Radical anionic versus neutral 2,2′-bipyridyl coordination in uranium complexes supported by amide and ketimide ligands

Abstract

A series of 2,2′-bipyridyl uranium complexes shows that bipy is found as a radical anion or a neutral ligand such that uranium is in the +4 oxidation state.

[U(bipy)4]: A Mistaken Case of U0?

“…The C py −C py distances of 1.425(4)–35(5) Å in 1 are clear‐cut and all within the 1.43 Å range established for the bipy .− anion and comparable to those found in [U IV [N( t Bu)Ar] 2 (bipy .− ) 2 ] (C py −C py =1.429(7), 1.426(7) Å) . Moreover, the NCCN dihedral angles in 1 range from 0.4–2.2°, consistent with the increased planarity that occurs within the bipy ligand upon one‐electron reduction .…”

“…The diagnostic, oxidation-stated ependentf !ft ransition bands of uranium are not visible in the UV/Vis/NIR spectra of 1 and 2.I nstead, in both spectra of 1 and 2,t hese bands are obscured by ac omparatively intense, broad absorption in the 600-1000 nm region with an accompanying band found between 300-400 nm, [16] signature features of the bipy À C p!p* transitions. [15] The magnetic properties of 1 and 2 werei nvestigated using aS QUID magnetometer.T he dc magnetic susceptibility measurements of 1 and 2 were performed on crushed polycrystal-line samples in the temperature range of 1.9-300 K, under an appliedd cf ield of 1000 Oe (Figure 3). The room temperature c M T value of 2.37 cm 3 Kmol À1 for 1 strongly suggests the presence of aU IV ion ( 3 H 4 , S = 1, L = 5) with four bipyC À radicals.…”

“…Instead, in both spectra of 1 and 2 , these bands are obscured by a comparatively intense, broad absorption in the 600–1000 nm region with an accompanying band found between 300–400 nm, signature features of the bipy −. π→π* transitions …”

“…The C py ÀC py distances of 1.425(4)-35(5) in 1 are clear-cut and all within the 1.43 r ange established for the bipyC À anion and comparable to those found in [U IV [N(tBu)Ar] 2 (bipyC À ) 2 ]( C py À C py = 1.429(7), 1.426 (7) ). [15] Moreover,t he NCCN dihedral angles in 1 range from 0.4-2.28,c onsistent with the increased planarity that occurs within the bipy ligand upon one-electron reduction. [10a, 15] Thus, based upon the structurale vidence alone, 1 is bestd escribed not as U 0 but as U IV ligatedb yf our radicalb ipy monoanions in the form [U IV (bipyC À ) 4 ].…”

Double Reduction of 4,4′‐Bipyridine and Reductive Coupling of Pyridine by Two Thorium(III) Single‐Electron Transfers

“…[14] Germane to this study Th IV complexes containing direduced 2,2′-bipy have been reported by Walter and Zi [15][16][17] and Arnold, [18] and uranium complexes containing mono-reduced 2,2′-bipy have been reported by Bart [19] and Cummins. [20] Two equivalents of 1 doubly reduce 4,4′-bipy and reductively couple two equivalents of pyridine to yield [{Th(Cp′′)3}2{µ-(NC5H4)2}] (2) and [{Th(Cp′′)3}2{µ-(NC5H5)2}] (3), respectively (Scheme 1). Although selected Ln II (Ln = Sm, Tm) [12c-e] and Sc III arene [21] complexes can reductively couple pyridine, Ephritikhine and co-workers have shown by single crystal XRD studies that the comparable U III complexes [U(Cp′)3] and [U(Cp t )3] (Cp t = {C5H4 t Bu}) do not reduce pyridine [14,22] or 4,4′-bipy.…”