Bhunia Pradip, Gomila Rosa M, Frontera Antonio, Ghosh Ashutosh
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009, India.
Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca, Baleares, Spain.
Dalton Trans. 2024 Jul 23;53(29):12316-12330. doi: 10.1039/d4dt00953c.
With the objective of gaining insight into the modulation of the reduction potential of the Ni(II/I) couple, we have synthesized two mononuclear nickel(II) complexes, NiL (HL = ,'-bis(3-methoxysalicylidene)-1,2-diamino-2-methylpropane) and NiL (HL = ,'-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) of two NO donor ligands and recorded their cyclic voltammograms. Both the nickel complexes show reversible reduction processes for the Ni(II/I) couple in acetonitrile solution but the reduction potential of NiL ( = -1.883 V) is 188 mV more positive than that of NiL ( = -2.071 V). In the presence of redox inactive metal ions (Li, Na, K, Mg, Ca and Ba), the reduction potentials are shifted by 49-331 mV and 99-435 mV towards positive values compared to NiL and NiL, respectively. The shift increases with the decrease of the p of the respective aqua-complexes of the metal ion but is poorly co-linear; however, better linearity is found when the shift of the mono- and bi-positive metal ion aqua complexes is plotted separately. Spectrophotometric titrations of these two nickel complexes with the guest metal ions in acetonitrile showed a well-anchored isosbestic point in all cases, confirming the adduct formation of NiL and NiL with the metal ions. Structural analysis of single crystals, [(NiL)Li(HO)]·ClO (1), [(NiL)Li(HO)]·ClO (2), [(NiL)Na]·BF (3) and [(NiL)Ba(HO)(ClO)]·ClO (4), also corroborates the heterometallic adduct formation. The orbital energies of the optimised heterometallic adducts from which electron transfers originated were calculated in order to explain the observed reduction process. A strong linear connection between the calculated orbital energies and the experimental values was observed. According to MEP and 2D vector field plots, the largest shift for divalent metal ions is most likely caused by the local electric field that they impose in addition to Lewis acidity.
为深入了解Ni(II/I)电对还原电位的调制情况，我们合成了两种单核镍(II)配合物，即两种含NO供体配体的NiL（HL = ，'-双(3 - 甲氧基水杨醛)-1,2 - 二氨基 - 2 - 甲基丙烷）和NiL（HL = ，'-双(3 - 甲氧基水杨醛)-1,3 - 二氨基 - 2,2 - 二甲基丙烷），并记录了它们的循环伏安图。两种镍配合物在乙腈溶液中均显示出Ni(II/I)电对的可逆还原过程，但NiL（ = -1.883 V）的还原电位比NiL（ = -2.071 V）正188 mV。在存在氧化还原惰性金属离子（Li、Na、K、Mg、Ca和Ba）的情况下，与NiL和NiL相比，还原电位分别向正值方向移动了49 - 331 mV和99 - 435 mV。这种移动随着金属离子相应水合配合物p值的降低而增加，但线性关系较差；然而，当分别绘制单正和双正金属离子水合配合物的移动情况时，发现线性关系更好。在乙腈中用客体金属离子对这两种镍配合物进行分光光度滴定，在所有情况下都显示出一个固定良好的等吸收点，证实了NiL和NiL与金属离子形成了加合物。单晶[(NiL)Li(HO)]·ClO (1)、[(NiL)Li(HO)]·ClO (2)、[(NiL)Na]·BF (3)和[(NiL)Ba(HO)(ClO)]·ClO (4)的结构分析也证实了异金属加合物的形成。计算了发生电子转移的优化异金属加合物的轨道能量，以解释观察到的还原过程。观察到计算出的轨道能量与实验值之间有很强的线性关系。根据分子静电势（MEP）和二维矢量场图，二价金属离子的最大移动最有可能是由它们除了路易斯酸性之外施加的局部电场引起的。
