Low temperature isolation of a dinuclear silver complex of the cyclotetraphosphane [ClP(μ-PMes*)]₂

Abstract

The reaction of the cyclotetraphosphane [ClP(μ-PMes*)]₂ (1, Mes* = 2,4,6-tri-tert-butylphenyl) with Ag[Al(OR^F)₄] (R^F = CH(CF₃)₂) resulted in a labile, dinuclear silver complex of 1, which eliminates AgCl above −30 °C. Its properties were investigated by spectroscopic methods, single crystal X-ray diffraction and DFT calculations.

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Introduction

Ring systems composed of Group 15 elements (pnictogens) are an intriguing aspect of main group chemistry.¹ Especially, those comprising a binary N₂E₂ (E = P, As, Sb, Bi) scaffold represent versatile reagents in pnictogen chemistry.² Metal complexes of such ring systems were investigated to quite some extent,^3–5 including studies about their potential application in catalysis⁶ and even anti-tumour studies.⁷ Interestingly, despite the indisputable importance of phosphorus compounds as ligands in catalysis,⁸ the coordination chemistry of homologous P₄ ring systems (cyclotetraphosphanes) has received much less attention. Examples include mainly P₄R₄ systems (A, Scheme 1; R = organic substituent) which coordinate to transition metal fragments such as metal carbonyls,^9–13 metal halides¹⁴ or phosphane substituted metal complexes.¹⁵ The coordination chemistry of unsubstituted P_n scaffolds, on the other hand, is more versatile;^16,17 in particular, there are numerous examples of coordination to P₄, which can adopt either a tetrahedral (B1, B2),^18–22 bicyclic (C1, C2)^23–26 or rectangular/square planar structure (D1–D3)^27–31 in the complex (Scheme 1). Likewise, the coordination chemistry of the cyclo-P₅⁻ moiety was well investigated; it found application in a variety of syntheses ranging from “simple” molecules such as Cp*FeP₅ (Cp* = pentamethylcyclopentadiene)^32,33 to chain-like poly-cations^34,35 or spherical macromolecules.^36–39

Scheme 1 Common coordination patterns of various P₄ scaffolds.

Results and discussion

Pursuing our interest in the chemistry of the recently reported cyclotetraphosphane [ClP(μ-PMes*)]₂ (1),⁴⁰ we explored its reactivity towards silver salts of the weakly coordinating anions [Al(OR^F)₄]⁻ (R^F = CH(CF₃)₂) and [B(C₆F₅)₄]⁻.^41,42 Taking advantage of the bulky, weakly coordinating anion and naked Ag⁺ ion – in terms of both stabilization and halide abstraction abilities – we hoped to find access to the dark red cyclotetraphosphenium ion [ClP(μ-PMes*)₂P]⁺ (2⁺), which had previously been observed as an intermediate during the reaction of 1 and the Lewis acid GaCl₃ (Scheme 2, top).⁴³ However, when mixing Ag[Al(OR^F)₄] (4) and 1 in a 1 : 1 ratio in CH₂Cl₂ at low temperatures, no precipitation of AgCl was observed. Instead, crystallization at −80 °C afforded colourless crystals that were identified as CH₂Cl₂ solvate of a dimeric silver complex of 1 (5[Al(OR^F)₄]₂, Scheme 2, bottom; yield of isolated substance: 28%).

Scheme 2 Top: the reaction of cyclophosphane 1 with GaCl₃ led to the formation of the intermediate 2⁺. Bottom: the reaction of 1 and 4 at low temperatures yielded the dinuclear silver complex 5²⁺ (R = Mes*).

Molecular structure

Most interestingly, both chlorine atoms remained at the P₄ scaffold; nonetheless two rather long Ag⋯Cl contacts were observed, which are shorter than the sum of van der Waals radii (Ag1–Cl2A 3.496(1) Å, Ag1–Cl2A′ 3.641(2) Å; cf. ∑r_vdW = 4.35 Å)⁴⁴ as revealed by single crystal X-ray diffraction (Fig. 1). The P–Ag bond lengths (P1A–Ag1 2.394(2) Å, P3a–Ag1′ 2.391(2) Å) compare well to the sum of the covalent radii (2.39 Å),⁴⁵ whereas the Ag–Ag distance (3.0511(7) Å) lies between the sum of the covalent (2.56 Å)⁴⁵ and van der Waals radii (5.06 Å).⁴⁴ The silver atoms are almost linearly coordinated (177.52(5)°) and lie in a perfect plane with the Mes* substituted P atoms. Strikingly, the configuration at P2 is inverted in comparison with the starting material, where both chlorine atoms are arranged in an equatorial position with respect to the P₄ ring system. The dinuclear silver complex is nicely shielded by all four Mes* substituents: the Ag atoms and the equatorial Cl atoms (Cl2A, Cl2A′) are protected by the ortho-tBu groups, while the axial Cl atoms (Cl1A, Cl1A′) are sandwiched between the phenyl rings (Fig. S1, ESI†). A similar coordination pattern was previously observed in [(Cy₄P₄)₂Sb₂Cl₂]²⁺ (Cy = cyclohexyl), where a planar Sb₂Cl₂ scaffold is coordinated by two Cy₄P₄ rings.⁴⁶ To the best of our knowledge, it is the only other example of a dinuclear metal complex capped by two P₄ ring systems.

Fig. 1 Molecular structure of 5²⁺. Ellipsoids are set at 50% probability (173 K). The cation is situated on a crystallographic inversion centre and thus displays C_i symmetry. Selected bond lengths [Å] and angles [°]: P1a–Ag1 2.394(2), P1a–P2a 2.235(2), P1a–P4a 2.242(2), P2a–Cl1a 2.059(2), P2a–P3a 2.232(2), P3a–Ag1′ 2.391(2), P3a–P4a 2.252(2), P4a–Cl2a 2.069(2), Ag1–Ag1′ 3.0511(7), P3a′–Ag1–P1a 177.52(5), P1a–Ag1–Ag1′ 90.68(4), P3a′–Ag1–Ag1′ 87.24(4).

Spectroscopic characterization

The low temperature ³¹P NMR spectrum (−60 °C) of 5[Al(OR^F)₄]₂ showed a complex AA′BB′MM′M′′M′′′XX′ spin system (Fig. 2). The A and B part were assigned to the inequivalent Cl substituted P atoms (axially substituted: 126.9 ppm, equatorially substituted: 122.1 ppm), the M part to the Mes* substituted P atoms (16.8 ppm) and the X part to the Ag nuclei. The ¹J(³¹P–³¹P) coupling constants amount to −205 (¹J_AM) and −248 Hz (¹J_BM); the ¹J(³¹P–¹⁰⁷Ag) coupling is −468 Hz (¹J_MX), which compares to reported (unsigned) coupling constants of other two-coordinate complexes such as [(Ph₃P)₂Ag]⁺ (552 Hz) or [(p-Tol₃P)₂Ag]⁺ (496 Hz).^47,48 Due to the complex multiplet structure and generally low solubility of 5[Al(OR^F)₄]₂, the signal to noise ratio of the NMR spectrum was rather poor, which is why only the large ¹J coupling constants could be determined unambiguously, while all smaller coupling constants have higher uncertainties. Nonetheless, the experimental data agree well with calculated NMR shifts and coupling constants (Table S3†).

Fig. 2 Experimental (up) and simulated (down) ³¹P NMR spectrum of 5²⁺. Starred lines are due to slow elimination of AgCl even at low temperatures.

In the solid state Raman spectrum (at −60 °C), characteristic bands of the [P₄Ag]₂ fragment are observed at 464 cm⁻¹ (P–P stretching within the P₄ scaffold), 493 cm⁻¹ (P–Cl stretching) and 514 cm⁻¹ (P–C stretching). Further intense signals in the Raman spectrum can be attributed to the Mes* moieties (e.g. 563, 603 cm⁻¹: tBu deformation; 739 cm⁻¹: phenyl ring deformation; 1007, 1018 cm⁻¹: Me deformation; 1130 cm⁻¹: combination of ring and CH deformation; 1582 cm⁻¹: ring stretching; 2900–3000 cm⁻¹: CH stretching of Me groups) and the [Al(OR^F)₄]⁻ anion (e.g. 563 cm⁻¹: CF deformation; 746 cm⁻¹: symmetrical AlO₄ stretching; 818 cm⁻¹: combination of deformation at O and CF₃; 1364, 1386 cm⁻¹: CH deformation; 2873 cm⁻¹: CH stretching). The assignments of the signals were made on the basis of computed vibrational data.

Computational study

To further investigate the bonding situation in 5²⁺, density functional theory (DFT) calculations were performed.† According to NBO analysis,⁴⁹ there is no significant bonding interaction between the two Ag atoms, as already indicated by the nearly linear coordination of the Ag centres. Furthermore, both Wiberg bond index (0.31) and natural bond index (0.56) indicate a low covalent character of the Ag–P bonds; accordingly, the natural Lewis representation comprises two distinguished Ag⁺ cations and two neutral cyclophosphane moieties. Second order perturbation analysis reveals two stabilizing interactions per Ag⁺ cation between the empty s-orbital and the lone pairs (LPs) of the flanking P atoms (346.2 kJ mol⁻¹ each), which is consistent with a classical dative bond from P to Ag (Fig. 3). The natural partial charge of each Ag centre is +0.59e, while each of the four coordinating P atoms bears a charge of +0.17e. Hence, the formal charge transfer amounts to −0.41e per Ag⁺ ion.

Fig. 3 Left: molecular orbitals (MOs) of 5²⁺. The HOMO shows contributions to the LPs at the coordinating P atoms, the HOMO−8 encompasses the LPs at Cl and the other P atoms as well as bonding within the P₄ ring systems. Right: natural bond orbital (NBO) representation of a LP at phosphorus and the corresponding natural localized molecular orbital (NLMO), which consists mainly of a formal sp hybrid orbital at P (86%) and an s orbital at Ag (11%), thus illustrating the bonding between these two centres.

Intramolecular elimination of AgCl

When the reaction mixture of the cyclophosphane 1 and the silver salt 4 was allowed to warm to temperatures above −30 °C, precipitation of a white solid was observed, indicating elimination of AgCl. In situ³¹P NMR spectroscopy revealed that the intermediately formed silver complex 5²⁺ decomposed above that temperature (Fig. S3†), yielding the bicyclic cation [Mes*P₄(Cl)Mes*]⁺ (3⁺, Scheme 3), which had previously been obtained by reacting 1 with GaCl₃ (Scheme 2, top).⁴³ The same reaction outcome was observed when warming a solution of pure 5[Al(OR^F)₄]₂. In this respect, 5[Al(OR^F)₄]₂ can be viewed as an isolable intermediate, which demonstrates that the eventual AgCl elimination does not occur via direct attack of a silver ion, but rather via complexation and a subsequent intramolecular elimination reaction.

Scheme 3 Above −30 °C, the silver complex 5²⁺ eliminates AgCl, which leads to the formation of the bicyclic phosphino–phosphonium cation 3⁺.

Interestingly, when treating 1 with Ag[B(C₆F₅)₄] (6) under the same reaction conditions, precipitation of AgCl was observed even at −80 °C, leading once more to the formation of 3⁺, as indicated by ³¹P NMR spectroscopy. However, an intermediate similar to the cyclotetraphosphenium cation 2⁺ could not be observed in any of these reactions.

Conclusions

In conclusion, we present a thermally labile dinuclear silver complex capped by two P₄ ring systems, which eliminates AgCl at temperatures above −30 °C. Thus it can be considered an intermediate of the chloride abstraction from [ClP(μ-PMes*)]₂, demonstrating that the reaction occurs via an intramolecular rather than an intermolecular process.

Acknowledgements

We would like to thank Dr Dirk Michalik for low temperature NMR measurements as well as the Fonds der Chemischen Industrie (scholarship for JB) and Deutsche Forschungsgemeinschaft (SCHU 1170/11-1) for funding.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details, crystallographic and spectroscopic data. CCDC 1417701. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03928b

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