Reversible P–P bond cleavage at an iridium(III) metal centre

Abstract

Treatment of a κ¹-P-monodentate bicyclic diphosphane iridium(III) complex with a labile gold(I) precursor afforded an unusual Ir^III/Au^I complex in which the P–P single bond has been cleaved. This reaction was cleanly reversed upon addition of tertiary phosphine. Carbon–carbon bond activation, across neighbouring P₂C₂N rings of the coordinated bicyclic diphosphane, occurred upon thermolysis of the Ir^III/Au^I complex.

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Phosphorus based compounds underpin many significant advances that have been achieved in the fields of synthesis, reactivity and mechanistic understanding, and in catalysis. In this context, the ubiquitous tertiary phosphines stand out as excellent examples of highly tuneable ligands for various transition metals. By contrast, the coordination chemistry of P-based compounds such as terminal/bridging phosphides (PR₂⁻)¹ and, to a lesser extent, diphosphanes (R₂P–PR₂),² have witnessed relatively few studies. In spite of this, there is a growing interest³ in new synthetic routes to R₂P–PR₂ compounds given also the importance of P–P bonded species in organophosphorus chemistry. Metal bound phosphide ligands can undergo P–P radical coupling leading to dimeric products,⁴ in a few instances this process is reversible (Chart 1).⁵ Furthermore, diphosphanes have been shown to undergo homolytic P–P bond cleavage affording R₂P˙ radicals.⁶ The reactivity of R₂P–PR₂ towards soft transition metals has been shown to result in rapid P–P bond cleavage and formation of terminal or bridging phosphido ligands (Chart 1). A terminal phosphido Rh^V intermediate has been implicated in the dehydrocoupling of HPR₂ affording R₂P–PR₂.⁷ Tsipis and co-workers also reported the reversible transformation of two bridging PPh₂⁻ ligands to a neutral, μ-Ph₂P–PPh₂ ligand at a trinuclear Pt₂Pd core.⁸ Herein we describe the intramolecular P–P cleavage of a well-defined Ir^III(κ¹-R₂P–PR₂) complex upon treatment with a labile gold(I) source and, moreover, shown this to be a reversible process. Under thermal conditions we also observed an unexpected C–C bond activation across both P₂C₂N rings of a coordinated bicyclic diphosphane.

Chart 1 Interplay between PR₂⁻ and R₂P–PR₂ (R = alkyl, aryl) metal chemistry via oxidative P–P coupling/cleavage reactions.

Various groups^2,3,6,9–12 have reported the synthesis and reactivity of several types of R₂P–PR₂, with acyclic variants being most common. We reasoned a “rigid” R₂P–PR₂ would reduce P–P bond rotation/dissociation yet still function as a bridging ligand. Accordingly, the new air stable bicyclic diphosphane, P–P(OMe), was readily synthesised in 44% yield by a one-step reaction between [P(CH₂OH)₄]Cl and 4-H₂NC₆H₄OMe (see ESI† for details).^13,14 The ³¹P{¹H} NMR spectrum of a CDCl₃ solution of P–P(OMe) displayed a singlet at δ −34.7 ppm and the single crystal X-ray structure (Fig. 1) confirmed a bicyclic five-membered arrangement with a typical P–P bond length of 2.2042(7) Å.¹² The nitrogen atoms are nearly planar (sum of angles approx. 350°) whilst the C(1)–P(1)–P(1′)–C(1′) torsion angle between both five-membered rings is 118.65(6)°. Conformational restriction imposed by the two adjacent, five-membered, P₂C₂N rings dictates the orientation of the two phosphorus lone pairs are syn disposed.

Fig. 1 Molecular structure of P–P(OMe) in the solid state. Thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms have been omitted for clarity. Selected bond length [Å]: P(1)–P(1′) 2.2042(7) Å. Symmetry operator: 1 − x, 1 y, +z.

Reaction of 2 equiv. of P–P(OMe) and [IrCl(μ-Cl)(η⁵-C₅Me₅)]₂ in CH₂Cl₂ gave 1 in excellent yield (88%) as a yellow solid (Scheme 1). The ³¹P{¹H} NMR spectrum showed the expected AX spectrum consistent with κ¹-mode (Ir^III–P_coord, δ 15.8 ppm; P_free, δ −56.3 ppm, J_PP = 230 Hz) and was further supported by the appearance of two ¹³C signals for the methylene carbon atoms (δ 54.5 ppm, J_PC = 28 Hz; δ 52.1 ppm, J_PC = 24 Hz). Reaction of 1 equiv. of P–P(OMe) and [IrCl(μ-Cl)(η⁵-C₅Me₅)]₂ in CH₂Cl₂ gave the dinuclear Ir^III complex 1′ which showed a singlet at δ −7.2 ppm consistent with a highly symmetric bridged structure and two P-bound {IrCl₂(η⁵-C₅Me₅)} groups. The X-ray structure of 1 (Fig. 2) shows a slightly contracted P–P bond length of 2.1836(19) Å, with respect to P–P(OMe), whilst the nitrogen atoms in 1 are nearly planar [sum of angles 351° around N(1); 345° around N(2)].

Scheme 1 Synthesis of P–P(OMe), 1, 1′ and 2. Reaction conditions: (i) 2 equiv. P–P(OMe)/[IrCl(μ-Cl)(η⁵-C₅Me₅)] (ii) 1 equiv. P–P(OMe)/[IrCl(μ-Cl)(η⁵-C₅Me₅)] (iii) AuCl(tht) (iv) 1 equiv. PPh₃, – AuCl(PPh₃). R = 4-C₆H₄OMe; Cp* = η⁵-C₅Me₅.

Fig. 2 Molecular structure of 1 in the solid state. Thermal ellipsoids drawn are at the 50% probability level. All hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Ir(1)–P(1) 2.2705(13), Ir(1)–Cl(1) 2.4034(13), Ir(1)–Cl(2) 2.4155(15), P(1)–P(2) 2.1836(19); Cl(1)–Ir(1)–P(1) 89.25(5), Cl(2)–Ir(1)–P(1) 82.33(5), Cl(1)–Ir(1)–Cl(2) 89.49(5).

Given the available free P-coordination site in 1 we sought to explore whether a second, different, soft metal centre could be supported on a R₂P–PR₂ ligand. Hence, reaction of 1 with AuCl(tht) (tht = tetrahydrothiophene) in CH₂Cl₂ at r.t. gave, instead, compound 2 in excellent yield (83%) as a pale yellow solid. The ³¹P NMR spectrum of 2 showed an AX pattern, with a significantly reduced J_PP coupling of 53.4 Hz, consistent with two distinct, non-bonded, P centres and further supported by the ¹H NMR spectrum which showed two sets of aromatic resonances consistent with non-equivalent –C₆H₄OMe rings.

The most significant feature of the X-ray structure of 2 (Fig. 3) is insertion¹⁵ of an “Ir(η⁵-C₅Me₅)Cl” fragment, coupled with P–P bond cleavage [P⋯P interatom separation is 2.8839(18)/2.8941(18) Å] and migration of Cl to P(1)¹⁶ and a terminal AuCl group on P(2). The nitrogen atoms are significantly more distorted (sum of angles 340–346°) and there is a strong aurophilic Au⋯Au interaction [2.9278(3) Å] between neighbouring molecules.¹⁷ The bond lengths for Ir(1)–P(2)/P(2)–Au(2)/Au(1)–Cl(3) are similar to reported examples^18,19 whilst the Ir(1)–P(1)/Ir(1)–P(2) bond distances differ reflecting the bonding arrangements of both phosphorus donors [P(2) bridges both Au^I and Ir^III centres].

Fig. 3 Molecular structure of 2·CDCl₃ in the solid state. Thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms and solvent have been omitted for clarity. Selected bond lengths [Å] and angles [°] (values in parentheses are for the second independent molecule): Ir(1)–P(1) 2.2111(13) [2.2095(14)], Ir(1)–Cl(2) 2.3965(12) [2.3967(13)], Ir(1)–P(2) 2.3318(13) [2.3381(13)], Au(1)–P(2) 2.2429(12) [2.2525(13)]; Cl(3)–Au(1) 2.3252(12) [2.3273(13)], P(1)–Cl(1) 2.0389(17) [2.0510(18)]; P(1)–Ir(1)–P(2) 78.76(5) [78.99(5)], Cl(2)–Ir(1)–P(1) 92.21(5) [95.04(5)], Cl(2)–Ir(1)–P(2) 88.83(5) [87.70(5)], Cl(3)–Au(1)–P(2) 172.32(5) [174.09(5)].

In order to gain more insight into the reaction pathway leading to 2 from 1via2′, we undertook density functional theory calculations (see ESI† for details). An accessible transition state between 2′ and 2 (TS, 18.6 kcal mol⁻¹), following initial migration of a chlorine atom to P(1), with the P–P bond intact was located and optimised (Fig. 4). The formation of 2 from 2′ is exothermic by −3.5 kcal mol⁻¹. It is unlikely steric factors are solely important here given that P–P(OMe) can accommodate two, bulky, {IrCl(η⁵-C₅Me₅)} groups (as in 1′) across the P–P single bond vector suggesting electronic effects contribute. Natural bond orbital (NBO) analysis for 2 (see ESI† for details) confirmed a near perfect sp³ hybridisation of P(2) as a phosphido with a dative bond to AuCl. Conversely, in 2′ enhanced p orbital character is associated to the P–P single bond, leaving the P(2) lone pair sp hybridised and unable to form a strong Au–P bond.

Fig. 4 Energy profile (kcal mol⁻¹) for the P–P cleavage of 2.

When 2 was treated with 1 equiv. of PPh₃, in CDCl₃, clean conversion to 1 occurred (as monitored by in situ³¹P{¹H} NMR, Fig. 5), accompanied by concomitant formation of AuCl(PPh₃) (δ 33.8 ppm). No reaction took place between 1 and AuCl(PPh₃) acting as the source of “AuCl”.

Fig. 5 ³¹P{¹H} NMR spectra of (a) complex 1 (b) complex 2 (c) in situ addition of 1 equiv. PPh₃ to 2 (d) AuCl(PPh₃).

Whilst complex 1 shows good thermal stability (C₇H₈, reflux, 24 h), heating a CDCl₃ solution of 2 for 4 d at ca. 50 °C did not result in any observable formation of 1. Instead, in situ monitoring by ³¹P{¹H} NMR (Scheme 2) revealed the appearance of a new AX pattern (δ 50.5 ppm, −30.2 ppm, J_PP = 55.2 Hz), along with a second minor species and significant amounts of decomposition products. The ¹H NMR spectrum of 3, after fractional crystallisation (∼20% isolated yield) from CDCl₃ solution, showed seven unique aliphatic hydrogens suggesting formation of a new C–C bond across neighbouring P₂C₂N rings. The X-ray structure of 3 (Fig. 6) again confirmed the absence of a P–P bond [P⋯P separation 2.943(3) Å] and on P(1) a chlorine atom and P(2) a terminal AuCl group. The nitrogen atoms are significantly more distorted [sum of angles 343° around N(1); 346° around N(2)]. Most significantly a new C–C single bond [1.511(11) Å] leads to a unusual bicyclic ligand framework. The closest Au⋯Au interaction between neighbouring molecules is approx. 10 Å, significantly different form that observed for 2. Treatment of 3 with 1 equiv. of PPh₃ afforded cleanly 4, as two diastereomers (ca. 50 : 50), as shown by their diagnostic AX patterns (in situ³¹P{¹H} NMR studies, see ESI† for details).

Scheme 2 Thermal conversion of 2 to 3 and reversible P–P coupling to generate 4 as a pair of diastereomers. Reaction conditions: (i) heat, CDCl₃ (ii) 1 equiv. PPh₃, – AuCl(PPh₃). Cp* = η⁵-C₅Me₅.

Fig. 6 Molecular structure of 3·CDCl₃ in the solid state. Thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms and solvent have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Ir(1)–P(1) 2.233(2), Ir(1)–Cl(2) 2.403(2), Ir(1)–P(2) 2.328(2), Au(1)–P(2) 2.245(2), Au(1)–Cl(3) 2.302(2) P(1)–Cl(1) 2.063(3); P(1)–Ir(1)–P(2) 80.34(8), Cl(2)–Ir(1)–P(1) 91.32(8), Cl(2)–Ir(1)–P(2) 90.75(8), Cl(3)–Au(1)–P(2) 177.60(8).

In conclusion, this study highlights the reversible, intramolecular insertion of a late transition metal entity across a P–P single bond of a coordinated diphosphane. By virtue of flexible, neighbouring arene substituents on the diphosphane, we observed an unprecedented C_sp2–C_sp3 coupling affording a highly functionalised P-ligand. Further studies will be directed towards exploring the ligand scope, reactivity and P–P/C–C mechanistic aspects of these transformations.

We thank Johnson Matthey for their donation of IrCl₃·nH₂O, Solvay for kindly providing tetrahydroxymethylphosphonium chloride (THPC) and the UK National Crystallography Service at the University of Southampton for all four data collections and processing. We acknowledge the Lovelace high-performance computing cluster at Loughborough University.

Conflicts of interest

There are no conflicts of interest to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, spectroscopic/analytical data, single crystal X-ray crystallographic data, and computational details. CCDC 2143419, 2143421, 2143424 and 2143445. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2cc00706a

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