Here described are compounds of formula I: wherein R1 and R2 are independently selected from C10 to C18 alkyl, C12 to C18 alkenyl, and oleoyl; R3 and R4 are independently selected from C1 to C6 alkyl and C2 to C6 alkanol; X is selected from —CH2—, —S—, and —O—, or X is absent; Y is selected from —(CH2)n, —S(CH2)n—, —O(CH2)n—, -thiophene-, —SO2(CH2)n—, and ester; n=1-4; a=1-4; b=1-4; c=1-4; and Z− is a counterion. Also described herein are compositions and pharmaceutical formulations including compounds of formula I which are useful for the delivery of therapeutic agents, and methods of using these compositions and formulations.