The effect of fluorinated aryl substituents on the crystal structures of 1,2,3,5-dithiadiazolyl radicals

Abstract

The crystal structures of six new partially fluorinated aryl 1,2,3,5-dithiadiazolyls [ArCNSSN]˙ are reported [Ar = 2,6-F₂C₆H₃ (4); Ar = 3,4-F₂C₆H₃ (5); Ar = 3,5-F₂C₆H₃ (6); Ar = 2,3,6-F₃C₆H₂ (9); Ar = 2,4,6-F₃C₆H₂ (11); and Ar = 3,4,5-F₃C₆H₂ (12)] and compared with three previously reported structures in this series (Ar = 2,3-F₂C₆H₃ (1); Ar = 2,5-F₂C₆H₃ (3); Ar = 2,3,4-F₃C₆H₂ (7)]. Radical 4 is shown to be polymorphic. Molecular electrostatic potential maps have been used to rationalise these structures. These reveal that whilst single F atoms appear to have little structure-directing influence, combinations of an ortho-F on the phenyl ring and a heterocyclic ring N, or several F atoms located in mutually ortho positions with respect to each other, generate significant regions of negative charge which have a substantial structure-directing influence.

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Introduction

For some time, our group has been interested in the properties of 1,2,3,5-dithiadiazolyl (DTDA) radicals (Scheme 1). These neutral 7π-electron radicals are of great interest due to their potential as magnetic¹ or conducting² materials. The physical properties of these radicals are intimately linked to their solid state structures, and progress in the development of these radicals as molecular materials is strongly dependent upon the ability to control the structure at both the molecular and supramolecular levels.

Scheme 1 The 1,2,3,5-dithiadiazolyl radical.

At the molecular level EPR studies have revealed that the spin density distribution in these radicals appears insensitive to substituent R,^3,4 and the redox behaviour of aryl derivatives has been shown to follow a Hammett-type correlation, although the coefficient indicates only very modest changes in redox behaviour over a wide range of substituents.⁵ Conversely, few attempts have been made to undertake a systematic analysis of the factors which affect the supramolecular structure of these materials in the solid state, with perhaps the most notable exceptions being the identification⁶ of the CN^δ−⋯S^δ+ structure-directing functionality in cyano-substituted DTDAs, and an analysis of polymorphism in ClCNSSN˙ and HCNSSN˙, both of which exhibit heterocyclic S^δ+⋯N^δ− contacts.⁷

In the majority of cases DTDA derivatives adopt π*–π* dimeric structures (Fig. 1) with strong interactions between singly-occupied molecular orbitals on the monomers. The dimerisation energy in solution has been estimated at ca. 35 kJ mol⁻¹ for a range of derivatives (R = Ph, ^tBu and CF₃)⁸ and renders the majority of DTDAs diamagnetic in the solid state. This dimerisation energy is significant when placed in the context of hydrogen bonds (which lie approximately in the range 2–167 kJ mol⁻¹)⁹ and other non-covalent intermolecular interactions. One group of perfluorophenyl DTDA derivatives (R = p-XC₆F₄) has attracted particular attention^3a,6b,10,11 since a number retain their paramagnetic nature in the solid state. Of these, β-p-NCC₆F₄CNSSN˙ undergoes a phase transition to a canted antiferromagnetic state below 36 K,¹⁰ whereas p-O₂NC₆F₄CNSSN˙ orders as a ferromagnet below 1.3 K.^6b

Fig. 1 Modes of association in dithiadiazolyl radicals; (a) cis-cofacial; (b) twisted; (c) trans-antarafacial; (d) trans-cofacial.

The insensitivity of the spin density distribution on the heterocyclic ring to substituent R makes DTDAs particularly attractive for a systematic structural study since modification of the R group can lead to variation in solid state structure without influencing the spin distribution.⁴ As part of our on-going research in this area we have investigated the structural variations in a series of di- and tri-fluorophenyl-1,2,3,5-dithiadiazolyls (Scheme 2) to probe the ways in which the fluorine atom may hinder the dimerisation process. The role of fluorine in determining crystal structure has been a matter of some debate,¹² and this study offers an opportunity to clarify the effect of F, and its substitution patterns, on the observed crystal structures of DTDA molecules. These structures reveal that an examination of interactions between individual structure-directing groups is not always an appropriate methodology, and that a more holistic approach is required.

Scheme 2 The isomers of di- and trifluorophenyl-1,2,3,5-dithiadiazolyl.

Experimental

All the dithiadiazolyls 1–12 were synthesized using standard literature methods⁴ from commercially available starting materials. All reagents {Li[N(SiMe₃)₂], SCl₂, Zn/Cu (Aldrich); benzonitriles (Apollo, Avocado, Fluorochem, Lancaster)} were used as received. All reactions were carried out using standard double-manifold techniques and a glovebox (Saffron Scientific) which typically provided an atmosphere containing less than 10 ppm H₂O. Et₂O was freshly distilled off Na prior to use and THF freshly distilled off CaH₂. SO₂ (Aldrich) was stored over P₄O₁₀ before use. The in-house supply of nitrogen, dried by passing through a P₄O₁₀ column, was used in experimentation. The preparation of 1 is described as a typical protocol, though reductions were undertaken in either THF or liquid SO₂, and the chloride salt of 11 was synthesized in hexane.

Synthesis of 1

Li[N(SiMe₃)₂] (1.39 g, 8.3 mmol) and 2,3-difluorobenzonitrile (1.30 g, 8.3 mmol) were stirred in Et₂O (40 ml) overnight to yield a pale yellow solution. The solution was cooled to 0 °C and SCl₂ (1.5 ml, 24.45 mmol) added slowly with stirring. The resultant orange precipitate was stirred (3 h), filtered and pumped to dryness in vacuo. Zn/Cu couple (0.50 g, 7.69 mmol) was added and the mixture stirred in dry THF (20 ml) overnight and the resultant black suspension dried in vacuo. The solid residue was heated to 90 °C and sublimed to yield black/green blocks of the product on the cold finger. The crystals were removed from the cold finger and the process repeated until no new product could be sublimed. Yield 0.54 g, 28%. Found: C 37.79, H 1.34, N 12.51%. Required for C₇H₃F₂N₂S₂: C 38.71, H 1.38, N 12.90%. +EI-MS: 216.9706 [(C₆F₂H₃)CNSSN]⁺, 171 [(C₆F₂H₃)CNS]⁺, 139 [(C₆F₂H₃)CN]⁺, 112 [C₆F₂H₃]⁺, 78 [S₂N]⁺.

Yields were not optimized for this structural study, but typically fell in the range 10–35%. Recovered yields, microanalytical data, mass spectral data and sublimation conditions for 1–12 (excluding 1, 8 and 10) are available as ESI.†

X-Ray diffraction

Crystals of 1, 3, 4, 5, 6, 9, 11 and 12 for single crystal X-ray diffraction studies were grown by sublimation in vacuo and/or under ca. 1/3 atm of N₂. The single crystal structures of 1 and 3 have been reported previously.^13,14 We identified two different morphologies of 4 (4α and 4β) which were dependent upon sublimation conditions.¹⁵ Radicals 2, 7, 8 and 10 provided only polycrystalline samples despite multiple attempts at sublimation using a variety of experimental conditions (temperature and pressure). We have successfully determined the structure of one of these isomers (7) from powder diffraction using synchrotron radiation.¹⁶

Single crystals were mounted on the end of glass fibres using perfluorinated polyether oil.¹⁷ XRD data for 4α, 9 and 12 were obtained using a Siemens SMART 1K CCD three-circle diffractometer. Data for 5 and 6 were collected on a RIGAKU AFC7-R four circle diffractometer, whilst data for 4β and 11 were obtained on a Nonius Kappa CCD diffractometer. All diffractometers utilised monochromatic Mo Kα radiation (λ = 0.71073 Å) and the temperature was controlled using an Oxford Cryosystems cryostream device. Structures were solved and refined using SHELX97.¹⁸ The esds on intermolecular contacts were calculated using SHELXL97¹⁸ implemented through WinGX.¹⁹ Graphics were prepared using Mercury.²⁰

Selected crystal data for 1–12 are presented in Table 1, and selected structural parameters are collected in Table 2. Intramolecular bond lengths and angles are supplied as ESI.†

Table 1 Selected structural data for a series of fluorinated dithiadiazolyl radicals

	1	3	4α	4β	5	6	7	9	11	12
Chemical formula	C7H3F2N2S2	C7H3F2N2S2	C7H3F2N2S2	C7H3F2N2S2	C7H3F2N2S2	C7H3F2N2S2	C7H2F3N2S2	C7H2F3N2S2	C7H2F3N2S2	C7H2F3N2S2
Colour	Black/green	Black	Red	Red	Red	Red	Green-black	Green-black	Brown	Brown
Appearance	Blocks	Needles	Blocks	Needles	Blocks	Blocks	Polycrystalline bundles	Prisms	Needles	Blocks
Formula weight	217.2	217.2	217.23	217.23	217.23	217.23	234.96	235.23	235.23	235.23
Crystal system	Triclinic	Tetragonal	Monoclinic	Tetragonal	Monoclinic	Triclinic	Monoclinic	Tetragonal	Monoclinic	Triclinic
Space group	P-1	I41/a	P21/c	I41/a	P21/n	P-1	P21/n	I41/a	P21/c	P-1
a/Å	6.637(6)	30.214(2)	16.885(4)	30.168(1)	7.543(2)	7.058(3)	11.543(3)	30.483(2)	7.1195(1)	5.9462(1)
b/Å	8.768(11)	30.214(2)	11.989(4)	30.168(1)	11.193(2)	15.045(5)	20.6570(5)	30.483(2)	27.2091(3)	10.2733(2)
c/Å	13.546(5)	7.1829(1)	8.207(4)	7.1749(2)	18.688(4)	16.258(6)	7.0510(1)	7.117(1)	25.1504(4)	14.3236(2)
α/°	88.79(6)	90	90.00	90.00	90.00	66.79(3)	90	90.00	90.00	76.255(1)
β/°	86.37(5)	90	95.51(3)	90.00	93.87(3)	84.17(3)	100.367(1)	90.00	92.614(1)	79.899(1)
γ/°	80.40(9)	90	90.00	90.00	90.00	80.40(3)	90	90.00	90.00	78.649(1)
V/Å^3	776(1)	6556.3(11)	1653.7(11)	6529.9(4)	1574.2(6)	1563.2(10)	1654.14(9)	6613.2(11)	4866.95(12)	825.67(2)
Z	4	32	8	32	8	8	8	32	24	4
D calcd/g cm^−3	1.86	1.761	1.745	1.768	1.833	1.846	1.887	1.890	1.926	1.892
Temperature/K	150(2)	293	150(2)	180(2)	150(2)	200(2)	293	150(2)	180(2)	150(2)
µ/mm^−1	—	—	0.623	0.631	0.655	0.659	—	0.648	0.661	0.649
R int	—	—	0.0382	0.0329	0.0863	0.0496	—	0.0385	0.0899	0.0250
R1 [I > 2σ(I)]	—	—	0.0589	0.0337	0.0464	0.0727	—	0.0274	0.0488	0.0321
Reference	13	14	This work	This work	This work	This work	16	This work	This work	This work

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Table 2 Selected structural parameters for a series of fluorinated dithiadiazolyl radicals

	Twist angle between aryl and heterocyclic ring planes (°)	Intradimer S⋯S (Å)	Interdimer S⋯S (Å) (stacks)	‘In-plane’ S⋯S (Å) sum of vdW radii 3.20–4.06 Å ^29	S⋯N sum of vdW radii 3.20–3.63 ^29	S⋯F sum of vdW radii 2.90–3.41 Å ^29	Ref.
a contact distances measured using Mercury, a 1 + x, y, z, b 1 + x, −1 + y, z, c x, −1 + y, z, d x, y, −1 + z, e −0.25 + y, 0.75 − x, −0.25 + z, f −0.25 + y, 0.75 − x, 0.75 + z, g 0.75 − y, 0.25 + x, 0.25 + z, h 0.75 − y, 0.25 + x, −0.75 + z, i x, 1.5 − y, 0.5 + z, j −x, −0.5 + y, 0.5 − z, k −x, 1 − y, 1 − z, l x, 1.5 − y, −0.5 + z, m 1 − x, −y, 1 − z, n − 0.5 + x, 0.5 − y, 0.5 + z, o − x, −y, 1 − z, p −1 + x, y, z, q 1 − x, 1 − y, −z, r 1 − x, 1 − y, 1 − z, s x, y, 1 + z, t 1 − x, −y, 2 − z, u 0.5 − x, −0.5 + y, 1.5 − z, v 0.5 − x, −0.5 + y, 0.5 − z, w 0.75 − y, −0.25 + x, 0.75 − z, x 0.75 − y, −0.25 + x, 1.75 − z, y 1 − x, 0.5 + y, 1.5 − z, z x, 0.5 − y, 0.5 + z, aa −x, 0.5 + y, 1.5 − z, bb −x, 1 − y, −z.
1	5.9	3.020	S2⋯S4 ^a…3.628	—	—	F2⋯S4^b 3.072	13
	5.7					F2⋯S3 ^b 3.549
						F4⋯S2^c 3.028
						F4⋯S1 ^c 3.382
						F4⋯S4 ^c 3.247
3	21.2	3.203	S1⋯S3^d 4.006	S1⋯S1^e 3.517	S1⋯N1^g…3.447	F2b⋯S2^e 3.062	14
	18.3	3.119	S2⋯S4^d 4.084	S1⋯S3^e 3.554	S2⋯N1^g…3.322	F2b⋯S4^e 3.064
				S3⋯S3^e 3.578	S3⋯N3^g…3.776	F4b⋯S2^f 3.115
				S3⋯S1^f 3.587	S4⋯N3^g…3.390	F4b⋯S4^e 3.411
					S1⋯N3^h 3.864
					S2⋯N3^h 3.763
					S3⋯N1^g 3.777
					S4⋯N1^g 3.639
4α	48.72(27)	3.069(3)	—	S12⋯S11^i 3.361(3)	S11⋯N11^i 2.974(6)	F12⋯S11^j 3.376(5)	This work
	47.69(28)	3.129(3)		S22⋯S21^i 3.435(3)	S12⋯N11^i 3.102(7)	F12⋯S12^k 3.345(6)
				S22 S11^i 3.694(3)	S21⋯N21^i 3.284(7)	F16⋯S21^l 3.088(5)
					S22⋯N21^i 3.498(7)	F16⋯S22^d 3.234(5)
4β	29.42(4)	3.2167(7)	S11⋯S21^d 3.9939(8)	S11⋯S11^g 3.4885(6)	S11⋯N11^g 3.5217(17)	F12⋯S12^e 3.2297(13)	This work
	24.93(4)	3.0692(8)	S12⋯S22^d 4.1258(8)	S11⋯S21^h 3.5191(7)	S12⋯N11^g 3.2139(17)	F12⋯S22^e 3.1052(12)
				S21⋯S11^g 3.5491(7)	S11⋯N21^h 3.9402(17)	F22⋯S12^f 3.2407(12)
				S21⋯S21^g 3.5363(6)	S12⋯N21^h 3.8125(17)	F22⋯S22^e 3.7534(14)
					S21⋯N11^g 3.9081(17)
					S22⋯N11^f 3.6066(17)
					S21⋯N21^g 3.9046(17)
					S22⋯N21^g 3.2715(17)
5	11.99(18)	3.074(2)	—	—	S11⋯N21^m 3.262(4)	F14⋯S11^n 3.079(3)	This work
	8.46(17)	3.172(2)			S21⋯N11^m 3.284(4)	F14⋯S12^n 2.986(3)
						F13⋯S12^n 3.265(3)
						F24⋯S21^n 3.114(3)
						F24⋯S22^n 3.194(3)
						F23⋯S22^n 3.168(3)
						S11⋯F13° 3.203(3)
6	10.93(13)	3.113(3)	S11⋯S21^p 3.965(2)	S11⋯S32 3.666(3)	S11⋯N32 3.376(6)	F35⋯S21^r 3.255(5)	This work
	11.11(15)	3.111(3)	S12⋯S22^p 3.982(3)	S12⋯S32 3.640(3)	S11⋯N42 3.684(6)	F25⋯S31^q 3.062(4)
	9.45(13)	3.150(3)	S31⋯S41^p 3.945(3)	S11⋯S42 4.008(3)	S21⋯N42 3.205(6)	F25⋯S32^q 3.017(5)
	11.09(16)	3.195(3)	S32⋯S42^p 3.927(3)	S12⋯S42 3.769(3)	S21⋯N32^a 3.763(6)	F15⋯S41^q 2.938(4)
				S21⋯S32^a 3.858(3)	S12⋯N22^q 3.276(6)	F15⋯S42^q 2.936(5)
				S22⋯S32^a 3.744(3)	S22°⋯N12 3.284(6)	F15⋯S31^b 3.361(4)
				S21⋯S42 3.804(3)
				S22⋯S42 3.461(3)
7	11.8	3.295	S1⋯S4^s 3.839	—	S1⋯N4^m 3.237	(within chains)	16
	23.6	3.249	S2⋯S3^s 3.780		S4⋯N1^m 3.373	F2⋯S2^u 3.017
					S4⋯N4^m 3.820	F3⋯S1^u 3.248
					S1⋯N1^t 3.742	F3⋯S2^u 3.374
						F5⋯S3^v 3.157
						F6⋯S4^v 3.115
						F6⋯S3^v 3.365
						(between chains)
						F1⋯S4^m 3.264
						F4⋯S4^m 3.232
						F4⋯S1^m 3.352
9	30.37(3)	3.1893(7)	S11⋯S21^d 3.9832(7)	S11⋯S11^e 3.5901(5)	S11⋯N21^h 3.8346(12)	F12⋯S12^e 3.0245(9)	This work
	26.57(3)	3.0748(7)	S12⋯S22^d 4.0810(8)	S11⋯S21^e 3.6488(5)	S12⋯N21^h 3.9361(12)	F12⋯S22^e 3.1447(9)
				S21⋯S21^e 3.5290(4)	S11⋯N11^g 3.4516(12)	F22⋯S12^f 3.1729(9)
				S21⋯S11^f 3.5141(5)	S12⋯N11^g 3.3409(12)	F22⋯S22^e
					S21⋯N11^g 3.9241(12)	3.4884(10)
					S22⋯N11^g 3.8386(12)	F13⋯S12^w 3.456(3)
					S21⋯N21^g 3.7735(11)	F23⋯S22^x 3.4572(11)
					S22⋯N21^g 3.2313(11)
11	29.24(6)	3.2020(12)	S11⋯S21^a 3.9936(12)	(in pinwheels)	S11⋯N52 3.347(3)	(in pinwheels)	This work
	30.57(6)	3.1455(12)	S12⋯S22^a 4.0266(12)	S11⋯S52 3.3929(12)	S12⋯N52 3.567(3)	F12⋯S61^r 2.9420(19)
	27.98(6)	3.2552(11)	S31⋯S41^p 3.9318(11)	S21⋯S62 3.6130(12)	S21⋯N62 3.121(3)	F12⋯S51^r 3.346(2)
	30.09(5)	3.1320(11)	S32⋯S42^p 4.0626(11)	S11⋯S51^r 3.7249(12)	S22⋯N62 3.895(3)	F22⋯S51^r 3.3221(19)
	28.58(5)	3.1011(12)	S51⋯S61^a 4.0669(11)	S11⋯S52^r 3.5469(12)	S51⋯N11^r 3.097(3)	F22⋯S61^k 3.2416(19)
	33.23(6)	3.2222(12)	S52⋯S62^a 3.9680(12)	S21⋯S61^k 4.4280(12)	S52⋯N11^r 3.871(3)	F66⋯S22 2.9208(19)
				S21⋯S62^k 3.7510(13)	S61⋯N11^r 3.517(3)	F66⋯S12 3.2441(18)
					S61⋯N21^k 3.468(3)	F56⋯S12 3.273(2)
				(between pentamers)	S62⋯N21^k 3.619(3)	F56⋯S22^a
				S31⋯S41°		3.2638(18)
				3.4777(12)
				S31⋯S42°		(between pinwheels)
				3.8709(12)		F14⋯S41^y 3.257(2)
				S32⋯S41°		F14⋯S42^z 3.079(2)
				3.8168(12)		F24⋯S31^aa 3.046(2)
				S31⋯S31°		F24⋯S32^z 3.151(2)
				3.2928(15)		F54⋯S42 3.317(2)
				S31⋯S32°		F54⋯S32^a 3.253(2)
				3.7034(11)		F64⋯S32 3.462(2)
				S41⋯S41^m		F64⋯S42 3.053(2)
				3.8037(16)
12	9.07(8)	3.1189(6)	—	S11⋯S12^a 4.2330(6)	S12⋯N11^p 3.5659(15)	(within chains)	This work
	9.05(8)	3.0881(7)		S21⋯S12^a 4.0561(7)	S12⋯N21^p 3.6184(15)	F14⋯S11^b 3.0522(12)
				S21⋯S22^a 4.0794(7)	S22⋯N21^p 3.6531(15)	F14⋯S12^b 3.2170(12)
						F24⋯S22^b 2.9404(11)
						(between chains)
						F13⋯S11^r 3.2878(13)
						F15⋯S21^c 3.2067(11)
						F25⋯S22^bb 3.1889(12)

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Theoretical calculations

Molecular electrostatic isopotential maps (MEPs) for 1–12 were determined using restricted Hartree–Fock semi-empirical PM5 methods within the Quantum Cache programme.²¹ Whilst there are certain limitations to the semi-empirical methods, previous studies on HCNSSN˙ and ClCNSSN˙ inter alia indicate that this method provides a good analysis of both the spin density and charge distribution in these molecules.⁷ In addition, excellent correlations have been observed between the redox behaviour of phenyl-substituted DTDA radicals and the calculated HOMO/LUMO energies of the DTDA ring using semi-empirical methods.^5c

Calculations of the energy minima and barrier to rotation between phenyl and dithidiazolyl rings have been undertaken previously,²² with both double zeta (UB3LYP/6-31G*) and triple zeta (UB3LYP/6-311G) methods offering similar energetic barriers to rotation of the phenyl ring (25 and 26 kJ mol⁻¹ respectively), indicating reasonable convergence at this level. In this study, energies of 4′-(2-fluorophenyl)- and 4′-(2,6-difluorophenyl)-dithiadiazolyl radicals were determined using single point calculations at 10° intervals in the range 0–90° for the torsion angles between dithiadiazolyl and aryl rings. In all cases the ring geometries were otherwise fixed at those determined from a full geometry optimization. Calculations employed identical split-valence double zeta (UB3LYP/6-31G*) methods within GAMESS-UK²³ to those applied previously.²²

Results

The twelve possible difluorophenyl and trifluorophenyl dithiadiazolyl radicals are shown in Scheme 2. These were prepared according to the standard synthetic methods.⁴ Vacuum sublimation of 2, 7, 8 and 10 did not afford crystals of sufficient size or quality for single crystal diffraction despite multiple attempts. Nevertheless an analysis of the remaining eight isomers (plus polymorph) has proved instructive. A number of dithiadiazolyl radicals have been shown to exhibit polymorphism or form host–guest complexes with small molecules such as N₂ or CO₂.^7e,24 For this reason, crystalline samples were carefully scrutinised for different polymorphs and sublimation of 1–12 was carried out in vacuo (10⁻¹ Torr) and under ca. 1/3 atm of N₂ in order to probe for the presence of additional polymorphs. In our hands, with the exception of 4, each compound appeared phase pure.¹⁵ Compound 4 produced two different phases dependent upon sublimation conditions. Sublimation in vacuo produced 4α whereas sublimation under a partial atmosphere of N₂ yielded 4β.

The crystal structures of 4α, 4β, 5, 6, 9, 11 and 12 are described in relation to the previously reported^13–15 structures of 1, 3 and 7.

Molecular structure of the dithiadiazolyl ring

The heterocyclic ring geometries (see ESI†) are similar and consistent with those previously reported in other DTDA radicals e.g. ClCNSSN˙ and HCNSSN˙.⁷ However, the twist angles (θ) between the DTDA ring and the phenyl ring vary considerably (5.7–48.7°) (Table 2). In cases where there are two H atoms in the ortho positions (5, 6 and 12), θ is small spanning a range between 8.5 and 11.9°. These values are reflected in other phenyl-substituted dithiadiazolyls bearing two ortho–H atoms such as p-XC₆H₄CNSSN˙ (X = H, Cl, I, CN, CNSSN˙) (θ in range: 5.0–11.8°).^5c,6a,25 Conversely, when two F atoms are positioned in the ortho positions, large twist angles are observed (24.9–48.7°). Again similar behaviour has been observed in perfluorophenyl derivatives such as p-XC₆F₄CNSSN˙ (X = Br, CN, NO₂ and p-NCC₆F₄) (θ range: 32.2–68.7°).^3,10,11,26 Of the derivatives containing one ortho–H and one ortho-F the behaviour is rather less well-defined. Whilst only three of the potential six derivatives containing this functionality produced crystals of a suitable quality for diffraction studies (1, 3 and 7), θ in these three structures spans the range 5.7–23.6°. The variation in θ value within each of these groups of compounds suggests a shallow minimum on the potential energy surface for rotation about the C–C bond which links the DTDA and phenyl rings.

Previous DFT (UB3LYP/6-31G*) studies on PhCNSSN˙²² reveal a shallow minimum at θ = 0° with an energy barrier of 25 kJ mol⁻¹ for complete rotation about the C–C bond between the two rings. A distortion of ±25° from co-planarity corresponds to ca. 3 kJ mol⁻¹. Replacement of one ortho-H by F reveals a similar energy minimum at 0° but with a reduced energy barrier for complete rotation (15 kJ mol⁻¹) and a distortion of ±35° corresponding to ca. 3 kJ mol⁻¹. In the case of the 2,6-difluorophenyl derivative, the two ortho fluorines destabilise the planar configuration and an energy minimum occurs at θ ∼ 50° with an even lower energy barrier to rotation (11 kJ mol⁻¹) and a nominal 3 kJ mol⁻¹ distortion offering a range of angles in the range 30–90°. With a non-planar energy minimum, two different mechanisms arise for a 180° rotation; either via a transition state in which the two rings become coplanar, or one in which they are orthogonal. In the current case, rotation via a transition state with mutually orthogonal rings is more favourable by ca. 9.5 kJ mol⁻¹ than one in which the two rings are coparallel. [The angular dependences of the energies (UB3LYP/6-31G*) of PhCNSSN˙, 2-FC₆H₄CNSSN˙ and 2,6-F₂C₆H₃CNSSN˙ are available as ESI†].

Both the experimental observations and theoretical studies indicate that the energy barrier to distortion is low in aryl substituted DTDAs, and decreases markedly with increasing fluorination in the 2-position. This leads to larger deviations from coplanarity in the case of the fluorinated derivatives. As a consequence the torsion angle θ is likely to be rather flexible and amenable to distortions to accommodate crystal packing forces. This is particularly emphasised in the two polymorphs of 4: in 4α both θ angles (47.7 and 48.7°) fall in the conventional range for a derivative bearing two ortho-F atoms, but these angles appear unusually small in polymorph 4β (24.9 and 29.4°).

The vast majority of dithiadiazolyl radicals associate in the solid state through a π*–π* interaction between singly occupied molecular orbitals. The local a₂ symmetry of this orbital allows a number of possible bonding modes to be adopted (Fig. 1).²⁷ Whilst the majority of DTDA radicals adopt the cis-cofacial conformation, twisted and trans conformations are not unprecedented. Previous MO calculations have indicated that while the enthalpy of dimerisation is significant, the energy difference between these conformations is small.²⁸ Indeed amongst the five reported polymorphs of ClCNSSN˙,^7a–c three adopt twisted conformations whereas two adopt cis conformations. Clearly there is little energetic preference between the conformers and the mode of dimerisation is likely to be driven by other packing factors. With the exception of 1, all the di- and tri-fluorophenyl derivatives reported here adopt cis-cofacial geometries. The intradimer S⋯S distances in the cofacial dimers fall in the range 3.069–3.295 Å. Notably those structures which form ‘isolated’ dimers exhibit slightly shorter S⋯S contacts (3.069–3.172 Å, average 3.11 Å) than those which adopt π-stacked structures (3.0692–3.295 Å, average 3.17 Å). This increase in S⋯S distance for π-stacked systems presumably arises out of a bonding interaction between dimers which leads to a concomitant weakening of the intra-dimer S⋯S interaction.

Crystal structures of 4-(difluorophenyl)-1,2,3,5-dithiadiazolyls

The importance of an intermolecular contact is typically made in comparison with the sum of the van der Waals radii. A systematic study of the Cambridge Structural Database (CSD) by Nyburg and Faerman²⁹ revealed that the van der Waals radii need not necessarily be spherical and, particularly for heavier p-block elements, may exhibit significant anisotropy. The minor radii correspond to contacts close to the molecular plane whereas major radii contacts are formed perpendicular to the plane. Within the context of the current study it is worth noting Nyburg and Faerman's estimates of the minor and major radii of N, F and S; for both N and F the radii are essentially isotropic (1.60 Å for N and 1.30–1.38 Å for F), whilst there is significantly greater anisotropy for S (1.60–2.03Å). Relevant intermolecular close contact distances for all the structures discussed here are collected in Table 2.

Crystal structure of 4-(2′,3′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 1. The structure of this radical was originally reported¹³ by Banister et al. It crystallises as a twisted dimer (Fig. 1b) in the triclinic space group P-1, with one dimer in the asymmetric unit.

The dimers stack along the a-axis, with alternating short intra-dimer (3.020 Å) and long inter-dimer (3.628 Å) S⋯S contacts. Individual molecules of 1 form contacts from the meta-F atom to S. Both single (3.247 Å) and bifurcated S⋯F contacts (3.028–3.549 Å) are comparable with the sum of the van der Waals radii (2.90–3.41 Å) and link dimers together along the crystallographic b-axis (Fig. 2a).

Fig. 2 Packing in 1 (a) Chain-like motif parallel to the crystallographic b–axis generated from S⋯F contacts; (b) antiparallel alignment of chains generated through a bifurcated interaction between the p-H atom and the electronegative N and F atoms. The second molecule of each twisted dimer is shown in grey in order to emphasize the in-plane inter-dimer interactions (view is down the a-axis).

Neighbouring chains are oriented antiparallel along the b-axis and related via an inversion centre which generates close contacts between the electronegative N and o-F atoms and the electropositive p-H on the aryl ring (N⋯H 2.544 and 2.530 Å, F⋯H 2.496 and 2.617 Å; 2.609 and 2.623 Å). This bifurcated C–H⋯N/C–H⋯F contact to DTDA derivatives bearing ortho-F phenyl rings is a common motif in these structures and is also observed in 3, 4β and 9.

Crystal structure of 4-(2′,5′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 3. The crystal structure of this compound has been described previously.¹⁴ Initial studies indicated a regular π-stacked structure with interlayer spacing of 3.544 Å, which appeared at odds with the observed diamagnetism of the sample.¹³ Subsequent studies by Oakley identified a super-cell consistent with a doubling of the crystallographic c-axis.¹⁴ Further structural refinement confirmed that this radical crystallises in the tetragonal space group I4₁/a with a single cis-cofacial dimer in the asymmetric unit which adopts a distorted π-stacked structure with alternating short (3.119 and 3.203 Å) and long (4.006 and 4.084 Å) contacts along the c-axis.

In the ab plane radicals are arranged in pinwheels (labelled A in Fig. 3). The S⋯N separations in the centre of the pinwheel range from 3.322 to 3.864 Å (cf. sum of the van der Waals radii: 3.20–3.63 Å) whereas the S⋯S separations range from 3.517–3.587 Å (cf. sum of the van der Walls radii 3.20–4.06 Å). Some structural disorder of the two fluorine positions, equivalent to a 180° flip of the phenyl ring, is observed. The major component of the disorder (Fig. 3) places the ortho-F atoms ‘exo’ to the pin-wheel and supports a bifurcated interaction between the p-H and the N and F atoms analogous to that observed in 1; the F⋯H contacts are 2.542 and 2.624 Å and N⋯H contacts are 2.584 and 2.782 Å. In the minor ‘endo’ position, there is an interaction between the N and F atoms and a heterocyclic S atom, with S⋯F contacts ranging between 3.062 and 3.411 Å (sum of the van der Waals radii 2.90–3.41 Å).

Fig. 3 Packing of 3 viewed down the c-axis showing the pinwheel formation (A) and channel (B) along fourfold axes. The fluorine positional disorder has been omitted for clarity and the substituents shown are the major component.

Notably, the major component of the disorder also generates close F⋯F contacts of 2.410–2.814 Å between meta-F atoms and a channel at the centre of the second ‘aryl’ pin wheel (labelled B in Fig. 3). Disorder in the position of these meta-F atoms alleviates F⋯F contacts and simultaneously partially fills the void at B. This should be compared with the isostructural derivative 4β which has no meta-F atoms (Fig. 5b).

Crystal structures of 4-(2′,6′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 4. This radical crystallises in one of two polymorphs dependent upon sublimation conditions: crystals of 4α were grown under vacuum at 110 °C, whilst crystals of 4β where grown under 1/3 of an atmosphere of nitrogen at 95 °C.
Crystal structure of α-4-(2′,6′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 4α. Radical 4α crystallises in the monoclinic space group P2₁/c with one cis-cofacial dimer in the asymmetric unit. The dimers form herring-bone chains, oriented approximately along the c-axis, linked via close S⋯N interactions (2.974–3.498 Å, cf. van der Waals radii 3.20–3.63 Å). These chains align coparallel to form a two-dimensional sheet (Fig. 4) in which the para-H atoms of one molecule form close contacts to the aryl ring of a symmetry-related molecule (C⋯centroid 4.126 Å, C–H-centroid angle 171°) i.e. a C–H⋯π type interaction commonly observed in aromatic molecules.³⁰

Fig. 4 Crystal packing in 4α illustrating chain-like motif along the crystallographic c-axis.

Crystal structure of β-4-(2′,6′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 4β. The second polymorph of 4 crystallises in the tetragonal space group I4₁/a and is isostructural with both 3 and 9 (Fig. 5a). The dimers are arranged in pinwheels, with close S⋯N (3.214–3.940 Å) and S⋯F contacts (3.105–3.753 Å) within the pinwheel. Both these sets of contacts are similar to those observed in 3. Exo to the pinwheel the para-H forms F⋯H (2.544 and 2.574 Å) and N⋯H (2.591 and 2.571 Å) contacts. The absence of F atoms in the meta-position (cf. radical 3) eliminates close F⋯F contacts, but also results in channels in this structure which run parallel to the c-axis (Fig. 5b). The cross-sectional area of this void is estimated at ca. 2.6 Ų and thus is sufficiently small to preclude accommodation of guest molecules: no significant residual electron density was observed in the channel region. This should be contrasted with the comparatively diverse range of host–guest behaviour displayed by 4-(4′-trifluoromethyl-3′-fluorophenyl)-1,2,3,5-dithiadiazolyl.²⁴

Fig. 5 (a) Superimposition of the isostructural DTDA derivatives 3 (red), 4β (blue) and 9 (green) viewed in the ab plane. (b) Space-filling diagram of 4β showing channels.

Crystal structure of 4-(3′,4′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 5. This radical crystallises in the monoclinic space group P2₁/n with one cis-cofacial dimer in the asymmetric unit. The dimers form ribbons linked through S⋯F interactions (2.986–3.265 Å, cf. sum of the van der Waals radii 2.90–3.41 Å) approximately parallel to the ac diagonal. The ribbons are related via an inversion centre generating antiparallel chains which are linked by lateral S⋯N (3.262 and 3.284 Å, cf. sum of the van der Waals radii 3.20–3.63 Å) and S⋯F contacts (3.203 Å) to form two-dimensional sheets (Fig. 6). Perpendicular to the ac plane sheets pack in a stepped manner.

Fig. 6 Crystal structure of 5 (a) with in-plane close S⋯F and S⋯N contacts emphasized and (b) showing stepped packing of sheets/ribbons.

Crystal structure of 4-(3′,5′-difluorophenyl)-1,2,3,5-dithiadiazolyl, 6. This radical crystallises in the triclinic space group P-1 with four molecules in the asymmetric unit comprising two crystallographically independent cis-cofacial dimers. These dimers form sheets perpendicular to the crystallographic a axis (Fig. 7), with neighbouring dimers linked via S⋯N and S⋯S contacts into discrete centrosymmetric tetramers. Four dimers (C, D, E and F) in one such unit are marked in Fig. 7. The centrosymmetric pair (dimers D and E) are linked via ‘side-on’ S⋯N contacts (3.276 and 3.284 Å, cf. sum of the van der Waals radii 3.20–3.63 Å). These are similar to those seen in 5 (3.262 and 3.284 Å). The S⋯N and S⋯S contacts between C and D (equivalent to E and F) are 3.205 and 3.376, and 3.461 and 3.640 Å, respectively. These are comparable to similar S⋯N and S⋯S contacts located at the centre of the pin-wheel structures of 3, 4β and 9 as well as those generating the chain-motif in 4α. In addition molecules C and E (and symmetry equivalent D and F) are linked through a set of close intermolecular S⋯F contacts (Fig. 7). These fall in the range 2.936 to 3.062 Å (cf. sum of the van der Waals radii of 2.90–3.41 Å). Similar S⋯F contacts to the meta-F are observed in 1 but they are less symmetric than those observed in 6 [short S⋯F = 3.072 Å; long S⋯F = 3.549 Å]. The tetramers are linked to one another via N⋯H interactions (2.602–2.681 Å) from the para-H (Fig. 7). These are comparable to the interactions seen in 1 and 3, although the contacts in 6 are slightly longer than those in both 1 and 3.

Fig. 7 Sheets of dimers in 6 viewed down the a-axis.

The sheets of tetramers stack along the c-axis with alternating short intradimer (∼3.1 Å) and long interdimer (∼4.0 Å) S⋯S contacts.

The structure of 6 is closely related to the pinwheel structures of the three isomorphous radicals 3, 4β and 9 (Fig. 8). If the layer structure in 6 is dissected into slabs as shown in Fig. 8, then application of a translation as shown leads to cleavage of the two centrosymmetric S⋯N contacts (between D and E in Fig. 7) and cleavage of two sets of bifurcated S⋯F contacts. For 6, such a translation would generate two sets of (favourable) S⋯S and S⋯N pinwheel contacts but also two sets of (disfavoured) F⋯F contacts.

Fig. 8 Relationship between the layer-like structures of 6 (left) with isostructural 3, 4β and 9via translation (right, 4β shown here).

Crystal structures of 4-(trifluorophenyl)-1,2,3,5-dithiadiazolyls

Crystal structure of 4-(2′,3′,4′-trifluorophenyl)-1,2,3,5-dithiadiazolyl, 7. The structure of this radical was solved by powder diffraction using high resolution synchrotron data as described previously.¹⁶ It crystallises as a cis-cofacial dimer in the monoclinic space group P2₁/n. The dimers pack in ribbons parallel to the b-axis via S⋯F contacts (3.017–3.248 Å) (Fig. 9.). These ribbons are related via an inversion centre with centrosymmetric S⋯N contacts (3.237 and 3.373 Å) across the inversion centre between ribbons. These layers pack together such that stacks of dimers are formed, with interdimer S⋯S distances of 3.780 and 3.839 Å. This compound has the same packing as 5 within the layers, but the packing between the layers differs slightly in that 5 forms slipped stacks. Notably, the ribbons in 5 are formed between dimers, whereas the ribbons in 7 are formed between radicals in different dimers, causing an undulation in 7 that is not seen in 5. (cf.Fig. 6b and 9b).

Fig. 9 (a) Layerlike structure of dimers in 7 observed in the ab plane. (b) Side-on view showing stacking of the layers (cf. 5, Fig. 6).

Crystal structure of 4-(2′,3′,6′-trifluorophenyl)-1,2,3,5-dithiadiazolyl, 9. This radical crystallises in the tetragonal space group I4₁/a, and is isostructural with 3 and 4β (see Fig. 5). As in 3 there is positional disorder of the meta-F atoms. For both molecules in the asymmetric unit, the major component of the disorder allows the F to adopt a position ‘exo’ to the pinwheel in an analogous fashion to 3. This again (cf. 3) leads to blocking of the channels observed in 4β.

Crystal structure of 4-(2′,4′,6′-trifluorophenyl)-1,2,3,5-dithiadiazolyl, 11. This radical crystallises in the monoclinic space group P2₁/c, with six molecules in the asymmetric unit. The radicals form cis-cofacial dimers, which stack along the a-axis with alternating short intradimer (3.101–3.255 Å) and long interdimer (3.932–4.067 Å) contacts. Two of the crystallographically independent dimers form slightly distorted pinwheels, comparable with 3, 4β and 9, in which the S⋯S contacts fall in the range 3.393–4.428 Å and the S⋯N contacts span 3.097 to 3.895 Å. The corresponding S⋯ortho-F contacts are 2.921–3.346 Å. (Fig. 10a). These tetramers are then linked via head-to-tail S⋯F interactions (3.046–3.462 Å, Fig. 10b) to the third crystallographically independent dimer generating a pentamer in which the ortho-F, meta–H and para-F atoms form an extensive set of C–H⋯F–C contacts (approximately 2.55–2.65 Å). These pentamers are linked together to form a bimolecular tape (via blue and red molecules in Fig. 10b), which propagates along the crystallographic c-axis. These arrays are linked in the bc plane via an additional web of S⋯F, N⋯F (2.723 and 2.579 Å) and CH⋯F contacts (Fig. 10c).

Fig. 10 Packing in 11 showing (a) formation of slipped pinwheels, (b) generation of pentamers from pinwheels and their subsequent linkage to form bimolecular tapes and (c) packing of tapes in the bc plane.

Crystal structure of 4-(3′,4′,5′-trifluorophenyl)-1,2,3,5-dithiadiazolyl, 12. This compound crystallises in the triclinic space group P-1, with two molecules in the asymmetric unit forming a cis-cofacial dimer. Chains of molecules are formed through S⋯F interactions (2.940 Å; bifurcated 3.052 and 3.217 Å). The chains are aligned approximately parallel to the ab diagonal. Parallel chains align to form a stepped sheet, with long S⋯N contacts between chains (3.566–3.653 Å). Stepped sheets then stack in an antiparallel manner. There are several interchain S⋯F contacts (3.189–3.288 Å) (Fig. 11).

Fig. 11 Chain-like motif in 12 generated through bifurcated S⋯F contacts.

Discussion

All the radicals described in this paper are dimeric in the solid state, with singlet diamagnetic ground states due to strong radical–radical interactions, and all but one (1) form cis-cofacial dimers. Whilst some derivatives are isostructural, there is a great diversity in packing arrangements, i.e. small alterations to the fluorine substitution pattern result in significant changes to the packing motif. We now investigate these structures through an analysis of their intermolecular contacts in relation to their charge distribution determined through theoretical calculations.

It is generally accepted that neutral planar aromatic molecules tend to adopt either of two packing arrangements in the solid state; π-stacked or herringbone motifs.³⁰ Within these two broad categories there is some structural diversity e.g. slipped π-stacks, distorted π-stacks (with a commensurate doubling of the ‘short’ stacking axis), sandwich herringbone motifs etc. For neutral aromatic hydrocarbons the stronger nature of the C–H⋯π interaction over the π–π interaction favours herringbone structures. Indeed in the case of the neutral DTDA radicals (PhCNSSN˙)₂^25a and (p-ClC₆H₄CNSSN˙)₂^5c sandwich herringbone motifs are observed between dimers. Solid state structures based upon planar π-stacked arrangements become favoured when there are directional intermolecular forces which may be dipolar, covalent or electrostatic in origin. In the case of 1–12 every structure is based upon a layer-like motif consistent with the presence of strong directional in-plane forces. We have previously rationalised the intermolecular contacts in some of the simplest DTDA derivatives (HCNSSN˙ and ClCNSSN˙) through an analysis of the electrostatic contribution to bonding.^7a,b We now extend this approach to this more complex family of fluoro-phenyl DTDAs.

Electrostatic contribution to bonding

It is our hypothesis that the majority of the important structure-directing intermolecular interactions involved in determining the crystal structures of these compounds are electrostatic in nature and optimise contacts between electronegative and electropositive regions in neighbouring molecules.

A simple analysis of the electronegativities of S and N clearly reveals that the S–N bond should be considered to be polarized in the sense S^δ+—N^δ−. Therefore the positioning of two electropositive S atoms adjacent to each other generates a strongly electropositive region in every DTDA radical near the S atoms. As a consequence close intermolecular contacts between S and N (or indeed S and other electronegative elements, such as F) should be electrostatically favourable.

Previous theoretical and experimental analysis of intermolecular contacts in ClCNSSN˙ and HCNSSN˙ revealed four favourable in-plane modes of association between DTDA rings. These classifications are depicted in Fig. 12, and employed in this dicussion. The SN-I interaction occurs in all five polymorphs of ClCNSSN˙,^7a–c and in HCNSSN˙^7d–e and CF₃CNSSN˙²⁸inter alia. However in the presence of simple aryl substituents SN-I tends to become disfavoured to accommodate the sterics of the near-coplanar R group.³¹ As a consequence this interaction often appears less symmetric, with one contact longer than the other, generating an SN-IV motif which is more amenable to close-packing. The loss in electrostatic S^δ+⋯N^δ− attraction and increased S^δ+⋯S^δ+ repulsion may be partially balanced by a greater dispersion term between S atoms which are in close proximity.

Fig. 12 Electrostatically favourable S^δ+⋯N^δ− motifs in DTDA derivatives.

Similarly with simple aryl substituents, the steric demands of R will typically disfavour SN-III in relation to SN-II. As a consequence the SN-II and SN-IV interactions are likely to play an important role in the structures of many aryl-substituted DTDA derivatives. In the current sample 3, 4α, 4β, 6, 9 and 11 all display SN-IV type interactions whereas 5, 6 and 7 display SN-II contacts. Only 1 and 12 do not appear to exhibit close in-plane contacts between DTDA rings.

The incorporation of fluorinated aromatic groups into the DTDA derivatives 1–12 leads to the additional possibility of S^δ+⋯F^δ− interactions which may compete with or additionally stabilise the favourable S⋯N interactions. The placement of F in the 2′ position of the phenyl ring is expected to enhance the stability of the SN-IV interaction described previously. Indeed, with the exception of 6, all those structures exhibiting an SN-IV contact also possess an ortho-F atom.

When two F atoms are placed ortho with respect to each other then this leads to the potential to form pairs of S⋯F contacts as in 5 and 7. Notably in 1 and 12, although these do not exhibit S⋯N contacts, they do exhibit pairs of S⋯F contacts.

Molecular electrostatic potential maps

One method to analyse the electrostatic contribution to bonding (and thereby assessing the relative strengths of S^δ+⋯N^δ−vs S^δ+⋯F^δ−) is through the use of molecular electrostatic isopotential (MEP) maps. We have previously employed this method for successfully interpreting the packing motifs in ClCNSSN˙ and HCNSSN˙^7a,b as well as in dithiazolyl radicals³² and dithiatetrazocines.³³ Others have utilized similar methods for other C/N/S heterocycles.³⁴ The calculated MEP for each radical based on its geometry in the crystal is shown in Table 3. Red regions denote areas of an electropositive nature whilst blue regions have electronegative character. An analysis of these MEPs reinforces the arguments proposed above. They reveal electropositive regions near the heterocyclic S atoms and aromatic H atoms, whereas those regions near heterocyclic N and aromatic F atoms reflect a build-up of partial negative charge as anticipated from a simple consideration of electronegativity values. Favourable electrostatic interactions therefore comprise S^δ+⋯N^δ−, S^δ+⋯F^δ−, H^δ+⋯N^δ− and H^δ+⋯F^δ−. Importantly the relative positions of F atom substitution on the phenyl ring have a strong influence on both the charge distribution itself and also the relative magnitudes of the charges. Specifically, the location of electronegative F atoms on adjacent C atoms, i.e. mutually ortho with respect to each other e.g. in 1, 5, 7, 9 and 12, leads to an enhancement of the electronegative region near those F atoms. An examination of the electronegative regions in 1–12 clearly identifies that S⋯N contacts are likely to dominate the structures of 3, 4, 9 and 11 whereas S⋯F are likely to be structure directing in 1, 5, 7 and 12. Only in the case of 6 would there appear to be a fine balance between these two competing sets of interactions.

Table 3 Molecular electrostatic potential maps for a series of fluorinated dithiadiazolyl radicals

	MEP	Packing summary	SN motif		MEP	Packing summary	SN motif
1		Twisted dimer, stacks of molecules with closest favourable intermolecular interaction S⋯F contact.	none	7		cis-Cofacial dimers packed in head-to-tail ribbons linked via intra-ribbon S⋯F contacts, aligned antiparallel with lateral inter-ribbon S⋯N contacts.	SN-II
3		cis-Cofacial dimers packed in S-centred pinwheels. Isostructural with 4β and 9.	SN-IV	9		cis-Cofacial dimers packed in S-centred pinwheels. Isostructural with 3 and 4β.	SN-IV
4		4α: cis-Cofacial dimers, one-dimensional chains linked by intrachain S⋯N contacts.	4α SN-IV	11		cis-Cofacial dimers packed in distorted S-centred pinwheels with extra dimer in between pinwheels, S⋯F and S⋯N contacts.	SN-IV
		4β: cis-Cofacial dimers packed in S-centred pinwheels. Isostructural with 3 and 9.	4β SN-IV
5		cis-Cofacial dimers packed in head-to-tail ribbons linked via intra-ribbon S⋯F contacts, aligned antiparallel with lateral inter-ribbon S⋯N contacts.	SN-II	12		cis-Cofacial dimers packed in head-to-tail chains linked via S⋯F contacts, interchain S⋯F and long S⋯N contacts.	SN-III-type (long)
6		cis-Cofacial dimers packed in off-set S-centred pinwheels, S⋯F and S⋯N contacts.	SN-II, SN-IV

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All structures presented here were examined for close contacts (less than or equal to the sum of the van der Waals radii) between S and F and between S and N. These are summarised in Table 3. It is noteworthy that 3, 4, 9 and 11 all exhibit the SN-IV motif in excellent agreement with the MEP analysis, whereas 1, 5, 7 and 12 all exhibit S⋯F contacts. Notably the chain-like motifs of 5, 6 and 7 also support some SN-II type contacts between neighbouring chains or ribbons. In addition to these general remarks, some specific observations are worthy of further comment.

Comparison of 3, 4β and 9

These three compounds are isostructural, consistent with the presence of a strongly structure-directing motif common to all molecules. The varied F atom positions on the aryl ring, coupled with positional disorder of the F atoms, suggests that here the SN-IV contact is structure-directing. This is supported by the observation that radical 4 is polymorphic, but both 4α and 4β exhibit the same SN-IV type interactions. Whilst they generate a pinwheel in 4β, they form chains in 4α. The observation of the same packing motif in two different polymorphs is also consistent with its structure-directing nature (cf. chains and hexameric rings in polymorphs of HCNSSN˙ linked viaSN-I type interactions).⁷

On the behaviour of 11

Given the propensity for the SN-IV motif in isostructural 3, 4β and 9, it is initially surprising that 11 does not also adopt this tetragonal packing arrangement since its MEP clearly favours a structure dominated by S⋯N interactions. Indeed 11 does exhibit pinwheel motifs but these are distorted and accommodate additional molecules between the pinwheels. A comparison of the structure of 11 with isostructural 3, 4β and 9 resolves the anomalous behaviour of 11. If 11 adopted an isomorphous lattice to 4β, the additional para-F atom would be located in close proximity to the electronegative N/F ‘pocket’ of a neighbouring pinwheel resulting in structural destabilisation. Instead the pinwheel motif is retained (further evidence for the structure directing nature of SN-IV contacts), but with additional dimers packing between pinwheels. These additional dimers located between the pinwheels provide a network of C–H⋯F–C contacts to two neighbouring pinwheels and an S⋯F contact to a third pinwheel (Fig. 10).

On the behaviour of 6

The MEP of radical 6 clearly reflects competition between S⋯N and S⋯F contacts. The lack of ortho-F atoms reduces the negative charge build-up associated with the N/F ‘pocket’ observed in 3, 4, 9 and 11. However in 6 the F atoms are not ortho to each other and do not offer a combined strong structure-directing influence. Earlier we emphasised the relationship between the structure of 6 and the isomorphous set of 3, 4β and 9. In 6, as in 11, the tetragonal pinwheel motif is destabilised by the presence of a repulsive F⋯F contact, this time between meta-F atoms. This results in an alternative packing strategy comprising some residual SN-IV contacts coupled with SN-II contacts.

Propagation of S⋯F contacts

In the case of 1, 5, 7 and 12 the MEPs reveal that chain-like motifs with S⋯F contacts should be favoured. In the case of 5, 7 and 12 the linear or near-linear nature of the ribbon motifs favours chain formation. Interchain electrostatic contacts in 5 are few, although the SN-II motif is clearly present and favours an antiparallel arrangement of chains. Longer C–H⋯F–C and C–H⋯N contacts are also discernable. The inter-chain contacts in 7 are very similar to those observed in 5 (Fig. 13a) though the way in which the dimer distortion occurs differs in 5 and 7 (see Fig. 13b,c).

Fig. 13 Comparison of interchain interactions in 5 (red) and 7 (blue). (a) Overlay of chains. (b) Overlay of chains viewed side-on; note slippage of red chains. (c) Relationship between chain formation and dimers in 5 (top) and 7 (bottom).

Conclusions

The results presented here show clearly that replacement of hydrogen by fluorine in fluorinated aromatic dithiadiazolyl derivatives has a very marked influence on solid state structure. The outcome of crystallization is extremely sensitive to the precise position and number of fluorine substituents. In the current sample of structures, the effects can be understood by considering the molecule as an entity, rather than being comprised of a series of different functionalities. Molecular electrostatic potential maps are extremely informative in identifying which of the possible combinations of electrostatic interactions are likely to dominate in the structure. In all cases the disulfide bond in the dithiadiazolyl ring bears a large partial positive charge and appears best stabilized by close contacts to large partial negative charges. The presence of fluorine substitution ortho to the dithiadiazolyl ring leads to highly electronegative regions near the dithiadiazolyl ring nitrogen. This ‘N/F pocket’ enhances S⋯N contacts which favours an SN-IV packing motif. Placement of two F atoms ortho with respect to each other generates large regions of electronegative character which can often lead to structure-directing S⋯F contacts in preference to the conventional S⋯N interaction.

Importantly, whilst there is a strong tendency to examine observed crystal structures to identify structure-directing influences, much can also be gained through a study of hypothetical alternative structures or polymorphs to identify structure-destabilising motifs.

Acknowledgements

D.A.H. thanks the Cambridge Commonwealth Trust, the ORS and the Cecil Renaud Trust for funding. S.I.P. thanks the Royal Society for funding (URF). C.S.C. thanks the Natural Environment Research Council for funding.

References

1. J. M. Rawson, A. Alberola and A. Whalley, J. Mater. Chem., 2006, 16, 2560.
2. (a) R. C. Haddon, Nature, 1975, 256, 394; (b) A. Cordes, R. Haddon and R. Oakley, Phosphorus, Sulfur Silicon Relat. Elem., 2004, 179, 673; (c) R. T. Oakley, Can. J. Chem., 1993, 71, 1775 see also ref. 1.
3. A. J. Banister, N. Bricklebank, W. Clegg, M. R. J. Elsegood, C. I. Gregory, I. Lavender, J. M. Rawson and B. K. Tanner, J. Chem. Soc., Chem. Commun., 1995, 679.
4. J. M. Rawson, A. J. Banister and I. Lavender, Adv. Heterocycl. Chem., 1995, 62, 137.
5. (a) C. M. Aherne, A. J. Banister, T. G. Hibbert, A. W. Luke and J. M. Rawson, Polyhedron, 1997, 16, 4239; (b) C. M. Aherne, A. J. Banister, I. B. Gorrell, M. I. Hansford, Z. V. Hauptman, A. W. Luke and J. M. Rawson, J. Chem. Soc., Dalton Trans., 1993, 967; (c) R. T. Boeré, K. H. Moock and M. Parvez, Z. Anorg. Allg. Chem., 1994, 620, 1589.
6. (a) A. W. Cordes, R. C. Haddon, R. G. Hicks, R. T. Oakley and T. T. M. Palstra, Inorg. Chem., 1992, 31, 1802; (b) A. Alberola, R. J. Less, F. Palacio, C. M. Pask and J. M. Rawson, Molecules, 2004, 9, 771; (c) G. Antorrena, S. Brownridge, T. S. Cameron, F. Palacio, S. Parsons, J. Passmore, L. K. Thompson and F. Zarlaida, Can. J. Chem., 2002, 80, 1568; (d) A. W. Cordes, C. M. Chamchoumis, R. G. Hicks, R. T. Oakley, K. M. Young and R. C. Haddon, Can. J. Chem., 1992, 70, 919.
7. (a) A. D. Bond, D. A. Haynes, C. M. Pask and J. M. Rawson, J. Chem. Soc., Dalton Trans., 2002, 2522; (b) C. S. Clarke, S. I. Pascu and J. M. Rawson, CrystEngComm, 2004, 6, 79; (c) C. Knapp, E. Lork, K. Gupta and R. Mews, Z. Anorg. Allg. Chem., 2005, 631, 1640; (d) A. W. Cordes, C. D. Bryan, W. M. Davis, R. H. de Laat, S. H. Glarum, J. D. Goddard, R. C. Haddon, R. G. Hicks, D. K. Kennepohl, R. T. Oakley, S. R. Scott and N. P. C. Westwood, J. Am. Chem. Soc., 1993, 115, 7232; (e) C. D. Bryan, A. W. Cordes, R. C. Haddon, R. G. Hicks, D. K. Kennepohl, C. D. MacKinnon, R. T. Oakley, T. T. M. Palstra, A. S. Perel, S. R. Scott, L. F. Schneemeyer and J. V. Waszczak, J. Am. Chem. Soc., 1994, 116, 1205.
8. S. A. Fairhurst, K. M. Johnson, L. H. Sutcliffe, K. F. Preston, A. J. Banister, Z. V. Hauptman and J. Passmore, J. Chem. Soc., Dalton Trans., 1986, 1465.
9. G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond, 2001, Oxford University Press.
10. A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson, C. I. Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood and F. Palacio, Angew. Chem., Int. Ed. Engl., 1996, 35, 2533.
11. G. Antorrena, J. E. Davies, M. Hartley, F. Palacio, J. M. Rawson, J. N. B. Smith and A. Steiner, Chem. Commun., 1999, 1393.
12. See for example J. D. Dunitz and R. Taylor, Chem.–Eur. J., 1997, 3, 89; A. R. Choudhury and T. G. Row, CrystEngComm, 2006, 8, 265.
13. A. J. Banister, A. S. Batsanov, O. G. Dawe, P. L. Herbertson, J. A. K. Howard, S. Lynn, I. May, J. N. B. Smith, J. M. Rawson, T. E. Rogers, B. K. Tanner, G. Antorrena and F. Palacio, J. Chem. Soc., Dalton Trans., 1997, 2539.
14. L. Beer, A. W. Cordes, D. J. T. Myles, R. T. Oakley and N. J. Taylor, CrystEngComm, 2000, 2, 109.
15. In addition, K. Preuss (University of Guelph) has identified a third polymorph of 4 [personal communication].
16. A. M. Bell, J. N. B. Smith, J. P. Attfield, J. M. Rawson, K. Shankland and W. I. F. David, New J. Chem., 1999, 23, 565.
17. T. Kottke and D. Stalke, J. Appl. Crystallogr., 1993, 26, 615.
18. G. M. Sheldrick, University of Göttingen, Germany, 1997.
19. L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
20. I. J. Bruno, J. C. Cole, P. R. Edgington, M. K. Kessler, C. F. Macrae, P. McCabe, J. Pearson and R. Taylor, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 389.
21. Quantum Cache, version 5.0, Fujitsu Co., 2001.
22. J. M. Rawson, C. S. Clarke and D. W. Bruce, Magn. Reson. Chem., 2009, 47, 3.
23. M. F. Guest, I. J. Bush, H. J. J. van Dam, P. Sherwood, J. M. H. Thomas, J. H. van Lenthe, R. W. A. Havenith and J. Kendrick, Mol. Phys., 2005, 103, 719 . GAMESS-UK is a package of ab initio programs. See: http://www.cfs.dl.ac.uk/gamess-uk/index.shtml.
24. A. D. Bond, C. S. Clarke, D. A. Haynes and J. M. Rawson, Chem. Commun., 2003, 2774.
25. (a) A. Vegas, A. Pérez-Salazar, A. J. Banister and R. G. Hey, J. Chem. Soc., Dalton Trans., 1980, 1812; (b) A. W. Cordes, R. C. Haddon, R. T. Oakley, L. F. Schneemeyer, J. V. Waszczak, K. M. Young and N. M. Zimmerman, J. Am. Chem. Soc., 1991, 113, 582; (c) N. Bricklebank, S. Hargreaves and S. E. Spey, Polyhedron, 2000, 19, 1163.
26. A. Alberola, R. J. Less, C. M. Pask, J. M. Rawson, F. Palacio, P. Oliete, C. Paulsen, A. Yamaguchi, R. D. Farley and D. M. Murphy, Angew. Chem., Int. Ed., 2003, 42, 4782.
27. A. Alberola, C. S. Clarke, D. A. Haynes, S. I. Pascu and J. M. Rawson, Chem. Commun., 2005, 4726.
28. H. U. Hofs, J. W. Bats, R. Gleiter, G. Hartmann, R. Mews, M. Eckert-Maksic, H. Oberhammer and G. M. Sheldrick, Chem. Ber., 1985, 118, 3781.
29. S. C. Nyburg and C. H. Faerman, Acta Crystallogr., Sect. B: Struct. Sci., 1985, 41, 274.
30. G. R. Desiraju, Crystal Engineering: The Design of Organic Solids, Materials Science Monographs54, Elsevier Press, Amsterdam, 1989.
31. Notably with bulky substituents such as the 2,4,6-(CF₃)₃C₆H₂CNSSN the bulky trifluoromethyl group destabilises a coplanar arrangement of phenyl and dithiadiazolyl groups and the SN-I interaction is favoured. See ref. 27.
32. G. D. McManus, J. M. Rawson, N. Feeder, J. van Duijn, E. J. L. McInnes, J. Novoa, R. Burriel, F. Palacio and P. Oliete, J. Mater. Chem., 2001, 11, 1992.
33. A. D. Bond, D. A. Haynes and J. M. Rawson, Can. J. Chem., 2002, 80, 1507.
34. C. Knapp, E. Lork, T. Borrmann, W. D. Stohrer and R. Mews, Eur. J. Inorg. Chem., 2003, 3211.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, analysis, tables of bond lengths and angles, details of theoretical calculations. CCDC reference numbers 736245–736251. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b911636b

‡ Present address: Department of Chemistry and Polymer Science, Stellenbosch University, P. Bag X1, Matieland, 7602, Republic of South Africa. E-mail: dhaynes@sun.ac.za.

§ Present address: Department of Chemistry, University of Bath, Bath, UK BA2 7AY.

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