The tropolone–iso­butyl­amine complex: a hydrogen-bonded troponoid without dominant π–π inter­actions

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This mol­ecule, which juxtaposes ketonic, hy­droxy­lic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing inter­molecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π–π, C—H...π, or ion–π inter­actions. The organic salt (TrOH·iBA) formed by a facile proton-transfer reaction between tropolone (TrOH) and iso­butyl­amine (iBA), namely iso­butyl­ammonium 7-oxo­cyclo­hepta-1,3,5-trien-1-olate, C₄H₁₂N⁺·C₇H₅O₂⁻, has been investigated by X-ray crystallography, with complementary quantum-chemical and statistical-database analyses serving to elucidate the nature of attendant inter­molecular inter­actions and their synergistic effects upon lattice-packing phenomena. The crystal structure deduced from low-temperature diffraction measurements displays extensive hydrogen-bonding networks, yet shows little evidence of the aryl forces (viz. π–π, C—H...π, and ion–π inter­actions) that typically dominate this class of compounds. Density functional calculations performed with and without the imposition of periodic boundary conditions (the latter entailing isolated subunits) documented the specificity and directionality of noncovalent inter­actions occurring between the proton-donating and proton-accepting sites of TrOH and iBA, as well as the absence of aromatic coupling mediated by the seven-membered ring of TrOH. A statistical comparison of the structural parameters extracted for key hydrogen-bond linkages to those reported for 44 previously known crystals that support similar binding motifs revealed TrOH·iBA to possess the shortest donor–acceptor distances of any troponoid-based complex, combined with unambiguous signatures of enhanced proton-delocalization processes that putatively stabilize the corresponding crystalline lattice and facilitate its surprisingly rapid formation under ambient conditions.

Computing details

Data collection: CrystalClear-SM Expert (Rigaku, 2011); cell refinement: CrystalClear-SM Expert (Rigaku, 2011); data reduction: CrystalClear-SM Expert (Rigaku, 2011); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: CIFTAB (Sheldrick, 2015b).

Isobutylammonium 7-oxocyclohepta-1,3,5-trien-1-olate

Crystal data

C4H12N^+·C7H5O2^−	F(000) = 424
Mr = 195.25	Dx = 1.163 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54178 Å
a = 6.5770 (2) Å	Cell parameters from 26705 reflections
b = 9.5518 (3) Å	θ = 2.5–68.0°
c = 17.8425 (13) Å	µ = 0.64 mm^−1
β = 96.026 (5)°	T = 93 K
V = 1114.71 (10) Å^3	Needle, colorless
Z = 4	0.40 × 0.04 × 0.04 mm

Data collection

Rigaku Saturn 944+ CCD diffractometer	2027 independent reflections
Radiation source: Rotating Anode	1761 reflections with I > 2σ(I)
Detector resolution: 22.2 pixels mm^-1	Rint = 0.079
ω scans	θmax = 68.1°, θmin = 5.0°
Absorption correction: multi-scan (REQAB; Jacobson, 1998)	h = −7→7
Tmin = 0.773, Tmax = 0.975	k = −11→11
34893 measured reflections	l = −21→21

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F^2 > 2σ(F^2)] = 0.036	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.096	w = 1/[σ^2(Fo^2) + (0.0529P)^2 + 0.1575P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
2027 reflections	Δρmax = 0.14 e Å^−3
141 parameters	Δρmin = −0.21 e Å^−3
0 restraints

Special details

Experimental. Rigaku MicroMax-007HF, Rigaku Saturn 944+ CCD

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2sigma(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

The hydrogen atoms were first found in the difference map, then generated geometrically and refined as riding atoms with C-H distances = 0.95- 0.99 Å and U_iso(H) = 1.2 times U_eq(C) for CH and CH₂ groups and U_iso(H) = 1.5 times U_eq(C) for CH₃ groups. The only exceptions are H1a, H1b, and H1c, which are freely refining. These hydrogen atoms are also a part of refined hydrogen bonds which are reported in the RES file appended to this CIF.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.42316 (13)	0.39405 (9)	0.57684 (5)	0.0346 (2)
O2	0.08069 (12)	0.47078 (8)	0.62236 (4)	0.0280 (2)
N1	0.78054 (16)	0.39758 (10)	0.51132 (6)	0.0265 (2)
H1A	0.868 (2)	0.4389 (15)	0.5499 (9)	0.043 (4)*
H1B	0.649 (3)	0.3858 (16)	0.5340 (9)	0.054 (5)*
H1C	0.763 (2)	0.4608 (17)	0.4696 (9)	0.050 (4)*
C1	0.37445 (17)	0.33118 (11)	0.63515 (6)	0.0262 (3)
C2	0.51098 (18)	0.23298 (12)	0.67252 (7)	0.0312 (3)
H2	0.6373	0.2246	0.6515	0.037*
C3	0.4945 (2)	0.14685 (12)	0.73408 (7)	0.0334 (3)
H3	0.6116	0.0914	0.7490	0.040*
C4	0.3332 (2)	0.12959 (12)	0.77701 (7)	0.0342 (3)
H4	0.3497	0.0611	0.8158	0.041*
C5	0.1501 (2)	0.20208 (13)	0.76920 (7)	0.0340 (3)
H5	0.0553	0.1750	0.8032	0.041*
C6	0.08340 (19)	0.30809 (12)	0.71946 (7)	0.0302 (3)
H6	−0.0473	0.3445	0.7270	0.036*
C7	0.17364 (17)	0.37196 (11)	0.65968 (6)	0.0245 (3)
C8	0.87004 (18)	0.26312 (12)	0.48895 (7)	0.0285 (3)
H8A	0.8741	0.1968	0.5317	0.034*
H8B	1.0125	0.2797	0.4780	0.034*
C9	0.75123 (19)	0.19696 (12)	0.42037 (7)	0.0315 (3)
H9	0.7539	0.2628	0.3769	0.038*
C10	0.8596 (2)	0.06189 (14)	0.40150 (8)	0.0430 (4)
H10A	1.0035	0.0820	0.3962	0.065*
H10B	0.7939	0.0233	0.3541	0.065*
H10C	0.8511	−0.0063	0.4421	0.065*
C11	0.5289 (2)	0.16975 (14)	0.43255 (8)	0.0415 (3)
H11A	0.5232	0.1124	0.4778	0.062*
H11B	0.4609	0.1202	0.3888	0.062*
H11C	0.4595	0.2591	0.4389	0.062*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0344 (5)	0.0353 (5)	0.0359 (5)	0.0059 (3)	0.0120 (4)	0.0122 (4)
O2	0.0320 (5)	0.0250 (4)	0.0268 (4)	0.0036 (3)	0.0027 (3)	0.0004 (3)
N1	0.0293 (5)	0.0231 (5)	0.0273 (6)	0.0017 (4)	0.0033 (4)	0.0022 (4)
C1	0.0297 (6)	0.0233 (5)	0.0257 (6)	−0.0021 (4)	0.0028 (5)	0.0003 (5)
C2	0.0297 (6)	0.0297 (6)	0.0342 (7)	0.0017 (5)	0.0024 (5)	0.0018 (5)
C3	0.0391 (7)	0.0273 (6)	0.0322 (7)	0.0030 (5)	−0.0047 (5)	0.0024 (5)
C4	0.0506 (8)	0.0261 (6)	0.0250 (7)	−0.0012 (5)	−0.0001 (6)	0.0041 (5)
C5	0.0474 (8)	0.0302 (6)	0.0256 (6)	−0.0048 (5)	0.0095 (5)	0.0011 (5)
C6	0.0334 (6)	0.0294 (6)	0.0286 (7)	−0.0002 (5)	0.0066 (5)	−0.0006 (5)
C7	0.0302 (6)	0.0204 (5)	0.0224 (6)	−0.0015 (4)	0.0006 (4)	−0.0035 (4)
C8	0.0315 (6)	0.0240 (6)	0.0296 (6)	0.0032 (5)	0.0014 (5)	0.0002 (5)
C9	0.0384 (7)	0.0272 (6)	0.0279 (7)	−0.0015 (5)	−0.0005 (5)	0.0026 (5)
C10	0.0547 (9)	0.0335 (7)	0.0395 (8)	0.0012 (6)	−0.0010 (6)	−0.0079 (6)
C11	0.0402 (7)	0.0355 (7)	0.0471 (8)	−0.0080 (5)	−0.0032 (6)	0.0050 (6)

Geometric parameters (Å, º)

O1—C1	1.2710 (14)	C5—H5	0.9500
O2—C7	1.2737 (13)	C6—C7	1.4125 (17)
N1—C8	1.4848 (14)	C6—H6	0.9500
N1—H1A	0.938 (16)	C8—C9	1.5187 (16)
N1—H1B	1.001 (17)	C8—H8A	0.9900
N1—H1C	0.956 (17)	C8—H8B	0.9900
C1—C2	1.4161 (16)	C9—C11	1.5231 (18)
C1—C7	1.4863 (16)	C9—C10	1.5287 (18)
C2—C3	1.3856 (18)	C9—H9	1.0000
C2—H2	0.9500	C10—H10A	0.9800
C3—C4	1.3822 (19)	C10—H10B	0.9800
C3—H3	0.9500	C10—H10C	0.9800
C4—C5	1.3835 (18)	C11—H11A	0.9800
C4—H4	0.9500	C11—H11B	0.9800
C5—C6	1.3868 (17)	C11—H11C	0.9800
C8—N1—H1A	109.3 (9)	C6—C7—C1	124.47 (10)
C8—N1—H1B	113.3 (9)	N1—C8—C9	113.05 (9)
H1A—N1—H1B	104.2 (13)	N1—C8—H8A	109.0
C8—N1—H1C	110.8 (10)	C9—C8—H8A	109.0
H1A—N1—H1C	108.7 (13)	N1—C8—H8B	109.0
H1B—N1—H1C	110.3 (13)	C9—C8—H8B	109.0
O1—C1—C2	119.77 (11)	H8A—C8—H8B	107.8
O1—C1—C7	115.17 (10)	C8—C9—C11	112.12 (11)
C2—C1—C7	125.04 (10)	C8—C9—C10	108.44 (10)
C3—C2—C1	131.92 (12)	C11—C9—C10	111.37 (11)
C3—C2—H2	114.0	C8—C9—H9	108.3
C1—C2—H2	114.0	C11—C9—H9	108.3
C4—C3—C2	129.69 (12)	C10—C9—H9	108.3
C4—C3—H3	115.2	C9—C10—H10A	109.5
C2—C3—H3	115.2	C9—C10—H10B	109.5
C3—C4—C5	126.53 (12)	H10A—C10—H10B	109.5
C3—C4—H4	116.7	C9—C10—H10C	109.5
C5—C4—H4	116.7	H10A—C10—H10C	109.5
C4—C5—C6	130.04 (12)	H10B—C10—H10C	109.5
C4—C5—H5	115.0	C9—C11—H11A	109.5
C6—C5—H5	115.0	C9—C11—H11B	109.5
C5—C6—C7	131.98 (12)	H11A—C11—H11B	109.5
C5—C6—H6	114.0	C9—C11—H11C	109.5
C7—C6—H6	114.0	H11A—C11—H11C	109.5
O2—C7—C6	119.88 (11)	H11B—C11—H11C	109.5
O2—C7—C1	115.65 (10)
O1—C1—C2—C3	−177.01 (12)	C5—C6—C7—C1	1.8 (2)
C7—C1—C2—C3	4.6 (2)	O1—C1—C7—O2	−4.22 (14)
C1—C2—C3—C4	1.5 (2)	C2—C1—C7—O2	174.21 (10)
C2—C3—C4—C5	−3.1 (2)	O1—C1—C7—C6	175.13 (10)
C3—C4—C5—C6	−1.2 (2)	C2—C1—C7—C6	−6.45 (18)
C4—C5—C6—C7	3.0 (2)	N1—C8—C9—C11	−57.99 (13)
C5—C6—C7—O2	−178.90 (12)	N1—C8—C9—C10	178.63 (10)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O2^i	0.938 (16)	1.826 (16)	2.7368 (13)	163.0 (14)
N1—H1B···O1	1.001 (17)	1.740 (18)	2.7330 (13)	170.7 (14)
N1—H1C···O1^ii	0.956 (17)	1.974 (17)	2.7907 (13)	142.1 (14)
N1—H1C···O2^ii	0.956 (17)	2.128 (17)	2.9258 (14)	140.0 (13)

Symmetry codes: (i) x+1, y, z; (ii) −x+1, −y+1, −z+1.