Synthesis and magnetic properties comparison of M-Cu(II) and M-VO(II) Schiff base-porphyrazine complexes: What is the mechanism for spin-coupling?

Article abstract of DOI:10.1021/ja0438112

Dimetallic Schiff base-porphyrazine (pz) compounds, denoted 1[M ¹; M²; R], have been prepared, where metal ion M ¹ is incorporated into the pz core, and metal ion M² is bound to a bis(5-tert-butylsalicylidenimine) chelate built onto two amino nitrogens attached to the pz periphery; R is a solubilizing group (either propyl (Pr) or 3,4,5-trimethoxyphenyl (TMP)) attached to the remaining carbons of the pz periphery. The synthesis of 1[Cu; Cu; R], 1[Cu; VO; R], 1[CIMn; Cu; Pr], and 1[CIMn; VO; Pr] is discussed, the crystal structures of 1[Cu; Cu; TMP] and 1[CIMn; VO; Pr] are presented, and the magnetic properties of these compounds are compared. The pattern of ligand-mediated exchange coupling in these complexes is startling: for the Cu-M² complexes 1[Cu; VO; R] and 1[Cu; Cu; R], 2 × 10² ≤ | J(Cu-VO)/J(Cu-Cu)|; for the CIMn-M² complexes 1[CIMn; Cu; Pr] and 1[CIMn; VO; Pr], J(CIMn-VO)/J(CIMn-Cu) ≈ 1/3, an inverse ratio from that of the Cu-M ² complexes, but with lesser discrimination. This coupling pattern is explained in terms of a novel orientation relative to the M¹-M ² direction: the square-planar Schiff base ligand set of M² is rotated in-plane by 45° relative to the effectively coplanar pz ligand set of M¹.

Full text of DOI:10.1021/ja0438112

Published on Web 06/18/2005
Synthesis and Magnetic Properties Comparison of M-Cu(II)
and M-VO(II) Schiff Base-Porphyrazine Complexes: What Is
the Mechanism for Spin-Coupling?
Min Zhao,† Chang Zhong,† Charlotte Stern,† Anthony G. M. Barrett,*,‡ and
Brian M. Hoffman*,†
Contribution from the Departments of Chemistry, Northwestern UniVersity,
EVanston, Illinois 60208-3113, and Imperial College of Science, Technology, & Medicine,
London SW7 2AY, U.K.
Received October 11, 2004; E-mail: agmb@ic.ac.uk; bmh@northwestern.edu
Abstract: Dimetallic Schiff base-porphyrazine (pz) compounds, denoted 1[M1; M2; R], have been prepared,
where metal ion M1 is incorporated into the pz core, and metal ion M2 is bound to a bis(5-tert-
butylsalicylidenimine) chelate built onto two amino nitrogens attached to the pz periphery; R is a solubilizing
group (either propyl (Pr) or 3,4,5-trimethoxyphenyl (TMP)) attached to the remaining carbons of the pz
periphery. The synthesis of 1[Cu; Cu; R], 1[Cu; VO; R], 1[ClMn; Cu; Pr], and 1[ClMn; VO; Pr] is discussed,
the crystal structures of 1[Cu; Cu; TMP] and 1[ClMn; VO; Pr] are presented, and the magnetic properties
of these compounds are compared. The pattern of ligand-mediated exchange coupling in these complexes
is startling: for the Cu-M2 complexes 1[Cu; VO; R] and 1[Cu; Cu; R], 2 × 102 e |J(Cu-VO)/J(Cu-Cu)|;
for the ClMn-M2 complexes 1[ClMn; Cu; Pr] and 1[ClMn; VO; Pr], J(ClMn-VO)/J(ClMn-Cu) ≈ 1/3, an
inverse ratio from that of the Cu-M2 complexes, but with lesser discrimination. This coupling pattern is
explained in terms of a novel orientation relative to the M1-M2 direction: the “square-planar” Schiff base
ligand set of M2 is rotated in-plane by 45° relative to the effectively coplanar pz ligand set of M1.
Introduction
groups, especially with S, N, or O heteroatom peripheral
functionalization, and to bind metal ions at the periphery through
Magneto-structural studies of the interactions between metal
centers in spin-coupled dimetallic systems are a foundation of
efforts to generate high-spin molecules. Homobinuclear com-
plexes of copper(II) have been intensively used to derive
magneto-structural correlations: according to a recent review,
more than 900 of such complexes have been structurally
characterized.1 Only a few heterobinuclear complexes have been
synthesized because of the relative difficulty of their synthesis.
In general, such heterobimetallic compounds have involved a
metal complex as a ligand for a simple metal salt,2-4 or the
stepwise reaction of a ligand having two different coordination
sites with two different metal ions.5,6
the heteroatoms.7-15 We have developed a new type of
binucleating ligand, porphyrazines with a Schiff base appended
to the periphery, and reported the synthesis of dimetallic
compounds M1[pz]-M2[Schiff base].16
To define the systematics of spin-coupling within the Schiff
base-porphyrazine system, herein we present the synthesis and
characterization of binuclear Cu(II)-Cu(II), Cu(II)-VO(II),
ClMn(III)-Cu(II), and ClMn(III)-VO(II) compounds, 1[M1;
M2; R] (Chart 1), derived from this ligand system. We report a
surprising pattern to the strength of spin-coupling, and explain
(7) Vela´zquez, C. S.; Fox, G. A.; Broderick, W. E.; Andersen, K. A.; Anderson,
O. P.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1992, 114,
7416-7424.
Porphyrazines (pz’s) have been shown to be a template to
rigidly organize multiple metal ions. The unique synthetic route
of porphyrazine, by template cyclization of maleonitrile deriva-
tives, allows us to prepare pz’s with a variety of peripheral
(8) Vela´zquez, C. S.; Baumann, T. F.; Olmstead, M. M.; Hope, H.; Barrett, A.
G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1993, 115, 9997-10003.
(9) Baumann, T. F.; Nasir, M. S.; Sibert, J. W.; White, A. J. P.; Olmstead, M.
M.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc.
1996, 118, 10479-10486.
(10) Michel, S. L. J.; Goldberg, D. P.; Stern, C. L.; Barrett, A. G. M.; Hoffman,
B. J. Am. Chem. Soc. 2001, 123, 4741-4748.
† Northwestern University.
‡ Imperial College of Science, Technology, & Medicine.
(1) Melnik, M.; Kabesova, M.; Koman, M.; Macaskova, L.; Garaj, J.; Holloway,
C. E.; Valent, A. J. Coord. Chem. 1998, 45, 147-359.
(2) Kahn, O.; Tola, P.; Galy, J.; Coudanne, H. J. Am. Chem. Soc. 1978, 100,
3931-3933.
(11) Goldberg, D. P.; Michel, S. L. J.; White, A. J. P.; Williams, D. J.; Barrett,
A. G. M.; Hoffman, B. M. Inorg. Chem. 1998, 37, 2100-2101.
(12) Goldberg, D. P.; Montalban, A. G.; White, A. J. P.; Williams, D. J.; Barrett,
A. G. M.; Hoffman, B. M. Inorg. Chem. 1998, 37, 2873-2879.
(13) Lange, S. J.; Nie, H.; Stern, C. L.; Barrett, A. G. M.; Hoffman, B. M.
Inorg. Chem. 1998, 37, 6435-6443.
(3) Cortes, R.; Urtiaga, M. K.; Lezama, L.; Arriortua, M. I.; Rojo, T. Inorg.
Chem. 1994, 33, 829-832.
(4) O’Connor, C. J.; Freyberg, D. P.; Sinn, E. Inorg. Chem. 1979, 18, 1077-
1088.
(5) Tomlonovic, B.; Hough, R. L.; Glick, M. D.; Lintvedt, R. L. J. Am. Chem.
Soc. 1975, 97, 2925-2927.
(6) Lintvedt, R. L.; Glick, M. D.; Tomlonovic, B. K.; Gavel, D. P. Inorg. Chem.
1976, 15, 1646-1654.
(14) Zhao, M.; Stern, C.; Barrett, A. G. M.; Hoffman, B. M. Angew. Chem.,
Int. Ed. 2003, 42, 462-465.
(15) Cook, A. S.; Williams, D. B. G.; White, A. J. P.; Williams, D. J.; Lange,
S. J.; Barrett, A. G. M.; Hoffman, B. M. Angew. Chem., Int. Ed. 1997, 36,
760-761.
(16) Zhao, M.; Zhong, C.; Stern, C.; Barrett, A. G. M.; Hoffman, B. M. Inorg.
Chem. 2004, 43, 3377-3385.
9
10.1021/ja0438112 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 9769-9775
9769

A R T I C L E S
Chart 1
Zhao et al.
Cu[Pz(A; B3)], A ) Copper(II) Bis(5-tert-butylsalicylidenimine),
B ) (3,4,5-Trimethoxyphenyl)2, 1[Cu; Cu; TMP]. Compound 2[Cu;
TMP] and 50-fold excess 5-tert-butyl-2-hydroxybenzaldehyde were put
into a three-neck round-bottom flask, 20 mL of anhydrous pyridine
was added via syringe, and H2S was bubbled through the reaction
mixture for 5 min, during which time the blue solution turned to bluish
purple. The solvent was evaporated under reduced pressure, and the
residue was chromatographed using 4% methanol in chloroform.
Without further purification, the crude ligand was dissolved in 20 mL
of methanol and 20 mL of chloroform, with the addition of 10-fold
excess anhydrous CuCl2 and trace amounts of triethylamine. The
reaction was brought to reflux for 2 h until there was no further change
in the UV-vis spectra. The expected product was then purified by
column chromatography (4% methanol in methylene chloride). Yield:
45%. UV-vis (CH2Cl2): λmax 349, 532, 645. ESI-MS: m/z 1785.7 (M
+ H+), calcd for C92H93N10O20Cu2 1785.9.
it with an analysis based on the fact that the metal ligands of
the pz and Schiff base have a novel orientation relative to the
M-M direction: the “square-planar” ligand set of M2 is rotated
in-plane by 45° relative to the effectively coplanar ligand set
of M1.
Cu[Pz(A; B3)], A ) Vanadium(II) Oxide-bis(5-tert-butylsali-
cylidenimine), B ) (n-Propyl)2, 1[Cu; VO; Pr]. The compound was
prepared using the same procedure as for 1[Cu; Cu; Pr]. Yield: 54%.
UV-vis (CH2Cl2): λmax 339, 592, 654. APCI-MS: m/z 1043.4 (M +
H+), calcd for C56H69N10O3CuV 1043.4.
Cu[Pz(A; B3)], A ) Vanadium(II) Oxide-bis(5-tert-butylsali-
cylidenimine), B ) (3,4,5-Trimethoxyphenyl)2, 1[Cu; VO; TMP].
This compound was synthesized by a procedure similar to that used
for 1[Cu; Cu; TMP], except that NaOMe was used as the base and the
reaction was stirred at room temperature overnight. Yield: 43%. UV-
vis (CH2Cl2): λmax 348, 537, 679. APCI-MS: m/z 1788.4 (M + H+),
calcd for C92H93N10O21CuV 1788.5.
Experimental Section
Materials and Methods. All starting materials were purchased from
Aldrich Chemical and used as received, with the exception of vanadium-
(IV) oxide bis(2,4-pentanedionate) (VO(acac)2), which was purchased
from Alfa Aesar and used as received. All solvents were used as
supplied. Silica gel used for chromatography was Whatman silica gel
60 Å (230-400 mesh) from VWR. Porphyrazines 2[Cu; Pr],16 2[Cu;
TMP],16 2[Mg; Pr],17 and 1[ClMn; Cu; Pr]14,16 were prepared as
previously reported. [N,N′-Ethylenebis(salicylideneaminate)]copper(II)18
and [N,N′-ethylenebis(salicylideneaminate]oxovanadium(IV)19 were
prepared and purified as described previously. Schlenk-line manipula-
tions were performed on an apparatus purchased from Chemglass and
with dry nitrogen.
ClMn[Pz(A; B3)], A ) Vanadium(II) Oxide-bis(5-tert-butylsali-
cylidenimine), B ) (n-Propyl)2, 1[ClMn; VO; Pr]. H2S was bubbled
through a solution of 2[Mg; Pr] (28.0 mg, 0.0403 mmol) and 5-tert-
butyl-2-hydroxybenzaldehyde (353 mg, 50-fold excess) in 10 mL of
pyridine for 5 min, during which time the blue color changed to violet.
The solvent was evaporated, and the residue was purified via column
chromatography in 4% methanol in chloroform (with 0.5% triethyl-
amine) and then redissolved in methanol. Ten equivalent of VO(acac)
and trace amount of triethylamine were then added. The reaction was
stirred at room temperature overnight. The solvent was then evaporated,
and column chromatography was applied. The major blue fraction was
collected and dissolved in 10 mL of DMF and 20 mL of chlorobenzene.
Excess MnCl2 (10-fold) was added, and the reaction was brought to
100 °C; it was stopped when there were no further changes in the optical
spectrum. The solvent was evaporated, the residue was dissolved in
methylene chloride, and the solution was stirred with brine for 30 min.
The organic phase was separated, and column chromatography (4%
methanol in methylene chloride) afforded the green product, 20.1 mg.
Yield: 46.6%. UV-vis (CH2Cl2): λmax 337, 382, 573, 642, 685. FAB-
MS: m/z 1035.4 (M - Cl)+, calcd C56H68N10O3MnV 1035.1.
X-ray Structure Determination. A summary of the crystal data
and structure refinement parameters for compound 1[Cu; Cu; TMP]
and 1[ClMn; VO; Pr] is provided in Table 1. Both structures were
solved by direct methods and expanded using Fourier techniques and
were refined by full-matrix least-squares based on F 2. All of the non-
hydrogen atoms of both complexes were refined anisotropically.
Hydrogen atoms were included but not refined. For 1[Cu; Cu; TMP],
a structural model consisting of the host molecule plus disordered
solvate molecules was developed; since positions for the solvate
molecules were poorly determined, a second structural model was
refined with contributions from the solvate molecules removed from
diffraction data using the bypass procedure in PLATON (Spek, 1990).
No positions for the host network differed by more than two significant
units between these two refined models. The electron count from the
“squeeze” model converged to 897 electrons in a total potential solvent-
accessible area volume of 3176.7 Å3. For 1[ClMn; VO; Pr], the other
disordered components to C24, C31, and C34 were not found in the
Electronic absorption spectra were recorded using a Hewlett-Packard
HP8452A diode array spectrophotometer. Fast atom bombardment mass
spectra (FAB-MS) were recorded by the Mass Spectrometry Laboratory
in University of Illinois at Urbana-Champaign. Atmospheric-phase
chemical ionization mass spectra (APCI-MS) and electrospray ionization
mass spectra (ESI-MS) were recorded using a Finnigan LCQ Advantage
mass spectrometer. Electron paramagnetic resonance (EPR) spectra were
measured by using a modified Varian E-4 X-band spectrometer. Solid-
state magnetic susceptibility measurements were made for a polycrys-
talline sample by using a Quantum Design MPMS SQUID susceptom-
eter operating in the temperature range 2-300 K and with a 500 H
magnetic field.
Cu[Pz(A; B3)], A ) Copper(II) Bis(5-tert-butylsalicylidenimine),
B ) (n-Propyl)2, 1[Cu; Cu; Pr]. H2S was bubbled into a suspension
of 2[Cu; Pr] in anhydrous pyridine with 50-fold excess 5-tert-butyl-
2-hydroxybenzaldehyde for 10 min, during which time the solid was
totally dissolved and the blue solution turned to bluish purple. The
solvent was then removed under reduced pressure. The remaining solid
was washed with methanol until the eluate was colorless and then
dissolved in chloroform and methanol (4:1). Anhydrous CuCl2 (10-
fold) was added, and the reaction was stirred at room temperature
overnight. The desired product was separated through column chro-
matography with chloroform and dried as a blue powder. Yield: 50%.
UV-vis (CH2Cl2): λmax 346, 600, 666. FAB-MS: m/z 1039.6 (M +
H+), calcd for C56H69N10O2Cu2 1039.4.
(17) Baum, S. M.; Trabanco, A. A.; Andres, A.; Montalban, A.; Micallef, A.
S.; Zhong, C.; Meunier, H. G.; Suhling, K.; Phillips, D.; White, A. J. P.;
Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Org. Chem. 2003,
68, 1665-1670.
(18) Bhadbhade, M. M.; Srinivas, D. Inorg. Chem. 1993, 32, 6122-6130.
(19) Bonadies, J. A.; Carrano, C. J. J. Am. Chem. Soc. 1986, 108, 4088-4095.
9
9770 J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005

Dimetallic Schiff Base−Porphyrazine Complexes
A R T I C L E S
Table 1. Crystallographic Data for 1[Cu; Cu; TMP] and 1[ClMn;
coordination sphere are listed in Table 2. The Cu(1) in the pz
core adopts an essentially square-planar coordination geom-
etry: the four Cu-N bonds lie between 1.926 and 1.937 Å,
and the four N-Cu-N angles are in the range of 89.48°-
90.31°. The structure of the Cu(2) ion in the Schiff base site
closely resembles that of [Cu(5-CH3O-salen)].18 The copper and
its coordinating atoms are planar, with the sum of angles at
copper equalling 360.1° and Cu displaced by only 0.0252 Å
from the least-squares N2O2 plane (mean deviation of all atoms
from the plane is 0.047 Å). The major difference in the two
similar structures is the dimension of the Cu-NCCN chelate
ring. C(14)-N(9) and C(91)-N(10) (1.365(7) and 1.373(6) Å,
respectively) are much shorter in 1[Cu; Cu; TMP] than the
equivalent bonds in [Cu(5-CH3O-salen)] (1.47(2) and 1.49(1)
Å, respectively). Also the C(14)-C(91) bond (1.397(7) Å)
within the pz conjugated system is significantly shorter than
the [Cu(5-CH3O-salen)] C-C bond (1.49(1) Å), as expected.
X-ray Structure of 1[ClMn; VO; Pr]. A dark block crystal
was grown from methylene chloride/methanol solution; Figure
2 shows the crystal structure. The manganese ion adopts a
distorted square-pyramidal coordination geometry and is 0.2933
Å out of the N4 plane (mean deviation of all atoms from the
plane is 0.0394 Å), as with other ClMn(III) porphyrazines.12,14,21
The Mn-Cl distance is 2.3408 Å, nearly the same as the value
for 1[ClMn; Cu; Pr].14 The coordination geometry around the
vanadium ion is also a distorted square pyramid. The V atom
is displaced 0.5863 Å from the equatorial coordination plane
N2O2 (planar to within 0.0148 Å), and the VdO distance is
1.591 Å. These two values are comparable with those of
monomeric tetradentate Schiff base-oxovanadium(IV) com-
plexes.22,23
VO; Pr]a
1[Cu; Cu; TMP]
1[ClMn; VO; Pr]
empirical formula
formula weight
crystal size/mm
lattice type
space group
a/Å
C
92H92N10O20Cu2
C
55H63.25N10.25O3MnVCl
1784.84
1057.25
0.316 × 0.302 × 0.142 0.428 × 0.344 × 0.144
monoclinic
P21/n
triclinic
P1h
13.959(3)
42.297(15)
18.391(5)
90
94.814(18)
90
11.1722(17)
13.105(2)
19.514(3)
92.328(2)
98.748(2)
94.309(3)
2812.0(7)
2, 1.253
0.489
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
10820(5)
4, 1.095
Z, Dc/g cm-3
absorption coefficient/mm-1 0.455
F(000)
3728
1112
θ range for data
collection/deg
1.21-29.20
1.56-28.97
reflections collected/unique 96 607/26 468
25 165/13 127
[R(int) ) 0.0389]
88.0
integration
13 127/0/694
1.087
[R(int) ) 0.1309]
completeness to θmax/%
absorption correction
data/restraints/parameters
goodness-of-fit on F 2
90.3
none
26 468/0/1141
0.945
0.1137
0.2812
b
R1
0.0818
0.2388
c
wR2
a Details in common: graphite-monochromated Mo KR radiation,
wavelength 0.71073 Å, Bruker SMART-1000 CCD area detector, 153(2)
K, full-matrix least-squares refinement based on F 2. b R1 ) ∑||Fo| - |Fc||/
∑|Fo|. c wR2 ) [∑(w(Fo - Fc )2)/∑w(Fo )2]1/2
.
2
2
2
difference map. A group anisotropic displacement parameter was refined
for the disordered C25 atom. The solvent atom was refined to quarter
occupancy.
Results and Discussions
Synthesis. The preparation of porphyrazines 1[Cu; M2; R]
begins with the reductive deselenation of 2[Cu; R] in pyridine
by hydrogen sulfide to form a free amino group terminal
(Scheme 1), a strategy developed by Ercolani et al.20 In the
presence of a large excess of 5-tert-butyl-2-hydroxybenzalde-
hyde (50-fold), the condensation reaction between the carbonyl
group and the amino group leads to 3[Cu; 2H; R], which reacts
with metal salts to give the dimetallic Schiff base pz’s. The
availability of both R ) n-propyl (Pr) and R ) 3,4,5-
trimethoxyphenyl (TMP) groups as substituents on the other
pyrroles allows us to optimize solubility: compounds 1[ClMn;
M2; Pr] are soluble in organic solvents and solutions of these
are readily studied, while compounds 1[Cu; M2; Pr] are not. In
contrast, compounds 1[Cu; M2; TMP] are readily soluble in most
common organic solvents.
The preparation of compound 1[ClMn; VO; Pr] started from
2[Mg; Pr] instead of 1[ClMn; Pr] (Scheme 2), because of the
sensitivity of Mn(III) to basic conditions. Deprotection (H2S,
pyridine)14,16,20 of 2[Mg; Pr] in the presence of 5-tert-butyl-2-
hydroxybenzaldehyde afforded the ligand 3[2H; 2H; Pr].
Sequential metalation of VO(II) in the reactive peripheral Schiff
base and ClMn(III) in the porphyrazine core gave the target
compound, 1[ClMn; VO; Pr].
Magnetic and EPR Studies. The EPR spectrum of 1[Cu;
Cu; Pr] in frozen solution could not be collected because of its
poor solubility in organic solvents. However, the enhanced
solubility of 1[Cu; Cu; TMP] enabled us to record its 77 K
spectrum dissolved in 10% chloroform/methylene chloride
(Figure 3). For comparison, the figure includes a spectrum of a
solid powder of 2[Cu; Pr] magnetically diluted in its free-base
diamagnetic host as a model for the copper core of 1[Cu; Cu;
TMP]; for a model of the peripheral copper, the figure includes
the spectrum of a frozen solution of a very common and well-
studied copper Schiff base compound, [N,N′-ethylenebis(sali-
cylideneaminate)]copper(II) (Cu(salen)). The EPR spectrum of
2[Cu; Pr] has the axial symmetry typical of monomeric square-
planar Cu(II) complexes: g ) 2.05, g| ) 2.14, A|Cu ) 220.0
G.16 The d9 Cu(II) ion in Cu(salen) behaves quite similarly,
with g| ≈ 2.2, g ≈ 2.05, and A|Cu ≈ 197 G.24
The EPR spectrum of the dimetallic compound 1[Cu; Cu;
TMP] is not a simple addition of the spectra of the two
individual spin centers, as would be seen if the two copper units
were independent. Instead, the g| region of the spectra shows a
superposition of two seven-line hyperfine patterns. Within a
Cu
pattern, the average splitting, Ah| ≈ [(A|Cu(pz) + A|Cu(Schiff
base))/2]/2 ≈ 92 G; the two patterns are offset around g| by
2D/g|â ≈ 180 G; D is the dipolar zero-field splitting parameter,
X-ray Structure of 1[Cu; Cu; TMP]. Dark blue, platelike
crystals of 1[Cu; Cu; TMP] were grown from ethyl acetate/
methanol solution. The crystal structure is illustrated in Figure
1, and bond distances and angles relevant to the copper
(21) Ricciardi, G.; Bavoso, A.; Bencini, A.; Rosa, A.; Lelj, F.; Bonosi, F. J.
Chem. Soc., Dalton Trans. 1996, 2799-2807.
(22) Hoshina, G.; Tsuchimoto, M.; Ohba, S.; Nakajima, K.; Uekusa, H.; Ohashi,
Y.; Ishida, H.; Kojima, M. Inorg. Chem. 1998, 37, 142-145.
(23) Ryuichi, K.; Tsuchimoto, M.; Ohba, S.; Nakajima, K.; Ishida, H.; Kojima,
M. Inorg. Chem. 1996, 35, 7661-7665.
(20) Bauer, E. M.; Ercolani, C.; Galli, P.; Popkova, I. A.; Stuzhin, P. A. J.
Porphyrins Phthalocyanines 1999, 3, 371-379.
(24) Tauber, C. E.; Allen, H. C., Jr. J. Phys. Chem. 1979, 83, 1391-1393.
9
J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005 9771

A R T I C L E S
Zhao et al.
Scheme 1. Synthesis of 1[Cu; M2; R]
Scheme 2. Synthesis of 1[ClMn; VO; Pr]
Table 2. Interatomic Distances (Å) and Angles (deg) Relevant to
the Copper Coordination Sphere in 1[Cu; Cu; TMP]
Cu(1)-N(1)
Cu(1)-N(5)
Cu(2)-O(21)
Cu(2)-N(9)
C(14)-N(9)
C(14)-C(91)
1.926(5)
1.930(5)
1.899(4)
1.973(5)
1.365(7)
1.397(7)
Cu(1)-N(3)
Cu(1)-N(7)
Cu(2)-O(22)
Cu(2)-N(10)
C(91)-N(10)
1.933(5)
1.937(5)
1.904(4)
1.948(5)
1.373(6)
N(1)-Cu(1)-N(3)
N(5)-Cu(1)-N(3)
N(1)-Cu(1)-N(5)
89.48(18)
90.31(19)
179.1(2)
N(1)-Cu(1)-N(7)
N(5)-Cu(1)-N(7)
N(3)-Cu(1)-N(7)
89.92(19)
90.2(2)
175.5(2)
O(21)-Cu(2)-O(22) 88.08(16)
O(22)-Cu(2)-N(10) 93.31(17)
O(21)-Cu(2)-N(9) 93.26(17)
N(10)-Cu(2)-N(9) 85.45(18)
O(21)-Cu(2)-N(10) 175.49(19) O(22)-Cu(2)-N(9) 178.17(19)
Figure 2. X-ray structure of 1[ClMn; VO; Pr].
Hˆ ) JS1‚S2
(1)
doublets splits the spin manifold into states with total spin of S
) 0 and S ) 1, separated by J; for positive J the spin triplet is
the higher level. To determine J, the temperature dependence
of the magnetic susceptibility for complex 1[Cu; Cu; Pr] was
measured (Figure 4). The susceptibility monotonically increases
with decreasing the temperature in the range of 300 to 2 K,
and the plot of the øm vs T is well described by the Curie-
Weiss law: øm ) B + C/(T - Θ), with B ) -0.00566 cm3/
mol, C ) 0.808 (cm3 K)/mol, and Θ ) -1.00 K. Compound
1[Cu; Cu; TMP] shows the same magnetic behavior as 1[Cu;
Cu; Pr], and the Curie-Weiss law fit gives B ) -0.00988 cm3/
mol, C ) 0.796 (cm3 K)/mol, and Θ ) -0.08 K, where the
Weiss constant, Θ, corresponds to the sum of intra- and
intermolecular exchange between spins. Thus we have very
weak Cu-Cu coupling in 1[Cu; Cu; R]: gâAh/kB , |J|/kB ,
|Θ| ) 0.08 K. The value is too small to decide whether the
coupling is antiferromagnetic or ferromagnetic, but in either
Figure 1. X-ray structure of 1[Cu; Cu; TMP].
and its value is close to that expected for the dipole-dipole
interaction.25 This spectrum indicates that the two Cu(II) centers
experiences a Heisenberg exchange coupling (eq 1), with |J|
. gâAh.
The exchange interaction between the two single-ion spin
(25) Bencini, A.; Gatteschi, D. Electron Paramagnetic Resonance of Exchange
Coupled Systems; Springer-Verlag: Berlin, Heidelberg, New York, 1990.
9
9772 J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005

Dimetallic Schiff Base−Porphyrazine Complexes
A R T I C L E S
Figure 3. X-band EPR spectra of 2[Cu; Pr], Cu(salen), and 1[Cu; Cu;
TMP].
Figure 5. X-band EPR spectra of 2[Cu; Pr], VO(salen), and 1[Cu; Cu;
TMP].
Variable-temperature susceptibility measurements were car-
ried out on 1[Cu; VO; Pr] and 1[Cu; VO; TMP]. The
experimental data for 1[Cu; VO; Pr] are illustrated in Figure 4;
it shows a sharp maximum in øm at ∼8 K, the result of
antiferromagnetic Cu-VO exchange coupling. The data were
fit to the Bleaney-Bowers equation (eq 2)32 for exchange-
coupled pairs of Cu(II) and VO(II) ions on the basis of the
isotropic exchange Hamiltonian (eq 1) for two interacting S )
1/2 centers.
Nâ2g2
3kB(T - Θ)
1
3
øm )
1 + exp(J/kBT) -1(1 - F) +
[
]
Figure 4. Plot of the molar magnetic susceptibility of a powdered sample
of 1[Cu; Cu; Pr] (9) and 1[Cu; VO; Pr] (b) versus temperature. The solid
lines are a fit by Curie-Weiss law to 1[Cu; Cu; Pr], B ) -0.00566 cm3/
mol, C ) 0.808 (cm3 K)/mol, Θ ) -1.00 K, and a fit to 1[Cu; VO; Pr] by
eq 1, g ) 2.03, J/kB ) 13.8 K, F ) 0.07, NR ) -0.00213 cm3 mol-1, Θ
) -2.33 K.
[Nâ2g2]F
4kBT
+ NR (2)
In eq 2, øm is expressed as cubic centimeters kelvin per mole
of M(II) ion; the Curie term allows for a fraction of monomeric
impurity spins F, while NR describes the temperature-indepen-
dent susceptibility. To account for possible interdimer magnetic
interactions, a Weiss-like correction term Θ is introduced; the
other parameters have their usual meaning. The solid lines
correspond to the best fit of the data for 1[Cu; VO; Pr] to eq 2:
case, it is negligible. This result is different from that obtained
for the previously reported binuclear copper complexes: in most
of those cases there is a strong antiferromagnetic coupling
between the two copper ions.26-31
Again, we could not record the frozen-solution EPR spectrum
of 1[Cu; VO; Pr] because of the poor solubility, but 1[Cu; VO;
TMP] dissolved well in 10% chloroform/dichloromethane and
afforded a good EPR spectrum in frozen solution at 77 K (Figure
5). The figure displays the reference spectra of 2[Cu; Pr] and
[N,N′-ethylenebis(salicylideneaminate)]oxovanadium(IV) (VO-
(salen)); the spin-Hamiltonian parameters of VO(salen) in frozen
g ) 2.03, J/k ) 13.8 K, F ) 0.07, NR ) -0.00213 cm3 mol-1
,
Θ ) -2.33 K. For 1[Cu; VO; TMP], g ) 2.01, J/kB ) 12.5 K,
F ) 0.11, NR ) -0.00059 cm3 mol-1, and Θ ) -4.02 K. Thus,
the ratio of Cu-M2 coupling constants for M2 ) VO, Cu is
extraordinarily large: if J(Cu-Cu) is antiferrromagnetic, 2 ×
102 e |J(Cu-VO)/J(Cu-Cu)|.
solution are g ) 1.985, g| ) 1.951, |A | ) 59 × 10-4 cm-1
,
and |A|| ) 159 × 10-4 cm-1. The spectrum of 1[Cu; VO; TMP]
is complicated by hyperfine splittings from exchange-coupled
51V and 63,65Cu centers, which again shows a coupling where J
. gâAh, along with a zero-field splitting interaction.
X-band EPR spectra of 1[ClMn; Cu; Pr] and 1[ClMn; VO;
Pr] taken at 77 K are shown in Figure 6. As we discussed
previously, both the spectra of 1[ClMn; Cu; Pr]14,16 and 1[ClMn;
VO; Pr] are the result of strong Heisenberg exchange coupling
between the S ) 2 and S ) 1/2 spins, which produces two total-
spin manifolds with S ) 3/2 and 5/2, separated by ∆ ) 5J. Both
spectra show “perpendicular” features at g ≈ 6 and g ≈ 4,
(26) Kogan, V. A.; Lukov, V. V. Russ. J. Coord. Chem. 2004, 30, 205-213.
(27) Melnik, M. Coord. Chem. ReV. 1982, 42, 259-293.
(28) Hasty, E. F. W.; Lon, J.; Hendrickson, D. N. Inorg. Chem. 1978, 17, 1834-
1841.
(29) Lambert, S. L. H.; David, N. Inorg. Chem. 1979, 18, 2683-2686.
(30) Thompson, L. K. T.; Santokh, S. Comm. Inorg. Chem. 1996, 18, 125-
144.
5
the former characteristic of the S ) /2 spin state, the latter of
3
the S )
/ state, both with axial zero-field splitting (zfs)
2
(31) Thompson, L. K.; Mandal, S. K.; Tandon, S. S.; Bridson, J. N.; Park, M.
K. Inorg. Chem. 1996, 35, 3117-3125.
(32) Bleaney, B. B., K. D. Proc. R. Soc. London 1952, 451.
9
J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005 9773

A R T I C L E S
Zhao et al.
Figure 7. Plot of the molar magnetic susceptibility of a powdered sample
of 1[ClMn; VO; Pr], plotted as (øT). The solid line is a fit to the data by
eq 2, with θ ) 0.18 K, g ) 1.95, D3/2 ) (3/5)DMn ) -1.38 cm-1, and
∆/kB ) 7 K.
Figure 6. X-band EPR spectra of 1[ClMn; Cu; Pr] and 1[ClMn; VO; Pr].
spins, each having intrinsic g values of 1.95. The temperature
dependence of ø(T) was fit to eq 3,
parameter (DS) large compared to the microwave quantum.33
The spectrum of 1[ClMn; Cu; Pr] shows resolved sextet
2
35 e-∆/k T
B
ø ) ø3/2
+ (C/4)
(3)
5
hyperfine patterns from interaction with 55Mn(I ) /2) in both
2 + 3 e-∆/k T
2 + 3 e-∆/k T
B
B
5/2
g ≈ 4 and g ≈ 6 features: AMn3/2 ) (6/5)AMn ) 80 G, AMn
) (4/5)AMn ) 53 G, and AMn is the intrinsic value for the
uncoupled Mn(III) (S ) 2) ion. The 55Mn splitting patterns are
clearly present but unresolved in 1[ClMn; VO; Pr]. The
spectrum of 1[ClMn; Cu; Pr] further shows unresolved intensity
from the associated g| features around g ) 2, that from 1[ClMn;
VO; Pr] must do so as well, but the region is partially obscured
by a signal from impurity vanadyl ion.
3kBT
D3/2
4(1 + e-2D
3/2/kBT
4 +
(1 - e-2D
)
3/2/kBT
1 1 + 9 e-2D
2
3
ø3/2 ) C
+
-2D3/2/kBT
3/2/kBT
[
]
3
4(1 + e
)
)
where C ) Nâ2g2/kB(T - θ) and the symbols have their usual
meanings. The function ø3/2 is associated with the S )
/
3
2
manifold and incorporates the effects of the zero-field split-
ting: D3/2 ) (3/5)DMn, where DMn ) -2.3 cm-1 34
The fits
The difference in the resolution of the M2 ) Cu and M2 )
VO complexes is caused by unresolved hyperfine splittings from
M2. Simple calculation shows14 that the M2 hyperfine coupling
is sharply reduced in the total-spin manifolds generated by the
spin exchange: A (M2)3/2 ) A (M2)5/2 ) ((1/5)A (M2). Each
line of the Mn sextet of 1[ClMn; Cu; Pr] must contain the
.
give ∆/kB ) 7 K and the Curie-Weiss constant, θ ) 0.18 K.
Thus, the ratio of ClMn-M2 exchange parameters, J(ClMn-
VO)/J(ClMn-Cu) ≈ 1/3, is an inverse behavior from that of
the Cu-M2 complexes, and with a lesser discrimination.
Mechanism of the Exchange Coupling. We have seen that
when an M1 ) Cu(II) ion is in the pz core, the coupling with
an M2 ) Cu(II) ion at the periphery is extremely weak compared
to the coupling with an M2 ) VO(II) ion at the periphery: 2 ×
102 e |J(Cu-VO)/J(Cu-Cu)|. Conversely, when M1 ) ClMn-
(III) ion is in the core, the intramolecular interaction is stronger
to M2 ) Cu at the periphery: J(ClMn-VO)/J(ClMn-Cu) ≈
1/3, but the discrimination between the M2 ) Cu, VO is not as
great. This dramatic difference can be explained by considering
the relative symmetries of the “magnetic orbitals”, the dσ orbitals
3
hyperfine splitting of Cu (I ) /2), which would broaden the
line by about (2I)A S ) 3 A S ) 3/5 A (Cu). As A (Cu) ≈ 30
G is small, the broadening by Cu is small, and the sextet from
55Mn is well resolved. Likewise for 1[ClMn; VO; Pr], A (VO)3/2
7
) A (VO)5/2 ) ((1/5)A (VO), but in this case the V (I ) /2)
S
S
broadens the Mn lines by (2I)A ) 7 A ) 7/5 A (V). As
A (V) ≈ 70 G, the broadening is appreciable and the Mn sextet
no longer is resolved.
The observation of both signals at 77 K implies a thermal
population of the two spin manifolds at this temperature. In
1
2
φM and φM (Figure 8), which contain the single unpaired
electron of the S ) 1/2 Cu(II) and VO(II) centers and the single
dσ orbital of the S ) 2 ClMn(III).
5
1[ClMn; VO; Pr], the S ) /2 signal is more intense than that
3
of S ) /2 compared with compound 1[ClMn; Cu; Pr], which
indicates that the J value is smaller for the M2 ) VO complex.
We previously reported that ∆/kB ) 23 K for 1[ClMn; Cu; Pr].14
To determine the exchange parameter ∆ in 1[ClMn; VO; Pr],
the magnetic susceptibility was measured over the temperature
range from 2 to 300 K using a SQUID magnetometer (Figure
7). The high-temperature limiting value, øT ≈ 3.2 cm3 mol-1
K, is consistent with the presence of S ) 2 and S ) 1/2 partner
The magnetic orbital φCu centered on a Cu(II) ion in the pz
2
2
core environment is dσ ) dx -y , partially delocalized over the
nitrogen atoms surrounding the copper to form a σ-antibonding
molecular orbital. This orbital is symmetric with regard to the
xz mirror plane (as defined in Figure 8). However, unlike the
situation in most dimetallic complexes, the “square plane” of
ligands of the peripheral Schiff base is rotated by 45° relative
(33) Weltner, W., Jr. Magnetic Atoms and Molecules; Dover: New York, 1983;
pp 266-277.
(34) Krzystek, J.; Telser, J.; Pardi, L. A.; Goldberg, D. P.; Hoffman, B. M.;
Brunel, L.-C. Inorg. Chem. 1999, 38, 6121-6129.
9
9774 J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005

Dimetallic Schiff Base−Porphyrazine Complexes
A R T I C L E S
Figure 8. Relative symmetries of the magnetic orbitals in the compound pair 1[Cu; Cu; R] and 1[Cu; VO; R], and the compound pair 1[ClMn; Cu; Pr] and
1[ClMn; VO; Pr].
to that in the pz core, so the magnetic orbital φCu′ centered on
the Cu(II) ion in the Schiff base is the dσ ) dxy, σ-antibonding
orbital, which is antisymmetric with regard to the xz mirror
plane. Thus, dσ(Cu) and dσ(Cu′) are orthogonal, and the exchange
parameter J for the Cu-Cu pair, which is proportional to the
overlap integral, φCu|φCu′ , should vanish as observed. In
in Figure 8 leads to Jσ(ClMn-VO) ≈ 0, and thus J(ClMn-
VO) ≈ Jπ(ClMn-VO); as a result, J(ClMn-VO)/J(ClMn-Cu)
≈ Jπ(ClMn-VO)/([Jσ(ClMn-Cu) + Jπ(ClMn-Cu)]. It is
reasonable to take Jπ(ClMn-M2) as being roughly invariant with
M2, in which case the value, J(ClMn-VO)/J(ClMn-Cu) ≈ 1/3,
implies Jσ(ClMn-M2) ≈ 2Jπ(ClMn-M2).
2
2
contrast, φVO is the dσ ) dx -y molecular orbital, partially
delocalized over the nitrogen and oxygen ligand atoms through
in-plane π-type overlap, and therefore it also is symmetric with
regard to the xz mirror plane. Thus, while the overlap integral
φCu|φCu′ is identically zero by symmetry for the Cu(II)-Cu(II)
pair, the magnetic orbitals of the Cu(II)-VO(II) pair are of the
same symmetry and interact through intervening atoms. This
difference in symmetry clearly underlies the large ratio of J(Cu-
VO)/J(Cu-Cu) we observe.
Conclusions
Two new homobinuclear Cu-Cu complexes, 1[Cu; Cu; R],
two new heterobinuclear Cu-VO complexes, 1[Cu; VO; R],
and a heterobinuclear ClMn-VO complex, 1[ClMn; VO; Pr],
have been synthesized though use of kinetic control in metal
ion incorporation to Schiff base-porphyrazine ligands. 1[Cu;
Cu; TMP] and 1[ClMn; VO; Pr] have been structurally
characterized. The disposition of the ligand fields in the pz and
Schiff base fragments in this system causes exchange between
M1-M2 ) Cu-Cu to be negligible compared to that in Cu-
VO complexes, wherever exchange between M1-M2 ) ClMn-
Cu complex is 3-fold stronger than in ClMn-VO. The results
clearly indicate that the exchange coupling for Cu and VO ions,
each with a single dσ odd electron, is rigorously controlled by
the novel symmetries of the metallic orbitals and occurs through
σ-delocalization. On the other hand, π-delocalization to a σ
electron on M2 from the M1 ) ClMn (S ) 2) is deduced to be
half as effective as σ-σ coupling between M1 and M2 spins.
When a high-spin d4 M1 ) ClMn(III) ion (S ) 2) is in the
2
pz core, the four unpaired electrons occupy dxy, dxz, dyz, and dz .
2
2
The only σ orbital is dxy, not dx -y as with Cu(II), and it has
the same symmetry as the magnetic orbital of the M2 ) Cu(II)
2
2
ion (dxy) in the Schiff base, not that of the VO(II) ion (dx -y ) in
the peripheral site (Figure 8). This explains the fact that the
coupling is stronger in the ClMn(III)-Cu(II) pair than the ClMn-
(III)-VO(II) pair: J(ClMn-VO)/J(ClMn-Cu) ≈ 1/3.
Why is the M2 discrimination so much less in the ClMn-
M2 complexes than in the Cu-M2 complexes? The large
discrimination between the Cu-M2, M2 ) Cu, VO pair, 2 ×
102 e J(Cu-VO)/J(Cu-Cu) e 104, clearly indicates that the
exchange involves σ-delocalization only, and that this is
rigorously controlled by the symmetry constraints we have
described. The dominance of σ coupling also explains the
“inverse” value of the ratio, J(ClMn-VO)/J(ClMn-Cu) ≈ 1/3.
However, this ratio might have been expected to have a much
smaller value, ∼10-2-10-4, if σ coupling along were operating,
not 1/3. We suggest that for complexes with high-spin Mn(III),
π-exchange plays a significant role: J(ClMn-M2) ) Jσ(ClMn-
M2) + Jπ(ClMn-M2). When M2 ) VO, the symmetry shown
Acknowledgment. This work was supported by the MRSEC
program of the National Science Foundation (DMR-0076097)
at the materials Research Center of Northwestern University.
We also thank the NSF (Grant CHE-0091364, B.M.H.) and the
EPSRC (A.G.M.B.) for fundings.
Supporting Information Available: Crystallographic infor-
mation files (CIF) of compounds 1[Cu; Cu; TMP] and 1[ClMn;
VO; Pr]. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0438112
9
J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005 9775