Takamuku Toshiyuki, Ogawa Akira, Tsutsui Sou, Sadakane Koichiro, Iwase Hiroki, Mayumi Koichi, Ozutsumi Kazuhiko
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan.
Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan.
J Phys Chem B. 2024 Sep 5;128(35):8567-8577. doi: 10.1021/acs.jpcb.4c04490. Epub 2024 Aug 27.
Understanding the complex formation of metal ions in room-temperature ionic liquids (ILs) is essential for the application of ILs in solvent extraction. Nevertheless, the research on metal complex formation in ILs lags behind other applications. The complex formation equilibria may be influenced by specific interactions among the metal ion, ligand molecule, and IL cation and anion. In the present investigation, the complex formation of Ni with ethanol (EtOH) and methanol (MeOH) molecules in ILs, 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Cmim][TFSA], where represents the alkyl chain lengths of 2 and 8) was discussed in terms of the microscopic interactions among alcohol molecules, [Cmim] and [TFSA], and the mesoscopic mixing states of alcohols in [Cmim][TFSA], with = 2-12. The microscopic interaction of alcohol molecules with the imidazolium ring H atoms in the ILs was evaluated by using ATR-IR and H and C NMR spectroscopies. The self-hydrogen bonding of alcohol molecules was clarified from the O-H stretching vibration of alcohol molecules. MeOH molecules can be more strongly hydrogen-bonded with themselves than EtOH molecules due to the less steric hindrance and the weaker dispersion force of MeOH with the IL cation's alkyl chain. In fact, small-angle neutron scattering (SANS) experiments revealed the more heterogeneous mixing of MeOH with the ILs by the self-hydrogen bonding among MeOH molecules than EtOH. The longer the IL cation's alkyl chain, the more the MeOH clusters significantly form. In contrast, the formation of EtOH clusters becomes weaker with elongating the alkyl chain. Ultraviolet (UV)-visible spectroscopic measurements on Ni-[Cmim][TFSA]-alcohol solutions with = 2 and 8 revealed that di-, tetra-, and hexa-alcohol-Ni complexes are formed in both the ILs. With = 2, the stabilities of Ni-EtOH and Ni-MeOH complexes are comparable in the IL. However, with = 8, the complexes are more stable in the EtOH solutions than in the MeOH solutions. This is because the less heterogeneous mixing of EtOH molecules with the IL results in the larger enthalpic contribution in the complex formation, as shown by the thermodynamic parameters estimated by the van't Hoff plots on the stability constants at several temperatures.
了解室温离子液体（ILs）中金属离子的络合物形成对于ILs在溶剂萃取中的应用至关重要。然而，关于ILs中金属络合物形成的研究落后于其他应用。络合物形成平衡可能会受到金属离子、配体分子以及IL阳离子和阴离子之间特定相互作用的影响。在本研究中，从醇分子、[Cmim]和[TFSA]之间的微观相互作用以及醇在[Cmim][TFSA]（其中 代表2和8的烷基链长度）中的介观混合状态方面，讨论了镍与乙醇（EtOH）和甲醇（MeOH）分子在ILs中的络合物形成情况， = 2 - 12。通过衰减全反射红外光谱（ATR - IR）以及氢和碳核磁共振光谱（H和C NMR）评估了醇分子与ILs中咪唑环氢原子的微观相互作用。从醇分子的O - H伸缩振动中明确了醇分子的自氢键作用。由于空间位阻较小以及甲醇与IL阳离子烷基链之间的色散力较弱，甲醇分子之间的氢键作用比乙醇分子更强。实际上，小角中子散射（SANS）实验表明，甲醇分子通过自氢键作用与ILs的混合比乙醇更不均匀。IL阳离子的烷基链越长，甲醇簇形成得越显著。相反，随着烷基链的延长，乙醇簇的形成变得更弱。对 = 2和8的镍 - [Cmim][TFSA] - 醇溶液进行紫外可见光谱测量表明，在两种ILs中都形成了二、四和六醇 - 镍络合物。当 = 2时，镍 - 乙醇和镍 - 甲醇络合物在IL中的稳定性相当。然而，当 = 八时，这些络合物在乙醇溶液中比在甲醇溶液中更稳定。这是因为乙醇分子与IL的混合不均匀性较小，导致在络合物形成过程中焓的贡献更大，这从通过范特霍夫图在几个温度下对稳定常数估计的热力学参数可以看出。
