Synthesis and site-selective metalation of a benzene-o-dithiol/ pyridylimine ligand

Article abstract of DOI:10.1515/znb-2010-0405

The benzene-o-dithiol/pyridylimine (S-S)/(N-N) ligand H₂-7, representing the first example of a new type of bis(bidentate) heterodonor ligands with a mixed donor set, has been synthesized by Schiff base condensation of pyridine-2-carbaldehyde w

Full text of DOI:10.1515/znb-2010-0405

Synthesis and Site-selective Metalation of a Benzene-o-dithiol/
Pyridylimine Ligand
Johannes Do¨mer, Tania Pape, and F. Ekkehardt Hahn
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Corrensstraße 30, 48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. F. Ekkehardt Hahn. Fax: +49-25183-33013.
E-mail: fehahn@uni-muenster.de
Z. Naturforsch. 2010, 65b, 470 – 474; received January 7, 2010
The benzene-o-dithiol/pyridylimine (S-S)/(N-N) ligand H2-7, representing the first example of a
new type of bis(bidentate) heterodonor ligands with a mixed donor set, has been synthesized by Schiff
base condensation of pyridine-2-carbaldehyde with an amine-substituted benzene-o-dithiol moiety.
Bis(cyclopentadienyl)titanium dichloride reacts selectively at the S-S donor group of H2-7 to yield
complex [Cp2Ti(7)].
Key words: Heterodonor Ligands, S-S Ligands, N-N Ligands, Titanium, Schiff Bases
Introduction
Metallosupramolecular chemistry is directed to-
Results and Discussion
The synthesis of ligand H2-7 was achieved us-
wards the generation of molecular structures based on ing a methodology developed for the preparation of
non-covalent metal-ligand interactions. Reactions pro- unsymmetrical benzene-o-dithiol/salicylimine ligands
ceeding under self-organization to metallosupramolec- (Scheme 1) [9]. Amine 1 bearing an additional tert-
ular structures have initially been studied for simple butoxycarbonyl (Boc) protected amine function was
molecular architectures. Among the most important obtained by reaction of 4,4ꢀ-diaminodiphenylmethane
metallosupramolecular structures are synthetic heli- with one equivalent of Boc-ON. Reaction of 1 with
cates which can be considered to be models of the nat- the acid chloride 2, which was obtained from 2,3-
ural DNA helices [1]. A large number of polynuclear di(isopropylmercapto)benzoic acid [10] and oxalyl
double- or triple-stranded helicates derived from poly- chloride, gave compound 3. Compound 3 reacts af-
dentate ligands with oligopyridine- [2], catecholato- ter removal of the Boc protection group in a Schiff
[3], imine- [4], and most recently benzene-o-dithiolato base condensation with pyridine-2-carbaldehyde in
donor groups [5] have been described. Studies on methanol to give ligand 5 containing a bidentate
heterometallic helicates are less common [6]. The pyridylimine donor group linked by a spacer to the S,S-
main reason for this situation is the normally com- diisopropyl-protected S-S donor group. All attempts
plicated synthesis of suitable polydentate ligands con- to cleave the S-iPr bonds in 5 with sodium/naph-
taining different donor groups for the selective bind- thalene [10] led to decomposition or to reduction of
ing of different metal ions. We have recently de- the imine function. Crystals of 5 suitable for an X-
scribed the preparation of unsymmetrical benzene-o- ray diffraction analysis have been obtained by evap-
dithiol/catechol ligands [7] which allowed the syn- oration of the solvents from a saturated solution of 5
thesis of triple-stranded heterodinuclear helicates [8]. in dichloromethane/n-hexane(1:1, v:v). The molecular
Based on these results we have prepared unsymmet- structure of 5 is depicted in Fig. 1. All metric parame-
ric ligands containing benzene-o-dithiol/salicylimine ters in 5 are unspectacular and resemble the values for
donor groups and used these for the generation of dinu- equivalent parameters in related compounds [10b, 11].
clear nickel complexes [9]. Here we present the synthe-
From the results described above it becomes ob-
sis of a novel benzene-o-dithiol/pyridylimine(S-S)/(N- vious that the S-iPr bonds of the dimercaptobenzene
N) ligand H2-7 and its reaction with titanocene dichlo- group have to be cleaved before the imine group is
ride leading to complex [Cp2Ti(7)] containing a meta- generated by Schiff base condensation. To this end
lated benzene-o-dithiolato donor group.
compound 4 was reacted with sodium/naphthalene
c
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J. Do¨mer et al. · Benzene-o-dithiol/Pyridylimine Ligand
471
Scheme 1. Synthesis of ligands 5 and H2-7.
tive H2-6 containing an amine substituted spacer. This
amine function can be used to generate the pyridylim-
ine function by reaction with pyridine-2-carbaldehyde
in a Schiff base condensation. Compound H2-6 does
indeed react with pyridine-2-carbaldehyde to give
the unsymmetrical (S-S)/(N-N) ligand H2-7 in good
(81 %) yield (Scheme 1). The free thiol donors in the
precursor H2-6 did not interfere in this reaction.
Ligand H2-7 was reacted with one equiva-
lent of titanocene dichloride in the presence of
NEt3 [12] to give the titanocene complex [Cp2Ti(7)]
in good (86%) yield (Scheme 2). The dark-
green complex [Cp2Ti(7)] is stable in air and
can be purified by column chromatography (SiO2,
dichloromethane/methanol 20 : 1, v : v).
Complex [Cp2Ti(7)] was crystallized by vapor dif-
fusion of n-hexane into a benzene/dichloromethane
(1 : 1, v : v) solution of the complex. The X-ray diffrac-
tion analysis confirmed that only one of the bidentate
Fig. 1. Molecular structure of 5 (hydrogen atoms omitted for
clarity, displacement ellipsoids at the 50 % probability level).
˚
Selected bond lengths (A) and angles (deg): S1–C1 1.781(2),
S2–C2 1.774(2), S1–C27 1.831(3), S2–C30 1.835(2), O1–
C7 1.216(2), N1–C7 1.351(3), N2–C21 1.269(3); C1–S1–
C27 100.37(11), C2–S2–C30 101.51(10).
in THF as we have described previously for related donor groups has been metalated by a titanocene moi-
compounds [10]. Protonation of the initial reaction ety (Fig. 2). Bond lengths and angles in [Cp2Ti(7)]
product with HCl gave the benzene-o-dithiol deriva- fall in the range previously reported for [Cp2Ti(bdt)]
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472
J. Do¨mer et al. · Benzene-o-dithiol/Pyridylimine Ligand
Fig. 2. Molecular structure of [Cp2Ti(7)] (hydrogen atoms
omitted for clarity, displacement ellipsoids at the 50 % prob-
Scheme 2. Synthesis of the titanocene complex [Cp2Ti(7)].
˚
ability level). Selected bond lengths (A) and angles (deg):
Ti–S1 2.405(2), Ti–S2 2.394(2), S1–C1 1.754(5), S2–C2
1.758(5); S1–Ti–S2 82.83(5), C1–S1–Ti 97.8(2), C2–S2–Ti
96.9(2).
complexes (bdt2− = benzene-o-dithiolato anion) [13].
The five-membered C2S2Ti ring is significantly bent
along the S···S vector, a property regularly seen for ti-
tanium complexes with bdt2− ligands [5a, 13, 14], but
uncommon for bdt2− complexes of other transition
metals [10a, 13a, 15]. The selective metalation of the
benzene-o-dithiolato donor group and the presence of
an uncoordinated pyridylimine group was initially sur-
prising as the hard titanium center was not expected
to bind preferably to the soft sulfur donors but in-
stead to the harder nitrogen donors. The reason for
the selective metalation of the thiolato donors is most
likely the neutralization of the charges both at titanium
and sulfur while coordination of the nitrogen donors
AMX 6400 spectrometers. MALDI mass spectra were ob-
tained with a Varian MAT 212 spectrometer. Elemental anal-
yses were performed with a Vario EL III CHNS Elemental
Analyzer at the Institut fu¨r Anorganische und Analytische
Chemie, Westfa¨lische Wilhelms-Universita¨t Mu¨nster. If not
noted otherwise, all preparations were carried out in Schlenk
flasks under an argon atmosphere. Compounds 2 [10], 1 – 4
and H2-6 [9] were prepared according to published proce-
dures.
4-[2,3-Di(isopropylmercapto)benzamido]-4ꢀ -[2-pyridyl-
methyleneamino]diphenyl-methane (5)
2+
would lead to a dicationic {Cp2Ti(N-N)} moiety.
Samples of compound 4 (2.00 g, 4.4 mmol) und pyridine-
2-carbaldehyde (566 mg, 5.3 mmol) were suspended in
methanol (20 mL). The reaction mixture was stirred at ambi-
ent temperature for 12 h with formation of an off-white solid.
The solid was isolated by filtration and dried in vacuo. Yield:
2.0 g (84 %). – 1H NMR (400 MHz, CDCl3, ppm): δ = 8.94
(s, 1 H, NH), 8.65 (m, 1 H, Ar-H), 8.58 (s, 1 H, N=CH),
8.16 – 7.10 (m, 14 H, Ar-H), 3.97 (s, 2 H, CH2), 3.46 (sept,
3J = 6.8 Hz, 1 H, CH(CH3)2), 3.41 (sept, 3J = 6.5 Hz, 1
H, CH(CH3)2), 1.37 (d, 3J = 6.8 Hz, 6 H, CH3), 1.20 (d,
3J = 6.5 Hz, 6 H, CH3). – 13C NMR (100 MHz, CDCl3,
ppm): δ = 166.1 (C=O), 159.8 (C=N), 154.5, 149.5, 148.8,
The selective metalation of the benzene-o-dithiolato
donor observed in the reaction of H2-7 with titanocene
dichloride constitutes a desirable und useful property
which will be important for our ongoing studies con-
cerning the preparation of heterobimetallic complexes
with benzene-o-dithiolato/pyridylimine ligands.
Conclusion
With the benzene-o-dithiol/pyridylimine H2-7 we
present a new directional heterodonor ligand. First
studies of its coordination chemistry with titanocene
derivatives revealed a selective metalation of the 145.7, 142.3, 139.8, 137.0, 136.6, 136.1, 129.6, 129.4, 129.0,
128.8, 128.1, 125.9, 124.9, 121.7, 121.2, 120.2 (Ar-C), 41.1
(C(CH3)3), 40.9 (CH2), 36.1 (C(CH3)3), 23.0 (CH3), 22.6
(CH3). – C32H33N3OS2 (539.8): calcd. C 71.21, H 6.16,
N 7.79, S 11.88; found C 71.25, H 6.37, N 7.67, S 11.68.
benzene-o-dithiolato donor group most likely due to
charge minimization. Ongoing studies are directed
towards using this property for the generation of
heterobimetallic helical complexes with (S-S)/(N-N)
benzene-o-dithiolato/pyridylimine ligands.
4-[2,3-Dimercaptobenzamido]-4ꢀ-[2-pyridylmethylene-
amino]diphenyl-methane (H -7)
Experimental Section
2
Chemicals and solvents were purchased from Aldrich.
NMR spectra were recorded on Bruker AC 200 and 2-carbaldehyde (120 mg, 1.1 mmol) were suspended in
Compound H2-6 (400 mg, 1.1 mmol) and pyridine-
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J. Do¨mer et al. · Benzene-o-dithiol/Pyridylimine Ligand
473
˚
methanol (15 mL). The reaction mixture was stirred at am- (λ = 1.54178 A, for [Cp2Ti(7)]) radiation. Data were col-
bient temperature for 3 d with formation of a yellow solid, lected over the full sphere in the range 3.3 ≤ 2θ ≤ 53◦
which was isolated by filtration, washed with methanol and for 5 and 36.9 ≤ 2θ ≤ 130.0◦ for [Cp2Ti(7)]. Structure so-
dried in vacuo. Yield: 404 mg (81 %). – 1H NMR (200 MHz, lution [16] and refinement [17] were achieved with stan-
[D7]DMF, ppm): δ = 10.45 (s, 1 H, NH), 8.64 (s, 1 H, dard Patterson and Fourier techniques, respectively. All non-
N=CH), 7.88 – 6.66 (m, 15 H, Ar-H), 4.81 (s br, 2H, SH), hydrogen atoms were refined with anisotropic displacement
3.93 (s, 2 H, CH2). – 13C NMR (50 MHz, [D7]DMF, parameters. Hydrogen atoms were added to the structure
ppm): δ = 163.3 (C=O), 162.7 (HC=N), 150.0, 149.9, 138.7, models in calculated positions.
137.5, 137.4, 137.1, 130.6, 130.5, 130.3, 130.1, 129.7, 129.4,
Selected crystallographic data for 5: Formula C32H33-
124.2, 122.2, 121.2, 120.5, 115.9, 115.7, 115.6 (Ar-C), 40.8 N3OS2, M = 539.73, colorless crystal, 0.32 × 0.24 ×
(CH2). – C26H21N3OS2 (455.6): calcd. C 68.54, H 4.65, 0.05 mm3, monoclinic, space group P21/c, Z = 4, a =
◦
˚
N 9.22, S 14.08; found C 68.63, H 4.78, N 9.11, S 13.72.
8.8274(15), b = 24.239(4), c = 13.418(2) A, β = 90.083(4) ,
V = 2881.7(8) A , ρcalcd. = 1.24 g cm−3, µ = 0.2 mm−1
,
3
˚
[Cp Ti(7)]
2
empirical absorption correction (0.9346 ≤ T ≤ 0.9894),
25894 intensities collected ( h, k, l), 5972 independent
(Rint = 0.0426) and 4698 observed intensities [I ≥ 2σ(I)],
347 refined parameters refined against F2, residuals for all
Ligand H2-7 (230 mg, 0.64 mmol) und bis(cyclopenta-
dienyl)titanium dichloride (164 mg, 0.64 mmol) were sus-
pended in THF (20 mL). Triethylamine (0.9 mL) was
added to the reaction mixture which was then stirred
at ambient temperature for 12 h. Subsequently, all sol-
vents were removed in vacuo. A green solid was ob-
tained, composed of [Cp2Ti(7)], triethylammonium chloride,
and excess titanocene dichloride, and out of this mixture
[Cp2Ti(7)] was separated by column chromatography (SiO2,
dichloromethane/methanol, 20 : 1, v : v). Dark-green single
crystals of [Cp2Ti(7)] were obtained by diffusion of n-hexane
into a solution of the complex in benzene/dichloromethane
(1 : 1, v : v). Yield: 350 mg (86 %). – 1H NMR (200 MHz,
CDCl3, ppm): δ = 9.13 (s, 1 H, NH), 8.70 (m, 1 H, Ar-H)
8.61 (s, 1 H, HC=N), 8.21 – 7.18 (m, 14 H, Ar-H), 6.08 (s
br, 10H, C5H5), 3.99 (s, 2 H, CH2). – 13C NMR (50 MHz,
CDCl3): δ = 164.3 (C=O), 159.9 (HC=N), 159.1, 146.6,
140.1, 137.6, 136.8, 136.7, 132.7, 132.1, 129.6, 129.4, 128.8,
126.4, 125.0, 124.7, 123.5, 122.8, 121.9, 121.3, 120.3 (Ar-
C), 113.2 (C5H5), 40.9 (CH2). – C36H29N3OS2Ti (631.6):
calcd. C 68.46, H 4.63, N 6.65, S 10.15; found C 68.26,
H 4.57, N 6.83, S 10.24.
data: R = 0.0673, wR2 = 0.1220; largest−p3eak / hole in last
˚
Difference Fourier map 0.46 / −0.27 e A
.
Selected crystallographic data for [Cp2Ti(7)]: Formula
C36H29N3OS2Ti, M = 631.64, green crystal, 0.16 × 0.06 ×
3
¯
0.04 mm , triclinic, space group P1, Z = 2, a = 10.6495(6),
˚
b = 11.1328(6), c = 14.4977(7) A, α = 112.052(3), β =
◦
3
˚
102.214(4), γ = 100.002(4) , V = 1495.01(14) A , ρcalcd.
=
1.40 g cm−3, µ = 4.0 mm−1, empirical absorption correction
(0.5668 ≤ T ≤ 0.8563), 7748 intensities collected ( h, k,
l), 4526 independent (Rint = 0.0345) and 2874 observed
intensities [I ≥ 2σ(I)], 388 refined parameters against F2,
residuals for all data: R = 0.1007, wR2 = 0.1773; larges−t p3eak
˚
/ hole in last Difference Fourier map: 0.58 / −0.26 e A
.
CCDC 755366 (5) and CCDC 691378 ([Cp2Ti(7)])
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
X-Ray structure determinations for compounds 5 and
Acknowledgement
[Cp Ti(7)]
2
Financial support by the Deutsche Forschungsgemein-
schaft (SFB 424 and IRTG 1444) is gratefully acknowledged.
Diffraction data for 5 and [Cp2Ti(7)] were measured
˚
at 153(2) K using MoKα (λ = 0.71073 A, for 5) or CuKα
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