Syntheses and structural studies of heterobimetallic thiocarboxylate complexes containing zinc and copper

Article abstract of DOI:10.1016/j.ica.2012.11.022

Anionic zinc(II) complexes, [Zn(SCOR)₃]^- [R = th (1)] have been used for the synthesis of heterobimetallic complexes, [(PPh ₃)Cu(μ-SCOR)₃Zn(PPh₃)] [R = Ph (2) and th (3)]. All the three complexes have been characterized by FTIR, ¹H and ¹³C NMR spectroscopy. The synthetic reactions involved phosphine migration from Cu(I) to Zn(II) in the cases of 2 and 3. Molecular structures of the complexes have been determined by single crystal X-ray diffraction and the structural features have been explained on the basis of NBO calculations. The Cu-Zn distance in these molecules are shorter than the sum of their covalent radii indicating the existence of a bond between the metals. Electronic spectral behavior of the complexes have been explained by TDDFT calculations. Luminescence properties of 3 has also been studied in solid state.

Full text of DOI:10.1016/j.ica.2012.11.022

Inorganica Chimica Acta 396 (2013) 6–9
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Inorganica Chimica Acta
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Note
Syntheses and structural studies of heterobimetallic thiocarboxylate complexes
containing zinc and copper
Suryabhan Singh, Jyotsna Chaturvedi 1, A.S. Aditya, N. Rajasekhar Reddy, Subrato Bhattacharya
⇑
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Anionic zinc(II) complexes, [Zn(SCOR)3]À [R = th (1)] have been used for the synthesis of heterobimetallic
complexes, [(PPh3)Cu( -SCOR)3Zn(PPh3)] [R = Ph (2) and th (3)]. All the three complexes have been char-
Article history:
Received 26 October 2012
Received in revised form 23 November 2012
Accepted 26 November 2012
Available online 3 December 2012
l
acterized by FTIR, 1H and 13C NMR spectroscopy. The synthetic reactions involved phosphine migration
from Cu(I) to Zn(II) in the cases of 2 and 3. Molecular structures of the complexes have been determined
by single crystal X-ray diffraction and the structural features have been explained on the basis of NBO
calculations. The Cu–Zn distance in these molecules are shorter than the sum of their covalent radii indi-
cating the existence of a bond between the metals. Electronic spectral behavior of the complexes have
been explained by TDDFT calculations. Luminescence properties of 3 has also been studied in solid state.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Zinc
Copper
Thiocarboxylate
Heterobimetallic
Density functional calculations
For more than past two decades heterobimetallic complexes
have been of interest as they find applications in different fields
of research and development because of the fact that the different
metal centers could exhibit cooperative behavior [1,2]. Apart from
the tuned reactivities such heterobimetallic systems have been
found extremely useful as precursors of electronic and spintronic
materials [3–6]. During recent years, heterobimetallic complexes
containing sulfur/selenium ligands are being explored to prepare
ternary metal sulfides/selenides by energy efficient soft chemical
routes [7].
Complex anions, tris(thiocarboxylato)zincate(II) were used as
the precursors for all the complexes reported here. The tris(thio-
phen-2-thiocarboxylato)zincate(II) (1) was however, isolated by
the cation exchange of sodium tris(thiophene-2-thiocarboxyla-
to)zincate(II) with tetraphenylphosphonium bromide. The com-
plexes 2 and 3 were synthesized by reacting the nitrate salt of
bis(triphenylphosphine)copper(I) with tris(thiobenzoato)zincate
and tris(thiophene-2-thiocarboxylato)zincate complexes, respec-
tively (Scheme 1).
The formation of complexes 2 and 3 is quite interesting as there
is migration of triphenylphosphine ligand from Cu(I) to Zn(II). It is
also worth mentioning here that a number of Zn(II)/Cu(II) com-
plexes are reported in the literature but Zn(II)/Cu(I) complexes
are uncommon and there are only a few such compounds that
contain a sulfur ligand. To the best of our knowledge the only
structurally characterized Zn(II)/Cu(I) complexes are those in
which a (N-donor) macrocyclic complex of Zn(II) is bounded to
Cu(I) [15,16] through the sulfur atoms present in the pendent arms
of the macrocyclic ligand.
Complex 1 crystallized in trigonal system with P31 space group
(Table S1, Supporting information). The molecular structure is de-
picted in Fig. 1. It has discrete cations and anions in the lattice. One
of the phenyl rings of the tetraphenylphosphonium cation and all
the three thiophene rings of the thiocarboxylate ligand are disor-
dered. As there is no unusual feature in the cationic part its struc-
tural details will not be discussed further. At a glance the structure
of the anion is similar to that of the reported complex,
[Zn(SCOPh)3]À [11], however, a closer look revealed a few signifi-
cant differences between the two. In the anionic thiobenzoate
Very recently, we have reported the synthesis and structures of
two Cd/Cu heterobimetallic thiocarboxylate complexes, (PPh3)2
Cu(l-SCOR)2Cd(SCOR) [8]. Surprisingly, our attempts to crystallize
analogous heterobimetallic complex containing Cu and Hg led to
disproportionation of the complex into several monometallic thio-
carboxylates depending on the solvent used for crystallization [9].
A search of literature revealed the fact that though mononuclear
Zn(II) thiocarboxylates [10–13] are known no heterobinuclear
thiocarboxylate containing a Zn(II) atom has ever been reported.
In view of these facts we have taken up the synthesis and char-
acterization of Zn/Cu thiocarboxylate complexes. We have chosen
thiophene-2-thiocarboxylate because of the fact that it contains
an additional sulfur atom at the peripheral part of the ligand which
could also affect the structure and bonding of the complexes [14].
⇑
Corresponding author. Tel.: +91 542 6702451.
E-mail address: s_bhatt@bhu.ac.in (S. Bhattacharya).
Present address: IPC Department, I.I.Sc., Bangalore 560012, India.
1
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.11.022

S. Singh et al. / Inorganica Chimica Acta 396 (2013) 6–9
7
Scheme 1.
Fig. 1. Thermal ellipsoid plot of 1 at 30% probability level. One of the phenyl rings
and all the thiophene rings are disordered (hydrogen atoms are omitted for clarity).
Selected metric data (bond lengths in Å and angles in °): Zn1–S1 2.488(2), Zn1–S3
2.404(2), Zn1–S5 2.425(3), Zn1–O1 2.251(5), Zn1–O2 2.434(4), Zn1–O3 2.370(5).
S1–Zn1–S3 105.89(8), S1–Zn1–S5 105.91(8), S3–Zn1–S5 114.51(9), O1–Zn1–O2
92.00(15), O1–Zn1–O3 87.13(17), O2–Zn1–O3 87.07(15), S1–Zn1–O1 64.50(11),
S3–Zn1–O1 98.67(12), S5–Zn1–O1 146.75(12), S1–Zn1–O2 153.32(14), S3–Zn1–O2
63.35(12), S5–Zn1–O2 100.71(14), S1–Zn1–O3 103.25(13), S3–Zn1–O3 149.91(13),
S5–Zn1–O3 63.28(14).
Fig. 3. Thermal ellipsoid plot of 3 at 30% probability level. Thiophene rings are
disordered (hydrogen atoms are omitted for clarity). Selected metric data (bond
lengths in
Å and angles in °): Zn1–S1 2.3162(12), Zn1–O2 1.961(3), Zn1–O3
1.979(3), Zn1–P1 2.4568(11), Zn1–Cu1 2.9140(8), Cu1–S1 2.4191(11), Cu1–S3
2.2772(12), Cu1–S5 2.2917(13), Cu1–P2 2.2606(11). S1–Zn1–O2 129.23(8), S1–
Zn1–O3 109.80(9), S1–Zn1–P1 109.48(4), O2–Zn1–O3 98.98(12), O2–Zn1–P1
99.91(8), O3–Zn1–P1 107.52(8), S1–Zn1–Cu1 53.63(3), O2–Zn1–Cu1 86.34(8),
O3–Zn1–Cu1 90.45(8), P1–Zn1–Cu1 159.59(3), S1–Cu1–S3 112.68(4), S1–Cu1–S5
104.07(5), S3–Cu1–S5 116.01(5), S1–Cu1–P2 107.93(4), S3–Cu1–P2 108.36(5), S5–
Cu1–P2 107.40(5), S1–Cu1–Zn1 50.44(3), S3–Cu1–Zn1 83.18(4), S5–Cu1–Zn1
82.31(4), P2–Cu1–Zn1 158.34(3).
complex, Zn(II) lies in the plane constituted by the three sulfur
atoms where as in 1 the zinc atom is tipped above (0.838 Å) the
S3 plane. All the three Zn–S distances are comparable to those
found in [Zn(SCOPh)3]À. In contrast to [Zn(SCOPh)3]À where the
Zn–O distances are unequal the three Zn–O distances in 1 are com-
parable with one another. These are slightly longer than the sum of
the covalent radii of Zn and O atoms (1.90 Å), however, the angles
subtended at Zn by the three sulfur and three oxygen atoms reveal
a six coordination environment around Zn. The structure can pos-
sibly be best described as octahedral with considerable distortion
due to small ligand bite. It may be noted that in the [Zn(SCOPh)3]-
À
complex the Zn–O distances are larger than those found in 1
ranging between 2.328 and 3.156 Å and the geometry has been de-
scribed as trigonal bipyramidal with ZnS3O2 core. Vittal and Dean
have calculated [11] Bond–valence parameters for [Zn(SCOPh)3]À
complex and found the contributions of Zn–S bond 82.6% and
Zn–O bond 17.4%. Similar calculations on 1 about ZnS3O3 unit re-
vealed that the total bond valence is 1.707 in which Zn–S and
Zn–O bonds contributions are 68.83% and 31.17% respectively.
The latter evinces the significance of the Zn–O bonds in 1.
Fig. 2. Thermal ellipsoid plot of 2 at 30% probability level (hydrogen atoms are
omitted for clarity). Selected metric data (bond lengths in Å and angles in °): Zn1–S
1 2.2823(24), Zn1–O2 1.976(3), Zn1–O3 1.965(3), Zn1–P2 2.5385(14), Zn1–Cu1
2.9415(8), Cu1–S1 2.4153(13), Cu1–S2 2.3159(13), Cu1–S3 2.2925(12), Cu1–P1
2.2885(12). S1–Zn1–O2 112.11(11), S1–Zn1–O3 124.47(9), S1–Zn1–P2 123.41(5),
O2–Zn1–O3 103.95(15), O2–Zn1–P2 90.38(10), O3–Zn1–P2 96.14(9), S1–Zn1–Cu1
53.28(3), O2–Zn1–Cu1 86.88(10), O3–Zn1–Cu1 89.66(9), P2–Zn1–Cu1 174.05(4),
S1–Cu1–S2 104.32(5), S1–Cu1–S3 111.14(5), S2–Cu1–S3 116.61(5), S1–Cu1–P1
112.18(5), S2–Cu1–P1 104.69(4), S3–Cu1–P1 107.82(4), S1–Cu1–Zn1 49.24(4), S2–
Cu1–Zn1 83.74(3), S3–Cu1–Zn1 83.06(3), P1–Cu1–Zn1 161.40(4).
The complex 2, [(PPh3)Cu(l-SCOPh)3Zn(PPh3)] crystallized in
the orthorhombic crystal system with the P212121 space group.
The molecular structure is shown in Fig. 2. In this complex two

8
S. Singh et al. / Inorganica Chimica Acta 396 (2013) 6–9
Fig. 4. Selected orbital transitions for 2 (orbital contour value 0.05).
different types of binding modes of thiobenzoate ligands are ob-
served; O, S (bidentate) bridging and S bridging. In the former case
which represents the classical binding mode of the ligands, oxygen
atom is bonded to Zn(II), a comparatively hard metal center where
as the sulfur atom is bounded with the soft Cu(I) atom. Besides, the
three bridging thiobenzoate ligands a strong Cu–Zn interaction is
also responsible for holding the two metals together (the Cu–Zn
distance [2.9415(8) Å] is slightly shorter than the sum of the cova-
lent radii of the two metals (2.97 Å)).
The coordination environment around Zn(II) seems to be dis-
torted tetrahedral if the Zn–Cu contact is ignored. However, the an-
gles subtended at Zn are highly deviated from ideal tetrahedral
angle. Two P–Zn–O angles are closer to 90° while the two O–Zn–
S angles are rather obtuse (112.11° and 124.47°). A better descrip-
tion of the geometry around Zn would be trigonal bipyramidal if
the Cu atom is considered to be within the coordination sphere
of Zn (as it appears by short Cu–Zn distance). The P–Zn–Cu axial
angle (174.05°) is quite close to the ideal value of 180° while the

S. Singh et al. / Inorganica Chimica Acta 396 (2013) 6–9
9
equatorial O–Zn–S angles are closer to 120°. The geometry around
Cu atom is highly deviated from that of a trigonal bipyramid. The
angles subtended at the Cu center by three S and one P atoms
are closer to 109°. Though two S–Cu–Zn angles are closer to 90°,
the third one (S1–Zn–Cu) is very small. The Zn–Cu–P angle
(161.39°) is also highly deviated from the expected value of 180°.
During recent years four papers have appeared describing com-
pounds containing Cu(II) and Zn(II) together [17–20]. In all these
compounds Zn and Cu centers are bridged by two oxygen atoms
of the ligands with a Cu–Zn distance varying within a range of
2.95–3.23 Å which is very close to the sum of their covalent radii.
Notably, in none of the reports there is any comment on the nature
of Cu–Zn bond and the coordination geometries of the metals have
been described without considering the existence of Cu–Zn bond.
Possibly, the short M–M distance in these molecules is a conse-
quence of O bridging which cannot be stretched beyond a limit. In
the present case, however, the bridging are through OCS units
(which can open wide to a larger extent) and through sulfur atom
which has a substantially larger covalent radius than that of oxygen.
One would expect the angle subtended at sulfur (Zn–S–Cu) to be lar-
ger than 90° considering its hybridization and number of bonds. In
that case the Zn–Cu distance should be larger than 3.32 Å based on
purely geometric considerations. Evidently, the short Cu–Zn dis-
tance in the present case is not a consequence of the strain imposed
by the bridging atoms. On the contrary the acute Zn–S–Cu angle
(77.48°) is possibly a consequence of strong Zn–Cu interaction.
While comparing with the earlier reported data it should also be
kept in mind that the present complex is the first example where
Cu is in +1 oxidation state and obviously will have a larger radius
than those reported in the literature having +2 oxidation state.
To get further insight into the nature of Cu–Zn interaction we
have carried out density functional calculations. The results of sec-
ond order perturbation theory analysis of Fock matrix (NBO) reveal
that there are significant intramolecular interactions. Electron
transfers from Lp⁄Zn to Lp⁄Cu and various Ry⁄Cu orbitals amount
to 50.17 and 8.01 kcal/mol of energy lowering. Similarly, there
are significant Cu ? Zn charge transfers also. The total energy in-
volved in the electron transfer from the LpCu to various antibond-
ing orbitals of Zn is À8.80 kcal/mol while the same from Lp⁄Cu is
À10 kcal/mol. It may therefore, be concluded that the short Cu–
Zn distance is not merely a consequence of steric constrains im-
posed by the ligands but is a result of strong bonding interactions
between the two atoms.
transfers while peak below 300 nm are due to the inter- or intra-
ligand charge transfers.
For unambiguous assignment of the absorption bands time
dependent density functional theory (TDDFT) calculations have
been performed at PBE1 level. The orbital transition plots of 2
are shown in Fig. 4. The calculated absorptions at 427 nm and
419 nm are due to electron transfers from sulfur and copper atoms
to the bridging thiobenzoate phenyl ring (n ? p⁄) involving HOMO
to LUMO, LUMO+2, LUMO+3 and HOMOÀ1 to LUMO and LUMO+3
orbitals of the molecule. Other absorption peaks at 368 nm,
336 nm, 333 and 325 nm are due to intra- and inter-ligand charge
transfers.
To the best of our knowledge there is no literature available on
the photoluminescence properties of Zn(II)/Cu(I) heterobimetallic
compounds. Emission spectrum of Complex 3 has been recorded
in solid state. When exited at 400 nm the complex 3 shows a
strong emission at 423 nm due to inter-ligand transitions (Fig-
ure S4, Supporting information). The weak intensity emissions at
485 and 546 nm are possibly due to ligand centered (involving
thiocarboxylate ligands) charge transfer.
Acknowledgments
The authors are grateful to Professor R.J. Butcher, Howard Uni-
versity, USA for his help in refining the structures (particularly of
compound 1). Financial supports in the form of a project to S.B.
by the University Grants Commission, India, SRF to S.S. and J.C.
by the Council of Scientific and Industrial Research, India are grate-
fully acknowledged.
Appendix A. Supplementary material
Experimental details, Figures S1–S4, Table S1 and the crystallo-
graphic data in CIF format. CCDC 772374, 772372 and 772373 con-
tains the supplementary crystallographic data for compounds 1–3,
respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.ca-
m.ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.ica.2012.11.022.
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