Dinuclear copper(II) complexes of two homologous pyrazine-based bis(terdentate) diamide ligands

Article abstract of DOI:10.1002/ejic.200400908

The 1:1 reactions of the new bis(terdentate) diamide ligand N, N′-bis(2-pyridylmethyl)pyrazine-2,5-dicarboxamide (H₂L ¹) and its higher homologue N,N′-bis[2-(2-pyridyl)ethyl] pyrazine-2,5-dicarboxamide (H₂L²) with Cu(BF ₄)₂·4H₂O in the absence of added base have consistently afforded dicopper(II) complexes of the doubly deprotonated ligands (L¹)^2- and (L²)^2-. The complex [Cu^II₂(L²)(H₂O) ₂(MeCN)₂](BF₄)₂ (2a) has been structurally characterised. Subsequently, reactions employing a correct stoichiometric 2:1 metal-to-ligand ratio in MeCN have afforded bulk samples of the dinuclear complexes. The compounds [Cu^II₂(L ¹)(MeCN)₂(H₂O)₂)(BF ₄)₂·H₂O (1a·H₂O) and [Cu^II₂(L²)(H₂O)₄(BF ₄)₂]-2H₂O (2b-2H₂O) have been structurally characterised. While complex 1a-H₂O of the lower ligand homologue exhibits very weak antiferromagnetic spin coupling (J = -0.24 cm ^-1), complex 2b of the higher ligand homologue exhibits very weak ferromagnetic spin coupling (J = +0.67 cm^-1). EPR studies have been carried out on polycrystalline powders and frozen DMF solutions of 1a-H ₂O and 2b·2H₂O. The EPR spectra of the polycrystalline powders indicate the presence of dipolar broadening and weak intermolecular exchange, while those of the frozen DMF solutions are characteristic of dipolar-coupled Cu^II pairs within the dinuclear molecules, with no evidence of intraor intermolecular exchange. The spectral simulations confirm that the binuclear structure and the Cu...Cu distances are retained in frozen solution. Dinuclear SiF₂^2- containing compounds, [Cu^II₂(L¹)(H₂O) ₄](SiF₆) (1b) and [[Cu^II₂(L ¹)(H₂O)₂(μSiF₆))·4H ₂O]_∞ (1c-4H₂O). were obtained serendipitously, in nearly quantitative yield, by the 2:1 reaction of Cu(BF ₄)₂· 4H₂O with H₂L ¹ in H₂O. The unexpected SiF₆^2- anions were generated in the course of the reaction by partial hydrolysis of the BF₄^- anions employed, thus forming traces of HF which reacted with the glassware.

Full text of DOI:10.1002/ejic.200400908

FULL PAPER
Dinuclear Copper(II) Complexes of Two Homologous Pyrazine-Based
Bis(terdentate) Diamide Ligands
Julia Klingele (née Hausmann),[a] Boujemaa Moubaraki,[b] Keith S. Murray,[b]
John F. Boas,[c] and Sally Brooker*[a]
Keywords: N ligands / Copper / Amides / Pyrazine / Hexafluorosilicate
The 1:1 reactions of the new bis(terdentate) diamide ligand
N,NЈ-bis(2-pyridylmethyl)pyrazine-2,5-dicarboxamide
(H2L1) and its higher homologue N,NЈ-bis[2-(2-pyridyl)ethyl]-
pyrazine-2,5-dicarboxamide (H2L2) with Cu(BF4)2·4H2O in
the absence of added base have consistently afforded dicop-
per(II) complexes of the doubly deprotonated ligands (L1)2–
and (L2)2–. The complex [CuII2(L2)(H2O)2(MeCN)2](BF4)2 (2a)
has been structurally characterised. Subsequently, reactions
employing a correct stoichiometric 2:1 metal-to-ligand ratio
in MeCN have afforded bulk samples of the dinuclear com-
plexes. The compounds [CuII2(L1)(MeCN)2(H2O)2](BF4)2·H2O
(1a·H2O) and [CuII2(L2)(H2O)4(BF4)2]·2H2O (2b·2H2O) have
been structurally characterised. While complex 1a·H2O of the
lower ligand homologue exhibits very weak antiferromag-
netic spin coupling (J = –0.24 cm–1), complex 2b of the higher
ligand homologue exhibits very weak ferromagnetic spin
coupling (J = +0.67 cm–1). EPR studies have been carried out
on polycrystalline powders and frozen DMF solutions of
1a·H2O and 2b·2H2O. The EPR spectra of the polycrystalline
powders indicate the presence of dipolar broadening and
weak intermolecular exchange, while those of the frozen
DMF solutions are characteristic of dipolar-coupled CuII pairs
within the dinuclear molecules, with no evidence of intra-
or intermolecular exchange. The spectral simulations confirm
that the binuclear structure and the Cu···Cu distances are re-
2–
tained in frozen solution. Dinuclear SiF6 containing com-
pounds, [CuII2(L1)(H2O)4](SiF6) (1b) and {[CuII2(L1)(H2O)2(μ-
SiF6)]·4H2O}ϱ (1c·4H2O), were obtained serendipitously, in
nearly quantitative yield, by the 2:1 reaction of Cu(BF4)2·
4H2O with H2L1 in H2O. The unexpected SiF62– anions were
generated in the course of the reaction by partial hydrolysis
of the BF4– anions employed, thus forming traces of HF which
reacted with the glassware.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
systems as amides can usually be synthesised relatively eas-
ily. Apart from their ability to coordinate to metal ions,
amide groups also reduce the degrees of freedom in a given
ligand system.[4]
Introduction
For the formation of grid-type complexes relatively rigid
ligand systems with a repeating linear array of bi- or ter-
dentate binding pockets are necessary.[1] With regard to ad-
vanced materials, grid-type complexes containing octahe-
dral rather than tetrahedral metal ions are expected to give
rise to a broader range of potentially useful properties. The
design of ligands with terdentate rather than bidentate
binding pockets is therefore desirable. Grid-type complexes
of relatively simple pyrimidine-bridged bis(terdentate) li-
gands, which exhibit intriguing electronic[2] or magnetic
properties,[3] for example, have been reported by Lehn and
co-workers. The use of amide-based ligands for the forma-
tion of grid-type complexes offers advantages over other
The two homologous pyrazine-based diamide ligands
N,NЈ-bis(2-pyridylmethyl)pyrazine-2,5-dicarboxamide
(H2L1) and N,NЈ-bis[2-(2-pyridyl)ethyl]pyrazine-2,5-dicarb-
oxamide (H2L2) are relatively rigid, bis(terdentate) chelates
with antiparallel coordinate vectors, and in theory they
should be able to form [2×2] grid-type complexes of octa-
hedral transition metal ions. Indeed, we have recently re-
ported a cobalt(iii) [2×2] grid-type complex of the doubly
deprotonated ligand (L2)2–.[5] Likewise, copper(ii)[6] and
nickel(ii)[7] [2×2] grid-type complexes of the ligand N,NЈ-
bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide
(H2L3),
which is isomeric to the lower ligand homologue H2L1, have
been reported recently. However, it has been found that
attempts to isolate similar copper(ii) [2×2] grid-type com-
plexes of either H2L1 or H2L2, in the presence of base, con-
sistently produced amorphous solids, which analysed as
[a] Department of Chemistry, University of Otago,
P. O. Box 56, Dunedin, New Zealand
Fax: +64-3-479-7906
E-mail: sbrooker@alkali.otago.ac.nz
[a] School of Chemistry, Building 23, Monash University,
Clayton, Victoria 3800, Australia
[CuHL]n+ (1 equivalent of base per ligand strand) and
[a] School of Physics and Materials Engineering, Monash Univer-
sity,
n
[CuxLx] (2 equivalents of base per ligand strand) species.[8]
These rather intractable solids have not been further charac-
terised. In contrast, in the absence of added base, reactions
P. O. Box 27, Clayton, Victoria 3800, Australia
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400908
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Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
Scheme 1. Synthesis of the ligands H2L1 and H2L2. Reagents and conditions: (i) SeO2, pyridine/H2O, (10:1), reflux; (ii) H2O, filtration;
(iii) SOCl2, MeOH, reflux; (iv) 2.2 equiv. 2-(aminomethyl)pyridine, MeOH, 80–90°, open flask; (v) 2.2 equiv. 2-(2-aminoethyl)pyridine,
MeOH, 80–90°C, open flask; (vi) filtration.
of copper(ii) with these ligands consistently resulted in the and CH2Cl2 at room temperature and in MeOH, MeCN
isolation of crystalline samples of dinuclear complexes of a and DMF at reflux. However, the poor solubility of these
single ligand strand, of the type [CuII2(L1)(co-ligand)n]2+ or two ligands did not preclude successful complexations be-
[CuII2(L2)(co-ligand)n]2+, which are related to the dinuclear ing carried out (see below).
copper(ii) complexes obtained by Fleischer and co-workers
with the isomeric ligand N,NЈ-bis[2-(2-pyridyl)ethyl]pyr-
azine-2,3-dicarboxamide (H2L4).[9,10] In this paper, we re-
port the synthesis of the homologous diamide ligands H2L1
and H2L2 and describe the X-ray crystal structures of the
five dinuclear copper(ii) complexes [CuII2(L1)(MeCN)2-
(H2O)2]-(BF4)2·H2O (1a·H2O), [CuII2(L1)(H2O)4](SiF6)
Dinuclear Copper(II) Complexes
Complexations of H2L1 and H2L2 with Cu(BF4)2·4H2O
in a metal-to-ligand ratio of 1:1 in MeCN resulted in the
isolation of 2:1 complexes of the doubly deprotonated li-
gands. Single crystals of [CuII2(L2)(H2O)2(MeCN)2](BF4)2
(2a) suitable for an X-ray crystal structure determination
were obtained by the vapour diffusion of Et2O into the 1:1
MeCN reaction solution (Figure 1, Table 1, Table 2, Table
3 and Table 4).
(1b),
{[CuII2(L1)(H2O)2(μ-SiF6)]·4H2O}ϱ
(1c·4H2O),
[CuII2(L2)-(H2O)2(MeCN)2](BF4)2 (2a) and [CuII2(L2)-
(H2O)4(BF4)2]·2H2O (2b·2H2O). The magnetic properties
and EPR data of 1a·H2O and 2b are also presented.
Complex 2a features two crystallographically indepen-
dent but chemically very similar centrosymmetric dinuclear
molecules. In each molecule the doubly deprotonated ligand
acts as an (N3)2 bis(terdentate) chelate. Each copper(ii) cen-
tre is in a distorted N4O square-pyramidal coordination en-
Results and Discussion
Ligand Syntheses
The ligand precursor, dimethyl pyrazine-2,5-dicarboxyl- vironment (τ[14] = 0.24, 0.41). The coordination sphere is
ate (II),[11,12] was synthesised, by modifying the literature made up of the equatorially coordinating N3 terdentate co-
procedure, from commercially available 2,5-dimethylpyr- ordination site of the deprotonated amide ligand, an H2O
azine in a one-pot two-step procedure (Scheme 1). The first co-ligand in the remaining equatorial position and a MeCN
step consisted of the oxidation of the α-methyl groups with co-ligand at the apex. As observed in related amide com-
SeO2 in aqueous pyridine to afford pyrazine-2,5-dicarb- plexes,[6,7,10,15–18] the Cu–Namide distances of 2a [Cu(1)–
oxylic acid (I).[11–13] The diacid I was not isolated in pure N(2) 1.936(2) Å and Cu(2)–N(12) 1.929(2) Å] are shorter
form as the crude product could be esterified with MeOH than the Cu–Nheterocycle distances [Cu–Npy: Cu(1)–N(3)
and SOCl2 yielding analytically pure diester II as a crystal- 2.011(3) Å, Cu(2)–N(13) 1.992(3) Å; Cu–Npz: Cu(1)–N(1)
line material.
2.027(2) Å, Cu(2)–N(11) 2.021(2) Å]. The canting angles,
The potentially bis(terdentate) diamide ligands H2L1 and formed between the mean plane of the pyridine ring and
H2L2 were obtained by reacting the diester II with 2-(ami- the mean plane of the pyrazine ring, are 33.4(1)° and
nomethyl)pyridine or 2-(2-aminoethyl)pyridine, respec- 39.5(1)° for the two independent molecules Cu(1) and
tively, in a 1:2.2 molar ratio (Scheme 1). The reactions were Cu(2), respectively. The intramolecular Cu···Cu distances
carried out in MeOH solutions in open flasks, thus allowing [Cu(1)···Cu(1A) 6.784(2) Å and Cu(2)···Cu(2B) 6.756(2) Å]
most of the solvent to evaporate. The ligands H2L1 and across the pyrazine bridge are about 0.1 Å shorter than that
H2L2 were obtained by filtration in ca. 80 and 70% yield, found in a related dicopper(ii) complex of the chemically
respectively, as colourless solids. Both compounds proved related ligand N,NЈ-bis(2-pyridylmethyl)pyrazine-2,3-di-
to be virtually insoluble in all common solvents at room carboxamide (H2L3).[7] In complex 2a the independent di-
temperature, but some solubility was observed in CHCl3 nuclear subunits are Owater–H···Oamide intermolecularly hy-
Eur. J. Inorg. Chem. 2005, 1530–1541
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J. Klingele (née Hausmann), B. Moubaraki, K. S. Murray, J. F. Boas, S. Brooker
FULL PAPER
Figure 1. Molecular structure of one of the two independent cations, [CuII2(L2)(H2O)2(MeCN)2]2+, of complex 2a. Hydrogen atoms,
except those of the H2O co-ligands, have been omitted for clarity. Symmetry operation used to generate equivalent atoms: (A) 1 – x,
2 – y, 1 – z.
drogen-bonded, forming
a 2D ribbon-like structure
[Owater···Oamide 2.598 Å and 2.595 Å]. Parallel ribbons are
connected through Owater–H···F–BF2–F···H–Owater hydro-
gen bonds (Figure S1, Supporting Information; Owater···F
2.738 Å and 2.801 Å).
As reactions, in the absence of base, employing metal-to-
ligand ratios of 1:1 consistently afforded 2:1 products, the
complexations of H2L1 and H2L2 were repeated using 2:1
molar metal-to-ligand ratios to isolate bulk materials
(Scheme 2).
Vapour diffusion of Et2O into the reaction solution con-
taining the lower ligand homologue H2L1 and Cu(BF4)2·
4H2O in a 1:2 molar ratio in MeCN afforded the dinu-
clear compound [CuII2(L1)(H2O)2](BF4)2 (1) in 60% yield
as a dark turquoise microcrystalline solid (Scheme 2). Sin-
gle crystals of [CuII2(L1)(MeCN)2(H2O)2](BF4)2·H2O
(1a·H2O) (Figure 2, Table 1, Table 2, Table 3 and Table 4)
suitable for an X-ray crystal structure determination were
obtained by slow evaporation of the reaction solution.
The copper(ii) coordination sphere in the centrosymmet-
ric dinuclear complex in 1a·H2O is N4O square-pyramidal,
exhibiting only a slight distortion from the perfect geometry
(τ[14] = 0.06). As in 2a, the doubly deprotonated (N3)2 bis-
(terdentate) ligand (L1)2– in 1a·H2O encapsulates each cop-
per(ii) ion in an equatorial manner. Again, the copper(ii)
coordination sphere is completed by a MeCN and an H2O
co-ligand. Compared to complex 2a, however, these co-li-
gands have switched their positions, so that in complex
1a·H2O the H2O co-ligand coordinates axially and the
MeCN co-ligand coordinates in the equatorial position.
Scheme 2. Synthesis of the complexes 1, 1b, 1c, 2 and 2b. Reagents
and conditions: (i) 2 equiv. Cu(BF4)2·4H2O, MeCN, room temp.;
(ii) Et2O (vapour diffusion); (iii) 2 equiv. Cu(BF4)2·4H2O, H2O,
room temp.; (iv) 2 equiv. NEt3; (v) slow evaporation; (vi) 2 equiv.
Cu(BF4)2·4H2O, MeCN, 60 °C; (vii) room temp., 10 h; (viii) fil-
tration; (ix) MeCN/EtOH (1:1) (slow evaporation).
Owing to the increase in ligand rigidity due to the methyl- Cu···Cu distance of the equivalent copper(ii) centres
ene instead of the ethylene linkers, the ligand backbone of [Cu(1)···Cu(1A) 6.811(3) Å] is slightly longer than that
complex 1a·H2O is much flatter than that in 2a, showing a found in complex 2a. A stair-like chain structure is formed
mean deviation of only 0.048 Å from the mean plane made through hydrogen bonding of the axial H2O co-ligand to
up of all non-hydrogen atoms of the ligand, and a maxi- the amide oxygen atom of a neighbouring dinuclear subunit
mum deviation of only 0.082(4) Å for C(10)(4-py). The [Owater···Oamide 2.714 Å]. The H2O co-ligand forms a second
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Eur. J. Inorg. Chem. 2005, 1530–1541

Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
Figure 2. Molecular structure of [CuII2(L1)(MeCN)2(H2O)2]2+, the cation of complex 1a·H2O. Hydrogen atoms, except those of H2O co-
ligands, have been omitted for clarity. Symmetry operation used to generate equivalent atoms: (A) 2 – x, 1 – y, 1 – z.
Table 1. Crystallographic data for the complexes 1a·H2O, 1b, 1c·4H2O, 2a and 2b·2H2O.
1a·H2O
1b
1c·4H2O
2a
2b·2H2O
Molecular formula
C22H26B2Cu2F8N8O5 C18H22Cu2F6N6O6Si C18H26Cu2F6N6O8Si C24H28B2Cu2F8N8O4 C20H30B2Cu2F8N6O8
Mr [gmol–1
T [K]
]
783.21
150(2)
triclinic
687.59
200(2)
monoclinic
C2/c
723.62
200(2)
triclinic
793.24
168(2)
triclinic
783.20
168(2)
triclinic
Crystal system
Space group
a [Å]
b [Å]
c [Å]
¯
¯
¯
¯
P1
P1
P1
P1
8.29650(10)
10.0428(2)
10.1034(2)
74.5940(10)
69.3420(10)
84.3250(10)
759.37(2)
1
21.1697(3)
7.55260(10)
15.1687(2)
90
94.4300(10)
90
8.00020(10)
8.96160(10)
10.2285(2)
71.3330(10)
77.7600(10)
69.0230(10)
644.760(17)
1
9.312(3)
13.156(4)
13.802(4)
106.649(4)
101.489(4)
99.121(4)
1545.0(8)
2
7.898(16)
8.101(20)
11.19(3)
84.98(6)
80.11(6)
85.62(8)
701(3)
α [°]
β [°]
γ [°]
V [Å3]
Z
2418.02(6)
4
1
ρcalcd. [g cm–3
]
1.713
1.889
1.864
1.705
1.855
μ [mm–1
F(000)
]
1.498
1.904
1.796
1.471
1.628
394
1384
366
800
396
Crystal colour and shape
Crystal size [mm3]
θ range [°]
Reflections collected
Independent reflections
Completeness to θ
Data/restraints/parameters
GOF on F2
green block
0.38×0.26×0.08
2.10/25.48
6914
green block
0.20×0.15×0.08
0.96/27.16
14503
green block
0.40×0.20×0.08
2.11/26.42
6211
green block
0.50×0.40×0.35
1.59/26.37
19871
green block
0.70×0.20×0.10
2.53/26.38
3172
2803 [Rint = 0.0235] 2637 [Rint = 0.0380] 2598 [Rint = 0.0359] 6216 [Rint = 0.0432] 2497 [Rint = 0.1674]
99.5%
2803/0/226
1.046
98.0%
2637/0/189
1.072
98.5%
2598/0/205
1.129
98.3%
6216/0/451
1.027
87.4%
2497/0/232
1.108
R indices [I Ͼ 2σ(I)]
R1 = 0.0464,
wR2 = 0.1210
R1 = 0.0507,
wR2 = 0.1244
] 1.294/–1.364
R1 = 0.0625,
wR2 = 0.1683
R1 = 0.0728,
wR2 = 0.1771
2.622/–0.858
R1 = 0.0277,
wR2 = 0.0766
R1 = 0.0294,
wR2 = 0.0775
0.694/–0.524
R1 = 0.0426,
wR2 = 0.1127
R1 = 0.0538,
wR2 = 0.1194
1.212/–0.758
R1 = 0.0470,
wR2 = 0.1250
R1 = 0.0542,
wR2 = 0.1302
0.750/–0.728
R indices (all data)
Largest diff. peak/hole [e·A–3
–
hydrogen bond, to the BF4 anion [Owater···F 2.745 Å]. The crystal structure determination (Figure 3, Table 1, Table 2,
amide oxygen atom is also involved in another hydrogen Table 3 and Table 4).
bond, to the H2O solvate molecule [Oamide···Osolvate
The overall molecular structure of the centrosymmetric
2.907 Å] (Figure S2, Supporting Information).
molecule of complex 2b·2H2O is very similar to that of
The 2:1 molar reaction of Cu(BF4)2·4H2O with the complex 2a. The major difference is the N3O2F distorted
higher ligand homologue H2L2 in MeCN afforded the com- octahedral coordination environment about the copper(ii)
plex [CuII2(L2)(solvent)n](BF4)2 (2) in ca. 50% yield in the ions in 2b·2H2O, the two axial positions being elongated
–
form of a bottle-green microcrystalline solid, that precipi- and occupied by an H2O and a BF4 co-ligand. The Cu–
tated from the reaction mixture after several hours of stir- Fax distance [Cu(1)–F(13B) 2.598(3) Å] is, as expected,
ring (Scheme 2). Recrystallisation of complex 2 from somewhat longer than the Cu–Oax distance [Cu(1)–O(60)
MeCN/EtOH (1:1) afforded single crystals of [CuII2(L2)- 2.516(5) Å]. Although BF4– ions are commonly regarded as
(H2O)4(BF4)2]·2H2O (2b·2H2O) suitable for an X-ray “non-coordinating” a number of structures with coordi-
Eur. J. Inorg. Chem. 2005, 1530–1541
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J. Klingele (née Hausmann), B. Moubaraki, K. S. Murray, J. F. Boas, S. Brooker
Table 2. Selected distances [Å] for the complexes 1a·H2O, 1b, 1c·4H2O, 2a and 2b·2H2O.
FULL PAPER
1a·H2O[a]
1b[a]
1c·4H2O[a]
Cu–Nam
Cu–Npy
Cu–Npz
Cu–Leq
Cu–Lax
Cu···Cu
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–N(100)
Cu(1)–O(100)
Cu(1)···Cu(1A)
1.906(3)
1.999(3)
2.044(3)
1.972(3)
2.201(3)
6.811(3)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–O(20)
Cu(1)–O(30)
Cu(1)···Cu(1A)
1.904(4)
2.007(5)
2.030(4)
1.949(4)
2.271(5)
6.775(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–O(10)
Cu(1)–F(1)
Cu(1)···Cu(1A) (intra) 6.827(1)
Cu(1)···Cu(1B) (inter) 7.166(1)
1.912(2)
2.012(2)
2.052(2)
1.951(2)
2.235(1)
2a[a]
2b·2H2O[a]
Cu–Nam
Cu–Npy
Cu–Npz
Cu–Leq
Cu–Lax
Cu···Cu
Cu(1)–N(2)
Cu(2)–N(12)
Cu(1)–N(3)
Cu(2)–N(13)
Cu(1)–N(1)
Cu(2)–N(11)
Cu(1)–O(50)
Cu(2)–O(60)
Cu(1)–N(50)
Cu(2)–N(60)
Cu(1)···Cu(1A)
Cu(2)···Cu(2B)
1.936(2)
1.929(2)
2.011(3)
1.992(3)
2.027(2)
2.021(2)
1.972(2)
1.996(2)
2.321(3)
2.299(3)
6.784(2)
6.756(2)
Cu(1)–N(2)
1.946(4)
1.981(4)
2.021(4)
1.961(4)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–O(50)
Cu(1)–O(60)
Cu(1)–F(13B)
Cu(1)···Cu(1A)
2.516(5)
2.598(3)
6.764(1)
[a] Symmetry operations used to generate equivalent atoms: 1a·H2O: (A) 2 – x, 1 – y, 1 – z. 1b: (A) 1 – x, 2 – y, 1 – z. 1c·4H2O: (A)
–x, –y – 1, –z; (B) –x, –y, –z. 2a: (A) 1 – x, 2 – y, 1 – z; (B) –x, 2 – y, –z. 2b·2H2O: (A) 1 – x, 1 – y, 1 – z; (B) x – 1, y, z.
Table 3. Selected bond angles [°] for the complexes 1a·H2O, 1b, 1c·4H2O, 2a and 2b·2H2O.
1a·H2O
1b
1c·4H2O
Npz–Cu–Nam N(1)–Cu(1)–N(2)
Npz–Cu–Npy N(1)–Cu(1)–N(3)
Npz–Cu–Leq N(1)–Cu(1)–N(100)
Npz–Cu–Lax N(1)–Cu(1)–O(100)
Nam–Cu–Npy N(2)–Cu(1)–N(3)
81.1(1)
161.7(1)
95.9(1)
94.5(1)
81.9(1)
165.2(1)
103.5(1)
99.0(1)
95.8(1)
95.8(1)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(20)
N(1)–Cu(1)–O(30)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(20)
N(2)–Cu(1)–O(30)
N(3)–Cu(1)–O(20)
N(3)–Cu(1)–O(30)
O(20)–Cu(1)–O(30)
81.6(2)
162.9(2)
92.0(2)
96.4(2)
82.2(2)
161.0(2)
106.2(2)
101.7(2)
93.4(2)
92.2(2)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(10)
N(1)–Cu(1)–F(1)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(10)
N(2)–Cu(1)–F(1)
N(3)–Cu(1)–O(10)
N(3)–Cu(1)–F(1)
O(10)–Cu(1)–F(1)
81.27(7)
163.58(7)
97.53(7)
87.19(6)
82.40(7)
163.68(8)
105.31(6)
97.68(7)
98.75(6)
90.84(6)
Nam–Cu–Leq N(2)–Cu(1)–N(100)
Nam–Cu–Lax N(2)–Cu(1)–O(100)
Npy–Cu–Leq N(3)–Cu(1)–N(100)
Npy–Cu–Lqx N(3)–Cu(1)–O(100)
Leq–Cu–Lax N(100)–Cu(1)–O(100)
2a
2b·2H2O[a]
Npz–Cu–Nam N(1)–Cu(1)–N(2)
N(11)–Cu(2)–N(12)
Npz–Cu–Npy N(1)–Cu(1)–N(3)
N(11)–Cu(2)–N(13)
Npz–Cu–Leq N(1)–Cu(1)–O(50)
N(11)–Cu(2)–O(60)
Npz–Cu–Lax N(50)–Cu(1)–N(1)
N(60)–Cu(2)–N(11)
Nam–Cu–Npy N(2)–Cu(1)–N(3)
N(12)–Cu(2)–N(13
Nam–Cu–Leq N(2)–Cu(1)–O(50)
N(12)–Cu(2)–O(60)
82.1(1)
82.3(1)
161.2(1)
175.7(1)
93.5(1)
93.7(1)
91.4(1)
84.9(1)
92.3(1)
94.3(1)
175.6(1)
51.4(1)
92.8(1)
102.8(1)
91.9(1)
90.5(1)
106.9(1)
93.3(1)
87.1(1)
105.0(1)
N(1)–Cu(1)–N(2)
81.1(2)
173.5(1)
91.4(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(50)
N(1)–Cu(1)–O(60)
N(1)–Cu(1)–F(13B)
N(2)–Cu(1)–N(3)
85.5(2)
91.6(1)
95.0(2)
N(2)–Cu(1)–O(50)
165.7(1)
Nam–Cu–Lax N(2)–Cu(1)–N(50)
N(12)–Cu(2)–N(60)
N(2)–Cu(1)–O(60)
N(2)–Cu(1)–F(13B)
N(3)–Cu(1)–O(50)
102.0(2)
91.4(1)
93.5(2)
Npy–Cu–Leq N(3)–Cu(1)–O(50)
N(13)–Cu(2)–O(60)
Npy–Cu–Lqx N(3)–Cu(1)–N(50)
N(13)–Cu(2)–N(60)
Leq–Cu–Lax N(50)–Cu(1)–O(50)
O(60)–Cu(2)–N(60)
N(3)–Cu(1)–O(60)
N(3)–Cu(1)–F(13B)
O(50)–Cu(1)–O(60)
O(50)–Cu(1)–F(13B)
O(60)–Cu(1)–F(13B)
90.3(2)
93.7(1)
89.5(2)
76.5(1)
165.7(1)
Lax–Cu–Lax
[a] Symmetry operations used to generate equivalent atoms: 2b·2H2O: (B) x – 1, y, z.
1534
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Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
Table 4. Selected hydrogen bond lengths [Å] for the complexes 1a·H2O, 1b, 1c·4H2O, 2a and 2b·2H2O.
1a·H2O[a]
1b[a]
1c·4H2O[a]
Oamide···Oco-lig.
Oamide···Osolv.
Oco-lig.···Osolv.
Oco-lig.···Fanion
O(1B)···O(100)
O(1)···O(110)
2.714
2.907
O(1A)···O(20)
2.603(7)
O(1C)···O(10)
2.662
2.571
O(10)···O(50)
O(100)···F(11)
2.745
O(20)···F(11)
O(20)···F(12)
O(30)···F(11B)
O(30)···F(13C)
2.760(7)
2.792(7)
3.169(7)
3.377(7)
Osolv.···Osolv.
Osolv.···Fanion
O(50)···O(60)
O(50)···F(2D)
O(60)···F(3E)
O(60)···O(1F)
2.670
2.736
2.714
2.845
2a[a]
2b·2H2O[a]
Oamide···Oco-lig.
O(10B)···O(50)
O(1)···O(60)
2.598
2.595
O(1D)···O(60)
O(1E)···O(60)
2.828
2.747
Oamide···Osolv.
Oco-lig.···Osolv.
Oco-lig.···Fanion
O(50)···F(11)
O(60)···F(12C)
2.738
2.801
O(50)···F(12C)
2.771
Osolv.···Osolv.
O(50)···O(70)
O(60F)···O(70)
O(70)···(F11)
2.620
2.794
2.756
Osolv.···Fanion
[a]
Symmetry operations used to generate equivalent atoms: 1a·H2O: (B) –x + 1, –y + 1, –z + 1. 1b: (A) x, y – 1, z; (B) 1 – x, y, 0.5 – z;
(C) –x + 1, y + 1, –z + 0.5. 1c·4H2O: (C) x + 1, y, z; (D) –x + 1, –y – 1, –z; (E) x, y – 1, z; (F) x + 1, y – 1, z. 2a: (B) x + 1, y, z + 1;
(C) x, y, z – 1. 2b·2H2O: (C) –x + 2, –y + 1, –z + 2; (D) x + 1, y, z; (E) x + 1, –y + 2, –z + 1; (F) –x + 2, –y + 1, –z + 1.
Figure 3. Molecular structure of the complex [CuII2(L2)(H2O)4(BF4)2] (2b·2H2O). The H2O solvates and hydrogen atoms, except those
of H2O co-ligands, have been omitted for clarity. Symmetry operations used to generate equivalent atoms: (A) 1 – x, 1 – y, 1 – z; (B)
x – 1, y, z.
–
nated BF4 ions are known.[19,20] In copper(ii) complexes evaporation of the solvent in air). Reactions carried out
–
with coordinated BF4 ions, Cu–F distances between without base resulted in the isolation of blue-green crystal
2.07 Å[21] and 2.85 Å[22] have been observed, but mostly the blocks, along with yellow crystals which, on the basis of
distances lie in the range 2.40–2.71 Å. In complex 2b·2H2O elemental and IR analyses, are believed to be (H4L1)-(BF4)2.
a 3D network is formed by hydrogen bonds, involving the The blue-green crystal blocks were identified as a mixture
–
amide oxygen atoms, both H2O co-ligands, the BF4 co- of the complexes [CuII2(L1)(H2O)4](SiF6) (1b) and
ligands and the H2O solvates (Figure S3, Supporting Infor- {[CuII2(L1)(H2O)2(μ-SiF6)]·4H2O}ϱ (1c·4H2O) by X-ray
mation).
crystallography (Figure 4 and Figure S4, Supporting infor-
Dinuclear copper(ii) compounds unexpectedly incorpo- mation, Table 1, Table 2, Table 3 and Table 4). The forma-
2–
rating SiF6 as counterions were obtained by reacting tion of the yellow crystals could be prevented by the ad-
Cu(BF4)2·4H2O with the lower ligand homologue H2L1, in dition of two equivalents of NEt3 as a base (Scheme 2),
a 2:1 metal-to-ligand ratio, using H2O as the solvent and which also resulted in an increased yield (ca. 70–80% yield)
2–
2–
a glass vial as the reaction and crystallisation vessel (slow of the desired SiF6 compounds. The occurrence of SiF6
Eur. J. Inorg. Chem. 2005, 1530–1541
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J. Klingele (née Hausmann), B. Moubaraki, K. S. Murray, J. F. Boas, S. Brooker
FULL PAPER
Figure 4. View of the [CuII2(L1)(H2O)2(μ-SiF6)] subunit of the polymeric chain structure of the cation of complex 1c·4H2O, emphasising
the coordination environment about Cu(1). Hydrogen atoms, except those of H2O co-ligands, have been omitted for clarity. Symmetry
operations used to generate equivalent atoms: (A) –x, –y – 1, –z; (B) –x, –y, –z.
–
ions in these compounds, rather than BF4 ions, was unex- tal ions and mononuclear subunits.[24,29–31] To the best of
pected but was readily confirmed by the negative ion ESI our knowledge, complex 1c·4H2O is the first polymeric
mass spectrum, which showed a peak at m/z = 123.0 corre- chain compound incorporating square-pyramidal metal
–
2–
sponding to the SiF5 anion, and the IR spectrum, which ions bridged by SiF6 ions and, at the same time, it is the
showed an intense broad split band (with peaks at 780 and first polymeric compound featuring discrete dinuclear sub-
750 cm–1) typical of the SiF6 ion. It has been reported units that are bridged by SiF62– ions. The Cu–F bond length
that SiF6 anions can arise in compounds originally con- in complex 1c·4H2O [Cu(1)–F(1) 2.235(1) Å] is around
2–
2–
–
taining BF4 ions as partial hydrolysis of the latter in the 0.3 Å shorter than in the dimeric complexes
presence of moisture can generate traces of HF, which then [{CuII(Hsabh)(H2O)}2(SiF6)]·2H2O (H2sabh = salicylalde-
2–
react with the glassware to form SiF6 ions.[23–27]
hyde benzoylhydrazone) (2.522 Å)[32] or [{CuII(Hspca)-
The molecular structure of the hexafluorosilicate com- (H2O)}2(SiF6)]·2H2O (H2spca = N3-salicyloylpyridine-2-
plex 1b (Figure S4, Supporting Information, Table 1, carboxamidrazone) (2.528 Å),[25] the only crystal structures
Table 2, Table 3 and Table 4) is very similar to the structure reported to date that contain square-pyramidal copper(ii)
2–
of the analogous tetrafluoroborate complex 1a·H2O. In the ions bridged by the apical positions by SiF6 ions. The
centrosymmetric complex 1b an N3O2 distorted square-py- intermolecular Cu···Cu distance in complex 1c·4H2O is
ramidal coordination environment about the copper(ii) ion 7.166(1) Å and thus is in between the distances reported for
is achieved by equatorially and axially coordinated H2O co- [{CuII(Hsabh)(H2O)}2(SiF6)]·2H2O
(6.04 Å)[32]
and
[14]
ligands (τCu(1) = 0.03). Hydrogen bonding, involving the [{CuII(Hspca)(H2O)}2(SiF6)]·2H2O (7.951 Å).[25] The Cu–
amide oxygen atom and the equatorial H2O co-ligand of F–Si angle in complex 1c·4H2O is only 130.45(7)° and is
two neighbouring subunits [Oamide···Owater 2.603 Å] and thus smaller than in both [{CuII(Hsabh)(H2O)}2(SiF6)]·
2–
both of the H2O co-ligands with the SiF6
anions 2H2O (144.0°)[32] and [{CuII(Hspca)(H2O)}2(SiF6)]·2H2O
[Fanion···Owater 2.760–3.377 Å], is also a feature of this struc- (140.1°)[25] and considerably smaller than in the six-coordi-
ture.
nate polymeric chain structures, where the M–F–Si angles
The related compound 1c·4H2O, obtained from the same range from 152° to 180°.[24,28,33] In complex 1c·4H2O the
2–
reaction mixture, features bridging SiF6 co-ligands. The parallel polymeric strands are interconnected through hy-
coordination environment about the copper(ii) ion is there- drogen bonds involving the equatorial H2O co-ligand and
fore best described as N3OF square-pyramidal (Figure 4; the amide oxygen atom [O(10)···O(1C) 2.662 Å]. Further-
τ[14] = 0.00).
more, the two H2O solvates per asymmetric unit are in-
A polymeric chain structure is formed by the SiF62– brid- volved in a network of hydrogen bonds involving all non-
2–
ges, which connect two apical positions of two neighbour- coordinated fluorine atoms of the SiF6 ligand and the
ing dinuclear subunits (Figure 5). The first crystal structure equatorial H2O co-ligand (Figure 5).
of a chain compound containing bridging SiF62– co-ligands,
In all of the above structures the Cu–Nligand bond lengths
[CoII(viz)4(SiF6)]ϱ (viz = N-vinylimidazole), was reported in follow the same general trend, specifically Cu–Nam Ͻ Cu–
1982 by Reedijk and co-workers.[28] Even now polynuclear Npy Ͻ Cu–Npz (Table 2). Another general, and expected,
compounds with bridging SiF62– ions are not very common trend is that in each case on deprotonation and coordina-
and all reported compounds incorporate six-coordinate me- tion to copper(ii) the υCO band of the free ligand moves to
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Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
Figure 5. Stereo view of a section of the 3D structure of {[CuII2(L1)(H2O)2(SiF6)]·4H2O}ϱ (1c·4H2O). Hydrogen atoms not involved in
hydrogen bonds have been omitted for clarity.
lower energy [1681 cm–1 for H2L1 to 1628–1634 cm–1 for the netically coupled, there is no clearly identifiable feature
complexes of (L1)2–; 1659 cm–1 for H2L2 to 1607 cm–1 for which explains this difference in J values. For complex 2b
the complexes of (L2)2–].
the χm values follow Curie behaviour while μeff is constant
at 1.86 μB down to 50 K, then increases to 1.92 μB at 4 K.
Fitting to a dinuclear model gave g = 2.14 and J =
+0.67 cm–1. The observed very weak ferromagnetic spin
coupling was unexpected, as the main difference between
the complexes 2b and 1a·H2O is only the more flexible eth-
ylene containing ligand arm and weak Cu–F coordination
in complex 2b. Furthermore, antiferromagnetic spin coup-
ling has been observed for various copper(ii) complexes of
the ligands N,NЈ-bis(2-pyridylmethyl)pyrazine-2,3-dicar-
boxamide (H2L3)[7] and N,NЈ-bis[2-(2-pyridyl)ethyl]pyr-
azine-2,3-dicarboxamide (H2L4),[35] which are isomeric to
H2L1 and H2L2, respectively.
Magnetic and EPR Studies
The temperature dependence of the magnetic suscep-
tibility was measured for the two complexes 1a·H2O and
2b over the range 300–4.2 K. The plots of the temperature
dependence of μeff and χm are given in the Figures S5–S8
(Supporting Information).
Complex 1a·H2O showed Curie-like behaviour at higher
temperatures (μeff = 1.91 μB) and only at temperatures be-
low 5 K a small, rapid decrease was observed. Conse-
quently, χm increased and no maximum was observed in the
plot χm vs. T. The best fit to the Bleaney–Bowers equa-
tion[34] was obtained from the parameters J = –0.24 cm–1,
The EPR spectrum of a polycrystalline powder of
g = 2.2, TIP = 60×10–6 cm3 mol–1. Setting J at zero gave a 1a·H2O at 120 K shows a strong resonance near g = 2, with
calculated line that did not reproduce the decrease in μeff no hyperfine structure being resolved on the parallel feature
values below 5 K. Calculations of μeff using the thermo- (Figure S9, Supporting Information). Simulations give g|| =
dynamic form of susceptibility, applicable to low tempera- 2.24 and gЌ = 2.06. An extremely weak resonance of similar
ture/high field combinations, rather than the Bleaney–Bow- shape is observed at g Ϸ 4 (Figure S10, Supporting Infor-
ers equation, show that the small decrease is partly, but not mation). The spectrum of a polycrystalline powder of
wholly, due to Zeeman level depopulation (saturation) ef- 2b·2H2O at both 260 K and 150 K (Figure S11, Supporting
fects in the field used (1 T). Nevertheless, the antiferromag- Information) shows an asymmetric resonance near g = 2,
netic coupling interaction was less than that (–7.5 cm–1) of with the features associated with g|| being unresolved and
a related dicopper(ii) complex of the isomeric ligand N,NЈ- tailing off to low field. The g-values are estimated as g|| =
bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H2L3).[7] 2.23 and gЌ = 2.05. Again, a weak resonance is observed
In the 2,3-dicarboxamide compound, an internal near g = 4 (Figure S12, Supporting Information). None of
O···H···O=C hydrogen-bond was present in the (HL3)– form the polycrystalline powder spectra showed a dependence on
of the bridging ligand and the terminal ligands to each cop- temperature between 120 K and 295 K. The appearance of
per(ii) centre were different to those in 1a·H2O. However, the resonances in the g Ϸ 2 and g Ϸ 4 regions is indicative
the Cu···Cu distances, τ values and trans-axial copper(ii) of the presence of dipolar and weak exchange interactions
geometries were very similar in both cases (vide infra). DFT between the copper(ii) ions.[36] The former give rise to spec-
calculations made on the 2,3-dicarboxamide complex tral broadening while the latter result in exchange nar-
yielded spin densities and spin populations on the pyrazine rowing, as is observed here. The effect of the different mag-
N-donor atoms compatible with weak antiferromagnetic nitudes of exchange interaction on the spectra is shown by
superexchange coupling of the CuII(dx2–y2) orbitals across the collapse of the hyperfine interaction in 1a·H2O and the
the pyrazine ring.[7] Thus, while 1a·H2O and this 2,3-dicarb- smearing out of the differences in g-value due to a larger
oxamide analogue are both very weakly antiferromag- exchange interaction in 2b·2H2O.
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J. Klingele (née Hausmann), B. Moubaraki, K. S. Murray, J. F. Boas, S. Brooker
FULL PAPER
The observation of resonances at g Ϸ 4 in the polycrys-
talline powder systems is additional evidence for interaction
between the copper(ii) ions. These resonances are not due
to FeIII impurities in either the cavity background or in the
powders themselves. Although in a three-dimensional ex-
tended lattice the g Ϸ 4 resonances are expected to be
[37]
smeared out unless Ed, Eex ϽϽ E0,
this inequality does
not hold in the present case, where the Zeeman energy E0
Ϸ 0.3 cm–1, the dipolar coupling energy Ed Ϸ 0.01 cm–1 and
Eex is –0.24 cm–1 for 1a·H2O and +0.67 cm–1 for 2b·2H2O.
However, this condition is relaxed for low-dimensional
magnetic systems. Thus the observation of resonances near
g = 4 for both complexes implies the existence of low-di-
mensional intermolecular exchange interactions in the solid
state.[37,38]
The EPR spectra of 1a·H2O and 2b·2H2O in frozen
DMF solution (ca. 1 mm) at 130 K were identical and
showed well resolved resonances at g Ϸ 2 and g Ϸ 4 as
shown in Figure 6 and Figure 7 respectively. These reso-
nances are typical of those observed for many dinuclear
copper(ii) complexes in dilute frozen solution[36] and are at-
tributed to the coupling of two copper(ii) ions, each with
electron spin S = ½, by dipole–dipole and exchange interac-
tions to give a triplet state (S = 1) and a singlet state (S =
0). The resonances in the g Ϸ 2 region are due to the ΔMs
= 1 transitions within the triplet state and those near g Ϸ
4 (“half field”) to the “forbidden“ or ΔMs = 2 transitions
between the Ms = 1 levels of the triplet state.
Figure 7. Half-field portion of EPR spectrum of 2b·2H2O in frozen
DMF solution at 130 K. Experimental spectrum (top): microwave
frequency 9.424 GHz; microwave power 10 mW; 100 kHz modula-
tion amplitude 10 G; receiver gain 5.0×105; scan time 84 s; time
constant 328 ms; average of 17 scans. Simulated spectrum (bottom):
computed using the parameters referred to in the text.
sent the range of parameters for which acceptable agree-
ment is found between experimental and simulated spectra.
There is no inherent contradiction between the value of J
= 0 found from the simulation of the frozen solution spectra
and the small values of J found by magnetic susceptibility
measurements, as the latter may arise from intermolecular
interactions in the solid state. Intermolecular exchange is
not expected to be significant in dilute frozen solution.
The spin Hamiltonian parameters from the simulation of
the frozen solution spectra can be taken as being those of
the individual copper(ii) ions and give gav = 2.14. This value
may be compared with the values of gav from the solid-state
powder spectra of 2.12 (1a·H2O) and 2.11 (2b·2H2O) and
from magnetic susceptibility of 2.20 (1a·H2O) and 2.14
(2b·2H2O). The g values of the solid-state powder spectra
and those from magnetic susceptibility measurements are
subject to significant sources of uncertainty. Nevertheless,
the magnetic susceptibility and EPR measurements show
that the copper(ii) ions retain the CuII(dx2–y2) orbital ground
state even in solution, consistent with the distorted square
pyramidal or octahedral coordination shown by the X-ray
crystallographic analysis.
Figure 6. EPR spectrum in the g = 2 region of 2b·2H2O in frozen
DMF solution at 130 K (approximately 1 mm). Experimental spec-
trum (top): microwave frequency 9.424 GHz; microwave power
2 mW; 100 kHz modulation amplitude 1 G; receiver gain 2.0×104;
scan time 167 s; time constant 82 ms. Simulated spectrum (bottom):
computed using the parameters referred to in the text.
Conclusions
The pyrazine-based bis(terdentate) diamide ligands H2L1
and H2L2 can be used for the formation of grid-type metal
The spectra can be simulated as shown in Figures 6 and complexes.[5] However, it has been found that on complex-
7 with the following spin Hamiltonian parameters g|| = ation with Cu(BF4)2·4H2O both ligands prefer to form di-
2.290 ( 0.005), gЌ = 2.080 ( 0.005), A|| = 170 copper(ii) complexes of the type [CuII2(L1)(co-ligand)4]2+ or
(
5)×10–4 cm–1, AЌ = 5 ( 5)×10–4 cm–1. A CuII···CuII dis- [CuII2(L2)(co-ligand)4]2+ featuring doubly deprotonated li-
tance of 6.9 ( 0.1) Å is obtained with the inter-nuclear gands and varying co-ligands. Unlike their isomeric ligands
vector in the xy plane of the magnetic axes of the copper(ii) N,NЈ-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide
ions (to within 15°), i.e. perpendicular to the z or symmetry (H2L3)[6] and N,NЈ-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-di-
axis of the individual ions. The best fit of experimental and carboxamide (H2L4),[35] where the addition of base is re-
simulated spectra is found with an isotropic exchange inter- quired to even monodeprotonate the ligand for complex-
action J = 0. The uncertainties, given in parentheses, repre- ation, no additional base is required to form complexes of
1538
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Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
the doubly deprotonated ligand species (L1)2– and (L2)2–.
All of the complexes described in this paper, except com-
pound 2b·2H2O, feature distorted square-pyramidal coor-
dination spheres for the copper(ii) ions. A distorted octahe-
treated with solid SeO2 (37.5 g, 337.5 mmol) and the resulting sus-
pension was refluxed for 18 hours. The resulting dark red-brown
mixture was evaporated to dryness under reduced pressure. H2O
(250 mL) was added and the solid elemental selenium was filtered
off. After evaporation of the red solution to dryness, the resulting
dark red brown solid was taken up in MeOH (150 mL), treated
with SOCl2 (3.5 mL) and refluxed for 8 hours. The resulting sus-
pension was then filtered whilst hot and the solid was washed with
CH2Cl2 (5×20 mL). The combined organic layers were reduced in
volume under reduced pressure (to ca. 100 mL) to give the product
–
dral coordination sphere with BF4 coordination is ob-
served in complex 2b·2H2O. In all cases the central pyrazine
ring bridges the metal ions. Very weak antiferromagnetic
spin coupling is observed in magnetic studies on complex
1a·H2O which features N4O five-coordinate copper(ii) ions,
whereas very weak ferromagnetic spin coupling is observed in the form of pale yellow-orange feathery crystals. The solid was
filtered off and washed with ice-cold MeOH (20 mL) to give 8.98 g
(45.8 mmol, 61%) of analytically pure II. M.p. 167–168 °C.
C8H8N2O4 (196.16): calcd. C 48.98, H 4.11, N 14.28; found C
49.24, H 3.87, N 14.25. TLC (SiO2, CH2Cl2/10% MeOH): Rf =
in studies of complex 2b, which features N3O2F six-coordi-
nate copper(ii) ions. X-band EPR studies confirm the very
weak exchange coupling in the solid state. The frozen solu-
tion spectra are interpreted in terms of dipolar coupling
with no intramolecular exchange and intra-cluster Cu···Cu
separations similar to those obtained by crystallography.
1
0.83. H NMR (300 MHz, CDCl3): δ = 9.39 (s, 2 H, pzH), 4.07 (s,
6 H, CH3) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 163.6
(pzCO), 145.6 (pzH), 145.3 (pzCO), 53.6 (OCH3) ppm. IR (KBr,
2–
Serendipitous formation of SiF6 ions, and the almost
disk): ν = 3549, 3472, 3417, 3077, 3015, 2960, 2853, 1720, 1637,
˜
2–
quantitative crystallisation of the corresponding SiF6
1618, 1472, 1431, 1359, 1279, 1202, 1181, 1144, 1020, 959, 824,
759, 616 495, 466, 425 cm–1.
compounds 1b and 1c·4H2O, has been observed when re-
acting H2L1 with Cu(BF4)2·4H2O in H2O as the solvent
and using a glass vial as both the reaction and crystallis-
ation vessel. Complex 1c·4H2O was found to be a rare ex-
ample of a structure featuring five-coordinate dicopper(ii)
N,NЈ-Bis(2-pyridylmethyl)pyrazine-2,5-dicarboxamide (H2L1):
A
suspension of II (1.96 g, 10.0 mmol) in MeOH (70 mL) was treated
with a solution of 2-(aminomethyl)pyridine (2.38 g, 22.0 mmol) in
MeOH (30 mL) and the resulting solution was kept in an open
flask at 80–90 °C for 4 hours, allowing most of the solvent to evap-
orate. Filtration of the resulting suspension gave 2.92 g (8.38 mmol,
84%) of H2L1 in the form of an analytically pure colourless pow-
der. M.p. 205–206 °C. C18H16N6O2 (348.36): calcd. C 62.06, H
4.63, N 24.12; found C 61.94, H 4.66, N 24.30. TLC (SiO2, CH2Cl2/
2–
subunits bridged by SiF6 ions.
Experimental Section
1
General Remarks: Elemental analyses were performed by the
Campbell Microanalytical Laboratory at the University of Otago.
Melting points were determined with a Gallenkamp melting point
10% MeOH): Rf = 0.60. H NMR (500 MHz, CDCl3): δ = 9.39 (s,
3
2 H, 2×pzH), 8.88 (t, J = 5.0 Hz, 2 H, 2×NH), 8.62 (ddd, J6,5
=
=
4
5
3
5.0, J6,4 = 2.0, J6,3 = 1.0 Hz, 2 H, 2×6-pyH), 7.69 (dt, J4,5
apparatus in open-glass capillaries and are uncorrected. 1H and 13
C
3J4,3 = 7.5, J4,6 = 2.0 Hz, 2 H, 2×4-pyH), 7.34 (td, J3,4 = 7.5,
4
3
4J3,5 = J3,6 = 1.0 Hz, 2 H, 2×3-pyH), 7.23 (ddd, J5,4 = 7.5, J5,6
5
3
3
NMR spectra were recorded with a Varian INOVA-300 or with a
Varian INOVA-500 spectrometer at 25 °C. Chemical shifts are
given relative to tetramethylsilane (TMS). IR spectra were recorded
with a Perkin–Elmer Spectrum BX FT-IR spectrophotometer over
the range 4000–400 cm–1. UV/Vis/NIR spectra were recorded with
a Varian CARY 500 Scan UV/Vis/NIR spectrophotometer over the
range 200–1400 nm. Molar conductivities were measured using
1 mm solutions with a Suntex SC-170 conductivity meter. ESI mass
spectra were recorded with a MicroMass LCT spectrometer. For
all compounds MeCN was used as the solvent. Magnetic data were
recorded over the range 300–4.2 K using a Quantum Design
MPMS5 SQUID magnetometer with an applied field of 1 T. Single-
crystal X-ray data were collected with a Bruker SMART CCD area
detector diffractometer (λ = 0.71073 Å). The structures were solved
by direct methods using SHELXS-97[39,40] and refined against F2
using full-matrix least-squares techniques with SHELXL-97.[41]
Continuous wave (CW) EPR spectra were obtained with a Bruker
ESP380FT/CW X-band spectrometer at Monash University, using
the standard rectangular TE012 cavity. Temperatures below room
temperature (295 K) down to 120 K were achieved with a Bruker
nitrogen flow insert in the cavity. The microwave frequency was
measured with an EIP microwave 548A frequency counter and the
g factors were determined by proton NMR and with reference to
the F+ line in CaO (2.0001 0.0001).[42] Spectrum simulations were
performed with either the Bruker SIMFONIA software or (for Fig-
ure 6 and Figure 7) with the SOPHE software described by Griffin
and co-workers.[43]
4
= 5.0, J5,3 = 1.0 Hz, 2 H, 2×5-pyH), 4.82 (d, J = 5.0 Hz, 4 H,
2×pyCH2) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 162.6
(pzCO), 156.0 (2-py), 149.5 (6-py), 146.4 (pzCO), 142.4 (pzH), 136.9
(4-py), 122.6 (5-py), 122.1 (3-py), 44.7 (CH2) ppm. IR (KBr, disk):
ν = 3458, 3417, 3338, 2922, 1681, 1572, 1522, 1461, 1434, 1363,
˜
1325, 1208, 1177, 1027, 997, 902, 758, 726, 648, 504, 460 cm–1.
N,NЈ-Bis[2-(2-pyridyl)ethyl]pyrazine-2,5-dicarboxamide
(H2L2):
This compound was synthesised in an analogous manner to the
preparation of H2L1, using 2-(2-aminoethyl)pyridine (2.69 g,
22.0 mmol) instead of 2-(aminomethyl)pyridine. Yield: 2.68 g
(7.12 mmol, 71%). M.p. 225–227 °C. C20H20N6O2 (376.42): calcd.
C 63.82, H 5.36, N 22.33; found C 63.51, H 5.42, N 22.69. TLC
(SiO2, CH2Cl2/10% MeOH): Rf = 0.70. 1H NMR (500 MHz,
CDCl3): δ = 9.28 (s, 2 H, 2×pzH), 8.59 (d, J = 5.0 Hz, 2 H, 2×6-
3
4
pyH), 8.54 (s, 2 H, 2×NH), 7.62 (dt, J4,5
= =
3J4,3 = 7.8, J4,6
2.0 Hz, 2 H, 2×4-pyH), 7.20–7.16 (m, 4 H, 2×3-pyH and 2×5-
pyH), 3.92 (q, J = 6.5 Hz, 4 H, 2×NHCH2), 3.13 (t, J = 6.5 Hz, 4
H, 2×pyCH2) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 162.5
(pzCO), 159.1 (2-py), 149.5 (6-py), 146.4 (pzCO), 142.2 (pzH), 136.7
(4-py), 123.5 (3-py), 121.8 (5-py), 38.8 (HNCH2), 37.0 (pyCH2)
ppm. IR (KBr, disk): ν = 3369, 3092, 3036, 3013, 1985, 2973, 1659,
˜
1590, 1569, 1535, 1475, 1461, 1441, 1367, 1321, 1286, 1263, 1196,
1169, 1049, 1036, 1017, 993, 951, 884, 857, 763, 745, 654, 633, 614,
512, 494, 472, 449 cm–1.
[CuII2(L1)(H2O)2](BF4)2 (1):
A solution of Cu(BF4)2·4H2O
Dimethyl Pyrazine-2,5-dicarboxylate (II): A solution of 2,5-dimeth-
ylpyrazine (8.11 g, 75.0 mmol) in pyridine/H2O (10:1) (165 mL) was
(149 mg, 482 μmol) in MeCN (10 mL) was added to a hot (60 °C)
solution of H2L1 (84.0 mg, 241 μmol) in MeCN (10 mL). The re-
Eur. J. Inorg. Chem. 2005, 1530–1541
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J. Klingele (née Hausmann), B. Moubaraki, K. S. Murray, J. F. Boas, S. Brooker
FULL PAPER
sulting dark bottle-green solution was left at this temperature for
10 minutes and was then cooled to room temperature. By vapour
diffusion of Et2O into the reaction mixture, 106 mg (155 μmol,
64%) of complex 1 were obtained in the form of a dark turquoise
the molecular structure of complex 1b (Figure S4), the curves for
the molar magnetic susceptibilities and effective magnetic moments
of complexes 1a and 2b over the range 300–4.2 K (Figures S5–S8),
the EPR spectra in g = 2 and in low-field regions of powdered
solid. Single crystals of [CuII2(L1)(MeCN)2(H2O)2](BF4)2·H2O samples of 1a·H2O and 2b·2H2O (Figures S9–S12) and some back-
(1a·H2O) suitable for X-ray analysis were obtained by slow evapo-
ground information on the effect on EPR spectra of dipole–dipole
and exchange interactions. CCDC-252079 (for 1a·H2O), -252080
(for 1b), -252081 (for 1c·4H2O), -252082 (for 2a) and -252083 (for
2b·2H2O) contain the supplementary crystallographic data for this
ration of the reaction solution, obtained as described above. [CuII
-
2
(L1)(H2O)2](BF4)2 (C18H18B2N6O4F8Cu2) (683.08): calcd. C 31.65,
H 2.66, N 12.30; found C 31.86, H 2.23, N 12.30. IR (KBr, disk):
ν = 3424, 3110, 2897, 1634, 1567, 1485, 1449, 1436, 1416, 1361, paper. These data can be obtained free of charge from The Cam-
˜
1350, 1335, 1288, 1214, 1190, 1167, 1083, 1062, 923, 772, 743, 715, bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
656, 635, 585, 556, 533, 521, 465, 415 cm–1. ESI-MS (pos, MeCN):
m/z (fragment) = 571.5 ([CuICuII(L1)(H2O)(MeCN)2]+), 531.7
([CuICuII(L1)(H2O)(MeCN)]+), 450.7 ([CuII(HL1)(MeCN)]+),
276.9 ([CuII2(L1)(MeCN)2]2+), 237.9 ([CuI2(H2L1)]2+), 225.9
([CuII(H2L1)(MeCN)]2+). UV/Vis/NIR (MeCN): λmax. (ε) = 212
(34100), 261 (20500), 349 (4200), 641 nm (204 m–1 cm–1). Λm
(MeCN) = 231 Ω–1 mol–1 cm2.
data_request/cif.
Acknowledgments
This work was supported by grants from the University of Otago
(Postgraduate Scholarship to J. K. and a Bridging Grant) and the
Australian Research Council (Discovery Grant). J. F. B. acknowl-
edges support from the School of Physics and Materials Engineer-
ing, Monash University. J. K. thanks Dr. M. H. Klingele (Univer-
sity of Otago) for many helpful discussions. S. B. is grateful to the
University of Otago for the granting of sabbatical leave which has
facilitated the completion of this manuscript and thanks her host
Prof. A. K. Powell (University of Karlsruhe) for helpful discussions
and the DFG Research Centre for Functional Nanostructures
(CFN) for the award of a visiting Professorship. We thank Prof. W.
T. Robinson and Dr. J. Wikaira (University of Canterbury) and
Prof. G. Clark, Assoc. Prof. P. D. W. Boyd and T. Groutso (Univer-
sity of Auckland) for the X-ray data collections.
[CuII2(L1)(H2O)4](SiF6)
(1b)
and
{[CuII2(L1)(H2O)2(μ-
SiF6)]·4H2O}ϱ (1c·4H2O): Solid H2L1 (40.1 mg, 115 μmol) was
treated with a solution of Cu(BF4)2·4H2O (71.1 mg, 230 μmol) in
H2O (5 mL) and the resulting dark bottle green solution was fur-
ther treated with a solution of NEt3 (23.3 mg, 230 μmol) in H2O
(3 mL). The resulting blue-green solution was transferred into a
sample vial (SAMCO specimen tubes, soda glass, 75×25/26 mm,
ISO 9002) and was left to slowly evaporate. After 4 weeks 66 mg
(96 μmol, 83%) of huge blue-green crystal blocks were isolated by
filtration. The crystal blocks analysed as [CuII2(L1)](SiF6)·4H2O.
Single crystals suitable for X-ray crystal structure determinations,
[CuII2(L1)(H2O)4](SiF6) (1b) and {[CuII2(L1)(H2O)2(μ-SiF6)]
·4H2O}ϱ (1c·4H2O), were selected from these blocks before fil-
tration. [CuII2(L1)](SiF6)·4H2O (C18H22N6O6F6SiCu2) (687.58): C
31.44, H 3.23, N 12.22; found C 31.45, H 3.23, N 12.08. IR (KBr,
disk): ν = 3442, 3084, 2872, 1628, 1563, 1481, 1449, 1407, 1352,
˜
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Supporting Information: (See also footnote on the first page of this
article) A PDF file (5 pages) with supporting information for this
article is available on the WWW under http://www.eurjic.org or
from the authors. It contains a view of the extended 3D structures
of complexes 2a, 1a·H2O and 2b·2H2O (Figures S1–S3), a view of
1540
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 1530–1541

Dinuclear Copper(ii) Complexes of Pyrazine-Based Bis(terdentate) Diamide Ligands
FULL PAPER
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Received: October 27, 2004
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