Synthesis and Coordination Chemistry of the Bis(imidazole) Ligand, Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane

Article abstract of DOI:10.1021/ic961204c

The synthesis and characterization of the new Lmidazole ligand bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane (BimOBz) and its copper(II) complexes [Cu(BimOBz)₂][BF₄]₂·2H₂O), (1·2H₂O), [Cu(BimOBz)(NO₃)₂]·CH₂Cl₂ (2·CH₂Cl₂) and [Cu(BimOBz)Cl₂]·2EtOH (3·2EtOH) are presented. The X-ray crystallographic parameters determined are as follows: 1·solv (solv = petroleum ether) C₈₄H₆₆N₈B₂CuF₈O ₂, 1456.65, monoclinic space group (P2₁/a), a = 20.862(8) A?, b = 19.110(5) A?, c = 22.452(9) A?, β= 110.09(3)°, Z = 4, R = 0.129, and R_w = 0.139. 2·CH₂Cl₂, C₄₁H₃₆N₆Cl₂O₇, 859.22, monoclinic space group (P2₁/c), a = 9.748(1) A?, b= 14.157(5) A?, c = 17.209(2) A?, β= 103.709(8)°, Z = 4, R = 0.066, and R_w = 0.087. 3·2EtOH, C₄₄H₃₈N₄Cl₂CuO₂, 789.26, monoclinic space group (P2₁/a), a = 17.171(5) A, b = 13.988(3) A?, c = 17.897(5) A?, β= 112.52(2)°, Z = 4, R = 0.084, and R_w = 0.092. The geometry at these Cu^II centers is distorted tetrahedral in 1 and 3 and essentially square-planar in 2, and this difference is reflected in their EPR spectra, especially the value of A_z; 1 (solid, g_x = 2.06. g_y = 2.13, g_z = 2.30, A_z = 94 × 10^-4 cm^-1), 3 (CH₂Cl₂, 77 K, g_x = g_y = 2.06, g_z = 2.37, A_z = 75 × 10^-4 cm^-1), 2 (solid, g_x = g_y = 2.06, g_z = 2.28, A_z = 169 × 10^-4cm^-1). 1 displays a reversible, one-electron Cu^II/Cu^I couple in CH₂Cb and MeCN at potentials of +0.62 and +0.49 V vs SCE, respectively.

Full text of DOI:10.1021/ic961204c

2944
Inorg. Chem. 1997, 36, 2944-2949
Synthesis and Coordination Chemistry of the Bis(imidazole) Ligand,
Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane
Rajiv Bhalla, Madeleine Helliwell, and C. David Garner*
The Chemistry Department, University of Manchester, Oxford Road, Manchester, M13 9PL U.K.
ReceiVed October 2, 1996X
The synthesis and characterization of the new imidazole ligand bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)-
methane (BimOBz) and its copper(II) complexes [Cu(BimOBz)2][BF4]2‚2H2O (1‚2H2O), [Cu(BimOBz)(NO3)2]‚CH2-
Cl2 (2‚CH2Cl2) and [Cu(BimOBz)Cl2]‚2EtOH (3‚2EtOH) are presented. The X-ray crystallographic parameters
determined are as follows: 1‚solv (solv ) petroleum ether) C84H66N8B2CuF8O2, 1456.65, monoclinic space group
(P21/a), a ) 20.862(8) Å, b ) 19.110(5) Å, c ) 22.452(9) Å, â ) 110.09(3)°, Z ) 4, R ) 0.129, and Rw
)
0.139. 2‚CH2Cl2, C41H36N6Cl2O7, 859.22, monoclinic space group (P21/c), a ) 9.748(1) Å, b ) 14.157(5) Å, c
) 17.209(2) Å, â ) 103.709(8)°, Z ) 4, R ) 0.066, and Rw ) 0.087. 3‚2EtOH, C44H38N4Cl2CuO2, 789.26,
monoclinic space group (P21/a), a ) 17.171(5) Å, b ) 13.988(3) Å, c ) 17.897(5) Å, â ) 112.52(2)°, Z ) 4,
R ) 0.084, and Rw ) 0.092. The geometry at these CuII centers is distorted tetrahedral in 1 and 3 and essentially
square-planar in 2, and this difference is reflected in their EPR spectra, especially the value of Az; 1 (solid, gx )
2.06, gy ) 2.13, gz ) 2.30, Az ) 94 × 10-4 cm-1), 3 (CH2Cl2, 77 K, gx ) gy ) 2.06, gz ) 2.37, Az ) 75 × 10-4
cm-1), 2 (solid, gx ) gy ) 2.06, gz ) 2.28, Az ) 169 × 10-4 cm-1). 1 displays a reversible, one-electron CuII/CuI
couple in CH2Cl2 and MeCN at potentials of +0.62 and +0.49 V vs SCE, respectively.
Introduction
histidine imidazole. While imidazole is the obvious choice, the
difficulty of incorporating the imidazole group into a chelating
The role of the metal center at the active sites of copper
proteins has been the focus of much attention for both synthetic
and biological chemists. The active site structures of these
proteins usually involve copper coordinated to at least two
histidine imidazole groups.1-7 The oxidized form of Cu/Zn
bovine superoxide dismutase possesses a copper center coor-
dinated to three histidine imidazoles and one histidine imida-
zolate group; the imidazolate group bridges the copper to a zincII
ion.4 The geometry of the CuIIN4 unit is considerably removed
from square-planar; X-ray crystallographic studies have shown
that the geometry of the copper center is essentially retained
upon reduction in the solid state,8 but NMR9 and EXAFS10
studies indicate that the CuI center is three-coordinate in solution.
These naturally occurring copper centers have stimulated
much research into the design and synthesis of chemical analogs.
The ubiquity of the imidazole groups of the amino acid histidine
in the coordination sphere of copper proteins1-7 has necessitated
that ligands should contain a donor group that mimics the
ligand has often resulted in the use of other nitrogen donors,
especially aromatic nitrogen donors (pyridine, pyrazole).11-13
Although pyridine has electronic properties similar to imidazole,
it is larger than imidazole. In contrast, the pyrazole group is
of a similar size, but is considerably more basic (pKa ) 14.2 vs
7.2). The abiological nature of the pyrazole (and pyridine) group
has prompted the synthesis of poly(imidazole) ligands.14-18 As
a contribution to this topic, we have synthesized19-21 a series
of bis- and tris(imidazole) ligands. Also, we have recently
reported the synthesis of bis(1-methyl-4,5-diphenylimidaz-2-
oyl)carbinol (BimOH) and the tetracopper complex [Cu4-
(BimOH)2(BimO)2][PF6]4.21b For the work reported herein, the
conversion of the alcohol functionality of BimOH to produce
bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane Bi-
mOBz (Scheme 1) was accomplished in order to remove the
option of the deprotonation to give an alkoxide, which could
be a strong competitor of the imidazole for coordination sites
on a metal ion such as CuII.
X Abstract published in AdVance ACS Abstracts, May 15, 1997.
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M.; Hodgson, K. O.; Freeman, H. C. Science 1988, 241, 806.
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M.; Gatti, G.; Manchesini, A.; Petruzelli, R.; Finazzo-Agno´, A. J. Mol.
Biol. 1989, 206, 513.
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J. A. A.; Leduc, P.; Mulliez, E.; Boumchita, H.; Reedijk, J.; Chottard,
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Soc. 1994, 116, 11030.
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1996, 2101.
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M.; Garner, C. D. Chem. Eur. J. 1996, 2, 685.
(21) (a) Bhalla, R.; Collison, D.; Helliwell, M.; Garner, C. D., submitted
to Inorg. Chim. Acta. (b) Bhalla, R.; Helliwell, M.; Garner, C. D. J.
Chem. Soc., Chem. Commun. 1996, 921.
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S0020-1669(96)01204-9 CCC: $14.00 © 1997 American Chemical Society

Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane
Inorganic Chemistry, Vol. 36, No. 14, 1997 2945
Scheme 1. Synthesis of Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane
Table 1. Crystallographic Data for 1‚solv, 2‚CH2Cl2, and 3‚2EtOH
C84H66N8B2CuF8O2
Experimental Section
Synthesis. Bis(1-methyl-4,5-diphenylimidaz-2-oyl)carbinol was pre-
pared according to procedures described by Bhalla et al.21a All other
reagents were obtained from Aldrich Chemical Co. and used without
further purification. Tetrahydrofuran was distilled from sodium/
benzophenone.
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
20.862(8) space group
P21/a
296
1.541 78
1.151
8.83
0.129
0.139
19.110(5)
22.452(9)
110.09(3)
8407(6)
4
T, K
Cu KR (λ, Å)
D
calcd, g cm-1
µ, cm-1
Ra
Rw
Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane (Bi-
mOBz). Bis(1-methyl-4,5-diphenylimidaz-2-oyl)carbinol (3.0 g, 6.0
mmol) was placed in a 3-necked 500 cm3 round-bottomed flask. The
flask was purged and filled with argon. Dry, degassed THF (80 cm3)
was then added via a syringe. Sodium hydride (80% dispersion in
mineral oil, 0.36 g, 12.0 mmol) was added to the suspension, which
was stirred at room temperature for 30 min, in order to effect complete
deprotonation, during which time a pale yellow solution formed.
Benzyl bromide (1.5 cm3, 12.6 mmol) was then carefully added to the
reaction via syringe. The reaction mixture was stirred at room
temperature overnight, during which time a fine white precipitate
formed. The excess NaH was destroyed by the careful addition of
i-PrOH/EtOH (2:1 v/v) (20 cm3). The pale yellow solution was
transferred to a separating funnel and washed with distilled water (2
× 50 cm3). The aqueous phase was then washed with CH2Cl2 (2 ×
20 cm3). The organic fractions were combined and the solvents
removed under vacuum, yielding a pale yellow oil. This was dissolved
in CH2Cl2 (100 cm3) and dried over anhydrous MgSO4, which was
collected by filtration and washed with CH2Cl2 (2 × 20 cm3). The
filtrate and the washings were combined, and the volume was reduced
by half under vacuum. The solution was placed in a crystallizing dish,
together with absolute EtOH (20 cm3). Colorless crystals formed which
were collected by filtration, washed with absolute EtOH (20 cm3), and
dried under vacuum. Yield: 2.8 g (79%). Mp: 208-210 °C. 1H NMR
(300 MHz, CDCl3): δ 3.5 (s, 6 H, NCH3), 4.8 (s, 2 H, CH2), 6.2 (s, 1
H, CHOBz), 7.1-7.65 (m, 25 H, ArH). Anal. Calcd for C40H34N4O
(MW 586.73): C, 81.9; H, 5.8; N, 9.6. Found: C, 80.1; H, 6.4; N,
9.2. MS observed parent (EI/CI) 586 (M+ 586).
[Cu(BimOBz)2][BF4]2‚2H2O (1‚2H2O). A solution of Cu(BF4)2‚
4.5H2O (0.055 g, 0.17 mmol) in absolute EtOH (10 cm3) was added
dropwise to a colorless solution of BimOBz (0.20 g, 0.34 mmol) in
CH2Cl2 (20 cm3). This resulted in the instantaneous formation of an
intense green solution which was stirred at room temperature for 1 h,
at the end of which time a blue/green solution was obtained. Blue
crystals formed overnight which were collected by filtration, washed
with absolute EtOH (20 cm3), and dried under vacuum. Yield: 0.19
g (77%). Anal. Calcd for C80H72N8B2CuF8O4 (MW 1446.65): C, 66.4;
H, 5.0; N, 7.7; B, 1.5; Cu, 4.4. Found: C, 64.2; H, 4.9; N, 7.4; B, 1.5;
Cu, 3.9. MS (+FAB) (BimOBz)+, 586; [Cu(BimOBz)]+, 649; [Cu-
(BimOBz)2]+, 1237.
b
MW
1456.65
C41H36N6Cl2CuO7
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
9.748(1)
14.157(5)
17.209(2)
103.709(8)
3937(1)
4
P21/c
296
1.541 78
1.449
25.21
0.066
0.087
T, K
Cu KR (λ, Å)
D
calcd, g cm-1
µ, cm-1
Ra
Rw
b
MW
859.22
C44H38N4Cl2CuO2
17.171(5) space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
P21/a
296
1.541 78
1.320
23.44
0.084
0.092
13.988(3)
17.897(5)
112.52(2)
3971(2)
4
T, K
Cu KR (λ, Å)
D
calcd, g cm-1
µ, cm-1
Ra
Rw
b
MW
789.26
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [(∑w(|Fo| - |Fc|)2/∑wFo )]1/2; w
2
2
) 4Fo /σ2(Fo ).
resulted in the formation of a red/brown solution initially which changed
to green color on complete addition. The solution was allowed to stir
at room temperature for 1 h. Orange crystals formed on standing at
room temperature overnight which were collected by filtration, washed
with absolute EtOH (20 cm3), and dried under vacuum. Yield: 0.19 g
(75%). Anal. Calcd for C40H34N4Cl2CuO (MW 721.20): C, 66.6; H,
5.0; N, 7.8; Cl, 9.8; Cu, 8.8. Found: C, 65.3; H, 4.8; N, 7.6; Cl, 9.6;
Cu, 7.9. MS (+FAB) (BimOBz)+, 586; [Cu(BimOBz)]+, 649; [Cu-
(BimOBz)Cl]+, 684.
Crystallographic Data Collection and Refinement of the Struc-
tures. A summary of the crystallographic data is presented in Table
1. Data were collected on a Rigaku AFC5R diffractometer with
graphite-monochromated Cu KR radiation and a 12 kW rotating anode
generator at room temperature using the ω/2θ scanning technique. The
structures were solved by Patterson methods using the SHELXS-86
program.22 The data in each case were corrected for absorption, Lorentz
and polarization effects using the DIFABS program.23 The intensities
of three representative reflections were measured after every 150
reflections, and a linear correction factor was included to account for
any decay. In each case, the function minimized during full-matrix,
least-squares refinement was ∑w(|Fo| - |Fc|)2, using standard neutral-
atom dispersion factors and anomalous dispersion corrections.24 Hy-
drogen atoms were placed in idealized positions (C-H ) 0.95 Å) and
were assigned isotropic thermal parameters that were 20% greater than
[Cu(BimOBz)(NO3)2]. CH2Cl2 (2‚CH2Cl2). A solution of Cu-
(NO3)2‚3H2O (0.08 g, 0.33 mmol) in absolute EtOH (10 cm3) was added
dropwise to a colorless solution of BimOBz (0.20 g, 0.34 mmol) in
CH2Cl2 (20 cm3). Initially, this resulted in the formation of a pale
blue solution which eventually changed to an intense green on stirring
at room temperature for 1 h. Pale blue crystals formed which were
collected by filtration, washed with absolute EtOH (20 cm3), and dried
under vacuum. Yield: 0.20 g (68%). Anal. Calcd for C41H36N6Cl2-
CuO7 (MW 859.22): C, 57.3; H, 4.2; N, 9.8; Cu, 7.4. Found: C, 57.8;
H, 4.3; N, 9.5; Cu, 7.1. MS (+FAB) (BimOBz)+, 586; [Cu-
(BimOBz)]+, 649; [Cu(BimOBz)(NO3)]+, 711.
(22) Sheldrick, G. M. SHELXS-86 in Crystallographic Computing 3;
Sheldrick, G. M., Kreuger, C., Goddard, R., Eds.; Oxford University
Press: New York, 1985; p 175.
(23) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(24) (a) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974;
Vol. IV, Table 2.2A. (b) Cromer, D. T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974;
Vol. IV, Table 2.3.1.
[Cu(BimOBz)Cl2] (3). A solution of CuCl2‚3H2O (0.08 g, 0.33
mmol) in absolute EtOH (10 cm3) was added dropwise to a colorless
solution of BimOBz (0.20 g, 0.34 mmol) in CH2Cl2 (20 cm3). This

2946 Inorganic Chemistry, Vol. 36, No. 14, 1997
Bhalla et al.
Table 2. Selected Bond Distances (Å) and Angles (Deg) (and
the equivalent B value of the atom to which they were bonded. Least-
squares planes were calculated using the PLATON program.25
ESDs) of 1‚solv
Crystals of 1‚solv were grown by solvent layering. The compound
was dissolved in CH2Cl2 and transferred to a Pasteur pipet, flame sealed
at one end. Five times the volume of 60-80 petroleum ether was then
carefully layered onto the solution. The pipet was then completely
sealed. After several days, blue blocklike crystals suitable for X-ray
diffraction studies formed at the interface. A crystal was selected and
mounted on a glass capillary and coated with an epoxy resin due to its
solvent sensitivity. The structure was refined in the non-standard setting
P21/a of the space group P21/c. The asymmetric unit consists of the
copper-containing cation and two [BF4]- anions. Also present were a
number of solvent fragments (probably disordered 60-80 petroleum
ether) with an atom occupancy of a 0.5. The phenyl ring carbons and
the non-hydrogen atoms of the solvent fragments were refined
isotropically since there were insufficient data to refine all of the atoms
anisotropically. Initially, the [BF4]- anions were treated as rigid groups,
but in the later stages of refinement, the fluorine atoms were refined
anisotropically with the boron atoms fixed in their idealized positions
and refined isotropically. The remaining non-hydrogen atoms were
refined anisotropically. The final full-matrix least-squares refinement
converged with R ) 0.129 and Rw ) 0.139. The high R value is a
attributed primarily to the disordered [BF4]- anions and the solvent
fragments. The maximum and minimum peaks on the final difference
Bond Distances
Cu1-N1
Cu1-N5
1.94(1)
2.00(1)
Cu1-N3
Cu1-N7
2.02 (1)
1.92 (1)
Bond Angles
N1-Cu1-N3
N1-Cu1-N7
N3-Cu1-N7
93.7(6)
142.9(6)
105.5(6)
N1-Cu1-N5
N3-Cu1-N5
N5-Cu1-N7
105.2(6)
117.0(6)
94.1(6)
Table 3. Selected Bond Distances (Å) and Angles (Deg) (and
ESDs) of 2‚CH2Cl2
Bond Distances
Cu1-N1
Cu1-O2
Cu1-O3
1.997(4)
1.968(4)
2.603(4)
Cu1-N3
Cu1-O5
Cu1-O6
1.966(4)
2.006(4)
2.492(4)
Bond Angles
O2-Cu1-O5
O2-Cu1-N3
O5-Cu1-N3
86.9(2)
174.5(2)
90.3(2)
O2-Cu1-N1
O5-Cu1-N1
N1-Cu1-N3
92.9(2)
165.1(1)
91.0(2)
Table 4. Selected Bond Distances (Å) and Angles (Deg) (and
ESDs) of 3‚2EtOH
Fourier map corresponded to 0.59 and -0.44 e Å-3
.
Bond Distances
Cu1-Cl1
Cu1-N1
2.212(3)
1.997(7)
Cu1-Cl2
Cu1-N3
2.245(3)
1.989(7)
Green tabular crystals of 2‚CH2Cl2 suitable for crystallographic
investigations were grown by evaporation from a CH2Cl2/EtOH solution.
In addition to one molecule of [Cu(BimOBz)(NO3)2], a CH2Cl2
molecule was present in the asymmetric unit. All the non-hydrogen
atoms were refined anisotropically. The final full-matrix least-squares
refinement converged with R ) 0.066 and Rw ) 0.087. The maximum
and minimum peaks on the final difference Fourier map corresponded
Bond Angles
Cl1-Cu1-Cl2
Cl1-Cu1-N3
Cl2-Cu1-N3
104.9(1)
132.5(2)
100.0(2)
Cl1-Cu1-N1
Cl2-Cu1-N1
N1-Cu1-N3
103.9(2)
125.8(2)
92.9(3)
to 0.68 and -0.73 e Å-3
.
BimOH was capable of deprotonating in the presence of copper
and resulted in the formation of alkoxide bridged copper clusters.
Thus, conversion of the alcohol group to an ether functionality
was considered desirable to promote the synthesis of monomeric
copper compounds. The synthesis of BimOBz involved treating
the sodium salt of BimOH with benzyl bromide at room
temperature (Scheme 1). Subsequent coordination chemistry
of BimOBz resulted in the synthesis of the monomeric copper
complexes [Cu(BimOBz)2][BF4]2 (1), [Cu(BimOBz)(NO3)2] (2),
and [Cu(BimOBz)Cl2] (3), which have been characterized by
crystallography. The coordination geometry of the copper
centers of these compounds are presented in Tables 2-4 .
1‚solv consists of monomeric [Cu(BimOBz)2]2+ cations
(Figure 1) and [BF4]- anions. Each copper has a CuN4
coordination (Cu-N1 1.94(1) Å, Cu1-N3 2.02(1) Å, Cu1-
N5 2.00(1) Å, Cu1-N7 1.92(1) Å), resulting from the ligation
of two BimOBz ligands. The angles associated with coordina-
tion sphere (N1-Cu1-N3 93.7(6)°, N1-Cu1-N5 105.2(6)°,
N1-Cu1-N7 142.9(6)°, N3-Cu1-N5 117.0(6)°, N3-Cu1-
N7 105.5(6)°, N5-Cu1-N7 94.1(6)°) indicate that the geometry
of the CuN4 unit is considerably removed from square-planar
with the interligand dihedral angle (ω) between the Cu1, N1,
N3 and Cu1, N5, N7 planes being 73.0°. The bite angles of
the ligands, N1-Cu1-N3 93.7(6)° and N5-Cu1-N7 94.1(1)°,
impose one of the constraints upon the geometry. Another
governing factor is the presence of considerable steric bulk of
the ligands; the ligands appear to adopt a conformation in which
the intramolecular steric interactions, most notably those
between phenyl groups on the 4-positions of the imidazole
groups, are minimized.
Orange platelike crystals of 3‚2EtOH suitable for X-ray crystal-
lographic study were grown by evaporation from a CH2Cl2/EtOH
solution. A crystal was selected, mounted on a glass fiber, and quickly
coated with an epoxy resin in order to avoid contact with the air, since
the crystal quality was sensitive to the loss of solvent. As well as the
copper-containing molecule, the asymmetric unit contains a molecule
of EtOH and also a two-atom fragment which gave a four-atom
fragment upon expansion about a center of symmetry and was
considered to be a disordered EtOH molecule. The non-hydrogen atoms
were refined anisotropically, except those of the disordered EtOH
molecule, which were refined isotropically. The final full-matrix least-
squares refinement converged with R ) 0.084 and Rw ) 0.092. The
maximum and minimum peaks on the final difference Fourier map
corresponded to 0.74 and -0.69 eÅ-3
.
Physical Measurements. The 1H NMR spectrum of BimOBz was
recorded on a Bruker AC 300 spectrometer. Mass spectra using EI
and CI ionisation were obtained using an IC Kratos MC25 instrument;
+FAB spectra were recorded on a Kratos Concept 1S spectrometer.
Electronic spectra were recorded on a Shimadzu UV-260 spectrometer;
electronic spectra of solid samples were obtained by reflectance with
a finely ground sample of the compound using BaSO4 (99.7%) as both
the reference and the base material onto which the sample was loaded.
EPR spectra were recorded on a Varian E112 spectrometer at X- and
Q-band frequencies. Cyclic voltammetric studies were performed using
an EG&G PAR 173 potentiostat and universal programmer. A glassy
carbon working electrode, platinum wire secondary electrode, saturated
calomel reference electrode (SCE), and the electrolyte [NBun ][BF4]
4
were used in the cell.
Results and Discussion
Synthesis and Structure. The synthesis of imidazole
chelates as chemical analogs for the histidine side chain has
led us to synthesize a series of poly(imidazole) chelates.19-21
The synthesis of BimOBz is a result of earlier investigation of
the copper chemistry of the precursor ligand BimOH. These
investigations demonstrated that the alcohol functionality of
A comparison of the geometry of the CuII ion in 1 with other
[Cu(bis(imidazole))2]2+ complexes is presented in Table 5. The
similarity of the interligand dihedral angle, ω, and the average
intraligand angle, D, indicate that the geometry of the CuII ion
in 1 is similar to that found in [Cu(BimKet)2]2+ (BimKet )
bis(1-methyl-4,5-diphenylimidaz-2-oyl)ketone).20 The value of
the ω (86.3°) in [Cu(BiPhen)2]2+ (BiPhen ) 2,2′-bis(2-imida-
(25) Spek, A. L. Acta Crystallogr. 1990, A46, C34.

Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane
Inorganic Chemistry, Vol. 36, No. 14, 1997 2947
Figure 1. ORTEP view of [Cu(BimOBz)2]2+. The 30% probability
ellipsoids are shown, and selective atomic labeling has been used.
Table 5. Comparison of the Geometry of the CuII Ion and Copper
Hyperfine Coupling Constants in a Selection of
[Cu(bis(imidazole))2]2+ Complexes
Azb/10-4 cm-1
Figure 3. ORTEP view of [Cu(BimOBz)(NO3)2]. The 30% probability
ellipsoids are shown, and selective atomic labeling has been used.
ligand
ωa/deg Da/(deg) solid CH2Cl2 MeCN ref
BimOBz
BimKet
BiPhen
73.0
68.2
86.3
93.9
94.2
141.9
94
112
130
104
114
d
107
121
147
c
20
26
nitrato group is anisobidentate (Cu1-O5 2.006(4) Å, Cu1-O6
2.492(4) Å). Therefore, strictly speaking, the CuII is five-
coordinate (CuN2O3). However, since Cu1-O6 is relatively
long, the primary coordination of the CuII can be approximated
to CuN2O2, the geometry of which is almost square-planar. N1-
Cu1-N3 (91.0(2)°), O2-Cu1-O5 (86.9(2)°), O5-Cu1-N3
(90.3(2)°), and O2-Cu1-N1 (92.9(2)°) each approach 90°, and
O2-Cu1-N3 (174.5(2)°) and O5-Cu1-N1 (165.1(1)°) ap-
proach 180°; the dihedral angle between the CuN2 and CuO2
planes is 15.6°. The imidazole rings are twisted away from
the CuN2 plane; the dihedral angle between CuN2 plane and
imidazole rings N1,C2,N2,C3,C4 and N3,C18,N4,C19,C20 are
35.9 and 27.5°, respectively. The dihedral angle between the
imidazole groups within the ligand is 32.1°. The nitrato groups
are almost mutually perpendicular with a dihedral angle of 77.7°.
a ω, interligand dihedral angle; D, average intraligand angle. b EPR
spectra recorded at 77 K. c This work. d Not available.
Figure 2. Bis(imidazole) ligands BiPhen26 and BimKet.20
zoyl)biphenyl) infers that the CuII center in this compound is
very close to tetrahedral.26 However, the value of D for [Cu-
(BiPhen)2]2+ (141.9°) indicates a greater distortion toward
square-planar than that of both 1 and [Cu(BimKet)2]2+, and this
is reflected in the electronic properties of this compound (Vide
infra).
2‚CH2Cl2 (Figure 3) is comprised of monomeric [Cu-
(BimOBz)(NO3)2] units, in each of which CuII is coordinated
to two imidazole groups from the bis(imidazole) ligand (Cu1-
N1 1.997(4) Å, Cu1-N3 1.966(4) Å) and two nitrate groups.
According to the criteria suggested by Reedijk et al.,27 the nitrato
group N6,O2,O3,O4 binds as a unidentate ligand to the copper
(Cu1-O2 1.968(4) Å, Cu-O3 2.603(4) Å), whereas the other
3‚2EtOH (Figure 4) consists of monomeric [Cu(BimOBz)-
Cl2] units in which each copper atom is coordinated to two
imidazole nitrogens (Cu1-N1 1.997(7) Å, Cu1-N3 1.989(7)
Å) from BimOBz and to two chlorine atoms (Cu1-Cl1 2.212-
(3) Å, Cu1-Cl2 2.245(3) Å). The geometry of the CuN2Cl2
unit is intermediate between square-planar and tetrahedral. The
angles associated with the coordination sphere of the CuII (N1-
Cu1-N3 92.9(3)°, N1-Cu1-Cl1 103.9(2)°, N1-Cu1-Cl2
125.8(2), N3-Cu1-Cl1 132.5(2)°, N3-Cu1-Cl2 100.0(2),
Cl1-Cu1-Cl2 104.9(1)°) and the dihedral angle between the
CuN2 and CuCl2 planes (68.5°) indicates that the coordination
geometry is closer to tetrahedral than square-planar. The
imidazole rings are twisted away from the CuN2 plane; the
dihedral angle between the CuN2 plane and the imidazoles N1,-
C2,N2,C3,C4 and N3,C18,N4,C19,C20 are 15.1 and 12.8°,
respectively. The dihedral angle between the imidazole groups
within the ligand is 22.8°.
(26) Knapp, S; Keenen, T. P.; Zhang, X; Fikar, R; Potenza, J. A.; Schugar,
H. J. J. Am. Chem. Soc. 1990, 112, 3452.
(27) Kleyweg, G. J.; Wiesmeijer, W. G. R.; Van Driel,G. J.; Driessen, W.
L.; Reedijk, J.; Noordik, J. H. J. Chem. Soc., Dalton Trans. 1985,
2177.

2948 Inorganic Chemistry, Vol. 36, No. 14, 1997
Bhalla et al.
Figure 6. X-band EPR spectrum of 3 in CH2Cl2 at 77 K.
coupling constant, Az, of 1 increases in CH2Cl2 and MeCN
(recorded at 77 K), indicating that the geometry at the copper
center becomes less tetrahedral in solution; however, this
difference is small (<13 × 10-4 cm-1 in MeCN), suggesting
that the solid state structure is essentially retained in solution.
The metal hyperfine splittting value of 1 as both a solid and
frozen glass (Az < 110 × 10-4 cm-1) is considerably smaller
than those observed for other [Cu(imidazole)4]2+ centers.19,21a,29
In fact, the value of Az in 1 is even smaller than that for the
20
near tetrahedral CuN4 compounds [Cu(BimKet)2][BF4]2 and
[Cu(BiPhen)2][ClO4]2.26 This observation is significant because
studies have demonstrated that the value of Az in compounds
with CuN4 centers decreases as the distortion from square-planar
to tetrahedral increases.30 This correlation between the inter-
ligand dihedral angle, ω, and Az is in the sense that as ω
increases Az decreases. On this basis, it seems surprising that
the value of Az in [Cu(BiPhen)2][ClO4]2 (ω ) 86.3°, Az ) 130
Figure 4. ORTEP view of [Cu(BimOBz)Cl2]. The 30% probability
ellipsoids are shown, and selective atomic labeling has been used.
× 10-4 cm-1 26
) is greater than that found in 1 (ω ) 73.0°, Az
) 94 × 10-4 cm-1) and [Cu(BimKet)2][BF4]2 (ω ) 68.2°, Az
) 112 × 10-4 cm-1).20 However, this correlation does not take
account of the intraligand angles which are substantially larger
in [Cu(BiPhen)2][ClO4]2 (141.9°) than in 1 (93.9°) and [Cu-
(BimKet)2][BF4]2 (94.2°), and this will affect the electronic
structure of the CuII center.
The EPR powdered X- and Q-band spectra of 2 are axial in
character (gx,y ) 2.06, gz ) 2.28); three of the four metal
hyperfine peaks are observed at X-band, and all are seen at
Q-band. The value of the metal hyperfine splitting (Az ) 169
× 10-4 cm-1) is typical of that observed for CuII in an essentially
planar environment.28
A full analysis of the X- and Q-band powder EPR spectra of
3 at room temperature (and 77 K) was not possible because of
line-broadening, but gx,y ) 2.08, gz ) 2.38 were read from the
Q-band spectrum. The X-band spectrum of 3 in CH2Cl2 at 77
K gave gx,y ) 2.06 and gz ) 2.37 with all four of the Cu
hyperfine lines apparent in the gz region (Figure 6); the Az value
of 75 × 10-4 cm-1 is even lower than that observed in the EPR
spectrum of 1 in CH2Cl2 at 77 K. The geometries of the CuII
center in both 1 and 3 are similar (distorted tetrahedral), and
the additional decrease in the metal hyperfine value of 3 may
be attributed to the covalency of the Cu-Cl bonds.
Figure 5. X-band EPR spectra of 1: (a) powder at 298 K; (b) in CH2-
Cl2 at 77 K; (c) in MeCN at 77 K.
Electronic Structure. The X- and Q-band powder and
frozen glass EPR spectra of 1 (Figure 5) are rhombic and
characteristic of CuN4 complexes with a dxy ground state.28 The
X-band powder spectrum, recorded at both room temperature
and 77 K, shows 63,65Cu hyperfine splitting in the gz region;
three of the four lines are easily observed and the fourth is
partially hidden in the gx,y region. All four Cu hyperfine lines
are observed in the Q-band powder spectrum (gx ) 2.06, gy )
2.13, gz ) 2.30, Az ) 94 × 10-4 cm-1). The metal hyperfine
The UV-visible spectra of 1-3 are presented in Table 6.
The assignments of the bands in the spectra of these complexes
were made by comparison with the UV-visible spectra of the
free ligand (π f π*, ν ) 37 700-37 400 cm-1) and [Cu-
(29) Prochaska, H. J.; Scwindinger, W. F.; Schwartz, M.; Burk, M. J.;
Bernarducci, E; Lalancette, R. A.; Potenza, J. A., Schugar, H. J. J.
Am. Chem. Soc. 1981, 103, 3446.
(30) Addison, A. W. In Copper Chemistry: Biochemical and Inorganic
PerspectiVes; Karlin, K. D., Zubieta, J., Eds.; John Wiley & Sons:
New York, 1983, p 109.
(28) Mabbs, F. E.; Collison, D. Electron Paramagnetic Resonance of
d-Transition Metal Complexes; Elsevier: Amsterdam, 1992.

Bis(1-methyl-4,5-diphenylimidaz-2-oyl)(benzyloxy)methane
Inorganic Chemistry, Vol. 36, No. 14, 1997 2949
Table 7. Comparison of the Redox Potentials of a Selection of
[Cu(bis(imidazole))2]2+ Complexes
ligand ωa/deg Da/deg E° (CH2Cl2)/V E° (MeCN)/V
ref
BimOBz 73.0
93.9
94.2
141.9
0.62
0.80
b
0.49
0.59
0.11
this work
20
26
BimKet
BiPhen
68.2
86.3
a ω, interligand dihedral angle; D, average intraligand angle. b Not
available.
could accommodate CuI;34 i.e., electron transfer between the
CuII and CuI states should not require a significant reorganization
of the metal. [Cu(BimOBz)2]+ has been synthesized electro-
chemically; reduction of [Cu(BimOBz)2]2+ at +0.1 V (in both
CH2Cl2 and MeCN) resulted in the decoloration of the green/
blue solution to produce a colorless solution over a period of 1
h at room temperature. The cyclic voltammogram of the
colorless solution was identical to that of the CuII complex.
Oxidation to the CuII derivative was achieved rapidly, either
by controlled-potential electrolysis at a potential of +1.0 V under
an inert atmosphere or by exposure to air.
Figure 7. Cyclic voltammogram of a 1 mM solution of [Cu(BimOBz)2]-
[BF4]2 recorded at 298 K in a 0.2 M solution of [NBun ][BF4] in CH2-
4
Cl2 using a glassy carbon electrode, a scan rate of 200 mV s-1, and an
SCE reference electrode.
Table 6. Electronic Absorption Spectra with Band Assignments for
1-3
ν/cm-1 (ꢀ/M-1 cm-1
)
CH2Cl2
MeCN
solid
assignment
Examples of reversible CuII/CuI redox couples of CuN4
complexes are rare20,26 since CuIIN4 centers generally tend to
adopt a square-planar geometry and CuIN4 centers favor
tetrahedral geometry.34 Table 7 presents a comparison of the
redox potential of the CuII/CuI couple and the overall geometry
of 1 with those for two other [Cu(bis(imidazole))2]2+. The
positive redox potential of the [Cu(BimOBz)2]2+/[Cu(Bi-
mOBz)2]+ couple is attributed to the geometry of the CuII center,
for which interligand steric interactions prevent adoption of a
tetragonal geometry, and to the presence of ligands which are
capable of being π-acceptors. Thus, although the geometry of
the the Cu center in [Cu(BimKet)2]2+ is similar to that of 1, the
more positive redox potential of the former may be a result of
an increased π-acceptor ability of the BimKet ligand, arising
from the extended conjugation of this ligand.
Compound 1
38 800 (69 400)
26 200 (1930)
16 050 (227)
12 400 (202)
38 600 (60 500)
27 100 (1910)
16 900 (267)
12 600 (202)
38 200
25 400
15 900
12 200
π f π*
im f Cu LMCT
d-d
d-d
Compound 2
38 900 (39 300)
39 400 (42 800)
37 900
25 500
16 300
14 000
π f π*
im f Cu LMCT
d-d
15 400 (56)
13 300 (61)
16 600 (45)
13 700 (61)
d-d
Compund 3
37 800 (36 200)
25 500 (1021)
37 900 (39 700)
25 100 (1230)
18 350 (74)
37 300
24 600
18 250
11 500
π f π*
im f Cu LMCT
d-d
11 400 (76)
11 100 (81)
d-d
(imidazole)4]2+ complexes.19-21a,26,31,32 The similarity of the
the solid and solution UV-visible spectra of these complexes
indicates that the solid state structures of 1-3 are essentially
retained in CH2Cl2 and MeCN solution.
Conclusion
The syntheses of a new sterically hindered imidazole chelate
(BimOBz) and three monomeric copper(II) complexes have been
achieved. The coordination number at the copper center has
been resticted to four by the use of this sterically hindered ligand.
Furthermore, we have shown that the small values of copper
hyperfine splitting and high redox potentials, characteristic of
type I copper proteins, can be reproduced by complexes of
alkylated imidazole ligands without the presence of thiolate
coordination. Thus, 1 and 3 have Cu hyperfine splitting (Az)
values of 94 × 10-4 and 75 × 10-4 cm-1, respectively. 1
displays a reversible, one-electron CuII/CuI couple in CH2Cl2
and MeCN at potentials of +0.62 and +0.49 V vs SCE,
respectively. The reversibility of the [Cu(BimOBz)2]2+/[Cu-
(BimOBz)2]+ couple demonstrates that electron transfer proteins
could possess a [Cu(histidine)4] core.
Electrochemistry. A cyclic voltammogram for 1 in CH2-
Cl2 over the potential range 0 to +1.0 V at a scan rate of 200
mV s-1 is presented in Figure 7. Analysis over this range at a
variety of scan rates (500-50 mV s-1) showed the presence of
a redox process at +0.62 V in CH2Cl2 and +0.49 V in MeCN
vs a SCE, respectively, which is assigned to the [Cu(Bi-
mOBz)2]2+/[Cu(BimOBz)2]+ couple. The ratio of ip /ip over
the scan rates 500-50 mV s-1 is ∼1 in both CH2Cl2 and MeCN;
the peak-peak separation, ∆E, decreased from 110 mV at 500
mV s-1 to 80 mV at 50 mV s-1 in CH2Cl2; ∆E decreased from
90 mV at 500 mV s-1 to 70 mV at 50 mV s-1 in MeCN; a plot
of ipc vs ν1/2 is a straight line for both solvents. Under identical
conditions, the cyclic voltammogram of ferrocene displayed a
reversible couple in both CH2Cl2 and MeCN with ∆E ) 70
mV. Therefore, the [Cu(BimOBz)2]2+/[Cu(BimOBz)2]+ couple
may be described as reversible.33
c
a
Acknowledgment. We thank the University of Manchester
for the provision of a scholarship (R.B), the funds provided by
the EC Human Capital and Mobility Framework III MASIMO
Network, and Drs. T. C. Higgs, J. McMaster, D. Collison, and
E. J. L. McInnes for their valuable contributions.
The reversibility of the [Cu(BimOBz)2]2+/[Cu(BimOBz)2]+
couple can be rationalized on the basis that the geometry of the
copper center in 1 is a flattened tetrahedral, a geometry that
Supporting Information Available: Three X-ray crystallographic
files, in CIF format, are available on the Internet only. Access
information is given on any current masthead page.
(31) Bernarducci, E; Bharadawaj, P. K.; Lalancette, R. A.; Krogh-Jesperson,
K; Potenza, J. A., Schugar, H. J. Inorg. Chem. 1983, 22, 3911.
(32) Bernarducci, E; Bharadawaj, P. K.; Krogh-Jesperson, K.; Potenza, J.
A.; Schugar, H. J. J. Am. Chem. Soc. 1983, 105, 3860.
IC961204C
(33) Southampton Chemistry Group. Instrumental Methods in Electro-
chemistry, John Wiley & Sons: New York, 1985.
(34) Hathaway, B. J.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143.