Compounds are provided having the following structure (I) or (II): or a pharmaceutically acceptable salt, tautomer or stereoisomer thereof, wherein R, R1, R2, L, X, L, G1, G2, Z, a1, a2 and n are independently as defined herein for each of structures (I) and (II). Use of the compounds as a component of lipid nanoparticle formulations for delivery of a therapeutic agent, compositions comprising the compounds and methods for their use and preparation are also provided.
