Using carbon dioxide (CO 2 ) as a feedstock for commodity synthesis is an attractive means of reducing greenhouse gas emissions and a possible stepping-stone towards renewable synthetic fuels1,2. A major impediment to synthesizing compounds from CO 2 is the difficulty of forming carbon–carbon (C–C) bonds efficiently: although CO 2 reacts readily with carbon-centred nucleophiles, generating these intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon–heteroatom bonds or relatively acidic carbon–hydrogen (C–H) bonds3,4,5. These requirements negate the environmental benefit of using CO 2 as a substrate and limit the chemistry to low-volume targets. Here we show that intermediate-temperature (200 to 350 degrees Celsius) molten salts containing caesium or potassium cations enable carbonate ions (CO 3 2–) to deprotonate very weakly acidic C–H bonds (pK a > 40), generating carbon-centred nucleophiles that react with CO 2 to form carboxylates. To illustrate a potential application, we use C–H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)—a highly desirable bio-based feedstock6 with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET)7,8. Since 2-furoic acid can readily be made from lignocellulose9, CO 3 2–-promoted C–H carboxylation thus reveals a way to transform inedible biomass and CO 2 into a valuable feedstock chemical. Our results provide a new strategy for using CO 2 in the synthesis of multi-carbon compounds.