Mono- to Tetra-nuclear Iron Dichloride Complexes Bearing <i>N</i>-(Fluoro)-aryl-Substituted Iminopyridine Ligands: Synthesis, Structures, Magnetic Properties, and Polymerization of Isoprene

The reaction of FeCl₂ with the iminopyridine ligand containing fluorinated <i>N</i>-aryl groups {2-[(Ar)N = C(R)]C₅H₄N, where Ar = 3,5-(CF₃)₂C₆H₃ and R = H (<b>L1</b>) or CH₃ (<b>L3</b>); Ar = C₆F₅ and R = H (<b>L2</b>) or CH₃ (<b>L4</b>); and iminoquinoline 2-[(3,5-(CF₃)₂(C₆H₃)N = CH)]C₉H₆N (<b>L5</b>)} resulted in distinct iron coordination complexes whose structures were determined by single-crystal X-ray diffraction. Ligands <b>L1</b> and <b>L2</b> yielded mononuclear bis-ligated iron dichlorides (<b>L1</b> or <b>L2</b>)₂FeCl₂ (complexes <b>1</b> and <b>2</b>), while <b>L3</b> and <b>L5</b> formed dinuclear complexes (<b>L3</b> or <b>L5</b>)₂Fe(μ-Cl₂)FeCl₂ (complexes <b>3</b> and <b>5</b>). Ligand <b>L4</b> produced an unprecedented tetranuclear dimer [(<b>L4</b>)₂FeCl(μ-Cl₂)FeCl₂]₂ (complex <b>4</b>). Mössbauer spectroscopy revealed single quadrupole doublets for mononuclear complex <b>2</b> and two sets of doublets for dinuclear complexes <b>3</b> and <b>5</b> and tetranuclear complex <b>4</b>, confirming Fe(II) in high-spin states. Magnetic susceptibility and Curie-Weiss analyses elucidated magnetic interactions, showing reduced effective magnetic moments correlated with increasing Fe-(μ-Cl)-Fe bridges. DFT calculations and NBO analyses supported these findings, highlighting lower charge density on bridging compared to terminal chlorides. Activated by Al^<i>i</i>Bu₃/[CPh₃][B(C₆F₅)₄], complexes <b>1</b> and <b>2</b> showed lower activities (TOF = 1,000-1,200 h^-1) than complexes <b>3</b> and <b>5</b> (TOF = 90,000 h^-1) for isoprene polymerization, producing polymers with enriched <i>cis</i>-1,4-<i>alt</i>-3,4 microstructures.