Synthesis and crystal structures of N,N′-disubstituted piperazines

Article abstract of DOI:10.1007/s10870-012-0346-1

The structure of 1,4-diphenylpiperazine (1) was determined; it crystallized in the orthorhombic space group Pbca, a = 8.6980(7), b = 8.4287(7), c = 17.6359(15), V = 1292.94(19), Z = 4. Three novel N,N′-disubstituted piperazines were synthesized via reduct

Full text of DOI:10.1007/s10870-012-0346-1

J Chem Crystallogr (2012) 42:981–987
DOI 10.1007/s10870-012-0346-1
ORIGINAL PAPER
Synthesis and Crystal Structures of N,N0-Disubstituted
Piperazines
•
Jason P. Safko Robert D. Pike
Received: 20 February 2012 / Accepted: 16 June 2012 / Published online: 4 July 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The structure of 1,4-diphenylpiperazine (1) was
determined; it crystallized in the orthorhombic space group
Pbca, a = 8.6980(7), b = 8.4287(7), c = 17.6359(15),
V = 1292.94(19), Z = 4. Three novel N,N0-disubstituted
piperazines were synthesized via reductive amination of
piperazine or N-diphenylmethylpiperazine. The products
were characterized by NMR and X-ray crystallography. 1,4-
Diphenethylpiperazine (2) crystallized in the monoclinic
space group C2/c, a = 17.9064(13), b = 6.2517(5), c =
14.9869(11), b = 90.613(4), V = 1677.6(2), Z = 4.
1-Benzhydryl-4-benzylpiperazine (3) crystallized in the
monoclinic space group Pn, a = 5.9450(2), b =
19.0722(4), c = 8.6084(2), b = 96.4600(10), V =
98.1790(10), Z = 2. 1-Benzhydryl-4-(pyridin-2-ylmethyl)
piperazine (4) crystallized in the monoclinic space group
P2/c, a = 13.5637(2), b = 5.82170(10), c = 24.0645(4),
b = 90.613(4), V = 1888.16(5), Z = 4. Comparison of the
structures showed significant sp2 character for the piperazine
nitrogen atoms in 1. Each structure showed multiple inter-
molecular non-bonding interactions.
Introduction
N,N0-Disubstituted piperazines have found application as
ligands in metal complexes and form the basis of various
natural products that exhibit favorable pharmacological
properties. Piperazine is conveniently substituted at the
N and N0 positions via reductive amination and nucleo-
philic substitution reactions. Synthetic piperazines are
important in biomedical applications as ion channel and
anticancer agents [1, 2]. The substitutional flexibility of
piperazines also makes them tunable ligands. The behavior
of piperazine ligands in metal complexes is also of interest
because they can act either as chelating or bridging ligands.
Although the cyclohexane-type ring is relatively flexible,
the chelate bite of the nitrogen atoms requires adoption of a
boat conformation leading to significant ring strain.
Therefore, the piperazine is at least as likely to engage in
bridging behavior between metal centers despite the well-
known thermodynamic preference for chelation [3–11].
Many, but not all, chelated piperazine complexes feature
ancillary coordination from pendant groups. A variety of
bridging piperazine complexes has been reported, includ-
ing metal dimers and tetramers, polymeric chains, and 2-D
and 3-D frameworks [3, 12–22].
Keywords Piperazine ꢀ X-ray ꢀ Crystal structure ꢀ
Resonance ꢀ Amination
The four disubstituted piperazine ligands whose struc-
tures are reported herein, and which are shown in Chart 1,
were prepared with the intention of producing photolumi-
nescent network complexes with copper(I) salts. Cop-
per(I) halide complexes tend to form oligomeric units or
metal organic networks having copper coordination num-
bers of 2, 3, and/or 4 [20–26]. We have shown that certain
low-coordinate copper(I) systems are reactive toward vol-
atile nucleophiles, forming the basis for potential lumi-
nescence detectors [23–25]. Therefore, by controlling
substituent size at the piperazine N and N0 positions we
J. P. Safko ꢀ R. D. Pike (&)
Department of Chemistry, College of William and Mary,
Williamsburg, VA 23187-8795, USA
e-mail: rdpike@wm.edu
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J Chem Crystallogr (2012) 42:981–987
Synthesis and Crystallization
1,4-Diphenylpiperazine (1)
Commercial 1 (0.477 g, 2.00 mmol) was heated with CuI
(0.190 g, 1 mmol) in 30 mL MeCN solution to 100 °C for
3 days in a sealed tube. No reaction took place, but 1
recrystallized, forming X-ray quality crystals.
1,4-Diphenethylpiperazine (2)
Piperazine (0.431 g, 5.00 mmol) and phenylacetaldehyde
(1.204 g, 10.00 mmol) were dissolved in 30 mL of CH2Cl2
and stirred under Ar at room temp. To this solution a few
drops of trifluoroacetic acid (0.2 mL) were added, and
the mixture was allowed to stir for 30 min, producing a
clear yellow solution. Next, NaBH(OAc)3 (2.109 g,
10.00 mmol) dissolved in 25 mL CH2Cl2 was added, and
the solution allowed to stir overnight under Ar. The
resulting solution was washed with 1 M NaOH (aq), then
saturated NaHCO3 (aq), and finally deionized water. The
organic layer was placed in the freezer and allowed to
crystallize. The crystalline precipitate was collected via
filtration (1.174 g, 79.2 %). X-ray quality crystals were
grown by cooling a 0.15 M solution in CH2Cl2 to -5 °C.
1H NMR (400 MHz, CDCl3) d 2.62 (m, 12H, CH2A, CHB2 ),
2.81 (dd, J = 11.7, 7.7 Hz, 4H, CHC2 ), 7.20 (m, 6H, Pho,p),
7.27 (t, J = 7.4 Hz, 4H, Phm). 13C{1H} NMR (100 MHz,
CDCl3) d 33.82, 53.38, 60.73, 126.28, 128.62, 128.92, and
140.53. Anal. Calcd. for C20H26N2: C, 81.59; H, 8.89; N,
9.52. Found: C, 81.12; H, 8.73; N, 8.74.
Chart 1 Disubstituted piperazines reported herein
hope to exert control over polymerization and cop-
per(I) coordination number. Here we report the structures
of four N,N0-disubstituted piperazine molecules.
1-Benzhydryl-4-benzylpiperazine (3)
N-Diphenylmethylpiperazine (3.061 g, 12.00 mmol) and
benzaldehyde (1.281 g, 12.00 mmol) were dissolved in
30 mL CH2Cl2 and stirred under Ar at room temp. To this
solution a few drops of trifluoroacetic acid (0.2 mL) were
added, and the mixture was allowed to stir for 30 min,
producing a clear solution. Next, NaBH(OAc)3 (2.539 g,
12.00 mmol) dissolved in 25 mL CH2Cl2 was added, and
the mixture was allowed to stir overnight under Ar. The
resulting solution was washed with 1 M NaOH (aq), then
saturated NaHCO3 (aq), and finally deionized water. The
organic layer was passed through a plug of activated Al2O3
and evaporated in vacuo, resulting in a white powder that
was dried overnight under vacuum (1.340 g, 59.29 %).
X-ray quality crystals were obtained through slow evapo-
Experimental
General
All reagents were purchased from Aldrich or Acros and
were used as received including, piperazine, phenylacet-
aldehyde, diphenylmethylpiperazine, 2-pyridinecarboxal-
dehyde, and sodium triacetoxyborohydride (NaBH(OAc)3),
except for benzaldehyde, which was distilled prior to use.
1,4-Diphenylpiperazine (1) was purchased from MP Bio-
medical. Piperazines (2–4) were prepared via reductive
amination as described below [20–27]. NMR data were
recorded on a Varian Mercury 400 instrument (s = singlet,
d = doublet, t = triplet, br = broad, Ph = phenyl,
Py = pyridyl). Analyses for C, H, and N were carried out
by Atlantic Microlabs, Norcross, GA or using a Thermo
Scientific Flash 2000 Organic Elemental Analyzer with a
Mettler Toledo XP6 Microbalance.
1
ration of a pentane solution. H NMR (400 MHz, CDCl3)
d 2.46 (m, 8H, CHA2 , CH2B), 3.51 (s, 2H, CHC2 ), 4.22 (s, 1H,
CH), 7.15 (t, J = 7.0 Hz, 2H, CHPhp2), 7.25 (m, 9H, Ph),
7.39 (d, J = 7.0 Hz, 4H, CHPho2). 13C{1H} NMR
(100 MHz, CDCl3) d 52.14, 53.59, 65.30, 76.44, 127.06,
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J Chem Crystallogr (2012) 42:981–987
983
Table 1 Crystal and structure refinement data
1,4-Diphenylpiperazine
(1)
1,4-Diphenethyl-
piperazine (2)
1-Benzhydryl-4-
benzylpiperazine (3)
1-Benzhydryl-4-(pyridin-
2-ylmethyl)-piperazine (4)
CCDC deposit no.
Color and habit
Size, mm
863662
863658
863660
863661
Colorless plate
0.28 9 0.27 9 0.14
Colorless prism
0.32 9 0.12 9 0.07
C20N2H26
296.46
Colorless plate
0.44 9 0.16 9 0.06
C24N2H26
342.48
Colorless plate
0.44 9 0.19 9 0.07
C23N3H25
343.47
Formula
C16H2N18
Formula weight
Space group
238.67
Pbca
C2/c
Pn
P2/c
˚
a, A
8.6980(7)
8.4287(7)
17.6359(15)
90
17.9064(13)
6.2517(5)
14.9869(11)
90.613(4)
1677.6(2)
4
5.9450(2)
19.0722(4)
8.6084(2)
98.1790(10)
966.13(4)
2
13.5637(2)
5.82170(10)
24.0645(4)
96.4600(10)
1888.16(5)
4
˚
b, A
˚
c, A
b, °
Volume, A
3
˚
1292.94(19)
4
Z
q
calc, g cm-3
1.224
1.174
1.177
1.208
F000
l(Cu Ka), mm-1
512
648
368
736
0.556
0.516
0.522
0.551
˚
˚
˚
˚
Radiation
Cu Ka (k = 1.54178 A)
Cu Ka (k = 1.54178 A)
Cu Ka (k = 1.54178 A)
Cu Ka (k = 1.54178 A)
Temperature, K
Residuals:a R; Rw
Goodness of fit
a
100
100
100
100
0.0353, 0.0906
1.071
0.0374, 0.0943
1.075
0.0279, 0.0684
1.055
0.0312, 0.0801
1.044
R = R1 = R||Fo| - |Fc||/R|Fo| for observed data only. Rw = wR2 = {R[w(F2o - Fc2)2]/R[w(Fo2)2]}1/2 for all data
127.18, 128.21, 128.36, 128.62, 129.45, 138.37, 143.04.
Anal. Calcd. for C24H26N2: C, 84.17; H, 7.65; N, 8.18.
Found: C, 83.89; H, 7.53; N, 7.96.
1H, PyF), 8.54 (d, J = 5.1 Hz, 1H, PyG). 13C{1H} NMR
(100 MHz, CDCl3) d 51.81, 53.57, 64.57, 76.17, 121.88,
123.13, 126.82, 127.94, 128.38, 136.21, 142.75, 149.21,
158.64. Anal. Calcd. for C23H25N3: C, 80.43; H, 7.33; N,
12.24. Found: C, 79.81; H, 7.30; N, 11.90.
1-Benzhydryl-4-(pyridin-2-ylmethyl)piperazine (4)
N-Diphenylmethylpiperazine (2.526 g, 10.00 mmol) and
2-pyridinecarboxaldehyde (1.070 g, 10.00 mmol) were
dissolved in 30 mL CH2Cl2 and stirred under Ar at room
temp. To this solution a few drops of trifluoroacetic acid
(0.2 mL) were added and the mixture was allowed to stir
for 30 min, producing a clear yellow solution. Next,
NaBH(OAc)3 (2.158 g, 10.00 mmol) dissolved in 25 mL
CH2Cl2 was added, and the mixture was allowed to stir
overnight under Ar. The resulting solution was washed
with 1 M NaOH (aq), then saturated NaHCO3 (aq), and
finally deionized water. The organic layer was passed
through a plug of activated Al2O3 and evaporated in vacuo.
The resulting thick yellow oil solidified into a beige pow-
der under vacuum (1.34 g, 59.3 %). X-ray quality crystals
were obtained by layering a 20 mM solution in CH3CN
X-ray Crystallography
Crystals were mounted on glass fibers. All measurements
were made using graphite-monochromated Cu Ka radiation
on a Bruker-AXS three-circle diffractometer, equipped
with a SMART Apex II CCD detector. Initial space group
determination was based on a matrix consisting of 120
frames. The data were reduced using SAINT? [28], and
empirical absorption correction applied using SADABS
[29].
Structures were solved using direct methods. Least-
squares refinement for all structures was carried out on F2.
The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms in were located in the Fourier difference
map and then allowed to refine isotropically. Structure
solution, refinement and the calculation of derived results
were performed using the SHELXTL package of computer
programs [30]. Details of the X-ray experiments and crystal
data are summarized in Table 1. Selected bond lengths and
bond angles are given in Table 2.
1
with ether in a 5 mm diameter tube. H NMR (400 MHz,
CDCl3) d 2.44 (br s, 4H, CHA2 ), 2.54 (br s, 4H, CH2B), 3.66
(s, 2H, CHC2 ), 4.23 (s, 1H, CH), 7.15 (m, 3H, Php, PyE),
7.25 (t, J = 7.8 Hz, 4H, Phm), 7.37 (d, J = 7.8, 1H, PyD),
7.40 (d, J = 6.3 Hz, 4H, Pho), 7.61 (td, J = 7.8, 1.6 Hz,
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J Chem Crystallogr (2012) 42:981–987
Table 2 Selected bond lengths and angles
1,4-Diphenylpiperazine (1)
1,4-Diphenethylpiperazine (2)
Bond lengths
N(1)–C(1)
1.4589(15)
1.4695(15)
1.4157(15)
N–C(1)
N–C(2)
N–C(3)
1.4589(17)
1.4617(16)
1.4611(16)
N(1)–C(2)
N(1)–C(3)
Bond angles
C(3)–N(1)–C(1)
C(3)–N(1)–C(2)
C(1)–N(1)–C(2)
116.84(9)
115.48(9)
110.18(9)
C(1)–N–C(2)
C(1)–N–C(3)
C(2)–N–C(3)
108.67(10)
110.58(10)
112.14(10)
1-Benzhydryl-4-benzylpiperazine (3)
1-Benzhydryl-4-(pyridin-2-ylmethyl)piperazine (4)
Bond lengths
N(1)–C(8)
1.459(2)
1.4600(18)
N(1)–C(4)
N(1)–C(1)
N(1)–C(18)
N(2)–C(3)
N(2)–C(2)
N(2)–C(5)
C(3)–C(4)
C(1)–C(2)
1.4583(14)
1.4585(13)
N(1)–C(10)
N(1)–C(7)
1.463(2)
1.4686(18)
1.4690(19)
1.477(2)
1.4600(13)
1.4678(13)
1.4699(13)
1.4807(13)
1.5161(15)
1.5153(15)
N(2)–C(11)
N(2)–C(9)
N(2)–C(12)
C(10)–C(11)
1.514(2)
C(8)–C(9)
1.518(2)
Bond angles
C(8)–N(1)–C(10)
C(8)–N(1)–C(7)
C(10)–N(1)–C(7)
C(11)–N(2)–C(9)
C(11)–N(2)–C(12)
C(9)–N(2)–C(12)
N(1)–C(8)–C(9)
N(2)–C(9)–C(8)
N(1)–C(10)–C(11)
N(2)–C(11)–C(10)
108.61(11)
111.86(12)
110.26(12)
107.92(11)
110.04(12)
110.60(11)
110.67(12)
111.23(12)
110.27(12)
110.73(13)
C(4)–N(1)–C(1)
C(4)–N(1)–C(18)
C(1)–N(1)–C(18)
C(3)–N(2)–C(2)
C(3)–N(2)–C(5)
C(2)–N(2)–C(5)
N(1)–C(1)–C(2)
N(2)–C(2)–C(1)
N(2)–C(3)–C(4)
N(1)–C(4)–C(3)
108.69(8)
111.64(8)
111.32(8)
108.83(8)
108.81(8)
110.93(8)
110.20(9)
110.47(9)
111.99(9)
109.82(9)
Results and Discussion
as indicated in Scheme 1. For the homo-disubstituted 2,
piperazine was reacted with two equivalents of phenyl-
acetaldehyde. The hetero-disubstituted 3 and 4 were pre-
pared by reacting N-diphenylmethylpiperazine with
Compounds 2–4 were prepared through reductive amina-
tion by stirring overnight in the presence of reducing agent,
Scheme 1 Syntheses of
disubstituted piperazines
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J Chem Crystallogr (2012) 42:981–987
985
Fig. 1 Thermal ellipsoid (50 % probability) drawings of 1, 2, 3, and 4 (A–D)
Scheme 2 Resonance in 1,4-diphenylpiperazine
benzaldehyde and 2-pyridinecarboxaldehyde, respectively.
Fig. 2 Wireframe representations of compounds 1–6 overlaid using
the piperazine rings. Ring nitrogen atoms are shown as spheres
The reactions produced beige to brown solids that precip-
itated out of solution upon cooling or evaporation. The
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J Chem Crystallogr (2012) 42:981–987
analytically pure compounds were then crystallized for
X-ray diffraction. For structural comparison, commercial
1,4-diphenylpiperazine (1) was also crystallized for X-ray
diffraction.
Conclusions
We have synthesized and characterized three N,N0-disub-
stituted piperazines in which the substituents are linked to the
ring by a saturated carbon. Comparison of these and related
structures to the newly presented N,N0-diphenylpiperazine
reveals flattening in the latter, indicative of resonance
p-bond character for the N–Ph substituents. Non-bonding
interactions are present in all four new structures.
The novel structures of 1–4 are shown in Fig. 1. Crys-
tallographic determination information is given in Table 1
and selected bond lengths and angles in Table 2. For
compounds 1 and 2, the molecules were situated around an
inversion center, located at the center of the piperazine
ring. For compounds 3 and 4 the entire molecule comprised
the asymmetric unit.
Numerous related N,N0-disubstituted piperazines have
been reported, including ring-substituted N,N0-diphenyl-
piperazines [31–36], piperazines with the N-CHPh2 sub-
stituent [37–42], the simple N,N0-dibenzylpiperazine (5)
[43], and 1,4-bis(pyridin-2-ylmethyl)piperazine (6) [3, 4].
In the present cases all piperazine carbon–nitrogen and
carbon–carbon bond lengths were found to be within the
expected range. However, for compound 1 the phenyl
groups appear to force a more planar conformation with
bond angles about the nitrogen atoms, approaching the
120° sp2 value. This effect may be attributed to the delo-
calization of the nitrogen lone pair caused by resonance
with the aromatic system, as shown in Scheme 2. In
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