Compounds are provided having the following structure:




or a pharmaceutically acceptable salt, tautomer, or stereoisomer thereof, wherein a, b, c, d, G1, G2, L1, L2, R1a, R1b, R2a, R2b, R3a, R3b, R4a, R4b, R5, R6, R7, R8 and X are as defined herein. Use of the compounds as a component of lipid nanoparticle formulations for delivery of a therapeutic agent, nanoparticles comprising the compounds and methods for their use and preparation are also provided.