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

Abstract

The redox chemistry of uranium is burgeoning and uranium(III) complexes have been shown to promote many interesting synthetic transformations. However, their utility is limited by their reduction potentials, which are smaller than many non-traditional lanthanide(II) complexes. Thorium(III) has a greater redox potential so it should present unprecedented opportunities for actinide reactivity but as with uranium(II) and thorium(II) chemistry, these have not yet been fully realized. Herein we present reactivity studies of two equivalents of [Th(Cp'') ] (1, Cp''={C H (SiMe ) -1,3}) with 4,4'-bipyridine or two equivalents of pyridine to give [{Th(Cp'') } {μ-(NC H ) }] (2) and [{Th(Cp'') } {μ-(NC H ) }] (3), respectively. As relatively large reduction potentials are required to effect these transformations we have shown that thorium(III) can promote reactions that uranium(III) cannot, opening up promising new reductive chemistry for the actinides.

Delivery of a Masked Uranium(II) by an Oxide‐Bridged Diuranium(III) Complex

“…The value of the C py ‐C py bond distance of 1.380(5) Å in 6 is diagnostic of the inter‐pyridyl double bond found in doubly reduced 4,4′‐bpy [32] . This value is similar to the value (1.376(10) Å) found in the Th IV /(4,4′‐bipyridinyl) 2− complex [{(C 5 H 3 (SiMe 3 ) 2 ) 3 Th} 2 {μ‐(4,4′‐bpy)}] recently obtained from the reduction of 4,4′‐bpy by a Th III complex [15] . The C py ‐C py bond distance in 6 is also significantly shorter than the 1.47–1.48 Å range of C py ‐C py bond distances reported for An IV adducts of neutral 4,4′‐bpys [17a, 33] …”

“…Therefore, the coordination of pyridine induces a formal disproportionation of the U III ‐O‐U III complex into a U IV and a formal U II complex that reduces pyridine to afford the radical coupling of two pyridine rings. Radical coupling of two reduced pyridine rings was reported previously in low oxidation state lanthanide [13f] and thorium [15] chemistry.…”

“…Pyridine reductive coupling by metal complexes is more rarely observed than bipyridine reduction [27] . Examples of pyridine reduction by Sc‐arene complexes, [28] samarium(II), [13f] thulium(II), [14] and thorium(III) [15] have been reported, but no examples of pyridine reduction by uranium complexes are known. Interestingly, complex 3 reduces pyridine, making it the first U III complex able to effect this type of reactivity.…”

“…F‐element complexes of reduced N‐heterocycles have attracted attention for their ability to promote magnetic coupling between lanthanide centers, [11] and their ability to store electrons that can become available for small molecule activation [12] . Several examples of N‐heterocycles reduction including pyridine and bipyridine have been reported in lanthanide(II) [13, 14] and in thorium(III) [15] chemistry. Uranium(III) complexes have been reported to promote the reductive coupling of polyazines such as pyrazine and 2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine, [16] but reduction of pyridine, 2,2′‐bipyridine (2,2′‐bpy) and 4,4′‐bipyridine (4,4′‐bpy) was not observed for U III complexes supported by cyclopentadienyl or hydrotris(3,5‐dimethylpyrazolyl) borate ligands [12c, 17] .…”

Actinide Organometallic Complexes with π‐Ligands

“…The low oxidation state compounds of the actinides show of course ah ighr eactivity towards electrophiles. [19] So Th(Cp") 3 is able to activate and to reactw ith white phosphorous, transferring electrons to it;o ri tr eacts with CO 2 or CS 2 as well under electron transfer,r educing the CO 2 to oxalate in the coordination sphere of the metal.E lectrons also can be transferred to organic molecules such as 4,-4'-bipyridine or to pyridine itself, the latter one then undergoing CÀCc oupling reaction.…”