Supramolecular coordination polymers of silver(I) with 2-isocyanopyridine or 1,2-phenylenediisocyanide

Article abstract of DOI:10.1016/j.ica.2010.02.007

The silver(I) complexes [Ag{C₅H₄N(NC)}]_n(BF₄) _n (1), [Ag{C₅H₄N(NC)}₂]_n (BF₄)_n (2), [Ag{C₆H₄(NC)₂}]_n (BF₄)_n (3), and [Ag{C₆H₄(NC)₂}₂]_n (BF₄)_n (4) have been synthesized using different Ag:L ratios of 2-isocyanopyridine (or 2-pyridylisocyanide, CNPy-2) or 1,2-phenylenediisocyanide ligands. The polymeric complex 2 has been characterized by X-ray diffraction revealing a polymeric chain structure. Breaking the polymeric structure of [Ag{C₆H₄(NC)₂}]_n (BF₄)_n (3) with acetonitrile, the dimeric complex [Ag{(CN)₂C₆H₄)}(NCMe)₂] ₂(BF₄)₂ (5) is formed, which has been also characterized by X-ray diffraction.

Full text of DOI:10.1016/j.ica.2010.02.007

Inorganica Chimica Acta 363 (2010) 1864–1868
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Supramolecular coordination polymers of silver(I) with 2-isocyanopyridine
or 1,2-phenylenediisocyanide
a
b
Camino Bartolomé a, Pablo Espinet a, , Jose M. Martín-Alvarez , Katerina Soulantica ,
*
Jonathan P.H. Charmant c
a IU CINQUIMA/Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, E-47071 Valladolid, Spain
b Université de Toulouse; INSA, UPS; LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France; CNRS; LPCNO, F-31077 Toulouse, France
c School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1T, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
The silver(I) complexes [Ag{C5H4N(NC)}]n(BF4)n (1), [Ag{C5H4N(NC)}2]n(BF4)n (2), [Ag{C6H4(NC)2}]n(BF4)n
(3), and [Ag{C6H4(NC)2}2]n(BF4)n (4) have been synthesized using different Ag:L ratios of 2-isocyanopyri-
dine (or 2-pyridylisocyanide, CNPy-2) or 1,2-phenylenediisocyanide ligands. The polymeric complex 2
has been characterized by X-ray diffraction revealing a polymeric chain structure. Breaking the polymeric
structure of [Ag{C6H4(NC)2}]n(BF4)n (3) with acetonitrile, the dimeric complex [Ag{(CN)2C6H4)}(NC-
Me)2]2(BF4)2 (5) is formed, which has been also characterized by X-ray diffraction.
Received 20 November 2009
Received in revised form 28 January 2010
Accepted 8 February 2010
Available online 12 February 2010
Keywords:
Silver
Ó 2010 Elsevier B.V. All rights reserved.
Helical coordination polymers
Diisocyanides
2-Isocyanopyridines
1. Introduction
derivatives have been reported [7]. By contrast, isocyanides, which
are known to bind more strongly to many transition metals, have
Supramolecular polymer chemistry is an area of current inter-
est, focusing on the formation of polymers by the connection of
molecular monomers [1,2]. The engineering of this kind of poly-
mers is based on the concepts of supramolecular interactions and
supramolecular synthons [3]. Thus, a supramolecular polymer is
generated by self-assembly of complementary monomeric com-
pounds through reversible intermolecular interactions (e.g. labile
coordination bonds and hydrogen bonds). The reversible bonds
connecting the building blocks allow for the polymer to rearrange
and adopt the supramolecular self-assembled structure. Reversible
formation of bonds of moderate energy often allows for solubiliza-
tion of the polymers in coordinating solvents and then crystalliza-
tion of the resulting polymers for single-crystal X-ray diffraction
structure investigation. In other cases, complexes derived from
fragmentation of the original polymer are obtained [4].
A very fruitful area of coordination polymers is based on sil-
ver(I) metal centers connected in simple geometric dispositions
to bridging ligands with nitrogen donors [5]. The most common
N-donor fragments are the pyridyl ligand, with bond energy of
about 47.0 kJ molÀ1 to silver(I) [6], and the much weaker bonding
cyano group. Very recently polymers or oligomers derived from
bis-bidentate imino-pyridine ligands and their reduced N-methyl
been little used. Silver polymers based on 1,8-diisocyano-p-men-
thane [8], similar to those produced with the weakly bonding but
structurally similar bis or tris(nitrile) ligands, have been reported
[9]. Mixed donors 1,3- and 1,4-cyanoisocyanoarenes have been
used to obtain Pd/Cu mixed complexes [10] and Ag polymers
[11]. However, 2-isocyanopyridine or 1,2-phenylenediisocyanide
(Fig. 1) has not been used.
Both ligands, defining identical bonding directions at 60° but
with different structural constraints, show some other differences.
The 2-isocyanopyridine ligand has two coordinating functions with
different bond strengths, and can be expected to act either as
bidentate or as monodentate with only the isocyanide group coor-
dinated to the metal. This selective behavior cannot be expected
from 1,2-phenylenediisocyanide. On the other hand, the 1,2-phen-
ylenediisocyanide is a symmetric ligand, whereas the 2-isocyano-
pyridine is less symmetric, imposing two different M-ring
* Corresponding author.
E-mail address: espinet@qi.uva.es (P. Espinet).
Fig. 1. Ligands used as building blocks in the synthesis of the silver complexes.
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.02.007

C. Bartolomé et al. / Inorganica Chimica Acta 363 (2010) 1864–1868
1865
Fig. 2. Possible trimeric structures for linear coordination geometry of the metal
center. The structure at the right has been found for M = trans-Pt(C6F5)2.
lengths. Possible structural consequences of this are illustrated in
(Fig. 2) for the event of the formation of metallatriangles from me-
tal centers imposing linear coordination, as reported for
[Pt(C6F5)2{l-C6H4(NC)2}]3 [12], where two trans positions of the
square-planar Pt(II) have been blocked with C6F5 before the diiso-
cyanide is coordinated.
The coordination sphere of Ag(I) is far more flexible than that of
Pt(II) and can adopt coordination numbers between two and six
and various coordination modes from linear through trigonal to
tetrahedral, trigonal pyramidal or even octahedral. This structural
flexibility could give rise to other structural outcomes, including
coordination polymers. The aim of this work is to study the behav-
ior of the structurally related 2-isocyanopyridine (CNPy-2) and 1,2-
phenylenediisocyanide ligands in the construction of silver-based
polymers.
2. Results and discussion
Fig. 3. Above: molecular structure of [Ag{C5H4N(NC)}2]n(BF4)n (2) showing 30%
thermal ellipsoids. Below: cationic infinite chain of 2 (anions omitted for clarity).
Bond lengths (Å): Ag(1)–C(2), 2.125(6); Ag(1)–C(1), 2.137(6); Ag(1)–N(4), 2.465(4);
Ag(1)–N(3), 2.482(4). Bond angles (°): N(4)–Ag(1)-N(3), 87.68(14); C(2)–Ag(1)–
C(1), 129.4(2); C(2)–Ag(1)–N(4), 110.82(18); C(1)–Ag(1)–N(4), 106.0(2); C(2)–
Ag(1)–N(3), 106.56(19); C(1)–Ag(1)–N(3), 108.40(18). Symmetry transformations
used to generate equivalent atoms: (A) Ày + 1, x, z + 1 4; (B) y, Àx + 1, z À 1/4.
The 1:1 reaction of 2-isocyanopyridine and AgBF4 in acetone
leads to the formation of [Ag{C5H4N(NC)}]n(BF4)n (1), whereas the
treatment of AgBF4 with 2-isocyanopyridine (1:2 molar ratio) in
acetone affords [Ag{C5H4N(NC)}2]n(BF4)n (2). Similar results are ob-
tained with 1,2-phenylene diisocyanide and AgBF4 affording
[Ag{C6H4(NC)2}]n(BF4)n (3) and [Ag{C6H4(NC)2}2]n(BF4)n (4), respec-
tively (Scheme 1).
and each silver atom coordinates with two pyridyl nitrogen atoms
and two isocyanide carbon atoms, giving rise to a four-coordinate
silver(I) complex with distorted tetrahedral coordination. The
average plane described by each pair of 2-isocyanopyridine ligands
making a double-bridge is tilted by 86.7° with respect to the previ-
ous or the next double-bridge plane in the polymeric chain.
The Ag–C distances found in 2 do not differ significantly from
those found in other silver isocyanide complexes in the Cambridge
Data Base. The Ag–N distances are longer than those found for the
In the four complexes the C, H, and N analyses are consistent
with the proposed empirical formula. These, as well as relevant
IR data for all the complexes, are given in the experimental part.
All the complexes show typical IR absorptions of coordinated iso-
cyanide at higher wavenumbers (ca. 100 cmÀ1) than the free li-
gands [13].
All the complexes are poorly soluble in most organic solvents,
which can be a consequence of the plausible polymeric structure
of the complexes. Attempts to obtain crystals suitable for X-ray
diffraction the polymers, 1–4, from extremely diluted solutions
of the complexes in acetone, were unsuccessful, except for
[Ag{C5H4N(NC)}2]n(BF4)n (2). Views of the asymmetric unit of 2
and its extended polymeric structure are given in Fig. 3.
The polymeric structure of the cation of 2 consists of helical
[Ag{C5H4N(NC)}2]n chains. Each ligand is bridging two silver atoms,
silver coordination polymers [Ag2(l-2-cyanopyridine)2(NO3)2]n
and [Ag( -2-cyanopyridine)2(BF4)2]n [14,15], in which the silver
l
atoms occupy a distorted trigonal-planar environment and the an-
ions support the polymer chain.
Fig. 4 shows the space-fill representation of a fragment of the
infinite chain, and a view of the chain along the helix axis. All
Scheme 1. Synthesis of Ag(I) polymers based on 2-isocyanopyridine or 1,2-phenylene diisocyanide.

1866
C. Bartolomé et al. / Inorganica Chimica Acta 363 (2010) 1864–1868
compatible with linear coordination for silver, which should lead
to a zig-zag polymeric structure for both complexes [18]. This
hypothesis is consistent with the result found in the X-ray diffrac-
tion study of [Ag{(CN)2C6H4)}(NCMe)2]2(BF4)2 (5), the crystals of
which were obtained by the addition of a mixture of toluene and
n-hexane to a solution of 3 in acetonitrile and cooling for several
days. The molecular structure shows a rearrangement of the coor-
dination mode of the diisocyanide (see below) that necessarily in-
volves the splitting by acetonitrile of the initial silver polymer
(Scheme 2). This is supported by the observation, in the infrared
spectrum of an acetonitrile solution of 3, of a m(C„N) at a fre-
quency corresponding to the uncoordinated isocyanide
(2128 cmÀ1), which would correspond to intermediates on the
way from 3 to 5.
The structure of the dication of 5 determined by X-ray diffrac-
tion shows a dimer with each silver(I) center in a distorted tetrahe-
dral environment involving two bridging 1,2-phenylene
diisocyanide ligands and two acetonitriles (Fig. 5). The Ag–C dis-
tances are similar to those found for silver bis(isocyanide) com-
plexes in the Cambridge Crystallographic Data Base and in
complex (2). The main angular distortions from tetrahedral, associ-
ated to the C–Ag–C angles, are close to 130°.
Although dimer 5 derives from polymer 3 (both have an Ag:diis-
ocyanide = 1:1 ratio), the structure found is a good model for 4
(Ag:diisocyanide = 1:2 ratio), which can be generated by replacing
the four acetonitrile ligands by two bridging diisocyanides. This
would produce a polymeric structure entirely similar to that found
for 2.
Fig. 4. Above: View of [Ag{C5H4N(NC)}2]n(BF4)n (2) running along the helical axis (in
the
c
direction). Below: Space-filling representation of the helix in
[Ag{C5H4N(NC)}2]n(BF4)n (2) with the tetrafluoroborate anion omitted for clarity.
In conclusion, the two bidentate ligands behave rather similarly
toward silver(I) and give either single-bridged or double-bridged
polymers, depending on the Ag:ligand ratio. These structures can
be split in coordinating solvents.
the chains in the crystal structure have the same helicity, so the
compound crystallizes as a racemic conglomerate of enantiomeri-
cally pure crystals [16]. The structure of 2 is one of the various
examples of metallohelices where a 1D silver coordination poly-
mer spontaneously assumes a helical conformation [17].
3. Experimental
Although the four silver polymeric complexes 1–4 are poorly
soluble in most organic solvents, all of them are soluble in acetoni-
trile. This gain in solubility in a coordinating solvent suggests that
the polymeric structures of the silver complexes are being broken
due to the coordination of NCMe to the metal center. Unfortu-
nately, all the attempts to obtain suitable crystals from solutions
of the rest of the complexes in MeCN were unsuccessful. Due to
lack of experimental data, the polymeric structures of 1 and 3
can only be speculative. However, the stoichiometry seems only
3.1. General remarks
The reactions of AgBF4 with 1,2-phenylenediisocyanide or 2-
isocyanopyridine were carried out under nitrogen, using standard
Schlenk-type flasks and without exposure to light. Workup proce-
dures were done in air. All solvents were dried and purified accord-
ing to standard methods. The ligand 2-isocyanopyridine was
obtained according to the reported procedure [19]. 1,2-Phenylene-
Scheme 2. Synthesis of 5 upon crystallization of a solution of 3 in acetonitrile.

C. Bartolomé et al. / Inorganica Chimica Acta 363 (2010) 1864–1868
1867
Calc. for C12H8N4AgBF4: C, 35.77; H, 2.00; N, 13.91. Found: C, 36.11;
H, 2.05; N, 13.50%.
3.2.3. [Ag{C6H4(NC)2}]n(BF4)n (3)
A solution of o-(CN)2C6H4 (0.0623 g, 0.49 mmol) in acetone
(10 mL) was added to a solution of AgBF4 (0.0946 g, 0,49 mmol)
in acetone (10 mL) at room temperature and a pale orange solid
was formed. The volatiles were pumped off and the pale brown
residue was washed with n-hexane (3 Â 5 mL) yielding 0.126 g
(80%). IR: 2204, 2175
m
(C„N). 1H NMR (300 MHz, Me2CO-d6,
298 K): d 8.12 (m, 2H, C6H4), 7.93 (m, 2H, C6H4). Anal. Calc. for
C8H4N2AgBF4: C, 29.77; H, 1.25; N, 8.68. Found: C, 29.38; H, 1.24;
N, 8.35%. Suitable crystals of [Ag{(CN)2C6H4)}(NCMe)2]2(BF4)2 (5)
were grown by slow diffusion of a concentrated acetonitrile solu-
tion of the complex into n-hexane and toluene at À20 °C.
3.2.4. [Ag{C6H4(NC)2}2]n(BF4)n (4)
A solution of o-(CN)2C6H4 (0.1240 g, 0.97 mmol) in acetone
(10 mL) was added to a solution of AgBF4 (0.0942 g, 0.484 mmol)
in acetone (10 mL) at room temperature and a brown solid was
formed. The volatiles were pumped off and the pale brown residue
was washed with n-hexane (3 Â 5 mL) yielding 0.176 g (81%). IR:
À
Fig. 5. Molecular structure [Ag{(CN)2C6H4)}(NCMe)2]2(BF4)2 (5) (BF4 anions omit-
ted for clarity) showing 30% thermal ellipsoids. Bond lengths (Å): Ag(1)–C(5),
2.133(3); Ag(1)–C(12), 2.148(3); Ag(1)–N(1), 2.365(3); Ag(1)–N(2), 2.399(3). Bond
angles (°): N(1)–Ag(1)–N(2), 91.81(9); C(5)–Ag(1)–C(12), 129.57(10); C(5)–Ag(1)–
N(1), 107.28(10); C(12)–Ag(1)–N(1), 109.00(10); C(5)–Ag(1)–N(2), 115.42(10);
C(12)–Ag(1)–N(2), 97.24(10); C(1)–N(1)–Ag(1), 163.4(3); C(3)–N(2)–Ag(1),
159.0(3).
2174 m
(C„N). 1H NMR (300 MHz, Me2CO-d6, 298 K): d 7.85 (m,
2H, C6H4), 7.75 (m, 2H, C6H4). Anal. Calc. for C16H8N4AgBF4: C,
42.62; H, 1.79; N, 12.42. Found: C, 42.81; H, 1.83; N, 11.94%.
4. X-ray crystallography
diisocyanide was obtained by dehydrating the corresponding
diformamide [20] with triphosgene [21]. Infrared spectra were re-
corded in a Perkin–Elmer 883 apparatus as Nujol mulls between
polyethylene films. NMR spectra were recorded on a Bruker AC-
300 or ARX-300 instrument. NMR spectra are referred to TMS. Ele-
Suitable single crystals of 2 were obtained from an acetone/n-
hexane solution. Crystals of 5 were obtained by adding a mixture
of toluene and n-hexane to a solution of 3 in acetonitrile. Crystals
were mounted on glass fibers, and diffraction measurements were
made using a Bruker SMART CCD area-detector diffractometer with
mental analyses were performed on
microanalyzer.
a Perkin–Elmer 2400B
Mo K
from several series of exposures, each exposure covering 0.3° in
, the total data set being almost a sphere for compound 5 and al-
a radiation (k = 0.71073 Å) [22]. Intensities were integrated
x
3.2. Synthesis
most a hemisphere for compound 2 [23]. Absorption corrections
were applied based on multiple and symmetry-equivalent mea-
surements [24]. The structures were solved by direct methods
The silver polymers prepared are sensitive to light under long
exposure, but can be stored indefinitely in the dark at low temper-
ature (253 K, in the freezer).
Table 1
Crystal Data and Structure Refinement for 2 and 5.
3.2.1. [Ag{C5H4N(NC)}]n(BF4)n (1)
2
5
2-CNPy (13.6 mL, 1.9 mmol, 0.144 M solution in acetone) was
added to a solution of AgBF4 (0.371 g, 1.9 mmol) in acetone
(30 mL). After stirring for 30 min at room temperature the volatiles
were pumped off and the pale brown residue was washed with n-
hexane (3 Â 5 mL) and vacuum dried yielding 0.153 g (78%). IR:
Empirical formula
Formula weight
Temperature (K)
Wavelength (k) (Å)
Crystal system
Space group
a (Å)
C12H8AgBF4N4
402.90
298(2)
0.71073
tetragonal
P4(1)
8.3597(12)
8.3597(12)
21.041(4)
90.00
1470.4(4)
4
1.820
1.413
784
C24H20Ag2B2F8N8
809.8
293(2)
0.71073
monoclinic
P21/c
5.918(1)
18.401(3)
14.052(3)
91.97(2)
1529.2(5)
2
1.759
1.359
792
0.3 Â 0.2 Â 0.1
1.82–27.49
9692
3505 (0.0236)
SADABS
2220, 2196 m
(C„N). 1H NMR (300 MHz, Me2CO-d6, 298 K): d 8.70
(dm, 1H, CNC5H4N), 8.25 (m, 1H, CNC5H4N), 7.98 (d, J = 8 Hz, 1H,
CNC5H4N), 7.82 (m, 1H, CNC5H4N). Anal. Calc. for C6H4N2AgBF4: C,
24.10; H, 1.35; N, 9.37. Found: C, 23.74; H, 1.67; N, 8.98%.
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dcalc (g cmÀ3
)
3.2.2. [Ag{C5H4N(NC)}2]n(BF4)n (2)
Absorption coefficient (mmÀ1
F(0 0 0)
)
A solution of 2-CNPy (0.9 mL, 0.13 mmol, 0.144 M in acetone) in
20 mL of acetone was added to a solution of AgBF4 (0.0126 g,
0.065 mmol) in acetone (20 mL). The orange solution was cooled
to À20 °C. The orange crystals thus formed were decanted, washed
with cold acetone, and dried, yielding 0.012 g (46%). Suitable crys-
tals of [Ag{C5H4N(NC)}2]n(BF4)n were grown by slow diffusion of a
diluted acetone solution of the complex into n-hexane at À20 °C. IR
Crystal size (mm)
Theta range for data collection (°)
Reflections collected
Independent reflections (Rint
0.22 Â 0.08 Â 0.07
2.44–25.36
8211
2700 (0.0262)
SADABS
)
Absorption correction
Data/restraints/parameters
Flack parameter
2700/1/200
À0.05(4)
1.064
0.0334
0.0928
3505/0/229
–
1.088
0.0313
0.0678
Goodness-of-fit (GOF) on F2
acetone: 2188 m
(C„N). 1H NMR (300 MHz, Me2CO-d6, 298 K): d
R1 [I > 2r(I)]
8.66 (d, J = 4 Hz, 1H, CNC5H4N), 8.18 (m, 1H, CNC5H4N), 7.89 (d,
J = 8.5 Hz, 1H, CNC5H4N), 7.76 (dd, J = 4 and 8, 1H, CNC5H4N). Anal.
wR2 (all data)

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C. Bartolomé et al. / Inorganica Chimica Acta 363 (2010) 1864–1868
and refined by least squares on weighted |F|2 values for all reflec-
tions (see Table 1) [25]. All non-hydrogen atoms were assigned
anisotropic displacement parameters and refined without posi-
tional constraints. All the hydrogen atoms were calculated with a
riding model. Complex neutral-atom scattering factors were used
[26].
[9] (a) D. Venkataraman, G.B. Gardner, S. Lee, J.S. Moore, J. Am. Chem. Soc. 117
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(b) B. Gardner, Y.-H. Kiang, S. Lee, A. Asgaonkar, D. Venkataraman, J. Am. Chem.
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(c) W. Choe, Y.-H. Kiang, Z. Xu, S. Lee, Chem. Mater. 11 (1999) 1776.
[10] A. Mayr, L.-F. Mao, Inorg. Chem. 37 (1998) 5776.
[11] M.-X. Li, K.-K. Cheung, A. Mayr, J. Solid State Chem. 152 (2000) 247.
[12] P. Espinet, K. Soulantica, J.P.H. Charmant, A.G. Orpen, Chem. Commun. (2000)
915.
[13] See for instance: (a) R. Bayón, S. Coco, P. Espinet, C. Fernández-Mayordomo,
J.M. Martin-Alvarez, Inorg. Chem. 36 (1997) 2329;
Acknowledgments
(b) R. Bayón, S. Coco, P. Espinet, Chem. Eur. J. 11 (2005) 1079;
(c) S. Coco, C. Cordovilla, C. Domínguez, P. Espinet, Dalton Trans. (2008) 6894.
[14] P. Lin, R.A. Henderson, R.W. Harrington, W. Clegg, C.-D. Wu, X.-T. Wu, Inorg.
Chem. 43 (2004) 181.
[15] A.J. Blake, N.R. Champness, J.E.B. Nicolson, C. Wilson, Acta Crystallogr., Sect. C
57 (2001) 1290.
This work was supported by the Spanish Comisión Interminis-
terial de Ciencia y Tecnología [Projects CTQ2008-03954/BQU, and
INTECAT Consolider Ingenio 2010 (CSD2006-0003)], and the Junta
de Castilla y León (Project VA012A08 and GR169).
[16] H.D. Flack, G. Bernardinelli, Acta Crystallogr., Sect. A 55 (1999) 908.
[17] For single silver helical polymers, see for example: (a) S.P. Argent, H. Adams, T.
Riis-Johannessen, J.C. Jeffery, L.P. Harding, W. Clegg, R.W. Harrington, M.D.
Ward, Dalton Trans. (2006) 4996;
Appendix A. Supplementary material
(b) D. Sun, R. Cao, Y. Sun, W. Bi, X. Li, M. Hong, Y. Zhao, Eur. J. Inorg. Chem.
(2003) 38;
(c) A. Erxleben, Inorg. Chem. 40 (2001) 2928;
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Chem. 34 (1995) 530;
(e) B. Wu, W.-J. Zhang, S.-Y. Yu, X.-T. Wu, J. Chem. Soc., Dalton Trans. (1997)
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(f) M.-L. Tong, X.-M. Chen, B.-H. Ye, S.W. Ng, Inorg. Chem. 37 (1998) 5278;
(g) P.K. Bowyer, K.A. Porter, A.D. Rae, A.C. Willis, S.B. Wild, Chem. Commun.
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Chem. 39 (2000) 3301.
CCDC 713334 and 713335 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.02.007.
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