Iron(II)-alkoxide and -aryloxide complexes of a tris(thioether)borate ligand: synthesis, molecular structures, and implications on the origin of instability of their iron(II)-catecholate counterpart

Abstract

The phenyltris[(tert-butylthio)methyl]borate ligand, [PhTt^tBu], has been studied extensively as a platform for coordination, organometallic, and bioinorganic chemistry, especially with 3d metals. While [PhTt^tBu]Co(3,5-DBCatH) (3,5-DBCatH is 3,5-di-tert-butylcatecholate), a Co^II-monoanionic catecholate complex, was successfully isolated to model the active site of cobalt(II)-substituted homoprotocatechuate 2,3-dioxygenase (Co-HPCD) [Wang et al. (2019). Inorg. Chim. Acta, 488, 49-55], its iron(II) counterpart, [PhTt^tBu]Fe(3,5-DBCatH), was not accessible via similar synthetic routes. Switching the nucleophile from catecholate to alkoxide or aryloxide, however, led to the successful isolation of three highly air-sensitive Fe^II-alkoxide and -aryloxide complexes, namely, (triphenylmethoxo){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₁₉H₁₅O)], (2), (2,6-dimethylphenolato){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₈H₉O)], (3), and bis{μ-tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S':S''}bis[(phenolato-κO)iron(II)] toluene disolvate, [Fe₂(C₂₁H₃₈BS₃)₂(C₆H₅O)₂]·2C₇H₈, (4). In the solid state, compounds (2) and (3) are monomeric, with [PhTt^tBu] acting as a tridentate ligand. In contrast, compound (4) crystallizes as a dimeric complex, wherein each [PhTt^tBu] ligand binds to an iron centre with two thioethers and binds to the other iron centre with the third thioether. The molecular structures of (2)-(4) demonstrate a diversity in the binding modes of [PhTt^tBu] and highlight its potential use for assembling multinuclear complexes. In addition, the successful isolation of (2)-(4), as well as the structural information of a [PhTt^tBu] modification product, namely, bis{μ-tris[(tert-butylsulfanyl)methyl](2-oxidophenolato)borato-κO,O',S,S':O'}dicobalt(II), [Co₂(C₂₁H₃₇BO₂S₃)₂], (5), obtained from the reaction of [PhTt^tBu]CoCl with potassium monoanionic catecholate, shed light on the origin of the instability of [PhTt^tBu]Fe(3,5-DBCatH).

Keywords: alkoxide; aryloxide; catecholate; cobalt; crystal structure; iron; thioether.