Neutral thioether and selenoether macrocyclic coordination to Group 1 cations (Li–Cs) – synthesis, spectroscopic and structural properties

Abstract

The complexes [M(L)][BAr^F] (BAr^F = tetrakis{3,5-bis(trifluoromethyl)-phenyl}borate), L = [18]aneO₄S₂ (1,4,10,13-tetraoxa-7,16-dithiacyclooctadecane): M = Li–Cs; L = [18]aneO₂S₄ (1,10-dioxa-4,7,13,16-tetrathiacyclooctadecane): M = Li, Na, K; L = [18]aneO₄Se₂ (1,4,10,13-tetraoxa-7,16-diselenacyclooctadecane): M = Na, K, as well as [Na(18-crown-6)][BAr^F], are obtained in good yield as crystalline solids by reaction of M[BAr^F] with the appropriate macrocycle in dry CH₂Cl₂. X-ray crystallographic analyses of [Li([18]aneO₄S₂)][BAr^F] and [Li([18]aneO₂S₄)][BAr^F] show discrete distorted octahedral cations with hexadentate coordination to the macrocycle. The heavier alkali metal complexes all contain hexadentate coordination of the heterocrown, supplemented by M⋯F interactions via the anions, producing extended structures with higher coordination numbers; Na: CN = 7 or 8; K: CN = 8; Rb: CN = 9; Cs: CN = 8 or 10. Notably, all of the structures exhibit significant M–S/Se coordination. The crystal structures of the potassium and rubidium complexes show two distinct [M(heterocrown)]⁺ cations, one with M⋯F interactions to two mutually cis [BAr^F]⁻ anions, and the other with mutually trans [BAr^F]⁻ anions, giving 1D chain polymers. Solution multinuclear (¹H, ¹³C, ⁷Li, ²³Na, ¹³³Cs) NMR data show that the macrocyclic coordination is retained in CH₂Cl₂ solution.

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Introduction

Thioethers and selenoethers are soft, moderate σ-donor ligands with a high affinity for medium and low oxidation state transition metal ions in particular, whilst a significant body of work concerning high oxidation state early transition metals, f- and p-block acceptors has emerged over the last decade or so.¹ In contrast, examples showing thioether or selenoether coordination towards the oxophilic s-block cations are much rarer.^2–4 Within Group 2, while there are no complexes with neutral acyclic thio- or seleno-ether coordination, a small number of examples containing Ca–S/Se and Sr–S/Se coordination, based upon 15- and 18-membered oxa-thia and oxa-selena macrocycles, have been reported recently.^2,3 These include [CaI₂([18]aneO₂S₄)] and [MI₂([18]aneO₄E₂)] (M = Ca, Sr; E = S, Se; [18]aneO₂S₄ = 1,10-dioxa-4,7,13,16-tetrathiacyclooctadecane, [18]aneO₄S₂ = 1,4,10,13-tetraoxa-7,16-dithiacyclooctadecane; [18]aneO₄Se₂ = 1,4,10,13-tetraoxa-7,16-diselenacyclooctadecane), all of which are eight-coordinate with the macrocycle hexadentate (d(Ca–S/Se) ∼ 3.0 Å) and the iodides mutually cis. The lower lattice energies of the alkaline earth diiodides (MI₂) compared to MCl₂ or M(CF₃SO₃)₂, etc., leads to MI₂ being suitable metal sources for this chemistry due to their higher solubilities in non-competitive solvents.³

The lower charge : radius ratio of the Group 1 cations is expected to lead to considerably lower affinity of these cations for soft thio- or seleno-ether coordination. This is supported by early data on binding constants for alkali metal cations towards the oxa-thia analogues of 18-crown-6. Thus, the stability constant for K⁺/[18]aneO₅S (1,4,7,10,13-pentaoxa-16-thiacyclooctadecane) and K⁺/[18]aneO₄S₂ are more than 10² and 10⁶ times lower than for K⁺/18-crown-6, respectively, in methanol.⁵ Unsurprisingly therefore, well-characterised complexes of the Group 1 metals with similar soft donor ligands are extremely rare. There are no complexes with simple neutral acyclic thioethers, and only two prior examples with oxa-thia macrocycles, the K⁺ and Na⁺ cations with [18]aneO₄S₂ are structurally characterised. These are based upon lamellar halo-cuprate species, in which the K⋯S distances are 3.322(5)–3.496(4) Å and Na⋯S = 3.11(1) Å, although the presence of Cu–S coordination in these structures clearly has a significant influence.⁴ We have exploited the high solubility of alkali metal salts with diffuse, weakly coordinating anions in weakly coordinating (non-competitive) solvents, as a source of ‘naked’ alkali metal cations, as a synthetic entry to promote their coordination towards soft, neutral donor ligands. Even so, it was surprising to us to find that this can lead to homoleptic octathia coordination to Na⁺ in the macrocyclic complex [Na([24]aneS₈)][BAr^F] ([24]aneS₈ = 1,4,7,10,13,16,19,22-octathiacyclotetracosane; BAr^F = tetrakis{3,5-bis(trifluoromethyl)-phenyl}borate), containing distorted dodecahedral S₈-coordination at the sodium centre, with d(Na–S) = 2.9561(15)–3.0524(15) Å.⁶ DFT calculations suggested that the pre-organisation of the [24]aneS₈ macrocycle contributed to the successful isolation of this unique cation. Subsequently, using a similar approach we have also found that using even softer, neutral diphosphines leads to homoleptic Li⁺ and Na⁺ hexaphosphine cations in [Li(L–L)₃][Al{OC(CF₃)₃}₄] and [Na(L–L)₃][BAr^F] (L–L = Me₂PCH₂CH₂PMe₂ or o-C₆H₄(PMe₂)₂).⁷ Further interest in caesium–sulfur coordination comes from the important role of Cs₂CO₃ in the formation of thioether macrocycles via high dilution cyclisation reactions in dmf solution.⁸ The key role in the cyclisation reaction is the formation of a Cs⁺–thiolate ion pair, however, despite the widespread application of this procedure for thioether macrocycle formation, evidence of caesium–thioether coordination, i.e. following ring-closure, has remained elusive.

We report here the results of a systematic study of the preparation, spectroscopic and structural features of a series of complexes of the Group 1 cations with hexadentate, 18-membered ring macrocycles containing both hard (O) and soft (S, Se) donor functions, which demonstrate that coordination of neutral thioether functions to alkali metal cations is not limited to just sodium and potassium, but extends to examples with all five cations from Li⁺ to Cs⁺. We also report the first examples of neutral selenoether coordination to a Group 1 cation, including structural authentication for [M([18]aneO₄Se₂)][BAr^F], M = Na and K.

Experimental

All preparations were carried out under a dry dinitrogen atmosphere using standard Schlenk and glove box techniques. [Li(thf)₄][BAr^F] and Na[BAr^F]·2thf were synthesised using a slight modification of the literature procedure.⁹ The lithium salt was isolated as [Li(OH₂)₄][BAr^F], which was converted to the thf adduct by stirring in thf for 16 h over 4 Å molecular sieves. Filtration and removal of solvents afforded the thf adduct. Crude Na[BAr^F] was recrystallised from thf/n-hexane to remove the last traces of coloured impurity, resulting in the isolation of Na[BAr^F]·2thf. K[BAr^F], Rb[BAr^F] and Cs[BAr^F] were synthesised via cation exchange of Na[BAr^F]·2thf in water at 95 °C with excess (5 mol equiv.) KNO₃, RbNO₃ or CsNO₃, respectively.¹⁰ The macrocycles [18]aneO₄S₂, [18]aneO₂S₄ and [18]aneO₄Se₂ were prepared using literature procedures.¹¹ 18-Crown-6 was purchased from Sigma and dried using SOCl₂. CH₂Cl₂ was dried by distillation from CaH₂ and n-hexane distilled from Na/K alloy. ¹H and ¹³C{¹H} NMR spectra were recorded in CD₂Cl₂ solution at 295 K using a Bruker AV II-400 spectrometer and are referenced to the residual CH₂Cl₂ resonance. ⁷Li, ²³Na and ¹³³Cs NMR spectra were obtained in CH₂Cl₂ solution on a Bruker AV II-400 spectrometer at 298 K (unless otherwise stated) and referenced to a 0.1 mol dm⁻³ solution of LiCl, NaCl or CsNO₃ in D₂O, respectively. NMR properties: ⁷Li: I = 3/2, N = 92.6%, R_c = 1.54 × 10³, Q = −3.7 × 10⁻³⁰ m², Ξ = 38.87 MHz; ²³Na: I = 3/2, N = 100%, R_c = 5.24 × 10², Q = 0.10 × 10⁻²⁸, Ξ = 26.43; ³⁹K: I = 3/2, N = 93.3%, R_c = 2.69, Q = 5.5 × 10⁻³⁰, Ξ = 4.67; ⁸⁵Rb: I = 5/2, N = 72.1%, R_c = 43.4, Q = 0.25 × 10⁻²⁸, Ξ = 9.69; ¹³³Cs: I = 7/2, N = 100%, R_c = 2.75 × 10², Q = −0.3 × 10⁻³⁰, Ξ = 13.21.¹² Microanalyses were undertaken at London Metropolitan University.

X-ray crystallography

Crystals were obtained as described below. Details of the crystallographic data collection and refinement are in Table 1. Diffractometer: Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum rotating anode generator (λ₁ = 0.71073 Å) with VHF Varimax optics (70 or 110 μm focus). Cell determination and data collection: CrystalClear-SM Expert 3.1 b27, data reduction, cell refinement, and absorption correction: CrystalClear-SM Expert 2.1 b29.¹³ Structure solution and refinement were carried out using Olex2 or WinGX and software packages within.¹⁴ Disorder in the CF₃ groups of the [BAr^F]⁻ anions was present in all of the structures, which is often observed in compounds containing [BAr^F]⁻,¹⁵ and this was satisfactorily modelled using DFIX, DANG, DELU and SIMU restraints. Positional disorder was also present in the macrocycle ligands in one of the crystallographically independent cations in each of [Na(L)][BAr^F] complexes (L = [18]aneO₄S₂, [18]aneO₄Se₂ and [18]aneO₂S₄) and was modelled similarly. H-atoms were placed in geometrically-assigned positions with C–H distances of 0.95 Å (CH) or 0.98 Å (CH₂) and refined using a riding model with U_iso(H) = 1.2U_eq(C). enCIFer was used to prepare material for publication.¹⁶

Table 1 Crystallographic parameters^a

Compound	[Li([18]aneO4S2)][BAr^F]	[Na([18]aneO4S2)][BAr^F]	[K([18]aneO4S2)][BAr^F]	[Rb([18]aneO4S2)][BAr^F]
Formula	C44H36O4BF24LiS2	C44H36O4BF24NaS2	C88H72O8B2F48K2S4	C44H36O4BF24RbS2
M/g mol^−1	1166.60	1182.65	2397.51	1245.13
Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group (no.)	Pc (7)	Pc (7)	P21/c (14)	P1̄ (2)
a/Å	12.753(3)	12.682(3)	16.9189(12)	13.923(3)
b/Å	22.349(5)	22.573(5)	13.6483(10)	16.765(3)
c/Å	17.450(4)	17.452(4)	44.153(3)	22.402(5)
α/°	90	90	90	97.029(4)
β/°	95.618(4)	97.460(3)	97.6620(10)	105.185(5)
γ/°	90	90	90	90.376(3)
U/Å^3	4950(2)	4954(2)	10 104.4(13)	5005(2)
Z	4	4	4	4
μ(Mo-Kα)/mm^−1	0.240	0.249	0.318	1.204
F(000)	2352	2384	4832	2488
Total reflections	65 730	37 772	79 011	58 188
Unique reflections	21 600	16 023	17 773	17 632
R int	0.064	0.038	0.102	0.071
Goodness-of-fit on F^2	1.022	1.015	0.961	1.023
R 1 [Io > 2σ(Io)]	0.069	0.042	0.077	0.055
R 1 (all data)	0.103	0.051	0.126	0.083
wR2^b [Io > 2σ(Io)]	0.165	0.096	0.194	0.132
wR2 (all data)	0.188	0.102	0.229	0.146

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Compound	[Cs([18]aneO4S2)][BAr^F]	[Li([18]aneO2S4)][BAr^F]	[Na([18]aneO4Se2)][BAr^F]	[Na([18]aneO2S4)][BAr^F]
Formula	C44H36O4BCsF24S2	C44H36BF24LiO2S4	C44H36BF24O4NaSe2	C44H36BF24NaO2S4
M/g mol^−1	1292.57	1198.72	1276.45	1214.77
Crystal system	Orthorhombic	Monoclinic	Monoclinic	Monoclinic
Space group (no.)	Pbca (61)	P21 (4)	Cc (9)	C2/c (15)
a/Å	17.974(5)	16.596(4)	12.7830(12)	22.415(5)
b/Å	19.582(5)	19.366(5)	67.748(7)	8.9368(17)
c/Å	28.567(7)	23.794(6)	17.4535(18)	25.072(5)
α/°	90	90	90	90
β/°	90	93.056(5)	96.603(3)	99.731(4)
γ/°	90	90	90	90
U/Å^3	10 055(5)	7637(3)	15 015(3)	4950.2(17)
Z	8	6	12	4
μ(Mo-Kα)/mm^−1	0.950	0.312	1.617	0.330
F(000)	5120	3624	7584	2448
Total reflections	68 140	42 404	70 507	13 830
Unique reflections	11 507	28 887	26 208	5638
R int	0.097	0.063	0.065	0.047
Goodness-of-fit on F^2	1.016	1.024	0.984	0.988
R 1 [Io > 2σ(Io)]	0.060	0.086	0.065	0.038
R 1 (all data)	0.118	0.145	0.079	0.056
wR2^b [Io > 2σ(Io)]	0.127	0.191	0.155	0.090
wR2 (all data)	0.155	0.234	0.167	0.096

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a Common items: T = 100 K; wavelength (Mo-Kα) = 0.71073 Å; θ(max) = 27.5°. b R 1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^2]^1/2.
Compound	[Na(18-crown-6)][BAr^F]	[K([18]aneO2S4)][BAr^F]	[K([18]aneO4Se2)][BAr^F]
Formula	C44H36BF24NaO6	C44H36BF24KO2S4	C88H72B2F48K2O8Se4
M/g mol^−1	1150.53	1230.88	2585.11
Crystal system	Triclinic	Monoclinic	Monoclinic
Space group (No.)	P1̄ (2)	Cc (15)	P21/c (14)
a/Å	10.495(2)	22.558(4)	16.622(3)
b/Å	11.085(2)	8.9230(14)	13.876(2)
c/Å	21.361(4)	25.072(4)	44.516(8)
α/°	83.314(10)	90	90
β/°	77.451(8)	99.950(2)	98.567(2)
γ/°	79.135(10)	90	90
U/Å^3	2374.7(8)	4970.8(14)	10 153(3)
Z	2	4	4
μ(Mo-Kα)/mm^−1	0.175	0.403	1.668
F(000)	1160	2480	5120
Total reflections	27 127	15 982	58 673
Unique reflections	8375	5069	17 383
R int	0.068	0.039	0.043
Goodness-of-fit on F^2	1.064	1.043	1.023
R 1 [Io > 2σ(Io)]	0.099	0.050	0.065
R 1 (all data)	0.149	0.061	0.104
wR2^b [Io > 2σ(Io)]	0.260	0.123	0.163
wR2 (all data)	0.303	0.131	0.193

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Preparations

General method. M[BAr^F] was suspended in CH₂Cl₂ (10 mL) and a solution of the macrocycle in CH₂Cl₂ (5 mL) was added. Complete dissolution occurred and the reaction was stirred for 16 h. After this time the solution was filtered, concentrated to ∼3 mL and layered with n-hexane (20 mL) to form crystals. These were isolated by decanting away the supernatant and drying in vacuo. Specific details for individual complexes are below.

[Na(18-crown-6)][BAr^F]. Na[BAr^F]·2thf (50 mg, 0.05 mmol) and 18-crown-6 (13 mg, 0.1 mmol). Yield: 40 mg, 70%. Required for C₄₄H₃₆BF₂₄NaO₆ (1150.53): C, 45.93; H, 3.15. Found: C, 45.82; H, 2.97%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.64 ([24H], s, OCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.36 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.40 (CH, BAr^F C2/6), 129.49 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.19 (OCH₂). ²³Na NMR (298 K, CH₂Cl₂) δ = −14.4.

[Li([18]aneO₄S₂)][BAr^F]. [Li(thf)₄][BAr^F] (116 mg, 0.1 mmol) and [18]aneO₄S₂ (31 mg, 0.1 mmol). Yield: 75 mg of an off-white solid, 61%. Required for C₄₄H₃₆BF₂₄LiO₄S₂ (1166.60): C, 45.30; H, 3.11. Found: C, 45.26; H, 3.13%. ¹H NMR (CD₂Cl₂) δ = 7.73 ([8H], s, H2/6), 7.57 ([4H], s, H4), 3.68–3.72 ([16H], m, OCH₂), 2.83 ([8H], t, J = 5.7 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.34 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.39 (CH, BAr^F C2/6), 129.49 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.21 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 68.07, 68.04 (OCH₂), 32.19 (SCH₂). ⁷Li NMR (298 K, CH₂Cl₂): δ = +0.07.

[Na([18]aneO₄S₂)][BAr^F]. Na[BAr^F]·2(thf) (100 mg, 0.1 mmol) and [18]aneO₄S₂ (31 mg, 0.1 mmol). Yield: 55 mg, 44%. Required for C₄₄H₃₆BF₂₄NaO₄S₂ (1182.64): C, 44.69; H, 3.07. Found: C, 44.55; H, 3.01%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.68 (m, [16 H], OCH₂), 2.84 ([8H], t, J = 5.0 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.34 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.40 (CH, BAr^F C2/6), 129.48 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.54, 68.20 (OCH₂), 31.75 (SCH₂). ²³Na NMR (298 K, CH₂Cl₂): δ = −1.9.

[K([18]aneO₄S₂)][BAr^F]. K[BAr^F] (45 mg, 0.05 mmol) and [18]aneO₄S₂ (15 mg, 0.05 mmol). Yield: 25 mg, 38%. Required for C₄₄H₃₆BF₂₄KO₄S₂ (1198.75): C, 44.09; H, 3.03. Found: C, 43.90; H, 3.12%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.65 ([16H], m, OCH₂), 2.83, 2.80 (each [4H], t, J = 7.0 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.34 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.39 (CH, BAr^F C2/6), 129.47 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 70.84, 68.93 (OCH₂), 32.29 (SCH₂).

[Rb([18]aneO₄S₂)][BAr^F]. Rb[BAr^F] (95 mg, 0.1 mmol) and [18]aneO₄S₂ (31 mg, 0.1 mmol). Yield: 97 mg of an off-white solid, 74%. Required for C₄₄H₃₆BF₂₄O₄RbS₂ (1245.12): C, 42.44; H, 2.91. Found: C, 42.34; H, 2.99%. ¹H NMR (CD₂Cl₂): δ = 7.73 ([8H], s, H2/6), 7.57 ([4H], s, H4), 3.61–3.68 ([16H], m, OCH₂), 2.81 ([8H], t, J = 5.1 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.35 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.41 (CH, BAr^F C2/6), 129.48 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.21 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 70.56, 69.33 (OCH₂), 32.91 (SCH₂).

[Cs([18]aneO₄S₂)][BAr^F]. Cs[BAr^F] (100 mg, 0.1 mmol) and [18]aneO₄S₂ (31 mg, 0.1 mmol). Yield: 91 mg of an off-white solid, 67%. Required for C₄₄H₃₆BCsF₂₄O₄S₂ (1292.56): C, 40.89; H, 2.81. Found: C, 40.72; H, 2.91%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], s, H2/6), 7.57 ([4H], s, H4), 3.65 ([8H], t, J = 5.1 Hz, OCH₂), 3.62 ([8H], s, OCH₂), 2.81 ([8H], t, J = 5.1 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.32 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.36 (CH, BAr^F C2/6), 129.42 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.16 (C, q, J_C–F = 272 Hz, CF₃), 118.05 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.79, 69.30 (OCH₂), 33.03 (SCH₂). ¹³³Cs NMR (298 K, CH₂Cl₂): δ = +45.2.

[Li([18]aneO₂S₄)][BAr^F]. [Li(thf)₄][BAr^F] (116 mg, 0.1 mmol) and [18]aneO₂S₄ (33 mg, 0.1 mmol). Yield: 75 mg of an off-white solid, 63%. Required for C₄₄H₃₆BF₂₄LiO₂S₄ (1198.72): C, 44.09; H, 3.03. Found: C, 44.00; H, 2.98%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], s, H2/6), 7.57 ([4H], s, H4), 3.70 ([8H], t, J = 5.5 Hz, OCH₂), 2.90 ([8H], s, SCH₂), 2.82 ([8H], t, J = 5.5 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.35 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.39 (CH, BAr^F C2/6), 129.47 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.07 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.06 (OCH₂), 31.54, 31.03 (SCH₂). ⁷Li NMR (298 K, CH₂Cl₂): δ = +1.85.

[Na([18]aneO₂S₄)][BAr^F]. Na[BAr^F]·2thf (100 mg, 0.1 mmol) and [18]aneO₂S₄ (33 mg, 0.1 mmol). Yield: 86 mg, 65%. Required for C₄₄H₃₆BF₂₄NaO₂S₄ (1214.76): C, 43.51; H, 2.99. Found: C, 43.57; H, 2.87%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.74 ([8H], t, J = 6.0 Hz, OCH₂), 2.89 ([8H], s, SCH₂), 2.80 ([8H], t, J = 6.0 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.34 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.40 (CH, BAr^F C2/6), 129.46 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.21 (C, q, J_C–F = 272 Hz, CF₃), 118.07 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 68.87 (OCH₂), 31.58, 30.61 (SCH₂). ²³Na NMR (298 K, CH₂Cl₂): δ = +2.4.

[K([18]aneO₂S₄)][BAr^F]. K[BAr^F] (45 mg, 0.05 mmol) and [18]aneO₄S₂ (16 mg, 0.05 mmol). Yield: 41 mg of an off-white solid, 67%. Required for C₄₄H₃₆BF₂₄KO₂S₄ (1230.87): C, 42.94; H, 2.95. Found: C, 42.86; H, 2.89%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], s, H2/6), 7.57 ([4H], s, H4), 3.72 ([8H], t, J = 5.1 Hz, OCH₂), 2.89 ([8H], s, SCH₂), 2.84 ([8H], t, J = 5.1 Hz, SCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.34 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.41 (CH, BAr^F C2/6), 129.46 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.69 (OCH₂), 32.94, 32.07 (SCH₂).

[Na([18]aneO₄Se₂)][BAr^F]. Na[BAr^F]·2thf (100 mg, 0.1 mmol) and [18]aneO₄Se₂ (38 mg, 0.1 mmol). Yield: 71 mg, 56%. Required for C₄₄H₃₆BF₂₄NaO₄Se₂ (1276.44): C, 41.40; H, 2.84. Found: C, 41.29; H, 2.74%. ¹H NMR (CD₂Cl₂): δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.70 ([16H], m, OCH₂), 2.86 ([8H], t, J = 6.0 Hz, SeCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.35 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.41 (CH, BAr^F C2/6), 129.49 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.08 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 69.48, 68.54 (OCH₂), 24.45 (SeCH₂). ²³Na NMR (298 K, CH₂Cl₂): δ = +2.3.

[K([18]aneO₄Se₂)][BAr^F]. K[BAr^F] (45 mg, 0.05 mmol) and [18]aneO₄Se₂ (19 mg, 0.05 mmol). Yield: 47 mg, 79%. Required for C₄₄H₃₆BF₂₄KO₄Se₂ (1292.55): C, 40.89; H, 2.81. Found: C, 40.93; H, 2.69%. ¹H NMR (CD₂Cl₂) δ = 7.72 ([8H], br s, H2/6), 7.57 ([4H], s, H4), 3.74 ([8H], t, J = 6 Hz, OCH₂), 2.89 ([8H], s, OCH₂), 2.80 ([8H], t, J = 6 Hz, SeCH₂). ¹³C{¹H} NMR (CD₂Cl₂): δ = 162.33 (C, q, J_C–B = 49.9 Hz, BAr^F C1), 135.39 (CH, BAr^F C2/6), 129.46 (C, qq, ²J_C–F = 31.6, 2.9 Hz, BAr^F C3/5), 125.20 (C, q, J_C–F = 272 Hz, CF₃), 118.07 (CH, septet, ³J_C–F = 4.0 Hz, BAr^F C4), 70.80, 69.89 (OCH₂), 24.76 (SeCH₂).

Results and discussion

Coordination of the Group 1 cations to a range of 18-membered oxa-thia and oxa-selena macrocyclic ligands was achieved by the direct reaction of M[BAr^F] with the appropriate macrocycle in anhydrous CH₂Cl₂ solution (Scheme 1). The isolated complexes are highly soluble in CH₂Cl₂ and were obtained in good yield as white powders or colourless crystals. Their formulations were confirmed by microanalysis, solution ¹H, ¹³C{¹H}, ⁷Li, ²³Na and ¹³³Cs NMR spectroscopic data as appropriate, and by single crystal X-ray structure analysis. No solvent incorporation was evident in any of the products. The ¹H and ¹³C{¹H} NMR spectra confirm a 1 : 1 ratio of the macrocycle and the [BAr^F]⁻ anion in each case, but were otherwise not very informative.

Scheme 1 Synthesis of the complexes reported in this work.

Lithium-7 NMR spectra recorded for [Li([18]aneO₄S₂)]⁺ and [Li([18]aneO₂S₄)]⁺ each contain a singlet, at +0.07 and +1.85 ppm respectively, i.e. very small differences from LiCl in water (δ = 0). This is not unexpected given the small chemical shift range observed for ⁷Li (typically ca. 15 ppm).¹² For the sodium complexes, ²³Na NMR spectra (CH₂Cl₂) show a shift of the resonance to higher frequency as the S/Se donor atoms are introduced to the macrocycle (18-crown-6: −14.4 ppm; [18]aneO₄S₂: −1.9 ppm; [18]aneO₄Se₂: +2.3 ppm; [18]aneO₂S₄: +2.4 ppm). These chemical shifts compare with those for [Na([24]aneS₈)][BAr^F] (S₈ donor set, δ²³Na = +4.0),⁶ [Na(thf)_x][BAr^F] (δ²³Na = −4.8),¹⁶ [Na(Me₃tacn)₂][BAr^F] (δ²³Na = +6.2; Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) and [Na(Me₄cyclam)(thf)][BAr^F] (δ²³Na = +11.4; Me₄cyclem = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane).¹⁷ Small positive chemical shifts were also seen for [Na{Me₂P(CH₂)₂PMe₂}₃]⁺ and [Na{o-C₆H₄(PMe₂)₂}₃]⁺,⁷ whereas δ²³Na for solutions of NaBPh₄ with 15-crown-5 in a variety of O-donor solvents are reported to be to low frequency of the reference irrespective of the concentration of the crown ether.¹⁸ The quadrupole moment (Q) for the ¹³³Cs nucleus is small, hence NMR spectra are readily observed. The ¹³³Cs NMR spectrum recorded for [Cs([18]aneO₄S₂)][BAr^F] (CH₂Cl₂) is a singlet at +45.2 ppm, significantly to high frequency of the chemical shifts observed for [Cs(crown)]⁺ (crown = 12-crown-4, 15-crown-5, 18-crown-6) in various solvents,¹⁹ whilst [Cs(Me₆[18]aneN₆)]⁺ shows a singlet +54 ppm at 298 K.²⁰ The large Q values for ³⁹K and ⁸⁵Rb result in fast relaxation in these low-symmetry environments and no resonances were seen. Overall, the spectroscopic data for the complexes reported herein are consistent with coordination of the macrocycles to the alkali metal cations, the complexes being dynamic in solution, most likely through ‘ring-whizzing’ within the complex cations (it is unlikely that the [BAr^F]⁻ coordination is retained in solution). However, these measurements do not provide unequivocal evidence for coordination of the soft S or Se donor atoms.

X-ray structures and comparisons

The paucity of complexes with thio- or seleno-ether coordination to a Group 1 cation in the literature means that structural authentication of the new complexes was essential to establish their identities. Furthermore, it is difficult to draw comparisons with other structures, and determining the significance of particular metal–ligand interactions is somewhat uncertain. Table 2 provides the sums of the relevant radii, on one hand the sum of the ionic radii of the cations and the covalent radii of the donor atoms (O, S, Se and F) and on the other the sum of the van der Waals radii for the relevant metal-donor atom combinations. Distances close to the former are considered to be ‘normal’ coordinate bonds, while in identifying weak interactions, for example, between the metal cation and the CF₃ groups from [BAr^F]⁻ anions, we have chosen to include any M⋯X distances at least 0.5 Å below the sum of the van der Waals radii. Although the precise cut-off is somewhat arbitrary, it seems to be a reasonable basis on which to describe the overall coordination in these complexes. It is also pertinent to note that due to the disorder present in the CF₃ groups of the [BAr^F]⁻ anions in the structures, while the gross coordination at the metal and the ligand conformations are secure, care should be exercised in any detailed analyses of bond lengths and angles.

Table 2 Sum of the M–X radii for Group 1 cations with various relevant donor atoms

M–X	X = O		X = S		X = Se		X = F
	∑(ionic + cov radii)^a	∑(vdW radii)^b	∑(ionic + cov radii)^a	∑(vdW radii)^b	∑(ionic + cov radii)^a	∑(vdW radii)^b	∑(ionic + cov radii)^a	∑(vdW radii)^b
a Ionic radii of Group 1 cations taken from ref. 21 and covalent radii for ligand donor atoms taken from ref. 22. b van der Waals radii taken from ref. 23. c Based upon CN = 6. d Based upon CN = 8.
M = Li^c	1.56	3.62	1.95	4.01	2.10	3.94	1.47	2.58
M = Na^d	1.98	4.00	2.37	4.39	2.52	4.32	1.89	3.96
M = K^d	2.31	4.23	2.70	4.54	2.85	4.55	2.22	4.19
M = Rb^d	2.41	4.71	2.80	5.10	2.95	5.03	2.32	4.67
M = Cs^d	2.54	4.98	2.93	5.37	3.08	5.30	2.45	4.94

---

The crystal structure of [Li([18]aneO₄S₂)][BAr^F] comprises discrete cations (Fig. 1) and anions, with two of each in the asymmetric unit. Each Li cation is six-coordinate with the hexadentate macrocycle, leading to a severely distorted octahedral geometry. The Li–O and Li–S bond distances show a range of values, with d(Li–O) = 2.055(12) to 2.349(13) Å and d(Li–S) = 2.724(11) to 2.788(11) Å. The angles, S1–Li1–S2 = 106.2(4)° and S3–Li2–S4 = 111.3(4)°, whilst the angles involved in the five-membered chelate rings all vary between ∼70–80°, indicating a significant degree of distortion from a regular octahedron. The irregularity of the coordination may in part reflect the poor size match of the 18-membered ring for the small Li⁺ centre, coupled with the packing of the large [BAr^F]⁻ anions around the cations in the crystal lattice.

Fig. 1 The structure of the Li1-centred cation in [Li([18]aneO₄S₂)][BAr^F] with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. The Li2 cation adopts a similar geometry, with some variation in the bond distances and angles. Selected bond lengths (Å) and angles (°): Li1–O4 = 2.055(12), Li1–O2 = 2.094(12), Li1–O3 = 2.150(13), Li1–O1 = 2.349(13), Li1–S1 = 2.745(12), Li1–S2 = 2.761(11), O4–Li1–O3 = 78.5(5), O2–Li1–O1 = 75.4(4), O4–Li1–S1 = 81.2(4), O1–Li1–S1 = 71.7(3), O2–Li1–S2 = 77.7(4), O3–Li1–S2 = 75.2(4), S1–Li1–S2 = 106.2(4).

Substituting two further S donor atoms into the macrocyclic ring, as in [Li([18]aneO₂S₄)][BAr^F], also leads to a structure comprising of discrete cations and anions, with three crystallographically independent, but structurally similar, variants of each in the asymmetric unit. Each Li is six-coordinate, encapsulated by the macrocycle which is folded to accommodate a distorted octahedral geometry at lithium (Fig. 2). The distortion from ideal octahedral is significantly less than in the [18]aneO₄S₂ analogue above, and the conformation of the coordinated macrocycle has the –S(CH₂)₂O(CH₂)₂S– linkages occupying meridional coordination sites, placing the CCOC torsion angles anti. The Li–O and Li–S distances vary between the different cations, and are ∼2.1 Å and ∼2.6 Å, respectively. The latter appear to be slightly shorter than those in [Li([18]aneO₄S₂)]⁺ above.

Fig. 2 The structure of the Li1-centred cation in [Li([18]aneO₂S₄)][BAr^F] with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Li1–O2 = 2.082(18), Li1–O1 = 2.118(18), Li1–S3 = 2.539(16), Li1–S1 = 2.573(16), Li1–S2 = 2.593(16), Li1–S4 = 2.604(15), O2–Li1–S3 = 80.9(6), O1–Li1–S1 = 82.1(6), O2–Li1–S2 = 84.2(6), S1–Li1–S2 = 81.5(4), O1–Li1–S4 = 79.9(5), S3–Li1–S4 = 85.6(5).

The crystals of [Na([18]aneO₄S₂)][BAr^F] are isomorphous, but not isostructural, with their direct Li analogue, again with two crystallographically independent cations (the Na1-centred one of which is partly disordered, hence the structural description below is concerned with the Na2-centred cation) and two anions in the asymmetric unit. In this case, the Na-macrocycle coordination is quite similar to that in the Li analogue, with d(Na–O) in the range 2.355(4)–2.395(4) Å and d(Na2–S) (2.849(2) and 2.890(2) Å) all somewhat elongated. This is a consequence of the larger ionic radius of the Na⁺ centre, hence the ligand is less puckered, and the angles involved in the five-membered chelate rings span a narrower range (∼71–74°), with <S3–Na2–S4 = 129.77(8)°. Notably in this species there is an additional weak interaction to one F atom derived from a CF₃ group in the anion, Na2⋯F26ⁱ = 3.306(3) Å (Fig. 3), leading to overall seven-coordination at the sodium centre through weakly associated ion pairs.

Fig. 3 The structure of the Na2-centred cation in [Na([18]aneO₄S₂)][BAr^F] showing the interaction to a fluoride from the anion, with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Na2–O5 = 2.389(4), Na2–O6 = 2.355(4), Na2–O7 = 2.394(4), Na2–O8 = 2.395(4), Na2⋯F26ⁱ = 3.306(3), Na2–S3 = 2.849(2), Na2–S4 = 2.890(2), O6–Na2–O5 = 71.64(12), O7–Na2–O8 = 71.55(12), O5–Na2–S3 = 72.14(10), O8–Na2–S3 = 73.73(10), O6–Na2–S4 = 73.64(10), O7–Na2–S4 = 70.74(9), S3–Na2–S4 129.77(8). Symmetry operation: (i) −1 + x, −1 + y, z.

[Na([18]aneO₄Se₂)][BAr^F] crystallises with three crystallographically distinct cations (the Na3-centred cation shows some disorder and is therefore excluded from the structural description) and three [BAr^F]⁻ anions in the asymmetric unit. The Na1 centre is seven-coordinate through a puckered hexadentate macrocycle and one long, weak Na⋯F interaction (this is of borderline significance based on our criteria stated above; however the corresponding distances in the Na2 and Na3-centred cations are slightly shorter) (Fig. 4), with a near planar O₃Se donor set and the remaining donor atoms above and below that plane. Similar coordination environments are present in the other cations, although there is a spread of bond distances, d(Na–O) = 2.315(7)–2.479(7), d(Na–Se) = 2.942(2)–2.986(4) Å. The latter are in line with the increased covalent radius of Se over S; otherwise the Na coordination environments are very similar to [Na([18]aneO₄S₂)][BAr^F] above.

Fig. 4 The structure of the Na1-centred cation in [Na([18]aneO₄Se₂)][BAr^F] showing the interaction to fluoride from the anion, with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Na1–O3 = 2.332(7), Na1–O1 = 2.380(7), Na1–O4 = 2.393(6), Na1–O2 = 2.396(7), Na1–Se1 = 2.942(3), Na1–Se2 = 2.967(3), Na1⋯F5 = 3.501(6), O3–Na1–O4 = 72.3(2), O1–Na1–O2 = 71.9(2), O1–Na1–Se1 = 74.90(18), O4–Na1–Se1 = 73.16(17), O3–Na1–Se2 = 74.54(17), O2–Na1–Se2 = 71.74(16), Se1–Na1–Se(2) = 121.13(13).

The structure of [Na([18]aneO₂S₄)][BAr^F] is markedly different from its Li analogue above. It crystallises as a 1D chain polymer, with the Na in an eight-coordinate environment (Fig. 5a), and only one centrosymmetric cation and one anion (with crystallographic 2-fold symmetry) in the asymmetric unit. The macrocycle adopts a chair conformation with all four S atoms coordinated and the S₄ donor set planar, d(Na–S) = 2.8823(6), 3.0718(7) Å, with the two oxygen atoms above and below that plane, d(Na–O) = 2.5526(13) Å. Two weak Na⋯F interactions (2.8766(13) Å) from bridging [BAr^F]⁻ anions on opposite sides of the macrocycle give a distorted dodecahedral geometry overall at sodium, and give rise to the polymeric chain structure (Fig. 5b).

Fig. 5 (a) The structure of the centrosymmetric Na1-centred cation in [Na([18]aneO₂S₄)][BAr^F] showing the interactions to fluorides from the anions, with ellipsoids drawn at the 50% probability level. The other crystallographically independent cation adopts a similar structure. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Na1⋯F1 = 2.8766(13), Na1–O1 = 2.5526(13), Na1–S1 = 2.8823(6), Na1–S2 = 3.0718(7), O1–Na1–S1 = 67.26(3), O1–Na1–S2 = 66.22(3), S1ⁱ–Na1–S2 = 69.427(14), S1–Na1–S2ⁱ = 69.426(14). Symmetry operation: (i) 0.5 − x, 0.5 − y, 1 − z. (b) View of a portion of the polymeric chain with the [BAr^F]⁻ anions greyed out for clarity. Colour key: gold = Na, yellow = S, red = O, green = F, black = C.

To provide a benchmark comparison for the mixed donor macrocyclic complexes in this work, we also prepared and determined the structure of [Na(18-crown-6)][BAr^F]. The structure also shows a 1D chain polymer with eight-coordinate Na. There are two 50% occupancy (centrosymmetric) Na environments in the asymmetric unit, one of which (Na2-centred cation) shows severe rotational disorder of the macrocycle, while the other refines much better and is therefore the focus of the structural description and illustrated in Fig. 6a and b. The crown ether provides hexagonal planar coordination at Na, with irregular Na–O bond distances spanning >0.3 Å, and with a slightly puckered conformation as expected (<O–Na–O ∼ 62–65°), with two axial Na⋯F interactions (2.435(4) Å), similar motifs are evident in other salts containing the sodium-18-crown-6 cation.¹ The Na⋯F distances are very much shorter than in the mixed donor macrocyclic cations described above.

Fig. 6 (a) View of the structure around Na1 of the centrosymmetric [Na(18-crown-6)][BAr^F], with ellipsoids drawn at the 50% probability level. The second crystallographically independent cation in the asymmetric unit contains a disordered 18-crown-6, but is otherwise similar. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Na1–O1 = 2.804(4), Na1–O2 = 2.489(4), Na1–O3 = 2.605(4), Na1⋯F1 = 2.435(4). Symmetry operation: (i) 2 − x, 2 − y, −z. (b) View of a portion of the polymeric chain formed through the bridging [BAr^F]⁻ anions with the [BAr^F]⁻ anions greyed out for clarity. Colour key: gold = Na, red = O, green = F, black = C.

[K([18]aneO₂S₄)][BAr^F] is isostructural and isomorphous with [Na([18]aneO₂S₄)][BAr^F] described above, hence also forming a chain polymer with the K⁺ in an eight-coordinate environment via the O₂S₄ donor set from the hexadentate macrocycle (endocyclic) and weak axial interactions to CF₃ groups from the bridging [BAr^F]⁻ anions (Fig. 7). All of the bond distances at K1 are elongated by ca. 0.1 Å compared to the Na analogue. This is less than expected based purely on the difference in the ionic radii of the metal ions (0.33 Å for CN = 8), possibly reflecting a better size match between K⁺ and the [18]aneO₂S₄ binding cavity (although caution is required to avoid over-interpretation of such differences given that there is some disorder in the anion).

Fig. 7 View of the structure of the K1-centred cation in [K([18]aneO₂S₄)][BAr^F] showing the interactions to fluoride from the anions, with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): K1–O1 = 2.6253(18), K1⋯F7 = 2.9589(19), K1–S1 = 3.0445(8), K1–S2 = 3.1399(8), O1–K1–S1 = 65.74(4), O1–K1–S2 = 65.98(4), S1ⁱ–K1–S2 = 68.880(19). Symmetry operation: (i) −0.5 − x, 0.5 − y, −z.

The structure of [K([18]aneO₄S₂)][BAr^F] is formed of a 1D chain polymer with three crystallographically independent cations in the asymmetric unit (as well as three [BAr^F]⁻ anions). The K centres are eight-coordinate, through hexadentate endocyclic coordination of the macrocycle, with d(K–O) ∼ 2.7 Å and d(K–S) ∼ 3.2 Å, with two quite short K–F interactions (each ∼2.7 Å) completing the coordination environment (Fig. 8). The centrosymmetric K1- and K3-centred cations have significant K⋯F interactions on opposite sides of the macrocycle, while in the K2-centred cation the K⋯F interactions lie mutually cis, and <S3–K2–S2 = 154.90(6)°. Some puckering of the rings is evident from the angles subtended at K, which are all ∼64°.

Fig. 8 The structures of (a) the K1-centred cation and (b) the K2-centred cation in the asymmetric unit of [K([18]aneO₄S₂)][BAr^F], showing the interaction to fluoride from the anions, with ellipsoids drawn at the 50% probability level. The K3-centred cation is very similar to the K1-centred cation. Hydrogen atoms are omitted for clarity. The macrocycle around K1 is disordered positionally (up/down) around O2 with an occupancy split of roughly 3 : 2; the major component is shown. Selected bond lengths (Å) and angles (°): K1–F1 = 2.744(3), K1–S1 = 3.2161(12), K1–O1 = 2.703(3), K1–O2A = 2.706(7), K2–F7 = 2.690(6), K2–F25 = 2.733(3), K2–S2 = 3.2119(15), K2–S3 = 3.2075(16), K2–O5 = 2.709(3), K2–O3 = 2.731(3), K2–O4 = 2.744(3), K2–O6 = 2.750(3), S3–K2–S2 = 154.90(6). Symmetry operation: (i) 1 − x, 1 − y, −z. (c) View of a portion of the polymer chain with the [BAr^F]⁻ anions greyed out for clarity. Colour key: blue = K, yellow = S, red = O, green = F, black = C.

[K([18]aneO₄Se₂)][BAr^F] (Fig. 9) is isomorphous and isostructural with the tetraoxa-dithia analogue, and presents the first known complex containing potassium–selenoether coordination, d(K–Se) ca. 3.3 Å.

Fig. 9 The structures of the K1- and K2-centred cations in the asymmetric unit for [K([18]aneO₄Se₂)][BAr^F] showing the interactions to fluoride from the anions, with ellipsoids drawn at the 50% probability level. The K3-centred cation is very similar to the former. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): K1–O1 = 2.663(4), K1–O2 = 2.686(4), K1–F1 = 2.714(4), K1–Se1 = 3.3123(7), K2–O3 = 2.712(5), K2–O4 = 2.734(4), K2–O5 = 2.642(17), K2–O6 = 2.757(5), K2⋯F7 = 3.040(9), K2–F25 = 2.674(8), K2–Se2 = 3.3065(13), K2–Se3 = 3.307(4). Symmetry code: (i) −x, −y, 1 − z.

The structure of [Rb([18]aneO₄S₂)][BAr^F] is also a 1D chain polymer, very similar to the K analogue, with the macrocycle hexadentate in each form of the cation, d(Rb–S) ∼ 3.3 Å (Fig. 10), although the crystal is not isomorphous. All the Rb–F distances are comparable to the Rb–O distances, indicating significant interactions that lie well within the sum of vdW radii. The geometry at Rb1 is higher than in the K analogue, as there are now three CF₃ groups interacting with the Rb centre which appears to be ten-coordinate. The Rb2 (and Rb3: two half-occupancy centrosymmetric Rb centres in the asymmetric unit) are both 8-coordinate. The macrocycle donor set is closer to planar than in the K analogue.

Fig. 10 The structures of the two independent cations in [Rb([18]aneO₄S₂)][BAr^F] showing the interactions to fluoride from the anions, with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Rb1–O1 = 2.944(3), Rb1–O2 = 3.026(3), Rb1–O3 = 2.981(3), Rb1–O4 = 3.030(7), Rb1–S1 = 3.35564(12), Rb1–S2 = 3.2897(13), Rb1⋯F7 = 2.965(3), Rb1⋯F8 = 3.269(2), Rb1⋯F37 = 3.249(2), Rb1⋯F44 = 3.269(2), Rb2–O5 = 2.901(3), Rb2–O6 = 2.907(2), Rb2–S3 = 3.2690(12), Rb2⋯F23 = 2.818(7); O1–Rb1–O2 = 56.42(7), S2–Rb1–S1 = 146.62(3), O2–Rb1–S2 = 58.98(6), O3–Rb1–O4 = 57.34(14), O4–Rb1–S1 = 58.25(14), O1–Rb1–S1 = 57.99(6), O3–Rb1–S2 = 58.10(6), O5–Rb2–O6 = 59.10(8), O5–Rb2–S3 = 62.33(6), O6–Rb2–S3ⁱ = 61.72(6). Symmetry operation: (i) 1 − x, 1 − y, −z.

Finally, the structure of [Cs([18]aneO₄S₂)][BAr^F] (Fig. 11) shows one cation and one anion in the asymmetric unit, and forms a 2D sheet polymer. The hexadentate macrocycle occupies one face of the Cs⁺ cation (Cs–S ∼ 3.5 Å), with four CF₃ groups (each from a different [BAr^F]⁻ anion) also coordinated through the other face, although the precise coordination at Cs cannot be confirmed due to disorder of the CF₃ groups. There is also one longer Cs⋯F interaction through the centre of the macrocycle of ∼4.2 Å.

Fig. 11 The structure of the cation in [Cs([18]aneO₄S₂)][BAr^F] showing the interactions to fluoride from the anions, with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Cs1–O1 = 3.374(7), Cs1–O2 = 3.068(9), Cs1–O3 = 3.083(9), Cs1–O4 = 3.092(9), Cs1–S1 = 3.568(4), Cs1–S2 = 3.505(3), Cs1⋯F5 = 3.235(3), Cs1⋯F8ⁱⁱ = 3.525(3), Cs1⋯F10ⁱⁱⁱ = 3.448(7), Cs1⋯F12ⁱⁱⁱ = 3.319(4), Cs1⋯F20ⁱ = 3.345(4), Cs1⋯F14^iv = 4.27(2). Symmetry operations: (i) 1 − x, −0.5 + y, 1.5 − z; (ii) −0.5 + x, y, 1.5 − z; (iii) 1.5 − x, −0.5 + y, z; (iv) x, 0.5 − y, −0.5 + z.

Conclusions

We have isolated and fully characterised a series of alkali metal complexes (Li⁺ to Cs⁺) with soft, neutral thioether and selenoether coordination via saturated 18-membered oxa-thia and oxa-selena macrocycles, and containing the large, weakly coordinating [BAr^F]⁻ anion. There are no previously reported examples of lithium-, rubidium- or caesium-thioether coordination, and no Group 1 cation-selenoether complexes.

The solution lability of the Group 1 complex cations limits the information that can be obtained from spectroscopic studies on these complexes, hence we have determined their crystal structures to establish unequivocally the metal coordination environments present. The M–S and M–Se distances determined, together with the endocyclic conformations of the macrocycles in all of the new complexes (which contrast with the exocyclic conformations adopted by the metal-free macrocycles^3,24) are strongly indicative of significant, directional M–S/Se bonding interactions. Thus, in all the complexes reported, the macrocycle is hexadentate, while the flexibility of the saturated ring allows for very different degrees of ring puckering as the metal ion radius varies. Also notable, is the presence of M⋯F–CF₂– interactions of variable significance across all examples, except with the smallest Li⁺ cation. These contacts serve to fill available spaces in the metal coordination spheres. Hence, while discrete distorted octahedral cations with O₄S₂ and O₂S₄ donor sets are present in the Li⁺ salts, higher coordination numbers are observed in all of the other cations; Na: CN = 7 or 8; K: CN = 8; Rb: CN = 9; Cs: CN = 8 or 10.

Although unexpected, these M–S/Se interactions clearly make a favourable contribution to the stability of the cations since other conformations of the macrocycles with O-only coordination are possible.¹

Acknowledgements

We thank the EPSRC for supporting the SCFED project through a Programme Grant (EP/I033394/1). The SCFED Project (http://www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids.

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Footnote

† Additional Data Available: Original NMR spectra for all complexes are available (DOI: 10.5258/SOTON/380541) and can be downloaded via <http://dx.doi.org/10.5258/SOTON/380541>. CCDC 1410981 ([Na(18-crown-6-][BAr^F]), 1410982 ([Li([18]aneO₄S₂)][BAr^F]), 1410983 ([Na([18]aneO₄S₂)][BAr^F]), 1410984 ([K([18]aneO₄S₂)][BAr^F]), 1410985 ([Rb([18]aneO₄S₂)][BAr^F]), 1410986 ([Cs([18]aneO₄S₂)][BAr^F]), 1410987 ([Li([18]aneO₂S₄)][BAr^F]), 1410988 ([Na([18]aneO₂S₄)][BAr^F]), 1410989 ([K([18]aneO₂S₄)][BAr^F]), 1410990 ([Na([18]aneO₄Se₂)][BAr^F]) and 1410991 ([K([18]aneO₄Se₂)][BAr^F]). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt02583d

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