Synthesis of the ligand N,N′-bis(2-tert-butylthiobenzenylidene)-diethylenetriamine; Its reactivity with nickel(II) salts

Article abstract of DOI:10.1016/S0020-1693(98)00406-X

The synthesis of the Schiff base ligand N,N′-bis(2-tert-butylthiobenzenylidene)-diethylenetriamine (^tBu₂L1) and its reactivity with nickel(II) salts is described. Reaction of ^tBu₂L1 with nickel chloride results in two different complexes, depending on the temperature of the reaction mixture. The green paramagnetic five-coordinate complex [Ni(^tBu₂L1)Cl₂] forms on reaction at room temperature, whereas at reflux temperature the red diamagnetic [Ni(L1*)]Cl · H₂O is formed. In the ligand L1* one of the imine groups is hydrolysed, and the protecting tert-butyl group of the remaining thiobenzylidene is lost, resulting in a new tetradentate N₃S thiolato ligand. [Ni(L1*)]Cl · H₂O crystallises in the monoclinic space group P2₁/c, with Ni-N distances of 1.860(7), 1.941(8) and 1.914(7) A and a Ni-S distance of 2.131(3) A.

Full text of DOI:10.1016/S0020-1693(98)00406-X

Inorganica Chimica Acta 287 (1999) 105–108
Note
Synthesis of the ligand
N,N%-bis(2-tert-butylthiobenzenylidene)-diethylenetriamine;
its reactivity with nickel(II) salts
Elisabeth Bouwman a,*, Richard K. Henderson a, Jan Reedijk a,
w
Nora Veldman b, , Anthony L. Spek b
a Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden Uni6ersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b Bij6oet Centre for Biomolecular Research, Crystal and Structural Chemistry, Utrecht Uni6ersity, Padualaan 8,
3584 CH Utrecht, The Netherlands
Received 17 July 1998; accepted 3 November 1998
Abstract
The synthesis of the Schiff base ligand N,N%-bis(2-tert-butylthiobenzenylidene)-diethylenetriamine (tBu2L1) and its reactivity
t
with nickel(II) salts is described. Reaction of Bu2L1 with nickel chloride results in two different complexes, depending on the
temperature of the reaction mixture. The green paramagnetic five-coordinate complex [Ni(tBu2L1)Cl2] forms on reaction at room
temperature, whereas at reflux temperature the red diamagnetic [Ni(L1*)]Cl · H2O is formed. In the ligand L1* one of the imine
groups is hydrolysed, and the protecting tert-butyl group of the remaining thiobenzylidene is lost, resulting in a new tetradentate
N3S thiolato ligand. [Ni(L1*)]Cl · H2O crystallises in the monoclinic space group P21/c, with Ni–N distances of 1.860(7), 1.941(8)
˚
˚
and 1.914(7) A and a Ni–S distance of 2.131(3) A. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Nickel complexes; Five-coordinate complexes; Schiff base complexes
1. Introduction
uclear Ni–Fe active site of hydrogenases. It was
decided that ligands containing mixed N/S donor
groups would be used preferentially to provide greater
synthetic control. Previous work resulted in mononu-
clear compounds with square planar cis NiN2S2 chro-
mophores [4,5]. However, reaction of these complexes
with suitable iron complexes did not result in the
desired Ni–Fe heterodinuclear compounds; therefore,
several new ligands were designed and synthesised.
This has resulted in an interesting dinuclear iron com-
pound as a good structural model for Fe-only hydro-
genases [6]. A schematic drawing of the target ligand
described in this paper is given in Fig. 1. Loss of the
protecting tertiary butyl groups upon complexation
with nickel salts should lead to a five-coordinated
nickel complex bound to two imino-nitrogens, one sec-
ondary amine and two thiolato-sulfur donors, which
then would be available for bridging to a second metal
ion.
The structural characterisation of the active site of
the [Ni–Fe] hydrogenase enzyme, extracted from
Desulfo6ibrio Gigas [1], by which the catalytic site was
revealed to contain a heterodimetallic nickel–iron clus-
ter in a sulfur rich environment, has renewed interest
in the chemical modelling of [Ni–Fe] hydrogenases. A
large amount of modelling chemistry had already been
reported prior to the publication of the X-ray struc-
ture [2,3], when it was believed that the active site
contained mononuclear nickel.
The compounds described in this paper were synthe-
sised as part of a research in modelling the heterodin-
* Corresponding author. Tel.: +31-71-527 4550; fax: +31-71-527
4551.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman)
w Deceased.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00406-X

106
E. Bouwman et al. / Inorganica Chimica Acta 287 (1999) 105–108
bright green precipitate began to form. The precipitate
was collected on microfine filter paper (1.12 g, 85% yield
based on NiCl2 · 6H2O). Anal. Calc. for C26H37Cl2N3-
NiS2: C, 53.35; H, 6.37; N, 7.18; Ni, 10.0; S, 10.96.
Found: C, 53.23; H, 6.55; N, 7.07; Ni, 10.0; S, 10.6%. IR
(KBr) w (cm−1): 3266 (w), 2960 (m), 2929 (m), 2863 (m),
1636 (vs), 1584 (s), 1458 (s), 1434 (m), 1423 (w), 1385 (s),
1363 (s), 1328 (w), 1292 (w), 1274 (w), 1206 (w), 1166
(vs), 1126 (m), 1104 (m), 1062 (w), 1044 (m), 1032 (m),
972 (w), 955 (m), 929 (vs), 849 (w), 817 (w), 764 (vs), 700
(w), 627 (w), 572 (w), 500 (m), 444 (w). Vis-NIR
(solid-state reflectance, D2 lamp): bands at 5.8×103,
Fig. 1. The protected ligand N,N%-bis(2-tert-butylthiobenzenylidene)-
diethylenetriamine.
2. Experimental
2.1. Materials
Starting materials were obtained from Aldrich. All
procedures were performed in a nitrogen or argon
atmosphere using solvents that were degassed on a
vacuum system prior to use. 2-Tert-butyl-thiobenzalde-
hyde was prepared according to a literature procedure
[7].
8.8×103, 15.4×103, and 24.9×103 cm−1
.
2.4.2. [Ni(L1*)]Cl · H2O
The yellow oil of ligand N,N%-bis(2-tert-butylthioben-
zenylidene)-diethylenetriamine was dissolved in 10 ml
ethanol (98%) and a solution of NiCl2 · 6H2O (0.61 g,
2.58 mmol) in 10 ml ethanol (98%) was added. The
mixture was heated under reflux conditions for 1 h.
After 5 min a bright green precipitate formed, but this
redissolved after half an hour of further heating, during
which time the solution turned a deep red colour. The
solution was left to cool and on slow evaporation, red
crystals of [Ni(L1*)]Cl · H2O were formed (0.53 g, 63%
yield based on NiCl2 · 6H2O). Anal. Calc. for
C11H18ClN3NiOS: C, 39.50; H, 5.42; N, 12.56; S, 9.59
Found: C, 39.57; H, 5.40; N, 12.21; S, 9.75%. IR (KBr)
w (cm−1): 3425 (m), 3363 (m), 3212 (m), 3157 (m), 1608
(vs), 1587 (s), 1535 (s), 1462 (m), 1428 (w), 1409 (m),
1325 (w), 1250 (m), 1220 (m), 1190 (s), 1128 (w), 1096
(m), 1058 (m), 1050 (s), 981 (s), 964 (m), 879 (w), 828
(w), 768 (s), 722 (w), 645 (w), 587(m), 494 (w), 448 (w).
Vis-NIR (solid-state reflectance, D2 lamp): bands at
2.2. Physical methods
Infrared spectra (KBr pellets) were recorded in the
range 4000–400 cm−1 using a Perkin–Elmer FT-IR
Paragon spectrophotometer controlled by a PC using
PE Grams analyst software. Nuclear magnetic reso-
nance spectra were recorded on a Bruker dpx300 MHz
spectrometer. Ligand field spectra of the solids (300–
2000 nm, diffuse reflectance) were taken on a Perkin–
Elmer 330 spectrophotometer equipped with a data
station. Microanalysis measurements were performed at
the Microanalytical Laboratory of the University Col-
lege, Dublin.
2.3. Ligand synthesis
1
23.4×103 and 18.9×103 cm−1. H NMR (CD3OD) l
To a solution of 2-tert-butyl-thiobenzaldehyde (1.0 g,
5.15 mmol) in 40 ml ethanol (98%) was added a solution
of diethylenetriamine (0.28 ml, 2.58 mmol) in 20 ml
ethanol (98%) and the mixture was heated under reflux
for 1 h in the presence of CaSO4 as a drying agent. The
resultant bright yellow solution was dried over Na2SO4,
filtered and the solvent evaporated, leaving a yellow oil
(tBu2L1). The yellow oil could only be isolated in a
crude form, and was used for further reactions in this
(ppm)=8.14 (1H, s, HC=N), 7.64 (1H, d, J=8.1 Hz),
7.46 (1H, dd, J=8.0 and 1.3 Hz), 7.23 (1H, td, J=7.2
and 1.5 Hz), 7.04 (1H, td, J=8.0 and 1.1 Hz), 4.00–
3.87 (2H, m), 3.1–2.5 (6H, several multiplets).
This type of complex, [Ni(L1*)]X, can be synthesised
using different nickel salts (X=ClO4, BF4, and with
PF6 after ion exchange). [Ni(L1*)](BF4): Anal. Calc. for
C11H16BF4N3NiS: C, 35.92; H, 4.38; N, 11.42; S, 8.72.
Found: C, 35.69; H, 4.35; N, 11.01; S, 8.94%.
[Ni(L1*)](PF6): Anal. Calc. for C11H16F6N3NiPS: C,
31.0; H, 3.8; N, 9.9; S, 7.5. Found: C, 30.98; H, 3.73; N,
9.87; S, 7.43%.
1
form. H NMR (CDCl3) l=9.11 (2H, s, HCꢀN), 8.01
(2H, m), 7.53 (2H, m), 7.38 (4H, m), 3.80 (4H, t, CH2),
t
3.02 (4H, t, CH2), 1.24 (18H, s, Bu).
2.4. Preparation of the complexes
2.5. Crystal structure determination
2.4.1. [Ni(tBu2L1)Cl2]
2.5.1. Crystal data
The yellow oil of crude ligand N,N%-bis(2-tert-butyl-
thiobenzenylidene)-diethylenetriamine was dissolved in
10 ml ethanol (98%) and a solution of NiCl2 · 6H2O
(0.61 g, 2.58 mmol) in 10 ml ethanol (98%) was added.
The mixture was stirred for 10 min during which time a
C11H18ClN3NiOS, M=334.5, monoclinic space
group P21/c, a=11.285(2), b=12.980(2), c=10.092(2)
3
˚
˚
A, i=110.356(12)°, V=1386.0(4) A , Z=4, Dcalc
=
1.603 g cm−3, T=150 K, v=17.3 cm−1, F(000)=
˚
696.0, Mo Ka radiation, u=0.71073 A.

E. Bouwman et al. / Inorganica Chimica Acta 287 (1999) 105–108
107
2.5.2. Data collection and refinement
[Ni(tBu2L1)Cl2]. Assuming that the unprotected
thioether groups are too weak donors to be coordinated
to the nickel ion, a five-coordinate environment consist-
ing of three nitrogen donors from the ligand and two
chloride ions is anticipated. The bulkiness of the pen-
dant 2-tert-butylbenzenylidene groups would thereby
prevent the formation of an octahedral coordination
geometry. Several structures of five-coordinate nickel
ions with sterically demanding tridentate ligands have
been reported [14,16].
An orange crystal of [Ni(C11H16N3S)]Cl · H2O suit-
able for X-ray structure determination was mounted on
a Lindemann-glass capillary and transferred into the
cold nitrogen stream on an Enraf–Nonius CAD4-T
diffractometer on rotating anode. The data were col-
lected using Mo Ka radiation, with
a graphite
˚
monochromator (u=0.71073 A). Accurate unit cell
parameters and an orientation matrix were determined
by least-squares refinement of the setting angles of 25
well-centered reflections (SET4) in the range 8.3BqB
15.2°. Data were collected in ꢀ–2q scan mode with
scan angle Dꢀ=1.18+0.35 tan q°. Intensity data of
4373 reflections were measured, of which 2437 were
independent (Rint=0.10). Data were corrected with the
DELABS option in PLATON [8]. The structure was solved
with automated Patterson techniques (DIRDIF92 [9])
and refined on F2 using SHELXL93 [10]. Non-hydrogen
atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were included in the refin-
ement on calculated positions, riding on their carrier
atoms except those on H2O that were located from a
difference map and refined freely. Isotropic displace-
ment parameters for hydrogen atoms were fixed on
values related to the equivalent isotropic displacement
parameter of the atom they are attached to. Conver-
On repeating the synthesis but this time heating the
mixture under reflux conditions in order to remove the
protecting tertiary butyl groups, the same green precip-
itate was observed initially, but after half an hour of
heating, the precipitate had redissolved and a red solu-
tion had formed. On cooling and after slow evapora-
tion of the solvent, red crystals of [Ni(L1*)]Cl · H2O
were formed. The infrared spectra of this complex
showed that the tertiary butyl groups had indeed been
removed. In addition, broad peaks in the 3500–3100
cm−1 region of the infrared spectrum, indicative of OH
or NH groups were apparent. The red complex is
diamagnetic and its NMR spectrum in MeOD was
recorded, which confirmed the absence of tert-butyl
groups, but which showed the presence of only one
aromatic group per C4 backbone. Single crystal diffrac-
tion studies showed that the ligand was partially de-
composed, with one of the imine groups being
hydrolysed. A projection of the crystal structure of
[Ni(L1*)]Cl · H2O is given in Fig. 2, and selected bond
distances and angles are given in Table 1. The structure
shows that the nickel ion is in a square planar environ-
ment, formed by a primary and a secondary amine, an
imino nitrogen and a thiolate group. The bond dis-
tances and angles are in the normal range for low-spin
divalent nickel. The bond distances in the aliphatic
chain of the ligand are somewhat shortened, due to
gence was reached at wR2=0.1416, w−1=|2(Fo )+
2
(0.0318P)2,
where
P=(max(Fo , 0)+2Fc )/3,
2
2
R1=0.0751 for 1230 Fo\2|(Fo), S=1.02 for 169
parameters. No residual density was found outside
−0.50 and 0.53 e A−3. Neutral atom scattering factors
˚
and anomalous dispersion corrections were taken from
the International Tables for Crystallography [11]. Geo-
metrical calculations and illustrations were performed
with PLATON [8].
3. Results and discussion
t
The ligand Bu2L1 was synthesised and reacted with
nickel salts in an attempt to form a five-coordinate
species which contained two cis thiolato groups, as has
been reported for a similar ligand with aliphatic thiolate
groups [12]. The thiolate groups in L1 would be formed
by deprotection of the thioether groups as has been
observed in the synthesis of [N,N%-bis(2-thiobenzenyli-
dene)ethylenediaminato]nickel(II) [4,5]. On mixing an
alcoholic solution of nickel chloride and the ligand
tBu2L1, a bright green precipitate formed and was
collected. The infrared spectrum of this compound
suggests that the tertiary butyl groups are still present.
No NMR spectrum could be recorded of this complex
because of paramagnetism. The ligand field data of this
compound show that the nickel ion probably is in
a distorted square pyramidal environment [13–15].
Fig. 2. A PLUTON projection of the compound [Ni(L1*)]Cl · H2O.
Microanalysis data suggest
a
formulation of

108
E. Bouwman et al. / Inorganica Chimica Acta 287 (1999) 105–108
Table 1
˚
Selected bond lengths (A) and angles (°) for [Ni(L1*)]Cl · H2O
Ni(1)–S(10)
Ni(1)–N(18)
2.131(3)
1.860(7)
Ni(1)–N(21)
Ni(1)–N(24)
1.941(8)
1.914(7)
S(10)–Ni(1)–N(18)
98.5(2)
N(18)–Ni(1)–N(21)
86.1(3)
S(10)–Ni(1)–N(21) 175.2(2)
N(18)–Ni(1)–N(24) 171.1(3)
N(21)–Ni(1)–N(24) 86.3(3)
S(10)–Ni(1)–N(24)
89.2(2)
Table 2
˚
Hydrogen bond distances (A) and angles (°) in [Ni(L1*)]Cl · H2O
D–H···A D···A D–H H···A D–H···A
N(24)–H(24a)–O(30) 2.895(10) 0.92(1)
N(24)–H(24b)–Cl(2) 3.280(7)
O(30)–H(31)–Cl(2)a 3.166(7)
O(30)–H(32)–Cl(2)b 3.208(7)
Fig. 3. An impression of the two-dimensional hydrogen bonding
network in [Ni(L1*)]Cl · H2O.
2.005(10) 162.4(8)
0.921(9) 2.385(7) 164.1(8)
0.98(2)
0.98(8)
2.21(4)
2.27(8)
164(9)
160(7)
from the European Union. The research of E.B. has
been made possible by a Fellowship of the Royal
Netherlands Academy of Arts and Sciences. The work
was in part (N.V. and A.L.S.) supported by The
Netherlands Foundation of Chemical Research (SON)
with financial aid from the Netherlands Foundation for
Scientific Research (NWO).
a Symmetry operations: x, 1/2−y, −1/2+z.
b Symmetry operations: 1−x, −1/2+y, 1/2−z.
positional disorder (puckering), resulting in a rather
high R-value. However, no attempt was made to calcu-
late a disorder model. The thermal motion observed in
the aliphatic chain explains the complex splitting pat-
terns observed in the NMR spectra of this compound.
The hydrogen atoms originating from the water
molecule and the primary amine are all involved in an
extended hydrogen bonding network. The distances and
angles in these hydrogen bonds are given in Table 2.
The non-coordinated chloride ion is held in its position
by three H-bonds, two from the water molecule, and
one from the primary amine. An impression of the
two-dimensional network which is formed by the hy-
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Acknowledgements
Financial support for this project has been provided
by a Biotech grant (Contract no. BIO2-CT94-2041)