Tetranuclear copper(n) complexes incorporating short and long metal-metal separations: Synthesis, structure and magnetism 1

Article abstract of DOI:10.1039/a908177a

Synthesis, structural and magnetic characterization of tetranuclear [(Cu₂L₂B] complexes [where H₃L is the Schiffs-base ligand, l,3-bis(salicylideneamino)propan-2-of; B is the dipyrazolate entities: methylenebis(3,5-dimethylpyrazolate) (mbdpz) or 4,4′-bi-3,5-dimethylpyrazolate (mdpz)] are reported. The structures reveal short intra- and inter-molecular Cu ... Cu separations (ca. 3.4 A), in combination with long intramolecular copper distances (ca. 10 A). The magnetic behaviour of [(Cu₂L)₂(mbdpz)] (1) is indicative of an intramolecular antiferromagnetic interaction between the copper(n) centres [2J = -460 cm^-1], whilst that of [(Cu₂L)₂(mdpz)] (2) is characteristic of an antiferromagnetically coupled system with intramolecular coupling in the order of 2J= -380 cm^-1, with the presence of a further intermolecular interaction indicated. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908177a

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Tetranuclear copper(II) complexes incorporating short and long
metal–metal separations: synthesis, structure and magnetism†
a
b
b
b
Paul E. Kruger,* Boujemaa Moubaraki, Gary D. Fallon and Keith S. Murray
a
Department of Chemistry, Trinity College, Dublin 2, Ireland. E-mail: Paul.Kruger@tcd.ie
Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia
b
Received 12th October 1999, Accepted 12th January 2000
Synthesis, structural and magnetic characterization of tetranuclear [(Cu L) B] complexes [where H L is the
2
2
3
Schiff’s-base ligand, 1,3-bis(salicylideneamino)propan-2-ol; B is the dipyrazolate entities: methylenebis(3,5-
dimethylpyrazolate) (mbdpz) or 4,4Ј-bi-3,5-dimethylpyrazolate (mdpz)] are reported. The structures reveal
short intra- and inter-molecular Cu ؒ ؒ ؒ Cu separations (ca. 3.4 Å), in combination with long intramolecular
copper distances (ca. 10 Å). The magnetic behaviour of [(Cu L) (mbdpz)] (1) is indicative of an intramolecular
2
2
Ϫ1
antiferromagnetic interaction between the copper() centres [2J = Ϫ460 cm ], whilst that of [(Cu L) (mdpz)] (2)
2
2
is characteristic of an antiferromagnetically coupled system with intramolecular coupling in the order of
Ϫ1
2
J = Ϫ380 cm , with the presence of a further intermolecular interaction indicated.
Protein crystallographic and detailed spectroscopic studies of
a number of enzymes containing polymetallic active sites have
shown the occurrence of small clusters of metal ions, with short
metal–metal separations (e.g. <3.6 Å), together with other clus-
ters or single-metal ion sites, situated a much larger distance
1
away in the protein matrix (e.g. >10 Å). Such structural motifs
2
have been identified within the multi-copper oxidases. Laccase
and ascorbate oxidase possess oxygen binding trinuclear clus-
ters, T2 plus T3, a large distance away (ca. 13 Å) from the
substrate oxidation/electron transfer T1 copper site. The hexa-
copper ceruloplasmins possess a similar topology but contain
two additional T1 sites ca. 18 Å from T1. In addition, the 8-Fe
ferrodoxins contain two cubane clusters spaced ca. 12 Å apart
and, more recently identified, the Fe–Mo cofactor of the nitro-
genase enzyme contains a metal cluster possessing both short
3
and long metal–metal separations. We have been developing a
strategy aimed at incorporating metal centres into multi-
metallic complexes such that they are both in close proximity to
each other and at much further distances away (within 3–4 Å
and at ≥10 Å). This is in an attempt to mimic the topology
found in the above native systems.
Experimental
Materials and methods
Solvents and chemicals were of laboratory grade and were used
as received. Acquisition of infrared, ultraviolet-visible, mass,
Several methods may be employed in the synthesis of poly-
metallic coordination compounds and include, amongst
others, the self-assembly or aggregation of metal centres and
ligand species, both coordinating and bridging, into poly-
6
δ and δ NMR spectra were performed as detailed previously.
H
C
Abbreviations have their usual meaning. Chemical analyses; C,
H, N were performed by the Commonwealth Micro-Analytical
Services, Melbourne, Australia. Magnetic measurements at
room temperature were performed using a Faraday balance
which incorporated a Newport electromagnet fitted with
Faraday-profile pole faces. Diamagnetic corrections for ligand
susceptibilities were made using Pascal’s constants. Variable-
temperature magnetic susceptibility measurements (300–4.2 K)
were performed on powdered samples at a field strength of
4
metallic arrays; or the direct incorporation of metal ions into
5
preformed polydentate ligands. This second approach offers
many potential advantages over the self-assembly route in that
it enables more stringent control over the course of the reaction
and upon the products that form. We have favoured a ‘pair-of-
dimers’ approach to form tetranuclear complexes through the
employment of preformed dinucleating ligands (L). In this
1
0 000 G (1 T) using a Quantum Design M.P.M.S. Squid
method [M L] units are connected through bridging ligands to
8
2
magnetometer.
form [M L] compounds. We have utilized a range of dinucleat-
2
2
ing Schiff’s-base ligands built on a propan-2-ol backbone and
Crystallographic measurements on 1 and 2
successfully incorporated first row transition metals within their
6
Crystal data and experimental details are summarized in Table
1. The structures of 1 and 2 were solved by direct methods.
CCDC reference number 186/1801.
See http://www.rsc.org/suppdata/dt/a9/a908177a/ for crystal-
lographic files in .cif format.
coordination cavities. Linking [M L] units with dipyrazolate
2
9
7
bridging ligands gives ‘pairs-of-dimers’ [M L] . We extend this
2
2
series in the present study and report full preparative, structural
and magnetic details of tetranuclear, [(Cu L) (mbdpz)] (1) and
2
2
[
(Cu L) (mdpz)] (2).
2 2
Synthesis of 1,3-bis(salicylideneamino)propan-2-ol, H L
3
†
Dedicated to the memory of Professor Olivier Kahn, a leading light in
10
the field of molecular magnetism.
The method of Mazurek was employed in the synthesis of
DOI: 10.1039/a908177a
J. Chem. Soc., Dalton Trans., 2000, 713–718
This journal is © The Royal Society of Chemistry 2000
713

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Table
1
2
Crystallographic details for [(Cu L) (mbdpz)] (1) and
(Nujol): 3185vs, 3130vs, 3081vs, 1620w (sh), 1589s, 1516w,
1418s, 1300s, 1207w, 1152m, 1079w, 1030m, 1000w (sh), 876w
2
2
[
(Cu L) (mdpz)] (2)
2
ϩ
(
sh), 840s, 798m and 750m. m/z 204 (M , 50%), 189 (30), 108
100) and 97 (30). δ (DMSO-d , 200 MHz): 2.0 (12H, s, CH ),
1
2
(
H
6
3
3
.35 (2H, s, CH ) and 11.9 (2H, s, br, NH). δ (DMSO-d , 50
2
C
6
Chemical formula
Formula weight
Crystal system
Space group
Cu C H N O
Cu C H N O
1033.1
4
45 44
8
6
4
44 42
8
6
MHz): 146 (C2), 112 (C3), 17 (C4) and 11 (C1).
1047.1
Monoclinic
C2/c
Triclinic
Synthesis of 4,4Ј-bi-3,5-dimethylpyrazole (H mdpz)
¯
2
P1
Ϫ1
µ(Cu-Kα)/cm
28.8
27.9
4
,4Ј-Bi-3,5-dimethylpyrazole was prepared from the tetra-
a/Å
b/Å
c/Å
α/Њ
14.495(3)
17.395(2)
16.334(2)
—
100.98(1)
—
4043(1)
4
293.2
18.630(2)
12.459(1)
9.319(1)
81.546(8)
86.965(6)
76.897(8)
2083.4(4)
2
293.2
0.019
0.039, 0.049
0.055, 0.064
ketone intermediate, 3,4-diacetylhexane-2,5-dione, by a slight
modification to the method of Mosby.
12
β/Њ
3,4-Diacetylhexane-2,5-dione. Acetylacetone (100 g, 1.0 mol)
was added dropwise, over a period of 6 h under a nitrogen
atmosphere, to a diethyl ether slurry of NaH (24 g, 1.0 mol). A
further amount of ether was added and the mixture heated so
as to extract iodine (127 g, 1.0 mol) from a Soxhlet thimble
placed below the condenser. On completion of the iodine
extraction the mixture was allowed to cool, and was filtered and
the solid washed with ether so as to remove any excess iodine
present. The resultant white solid was recrystallized from acetic
acid to retrieve a white crystalline product in 65% yield. Mp
γ/Њ
V/Å
3
Z
T/K
Rint
0.024
0.039, 0.048
0.057, 0.065
R, wR2 (obs. data)
R, wR2 (all data)
Reflections:
measured
independent
observed
6515
3011
2287
6546
6185
4682
1
6
90–191 ЊC. Found: C, 60.4; H, 7.0%. C H O : requires C,
10 14 4
Ϫ1
0.6; H, 7.1%. νmax/cm (KBr) 1630–1520 s (br), 1410m, 1370w
ϩ
H L, the product being recrystallized from methanol. Full
(sh), 1260s, 1025vs and 1000s. m/z 198 (M 50%), 180 (70), 165
(100), 123 (50) and 113 (20). δ (CDCl , 200 MHz): 2.0 (12H, s,
CH ). δ (CDCl , 50 MHz): 192 (C2), 108 (C3) and 23.5 (C1).
3
characterisation is reported here. Mp 101–103 ЊC. Found: C,
H
3
6
9
8.3; H, 6.3; N, 9.2%. C H N O : requires C, 68.4; H, 6.1; N,
.4%. λmax/nm (MeOH) 215 (ε/dm mol cm 41500), 250
1
7
18
2
3
3 C 3
3
Ϫ1
Ϫ1
Ϫ1
(
22000), 315 (7200) and 400 (2350). νmax/cm (Nujol) 3394s
4,4Ј-Bi-3,5-dimethylpyrazole (H mdpz). 3,4-Diacetylhexane-
2
(
br), 1635s, 1611m, 1579m, 1497m, 1339w, 1275s, 1207w,
2,5-dione (9.9 g, 0.05 mol) was added portionwise to hydrazine
hydrate (15 ml of an 85% aqueous solution) as expeditiously as
possible (CAUTION! highly exothermic reaction). The mixture
was cooled in ice, the precipitate filtered off and washed with ice
cold water. The solid was dissolved in hot methanol and water
added to induce reprecipitation of a microcrystalline white
powder which was collected and dried under high vacuum.
Yield 5.5 g, 58%. Mp 300 ЊC. Found: C, 62.9; H, 7.2; N, 29.2%.
1
156w, 1104w, 1084w, 1048m, 1036w, 1025m, 973w, 940w, 844m
ϩ
(
1
br), 771m, 752s, 738m, 661w. m/z 298 (M , 30%), 164 (100),
35 (70). δH (CDCl , 300 MHz): 8.25 (2H, s, imine), 7.30 (4H,
3
m, phenyl), 6.80 (4H, m, phenyl), 4.15 (1H, m, CH–O) and 3.60
(
(
(
4H, m, CH ). δ (CDCl , 75 MHz): 167 (C7), 161 (C1), 133
C3), 131 (C5), 120 (C6), 118 (C4), 116 (C2), 69 (C9) and 63
C8).
2 C 3
Ϫ1
C H N : requires C, 63.1; H, 7.4; N, 29.4%. νmax/cm (Nujol)
3
10
14
4
Synthesis of methylenebis(3,5-dimethylpyrazole) (H mbdpz)
2
172s (br), 3050s, 1685w (br), 1618m, 1576m, 1532m, 1420s,
Methylenebis(3,5-dimethylpyrazole) was prepared from 3,5-
1302s, 1258m, 1157m, 1138m, 1063w, 1017vs, 972 (sh), 830 (sh)
(br), 783vs, 700sh (br) and 625w. m/z 190 (M , 100%), 175 (20)
ϩ
diacetylheptane-2,6-dione which in turn was prepared via a
11
modification of the method of Knövenagel from acetyl-
and 148 (35). δ
12.7 (2H, s, br, NH). δ
C2, br), 108 (C3) and 11 (C1).
H
(DMSO-d
6
, 200 MHz): 2.0 (12H, s, CH
3
) and
acetone and formaldehyde.
C
(DMSO-d
, Cr(acac) , 50 MHz): 141
6
3
(
3
,5-Diacetylheptane-2,6-dione. To an ice cold solution of
acetylacetone (20 g, 0.20 mol) and formaldehyde (7.2 g, 0.24
mol), was added with vigorous stirring, four drops of diethyl-
amine. The mixture was stirred for 24 h at room temperature
and an additional four drops of diethylamine added. This pro-
cedure was repeated for the following five days. After this time
an aqueous and organic phase were evident. The organic phase
was separated and extracted with diethyl ether and dried over
Synthesis of (1) and (2)
The compounds were prepared by using a modification to the
procedure reported previously by Mazurek. The dipyrazole
entities were used in place of pyrazole, with relevant adjustment
being made for reactant stoichiometry. The following prepar-
ation of 1 is representative.
13
A methanolic solution of H L (2.98 g, 0.01 mol) was slowly
MgSO . The ethereal solution was cooled in an acetone/dry ice
3
4
added to a stirred methanolic solution containing H mbdpz
bath to precipitate a white solid which was filtered off at low
temperature. A pale yellow oil results on standing the filtrate at
room temperature. Yield 90%. Found: C, 62.4; H, 7.8%.
2
(
0
1.02 g, 0.005 mol) and copper() nitrate trihydrate (4.832 g,
.02 mol). After dissolution was complete, a methanolic solu-
Ϫ1
tion of potassium hydroxide (2.24 g, 0.04 mol) was added with
vigorous stirring. Stirring continued for a further 20 min, result-
ing in the full development of a deep blue-green coloured pre-
cipitate. The mixture was filtered to remove the precipitate,
which was subsequently washed with water (2 × 50 ml), meth-
anol (2 × 20 ml), acetone (2 × 20 ml) and air dried. The solid
was purified by Soxhlet extraction (CHCl /CH Cl ) over a
C H O : requires C, 62.2; H, 7.6%. νmax/cm (Nujol): 1720–
11
16
4
1
1
680vs (br), 1430s, 1355m, 1330sh, 1315m, 1290m, 1155vs,
030sh, 960m (br) and 740w. δH (CDCl , 200 MHz): 2.1–2.4
3
(
14H, m, CH and CH ) and 3.8 (2H, t, CH).
3 2
4
,4Ј-Methylenebis(3,5-dimethylpyrazole) (H mbdpz). 3,5-
2
Diacetylheptane-2,4-dione (12.0 g, 0.055 mol) dissolved in 20
ml of ethanol was added cautiously to 13 ml of hydrazine
hydrate (98% soln.) with stirring at ambient temperature. The
reaction mixture was then refluxed for 45 min and stirred at
room temperature for an additional 18 h. The resultant white
solid was filtered off, washed with water and dried in vacuo.
3
2
2
period of 3 days. The resultant solution was reduced in volume
to complete the precipitation of the blue-green coloured micro-
crystalline solid, which was collected, washed with diethyl ether
and air dried. Recrystallization of the products from dimethyl-
formamide yielded dark aquamarine coloured crystalline com-
pounds. The crystals used for the structural determinations of 1
and 2 were grown by vapour diffusion of diethyl ether into a
Yield 10 g, 90%. Mp 275–277 ЊC. Found: C, 64.5; H, 8.1; N,
Ϫ1
2
7.2%. C H N : requires C, 64.7; H, 7.9; N, 27.4%. νmax/cm
11 16 4
7
14
J. Chem. Soc., Dalton Trans., 2000, 713–718

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L) (mbdpz)]
2 2
dimethylformamide/dimethyl sulfoxide solution of the respect-
ive compounds.
Table 2 Selected bond lengths (Å) and angles (Њ) for [(Cu
1)
(
Cu(1)–O(1)
Cu(1)–N(2)
Cu(2)–O(1)
Cu(2)–N(1)
1.895(3)
1.972(4)
1.901(3)
1.964(4)
3.391(1)
10.446(1)
3.431(1)
2.590(2)
172.2(1)
82.3(1)
85.9(1)
91.3(1)
101.3(1)
162.9(2)
126.5(2)
Cu(1)–O(2)
Cu(1)–N(3)
Cu(2)–O(3)
Cu(2)–N(4)
Cu(1) ؒ ؒ ؒ Cu(2A)
Cu(2) ؒ ؒ ؒ Cu(2A)
Cu(2) ؒ ؒ ؒ Cu(2B)
1.923(3)
2.020(4)
1.910(3)
1.989(4)
9.456(1)
9.459(2)
4.063(1)
[
(Cu L) (mbdpz)] (1). Yield 73%. Found: C, 51.6; H, 4.2; N,
2 2
1
0.7%. Cu C H N O : requires C, 51.4; H, 4.3; N, 10.8%. λmax/
4
45 44
8
6
3
Ϫ1
Ϫ1
nm (CHCl ) 378 (ε/dm mol cm 26050) and 600 (1189). νmax
/
3
Ϫ1
cm (Nujol) 1630vs, 1598s, 1532m, 1448vs, 1392m, 1345m,
Cu(1) ؒ ؒ ؒ Cu(2)
Cu(1) ؒ ؒ ؒ Cu(1A)
Cu(1) ؒ ؒ ؒ Cu(1B)
Cu(1A) ؒ ؒ ؒ O(2B)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(3)
O(2)–Cu(1)–N(2)
O(2)–Cu(1)–N(3)
N(2)–Cu(1)–N(3)
Cu(1)–O(1)–Cu(2)
1
1
308s, 1210w (sh), 1195m, 1144m, 1125m, 1080w, 1058m,
030w, 990w, 967w, 911w (sh), 893m, 858w, 793m, 755vs, 720m
and 670m. µeff (295 K) = 1.16 µ per Cu.
B
O(1)–Cu(2)–O(3)
O(1)–Cu(2)–N(1)
O(1)–Cu(2)–N(4)
O(3)–Cu(2)–N(1)
O(3)–Cu(2)–N(4)
N(1)–Cu(2)–N(4)
170.7(1)
82.6(2)
86.5(1)
90.6(2)
101.5(1)
162.9(2)
[
(Cu L) (mdpz)] (2). Yield 68%. Found: C, 51.1; H, 4.0; N,
2 2
1
0.8%. Cu C H N O : requires C, 50.9; H, 3.8; N, 11.0%. λmax/
4
44 42
8
6
3
Ϫ1
Ϫ1
nm (CHCl ) 376 (ε/dm mol cm 26428) and 601 (1211). νmax
/
3
Ϫ1
cm (Nujol) 1630vs, 1599s, 1537s, 1448vs, 1385m, 1330m,
1
1
315s, 1309s, 1280w, 1240w, 1210w (sh), 1194m, 1149m, 1126m,
088w, 1050m (sh), 1039s, 961w, 908w, 894m, 790w, 750s, 719s
and 668m. µeff (295 K) = 1.22 µ per Cu.
B
Results and discussion
Synthesis and characterization
The synthesis of the tetranuclear copper complexes followed a
general procedure in which methanolic solutions of the divalent
metal salts, H L and the respective dipyrazole entity, were
3
treated with a methanolic solution of potassium hydroxide,
resulting in the immediate precipitation of the product.
Recrystallization was achieved through Soxhlet extraction, over
a period of days, of the solids obtained into a CH Cl /CHCl
2
2
3
solvent mix. The tetranuclear Cu() complexes are blue-green
in appearance giving similarly coloured solutions when dis-
solved in DMF or chloroform and this is in accord with planar
14
geometry around the metal atoms.
Crystal structure of 1
The dinucleating pentadentate character of the ligand and the
desired pair-of-dimers arrangement, through the incorporation
of the dipyrazolate moiety within the exogenous bridge, is
clearly evident and is shown in Fig. 1. The molecule possesses a
two-fold axis of symmetry through C(23) and each dinucleating
half is rotated by 10Њ relative to the other at this atom. This is
probably due to non-bonded interactions between the methyl
15
groups and the protons of the methylene bridge. The copper
atoms are coordinated in slightly distorted square planar
arrangements within each dinucleating half of the complex and
bridged mono-atomically by the secondary alkoxo oxygen of
the ligand and di-atomically by the pyrazolato moiety. The
dihedral angle between the [CuN O ] chromophores is 169.0Њ
and consequent bending of the molecule from coplanarity is
evident. The intramolecular distance between Cu(1) and Cu(2)
is 3.391(1) Å, which is slightly longer than those within 2
2
2
Fig. 1 (a) Molecular structure and atomic numbering scheme for 1.
Thermal ellipsoids are shown at the 40% level. (b) The coordination
chromophores and bridging ligand (mbdpz, methyl groups omitted for
clarity) of 1 showing the intermolecular association of Cu(1A) and
Cu(1B) through the phenoxy oxygens O(2A) and O(2B).
(
3.373(1) Å see below), but falls within the range of distances
found for this and related pyrazolato-bridged dinuclear copper
13,16
species.
The Cu(1)–O(1)–Cu(2) bridging angle is 126.5(2)Њ
shown in Fig. 2. Important bond lengths and angles are given in
Table 3. The Cu ؒ ؒ ؒ Cu separations within each half are
Cu(1) ؒ ؒ ؒ Cu(2) 3.373(1) Å and Cu(3) ؒ ؒ ؒ Cu(4) 3.368(1) Å, and
are slightly shorter than those found in 1 (see above). The
complex possesses a distinct twisting around the central C(20)–
C(42) bond, (≈68Њ), generated by an interaction of the 3,3Ј and
and the angle at which the plane of the pyrazolate ring of the
mbdpz molecule intersects the plane encompassing the Cu(1)–
O(1)–Cu(2) bridge is 163.1Њ. The pertinent across-mbdpz
intramolecular copper-to-copper separations are, Cu(1) ؒ ؒ ؒ
Cu(1A) 10.446(1) Å, Cu(2) ؒ ؒ ؒ Cu(2A) 9.459(2)
Å and
Cu(1) ؒ ؒ ؒ Cu(2A) 9.456(1) Å. Intermolecular copper-to-copper
separations are significantly shorter than the across dipyrazole
5
,5Ј methyl groups on the mdpz rings, and compares with the
1 7
rotation of ≈58Њ found in [(Ni L ) (mdpz)]. The overall
2
2
(
mbdpz) Cu ؒ ؒ ؒ Cu separations. This is brought about by the
molecular geometry consists of two non-identical halves
bridged by the mdpz moiety. The disparity is made evident
through noting the following: (i) the bridging angle, Cu–O–Cu,
within each half of the complex is slightly different,
Cu(1)O(1)Cu(2) 127.0(1)Њ, Cu(3)O(4)Cu(4) 126.0(1)Њ; (ii) the
dihedral angle between each [CuN O ] chromophore is differ-
phenoxy oxygens, O(2A) and O(2B), bridging between Cu(1A)
and Cu(1B) and forming a tetranuclear copper centre Fig. 1(b).
Further important bond lengths and angles are given in Table 2.
Crystal structure of 2
2
2
The desired pair-of-dimers arrangement is clearly evident and
ent, [Cu(1)N O –Cu(2)N O ] 176.6Њ and [Cu(3)N O –Cu(4)-
2 2 2 2 2 2
J. Chem. Soc., Dalton Trans., 2000, 713–718
715

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Fig. 3 Temperature dependence of the magnetic susceptibility (ᮀ) and
magnetic moment (᭺) for 1. The solid line is that calculated using
eqn. (1) using the parameters outlined in the text.
χCu
=
2
2
Ϫ1
2 2
Ng β
Ϫ2J
Ng β P
4kT
ͫ
3 ϩ expͩ ͪͬ (1 Ϫ P) ϩ
ϩ Nα (1)
kT
kT
of a percentage of monomeric impurity that was assumed to
have the same g-value as the complex.
All the symbols have their usual meaning and P is the
fraction of monomeric impurity.
[
(Cu L) (mbdpz)] (1). The µ values decrease from 1.16 µ at
2 2 Cu B
3
00 K to 0.1 µ at 4.2 K and are shown graphically in Fig. 3. It
B
is evident from Fig. 3 that the compound is experiencing a
moderately strong antiferromagnetic interaction between the
metal centres within the [Cu L] fragments. The data were fitted
2
first to eqn. (1) and this assumed that the two dinuclear moieties
had no cross-mbdpz coupling as would be expected. An excel-
lent fit was obtained (Fig. 3) using the parameter values
Fig. 2 (a) Molecular structure and atomic numbering scheme for 2.
Thermal ellipsoids are shown at the 40% level. (b) The coordination
chromophores and bridging ligand (mdpz, methyl groups omitted for
clarity) of 2 showing the intermolecular stacking association of Cu(1)
and Cu(2) with Cu(1A) and Cu(2B), respectively and of Cu(3B)
and Cu(4B) with Cu(4C) and Cu(3C), respectively.
Ϫ1
Ϫ6
3
Ϫ1
g = 2.10, 2J = Ϫ460 cm , P = 0.016, N = 60 × 10 cm mol
α
per Cu. This 2J value is larger than that of the related µ-
pyrazolato bridged dinuclear complex [Cu L(pz)] (2J = Ϫ240
2
Ϫ1
cm ) which has r(Cu–Cu) = 3.359(4) Å and a Cu(1)–O–Cu(2)
18
angle of 125.1(7)Њ, both slightly smaller than in 1. In order to
see if any intermolecular coupling occurs in 1, between Cu(1A)
and Cu(1B) through the phenoxy oxygens O(2A) and O(2B),
the data were fitted to a tetramer model developed by Hatfield
N O ] 169.9Њ; (iii) the angle at which the plane of each mdpz
2
2
pyrazolate ring intersects the plane encompassing the Cu–O–
Cu bridge differs by some 15Њ, [N(3)N(4)C(19)C(20)C(21)]–
19
and Inman, to treat dinuclear copper() Schiff-base com-
[
[
Cu(1)O(1)Cu2)] 175.1Њ and [N(7)N(8)C(41)C(42)C(43)]–
Cu(3)O(4)Cu(4)] 160.3Њ, thus the mdpz bridging between
plexes paired weakly together via 90Њ phenoxo bridges. The
intermolecular J value of interest in their model (J12 short diag-
onal), is equivalent to J1A,1B in 1. The other intermolecular
couplings (J14 and J34 in ref. 19) were set to zero since there are
no direct bridging pathways. It was found upon varying this
Cu(1) and Cu(2) is essentially coplanar with the dinuclear
fragment containing them, whereas the bridging between Cu(3)
and Cu(4) is quite removed from coplanarity and; (iv) the
intermolecular interactions experienced by Cu(1) and Cu(2) are
different to those of Cu(3) and Cu(4) as shown in Fig. 2(b).
This results in the intermolecular Cu ؒ ؒ ؒ Cu separations
being smaller (ca. 3.4–4.3 Å) than across mdpz separations,
which are in the order of 10 Å. Further distortions about
the metal centres are also evident and are indicated within
Table 3.
Ϫ1
Ϫ1
J1A,1B parameter from ϩ50 cm to Ϫ50 cm , while keeping the
intra-dinuclear parameter J and g values constant at the above
values, or when allowing J and g also to vary, that J1A,1B had
only a very small effect on the quality of the fit. Thus J1
A,1B
Ϫ1
equal to ϩ50 cm gave calculated χ values a little larger than
Ϫ1
observed above 200 K, while use of J1A,1B of Ϫ50 cm gave
calculated values slightly less than observed above 200 K. Thus
J1A,1B = 0 remains the best-fit. Other related ‘pair-of-dimer’
species, having weak phenoxo bridges also show either positive
Magnetic exchange in 1 and 2
20
Variable temperature magnetic susceptibility studies were
carried out on powdered samples of the complexes over the
temperature range 4.2–300 K. As is commonly observed within
complexes possessing an antiferromagnetic interaction between
metal centres, the presence of a monomeric impurity is evident
or negative values close to zero.
[(Cu L) (mdpz)] (2). The µ values decrease from 1.22 µ at
2
2
B
B
300 K to 0.1 µ at 4.2 K. Though a maximum is not evident
B
below 300 K, within the corresponding susceptibility plot, the
general appearance is that of a compound possessing a medium
to strong antiferro-magnetic interaction between the para-
magnetic centres. In attempting to fit the data, employment of
an isolated dimer model (single J value) of the type described
at low temperatures, through a slight increase in χ . The data
Cu
17
were first treated by applying a modified Bleaney–Bowers
equation, calculated for two S = 1/2 centres under a Ϫ2JS × S
1
2
spin Hamiltonian, using a non-linear least-squares fitting
routine. The susceptibility equation (1) allowed for the presence
Ϫ1
above gave a reasonable fit using 2J of ca. Ϫ380 cm , with
7
16
J. Chem. Soc., Dalton Trans., 2000, 713–718

View Online
Table 3 Selected bond lengths Å and angles (Њ) for [(Cu L) (mdpz)] (2)
2
2
Cu(1)–O(1)
Cu(3)–O(4)
Cu(1)–O(2)
Cu(3)–O(5)
Cu(1)–N(1)
Cu(3)–N(5)
Cu(1)–N(3)
Cu(3)–N(7)
Cu(1) ؒ ؒ ؒ Cu(2)
Cu(1) ؒ ؒ ؒ Cu(1A)
Cu(3B) ؒ ؒ ؒ Cu(3C)
Cu(3B) ؒ ؒ ؒ Cu(4C)
O(2)–Cu(1)–O(1)
O(4)–Cu(3)–O(5)
O(2)–Cu(1)–N(1)
O(4)–Cu(3)–N(5)
O(2)–Cu(1)–N(3)
O(4)–Cu(3)–N(7)
O(1)–Cu(1)–N(1)
O(5)–Cu(3)–N(5)
O(1)–Cu(1)–N(3)
O(5)–Cu(3)–N(7)
N(1)–Cu(1)–N(3)
N(5)–Cu(3)–N(7)
Cu(1)–O(1)–Cu(2)
Cu(2)–Cu(1)–Cu(1A)
Cu(4B)–Cu(3B)–Cu(3C)
Cu(3C)–Cu(3B)–Cu(4C)
1.884(3)
1.883(3)
1.880(3)
1.901(3)
1.961(4)
1.957(4)
2.005(4)
1.985(3)
3.373(1)
3.772(1)
3.477(1)
3.701(1)
Cu(2)–O(1)
Cu(4)–O(4)
Cu(2)–O(3)
Cu(4)–O(6)
Cu(2)–N(2)
Cu(4)–N(6)
Cu(2)–N(4)
Cu(4)–N(8)
Cu(3) ؒ ؒ ؒ Cu(4)
Cu(2) ؒ ؒ ؒ Cu(2B)
Cu(4B) ؒ ؒ ؒ Cu(4C)
Cu(4B) ؒ ؒ ؒ O(5C)
O(3)–Cu(2)–O(1)
O(4)–Cu(4)–O(6)
O(3)–Cu(2)–N(2)
O(4)–Cu(4)–N(6)
O(3)–Cu(2)–N(4)
O(4)–Cu(4)–N(8)
O(3)–Cu(2)–N(2)
O(6)–Cu(4)–N(6)
O(1)–Cu(2)–N(4)
O(6)–Cu(4)–N(8)
N(2)–Cu(2)–N(4)
N(6)–Cu(4)–N(8)
Cu(3)–O(4)–Cu(4)
Cu(1)–Cu(2)–Cu(2B)
Cu(4B)–Cu(3B)–Cu(4C)
Cu(3B)–Cu(4B)–Cu(3C)
1.889(3)
1.897(3)
1.886(3)
1.886(3)
1.945(4)
1.955(3)
1.998(3)
2.006(3)
3.368(1)
3.693(1)
4.284(1)
2.731(1)
175.1(1)
174.3(1)
166.8(1)
92.7(2)
82.6(1)
97.6(1)
87.0(1)
82.8(1)
91.8(1)
86.9(1)
100.7(1)
169.3(2)
164.5(1)
127.0(1)
87.20(3)
65.45(2)
55.86(2)
174.8(1)
92.8(2)
82.5(1)
97.7(1)
86.5(1)
92.8(1)
92.3(1)
86.9(1)
98.7(1)
169.4(2)
163.5(2)
126.0(1)
80.93(3)
121.30(2)
58.70(2)
g = 2.05 and P = 0.01, but there were divergences between the
observed and calculated χ values. Various attempts were made
to improve the fit while taking into account both the small
structural differences between each dinuclear half of 2 and the
intermolecular interactions. While the latter interactions are
complex (see above), a first approximation to probing their
plexes 1 and 2 are generally very similar with only some small
differences observed in the shape of the χ/T plot above 250 K
and with a larger 2J value deduced for 1. We will report separ-
ately on similar structure/magnetism investigations of mixed-
metal di- a1nd tetra-nuclear derivatives of these ligand
2
frameworks.
19
effect was to use the Hatfield and Inman tetramer model,
described above. This is appropriate to the parallelogram inter-
action between Cu(3B) ؒ ؒ ؒ Cu(4C)/Cu(4B) ؒ ؒ ؒ Cu(3C) but not
to the polymer stepwise interactions involving Cu(1) ؒ ؒ ؒ
Cu(1A)/Cu(2) ؒ ؒ ؒ Cu(2B) [Fig. 2(b)]. Once more cross-
bipyrazole couplings within 2 are zero. Systematic and careful
variation of the three intermolecular ‘cross’ J values were made
Acknowledgements
This research was supported by funds from the Australian
Research Council and Trinity College Academic Development
Research fund.
and small improvement over eqn. (1) was obtained for
Ϫ1
2
J3B,4C = Ϫ20 cm , that is one of the sides of the parallelogram
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Paper a908177a
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J. Chem. Soc., Dalton Trans., 2000, 713–718