A Second-generation Janus Scorpionate Ligand: Controlling Coordination Modes in Iron(II) Complexes by Steric Modulation


The second-generation Janus scorpionate ligand [HB(mtdaMe)3]− (mtdaMe = 2-mercapto-5-methyl-1,3,4-thiadiazolyl) with conjoined (N,N,N-) and (S,S,S-) donor faces has been prepared. This second-generation Janus scorpionate ligand [HB(mtdaMe)3]− differs from the first-generation [HB(mtda)3]− ligand by the replacement of hydrogens on the heterocyclic rings proximal to the nitrogenous face with methyl groups. This study probed whether steric interactions introduced by such methyl group substitution could modulate the reactivity and coordination preferences of these ambidentate ligands. The crystal structures of a sodium complex Na[HB(mtdaMe)3]·3(MeOH), the potassium complexes K[HB(mtda)3]·MeOH, and K2[HB(mtdaMe)3]2·3MeOH, and several iron complexes were obtained. The difference between first- and second-generation Janus scorpionate ligands is most obvious from the discrepancy between the properties and structures of the two iron(II) compounds with the formula Fe[HB(mtdaR)3]2·4DMF (R = H or Me). The complex with the first-generation ligand (R = H) is pink and diamagnetic. An X-ray structural study revealed two facially coordinated κ3N-scorpionates with no bound solvent molecules. The average Fe−N bond distance of 1.97 Å is indicative of the low-spin t2g6eg*0 electron configuration. In contrast, the iron(II) complex of the second-generation ligand (R = Me) is yellow and paramagnetic. This structure shows two trans-κ1S-scorpionates and four equatorial-bound DMF where the average Fe−O and Fe−S distances of 2.12 and 2.51 Å, respectively, are indicative of the high-spin t2g4eg*2 electron configuration. The discrepancy in binding modes and spin-states of iron(II) is carried over to the solvent-free Fe[HB(mtdaR)3]2 (R = H, Me) complexes, as determined from Mössbauer spectral studies. The Mössbauer spectral parameters for Fe[HB(mtda)3]2 are fully consistent with low-spin iron(II) in a FeN6 environment, whereas those for Fe[HB(mtdaMe)3]2 are most consistent with high-spin iron(II) in a FeS6 environment. Interestingly, when either complex is dissolved in highly polar solvents (DMF, DMSO, or H2O), the ligand completely dissociates forming [Fe(solvent)6][HB(mtdaR)3]2 (R = H, Me).




National Science Foundation of Belgium
Marquette University
Petroleum Research Fund

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