Synthesis and characterization of cobalt(II)-salicylate complexes derived from N4-donor ligands: Stabilization of a hexameric water cluster in the lattice host of a cobalt(III)-salicylate complex

Article abstract of DOI:10.1016/j.ica.2011.09.008

The syntheses and structural characterization of four cobalt(II)-salicylate complexes, [(TPA)Co^II(HSA)](ClO₄) (1), [(isoBPMEN)Co ^II(HSA)](BPh₄) (2), [(TPzA)Co^II(HSA)](ClO ₄) (3) and [(6Me

Full text of DOI:10.1016/j.ica.2011.09.008

Inorganica Chimica Acta 378 (2011) 231–238
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and characterization of cobalt(II)–salicylate complexes derived from
N4-donor ligands: Stabilization of a hexameric water cluster in the lattice host
of a cobalt(III)–salicylate complex
⇑
Biswarup Chakraborty, Tapan Kanti Paine
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
The syntheses and structural characterization of four cobalt(II)–salicylate complexes, [(TPA)CoII(HSA)]
(ClO4) (1), [(isoBPMEN)CoII(HSA)](BPh4) (2), [(TPzA)CoII(HSA)](ClO4) (3) and [(6Me3TPA)CoII(HSA)](BPh4)
(4) [TPA = tris(2-pyridylmethyl)amine, isoBPMEN = N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-
1,2-diamine, TPzA = tris((3,5-dimethyl-1H-pyrazole-1-yl)methyl)amine and 6Me3TPA = tris(6-methyl-2-
pyridylmethyl)amine] are described. While 2, 3 and 4 are unreactive towards dioxygen, 1 reacts slowly
with molecular oxygen to a cobalt(III)–salicylate complex, [(TPA)CoIII(SA)](ClO4) (1a). Two different crys-
talline forms, 1a and 1aꢀ4H2O were isolated depending upon the condition of oxidation and crystallization.
The solid-state structures of cobalt(III)–salicylate unit in both 1a and 1aꢀ4H2O show a six-coordinate dis-
torted octahedral coordination geometry at the cobalt(III) center ligated by the tetradentate ligand (TPA)
where the dianionic salicylate (SA) binds in a bidentate fashion through one carboxylate and one phenolate
oxygen. The hydrated form 1aꢀ4H2O reveals a hexameric water cluster formation in the inorganic lattice
host. The complex cation and the perchlorate counterion are involved in stabilizing the (H2O)6 cluster in
a rare ‘pentamer planar+1’ conformation. A one-dimensional water tape consisting of edge-shared water
hexamers is observed. The water tape represents a subunit of ice structure.
Article history:
Received 15 June 2011
Received in revised form 26 August 2011
Accepted 2 September 2011
Available online 8 September 2011
Keywords:
Cobalt complexes
Salicylate
Nitrogen donor ligands
X-ray crystal structures
Dioxygen reactivity
Water cluster
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
water [1]. A cyclic hexamer structure in liquid helium has been pro-
posed to be the building block of Ih structure of ice [77]. The hexa-
Water plays an important role in the structure and function of
biological macromolecules. The unusual properties and importance
of water in life have fueled the interest to understand the structure
of water in solid and liquid state [1–6]. The structures of different
hydrogen bonded networks in various environments help in under-
standing the structure and behavior of water [7]. In synthetic
chemistry, one approach is to stabilize water cluster in organic
and/or inorganic lattice host. The host molecules stabilize the guest
water cluster through hydrogen bonding interaction giving rise to
one-dimensional chains [8–13], tapes [13–17], two-dimensional
layers [13,18–26] and three-dimensional networks [27–30]. The
structure of water cluster depends on various non-covalent inter-
actions that the host molecule can render in a lattice. A large num-
ber of water clusters stabilized in various crystal lattice hosts have
been structurally characterized [23,31–76]. The crystalline Ih and Ic
structures of ice show chair and boat conformation of hexameric
meric water cluster has been proposed to be the smallest unit in
bulk water [1]. Water co-crystallized as a self-assembled cluster
with chair, boat and cyclic planar hexameric conformation in inor-
ganic lattice host has also been reported [47–49,61,65–67,78–85].
Very recently, it has been shown theoretically that 24 different
conformations are possible for hexameric water cluster [86].
However the structure of hexameric water cluster with high energy
conformations like pentamer planar+1, twisted boat, pentamer+1,
closed chair down, tetramer+2, triple trimer and trimer/trimer+1
are rare.
In this paper, we report the synthesis and structural character-
ization of four cobalt(II)–salicylate complexes of different tripodal
N4-donor ligands (Scheme 1). A hexameric water cluster is stabi-
lized in the lattice host of [(TPA)CoIII(SA)](ClO4) (1a) formed upon
oxidation of the corresponding cobalt(II)–salicylate complex,
[(TPA)CoII(HSA)](ClO4) (1) in the presence of dioxygen in moist sol-
vent. The water hexamer shows a rare ‘pentamer planar+1’ type
structure and forms an infinite one-dimensional water tape. The
controlled crystallization of hydrated cobalt(III)–salicylate com-
plex and the role of anion in stabilizing the hexameric water clus-
ter are described.
⇑
Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805.
E-mail address: ictkp@iacs.res.in (T.K. Paine).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.008

232
B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
(m), 1483 (s), 1442 (m), 1389 (s), 1253 (m), 1144 (m), 1107 (s),
1088 (s), 764 (m). ESI-MS (in positive ion mode, CH3CN):
m/z = 486.06 (100%, [(TPA)Co(HSA)]+). UV–Vis (in CH3CN):
465 nm (200 Mꢁ1 cmꢁ1), 605 nm (160 Mꢁ1 cmꢁ1), 622 nm
(140 Mꢁ1 cmꢁ1). leff (298 K): 3.96 BM.
2.2.2. [(isoBPMEN)CoII(HSA)](BPh4) (2)
Complex 2 was synthesized according to the procedure described
for 1 except that isoBPMEN ligand was used instead of TPA. The
resulting green solution was stirred for 2 h and then treated with
sodium tetraphenylborate (0.34 g, 1 mmol). After 1 h a green solid
was precipitated which was isolated by filtration and dried. X-ray
quality single crystals were obtained by vapor diffusion of
diethylether into a THF–MeOH solution of the complex. Yield:
0.60 g (76%). Anal. Calc. for C47H47BCoN4O3 (785.63 g/mol): C,
71.85; H, 6.03; N, 7.13. Found: C, 71.6; H, 5.9; N, 6.9%. IR (KBr,
cmꢁ1): 3408 (br), 3051 (m), 2988 (m), 2926 (m), 1605 (s), 1571
(m), 1481 (s), 1443 (m), 1391 (s), 1257 (m), 762 (s), 739 (m), 706
(m). ESI-MS (in positive ion mode, CH3CN): m/z = 465.80 (100%,
[(isoBPMEN)Co(HSA)]+). UV–Vis (in CH3CN): 472 nm (110 Mꢁ1
cmꢁ1), 601 nm (100 Mꢁ1 cmꢁ1).
leff (298 K): 4.21 BM.
Scheme 1. Ligands.
2.2.3. [(TPzA)CoII(HSA)](ClO4) (3)
2. Experimental
Complex
3 was synthesized according to the procedure
described for 1 except that TPzA ligand was used instead of TPA.
X-ray quality pink crystals were obtained by ether diffusion into
a THF–MeOH solution of the complex. Yield: 0.43 g (68%). Anal.
Calc. for C25H32ClCoN7O7 (636.95 g/mol): C, 47.14; H, 5.06; N,
15.39. Found: C, 47.3; H, 4.9; N, 15.2%. IR (KBr, cmꢁ1): 3437 (m),
2928 (m), 1626 (m), 1559 (s), 1483 (s), 1468 (s), 1404 (s), 1365
(s), 1254 (m), 1109 (s), 1092 (s), 768 (m). ESI-MS (in positive ion
mode, CH3CN): m/z = 537.09 (100%, [(TPzA)Co(HSA)]+). UV–Vis (in
CH3CN): 497 nm (150 Mꢁ1 cmꢁ1), 577 nm (164 Mꢁ1 cmꢁ1). leff
(298 K): 4.29 BM.
2.1. General considerations and physical methods
Commercial grade chemicals were used for the synthetic pur-
poses and solvents were distilled and dried before use. Although
no problem was encountered during the synthesis of the ligand
and the complex, perchlorate salts are potentially explosive and
should be handled with care [87]. The air sensitive complexes were
prepared and stored in an inert-atmosphere glove box. Ligands
were prepared according to the reported procedures [88,89].
Fourier transform infrared spectroscopy on KBr pellets was per-
formed on a Shimadzu FT-IR 8400S instrument. Elemental analyses
were performed on a Perkin–Elmer 2400 series II CHN analyzer.
Electro-spray ionization mass spectra were recorded with a Waters
QTOF Micro YA263. 1H NMR spectra were measured at room tem-
perature using Bruker DPX-300 spectrometer. Solution electronic
spectra were measured on an Agilent 8453 diode array spectropho-
tometer. Room temperature magnetic data were collected on a
Gouy balance (Sherwood Scientific, Cambridge, UK). Diamagnetic
contributions were estimated for each compound by using Pascal’s
constants. TGA measurements were carried out on a TA instru-
ments SDT Q 600 thermal analyzer. Powder XRD measurements
2.2.4. [(6Me3TPA)CoII(HSA)](BPh4) (4)
Complex 4 was synthesized according to the procedure de-
scribed for 2 except that 6Me3TPA ligand was used instead of
isoBPMEN. Single crystals were obtained from a solvent mixture
of dichloromethane and methanol. Yield: 0.55 g (65%). Anal. Calc.
for C52H49BCoN4O3 (847.71 g/mol): C, 73.68; H, 5.83; N, 6.61.
Found: C, 72.7; H, 5.6; N, 6.3%. IR (KBr, cmꢁ1): 3053–2926 (br),
1603 (s), 1580 (m), 1464 (s), 1440 (m), 1252 (m), 787–737 (m),
706 (m). ESI-MS (in positive ion mode, CH3CN): m/z = 528.24
(100%, [(6Me3TPA)Co(HSA)]+). UV–Vis (in CH3CN): 476 nm
(50 Mꢁ1 cmꢁ1), 515 nm (40 Mꢁ1 cmꢁ1), 557 nm (50 Mꢁ1 cmꢁ1). leff
(298 K): 4.60 BM.
were carried out on a Bruker D8 Advance Powder (CuKa1 radiation,
k = 1.5406 Å) instrument.
2.2. Syntheses
2.2.5. [(TPA)CoIII(SA)](ClO4) (1a)
Complex 1 (15 mg, 0.025 mmol) was dissolved in dry acetoni-
trile and dioxygen was bubbled through the solution. The solution
was kept under oxygen environment for 2 weeks whereupon the
green solution changed to purple. A deep purple crystalline com-
pound was obtained from the reaction solution upon slow evapo-
ration of the solvent. Single crystals suitable for X-ray diffraction
were obtained by vapor diffusion of diethylether into a THF–MeOH
solution of the purple solid. Yield: 13.9 mg (95%). Anal. Calc. for
2.2.1. [(TPA)CoII(HSA)](ClO4) (1)
Cobalt(II) perchlorate hexahydrate (0.37 g, 1 mmol) was added
to a methanolic (5 mL) solution of the ligand (0.29 g, 1 mmol).
The resulting orange solution turned to green immediately upon
addition of a mixture of salicylic acid (0.14 g, 1 mmol) and 1 eq.
of Et3N (140 lL, 1 mmol) in 1 mL methanol. The solution was al-
lowed to stir at room temperature for 3 h whereupon a green crys-
talline solid was precipitated. The solid was isolated by filtration
and dried. Single crystals suitable for X-ray diffraction were
obtained by vapor diffusion of diethyl ether into a THF–MeOH
solution of the complex. Yield: 0.34 g (58%). Anal. Calc. for
C
25H22ClCoN4O7 (584.85 g/mol): C, 51.34; H, 3.79; N, 9.58. Found:
C, 52.2; H, 3.9; N, 9.2%. IR (KBr, cmꢁ1): 3398 (br), 3110–2924
(m), 1606 (s), 1574 (m), 1468 (s), 1454 (s) 1359 (s), 1248 (m),
1142 (m), 1090 (s), 767 (m), 625 (m). ESI-MS (in positive ion mode,
CH3CN): m/z = 485.23 (100%, [(TPA)Co(SA)]+). UV–Vis (in CH3CN):
405 nm (330 Mꢁ1 cmꢁ1), 561 (275 Mꢁ1 cmꢁ1).
C
25H23ClCoN4O7 (585.86 g/mol): C, 51.25; H, 3.96; N, 9.56. Found:
C, 50.9; H, 3.8; N, 9.4%. IR (KBr, cmꢁ1): 3425 (m), 1610 (s), 1566

B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
233
2.2.6. [(TPA)CoIII(SA)](ClO4)ꢀ4H2O (1aꢀ4H2O)
Complex 1 (15 mg, 0.025 mmol) was dissolved in moist acetoni-
trile and the solution was saturated with dioxygen. The solution
was kept under air for 2 weeks whereupon the color of the solution
was changed from green to purple. A purple crystalline solid of
1aꢀ4H2O was isolated from the purple solution by slow evaporation
of solvent. Yield: 14.7 mg (89%). Anal. Calc. for C25H30ClCoN4O11
(656.91 g/mol): C, 45.71; H, 4.60; N, 8.53. Found: C, 45.8; H, 4.3;
N, 8.3%. IR (KBr, cmꢁ1): 3410 (br), 3074 (m), 2930 (m), 1605 (s),
1576 (m), 1462 (s), 1356 (s), 1248 (m), 1142 (m), 1090 (s), 770
(m), 629 (m).
Scheme 2. Syntheses of cobalt(II)–salicylate complexes.
2.3. X-ray crystallographic data collection and refinement of the
structures
The complexes were further characterized by X-ray single crys-
tal diffraction studies. The cobalt(II)–salicylate complexes 1, 2 and 3
show a five-coordinate distorted trigonal bipyramidal coordination
geometry at the metal center. The metal ion is coordinated by four
nitrogen donors from the ligand and one carboxylate oxygen (O1) of
monoanionic salicylate (Figs. 1 and 2a). The nitrogen donors (N1,
N3, N4) from ligand occupy the equatorial plane while the amine
nitrogen (N2) from ligand and the carboxylate oxygen (O1) of salic-
ylate occupy the axial positions. This arrangement of the ligand
forces the metal ion to adopt a distorted trigonal bipyramidal
coordination geometry. The average M–N bond distance (Å) in the
equatorial plane is shorter compared to that in the axial position
(Table 2). The noncoordinated phenolic oxygen (O3) forms a strong
intramolecular hydrogen bond with carboxylate oxygen (O2).
The solid-state structure of 4 reveals one asymmetric unit con-
taining one mononuclear complex cation and tetraphenylborate
counterion. The metal ion in the complex cation is coordinated
by four nitrogen donors from ligand and two carboxylate oxygen
atoms of monoanionic salicylate forming a distorted octahedral
coordination geometry (Fig. 2b). The Co–Npy bond distances in 4
are in the range of 2.129(3)–2.226(4) Å, longer than Co–Npy dis-
tances in 1, 2 and 3. Steric interactions among three methyl groups
on the ligand give rise to longer M–Npy distances in 4. However, the
Co–Namine distance at 2.112(3) Å in 4 is close comparable to that
observed in 1, 2 and 3 (Table 3). The phenolic OH group of the
Diffraction data were collected at 120(2) K for 1, 1a, 1aꢀ4H2O, 2
and 3 and at 298(2) K for 4 on a Bruker Smart APEX II (Mo Ka radi-
ation, k = 0.71073 Å). Crystallographic data are summarized in Ta-
ble 1. Cell refinement, indexing and scaling of the data set were
carried out using the APEX2 v2.1-0 software [90]. The structures
were solved by direct methods and subsequent Fourier analyses
and refined by the full-matrix least-squares method based on F2
with all observed reflections [91]. One solvent molecule crystal-
lizes with the asymmetric unit of complex 1a but due to structural
disorder that could not be modeled well and squeezed it out.
3. Results and discussion
The cobalt(II)–salicylate complexes (1–4) were isolated either
as perchlorate or tetraphenylborate salt by reacting the ligands
with equimolar amounts of cobalt(II) perchlorate, salicylic acid
(H2SA) and triethylamine in methanol (Scheme 2). All the cobalt(II)
complexes were characterized by different spectroscopic and ana-
lytical tools (see Section 2). All the cobalt(II)–salicylate complexes
show d–d transition in the visible region of the optical spectra and
exhibit room temperature magnetic moment value between 3.96
and 4.60 BM indicative of high-spin cobalt(II) species.
Table 1
Crystallographic information for 1, 2, 3, 4, 1a and 1aꢀ4H2O.
1
2
3
4
1a
1aꢀ4H2O
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
25H23ClCoN4O7
C47H47BCoN4O3
785.63
C25H32ClCoN7O7
636.96
monoclinic
P2(1)/c
17.3907(16)
12.3850(12)
13.9826(13)
90.00
104.180(2)
90.00
2919.9(5)
4
1.449
0.734
1324
C53H51BCl2CoN4O3
932.62
monoclinic
P2(1)/c
17.152(4)
9.653(2)
29.646(7)
90.00
98.233(5)
90.00
4858(2)
4
1.275
0.510
1948
C25H22ClCoN4O7
584.85
C25H30ClCoN4O11
656.91
monoclinic
P2(1)/c
17.7339(16)
8.3365(8)
22.5448(17)
90.00
123.177(5)
90.00
2789.7(4)
4
1.564
0.779
1360
585.85
monoclinic
P2(1)/c
18.9021(19)
9.4210(10)
14.4765(14)
90.00
93.816(3)
90.00
2572.2(5)
4
1.513
0.824
1204
triclinic
triclinic
ꢀ
ꢀ
P1
P1
10.160(3)
10.584(3)
19.416(5)
76.312(5)
77.442(5)
79.518(5)
1961.3(8)
2
1.330
0.486
826
1.10–25.00
18095
8.3643(9)
11.8532(12)
14.8614(14)
69.416(2)
84.133(3)
73.622(3)
1323.4(2)
2
1.468
0.800
600
1.46–25.00
12493
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
l
MoK
F(000)
h range (°)
a
(mmꢁ1
)
2.16–24.53
22613
1.21–25.00
26778
1.20–24.39
41630
1.37–25.31
25909
Reflections collected
Reflections unique
4273 (0.0392)
6883 (0.0622)
5150 (0.0309)
7810 (0.0477)
4639 (0.0274)
5060 (0.0446)
(Rint
)
Data [I > 2
r
(I)]
3285
338
1.268
5883
508
1.034
4548
377
1.221
5865
581
1.441
3837
343
0.976
4087
387
1.013
Parameters refined
Goodness-of-fit (GOF)
on F2
[I > 2
r
(I)]
R1 = 0.0545,
wR2 = 0.1688
ꢁ0.522, 1.243
R1 = 0.0526,
wR2 = 0.1496
ꢁ1.513, 1.132
R1 = 0.0396,
wR2 = 0.1394
ꢁ0.428, 1.447
R1 = 0.0682,
wR2 = 0.2041
ꢁ0.690, 1.352
R1 = 0.0486,
wR2 = 0.1441
ꢁ0.572, 0.807
R1 = 0.0452,
wR2 = 0.1098
ꢁ0.553, 1.095
Residuals (e Åꢁ3
)

234
B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
Fig. 1. Crystal structures of the cationic part of 1 (a) and 2 (b). All H atoms except that attached to O3 are omitted for clarity.
Fig. 2. Molecular structures of the cationic part of 3 (a) and 4 (b). All H atoms except that attached to O3 are omitted for clarity.
salicylate is in intramolecular hydrogen bonding interaction with
the metal bound carboxylate oxygen O1 at a distance of 2.587 Å.
The asymmetric binding of carboxylate group in 4 is comparable
to that in iron(II)–benzoate complex of the same ligand [92].
The cobalt(II)–salicylate complexes, 2, 3 and 4 are unreactive
towards dioxygen, while complex 1 slowly reacts with aerial oxy-
gen. The reaction was monitored by optical spectroscopy. The opti-
cal spectrum of 1 in acetonitrile exhibits bands at 465, 605 and
622 nm due to d–d transition. The d–d bands slowly vanish upon
reaction of the cobalt–salicylate complex with dioxygen and a
new charge transfer transition band appears at 560 nm. The latter
band is attributable to the phenolate-to-cobalt(III) charge-transfer
transition. The reaction of 1 with dioxygen is slow and takes place
over a period of 2 weeks (Fig. 3). However, the reaction takes about
4 days when the same is carried out in the presence of a base, tri-
ethylamine. The ESI-MS of the purple oxidized solution shows peak
at m/z = 485.23 with expected isotope distribution pattern calcu-
lated for [(TPA)Co(SA)]+ ion. The formation of a mononuclear
Table 2
Selected bond lengths (Å) and bond angles (°) for complexes 1, 2 and 3.
Distance (Å)/angle (°)
1
2
3
Co(1)–O(1)
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
1.970(3)
2.048(3)
2.189(3)
2.070(4)
1.9712(17)
2.073(2)
2.217(2)
2.100(2)
2.070(2)
110.33(8)
98.69(8)
170.08(7)
93.46(8)
77.87(8)
112.17(8)
116.19(8)
82.73(8)
77.59(8)
121.67(8)
1.9766(18)
2.059(2)
2.327(2)
2.053(2)
2.050(2)
105.61(8)
104.40(8)
176.76(7)
101.01(8)
75.42(7)
125.27(8)
111.97(8)
77.16(7)
75.79(7)
105.73(8)
Co(1)–N(4)
2.076(3)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(3)
O(1)–Co(1)–N(2)
O(1)–Co(1)–N(4)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(4)
N(2)–Co(1)–N(3)
N(2)–Co(1)–N(4)
N(3)–Co(1)–N(4)
111.29(14)
100.91(13)
170.59(13)
95.33(14)
77.43(12)
115.35(14)
117.66(13)
77.56(13)
76.99(12)
112.95(14)

B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
235
Table 3
Selected bond lengths (Å) and bond angles (°) for complexes 4, 1a and 1aꢀ4H2O.
Distance (Å)/angle (°)
4
1a
1aꢀ4H2O
Co(1)–O(1)
2.246(3)
1.882(2)
1.889(2)
Co(1)–O(2)
2.068(3)
1.848(2)
1.860(2)
Co(1)–N(1)
2.226(4)
1.918(3)
1.922(3)
Co(1)–N(2)
2.112(3)
1.953(3)
1.957(2)
Co(1)–N(3)
2.159(4)
1.930(3)
1.926(3)
Co(1)–N(4)
2.129(3)
1.929(3)
1.925(3)
O(1)–Co(1)–O(2)
N(1)–Co(1)–O(2)
O(1)–Co(1)–N(1)
O(2)–Co(1)–N(3)
O(1)–Co(1)–N(3)
N(1)–Co(1)–N(3)
O(2)–Co(1)–N(4)
O(1)–Co(1)–N(4)
N(1)–Co(1)–N(4)
N(3)–Co(1)–N(4)
O(2)–Co(1)–N(2)
O(1)–Co(1)–N(2)
N(1)–Co(1)–N(2)
N(3)–Co(1)–N(2)
N(4)–Co(1)–N(2)
60.78(11)
101.87(13)
88.75(12)
96.43(12)
93.18(13)
157.35(13)
115.12(12)
171.74(12)
85.13(12)
99.16(13)
162.36(12)
102.04(12)
80.41(13)
78.15(13)
82.43(12)
95.93(10)
96.32(11)
89.39(11)
94.89(12)
88.09(11)
168.71(11)
89.15(10)
174.39(10)
92.42(11)
89.11(11)
175.19(10)
88.84(10)
84.33(11)
84.62(11)
86.06(11)
96.12(9)
95.45(11)
89.74(10)
94.24(11)
87.97(10)
170.22(11)
89.56(10)
174.18(10)
90.98(11)
90.37(11)
175.98(10)
87.70(10)
85.78(11)
84.63(10)
86.59(10)
Fig. 4. Structure of the cationic complex 1aꢀ4H2O. The perchlorate anion and H
atoms are omitted for clarity.
donor O1 form the equatorial plane with the average Co1–Npy dis-
tances of 1.93 Å and Co1–O1 distance of 1.889(2) Å (Table 3). The
amine nitrogen (N2) of the ligand and the phenolate oxygen (O2)
of salicylate occupy the axial position with the Co1–N2 and Co1–
O2 distances of 1.957(2) and 1.860(2) Å, respectively. The oxida-
tion of cobalt(II) to cobalt(III) leads to a rearrangement in the bind-
ing of salicylate anion. In the cobalt(II)–salicylate complex (1), the
carboxylate oxygen O1 binds to the metal center opposite to the
amine nitrogen of the ligand. Upon oxidation, the carboxylate oxy-
gen occupies a position trans to a pyridine donor while the pheno-
late oxygen O2 binds to the metal center opposite to the amine
nitrogen.
The carboxylate oxygen of monoanionic salicylate in 1 forms an
intramolecular hydrogen bonding with the phenolic OH group. On
the other hand, salicylate in 1a (and in 1aꢀ4H2O) binds to the metal
center in a bidentate fashion leaving one carboxylate oxygen atom
for intermolecular hydrogen bonding interaction with guest sol-
vent molecules. In the hydrated complex, 1aꢀ4H2O, one of the
water molecules (O8) is in direct hydrogen bonding interaction
with the cationic complex as well as with other three water mole-
cules. The hydrogen boding motif associated with water molecules
forms (H2O)6 clusters in the crystal lattice (Fig. 5).
Fig. 3. Optical spectra of 1 and 1a (conc. = 0.1 mM) in acetonitrile.
low-spin cobalt(III)–salicylate complex, [(TPA)CoIII(SA)](ClO4) (1a)
was further confirmed from the diamagnetic 1H NMR spectrum.
Two different crystalline forms of the cobalt(III)–salicylate com-
plex, 1a and 1aꢀ4H2O were isolated depending upon the condition
of oxidation reaction and subsequent crystallization. Oxidation of 1
in dry acetonitrile or dry THF/MeOH leads to 1a. Single crystals of
1a were grown in dry THF/MeOH-diethyl ether mixture and crys-
ꢀ
tallize in triclinic system with space group P1. On the other hand,
single crystals of 1aꢀ4H2O, crystallized in a monoclinic unit cell
with space group P21/c, were isolated from a moist acetonitrile
solution of 1a. Crystalline 1aꢀ4H2O could also be isolated from a
reaction mixture containing equimolar amounts of TPA, cobalt(II)
salt, salicylic acid and triethylamine, under air in moist acetoni-
trile. The solid state structures of 1a and its hydrated form
(1aꢀ4H2O) both reveal a similar complex cation with similar bond
parameters within their estimated standard deviations (Fig. 4).
The complex cation comprises a mononuclear distorted octahedral
cobalt coordinated by four nitrogen donors of the N4 ligand and
two oxygen donors of dianionic salicylate. The salicylate group
binds to the metal center in a bidentate fashion through one car-
boxylate oxygen atom (O1) and one phenolate oxygen atom (O2).
The pyridine nitrogens (N1, N3 and N4) of the ligand and oxygen
Fig. 5. Hexameric water cluster with ‘pentamer planar+1’ conformation.

236
B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
Table 4
Hydrogen bonding parameters of 1aꢀ4H2O.
Dꢀ ꢀ ꢀHꢀ ꢀ ꢀA
Symmetry
Dꢀ ꢀ ꢀH (Å)
Hꢀ ꢀ ꢀA (Å)
Dꢀ ꢀ ꢀA (Å)
Dꢀ ꢀ ꢀHꢀ ꢀ ꢀA (°)
O(8)ꢀ ꢀ ꢀH(8A)ꢀ ꢀ ꢀO(3)
O(8)ꢀ ꢀ ꢀH(8B)ꢀ ꢀ ꢀO(10)
O(9)ꢀ ꢀ ꢀH(9B)ꢀ ꢀ ꢀO(8)
O(10)ꢀ ꢀ ꢀH(10A)ꢀ ꢀ ꢀO(8)
O(10)ꢀ ꢀ ꢀH(10B)ꢀ ꢀ ꢀO(11)
O(11)ꢀ ꢀ ꢀH(11B)ꢀ ꢀ ꢀO(9)
1 ꢁ x, 1 ꢁ y, 2 ꢁ z
x, y, z
x, ꢁ1 + y, z
1ꢁx, ꢁ1/2 + y, 3/2 ꢁ z
x, y, z
0.80
0.98
1.03
1.00
0.85
1.14(11)
1.90
1.75
1.71
1.81
1.91
1.69(11)
2.695(3)
2.733(4)
2.736(5)
2.801(4)
2.750(5)
2.766(5)
170
176
173
175
167
155(10)
x, y, z
Fig. 6. Propagation of hexameric water clusters in 1aꢀ4H2O as one-dimensional tape along the crystallographic b axis.
Fig. 7. TGA patterns of (a) 1aꢀ4H2O and (b) 1a.
Six water molecules in the cyclic repeating unit form strong
It is clear from the structural motif of 1aꢀ4H2O that the complex
cation and perchlorate ions are involved in stabilizing the water
cluster (Fig. 6). The salicylate oxygen (O3) strongly interacts with
hydrogen bonding with the Oꢀ ꢀ ꢀO distances in the range of
0
0
2.733–2.801 ÅA (Table 4). The average Oꢀ ꢀ ꢀO distance (2.76 ÅA) is very
close to that found in the Ih structure of ice [1]. The average
Oꢀ ꢀ ꢀOꢀ ꢀ ꢀO angle of 116.8° deviates considerably from tetrahedral
angle in ice Ih form. Among the six water molecules, oxygen atoms
of five waters are in the same plane whether the rest one is out of
the plane and makes an open envelop like ‘pentamer planar+1’
conformation with the dihedral angle of 36.8° (Fig. 5). Hexameric
water clusters in complex lattice hosts with boat or chair like con-
formation have been reported in the literature. However, the water
cluster in this case is one of the rare forms of ‘pentamer planar+1’
type conformation [86].
the water molecule (O8) of the cluster through H-bonding with a
0
distance of 2.695 ÅA. On the other side, one oxygen atom of the per-
chlorate ion interacts with another water molecule (O11) of the
cluster via a strong H-bonding interaction. Five water molecules,
either act as two hydrogen bond acceptor or single hydrogen bond
donor as well as acceptor, form a planar pentamer structure. The
sixth water molecule interacting via two donors with two lone
pairs of neighboring water molecules leads to tetrahedral angles
and deviates from the pentameric plane [93]. This arrangement
of water molecules gives rise to hexameric water clusters that

B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
237
a one dimensional edge-shared water tape and represent a subunit
of ice structure.
Acknowledgments
We thank the Department of Science and Technology (DST),
Government of India for the financial support. Crystal structure
determination was performed at the DST-funded National Single
Crystal Diffractometer Facility at the Department of Inorganic
Chemistry, IACS. B.C. acknowledges the Council of Scientific and
Industrial Research (CSIR), India for a Fellowship.
Appendix A. Supplementary material
Fig. 8. XRPD patterns of 1, 1aꢀ4H2O and 1a.
CCDC 818490, 818491, 818492, 818493, 818494, and 818495
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2011.09.008.
grow along the crystallographic b axis and form a one-dimensional
tape (Fig. 6). In the tape each hexamer shares two water molecules
(O8 and O10) and the overall cluster may be represented by T6(2)
according to water cluster notation. T6(2) signifies a tape consist-
ing of six-membered water rings involved in one-dimensional
propagation along a particular direction where two water mole-
cules are shared between the adjacent rings [94]. The edge-shared
one-dimensional water tape observed in the small water clusters is
considered to be a simplified model of ice [95,96].
Thermogravimetric analysis of 1aꢀ4H2O in the temperature
range 30–200 °C reveals a weight loss of 7.95%, close to a value
for three water molecules (Fig. 7a). The difference between exper-
imental and calculated values may be attributed to the release of
some lattice water molecules below 30 °C. Thermogravimetric
analysis of 1a also shows a step-wise weight loss of about 10% in
the temperature range 30–200 °C that correspond to the loss of
disordered solvent molecules (Fig. 7b).
The purity of bulk crystalline compounds was checked by com-
paring the experimental XRPD pattern with that of simulated pat-
tern obtained from the single-crystal data (Fig. 8). The peaks for 1
and 1aꢀ4H2O do match well with the corresponding simulated pat-
terns suggesting the purity of bulk crystalline compounds. It is
important to mention here that the XRPD patterns of the sample
after heating 1aꢀ4H2O up to 200 °C do not change much, suggesting
that the lattice structure of the host remains same after removal of
water molecules. On the other hand, the experimental XRPD data
of 1a do not match well with the simulated patterns. The experi-
mental data for 1a, however, match with the simulated pattern
of 1aꢀ4H2O. This may be attributed to the fact that the crystals of
1a lose the disordered solvent molecules present in the crystal lat-
tice resulting in the formation of polycrystalline form with a very
similar lattice structure as in 1aꢀ4H2O.
References
[1] R. Ludwig, Angew. Chem., Int. Ed. 40 (2001) 1808.
[2] D. Eisenberg, W. Kauzmann, The Structure and Properties of Water, Oxford
University Press, Oxford, 1969.
[3] K. Mitsuoka, K. Murata, T. Walz, T. Hirai, P. Agre, J.B. Heymann, A. Engel, Y.J.
Fujiyoshi, J. Struct. Biol. 128 (1999) 34.
[4] E. Westhoff, Water and Biological Macromolecules, CRC Press, Boca Raton, FL,
1993.
[5] E. Tajkhorshid, P. Nollert, M.Ø. Jensen, L.J.W. Miercke, J. O’Connell, R.M. Stroud,
K. Schulten, Science 296 (2002) 525.
[6] M. Matsumoto, S. Saito, I. Ohmine, Nature 416 (2002) 409.
[7] S. Pal, N.B. Sankaran, A. Samanta, Angew. Chem., Int. Ed. 42 (2003) 1741.
[8] A. Mukherjee, M.K. Saha, M. Nethaji, A.R. Chakravarty, Chem. Commun. (2004)
716.
[9] Z. Fei, D. Zhao, T.J. Geldbach, R. Scopelliti, P.J. Dyson, S. Antonijevic, G.
Bodenhausen, Angew. Chem., Int. Ed. 44 (2005) 5720.
[10] B.K. Saha, A. Nangia, Chem. Commun. (2005) 3024.
[11] Y. Cui, M.L. Cao, L.F. Yang, Y.L. Niu, B.H. Ye, CrystEngComm 10 (2008) 1288.
[12] R. Natarajan, J.P.H.A. Charmant, G. Orpen, A.P. Davis, Angew. Chem., Int. Ed. 49
(2010) 5125.
[13] D. Sun, H.-R. Xu, C.-F. Yang, Z.-H. Wei, N. Zhang, R.-B. Huang, L.-S. Zheng, Cryst.
Growth Des. 10 (2010) 4642.
[14] B.Q. Ma, H.L. Sun, S. Gao, Chem. Commun. (2005) 2336.
[15] M. Tadokoro, S. Fukui, T. Kitajima, Y. Nagao, S. Ishimaru, H. Kitagawa, K. Isobee,
K. Nakasuji, Chem. Commun. (2006) 1274.
[16] R. Custelcean, C. Afloroaei, M. Vlassa, M. Polverejan, Angew. Chem., Int. Ed. 39
(2000) 3094.
[17] X.F. Shi, H.B. Song, L. He, W.Q. Zhang, CrystEngComm 11 (2009) 542.
[18] C. Janiak, T.G. Scharamann, S.A. Mason, J. Am. Chem. Soc. 124 (2002) 14010.
[19] N.S. Oxtoby, A.J. Blake, N.R. Champness, C. Wilson, Chem. Eur. J. 11 (2005)
4643.
[20] Z.F. Fei, T.J. Geldbach, D.B. Zhao, R. Scopelliti, P.J. Dyson, Inorg. Chem. 44 (2005)
5200.
[21] P. Rodríguez-Cuamatzi, G. Vargas-Díaz, H.A. Höpfl, Angew. Chem., Int. Ed. 43
(2004) 3041.
[22] J.P. Zhang, X.C. Huang, Y.Y. Lin, X.M. Chen, Inorg. Chem. 44 (2005) 3146.
[23] X.J. Luan, Y.C. Chu, Y.Y. Wang, D.S. Li, P. Liu, Q.Z. Shi, Cryst. Growth Des. 6
(2006) 812.
[24] P.S. Lakshminarayanan, E.I. Suresh, P. Ghosh, J. Am. Chem. Soc. 127 (2005)
13132.
4. Conclusion
[25] A.H. Yang, H. Zhang, H.L. Gao, W.Q. Zhang, L. He, J.Z. Cui, Cryst. Growth Des. 8
(2008) 3354.
[26] Y.-P. Wu, F. Fu, D.-S. Li, Z.-H. Yang, K. Zou, Y.-Y. Wang, Inorg. Chem. Commun.
11 (2008) 621.
[27] R. Carballo, B. Covelo, C. Lodeiro, E.M. Vázquez-López, CrystEngComm 7 (2005)
294.
[28] R. Carballo, B. Covelo, F.-H.N.E. García-Martínez, A.B. Lago, M. Vázquez, E.M.
Vázquez-López, Cryst. Growth Des. 6 (2006) 629.
[29] C.H. Li, K.L. Huang, J.M. Dou, Y.N. Chi, Y.Q. Xu, L. Shen, D.Q. Wang, C.W. Hu,
Cryst. Growth Des. 8 (2008) 3141.
[30] C.-M. Jin, Z. Zhu, Z.-F. Chen, Y.-J. Hu, X.-G. Meng, Cryst. Growth Des. 10 (2010)
2054.
[31] S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 6887.
[32] S.K. Ghosh, J. Ribas, M.S.E. Fallah, P.K. Bharadwaj, Inorg. Chem. 44 (2005) 3856.
[33] R.L. Sang, L. Xu, CrystEngComm 12 (2010) 1377.
[34] K. Raghuraman, K.K. Katti, L.J. Barbour, N. Pillarsetty, C.L. Barnes, K.V. Katti, J.
Am. Chem. Soc. 125 (2003) 6955.
We have synthesized and structurally characterized four
mononuclear cobalt(II)–salicylate complexes of different tripodal
N4-donor ligands. The cobalt(II)–salicylate complex of tris(2-pyri-
dylmethyl)amine (TPA) ligand reacts with aerial oxygen to form
the corresponding six-coordinate cobalt(III)–salicylate complex
(1a) whereas other three complexes are unreactive towards dioxy-
gen. Two different crystalline forms (hydrated and non-hydrated)
of the cobalt(III)–salicylate complex were isolated depending upon
the reaction/crystallization condition. In the lattice of hydrated
crystalline form 1aꢀ4H2O, a hexameric water cluster is stabilized.
The formation of the water cluster could be controlled during the
crystallization process. The water hexamers in the crystal lattice
adopt a rare ‘pentamer planar+1’ type conformation resulting in

238
B. Chakraborty, T.K. Paine / Inorganica Chimica Acta 378 (2011) 231–238
[35] Y. Bi, W. Liao, H. Zhang, D. Li, CrystEngComm 11 (2009) 1213.
[36] B.-Q. Ma, H.-L. Sun, S. Gao, Angew. Chem., Int. Ed. 43 (2004) 1374.
[37] S.K. Ghosh, P.K. Bharadwaj, Angew. Chem., Int. Ed. 43 (2004) 3577.
[38] X. Wang, H. Lin, B. Mu, A. Tian, G. Liu, Dalton Trans, 39 (2010) 6187.
[39] S. Neogi, G. Savitha, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 3771.
[40] H.H. Song, B.Q. Ma, CrystEngComm 9 (2007) 625.
[41] L.J. Barbour, G.W. Orr, J.L. Atwood, Nature 393 (1998) 671.
[42] M. Yoshizawa, T. Kusukawa, M. Kawano, T. Ohhara, I. Tanaka, K. Kurihara, N.
Niimura, M. Fujita, J. Am. Chem. Soc. 127 (2005) 2798.
[65] X.-M. Zhang, R.-Q. Fang, H.-S. Wu, Cryst. Growth Des. 5 (2005) 1335.
[66] Y.-C. Liao, Y.-C. Jiang, S.-L. Wang, J. Am. Chem. Soc. 127 (2005) 12794.
[67] U. Mukhopadhyay, I. Bernal, Cryst. Growth Des. 5 (2005) 1687.
[68] S.K. Ghosh, P.K. Bharadwaj, Eur. J. Inorg. Chem. (2005) 4886.
[69] S. Das, P.K. Bharadwaj, Cryst. Growth Des. 6 (2006) 187.
[70] T.K. Prasad, M.V. Rajasekharan, Cryst. Growth Des. 6 (2006) 488.
[71] R.J. Doedens, E. Yohannes, M.I. Khan, Chem. Commun. (2002) 62.
[72] W.B. Blanton, S.W. Gordon-Wylie, G.R. Clark, K.D. Jordan, J.T. Wood, U. Geiser,
T.J. Collins, J. Am. Chem. Soc. 1221 (1999) 3551.
[43] L.J. Barbour, G.W. Orr, J.L. Atwood, Chem. Commun. (2000) 859.
[44] J.L. Atwood, L.J. Barbour, T.J. Ness, C.J. Raston, P.L. Raston, J. Am. Chem. Soc. 123
(2001) 7192.
[45] S. Khatua, J. Kang, J.O. Huh, C.S. Hong, D.G. Churchill, Cryst. Growth Des. 10
(2010) 327.
[73] R.D. Bergougnant, A.Y. Robin, K.M. Fromm, Cryst. Growth Des. 5 (2005) 1691.
[74] G.-G. Luo, H.-B. Xiong, J.-C. Dai, Cryst. Growth Des. 11 (2011) 507.
[75] C. Massera, M. Melegari, F. Ugozzoli, E. Dalcanale, Chem. Commun. 46 (2010)
88.
[76] S. Supriya, S.K. Das, J. Cluster Sci. 14 (2003) 337.
[46] C. Maxim, L. Sorace, P. Khuntia, A.M. Madalan, V. Kravtsov, A. Lascialfari, A.
Caneschi, Y. Journaux, M. Andruh, Dalton Trans. 39 (2010) 4838.
[47] J. Wang, L.-L. Zheng, C.-J. Li, Y.-Z. Zheng, M.-L. Tong, Cryst. Growth Des. 6
(2006) 357.
[77] K. Nauta, R.E. Miller, Science 287 (2000) 293.
[78] B.-H. Ye, B.-B. Ding, Y.-Q. Weng, X.-M. Chen, Inorg. Chem. 43 (2004) 6866.
[79] R. Custecean, C. Afloroaei, M. Vlassa, M. Polverejan, Angew. Chem., Int. Ed. 39
(2000) 3094.
[48] S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 42 (2003) 8250.
[49] R. Luna-García, B.M. Damián-Murillo, V. Barba, H. Höpfl, H.I. Beltrán, L.S.
Zamudio-Rivera, Chem. Commun. (2005) 5527.
[50] L.S. Long, Y.R. Wu, R.B. Huang, L.S. Zheng, Inorg. Chem. 43 (2004) 3798.
[51] M. Zuhayra, W.U. Kampen, E. Henze, Z. Soti, L. Zsolnai, G. Huttner, F.A.
Oberdorfer, J. Am. Chem. Soc. 128 (2006) 424.
[52] M.H. Mir, J.J. Vittal, Angew. Chem., Int. Ed. 46 (2007) 5925.
[53] B.Q. Ma, H.L. Sun, S. Gao, Chem. Commun. (2004) 2220.
[54] M.B. Day, K.N. Kirschner, G.C. Shields, J. Phys. Chem. A 109 (2005) 6773.
[55] L.R. MacGillivray, J.L. Atwood, J. Am. Chem. Soc. 119 (1997) 2592.
[56] S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 44 (2005) 5553.
[57] Q.H. Pan, J.Y. Li, K.E. Christensen, C. Bonneau, X.Y. Ren, L. Shi, J.L. Sun, X.D. Zou,
G.H. Li, J.H. Yu, R.R. Xu, Angew. Chem., Int. Ed. 47 (2008) 7868.
[58] M.C. Das, P.K. Bharadwaj, Eur. J. Inorg. Chem. (2007) 1229.
[59] Y.S. Xie, J. Ni, F.K. Zheng, Y. Cui, Q.G. Wang, S.W. Ng, W.H. Zhu, Cryst. Growth
Des. 9 (2009) 118.
[80] J.N. Moorthy, R. Natarajan, P. Venugopalan, Angew. Chem., Int. Ed. 41 (2002)
3417.
[81] Y.-P. Ren, L.-S. Long, B.-W. Mao, Y.-Z. Yuan, R.-B. Huang, L.-S. Zheng, Angew.
Chem., Int. Ed. 42 (2003) 532.
[82] S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 5180.
[83] M.N. Kopylovich, E.A. Tronova, M. Haukka, A.M. Kirillov, V.Y. Kukushkin, J.J.R.
Fraústo da Silva, A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2007) 4621.
[84] A. Michaelides, S. Skoulika, E.G. Bakalbassis, J. Mrozinski, Cryst. Growth Des. 3
(2003) 487.
[85] B.-Q. Ma, H.-L. Sun, S. Gao, Eur. J. Inorg. Chem. (2005) 3902.
[86] G. Hincapié, N. Acelas, M. Castaño, J. David, A. Restrepo, J. Phys. Chem. A 114
(2010) 7809.
[87] W.C. Wolsey, J. Chem. Educ. 50 (1973) A335.
[88] G.J.P. Britovsek, J. England, A.J.P. White, Inorg. Chem. 44 (2005) 8125.
[89] I. Bertini, G. Canti, C. Luchinat, F. Mani, Inorg. Chem. 20 (1981) 1670.
[90] APEX 2v2.1-0, Bruker AXS, Madison, WI, 2006.
[60] P.S. Lakshminarayanan, E. Suresh, P. Ghosh, Angew. Chem., Int. Ed. 45 (2006)
3807.
[91] G.M. Sheldrick, SHELX97 Programs for Crystal Structure Analysis (Release 97-2),
University of Göttingen, Germany, 1998.
[61] H. Xu, J.-L. Mi, Z.-J. Hu, X.-K. Guo, Q. Chen, Acta Crystallogr., Sect. C 66 (2010)
o151.
[92] Y. Zang, T.E. Elgren, Y. Dong, L. Que Jr., J. Am. Chem. Soc. 115 (1993)
811.
[62] W.Z. Xu, J.L. Sun, Z.T. Huang, Q.Y. Zheng, Chem. Commun. (2009) 171.
[63] F.N. Dai, H.Y. He, D.F. Sun, J. Am. Chem. Soc. 130 (2008) 14064.
[64] A. Hazra, P. Kanoo, S. Mohapatra, G. Mostafa, T.K. Maji, CrystEngComm 12
(2010) 2775.
[93] R. Ludwig, ChemPhysChem 1 (2000) 53.
[94] M. Mascal, L. Infantes, J. Chisholm, Angew. Chem., Int. Ed. 45 (2006) 32.
[95] L. Infantes, S. Motherwell, CrystEngComm 4 (2002) 454.
[96] L. Infantes, J. Chisholm, S. Motherwell, CrystEngComm 5 (2003) 480.