New zinc phosphates decorated by imidazole-containing ligands

Article abstract of DOI:10.1021/ic0487528

Four new zinc phosphates [Zn(HPO₄)(C₆H ₉N₃O₂)] (1), [Zn(HPO₄)(C ₄H₆N₂)]·H₂O (2), [Zn ₂(HPO₄)₂(C₁₄H₁₄N ₄)]·2H₂O (3), and [Zn(HPO₄)(C ₁₄H₁₄N₄)] (4) were synthesized in the presence of D-histidine, 1-methylimidazole, 1,4-bis-(imidazol-1-ylmethyl)benzene (L1), and 1,2-bis(imidazol-1-ylmethyl)benzene (L2), respectively, and their structures were determined by X-ray crystallography. The inorganic framework of compounds 1, 2, and 3 is composed of vertex-shared ZnO₃N and HPO₄ tetrahedra that form four rings, which, in turn, are linked to generate a one-dimensional ladder structure. In 1 and 2 the organic groups (monoimidazole ligand) are located at each side of the ladders, while in 3 the bisimidazole ligand, 1,4-bis(imidazol-1-ylmethyl)benzene, links the ladders together to form a novel 2D structure. Compound 1 is the first zinc phosphate framework to be templated by an N-bonded chiral amino acid. In 4 the zero-dimensional four rings are joined together by the linear bridging ligand, 1,2-bis(imidazol-1-ylmethyl) benzene, to generate a one-dimensional framework with a new face-to-face structural motif. The 3D structure of compound 4 is stabilized by hydrogen-bonding, π-π interactions, and C-H...π interactions. The approach of incorporating multifunctional ligands into zinc phosphate frameworks and linking the inorganic zinc phosphates subunits by an organic ligand provides opportunities for the design of new inorganic-organic open frameworks.

Full text of DOI:10.1021/ic0487528

Inorg. Chem. 2005, 44, 552−558
New Zinc Phosphates Decorated by Imidazole-Containing Ligands
Jian Fan,† Carla Slebodnick,† Ross Angel,‡ and Brian E. Hanson*,†
Departments of Chemistry and Geosciences, Virginia Polytechnic Institute and State UniVersity,
Blacksburg, Virginia 24061
Received September 6, 2004
Four new zinc phosphates [Zn(HPO4)(C6H9N3O2)] (1), [Zn(HPO4)(C4H6N2)]
(3), and [Zn(HPO4)(C14H14N4)] (4) were synthesized in the presence of
‚
D
H2O (2), [Zn2(HPO4)2(C14H14N4)]
-histidine, 1-methylimidazole, 1,4-bis-
‚
2H2O
(imidazol-1-ylmethyl)benzene (L1), and 1,2-bis(imidazol-1-ylmethyl)benzene (L2), respectively, and their structures
were determined by X-ray crystallography. The inorganic framework of compounds 1, 2, and 3 is composed of
vertex-shared ZnO3N and HPO4 tetrahedra that form four rings, which, in turn, are linked to generate a one-
dimensional ladder structure. In 1 and 2 the organic groups (monoimidazole ligand) are located at each side of the
ladders, while in 3 the bisimidazole ligand, 1,4-bis(imidazol-1-ylmethyl)benzene, links the ladders together to form
a novel 2D structure. Compound 1 is the first zinc phosphate framework to be templated by an N-bonded chiral
amino acid. In 4 the zero-dimensional four rings are joined together by the linear bridging ligand, 1,2-bis(imidazol-
1-ylmethyl)benzene, to generate a one-dimensional framework with a new face-to-face structural motif. The 3D
structure of compound 4 is stabilized by hydrogen-bonding, π−π interactions, and C−H‚‚‚π interactions. The
approach of incorporating multifunctional ligands into zinc phosphate frameworks and linking the inorganic zinc
phosphates subunits by an organic ligand provides opportunities for the design of new inorganic
−organic open
frameworks.
Scheme 1. Schematic Drawings of Monomer and Condensed Four
Introduction
Rings Are Showna
Interest in zinc phosphate materials stems from their
potential application in the areas of coatings, membranes,
absorbents, and catalysis.1 As a result, in recent years
templated and ligand-modified zinc phosphates have been
shown to have a rich structural chemistry.1-7 One common
feature in the observed structures is the four ring, which may
be isolated as a monomer, as, for example, in [N(CH3)4Zn-
(H2PO4)3] (Scheme 1),2 [Zn(2,2′-bipy)]2(H2PO4)4,3 and
[C6N2H18][Zn(H2PO4)2(HPO4)].4 Furthermore, the monomers
* To whom correspondence should be addressed. E-mail: hanson@vt.edu.
† Department of Chemistry.
‡ Department of Geosciences.
(1) (a) Gier, T. E.; Stucky, G. D. Nature 1991, 349, 508. (b) Cheetham,
A. K.; Fe´rey, G.; Loiseau, T. Angew. Chem., Int. Ed. 1999, 38, 3268.
(c) Rao, C. N. R.; Natarajan, S.; Choudhury, A.; Neeraj, S.; Ayi, A.
A. Acc. Chem. Res. 2001, 34, 80. (d) Harrison, W. T. A. Curr. Opin.
Solid State Mater. Sci. 2002, 6, 407. (e) Yang, G.-Y.; Sevov, S. C. J.
Am. Chem. Soc. 1999, 121, 8389. (f) Johnstone, J. A.; Harrison, W.
T. A. Inorg. Chem. 2004, 43, 4567.
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2003, 2212.
a The black dots represent zinc atoms.
may condense to form a variety of one- and two-dimensional
structures and in some cases three-dimensional frameworks.2b,4,5
Some of the observed condensation pathways are indicated
in Scheme 1. There is remarkable variety in the manner in
(3) Lin, Z.-E.; Zhang, J.; Zheng, S.-T.; Yang, G.-Y. Inorg. Chem. Chem.
2003, 6, 1035.
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122, 2810. (b) Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Solid. State
Chem. 2000, 150, 417.
552 Inorganic Chemistry, Vol. 44, No. 3, 2005
10.1021/ic0487528 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/13/2005

New Zinc Phosphates
2.0 mmol), sodium hydroxide (80.0, 2.0 mmol), D-histidine
hydrochloride (383.2 mg, 2.0 mmol), and water (6 mL) for 10 days
at 125 °C yields a colorless needle crystalline product. Yield: 82%
on the basis of zinc source. FT-IR (cm-1): 3133(w), 3002(w),
2926(w), 1618(m), 1586(m), 1492(m), 1414(m), 1344(m), 1310(m),
1310(m), 1270(m), 1101 (s), 1082(s), 992(s), 971(s), 911(s), 847(s),
770(m), 752(m), 683(s), 664(s). Anal. Calcd for C6H10N3O6PZn:
C, 22.77; H, 3.18; N, 13.28. Found: C, 22.52; H, 3.43; N, 13.09.
[Zn(HPO4)(C4H6N2)]‚H2O (2). Hydrothermal treatment of zinc
nitrate hexahydrate (297.5 mg, 1.0 mmol), phosphoric acid (85 wt
%, 230.6 mg, 2.0 mmol), 1-methylimidazole (492.7 mg, 6.0 mmol),
and water (6 mL) for 10 days at 125 °C yields a colorless
microcrystalline product. This sample was allowed to stand in the
mother liquid for several weeks, and then needle crystals were
collected. Yield: 31% on the basis of zinc source. FT-IR (cm-1):
3377(w), 3125(m), 2929(w), 1544(m), 1530(m), 1290(w), 1254-
(w), 1207(w), 1141(m), 1021(s), 955(m), 911(m), 861(m), 848-
(m), 780(m), 770(m), 675(m). Anal. Calcd for C4H9N2O5PZn: C,
18.37; H, 3.47; N, 10.71. Found: C, 18.16; H, 3. 49; N, 10.58.
The weight loss (26 to ∼159 °C) corresponds to the loss of lattice
water molecules (obsd 7.0%, 1H2O calcd 6.9%). This compound
is stable up to 197 °C as shown in the TGA curve.
[Zn2(HPO4)2(C14H14N4)]‚2H2O (3). Hydrothermal treatment of
zinc acetate dihydrate (65.8 mg, 0.3 mmol), potassium dihydro-
genphosphate (81.6 mg, 0.6 mmol), 1,4-bis(imidazol-1-ylmethyl)-
benzene (71.4, 0.3 mmol), and water (6 mL) for 9 days at 130 °C
yields a needle crystalline product. Yield: 89% on the basis of
zinc source. FT-IR (cm-1): 3357(w), 3146(w), 3130(w), 1526(m),
1440(m), 1238(m), 1132(m), 1088(s), 1021(s), 952(m), 911(s),
844(m), 805(m), 735(s). Anal. Calcd for C14H20N4O10P2Zn2: C,
28.16; H, 3.38; N, 9.38. Found: C, 27.73; H, 3.56; N, 9.22. The
weight loss (23 to ∼163 °C) corresponds to the loss of lattice water
molecules (obsd 5.9%, 2H2O calcd 6.0%). This compound is stable
up to 312 °C as shown in the TGA curve.
which the four rings may be ligated and linked. Yet, although
many of the structures can be readily rationalized as deriving
from the four rings, there are no rational syntheses available
for a given structural motif. Indeed, the same templating or
ligating group can lead to several different structures
depending on temperature, concentration, and pH.6 However,
it is recognized that the construction of extended structures
via molecular building blocks offers great potential for the
design of materials.7 For example, very recently the Natarajan
group combined the principles of supramolecular organic
chemistry with inorganic building units to form new zinc
phosphate solids.5c
The goal of our research is to synthesize zinc phosphates
decorated by ancillary ligands and at the same time try to
understand the role of the ancillary ligands in the formation
of these solids. According to this approach, D-histidine,
1-methylimidazole, 1,4-bis(imidazol-1-ylmethyl)benzene, and
1,2-bis(imidazol-1-ylmethyl)benzene were used as ancillary
ligands. Here, we report the synthesis, structure, and
characterization of four new zinc phosphates: [Zn(HPO4)-
(C6H9N3O2)] (1), [Zn(HPO4)(C4H6N2)]‚H2O (2), [Zn2(HPO4)2-
(C14H14N4)]‚2H2O (3), and [Zn(HPO4)(C14H14N4)]‚H2O (4).
Compounds 1 and 2 have the same framework backbone,
an edge-shared ladder structure. In 3 edge-shared ladder
structures, as observed in 1 and 2, are linked by 1,4-bis-
(imidazol-1-ylmethyl)benzene to form a novel 2D structure.
In 4 the linear ligand 1,2-bis(imidazol-1-ylmethyl)benzene
connects the four-ring building blocks to generate a new
structural motif with the four rings linked face-to-face.
Experimental Section
[Zn(HPO4)(C14H14N4)]‚H2O (4). Hydrothermal treatment of zinc
acetate dihydrate (65.8 mg, 0.3 mmol), potassium dihydrogenphos-
phate (81.6 mg, 0.6 mmol), 1,2-bis(imidazol-1-ylmethyl)benzene
(71.4, 0.3 mmol), and water (6 mL) for 5 days at 125 °C yields a
block crystalline product. Yield: 81% on the basis of zinc source.
FT-IR (cm-1): 3422(w), 3127(m), 1529(m), 1520(m), 1455(m),
1445(m), 1364(m), 1280(w), 1250(w), 1106(s), 1071(s), 1028(m),
979(m), 952(s), 890(m), 844(m), 764(s), 748(s). Anal. Calcd for
C14H17N4O5PZn: C, 40.26; H, 4.10; N, 13.41. Found: C, 39.54;
H, 4.37; N, 13.17. The weight loss (23 to ∼255 °C) corresponds
to the loss of lattice water molecules (obsd 4.4%, 1H2O calcd 4.3%).
This compound is stable up to 311 °C as shown in the TGA curve.
Crystallographic Analyses. Low-temperature (100 K) single-
crystal X-ray diffraction measurements for complexes 1-4 were
collected on a Oxford Diffraction Xcalibur2 diffractometer equipped
with the Enhance X-ray source and a Sapphire 2 CCD detector.
The data collection routine, unit cell refinement, and data processing
were carried out with the program CrysAlis.10 The structure was
solved by direct methods using SHELXS-97 and refined by full-
matrix least-squares.11 The final refinements involved an anisotropic
model for all non-hydrogen atoms. Hydrogen atoms were either
located from the residual e- density map and refined independently
or located by a riding model. The crystal parameters, data collection,
and refinement results for compounds 1-4 are summarized in Table
Materials and Measurements. All commercially available
chemicals are of reagent grade and used as received without further
purification. The ligands 1,4-bis(imidazol-1-ylmethyl)benzene8a,b
and 1,2-bis(imidazol-1-ylmethyl)benzene8c were synthesized by the
reaction of imidazole with R,R′-dibromo-p-xylene and R,R′-
dibromo-o-xylene, respectively, with the same procedures reported
for preparation of 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethyl-
benzene.9 Elemental analyses of C, H, and N were performed by
Galbraith Laboratories, Inc. Thermogravimetric measurements were
performed on a TA Instruments Q500 thermal analyzer in N2 with
the heating rate of 10 °C min-1
.
[Zn(HPO4)(C6H9N3O2)] (1). Hydrothermal treatment of zinc
oxide (81.4 mg, 1.0 mmol), phosphoric acid (85 wt %, 230.6 mg,
(5) (a) Liu, W.; Liu, Y.; Shi, Z.; Pang, W. J. Mater. Chem. 2000, 10,
1451. (b) Ayi, A. A.; Choudhury, A.; Natarajan, S.; Rao, C. N. R. J.
Mater. Chem. 2001, 11, 1181. (c) Natarajan, S.; Wullen, L. V.; Klein,
W.; Jansen, M. Inorg. Chem. 2003, 42, 6265. (d) Choudhury, A.;
Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Mater. Chem. 2001, 11,
1537.
(6) (a) Neeraj, S.; Natarajan, S. Chem. Mater. 2000, 12, 2753. (b)
Choudhury, A.; Natarajan, S.; Rao, C. N. R. Inorg. Chem. 2000, 39,
4295.
(7) (a) Fe´rey, G. J. Solid State Chem. 2000, 152, 37. (b) Murugavel, R.;
Walawalkar, D. M.; Roesky, H. W.; Rao, C. N. R. Acc. Chem. Res.
2004, in press.
(8) (a) Dahl, P. K.; Arnold, F. H. Macromolecules 1992, 25, 7051. (b)
Abrahams, B. F.; Hoskins, B. F.; Robson, R.; Slizys, D. A. Crys-
tEngComm 2002, 4, 478. (c) Tan, H. Y.; Zhang, H. X.; Ou, H. D.;
Kang, B. S. Inorg. Chim. Acta 2004, 357, 869.
(10) CrysAlis v1.171; Oxford Diffraction: Wroclaw, Poland, 2004.
(11) (a) Sheldrick, G. M. SHELXS97, Program for Crystal Structure
Determination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(b) Sheldrick, G. M. SHELXL97, Program for Crystal Structural
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(9) (a) Sun, W. Y.; Fan, J.; Okamura, T.-a.; Xie, J.; Yu, K. B.; Ueyama,
N. Chem. Eur. J. 2001, 7, 2557. (b) Liu, H. K.; Sun, W. Y.; Zhu, H.
L.; Yu, K. B.; Tang, W. X. Inorg. Chim. Acta 1999, 295, 129.
Inorganic Chemistry, Vol. 44, No. 3, 2005 553

Fan et al.
Table 1. Crystal Data and Structure Refinement Parameters for 1-4
structure
parameter
1
2
3
4
empirical
formula
fw
C6H10N3-
O6PZn
316.51
100
C4H9N2-
O5PZn
261.47
100
C
14H20N4-
C
14H17N4-
O
10P2Zn2
O5PZn
597.02
100
417.66
100
T/K
space group
a/Å
b/Å
P-1
P21/n
P-1
P-1
5.191(2)
9.535(4)
10.775(3)
86.18(3)
84.58(3)
75.79(3)
514.1(3)
2
2.044
2.567
0.0529
0.1029
11.189(1)
5.226(1)
14.579(1)
9.419(1)
9.520(1)
13.247(1)
76.692(9)
78.841(10)
66.443(10)
1052.71(19)
2
1.883
2.492
0.0392
0.0759
8.536(3)
9.914(3)
10.631(4)
81.33(3)
74.62(3)
69.10(3)
808.7(5)
2
1.715
1.653
0.0340
0.1032
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
95.930(7)
847.9(1)
4
2.048
3.077
0.0238
0.0269
D
calc/g cm-3
µ/mm-1
R1a [I > 2σ(I)]
wR2b [I > 2σ(I)]
Figure 1. Ladder structure of compound 1 showing the connectivity of
the PO4 (yellow) and ZnO3N (blue) tetrahedra to four-ring loops.
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc )2]/∑[w(Fo )2]}1/2
,
where w ) 1/[σ2(Fo)2 + (aP)2 + bP], P ) [(Fo)2 + 2(Fc)2]/3.
complete its tetrahedral coordination environment. A similar
arrangement, plus an extra framework water molecule, is seen
in compound 2 (see Figure S2 in the Supporting Information.
Structural representations for 2 are given in the Supporting
Information due to its close structural similarity to the
histidine structure 1.) This kind of ZnO3N binding set is
characteristic of several nitrogen-donor-ligand-modified zinc
phosphates.5b,6b,12 The tetrahedral coordination modes in
compounds 1 and 2 have typical geometrical parameters.
Assuming the usual valences for Zn, P, and O as +2, +5,
and -2, respectively, and a neutral zwitterionic histidine in
1, then the asymmetric unit has a net charge of -1.
Accordingly, for charge balance a proton must reside on
either the phosphate or carboxylate group. Because the pKa
of HPO42- (12.3) is larger than that of the carboxylate group
of histidine (1.8), it is the phosphate group that is protonated,
making it monohydrogen phosphate. The identical situation
occurs in 2; the additional proton can only be placed on a
phosphate oxygen atom. To confirm these assignments the
hydrogen was located in the residual e- density map and
refined independently. Thus, each phosphorus atom makes
three connections to neighboring zinc atoms through the
bridging oxygen, and the terminal oxygen atom is formulated
as a -OH group not an unsaturated dO atom. The longest
P-O bond length in 1, 1.574(5) Å, corresponds to the
terminal oxygen, consistent with this assignment. The ZnO3N
and HPO4 groups are connected to form an edge-shared
ladder structure composed of four rings that propagates along
the [100] direction (Figure 1). The framework of compound
2 also consists of ladders composed of four rings onto which
the 1-methylimidazole molecules are grafted (Figure S3,
Supporting Information).
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1-4a
1
Zn1-O4#1
Zn1-O1#2
1.926(4)
1.915(4)
Zn1-N1
Zn1-O2
1.992(5)
1.966(4)
O1#2-Zn1-O4#1
O4#1-Zn1-O2
O4#1-Zn1-N1
116.16(18)
111.01(19)
103.2(2)
O1#2-Zn1-O2
O1#2-Zn1-N1
O2-Zn1-N1
111.98(19)
108.5(2)
105.0(2)
2
3
Zn1-O2
1.929(1)
1.953(1)
Zn1-O4#3
Zn1-N2
1.936(1)
1.998(2)
Zn1-O1#4
O2-Zn1-O4#3
O4#3-Zn1-O1#4
O4#3-Zn1-N2
115.34(5)
107.46(5)
112.32(6)
O2-Zn1-O1#4
O2-Zn1-N2
O1#4-Zn1-N2
110.73(5)
104.08(6)
106.61(6)
Zn1-O7
Zn1-O4
Zn2-O3
Zn2-O8
1.925(2)
1.959(2)
1.915(2)
1.950(2)
Zn1-O2#5
Zn1-N42#6
Zn2-O5#7
Zn2-N12
1.921(2)
2.005(3)
1.944(2)
1.993(3)
O2#5-Zn1-O7
O7-Zn1-O4
109.74(9)
110.60(9)
120.47(9)
103.75(9)
105.82(9)
112.08(9)
O2#5-Zn1-O4
O2#5-Zn1-N42#6
O4-Zn1-N42#6
O3-Zn2-O8
O3-Zn2-N12
O8-Zn2-N12
109.98(9)
105.31(9)
100.12(10)
122.63(9)
108.71(10)
103.93(10)
O7-Zn1-N42#6
O3-Zn2-O5#7
O5#7-Zn2-O8
O5#7-Zn2-N12
4
Zn1-O4
1.926(2)
1.936(2)
Zn1-N12
1.983(2)
1.998(2)
Zn1-O2#8
Zn1-N22#9
O4-Zn1-O2#8
O2#8-Zn1-N12
O2#8-Zn1-N22#9
108.89(7)
105.84(7)
108.05(7)
O4-Zn1-N12
O4-Zn1-N22#9
N12-Zn1-N22#9
117.54(7)
98.72(8)
117.37(8)
a Symmetry transformations used to generate equivalent atoms: #1, -x,
1 - y, -z; #2, 1 - x, 1 - y, -z; #3, 1 - x, 1 - y, -z; #4, 1 - x, -y, -z;
#5, -x + 1, -y + 1, -z + 1; #6, x, y - 1, z - 1; #7, -x, -y + 2, -z +
1; #8, 2 - x, 1 - y, z; #9, x, -1 + y, 1 + z.
1. Selected bond length and angles are listed in Table 2. Further
details are provided in the Supporting Information.
The histidine molecules in 1 and the methylimidazole
ligands in 2 are attached to zinc atoms, extending to each
side of the ladder structure. It is noteworthy that in 1 the
histidine molecules on one side have the R absolute config-
Results and Discussion
Description of the Crystal Structures. [Zn(HPO4)-
(C6H9N3O2)] (1) and [Zn(HPO4)(C4H6N2)]‚H2O (2). The
asymmetric unit of compound 1 contains one zinc atom, one
phosphate, and one histidine as shown in Figure S1 (Sup-
porting Information). Each zinc atom in compound 1 is
coordinated by three oxygen atoms from three different
phosphate groups and one nitrogen atom from histidine to
(12) (a) Harrison, W. T. A.; Phillips, M. L. F.; Stanchfield, J.; Nenoff, T.
M. Inorg. Chem. 2001, 40, 895. (b) Drumel, S.; Janvier, P.; Deniaud,
D.; Bujoli, B. J. Chem. Soc., Chem. Commun. 1995, 1051. (c) Cui,
A.; Yao, Y. Chem. Lett. 2001, 1148. (d) Gordon, L. E.; Harrison, W.
T. A. Inorg. Chem. 2004, 43, 1808. (e) Neeraj, S.; Natarajan, S.; Rao,
C. N. R. New J. Chem. 1999, 303.
554 Inorganic Chemistry, Vol. 44, No. 3, 2005

New Zinc Phosphates
Table 3. Distances (Å) and Angles (deg) of Hydrogen Bonding for
1-4a
distance
(D‚‚‚A)
angle
(D-H-A)
D-H‚‚‚Ab
D-H-A
1
N3-H6‚‚‚O10#1
N3-H7‚‚‚O10#2
N3-H8‚‚‚O2#2
N2-H9‚‚‚O3#3
O3-H10‚‚‚O11#1
C2-H2‚‚‚O11#4
C4-H4‚‚‚O4#5
C5-H5‚‚‚O2
2.901(7)
2.907(7)
2.859(7)
2.821(7)
2.528(6)
3.325(9)
3.275(8)
3.276(8)
N3-H6-O10#1
N3-H7-O10#2
N3-H8-O2#2
N2-H9-O3#3
O3-H10-O11#1
C2-H2-O11#4
C4-H4-O4#5
C5-H5-O2
153
172
150
153
172
133
132
169
2
3
Figure 2. Crystal packing diagram for compound 1 in the bc plane. Dashed
lines represent the various possible hydrogen-bond interactions. The eclipsed
nature of the four rings is evident in this view, showing that the sheets are
stacked in an AAA fashion.
O5-H5A‚‚‚O1#6
O5-H5B‚‚‚O3#7
O5-H5B‚‚‚O4#7
O3-H6‚‚‚O5#8
C1-H1‚‚‚O5#9
C4-H4B‚‚‚O4#10
2.810(2)
3.164(2)
2.836(2)
2.614(2)
3.372(2)
3.360(2)
O5-H5A-O1#6
O5-H5B-O3#7
O5-H5B-O4#7
O3-H6-O5#8
C1-H1-O5#9
C4-H4B-O4#10
172
135
158
171
159
141
uration and are related by an inversion center, as required
by the P-1 space group, to the groups of S configuration on
the other side of the ladder. As a result, this compound is
achiral, though the starting histidine is homochiral (See
Experimental Section). It is well known that histidine is
racemized under both alkaline and acidic conditions at high
temperature.13 Thus far, there are some reports of zinc
phosphite or zinc phosphate templated by chiral organic
groups,12d,14 but to the best of our knowledge, compound 1
is the first zinc phosphate phase to be templated by a chiral
amino acid, although the resulting compound is obtained with
racemic histidine.
Hydrogen bonding is an important factor in defining the
molecular packing in zinc phosphates.12d For example, Yu
reported recently that racemic [Co(en)3]3+ imparts chirality
into the zinc phosphate framework via hydrogen bonding.15
As shown in Figure 2, the neutral [100] ladders in compound
1 are connected by means of hydrogen bonds, N2-H9‚‚‚
O3(1 + x, -1 + y, z), to form the pseudo-sheets along the
[010] direction (Table 3). The sheets are further linked by
hydrogen-bond interactions to form the 3D structure. The
sheets configure themselves in an AAA arrangement along
the [001] direction, and because the inversion center is
contained within the core ladder motif, there are no chiral
components to the structure. The packing diagram for 2 also
shows the formation of sheets connected by hydrogen bonds
in an AAA arrangement; however, the potential for hydrogen
bonding in 2 is limited compared to the histidine structure,
1 (Figure S4, Supporting Information).
O6-H1‚‚‚O2W
2.590(4)
O6-H1-O2W
166
174
162
173
159
165
146
134
O1W-H1A‚‚‚O4#11 2.965(4)
O1W-H1A-O4#11
O1W-H1B-O8#12
O1-H2-O1W
O1W-H1B‚‚‚O8#12 2.781(3)
O1-H2‚‚‚O1W
2.602(4)
O2W-H4A‚‚‚O7#13 2.833(4)
O2W-H4B‚‚‚O4#14 2.792(3)
C12-H12A‚‚‚O1#14 3.308(3)
C41-H41B‚‚‚O3#15 3.282(4)
O2W-H4A-O7#13
O2W-H4B-O4#14
C12-H12A-O1#14
C41-H41B-O3#15
4
O5-HW1‚‚‚O1#16
O5-HW2‚‚‚O2
O3-H20‚‚‚O1#16
C3-H3‚‚‚O5#17
C12-H12‚‚‚O2
C14-H14‚‚‚O1#18
2.892(3)
2.818(3)
2.558(3)
3.318(3)
3.327(3)
3.289(3)
O5-HW1-O1#16
O5-HW2-O2
156
166
178
139
147
171
160
O3-H20-O1#16
C3-H3-O5#17
C12-H12-O2
C14-H14-O1#18
C21-H21B-O3#19
C21-H21B‚‚‚O3#19 3.249(3)
a Symmetry transformation used to generate equivalent atoms: #1, -x,
1 - y, 1 - z; #2, -1 + x, y, z; #3, 1 + x, -1 + y, z; #4, -x, -y, 1 - z;
#5, -x, 1 - y, -z; #6, 1/2 + x, 1/2 - y, 1/2 + z; #7, 1 - x, -y, -z; #8,
1 - x, 1 - y, -z; #9, 3/2 - x, -1/2 + y, 1/2 - z; #10, 1/2 - x, -1/2 +
y, 1/2 - z; #11, -x, 1 - y, 1 - z; #12, x, -1 + y, z; #13, 1 - x, 2 - y,
1 - z; #14, x, 1 + y, z; #15, -x, 2 - y, 2 - z; #16, 3 - x, 1 - y, -z; #17,
2 - x, 2 - y, -1 - z; #18, -1 + x, 1 + y, z; #19, 2 - x, 2 - y, -z. b D
) donor; A ) acceptor.
zene, L1, and 1,2-bis(imidazol-1-ylmethyl)benzene, L2,
leading to the formation of compounds 3 and 4, respectively.
In 3 the observed structure forms sheets in the manner
expected. However, in 4 a new linear structure was obtained
in which the four-ring units are linked in a face-to-face
fashion by L2.
[Zn2(HPO4)2(C14H14N4)]‚2H2O (3) and [Zn(HPO4)-
(C14H14N4)]‚H2O (4). The asymmetric unit of compound 3
contains two zinc atoms, two monohydrogen phosphate units,
and one L1, which repeat to form the framework, and two
extra framework water molecules (Figure S5, Supporting
Information). The two independent zinc atoms are both
tetrahedrally coordinated with the same O3N binding set. For
Zn1, (Zn-O)av ) 1.94 Å, O-Zn-O bond angles are in the
range of 109.7-110.6° with an average value of 110.1°, and
O-Zn-N bond angles are in the range 100.1-120.5° with
an average of 108.6°. For Zn2, (Zn-O)av ) 1.93 Å,
O-Zn-O bond angles are in the range of 103.8-122.6° with
an average value of 110.7°, and O-Zn-N bond angles are
in the range 103.9-112.1° with an average of 108.2°. Within
each hydrogen phosphate unit three oxygen atoms bond to
The ladder structures observed in 1 and 2 are stabilized
by the imidazole-containing pendant ligands that bind zinc.
To enhance our understanding of the various pathways for
the formation of new solids, we wished to keep the basic
four-ring unit intact while modifying the ligand to influence
the assembly process. Thus, we tried to knit the ladder motif
together into sheets by connecting them with bifunctional
ligands, specifically bisimidazole ligands. We performed this
reaction with the ligands 1,4-bis(imidazol-1-ylmethyl)ben-
(13) Greenstein, J. P.; Winitz, M. Chemistry of the Amino Acids; John Wiley
& Sons: New York, 1961; Vol. 3, Chapter 27.
(14) Nenoff, T. M.; Thoma, S. G.; Provencio, P.; Maxwell, R. S. Chem.
Mater. 1998, 10, 3077.
(15) Wang, Y.; Yu, J.; Shi, Z.; Xu, R. Chem. Eur. J. 2003, 9, 5048.
Inorganic Chemistry, Vol. 44, No. 3, 2005 555

Fan et al.
between layers by forming two O-H‚‚‚O(water) and four
O(water)-H‚‚‚O hydrogen bonds.
Compounds 3 and 4 are synthesized under similar condi-
tions; however, the structures are quite different due to the
subtle difference between the ligands, L1 and L2. The
asymmetric unit of compound 4 contains one zinc, one
hydrogen phosphate, one bisimidazole benzene ligand, L2,
and one extra framework water molecule (Figure S8,
Supporting Information). Two nitrogen atoms from the two
L2 units [(Zn-N)av ) 1.99 Å] and two oxygen atoms from
two phosphate groups [(Zn-O)av ) 1.93 Å] tetrahedrally
coordinate each zinc atom in 4. The average bond angles
for O-Zn-O, N-Zn-N, and O-Zn-N are 108.9°, 117.4°,
and 107.5°, respectively. This kind of ZnO2N2 ligand set for
zinc phosphates is rare.12c,16 Two hydrogen phosphate oxygen
atoms bond to zinc to form bridges, and two oxygen atoms,
a -OH group and an unsaturated dO atom, are terminally
bonded to phosphorus. The P-O distances are in the range
of 1.51-1.59 Å, which is similar to that in compounds 1-3.
The longest P-O distance of P1-O3 ) 1.587(2) Å suggests
that O3 is protonated. The presence of a hydroxyl group is
confirmed by the observation of a peak in the difference
Fourier map corresponding to the hydrogen atom on the O3
atom. Unlike the structures in 1-3, the four rings in 4 are
not fused to form a one-dimensional ladder structure. Instead,
the four rings are bridged in one dimension by L2 ligands
(see below).
Figure 3. 2D structure of compound 3 showing the connectivity of PO4
(yellow) and ZnO3N (blue) tetrahedra to four-ring loops.
Figure 4. Polyhedral representation of the one-dimensional structure of
compound 4 showing the connectivity of the PO4 (yellow) and ZnO2N2
(blue) tetrahedra to four-ring loops. The face-to-face π-π interaction is
indicated by the red line.
three zinc atoms and one -OH group is terminally bonded
to phosphorus. The presence of a hydroxyl group is
confirmed by the observation of a peak in the difference
Fourier map corresponding to the hydrogen atom on the O1
and O6 atoms. The P-O distances are in the range of 1.51-
1.58 Å for both hydrogen phosphate units.
In 4 the imidazole groups are essentially perpendicular to
the benzene ring with dihedral angles of 85.9° and 90.5° for
the two rings. In general, the pendant imidazol-1-ylmethyl
groups of the L2 ligand adopt either a trans-conformation
or a cis-conformation relative to the benzene ring.8c It is
interesting that in 4 both C(methylene group)-N(imidazole
group) bonds are eclipsed, i.e., they extend nearly parallel
to the benzene ring with torsion angles C2-C1-C11-N11
) 160.82(18)° and C1-C2-C21-N21 ) -160.16(18)°.
Thus, the L2 group can be regarded as a linear bridging
ligand. This is a unique geometry for the L2 ligand. As
illustrated in Figure 4 the strictly alternating ZnO2N2 and
HPO4 tetrahedral units give rise to the formation of four rings
which are linked by the linear ligand to generate a new one-
dimensional structure with the face-to-face structural motif.
Thus, the Zn2P2 planes are parallel to each other, as required
by symmetry, and separated by two L2 ligands. As mentioned
above, within most zinc phosphate structures the four rings
are connected by Zn-O-P linkages to generate higher
dimensional frameworks. Recently, many zinc organophos-
phates have been synthesized in which the inorganic scaffold
is connected by an organic group attached to the phosphorus
atom.17 However, so far there are only a few examples of
In compound 3 the ZnO3N and HPO4 groups are connected
to form an edge-shared ladder structure composed of four
rings, as observed in 1 and 2 (Figure 3). Because there are
two independent zinc atoms and two hydrogen phosphate
groups within the one asymmetric unit as mentioned above,
the four rings within one ladder structure are not identical
and repeat in the ABCDABCD sequence as shown in Figure
S6 (Supporting Information). In 3 the dihedral angles
between the imidazole groups and the benzene ring are 77.6°
and 77.2°, respectively, and the imidazole groups extend to
each side of the benzene ring, in the opposite direction. Thus,
the L1 ligand in 3 adopts the trans-conformation in a Z-type
mode.8b
In compounds 1 and 2 the ladder structures are linked by
hydrogen bonds to generate the pseudo-sheet. These hydro-
gen bonds are derived from the interaction between the
ancillary organic ligand within one ladder and the hydrogen
phosphate group within the adjacent ladder (Figure 2 and
Figure S4 (Supporting Information)). In 3 the inorganic
ladders within a layer are parallel and linked together by L1
to form the 2D structure as desired. Two L1 units and four
four rings form a macrocycle of 34 atoms. The layers of 3
are connected by hydrogen bonding to form the 3D structure
in an AAA arrangement (Figure S7, Supporting Information).
The solvated water molecules are located in the 3D structure
(16) Ayi, A. A.; Neeraj, S.; Choudhury, A.; Natarajan, A.; Rao, C. N. R.
J. Phys. Chem. Solids. 2001, 62, 1481.
(17) (a) Mao, J. G.; Wang, Z.; Clearfield, A. New J. Chem. 2002, 26, 1010.
(b) Mao, J. G.; Clearfield, A. Inorg. Chem. 2002, 41, 2319. (c) Mao,
J. G.; Wang, Z.; Clearfield, A. Inorg. Chem. 2002, 41, 2334. (d) Lei,
C.; Mao, J. G.; Sun, Y. Q.; Zeng, H. Y.; Clearfield, A. Inorg. Chem.
2003, 42, 6157. (e) Zhang, B.; Poojary, D. M.; Clearfield, A. Inorg.
Chem. 1998, 37, 1844. (f) Poojary, D. M.; Zhang, B.; Bellinghausen,
P.; Clearfield, A. Inorg. Chem. 1996, 35, 5254.
556 Inorganic Chemistry, Vol. 44, No. 3, 2005

New Zinc Phosphates
Figure 5. Pseudo-2D network of compound 4 with the C-H‚‚‚π interactions indicated by dashed lines.
Scheme 2. Schematic Drawing of Compounds 1-4a
the hydrogen atom (H13) bound to C13 (imidazole group)
and the benzene ring of a neighboring chain. The C13-
H13-X (X refers to the centroid of the benzene ring) angle
is 155.7°, and the H‚‚‚X distance is 2.98 Å. The solvated
water molecules are located in the 3D structure by forming
one C-H‚‚‚O (water) and two O(water)-H‚‚‚O hydrogen
bonds with the neutral backbone.
Conclusion
Four new zinc phosphates have been synthesized in the
presence of D-histidine, 1-methylimidazole, 1,4-bis(imidazol-
1-ylmethyl)benzene, and 1,2-bis(imidazol-1-ylmethyl)ben-
zene. Each structure is constructed from four-ring secondary
building units that consist of two tetrahedral zinc atoms and
two tetrahedral phosphate groups. In compounds 1 and 2
the monoimidazole ligands simply decorate the ladder
structures that are formed from edge-fused four rings;
however, in 3 the bisimidazole ligand L1 links the ladder
structures together to form layers. In 4 the bisimidazole
ligand L2 not only completes the zinc coordination sphere
but also separates adjacent inorganic four rings to form chains
(Scheme 2). The 3D structures of 1-3 are stabilized by
hydrogen bonding, while in 4 three kinds of weak interac-
tions, hydrogen bonding, π-π interactions, and C-H‚‚‚π
interactions, are involved and play a subtle role in the
formation of its 3D structure. The ability to design and
control these interactions will enable supramolecular crystal
engineering of metal phosphate materials. We are currently
investigating additional bifunctional ligands in the construc-
tion of metal phosphate materials.
a The zinc phosphate four rings are depicted as parallelograms, and the
bridging bisimidazole ligands are represented as bars.
zinc phosphate or zinc phosphite fragments directly con-
nected by ligands bound to zinc.18
The two four rings that are connected by two L2 units in
4 form a macrocycle of 30 atoms. A centroid-centroid
distance of 4.11 Å between two adjacent benzene rings within
one macrocycle indicates the presence of face-to-face π-π
interactions.19 Another striking feature of compound 4 is that
C-H‚‚‚π interactions20 associate the one-dimensional chains
into a pseudo-2D structure as illustrated in Figure 5, which,
in turn, is connected to form a 3D structure in an AAA
arrangement by hydrogen-bonding interactions (Figure S9,
Supporting Information). Such an interaction occurs between
(18) (a) Halasyamani, P. S.; Drewitt, M. J.; O’Hare, D. Chem. Commun.
1997, 867. (b) Lin, Z.; Zhang, J.; Zheng, S. T.; Yang, G. Y.
Microporous Mesoporous Mater. 2004, 68, 65. (c) Rodgers, J. A.;
Harrison, W. T. A. Chem. Commun. 2000, 2385.
(19) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
(20) (a) Zhu, H. F.; Kong, L. Y.; Okamura, T.-a.; Fan, J.; Sun, W. Y.;
Ueyama, N. Eur. J. Inorg. Chem. 2004, 1465. (b) Caradoc-Davies, P.
L.; Gregory, D. H.; Hanton, L. R.; Turnbull, J. M. J. Chem. Soc.,
Dalton Trans. 2002, 1574. (c) Jitsukawa, K.; Iwai, K.; Masuda, H.;
Ogoshi, H.; Einaga, H. J. Chem. Soc., Dalton Trans. 1997, 3691.
Acknowledgment. We thank R. J. Reynolds for support
of this work through a McNair Postdoctoral Fellowship to
J.F. We thank the NSF (grant CHE-0131128) for funding
the purchase of the Oxford Diffraction Xcalibur2 single-
crystal diffractometer. We thank Michael Vadala and Prof.
J. Riffle for the TGA measurements.
Inorganic Chemistry, Vol. 44, No. 3, 2005 557

Fan et al.
Supporting Information Available: Figure S1, showing the
asymmetric unit of 1, Figure S2, showing the asymmetric unit of
1, Figure S3, showing the ladder structure of 2, Figure S4, showing
the packing diagram of 2, Figure S5, showing the asymmetric unit
of 3, Figure S6, zigzag arrangement of four rings in 3, Figure S7,
showing the packing diagram of 3, Figure S8, showing the ladder
structure of 4, Figure S9, showing the packing diagram of 4, and
X-ray crystallographic files in CIF format. This material is available
free of charge via Internet at http://pubs.acs.org.
IC0487528
558 Inorganic Chemistry, Vol. 44, No. 3, 2005