A novel straightforward synthetic route to monocyclic 1,3,2,4-diazadiborinines has been developed by the sequential reaction of the NHC-coordinated iminoborane with bases and haloboranes (or borate). The first examples of monocyclic 1,3,2,4-diazadiborinines featuring different functional groups on the two B atoms have been synthesized and structurally characterized. Further derivatization of 4-bromophenyl-substituted 1,3,2,4-diazadiborinine has also been achieved, giving the biphenyl-substituted 1,3,2,4-diazadiborinine. The aromaticity of these newly synthesized 1,3,2,4-diazadiborinines was also studied by theoretical calculations.