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The dinuclear NiII complex [Ni2(L²)][ClO4]2 (3) supported by the 28-membered hexaaza-dithiophenolate macrocycle (L²)2- binds the N3- ion specifically end-on yielding [Ni2(L²)(μ1,1-N3)][ClO4] (7) or [Ni2(L²)(μ1,1-N3)][BPh4] (8), while the previously reported complex [Ni2L¹(μ1,3-N3)][ClO4] (2) of the 24-membered macrocycle (L¹)2- coordinates it in the end-to-end fashion. A comparison of the X-ray structures of 2, 3, and 7 reveals the form-selective binding of complex 3 to be a consequence of its preorganized, channel-like binding pocket, which accommodates the azide anion via repulsive CH•••π interactions in the end-on mode. In contrast to [Ni2L¹(μ1,3-N3)][ClO4] (2), which features a S = 0 ground state, [Ni2(L²)(μ1,1-N3)][BPh4] (8) has a S = 2 ground state that is attained by competing antiferromagnetic and ferromagnetic exchange interactions via the thiolato and azido bridges with a value for the magnetic exchange coupling constant J of 13 cm-1 (H = −2JS1S2). These results are further substantiated by density functional theory calculations. The stability of the azido-bridged complex determined by isothermal titration calorimetry in MeCN/MeOH 1/1 v/v (log K11 = 4.88(4) at I = 0.1 M) lies in between those of the fluorido- (log K11 = 6.84(7)) and chlorido-bridged complexes (log K11 = 3.52(5)). These values were found to compare favorably well with the equilibrium constants derived at lower ionic strength (I = 0.01 M) by absorption spectrophotometry (log K11 = 5.20(1), 7.77(9), and 4.13(3) for N3-, F-, and Cl- respectively). [ABSTRACT FROM AUTHOR]

Brønsted superbases have wide applications in organic chemistry due to their ability to activate C-H bonds. The strongest neutral bases to date are substituted aminophosphazenes developed in the late 1980s by Reinhard Schwesinger. Since then, much effort has been expended to create even stronger neutral bases. In this article, the reasons for the instability of very basic compounds are investigated by means of high-level quantum-chemical calculations. Theoretical basicity limits are suggested for solutions as well as for the gas phase. A record-breaking superbase most likely to be synthesizable and stable at ambient conditions is proposed. Hexamethylphosphoramide is considered a reliable ionizing solvent for superbases.

The dinuclear Ni(II) complex [Ni2(L(2))][ClO4]2 (3) supported by the 28-membered hexaaza-dithiophenolate macrocycle (L(2))(2-) binds the N3(-) ion specifically end-on yielding [Ni2(L(2))(μ(1,1)-N3)][ClO4] (7) or [Ni2(L(2))(μ(1,1)-N3)][BPh4] (8), while the previously reported complex [Ni2L(1)(μ(1,3)-N3)][ClO4] (2) of the 24-membered macrocycle (L(1))(2-) coordinates it in the end-to-end fashion. A comparison of the X-ray structures of 2, 3, and 7 reveals the form-selective binding of complex 3 to be a consequence of its preorganized, channel-like binding pocket, which accommodates the azide anion via repulsive CH···π interactions in the end-on mode. In contrast to [Ni2L(1)(μ(1,3)-N3)][ClO4] (2), which features a S = 0 ground state, [Ni2(L(2))(μ(1,1)-N3)][BPh4] (8) has a S = 2 ground state that is attained by competing antiferromagnetic and ferromagnetic exchange interactions via the thiolato and azido bridges with a value for the magnetic exchange coupling constant J of 13 cm(-1) (H = -2JS1S2). These results are further substantiated by density functional theory calculations. The stability of the azido-bridged complex determined by isothermal titration calorimetry in MeCN/MeOH 1/1 v/v (log K11 = 4.88(4) at I = 0.1 M) lies in between those of the fluorido- (log K11 = 6.84(7)) and chlorido-bridged complexes (log K11 = 3.52(5)). These values were found to compare favorably well with the equilibrium constants derived at lower ionic strength (I = 0.01 M) by absorption spectrophotometry (log K11 = 5.20(1), 7.77(9), and 4.13(3) for N3(-), F(-), and Cl(-) respectively).