Compounds of formula I: 1 wherein R1, R2, R3, R4 and n have any of the values defined in the specification, and their pharmaceutically acceptable salts, are useful for inhibiting null-lactamase enzymes, for enhancing the activity of null-lactam antibiotics, and for treating null-lactam resistant bacterial infections in a mammal. The invention also provides pharmaceutical compositions, processes for preparing compounds of formula I, and novel intermediates useful for the synthesis of compounds of formula I.