Posts Tagged ‘metal centres’

Molecule of the year (month/week)?

Monday, December 12th, 2016

Chemical and engineering news (C&EN) is asking people to vote for their molecule of the year from six highlighted candidates. This reminded me of the history of internet-based “molecules of the moment“. It is thought that the concept originated in December 1995 here at Imperial and in January 1996 at Bristol University by Paul May and we were joined by Karl Harrison at Oxford shortly thereafter. Quite a few more such sites followed this concept, differentiated by their time intervals of weeks, months or years. The genre is well suited for internet display because of plugins or “helpers” such as Rasmol, Chime, Jmol and now JSmol which allow the three dimensions of molecular structures to be explored by the reader. Here I discuss a second candidate from the C&EN list; a ferrocene-based Ferris wheel[1],[2] (DOI for 3D model: 10.5517/CCDC.CSD.CC1JPKYQ) originating from research carried out at Imperial by Tim Albrecht, Nick Long and colleagues.

The chemical interest was the redox chemistry of the six metal centres, and the interactions between these centres, expressed more succinctly as “do the iron centres talk to each other?”. The suggestion was that the charges in the molecules originating from oxidation move between ferrocene centres at a rate that is fast compared to the electrochemical timescale. An analogy is drawn to the nanoscale and uniformly charged conductive rings.

I was interested to compare this system with any similar Fe compounds that might also be known in the CSD (Cambridge structure database). Here are some that I found:

  1. CEFDOG[3] with two cyclic ferrocene units with both neutral Fe and Fe(+) present
  2. EZEVIO[4], 3D: 10.5517/CC805N2  with Fe and Ge as the metals.
  3. FULVFE[5] from 1969 with two Fe centres.
  4. PETTUD and PETVAL[6] with two Fe centres.
  5. PETVEP and PETVIT[6] with Fe and Zr centres
  6. URAFUQ and URAGAX (3D: 10.5517/CCDC.CSD.CC1JPKZR), the system shown above.
  7. VOKXOI[7] with one Fe and one Fe+.
  8. VOKXUO[7] with one Fe and one Co+.
  9. WOJDOQ[8], 3D : 10.5517/CC133PGC from 2014 with three Fe units.
  10. ZECTOQ[9] with one Fe and one Th.

Returning to the communication between ferrocene units, the six-unit ferris wheel noted above has four sets differentiated from the other two in the solid state, although in solution by NMR they are all seen as equalised by exchange. The twist angle between four pairs is ~47° (C-C distance 1.471Å) and for the other two it is ~18° (C-C distance 1.466Å) which allows a fair measure of π-π conjugation to operate between the rings. Contrast this with the smaller WOJDOQ[10], where the torsions between the rings are closer to 80° (C-C distance 1.486Å) thus inhibiting π-π conjugation. It would certainly be interesting to compare e.g. the cyclic voltammetry for these two species to see if electronic communication between the rings is affected by this structural difference.

WOJDOQ

In regard to the D3-symmetric WOJDOQ[10], this is of course chiral and here its chiroptical properties intrigue,‡ along with questions of whether the two enantiomers are configurationally stable at room temperatures. If so, perchance they might be capable of acting as asymmetric catalysts?

Finally I speculate whether these sorts of rings can be constructed as Möbius strips or perhaps even as trefoil knots. It is certainly nice to see new molecules that spark all sorts of interesting new ideas!

‡The calculated optical rotation of WOJDOQ (TPSSh/6-311G(d,p)/SCRF=dichloromethane) is 427° at 800 nm and 1077° at 589 nm (doi: 10.14469/hpc/1971); the VCD (ωB97XD/6-311G(d,p)/SCRF=dcm) is shown below (doi: 10.14469/hpc/1970);

the  ECD (doi: 10.14469/hpc/1972 ):

References

  1. M.S. Inkpen, S. Scheerer, M. Linseis, A.J.P. White, R.F. Winter, T. Albrecht, and N.J. Long, "Oligomeric ferrocene rings", Nature Chemistry, vol. 8, pp. 825-830, 2016. https://doi.org/10.1038/nchem.2553
  2. Inkpen, Michael S.., Scheerer, Stefan., Linseis, Michael., White, Andrew J.P.., Winter, Rainer F.., Albrecht, Tim., and Long, Nicholas J.., "CCDC 1420914: Experimental Crystal Structure Determination", 2016. https://doi.org/10.5517/ccdc.csd.cc1jpkyq
  3. M. Hillman, and A. Kvick, "Structural consequences of oxidation of ferrocene derivatives. 1. [0.0]Ferrocenophanium picrate hemihydroquinone", Organometallics, vol. 2, pp. 1780-1785, 1983. https://doi.org/10.1021/om50006a013
  4. M. Joudat, A. Castel, F. Delpech, P. Rivière, A. Mcheik, H. Gornitzka, S. Massou, and A. Sournia-Saquet, "Synthesis, Structures, and Reactivity of Mono- and Bis(ferrocenyl)-Substituted Group 14 Metallocenes", Organometallics, vol. 23, pp. 3147-3152, 2004. https://doi.org/10.1021/om0400393
  5. M.R. Churchill, and J. Wormald, "Crystal and molecular structure of bis(fulvalene)diiron", Inorganic Chemistry, vol. 8, pp. 1970-1974, 1969. https://doi.org/10.1021/ic50079a030
  6. P. Scott, U. Rief, J. Diebold, and H.H. Brintzinger, "ansa-Metallocene derivatives. 28. Homo- and heterobimetallic bis(fulvalene) complexes from bis(cyclopentadienyl)- and bis(indenyl)-substituted ferrocenes", Organometallics, vol. 12, pp. 3094-3101, 1993. https://doi.org/10.1021/om00032a036
  7. P. Brüggeller, P. Jaitner, and H. Schottenberger, "Kristallographische Gegenüberstellung der Monokationen von Bis(fulvalen)dieisien und Bis(fulvalen) eisen-cobalt mit identischem Gegenion (PF6−)", Journal of Organometallic Chemistry, vol. 417, pp. C53-C58, 1991. https://doi.org/10.1016/0022-328x(91)80206-y
  8. R. Shekurov, V. Miluykov, O. Kataeva, A. Tufatullin, and O. Sinyashin, "Crystal structure of cyclic tris(ferrocene-1,1′-diyl)", Acta Crystallographica Section E Structure Reports Online, vol. 70, pp. m318-m319, 2014. https://doi.org/10.1107/s1600536814017346
  9. P. Scott, and P.B. Hitchcock, "Synthesis, structure and electrochemistry of the first fulvalene derivative of an actinide", Journal of Organometallic Chemistry, vol. 497, pp. C1-C3, 1995. https://doi.org/10.1016/0022-328x(95)00108-3

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Multi-centre bonding in the Grignard Reagent

Tuesday, December 1st, 2009

The Grignard reaction is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction:

2PhBr + 2Mg → 2PhMgBr ↔ MgBr2 + Ph2Mg

The reagent itself exists as part of an equilibrium, named after Schlenk, in which a significant concentration of a dialkyl or diarylmagnesium species is formed. The topic of this blog entry is to analyse the structure and bonding in this latter species.

First, the structure is shown below (for 2,6-diethylphenyl magnesium). This reveals a dimeric structure with a four membered ring core, comprising two  Mg atoms  connected by two bridging  aryl groups.

The crystal structure of a di-aryl magnesium. Click to view 3D

The crystal structure of a di-aryl magnesium. Click to view 3D

The question to be addressed here is the nature of the aryl groups. Put simply, it seems as if their bridging role means that one of the six carbons involved in the benzene ring has become sp3 hybridized. This would in turn mean that the cyclic conjugation of the benzene ring is interrupted, and a species akin to the Wheland intermediate is formed in which the aromaticity of two of the benzene rings is no longer sustained. This situation could be depicted thus;

A Simple bonding representation in Ph2Mg dimer

A Simple bonding representation in Ph2Mg dimer

Is this really the best way of depicting the bonding in this species? A more subtle analysis of the bonding can be achieved using a technique known as ELF (involving analysis of the electron localization function). This reveals bonds as so-called synaptic basins, which come in two varieties; disynaptic basins corresponding to two-centre bonds, and trisynaptic basins which reveal three-centre bonds (there is also a monosynaptic basin which corresponds to electron lone pairs). Such an ELF analysis (based on a B3LYP/6-311G(d,p) computed wavefunction for Ph2Mg dimer) is shown below;

ELF analysis of the bonding in Ph2Mg dimer

ELF analysis of the bonding in Ph2Mg dimer. Click for 3D model

The small purple dots represent synaptic basins. Several of these are circled. The  ones circled in orange are conventional disynaptic forms, and the basins can be integrated to to 2.48 electrons each. The red basin however is clearly revealed as a trisynaptic form (covering both metal centres and the carbon) and integrating to  2.7 electrons. The  three basins surrounding each Mg atom integrate to 7.91 electrons, which reveal the metal to have a conventional octet of electrons in its valence shell. The bonding in the central region could therefore be described as comprising two three-centre-three-electron bonds. The key aspect of this is that the two bridging phenyl groups do not break their aromaticity, ie all four phenyl/aryl groups largely retain their aromaticity! Thus the disynaptic basins for  the normal non-bridging phenyl group and  circled in green integrates to 2.6 electrons and the blue to 2.8 (an ideal aromatic bond would of course integrate to 3.0 electrons), whereas the equivalent basins for the bridging phenyl (brown and purple, 2.5 and  2.8) are virtually the same.

It is interesting how a veritable mainstay of most taught chemistry courses, the Grignard reagent,  can have such subtle aspects of the bonding surrounding both the metal atom and the aromatic groups, and how rarely this bonding is actually dissected in most text books.

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