The effect of different ligand substituents on the chemistry of a zinc–pyrazole anion host

Abstract

The influence of the pyrazole ligand on the previously reported anion host [ZnCl(Hpz^tBu)₃]⁺ (Hpz^tBu = 5-tert-butylpyrazole) has been investigated. Reaction of ZnCl₂ with 3 equiv. of 3{5}-cyclohexylpyrazole (Hpz^Cy) or 3{5}-phenylpyrazole (Hpz^Ph) affords [ZnCl(Hpz^R)₃]Cl (R = Cy or Ph), both of which undergo decomposition to [ZnCl₂(Hpz^R)₂] upon recrystallisation. A similar reaction using 3{5}-(thien-2-yl)pyrazole (Hpz^Tn) affords [ZnCl₂(Hpz^Tn)₂] only. The salts [ZnCl(Hpz^Ph)₃]BF₄, [ZnCl(Hpz^Cy)₃]X (X⁻ = NO₃⁻, ClO₄⁻, CF₃SO₃⁻ or ½SO₄²⁻) and [ZnBr(Hpz^Cy)₃]NO₃·H₂O have been isolated, by performing the above reactions in the presence of 1 equiv. of AgX. The cations in [ZnCl(Hpz^Cy)₃]NO₃ associate into a dimeric capsule encapsulating two nitrate anions. In contrast, [ZnBr(Hpz^Cy)₃]NO₃·H₂O contains hydrogen-bonded [NO₃·H₂O]_nⁿ⁻ chains enclosed within channels formed by the complex cations, while [ZnCl(Hpz^Cy)₃]ClO₄ forms a different hydrogen-bonded dimer with the anions occupying two shallow cavities in its surface. These data imply that anion binding by [ZnCl(Hpz^R)₃]⁺ depends strongly on the steric and inductive properties of the pyrazole ‘R’ substituents.

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Introduction

The continuing interest in anion coordination chemistry^1–5 has been promoted by its relevance to biochemistry and medicine.^6,7 Metal complexes are an important class of anion hosts, in which the metal ion can impart various properties to the system.^2–4,8–10 As well as playing a structural role in maintaining the shape of the anion-binding cavity, the metal ion usually provides a positive charge to the anion receptor that promotes guest binding on Coulombic grounds. In more specific examples the metal ion can itself form part of the anion-binding site by accepting coordinative bonds from the guest,¹⁰ or it can serve as an electrochemical, photometric or fluorescent reporter group in an anion sensor.² In a related application, organic ligands designed to coordinate cations and anions simultaneously can serve as extractants for metal salts in liquid–liquid extraction processes.¹¹

We and others have described anion hosts based on the [L_nM(Hpz)₃]⁺ motif, where [L_nM]⁺ is a transition metal fragment and Hpz is pyrazole or a substituted derivative of it (Scheme 1).⁹ While several anion complexes based on this motif have been crystallographically characterised,^12–20 two systems have been studied particularly thoroughly. First is [ZnCl(Hpz^tBu)₃]⁺ (Hpz^tBu = 5-tert-butyl-1H-pyrazole), where the three pyrazole NH groups and tert-butyl substituents occupy one face of the coordination tetrahedron, forming a bowl-shaped cavity.^14–16 Inorganic anions smaller than PF₆⁻ form crystalline 1 : 1 adducts with this cation, in which the anions are bound within the cavity by three N–H⋯X hydrogen bonds (Scheme 1A).^14,15 Larger anions such as PF₆⁻ and [Co(C₂B₉H₁₁)₂]⁻ are too big for the cavity and instead form 2 : 2 adducts with the complex, which dimerises through intermolecular N–H⋯Cl hydrogen bonding (Scheme 1B and C).^15,16 Rotation of one or two of the pyrazole groups at each zinc centre about their Zn–N bonds enlarges the cavities, allowing them to bind the larger anions.

Scheme 1 Two previously reported metal–pyrazole anion hosts, and their modes of guest binding. A, B and C: structures adopted by [ZnCl(Hpz^tBu)₃]⁺ in the presence of differently sized guests X⁻. D: one type of organometallic anion host.

Second, Pérez et al. have studied a series of octahedral compounds including [M(CO)₃(HPz^R)₃]⁺ (M = Mn or Re) and [Mo(η³-C₃H₅)(CO)₂(HPz^R)₃]⁺ (HPz^R = Hpz^tBu, 3,5-dimethyl-1H-pyrazole {Hpz′}, 5-phenyl-1H-pyrazole {Hpz^Ph} or another 1H-pyrazole derivative).^18–20 Where available, crystal structures imply that guest anions associate with only two of the three N–H donors at the complex cation (Scheme 1D). This may be a consequence of the small cavity afforded by three pyrazole groups linked by N–M–N angles of ∼90° at an octahedral metal ion, which requires severe structural distortions to bind even small guests. In contrast, the pyrazole rings in tetrahedral [ZnCl(Hpz^tBu)₃]⁺ are separated by a N–Zn–N angle of ca. 109.5°, which affords a larger three-fold cavity that can bind the smaller inorganic anions without any large structural changes (Scheme 1A). In solution, [ZnCl(Hpz^tBu)₃]⁺ and [Re(CO)₃(Hpz^R)₃]⁺ behave as typical anion hosts²¹ in that their affinities for different anions depend predominantly on the hydrogen-bond acceptor capability of the guests.^15,18,19 Accurate anion-binding constants were measured for the organometallic receptors, which fall in the range 2 × 10¹–6 × 10³ dm³ mol⁻¹ in CD₃CN.¹⁸

Anion-binding constants in solution for [Re(CO)₃(Hpz^R)₃]⁺ depend significantly on the pyrazole used. For example, the binding constants for Br⁻ by this receptor in CD₃CN follow the trend in Hpz^R:^18,19

Hpz′ > Hpz^Ph > Hpz^tBu ≈ pyrazole > indazole

This presumably reflects a combination of steric and electronic factors. Crystal structures of [Re(CO)₃(Hpz^R)₃]X salts with different pyrazole substituents show only small differences in their modes of anion binding (Scheme 1D), however, although more nucleophilic anions can displace pyrazole groups from the metal centre leading to structurally isomeric products [ReX(CO)₃(Hpz^R)₂]·Hpz^R.^18,19 To similarly determine the influence of pyrazole substituents on [ZnCl(Hpz^R)₃]⁺, we now describe a study of three such compounds with different 1H-pyrazole ligands Hpz^R (R = cyclohexyl {Cy}, phenyl {Ph} and thien-2-yl {Tn}).

Results and discussion

Reaction of zinc(II) halides with Hpz^R

Reaction of ZnCl₂ with 3 equiv. of Hpz^Cy or Hpz^Ph in MeOH affords a colourless solution, which gives an oily residue upon evaporation to dryness. Crystallisation of the residue from dichloromethane–pentane affords colourless crystalline solids [ZnCl(Hpz^R)₃]Cl (R = Cy or Ph), with the Hpz^Cy complex crystallising as a monohydrate. However, recrystallisation of both these complexes from chloroform–diethyl ether led to their partial decomposition, giving a mixture of the starting compounds and the new products [ZnCl₂(Hpz^R)₂]. When R = Ph, these two compounds could be isolated separately in pure form by crystallisation under the appropriate conditions. When R = Cy the compounds always co-crystallised, but small amounts of crystalline [ZnCl₂(Hpz^Cy)₂] were separated manually from powder [ZnCl(Hpz^Cy)₃]Cl·H₂O for microanalysis. Treatment of ZnCl₂ with 2 or 3 equiv. of Hpz^Tn under the same conditions gave [ZnCl₂(Hpz^Tn)₂] only.

A crystal structure analysis of [ZnCl(Hpz^Ph)₃]Cl confirmed that it adopts the expected connectivity (Scheme 1A). The complex cation contains a tetrahedral zinc centre, with a non-coordinated chloride ion bound in a pocket formed by the Hpz^Ph ligands through three N–H⋯Cl hydrogen bonds (Fig. 1). The bond lengths and angles about the zinc centre form a more regular tetrahedral coordination geometry compared to [ZnCl(Hpz^tBu)₃]Cl (Table 1).^14,15 The weak steric influence of the ligand phenyl substituents on the bound chloride guest is illustrated by the two phenyl rings C(8)–C(13) and C(19)–C(24) in Fig. 1, which are both oriented almost edge-on to the bound anion. The closest C–H⋯Cl contact between Cl(36) and these phenyl groups is 3.0 Å, outside the sum of the covalent radii of a H and a Cl atom.²²

Fig. 1 View of the asymmetric unit of [ZnCl(Hpz^Ph)₃]Cl. All H atoms not involved in hydrogen bonding have been omitted for clarity, and displacement ellipsoids are at the 50% probability level.

Table 1 Selected bond lengths and angles in the crystal structure of [ZnCl(Hpz^Ph)₂]Cl (Å, °)

Zn(1)–Cl(2)	2.2286(6)	Zn(1)–N(14)	2.0279(15)
Zn(1)–N(3)	2.0169(16)	Zn(1)–N(25)	2.0187(16)
Cl(2)–Zn(1)–N(3)	108.37(5)	N(3)–Zn(1)–N(14)	107.21(6)
Cl(2)–Zn(1)–N(14)	107.85(5)	N(3)–Zn(1)–N(25)	114.65(6)
Cl(2)–Zn(1)–N(25)	106.75(5)	N(14)–Zn(1)–N(25)	111.79(7)

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Crystal structures of all three [ZnCl₂(Hpz^R)₂] complexes were obtained (R = Cy, Ph and Tn). Although they are not isostructural, each compound crystallises as a hydrogen-bonded dimer, linked by two pairs of N–H⋯Cl hydrogen bonds (Fig. 2 and ESI†). The chloride ligand not involved in the dimerisation interaction accepts a short C–H⋯Cl interaction from a chloroform molecule in the solvate [ZnCl₂(Hpz^Ph)₂]·CHCl₃ (Fig. 2). In [ZnCl₂(Hpz^Cy)₂] and [ZnCl₂(Hpz^Tn)₂], which are both unsolvated, there is a longer but still significant contact between this Cl atom and a pyrazole H4 atom from a neighbouring molecule (ESI†). Interestingly, although there are several other known complexes of the type [ZnY₂(Hpz^R)₂] (Y⁻ = halide),^23,24 the hydrogen-bonded dimer structure adopted by these three compounds has only been observed in one previous example.²⁴ The coordination geometries about the zinc centres in the three compounds are somewhat flattened (Table 2 and ESI†). The origin of this distortion is unclear, since it is the compound with the bulkiest pyrazole substituents [ZnCl₂(Hpz^Cy)₂] that exhibits the least distorted coordination geometry (ESI†).

Fig. 2 View of the hydrogen-bonded dimer in the crystal structure of [ZnCl₂(Hpz^Ph)₂]·CHCl₃. All H atoms not involved in hydrogen bonding have been omitted for clarity, and displacement ellipsoids are at the 50% probability level. Symmetry code: (i) 1 − x, 1 − y, 1 − z.

Table 2 Selected bond lengths and angles in the crystal structure of [ZnCl₂(Hpz^Ph)₂]·CHCl₃ (Å, °). Comparable data for [ZnCl₂(Hpz^Tn)₂] and [ZnCl₂(Hpz^Cy)₂] are given in the ESI†

Zn(1)–Cl(2)	2.3189(6)	Zn(1)–N(4)	2.0355(17)
Zn(1)–Cl(3)	2.2470(5)	Zn(1)–N(15)	2.0105(17)
Cl(2)–Zn(1)–Cl(3)	119.86(2)	Cl(3)–Zn(1)–N(4)	106.80(5)
Cl(2)–Zn(1)–N(4)	101.94(5)	Cl(3)–Zn(1)–N(15)	104.38(5)
Cl(2)–Zn(1)–N(15)	103.06(5)	N(4)–Zn(1)–N(15)	121.96(7)

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These results contrast with the chemistry of [ZnCl(Hpz^tBu)₃]Cl, which is recrystallised intact from organic solvents. Clearly, the [ZnCl(Hpz^R)₃]⁺ cation is stabilised against nucleophilic attack by free Cl⁻ when R = tBu, compared to when R = Cy, Ph or Tn. This probably reflects a combination of steric factors, where the larger tBu substituents shield the zinc ion from attack; and, the relative basicities of these four monodentate pyrazoles. It is reasonable that the most electron-rich pyrazole ligand in this series, Hpz^tBu, should be the most difficult to substitute by other nucleophiles. Consistent with this suggestion, [ZnBr(Hpz^Cy)₃]Br (prepared from ZnBr₂ + 3 equiv. HPz^Cy as before), containing the less nucleophilic bromide ion, is stable to decomposition in solution.

Complexes of [ZnY(Hpz^R)₃]⁺ (Y⁻ = Cl⁻ or Br⁻) with other anions

A general route to complexes of [ZnY(Hpz^R)₃]X with different X⁻ anions is to react [ZnY(Hpz^R)₃]Y with 1 equiv. of AgX.^14–16 While we investigated several such reactions using all three of the substituted pyrazoles listed above, the high solubility of many of the products made isolating them as pure solids a challenge. The following compounds were obtained in pure form: [ZnCl(Hpz^Ph)₃]BF₄, [ZnCl(Hpz^Cy)₃]X (X = NO₃⁻, ClO₄⁻, CF₃SO₃⁻ and ½SO₄²⁻) and [ZnBr(Hpz^Cy)₃]NO₃·H₂O, three of which were crystallographically characterised.

The structures of the two nitrate salts [ZnY(Hpz^Cy)₃]NO₃ (Y⁻ = Cl⁻ and Br⁻) are notable, in being different from each other and from [ZnCl(Hpz^tBu)₃]NO₃, which adopts the connectivity in Scheme 1A.¹⁵ Both the Hpz^Cy complexes contain (approximately) tetrahedral [ZnY(Hpz^Cy)₃]⁺ cations with in-cavity nitrate anions (Fig. 3 and Table 3). However, the larger cavities afforded by the cyclohexyl, rather than tert-butyl, pyrazole substituents can accommodate additional guest species in addition to a nitrate ion, which manifests itself in different ways in the two structures. The molecules in [ZnCl(Hpz^Cy)₃]NO₃ associate into centrosymmetric, dimeric capsules encapsulating two nitrate guests, with each cation donating two N–H⋯O hydrogen bonds to one anion and one hydrogen bond to the other (Fig. 3 and Fig. 4). As well as placing both anions in the cavities of both cations, the capsule is formed by interdigitation of the cyclohexyl groups from each half of the supramolecule. Both features are evidence of a more open bowl-shaped cavity in [ZnCl(Hpz^Cy)₃]⁺, compared to [ZnCl(Hpz^tBu)₃]⁺.¹⁵ The capsule has approximate internal dimensions of 3.9 × 5.9 × 6.9 Å, although there are several openings in its walls (Fig. 4). The two coplanar nitrate guests are separated by 3.155(6) Å, slightly greater than the sum of their van der Waals radii (Fig. 3 and Fig. 4).

Fig. 3 Views of the formula units in the crystal structures of [ZnCl(Hpz^Cy)₃]NO₃ (top) and [ZnBr(Hpz^Cy)₃]NO₃·H₂O (bottom). All H atoms not involved in hydrogen bonding have been omitted for clarity, and displacement ellipsoids are at the 50% probability level. Only one orientation of the disordered cyclohexyl groups in [ZnCl(Hpz^Cy)₃]NO₃ is shown. Symmetry codes: (vi) 1 − x, −y, −z; (vii) x, −y + ½, z; (viii) ½ + x, ½ − y, ¾ − z.

Fig. 4 View of the hydrogen-bonded dimer in [ZnCl(Hpz^Cy)₃]NO₃, showing the pair of nitrate ions (space-filling) encapsulated by two [ZnCl(Hpz^Cy)₃]⁺ cations (50% displacement ellipsoids). Only one orientation of the disordered cyclohexyl groups in the structure is shown.

Table 3 Selected bond lengths and angles in the crystal structures of [ZnCl(Hpz^Cy)₃]NO₃ and [ZnBr(Hpz^Cy)₃]NO₃·H₂O (Å, °). Symmetry code: (vii) x, –y + ½, z

[ZnCl(Hpz^Cy)3]NO3		[ZnBr(Hpz^Cy)3]NO3·H2O
Zn(1)–Cl(2)	2.2493(7)	Zn(1)–Br(2)	2.4276(5)
Zn(1)–N(3)	2.020(2)	Zn(1)–N(3)	2.0381(19)
Zn(1)–N(14)	2.0229(19)	Zn(1)–N(14)	2.060(3)
Zn(1)–N(25)	2.034(2)	—	—
Cl(2)–Zn(1)–N(3)	108.52(6)	Br(2)–Zn(1)–N(3)	105.49(5)
Cl(2)–Zn(1)–N(14)	105.17(6)	Br(2)–Zn(1)–N(14)	102.82(7)
Cl(2)–Zn(1)–N(25)	108.92(7)	—	—
N(3)–Zn(1)–N(14)	113.61(8)	N(3)–Zn(1)–N(14)	112.28(6)
N(3)–Zn(1)–N(25)	109.70(8)	—	—
N(14)–Zn(1)–N(25)	110.72(8)	N(3)–Zn(1)–N(3^vii)	116.94(10)

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The complex molecule in [ZnBr(Hpz^Cy)₃]NO₃·H₂O has crystallographic m symmetry, with the mirror plane bisecting the cation, the nitrate ion and water molecule (Fig. 3 and Table 3). In contrast to its chloro analogue, the bowl-shaped cavities in [ZnBr(Hpz^Cy)₃]NO₃ are occupied by the molecule of water, as well as by the nitrate ion. The cation donates two N–H⋯O hydrogen bonds to the water molecule and one to the nitrate ion, while the water and nitrate are themselves linked by two O–H⋯O hydrogen bonds (Fig. 3). The latter interactions associate the guest species into polymeric [NO₃·H₂O]_nⁿ⁻ zig-zag chains, lying within channels formed by the complex cations (Fig. 5). The channels contain cavities of approximate internal dimensions 4.5 × 5.0 Å, separated by constrictions of 1.9 × 3.6 Å.

Fig. 5 Partial packing diagram of [ZnBr(Hpz^Cy)₃]NO₃·H₂O, showing the [NO₃·H₂O]_nⁿ⁻ chains (space-filling) within channels formed by the [ZnBr(Hpz^Cy)₃]⁺ cations (50% displacement ellipsoids). The view is approximately along the crystallographic (010) plane, with c vertical.

The crystal structure of [ZnCl(Hpz^Cy)₃]ClO₄ is different again from the two nitrate compounds, in that the cations associate into dimers through N–H⋯Cl hydrogen bonding (Scheme 1B; Fig. 6, ESI†). This dimerisation is probably sterically driven, by the binding of larger anions in the host cavity. Any steric clashes between the host and guest are relieved by rotation of one pyrazole ring on each zinc centre about its Zn–N bond, so that it presents the face of its five-membered ring to the bowl-shaped cavity rather than its N–H group. The same dimeric structure is exhibited by [ZnCl(Hpz^tBu)₃]PF₆, but not by [ZnCl(Hpz^tBu)₃]ClO₄ which adopts a mononuclear structure in the crystal (Scheme 1A).¹⁵ However, since the radius of the ClO₄⁻ ion is only 5% smaller than the PF₆⁻ ion,²⁵ we propose that the ClO₄⁻ guest must be around the maximum size that can be accommodated by a [ZnCl(Hpz^R)₃]⁺ host (Scheme 1A) without distorting itself.

Fig. 6 View of the hydrogen-bonded dimer in [ZnCl(Hpz^Cy)₃]ClO₄, showing the pair of perchlorate ions (space-filling) lying in the bowl-shaped cavities of two [ZnCl(Hpz^Cy)₃]⁺ cations (50% displacement ellipsoids).

In the light of these results, the synthesis of [ZnY(Hpz^Cy)₃]₂SO₄ (Y⁻ = Cl⁻ or Br⁻) was pursued, which could contain a sulfate ion inside a [ZnY(Hpz^Cy)₃]₂²⁺ capsule. A highly soluble solid [ZnCl(Hpz^Cy)₃]₂SO₄ was isolated from such a reaction, but single crystals of this product could not be obtained.

The electrospray (ES) mass spectra of [ZnCl₂(Hpz^R)₂] (R = Ph, Tn) and of [ZnCl(Hpz^Ph)₃]X (X⁻ = Cl⁻ and BF₄⁻) all show predominant mass peaks corresponding to [⁶⁴Zn(Hpz^R)₂]⁺ (R = Ph, m/z = 352; R = Tn, m/z = 364). In contrast, the [ZnCl(Hpz^Cy)₃]X (X⁻ = Cl⁻, NO₃⁻, ClO₄⁻ and ½SO₄²⁻) and [ZnBr(Hpz^Cy)₃]NO₃·H₂O, all exhibit a strong peak for the intact cation [⁶⁴ZnY(Hpz^Cy)₃]⁺ (Y⁻ = ³⁵Cl⁻, m/z = 549; Y⁻ = ⁷⁹Br⁻, m/z = 593). This is in agreement with our earlier observations about the higher stability of [ZnCl(Hpz^Cy)₃]⁺ when R = Cy, compared to the two aryl-substituted pyrazole complexes. There is no peak corresponding to substitution of the Y⁻ ligand by X⁻ in any of the [ZnY(Hpz^Cy)₃]X mass spectra, although some spectra show peaks assignable to [Zn(O₂CH)(Hpz^Cy)₃]⁺ and related species from the formate ion present in the ES carrier solution.

The ¹H and ¹³C NMR spectra of all the compounds in CDCl₃ show peaks from a single Hpz^R environment. ¹³C NMR spectra can indicate the solution stability of pyrazole complexes, since the pyrazole C3 and C5 peaks are often broadened in free pyrazoles owing to a tautomeric equilibrium²⁶ that is suppressed when the pyrazole is metal-coordinated.¹⁵ By this measure, all the compounds in this work except the two [ZnY(Hpz^Cy)₃]Y complexes (Y⁻ = Cl⁻ and Br⁻) undergo significant solvolysis in CDCl₃ since their pyrazole C3 and C5 NMR peaks were very broad, often to the point of being unobservable. This contrasts with [ZnCl(Hpz^tBu)₃]X salts, which are all stable in CDCl₃ by this criterion in the absence of a large excess of X⁻ anion.¹⁵

Conclusions

We have previously shown that [ZnCl(Hpz^tBu)₃]⁺ acts as a cavitand host towards a variety of inorganic anions, ranging from Cl⁻ to [Co(C₂B₉H₁₁)₂]⁻.^14–16 The geometry of the host–guest interaction depends on the size of the guest anion, with large guests causing the compound to lose its (approximate) C₃ symmetry. This leads to dimerisation of the complex, and enlargement of the guest-binding cavities on the surface of the dimer (Scheme 1B and C).^15,16 Importantly, however, the complex retains its integrity in the solid state and in weakly associating solvents, even in the presence of nucleophilic halide and nitrate ions.¹⁵ This study has shown that this unusually consistent behaviour in the [ZnCl(Hpz^R)₃]⁺ system is only observed when R = tBu.

The different stabilities of the chloride adducts [ZnCl(Hpz^R)₃]Cl with different ‘R’ substituents may relate to the basic pK_as of the different Hpz^R ligands used:

Hpz^tBu (pK_a = 3.3)²⁷ ≥ Hpz^Cy > Hpz^Ph (2.1)²⁷ ≥ Hpz^Tn

The positioning of Hpz^Cy and Hpz^Tn on the list are inferred from the inductive properties of their ‘R’ groups, since pK_a measurements for these pyrazoles are not available. It is reasonable that the more basic pyrazoles, which form stronger Zn–N bonds, should be more resistant to being nucleophilically displaced by the chloride guest, as observed. Alternatively, consideration of the degree of steric protection afforded by the pyrazole ‘R’ substituents leads to the same ordering. In either case, the stabilities of the different [ZnCl(Hpz^R)₃]Cl products to recrystallisation from chlorinated solvents perfectly follows the above trend (indefinitely stable when R = tBu, partial decomposition for R = Cy, complete decomposition when R = Ph while the compound could not be made for R = Tn). The ES mass spectra and ¹³C NMR spectra of the compounds are also broadly in agreement with these observations.

The steric influence of the pyrazole ‘R’ substituents is apparent in the nitrate adducts [ZnCl(Hpz^R)₃]NO₃ (R = tBu¹⁵ and Cy) and [ZnBr(Hpz^Cy)₃]NO₃ (ESI†). The tert-butyl groups in [ZnCl(Hpz^tBu)₃]⁺ are effectively cone-shaped, and form a consistent binding pocket that can only accommodate one anion guest. The same substituents are also large enough to prevent dimerisation of [ZnCl(Hpz^tBu)₃]⁺ centres through their cavities, so that all complexes of this host with smaller anions adopt the mononuclear structure (Scheme 1A) in the solid state.^14,15 In contrast, the disc-shaped cyclohexyl substituents in [ZnY(Hpz^Cy)₃]⁺ (Y⁻ = Cl⁻ or Br⁻) are conformationally flexible, and the crystal structures show there is apparently free rotation about the ipso-C{pyrazole}–C{cyclohexyl} bond when the guest-binding cavity is occupied. This allows the [ZnY(Hpz^Cy)₃]⁺ host to interact with two guest species, either two nitrate ions or a nitrate and a water molecule (ESI†).

In conclusion, any future anion host based on the [LM(Hpz^R)₃]⁺ design, containing a tetrahedral metal fragment, may require tert-butyl or similarly large pyrazole substituents to afford clean 1 : 1 host–guest chemistry (Scheme 1A).

Experimental

3{5}-Cyclohexylpyrazole,²⁸ 3{5}-phenylpyrazole²⁹ and 3{5}-(thien-2-yl)pyrazole³⁰ were prepared by the literature procedures, while other reagents and metal salts were purchased commercially and used as supplied. All manipulations were carried out in air using as-supplied AR grade solvents.

Synthesis of the complexes

[ZnCl(Hpz^Ph)₃]Cl and [ZnCl₂(Hpz^Ph)₂]. A mixture of ZnCl₂ (0.16 g, 1.2 mmol) and 3{5}-phenylpyrazole (0.50 g, 3.5 mmol) in methanol (50 cm³) was stirred at room temperature for 1 h, filtered and then evaporated to dryness. The oily residue was extracted into a minimum volume of dichloromethane and filtered. Addition of a large excess of pentane, and storage at −20 °C, afforded colourless needles of [ZnCl(Hpz^Ph)₃]Cl. Recrystallisation of this compound from chloroform–diethyl ether instead yielded crystalline [ZnCl₂(Hpz^Ph)₂]·CHCl₃, which loses its lattice solvent upon drying in vacuo.

For [ZnCl(Hpz^Ph)₃]Cl: found C, 57.2; H, 4.2; N, 14.9%. Calcd for C₂₇H₂₄Cl₂N₆Zn C, 57.0; H, 4.3; N, 14.8%. Electrospray mass spectrum: m/z = 145 [H₂pz^Ph]⁺, 352 [⁶⁴Zn(Hpz^Ph)₂]⁺. ¹H NMR spectrum (CDCl₃): δ 6.62 (d, 2.0 Hz, 3H, Pz H⁴), 7.47 (m, 9H, Ph H^3-5), 7.69 (d, 7.5 Hz, 6H, Ph H^2/6), 7.96 (d, 2.0 Hz, 3H, Pz H³), 13.0 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 104.5 (3C, Pz C⁴), 127.3 (3C, Ph C⁴), 127.8 (6C, Ph C^3/5), 128.7 (6C, Ph C^2/6), 129.6 (3C, Ph C¹), 141 (v br, 3C, Pz C³). No peak for Pz C⁵ was observed. IR (nujol): 3248m, 3174w, 3140w, 1610w, 1584w, 1569m, 1479s, 1351m, 1318w, 1304m, 1266m, 1213w, 1171w, 1132s, 1092s, 1031w, 969s, 946m, 923w, 915m, 896w, 847w, 807s, 802s, 765s, 703m, 692s, 673s, 611w cm⁻¹.

For [ZnCl₂(Hpz^Ph)₂]: found C, 50.9; H, 3.8; N, 13.3%. Calcd for C₁₈H₁₆Cl₂N₄Zn C, 50.9; H, 3.8; N, 13.2%. Electrospray mass spectrum: m/z = 145 [H₂pz^Ph]⁺, 352 [⁶⁴Zn(Hpz^Ph)₂]⁺, 392 [⁶⁴Zn(pz^Ph)(Hpz^Ph)(NCCH₃)]⁺. ¹H NMR spectrum (CDCl₃): δ 6.71 (d, 2.1 Hz, 3H, Pz H⁴), 7.48 (m, 9H, Ph H^3-5), 7.87 (d, 7.4 Hz, 6H, Ph H^2/6), 8.11 (d, 2.1 Hz, 3H, Pz H³), 14.4 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 103.8 (3C, Pz C⁴), 126.6 (6C, Ph C^3/5), 127.9 (3C, Ph C¹), 129.7 (6C, Ph C^2/6), 130.2 (3C, Ph C⁴), 142 (v br, 3C, Pz C³), 146 (v br, 3C, Pz C⁵). IR (nujol): 3141m, 3094w, 3061w, 1611w, 1588m, 1569m, 1483s, 1349m, 1316w, 1309m, 1271m, 1159w, 1133s, 1093s, 1030w, 964s, 944m, 916m, 896w, 837w, 812s, 801s, 763s, 693s, 683s, 672s, 635w cm⁻¹.

[ZnCl(Hpz^Cy)₃]Cl·H₂O and [ZnCl₂(Hpz^Cy)₂]. Method as above, using 3{5}-cyclohexylpyrazole (0.54 g, 3.5 mmol). Crystallisation of the crude residue from dichloromethane–pentane afforded white microcrystals of [ZnCl(Hpz^Cy)₃]Cl·H₂O. Recrystallisation of this material from chloroform–diethyl ether afforded the same white microcrystals, contaminated by colourless single crystals of [ZnCl₂(Hpz^Cy)₂]. A small amount of the latter compound was isolated by a Pasteur separation for microanalysis.

For [ZnCl(Hpz^Cy)₃]Cl·H₂O: found C, 53.5; H, 7.0; N, 14.0%. Calcd for C₂₇H₄₂Cl₂N₆Zn·H₂O C, 53.6; H, 7.3; N, 13.9%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 549 [⁶⁴Zn³⁵Cl(Hpz^Cy)₃]⁺, 559 [⁶⁴Zn(O₂CH)(Hpz^tBu)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.78 (m, 3H, Cy H¹), 6.20 (d, 2.0 Hz, 3H, Pz H⁴), 7.91 (d, 2.0 Hz, 3H, Pz H³), 13.4 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.0 (3C, Cy C⁴), 26.2 (6C, Cy C^3/5), 32.7 (6C, Cy C^2/6), 35.4 (3C, Cy C¹), 103.8 (3C, Pz C⁴), 141.2 (3C, Pz C³), 152.5 (3C, Pz C⁵). IR (nujol): 3154w, 3131w, 3059m, 1570m, 1485s, 1340w, 1315m, 1282m, 1237w, 1221w, 1182w, 1136m, 1111m, 1057w, 1033w, 983m, 956s, 908w, 893m, 855w, 823s, 808m, 795m, 754w, 639w cm⁻¹.

For [ZnCl₂(Hpz^Cy)₂]: found C, 49.4; H, 6.5; N, 12.8%. Calcd for C₁₈H₂₈Cl₂N₄Zn C, 49.5; H, 6.5; N, 12.8%.

[ZnCl₂(Hpz^Tn)₂]. Method as above, using 3{5}-(thien-2-yl)pyrazole (0.53g, 3.5 mmol). Crystallisation of the crude residue from dichloromethane–pentane afforded colourless crystals of the product in analytical purity. Yield 0.34 g, 64%. Found C, 38.7; H, 2.7; N, 13.0%. Calcd for C₁₄H₁₂Cl₂N₄S₂Zn C, 38.5; H, 2.8; N, 12.8%. Electrospray mass spectrum: m/z = 151 [H₂pz^Tn]⁺, 364 [⁶⁴Zn(Hpz^Tn)₂]⁺. ¹H NMR spectrum (CDCl₃): δ 6.52 (d, 2.2 Hz, 3H, Pz H⁴), 7.12 (dd, 3.7 and 5.0 Hz, 3H, Tn H⁴), 7.42 (d, 5.0 Hz, 3H, Tn H³), 7.55 (d, 3.7 Hz, 3H, Tn H⁵), 7.89 (d, 2.2 Hz, 3H, Pz H³), 12.9 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 104.7 (3C, Pz C⁴), 127.3, 127.8, 128.7 (all 3C, Tn C³-C⁵). No peaks for Pz C³ and C⁵ and Tn C1 were observed. IR (nujol): 3186m, 1797m, 1733w, 1663w, 1592s, 1508m, 1419s, 1300s, 1229m, 1171m, 1119s, 1090m, 1073s, 1043m, 960s, 900m, 850s, 837m, 808s, 794m, 777m, 705s, 667m, 643w, 603m cm⁻¹.

[ZnBr(Hpz^Cy)₃]Br. A solution of ZnBr₂ (0.27 g, 1.2 mmol) and 3{5}-cyclohexylpyrazole (0.54 g, 3.5 mmol) in MeOH (50 cm³) was stirred at room temperature for 1 h, then filtered and evaporated to dryness as before. Crystallisation of the residue from chloroform–pentane gave large colourless crystals of the compound that proved to suffer from twinning. Yield 0.53 g, 66%. Found C, 48.1; H, 6.3; N, 12.5%. Calcd for C₂₇H₄₂Br₂N₆Zn C, 48.0; H, 6.3; N, 12.4%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 363 [⁶⁴Zn(pz^tBu)(Hpz^Cy)]⁺, 409 [⁶⁴Zn(O₂CH)(Hpz^Cy)₂]⁺, 593 [⁶⁴Zn⁷⁹Br(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.80 (m, 3H, Cy H¹), 6.21 (d, 2.1 Hz, 3H, Pz H⁴), 8.01 (d, 2.1 Hz, 3H, Pz H³), 11.9 (v br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.1 (3C, Cy C⁴), 26.3 (6C, Cy C^3/5), 32.9 (6C, Cy C^2/6), 35.6 (3C, Cy C¹), 103.4 (3C, Pz C⁴), 141.7 (3C, Pz C³), 153.0 (3C, Pz C⁵). IR (nujol): 3152w, 3130w, 3061m, 1570m, 1485s, 1312m, 1279m, 1221w, 1181w, 1136m, 1108m, 1056w, 1034w, 984m, 955s, 909w, 894m, 855w, 817s, 795s, 758m, 635m cm⁻¹.

[ZnCl(Hpz^Ph)₃]BF₄. A mixture of ZnCl₂ (0.16 g, 1.2 mmol) and AgBF₄ (0.23 g, 1.2 mmol) was stirred in methanol (50 cm³) for 1 h, then filtered. 3{5}-Phenylpyrazole (0.50 g, 3.5 mmol) was then added, and the mixture stirred until the solid had dissolved. Filtration and evaporation of the solution gave an oily residue that was extracted into a minimum volume of dichloromethane and filtered. Addition of a large excess of pentane, and storage at −20 °C, afforded colourless needles. Yield 0.23 g, 31%. Found C, 52.6; H, 3.9; N, 13.9%. Calcd for C₂₇H₂₄BClF₄N₆Zn C, 52.3; H, 3.9; N, 13.6%. Electrospray mass spectrum: m/z = 145 [H₂pz^Ph]⁺, 352 [⁶⁴Zn(Hpz^Ph)₂]⁺, 387 [⁶⁴Zn³⁵Cl(Hpz^Ph)₂]⁺. ¹H NMR spectrum (CDCl₃): δ 6.69 (d, 2.2 Hz, 3H, Pz H⁴), 7.30 (m, 9H, Ph H^3-5), 7.70 (dd, 2.4 and 7.8 Hz, 6H, Ph H^2/6), 8.11 (d, 2.2 Hz, 3H, Pz H³). No pyrazole NH peak was observed. ¹³C NMR spectrum (CDCl₃): δ 104.3 (3C, Pz C⁴), 126.5 (6C, Ph C^3/5), 127.9 (3C, Ph C¹), 129.7 (6C, Ph C^2/6), 130.3 (3C, Ph C⁴). No peaks for Pz C³ and C⁵ were observed. IR (nujol): 3322m, 3145w, 1611w, 1587w, 1569m, 1481s, 1351m, 1302m, 1272m, 1132s, 1097, 1060vs, 963s, 944m, 916m, 893w, 839w, 802s, 762s, 691m, 682m, 667w, 655w cm⁻¹.

[ZnCl(Hpz^Cy)₃]X (X⁻ = NO₃⁻, ClO₄⁻, CF₃SO₃⁻ or ½SO₄²⁻). The syntheses of all of these complexes followed the same basic procedure, as described here for the nitrate salt. A mixture of ZnCl₂ (0.16 g, 1.2 mmol) and AgNO₃ (0.20 g, 1.2 mmol) was stirred in methanol (50 cm³) for 1 h, then filtered. 3{5}-Cyclohexylpyrazole (0.54 g, 3.5 mmol) was then added, and the mixture stirred for 30 min. Filtration and evaporation of the solution gave an oily residue that was extracted into a minimum volume of dichloromethane and filtered. Addition of a large excess of pentane, and storage at −20 °C, afforded colourless microcrystals. Analogous procedures using AgClO₄ (0.25 g, 1.2 mmol), AgCF₃SO₃ (0.31 g, 1.2 mmol) and Ag₂SO₄ (0.17 g, 0.6 mmol) afforded the corresponding salts of these anions. If necessary, the products were recrystallised from chloroform–diethyl ether. Final crystalline yields were ca. 55%.

For [ZnCl(Hpz^Cy)₃]NO₃: found C, 52.6; H, 6.7; N, 15.4%. Calcd for C₂₇H₄₂ClN₇O₃Zn C, 52.9; H 6.9; N 16.0%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 409 [⁶⁴Zn(O₂CH)(Hpz^Cy)₂]⁺, 549 [⁶⁴Zn³⁵Cl(Hpz^Cy)₃]⁺, 559 [⁶⁴Zn(O₂CH)(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.82 (m, 3H, Cy H¹), 6.22 (d, 2.0 Hz, 3H, Pz H⁴), 7.93 (d, 2.0 Hz, 3H, Pz H³), 12.7 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.0 (3C, Cy C⁴), 26.2 (6C, Cy C^3/5), 32.7 (6C, Cy C^2/6), 35.6 (3C, Cy C¹), 103.6 (3C, Pz C⁴), 142 (v br, 3C, Pz C³), 153 (v br, 3C, Pz C⁵). IR (nujol): 3176m, 3137w, 3080w, 1572m, 1487s, 1309w, 1289m, 1234w, 1179w, 1153w, 1136m, 1116s, 1065w, 1031w, 985m, 958s, 893m, 854w, 831m, 797m, 752w cm⁻¹.

For [ZnCl(Hpz^Cy)₃]ClO₄: found C, 49.9; H, 6.5; N, 13.0%. Calcd for C₂₇H₄₂Cl₂N₆O₄Zn C, 49.8; H, 6.5; N, 12.9%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 409 [⁶⁴Zn(O₂CH)(Hpz^Cy)₂]⁺, 549 [⁶⁴Zn³⁵Cl(Hpz^Cy)₃]⁺, 559 [⁶⁴Zn(O₂CH)(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.81 (m, 3H, Cy H¹), 6.25 (d, 2.0 Hz, 3H, Pz H⁴), 8.03 (d, 2.0 Hz, 3H, Pz H³), 11.8 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.0 (3C, Cy C⁴), 26.2 (6C, Cy C^3/5), 32.7 (6C, Cy C^2/6), 35.6 (3C, Cy C¹), 103.8 (3C, Pz C⁴), 142.4 (br, 3C, Pz C³), 153.6 (br, 3C, Pz C⁵). IR (nujol): 3244m, 3140w, 3114w, 1569m, 1490s, 1351w, 1309m, 1278m, 1228w, 1180w, 1120vs, 1108m, 1067m, 1049m, 1030w, 986m, 961s, 931w, 893w, 857w, 825m, 796s, 752w, 620m cm⁻¹.

For [ZnCl(Hpz^Cy)₃]CF₃SO₃: found C, 47.3; H, 5.9; N, 11.9%. Calcd for C₂₈H₄₂ClF₃N₆O₃SZn C, 48.0; H, 6.0; N, 12.0%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 549 [⁶⁴Zn³⁵Cl(Hpz^Cy)₃]⁺, 559 [⁶⁴Zn(O₂CH)(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.81 (m, 3H, Cy H¹), 6.21 (d, 2.0 Hz, 3H, Pz H⁴), 7.97 (d, 2.0 Hz, 3H, Pz H³), 12.1 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.0 (3C, Cy C⁴), 26.2 (6C, Cy C^3/5), 32.8 (6C, Cy C^2/6), 35.5 (3C, Cy C¹), 103.7 (3C, Pz C⁴), 142.1 (br, 3C, Pz C³), 154 (v br, 3C, Pz C⁵). IR (nujol): 3238m, 3135w, 3108w, 1571m, 1487m, 1301m, 1277w, 1233m, 1167m, 1133w, 1113m, 1030s, 986m, 960m, 893w, 854w, 800m, 769w, 637m cm⁻¹.

For [ZnCl(Hpz^Cy)₃]₂SO₄: found C, 53.8; H, 7.0; N, 14.0%. Calcd for C₅₄H₈₄Cl₂N₁₂O₄SZn₂ C, 54.1; H, 7.1; N, 14.0%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 399 [⁶⁴ZnCl(Hpz^Cy)₂]⁺, 549 [⁶⁴Zn³⁵Cl(Hpz^Cy)₃]⁺, 683 [⁶⁴Zn₂³⁵Cl₃(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.81 (m, 3H, Cy H¹), 6.19 (d, 2.0 Hz, 3H, Pz H⁴), 7.93 (d, 2.0 Hz, 3H, Pz H³), 13.6 (br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.1 (3C, Cy C⁴), 26.3 (6C, Cy C^3/5), 32.8 (6C, Cy C^2/6), 35.6 (3C, Cy C¹), 103.2 (3C, Pz C⁴). No peaks for Pz C³ and C⁵ were observed. IR (nujol): 3153w, 3130w, 1569m, 1485m, 1339w, 1314m, 1280m, 1221w, 1181w, 1136n, 1111s, 1057w, 1032m, 983m, 955s, 908w, 893m, 854w, 820m, 795m, 752w, 640m cm⁻¹.

[ZnBr(Hpz^Cy)₃]NO₃·H₂O. Method as above, using ZnBr₂ (0.27 g, 1.2 mmol) and AgNO₃ (0.20 g, 1.2 mmol). Slow diffusion of pentane vapour into a chloroform solution of the crude residue yielded single crystals of the product. Yield 0.38 g, 47%. Found C, 48.2; H, 6.4; N, 14.5%. Calcd for C₂₇H₄₄BrN₇O₄Zn C, 48.0; H, 6.6; N, 14.5%. Electrospray mass spectrum: m/z = 151 [H₂pz^Cy]⁺, 363 [⁶⁴Zn(pz^tBu)(Hpz^Cy)]⁺, 409 [⁶⁴Zn(O₂CH)(Hpz^Cy)₂]⁺, 559 [⁶⁴Zn(O₂CH)(Hpz^Cy)₃]⁺, 593 [⁶⁴Zn⁷⁹Br(Hpz^Cy)₃]⁺. ¹H NMR spectrum (CDCl₃): δ 1.2–2.0 (m, 30H, Cy H^2-6), 2.80 (m, 3H, Cy H¹), 6.20 (d, 2.1 Hz, 3H, Pz H⁴), 7.97 (d, 2.1 Hz, 3H, Pz H³), 11.9 (v br, 3H, Pz NH). ¹³C NMR spectrum (CDCl₃): δ 26.1 (3C, Cy C⁴), 26.3 (6C, Cy C^3/5), 32.7 (6C, Cy C^2/6), 35.6 (3C, Cy C¹), 103.5 (3C, Pz C⁴), 142.0 (3C, Pz C³), 153.4 (3C, Pz C⁵). IR (nujol): 3186m, 3139w, 3081w, 1566m, 1487s, 1351w, 1337w, 1289m, 1226w, 1182w, 1136m, 1111s, 1060w, 1028w, 984m, 957s, 909w, 894w, 853w, 832m, 795m, 752w cm⁻¹.

Single crystal X-ray structure determinations

All diffraction data were measured using a Bruker X8 Apex diffractometer, with graphite-monochromated Mo-K_α radiation (λ = 0.71073 Å) generated by a rotating anode. The diffractometer is fitted with an Oxford Cryostream low temperature device. Experimental details of the structure determinations in this study are given in Table 4. All the structures were solved by direct methods (SHELXS97³¹), and developed by full least-squares refinement on F² (SHELXL97³¹). Crystallographic figures were prepared using XSEED,³² which incorporates POVRAY.³³

Table 4 Experimental details for the single crystal structure determinations in this study

	[ZnCl2(Hpz^Ph)2]·CHCl3	[ZnCl2(Hpz^Tn)2]	[ZnCl2(Hpz^Cy)2]	[ZnCl(Hpz^Ph)3]Cl	[ZnCl(Hpz^Cy)3]NO3	[ZnBr(Hpz^Cy)3]NO3·H2O	[ZnCl(Hpz^Cy)3]ClO4
a R = Σ[|Fo| − |Fc|]/Σ|Fo|. b wR = [Σw(Fo^2 − Fc^2)/ΣwFo^4]^1^/2.
Molecular formula	C19H17Cl5N4Zn	C14H12Cl2N4S2Zn	C18H28Cl2N4Zn	C27H24Cl2N6Zn	C27H42ClN7O3Zn	C27H44BrN7O4Zn	C27H42Cl2N6O4Zn
M r	543.99	436.67	436.71	568.79	613.50	675.97	650.94
Crystal class	Triclinic	Monoclinic	Monoclinic	Orthorhombic	Triclinic	Orthorhombic	Monoclinic
Space group	P1̄	P21/c	P21/n	Pbca	P1̄	Pnma	C2/c
a/Å	8.8197(11)	13.0349(13)	10.6333(9)	17.685(2)	11.9059(9)	11.5875(16)	24.414(3)
b/Å	12.0309(13)	20.252(2)	11.5074(11)	17.185(2)	12.3024(9)	15.191(3)	16.933(2)
c/Å	12.1267(15)	14.7727(17)	17.8610(17)	17.758(3)	12.4108(9)	18.270(3)	19.071(4)
α/°	98.231(6)	—	—	—	66.546(3)	—	—
β/°	104.297(6)	113.506(5)	97.665(5)	—	74.875(4)	—	126.019(5)
γ/°	104.816(6)	—	—	—	71.417(4)	—	—
V/Å^3	1175.8(2)	3576.1(7)	2166.0(3)	5397.0(13)	1561.8(2)	3215.9(8)	6376.6(18)
Z	2	8	4	8	2	4	8
μ (Mo-Kα)/mm^−1	1.626	1.908	1.389	1.135	0.910	2.048	0.979
T/K	150(2)	150(2)	150(2)	150(2)	150(2)	150(2)	150(2)
Measured reflections	16 132	31 466	37 946	124 368	55 714	110 627	113 573
Independent reflections	6145	8216	5178	6562	7719	4191	7384
R int	0.034	0.065	0.027	0.061	0.039	0.054	0.108
R(F)^a	0.034	0.044	0.020	0.030	0.043	0.032	0.040
wR(F^2)^b	0.081	0.116	0.053	0.077	0.108	0.093	0.089
Goodness of fit	1.066	1.023	1.038	1.052	1.061	1.118	1.028

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[ZnCl₂(Hpz^Ph)₂]·CHCl₃, [ZnCl₂(Hpz^Cy)₂], [ZnCl(Hpz^Ph)₃]Cl and [ZnCl(Hpz^Cy)₃]ClO₄. No disorder was detected during refinement of these structures, and no restraints were applied. All non-H atoms were refined anisotropically, and all H atoms were placed in calculated positions and refined using a riding model.

[ZnCl₂(Hpz^Tn)₂]. There are two molecules in the asymmetric unit, labelled ‘A’ and ‘B’, that are associated into a unique dimer by N–H⋯Cl hydrogen bonding. Two of the four unique thienyl groups in the model are disordered over two sites: C(9A)–C(13A) (refined occupancy 0.73) and C(9A), S(10C)–C(13C) (0.27); and, C(9B)–C(13B) (0.80) and C(9B), S(10D)–C(13D) (0.20) so the ipso C atoms C(9A) and C(9B) are wholly occupied and shared between both disorder sites. The following restraints were applied to these residues: C–S = 1.72(2), thienyl C2–C3 = C4–C5 = 1.36(2) and thienyl C3–C4 = 1.42(2) Å. All non-H atoms with occupancy >0.5 were refined anisotropically, while all H atoms were placed in calculated positions and refined using a riding model.

[ZnCl(Hpz^Cy)₃]NO₃. Two of the three cyclohexyl substituents in the complex are disordered, both over two sites, labelled ‘A’ and ‘B’: C(19)–C(24) (refined A : B occupancies 0.61 : 0.39) and C(30)–C(35) (0.50 : 0.50). All disordered C–C bonds were restrained to 1.52(2) Å, and non-bonded 1,3-C⋯C contacts within each partial residue to 2.48(2) Å. The disorder sites C(19A)–C(24A) and C(30A^vi)–C(35A^vi) (symmetry code (vi): 1 − x, −y, −z) probably cannot be simultaneously occupied in the centrosymmetric dimer, owing to a steric contact C(23A)⋯C(32A^vi) = 3.33 Å. All crystallographically ordered non-H atoms were refined anisotropically, while all H atoms were placed in calculated positions and refined using a riding model.

[ZnBr(Hpz^Cy)₃]NO₃·H₂O. The asymmetric unit contains half a molecule, with Zn(1), Br(2), N(14)–C(19), C(22), nitrate ion N(23)–O(26) and water molecule O(27) all lying on the crystallographic mirror plane x, ½, z. No disorder was detected during refinement. All non-H atoms were refined anisotropically, while H atoms were placed in calculated positions and refined using a riding model. The positions of the H atoms in the half-water molecule O(27) could not be refined successfully, so these were also placed in calculated positions and refined with a riding model, assuming a tetrahedral disposition of H atoms and H bonds to O(27). This calculation procedure used means that the two O(27)–H bonds are artificially long, at 0.99 Å (0.85 Å would be a more meaningful value). This accounts for the apparent steric clash between H(9A) and H(27A) (which are only 2.3 Å apart in the model), and means that the metric parameters for the two hydrogen bonds for the two hydrogen bonds donated by O(27) should be treated with care.

There are three residual Fourier peaks >1.0 e Å⁻³ in the final difference map. Two of these lie in the nitrate ion, and may indicate a weak alternative disorder site for that group. The third peak lies 1.2 Å from Br(2). The deepest Fourier hole of −1.2 e Å⁻³ is <0.1 Å from Zn(1).

Other measurements

Electrospray (ES) mass spectra were obtained with a Waters ZQ4000 TOF spectrometer using a MeCN feed solution. All peaks show correct isotopic distributions for their assigned molecular ions. CHN microanalyses were performed by the University of Leeds School of Chemistry microanalytical service. ¹H NMR spectra were run on a Bruker ARX250 spectrometer, operating at 250.1 MHz. Infrared spectra were run as nujol mulls between NaCl winows, using a Nicolet Avatar 360 spectrophotometer.

Acknowledgements

This work was funded by the EPSRC and the University of Leeds. MAH thanks the Research Centre for Materials Science, Nagoya University, for a sabbatical that allowed the writing of this paper.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic figures and tables for the compounds in this work. CCDC reference numbers 744620–744626. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b9nj00412b

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