Ultralarge 3d/4f Coordination Wheels: From Carboxylate/Amino Alcohol-Supported {Fe<sub>4</sub>Ln<sub>2</sub>} to {Fe<sub>18</sub>Ln<sub>6</sub>} Rings

A family of wheel-shaped charge-neutral heterometallic {Fe<sup>III</sup><sub>4</sub>Ln<sup>III</sup><sub>2</sub>}- and {Fe<sup>III</sup><sub>18</sub>M<sup>III</sup><sub>6</sub>}-type coordination clusters demonstrates the intricate interplay of solvent effects and structure-directing roles of semiflexible bridging ligands. The {Fe<sub>4</sub>Ln<sub>2</sub>}-type compounds [Fe<sub>4</sub>Ln<sub>2</sub>(O<sub>2</sub>CCMe<sub>3</sub>)<sub>6</sub>­(N<sub>3</sub>)<sub>4</sub>(Htea)<sub>4</sub>]·2­(EtOH), Ln = Dy (<b>1a</b>), Er (<b>1b</b>), Ho (<b>1c</b>); [Fe<sub>4</sub>Tb<sub>2</sub>(O<sub>2</sub>CCMe<sub>3</sub>)<sub>6</sub>­(N<sub>3</sub>)<sub>4</sub>(Htea)<sub>4</sub>] (<b>1d</b>); [Fe<sub>4</sub>Ln<sub>2</sub>(O<sub>2</sub>CCMe<sub>3</sub>)<sub>6</sub>­(N<sub>3</sub>)<sub>4</sub>(Htea)<sub>4</sub>]·2­(CH<sub>2</sub>Cl<sub>2</sub>), Ln = Dy (<b>2a</b>), Er (<b>2b</b>); [Fe<sub>4</sub>Ln<sub>2</sub>(O<sub>2</sub>CCMe<sub>3</sub>)<sub>4</sub>­(N<sub>3</sub>)<sub>6</sub>(Htea)<sub>4</sub>]·2­(EtOH)·2­(CH<sub>2</sub>Cl<sub>2</sub>), Ln = Dy (<b>3a</b>), Er (<b>3b</b>) and the {Fe<sub>18</sub>M<sub>6</sub>}-type compounds [Fe<sub>18</sub>M<sub>6</sub>(O<sub>2</sub>CCHMe<sub>2</sub>)<sub>12</sub>­(Htea)<sub>18</sub>(tea)<sub>6</sub>(N<sub>3</sub>)<sub>6</sub>]·<i>n</i>(solvent), M = Dy (<b>4</b>, <b>4a</b>), Gd (<b>5</b>), Tb (<b>6</b>), Ho (<b>7</b>), Sm (<b>8</b>), Eu (<b>9</b>), and Y (<b>10</b>) form in ca. 20–40% yields in direct reaction of trinuclear Fe<sup>III</sup> pivalate or isobutyrate clusters, lanthanide/yttrium nitrates, and bridging triethanolamine (H<sub>3</sub>tea) and azide ligands in different solvents: EtOH for the smaller {Fe<sub>4</sub>Ln<sub>2</sub>} wheels and MeOH/MeCN or MeOH/EtOH for the larger {Fe<sub>18</sub>M<sub>6</sub>} wheels. Single-crystal X-ray diffraction analyses revealed that <b>1</b>–<b>3</b> consist of planar centrosymmetric hexanuclear clusters built from Fe<sup>III</sup> and Ln<sup>III</sup> ions linked by an array of bridging carboxylate, azide, and aminopolyalcoholato-based ligands into a cyclic structure with a cavity, and with distinct sets of crystal solvents (2 EtOH per formula unit in <b>1a</b>–<b>c</b>, 2 CH<sub>2</sub>Cl<sub>2</sub> in <b>2</b>, and 2 EtOH and 2 CH<sub>2</sub>Cl<sub>2</sub> in <b>3</b>). In <b>4</b>–<b>10</b>, the largest 3d/4f wheels currently known, nearly linear Fe<sub>3</sub> fragments are joined via mononuclear Ln/Y units by a set of isobutyrates and amino alcohol ligands into virtually planar rings. The magnetic properties of <b>1</b>–<b>10</b> reveal slow magnetization relaxation for {Fe<sub>4</sub>Tb<sub>2</sub>} (<b>1d</b>) and slow relaxation for {Fe<sub>4</sub>Ho<sub>2</sub>} (<b>1c</b>), {Fe<sub>18</sub>Dy<sub>6</sub>} (<b>4</b>), and {Fe<sub>18</sub>Tb<sub>6</sub>} (<b>6</b>).