2,6-Bis(hydroxymethyl)phenols for the synthesis of high-nuclearity clusters

Ligands based on 2,6-bis(hydroxymethyl)phenol H3LR possess three potential ligating groups either deprotonated or non-deprotonated: one phenol and two benzylalcohol groups. One important property of the benzylalcoholato function is its strong basicity mainly due to the lack of resonance stabilization of the corresponding anion. This leads to their pronounced tendency to bridge metal centers rather than to bind to one metal center in a terminal fashion. Therefore, ligands based on 2,6-bis(hydroxymethyl)phenol H3LR are well suited for the preparation of high-nuclearity complexes. The possible coordination modes of 2,6-bis(hydroxymethyl)phenol ligands H3LR have been summarized. Only the use of strong Lewis-acidic metal ions like TiIV enables a terminal coordination mode of the benzylalcoholato function as evidenced in the dinuclear complex [(Lt-Bu)3Ti2]−, [Ti2]−. This complex is in protic solvents in an equilibrium with the tetranuclear complex [(Lt-Bu)2(HLt-Bu)4Ti4O2]2−, [Ti4]2−. In the latter, the deprotonated benzylalcohol function of the ligand (HLt-Bu)2− bridges two TiIV ions demonstrating a small energy difference for a terminal and a bridging coordination mode of the benzylalcoholate to TiIV. For the less Lewis-acidic FeIII ion, only a bridging coordination of benzylalcoholate has been found in a tetranuclear complex, [(HLt-Bu)6FeIII4(acac)2]2−, [FeIII4]2−, a decanuclear complex, [(HLt-Bu)12Fe10Na4(μ3-O)4(μ3-OH)2(dme)2(EtOH)2], [FeIII10], and a dinuclear complex, [(HLt-Bu)4FeIII(μ2-CO3)]4−, [FeIII2]4−. The exchange interactions in these complexes are all antiferromagnetic leading to diamagnetic St = 0 spin ground states. On the other hand, the reaction of H3LMe with MnII in the presence of N3− results in the nonadecanuclear mixed valence complex [(HLMe)12MnIII12MnII7(μ4-O)8(μ3,η1-N3)8(MeCN)6]2+, [MnIII12MnII7]2+, which exhibits ferromagnetic interactions with the maximum St = 83/2 spin ground state—the highest spin yet established in a molecule. These results demonstrate the potential of 2,6-bis(hydroxymethyl)phenol ligands for the synthesis of high-nuclearity clusters with interesting magnetic properties.