Download Electronic Structure of Pseudotetrahedral Copper(II)

Electronic Structure of Pseudotetrahedral Copper(II)
Schiff Base Complexes
J o h n R . W a ss o n , H . W a y n e R ic h a r d s o n , a n d W illia m
E. H a tfie ld
Department of Chemistry, University of North Carolina, Chapel Hill, N. C. 27514, USA
(Z. Naturforsch.
32b, 551-561
[1977); received December 2, 1976/February 12, 1977)
Copper(II) Complexes, Electron Paramagnetic Resonance, Schiff Base Complexes,
Electronic Spectroscopy, Hyperfine Coupling Constants
Copper(II) complexes with the Schiff bases formed by condensing i-butyl-, isopropyl-,
cyclohexyl- and diphenylmethylamines with salicylaldehyde and 3-methoxy- and 5-nitrosalicyl -aldehyde have been prepared and characterized. The isotropic nuclear hyperfine coup ling constants increase in the order t -butyl < isopropyl < cyclohexyl < diphenylmethyl for
the N-substituents in the three series of complexes while isotropic g-values increased in the
opposite direction. Spectra structure correlations were examined and discussed.
Introduction
Pseudotetrahedral copper(II) compounds have
been known for several years and have been
investigated with varying degrees of thorough­
ness1-31. In principle, operation of the Jahn-Teller
effect32 precludes the existence of purely tetrahedral
(Ta) copper(II) complexes and lower symmetries
are found for CuLa (L = monodentate ligand) species,
e.g., CuCl2- and CuBr|- 11. Molecular orbital
studies of distortions of the CuCU2- ion have shown
that total energy versus distortion angle curves are
rather flat bottomed8. This has been interpreted to
mean that a variety of geometries (“distortion
isomers” 30-33) are possible for a given tetrahedral
CuLi complex depending on ionic and van der Waals
forces as well as steric factors present in a given
compound. The tetrachlorocuprate(II) anion is
well-known to exhibit square planar and a variety
of “tetrahedral” geometries depending on the nature
of the cation10-28’34. The rather general occurence
of distortion isomerism in copper(II) compounds
has been reviewed33. For pseudotetrahedral copper(II) compounds of the type CuL2L2' (L ^ L') the
possibility of distortion isomerism is an additional
complication33 to an already lowr-symmetry prob­
lem.
Copper(II) Schiff base complexes have been
employed as enzyme models35’36. Inspection of
Requests for reprints should be sent to Professor
E. H a t f i e l d , Department of Chemistry,
University of North Carolina, Chapel H ill, North
Carolina 27514, USA.
W illia m
available ESR data suggests that pseudotetrahedral
Schiff5’ 19- 25’ 29’ 37 base compounds might serve as
models for the “blue” copper(II) proteins 38-39. This
study was initiated to further explore that pos­
sibility as well as more generally probe the electronic
structures of pseudotetrahedral Schiff base com­
plexes. It is well-established that the “d -p ” mixing
is the principal mechanism giving rise to the elec­
tronic spectral intensities of d-d bands of tetra­
hedral transition metal compounds. The d-d band
intensities can arise from metal 4p orbital partici­
pation in the ground and/or excited states40. ESR
studies of pseudotetrahedral copper(II) compounds
have shown that they exhibit much reduced electron
spin-nuclear spin hyperfine coupling constants
compared to complexes with other geometries. The
small values of the hyperfine coupling constants
have been associated with metal 4p orbital partici­
pation in the ground state41-44. These considerations
suggested to us that there should be a relation
between the oscillator strengths of the d-d absorp­
tion bands and the nuclear hyperfine coupling
constants of pseudotetrahedral copper(II) com­
plexes. To investigate this matter we prepared a
variety of copper(II) Schiff base complexes (1) and
characterized them spectroscopically.
H
'C = N=
I
R = (CH3 )2 C-, (CH3 )3 C-, C6 H n H
I
and (C6 H 5 )2 CX — H, 3-methoxy, 5-nitro
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J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes
Isopropyl (P), £-butyl (T), cyclohexyl (C) and diphenylmethyl (D) groups substituted on the azomethine nitrogen atom were chosen since crystallographic studies 3 ’ 17 have shown that bulky groups
lead to pseudotetrahedral geometries for copper(II)
salicylaldimine complexes. For convenience, the
complexes are designated Cu(TsalOCH3 )2, etc.,
where the capital letter indicates the substituent
group and the latter part of the abbreviation
indicates the appropriate salicylaldimine.
Experimental
The complexes employed in this study were
prepared and purified by standard methods and
gave satisfactory elemental analyses (C, H and N
from Chemalytics, Inc., Tempe, Arizona and copper
by EDTA titration). Crystals of most of the
complexes could be growrn from acetone or chloro­
form solution.
Electronic spectra were obtained with a Cary Mod­
el 17 recording spectrophotometer using matched
1.0 cm cells. Mull (transmittance) spectra were
obtained by a technique described previously45.
Electron spin resonance spectra were recorded with
a Varian E-3 spectrometer at room temperature or
at 77 °K. Quartz sample tubes were employed for
powders and chloroform solutions. Spectra were
calibrated using diphenylpicrylhydrazyl (DPPH,
g = 2.0036) as a field marker. A sample of polycrystalline DPPH taped to a tube containing
oxobis(2,4-pentane-dionato)vanadium(IV) in ben­
zene served as a double standard for checking field
strength, frequency and sweep rate settings37.
Table I. Electronic spectral data
(chloroform solutions).
0
4
3
1
6
8
7
0
10
12
11
9
Compound 3
v(kK)b
E mc
f X 10+3d
Cu(Psal) 2
Cu(Csal ) 2
Cu(Tsal) 2
Cu(Dsal)2
Cu(PsalOCH 3 ) 2
Cu(CsalOCH3 ) 2
Cu(TsalOCH3 ) 2
Cu(DsalOCH 3 ) 2
15.63
16.21
13.16
16.75
15.15
15.63
13.64
16.67
15.77
16.18
13.30
16.55
165
157
4.17
4.56
3.64
3.96
3.90
4.63
5.18
4.90
4.92
5.10
4.74
4.70
Cu(PsalN 0 2 ) 2
Cu(CsalN 0 2 ) 2
Cu(TsalN 0 2 ) 2
Cu(DsalN 0 2 ) 2
(15.27)
(16.53)
2 0 2
(16.58)
(16.67)
157
166
188
234
189
2 0 2
(16.45)
(16.53)
189
261
192
a P, C, T and D refer to isopropyl, cyclohexyl, <-butvl
and diphenylmethyl groups attached to the azometliine nitrogen atom (1 ). sal, salOCH 3 and salN 0 2
refer to the salicylaldimine, 3-methoxysalicylaldimine and 5-nitrosalicylaldimine groups, respec­
tively.
b 1 kK — 1000 cm - 1 values in parentheses are the
band maxima in the mulled solids.
c Molar absorptivity.
d Oscillator strengths, f, were calculated using the
expression: f = 4.60 X 10- 9 E max ^1 / 2 where E max is
the molar absorptivity of the band maximum and
i>i/ 2 is the band width at half-height expressed in
wave numbers: C. J. B a llh a u s e n , Proer. Inorg.
Chem. 2, 251 [I960].
Results and Discussion
Electronic spectra
Table I summarizes the electronic spectra of the
copper(II) Schiff base complexes in chloroform
solution and representative spectra are shown in
Fig. 1. Apparent oscillator strengths for the mani­
fold of d-d transitions, which provide a better
measure of band intensity than the molar ab­
sorptivity of the band maximum, were determined
in order to obtain a comparison, discussed below, of
band intensities and isotropic hyperfine coupling
constants. The molar absorptivities of the Schiff
base complexes are all about 200 M-1 cm-1 and the
oscillator strengths are in the range 3-5 X 10-3. For
pseudotetrahedral complexes the major contribution
to the intensities of the d-d bands is expected to be
,ld -p ” mixing whereas the vibronic mechanism is
operative in planar complexes. Although the 4p
orbital participation in the ground and excited
states might be anticipated to increase on going
Fig.
( . . . .
V (kK)
, Electronic spectra ofCu(Dsal ) 2 (— —), Cu(Csal)2
, Cu(Psal) 2 ( - • ---- ) and Cu(Tsal)2 (- - - ) in chloroform.
from effective planar (~D 2h or D411) to pseudotetra­
hedral (C2, C2V or D2d) geometries, the degree of
mixing is not necessarily a smooth function of the
degree of distortion. Iterative extended Huckel
molecular orbital calculations8 for the CuCU2- anion
show that small distortions from D4h toward Td
symmetry are more likely to introduce appreciable
metal 4p orbital character into the ground state
than are larger distortions. Crystal structure data
for pseudotetrahedral Schiff base complexes, sum­
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J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase Com plexes
marized by W e i 17, show that Cu(Psal)2 is more
tetrahedral than Cu(Tsal)2. Just the opposite
conclusion is implied by the band maxima and
oscillator strength data presented in Table I. The
oscillator strengths are not always in accord with
the expectation that isopropyl-substituted salicylaldimines are more tetrahedral than t-butyl
substituted compounds. Apparently, the solution
species are not always identical to those existing in
the solid state as evidenced by the differences in the
electronic spectra of solutions and mulled solids
(Table I). Unfortunately, crystal structures are not
available for compounds other than Cu(Psal)2 and
Cu(Tsal)2. Since the apparent oscillator strengths
involve all of the bands (d-d, intraligand and chargetransfer tail contributions) it is not surprising that
they do not yield a simple spectra-structure correla­
tion although, generally, the £-butyl derivatives
have larger molar absorptivities at the band maxima
and larger oscillator strengths. The spread in the
d-d band centers is probably the dominant contribu­
tion to the failure of oscillator strength-structure
correlations. The band maxima for each series of
compounds indicate that the degree of tetrahedral
distortion of the complexes in solution decreases in
the order: £-butyl > isopropyl > cyclohexyl > diphenylmethyl. There is a general increase in inten­
sities on going from sal to salOCH3 to salN 0 2 groups
suggesting covalency contributions to the oscillator
strengths are also of significance. For a complex
with Td, D2d. C2 or lower symmetry the wave
function for the ground state can be expressed by
yg.s. == a3d ~f- b4s -f- c4p —
j- dL
(1)
where a, b, c, and d are ground state orbital
coefficients for metal 3d, 4s, and 4p and ligand L
orbitals, respectively. A given excited state might
have an eigenfunction given b y :
Vex = a'3 d -f b '4s + c'4p + d'L.
(2)
The transition moments will then consist of a sum of
terms of the types ac'(3d|f|4p), a'c (4p|r |3d>,
be'(4s|r|4p), b'c(4p[f|4s> and (metal orbital|r|L>
and dd' (L|r jL). Simple calculations 40 show that d-p
terms can readily lead to oscillator strengths of 1 0 -5
or larger, Fig. 2 shows results of some calculations
of transition moment integrals for copper com­
pounds. The integrals were evaluated using the
general expression of M c G l y n n et a l .46. The effective
nuclear charges for copper 3d, 4s and 4p orbitals
were estimated from the overlap matched orbital
553
% Effective Nuclear Charge
Fig. 2. Transition moment integrals and their squares
as a function of effective nuclear charge on copper.
exponents of Z e r n e r and G o u t e r m a n 47 by multi­
plying the exponents by the appropriate principal
quantum number. These were then reduced in
increments of 1 0 % in order to simulate the effect of
charge reduction by complexation of the metal ion.
Using the expression48
f — 1.096 x 10u v(cm_1) <yex|r|y>g.r) 2
(3)
it is easily shown that for a d-d transition appearing
at 1 0 ,0 0 0 cm-1 the d-p contribution to the oscillator
strength can range from 0.016 to 0.062 while the s-p
contribution only varies from 6.3 X 10- 5 —4.4 x 10-4
for a corresponding 50% reduction in the effective
nuclear charge on the copper ion. It is noted that
the oscillator strengths of the electronic absorption
bands of the “blue” copper proteins range 38 from
1 0 _3 to 1 0 -1 which has led to the suggestion of the
presence of a pseudotetrahedral chromophore. Data
for planar copper(II) complexes with sulfur donors49
implies that the presence of sulfur atoms leads to a
predominance of (metal orbital|r|S) and (LjrlL'}
contributions, particularly the former, to the
transition moments. By considering “d -p ” mixing
and covalency contributions to the transition
moments, a semiquantitative account of the “d-d”
band intensities of pseudotetrahedral copper(II)
complexes is attainable.
Crystal field calculations were performed using
the explicit approach of C o m p a n io n and K o m a -
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554
J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase Complexes
and a computer program described else­
where51. Calculations were performed for a CuXj
complex having the geometry (2 ) with the angles
X 1CUX2 and X 3CUX4 being 90°.
1
z
r y n s k y 50
j /*1
Cu— —x
■ * / 1\ ( 3
a>
X2 —
Cu
^
X, — x
^
2
3
The ligands Xi and X 2 were held in the x z plane
while the X 3CUX4 plane was rotated about the
2 -axis: the angle between the XiCuX 2 and X 3CUX4
planes was designated as to (3). The calculations
were performed for various values of the crystal
field splitting parameter a 4 (014 = 6 Dq for a cubic
complex) for each of the ligands while holding the
a 2/a4 ratio equal to 0.9. Fig. 3 shows that small
distortions from planar to tetrahedral type geo­
metries result in a slight increase in the d-d transi­
tion energies but these generally decrease as co
approaches 90° (D2d geometry). Increasing the
tetrahedral geometries to complexes in the order
of /-butyl > isopropyl > cyclohexyl > diphenylmethyl substituents is in accord with the phenome­
nological crystal field results. It is noted that
changes in geometry change the populations of the
various d-orbitals in the ground and excited states.
Thus, to discuss the electronic energy levels of a
particular compound in great detail requires
extensive calculational and experimental effort.
Fig. 4. Calculated electronic transition energies as a
function of 0 4 parameters for two C2 geometries.
Electron Spin Resonance Spectra
Fig. 5 compares the ESR spectra of polycrystalline Cu(Tsal)2 and Cu(Psal)2. Although qualitatively
similar, the g-values are better resolved for Cu(Psal)2
which is more tetrahedral in the solid state. Fig. 6
compares the ESR spectra of polycrystalline samples
of salOCH3 derivatives; again, the isopropyl
derivative shows the best resolved g-values. The
ESR spectra were interpreted by standard proce-
Fig. 3. Calculated electronic transition energies as a
function of dihedral distortion.
crystal field strength of the ligands increases the
band separations. In optimal cases where the band
widths are rather small ( < ~2kK ) and the bands
are 2-3 kK apart it may be possible to resolve three
of the four d-d bands. Fig. 4 which shows calculated
transition energies as a function of cu for two
distorted (C2) geometries shows that resolution of
more than three of the four absorption bands cannot
be readily anticipated. Although resolution of d-d
bands were not observed for the copper(II) Schiff'
base complexes, the assignment of decreasingly
Fig. 5. Electron spin resonance spectra of p oly­
crystalline Cu(Psal) 2 (— ) and Cu(Tsal)2 (----- ) at room
temperature.
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J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase Com plexes
dures. In several instances the A|| or A z nuclear
hyperfine coupling constants were estimated by
taking one third of the peak width at half-height.
Experiments with copper-doped samples of the
corresponding zinc complexes did not appear to
result in improved resolution of g and A values and
TsalOCI
HlOOoe^
Fig. 6 . Electron spin resonance spectra of poly­
crystalline o-vanillin Schiff base complexes with
copper(II). Room temperature.
555
were thus not performed for all of the complexes.
The spin-Hamiltonian parameters for the Schiff
base complexes are summarized in Table II. The
average g-values calculated from the anisotropic
powder data generally tend to be lower than the
isotropic values obtained from the chloroform
solutions. This strongly supports the conclusion that
the species present in solution are not those found
in the solid state and is in accord with the electronic
spectral data discussed in the preceding section (see
also, Table I). The isotropic nuclear hj^perfine
coupling constants increase in the order £-butyl <
isopropyl < cyclohexyl < diphenylmethyl for the
N-substituents in the three series of complexes
while isotropic g-values increased in the opposite
direction. Thus, as a complex becomes more tetra­
hedral, the isotropic g-values increases and the Ao
value decreases.
Table II. Electron spin resonance data.
j
X 104 cm- \
Compound
Solvent
go
g||
gjAi gX)
gyJ
Ao
Cu(Psal )o
Chloroform
powder
Chloroform
2.118
2.094
2.116
2.092
2.138
_
_
2.180
2.051
61.4
64.8
2.169
2.053
Cu(Csal)-2
Cu(Tsal) 2
Cu(Dsal) 2
Cu(PsalOCH 3 ) 2
Cu(CsalOCH3 ) 2
Cu(TsalOCH3 ) 2
Cu(DsalOCH 3 ) 2
Cu(PsalN 0 2 ) 2
Cu(CsalN 0 2 ) 2
Cu (TsalN O2 ) 2
Cu(DsalN 0 2 ) 2
Chloroform
Zn(II)
complex
powder
Chloroform
Zn(II)
complex
powder
Chloroform
powder
2 .1 2 0
2.090
2.094
-
2.276
2.167
-
2.132
2.217
2.151
2.262
Chloroform
powder
2.116
2.104
2.205
Chloroform
powder
Chloroform
powder
Chloroform
powder
Chloroform
powder
Chloroform
powder
2.143
2.097
2.115
2.075
2.119
2.116
2.118
2.182
2.116
2.224
Chloroform
powder
2.114
2.086
2 .1 2 1
2 .1 0 2
2.141
2.132
2.114
2.083
41.6
2.028
2.056
2.051
2.062
2.054
2.047
2.086
2.044
2.062
2.055
2.055
-
6 8 .0
2.178
2.064
-
-
2.256
2.054
2.086
-
2.051
_
9.3
-
17.6
-
2.147
-
61.9
2.062
-
_
-
69.2
67.9
-
50.0
A l( t )
_
118.2
59.0
-
-
50.0
68.9
64.9
-
-
A||
- ’
42.9
36.1
40.4
-
49.4
-
36.6
-3 2
-3 2
25.5
13.4
17.6
1 2 .2
-
6 .0
9.4
24.0
12.7
-
1 0 .0
-
1A
-
8 .0
17.6
69.9
-
-
-
-
18.6
* See Table I for abbreviations.
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J. R. Wasson et al. • Electronic Structure of Copper(II) Schiff Base Complexes
Table III. ESR data for pseudotetrahedral copper(II) complexes.
|— X 104 cm - 1
ZnW 0 4
Reference g||(g*)
2.072
53
2.072
57, 58
2 .0 0 1
CdW 0 4
58
2 .0 1 2
M gW 0 4
58
2.013
BeO
ZnO
54
55
CdS
52
ZuS
N H 4F
59
2 2
1.709
0.7392
0.7392
0.74
2.240
1.930
2.470
Cs2 CuCl4
2 0
2.384
Cs2 ZnCl4
2 0
2.445
Compound or host lattice
CaW 0 4
g i \gvJ
2.286
2.286
2.341
2.382
2.302
2.496
2.334
2.385
2.379
1.5182
1.5182
1.531
1.75
2.14
2.077
2.104
2.083
2.105
2.083
Ax ( Ay)
All
55.6
38.5
59.5
41.1
76.5 - - 9.2
76.5 - -1 9 .3
82
15
2 0
2.462
2.078
Comment
for 63Cu
for 65Cu
0
76
0
18
50
108
199.6
224.1
240.1
213.8
231
195
99 ± 5 28.1
for 63Cu
for 65Cu
2 1 .1
< 4
<4
-
-
25 + 5
2 .1 0 0
[(CH3 )4 N] 2 ZnCl4
|
17 + 5
several centers
51 + 5
46 + 5
51+5
2 .1 0 1
[(C4H 9 )4N ]2CuB r4
Cu(dmp) (CH3 C0
2 )2
4
0 0
2 .2 2 0
2.324
Cubich (CH2 C 0 2 ) 2
60
2.333
Cu2+ penicillamine
62
2.27
2.14
Cu(II)-doped
L-histidine hydrochloride
monohydrate
64
2.013
Cu(MPG) (violet)
Cu(MPG) (green)
Cu(dapy)2 (C104 ) 2 (red)
Cu(dapv)2 (C104 ) 2 (orange) -
39
34
30
30
2.185
. 2.259
2.467
2.451
Cu(dapy)2 (BF 4 ) 2
30
2.455
Cu(dapyp)2 (C104 ) 2
30
2.465
Cu(dapyp)2 (BF 4 ) 2
30
2.399
6 6
2.291
Cu(OAs{C6 H 5 }3 )2 Br2
Cu (OP{CßHö }3 )2 Br2
6 6
2.224
ZnHg(SCN ) 4
ZnHg(SCN ) 4
CdHg(SCN ) 4
ZnCd(SCN ) 4
CdCd(SCN) 4
13
56
56
56
56
2.40
2.440
2.447
2.270
2.245
2.053 < 145
2.068
168
2.068
2.078
2 .1 1
2.03
2.262
2.153
2.037
2.040
2 .1 0 1
2.061
2.093
2.078
2.107
2.053
2.145
2.046
2.076
2 .0 2 1
2.070
2.096
2.167
2.07
2.095
2.095
2.036
2.036
129
149
140
131.7
171
8 6
128.3
135.4
135.0
Ao — 54.1 x 10'4 cm _ 1
25.9
_
dmp = 2,9-dimethyl1 , 1 0 -phenanthroline
A0 = 30 + 0.5 X 10~4cm
bich = 2 ,2 '-biquinoline
Solution species.
Solution species.
62.4
~31
_
11.7
12.3
17.4
MPG = u-merceptopropionylglycine
dapy = diantipyrylmethane
8 .1
18.0
108.0
2 0 .6
30.5
165.2
1 0 .2
dapyp = diantipvrvlpropylmethane
11.3
Zn(II) lattice
170.5
«7.3
74
77
1 0 C
14 7
Z n(ll) lattice
11.5
29
29
13
60
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J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes
557
Table III (continued)
I— X 104 cm - 1 —|
Compound or host lattice
Reference g||(gz)
Catena-// -bis (1,2 dipheny 1-phosphinyl)ethan dichloiocopper(II)
6
2.413
Cu({C6 H 5 }sPO)2 Cl2
65
2.408
Cu({C6 H 5 }3 PO)2 Cl2
6 6
2.292
Zn(l,10-phenanthroline)Cl2
23
2.297
Cu( 1 -sparteine)Cl2
24
2.299
Cu( 1-sparteine)Br 2
24
2.27
Cu( 1-benzene-azo-N-phenyl2 -naphtylamine )2
Cu(2 ,2 /-bipyridvlamine) 2 (C1 0 4 ) 2
7
14
2.158
2.244
Cu(N-<-butylpyrole2 -carboxaldimine ) 2
„tetragonal form
37
29
2.156
2.209
,,triclinic form
29
2.266
CuL2 Cl2
L = [(C6 H 5 )2 PO] 2 CH2
= [(CjHg^PO^CH^
= [(C4 H 9 0)2PO]2 CH2
= [(C4 H 9 )2 PO](CH 2 ) 6
63
63
63
63
2.538
2.495
2.445
2.422
Dipyrromethanes
3,3',4,4'-tetramethyl
3,3\5-trim ethyl
3,4,5-trimethyl
3,3',5,5'-tetramethyl
3,3',4,4',5,5'-hexamethyl
5,5'-diphenyl
5,5'-dibromo
Cu(N -R -sal ) 2
R = isopropyl
= <-butyl
= s-butyl
= cyclohexyl
21
21
21
21
21
21
41
25, 5
19
25, 5
19
25
25
Cu(PBO ) 2 a
Cu(PBS ) 2 b
67
67
Cu(Im ) 2 c
Cu(Benzim ) 2 d
Cu(II) in:
Zn(a-picoline)2 (NCS ) 2
Zu(pyridine) 2 (NCS ) 2
Zn(ß-picoline)2 (NCS ) 2
Zn(y-picoline)2 (NCS ) 2
6 6
Cu{Me2 Ga(dmpz) 2 } 2 e
69
6 6
68
6 8
6 8
6 8
, 61
g±
2.067
2.073
2.064
2.071
2.026
2.079
2.058
2.062
2.075
2.050
2.071
2.042
2.041
2.059
2.069
2.089
2.086
2.045
2.090
2.065
2 .1 1 0
2 .1 0 0
2.091
2.103
A |
Pure crystal studied
Zn(II) lattice
170.6
123 ± 4
131.4
123 ± 0
-
Zn(II) host
20.1
Adopted inNi (II)complex
Pure crystal studied
A 0 - 2 0 .9 x 1 0 - 4 cm Pure crystal studied
1
Pure crystal studied
67.4
74.9
91.2
118
2.053
2.067
9.068
156
164.9
145
119.3
156
156
2.117
2.162
2.135
2.108
2.041
Zn(II) host
29.8
-
-
2.253
2.229
2.270
2.273
2.253
2.253
2.155
2.181
2.426
2.406
2.385
2.370
2.316
9.4
14.5
8.4
14.9
-
-1 7 .1
154
136
136
108
108
103
2.198
2.178
9±4
97.0
2.039
2.050
2.057
2.062
2.062
2.070
2.084
2.069
2.055
2.062
2.075
2.079
2.050
Comment
Pure crystal studied
2.224
2.248
2.245
2.272
2.278
2.279
2.283
-
Aj^ \AjJ
-1 4
-
-
-
-
-
158
183
197
57
1 1 .8
8
-
-1 7
-1 6
30
29
44
-
81
97
95
—50
63
63
63
6 8
6 8
-
-1 6
-
a>
73
-
<50
< 16
< 18
<30
6 8
Ao X 104 cm - 1
55.6
56.1
56.2
82.3
83.1
-
Toluene
1 , 1 , 2 ,2 -tetrachlorothene
Toluene
1 , 1 , 2 ,2 -tetrachloroethene
Toluene
Toluene
Zn(II) lattice
Zn(II) lattice
Zn(II) lattice
Zn(II) lattice
a PBO = anion from 2-(0-hydroxyphenyl)benzoxazole, b PBS = anion from 2-(0-hydroxyphenyl)benzoxazole, CIM = imidazolate anion, d benzim = benzimidazolate, e Me2 Ga(dmpz ) 2 = anion of 3,5-dimethylpyrazoyldimethylgallium.
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558
J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase C om plexes
Table III summarizes ESR data for pseudotetrahedral copper(II) complexes. The ESR spectra of
Cu(II) doped into binary compounds such as BeO
pose special problems associated with the JahnTeller effect. In tungstates and other compounds
the g-values vary widely with the g, or g|| values
becoming as large as 2.5. The copper nuclear
hyperfine constants of pseudotetrahedral com­
pounds also vary widely but they take values less
than 150 X 10-4 cm-1 which are significantly smaller
than A2 or A 11 values found for planar and pseudotetrahedral Cu(II) compounds. Table III provides
a brief review of ESR studies of pseudotetrahedral
Cu(II) compounds and is included for the reader’s
convenience as well as to demonstrate the observed
variation of spin-Hamiltonian parameters.
The large value of g2 or g|| of many pseudotetra­
hedral Cu(II) compounds is readily accounted for
in terms of the theoretical expressions for the
g-values70. The mixing of d levels, as noted in the
crystal field results, can be such that explicit
expressions for the g-values must be derived for a
particular compound. Table VIII of reference 70
lists g-value expressions for a number of geometries
having various ground states. Taking selected
expressions from that tabulation many ground
states and geometries as well as intermediate
situations can be considered.
Fig. 7 shows how g2 or g|| varies with the lowest
electronic transition energy and the effective spinorbit coupling constant. The particular label for the
electronic transition applies to a C2 complex with a
d rz ground state24. However, the results in Fig. 7
are applicable to a great number of complexes
possessing a non-degenerate and “unmixed” elec-
Fig. 7. gz or g11 as a function of electronic transition
energies and spin-orbit coupling constants.
tronic ground state70. An alternate expression for
gzis‘2 k ,2A
g2 = 2 .0 0 2
(4)
AE
where k 22 is the orbital reduction factor, kz2/
replacing ?. the effective spin-orbit coupling constant.
k -2 can be expressed in terms of molecular orbital
coefficients70-71 and it expresses the well-known
reduction in the spin-orbit coupling constant when
a metal ion is complexed. Keeping in mind that X is
negative for Cu(Il) and rearranging the preceding
expression, it is found th a t:
ZlgZlE
k 2A
(5)
Table IV. Orbital reduction factors for pseudotetrahedral copper(II) Schiff base complexes 3
Part A
k z2
k?
Ligand
1.63
1.31
1.53
1.30
1.28
1.14
1.24
PsalOCHg
CsalOCHg
TsalOCHg
DsalOCHg
Ligand
k,2
k2
Ligand
Psal
Csal
Tsai
Dsal
0.42
0.41
0.33
0.38
0.65
0.63
0.57
0.62
PsalOCHg
CsalOCHg
TsalOCHg
DsalOCHg
Ligand
Psal
Csal
Tsai
Dsal
k?2
1 .6 8
2.38
1.92
1.48
1.15
Part B
k, 2
0.60
0.48
0.37
0.29
kz
Ligand
kz 2
1.54
1.39
2 .1 1
1.07
P sa lN 0 2
CsalN 0 2
T salN 0 2
D sa lN 0 2
1.72
2.04
1.45
kz
Ligand
kz2
k2
0.77
0.69
0.61
0.54
PsalOCHg
CsalOCHg
TsalOCHg
DsalOCHg
0.53
0.43
0.51
0.36
0.73
1 .2 2
a Values found using data from Tables I and II and Equation (5)
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k2
1.45
1.31
1.43
1 .2 0
0 .6 6
0.71
0.60
J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase C om plexes
Table IV, Part A shows that the optical and ESR
data in Tables I and II lead to orbital reduction
factors which are significantly greater than a value
of 1 .0 which is considered to be the orbital reduction
factor when the unpaired electron is completely
localized on the metal ion. As covalency increases,
kz2 normally decreases. Since the 10-20% reduction
(more if sulfur and selenium ligands are involved)
of spin-orbit coupling constants in complexed ions
is a well-established phenomenon, the anomalous
results listed in Table IV need to be accounted for.
An alternate expression for g-values also covering
many pseudotetrahedral geometries having non­
degenerate and “unmixed” ground states is:
* ■ "2002 ~ ^
(fi)
where the symbols have their usual meanings.
Similarly to equation (5), the orbital reduction
factor can be evaluated using
k 2 A=
zlgzlE
(7 )
Fig. 8 demonstrates how equation (6 ) accounts for
many of the rat her large gz or g| | values encountered.
Table IV, Part B, shows how equation (7) gives a
more reasonable account of the data for the com­
plexes encountered herein.
559
d xy orbitals. However, it is common practice for a
complex to be treated as though it had a higher
symmetry and a pure “d” ground state. This
practice is convenient and will undoubtedly con­
tinue. Two other common sources of error in the
interpretation of the ESR spectra are: a) electronic
band assignments and b) higher order contributions
to the g-values. Fig. 4 show's that the assignment of
the band maximum in the electronic absorption
spectrum to ZlE employed in equations (4) to (7) is
a very tenuous practice. For a complex having
electronic transitions below 10 kK the errors in k 22
can be enormous. Hence, estimation of meaningful
orbital reduction (or covalency factors when a
molecular orbital approach is involved) necessitates
accurate band assignments. When very small
electronic transition energies are encountered,
higher order contributions of the type (A/zIE) 2 to
the g-values must be considered72. Fig. 9 shows that
these contributions became highly significant for
only very small transition energies and can probably
be neglected in all but the most exacting wrork and
wrhen very weak field ligands are of concern. For
pseudotetrahedral Cu(II) complexes writh Schiff base
Fig. 9. Second-order corrections to g-values as a
function of electronic transition energies and spinorbit coupling constants. ?. — -— 828 cm-1.
(a) 0.5A, (b) 0.62, (c) 0.7Ä, (d) 0.8A, (e) 0.9A, (f) 1.0A.
Fig. 8 . gz or g|| as a function of electronic transition
energies and spin-orbit coupling constants.
For lowr-symmetry complexes expressions for the
g-values frequently have to be derived for a
particular situation, e.g., a complex with C2 sym­
metry and a ground state made up of dr2, d r2_)/2 and
ligands the above considerations show that accurate
electronic band assignments are vital to meaningful
discussions of electronic structures. Several cor­
relations of spectral data for the compounds listed
in Table I wrere attempted. A plot of emax vs Ais 0
showed a general increase of £max with decreasing
Also but no quantitative relationship. A plot of
giso vs Also showed that giS0 increases with decreasing
Aiso but, again, no quantitative correlations w^ere
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560
J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes
found and. similarly, for a plot of f vs AjS0. Fig. 10
shows that a plot of the band maximum vs the
isotropic hyperfine coupling constants is reasonably
linear for each family of Schiff base complexes where
the azomethine nitrogen R groups are being
substituted. In the absence of extensive crvstallo-
Fig. 10. Electronic band maxima in chloroform vs
isotropic nuclear hyperfine coupling constants for three
groups of copper(II) Schiff base compleses.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
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graphic data additional structure-spectra correla­
tions were not attempted.
Estimation of isotropic nuclear hyperfine coupling
constants for the “blue” copper proteins 73 from
available data 38 shows that they should lie in the
range 20-40 X 10~4 cm-1. The ESR data in Table II
shows that the butyl salicylaldimine chelates have
isotropic coupling constants at the upper end of this
range. Hence, the complexes discussed herein are
not good models for the “blue” copper proteins - the
hyperfine coupling constants are too large and the
“d -d ” band intensities are too weak. By going to
the corresponding sulfur substituted derivatives we
expect to remedy both of these deficiencies and the
next step for validation of the compounds as
reasonable models for the “blue” copper proteins
will be an investigation of the redox reactions. Such
work is in progress and will be described subse­
quently.
This research was supported by the National
Science Foundation through Grant MPS 74-11495
AO 1 and by Materials Research Center of the Uni­
versity of North Carolina under Grant GH-33632
from the National Science Foundation.
18
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J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase Com plexes
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