Transmetalation of Chromocene by Lithium-Amide, -Phosphide, and -Arsenide Nucleophiles

The
pnictogen-centered nucleophiles LiE­(SiMe3)2 (E
= N, P, or As) substitute a cyclopentadienide ligand of chromocene
(Cp2Cr), with elimination of lithium cyclopentadienide,
to give the series of pnictogen-bridged compounds [(μ:η2:η5-Cp)­Cr­{μ-N­(SiMe3)2}2Li] (1) and [(η5-Cp)­Cr­{μ-E­(SiMe3)2}]2, with
E = P (2) or E = As (3). Whereas 1 is a heterobimetallic coordination polymer, 2 and 3 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 1, the experimental
magnetic susceptibility data were accurately reproduced by a single-ion
model based on high-spin chromium­(II) (S = 2), which
gave a g-value of 1.93 and an axial zero-field splitting
parameter of D = −1.83 cm–1. Determinations of phosphorus- and arsenic-mediated magnetic exchange
coupling constants, J, are rare: in the dimers 2 and 3, variable-temperature magnetic susceptibility
measurements identified strong antiferromagnetic exchange between
the chromium­(II) centers, which was modeled using the spin Hamiltonian H = −2J(SCrA·SCrB), and produced large coupling
constants of J = −166 cm–1 for 2 and −77.5 cm–1 for 3.