Tin(iv) and lead(iv) complexes with a tetradentate redox-active ligand

Article abstract of DOI:10.1039/c2dt30656e

The coordination chemistry of a tetradentate redox-active ligand, glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil) (H₂L), was investigated with the diorganotin(iv) and diphenyllead(iv) moieties. Complexes R ₂SnL (R = Me (1), Et (2), ^tBu (3), Ph (4)) and Ph ₂PbL (5) have been prepared and characterized. The molecular structures of compounds 1, 3 and 5 have been determined by single crystal X-ray diffraction. The diamagnetic octahedral complexes bear a tetradentate O,N,N,O redox-active ligand with a nearly planar core. Complexes 1-5 demonstrate solvatochromism in solution. The CV of complexes 1-5 reveals four one-electron redox processes. The spin density distribution in the chemically generated cations and anions of 1-5 was studied by X-band EPR spectroscopy. The experimental data agree well with the results of DFT calculations of electronic structures for 1, its pyridine adduct 1·Py, cation 1⁺ and anion 1^-.

Full text of DOI:10.1039/c2dt30656e

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PAPER
Tin(IV) and lead(IV) complexes with a tetradentate redox-active ligand†
Alexandr V. Piskunov,*a Olesya Yu. Trofimova,a Georgy K. Fukin,a Sergei Yu. Ketkov,a Ivan V. Smolyaninovb
and Vladimir K. Cherkasova
Received 22nd March 2012, Accepted 25th April 2012
DOI: 10.1039/c2dt30656e
The coordination chemistry of a tetradentate redox-active ligand, glyoxal-bis(2-hydroxy-3,5-di-tert-
butylanil) (H2L), was investigated with the diorganotin(IV) and diphenyllead(IV) moieties. Complexes
R2SnL (R = Me (1), Et (2), tBu (3), Ph (4)) and Ph2PbL (5) have been prepared and characterized. The
molecular structures of compounds 1, 3 and 5 have been determined by single crystal X-ray diffraction.
The diamagnetic octahedral complexes bear a tetradentate O,N,N,O redox-active ligand with a nearly
planar core. Complexes 1–5 demonstrate solvatochromism in solution. The CVof complexes 1–5 reveals
four one-electron redox processes. The spin density distribution in the chemically generated cations and
anions of 1–5 was studied by X-band EPR spectroscopy. The experimental data agree well with the
results of DFT calculations of electronic structures for 1, its pyridine adduct 1·Py, cation 1+ and anion 1−.
be reduced by one electron to form a monoradical trianionic
Introduction
ligand which has been experimentally characterized as an iron
coordinated species.5 Not long ago the cobalt(III) derivative with
The chemistry of metal complexes with redox-active o-quinonato
and o-iminoquinonato ligands has been extensively developed
over the last forty years. The most important features of this
chemistry based on transition metals have been collected in
several reviews.1 The chemistry of non-transition metal com-
plexes with redox-active ligands has progressed in recent years.2
Such types of ligands can reversibly accept one, two, or more
electrons without loss of coordination to the metal. Thus a
redox-active ligand bonded to a main-group-metal ion is able to
reduce or oxidize a substrate coordinated to the metal. It is poss-
ible to simulate the specific reactivity of transition-metal coordi-
nation and organometallic compounds by non-transition-metal
derivatives by including a redox-active ligand in a main-group-
metal complex.3 Tetradentate redox-active ligands are of special
interest due to their ability to form a more extended range of
redox and spin states. Five oxidation levels have been confirmed
for metal complexes based on N,N′-bis(3,5-di-tert-butyl-2-hydro-
xyphenyl)-1,2-phenylenediamine.4 Recently the dianion of cis-
glyoxalbis(2-mercaptoanil) (cis-gma) was shown to be a redox-
active tetradentate ligand. The diimine fragment of cis-gma can
the dianion of cis-glyoxalbis(2-hydroxyanil) (cis-gha) was exam-
ined in the course of redox activity of the ligand moiety.6 The
sterically hindered tetra-tert-butyl substituted analogue of cis-
gha – glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil) (H2L) – has
been known since 19647 but its coordination chemistry has not
been explored adequately to date. This type of ligand in its
deprotonated form may potentially be involved in metal com-
plexes in five different redox states (Scheme 1). The dianion
(L−2) and the trianion (L−3) forms have a number of resonance
structures. The neutral (L0) and the tetraanion (L−4) forms are
diamagnetic while the monoanion (L−1) and the trianion (L−3
)
are radicals. In addition, the dianion (L−2) can exist both in
singlet (L−2A, L−2B) and triplet (L−2C) states. The first attempts
to synthesize metal derivatives with the deprotonated form of
H2L were performed by Wieghardt and co-workers.8 The nickel(II)
complex Ni(HL)2 containing two tridentate mono-deprotonated
HL ligands was prepared. Unfortunately the authors8 were
unable to obtain metal derivatives with the tetradentate form of
this ligand.
In the present study we have synthesized diorganotin(IV) and
diphenyllead(IV) complexes containing the tetradentate dianion
of glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), and examined
their molecular and electronic structure and activity in redox
processes.
aG.A. Razuvaev Institute of Organometallic Chemistry of the Russian
Academy of Sciences, Tropinina str. 49, 603950 Nizhny Novgorod,
Russia. E-mail: pial@iomc.ras.ru; Fax: +78314 627497;
Tel: +7 8314 627682
bSouthern Scientific Centre of the Russian Academy of Sciences,
Toxicology Research Group, Tatischeva str. 16, 414025 Astrakhan,
Russia. E-mail: thiophen@mail.ru; Fax: +78512 250923;
Tel: +7 8512 553651
Experimental
†Electronic supplementary information (ESI) available: Optimized co-
ordinates for cis-H2L, trans-H2L, 1, 1·Py, 1+ and 1−. CCDC reference
numbers 872115–872117 for complexes 1, 3 and 5 respectively. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30656e
General
The infrared spectra of the complexes in the 4000–400 cm−1
range were recorded on an FSM 1201 Fourier-IR spectrometer in
10970 | Dalton Trans., 2012, 41, 10970–10979
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Scheme 1 The possible redox states and resonance structures for the L ligand in the metal complexes.
nujol. NMR spectra were recorded using a Bruker DPX-200
spectrometer and a Bruker Avance III 400 MHz instrument with
Me4Si as an internal standard. UV-Vis spectra were recorded on
a Perkin-Elmer λ 25 spectrometer. EPR investigations were
carried out using a Bruker EMX (working frequency ∼ 9.75 GHz)
spectrometer. The gi values were determined using DPPH as the
reference (gi = 2.0037). HFS constants were obtained by simu-
lation with the WinEPR SimFonia Software (Bruker).
scaling and absorption corrections. The structures were solved
by direct methods and were refined on F2 using all reflections
with the SHELXTL package.13 All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms in 1, 3 and 5 were
placed in calculated positions and were refined in the riding
model. The crystal data collection and structure refinement data
are listed in Table 1.
Electrochemical studies were carried out using an IPC-Pro
potentiostat in three-electrode mode. The stationary glassy
carbon (d = 2 mm) disk was used as working electrode, and the
auxiliary electrode was a platinum-flag electrode. The reference
electrode was Ag/AgCl/KCl (sat.) with a watertight diaphragm
Density functional theory calculations
DFT calculations were performed with the GAUSSIAN 0314
program package using the B3LYP/DGDZVP (complexes 1, 1+,
1− and 1·Py) and B3LYP/6-31G(d) (cis-H2L and trans-H2L)
levels of theory. The absence of imaginary frequencies after the
optimization procedure suggests that the molecular geometries
correspond to the energy minima.
and ferrocene was added as an internal standard (EFc+/Fc
=
0.42 V vs. Ag/AgCl). All measurements were carried out in CH2Cl2
under argon. The samples were dissolved in the pre-deaerated
solvent. The scan rate was 200 mV s−1. The supporting electro-
lyte 0.1 M Bu4NClO4 (99%, “Acros”) was twice recrystallized
from aqueous EtOH and then it was dried in vacuum (48 h). The
elemental analysis was made using Elemental Analyzer Euro EA
3000 instrument.
Reagents were purchased from Aldrich. H2L was synthesised
according to the reported procedure.4d Solvents were purified fol-
lowing standard methods.9 Organotin(IV) chlorides and diphenyl-
lead dichloride were synthesized according to known methods.10
All experiments concerning reduction and oxidation of com-
plexes were carried out under conditions excluding oxygen and
moisture.
Synthesis
General procedure for the preparation of complexes 1–5. An
equimolar mixture of diorganotin dichloride (or diphenyllead
dichloride) and H2L was suspended in methanol (50 ml) under
aerobic conditions. The addition of Et3N (0.5 ml) led to the
homogenization and the solutions became dark green. The reac-
tion mixture was stirred for 30 min and a dark colored solid pre-
cipitated. The crystalline samples of complexes 1–5 were
collected by filtration and washed with 5 ml of methanol.
Me2SnL2 (1). 0.46 g (1 mmol) of H2L and 0.22 g (1 mmol)
of Me2SnCl2. X-ray suitable crystals of 1·CH2Cl2 were obtained
from a CH2Cl2–methanol mixture. The total yield of the
complex is 0.34 g (56%).
X-ray crystallographic study of 1, 3 and 5
Intensity data for 1, 3 and 5 were collected at 100 K on a Bruker
Smart Apex diffractometer with graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å) in the φ–ω scan mode (ω = 0.3°, 10 sec
on each frame). The intensity data were integrated by the SAINT
program.11 SADABS12 was used to perform area-detector
Anal. calc for C32H48SnN2O2 C, 62.86; H, 7.91; Sn, 19.41;
1
N, 4.58%. Found: C, 62.53; H, 7.98; Sn, 19.52; N, 4.54%. H
NMR (400 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 0.61 (s, 6H,
J (119Sn-1H) = 102.6, Sn-CH3), 1.33, (s, 18H, t-Bu), 1.41 (s,
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Table 1 The crystal data collection and structure refinement data for complexes 1, 3 and 5
Complex
1·CH2Cl2
3
5·CH2Cl2
Empirical formula
Formula weight
C33H50Cl2SnN2O2
696.34
C38H60SnN2O2
695.57
C43H54Cl2PbN2O2
908.97
T (K)
Wavelength (Å)
100(2)
0.71073
100(2)
0.71073
100(2)
0.71073
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Triclinic
P1
ˉ
a (Å)
b (Å)
10.9657(4)
18.2113(6)
16.2139(5)
25.7750(8)
10.1974(3)
13.8984(4)
c (Å)
α (°)
17.0576(6)
90
18.4703(5)
90
15.4418(4)
69.3860(10)
β (°)
98.1810(10)
90
3371.7(2)
103.8960(10)
90
7493.1(4)
80.3170(10)
89.5930(10)
2016.00(10)
γ (°)
Volume (Å3)
Z
4
8
2
Density (calculated) [g cm−3
]
1.372
0.947
0.20 × 0.05 × 0.04
2.18–26.00
28 608
1.233
0.714
0.65 × 0.60 × 0.41
1.58–27.00
48 606
1.497
4.354
0.42 × 0.18 × 0.04
2.27–26.50
12 739
Absorption coefficient (mm−1
Crystal size (mm)
Θ range for data collection [°]
Reflections collected
)
Independent reflections
Completeness (to Θ)
Absorption correction
Max. and min. transmission
Refinement method
6605 [R(int) = 0.0498]
99.6 (26.00)
Semi-empirical from equivalents
0.9631 and 0.8332
Full-matrix least-squares on F2
6605/0/375
R1 = 0.0321, wR2 = 0.0674
R1 = 0.0523, wR2 = 0.0725
1.000
0.848 and −0.520
16 333 [R(int) = 0.0195]
99.9% (27.00)
Semi-empirical from equivalents
0.7584 and 0.6539
Full-matrix least-squares on F2
16 333/0/811
R1 = 0.0284, wR2 = 0.0680
R1 = 0.0362, wR2 = 0.0711
1.064
8283 [R(int) = 0.0114]
99.1% (26.50)
Semi-empirical from equivalents
0.8451 and 0.2621
Full-matrix least-squares on F2
8283/0/471
R1 = 0.0200, wR2 = 0.0513
R1 = 0.0216, wR2 = 0.0519
1.068
Data/restraints/parameters
Final R indices [I > 2σ(I)]
R indices (all data)
Goodness-of-fit on F2
Largest diff. peak and hole [e Å−3
]
1.345 and −0.703
1.616 and −1.312
18H, t-Bu), 7.22 (d, 2H, 2.09, aromatic), 7.29 (d, 2H, 2.09, aro-
matic), 8.35 (s, 2H, J (119Sn–1H) = 22.40, CH). 13C NMR
(100 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 6.77 (Sn-CH3, J
NMR (400 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 0.87 (t, 6H, J
119Sn-1H) = 155.6, CH3(Et)), 1.32 (s, 18H, t-Bu), 1.33 (quartet,
(
4H, CH2(Et)), 1.41 (s, 18H, t-Bu), 7.20 (d, 2H, 2.14, aromatic),
7.26 (d, 2H, 2.14, aromatic), 8.37 (s, 2H, J (119Sn–1H) = 17.20,
CH). 13C NMR (100 MHz, CDCl3, 293 K, δ (ppm)): 9.72
(CH3(Et)), 19.24 (CH2(Et)), 29.16, 31.20 (CH3(t-Bu)), 34.50,
35.40 (C(t-Bu)), 109.39, 128.13, 129.80, 133.10,137.47,141.57
(aromatic), 165.54 (CvN). 119Sn NMR (149 MHz, CDCl3,
293 K, δ (ppm)): −270.92. IR (KBr) cm−1: 1561(m), 1533(m),
1516(s), 1403(s), 1361(s), 1343(m), 1285(m), 1266(m), 1251(s),
1236(s), 1200(m), 1158(s), 1115(s), 1072(m), 1036(m), 1022
(m), 1008(m), 966(w), 931(w), 909(s), 841(m), 830(w), 779(w),
748(w), 732(m), 669(w), 635(m), 582(w), 528(m). λmax
(toluene, 293 K)/nm 432 (ε/dm3 mol−1 cm−1 7200), 787
(24 900).
(
119Sn–13C) = 929.5), 29.11, 31.18 (CH3(t-Bu)), 34.51, 35.47 (C
(t-Bu)), 109.56, 128.33, 129.03, 133.01, 137.71, 141.88 (aro-
matic), 164.65 (CvN, J (119Sn–13C) = 26.6). 119Sn NMR
(149 MHz, CDCl3, 293 K, δ (ppm)): −254.90. 1H NMR
(400 MHz, d-Py, 293 K, δ (ppm), J/Hz): 0.65 (s, 6H, J
(
119Sn–1H) = 114.9, Sn–CH3), 1.30, (s, 18H, t-Bu), 1.54 (s,
18H, t-Bu), 7.63 (d, 2H, 2.19, aromatic), 8.68 (s, 4H, aromatic
and CH). 13C NMR (100 MHz, d-Py, 293 K, δ (ppm), J/Hz):
12.31 (Sn–CH3, J (119Sn–13C) = 1205.7), 29.33, 31.31 (CH3(t-
Bu)), 34.28, 35.39 (C(t-Bu)), 110.81, 127.30, 130.55, 136.13,
136.24, 140.79 (aromatic), 163.83 (CvN, J (119Sn-13C) = 26.7).
119Sn NMR (149 MHz, d-Py, 293 K, δ (ppm)): −417.97. IR
(KBr) cm−1: 1562(m), 1520(m), 1516(s), 1483(m), 1408(s),
1379(s), 1361(s), 1341(s), 1288(m), 1266(m), 1253(s), 1239(s),
1200(s), 1158(s), 1116(m), 1071(w), 1038(m), 1023(m), 1009
(w), 968(w), 932(w), 910(s), 897(w), 843(s), 832(w), 780(w),
748(w), 727(m), 636(m), 583(m), 570(m), 527(m). λmax
(toluene, 293 K)/nm 432 (ε/dm3 mol−1 cm−1 6500), 789
(22 100). λmax (acetonitrile, 293 K)/nm 418 (ε/dm3 mol−1 cm−1
8600), 738 (24 000). λmax (THF, 293 K)/nm 415 (ε/dm3 mol−1
cm−1 8200), 723 (23 400). λmax (MeOH, 293 K)/nm 409
(ε/dm3 mol−1 cm−1 7200), 706 (19 500). λmax (pyridine,
293 K)/nm 399 (ε/dm3 mol−1 cm−1 9000), 679 (23 400).
t-Bu2SnL2 (3). 0.46 g (1 mmol) of H2L and 0.30 g (1 mmol)
of t-Bu2SnCl2. X-ray suitable crystals were obtained from a
CH2Cl2–methanol mixture. The total yield of the complex is
0.45 g (65%).
Anal. calc for C38H60SnN2O2 C, 65.61; H, 8.69; Sn, 17.07;
1
N, 4.03%. Found: C, 65.54; H, 8.76; Sn, 17.16; N, 4.01%. H
NMR (400 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 1,08 (s, 18H, J
(
119Sn–1H) = 129.9, t-Bu(Sn)), 1.31 (s, 18H, t-Bu), 1.47 (s, 18H,
t-Bu), 7.15 (d, 2H, 2.08, aromatic), 7.25 (d, 2H, 2.08, aromatic),
8.34 (s, 2H, J (119Sn–1H) = 12.55, CH). 13C NMR (100 MHz,
CDCl3, 293 K, δ (ppm)): 29.55, 30.05, 31.25 (CH3(t-Bu)),
34.47, 35.37, 43.84 (C(t-Bu)), 109.17, 128.05, 131.22, 132.42,
137.41, 140.90 (aromatic), 166.70 (CvN). 119Sn NMR
Et2SnL2 (2). 0.46 g (1 mmol) of H2L and 0.25 g (1 mmol) of
Et2SnCl2. The total yield of the complex is 0.27 g (42%).
Anal. calc for C34H52SnN2O2 C, 63.86; H, 8.20; Sn, 18.56;
(149 MHz, CDCl3, 293 K, δ (ppm)): −344.58. IR (KBr) cm−1
:
1
N, 4.38%. Found: C, 63.75; H, 8.27; Sn, 18.62; N, 4.33%. H
1559(m), 1530(m), 1514(s), 1405(s), 1359(s), 1335(s), 1284(s),
10972 | Dalton Trans., 2012, 41, 10970–10979
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1264(m), 1250(m), 1233(s), 1197(s), 1155(s), 1117(s), 1066(m),
1037(m), 1023(m), 1009(m), 966(w), 928(w), 909(s), 894(w),
845(s), 828(w), 781(w), 746(w), 730(m), 671(w), 637(m), 584
(w), 527(m). λmax (toluene, 293 K)/nm 434 (ε/dm3 mol−1 cm−1
7800), 800 (25 900).
metals make it possible to observe pure redox behavior of the
coordinated ligand which is not complicated with oxidation or
reduction of the metallic center. In addition, glyoxal-bis(2-
hydroxy-3,5-di-tert-butylanil) is known to prefer tridentate
coordination with the small cation of nickel.4 A large coordi-
nation sphere of diorganotin or diorganolead dications should
provide the tetradentate coordination of the organic ligand.
The reaction of H2L with organometallic chlorides (ratio 1 : 1)
in methanol in the presence of triethylamine at room temperature
resulted in the formation of dark colored precipitates of com-
plexes 1–5 (Scheme 2).
Ph2SnL2 (4). 0.46 g (1 mmol) of H2L and 0.34 g (1 mmol) of
Ph2SnCl2. The total yield of the complex is 0.29 g (40%).
Anal. calc for C42H52SnN2O2 C, 68.58; H, 7.13; Sn, 16.14;
1
N, 3.81%. Found: C, 68.52; H, 7.19; Sn, 16.25; N, 3.78%. H
NMR (400 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 1,34 (s, 18H,
t-Bu), 1.48 (s, 18H, t-Bu), 7.14 (m, 6H, m-p-H(Sn-Ph)), 7.19 (d,
2H, 2.20, aromatic), 7.40 (d, 2H, 2.20, aromatic), 7.54 (dd, 4H,
1.20, 8.04, J (119Sn–1H) = 100.14, o-H(Sn-Ph)), 8.31 (s, 2H, J
(
119Sn–1H) = 24.40, CH). 13C NMR (100 MHz, CDCl3, 293 K,
δ (ppm)): 29.45, 31.21 (CH3(t-Bu)), 34.55, 35.46 (C(t-Bu)),
109.99, 128.06, 128.74, 129.05, 129.89, 133.38, 136.04, 138.34,
141.19, 146.05 (aromatic), 165.33 (CvN). 119Sn NMR
(149 MHz, CDCl3, 293 K, δ (ppm)): −421.30. IR (KBr) cm−1
:
1585(s), 1540(w), 1524(s), 1478(s), 1430(m), 1406(m), 1377(s),
1359(s), 1326(s), 1319(s), 1280(s), 1259(s), 1234(s), 1199(s),
1160(s), 1117(s), 1078(w), 1063(m), 1038(s), 1026(m), 1011(s),
960(w), 930(w), 910(s), 872(m), 860(m), 846(m), 831(m), 779
(w), 740(s), 694(s), 658(w), 640(w), 590(w), 569(m), 529(m).
λmax (toluene, 293 K)/nm 429 (ε/dm3 mol−1 cm−1 7500), 758
(19 300).
Scheme 2 Synthesis of 1–5.
X-ray structures
The molecular structures of complexes 1, 3 and 5 were examined
with X-ray diffraction analysis. X-ray suitable crystals of the
complexes were obtained by prolonged crystallization from a
CH2Cl2–methanol mixture. Crystals of complexes 1 and 5 have
one CH2Cl2 solvated molecule per complex molecule. There are
two crystallographically unique molecules in the asymmetric
unit of 3 but their geometries around the metal center are similar
and only one molecule is discussed. Fig. 1–3 show the molecular
Ph2PbL2 (5). 0.46 g (1 mmol) of H2L and 0.43 g (1 mmol) of
Ph2PbCl2. X-ray suitable crystals of 5·CH2Cl2 were obtained
from a CH2Cl2–methanol mixture. The total yield of the
complex is 0.57 g (69%).
Anal. calc for C42H52PbN2O2 C, 61.21; H, 6.36; Pb, 25.14;
1
N, 3.40%. Found: C, 61.20; H, 6.42; Pb, 25.19; N, 3.36%. H
NMR (400 MHz, CDCl3, 293 K, δ (ppm), J/Hz): 1.31 (s, 18H,
t-Bu), 1.48 (s, 18H, t-Bu), 7.08 (d, 2H, 2.40, aromatic), 7.29 (m,
6H, m-p-H(Pb-Ph))7.34 (d, 2H, 2.40, aromatic), 7.89 (dd, 4H,
1.20, 8.13, o-H(Pb-Ph)), 8.31 (s, 2H, CH). 13C NMR (100 MHz,
CDCl3, 293 K, δ (ppm)): 29.47, 31.23 (CH3(t-Bu)), 34.38,
35.43 (C(t-Bu)), 110.65, 128.36, 129.81, 129.87, 131.91.
135.47, 135.77, 136.55, 142.68, 162.14 (aromatic), 167.05
(CvN). IR (KBr) cm−1: 1576.1(s), 1531.0(w), 1516.0(s),
1481.5(s), 1403.4(s), 1380.9(s), 1362.9(m), 1329.9(m), 1280.4
(m), 1260.9(s), 1247.3(s), 1233.8(s), 1199.3(s), 1151.3(s),
1121.3(m), 1089.7(w), 1064.2(m), 1025.2(m), 1016.2(m),
1008.7(m), 993.7(m), 963.7(w), 933.6(w), 909.6(s), 896.1(w),
878.1(w), 858.6(m), 849.6(m), 828.6(w), 780.5(w), 740.0(m),
726.5(s), 686.0(m), 631.9(m), 580.9(w), 523.9(m), 507.4(w).
λmax (toluene, 293 K)/nm 420 (ε/dm3 mol−1 cm−1 7500), 770
(22 200). λmax (Py, 293 K)/nm 402 (ε/dm3 mol−1 cm−1 7000),
707 (17 400).
Fig. 1 The molecular structure of 1 with 50% ellipsoid probability. H
atoms and the solvated CH2Cl2 molecule are omitted for clarity.
Results and discussion
Synthesis
Interest in the synthesis of complexes containing the glyoxal-bis-
(2-hydroxy-3,5-di-tert-butylanil) was caused by the necessity to
establish the potential of this ligand to be involved in redox
transformations. The decision to obtain complexes with group
14 elements was made based on two reasons. The redox inactive
Fig. 2 The molecular structure of 3 with 50% ellipsoid probability. H
atoms are omitted for clarity.
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Table 2 Selected bond lengths (Å) and angles (°) for 1, 3 and 5
Bonda
1
3
5
M(1)–O(1)
M(1)–N(1)
M(1)–O(2)
M(1)–N(2)
O(1)–C(1)
O(2)–C(17)
N(1)–C(15)
N(1)–C(2)
N(2)–C(16)
N(2)–C(18)
C(1)–C(2)
C(1)–C(6)
C(2)–C(3)
2.2518(16)
2.2626(19)
2.2649(16)
2.2523(19)
1.295(3)
1.308(3)
1.301(3)
1.386(3)
1.306(3)
1.388(3)
1.426(3)
1.439(3)
1.410(3)
1.371(3)
1.426(3)
1.372(3)
1.419(3)
1.422(3)
1.439(3)
1.405(3)
1.372(3)
1.415(3)
1.377(3)
2.115(2)
2.122(2)
—
2.2544(12)
2.2668(14
2.2707(12)
2.2626(14)
1.301(2)
1.311(2)
1.300(2)
1.389(2)
1.305(2)
1.389(2)
1.433(2)
1.435(2)
1.404(2)
1.378(2)
1.413(2)
1.383(2)
1.430(2)
1.428(2)
1.443(2)
1.406(2)
1.372(2)
1.422(2)
1.375(3)
2.2003(18)
—
2.4187(13)
2.4196(16)
2.3522(14)
2.3749(15)
1.302(2)
1.307(2)
1.298(2)
1.387(2)
1.296(2)
1.392(2)
1.431(3)
1.444(3)
1.411(3)
1.369(3)
1.419(3)
1.378(3)
1.437(3)
1.427(3)
1.437(3)
1.403(3)
1.380(3)
1.412(3)
1.383(3)
2.180(2)
—
Fig. 3 The molecular structure of 5 with 50% ellipsoid probability. H
atoms and the solvated CH2Cl2 molecule are omitted for clarity.
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(15)–C(16)
C(17)–C(18)
C(17)–C(22)
C(18)–C(19)
C(19)–C(20)
C(20)–C(21)
C(21)–C(22)
M(1)–C(31)
M(1)–C(32)
M(1)–C(35)
M(1)–C(37)
structures of 1, 3 and 5 respectively. Selected bond lengths and
angles for 1, 3 and 5 are summarized in Table 2.
The distorted octahedral coordination sphere of the metals in
complexes 1, 3 and 5 is formed by two alkyl (or aryl) substitu-
ents and the tetradentate chelated L ligand. Oxygen and nitrogen
atoms of the latter lie in the equatorial plane while the hydro-
carbon fragments are in the apical positions. The tin (or lead)
atoms are strongly deviated from the centre of the tetragon
O(1)N(1)N(2)O(2). The geometry of the ONNO ligand in 1, 3
and 5 is close to planar. The dihedral angles between the C(1)–
C(6) and C(17)–C(22) rings are 24.62°, 12.32° and 20.39° for 1,
3 and 5 respectively.
One of the chlorine atoms of the solvated CH2Cl2 molecule in
1 and 5 is disposed in the plane of the eleven membered metallo-
cycle. The Sn⋯Cl and Pb⋯Cl distances in 1 and 5 are 3.977 Å
and 3.396 Å respectively. Such Sn(Pb)⋯Cl distances allow to
suppose the presence of van der Waals (vdW) interactions
between these atoms. The sum of vdW radii for Sn and Cl (Pb
and Cl) atoms is 4.0 Å (4.1 Å). This vdW contact is normal in 1
whereas it is significantly shortened in 5. The C(Ph)PbC(Ph)
angle (158.15(7)°) has a maximal value in comparison with ana-
logous angles in 1 (144.75(10)°) and 3 (153.76(7)°) as a conse-
quence. It should be noted that there is no solvated CH2Cl2
molecule in 3. However the C(But)SnC(But) angle in 3 signifi-
cantly exceeds the equivalent one in 1. These observations
suggest a weakness of the intermolecular Sn⋯Cl interactions in
1 and strength of the non-bonding interactions between the tert-
butyl groups in 3.
The bond distances in the glyoxal bridging unit of the L
ligand in complexes 1, 3 and 5 are very informative in establish-
ing its electronic configuration. The C–C and C–N distances in a
neutral diimine are known to be sharply different from those in a
corresponding reduced diimine. The C–N and C–C distances in
the neutral diimine coordinated to the tin atom are 1.27 and
1.47 Å respectively,15 whereas the corresponding bond lengths
in the diimine radical-anion are 1.32 and 1.40 Å.16 The bond
parameters of the glyoxal fragment in 1, 3 and 5 are 1.301(3),
1.300(2), 1.298(2) Å and 1.419(3), 1.430(2), 1.437(3) Å for
C–N and C–C respectively. They are close to those observed for
the [Co(gha)(PPh3)2]+ cation6 and are intermediate between the
values typical for neutral diimine and its radical-anion. It is
important to note that complexes 1–5 exhibit CvN stretching
2.2009(18)
—
—
2.179(2)
—
Angle
C(31)–M(1)–C(32)
C(31)–M(1)–C(35)
C(31)–M(1)–C(37)
C(31)–M(1)–O(1)
C(32)–M(1)–O(1)
C(35)–M(1)–O(1)
C(37)–M(1)–O(1)
C(31)–M(1)–N(2)
C(32)–M(1)–N(2)
C(35)–M(1)–N(2)
C(37)–M(1)–N(2)
O(1)–M(1)–N(2)
C(31)–M(1)–N(1)
C(32)–M(1)–N(1)
C(35)–M(1)–N(1)
C(37)–M(1)–N(1)
O(1)–M(1)–N(1)
N(2)–M(1)–N(1)
C(31)–M(1)–O(2)
C(32)–M(1)–O(2)
C(35)–M(1)–O(2)
C(37)–M(1)–O(2)
O(1)–M(1)–O(2)
N(2)–M(1)–O(2)
N(1)–M(1)–O(2)
144.75(10)
—
—
83.47(8)
85.05(8)
—
—
153.76(7)
—
85.43(6)
—
86.96(6)
—
100.23(6)
—
100.74(6)
—
143.69(5)
104.04(6)
—
97.36(6)
—
72.00(5)
71.84(5)
87.06(6)
—
—
—
158.15(7)
88.49(6)
—
—
—
85.00(6)
99.71(6)
—
102.37(8)
107.03(9)
—
—
—
99.49(6)
135.75(5)
96.13(6)
—
142.20(6)
110.22(9)
97.26(9)
—
—
—
100.51(6)
67.29(5)
68.61(5)
87.50(6)
—
71.20(6)
71.78(7)
85.25(9)
86.01(8)
—
84.69(6)
—
144.54(4)
71.77(5)
143.28(5)
—
—
89.67(6)
155.10(4)
69.13(5)
137.59(5)
146.12(6)
71.53(6)
142.46(6)
a M = Sn for 1 and 3; M = Pb for 5.
vibrations at 1561–1585 cm−1 which is slightly less than the
stretching frequency of a coordinated aldimine (>1600 cm−1 17
)
indicating the elongation of the CvN bond in comparison with
diazabutadienes. This effect can be explained by the additional
conjugation of the diimine moiety π-system with the π-system of
the phenolate parts of the ligand. Unfortunately there are no
structural data for H2L or any similar ligands. We have per-
formed DFT calculations for H2L. The trans-configuration
appears to be 1.27 kcal mol−1 more stable than the cis geometry.
10974 | Dalton Trans., 2012, 41, 10970–10979
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The calculated C–N and C–C distances of the glyoxal fragment
in the free trans-H2L ligand are 1.289 and 1.446 Å, respectively,
which are very close to those obtained for complexes 1, 3 and 5.
On the other hand, the structural parameters of the L ligand
show a quinoid-type distortion in the two aromatic phenolic
rings. The C(3)–C(4) and C(5)–C(6) bond distances are shorter
than the average bond distances of C(1)–C(2), C(2)–C(3), C(4)–
C(5) and C(1)–C(6). The same trend is also evident in the other
ring where both the C(19)–C(20) and C(21)–C(22) bonds are
shorter than the other ones. The C–O distances are also shorter
than the C–O bonds in reported tin(IV) o-amidophenolate com-
plexes18a,b and are closer to those obtained for the corresponding
o-iminosemiquinolate derivatives.3e,18b–e The above mentioned
structural features indicate the importance of resonance structures
L−2B and L−2C (Scheme 1) on consideration of complexes 1–5.
solutions of complexes 1–5 in toluene are green while in pyri-
dine they are blue.
According to the structure of complexes 1, 3 and 5 the meta-
llic center has a vacant place between alkyl (or aryl) substituents
which is sufficient for the coordination of an extra ligand. Such
coordination has to be accompanied by an increase of the coordi-
nation number of the metal up to seven and an increase of the
C–M–C angle. It is evidenced by the NMR spectroscopy exper-
iments. A δ value in the range from −254.9 to −421.3 ppm for
the 119Sn NMR spectra was obtained for complexes 1–4 in
CDCl3. The chemical shifts are in agreement with the range of
ca. −125 to −525 ppm reported for six-coordinated tin(IV) com-
pounds.19 It is well known that an increase in the coordination
number of tin should give rise to a high field shift of δ (119Sn).20
Indeed the chemical shift δ (119Sn) for complex 1 increases from
−254.90 to −417.97 ppm when the 119Sn NMR spectrum is
recorded in Py-d5. In addition the magnitude of the tin–carbon J
coupling, (1J(119Sn,13C)), depends linearly on the Me–Sn–Me
angle in compliance with the known equation.21 The Me–Sn–Me
angle of complex 1 in the course of pyridine coordination
increases by 30° as can be estimated from the differences
Electronic spectra
The electronic absorption data of complexes 1–5 are summarized
in Table 3 and shown in Fig. 4 for complex 1. The spectrum of 1
appears to be dominated by two intense ligand-to-ligand charge
transfers (LLCT). The positions of these bands were found to be
strongly influenced by solvent. This influence is due to the
coordination capability of the solvent molecule rather than its
polarity. It is clear from the comparison of the spectra in aceto-
nitrile (strongly polar and weak donor) and pyridine (strong
donor and medium polarity) that the shift of the charge transfer
bands is much larger in pyridine than in acetonitrile. The
1
between the J(119Sn,13C) values for 1 recorded in CDCl3 and
Py-d5 on the basis of this equation.21
Cyclic voltammetry
The electronic structure of compounds 1–5 was assessed by per-
forming cyclic voltammetry in dichloromethane solution. The
relatively high scan rate (200 mV s−1) was chosen to increase
the reversibility of the redox waves. The CV experiments with a
lower scan rate (50 mV s−1) were complicated with the adsorp-
tion–desorption processes. Fig. 5 displays the cyclic voltammo-
gram (CV) of complex 1. Electrochemical data for all complexes
1–5 are summarized in Table 4.
The CV of 1, 2 and 4 displays four reversible one-electron
transfer waves due to the change of the ligand redox states
(Scheme 3). The redox potentials of complexes 1–4 are
influenced by the nature of the hydrocarbon substituent bonded
to the tin. The donor groups shift the position of E1/2 into the
cathodic region (3 < 2 < 1) while electron-withdrawing phenyls
Table 3 Electronic spectra of complexes at 293 K
Complex
Solvent
λmax, nm (ε, dm3 mol−1 cm−1
)
1
Toluene
Acetonitrile
THF
Methanol
Pyridine
Toluene
Toluene
Toluene
Toluene
Pyridine
423 (6500), 789 (22 100)
418 (8600), 738 (24 000)
415 (8200), 723 (24 400)
409 (7200), 706 (19 500)
399 (9000), 679 (23 400)
432 (7200), 787 (24 900)
434 (7800), 800 (25 900)
429 (7500), 758 (19 300)
420 (7500), 770 (22 200)
402 (7000), 707 (17 400)
2
3
4
5
Fig. 4 UV-vis spectra of 1 in toluene (1), acetonitrile (2), THF (3),
methanol (4) and pyridine (5). C = 1 × 10−4 M.
Fig. 5 CVof compound 1.
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Table 4 The electrochemical dataa for complexes 1–5
the reverse scan of the CV. A similar situation occurs in the
anodic region. The first oxidation wave responds to the formation
of the relatively stable cation (Ic/Ia = 0.45) while the second ir-
reversible redox process (E1/2 = 1.32 V) is shifted to the anodic
region in comparison with 1, 2 and 4.
The additional one electron oxidation wave is observed at
1.45 V. This value is very close to the oxidation potential of
sterically hindered o-iminobenzoquinones22 and is responsible
for the oxidation of the imine nitrogen atom in L0. Two irrevers-
ible anodic processes at E1/2 1.32 and 1.45 V result in the
destruction of 3 and the reversibility of the first anodic redox
wave disappears on the reverse scan of the CV.
Complex
ERed.11/2, V
ERed.21/2, V
EOx.11/2, V
EOx.21/2, V
1
2
3
4
5
−0.83
−0.88
−0.88
−0.70
−0.89
−1.41
−1.48
−1.60b
−1.30
−1.37b
0.65
0.65
0.63
0.78
0.62
1.10
1.11
1.32b
1.23
0.98
a CH2Cl2 solution containing 0.1 M n-Bu4NClO4 supporting electrolyte,
scan rate = 200 mV s−1, vs. Ag/AgCl/KCl, C = 1–3 × 10−3 M, glassy
carbon working electrode, Ar, 0.1 M Bu4NClO4. b The value of peak
potential for irreversible process.
The CV of 5 displays four one-electron transfer waves. Both
the anodic waves are reversible while only the first redox process
in the cathodic region is reversible. The nature of the metal ion
has an effect on the values of the redox potentials. They are
shifted into the cathodic region for complex 5 in comparison
with the tin complex 4.
The cations and anions of complexes 1–5 were generated
chemically by oxidation with AgBF4 or reduction with metallic
potassium respectively. The obtained CH2Cl2 (for cations) and
THF (for anions) solutions were examined with X-band EPR
spectroscopy.
The EPR spectra of cations 1+–5+ are wide singlets (ΔH = 2–3
G) with satellite splitting due to the interaction of unpaired elec-
trons with the magnetic isotopes of tin (117Sn (7.68%, I = 1/2,
μN = 1.000), 119Sn (8.58%, I = 1/2, μN = 1.046))23 or lead (207Pb
(22.1%, I = 1/2, μN = 0.5926))23 (Fig. 6). The EPR spectra para-
meters for cations 1+–5+ are summarized in Table 5.
Fig. 6 An EPR spectrum of 5+ in CH2Cl2 at 290 K.
Table 5 EPR spectra parametersa for cations 1+–5+
Scheme 3 Redox states of the complexes 1–5.
Cation
ai(Mb), G
ΔH, G
gi
(4) complicate the oxidation and shift the potentials in the
anodic direction.
1+
2+
3+
4+
5+
—
2.55
3.00
2.85
1.90
2.06
2.0058
2.0042
2.0026
2.0035
2.0032
c
5.80, 5.54
9.20, 8.80
6.80, 6.50
12.2
Complex 3 slightly differs from the other tin compounds. The
tert-butyl substituent lead to the destabilization of doubly
charged ions. The first cathodic process gives the anion which is
relatively stable on the electrochemical time scale. The second
one-electron reduction is irreversible and leads to the additional
chemical transformations following the electrochemical stage. It
is evidenced by the presence of additional oxidation waves on
a CH2Cl2. T = 290 K. b M = 119Sn, 117Sn for 1+–4+ and M = 207Pb for
5+. c The satellite splitting in the EPR spectra of 1+ is too small for clear
determination and appears as shoulders on the central line.
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The EPR spectra of anions 1−–4− are well resolved. They are
quintets (1 : 2 : 3 : 2 : 1) of triplets (1 : 2 : 1) of quintets
(1 : 4 : 6 : 4 : 1) with satellite splitting. The hyperfine structure of
the EPR spectra of anions 1−–4− arises from the hyperfine coup-
ling of unpaired electron with two and four equivalent magnetic
1
H nuclei (99.98%, I = , μN = 2.7928),23 two equivalent nitro-
1
2
gen nuclei 14N (99.63%, I = 1, μN = 0.4037)23 and the magnetic
isotopes of tin 117,119Sn (Fig. 7). The EPR spectra parameters for
anions 1−–4− are summarized in Table 6. We were unable to
Scheme 4 The assignment of hfs constants in 1−. The hfs constants
are given in Gauss.
obtain under the EPR experimental conditions stable paramag-
netic derivative 5−.
The hyper fine splitting (hfs) constants with magnetic isotopes
119,117Sn in the EPR spectra of anions 1−–4− are four times
lower than the values characteristic for tin(II) derivatives with
paramagnetic ligands24 and lie in the same region as for o-imino-
semiquinolate tin(IV) complexes.18b,d It unequivocally confirms
that the reduction of complexes 1–4 does not concern the met-
allic center.
The hfs with two equivalent protons ai(2H) = 3.47–3.56 G in
EPR spectra of anions 1−–4− should to be attributed to the
protons of the glyoxal fragment. The values of hfs constants on
two equivalent nitrogen atoms in anions 1−–4− are slightly
lower than those observed for tin derivatives containing the
radical-anion of diazabutadiene.24 It indicates that most of the
spin density in 1−–4− is localized on the glyoxal fragment of the
ligand. The delocalization of spin density into the two phenolic
moieties of the ligand calls for the additional hfs with four equiv-
alent protons with constants ai(4H) ≈ 1.5 G. The assignment of
hfs constants in 1− is depicted in Scheme 4. Thus anions 1−–4−
have to be considered as containing a trianion of the L ligand in
the L−3B rather than L−3A resonance form (Scheme 1).
DFT calculations
We have performed DFT calculations for complex 1 and its pyri-
dine adduct 1·Py in the closed-shell singlet ground state. The
doublet cation and anion of 1 were computed also. The opti-
mized structural parameters are given in Table 7 along with the
corresponding experimental values (atom labelling follows that
in the crystal structure, see Fig. 1).
It is seen that the calculated structural parameters are in good
agreement with the experimental values. Though the trends in
C–C bond lengths are well described by the chosen level of
theory, the Sn–N, Sn–O and Sn–C bond lengths are slightly
overestimated. Computations of 1, 1·Py, 1− and 1+ were per-
formed with no symmetry restrictions but all optimized struc-
tures have a two-fold axis passing through the tin atom and the
middle of the C(15)–C(16) bond. The optimized L ligand in 1,
1·Py, 1− and 1+ is planar.
The redox activity of complex 1 is specified with frontier orbi-
tals. The HOMO (−5.07 eV), HOMO-1 (−5.77 eV) and
HOMO-2 (−6.44 eV) orbitals are localized on the L ligand and
they contain no contribution of the tin atom (Fig. 8).
Fig. 7 The experimental EPR spectrum of 1− (1), its simulation (2)
and stick-diagram (3) (117,119Sn satellite splitting is omitted for the stick-
diagram). T = 290 K, THF.
Table 6 EPR spectra parametersa for anions 1−–4−
ai(214N)/G ai(21H)/G ai(41H)/G ai(119,117Sn)/G gi
1− 4.44
2− 4.47
3− 4.46
4− 4.48
3.56
3.47
3.54
3.49
1.54
1.53
1.56
1.58
51.10, 48.80
56.20, 53.72
65.40, 62.50
43.20, 41.30
2.0024
2.0025
2.0026
2.0019
The HOMO-3 (−6.75 eV) is the highest occupied orbital
responsible for metal–ligand bonding in the complex. The
LUMO (−3.16 eV) is a ligand-localized orbital. Its small part
a THF, T = 290 K.
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Table 7 Experimental bond distances (Å) and angles (°) of 1 in
comparison with the calculated values for 1, 1·Py, 1− and 1+
Bond
1 (Exp.)
1 (Calc.) 1·Py
1−
1+
Sn(1)–O(1)
Sn(1)–N(1)
O(1)–C(1)
N(1)–C(15)
N(1)–C(2)
C(1)–C(2)
C(1)–C(6)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(15)–C(16)
Sn(1)–C(31)
Sn(1)–N(Py)
Angle
O(1)–Sn(1)–O(2)
N(1)–Sn(1)–N(2)
2.2518(16)
2.2626(19)
1.295(3)
1.301(3)
1.386(3)
1.426(3)
1.439(3)
1.410(3)
1.371(3)
1.426(3)
1.372(3)
1.419(3)
2.115(2)
2.285
2.304
1.299
1.315
1.378
1.443
1.446
1.416
1.381
1.428
1.385
1.429
2.166
2.285
2.316
2.252
1.311
1.344
1.388
1.441
1.433
1.407
1.396
1.412
1.399
1.399
2.174
2.349
2.390
1.306
1.307
1.380
1.440
1.445
1.415
1.382
1.424
1.387
1.437
2.178
2.554
2.296
1.272
1.337
1.360
1.475
1.458
1.414
1.381
1.445
1.373
1.402
2.157
Fig. 9 Spin density isosurfaces for cation 1+ (left) and anion 1−
(right). The isovalue is 0.0015 a.u. H atoms are omitted for clarity.
distortion was predicted by the NMR spectroscopy experiments
mentioned above. The shape of the frontier orbitals is not
influenced by the pyridine coordination. At the same time the
HOMO (−4.84 eV)–LUMO (−2.72 eV) and HOMO-2
(−6.18 eV)–LUMO (−2.72 eV) gaps (2.12 and 3.46 eV respec-
tively) increase in comparison with the six-coordinated complex
1. This accounts for a hypsochromic shift in the electronic
absorption spectra of 1 in coordinating solvents.
146.12
71.78
146.55
71.77
148.74
152.03 144.77 149.05
69.03 73.09 72.24
174.90 142.00 144.05
C(31)–Sn(1)–C(32) 144.75
The reduction of 1 is accompanied with population of the
LUMO. The bond distance redistribution affects both the pheno-
lic and glyoxal part of the ligand. The quinoid-type distortion in
the two phenolic parts disappears while the C–C and C–N bond
distances of the glyoxal fragment (1.400 and 1.344 Å respect-
ively) become very close to those obtained for the diimine
radical-anion.16 The Me2Sn moiety moves into the chelate
ONNO semi-ring during the reduction of complex 1 which is
clear from the Sn–O and Sn–N bond lengths changing. It leads
to an increase in sterical hindrance around the metal atom in 1−.
This effect, along with the population of the antibonding orbital
for the M–C bond, accounts for the instability (according to the
EPR and electrochemical experiments) of anions 3− and 5− con-
taining bulky substituents at the metal atom. The spin density in
the anion 1− is essentially localized on the NCCN fragment
(Fig. 9). The Mulliken spin densities on both the N and C atoms
are 0.22 and 0.12 respectively.
The isotropic Fermi constants from the DFT calculations
(ai(214N) = 5.7 G, ai(21H) = 3.5 G, ai(21H) = 1.8 G, ai(21H) =
1.9 G, ai(117Sn) = 48.1 G) are in good agreement with the hfs
constants obtained in the EPR experiment. Thus the calculations
confirm the anion 1− structure as that containing an L trianion
ligand in the L−3B resonance form (Scheme 1).
The oxidation of 1 is accompanied by the sharpening of the
quinoid-type distortion in the two phenolic rings. The C(1)–
O(1), C(2)–C(3) and C(5)–C(6) bonds of 1+ are shorter in com-
parison with neutral 1. The examination of the spin distribution
(Fig. 9) in cation 1+ demonstrates that the unpaired electron is
delocalized between two phenoxy moieties of the ligand predo-
minantly. The Mulliken spin densities on the O(1), C(2), C(4)
and C(6) atoms are 0.17, 0.19, 0.19 and 0.06 respectively. Such
a spin distribution in cation 1+ leads to the small hyperfine coup-
ling of the unpaired electron with the hydrogen and nitrogen
atoms of both the phenolic and glyoxal parts of the ligand. As a
result the hyperfine structure in the EPR spectrum of 1+ is
absent.
Fig. 8 Representation of the frontier orbitals, HOMO-1, HOMO-2 and
HOMO-3 of 1 from the DFT calculation (the isovalue is 0.02 a.u.). H
atoms are omitted for clarity.
formed by the Sn and Me contributions is antibonding for the
Sn–Me bond. The HOMO–LUMO gap (1.92 eV) is in reason-
able agreement with the experimental values which can be esti-
mated from the electrochemical (1.48 eV) or UV-vis
spectroscopy (1.57 eV for the long-wavelength band) exper-
iments. The second intense band in the UV-visible spectrum at
∼400 nm (∼3.1 eV) for complex 1 should be attributed to the
another intraligand HOMO-2–LUMO (3.28 eV) transition.
The coordination of the pyridine molecule to the tin atom
leads to the weakening of the metal–glyoxal coordination in
1·Py. This results in elongation of the Sn–N(glyoxal) bonds fol-
lowed by redistribution of bond distances in the glyoxal fragment
which tend towards the free diimine values. Pyridine coordi-
nation causes an increase of the Me–Sn–Me angle by 30°. This
Conclusions
We reported the first examples of metal complexes containing a
tetradentate redox active ligand – glyoxal-bis(2-hydroxy-3,5-di-
10978 | Dalton Trans., 2012, 41, 10970–10979
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10 K. A. Kocheshkov, N. N. Zemlyanskii, N. I. Sheverdina and
E. M. Panov, Methods of Elementoorganic Chemistry. Germanium, Tin,
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11 Bruker, SAINTPLUS Data Reduction and Correction Program v.6.02a,
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12 G. M. Sheldrick, SADABS v.2.01, Bruker/Siemens Area Detector Absorp-
tion Correction Program, Bruker AXS, Madison, WI, USA, 1998.
13 G. M. Sheldrick, SHELXTL v.6.12, Structure Determination Software
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tert-butylanil). Complexes 1–5 give the unique opportunity to
investigate the redox behavior of this ligand which is not compli-
cated by redox transformations of the metallic center. The possi-
bility of the dianionic form of the ligand undergoing redox
transformations was shown using electrochemistry, EPR spec-
troscopy and DFT calculations.
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Rob, J. R. Cheeseman,
J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant,
J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,
Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox,
H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
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P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
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Acknowledgements
We are grateful to the FSP “Scientific and Scientific-Pedagogical
Cadres of Innovation Russia” for 2009–2013 years (GK P839
from 25.05.2010), Russian Foundation for Basic Research (grant
11-03-97041-r_povolzh’e_a, 12-03-00513-a), Russian President
Grants (NSh-1113.2012.3, MK-614.2011.3, MK-1156.2011.3)
for financial support of this work.
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