Sterically Crowded Tin Acenaphthenes

The synthesis of crowded peri-5-bromo-6-(organostannyl)­acenaphthenes
is described. Reaction of 5,6-dibromoacenaphthene with 1 equiv of n-BuLi at −40 °C in diethyl ether followed by
addition of the appropriate organotin reagent at 0 °C gave 5-bromo-6-(triphenylstannyl)­acenaphthene
(1), 5-bromo-6-(chlorodiphenylstannyl)­acenaphthene (2), bis­(6-bromoacenaphthen-5-yl)­diphenylstannane (3), bis­(6-bromoacenaphthen-5-yl)­dibenzylstannane (4),
bis­(6-bromoacenaphthen-5-yl)­dibutylstannane (6), and
bis­(6-bromoacenaphthen-5-yl)­dichlorostannane (7) in low
to medium yields (10–56%). 4 was converted into
5-iodo-6-bromoacenaphthene (5) by stirring overnight
in the presence of a large excess of iodine. The new compounds were
fully characterized spectroscopically. 119Sn NMR spectra
suggest and interaction between the tin atoms and the neighboring
peri halogen atoms. Single-crystal X-ray studies on 1–4 and 6–8 revealed
Sn···X distances which are significantly less than
the sum of the van der Waals radii, while DFT calculations indicate
Wiberg bond indices of up to 0.11. Furthermore, there is evidence
of the onset of 3c–4e bonding, though according to natural
population analysis, the charge on tin is close to +2 in all compounds
studied. Electrostatic interactions may thus be another important
driving force for the close Br···Sn interactions, along
with the small covalent (donor–acceptor) contributions.