Influence of the chelate effect on the electronic structure of one-electron oxidized group 10 metal(ii)-(disalicylidene)diamine complexes

Abstract

The neutral and one-electron oxidized group 10 metal, Ni(II), Pd(II) and Pt(II), six-membered chelate Salpn (Salpn = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,3-propanediamine) complexes have been investigated and compared to the five-membered chelate Salen (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethanediamine) and Salcn (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) complexes. Reaction of the Salpn complexes with 1 equivalent of AgSbF(6) affords the oxidized complexes which exist as ligand radical species in solution and in the solid state. The solid state structures of the oxidized complexes have been determined by X-ray crystal structure analysis. While the Ni and Pt analogues exhibit an essentially symmetric coordination sphere contraction upon oxidation, the oxidized Pd derivative exhibits an asymmetric metal binding environment demonstrating at least partial ligand radical localization. In comparison to the oxidized Salen and Salcn complexes, the propyl backbone of the Salpn complexes leads to a larger deviation from a planar geometry in the solid state. The electronic structure of the oxidized Salpn complexes was further probed by UV-vis-NIR measurements, electrochemistry, EPR spectroscopy, and theoretical calculations. The intense NIR band for the one-electron oxidized Salpn complexes shifts to lower energy in comparison to the 5-membered chelate analogues, which is attributed to lower metal d(xz) character in the β-LUMO for the Salpn series. The reactivity of the one-electron oxidized Salpn complexes with exogenous ligands was also studied. In the presence of pyridine, the oxidized Ni analogue exhibits a shift in the locus of oxidation to a Ni(III) species. The oxidized PtSalpn complex rapidly decomposes in the presence of pyridine, even at low temperature. Interestingly, electronic and EPR spectroscopy suggests that the addition of pyridine to the oxidized Pd analogue results in initial dissociation of the phenoxyl radical ligand, likely due to the increased flexibility of the propyl backbone.

Oxidation Chemistry of Metal(II) Salen-Type Complexes

“…The X-ray crystal structure analyses of one-electron oxidized group 10 metal salen-type complexes are shown in Figure 5 [41][42][43]. The structures of all these complexes were found to be similar to those of the corresponding complexes before oxidation, which supports the CV results that significant structure changes did not occur in the course of the oxidation.…”

“…These structural features of the phenolate moiety in the oxidized Pd complex are in good agreement with the characteristics of the phenoxyl radical, which showed the quinoid form due to delocalization of the radical electron on the phenolate moiety as shown in Scheme 2 and reference [53]. Such properties were also detected for the Pd complex with the 6-membered dinitrogen chelate back bone, [Pd(Salpn)]SbF6 [43]. In addition, the SbF6 -counterion was positioned close to the quinoid moiety of this complex; the closest distance between the SbF6 -and the C-O carbon atom of the phenoxyl ligand was 3.026 Å.…”

“…Such a trend is also applied to the salen-type complexes, and most of the Cu(II) and group 10 metal(II) salen-type complexes exhibited two reversible redox waves in the range of 0 to 1.5 V vs NHE, due to having two phenolate moieties [29][30][31][39][40][41][42][43][44][45][46][47]. On the other hand, the metal centered one-and two-electron oxidized complexes may possibly be generated in the similar potential range.…”

“…The small potential differences are due to the substitution of the phenolate moiety and the ligand structure. However, there are some different electronic structures among the one-electron oxidized forms of the complexes in Table 1, i.e., a M III -phenolate ground state complex, a M II -phenoxyl radical where the radical electron is fully delocalized on two phenolate moieties, a relatively localized M II (phenolate)(phenoxyl) complex, and a M II (dinitrogen ligand radical)(phenolate)2 species [39][40][41][42][43][44][45][46][47]. The similarity of the potentials indicates that formation of all of the oxidized species is due to a simple electron transfer without significant structural changes.…”

“…The present argument is focused on recent advances in the chemistry related with the synthesis, characterization, and reactivity of some of the one-electron oxidized metal(II)-salen type complexes, especially the complexes of group 10 metal(II) ions, Ni(II), Pd(II), and Pt(II) [39][40][41][42][43], and Cu(II) complexes [44][45][46][47] (Figure 3). Various useful and interesting salen-type complexes have been synthesized and characterized until now [48], and detailed electronic structural studies have recently been reported for a dinuclear chelating salen-type ligand having a catechol and a tetra(amino)tri(hydroxy)phenyl moiety [49,50].…”