On the Reaction of Pacman‐Phosphanes with Lewis Acids

Abstract

The reactions of Pacman phosphanes (1a) with Lewis acids such as R₃B (R = C₆H₅, C₆F₅) and GaCl₃ were investigated. While equilibria between mono‐ and di‐adducts were found in the reaction with BPh₃, B(C₆F₅)₃ preferentially formed the di‐adduct (1a·2B(C₆F₅)₃), which could be isolated and fully characterized. density functional theory (DFT) calculations showed that the N‐bonded adducts are thermodynamically more stable than the P‐bonded adducts. In contrast to the reaction of the Pacman phosphane with boranes, GaCl₃ reacted with 1a to give an unusual [Pacman‐GaCl₂]⁺[GaCl₄]⁻ salt, featuring a nearly planar GaN₂C₂ heterocycle as part of the Pacman macrocycle. In addition, the coordination geometry on one P atom changes from triple to an unusual quadruple coordination, although it is still a formal P atom in the + III oxidation state, as shown by quantum chemical calculations.

The reactivity of a Pacman phosphane (a macrocyclic, multidentate phosphane ligand) with Lewis acids such as BPh₃, B(C₆F₅)₃, and GaCl₃ were investigated, revealing distinct bonding motifs and coordination behaviors. While B(C₆F₅)₃ forms a stable di‐adduct, GaCl₃ induces an unusual structural rearrangement with the formation of a [Pacman‐GaCl₂]⁺[GaCl₄]⁻ salt and a rare tetracoordinated phosphorus center in the + III oxidation state.

1. Introduction

Pacman ligands, first introduced in the late 1970s to mimic enzymes and study metal‐metal interactions, are a class of nitrogen‐based heterocycles widely used as chelating ligands in coordination chemistry (Scheme 1, top left).^{[ 1} , ^{2 ]} The term “Pacman ligand” is a colloquial reference to their unique structural characteristics, which resemble the shape of the popular arcade character “Pac‐Man.”^{[ 3} , ^{4 ]} These ligands are typically large, multidentate molecules that can wrap around or “capture” metal ions in a specific, constrained geometry, resembling Pac‐Man's open mouth. This ability to coordinate with (mostly) two metal centers in a precise and selective manner makes Pacman ligands particularly interesting in various applications, such as in catalysis, materials science, and bioinorganic chemistry.^{[ 5} , ^{6 ]} Pacman ligands can be based on porphyrins or calix[4]pyrroles,^{[ 7} , ^{8 ]} featuring mostly one metal ion in both halves of the multidentate Pacman ligand.

Scheme 1.

Top: Pacman metal complex^{[ 20 ]} (left), Pacman phosphane metal complex^{[ 21 ]} (right), and bottom: Pacman phosphane Lewis acid (LA) complexes.

Only recently, Pacman phosphanes were introduced, which are a specific subclass of Pacman ligands that incorporate two phosphorus‐based donor atoms into their structure,^{[ 9} , ^{10 ]} however, in contrast to classical diphosphanes without a direct P‐P bond (Scheme 1, top right).^{[ 11} , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ^{19 ]} Phosphanes are well‐known in coordination chemistry for their ability to donate electron pairs to metal centers, forming stable metal‐phosphane complexes. What makes Pacman phosphanes distinctive is the way two phosphane groups are arranged within the ligand to create a rigid, “C‐shaped” structure that can enclose and “capture” a metal ion in a specific, sterically confined manner.^{[ 9 ]} Typically, Pacman phosphanes consist of two central, unlinked phosphane fragments, each attached to two nitrogen atoms of the Pacman heterocycle, which is structured to create a cavity around both P donor atoms. As a result, when Lewis acidic metal cations are incorporated into the Pacman system, they are usually coordinated by the two phosphorus atoms, but the iminic nitrogen atoms of the Pacman ligand can also act as additional donor atoms ^{[ 9 ]} Therefore, Pacman phosphanes can bind to metal ions through multiple donor sites. The phosphorus atoms provide electron density to the metal center, while the rest of the ligand's structure helps maintain a controlled and stable coordination environment.

We were intrigued by the idea of studying the reaction of Pacman phosphanes with neutral, classical main group element‐based Lewis acids (Scheme 1, bottom), focusing on the adduct formation that can occur via either the embedded central P atoms or the N atoms of the heterocycles. In addition, the formation of FLPs (frustrated Lewis acid‐base pair)^{[ 22} , ²³ , ^{24 ]} is still possible if the steric demand is high enough, so that in this case a corresponding activation chemistry could be possible.

2. Results and Discussion

2.1. Synthesis of Pacman Phosphanes.^{[ 9 ]}

A yellow solution of Pacman ligand dissolved in tetrahydrofuran (THF) is treated with triethylamine at ambient temperature, then cooled down to − 80 °C. At this temperature, PhPCl₂ is added dropwise, forming P‐N bonds in Pacman phosphane 1 as HCl is eliminated. After removal of the THF and extraction with diethyl ether, a mixture of the exo‐exo‐isomer (1a) and endo‐exo‐isomer (1b) is obtained. To separate the exo‐exo‐isomer (as shown in Scheme 2) from the endo‐exo‐isomer, the orange powder is dissolved in THF, concentrated in vacuo, and left overnight for crystallization at 5 °C, resulting in the deposition of orange crystals of the exo‐exo‐isomer with a yield of ca. 40%. Pure (exo‐exo) 1a decomposes at 284 °C and shows a singlet ³¹P{¹H} nuclear magnetic resonance (NMR) signal in CD₂Cl₂ at 68.1 ppm (s).

Scheme 2.

Synthesis of Pacman phosphanes 1.

2.2. Reaction of Pacman Phosphane 1a with Phenyl Boranes

2.2.1. Synthesis

The reaction was first carried out in an NMR tube, which was filled with 1a and one equivalent of colorless BPh₃ dissolved in deuterated dichloromethane, resulting in a clear yellow solution (Scheme 3). The ³¹P NMR spectrum revealed the formation of mono‐adduct 1·BPh₃ (δ[³¹P] = 68.6 and 60.2, doublet) and small amounts of di‐adduct 1·2BPh₃ (δ[³¹P] = 58.4, singlet), with uncoordinated 1a (δ[³¹P] = 68.1, singlet) remaining the major component (Figure 1). After 24 hours another equivalent of BPh₃ was added, and another 24 hours later, four additional equivalents of BPh₃ were added. In both experiments, signals of all three species (1a, 1a·BPh₃, and 1·2BPh₃) were observed; however, mono‐adduct 1a·BPh₃ was now the dominant species, and there was a slightly increased intensity for 1a·2BPh₃.

Scheme 3.

Reaction of 1a with BR₃ (R = Ph, C₆F₅).

Figure 1.

³¹P{¹H} NMR spectrum of 1 : n BPh₃ mixtures (n = 1–6).

Since in the reaction of 1a with BPh₃ only equilibria between 1a, mono‐ and di‐adducts were found, we used a stronger Lewis acid, namely B(C₆F₅)₃, in a second series of experiments to favor adduct formation. Indeed, when one or two or more equivalents of B(C₆F₅)₃ were added, the formation of the novel di‐adduct (1a·2 B(C₆F₅)₃, δ[³¹P] = 61.0, singlet) was observed in the ³¹P NMR experiment almost exclusively alongside traces of mono‐adduct (1a·2 B(C₆F₅)₃, (δ[³¹P] = 72.1 and 21.0 doublet). Even when purified crystals of the di‐adduct were dissolved again, only very small traces of mono‐adduct were found, but no uncoordinated starting material (δ[³¹P] = 68.1). The optimized reaction of 1a and B(C₆F₅)₃ in CH₂Cl₂ yielded a highly pure, crystalline di‐adduct in yields between 50 and 60%, which could be isolated in the form of yellow crystals. 1a·2B(C₆F₅)₃ decomposes above 254 °C and dissolves completely in CH₂Cl₂, albeit with the formation of small amounts of mono‐adduct.

Single‐crystal X‐ray diffraction studies (SC‐XRD) clearly showed the presence of the di‐adduct 1a·2B(C₆F₅)₃ in solid state, but both Lewis acid molecules are bound to the two iminic N atoms of the Pacman ligand, but not to the P atoms of the two phosphane moieties (Figure 2). The typical Pacman structure as illustrated in Scheme 2 is maintained. 1a·2B(C₆F₅)₃ crystallizes in the monoclinic space group P2₁/c with Z = 4. In accord with computations (vide infra), both B(C₆F₅)₃ molecules are attached to iminic N atoms outside the “Pacman mouth.” The B‐N distances of 1.630(4) and 1.637(4) Å are in the expected range of N‐bound B(C₆F₅)₃ donor‐acceptor complexes (cf. Σr _cov(B‐N)  =  1.56 Å,^{[ 25 ]} 1.556(2) and 1.564(2) in the silver dicyanamide salt [Ag(Et₂O)₃][N{CN·B(C₆F₅)₃}₂], 1.611(2) in HCN·B(C₆F₅)₃)₂, and N1–B1 1.664(2) in HN₃·B(C₆F₅)₃).^{[ 26} , ²⁷ , ²⁸ , ²⁹ , ^{30 ]} The formation of the P‐bound adduct would lead to significantly longer P‐B bonds (Σr _cov(P‐B)  =  1.96 Å)^{[ 25 ]} and is thermodynamically less favored according to computations (see below). The distorted pyramidal P atom displays two P‐N bonds in the range of 1.75 – 1.83 Å which corresponds to a P‐N single bond (Σr _cov(P‐N) = 1.82 Å).^{[ 25 ]} The P‐N distances to the not directly bound N4 / N8 atoms lie both around 2.50 Å, which is considerably smaller than the sum of the van der Waals radii (Σr _vdW(P‐N) = 3.32 Å), indicative of weak interactions.^{[ 31 ]}

Figure 2.

Two different views of the molecular structure of 1a·2B(C₆F₅)₃ in the crystal. ORTEPs are shown of all noncarbon ring atoms and ring carbon atoms (T = 123(2) K, ellipsoid probability = 50%). Color code: N blue, C grey, H light grey, P orange, F light blue, and B brown. Selected structural parameters, distances in Å: P1···P2 3.9433(9), P1‐N2 1.827(2), P2‐N6 1.814(2), P1‐N1 1.754(2), P2‐N5 1.750(2), P1···N8 2.499(2), P2···N4 2.496(2), B1‐N3 1.630(3), B2‐N7 1.637(3).

Finally, we tried to use the borane‐Pacman adducts for the activation of small molecules such as CS₂, EtBr, and C₂H₂, but in all these cases no activation was observed (see Supporting Information).

2.2.2. Computations–Structure and Bonding

To shed light on the thermodynamics of the adduct formation, quantum chemical computations were carried out. As we encountered a variety of different isomers with a large number of conformers and rotamers within the Pacman‐phosphane macrocycle, we always conducted an isomer search calculation for each species first, employing the xTB software (GFN2‐xTB level of theory)^{[ 32} , ^{33 ]} as well as the CREST computer code^{[ 34} , ^{35 ]} and CENSO algorithm^{[ 36 ]} (for details, see Supporting Information) to better understand the complex 3D structure of 1 and its Lewis acid adducts. Only the lowest energy isomers/conformers are discussed below. Starting with the Pacman phosphane macrocycle, we have essentially found only two important isomers, namely the exo‐exo (1a) and endo‐exo species (1b), which are separated by only 2.4 kcal/mol, with 1a being the most thermodynamically stable isomer (Scheme 2). Note: The endo‐endo species is optimized to the exo‐exo species 1a. Starting from the isomers 1a and 1b, we have now calculated all possible mono‐ and di‐adducts (Scheme 4). For comparison, in addition to the BPh₃ and B(C₆F₅)₃ adducts, we also calculated BH₃ adducts to account for the possible problem of steric hindrance within the Pacman phosphane.

Scheme 4.

Structural sketch of the mono‐ and di‐adducts under consideration.

The relative stability of the P‐ and N‐bonded mono‐adducts of 1a and 1b (Table 1) shows that the N‐bonded adducts are thermodynamically much more stable in all cases, whereas in the P‐bonded adducts only the BH₃ adduct is a true adduct, while in the BPh₃ and B(C₆F₅)₃ P‐bonded adducts the borane dissociates off to form a weakly bonded van der Waals complex with the Pacman phosphane, which can be attributed to the increased steric demand of these two boranes compared to the small BH₃. In addition, the exo‐exo adducts 1a· N‐BR₃ are thermodynamically the most stable in all cases (Table 1). For this reason, to save CPU time, starting from 1a, only the NN’‐bonded di‐adducts, 1a· NN‐2BR₃, were investigated further in the following. Stable 1a· NN‐2BR₃ isomers were found for all three boranes (see Supporting Information, Table S3). As expected, the formation of the mono‐adduct 1a·N‐BR₃ according to eq. 1 is exergonic for all three boranes, whereby the exergonicity along 1a·N‐BH₃ [‐25.56] > 1a·N‐B(C₆F₅)₃ [‐7.13] > 1a·N‐BPh₃ [‐3.29 kcal/mol] becomes significantly lower (Scheme 5). The same trend can also be observed in the formation of di‐adducts according to eqs. 2 and 3. Since the reaction enthalpies for adduct formation in the case of BPh₃ and B(C₆F₅)₃ are relatively small (between ca. −2 and −11 kcal/mol), it is not surprising that traces of mono‐adduct are always observed upon addition of two or more equivalents of Lewis acid or upon dissolution of di‐adduct crystals.

Table 1.

Relative energies (in kcal/mol) of the N‐ and P‐bound mono‐adducts of 1a and 1b.

species/ R =	BH3	BPh3	B[C6F5]3
1a‐N‐BR3	0.00	0.00	0.00
1a‐P‐BR3	23.44	16.41[ a ]	20.48[ a ]
1b‐N‐BR3	1.63	7.14	6.76
1b‐P‐BR3	22.76	11.75[ a ]	15.29[ a ]

^[a] no true adduct since the B‐P distance is far too long

Scheme 5.

Computed thermodynamical data (in kcal/mol) of the mono‐ and di‐adduct formation in the reaction between 1a and BR₃ (level of theory: b97‐3c + SMD[CH₂Cl₂] + GmRRHO(GFN2[alpb]‐bhess) // b97‐3c[SMD], for details see Supporting Information). All adducts are N‐bonded, but the notation is omitted for clarity.

According to Natural Bond Orbital (NBO) analysis^{[ 37} , ³⁸ , ^{39 ]} both P atoms possess two strongly polarized P‐N and one weakly polarized P‐C bonds along with one lone pair localized at each P atom (Table S9). The B‐N bonds are similarly polarized like the P‐N bonds, and the charge transfer amounts to 0.367e per B(C₆F₅)₃ molecule.

2.3. Reaction of Pacman Phosphane 1a with GaCl₃

2.3.1. Synthesis

In a third series of experiments, we studied the reaction of 1a with the Lewis acid GaCl₃ (Scheme 6). For this purpose, the orange Pacman ligand was dissolved together with colorless crystalline GaCl₃ in dichloromethane, and the solution was stirred for 1 h at room temperature to form an orange solution. The solution was layered with n‐heptane. After 4 days, orange‐colored crystals could be isolated (yield 40%). The ¹H, ¹³C, and ³¹P NMR data showed that it was not a classical di‐adduct as in 1a·2B(C₆F₅)₃ (vide supra), which was isolated, because two singlet signals were found in the ³¹P NMR spectra (δ[³¹P] = 70.3 and 52.2), and a fourfold splitting of the classical Pacman ring signals was observed in the ¹H/¹³C spectra (see Figure S2). The isolated crystals (2GaCl₄, Scheme 6) decomposed above 175 °C but are long‐term stable when stored under argon in a sealed glass ampoule.

Scheme 6.

Reaction of 1a with GaCl₃.

The exact molecular structure of the isolated crystals (2GaCl₄) could be determined using SC‐XRD. 2GaCl₄ crystallized in the monoclinic space group P2₁/n with Z = 8 and two independent molecules per cell. Only one of the two (very similar) independent molecules is depicted in Figure 3, revealing a macrocyclic cation (2⁺ ) featuring a GaN₂C₂ five‐membered heterocycle with [GaCl₄]⁻ as counter ion. This means that presumably the two GaCl₃ molecules formally reacted to give a [GaCl₂]⁺ and [GaCl₄]⁻, with the [GaCl₂]⁺ ion now coordinating to an iminic N atom and a neighboring pyrrolic N atom that was previously bound to a P atom, forming a [Pacman‐GaCl₂]⁺ cation (2⁺ ), featuring the GaN₂C₂ five‐membered heterocycle now as part of the Pacman‐phosphane and with the [GaCl₄]⁻ ion as the counterion. It can be assumed that in the first step the mono‐adduct 1a·GaCl₃ is formed (Scheme 6), from which the second GaCl₃ molecule abstracts a chloride ion, affording a cation that rearranges to give 2GaCl₄. It is worth noting that in solution, in addition to the main NMR signals in ¹H, ¹³C, and ³¹P spectra that can be assigned to the structure of 2GaCl₄ as shown in Figure 3, another set of broadened signals can be observed (Figure S2). These signals also occur when pure crystals of 2GaCl₄ are dissolved, indicating a dynamic behavior of 2GaCl₄ in solution.

Figure 3.

Molecular structure of 2GaCl₄ in the crystal. Only one of the two independent molecules is shown. [GaCl₄]⁻ omitted for clarity. ORTEPs are shown of all noncarbon ring atoms and ring carbon atoms (T = 123(2) K, ellipsoid probability = 50%). Color code: N blue, C grey, H light grey, P orange, Cl green, and Ga violet. Selected structural parameters, distances in Å: P1···P2 5.385(1), P1‐N3 1.744(2), P1‐N4 1.782(2), P2‐N6 1.825(2), P2‐N7 1.855(2), P2‐N5 2.137(2), P1‐C47 1.826(3), P2‐C53 1.844(2), Ga1‐N8 1.914(2), Ga1‐N1 1.954(2), Ga1‐Cl2 2.1473(7), Ga1‐Cl1 2.1497(7), N1‐C46 1.321(3), N8‐C45 1.395(3), C45‐C46 1.400(3).

A closer look at the molecular structure in the crystal shows no significant interionic interactions and that the typical Pacman structure is no longer present (d(P1···P2) = 5.385(1), cf. 3.9433(9) Å in 1a·B(C₆F₅)₃), that is, it is only a macrocycle without the typical Pacman mouth with a formal 3‐coordinated and a 4‐coordinated P atom in the center of the overall cationic macrocycle. While the two P‐N bonds on the 3‐coordinated P1 atom (d(P1‐N3)  =  1.744(2), d(P1‐N4)  =  1.782(3) Å) are in the range of typical single bonds (vide supra), the three P‐N bonds on P2 are significantly elongated (d(P2‐N6)  =  1.825(2), d(P2‐N7)  =  1.856(2), d(P2‐N5)  =  2.137(3) Å, cf. Σr _cov(P‐N) = 1.82 Å,^{[ 25 ]} Σr _vdW(P‐N) = 3.32 Å).^{[ 31 ]} Therefore, the environment of P1 is best described as a distorted trigonal pyramid and that of P2 as a highly distorted bisphenoid (vide infra). Accordingly, the (N5‐P2‐N7) axial angle is 160.85(9)°, the (N6‐P2‐N7) angle is 83.49(9)° and the (N6‐P2‐C53) angle is 95.6(1)°. The five‐membered GaN₂C₂ heterocycle is almost planar (deviation from planarity < 5°) with Ga‐N distances of 1.914(2) and 1.954(2) Å (cf. Σr _cov(Ga‐N) = 1.95 Å).^{[ 25 ]}

2.3.2. Computations–Structure and Bonding

The same methodology as discussed before for the boranes was used to calculate the structures and thermodynamic data of the gallium species discussed below (for more details, see Supporting Information). As already calculated for the boranes, both the mono‐ (Δ_eq.5 G° = −26.8/mol) and the di‐adduct formation (Δ_eq.7 G° = −55.42 kcal/mol) between 1a and GaCl₃ are exergonic (Scheme 7). However, the formation of 2GaCl₄ is also exergonic (Δ_eq.8 G° = −56.51 kcal/mol) and even more thermodynamically stable than the di‐adduct 1a·2GaCl₃ by −1.1 kcal/mol, which explains the formation of this salt and not the di‐adduct in the experiment (vide supra).

Scheme 7.

Computed thermodynamical data (in kcal/mol) for the reaction of 1a with GaCl₃ (Level of theory: b97‐3c + SMD[CH₂Cl₂] + GmRRHO(GFN2[alpb]‐bhess) // b97‐3c[SMD], for details see Supporting Information).

The generation of 2GaCl₄, with its unique structure around the phosphorus atoms and the GaN₂C₂ five‐membered heterocycle, warrants further investigation to better understand its structure and bonding. According to the NBO analysis, a lone pair is localized at each of the two P atoms, both of which have a dominant s‐character (P1: HO = h = sp^0.77, P2: h = sp^0.58; HO = h = hybrid orbital). While the NBO analysis finds two polarized N‐P bonds (with a N: 77 / P: 23% localization) and one weakly polarized P‐C bond (C: 62 / P: 38%) for P1, three very strongly polarized P2‐N bonds (P: 20, 15, and 8%) and one weakly polarized P‐C (P: 38%) bond are found for P2 (Tables S10). In agreement with the structural data (see above), this means that P1 adopts a distorted trigonal pyramidal structure, while P2 has a distorted bisphenoidal structure (Scheme 8). The unusual coordination geometry on the P2 requires further examination. A closer study of delocalization effects using second‐order perturbation theory within the NBO methodology shows very strong hyperconjugative effects, especially along the N‐P‐N axis [σ(P2‐N5) → σ*(P2‐N7): E ²  =  48 kcal/mol and σ(P2‐N7) → σ*(P2‐N5): E ² = 81 kcal/mol], corresponding to resonance between covalent and ionic Lewis formulae (Scheme 8). In other words, the bond along the N‐P‐N axis in this pseudo‐trigonal bipyramidal structure is best understood as a classic 4‐electron 3‐center (4e, 3c) bonding unit, commonly observed in trigonal bipyramidal PX₅ systems with P in the + V oxidation state. However, in Pacman phosphane cation 2⁺, the P2 atom has a lone pair, indicating the formal oxidation state + III. Both the Ga‐N and the Ga‐Cl bonds are highly polarized, with only 18% (Ga‐Cl) and 11% (Ga‐N) localization of the bonds at the gallium center.

Scheme 8.

NBO Lewis formulae describing the 4‐electron 3‐center bond along the N5‐P2‐N7 axis in 2⁺ . Only the distorted pseudo‐trigonal bipyramidal environment around P2 is shown. A certain degree of ionic bonding can also be considered for the equatorial P‐N and P‐C bonds but is not shown here. The grey line represents only connectivity, and the black line a 2‐electron bond.

3. Conclusion

The reactions of Pacman phosphanes (1a) with Lewis acids such as R₃B (R = C₆H₅, C₆F₅) and GaCl₃ led to the formation of equilibria between free 1a, mono‐ and di‐adducts. In the case of B(C₆F₅)₃, it was possible to isolate the N,N’‐bonded di‐adduct (1a·2B(C₆F₅)₃), which could be fully characterized. DFT calculations indicated that the N‐bonded adducts are thermodynamically more stable than the P‐bonded adducts.

In contrast to the reaction of the Pacman phosphane with boranes, GaCl₃ reacted with 1a to produce an intriguing [Pacman‐GaCl₂]⁺[GaCl₄]⁻ salt, featuring a nearly planar GaN₂C₂ heterocycle as part of the Pacman macrocycle. Additionally, the coordination geometry on one P atom changes from triple to an unusual quadruple coordination, although it remains a formal P atom in the + III oxidation state, as demonstrated by quantum chemical calculations. The formation of [Pacman‐GaCl₂]⁺[GaCl₄]⁻ salt is thermodynamically favored over the di‐adduct 1a·2GaCl₃ by 1.1 kcal/mol, consistent with experimental results.

4. Experimental Section

General

All manipulations were carried out under an argon atmosphere using standard Schlenk line or Glovebox techniques (oxygen‐and moisture‐free conditions). All starting materials, reactants, and solvents were purified and/or synthesized according to literature procedures, which are described in further detail in the Supporting Information. The removal of solvents in vacuo was carried out at 1 × 10⁻³ mbar and at 25 °C if not stated otherwise. Further information on experimental procedures, data acquisition and processing, purification of starting materials and solvents, as well as a full set of analytical data for each compound and crystallographic information can be found in the Supporting Information.

Synthesis of 1a·2B(C₆F₅)₃

A 25 mL Schlenk flask is filled with colorless B(C₆F₅)₃ (215 mg, 0.42 mmol) and orange 1a (217 mg, 0.21 mmol). The solids are dissolved in dichloromethane (12 mL) and left to stand without stirring overnight, resulting in the formation of yellow crystals. The supernatant solution is removed with a syringe and kept as a second fraction. The crystals are rinsed with dichloromethane (0.5 mL) at − 80 °C. The crystals are dried in vacuo (1 × 10⁻³ mbar) for 2 hours at 60 °C. The solution of the second fraction is concentrated in vacuo (1 × 10⁻³ mbar) and left to stand in a water bath overnight at 5 °C. The crystals of the second fraction are rinsed twice with dichloromethane (0.5 mL) at − 80 °C and dried in vacuo (1 × 10⁻³ mbar) for 2 hours at 60 °C. Yield: 223 mg (0.11 mmol, 55%).

Mp.: 254 °C (dec.); EA calc. (incl. 0.5 eq. CH₂Cl₂) (found) in %: C 56.88 (56.19), H 2.98 (2.92), N 5.62 (5.66); ³¹P{¹H} NMR (CD₂Cl₂, 202.5 MHz): δ  =  61.0 ppm (s); MS: (ESI, m/z, CH₂Cl₂): 929.4 [PacPPh]⁺.*

Synthesis of 2GaCl₄

A 10 mL Schlenk tube is filled with orange crystalline 1a (520 mg, 0.50 mmol) and colorless crystalline GaCl₃ (180 mg, 1.02 mmol). The solids are dissolved in dichloromethane (10 mL). The solution is stirred for 1 hour at ambient temperature, resulting in an orange solution. The solution is layered with n‐heptane (5 mL) and left to stand for 4 d, resulting in the formation of orange crystals. The supernatant solution is removed via a syringe. The crystals are rinsed with dichloromethane (0.5 mL) at − 80 °C and dried in vacuo (1 × 10⁻³ mbar) for 1 hour. Yield: 254 mg (0.2 mmol, 40%).

Mp.: 176 – 187 °C (dec.); EA calc. (found) in %: C 54.37 (53.83), H 4.56 (4.90), N 8.75 (8.83); ³¹P{¹H} NMR (CD₂Cl₂, 202.5 MHz): δ  =  70.3 (s ppm (s, 1 P), 52.2 (s, 1 P); MS: (ESI, m/z, CH₂Cl₂): 1069.3 [K]⁺, 929.4 [PacPPh]⁺.*

* The full set of analytical data including assignments etc. can be found in the Supporting Information.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Data Availability Statement

The data that support the findings of this study are available in the supplementary information of this article.