Oxovanadium(IV), nickel(II) and palladium(II) complexes of tridentate salicylaldiminates derived from 2,4-di-tert-butyl-6-aminophenol

Article abstract of DOI:10.1515/znb-2001-0308

Several new binuclear nickel(II), oxovanadium(IV) and palladium(II) complexes (ML^x) of tridentate salicylaldimines (L^xH₂) obtained from substituted salicylaldehydes and 2,4-di-tert-butyl-6-aminophenol, as well as mononucle

Full text of DOI:10.1515/znb-2001-0308

Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes of Tridentate
Salicylaldiminates Derived from 2,4-Di-terMmtyl-6-aminophenol
Veli T. Kasumov
Department of Chemistry, Faculty of Arts and Sciences, Harran University, Sanhurfa, Turkey
Reprint requests to Dr. V. T. Kasumov. E-mail: vkasumov@harran.edu.tr
Z. Naturforsch. 56 b, 263-270 (2001); received July 27, 2000
Tridentate Salicylaldiminates, ESR, Catalytic Activity
Several new binuclear nickel(II), oxovanadium(IV) and palladium(II) complexes (MLX) of
tridentate salicylaldimines (LXH2) obtained from substituted salicylaldehydes and 2,4-di-tert-
butyl-6-aminophenol, as well as mononuclear nickel(II) and oxovanadium(IV), MQx nH20,
complexes with hydrogenated LXH2 ligands (QXH2) have been prepared and their spectroscopic,
magnetic and catalytic activity (for PdLx) have been investigated. NiLx and VOLx complexes
when dissolved in pyridine, take up three or two pyridine molecules to form six coordinated
complexes, respectively. The Ni(II) and VO(IV) complexes obtained from QXH2 can be formu-
lated as NiQx-3H20 and VOQx H2O. All VO(IV) complexes have been characterized by ESR
parameters. It has been found that some PdLx complexes without any preliminary activation,
in EtOH, THF and DMF exhibit high catalytic activity in the hydrogenation of nitrobenzene or
cyclohexene at room temperature and at 30 - 40 °C under 760 torr H2. When this reaction has
been carried out in the ESR cavity at room temperature the triplet of 12 line multiplets due to
the nitroxyl type coordinated radical (g = 2.013, AN= 10 G, A = 0.8 G) was detected.
Introduction
In recent years there has been considerable inter-
est in the chemistry of transition metal complexes of
tridentate dibasic salicylaldimines [1 - 3]. These lig-
ands force the metal ions into close proximity lead-
ing to metal complexes with unusual magnetic and
structural properties. Particular interest has been fo-
cused towards the magnetic coupling phenomena
and the catalytic activities in biological model dinu-
clear complexes [4]. The palladium complexes were
found to be effective catalysts for promoting spe-
cific carbon-carbon, carbon-oxygen, carbon-nitro-
gen and nitrogen-hydrogen bond formation under
mild conditions [7].
M=Ni(II), VO(IV), Pd(II)
The coordination chemistry of some transition
metals with potentially redox-active chelating lig-
ands such as bi-, tri- and tetradentate salicy-
laldimines, naphthaldimines, azo ligands, /3-keto-
amines and other ligands containing sterically hin-
dered phenol fragments have been studied by us
[8 - 10]. In these studies complexes along with un-
expected structures, coordinated radical ligand in-
termediates, as well as high redox-reactivity of
Cu(II) and Pd(II) salicylaldiminates towards PbC>2
and triarylphosphines have been found.
LxH2: X= H (L'Hj); 5-Br (L2H2); 3,5-di-Br (L3H2); 5-N02(L4H2); 3-N02-5-Br (L5H2)
QxH2: X= H (Q‘H2); 5-Br (Q2H2); 3,5-di-Br (Q3H2)
Scheme 1.
The present work is part of a research plan to
study the syntheses of transition metal complexes
with redox-active ligands containing sterically hin-
dered phenol groups and their structure, spectro-
scopic behavior, redox-reactivity, and catalytic ac-
tivity in various reactions. We prepared and charac-
terized a series of binuclear metal complexes, MLX
K
0932-0776/01/0300-0263 $ 06.00 © 2001 Verlag der Zeitschrift für Naturforschung, Tübingen ■www.znaturforsch.com
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Table 1. Analytical data for the QXH ligands and the MLXand MQx nH20 complexes.
Elemental analyses, Found/Calcd [%]
V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
Compound
Yield
M.p. °C
Stoichiometry
C
H
N
NiL1
NiL2
NiL3
NiL4
NiL5
VOL1
VOL2
VOL3
VOL4
VOL5
PdL1
PdL2
PdL4
PdL5
QlH2
Q H 2
Q3H2
NQ'-3H20
NiQ2-3H20
NiQ3-3H20
v o q ' h 2o
VOQ2 H20
VOQ3 H20
65.54/65.84
54.91/54.46
49.51/49.63
46.78/46.60
49.24/49.83
58.88/58.68
49.06/48.96
70.12/71.49
42.85/42.75
45.95/45.46
58.76/58.27
63.78/63.40
50.34/49.36
52.34/52.77
77.14/77.01
62.15/62.03
51.83/51.93
57.34/57.54
48.68/48.75
42.43/42.28
58.73/61.43
51.43/51.52
44.58/44.35
92
93
96
94
95
98
96
76
94
95
75
69
74
76
87
88
82
74
71
73
76
68
72
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
113
6.91/6.79
5.68/5.40
4.81/4.72
5.34/4.25
4.24/4.54
5.49/5.82
4.24/4.71
6.46/7.09
3.80/3.90
4.44/4.14
6.21/5.54
5.56/6.04
4.11/4.51
4.24/4.81
8.66/8.93
6.74/6.94
5.53/5.61
7.42/7.59
6.43/6.23
5.27/5.24
6.59/7.13
5.65/5.76
4.82/4.79
C42H5oN204Ni2
C42H48N2Br204Ni2
C42H4öN2Br404Ni2
C42H48N408Ni2
C42H48N4Br20sNi2
C42H50N2O5V2
C42H48N2Br205V2
C42H46N2Br40 5V2
C42H48N4O9V2
C42H46N4Br209V2
C42H5oN204Pd2
C42H48N2BT204 Pd2
C42H48N408Pd2
C42H46N4Br208 Pd2
C21H29N02
C2iH28NBr02
C2iH27NBr202
C2iH33N05Ni
C21H32NBrOsNi
C2iH3iNBr205Ni
C21H29NO4V
C2iH28NBr04V
C21H27NBT204V
3.82/3.65
3.23/3.03
5.61/5.51
2.96/2.58
5.75/5.53
3.44/3.20
2.84/2.74
3.32/3.97
2.57/2.37
5.83/5.51
3.76/3.23
7.56/7.04
2.65/2.74
5.34/5.86
4.22/4.28
3.32/3.45
3.02/2.88
3.24/3.19
2.64/2.71
2.12/2.35
3.28/3.41
2.64/2.86
2.57/2.46
124
132
>250
>250
>250
>250
>250
>250
(M = VO, Ni, Pd), where Lx represents the dianions
of tridentate N-(3,5-di-ter?-butyl-2-hydroxyphnyl)
salicylaldimines, (formed by the condensation of
substituted salicylaldehydes and 2,4-di-terf-butyl-
6 -aminophenol) and their hydrogenated MQX«H20
analogues, with QXH2 obtained by the hydrogena-
tion of the appropriate LXH using NaBH4 as pre-
sented in Scheme 1. Binuclear CuLx complexes
have previously been reported [9b], while our at-
tempts to prepare the similar binuclear complexes
of Co(II) with LXH2 ligands were not successful.
external standard. The ESR parameters of the complexes
were determined directly from the spectra. The magnetic
field was calibrated using a standard Mn+2 sample. Re-
ported values involve errors within ± 0.002 for g-values
and ±0.05 G for A-values of VOLx. The errors for g-
factor and hyperfine splitting constants (hfsc) (APd and
AH) are ±0.0005 and ±0.005 G, respectively.
Preparation of the compounds
Salicylaldehyde, all solvents and metal acetates were
commercial products (Merck) and used without fur-
ther purification. N-(3,5-di-ter/-butyl-2-hydroxyphenyl)-
salicylaldimine (LXH2) and MLX(M = Ni(II), VO(IV),
Pd(II)) compounds were synthesized as previously de-
scribed [9, 11(a), (b)] by mixing equivalent amounts of
ligands and metal acetates in a 1:1 ligand to metal mo-
lar ratio. The substituted salicylaldehydes were prepared
according to the literature [11(c)], using commercially
available reagents.
Experimental
The IR spectra were obtained on a Carl Zeiss Jena
Specord M 80 spectrophotometer in the range of 4000 -
400 cm- 1on KBr discs. Electronic spectra were recorded
on a Carl Zeiss Jena Specord M 40 spectrophotometer in
the range of 200 - 900 nm. Magnetic susceptibilities were
measured by the Faraday technique, from liquid nitrogen
temperature to room temperature, and diamagnetic cor-
rections were made using Pascal’s constants. The appara-
tus was calibrated by the use of Hg[Co(NCS)4]. X-Band
ESR spectra were recorded in solid state and in solution
For comparative purposes, the nickel(II) and oxovana-
dium(IV) complexes (MQx nH2 0 ) with hydrogenated
LXH (abbreviated as QXH) were prepared according
to [10(c)],
N-(3,5-di-terf-butyl-2-hydroxyphenyl)-2’-hydroxy-
at 300 and 77 K on a Radiopan model SE/X-2547 X- phenylbenzylamines (QXH) were prepared from LXH (x =
Band spectrometer. DPPH (g = 2.0036) was used as an
1, 2, 3) by the following procedure. NaBH4 (0.05 mol)
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V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
265
Table 2. Magnetic and spectroscopic characteristics of MLX, MQxnH2Ü and QxHi compounds.
Compounds
— IR spectra [cm !] —
Solvent
Electronic spectra Amax [nm]
e (L mol-1cm-1)
/^eff
B.M.
^C=N
1600
Z'N-H
i'M-N
^M-O
570
NiL1
NiL2
dia
550
THF
Py
EtOH
Py
EtOH
Py
Toluene
Py
Toluene
Py
300, 340, 370, 480
-
350, 440, 550 (8), 870 (6)
220, 240, 270, 330, 360, 480
355, 460, 560 (7.5), 860 (9.8)
220, 263, 270, 333, 365, 580
360, 470, 560 (6.9), 875 (9.5)
335, 352, 406,510, 625,848
486, 623, 875
335,354, 408,514, 628, 836
490, 624, 885
343, 358,412,514, 632, 828
490, 628, 852
3.42
3.06
1.57
1.59
1.63
1.58
1604
1620
1608
1604
1610
1605
480
470
490
520
535
530
580
580
580
565
585
580
—
—
—
—
—
—
NiL4
VOL1
VOL2
VOL3
VOL5
Toluene
Py
Toluene
338, 356,410,516, 630, 838
492, 625, 880
Py
-
-
-
-
PdL1
PdL2
PdL3
PdL4
Q'H2
q 2h 2
Q H2
NiQ'-3H20
NiQ2-3H20
NiQ3 3H20
VOQ1H20
VOQ2 H20
VOQ3 H20
Dia
1600
1604
1605
1600
-
-
-
-
-
-
-
-
440
430
415
420
-
520
530
525
540
-
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
CHC13
CHC13
CHC13
c h c i3
c h c i3
c h c i3
255,300,314,415,440
260,310, 360, 435
240, 335,360,410, 450
260, 300, 335, 410, 450
210, 240, 280, 434
212, 238,284, 430
205, 232, 266, 434
285, 342, 390, 660, 859
274, 325, 385, 650, 868
265,315, 378,640, 840
264, 289, 372, 464, 650
258, 295, 367, 465, 664
268, 309, 382, 490, 658
-
-
-
3345
3323
3315
3250
3230
3235
3240
3245
3238
-
-
-
-
2.95
3.21
3.26
1.69
1.72
1.73
543
534
556
465
456
468
586
572
588
535
545
532
-
was added slowly (small portions of about 10-15 mg) and / or DMF. Nitrobenzene (1 - 4 1 0 -4 mol) in 15 ml
to 0.01 mol stirred solutions of LXH in 40 - 50 ml of
isopropanol for 15 - 20 min. Stirring was continued for
another 30 - 40 min and the cooled mixture was then
poured into 150 - 200 ml water. After cooling to room
temperature the solution was stirred for 1 h and allowed
to stand for 3 - 4 h. The precipitated white crystals were
collected by filtration and then washed with water, dried
under vacuum and recrystallized from a hexane-acetone
mixture, yield 85 - 95%. Analysis for Q'H: M. p. 113 °C.
- UV/vis (C2H5OH): Amax = 210, 240, 280, 450 nm. -
IR (pellet): v = 3345 cm“ 1 (N-H, O-H). - *H NMR
(200 MHz, CDCI3): 6 = 1.31(s, 9 H, C(CH3)3) 1.43 (s, 9
solvent was transferred into a vessel, while 2 - 6T0~6mol
PdLx was added through the side bend. Then the reaction
mixture was deoxygenated and hydrogen gas was pumped
into the vessel. After stirring for 15 - 20 min, an excess
of H2 was added and the volume of absorbed H2 was
measured periodically.
Results and Discussion
The analytical data of the MLXcomplexes indi-
cate 1 : 1 metal-ligand ratio stoichiometry, and hence
the ligands (LXH2) act as tridentate dibasic ligands
(Table 1). As confirmed by the ESR examination the
samples of MLXand MQx nH20 except the Co(II)
compounds did not exhibit evidence for the forma-
tion of radical species during complexation.
H,
C(CH3)3), 3.35 (s, 2 H, CH2), 4.38 (s, 1 H OH), 6.72,
6.93, 7.03, 7.28 (4xd, J = 2.4 Hz, 4 H, Ar-H), 6.75, 6.85
(2 x t,J = 12,1 Hz, 2 H, Ar-H), very broad centered at 8.5.
- C21H29NO2, calcd. C 77.01, H 8.93, N 4.28; found C
77.14, H 8.66, N 4.22.
The stretching vibrations z/(CH=N) in the IR
spectra of MLX were detected in the 1604 -
1612 cm- 1 region and shifted to lower frequen-
cies relative to free LXH2 (1620 - 1630 cm-1) in-
The m. p’s of all complexes exceed 250 °C. Elemental
analyses, melting points and yields are given in Table 1.
Hydrogenation of nitrobenzene was carried out in a
thermostatic glass vessel (25 cm3) at 30 - 40 °C under
760 torr H2 and with vigorous stirring in EtOH, THF dicating that coordination has taken place through
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V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
266
the azomethine nitrogen atom of the ligands. The
NiLx-3Py adduct type octahedral complexes. The
stretching frequency of the phenolic OH of LXH2 bands observed in the regions of 440 - 470, 550 -
appeared at 3520 - 3530 cm- 1 and disappeared 560 and 860 - 875 nm in pyridine are assigned to
in the IR spectra of MLX and MQX (Table 2).
spin-allowed transitions 3A2g(F) —>3T2g(P), 3A2g(F>
The v(V = 0) bands of the VOLx complexes oc- —> 3Tig(F) and 3A2g —> 3T2g(F), respectively, for
octahedral Ni(II) complexes [12, 15].
It is of interest to compare the complexation abil-
ity of tridentate LXH2 with their hydrogenated po-
cur at 910 - 960 cm- 1 which is in the usual
range (960±50 cm-1) observed for the majority
of VO(IV) complexes [13]. New bands, which are
absent in the spectra of all the free ligands, were tentially tridentate QXH2 analogs (Scheme 1). The
observed in the 600 - 400 cm- 1 region, and were analytical and spectral data of QXH2 and their Ni(II)
assigned to z^(M-O) and z^(M-N) (Table 2) [14]. chelates (NiQx-3 H2 0 ) are presented in Tables 1
In the IR spectra of MQxn(H2 0 ) compounds ob- and 2. The results of elemental analyses and the
tained from hydrogenated tridentate benzylamines, IR and electronic spectral data of Ni(II) complexes
the i/(OH) and i'(NH) stretching modes appear as with QXH2 indicate to the formation of monomeric
overlapping broad absorptions centered in the 3238 octahedral NiQx 3 H2O complexes. The room tem-
perature magnetic moments and electronic spectra
in chloroform are typical for octahedral Ni(II) com-
plexes and differ from those of NiLx in CHCI3 and
EtOH. These complexes exhibit three low intensity
bands in the 375 - 870 nm range which are attributed
to d-d transitions in octahedral field.
- 3250 cm- 1 range.
Complexes of Ni(II)
The NiL1 complex is diamagnetic in agreement
with a square planar geometry for the metal atom
in the dimer. However, the 5-substituted NiL2 and
NiL4 complexes show magnetic moments in the
3.06 - 3.42 B.M. range, which supports octahedral
structures [15]. Note that the magnetic moments
Complexes of VO(IV)
The oxovanadium(IV) complexes belong to the
of these compounds remain essentially unchanged S = 1/2 system and it is expected that the mag-
netic moments of the magnetically dilute complexes
would be very close to the spin-only magnetic mo-
ment, as the spin-orbit coupling constant is positive
and the orbital contribution is almost completely
quenched [13]. The room temperature magnetic mo-
over the temperature range 80 - 300 K. It is known
that in Oh symmetry Ni(II) complexes have a low
orbital contribution (//eff = 2.83 - 3.4 B.M.), while
those having a Td symmetry are subject to a much
higher orbital contribution (//eff = 3.5 - 4.2 B.M.),
due to the presence of orbitally degenerate ground ments of VOLx (1.57 - 1.63 B.M.) are slightly lower
states [13]. Consequently, considering the small or- than the expected spin-only moment 1.73 B.M for
the d1 system [13], showing a weak antiferromag-
netic exchange interaction between the neighboring
VO(IV) ions. The magnetic moments of the com-
plexes remained practically unchanged in the range
bital contribution to the effective magnetic moments
and the temperature independent behavior of /ueff,
we suggest a pseudo-octahedral structure for the
metal environment in the solid state. This configu-
ration can be reached by axial Ni-O-Ni bridges of 80 - 300 K and obey the Curie-Weiss law with Weiss
planar dimer moieties.
constants 9 in the range of -1.2 to -0.9 K.
Interestingly the ß eff values of these complexes as
estimated from ESR spectra by using the equation:
Peff = 1/2 (g2 + 2 g^ ) 1/2 [16] are in the range of
1.71 - 1.74 B.M. They agree well with spin-only
values expected for magnetically diluted VO(IV)
The electronic spectra of the NiLx complexes re-
veal very similar spectral patterns in THF or EtOH
solution with a single ligand field band at 480 for
NiL1 and NiL2, and at 560 nm for NiL4 characteris-
tic of diamagnetic square-planar Ni(II) complexes
(Table 2). These bands may be assigned to *Aig complexes.
—* 1A2g and !Aig —> 1Big transitions [1 2 ], respec-
tively. Three new d-d bands appear in the 500 -
The electronic spectra of VOLx complexes in
chloroform (Table 2) exhibit three absorption bands
900 nm range with molar extinction coefficient in at 828 - 848 (I), 625 - 632 (II), and 510 - 516
(III) nm, which are assigned to the dxy —> dXZ)J/2,
the range 6 -1 0 M- 1cm- 1 generated on dissolving
of NiLx in pyridine (Table 2) probably caused by dxy —►dx2_ y2, djy —» d.2 transitions, respectively,
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267
and
V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
Table 3. ESR parameters of the VOLx
VOQx H20 complexes.
Compound Solvent
A±
Aj$o
giso
A||
g-L
Sn
VOL1
Toluene 1.998 1.941 2.027 99.4 182.2 57.9
1.957 1.999 75.2 45.2
1.985
Toluene 1.996 1.985
135.1
CHCI3
VOL2
VOL3
2.002 88.4
172.6 46.3
1.987
Toluene
1.956 2.003 79.2 121.5 58.1
1.994
1.983 1.965
69.6 140.9 31.8
CHCb
VOL5
Toluene 2.003 1.948 2.031 99.6 185.2 56.8
CHCI3 1.992 1.971 2.003 105.7 184.9 66.1
v o q 1 h 2o CHCI3 1.959 1.942 1.967 105.7 187.7 64.7
1.964 1.985 104.8 57.1
V0Q 2 H20 CHCI3 1.978
190.3
v o q 3 h 2o CHCI3 1.995 1.975 1.975 105.1 191.2 62.1
according to the Ballhausen - Gray energy level
scheme [17]. Note that although this sequence of en-
ergy levels was proposed by Ballhausen and Gray
in 1962, the same sequence of energy levels for
VO(IV) complexes was referenced by A.Syamal
and co-workers, in all of their paper, to Vanquick-
enbome and McGlynn [18]. A comparison of the
spectra recorded in pyridine with those obtained
in toluene indicates that upon going from non-co-
ordinating toluene to strongly coordinating pyri-
dine, the band I shifts to lower energy and band
III to higher energy. A minor blue shift was de-
tected also in the positions of band II upon going
from toluene to pyridine (Table 2). Thus, our inves-
tigation indicates that the 2B2 (dxy) —►2E(dX2, dyz)
(I) and 2B2(dxy) —>2Aj (dz2) (III) transitions shift
to lower and higher energies, respectively, upon go-
ing from non-coordinating toluene to coordinating
pyridine solvent. This fact seems to indicate that
the oxovanadium(IV) complexes, when dissolved in
pyridine, combine with two molecules of pyridine to
form six-coordinated adducts. The electronic spec-
tral changes on adduct formation of VOLx with pyri-
dine can be interpreted according to the Ballhausen-
Gray scheme. The axial and equatorial coordination
of pyridine molecules leads to the decrease of Exy
—> Exz,yz and increase Exy —» Ez2 energy separa-
tions, respectively. Since the energy of the Exy —*■
Exz,yz transition decreases, the increased axial 7r-
bonding will stabilize the dxz>yz pair more than the
in-plane 7r-bonding of dxy.
The ESR spectrum of VOL1 in polycrystalline
state exhibits a single broad line at gav= 1.99 (300 K)
and a low intensity resonance at g = 4.1 (77 K) due
to the A Ms = ± 2 transition. For other VOLx com-
pounds at 77 K along with a weak signal at g = 4.32,
Fig. 1. ESR spectra of VOLx at 77 K: spectrum of VOL1
in CHCI3 (a) and in toluene (b); spectrum of VOL5 in
CHCI3 (c).
isotropic ESR spectra of VOLx in chloroform and
complicated signal consisting of 1 0 - 1 2 broad lines toluene consist of the usual eight hyperfine lines re-
at ca. g = 2.0 was observed. The room-temperature sulting from coupling of the unpaired electron to the
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268
V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
51V (I = 7/2) nucleus. However, the frozen-solution
spectra of these complexes in CHCI3 were quite
different from the corresponding spectra in toluene
(Fig. 1). For example, in the spectrum of VOL1 in
CHCI3 at 77 K, the gy components appeared with a
well-resolved hyperfine structure (Ay = 135 G), but
the g|| components were not resolved. The spectrum
of the same complex in toluene shows well resolved
gHcomponents, but along with some other hyperfine
lines the broadening and splitting of high field g||
components were also observed (Figs. la, b). Sim-
ilar features were observed for VOL3 and VOL5.
Such behavior of the spectra may be caused by
structures having almost identical ESR parameters
or from anisotropic g and hyperfine tensors [18,19].
Some low intensity satellite lines appeared in the
frozen solution spectra, probably indicate the pres-
ence of a dimeric structure [20]. Although the ESR
spectra of VOL5 in toluene and CHCI3 are typical
for monomeric VO(IV) and all gy and g_L compo-
nents are well resolved (Fig. 1 c), the value of A_l
calculated from formula (3AiS0 - A||)/2 was in dis-
agreement with that determined directly from the
experimental spectrum. Analysis of the ESR spec-
tra of VOLx complexes showed that in toluene they
have g± values of ca. 2 and g|| < 2 with A~ > A x
= A y, and gz < gx = gy < 2 with A z > A x = A y
in CHC13, which typical for axial symmetry [21].
There was no splitting observed of the x and y fea-
tures.
tively, were observed. The intense band observed
in the UV region at 310 - 335 nm has undoubt-
edly charge transfer nature. In the electronic spec-
tra of PdLx in pyridine, only PdL1 and PdL4 com-
plexes show a new single absorption maximum at
673 nm with molar extinction coefficients of 104 and
102 M_ 1cm_1, respectively. These two complexes
exhibit high catalytic activity in the hydrogenation
of nitrobenzene.
Nitrobenzene hydrogenation catalyzed by
complexes PdU
Palladium-mediated catalysis in organic synthe-
sis has received much attention [7, 22]. It is known
that some Pd(II) complexes (Pd-salen, Pd-azocom-
plexes, Pd-nitroso complexes and Pd-alizarin) show
catalytic activity in the hydrogenation of aromatic
nitro compounds and of olefins [23, 24], Our inves-
tigation has shown that among the PdLx complexes
i
------------- T------------- *An
It is interesting to compare the VOLx com-
plexes with corresponding compounds obtained
from hydrogenated LXH2, abbreviated as QXH2 and
VOQx H2O. As can be seen from Tables 2 and 3
the electronic spectral data and ESR parameters
of VOQ1 H20 are different from those for VOLx
complexes. The magnetic and spectral behavior of
VOQ1 H2O are typical for mononuclear VO(IV)
compounds. In the lattercomplexes the values of the
g-factors are reduced and the values of the d-d tran-
sition energy are increased. This effect is probably
caused by the stronger ligand fields of QXH2 ligands
compared to their salicylaldimine LXH2 analogs.
Complexes of Pd(II)
In the electronic spectra of the PdLx compounds
in THF or ethanol solutions low intensity bands at
410 - 145, 435 - 450 nm, attributable to the transi- at 300 K and under H2 recorded after 5 min; (b) spectrum
of this system at 77 K.
tions JAig —> ’Big and *Aig —> ’A2g [1 2 ], respec-
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V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
Table 4. Hydrogenation of nitrobenzene in the presence of PdLx complexes under 760 torr H2 in various solvents.
269
C
*
Complex
PdL1
Temp.
°C
Solvent
CPhN02
[10~4 mol/L]
Specific activity
Mol H2/mol M-min
Absorption rate W
[ml H2/min]
'-c a t
[10~6mol/L]
30
40
30
40
30
40
30
40
40
30
40
30
40
30
40
40
c 2h 5o h
c 2h 5o h
DMF
DMF
THF
THF
THF
C2H5OH
DMF
C2H5OH
C2H5OH
DMF
3.61
5.82
2.85
3.9
1.26
1.16
1.28
1.33
1.17
1.28
1.44
1.71
1.32
1.66
0.95
1.42
1.06
1.42
1.42
1.77
6.81
8.89
0.22
0.16
2.64
3.99
-
0.55
1.16
0.0142
0.0125
0.22
0.32
-
3.72
3.58
3.07
4.70
2.62
2.65
4.42
3.62
2.95
2.85
2.16
4.52
PdL2
PdL4
-
0.01
1.70
4.21
1.52
14.79
15.13
0.78
—
0.25
0.15
1.20
1.00
0.05
-
DMF
THF
DMF
c 2h 5o h
PdL5
1.14
0.055
* Ccat: concentration of catalyst PdLx.
only PdL1, PdL4 and PdL5 complexes, without any mixture a new band at 600 nm appeared. This band
preliminary activation with NaBH4 or NaOH, ex-
hibit catalytic activity in the hydrogenation of ni-
probably is caused by the coordination of nitroben-
zene to the Pd(II) ion. The ESR spectrum of this
trobenzene under H2 pressure, 760 torr, in polar mixture shows a weak and poorly resolved signal at
solvents such as EtOH, THF and DMF at 30 and g = 2.0185, which can be assigned to a coordinated
40 °C. This reaction also takes place at room temper-
phenoxyl type radical. This signal disappears under
ature. The data of the absorption of H2 with various vacuum within 2 - 3 min. When H2 was introduced
concentrations of catalysts are presented in Table 4. into this reaction mixture, a strong triplet pattern due
The highest catalytic activity has been observed for to coupling to the 14N (I = 1) nucleus with a spac-
PdL1 and PdL4 in DMF (Table 1). Note that hydro- ing of A£0 = 10.66 G at g = 2.0125 was observed.
genation of PhN02 under the above conditions us- There is additional, well-resolved hyperfine split-
ing Pd(II) chelates obtained from bi- and tridentate ting (A = 0.8 G) for each component of the triplet
salicylaldimines without tert-butyl groups, were un- consisting of 12 lines (Fig. 2a). This is most likely
successful.
due to the interaction of the unpaired electron with
In order to identify the nature of any paramag- ring protons of the coordinated PhN0H(0 ) radi-
netic intermediates formed in the process of hy- cal. The increased value of the g-factor compared to
drogenation, the interaction of PdL1 with PhN02 free radicals (g = 2.003 - 2.008), undoubtedly indi-
was carried out in the presence and absence of H2 cates that there is a noticeable contribution from the
in THF and examined by means of ESR and elec- metal d-orbitals to the molecular orbital containing
the unpaired electron. The frozen glass spectrum
of this mixture at 77 K exhibited an anisotropic
tronic spectroscopy. The PdL1 complex is stable un-
der H2 atmosphere in the absence of PhN02 and this
mixture does not show any radical species during spectrum typical for Pd(I), with parameters gy =
2.0436, gy = 2.0269, Ay = 57.2, A± = 35.2 G due
to hyperfine coupling with 105Pd (I = 5/2, natural
3 h at room temperature. With the PdL2 complex,
which does not possesses catalytic activity in THF
under the above-mentioned conditions, on mixing abundance 22.2%) [25], (Fig. 2b). The increased
values of the g-factors and the appearance of hyper-
diates were observed. By mixing solutions of PdL1 fine splitting from 14N, ]H, and 105Pd nuclei indicate
that the unpaired electron spin density is localized
the yellow solution of complex immediately turned on the PhN0H(0 ) fragment as well as at the Pd(I)
center.
with PhN02 no color changes and radical interme-
and PhN02 in the absence of H2 under vacuum,
dark green, and in the electronic spectrum of this
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270
V. T. Kasumov • Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes
Some bis[N-(2,6-di-tert-butyl-l-hydroxyphenyl- exhibit catalytic activity in the hydrogenation
salicylaldiminato)]palladium(II) complexes unlike of PI1NO2 and cyclohexene under the above-
their unhindered salicylaldiminate analogs also mentioned conditions [26].
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