Stepwise Reduction of Dinitrogen by a Uranium–Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

Abstract

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N3–), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K2{[UV(OSi­(OtBu)3)3]2(μ-O)­(μ-η2:η2-N2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K6{(OSi­(OtBu)3)2UIV}3­{(OSi­(OtBu)3)2UVI}­(μ4-N)3­(μ3-N)­(μ3-O)2], 1, a novel example of N2 cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-NUVIN core analogue of UO2 2+. Experimental and computational studies indicate that the formation of the U­(IV)/U­(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N2 4– ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium­(V) bis-nitride [K4{[UV(OSi­(OtBu)3)3]2­(μ-O)­(μ-N)2}], X. Additionally, cooperative potassium binding to the U-bound N2 4– ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93–97% yield and with excess 13CO to yield K13CN and KN13CO. The structures of two tetranuclear U­(IV)/U­(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Heterometallic Clusters with Uranium–Metal Bonds Supported by Double-Layer Nitrogen–Phosphorus Ligands

“…Molecular uranium complexes exhibit an attractive ability for small-molecule activation. , Although a series of examples of N 2 binding or activation by molecular uranium species have been reported in recent decades, − only one example of N 2 cleavage has been achieved . Recently, we reported an example of N 2 cleavage by a U­(III)–P­(III) synergetic effect under ambient temperature and pressure .…”

Cation assisted binding and cleavage of dinitrogen by uranium complexes

“…As a result, the dinitrogen chemistry of molecular uranium compounds remains less developed 1 j , m ,3 than d-block metals, and only three examples of dinitrogen cleavage were reported so far (complexes A–C in Chart 1) where multimetallic cooperativity between uranium and s- or d-block metals is thought to play an important role. 4…”

“…The few multimetallic uranium–alkali ion systems reported so far that effect the four-electron reduction of N 2 , 1 j , m ,3 h or even its complete cleavage to nitride, 4 all contain the potassium cation. It should be noted that although the number of reported uranium nitride complexes is rapidly increasing 12,13 only three examples of uranium nitrides have been obtained from N 2 reduction (Chart 1).…”

Uranium–nitride chemistry: uranium–uranium electronic communication mediated by nitride bridges

“… 8 To date, however, mid- or low-oxidation state ‘diamond-core’ dinitrides are rare, being restricted to trivalent binary U III 2 N 2 studied in cryogenic matrix isolation experiments, 9 a molecular tetrauranium( iv )–dirhodium cluster, 10 a mixed-valent diuranium( IV/V )–calix[4]tetrapyrrole complex, 11 and a tetrauranium( iv ) cluster. 12 There are, however, no examples of diuranium( iv ) complexes exhibiting the ‘diamond-core’ dinitride motif.…”