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NJC
Scheme 1 Synthesis of compounds 2-As and 3-As.
Fig. 1 Pnictogen alkoxide cage structures and dimer depicting the
[Pn–O]2 supramolecular synthon in blue.
along with a marked shift in the resonances of the cage
methylene protons. This is consistent with what was observed
upon cage formation for the analogous antimony compounds.10
The FTIR spectrum was consistent with the loss of H from the
alcohols as well as the formation of a new As–O bond with
stretching frequencies of 583 and 581 cmꢀ1 for 2-As and 3-As,
respectively. These vibrations are found at a lower energy than
the corresponding one in the starting compound, As(OEt)3,
which absorbs at 604 cmꢀ1. It has been previously noted for
antimony compounds, that the vibrational frequencies are
affected by the formation of pnictogen bonds.10,33–35 This can
be rationalized by applying an orbital model in which the low-
lying s* orbital of the pnictogen–oxygen primary bond is
populated during formation of the pnictogen bond, leading
to a weakening of the primary bond. The resulting shift to lower
energy in the IR is, therefore, an important spectroscopic
signature of pnictogen bond formation. DFT calculations pre-
dict vibrational frequencies of 599 cmꢀ1 for monomeric 2-As
or 3-As. Computationally, dimerization through pnictogen
bond formation is predicted to shift the vibrational energy to
583 cmꢀ1. This is consistent with the observed experimental
stretching frequency and evidence of pnictogen bond formation
in the solid state for 2-As and 3-As. It is notable that the predicted
change in the vibrational frequency is smaller for arsenic
(16 cmꢀ1) than was predicted for antimony (47 cmꢀ1). This would
be consistent with a much weaker pnictogen bond in the case of As.
polar primary bonds directed the formation of three strong
pnictogen bonds, each with an experimentally estimated energy
of 33 kJ molꢀ1 10
The design was chosen to provide sufficient
.
space around the pnictogen to accommodate the lone pair and
the formation of three pnictogen bonds opposite to each of
the three polar primary bonds. Based on electronegativity
arguments alone, it is expected that the other pnictogen atoms
should be able to form strong pnictogen bonds when paired
with oxygen; the polarity of the bond between antimony and
oxygen should be similar to that of an arsenic oxygen bond.
Only one such arsenic alkoxide cage has been crystallographi-
cally characterized and it is observed to participate in only one
weak PnB.31 The length of the appended alkyl chain could,
perhaps be a contributing factor to the absence of further
pnictogen bonds. Pnictogen bonding in the phosphorus con-
gener is rare, and the bismuth analogue has only been studied
by elemental analysis.
This study reports the preparation of arsenic alkoxide cages
along with spectroscopic and structural characterization. In contrast
to antimony, only a single pnictogen bond is observed to form at
each arsenic centre. Analogous bismuth alkoxides were prepared
using similar approaches. Structural characterization remained
elusive, but vibrational spectroscopic studies hint at the possibility
of strong pnictogen bonds that prevent solubilization of the product.
A detailed DFT study is used to provide insight into this deference as
well as explore the trends in pnictogen bonding from P to Bi.
Attempted preparations and characterizations of bismuth
congener
Several attempts to prepare 2-Bi and 3-Bi using different bismuth
starting materials, such as BiCl3, Bi2O3, Bi(OtBu)3, Bi(NMe2)3, were
made (Scheme 2). All attempts resulted in the formation of
amorphous solids or gels depending on the reactions conditions
(for additional details see Table S1 in the ESI†). Controlling the
rate of the reaction, either by slowing down the addition of a base
Results and discussion
Synthesis and characterization of arsenic alkoxide cages
Arsenic alkoxide cages, 2-As and 3-As, were prepared under
mild conditions by treating arsenic(III) ethoxide with the appro-
priate triol (Scheme 1) following a similar procedure used to
prepare the analogous antimony cages.10 Both compounds
have been previously reported, preparation of 2-As by treatment
of the triol 4 with AsCl3 and 3-As by treatment of triol 5 with
As2O3.31,32 In contrast to the antimony cages, the arsenic cages
did not precipitate out of THF. This higher solubility is at odds
with the formation of multiple strong pnictogen bonds.
Pure materials, established by combustion analysis, were
obtained by recrystallization from hexanes. The 1H NMR of
both 2-As and 3-As revealed a loss of the hydroxyl proton signals
Scheme 2 Synthesis of compounds 2-Bi and 3-Bi.
New J. Chem.
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