I have synthesized N-benzyl-2-(2'-pyridinyl)benzimidazole and their ruthenium complexes. The 1H NMR spectrum of the free ligand shows a singlet for the methylene protons which are interposed by a phenyl ring and the imidazole nitrogen of the 2-(2'-pyridinyl)benzimidazole moiety. As these protons are diastereotopic, they are expected to show a multiplet (1:3:3:1 intensity). But, in the free ligand a singlet is observed for these protons whereas complexation to form ruthenium(II) complexes lead to splitting of this signal. Is this because of any restriction in the rotation upon complexation? Could recording a DEPT spectra give any hint on this?
Hello. If I'm not getting the structure wrong, the methylene protons you are referring to are enantiotopic, not diastereotopic, and should be magnetically equivalent, unless when put in a chiral environment, thus giving the singlet you are observing. Regarding the complex, if there is one (or more) asymmetric centre in the ruthenium reactant you are using, then these methelyne hydrogens become diastereotopic when the complex is formed, which would explain the splitting pattern mentioned.
I hope this helps.
Please see this reference from Proceeding of the Indian Academy of Sciences: Chemical Science (currently renamed as Journal of Chemical Sciences)
Volume 96 Issue 1-2 January 1986 pp 21-58 Organic
Anisochrony of O-methylene protons of ethyl ester functions and structural factors in the connected chiral entities
Check if the Ru complex (known structure? Octahedral, square planar?.....) is on the whole asymmetric (mirror images not superposable). In that case the CH2 will become CHACHB. They will then exhibit an AB sub-spectrum. The sub-spectrum will not necessarily have 1:3:3:1 branch relationships. Delta AB will depend on the local anisotropic gradient.
one of the methylene hydrogen atoms may be having some interactions that makes two hydrogen atoms dissimilar and hence possibly they become magnetically inequivalent. For example, in your complex, the ligand may be coordinated to ruthenium atom through imidazole nitrogen having benzyl group along with pyridyl nitrogen, which brings one of the hydrogen atoms of methylene in close proximity to establish agostic or anagostic interactions, which obviously induces non-equivalence among those hydrogen atoms.
Do more NMR experiments to get more insight.
@Vanderson Barbosa Bernardo Dear Vanderson, Greetings! Thank you for replying. Your observation is correct, the methylene protons are enantiotopic. The resulting complex formed is of the type [Ru(A-A)2(A-B)]. if I’m not wrong, the complex is inherently chiral. Now, would these enantiotopic protons become diastereotopic with the introduction of an adjacent chiral center?
@Saraswathi N. Balasubrahmanyam Professor Saraswathi, Greetings! Thank you for the reply and the reference article. The Ruthenium complex is octahedral of the type [Ru(A-A)2(A-B)] and it constitute nonsuperimposable mirror images. The structure in the solid state has been confirmed by single crystal XRD. The other interesting observation I have in connection with this is the absence of such a splitting in the trinuclear system with a mesitylene based linker (structure is attached). Is it denoting any mutual cancellation of optical rotation due to symmetry?
@Balakrishna Maravanji Professor Balakrishna, Greetings! Thank you for sharing your observations. The benzyl substituted nitrogen of imidazole is not involved in binding with ruthenium which is coordinatively saturated. There are no agostic or anagostic interactions observed in the solid state (as inferred from the single crystal XRD and J = 23 Hz). Does the solvent play any role? The ligand spectrum is recorded in CDCl3 while the complexes are in DMSO-d6.
I hope the discussion in the link (https://www.researchgate.net/post/When_does_N-Benzyl_methylene_proton_show_geminal_coupling_splitting_in_H1_NMR_spectroscopy) would throw more information into this topic. May I have a general clarification on the concept? Are the methylene-protons adjacent to a chiral center diastereotopic and anisochronous always? Is it advisable to record the circular dichroism spectra for the ligand and complexes in order to justify the introduction of chiral center adjacent to methylene protons and hence the splitting? @Walter Bauer @Ludger Ernst @Krishna Verma @Anthony Foris @Deokar Rhushikesh Chandrabhan
Vanderson has given the correct suggestion to your neat explanation. Best Balakrishna
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