Influence of the counter anion and solvent in the structure of copper derivatives with the 2,3-bis(2-pyridyl)pyrazine ligand

Article abstract of DOI:10.1016/j.ica.2010.03.080

Several compounds have been isolated from the reaction between different copper bis(acetylacetonato) derivatives and the potentially bridging ligand 2,3-bis(2-pyridyl)pyrazine (bppz). A compound of formula [Cu(tfacac) ₂(bppz)] (1) is obtained when the substituted trifluoromethylacetylacetonato is used. The use of different anions and the unsubstituted acetylacetonato give rise to new derivatives of general formula [{Cu(acac))₂(μ-bppz)₂]X₂ (X^-- BF₄^-, 2; PF₆^-, 3; BPh ₄^-, 4). In these compounds the bppz ligand is acting as a bridge by chelating one copper atom and bonding monodentate a second copper atom. The presence of anions with different coordination abilities introduces variations in the copper environment and geometry. When the non-coordinating tetraphenylborate is used different compounds depending on the nature of the solvent are obtained. The dimer 4 was isolated from a methanol/chloroform mixture, while in the absence of chloroform the monomeric compound of formula [Cu(acac)(bppz)(ROH)](BPh₄)-ROH (ROH=MeOH, 5) was obtained. When ethanol was used instead of methanol the analogous derivative 6 (R=EtOH) was isolated. Both species show a mononuclear structure with the copper atom five-coordinated by the chelating acac and bppz ligands and one hydroxo group occupying the apical position. A similar environment for the copper appears in [Cu(tfacac)(bppz)(MeOH)](BPh₄), 7, which shows a dimeric structure through hydrogen bonds interactions. The magnetic susceptibility data of the dimeric compounds show very weak antiferromagnetic interactions between the copper atoms, an expected fact since the bridging bppz ligand is not planar but the monodentate pyridine is more or less perpendicular to the other two aromatic rings, precluding the spin exchange via the it ligand electrons.

Full text of DOI:10.1016/j.ica.2010.03.080

Inorganica Chimica Acta 363 (2010) 2443–2451
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Influence of the counter anion and solvent in the structure of copper
derivatives with the 2,3-bis(2-pyridyl)pyrazine ligand
a
a
a
Angel Gutiérrez a, , M. Felisa Perpiñán , Ana E. Sánchez , M. Carmen Torralba ,
*
M. Rosario Torres b, M. Pilar Pardo c
a Departamento de Química Inorgánica I, Universidad Complutense, 28040 Madrid, Spain
b CAI de Difracción de Rayos X, Universidad Complutense, 28040 Madrid, Spain
c Departamento de Química, Facultad de Farmacia, Universidad CEU San Pablo, Boadilla del Monte, 28668 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Several compounds have been isolated from the reaction between different copper bis(acetylacetonato)
derivatives and the potentially bridging ligand 2,3-bis(2-pyridyl)pyrazine (bppz). A compound of formula
[Cu(tfacac)2(bppz)] (1) is obtained when the substituted trifluoromethylacetylacetonato is used. The use
of different anions and the unsubstituted acetylacetonato give rise to new derivatives of general formula
Received 15 January 2010
Received in revised form 25 March 2010
Accepted 31 March 2010
Available online 14 April 2010
[{Cu(acac)}2(l
-bppz)2]X2 (Xꢀ = BF4ꢀ, 2; PF6ꢀ, 3; BPh4ꢀ, 4). In these compounds the bppz ligand is acting as
a bridge by chelating one copper atom and bonding monodentate a second copper atom. The presence of
anions with different coordination abilities introduces variations in the copper environment and geom-
etry. When the non-coordinating tetraphenylborate is used different compounds depending on the nat-
ure of the solvent are obtained. The dimer 4 was isolated from a methanol/chloroform mixture, while in
the absence of chloroform the monomeric compound of formula [Cu(acac)(bppz)(ROH)](BPh4)ꢁROH
(ROH = MeOH, 5) was obtained. When ethanol was used instead of methanol the analogous derivative
6 (R = EtOH) was isolated. Both species show a mononuclear structure with the copper atom five-coordi-
nated by the chelating acac and bppz ligands and one hydroxo group occupying the apical position. A
similar environment for the copper appears in [Cu(tfacac)(bppz)(MeOH)](BPh4), 7, which shows a dimeric
structure through hydrogen bonds interactions. The magnetic susceptibility data of the dimeric com-
pounds show very weak antiferromagnetic interactions between the copper atoms, an expected fact since
the bridging bppz ligand is not planar but the monodentate pyridine is more or less perpendicular to the
Keywords:
Crystal engineering
Bridging ligands
Hydrogen bonds
Non-covalent interactions
other two aromatic rings, precluding the spin exchange via the
p
ligand electrons.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
bipyridine based rod-like ligands [21,27], whereas bent ligands
are relatively less used as building blocks, probably due to the
greater difficulty in predicting the resulting structures [28]. It is
to note that among the multifunctional ligands, the poly(pyridyl)
ligands that can bridge two or more remote metal centres and that
The use of unsaturated transition metal coordination complexes
and organic bidentate or multifunctional ligands as precursors to
organic/inorganic hybrid materials is one of the most important
current developments in chemical research, not only because of
their novel topologies and structural diversities, but also for their
potential applications in optical, electric, magnetic, and micropo-
rous materials [1–13]. Generally, some control over the type and
topology of the product generated from the self-assembly of inor-
ganic metal species and organic ligands can be achieved by careful
choice of the building blocks (metal complex and organic ligand),
solvent system, inorganic counter ion or metal-to-ligand ratio
[14–23]. Many examples of polymer compounds connected by
bridging ligands have been reported [24–26]. In particular, the re-
search has been concentrated on the use of pyrazine and 4,40-
also contain a delocalized
p system have received considerable
attention in recent coordination chemistry [29–34].
On the other hand, the unsaturated [M(b-diketonato)2] com-
plexes have been extensively used in the formation of metal–or-
ganic frameworks, either as precursors for the synthesis of metal
coordination clusters with interesting magnetic properties [35–
37] or as coordination synthons for the creation of extended supra-
molecular architectures [38–40].
With these considerations in mind, the aim of this work is to
study the interaction between the unsaturated metal complexes,
[Cu(acac)2] (acac = acetylacetonato) or [Cu(tfacac)2] (tfacac = tri-
fluoromethylacetylacetonato), and the bent poly(pyridyl) ligand,
2,3-bis(2-pyridyl)pyrazine (bppz). The latter is a potential tetra-
dentate ligand that can both chelate and bridge metal atoms
* Corresponding author. Tel.: +34 91 394 4303; fax: +34 91 394 4352.
E-mail address: alonso@quim.ucm.es (A. Gutiérrez).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.03.080

2444
A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
due to its appropriate geometry and conformational non-rigidity.
Moreover, the ligand would not be constrained to a planar confor-
mation allowing the interconnection of chains and propagation of
the structure in two or three dimensions. In contrast to rigid poly-
pyridyl systems, systematic studies on the metal complexes of the
bppz and related ligands are relatively rare. The bppz ligand is a
non-innocent ligand that can show several bonding modes. The
X-ray crystal structures of transition metal complexes with bppz
and related ligands reveal that these ligands can adopt not only
the anticipated terminal five-membered bidentate (I) [41–56]
and bridging bis-bidentate (II) [46,49,54,57–67] coordination
modes, but also the less common terminal seven-membered
bidentate (III) [47,68–74], the bidentate/monodentate (IV)
[46,47,62,75,76], bidentate/bis-monodentate (V) [62], and bis-
monodentate (VI) [75,76] bridging modes shown in Scheme 1.
Delicate tuning of the bonding mode can contribute to the devel-
opment of metal complexes that exhibit different structures and
desirable properties.
The comparative study of the reactions of [Cu(acac)2] and
[Cu(tfacac)2] can show the different Lewis acidic character of the
acetylacetonate complex used. It is to note that very few com-
plexes of [Cu(tfacac)2] with N-donor bridging species have been re-
ported [28,77–81].
The study of the interaction of [Cu(acac)2] and [Cu(tfacac)2]
with bppz was also carried out in presence of different anions
(BF4ꢀ, PF6ꢀ, BPh4ꢀ). These anions show differences in volume,
shape, and coordinative ability (BF4ꢀ, is a moderately coordinating
anion, while PF6ꢀ and BPh4ꢀ are practically non-coordinating), that
may be important in the synthetic process to determine the prop-
erties and the structures of the final assemblies [24,28,82,83].
2.2. X-ray structures determination
Good quality crystals of the new compounds have been col-
lected directly from the synthetic reaction. A summary of the fun-
damental crystal data is given in Table 1. The crystal data were
collected in the ‘‘CAI de Difracción de Rayos X, UCM”. In all the
cases a green (orange for 2) crystal was resin epoxy coated and
mounted on a Bruker Smart CCD diffractometer using graphite
monochromated Mo K
a radiation (k = 0.71073 Å) operating at
50 kV and 25 mA. Data were collected over a reciprocal space
hemisphere by combination of three exposure sets. Each frame
exposure time was 20 s covering 0.3° in
x. The cell parameters
were determined and refined by least-squares fit of all reflections
collected. The first 50 frames were recollected at the end of the
data collection to monitor crystal decay, and no appreciable decay
was observed. No absorption correction was applied. All the struc-
tures were solved by direct methods and refined by applying full-
matrix least-squares on F2 with anisotropic thermal parameters for
the non-hydrogen atoms. The hydrogen atoms were included with
fixed isotropic contributions at their calculated positions deter-
mined by molecular geometry, except for the hydroxo groups in
4–7 whose hydrogen atoms were located from the Fourier map
and refined riding on the corresponding oxygen atoms. All the cal-
culations were carried out with the SHELX97 software package [84].
2.3. Synthesis of [Cu(tfacac)2(bppz)] (1)
A solution of bppz (0.117 g, 0.5 mmol) in chloroform (8 mL) was
added to a stirred solution of [Cu(tfacac)2] (0.185 g, 0.5 mmol) in a
mixture of ethanol (5 mL)/chloroform (5 mL). The dark green
solution was stirred for 1 h and cooled to 0 °C for 15 d affording
yellow-green crystals of 1. Yield: 0.190 g, 63%. Anal. Calc. for
C
24H18CuF6N4O4: C, 47.73; H, 3.00; N, 9.28. Found: C, 47.64; H,
2. Experimental
3.01; N, 9.33%. IR (KBr, cmꢀ1): 3071 w, 1650 vs, 1630 vs, 1526 s,
1474 s, 1404 m, 1359 m, 1301 m, 1279 vs, 1218 m, 1179 s, 1131
vs, 1061 w, 1036 w, 933 w, 925 w, 876 w, 851 m, 823 w, 789 s,
773 s, 742 m, 726 m, 666 w, 642 w, 620 w, 584 w, 560 m, 463
m, 453 w, 410 w.
2.1. Physical measurements
Elemental analyses were carried out by the ‘‘Servicio de Micro-
análisis” of the Universidad Complutense de Madrid. Infrared spec-
tra were recorded as KBr pellets on a Thermo Nicolet 200 FT-IR
spectrophotometer. Magnetic experiments were made on poly-
crystalline samples using a SQUID magnetometer MPMS-XL-5
manufactured by Quantum Design. The temperature dependence
of the magnetisation in the range between 2 and 300 K was re-
corded using a constant magnetic field of 0.5 T. The experimental
data have been corrected for the magnetisation of the sample
holder (gelatine) and for atomic diamagnetism as calculated from
the known Pascal’s constants.
2.4. Synthesis of [{Cu(acac)}2(l-bppz)2](BF4)2 (2)
An orange solution of bppz (0.234 g, 1 mmol) and acetylacetone
(0.1 mL, 1 mmol) in acetonitrile (40 mL) was added to a stirred
solution of Cu(BF4)2ꢁH2O (0.237 g, 1 mmol) in 20 mL of acetonitrile.
The green solution was stirred for 1.5 h. Evaporation at room tem-
perature to almost dryness yielded orange crystals of 2. Yield:
0.193, 40%. Anal. Calc. for C38H34B2Cu2F8N8O4: C, 47.18; H, 3.54;
N, 11.58. Found: C, 47.09; H, 3.47; N, 11.62%. IR (KBr, cmꢀ1):
3080 w, 3001 w, 2967 w, 2922 w, 1580 vs, 1525 vs, 1477 m,
1407 s, 1378 vs, 1301 m, 1285 s, 1242 w, 1175 s, 1070 vs, 933 s,
880 m, 823 m, 798 s, 787 s, 778 s, 759 m, 730 w, 693 w, 665 w,
648 w, 624 w, 598 m, 581 m, 521 m, 463 m, 447 m, 421 m.
N
N
N
N
N
N
N
N
M
M
M
N
N
N
N
2.5. Synthesis of [{Cu(acac)}2(l-bppz)2](PF6)2 (3)
M
I
II
III
An orange solution of bppz (0.117 g, 0.5 mmol) and KPF6
(0.184 g, 0.5 mmol) in methanol (40 mL) was added to a stirred
suspension of [Cu(acac)2] (0.131 g, 0.5 mmol) in 20 mL of metha-
nol. The dark green reaction mixture was stirred for 3 d and fil-
tered. Evaporation at room temperature to dryness yielded a
mixture of products, which was extracted with 10 mL of chloro-
form to remove the unreacted [Cu(acac)2]. The remaining green so-
lid was washed with water and dried under vacuum. Yield: 0.130 g,
48%. Anal. Calc. for C38H34Cu2F12N8O4P2: C, 42.11; H, 3.16; N, 10.34.
Found: C, 42.13; H, 3.22; N, 10.50%. IR (KBr, cmꢀ1): 3130 w, 3108
w, 3077 w, 3065 w, 2999 w, 2965 w, 2920 w, 1578 vs, 1546 w,
M
N
N
N
N
N
N
M
M
M
M
M
M
N
N
N
N
N
N
VI
IV
V
Scheme 1. Different coordination modes observed for the bppz ligand.

A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
2445
Table 1
Crystallographic data for compounds 1–7.
1
2
3
4
5
6
7
Formula
C24H18CuF6N4O4 C38H34B2Cu2F8N8O4 C38H34Cu2F12N8O4P2 C87H78B2Cu2N8O5 C45H45BCuN4O4 C47H49BCuN4O4 C44.25H39.25BCuF3N4O3.25
Formula weight 603.96
967.44
293
1083.76
293
1464.28
293
780.16
293
808.25
293
810.40
293
T (K)
293
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c (14)
8.7441(7)
8.5873(7)
34.655(3)
90
95.308(2)
90
2591.1(4)
4
Monoclinic
P21/n (14)
13.0707(16)
10.8279(13)
13.9376(17)
90
95.877(2)
90
1962.2(4)
2
Triclinic
P1 (2)
Triclinic
P1 (2)
Monoclinic
P21/n (14)
16.8020(14)
12.5212(11)
19.7715(17)
90
92.800(2)
90
4154.6(6)
4
Triclinic
P1 (2)
Monoclinic
P21/n (14)
13.1997(16)
13.5873(16)
22.837(3)
90
97.737(2)
90
4058.5(8)
4
ꢀ
ꢀ
ꢀ
9.0476(11)
9.6927(12)
13.5708(17)
104.082(2)
103.490(2)
99.655(2)
1090.2(2)
1
1.651
1.150
0.0488
0.0994
12.3222(18)
12.5984(18)
15.023(2)
111.377(3)
91.744(3)
115.242(3)
1915.9(8)
1
1.269
0.613
0.06136
0.1301
10.9641(16)
12.9280(19)
15.963(2)
79.828(3)
87.686(3)
74.775(3)
2148.8(5)
2
1.249
0.555
0.0509
0.1081
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g cmꢀ3
)
1.548
0.924
0.0483
0.0944
1.637
1.175
0.0418
0.1051
1.247
0.572
0.0479
0.1019
1.326
0.597
0.0490
0.1137
l
(mmꢀ1
)
r
R1,a [I > 2
(I)]
wR2,b [I > 2
r(I)]
a
R1
wR2 = {
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
2
R
[w(Fo ꢀ Fc2)2]/
R
[w(Fo2)2]}1/2
.
1520 vs, 1477 m, 1411 m, 1376 vs, 1297 m, 1277 m, 1250 w, 1202
w, 1164 s, 1125 w, 1105 w, 1072 m, 1063 m, 1005 m, 907 m, 873
m, 840 vs, 777 m, 752 m, 681 w, 661 w, 647 w, 633 w, 592 w, 577
w, 557 s, 450 m, 416 m.
3001 w, 2983 m, 2925 w, 2824 w, 1575 vs, 1519 vs, 1477 s, 1451
w, 1424 m, 1407 s, 1375 vs, 1303 m, 1279 m, 1249 w, 1174 s,
1162 m, 1134 w, 1124 w, 1105 w, 1068 m, 1031 s, 1023 s, 998
m, 935 m, 874 w, 862 w, 843 w, 820 w, 788 m, 770 m, 748 s,
734 vs, 706 vs, 667 w, 612 s, 574 w, 559 m, 468 w, 454 w, 444
m, 423 w.
2.6. Electrochemical synthesis of 3
The electrochemical procedure used in this synthesis was sim-
ilar to that described by Tuck and co-workers [85]. The cell was a
tall-form beaker (100 mL) fitted with a rubber bung through which
the electrochemical leads entered the cell. The anode was a copper
wire and the cathode was a platinum wire. The electrolysis of an
acetonitrile solution (60 mL) containing acetylacetone (0.1 mL,
1 mmol), bppz (0.234 g, 1 mmol), and KPF6 (0.184 g, 1 mmol), act-
ing the last one as reactive and as supporting electrolyte, using a
d.c. current of 20 V and 20 mA for 1 h and 20 min dissolved
0.64 g of copper (Ef ꢂ 1 mol Fꢀ1). Gaseous nitrogen was bubbled
during the reaction time. Hydrogen was evolved at the cathode,
the colour of the solution changed from pale yellow to dark red
and a deposit of copper metal was formed in the bottom and walls
of the vessel and on the cathode. The cell can be summarized as
follow: Pt(ꢀ)/CH3CN + acacH + bppz + KPF6/Cu(+). At the end of
the experiment the reaction mixture was filtered and the filtrate
was cooled to 0 °C for 2 weeks affording a mixture of blue and
green crystals. The mixture was filtered, and the blue crystals,
identified as [Cu(acac)2], were eliminated of the solid obtained by
repeated washing with chloroform. The green crystals were
washed with acetonitrile and dried under vacuum affording a yield
of 0.119 g, 22%. These crystals were suitable for X-ray crystallo-
graphic investigation.
2.8. Synthesis of [Cu(acac)(MeOH)(bppz)](BPh4)ꢁMeOH (5)
An orange solution of bppz (0.117 g, 0.5 mmol) and NaBPh4
(0.171 g, 0.5 mmol) in methanol (25 mL) was added to a stirred
suspension of [Cu(acac)2] (0.131 g, 0.5 mmol) in methanol
(25 mL). The dark green reaction mixture was stirred for 2.5 h
and filtered. The filtrate was kept at room temperature for 4 d
affording green crystals of 5, which were filtered off and washed
with methanol/water (1:1) and dried under vacuum. Yield:
0.300 g, 77%. Anal. Calc. for C45H45BCuN4O4: C, 69.27; H, 5.81; N,
7.18. Found: C, 69.00; H, 5.64; N, 7.19%. IR (KBr, cmꢀ1): 3612 w,
3326 m, 3155 m, 3055 m, 2999 w, 2981 w, 2938 w, 2824 w,
1578 vs, 1522 vs, 1478 s, 1451 m, 1424 s, 1377 vs, 1302 m, 1281
m, 1250 w, 1174 s, 1162 m, 1126 m, 1094 m, 1062 m, 1031 s,
1015 s, 1002 w, 933 m, 866 m, 843 w, 820 w, 791 s, 771 m, 747
m, 734 vs, 705 vs, 667 w, 612 s, 584 w, 562 w, 459 m, 421 w.
2.9. Synthesis of [Cu(acac)(EtOH)(bppz)](BPh4)ꢁEtOH (6)
A solution of [Cu(acac)2] (0.131 g, 0.5 mmol) in chloroform
(10 mL) was added to a stirred solution of NaBPh4 (0.171 g,
0.5 mmol) and bppz (0.117 g, 0.5 mmol) in a mixture of ethanol
(10 mL)/chloroform (5 mL). The dark green solution was stirred
for 1 h while its colour changed to brown-green. Cooling to 0 °C
for 3 d afforded green crystals of 6, which were filtered off and
washed with water, ethanol and finally with a small volume of
chloroform. Yield: 0.254 g, 63%. Anal. Calc. for C47H49BCuN4O4: C,
69.84; H, 6.11; N, 6.93. Found: C, 69.69; H, 5.98; N, 6.83%. IR
(KBr, cmꢀ1): 3600 w, 3118 w, 3057 m, 3037 m, 3002 w, 2983 m,
2914 w, 1574 vs, 1519 vs, 1477 s, 1451 w, 1424 m, 1407 s, 1374
vs, 1303 m, 1279 m, 1249 w, 1213 w, 1174 s, 1162 m, 1129 w,
1124 w, 1106 w, 1068 m, 1031 s, 1023 s, 998 m, 935 m, 878 w,
843 w, 820 w, 809 w, 789 m, 770 m, 761 m, 748 s, 734 vs, 706
vs, 664 w, 623 w, 611 s, 575 w, 559 m, 468 w, 453 w, 444 m,
423 m, 405 w.
2.7. Synthesis of [{Cu(acac)}2(
l
-bppz)2](BPh4)2ꢁMeOH (4)
A solution of [Cu(acac)2] (0.131 g, 0.5 mmol) in chloroform
(10 mL) was added to a stirred solution of NaBPh4 (0.171 g,
0.5 mmol) and bppz (0.117 g, 0.5 mmol) in a mixture of methanol
(10 mL)/chloroform (5 mL). The dark green solution was stirred for
24 h obtaining a brown solution which was cooled to 0 °C for 5 d
affording amber-green crystals of 4. The mixture was filtered and
the solid obtained was washed with water, methanol and finally
with a small volume of chloroform. Yield: 0.260 g, 71%. Anal. Calc.
for C87H78B2Cu2N8O5: C, 71.36; H, 5.37; N, 7.65. Found: C, 70.95; H,
5.47; N, 7.53%. IR (KBr, cmꢀ1): 3600 w, 3113 w, 3057 m, 3033 m,

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A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
1
2.10. Synthesis of [Cu(tfacac)(MeOH)(bppz)](BPh4)ꢁ 4MeOH (7)
[{Cu(acac)}2(
ligand connects two copper(II) ions in a bridging bidentate/mono-
dentate mode (IV). The analogous complex, [{Cu(acac)}2(
bppz)2](BF4)2 (2), cannot be obtained by this procedure, but it
has been isolated in the reaction of equimolecular amounts of
Cu(BF4)2ꢁH2O, Hacac, and bppz.
l
-bppz)2]X2 (Xꢀ = PF6ꢀ, 3; BPh4ꢀ, 4), where each bppz
A solution of [Cu(tfacac)2] (0.185 g, 0.5 mmol) in methanol
(25 mL) was added to a stirred solution of bppz (0.117 g, 0.5 mmol)
and NaBPh4 (0.171 g, 0.5 mmol) in methanol (25 mL). The green
solution obtained was stirred for a few minutes and kept at room
temperature for 2 h affording green crystals of 7. The solution
was filtered and kept at room temperature for 2 d affording a sec-
ond batch of crystals, which were washed with methanol and dried
under vacuum. Yield: 0.389 g, 96%. Anal. Calc. for C44.25H39BCuF3-
N4O3.25: C, 65.60; H, 4.85; N, 6.91. Found: C, 65.22; H, 4.90; N,
6.90%. IR (KBr, cmꢀ1): 3634 w, 3539 w, 3431 m, 3242 m, 3054 m,
1620 vs, 1592 m, 1580 m, 1560 w, 1528 m, 1477 m, 1455 m,
1405 m, 1365 w, 1296 vs, 1230 m, 1198 s, 1142 s, 1099 w, 1064
w, 1023 m, 1001 w, 951 w, 866 m, 788 m, 763 w, 750 s, 737 vs,
710 vs, 613 m, 593 w, 582 w, 563 w, 546 w, 446 w, 434 w, 422 w.
l-
In an attempt to get the polymeric related species, compound 3
has also been obtained by electrochemical synthesis, using a cop-
per anode and a platinum cathode in a solution of acetylacetone
and bppz ligands in acetonitrile. The electrochemical efficiency
for the process Ef, defined as the number of moles of metal dis-
solved per faraday of charge, was close to 1. This value shows that
the anodic oxidation leads initially to a CuI species, which is either
oxidized to CuII later in solution or is disproportioned to CuII and
Cu0. This behaviour has already been observed in other systems
in which species of low oxidation states are the initial electro-
chemical products [86,87]. The appearance of a copper metallic
mirror in the walls of the electrochemical cell supports the latter
process. This fact, along with the evolution of hydrogen at the cath-
ode, suggests that the possible mechanism that takes place is that
shown in Scheme 3. This type of synthesis had already showed to
be quite effective in obtaining mixed complexes in a single step by
simply adding a second ligand to the cell [80,86].
3. Results and discussion
3.1. Synthetic procedures
Scheme 2 shows the reactions carried out in this work. Different
types of compounds have been obtained depending on the exper-
imental conditions: starting copper complex, nature of the solvent
or presence of anions in the reaction mixture. All samples have
been characterised by analytical and IR spectroscopic techniques.
Details are given in Section 2. All the new compounds are essen-
tially discrete molecules as established by the following X-ray
crystal characterisation. They are air- and light-stable crystalline
solids.
The neutral and mononuclear complex, [Cu(tfacac)2(bppz)] (1),
has been obtained by direct reaction at room temperature of a mix-
ture of equimolecular amounts of [Cu(tfacac)2] and the bppz ligand
in a ethanol/chloroform mixture. All the attempts made with
[Cu(acac)2] to obtain the analogous derivative to 1, (direct chemi-
cal reaction, solvothermal synthesis in methanol at 140 °C or elec-
trochemical synthesis in acetonitrile) were unsuccessful. This fact
can be explained for the lower Lewis acidic character of the [Cu(a-
cac)2] relative to the [Cu(tfacac)2], that favours the increase of the
copper coordination number in the latter compound.
It is to note that, depending on the experimental conditions, the
ꢀ
reaction with BPh4 as counter anion affords either the dimeric 4
or a second type of derivatives with general formula [Cu(R-aca-
c)(ROH)(bppz)](BPh4)ꢁROH (5–7), in which one acac or tfacac li-
gand has been displaced from the copper environment. In these
compounds the alcohol molecule occupies the coordination posi-
tion that was available for the bridging bppz ligand in 2–4, giving
rise to the corresponding mononuclear derivative.
Since the only difference in the synthesis of 4 and 5 is the pres-
ence of chloroform as solvent in the former, we can assume a com-
petition in the coordination to the apical position of the copper
atom between the pyridine of the bridging bppz ligand and the
alcohol present in the mixture, the increase in the concentration
seems to be the reason for the coordination of the alcohol and
the stabilization of the monomeric species.
3.2. IR spectra
Nevertheless, [Cu(acac)2] reacts with bppz in presence of a
counter anion, in a 1:1:1 M ratio, with displacement of one acetyl-
acetonate group and formation of the cationic cyclic dimers,
All the complexes show the characteristic bands of the corre-
sponding chelating ligands, around 1580 and 1520 cmꢀ1 for the
[{Cu(acac)}2(µ-bppz)2](BF4)2 (2)
[{Cu(acac)}2(µ-bppz)2](PF6)2 (3)
CH3CN
CH3CN
NaBF4
KPF6
MeOH
Cu(BF4)2·H2O + Hacac + bppz
MeOH
Cu + Hacac +bppz + KPF6
H3C
R
O
Cu
O
O
O
R
CH3
NaBPh4
[Cu(tfacac)2(bppz)] (1)
[{Cu(acac)}2(µ-bppz)2](BPh4)2·MeOH (4)
+
MeOH/CHCl3
EtOH/CHCl3
NaBPh4
N
N
N
N
NaBPh4
MeOH
MeOH
EtOH/CHCl3
NaBPh4
[Cu(tfacac)(MeOH)(bppz)](BPh4)·1/4MeOH (7)
[Cu(acac)(MeOH)(bppz)](BPh4)·MeOH (5)
[Cu(acac)(EtOH)(bppz)](BPh4)·EtOH (6)
Scheme 2.

A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
2447
acac– + ½ H2
Cathode: acacH + e–
Cu + acac–
3.3.2. Crystal structures of the dimeric species 2–4
These compounds crystallise in the monoclinic P21/n group for
Anode:
[Cu(acac)] + e–
ꢀ
2 or the triclinic P1 for 3 and 4. Figs. 2 and 3 show ORTEP views
of 2 and 4, respectively.
4 [Cu(acac)] + bppz + 2 KPF6
[{Cu(acac)}2(µ–bppz)](PF6)2 + 2 Cu0 + 2 K+ + 2 acac–
The asymmetric unit is formed by one copper atom bonded to
one acetylacetonato group and one bidentate bppz, and bears
one positive charge that is neutralised by one anion. The molecular
entity is composed by two asymmetric units related by one inver-
sion centre. The free pyridine nitrogen of every bppz ligand is
pointing towards the copper atom of the symmetry related unit
giving rise to the formation of a pair of bonds that originate the di-
mer. The bppz ligand is, thus, acting as a bridge in the bidentate–
monodentate coordination mode IV.
The differences found between the three structures come from
the role played by the counter anions. In 2 the tetrafluoroborate is
bonded to the copper atom through one fluorine with a bond dis-
tance Cu–F2 = 2.755(2) Å. On the contrary, in 3 and 4 there are not
direct interactions between the hexafluorophosphate or tetraphen-
ylborate anions and the dimeric cation.
Scheme 3. Proposed mechanism for the electrochemical synthesis of 3.
unsubstituted acac, and around 1630 and 1520 cmꢀ1 for the triflu-
oromethylacetylacetonato, along with the typical vibrations of the
bppz ligand and those of the corresponding counter anion.
A sharp and weak absorption centred at ca. 3620 cmꢀ1 and a
medium and broad absorption at ca. 3200 cmꢀ1, corresponding to
the stretching m(OH), in the IR spectra of 5–7 are due to the pres-
ence of the both coordinated and crystallisation alcohol molecules
which are involved in hydrogen bonds, as confirmed by the crystal
structure determination.
3.3. Structure description
The formation of the Cu–F bond in 2 creates a distortion in the
tetrafluoroborate, as is shown by the longer B–F2 bond distance of
1.412(4) Å, while the three other boron–fluorine distances lie in
the 1.353(5)–1.367(4) Å range. The coordinative environment for
the copper atom is thus 4+2, forming a highly distorted octahedral
coordination with the metal atom lying in the equatorial plane
formed by the chelating acac and bppz ligands. A second interac-
tion between anion and cation can be observed in the less common
3.3.1. Crystal structure of 1
This compound crystallises in the monoclinic P21/c group. Fig. 1
shows an ORTEP view of the molecule.
The compound is mononuclear with the copper six-coordinated
to four oxygen atoms of two tfacac groups and two nitrogen atoms
of the bppz ligand that coordinates through one pyridine and one
pyrazine nitrogen atoms.
The copper environment shows the usual Jahn–Teller distortion
with the equatorial plane composed of the two nitrogen atoms at
distances of 1.998(3) Å for the pyridine and 2.042(3) Å for the pyr-
azine one and two oxygen atoms at 1.956(3) and 1.971(3) Å,
respectively. The other two oxygen atoms occupy the axial posi-
tions at 2.241(3) and 2.257(3) Å. As a consequence of this asym-
metric coordination of the tfacac groups the axial oxygen atoms
deviate from linearity with a O2–Cu–O3 angle of 168.6(1)°.
The bppz ligand is coordinated bidentate in the coordination
mode I (Scheme 1). The second pyridine ring is rotated relative
to the pyrazine plane, forming an angle of 50.8(1)°. This bending
of the ligand is expected for the uncoordinated bppz in order to
minimize short contacts between hydrogen atoms of different pyr-
idine rings [41–56].
p
interaction between F1 and the bonded pyrazine ring (Fig. 2)
with a distance between the fluorine atom and the ring centroid
of 2.908(5) Å, which falls in the range of previously described fluo-
rine-p interactions, either with fluoride or with tetrafluoroborate
anions [88–90].
In 3 and 4 the copper atom is five-coordinated with two oxygen
atoms of the acetylacetonato and the two chelating nitrogen atoms
of the bppz ligand, showing bond distances in the 1.89–2.00 Å
range, and one nitrogen atom from the bridging monodentate bppz
at 2.390(3) Å (3) or 2.458(8) Å (4). This environment corresponds
to a distorted square-pyramid with an Addison parameter [91]
s
= 0.32 for 3 and 0.08 for 4 (where s is 0 for the perfect square-
pyramidal and 1 for the trigonal-bipyramidal environments). The
One of the tfacac ligands shows a small disorder in the trifluo-
romethyl group. The fluorine atoms F1–F3 alternate with the
hydrogen methyl atoms between C5, with a 63% of occupancy,
and C4, for the remaining 37%. This disorder does not introduce
any special features to the crystal structure.
Fig. 1. ORTEP view, 50% probability ellipsoids, and labelling scheme of [Cu(tfa-
cac)2(bppz)] (1). The disordered fluorine and the hydrogen atoms are omitted for
clarity.
Fig. 2. ORTEP view, 50% probability ellipsoids, and labelling scheme for the dimeric
molecular unit of [{Cu(acac)}2(
l-bppz)2](BF4)2 (2). The fluorine-p contact described
in the text is represented as a dotted line.

2448
A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
Fig. 3. ORTEP view, 50% probability ellipsoids, and labelling scheme for the dimeric
cation [{Cu(acac)}2(
-bppz)2]2+ of 4. The hydrogen atoms were omitted. The
l
labelling scheme for 3 is identical.
Fig. 5. ORTEP view, 50% probability ellipsoids, and labelling scheme for the
cation in [Cu(acac)(EtOH)(bppz)](BPh4)ꢁEtOH (6). The intramolecular hydrogen
bonds are represented as dotted lines; bond distances (Å) and angles (°): O3–
H03 = 1.057(3), H03ꢁ ꢁ ꢁO4 = 1.676(3), O3–H03ꢁ ꢁ ꢁO4 = 162.6(2); O4–H04 = 0.996(3),
H04ꢁ ꢁ ꢁN4 = 1.997(3), O4–H04ꢁ ꢁ ꢁN4 = 143.3(2).
copper atom is displaced 0.245(1) Å out of the basal plane in com-
plex 3 (0.196(2) Å in 4) as is usual in the square-pyramidal
environment.
The crystal structures show some disorder with the fluorine
atoms of the hexafluorophosphate anion in 3 occupying two alter-
nate positions with occupancies of 50% each or the solvent metha-
nol in 4, which is found very close to an inversion centre, showing
disorder with respect to its centrosymmetric conformation that
has been modelled with 50% occupancy for each position.
3.3.3. Crystal structure of the monomeric 5–7
These compounds crystallise in the monoclinic P21/n group (5
ꢀ
and 7) or triclinic P1 group (6). Figs. 4–6 show an ORTEP view of
the corresponding cationic moiety.
The asymmetric unit consists of a monomeric copper complex
cation and one tetraphenylborate anion. The cation has a copper
atom in a five-coordinate environment formed by the two acetyl-
acetonato oxygen atoms and the two chelating bppz nitrogen
atoms in the basal plane of the square-pyramid (s = 0.03 for 5,
0.00 for 6, and 0.1 for 7) [91] and the oxygen atom of one alcohol
molecule in the apical position. The basal bond distances are in the
1.89–1.99 Å range, while the apical oxygen shows a longer bond
distance to the metal atom, 2.31–2.34 Å, as expected for a cop-
per(II) derivative.
Fig. 6. ORTEP view, 50% probability ellipsoids, and labelling scheme for the cationic
moiety in [Cu(tfacac)(MeOH)(bppz)](BPh4)ꢁ1/4MeOH (7). The hydrogen bonds that
form the dimer are represented as dotted lines; bond distances (Å) and angles (°):
O3–H03 = 1.009(3), H03ꢁ ꢁ ꢁN4 = 1.859(3), O3–H03ꢁ ꢁ ꢁN4 = 165.6(2).
The bppz ligand coordinates in the chelating mode I. The unco-
ordinated pyridine ring is rotated relative to the pyrazine ring an
angle of 51.1(1)° for 5, 45.6(1)° for 6, and 40.3(1)° for 7. In contrast
to compounds 2–4, where this pyridine was bonded to the second
copper atom, the ring is not constrained to point to the metal so its
rotation angle is lower and corresponds to the conformation that
minimizes repulsions between the orto pyridine hydrogen atoms
[92].
The uncoordinated pyridine in 5 and 6 forms a hydrogen bond
with one crystallisation alcohol molecule, which in turn is hydro-
gen bonded to the coordinated alcohol. This set of two hydrogen
bonds form a stable cyclic structure involving the copper atom,
the bppz ligand and two alcohol molecules.
In 7 the uncoordinated pyridine also forms a hydrogen bond,
but directly with the coordinated methanol of a neighbouring mol-
ecule, whose pyridine nitrogen is also hydrogen bonded to the
methanol of the former moiety, forming a double hydrogen bond
that leads to the presence of cationic dimers in the crystal.
There is some positional disorder in the ethyl group of the coor-
dinated ethanol molecule in 6 with two alternative positions for
the carbon atoms with relative occupancies of 70% and 30%.
Fig. 4. ORTEP view, 50% probability ellipsoids, and labelling scheme for the cation
in [Cu(acac)(MeOH)(bppz)](BPh4)ꢁMeOH (5). The intramolecular hydrogen bonds
are represented as dotted lines; bond distances (Å) and angles (°): O4-–
H04 = 1.116(3), H04ꢁ ꢁ ꢁN4 = 1.710(3), O4–H04ꢁ ꢁ ꢁN4 = 173.0(2); O3–H03 = 1.083(3),
H03ꢁ ꢁ ꢁO4 = 1.627(3), angle O3–H03ꢁ ꢁ ꢁO4 = 173.6(2).

A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
2449
In the crystal structure of 7 the solvent methanol has an occu-
pation of 30% and is located in a non polar void formed by two tri-
fluoromethyl groups and six phenyl rings of surrounding counter
anions. There is positional disorder between this solvent molecule
and its centrosymmetric analogue, which is also occupying the
same void. There is also disorder in the fluorine positions of the tri-
fluoromethyl group that has been modelled in two alternate posi-
tions for every atom with 50% occupancy.
is observed, with values of 65.9(1)° for 2, 64.2(1)° for 3, and
87.9(4)° for 4. The similitude in values, particularly between those
of 2 and 3, confirms that similar interactions can be observed for
both compounds.
A final point that should be discussed is the formation from the
same reagents of two different species, one dimeric, 4, and one
monomeric, 5. The only difference in both reactions is the nature
of the solvent, a mixture of chloroform/methanol for the former
and pure methanol for the latter, suggesting that the higher con-
centration of the methanol, and the change in polarity that implies,
in the second reaction was the responsible of the coordination of
one alcohol molecule to the copper leading to the monomeric com-
pound. The shorter copper–oxygen bond distance, compared to the
copper–nitrogen distance in 4 and previously attributed to the ste-
ric hindrance in the dimer, is an indication of a greater stability for
the monomeric species. This stabilization is partly due to the for-
mation of a set of hydrogen bonds involving the coordinated meth-
anol, one solvent methanol molecule and the uncoordinated
nitrogen of the pyridine ring.
3.4. Discussion of the crystal structures
The structures described show two different types of interac-
tions of the bppz ligand. The first one corresponds to the chelation
of the bppz ligand that coordinates to only one copper atom and it
is observed in 1, 5, 6, and 7, while in the second interaction mode
(2–4) the bppz bridges two copper atoms chelating the first one
and bonding monodentate the second one.
The bridging of the bppz ligand should be related to the avail-
ability of the fifth coordinative position on the copper atom. When
there is competition with other ligands for this position (tfacac in 1
or alcohol in 5–7) the bppz ligand only acts as a terminal chelating
ligand in the coordination mode I.
3.5. Magnetic properties
In the compounds where the copper is five-coordinated, 3–7,
the metal environment is square-pyramidal with the metal atom
displaced from the basal plane towards the apical ligand. Depend-
ing on the apical donor atom, the sum of the copper to ligand bond
distance plus the copper shift form the basal plane is kept approx-
imately constant at 2.65 Å in 3 and 4, with nitrogen as donor atom,
and 2.45 Å for 5–7, when the donor atom is oxygen. The difference
in size between oxygen and nitrogen is too small to account for the
difference in distances, which is more related to the steric hin-
The magnetic susceptibilities of 1–7 have been measured in the
temperature range 2–300 K.
The mononuclear compounds 1, 5, 6, and 7 follow the Curie–
Weiss law in the whole range of temperatures with magnetic mo-
ment values of 1.89 b (equivalent to g = 2.18) and h = ꢀ0.15(1) K for
1, 1.84 b (g = 2.12) and h = ꢀ0.05(2) K for 5, 1.97 b (g = 2.27) and
h = ꢀ0.39(1) K for 6, and 1.93 b (g = 2.23) and h = ꢀ0.27(2) K for
7. In all the cases the obtained values are typical for isolated
S = 1/2 spins, as expected for mononuclear CuII complexes.
The dimeric derivatives 2–4 also follow the Curie law above
30 K, but a slight decrease in the magnetic moment value is ob-
served below this temperature (Fig. 7). This deviation is better de-
scribed in terms of a weak antiferromagnetic coupling between the
S = 1/2 copper spins. The magnetic data can then be fitted to the
expression, shown in Eq. (1), based on the general isotropic ex-
change Hamiltonian for two interacting S = 1/2 spins, H = ꢀ2JS1S2
[94], where J is the exchanging coupling constant for the copper
ions, gCu is the Landé g factor for the copper ion and the other
parameters have their usual meanings.
drance caused by the
p overlap of the chelating pyridine and pyr-
azine rings in the dimer. The small alcohol molecule, on the
contrary, can get closer to the copper atom, reducing the bond
distance.
The formation of the dimeric species gives rise to the stacking of
the
for 2, 3.375(5) Å, for 3, and 3.29(2) Å, for 4, and corresponds to the
typical range for the overlap of aromatic clouds [93]. The steric
p rings of the bppz ligand with shortest contacts of 3.229(4) Å,
p
hindrance imposed by this overlap precludes the approach of the
pyridine ring to the copper atom.
The copper to nitrogen bond distance in 2 is very long,
3.036(2) Å, a fact that can be explained by two reasons. The first
one is a consequence of the six-coordination of the copper atom,
which is pulled to the basal plane by the interaction with the tet-
Ng2Cub2
kT 3 þ expðꢀ2J=kTÞ
2
v
¼
ð1Þ
rafluoroborate. The second reason is related to the p–p interactions
between neighboring bppz ligands. The chelating pyridine and pyr-
azine rings are almost coplanar with the acac in 2, pushing away
the other pyridine from the bridging copper due to steric reasons.
However, they are bent in 3 and 4 with a dihedral angle between
the copper-acac and the bppz planes of 26.6(1)° and 23.3(2)°,
respectively. This bending allows the approaching of the pyridine
nitrogen to the copper atom avoiding the hindrance imposed by
the overlapping.
In spite of the long distance copper–nitrogen in the compound
2, a real bond exists between both atoms and it is not just an inter-
molecular contact due to the proximity of the molecules in the di-
mer. The justification of the existence of this bond is based on the
variation of the dihedral angle between the pyrazine and the bridg-
ing pyridine rings. This angle is 50.8(1)° in 1, where no interactions
for the free pyridine nitrogen are found. This bending can be con-
sidered the value that minimizes the contacts between hydrogen
atoms of different pyridine rings [92]. Similar values are found in
5–7 with the dihedral angle ranging between 40.3° and 51.1°.
The formation of the copper–nitrogen bond, however, tilts the pyr-
idine ring to the opposite side and an increase of the dihedral angle
Fig. 7. Temperature dependence of
solid lines represent the best fits using equation and parameters described in the
text.
v
T for 2, squares, 3, circles, and 4, triangles. The

2450
A. Gutiérrez et al. / Inorganica Chimica Acta 363 (2010) 2443–2451
The best fit corresponds to gCu = 1.994(7) and J = ꢀ0.47(1) cmꢀ1 for
2, gCu = 2.141(2) and J = ꢀ0.15(1) cmꢀ1 for 3, and gCu = 2.241(3) and
J = ꢀ0.20(3) cmꢀ1 for 4, confirming the very weak coupling between
the copper atoms, as expected since the magnetic pathway through
the bridging bppz should be almost negligible due to the rotated
conformation found for this ligand. This coupling is, however, great-
er for 2 and we attribute this fact to a better coupling pathway be-
tween the copper atoms. The equatorial pyrazine ring of the bppz
ligand can interact with the magnetic orbital of the copper atom
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We gratefully acknowledge the Spanish Ministerio de Ciencia e
Innovación, project MAT2006-05417, the Comunidad Autónoma de
Madrid, project S0505-PPQ-0316, and the Universidad Complu-
tense de Madrid, project 921508, for financial support.
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