Synthesis and reactivity of a uranium(IV) complex supported by a monoanionic nitrogen–phosphorus ligand

Abstract

A monoanionic nitrogen–phosphorus ligand (CH₃)₂NCH₂CH₂NHPⁱPr₂ (L3) was designed and the corresponding U(IV) chloride complex {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂UCl₂} (1) and U(IV) iodide complex {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂UI₂} (2) were readily synthesized. Complexes 1 and 2 were fully characterized and the reactivity of complex 1 was further investigated. Complex 3 {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂U(C₁₂H₈)} with a uranium cyclopentadiene unit was constructed by the reaction of 1 with 2,2′-dilithiobiphenyl, which is a rare example of a homoleptic metallafluorene containing an actinide element. In addition, a U(IV) bi-alkyl complex {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂U[o-N(CH₃)(CH₂)C₆H₄CH₂]} (4) was isolated by the reaction of complex 1 with two equivalents of o-N(CH₃)₂C₆H₄CH₂K. Moreover, the reactivity of 1 with a series of transition metal precursors was also investigated, from which heterometallic clusters {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂UCl₂(μ-Cl)RuCl}₂ (5), {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂UCl₂Rh(μ-Cl)}₂ (6) and {[(CH₃)₂NCH₂CH₂NPⁱPr₂]₂UCl₂Ir(μ-Cl)}₂ (7) were isolated. The U–Rh and U–Ir single bonds were observed in complexes 6 and 7, respectively. This study further confirms that the nitrogen–phosphorus ligand is an effective platform for the construction of species with U–M bonds.

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Introduction

Ligands are important in organometallic chemistry because of their significant influence on the structure and properties of organometallic complexes.^1,2 The synthesis of ferrocene supported by the cyclopentadienyl (Cp) ligand was considered to be a starting point of modern organometallic chemistry.³ The first example of a uranium organometallic complex, Cp₃UCl, was also stabilized by the Cp ligand.⁴ Based on hard–soft acid–base theory, the N and O atoms are good donors to actinide elements. Consequently, a series of new ligands with N and O coordination sites were designed and used in the development of actinide organometallic chemistry.^5–18 For instance, since the first example of an actinide complex supported by a triamidoamine ([(RNCH₂CH₂)₃N]³⁻) ligand (Tren) was reported by Scott and co-workers in 1994,¹⁹ actinide organometallic chemistry with Tren ligands has flourished.^18,20–26 With these trianionic ligands, a series of trivalent uranium complexes were isolated.^24,25,27,28

In 2019, we found that a novel phosphine-substituted Tren ligand (L1, Fig. 1), namely a double-layer N–P ligand, can also be used to stabilize uranium(III) complexes and construct a series of heterometallic clusters with multiple U–M single or triple bonds.^29–31 In a subsequent study, we found that the dianionic double-layer N–P ligand (L2, Fig. 1) is an effective platform for the construction of heterometallic clusters with multiple U–M single or double bonds and could be used to synthesize neutral uranium(II) species.^32–34 In 2022, Layfield and co-workers reported the first example of a uranium(I) complex supported by a monoanionic ligand, η⁵-C₅ⁱPr₅.³⁵ Very recently, Mazzanti and co-workers found that the –OSiPh₃ ligand could be used to stabilize uranium(I) synthons.³⁶ These U(I) complexes are ionic-type species supported by multiple monoanionic ligands. Therefore, we are curious if a neutral U(I) species could be stabilized by a monoanionic ligand with additional donor coordination sites. Based on our continuing interest in uranium chemistry supported by N–P ligands,^37–43 herein, we have designed a new monoanionic double-layer N–P ligand with a pendant dimethylamino group as an additional coordination site (L3, Fig. 1). However, an unanticipated uranium precursor (1) supported by two L3 ligands was isolated. The reactivity of complex 1 was investigated in this study. Interestingly, the two P atoms from two L3 ligands in complex 1 could be coordinated with the same transition metal, which is totally different from the heterometallic clusters with multiple U–M bonds supported by L1 and L2.

Fig. 1 Design of the monoanionic N–P ligand L3.

Results and discussion

Synthesis of U(IV) complexes 1 and 2

The monoanionic ligand L3 was prepared from the reaction of N,N-dimethylethylenediamine with one equivalent of ⁱPr₂PCl in the presence of excess Et₃N (see the ESI for details†). L3 was deprotonated with ⁿBuLi at 25 °C in THF for 3 h and then treated with UCl₄ overnight at room temperature (RT) (Scheme 1), from which complex 1 was isolated as a bright-green solid in 78% yield after recrystallization from toluene at −30 °C. We found that different equivalents of L3 do not change the product in this reaction.

Scheme 1 Synthesis of complexes 1 and 2.

Under similar conditions, deprotonated L3 could also react with UI₃(THF)₄,⁴⁴ leading to the formation of U(IV) complex 2 in 37% yield as a crystalline product (Scheme 1). We proposed that a U(III) complex supported by L3 was formed and disproportionated into U(IV) complex 2 and U(0) species, a phenomenon which has been observed previously.^45–48 Complex 2 could also be synthesized in 91% yield by the reaction of complex 1 with TMSI. The reactions forming complexes 1 and 2 are different from the reactions leading to tri- and tetra-substituted U(IV) products containing isopropyl (ⁱPr) and mestrimethylphenyl (Mes)-substituted monoanionic N–P ligands, ⁱPrNHPPh₂ and MesNHPⁱPr₂, which were reported by Bart, Thomas and co-workers.⁴⁹ This difference is probably due to the additional NMe₂ group that could be coordinated with the U center, thus avoiding the formation of tri- and tetra-substituted U(IV) products.

The ¹H NMR spectra of complexes 1 and 2 exhibit five signals between +58.0 and −74.4 ppm, consistent with reported U(IV) complexes supported by dianionic N–P ligands.^32,33 The signals for N–CH₃ in complexes 1 and 2 were observed at −70.5 and −74.4 ppm, respectively. No phosphorus signals were observed between +1000 and −1000 ppm, probably due to the paramagnetic nature of U(IV) species.

The molecular structure of complexes 1 and 2 was confirmed by single-crystal X-ray diffraction. As shown in Fig. 2 and 3, the uranium centre of these species is octa-coordinated with four N atoms and two P atoms from two monoanionic N–P ligands, and two halogen atoms. In complex 1, the bond lengths of U–N_amine (U1–N1: 2.698(4) Å, U1–N3: 2.691(4) Å) are obviously longer than those of U–N_amido (U1–N2: 2.249(4) Å, U1–N4: 2.248(4) Å), reflecting the dative bonds between the U center and N1 and N3. The bond lengths of U1–P1 (3.0042(14) Å) and U1–P2 (2.9589(14) Å) are longer than the sum of the covalent single-bond radii of U and P (2.80 Å),⁵⁰ suggesting a weak coordination between the two P atoms and the U center. The structural parameters of complex 2 are comparable to those of complex 1.

Fig. 2 Molecular structure of 1 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.698(4), U1–N2 2.249(4), U1–N3 2.691(4), U1–N4 2.248(4), U1–P1 3.0042(14), U1–P2 2.9589(14), U1–Cl1 2.6890(13), U1–Cl2 2.6856(13).

Fig. 3 Molecular structure of 2 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.737(4), U1–N2 2.237(4), U1–P1 2.9694(11), U1–I1 3.1109(3).

Synthesis of uranium bi-aryl complex 3 and uranium bi-alkyl complex 4

Compared with uranium alkyl complexes, uranium aryl complexes remain extremely rare.^51–54 The first example of a homoleptic uranium aryl complex was reported by Arnold in 2016.⁵⁵ Reported uranium aryl complexes feature one or more mono-aryl units, whereas uranium bi-aryl complexes have not been reported. Heterofluorenes, in which the CH₂ unit in fluorene was replaced by a heteroatom, have been reported for several decades. Heteroatoms in the heterofluorenes could be main group elements,^56,57 transition metals,^58,59 and rare-earth metals.^60,61 With complex 1 in hand, we attempted to synthesize a uranium-containing metallafluorene species via salt metathesis. Treatment of complex 1 with one equivalent of 2,2′-dilithiobiphenyl⁶² in THF at RT overnight afforded a brown turbid solution, from which complex 3 was isolated in 76% yield as a yellow crystalline solid (Scheme 2). The ¹H NMR spectrum of complex 3 has fourteen peaks between +36.0 and −53.3 ppm, suggesting relatively tight binding of the dimethylamino pendant donor with the U center. The ³¹P{¹H} NMR spectrum of complex 3 showed a single peak at 540.0 ppm, which suggested that the two P atoms are equivalent in solution.

Scheme 2 Synthesis of uranium bi-aryl complex 3.

The metallafluorene unit in complex 3 was confirmed by X-ray diffraction (Fig. 4). The two six-membered rings and the five-membered ring of the uranium-containing fluorene unit are co-planar. The bond lengths of U1–C1 (2.50(2) Å) and U1–C2 (2.482(17) Å) are shorter than the U–C_aryl bond lengths in [Li]₂[U-(2,3-C₆H₃CH₂NMe₂)₂(2-C₆H₄CH₂NMe₂)₂] (2.609(4) Å),⁵¹ and [Li][U(2,3-C₆H₃CH₂NMe₂)(2-C₆H₄CH₂NMe₂)₃] (2.604(4), 2.650(7), 2.615(8) Å),⁵³ but are comparable to U–C_aryl bond lengths in [Li(Et₂O)₃][UCl₂(C₆Cl₅)₃] (2.497(13), 2.504(13), 2.505(14) Å).⁵² They are also close to those of the U–C_alkyl single bond in [Li(THF)₄][U(CH₂^tBu)₅] (2.47(1)–2.51(1) Å),⁶³ [Li(THF)₄][Li(THF)₂U(CH₃)₆] (2.500(5)–2.615(5) Å),⁶⁴ UCp*(TMTAA)(CH₂TMTS) (TMTAA = tetramethyl-tetra-aza-annulene) (2.48(1) Å),⁶⁵ [fc(NSi^tBuMe₂)₂U(CH₂Ph)(OEt₂)][BPh₄] (2.482(12) Å),⁴⁷ and [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U(η²-C₄Ph₂) (2.448(5)–2.475(5) Å).⁶⁶ The bond angle of C1–U1–C2 is 69.1(7)°, which is close to the C–Ln–C angles in [Li(DME)₃][(C₅Me₅)₂Ln(biphen)] (Ln = Ce, 69.0(7)°; Ln = La, 68.55(8)°)⁶⁰ but is smaller than the C–M–C angles in other reported metallafluorenes (74°–130°).^{56–61,67,68} The bond lengths of U–N_amine (average of 2.735 Å) and U–N_amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group. Complex 3 represents a new example of a heterofluorene containing an actinide element.

Fig. 4 Molecular structure of 3 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.73(2), U1–N2 2.260(17), U1–N3 2.739(18), U1–N4 2.37(2), U1–P1 3.15(3), U1–P2 3.226(12), U1–C1 2.50(2), U1–C2 2.482(17).

We further investigated the reactivity of complex 1 with o-NMe₂C₆H₄CH₂K via a salt metathesis reaction. Treatment of complex 1 with two equivalents of o-NMe₂C₆H₄CH₂K in THF afforded orange crystalline complex 4 in 68% yield (Scheme 3). The ¹H NMR spectrum of complex 4 has twenty-nine peaks between +119.3 and −125.1 ppm, which suggested that complex 4 has a low-symmetric structure. The ³¹P{¹H} NMR spectrum of complex 4 was not observed between +1000 and −1000 ppm.

Scheme 3 Synthesis of uranium bi-alkyl complex 4.

The molecular structure of complex 4 was determined by single-crystal X-ray diffraction, which was revealed to be a unique example of a uranium bi-alkyl metallacycle stabilized by an N-donor (N5) (Fig. 5). The lengths of U–C bonds are 2.461(4) and 2.553(4) Å, which are within the reported range for U–C single bonds (2.29–2.78 Å).^{63–66,69–75} The bond lengths of U–N_amine (U1–N1 2.734(4) Å, U1–N3 2.813(4) Å, U1–N5 2.568(3) Å) were obviously longer than those of U–N_amido (U1–N2 2.297(3) Å, U1–N4 2.285(4) Å), reflecting dative bonding of N1, N3 and N5 to the U center. The bond distances of U1–P1 (3.2145(11) Å) and U1–P2 (3.1286(11) Å) are longer than the sum of the covalent single-bond radii of U and P (2.80 Å), suggesting weak coordination between the two P atoms and the U center.

Fig. 5 Molecular structure of 4 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.734(4), U1–N2 2.297(3), U1–N3 2.813(4), U1–N4 2.285(4), U1–N5 2.568(3), U1–P1 3.2145(11), U1–P2 3.1286(11), U1–C1 2.553(4), U1–C2 2.461(4).

The sp³ C–H bond in the dimethylamino moiety of 4 is activated by the uranium centre. This phenomenon was observed previously in actinide organometallic chemistry,^76–78 such as for the formation of uranium hexamethyldisilazide metallaycles.^79,80 Inspired by the mechanism of generating {[N(CH₂CH₂NSiMe₂^tBu)]₂U[η²-(CH₂CH₂NSi^tBu(CH₃)(CH₂))]},⁸¹ two possible routes were proposed for the formation of complex 4 (Scheme 3). Firstly, complex 1 reacts with two equivalents of o-NMe₂C₆H₄CH₂K to produce intermediate A with the release of KCl. Then, the “U-CH₂” fragment in intermediate B coordinates with the C–H bond in the “NMe₂” unit, allowing the σ-bond metathesis to contribute to the production of 4 with the release of ortho-dimethylaminotoluene (o-NMe₂C₆H₅CH₃). On the other hand, complex 1 could react with one equivalent of o-NMe₂C₆H₄CH₂K to produce intermediate C, which could further react with another equivalent of o-NMe₂C₆H₄CH₂K to produce D with the release of o-NMe₂C₆H₅CH₃. Finally, complex 4 was formed by releasing KCl from intermediate D.

The UV-Vis-NIR electronic absorption spectra of 1–4 were measured in THF at RT (Fig. 6). Complexes 1 and 4 showed intense absorption peaks at 216 and 220 nm, respectively. Complexes 2 and 3 showed two absorption peaks at 217 and 233 nm and 214 and 247 nm, respectively. In addition, a weak absorption peak at 420 nm in the visible region was observed for complex 1. A set of low intensity absorptions were observed in the NIR region for these complexes. For instance, the peaks at 1112 and 1310 nm were found for complex 1, 1076 and 1305 nm for complex 2, 1037 and 1294 nm for complex 3, and 1114 and 1300 nm for complex 4. These low-intensity absorptions (ε < 150 M⁻¹ cm⁻¹) for complexes 1–4 in the NIR region were attributed to the 5f–5f transitions, which are typical features of U(IV) species.

Fig. 6 UV-visible absorption spectra of complexes 1–4 measured in THF at RT (inset: near-infrared absorption spectra).

Synthesis of heterometallic clusters 5–7

Clusters with U–M bonds involving transition metals or even main-group metal elements have flourished in recent decades.^82–94 Inspired by our previous isolation of heterometallic clusters with the U–TM bond supported by N–P ligands,³⁷ we attempted to synthesize heterometallic clusters from complex 1. Treatment of 1 with one equivalent of RuCl₂(PPh₃)₃ at RT in THF afforded a red-brown solution, from which a crystalline complex 5 was isolated in 42% yield (Scheme 4). Complex 5 is an example of a chlorine-bridged multimetallic cluster with U and Ru. The formal oxidation states of U and Ru in complex 5 are +4 and +2, respectively. The NMR characterization of complex 5 was hindered by the poor solubility of its crystalline species. Attempts to synthesize a heterometallic cluster with U–Ru bonds by the reduction of complex 5 with Na, Li, K or KC₈ were unsuccessful.

Scheme 4 Synthesis of heterometallic clusters 5–7.

The solid-state structure of 5 was confirmed by X-ray crystallographic analysis (Fig. 7). The centrosymmetric structure contains a heterometallic U–Ru–Ru–U core with four Cl atom substituents. The bond length of U–Cl3 in the U–Cl–Ru moiety (2.723(2) Å) is longer than the terminal U1–Cl1 (2.707(2) Å) and U1–Cl2 (2.674(3) Å) bond lengths, suggesting the bridged nature of the Cl3 atom. The two P atoms (P1 and P2) are coordinated to the same Ru centre in this species, which is similar to the formation of the U–M bond supported by a dianionic N–P ligand {[CH₂O(CH₂)₂NHPⁱPr₂]₂}.³² The penta-coordinated Ru centre has an approximately square pyramidal geometry with one of the P atoms at the apical position. The U–Ru distances (3.6349(8) Å) are considerably longer than the sum of the covalent single-bond radii of U and Ru (2.95 Å),⁵⁰ suggesting the absence of any bonding interaction between these metal atoms.

Fig. 7 Molecular structure of 5 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.670(10), U1–N2 2.341(9), U1–N3 2.772(11), U1–N4 2.291(9), U1–Cl1 2.707(2), U1–Cl2 2.674(3), U1–Cl3 2.723(2), Ru1–Cl3 2.3769(19), Ru1–Cl4 2.3899(19), Ru1′–Cl4 2.498(2).

To synthesize heterometallic clusters containing U–M bonds, we further attempted the reaction of complex 1 with [M(COD)Cl]₂ (M = Rh, Ir). Treatment of 1 with 0.5 equivalents of [Rh(COD)Cl]₂ at RT in THF for 10 h afforded complex 6, which was isolated as a yellow crystalline solid in 57% yield (Scheme 4). Under the same conditions, complex 7 was synthesized by the reaction of complex 1 with 0.5 equivalents of [Ir(COD)Cl]₂, and was isolated as a yellow crystalline solid in 45% yield after storing at −30 °C (Scheme 4). Complexes 6 and 7 exhibited extremely low solubility after crystallization and no signals could be observed in the ¹H NMR spectra of these clusters even when different deuterated solvents were used.

The molecular structures of 6 and 7 in the solid state were determined by single-crystal X-ray diffraction (Fig. 8 and 9). The crystal systems and space groups of 6 and 7 are monoclinic P2₁/n. The coordination environment of the U centers in 6 and 7 is very similar, and comprises a hepta-coordinated geometry with one Rh/Ir atom, two Cl atoms, and four N atoms. The Rh and Ir atoms in 6 and 7 adopt almost the same coordination geometry with one U atom, two P atoms, and two Cl atoms. The most remarkable feature of these complexes is the two U–Rh/Ir bonds bridged by two Cl atoms. In complex 6, the U–Rh bond length is 2.7910(6) Å, which is obviously shorter than the sum of the covalent single-bond radii of U and Rh (2.95 Å).⁵⁰ The U–Rh bond length in 6 is longer than the U–Rh single-bond lengths reported in [U^IVI₂(μ-OAr^P-1κ¹O,2κ¹P)₂Rh^I(μ-I)]₂ (Ar^PO⁻ = 2-(diphenylphosphino)-6-tert-butyl-4-methylphenoxide) (2.7601(5) Å), U^IVI(μ-I)(μ-OAr^P-1κ¹O,2κ¹P)₃Rh^I (2.7630(5) Å)⁹⁵ and U[N(CH₃)(CH₂CH₂NPⁱPr₂)₂](μ-Me)₂Rh₂(μ-Me)₄Mg(C₄H₈O) (2.6612(5) Å).³⁴ The U–Ir bond length (2.8656(3) Å) is shorter than the sum of the covalent single-bond radii of U and Ir (2.92 Å), but longer than the U–Ir single-bond length in complex U[N(CH₃)(CH₂CH₂NPⁱPr₂)₂] [(μ-Me)₂Ir₂(μ-Me)₄Mg(C₄H₈O)] (2.6968(4) Å).³⁴ Nevertheless, the U–Rh/Ir bond lengths observed in 6 and 7 are consistent with the presence of a U–M bonding interaction in these clusters.

Fig. 8 Molecular structure of 6 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.700(8), U1–N2 2.348(7), U1–N3 2.793(8), U1–N4 2.298(7), U1–Cl1 2.651(2), U1–Cl2 2.696(2), U1–Rh1 2.7910(6), Rh1–Cl3 2.4487(19), Rh1–Cl3′ 2.473(2).

Fig. 9 Molecular structure of 7 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. The ⁱPr groups on P atoms are simplified into lines. Selected bond distances (Å): U1–N1 2.715(4), U1–N2 2.293(4), U1–N3 2.763(5), U1–N4 2.329(4), U1–Cl1 2.7378(14), U1–Cl2 2.6419(13), U1–Ir1 2.8656(3), Ir1–Cl3 2.4019(12), Ir1–Cl3′ 2.4377(12).

The UV-Vis-NIR electronic absorption spectra of 5–7 were measured in THF at RT (Fig. 10). Complex 5 exhibited an intense peak at 223 nm and a weak peak at 313 nm in the UV-Vis region. Two peaks at 215 and 326 nm were observed for complex 6 and one intense peak at 219 nm was observed for complex 7. In the NIR region, complexes 5, 6 and 7 exhibited similar absorption peaks at 1070 and 1286 nm, 1072 and 1293 nm, and 1067 and 1294 nm, respectively. These weak absorptions (ε < 100 M⁻¹ cm⁻¹) were attributed to the f–f transitions expected for U(IV) complexes.

Fig. 10 UV-visible absorption spectra of heterometallic clusters 5–7 measured in THF at RT (inset: near-infrared absorption spectra).

Variable-temperature magnetic data of 5–7 in the solid state were collected using a superconducting quantum interference device (SQUID) (Fig. 11). The effective moments of 5, 6 and 7 at 300 K are 4.38, 4.77 and 4.54 μ_B per molecule, respectively, which are lower than the theoretical value (5.06 μ_B) for two independent 5f² U(IV) ions. This phenomenon was probably due to the quenching of spin–orbit coupling and was observed for the reported U(IV) complexes.⁹⁶ The magnetic moments of 6 and 7 decline slowly from 300 K to 50 K, and then drop sharply to 0.74 μ_B and 0.76 μ_B at 1.8 K with a trend toward zero. However, the magnetic moment of 5 declines much more obviously and starts to decline from 300 K persistently. At 1.8 K, the magnetic moment of 5 is 0.39 μ_B per molecule. The magnitude of μ_eff and temperature dependence of complexes 5, 6 and 7 are comparable to those observed for the reported U(IV) complexes.

Fig. 11 Variable-temperature effective magnetic moment data. Magnetic moment per molecule for clusters 5–7.

Conclusions

A novel monoanionic N–P ligand L3 and the corresponding U(IV) complexes 1 and 2 were synthesized. An actinide-containing metallafluorene 3 was synthesized by the reaction of complex 1 and 2,2′-dilithiobiphenyl by a salt metathesis method. Using a similar strategy, U(IV) bi-alkyl complex 4 was isolated by the reaction of complex 1 with o-NMe₂C₆H₄CH₂K. The monoanionic N–P ligand L3 also proved to be an effective platform for the construction of heterometallic clusters with U–M bonds. For example, the heterometallic clusters 6 and 7 containing U–Rh or U–Ir bonds were synthesized by the reaction of complex 1 with [M(COD)Cl]₂ (M = Rh, Ir). Further studies on the reactivity of the U(IV) bi-aryl complex 3, the bi-alkyl complex 4 and the heterometallic clusters 5–7 are in progress.

Experimental section

All manipulations were performed under an N₂ atmosphere using standard Schlenk techniques or in an N₂ glovebox (<1 ppm O₂/H₂O). Solvents were dried and deoxygenated by distillation under nitrogen and further dried over 4 Å molecular sieves before use. Benzene-d₆ and THF-d₈ were dried over Na/K and stored under an N₂ atmosphere prior to use. 2,2′-Dilithiobiphenyl,⁶²o-NMe₂C₆H₄CH₂K,⁹⁷ UCl₄ ⁹⁸ and UI₃(THF)₄ ⁴⁴ were prepared using published procedures. Other reagents were purchased and used without further purification. Nuclear magnetic resonance (NMR) spectroscopy was performed using a Bruker AVIII-400 or a Bruker AVIII-500 spectrometer at room temperature (RT). Absolute values of the coupling constants (δ) are provided in Hertz (Hz). Multiplicities of peaks are abbreviated as singlet (s), doublet (d), triplet (t), multiplet (m) or broad (br). Elemental analyses (C, H, N) were performed on a Vario MICRO elemental analyzer at the Center of Modern Analysis of Nanjing University.

Synthesis of L3

A solution of ⁱPr₂PCl (4.20 g, 27.5 mmol) in THF (10 mL) was added dropwise to a solution of (CH₃)₂NCH₂CH₂NH₂ (2.40 g, 27.5 mmol) and Et₃N (15 mL, 110.0 mmol) in THF (20 mL), resulting in the immediate formation of a white precipitate. The mixture was stirred overnight before being dried in vacuo. The white solid was extracted with n-hexane (20 mL) and then filtered through Celite. Volatiles were removed under reduced pressure to give L3 as a colorless oil. Yield: 5.00 g (89%). ¹H NMR (C₆D₆, 500 MHz, ppm) 2.91–2.95 (m, 2H, NCH₂CH₂), 2.18 (t, ³J_HH = 6.1 Hz, 2H, NCH₂CH₂), 2.03 (s, 6H, N(CH₃)₂), 1.43–1.52 (m, 2H, CH(CH₃)₂), 0.98–1.03 (m, 12H, CH(CH₃)₂). ¹³C{¹H} NMR (C₆D₆, 125 MHz, ppm) 61.86 (d, J_PC = 6.1 Hz, NCH₂CH₂), 46.03 (d, J_PC = 23.9 Hz, NCH₂CH₂), 45.07 (s, N(CH₃)₂), 26.42 (d, J_PC = 13.0 Hz, CH(CH₃)₂), 19.10 (d, J_PC = 21.0 Hz, CH(CH₃)₂), 17.29 (d, J_PC = 8.2 Hz, CH(CH₃)₂). ³¹P{¹H} NMR (C₆D₆, 202 MHz, ppm) 63.68. HRMS (ESI) calcd for C₁₀H₂₆N₂P, [M + H]⁺, m/z = 205.1828. Found 205.1837.

Synthesis of complex 1

Method A. A 2.4 M solution of ⁿBuLi in n-hexane (0.2 mL, 0.5 mmol) was added dropwise to a solution of L3 (102.2 mg, 0.5 mmol) in THF (4 mL) at −30 °C. The mixture was allowed to warm to RT and stirred for 3 h, and then a solution of UCl₄ (190.0 mg, 0.5 mmol) in THF (5 mL) was added to this mixture. The solution color changed from green to dark green. This solution was stirred at RT for 10 h and then the solvents were removed under reduced pressure. Next, the residues were extracted with toluene (1 mL) and stored at −30 °C overnight to afford green crystalline solid 1 (37.6 mg, 21%).

Method B. A 2.4 M solution of ⁿBuLi in n-hexane (0.8 mL, 2.0 mmol) was added dropwise to a solution of L3 (408.6 mg, 2.0 mmol) in THF (8 mL) at −30 °C. The mixture was allowed to warm to RT and stirred for 3 h, and then a solution of UCl₄ (380.0 mg, 1.0 mmol) in THF (10 mL) was added to this mixture. The solution color changed from green to dark green. This solution was stirred at RT for 10 h and then the solvents were removed under reduced pressure. Next, the residues were extracted with toluene (3 mL) and the filtrate was dried in vacuo and then washed with n-hexane (1 mL) to afford complex 1 as a pure green solid (558.1 mg, 78%). Crystals of 1 compatible for X-ray diffraction were grown from a saturated solution in toluene (2 mL) stored at −30 °C. ¹H NMR (C₆D₆, 500 MHz, ppm) 54.5 (s, 4H, CH₂), 35.8 (s, 4H, CH₂), 16.5 (s, 14H, CH(CH₃)₂), 14.4 (s, 14H, CH(CH₃)₂), −70.5 (s, 12H, N(CH₃)₂). ³¹P{¹H} NMR (C₆D₆, 162 MHz, ppm) was not observed in the range of +1000 to −1000 ppm. Anal. calcd (%) for C₂₀H₄₈Cl₂N₄P₂U: C, 33.57; H, 6.76; N, 7.83. Found: C, 33.82; H, 6.71; N, 7.79.

Synthesis of complex 2

Method A. A 2.4 M solution of ⁿBuLi in n-hexane (0.4 mL, 1.0 mmol) was added dropwise to a solution of L3 (204.3 mg, 1.0 mmol) in THF (4 mL) at −30 °C. The mixture was stirred for 3 h, and was then added to a solution of UI₃(THF)₄ (2/3 equiv., 602.1 mg, 0.7 mmol) in THF (5 mL). The mixture was stirred at RT for 10 h and then the solvents were removed under reduced pressure. Next, the residues were extracted with toluene (1 mL) and the filtrate was stored at −30 °C to afford complex 2 as red-orange crystals (166.2 mg, 37% based on ligand).

Method B. Trimethylsilyl iodide (80.0 mg, 0.4 mmol) dissolved in toluene (0.5 mL) was added to a solution of complex 1 (143.1 mg, 0.2 mmol) in toluene (2 mL). The solution immediately changed from green to red-orange. The mixture was stirred at RT for 6 h and then the solvents were removed under reduced pressure to afford complex 2 as a pure red-orange solid without subsequent purification. Yield: 2 (163.5 mg, 91%). Crystals of 2 compatible for X-ray diffraction were grown from a saturated solution in toluene (1 mL) stored at −30 °C. ¹H NMR (C₆D₆, 400 MHz, ppm) 58.0 (s, 4H, CH₂), 35.1 (s, 4H, CH₂), 24.3 (s, 14H, CH(CH₃)₂), 12.8 (s, 14H, CH(CH₃)₂), −74.4 (s, 12H, N(CH₃)₂). ³¹P{¹H} NMR (C₆D₆, 162 MHz, ppm) was not observed in the range of +1000 to −1000 ppm. Anal. calcd (%) for C₂₀H₄₈I₂N₄P₂U: C, 26.74; H, 5.39; N, 6.24. Found: C, 26.61; H, 5.49; N, 6.38.

Synthesis of complex 3

Complex 1 (143.1 mg, 0.2 mmol) and 2,2′-dilithiobiphenyl (33.2 mg, 0.2 mmol) were added to a 5 mL flask and dissolved in THF (2 mL). The solution immediately changed from green to red-orange. The mixture was stirred at RT for 10 h and then the solvent was removed under reduced pressure and the residue was extracted with ether (3 mL). The filtrate was dried in vacuo to afford complex 3 as a pure yellow solid (121.1 mg, 76%). Crystals of 3 compatible for X-ray diffraction were grown from a saturated solution in ether (2 mL) stored at RT. ¹H NMR (C₆D₆, 400 MHz, ppm) 36.0 (d, J = 18.7 Hz, 4H, CH₂), 26.6 (d, J = 8.7 Hz, 2H, CH(CH₃)₂), 25.8 (d, J = 34.1 Hz, 4H, CH₂), 21.3 (s, 2H, biphenyl), 17.5 (t, J = 6.5 Hz, 2H, CH(CH₃)₂), 15.7 (s, 2H, biphenyl), 13.0 (s, 6H, CH(CH₃)₂), 9.4 (s, 6H, CH(CH₃)₂), 8.9 (s, 6H, CH(CH₃)₂), 6.8 (s, 6H, CH(CH₃)₂), −24.5 (s, 2H, biphenyl), −26.0 (s, 6H, N(CH₃)₂), −43.7 (s, 2H, biphenyl), −53.3 (s, 6H, N(CH₃)₂). ³¹P{¹H} NMR (C₆D₆, 162 MHz, ppm) 540.0. Anal. calcd (%) for C₃₂H₅₆N₄P₂U: C, 48.24; H, 7.08; N, 7.03. Found: C, 47.01; H, 6.91; N, 6.87. No satisfactory result for the C% value was obtained despite repeated attempts, which is probably due to the inadequate combustion of this air- and moisture-sensitive species.

Synthesis of complex 4

Complex 1 (71.5 mg, 0.1 mmol) and o-NMe₂C₆H₄CH₂K (34.7 mg, 0.2 mmol) were added to a 5 mL flask and dissolved in THF (2 mL). The solution immediately changed from green to orange. The mixture was stirred at RT overnight. Then the solvent was removed under reduced pressure and the residue was extracted with n-hexane (3 mL). Yellow crystals of 4 were grown from a solution of ether (2 mL) at room temperature. Yield: 4 (52.9 mg, 68%). ¹H NMR (d₈-THF, 400 MHz, ppm) 119.3 (s, 1H), 98.4 (s, 1H), 95.2 (s, 3H), 88.7 (s, 1H), 67.4 (s, 1H), 48.5 (s, 1H), 43.5 (s, 3H), 38.9 (s, 3H), 31.4 (s, 4H), 23.2 (s, 4H), 20.8 (s, 1H), 8.2 (s, 3H), −1.3 (s, 2H), −1.9 (s, 3H), −6.9 (s, 3H), −12.5 (s, 1H), −16.8 (s, 4H), −28.4 (s, 1H), −30.7 (s, 1H), −34.5 (s, 1H), −41.1 (s, 1H), −45.8 (s, 3H), −48.0 (s, 1H), −50.8 (s, 1H), −63.2 (s, 3H), −67.4 (s, 3H), −74.1 (s, 1H), −96.2 (s, 3H), −125.1 (s, 1H). ³¹P{¹H} NMR (C₆D₆, 162 MHz, ppm) was not observed in the range of +1000 to −1000 ppm. Anal. calcd (%) for C₂₉H₅₉N₅P₂U: C, 44.78; H, 7.65; N, 9.00. Found: C, 44.70; H, 7.55; N, 8.91.

Synthesis of complex 5

A solution of RuCl₂(PPh₃)₃ (95.9 mg, 0.1 mmol) in THF (2 mL) was added to a solution of complex 1 (71.5 mg, 0.1 mmol) in THF (2 mL) at RT. The mixture was left undisturbed for 1 day at RT. Then, black block crystals of 5 (74.6 mg, 42%) were obtained. NMR characterization of this species was prevented by its poor solubility after recrystallization. Anal. calcd (%) for C₄₀H₉₆Cl₈N₈P₄Ru₂U₂·2C₄H₈O: C, 30.04; H, 5.88; N, 5.84. Found: C, 30.27; H, 5.65; N, 5.97.

Synthesis of complex 6

A solution of [Rh(COD)Cl]₂ (33.7 mg, 0.1 mmol) in THF (2 mL) was added to a solution of complex 1 (143.1 mg, 0.2 mmol) in THF (2 mL) at RT. The solution immediately changed from green to orange. The mixture was stirred at RT overnight and then the solvents were removed. The residue was extracted with dichloromethane (2 mL). Storing the filtrate at −30 °C for 18 h yielded product 6 as orange block crystals (97.3 mg, 57%). NMR characterization of this species was prevented by its poor solubility after recrystallization. Anal. calcd (%) for C₄₀H₉₆Cl₆N₈P₄Rh₂U₂: C, 28.13; H, 5.67; N, 6.56. Found: C, 28.34; H, 5.73; N, 6.44.

Synthesis of complex 7

A solution of [Ir(COD)Cl]₂ (51.5 mg, 0.1 mmol) in THF (2 mL) was added to a solution of complex 1 (143.1 mg, 0.2 mmol) in THF (3 mL). The color of the solution immediately changed from green to orange. The mixture was stirred at RT overnight and then the solvents were removed. The residue was extracted with dichloromethane (2 mL). Storing the filtrate at −30 °C for 24 h yielded product 7 as orange block crystals (84.9 mg, 45%). NMR characterization of this species was prevented by its poor solubility after recrystallization. Anal. calcd (%) for C₄₀H₉₆Cl₆N₈P₄Ir₂U₂: C, 25.47; H, 5.13; N, 5.94. Found: C, 25.76; H, 5.14; N, 5.87.

Single-crystal X-ray diffraction

The crystallographic data of complexes 1, 2, 3, 4, 5, 6 and 7 were collected on Bruker D8 venture photon II detectors at 193 K or 296 K with a radiation source of Ga(Kα) (λ = 1.34139 Å) or Mo(Kα) (λ = 0.71073 Å) using the ω-scan technique. Multiscan or empirical absorption corrections (SADABS) were applied.⁹⁹ The structures were solved by direct methods, expanded by difference Fourier syntheses, and refined by full-matrix least squares on F² using Olex2.¹⁰⁰ All non-hydrogen atoms were refined on F² by full-matrix least-squares procedures with the use of anisotropic displacement parameters.¹⁰¹ Hydrogen atoms were introduced at their geometric positions and refined as riding atoms. In complex 3, the “CH₂NPⁱPr₂” unit was split into two parts with 43.3% and 56.7% occupancies. The X-ray crystal structures have been deposited in the Cambridge Crystallographic Data Centre (CCDC 2254129 (1), 2254126 (2), 2254132 (3), 2254127 (4), 2254131 (5), 2254128 (6), and 2254130 (7)†). Details of the data collection and refinement for complexes 1, 2, 3, 4, 5, 6 and 7 are given in Tables S1, S2 and S3 in the ESI.†

Author contributions

Kai Li: investigation, writing – original draft. Jialu He: investigation. Yue Zhao: software. Congqing Zhu: resources, supervision, funding acquisition, writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This research was supported by the National Key R&D Program of China (2021YFA1502500), the National Natural Science Foundation of China (No. 91961116 and 22271138), and the Natural Science Foundation of Jiangsu Province (BK20220065), Programs for High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2254126–2254132. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01447a

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