Changes in aromaticity of spin-crossover complexes: a signature for non-innocent ligands

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(ii)tris-bipyridine (Fe(bpy)32+), and Fe(ii)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands.


Experimental details
All manipulations were carried out under nitrogen using standard glovebox and Schlenk-line techniques.THF was dried by percolation over a column of Al2O3 (Fluka).Compound 3 was prepared as described in the literature. 1 Solution IR spectra were acquired using a Mettler Toledo ReactIR 700 instrument equipped with a DiComp (diamond) probe and a AgX Fiber Conduit connected to a liquid N2-cooled MCT detector.Spectra were recorded with 4 nm -1 resolution.The temperature of the solution was controlled by immersing the flask in an ethanol bath (for temperatures below room temperature).The bath temperature was regulated using a Julabo FT902 immersion chiller, and the internal temperature of the solution of 3 was monitored using the integrated temperature sensor in the IR probe.For temperatures above room temperature, the flask was immersed in an oil bath that was heated to the desired temperature on a hotplate.A Schlenk flask with a 50 mM solution of 3 in anhydrous THF was prepared inside a glovebox, and subsequently transferred to the ReactIR setup.Under a counterflow of N2, the probhead was inserted into the flask.The IR measurements were started around 15 °C, and the flask was subsequently cooled to -45 °C while continuously measuring the IR spectrum (every ~10 seconds).The spectra were analyzed using iC IR 7.1.
The experiment was repeated but then with only pure THF to collect IR spectra for background subtraction.Spectra recorded at selected temperatures were imported into Spectragryph-on 1.2.16, and after base-line correction the appropriate THF spectrum was subtracted from that of 3/THF collected at the same temperature.
The largest changes are observed for the spectra at the extremes of the temperature range (in this case -45 and + 53 °C); these are used for the discussion in the main text.

Figure S1 .
Figure S1.Calculated IR spectrum for the different spin states of 1 S14 Figure S2.Plots of the induced current π-density using PBE0 for the different spin states of 2 S14

Figure S1 .
Figure S1.Calculated IR spectrum for the singlet, triplet, and quintet spin states of 1.