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1,1'-bis(α-methylmethanol)ferrocene

Upon assigning $\ce{^{13}C}$-NMR for 1,1'-bis(α-methylmethanol)ferrocene, we discovered the substituted FcC-CH peak (95.14 ppm) was more downshifted than the CH-OH peak (65.59 ppm) as was confirmed by HSQC. The other four FcCH peaks were found at 67.77, 67.67, 66.12 and 66.07 ppm.

I can't rationalise why the FcC-CH peak is so downshifted as the lone pairs of oxygen can't donate in via resonance and if it was purely inductive, surely the CH-OH peak would be further downshifted.

Upon transformation to 1,1'-bis(α-methoxyethyl)ferrocene and then 1,1'-bis((α-methyl)methylpropanoate)ferrocene, the FcC-CH peak was still seen to be further downshifted than I expected.

If anyone can offer an explanation, I'd be very grateful!

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1 Answer 1

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First of all, electronic effect on $\ce{^{13}C}$-chemical shifts of 1,3-cyclopentadiene $(\pu{132.2 ppm}, \pu{132.8 ppm})$ is well evident due to the iron metal in ferrocene $(\pu{67.7 ppm})$. Yet, when ferrocene is substituted with any group, the $\ce{^{13}C}$-chemical shift of the carbon attached to the substituent shows a blue shift. I think this seemingly large blue shift in $\ce{C}$1 of the substituted ferrocenes is due to the sheilding-deshielding effects due to steric hindrance (similar to chemical shift difference shown in cis- and trans-alkenes). The same effect we have usually shown in substituted benzenes as well. I also think this chemical shift difference in $\ce{C}$1 of substituted ferrocenes is not too much of a difference than that shown in substituted benzenes:

substituted ferrocene vs substituted benzene

For example, when $\ce{R = CH3}$ (see above structures), $\ce{^{13}C}$-chemical shift of $\ce{C}$1 of ferrocene is $\pu{83.4 ppm}$ $(\Delta\delta = \pu{15.7 ppm})$ while that of substituted benzene is $\pu{137.4 ppm}$ $(\Delta\delta = \pu{9.3 ppm})$ where $\Delta\delta =$ $\ce{C}$1 of unsubstituted - $\ce{C}$1 of substituted (Ref.1).
Similarly, when stereo bulk is increased, say $\ce{R = C(CH3)3}$, then $\ce{^{13}C}$-chemical shift of $\ce{C}$1 of ferrocene is increased to $\pu{101.7 ppm}$ $(\Delta\delta = \pu{34.0 ppm})$ while that of substituted benzene is $\pu{150.5 ppm}$ $(\Delta\delta = \pu{22.4 ppm})$ (Ref.1).
Also, in a compound related to OP's mentioned compound where $\ce{R = CH2OH}$, the $\ce{^{13}C}$-chemical shift of $\ce{C}$1 of ferrocene is given as $\pu{87.7 ppm}$ $(\Delta\delta = \pu{20.0 ppm})$ while that of the substituted benzene is $\pu{141.1 ppm}$ $(\Delta\delta = \pu{13.0 ppm})$ (Ref.1).

There are few other examples in Ref.1 suggests that the difference in this chemical shift is due to the stereo hindrance. Also keep in mind that extra large $\Delta\delta$ shown in ferrocenes are due to the electronic influence from the iron metal.

References:

  1. A. A. Koridze, P. V. Petrovskii, A. I. Mokhov, A. I. Lutsenko, "Electronic effects in the cyclopentadienyl ring. The $\ce{^{13}C}$ NMR spectra of monosubstituted ferrocenes," Journal of Organometallic Chemistry 1977, 136(1), 57-63 (DOI: https://doi.org/10.1016/S0022-328X(00)87967-X).
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