The syntheses of selenenate/seleninate esters and related derivatives by aromatic nucleophilic substitution (SNAr) reactions of 2-bromo-3-nitrobenzylalcohol () and 2-bromo-3-nitrobenzaldehyde () with Na2Se2/nBuSeNa are described. The reaction of with Na2Se2 at room temperature afforded 7-nitro-1,2-benzisoselenole(3 H) () instead of the desired diaryl diselenide . Oxidation of selenenate ester with hydrogen peroxide afforded the corresponding selenium(IV) derivative, 7-nitro-1,2-benzisoselenole(3 H) selenium oxide (). 2-(Butylselanyl)-3-nitrobenzaldehyde () was synthesized by treating compound with in situ generated nBuSeNa. The bromination reaction of selenide did not afford the expected arylselenenyl bromide 20, instead, it resulted in the formation of the unexpected 7-nitro-1,2-benzisoselenol(3 H)-3-ol () and 3,3′-oxybis(7-nitro-1,2-benzisoselenole(3 H)) (), respectively. The facile formation of heterocycles and is rationalized in terms of the aromatic ring strain in selenenyl bromide 20. The presence of intramolecular secondary Se⋅⋅⋅O interactions in esters , , , , and selenenic anhydride has been confirmed by single-crystal X-ray diffraction studies as well as computational studies. The presence of an intramolecular Se⋅⋅⋅O interaction in esters , , , , , and has been further proved by natural bond orbital (NBO) and atoms in molecules (AIM) calculations. Glutathione peroxidase-like (GPx) antioxidant activities of , , , , and related heterocycles such as 7-nitro-1,2-benzisoselenol(2 H)-3-one selenium oxide (), 7-nitro-1,2-benzisoselenol(2 H)-3-one (), and have been determined by the coupled reductase assay.