Synthesis and structure of rare zwitterionic complexes involving the presence of N(py)MCl3? moieties (Mu202f=u202fPt(II), Pd(II))

Article abstract of DOI:10.1016/j.ica.2018.04.031

The monocation 2,3-dicyano-5-[(N-methyl)-2-pyridyl}-6-(2-pyridyl)pyrazine, [(CN)₂(2-Mepy)PyPyz]⁺ (1) reacts, as the iodide salt (2), under mild experimental conditions, with dichloro-di(benzonitrile)platinum(II), [(C₆H₅CN)₂PtCl₂], forming trans-diiodo-di(benzonitrile)platinum(II), [(C₆H₅CN)₂PtI₂] (3), and the complex [(CN)₂Py(2-Mepy)PyzPtCl₃]?CH₃CN (4), both structurally elucidated by single-crystal X-ray work. Using a parallel procedure, reaction of 2 with dichloro-di(benzonitrile)palladium(II), [(C₆H₅CN)₂PdCl₂], results in the formation of complex 5, [(CN)₂Py(2-Mepy)PyzPdCl₃]?CH₃CN, isostructural with 4. Complex 5 is also formed by reaction of the same reactant 2 with PdCl₂. Complexes 4 and 5, further characterized by IR and UV–vis spectral data, are neutral stable zwitterions presenting both a +1 pyridinium component neutralized by rare N_(py)PtCl₃^? and N_(py)PdCl₃^? anionic moieties, respectively (py = 2-pyridyl group). DFT calculations fit reasonably well with the observed UV–vis absorptions and provide information on the localization of the HOMO and LUMO within the species 4 and 5.

Full text of DOI:10.1016/j.ica.2018.04.031

Accepted Manuscript
Research paper
Synthesis and structure of rare zwitterionic complexes involving the presence
of N(py)MCl3 - moieties (M = Pt(II), Pd(II))
Elisa Viola, Maria Pia Donzello, Claudio Ercolani, Corrado Rizzoli, A.B.P.
Lever
PII:
S0020-1693(18)30238-X
https://doi.org/10.1016/j.ica.2018.04.031
ICA 18222
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
11 February 2018
12 April 2018
18 April 2018
Please cite this article as: E. Viola, M.P. Donzello, C. Ercolani, C. Rizzoli, A.B.P. Lever, Synthesis and structure
-
of rare zwitterionic complexes involving the presence of N(py)MCl3 moieties (M = Pt(II), Pd(II)), Inorganica
Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.04.031
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Synthesis and structure of rare zwitterionic complexes involving the presence of
-
N(py)MCl3 moieties (M = Pt(II), Pd(II))
Elisa Viola†, Maria Pia Donzello†*, Claudio Ercolani†, Corrado Rizzoli*, and A.B.P. Lever&*
† Dipartimento di Chimica, Università di Roma Sapienza, P.le Aldo Moro, 5 – 00185 Roma, Italy
 Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università degli
Studi di Parma, Parco Area delle Scienze 17/A, I-43124 Parma, Italy
& Department of Chemistry, York University, 4700 Keele St., Toronto, Ont. Canada M3J 1P3
Keywords: Pt(II) and Pd(II) zwitterionic complexes; X-ray crystallography; electronic spectra
Abstract
The
monocation
2,3-dicyano-5-[(N-methyl)-2-pyridyl}-6-(2-pyridyl)pyrazine,
[(CN)2(2-
Mepy)PyPyz]+ (1) reacts, as the iodide salt (2), under mild experimental conditions, with dichloro-
di(benzonitrile)platinum(II), [(C6H5CN)2PtCl2], forming trans-diiodo-di(benzonitrile)platinum(II),
[(C6H5CN)2PtI2] (3), and the complex [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4), both structurally
elucidated by single-crystal X-ray work. Using a parallel procedure, reaction of 2 with dichloro-
di(benzonitrile)palladium(II), [(C6H5CN)2PdCl2], results in the formation of complex 5,
[(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN, isostructural with 4. Complex 5 is also formed by reaction of
the same reactant 2 with PdCl2. Complexes 4 and 5, further characterized by IR and UV-vis
spectral data, are neutral stable zwitterions presenting both a +1 pyridinium component neutralized
-
-
by rare N(py)PtCl3 and N(py)PdCl3 anionic moieties, respectively (py = 2-pyridyl group). DFT
calculations fit reasonably well with the observed UV-vis absorptions and provide information on
the localization of the HOMO and LUMO within the species 4 and 5.
1. Introduction
The multifaceted modes of binding of metal ions by polyazine N donors such as 2,3-di(2-
pyridyl)pyrazine (dpp, Scheme 1A) and derivatives carrying open chain or annulated ring
substituents in 5,6 positions have been recently reviewed.1 As extensively demonstrated by
crystallographic work, for a variety of mononuclear metal complexes bidentate chelation of the
donor ligand takes place either by using both the N atoms of the vicinal pyridine rings (“py-py“
coordination, Scheme 1B) or involving one pyridine N atom and one pyrazine N atom (“py-pyz”
coordination, Scheme 1C), depending on the type and oxidation state of the metal center. With
specific reference to mononuclear derivatives of unsubstituted dpp, of formula [M(dpp)X2] (M =
1

PdII or PtII; X = Cl-, Br-, I-) numerous complexes were recently reported and structurally
characterized predominantly by the same group of authors.2 Among the complexes described, either
“py-py”2a-e or “py-pyz”2f-j modes of coordination are well represented, the two modes even being
characterized for the same species, e.g. [PdCl2(dpp)]2d,g and [PtBr2(dpp)]2c,h in separate samples
(see Figure S1 for the PtII species).
N
N
2
3
N
1 N
4
6
5
A: dpp
M
N
N
N
N
N
N
N
6
5
5
6
M
C: py-pyz
B: py-py
Scheme 1
2

Our previous work demonstrated that the dicyano derivative of dpp, 2,3-dicyano-5,6-di(2-
pyridyl)pyrazine, [(CN)2Py2Pyz], reacts with dichloro-di(benzonitrile)palladium(II),
[(C6H5CN)2PdCl2], at room temperature forming the mononuclear complex [(CN)2Py2PyzPdCl2].3
+
CH3
N
M
N
N
NC
CN
The analogous PtII complex, [(CN)2Py2PyzPtCl2] (Figure S2) could also be similarly obtained.4 The
two complexes proved to be isostructural by single-crystal X-ray work, with “py-py” coordination.
In light of this, we considered that the dpp +1 charged derivative of formula [(CN)2Py(2-
Mepy)Pyz]+ (1), obtained as the iodide salt [(CN)2Py(2-Mepy)Pyz](I) (2) by reaction of
[(CN)2Py2Pyz] with CH3I,5 could be the appropriate dpp derivative able to promote the formation of
PtII and PdII mononuclear complexes with a “py-pyz” mode of coordination. In fact, as was
established by crystallographic work on 25 (Figure 1A), the unquaternized pyridine ring and the
pyrazine ring in the cation [(CN)2Py(2-Mepy)Pyz]+ are almost coplanar (dihedral angle: 18.3(3)°),
this arrangement appearing favourable to promote the “py-pyz” mode of coordination (Figure 1B);
the other N-methylated pyridine ring is positioned almost perpendicular to the plane of the pyrazine
ring (dihedral angle: 76.2(4)°). As shown by the data presented here below, reaction of the iodide
salt 2 with appropriate PtII and PdII reagents leads in both cases to interesting new species different
from those which motivated this work.
A
B
Figure 1. A: Structure of the cation [(CN)2Py(2-Mepy)Pyz]+ (1) with displacement ellipsoids drawn
at the 50% probability level; B: Schematic “py-pyz” mode of coordination of the same cation to a
metal center (M = PtII, PdII).
2. Results and Discussion
2.1 Synthesis of new PtII and PdII complexes of cation 1.
3

The synthesis of the PtII derivative of cation 1 was accomplished by reaction of 2 with
[(C6H5CN)2PtCl2] under experimental conditions close to those used for the synthesis of the two
above cited species [(CN)2Py2PyzMCl2] (M = PdII, PtII).3,4 The reaction followed an unexpected
pathway leading to the formation of two new species, ie. trans-diiodo-di(benzonitrile)-
platinum(II)), trans-[(C6H5CN)2PtI2] (3) and the PtII complex of formula [(CN)2Py(2-
Mepy)PyzPtCl3]∙CH3CN (4), both structurally elucidated by single-crystal X-ray work (Figure
2). Details about the synthetic procedure are given in the Experimental Section.
Figure 2. ORTEP view of: (A) trans-[(C6H5CN)2PtI2] (3), with displacement ellipsoids
drawn at the 50% probability level; (B) [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4), with
Figure 2. ORTEP view of: (A) trans-[(C6H5CN)2PtI2] (3), with displacement ellipsoids
displacement ellipsoids drawn at the 40% probability level.
drawn at the 50% probability level; (B) [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4), with
displacement ellipsoids drawn at the 40% probability level.
Formation of 3 is obviously determined by the release of Cl- ions from [(C6H5CN)2PtCl2] and
assumption of the free I- ions present in solution. In a concomitant process, PtCl2, released from
(A)
(B)
[(C6H5CN)2PtCl2], coordinates to the N atom of the unquaternized pyridine ring present in 1 and,
-
upon capture of one of the available Cl- ions, generates the -1 charged N(py)PtCl3 coordination site,
-
resulting in the formation of complex 4. As can be seen (Figure 2B), the fragment N(py)PtCl3 in the
complex is neutralized intramolecularly by the +1 charge located on the methylated N atom of the
remote pyridine ring. This definitely identifies complex 4 as a neutral stable zwitterionic complex.
4

In an effort to further validate the process illustrated above for PtII, a parallel type of reaction
was attempted involving as reactants the iodide salt 2 and the PdII species [(C6H5CN)2PdCl2] (see
details in the Experimental Section). The reaction led to the formation of the zwitterionic PdII
complex [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5; Figure S3), isostructural with the PtII analog 4,
as unequivocally demonstrated by a full single-crystal X-ray work (Table 1), thus also characterized
-
by the presence of the N(py)PdCl3 fragment formed in a process similar to that mentioned above for
the PtII species 4. No trace of other compounds could be isolated in this reaction.
Table 1. Experimental Data for the X-ray Diffraction Studies on Crystalline Compounds 3, 4 and 5.
3
4
5
Formula
C14H10I2N2Pt
C17H11Cl3N6Pt.
C2H3N
C17H11Cl3N6Pd. C2H3N
Formula weight
Crystal habit
Crystal colour
Crystal dimension, mm
Crystal system
Space group
Cell parameters:
a, Å
655.13
prism
yellow
0.12x0.14x0.22
monoclinic
C2/c
641.81
block
orange
0.18x0.21x0.30
monoclinic
P21/n
553.12
block
brown
0.16x0.23x0.28
monoclinic
P21/n
11.281(2)
20.270(4)
7.3389(12)
90
97.998(3)
90
1661.8(5)
4
2.618
11.718(4)
14.745(5)
12.795(4)
90
97.986(5)
90
2189.3(13)
4
1.947
11.6604(6)
14.7378(8)
12.7443(7)
90
98.1562(8)
90
2167.9(2)
4
1.695
b, Å
c, Å
, °
, °
, °
V, Å3
Z
D
, Mg m-3
calc
Linear absorption coefficient, 12.147
6.796
1.247
mm-1
Diffractometer
Bruker
CCD
APEX-II Bruker APEX-II CCD Bruker APEX-II CCD
Temperature, K
Radiation, Å
Data collection range of 2,
deg
294(2)
0.71073
2.0-25.5
294(2)
0.71073
2.1-25.5
294(2)
0.71073
2.1-25.5
Reflections measured
Total data collected
±h, ±k, ±l
9107
±h, ±k, ±l
22667
±h, ±k, ±l
15467
5

Unique total data
Unique observed data
Criterion for observation
1548
1349
I > 2 (I)
4078
2723
I > 2 (I)
4047
3566
I > 2 (I)
Unique data used in the 1548
refinement (NO)
4078
4047
Number of parameters refined 91
(NV)
273
272
Overdetermination
(NO/NV)
ratio 17.0
14.9
14.9
Transmission coefficients
0.112-0.408
0.039
= 0.108
0.376-0.745
0.046
0.144
0.337-0.618
0.035
0.105
R = FFoa
wR2
2
2
2
2
b
½
[wF  /wFo  ]
2
2
1.125
1.092
1.053

GOF=[wF
/
(NO-
½
NV)]
Largest shift/esd, final cycle
Largest peak, e/Å3
<0.001
3.01c
0.001
2.26d
0.001
1.16e
b
c
a calculated on the unique observed data; calculated on the unique data used in the refinement; located at 0.96 Å
d
e
from the Pt1 atom; located at 0.95Å from the Pt1 atom; located at 0.72Å from the Cl3 atom.
2.2 Structural features of 4 and 5.
Crystal data of 4 (Figure 2B) and 5 (Figure S3) and details associated with the structure
refinement are given in Table 1; selected bond lengths (Å) and angles (°) are listed in Table 2. The
asymmetric unit for the two species 4 and 5 consists of a complex molecule and a CH3CN molecule
in the stoichiometric molar ratio of 1:1. The square-planar NCl3 coordination geometry at the metal
centres PtII and PdII for 4 and 5, respectively, is slightly distorted (maximum deviation 0.071(9) and
0.104(3) Å for the atom N6 in the two species), with the metal displaced by only 0.0557(5) and
0.0608(3) Å for PtII and PdII, respectively. This plane forms with the coordinated pyridine ring a
dihedral angle of 58.2(2)° in 4 and 58.66(8)° in 5 and is approximately parallel to the
methylpyridine ring, forming dihedral angles of 12.5(3)° in 4 and 12.12(8)° in 5. In this
conformation, one chloride anion (Cl1) overlaps the centroid of the methylpyridine ring at a distance
of 3.353(5) and 3.3401(18) Å in 4 and 5, respectively. The pyrazine ring (N1/C1/C2/N2/C3/C4)
assumes a flattened twist-boat conformation, with puckering parameters QT = 0.085(10) Å,  =
101(7)°,  = -86(6)° in 43, and QT = 0.085(3) Å,  = 97(2)°,  = -77(2)° in 5. The least-square mean
plane through the pyrazine ring forms with the aromatic rings of the pyridine and methylpyridine
substituents dihedral angles of 48.7(3) and 46.0(3)° in 4, and 45.94(10) and 48.12(12)° in 5. The
dihedral angle between the pyridine rings is 60.4(4) and 59.41° for 4 and 5, respectively. A
comparison of the MCl (mean value 2.332(12) Å in 4 and 2.309(8) Å in 5) and MN (2.044(8) Å
6

in 4 and 2.047(3) Å in 5) bond distances indicates that their values are not significantly different and
are not affected by the nature of the metal. In the crystal, complex molecules for both species are
linked by CH...N and CH...Cl hydrogen bonds (Table S1) into supramolecular tubes aligned
along the a axis having cavities (estimated volume of 185(3) and 179(3) Å3 for 4 and 5,
respectively) able to host centrosymmetric pairs of acetonitrile molecules.
Complexes 4 and 5 are almost exclusive examples of zwitterionic compounds characterized by
-
the presence of a NMCl3 fragment (M = PtII, PdII) in which the N atom belongs to a pyridine ring,
-
N(py)PtCl3 . To our knowledge, only one couple of analogs of formula [M(2,4’)-mbipy)Cl3]2H2O
(M = PtII, PdII) presenting the same fragment has been reported, the structure of the PtII derivative
being elucidated by X-ray work.6 The complex shows bond distances and angles for the fragment
-
-
N(py)PtCl3 closely approaching those of 4 and 5 (Table 2), with the PtCl3 coordinated to the N atom
of one of the two pyridine rings of the mbipy ligand. The +1 internal site is located on the
quaternized remote pyridine ring of mbipy. Three additional similar PtII complexes of known crystal
structure with molecular features typical of zwitterionic species are trichloro(thiamin)platinum(II)-
mono-aquo,7
trichloro(9-methyl-adeninium)platinum(II),8
and
trichloro(9-methyl-
-
guaninium)platinum(II)mono-aquo,9 in which the N atom of the NPtCl3 coordination site belongs
to a heterocyclic ring different from pyridine.
Table 2. Selected Bond Distances (Å) and Angles (°) for Compounds 3, 4, 5.
3
4
5
2.6052(8)
1.930(9)
1.943(9)
2.327(3)
2.311(3)
2.353(3)
2.044(8)
2.3165(10)
Pt1I1
Pt1N1
Pt1N2
Pt1Cl1
Pt1Cl2
Pt1Cl3
Pt1N6
Pd1Cl1
Pd1Cl2
Pd1Cl3
Pd1N6
2.2935(9)
2.3288(11)
2.047(3)
180(-)
178.97(3)
Symmetry code: (i) 1-x, y, 3/2-z.
176.82(13)
178.0(2)
175.93(5)
178.14(9)
N1Pt1N2
I1Pt1I1i
Cl1Pt1Cl3
Cl2Pt1N6
Cl1Pd1Cl3
Cl2Pd1N6
In an attempt to verify the possibility of formation of a “py-pyz” coordination site of the type
proposed in Figure 1B and isolation of the related salt-like species [(CN)2(2-
Mepy){Py(PdCl2)}Pyz](I), a reaction of the +1 charged ion 1 (as iodide salt) was conducted using
PdCl2 instead of [(C6H5CN)2PdCl2] (see Experimental Section). The observed reaction pathway
7

allowed isolation of crystals suitable for X-ray analysis which unequivocally identified the complex
again as the zwitterionic species 5. This result suggests that in the reaction performed, release of Cl-
-
ions by PdCl2 might lead to formation of the PdCl3 anion, which is converted in the presence of
-
cation 1 into the N(py)PdCl3 fragment, a process therefore concluded with formation of the
zwitterionic species 5. No trace of other compounds were isolated in the process. The observed
formation of the isostructural complexes 4 and 5 is of particular interest as indicative that the
overall reaction process of the cation 1 with the species [(C6H5CN)2MCl2] (M = PtII, PdII) or with
PdCl2 undergoes a reproducible evolution, no matter which metal center or species involved.
2.3 Spectroscopic features of 3, 4 and 5.
Some aspects of the IR and UV-visible spectroscopic features of the complexes 3, 4 and 5
deserve some illustration. The IR spectra of the complexes 3-5 were examined in the range 4000-
300 cm-1. Of interest is particularly the region 400-300 cm-1, where νPt-Cl and νPd-Cl are expected to
occur. In fact, split νPt-Cl and νPd-Cl (not significantly moving as a function of metal ion) have
previously been reported in that region for [(CN)2Py2PyzPtCl2] (350/340 cm-1),4
[(CN)2Py2PyzPdCl2] (340/335 cm-1),3 with the related closely lying modes occurring for the
zwitterions 4 and 5 at 341/330 and 339/334 cm-1, respectively. DFT calculations for complex 5 (gas
phase) gave a pair of bands at 332 and 317 cm-1, assigned to the Pd-Cl stretching vibration trans to
the coordinating pyridine and the asymmetric stretch of the trans Cl pair of atoms, respectively. For
complex 3, trans-[(C6H5CN)2PtI2], νPt-I is observed at lower frequencies (332/326 cm-1) in a
comparison with the value of νPt-Cl for [(C6H5CN)2PtCl2] (356/344 cm-1). Figure S4 shows the
spectra in the range 1800-300 cm-1 of 3 and the two isostructural 4 and 5 which appear quite similar
as expected.
The UV spectroscopic data of the complexes 3, 4 and 5 in solution of CH3CN are listed in Table
3. The complexes exhibit three main absorption bands in the region 200-300 nm (with evidence for
further weak unresolved absorptions in the range 300-500 nm).
Table 3.
UV Spectroscopic data in CH3CN of [(C6H5CN)2PtCl2],[(CN)2(2-
Mepy)PyPyz](I) (2), trans-[(C6H5CN)2PtI2] (3), [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN
8
(4) and [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5).

Compound
λ, nm (log ε)
267(sh) (4.41)
269 (4.20)
trans-[(C6H5CN)2PtI2] (3)
244 (4.63)
240 (4.30)
245 (4.45)
299(sh) (4.11)
298 (4.19)
[(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4)
[(CN)2Py(2Mepy)PyzPdCl3]∙CH3CN (5)
280(sh) (4.33)
299 (4.35)
The UV-Vis spectra for 3 and 4 are shown in Figure 3. The spectrum of 5 reported in Figure 4 is
similar to that of 4 in terms of peak positions and ε values. A brief DFT analysis was carried out on
the PdII complex 5 using a geometry optimized structure. The DFT predicted gas phase structure
had coordinates closely similar to the X-ray structure (see Experimental section). The LUMO
(#113) (see below) and LUMO+1 (#114) are delocalised primarily over the pyrazinoid π* ligand
framework incorporating the pyridinium ring in the former and more the coordinating pyridine ring
-
in the latter. The frontier filled MOs from #108 to HOMO #112 are all localized on the PdCl3
fragment, being a mix of metal d orbitals and chloride π orbitals. The first ligand based frontier
orbital is then #107 (HOMO-5). A time dependent DFT analysis10 (of an acetonitrile solution
optimized structure) was employed to predict the UV-Vis spectrum of 5. While b3lyp has been
criticized when used to predict long range transition energies11 it seems quite adequate for short
range intramolecular charge transfer.12 The predicted UV-Vis spectrum agreed quite reasonably
well with the experimental spectrum (Figure 4) albeit some absorption shifted to longer
wavelengths. Although there appear to be three main ‘peaks’, there are in fact a large number of
weak transitions underlying the absorption envelope as shown by the oscillator strength bars.
9

Figure 3. UV-Vis spectroscopic data in CH3CN of trans-
[(C6H5CN)2PtI2] (3) (A) and [(CN)2Py(2-
Mepy)PyzPtCl3]∙CH3CN (4) (B).
Figure 4. Time dependent DFT predicted electronic spectrum of the PdII species 5. A halfwidth at
halfbandheight of 1500 cm-1 was used to simulate the DFT spectrum. The species was geometry
optimized using CH3CN as solvent, and the spectrum derived using the Polarized Continuum
Model. Blue vertical bars are locations of the electronic transitions scaled to their oscillator
strengths. The HOMO to LUMO transition at 594 nm is too weak to be identified.
Essentially all the transitions in the visible and near UV region, up to and including the peak at
-
ca. 310 nm are described as MLCT/L’LCT transitions from the anionic PdCl3 fragment to the
positively charged ligand core. Indeed the lowest energy MLCT is the HOMO to LUMO transition
predicted to lie as a very weak transition near 594 nm. The more obvious absorption around 380 -
450 nm in Figure 4 arises from MLCT/L’LCT involving deeper lying d+Cl orbital MOs. The higher
energy absorption below 300 nm is a mix of π → π* and additional MLCT transitions.
10

It is of interest to take a closer look at the HOMO orbital (gas phase) shown in Figure 5A
(isosurface 0.02). It is clearly a metal d orbital with chloride π* admixture. Note however that it is
oriented towards the adjacent, almost parallel, pyridinium ring. Figure 5B shows the same orbital
but using an expanded isosurface of 0.007. This clearly demonstrates a small contribution, to the
HOMO, of a pyridinium π* orbital, indicative of a direct pathway for charge transfer between the
ionic components of the zwitterion. However, as noted below, the quantity of charge transferred is
not significant. Similarly, the complex 5 LUMO (gas phase; isosurface 0.02), shown in Figure 5C,
is localized on the pyridinium ring and the pyrazine fragment but not significantly on the
-
coordinating pyridine ring. This orbital also has a very small contribution from the PdCl3 unit with
a fractional contribution from a PdII d-orbital and from the π-orbital of the chloride that lies over
the pyrazine ring centre.
A
B
C
Figure 5. HOMO of the gas phase optimized PdII structure 5 (A; isosurface 0.02); HOMO of gas
phase optimized PdII structure 5 (B; isosurface 0.007). LUMO of the gas phase optimized PdII
structure 5 (C; isosurface 0.02). Note the orientation looking along the plane of the pyridinium ring
with the methyl group facing outwards towards the viewer.
The charge distribution within the molecule is of some interest. The natural bond orbital
approach13 was used to show that in the free ligand, the net +1 charge resides almost 100% on the
pyridinium ring while the other pyridine and the pyrazine fragments have equal and opposite
-
charges of ca +0.08 and -0.07 esu respectively. Upon coordination of the PdCl3 group, there is net
-
charge donation from pyridine to PdCl3 of ca 0.19 esu, mostly σ but with some back donation.
11

This charge comes almost exclusively from the pyridine unit and the net charge on the pyrazine (-
0.07 esu) and pyridinium units (0.99 esu) remains essentially the same as in the free ligand. Thus
there is no measurable through space transfer between the anion and cation in the ground state.
The lowest lying excited state will be the spin triplet generated by the HOMO to LUMO
transition. This was optimized to briefly explore its charge distribution. Formally an electron moves
-
from the PdCl3 site to the pyridinium site and so one might expect that the pyridinium unit would
be neutralized. However, once the positive charge on the pyridinium unit is diminished, it is the
pyrazine moiety that becomes the better acceptor, and it is there that the negative charge builds up.
-
The NBO charges on PdCl3 , pyrazine, pyridinium and coordinating pyridine are then respectively -
0.25, -0.45, +0.53 and +0.17 esu.
3. Conclusions
The reaction of the iodide salt of 2,3-dicyano-5-[(N-methyl)-2-pyridyl}-6-(2-pyridyl)pyrazine,
[(CN)2(2-Mepy)PyPyz](I) (2), with dichloro-di(benzonitrile)platinum(II), [(C6H5CN)2PtCl2], leads
to formation of the complex [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4) under mild experimental
conditions. In a parallel reaction, the same iodide salt 2 with the PdII complex [(C6H5CN)2PdCl2]
produces the complex [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5) which is also obtained using the
PdII reactant PdCl2. X-ray data reveal that species 4 and 5 are isostructural and are characterized by
-
the presence of MCl3 groups attached to the neutral pyridine N atom of the cation [(CN)2Py(2-
-
Mepy)Pyz]+ (1) thus forming the N(py)MCl3 anionic fragments with M = PtII or PdII,
intramolecularly neutralized by the N-methylated +1 charged remote pyridine ring in 1. The two
complexes are rare examples of zwitterionic PtII and PdII complexes typically presenting the anionic
-
moiety N(py)MCl3 and structural data are discussed in relationship with those reported in the
literature for similar species. The reaction studied of 2 with [(C6H5CN)2PtCl2] led to the new
complex trans-diiododi(benzonitrile)platinum(II), [(C6H5CN)2PtI2] (3) the structure of which has
also been elucidated by X-ray work. The IR and UV spectroscopic behaviour of the complexes 3, 4
and 5 are reported. As to the PdII complex 5, the DFT analysis nicely reproduces the main features
-
of the PdCl3 fragment and suggests a direct through space pathway for charge flow between
12

anionic and cationic components. This pathway does not seem important in the ground state but is
likely a key feature of the excited state properties of these species. The prevalence of the binding of
the metal centers (M = PtII, PdII) in 4 and 5 to the N atom of the uncharged pyridine ring with
-
formation of the NMCl3 fragments rather than the binding to the potentially bidentate coordination
site formed by the pyrazine and pyridine N donor atoms is most likely determined by the weak
donor properties of the dicyanopyrazine ring. Note that if a dihydropyrazine ring is used as in dhp
(di(2-pyridyl)dihydropyrazine), coordination of PtCl2 takes place at the chelating site, as
1
unequivocally proven by H NMR spectroscopic data,14 leaving the neutral pyridine ring
-
unchanged, hence with the exclusion of the formation of the NPtCl3 fragment. Further studies of
this problem are underway.
4. Experimental section
4.1 Solvents and reagents
Solvents and reagents were commercially obtained and used as received. The synthesis of
[(CN)2Py(2-Mepy)Pyz](I) (2) by reaction of [(CN)2Py2Pyz] with CH3I was performed as reported
previously.5 [(C6H5CN)2PtCl2] and [(C6H5CN)2PdCl2] were prepared as described elsewhere.15
4.2 Synthesis of trans-[(C6H5CN)2PtI2] (3) and [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4).
A solution of the iodide salt 2 (142 mg, 0.330 mmol) and [(C6H5CN)2PtCl2] (172 mg, 0.364
mmol; molar ratio 1/1.1) in CH3CN (30 mL) was heated at 70 °C for 2 h. After cooling, the orange
solution was kept in the refrigerator and needle-like light-orange crystals were formed and isolated
in two steps over a few days. The crystals were filtered off, washed with CH3CN, and dried under
vacuum (14.0 mg; yield6.5%). Single crystal X-ray work established the crystals to be trans-
[(C6H5CN)2PtI2] (3) (shown in Figure 2A) not previously reported in the literature. Calcd for 3,
C14H10I2N2Pt: C, 25.67; H, 1.54; N, 4.28. Found: C, 25.97; H, 1.52; N, 4.30%. IR (KBr, nujol/CsI,
cm-1): 3053 vw, 2285 w (νCN), 1697 vw, 1622 w, 1591 w, 1572 w, 1508 vw, 1485 w, 1444 s, 1381
w, 1315 vw, 1286 vw, 1249 vw, 1201 w-m, 1176 w, 1155 w, 1091 w, 1070 w, 1022 w, 999 w, 926
vw, 785 vw, 754 vs, 681 s, 577 vw, 538 s, 420 w, 393 w, 332 w, 326 w-m (split νPt-I). UV
spectroscopic data in CH3CN (, nm (logε)): 244 (4.63), 267(sh) (4.41), 299(sh) (4.11). Long
13

standing of the mother liquors in the refrigerator (weeks) led to the formation of dark-orange
twinned crystals of a compound assigned, on the basis of the structure elucidated by single-crystal
X-ray work, the formula [(CN)2Py(2-Mepy)PyzPtCl3]∙CH3CN (4), shown in Figure 2B (35.5 mg;
yield: 17%). Calcd for C19H14Cl3N7Pt: C, 35.56; H, 2.20; N, 15.28. Found: C, 35.16; H, 1.98; N,
15.14%. IR (KBr, nujol/CsI, cm-1): 3049 m, 2241 vw(νCN), 1618 m, 1599 w, 1581 w, 1527 w, 1508
w-m, 1483 w, 1460 w, 1446 w, 1435 w, 1387 vs, 1284 m, 1250 w-m, 1176 m, 1155 vw, 1140 w,
1130 w, 1111 vw, 1091 w-m, 1066 w, 1032 vw, 953 vw, 902 vw, 812 vw, 798 vw, 785 s, 771 m,
748 s, 700 w, 667 vw, 577 w-m, 551 vw, 528 m, 501 vw, 465 vw, 442 w, 432 w, 382 w-m, 341 m,
330 m (split νPt-Cl). UV spectroscopic data in CH3CN (, nm (logε)): 240 (4.30), 269 (4.20), 298
(4.19).
4.3 Synthesis of [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5) from 2 and [(C6H5CN)2PdCl2]
A solution of 2 (50.9 mg, 0.119 mmol) and [(C6H5CN)2PdCl2] (50.4 mg, 0.131 mmol) in CH3CN
(2.0 ml) was kept at room temperature. After 2 h the formation of a brown precipitate from the
dark-red solution was observed. The microcrystalline solid (20.5 mg) was separated by
centrifugation, washed with CH3CN, and brought to a constant weight under vacuum (10-2mmHg).
Long standing in the refrigerator of the mother liquors (several weeks) gave a few brown crystals
(8.1 mg) suitable for X-ray analysis. The elucidated structure indicated the formation of a PdII
compound of formula [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5), isostructural with the PtII analog
4. The initial microcrystalline solid was found by X-ray powder patterns to be isomorphous with the
crystals of compound 5 (total yield: 43%). No trace of other compounds could be isolated. Calcd for
C19H14Cl3N7Pd: C, 41.26; H, 2.55; N, 17.72. Found: C, 40.92; H, 2.38; N, 17.55%. IR
(KBr,nujol/CsI, cm-1): 3050 m-s, 2245 vw (νCN), 1618 m, 1594 w, 1581 w, 1529 w, 1506 w-m,
1483 w, 1462 w, 1446 w, 1435 w, 1387 vs, 1284 m, 1246 w-m, 1176 m, 1155 vw, 1140 w, 1130 w,
1108vw, 1091 w-m, 1063 w, 1032 w, 953 vw, 908 vw, 812 vw, 798 w, 789 s, 771 m, 744 s, 698 w,
667 vw, 650 vw, 578 w-m, 550 vw, 528 w-m, 501 vw, 459 vw, 442 w, 434 w, 380 w, 339 s, 334 s
(split νPd-Cl). UV spectroscopic data in CH3CN (, nm (logε)): 245 (4.45), 280(sh) (4.33), 299
(4.35).
14

4.4 Synthesis of [(CN)2Py(2-Mepy)PyzPdCl3]∙CH3CN (5) from 2 and PdCl2.
A solution of 2 (20.5 mg, 0.047 mmol) and PdCl2 (14.0 mg, 0.079 mmol) in CH3CN (3 mL) was
kept at room temperature. Formation of red-brown crystals from the orange solution was observed
after some days. The crystals were filtered off, washed with CH3CN, and brought to constant weight
under vacuum (10-2 mmHg; 12 mg, yield 46% ). Calcd for C19H14Cl3N7Pd: C, 41.26; H, 2.55; N,
17.72. Found: C, 41.15; H, 2.35; N, 17.55%.
4.5 X-ray Diffraction Data
Data on the compounds 3, 4 and 5 were collected on a Bruker APEX-II CCD diffractometer
using graphite-monochromatized Mo-K radiation at 294(2) K. Unit cell parameters were
determined by using the APEX21 program. Data reduction was carried out by the SAINT1 program.
Correction for absorption was performed using the SADABS16 program. The function
minimized during the least-square refinement was w(F2)2. Anomalous scattering
corrections were included in structure factor calculations. Structure solution was based on the
observed reflections [I>2(I)] while the refinements were based on the unique reflections
having I>0. The structure was solved by direct methods using SHELXT.17 Refinement was
done anisotropically by full matrix least-squares for all non-H atoms using SHELXL-
2014/7.18 The hydrogen atoms were placed in idealized calculated positions with C-H = 0.93-
0.96 Å and refined using a riding model approximation, with Uiso(H) = 1.2 Ueq(C) or 1.5 Ueq(C) for
methyl H atoms. A rotating group model was adopted for the methyl groups in 3 and 4. In3, the
displacement parameters of the acetonitrile molecule were restrained to be similar and nearly
isotropic (SIMU and ISOR commands in SHELXL-2014/7).
4.6 Density Functional Theory (DFT) calculations.
The PdII complex 5 was geometry optimized using Gaussian19 both as a gas phase species and
in a computed solvent environment (CH3CN) using the Polarized Continuum Model.20 The b3lyp
functional was employed21 with lanl2dz basis set22 for hydrogen, carbon, and nitrogen and def2-
tzvp basis sets for palladium and chlorine, the latter two obtained from the EMSL Basis Set
exchange (https://bse.pnl.gov/bse/portal). Tight convergence was employed and frequency
calculation employed to ensure the structure was at a potential energy minimum. Structural x,y,z
coordinates are provided in the supplementary file. The gas phase structure yielded a Pd-N distance
15

of 2.08 Å and Pd-Cl distances of 2.35 (trans halides) and 2.30 Å. These values are very closely
similar to the X-ray distances listed in Table 2.
4.6 Other Physical measurements
IR spectra of the solid materials as KBr pellets or by using Nujol mulls and CsI windows were
recorded in the range of 4000-300 cm-1 on a Varian 660-IR FT-IR spectrometer. UV-visible solution
spectra were recorded on a Varian Cary 5E spectrometer by using 1-cm quartz cuvettes. Elemental
analyses for C, H, and N were provided by the “Servizio di Microanalisi” at the Dipartimento di
Chimica, Università “Sapienza” (Rome) on an EA 1110 CHNS-O instrument.
ASSOCIATED CONTENT
Supporting Information
Molecular structure of compounds [PtBr2dpp], [(CN)2Py2PyzPtCl2 and complex 5. IR spectra of 3, 4
and 5. Packing diagrams for 3,4 and 5 ; hydrogen bonding interactions for 4 and 5. The x,y,z,
coordinate listing for the X-ray determined structures of 3, 4 and 5 and of the DFT derived
geometries of complex 5 (gas phase, CH3CN solution). X-ray crystallographic information files:
CCDC 1522248 (3), 1522249 (4), and 1522250 (5). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/x0xx00000x.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mariapia.donzello@uniroma1.it
*E-mail corrado.rizzoli@unipr.it
*E-mail: blever@yorku.ca
Author Contributions
The manuscript was written through contribution of all authors.
Notes
The authors declare no competing financial interests.
16

ACKNOWLEDGMENT
Financial support by the University of Rome La Sapienza (Ateneo 2016, B82F16003510005) is
gratefully acknowledged.
References
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SYNOPSIS TOC
Synthesis and structure of the stable zwitterions [(CN)2Py(2-Mepy)PyzMCl3]∙CH3CN (M = PtII,
-
-
PdII) characterized by the presence of the rare N(py)PtCl3 and unprecedented N(py)PdCl3 fragments
(py = 2-pyridyl group).
18

PtII complex
19

SYNOPSIS TOC
Synthesis and structure of the stable zwitterions [(CN)2Py(2-Mepy)PyzMCl3]∙CH3CN (M = PtII,
-
-
PdII) characterized by the presence of the rare N(py)PtCl3 and unprecedented N(py)PdCl3 fragments
(py = 2-pyridyl group).
PtII complex
20

Highlights
-
Rare zwitterionic complexes involving the presence of N(py)MCl3 moieties (M =
Pt(II), Pd(II)) have been structurally elucidated by single-crystal X-ray work.
The two complexes, further characterized by IR and UV-visible spectral data, are neutral stable
zwitterions presenting both a +1 pyridinium component neutralized by rare N(py)PtCl3- and N(py)PdCl3-
anionic moieties (py = 2-pyridyl group).
DFT calculations fit reasonably well with the observed UV-visible absorptions and provide
information on the localization of the HOMO and LUMO within the two species.
21