Synthesis, Structure, and Magnetic Properties of Phosphinate‐Bridged Hexanuclear Fe III Complexes Containing Two Butterfly‐Shaped Fe 3 O Cores

The reaction of Fe III salts with 1,1,2,3,3‐pentamethyltrimethylenephosphinic acid [ cyc PO 2 H] or dicyclohexylphosphinic acid [(C 6 H 11 ) 2 PO 2 H] along with pivalic acid in the presence of triethylamine leads to the formation of two hexanuclear iron(III) clusters: [Fe III 6 (μ 3 ‐O) 2 (μ‐OH) 2 (μ ‐cyc PO 2 ) 6 (μ‐ t BuCO 2 ) 6 (η 1 ‐ t BuCO 2 H) 2 ] · 4CH 3 CN · 3CH 2 Cl 2 ( 1 ) and [Fe III 6 (μ 3 ‐O) 2 (μ‐OH) 2 {μ‐(C 6 H 11 ) 2 PO 2 } 6 (μ‐ t BuCO 2 ) 6 (η 1 ‐OH 2 ) 2 ] · 2CH 3 CN · CH 2 Cl 2 ( 2 ). These complexes are made up of two butterfly‐shaped {Fe III 3 O} subunits that are joined to each other by two bridging phosphinate ligands. In addition, the formation of 1 and 2 is assisted by μ 3 ‐O, μ‐OH, and pivalate ligands. Variable‐temperature magnetic susceptibility measurements of complexes 1 and 2 reveal medium to strong antiferromagnetic interactions between the Fe III ions inside the Fe 3 O subunits and weak inter‐trinuclear antiferromagnetic interactions through the bridging phosphinate ligands. The triangular topology of the Fe 3 O subunits gives rise to competing antiferromagnetic interactions and spin frustration. The different relative magnitudes of the competing interactions leads to S = 5/2 and S = 1/2 ground spin states for the Fe 3 O subunits in 1 and 2 , respectively.