Functionalization of P₄ in the coordination sphere of coinage metal cations

Abstract

Selective functionalization of white phosphorus is achieved by addition of ArLi to unique cationic coinage metal η²–P₄ complexes. This novel approach allows controlled P–C bond formation using the bulky DmpLi (Dmp = 2,6-Mes₂C₆H₃) and the unencumbered MesLi, giving sterically diverse doubly complexed RP₄ butterfly derivatives in a single step.

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Controlling direct P–C bond formation using P₄ as starting material is of interest in avoiding chlorinated intermediates, such as PCl₃, for the production of organophosphorus compounds. Yet this task is extremely challenging due to the highly reactive nature of the P₄ tetrahedron.¹ Currently, several selective methods have been developed, like the use of ambiphilic carbenes pioneered by the group of Bertrand,² and the metal-mediated radical functionalization of P₄ reported by Scheer et al. (A; R = Cp^R, Scheme 1)³ as well as by Cummins and co-workers (R = Dmp),⁴ who also demonstrated facile P-functionalization chemistry by embedding photochemically generated P₂ fragments into organic frameworks (B).⁵ In contrast, conventional methods for the formation of P–C bonds,⁶ such as the use of organolithium and Grignard reagents, have been less fruitful due to the low selectivity and complex product distributions associated with their reactions with P₄.⁷ An intriguing exception was recently described by Hill, who achieved selective activation of P₄ using a β-diketiminato organomagnesium compound, producing the [nBu₂P₄]²⁻ dianion C,⁸ which is related to the thallium tetraphosphabutadienediide [Ar₂P₄]²⁻ salt D reported by Power et al.⁹ We showed that the reactivity of bulky ArLi reagents toward P₄ can be controlled in the presence of Lewis acids (B(C₆F₅)₃ and BPh₃), giving the LA-stabilized bicyclo[1.1.0]tetraphosphabutanides [ArP₄·LA]⁻E that can subsequently be functionalized selectively generating the neutral disubstituted bicyclic phosphanes ArP₄R (type A) and the doubly coordinated tetraphosphides [ArP₄·(LA)₂]⁻F.¹⁰ Key in this approach is the irreversible formation of the transient phosphide [RP₄]⁻ that is directly trapped by the Lewis acid. Note that P₄ does not form an adduct with BPh₃ or even B(C₆F₅)₃,¹⁰ and therefore requires the use of sterically encumbered FLP-type ArLi/LA combinations to avoid quenching. In this work, we present an alternative strategy by using novel cationic coinage metal based Lewis adducts of P₄ as synthon that now tolerate varied bulk on the ArLi reagents, as demonstrated by the selective addition of Dmp (Dmp = 2,6-dimesitylphenyl) and mesityl lithium, resulting in the formation of unique doubly complexed RP₄ butterfly cations.

Scheme 1 Methods allowing selective direct P–C bond formation using P₄. Dmp = 2,6-dimesitylphenyl; BDI^dipp = HC{C(Me)N(2,6-iPr₂C₆H₃)}₂; Mes* = 2,4,6-tBu₃C₆H₂; Cp^R = Cp^BIG, Cp′′′, Cp*, Cp^4iPr.

Commercially available IPrMCl (M = Cu, Au; IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene) in combination with Li⁺ [Al(pftb)₄]⁻ (pftb = perfluoro-tert-butoxy)^11,12 as chloride scavenger were found to be suitable starting materials allowing the isolation of readily available LA–P₄ adducts. The complexation of P₄ was achieved by dropwise addition of a solution of IPrMCl (1 equiv.; M = Cu, Au) in DCM to a suspension of white phosphorus (1.1 equiv.) and Li[Al(pftb)₄] (1 equiv.) in DCM at 0 °C (Scheme 2), which resulted in a sharp downfield shifted singlet in the ³¹P{¹H} NMR spectrum in the case of Cu(I) (−483.1 ppm), and a lower field and broadened singlet for Au(I) (−464.4 ppm), indicating both P₄ tetrahedra to be coordinated dynamically to the cationic metal centers (free P₄ in CD₂Cl₂: −522.0 ppm). The dynamics were confirmed by VT NMR spectroscopy at −90 °C,¹³ revealing broadening of the ³¹P signal for Cu–P₄ complex 1a, and two broad triplets for Au–P₄ analogue 1b (δ³¹P: −453.3 and −462.1 ppm, 2 : 2 ratio; ¹J_P,P = −209.8 Hz). Both novel complexes were isolated as white powders in 92% (1a) and 87% (1b) yield, respectively, and are unique examples of heteroleptic cationic P₄ coinage metal complexes, complementing the homoleptic series [M(η²–P₄)₂]⁺ reported by Krossing^14a–d (M = Ag, Cu) and Slattery et al.^14e (M = Au), and the neutral copper complex [NacnacCu(η²–P₄)] isolated by Scheer and coworkers.^14f

Scheme 2 Synthesis of cationic η²–P₄ complexes of copper and gold (pftb = OC(CF₃)₃; dipp = 2,6-diisopropylphenyl).

The A₂B₂ spin-system of gold(I) complex 1b observed at low temperature by ³¹P NMR spectroscopy is indicative of η²–P₄ coordination, which was confirmed by a single-crystal X-ray analysis (Fig. 1)¹⁵ that showed nearly equal Au1–P1 (2.4043(17) Å) and Au1–P2 distances (2.4286(19) Å), a distorted trigonal planar Au center with a short Au1–C1 bond (2.037(5) Å), and an acute P1–Au1–P2 angle (57.79(7)°). A comparison of the P–P bonds in “free” P₄ (2.1994(3) Å, determined by gas-phase electron diffraction¹⁶) with those in 1b⁺ shows a contraction of the P3–P4 bond (2.148(3) Å), as well as shortened P1/P2–P3/P4 bonds (2.155(3)–2.167(4) Å), but an elongated P1–P2 bond (2.335(3) Å) due to coordination to gold, albeit less pronounced than the one found in [Au(η²–P₄)₂][GaCl₄] (i.e. 2.410(1) Å^14e).

Fig. 1 Molecular structure of 1b⁺ in the crystal¹⁵ (ellipsoids are set at 50% probability; [Al(OC(CF₃)₃)₄]⁻ counter-ion and CH₂Cl₂ solvent molecule omitted). Selected bond lengths [Å] and angles [°]: P1–P2 (2.335(3)), P3–P4 (2.148(3)), P1–P3/P4 (2.167(4)/2.164(3)), P2–P3/P4 (2.156(4)/2.155(3)), Au1–P1/P2 (2.4043(17)/2.4286(19)), C1–Au1 (2.037(5)); C1–Au1–P1 (156.75(14)), C1–Au1–P2 (141.92(14)), P1–Au1–P2 (57.79(7)).

To analyze the bonding situation of 1 in more detail, we resorted to AIM analyses^17,18 on the gas-phase optimized structures of 1a⁺ and 1b⁺,¹⁹ which revealed bond critical points (BCP) between P1 and P2 (ρ = 0.079 a.u. (ε = 1.10) in 1a⁺ and 0.074 a.u. (ε = 0.93) in 1b⁺) with only a slightly lower electron density compared to that computed for the naked P₄ (ρ = 0.105 a.u.; ε = 0.10),¹³ confirming the coordinating P₄ fragments to remain intact, disfavoring oxidative addition by P–P bond cleavage. Interestingly, examination of the Laplacian of the electron densities (∇²ρ) in the P1–P2 BCPs indicated a stronger P₄–M⁺ interaction in gold complex 1b⁺ (0.056 a.u.) than in Cu derivative 1a⁺ (0.033 a.u.), which is in agreement with the observed ³¹P{¹H} NMR shifts (−483.1 vs. −464.4 ppm for 1a and 1b, respectively). ETS-NOCV²⁰ analyses of the M⁺–P₄ bonds concur with these observations,¹⁸ revealing indeed a higher bonding energy for the Au complex (ΔΔE = 1.2 kcal mol⁻¹), with the most prominent difference found for the orbital interactions, showing larger contributions for σ donation (1b⁺ −36.7; 1a⁺ −25.9 kcal mol⁻¹) and concurrent π back-donation (1b⁺ −21.4; 1a⁺ −20.7 kcal mol⁻¹), attributable to the influence of relativistic effects on the valence shell of Au(I).^21,22

This difference in bonding energy is also reflected in the stability of 1avs.1b. Namely, dissolving 1a in toluene directly led to complete displacement of P₄ at room temperature, whereas 1b is indefinitely stable under those conditions,²³ rendering Au complex 1b a suitable building block for the functionalization of P₄. As proof of concept, we first selected the bulky DmpLi to react with 1b, which proved successful in the synthesis of the LA-stabilized Li⁺ [DmpP₄·B(C₆F₅)₃]⁻.^10a Hence, a solution of DmpLi (1 equiv.) in toluene was slowly added to a solution of 1b (1 equiv.) in toluene at −78 °C, revealing an AMX₂ spin system in the ³¹P{¹H} NMR spectrum (−105.5 (P1), −118.7 (P4) and −327.9 (P2, P3) ppm in a 1 : 1 : 2 ratio, respectively), indicative for a non-symmetrically substituted P₄ butterfly.^10,25 Interestingly, ¹H NMR analysis revealed the presence of two NHC moieties instead of only one needed for the anticipated neutral DmpP₄AuIPr 2 (Scheme 3), which suggests the formation of a doubly coordinated RP₄ complex.

Scheme 3 Functionalization of P₄ by reaction of ArLi with 1b, with the proposed intermediate 2 in brackets (pftb = OC(CF₃)₃; dipp = 2,6-diisopropylphenyl).

Indeed, X-ray crystal structure determination of colorless crystals obtained by layering a DCM solution with n-pentane, displayed the non-symmetrical [DmpP₄·(AuIPr)₂][Al(pftb)₄] 3 (Fig. 2) featuring a unique bimetallic gold fragment, with similar P4–Au1/Au2 distances (2.2924(7)/2.2860(7) Å) and a Au1–P4–Au2 angle of 128.02(3)°, which is larger than found in the triaurated cation [RP(AuPPh₃)₃]⁺ (av. 106°),²⁴ likely due to the steric repulsion between the large NHC ligands. The P4–P2/P3 bonds (2.1919(10)/2.2077(10) Å) are slightly contracted compared to the P1–P2/P3 bonds (2.2140(10)/2.2240(11) Å), and are similar in length to the bridgehead P2–P3 bond (2.1992(11) Å). These structural parameters are akin to those reported for the cationic [Mes*₂P₄Cl]⁺ of Schulz et al.^25a as well as to those of the bis-LA complexed anions [Mes*P₄·(LA)₂]⁻ (LA = BH₃, W(CO)₅) reported by us.^10b Intriguingly, the bicyclic P₄ entity in 3⁺ is sterically highly shielded, as illustrated by a space-filling model (Fig. 2, right), reminiscent of the incorporation of white phosphorus in the self-assembled [Fe₄L₆]⁸⁺ container reported by Nitschke and co-workers.²⁶

Fig. 2 Left: Molecular structure of 3⁺ in the crystal¹⁵ (ellipsoids are set at 50% probability; [Al(OC(CF₃)₃)₄]⁻ counter-ion and disordered solvent molecules omitted). Selected bond lengths [Å], angles and torsion angle [°]: P1–P2/P3 (2.2140(10)/2.2240(11)), P4–P2/P3 (2.1919(10)/2.2077(10)), P2–P3 (2.1992(11)), Au1–P4 (2.2924(7)), Au2–P4 (2.2860(7)), C13–P1 (1.865(3)); Au1–P4–Au2 (128.02(3)); P1–P2–P3–P4 (100.98(4)). Right: Space-filling model of 3⁺.

The formation of 3 could be optimized by using two equivalents of 1b, which allowed its isolation in 67% yield. Bis-gold complex 3 is likely formed via neutral exo,exo-ArP₄AuIPr 2 (Scheme 3) that displaces a P₄ molecule from a second equivalent of gold complex 1b, which was computed to be energetically favorable by −43.1 kcal mol⁻¹,²⁷ and acts as a monodentate ligand (via P4) for [IPrAu]⁺, displaying reactivity analogous to the recent coordination of bicyclic Mes*₂P₄ to GaCl₃^25b shown by Schulz et al., and of [{Cp′′′Fe(CO)₂}₂(μ,η^1:1-P₄)] toward [Cu(MeCN)]⁺ presented by the group of Scheer.²⁸

Next, we assessed the reactivity of 1b toward the less encumbered nucleophile MesLi,²⁹ which was not feasible in our original approach (E, Scheme 1)¹⁰ as combining MesLi with P₄ in the presence of BPh₃ exclusively produces Li⁺ [MesBPh₃]⁻.¹³ Gratifyingly, formation of the bicyclic tetraphosphane [MesP₄·(AuIPr)₂][Al(pftb)₄] (4) proceeded readily upon mixing MesLi and 1b (2 equiv.) in toluene at −78 °C, showing a distinct set of three ³¹P{¹H} resonances at −110.6 (P1), −119.9 (P4) and −314.5 (P2, P3) ppm (1 : 1 : 2 ratio), and an additional signal for free P₄. The product could be isolated in 62% yield, and was confirmed to contain only one mesityl unit by mass spectrometry (ESI) and ¹H NMR spectroscopy, and two flanking IPrAu moieties.¹³ In contrast to related Aryl₂P₄ species, which feature either bulky 2,4,6-tBu₃C₆H₃ (Mes*)^7c,10,25a,b or terphenyl^4,9,10a groups, 4 is the first example of a mesityl-substituted P₄ butterfly, which illustrates the merit of this novel P₄-functionalization strategy in controlling direct P–C bond formation using organolithium reagents.

In summary, addition of Dmp or mesityl lithium to the coinage metal based P₄–LA adduct 1b gives the unique bimetallic ArP₄-butterfly cations 3 and 4. This novel approach allows for varied bulk on the organosubstituents in a single controlled step, showing facile functionalization of P₄. Currently, we are defining the scope of this new methodology and are exploring the application of 1 in new P₄ transformations.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1440355 and 1440356. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10037b

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