Compounds are provided having the following structure: or a pharmaceutically acceptable salt, tautomer or stereoisomer thereof, wherein R1a, R1b, R2a, R2b, R3a, R3b, R4a, R4b, R5, R6, R7, R8, R9, L1, L2, G1, G2, G3, a, b, c and d are as defined herein. 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.