Experiments were carried out with Mo Kα radiation using a Rigaku Xcalibur Sapphire3 diffractometer. Absorption was corrected for by multi-scan methods (CrysAlis PRO; Rigaku OD, 2020 ▸).

	I	III	IV
Crystal data
Chemical formula	C4H4O2	C4H5O2+·AsF6−	C6H8O4
M r	84.07	274.00	144.12
Crystal system, space group	Monoclinic, P21/n	Monoclinic, P21/n	Triclinic, P
tem­per­a­ture (K)	111	111	112
a, b, c (Å)	3.8409 (5), 15.593 (2), 7.6149 (10)	6.9609 (5), 8.9319 (7), 13.7189 (9)	3.8726 (11), 5.6546 (10), 8.3778 (18)
α, β, γ (°)	90, 99.910 (12), 90	90, 91.664 (7), 90	100.642 (16), 100.42 (2), 105.73 (2)
V (Å3)	449.27 (10)	852.60 (11)	168.30 (7)
Z	4	4	1
μ (mm−1)	0.10	4.06	0.12
Crystal size (mm)	1.00 × 0.56 × 0.31	0.90 × 0.21 × 0.15	0.52 × 0.46 × 0.35
Data collection
Tmin, Tmax	0.457, 1.000	0.582, 1.000	0.579, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	3885, 1060, 891	4205, 2303, 1900	1403, 825, 703
R int	0.023	0.029	0.015
θmax (°)	27.9	29.1	28.3
(sin θ/λ)max (Å−1)	0.658	0.685	0.667
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.036, 0.099, 1.05	0.039, 0.099, 1.05	0.048, 0.144, 1.04
No. of reflections	1060	2303	825
No. of parameters	71	123	51
No. of restraints	0	0	0
H-atom treatment	All H-atom parameters refined	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.18, −0.15	0.87, −0.92	0.52, −0.24