Multi-electron redox reactivity of a samarium(II) hydrido complex

Abstract

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb²⁺ and Eu²⁺ centers. Here, we report the first example of the divalent samarium(II) hydrido complex [(Cp^Ar5)Sm^II(μ-H)(DABCO)]₂ (4) (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(II) alkyl complex [(Cp^Ar5)Sm^II{CH(SiMe₃)₂}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO₂ results in the formation of the trivalent samarium(III) mixed-bis-formate/carbonate complex [(Cp^Ar5)Sm^III(μ-η²:η¹-O₂CH)(μ-η²:η²-CO₃)(μ-η¹:η¹-O₂CH)Sm^III(Cp^Ar5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS₂ to afford the trivalent samarium(III) bis-trithiocarbonate complex [(Cp^Ar5)Sm^III(μ-η²:η²-CS₃)(DABCO)]₂ (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.

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Introduction

The chemistry of molecular hydrides of rare-earth metals has been extensively studied in the past four decades.¹ However, despite their significant roles in various stoichiometric and catalytic reactions, such complexes are dominated by trivalent rare-earth metal centers.² The difficulties in isolating discrete divalent rare-earth metal hydrides are mainly attributed to their greater ionic radii and higher reactivity levels, compared with trivalent rare-earth metals. So far, structurally characterized divalent rare-earth metal hydride species are scarce, consisting of a handful of ytterbium(II) hydrides^3–9 and three examples of europium(II) hydride complexes,¹⁰ although all lanthanides(II), except for the radioactive promethium, are accessible in soluble molecules.¹¹ The Takats group reported the first example of a divalent ytterbium hydride complex [(Tp^tBu,Me)Yb(μ-H)]₂ (Tp^tBu,Me = hydrotris(3-tert-butyl-5-methyl-pyrazolyl)borate) (Fig. 1, I).³ The first mononuclear divalent ytterbium hydride complex bearing a terminal hydrido ligand—namely [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (Fig. 1, II)—was produced.⁴ Three other ancillary ligand types (Fig. 1, III–VI) have been shown to stabilize neutral dimeric ytterbium hydrides, including the bulky ketiminate (BDI^Dipp = (2,6-ⁱPr₂C₆H₃)NC(Me)C(H)C(Me)N(2,6-ⁱPr₂C₆H₃)),^5,6 amidinate (^tBuC(N-Dipp)₂ = (2,6-ⁱPr₂C₆H₃)NC(^tBu)N(2,6-ⁱPr₂C₆H₃)),⁷ and β-diketiminato-based tetradentate ligand (MeC(NDipp)CHC(Me)NCH₂CH₂N(Me)CH₂CH₂NMe₂, Dipp = 2,6-ⁱPr₂-C₆H₃).⁸ Recently, Okuda and co-worker disclosed the first examples of cationic divalent ytterbium hydrides [(Me₄TACD)₂Yb₂(μ-H)_(2+n)]⁽²⁻ⁿ⁾⁺ (n = 0, 1) (Me₄TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) (Fig. 1, VII).⁹ Very recently, the bulky β-diketiminate ligand frameworks {BDI^DCHP}⁻ and [BDI^Dipp/Ar]⁻ (BDI = [HC{C(Me)₂N-Dipp/Ar}₂]⁻ (Dipp = 2,6-ⁱPr₂-C₆H₃; Ar = 2,6-Cy₂-C₆H₃ (DCHP) or 2,4,6-Cy₃-C₆H₂ (TCHP)) were developed for the kinetic stabilization of the first europium(II) hydride complexes (Fig. 1, VIII).¹⁰

Fig. 1 Selected examples of divalent rare-earth metal hydride complexes.

Divalent ytterbium hydrides have been used in a variety of organic transformations, and the oxidation state of ytterbium center remains unchanged in most cases.^3–9,12 A more versatile chemistry can confidently be predicted for rare-earth metal hydrides with a highly reducing metal center.

Sm²⁺, Yb²⁺ and Eu²⁺ have been classically regarded as the three most common Ln²⁺ examples in molecular lanthanide complexes in solution.¹³ The Ln³⁺/Ln²⁺ redox potentials for Sm²⁺, Yb²⁺ and Eu²⁺ are, respectively, −1.55, −1.15 and −0.35 V versus NHE.¹⁴ Divalent samarium complexes, for example, Sm^III₂(THF)_x (ref. 15) and (Cp*)₂Sm^II(THF)_x (Cp* = C₅Me₅),¹⁶ have been widely used as powerful electron-transfer reagents in various organic transformations. Compared with divalent ytterbium hydrides, the synthesis and structural characterization of divalent samarium hydrides are more challenging, because of the larger ionic radius (Yb²⁺ 1.02 Å, Sm²⁺ 1.17 Å; CN = 6),¹⁷ and more reducing metal center and paramagnetic nature (Yb²⁺: diamagnet; Sm²⁺: 3.5–3.8 μ_B) of divalent samarium.¹⁸ Molecular divalent samarium hydride complexes [Sm^IIH₂(THF)₂]¹⁹ and [(Cp*)Sm^IIH(THF)₂]²⁰ have been studied for decades, but no crystal structure has to date been obtained. Gambarotta et al. described an anionic divalent samarium hydride species, namely tetranuclear {([Ph₂C(C₄H₃N)₂]Sm^II)₄(H)(THF)₂}⁻, obtained serendipitously from the reduction of trivalent samarium precursor {[Ph₂C(C₄H₃N)₂]Sm^IIICl} with sodium in THF. However, the hydride position in the crystal structure is elusive.²¹ No further reactivity studies have been presented, as far as we know.

As the ionic radii of Sr²⁺ and Sm²⁺ are very similar (Sr²⁺ 1.18 Å, Sm²⁺ 1.17 Å; CN = 6),¹⁷ there are strikingly similar interatomic parameters in their crystal structures.²² Recently, our group found that the sterically demanding penta-arylcyclopentadienyl ligand Cp^Ar5 (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃)²³ can serve as an excellent ligand with the dinuclear structure.^23a Encouraged by this success, we turned our attention to the synthesis of divalent samarium hydrides. Herein, we report the first example of a structural characterization of a neutral divalent samarium(II) hydrido complex, namely [(Cp^Ar5)Sm^II(μ-H)(DABCO)]₂ (4), generated from a moderate-pressure (20 atm) hydrogenolysis of the corresponding samarium(II) alkyl precursor. Preliminarily determined reactivities of divalent samarium(II) hydrides with Se, azobenzene (PhN=NPh), CO₂, and CS₂ are presented as well.

Results and discussion

As the precursor of reported strontium hydrides, the strontium alkyl complex [(Cp^Ar5)Sr{CH(SiMe₃)₂}(THF)]^23a was generated from the reaction of (Cp^Ar5)H with [Sr{CH(SiMe₃)₂}₂(THF)₂].²⁴ However, the samarium analogue “[Sm^II{CH(SiMe₃)₂}₂(THF)₂]” is inaccessible. Protonolysis reactions of (Cp^Ar5)H with samarium benzyl complexes, for example [Sm^II₂(CH₂Ph)₄(THF)₃]²⁵ and [(DMAT)₂Sm^II(THF)] (DMAT = 2-Me₂N-α-Me₃Si-benzyl),^24,26 failed. Thus, two-step salt metathesis reactions were carried out (Scheme 1). Treatment of (Cp^Ar5)K(THF)₂ (1) with [Sm^III₂(THF)₅]²⁷ in tetrahydropyran (THP), followed by the exchange of coordinated THP with DABCO, afforded the heteroleptic divalent samarium(II) iodide complex [(Cp^Ar5)Sm^II(μ-I)(DABCO)]₂ (2) as dark-green crystals in 92% isolated yield. Reaction of complex 2 with KCH(SiMe₃)₂ (ref. 28) in benzene gave the half-sandwich samarium(II) alkyl complex [(Cp^Ar5)Sm^II{CH(SiMe₃)₂}(DABCO)] (3) as a dark-brown solid in 86% isolated yield. Due to the high solubility of complex 3 in hydrocarbon solvents such as hexane, no crystallographically definable product could be isolated.

Scheme 1 Synthesis of the divalent samarium(II) hydride 4.

Hydrogenolysis of complex 3 in hexane under 20 atm H₂ successfully afforded the corresponding hydride complex [(Cp^Ar5)Sm^II(μ-H)(DABCO)]₂ (4) as dark-brown crystals in 78% isolated yield (Scheme 1), concomitant with CH₂(SiMe₃)₂. Complex 4 was found to be slightly soluble in hexane, and quite soluble in benzene. Complex 4 decomposed completely in C₆D₆ within two days and converted quickly to samarocene [(Cp^Ar5)₂Sm^II] (5) in THF through ligand redistribution at room temperature. The molecular structure of complex 5 (ESI, Fig. S32†) was observed to be similar to those of other deca-arylsamarocenes [{C₅(4-R-C₆H₄)₅}₂Sm^II] (R = Et, n-Bu).^22,29

The ¹H NMR spectrum of complex 4 showed very broad signals, attributed to the paramagnetic nature of the Sm²⁺ ion, and hence could not be fully assigned. However, a signal attributed to bridging hydrides was observed at 1.63 ppm, corroborated by a ²H NMR spectrum for the deuterated analogue [(Cp^Ar5)Sm^II(μ-D)(DABCO)]₂ (4-D). Based on magnetic measurements, the magnetic moments of complexes 2 and 4 at 300 K were measured to be 3.2 μ_B and 3.9 μ_B, respectively, consistent with divalent samarium derivatives.¹⁸

The dimeric nature of complex 4 in the solid state was revealed by X-ray crystallographic studies to be isostructural to [(Cp^Ar5)Sr(μ-H)(DABCO)]₂. Each Sm²⁺ center coordinated with one η⁵-Cp^Ar5 ligand, one DABCO ligand and two bridging hydrido ligands. The bridging hydrido ligand H1 was located using difference Fourier syntheses and refined isotopically (Fig. 2). Analysis of the structure showed Sm1–N1 (2.652(4) Å), Sm1–Cent1 (2.594 Å) and Sm1–Sm1ⁱ (3.7966(5) Å) bond distances almost identical to those in [(Cp^Ar5)Sr(μ-H)(DABCO)]₂,^23a and showed Sm–H distances, of 2.26(6) Å and 2.48(5) Å, in the range (2.26(5)–2.86(6) Å) found in the molecular strontium hydride complexes.^23a,30 Thus, our work with complex 4 yielded the first example of a structurally characterized Sm(II) hydride complex, in comparison to the dozen previously reported examples of trivalent samarium(III) hydrides.³¹

Fig. 2 Molecular structure of complex 4. All the hydrogen atoms except the bridging hydrides (H1 and H1ⁱ) are omitted for clarity. Selected interatomic distances [Å] and angles [deg]: Sm1–H1 2.26(6), Sm1–N1 2.652 (4), Sm1–Cent1 2.594, Sm1–Sm1ⁱ 3.7966 (5), Sm1–H1–Sm1ⁱ 106 (2), H1–Sm1–H1ⁱ 74 (2). Cent: center of the Cp^Ar5 ring.

As shown in Scheme 2, complex 4 readily reduced two equivalents of elemental Se in an apparent four-electron reduction to afford the corresponding trivalent samarium complex [(Cp^Ar5)Sm^III(μ-Se)(DABCO)]₂ (6) in 77% isolated yield, with the four-electron reduction formally involving two one-electron hydride reductions and two one-electron Sm²⁺/Sm³⁺ reductions. Here, a distinct change in the color of the benzene solution from dark brown to pale brown was observed instantly, in company wih the release of H₂.

Scheme 2 Reactions of complex 4 with Se and azobenzene.

Structural analyses showed the molecular framework of complex 6 to be similar to that of complex 2, except for the bridging atoms and the oxidation state of Sm centers (ESI, Fig. S30 and S33†), and showed the bond lengths of Sm–Se (2.7349(7)∼2.7524(6) Å) in complex 6 to be comparable to those in samarium complex [(Cp*)₂Sm^III(THF)]₂(μ-Se) (2.782(1) and 2.779(1) Å).³²

Complex 4 also apparently acted as a four-electron reductant in the reduction of 2 equivalents of PhN=NPh to give [(Cp^Ar5)Sm^III(μ-PhN-NPh)]₂ (7) as a red-brown solid in a measured 89% isolated yield. As shown in Fig. S34,† the N1–N2 bond length (1.462(3) Å) in the [PhN-NPh]²⁻ unit was found to be comparable to that of the previously reported samarium(III) complex [(Cp*)Sm^III(μ-PhN-NPh)(THF)]₂ (1.44(1) Å).^33a In contrast, Evans and coworkers reported the reductive cleavage of azobenzene into uranium(VI) bis(imido) complex (Cp*)₂U(NPh)₂.^33b

Rare-earth metal complexes have been found to engage in extensive chemistry with carbon dioxide (CO₂)³⁴ and carbon disulfide (CS₂).³⁵ Exposure of a solution of complex 4 in toluene to 1 atm of CO₂ resulted in a decolorization to pale yellow in 30 minutes. The trivalent-samarium mixed-bis-formate/carbonate complex [(Cp^Ar5)Sm^III(μ-η²:η¹-O₂CH)(μ-η²:η²-CO₃)(μ-η¹:η¹-O₂CH)Sm^III(Cp^Ar5)(DABCO)] (8) was isolated from a hexane solution as pale-yellow crystals in 75% yield (Scheme 3).

Scheme 3 Reactions of complex 4 with CO₂ and CS₂.

Due to the paramagnetic nature of complex 8, no meaningful data could be obtained from its ¹H and ¹³C NMR spectra. Accordingly, the solid-state structure of compound 8 was determined using X-ray crystallography and is highlighted in Fig. 3. Inspection of this structure showed two [(Cp^Ar5)Sm^III] fragments bridged by one carbonate [CO₃]²⁻ ligand and two formate [O₂CH]⁻ moieties. It specifically showed the bridging carbonate [CO₃]²⁻ unit symmetrically bonded to the metal centers in a μ-η²:η² mode, with similar Sm–O bond distances (Sm1–O3, 2.397(3) Å; Sm2–O3, 2.398(3) Å) and Sm–O–C bond angles (Sm1–O3–C1, 88.2(3)°; Sm2–O3–C1, 91.0(3)°)—but showed unequal C–O bond lengths in the carbonate unit, specifically with C1–O1 (1.204(6) Å) and C1–O2 (1.251(6) Å) being significantly shorter than C1–O3 (1.349(8) Å). Two different coordination patterns for the [O₂CH]⁻ moieties were observed: one being a μ-η¹:η¹ mode, and the other a μ-η²:η¹ mode, different from previously reported rare-earth metal formate complexes.³⁶ The coordination sphere of Sm1 was completed by a DABCO ligand, to have the same coordination number as that of the Sm2 center. Other interatom bond distances, e.g., Sm1–N1 (2.605(3) Å) and Sm–Cent (2.523 and 2.452 Å), were found to be in agreement with trivalent samarium(III) complexes 6 and 7.

Fig. 3 Molecular structure of complex 8. All the hydrogen atoms, except H2 and H3, are omitted for clarity. Selected interatomic distances [Å] and angles [deg]: Sm1–O2 2.354(2), Sm1–O3 2.397(3), Sm1–O5 2.338(2), Sm1–O7 2.426(2), Sm1–N1 2.605(3), Sm2–O1 2.373(3), Sm2–O3 2.398(3), Sm2–O4 2.346(2), Sm2–O6 2.406(3), Sm2–O7 2.644(2), C1–O1 1.251 (6), C1–O2 1.204(6), C1–O3 1.349(8), C2–O4 1.248(7), C2–O5 1.206(7), C3–O6 1.249(5), C3–O7 1.271(4), Sm1–Cent1 2.523, Sm2–Cent2 2.453, O2–C1–O1 133.6(6), O2–C1–O3 112.1(5), O1–C1–O3 113.8(5), O5–C2–O4 133.1(7), O6–C3–O7 121.7(4). Cent: center of the Cp^Ar5 ring.

Apparently, four equivalents of CO₂ were involved in this reaction. Two CO₂ molecules inserted into Sm–H bonds to form two formate units with different bonding modes. The other two CO₂ molecules likely underwent two-electron reduction and disproportionation to generate carbonate species [CO₃]²⁻ with the release of carbon monoxide (CO), which has been proposed in the formation of other rare-earth metal carbonate complexes.³⁷

Treatment of complex 4 with CS₂ in toluene gave a red-brown solution in 30 minutes. Trivalent samarium bis-trithiocarbonate complex [(Cp^Ar5)Sm^III(μ-η²:η²-CS₃)(DABCO)]₂ (9) was isolated as red-brown crystals in 41% yield. The whole molecule was determined according to X-ray crystallographic studies to adopt a binuclear structure composed of two Cp^Ar5-supported samarium trithiocarbonate units (Fig. 4), with each Sm³⁺ center coordinated with one η⁵-Cp^Ar5 ligand, one DABCO ligand and two bridging μ-η²:η²-CS₃ ligands. It was obvious that this reaction occurred with the oxidation of both Sm(II) and hydrido centers, and the reductive disproportionation of four equivalents of CS₂ to give two trithiocarbonate [CS₃]²⁻ units with the release of carbon monosulfide (CS). Note the generation of [CS₃]²⁻ species through a two-electron reduction in complex 4 being comparable to that previously reported for a rare-earth metal trithiocarbonate complex.³⁸

Fig. 4 Molecular structure of complex 9. All the hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [deg]: Sm1–S1 2.9774(8), Sm1–S2 2.8243(8), Sm1–S3ⁱ 2.8393(8), C1–S1 1.726(3), C1–S2 1.698(3), C1–S3 1.705(3), Sm1–N1 2.581(3), Sm1–Cent1 2.518, S1–C1–S2 118.35(17), S1–C1–S3 119.28(17), S2–C1–S3 122.29(19). Cent1: center of the Cp^Ar5 ring.

In order to gain more insights into the previously unobserved reactivity of complex 4 toward CO₂, DFT calculations (B3PW91) were carried out (Fig. 5). According to these calculations, the reaction begins with the insertion of a CO₂ molecule into a Sm–H bond, viaTS1 and an activation barrier of 12.2 kcal mol⁻¹, a value typical of an insertion of a double-bond into to a Ln–H bond. The reaction then yields a very stable formate-hydride di-samarium(II) intermediate (Int2), with the formate ligand in this complex μ²-bonded to the two Sm centers. A second CO₂ molecule is readily inserted into this intermediate with a very low barrier (5.1 kcal mol⁻¹) to form the bisformate di-samarium(II) intermediate (Int3), calculated to be thermodynamically highly stable (−113.4 kcal mol⁻¹). Int3 then reacts with a third molecule of CO₂ in an expected redox fashion. Indeed, each of the two Sm(II) centers undergoes a single electron transfer (SET) to CO₂, which becomes doubly reduced, and each Sm center becomes oxidized to +III. This intermediate (Int4), usually called the “key intermediate” in SET reactions, adopts a dioxocarbene form as previously reported by Paparo et al. in titanium chemistry.³⁹ This step is slightly endothermic, by 5.7 kcal mol⁻¹, so that Int4 is expected to be quite reactive. Thus, Int4 readily reacts with a fourth molecule of CO₂, with this reaction implying the release of one DABCO molecule to allow CO₂ to bind to one Sm(III) center (Int5). This ligand exchange is endothermic by 9.9 kcal mol⁻¹ from Int4, that is 15.6 kcal mol⁻¹ from Int3. Then from Int5, a C–O bond formation occurs between the doubly reduced CO₂ and the other CO₂ molecule (bonded to Sm) viaTS3—with an associated barrier of 12.1 kcal mol⁻¹ from Int5, that is 27.7 kcal mol⁻¹ from Int3, in line with a kinetically accessible reaction. Finally according to the calculations, the formation of the C–O bond allows the formation of the carbonate ligand and the release of a CO molecule, as observed in complex 8, with the formation of this complex being highly exothermic (−131.6 kcal mol⁻¹).

Fig. 5 Computed enthalpy profile for the reaction of 4 with four equivalents of CO₂ at room temperature. The energies are given in kcal mol⁻¹.

Conclusions

In summary, we have demonstrated a synthesis, isolation and structural characterization of dimeric divalent samarium hydrido complex 4 bearing a super-bulky penta-arylcyclopentadienyl ligand. Complex 4 showed four-electron reduction toward various substrates, including Se, PhN=NPh and CS₂, to afford the corresponding double Se²⁻, [PhN-NPh]²⁻, and [CS₃]²⁻ bridged trivalent samarium complexes, respectively, without suffering from ligand loss or ligand scrambling. For comparison, hydride insertion and two-electron reductive disproportionation of CO₂ were observed in the formation of mixed-bis-formate/carbonate complex 8, corroborated by DFT calculations. These preliminarily determined reactivities indicate that complex 4 can provide an entry into an exciting new area involving the cooperation of a highly reactive hydride and a strong reducing metal center.

Data availability

All data (experimental procedures and characterization) that support the findings of this study are available within the article and its ESI.† Crystallographic data for complexes 2, 4–9 have been deposited with the Cambridge Crystallographic Data Centre under CCDC 2104460–2104466.

Author contributions

X. S. carried out the synthesis and characterization of complexes 1–9. P. D. assisted in the NMR spectroscopy experiment and crystal structure analysis. T. R. and L. M. performed the theoretical calculations. J. C. conceived this project and provided guidance. The manuscript was written with contributions from all the authors.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21901238 and 22271272). L. M. is a senior member of the Institut Universitaire de France. The Chinese Academy of Science is acknowledged for financial support (LM) and CalMip for a generous grant of computing time.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, spectroscopic characterization. CCDC 2104460–2104466. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc03104k

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