Fragmentation, catenation, and direct functionalisation of white phosphorus by a uranium(IV)–silyl–phosphino–carbene complex

Abstract

Room temperature reaction of the uranium(IV)–carbene [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)_0.5]₂ (1, BIPM^TMS = C(PPh₂NSiMe₃)₂) with white phosphorus (P₄) produces the organo-P₅ compound [P₅{C(SiMe₃)(PPh₂)}₂][Li(TMEDA)₂] (2) and the uranium(IV)–methanediide [U{BIPM^TMS}{Cl}{μ-Cl}₂{Li(TMEDA)}] (3). This is an unprecedented example of cooperative metal–carbene P₄ activation/insertion into a metal–carbon double bond and also an actinide complex reacting with P₄ to directly form an organophosphorus species. Conducting the reaction at low temperature permits the isolation of the diuranium(IV) complex [{U(BIPM^TMS)([μ-η²:η²-P₂]C[SiMe₃][PPh₂])}₂] (4), which then converts to 2 and 3. Thus, surprisingly, in contrast to all other actinide P₄ reactivity, although this reaction produces catenation overall it proceeds via P₄ cleavage to functionalised P₂ units. Hence, this work establishes a proof of concept synthetic cycle for direct fragmentation, catenation, and functionalisation of P₄.

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Organophosphorus compounds have numerous vital uses.¹ Industrially, organophosphorus compounds are derived from PCl₃, which is in turn formed by chlorination of white phosphorus (P₄). Thus, there is significant interest in discovering new, direct paths to the derivatisation of P₄. Indeed, the activation and functionalisation of P₄ by complexes of the transition and lanthanide metals² and main group elements,³ and even by free singlet carbenes,⁴ has received significant attention. However, the catalytic derivatisation/functionalisation of P₄ in processes which directly yield organophosphorus compounds is challenging and only beginning to be realised.^2c

In contrast to burgeoning studies of P₄ activation by transition metal and main group compounds, reports of P₄ activation by actinide elements are scarce, being limited to six examples.⁵ Also, although P–P bonds are broken in almost all cases, P₄ is not fragmented into smaller phosphorus-containing units which are suited for functionalisation. In previous work, we reported uranium-mediated aggregation of P₄, generating the triuranium-[P₇] Zintl complex [{U(Ts^Tol)}₃(μ₃-η²:η²:η²-P₇)] (I, Ts^Tol = HC(SiMe₂NC₆H₄-4-Me)₃),^5d as well as the diuranium inverted-sandwich cyclo-P₅ complex [{U(Tren^TIPS)}₂(μ-η⁵:η⁵-cyclo-P₅)] (II, Tren^TIPS = N(CH₂CH₂NSiPrⁱ₃)₃).^5c Complex I is the only actinide-mediated P₄ activation product which has been functionalised further, in two steps overall, to generate compounds containing P–C and P–Si bonds.^5d Recently, we described the synthesis of a family of uranium(IV)–silyl–phosphino–carbene complexes,⁶ which are the first actinide carbene complexes to be free of carbene-stabilising phosphorus(V)-substituents.⁷ We have found the silyl–phosphino–carbene ligand to be nucleophilic and readily transferred, allowing the formation of unusual bonding linkages, such as a uranium(V)–dinitrogen complex, or a uranium–rhodium double-dative bond.^8,9 Therefore, we set out to examine whether the nucleophilic-nature of the silyl–phosphino–carbene ligand would enable direct functionalisation of P₄.

Here, we report activation and direct functionalisation of P₄ by a uranium–silyl–phosphino–carbene complex. This is an unprecedented example of cooperative metal–carbene P₄ activation/insertion into any metal–carbon double bond and also of an actinide complex reacting with P₄ to directly generate an organophosphorus species.^3e Overall, a diorgano-P₅ species is formed. Surprisingly however, by isolation of a reaction intermediate at low temperature, we find that this net catenation reaction actually initially proceeds by fragmentation of P₄, yielding functionalised P₂ units that subsequently aggregate at room temperature. The main uranium by-product is also found to be a precursor to the active uranium–carbene starting complex, thereby establishing the components of a proof of principle synthetic cycle.

Treatment of the uranium(IV)–silyl–phosphino–carbene complex [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)_0.5]₂ (1, BIPM^TMS = C(PPh₂NSiMe₃)₂)⁶ with one molar equivalent of finely-divided P₄ in toluene at room temperature afforded orange crystals of the diorgano-P₅ compound [P₅{C(SiMe₃)(PPh₂)}₂][Li(TMEDA)₂] (2) in 32% isolated yield after work-up (Scheme 1).¹⁰ Decanting and slow evaporation of the mother liquor resulted in isolation of yellow crystals of the uranium-containing product of this reaction, the methanediide complex [U{BIPM^TMS}{Cl}{μ-Cl}₂{Li(TMEDA)}] (3), isolated in 26% yield.¹⁰

Scheme 1 Activation of P₄ by the uranium(IV)–silyl–phosphino–carbene complex 1, yielding complexes 2, 3, and 4.

The solid-state structure of 2 (Fig. 1) reveals it consists of a central cyclo-P₅ core, which resembles the envelope conformation of cyclopentane. The P₅-ring is non-planar and does not show signs of aromaticity, in contrast to the cyclo-P₅ unit in II.^5c Two adjacent phosphorus atoms of the P₅-ring are each coordinated by an individual silyl–phosphino–carbene ([SiPC]) ligand (C1-P3/C2-P4). The phosphine-substituent of each [SiPC] ligand coordinates to the P₅-phosphorus atom adjacent to that coordinated by the other [SiPC] α-carbon (P1–P5/P2–P7). The P–P distances of the P₅-ring (P3 to P7) (av. 2.212(2) Å) are consistent with P–P single bonds.¹¹ The only two-coordinate phosphorus centre of the P₅-ring (P6), features the two shortest P–P distances (P5–P6 : 2.174(2) Å; P6–P7 : 2.171(2) Å), with the other P–P distances ranging from 2.223(1) – 2.249(2) Å. The P1–P5 and P2–P7 interactions, between the carbene–phosphorus substituents and phosphorus atoms of the cyclo-P₅ unit, of 2.1966(14) and 2.1984(15) Å, respectively, are also consistent with P–P single bonds. The C1-P3 and C2-P4 distances are 1.786(4) and 1.776(4) Å, respectively. These values are in-between the sum of Pyykkö's covalent single- and double-bond radii for carbon and phosphorus (1.86 and 1.69 Å, respectively).¹¹ However, the C1-P1 and C2-P2 distances are even shorter, at 1.710(4) and 1.705(4), respectively; suggesting significant P–C multiple bonding or dipolar bond-shortening effects. The geometry of C1 and C2 are near-perfect trigonal planar, with the sum of the angles around both carbon atoms totalling 359.8(12)°.

Fig. 1 Molecular structure of 2 at 100 K with displacement ellipsoids set at 30% probability. Hydrogen atoms and [Li(TMEDA)₂]⁺ counterion are omitted for clarity.

The solid-state structure of 3 was also determined.¹⁰ The structural parameters of this complex are typical for uranium(IV)-BIPM^TMS complexes,^7b and very similar to those of the closely related complex [U(BIPM^TMS)(Cl)(μ-Cl)₂Li(THF)₂],¹² which is the starting material used to prepare 1.⁶

Once isolated in crystalline form, 2 is insoluble in aromatic solvents and it decomposes in ethers and other donor solvents, forming oily residues, which precluded its characterisation by multinuclear NMR and UV/Vis/NIR spectroscopies. It would appear that 2 is formed via the catenation/aggregation of P₄, induced by 1, with formation of two P–C bonds and two new P–P bonds.⁵

In order to gain greater insight into the formation of 2, we set out to isolate uranium-containing intermediates in its formation. Accordingly, reaction of 1 with a single molar equivalent of P₄ in toluene was conducted at −35 °C. The mixture was stirred for 4 hours, then left to stand at −35 °C for 4 hours (Scheme 1), resulting in the formation of red crystals of the diuranium(IV) complex [{U(BIPM^TMS)([μ-η²:η²-P₂]C[SiMe₃][PPh₂])}₂] (4), isolated in 46% yield.¹⁰

The formulation of 4 was confirmed by its solid-state structure (Fig. 2), revealing the salient feature of two uranium ions bridged by two cyclo-1,2-diphosphapropan-1,2-diide [(μ-η²:η²-P₂)C(SiMe₃)(PPh₂)]²⁻ groups. The formally dianionic R₂CP₂ unit features a three-membered CP₂-ring, formed by insertion of 0.5 equivalents of P₄ into the U=C_carbene double bond. Additionally, each uranium centre in 4 is coordinated by the [BIPM^TMS]²⁻ methanediide and a κ¹-phosphino group.

Fig. 2 Molecular structure of 4 at 100 K with displacement ellipsoids set at 50% probability. Hydrogen atoms and lattice solvent are omitted for clarity.

The P2–P3 distance within the CP₂ ring of 4 (2.1762(11) Å) is similar to the average measured for the cyclo-P₅ unit in 2 (2.212(2) Å), and typical of a P–P single bond.¹¹ Additionally, the C2-P2/C2-P3 distances in 4 (1.889(3) Å and 1.892(3) Å, respectively) are typical of P–C single bonds, and somewhat longer than the C1-P3 (1.786(4) Å) and C2-P4 (1.776(4) Å) distances in 2. In 4 the U1–C1 distance is 2.309(3) Å – somewhat shorter than the 2.405(9) Å measured for the parent uranium(IV) complex 1.⁶ This may be due to the loss of the strongly-donating silyl–phosphino–carbene ligand, allowing for an increase in the magnitude of the U=C_BIPM bonding interaction. The U=C bond distance within 4 is typical of a uranium(IV) complex,^7b with this oxidation state assignment supported by charge balancing of the coordinated ligands, and further evidenced by magnetometric measurements performed on 4 (vide infra). Within 4, P1 is clearly angled towards U1, with small P1-C2-P2 and P1-C2-P3 angles (100.84(15) and 102.94(15)°, respectively), and larger Si3-C2-P2/Si3-C2-P3 angles (117.01(16) and 117.20(16)°, respectively). The orientation of the phenyl substituents on P1 suggest that the lone pair of this phosphorus atom is directed towards U1, allowing for a dative P → U interaction. At 3.0748(7) Å, the U1–P1 distance is notably greater than the sum of the single bond covalent radii for uranium and phosphorus (2.81 Å).¹¹ However, values between 2.9 and 3.1 Å are typical of dative P(III) → U(IV) interactions.¹³ The U1-P2/P2’ and U1-P3/P3’ distances within 4 (2.9753(7)/2.9565(7) and 3.0459(7)/2.9974(8) Å, respectively) are somewhat longer than expected for U–P single bonds.¹¹ This is likely due to the bridging nature of the [(μ-η²:η²-P₂)C(SiMe₃)(PPh₂)]²⁻ moieties between both uranium centres.

Variable temperature SQUID magnetometric measurements on a powdered sample of 4 in a 0.5 T magnetic field confirm the diuranium(IV) formulation (Fig. S7, ESI†).^10,14 At 300 K the effective magnetic moment is 3.81 μ_B per molecule (2.70 μ_B per uranium ion). The effective magnetic moment decreases slowly with decreasing temperature from 300 to 50 K, at which point a much more rapid decrease in μ_eff occurs to 1.06 μ_B per molecule (0.75 μ_B per uranium ion) at 2 K. The gradual decrease in effective magnetic moment between 300 and 50 K followed by a sharper decrease is indicative of the binding of strongly donating ligand(s) to uranium(IV), and this behaviour has been observed for 1 and a growing range of uranium(IV) complexes with strong donor ligands.^6,7a,9,14,15

As for 2, 4 decomposes in ethers and is insoluble in aromatic solvents once isolated in crystalline form, which precluded its solution-phase characterisation. To examine whether 4 is an intermediate in the formation of 2, isolated crystalline 4 was combined with an excess of LiCl and TMEDA in toluene and allowed to stand for 24 hours (Scheme 1). After workup, both 2 and 3 were isolated in higher yields (66 and 54%, respectively) than from reaction of 1 with P₄ at room temperature. This suggests that the formation of 2 does not involve the reaction of 4 with unreacted P₄, and is perhaps instead an intermolecular process between molecules of 4 and LiCl/TMEDA. Attempts to synthesise alternative organophosphorus products from the reaction of 4 with various molar quantities of P₄ led to the isolation of 2 and 3, albeit in reduced yields.

To examine whether a closed synthetic cycle for the formation of 2 could be devised, the use of 3 as an alternative precursor to 1 was examined. Straightforward addition of [{Li(TMEDA)}C(H)(Ph)(SiMe₃)], followed by [{Li(THF)}C(H)(PPh₂)(SiMe₃)], to 3 at −78 °C generates 1 in comparable isolated yield (39%) to the published procedure (36%).⁶ Thus, the synthesis of 2 can be carried out within a synthetic cycle, with reuse of the uranium-containing products of the reaction.

To conclude, reaction of 1 with P₄ at room temperature produces the organo-P₅ compound 2, as well as the uranium methanediide complex 3. If the reaction of 1 with P₄ is carried out at low temperature an intermediate complex 4, which results from P₄ fragmentation/insertion across the U=C bond, can be isolated and then converted to 2 and 3, which is likely driven by the formation of strong P–C and U–Cl bonds. Furthermore, a proof of principle synthetic cycle has been established. This work reports the first example of cooperative metal–carbene P₄ activation/insertion into any metal–carbon double bond and also of an actinide complex generating an organophosphorus compound directly from P₄. Whereas actinide reactivity with P₄ usually results only in reductive ring-opening of P₄ and/or catenation, this work reveals a catenation that surprisingly proceeds via fragmentation of P₄ to functionalised P₂ units that subsequently aggregate. More broadly, these results suggest that the reactivity of metal–carbene complexes, particularly those of the early metals with polarised M=C bonds, with P₄ should be investigated. Such studies could provide a new, divergent, approach from the traditional two-step method of reduction followed by functionalisation, to develop the synthetic strategy of direct preparation of organophosphorus compounds from P₄ that would be a basis for further derivatisation.

We gratefully acknowledge the UK EPSRC (grants EP/M027015/1, EP/P001386/1, and EP/S033181/1), ERC (grant CoG612724), Alexander von Humboldt Foundation (Friedrich Wilhelm Bessel Award to STL), and The University of Manchester for funding and support.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic, structural, and spectroscopic details. CCDC 2060876–2060878. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc01741a

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