Tris(2-pyridyl)phosphine oxide, (I), C
15H
12N
3OP, is isomorphous with tris(2-pyridyl)phosphine. Because of a combination of C—H
O and C—H
N interactions, the crystal packing is denser in the title compound than in the related compounds triphenylphosphine oxide and tris(2-pyridyl)phosphine.
Supporting information
CCDC reference: 237938
Tris(2-pyridyl)phosphine oxide was obtained as a by-product during the attempted synthesis of phenyl tris(2-pyridyl)phosphonium bromide from tris(2-pyridyl)phosphine and bromobenzene under reflux conditions. Crystals suitable for X-ray analysis were obtained by dissolving the crude product in methanol and cooling it to 213 K.
H atoms were positioned geometrically and allowed to ride on their parent atoms, with C—H bond lengths of 0.95 Å and Uiso values equal to 1.2Ueq of the parent atom.
Data collection: SMART (Bruker, 1998); cell refinement: SAINT-Plus (Bruker, 1999); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Bruker, 1999); program(s) used to refine structure: SHELXTL; molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXTL.
Crystal data top
C15H12N3OP | F(000) = 584 |
Mr = 281.25 | Dx = 1.422 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 1007 reflections |
a = 9.0807 (9) Å | θ = 2.3–28.2° |
b = 9.1550 (9) Å | µ = 0.21 mm−1 |
c = 16.0629 (16) Å | T = 173 K |
β = 100.409 (2)° | Prism, light brown |
V = 1313.4 (2) Å3 | 0.24 × 0.20 × 0.17 mm |
Z = 4 | |
Data collection top
Bruker SMART 1K CCD area-detector diffractometer | 2589 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.026 |
Graphite monochromator | θmax = 28.3°, θmin = 2.3° |
ϕ and ω scans | h = −11→12 |
8966 measured reflections | k = −9→12 |
3251 independent reflections | l = −21→21 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.036 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.096 | H-atom parameters constrained |
S = 1.02 | w = 1/[σ2(Fo2) + (0.0454P)2 + 0.563P] where P = (Fo2 + 2Fc2)/3 |
3251 reflections | (Δ/σ)max = 0.001 |
181 parameters | Δρmax = 0.41 e Å−3 |
0 restraints | Δρmin = −0.36 e Å−3 |
Crystal data top
C15H12N3OP | V = 1313.4 (2) Å3 |
Mr = 281.25 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 9.0807 (9) Å | µ = 0.21 mm−1 |
b = 9.1550 (9) Å | T = 173 K |
c = 16.0629 (16) Å | 0.24 × 0.20 × 0.17 mm |
β = 100.409 (2)° | |
Data collection top
Bruker SMART 1K CCD area-detector diffractometer | 2589 reflections with I > 2σ(I) |
8966 measured reflections | Rint = 0.026 |
3251 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.036 | 0 restraints |
wR(F2) = 0.096 | H-atom parameters constrained |
S = 1.02 | Δρmax = 0.41 e Å−3 |
3251 reflections | Δρmin = −0.36 e Å−3 |
181 parameters | |
Special details top
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 > σ(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. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
P | 0.86663 (4) | 0.29096 (4) | 0.11854 (2) | 0.01960 (11) | |
O | 1.02875 (12) | 0.29006 (13) | 0.15391 (7) | 0.0285 (3) | |
C11 | 0.82427 (17) | 0.19197 (16) | 0.01882 (9) | 0.0217 (3) | |
N12 | 0.69184 (15) | 0.12300 (16) | 0.00009 (8) | 0.0286 (3) | |
C13 | 0.66551 (19) | 0.0480 (2) | −0.07296 (10) | 0.0327 (4) | |
H13 | 0.5728 | −0.0022 | −0.0874 | 0.039* | |
C14 | 0.7654 (2) | 0.03949 (19) | −0.12839 (10) | 0.0311 (4) | |
H14 | 0.7414 | −0.0150 | −0.1794 | 0.037* | |
C15 | 0.8996 (2) | 0.1112 (2) | −0.10839 (11) | 0.0364 (4) | |
H15 | 0.9700 | 0.1077 | −0.1455 | 0.044* | |
C16 | 0.9309 (2) | 0.1892 (2) | −0.03311 (11) | 0.0329 (4) | |
H16 | 1.0233 | 0.2394 | −0.0175 | 0.039* | |
C21 | 0.78892 (16) | 0.47360 (16) | 0.09830 (9) | 0.0201 (3) | |
N22 | 0.82838 (14) | 0.56238 (14) | 0.16552 (8) | 0.0220 (3) | |
C23 | 0.78126 (17) | 0.70115 (17) | 0.15693 (10) | 0.0247 (3) | |
H23 | 0.8095 | 0.7658 | 0.2033 | 0.030* | |
C24 | 0.69341 (18) | 0.75533 (18) | 0.08376 (11) | 0.0288 (4) | |
H24 | 0.6622 | 0.8545 | 0.0805 | 0.035* | |
C25 | 0.65196 (19) | 0.66228 (19) | 0.01564 (11) | 0.0312 (4) | |
H25 | 0.5907 | 0.6961 | −0.0350 | 0.037* | |
C26 | 0.70164 (18) | 0.51843 (18) | 0.02264 (10) | 0.0269 (3) | |
H26 | 0.6764 | 0.4523 | −0.0233 | 0.032* | |
C31 | 0.75141 (16) | 0.20849 (16) | 0.18732 (9) | 0.0189 (3) | |
N32 | 0.61072 (14) | 0.26016 (14) | 0.18119 (8) | 0.0227 (3) | |
C33 | 0.52699 (17) | 0.19927 (17) | 0.23244 (10) | 0.0252 (3) | |
H33 | 0.4279 | 0.2347 | 0.2298 | 0.030* | |
C34 | 0.57598 (18) | 0.08765 (17) | 0.28908 (10) | 0.0259 (3) | |
H34 | 0.5113 | 0.0473 | 0.3234 | 0.031* | |
C35 | 0.72075 (18) | 0.03610 (17) | 0.29466 (10) | 0.0248 (3) | |
H35 | 0.7576 | −0.0402 | 0.3329 | 0.030* | |
C36 | 0.81123 (16) | 0.09871 (16) | 0.24287 (9) | 0.0220 (3) | |
H36 | 0.9116 | 0.0670 | 0.2455 | 0.026* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
P | 0.02009 (19) | 0.0208 (2) | 0.01789 (18) | −0.00168 (14) | 0.00331 (13) | −0.00064 (14) |
O | 0.0223 (6) | 0.0320 (6) | 0.0303 (6) | −0.0016 (5) | 0.0023 (4) | 0.0006 (5) |
C11 | 0.0250 (7) | 0.0202 (7) | 0.0201 (7) | 0.0004 (6) | 0.0049 (6) | −0.0004 (5) |
N12 | 0.0272 (7) | 0.0335 (8) | 0.0257 (7) | −0.0044 (6) | 0.0063 (5) | −0.0078 (6) |
C13 | 0.0320 (9) | 0.0348 (9) | 0.0306 (9) | −0.0059 (7) | 0.0037 (7) | −0.0096 (7) |
C14 | 0.0430 (10) | 0.0291 (9) | 0.0210 (8) | 0.0010 (7) | 0.0055 (7) | −0.0053 (6) |
C15 | 0.0430 (10) | 0.0407 (10) | 0.0302 (9) | −0.0058 (8) | 0.0197 (8) | −0.0071 (7) |
C16 | 0.0320 (9) | 0.0374 (10) | 0.0318 (9) | −0.0099 (7) | 0.0129 (7) | −0.0078 (7) |
C21 | 0.0201 (7) | 0.0210 (7) | 0.0193 (7) | −0.0025 (5) | 0.0035 (5) | 0.0015 (5) |
N22 | 0.0220 (6) | 0.0239 (7) | 0.0201 (6) | −0.0026 (5) | 0.0038 (5) | −0.0018 (5) |
C23 | 0.0244 (7) | 0.0234 (8) | 0.0278 (8) | −0.0038 (6) | 0.0084 (6) | −0.0041 (6) |
C24 | 0.0269 (8) | 0.0236 (8) | 0.0369 (9) | 0.0025 (6) | 0.0089 (7) | 0.0053 (7) |
C25 | 0.0306 (9) | 0.0331 (9) | 0.0278 (8) | 0.0014 (7) | −0.0008 (7) | 0.0085 (7) |
C26 | 0.0296 (8) | 0.0288 (8) | 0.0206 (7) | −0.0025 (7) | −0.0001 (6) | 0.0002 (6) |
C31 | 0.0206 (7) | 0.0190 (7) | 0.0168 (7) | −0.0011 (5) | 0.0028 (5) | −0.0029 (5) |
N32 | 0.0219 (6) | 0.0228 (6) | 0.0235 (6) | 0.0018 (5) | 0.0045 (5) | 0.0013 (5) |
C33 | 0.0226 (7) | 0.0267 (8) | 0.0272 (8) | 0.0025 (6) | 0.0070 (6) | 0.0001 (6) |
C34 | 0.0288 (8) | 0.0252 (8) | 0.0253 (8) | −0.0027 (6) | 0.0092 (6) | 0.0012 (6) |
C35 | 0.0311 (8) | 0.0208 (7) | 0.0219 (7) | 0.0025 (6) | 0.0034 (6) | 0.0024 (6) |
C36 | 0.0215 (7) | 0.0224 (7) | 0.0213 (7) | 0.0018 (6) | 0.0021 (6) | −0.0016 (6) |
Geometric parameters (Å, º) top
P—O | 1.4792 (11) | C23—C24 | 1.387 (2) |
P—C31 | 1.8173 (15) | C23—H23 | 0.9500 |
P—C11 | 1.8200 (15) | C24—C25 | 1.384 (2) |
P—C21 | 1.8211 (15) | C24—H24 | 0.9500 |
C11—N12 | 1.343 (2) | C25—C26 | 1.390 (2) |
C11—C16 | 1.388 (2) | C25—H25 | 0.9500 |
N12—C13 | 1.343 (2) | C26—H26 | 0.9500 |
C13—C14 | 1.383 (2) | C31—N32 | 1.3492 (18) |
C13—H13 | 0.9500 | C31—C36 | 1.388 (2) |
C14—C15 | 1.371 (2) | N32—C33 | 1.3389 (19) |
C14—H14 | 0.9500 | C33—C34 | 1.387 (2) |
C15—C16 | 1.389 (2) | C33—H33 | 0.9500 |
C15—H15 | 0.9500 | C34—C35 | 1.384 (2) |
C16—H16 | 0.9500 | C34—H34 | 0.9500 |
C21—N22 | 1.3473 (19) | C35—C36 | 1.394 (2) |
C21—C26 | 1.388 (2) | C35—H35 | 0.9500 |
N22—C23 | 1.340 (2) | C36—H36 | 0.9500 |
| | | |
O—P—C31 | 113.99 (7) | N22—C23—H23 | 118.3 |
O—P—C11 | 111.88 (7) | C24—C23—H23 | 118.3 |
C31—P—C11 | 105.95 (7) | C25—C24—C23 | 118.83 (15) |
O—P—C21 | 113.60 (7) | C25—C24—H24 | 120.6 |
C31—P—C21 | 104.06 (7) | C23—C24—H24 | 120.6 |
C11—P—C21 | 106.65 (7) | C24—C25—C26 | 118.76 (15) |
N12—C11—C16 | 123.39 (14) | C24—C25—H25 | 120.6 |
N12—C11—P | 118.00 (11) | C26—C25—H25 | 120.6 |
C16—C11—P | 118.60 (12) | C21—C26—C25 | 118.47 (15) |
C13—N12—C11 | 116.54 (14) | C21—C26—H26 | 120.8 |
N12—C13—C14 | 123.87 (16) | C25—C26—H26 | 120.8 |
N12—C13—H13 | 118.1 | N32—C31—C36 | 123.80 (13) |
C14—C13—H13 | 118.1 | N32—C31—P | 116.84 (11) |
C15—C14—C13 | 118.74 (15) | C36—C31—P | 119.36 (11) |
C15—C14—H14 | 120.6 | C33—N32—C31 | 116.40 (13) |
C13—C14—H14 | 120.6 | N32—C33—C34 | 124.03 (14) |
C14—C15—C16 | 118.95 (16) | N32—C33—H33 | 118.0 |
C14—C15—H15 | 120.5 | C34—C33—H33 | 118.0 |
C16—C15—H15 | 120.5 | C35—C34—C33 | 118.81 (14) |
C11—C16—C15 | 118.51 (16) | C35—C34—H34 | 120.6 |
C11—C16—H16 | 120.7 | C33—C34—H34 | 120.6 |
C15—C16—H16 | 120.7 | C34—C35—C36 | 118.46 (14) |
N22—C21—C26 | 123.41 (14) | C34—C35—H35 | 120.8 |
N22—C21—P | 111.87 (10) | C36—C35—H35 | 120.8 |
C26—C21—P | 124.72 (12) | C31—C36—C35 | 118.50 (13) |
C23—N22—C21 | 117.12 (13) | C31—C36—H36 | 120.8 |
N22—C23—C24 | 123.39 (15) | C35—C36—H36 | 120.8 |
| | | |
O—P—C11—N12 | 148.29 (12) | P—C21—N22—C23 | 178.00 (10) |
C31—P—C11—N12 | 23.51 (14) | C21—N22—C23—C24 | 1.1 (2) |
C21—P—C11—N12 | −86.93 (13) | N22—C23—C24—C25 | −0.2 (2) |
O—P—C11—C16 | −30.52 (15) | C23—C24—C25—C26 | −0.9 (2) |
C31—P—C11—C16 | −155.30 (13) | N22—C21—C26—C25 | −0.1 (2) |
C21—P—C11—C16 | 94.26 (14) | P—C21—C26—C25 | −178.89 (12) |
C16—C11—N12—C13 | 0.3 (2) | C24—C25—C26—C21 | 1.0 (2) |
P—C11—N12—C13 | −178.41 (12) | O—P—C31—N32 | 150.58 (11) |
C11—N12—C13—C14 | −0.4 (3) | C11—P—C31—N32 | −85.96 (12) |
N12—C13—C14—C15 | 0.1 (3) | C21—P—C31—N32 | 26.30 (13) |
C13—C14—C15—C16 | 0.3 (3) | O—P—C31—C36 | −28.91 (14) |
N12—C11—C16—C15 | 0.0 (3) | C11—P—C31—C36 | 94.55 (12) |
P—C11—C16—C15 | 178.77 (14) | C21—P—C31—C36 | −153.18 (11) |
C14—C15—C16—C11 | −0.4 (3) | C36—C31—N32—C33 | −0.2 (2) |
O—P—C21—N22 | −49.20 (12) | P—C31—N32—C33 | −179.65 (11) |
C31—P—C21—N22 | 75.33 (11) | C31—N32—C33—C34 | −0.8 (2) |
C11—P—C21—N22 | −172.92 (10) | N32—C33—C34—C35 | 1.0 (2) |
O—P—C21—C26 | 129.72 (13) | C33—C34—C35—C36 | −0.1 (2) |
C31—P—C21—C26 | −105.75 (14) | N32—C31—C36—C35 | 1.0 (2) |
C11—P—C21—C26 | 6.00 (15) | P—C31—C36—C35 | −179.54 (11) |
C26—C21—N22—C23 | −0.9 (2) | C34—C35—C36—C31 | −0.8 (2) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C23—H23···Oi | 0.95 | 2.50 | 3.313 (2) | 144 |
C35—H35···Oii | 0.95 | 2.46 | 3.202 (2) | 134 |
C36—H36···N22ii | 0.95 | 2.53 | 3.353 (2) | 145 |
Symmetry codes: (i) −x+2, y+1/2, −z+1/2; (ii) −x+2, y−1/2, −z+1/2. |
Experimental details
Crystal data |
Chemical formula | C15H12N3OP |
Mr | 281.25 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 173 |
a, b, c (Å) | 9.0807 (9), 9.1550 (9), 16.0629 (16) |
β (°) | 100.409 (2) |
V (Å3) | 1313.4 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.21 |
Crystal size (mm) | 0.24 × 0.20 × 0.17 |
|
Data collection |
Diffractometer | Bruker SMART 1K CCD area-detector diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 8966, 3251, 2589 |
Rint | 0.026 |
(sin θ/λ)max (Å−1) | 0.667 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.036, 0.096, 1.02 |
No. of reflections | 3251 |
No. of parameters | 181 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.41, −0.36 |
Selected geometric parameters (Å, º) topP—O | 1.4792 (11) | P—C11 | 1.8200 (15) |
P—C31 | 1.8173 (15) | P—C21 | 1.8211 (15) |
| | | |
O—P—C31 | 113.99 (7) | O—P—C21 | 113.60 (7) |
O—P—C11 | 111.88 (7) | C31—P—C21 | 104.06 (7) |
C31—P—C11 | 105.95 (7) | C11—P—C21 | 106.65 (7) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C23—H23···Oi | 0.95 | 2.50 | 3.313 (2) | 144 |
C35—H35···Oii | 0.95 | 2.46 | 3.202 (2) | 134 |
C36—H36···N22ii | 0.95 | 2.53 | 3.353 (2) | 145 |
Symmetry codes: (i) −x+2, y+1/2, −z+1/2; (ii) −x+2, y−1/2, −z+1/2. |
Cell parameters for the various forms of triphenyl phosphine oxide topForm | SG | a/Å | b/Å | c/Å | β/° | V/Å3 | T/K |
Ia | Pbca | 28.898 (3) | 9.094 (2) | 11.138 (2) | - | 2929.0 | 153 |
IIa | P21/c | 10.952 (2) | 8.687 (2) | 16.221 (6) | 108.78 (2) | 1461.1 | 153 |
IIIb | P21/c | 15.066 (1) | 9.037 (2) | 11.296 (3) | 98.47 (1) | 1521.2 | 295 |
aBrock et al. (1985); bSpek (1987); |
Tris(2-pyridyl)phosphine oxide has been used as a ligand in the coordination chemistry of transition metals and a number of complexes have been characterized by X-ray crystallography [Cambridge Structural Database (Allen, 2002) refcodes XAMNUU (Anderson et al., 2000), QATYUF and QATZAM (Casares et al., 2001), VOPREX (Keene et al., 1991), and MAWYOY (Espinet et al., 2000)]. The solid-state structure of the free tris(2-pyridyl)phosphine oxide (I) is, however, still outstanding and crystallographic data are presented in this paper.
Compound (I) is isomorphous with the parent compound tris(2-pyridyl)phosphine (TP; GEKTIZ; Keene et al., 1988), with the O atom replacing the lone pair in the phosphine [a = 9.162 (1) Å, b = 9.163 (1) Å, c = 16.071 (2) Å and β = 100.92 (1)°; space group P21/c]. With the exception of the O atom, the coordinates of the two structures are related by (1 − x, y, 1/2 − x). The molecular geometry of (I) (Fig. 1) is pyramidal, with a propeller-type arrangement of the three pyridyl rings, and shows considerable deviation from C3v symmetry. One of the N atoms points to the same side as the O atom, while the remaining two N atoms point in the opposite direction. The P—C bond distances (Table 1) are comparable to those in TP [mean 1.828 (3) Å] but slightly longer than those in triphenylphosphine oxide (TPO) (mean 1.800 Å; Brock et al., 1985). An opposite trend is observed for the P=O bond, which is slightly shorter in (I) [1.479 (1) Å] than in triphenylphosphine oxide [1.491 (2) and 1.494 (2) Å; Brock et al., 1985].
No classical (strong) hydrogen bonding occurs in the structure of (I). Molecules are bound together by weak C—H···O and C—H···N interactions (Table 2 and Fig. 2). The same C—H···N interaction exists in TP, the intermolecular C···N distances being 3.341 and 3.353 (2) Å in TP and (I), respectively.
Three modifications of TPO, viz. one orthorhombic (Form I) and two monoclinic (Forms II and III), have been described in the literature (Brock et al., 1985; Spek, 1987; Thomas & Hamor, 1993; Table 3). Unique to Form II is a pair of molecules connected by C—H···O interactions, forming a ring described by the graph set motif R22(12) (Etter, 1990; Bernstein et al., 1995). Extending the C—H···O network produces a chain of rings (Bernstein et al., 1995) running slomg the a axis (in P21/c, e.g. TPEPHO06; Brock et al., 1995). A similar chain of R22(12) C—H···O rings runs along the b axis in (I). In the structure of (I), the additional C—H···N interaction seems to reinforce the R22(12) C—H···O ring, leading to C—H···O contacts that are significantly shorter than those found in Form II of TPO. Intermolecular C···O distances in TPO are in the range 3.488–3.586 Å (TPEHO10 and TPEPHO11; Falvello et al., 2002), while they are considerably shorter in (I) [3.202 (2) and 3.313 (2) Å; Table 2].
The three polymorphs of TPO have calculated densities in the range 1.215–1.285 Mg m−3, considerably lower than that of (I) (1.422 Mg m−3). Molecular volumes range from 380.3 Å3 (at 295 K) in Form III (Spek, 1987) to 365.9 Å3 (at 153 K; Brock et al., 1985) in Form I and 365.3 Å3 (at 153 K; Brock et al., 1985) in Form II. The molecular volume tfound in (I) [328.35 (5) Å3] is considerably smaller, possibly as a consequence of the reduced number of H atoms (three per molecule) and/or the more efficient packing in (I) as a result of there being additional C—H···N contacts (Fig. 2) that are not found in the carbon analogue.
The cell volume and calculated crystal density in TP are 1325 (2) Å3 and 1.330 Mg m−3 (at 283–303 K). The fact that TPO and TP have almost identical molecular volumes indicates that there is little cost in inserting the extra O atom into the TP structure. Thus it seems logical that the extra C—H···O interactions in (I) promote more efficient packing. Since (I) and TP are isomorphous and the crystal packing of (I) is directed by weak interactions, it is likely that the structure of TP is also dominated by weak interactions. It is therefore possible that in place of the C—H···O interactions, the TP structure has interactions between the equivalent C/H groups and the phosphine lone pair. The two closest C···P (D···A) distances in the two structures are 4.319 and 4.249 Å for TP, and 4.443 and 4.374 Å for (I). Although rarely mentioned in the literature, such interactions have been described previously (Desiraju and Steiner, 1999). However, since these C···P distances in TP are somewhat longer than examples cited in the above reference (3.19–3.83 Å) the interaction must be considered very weak, certainly when compared with the C—H···O interaction in (I). Note that this C—H···P interaction is not unreasonable given that a C···O distance of 4.0 Å is regarded as a reasonable upper limit for a C—H···O interaction (Desiraju, 1991; Desiraju, 1996; Taylor & Kennard, 1982).