Molecular structure and spectroscopy of divalent first row transition metals, Mn-Zn, with salicylaldiminate ligands

Article abstract of DOI:10.1016/j.poly.2013.02.011

The synthesis and spectroscopy of divalent first row transition metals bearing two monoanionic salicylaldiminate ligands is reported. The reaction of MnCl₂, FeCl₂, CoBr₂, 1,2-(Ph ₂PCH₂CH₂PP

Full text of DOI:10.1016/j.poly.2013.02.011

Polyhedron 54 (2013) 300–308
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Molecular structure and spectroscopy of divalent first row transition metals,
Mn–Zn, with salicylaldiminate ligands
Agnes Mrutu, Andrew C. Lane, Jonathan M. Drewett, Steven D. Yourstone, Charles L. Barnes,
⇑
Christopher M. Halsey, Jason W. Cooley, Justin R. Walensky
Department of Chemistry, University of Missouri, Columbia, MO 65211, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and spectroscopy of divalent first row transition metals bearing two monoanionic salicyl-
aldiminate ligands is reported. The reaction of MnCl2, FeCl2, CoBr2, 1,2-(Ph2PCH2CH2PPh2)NiCl2, and CuCl2
with 2 equiv. of the alkali metal salt of [OC6H2tBu2C(H)@N(C6H3Me2)]1ꢀ produces the corresponding
M[OC6H2tBu2C(H)@N(C6H3Me2)]2, M = Mn, Fe, Co, Ni, and Cu. Reaction of ZnEt2 with 2 equiv. of the pro-
tonated ligand affords Zn[OC6H2tBu2C(H)@N(C6H3Me2)]2. The molecular structure of each complex has
been analyzed using IR spectroscopy and X-ray crystallography while UV–Vis, CW-EPR, solution magnetic
susceptibilities, and DFT calculations were also used to probe their electronic structure.
Published by Elsevier Ltd.
Received 27 December 2012
Accepted 7 February 2013
Available online 16 February 2013
Keywords:
Schiff base
First row transition metal complexes
X-ray crystallography
1. Introduction
dentate ligands and, typically, transition metal complexes with sal-
icylaldiminate ligands have been used for olefin polymerization
The importance of first row transition metals in the divalent
oxidation state for a variety of applications such as enzymatic [1]
and polymerization [2] catalysis cannot be overstated. The molec-
ular and electronic structure of transition metal complexes corre-
spond to their chemical properties and reactivity, therefore,
examining the coordination chemistry of transition metals pro-
vides insight into their current and future applications. The influ-
ence of steric and electronic properties to control structure and
reactivity continues to be an exciting avenue of interest for main
group [3], transition metals [4], and f elements [5]. This has been
activity [9a,b–e,10] as well as epoxide and CO2 copolymerization
[9a,11].
Our focus was to create
a sterically demanding ligand
framework to isolate transition metal complexes from which fur-
ther reactivity could be pursued. To our surprise, van der Waals
interactions between each salicylaldiminate chelate shields the
metal center. Herein, we report the synthesis and structural
characterization of new bis(salicylaldiminate) complexes of mid-
dle to late transition metal complexes with the general formula
t
M[OC6H2 Bu2C(H)@N(C6H3Me2)]2, where M = Mn, Fe, Co, Ni, Cu,
beautifully illustrated with
(C5Me5)1ꢀ to (C5Me4H)1ꢀ by the Chirik group in the complex [(C5-
Me4H)2Zr]2( -N2) which was shown to convert dinitrogen to
a
simple change of ligand from
Zn.
l
ammonia while this reactivity did not occur with the analogous
(C5Me5)1ꢀ ligated complex [6].
2. Experimental section
Similar to cyclopentadienyl, Schiff base ligands are ubiquitously
employed in coordination chemistry and used for a variety of
applications [7] mainly because of the ease of their preparation
as well as their steric and electronic properties can be readily var-
ied [8]. However, while traditional Schiff base ligands take two va-
lence and four coordination sites, we aimed to prepare complexes
with more steric influence around the metal center using two li-
gands that would produce the same valence and coordination,
hence one subset of Schiff base ligands, salicylaldiminates, seemed
a logical conclusion. Salicylaldiminates [9] are monoanionic, poly-
2.1. General considerations
All manipulations were carried out inside an argon atmosphere
Vacuum Atmosphere OMNI glove box with rigorous exclusion of
air and water unless otherwise specified. Solvents (Aldrich) were
purchased anhydrous and stored over molecular sieves. Benzene-
d6 (Cambridge Isotopes) was dried over molecular sieves. Anhy-
drous MnCl2, FeCl2, 1,2-(Ph2PCH2CH2PPh2)NiCl2, and CuCl2 (Strem),
as well as CoBr2 and ZnEt2 (Fisher) were used as received. The pro-
t
t-
tonated ligands, HOC6H2 Bu2C(H)@N(C6H3Me2), 1, and HOC6H2
i
⇑
Bu2C(H)@N(C6H3 Pr2), 2, were synthesized following their
Corresponding author.
E-mail address: walenskyj@missouri.edu (J.R. Walensky).
literature procedures [12].
0277-5387/$ - see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2013.02.011

A. Mrutu et al. / Polyhedron 54 (2013) 300–308
301
2.2. General synthesis
To stirred solution of HOC6H2 Bu2C(H)@N(C6H3Me2), 1,
(300 mg, 0.89 mmol) in 10 mL THF was added NaH (23 mg,
0.89 mmol) presumably producing the sodium salt, 3. As the reac-
tion progressed, effervescing was observed. After 30 min, half an
equiv of metal halide was added. After 12 h, insoluble material
was removed by centrifugation the supernatant decanted and the
solvent removed under vacuum.
NMR (C6D6): d 174.9 (imine C), 170.4 (ArC-O), 148.7 (Ar-C), 142.1
(Ar-C), 135.5 (Ar-C), 131.4 (Ar-C), 131.2 (Ar-C), 131.0 (Ar-C),
130.3 (Ar-C), 129.0 (Ar-C), 128.5 (Ar-C), 117.4 (Ar-C), 35.7
(C(CH3)3), 33.8 (C(CH3)3), 31.3 (C(CH3)3), 29.5 (C(CH3)3), 18.7
(CH3), 17.0 (CH3). IR (KBr): vN@C stretch 1602 cmꢀ1. UV–Vis (THF,
10ꢀ4 M): kmax = 401 nm (3200 Mꢀ1 cmꢀ1). Anal. Calc. for C53H68N2-
O2Zn: C, 76.65; H, 8.25; N, 3.85. Found: C, 76.47; H, 8.13; N, 3.27%.
t
a
t
i
2.2.7. Cu[OC6H2 Bu2C(H)@N(C6H3 Pr2)]2, 10
t
i
To
a stirred solution of HOC6H2 Bu2C(H)@N(C6H3 Pr2), 2,
t
2.2.1. Mn[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 4
(300 mg, 0.762 mmol) in 10 mL THF was added NaH (20 mg,
0.83 mmol) presumably producing the sodium salt. As the reaction
progressed, effervescing was observed. After 30 min, CuCl2
(102 mg, 0.759 mmol) was added. After 12 h, insoluble material
was removed by centrifugation the supernatant decanted and the
solvent removed under vacuum yielding a red powder (504 mg,
79%). 1H NMR (C6D6): d 7.46 (s, 18H, C(CH3)3), 2.58 (s, 18H,
C(CH3)3), 0.55 (s, 12H, CH(CH3)2). UV–Vis (THF, 10ꢀ4 M):
kmax = 346 nm (2920 Mꢀ1 cmꢀ1), kmax = 394 nm (2560 Mꢀ1 cmꢀ1),
kmax = 548 nm (140 Mꢀ1 cmꢀ1). Anal. Calc. for C68H92N2O2Cu: C,
79.06; H, 8.98; N, 2.71. Found: C, 78.94; H, 8.76; N, 2.86%.
Orange crystals suitable for X-ray diffraction analysis were
grown from a saturated toluene solution at ꢀ35 °C (223 mg,
70%). 1H NMR (C6D6): d 1.34 (s, C(CH3)3,
m1/2 = 11.0 Hz), 1.41 (s,
C(CH3)3,
1/2 = 16.0 Hz). IR (KBr): vN@C 1613 cmꢀ1. UV–Vis (THF, 10ꢀ4 M):
338 nm (831 Mꢀ1 cmꢀ1), 487 nm (130 Mꢀ1 cmꢀ1). Anal. Calc. for
46H60MnN2O2: C, 75.90; H, 8.31; N, 3.85. Found: C, 75.89; H,
8.67; N, 3.67%.
m1/2 = 13.3 Hz), 2.00 (s, CH3, m1/2 = 21.0 Hz), 2.13 (s, CH3,
m
C
t
2.2.2. Fe[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 5
Red crystals suitable for X-ray diffraction analysis were grown
from a saturated toluene solution at 0 °C (210 mg, 69%). 1H NMR
2.3. Crystallographic data collection and structure determination
(C6D6): d 1.24 (s, C(CH3)3,
m
1/2 = 39.8 Hz), 1.34 (s, C(CH3)3, m1/
1/2 = 82.9 Hz). IR (KBr): vN@C
2 = 75.1 Hz), 3.74 (s, CH3,
m
The selected single crystal was mounted on nylon cryoloops
using viscous hydrocarbon oil. X-ray data collection was performed
at 173(2) K. The X-ray data were collected on a Bruker CCD diffrac-
1611 cmꢀ1 UV–Vis (THF, 10ꢀ4 M): kmax = 353 nm (1500 Mꢀ1
. -
cmꢀ1), 517 nm (180 Mꢀ1 cmꢀ1). Anal. Calc. for C46H60FeN2O2: C,
75.81; H, 8.30; N, 3.84. Found: C, 75.93; H, 8.54; N, 3.24%.
tometer with monochromated Mo Ka radiation (k = 0.71073 Å).
The data collection and processing utilized Bruker APEX2 suite of
programs [13]. The structures were solved using direct methods
and refined by full-matrix least-squares methods on F2 using Bru-
ker SHELEX-97 program [14]. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were added on idealized positions and not allowed to vary. Ther-
mal ellipsoid plots were prepared by using X-seed [15] with 50%
of probability displacements for non-hydrogen atoms. Crystal data
and detail for data collection for complexes 4–9 are provided in Ta-
ble 1, and significant bond distances and angles are gathered in
Table 2.
t
2.2.3. Co[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 6
Red crystals suitable for X-ray diffraction analysis were grown
from a saturated toluene solution at 0 °C (205 mg, 67%). 1H NMR
(C6D6): d 0.93 (s, C(CH3)3,
m1/2 = 181.1 Hz), 1.32 (s, C(CH3)3, m1/
2 = 141.5 Hz), 3.78 (s, CH3,
m
1/2 = 162.0 Hz). IR (KBr): vN@C
1613 cmꢀ1 UV–Vis (THF, 10ꢀ4 M): kmax = 385 nm (2400 Mꢀ1
. -
cmꢀ1), 515 nm (170 Mꢀ1 cmꢀ1). Anal. Calc. for C53H68N2O2Co: C,
77.25; H, 8.32; N, 3.40. Found: C, 76.89; H, 8.29; N, 3.35%.
t
2.2.4. Ni[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 7
Red crystals suitable for X-ray diffraction analysis were grown
from a saturated THF solution at room temperature (162 mg,
2.4. Computational details
49%). 1H NMR (C6D6): d 1.68 (s, C(CH3)3,
m
1/2 = 4.5 Hz,), 3.78 (s,
1/2 = 2.0 Hz). IR (KBr): vN@C stretch 1612 cmꢀ1. UV–Vis
(THF, 10ꢀ4 M): kmax = 390 nm (1920 Mꢀ1 cmꢀ1), 514 nm (200 Mꢀ1
cmꢀ1).
eff = 2.25 BM. Anal. Calc. for C54H76N2NiO4: C, 74.05; H,
CH3,
m
Calculations were performed using the B3LYP [16] (Becke-3 ex-
change [17] and Lee–Yang–Parr correlation [18] functional) in the
GAUSSIAN09 suite of software [19]. Geometry optimizations were ini-
tiated from the crystal structure coordinates and found to be min-
ima with no imaginary frequencies. The Stuttgart triple-f quality
basis set with corresponding effective core potential (ECP) [20]
was used for each metal atom while the Pople double-f quality ba-
sis set, 6-31G(d’) [21], was used for the remaining atoms.
-
l
8.75; N, 3.20. Found: C, 73.69; H, 8.53; N, 3.05%.
t
2.2.5. Cu[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 8
Red crystals suitable for X-ray diffraction analysis were grown
from a saturated toluene solution at 0 °C (269 mg, 85%). 1H NMR
(C6D6): d 1.18 (s, C(CH3)3),
2 = 30.3 Hz), 3.58 (s, CH3
1615 cmꢀ1 UV–Vis (THF, 10ꢀ4 M): kmax = 407 nm (2800 Mꢀ1
cmꢀ1), 535 nm (100 Mꢀ1 cmꢀ1).
eff = 1.06 BM. Anal. Calc. for C67-
H84N2O2Cu: C, 79.44; H, 8.36; N, 2.77. Found: C, 78.67; H, 8.24;
N, 3.06%.
m
1/2 = 37.5 Hz), 1.42 (s, C(CH3)3, m1/
m
1/2 = 25.2 Hz). IR (KBr): vN@C stretch
2.5. Physical measurements
.
-
l
Elemental analyses services were provided by Atlantic Microlab,
Inc. (Norcross, Georgia) for C, H and N. Infrared spectra were ac-
quired on a Thermo Nicolet Nexus 670 FT-IR spectrophotometer.
Samples were run as KBr pellets prepared under argon in the glove
box. 1H and 13C NMR spectra were obtained on Bruker AMX-
300 MHz superconducting NMR spectrometer. Magnetic suscepti-
bilities in C6D6 were determined using Evans method [22]. Contin-
uous wave electron paramagnetic resonance (CW-EPR) spectra
were acquired using an X-band Bruker ESP-300E spectrometer
equipped with an Oxford Instruments ESR-9 helium cryostat, using
a modulation frequency was 100 kHz. Spectra were processed and
simulated in the Matlab environment (The Mathworks, Natick, NJ)
using the easyspin toolbox [23].
t
2.2.6. Synthesis of Zn[OC6H2 Bu2C(H)@N(C6H3Me2)]2, 9
ZnEt2 (0.3 mL, 0.296 mmol) was added to a stirred solution of 1
(300 mg, 0.668 mmol) in 8 mL THF. The color changed immediately
from yellow to orange. After 1 h, the volatiles were removed under
vacuum to yield an orange microcrystalline solid (310 mg, 94%).
Orange crystals suitable for X-ray diffraction analysis were grown
from a saturated toluene solution at 0 °C. 1H NMR: (C6D6): d 7.83
(s, imine H, 2H), 7.50–6.72 (m, Ar-H, 10H), 2.46 (s, CH3, 6H), 1.81
(s, C(CH3)3, 18H) 1.40 (s, C(CH3)3, 18H), 1.18 (s, CH3, 6H). 13C

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A. Mrutu et al. / Polyhedron 54 (2013) 300–308
Table 1
X-ray crystallographic data for complexes 4–10.
4
5
6
7
8
9
10
CCDC Deposit
Number
916087
884459
884457
884460
884458
884461
887691
Empirical formula
Formula weight (g/
mol)
C53H68N2O2Mn
820.05
C53H68N2O2Fe
820.94
C53H68N2O2Co
824.02
C52H68N2O3Ni
875.88
C67H84N2O2Cu
1012.90
C53H68N2O2Zn
830.46
C68H92N2O2Cu
1032.98
Crystal habit, color
Temperature (K)
Space group
Crystal system
Volume (Å3)
a (Å)
plate, orange
173(2)
P21/c
monoclinic
4791.85
18.135(2)
14.4053(15)
19.658(2)
90
plate, red
173(2)
P21/c
monoclinic
4766.8(10)
18.214(2)
14.3298(17)
19.577(2)
90
plate, red
173(2)
P21/c
monoclinic
4756.4(17)
18.250(4)
14.272(3)
19.551(4)
90
block, red
173(2)
Pbcn
orthorhombic
4962.3(9)
19.550(2)
11.8225(12)
21.470(2)
90
block, red
173(2)
I2/a
monoclinic
5920(2)
20.931(6)
12.2908(19)
23.030(4)
90
block, orange
173(2)
P21/c
monoclinic
4767.4(14)
18.279(3)
14.268(2)
19.587(3)
90
block, red
173(2)
Pca21
orthorhombic
6111.0(14)
37.623(5)
13.3634(17)
12.1548(16)
90
b (Å)
c (Å)
a
(°)
b (°)
111.0780(10)
90
111.1090(10)
90
110.931(2)
90
90
90
92.3130(10)
90
111.054(2)
90
90
90
c
(°)
Z
4
4
4
4
4
4
4
Calculated density
1.137
1.144
1.151
1.172
1.137
1.157
1.123
(Mg/m3)
Absorption
coefficient
0.315
0.356
0.401
0.436
0.413
0.555
0.401
(mmꢀ1
)
Final R indices
[I > 2 (I)]
R1 = 0.0452,
wR2 = 0.1292
R1 = 0.0396,
wR2 = 0.1039
R1 = 0.0456,
wR2 = 0.1200
R1 = 0.0554,
wR2 = 0.1590
R1 = 0.0676,
wR2 = 0.1914
R1 = 0.0405,
wR2 = 0.1035
R1 = 0.0456,
wR2 = 0.1128
r
Table 2
Selected bond distances (Å) and angles (°) for complexes 4–10.
Bond distance/angle
4, Mn
5, Fe
6, Co
7, Ni
8, Cu
9, Zn
10, Cu
M–O(1)
M–O(2)
M–N(1)
M–N(2)
O(1)–M–N(1)
O(1)–M–N(2)
O(2)–M–N(1)
O(2)–M–N(2)
O(1)–M–O(2)
N(1)–M–N(2)
1.968(2)
1.971(1)
2.115(1)
2.107(2)
89.54(6)
133.03(6)
121.28(6)
89.11(6)
112.59(6)
114.85(6)
1.907(1)
1.910(1)
2.036(1)
2.030(2)
91.35(5)
129.23(6)
121.93(5)
91.17(5)
112.64(5)
113.78(5)
1.897(1)
1.898(2)
1.984(2)
1.986(1)
94.83(6)
119.85(6)
124.83(7)
94.82(6)
110.41(6)
114.19(6)
1.896(2)
1.896(2)
1.970(2)
1.970(2)
92.65(8)
129.26(8)
129.26(8)
92.65(8)
108.26(8)
109.16(8)
1.902(2)
1.902(2)
1.966(2)
1.966(2)
93.68(8)
145.36(9)
145.36(9)
93.68(8)
90.33(8)
102.05(9)
1.918(1)
1.893(2)
1.902(2)
1.974(2)
1.988(2)
92.92(8)
96.02(9)
94.62(9)
92.81(9)
152.64(8)
144.89(9)
1.914(2)
1.991(2)
2.000(1)
95.43(6)
119.54(6)
125.80(7)
95.24(6)
108.35(6)
114.39(6)
were done in situ with the corresponding metal halides to yield
the Mn, 4, Fe, 5, Co, 6, Ni, 7, and Cu, 8, complexes in good to excel-
lent yields, Eq. (1). In the case of nickel, the insolubility of anhy-
drous NiCl2 in THF did not lead to isolation of 7 and only 3 was
recovered from this mixture (see Supporting Information). No fur-
ther characterization was done on 3. Instead, (dppe)NiCl2 was used
which provided a soluble starting material in THF. The Zn complex,
9, was made directly from 1 and ZnEt2, Eq. (1). Each complex is sol-
uble in THF, CH3CN, and arene solvents and only sparingly soluble
in aliphatic solvents.
3. Results and discussion
3.1. Synthesis
t
The salicylaldimine ligands, HOC6H2 Bu2C(H)@N(C6H3Me2), 1,
t
and HOC6H2 Bu2C(H)@N(C6H3Me2), 2, were synthesized by con-
densation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde and 2,6-
dimethylaniline in refluxing ethanol for 12 h with no addition of
formaldehyde as previously reported [12]. Reaction of 1 with
NaH generates the sodium salt, 3. The ionic metathesis reactions
ð1Þ

A. Mrutu et al. / Polyhedron 54 (2013) 300–308
303
Fig. 3. Molecular structure of 6 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
Fig. 1. Molecular structure of 4 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
Fig. 4. Molecular structure of 7 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
O1 and O2, from the phenolate group in such a way that the
phenolate oxygen atoms are cis to one another; so is the case
for the imine nitrogen atoms. Complexes 4, 5, 6, and 9 are iso-
morphous having nearly identical unit cell parameters with
one co-crystallized toluene molecule, while complex 7 co-crys-
tallized with one THF molecule, and 8 with two distorted tolu-
ene molecules.
Fig. 2. Molecular structure of 5 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
The Mn2+ derivative, 4, shows a distorted tetrahedral geometry
about the metal center, Fig. 1. With Mn–O bond distances of
1.968(2) and 1.971(1) Å, these are shorter than other Mn2+ Schiff
base complexes which are between 2.110(4) and 2.173(4) Å [24]
and have Mn–N dative bonds 2.269(5)–2.380(4) Å while 4 has
Mn–N bonds of 2.107(2) and 2.115(1) Å.
Complex 5 crystallizes in a monoclinic crystal system, space
group P21/c. While Fe2+ Schiff base complexes are known [23,24],
this appears to be the first monometallic complex with a salicyl-
aldiminate ligand. Complex 5 has a distorted tetrahedral geometry,
3.2. Molecular structure
The solid-state molecular structures of complexes 4–9 were
determined using single crystal X-ray diffraction. The solid struc-
tures for 4–9 are shown in Figs. 1–6, respectively, and have sim-
ilar solid state structures. The metal center of each complex is
chelated by two salicylaldiminate ligands in a bidentate (j2
)
fashion. The ligands are coordinated to the metal through nitro-
gen atoms, N1 and N2, from the imine group and oxygen atoms,

304
A. Mrutu et al. / Polyhedron 54 (2013) 300–308
Single crystal analysis of 7 indicates that the Ni2+ is in a dis-
torted tetrahedral geometry (Fig. 4). Complex 7 crystallizes in
orthorhombic space group Pbcn. The two ligands are arranged
in
a twisted fashion around the metal with O1–Ni1–O1 of
108.26(8)° and N1–Ni1–N1 of 109.16(8)°. The Ni1–O1 and Ni1–
N1 bond lengths are 1.896(2) Å and 1.970(2) Å, respectively, are
shorter than those reported by Jarjayes, Thomas, and co-workers
[9j] by 0.125 Å (Ni–O) and 0.153 Å (Ni–N) for a high spin octahe-
dral nickel complex with two tridentate salicylaldiminate ligands.
In addition, the bond distances in 7 are shorter compared to
t
Ni[OC6H2 Bu2C(H)@N(CH2NMe2)]2, another pseudo-octahedral
complex coordinating two tridentate ligands, in which Ni–O bond
distances of 2.0185(16) and 2.0252(14) Å and Ni–N bond dis-
tances of 1.9958(17) and 2.0043(17) Å were reported [27]. How-
ever, when comparing the bond lengths of 1.905(2)° for Ni–O
and 1.924(2)° for Ni–N in [(iPr2H3C6)N@C(H)C6H2 Bu2-
t
O]NiPh(NHC), NHC = 1,3-diisoproylimidazole [9i], the bond dis-
tances are similar.
Fig. 5 shows complex 8 that crystallizes in monoclinic space
group I2/a and is best described as a distorted square planar as evi-
denced by the O1–Cu1–O1 bond angle of 90.33(8)° and 102.05(9)°
angle for N1–Cu1–N1. The Cu1–O1 and Cu1–N1 bond length of
1.902(2) Å and 1.966(2) Å, respectively, are geometrically similar
to a phenolate–pyrazole Cu2+ complex recently reported [28] with
Cu–O and Cu–N bond lengths of 1.9233(14) Å and 1.9092(19) Å,
respectively. The shortening of the bond distances in the pheno-
late-pyrazole Cu2+ complex is most likely due to intramolecular
hydrogen-bonding.
Fig. 5. Molecular structure of 8 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
The geometry around Zn2+ metal center is also distorted tetra-
hedral with N1–Zn1–N2 and O1–Zn1–O2 bond angles of
114.39(6)° and 108.35(6)°, respectively. Single crystal analysis re-
vealed that 9 (Fig. 6) crystallizes in monoclinic space group P21/c.
t
i
The 2,6-diisopropyl derivative, Zn[OC6H2 Bu2C(H)@N(C6H3 Pr2)]2,
11, has been reported [9d]. Instead of 9 having a pseudo-square
planar conformation as observed in 11, it is distorted tetrahedral
like complexes 4–8, Fig. 7. The 1.914(2) and 1.918(1) Å Zn–O bond
distances in 9 are slightly shorter than those in 11 of 1.9413(22)
and 1.9373(22) Å. This is also the case of the Zn–N bond distances
which are approximately 0.03 Å shorter in 9 than 11.
Following the tetrahedral versus square planar moieties ob-
served between ligands 1 and 2 with Zn2+, we presumed that
Cu2+, with a smaller ionic radius, would produce a similar result.
t
i-
The X-ray crystallography analysis of Cu[OC6H2 Bu2C(H)@N(C6H3
Pr2)]2, 10, confirmed the pseudo-square planar conformation,
Fig. 8. Comparing 8 and 10, the Cu–N bond distances are similar,
elongating slightly in 10: 1.966(2) Å in 8, and 1.974(2) and
1.988(2) Å in 10, while the Cu–O bonds are statistically equivalent:
1.902(2) Å in 8, and 1.893(2) and 1.902(2) Å in 10. The change in
orientation between 8 and 10, as well as 9 and 11, is due to the iso-
propyl group imposing steric demand on the complex and this is
relieved with the pseudo square planar conformation. It should
also be noted that similar Ni2+ and Cu2+ complexes with isopropyl
groups [29], but without tert-butyl groups on the phenoxide also
showed a distorted square planar orientation around the metal
center [30], indicating that the isopropyl substituents may
necessitate this conformation.
Fig. 6. Molecular structure of 9 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
Fig. 2, with O1–Fe1–O2 of 112.64(5)° and N1–Fe1–N2 of
113.78(5)°. Bond lengths, Fe1–N1 and Fe1–N2 are 2.036(1) Å and
2.030(2) Å, respectively while the Fe1–O1 and Fe1–O2 distances
are 1.907(1) Å and 1.910(1) Å, respectively. These bond lengths
are slightly shorter than those reported on Fe2+ complexes with
N4O dinuclear Schiff base ligands [25,26].
Complex 6, Fig. 3, crystallizes in monoclinic space group P21/
c. The coordination environment of the cobalt ion is best de-
scribed as distorted tetrahedral with N1–Co1–N2 of 114.19(6)°
and O1–Co1–O2 of 110.41(6)°. The bond lengths, Co1–N1 and
Co1–N2 are 1.984(2) Å and 1.986(1) Å, respectively. Co1–O1
and Co1–O2 bond distances are 1.897(1) Å and 1.898(2) Å,
respectively.
Based on the crystal structure data the deviation of each com-
plex from an ideal tetrahedral geometry can be calculated using
the four-coordinate geometry index, s4 [31]. This index is de-
360ꢁꢀð
a
scribed by the equation: s4
¼
þbÞ, where
a and b represent
141ꢁ
the two largest bond angles observed in the crystal structure. Per-
fectly tetrahedral complexes afford s4 of 1.00 while square planar
will give zero. As shown in Table 3, complexes 4, 5, 6, 7, and 9
can all be described as pseudo- or distorted tetrahedral while 8
and 10, the Cu2+ compounds, are best described as distorted square
planar.

A. Mrutu et al. / Polyhedron 54 (2013) 300–308
305
Fig. 7. Structural difference between the 2,6-dimethylphenyl, 9, and 2,6-diisopropylphenyl, 11, derivatives is depicted.
3.4. Spectroscopy
The IR spectra of complexes 4–9 show a strong and sharp band
assigned as the C@N stretch at 1613, 1611, 1613, 1612, 1615 and
1612 cmꢀ1, respectively. These are red shifted from the free ligand,
which has a stretching frequency of 1625 cmꢀ1, demonstrating the
weakening of the imine bond with electron density being donated
to the metal. The absorptions observed for 4–9 are consistent with
the analogous aluminum salicylaldiminate metal complex
(1613 cmꢀ1) [12].
1H NMR spectra obtained in C6D6 for complexes 4–8 suggest
these complexes are paramagnetic as evidenced by broad 1H
NMR signals, however two resonances were found for each –
C(CH3)3 group and one for the –CH3 groups. Each singlet is lo-
cated near the resonances observed for diamagnetic 9 and this
is probably due to the distance of the groups from the paramag-
netic metal center. For 9, the 1H NMR spectrum reveals singlet
resonances at 1.81 and 1.40 ppm consistent with –C(CH3)3
groups. In addition, the resonances at 1.18 and 2.46 ppm can be
assigned to –CH3 groups while the singlet observed at 7.83 ppm
can be assigned to proton attached to imine carbon. The 13C
NMR spectrum showed resonances at 29.5, 31.3, 33.8, and
35.7 ppm which can be assigned to the –C(CH3)3 groups. 13C
NMR also revealed a resonance at 174.9 ppm that is characteristic
of an imine carbon.
Fig. 8. Molecular structure of 10 with thermal ellipsoids projected at 50% level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
Table 3
Four-coordinate geometry index, s4, calculated for 4–10.
4
5
6
7
8
9
10
a
b
133.03(6) 129.23(6) 124.83(7) 129.26(8) 145.36(9) 125.80(7) 152.64(9)
121.28(6) 121.93(5) 119.85(6) 129.26(8) 145.36(9) 119.54(6) 144.89(9)
The UV–Vis electronic spectra of complexes 4–9 depicted in
Fig. 10 were recorded in THF solution. The electronic spectra show
intense UV absorption bands at 338, 353, 385, 390, 407 and
401 nm for complexes 4–9, respectively. Due to the similarity in
absorption bands, especially with respect to the zinc complex, 9,
we attribute these to transitions within the ligand. Another set of
transitions were observed between 500 and 540 nm (Fig. 10 inset)
with small molar absorptivity and we attribute these to forbidden
d ? d transitions. However, since the paramagnetic complexes, i.e.
5–8, are all red in color (4 is orange), this would suggest that the
transitions located in the 500–540 nm region have some ligand
character as well.
CW-EPR spectra was obtained for complex 4 in THF at 4 K which
showed the typical six-line pattern for a S = 5/2 ground state of tet-
rahedral Mn2+, Fig. 11. Hyperfine splitting was observed with 14N.
CW-EPR spectra were also obtained for the two Cu2+ complexes, 8
[34] and 10, Fig. 12, in THF at 77 K. Each spectrum exhibits a char-
acteristic Cu2+ axial signal for an S = 1/2 ground state. With an iso-
tropic superhyperfine coupling constant, A||, of 155 G and g|| value
of 2.24 and 2.25, for 8 and 10, respectively, this is typical of a dis-
torted square planar geometry [35]. Based on the parameters ob-
tained from the EPR spectra we can calculate the covalency
parameter, a2 [36], using the equation:
s4
0.75
0.77
0.82
0.72
0.49
0.81
0.44
3.3. Non-covalent interactions
It must be noted in bis(Schiff base) divalent transition metal
complexes, the geometry adopted around the metal center is nor-
mally square planar with the two 3,5-di-tert-butylphenoxide
groups on opposite sides to each other. For example, the 2,6-
dimethylphenyl derivative has been made without tert-butyl
groups and its Cu2+ and Ni2+ complexes have a square planar
arrangement [30]. In addition, removing the methyl groups and
leaving the phenyl off the imine also affords square planar
geometries [32]. The reason for these structural inconsistencies
are reasoned that in complexes 4–9, the proximity of the phenyl
groups stemming from the nitrogen atoms of each ligand may pro-
vide stabilization through non-covalent interactions. Indeed, the
closest carbon–carbon distances found are 3.583, 3.528, 3.437,
3.403, and 3.577 Å for 4–9, respectively, and correlate to the ionic
radius of the metal ion. These lengths are within the range found in
complexes involving
ser inspection reveals van der Waals interaction between a methyl
proton, H(15B), and the -system, C(8)–C(13), of the other phenyl
p–p stacking interactions [33], however clo-
p
ring, Fig. 9, at distances of approximately 2.640 Å. These interac-
tions could stabilize the tetrahedral arrangement in each structure.
a2 ¼ ꢀðA =0:036Þ þ ðg ꢀ 2:0023Þ þ 3=7ðg? ꢀ 2:0023Þ þ 0:04
k
k

306
A. Mrutu et al. / Polyhedron 54 (2013) 300–308
Fig. 11. X-band EPR spectrum of 4 in THF at 4 K is shown. Asteric (ꢂ) indicate
spectral features derived solely from 14N hyperfine splittings.
the IR spectra that indicate altered C@N bonding in the metal com-
plexed ligands [9b,36,37]. While 8 shows obvious hyperfine inter-
actions in the g? transition consistent with two inequivalent or
trans-nitrogen ligands, only a subtle feature is seen in 10 where
these nitrogen ligands should be roughly equivalent. However,
we cannot rule out that the differences in the hyperfine interaction
could also be the result of interactions of the Cu2+ z-molecular axis
with the differing functional groups of each ligand type.
Fig. 9. Thermal ellipsoid plot of complex 7 shown at the 50% probability level
showing H(15B) interaction with opposing phenyl ring, C(8)–C(13).
3.5. Density functional calculations
One measure of electron density for paramagnetic complexes
can be derived from the Mulliken atomic spin densities. This is a
more reliable indication of the localization of electron density than
Mulliken atomic charges and population analysis since it is basis
set independent. The spin density should equate to the total
Substitution provides
ence,
2 = 0.50 suggests complete covalent bonding while 1.0 indi-
a
2 = 0.75 for 8 and 0.77 for 10. For refer-
a
cates completely ionic character. Our values are similar to other
reported Cu2+ salicylaldiminate complexes and is consistent with
Fig. 10. UV-Vis spectra of salicylaldimine ligand, 1, and complexes 4–9 are shown.

A. Mrutu et al. / Polyhedron 54 (2013) 300–308
307
Fig. 12. X-band EPR spectrum of 8 and 10 in THF at 77 K are shown. Experimental spectrum is in black and simulated spectra in dashed grey. Spectrometer settings:
modulation amplitude, 10 G, power was attenuated to 5 mW For 8 and 10 mW for 10.
seen in the Cu2+ complexes, 8 and 10. We believe the tetrahedral
Table 4
Mulliken atomic spin densities of the metal, oxygen, and nitrogen atoms in the
paramagnetic complexes, 4–8, are shown.
conformation is stabilized by van der Waals interactions between
methyl C–H and -system of the neighboring phenyl ring. This
p
demonstrates the ability for chelating ligand to adopt unusual
Mulliken atomic spin density
4
5
6
7
8
binding modes with the assistance of non-covalent interactions.
Metal
4.793
3.747
2.704
1.636
0.603
O (per atom)
N (per atom)
<S2> = S(S + 1)
S
0.0464 0.0868 0.0848 0.0930 0.0977
0.0013 0.0390 0.0483 0.0765 0.0766
8.7573 6.0264 3.7614 2.0085 0.7543
2.5000 2.0053 1.5028 1.0028 0.5021
Acknowledgments
We gratefully acknowledge financial support from the Nuclear
Forensics Education Award Program for startup funds and the Uni-
versity of Missouri College of Arts & Sciences Alumni Association
Faculty Incentive Award. A.C.L. thanks the Howard Hughes Fellow-
ship Program at the University of Missouri.
number of unpaired electrons, on the metal center in this case, and
should be an integer value. For example, a tetrahedral Fe2+ complex
should have four unpaired electrons and therefore a spin density of
4.000. Shown in Table 4 are spin densities for each of the paramag-
netic complexes, 4–8, and Fe2+ has a spin density of 3.747. As one
moves across the series, there is a clear trend of increasing devia-
tion from the integer value, attributed to more electron density
being removed from the metal center and donated to the ligand,
which is observed in the increase in spin density on the oxygen
and nitrogen atoms which should be zero. This is credited to the
increasingly electron-rich metal centers donating to the ligand,
causing the corresponding increases in ligand atom spin densities.
This equates to more covalent bonding from manganese to copper.
The amount of Cu–L bonding was estimated using the covalency
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.02.011.
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