Structure and multinuclear solid-state NMR of a highly birefringent lead-gold cyanide coordination polymer

Article abstract of DOI:10.1021/ja0566634

The coordination polymer Pb(H₂0)[Au(CN)₂]a (1) was synthesized by the reaction of KAu(CN)₂ and Pb(NO₃) ₂. The structure contains 1-D chains of lead(II)-OH₂ linked via Au(CN)₂^- moieties, generating a 2-D slab; weak aurophilic interactions of 3.506(2) and 3.4885(5) A occur within and between slabs. The geometry about each lead(II) is bicapped trigonal prismatic, having six N-bound cyanides at the prism vertices and waters at two of the faces. Dehydration at 175 °C yields microcrystalline Pb[Au(CN) ₂]₂ (2), which, along with 1, was examined by ¹³C, ¹⁵N, ¹H, and ²⁰⁷Pb solid-state NMR methods. TwO ¹⁵N resonances are assigned to the μ₂-bridging and hydrogen-bonding cyanides in 1. Upon dehydration, the ²⁰⁷Pb NMR spectrum becomes axially symmetric and yields a reduced shielding span, indicating higher site symmetry, while the ¹³C and ¹⁵N spectra reveal a single cyanide. Although no single-crystal X-ray structure of 2 could be obtained, a structure is proposed on the basis of the NMR and X-ray powder data, consisting of a lead(II) center in a distorted square-prismatic environment, with cyanides present at each corner. The birefringence of single crystals of 1 is found to be 7.0 × 10^-2 at room temperature. This value is large compared to that of most optical materials and can be attributed to the anisotropy of the 2-D slabs of 1, with all C≡N bonds aligned in the same direction by the polarizable lead(II) center.

Full text of DOI:10.1021/ja0566634

Published on Web 02/22/2006
Structure and Multinuclear Solid-State NMR of a Highly
Birefringent Lead-Gold Cyanide Coordination Polymer
Michael J. Katz,† Pedro M. Aguiar,‡ Raymond J. Batchelor,† Alexei A. Bokov,†
Zuo-Guang Ye,*,† Scott Kroeker,*,‡ and Daniel B. Leznoff*,†
Contribution from the Department of Chemistry, Simon Fraser UniVersity,
8888 UniVersity DriVe, Burnaby, BC, Canada V5A 1S6, and
Department of Chemistry, UniVersity of Manitoba, Winnipeg, MB, Canada R3T 2N2
Received September 28, 2005; E-mail: dleznoff@sfu.ca; scott_kroeker@umanitoba.ca; zye@sfu.ca
Abstract: The coordination polymer Pb(H2O)[Au(CN)2]2 (1) was synthesized by the reaction of KAu(CN)2
and Pb(NO3)2. The structure contains 1-D chains of lead(II)-OH2 linked via Au(CN)2- moieties, generating
a 2-D slab; weak aurophilic interactions of 3.506(2) and 3.4885(5) Å occur within and between slabs. The
geometry about each lead(II) is bicapped trigonal prismatic, having six N-bound cyanides at the prism
vertices and waters at two of the faces. Dehydration at 175 °C yields microcrystalline Pb[Au(CN)2]2 (2),
which, along with 1, was examined by 13C, 15N, 1H, and 207Pb solid-state NMR methods. Two 15N resonances
are assigned to the µ2-bridging and hydrogen-bonding cyanides in 1. Upon dehydration, the 207Pb NMR
spectrum becomes axially symmetric and yields a reduced shielding span, indicating higher site symmetry,
while the 13C and 15N spectra reveal a single cyanide. Although no single-crystal X-ray structure of 2 could
be obtained, a structure is proposed on the basis of the NMR and X-ray powder data, consisting of a
lead(II) center in a distorted square-prismatic environment, with cyanides present at each corner. The
birefringence of single crystals of 1 is found to be 7.0 × 10-2 at room temperature. This value is large
compared to that of most optical materials and can be attributed to the anisotropy of the 2-D slabs of 1,
with all CtN bonds aligned in the same direction by the polarizable lead(II) center.
(NLO),5 ferroelectric,6 semiconductor,7,8 and birefringent9 ma-
terials. Indeed, the molecular coordination chemistry of lead-
Introduction
Research into the chemistry of supramolecular coordination
polymers has rapidly grown in recent years due to an increasing
demand for functional materials with conducting, magnetic,
nonlinear optical, or zeolitic properties.1 The modular nature
of coordination polymer synthesis2 allows the properties of each
building block to be incorporated into the polymer. This concept
has been used to great effect to target coordination polymers
with interesting magnetic properties, in particular, by employing
the very popular cyanometalate building blocks (e.g., Prussian
blue-type systems).3 In this light, it seems surprising that
literature describing the incorporation of highly polarizable metal
cations, such as lead(II), as building blocks in coordination
polymers is unusually sparse,4 despite the fact that such soft
metal centers are key components in a range of nonlinear optical
(II) is well understood, featuring a vast number of binding modes
and geometries ranging from three- to twelve-coordinate not
often observed among transition metals.10,11 As well, lead(II)
(4) For selected Pb(II) coordination polymers, see: (a) Yilmaz, V. T.; Caglar,
S.; Harrison, W. T. A. Z. Anorg. Allg. Chem. 2004, 630, 948. (b) Foreman,
M. R.; Plater, M. J.; Skakle, J. M. S. J. Chem. Soc., Dalton Trans. 2001,
1897. (c) Zhu, L.-H.; Zeng, M.-H.; Ye, B.-H.; Chen, X.-M. Z. Anorg. Allg.
Chem. 2004, 630, 952. (d) Li, G.; Hou, H.; Li, L.; Meng, X.; Fan, Y.; Zhu,
Y. Inorg. Chem. 2003, 42, 4995. (e) Shi, Y.-J.; Li, L.-H.; Li, Y.-Z.; Chen,
X.-T.; Xue, Z.; You, X.-Z. Polyhedron 2003, 22, 917.
(5) (a) Ye, Q.; Li, Y.-H.; Wu, Q.; Song, Y.-M.; Wang, J.-X.; Zhao, H.; Xiong,
R.-G.; Xue, Z. Chem.sEur. J. 2005, 11, 988. (b) Singh, N. B.; Suhre, D.
R.; Green, K.; Fernelius, N.; Hopkins, F. K. J. Cryst. Growth 2005, 274,
132. (c) Sui, B.; Zhao, W.; Ma, G.; Okamura, T.-A.; Fan, J.; Li, Y.-Z.;
Tang, S.-H.; Sun, W.-Y.; Ueyama, N. J. Mater. Chem. 2004, 14, 1631. (d)
Meng, X.; Song, Y.; Hou, H.; Fan, Y.; Li, G.; Zhu, Y. Inorg. Chem. 2003,
42, 1306. (e) Guloy, A. M.; Tang, Z.; Miranda, P. B.; Srdanov, V. I. AdV.
Mater. 2001, 13, 833. (f) Kondo, T.; Iwamoto, S.; Hayase, S.; Tanaka, K.;
Ishi, J.; Mizuno, M.; Ema, K.; Ito, R. Solid State Commun. 1998, 105,
503.
† Simon Fraser University.
‡ University of Manitoba.
(1) (a) Janiak, C. Dalton Trans. 2003, 2781. (b) Cassoux, P.; Valade, L.;
Kobayashi, H.; Kobayashi, A.; Clark, R. A.; Underhill, A. E. Coord. Chem.
ReV. 1991, 110, 115. (c) Ouahab, L. J. Chem. Soc., Dalton Trans. 1997, 9,
1909. (d) Decurtins, S.; Pellaux, R.; Antorrena, G.; Palacio, F. Coord. Chem.
ReV. 1999, 190-192, 841. (e) Roundhill, D. M.; Fackler, J. P., Jr.
Optoelectronic Properties of Inorganic Compounds; Plenum Press: New
York, 1999.
(6) Newnham, R. MRS Bull. 1997, XXII, 20.
(7) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286,
945.
(8) Mitzi, D. B.; Wang, S.; Feild, C. A.; Chess, C. A.; Guloy, A. M. Science
1995, 267, 1473.
(9) Sigman, M. B. J.; Korgel, B. A. J. Am. Chem. Soc. 2005, 127, 10089.
(10) (a) Holloway, C. E.; Melnik, M. Main Group Metal Chemistry 1997, 20,
399. (b) Parr, J. In ComprehensiVe Coordination Chemistry II: From
Biology to Nanotechnology; Parkin, G. F. R., Ed.; Elsevier Pergamon:
Boston, 2004; Vol. 3, p 545. (c) Engelhardt, L. M.; Patrick, J. M.; Whitaker,
C. R.; White, A. H. Aust. J. Chem. 1987, 40, 2107. (d) Engelhardt, L. M.;
Harrowfield, J. M.; Miyamae, H.; Patrick, J. M.; Skelton, B. W.; Soudi,
A. A.; White, A. H. Aust. J. Chem. 1996, 49, 1111.
(2) (a) Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi,
O. M. Acc. Chem. Res. 2001, 34, 319. (b) Chesnut, D. J.; Hagrman, D.;
Zapf, P. J.; Hammond, R. P.; LaDuca, R.; Haushalter, R. C.; Zubieta, J.
Coord. Chem. ReV. 1999, 192, 737.
(3) (a) Miller, J. S. Inorg. Chem. 2000, 39, 4392. (b) Verdaguer, M.; Bleuzen,
A.; Marvaund, V.; Vaissermann, J.; Seuleiman, M.; Desplanches, C.;
Scuiller, A.; Train, C.; Garde, R.; Gelly, G.; Lomenech, C.; Rosenman, I.;
Veillet, P.; Cartier, C.; Villain, F. Coord. Chem. ReV. 1999, 190, 1023.
(11) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998, 37,
1853.
9
10.1021/ja0566634 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 3669-3676
3669

A R T I C L E S
Katz et al.
compounds can have a stereochemically active lone pair of 6s-
electrons, which, depending upon their degree of p character,
can result in a more anisotropic geometry;11 this effect can have
a significant impact on the bulk properties of the material.
In addition to standard diffraction methods, lead-containing
materials can be investigated by solid-state 207Pb NMR. Lead-
207 (I ) 1/2, 22.1% natural abundance) NMR chemical shifts
are extremely sensitive to small deviations in local structure.
For example, solid Pb(NO3)2 is often used in solid-state NMR
to calibrate probe temperature since its chemical shift changes
by 0.70 ppm/K.12,13 This sensitivity also manifests itself in very
anisotropic chemical shieldings. Lead(II) compounds of higher
point group symmetry exhibit shielding spans of 55-900 ppm,
while those possessing lower symmetry tend to exhibit spans
of 600-3800 ppm.14-16 Although this can make it difficult to
acquire 207Pb NMR spectra of solids, it also makes such studies
very attractive to characterize site symmetry.
The majority of reported lead(II)-based coordination polymers
contain bridging halide ligands.17 The [(PbBr2)2(µ-pyrazine)]
polymer contains 2-D Pb-Br sheets, which are distorted due
to the stereochemical lone pair. The sheets are linked to one
another via pyrazine ligand connectors.17a Such halide-bridged
polymers, particularly those with iodide units, show a large third-
harmonic NLO response with short response time (e.g., the
layered polymer (C6H13NH3)2PbI4).5f By incorporating a chiral
organic fragment, large second-order NLO properties have been
observed.5e Similar polymers containing tin(II) ions, which are
less polarizable, yet still contain a potentially stereochemically
active lone pair of electrons, have also been investigated.7 These
organic-inorganic hybrid materials show properties comparable
to the best organic semiconductors made by vacuum evapora-
tion, but allow a synthetically simpler route to highly ordered
films. Conducting materials have also been synthesized with
these tin(II) polymers by simply changing the organic portion
of the polymer.8
exhibiting interesting luminescence behavior.24 We have been
investigating Au(CN)2-based coordination polymers25-28 in an
effort to utilize the unique ability of d10-metal centers, such as
gold(I), to form attractive metallophilic bonds.29-35 Motivated
by our continuing interest in such polymers and the potentially
useful properties accessible via the addition of a lead(II) building
block, we have studied the aforementioned Pb[Au(CN)2]2(H2O)x
(x ) 0, 1) system and hereby report its crystal structure,
multinuclear solid-state NMR spectra, and high optical bire-
fringence.
Experimental Section
General Procedures and Physical Measurements. All manipula-
tions were performed in air. All reagents were obtained from com-
mercial sources and used as received. Infrared spectra were recorded
as KBr pressed pellets on a Thermo Nicolet Nexus 670 FT-IR
spectrometer. Raman spectra were recorded on a Thermo Nicolet Nexus
670 FT-Raman spectrometer, equipped with a Nd:YAG laser (1064
nm). Microanalyses (C, H, N) were performed at Simon Fraser
University by Mr. Miki Yang. Thermogravimetric analysis (TGA) data
were collected using a Shimadzu TGA-50 instrument heating at 5 °C/
min in an air atmosphere. Solid-state luminescence data were collected
at room temperature on a Photon Technology International (PTI)
fluorometer, using a Xe arc lamp, and a photomultiplier detector. X-ray
powder patterns were collected on a Rigaku RAXIS rapid curved image
plate area detector with graphite monochromator, utilizing Cu KR
radiation. One-hour scans were taken with a 0.3 µm collimator and a
æ spinning speed of 5°/s; ω was held at 90°, and ø was held at 0°. The
powder was adhered to a glass fiber with grease. Peak positions for 2
were located in WinPlotr.36 Cell parameters were determined using
Dicvol37 and Treor.38 Further refinement of the lattice parameters was
performed using the FullProf package in WinPlotr.36 Structural models
for 2 were produced with Powder Cell39 using triclinic symmetry. The
final atomic positions were placed in a crystal data file and analyzed
for existing symmetry elements using the MISSYM program,40 thereby
uniquely identifying the space group.
1
Solid-State NMR. 207Pb, H, 13C, and 15N NMR experiments were
run on a Varian Inova 600 (B0 ) 14.1 T) at 195.08, 599.68, 150.79,
Another common bridging unit is the family of cyanometa-
lates [M(CN)x]n-, but within the vast cyanometalate coordination
polymer literature,18,19 the only lead-containing materials include
organolead(IV) polymers with [M(CN)6]2-/3- (M ) Fe, Ru, Co)
linkers,20,21 a Pb2[Fe(CN)6] semiconducting material whose
structure was resolved only by powder X-ray diffraction,22,23
and a lead(II) cyanoaurate system of undetermined structure
(22) Zubkov, V. G.; Tyutyunnik, A. P.; Berger, I. F.; Maksimova, L. G.;
Denisova, T. A.; Polyakov, E. V.; Kaplan, I. G.; Voronin, V. I. Solid State
Sci. 2001, 3, 361.
(23) Zhukov, V. P.; Zainullina, V. M.; Zubkov, V. G.; Tyutyunnik, A. P.;
Denisova, T. A. Solid State Sci. 2001, 3, 539.
(24) Patterson, H. H.; Bourassa, J.; Shankle, G. Inorg. Chim. Acta 1994, 226,
345.
(25) Leznoff, D. B.; Xue, B.-Y.; Patrick, B. O.; Sanchez, V.; Thompson, R. C.
Chem. Commun. 2001, 3, 259.
(26) Leznoff, D. B.; Xue, B.-Y.; Stevens, C. L.; Storr, A.; Thompson, R. C.;
Patrick, B. O. Polyhedron 2001, 20, 1247.
(12) van Gorkom, L. C. M.; Hook, J. M.; Logan, M. B.; Hanna, J. V.;
Wasylishen, R. E. Magn. Reson. Chem. 1995, 33, 791.
(13) Bielecki, A.; Burum, D. P. J. Magn. Reson. A 1995, 116, 215.
(14) Nolle, A. Z. Naturforsch. 1977, 32A, 964.
(15) Sebald, A.; Harris, R. K. Organometallics 1990, 9, 2096.
(16) Fayon, F.; Farnan, I.; Bessada, C.; Coutures, J.; Massiot, D.; Coutures, J.
P. J. Am. Chem. Soc. 1997, 119, 6837.
(17) (a) Shi, Y.-J.; Chen, X.-T.; Cai, C.-X.; Zhang, Y.; Xue, Z.; You, X.-Z.;
Peng, S.-M.; Lee, G.-H. Inorg. Chem. Commun. 2002, 5, 621. (b) Shi, Y.-
J.; Xu, Y.; Zhang, Y.; Huang, B.; Zhu, D.-R.; Jin, C.-M.; Zhu, H.-G.; Yu,
Z.; Chen, X.-T.; You, X.-Z. Chem. Lett. 2001, 678. (c) Calabrese, J.; Jones,
N. L.; Harlow, R. L.; Herron, N.; Thorn, D. L.; Wang, Y. J. Am. Chem.
Soc. 1991, 113, 2328. (d) Cui, Y.; Ren, J.; Chen, G.; Yu, W.; Qian, Y.
Acta Crystallogr. C 2000, 56, e552. (e) Nordell, K. J.; Schultz, K. N.;
Higgins, K. A.; Smith, M. D. Polyhedron 2004, 23, 2161. (f) Morsali, A.;
Mahjoub, A. Polyhedron 2004, 23, 2427.
(27) Leznoff, D. B.; Xue, B.-Y.; Batchelor, R. J.; Einstein, F. W. B.; Patrick,
B. O. Inorg. Chem. 2001, 40, 6026.
(28) Lefebvre, J.; Batchelor, R. J.; Leznoff, D. B. J. Am. Chem. Soc. 2004,
126, 16117.
(29) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H. J. Am. Chem.
Soc. 2000, 122, 10371.
(30) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P.,
Jr. J. Am. Chem. Soc. 2001, 123, 11237.
(31) Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.;
Perez, J.; Laguna, A. J. Am. Chem. Soc. 2002, 124, 5942.
(32) Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.;
Perez, J.; Laguna, A.; Mohamed, A. A.; Fackler, J. P., Jr. J. Am. Chem.
Soc. 2003, 125, 2022.
(33) Wang, S.; Garzon, G.; King, C.; Wang, J.-C.; Fackler, J. P., Jr. Inorg. Chem.
1989, 28, 4623.
(18) Dunbar, K. R.; Heintz, R. A. Prog. Inorg. Chem. 1997, 45, 283.
(19) (a) Lefebvre, J.; Leznoff, D. B. In Metal and Metalloid-Containing
Polymers; Wiley: New York, 2005; Vol. 5, pp 155-208. (b) Iwamoto, T.
In ComprehensiVe Supramolecular Chemistry; Lehn, J. M., Atwood, J. L.,
Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Alberti, G., Bein, T., Eds.;
Pergamon Press: Oxford, 1996. (c) Ohba, M.; Okawa, H. Coord. Chem.
ReV. 2000, 198, 313.
(34) Bardaj´ı, M.; Laguna, A. J. Chem. Educ. 1999, 76, 201.
(35) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412.
(36) Roisnel, T.; Rodr´ıguez-Carvajal, J. In Materials Science Forum, Proceedings
of the Seventh European Powder Diffraction Conference (EPDIC 7);
Delhez, R., Mittenmeijer, E. J., Eds.; 2000; p 118.
(37) Boultif, A.; Loue¨r, D. J. Appl. Crystallogr. 1991, 24, 987.
(38) Werner, P.-E.; Eriksson, L.; Westdahl, M. J. Appl. Crystallogr. 1985, 18,
367.
(20) Brimah, A. K.; Schwarz, P.; Fischer, R. D.; Davies, N. A.; Harris, R. K. J.
Organomet. Chem. 1998, 568, 1.
(39) Kraus, W.; Nolze, G. J. Appl. Crystallogr. 1996, 29, 301.
(40) Gabe, E. J.; Page, Y. L.; Charland, J. P.; Lee, F. L.; White, P. S. J. Appl.
Crystallogr. 1989, 22, 384.
(21) Behrens, U.; Brimah, A. K.; Soliman, T. M.; Fischer, R. D.; Apperley, D.
C.; Davies, N. A.; Harris, R. K. Organometallics 1992, 11, 1718.
9
3670 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006

Birefringence and Solid-State NMR of Pb(H2O)[Au(CN)2]2
A R T I C L E S
Table 1. Crystallographic Data for 1
and 60.77 MHz, respectively, using 3.2 and 5 mm double-resonance
magic-angle spinning (MAS) probes.
1
A. Pb-207. A single-pulse experiment was employed for the MAS
spectra with a 1 µs pulse (18° tip angle), using recycle delays of 5 and
10 s for 1 and 2, respectively. Nonspinning experiments were acquired
using a Hahn-echo, with pulse lengths of 5 and 10 µs and recycle delays
of 10 and 25 s for 1 and 2, respectively. Chemical shifts are reported
with respect to Pb(CH3)4 using 1.0 M lead nitrate (-2961.2 ppm) as a
secondary reference.41
To ensure that the measured chemical shifts are reliable, the effects
of sample heating due to fast spinning were measured for spinning
rates up to 11 kHz. The chemical shift was found to vary <0.1 ppm/
K, as calibrated by Pb(NO3)2.12,13 The small peak shift and symmetric
line shape verify that the temperature dependence is too small to
influence the data presented here.
empirical formula
formula weight
crystal system
space group
a, Å
C4H2N4Au2OPb
723.22
orthorhombic
Pcma
6.8938(8)
9.8074(9)
13.1773(19)
890.92(18)
4
293
0.70930
5.392
51.725
0.049
b, Å
c, Å
V, Å3
Z
T, K
λ, Å
F
calcd, g‚cm-3
µ, mm-1
Ra (I > 2.5σ(I))
Rwa (I > 2.5σ(I))
goodness of fit
0.058
1.25
B. H-1. Proton spectra were acquired using 1 µs pulse lengths (B1
) 71.4 kHz), employing spinning speeds of 5 and 9 kHz and 20 s
1
a Function minimized ∑w(|Fo| - |Fc|)2, where w-1 ) [σ2(Fo) +
recycle delays. H chemical shifts are relative to TMS, using water
(0.03*Fo)2], R ) ∑||Fo| - |Fc||/Σ|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
2
(+4.8 ppm) as a secondary reference.
C. C-13. 13C{1H} cross-polarization was employed for the MAS
and nonspinning spectra of 1, with recycle delays of 30 s. For 2, a
Bloch-decay sequence with 1 µs pulse (18° tip angle) and 60 s recycle
delay was used. Nonspinning experiments were conducted with an echo
pulse sequence to eliminate baseline distortions. 13C chemical shifts
are relative to TMS, using adamantane (29.5 and 38.6 ppm) as a
secondary reference.
D. N-15. Spectra were acquired with single-pulse experiments using
2 µs pulse lengths (35° tip angle) and recycle delays of 720 s for 1.
Spectra of 2 were acquired with a 1 µs pulse length (18° tip angle) and
a 120 s recycle delay. 15N chemical shifts are relative to NH3(l), using
15NH415NO3 (23.8 ppm, 15NH4) as a secondary reference.
Birefringence. The birefringence was measured by means of
polarized light microscopy utilizing an Olympus BX60 microscope,
on plate-shaped single crystals of 1. The optical retardation was
measured using a tilting Berek compensator (3λ) at the wavelength
of 546.1 nm. The birefringence was calculated by dividing the mea-
sured retardation by the crystal thickness of 26(2) µm. A heating/
cooling stage (Linkam HTMS600) mounted on the microscope was
used for studying the temperature dependence (150-340 K) of the
birefringence.
Synthesis of 13C15N-Enriched K[Au(CN)2]. An aqueous solution
of K13C15N/KCN (18 mg of K13C15N, 0.27 mmol; 41 mg of KCN, 0.63
mmol) was added to a suspension of AuCN (200 mg, 0.89 mmol) in
30 mL of water. The mixture was stirred and heated to 60 °C for 3 h,
after which the remaining AuCN was filtered off. The water was
removed in vacuo to yield a white precipitate of 15% 13C15N-enriched
K[Au(CN)2]. Yield: 90 mg (35%). Anal. Calcd for C2N2AuK: C,
8.42%; H, 0.00%; N, 9.80%. Found: C, 8.11%; H, 0.00%; N, 9.47%.
IR (KBr, cm-1): 2152 (s), 2140 (s), 2101 (w), 2071 (s), 2063 (s).
Pb(H2O)[Au(CN)2]2 (1). The addition of a 0.5 mL aqueous solution
of K[Au(CN)2] (58 mg, 0.2 mmol) to a 0.5 mL aqueous solution of
Pb(NO3)2 (33 mg, 0.1 mmol) generated an immediate pale yellow
precipitate of Pb(H2O)[Au(CN)2]2 (1), which was filtered and dried.
Yield: 60 mg (83%). Anal. Calcd for C4H2N4Au2OPb: C, 6.64%; H,
0.28%; N, 7.75%. Found: C, 6.57%; H, 0.48%; N, 7.55%. IR (KBr,
cm-1): 3556 (w), 3476 (br), 2142 (s), 2131 (s), 1633 (w), 1593 (w).
Raman (cm-1): 2163 (s), 2153 (m).
Isotopically Enriched 1. Reaction of 15% 13C15N-enriched K[Au-
(CN)2] with Pb(NO3)2 as above generated a partially 13C15N-labeled 1
for solid-state NMR analysis. Anal. Calcd for C4H2N4Au2OPb: C,
6.71%; H, 0.28%; N, 7.81%. Found: C, 6.65%; H, 0.29%; N, 7.64%.
IR (KBr, cm-1): 3469 (br), 2152 (s), 2140 (s), 2070 (s), 2062 (s), 1633
(br), 1591 (m), 1386 (m).
Pb[Au(CN)2]2 (2). Solid 1 (60 mg, 0.083 mmol) was heated at 175
°C for 18 h, forming a powder of 2. Yield: 59 mg (100%). Anal. Calcd
for C4N4Au2Pb: C, 6.81%; H, 0.00%; N, 7.94%. Found: C, 6.70%; H,
0.00%; N, 7.66%. IR (KBr, cm-1): 2130 (s), 1631 (br), 1385 (m).
Raman (cm-1): 2154.
Isotopically Enriched 2. Labeled 2 was synthesized by heating
labeled 1 at 175° for 18 h.
X-ray Crystallographic Analysis of Pb(H2O)[Au(CN)2]2 (1).
Crystallographic data for 1 are tabulated in Table 1. A pale yellow
crystal of 1 having dimensions of 0.18 × 0.17 × 0.05 mm3 was glued
onto a glass fiber. The data range 4° e 2θ e 62° was recorded with
the diffractometer control program DIFRAC42 and an Enraf Nonius
CAD4F diffractometer with Mo KR radiation. The data were cor-
rected by integration for the effects of absorption with a transmission
range of 0.073-0.162. Data reduction included corrections for Lorentz
and polarization effects. Final unit-cell dimensions were determined
on the basis of 53 well-centered reflections with range 40° e 2θ e
45°.
The programs used for the absorption correction and data reduction
were from the NRCVAX Crystal Structure System.40 The structure was
solved and refined using CRYSTALS.43 Diagrams were made using
ORTEP-344 and POV-RAY.45 Complex scattering factors for neutral
atoms were used in the calculation of structure factors.46
The coordinates and anisotropic displacement parameters for the non-
hydrogen atoms of 1 were refined. Hydrogen atoms were placed in
geometrically calculated positions and refined using a riding model
and a constrained isotropic thermal parameter. The final refinement
using observed data (Io g 2.50σ(Io)) and statistical weights included
75 parameters for 805 unique reflections. Selected bond lengths and
angles are given in Table 2.
The structure of 1 appears to exhibit some disorder, which is partially
modeled by 4% occupancy alternate sites for the lead and gold atoms
Single crystals of 1 were synthesized as above, with both reactants
dissolved in 10 mL of solvent. Slow evaporation yielded pale yellow
crystals near dryness. The elemental analysis and IR spectra of the
crystals were comparable with the powder.
Single crystals were also synthesized hydrothermally by heating the
reactants to 125 °C, followed by slow cooling to room temperature
over a three-day period.
(42) Gabe, E. J.; White, P. S.; Enright, G. D. DIFRAC: A Fortran 77 Control
Routine for 4-Circle Diffractometers; N. R. C.: Ottawa, 1995.
(43) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J.
J. Appl. Crystallogr. 2003, 36, 1487.
(44) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(45) Fenn, T. D.; Ringe, D.; Petsko, G. A. J. Appl. Crystallogr. 2003, 36, 944
(Persistence of Vision Raytracing: http://www.povray.org).
(46) International Tables for X-ray Crystallography; Kynoch Press: Birming-
ham, U.K. (present distributor Kluwer Academic Publishers: Boston, MA),
1975; Vol. IV.
(41) Maciel, G. E.; Dallas, J. L. J. Am. Chem. Soc. 1973, 95, 3039.
9
J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3671

A R T I C L E S
Katz et al.
Table 2. Selected Bond Lengths (Å) and Angles (°) for 1a
Pb(1)-N(1)
Pb(1)-N(1′)
Pb(1)-N(1*)
Pb(1)-O(1*)
Au(1)-Au(1′′)
2.595(18) Pb(1)-N(2)
2.595(18) Pb(1)-N(2′)
2.65(2)
2.65(2)
2.57(2)
3.541(2)
3.454(2)
2.87(2)
2.83(2)
Pb(1)-O(1)
Au(1)-Au(2)
3.5222(8) Au(2)-Au(1′′)
Pb(1)-N(1)-C(1)
138(2)
Pb(1)-N(2)-C(2)
N(1)-Pb(1)-N(2)
N(1)-Pb(1)-N(1′)
N(1)-Pb(1)-N(1*)
N(2)-Pb(1)-N(2′)
132.1(19)
83.1(6)
87.2(9)
132.4(3)
83.5(8)
Pb(1)-N(1′′)-C(1′′) 125(2)
N(1)-Pb(1)-N(1′′)
N(1)-Pb(1)-N(2′)
N(2)-Pb(1)-N(1′′)
N(2)-Pb(1)-N(1*)
O(1)-Pb(1)-O(1*)
79.2(4)
143.0(7)
131.9(6)
80.6(6)
N(1*)-Pb(1)-N(1′′) 77.2(7)
122.7(7)
a Symmetry transformations: ′, x, -y - 1, -z; *, x - 1/2, -y + 1, -z
+ 1/2; ′′, x - 1/2, y, -z + 1/2.
only. The alternate (4%) sites for the cyanide and water groups were
not modeled due to the very low electron density expected for these
sites.
Results and Discussion
Structure of Pb(H2O)[Au(CN)2]2 (1). The addition of 2
equiv of K[Au(CN)2] to a concentrated aqueous solution of Pb-
(NO3)2 resulted in the precipitation of the microcrystalline
coordination polymer Pb(H2O)[Au(CN)2]2 (1). The IR spectrum
of 1 reveals two cyanide bands at 2140 and 2130 cm-1, both of
which are lower than the νCN of 2141 cm-1 for K[Au(CN)2],
indicating bridging to another metal. Although νCN bands in
most cyanometalates blue-shift upon nitrile binding,18,47 the red-
shift in 1 can be attributed to substantial back-bonding from
the lead(II) center. The Raman spectrum also shows two νCN
peaks at 2163 and 2153 cm-1. The room temperature lumines-
cence of 1 shows emission and excitation maxima at 520 and
420 nm, respectively, as previously reported by Patterson.24
The structure of 1 contains a Pb(II) center in a bicapped
trigonal prism coordination geometry, defined by six cyanide
nitrogens forming the vertices of the prism and two capping
water molecules (Figure 1a). The water molecules bridge
neighboring lead centers to form 1-D chains which are aligned
down the 2-fold screw along the a-axis. The 1-D chains link to
one another via Au(CN)2- units, generating a 2-D slab of edge-
sharing trigonal prismatic columns (Figure 1b). Weak aurophilic
interactions of 3.506(2) Å occur within each slab (Figure 1,
dashed lines). These slabs stack upon each other via hydrogen
bonding (O(1)-N(2) 2.86(3) Å) and aurophilic interactions (Au-
(1)-Au(2) 3.488(5) Å). The layer of gold atoms shows a nearly
Figure 1. (a) Local site geometry about the Pb(II) center of 1. (b) Two-
dimensional slab of 1 viewed down the c-axis.
nitrile also accepts a hydrogen bond from a water molecule of
the adjacent slab.
The eight-coordinate lead center shows an asymmetry in the
bond lengths toward one hemisphere, which has been attributed
to the presence of a stereochemically active lone pair on the
lead(II), producing a hemi-directional structure (Table 2).11,51
This asymmetry is exemplified by Pb(1)-N(1) and Pb(1)-
N(1*) bond lengths of 2.62(3) and 2.87(3) Å, respectively; these
are similar to Pb(dca)2, which has Pb-N bond lengths of 2.679-
(6) and 2.868(10) Å.49,50 The Pb(1)-OH2 bonds also show an
asymmetry associated with the lone pair’s presence, with Pb-
(1)-O(1) and Pb(1)-O(1*) distances of 2.57(2) and 2.83(2)
Å, respectively; these lengths are comparable to that of other
water-bound lead complexes.52
In fact, there are relatively few compounds in which there
are two H2O units bound to a single lead(II) center; this paucity
has generally been attributed to the low aqueous solubility of
lead compounds.11 The TGA of 1 demonstrates the loss of bound
water over the broad temperature range of 100-200 °C,
suggesting relatively strong H2O binding to the lead center. The
dehydrated structure is then stable up to 300 °C, at which point
it decomposes via loss of cyanogen in a single step.53,54
The dehydrated compound Pb[Au(CN)2]2 (2) was synthesized
in bulk by heating 1 in an oven overnight at 175 °C. The
resulting bright yellow powder exhibits a single νCN peak at
2130 cm-1 in the IR and one at 2154 cm-1 in the Raman
spectrum. In contrast to 1, no room temperature emission was
observed for 2. Once dehydrated, 2 does not readily rehydrate.
Single crystals of 1 degraded upon dehydration, precluding a
structural investigation of 2 via single-crystal X-ray diffraction.
-
close-packed environment. The Au(CN)2 groups are nearly
eclipsed, with a dihedral angle of <1°.30,48 Selected bond lengths
and angles can be found in Table 2.
Slight (4%) disorder in the structure was observed and appears
to arise from an alternate choice of trigonal prismatic interstices,
in which the 1-D chain of Pb-OH2 is shifted by half a cell in
the a-direction, effectively switching the location of the lead
and water; this forces the gold atoms to reorganize slightly to
accommodate the shifted chain.
Consistent with the IR data, there are two distinct cyanide
units in the structure: CN(1) bridges two lead centers (µ2-nitrile),
while CN(2) binds only to one lead atom. A similar situation
was observed in Pb(dca)2 (dca ) dicyanamide).49,50 The CN(2)
(49) Shi, Y.-J.; Chen, X.-T.; Li, Y.-Z.; Xue, Z.; You, X.-Z. New J. Chem. 2002,
26, 1711.
(50) Ju¨rgens, B.; Ho¨ppe, H. A.; Schnick, W. Solid State Sci. 2002, 4, 821.
(51) Morsali, A.; Mahjoub, A. R. HelV. Chim. Acta 2004, 87, 2717.
(52) Dale, S. H.; Elsegood, M. R. J.; Kainth, S. Acta Crystallogr. C 2004, 60,
m76.
(53) Chomic, J.; Cerna`k, J. J. Thermochim. Acta 1985, 93, 93.
(54) Cartraud, P.; Cointot, A.; Renaud, A. J. J. Chem. Soc., Faraday Trans.
1981, 77, 1561.
(47) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals;
Academic Press: London, New York, San Francisco, 1976.
(48) Colacio, E.; Lloret, F.; Kiveka¨s, R.; Ruiz, J.; Sua´rez-Varelaa, J.; Sundberg,
M. R. Chem. Commun. 2002, 592.
9
3672 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006

Birefringence and Solid-State NMR of Pb(H2O)[Au(CN)2]2
A R T I C L E S
Figure 3. 15N MAS NMR of (a) 1 and (b) 2 spinning at 18 and 17 kHz,
respectively.
Figure 2. 207Pb nonspinning NMR spectra for (a) 1 and (b) 2 showing the
principal components (δ11 > δ22 > δ33) of the chemical shift tensor.
where it is evident that the Pb center occupies a site of higher
symmetry after loss of the structural water. Not only is the span
(Ω ) 91 ppm) much smaller than that in the hydrated sample,
the skew (κ ) 0.9 ( 0.1) indicates that the lead possesses
essentially axial symmetry. Comparing this with the 207Pb NMR
spectrum of the hydrated sample, it appears that the most
shielded component in 1 (δ33 ) -3014 ppm) is changed little
upon dehydration, whereas the least shielded component (δ11
) -2672 ppm) shifts to align with the intermediate component,
δ22 (vide infra).
Table 3. NMR Data for 1 and 2 from MAS Spectra (except where
noted)
δiso (ppm)
Ω
(ppm)
κ
13C
15N
1
2
167 ( 1
173 ( 1
266 ( 1
259 ( 1
276 ( 1
346 ( 4
353 ( 4
480 ( 10
450 ( 30
n.d.
>0.95
>0.95
>0.8
>0.7
n.d.
1 (CN(1))
1 (CN(2))
2
1
2
207Pb
-2850 ( 1
-2921 ( 1
342 ( 5a
91 ( 4a
-0.16 ( 0.07a
0.9 ( 0.1a
15N MAS NMR. Two sites appear in a 1:1 ratio at 259 and
266 ppm in the 15N MAS NMR spectrum (Figure 3a) of
isotopically enriched 1, in agreement with the crystal structure.
Since hydrogen bonding has been shown to increase nitrogen
shielding,58,59 the peak at 259 ppm is probably assignable to
CN(2), although bonding to one versus two lead atoms may
also have an effect on the observed chemical shift. Simulation
of the slow-spinning 15N MAS spectra of 1 (not shown) indicates
axial symmetry and typical shielding spans for both cyanides
a From nonspinning spectra.
Solid-State NMR. To probe the structure of microcrystalline
2, multinuclear magic-angle spinning (MAS) and nonspinning
NMR of 1 and 2 were performed. 13C and 15N NMR experiments
were done on samples prepared with 15% doubly labeled 13C15N
to enhance sensitivity. These samples afford the additional
advantage that 13C signals are coupled to spin-1/2 15N, and not
the quadrupolar 14N, resulting in higher resolution and less
complicated spectra.55-57
1
(Table 3).56,60 The expected J(13C,15N) of ca. 16 Hz is not
observable due to the 130-180 Hz 15N line widths.61 One-bond
J-couplings to 207Pb are known to be in the range of 100-300
Hz.62,63 This would be expected to give rise to low-intensity
(10%) satellites; however, no such satellites could be confidently
assigned.
The dehydrated sample produces a single 15N peak, slightly
higher in frequency than CN(1), indicating a single cyanide site
in 2 (Figure 3b).
13C MAS NMR. A single site at 167 ppm is observed in the
13C MAS NMR of isotopically enriched 1 (Figure S2), despite
the presence of two crystallographically inequivalent cyanides.
This observation indicates that the chemical shielding is
dominated by bonding to the gold and is little affected by the
presence of water in the second coordination sphere. Upon
dehydration, the peak shifts to 174 ppm in 2 (Figure S3),
presumably due to the structural adjustment of the cyanides with
respect to Pb and Au. Both compounds possess axially sym-
1H MAS NMR. The 1H MAS NMR spectrum of the hydrated
sample 1 possesses signals at 3.7 and 4.9 ppm (Figure S1),
corresponding to the H-bonded and non-H-bonded sites present
in the crystal. 1H chemical shift systematics for waters H-bonded
to N-donor groups are poorly defined and do not permit reliable
assignments of these two peaks. Nevertheless, both peaks
disappear upon dehydration, thus confirming the loss of water.
207Pb NMR. The hydrated sample, 1, has a surprisingly small
207Pb chemical shielding span (Ω ) δ11 - δ33) for a nucleus
that commonly logs spans up to 3800 ppm (Figure 2a, Table
3).16 The measured shielding, spanning 342 ppm, is consistent
with the relatively regular trigonal prism formed by the N-bound
cyanides. The skew (κ ) 3(δ22 - δiso)/Ω) of -0.16 differs
significantly from axial symmetry (given by κ ) +1 or -1),
indicating that all three components of the shielding are
markedly different.
After dehydration, the 207Pb MAS NMR spectrum exhibits
much broader peaks (FWHM ) 840 vs 2000 Hz), indicating a
lower degree of crystallinity. There is a relatively small change
of 71 ppm in the isotropic chemical shift to lower frequency,
which can be understood in terms of the chemical shift
components measured from the nonspinning spectra (Figure 2b),
(58) Lorente, P.; Shenderovich, I. G.; Golubev, N. S.; Denisov, G. S.;
Buntkowsky, G.; Limbach, H.-H. Magn. Reson. Chem. 2001, 39, s18.
(59) Shenderovich, I. G.; Buntkowsky, G.; Schreiber, A.; Gedat, E.; Sharif, S.;
Albrecht, J.; Golubev, N. S.; Findenegg, G. H.; Limbach, H.-H. J. Phys.
Chem. B 2003, 107, 11924.
(60) Duncan, T. M. Chemical Shift Tensors, 2nd ed.; The Farragut Press:
Madison, WI, 1997.
(61) Wasylishen, R. E.; Muldrew, D. H.; Friesen, K. J. J. Magn. Reson. 1980,
41, 341.
(55) Curtis, R. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Chem. Soc., Chem.
Commun. 1992, 1800.
(62) Kennedy, J. D.; McFarlane, W.; Pyne, G. S.; Wrackmeyer, B. J. Organomet.
Chem. 1980, 195, 285.
(56) Kroeker, S.; Wasylishen, R. E.; Hanna, J. V. J. Am. Chem. Soc. 1999,
121, 1582.
(63) Kennedy, J. D.; McFarlane, W.; Wrackmeyer, B. Inorg. Chem. 1976, 15,
1299.
(57) Kroeker, S.; Wasylishen, R. E. Can. J. Chem. 1999, 77, 1962.
9
J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3673

A R T I C L E S
Katz et al.
Figure 4. X-ray powder diffractogram of 1 (green), 2 (blue), and simulated
peaks for 2 (red).
metric 13C shielding tensors and spans comparable to other
diamagnetic metal cyanides (Table 3).56,60,64
Structure of Pb[Au(CN)2]2 (2). The powerful combination
of the multinuclear solid-state NMR and X-ray powder diffrac-
tion data permits a reasonable proposal for the structure of 2,
despite the lack of single-crystal diffraction data. Indeed,
considering the high sensitivity of chemical shifts to local
structure, the 207Pb NMR results are crucial in postulating the
structure of 2. For example, 207Pb isotropic chemical shifts of
tetrahedral lead in lead oxides range from 1940 to -1300 ppm,
whereas eight-coordinated lead sites have shifts of -2000 to
-2100.16 Although the removal of solvents within coordination
polymers can yield quite different structures,28,65-67 the isotropic
207Pb chemical shift of 2 is quite similar to that observed in 1,
implying that the coordination geometry of the Pb(II) is not
greatly altered by dehydration. The coordination geometry of
the Pb(II) center is also more symmetric than in 1 since the
nonspinning 207Pb NMR spectrum of 2 (Figure 2b) indicates
essentially axial symmetry (κ ) 0.9 ( 0.1). In addition, the
15N MAS NMR data show only a single site at 276 ppm (Figure
3b), indicating that all cyanide groups interact equally with the
lead. Using the structure of 1 as a reference point, the loss of
the H-bond at CN(2) via the removal of the water molecule
would result in a CN(2) environment similar, but not identical,
to that of CN(1), illustrating that only a small structural
rearrangement is necessary to render the two CN groups
equivalent. Taking into account the above constraints, the Pb-
[Au(CN)2]2 stoichiometry, and also the presence of one down-
shifted cyanide in the IR spectrum, the structure of 2 must
contain a Pb(II) center surrounded by either eight or four
N-bound cyanides. Considering the similar 207Pb chemical shifts
in 1 and 2, relative to the large changes documented for
significant changes in the lead coordination number,16 it may
be inferred that a strictly four-coordinate Pb(II) center is highly
unlikely in the dehydrated product.
Figure 5. (a) View of 2 showing the column aligned along the c-axis,
highlighting the Pb(II) coordination sphere and intra-column aurophilic
interactions (dashed lines). (b) Single layer of Au(I) atoms in the ab plane
with Pb(II) atoms in the square channels; dashed lines are aurophilic
interactions. (c) Local site geometry of Pb(II) showing the distorted square
antiprism.
6.40105(13), c ) 18.3662(6) Å. Note that starting from 1,
compression of all axes followed by a simple transformation
yields the observed cell for 2 (a1 f a2, 2b1 f c2, 1/2c1 f b2),
reinforcing the NMR-based supposition that dehydration of 1
does not radically alter the structure. Thus, using the structure
of 1 as a starting point, lead and gold atoms were placed at
specific fractional coordinates inside the tetragonal unit cell for
2, and the resulting crystal symmetry and calculated powder
patterns were compared with the experimental data. Several
different sets of positions for the gold and lead atoms in 2 were
considered, and those which did not conform to the NMR data
(which requires axial symmetry about the Pb(II) and equivalent
CN groups) were rejected. Of the remaining three models, which
differ slightly by either the geometry of the Au layer or the
position of the Pb(II) center, only one arrangement of atoms
generated a powder pattern comparable in calculated positions
and intensities to those observed (Figure 4, Table S2). On this
basis, the structure of 2 is proposed to consist of a Pb(II) cation
in an eight-coordinate distorted square prismatic geometry
(Figure 5a,c), in the space group I4/mcm. Indeed, this proposed
structure can be generated from the known structure of 1 with
minimal atomic rearrangement by bonding of the CN(2) nitrogen
to the Pb(II) of an adjacent slab (Figure 6b), followed by a minor
reorganization of the gold and lead layers. In 2, the Pb(II)-based
prisms stack upon one another, producing a column of prisms,
which edge-share to four neighboring columns (Figure 5b). As
a result, each gold center forms aurophilic interactions of 3.3
Å to four other gold centers (Figure 5, dashed lines), thereby
generating a 2-D array of interlinked gold atoms in the ab plane.
The aurophilic interactions in 2 are shorter than those observed
in 1. The cyanide units were placed in idealized positions (i.e.,
using standard, fixed bond lengths for the Au(CN)2 unit)
pointing at the Pb(II) in a fashion similar to that observed in 1
Further structural information can be gleaned from the powder
X-ray diffractogram of 2 (Figure 4 and Table S1), which can
be indexed to a tetragonal cell having dimensions a ) b )
(64) Wu, G.; Kroeker, S.; Wasylishen, R. E. Inorg. Chem. 1995, 34, 1595.
(65) Carlucci, L.; Ciani, G.; Proserpio, D. M. Chem. Commun. 2004, 380.
(66) Wang, Z.; Jin, C.-M.; Shao, T.; Li, Y.-Z.; Zhang, K.-L.; Zhang, H.-T.;
You, X.-Z. Inorg. Chem. Commun. 2002, 5, 642.
(67) Cui, Y.; Ngo, H. L.; White, P. S.; Lin, W. Inorg. Chem. 2002, 42, 652.
9
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Birefringence and Solid-State NMR of Pb(H2O)[Au(CN)2]2
A R T I C L E S
Figure 7. Single crystal of 1 under a polarizing microscope: (a) Single
polarizer, P, below the sample. (b) Crystal, in diagonal position, between
crossed polarizers, P and A. Directions of the polarizers are indicated by
black or white double-headed arrows. The direction of one crystallographic
axis (b or c) is shown by the blue arrow. The red ellipse denotes the bc
section of the indicatrix.
has been tabulated for many inorganic materials, there are very
few such measurements for coordination polymers.78-80
X-ray analysis reveals that the large faces of the plate-shaped
crystals of 1 are oriented perpendicular to the a-axis, suggesting
that the slowest crystal growth direction is along the a-axis.
The more rapid crystal growth occurring along the inter-slab
axis rather than via slab extension suggests that inter-slab
hydrogen bonding and aurophilic interactions may play an
important role in the kinetics of crystal growth in this system.
In addition, the stereochemical lone pair, whose hemisphere is
found oriented close to the a-axis, may also inhibit rapid bond
formation in that direction. A photograph of the crystal of 1
used to measure the birefringence is shown in Figure 7. The
fact that the crystal does not remain dark when rotated between
crossed polarizers (Figure 7b) indicates the existence of an
optical anisotropy, which is expected for orthorhombic crystals.
Between crossed polarizers, the crystal shows complete extinc-
tion at specific orientations. The dark streaks observed near the
right and bottom crystal edges in Figure 7b can be found in
Figure 7a also, and thus they are opaque regions, not part of
extinction. The extinction direction in an orthorhombic crystal
viewed down the a-axis should coincide with the other two
crystallographic axes.81 Thus, the position of the crystallographic
axes can be derived from optical observations (Figure 7a, blue
arrow); however, it cannot be determined from optical data if
the labeled axis is the b or c direction. With the crystal 45° off
of the extinction position, the crystal color appears homogeneous
(Figure 7b); the rainbow pattern observed is due to a slight
variation in thickness across the platelet. This homogeneity
implies that twins and significant internal stresses are absent.
This is also confirmed by the fact that the extinction is observed
in all parts of the crystal simultaneously; these preliminary
observations indicate that the crystal is suitable for a birefrin-
gence measurement with confidence.
Figure 6. Formation of squares in 2 (b) upon dehydration of 1 (a) via
adjacent slabs, including proposed chemical shift tensor orientations with
δ33 directed out of the plane of the page.
(Figures 1a and 5a), thereby preserving a similar coordination
geometry about the Pb(II). It should be noted, however, that
since the cyanides do not contribute greatly to the X-ray powder
pattern of 2 it is not possible to unambiguously determine
whether the cyanide groups bridge two eight-coordinate Pb(II)
centers symmetrically (as currently shown) or asymmetrically
(as in 1). In agreement with the NMR interpretation, a four-
coordinate lead center would be structurally unreasonable due
to the closeness of the gold and lead atoms. Based solely on
the placement of the cyanide groups, the Pb-N bond length is
approximately 2.8 Å, which is comparable to the 2.595(18),
2.65(2), and 2.87(2) Å in 1. Interestingly, the structure of 2 has
an Anyuiite-type structure (i.e., AuPb2); this mineral family68
crystallizes in the same space group as 2 (I4/mcm) with similar
fractional coordinates but different unit cells depending on the
constituent atoms.
The unique component of the 207Pb shielding tensor (δ33,
Figure 2) can only be reasonably directed along the local axis
running between the cyanides (Figure 6b), with the nearly
degenerate δ11 and δ22 lying in a plane bisecting the square
antiprism (Figure 5). Presumably, the most shielded component
also lies along the b-axis in 1 since the magnitudes of δ33 are
very similar and only subtle structural changes are observed
upon dehydration of this compound. Because of their structural
similarity, it is likely that the least-shielded components in 1
and 2 are confined to the ac mirror plane in 1. In 1, δ11 is
probably aligned with the Pb-OH2 bond, and δ22 is not directed
along any particular bond (Figure 6a). This interpretation
accounts for why the δ11 component shifts significantly with
the loss of water, while δ22 and δ33 remain virtually unaltered.
For the light propagating along the a-axis of 1, a birefringence
value of ∆n ) 7.0 × 10-2 at room temperature was measured.
Birefringence. To highlight the potential utility of highly
polarizable lead(II)-containing coordination polymers, the bi-
refringence of the plate-shaped single crystals of 1 was measured
(Figure 7). Birefringence (∆n), defined as the difference in the
refractive index between two directions, is an important optical
property often used for common-path profilometry systems,69
compact-disk readers,70 image processing, intraocular elements,71
and many other devices which would be more mechanically
cumbersome without birefringent materials.72-77 Although ∆n
(71) Lesso, J. P.; Duncan, A. J.; Sibbett, W.; Padgett, M. J. Appl. Opt. 2000,
39, 592.
(72) Davis, J. A.; Evans, G. H.; Crabtree, K.; Moreno, I. Appl. Opt. 2004, 43,
6235.
(73) Kikuta, H.; Iwata, K.; Shimomura, H. J. Opt. Soc. Am. A 1992, 9, 814.
(74) Sanyal, S.; Ghosh, A. Appl. Opt. 2000, 39, 2321.
(75) Park, J.-H.; Jung, S.; Choi, H.; Lee, B. Opt. Express 2003, 11, 1862.
(76) Waagaard, O. H.; Skaar, J. J. Opt. Soc. Am. A 2004, 21, 1207.
(77) Sanyal, S.; Bandyopadhyay, P.; Ghosh, A. Opt. Eng. 1998, 37, 592.
(78) Janiak, C.; Scharmann, T. G.; Albrecht, P.; Marlow, F.; Macdonald, R. J.
Am. Chem. Soc. 1996, 118, 6307.
(79) Draper, N. D.; Batchelor, R. J.; Sih, B. C.; Ye, Z.-G.; Leznoff, D. B. Chem.
Mater. 2003, 15, 1612.
(68) Havinga, E. E.; Damsma, H.; Hokkeling, P. J. Less-Common Metals 1972,
27, 169.
(80) Fromm, K. M.; Gueneau, E. D.; Rivera, J.-P.; Bernardinelli, G. Z. Anorg.
Allg. Chem. 2002, 628, 171.
(69) Chou, C.; Shyu, J.-C.; Huang, Y.-C.; Yuan, C.-K. Appl. Opt. 1998, 37,
4137.
(81) Wahlstrom, E. E. Optical Crystallography; John Wiley and Sons: New
York, Toronto, 1979.
(70) Kinnstatter, K.; Ojima, M.; Yonezawa, S. Appl. Opt. 1990, 29, 4408.
9
J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3675

A R T I C L E S
Katz et al.
This is a high value, much larger than the ∆n of 0.9 × 10-2 for
quartz, 2.7 × 10-2 for Al2O3, a material used for birefringent
coatings,82 and indeed larger than most other optical materials.83
The birefringence of 1 is still below that of commercially
important calcite (17.2 × 10-2), but is higher than the value of
6.38 × 10-2 that we recently obtained for a highly structurally
anisotropic Cu(II)/Hg(II) 2-D layered system.79
interactions with neighboring molecules. The electronic polar-
izability of covalent bonds in molecules is usually strongly
anisotropic and, if the directions of the bonds are ordered in
the crystal structure, this leads to the anisotropy of the
permittivity (i.e., to birefringence). This is best exemplified by
the coordination sphere around the lead, which orients all of
the [Au(CN)2]- moieties in only one direction (Figure 1); this
indicates that the polarizable lead(II) cation also plays a very
significant structural role in determining the observed optical
properties. Another possible reason for a large ∆n is the
anisotropy of local fields.85 However, due to the large number
of atoms and differently directed bonds in the structure of 1,
the anisotropy of total polarizability and the anisotropy of the
local fields (i.e., the main sources of birefringence) cannot be
even qualitatively estimated.
Orthorhombic crystals are optically biaxial, and their indi-
catrix (i.e., the surface showing the variation of the refractive
index, n, with the vibration direction of light) is represented by
a triaxial ellipsoid having its principal axes parallel to the
crystallographic axes.81 From this, any measured refractive index
can be broken down into components along the ellipsoid axes.
These three principal refractive indices can be designated as
na, nb, and nc for light propagating along the a, b, and c
crystallographic directions, respectively. The above measured
birefringence is ∆n ) ∆nbc ) |nb - nc|; two other values of
interest, ∆nab ) |na - nb| and ∆nac ) |na - nc|, remain unknown.
Unfortunately, to determine the remaining birefringence values,
and thereby to determine the maximum birefringence and the
full indicatrix, crystals with large enough surfaces perpendicular
to the b- or c-axes are required; these are currently unavailable
due to difficulties in altering the crystal growth direction.
The measured birefringence of 1 remains practically un-
changed upon heating to 340 K and upon cooling actually
increases gradually to 7.3 × 10-2 at 200 K, below which no
further change occurs, suggesting a “freezing” of the crystal
structure. This small temperature dependence of the birefrin-
gence indicates the absence of significant changes in the crystal
structure in the studied temperature range of 150-340 K since
any structural phase transformation would be indicated by an
abrupt variation of the birefringence.
Conclusion
The lead(II) cyanoaurate coordination polymer, Pb(H2O)[Au-
(CN)2]2, and its dehydrated analogue have been prepared and
structurally and optically characterized. This work illustrates
the power of solid-state NMR techniques for gaining structural
information about coordination polymers. Given the challenges
of obtaining single crystals of coordination polymers, this
multinuclear magnetic resonance approach deserves renewed
attention particularly for cyanometalate systems, which can be
readily 13C15N-labeled for convenient spectral acquisition. Pb-
(H2O)[Au(CN)2]2 was found to have a high optical birefringence
with a ∆n ) 7.0 × 10-2 at 300 K, attributable to the structural
anisotropy. Judicious modification of the modular building
blocks in 1 with other polarizable groups could potentially
generate materials with even higher birefringence.
Acknowledgment. Financial support from NSERC of Canada
and Natural Resources Canada (M.J.K.) is gratefully acknowl-
edged. The Varian Inova 600 and Thermo Nicolet Nexus 670
FT-Raman spectrometers (University of Manitoba) are supported
by the Canada Foundation for Innovation and the Province of
Manitoba.
The high optical anisotropy can be associated with the 2-D
layer crystal structure of 1. According to the Maxwell relation,
at optical frequency, the refractive index is defined by the
relative dielectric permittivity (ꢀ), n2 ) ꢀ.84 The only contribu-
tion to ꢀ at this frequency comes from the electronic polarization.
The electronic permittivity is proportional in turn to the
polarizability of the molecules in the material and to the local
electric fields. Local fields that act on the molecule can be very
different from the external field applied to the crystal, due to
Supporting Information Available: X-ray crystallographic
file (CIF) for Pb(H2O)[Au(CN)2]2 1. Powder X-ray data for 2
and additional solid-state NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
(82) Hodgkinson, I. J.; Wu, Q. H. Birefringent Thin Films and Polarizing
Elements; World Scientific Publishing Co. Pte. Ltd.: River Edge, 1997.
(83) Veber, M. J. Handbook of Optical Materials; CRC Press: Boca Raton,
FL, 2003.
(84) Chelkowski, A. Dielectric Physics; Elsevier Scientific Pub. Co: Amsterdam,
New York, 1980.
JA0566634
(85) Newnham, R. E. Structure-Property Relations; Springer-Verlag: New
York, Heidelberg, Berlin, 1975.
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3676 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006