Coordination chemistry and applications of N-heterocyclic imino complexes of boron and silicon

N-heterocyclic imines (NHI) are structurally related to N-heterocyclic olefines (NHO), as well as N-heterocyclic carbenes (NHC). While all of these ligand systems possess high sigma-electron-donating character the NHI group... [ view full abstract ]

N-heterocyclic imines (NHI) are structurally related to N-heterocyclic olefines (NHO), as well as N-heterocyclic carbenes (NHC). While all of these ligand systems possess high sigma-electron-donating character the NHI group stands out with its additional pronounced pi-electron-donating properties.¹ Moreover, the steric hindrance of this ligand can be conveniently fine-adjusted to meet individual requirements for kinetic stabilization of a target system. Consequently, NHIs are most suitable for the stabilization of electron-deficient metal(loid) centers as found in low-coordinate complexes of boron and silicon.

           In recent years our group has successfully implemented NHIs to furnish electron-rich boron hydrides that engage in the activation of CN-bonds or elemental sulfur (1-4, Fig. 1).^2,3 Moreover, we accomplished the synthesis and isolation of a novel acyclic silylene and of silyl cations which verifies the strong electron-donating character of the NHI ligand (5-7, Fig. 2).^4,5 With focus on catalytic applications of metal hydrides we demonstrated the activity of NHI-substituted aluminum hydrides (8, 9) as catalysts for the hydroboration of alkynes or carbonyl groups with pinacolborane (Fig. 3).⁶ Apart from catalytic applications these aluminum hydrides have proven valuable precursors for the synthesis of rare types of aluminum borohydride, as well as aluminum chalcogenide species.^7-9  Incited by our recent finding of the insertion of CO₂ into the BH-bond of 3 we have expanded our studies on the potential of NHI-substituted metal hydrides for the activation of CO₂ and other small molecules (10, Fig. 4). Very recently, we have isolated rare examples of a low-coordinate dicationic complex of boron and an asymmetric borinium cation (11, 12, Fig. 4).

[1] T. Ochiai, D. Franz, S. Inoue, Chem. Soc. Rev. 2016, 45, 6327.

[2] D. Franz, S. Inoue, Chem. Asian J. 2014, 9, 2083.

[3] D. Franz, E. Irran, S. Inoue, Angew. Chem. Int. Ed. 2014, 53, 14264.

[4] S. Inoue, K. Leszczyńska, Angew. Chem. Int. Ed. 2012, 51, 8589.

[5] T. Ochiai, T. Szilvási, S. Inoue, Molecules 2016, 21, 1155.

[6] D. Franz, L. Sirtl, A. Pöthig, S. Inoue, Z. Anorg. Allg. Chem. 2016, 642, 1245.

[7] D. Franz, E. Irran, S. Inoue, Dalton Trans. 2014, 43, 4451.

[8] D. Franz, S. Inoue, Chem. Eur. J. 2014, 20, 10645.

[9] D. Franz, T. Szilvási, E. Irran, S. Inoue, Nat. Commun. 2015, 6, 10037.