Assembly of two supramolecular structures by metal complexes with a urea-based pyridyl ligand

Article abstract of DOI:10.1016/j.inoche.2007.01.026

The supramolecular structures of two metal complexes, [CuL₄Cl₂] (1) and [ZnL₂Cl₂] (2), where L = N-(2,6-dimethylphenyl)-N′-(3-pyridy1)urea, are reported. Complex 1 features infinite tapes formed by intermolecular N-H?Cl hydrogen bonds with an R₂¹(6) motif, as well as a π-π stacking interaction. The solid-state structure of 2 shows a 2D network held together by two types of N-H?O R₂¹(6) motifs, and there is no significant interaction between the NH groups and the metal-bound Cl^- ions.

Full text of DOI:10.1016/j.inoche.2007.01.026

Inorganic Chemistry Communications 10 (2007) 563–566
www.elsevier.com/locate/inoche
Assembly of two supramolecular structures by metal complexes
with a urea-based pyridyl ligand
a,c,
a
a,e
Biao Wu *, Xiaojuan Huang a,e, Jianjun Liang , Yanyan Liu
,
b,c,
d
Xiao-Juan Yang *, Huai-Ming Hu
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
b
c
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
d
Department of Chemistry, Northwest University, Xi’an 710069, China
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
e
Received 17 January 2007; accepted 30 January 2007
Available online 8 February 2007
Abstract
The supramolecular structures of two metal complexes, [CuL4Cl2] (1) and [ZnL2Cl2] (2), where L = N-(2,6-dimethylphenyl)-N0-(3-
pyridy1)urea, are reported. Complex 1 features infinite tapes formed by intermolecular N–Hꢀ ꢀ ꢀCl hydrogen bonds with an R12(6) motif,
as well as a p–p stacking interaction. The solid-state structure of 2 shows a 2D network held together by two types of N–Hꢀ ꢀ ꢀO R12(6)
motifs, and there is no significant interaction between the NH groups and the metal-bound Clꢁ ions.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Pyridyl urea; Hydrogen bonding; p–p Stacking; Copper(II); Zinc(II)
Crystal engineering of inorganic superstructures has
become an approach of much interest in the design of
molecular materials [1]. In metal-directed assembly the
coordination chemistry of metal centers is often domi-
nant, but the non-covalent forces can also play an impor-
tant role in determining the supramolecular structure and
properties of the system [2]. Hydrogen bonding is one of
the most efficient and convenient ways to construct
supramolecular solid-state architectures with both organic
and metal-coordinated systems, and a variety of com-
pounds that are capable of hydrogen bond formation
have been developed [3]. Urea-based pyridyl ligands are
good candidates in this respect because in these bitopical
ligands the urea group can act as hydrogen bond donor
and/or acceptor, while the pyridyl function can coordi-
nate with metal ions. Thus, these ligands have the poten-
tial of simultaneously binding transition metal ions and
forming hydrogen bonds with other components such as
anions [4].
Recently we have established a facile synthetic route for
urea-containing pyridyl ligands, and prepared a series of
mono- and diureas as anion receptors. The co-binding of
H+/oxo-anion by two N-dimethylphenyl-N0-pyridylurea
ligands has been studied and will be reported elsewhere
[5]. We further tested the coordination chemistry of N-
(2,6-dimethylphenyl)-N0-(3-pyridyl)urea (L) with transition
metals, and herein report the synthesis [6] of two metal
complexes [CuL4Cl2] (1) and [ZnL2Cl2] (2), and their supra-
molecular structures [7] based on different hydrogen bond
interactions. It is interesting that although both com-
pounds feature R12(6) hydrogen bond motifs [8], the compo-
sition of the R1(6) ring is different: the Cu(II) complex
contains N–Hꢀ ꢀ2ꢀCl bonds, while the Zn(II) complex forms
N–Hꢀ ꢀ ꢀO contacts.
*
Corresponding authors. Tel.: +86 931 4968286 (B. Wu).
E-mail addresses: wubiao@lzb.ac.cn (B. Wu), yangxj@lzb.ac.cn (X.-J.
Yang).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.01.026

564
B. Wu et al. / Inorganic Chemistry Communications 10 (2007) 563–566
H
H
N
N
O
N
L
Reaction of ligand L with CuCl2 Æ 2H2O affords the 1:4
metal complex [CuL4Cl2] (1). This complex crystallizes in
the monoclinic space group P21/n. The coordination geom-
etry of the metal center is typical Jahn–Teller distorted
octahedral. The pyridyl N donors of four ligands are coor-
dinated to the Cu(II) atom in the equatorial plane, while
two chlorine atoms are located in the axial positions. The
˚
Cu–Cl bond length of 2.775 A is significantly longer than
the normal Cu–Cl bonds in the equatorial planes or in
the tetrahedral complexes, thus falling into the catalog of
weaker contacts [9]. For instance, these two types of Cu–
˚
˚
Cl bond have been found to be 2.30/2.33 A and 2.79 A in
(NH4)2[CuCl4] [9b]. The axial Cu–Cl bond lengths of sim-
ilar 4 + 2 complexes with nitrogen donor ligands,
[Cu(LN)4Cl2], have been reported to be still longer, in the
˚
range 2.80–3.08 A [9c]. The Cu(II) center in 1 resides at
an inversion center. Within a molecule, the four ureidopy-
ridyls are arranged into two pairs, each including two cis-
oriented ligands. Such a pair is linked by an intramolecular
˚
N–Hꢀ ꢀ ꢀO hydrogen bond (N(5)ꢀ ꢀ ꢀO(1), 2.943(2) A, \N(5)–
H(5A)ꢀ ꢀ ꢀO(1), 158.5°) between one of the urea NH groups
and the carbonyl oxygen from the other ligand (Fig. 1a).
The disubstituted aryl moieties of the cis-pair of ligands
are nearly parallel to each other (dihedral angle 6.76°),
but they stay out of significant p–p stacking because of
the lack of efficient overlapping between them. Instead, this
slippage of the two planes leads to a favorable arrangement
for a strong N–Hꢀ ꢀ ꢀp interaction to form between the
unbound NH group [N(6)H(6A)] and the aryl ring [C(7)–
C(12)] from the other ligand within a cis-pair (Fig. 1a).
The Hꢀ ꢀ ꢀCg (Cg represents the centroid of the aryl ring)
Fig. 1. (a) ORTEP drawing of a [CuL4Cl2] (1) molecule, showing the
intramolecular N–Hꢀ ꢀ ꢀO and N–Hꢀ ꢀ ꢀp interactions (thermal ellipsoid at
30% probability level); (b) Part of a tape structure formed by intermo-
lecular N–Hꢀ ꢀ ꢀCl (R12(6) motif) and p–p stacking interactions. Non-urea
˚
hydrogen atoms omitted for clarity. Selected bond lengths (A): Cu(1)–
N(1) 2.079(2), Cu(1)–N(4) 2.077(2), Cu(1)–Cl(1) 2.7747(6).
infinite tape along the crystallographic a axis (Fig. 1b).
Such tapes are parallel to each other in the b direction,
and are arranged in a zigzag fashion along the c axis. Nota-
bly, the cadmium(II) complex of a similar ligand,
˚
distance of this N–Hꢀ ꢀ ꢀp interaction is 2.66 A, the Nꢀ ꢀ ꢀCg
distance 3.40 A, the perpendicular distance of H atom to
[CdL04Cl2] Æ 2H2O Æ 2MeCN
(L0 = N-(p-tolyl)-N0-(3-pyr-
˚
˚
the plane 2.64 A, with a N–Hꢀ ꢀ ꢀCg angle of 144.2° [10].
idyl)urea) [4d], has been reported in which the enclathrated
water molecules act as bridging ligands between the Cl
atom and the urea group, thus breaking the intramolecular
N–Hꢀ ꢀ ꢀO hydrogen bond. Another difference is that in the
Cu(II) complex reported here the metal-coordinated Cl
atoms form merely N–Hꢀ ꢀ ꢀCl R12(6) rings, while in the
Cd(II) complex the Cl atoms also interact with the crystal-
line water molecules.
The zinc(II) complex [ZnL2Cl2] (2), obtained by the
reaction of L with ZnCl2, has a different structure from
the copper(II) analog. While 1 shows a metal to ligand
ratio of 1:4, the Zn:L ratio of compound 2 is 1:2. In con-
trast to the elongated octahedral configuration of the
Cu(II) complex, the zinc center is four-coordinated by
The typical N–Hꢀ ꢀ ꢀO R12(6) synthon of urea moieties [11],
which includes both NH groups, is absent in this complex.
In the extended structure, each of the axial Cl atoms
forms two intermolecular N–Hꢀ ꢀ ꢀCl hydrogen bonds
˚
˚
N(2)ꢀ ꢀ ꢀCl(1), 3.187(2) A, \N(2)–H(2A)ꢀ ꢀ ꢀCl(1), 124.0°;
(N(3)ꢀ ꢀ ꢀCl(1), 3.368(2) A, \N(3)–H(3A)ꢀ ꢀ ꢀCl(1), 141.2°)
in an R12(6) motif with the urea moiety of a neighboring
molecule. Two adjacent CuL4Cl2 units are held together
by two of the R12(6) rings, complemented by a rather strong
p–p stacking interaction between two perfectly parallel
(dihedral angle 0.0°) pyridine rings (centroid-centroid sep-
˚
aration 3.42 A, vertical displacement between ring cent-
˚
roids 1.14 A) [10]. Thus, the complex is packed into an

B. Wu et al. / Inorganic Chemistry Communications 10 (2007) 563–566
565
˚
two ureidopyridyl ligands and two Cl atoms with a tetrahe-
dral geometry. In a complex molecule of 2, the orientation
of the two ligands is in a ‘‘syn’’ fashion, and the Cl–Zn–Cl
angle is considerably widened (125.63(6)°), as shown in
Fig. 2a.
#1: N(6)–H(6A)ꢀ ꢀ ꢀO(2), 2.795(4) A, 154.3°; N(5)–H(5A)ꢀ ꢀ ꢀ
˚
O(2), 2.885(4) A, 149.0°; motif #2: N(2)–H(2A)ꢀ ꢀ ꢀO(1),
˚
˚
2.962(3) A, 140.2°; N(3)–H(3A)ꢀ ꢀ ꢀO(1), 2.748(3) A,
148.5°). Along the c axis, the two types of urea tapes repeat
alternatively, forming a hydrogen-bonded two-dimensional
network in ac plane (Fig. 2b), and these sheets are arranged
parallel to each other in the b direction. Interestingly, the
chloride ions act as coordination ligands rather than
hydrogen bond acceptors in this complex: they do not form
efficient intermolecular hydrogen bonding interactions with
The two ligand arms of each [ZnL2Cl2] unit participate
in intermolecular contacts with adjacent molecules. The
urea NH groups of one ligand form two N–Hꢀ ꢀ ꢀO hydro-
gen bonds with one carbonyl group of another molecule,
resulting in an R12(6) ring, while the other ligand of the same
ZnL2Cl2 unit donates two N–Hꢀ ꢀ ꢀO bonds to a third mol-
ecule, also with an R12(6) motif. Each of the hydrogen-
bonded rings is further expanded along the a axis to gener-
ate the urea tape synthon (i.e., the C(4)R12(6) motif [8]),
which is frequently seen in diarylureas [11]. This is in con-
trast to the absence of N–Hꢀ ꢀ ꢀO bonds in complex 1. It is
noteworthy that the two C(4)R12(6) tapes are different from
the viewpoints of orientation and geometry. First, one of
them is constructed by ligands which are roughly anti-par-
allel to each other (Fig. 2b, motif #1), while the other
includes staggered ligand molecules (Fig. 2b, motif #2).
Second, the hydrogen bond parameters are different (motif
˚
the NH units (the shortest N–Hꢀ ꢀ ꢀCl contact is 3.406 A,
but the \N–Hꢀ ꢀ ꢀCl angle is too acute, at 68.1°). This differs
significantly from the copper(II) complex 1 and a ZnL02Cl2
compound [4d], both of which feature two N–Hꢀ ꢀ ꢀCl
bonds of either R12(6) or R22(8) motif.
IR spectra of the two complexes are consistent with
hydrogen bond formation. It is known that upon associa-
tion by hydrogen bonding the urea NH stretch frequencies
would decrease (e.g. from above 3400 to around 3200 cmꢁ1
[12]). In the present work, the NH-stretch of 1 (3378 cmꢁ1
)
is considerably higher than that of 2 (3287 cmꢁ1), which
may be attributed to the different hydrogen bond types in
the two complexes. As described above, complex 1 has a
combination of N–Hꢀ ꢀ ꢀCl, N–Hꢀ ꢀ ꢀp and N–Hꢀ ꢀ ꢀO interac-
tions, while in 2 all the NH groups are involved in the
stronger N–Hꢀ ꢀ ꢀO bonds and thus displays less NH
stretch. Furthermore, the NH stretch of complex 2 is
almost identical to the hydrated ligand L Æ H2O
(3285 cmꢁ1), whose solid-state structure also features N–
Hꢀ ꢀ ꢀO hydrogen bonds between the NH groups and the
enclathrated water [5].
In conclusion, we report the supramolecular structures
of two metal complexes with a urea-based pyridyl ligand
and chloride ion. Both complexes contain R12(6) hydrogen
bond motifs that are further extended to an infinite tape
(1) or a 2D sheet structure (2). However, the metal-bound
Cl atoms in 1 are involved in intermolecular N–Hꢀ ꢀ ꢀCl
bonds, while in 2 they do not participate in any N–Hꢀ ꢀ ꢀCl
interactions.
Acknowledgement
The authors thank the ‘‘Bairen Jihua’’ program of Chi-
nese Academy of Sciences for funding.
Appendix A. Supplementary material
CCDC 634820 and 634821 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.inoche.2007.01.026.
Fig. 2. (a) ORTEP drawing of a [ZnL2Cl2] (2) molecule, showing the syn-
arranged ligand arms (thermal ellipsoid set at 30% probability level); (b)
The hydrogen-bonded 2D network in the ac plane with two different types
of N–Hꢀ ꢀ ꢀO R12(6) motifs. Non-urea hydrogens and some ligands are
˚
omitted for clarity. Selected bond lengths (A): Zn(1)–N(1) 2.068(3), Zn(1)–
N(4) 2.070(2), Zn(1)–Cl(1) 2.212(1), Zn(1)–Cl(2) 2.204(1).

566
B. Wu et al. / Inorganic Chemistry Communications 10 (2007) 563–566
(618.89): C, 54.34; H, 4.89; N, 13.58%; Found: C, 54.01; H, 4.46; N,
References
13.39%. 1H NMR (DMSO-d6, 400 MHz, ppm): d = 9.00 (s, 1H, NH),
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cmꢁ1): 3287 (N–H), 3036, 1640 (C@O), 1561, 1480, 1424, 1228, 770,
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solution (2 mL) of CuCl2 Æ 2H2O (17.7 mg, 0.10 mmol) under stirring.
The resulting greenish solution was allowed to evaporate at room
temperature for about 2 weeks to yield dark green crystals (60%).
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61.17; H, 5.50; N, 15.29%; Found: C, 60.99; H, 5.55; N, 15.20%. IR
(KBr, m/cmꢁ1): 3378 (N–H), 3064, 1703 (C@O), 1590, 1514, 1485, 805,
786. [ZnL2Cl2] (2): L (50.2 mg, 0.21 mmol) and ZnCl2 (13.6 mg,
0.10 mmol) were stirred in acetone/ethanol (1:1 v/v, 6 mL) for 3 h at
room temperature. The mixture was filtered, and the filtrate was
allowed to evaporate for several days to give colorless block crystals.
Yield: 58%. M.p.: 236–237 °C. Anal. Calc. for C28H30Cl2N6O2Zn
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