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 degrees C in diethyl ether followed by addition of the appropriate organotin reagent at 0 degrees C gave 5-bromo-6-(triphenylstannyl)acenaphthene (1), S-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. Sn-119 NMR spectra suggest and interaction between the tin atoms and the neighboring pen halogen atoms. Single-crystal X-ray studies on 1-4 and 6-8 revealed Sn center dot center dot center dot 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 center dot center dot center dot Sn interactions, along with the small covalent (donor-acceptor) contributions.