Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807043619/pk2043sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536807043619/pk2043Isup2.hkl |
CCDC reference: 663779
Key indicators
- Single-crystal X-ray study
- T = 290 K
- Mean (C-C) = 0.004 Å
- R factor = 0.047
- wR factor = 0.148
- Data-to-parameter ratio = 14.2
checkCIF/PLATON results
No syntax errors found
Alert level B PLAT063_ALERT_3_B Crystal Probably too Large for Beam Size ....... 1.00 mm
0 ALERT level A = In general: serious problem 1 ALERT level B = Potentially serious problem 0 ALERT level C = Check and explain 0 ALERT level G = General alerts; check 0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 0 ALERT type 2 Indicator that the structure model may be wrong or deficient 1 ALERT type 3 Indicator that the structure quality may be low 0 ALERT type 4 Improvement, methodology, query or suggestion 0 ALERT type 5 Informative message, check
The title compound was synthesized as outlined in the literature (Zaugg & Schaefer, 1965) and purified by column chromatography over silica gel using 1:1 n-pentane: n-hexane as the mobile phase. Crystals suitable for X-ray analysis were grown by slow evaporation from 1:1 n-pentane: n-hexane at 298 K. The crystals of 1 did not cleave very well and several attempts to break or cut them were unsuccessful. Therefore a larger than standard crystal (1 mm max. dimension) was used for this study. The beam tube that was used was large enough (2 mm i.d.) to ensure that the crystal was completely inside of the X-ray beam during the diffraction experiment.
H atoms were placed in calculated positions and allowed to ride during subsequent refinement, with Uiso(H) = 1.2Ueq(C) and C—H distances of 0.93 Å for H atoms attached to the aromatic rings, Uiso(H) = 1.5Ueq(C) and C—H distances of 0.96 Å for the methyl H atoms, and Uiso(H) = 1.2Ueq(C) and a C—H distance of 0.98 Å for the tertiary H atom.
Acetylacetone (2,4-pentanedione) and other β-diketones are molecules in dynamic equilibrium with both tautomers present in measurable concentrations at equilibrium. The keto-enol ratio in these systems is modulated by solvent polarity, hydrogen bonding capacity, and the concentration of dissolved salts (Pocker & Spyridis, 2002; Emsley, 1984). This ratio is also exquisitely sensitive to the size of the substituents on the carbon between the carbonyls, with the keto concentration increasing as the steric bulk rises (Emsley, 1984). In light of our recent investigations into the keto-enol ratio of acetylacetone in highly concentrated nonaqueous salt solutions (Pocker & Spyridis, 2002), we prepared and crystallized 3-triphenyl-2,4-pentanedione, 1.
The title compound exists exclusively in the diketo form with steric crowding producing an uncommonly long C3—C6 bond of 1.587 (3) Å. This is close to the adamantyl-diketone bond length of 1.595 (8) Å observed in 3-methyl-3-(1-adamantyl)-2,4-pentanedione, 2 (Moreno-Manas et al., 1992). The C2—C3 and C3—C4 linkages at 1.544 (3) and 1.531 (3) Å, respectively, are also slightly longer than the 1.50 Å typical for sp2-sp3 carbon-carbon bonds (Wiorkiewicz-Kuczera & Rabczenko, 1986), as are the phenyl-carbon bonds about C6. Steric congestion also produces bond angles about C3 that are unusually large for a sp3 hybridized carbon: C2—C3—C6 is 116.5 (2)° while C4—C3—C6 is 117.23 (19)°.
The main factor determining the conformation of the keto tautomer is the strong repulsion of the localized C═O bond dipoles (Emsley et al., 1986). For both 1 and 2 the C2—C3—C4 angle is between 105–107° while for the keto tautomer of acetylaceone (Lowery et al., 1971) the corresponding angle is 114°, possibly in an effort to minimize the interaction between the carbonyl bond dipoles. For β-diketones possessing large groups about C3 semiempirical MM2 calculations (Moreno-Manas et al., 1991) reveal that the conformations which predominate are those with a dihedral angle between the carbonyls of less than 90°, with their contribution increasing as the bulk of the substituent rises. For 1 only a single conformation is oberved in the crystalline state and it possesses a dihedral angle of 66.8 (2)° between the carbonyls.
For related literature, see: Emsley (1984); Emsley et al. (1986); Lowery et al. (1971); Moreno-Manas et al. (1991, 1992); Pocker & Spyridis (2002); Wiorkiewicz-Kuczera & Rabczenko (1986); Zaugg & Schaefer (1965).
Data collection: CAD-4-PC Software (Enraf–Nonius, 1993); cell refinement: CAD-4-PC Software (Enraf–Nonius, 1993); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1998); software used to prepare material for publication: publCIF (Westrip, 2007).
Fig. 1. The molecular structure of 1, with the atom-numbering scheme. Displacement ellipsoids for non-hydrogen atoms are drawn at the 50% probability level. Hydrogen atoms are not shown for clarity. |
C24H22O2 | F(000) = 728 |
Mr = 342.42 | Dx = 1.225 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 25 reflections |
a = 9.1906 (11) Å | θ = 8.1–13.5° |
b = 23.704 (3) Å | µ = 0.08 mm−1 |
c = 9.3251 (13) Å | T = 290 K |
β = 113.941 (12)° | Plate, colorless |
V = 1856.7 (4) Å3 | 1.00 × 0.48 × 0.17 mm |
Z = 4 |
Enraf–Nonius CAD-4 diffractometer | 1775 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.043 |
Graphite monochromator | θmax = 25.4°, θmin = 2.5° |
θ/2θ scans | h = 0→11 |
Absorption correction: ψ scan (North et al., 1968) | k = 0→28 |
Tmin = 0.868, Tmax = 0.991 | l = −11→10 |
3594 measured reflections | 3 standard reflections every 120 min |
3378 independent reflections | intensity decay: none |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.047 | H-atom parameters constrained |
wR(F2) = 0.148 | w = 1/[σ2(Fo2) + (0.0688P)2] where P = (Fo2 + 2Fc2)/3 |
S = 0.99 | (Δ/σ)max < 0.001 |
3378 reflections | Δρmax = 0.19 e Å−3 |
238 parameters | Δρmin = −0.16 e Å−3 |
0 restraints | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.014 (2) |
C24H22O2 | V = 1856.7 (4) Å3 |
Mr = 342.42 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 9.1906 (11) Å | µ = 0.08 mm−1 |
b = 23.704 (3) Å | T = 290 K |
c = 9.3251 (13) Å | 1.00 × 0.48 × 0.17 mm |
β = 113.941 (12)° |
Enraf–Nonius CAD-4 diffractometer | 1775 reflections with I > 2σ(I) |
Absorption correction: ψ scan (North et al., 1968) | Rint = 0.043 |
Tmin = 0.868, Tmax = 0.991 | 3 standard reflections every 120 min |
3594 measured reflections | intensity decay: none |
3378 independent reflections |
R[F2 > 2σ(F2)] = 0.047 | 0 restraints |
wR(F2) = 0.148 | H-atom parameters constrained |
S = 0.99 | Δρmax = 0.19 e Å−3 |
3378 reflections | Δρmin = −0.16 e Å−3 |
238 parameters |
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. |
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 > 2σ(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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.9972 (2) | 0.23311 (8) | 0.6900 (2) | 0.0575 (5) | |
O2 | 0.9543 (3) | 0.27619 (8) | 0.3716 (2) | 0.0740 (7) | |
C1 | 0.7447 (4) | 0.27723 (14) | 0.5824 (4) | 0.0776 (10) | |
H1A | 0.7873 | 0.2995 | 0.6764 | 0.116* | |
H1B | 0.6527 | 0.2571 | 0.5786 | 0.116* | |
H1C | 0.7152 | 0.3015 | 0.4926 | 0.116* | |
C2 | 0.8678 (3) | 0.23625 (11) | 0.5817 (3) | 0.0472 (7) | |
C3 | 0.8210 (3) | 0.20020 (10) | 0.4317 (3) | 0.0382 (6) | |
H3A | 0.7051 | 0.2037 | 0.3774 | 0.046* | |
C4 | 0.8882 (3) | 0.23122 (11) | 0.3279 (3) | 0.0457 (7) | |
C5 | 0.8630 (3) | 0.20898 (12) | 0.1699 (3) | 0.0583 (8) | |
H5A | 0.9423 | 0.2244 | 0.1387 | 0.087* | |
H5B | 0.7591 | 0.2197 | 0.0952 | 0.087* | |
H5C | 0.8714 | 0.1686 | 0.1743 | 0.087* | |
C6 | 0.8549 (3) | 0.13449 (9) | 0.4564 (3) | 0.0349 (6) | |
C7 | 0.8657 (3) | 0.11838 (10) | 0.6212 (3) | 0.0377 (6) | |
C8 | 0.9936 (3) | 0.09073 (11) | 0.7339 (3) | 0.0496 (7) | |
H8A | 1.0816 | 0.0817 | 0.7132 | 0.060* | |
C9 | 0.9922 (4) | 0.07632 (13) | 0.8774 (3) | 0.0633 (8) | |
H9A | 1.0796 | 0.0580 | 0.9522 | 0.076* | |
C10 | 0.8635 (4) | 0.08876 (14) | 0.9098 (3) | 0.0665 (9) | |
H10A | 0.8632 | 0.0788 | 1.0062 | 0.080* | |
C11 | 0.7344 (4) | 0.11606 (14) | 0.7995 (3) | 0.0627 (8) | |
H11A | 0.6468 | 0.1248 | 0.8212 | 0.075* | |
C12 | 0.7357 (3) | 0.13033 (12) | 0.6568 (3) | 0.0504 (7) | |
H12A | 0.6475 | 0.1484 | 0.5824 | 0.060* | |
C13 | 0.7134 (3) | 0.09979 (10) | 0.3388 (3) | 0.0365 (6) | |
C14 | 0.6992 (3) | 0.04335 (11) | 0.3709 (3) | 0.0495 (7) | |
H14A | 0.7752 | 0.0276 | 0.4617 | 0.059* | |
C15 | 0.5764 (3) | 0.00993 (12) | 0.2726 (3) | 0.0585 (8) | |
H15A | 0.5708 | −0.0278 | 0.2973 | 0.070* | |
C16 | 0.4627 (3) | 0.03230 (13) | 0.1384 (3) | 0.0591 (8) | |
H16A | 0.3789 | 0.0100 | 0.0726 | 0.071* | |
C17 | 0.4733 (3) | 0.08740 (12) | 0.1020 (3) | 0.0556 (8) | |
H17A | 0.3975 | 0.1025 | 0.0100 | 0.067* | |
C18 | 0.5965 (3) | 0.12122 (11) | 0.2015 (3) | 0.0445 (6) | |
H18A | 0.6009 | 0.1589 | 0.1757 | 0.053* | |
C19 | 1.0120 (3) | 0.11948 (10) | 0.4404 (3) | 0.0361 (6) | |
C20 | 1.1484 (3) | 0.15112 (11) | 0.5224 (3) | 0.0483 (7) | |
H20A | 1.1444 | 0.1802 | 0.5877 | 0.058* | |
C21 | 1.2892 (3) | 0.14001 (12) | 0.5082 (4) | 0.0568 (8) | |
H21A | 1.3793 | 0.1613 | 0.5651 | 0.068* | |
C22 | 1.2977 (3) | 0.09809 (12) | 0.4113 (3) | 0.0548 (7) | |
H22A | 1.3920 | 0.0915 | 0.3995 | 0.066* | |
C23 | 1.1649 (3) | 0.06562 (11) | 0.3315 (3) | 0.0493 (7) | |
H23A | 1.1701 | 0.0365 | 0.2670 | 0.059* | |
C24 | 1.0237 (3) | 0.07625 (10) | 0.3468 (3) | 0.0421 (6) | |
H24A | 0.9352 | 0.0538 | 0.2929 | 0.051* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0433 (11) | 0.0637 (12) | 0.0543 (12) | −0.0069 (9) | 0.0082 (10) | −0.0163 (9) |
O2 | 0.0818 (15) | 0.0550 (13) | 0.0785 (15) | −0.0209 (11) | 0.0257 (12) | 0.0060 (11) |
C1 | 0.0573 (19) | 0.078 (2) | 0.092 (2) | 0.0045 (16) | 0.0250 (18) | −0.0377 (19) |
C2 | 0.0375 (15) | 0.0458 (15) | 0.0561 (17) | −0.0059 (12) | 0.0168 (14) | −0.0075 (13) |
C3 | 0.0280 (12) | 0.0394 (13) | 0.0420 (13) | −0.0005 (10) | 0.0088 (11) | −0.0024 (11) |
C4 | 0.0377 (14) | 0.0385 (14) | 0.0538 (17) | 0.0031 (12) | 0.0113 (12) | 0.0090 (12) |
C5 | 0.0631 (19) | 0.0624 (17) | 0.0569 (17) | 0.0063 (15) | 0.0321 (15) | 0.0117 (15) |
C6 | 0.0282 (12) | 0.0387 (13) | 0.0339 (13) | 0.0014 (10) | 0.0085 (10) | 0.0008 (10) |
C7 | 0.0301 (13) | 0.0447 (14) | 0.0322 (13) | −0.0042 (11) | 0.0063 (11) | −0.0024 (11) |
C8 | 0.0403 (15) | 0.0570 (17) | 0.0422 (15) | 0.0008 (13) | 0.0071 (12) | 0.0028 (13) |
C9 | 0.0616 (19) | 0.076 (2) | 0.0379 (16) | 0.0029 (16) | 0.0055 (14) | 0.0109 (15) |
C10 | 0.078 (2) | 0.085 (2) | 0.0332 (16) | −0.0149 (18) | 0.0195 (16) | −0.0009 (15) |
C11 | 0.0549 (18) | 0.087 (2) | 0.0472 (17) | −0.0075 (16) | 0.0221 (15) | −0.0045 (16) |
C12 | 0.0375 (14) | 0.0686 (19) | 0.0419 (16) | −0.0021 (13) | 0.0129 (12) | −0.0005 (13) |
C13 | 0.0333 (13) | 0.0408 (14) | 0.0344 (13) | −0.0011 (10) | 0.0126 (11) | −0.0032 (11) |
C14 | 0.0499 (16) | 0.0475 (16) | 0.0437 (15) | −0.0022 (13) | 0.0112 (13) | −0.0008 (13) |
C15 | 0.0632 (19) | 0.0460 (16) | 0.0615 (18) | −0.0150 (14) | 0.0202 (16) | −0.0102 (14) |
C16 | 0.0455 (17) | 0.0642 (19) | 0.0583 (19) | −0.0119 (14) | 0.0114 (15) | −0.0238 (15) |
C17 | 0.0399 (15) | 0.0672 (19) | 0.0432 (16) | −0.0009 (14) | 0.0000 (13) | −0.0110 (14) |
C18 | 0.0380 (14) | 0.0473 (15) | 0.0418 (14) | 0.0004 (11) | 0.0096 (12) | −0.0014 (12) |
C19 | 0.0303 (13) | 0.0383 (13) | 0.0374 (13) | 0.0041 (10) | 0.0112 (10) | 0.0051 (11) |
C20 | 0.0358 (14) | 0.0486 (15) | 0.0583 (17) | 0.0003 (12) | 0.0167 (13) | −0.0069 (13) |
C21 | 0.0324 (15) | 0.0601 (18) | 0.074 (2) | −0.0009 (13) | 0.0181 (14) | −0.0084 (16) |
C22 | 0.0381 (15) | 0.0669 (19) | 0.0624 (19) | 0.0089 (13) | 0.0235 (14) | 0.0030 (15) |
C23 | 0.0503 (17) | 0.0562 (17) | 0.0440 (15) | 0.0122 (13) | 0.0219 (13) | −0.0011 (13) |
C24 | 0.0378 (14) | 0.0478 (15) | 0.0344 (13) | 0.0046 (11) | 0.0082 (11) | 0.0015 (11) |
O1—C2 | 1.211 (3) | C11—C12 | 1.378 (4) |
O2—C4 | 1.212 (3) | C11—H11A | 0.9300 |
C1—C2 | 1.493 (4) | C12—H12A | 0.9300 |
C1—H1A | 0.9600 | C13—C14 | 1.388 (3) |
C1—H1B | 0.9600 | C13—C18 | 1.391 (3) |
C1—H1C | 0.9600 | C14—C15 | 1.379 (3) |
C2—C3 | 1.544 (3) | C14—H14A | 0.9300 |
C3—C4 | 1.531 (3) | C15—C16 | 1.370 (4) |
C3—C6 | 1.587 (3) | C15—H15A | 0.9300 |
C3—H3A | 0.9800 | C16—C17 | 1.363 (4) |
C4—C5 | 1.492 (4) | C16—H16A | 0.9300 |
C5—H5A | 0.9600 | C17—C18 | 1.390 (3) |
C5—H5B | 0.9600 | C17—H17A | 0.9300 |
C5—H5C | 0.9600 | C18—H18A | 0.9300 |
C6—C7 | 1.547 (3) | C19—C24 | 1.379 (3) |
C6—C19 | 1.551 (3) | C19—C20 | 1.393 (3) |
C6—C13 | 1.554 (3) | C20—C21 | 1.379 (3) |
C7—C8 | 1.383 (3) | C20—H20A | 0.9300 |
C7—C12 | 1.393 (3) | C21—C22 | 1.366 (4) |
C8—C9 | 1.386 (4) | C21—H21A | 0.9300 |
C8—H8A | 0.9300 | C22—C23 | 1.378 (4) |
C9—C10 | 1.366 (4) | C22—H22A | 0.9300 |
C9—H9A | 0.9300 | C23—C24 | 1.385 (3) |
C10—C11 | 1.376 (4) | C23—H23A | 0.9300 |
C10—H10A | 0.9300 | C24—H24A | 0.9300 |
C2—C1—H1A | 109.5 | C10—C11—C12 | 119.7 (3) |
C2—C1—H1B | 109.5 | C10—C11—H11A | 120.2 |
H1A—C1—H1B | 109.5 | C12—C11—H11A | 120.2 |
C2—C1—H1C | 109.5 | C11—C12—C7 | 121.6 (3) |
H1A—C1—H1C | 109.5 | C11—C12—H12A | 119.2 |
H1B—C1—H1C | 109.5 | C7—C12—H12A | 119.2 |
O1—C2—C1 | 121.7 (2) | C14—C13—C18 | 116.5 (2) |
O1—C2—C3 | 122.7 (2) | C14—C13—C6 | 118.9 (2) |
C1—C2—C3 | 115.6 (2) | C18—C13—C6 | 124.7 (2) |
C4—C3—C2 | 105.75 (19) | C15—C14—C13 | 122.2 (3) |
C4—C3—C6 | 117.23 (19) | C15—C14—H14A | 118.9 |
C2—C3—C6 | 116.5 (2) | C13—C14—H14A | 118.9 |
C4—C3—H3A | 105.4 | C16—C15—C14 | 120.0 (3) |
C2—C3—H3A | 105.4 | C16—C15—H15A | 120.0 |
C6—C3—H3A | 105.4 | C14—C15—H15A | 120.0 |
O2—C4—C5 | 119.9 (3) | C17—C16—C15 | 119.5 (3) |
O2—C4—C3 | 119.1 (2) | C17—C16—H16A | 120.2 |
C5—C4—C3 | 120.9 (2) | C15—C16—H16A | 120.2 |
C4—C5—H5A | 109.5 | C16—C17—C18 | 120.5 (3) |
C4—C5—H5B | 109.5 | C16—C17—H17A | 119.7 |
H5A—C5—H5B | 109.5 | C18—C17—H17A | 119.7 |
C4—C5—H5C | 109.5 | C17—C18—C13 | 121.3 (2) |
H5A—C5—H5C | 109.5 | C17—C18—H18A | 119.4 |
H5B—C5—H5C | 109.5 | C13—C18—H18A | 119.4 |
C7—C6—C19 | 110.60 (18) | C24—C19—C20 | 117.6 (2) |
C7—C6—C13 | 105.55 (18) | C24—C19—C6 | 123.1 (2) |
C19—C6—C13 | 110.85 (18) | C20—C19—C6 | 119.3 (2) |
C7—C6—C3 | 108.44 (18) | C21—C20—C19 | 121.0 (3) |
C19—C6—C3 | 110.29 (18) | C21—C20—H20A | 119.5 |
C13—C6—C3 | 111.00 (17) | C19—C20—H20A | 119.5 |
C8—C7—C12 | 117.5 (2) | C22—C21—C20 | 120.7 (3) |
C8—C7—C6 | 124.3 (2) | C22—C21—H21A | 119.7 |
C12—C7—C6 | 118.1 (2) | C20—C21—H21A | 119.7 |
C7—C8—C9 | 120.8 (3) | C21—C22—C23 | 119.2 (3) |
C7—C8—H8A | 119.6 | C21—C22—H22A | 120.4 |
C9—C8—H8A | 119.6 | C23—C22—H22A | 120.4 |
C10—C9—C8 | 120.5 (3) | C22—C23—C24 | 120.3 (3) |
C10—C9—H9A | 119.7 | C22—C23—H23A | 119.9 |
C8—C9—H9A | 119.7 | C24—C23—H23A | 119.9 |
C9—C10—C11 | 119.8 (3) | C19—C24—C23 | 121.2 (2) |
C9—C10—H10A | 120.1 | C19—C24—H24A | 119.4 |
C11—C10—H10A | 120.1 | C23—C24—H24A | 119.4 |
Experimental details
Crystal data | |
Chemical formula | C24H22O2 |
Mr | 342.42 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 290 |
a, b, c (Å) | 9.1906 (11), 23.704 (3), 9.3251 (13) |
β (°) | 113.941 (12) |
V (Å3) | 1856.7 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.08 |
Crystal size (mm) | 1.00 × 0.48 × 0.17 |
Data collection | |
Diffractometer | Enraf–Nonius CAD-4 |
Absorption correction | ψ scan (North et al., 1968) |
Tmin, Tmax | 0.868, 0.991 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3594, 3378, 1775 |
Rint | 0.043 |
(sin θ/λ)max (Å−1) | 0.603 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.047, 0.148, 0.99 |
No. of reflections | 3378 |
No. of parameters | 238 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.19, −0.16 |
Computer programs: CAD-4-PC Software (Enraf–Nonius, 1993), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL (Bruker, 1998), publCIF (Westrip, 2007).
Acetylacetone (2,4-pentanedione) and other β-diketones are molecules in dynamic equilibrium with both tautomers present in measurable concentrations at equilibrium. The keto-enol ratio in these systems is modulated by solvent polarity, hydrogen bonding capacity, and the concentration of dissolved salts (Pocker & Spyridis, 2002; Emsley, 1984). This ratio is also exquisitely sensitive to the size of the substituents on the carbon between the carbonyls, with the keto concentration increasing as the steric bulk rises (Emsley, 1984). In light of our recent investigations into the keto-enol ratio of acetylacetone in highly concentrated nonaqueous salt solutions (Pocker & Spyridis, 2002), we prepared and crystallized 3-triphenyl-2,4-pentanedione, 1.
The title compound exists exclusively in the diketo form with steric crowding producing an uncommonly long C3—C6 bond of 1.587 (3) Å. This is close to the adamantyl-diketone bond length of 1.595 (8) Å observed in 3-methyl-3-(1-adamantyl)-2,4-pentanedione, 2 (Moreno-Manas et al., 1992). The C2—C3 and C3—C4 linkages at 1.544 (3) and 1.531 (3) Å, respectively, are also slightly longer than the 1.50 Å typical for sp2-sp3 carbon-carbon bonds (Wiorkiewicz-Kuczera & Rabczenko, 1986), as are the phenyl-carbon bonds about C6. Steric congestion also produces bond angles about C3 that are unusually large for a sp3 hybridized carbon: C2—C3—C6 is 116.5 (2)° while C4—C3—C6 is 117.23 (19)°.
The main factor determining the conformation of the keto tautomer is the strong repulsion of the localized C═O bond dipoles (Emsley et al., 1986). For both 1 and 2 the C2—C3—C4 angle is between 105–107° while for the keto tautomer of acetylaceone (Lowery et al., 1971) the corresponding angle is 114°, possibly in an effort to minimize the interaction between the carbonyl bond dipoles. For β-diketones possessing large groups about C3 semiempirical MM2 calculations (Moreno-Manas et al., 1991) reveal that the conformations which predominate are those with a dihedral angle between the carbonyls of less than 90°, with their contribution increasing as the bulk of the substituent rises. For 1 only a single conformation is oberved in the crystalline state and it possesses a dihedral angle of 66.8 (2)° between the carbonyls.