Evans Method with NMReady-60 for understanding 1H NMR of Paramagnetic Compounds

Due to the presence of unpaired d electrons in their metal ions, many transition metal complexes are paramagnetic. The unpaired electrons have a magnetic dipole moment due to their spin and act like tiny magnets, resulting in a small net attraction to an externally applied magnetic field. Unsurprisingly, the presence of paramagnetic ions has significant effects on both the chemical shift and lineshape of the ¹H NMR spectrum of transition metal complexes, with the chemical shift range being much wider along with broadening of the signals.

Furthermore, the presence of paramagnetic ions causes the chemical shifts of other compounds in the solution to move. This effect can be used to estimate the magnetic susceptibility of transition metal complexes and subsequently the electronic structure of transition metal ions. First demonstrated by Dennis Evans in 1959^[1] and described in subsequent publications,^[2-4] the change in chemical shift of an inert reference compound in the presence and absence of a paramagnetic transition metal complex is correlated to the magnetic susceptibility of the transition metal complex via Equation 1:

where Χ_m is the molar magnetic susceptibility of the transition metal complex, c is the concentration of the complex (mol/L), Δf is the shift in the frequency of the reference compound (Hz), and f is the frequency of the spectrometer (Hz).

Once the molar magnetic susceptibility is known, the effective magnetic moment can be calculated which can then be related to the number of unpaired electrons in the metal ion using Equation 2:

where Xm is the molar magnetic susceptibility determined earlier with the Evans method, T is the temperature in Kelvin, and n is the number of unpaired electrons in the metal ion.

In this blog post, I will demonstrate how the Evans method can be used to determine the electronic structure of manganese(III) acetylacetonate (Mn(acac)₃) using the NMReady-60. Mn(acac)₃ is an octahedral complex containing a manganese(III) ion ligated by three acetylacetonate ligands. As seen in Figure 1, there are two possible configurations, denoted as high spin and low spin, for the four d electrons of Mn(III) in an octahedral field. There are four unpaired electrons in the high spin configuration while the low spin configuration has only two unpaired electrons. Using Equation 2, the expected effective magnetic moment of each electronic configuration is calculated: high spin = 4.90 µ_B, low spin = 2.83 µ_B.

Figure 1. High spin and low spin electronic structures of Mn(III) in an octahedral field.

To perform the Evans method with the NMReady-60, tert-butanol is used as the reference compound in the form of a 10% v/v solution in chloroform. Five milligrams of Mn(acac)₃ was accurately weighed out and dissolved in 0.5 mL of the tert-butanol solution. Two separate melting point capillary tubes were then filled with the tert-butanol solution and the solution of Mn(acac)₃ and sealed. The capillary tubes were then inserted into separate NMR tubes and filled with deuterated chloroform. The ¹H NMR spectra of the two samples were then recorded and displayed as an overlay in Figure 2.

Figure 2. 1H NMR spectrum of the capillary tube filled with the reference solution (green) overlaid with the capillary tube filled with Mn(acac)3 dissolved in the reference solution (blue). The peaks are labelled in Hz. Inset: NMR tubes with the capillary tubes used for the Evans method.

The change in chemical shift of tert-butanol is apparent after the addition of Mn(acac)₃. Using Equation 1 followed by Equation 2, the effective magnetic moment was calculated to be 5.04 µ_B. As the effective magnetic moment is much closer to the value calculated for the high spin configuration (4.90 µ_B), the Mn(III) ion in Mn(acac)₃ is determined to be high spin.

As you can see, the Evans method can be performed easily on the NMReady-60, allowing for a quick and accessible experiment to determine the electronic structure of transition metal complexes suitable for undergraduate laboratories.

References

^[1]Evans, D. F. J. Chem. Soc. 1959, 2003.
^[2]Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
^[3]Piguet, C. J. Chem. Educ. 1997, 74, 815.
^[4]Nataro, C.; Fosbenner, S. M. J. Chem. Educ. 2009, 86, 1412.