Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear Macrocyclic Complex: Synthesis, Structure, and Properties of [Ni₂L(μ‑O₂PH₂)]BPh₄ (L = N₆S₂ Donor Ligand)

The dinickel­(II) complex [Ni₂L­(ClO₄)]­ClO₄ (<b>1</b>), where L^2– represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [<i>n</i>Bu₄N]­H₂PO₂ to form the hypophosphito-bridged complex [Ni₂L­(μ-O₂PH₂)]⁺, which can be isolated as an air-stable perchlorate [Ni₂L­(μ-O₂PH₂)]­ClO₄ (<b>2</b>) or tetraphenylborate [Ni₂L­(μ-O₂PH₂)]­BPh₄ (<b>3</b>) salt. <b>3</b>·MeCN crystallizes in the triclinic space group <i>P</i>1̅. The bisoctahedral [Ni₂L­(μ-O₂PH₂)]⁺ cation has a N₃Ni­(μ_1,3-O₂PH₂)­(μ-S)₂NiN₃ core structure with the hypophosphito ligand attached to the two Ni^II ions in a μ_1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni₂L]²⁺ dication for more basic oxoanions. Treatment of [Ni₂L­(μ-O₂PH₂)]­ClO₄ with H₂O₂ or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for <b>3</b> reveals the presence of ferromagnetic exchange interactions between the Niⁱⁱ (<i>S</i> = 1) ions with a value for the magnetic exchange coupling constant <i>J</i> of +22 cm^–1 (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂). These results are additionally supported by DFT (density functional theory) calculations

Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear Macrocyclic Complex: Synthesis, Structure, and Properties of [Ni₂L(μ‑O₂PH₂)]BPh₄ (L = N₆S₂ Donor Ligand)

The dinickel­(II) complex [Ni₂L­(ClO₄)]­ClO₄ (<b>1</b>), where L^2– represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [<i>n</i>Bu₄N]­H₂PO₂ to form the hypophosphito-bridged complex [Ni₂L­(μ-O₂PH₂)]⁺, which can be isolated as an air-stable perchlorate [Ni₂L­(μ-O₂PH₂)]­ClO₄ (<b>2</b>) or tetraphenylborate [Ni₂L­(μ-O₂PH₂)]­BPh₄ (<b>3</b>) salt. <b>3</b>·MeCN crystallizes in the triclinic space group <i>P</i>1̅. The bisoctahedral [Ni₂L­(μ-O₂PH₂)]⁺ cation has a N₃Ni­(μ_1,3-O₂PH₂)­(μ-S)₂NiN₃ core structure with the hypophosphito ligand attached to the two Ni^II ions in a μ_1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni₂L]²⁺ dication for more basic oxoanions. Treatment of [Ni₂L­(μ-O₂PH₂)]­ClO₄ with H₂O₂ or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for <b>3</b> reveals the presence of ferromagnetic exchange interactions between the Niⁱⁱ (<i>S</i> = 1) ions with a value for the magnetic exchange coupling constant <i>J</i> of +22 cm^–1 (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂). These results are additionally supported by DFT (density functional theory) calculations

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)

Encapsulation of the 4‑Mercaptobenzoate Ligand by Macrocyclic Metal Complexes: Conversion of a Metallocavitand to a Metalloligand

Complexation of the ambidentate ligand 4-mercaptobenzoate (4-SH-C₆H₄CO₂H, H₂mba) by the macrocyclic complex [Ni₂L­(μ-Cl)]­ClO₄ (L^2– represents a 24-membered macrocyclic hexaazadithiophenolate ligand) has been examined. The monodeprotonated Hmba^– ligand reacts with the Ni₂ complex in a selective manner by substitution of the bridging chlorido ligand to produce μ_1,3-carboxylato-bridged complex [Ni₂L­(Hmba)]⁺ (<b>2⁺</b>), which can be isolated as an air-sensitive perchlorate (<b>2</b>ClO₄) or tetraphenylborate (<b>2</b>BPh₄) salt. The reactivity of the new mercaptobenzoate complex is reminiscent of that of a “free” thiophenolate ligand. In the presence of air, <b>2</b>ClO₄ dimerizes via a disulfide bond to generate tetranuclear complex [{Ni₂L}₂(O₂CC₆H₄S)₂]²⁺ (<b>3²⁺</b>). The auration of <b>2</b>ClO₄ with [AuCl­(PPh₃)], on the other hand, leads to monoaurated complex [Ni^II₂L­(mba)­Au^IPPh₃]⁺ (<b>4⁺</b>). The bridging thiolate functions of the N₆S₂ macrocycle are deeply buried and are unaffected/unreactive under these conditions. The complexes were fully characterized by electrospray ionization mass spectrometry, IR and UV/vis spectroscopy, density functional theory, cyclic voltammetry, and X-ray crystallography [for <b>3</b>(BPh₄)₂ and <b>4</b>BPh₄]. Temperature-dependent magnetization and susceptibility measurements reveal an <i>S</i> = 2 ground state that is attained by ferromagnetic coupling between the spins of the Ni^II ions in <b>2</b>ClO₄ (<i>J</i> = +22.3 cm^–1) and <b>4</b>BPh₄ (<i>J</i> = +20.8 cm^–1; <i>H</i> = −2<i>JS</i>₁<i>S</i>₂). Preliminary contact-angle and X-ray photoelectron spectroscopy measurements indicate that <b>2</b>ClO₄ interacts with gold surfaces

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^–