Asymmetric urea, thiourea and squaramide receptors as minimal-anion binding motifs

Figures & data

Figure 1. Minimal anion-binding receptors 1-4.

Scheme 1. G-series chemical warfare agents and their hydrolysis products.

Table 1. Experimental parameters used for fluorescence scans of compounds 1-3.

	1	2	3
Excitation/nm	335	216	250
Start/nm	375	256	260
End/nm	575	456	450
Slit size/mm	20	20	5

Scheme 2. Synthesis of minimal receptors: 1 (X = O, R = NO₂), 2 (X = S, R = NO₂), 3 (X = O, R = CF₃) and 4.

Figure 2. Colour changes of receptors (from top) 1, 2, 3 (in acetonitrile) and 4 (in DMSO) with one equivalent of TBA anions (5 mM in acetonitrile): (l to r) free receptor, F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, CH₃CO₂⁻, H₂PO₄⁻, OH⁻, HSO₄⁻, BF₄⁻.

Table 2. Bathochromic shifts of the main absorption bands of 1 to 3 from the initial λ_max shown on addition of excess anion. *Not determined due to overlap of anion and receptor absorbance.

	1	2	3
Anion	Δλ 335 nm	Δλ 340 nm	Δλ 249 nm
F^−	22	26	7
Cl^−	9	13	2
Br^−	0	5	1
I^−	0	0	*
NO3^−	1	2	0
H2PO4^−	19	24	6
CH3CO2^−	25	28	9
OH^−	25	22	6

Figure 3. Examples of UV-visible titration responses of 2: F⁻ (top) and NO₃⁻ (bottom).

Table 3. Anion-binding constants calculated from UV-vis titrations for receptors 1-3 in acetonitrile for their respective TBA salts. *Not determined.

Anion	Association constant/ M^−1
	1	2	3
F^−	2340 ± 135	10110 ± 915	1740 ± 95
Cl^−	480 ± 45	*	30
Br^−	2850 ± 420	*	*
I^−	2825 ± 500	210 ± 10	*
NO3^−	3170 ± 500	*	*
CH3CO2^−	29770 ± 4460	32150 ± 7630	15760 ± 1180
H2PO4^−	2965 ± 150	2585 ± 185	2990 ± 180
OH^−	9035 ± 1060	85550 ± 9070	37270 ± 5550

Figure 4. Fluorescent response of receptor 2 to additions of TBAF (top) and TBAOH (middle) in acetonitrile with a comparison over 5 equivalents of anions (bottom).

Table 4. Gibbs’ free energies calculated by PM6.

Anion	Complex (ΔG/ kJmol^−1)
	1	2	3	4
F^−	−242.19	−266.29	−162.46	−272.27
Cl^−	−153.72	−169.64	−83.64	−177.28
Br^−	−157.02	−169.89	−88.39	−176.43
I^−	−118.65	−128.19	−51.19	−136.01
NO3^−	−103.41	−106.15	−32.14	−110.30
CH3CO2^−	−143.09	−159.86	−67.90	−155.23
H2PO4^−	−105.05	−117.87	−28.14	−118.32
OH^−	−319.95	−344.25	−237.89	−344.68
HCO3^−	−86.00	−102.41	−46.69	−92.68

Table 5. Experimental and computed structural data for receptors 1-3.

		1	2	3
Torsion angle C1N2C3N4/°	expt	176.97	4.77	177.15
	calcd	175.85	3.50	177.92
Bond length N2-H/Å	expt	0.892	0.860	0.844
	calcd	1.007	1.011	1.001
Bond length N4-H/Å	expt	0.892	0.859	0.860
	calcd	1.008	1.011	1.011
Bond length X···N2/N4/Å	expt	2.216/2.206	2.504/—	2.095/2.168

Figure 5. Structures of a) 1·CH₃CO₂⁻, b) (1·HCO₃⁻)₂, c) 1·F⁻ and d) 1·OH⁻ from DFT calculations.

Figure 6. Crystal structure of 1 showing the solid-state molecular structure (left) and hydrogen bonding within the crystal structure (right).

Table 6. Single crystal X-ray crystallographic details and parameters for 1, 1·TBA·CH₃CO₂, 1·TBACO₃H, 2 and 3.

Parameters	1	1·TBA·CH3CO2·H2O	1·TBA·CO3H·2 H2O	2	3
Formula	C8H9N3O3	C26H50N4O6	C25H50N4O8	C8H9N3O2S	C9H9F3N2O
Mr	195.18	514.70	534.70	211.25	218.18
Cryst. Syst.	monoclinic	triclinic	triclinic	monoclinic	monoclinic
Space group	P21/n	$\bar{P}$1	$\bar{P}$1	P21/n	P21/n
a/Å	4.3986(1)	8.4403(2)	8.1112(16)	8.2921(1)	14.7070(4)
b/Å	22.2681(5)	10.0978(2)	9.3645(19)	8.6878(1)	4.6270(1)
c/Å	8.9594(2)	18.3810(4)	19.959(4)	13.2480(2)	15.0549(4)
α/deg	90	79.768(2)	92.83(3)	90	90
β/deg	101.953(2)	87.405(2)	99.77(3)	106.759(2)	113.723(3)
γ/deg	90	71.295(2)	91.11(3)	90	90
V/Å^3	858.53(3)	1460.13(6)	1491.7(5)	913.85(2)	937.72(5)
T/K	100	100	100	100	100
Z	4	2	2	4	4
ρcalcd/g.cm^−3	1.510	1.171	1.190	1.535	1.545
λ/Å	1.54178	1.54178	0.71073	1.54184	1.54184
No. of indep reflns	1744	5209	8782	1799	1938
No. reflns with I > 2σ(I)	1586	4984	7152	1763	1822
No. of params	128	339	346	128	137
No. of restraints	0	0	0	0	0
Final R1	0.0423 (1586)	0.0351 (4984)	0.0431 (7152)	0.0338 (1736)	0.0362 (1822)
wR2 (all data)	0.1187 (1744)	0.0807 (5209)	0.1090 (8782)	0.0937 (1799)	0.1034 (1938
Goodness of fit	1.044	1.065	1.037	1.048	1.040
Largest residuals/e Å^−3	0.309	0.229	0.408	0.323	0.344

Figure 7. Solid-state molecular structure from single crystal X-ray diffraction of 1·TBA·CH₃CO₂·H₂O showing hydrogen bonding (left) and crystal packing (right) with TBA counterions removed for clarity.

Figure 8. Crystal structure of 1·TBA·HCO₃·2 H₂O showing hydrogen bonding (left) and the (HCO₃⁻)₂ dimer (right) with TBA counterions removed for clarity.

Figure 9. Crystal structure of 2 showing the solid-state molecular structure (left) and intermolecular hydrogen bonding within the crystal structure (right).

Figure 10. Crystal structure of 3 showing the solid-state molecular structure (left) and hydrogen bonding within the crystal structure (right).