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I want to understand why hydroquinone and 3H-pyrrole can or cannot act as ligands in transition metal complexes. If they are ligands, then what is their denticity?

hydroquinone 3H-pyrrole

I'm trying to understand the overall co-ordination behaviour of these two molecules, so please consider all atoms and rings as possible electron donors.

I think the denticity for the second molecule should be 1, since there's only 1 donor atom N, but would it be a weak ligand as the nitrogen is double bonded to a C?

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    $\begingroup$ Your second molecule is actually pyrrole in disguise. $\endgroup$ – Ivan Neretin May 2 '18 at 18:14
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    $\begingroup$ This is an under-specified question because you didn't specify the binding mode of each ligand. For example, in the hydroquinone, do you want an oxygen atom to be the electron pair donor or do you want the benzene ring? And if the benzene ring, do you want to coordinate 2, 4, or 6 carbon atoms? Same types of questions with the second structure. $\endgroup$ – Zhe May 2 '18 at 18:45
  • $\begingroup$ Would pyrrole even donate via the N-Atom? I think in pyridine the lone-pair on the nitrogen is orthogonal to the aromatic pi-system but in pyrrole it is parallel to the aromatic pi-system. Therefore I'd expect it to rather coordinate like in metallocenes. $\endgroup$ – Justanotherchemist Jun 15 '18 at 10:48
  • $\begingroup$ @Zhe I'd appreciate if you could explain donation from both the O and the benzene ring (and the N atom and the ring respectively). I'm just trying to understand the co-ordination behaviour, sorry for not making that clear. $\endgroup$ – vyb Jun 16 '18 at 11:48
  • $\begingroup$ Sorry, if my explanation doesn't make sense, I'm not sure trying to explain further will clear things up for you. You should look at a good reference text on coordination chemistry or organometallics. $\endgroup$ – Zhe Jun 16 '18 at 23:23
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  • Zaworotko, M. J.; Stamps, R. J.; Ledet, M. T.; Zhang, H.; Atwood, J. L. Heterocyclophane complexes of transition metals. 1. Synthesis and crystal structure of both the (.eta.5-2.2pyrroloparacyclophane)tricarbonylchromium and the (.eta.6-2.2pyrroloparacyclophane)tricarbonylchromium. Organometallics 1985, 4 (10), 1697-1700. DOI: 10.1021/om00129a001

A chromium tricarbonyl complex of a pyrrole is reported, here the pyrrole coordinates with all four carbon atoms and the nitrogen atom. I think that the pyrrole is a six electron donor to the chromium.

  • Bentivegna, B.; Mariani, C. I.; Smith, J. R.; Ma, S.; Rheingold, A. L.; Brunker, T. J. Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions. Organometallics 2014, 33 (11), 2820-2830. DOI: 10.1021/om500348u

A pyrrole binds with all five non hydrogen atoms to an iron which is also bonding to a pentamethyl cyclopentadienyl anion. This is forming a protonated version of CpFe(C4N) which looks very much like ferrocene. In this case the bonding is the same as it was in the chromium compound.

  • Yamamoto, K.; Kimura, S.; Murahashi, T. σ-π Continuum in Indole-Palladium(II) Complexes. Angew. Chem. 2016, 128 (17), 5408-5412. DOI: 10.1002/ange.201601992

A pyrrole binds with two carbons as a pi ligand to palladium. These selected results suggest to me that pyrrole can bind as an alkene or as if it was a Cp ligand to transition metals.

  • Son, S. U.; Reingold, J. A.; Carpenter, G. B.; Czech, P. T.; Sweigart, D. A. Charge-Assisted Hydrogen Bonding and Other Noncovalent Interactions in the Self-Assembly of the Organometallic Building Block [(η6-hydroquinone)Rh(P(OPh)3)2]+with a Range of Counteranions. Organometallics 2006, 25 (22), 5276-5285. DOI: 10.1021/om0604425

It reports the binding of a rhodium to all six carbon atoms of hydroquinone. Here it acts as a six electron donor.


So there is nothing to stop the molecules in the question acting as pi ligands to transition metals.

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