PPh₃ Propeller Diastereomers: Bonding Motif Ph_PPh3 Face-On π-Ar in Half-Sandwich Compounds [(π-Ar)LL′MPPh₃]

Abstract

Chiral-at-metal compounds (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ and (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ were prepared using anions 1O-2N^– and 2O-1N^– of the Schiff bases, derived from the hydroxynaphthaldehydes and (S)-1-phenylethylamine. The pure (R_Ru,S_C)-diastereomers were obtained by crystallization. In the unit cell of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, there are three independent molecules, which differ in the propeller sense of the PPh₃ ligand. Molecules [1] and [2] have (M_PPh3)-configuration and molecule [3] has (P_PPh3)-PPh₃ configuration. PPh₃ diastereoisomerism is discussed including other pairs of compounds, differing only in the PPh₃ configuration. A conformational analysis reveals an internal stabilization inside the PPh₃ ligand by a system of attractive CH/π interactions and a new bonding motif Ph_PPh3 face-on π-Ar, both characteristic features of [(π-Ar)LL′MPPh₃] compounds. The propeller diastereomers interconvert via a low-energy pathway and a high-energy pathway, corroborated by density functional theory calculations.

Introduction

In half-sandwich compounds of type [(π-Ar)LL′MPPh₃], π-Ar = η⁶-C₆H₆, η⁵-C₅H₅, the triphenylphosphine ligand accounts for about half of the molecule. Figure 1 shows a hypothetical staggered conformation A in a Newman projection looking along P-M, which differentiates the phenyl rings into gauche and trans with respect to π-Ar. In such a conformation, the inner ortho-hydrogen atoms of the phenyl rings would approximate each other to unacceptably short distances. The phenyl rings avoid this steric hindrance by rotation around their P–C_ipso bonds, adopting a propeller structure B in Figure 1. When steric hindrance disappears, weak attractive forces such as CH/π interactions in the internal PPh₃ stabilization (see below) come into play.

Figure 1.

Newman projection of [(π-Ar)LL′MPPh₃] looking along P-M. Hypothetical staggered conformation (A). Propeller conformation (B). Ph_PPh3 face-on π-Ar bonding conformation (C).

In a 1983 paper, we showed that in half-sandwich compounds [(π-Ar)LL′MPPh₃] there is an additional rotation about the P-M bond, differentiating the gauche phenyl rings into close and distant to π-Ar (C in Figure 1).¹ The phenyls close to π-Ar have rotation angles |0 < ρ < 60°|, and the phenyls distant to π-Ar have |60 < ρ < 120°|. Subsequently, we will show that this rotation is part of bonding motif Ph_PPh3 face-on π-Ar.

In the present paper, we describe the synthesis and characterization of compounds (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ (Cy = cymene, 1-isopropyl-4-methylbenzene, 1O-2N^– = (S)-2-[[(1-phenylethyl)imino]methyl]-1-naphthalenolate) and (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ (2O-1N^– = (S)-1-[[(1-phenylethyl)imino]methyl]-2-naphthalenolate). In the crystal, chiral-at-metal compound (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ forms diastereomers, which differ only in the configuration of the triphenylphosphine propeller. In this context, we discuss eight pairs of such propeller diastereomers, develop the Ph_PPh3 face-on π-Ar bonding concept, and reveal low- and high-energy pathways of the interconversion of the PPh₃ propeller.

Results and Discussion

Synthesis and X-ray Characterization

Diastereomerically pure compounds (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ and (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ (Scheme 1) were obtained in the following sequence of reactions. Deprotonated ligands 1OH-2N and 2OH-1N² were reacted with [CyRuCl]₂Cl₂ to give (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)Cl] and (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)Cl], respectively. Treatment with PPh₃ and NH₄PF₆ afforded products (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ and (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆. In both cases, the pure (R_Ru,S_C) diastereomers were obtained by crystallization from CH₂Cl₂ as red crystals suitable for X-ray analysis (Table S1 in the Supporting Information). In the unit cell of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, there are two different molecules [1] and [2] with the (M,M,M)-configuration of the PPh₃ ligand and one molecule [3] with the (P,P,P)-PPh₃ configuration.

Scheme 1. (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, 1O-2N^– = (S)-2-[[(1-phenylethyl)imino]methyl]-1-naphthalenolate, and (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆, 2O-1N^– = (S)-1-[[(1-phenylethyl)imino]methyl]-2-naphthalenolate.

Priority sequence,^3,4 Cy > PPh₃ > O > N.

The molecular structures will be discussed first for (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ and then for the diastereomers of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆. The π-stack between the substituted phenyl ring of the naphthyl system and one of the phenyl rings of the PPh₃ ligand is a striking feature in the structure of (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ (Figure 2, left side). The distance between the carbon atoms [(Np)C2-C_i(Ph) = 3.21 Å] is considerably shorter than the distance between the layers in graphite (3.35 Å). The distances (Np)C2-C_o(Ph) and (Np)C1-C_o(Ph) are 3.27 and 3.16 Å.

Figure 2.

Molecular structures of (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ (left side) and molecule (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ [3] with (P,P,P)-configuration of the PPh₃ propeller (right side). Hydrogen atoms omitted for clarity.

Two independent molecules [1] and [2] of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ with the same propeller sense show similar π-stacks with corresponding distances, e.g., (Np)C1-C_i(Ph) = 3.20 and 3.21 Å. On the other hand, there is no such π-stack in the third independent molecule [3] of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, having the opposite PPh₃ propeller configuration (Figure 2, right side).

Internal CH/π Stabilization within the PPh₃ Propeller

The architecture of the PPh₃ propeller is determined by CH/π interactions of the type found in the archetypal T-shaped benzene dimer.⁵⁻⁹ Contrary to those in the T-shaped benzene dimer, the CH/π interactions in PPh₃ are intramolecular and thus entropically almost neutral. In the PPh₃ ligand, there are six C_o-H bonds, three inside the propeller (ⁱⁿC_oH) and three outside (^outC_oH). It is the interaction between the ⁱⁿC_o-H bonds and C_i, ⁱⁿC_o, and ^outC_o atoms of neighboring phenyl rings (i/o/p = ipso/ortho/para) that adds up to an appreciable stabilization, as discussed in refs (10) and (11) (Figure 3).

Figure 3.

CH/π interactions Ph3 → Ph1, Ph2 → Ph3, and Ph1 → Ph2 looking along the P-M axis.

In the following discussion, the torsion angles |C_o-C_i-P-M|< 90° of phenyls Ph1, Ph2, and Ph3 will be called propeller angles τ. Table 1 contains these τ angles and distances C_oH-C_i and C_oH-C_o of the CH/π interactions inside the PPh₃ propeller. As in our former analyses,^10,11 we ordered the propeller angles according to the smallest angle in the phenyl ring called Ph1. In addition to the four new molecules of the present paper, we added the pair of diastereomers of (R_Ru,S_C)-[(π-C₆H₆)Ru(O-N)PPh₃]PF₆, O-N = anion of the Schiff base derived from salicylaldehyde and (S)-1-phenylethylamine, for which both diastereomers HEDYIY and HEDYOE differ only in the PPh₃ propeller sense.¹² (R_Ru,S_C)-[(π-C₆H₆)Ru(O-N)PPh₃]PF₆ is the parent benzene/phenyl compound of the cymene/naphthyl compounds of the present paper.

Table 1. Internal Stabilization in Compounds (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆, and (R_Ru,S_C)-[(π-C₆H₆)Ru(O-N)PPh₃]PF₆^a.

entry varianta	CSD symbol or CCDC numberb	formula		3 → 1 (Å)		1 → 2 (Å)		2 → 3 (Å)
			M-P-Ci-Co	inCoH-Ci	M-P-Ci-Co	inCoH-Ci	M-P-Ci-Co	inCoH-Ci
			Ph1 (deg)	inCoH-Coa	Ph2 (deg)	inCoH-Coa	Ph3 (deg)	inCoH-outCoa
1	1519531 [1]	(RRu,SC)-[CyRu(1O-2N)PPh3]PF6	6.4	2.75	89.5	2.78	44.7	2.77
A/B				2.67in		2.93out		2.56out
2	1519531 [2]	(RRu,SC)-[CyRu(1O- 2N)PPh3]PF6	14.2	2.71	82.6	2.77	44.0	2.72
B				2.77out		2.97out		2.58out
3	HEDYOEc	(RRu,SC)-[(π- C6H6)Ru(O-N)PPh3]PF6	–16.0	2.57	–67.6	2.80	–62.1	2.78
A				2.79in				2.64out
4	1519532	(RRu,SC)-[CyRu(2O- 1N)PPh3]PF6	25.6	2.62	86.2	2.74	44.3	2.77
A/B				2.79in		2.73out		2.63out
5	HEDYIYc	(RRu,SC)-[(π- C6H6)Ru(O-N)PPh3]PF6	23.9	2.58	77.9	2.63	53.7	2.76
A/B				2.86in		2.78out		2.54out
6	1519531 [3]	(RRu,SC)-[CyRu(1O- 2N)PPh3]PF6	–25.2	2.59	–61.9	2.58	–69.5	2.73
B						3.02out		2.54out

^aVariants A and B, torsion angles M-P-C_i-C_o < 90°, and distances ⁱⁿC_oH-C_i, ⁱⁿC_oH-ⁱⁿC_o, and ⁱⁿC_oH-^outC_o. Variant A refers to ⁱⁿC_oH-ⁱⁿC_o distances and variant B refers to ⁱⁿC_oH-^outC_o distances for Ph1 and Ph2, respectively (see refs (10) and (11)).

^bBrackets [ ] indicate independent molecules.

^cSee ref (12).

Each of the three phenyls plays a specific role in interactions Ph3 → Ph1 = ⁱⁿC_oH(3) → C_i/o(1), Ph2 → Ph3 = ⁱⁿC_oH(2) → C_i/o(3), and Ph1 → Ph2 = ⁱⁿC_oH(1) → C_i/o(2), represented by the arrows in Figure 3. The differentiation into dashed and bold ⁱⁿC_oH → C_o interactions is relevant.^10,11 All of the 18 ⁱⁿC_oH-C_i distances are appreciably below 3.0 Å, the sum of the van der Waals radii of the hydrogen atom and the sp²-hybridized carbon atom,^10,11,13 and thus within the bonding range of CH/π interactions. The same is true for 15 of the 18 ⁱⁿC_oH-C_o distances (Table 1). The approximation of the ortho-CH bonds to the ipso- and ortho-carbon atoms of neighboring phenyl rings to distances far below the sum of the van der Waals radii shows the internal stabilization in the PPh₃ ligands. The Ph1 propeller angles in Table 1 span a broad range from |6.4°| to |25.2°|. For all of these τ(Ph1) angles, phenyls Ph2 and Ph3 find propeller angles to establish the necessary CH/π interactions for the internal stabilization.^10,11

Propeller Chirality

Each of the three M-P-Ph systems in a PPh₃ ligand is an independent element of chirality.^10,11 The propeller angles C_o-C_i-P-M < 90° are measures of the chirality of the M-P-Ph entities. They define (P)/(M) chirality of the M-P-Ph blades of the PPh₃ propeller according to the helicity rule of the CIP system.¹⁴ Negative propeller angles correspond to (P) chirality, and positive propeller angles correspond to (M) chirality.

In (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆, the propeller angles C_o-C_i-P-M < 90° in the PPh₃ ligand are +25.6, +86.2, and +44.3° (Table 1). As all torsion angles are positive, the (M,M,M)-configuration has to be assigned to the PPh₃ ligand. In (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, the unit cell contains three independent molecules. Two of them, [1] and [2], have (M,M,M)-configuration due to positive torsion angles +14.2, +82.6, and +44.0° and +6.4, +89.5 (−88.0), and +44.7°. In the phenyl ring, with the highest torsion angle of the (M,M,M)-diastereomers of 2O-1N and 1O-2N, the two ortho positions are almost equivalent (large thermal ellipsoids). Use of one or the other will interchange the symbols (M) and (P). The torsion angles of [1] and [2] are very similar to those of (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆. However, the third molecule [3] in the unit cell of (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ is very different. Its PPh₃ ligand has (P,P,P)-configuration due to its negative torsion angles −25.2, −61.9, and −69.5° (Table 1).

Our recent analysis of 119 compounds of type [(π-Ar)LL′MPPh₃] had shown that propeller configurations can be divided into two subgroups (P,P,P)/(M,M,M) (∼90% abundance) and (M,P,P)/(P,M,M) (∼10% abundance).^10,11 As all of the new cymene/naphthyl compounds and their parent benzene/phenyl compounds HEDYOE and HEDYIY belong to the (P,P,P) or (M,M,M) type, we will subsequently use symbols (P_PPh3) and (M_PPh3) for the propeller configuration of the PPh₃ ligand.

Rotation Angles ρ

In ref (1), we demonstrated that 11 [(π-Ar)LLMPPh₃] compounds and 17 [(π-Ar)LL′MPPh₃] compounds adopted structures of type C, in Figure 1, with rotation angles far below 60°. This rotation brings the gauche phenyl |0 < ρ < 60°| close and face-on toward π-Ar, whereas the gauche phenyl |60 < ρ < 120°| becomes distant and edge-on toward π-Ar. It was argued that the steric hindrance of the π-Ar ligand with the ortho-CH bond of the edge-exposed phenyl is responsible for the rotation,¹ which is wrong (see below).

Figure 4 shows Newman projections of the six salicylaldiminato compounds. They clearly subdivide into two types, which have surprisingly similar conformations, irrespective of their π-Ar and O-N substituents. In all of the compounds, Ph3 is face-exposed to π-Ar with rotation angles ρ below |60°|, whereas Ph1 is edge-exposed with rotation angles ρ above |60°|. Taking into account the +/– signs of the rotation angles, the entire configurational symbols are (R_Ru,S_C,M_PPh3) for the compounds on the left and (R_Ru,S_C,P_PPh3) for those on the right of Figure 4. The rotation angles of Ph_face concentrate in the narrow range from |28.7°| to |47.0°|. With |89.2°| to |76.7°|, the rotation angles, ρ, of Ph_edge add up to 120° (Table S2 in the Supporting Information). The average of rotation angles ρ(Ph_face) is |40.0°|.

Figure 4.

Newman projections of (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆, (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ [1], [2], [3], HEDYIY, and HEDYOE looking along P-M.

In Table S2, we included another five pairs of diastereomers, which differ only in the propeller sense of the PPh₃ ligand: VOWTUW,¹⁵ GIRYIP,¹⁶ ZINXOJ,¹⁷ FOMZEN,¹⁸ and RCMXFE.¹⁹ These compounds are of types [CpFe(CO)(R)PPh₃] and [CpRe(NO)(R)PPh₃]X. In the unit cell of these compounds, there are two independent molecules with the same metal configuration and opposite PPh₃ configurations. This is similar to the four cymene/naphthyl compounds of the present paper, although they have an additional chiral center in the chelate ligand. For diastereomers HEDYIY and HEDYOE of (R_Ru,S_C)-[(π-C₆H₆)Ru(O-N)PPh₃]PF₆, however, the situation is different. We could isolate the diastereomers of this compound as separate single crystals.¹² Thus, HEDYOE and HEDYIY are two different modifications of (R_Ru,S_C)-[(π-C₆H₆)Ru(O-N)PPh₃]PF₆. We also included SEPZUI in Table S2. Its two diastereomers differ in the metal configuration, having the same PPh₃ propeller sense (P_PPh3).²⁰

The rotation angles of Ph_face and Ph_edge of the CpFe(CO) and CpRe(NO) compounds hook up with the salicyliminato compounds, except for (M_PPh3) diastereomer GIRYIP[1], which is not used for average calculations (Table S2). The overall average of rotation angles ρ(Ph_face) is −36.8° for the (P_PPh3) diastereomers and 40.0° for the (M_PPh3) diastereomers. The face-on approximation of Ph_face to π-Ar is an indication of a bonding attraction, considered next.

Bonding Motif Ph_PPh3 Face-On π-Ar

The bonding system Ph_PPh3 face-on π-Ar includes elements of the T-shape as well as of the π-stack benzene dimer, and it contains the Ru and the P atom. In addition to rotation angles ρ, angle φ between the planes of π-Ar and Ph_face is a measure of the π-Ar/Ph_face interaction. Figure 5 shows the arrangement of π-Ar and Ph_PPh3 in HEDYIY (φ = 27.6°, left side) and HEDYOE (φ =28.6°, right side). In HEDYIY, distances C_Ar-C_i 3.32 Å and C_Ar-C_o 3.30 Å are below the graphite distance of 3.35 Å, indicating a π-stack interaction. T-shape benzene dimer interactions show up in distances such as (π-Ar)CH-C_o = 2.76 Å and (π-Ar)CH-C_i = 2.91 Å. The corresponding distances of propeller diastereomer HEDYOE are similar. In HEDYIY and HEDYOE, rotation angles ρ = 37.3 and −28.7° of the face-on phenyls enforce rotation angles of ρ = −82.4 and 89.2° for the corresponding edge-on phenyls.

Figure 5.

Bonding system Ph_PPh3 face-on π-Ar in HEDYIY, HEDYOE, and PIGJOG.

In Figure 6, rotation angles ρ of the face-on and edge-on phenyls of the 18 diastereomers, differing only in the propeller configuration of the PPh₃ ligand, are shown as a function of π-Ar/Ph_face angles φ. They crowd around the averages of ρ and φ, which are ρ_av= −34.3° and φ_av = 23.6° for the (P_PPh3) diastereomers and ρ_av = 40.0° and φ_av = 24.2° for the (M_PPh3) diastereomers. The averages of ρ and φ of the edge-on phenyls are ρ_av = −81.0°/φ_av = 55.3° for the (P_PPh3) diastereomers and ρ_av = 81.8°/φ_av = 46.1° for the (M_PPh3) diastereomers.

Figure 6.

Plot of the rotation angle ρ versus angle φ plane Ph_face/edge/plane π-Ar for 18 diastereomers, differing only in the PPh₃ propeller configuration: Rotation angles ρ of P_Ph(face) (red ▲), M_Ph(face) (▲), P_Ph(edge) (red ●), and M_Ph(edge) (●) versus angles φ plane Ph_face/edge/plane π-Ar. Bottom: Pathways a and b for diastereomer interconversion.

The turning of Ph_PPh3 face-on to π-Ar is a general phenomenon. In the histogram of Figure 7, this is shown for 140 cases of 119 compounds of type [(π-C₆R₆)RuLL′PPh₃], obtained in a CSD search for [(π-C₆R₆)RuPPh₃].²¹ The sample points concentrate around averages ρ_av = −39.0° and φ_av = 27.7° for the (P_PPh3) diastereomers and ρ_av = 39.3° and φ_av = 25.7° for the (M_PPh3) diastereomers. This is surprising because L and L′ and the substituents in the π-Ar ligand of the (π-Ar)LL′Ru fragments vary considerably. In all of the 140 cases of Figure 7, there is no exception with ρ > 60° such as GIRYIP[1] in Figure 6.

Figure 7.

Plot of rotation angle ρ versus angle φ plane Ph_face/edge/plane π-Ar for 140 cases of 119 compounds of type [(π-C₆R₆)RuLL′PPh₃] according to Table S3 (Supporting Information): Rotation angles ρ of P_Ph(face) (red ▲), M_Ph(face) (▲), P_Ph(edge) (red ●), and M_Ph(edge) (●) versus angles φ plane Ph_face/edge/plane π-Ar. Bottom: Pathways a and b and transition states for the interconversion of HEDYIY and HEDYOE.

With a rotation angle of ρ = −5.1°, PIGJOG²² is almost in the middle between HEDYIY and HEDYOE (Figure 5). PIGJOG is even more perfectly stabilized than HEDYIY and HEDYOE, as apparent from distances C_Ar-C_i = 3.28 and 3.31 Å as well as (π-Ar)CH-C_o = 2.68 and 2.75 Å and (π-Ar)CH-C_i = 2.84 and 2.89 Å. When PIGJOG is very highly stabilized, the question arises, why do the rotation angles ρ of Ph_face concentrate around ±40° and not around 0°? The reason is the eclipsing interaction of the other two phenyls with substituents L and L′ in three-legged sandwich fragment (π-Ar)LL′M. PIGJOG’s substituent L = H₂BHNMe₃ is in a plane with Ar_cent, Ru, and P perpendicular to the plane of the paper, and the two phenyls stagger L perfectly (Figure 5). Thus, ±40° is a compromise of Ph_face to establish Ph_PPh3 face-on π-Ar stabilization and to avoid eclipsing of the other two phenyls with L and L′.

Interconversion of Propeller Diastereomers

The interconversion of diastereomers HEDYIY and HEDYOE, differing only in the propeller configuration, can occur by two different pathways: (a) Ph_face of HEDYIY is converted to Ph_face of HEDYOE via a transition state about ρ = 0° and vice versa and (b) Ph_face of HEDYIY is converted to Ph_edge of HEDYOE via a transition state about ρ = 60° and vice versa (Figures 6 and 7). Both pathways require only small intramolecular rotations of ρ and τ, far below full phenyl rotations.

Pathway a inverts the chirality of Ru-Ph_face from (M_Ph) in HEDYIY to (P_Ph) in HEDYOE and exchanges ⁱⁿC_o/m of Ph_face to ^outC_o/m. In addition, it brings Ph_trans of HEDYIY up into the position of Ph_edge of HEDYOE and it moves Ph_edge of HEDYIY down to the position of Ph_trans of HEDYOE. In pathway a, Ph_face passes through conformations with rotation angles ρ around 0° similar to the conformation of PIGJOG in Figure 5. Because these conformations are highly stabilized, pathway a would be energetically favorable for Ph_face. However, as discussed above, rotation angles of Ph_face around 0° imply the eclipsing of the other two phenyls with substituents L and L′, which makes the area of Ph_face around 0° a transition state.

Pathway b, although interchanging diastereomers HEDYIY and HEDYOE, does not change the (M_Ph) chirality, and it does not exchange ⁱⁿC_o/m of Ph_face to ^outC_o/m of the phenyl in question. In addition, this rotation brings Ph_edge of HEDYIY into the position of Ph_face of HEDYOE and it converts Ph_trans of HEDYIY to Ph_trans of HEDYOE. Thus, Ph_trans stays Ph_trans, but it inverts its chirality. Pathway b does not involve the eclipsing situation of pathway a.

It is well known that sample points of conformations, retrieved from the Cambridge Crystallographic Data file, concentrate in low-energy areas and thin out toward transition states.²³ Therefore, the high population of the areas at about ρ = |40°| in Figures 6 and 7 by sample points means that these structures are favorable molecular conformations. On the other hand, the thinning out of sample points on the two sides of the energy minimum ρ = |40°| indicates the approximation to transition states. Furthermore, the distribution of sample points in the areas of the two transition states allows a differentiation between pathways a and b of the (P_Ph)/(M_Ph) interconversion of Ph_face. At about ρ = 0°, sample points not only thin out but disappear completely (Figures 6 and 7). That means, rotation angles about ρ = 0° correspond to a high-lying transition state. On the other hand, sample points of (M_Ph)-Ph_face and (P_Ph)-Ph_edge about ρ = 60° overlap, indicating a low-lying transition state.

The process of diastereomer interconversion along pathways a and b is shown at the bottom of Figure 7 on the right side. In pathway b, starting with HEDYIY at ρ = 37.3°, the transition state is reached at about ρ = 60° to finally arrive at HEDYOE with ρ = 80°. The process on the right side of Figure 6 would be similar. This low-energy pathway, far below full rotations around the C_i-P and P-Ru axes, is corroborated by the experimental sample points in Figure 7. The use of such experimental data to find reaction pathways has been pioneered by Dunitz et al.²³

Whereas in the crystal, the PPh₃ propeller configurations are fixed, in solution, they rapidly interconvert. For the 18 propeller diastereomers, pathway b seems to be the easiest mechanism of interconversion. This discussion concentrated on Ph_face and did not take into account a detailed consideration of Ph_edge and Ph_trans. In addition, it must be kept in mind that each of the three phenyls has to carry out its duty in the internal stabilization of the PPh₃ propeller.

Density Functional Theory (DFT) Calculations

We checked the results, obtained in the analysis of CSD sample points, by DFT calculations²⁴ (RI²⁵-B3LYP²⁶/def2-TZVP^25b,27). Using the cif files, we calculated the ground-state structures of HEDYIY and HEDYOE. HEDYOE turned out to be more stable than HEDYIY by 2.68 kJ/mol. The energy difference of 2.68 kJ/mol would account for a ratio HEDYIY/HEDYOE = 1:3 at 20 °C. Going from the conformation in the crystal to the conformation in the gas phase, the rotation angle changes for HEDYIY from ρ = 37.3 to 47.4° and for HEDYOE from ρ = −28.7 to −21.6°.

Our sample point analysis had predicted a low-lying transition state for the conversion of Ph_face of HEDYIY to Ph_edge of HEDYOE, resulting in the interconversion of the two diastereomers. This low transition state was reached after a counter-clockwise rotation of the PPh₃ ligand in HEDYIY, which moved Ph_face from its position ρ = 37° in the crystal to 60° (pathway b). We calculated the relative energies of HEDYIY with the PPh₃ ligand rotated from its gas phase ground state ρ = 47.7 to 50.3° and 59.4°. The relative energies rose from 0 via 0.75 to 4.66 kJ/mol (Figure 8), supporting a low-lying transition state at 60°.

Figure 8.

Relative energies of HEDYIY in its ground state at rotation angle ρ = 47.4° and on its way to the low transition state at ρ = 60° and the high transition state at ρ = 0°.

In the sample point analysis, we had assigned a high-lying transition state to a clockwise rotation of Ph_face from ρ = 37.3 to 0°, which converts Ph_face of HEDYIY to Ph_face of HEDYOE (pathway a). The calculation of the relative energies of HEDYIY with the PPh₃ ligand rotated from ρ = 47.7 to 21.9° and 1.1° gave relative energies from 0 via 13.50 to 24.31 kJ/mol (Figure 8). Thus, the transition state of pathway a is much higher than that of pathway b and the results of sample point analysis and DFT calculations are fully in accord. As expected, the relative energies of the transition states are much higher than the ground state energies of HEDYIY and HEDYOE.

α- and β-Effects

In a recent paper, we reported CH/π interactions between cyclopentadienyl and phenyl rings in compounds of type CpM-L-E-Ph (Figure 9, left side), e.g., CpMo(CO)₂-amidinato and -thioamidato complexes.²⁸ These compounds were among the earliest examples, for which CH/π interactions have been observed. The Cp/Ph attraction had been termed the β-phenyl effect because of the β-position of Ph in the ligands.²⁹ In comparison, the Ph_PPh3 face-on π-Ar system of the present paper is an α-phenyl effect.

Figure 9.

β-Phenyl effect in CpM-L-E-Ph compounds and α- and β-phenyl effects in HEDYIY.

In new compounds (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆, (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ [1], [2], and [3] and in HEDYIY and HEDYOE, CH/π interactions are established between Ar and the phenyl ring of the CHMePh substituent, resulting in short (π-Ar)CH-C_i and (π-Ar)CH-C_o contacts far below the sum of the van der Waals radii. The dashed lines in Figure 9, right side, show the C₆H₆/Ph interactions in HEDYIY. An analysis according to ref (26) is given in Table S4 (Supporting Information).

The results in Table S4 reveal interesting differences between the compounds with and without π-stack stabilization. The two compounds (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ [3] and HEDYOE, lacking π-stacks, have appreciably shorter (π-Ar)CH-C_i and (π-Ar)CH-C_o distances than those in the four compounds containing π-stacks. Obviously, the π-stacks prevent a perfect build-up of the β-CH/π interactions and the better β-CH/π stabilization seems to be a compensation for the absence of π-stack formation.

Conclusions

Chiral-at-metal half-sandwich compounds [(π-Ar)LL′MPPh₃] form diastereomers, which differ in the propeller sense of the triphenylphosphine ligand. The inside of the PPh₃ ligand is stabilized by a system of attractive CH/π interactions, in which each phenyl ring plays a specific role. One of the phenyl rings orients face-on toward the π-arene ligand, establishing a ubiquitous Ph_PPh3 face-on π-Ar bonding motif. Interconversion of the propeller diastereomers occurs by a low-energy pathway, which exchanges Ph_face and Ph_edge of the diastereomers.

Experimental Section

General Methods

For IR, JASCO FT/IR4100ST was used. For ¹H/³¹P{¹H} NMR, Bruker Avance 400 (400/162 MHz, T = 293 K) or Bruker Avance III 500 (500/202 MHz, T = 293 K) were used. Tetramethylsilane was used as the internal standard, and H₃PO₄ was used as the external standard. For MS, Finnigan MAT 95 (EI, 70 eV) or ThermoQuest Finnigan TSQ 7000 was used. All manipulations were carried out in purified nitrogen or argon. The Cambridge Structural Database ver. 5.38 (update May 31, 2017) for the 140 compounds of type [(π-C₆R₆)RuLL′PPh₃] was used.²¹ The OLEX²,³⁰ Mercury CSD ver. 3.9,³¹ and ConQuest ver. 1.19³² programs were used for structural analyses.

Preparation and Characterization

(R_Ru,S_C)/(S_Ru,S_C)-Chloro[η⁶-1-methyl-4-(1-methylethyl)benzene][1-[[(1-phenylethyl)imino-κN]methyl]-2-naphthalenolato-κO]ruthenium, (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)Cl]

To a solution of (S)-1-[[(1-phenylethyl)imino]methyl]-2-naphthalenol² (200 mg, 0.73 mmol) in dichloromethane (20 mL) was added potassium t-butoxide (98 mg, 0.88 mmol). The solution was stirred for 1 h at room temperature and then cooled to −78 °C. [(η⁶-p-Cymene)RuCl]₂Cl₂ (250 mg, 0.36 mmol) was added to the cooled solution. The mixture was slowly warmed up to room temperature, stirred for 16 h, and then filtered on a short Celite column. After evaporation of the solvent, the residue was chromatographed on silica gel using EtOAc/hexane as an eluent. A reddish-brown band was collected and evaporated to give (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)Cl] 88:12 as a red powder in 70% yield (280 mg). Mp 125 °C (color changed from red to brown) > 200 °C. IR (KBr): ν 1614 cm^–1 (N=C). ¹H NMR (293 K, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets, if distinguishable): δ 8.75 (s, 1H, N=CH) [8.39 (s, 1H, N=CH)], 7.69 (d, 1H, ³J_H-H = 8.4 Hz, nap-H) [7.83 (d, 1H, ³J_H-H = 7.6 Hz, nap-H)], 7.62 (t, 2H, m-Ph-H), 7.56–7.07 (m, 8H, nap-H and Ph-H), 5.96 (q, 1H, ³J_H-H = 7.1 Hz, N-CH) [5.73 (q, 1H, ³J_H-H = 7.1 Hz, N-CH)], 5.25 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H) [5.52 (d, 1H, ³J_H-H = 6.4 Hz, Cy-H)], 5.10 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H) [5.44 (d, 1H, ³J_H-H = 6.4 Hz, Cy-H)], 5.02 (d, 1H, ³J_H-H = 5.7 Hz, Cy-H) [5.41 (d, 1H, ³J_H-H = 6.4 Hz, Cy-H)], 4.77 (d, 1H, ³J_H-H = 5.7 Hz, Cy-H) [5.20 (d, 1H, ³J_H-H = 6.4 Hz, Cy-H)], 2.63 (septet, 1H, ³J_H-H = 6.8 Hz, iPr-CH) [2.83 (septet, 1H, ³J_H-H = 7.0 Hz, iPr-CH)], 2.05 (s, 3H, Cy-CH₃) [2.13 (s, 3H, Cy-CH₃)], 1.81 (d, 3H, ³J_H-H = 7.1 Hz, CH₃), 1.14 (d, 3H, ³J_H-H = 6.9 Hz, iPr-CH₃) [1.14 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃)], 0.98 (d, 3H, ³J_H-H = 6.9 Hz, iPr-CH₃). MS (ESI, CH₂Cl₂/MeOH/NH₄OAc): m/z 510 ([CyRu(2O-1N)]⁺; 100). Anal. Calcd for C₂₉H₃₀ClNORu (545.1): C, 63.90; H, 5.55; N, 2.57. Found: C, 63.90; H, 5.58; N, 2.45.

(R_Ru,S_C)/(S_Ru,S_C)-Chloro[η⁶-1-methyl-4-(1-methylethyl)benzene]-[2-[[(1-phenylethyl)imino-κN]methyl]-1-naphthalenolato-κO]ruthenium, (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)Cl]

In a procedure as above, the reaction of (S)-2-[[(1-phenylethyl)imino]methyl]-1-naphthalenol² and [(η⁶-p-cymene)RuCl]₂Cl₂ gave (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)Cl] 86:14 as a red powder in 86% yield. Crystallization from dichloromethane/diethyl ether afforded red crystals of (R_Ru,S_C)-[CyRu(1O-2N)Cl]. Mp 146 °C (color changed from red to brown) > 200 °C. IR (KBr): ν 1596 cm^–1 (N=C). ¹H NMR (400 MHz, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets, if distinguishable): δ 8.64 (d, 1H, ³J_H-H = 7.6 Hz, nap-H) [8.61 (1H, ³J_H-H = 8.1 Hz, nap-H)], 8.01 (s, 1H, N=CH) [7.65 (s, 1H, N=CH)], 7.58 (d, 1H, ³J_H-H = 8.0 Hz, nap-H) [7.73 (d, 1H, ³J_H-H = 8.0 Hz, nap-H)], 7.55–7.35 (m, 6H, Nap-H and Ph-H), 6.99 (d, 1H, ³J_H-H = 8.6 Hz, nap-H) [7.75 (d, 1H, ³J_H-H = 9.0 Hz, nap-H)], 6.84 (d, 1H, ³J_H-H = 8.6 Hz, nap-H) [6.73 (d, 1H, ³J_H-H = 9.0 Hz, nap-H)], 5.93 (q, 1H, ³J_H-H = 7.0 Hz, N-CH) [5.66 (q, 1H, ³J_H-H = 7.0 Hz, N-CH)], 5.32 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H) [5.59 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.23 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H) [5.49 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.04 (d, 1H, ³J_H-H = 5.8 Hz, Cy-H) [5.18 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 4.87 (d, 1H, ³J_H-H = 5.8 Hz, Cy-H), 2.68 (septet, 1H, ³J_H-H = 7.0 Hz, iPr-CH) [2.84 (septet, 1H, ³J_H-H = 7.0 Hz, iPr-CH)], 2.12 (s, 3H, Cy-CH₃) [2.18 (s, 3H, Cy-CH₃)], 1.81 (d, 3H, ³J_H-H =7.1 Hz, CH₃) [2.03 (d, 3H, ³J_H-H = 6.9 Hz, CH₃)], 1.12 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃) [1.23 (d, 3H, ³J_H-H = 6.9 Hz, iPr-CH₃)], 0.97 (d, ³J_H-H = 6.9 Hz, iPr-CH₃) [1.08 (d, 3H, ³J_H-H = 6.9 Hz, iPr-CH₃)]. MS (EI): m/z 545 ([CyRu(1O-2N)Cl]⁺, 4), 510 ([CyRu(1O-2N)]⁺, 6). Anal. Calcd for C₂₉H₃₀ClNORu (545.1): C, 63.90; H, 5.55; N, 2.57. Found: C, 63.93; H, 5.46; N, 2.61.

(R_Ru,S_C)/(S_Ru,S_C)-[η⁶-1-Methyl-4-(1-methylethyl)benzene][1-[[(1-phenylethyl)imino-κN]methyl]-2-naphthalenolato-κO](triphenylphosphanyl)ruthenium hexafluorophosphate, (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆

To a solution of (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)Cl] (87 mg, 0.16 mmol) in chloroform (20 mL) was added PPh₃ (42 mg, 0.16 mmol). The mixture was stirred for 3 h at room temperature. Then, [NH₄]PF₆ (26 mg, 0.16 mmol) was added while stirring for 12 h. The reaction mixture was filtered on a short Celite column. After evaporation of the solvent, the residue was washed with diethyl ether to give (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ 97:3 in 70% yield (102 mg). Crystallization from dichloromethane afforded orange crystals of pure diastereomer (R_Ru,S_C)-[(CyRu(2O-1N)Ph)PPh₃]PF₆ suitable for X-ray structure analysis. Mp 143 °C (color changed from orange to brown) > 200 °C. IR (KBr): ν 1616 (N=C), 1435 (PPh₃), 838 cm^–1 (P-F). ¹H NMR (400 MHz, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets, if distinguishable): δ 8.68 (s, 1H, N=CH) [8.90 (s, 1H, N=CH)], 7.59–7.19 (m, 25H, nap-H and Ph-H), 6.93 (d, 1H, ³J_H-H = 9.1 Hz, nap-H), 5.56 (q, 1H, J = 7.0 Hz, N-CH), 5.49 (dd, 1H, ³J_H-H = 6.5 Hz, ³J_P-H = 1.3 Hz, Cy-H) [6.30 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.29 (d, 1H, ³J_H-H = 6.5 Hz, Cy-H) [6.11 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.26 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H), 4.71 (br d, 1H, ³J_H-H = 6.0 Hz, Cy-H), 2.35 (septet, 1H, ³J_H-H = 7.0 Hz, iPr-CH) [2.70 (septet, 1H, ³J_H-H = 7.0 Hz, iPr-CH)], 1.58 (s, 3H, Cy-CH₃) [1.76 (s, 3H, Cy-CH₃)], 1.39 (d, 3H, ³J_H-H = 7.0 Hz, CH₃) [2.09 (d, 3H, ³J_H-H = 6.7 Hz, CH₃)], 1.07 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃) [1.15(d, 3H, ³J_H-H = 7.2 Hz, iPr-CH₃)], 0.83 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃) [1.14 (d, 3H, ³J_H-H = 7.2 Hz, iPr-CH₃)]. ³¹P{¹H} NMR (162 MHz, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets): δ 32.80 (s, 1P, PPh₃) [29.60 (s, 1P, PPh₃)], −142.81 (septet, 1P, ¹J_P-F = 713.5 Hz, PF₆). MS (ESI, MeOH): m/z 772 ([(CyRu(2O-1N)Ph)PPh₃]⁺, 100), 510 ([CyRu(2O-1N)Ph]⁺, 10). Anal. Calcd for C₄₇H₄₅F₆NOP₂Ru (916.87): C, 61.57; H, 4.75; N, 1.53. Found: C, 61.37; H, 4.88; N, 1.40.

(R_Ru,S_C)/(S_Ru,S_C)-[η⁶-1-Methyl-4-(1-methylethyl)benzene][2-[[(1-phenylethyl)imino-κN]methyl]-1-naphthalenolato-κO](triphenylphosphanyl)ruthenium hexafluorophosphate, (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆

The reaction of (R_Ru,S_C)-[CyRu(1O-2N)Cl] (105 mg, 0.19 mmol) and PPh₃ (50 mg, 0.19 mmol) was carried out as described above. After filtration on a short Celite column and evaporation, the residue was chromatographed on silica gel using EtOAc/hexane as an eluent. The orange fraction gave (R_Ru,S_C)/(S_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ 96:4 in 68% yield (118 mg). Crystallization from dichloromethane afforded red crystals of pure diastereomer (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆ suitable for X-ray structure analysis. Mp 165 °C (color changed from red to brown) > 200 °C. IR (KBr): ν 1595 (N=C), 1431 (PPh₃), 839 cm^–1 (P-F). ¹H NMR (400 MHz, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets, if distinguishable): δ 8.30 (d, 1H, ³J_H-H = 8.4 Hz, nap-H), 7.83 (d, 1H, ⁴J_P-H = 2.1 Hz, N=CH) [8.05 (d, 1H, ⁴J_P-H = 2.0 Hz, N=CH)], 7.70–7.28 (m, 25H), 5.64 (d, 1H, ³J_H-H = 6.4Hz, Cy-H) [6.37 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.54 (q, 1H, ³J_H-H = 7.0 Hz, CH), 5.31 (d, 1H, ³J_H-H = 6.1 Hz, Cy-H) [6.07 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 5.25 (d, 1H, ³J_H-H = 6.1 Hz, Cy-H) [5.70 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 4.71 (d, 1H, ³J_H-H = 6.4 Hz, Cy-H) [4.95 (d, 1H, ³J_H-H = 6.0 Hz, Cy-H)], 2.31 (septet, 1H, ³J_H-H = 6.8 Hz, iPr-CH), 1.68 (s, 3H, Cy-CH₃) [1.77 (s, 3H, Cy-CH₃)], 1.35 (d, 3H, ³J_H-H = 7.0 Hz, CH₃) [2.05 (d, 3H, ³J_H-H = 7.0 Hz, CH₃)], 0.97 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃) [1.07 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃)], 0.74 (d, 3H, ³J_H-H = 6.9 Hz, iPr-CH₃) [1.06 (d, 3H, ³J_H-H = 7.0 Hz, iPr-CH₃)]. ³¹P{¹H} NMR (162 MHz, CDCl₃, major (R_Ru,S_C)-diastereomer, minor (S_Ru,S_C)-diastereomer in brackets): δ 34.22 (s, 1P, PPh₃) [31.06 (s, 1P, PPh₃)], −142.79 (septet, 1P, ¹J_P-F = 713.5 Hz, PF₆). MS (ESI, MeOH): m/z 772 ([CyRu(1O-2N)PPh₃]⁺, 100), 510 ([CyRu(1O-2N)]⁺, 10). Anal. Calcd for C₄₇H₄₅F₆NOP₂Ru (916.87): C, 61.57; H, 4.75; N, 1.53. Found: C, 61.53; H, 4.76; N, 1.62.

X-ray Analyses

Crystal and refinement data are given in Table S1 (the Supporting Information). X-ray data were collected on a Rigaku RAXIS-RAPID imaging plate diffractometer using Mo Kα (graphite monochromated, λ = 0.71073 Å, fine focus tube, ω-scan) radiation at 173 K or an Oxford Diffraction Gemini Ultra diffractometer (Cu Kα radiation, λ = 1.54184 Å, ω-scan) at 123 K. The structures were solved by SIR2004³³ or SIR97³⁴ and refined by full-matrix least squares on F² by SHELX 2016/6.³⁵ All H atoms were included at calculated positions. CCDC 1519530 {for (R_Ru,S_C)-[CyRu(1O-2N)Cl]}, 1519531 {for (R_Ru,S_C)-[(CyRu(1O-2N)PPh₃)PF₆]}, and 1519532 {for (R_Ru,S_C)-[(CyRu(2O-1N)PPh₃)PF₆]} contain the supplementary crystallographic data for this paper.

DFT Calculations

All calculations have been performed with the TURBOMOLE program package at the RI²⁴-B3LYP²⁵/def²⁶-TZVP^25b,27 level of theory. To speed up the geometry optimization, the Multipole Accelerated Resolution-of-the-Identity^24,36 approximation has been used. The relative energies have been calculated using the SCF energies without corrections.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01460.

• Table S1, crystallographic data of three complexes (R_Ru,S_C)-[CyRu(1O-2N)Cl], (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, and (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆; ¹H NMR spectra and ³¹P{¹H} NMR spectra of the complexes; Table S2, rotation angles ρ and angles φ plane Ph/plane π-Ar for the compounds in Table 1; Table S3, rotation angles ρ and angles φ plane Ph/plane π-Ar for the 140 cases of the 119 compounds [(π-C₆R₆)RuLL′PPh₃]; Table S4, CH/π interactions between π-Ar and Ph of the CHMePh substituent (β-phenyl effect) in compounds [(π-Ar)Ru(O-N)PPh₃]PF₆; Tables S5–S8, computational details (PDF)

• Crystallographic data of three complexes (R_Ru,S_C)-[CyRu(1O-2N)Cl], (R_Ru,S_C)-[CyRu(1O-2N)PPh₃]PF₆, and (R_Ru,S_C)-[CyRu(2O-1N)PPh₃]PF₆ (CIF)

The authors declare no competing financial interest.

Notes

CCDC 1519530–1519532 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/structures, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.