Co-ordination behaviour of a novel bisthiourea tripodal ligand: Structural, spectroscopic and electrochemical properties of a series of transition metal complexes

Article abstract of DOI:10.1039/c2dt11732k

The synthesis of a thiourea substituted derivative of tris(pyridyl-2- methyl)amine (TPA) is reported. Two of the three pyridine rings are substituted in the 6-position with benzoylthiourea groups. These thiourea groups undergo intramolecular hydrogen bonding to form six-membered rings which leaves one N-H group available to form hydrogen bonds with other molecules. This reports details how the complexation of this new ligand with transition metal ions yields complexes with differing geometries. Seven co-ordinate Mn(ii) and Cd(ii), six co-ordinate Ni(ii) and five co-ordinate Co(ii), Cu(ii) and Zn(ii) complexes have been isolated.

Full text of DOI:10.1039/c2dt11732k

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PAPER
Co-ordination behaviour of a novel bisthiourea tripodal ligand: structural,
spectroscopic and electrochemical properties of a series of transition metal
complexes†
Fawaz A. Saad, Niklaas J. Buurma, Angelo J. Amoroso,* James C. Knight and Benson M. Kariuki
Received 13th September 2011, Accepted 16th January 2012
DOI: 10.1039/c2dt11732k
The synthesis of a thiourea substituted derivative of tris(pyridyl-2-methyl)amine (TPA) is reported. Two
of the three pyridine rings are substituted in the 6-position with benzoylthiourea groups. These thiourea
groups undergo intramolecular hydrogen bonding to form six-membered rings which leaves one N–H
group available to form hydrogen bonds with other molecules. This reports details how the complexation
of this new ligand with transition metal ions yields complexes with differing geometries. Seven
co-ordinate Mn(II) and Cd(II), six co-ordinate Ni(II) and five co-ordinate Co(II), Cu(II) and Zn(II)
complexes have been isolated.
and the alkylated derivatives of these ligands8 as well as the ana-
Introduction
logous tertiary butyl amides (tppa, bppa and mppa).9 In the tri-
There is much interest in the design of new tripodal ligands
bearing additional functional groups or donor moieties. Such
ligands can modify the properties of the co-ordinated metal1 in
terms of their reactivity or even in terms of the metal geometry
they lead to. In addition, such ligands may co-ordinate a metal
ion in such a way as to form a site for anionic species.2 The
design of molecules which have specificity in their reactivity
remains a significant challenge and numerous approaches have
been undertaken. The products have possible application in
analytical assays by coupling to a sensing moiety3 or via dye dis-
placement4 or the chemical activation of guests upon binding
leading to catalytic behaviour.1
We are interested in developing the chemistry of tripodal tris
pyridine ligands5 and investigating the co-ordination chemistry
of the resulting ligands. One framework of interest is the tris
(pyridyl-2-methyl)amine (TPA) framework.6 Typically the
ligand has been shown to have a strong preference for five co-
ordinate trigonal bipyramidal geometries with the three pyridyl
donors taking equatorial positions and the axial donors being the
bridge-head nitrogen donor and one additional monodentate
ligand. The 6-position of the three pyridine rings in TPA may be
systematically substituted with a variety of derivatives. Such
derivatives have been extensively studied, with much attention
being given to the tris(6-amino-2-pyridylmethyl) amine (TAPA),
its bis and mono functionalised analogues (BAPA and MAPA)7
gonal bipyramidal metal complexes of these ligands, it was
shown that strong H-bonding can exist between the NH groups
in the 6 position and a monodentate guest ligand co-ordinating
to the vacant axial position of the metal. In fact, even in octa-
hedral Fe(III) complexes, similar hydrogen bonding has still been
observed.10 This hydrogen bonding has been found to greatly
modify the stability of the guest–host complex and also modify
the reactivity of the guest.7b,8,11 Also, it may modify the redox
behaviour of the metal centre.12 In addition, this ligand frame-
work has been substituted in the 6-position with guanidinium
groups and used as a host molecule for phosphate ions13 and it
has been capped by a calix-6-arene which allowed the complexes
to host anions, such as hydroxide, as well as neutral molecules.14
While the urea and thiourea group has previously been shown
to be very effective in hydrogen bonding to anions,15 this modifi-
cation of TPA has not been reported. Here we report an investi-
gation into the TPA ligand difunctionalised with benzoylthiourea
groups for a number of reasons. The new ligand could, for
example, lead to chemical behaviour different from previous
BAPA based structures and may be more like Anslyn’s guanidi-
nium substituted TPA framework,13 with the H bonding groups
being further removed from the metal centre. In addition,
although the molecule contains the thiourea moiety, each
benzoyl thiourea can cyclise through intramolecular hydrogen
bonding, leaving only one NH available for intermolecular H
bonding, but with the sulphur atoms potentially available for co-
ordination to the metal centre (Fig. 1). Thus we have conducted
an investigation into the basic co-ordination chemistry of the
ligand with a variety of metal ions. Of particular interest is the
variation of the binding modes with the covalent radii of the
metal centre along with the stereochemical preferences of the
different d electron configurations.
Cardiff University of Wales - Department of Chemistry, Main Building,
Park Place, Cardiff, CF10 3AT, United Kingdom. E-mail:
amorosoaj@cf.ac.uk; Fax: 02920874030; Tel: 02920874077
†Electronic supplementary information (ESI) available. CCDC reference
numbers 844367, 844368 and 844370–844373. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt11732k
4608 | Dalton Trans., 2012, 41, 4608–4617
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figures were drawn with ORTEP-3.0 for Windows (version
2.02).20 Unfortunately, only poor quality crystals could be
obtained for [MnL1(ClO4)][ClO4]·2H2O [1], despite numerous
attempts to grow better crystals. The resultant weak data did not
allow full anisotropic refinement leading to the lower precision
in the determined parameters.
Bis(6-benzoylthiourea-2-pyridylmethyl)(2-pyridylmethyl)
amine L1
Fig. 1 The thiourea derivative L1 (left) with potentially a larger guest
space than the BAPA ligand (right). In the case of L1, not only is the
cavity larger but the guest is more remote to the metal centre.
BAPA (5.453 g, 17 mmol) was dissolved in EtOH (50 mL) and
benzoyl isothiocyanate (4.58 mL, 34 mmol) was added drop-
wise. The mixture was heated up to 40 °C for 10 minutes then
allowed to cool to room temperature. The solvent was removed
under reduced pressure. Ether was used to precipitate the product
as a white powder (7.272 g, 66%). Recrystallisation from hot
acetonitrile yielded crystals as white plates. 1H-NMR δH
(400 MHz; CDCl3): 3.90 (2H, s); 3.97 (4H, s); 7.11 (2H, t, Ar, J
= 5.8 Hz); 7.41–7.70 (11H, m, Ar); 7.80 (2H, d, Ar, J = 7.6 Hz);
7.82 (2H, d, Ar, J = 7.6 Hz); 8.54 (1H, d, Ar, J = 4.1 Hz); 8.64
(2H, d, Ar, J = 7.9 Hz); 8.93 (2H, s, NH); 13.00 (2H, s, NH);
13C δC (62.5 MHz; CDCl3): 59.4, 60.2, 60.3 114.1, 120.7,
122.1, 123.2, 127.5, 129.2, 131.7, 133.7, 136.6, 138.2, 149.0,
150.5, 158.4, 166.3, 176.7; accurate ESMS (m/z): 645.1859
(100), 646.1887 (30) [L] and [L–H]+ [calculated 645.1855]. IR
KBr/cm−1: ν = 3318br, 3049br, 1673s, 1594s, 1454s, 1331s,
756s; UV/Vis [λmax, nm (εM, M−1 cm−1)] in CH3CN: 266
(50 300), 309 (28 600).
Experimental
General
NMR spectra were measured on a Bruker AM 400 MHz
FT-NMR spectrometer. Residual signals of solvent were used for
1
reference for H and 13C NMR. For infrared spectra, each com-
pound was pressed into a disk with an excess of dried KBr and
measured on a Jasco 660 FT-IR spectrophotometer. Electrospray
(ES) and high-resolution (HR) mass spectra were measured on a
Waters LCT Premier XE (oa-TOF) mass spectrometer. UV/Vis
absorption spectra were run in HPLC grade acetonitrile (Fisher)
and measured on a Jasco V-570 spectrophotometer from 280 to
1100 nm (optical path length 1.0 cm). Elemental analyses were
carried out by the Warwick Analytical Service, University of
Warwick.
General procedure for the synthesis of metal complexes
The cyclic voltammetry experiments were carried out with an
AUTOLAB PGSTAT12 potentiostat in conjunction with General
Purpose Electrochemical System software (GPES version 4.7 for
Windows) in a specially designed three-electrode glass cell with
a Teflon-coated cell cap. A bioanalytical platinum working elec-
trode (model no. MF2013) with a 1.6 mm disk was used for all
experiments. The counter electrode was a platinum wire and the
reference electrode an Ag–AgNO3 electrode. A 0.1 M [Bu4N]
[PF6] solution in CH3CN was used as supporting electrolyte. In
all cases, ferrocene was used as an internal reference. Solutions
were degassed with nitrogen and a nitrogen atmosphere was
maintained over the solution during the experiment. Bis(6-
amino-2-pyridylmethyl)(2-pyridylmethyl)amine (BAPA), was
prepared as reported by Harata et al.9,16
Ligand L1 (1 equivalent, typically 0.077 mmol) was dissolved in
the minimum amount of acetonitrile or acetone (typically 3 mL).
The solutions were warmed to ca. 60 °C to ensure that the ligand
fully dissolved. To this stirred solution, the metal perchlorate salt
(1 equivalent) dissolved in acetonitrile (ca. 2 mL) was added
dropwise. Crystallisation of the compounds typically involved
the diffusion of diethyl ether into acetonitrile solutions. The
yields from these reactions were moderate to high (44–94%).
The crystalline material was then subsequently used for all spec-
troscopic measurements.
Warning: Perchlorate salts of metal complexes are potentially
explosive. Care should be taken while handling such complexes.
All single crystal X-ray data was collected on a Bruker/Nonius
Kappa CCD diffractometer using graphite monochromated Mo-
Kα radiation (λ = 0.71073 Å), equipped with an Oxford Cryo-
stream cooling apparatus. The data was corrected for Lorentz
and polarisation effects and for absorption using SORTAV.17
Structure solution was achieved by direct methods (Sir-92
program system18) and refined by full-matrix least-squares on F2
(SHELXL-9719) with all non hydrogen atoms generally being
assigned anisotropic displacement parameters. Hydrogen atoms
attached to carbon atoms were placed in idealised positions and
allowed to ride on the relevant carbon atom. In the final cycles
of refinement, a weighting scheme that gave a relatively flat
analysis of variance was introduced and refinement continued
until convergence was reached. Molecular structures in the
[MnC34H30N8O2S2]·[ClO4]2 (1). Yellow block crystals (44%
yield); accurate ESMS (m/z): 709.1100 (100); [[Mn(L1)–H]+ cal-
culated as 700.1235]; IR KBr/cm−1: ν = 3424br, 1602s, 1541m,
1458s, 1260s, 1086s, 706s, 621s; UV/Vis [λmax, nm (εM, M−1
cm−1)] in CH3CN: 256 (17 300), 314 (10 600).
[CoC34H30N8O2S2]·[ClO4]2 (2). Brown-green needle crystals
(58% yield); accurate ESMS (m/z): 704.1196 (40), [Co(L1)–H]+,
[calculated 704.1186]; CoC34H30N8O2S2(ClO4)2 (found: C,
45.08; H, 3.47; N, 12.38. requires C, 45.18; H, 3.34; N,
12.40%); IR KBr/cm−1: ν = 3437br, 1613s, 3069m, 1551m,
1439s, 1258s, 726s, 1091s, 622s; UV/Vis [λmax, nm (εM, M−1
cm−1)] in CH3CN: 280 (33 200), 351 (7400), 400 (3100), 507
(80), 568 (55).
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[NiC34H30N8O2S2]·[ClO4]2 (3). Brown needle crystals (94%
yield); accurate ESMS (m/z): 703.1216 (100), [Ni(L1)–H]+; [cal-
using chloroform as a solvent, it was found that conducting the
reaction in ethanol gave the ligand in good yield (66%).
All complexes were readily formed by the addition of an
acetonitrile solution of the metal salt to an acetonitrile or acetone
solution of the ligand. As the ligand is not very soluble in these
solvents, the ligand solution was warmed to ensure the complete
solubilisation of the ligand. Upon addition, a colour change
occurs upon complex formation, but no precipitation of product
occurred. The complex may be isolated from solution, in crystal-
line form, by the solution diffusion of diethyl ether into the
acetonitrile solution.
The spectroscopic and spectrometric data is consistent with
the proposed structures for all complexes with one notable
exception. The mass spectrum of the Mn(II) complex has an
expected mass of 701.7 (801.1 including a ClO4 group),
however a mass of 709 was consistently observed. While we
have observed the conversion of thiourea to urea groups in
related Mn(II) complexes, no plausible fragmentation and/or urea
formation can explain the observed mass spectrum.
culated
703.1208];
NiC34H30N8O2S2(CH3CN)(H2O)(CH3-
COCH3)(ClO4)2 (found: C, 45.85; H, 4.12; N, 12.35. requires C,
45.92; H, 4.05; N, 12.36%); IR KBr/cm−1: ν = 3432br, 1610s,
3069m, 1566s, 1456s, 1262s, 709s, 1090s, 621s; UV/Vis [λmax
,
nm (εM, M−1 cm−1)] in CH3CN: 323 (15 800), 385 (1500), 549
(17), 849 (15), 940 (16).
[CuC34H30N8O2S2]·[ClO4]2 (4). Green block crystals (88%
yield); accurate ESMS (m/z): 708.1176 (100), [Cu(L1)–H]+ [cal-
culated 708.1151]; CuC34H30N8O2S2(ClO4)2 (found: C, 44.75;
H, 3.37; N, 12.34. requires C, 44.98; H, 3.33; N, 12.35%); IR
KBr/cm−1: ν = 3449br, 1609s, 3069m, 1546s, 1490s, 1260s,
726s, 1090s, 623s; UV/Vis [λmax, nm (εM, M−1 cm−1)] in
CH3CN: 315 (17 000), 623 (130), 793 (96).
[ZnC34H30N8O2S2]·[ClO4]2 (5). Colourless rectangle crystals
1
(85% yield); H-NMR (400 MHz; CD3CN): 4.35 (2H, s); 4.36
(4H, s); 7.58–7.80 (12H, m, Ar); 8.04 (2H, d, Ar, J = 7.9 Hz);
8.04 (2H, d, Ar, J = 7.9 Hz); 8.19 (1H, t, Ar, J = 7.6 Hz); 8.23
(2H, t, Ar, J = 8.4 Hz); 8.63 (1H, d, Ar, J = 8.7 Hz); 10.26 (2H,
s, NH); 13.58 (2H, s, NH); 13C NMR: (62.5 MHz; CD3CN):
56.1, 56.1, 122.3, 123.8, 125.0, 125.6, 128.7, 129.0, 130.9,
134.5, 141.9, 143.8, 148.2, 150.5, 154.3, 154.4, 169.4, 179.4;
accurate ESMS (m/z): 709.1176 (100) [Zn(L1)–H]+ [calculated
709.1146]; ZnC34H30N8O2S2(ClO4)2(CH3CN) (found: C, 45.41;
H, 3.45; N, 13.30. requires C, 45.51; H, 3.50; N, 13.27%); IR
KBr/cm−1: ν = 3394br, 1612s, 3069m, 1556s, 1442s, 1257s,
726s, 1092s, 622s; UV/Vis [λmax, nm (εM, M−1 cm−1)] in
CH3CN: 261 (29 300), 321 (10 500). Both the 1H and 13C NMR
spectra of the complexes may be found in the ESI.†
Spectroscopic properties of complexes
IR spectra. The amide carbonyl stretch for L1 appears at
1673 cm−1, typical for an amido group and it is red-shifted in all
its complexes. A strong peak at 1331 cm−1, assigned to the
ν(CvS) is also shifted to lower energy between
1257–1262 cm−1 indicating the co-ordination of the sulphur
groups. All compounds reveal two characteristic unsplit infrared
active bands at ∼1100 cm−1 and ∼623 cm−1 indicative of ionic
perchlorate (Td symmetry).22,23 These features are consistent
with the X-ray diffraction data for all complexes.
Cyclic voltammetry
[CdC34H30N8O2S2]·[ClO4]2 (6). Colourless block crystals
(82% yield); H-NMR (400 MHz; CD3CN): 4.16 (4H, s); 4.25
1
The electrochemical data for all the metal complexes were
measured as described in the Experimental section and are sum-
marised in Table 1.
(2H, s); 7.38 (2H, d, Ar, J = 7.4 Hz); 7.49 (2H, d, Ar, J = 7.5
Hz); 7.48–8.14 (15H, m, Ar); 8.70 (1H, d, Ar, J = 5.0 Hz);
10.36 (2H, s, NH); 13.72 (2H, s, NH); 13C NMR: (62.5 MHz;
CD3CN): 56.6, 56.7, 120.9, 123.6, 125.1, 125.2, 128.5, 128.7,
130.5, 134.2, 140.3, 142.2, 148.3, 150.2, 153.2, 153.4, 169.3,
179.3; accurate ESMS (m/z): 759.0936 (100), [Cd(L1)–H]+ [cal-
culated 759.0888]; CdC34H30N8O2S2(ClO4)2(CH3CN) (found:
C, 43.56; H, 3.31; N, 12.59. requires C, 43.24; H, 3.32; N,
12.61%). IR KBr/cm−1: ν = 3439br, 1609s, 3069m, 1541s,
1438s, 1261s, 704m, 1098s, 620s; UV/Vis [λmax, nm (εM, M−1
The cyclic voltammogram of the manganese compound, 1,
reveals two irreversible reductions −1.95 V and −1.70 V (vs.
Fc+–Fc). Discrete mononuclear complexes of manganese in a
hepta co-ordinated environment are quite rare.24 While other
seven co-ordinate Mn(II) complexes displayed an anodic Mn(II)/
Mn(III) redox couple process at +1.02 V and +1.1 V (vs. Fc+/
Fc),25,26 compound 1 reveals no peaks in the anodic region.
The cobalt complex, 2, undergoes an electrochemically revers-
ible27 redox process at +0.362 V (vs. Fc+/Fc) which we attribute
to the Co(II)/Co(III) redox couple. The 2 : 1 complexes of 2,6-bis
(pyrazol-1-ylmethyl)pyridine and its methylsubstituted deriva-
tives display a Co(II)/Co(III) process at 0.28 to 0.6 V (vs.
Fc+/Fc).28
1
cm−1)] in CH3CN: 261 (27 800), 321 (9900). Both the H and
13C NMR spectra of the complexes may be found in the ESI.†
Results and discussion
The voltammogram of the Ni(II) complex, 3, shows quite
similar electrochemical behaviour to the analogous Mn(II)
complex as it contains two similar irreversible reductions at
−1.78 Vand −1.37 V (vs. Fc+/Fc).
The copper complex, 4, contains a single reversible wave
process at the cathodic potential of −0.349 V (vs. Fc+–Fc),
which is reversible over the scan rate range of 100–1000 mV s−1
(Fig. 2). This is close to the standard potential of the Cu(II)/Cu(I)
couple, which occurs at −0.53V (vs. Fc+/Fc). It is also close to
Synthesis of ligand and complexes
The amino groups of the BAPA ligand were converted into
thiourea groups via the dropwise addition of 2 equivalents of
benzoyl isothiocyanate in ethanol with continuous stirring and
gentle heating to approximately 50 °C.21 After 10 minutes the
reaction mixture changed from a clear yellow solution to a
creamy suspension. The product was isolated as a white precipi-
tate. It is worth noting that after initial unsuccessful reactions
4610 | Dalton Trans., 2012, 41, 4608–4617
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Table 1 Electrochemical properties and electronic spectra assignments for the ligand and complexes 1, 2, 3, and 4
Cmpd
Ep/V (ΔE/mV)a
π–π* transitions/λ (nm)
MLCT/λ (nm)
d–d transitions/λ (nm)b
Δ/cm−1c
B′/cm−1c
L1
—
266 (50 300), 309 (28 600)
256 (17 300), 314 (10 600)
280 (33 200), 351 (7400)
323 (15 800)
—
—
—
—
—
—
—
10 640
—
—
—
—
575.8
—
1 (Mn)
2(Co)
3(Ni)
4(Cu)
−1.70, −1.95
+0.362 (79)
−1.37, −1.78
−0.349 (90)
400 (3100)
385 (1500)
—
507 (80), 568 (55)
549 (17), 849 (15), 940 (16)
623 (130), 793 (96)
315 (17 000)
a In acetonitrile solution (supporting electrolyte: [Bu4N][PF6] (0.1 mol dm−3); T = 20 °C). Measured at 0.1 V s−1. The potentials at which reversible
processes occur are calculated as the average of the oxidative and reductive peak potentials (Ep + Epred)/2. For irreversible processes, the anodic or
ox
cathodic peak potentials are given. Potentials are in volts vs. the Fc/Fc+ couple. b Performed in CH3CN solution at room temperature; numbers in
parentheses indicate molar absorption coefficients ε (M−1 cm−1). c Values calculated by assuming an octahedral geometry.
Fig. 3 Deconvolution of the electronic spectrum of the Cu(II) com-
pound, 4. The lower graph contains a trace revealing the individual com-
ponent peaks, while the upper graph has a trace which reflects the sum
curve of these peaks. Both the upper and lower graphs also contain a
trace of the raw input data.
10 640 cm−1, B = 575.8 cm−1. These values are in good agree-
ment with the related mononuclear compound [Ni(TPA)
Fig. 2 Isolation of the Cu(II/I) redox couple for 4.
(NCS)2]·H2O which has a similar absorption profile, containing
bands at 11 390, 17 540 and 26 900 cm−1 31
However compound
.
that of the Cu(II)/Cu(I) couple observed for the copper complex
of tris((6-phenyl-2-pyridyl)methyl)amine (TPPA)29 (−0.38 V vs.
Fc+/Fc). Indeed, the positioning of non-polar groups close to the
metal-ion is known to result in more stable Cu(I) species, and a
series of 6-substituted TPA ligands have been shown to produce
copper complexes where the Cu(I)/Cu(II) couple progressively
changes from −0.403 V for the TPA complex, to −0.280 V,
−0.060 V and +0.250 V for the mono, bis and trisbromosubsti-
tuted species.29
3 displays a lower racah B parameter due to the incorporation of
the bonding of the softer sulphur ligands resulting in a higher
degree of covalency in the metal bonding.
The electronic spectrum of the five-co-ordinate Cu(II)
complex, 4, (Fig. 3) reveals two d–d transitions in the visible
region at 12 790 and 16 285 cm−1. Both square planar and trigo-
nal bipyramidal Cu(II) complexes typically present similar
absorption patterns with two bands at comparable wavelengths.
It has been reported that in trigonal prismatic Cu(II) complexes
the absorption intensity of these transitions usually tends to
increase towards lower energy. This is the case for the essentially
regular trigonal bipyramidal structure Cu(NH3)2Ag(SCN)3
Electronic absorption spectra
which exhibits transitions at 12 800 and 14 500 cm−1 32
This is
.
The electronic spectra of L1 and its complexes have been
obtained and the data are presented in Table 1. The free ligand
shows typical π–π* transitions at high energy (∼265 and
310 nm). Similarly, the d10 metals, Zn2+ and Cd2+, form com-
plexes which also have absorptions at similar energies.
in good agreement with the continuous shape mapping calcu-
lations based on the solid state structure of 4 which indicate a
slightly distorted trigonal bipyramid, albeit with some square
planar character.
The Co(II) compound, 2, exhibits a predominantly trigonal
bipyramidal (N4S) ligand field in the solid state. In the case of a
high-spin trigonal bipyramidal geometry, three bands are typi-
Crystallographic studies
4
4
Crystal structure determination was performed as described in
the Experimental section. Relevant crystal data for all structures
are given in Table 2 along with relevant bond lengths and angles
in Tables 3 and 7–11.
cally observed corresponding to transitions from A2′(F) to E
4
4
(P), A2(P) and E(P), in ascending energy. The lowest energy
transition is usually found in the near-IR region. For example,
the compound Co(Et4dien)Cl2 (dien = diethylenetriamine)
reveals transitions at 10 500, 15 200 and 19 200 cm−1 30
.
The octahedral Ni(II) compound, 3, gives an electronic spec-
trum typical of octahedral geometry, with the final spin allowed
[C34H30N8O2S2] (L1)
3
3
transition, T1g(P) ← A2g, being obscured by a MLCT band at
∼26 315 cm−1
Using these assignments yields
The bisthiourea ligand crystallises in the triclinic space group
ˉ
.
Δ
=
P1. Bonds length are typical of organic molecules. CvS bonds
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Table 3 Selected bond lengths (Å) and angles (°) for 1
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(5)
Mn(1)–N(8)
2.335(15)
2.262(15)
2.298(16)
2.269(16)
Mn(1)–O(3)
Mn(1)–S(1)
Mn(1)–S(2)
2.508(17)
2.527(16)
2.560(12)
N(2)–Mn(1)–N(8)
N(2)–Mn(1)–N(5)
N(8)–Mn(1)–N(5)
N(2)–Mn(1)–N(1)
N(8)–Mn(1)–N(1)
N(5)–Mn(1)–N(1)
N(2)–Mn(1) –O(3)
N(8)–Mn(1)–O(3)
N(5)–Mn(1)–O(3)
N(1)–Mn(1)–O(3)
N(2)–Mn(1)–S(1)
104.2(5)
117.7(6)
116.5(6)
74.9(5)
72.3(6)
75.2(5)
79.6(6)
166.3(6)
71.7(5)
121.3(5)
75.8(5)
N(8)–Mn(1)–S(1)
N(5)–Mn(1)–S(1)
N(1)–Mn(1)–S(1)
O(3)–Mn(1)–S(1)
N(2)–Mn(1)–S(2)
N(8)–Mn(1)–S(2)
N(5)–Mn(1)–S(2)
N(1)–Mn(1)–S(2)
O(3)–Mn(1)–S(2)
S(1)–Mn(1)–S(2)
83.8(6)
148.7(6)
135.8(6)
84.4(5)
155.9(5)
79.9(5)
79.5(5)
128.0(5)
91.4(5)
81.2(5)
Fig. 4 The asymmetric unit of L1. Displacement ellipsoids are shown
at 50% probability. H atoms are of arbitrary size.
length are 1.675(10) Å and 1.670(10) Å. Similar structures show
CvS bond lengths ranging between 1.655 and 1.679 Å, and
similarly the hydrogen bond interactions in these examples range
between 1.85 and 2.0 Å.33 There are two intramolecular hydro-
gen bonds (Fig. 4) which help establish the molecular confor-
mation of L1. Specifically, these interactions occur between the
carbonyl oxygen atoms and the amine nitrogen atoms immedi-
ately adjacent to the pyridyl groups in the 2-position. These form
two six-membered rings which can be observed in each complex
discussed herein. Hydrogen bond lengths are ∼1.9 Å.
[Mn(L1)(ClO4)][ClO4]·2H2O (1). Crystallographic data for
this complex are listed in Tables 2 and 3. The manganese com-
pound crystallises in the monoclinic space group P21/n and con-
tains one complex within the asymmetric unit (asu).
The Mn(II) ion is surrounded by three types of donor atoms:
four nitrogen donors (N1, N2, N5 and N8), two sulphur donors
(S1 and S2) and a single oxygen donor (O3) located on a per-
chlorate counterion within the lattice framework (Fig. 5). Con-
tinuous Shape Mapping analysis (Table 4)34 indicates that the
structure is intermediate between a monocapped octahedron and
a mono capped trigonal prism (Fig. 6). The relationship between
monocapped octahedron and monocapped trigonal prismatic is
shown in Fig. 6 which also shows that interconversion requires a
surprisingly small translation of two atoms.35
The Mn(II)–N bond lengths range from 2.262(15) Å to 2.335
(15) Å, while the Mn(II)–O bond length is longer at 2.508(17) Å.
This is the first example of a complex in which a Mn(II) cation
is surrounded by this particular donor set; however the
4612 | Dalton Trans., 2012, 41, 4608–4617
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co-ordinative bond lengths are typical. The nitrogen donors N1,
N2, N5 and N8 display very similar values to those found in
similar TPA frameworks (Table 5). The Mn–S bond lengths in 1
are longer than the Mn–N distances but are similar to those in
related Mn complexes.36
Finally, while both benzoyl thiourea groups cyclise through
hydrogen bonding (N6–H6⋯O2 and N3–H3a⋯O1), the remain-
ing NH groups hydrogen bond to a water molecule (N4–
H4a⋯O11 and N7–H7⋯O11) which thereby links the two
pendant benzoyl thiourea groups.
Fig. 6 A view of the core geometry of 1 (top) and the structural
relationship between a monocapped octahedron and a monocapped tri-
gonal prism (below).
Fig. 5 The asymmetric unit of 1. Displacement ellipsoids are shown at
50% probability. H atoms are of arbitrary size. H-bonds are represented
by dashed lines.
Table 4 Continuous Shape Mapping results for complexes 1 and 6
Structure
HP-7
HPY
PBPY
OCF
TPRS
JPBP
L1_Mn (1)
L1_Cd (6)
35.142
36.154
20.029
18.225
6.509
6.776
0.831
1.726
0.671
0.606
10.339
11.475
HP-7: heptagon (D7h), HPY: hexagonal pyramid (C6v), PBPY:
pentagonal bipyramid (D5h), OCF: capped octahedron (C3v), TPRS:
capped trigonal prism (C2v), JPBP: Johnson pentagonal bipyramid
(D5h).
Fig. 7 The asymmetric unit of 2. Displacement ellipsoids are shown at
50% probability. H atoms are of arbitrary size. H-bonds are represented
by dashed lines.
Table 5 Co-ordination numbers of TPAwith first row transition metals and comparison in M–N bond lengths between L1 and other TPA complexes
TPA framework
Bond length/Å
N1-bridge head
% with co-ordination number
L1
M-pyridyl bond lengths
M(II)
Mn
Co
Ni
Cu
Zn
# hitsa
22
17
14
42
5
0
47
0
97.6
87.5
0
6
81.8
53
100
2.4
12.5
33.3
7
13.6
0
0
0
0
33.3
8
4.5
0
0
0
0
33.3
Co-ord. #
TPAa
2.341
2.182
2.102
2.040
2.233
2.49
L1
TPAa
2.296
2.104
2.078
2.066
2.083
2.42
L1 (N2)
L1 (N3)
2.298(16)
2.065(4)
2.050(4)
2.148(3)
2.131(5)
2.409(4)
L1 (N4)
2.269(16)
2.051(4)
2.086(4)
2.034(3)
2.058(5)
2.378(5)
7
5
6
5
5
7
2.335(15)
2.147(4)
2.072(8)
2.018(3)
2.181(5)
2.386(4)
2.262(15)
2.086(4)
2.237(4)
2.030(2)
2.078(5)
2.365(4)
16
3
Cd
a CCDC Refcodes for the structures used are listed in ref. 37.
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Table 6 Continuous shape mapping results for complexes 2, 4 and 5
Structure
PP
VOC
TBPY
SPY
JSPY
JTBP
L1_Co (2)
L1_Cu (4)
L1_Zn (5)
33.308
32.959
32.380
7.072
5.574
6.592
2.300
1.749
2.454
4.982
3.914
4.595
7.072
5.574
6.592
3.685
3.906
3.754
PP: pentagon (D5h), VOC: vacant octahedron (C4v), TBPY: trigonal
bipyramid (D3h), SPY: square pyramid (C4v), JSPY: Johnson square
pyramid (C4v), JTBP: Johnson trigonal bipyramid (D3h).
Table 7 Selected bond lengths (Å) and angles (°) for 2
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(5)
2.147(4)
2.086(4)
2.065(4)
Co(1)–N(8)
Co(1)–S(1)
2.051(4)
2.3170(14)
Fig. 8 The asymmetric unit of 3. Displacement ellipsoids are shown at
50% probability. H atoms are of arbitrary size. H-bonds are represented
by dashed lines.
N(8)–Co(1)–N(5)
N(8)–Co(1)–N(2)
N(5)–Co(1)–N(2)
N(8)–Co(1)–N(1)
N(5)–Co(1)–N(1)
116.53(16)
118.88(16)
112.89(15)
78.33(16)
78.39(15)
N(2)–Co(1)–N(1)
N(8)–Co(1)–S(1)
N(5)–Co(1)–S(1)
N(2)–Co(1)–S(1)
N(1)–Co(1)–S(1)
78.67(15)
99.42(12)
114.08(12)
91.04(12)
166.45(11)
Table 8 Selected bond lengths (Å) and angles (°) for [NiL1]2+ 3
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(5)
2.072(8)
2.237(4)
2.050(4)
Ni(1)–N(8)
Ni(1)–N(9)
Ni(1)–S(2)
2.086(4)
2.090(10)
2.345(3)
[CoL1]·[ClO4]2 (2). The cobalt compound crystallises in the
monoclinic space group P21/n and contains one complex within
the asu (Fig. 7). In an identical fashion to the analogous Cu(II)
(4) and Zn(II) (5) complexes, the Co(II) ion is co-ordinated by
four N-donors and one S donor in what is best described as a
highly distorted trigonal bipyramidal co-ordination geometry
(Table 6).
The co-ordinative pyridyl nitrogens (N2, N5 and N8) occupy
the equatorial positions and their bond lengths range from 2.051
(4) Å to 2.086(4) Å whereas the axial positions are occupied by
N1 and S1 with distances of 2.147(4) Å and 2.3170(14) Å
respectively (Table 7). This arrangement is similar to those
observed for other Co(II) trigonal bipyramidal complexes (see
Table 5). For example, the four nitrogen donors in the compound
[Co(TPA)Cl][ClO4] range between 2.046(6) and 2.184(6) Å.38
The Co(II)–S bond length is also typical when compared to com-
plexes with similar donor sets; for instance the complex [1,5-bis
N(5)–Ni(1)–N(1)
N(5)–Ni(1)–N(8)
N(1)–Ni(1)–N(8)
N(5)–Ni(1)–N(9)
N(1)–Ni(1)–N(9)
N(8)–Ni(1)–N(9)
N(5)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
85.9(3)
88.44(15)
82.1(2)
177.6(3)
94.4(3)
94.0(3)
N(8)–Ni(1)–N(2)
N(9)–Ni(1)–N(2)
N(5)–Ni(1)–S(2)
N(1)–Ni(1)–S(2)
N(8)–Ni(1)–S(2)
N(9)–Ni(1)–S(2)
N(2)–Ni(1)–S(2)
157.47(16)
82.7(3)
94.15(13)
179.8(3)
98.01(13)
85.6(2)
95.07(14)
75.9(2)
103.90(13)
the two sulphur atoms is co-ordinated to the metal centre in this
complex. The second sulphur donor remains un-co-ordinated
with a solvent molecule taking its place, probably due to strong
preference by the metal for an octahedral geometry.
The Ni(II)–N co-ordinative bond lengths of the five nitrogen
donors range from 2.050(4) Å to 2.237(4) Å, and the Ni(II)–S
bond length is significantly longer at 2.345(3) Å. These values
are similar to those of related compounds (Table 5). The asu also
contains two perchlorate counterions, one of which accepts three
H-bonds from the two lower NH groups of the thiourea groups
(N4–H4a⋯O3, N7–H7⋯O8 and N7–H7⋯O5).
(2-pyridylmethyl)-1,5-diazacyclooctane
Co(S-C6H4-p-CH3]
[BPh4] has a Co(II)–S bond length of 2.2670(12) Å.39
Complex 2 also shows extensive hydrogen bonding. In
addition to the typical intramolecular hydrogen bonding of the
thiourea arm to form a six-membered ring, hydrogen bond inter-
actions are also observed between the two arms i.e. N4–
H4a⋯O2 as well as hydrogen bonding to the perchlorate
counter-ion.
[CuL1][ClO4]2 (4). The copper compound crystallises in the
monoclinic space group P21/n and contains a single complex
within the asu (Fig. 9). Analysis of the core geometry reveals the
Cu(II) cation is situated at the centre of a trigonal bipyramidal
arrangement of donor atoms (S(TBPY) = 1.749) (Table 6). A
data search of the CSD,40 reveals that this geometry is very
common for Cu(II) complexes of TPA, with over 95% of the 42
examples having this geometry (Table 5). As with the analogous
Co(II) and Zn(II) complexes, the co-ordination sphere in this
instance also consists of four nitrogen donors (N1, N2, N5 and
N8) and one sulphur atom (S1). The equatorial sites are occupied
by the three pyridyl N-donors (N2, N5 and N8), while the axial
sites are occupied by N1 and S1. The bridge-head N1 and two
[Ni(L1)(CH3CN)]·[ClO4]20.5(H2O)0.5(CH3COCH3) (3). The
nickel compound crystallises in the monoclinic space group
P21/n and contains a single complex within the asu (Fig. 8). In
keeping with all other TPA complexes of Ni(II) (Tables 5 and 8),
this complex has an approximately octahedral geometry. The
Ni(II) cation lies at the centre of a slightly distorted octahedral
N5S co-ordination environment. The largest deviations are
observed in the angles N8–Ni1–N2 [157.47(16)°] and N1–Ni1–
N2 [75.9(2)°] where significant distortions from the classical
180° and 90° angles, respectively, occur. Note that just one of
4614 | Dalton Trans., 2012, 41, 4608–4617
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Fig. 9 The asymmetric unit of 4 and the core geometry. Displacement
ellipsoids are shown at 50% probability. H atoms are of arbitrary size. H-
bonds are represented by dashed lines.
Fig. 10 The asymmetric unit of 5. Displacement ellipsoids are shown
at 50% probability. H atoms are of arbitrary size. H-bonds are rep-
resented by dashed lines.
Table 9 Selected bond lengths (Å) and angles (°) for [CuL1]2+ 4
Table 10 Selected bond lengths (Å) and angles (°) for [ZnL1]2+ 5
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(5)
2.018(3)
2.030(2)
2.148(3)
Cu(1)–N(8)
Cu(1)–S(1)
2.034(3)
2.2605(9)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–N(4)
Zn(1)–S(2)
2.181(5)
2.078(5)
2.131(5)
2.058(5)
2.3716(15)
Zn(2)–N(9)
Zn(2)–N(11)
Zn(2)–N(10)
Zn(2)–N(12)
Zn(2)–S(3)
2.163(5)
2.066(5)
2.134(5)
2.054(5)
2.3653(17)
N(1)–Cu(1)–N(8)
N(2)–Cu(1)–N(8)
N(1)–Cu(1)–N(5)
N(2)–Cu(1)–N(5)
N(8)–Cu(1)–N(5)
83.17(11)
129.04(10)
80.11(10)
113.57(10)
111.11(10)
N(1)–Cu(1)–S(1)
N(2)–Cu(1)–S(1)
N(8)–Cu(1)–S(1)
N(5)–Cu(1)–S(1)
172.88(8)
92.79(8)
97.24(8)
106.23(7)
N(4)–Zn(1)–N(2)
N(4)–Zn(1)–N(3)
N(2)–Zn(1)–N(3)
N(4)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
N(3)–Zn(1)–N(1)
N(4)–Zn(1)–S(2)
N(2)–Zn(1)–S(2)
N(3)–Zn(1)–S(2)
N(1)–Zn(1)–S(2)
113.8(2)
121.05(19)
114.5(2)
80.6(2)
79.3(2)
77.16(19)
99.70(14)
114.35(14)
89.42(13)
164.19(15)
N(12)–Zn(2)–N(11)
N(12)–Zn(2)–N(10)
N(11)–Zn(2)–N(10)
N(12)–Zn(2)–N(9)
N(11)–Zn(2)–N(9)
N(10)–Zn(2)–N(9)
N(12)–Zn(2)–S(3)
N(11)–Zn(2)–S(3)
N(10)–Zn(2)–S(3)
N(9)–Zn(2)–S(3)
111.5(2)
122.8(2)
115.0(2)
79.3(2)
79.8(2)
78.1(2)
99.24(16)
115.22(15)
89.58(15)
163.92(15)
of the pyridyl nitrogens N2 and N8 form bonds with the copper
centre with lengths ranging from 2.018(3) Å to 2.034(3) Å,
whereas the nitrogen N5 that is located on the unsubstituted arm
has a longer distance at 2.148(3) Å. The sulfur bond is signifi-
cantly longer at 2.2605(9) Å (Table 9). A related compound [Cu
(TPA)2S2][ClO4]2[Et2O] also exhibits a trigonal bipyramidal co-
ordination environment and contains very similar bond lengths
ranging between 2.048(6) Å and 2.280(2) Å.41 There is an inter-
molecular H-bonding interaction between one of the two per-
chlorate counterions and the nitrogen atom N7. In addition, there
is an intra-molecular hydrogen bond within each of the two
thiourea arms (N3–H3a⋯O1) and N6–H6⋯O2) and an inter-
molecular hydrogen between the two arms (N4–H4a⋯O2).
[Cd(L1)(ClO4)]·[ClO4](CH3CN) (6). The cadmium compound
crystallises in the monoclinic space group P21/n and contains
one complex within the asu (Fig. 11). The Cd(II) cation is sur-
rounded by seven donor atoms (four N atoms, two S and one O
atom) and lies at the centre of a slightly distorted mono capped
trigonal prismatic co-ordination geometry, analogous to 1
(Fig. 12; Table 4). The bond lengths (Table 11) between the
pyridyl N-donors and the central Cd(II) cation lie within the
expected ranges on comparison to similar complexes (Table 5).
In the structure, one perchlorate counter ion is hydrogen
bonded between the two extended arms of the ligand via two
NH groups and intramolecular hydrogen bonding occurs within
the thiourea units.
[ZnL1]·[ClO4]20.5(CH3CN) (5). The zinc compound crystal-
lises in the monoclinic space group P21/c and contains two struc-
turally similar complex units within the asu (Fig. 10); just one is
discussed. In both instances the Zn(II) cation lies at the centre of
a slightly distorted trigonal bipyramid geometry (S(TBPY) =
2.454) Table 6) and reveals a similar co-ordination environment
to the analogous Co(II) and Cu(II) complexes (see Table 10 for
bond lengths and angles).
Preliminary anion binding studies
Hydrogen bond interactions are observed: an intra-molecular
hydrogen bond within each of the two thiourea arms (N5–
H5a⋯O1) and N7–H7⋯O2), an intra-molecular hydrogen
between the two arms (N8–H8⋯O1) and an intermolecular one
to a perchlorate counter-ion (N5–H5a⋯O7).
An initial study of the abilities of the ligand and complexes to
host anions was carried out. A 1H NMR titration of the host with
both fluoride ions and succinate ions was carried out, keeping
the concentration of the host constant at all time. Due to differ-
ences in solubilities, the ligand-based titration was carried out in
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with multiple sets of signals being generated at various different
ratios of host to ligand. (Stacked plots of these titrations are
included in the ESI†). The analysis of this data proved imposs-
ible, and it was assumed that the products of these titrations
might be a series of complexes with anions co-ordinated to both
the ligand and also directly to the metal centre. Clearly, the pro-
ducts generated were now in slow exchange. With this in mind,
future efforts will be the investigation of dye displacement reac-
tions of these complexes, by a series of molecular anions.
Conclusions
The tris(2-pyridylmethyl)amine (TPA) ligand has been studied
extensively, especially in co-ordination with first row transition
metals.6 While the ligand has a strong geometrical preference for
trigonal bipyramidal complexes, other geometries may be
adopted to suit the electronic preference of the metal. In con-
clusion, a novel TPA derivative has been synthesised and Mn(II),
Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes have been
investigated. A useful comparison between TPA metal com-
plexes based on structures in CCDC database and with those of
L1 is shown in Table 5. General conclusions are:
Fig. 11 The asymmetric unit of 6. Displacement ellipsoids are shown
at 50% probability. H atoms are of arbitrary size. H-bonds are rep-
resented by dashed lines.
a) All complexes display typical M–N bond lengths when
compared to previously reported M(TPA) complexes.
b) While most TPA complexes of Mn(II) are six co-ordinate,
the seven co-ordinate structure observed with L1 is not unusual.
c) The five co-ordinate, TBP, geometry observed for Zn(II) and
Cu(II) complexes of L1 is typical of that observed with TPA
ligands. ATBP geometry was also observed for Co(II), and while
it is less common it is not unusual.
Fig. 12 Aview of the core geometry of 6.
d) Typical of Ni(II), the metal resists the predisposition of the
ligand to TBP geometries and instead forms an octahedral
species.
e) There is limited structural data for Cd(II) complexes.
However, the larger size of the metal ions means that the metal
will readily form complexes with high co-ordination numbers.
(A search of the CDS shows approximately ∼7% of all the Cd
containing structures have a seven co-ordinate centre).
f) The five co-ordinate structures of Co(II), Cu(II) and Zn(II),
and the six co-ordinate Ni(II) complex, all have only one sulphur
atom co-ordinated to the metal centre. This results in two
thiourea groups coming close together, with hydrogen bonding
occurring between the two arms.
Table 11 Selected bond lengths (Å) and angles (°) for [CdL1]+2 6
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–N(5)
Cd(1)–N(8)
2.386(4)
2.365(4)
2.409(4)
2.378(5)
Cd(1)–O(5)
Cd(1)–S(1)
Cd(1)–S(2)
2.801
2.713(12)
2.5746(14)
N(2)–Cd(1)–N(8)
N(2)–Cd(1)–N(1)
N(8)–Cd(1)–N(1)
N(2)–Cd(1)–N(5)
N(8)–Cd(1)–N(5)
N(1)–Cd(1)–N(5)
N(2)–Cd(1)–S(2)
N(8)–Cd(1)–S(2)
117.83(15)
71.50(13)
70.20(16)
114.63(14)
99.55(16)
74.29(17)
138.58(11)
94.97(12)
N(1)–Cd(1)–S(2)
N(5)–Cd(1)–S(2)
N(2)–Cd(1)–S(1)
N(8)–Cd(1) –S(1)
N(1)–Cd(1)–S(1)
N(5)–Cd(1)–S(1)
S(2)–Cd(1)–S(1)
—
148.19(10)
81.08(13)
78.59(9)
83.04(11)
122.53(12)
162.33(13)
81.33(4)
—
g) The large metal ions, Mn(II) and Cd(II), form seven co-ordi-
nate species with both sulphur atoms co-ordinated. This appears
to cause the two thiourea arms to spread out, creating less steric
hindrance below the metal ion where a counter-ion (perchlorate)
or water molecule can reside and hydrogen bond with the
thiourea arms.
On considering these observations, one can consider how the
presence of different metals in the complex may affect the
binding of anions. From solid-state data, we can see that when
an octahedral complex is formed, the two nitrogens on the 6-pos-
ition of the two substituted pyridines are positioned ∼4.2 Å
apart. On forming a trigonal bipyramidal complex the two nitro-
gens are ∼4.8 Å apart. Finally, large metals with no stereochemi-
cal preference result in a seven-co-ordinate complex with the NH
groups on the pyridines being ∼5.7 Å apart. This ultimately
d6-acetone, while the complexes were dissolved in CD3CN. (Full
details and spectra are included in the ESI†). The titration curves
for anion–ligand binding were then fitted to a MIS model using
Origin, to yield the association constant for the guest–host mol-
ecule. Surprisingly, not only the fluoride, but also the succinate
appears to bind in a 2 : 1 ratio (guest : host) and the observed
binding was stronger in the case of the succinate (Ka = 16 190
5890 M−1) than in that of the fluoride (Ka = 4265 1785 M−1).
However, when the metal complexes were titrated with anion,
the anticipated increase in the binding constant could not be
observed. As the anion was titrated into the host, a different be-
haviour was observed. Rather than observing fast exchange and
hence one set of signals, we observed a complicated behaviour
4616 | Dalton Trans., 2012, 41, 4608–4617
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D. M. J. Doble, A. J. Blake, W.-S. Li and M. Schröder, J. Chem. Soc.,
Dalton Trans., 2001, 3137.
results in the H-bond donors being forced further apart meaning
a large anion will fit between the two donor arms. This appears
to offer interesting future possibilities of tuning a host to an
anion by suitable choice of cation.
25 O. Horner, J.-J. Girerd, C. Philouze and L. Tchertanov, Inorg. Chim.
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26 P. Gili, M. G. Martín Reyes, P. Martín Zarza, M. F. C. Guedes da Silva,
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XESVEX, XEXHEN, XIJXUJ. For Cu(II): BEJPOW, BIXMEA,
BIXMUQ, BIXPIH, BUBCIK, COHNIX, EBAWIN, EHIDII, EHIDOO,
EHIDUU, EHIFEG, EHIFIK, EXEPUS, GECRAH, GIHWOK,
HEQWEF, HEQXAC, KIWKEG, KIWKIK, MADZIB, MOQKAE,
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SIXLUH, SIZCAG, SOQJIS, TARMEF, UKACIS, VACJAL, WEGHOF,
XEMDOI, XUSPUW, YANNOR, YAVVOG, YIWKUL, ZEMXUK,
ZIFDUN, ZUBZIF. For Zn(II): CIWVAG, NABCAU, NABDOJ,
NABKIK, PEMBIS, PEMBOY, SINWES, SINWIN, SINWOC,
SINWUI, UMIQOW, WUCGOQ, ZERBIH, ZUBZAX, ZUBZEB,
ZUDHAH. For Cd(II): VIQLOW, ZUBZOL, ZUBZUR.
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structures, 474 (3.9%) were seven co-ordinate (Discounting species with
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Dalton Trans., 2012, 41, 4608–4617 | 4617