New sulfur-oxygen mixed-donor ligand N,N'-dimethyl-piperazine-3-oxo-2- thione (Me2pipto) and its Ni(ii) and Fe(ii) complexes

Article abstract of DOI:10.1039/c0dt00803f

Structural and electronic features of the novel title sulfur-oxygen donor ligand Me₂pipto (1), are discussed in comparison with those of the corresponding dithione ligand Me₂pipdt. A tuning of the electronic and coordination properties of the ligand, relatable to the soft/hard power of the donor atoms, is achieved. Coordination properties have been checked towards Ni(ii) and Fe(iii) cations obtaining [Ni(Me₂pipto) ₃](BF₄)₂ (2) and [Fe(Me₂pipto) ₃](BF₄)₂ (4) complexes, which show significant differences when compared with the corresponding reaction products when using Me₂pipdt.

Full text of DOI:10.1039/c0dt00803f

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New sulfur-oxygen mixed-donor ligand N,N’-dimethyl-piperazine-
3-oxo-2-thione (Me2pipto) and its Ni(II) and Fe(II) complexes†
Luca Pilia,a Flavia Artizzu,a Davide Espa,a Luciano Marchio`,b Maria Laura Mercuri,a Angela Serpea and
Paola Deplano*a
Received 29th March 2010, Accepted 15th July 2010
First published as an Advance Article on the web 6th August 2010
DOI: 10.1039/c0dt00803f
6,8
Structural and electronic features of the novel title sulfur-
oxygen donor ligand Me2pipto (1), are discussed in com-
parison with those of the corresponding dithione ligand
Me2pipdt. A tuning of the electronic and coordination
properties of the ligand, relatable to the soft/hard power
of the donor atoms, is achieved. Coordination properties
have been checked towards Ni(II) and Fe(III) cations ob-
taining [Ni(Me2pipto)3](BF4)2 (2) and [Fe(Me2pipto)3](BF4)2
(4) complexes, which show significant differences when com-
pared with the corresponding reaction products when using
Me2pipdt.
(CO)4 respectively, has been discussed on the basis of metrical
parameters and theoretical calculations.
With the aim to tune the electronic structure, it seemed
interesting to us to investigate the influence of varying the donor
atom (O vs. S) on coordination and electrodonating/withdrawing
properties of R2pipdt. Here we report the synthesis and charac-
terization of N,N’-dimethyl-piperazine-3-oxo-2-thione (Me2pipto,
1)† where one O atom substitutes one sulfur in Me2pipdt. The X-
ray structure‡ of Me2pipto is reported in Fig. 1a. The short C–S
(1.642(4) A) and C–O (1.218(5) A) bond distances are consistent
with a double bond character. On the other hand, the trigonal
planar geometry of the nitrogen atoms, the C(1)–C(2) long dis-
tance (1.538(6)), and the relatively short C–N distances (1.348(5)
˚
˚
Due to the softness of the donor atoms and of the chelating
properties of two vicinal sulfur atoms in the thioamidic groups, the
ligand N,N’-dialkyl-piperazine-2-,3-dithione (R2pipdt) has been
shown to possess remarkable coordinative properties towards nd8
transition metal ions. This capability has been employed to favour
the dissolution of the crude metals, including the noble ones
Pd and Pt, by using mild oxidation agents such as diiodine or
triiodide.1 The reaction products are salts, iodides-triiodides, of
[M(R2pipdt)2]2+ dications consisting of square-planar complexes.
These dications, which can also be obtained by reacting a salt
of the metal ion with R2pipdt, show four reversible reduction
processes and are the first example of positively charged metal-
dithiolenes obtained by chemical methods.2 These compounds
have been employed as precursors to prepare mixed-ligand
dithiolenes, and some of them show second-order nonlinear
optical (NLO) properties at the molecular level3 of interest for
optoelectronic applications.4 Optical properties are relatable to the
electrodonating/withdrawing properties of the two ligands as well
as the p-delocalization involving the dithiolene core (p-localised
with a dithione-dithiolate description or p-delocalized dithiolene
description for the two ligands).5 Potential as a second-order NLO
chromophore has been also observed in (Me2pipdt)Mo(CO)4.6
A possible dithiolate or dithione behaviour of dithio-oxamide
˚
and 1.361(5) A for C(1)–N(2) and C(2)–N(1), respectively) suggest
a certain degree of delocalization of the double bonds over the
nitrogen atoms. This type of structure exhibits similar features to
analogous sulfur or oxygen donor ligands previously reported.9,10
The cyclic voltammetry measurements§ performed on Me2pipdt
ligand in CH3CN solution, show an irreversible oxidation process
at +0.94 V and a quasi-reversible reduction one at -1.42 V.
EPR spectroelectrochemical studies performed on this ligand,
show that the unpaired electron of the reduced species is highly
delocalized on the p-LUMO orbital.11 In the case of Me2pipto,
both oxidation (+0.94 V) and reduction processes are irreversible,
and the reduction wave falls at very negative potential (-1.92 V).
Electronic structure DFT-calculations on Me2pipto and Me2pipdt
7
ligands in (R4N2C2S2)Mo(CO)2(PR3)2 or in (R4N2C2S2)Mo-
aDipartimento di Chimica Inorganica ed Analitica, Universita` di Cagliari, I-
09042, Monserrato, Italy. E-mail: deplano@unica.it; Fax: +39 0706754456;
Tel: +39 0706754456
bDipartimento di Chimica Generale ed Inorganica, Chimica Analitica,
Chimica Fisica, Universita` di Parma, Parco Area delle Scienze 17A, I-43100,
Parma, Italy
† Electronic supplementary information (ESI) available: Synthesis and
characterizations of 1, 2 and 4, Fig. S1–S2, Scheme S1 and RX data
and selected bond lengths and angles (Tables S3-S6), calculated relative
energies and selected bond distances (Table S1) and Mulliken spin
densities (Table S2). CCDC reference numbers 771140–771142. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00803f
Fig. 1 (a) Molecular drawing of Me2pipto with thermal ellipsoids at
the 30% probability level. (b) Diagram of the relative energy levels and a
drawing of the HOMO and LUMO for Me2pito and Me2pidt. Energies in
hartrees and isosurface contour value of 0.04 au are reported.
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for comparison¶ have been performed. The obtained frontier
orbitals (FO), which are energetically well separated from the
lower and higher lying orbitals, are reported in Fig. 1b and
Fig. S1†. As shown, the composition and energy of the FO in
the two ligands are different, and relatable to the low contribution
of the oxygen orbitals in Me2pipto. Accordingly, differences in
the electrochemical properties can be explained. In particular, the
higher p-delocalization and lower energy of the LUMO in the
Me2pipdt compared to the corresponding orbital in Me2pipto, are
in agreement with observed reduction processes: quasi-reversible
wave and lower potential for the former and irreversible for the
latter. Moreover, a different behaviour in the coordination with
metal-ions is predictable. In fact, by reacting Me2pipto with a
nickel salt, the pseudo-octahedral complex [Ni(Me2pipto)3](BF4)2
ligand compared to the dithione one, due to the reduced p-acceptor
capability of the former, explains the different stoichiometries of
the Ni complexes. In particular, in these cases, the p-back-donation
from the metal d-orbitals to the sulfur’s empty d-orbitals seems to
play a crucial role. This is in agreement with the fac configuration
of the complex, in fact, each sulfur is located trans to an oxygen
atom, which can not compete for the metal’s electrons. Further
work to investigate the magnetic properties of this compound,
as well as its potential to work as a precursor for mixed-ligand
oxo-thione-dithiolate complexes, is foreseen.
In addition, to further investigate the potential of this ligand
in tuning the properties of selected metal complexes due to the
soft/hard character of the donor atoms we have allowed Me2pipto
and Me2pipdt to react with FeCl3 in CH3CN solution, under
the same experimental conditions. The obtained products are:
[Fe(Me2pipto)2Cl2](BF4) and [Fe(Me2pipdt)3](BF4)2 (3).12 Using
the sulfate-salt as the source of Fe(III) to avoid the presence of
the more coordinating Cl- anion, the [Fe(Me2pipto)3](BF4)2 (4)
complex was obtained. The crystal structure of 4 is shown in
Fig. 3.‡ Three bidentate SO donor ligands define the octahedral
geometry of the metal. The complex lies on a binary axis so
that one ligand is found disordered in two equivalent positions.
For both of the images, the disposition of the sulfur and oxygen
atoms is such that the complex exhibits a mer configuration. The
considerations of the ligand in the nickel complex (e.g. bond
distances reflected by related vibrational markers) also apply in
the iron case.
(2) has been obtained in spite of the square-planar complex
9a,b
[Ni(Me2pipdt)2](BF4)2
obtained working at the same experi-
mental conditions with Me2pipdt. The molecular structure of the
cation in 2 is shown in Fig. 2.‡ The metal exhibits an octahedral
geometry achieved by three bidentate SO donor ligands. The
disposition of the sulfur and oxygen atoms is such that the complex
is in the fac configuration. The C–S and C–O bond distances
˚
˚
are in the range 1.669(2)–1.676(2) A and 1.241(2)–1.246(2) A,
respectively, suggesting a double bond character for these moieties.
Nevertheless, the planar nature of the thioamide-like or amide-like
functions within each ligand and the short C–N bonds of these
˚
groups (range 1.308(2)–1.321(2) A) point to a certain amount
of electron delocalization over the NCS or NCO atoms. As a
consequence of the coordination, a decrease (increase) of CO and
CS (CN) double bond character is observed with respect to the free
ligand (see Scheme S1†). This is reflected by the shift of the n(CO)
and n(CN) stretching vibrations to lower (1642 cm-1) and higher
(1559 cm-1) frequencies, respectively, with respect to the free ligand
(1669 and 1527 cm-1)ꢀ. The smaller stabilization energy of the SO
Fig. 3 Molecular drawing of 4 with thermal ellipsoids at the 30% proba-
-
bility level; the BF4 anions have been removed for clarity. Selected bond
˚
lengths (A): Fe–S(11) 2.465(2), Fe–O(21) 2.072(3), C(11)–S(11) 1.654(4),
C(21)–O(21) 1.222(4), C(11)–C(21) 1.493(5), C(11)–N(21) 1.288(5). Sym-
1
2
metry code ’ = 1 - x, y, - z.
Interestingly, the iron ion is reduced to the 2+ oxidation state
and preliminary magnetic measurements on these complexes show
Fig. 2 Molecular drawing of 2 with thermal ellipsoids at the 30%
probability level; the BF4 counter-ions have been removed for clarity.
-
8140 | Dalton Trans., 2010, 39, 8139–8142
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that, in the 400–5 K range, Fe(II) exhibits a high spin state in 4,
but low-spin magnetic behaviour in [Fe(Me2pipdt)3](BF4)2 (3). The
different magnetic behaviour can be explained by the difference in
the p-acceptor capability of the ligands. In fact, in the case of the
dithione the p-back-donation, which stabilizes the t2g orbitals of
the metal ion, is stronger than for the SO ligand, increasing the
10Dq separation energy between the d-orbitals and consequently
stabilizing the low-spin state. To support this, a DFT study on
complexes 3 and 4 was performed. In addition to the hybrid B3LYP
functional, the pure OLYP functional was used due to its good
performances in predicting the ground states of a number of iron
complexes.13,14 Accordingly, the geometries were optimized with
the OLYP functional for the singlet (S = 0) and quintet (S = 2) states
(see ESI†). From a comparison of the experimental and calculated
bond distances, the preferred ground states for complexes 4 and 3
are the S = 2 and S = 0, respectively. With the OLYP functional,
[Fe(Me2pipto)3]2+ exhibits an energy preference for the S = 2 state
over the S = 0 one of 60.14 kJ mol-1, whereas [Fe(Me2pipdt)3]2+
favours the S = 0 state over the S = 2 one by 31.88 kJ mol-1.
The spin density of 3 in the quintet state is located primarily on
the iron atom and to a minor extent over the sulfur and oxygen
atoms, Fig. 4. A different scenario is encountered when using the
B3LYP functional, since it decisively favours the S = 2 state for
both compounds. This is nevertheless in line with the property
of this density functional, which tends to overstabilize high spin
states. In order to obtain compounds that exhibit spin transition,
experiments to prepare mixed iron complexes of these ligands in
the 2 : 1 and 1 : 2 molar ratio are in progress. It can be anticipated
that [Fe(Me2pipto)2Cl2](BF4) will show high/low spin transition
at low temperature.
of Me2pipto and Me2pipdt ligands and in their coordination
properties towards metal ions, as shown in the cases reported
here: high spin vs. low spin in the iron(II) case, and octahedral vs.
square planar coordination in the nickel case.
Moreover the variation of the donor atom (O vs. S) will
allow evaluation of the influence of charge-separated character
by comparing the NLO-properties of mixed-ligand complexes of
the Nickel-triad15 based on dithiolate/R2pipdt or R2pipto ligands.
The authors are grateful to Prof. G. Concas, Dr F. Congiu and
Dr S. Sanna of the Dipartimento di Fisica, Universita` di Cagliari,
for the magnetic measurements on iron complexes.
This work has been supported by Universita` di Cagliari and
by COST Action D35, WG11 “Multifunctional and Switchable
Molecular Materials”.
Notes and references
‡ Crystal data for 1: C6H10N2OS, M = 158.22, orthorhombic, space group
3
˚
˚
Pna21, a = 9.758(4), b = 6.731(3), c = 11.686(6) A, V = 767.5(6) A , T =
293(2) K, Z = 4, 1067 measured reflections, 962 unique (Rint = 0.0320). R1 =
0.0431 (I > 2s(I)), wR2 = 0.0857(all data); CCDC 771140. Crystal data for
2: C18H30B2F8N6NiO3S3, M = 706.99, monoclinic, space group P21/n, a =
◦
˚
12.541(1), b = 12.255(1), c = 19.659(1) A, b = 101.198(3) , V = 2963.9(4)
3
˚
A , T = 293(2) K, Z = 4, 49228 measured reflections, 9916 unique (Rint
=
0.0355). R1 = 0.0397(I > 2s(I)), wR2 = 0.1226(all data); CCDC 771141.
Crystal data for 4: C18H30B2F8FeN6O3S3, M = 704.13, monoclinic,◦space
˚
group C2/c, a = 20.40(1), b = 10.992(8), c = 16.20(1) A, b = 129.60(2) , V =
3
˚
2799(3) A , T = 293(2) K, Z = 4, 13694 measured reflections, 2731 unique
(Rint = 0.0396). R1 = 0.0547(I > 2s(I)), wR2 = 0.1702(all data); CCDC
771142.
§ The cyclic voltammetry measurements were performed in CH3CN
solution, with Ag/AgCl 3 M as the reference electrode and fer-
rocene/ferrocenium couple (0.43 V in these conditions) as an internal
standard.
¶ The results on Me2pipdt are in agreement with those reported in ref. 6.
ꢀ Because of the presence of a strong broad peak centered near 1050 cm-1
-
for the BF4 anion, the peak of the n(CS) stretching vibration can not be
observed.
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