3 research outputs found
Azide Binding Controlled by Steric Interactions in Second Sphere. Synthesis, Crystal Structure, and Magnetic Properties of [Ni<sup>II</sup><sub>2</sub>(L)(μ<sub>1,1</sub>-N<sub>3</sub>)][ClO<sub>4</sub>] (L = Macrocyclic N<sub>6</sub>S<sub>2</sub> Ligand)
The
dinuclear Ni<sup>II</sup> complex [Ni<sub>2</sub>(L<sup>2</sup>)][ClO<sub>4</sub>]<sub>2</sub> (<b>3</b>) supported by the 28-membered
hexaaza-dithiophenolate macrocycle (L<sup>2</sup>)<sup>2–</sup> binds the N<sub>3</sub><sup>–</sup> ion specifically <i>end-on</i> yielding [Ni<sub>2</sub>(L<sup>2</sup>)(μ<sub>1,1</sub>-N<sub>3</sub>)][ClO<sub>4</sub>] (<b>7</b>)
or [Ni<sub>2</sub>(L<sup>2</sup>)(μ<sub>1,1</sub>-N<sub>3</sub>)][BPh<sub>4</sub>] (<b>8</b>), while the previously
reported complex [Ni<sub>2</sub>L<sup>1</sup>(μ<sub>1,3</sub>-N<sub>3</sub>)][ClO<sub>4</sub>] (<b>2</b>) of the 24-membered
macrocycle (L<sup>1</sup>)<sup>2–</sup> 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 [Ni<sub>2</sub>L<sup>1</sup>(μ<sub>1,3</sub>-N<sub>3</sub>)][ClO<sub>4</sub>] (<b>2</b>), which features a <i>S</i> =
0 ground state, [Ni<sub>2</sub>(L<sup>2</sup>)(μ<sub>1,1</sub>-N<sub>3</sub>)][BPh<sub>4</sub>] (<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<sup>–1</sup> (<b>H</b> = −2<i>JS</i><sub>1</sub><i>S</i><sub>2</sub>). 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><sub>11</sub> = 4.88(4) at <i>I</i> = 0.1 M) lies in between
those of the fluorido- (log <i>K</i><sub>11</sub> = 6.84(7))
and chlorido-bridged complexes (log <i>K</i><sub>11</sub> = 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><sub>11</sub> = 5.20(1), 7.77(9), and 4.13(3) for N<sub>3</sub><sup>–</sup>, F<sup>–</sup>, and Cl<sup>–</sup> 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-C<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>H, H<sub>2</sub>mba) by the macrocyclic complex
[Ni<sub>2</sub>L(μ-Cl)]ClO<sub>4</sub> (L<sup>2–</sup> represents a 24-membered macrocyclic hexaazadithiophenolate ligand)
has been examined. The monodeprotonated Hmba<sup>–</sup> ligand
reacts with the Ni<sub>2</sub> complex in a selective manner by substitution
of the bridging chlorido ligand to produce μ<sub>1,3</sub>-carboxylato-bridged
complex [Ni<sub>2</sub>L(Hmba)]<sup>+</sup> (<b>2<sup>+</sup></b>), which can be isolated as an air-sensitive perchlorate (<b>2</b>ClO<sub>4</sub>) or tetraphenylborate (<b>2</b>BPh<sub>4</sub>) 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>ClO<sub>4</sub> dimerizes via a disulfide bond
to generate tetranuclear complex [{Ni<sub>2</sub>L}<sub>2</sub>(O<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>S)<sub>2</sub>]<sup>2+</sup> (<b>3<sup>2+</sup></b>). The auration of <b>2</b>ClO<sub>4</sub> with [AuCl(PPh<sub>3</sub>)], on the other hand, leads to monoaurated
complex [Ni<sup>II</sup><sub>2</sub>L(mba)Au<sup>I</sup>PPh<sub>3</sub>]<sup>+</sup> (<b>4<sup>+</sup></b>). The bridging thiolate
functions of the N<sub>6</sub>S<sub>2</sub> 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>(BPh<sub>4</sub>)<sub>2</sub> and <b>4</b>BPh<sub>4</sub>]. 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<sup>II</sup> ions in <b>2</b>ClO<sub>4</sub> (<i>J</i> = +22.3 cm<sup>–1</sup>) and <b>4</b>BPh<sub>4</sub> (<i>J</i> = +20.8 cm<sup>–1</sup>; <i>H</i> = −2<i>JS</i><sub>1</sub><i>S</i><sub>2</sub>). Preliminary contact-angle and X-ray photoelectron
spectroscopy measurements indicate that <b>2</b>ClO<sub>4</sub> interacts with gold surfaces
Cavitands Incorporating a Lewis Acid Dinickel Chelate Function as Receptors for Halide Anions
The halide binding properties of
the cavitand [Ni<sub>2</sub>(L<sup>Me2H4</sup>)]<sup>2+</sup> (<b>4</b>) are reported. Cavitand <b>4</b> exhibits a chelating
N<sub>3</sub>Ni(μ-S)<sub>2</sub>NiN<sub>3</sub> moiety with
two square-pyramidal Ni<sup>II</sup>N<sub>3</sub>S<sub>2</sub> units
situated in an anion binding pocket of ∼4 Å diameter formed
by the organic backbone of the (L<sup>Me2H4</sup>)<sup>2–</sup> macrocycle. The receptor reacts with fluoride, chloride (in MeCN/MeOH),
and bromide (in MeCN) ions to afford an isostructural series of halogenido-bridged
complexes [Ni<sub>2</sub>(L<sup>Me2H4</sup>)(μ-Hal)]<sup>+</sup> (Hal = F<sup>–</sup> (<b>5</b>), Cl<sup>–</sup> (<b>6</b>), and Br<sup>–</sup> (<b>7</b>)) featuring
a N<sub>3</sub>Ni(μ-S)<sub>2</sub>(μ-Hal)NiN<sub>3</sub> core structure. No reaction occurs with iodide or other polyatomic
anions (ClO<sub>4</sub><sup>–</sup>, NO<sub>3</sub><sup>–</sup>, HCO<sub>3</sub><sup>–</sup>, H<sub>2</sub>PO<sub>4</sub><sup>–</sup>, HSO<sub>4</sub><sup>–</sup>, SO<sub>4</sub><sup>2–</sup>). The binding events are accompanied by discrete
UV–vis spectral changes, due to a switch of the coordination
geometry from square-pyramidal (N<sub>3</sub>S<sub>2</sub> donor set
in <b>4</b>) to octahedral in the halogenido-bridged complexes
(N<sub>3</sub>S<sub>2</sub>Hal donor environment in <b>5</b>–<b>7</b>). In MeCN/MeOH (1/1 v/v) the log <i>K</i><sub>11</sub> values for the 1:1 complexes are 7.77(9) (F<sup>–</sup>), 4.06(7) (Cl<sup>–</sup>), and 2.0(1) (Br<sup>–</sup>). X-ray crystallographic analyses for <b>4</b>(ClO<sub>4</sub>)<sub>2</sub>, <b>4</b>(I)<sub>2</sub>, <b>5</b>(F), <b>6</b>(ClO<sub>4</sub>), 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<sup>–</sup> to I<sup>–</sup> (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 [Ni<sub>2</sub>(L<sup>Me2H4</sup>)]<sup>2+</sup> receptor and a size fit mismatch
of the receptor binding cavity for anions larger than F<sup>–</sup>