NHS-Nb was made from commercially available 2-norbornenecarboxylic acid (endo:exo mixture, exo shown in schemes) by the well-known procedure using thionyl chloride. Briefly, 2-norbornenecarboxylic acid was refluxed with excess neat thionyl chloride for 2 hours, followed by evaporation of remaining SOCl 2 at 300 mTorr. The resulting acid chloride, a thick, oily liquid, was reacted in situ with 1.1 eq. NHS and triethylamine, with THF as solvent.