A R T I C L E S
Fan et al.
principle the most desirable geometry for the resting state of
the catalyst, as substrate uptake to afford a square-planar four-
coordinate reactant complex will be straightforward owing to
the empty coordination site at the reactive metal center. The
Y-shaped structure is expected to be less reactive toward reactant
complex formation in certain cases but may offer the advantage
of preventing unproductive side reactions in the later part of
the catalysis. Trigonal planar systems6 do, however, display
unparalleled reactivity in the context of three-coordinate group
4 imides given their ability to promote intermolecular C-H
activation of arenes and alkanes, which also includes methane.7
Likewise, three-coordinate group 5 systems supported by
sterically demanding silox ligands (silox ) -OSitBu3) have been
demonstrated to promote reductive cleavage of strong bonds
such as CO and the N-C linkage in anilines and pyridine.8
Three-coordinate tungsten systems complement the group 4
metals with their prominent role in intermolecular C-H
activation of arenes.9 This type of chemistry is not only restricted
to C-H bonds alone because C3 symmetric three-coordinate
molybdenum complexes supported by sterically encumbering
anilides can incite the activation and reductive cleavage of
atmospheric nitrogen under normal conditions (1 atm, 25 °C).10
When invoking the later metals, three-coordinate templates play
pivotal roles in catalytic and stoichiometric group transfer
reactions,11-13 in addition to providing useful models for the
reactive sites in metalloenzymes such as nitrogenase14 and type
1 copper biological electron-transfer manifolds.15 In unrelated
work, we demonstrated previously how controlling the coordi-
nation geometry of the metal site is critical for harnessing the
reactive power of transition metal complexes toward small
molecule activation.16 Intuitively, it is not clear which electronic
features could be exploited to control the formation of the Y-
versus T-shaped coordination geometry in three-coordinate
Cr(II) complexes. The goal of this study is to establish such
conceptual strategy combining theory and experiment, as well
as theoretically predict the geometrical preference for three-
coordinate fragments that remain unknown.17
Experimental Details
General Considerations. Unless otherwise stated, all experi-
ments were performed in a M. Braun Laboratory Master double-
dry box under an atmosphere of purified nitrogen or using high
vacuum standard Schlenk techniques under an argon atmosphere.18
Anhydrous n-hexane, pentane, toluene, and benzene were purchased
from Aldrich in sure-sealed reservoirs (18 L) and further dried by
passage through two columns of activated alumina and a Q-5
column.19 Diethylether and CH2Cl2 were dried by passage through
two columns of activated alumina.19 THF was distilled, under
nitrogen, from purple sodium benzophenone ketyl and stored over
sodium metal. Distilled THF was collected in a thick walled
collection flask under inert atmosphere and transferred to a glove
box. C6D6 was purchased from Cambridge Isotope Laboratory
(CIL), degassed, and dried over CaH2, and then vacuum transferred
to 4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves
were activated under vacuum overnight at 200 °C. Li(nacnac),20
t
22
LiCH2 Bu,21 and (THF)2LiSiH{2,4,6-Me3C6H2}2 were prepared
according to the literature. NaO{2,6-iPr2C6H3} was prepared by
addition of NaN{Si(CH3)3}2 to a -35 °C ether solution of HO{2,6-
iPr2C6H3}. The white solid was filtered, washed with ether, and
dried under reduced pressure. All other reagents were used as
received. CHN analyses were performed by Desert Analytics,
1
Tucson, AZ, and Midwest Microlabs, Indianapolis, IN. H NMR
spectra were recorded on Varian 400 or 300 MHz NMR spectrom-
eters. 1H NMR spectra are reported with reference to solvent
resonances (residual C6D5H in C6D6, 7.16 ppm). Electronic absorp-
tion spectra were obtained with a Perkin-Elmer Lamba 19 spec-
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