An Oligosilsesquioxane
Cage Functionalized with Molybdenum(II)
Organometallic Fragments
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Abstract
A silsesquioxane cage polymer functionalized with eight
chloropropyl
arms (<b>1</b>, T<sub>8</sub>-PrCl) reacted with 2,2′-dipyridiylamine
(DPA) to afford a new derivative with eight pendant linear chains
(<b>2</b>, T<sub>8</sub>-Pr-DPA). Further reaction with [Mo(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Br(CO)<sub>2</sub>(NCMe)<sub>2</sub>] afforded another derivative containing three molybdenum units (<b>3</b>, T<sub>8</sub>-Pr-DPA-Mo), after substitution of the two
nitrile ligands in each complex. These are the first silsesquioxane
species containing DPA and the Mo(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Br(CO)<sub>2</sub> fragment. The three materials were
characterized by <sup>1</sup>H, <sup>13</sup>C, <sup>29</sup>Si, and <sup>95</sup>Mo NMR, FTIR, XRD, and elemental analysis, and T<sub>8</sub>-PrCl (<b>1</b>) was also structurally characterized by single-crystal
X-ray diffraction. It was identified as a low-temperature polymorph
of this material. Elemental analysis indicated that all Cl atoms in
the parent material T<sub>8</sub>-PrCl (<b>1</b>) were substituted
by the deprotonated DPA group in T<sub>8</sub>-Pr-DPA (<b>2</b>). However, only three [Mo(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Br(CO)<sub>2</sub>(DPA)] units were detected in T<sub>8</sub>-Pr-DPA-Mo (<b>3</b>). A comprehensive NMR study, complemented
with DFT calculations, was carried out in order to detect the effect
of Mo coordination on the cage silicon and on the protons and carbons
of the propyl chain, but no significant effects were observed. Both <sup>1</sup>H and <sup>29</sup>Si chemical shifts vary upon introducing
DPA but remain the same after reaction with the Mo(II) precursor.
The <sup>95</sup>Mo NMR data reveal that the metal is not sensitive
to the cage. The catalytic activity of <b>3</b> was tested as
a precursor in the epoxidation of cyclooctene and styrene in the presence
of TBHP. Despite the high selectivity toward the epoxides, the conversion
and turnover frequencies were low, reflecting the behavior of the
[Mo(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Br(CO)<sub>2</sub>(DPA)] complex
