10.1021/om020229i.s001
Oliver M. Heigl
Oliver M.
Heigl
Eberhardt Herdtweck
Eberhardt
Herdtweck
Stefan Grasser
Stefan
Grasser
Frank H. Köhler
Frank H.
Köhler
Werner Strauss
Werner
Strauss
Harald Zeh
Harald
Zeh
Stable NMR- and EPR-Active 17-Electron Chromium(III)
Half-Sandwich Compounds
American Chemical Society
2002
ligand
electron
THF
triflate
2 H NMR spectroscopy
dication
Cp π systems
NMR signal shifts
CNCH 3
EPR
chromium
PF 6
Cp 2 Cr
BuCp
CF 3
2002-07-18 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Stable_NMR-_and_EPR-Active_17-Electron_Chromium_III_Half-Sandwich_Compounds/3772467
Formerly elusive 17-electron half-sandwiches of chromium(III) were isolated after
stabilization with isocyanide ligands by using different approaches. Thus, reaction of Cp<sub>2</sub>Cr
with CF<sub>3</sub>SO<sub>3</sub>H and CNCH<sub>3</sub> gave [CpCr(CNCH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> (<b>7</b><sup>2+</sup>) as a triflate, while [(R<i><sub>n</sub></i>Cp)CrCl<sub>2</sub>]<sub>2</sub>
(R = Me, H (<b>3</b> and <b>5</b>, respectively), <i>n</i> = 5; R = <i>t</i>-Bu, <i>n</i> = 1 (<b>8</b>)) reacted with Tl(CF<sub>3</sub>SO<sub>3</sub>) to
give <b>7</b><sup>2+</sup>, [(C<sub>5</sub>Me<sub>5</sub>)Cr(CNCH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> (<b>2</b><sup>2+</sup>), and [(<i>t</i>-BuCp)Cr(CNCH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> (<b>10</b><sup>2+</sup>) again as triflates
in up to 59% yield. The precursor <b>8</b> was synthesized from (<i>t</i>-BuCp)Na and CrCl<sub>3</sub>(THF)<sub>3</sub>.
The dication <b>2</b><sup>2+</sup> was also obtained electrochemically from [(C<sub>5</sub>Me<sub>5</sub>)Cr(CNCH<sub>3</sub>)<sub>4</sub>]<sup>+</sup>[PF<sub>6</sub>]<sup>-</sup>
(<b>2</b><sup>+</sup>[PF<sub>6</sub>]<sup>-</sup>), which was synthesized by reaction of [(C<sub>5</sub>Me<sub>5</sub>)Cr(C<sub>6</sub>H<sub>3</sub>Me<sub>3</sub>)]<sup>+</sup>[PF<sub>6</sub>]<sup>-</sup> with CNCH<sub>3</sub>
in 86% yield. Conversely, the dication <b>7</b><sup>2+</sup> could be reduced to <b>7</b><sup>+</sup> obtained as [PF<sub>6</sub>]<sup>-</sup> and
[B(C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>]<sup>-</sup> salts. The crystal structure of <b>10</b><sup>2+</sup>[CF<sub>3</sub>SO<sub>3</sub>]<sup>-</sup><sub>2</sub> confirmed the four-legged piano-stool structure of <b>10</b><sup>2+</sup>; no interaction of the chromium centers <b>10</b><sup>2+</sup> with the two triflates
per formula unit could be detected. Cyclic voltammetry showed a reversible conversion
between the cations <b>2</b><sup>2+/1+</sup>, <b>7</b><sup>2+/1+</sup>, and <b>10</b><sup>2+/1+</sup>. By contrast, further oxidation to the trication
and reduction to the neutral molecules proved to be irreversible. The dications were shown
by NMR and EPR spectroscopies to contain one unpaired electron in the chromium d<i><sub>z</sub></i><sup><sub>2</sub></sup> orbital.
With <sup>1</sup>H and <sup>2</sup>H NMR spectroscopy the remote protons of substituted Cps and (after labeling)
the deuterons of Cp and methylisocyanide ligands, respectively, were detected. The signs of
the NMR signal shifts established negative spin density in the Cp π systems.