five (t-Mn), three (t-Co), two (t-Ni), and one (t-Cu) unpaired electrons localized on M atoms. Several physical measurements and calculations revealed that the dimerized structures were maintained in MeCN, where cyclic voltammograms for t-M exhibited two-step oxidation and reduction attributed to Pt–M­(+2)–Pt···Pt–M­(+2)–Pt ↔ Pt–M­(+3)–Pt···Pt–M­(+2)–Pt ↔ Pt–M­(+3)–Pt···Pt–M­(+3)–Pt via mixed-valent states. Magnetic susceptibility measurements for t-M showed antiferromagnetic interaction, t-Mn: J = −0.9 cm^–1, t-Co: J = −3.5 cm^–1, t-Ni: J = −7.3 cm^–1, and t-Cu: J = 0.0 cm^–1, between the two M centers with distances of 9.0 Å through Pt···Pt bonds.