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

Abstract

Ligands based on 2,6-bis(hydroxymethyl)phenol H 3L R 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 H 3L R are well suited for the preparation of high-nuclearity complexes. The possible coordination modes of 2,6-bis(hydroxymethyl)phenol ligands H 3L R have been summarized. Only the use of strong Lewis-acidic metal ions like Ti IV enables a terminal coordination mode of the benzylalcoholato function as evidenced in the dinuclear complex [(L t-Bu ) 3Ti 2] −, [Ti 2 ] − . This complex is in protic solvents in an equilibrium with the tetranuclear complex [(L t-Bu ) 2(HL t-Bu ) 4Ti 4O 2] 2−, [Ti 4 ] 2− . In the latter, the deprotonated benzylalcohol function of the ligand (HL t-Bu ) 2− bridges two Ti IV ions demonstrating a small energy difference for a terminal and a bridging coordination mode of the benzylalcoholate to Ti IV. For the less Lewis-acidic Fe III ion, only a bridging coordination of benzylalcoholate has been found in a tetranuclear complex, [(HL t-Bu ) 6Fe III 4(acac) 2] 2−, [Fe III 4 ] 2− , a decanuclear complex, [(HL t-Bu ) 12Fe 10Na 4( μ 3-O) 4( μ 3-OH) 2(dme) 2(EtOH) 2], [Fe III 10 ], and a dinuclear complex, [(HL t-Bu ) 4Fe III( μ 2-CO 3)] 4−, [ Fe III 2 ] 4− . The exchange interactions in these complexes are all antiferromagnetic leading to diamagnetic S t = 0 spin ground states. On the other hand, the reaction of H 3L Me with Mn II in the presence of N 3 − results in the nonadecanuclear mixed valence complex [(HL Me) 12Mn III 12Mn II 7( μ 4-O) 8( μ 3, η 1-N 3) 8(MeCN) 6] 2+, [Mn III 12 Mn II 7 ] 2+ , which exhibits ferromagnetic interactions with the maximum S t = 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.