Posts Tagged ‘Bond length’

Impossible molecules.

Monday, April 1st, 2019

Members of the chemical FAIR data community have just met in Orlando (with help from the NSF, the American National Science Foundation) to discuss how such data is progressing in chemistry. There are a lot of themes converging at the moment. Thus this article[1] extolls the virtues of having raw NMR data available in natural product research, to which we added that such raw data should also be made FAIR (Findable, Accessible, Interoperable and Reusable) by virtue of adding rich metadata and then properly registering it so that it can be searched. These themes are combined in another article which made a recent appearance.[2]

One of the speakers made a very persuasive case based in part on e.g. the following three molecules which are discussed in the first article[1] (the compound numbers are taken from there). The question was posed at our meeting: why did the referees not query these structures? And the answer in part is to provide referees with access to the full/primary/raw NMR data (which almost invariably they currently do not have) to help them check on the peaks, the purity and indeed the assignments. I am sure tools that do this automatically from such supplied data by machines on a routine basis do exist in industry (and which is something FAIR is designed to enable). Perhaps there are open source versions available?

17 18 19

 
328[3] 348 713

Here I suggest a particularly simple and rapid “reality check” which I occasionally use myself. This is to compute the steric energy of the molecule using molecular mechanics. The mechanics method is basically a summation of simple terms such as the bond length, bond angle, torsion angle, a term which models non bonded repulsions, dispersion attractions and electrostatic contributions. The first three are close to zero for an unstrained molecule (by definition). The last three terms can be negative or positive, but unless the molecule is protein sized, they also do not depart far from zero. A suitable free tool that packages all this up is Avogadro.

The procedure is as follows

  1. Start from the Chemdraw representation of the molecule. If the publishing authors have been FAIR, you might be able to acquire that from their deposited data. Otherwise, redraw it yourself and save as e.g. a molfile or Chemdraw .cdxml file.
  2. Drop into Avogadro, which will build a 3D model for you using stereochemical information present in the Chemdraw or Molfile.
  3. In the  E tool (at the top on the left of the Avogadro menu) select e.g. the MMFF94 force field. This is a good one to use for “organic” molecules for which the total steric energy for “normal” molecules is likely to be < 200 kJ. Calculate that for your system; this normally takes less than one minute to complete. The values obtained for the three above are shown in the table. All three are well over 200 kJ/mol, which should set alarm bells ringing.
  4. A “more reasonable” structure for 17 is shown below. This has a steric energy of 152 kJ/mol, some 176 kJ/mol lower than the original structure. This does not of itself “prove” this alternative, but it is a starting point for showing it might be correct.Of course mis-assigned but otherwise reasonable structures are unlikely to be revealed by the steric energy test. But impossible ones will probably always be flagged as such using this procedure. 

Postscript: Hot on the heels of writing this, the molecule Populusone came to my attention.[4] On first sight, it seems to have some of the attributes of an “impossible molecule” (click on diagram below for 3D coordinates).

However, it has been fully characterised by x-ray analysis! The steric energy using the method above comes out at 384 kJ/mol, which in the region of impossibility! This can be decomposed into the following components: bond stretch 30, bend 51, torsion 32, van der Waals (including repulsions) 177, electrostatics 87 (+ some minor cross terms). These are fairly evenly distributed, with internal steric repulsions clearly the largest contributor. The C=C double bond is hardly distorted however, which is in its favour. Clearly a natural product can indeed load up the unfavourable interactions, and this one must be close to the record of the most intrinsically unstable natural product known!

References

  1. J.B. McAlpine, S. Chen, A. Kutateladze, J.B. MacMillan, G. Appendino, A. Barison, M.A. Beniddir, M.W. Biavatti, S. Bluml, A. Boufridi, M.S. Butler, R.J. Capon, Y.H. Choi, D. Coppage, P. Crews, M.T. Crimmins, M. Csete, P. Dewapriya, J.M. Egan, M.J. Garson, G. Genta-Jouve, W.H. Gerwick, H. Gross, M.K. Harper, P. Hermanto, J.M. Hook, L. Hunter, D. Jeannerat, N. Ji, T.A. Johnson, D.G.I. Kingston, H. Koshino, H. Lee, G. Lewin, J. Li, R.G. Linington, M. Liu, K.L. McPhail, T.F. Molinski, B.S. Moore, J. Nam, R.P. Neupane, M. Niemitz, J. Nuzillard, N.H. Oberlies, F.M.M. Ocampos, G. Pan, R.J. Quinn, D.S. Reddy, J. Renault, J. Rivera-Chávez, W. Robien, C.M. Saunders, T.J. Schmidt, C. Seger, B. Shen, C. Steinbeck, H. Stuppner, S. Sturm, O. Taglialatela-Scafati, D.J. Tantillo, R. Verpoorte, B. Wang, C.M. Williams, P.G. Williams, J. Wist, J. Yue, C. Zhang, Z. Xu, C. Simmler, D.C. Lankin, J. Bisson, and G.F. Pauli, "The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research", Natural Product Reports, vol. 36, pp. 35-107, 2019. https://doi.org/10.1039/c7np00064b
  2. A. Barba, S. Dominguez, C. Cobas, D.P. Martinsen, C. Romain, H.S. Rzepa, and F. Seoane, "Workflows Allowing Creation of Journal Article Supporting Information and Findable, Accessible, Interoperable, and Reusable (FAIR)-Enabled Publication of Spectroscopic Data", ACS Omega, vol. 4, pp. 3280-3286, 2019. https://doi.org/10.1021/acsomega.8b03005
  3. A.I. Savchenko, and C.M. Williams, "The Anti‐Bredt Red Flag! Reassignment of Neoveratrenone", European Journal of Organic Chemistry, vol. 2013, pp. 7263-7265, 2013. https://doi.org/10.1002/ejoc.201301308
  4. K. Liu, Y. Zhu, Y. Yan, Y. Zeng, Y. Jiao, F. Qin, J. Liu, Y. Zhang, and Y. Cheng, "Discovery of Populusone, a Skeletal Stimulator of Umbilical Cord Mesenchymal Stem Cells from <i>Populus euphratica</i> Exudates", Organic Letters, vol. 21, pp. 1837-1840, 2019. https://doi.org/10.1021/acs.orglett.9b00423

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Anomeric effects at boron, silicon and phosphorus.

Friday, July 1st, 2016

The anomeric effect occurs at 4-coordinate (sp3) carbon centres carrying two oxygen substituents and involves an alignment of a lone electron pair on one oxygen with the adjacent C-O σ*-bond of the other oxygen. Here I explore whether other centres can exhibit the phenomenon. I start with 4-coordinate boron, using the crystal structure search definition below (along with R < 0.1, no disorder, no errors).[1]anomeric-bo-sq

The result shows two prominent clusters, one with both torsion angles being 180°, and another with both being ~60°. This latter is the one that implies that there must be two lone pairs, one on each oxygen, that are anti-periplanar to the adjacent B-O bond. There are two more diffuse clusters where only one antiperiplanar alignment is seen. So yes, 4-coordinate boron can exhibit an anomeric effect!

anomeric-boThis compares to the carbon-anomeric plot which is shown here for comparison, where the top right cluster of 180° torsions contains proportionately few hits than with boron.

anomeric-coThe next centre is at 4-coordinate silicon. Again three significant clusters are seen; one with two antiperiplanar lone pair alignments with Si-O bonds, and two more with just one such alignment. The previous hotspot for which both measured torsions were 180° is largely absent. So here, the anomeric effect is much stronger. Notice also that whereas the torsions in the region of 60° for the carbon centre lie along a ridge coincident with the diagonal  (bottom left to top right), that for the silicon centre show a ridge running orthogonal to the diagonal. An interesting point to follow up perhaps?anomeric-sio

Since the off-diagonal clusters are relatively prominent, implying just one anomeric interaction, it is of interest to see if this results in any asymmetry in the two Si-O bond lengths. If its present, the effect is small.

anomeric-sio-distances

Finally 4-coordinate group 15 elements. Most of the hits are in fact for P; there are none for N. This shows four clusters; the two on the diagonal show respectively two and no antiperiplanar interactions. The two off-diagonal clusters show just one such orientation. As with  Si, the ridge in the 60° region run orthogonal to the diagonal.

anomeric-gp15-oSo this little exploration shows that the anomeric effect, best known for sugars and at a carbon centre, is in fact more general to the adjacent elements.

 

References

  1. H. Rzepa, "Anomeric effects at boron, silicon and phosphorus.", 2016. https://doi.org/10.14469/hpc/696

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A wider look at π-complex metal-alkene (and alkyne) compounds.

Monday, June 13th, 2016

Previously, I looked at the historic origins of the so-called π-complex theory of metal-alkene complexes. Here I follow this up with some data mining of the crystal structure database for such structures.

Alkene-metal "π-complexes" have what might be called a representational problem; they do not happily fit into the standard Lewis model of using lines connecting atoms to represent electron pairs. Structure 1 was the original representation used by Dewar intending the meaning of partial back donation from a filled metal orbital to the empty π* of the alkene. At the other extreme these compounds can be called metallacyclopropanes (2) in which only single bonds feature (these can be thought of as representing full back bonding from metal to alkene and full forward bonding from alkene to metal). Representations 3 and 4 are a more fuzzy blend of these, implying some sort of partial bond order for the metal-carbon bonds. Taken together, they imply that the formal bond order of the C-C bond might vary between single to double. Structures 1 and 2 in particular imply that there might be two distinct ways in arranging the bonding and that π-complexes and metallacyclopropanes might therefore be distinct valence-bond isomers, each potentially capable of separate existence.

Why do these representations matter? Well, I am going to mine the crystal structure database for these species to try to see if there is any evidence for a bimodal distribution in the C-C lengths, perhaps indicating evidence of the isomerism suggested above. Such a structural database is indexed against atom-pair connectivity in the first instance and then bond type; one can specify the following types of bond connecting any two atoms: single, double, triple, quadruple, polymeric, delocalised, pi and any. It is not entirely obvious which if any of these types apply to structure 1 (it is not possible to draw a bond ending at the mid-point of another bond using the Conquest structure editor); the dashed lines in structures 3 and 4 could be classed as delocalised, pi, or most generally any. The search query can be constructed thus, where the two carbons carry R which can be either H or C and all four C-R bonds are specified as acyclic (to try to avoid complications by excluding compounds such as cyclic metallacenes). Because representation 1 cannot be constructed in the editor, I am going to specify that each carbon carries four bonds of any type in the first instance. The torsion specified is defined as R-C-C-M and the full queries can be found deposited here.[1]

If the metallacyclopropane representation 2 is defined with explicit single bonds, one gets only 22 hits (no errors, no disorder, R < 0.1). The distribution of C-C bond lengths is shown below. Already one sees a representational problem emerging. A true metallacyclopropane might be expected to show a C-C single bond length, say > ~1.5Å. But only one or two of these examples actually have this value, the most probable value being ~1.4Å.

Using representation 3, one gets 1861 hits, but as before one sees a maximum at ~1.4Å with a tail reaching to both single and double bond values for the C-C distance.‡

If the C-C bond is also specified as "any", the hits increase to 3948, but the bond length distribution is still very similar, with no sign of any bimodal distribution.

Such a distribution is however found if the torsions between the R-C bond vector and the C-M bond vector are plotted (for all types of bond). A large number of the complexes have a torsion <90°, which suggests that in fact the substituent R is probably interacting with the metal (even though this would lead to formal cyclicity, specifying R-C as acyclic does not detect this interaction). Could this be masking a bimodal distribution in the C-C lengths?

If the previous search is repeated, but this time specifying that all four torsions must lie in the range 90-180° (the range expected for a "classical" alkene-metal complex and selecting only the top right hand side cluster in the plot above) the reduced value of 1051 hits are obtained, but the monomodal distribution remains.

For this last set, here is a plot of the two C-metal bond length, with colour indicating the C-C bond length, indicating the two C-metal bonds are clearly linearly correlated.

One final variation;  the atom on either C can only be H or a 4-coordinate (sp3) carbon; 645 hits. Again, a monomodal distribution centered at 1.4Å.

So this foray through metal alkene complexes suggests that there is a continuum between the formal metallacyclopropane with a C-C single bond and the only slightly perturbed alkene-metal complex with a C=C double bond. Whilst this would not prevent any one of these compounds existing as two distinctly different valence-bond isomers, it makes it very unlikely. I had noted in an earlier post that for molecules of the type RX≡XR (X=Si, Ge, Sn, Pb) that there was indeed a clear bimodal distribution of the X-X lengths evident in the crystal structures (for a relatively small sample number). The structures 1-4 shown at the start of this post are all simply just variations in a continuum and not distinct isomers.

POSTSCRIPT:  I noted above the bimodel distribution in compounds involving formal triple bonds. So I repeated the search above for π-complex metal-alkyne complexes. Specifying an acyclic C-R bond, and any for the CC bond type, one gets the following.

There is now a tantalizing suggestion of two clusters, one at 1.3 and another at 1.4Å. The torsional distribution shows that the latter distance appears to be associated with much smaller torsions, whereas the top right cluster is associated with shorter lengths.

If the torsions are restricted to the range 90-180, then the histogram looses the smaller cluster, and perhaps gains a second cluster at 1.22Å?  As I said, all quite tantalizing!

‡The tail in all the histograms extends into the 1.1-1.3Å region, which seems unreasonable for a carbon where four bonds are specified. This region probably represents errors in the crystallographic analysis or reporting. But who knows, perhaps some very unusual compounds are lurking there!

 

References

  1. H. Rzepa, "A wider look at the π-complex theory of metal-alkene compounds.", 2016. https://doi.org/10.14469/hpc/642

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Bond stretch isomerism. Did this idea first surface 100 years ago?

Tuesday, February 9th, 2016

The phenomenon of bond stretch isomerism, two isomers of a compound differing predominantly in just one bond length, is one of those chemical concepts that wax and occasionally wane.[1] Here I explore such isomerism for the elements Ge, Sn and Pb.

In one earlier post, I noted a form of bond stretch isomerism that can arise from a Jahn-Teller distortion ending in two different geometries in which one or more pairs of bonds swap short/long lengths. Examples include substituted cyclo-octatetraenes[2] and octahedral d9-Cu(II) complexes.[3] A more interesting seminal possibility was implied by G. N. Lewis a century ago when discussing the arrangement of electrons in a (carbon-carbon) triple bond.[4]

lewis1
*It took ~50 years to prove this assertion wrong.[5]

In a commentary, I reported the results of a search of the crystal structure database for the geometries associated with RX≡XR systems (X= C, Si, Ge, Sn, Pb). Here I focus the search[6] specifically for X=Sn,Ge; this version of bond stretch isomerism also allows angles to change (= rehybridisation at atoms) in order to provide a mechanism for a barrier separating the two forms.

For X=Sn, note the presence of up to three clusters, although the relatively low number of hits makes the statistics less certain.

  1. The hotspot cluster centered around angles of 125° and a Sn-Sn distance of ~2.6Å.
  2. Another with angles of <100° and Sn-Sn distances of ~3.3Å.
  3. A third with angles of <100° and Sn-Sn distances of 2.8Å, which may or may not be a genuine unique form of bonding.

This pattern was commented on in 2010 by Power[7], whose group synthesized most of the examples in the hits above.‡ A plot of compounds with Ge-Ge bonds reveals both similarity with (two, possibly three clusters) and difference from (the clusters are closely spaced in terms of the Ge-Ge bond length, but separated in terms of angle) Sn.†

GeGe

Time for some computations (which at least will remove random errors in the geometry). I selected the only known example of an RPb-PbR compound[8] as a seed and put it through a B3LYP+D3/Def2-TZVPP calculation (with 172 atoms and 2920 basis functions, this is a relatively large calculation!), which reproduces the known structure pretty well (table).

QIMQUY

So what about another bond stretch isomers? The Pb=Pb variation is indeed a stable minimum around 28.0 kcal/mol above the known structure, which seems to put this form out of experimental reach (with this ligand/aryl group at least). With Sn for the same aryl ligand, the energy difference is smaller (~15.8 kcal/mol for this ligand; Powers reports other systems where the energy difference may be only ~5 kcal/mol). Judging by the distribution of the 13 hits recovered from the CSD search, both bond stretch isomers may be accessible experimentally. The calculations show that the GeGe bond isomers are much closer in energy than SnSn (for this ligand). For all three metals however, the calculated difference in the metal-metal length for the two isomers is ~0.45 – 0.52Å. This strongly suggests that whereas the SnSn plot above is demonstrating bond length isomerism, the GeGe plot may not be; at least not of the same type that the calculations here are revealing (via the Wiberg bond orders).

System Relative energy XX distance RXX angle Wiberg bond order DataDOI
Pb=Pb +28.0 2.767 118.7 1.666 [9]
Pb-Pb 0.0 3.215 (3.188)[8] 93.7 (94.3)[8] 0.889 [10]
Sn=Sn +15.8 2.640 123.1 1.911 [11]
Sn-Sn 0.0 3.126 95.5 0.892 [12]
Ge=Ge +0.5 2.263 125.2 2.138 [13]
Ge-Ge 0.0   2.777 99.7 0.866 [14]

No doubt the particular bond length form is being facilitated by the nature of the ligand and the steric interactions therein imparted, both repulsive AND attractive. These interactions can be visualised via NCI (non-covalent-interaction) plots (click on the image to obtain a rotatable 3D model). First Pb-Pb followed by Pb=Pb. Note how in both cases, the PbPb region is enclosed in regions of weak attractive dispersion interactions, which however avoid the "hemidirected" inert Pb lone pairs.[15]

Pb-Pb Pb=Pb

So in the end we have something of a mystery. There is evidence from crystal structures that at least two bond-stretch isomers of RSnSnR compounds can form, but the calculations indicate that the Sn=Sn form is significantly higher in energy (although not impossibly so for thermal accessibility). Conversely, the Ge=Ge equivalent is very similar in energy to a Ge-Ge form with a significantly longer bond length, but there seems no crystallographic evidence for such a big difference in bond lengths. Perhaps the answer lies with the ligands?

It seems particularly appropriate on the centenary of G. N. Lewis' famous paper in which he clearly notes the possibility of three isomeric forms for the triple bond, to pay tribute to the impact his suggestions continue to make to chemistry.

‡The individual entries can be inspected via the following dois: [16],[17],[18],[19],[20],[21],[22],[23],[24],[25]

†You can view individual entries via the following DOIs: [26],[27],[28],[29],[30],[31],[32],[33],[34],[35]

References

  1. J.A. Labinger, "Bond-stretch isomerism: a case study of a quiet controversy", Comptes Rendus. Chimie, vol. 5, pp. 235-244, 2002. https://doi.org/10.1016/s1631-0748(02)01380-2
  2. J.E. Anderson, and P.A. Kirsch, "Structural equilibria determined by attractive steric interactions. 1,6-Dialkylcyclooctatetraenes and their bond-shift and ring inversion investigated by dynamic NMR spectroscopy and molecular mechanics calculations", Journal of the Chemical Society, Perkin Transactions 2, pp. 1951, 1992. https://doi.org/10.1039/p29920001951
  3. W. Zhang, L. Chen, R. Xiong, T. Nakamura, and S.D. Huang, "New Ferroelectrics Based on Divalent Metal Ion Alum", Journal of the American Chemical Society, vol. 131, pp. 12544-12545, 2009. https://doi.org/10.1021/ja905399x
  4. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. https://doi.org/10.1021/ja02261a002
  5. F.A. Cotton, "Metal-Metal Bonding in [Re<sub>2</sub>X<sub>8</sub>]<sup>2-</sup> Ions and Other Metal Atom Clusters", Inorganic Chemistry, vol. 4, pp. 334-336, 1965. https://doi.org/10.1021/ic50025a016
  6. H. Rzepa, "Crystal structures containing Sn...Sn bonds", 2016. https://doi.org/10.14469/hpc/249
  7. Y. Peng, R.C. Fischer, W.A. Merrill, J. Fischer, L. Pu, B.D. Ellis, J.C. Fettinger, R.H. Herber, and P.P. Power, "Substituent effects in ditetrel alkyne analogues: multiple vs. single bonded isomers", Chemical Science, vol. 1, pp. 461, 2010. https://doi.org/10.1039/c0sc00240b
  8. L. Pu, B. Twamley, and P.P. Power, "Synthesis and Characterization of 2,6-Trip<sub>2</sub>H<sub>3</sub>C<sub>6</sub>PbPbC<sub>6</sub>H<sub>3</sub>-2,6-Trip<sub>2</sub> (Trip = C<sub>6</sub>H<sub>2</sub>-2,4,6-<i>i</i>-Pr<sub>3</sub>):  A Stable Heavier Group 14 Element Analogue of an Alkyne", Journal of the American Chemical Society, vol. 122, pp. 3524-3525, 2000. https://doi.org/10.1021/ja993346m
  9. H.S. Rzepa, "C 72 H 98 Pb 2", 2016. https://doi.org/10.14469/ch/191856
  10. H.S. Rzepa, "C 72 H 98 Pb 2", 2016. https://doi.org/10.14469/ch/191873
  11. https://doi.org/
  12. H.S. Rzepa, "C 72 H 98 Sn 2", 2016. https://doi.org/10.14469/ch/191881
  13. H.S. Rzepa, "C 72 H 98 Ge 2", 2016. https://doi.org/10.14469/ch/191882
  14. H.S. Rzepa, "C 72 H 98 Ge 2", 2016. https://doi.org/10.14469/ch/191883
  15. M. Imran, A. Mix, B. Neumann, H. Stammler, U. Monkowius, P. Gründlinger, and N.W. Mitzel, "Hemi- and holo-directed lead(<scp>ii</scp>) complexes in a soft ligand environment", Dalton Transactions, vol. 44, pp. 924-937, 2015. https://doi.org/10.1039/c4dt01406e
  16. Jones, C.., Sidiropoulos, A.., Holzmann, N.., Frenking, G.., and Stasch, A.., "CCDC 892557: Experimental Crystal Structure Determination", 2012. https://doi.org/10.5517/ccyys5t
  17. Phillips, A.D.., Wright, R.J.., Olmstead, M.M.., and Power, P.P.., "CCDC 187521: Experimental Crystal Structure Determination", 2002. https://doi.org/10.5517/cc6942p
  18. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771267: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwklt
  19. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771268: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkmv
  20. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771270: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkpx
  21. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771271: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkqy
  22. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771272: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkrz
  23. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771274: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkt1
  24. Fischer, R.C.., Pu, Lihung., Fettinger, J.C.., Brynda, M.A.., and Power, P.P.., "CCDC 624216: Experimental Crystal Structure Determination", 2007. https://doi.org/10.5517/ccnyk04
  25. Pu, Lihung., Phillips, A.D.., Richards, A.F.., Stender, M.., Simons, R.S.., Olmstead, M.M.., and Power, P.P.., "CCDC 221953: Experimental Crystal Structure Determination", 2004. https://doi.org/10.5517/cc7fysc
  26. Sasamori, Takahiro., Sugahara, Tomohiro., Agou, Tomohiro., Guo, Jing-Dong., Nagase, Shigeru., Streubel, Rainer., and Tokitoh, Norihiro., "CCDC 1035078: Experimental Crystal Structure Determination", 2014. https://doi.org/10.5517/cc13r2mk
  27. Sidiropoulos, A.., Jones, C.., Stasch, A.., Klein, S.., and Frenking, G.., "CCDC 749451: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cct4vvm
  28. Shan, Yu-Liang., Yim, Wai-Leung., and So, Cheuk-Wai., "CCDC 1019495: Experimental Crystal Structure Determination", 2015. https://doi.org/10.5517/cc136vy3
  29. Sugiyama, Y.., Sasamori, T.., Hosoi, Y.., Furukawa, Y.., Takagi, N.., Nagase, S.., and Tokitoh, N.., "CCDC 297827: Experimental Crystal Structure Determination", 2006. https://doi.org/10.5517/cc9zxbh
  30. Stender, M.., Phillips, A.D.., Wright, R.J.., and Power, P.P.., "CCDC 180660: Experimental Crystal Structure Determination", 2002. https://doi.org/10.5517/cc61zry
  31. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771273: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwks0
  32. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771269: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwknw
  33. Peng, Yang., Fischer, R.C.., Merrill, W.A.., Fischer, J.., Pu, Lihung., Ellis, B.D.., Fettinger, J.C.., Herber, R.H.., and Power, P.P.., "CCDC 771266: Experimental Crystal Structure Determination", 2010. https://doi.org/10.5517/cctwkks
  34. Jones, C.., Sidiropoulos, A.., Holzmann, N.., Frenking, G.., and Stasch, A.., "CCDC 892556: Experimental Crystal Structure Determination", 2012. https://doi.org/10.5517/ccyys4s
  35. Jones, C.., Sidiropoulos, A.., Holzmann, N.., Frenking, G.., and Stasch, A.., "CCDC 892555: Experimental Crystal Structure Determination", 2012. https://doi.org/10.5517/ccyys3r

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