First Dinuclear B(II) Monocations with Bridging Guanidinate Ligands: Synthesis and Properties

Reaction between the diborane (4) B2Cl2(NMe2)2 and Li(hpp) (hpp− = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) leads to [(Me2N)B(μ-hpp)]2. This species can be protonated by HCl·OEt2 to give [(Me2HN)B(μ-hpp)]2Cl2featuring two B(II) cations with direct B−B bonding. The unsymmetrical monocation [(Me2N)B2(μ-hpp)2(NHMe2)]+ is also obtained. [(Me2HN)B(μ-hpp)]2Cl2 eliminates NHMe2 in a slow reaction leading to [ClB2(μ-hpp)2(NHMe2)]Cl and ultimately, presumably, to [ClB(μ-hpp)]2. We report the crystal structures of the two monocations [ClB2(μ-hpp)2(NHMe2)]Cl and [(Me2N)B2(μ-hpp)2(NHMe2)]Cl. The experimental results are accompanied by some quantum chemical density-functional theory calculations.