Azide Binding Controlled by Steric Interactions in Second Sphere. Synthesis, Crystal Structure, and Magnetic Properties of [Ni^II₂(L)­(μ_1,1-N₃)]­[ClO₄] (L = Macrocyclic N₆S₂ Ligand)

The dinuclear Ni^II complex [Ni₂(L²)]­[ClO₄]₂ (<b>3</b>) supported by the 28-membered hexaaza-dithiophenolate macrocycle (L²)^2– binds the N₃^– ion specifically <i>end-on</i> yielding [Ni₂(L²)­(μ_1,1-N₃)]­[ClO₄] (<b>7</b>) or [Ni₂(L²)­(μ_1,1-N₃)]­[BPh₄] (<b>8</b>), while the previously reported complex [Ni₂L¹­(μ_1,3-N₃)]­[ClO₄] (<b>2</b>) of the 24-membered macrocycle (L¹)^2– coordinates it in the <i>end-to-end</i> fashion. A comparison of the X-ray structures of <b>2</b>, <b>3</b>, and <b>7</b> reveals the form-selective binding of complex <b>3</b> to be a consequence of its preorganized, channel-like binding pocket, which accommodates the azide anion via repulsive CH···π interactions in the <i>end-on</i> mode. In contrast to [Ni₂L¹­(μ_1,3-N₃)]­[ClO₄] (<b>2</b>), which features a <i>S</i> = 0 ground state, [Ni₂(L²)­(μ_1,1-N₃)]­[BPh₄] (<b>8</b>) has a <i>S</i> = 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 <i>J</i> of 13 cm^–1 (<b>H</b> = −2<i>JS</i>₁<i>S</i>₂). 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 <i>K</i>₁₁ = 4.88(4) at <i>I</i> = 0.1 M) lies in between those of the fluorido- (log <i>K</i>₁₁ = 6.84(7)) and chlorido-bridged complexes (log <i>K</i>₁₁ = 3.52(5)). These values were found to compare favorably well with the equilibrium constants derived at lower ionic strength (<i>I</i> = 0.01 M) by absorption spectrophotometry (log <i>K</i>₁₁ = 5.20(1), 7.77(9), and 4.13(3) for N₃^–, F^–, and Cl^– respectively)

Cavitands Incorporating a Lewis Acid Dinickel Chelate Function as Receptors for Halide Anions

The halide binding properties of the cavitand [Ni₂(L^Me2H4)]²⁺ (<b>4</b>) are reported. Cavitand <b>4</b> exhibits a chelating N₃Ni­(μ-S)₂NiN₃ moiety with two square-pyramidal Ni^IIN₃S₂ units situated in an anion binding pocket of ∼4 Å diameter formed by the organic backbone of the (L^Me2H4)^2– macrocycle. The receptor reacts with fluoride, chloride (in MeCN/MeOH), and bromide (in MeCN) ions to afford an isostructural series of halogenido-bridged complexes [Ni₂(L^Me2H4)­(μ-Hal)]⁺ (Hal = F^– (<b>5</b>), Cl^– (<b>6</b>), and Br^– (<b>7</b>)) featuring a N₃Ni­(μ-S)₂(μ-Hal)­NiN₃ core structure. No reaction occurs with iodide or other polyatomic anions (ClO₄^–, NO₃^–, HCO₃^–, H₂PO₄^–, HSO₄^–, SO₄^2–). The binding events are accompanied by discrete UV–vis spectral changes, due to a switch of the coordination geometry from square-pyramidal (N₃S₂ donor set in <b>4</b>) to octahedral in the halogenido-bridged complexes (N₃S₂Hal donor environment in <b>5</b>–<b>7</b>). In MeCN/MeOH (1/1 v/v) the log <i>K</i>₁₁ values for the 1:1 complexes are 7.77(9) (F^–), 4.06(7) (Cl^–), and 2.0(1) (Br^–). X-ray crystallographic analyses for <b>4</b>(ClO₄)₂, <b>4</b>(I)₂, <b>5</b>(F), <b>6</b>(ClO₄), and <b>7</b>(Br) and computational studies reveal a significant increase of the intramolecular distance between two propylene groups at the cavity entrance upon going from F^– to I^– (for the DFT computed structure). In case of the receptor <b>4</b> and fluorido-bridged complex <b>5</b>, the corresponding distances are nearly identical. This indicates a high degree of preorganization of the [Ni₂(L^Me2H4)]²⁺ receptor and a size fit mismatch of the receptor binding cavity for anions larger than F^–