Bonding Interactions in Uranyl α-Diimine Complexes: A Spectroscopic and Electrochemical Study of the Impacts of Ligand Electronics and Extended Conjugation

Abstract

Uranyl complexes of aryl-substituted α-diimine ligands gbha (UO 2 -1a−f) and phen-BIAN (UO 2 -2a-f) [gbha (1) = glyoxal bis(2hydroxyanil); phen-BIAN (2) = N,N′-bis(iminophenol)acenaphthene; R = OMe (a), t-bu (b), H (c), Me (d), F (e), and naphthyl (f)] were designed, prepared, and characterized by X-ray diffraction, FT-IR, NMR, UV−vis, and electrochemical methods. These ligand frameworks contain a salen-type O− N−N−O binding pocket but are redox-noninnocent, leading to unusual metal complex behaviors. Here, we describe three solid-state structures of uranyl complexes UO 2 -1b, UO 2 -1c, and UO 2 -1f and observe manifestations of ligand noninnocence for the U(VI) complexes UO 2 -1b and UO 2 -1c. The impacts of accessible π-systems and ligand substitution on the axial uranium−oxo interactions were evaluated spectroscopically via the intraligand charge-transfer (ILCT) processes that dominate the absorption spectra of these complexes and through changes to the asymmetric (ν 3 ) OUO stretching frequency. This, in combination with electrochemical data, reveals the effects of the inclusion of the conjugated acenaphthene backbone and the importance of ligand electronic structure on uranyl's bonding interactions.

Utility of redox-active ligands for reversible multi-electron transfer in uranyl(vi) complexes

“…Although CH 2 Cl 2 is incorporated within the crystal lattice, it merely provides outer sphere solvation proximal to the [U VI O 2 (L3)] unit forming the closest contact between them (2.37 Å). The bond lengths between U and O ax (U(1)-O(1) and U(1)-O(2) = 1.789-1.794 Å) and are again comparable to those typically found in ordinary U VI O 22+ complexes reported elsewhere [43][44][45][46]57,76. The U(1)-O(3) and U(1)-O(4) bond lengths (2.235-2.250 Å) were slightly shorter than those in [U VI O 2 (L1) C 5 H 5 N] and [U VI O 2 (L2)H 2 O], whereas the azomethine bonds (C(7)-N(1) and C(13)-N(3) = 1.286 Å) of [U VI O 2 (L3)] were quite commonly found in the CvN double bond lengths of U VI O 2 2+…”

“…All the previous structural features agree with those of the U VI O 2 2+ complexes with the glyoxal-bis(2-hydroxyanil) derivatives reported previously. 57,76…”

“…All the previous structural features agree with those of the U VI O 2 2+ complexes with the glyoxal-bis(2-hydroxyanil) derivatives reported previously. 57,76 The reaction of N,N′-bis-(3,5-di-tert-butyl-2-hydroxyphenyl)- As summarized in Table S2 (ESI †), all the bond angles around U(1) of this complex and deviations of the coordinating atoms from the mean equatorial plane were quite similar to those of [U VI O 2 (L1)C 5 H 5 N], which indicated that the introduction of the phenyl backbone to the diimine-based systems (Scheme 1(a) and (b)) had little impact on the actual coordination structures of their U VI 64 Indeed, the 1 H-NMR and IR spectra of [U VI O 2 (L3)] in DMSO-d 6 (Fig. S8 and S9, ESI †) were in good agreement with those in our previous report.…”

Synthesis, Covalency Sequence, and Crystal Features of Pentagonal Uranyl Acylpyrazolone Complexes along with DFT Calculation and Hirshfeld Analysis

“…Thus, asymmetric and symmetric uranyl stretching frequencies are an important concept in identifying the covalency order in uranyl complexes. 35 , 36 Figures S1–S3 show the FTIR spectra.…”

“…The relative effect of the U–O bond strength of the pentagonal plane in reverse order created due to these uranyl bonds is responsible for the covalency order of II > III > I for complexes. Thus, asymmetric and symmetric uranyl stretching frequencies are an important concept in identifying the covalency order in uranyl complexes. , Figures S1–S3 show the FTIR spectra.…”