Metal-organodiphosphonate chemistry: Hydrothermal syntheses and structures of Zn(II) and Cd(II) coordination polymers with xylyldiphosphonate ligands

Article abstract of DOI:10.1016/j.ica.2013.12.003

Hydrothermal reactions of zinc(II) acetate dihydrate or cadmium(II) nitrate tetrahydrate with xylyldiphosphonic acid isomers in the presence of an organonitrogen chelating ancillary ligand provided a series of materials of the (M(II)-organonitrogen chelate)/xylyldiphosphonate family. One-dimensional structures were observed for [Zn(H₂O)(bpy)(1,2-HO₃PC ₈H₈PO₃H)] (1), [Zn(bpy)(1,3-HO ₃PC₈H₈PO₃H)]·H₂O (2·H₂O), [M(o-phen)(1,3-HO₃PC₈H ₈PO₃H)] (M = Zn (5·H₂O), M = Cd (11), and [Zn(tpyprz)(1,2-HO₃PC₈H₈PO₃H)] (7), while the phases [M(bpy)₂(1,4-H₂O₃PC ₈H₈PO₃H₂)(1,4-HO₃PC ₈H₈PO₃H)] (M = Zn (3) and Cd(9)) exhibited virtual layered structures constructed from {M(_bpy)2(1,4-H ₂O₃PC₈H₈PO₃H ₂)}n2n+ chain hydrogen-bonded to the {HO₃PC ₈H₈PO₃H}^2- charge compensating anions. The two-dimensional structures are represented by [M(o-phen)(1,2-HO ₃PC₈H₈PO₃H)] (M = Zn (4) and Cd (10)) and [Cd(bpy)(1,3-HO₃PC₈H₈PO ₃H)] (8). Expansion into three-dimensions occurs only with the spatially extended 1,4-xylyldiphosphonate ligand in [Zn₄(o-phen) ₄(1,4-HO₃PC₈H₈PO₃) ₂(1,4-HO₃PC₈H₈PO₃H)] ·3H₂O (6·3H₂O) and [Cd(o-phen)(1,4-HO ₃PC₈H₈PO₃H)] (12).

Full text of DOI:10.1016/j.ica.2013.12.003

Inorganica Chimica Acta 411 (2014) 172–187
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Metal-organodiphosphonate chemistry: Hydrothermal syntheses
and structures of Zn(II) and Cd(II) coordination polymers
with xylyldiphosphonate ligands
⇑
⇑
Tiffany M. Smith, Diona Symester, Kathryn A. Perrin, Bruce S. Hudson , Jon Zubieta
Department of Chemistry, Syracuse University, Syracuse, NY 13244, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 October 2013
Received in revised form 2 December 2013
Accepted 8 December 2013
Available online 12 December 2013
Hydrothermal reactions of zinc(II) acetate dihydrate or cadmium(II) nitrate tetrahydrate with xylyldi-
phosphonic acid isomers in the presence of an organonitrogen chelating ancillary ligand provided a series
of materials of the (M(II)-organonitrogen chelate)/xylyldiphosphonate family. One-dimensional struc-
tures were observed for [Zn(H2O)(bpy)(1,2-HO3PC8H8PO3H)] (1), [Zn(bpy)(1,3-HO3PC8H8PO3H)]ꢀH2O
(2ꢀH2O), [M(o-phen)(1,3-HO3PC8H8PO3H)] (M = Zn (5ꢀH2O), M = Cd (11), and [Zn(tpyprz)(1,2-HO3PC8H8-
PO3H)] (7), while the phases [M(bpy)2(1,4-H2O3PC8H8PO3H2)(1,4-HO3PC8H8PO3H)] (M = Zn (3) and
Keywords:
2nþ
Cd(9)) exhibited virtual layered structures constructed from fMðbpyÞ2ð1; 4 ꢁ H2O3PC8H8PO3H2Þgn
Zn(II) and Cd(II) coordination polymers
Metal-diphosphonate compounds
Low dimensional metal-diphosphonates
through ancillary ligand incorporation
Crystal structures.
chain hydrogen-bonded to the {HO3PC8H8PO3H}2- charge compensating anions. The two-dimensional
structures are represented by [M(o-phen)(1,2-HO3PC8H8PO3H)] (M = Zn (4) and Cd (10)) and
[Cd(bpy)(1,3-HO3PC8H8PO3H)] (8). Expansion into three-dimensions occurs only with the spatially
extended 1,4-xylyldiphosphonate ligand in [Zn4(o-phen)4(1,4-HO3PC8H8PO3)2(1,4-HO3PC8H8PO3H)]ꢀ3H2-
O (6ꢀ3H2O) and [Cd(o-phen)(1,4-HO3PC8H8PO3H)] (12).
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
The family of a, x-alkyldiphosphonates has been extensively
studied and shown to typically form three-dimensional
frameworks, characterized as ‘‘pillared layer’’ materials whose
interlamellar separations may be modified by variations of tether
lengths. In pursuing our systematic investigations of metal-organ-
ophosphonate chemistry [39–59], we have recently turned our
attention to the relatively unexplored xylyldiphosphonate ligands
[60–67]. In an effort to avoid the common pillared layer structural
prototypes such as [M(HO3PRPO3H)] or [M2(O3PRPO3)] for divalent
metal cations, we have exploited the introduction of ancillary
ligands such as bipyridine (bpy), o-phenanthroline (o-phen) or
tetra-2-pyridylpyrazine (tpyprz). In this study, we present the
syntheses and structures of a series of Zn(II) and Cd(II) complexes
of 1,2-, 1,3- and 1,4-xylyldiphosphonates with bpy, o-phen or
tpyprz ancillary ligands: [Zn(bpy)(H2O)(1,2-HO3PC8H8PO3H)] (1),
[Zn(bpy)(1,3-HO3PC8H8PO3H)]ꢀH2O (2ꢀH2O), [Zn(bpy)2(1,4-HO3
Organic–inorganic hybrid materials have enjoyed significant
contemporary interest because of the diversity of their structural
chemistry [1–3] and their potential applications to catalysis, optics,
sorption, separation and magnetism [4–9]. The designed synthesis
of such hybrid materials requires appropriate ligands, and carbox-
ylates and polypyridyl ligands have been exploited most
commonly in this respect [10–15]. However, while organophosph-
onate ligands have not been as extensively explored, metal-organ-
ophosphonate complexes represent prototypical organic–inorganic
hybrid materials. Since the first reports of layered metal-organ-
ophosphonates in the early 1970s, Clearfield has pioneered the
use of phosphonate ligands in transition metal chemistry
[16–18]. These studies and those of other researchers have estab-
lished an unusual compositional range and rich structural chemis-
try [19–30] and potential and realized applications to sorption,
catalysis, cation exchange, sensors and non-linear optics [31–35].
The magnetic properties of metal-organophosphonate materials
have also received considerable attention [36–38].
PC8H8PO3H)(1,4-H2O3PC8H8PO3H2)]
(3),
[Zn(o-phen)(1,2-HO3
PC8H8PO3H)] (4), [Zn(o-phen)(1,3-HO3PC8H8PO3H)]ꢀH2O (5ꢀH2O),
[Zn4(o-phen)4(1,4-HO3PC8H8PO3)2(1,4-HO3PC8H8PO3H)]ꢀ3H2O
(6ꢀ3H2O), [Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7), [Cd(bpy)(1,3-HO3
PC8H8PO3H)] (8), [Cd(bpy)2(1,4-HO3PC8H8PO3H)(1,4-H2O3PC8H8
PO3H2)] (9), [Cd(o-phen)(1,2-HO3PC8H8PO3H)] (10), [Cd(o-phen)
(1,3-HO3PC8H8PO3H)] (11), and [Cd(o-phen)(1,4-HO3PC8H8PO3H)]
(12).
⇑
Corresponding authors. Tel.: +1 315 443 2547; fax: +1 315 443 4070 (J. Zubieta).
Tel.: +1 315 443 5805; fax: +1 315 443 4070 (B.S. Hudson).
E-mail addresses: bshudson@syr.edu (B.S. Hudson), jazubiet@syr.edu
(J. Zubieta).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.12.003

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
173
2.5. Synthesis of [Zn(o-phen)(1,2-HO3PC8H8PO3H)] (4)
2. Experimental
A solution of zinc(II) acetate dihydrate (0.092 g, 0.42 mmol),
1,10 phenanthroline (0.039 g, 0.216 mmol), o-xylene-diphosphon-
ic acid (0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in the
mole ratio 1:0.51:0.71:1324 was stirred briefly before heating to
120 °C for 48 h. The initial and final pH values were 1 and 1, respec-
tively. Colorless blocks suitable for X-ray diffraction were isolated
in 60% yield. IR (KBr pellet, cmꢁ1): 3434(m), 1422(m), 1237(w),
1214(m), 1165(w), 1135(s), 1047(w), 1041(s), 937(m), 889(m),
851(m), 791(m), 727(m), 543(m). Anal. Calc. for C20H18N2O6P2Zn:
C, 47.1; H, 3.53; N, 5.49. Found: C, 47.1; H, 3.62; N, 5.22%.
2.1. General procedures
All chemicals were used as obtained without further purifica-
tion with the exception of p-xylene diphosphonic acid, ortho-xy-
lene diphosphonic acid and meta-xylene diphosphonic acid
which were synthesized in a similar fashion to the literature meth-
od [62] using the respective dibromide starting materials. Zinc(II)
acetate dihydrate (98%), cadmium(II) nitrate tetrahydrate (98%),
2,20-bipyridine (99%), 1,10-phenanthroline (99%), and tetra-2-
pyridinylpyrazine (97%) were purchased from Sigma–Aldrich. All
syntheses were carried out in 23-mL poly(tetrafluoroethylene)-
lined stainless steel containers under autogeneous pressure. The
pH values of the solutions were measured prior to and after
heating using pHydrion vivid 1-11Ò pH paper. Water was distilled
above 3.0 MO in-house using a Barnstead Model 525 Biopure
Distilled Water Center.
2.6. Synthesis of [Zn(o-phen)(1,3-HO3PC8H8PO3H)]ꢀH2O (5ꢀH2O)
A solution of zinc(II) acetate dihydrate (0.10 g, 0.46 mmol), 1,10
phenanthroline (0.040 g, 0.22 mmol), m-xylene-diphosphonic acid
(0.080 g, 0.30 mmol), and H2O (5 mL, 278 mmol) in the mole ratio
1:0.48:0.65:604 was stirred briefly before heating to 120 °C for
48 h. The initial and final pH values were 3 and 3, respectively. Col-
orless blocks suitable for X-ray diffraction were isolated in 30%
yield. IR (KBr pellet, cmꢁ1): 3503(m), 3058(w), 2911(w), 2368(w),
2344(w), 1637(w), 1623(w), 1607(w), 1586(w), 1517(m),
1489(m), 1427(m), 1267(m), 1186(m), 1135(s), 1095(m), 1051(s),
931(s), 866(w), 851(m), 807(m), 728(m), 706(m), 58(w), 594(w),
522(s), 481(m). Anal. Calc. for C20H20N2O7P2Zn: C, 45.5; H, 3.79;
N, 5.31. Found: C, 45.6; H, 3.57; N, 5.71%.
2.2. Synthesis of [Zn(H2O)(bpy)(1,2-HO3PC8H8PO3H)] (1)
A solution of zinc(II) acetate dihydrate (0.092 g, 0.42 mmol),
2,20bipyridine (0.078 g, 0.50 mmol), o-xylene-diphosphonic acid
(0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in the mole ratio
1:1.2:0.7:1324 was stirred briefly before heating to 120 °C for 72 h.
The initial and final pH values were 4 and 4, respectively. Colorless
blocks suitable for X-ray diffraction were isolated 90% yield. IR (KBr
pellet, cmꢁ1): 3422(m), 3105(w), 3073(w), 2913(w), 2775(w),
2344(m), 1654(w), 1610(m), 1598(m), 1577(w), 1493(m),
1477(m), 1446(m), 1406(w), 1255(m), 1235(m), 1162(m),
1140(s), 1109(s), 1090(s), 1060(m), 1012(m), 956(m), 938(s),
875(w), 788(m), 768(s), 734(m), 657(w), 597(m), 517(s), 480(s).
Anal. Calc. for C18H20N2O7P2Zn: C, 42.9; H, 3.97; N, 5.56. Found:
C, 42.6; H, 3.88; N, 5.69%.
2.7. Synthesis of [Zn4(o-phen)4(1,4-HO3PC8H8PO3)2(1,4-
HO3PC8H8PO3H)]ꢀ3H2O (6ꢀ3H2O)
A solution of zinc(II) acetate dihydrate (0.092 g, 0.42 mmol),
1,10 phenanthroline (0.040 g, 0.222 mmol), p-xylene-diphosphon-
ic acid (0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in the
mole ratio 1:0.53:0.714:1324 was stirred briefly before heating
to 120 °C for 72 h. The initial and final pH values were 5 and 4,
respectively. Colorless blocks suitable for X-ray diffraction were
isolated in 90% yield. IR (KBr pellet, cmꢁ1): 3482(m), 3054(m),
2908(m), 2344(w), 1660(w), 1623(m), 1579(m), 1513(s), 1425(s),
1250(s), 1200(s), 1162(s), 1078(s), 933(s), 857(s), 726(s), 596(w),
563(s), 518(m), 467(w). Anal. Calc. for C72H68N8O21P6Zn4: C, 47.2;
H, 3.72; N, 6.12. Found: C, 46.6; H, 3.55; N, 6.11%.
2.3. Synthesis of [Zn(bpy)(1,3-HO3PC8H8PO3H)]ꢀH2O (2ꢀH2O)
A solution of zinc(II) acetate dihydrate (0.10 g, 0.46 mmol),
2,20bipyridine (0.040 g, 0.26 mmol), m-xylene-diphosphonic acid
(0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in the mole ratio
1:0.57:0.65:1209 was stirred briefly before heating to 120 °C for
48 h. The initial and final pH values were 2 and 2, respectively.
Colorless blocks suitable for X-ray diffraction were isolated in
40% yield. IR (KBr pellet, cmꢁ1): 3537(m), 3454(s), 3059(w),
3029(w), 2924(w), 2344(w), 1637(w), 1606(m), 1597(m),
1442(s), 1272(m), 1199(m), 1175(m), 1125(s), 1046(s), 1022(w),
935(s), 765(s), 736(m), 699(m), 528(s), 482(m). Anal. Calc. for
2.8. Synthesis of [Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7)
A solution of zinc(II) acetate dihydrate (0.092 g, 0.42 mmol),
tetra-2-pyridinylpyrazine (0.085 g, 0.219 mmol), o-xylene-diphos-
phonic acid (0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in
the mole ratio 1:0.52:0.71:1324 was stirred briefly before heating
to 120 °C for 72 h. The initial and final pH values were 4 and 2,
respectively. Colorless blocks suitable for X-ray diffraction were
isolated in 80% yield. IR (KBr pellet, cmꢁ1): 3061(w), 3020(w),
2970(w), 2921(w), 2737(w), 2362(w), 2344(w), 1587(m),
1517(w), 1541(w), 1446(m), 1401(m), 1237(m), 1150(s), 1072(s),
1061(s), 1007(m), 926(s), 797(m), 773(s), 744(m), 634(w),
551(m), 529(s), 494(s). Anal. Calc. for C32H26N6O6P2Zn: C, 53.5; H,
3.62; N, 11.7. Found: C, 53.6; H, 3.66; N, 11.5%.
C18H20N2O7P2Zn: C, 42.9; H, 3.97; N, 5.56. Found: C, 43.1; H,
3.67; N, 5.44%.
2.4. Synthesis of [Zn(bpy)2(1,4-H2O3PC8H8PO3H2)(1,4-HO3PC8H8PO3H)]
(3)
A solution of zinc(II) acetate dihydrate (0.092 g, 0.42 mmol),
2,2’bipyridine (0.039 g, 0.250 mmol), p-xylene-diphosphonic acid
(0.080 g, 0.30 mmol), and H2O (10 mL, 556 mmol) in the mole ratio
1:0.6:0.716:1324 was stirred briefly before heating to 120 °C for
72 h. The initial and final pH values were 3 and 3, respectively. Col-
orless blocks suitable for X-ray diffraction were isolated in 90%
yield. IR (KBr pellet, cmꢁ1): 3490(s), 3408(s), 3056(w), 2917(w),
2344(w), 1580(s), 1520(m), 1440(s), 1421(w), 1311(w), 1259(m),
1187(m), 1142(m), 1080(s), 990(s), 856(m), 771(m), 566(s),
509(s). Anal. Calc. for C36H38N4O12P4Zn: C, 47.6; H, 4.19; N, 6.17.
Found: C, 47.2; H, 4.27; N, 5.99%.
2.9. Synthesis of [Cd(bpy)(1,3-HO3PC8H8PO3H)] (8)
A
solution of cadmium(II) nitrate tetrahydrate (0.098 g,
0.317 mmol), 2,20bipyridine (0.071 g, 0.46 mmol), o-xylene-
diphosphonic acid (0.080 g, 0.30 mmol), and H2O (10 mL,
556 mmol) in the mole ratio 1:0.69:0.95:1753 was stirred briefly
before heating to 120 °C for 72 h. The initial and final pH values
were 1 and 1, respectively. Colorless rods suitable for X-ray

174
T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
diffraction were isolated in 90% yield. IR (KBr pellet, cmꢁ1):
3422(m), 3059(m), 2945(m), 2364(w), 2343(w), 1595(m),
1576(w), 1487(m), 1439(s), 1411(w), 1321(s), 1247(m), 1205(m),
1174(s), 1078(w), 1052(s), 1034(s), 915(s), 896(m), 805(m),
762(m), 736(m), 700(m), 649(m), 522(s), 478(m). Anal. Calc. for
2.14. X-ray crystallography
Crystallographic data for all compounds were collected at low
temperature (90 K) on a Bruker KAPPA APEX DUO diffractometer
equipped with an APEX II CCD system using Mo K
a radiation
C
18H12N2O6P2Cd: C, 40.5; H, 3.38; N, 5.26. Found: C, 40.3; H,
(k = 0.71073 Å) [68,69]. The data were corrected for Lorentz and
polarization effects [70], and adsorption corrections were made
using SADABS [71]. The structures were solved by direct methods,
and refinements were made using the SHELXTL [72] crystallographic
software. After first locating all of the nonhydrogen atoms from the
initial solution of each structure, the models were refined against
F2 using first isotropic and then anisotropic thermal displacement
3.41; N, 5.42%.
2.10. Synthesis of [Cd(bpy)2(1,4-H2O3PC8H8PO3H2)(1,4-
HO3PC8H8PO3H)] (9)
A
solution of cadmium(II) nitrate tetrahydrate (0.098 g,
0.317 mmol), 2,20bipyridine (0.070 g, 0.45 mmol), p-xylene-
diphosphonic acid (0.080 g, 0.30 mmol), and H2O (10 mL,
556 mmol) in the mole ratio 1:1.42:0.95:1754 was stirred briefly
before heating to 120 °C for 72 h. The initial and final pH values
were 2 and 1, respectively. Colorless blocks suitable for X-ray dif-
fraction were isolated in 90% yield. IR (KBr pellet, cmꢁ1):
2950(w), 2914(w), 2373(w), 1650(m), 1565(m), 1470(m),
1279(m), 1254(w), 1139(m), 1125(s), 1100(s), 985(s), 926(s),
770(s), 736(w), 566(s), 510(m). Anal. Calc. for C36H38N4O12P4Cd:
C, 45.2; H, 3.98; N, 5.86. Found: C, 44.2; H, 3.88; N, 5.71%.
parameters until the final value of D/rmax was less than 0.001.
Hydrogen atoms were introduced in calculated positions and re-
fined isotropically. Neutral atom scattering coefficients and anom-
alous dispersion corrections were taken from the International
Tables, Vol. C. Crystallographic details for the structures of 1–12
are summarized in Table 1. Selected bond lengths and angles for
the structures are given in Table S77. Images of the crystal struc-
tures were generated using CrystalMakerÒ [73].
2.15. Thermogravimetric analyses
2.11. Synthesis of [Cd(o-phen)(1,2-HO3PC8H8PO3H)] (10)
TGA data were collected on a TA instruments Q500 v6.7 Ther-
mogravimetric Analyzer. Data was collected on samples that ran-
ged between 5 and 10 mg, ramping the temperature at 10 °C/
minute between ꢂ25–800 °C.
A
solution of cadmium(II) nitrate tetrahydrate (0.098 g,
0.317 mmol), 1,10 phenanthroline (0.042 g, 0.23 mmol), o-xy-
lene-diphosphonic acid (0.080 g, 0.30 mmol), and H2O (10 mL,
556 mmol) in the mole ratio 1:0.73:0.95:1754 was stirred briefly
before heating to 120 °C for 72 h. The initial and final pH values
were 2 and 2, respectively. Colorless blocks suitable for X-ray dif-
fraction were isolated in 70% yield. IR (KBr pellet, cmꢁ1):
3447(w), 3054(w), 2362(w), 1635(w), 1557(w), 1519(w),
1448(w), 1424(m), 1278(m), 1231(m), 1164(w), 1132(s), 1024(s),
939(m), 890(s), 852(m), 789(m), 728(m), 542(m), 497(m). Anal.
Calc. for C20H18N2O6P2Cd: C, 43.1; H, 3.23; N, 5.03. Found: C,
42.6; H, 3.09; N, 4.85%.
2.16. Luminescence studies
Spectra were obtained using a Photon Technologies Interna-
tional (Canada) QuantaMaster series spectrofluorometer modular
system with an arc for steady state measurements and a pulsed Xe-
non arc lamp for measuring phosphorescence lifetimes based on a
traveling wave gated PMT.
3. Results and discussion
2.12. Synthesis of [Cd(o-phen)(1,3-HO3PC8H8PO3H)] (11)
3.1. Syntheses
A
solution of cadmium(II) nitrate tetrahydrate (0.098 g,
0.317 mmol), 1,10 phenanthroline (0.042 g, 0.23 mmol), m-xy-
lene-diphosphonic acid (0.080 g, 0.30 mmol), and H2O (10 mL,
556 mmol) in the mole ratio 1:0.73:0.95:1754 was stirred briefly
before heating to 120 °C for 72 h. The initial and final pH values
were 2 and 1, respectively. Colorless plates suitable for X-ray dif-
fraction were isolated in 50% yield. IR (KBr pellet, cmꢁ1):
3448(m), 3006(w), 2945(w), 2919(w), 1513(m), 1428(m),
1231(m), 1171(s), 1136(w), 1052(m), 1031(s), 923(s), 909(s),
850(m), 804(m), 728(s), 698(m), 523(s). Anal. Calc. for C20H18N2O6-
P2Cd: C, 43.1; H, 3.23; N, 5.03. Found: C, 43.3; H, 3.68; N, 4.98%.
Hydrothermal methods, which were first applied for the syn-
thesis of zeolites and metal phosphates [74,75], have been
extended to the routine synthesis of metal oxides and organic–
inorganic composite materials [76–79]. The technique, conven-
tionally carried out in water at 120–250 °C at autogenous pressure,
exploits ‘‘self-assembly’’ of a solid phase from soluble precursors at
moderate temperatures. Product composition depends on a num-
ber of critical conditions, including pH of the medium, temperature
and hence pressure, the presence of structure-directing cations,
and the use of mineralizers.
Hydrothermal methods have been exploited for decades in the
preparation of metal-diphosphonate materials with pillared layer
structures, such as that of the prototypical pillared layer compound
[Cu2(H2O)2(1,4-O3PC8H8PO3)] [80]. The introduction of auxiliary
ligands, such as o-phenanthroline and 2,20-bipyridyl, provides a
method for reducing the overall dimensionality of the phase by
blocking coordination sites at the metal and preventing structural
expansion in one or more dimensions. The chain structures of
compounds 1, 2ꢀH2O, 5ꢀH2O, 7 and 11, the layers associated with
compounds 4, 8 and 10, and the one-dimensional substructures
of the virtual two-dimensional materials 3 and 9 are characteristic
examples of the use of simple auxiliary chelates in the design of
lower dimensional materials. However, even in the presence of
such chelating ancillary ligands, three-dimensional structures
were observed for compounds 6ꢀ3H2O and 12, where the 1,
2.13. Synthesis of [Cd(o-phen)(1,4-HO3PC8H8PO3H)] (12)
A
solution of cadmium(II) nitrate tetrahydrate (0.098 g,
0.317 mmol), 1,10 phenanthroline (0.042 g, 0.22 mmol), p-xy-
lene-diphosphonic acid (0.080 g, 0.30 mmol), and H2O (10 mL,
556 mmol) in the mole ratio 1:0.69:0.95:1754 was stirred briefly
before heating to 120 °C for 72 h. The initial and final pH values
were 2 and 1, respectively. Colorless blocks suitable for X-ray dif-
fraction were isolated in 50% yield. IR (KBr pellet, cmꢁ1):
3005(w), 2945(w), 2367(w), 1573(w), 1513(m), 1443(w),
1428(m), 1231(s), 1190(s), 1052(s), 1029(s), 923(s), 909(s),
862(m), 728(s), 697(s), 582(w), 575(s), 478(m). Anal. Calc. for C20-
H18N2O6P2Cd: C, 43.1; H, 3.23; N, 5.03. Found: C, 43.4; H, 3.55;
N, 5.11%.

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
175
4-xylyldiphosphonate ligand provides sufficient spatial extension
to encapsulate the chelating organonitrogen ligands within the
framework void spaces.
Such naïve design principles also did not extend to the chemistry
of the Zn(II)/tpyprz/xylyl-diphosphonate system. Our expectation
was that the dipodal tpyprz would bridge metal sites of zinc-
diphosphonate chains to expand the structure into two-dimensions.
However, the tpyprz coordinates to a single metal site so as to dec-
orate the surface of the chain, projecting one uncoordinated
pyrazine face and two uncoordinated pyridyl groups into the ex-
tra-linkage domain. Similar results were observed for the previously
reported Mn(II), Co(II) and Ni(II) complexes of the xylyldiphospho-
nates when tpyprz was incorporated into the chain [81].
It is also noteworthy that xylyl-diphosphonate ligands may
adopt a variety of protonated forms, in addition to the more com-
mon fully deprotonated tetranegative anion {O3P-R-PO3}4ꢁ. Under
the conditions of this study, the symmetrically protonated form
{HO3PC8H8PO3H}2ꢁ predominates.
The infrared spectra of compounds 1–12 exhibit a pattern of
two or three medium to strong bands in the 1000–1200 cmꢁ1
range attributed to m(P–O) of the phosphonate ligands. In addition,
compounds 2ꢀH2O, 5ꢀH2O and 6ꢀ3H2O exhibit an intense broad
band at ca. 3100 cmꢁ1 assigned to
molecules.
m(O–H) of the coordinated water
3.2. Descriptions of the structures of this study
As shown in Fig. 1, the structure of [Zn(bpy)(H2O)(1,2-HO3
PC8H8PO3H)] (1) is one-dimensional with the {Zn(II)-diphospho-
nate}n segment describing a shallow helix. The zinc sites exhibit
distorted trigonal bipyramidal geometry through bonding to a
nitrogen donor of the bpy chelate and oxygen donors from two
diphosphonate ligands in the equatorial plane and an aqua oxygen
and the second bipyridyl nitrogen in the axial positions. Each
diphosphonate ligand in turn bridges two zinc centers through
sharing of a single oxygen donor at each {–PO3} terminus of the li-
gand. Each {–PO3} terminus is protonated at a single oxygen site,
readily located on the final difference Fourier and also apparent
from P–O and P–O(H) average distances of 1.514(1) and
1.567(1) Å, respectively.
It is noteworthy that the 1,2-xylyldiphosphonate ligand with
Mn(II) and bpy yields the previously described two-dimensional
material [Mn(bpy)(1,2-HO3PC8H8PO3H)] where the square pyrami-
dal Mn sites are paired in the characteristic di-
bridged binuclear units [81].
l
(O,O0) phosphonate
When o-phenanthroline is substituted for bpy, the two-dimen-
sional [Zn(o-phen)(1,2-HO3PC8H8PO3H)] (4) is obtained (Fig. 2).
The structure is constructed from the afore-mentioned units of
di-l
(O,O0) phosphonate bridging two metal square pyramids in a
{Zn2P2O4} eight-membered ring. A secondary building unit consist-
ing of two zinc square pyramids and four phosphorous tetrahedra
is connected to four adjacent {Zn2(o-phen)2(RPO3H)4} clusters
through the xylyl spacers to provide the two-dimensional network
structure.
The basal plane of each Zn(II) site is defined by the oxygen do-
nors of the two bridging diphosphonate tetrahedra and the nitro-
gen donors of the o-phen chelate. The apical site is occupied by
an oxygen donor from one terminus of a diphosphonate ligand
linking to an adjacent binuclear zinc site of the layer. Thus, each
diphosphonate ligand uses one terminus in bridging the metal sites
of one cluster while the second bonds to a single metal site of an
adjacent cluster. The linkage pattern produces intralamellar cavi-
ties of approximate dimensions 12.7 ꢃ 9.4 Å whose boundaries
are defined by four {Zn2(o-phen)2(RPO3H)4} clusters. The two-
dimensional structure of 4 is isomorphous with the previously
described [Mn(bpy)(1,2-HO3PC8H8PO3H)], [Mn(o-phen)(1,2-HO3

176
T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
Fig. 1. A mixed polyhedral and ball-and-stick representation of the structure of [Zn(H2O)(bpy)(1,2-HO3PC8H8PO3H)] (1). Color scheme: Zn, grey polyhedra; P, yellow
polyhedra; oxygen, red spheres, nitrogen, light blue spheres; carbon, black spheres. This scheme is used throughout. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
PC8H8PO3H)] and [Co(o-phen)(1,2-HO3PC8H8PO3H)]. This recurrent
network structure is also observed for the Cd(II) analogue
[Cd(o-phen)(1,2-HO3PC8H8PO3H)] (10).
In common with structures of this general type, the distances
from the metal to the basal oxygen donors are significantly shorter
than the metal-apical oxygen distances: Zn–Obasal (ave): 1.977(2) Å;
The isomorphous 1,4-xylyldiphosphonate derivatives [M(bpy)2
(1,4-H2O3PC8H8PO3H2)(1,4-HO3PC8H8PO3H)] (M = Zn (3), Cd (9))
exhibit the unusual, but apparently common, virtual
two-dimensional network structure of Fig. 5. The structures are
similar to those previously reported for [M(NꢂN)2(1,4-H2O3PC8H8-
PO3H2)(1,4-HO3PC8H8PO3H)] (M = Mn, NꢂN = bpy; M = Co, Ni, and
NꢂN = o-phen). The structure is constructed from fMðN ꢂ NÞ2
Zn–Oapical
:
2.057(2) Å; Cd–Obasal(ave)
:
2.162(1) Å; Cd–Oapical:
2nþ
2.219(1) Å. The protonation sites on the phosphate oxygen atoms
are again revealed in the bond distances: P–O (ave): 1.512(2) Å
(4) and 1.506(2) Å (10); P–O(H) (ave): 1.578(2) Å (4) and
1.571(2) Å (10).
ð1; 4 ꢁ HO3PC8H8PO3HÞgn chains that are distinct from the one-
dimensional motifs adopted by compounds 1, 2ꢀH2O, 8 and 11 of
this study. In the case of 3 and 9, each metal bonds to two bpy
or o-phen ligands and to two fully protonated diphosphonate li-
gands, H2O3PC8H8PO3H2, to give cis-{MO2N4} coordination geome-
try. Adjacent metal octahedra of the chain are linked through the
1,4-xylyldiphosphonic acid ligands, each bonding to one metal site
through one oxygen of each terminus. The remaining oxygen
While compound 2ꢀH2O, [Zn(bpy)(1,3-HO3PC8H8PO3H)]ꢀH2O
(2ꢀH2O) is one-dimensional, the structure is quite distinct form that
of 1. As shown in Fig. 3, the building unit in this case is the {M2P2O4}
ring of di-l
(O,O0) phosphonate groups bridging metal square pyra-
mids. This linkage provides the {Zn2(o-phen)2(RPO3H)4} cluster
building block, each of which is linked to two adjacent clusters of
the chain. Each diphosphonate ligand provides two oxygen atoms
of one {–PO3H} terminus to bridge the zinc sites of a cluster unit,
while one oxygen donor of the second terminus bonds to a metal
site of an adjacent cluster.
The zinc geometry is defined by the nitrogen donors of the che-
late and two oxygen donors in the basal plane while an oxygen
from a third diphosphonate ligand occupies the apical position.
atoms are protonated, such that the one-dimensional substructure
2nþ
is positively charged, fMðN ꢂ NÞ2ð1; 4 ꢁ H2O3PC8H8PO3H2Þgn
.
The structure expands into a virtual two-dimensional network
through strong multipoint hydrogen-bonding between the
P–O(H) groups of the chains and the unprotonated oxygen atoms
of the associated but uncoordinated {HO3PC8H8PO3H}2ꢁ anions.
The hydrogen-bonding interactions, shown in Fig. 5b, result in a
two-dimensional network of cationic chains linked through
diphosphonate anions. The {M(NꢂN)2}2+ groups of the layers pro-
ject above and below the plane into the interlamellar domain.
These projecting units from adjacent layers interdigitate to pro-
duce a dense structure with minimal void space.
The common bond length pattern is observed: Zn–Obasal
:
1.953(1) Å; Zn–Oapical: 2.035(1) Å. The protonation sites are again
obvious with P–O and P–O(H) average distances of 1.512(2) and
1.576(2) Å, respectively.
The structures of the o-phen derivatives [Zn(o-phen)(1,3-HO3
PC8H8PO3H)]ꢀH2O (5ꢀH2O) and [Cd(o-phen)(1,3-HO3PC8H8PO3H)]
(11) are similar to that of compound 2ꢀH2O. A nearly identical double
chain structure is observed with projecting o-phenanthroline li-
gands replacing the bpy ligands of 2ꢀH2O. The usual trends in bond
distances are observed, as shown in Table S77.
Low dimensional structures may be designed by introducing
coligands of higher denticity, such as tetra-2-pyridinylpyrazine,
and the one-dimensional theme is observed for [Zn(tpyprz)(1,2-
HO3PC8H8PO3H)] (7), shown in Fig. 4. The structure consists of dis-
torted trigonal bipyramidal {ZnN3O2} sites linked through the
xylyldiphosphonate ligands. The equatorial plane is defined by
oxygen donors from {–PO3H} groups of two xylyldiphosphonate li-
gands and the nitrogen donor of the pyrazine group. The axial posi-
tions are occupied by the pyridyl nitrogen donors of one side of the
tpyprz ligands. In common with the previously reported one-
dimensional structure [M(tpyprz)(1,2-HO3PC8H8PO3H)] (M = Co,
Ni), the other pyrazine nitrogen and two pyridyl arms of the tpyprz
ligand are uncoordinated. However, the metal sites of the Co and Ni
derivatives adopt {MO3N3} distorted octahedral coordination
geometries that result in the common {M2(tpyprz)2(RPO3H)2} clus-
ter as the secondary building unit.
The protonation sites of 3 and 9 are revealed by the appearance
of peaks at appropriate locations in the Fourier maps and by the
usual trends in P–O and P–O(H) bond distances. As shown in
Table S77, the metrical parameters are unexceptional.
The two-dimensional theme is recurrent in the structure of
[Cd(bpy)(1,3-HO3PC8H8PO3H)] (8), shown in Fig. 6. However, the
juxtaposition of the common {M2(NꢂN)(RPO3H)2} clusters in 8 is
quite distinct from that of network structures of 4 and 10 of this
study. Rather than the isolated clusters with radiating xylyl tether-
ing arms of the latter, 8 exhibits chains of {Cd2(NꢂN)2(RPO3H)2}
units running parallel to the a or c axis, respectively. Adjacent
chains are linked through the xylyl tethers of the diphosphonate li-
gands to establish the two-dimensional connectivity.
The Cd(II) sites of 8 exhibit trigonally distorted {CdO4N2} octa-
hedral geometries. As illustrated by compound 8, the {CdO3} face
of the polyhedra exhibits an average bond distance of 2.246(1) Å
while the {MON2} face has an average bond distance of
2.352(1) Å. Once again, the protonation sites are apparent from
the P–O and P–O(H) distances.
Curiously, there is a single example of a three-dimensional
structure for the Zn(II) series, [Zn4(o-phen)4(1,4-HO3PC8H8PO3)2
(1,4-HO3PC8H8PO3H)]ꢀH2O (6ꢀH2O), shown in Fig. 7. The structure
is constructed from the common {Zn2(o-phen)2(RPO3)2} subunits

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
177
linked into a one-dimensional chain through {RPO3} tetrahedra.
These chains are distinct from those observed for 8 and 12 which
consist of {M2(NꢂN)(RPO3H)2} units linked by two {RPO3H} tetra-
hedra at each metal site to the two adjacent binuclear building
units of the chain. In effect, each M(II) site shares a vertex with four
{RPO3H} tetrahedra. In the case of compound 6ꢀH2O, each {Zn2(o-
phen)2(RPO3Hx)2} unit is linked to adjacent clusters through a sin-
gle {RPO3} group, such that each Zn(II) site is associated with three
phosphonate tetrahedra rather than four.
The {Zn2(o-phen)2(RPO3Hx)}n chains run parallel to the crystal-
lographic c-axis and the xylyl tethers of the diphosphonate ligands
radiate outward along the a and b directions connecting each chain
to four adjacent chains. The xylyl tethers provide sufficient spacing
between chains to generate channels of rhombic profile of dimen-
sions 11.5 by 8.4 Å, accommodating the o-phen groups that project
outward from the chain axes.
The zinc sites exhibit distorted square pyramidal {ZnO3N2}
geometry with the basal plane occupied by the o-phen nitrogen
Fig. 2. (a) A view of the two-dimensional structure of [Zn(o-phen)(1,2-HO3PC8H8PO3H)] (4); (b) a view of the structure in the ac plane. The structure of [Cd(o-phen)(1,2-
HO3PC8H8PO3H)] (10) is isomorphous.

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T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
Fig. 3. A view of the one-dimensional structure of [Zn(bpy)(1,3-HO3PC8H8PO3H)]ꢀH2O (2ꢀH2O). The structures of [M(o-phen)(1,3-HO3PC8H8PO3H)] (M = Zn (5) and Cd (11)
exhibit similar connectivities.
Fig. 4. A view of the one-dimensional structure of [Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7).
Fig. 5. (a) A view of the one-dimensional substructure of [Zn(bpy)2(1,4-H2O3PC8H8PO3H2)(1,4-HO3PC8H8PO3H)] (3) in the bc plane; (b) a view of the structure of 3 in the ab
2nþ
plane, showing the hydrogen bonding between fZnðbpyÞðH2O3PC8H8PO3H2Þgn chains and the {HO3PC8H8PO3H}2ꢁ anions. The structure of [Cd(bpy)2(1,4-H2O3PC8H8PO3-
H2)(1,4-HO3PC8H8PO3H)] (9) is isomorphous. Color scheme: Cd, blue polyhedra; other atoms as above. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
donors and two phosphonate oxygen atoms; the oxygen donor
from a third phosphonate group adopts the apical position. There
are two distinct xylyldiphosphonate ligands. The first is in the sym-
metrically singly protonated form {HO3PC8H8PO3H}2ꢁ which
bridges two zinc sites of a {Zn2(o-phen)2(RPO3H)2} cluster at either
the pendant oxo-group and 1.541(2) and 1.542(2) Å for the metal
ligated oxygen atoms.
The second example of a three-dimensional structure is pro-
vided by compound 12, [Cd(o-phen)(1,4-HO3PC8H8PO3H)] (12).
Once again, the common {M2(NꢂN)2(HO3PR)2} secondary building
unit is encountered. As shown in Fig. 8, these clusters fuse into
chains in a fashion reminiscent of the structure of compound 8.
However, in this case, each chain is linked through the 1,4-diphos-
phonate ligand to four adjacent chains, rather than two as for 8, to
provide expansion into the third dimension. An unusual feature of
the structure is the large cavities running parallel to the c-axis,
formed by four {M2(HO3PR)2} clusters and the four bridging
1,4-xylyl groups, which are occupied by the o-phen ligands.
terminus and is aligned parallel to the
b axis. The second
xylyldiphosphonate ligand employs one terminus as a bridge with-
in a {Zn2(o-phen)2(RPO3H)2} cluster with the second terminus
present as a fully deprotonated {RPO3}2ꢁ unit bridging two
{Zn2(o-phen)2(RPO3H)2} clusters of an adjacent chain. The P–O
and P–O(H) bond lengths for the three {RPO3H} termini of the
two ligands are 1.507(2) Å (ave) and 1.564(2) Å, respectively, while
the P–O distances for the fully deprotonated unit are 1.516(2) Å for

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
179
Fig. 6. (a) A view of the two-dimensional structure of [Cd(bpy)(1,3-HO3PC8H8PO3H)] (8) in the ab plane; (b) the structure shown in the bc plane.
Consequently, the o-phen groups, which decorate the surfaces of
the layers in the previously described two-dimensional series,
are incorporated into organic–inorganic capsules within the
three-dimensional framework of 12.
species also reveal the pronounced tendency of the copper in these
compounds to adopt ‘4 + 1’ axially elongated square pyramidal
geometry, a common structural feature of d9-Jahn–Teller distorted
Cu(II) complexes.
The denticity of the secondary ligand also results in dramatic
structural consequences. The tpyprz ligand was introduced as a tri-
dentate, binucleating ligand in the naive expectation that this
would provide expansion into three-dimensional structures.
However, only molecular [Cu2(tpyprz)(1,2-xdpH2)2] and [Cu4Cl4
(tpyprz)2(1,4-xdpH3)2]Cl2, and one-dimensional, [M(tpyprz)(1,2-
xdpH2)] (M = Co, Ni, Zn) and [Ni2(tpyprz)(1,4-xdpH2)] structures
were observed. In all cases but that of [Ni2(tpyprz)(1,4-xdpH2)],
the tpyprz ligand binds to a single site, projecting two pendant pyr-
idyl groups.
The coordination preferences of the metals are also contribu-
tory structural determinants. For example, the one-dimensional
structures of [Cu(bpy)(1,3-xdpH2)] and [Cu(o-phen)(1,3-xdpH2)]
are similar to those of Zn(II) and Cd(II) analogues [M(NꢂN)(1,3-
xdpH2)] (NꢂN = bpy and o-phen) with {MN2O3} square pyramidal
coordination geometries and quite distinct from the two-dimen-
sional series of 1,3-xdpH2 analogues, [M(NꢂN)(1,3-xdpH2)]
(M = Mn, Co, Ni; NꢂN = bpy, o-phen), where octahedral {MN2O4}
coordination is observed.
3.3. Structural tends and observations
The consequences of incorporation of ancillary chelating ligands
are evident in the structures of this study and those of the Mn(II),
Co(II), Ni(II) and Cu(II) series previously described [77,78]. The
expectation that occupation of coordination sites on the metal cen-
ters by the organonitrogen chelates would effect reduction of the
overall dimensionality of the materials has been realized. In the ab-
sence of this secondary chelating ligand, pillared layer structures
are the rule in general and specifically in the xylyldiphosphonate
derivative [Cu2(H2O)2(1,4-O3PC8H8PO3)] [82]. However, as noted
in Table 2, in the presence of the secondary ligands, there are only
three instances of three-dimensional structures [M(o-phen)
(1,4-xdpH2)] (M = Co, Cd) and the unusual [Zn4(o-phen)4
(1,4-xdpH2)(1,4-xdpH3)2], which do not exhibit the prototypical
pillared layer structure but one derived from the recurrent
{M2(RPO3)2} building unit (xdpH2 = xylyldiphosphonic acid) shown
in Fig. 9.
It is noteworthy that the multidentate capability of diphospho-
nate ligands favors the formation of extended structures and often
of insoluble polymeric materials [58]. Molecular or zero-dimen-
sional phosphonates are relatively rare [83]. Curiously, the molec-
ular species encountered in our studies are exclusively Cu(II)
species, although the reactions conditions were similar for the
preparation of all phases described in the table. The molecular
While there is a predominance of structures exhibiting square
pyramidal coordination geometries, six coordination is also com-
mon. For example, the Cd(II) derivatives [Cd(bpy)(1,2-xdpH2)]
and [Cd(o-phen)(1,4-xdpH2)] exhibit {CdN2O4} octahedral geome-
try, whose building units, {Cd2(RPO3)2} rings fused into one-
dimensional chains, are similar to those of the two-dimensional
phases [M(NꢂN)(1,3-xdpH2)] (M = Mn, Co, Ni and NꢂN = bpy and

180
T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
Fig. 7. (a) A view of the three-dimensional structure of [Zn4(o-phen)4(1,4-HO3PC8H8PO3)2(1.4-HO3PC8H8PO3H)]ꢀ3H2O (6ꢀH2O) in the ab plane; (b) a view of the structure of 6
in the bc plane showing the {Zn2(o-phen)2(RPO3Hx)3}n one-dimensional substructures and the linkage through the xylyl chains.
o-phen) where a different xylyl ligand is employed. Thus, the
[Cd(bpy)(1,2-xdpH2)] and [M(NꢂN)(1,3-xdpH2)] series of com-
pounds exhibit similar overall two-dimensional connectivities de-
spite the incorporation of 1,2- or 1-3-xylyldiphosphonate ligands.
In general, this is not the case, as the relative locations of the
methylenephosphonate groups on the ligand phenyl ring result
in distinctive structural types. Consequently, the two-dimensional
structures [M(NꢂN)(1,2-xdpH2)] (M = Mn, Co, Cu, Zu, Cd) are quite
distinct from the network structures [M(NꢂN)(1,3-xdpH2)]
(M = Mn, Co, Ni) and from the virtual two-dimensional structures
of the [M(NꢂN)2(1,4-xdpH2)(1,4-xdpH4)] class.
In the absence of crystal field coordination preferences, the d10
Zn(II) structures exhibit coordination modes that approach both
the square pyramidal and trigonal bipyramidal idealized

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
181
Fig. 8. (a) A view of the three-dimensional structure of [Cd(o-phen)(1,4-HO3PC8H8PO3H)] (12) in the ab plane; (b) a view of the linking of one-dimensional substructures
through the xylyl tethers of the diphosphonate ligands in the ac plane.

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T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
Table 2
Summary of Selected Structural Characteristics of Xylyldiphosphonates with M(II)-organonitrogen chelate Moieties.
Molecular speciesa
M(II) coordination
Phosphonate bonding
Metal-phosphonate SBU
di-
(O,O0) bridged {Cu2(RPO3)2} ring
[Cu(H2O)(o-phen)(1,4-xdpH2)]
{CuN2O3} square pyramidal
one terminus bridges 2 Cu sites; one is
pendant
l
[Cu2(H2O)2(bpy)(1,4-xdpH3)2(1,4-xdpH2)]
{CuN2O3}square pyramidal
one terminus bonds to a single Cu site; one {Cu(RPO3)2} unit
is pendant
[Cu2(tpyprz)(1,2-xdpH2)2]
[Cu4Cl4(tpyprz)2(1,4-xdpH3)2]Cl2
{CuN2O3} square pyramidal
{CuN3OCl} square pyramidal
each terminus bonds to a single Cu site
one terminus bridges 2 Cu sites; one is
pendant
one terminus bridges 2 Cu sites; one is
pendant
{Cu(RPO3)2} unit
l
(O,O0) bridged {Cu2(RPO3)} unit
[Cu(2,20-dpa)(1,3-xdpH2)]
{CuN2O2} square planar
{CuN2O3} square pyramid
di-
l
l
(O,O0) bridged {Cu2(RPO3)2} ring
(O,O0) bridged {Cu2(RPO3)2}
One-dimensional structures
[Cu2(H2O)2(o-phen)2(1,4-xdpH2)]
each terminus bonds to two Cu sites
di-
rings
[Zn(bpy)(H2O)(1,2-xdpH2]
{ZnN2O3} trigonal bipyramid
{MN2O3} square pyramid
each terminus bonds to one Zn site
one terminus bridges two M sites; the
second bonds to a single M site
{Zn(RPO3)2} unit
[M(NꢂN)(1,3-xdpH2] M = Cu, NꢂN = bpy,
o-phen; M = Zn, NꢂN = bpy o-phen;
M = Cd, NꢂN = o-phen
di-l
(O,O’) bridged {M2(RPO3)2} ring
[M(tpyprz)(1,2-xdpH2)] M = Co, Ni M = Zn
{MN3O3} octahedron {MN3O2}
trigonal bipyramid
one terminus bridges two M sites; the
second bonds to a single M site; each
terminus bonds to one Zn site
di-l
(O,O0) bridged {M2(RPO3)2 ring
{Zn(RPO3)2} unit
[Ni2(tpyprz)(1,4-xdpH2)]
{MN3O3} octahedron
one terminus bridges two Ni sites; the
second bonds to a single Ni site
di-
di-
di-
l
l
l
(O,O’) bridged {Ni2(RPO3)2 ring
(O,O’) bridged {M2(RPO3)2} ring
(O,O’) bridged {Cu2(RPO3)2 ring
Two-dimensional structures
[M(NꢂN)(1,2-xdpH2)]
{MN2O3} square pyramid
one terminus bridges two M sites; the
second bonds to a single M site
M = Mn, Co, Cu, Zn, Cd; NꢂN = o-phen.
M = Mn; NꢂN = bpy
[Cu(NꢂN)(1,4-xdpH2)] NꢂN = bpy
NꢂN = o-phen
{CuN2O3} square pyramid
{CuN2O3} square pyramid
each terminus of one diphosphonate
bridges two Cu sites; each terminus of a
second diphosphonate bonds to one Cu site
One terminus bridges two Cu sites; the
second bonds to a single Cu site
[M(NꢂN)(1,3-xdpH2)] M = Mn, Co, Ni;
NꢂN = bpy.
{MN2O4} octahedron
each terminus bridges two M sites
di-
l
(O,O0)bridged {M2(RPO3)2} rings
fused into a 1-D chain
M = Mn, Co Ni; NꢂN = o-phen
[Cd(NꢂN)(xdpH2)] NꢂN = bpy; xdp = 1,3 xdp
{Cd(N2O4} octahedron
{M(N4O2} octahedron
each terminus bridges two Cd sites
di-
rings fused into a 1D chain
l
(O,O0) bridged {Cd2(RPO3)2}
[M(NꢂN)2(1,4-xdpH2)(1,4-xdpH4)] M = Mn,
Zn, Cd; NꢂN = bpy. M = Co, Ni;
NꢂN = o-phen
each terminus of one diphosphonate bonds {M(RPO3)2} unit
to one M site; the second diphosphonate is
involved in H-bonding only
Three-dimensional structures
[Zn4(o-phen)4(1,4-xdpH2)(1,4-xdpH)2]
{Zn(N2O3)} distorted square
pyramid
{MN2O4} octahedron
each terminus of both unique
diphosphonates bridge two Zn sites
each terminus bonds to two metal sites
di-
l
l
(O,O0) bridged {Zn2(RPO3)2 rings.
(O,O0) bridged {M2(RPO3)2} rings
[M(o-phen)(1,4-xdpH2)] M = Co, Cd
xdpH4 = xylyldiphosphonate acid.
di-
fused into a 1D chain
a
geometries. As expected, Cd(II) provided a number of materials
where the coordination number expanded to six in distorted octa-
hedral geometries of {CdN2O4} and {CdN5O2} types.
phenyl rings while in the materials 2–4 and 9, the substituents
adopt the anti limiting geometry (Scheme 1). While these limiting
geometries predominate in this study and are also apparent in the
previously reported manganese, cobalt, nickel and copper series of
xylyldiphosphonates [77,78], the rotational versatility of the ligand
is manifested in the structure of compound 6 where one of the
structurally independent 1,4-xylyldiphosphonate ligands adopts
an intermediate geometry with one {PO3} group displaying the
conventional distance between the phenyl plane and the phospho-
rus atom of ca. 1.75 Å and the second folding into the phenyl plane
with a plane to phosphorus distance of ca. 0.45 Å.
A number of persistent structural prototypes can be identified
for specific xylyldiphosphonate geometries. The one-dimensional
phases of the type [M(NꢂN)(1,3-xdpH2)] are observed for Cu(II),
Zn(II) and Cd(II) with bpy and o-phen chelates. However, for
Mn(II), Co(II) and Ni(II), two dimensional phases of the same over-
all composition [M(NꢂN)(1,3-xdpH2)] are observed. In both series
of compounds, the core building unit is the {M2(di-l-O,O-phospho-
nate)2} ring. However, in the one-dimensional series, these second-
ary building units are linked only through the xylyl tethers of the
diphosphonate ligands, while in the two-dimensional series, these
rings are directly fused into a chain substructure.
A rather obvious structural determinant is the relative locations
of the methylenephosphonate groups on the ligand phenyl rings,
whether 1,2-, 1,3- or 1,4-substituted. Thus, three dimensional
structures are achieved only with the spatial expansion afforded
by the 1,4-xylyldiphosphonate ligand in compounds 6 and 12.
The structures of these series also reflect the free rotation of the
{PO3}2ꢁ or {PO3H}1ꢁ groups about the methylene carbon atoms.
For example, in compounds 1, 5–8, 11 and 12, the phosphate
groups are disposed in the syn configuration with respect to the
The two-dimensional phases provide two additional structural
prototypes, reflecting the identity of the xylyldiphosphonate
ligand: [M(NꢂN)(1,2-xdpH2)] and the unusual [M(NꢂN)2(1,
4-xdpH2)(2,4-xdpH2)]. The compounds incorporating the 1,2-
xylyldiphosphonate ligand again exhibit the recurrent {M2(di-l-
O,O0-phosphonate)2} secondary building unit, and the network
structure is observed for all metals of this study except Ni(II) for
which no crystalline phases incorporating 1,2-xylyldiphosphonate
were isolated in our studies. It is noteworthy that Co, Ni and Zn
provide one-dimensional phases with 1,2-xylyldiphosphonate
with the general formula [M(NꢂN)(1,2-xdpH2)]. However, zinc
provides two distinct structural types, both based on isolated

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
183
Fig. 9. (a) The di-l
-O,O0-phosphonate bridged metal binuclear core, showing the {M2P2O4} ring; (b) the {M2(ligand)2(RPO3)4} building block common to the one-dimensional
compounds 2ꢀH2O, 5ꢀH2O, and 11 and to the two-dimensional materials 4 and 10; (c) the one-dimensional substructure of 6ꢀ3H2O; (d) the one-dimensional substructure of 8
and 12.
Scheme 1. The syn, anti and intermediate cinfiguration of the methylenephosphonate groups with respect to the phenyl rings of the xylyldiphodponate ligands.
{Zn(RPO3)2} units but with bpy and aqua coordination in one case
and tpyprz ligation in the second.
to provide sufficient extension to encapsulate the secondary che-
lating ligand within the framework structure. The most unusual
example is [Zn4(o-phen)4(1,4-xdpH2)(1,4-xdpH)2] which not only
exhibits two distinct ligand protonation modes but also adopts a
The most unusual structural prototype is the virtual network
adopted by the compounds [M(NꢂN)2(1,4-xdpH2)(1,4-xdpH4)]
2nþ
where fMðN ꢂ NÞ2ð1; 4 ꢁ H2O3PC8PO3H2Þgn
chains are linked
unique building unit consisting of {M2(di-
l
-O,O0-phosphonate)2}
a single l
-O,O0-phosphonate
through hydrogen-bonding to {HO3PC8H8PO3H}2ꢁ anions. The
series spans all the metals of this study except Cu(II) whose
coordination requirements are quite distinctive. In contrast to the
Jahn–Teller distorted coordination environments of Cu(II), the
other metals of this study readily adopt the more regular octahe-
dral geometries that are required for these structures.
clusters linked in turn through
tetrahedra.
3.4. Thermal analyses
The TGA for compound 1 exhibits stability up to ca. 215 °C fol-
lowed by a weight loss of ca. 37% at 425 °C, attributed to the loss of
the 2,20bipyridine ligand and the coordinated water molecule (35%,
Three-dimensional structures are restricted to compounds
incorporating the 1,4-xylyldiphosphonate ligand which appears

184
T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
Fig. 10. The steady state total emission spectra at room temperature (purple) and 77 K (lightblue) compared to the gated detection spectra at 77 K with delay times of 1 ms
(blue), 3 ms (red) or 10 ms(green) for the (a) 2,20-bipyrdine and (b) 1,10-phenanthroline compounds of this study. The compounds are identified by their assigned number on
the plots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
theoretical). The compound is then stable up to 580 °C, followed by
an additional weight loss of nearly 10% between 580 and 600 °C.
This weight loss is most likely attributed to the partial decomposi-
tion of the diphosphonate ligand. The amorphous residue, most
likely a zinc phosphate, is stable from about 610 °C to the temper-
ature limit of the scan. Compound 6ꢀ3H2O exhibits similar features
in its respective TGA plot.
residue is stable from 620 to 800 °C. The TGA of compound 7 exhib-
its similar features to that of compound 4.
Compound 5ꢀH2O experiences a gradual weight loss from 25 °C
to 200 °C of ca. 5% attributed to the water of crystallization (calcu-
lated, 4%). Subsequently, there is a plateau of stability up to about
390 °C, which is then followed by a weight loss of 26% in the range
390–800 °C. This loss is most likely due to the degradation of the
organonitrogen ligand, followed by the gradual decomposition of
the diphosphonate ligand.
Compound 8 is stable up to about 250 °C, and then experiences
a weight loss from 250 to 350 °C of about 35% attributed to the loss
of the organonitrogen ligand (30%, calculated). An additional
weight loss of about 20% occurs between 350 and 575 °C, which
is due to the partial degradation of the diphosphonate ligand.
The compound is then stable from about 575–800 °C. The TGA
plots of compounds 9 and 10 exhibit similar features.
Compound 11 is exhibits a gradual weight loss of about 20% be-
tween 375 and 800 °C which is attributed to the loss of the orga-
nonitrogen ligand. The TGA of compound 12 exhibits similar
features.
Compound 2ꢀH2O displays a weight decrease of about 4% attrib-
uted to the loss of the water of crystallization (calculated, 4%) be-
tween 25 °C to 205 °C. The compound is then stable until 300 °C,
whereupon from 300 °C to 515 °C, the organonitrogen ligand is
gradually lost, as shown by a weight loss of 31% (32%, theoretical).
At 515 °C, a loss in weight of less than 5% is observed, which is
most likely due to the initial decomposition of the diphosphonate
ligand. The compound is then relatively stable from 675 to 800 °C.
Compound 3 is stable from 25 to 210 °C, whereupon there is a
weight loss of 65% between 210 and 360 °C, which is attributed to
the decomposition of the organonitrogen ligands and the hydro-
gen-bonded diphosphonate ligand (64%, calculated). The amorphous
zinc phosphate residue is stable to the limit of the run, i.e., 800 °C.
Compound 4 exhibits no weight loss up to 310 °C. Between 310
and 620 °C a rapid weight loss is observed closely followed by a
more gradual weight loss, corresponding to a total of ca. 85% of
the original weight of sample. The weight loss is attributed to
the decompsition of the organonitrogen ligand, followed by the
initial degradation of the diphosphonate ligand. The amorphous
3.5. Luminescence studies
The spectra presented in Fig. 10 are sorted by ligand and then
by metal. The pairs of spectra shown side by side with Zn and Cd
share the same phosphonate structure. In each case steady state

T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
185
Fig. 10 (continued)
total emission spectra are shown at room temperature (purple)
and at 77 K (light blue). These are compared on an arbitrary scale
with spectra obtained by gated detection spectra obtained at
77 K following a delay of 1 ms (blue), 3 ms (red) or 10 ms (green).
These ‘‘time-gated’’ spectra reveal long lived emission probably
due to ligand phosphorescence or recombination luminescence.
The steady state emission spectra will have added the signal from
fluorescence of the ligand to this weak long-lived emission. In all
cases the time-gated emission is at lower energy than the prompt
steady state emission but only by at most 5000 cmꢁ1. In four of the
twelve cases the long lived emission, especially that obtained with
10 ms delay, exhibits structure that appears to be vibronic.
Comparison of the steady state spectra obtained at room temper-
ature and at 77 K shows diverse behavior. In the 2,20-bipyridine case
three of the five spectra are virtually superimposable at the two
temperatures, one shows a longer red emission at room tempera-
ture and the other at low temperature. All of these 2,20-bipyridine
steady state spectra exhibit what looks like structure that may be
due to the long lived emission. In the case of the o-phenanthroline
compounds, the steady state spectra are similar, indicative of simi-
lar vibronic structure. The case of the tetra-2-pyridinylpyrazine
structure, [Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7), is perhaps of most
interest in that the lower energy emission increases at low temper-
ature and the vibrational structure observed at room temperature is
repeated at low temperature.
the longest delay of 10 ms, appears to be red shifted relative to,
and superimposed on, a more continuous shorter lived emission.
When the longer delay changes the spectrum the new features
are structured and are predominantly at longer wave length.
A previous study on the behavior of 2,20-bipyridine in solution
reported the fluorescence of both 2,20-bipyridine and the com-
plexes with cadmium and zinc [84]. Upon complexation of 2,20-
bipyridine with the metal(II) species, the lowest excited singlet
state was assigned to the ligand. Consequently, there is no change
in spectral behavior when compared with that of the ligand in the
320–500 nm spectral region. This lowest excited singlet state is
due to a pp⁄ transition with a kcm = 328 nm. The complexes of
2,20-bipyridine with zinc(II) and cadmium(II) yield excitation max-
ima in the 365–370 nm range with the fluorescence maxima red
shifted to the 430–455 nm range.
The present set of spectra appear to be explained by the super-
position of ligand fluorescence with long lived structured ligand
phosphorescence. There are several considerations that should be
kept in mind in the description of emission spectra of crystalline
solids. The first is that energy transfer is likely to be rapid for the
singlet excitations and slow for the triplet excitations at low
temperature. This raises the point that it is always possible that
the emission is due to defect states of lower energy. The similarity
of the spectra for all compounds with a given ligand argues that this
last feature is not likely for the fluorescence component in these
cases. The present set of spectra appear to be explained as superpo-
sition of fluorescence and one or two components of a long-lived
emission. One of these is nearly continuous in its spectral shape
and does not depend appreciably on the initial time of the
In several cases, most clearly for the 2,20-bipyridine case
[Cd(bpy)(1,3-HO3PC8H8PO3H)](8) and for the tetra-2-pyridinylpyr-
azine case [Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7), the structured
part of the time-gated spectrum, especially that obtained with

186
T.M. Smith et al. / Inorganica Chimica Acta 411 (2014) 172–187
time-gate. This is the dominant component for most of the materi-
als studied. For [Cd(bpy)2(1,4-H2O3PC8H8PO3H2)(1,4-HO3PC8H8-
PO3H)] (9) this is the only component. This emission is close in
spectral position to the fluorescence. This is attributed to delayed
fluorescence due either to triplet recombination or to recombina-
tion luminescence. For [Zn(H2O)(bpy)(1,2-HO3PC8H8PO3H)] (1),
Cd(bpy)(1,3-HO3PC8H8PO3H)] (8), [Cd(o-phen)(1,2- HO3PC8H8PO3-
H)] (10), [Zn4(o-phen)4(1,4-HO3PC8H8PO3)2(1,4-HO3PC8H8PO3H)]ꢀ
3H2O (6ꢀ3H2O), [Cd(o-phen)(1,4-HO3PC8H8PO3H)] (12) and [Zn
(tpyprz)(1,2-HO3PC8H8PO3H)] (7) there is an additional structured
emission that is relatively enhanced by longer delay times. The
compound [Cd(bpy)(1,3-HO3PC8H8PO3H)] (8) is a case where the
emission clearly has two components. Furthermore, compound
[Zn(tpyprz)(1,2-HO3PC8H8PO3H)] (7) is a case where this structured
emission dominates and appears to have a lifetime in the 3 ms
range.
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2013.12.003.
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