Reversible carboxylation of N-heterocyclic carbenesElectronic supplementary information (ESI) available: crystal structure data and ORTEP for 1,3-bis(2,6-diisopropylphenyl)imidazolylium-2-carboxylate (3). See http://www.rsc.org/suppdata/cc/b3/b311350g/ | Synapse