N-Pyrrolyl phosphine ligands: an analysis of their size, conformation and supramolecular interactions

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Abstract

Analysis of the Cambridge Structural Database confirms the assertion that tri(N-pyrrolyl)phosphine, P(NC₄H₄)₃, is isosteric with triphenylphosphine and reveals that N-pyrrolyl rings have a greater propensity to exist in the solid state in certain conformations. Tri(N-pyrrolyl)phosphine ligands are able to interact with each other in a similar manner to PPh₃ giving rise to six-fold and four-fold pyrrolyl embraces. However, as a consequence of the five-membered pyrrolyl rings, vertex-to-face interactions are more common for P(NC₄H₄)₃ than for PPh₃.

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It has been recognised recently that C–H⋯π interactions involving multiple aryl groups can be an important factor in determining solid state structures.¹ Dance and Scudder have shown that these interactions, which they denote as multiple phenyl embraces, can occur in triphenylphosphine complexes,² and in addition to linking individual molecules together they can form the basis of extended networks such as hexagonal arrays.³ Although the energy of a single C–H⋯π interaction is small, calculations show that the energy of a six-fold phenyl embrace (6PE) can be as high as 11.3 kcal mol⁻¹.²

Recently, we have become interested in N-pyrrolyl phosphine ligands,⁴ in part due to the strong π-acceptor character of these ligands which ensures they are excellent electron-withdrawing ligands.⁵ In this paper, the Cambridge Structural Database (CSD)⁶ is used to investigate the size and conformation of N-pyrrolyl phosphines, and to determine whether the N-pyrrolyl groups can act in a similar manner to phenyl groups in forming supramolecular interactions.

Steric requirements

It has been reported previously that tri(N-pyrrolyl)phosphine, P(NC₄H₄)₃, is isosteric with PPh₃, and this is supported by comparison of individual compounds such as [Rh{(PPh₂SiMe₂)₂N}{P(NC₄H₄)₃}] and [Rh{(PPh₂SiMe₂)₂N}(PPh₃)].⁷ However, since the observed cone angle of a phosphine can vary considerably in the solid state, it was felt that a statistical analysis of a greater number of examples was needed to confirm that the two ligands are indeed isosteric. The crystallographic cone angle provides a means of obtaining a cone angle for a phosphine in a particular crystal structure.⁸ Thus, the cone angle Θ can be obtained from the three half angles θ_i through the equationwhereα being the P–M–H angle, r_H the radius of the hydrogen atom and d the M–H distance (Fig. 1). For each N-pyrrolyl group, the hydrogen atom giving the greatest value of θ_i is identified and used in calculating Θ. In this manner the cone angle Θ can be obtained solely from the structural data. The value of r_H used in the calculations was 1.00 Å in order to facilitate comparison with PPh₃ (see below), though it should be noted that the original report on this method used r_H⊕=⊕1.20 Å.

Fig. 1 Parameters involved in the calculation of crystallographic cone angles.

The CSD contains 14 compounds of P(NC₄H₄)₃, including 23 independent P(NC₄H₄)₃ ligands.⁹ The crystallographic cone angle was calculated using the method above for each of the P(NC₄H₄)₃ ligands, and the results are summarised in Table 1. The mean value was found to be 147(4)° with a range of 136–152°. This can be compared with a mean value obtained by Müller and Mingos for PPh₃ based on 2388 phosphines of 148(5)°, with a range of 129–168°.¹⁷ The crystallographic data from the CSD therefore clearly support the assertion that P(NC₄H₄)₃ is isosteric with PPh₃.

Table 1 Cone angles, torsion angles and conformations of the P(NC₄H₄)₃ ligands

CSD code	Complex	Cone angle, Θ/°	Torsion angles/°	Conformation	Ref.
CEHQIP (P1)	cis-[PtMe2{P(NC4H4)3}2]	149	81.0, 24.2, 4.6	Orthogonal flipper	10
CEHQIP (P2)	cis-[PtMe2{P(NC4H4)3}2]	150	78.2, 16.1, 11.1	Orthogonal flipper	10
GEQVUT	[RuH3(η^5-C5Me5){P(NC4H4)3}]	149	44.1, 41.1, 32.8	Rotor	11
GIZFOK	[Rh{(PPh2SiMe2)2N}{P(NC4H4)3}]	149	78.5, 32.2, 4.0	Irregular	7
GIZGEB	[Rh{(PPr^i2SiMe2)2N}{P(NC4H4)3}]	150	70.4, 35.4, 3.4	Irregular	7
HAQWAX	[Ru(CN)(η^5-C5Me5)(CNBu^t){P(NC4H4)3}]·0.5EtOH	148	49.7, 45.8, 24.3	Rotor	12
HAQWEB	[RuH2(η^5-C5Me5)(SiMe2Ph){P(NC4H4)3}]	141	77.8, 40.4, 21.5	Irregular	12
MAZBUK	[Co2(μ-HC2Ph)(CO)5{P(NC4H4)3}]	152	53.5, 38.4, 25.8	Rotor	13
MAZCAR (P1)	[Co2(μ-HC2Ph)(CO)4{P(NC4H4)3}2]	146	62.6, 41.7, 32.0	Rotor	13
MAZCAR (P2)	[Co2(μ-HC2Ph)(CO)4{P(NC4H4)3}2]	146	67.3, 40.5, 30.9	Rotor	13
NATCIU	[Rh(acac)(CO){P(NC4H4)3}]	151	67.2, 25.8, 21.8	Rotor	14
NATCOA (P1)	[Rh(acac){P(NC4H4)3}2]	150	62.0, 25.7, 24.6	Rotor	14
NATCOA (P2)	[Rh(acac){P(NC4H4)3}2]	149	72.9, 31.0, 25.6	Irregular	14
PULTEV	[RuCl2(η^6-p-cymene){P(NC4H4)3}]	144	76.9, 31.2, 12.2	Orthogonal flipper	15
TORJUF (P1)	[RuCl(η^5-C5Me5){P(NC4H4)3}2]	140	56.3, 47.1, 26.0	Rotor	16
TORJUF (P2)	[RuCl(η^5-C5Me5){P(NC4H4)3}2]	136	86.7, 44.2, 10.8	Irregular	16
ZAVYEA (P1)	trans-[RhCl(CO){P(NC4H4)3}2]	150	75.6, 26.8, 12.0	Orthogonal flipper	5
ZAVYEA (P2)	trans-[RhCl(CO){P(NC4H4)3}2]	151	77.9, 15.2, 14.9	Orthogonal flipper	5
ZAVYEA (P3)	trans-[RhCl(CO){P(NC4H4)3}2]	151	77.6, 6.7, 3.8	Orthogonal flipper	5
ZAVYEA (P4)	trans-[RhCl(CO){P(NC4H4)3}2]	150	73.9, 19.6, 10.7	Orthogonal flipper	5
ZAVYOK (P1)	[N(PPh3)2][Rh(CO){P(NC4H4)3}3]·THF	145	47.7, 43.4, 31.9	Rotor	5
ZAVYOK (P2)	[N(PPh3)2][Rh(CO){P(NC4H4)3}3]·THF	146	41.3, 39.5, 32.1	Rotor	5
ZAVYOK (P3)	[N(PPh3)2][Rh(CO){P(NC4H4)3}3]·THF	142	49.9, 45.1, 30.9	Rotor	5

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Data for the mixed N-pyrrolyl–phenyl phosphines PPh(NC₄H₄)₂ and PPh₂(NC₄H₄) are less abundant, with only three independent phosphines of each type in the CSD (Table 2). The mean calculated cone angles are 141(4)° for PPh(NC₄H₄)₂ and 141(7)° for PPh₂(NC₄H₄), neither of which is significantly different from the value obtained for P(NC₄H₄)₃.

Table 2 Cone angles, torsion angles and conformations of the PPh(NC₄H₄)₂ and PPh₂(NC₄H₄) ligands

CSD code	Complex	Cone angle, Θ/°	Torsion angles/°	Conformation	Ref.
PULTIZ	[RuCl2(η^6-p-cymene){PPh(NC4H4)2}]	144	84.7, 31.2, 14.4	Orthogonal flipper	15
TORKAM (P1)	[RuCl(η^5-C5Me5){PPh(NC4H4)2}2]	141	76.2, 40.5, 18.4	Orthogonal flipper	16
TORKAM (P2)	[RuCl(η^5-C5Me5){PPh(NC4H4)2}2]	136	83.1, 53.9, 9.5	Irregular	16
DORVAH	[Rh(acac)(CO){PPh2(NC4H4)}]	149	62.5, 37.1, 36.0	Rotor	18
TORKEQ (P1)	[RuCl(η^5-C5Me5){PPh2(NC4H4)}2]	135	87.2, 56.1, 2.3	Irregular	16
TORKEQ (P2)	[RuCl(η^5-C5Me5){PPh2(NC4H4)}2]	140	66.8, 51.3, 6.5	Irregular	16

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Conformations of P(NC₄H₄)₃ ligands

Although the most favourable conformation for an isolated triarylphosphine ligand is a ‘propeller’, or rotor, a recent analysis of the CSD by Dance and Scudder showed that for PPh₃ only 40% of phosphines adopted this conformation.² In their analysis, Dance and Scudder used M–P–C–C torsion angles (T) to characterise each phenyl ring on a PPh₃ ligand as orthogonal (0⊕≤⊕|T|⊕<⊕20°), parallel (70⊕<⊕|T|⊕≤⊕90°), or staggered (20⊕≤⊕|T|⊕≤⊕70°), with the average value of the two M–P–C–C torsion angles for each phenyl ring used. A rotor conformation for a PPh₃ ligand was defined on the basis of the ligand having three staggered phenyl rings with the same sign for the torsion angles.

A similar procedure, based on M–P–N–C torsion angles, was used to analyse the P(NC₄H₄)₃ ligands and the results are summarised in Table 1. Eleven of the 23 independent P(NC₄H₄)₃ ligands were characterised in this manner as rotors, the same proportion within experimental error as for PPh₃. An example of this conformation is illustrated in Fig. 2. The number of P(NC₄H₄)₃ ligands containing orthogonal pyrrolyl rings (12 of the 23) is considerably higher than for PPh₃ (34.4%), this difference being significant at the 95% probability level. Furthermore, for six of these orthogonal pyrrolyl groups the phosphorus atom lies out of the plane of the pyrrolyl ring, as evidenced by a pyramidal distortion at the nitrogen atom with the sum of angles at the nitrogen atom being less than 358°. Consideration of individual structures suggests a number of reasons for these distortions, such as involvement in C–H⋯π hydrogen bonds and π–π interactions (Fig. 3). Although such distortions from planarity appear small and are of relatively low energy, they are significant as their occurrence can lead to the displacement of the far end of a pyrrolyl group by ca. 0.5 Å.

Fig. 2 Rotor conformation of the P(NC₄H₄)₃ ligand in [RuH₃(η⁵-C₅Me₅){P(NC₄H₄)₃}] (GEQVUT) (side and top views).

Fig. 3 Intramolecular C–H⋯π (shown in blue and white candystripe) and π–π interactions (shown in red and white) involving the orthogonal N-pyrrolyl groups in cis-[PtMe₂{P(NC₄H₄)₃}₂] (CEHQIP).

Of the 12 non-rotors, three are in the conformation denoted by Dance and Scudder as orthogonal flippers. This conformation contains one orthogonal ring, with the other two possessing torsion angles of similar magnitude (±20°) but opposite sign. An example of this conformation is illustrated in Fig. 4. Given the level of occurrence of orthogonal pyrrolyl groups in structures containing P(NC₄H₄)₃, it is somewhat surprising that in the two crystal structures containing PPh(NC₄H₄)₂ ligands – [RuCl₂(η⁶-p-cymene){PPh(NC₄H₄)₂}] (PULTIZ) and [RuCl(η⁵-C₅Me₅){PPh(NC₄H₄)₂}₂] (TORKAM) (Table 2) – both contain an orthogonal phenyl group and a parallel pyrrolyl group.

Fig. 4 Orthogonal flipper conformation of the P(NC₄H₄)₃ ligand in trans-[RhCl(CO){P(NC₄H₄)₃}₂] (ZAVYEA) (side and top views).

Supramolecular interactions

Triphenylphosphine ligands are able to interact through multiple phenyl embraces to give extended structures. Having established the similarities in size and conformation of tri(N-pyrrolyl)phosphine to PPh₃, the existence of multiple pyrrolyl embraces has been investigated.

Dance and Scudder used three criteria to define a six-fold phenyl embrace:¹

rotor conformations;
a P⋯P separation of less than 7.5 Å;
both M–P⋯P angles in the range 160–180°.

Of the 23 independent tri(N-pyrrolyl)phosphine ligands, three fulfil all these criteria and can be said to contain six-fold pyrrolyl embraces. These are the phosphine ligand in [Rh(acac)(CO){P(NC₄H₄)₃}] (NATCIU) and the two phosphine ligands in [Co₂(μ-HC₂Ph)(CO)₄{P(NC₄H₄)₃}₂] (MAZCAR). In the former the embrace occurs to link molecules into dimers (Fig. 5), whereas in the latter, the two embraces serve to link the molecules into one-dimensional chains (Fig. 6). None of these three six-fold pyrrolyl embraces involves the six edge-to-face arrangements that are observed in regular six-fold phenyl embraces.¹ In each case there are four interactions with C⋯C⊕<⊕4.0 Å, two of which are edge-to-face, the other two vertex-to-face. The occurrence of the latter is a consequence of the five-membered pyrrolyl ring and the resultant relative orientations of the CH vectors. It is noteworthy that vertex-to-face interactions are more frequent in interactions between P(NC₄H₄)₃ ligands than in those between PPh₃ ligands, for which edge-to-face interactions are more prevalent.

Fig. 5 Six-fold pyrrolyl embraces in [Rh(acac)(CO){P(NC₄H₄)₃}] (NATCIU) linking the molecules into dimers (side and end views).

Fig. 6 Six-fold pyrrolyl embraces in [Co₂(μ-HC₂Ph)(CO)₄{P(NC₄H₄)₃}₂] (MAZCAR) linking the molecules into one-dimensional chains.

Distorted six-fold phenyl embraces are possible for PPh₃ ligands in conformations other than rotors. This is also observed for P(NC₄H₄)₃ ligands, with [Rh{(PPh₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZFOK) and one of the phosphines in cis-[PtMe₂{P(NC₄H₄)₃}₂] (CEHQIP) providing examples (Fig. 7). In [Rh{(PPh₂SiMe₂)₂N}{P(NC₄H₄)₃}] the distorted six-fold N-pyrrolyl embrace links the molecules into dimers, whereas in [PtMe₂{P(NC₄H₄)₃}₂] the extended structure is more complex as the second phosphine ligand does not show a regular multiple pyrrolyl embrace.

Fig. 7 Distorted six-fold pyrrolyl embraces in [Rh{(PPh₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZFOK).

Two classes of four-fold phenyl embrace have been identified for PPh₃ compounds, defined as orthogonal (O4PE) or parallel (P4PE) depending on the relative orientations of the C–P–C planes. Dance and Scudder reported the latter to be more common for PPh₃, and for the offset face-to-face interaction component observed for PPh₄⁺ cations to not always be present.² Very similar observations are made for the tri(N-pyrrolyl)phosphine structural series. Six of the 23 independent P(NC₄H₄)₃ ligands are involved in parallel four-fold pyrrolyl embrace interactions, whereas none are involved in orthogonal four-fold pyrrolyl embrace interactions. Of these six, only one – [Ru(CN)(η⁵-C₅Me₅)(CNBu^t){P(NC₄H₄)₃}] (HAQWAX), Fig. 8 – contains a face-to-face interaction of pyrrolyl rings in addition to a pair of edge-to-face interactions. The others – [Rh{(PPrⁱ₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZGEB), [RuH₂(η⁵-C₅Me₅)(SiMe₂Ph){P(NC₄H₄)₃}] (HAQWEB), [RuCl₂(η⁶-p-cymene){P(NC₄H₄)₃}] (PULTEV) and [RuCl(η⁵-C₅Me₅){P(NC₄H₄)₃}₂] (TORJUF), Fig. 9 – contain only a pair of vertex-to-face interactions. In the case of [RuCl(η⁵-C₅Me₅){P(NC₄H₄)₃}₂] (TORJUF), parallel four-fold pyrrolyl embraces serve to link the molecules into chains.

Fig. 8 Parallel four-fold pyrrolyl embrace present in [Ru(CN)(η⁵-C₅Me₅)(CNBu^t){P(NC₄H₄)₃}] (HAQWAX). The shortest H⋯C interactions are marked in blue and white and the shortest π⋯π interactions are marked in red and white.

Fig. 9 Parallel four-fold pyrrolyl embraces comprising vertex-to-face interactions present in (a) [Rh{(PPrⁱ₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZGEB), (b) [RuH₂(η⁵-C₅Me₅)(SiMe₂Ph){P(NC₄H₄)₃}] (HAQWEB), (c) [RuCl₂(η⁶-p-cymene){P(NC₄H₄)₃}] (PULTEV) and (d) [RuCl(η⁵-C₅Me₅){P(NC₄H₄)₃}₂] (TORJUF). The shortest H⋯C interactions are marked in blue and white.

Despite containing seven independent P(NC₄H₄)₃ ligands between them, neither trans-[RhCl(CO){P(NC₄H₄)₃}₂] (ZAVYEA) nor [N(PPh₃)₂][Rh(CO){P(NC₄H₄)₃}₃]·THF (ZAVYOK) contain any good multiple aryl embraces. The supramolecular structure of trans-[RhCl(CO){P(NC₄H₄)₃}₂] is complex, with edge-to-face or vertex-to-face interactions involving all four independent P(NC₄H₄)₃ ligands. The most noteworthy of these interactions are a five-fold embrace involving P(2) and P(3), and pairs of four-fold embraces involving P(1) and P(2), and P(3) and P(4), which link the independent molecules into dimers. However, none of the edge-to-face or vertex-to-face interactions involved in these multiple N-pyrrolyl embraces comprise particularly short or directional C–H⋯π interactions. The structure of [N(PPh₃)₂][Rh(CO){P(NC₄H₄)₃}₃]·THF is dominated by interactions between the cations and anions. These include C–H⋯π interactions in which the N-pyrrolyl groups act as both donors and acceptors, though no regular multiple embraces between tri(N-pyrrolyl)phosphine and triphenylphosphine groups are observed.

Although not all of the P(NC₄H₄)₃ complexes have PPh₃ analogues that have been structurally characterised, a comparison for those that do with these compounds reveals the intermolecular interactions to be largely similar in both structures. Thus [Rh(acac)(CO)(PPh₃)] (ACRHCP), like [Rh(acac)(CO){P(NC₄H₄)₃}] (NATCIU), shows a six-fold embrace, whereas [RuCl(η⁵-C₅Me₅)(PPh₃)₂] (GOLTAC) and [Rh{(PPrⁱ₂SiMe₂)₂N}(PPh₃)] (GIZGAX), like [RuCl(η⁵-C₅Me₅){P(NC₄H₄)₃}₂] (TORJUF) and [Rh{(PPrⁱ₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZGEB), show parallel four-fold embraces. The biggest exception is [Rh{(PPh₂SiMe₂)₂N}(PPh₃)] (GIZFIE) for which a P4PE is observed: its P(NC₄H₄)₃ analogue – [Rh{(PPh₂SiMe₂)₂N}{P(NC₄H₄)₃}] (GIZFOK) – shows an irregular six-fold pyrrolyl embrace.

Of the six independent phenyldi(N-pyrrolyl)phosphine or diphenyl(N-pyrrolyl)phosphine ligands, one ([Rh(acac)(CO){PPh₂(NC₄H₄)}], DORVAH) fulfils the criteria for a six-fold embrace, and the mixed pyrrolyl–phenyl embrace links the molecules into dimers (Fig. 10).

Fig. 10 Six-fold phenyl–pyrrolyl embrace in [Rh(acac)(CO){PPh₂(NC₄H₄)}] (DORVAH) (side and end views).

In conclusion, despite the large difference in the electronic properties of the two ligands, the size, conformation and supramolecular structure observed for P(NC₄H₄)₃ are broadly similar to those previously reported for PPh₃. However, for P(NC₄H₄)₃ there is an increased frequency in the occurrence of conformations containing orthogonal rings, which is in part a consequence of the greater propensity of the pyrrolyl ring to bend away from the P–N vector. In addition, vertex-to-face C–H⋯π hydrogen bonds are a greater factor in the interactions between P(NC₄H₄)₃ ligands than they are for PPh₃, for which edge-to-face C–H⋯π hydrogen bonds dominate.

Acknowledgements

Use of the EPSRC's Chemical Database Service at Daresbury is acknowledged, and the EPSRC is thanked for financial support.

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