Large, weakly basic bis(carboranyl)phosphines: an experimental and computational study

Abstract

The bis(carboranyl)phosphines [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I) and [μ-2,2′-PEt-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (1) have been prepared and spectroscopically and structurally characterised. Crystallographic and DFT computational studies of 1 suggest that the orientation of the ethyl group, relative to the bis(carborane), is the result of intramolecular dihydrogen bonding. This orientation is such that the magnitudes of the ²J_PH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed ²J_PH close to zero. The steric properties of I, 1 and their derivatives [μ-2,2′-P(Ph)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (2) and [μ-2,2′-P(Et)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (3) have been assessed by Tolman cone angle and percent buried volume calculations, from which it is concluded that the bis(carboranyl)phosphines I and 1 are comparable to PCy₃ in their steric demands. The selenides [μ-2,2′-P(Ph)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (4) and [μ-2,2′-P(Et)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (5) have also been prepared and characterised. The ¹J_PSe coupling constants for 4 and 5 are the largest reported so far for carboranylphosphine selenides and indicate that I and 1 are very weakly basic.

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Introduction

Currently, one of the most active areas of carborane chemistry concerns the compound [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-1,2-closo-C₂B₁₀H₁₁], two ortho-carborane units connected by a C–C bond¹ and commonly referred to as 1,1′-bis(o-carborane) (Fig. 1). Although it was first reported >50 years ago,² the chemistry of 1,1′-bis(o-carborane) was not fully explored for long periods because of the lack of a reliable, high-yielding synthesis. However, this has now been achieved³ and consequently a significant amount of new chemistry of 1,1′-bis(o-carborane) has recently appeared.^4–20

Fig. 1 1,1′-Bis(o-carborane).

Several authors have taken advantage of the functionality of its C_cageH units to use 1,1′-bis(o-carborane) as a κ² ligand, binding transition-metals to the cage through two M–C σ-bonds, with recent studies^7,8,13,19 building on the pioneering work of Hawthorne and co-workers.²¹ In marked contrast, very little has been reported concerning main group elements bound to 1,1′-bis(o-carborane). Zakharkin reported [μ-2,2′-AsMe-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] and [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I), although the latter was only poorly characterised,^22,23 and Johnson and Knobler described [μ-2,2′-PX-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}], X = Cl and F.²⁴ Very recently, Peryshkov and co-workers isolated an interesting 12-vertex closo/12-vertex nido species with both bridging P(i-Pr)₂ and non-bridging PH(i-Pr)₂ units when attempting to add two P(i-Pr)₂ groups onto 1,1′-bis(o-carborane), one on each cage.¹⁷

In this contribution we expand on Zakharkin's early work, resynthesising I (in higher yield) and preparing the related μ-2,2′-PEt compound (1), with full characterisation of both bis(carboranyl)phosphines. All phosphine have potential as ligands in homogeneous catalysis, and consequently we have explored the steric and electronic properties of I and 1via the synthesis and study of derivatives in which the P lone pair is bound to {–AuCl} and {=Se} fragments.

Results and discussion

Bis(carboranyl)phosphines

Double deprotonation of 1,1′-bis(o-carborane) in Et₂O with n-BuLi in hexanes followed by treatment with PPhCl₂ affords [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I) as a white solid in 72% isolated yield, following work-up involving flash chromatography on silica. Although this bis(carboranyl)phosphine has been reported before it was previously characterised only by elemental analysis.²² We have studied the compound by mass spectrometry, multinuclear NMR spectroscopy and single-crystal X-ray diffraction.

The mass spectrum of I is dominated by a typical carborane envelope centred at m/z 392 corresponding to the molecular ion. The ¹¹B{¹H} NMR spectrum contains resonances from δ ca. 0 to −13 ppm but is relatively uninformative in terms of the detailed structure because of the overlap of the lower-frequency resonances. The ¹H NMR spectrum confirms that no C_cageH resonances are present and reveals the expected multiplets for ortho, meta and para H atoms which collapse to a doublet, apparent triplet and triplet, respectively, on ³¹P decoupling. In the ³¹P{¹H} spectrum there is a simple singlet at δ 40.35 ppm.

The molecular structure of I is shown in Fig. 2. As anticipated the {PPh} fragment binds symmetrically to the bis(carborane) (the P1–C2 and P1–C2′ distances are identical within experimental error), and only a small rotation of the Ph substituent about the P1–C11 bond would afford the molecule C_s symmetry, the likely time-averaged symmetry in solution. The formation of the P1C2C1C1′C2′ ring results in a slight bending of the spine of the bis(carborane) with angles B12⋯C1–C1′ 169.14(16)° and B12′⋯C1′–C1 170.17(15)°, cf. 175.14(5)° in 1,1′-bis(o-carborane) itself.¹

Fig. 2 Perspective view of I with the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level except for H atoms. Selected interatomic distances (Å): P1–C2 1.893(3), P1–C2′ 1.887(3), P1–C11 1.819(3), and C1–C1′ 1.534(3).

An analogous compound, [μ-2,2′-PEt-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (1), was similarly prepared. The ¹¹B{¹H} NMR spectrum of 1 reveals a 2 : 2 : 6 : 2 : 2 : 6 pattern (high frequency to low frequency) between δ ca. 0 and −11 ppm, consistent with time-averaged C_s molecular symmetry. The ¹H{³¹P} NMR spectrum affords the expected quartet and triplet for the PCH₂CH₃ and PCH₂CH₃ resonances, respectively, and a singlet at δ 42.75 ppm is observed in the ³¹P{¹H} spectrum.

There are two crystallographically-independent molecules of compound 1 in the asymmetric fraction of the unit cell, and Fig. 3 shows a perspective view of one of them, 1AB (the other is 1CD). Using the Structure Overlay tool in Mercury²⁵ the overall root-mean-square (rms) misfit between the {P(C₂B₁₀)₂} fragments of the two independent molecules (25 atoms) is 0.024 Å, with the greatest individual misfit between B4B and B4D, 0.049 Å. Including the C atoms of the Et groups results in only a marginal increase in the rms misfit to 0.029 Å (the greatest individual misfit 0.073 Å between C12A and C12C) since the orientation of the ethyl group relative to the bis(carborane) in the two independent molecules is effectively the same.

Fig. 3 Perspective view of one of the two crystallographically-independent molecules of compound 1 (1AB) with the atomic numbering scheme. Molecule 1CD is practically superimposable. Displacement ellipsoids are as in Fig. 2. Selected interatomic distances (Å): P1A–C2A 1.870(5), P1A–C2B 1.890(5), P1A–C11A 1.844(5), C1A–C1B 1.533(7); P1C–C2C 1.886(5), P1C–C2D 1.890(5), P1C–C11C 1.818(5), and C1C–C1D 1.530(6).

This ethyl orientation is significant because of its influence on the ¹H NMR spectroscopic properties of 1 (vide infra). We believe that the origin of the orientation is that it maximises intramolecular dihydrogen bonding between the weakly protonic CH atoms and the weakly hydridic BH atoms of the carborane cage, with the key CH⋯HB contacts shown in Fig. 4, an alternative view of 1AB.

Fig. 4 Alternative view of 1AB showing dihydrogen bonds as red lines. Interatomic distances (Å): H11P⋯H3A 2.45(4), 2.39(4), 2.28; H11P⋯H6B 2.28(5), 2.29(5), 2.10; H11Q⋯H7A 2.46(5), 2.47(4), 2.36; H12R⋯H11B 2.51(5), 2.40(5), 2.31. Values in italics are the equivalent distances in molecule 1CD, and values in bold are those from the DFT study of 1.

To investigate this further we have undertaken DFT calculations on compound 1. The optimised (BP86-D3/def2-TZVP/def2-SVP) geometry, 1_DFT (Fig. 5), is in excellent agreement with the structure determined crystallographically (see the ESI‡ for key optimised bond parameters), including the orientation of the ethyl group. Thus the overall rms misfit between the computed and experimental {P(C₂B₁₀)₂} fragments (using molecule 1AB) is only 0.019 Å (the greatest individual misfit 0.031 Å for P1), rising to only 0.029 Å if the ethyl C atoms are included (the greatest individual misfit 0.081 Å for C12A). In terms of the ethyl group orientation, the experimental lp–P1–C11–C12 torsion angles are 38.8(4)° and 42.5(4)° for 1AB and 1CD, respectively, and the computed torsion angle is 38.4°. Moreover, the DFT study provides strong support for the presence of dihydrogen bonding. A topological analysis of the electron density in 1_DFT using the QTAIM methodology reveals bond critical points (BCP) between H11PX and the cage hydrogen atoms H3 and H6′, located roughly at the centre of each bond vector (see Fig. S1‡ for a molecular graph). Additional, albeit somewhat weaker interactions, are present for H11QY/H7 and H12R/H11 (see Fig. S1‡). Both the electron density ρ(r) (0.007–0.011 a.u.) and its Laplacian ∇²ρ(r) (0.022–0.038 a.u.) at the relevant BCPs are diagnostic of typical closed-shell interactions and fall within the ranges proposed for dihydrogen bonds.²⁶

Fig. 5 DFT-optimised structure of 1. Note the excellent agreement with the experimentally-determined structure (Fig. 3), including the orientation of the Et group. Selected interatomic distances (Å): P1–C2 1.889, P1–C2′ 1.892, P1–C11 1.852, and C1–C1′ 1.528.

An interesting feature of the ¹H NMR spectrum of compound 1 is the lack of observable coupling between the PCH₂CH₃ H atoms and the P atom. Thus, whilst the PCH₂CH₃ H atoms appear as a doublet (³J_PH = 23.0 Hz) of triplets (³J_HH = 7.9 Hz) the PCH₂CH₃ H atoms appear as only a simple quartet (³J_HH = 7.9 Hz). A ¹H–³¹P HMBC experiment shows the presence of the expected coupling between the P atom and the CH₂CH₃ H atoms but, additionally, a weak correlation between signals due to P and CH₂CH₃. The lack of any observable splitting in the 1D ¹H spectrum suggests that the value of ²J (¹H–³¹P) is of the order of the ¹H resonance linewidth (estimated to be ca. 1 Hz).

An understanding of the origin of this small 2-bond P–H coupling comes from the DFT calculations on compound 1. The computed energy profile for ethyl rotation about the P1–C11 vector (Fig. 6) establishes that 1_DFT and its isoenergetic rotational conformer 1′_DFT dominate the equilibrium distribution, whilst the Boltzmann population of the symmetrical isomer 1′′_DFT (+4 kcal mol⁻¹) is near zero at room temperature. Importantly, due to the low energetic barrier associated with TS(1_DFT–1′_DFT) separating 1_DFT and 1′_DFT (ΔG^‡ = +1.8 kcal mol⁻¹) the methylene protons can undergo rapid exchange in solution. The calculated averaged ³J_PH (calc. 26.2 Hz; exp. 23.0 Hz) and ³J_HH (calc. 9.7 Hz; exp. 7.9 Hz) coupling constants are in good agreement with the experimental values. The values of ²J_PH for coupling to the individual methylene hydrogen atoms are of the same order of magnitude but, importantly, they are of opposite sign (for H11PX ²J_PH = −5.4 Hz whilst for H11QY ²J_PH = +5.6 Hz). Hence, fast exchange between these hydrogen atoms reduces the observed ²J_PH to approximately zero, in line with the experimental upper limit of ∼±1 Hz. The predominant contribution to the total coupling constant is associated with the Fermi contact term.

Fig. 6 Calculated Gibbs free energy profile (kcal mol⁻¹) for ethyl rotation in 1_DFT (BP86-D3/def2-TZVP/def2-SVP). Boltzmann populations: 1_DFT 50%, 1′_DFT 50%, and 1′′_DFT 0%.

With the characterisation of I and 1 now complete, we turn to their derivatives. Phosphines are ligands of great importance in homogeneous catalysis by transition-metal complexes,²⁷ and carboranylphosphines have been used extensively in a variety of catalytic applications.²⁸ The two key characteristics of phosphines as ligands in catalysis are their size and basicity, and we have targeted and studied derivatives of I and 1 specifically to assess these features.

Steric properties

The steric demand of a phosphine is classically assessed using its Tolman cone angle (θ),²⁹ and we have used the crystallographically-determined structures of I and 1 to measure these. In addition, a recent alternative to θ is Cavallo and Nolan's percent buried volume parameter, %V_bur.³⁰ Although originally developed for NHCs, %V_bur has been shown to scale linearly with θ for a range of phosphines and, furthermore, there is a strong linear relationship between θ of PR₃ and %V_bur of the adduct R₃PAuCl.³¹ For this reason we have prepared and studied the gold–chloride adducts of compounds I and 1.

Treatment of a solution of I or 1 in DCM with an equimolar amount of (tht)AuCl affords the new species [μ-2,2′-P(Ph)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (2) and [μ-2,2′-P(Et)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (3) as white solids in 81 and 32% yields, respectively.

Compounds 2 and 3 were initially characterised by mass spectrometry and ¹H, ¹¹B and ³¹P NMR spectroscopies. The ¹¹B{¹H} spectrum of 2 is largely uninformative because of multiple overlapping resonances, but that of 3 is sufficiently well-resolved to allow integration, which is consistent with time-averaged C_s molecular symmetry in solution at room temperature. In both 2 and 3 the ³¹P{¹H} spectrum reveals a simple singlet, shifted ca. 28 and 33 ppm, respectively, to a high frequency relative to those in I and 1. The ¹H NMR spectrum of 2 shows the anticipated multiplets for the phenyl protons, whilst for 3 the H atoms of the ethyl group appear as a doublet of quartets (PCH₂CH₃) and a doublet of triplets (PCH₂CH₃).

The molecular structures of 2 and 3, as determined crystallographically, are shown in Fig. 7 and 8, respectively. In both cases co-ordination to the {AuCl} fragments causes minimal change in the structures of the bis(carboranyl)phosphines, with the same orientations of the Ph groups or Et groups being effectively maintained between I and 2 and between 1 and 3. Moreover, a DFT-optimised study of 3 was fully consistent with its crystallographic counterpart; a structure overlay of 3 and 3_DFT yielded an overall rms misfit of 0.018 Å for {P(C₂B₁₀)₂} fragments (the greatest individual misfit 0.027 Å for B5′) rising to only 0.020 Å if the ethyl carbon atoms were included (the greatest misfit 0.033 Å for C11). The overlay is somewhat poorer if the {AuCl} fragment is included, with the overall misfit increasing to 0.077 Å and the Cl atoms misfitting by 0.345 Å.

Fig. 7 Perspective view of compound 2 with the atomic numbering scheme. Displacement ellipsoids are as in Fig. 2. Selected interatomic distances (Å): P1–Au1 2.2217(8), Au1–Cl1 2.2698(8), P1–C2 1.878(3), P1–C2′ 1.879(3), P1–C11 1.797(3), and C1–C1′ 1.523(4).

Fig. 8 Perspective view of compound 3 with the atomic numbering scheme. Displacement ellipsoids are as in Fig. 2. Selected interatomic distances (Å): P1–Au1 2.2181(11), Au1–Cl1 2.2910(11), P1–C2 1.860(4), P1–C2′ 1.883(4), P1–C11 1.816(4), and C1–C1′ 1.538(5).

The Tolman cone angle for I was calculated to be 172.5°, whilst that for 1 is 171.6° (Table 1). Although Ph is a larger substituent than Et, the similarity of the cone angles for I and 1 reflects the fact that the vast majority of the steric bulk of I and 1 comes from the common bis(carborane) fragment. Percent buried volumes for I and 1 were calculated to be 32.0 and 30.8 (average), respectively. Co-ordination to {AuCl} to afford 2 and 3, respectively, results in small increases in both θ and %V_bur. This arises because the stereochemical influence of the P lone pair of electrons on the bis(carborane) and Ph/Et substituents is reduced on co-ordination, allowing small increases in the C–P–C angles and producing a slightly bulkier ligand.

Table 1 Tolman cone angles (θ/°) and %V_bur for bis(carboranyl)phosphines I and 1 and their complexes with {AuCl}, 2 and 3 respectively

	θ	%Vbur
I	172.5	32.0
1AB	171.6	30.9
1CD	171.6	30.7
2	176.2	33.2
3	176.5	32.0

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Comparing the θ and %V_bur values for I, 1, 2 and 3 with analogous species in the literature, we conclude that the bis(carboranyl)phosphines I and 1 are, in terms of their size, the most comparable to (but slightly larger than) tricyclohexylphosphine, PCy₃.³¹

Electronic properties

The basicity of phosphines PR₃ is conveniently assessed by the measurement of the 1-bond P–Se coupling constant, ¹J_PSe, of the corresponding selenide R₃P=Se; the more electron-withdrawing the substituents (i.e. the less basic the phosphine), the greater the degree of 3s character in the phosphorus lone pair and the greater the magnitude of ¹J_PSe.³² Accordingly, we have synthesised [μ-2,2′-P(Ph)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (4) and [μ-2,2′-P(Et)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (5), the selenides of I and 1 respectively, by the simple process of heating the bis(carboranyl)phosphine with an excess of elemental Se in toluene. Scheme 1 summarises all the syntheses reported in this paper.

Scheme 1 Generalised reaction scheme for compounds I and 1–5.

Compounds 4 and 5 are afforded as pale-pink solids in reasonable isolated yields. Both show the anticipated molecular ion peaks in their mass spectra. Their ¹¹B{¹H} NMR spectra are relatively uninformative due to considerable overlap of resonances. In the ³¹P{¹H} NMR spectra are singlets at δ ca. 50 and 58 ppm, respectively, at higher frequency than in the corresponding bis(carboranyl)phosphine but not as deshielded as in the AuCl compounds 2 and 3. The ¹H NMR spectra of 4 and 5 show the expected resonances for the Ph or Et substituents, including a doublet of quartets for the PCH₂CH₃ resonance of 5 with ²J_PH = 11.6 Hz, and the re-emergence of this 2-bond P–H coupling can be understood in terms of the rehybridisation of the phosphorus orbitals induced by coordination of the P lone pair to the {=Se} fragment.³³ These changes closely follow the general trends observed for P(III) and P(V) compounds.³⁴ The computed averaged geminal ²J_PH coupling constant in 5_DFT increases to −14.1 Hz, a value in good agreement with experiment (²J_PH = 11.6 Hz). We note that the phase information of the coupling constant is invisible in first-order NMR-spectra, and thus all coupling constants appear to be positive. The observed decrease of ²J_PH (or increase of the modulus |²J_PH|) can be correlated with the admixture of a higher degree of 3s character into the P bonds, rendering the hybridisation around P close to sp³, as borne out in the NBO analysis of the associated P–C bonding orbitals (21–26% s, 73–78% p). This is paralleled by a notable reduction of P 3s character in the P–Se σ-bond (32% s, 67% p), compared to 52% 3s character of the phosphorus lone pair in 1_DFT. This change of the electron distribution around P as the lone pair is replaced by Se in 5_DFT in turn reinforces the direct Fermi contact contribution to the 1- and 2-bond coupling pathways (see the ESI‡ for more details).

Of relevance to the basicities of the bis(carboranyl)phosphine parent compounds I and 1, the ³¹P{¹H} spectra of 4 and 5 show clear Se satellites with ¹J_PSe = 891 and 894 Hz, respectively.³⁵ These coupling constants are considerably larger than those of a range of common phosphines [typically 670–750 Hz, Table 1 of ref. 32d], suggesting that I and 1 are very weakly basic. There are few carboranylphosphine selenides in the literature, but those that are known all have relatively large values of ¹J_PSe. In [1-P(Se)Ph₂-2-PPh₂-1,2-closo-C₂B₁₀H₁₀] ¹J_PSe = 807 Hz, whilst in the 2-Me and 2-Ph analogues ¹J_PSe = 804 and 812 Hz, respectively.³⁶ Clearly, these large couplings are the result of the strong electron-withdrawing property of carboranes relative to alkyl or aryl substituents, and we attribute the even larger ¹J_PSe values in 4 and 5 to the fact that here the {P=Se} fragment is directly bonded to two carborane cages. Consistent with this, Viñas and co-workers have described species [μ-1,1′-P(R)Se-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ in which the {P(R)Se} fragment is also attached to two heteroborane cages and reported ¹J_PSe values of 833 Hz (R = Ph) and 679 Hz (R = t-Bu).³⁷ In the related species [8,8′-μ-(1′′,2′′-C₆H₄)-μ-1,1′-P(R)Se-3,3′-Co(1,2-C₂B₉H₉)₂]⁻, ¹J_PSe is 848 Hz (R = Ph) and 692 Hz (R = t-Bu), respectively.³⁷

Overall, the magnitudes of ¹J_PSe recorded for 4 and 5 are currently the largest reported for carboranylphosphine selenides, implying that the parent phosphines I and 1 are very weakly basic. Note that, in principle, the degree of s character in the phosphorus lone pair in PR₃ can be influenced not only by the electron-withdrawing properties but also by the size of the substituents. In this respect we recall that I and 1 are comparable in their steric demands to PCy₃. They are, however, much less basic, since ¹J_PSe in Cy₃P=Se is only 673 Hz.³⁸

Compounds 4 and 5 were also studied crystallographically, and perspective views of single molecules are shown in Fig. 9 and 10, respectively. As noted above, compound 5 was also studied computationally, again affording excellent agreement with the crystallographic structure; for the {P(C₂B₁₀)₂} fragments of 5 and 5_DFT the rms misfit is 0.020 Å (the worst individual misfit 0.034 Å for B11′), rising to only 0.034 Å (the poorest individual fit 0.100 Å for C11) when the ethyl C atoms are included and 0.036 Å (the worst fit 0.098 Å, C12) when the Se atom is added. Once again, the general orientation of the Ph or Et substituent relative to the bis(carborane) moiety is maintained, with torsion angles lp/Au/Se–P–C11–C12 broadly consistent for I, 2 and 4 (Ph substituents) and for 1, 1_DFT, 3, 3_DFT, 5 and 5_DFT (Et substituents) albeit with a ca. 10° increase for the selenides 4 and 5 (Table 2). As previously noted, for the ethyl species 1, 3 and 5 this orientation is traced to intramolecular dihydrogen bonding involving H11A (H11P and H11X in 1) with H6′ and with H3, whilst for the phenyl compounds I, 2 and 4 H16 of the phenyl ring is seen to interact primarily with H6′ and to a lesser extent with H3.

Fig. 9 Perspective view of compound 4 with the atomic numbering scheme. Displacement ellipsoids are as in Fig. 2. Selected interatomic distances (Å): P1–Se1 2.0798(6), P1–C2 1.8860(19), P1–C2′ 1.896(2), P1–C11 1.810(2), and C1–C1′ 1.536(3).

Fig. 10 Perspective view of compound 5 with the atomic numbering scheme. Displacement ellipsoids are as in Fig. 2. Selected interatomic distances (Å): P1–Se1 2.0801(7), P1–C2 1.879(2), P1–C2′ 1.893(2), P1–C11 1.822(2), and C1–C1′ 1.543(3).

Table 2 X–P–C11–C12 torsion angles (/°) and key dihydrogen bonding (/Å) in bis(carboranyl)phosphines and related compounds

Compound	X	X–P–C11–C12	Dihydrogen bond	Distance	Dihydrogen bond	Distance
(i) Phenyl substituent
I	lp	−29.4(2)	H16⋯H6′	2.15(3)	H16⋯H3	2.48(2)
2	Au	−28.8(3)	H16⋯H6′	2.20(3)	H16⋯H3	2.49(3)
4	Se	−40.0(2)	H16⋯H6′	2.19(2)	H16⋯H3	2.84(2)
(ii) Ethyl substituent
1AB	lp	38.8(4)	H11P⋯H6B	2.28(5)	H11P⋯H3A	2.45(5)
1CD	lp	42.5(4)	H11X⋯H6D	2.29(5)	H11X⋯H3C	2.39(4)
1DFT	lp	38.4	H11PX⋯H6′	2.10	H11PX⋯H3	2.28
3	Au	37.7(3)	H11A⋯H6′	2.32(3)	H11A⋯H3	2.52(4)
3DFT	Au	39.8	H11A⋯H6′	2.17	H11A⋯H3	2.34
5	Se	48.6(3)	H11A⋯H6′	2.40(3)	H11A⋯H3	2.46(2)
5DFT	Se	39.9	H11A⋯H6′	2.14	H11A⋯H3	2.29

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The P=Se distances in 4 and 5 are 2.0798(6) and 2.0801(7) Å, respectively. These are significantly shorter than that in Cy₃P=Se, 2.108(1) Å,³⁹ and those in recent compilations of P=Se distances.^38,40 In fact, the P=Se distances in 4 and 5 are amongst the shortest yet reported; of 378 structures containing the fragment {R¹R²R³P=Se} in the Cambridge Structural Database⁴¹ only seven have P=Se distances <2.08 Å.⁴² Short P=Se distances in 4 and 5 are fully consistent with a significant degree of s character in the phosphorus lone pair of I and 1, as implied from the ¹J_PSe values.

Conclusions

The bis(carboranyl)phosphines I and 1 are comparable in their steric demands to PCy₃ but are much less basic. In fact, in terms of their ¹J_PSe values, I and 1 are the least basic of any carboranylphosphines so far reported. As large, weakly basic phosphines I and 1 have potential as ligands in homogeneous catalysis, and future contributions will develop this theme.

Experimental

Synthesis

Experiments were performed under dry, oxygen free N₂ using standard Schlenk techniques, although subsequent manipulations were sometimes performed in an open laboratory. Solvents were either freshly distilled under nitrogen from the appropriate drying agent [THF, Et₂O and 40–60 °C petroleum ether (petrol); sodium wire: CH₂Cl₂ (DCM); calcium hydride] or were purified by using an MBRAUN SPS-800 and stored over 4 Å molecular sieves, and all were degassed (3× freeze–pump–thaw cycles) before use. Preparative TLC employed 20 × 20 cm Kieselgel F₂₅₄ glass plates and for column chromatography we used 60 Å silica as the stationary phase. NMR spectra at 400.1 MHz (¹H), 128.4 MHz (¹¹B) or 162.0 MHz (³¹P) were recorded on a Bruker DPX-400 spectrometer from CDCl₃ solutions at 298 K, using CDCl₃ stored over 4 Å molecular sieves. Electron impact mass spectrometry (EIMS) was carried out using a Finnigan (Thermo) LCQ Classic ion trap mass spectrometer (University of Edinburgh). The starting materials 1,1′-bis(o-carborane)³ and (tht)AuCl⁴³ were prepared by literature methods or slight variations thereof. All other reagents were supplied commercially.

[μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I). n-BuLi (2.79 mL of 2.5 M solution in hexanes, 6.975 mmol) was added dropwise to a cooled (0 °C) solution of 1,1′-bis(o-carborane) (1.000 g, 3.491 mmol) in Et₂O (20 mL) and the products were stirred for 1 h at room temperature. The pale yellow solution was cooled to 0 °C and then PPhCl₂ (0.47 mL, 3.491 mmol) in Et₂O (10 mL) was added over 30 min to afford a green-yellow solution, which was subsequently heated to reflux for 2 h to produce a pale yellow solution. Once cooled this was filtered and the solvent was removed in vacuo. Purification by flash chromatography (petrol) yielded a white solid, subsequently identified as [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I) (0.981 g, 2.499 mmol, 72%). ¹¹B{¹H} NMR, δ −0.1 (2B), −3.9 to −11.2 [overlapping resonances with maxima at −3.9, −6.4, −8.7, −9.8, −11.2 (18B)]. ¹H NMR, δ 7.77–7.73 (m, 2H, C₆H₅), 7.65–7.62 (m, 1H, C₆H₅), 7.59–7.54 (m, 2H, C₆H₅). ¹H{³¹P} NMR, δ 7.76 (d, ³J_HH = 7.1 Hz, 2H, C₆H₅), 7.64 (t, ³J_HH = 7.4 Hz, 1H, C₆H₅), 7.57 (app t, ³J_HH = 7.1 Hz, 2H, C₆H₅). ³¹P{¹H} NMR, δ 40.35 (s). EIMS, envelope centred on m/z 392.3 (M⁺).

[μ-2,2′-PEt-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (1). n-BuLi (0.56 mL of a 2.5 M solution in hexanes, 1.396 mmol) was added dropwise to a cooled (0 °C) solution of 1,1′-bis(o-carborane) (0.200 g, 0.698 mmol) in THF (10 mL) and the products were stirred for 1 h. The pale yellow solution was frozen at −196 °C and then PEtCl₂ (0.70 mL of a 1.0 M solution in THF, 0.698 mmol) was added and the reaction mixture was stirred overnight at room temperature to give a colourless solution. Volatiles were removed in vacuo and the crude mixture was dissolved in DCM and filtered. Purification by flash chromatography (petrol) followed by preparative TLC (DCM : petrol, 1 : 19) yielded a white solid (R_f = 0.97), subsequently identified as [μ-2,2′-PEt-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (1) (0.154 g, 0.447 mmol, 64%). ¹¹B{¹H} NMR, δ −0.3 (2B), −3.8 (2B), −6.6 (6B), −7.6 (2B), −8.5 (2B), −10.6 (6B). ¹H NMR, δ 1.98 (q, 2H, PCH₂CH₃, ³J_HH = 7.9 Hz), 1.31 (dt, 3H, PCH₂CH₃, ³J_HH = 7.9 Hz, ³J_PH = 23.0 Hz). ¹H{³¹P} NMR, δ 1.99 (q, 2H, PCH₂CH₃, ³J_HH = 7.9 Hz), 1.32 (t, 3H, PCH₂CH₃, ³J_HH = 7.9 Hz). ³¹P{¹H} NMR, δ 42.75 (s). EIMS, envelope centred on m/z 344.3 (M⁺).

[μ-2,2′-P(Ph)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (2). A DCM (5 mL) solution of I (0.100 g, 0.255 mmol) was transferred via a cannula to a DCM (10 mL) solution of (tht)AuCl (0.082 g, 0.256 mmol) at 0 °C. The colourless solution was stirred at 0 °C for 30 min, and then reduced to ca. 3 mL in vacuo. Petrol (10 mL) was added to afford a white precipitate which was collected by filtration and washed with petrol (10 mL) to give a white solid, subsequently identified as [μ-2,2′-P(Ph)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (2) (0.129 g, 0.206 mmol, 81%). ¹¹B{¹H} NMR, δ −1.87 (2B), −0.7 to −13.9 [overlapping resonances with maxima at −3.6, −5.6, −7.6, −9.2, −10.9 (18B)]. ¹H NMR, δ 8.15–8.09 (m, 2H, C₆H₅), 7.86–7.81 (m, 1H, C₆H₅), 7.75–7.70 (m, 2H, C₆H₅). ³¹P{¹H} NMR, δ 68.96 (s). EIMS, envelope centred on m/z 624.1 (M⁺).

[μ-2,2′-P(Et)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (3). Similarly, from compound 1 (0.100 g, 0.290 mmol) and (tht)AuCl (0.093 g, 0.290 mmol) was isolated [μ-2,2′-P(Et)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (3) (0.053 g, 0.092 mmol, 32%) as a white solid. ¹¹B{¹H} NMR, δ 1.7 (2B), −3.3 (2B), −5.7 (6B), −7.6 (4B), −10.4 (6B). ¹H NMR, δ 2.48 (dq, 2H, PCH₂CH₃, ²J_PH = 10.9 Hz, ³J_HH = 7.7 Hz), 1.52 (dt, 3H, PCH₂CH₃, ³J_PH = 27.9 Hz, ³J_HH = 7.7 Hz). ³¹P{¹H} NMR, δ 75.76 (s). EIMS, envelope centred on m/z 577.3 (M⁺).

[μ-2,2′-P(Ph)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (4). Elemental selenium (0.201 g, 2.546 mmol) was added to a toluene (10 mL) solution of I (0.100 g, 0.225 mmol) which was then heated to reflux for 72 h. Excess selenium was removed by filtration and ³¹P{¹H} NMR spectroscopy revealed a 74% conversion of I to a new species. Preparative TLC (DCM : petrol, 1 : 9) yielded a colourless band at R_f = 0.59 from which the product, [μ-2,2′-P(Ph)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (4), was isolated as a pale-pink solid (0.057 g, 0.120 mmol, 47%). ¹¹B{¹H} NMR, δ 1.2 (2B), −4.1 to −11.2 [overlapping resonances with maxima at −4.1, −6.0, −6.4, −8.5, −9.9, −11.2 (18B)]. ¹H NMR, δ 8.31–8.25 (m, 2H, C₆H₅), 7.73–7.68 (m, 1H, C₆H₅), 7.63–7.57 (m. 2H, C₆H₅). ³¹P{¹H} NMR, δ 50.52 (s + Se satellites, ¹J_PSe = 891 Hz). EIMS, envelope centred on m/z 471.3 (M⁺).

[μ-2,2′-P(Et)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (5). Similarly, from Se (0.241 g, 3.052 mmol) and compound 1 (0.100 g, 0.145 mmol) was prepared [μ-2,2′-P(Et)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (5), a pale-pink solid (71% conversion by ³¹P{¹H} NMR, isolated yield 0.049 g, 0.116 mmol, 38%). ¹¹B{¹H} NMR, δ 0.9 (2B), −3.7 (2B), −6.2 to −10.6 [overlapping resonances with maxima at −6.2, −7.6, −8.7, −9.2, −10.6 (16B)]. ¹H NMR, δ 2.72 (dq, 2H, PCH₂CH₃, ²J_PH = 11.6 Hz, ³J_HH = 7.6 Hz), 1.52 (dt, 3H, PCH₂CH₃, ³J_PH = 25.9 Hz, ³J_HH = 7.6 Hz). ³¹P{¹H} NMR, δ 58.05 (s + Se satellites, ¹J_PSe = 894 Hz). EIMS, envelope centred on m/z 423.4 (M⁺).

Crystallography

Diffraction-quality crystals of all compounds were obtained by slow evaporation of a solution of the appropriate compound: I, 1 and 4; petrol: 2 and 3; DCM: 5; CDCl₃. Intensity data were collected on a Bruker X8 APEXII diffractometer using Mo-K_α X-radiation, with crystals mounted in inert oil on a cryoloop and cooled to 100 K by using an Oxford Cryosystems Cryostream. Indexing, data collection and absorption correction were performed using the APEXII suite of programs.⁴⁴ Using OLEX2,⁴⁵ structures were solved by direct methods using the SHELXS⁴⁶ or SHELXT⁴⁷ programme and refined by full-matrix least-squares (SHELXL).⁴⁶

All crystals were single except those of 5 which was treated as a two-component twin. All crystals were also fully ordered and solvate-free, except those of compound 3. In 3 the gold carboranylphosphine complex is fully ordered but there is a disordered solvent in the lattice that was impossible to satisfactorily model. Hence for this structure the intensity contribution of the disordered solvent was removed using the BYPASS procedure⁴⁸ implemented in OLEX2. The total electron count of the solvent per cell was 420e, which corresponds to 10 DCM molecules. These disordered solvent molecules predominantly occupy four voids of ca. 270 ų each.

For all structures H atoms bound to cage B atoms were allowed to refine positionally whilst H atoms bound to C atoms were constrained to idealised geometries; C_phenyl–H = 0.95 Å, C_methyl–H = 0.98 Å, C_methylene–H = 0.99 Å. All H displacement parameters, U_iso, were constrained to be 1.2 × U_eq (bound B or C) except for Me H atoms [U_iso(H) = 1.5 × U_eq C(Me)]. Table 3 contains further experimental details.

Table 3 Crystallographic data

	I	1	2	3·1.25CH2Cl2	4	5
CCDC	1527028	1527029	1527030	1527031	1527032	1527033
Formula	C10H25B20P	C6H25B20P	C10H25AuB20ClP	C6H25AuB20ClP	C10H25B20PSe	C6H25B20PSe
M	392.47	344.43	624.89	683.00	471.43	423.39
Crystal system	Triclinic	Monoclinic	Monoclinic	Orthorhombic	Monoclinic	Triclinic
Space group	P1̄	P21/n	C2/c	Pbca	P21/n	P1̄
a/Å	7.1139(10)	23.441(2)	17.0588(8)	10.4380(13)	10.3457(12)	7.1663(7)
b/Å	11.4378(14)	7.2690(6)	14.2499(7)	17.711(2)	16.9431(17)	11.0308(15)
c/Å	13.3912(15)	24.083(2)	20.0698(9)	27.475(3)	13.6941(14)	14.573(2)
α/°	81.884(5)	90	90	90	90	111.478(9)
β/°	84.954(5)	108.290(5)	102.497(3)	90	105.612(6)	92.125(9)
γ/°	81.231(6)	90	90	90	90	104.679(6)
U/Å^3	1063.6(2)	3896.2(6)	4763.1(4)	5079.3(10)	2311.9(4)	1026.2(2)
Z, Z′	2, 1	8, 2	8, 1	8, 1	4, 1	2, 1
F(000)/e	400	1408	2368	2596	936	420
D calc./Mg m^−3	1.225	1.174	1.743	1.786	1.354	1.370
μ(Mo-Kα)/mm^−1	0.127	0.128	6.357	6.224	1.693	1.898
θ max/°	26.46	23.77	32.01	27.24	28.95	30.14
Data measured	15 395	44 057	58 934	33 633	43 105	40 190
Unique data, n	4334	5928	8266	5651	6088	5691
R int	0.0636	0.1415	0.0796	0.0526	0.0740	0.0763
R, wR2 (obs. data)	0.0685, 0.1260	0.0795, 0.1841	0.0342, 0.0602	0.0297, 0.0585	0.0365, 0.0767	0.0410, 0.0861
S (all data)	1.135	1.053	1.015	1.020	1.034	1.036
Variables	340	609	358	323	349	315
E max, Emin/e Å^−3	0.31, −0.35	0.48, −0.41	0.93, −1.81	0.69, −0.65	0.35, −0.42	0.61, −0.58

---

Cone angle and percent buried volume calculations

Bis(carborane)phosphine cone angles were calculated from the crystallographically-determined structures using both the free bis(carborane)phosphines (compounds I and 1) and their AuCl complexes (compounds 2 and 3), using the method of Müller and Mingos.⁴⁹ The P–M distance was set at 2.28 Å. %V_bur calculations were performed (again on all compounds I, 1–3) using the SambVca software of Cavallo and co-workers,⁵⁰ with a sphere radius of 3.5 Å, a P–M distance of 2.28 Å and scaled Bondi radii.

Computational methods

All electronic structure calculations were carried out using the Gaussian 09 (Revision D.01)⁵¹ program suite at the DFT level of theory. Geometries of all compounds were fully optimised without imposing symmetry constraints (C₁ symmetry), employing the BP86 functional.⁵² Ahlrich's def2-TZVP basis of triple-ζ quality was used on P, Au, Cl, Se, and C and all hydrogen atoms of the ethyl group, while all B–H units of the carborane cage were described with the def2-SVP basis set.⁵³ The core electrons in Au were replaced by the Stuttgart–Dresden scalar relativistic effective core potential (SDD, ECP60MWB).⁵⁴ Optimised stationary points were characterised by analysis of their analytical second derivatives, with minima having only positive eigenvalues and transition states having exactly one imaginary eigenvalue. Subsequent geometry optimisations in both directions of the reaction coordinate were performed to confirm the minima linked by each transition state. The frequency calculations also provided thermal and entropic corrections to the total energy in the gas phase at T = 298.15 K and p = 1 atm within the rigid-rotor/harmonic oscillator (RRHO) approximation. Dispersion effects were accounted for by applying Grimme's van der Waals correction (D3 parameterization) protocol including Becke–Johnson damping during the optimisations.⁵⁵ The topology of the electron density was analysed by means of QTAIM (quantum theory of atoms in molecules),⁵⁶ as implemented in the AIMAll package.⁵⁷ For 3-DFT, inner shell electrons on Au modelled by the ECP were fitted by core density functions. Isotropic NMR spin–spin coupling constants were calculated using the coupled-perturbed SCF⁵⁸ method with the BHandHLYP⁵⁹ functional, which was found to yield the best agreement with experimental couplings. The J-couplings were obtained as the sum of all four Ramsey terms, i.e. Fermi contact (FC), spin–dipolar (SD), paramagnetic spin–orbit (PSO), and diamagnetic spin–orbit (DSO). The reported coupling constants have been averaged assuming free internal molecular rotation. The basis sets stated above were replaced by aug-cc-pVTZ-J⁶⁰ on P, and ethyl (C, H), as well as the cc-pVDZ⁶¹ basis set (and the associated ECP for Au) on all other atoms. Effects due to the presence of a solvent were treated implicitly with a polarisable dielectric model, using the IEFPCM formalism in conjunction with Truhlar's SMD model.⁶² The chosen dielectric constant (ε = 4.71) corresponds to that of chloroform. The compositions of molecular orbitals were analysed within the framework of localised natural bond orbitals, using the NBO 6.0 software.⁶³

Acknowledgements

We thank the EPSRC for support of LER through grant no. EP/I031545/1. TK wishes to thank Prof. Stuart A. Macgregor for providing access to high-performance computing facilities and helpful discussions.

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Footnotes

† Dedicated to Professor Evamarie Hey-Hawkins on the occasion of her 60^th birthday.

‡ Electronic supplementary information (ESI) available: NMR spectra of all new compounds reported. Full details of computational studies. CCDC 1527028–1527033. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt00485k

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