Transmetalation
of Chromocene by Lithium-Amide, -Phosphide,
and -Arsenide Nucleophiles
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Abstract
The
pnictogen-centered nucleophiles LiE(SiMe<sub>3</sub>)<sub>2</sub> (E
= N, P, or As) substitute a cyclopentadienide ligand of chromocene
(Cp<sub>2</sub>Cr), with elimination of lithium cyclopentadienide,
to give the series of pnictogen-bridged compounds [(μ:η<sup>2</sup>:η<sup>5</sup>-Cp)Cr{μ-N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>Li] (<b>1</b>) and [(η<sup>5</sup>-Cp)Cr{μ-E(SiMe<sub>3</sub>)<sub>2</sub>}]<sub>2</sub>, with
E = P (<b>2</b>) or E = As (<b>3</b>). Whereas <b>1</b> is a heterobimetallic coordination polymer, <b>2</b> and <b>3</b> are homometallic dimers, with the differences being due
to a structure-directing influence of the hard or soft character of
the bridging group 15 atoms. For compound <b>1</b>, the experimental
magnetic susceptibility data were accurately reproduced by a single-ion
model based on high-spin chromium(II) (<i>S</i> = 2), which
gave a <i>g</i>-value of 1.93 and an axial zero-field splitting
parameter of <i>D</i> = −1.83 cm<sup>–1</sup>. Determinations of phosphorus- and arsenic-mediated magnetic exchange
coupling constants, <i>J</i>, are rare: in the dimers <b>2</b> and <b>3</b>, variable-temperature magnetic susceptibility
measurements identified strong antiferromagnetic exchange between
the chromium(II) centers, which was modeled using the spin Hamiltonian <i>H</i> = −2<i>J</i>(<i>S</i><sub>CrA</sub>·<i>S</i><sub>CrB</sub>), and produced large coupling
constants of <i>J</i> = −166 cm<sup>–1</sup> for <b>2</b> and −77.5 cm<sup>–1</sup> for <b>3</b>
