N-Donor

ARTICLE pubs.acs.org/crystal

2. The Multiple Phenyl Embrace as a Synthon in Cu(I)/PPh3/N-Donor Ligand Coordination Polymers Femke F. B. J. Janssen,† Rene de Gelder,*,‡ and Alan E. Rowan*,† †

Molecular Materials and ‡Solid State Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands ABSTRACT: The multiple phenyl embrace is a supramolecular motif comprised of phenylphenyl interactions, which can, like hydrogen bonds, form extended networks between molecules in the solid state. The analysis of 23 crystal structures of coordination polymers based on the M/PPh3/Ndonor ligand system (M = Cu(I) or Ag(I)) showed that 71% of the independent MPPh3 groups are involved in a 6-fold phenyl embrace (6PE). Strong 6PE interactions are obtained when the geometry of the PPh3 group can be described as a rotor. The analysis of these groups showed that 83% of the PPh3 groups have their phenyl groups in the rotor conformation. It is shown, however, that these good rotors are not necessarily involved in the 6PE and that the 6PE can also be formed by nonrotors. In the Cu(I)/PPh3/N-donor ligand system, the 6PE interactions form an independent connection (often) perpendicular to the backbone of the coordination polymer. In many cases, the 6PE increases the dimensionality of the network formed between Cu(I) and N-donor ligands. Therefore, the multiple phenyl embrace seems to be a useful synthon in crystal engineering of stable networks.

’ INTRODUCTION The field of crystal engineering aims at gaining control and understanding of noncovalent interactions in crystalline materials. The most important way to achieve this understanding is by analyzing crystal structures in the Cambridge Structural Database (CSD) for structural motifs. Although there are many different types of noncovalent interactions that can form structural motifs (e.g., van de Waals forces and aromatic interactions), the hydrogen bond is probably the most studied. Its strength and directionality lead to the formation of networks with wellunderstood motifs. Another important intermolecular interaction in supramolecular chemistry is the interaction between aromatic rings, but its supramolecular motifs are less well understood. The face-to-face interaction (Chart 1a), also known as stacking, is the most encountered orientation between aromatic rings. Other synthons found with aromatic rings are the edge-toface and vertex-to-face type of interactions (Chart 1b,c).13 When phenyl rings are involved in multiple aromatic interactions, like in structures containing PPh3 moieties and [PPh4]+ cations, we deal with multiple phenyl embraces (MPEs), which is the subject of this paper. The multiple phenyl embrace (MPE) or more generally multiple aryl embrace (MAE) was first described in 1995 by Dance and Scudder.4 It is a supramolecular motif comprised of a set of concerted phenylphenyl attractions between XPh2, XPh3, and XPh4 peripheral groups (X = any atom with a tetrahedral geometry).5,6 Detailed studies of compounds with peripheral PPh3 groups have revealed different types of MPEs r 2011 American Chemical Society

depending on the number of phenyl groups involved: 4-, 6-, 8-, and 12-fold phenyl embraces (abbreviated with 4PE, 6PE, 8PE, and 12PE, respectively). The 6PE is the most common structural motif and has been investigated in great detail. It consists of a concerted cycle of six edge-to-face interactions (Figure 1a).4 Calculations of the interaction energy show that the strength of the 6PE lies between 50 and 80 kJ/mol, which is comparable with very strong hydrogen bonds and weaker coordination bonds.7 The other commonly encountered motif is the 4PE, which is mostly found in compounds containing phosphonium cations. The 4PE can be divided into two types depending on the orientation of the interacting phenyl groups. The orthogonal 4PE (Figure 1b) consists of four phenyl rings that are engaged in four edge-to-face interactions. In the parallel 4PE (not shown), the phenyl rings have two offset face-to-face interactions and two edge-to-face interactions.8 Although the MPE has been extensively studied for crystal structures containing MPPh3 and [PPh4]+ moieties, it can also be found between substituted phenyl,911 fluorinated phenyl groups,12 derivatives of the phosphonium cation,1315 bipyridine complexes,16 or any other molecule that contains multiple phenyl groups.1722 In this article, the occurrences of the multiple phenyl embrace in coordination polymers based on the Cu(I)/PPh3/N-donor ligand system are studied. The MPEs are of a strong attractive Received: May 23, 2011 Revised: July 17, 2011 Published: September 13, 2011 4326

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Chart 1. Different Interactions between Phenyl Ringsa

a

(a) Face-to-face, (b) edge-to-face, (c) vertex-to-face.

Figure 2. Labeling scheme of the atoms used for the torsion angles T1T3.

Figure 1. The two most common encountered types of phenyl embraces. (a) 6-Fold phenyl embrace (CSD refcode: ACRHCP). Cu, green; P, yellow; C, black; covalent bond, bronze. (b) 4-Fold phenyl embrace (CSD refcode: DEMYEZ02). P, green; C, gray; covalent bond, bronze.

nature, and they could, therefore, dominate solid state structures, which might lead to the formation of networks in one, two, and three dimensions.22,24 The possibility of using this MPE as a supramolecular synthon in these polymers is discussed in terms of the probability of its occurrence. Steiner argued that in general the occurrence of the MPE is very low and that, accordingly to him, it is not a very good synthon.5 The analysis of the polymers will mainly focus on the 6PE, which is the most common type of embrace. The first stage of the analysis comprises the search for intermolecular PP distances and MP---PM colinearities, which are parameters that can identify the presence of a multiple phenyl embrace. The second stage is to describe the intramolecular geometry of the PPh3 group, which tells us something about the strength of the embrace. In the last part of this article, the supramolecular motifs of the multiple phenyl embrace in Cu(I)/PPh3/N-donor ligand coordination polymers are discussed.

’ EXPERIMENTAL SECTION Crystal Structures. In this paper, 23 crystal structures of coordination polymers based on the M/PPh3/N-donor system (M = Cu(I) or Ag(I)) have been analyzed. The crystal structures of the compounds {(Cu 2 (PPh 3 )2 (μ-Cl)2 (μ-pyz)}∞ (1) (CSD refcode RINLOP), {(Cu2(PPh3)2(μ-X)2(μ-4,40 -bipy)}∞ (2: X = Cl, 3: X = Br, 4: X = I) (CSD refcodes respectively ROQDIK, SIPYAS, and IKETEX01), {(Ag2(PPh3)2(μ-I)2(μ-4,40 -bipy)}∞ (5) (CSD refcode LONKUU), {Ag2(PPh3)2(μ-ONO2)2(μ-4,40 -bipy)}∞ (6) (CSD refcode LONLAB), and [Ag2(PPh3)2(μ-4,40 -bipy)][ClO4]2 (15) (CSD refcode LONKOO) have been described elsewhere.2535 The preparation and crystal structures of the compounds {[Cu(μ-pyz)(pyz)(PPh3)][X] 3 Solv}∞ (7: X = BF4, Solv = CH2Cl2, 8: X = BF4, Solv = THF, 9: X = ClO4, Solv = CH2Cl2, 10: X = PF6, Solv = THF), {[Cu(μ-pyz)(PPh3)(OClO3)] 3 CHCl3}∞ (14), {[Cu(μ-4,40 -bipy)(PPh3)2][BF4] 3 4CHCl3}∞ (16), {[Cu(μ-4,40 -bipy)(PPh3)2][PF6] 3 4CHCl3}∞ (17), {[Cu(μ-4,40 -bipy)1.5(PPh3)][X] 3 2CH2Cl2}∞ (19: X = BF4, 20: X = ClO4, 21: X = PF6), {[Cu(μ-4,40 -bipy)1.5(PPh3)][BF4] 3 1.33THF}n (22b), {[Cu(μ-4,40 bipy)1.5(PPh3)][ClO4] 3 1.33THF}n (23b), {[Cu(3,40 -bipy)(PPh3)2][X] 3 3Solv}∞ (25: X = BF4, Solv = THF, 26: X = ClO4, Solv = CHCl3,

Figure 3. Scatterplot of the intermolecular interaction between PPh3 groups. The red oval captures the points that represent the 6PE. The majority of the other points do not represent a phenylphenyl interaction. 27: X = ClO4, Solv = THF), and {[Cu(3,40 -bipy)(PPh3)2][PF6] 3 THF}∞ (28) have been described elsewhere, and X-ray crystallographic information files (CIF) can be found in the Supporting Information accompanying that article.36 Crystal Structure Analysis. The crystal structures were analyzed via an in-house database using the program PreQuest.37 The software Conquest38 was used to search the crystal structures for PP interactions up to 10.0 Å and MP---PM colinearities (half the sum of the MP---P and P---PM angles) in the range of 0180°. The results are summarized in a scatter plot. The intramolecular geometry of the PPh3 groups was determined by selecting the three MPCipsoC torsion angles (Ti, i = 13) (Figure 2), which lie in the range 90 to +90° and ordering them in such a way that T1 < T2 < T3. The criteria for good rotor symmetry are that the three MPCipsoC torsion angles lie in the range of 2070° and are of the same sign. Symmetric rotors have T2T1 and T3T2 values between 020°. Finally, the structures have been analyzed in terms of the topology formed by the PPh3 groups, which participate in a 6PE.

’ RESULTS AND DISCUSSION Identifying the Intermolecular Interactions. In Figure 3, a scatterplot is shown of the intermolecular PP distances in the 4327

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Table 1. MPCipsoC Torsion Angles (°) for the Structures That Form a 6PE, but Do Not Have a Good Rotor Symmetry torsion angle

7

8

9

14

T1

17.55

18.62

18.40

12.68

T2

48.34

42.96

47.42

40.85

T3

49.42

44.86

50.97

54.05

Table 2. MPCipsoC Torsion Angles (°) of the PPh3 Groups with Good Rotor Symmetry but that Do Not Form a 6PEa torsion angle

10

16

17

22b

22b

23b

23b

T1

31.87 20.67 25.28 29.11

48.24 41.44 28.30

T2

34.65 43.08 32.77 51.37

50.69 48.80 54.23

T3 T2T1

41.88 62.98 69.24 53.55 2.78

41.01 53.48 58.29b 2.45 7.36

T3T2

7.23

9.68

4.68

The values of T2T1 and T3T2 are only given for symmetric rotors. b This PPh3 group has a disorder in one of the phenyl groups. The MPCipsoC torsion angle for the other orientation of the phenyl ring is 40.75°. a

Table 3. MPCipsoC Torsion Angles (°) of One of the PPh3 Groups of 22b and 23b, Which Are of the Flipper Conformation torsion angle

22b

23b 16.67

T1

13.89

T2

19.19

16.14

T3

78.69

76.69

range of 6.010 Å versus the MP---PM colinearity (half the sum of the MP---P and P---PM angles) in the range of 0180° as found in the 23 crystal structures. The criteria for a 6PE are a PP distance in the range of 6.07.5 Å and MP--PM colinearities between 160 and 180°.4 Figure 3 clearly shows an island of 24 points falling within the range of these criteria (red oval). The other points in Figure 3 have not been analyzed in detail, but the majority of these points do not represent any interaction between the phenyl rings because the PP distance is too large and/or the MP---PM colinearity is too small. Intramolecular Geometry of the PPh3 Group. The strength of the 6PE is determined by the PP distances between the PPh3 groups and the geometry of the phenyl rings around the phosphorus atom. The strongest phenyl embraces are obtained when the PPh3 group has 3-fold symmetry (perfect rotor); however, there are very few examples of PPh3 groups with this perfect 3-fold symmetry.45 Therefore, the criteria for rotor symmetry have been expanded in such a way that a 6PE formed by these groups still has a very strong interaction and approach an ideal situation. A good rotor has been defined as a PPh3 group with the three MPCipsoC torsion angles lying in the range of 2070° and of the same sign.39 Dance and Scudder analyzed 8663 independent MPPh3 groups (in 5089 crystal structures) for the presence of these good rotors. They found that about 40% (3462 M-PPh3 groups) had a good rotor conformation,39 and furthermore that only 36 out of the 5089 structures have one or more MPPh3 groups possessing exact 3-fold symmetry (0.7%). We have analyzed 23 crystal structures, which have a total of 33 independent MPPh3 groups. Of these 33 MPPh3 groups, 27 can be described as having good rotor symmetry (82%). Two of the MPPh3 groups can be considered to be of the flipper

Figure 4. (a) The multiple phenyl embrace forms an intermolecular link between different strands of 1. (b) The multiple phenyl embrace is perpendicular to the polymer backbone and creates a 2D network grid if this intermolecular interaction is combined with the chain topology of the coordination polymer. This structure is representative for the polymers 16. The polymer backbone is gray and the PPh3 groups are red. 4328



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conformation,39 and four M-PPh3 groups are too ill defined to be tagged. Predictability of the Multiple Phenyl Embrace. The analysis performed in the previous section could tell us something about the chance that an MPPh3 group is involved in a 6PE. A question one could ask is whether or not a 6PE is always associated with PPh3 groups with good rotor symmetry? The answer, based on our 23 crystal structures, has to be negative. Of the 33 independent PPh3 groups only 24 form a 6PE (71%), and only 20 have good rotor symmetry (83%). The isostructural 1D coordination polymers 7, 8, and 9 form a 6PE, but the T1 values are slightly lower than the requirement of 20° for a good rotor (Table 1). The same is seen for the 1D coordination polymer 14, although the T1 value deviates more from the 20° than the T1 values in the polymers 79. Of course, the aforementioned question can also be reversed; does a good rotor always form a 6PE? Again the answer has to be negative but less negative since of the 33 independent PPh3 groups only nine do not form a 6PE. The analysis of the geometry of these PPh3 groups shows that seven out of the nine groups (21%) have good rotor symmetry (Table 2). The 1D coordination polymer 10, which has the same connectivity as 79 but crystallizes in a different packing, has a symmetric rotor. Symmetric rotors are PPh3 groups which have T2T1 and T3T2 values in the range of 020° (Table 2). The geometry of these PPh3 groups is almost ideal for a very strong 6PE, but the packing of the polymer in the crystal structure of 10 does not allow for the PPh3 groups to approach each other in the right direction. Table 4. PP Distances (Å) and the MP---PM Colinearities (°) for 16 1 PP distance colinearity

2

3

4

5

6

6.88

7.15

7.11

7.11

7.04

7.13

175.88

174.38

172.92

170.73

171.72

171.77

Clearly, the geometry of the PPh3 alone is not decisive for the presence of a 6PE. The 1D coordination polymers 16 and 17 have two independent PPh3 groups per copper atom. Although both PPh3 groups have good rotor symmetry, only one of these groups is involved in a 6PE (Table 2). The other exceptions are the 2D coordination polymers 22b and 23b, which have three crystallographic independent copper atoms per asymmetric unit. Two of these independent PPh3 groups have good rotor symmetry (one of them is almost ideal), but neither of them is involved in a 6PE (Table 2). The third independent PPh3 group has a different geometry as can be seen in Table 3. The T1 and T3 torsion angles have the same sign, but the T2 torsion angle is of the opposite sign. Furthermore, none of the torsion angles fall within the criteria set for a good rotor. Another conformation of the PPh3 groups is the flipper conformation.23,39 The criteria for a flipper conformation is that one of the |Ti| values is either between 0 and 20° or 7090° and the Ti values for the mirror-related rings differ in magnitude by no more than 20°.39 The values of the mirrorrelated T1 and T2 torsion angles differ by 5.3°, which is well within the range of 020°. This flipper conformation could indicate that this PPh3 group is involved in a 4PE, but this has not been investigated. Networks Based on the Multiple Phenyl Embrace. As mentioned before, the 6PE is a supramolecular motif, which could lead to the formation of networks in one, two, and three dimensions. With our type of coordination polymers, it is possible that the MPE interactions lie in a different direction than the backbone of the coordination polymer and that it thus increases the dimensionality and stability of the total network.8,24,4044 In this section, we will describe the topology of the 6PE network in the different crystals. Of the 23 examined crystal structures, only three structures do not form a 6PE at all. These are the 2D pentagon structures 22b and 23b and the 1D coordination polymer 10.

Figure 5. (a) The multiple phenyl embrace forms an intermolecular link between different strands of 7. (b) The multiple phenyl embrace propagates perpendicular to the backbone of 7 creating a 2D network with grid topology. The polymer backbone is gray, and the PPh3 groups are red. 4329



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1D Coordination Polymers with One MPPh3 Group. Analysis of the compounds with a single MPPh3 group (CSD) showed that they form in about 25% of the cases a 6PE.23 Our in-house database consists of 17 structures with a single PPh3 group per copper atom and 14 of these PPh3 groups form a 6PE (82%). It has to be remarked that our database is of course much smaller and less diverse than the database collected from the CSD. The 1D coordination polymers 26 are isostructural and will have the same intermolecular interactions. The 1D coordination polymer 1 has the same connectivity but crystallizes in a different space group. Nevertheless, these polymers show the same type of topology created by the 6PE (Figure 4). Figure 4a shows a view of the crystal structure along the backbone of the coordination polymer, and it shows that the 6PE forms linear columns. The PP distance in 26 lies between 7.04 and 7.15 Å (Table 4). The PP distance in 1 is slightly shorter (6.88 Å), which could be attributed to the shorter linker ligand, which induces a shorter interchain separation of the polymer strands. When viewed from a different direction, it can be seen that these interactions are perpendicular to the backbone of the coordination polymer (Figure 4). If the interactions of both the coordination bonds and the 6PE are combined, the network can be described as having a 2D grid-like topology. The 1D coordination polymers 79 are structurally very similar to the 1D coordination polymers 16. The 6PE is again perpendicular to the polymer backbone (Figure 5a). Combining the network of the 6PE with the coordination backbone leads to an overall 2D topology (Figure 5b). The PP distance is comparable to the PP distance in the polymer 26 (Table 5). The 1D coordination polymer 14 has a similar topology as the polymers 79, although the monodentate pyz has been replaced with a coordinating anion. The 6PE is again perpendicular to the backbone of the coordination polymer and creates an overall 2D topology. The PP distance is slightly shorter than this distance in 79. The MP---PM colinearity in 14 is much smaller than in 79. The coordination perchlorate anion in 14 forces the polymer to pack in a different way, creating slightly different interchain interactions.

The preferred coordination geometry of a metal atom is very important for the directionality of the intermolecular interactions. The above examples all have copper(I) as a metal, which prefers a tetrahedral coordination geometry. Although silver has a flexible coordination number, it prefers being two-coordinated with a linear geometry. This can be seen in the 0D complex 15 (Figure 6). The PPh3 group is involved in a 6PE, but it is not perpendicular to the backbone of the complex. The PPh3 group lies in the same plane as the linker ligand and thus becomes this 0D complex a 1D polymer with respect to the multiple phenyl embrace. 1D Coordination Polymers with a M-(PPh3)2 Group. The 1D coordination polymers 16 and 17 have two PPh3 groups per copper atom (Figure 7). Only one of these groups is involved in a 6PE and forms a connection between different strands. The zigzag motif of the coordination polymer backbone prohibits both PPh3 groups from being involved in the 6PE simultaneously. The PP distances in these polymers are short compared to the previously mentioned polymers indicating a strong interaction in this polymer (Table 6). Again the 6PE interactions are in line forming linear columns. The 1D coordination polymers 2528, which have nonlinear N-donor ligands, also have two PPh3 groups per copper atom. But in these polymers both PPh3 groups are involved in a 6PE. This creates large density of 6PE interactions (Figure 8a). The PP distances are again very short indicating a strong interaction (Table 7). Viewing along the direction of the backbone of the coordination polymer shows that the 6PE propagates via a zigzag motif.23 2D Coordination Polymers. The 2D coordination polymers 1921 have one PPh3 group per copper atom. In Figure 9a, the individual 2D coordination grids are each given a different color. If we focus on the blue polymer, we see that this grid is connected to the green grid via a 6PE. The red polymer is interwoven between the blue and green polymer, and is connected to the violet and yellow polymers via again the 6PE. If the copper atoms in this plane are schematically connected with one another via both the N-donor linker ligands as well as the 6PE, a hexagon topology is formed (Figure 9b). The blue lines in this schematic representation are formed by the blue and green polymers in Figure 9a. The red lines are

Table 5. PP Distances (Å) and the MP---PM Colinearities (°) for 710, 14, and 15

Table 6. PP Distances (Å) and the MP---PM Colinearities (°) for 16 and 17

7 PP distance Colinearity

8

9

14

15

7.03

7.16

7.03

6.90

6.98

171.46

171.11

171.14

165.18

172.95

16 PP distance colinearity

17

6.73

6.67

173.20

179.52

Figure 6. The 0D dinuclear complex 15 becomes a 1D polymer via the multiple phenyl embrace. The polymer backbone is gray, and the PPh3 groups are red. 4330



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formed by the violet, red, and yellow coordination polymers from Figure 9a. The schematic representation shows clearly that the blue and red lines form two independent networks, which are interpenetrating. The overall topology of this network becomes 3D; this is a combination of the 2D coordination and the 1D MPE network. The PP distances in 1921 are very short, although the length in 21 is somewhat longer than the distance in 19 and 20 (Table 8). The MP---PM colinearity in 21 is also larger than in 19 and 20. So, although the polymers are isostructural, the different geometry of the anion (octahedral versus tetrahedral) induces slight differences in the interchain interactions and, hence, in the strength of the 6PE. Table 7. PP Distances (Å) and the MP---PM Colinearities (°) for 2528 Figure 7. The coordination polymer 17 has two PPh3 groups per copper atom. Only one of the PPh3 groups is involved in a 6PE. The polymer backbone is gray, and the PPh3 group in the 6PE and the P atom of the other PPh3 group are red.

PP

25

25

26

26

27

27

28

28

6.55

6.70

6.54

6.77

6.66

6.47

6.58

6.58

distance colinearity 177.50 178.26 174.14 170.62 178.61 177.46 179.69 178.66

Figure 8. In 25 both PPh3 groups are involved in the 6PE. (a) The density of 6PEs between the polymers strands creates a strong interaction. (b) The zigzag motif of the 6PE. This is representative for 2628. The polymer backbone is gray, and the PPh3 groups are red. 4331



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135, 6525 AJ Nijmegen, The Netherlands. Tel: +31 (0) 24 36 52842. Fax: +31 (0)24 35 53450. E-mail: [email protected]. nl. Web: http://www.xtal.science.ru.nl. (A.E.R.) Mailing address: Molecular Materials, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands. Tel: +31 (0) 24 36 52323. Fax: +31 (0)24 36 53393. E-mail: [email protected]. Web: http:// www.molchem.science.ru.nl.

’ REFERENCES

Figure 9. (a) The packing of the 2D coordination polymer via the 6PE. Each 2D coordination grid has been given a different color. (b) Schematic representation of the interpenetration of the 2D coordination polymer via the 6PE.

Table 8. PP Distances (Å) and the MP---PM Colinearities (°) for 1921 PP distance colinearity

19

20

21

6.31 167.64

6.31 167.90

6.65 170.40

’ CONCLUSIONS The analysis of 23 coordination polymers of the Cu(I)/PPh3/ N-donor system with 33 independent MPPh3 groups shows that 24 of these MPPh3 moieties are involved in a 6PE (71%). This high percentage indicates that the 6PE is a good synthon for our type of coordination polymers, in contrast to the general observation for this interaction based on an analysis of the CSD. It is shown that the geometry of the PPh3 group alone is not decisive for the presence of a strong interaction. The specific packing of a polymer can still hamper the formation of a strong multiple phenyl embrace, showing that the MPE is not a dominant interaction. Although it is not possible to predict a priori the presence/ absence of the 6PE, the specific orientation of the PPh3 group in our system seems to facilitate the formation of 6PE interactions perpendicular to the backbone of polymers formed by Cu(I)/Ndonor ligand interactions. The advantage of this situation is that in general such a perpendicular interaction increases the dimensionality of the overall network. Therefore, the main conclusion is that the multiple phenyl embrace seems to be a very useful synthon for increasing the stability of coordination polymers once the system is properly chosen. ’ AUTHOR INFORMATION Corresponding Author

*(R.d.G.) Address: Solid State Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg

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(35) Prest, P.-J.; Moore, J. S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, C52 (9), 2176–2178. (36) Janssen, F. F. B. J.; Veraart, L. P. J.; Smits, J. M. M.; de Gelder, R.; Rowan, A. E., Cryst. Growth Des., 2011, DOI: 10.1021/cg2006502. (37) PreQuest; Cambridge Crystallographic Data Centre: Cambridge, England. (38) Bruno, I. J; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58 (3, No.1), 389–397. (39) Dance, I.; Scudder, M. J. Chem. Soc., Dalton Trans. 2000, No. 10, 1579–1585. (40) Scudder, M.; Dance, I. J. Chem. Soc., Dalton Trans. 1998, No. 3, 329–344. (41) Lorenzo, S.; Horn, C.; Craig, D.; Scudder, M.; Dance, I. Inorg. Chem. 2000, 39 (3), 401–405. (42) Dance, I.; Scudder, M. New J. Chem. 2001, 25 (12), 1500–1509. (43) Dance, I.; Scudder, M. New J. Chem. 2001, 25 (12), 1510–1515. (44) Scudder, M.; Dance, I. Chem.—Eur. J. 2002, 8 (23), 5456–5468. (45) An example of a 6PE with 3-fold symmetry can for instance be found in the crystal structure with the CSD refcode VEDZOT.

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