Synthesis of Uranium Isocyanoimido Complexes

Heating a toluene solution of (trimethylsilyl)diazomethane, Me₃SiCHN₂, and the uranium terminal imido metallocene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (1) at 80 °C, gives after recrystallization from an n-hexane solution the bimetallic isocyanoimido uranium complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(=NNC)(μ-CNN=)U(dmap)[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂ (2) in 76 % isolated yield (Scheme 2). Furthermore, complex 2 shows notable thermal stability with a decomposition temperature in the solid state above 230 °C. Two mechanistic scenarios accounting for the formation of 2 can be envisioned: (A) Similar to the reactivity of the thorium imido metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=N(p-tolyl) with Me₃SiCHN₂,^8d an initial deprotonation occurs between 1 and Me₃SiCHN₂ yielding a nitrilimido intermediate concomitant with dmap release (Scheme 2). In a second step, the nitrilimido complex converts via p-tolylNHSiMe₃ elimination to a terminal isocyanoimido complex, which dimerizes to the product 2 in the presence of dmap (Scheme 2; left). (B) Alternatively, after formation of the nitrilimido intermediate a [1,3]-Si migration may occur to yield an unstable isocyanoamido intermediate,^{5b, 7, 10} eliminating p-tolylNHSiMe₃ to give the terminal isocyanoimido compound, which in the presence of dmap readily dimerizes to the final product 2 (Scheme 2; right). To evaluate the feasibility of both pathways, DFT studies were performed. After initial dmap dissociation from 1 the imido intermediate INTa is formed (Figure 1). Coordination of Me₃SiCHN₂ to INTa forms the adduct COM, followed by intramolecular deprotonation via transition state TSa to yield the nitrilimido intermediate INTb. The two sterically encumbered 1,2,4-(Me₃Si)₃C₅H₂ ligands prevent the [1,3]-Si migration, but instead INTb eliminates p-tolylNHSiMe₃ via the transition state TSb to give a terminal isocyanoimido intermediate INTc, which gives in the presence of dmap the product 2. The formation of 2 is exergonic with ΔG(353 K)=−120.8 kJ mol⁻¹ and the reaction barrier is ΔG^≠(353 K)=125.8 kJ mol⁻¹ (Figure 1), that can be overcome under the experimental reaction conditions. Furthermore, DFT computations also predict that the dimer formation is energetically much more favorable than a hypothetic monomeric dmap adduct [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(=NNC)(dmap) (2′) (ΔG(353 K)=−95.7 kJ mol⁻¹) (see the Supporting Information for details), which is also consistent with the experiment. The molecular structure of 2 is shown in Figure 2, and represents, to the best of our knowledge, the first structurally authenticated isocyanoimido complex. The dimeric complex 2 contains a μ-κN : κC bridging isocyanoimido group and a terminal κN-isocyanoimido ligand. The U(1) atom is η⁵-bound to two eclipsed Cp-rings, and it coordinates κN to the terminal nitrogen atom of a [NNC]²⁻ group and κC to the carbon atom of the other [CNN]²⁻ group in a distorted-tetrahedral fashion. The average U(1)−C(ring) distance is 2.788(15) Å, and the Cp(cent)−U(1)−Cp(cent) and N(1)−U(1)−C(58) angles are 132.8(2)° and 88.6(2)° (Table 1), respectively. The U(2) atom is also η⁵-coordinate to two eclipsed Cp-rings, but it is bound to two nitrogen atoms, one nitrogen atom of a [NNC]²⁻ group and the pyridine nitrogen atom of the dmap ligand forming a distorted-tetrahedral structure. The average U(2)-C(ring) distance of 2.786(15) Å is essentially identical to that at U(1), whereas slight deviations are observed for the Cp(cent)−U(2)−Cp(cent) and N(3)−U(2)−N(5) angles of 131.1(2)° and 92.6(2)°, respectively. The short U(1)−N(1) and U(2)−N(3) distances of 2.035(4) Å and 2.067(4) Å, respectively, are comparable to the U=N distances found in the uranium imido complex 1 (2.021(5) Å),⁹ and the diazoalkyl compounds [(2-ⁱPr₂P-4-MePh)₂N]₂U=N₂CPh₂ (2.097(5) Å),^11a (η⁵-C₅Me₅)₂U(=N₂CPh₂)₂ (1.999(4) and 2.018(4) Å),^11a [ⁱPrNC(NⁱPr₂)NⁱPr]₃U=N₂CPh₂ (2.060(3) Å),^11b and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(bipy)(=N₂CHSiMe₃) (2.014(4) Å).^11c Nevertheless, the significantly longer U(2)-N(5) distance of 2.481(5) Å between the dmap ligand and the U(IV) atom is comparable to the U−N(dmap) distance established in 1 (2.426(6) Å),⁹ which is in line with a dative bond. Furthermore, the U(1)−C(58) distance of 2.548(5) Å is markedly longer than the U−C(Me) bonds found in [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂UMe₂ (2.345(11) and 2.399(8) Å)^8a but comparable to those observed in {[(Me₃SiNCH₂CH₂)₃N]U[μ-N(SiMe₃)NC]}₂ (2.540(2) Å)^10c and [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(bipy)(2,6-Me₂PhNC) (2.582(6) Å),¹² consistent with a dative coordination of the carbon atom to the U atom. In addition, the N(1)−N(2) (1.300(6) Å) and N(3)−N(4) (1.284(6) Å) distances are elongated compared to that in free diazoalkanes (1.12–1.13 Å),¹³ but similar to those found in diazoalkyl compounds [(2-ⁱPr₂P-4-MePh)₂N]₂U=N₂CPh₂ (1.359(7) Å),^11a (η⁵-C₅Me₅)₂U(=N₂CPh₂)₂ (1.315(6) and 1.324(5) Å),^11a [ⁱPrNC(NⁱPr₂)NⁱPr]₃U=N₂CPh₂ (1.299(4) Å),^11b and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(bipy)(=N₂CHSiMe₃) (1.332(6) Å).^11c Moreover, the C(57)−N(2) and C(58)-N(4) distances of 1.169(8) Å and 1.172(7) Å, respectively, are identical within the 3σ-criterion. These structural parameters are consistent with the structure C (Scheme 1). Furthermore, the N(1)−N(2)−C(57) and N(3)−N(4)−C(58) angles are close to linearity with 177.9(6) and 179.0(6)°, respectively, whereas the U(1)−N(1)−N(2), U(2)−N(3)−N(4) and U(1)−C(58)−N(4) angles are slightly smaller with 175.9(4), 168.7(4) and 167.9(4)°, respectively.

In contrast, the less sterically hindered the uranium imido metallocene [η⁵-1,3-(Me₃C)₂C₅H₃]₂U=N(p-tolyl)(dmap) (4), derived from [η⁵-1,3-(Me₃C)₂C₅H₃]₂UMe₂ (3) and p-tolylNH₂ in the presence of dmap, reacts with Me₃SiCHN₂ in toluene at 50 °C to form a different product. Instead of a bimetallic complex, the hexametallic isocyanoimido uranium cluster {[η⁵-1,3-(Me₃C)₂C₅H₃]₂U[μ-(=NNC)]}₆ (5) is isolated in good yield (Scheme 3). This difference is attributed to the reduced steric hindrance at the uranium atom in 4. Figure 3 shows the molecular structure of 5. Each [η⁵-1,3-(Me₃C)₂C₅H₃]₂U fragment coordinates κN to to the terminal nitrogen atom of a [NNC]²⁻ group and κC to the carbon atom of another [CNN]²⁻ group, resulting in a self-assembly of six {[η⁵-1,3-(Me₃C)₂C₅H₃]₂U}²⁺ cations and six [NNC]²⁻ anions to form the hexameric macro-ring¹⁴ with U(1)−U(3A), U(1)−U(2) and U(2)−U(3) distances of 6.933(1), 6.935(1) and 6.941(1) Å, respectively. The coordination environment at the uranium atom is best described as distorted-tetrahedral. The U(1)−N(6A), U(2)−N(2) and U(3)−N(4) distances are 2.130(18), 2.086(19) and 2.143(19) Å (Table 1), respectively, whereas the U(1)−C(27), U(2)−C(54) and U(3)−C(81) distances are 2.48(3), 2.43(2) and 2.380(17) Å, respectively. Overall, these structural parameters are comparable to those observed in complex 2. Furthermore, complexes 2 and 5 exhibit the ν(C=N=N) stretching frequencies at 2117 cm⁻¹ and 1990 cm⁻¹, respectively, which are in line with those (1950–2035 cm⁻¹) detected in other diazoalkyl complexes.^5b

Bonding Studies

Density functional theory (DFT) computations at the B3PW91 level of theory were performed on the model complex [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(=NNC) (5′) to evaluate the bonding between the [η⁵-1,3-(Me₃C)₂C₅H₃]₂U²⁺ and [NNC]²⁻ fragments. These studies identified one U−N σ-bond and two U−N π-bonds as illustrated in Figure 4. The natural bond orbital (NBO) analysis reveals that U−N σ-bond, σ(U=N), is formed by a nitrogen hybrid lone pair (63.8 % 2 s and 36.2 % 2p) donating to a vacant U orbital. The two orthogonal U−N π bonds, π₁ and π₂, have similar compositions and consist of a pure 2p nitrogen-based orbital (78.3 %) and a uranium hybrid orbital (21.7 %; 1.7 % 7p+52.6 % 6d+45.7 % 5f) and a pure 2p nitrogen-based orbital (74.4 %) and a uranium hybrid orbital (25.6 %; 0.4 % 7p+25.7 % 6d+73.9 % 5f), respectively. These values are comparable to those computed for the imido metallocene (η⁵-C₅H₅)₂U=NMe.^15a Within this description additional electron density is transferred from the π-orbitals of [NNC]²⁻ fragment to the electron deficient metallocene unit [η⁵-1,3-(Me₃C)₂C₅H₃]₂U²⁺. Moreover, the calculated natural charges for U, N_α, N_β and C_γ are 1.44, −0.72, −0.28 and 0.08, respectively, whereas the averaged natural charge for the two 1,3-(Me₃C)₂C₅H₃ ligands is −0.26. The Wiberg bond orders are 1.51 for U−N_α, 1.17 for N_α-N_β, and 2.32 for N_β-C_γ, respectively, consistent with an isocyanidoimido resonance structure (Scheme 1, C). Based on these computations a significant involvement of the uranium 5 f and 6d orbitals in the bonding between the metallocene [η⁵-1,3-(Me₃C)₂C₅H₃]₂U²⁺ and [NNC]²⁻ fragment can be assumed as previously shown in imido metallocene (η⁵-C₅H₅)₂U=NMe,^15a indicating the 5 f as well as the 6d orbitals participate in the bonding of actinide complexes and that the U=N bonds are generally more polarized than those in related d-transition metals.^15b Furthermore, a multiconfigurational, CASSCF computation on the geometry optimized at the DFT level of theory gives a triplet electronic ground state comparable to that found in (η⁵-C₅H₅)₂U=NMe.^15a Similar to (η⁵-C₅H₅)₂U=NMe^15a the triplet ground state is composed of two determinants (85 % and 15 %), indicating that this state is properly described by a single reference configuration.^15a Moreover, the geometry optimization at the CASSCF level gives a geometry identical to that obtained at the DFT level of theory, further validating the use of DFT methods for [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(=NNC) (5′).

Alternatively, the bonding in complex 5′ may also be analyzed using the quantum theory of atoms in molecules (QTAIM), which is based on the topology of the electron density at a bond critical point (BCP).⁹ QTAIM data (ρ_b=0.156, H_b=−0.078) are in line with a polarized U⁺−N⁻ bond, but also possesses covalent character. Moreover, the calculated delocalization indices (DI=2.29) indicate significantly accumulated electron density in the U−N bonding region of the complex 5′. Hence, the QTAIM and NBO analyses arrive at similar conclusions.

Reactivity Studies

The polarized nature of the U=NNC moiety may also impact its reactivity, and it is instructive to compare this reactivity to that of the imido compound [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (1).⁹ The bimetallic isocyanoimido uranium complex 2 is surprisingly stable and has so far proven to be inert to a series of small molecules such as Me₃SiX (X=Cl, I), CS₂ and RC≡CR (R=Ph, Me) even when heated to 100 °C for one week. This lack of reactivity is presumably a consequence of the effective shielding of the uranium atoms by the Cp ligands hindering the substrate approach to the uranium atom.

In contrast, treatment of 5 with Me₃SiI gives the diiodido complex [η⁵-1,3-(Me₃C)₂C₅H₃]₂UI₂ (6) concomitant with (Me₃Si)₂CN₂ loss (Scheme 3). Most likely, this reaction proceeds via a monoiodido intermediate [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(NNCSiMe₃)(I) but it readily converts in the presence of a second molecule of Me₃SiI to yield 6 and (Me₃Si)₂CN₂ (Scheme 3). The U⁴⁺ atom in 6 is η⁵-bound to two eclipsed Cp-rings and coordinates to two iodide ligands in a distorted-tetrahedral fashion (Figure 5) with an average U−C(ring) distance of 2.721(18) Å, the Cp(cent)−U−Cp(cent) angle of 127.8(2)° and the I−U−I angle of 101.2(1)° (Table 1). The U−I(1) and U−I(2) distances are rather asymmetric with 2.986(1) and 2.936(1) Å, respectively, but they are comparable to those found in (η⁵-C₅Me₅)₂UI₂ (2.981(1) Å and 2.987(1) Å),^16a (η⁵-C₅ⁱPr₄H)₂UI₂ (2.964(1) Å and 2.973(1) Å),^16b [η⁵-C₅(CH₂Ph)₅]₂UI₂ (2.962(1) Å and 2.948(1) Å),^16c and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UI₂ (2.966(1) Å and 2.952(1) Å).^11c

Moreover, like the uranium imido compound [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (1),⁹ complex 5 is reactive toward hetero-unsaturated organic substrates. For example, addition of a toluene solution of CS₂ to 5 in the presence of dmap forms the bis(isothiocyanate) uranium metallocene, [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(NCS)₂(dmap)₂ (7) in good yield (Scheme 4).

We propose that under these conditions an initial dissociation of 5 to form monomeric intermediate [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(=NNC) (5′) occurs, followed by a reaction with CS₂ proceeding via a [2+2] cycloaddition to give a four-membered intermediate A (Scheme 4; down), which then converts to the thioadiazolonediide intermediate B. In a next step, intermediate B undergoes N−N and C−S bond cleavage to yield a bis-isothiocyanate complex C, that rapidly reacts with dmap to give the final product 7 (Scheme 4; down). Alternatively, complex 5 (5′) may directly convert with CS₂ via a [2+3] cycloaddition to give the thioadiazolonediide intermediate B (Scheme 4; up), and cleavage of the N−N and C−S bonds result in the bis-isothiocyanate complex C, which then forms adduct 7 in the presence of dmap (Scheme 4; up). To differentiate both pathways DFT computations were performed revealing that in contrast to the reactivity of the diazomethanediide-bridged platinum(II) complex [(^tBu₂PCHCH)₂NPt]₂(μ-κN : κC-CNN) with CO₂,⁷ a direct approach of CS₂ to the [U=NNC] moiety in [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(=NNC) (5′) does not initiate a [2+3] cycloaddition, instead, complex 5′ initially engages in a [2+2] cycloaddition with CS₂ to give a four-membered intermediate INT7a (Figure 6), which then converts to the thioadiazolonediide intermediate INT7b. In a next step, intermediate INT7b isomerizes barrierless to INT7c, that undergoes simultaneous N−N and C−S cleavage to yield the bis-isothiocyanate INT7d, which forms 7 in the presence of dmap. The formation of 7 is energetically extremely favorable (ΔG(298 K)=−513.3 kJ mol⁻¹) and occurs with a low activation barrier of only ΔG^≠(298 K)=7.1 kJ mol⁻¹ (Figure 6). The U⁴⁺ atom in 7 is η⁵-bound to two eclipsed Cp-rings and κN-coordinated to four nitrogen atoms to form a distorted-octahedral coordination sphere (Figure 7) with an average U−C(ring) distance of 2.808(26) Å, the Cp(cent)−U−Cp(cent) angle of 111.5(3)° and the N(1)−U−N(1A) angle of 145.2(4)° (Table 1). The U−N(1) distance to the ionic isothiocyanate ligands is 2.388(8) Å, whereas the U−N(2) distance to the neutral dmap ligands is much longer with 2.646(8) Å. Moreover, the N(1)−C(14) and C(14)−S(1) distances are 1.171(12) and 1.611(11) Å, respectively, and the angles of U(1)−N(1)−C(14) and N(1)−C(14)−S(1) are 172.2(8) and 177.6(9)°, respectively. Furthermore, complex 7 exhibits the ν(N=C=S) stretching frequency at 2021 cm⁻¹, which is in line with those (1980–2200 cm⁻¹) established in isothiocyanate complexes,^17a and comparable to that (2047 cm⁻¹) found in [Bu₄N]₄[U(NCS)₈].^17b However, in contrast to the imido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (1),^9b no reaction occurs between complex 5 and internal alkynes RC≡CR or RC≡CC≡CR (R=Ph, Me) even when heated to 100 °C for one week, presumably due to the more polarized nature of the uranium-isocyanoimido bond U⁺-[NNC]⁻.^15b