NH...O and NH...N hydrogen bonded networks in imidazole-based phosphane oxides: Structures of tris(2-isopropylimidazol-4(5)-yl)phosphane oxide, bis(2-isopropylimidazol-4(5)-yl)phosphane oxide and diphenyl-2- isopropylimidazol-4(5)-yl phosphane oxide

Article abstract of DOI:10.1007/s10870-010-9845-0

The three title compounds show extensive hydrogen bonding networks in the solid state. The structure of diphenyl-2-isopropylimidazol-4(5)yl phosphane oxide (3) is dominated by N-H...OP hydrogen bonds, whereas in bis(2-isopropylimidazol-4(5)-yl)phenyl- (2) and tris(2-isopropylimidazol-4(5)yl) phosphane oxide (1) both, N-H...N and N-H...OP hydrogen bonds determine the solid-state structures. Compound 1 crystallises in the monoclinic space group Cc with cell parameters a = 19.5447(6) A, b = 10.45764(16) A, c = 10.8549(3) A and β = 121.418(4)°; 2 in the orthorhombic space group Pna2₁, with a = 11.5997(3) A, b = 9.5836(2) A, c = 16.1860(4) A and 3 in the orthorhombic space group Pca2₁, with a = 10.8430(2) A, b = 10.9277(2) A and c = 27.7088(6) A.

Full text of DOI:10.1007/s10870-010-9845-0

J Chem Crystallogr (2011) 41:105–110
DOI 10.1007/s10870-010-9845-0
ORIGINAL PAPER
NH_O and NH_N Hydrogen Bonded Networks
in Imidazole-Based Phosphane Oxides: Structures
of Tris(2-Isopropylimidazol-4(5)-yl)Phosphane Oxide,
Bis(2-Isopropylimidazol-4(5)-yl)Phosphane Oxide
and Diphenyl-2-Isopropylimidazol-4(5)-yl Phosphane Oxide
•
Peter C. Kunz Wilhelm Huber Bernhard Spingler
•
Received: 17 March 2010 / Accepted: 28 June 2010 / Published online: 17 July 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The three title compounds show extensive
hydrogen bonding networks in the solid state. The structure
of diphenyl-2-isopropylimidazol-4(5)yl phosphane oxide
(3) is dominated by N–H_OP hydrogen bonds, whereas in
bis(2-isopropylimidazol-4(5)-yl)phenyl- (2) and tris(2-iso-
propylimidazol-4(5)yl)phosphane oxide (1) both, N–H_N
and N–H_OP hydrogen bonds determine the solid-state
structures. Compound 1 crystallises in the monoclinic space
e.g. N,N,N-, N,P- or P-ligands as well as bridging two or
more metal atoms. Tris(imidazolyl)phosphane complexes,
usually bearing imidazol-2-yl moieties, have been used to
mimic the active site of carbonic anhydrase and as water
soluble ligands for gold based anti-tumour drugs [1–8].
We are especially interested in the coordination chem-
istry of the not so common imidazolyl phosphane regioi-
somers where the phosphorous atom is bound to the
heteroaromatic ring via the 4- or 5-position. Although a
number of metal complexes of these ligands have already
been reported, only a few structures of ligands or their
oxidation products were determined so far [9–11].
Here, we report on the solid state structure of the series
of imidazol-4(5)yl phosphane oxide which, due to their
P=O and N–H functionalities, show distinct hydrogen
bonding in the solid state.
˚
group Cc with cell parameters a = 19.5447(6) A, b =
˚
˚
10.45764(16) A, c = 10.8549(3) A and b = 121.418(4)°; 2
in the orthorhombic space group Pna21, with a =
˚
11.5997(3) A, b = 9.5836(2) A, c = 16.1860(4) A and 3 in
˚
˚
the orthorhombic space group Pca21, with a =
˚
10.8430(2) A, b = 10.9277(2) A and c = 27.7088(6) A.
˚
˚
Keywords Tris(imidazolyl)phosphane oxide Á
Imidazolyl phosphane oxide Á
Heterocyclic phosphane oxide
Experimental
Introduction
Crystallography
Tris(azolyl)phosphanes show a plethora of interesting
properties as ligands in coordination chemistry. Most
notably, they can adopt different coordination modes as,
Crystallographic data were collected at 183(2) K on an
Oxford Diffraction Xcalibur system with a Ruby detector
˚
using Mo Ka radiation (k = 0.7107 A) that was graphite-
monochromated. Suitable crystals were covered with oil
(Infineum V8512, formerly known as Paratone N), moun-
ted on top of a glass fibre and immediately transferred to
the diffractometer. The program suite CrysAlisPro was used
for data collection, semi-empirical absorption correction
and data reduction [12]. Structures were solved with direct
methods using SIR97 [13] and were refined by full-matrix
least-squares methods on F2 with SHELXL-97 [14]. The
structures were checked for higher symmetry with help of
the program Platon [15].
P. C. Kunz (&) Á W. Huber
¨
Heinrich-Heine-Universitat Dusseldorf,
Universitatsstr. 1, 40225 Dusseldorf, Germany
¨
¨
¨
e-mail: peter.kunz@uni-duesseldorf.de
B. Spingler
Anorganisch-Chemisches Institut, Universitat Zurich-Irchel,
Winterthurerstr. 190, 8057 Zu
¨
¨rich, Switzerland
¨
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J Chem Crystallogr (2011) 41:105–110
General Instrumentation
All preparations were carried out in Schlenk tubes under an
atmosphere of dry nitrogen using anhydrous solvents
purified according to standard procedures. All chemicals
1
were used as purchased. H and 31P NMR spectra were
1
recorded on a Bruker DRX 200 spectrometer. The H and
13C NMR spectra were calibrated against the residual
proton signals and the carbon signals of the solvents as
internal references ([D1]chloroform: dH = 7.24 ppm and
dC = 77.0 ppm; [D4]methanol: dH = 5.84 ppm and
dC = 49.1 ppm) while the 31P{1H} NMR spectra were
referenced to external 85% H3PO4. Infrared spectra were
recorded with a Bruker IFS 66 FT-IR spectrometer. Time-
of-flight mass spectra were recorded on a Bruker Ultraflex
TOF, ESI mass spectra on an ion-trap mass spectrometer
Fig. 3 View of the molecular structure of 2, showing 50% proba-
bility ellipsoids (non acidic H-atoms are omitted for clarity)
Fig. 4 View of the molecular structure of 3, showing 50% proba-
bility ellipsoids (non acidic H-atoms and second molecule are omitted
for clarity)
Fig. 1 Imidazoly-4(5)-yl phosphane oxides used in this study
˚
Table 1 Selected bond lengths (A) and angles (°) of 1–3
Compound
1
2
3
d(P-O)
d(P-C)
1.5014(10)
1.7694(15)
1.7706(13)
1.7765(13)
104.40(6)
107.52(6)
106.83(6)
1.4957(10)
1.7793(12)
1.7792(13)
1.8001(12)
106.34(6)
109.63(6)
103.81(6)
1.4956(13)
1.7693(19)
1.802(2)
1.802(2)
\(C-P-C)
105.74(9)
106.70(9)
107.95(9)
Fig. 5 Columnar arrangement in the hydrogen bonding network of 3
(view along the b axis)
Finnigan LCQ Deca and EI mass spectra on a GC/MS-
system Thermo Finnigan Trace DSQ.
Tris(2-Isopropylimidazol-4(5)-yl)Phosphane Oxide (1)
Fig. 2 View of the molecular structure of 1, showing 50% proba-
bility ellipsoids (non acidic H-atoms are omitted for clarity). The
isopropyl group consisting of C16, C17, C18 shows a twofold
disorder, only one conformer is shown
The compound 1 was prepared as previously described [1]
and crystallised upon slow evaporation of a ethanolic
solution to yield colourless needles.
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J Chem Crystallogr (2011) 41:105–110
107
Bis(2-Isopropylimidazol-4(5)-yl)Phenyl Phosphane
Oxide (2)
oily residue was re-dissolved in 100 mL of acetone/water
10:1 and heated to reflux for 12 h. The resulting white
precipitated was collected and recrystallized from metha-
nol/acetone. 1H NMR (200MHz, CDCl3): d = 1,39 (d,
To a solution of 1-diethoxymethyl-2-isopropylimidazole
(10.6 g, 50.0 mmol) in 300 mL of diethyl ether 33 mL
(53 mmol) of 1.6 M n-butyl lithium in hexane were added
at -78 °C. The solution was stirred at -78 °C for 30 min
and additionally for 30 min at room temperature. After
cooling to -78 °C PhPCl2 (3.39 mL, 25 mmol) in 10 mL
diethyl ether was added. The white suspension was stirred
over night at room temperature, 75 mL of conc. NH4OH
were added the organic layer separated, washed with water
and dried over MgSO4. After removal of the solvent the
3
3JHH = 7 Hz, 12H), 3,12 (sept., JHH = 7 Hz, 2H), 7,24
(m, 5H), 7,43 (d, JPH = 0,8 Hz). 31P{1H}-NMR (81 MHz,
CDCl3): d = -63 (s). EI MS (70 eV, 240 °C): m/z
(%) = 326 (100) [M]?, 249 (18) [M-Ph]?, 217 (61) [M-
imiPr]?. C18H23N4P (326.38): calc. C 66.2 H 7.1 N 17.2,
found C 65.6 H 7.1 N 17.1. A solution of the phosphane in
ethanol is treated with H2O2 and stirred over night at
ambient temperature. The solvent is removed in vacuo and
the residue crystallised from ethanol/water. 1H NMR
Table 2 Crystal data and structure refinement for 1–3
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
C18H27N6OP
374.43
C18H23N4OP
342.37
C18H19N2OP
310.32
Monoclinic
Cc
Orthorhombic
Pna21
Orthorhombic
Pca21
˚
a (A)
19.5447(6)
10.45764(16)
10.8549(3)
90
11.5997(3)
9.5836(2)
16.1860(4)
90
10.8430(2)
10.9277(2)
27.7088(6)
90
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
121.418(4)
90
90
90
90
90
3
˚
Volume (A )
1893.36(8)
4
1799.35(7)
4
3283.19(11)
8
Z
Density (calculated) (Mg m-3
)
1.314
1.264
1.256
Absorption coefficient (mm-1
)
0.166
0.165
0.171
F(000)
Crystal size (mm3)
Crystal description
800
728
1312
0.53 9 0.25 9 0.21
Colourless plate
2.44–30.51
0.44 9 0.24 9 0.18
Colourless needle
2.76–30.50
0.30 9 0.20 9 0.10
Colourless plate
2.37–30.50
Theta range for data collection (°)
Index ranges
-27 B h B 27, -14 B k B 14,
-15 B l B 15
-9 B h B 16, -12 B k B 13,
-23 B l B 21
-15 B h B 14, -15 B k B 15,
-29 B l B 39
Reflections collected
14844
12583
25878
Independent reflections
Reflections observed
5418 [R(int) = 0.0229]
4983
5284 [R(int) = 0.0245]
4692
9065 [R(int) = 0.0375]
6701
Criterion for observation
Completeness to H
[2r(I)
[2r(I)
[2r(I)
100.0% to 30.51°
0.9660 and 0.9083
5418/2/259
99.9% to 30.50°
0.9709 and 0.9026
5284/1/221
100.0% to 30.50°
0.9831 and 0.8663
9065/1/401
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I [ 2r(I)]
R indices (all data)
1.035
0.983
0.941
R1 = 0.0351, wR2 = 0.0898
R1 = 0.0386, wR2 = 0.0913
0.16(6)
R1 = 0.0336, wR2 = 0.0782
R1 = 0.0384, wR2 = 0.0797
-0.01(7)
R1 = 0.0450, wR2 = 0.0810
R1 = 0.0689, wR2 = 0.0859
0.00(6)
Absolute structure parameter
-3
)
˚
Largest diff. peak and hole (e A
0.472 and -0.204
0.310 and -0.175
0.370 and -0.292
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J Chem Crystallogr (2011) 41:105–110
3
Diphenyl-2-Isopropylimidazol-4(5)-yl Phosphane
(200 MHz, [D4]methanol): d = 1,37 (d, JHH = 7 Hz,
12H), 3,17 (sept., 3JHH = 7 Hz, 1H), 7,37 (s, 2H), 7,74 (m,
5H). 31P{1H}-NMR (81 MHz, [D4]methanol): d = 12 (s).
EI MS (70 eV, 290 °C): m/z (%) = 342 (100) [M]?, 327
(99) [M-O]?, 233 (49) [M-imiPr]?, 217 (72) [M-O-imiPr]?.
C18H23N4OPÁ2H2O (378.41): calc. C 57.1 H 7.2 N 14.8,
found C 57.1 H 6.6 N 14.8.
Oxide (3)
Diphenyl-2-isopropylimidazol-4(5)-yl phosphane (4-MIPiPr)
was prepared as previously reported [3]. For the oxidation of
4-MIPiPr to 4-MIPOiPr (3) diphenylimidazol-4(5)-yl phos-
phane was dissolved in ethanol and hydrogen peroxide was
Fig. 6 Hydrogen bonding in the solid state of 2 (view along the b axis). a PO_HN chain along a axis and b NH_N chain along c axis
Fig. 7 Hydrogen bonding network in the solid state of 2. The two independent networks R66(37)R66(35) are shown
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J Chem Crystallogr (2011) 41:105–110
109
added until no more signal of the phosphane was observed in
31P{1H} NMR. The mixture was stirred over-night, the sol-
vent removed in vacuo, the resulting solid washed with ace-
tone and diethyl ether and dried in vacuo. The solid thus
obtainedwasdissolvedinaminimumamountofmethanoland
crystallized by slow diffusion of diethyl ether into this solu-
1
tion. H NMR (200 MHz, [D4]methanol): d = 1.35 (d, 6H,
JHH = 7 Hz, CH(CH3)2), 3.16 (sept, 1H, JHH = 7 Hz,
CH(CH3)2), 7.32 (s, br, 1H, Him), 7.5–7.9 (m, 10H, Ph).
31P{1H} NMR (81 MHz, [D4]methanol): d = 24. EI MS
(70 eV, 220 °C): m/z (%) = 310 (100) [M]?, 295 (84)
[M-Me]?, 233 (17) [M-Ph]?. C18H19N2OP (310.34): calc. C
69.7 H 6.2 N 9.0, found C 69.3 H 6.0 N 8.9.
Results and Discussion
The three phosphane oxides 1–3 (Fig. 1) are easily
obtained by oxidising the corresponding phosphanes using
hydrogen peroxide. Selected bond lengths and angles are
summarised in Table 1.
The P=O bond lengths in compounds 1–3 is significantly
longer than the P=O bond length found in triphenylphos-
˚
phane oxide (1.479(2) A) [16], even when taken in
account, that the P=O bond is elongated when involved in
hydrogen bonding [17, 18] (Figs. 2, 3, 4).
Compound 1 crystallises in the monoclinic space group
Cc whereas 2 and 3 crystallise in the orthorhombic space
groups Pna21 and Pca21, with one and two molecules in the
asymmetric unit respectively. In all three compounds the
aromatic substituents exhibit a comparable propeller
arrangement as found for the phenyl substituents in PPh3
[19, 20]. The conformation of the 2-isopropylimidazolyl
substituent in
3 resembles the one found in the
Au(I)chlorido complex of this ligand [3]. In non of these
compounds interactions between the aryl substituents
similar to the sextuple phenyl embrace (SPE) [21] com-
monly found in crystal structures containing PPh3 are
found, here hydrogen bonds are the more dominant inter-
molecular forces.
In the solid state structures of compounds 1–3 hydro-
gen bonding networks are found. Columns of 3 are ori-
entated pair-wise parallel and anti-parallel to the a axis
(Fig. 5). The graph set description [22] of NH_OP
hydrogen bonding is C(6). The rungs of these zick-zack
ladder-like columns are formed by the imidazolyl sub-
stituents and the stiles by the phenyl and isopropyl groups
(Table 2).
Fig. 8 Hydrogen bonding network in the solid state of 1. The three
independent networks R44(27)R44(23)R43(22) are shown
The ladder motif is also found in 2 (Fig. 6). Additionally
to the NH_OP hydrogen bonds the second imidazolyl
substituents forms NH_N hydrogen bridges to the imine N
atom of the imidazolyl ring involved in the NH_OP
system. This leads to the formation of a set of hydrogen
bonds in the solid-state best described by the graph set
R66(37)R66(35) (Fig. 7).
123

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J Chem Crystallogr (2011) 41:105–110
¨
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The situation in 1 is much more complicated due to the
three imidazolyl substituents. Here a three-dimensional
hydrogen bonded network is found in which one molecule
is hydrogen bonded to five others. The PO group forms
hydrogen bonds to the nitrogen atoms of two imidazolyl
groups N2 and N4 of different molecules of 1. The phos-
phorus atoms of four molecules can be placed on the cor-
ners of a distorted cube. Then, one each face a closed
hydrogen-bonded pathway of four hydrogen bridges
determines the network. The motif is described by the
graph set R44(27)R44(23)R43(22) (Fig. 2). Interestingly, only
hydrogen bridges between the atom pairs N4/O1, N6/O1
and N2/N5 describe these networks (Fig. 8).
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In all three compounds only the imidazol-4-yl tautomer
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the NH proton and the PO oxygen atom are located in a
more or less transoid conformation, which contributes to
the formation of the intermolecular NH_OP bonds in one
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Supplementary Material
Crystallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC 769495–769497 for compounds 1–3. Copies of
this information can be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: ?44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or http://www.ccdc.cam.ac.uk).
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