Second generation O-alkyldithiocarbonates: Easy access to a new class of metalloligands

Article abstract of DOI:10.1016/j.ica.2017.08.062

The second-generation xanthate ligand 4-PyCH₂OCS₂Na, consisting of a backbone functionalized O-alkyldithiocarbonate donor set, has been prepared. The reaction of this ligand with either Ph₃SnCl or (Ph₃P)₂CuNO₃ yields the neutral monometallic complexes [4-PyCH₂OCS₂SnPh₃] and [4-PyCH₂OCS₂Cu(PPh₃)₂], respectively. These compounds were characterized by ¹H NMR and UV–Vis spectroscopies, and in solid state by single crystal X-ray diffraction. The structure of [4-PyCH₂OCS₂SnPh₃] consists of a mono-dimensional coordination polymer, formed by the monodentate coordination of the –O–CS₂^? group to the Sn(IV) center, in combination with the coordination of the pyridine group to an adjacent tin atom. In the case of [4-PyCH₂OCS₂Cu(PPh₃)₂], the xanthate ligand coordinates to the Cu(I) center in an isobidentate fashion, leaving the pyridine moiety free. These monometallic complexes act as metalloligands toward the [Rh₂(OAc)₄] dimer, producing the tetrametallic derivatives [Rh₂(OAc)₄]·2[4-PyCH₂OCS₂SnPh₃] and [Rh₂(OAc)₄]·2[4-PyCH₂OCS₂Cu(PPh₃)₂], respectively. These compounds were investigated in solution phase using ¹H NMR and UV–Vis techniques, and it is proposed that the formation of the first compound proceeds via a two-step process, while the formation of the latter is a one-step process.

Full text of DOI:10.1016/j.ica.2017.08.062

Accepted Manuscript
Second generation O-alkyldithiocarbonates: Easy access to a new class of met-
alloligands
Radu F. Semeniuc , .Thomas J. Reamer, Kaitlyn A. Hammock, Holly B. Jones,
Mark D. Smith, Kraig A. Wheeler
PII:
S0020-1693(17)30939-8
http://dx.doi.org/10.1016/j.ica.2017.08.062
ICA 17859
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
22 June 2017
27 August 2017
31 August 2017
Please cite this article as: R.F. Semeniuc, ..J. Reamer, K.A. Hammock, H.B. Jones, M.D. Smith, K.A. Wheeler,
Second generation O-alkyldithiocarbonates: Easy access to a new class of metalloligands, Inorganica Chimica
Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.08.062
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Second generation O-alkyldithiocarbonates: Easy access to a new class of metalloligands
a,
Radu F. Semeniuc, * Thomas J. Reamer, Kaitlyn A. Hammock, Holly B. Jones, Mark D. Smith,
a
a
a
b
a
and Kraig A. Wheeler
a
Department of Chemistry and Biochemistry, Eastern Illinois University, Charleston, IL 61920, USA.
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA.
b
Corresponding author:
Radu F. Semeniuc
Department of Chemistry, Eastern Illinois University
Charleston, IL 61920, USA.
Tel: +1 217 581 5422; E-mail: rsemeniuc@eiu.edu
th
Dedicated to Professor Ionel Haiduc on the occasion of his 80 birthday. R. F. Semeniuc wishes to
express his gratitude to Professor Ionel Haiduc for outstanding guidance during his undergraduate and
graduate studies at Babes-Bolyai University, Chemistry Department.
Abstract
The second-generation xanthate ligand 4-PyCH
alkyldithiocarbonate donor set, has been prepared. The reaction of this ligand with either Ph
Ph P) CuNO yields the neutral monometallic complexes [4-PyCH OCS SnPh and [4-
PyCH OCS Cu(PPh ], respectively. These compounds were characterized by H-NMR and UV-Vis
spectroscopies, and in solid state by single crystal X-ray diffraction. The structure of [4-
PyCH OCS SnPh ] consists of a mono-dimensional coordination polymer, formed by the monodentate
coordination of the -O-CS ⁻ group to the Sn(IV) center, in combination with the coordination of the
pyridine group to an adjacent tin atom. In the case of [4-PyCH OCS Cu(PPh ], the xanthate ligand
coordinates to the Cu(I) center in an isobidentate fashion, leaving the pyridine moiety free. These
monometallic complexes act as metalloligands toward the [Rh (OAc) ] dimer, producing the tetrametallic
derivatives [Rh (OAc) ]·2[4-PyCH OCS SnPh and [Rh (OAc) ]·2[4-PyCH OCS Cu(PPh ],
2
OCS
2
Na, consisting of a backbone functionalized O-
3
SnCl or
(
3
2
3
2
2
3
]
1
2
2
)
3 2
2
2
3
2
2
2
)
3 2
2
4
2
4
2
2
3
]
2
4
2
2
3 2
)
1
respectively. These compounds were investigated in solution phase using H-NMR and UV-Vis
techniques, and it is proposed that the formation of the first compound proceeds via a two-step process,
while the formation of the latter is a one-step process.
Keywords: O-Alkyldithiocarbonates; X-ray crystal structures; Tin complexes; Copper complexes;
Supramolecular Chemistry.
Highlights:
•
•
•
•
A backbone functionalized xanthate ligand has been prepared.
Its propensity to coordinate to Sn(IV) and Cu(I) centers has been explored.
Neutral metalloligands have been obtained.
Hetero-tetrametallic complexes have been prepared.
1

1. Introduction
A major direction that has emerged within chemical research around the world involves applying the
principles of supramolecular chemistry in a quest for new materials with useful properties. Promising
results have been accomplished and startling applications are being developed by preparing well-defined,
self-assembled superstructures with specific properties [1]. Organizational control is achieved by
incorporating specific recognition sites within the structures of the building blocks that will drive the self-
assembling process toward the desired goal. Dative bonds [2], secondary bonds [3], hydrogen bonding
[2e, 4], and π–π stacking [2e, 5] are among the most frequently employed interactions to organize a large
variety of building blocks into higher-order arrays.
In this context, the development of new materials based on polynuclear metal complexes witnessed an
increased attention during the last years. Numerous compounds, ranging from discrete homo- and/or
hetero-nuclear architectures to metal-organic-frameworks with various topologies have been prepared and
characterized [6]. One of the most employed synthetic strategies in the preparation of these materials
utilizes metal complexes supported by ligands possessing additional functional groups that would allow
the expansion of the building blocks into the desired extended structure [7]. These species (also known as
metalloligands) are generated by a ligand having at least two donor sites: one for binding to a primary
metal ion in a first step, and the other(s) for subsequent coordination to additional metal center(s). In the
resulting architecture, the metal ions play two essential functions: a structural task, by directing and
supporting the solid-state architecture and a functional role, by bringing desired properties into the final
network [8].
To ensure the stability of the metalloligand and avoid the competition between the metal ions during
the subsequent steps, most of the metalloligands prepared to date were built around hard, chelating donor
groups such as catecholate, acetylacetonate, porphyrin, and Schiff-base scaffolds [6b]. These primary
coordination sites deliver relatively non-labile complexes; furthermore, these moieties can be easily
modified at several positions with additional rigid functionalities, thus offering a high degree of
predictability of the resulting polymetallic architectures.
Soft 1,1-dithiolates (-ES
2
⁻, E = C or P), such as dithiocarbamates (R
⁻, R[OR’]PS ⁻, and RO[R’O]PS
2
NCS
2
⁻), O-alkyldithiocarbonates
(
ROCS ⁻), and phosphor-1,1-dithiolates (R[R’]PS
2
2
2
2
⁻) represent an
important family of ligands that present both practical and academic interest [3e, 3f, 9]. Surprisingly, in
spite of years of chemistry, there are only a few reports on the incorporation of soft 1,1’-dithiolate ligands
into polyfunctional systems as a means to design heterometallic species [10-12]. Only recently, the Beer
group used the dithiocarbamate donor set to prepare supramolecular architectures such as macrocycles,
cages, catenanes, and resorcinarene-based assemblies [10]. Wilton-Ely prepared piperazine-based ditopic
dithiocarbamate ligands and used them in the preparation of catalytically active homo- and hetero
polymetallic species [11]. Polytopic dithiocarbamates were also used in the synthesis of diorganotin-
based cavitands and capsules [12]. These examples clearly demonstrate the potential of 1,1’-dithiolates to
support complex molecular edifices when combined with other “supramolecular functionalities”. While
the use of bridging ligands to tether together the same or different metallic centers has been ongoing for
several years, the self-assembly of heterometallic architectures built on heteroditopic ligands
incorporating the -ES
its full potential.
2
⁻ donor set represents a promising research area that has not been yet exploited to
2

O-alkyldithiocarbonate ligands (also known as xanthates) and their metal complexes have been
extensively studied. Following the pioneering work of Hoskins and Winter [9a, 9b, 13], systematic
structural analyses performed by Haiduc and Tiekink showed that these ligands can bind to metal centers
in a variety of coordination modes, as pictured in Scheme 1 [9f]. In addition, the rich structural features of
their metal complexes stem from the presence of the soft Lewis acidic centers on these ligands (i.e.
sulfur), thus offering the possibility of supramolecular association in the crystalline phase through
hypervalent interactions and/or secondary bonds [9e, 9f].
M
S
S
S
S
S
S
M
M
S
S
C
a
O
M
C
O
M
C
O
C
d
O
R
R
R
R
b
c
M
M
M
M
M
S
S
S
S
M
C
O
M
C
O
C
O
M
C
O
S
R
S
R
S
R
S
R
e
f
g
h
Scheme 1. Coordination modes of the xanthate ligands: a) monodentate; b) isobidentate; c)
anisobidentate; d) bimetallic biconnective; e) bimetallic triconnective; f) trimetallic tetraconnective; g)
bimetallic biconnective with oxygen involvement; h) bimetallic tetraconnective with oxygen involvement.
Given the scarce number of reports in the literature on xanthate ligands functionalized with additional
donor groups that would expand their coordination capabilities [14], we have started a project aimed at
the synthesis of “second-generation” O-alkyldithiocarbonate ligands (that is backbone-functionalized
xanthates) and their corresponding metal complexes.
By attaching to the backbone of the soft -O-CS ⁻ donor set other moieties with coordination
2
capabilities (for instance donors such as pyridine-based groups), it should be possible to take advantage of
the differences between the coordination preferences of sulfur atoms for “soft metals” and of pyridine-
based groups for other metals to design and prepare new heterometallic species. First, the negatively
charged -CS ⁻ group would selectively accept a suitable metal system at the “soft end” of the ligand
2
through a charge assisted process, while keeping the pyridine end free for further coordination
opportunities. Next, additional coordinatively unsaturated metal centers would connect to the resulting
metalloligand, generating the desired heterometallic complexes.
Following this strategy, we have prepared our first second-generation xanthate ligand, 4-
PyCH
2
OCS
CH
2
Na
and
reported
Ni·2[(4-PyCH
its
use
in
the
preparation
] (An = anisole) trinuclear complex,
of
a
luminescent
[
CH OCH
3
2
2
OP(An)S
2
]
2
2
OCS )Re(bipy)(CO)
2
3
supported by a heteroleptic xanthate-dithiophosphonate ligand system [15]. In this contribution, we wish
to report a more detailed characterization of this ligand, as well as its use in the preparation of two new
metalloligands incorporating Sn(IV) and Cu(I) metallic centers. Also reported here is the use of these
metalloligands in the preparation of tetranuclear Rh(II) – Sn(IV) and Rh(II) – Cu(I) heterometallic
complexes.
2. Results and Discussion
3

2.1. Syntheses and characterization
The syntheses of the second-generation xanthate ligand, its corresponding Sn(IV) and Cu(I)
metalloligands and the tetranuclear Rh(II) based heterometallic complexes are depicted in Scheme 2. The
4
-PyCH
methanol with NaH in dry THF, followed by the CS
ligand is stable for several weeks in air, soluble in water, alcohols, and acetone, and insoluble in
halogenated solvents and ethers. The reaction of 4-PyCH OCS Na with either Ph SnCl or (Ph P) CuNO
yields [4-PyCH OCS SnPh ] ([Py-Sn]) and [4-PyCH OCS Cu(PPh ] ([Py-Cu]), respectively. Both
2
OCS
2
Na ligand was prepared in excellent yields as reported [15], by deprotonating 4-pyridine-
2
reaction with the formed sodium alkoxide. The
2
2
3
3
2
3
2
2
3
2
2
)
3 2
compounds are air and moisture stable powders, and soluble in common organic solvents, but insoluble in
alcohols, ethers, and water. The reaction of two equivalents of [Py-Sn] and [Py-Cu] metalloligands with
one equivalent of [Rh
2
(OAc)
OCS SnPh
[Rh]-2[Py-Cu]), respectively. While the Sn(IV)-based complex is soluble in common organic solvents,
4
]·2MeOH dimer in acetone yields the tetrametallic derivatives
[Rh
2
(OAc)
4
]·2[4-PyCH
2
2
3
]
([Rh]-2[Py-Sn]) and [Rh (OAc) ]·2[4-PyCH OCS Cu(PPh ) ]
2
4
2
2
3
2
(
its Cu(I) congener shows a very low solubility in the same solvents.
N
N
N
S
S
[ M ]
i. NaH, THF, _ 20 o
, THF, _ 20 oC to r.t.
C
+
+
iv. + [ Rh (OAc)4
]
O
O
iii. + [ M ] ; - Na
2
O
O
Rh Rh
O
O
O
S
S
N
N
ii. CS
2
acetone, r.t., 2 hrs.
S Na+
acetone, r.t., 1 hr.
O
O
O
O O
OH
S
S
[ M ]
-
S
[
M ]
[ M ] = SnPh
3
: [ Rh ] - 2 [ Py - Sn ]
: [ Rh ] - 2 [ Py - Cu ]
[
M ] = Cu(PPh )
3 2
[
[
M ] = SnPh
3
: [ Py - Sn ]
: [ Py - Cu ]
M ] = Cu(PPh )
3 2
Scheme 2. The synthesis of Rh-based tetranuclear heterometallic complexes supported by the second-
generation 4-PyCH
2
OCS Na ligand via its corresponding Sn(IV) and Cu(I) metalloligands.
2
1
The H-NMR spectra (acetone-d
6
) of the synthesized compounds are pictured in Fig. 1. The formation
Na ligand from 4-PyCH OH is evident, as the -CH - peak is shifted downfield, from
.71 ppm to 5.59 ppm, due to the CS insertion. As expected, the pyridine signals are not affected by the
presence of the O-alkyldithiocarbonate group (see Fig. 1 a and b).
Upon coordination of the -O-CS ⁻ group to Sn(IV) and Cu(I) centers, the H-NMR spectra clearly
shows the formation of the corresponding metalloligands (Fig. 1 c and e). For [Py-Sn] the characteristic
Ph Sn peaks appear as multiplets in the 7.77-7.62 ppm and 7.54-7.44 ppm intervals, while for [Py-Cu]
the (Ph P) Cu signals appear between 7.49 and 7.25 ppm. Further, in the case of the [Py-Sn] complex
see Fig. 1b and Fig. 1c), the pyridine peaks experience a significant shift (from 8.51 to 8.41 ppm (∆δ =
of the 4-PyCH
2
OCS
2
2
2
4
2
1
2
3
3
2
(
0.1 ppm) and from 7.35 to 6.91 ppm (∆δ = 0.44 ppm)), suggesting the coordination of the pyridine group
to the tin centers. In contrast, upon the formation of the [Py-Cu] complex, the pyridine peaks do not shift
from their original position in 4-PyCH OCS Na (8.58 ppm vs. 8.51 ppm; the second pyridine signal
overlap with those from the Ph P moieties, but they are in the 7.49-7.25 ppm range, close to the original
2
2
3
position of 7.35 ppm in the ligand – compare Fig 1b with Fig. 1e).
The pyridine peaks of the tetrametallic [Rh]-2[Py-Sn] derivative (Fig. 1 d) are shifted downfield when
compared to the same peaks of the [Py-Sn] metalloligand (from 8.41 to 9.11 ppm (∆δ = 0.7 ppm) and
4

from 6.91 to 7.29 ppm (∆δ = 0.38 ppm)), indicative of pyridine coordination to the Rh centers. In the case
of [Rh]-2[Py-Cu] derivative, the low solubility of this complex in acetone precludes a detailed analysis;
however, its NMR spectrum (Fig. 1 f) suggests the formation of the desired tetrametallic derivative.
1
Fig. 1. Partial H-NMR spectra (acetone-d
Rh]-2[Py-Sn]; e) [Py-Cu]; f) [Rh]-2[Py-Cu].
6
) of a) 4-PyCH
2
OH; b) 4-PyCH
2
OCS Na; c) [Py-Sn]; d)
2
[
2.2. Solid state studies
Single crystals of 4-PyCH
2
OCS Na, [Py-Sn], and [Py-Cu] were grown by vapor phase diffusion of
2
diethyl ether into acetone solutions of the compounds and subjected to X-ray diffraction studies. Crystal
data and structure refinement details are collected in Table 1 and selected bond lengths and angles are
listed in Table 2.
2 2
2.2.1. Structural characterization of 4-PyCH OCS Na
The binding mode of 4-PyCH
pictured in Fig. 2. The -O-CS ⁻ group alone shows an unusual trimetallic pentaconnective coordination
mode. If we also consider the pyridine group positioned at the backbone of the -O-CS ⁻ donor set, the 4-
PyCH OCS ⁻ moiety acts as a bridging ligand, binding the Na ion in a tetrametallic hexaconnective
fashion.
The “classic” -O-CS
2
OCS Na and the coordination environment around the sodium ion are
2
2
2
2
2
2
⁻ donor set chelates one Na ion in a partial anisobidentate mode (S1-Na1 = 2.930
Å and S2-Na1 = 3.036 Å), the difference between the two S – Na bond lengths being 0.106 Å. In
addition, the S1 atom coordinates to two more sodium ions, Na1’ and Na1’’, the corresponding bond
lengths being 2.986 Å and 3.030 Å, respectively. The O atom of the xanthate ligand is also involved in
coordination to the metallic center, the O1-Na1’ bond length being 2.543 Å. A water molecule is also
present, bridging two Na ions, the O2-Na’ bond length being 2.476 Å. The pyridine ring of the ligand
5

coordinates to a third sodium, with a N1-Na1’’’ bond length of 2.468 Å. Overall, the geometry around the
Na1 center can be described as a distorted pentagonal bipyramid. The equatorial positions are occupied by
the S1, S2, S1’’’, O2’, and O1’’, and the axial positions by the N1’’ and S1’’’’ atoms. The average N1’’-
Na1-equatorial atoms angle is 93.94°, while the average S1’’’’-Na1-equatorial atoms angle is 85.93°, and
the N1’’-Na1-S1’’’’ angle is 166.66°. The bridging feature of the water molecule imposes the largest
distortion from the ideal geometry, the N-Na-O2 and S-Na-O2 angles being 120.13° and 73.20°,
respectively.
Fig. 2. Binding mode and coordination environment around the sodium center in 4-PyCH
2
OCS
2
Na.
This coordination mode of the ligand generates an undulating inorganic (Na ladder (see Fig. 3),
2
S
)
2 ∞
running along the c axis of the unit cell. This arrangement consist of two types of alternating Na-S-Na-S
metallacycles, one made by the coordination of S1 atom to the Na1 and Na1’ centers (and the symmetry
related S1’ atom to the same sodium ions), and the other by the S1’ and S1’’ coordination to the Na1 and
Na1’’ ions, respectively. The latter metallacycle also contains the bridging water molecule. The Na ··· Na
distances within the ladder fall in the range from 3.676 Å for the Na1-Na1’’ distance, to 4.642 Å for Na1-
Na1’, and 5.896 Å for Na1’-Na1’’. The corresponding angles are S1-Na1-S1’ = 77.73° and Na1-S1-Na1’
=
102.27° for the first metallacycle, and S1’-Na1-S1’’ = 97.01° and Na1-S1’-Na1’’ = 75.34° for the
second. These two metallacycles are connected by the bridging S1 atoms, the S1-Na1-S1’’ and Na1’-S1’-
Na1’’ angles being 141.35° and 170.52°, respectively.
6

Fig. 3. Two views of the Na
2
S
2
inorganic ladder in 4-PyCH
2
OCS Na.
2
As described above, each Na⁺ ion is also coordinated by the pyridine ring of the xanthate ligand. Due
to the bridging nature of the ligand, the overall structure of the 4-PyCH OCS Na complex consists of a
three-dimensional architecture, as pictured in Fig. 4. As each Na center is coordinated by one pyridine
moiety, the -O-CS ⁻ group (oriented in the opposite direction of the coordinate vector of the Npy atom)
2
2
2
participates in the formation of additional Na
shaped structure.
2
S ladders, all connected by the ligand in an overall W-
2
7

Fig. 4. The three-dimensional structure of the 4-PyCH
2
OCS Na complex.
2
2 2 3
2.2.2. Structural characterization of [4-PyCH OCS SnPh ] ([Py-Sn])
The coordination environment around the tin atom in [Py-Sn] is pictured at the top of Fig. 5. The
xanthate ligand binds to the Sn(IV) center in a monodentate fashion (S1-Sn1 = 2.593 Å), with the second
sulfur atom of the -O-CS
2
⁻ group oriented away from the metal. This monodentate coordination mode is
8

reflected in the different C-S bond lengths, their values (1.724 Å for C1-S1 and 1.648 Å for C1-S2)
suggesting some delocalization of the carbon – sulfur double bond. The oxygen atom of the ligand is
oriented toward the tin atom, but the O ··· Sn distance of 3.179Å does not suggest a significant interaction
between these two atoms. The Sn(IV) center is equatorially surrounded by the phenyl groups, with a C-Sn
bond length average of 2.135 Å. Interestingly, the pyridine atom of the ligand is also involved in
coordination, binding to a tin atom from an adjacent 4-PyCH
2
OCS
2
SnPh unit. As such, the geometry
3
around the tin center can be described as distorted trigonal bipyramid. In the case of five-coordinated
complexes, which can have either a trigonal bipyramidal or a square pyramidal geometry, the degree of
distortion from the ideal geometry is described by the angular parameter τ [16]. This parameter is
5
calculated by dividing the difference between the two largest angles by 60; a trigonal bipyramidal
structure would produce a value of 1, while a square pyramidal structure would produce a value of 0. For
this complex, the τ value is 0.7, clearly indicating a distortion from the ideal trigonal bipyramidal
5
geometry.
Fig. 5. The coordination environment around the Sn(IV) center (top) and the mono-dimensional
coordination polymer [Py-Sn] (bottom).
∞
The binding of the pyridine ring to the Sn(IV) atom leads to the formation of a mono-dimensional
coordination polymer, depicted at the bottom of Fig. 5. The -O-CH - spacer is bent over the Ph Sn group,
placing the pyridine ring in close proximity of two of the three phenyl rings attached to the tin center.
Another phenyl ring, from an adjacent Ph Sn moiety, also points toward the pyridine and phenyl rings.
2
3
3
Although several carbon – carbon distances are below 3.8 Å, the orientation of these aromatic regions do
not support the existence of C-H···π or π-π stacking interactions.
2 2 3 2
2.2.3. Structural characterization of [4-PyCH OCS Cu(PPh ) ] ([Py-Cu])
The molecular structure of [Py-Cu] is depicted in Fig. 6. There are two crystallographically
independent molecules per asymmetric unit, both having the same structural features, with only small
9

differences between their bond lengths and angles (see Table 2); therefore, only one molecule will be
described here. The -O-CS ⁻ end of the ligand coordinates to the Cu(I) center in an essentially
2
isobidentate mode with the two S – Cu bond lengths practically equal at S1A-Cu1 = 2.390 Å and S2A-
Cu1 = 2.427 Å, respectively. The C1A-S1A and C1A-S2A bond lengths, nearly equal at 1.696 Å and
1.689 Å, respectively, also support this fact. The coordination sphere around the metal is filled by two
PPh moieties, with P1A-Cu1 and P2A-Cu1 bond lengths of 2.246 Å and 2.233 Å, respectively. The
3
metal center is in a distorted tetrahedral environment, distortion imposed by the “bite” angle of the
xanthate ligand, the S1A-Cu1-S2A angle being 75.31°. The most important feature of this complex is the
fact that the pyridine group is not involved in the coordination of the ligand to the Cu(I) ion, thus
remaining available to bind to a second metallic center.
Fig. 6. The molecular structures of the two independent [Py-Cu] molecules.
2.3. Solution studies
As described above, the 4-PyCH
2
OCS Na ligand produces the [Py-Sn] and [Py-Cu] complexes,
2
mainly distinguished by the behavior of the pyridine ring: while in the first complex the Py moiety acts as
a donor group, in the latter it is not coordinated to the metal center. Considering the above results, two
questions immediately rise: can these metalloligands coordinate to additional metallic centers to form
heterometallic complexes and, in the case of [Py-Sn], how would the formation of a heterometallic
complex (if possible at all) be influenced by the coordination of the Npy atom to the Sn(IV) centers? To
answer these questions in the absence of crystallographic evidence for the formation of the dirhodium-
based tetrametallic complexes, we investigated the interaction between our metalloligands and the
dirhodium tetraacetate complex in solution phase.
In the [Rh
2
(OAc) ]·2MeOH starting material, the acetates are kinetically inert and they can be
4
exchanged only in harsh conditions. In contrast, the methanol molecules (or any other axial donor ligands
like acetone or water) are labile, and can be easily exchanged with stronger σ-donor ligands. The
electronic properties of the dirhodium tetraacetate dimer offer a unique possibility to study its interaction
with the [Py-Sn] and [Py-Cu] metalloligands. Scheme 3 depicts a qualitative MO diagram for a
dirhodium tetraacetate dimer of the general formula [Rh
2
(OAc)
4
]L
2
(where L is the axial ligand), built
from a [Rh (OAc) ] dimer (D4h overall symmetry) and two axial σ-donor ligands [17]. The σ-donor lone
2
4
pairs of the axial L ligands have two symmetry combinations, a1g and a2u. While the a1g combination will
stabilize the Rh-Rh σ bonding orbital, the a2u combination will raise the energy of the Rh-Rh σ* LUMO.
One of the consequences of this increase in energy is the change in the energy of the lowest energy
absorption in dirhodium complexes, the e π* → a2u σ* HOMO-LUMO transition, leading to a shift of the
g
10

λmax, depending on the nature of the axial ligands. For example, in the case of Rh
2
(TIPB) (TIPB = 2,4,6-
4
triisopropylbenzoate), a dirhodium tetracarboxylate complex that can be isolated without axial ligands,
addition of different (axial) ligands like water or acetone, changes the λmax from 760 nm in the “free”
complex to 670 nm for water, and to 610 nm for acetone [18].
b2u
b1g
a2u
eg
b1u
b
2g
eu
a
1g
a2u
a1g
[
Rh (OAc) ]
2
[Rh (OAc) ]L
2
2 x L
4
2
4
Scheme 3. Qualitative molecular orbital diagram for a [Rh
17].
2
(OAc)
4
]L complex; redrawn from ref.
2
[
The titration of [Rh
2
(OAc)
4
] with increasing amounts of [Py-Sn] (in 0.2 equivalent increments, up to
3.0 equivalents) was carried out in a 1 : 1 acetone – dichloromethane mixture and followed
spectrophotometrically, by monitoring the λmax shift of the lowest energy absorption, see Fig. 7. As more
equivalents of [Py-Sn] are added, the λmax shifts from 600 nm (the blue line in Fig. 7 – corresponding to
the “uncoordinated” dirhodium complex) to 532 nm (the purple line in Fig. 7 – corresponding to 3
equivalents of [Py-Sn] added to the bimetallic center).
Fig. 7. Absorption spectra collected during the titration of [Rh
2
(OAc) ] with [Py-Sn]; the dashed
4
arrows point toward two isosbestic points identified during the titration experiment.
We attempted to perform a similar experiment using the [Py-Cu] metalloligand. Unfortunately, a
detailed study was hampered not only by the low solubility of the [Rh]-2[Py-Cu] complex in the reaction
11

media, but also by the tail of a strong absorption of the complex in the UV region, overlapping with the
absorptions in the visible region. As it can be seen in Fig. 8, we were able to add only 0.7 equivalents (in
0.1 increments) of [Py-Cu] to the dirhodium center; the addition of one more 0.1 equivalent of [Py-Cu]
to [Rh] made any reading of the λmax value impossible.
Fig. 8. Absorption spectra collected during the titration of [Rh
2
(OAc) ] with [Py-Cu].
4
These experiments provided some insight into the solution structure of the complexes. To confirm the
: 2 stoichiometry of our tetrametallic complexes, we chose the Yoe-Jones’ mole-ratio method, which
1
consists of analyzing a series of solutions in which the concentration of one reactant is kept constant,
while increasing the concentration of the other reactant [19]. By plotting the change in a property of the
complex (in this case the change in λmax) against the mole-ratio of the two reactants, in addition to finding
the stoichiometry of a complex, one can also identify if the formation of a ML
or if it is a stepwise process. The results of these investigations are shown in Fig. 9. In the case of the
2
complex proceeds directly
[
Rh]-2[Py-Sn] system, the titration curve is indicated by the blue dots. As more and more equivalents of
Py-Sn] are added, the λmax shifts to lower wavelengths, until 1.0 equivalent of metalloligand is present in
[
solution, when the λmax value is 552 nm. Upon addition of 1.2 equivalents or more of [Py-Sn] to
Rh (OAc) ], a change in the slope is observed, but only up to 2.0 equivalents of [Py-Sn] (the
[
2
4
corresponding λmax value being 543 nm), when another change in the variation of λmax values becomes
evident. Addition of more metalloligand leads to a flatlining behavior at λmax = 532 nm, suggesting the
saturation of the system. These data imply that the formation of the [Rh]-2[Py-Sn] complex proceeds
stepwise, according to equations (1) and (2).
This stepwise process is also supported by a close inspection of the spectra of the [Rh]-2[Py-Sn]
system, which reveals the presence of two well-defined isosbestic points, shown by the dashed lines in
Fig. 7. Nevertheless, these two points do not appear simultaneously. The first isosbestic point (positioned
around 571 nm) is produced only by the first six spectral lines, corresponding to 0.0, 0.2, 0.4, 0.6, 0.8, and
1.0 equivalents of [Py-Sn] added to the rhodium starting material. The second point, located around 561
nm is formed by the intersection of the remaining spectral lines, from 1.2 to 3.0 equivalents of [Py-Sn]
added to the system. This suggests that during the titration process two absorbing species are present in
solution, e.g. the [Rh
Rh]-[Py-Sn] and [Rh]-2[Py-Sn] complexes in the second step, respectively.
For the reasons mentioned above (low solubility of the compound and overlapping absorption bands),
we were not able to gather sufficient data points to investigate the [Rh (OAc) ] / [Py-Cu] system in detail.
The available seven data points (shown as purple squares in Fig. 9) show a dramatic change in the λmax
2
(OAc) ] and the [Rh]-[Py-Sn] complexes in the first step of the reaction, and the
4
[
2
4
12

values (∆ λmax = 62 nm, from 600 nm to 538 nm), even after the addition of only 0.7 equivalents of
metalloligand. This suggests that the formation of [Rh]-2[Py-Cu] complex is a one step process, as
shown in equation (3).
To identify the λmax value for a 1 : 2 Rh : pyridine-based ligand complex, we added an excess of 4-
picoline to the dirhodium starting material, yielding a soluble [Rh]-2[4-picoline] compound. Its
absorption spectrum displays a λmax value at 520 nm, close to the one shown by our [Rh]-2[Py-Cu]
complex (538 nm), suggesting that the addition of 0.7 equivalents of [Py-Cu] produces directly the
desired Rh-Cu tetrametallic derivative. However, in the absence of more data points, we can only
speculate on the processes taking place within the [Rh
2
(OAc) ] / [Py-Cu] system.
4
Fig. 9. Plot of the λmax shift against the number of [Py-Sn] and [Py-Cu] equivalents added to [Rh].
[
[
[
Rh] + [Py-Sn]
[Rh]-[Py-Sn]
[Rh]-2[Py-Sn]
[Rh]-2[Py-Cu] (eq. 3)
(eq. 1)
Rh]-[Py-Sn] + [Py-Sn]
Rh] + 2 [Py-Cu]
(eq. 2)
3. Conclusions
We have described here a simple, high-yield, stepwise approach toward heterometallic complexes,
based on a second-generation, pyridine functionalized xanthate ligand. We have shown that the 4-
PyCH OCS Na ligand binds to Sn(IV) and Cu(I) centers to form metalloligands, that can further react
with coordinatively unsaturated metallic centers to produce heterometallic complexes, such as
Rh (OAc) ]·2[4-PyCH OCS SnPh ] and [Rh (OAc) ]·2[4-PyCH OCS Cu(PPh ]. This approach
2
2
[
2
4
2
2
3
2
4
2
2
3 2
)
opens the door toward the synthesis of a large variety of heterometallic architectures, either discrete
species (as described here) or extended networks, self-assembled via secondary bonds, supported by the
various polymetallic-polyconnective coordination modes of the -O-CS
addressed in forthcoming manuscripts from our group.
2
⁻ donor group, as will be
4. Experimental
13

General considerations: Unless otherwise noted, all operations were performed in ambient
atmosphere. Operations requiring inert (nitrogen) atmosphere were carried out using standard Schlenk
techniques. Solvents were dried by conventional methods and distilled under a dry N atmosphere
2
immediately prior to use. NMR spectra were recorded using a 400 MHz Bruker Avance FT-NMR
Spectrometer. UV-Vis spectroscopic investigations were performed on a Cary 100 Agilent
Spectrophotometer. (Ph
20, 21]. All other reagents were commercially available and were used without further purification.
-PyCH OCS Na: This compound was prepared following a reported procedure [15]. 4-
3
P)
2
CuNO
3
and [Rh
2
(OAc) ]·2MeOH were prepared as described in the literature
4
[
4
2
2
Pyridylcarbinol (1.09 g, 10 mmol) was dissolved in dry THF (50 mL) and added dropwise to a suspension
o
of NaH (0.24 g, 10 mmol) in dry THF (100 mL) under an inert atmosphere at -20 C (a salt-ice bath). The
mixture was stirred for 20 minutes, then excess carbon disulfide (1.0 mL, 1.6 excess) was added
dropwise, and the resulting mixture was stirred for one hour while allowed to return to room temperature.
The flask was disconnected from the Schlenk line, and the yellow solution was filtered to remove any
solid impurities. The THF was removed under reduced pressure, and the resulting yellow powder was
1
dried under vacuum; the solid (1.93 g, yield 93.1 %) was identified as 4-PyCH
acetone-d ): δ: 8.51 (d, J = 5.9 Hz, 2H, Py), 7.35 (d, J = 6.0 Hz, 2H, Py), 5.59 (s, 2H, Py-CH
4-PyCH OCS SnPh ] ([Py-Sn]): Ph SnCl (0.193 g, 0.5 mmol) was dissolved in CH Cl
and added to a solution of 4-PyCH OCS Na (0.104 g, 0.5 mmol) in acetone (15 mL). The mixture was
2
OCS
2
Na; H-NMR
(
6
2
).
[
2
2
3
3
2
2
(30 mL),
2
2
stirred at room temperature for two hours. The resulting NaCl was removed by filtration, and the clear
solution was concentrated under reduced pressure and kept in the refrigerator for two hours. The resulting
white solid was isolated by filtration, yielding 0.218 g (81.6 %) of the title compound. Anal. Calcd. for
1
C
25
H21NOS
2
Sn: C = 56.20, H = 3.96, N = 2.62. Found: C = 56.50, H = 4.26, N = 3.02. H-NMR (acetone-
d
6
): δ: 8.41 (d, J = 6.0 Hz, 2H, Py), 7.77-7.62 (m, 6H, Ph
Hz, 2H, Py), 5.35 (s, 2H, Py-CH ).
4-PyCH OCS Cu(PPh ] ([Py-Cu]): (Ph
20 mL), and added to a solution of 4-PyCH
mixture was stirred at room temperature for two hours. The resulting NaNO
and the solvent was removed under reduced pressure, yielding a light orange compound (0.322 g, 83.4
), identified as the title compound. Anal. Calcd. for C43 : C = 66.87, H = 4.70, N = 1.81.
): δ: 8.58 (d, J = 5.9 Hz, 2H, Py), 7.49-7.25
3
Sn), 7.54-7.44 (m, 9H, Ph Sn), 6.91 (d, J = 5.6
3
2
[
2
2
3
)
2
3
P)
OCS
2
CuNO
Na (0.104 g, 0.5 mmol) in acetone (20 mL). The
was removed by filtration,
3
(0.325 g, 0.5 mmol) was dissolved in acetone
(
2
2
3
%
H36CuNOP
S
2 2
1
Found: C = 66.51, H = 4.37, N = 1.68. H-NMR (acetone-d
m, 32H, Ph P overlap with Py), 5.54 (s, 2H, Py-CH ).
Rh (OAc) ]·2[4-PyCH OCS SnPh ] ([Rh]-2[Py-Sn]): [Py-Sn] (0.107 g, 0.2 mmol) was added as a
solid to an emerald green solution of [Rh (OAc) ]·2MeOH (0.051 g, 0.1 mmol) in acetone (75 mL) and
6
(
3
2
[
2
4
2
2
3
2
4
the mixture was stirred for one hour, producing a pink solution. The solvent was removed under vacuum,
yielding 0.135 g (89.4 %) of a pale pink solid. The solid was identified as the title compound. Anal.
Calcd. for C58
H-NMR (acetone-d
H
54
N
2
O
10Rh
2
S
4
Sn
): δ: 9.11 (d, J = 5.4 Hz, 2H, Py), 7.83-7.67 (m, 6H, Ph
Sn), 7.29 (d, J = 5.6 Hz, 2H, Py), 5.57 (s, 2H, Py-CH ).
Rh (OAc) ]·2[4-PyCH OCS Cu(PPh ] ([Rh]-2[Py-Cu]): [Py-Cu] (0.154 g, 0.2 mmol) was added
as a solid to an emerald green solution of [Rh (OAc) ]·2MeOH (0.051 g, 0.1 mmol) in acetone (50 mL)
2
: C = 46.12, H = 3.60, N = 1.85. Found: C = 46.51, H = 3.42, N = 1.54.
1
6
3
Sn), 7.56-7.47 (m, 9H,
Ph
3
2
[
2
4
2
2
3 2
)
2
4
and the mixture was stirred for one hour. During this time, a light orange-red precipitate formed, which
was collected on a frit by vacuum filtration, washed with diethyl ether and air-dried, yielding 0.179 g
(
89.9 %) of the title compound. Anal. Calcd. for C94
H
84Cu
2 2
N O
10
P
4
Rh
2
S : C = 56.83, H = 4.26, N = 1.41.
4
14

1
Found: C = 57.17, H = 3.92, N = 1.78. H-NMR (acetone-d
6
) (all peaks broad and very weak, no
assignments were made): δ: 8.73 (s), 7.74-7.37 (m), 5.59 (s).
2 2
Crystallography: For 4-PyCH OCS Na and [Py-Sn] X-ray intensity data were measured at 150(2) K
on a Bruker SMART APEX diffractometer (Mo Kα radiation, λ = 0.71073 Å) [22]. Raw area detector
data frame integration was performed with SAINT+ [22]. Final unit cell parameters were determined by
least-squares refinement. An absorption correction based on the multiple measurement of equivalent
reflections was applied to the data with SADABS [22]. Direct methods structure solution, difference
2
Fourier calculations and full-matrix least-squares refinement against F were performed with SHELXTL
[23]. For [Py-Cu] the X-ray data were collected at 100(2) K on a Bruker SMART APEXII CCD
diffractometer equipped with Cu-Kα radiation. Intensities were collected using phi and omega scans and
were corrected for Lorentz polarization and absorption effects. The X-SEED software platform [24],
equipped with SHELXS and SHELXL modules on a PC computer [25], was used for structure solution and
refinement calculations, and molecular graphics. The structures were solved by direct methods, and
refined by anisotropic full-matrix least-squares for all non-hydrogen atoms.
-
3
Titration studies: Stock solutions (1.5 ·10 M) of [Rh], [Py-Sn] and [Py-Cu] were prepared in an
acetone / dichloromethane (1 / 1, v / v) solvent system. Various volumes of [Py-Sn] or [Py-Cu],
corresponding to the desired number of equivalents, were added to a sample (750 µL) of [Rh] in a 1 cm
path-length cell. The final sample volume was kept constant at 3.0 mL and, accordingly, the final
-4
concentration of [Rh] was 3.75 · 10 M. The spectra were recorded in the 800 – 350 nm range.
Acknowledgments
The authors thank Eastern Illinois University for a Council on Faculty Research grant. The Single
Crystal Diffractometer at EIU was purchased using funds provided by an NSF-MRI grant (CHE-
0722547).
Appendix A. Supplementary Material
CCDC 1556155, 1556156, and 1556157contain 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.
15

2 2
Table 1. Crystal data and structure refinement details for 4-PyCH OCS Na and [Py-Sn], and [Py-Cu].
4
-PyCH
2
OCS
2
Na
[Py-Sn]
[Py-Cu]
2 2
36CuNOP S
Chemical formula
C
14
H
14
N
2
Na
2
O
3
S
4
25
C H
21NOS
2
Sn
C
43
H
M
r
432.49
534.24
772.33
Crystal system, space group
Orthorhombic, Pbcn Monoclinic, Cc
Triclinic, P-1
100
Temperature (K)
a (Å)
150
150
20.9667 (14)
9.9076 (6)
18.6634 (11),
13.2763 (8)
90
15.7555 (4)
16.1195 (4)
17.4752 (4)
71.275 (1)
87.843 (1)
62.904 (1)
3710.78 (16)
4
b (Å)
7.7314 (5)
c (Å)
11.1372 (7)
α (°)
90
β (°)
90
110.986 (1)
90
γ (°)
90
3
V (Å )
1805.4 (2)
4
2292.1 (2)
4
Z
−1
µ (mm )
0.59
1.31
2.97
Crystal size (mm)
0.42 × 0.38 × 0.30
0.863, 1.000
0.42 × 0.24 × 0.22
0.817, 1.000
0.46 × 0.32 × 0.22
0.558, 0.753
Tmin, Tmax
No. of measured, independent and 23089, 2250, 2102
17172, 5672, 5585 46739, 12843, 10448
observed [I > 2σ(I)] reflections
R
int
0.028
0.023
0.029
−1
(
sin θ/λ)max (Å )
0.667
0.667
0.602
2
2
2
R[F > 2σ(F )], wR(F ), S
No. of reflections
0.027, 0.074, 1.06
0.018, 0.046, 1.06
0.034, 0.103, 1.04
2250
118
5672
12843
901
No. of parameters
No. of restraints
271
2
−3
∆ρ
max, ∆ρmin (e Å )
0.46, −0.24
0.55, −0.27
0.68, −0.36
16

Table 2. Selected bond distances and angles for 4-PyCH
2
OCS
2
Na, [Py-Sn], and [Py-Cu].
4
-PyCH
2
OCS Na
Angles
2
[Py-Sn]
Angles
[Py-Cu]
Angles
Bonds
(Å)
(°)
Bonds
(Å)
(°)
Bonds
(Å)
(°)
Na1'''-N1
Na1'-O2
Na1'-O1
Na1-S1
Na1'-S1
Na1''-S1
Na1-S2
2.4682(11)
2.4761(9)
2.5425(9)
2.9306(6)
2.9856(6)
3.0301(5)
3.0357(6)
N1'-Na1'-O1
O2-Na1'-O1
N1'-Na1'-S1'
O2-Na1'-S1'
O1-Na1'-S1'
N1'-Na1'-S1
O2-Na1'-S1
O1-Na1'-S1
S1'-Na1'-S1
83.04(3)
122.87(3)
90.62(3)
137.009(19)
87.52(2)
88.01(3)
74.025(18)
53.968(19)
141.346(18)
Sn1-C31
Sn1-C11
Sn1-C21
Sn1-N1'
Sn1-S1
2.128(3)
2.136(3)
2.143(3)
2.529(2)
2.5932(7)
C31-Sn1-C11
C31-Sn1-C21
C11-Sn1-C21
C31-Sn1-N1'
C11-Sn1-N1'
C21-Sn1-N1'
C31-Sn1-S1
C11-Sn1-S1
C21-Sn1-S1
N1'-Sn1-S1
119.25(11)
116.62(11)
122.87(11)
89.81(9)
81.21(9)
88.08(9)
99.70(8)
83.90(8)
97.94(8)
164.89(6)
Cu1-P2A
Cu1-P1A
Cu1-S1A
Cu1-S2A
Cu2-P2B
Cu2-P1B
Cu2-S1B
Cu2-S2B
2.2331(6)
2.2460(6)
2.3899(6)
2.4273(6)
2.2307(6)
2.2359(6)
2.3758(6)
2.4388(6)
P2A-Cu1-P1A 126.08(2)
P2A-Cu1-S1A 109.44(2)
P1A-Cu1-S1A 112.39(2)
P2A-Cu1-S2A 119.22(2)
P1A-Cu1-S2A 103.28(2)
S1A-Cu1-S2A 75.31(2)
P2B-Cu2-P1B
P2B-Cu2-S1B
P1B-Cu2-S1B
P2B-Cu2-S2B
P1B-Cu2-S2B
S1B-Cu2-S2B
125.16(2)
108.47(2)
114.83(2)
117.32(2)
105.17(2)
75.31(2)
Na1-Na1'' 3.6763(9)
N1'-Na1'-S1'' 166.66(3)
O1-Na1'-S1''
S1'-Na1'-S1''
S1-Na1'-S1''
N1'-Na1'-S2
O2-Na1'-S2'
O1-Na1'-S2'
S1-Na1-S2
89.92(2)
77.725(13)
97.016(14)
87.92(3)
89.59(2)
146.36(3)
60.163 (12)
158.148 (19)
S1-Na1'-S2'
S1-Na1''-S2''' 91.787 (14)
17

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Graphical Abstract
Tetrametallic heteronuclear complexes were prepared from neutral metalloligands, comprising of a
second-generation xanthate ligand coordinated to Sn(IV) and Cu(I) centers.
21

Highlights:
•
•
•
•
A backbone functionalized xanthate ligand has been prepared.
Its propensity to coordinate to Sn(IV) and Cu(I) centers has been explored.
Neutral metalloligands have been obtained.
Hetero-tetrametallic complexes have been prepared.
22