Partial ordering of tripivaloylmethane at 110 K

Tripivaloylmethane [systematic name: 4-(2,2-dimethyl­pro­panoyl)-2,2,6,6-tetra­methyl­heptane-3,5-dione], C₁₆H₂₈O₃, is known to crystallize at room temperature in the space group R3m with three mol­ecules in the unit cell. The mol­ecules are conformationally chiral and pack so that each mol­ecular site is occupied with equal probability by the two enantiomers. Upon cooling to 110 K, the structure partially orders; two mol­ecules in the unit cell order into two different conformations of opposite chirality, while the third remains disordered. The symmetry of the resulting crystal is P3, with each of the mol­ecules lying about a different threefold rotation axis. This paper describes an unusual case of order-disorder phase transition in which the structure partially orders by changes of mol­ecular conformation in the single crystals. Such behaviour is of interest in the study of phase transitions and mol­ecular motion in the solid state.

S1. Comment

Triacylmethanes are a group of acyclic 1,3,3'-triketones which can be obtained by reaction of β-diketones (Lim et al., 2001), their salts (Rogers & Smith, 1955) and transition metal complexes (Murdoch & Nonhebel, 1962; Collman et al., 1963 or 1962??) with acyl halogenides and acid anhydrides. Although such compounds have been known since the late nineteenth century (Fischer & Bülow, 1885; Claisen, 1893), they have not received much attention to date. However, cyclical 1,3,3'-triketones have attracted some interest as a result of their application as herbicides (Lee et al., 1998, and references therein).

In a previous publication (Kaitner & Stilinović, 2007) we have reported the structure of tripivaloylmethane, (I), at room temperature. The three carbonyl groups of the triketo group define a helix, rendering the molecular conformation chiral. The compound has been found to crystallize in the space group R3m with a conformational enantiomeric disorder. The molecules are in the 3(a) special positions of the space group, so that each molecular site is occupied with equal probability by the two (M and P) enantiomers. It remained unclear whether the equal occupancy by the two enantiomers of the average molecular site was caused by spatial or temporal disorder or perhaps a combination of both.

Recently we have redetermined the crystal structure of tripivaloylmethane at 110 K. Although the molecular symmetry remains as C3, the space-group symmetry is reduced from R3m at ambient temperature to P3. The unit cell of tripivaloylmethane at 110 K contains of three independent molecules (Fig. 1) in the 1(a), 1(b) and 1(c) positions of the space group, i.e. all lying about threefold rotation axes. Two molecules are ordered: one as an M and the other as a P conformational enantiomer. In addition to the difference in the absolute sense of the triketo group twist, the two molecules also differ somewhat in the torsion of the tert-butyl groups to the respective carbonyl groups (the O11—C12—C13—C14 torsion angle is ca 44.22° and the O21—C22—C23—C24 angle is ca -38.41°). The third molecule remains disordered with M and P conformers occupying the same molecular sites. The occupancies of the two conformers were refined to 50% within the experimental error. The relative positions of all three molecules remain almost entirely unchanged from those in the room temperature structure (Fig. 2).

The ordered molecules stack to form homochiral columns along the [001] direction. Each column is neighboured by three columns of the opposite chirality and by three columns of disordered molecules. The structure could be viewed as a quasi-hexagonal packing of homochiral columns with the columns of disordered molecules in the voids (Fig. 3). The question remains whether the disordered molecules are placed at random within the columns themselves, or the `disordered' columns are in fact homochiral, but placed stochastically throughout the structure. However, since the intermolecular distances are the same for all three types of columns it would be surprising that only two out of three molecules are ordered, so we are inclined toward the latter structural model.

Comparing the packing diagrams of the room temperature structure with that of the 110 K one, it is obvious that the ordering i.e. the formation of the homochiral columns, had to include flipping of the triketo groups in (at least) one-third of the overall number of molecules. The flipping must include passing through a transition conformation of C_3v symmetry, which is more elongated along the threefold axis than the equilibrium C₃ conformation; the C1···centroid(O1, O2, O3) distance is ca 1.75 Å for the putative C_3v conformation, as compared with an average of ca 1.50 Å for the C₃ conformations.

Related literature

For related literature, see: Claisen (1893); Fischer & Bülow (1885); Flack (1983); Lee et al. (1998); Lim et al. (2001); Murdoch & Nonhebel (1962); Rogers & Smith (1955).

S2. Experimental

The preparation and crystallization of (I) were described in a previous publication (Kaitner & Stilinović, 2007).

S3. Refinement

In the absence of significant resonant scattering, the Flack (1983) parameter was indeterminate; hence the Friedel equivalent reflections were averaged. No restraints were used for modelling the structural disorder. Methyl H atoms were placed in calculated positions and treated as riding [C—H = 0.96 Å and U_iso(H) = 1.5U_eq(C)]. The methine H atoms of the ordered molecules were treated as riding atoms with C—H distances of 0.98 Å. The methine H atom of the disordered molecule was located in a difference map and refined isotropically, giving a C—H distance of 1.04 (9) Å.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2003); cell refinement: CrysAlis RED (Oxford Diffraction, 2003); data reduction: CrysAlis RED (Oxford Diffraction, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The molecular structures and the numbering schemes of the three independent molecules of tripivaloylmethane at 110 K. Displacement ellipsoids are drawn at the 50% probability level, and H atoms have been omitted for clarity.
	Fig. 2. Packing diagrams for the two polymorphs of tripivaloylmethane, viewed along the c axis: (a) the room-temperature polymorph and (b) the low-temperature polymorph. The M conformers of the disordered molecules are shown with open and the P conformers with full bonds.
	Fig. 3. Representation of the stacking of molecular columns for the low-temperature polymorph viewed along the c axis. The P conformers are shown in dark grey (red in the online version of the journal), the M conformers in pale grey (yellow), and the disordered molecules as CPK models.

4-(2,2-dimethylpropanoyl)-2,2,6,6-tetramethylheptane-3,5-dione

Crystal data

C16H28O3	Dx = 1.126 Mg m^−3
Mr = 268.38	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3	Cell parameters from 554 reflections
Hall symbol: P 3	θ = 4.6–32.0°
a = 15.5892 (5) Å	µ = 0.08 mm^−1
c = 5.6424 (3) Å	T = 110 K
V = 1187.52 (8) Å^3	Prism, colourless
Z = 3	0.65 × 0.29 × 0.22 mm
F(000) = 444

Data collection

Oxford Diffraction Xcalibur CCD diffractometer	1597 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
Graphite monochromator	θmax = 27.0°, θmin = 3.9°
ω scans	h = −19→19
9435 measured reflections	k = −19→17
1725 independent reflections	l = −7→7

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.055	w = 1/[σ^2(Fo^2) + (0.0498P)^2 + 0.7674P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.133	(Δ/σ)max = 0.004
S = 1.11	Δρmax = 0.34 e Å^−3
1728 reflections	Δρmin = −0.29 e Å^−3
220 parameters

Special details

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

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
C11	0	0	0.8437 (8)	0.0155 (9)
H11	0	0	0.67	0.071*
C13	0.1971 (2)	0.1009 (2)	0.8202 (5)	0.0241 (6)
O11	0.10198 (16)	0.12907 (16)	1.1109 (4)	0.0259 (5)
C12	0.10056 (19)	0.0804 (2)	0.9419 (5)	0.0187 (6)
C15	0.1847 (2)	0.0177 (3)	0.6552 (6)	0.0318 (8)
H15A	0.1627	−0.042	0.7451	0.048*
H15B	0.247	0.0359	0.5813	0.048*
H15C	0.1365	0.0069	0.5355	0.048*
C16	0.2320 (3)	0.1949 (3)	0.6735 (7)	0.0408 (9)
H16A	0.2401	0.2479	0.775	0.061*
H16B	0.1836	0.1834	0.5538	0.061*
H16C	0.2941	0.2124	0.5996	0.061*
C14	0.2726 (3)	0.1156 (4)	1.0120 (7)	0.0459 (11)
H14A	0.2488	0.0556	1.1015	0.069*
H14B	0.2817	0.1683	1.1158	0.069*
H14C	0.3347	0.1321	0.9391	0.069*
C21	0.3333	0.6667	0.1767 (8)	0.0143 (9)
H21	0.3333	0.6667	0.003	0.05 (2)*
C23	0.5302 (2)	0.7687 (2)	0.1522 (5)	0.0225 (6)
C22	0.4348 (2)	0.6890 (2)	0.2719 (5)	0.0167 (6)
O21	0.43720 (17)	0.64248 (17)	0.4412 (4)	0.0277 (5)
C26	0.5163 (3)	0.8396 (3)	−0.0087 (7)	0.0352 (8)
H26A	0.4673	0.8024	−0.1269	0.053*
H26B	0.578	0.884	−0.0846	0.053*
H26C	0.4949	0.877	0.0842	0.053*
C25	0.5651 (3)	0.7113 (3)	−0.0064 (7)	0.0414 (9)
H25A	0.5163	0.6759	−0.1262	0.062*
H25B	0.574	0.6653	0.0888	0.062*
H25C	0.6268	0.7571	−0.0802	0.062*
C24	0.6070 (3)	0.8248 (4)	0.3409 (7)	0.0484 (12)
H24A	0.6146	0.7787	0.4393	0.073*
H24B	0.5861	0.862	0.4363	0.073*
H24C	0.6692	0.8692	0.267	0.073*
C1	0.6667	0.3333	0.5087 (9)	0.0152 (9)
C2A	0.6491 (5)	0.4159 (5)	0.6115 (10)	0.0177 (13)	0.5
C3	0.5742 (2)	0.4380 (2)	0.4834 (5)	0.0213 (6)
C2B	0.5901 (5)	0.3587 (5)	0.6001 (9)	0.0165 (12)	0.5
O1B	0.5399 (3)	0.3127 (4)	0.7691 (8)	0.0292 (13)	0.5
O1A	0.6969 (3)	0.4637 (3)	0.7792 (8)	0.0246 (11)	0.5
C6B	0.4861 (8)	0.3859 (8)	0.3118 (19)	0.036 (2)	0.5
H6B1	0.438	0.3231	0.3769	0.054*	0.5
H6B2	0.509	0.3757	0.1621	0.054*	0.5
H6B3	0.4564	0.4264	0.2892	0.054*	0.5
C6A	0.5018 (6)	0.3565 (7)	0.3190 (17)	0.0267 (18)	0.5
H6A1	0.4625	0.2971	0.4087	0.04*	0.5
H6A2	0.5379	0.3441	0.199	0.04*	0.5
H6A3	0.4593	0.3768	0.2456	0.04*	0.5
C4B	0.5527 (9)	0.4962 (9)	0.6719 (19)	0.042 (2)	0.5
H4B1	0.5117	0.4518	0.7942	0.062*	0.5
H4B2	0.5191	0.5269	0.5994	0.062*	0.5
H4B3	0.614	0.5463	0.7397	0.062*	0.5
C4A	0.5104 (8)	0.4526 (8)	0.6807 (18)	0.034 (2)	0.5
H4A1	0.4726	0.3918	0.7667	0.051*	0.5
H4A2	0.4662	0.4707	0.6071	0.051*	0.5
H4A3	0.5536	0.504	0.7878	0.051*	0.5
C5A	0.6358 (8)	0.5311 (6)	0.3406 (17)	0.037 (2)	0.5
H5A1	0.6821	0.5827	0.4432	0.056*	0.5
H5A2	0.5932	0.5516	0.2682	0.056*	0.5
H5A3	0.6713	0.5183	0.2197	0.056*	0.5
C5B	0.6652 (7)	0.5125 (7)	0.333 (2)	0.041 (3)	0.5
H5B1	0.7111	0.4886	0.3188	0.061*	0.5
H5B2	0.6972	0.576	0.4098	0.061*	0.5
H5B3	0.6433	0.5187	0.1784	0.061*	0.5
H1	0.6667	0.3333	0.324 (16)	0.06 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C11	0.0158 (13)	0.0158 (13)	0.015 (2)	0.0079 (6)	0	0
C13	0.0190 (14)	0.0300 (16)	0.0190 (14)	0.0091 (12)	0.0007 (11)	0.0030 (12)
O11	0.0225 (11)	0.0259 (11)	0.0266 (12)	0.0101 (9)	−0.0046 (9)	−0.0079 (9)
C12	0.0157 (13)	0.0190 (14)	0.0179 (14)	0.0061 (11)	−0.0023 (10)	0.0021 (11)
C15	0.0225 (16)	0.0418 (19)	0.0382 (18)	0.0215 (15)	0.0040 (13)	−0.0034 (15)
C16	0.037 (2)	0.0262 (17)	0.041 (2)	0.0030 (15)	0.0124 (16)	0.0116 (15)
C14	0.0206 (17)	0.085 (3)	0.034 (2)	0.028 (2)	−0.0037 (14)	−0.005 (2)
C21	0.0156 (13)	0.0156 (13)	0.012 (2)	0.0078 (7)	0	0
C23	0.0145 (13)	0.0292 (15)	0.0197 (15)	0.0079 (12)	0.0011 (11)	−0.0001 (12)
C22	0.0166 (13)	0.0167 (13)	0.0200 (14)	0.0107 (11)	−0.0011 (10)	−0.0041 (10)
O21	0.0280 (12)	0.0326 (12)	0.0245 (12)	0.0166 (10)	−0.0043 (9)	0.0049 (9)
C26	0.0239 (16)	0.0331 (18)	0.042 (2)	0.0096 (14)	0.0108 (14)	0.0175 (15)
C25	0.035 (2)	0.050 (2)	0.041 (2)	0.0233 (18)	0.0160 (16)	0.0011 (17)
C24	0.0198 (17)	0.060 (3)	0.0296 (19)	−0.0069 (16)	−0.0052 (14)	−0.0051 (18)
C1	0.0128 (13)	0.0128 (13)	0.020 (2)	0.0064 (6)	0	0
C2A	0.019 (3)	0.018 (3)	0.017 (3)	0.010 (3)	0.003 (2)	0.002 (2)
C3	0.0254 (15)	0.0236 (14)	0.0226 (15)	0.0180 (12)	0.0009 (12)	0.0024 (11)
C2B	0.016 (3)	0.022 (3)	0.012 (3)	0.010 (3)	−0.003 (2)	−0.002 (2)
O1B	0.030 (3)	0.036 (3)	0.024 (3)	0.018 (2)	0.0127 (18)	0.0074 (18)
O1A	0.023 (2)	0.025 (2)	0.028 (3)	0.0142 (19)	−0.0047 (17)	−0.0094 (18)
C6B	0.034 (5)	0.034 (5)	0.038 (4)	0.016 (4)	−0.010 (3)	0.006 (4)
C6A	0.016 (4)	0.029 (5)	0.034 (4)	0.010 (3)	−0.004 (3)	0.004 (4)
C4B	0.063 (7)	0.048 (6)	0.030 (5)	0.041 (6)	0.000 (5)	−0.005 (5)
C4A	0.038 (5)	0.047 (6)	0.028 (4)	0.030 (4)	−0.004 (4)	−0.007 (5)
C5A	0.039 (5)	0.012 (3)	0.045 (5)	0.001 (3)	−0.011 (4)	0.004 (3)
C5B	0.028 (4)	0.031 (5)	0.071 (6)	0.020 (4)	0.002 (4)	0.023 (4)

Geometric parameters (Å, º)

C11—C12	1.540 (3)	C24—H24C	0.96
C11—C12^i	1.540 (3)	C1—C2B^v	1.523 (6)
C11—C12^ii	1.540 (3)	C1—C2B^vi	1.523 (6)
C11—H11	0.98	C1—C2B	1.523 (6)
C13—C16	1.526 (5)	C1—C2A^vi	1.556 (6)
C13—C15	1.528 (5)	C1—C2A^v	1.556 (6)
C13—C14	1.529 (4)	C1—H1	1.04 (9)
C13—C12	1.536 (4)	C2A—O1A	1.204 (8)
O11—C12	1.211 (4)	C2A—C3	1.551 (6)
C15—H15A	0.96	C3—C5A	1.512 (9)
C15—H15B	0.96	C3—C6A	1.522 (10)
C15—H15C	0.96	C3—C2B	1.526 (6)
C16—H16A	0.96	C3—C6B	1.539 (10)
C16—H16B	0.96	C3—C4B	1.540 (11)
C16—H16C	0.96	C3—C5B	1.559 (10)
C14—H14A	0.96	C3—C4A	1.584 (10)
C14—H14B	0.96	C2B—O1B	1.215 (8)
C14—H14C	0.96	C6B—H6B1	0.96
C21—C22^iii	1.536 (3)	C6B—H6B2	0.96
C21—C22^iv	1.536 (3)	C6B—H6B3	0.96
C21—C22	1.536 (3)	C6A—H6A1	0.96
C21—H21	0.98	C6A—H6A2	0.96
C23—C24	1.512 (4)	C6A—H6A3	0.96
C23—C26	1.527 (5)	C4B—H4B1	0.96
C23—C22	1.538 (4)	C4B—H4B2	0.96
C23—C25	1.545 (5)	C4B—H4B3	0.96
C22—O21	1.211 (4)	C4A—H4A1	0.96
C26—H26A	0.96	C4A—H4A2	0.96
C26—H26B	0.96	C4A—H4A3	0.96
C26—H26C	0.96	C5A—H5A1	0.96
C25—H25A	0.96	C5A—H5A2	0.96
C25—H25B	0.96	C5A—H5A3	0.96
C25—H25C	0.96	C5B—H5B1	0.96
C24—H24A	0.96	C5B—H5B2	0.96
C24—H24B	0.96	C5B—H5B3	0.96
C12—C11—C12^i	107.8 (2)	C23—C24—H24B	109.5
C12—C11—C12^ii	107.8 (2)	H24A—C24—H24B	109.5
C12^i—C11—C12^ii	107.8 (2)	C23—C24—H24C	109.5
C12—C11—H11	111.1	H24A—C24—H24C	109.5
C12^i—C11—H11	111.1	H24B—C24—H24C	109.5
C12^ii—C11—H11	111.1	C2B—C1—H1	109.8 (2)
C16—C13—C15	108.5 (3)	O1A—C2A—C3	122.0 (5)
C16—C13—C14	111.2 (3)	O1A—C2A—O1B	100.0 (4)
C15—C13—C14	108.7 (3)	C3—C2A—O1B	87.2 (4)
C16—C13—C12	106.5 (3)	C5A—C3—C6A	109.1 (5)
C15—C13—C12	113.6 (3)	C2B—C3—C6B	107.9 (5)
C14—C13—C12	108.4 (3)	C2B—C3—C4B	110.6 (5)
O11—C12—C13	120.8 (3)	C6B—C3—C4B	110.2 (7)
O11—C12—C11	119.0 (3)	C5A—C3—C2A	106.0 (5)
C13—C12—C11	120.1 (3)	C6A—C3—C2A	115.2 (5)
C13—C15—H15A	109.5	C2B—C3—C5B	112.5 (4)
C13—C15—H15B	109.5	C6B—C3—C5B	106.9 (6)
H15A—C15—H15B	109.5	C4B—C3—C5B	108.8 (6)
C13—C15—H15C	109.5	C5A—C3—C4A	112.3 (6)
H15A—C15—H15C	109.5	C6A—C3—C4A	106.9 (5)
H15B—C15—H15C	109.5	C2A—C3—C4A	107.5 (5)
C13—C16—H16A	109.5	O1B—C2B—C1	118.1 (5)
C13—C16—H16B	109.5	O1B—C2B—C3	119.9 (5)
H16A—C16—H16B	109.5	C1—C2B—C3	122.0 (4)
C13—C16—H16C	109.5	C3—C6B—H6B1	109.5
H16A—C16—H16C	109.5	C3—C6B—H6B2	109.5
H16B—C16—H16C	109.5	H6B1—C6B—H6B2	109.5
C13—C14—H14A	109.5	C3—C6B—H6B3	109.5
C13—C14—H14B	109.5	H6B1—C6B—H6B3	109.5
H14A—C14—H14B	109.5	H6B2—C6B—H6B3	109.5
C13—C14—H14C	109.5	C3—C6A—H6A1	109.5
H14A—C14—H14C	109.5	C3—C6A—H6A2	109.5
H14B—C14—H14C	109.5	H6A1—C6A—H6A2	109.5
C22^iii—C21—C22^iv	108.4 (2)	C3—C6A—H6A3	109.5
C22^iii—C21—C22	108.4 (2)	H6A1—C6A—H6A3	109.5
C22^iv—C21—C22	108.4 (2)	H6A2—C6A—H6A3	109.5
C22^iii—C21—H21	110.5	C3—C4B—H4B1	109.5
C22^iv—C21—H21	110.5	C3—C4B—H4B2	109.5
C22—C21—H21	110.5	H4B1—C4B—H4B2	109.5
C24—C23—C26	111.0 (3)	C3—C4B—H4B3	109.5
C24—C23—C22	108.9 (3)	H4B1—C4B—H4B3	109.5
C26—C23—C22	114.6 (2)	H4B2—C4B—H4B3	109.5
C24—C23—C25	109.6 (3)	C3—C4A—H4A1	109.5
C26—C23—C25	107.1 (3)	C3—C4A—H4A2	109.5
C22—C23—C25	105.4 (3)	H4A1—C4A—H4A2	109.5
O21—C22—C21	118.5 (3)	C3—C4A—H4A3	109.5
O21—C22—C23	121.5 (2)	H4A1—C4A—H4A3	109.5
C21—C22—C23	119.9 (2)	H4A2—C4A—H4A3	109.5
C23—C26—H26A	109.5	C3—C5A—H5A1	109.5
C23—C26—H26B	109.5	C3—C5A—H5A2	109.5
H26A—C26—H26B	109.5	H5A1—C5A—H5A2	109.5
C23—C26—H26C	109.5	C3—C5A—H5A3	109.5
H26A—C26—H26C	109.5	H5A1—C5A—H5A3	109.5
H26B—C26—H26C	109.5	H5A2—C5A—H5A3	109.5
C23—C25—H25A	109.5	C3—C5B—H5B1	109.5
C23—C25—H25B	109.5	C3—C5B—H5B2	109.5
H25A—C25—H25B	109.5	H5B1—C5B—H5B2	109.5
C23—C25—H25C	109.5	C3—C5B—H5B3	109.5
H25A—C25—H25C	109.5	H5B1—C5B—H5B3	109.5
H25B—C25—H25C	109.5	H5B2—C5B—H5B3	109.5
C23—C24—H24A	109.5

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