Two-Electron Reductive Carbonylation of Terminal Uranium(V) and Uranium(VI) Nitrides to Cyanate by Carbon Monoxide

Two-electron reductive carbonylation of the uranium(VI) nitride [U(TrenTIPS)(N)] (2, TrenTIPS=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(TrenTIPS)(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(TrenTIPS)] (4) and KNCO, or cyanate retention in [U(TrenTIPS)(NCO)][K(B15C5)2] (5, B15C5=benzo-15-crown-5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(TrenTIPS)(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(TrenTIPS)(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f-block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol−1 for uranium(VI) and uranium(V), respectively. A remarkably simple two-step, two-electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.

Method B: C6D6 (0.5 ml) was added to a Young's Tap NMR tube charged with [{U(Tren TIPS )(N)K}2] (6, see below) (15.3 mg, 8.5 µmol). The solution was freeze-thaw degassed, then exposed to an atmosphere of CO at -78 °C. As the suspension thawed, a dark blue-purple solution formed. 1 H NMR studies indicated 90% conversion to 4.
Control experiment: C6D6 (0.5 ml) was added to a Young's Tap NMR Tube charged with 4 (21.0 mg, 24.7 µmol) and KNCO (3.0 mg, 37.0 µmol). The sample was heated at 60 °C for 16 hours. 1 H NMR studies and no observable colour change indicated no reaction had taken place.
The identity of 4 was confirmed by comparison to previously reported methods. 1

Synthesis of [U(Tren TIPS )(NCO)][K(B15C5)2] (5)
Method A: C6D6 was added to a mixture of 3 (19 mg, 21 µmol) and KC8 (4 mg, 30 µmol). The mixture was heated to 50 °C for 1 min to afford a dark blue-purple suspension. Benzo-15-crown-5 (13 mg, 48 µmol) was added and an immediate colour change to dark green was observed. The mixture was filtered and volatiles removed in vacuo to afford 5 as a dark green oily solid. Yield:

g, 93%
Method B: Toluene (20 ml) was added to a stirring mixture of benzo-15-crown-5 (1.60 g, 5.95 mmol) and KNCO (0.241 g, 2.97 mmol). The mixture was added dropwise to a cold (-78 °C) stirring solution of 4 (2.53 g, 2.97 mmol) in toluene (20 ml). The mixture was allowed to warm to room temperature with stirring over 16 hours. The solution was filtered and volatiles were removed in vacuo to afford 5 as a dark green oil. Storage of this oil at room temperature for 72 hours yielded a small crop of dark green crystals of 5. Alternatively, suspension of the oil in hexanes (10 ml), followed by storage at -80 °C for 3 hours yielded 5 as a dark green solid. Yield 1.95 g, 45%.

S5
Method C: A solution of [U(Tren TIPS )(N)][K(B15C5)2] (7, see below) (0.79 g, 0.55 mmol) in toluene (10 ml) was degassed, cooled to -78 °C and exposed to an atmosphere of CO. The solution immediately turned dark green and was allowed to warm slowly to room temperature with stirring over 16 hours. Volatiles were removed in vacuo to yield a dark green oil. Hexanes (10 ml) were added and the suspension stored at -80 °C for 3 hours to yield 5 as a dark green solid. Yield: 0.76 g,

Synthesis of [{U(Tren TIPS )(N)K}2] (6)
Method A: A solution of 2 (0.12 g, 0.14 mmol) in toluene (15 ml) was added dropwise to KC8 (0.02 g, 0.15 mmol) at -78 °C with stirring. The mixture was allowed to warm to room temperature with stirring over 16 hours. The mixture was allowed to settle (1 hour) and carefully filtered to afford a dark brown solid that was washed with toluene (3 × 5 ml) until the washings obtained were colourless, at which point these were discarded. The solid was then extracted into hot benzene and quickly filtered through a frit to remove the graphite precipitate. The filtrate was stored at 7 °C for 16 hours to yield dark red crystals of 6 which were isolated by filtration and dried in vacuo. Yield: 0.20 g, 16%.
Method B: Benzene (5 ml) was added to a stirring mixture of [{U(Tren TIPS )(N)Na}2] (0.32 g, 0.18 mmol) and KO t Bu (0.04 g, 0.36 mmol). The mixture was heated to 80 °C for 2 hours and allowed to cool affording a red precipitate. The precipitate was isolated by filtration, extracted into boiling benzene and filtered through a frit. The filtrate was stored at 7 °C yield 6 as red/orange crystals.

Synthesis of [U(Tren TIPS )(N)][K(B15C5)2] (7)
Toluene (25 ml) was added dropwise to a cold (-78 °C) stirring mixture of 6 (1.63 g, 0.90 mmol) and benzo-15-crown-5 (0.97 g, 3.60 mmol). The red/brown mixture was allowed to warm to room temperature with stirring over 16 hours. The mixture was heated to ~80 °C for 5 mins, and allowed to cool slowly to room temperature and then filtered. The solvent was removed in vacuo to yield a brown solid. The product was washed with hexanes (2 x 10 ml) and dried vacuo to yield 7 as a brown powder. Red crystals of 7 were isolated from a concentrated solution of 7 in toluene. Yield:
Complexes 3, 5, 6, and 7 were characterized additionally by solid state and variable temperature magnetometry in order to confirm the oxidation states of the uranium ions (see below). The data for the uranium(V)-nitride potassium salts 6 and 7 are similar to those reported for their sodium analogues. 1

Computational Details
The assignment of N-bound cyanates in 3 and 5 was supported by DFT calculations. Unrestricted geometry optimizations were performed for the full models of 3 and the anion component of 5, using coordinates derived from the X-ray crystal structures of 3 and 5 either as the N-or O-bound cyanate isomers. No constraints were imposed on the structures during the geometry optimizations.
The calculations were performed using the Amsterdam Density Functional (ADF) suite version 2012.01. 4,5 The DFT geometry optimizations employed Slater type orbital (STO) triple-ζ-plus polarization all-electron basis sets (from the ZORA/TZP database of the ADF suite). Scalar relativistic approaches were used within the ZORA Hamiltonian for the inclusion of relativistic effects and the local density approximation (LDA) with the correlation potential due to Vosko et al 6 S12 was used in all of the calculations. Gradient corrections were performed using the functionals of Becke 7 and Perdew. 8 All the structures involved in the reductive carbonylation reaction profile were fully optimized with the Becke's 3-parameter hybrid functional combined with the non-local correlation functional provided by Perdew/Wang (denoted as B3PW91). 9,10 The basis set used for uranium is the Stuttgart-Dresden small core RECP (relativistic effective core potential) in combination with its adapted basis set. 11,12 Si atoms were treated with the corresponding Stuttgart-Dresden RECP in combination with their adapted basis sets, 13 each one augmented by an extra set of polarization functions. 14,15 For the rest of the atoms the 6-31G(d,p) basis set was used. [16][17][18] In all computations no constraints were imposed on the geometry. All stationary points have been identified for minimum (number of imaginary frequencies Nimag=0) or transition states (Nimag=1). Intrinsic Reaction Paths (IRPs) were traced from the various transition structures to verify the reactant to product linkage. 19,20 GAUSSIAN09 program suite was used in all the calculations of the structures involved in the reductive carbonylation reaction profile. 21