Exchange of pyridine and bipyridine ligands in trimethylplatinum(iv) iodide complexes: Substituent and solvent effects

Article abstract of DOI:10.1039/c5ra16541e

A series of mononuclear trimethylplatinum(iv) complexes of bipyridine ligands, [PtMe₃(L-L)I] (L-L = bipy, 4-Mebipy, 4-MeObipy and 4-Me₂Nbipy) has been synthesized by the reaction of trimethylplatinum(iv) iodide with bipyridine ligands L-L in an equimolar ratio. Also, treatment of mononuclear trimethylplatinum(iv) iodide complexes of pyridine ligands, [PtMe₃L₂I] (L = py, 4-Mepy, 4-MeOpy and 4-Me₂Npy) with the corresponding bipyridine ligands leads to the exchange of the pyridines by the bipyridine ligands, thereby resulting in the formation of the more stable chelate bipyridine complexes. The ligand-exchange reactions have been studied by ¹H NMR spectroscopy. The ¹H NMR spectra of a 1: 1 mixture of mononuclear pyridine complexes [PtMe₃L₂I] and corresponding bipyridine ligands L-L reveal the formation of two chelate bipyridine complexes, [PtMe₃(L-L)I] and [PtMe₃(L-L)L]I, in solution. Speciation of the pyridine and bipyridine complexes in solution was found to be dependent on the substituent as well as on the nature of the solvent. Furthermore, crystal structures of three bipyridine complexes [PtMe₃(L-L)I] (L-L = 4-Mebipy, 4-MeObipy and 4-Me₂Nbipy) have also been investigated here.

Full text of DOI:10.1039/c5ra16541e

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PAPER
Exchange of pyridine and bipyridine ligands in
trimethylplatinum(IV) iodide complexes: substituent
and solvent effects†
Cite this: RSC Adv., 2015, 5, 101900
ab
and Sabine Schlechtb
*
Biswa Nath Ghosh
A series of mononuclear trimethylplatinum(IV) complexes of bipyridine ligands, [PtMe3(L–L)I] (L–L ¼ bipy, 4-
Mebipy, 4-MeObipy and 4-Me2Nbipy) has been synthesized by the reaction of trimethylplatinum(IV) iodide
with bipyridine ligands L–L in an equimolar ratio. Also, treatment of mononuclear trimethylplatinum(IV)
iodide complexes of pyridine ligands, [PtMe3L2I] (L ¼ py, 4-Mepy, 4-MeOpy and 4-Me2Npy) with the
corresponding bipyridine ligands leads to the exchange of the pyridines by the bipyridine ligands, thereby
resulting in the formation of the more stable chelate bipyridine complexes. The ligand-exchange
reactions have been studied by 1H NMR spectroscopy. The 1H NMR spectra of a 1 : 1 mixture of
mononuclear pyridine complexes [PtMe3L2I] and corresponding bipyridine ligands L–L reveal the
formation of two chelate bipyridine complexes, [PtMe3(L–L)I] and [PtMe3(L–L)L]I, in solution. Speciation
of the pyridine and bipyridine complexes in solution was found to be dependent on the substituent as
well as on the nature of the solvent. Furthermore, crystal structures of three bipyridine complexes
[PtMe3(L–L)I] (L–L ¼ 4-Mebipy, 4-MeObipy and 4-Me2Nbipy) have also been investigated here.
Received 17th August 2015
Accepted 12th November 2015
DOI: 10.1039/c5ra16541e
www.rsc.org/advances
involvement as a key intermediate in Shilov type-chemistry for
C–H/C–X (X ¼ C, N, O) bond breaking or bond formation
processes.15–17 In order to study the mechanism for these PtII/
Introduction
The well-known nitrogen based heterocyclic ligands play an
important role in coordination and supramolecular chemistry
as well as material science.1–6 In particular, 2,20-bipyridine and
its derivatives have received much attention because of their
remarkable coordination chemistry. Extremely strong chelation
properties, due to the arrangement of two pyridinic nitrogen
atoms in addition to strong affinity towards most transition
metals ions, make bipyridines a highly useful ligand in
research. It has been found that 2,20-bipyridine and its
complexes with ruthenium(II), osmium(II) or platinum(II) exhibit
interesting electrochemical and photochemical properties.7–9 In
addition, the transition metal complexes of 2,20-bipyridine and
its derivatives are also used as building blocks in supra-
molecular assemblies,10,11 as electrochemical probes for DNA
detection,9,12 as electron transfer mediators for glucose
oxidase,13 or as sensitizers in solar energy conversion.14
PtIV mediated catalytic processes, several trimethylplatinum(IV)
complexes of nitrogen-based ligands have been synthesized.18–21
It was also found that trimethylplatinum(IV) complexes of
bipyridine ligands exhibit curious dynamic behavior in solu-
tion.22 In this regard, it is somewhat surprising that there exists
very few structural reports of trimethylplatinum(IV) complexes
of simple 2,20-bipyridine ligand22–26 and to the best of our
knowledge, no structural characterization of simple 4,40-
disubstituted 2,20-bipyridine ligands of trimethylplatinum(IV)
iodide complexes has yet been reported. Herein, in the present
paper, we report the synthesis and spectroscopic characteriza-
tion of
a series of mononuclear trimethylplatinum(IV)
complexes of 2,20-bipyridine ligands, [PtMe3(L–L)I] where L–L ¼
2,20-bipyridine (bipy), 4,40-dimethyl-2,20-bipyridine (4-Mebipy),
4,40-dimethoxy-2,20-bipyridine (4-MeObipy), 4,40-bis(dimethyla-
mino)-2,20-bipyridine (4-Me2Nbipy). The crystal structures of
three 4,40-disubstituted 2,20-bipyridine complexes, [PtMe3(L–L)
I] (L–L ¼ 4-Mebipy, 4-MeObipy and 4-Me2Nbipy) have also been
described. The presence of low-spin d6 electron conguration of
central metal ion makes ligand substitution reactions in PtIV
complexes very slow and thus ligand-exchange study was per-
formed in PtIV complexes having trimethylplatinum unit where
the leaving ligand is activated by the presence of methyl groups
having high trans effect.27–29 It was also observed that reaction of
mononuclear pyridine complex, [PtMe3(py)2I] (py ¼ pyridine)
with 2,20-bipyridine (bipy) leads to the substitution of pyridine
The chemistry of PtIV emerged as one of the leading research
interest in organometallic chemistry, because of the
aDepartment of Chemistry, Veer Surendra Sai University of Technology, Burla,
Sambalpur-768018,
Odisha,
India.
E-mail:
b.chem06@gmail.com;
Tel:
+91-8984710201
b
¨
¨
Institut fur Anorganische und Analytische Chemie, Justus-Liebig-Universitat, Heinrich-
Buff-Ring 58, 35392 Giessen, Germany
† Electronic supplementary information (ESI) available. CCDC 1418664–1418666.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra16541e
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was cooled to 0 ꢁC. Fuming nitric acid (20 mL) was carefully
added, and the mixture was stirred at 100 ꢁC for 8 h. The
solution was then cooled to 0 ꢁC and very cautiously poured
onto ice water (200 g). The resultant yellow product, 4,40-dinitro-
2,20-bipyridine N,N0-dioxide (O2N-bipy N-oxide), was ltered off
and washed with water until neutral. Yield: 9.6 g (34.5 mmol,
72%). 1H NMR (400 MHz, DMSO-d6, 27 ꢁC) d/ppm: 8.70 (d, 2H),
8.60 (d, 2H), 8.38 (dd, 2H). Aer that, a suspension of 1 g of O2N-
bipy N-oxide (3.59 mmol) in 50 mL of acetyl chloride was
reuxed under argon for 3 h. The resultant solid product, 4,40-
dichloro-2,20-bipyridine N,N0-dioxide (Cl-bipy N-oxide), was
ltered off, washed with 50 mL of diethyl ether, and dried under
vacuum. Yield: 700 mg of pale yellow powder (2.72 mmol, 76%).
H NMR (400 MHz, CDCl3, 27 C) d/ppm: 8.25 (d, 2H), 7.72 (d,
2H), 7.35 (dd, 2H). A suspension of 1 g of Cl-bipy N-oxide (3.89
mmol) in 150 mL DMF was reuxed under argon for 48 h. The
solvent was evaporated nearly completely, and the crude
product was dissolved in 100 mL of chloroform. Phosphorous
trichloride (8.5 mL, 97.4 mmol) was added drop wise to the
cooled solution at 0 ꢁC. The reaction mixture was then reuxed
for 3 h and then poured onto 250 mL of ice water. The chloro-
form layer was washed with 3 ꢂ 50 mL of water and the
combined aqueous extracts were concentrated under vacuum to
75 mL and made alkaline with saturated aqueous sodium
hydroxide. The resulting precipitate was recrystallized twice
from water/methanol (1.5 v/v) to give 150 mg of the beige
product 4,40-bis(dimethylamino)-2,20-bipyridine (4-Me2Nbipy)
(0.62 mmol, 16%). 1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC) d/
ppm: 8.37 (d, 2H), 8.01 (d, 2H), 6.50 (dd, 2H), 2.94 [s, 12H,
N(CH3)2].
Scheme
1
Different pyridines and corresponding 2,20-bipyridines
used to study the ligand-exchange reaction in trimethylplatinum(IV)
iodide system.
by the bipy ligand, resulting in the formation of the chelate
bipyridine complex. In order to explore the inuence of the
substituent on the above ligand-exchange reaction, we have
hereby carried out the exchange of different pyridine ligands (py,
4-methylpyridine (4-Mepy), 4-methoxypyridine (4-MeOpy),
4-dimethylaminopyridine (4-Me2Npy)) with the corresponding
bipyridine ligands (bipy, 4-Mebipy, 4-MeObipy and 4-Me2Nbipy)
in mononuclear trimethylplatinum(IV) iodide complexes
(Scheme 1). The substitution reactions were carried in two
different solvents (CDCl3, an apolar solvent and nitrobenzene-d5,
a polar solvent) to investigate the solvent effect on the chelation
reaction.
1
ꢁ
Experimental section
General methods and materials
Trimethylplatinum(IV) iodide was purchased from Strem
Chemicals. The ligand 4,40-bis(dimethylamino)-2,20-bipyridine
(4-Me2Nbipy) was prepared following the modied literature
procedure.30,31 Other bipyridine ligands, pyridine ligands and
solvents were of analytical reagent grade, purchased commer-
cially and used as received. The mononuclear pyridine
complexes [PtMe3L2I] (L ¼ py, 4-Mepy, 4-MeOpy and 4-Me2Npy)
and bipyridine complex [PtMe3(bipy)I] were synthesized
following literature methods.32–34 1H NMR spectra recorded on
a Bruker Avance II 400 MHz spectrometer at room temperature.
Spin–spin coupling constants are considered accurate to ꢀ0.5
Hz and d values to ꢀ0.01 ppm. Elemental analyses were per-
formed on a Thermo Electron Flash EA 1112 series. Mass
Synthesis of [PtMe3(4-Mebipy)I]
Trimethylplatinum(IV) iodide (200 mg, 0.54 mmol) was dis-
solved in benzene (15 mL) and 4-Mebipy (100 mg, 0.54 mmol)
added to it. The solution was stirred for 1.5 h and the resulting
pale yellow solid [PtMe3(4-Mebipy)I] was ltered off, washed
several times with n-hexane and dried under vacuum. Yield: 258
mg (0.47 mmol, 86%). Also, diffraction quality single crystals
were obtained aer a few days, when a 1 : 1 mixture of trime-
thylplatinum iodide and 4-Mebipy were mixed in benzene
solution and le undisturbed. 1H NMR (400 MHz, CDCl3, 27 ꢁC)
d/ppm: 8.82 (d, 2H, 3JH–H: 5.8 Hz, 3JPt–H: 19.4 Hz), 8.01 (s, br, 2H),
7.42 (d, 2H), 2.57 (s, 6H, CH3), 1.51 [s, 6H, Pt–CH3 (trans to N),
2JPt–H: 70.3 Hz], 0.63 [s, 3H, Pt–CH3 (trans to I), 2JPt–H: 73.5 Hz].
1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC) d/ppm: 8.87 (d, 2H,
3JH–H: 5.6 Hz, 3JPt–H: 19.6 Hz), 8.05 (s, br, 2H), 7.42 (d, 2H), 2.52
spectra were recorded on
spectrometer.
a Finnigan MAT 95 mass
Synthesis of the ligand 4-Me2Nbipy
A 30 mL solution of 30% hydrogen peroxide was added to 2,20-
bipyridine (10.0 g, 64 mmol) in 50 mL of glacial acetic acid at
a rate that maintained the temperature between 70 and 80 C.
2
(s, 6H, CH3), 1.78 [s, 6H, Pt–CH3 (trans to N), JPt–H: 70.3 Hz],
2
0.81 [s, 3H, Pt–CH3 (trans to I), JPt–H: 72.9 Hz]. MS (ESI-TOF)
ꢁ
[PtMe3(4-Mebipy)]+ m/z 424.1342 (calcd 424.1347). Anal. calcd
for C15H21IN2Pt (551.33 g molꢃ1): C: 32.68, H: 3.84, N: 5.08.
Found: C: 32.46, H: 3.97, N: 4.98%.
The mixture was stirred at 75 ꢁC for an additional 8 h. The
solution was then cooled to room temperature, and a large
amount (750 mL) of acetone was added to precipitate the
product 2,20-bipyridine N,N0-dioxide as a white solid, which was
collected by ltration and air-dried. Yield: 11.3 g (60 mmol,
Synthesis of [PtMe3(4-MeObipy)I]
1
ꢁ
94%). H NMR (400 MHz, D2O, 27 C) d/ppm: d 8.47 (m, 2H), To a benzene solution (10 mL) of trimethylplatinum iodide (200
7.85 (m, 2H), 7.76 (m, 4H). Then a solution of 2,20-bipyridine mg, 0.54 mmol), the ligand 4-MeObipy (120 mg, 0.56 mmol) was
N,N0-dioxide (9.0 g, 47.8 mmol) in 26 mL of oleum–sulfuric acid added and the solution was stirred at room temperature for 2 h.
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1H NMR spectroscopic data of [PtMe3(4-Mepy)2I]
The pale yellow solid formed was then ltered off, washed
several times with n-hexane and dried under vacuum. Yield: 267
mg (0.46 mmol, 84%). X-ray quality single crystals were ob-
tained by slow diffusion of n-hexane into its chloroform solu-
1H NMR (400 MHz, CDCl3, 27 ꢁC) d/ppm: 8.61 (d, 4H, 3JH–H: 6.4
Hz, 3JPt–H: 19.0 Hz), 7.12 (d, 4H, 3JH–H: 6.0 Hz), 2.38 (s, 6H, CH3),
1.45 [s, 6H, PtCH3 (trans to N), 2JPt–H: 70.0 Hz], 1.17 [s, 3H, PtCH3
(trans to I), 2JPt–H: 70.0 Hz]. 1H NMR (400 MHz, nitrobenzene-d5,
27 ꢁC) d/ppm: 8.73 (d, 4H, 3JH–H: 6.4 Hz, 3JPt–H: 19.1 Hz), 7.12 (d,
4H, 3JH–H: 5.9 Hz), 2.28 (s, 6H, CH3), 1.69 [s, 6H, PtCH3 (trans to
N), 2JPt–H: 70.1 Hz], 1.28 [s, 3H, PtCH3 (trans to I), 2JPt–H: 69.2 Hz].
1
ꢁ
tion at room temperature. H NMR (400 MHz, CDCl3, 27 C) d/
ppm: 8.77 (d, 2H, 3JH–H: 6.4 Hz, 3JPt–H: 19.8 Hz), 7.58 (d, 2H), 7.08
(dd, 2H), 4.02 (s, 6H, OCH3), 1.48 [s, 6H, Pt–CH3 (trans to N),
2JPt–H: 70.2 Hz], 0.65 [s, 3H, Pt–CH3 (trans to I), 2JPt–H: 73.7 Hz].
1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC) d/ppm: 8.83 (d, 2H,
3JH–H: 6.4 Hz, 3JPt–H: 19.9 Hz), 7.61 (d, 2H), 7.09 (d, 2H), 3.96 (s,
6H, OCH3), 1.77 [s, 6H, PtCH3 (trans to N), 2JPt–H: 70.5 Hz], 0.86
[s, 3H, PtCH3 (trans to I), 2JPt–H: 73.0 Hz]. MS (ESI-TOF) [PtMe3(4-
1H NMR spectroscopic data of [PtMe3(4-MeOpy)2I]
1H NMR (400 MHz, CDCl3, 27 ꢁC) d/ppm: 8.58 (d, 4H, 3JH–H: 7.0
Hz, 3JPt–H: 19.0 Hz), 6.80 (d, 4H), 3.89 (s, 6H, OCH3), 1.42 [s, 6H,
PtCH3 (trans to N), 2JPt–H: 70.0 Hz], 1.15 [s, 3H, PtCH3 (trans to I),
MeObipy)]+ m/z 456.1238 (calcd 456.1245). Anal. calcd for C15
-
H21IN2O2Pt (583.33 g molꢃ1): C: 30.89, H: 3.63, N: 4.80. Found:
C: 30.78, H: 3.76, N: 4.72%.
2
1
ꢁ
JPt–H: 70.2 Hz]. H NMR (400 MHz, nitrobenzene-d5, 27 C) d/
ppm: 8.70 (d, 4H, 3JH–H: 6.8 Hz, 3JPt–H: 19.1 Hz), 6.79 (d, 4H), 3.81
(s, 6H, OCH3), 1.69 [s, 6H, PtCH3 (trans to N), JPt–H: 70.2 Hz],
2
Synthesis of [PtMe3(4-Me2Nbipy)I]
2
1.29 [s, 3H, PtCH3 (trans to I), JPt–H: 69.4 Hz].
132 mg of 4-Me2Nbipy (0.55 mmol) was added to a stirred
solution of trimethylplatinum iodide (200 mg, 0.54 mmol) in
benzene (15 mL) and the reactants were stirred at room
temperature for 1 h, aer which the resultant pale yellow solid
of [PtMe3(4-Me2Nbipy)I] was isolated, washed several times with
n-hexane and dried in vacuum. Yield: 286 mg (0.47 mmol, 87%).
Also, 1 : 1 mixture of trimethylplatinum iodide and 4-Me2Nbipy
in benzene on standing for few days afforded yellow colored
single crystals of the complexes. 1H NMR (400 MHz, CDCl3,
27 ꢁC) d/ppm: 8.46 (d, 2H, 3JH–H: 6.6 Hz, 3JPt–H: 20.0 Hz), 7.14 (d,
2H), 6.68 (dd, 2H), 3.16 [s, 12H, N(CH3)2], 1.40 [s, 6H, PtCH3
1H NMR spectroscopic data of [PtMe3(4-Me2Npy)2I]
1H NMR (400 MHz, CDCl3, 27 ꢁC) d/ppm: 8.29 (d, 4H, 3JH–H: 7.2
Hz, 3JPt–H: 19.6 Hz), 6.40 (d, 4H), 3.02 [s, 12H, N(CH3)2], 1.35 [s,
6H, PtCH3 (trans to N), 2JPt–H: 69.3 Hz], 1.14 [s, 3H, PtCH3 (trans
to I), 2JPt–H: 71.5 Hz]. 1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC)
3
3
d/ppm: 8.38 (d, 4H, JH–H: 6.9 Hz, JPt–H: 19.4 Hz), 6.28 (d, 4H),
2.89 [s, 12H, N(CH3)2], 1.70 [s, 6H, PtCH3 (trans to N), 2JPt–H: 69.3
2
Hz], 1.33 [s, 3H, PtCH3 (trans to I), JPt–H: 70.6 Hz].
(trans to N), 2JPt–H: 69.6 Hz], 0.68 [s, 3H, PtCH3 (trans to I), 2JPt–H
:
74.9 Hz]. 1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC) d/ppm:
8.50 (d, 2H, 3JH–H: 6.6 Hz, 3JPt–H: 20.0 Hz), 7.16 (d, 2H), 6.58 (dd,
Crystal structure determinations
2H), 3.04 [s, 12H, N(CH3)2], 1.76 [s, 6H, PtCH3 (trans to N), 2JPt–H
:
Diffraction data for the complexes [PtMe3(L–L)I] (L–L ¼ 4-
69.5 Hz], 0.97 [s, 3H, PtCH3 (trans to I), 2JPt–H: 74.2 Hz]. MS (ESI- Mebipy, 4-MeObipy and 4-Me2Nbipy) were collected on a STOE
TOF) [PtMe3(4-Me2Nbipy)]+ m/z 482.1871 (calcd 482.1878). Anal. IPDS diffractometer (Karlsruher Glastechnisches Werk). MoKa
calcd for C17H27IN4Pt (609.42 g molꢃ1): C: 33.51, H: 4.47, N: radiation (l ¼ 0.71073 A) was used. Frames were integrated with
˚
9.19. Found: C: 33.66, H: 4.29, N: 8.96%.
STOE soware package.35 Multi-scan absorption correction was
applied.36 The structures were solved with the program Super-
ip37 and rened by full-matrix least squares on F2 using the
WinGX38 soware equipped with SHELXL-2014.39 All non-
1H NMR spectroscopic data of [PtMe3(bipy)I]
1H NMR (400 MHz, CDCl3, 27 ꢁC) d/ppm: 9.02 (d, 2H, 3JH–H: 6.1 hydrogen atoms were rened with anisotropic thermal param-
Hz, 3JPt–H: 17.8 Hz), 8.23 (d, 2H), 8.07 (m, 2H), 7.64 (m, 2H), 1.56 eters. All hydrogen atoms were calculated to their optimal
2
[s, 6H, PtCH3 (trans to N), JPt–H: 70.5 Hz], 0.64 [s, 3H, PtCH3 positions and treated as riding atoms using isotropic displace-
(trans to I), 2JPt–H: 73.1 Hz]. 1H NMR (400 MHz, nitrobenzene-d5, ment parameters 1.2 larger than the respective host atoms. The
27 ꢁC) d/ppm: 9.08 (d, 2H, 3JH–H: 6.4 Hz, 3JPt–H: 18.5 Hz), 8.33 (d, asymmetric unit of [PtMe3(4-Me2Nbipy)I] contains disordered
2H), 8.08 (m, 2H), 7.67 (m, 2H), 1.81 [s, 6H, PtCH3 (trans to N), solvent molecule, which could not be modeled as discrete
2
2JPt–H: 70.5 Hz], 0.78 [s, 3H, PtCH3 (trans to I), JPt–H: 72.6 Hz].
atomic sites. So the structure was subjected to the SQUEEZE
procedure from the PLATON suite to calculate the diffraction
contribution of the solvent molecules and thereby produce a set
1H NMR spectroscopic data of [PtMe3(py)2I]
3
˚
of solvent-free diffraction intensities. A void volume of 138 A
1H NMR (400 MHz, CDCl3, 27 ꢁC) d/ppm: 8.80 (d, 4H, 3JH–H: 4.9 contains approximately 41 electrons. Prior to SQUEEZE, all non-
Hz, 3JPt–H: 17.3 Hz), 7.85 (t, 4H), 7.35 (t, 2H), 1.50 [s, 6H, PtCH3 hydrogen atoms were made anisotropic and all hydrogen atoms
(trans to N), 2JPt–H: 70.3 Hz], 1.20 [s, 3H, PtCH3 (trans to I), 2JPt–H
:
were inserted at their calculated positions. Details of the
69.6 Hz]. 1H NMR (400 MHz, nitrobenzene-d5, 27 ꢁC) d/ppm: SQUEEZE procedure are given in the CIF le. X-ray crystallo-
3
3
8.93 (d, 4H, JH–H: 5.5 Hz, JPt–H: 18.0 Hz), 7.88 (t, 4H), 7.37 (t, graphic data and structural renement parameters for the
2H), 1.70 [s, 6H, PtCH3 (trans to N), 2JPt–H: 70.4 Hz], 1.29 [s, 3H, complexes are listed in Table 1. The ORTEP gures were drawn
2
PtCH3 (trans to I), JPt–H: 68.9 Hz].
using the program Mercury 3.3.40†
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Table 1 Crystallographic data and structure refinement for [PtMe3(L–L)I] (L–L ¼ 4-Mebipy, 4-MeObipy and 4-Me2Nbipy) complexes
Complex
[PtMe3(4-Mebipy)I]$0.5(C6H6)
[PtMe3(4-MeObipy)I]
[PtMe3(4-Me2Nbipy)I]
CCDC no.
1418664
1418665
1418666
Empirical formula
Formula weight
T/K
C18H24IN2Pt
590.38
193(2)
C15H21IN2O2Pt
583.33
193(2)
C17H27IN4Pt
609.42
193(2)
˚
l/A
0.71073
0.71073
0.71073
Crystal color, shape
Crystal size/mm3
Crystal system
Space group
Yellow, block
0.330 ꢂ 0.300 ꢂ 0.290
Monoclinic
P21/c
11.8248(12)
8.3843(7)
19.403(2)
90
96.794(12)
90
1910.2(3)
4
2.053
8.961
1108
3.22 to 25.25
97.8%
Yellow, block
0.270 ꢂ 0.250 ꢂ 0.220
Monoclinic
P21/n
11.1515(11)
11.6097(15)
13.6162(14)
90
103.077(12)
90
1717.1(3)
4
2.256
9.975
1088
2.68 to 25.25
97.5%
Yellow, block
0.580 ꢂ 0.240 ꢂ 0.160
Orthorhombic
Pcan
13.7054(11)
17.6237(14)
18.1731(16)
90
90
90
4389.5(6)
8
1.844
˚
a/A
˚
b/A
˚
c/A
a/ꢁ
b/ꢁ
g/ꢁ
3
˚
V/A
Z
Dcalc. (g cmꢃ3
m/mmꢃ1
F(000)
)
7.804
2303
2.19 to 25.50
99.0%
20 808
q/ꢁ
Completeness to qfull
Reections collected
12 605
11 671
Independent reections
Data/restraints/parameters
Goodness-of-t on F2
Final R indices [I > 2s(I)]
3381 [Rint ¼ 0.0320]
3381/0/206
1.040
R1 ¼ 0.0236,
wR2 ¼ 0.0561
R1 ¼ 0.0269,
wR2 ¼ 0.0571
1.254, ꢃ0.664
3041 [Rint ¼ 0.0423]
3041/12/196
0.998
R1 ¼ 0.0321,
wR2 ¼ 0.0703
R1 ¼ 0.0381,
wR2 ¼ 0.0723
1.848, ꢃ2.262
4046 [Rint ¼ 0.0821]
4046/0/216
0.860
R1 ¼ 0.0521,
wR2 ¼ 0.1096
R1 ¼ 0.1110,
wR2 ¼ 0.1244
1.499, ꢃ1.060
R indices (all data)
ꢃ3
˚
Largest diff. peak and hole/e A
1H NMR spectroscopic studies on the reaction of
mononuclear pyridine complexes [PtMe3L2I] (L ¼ py, 4-Mepy,
4-MeOpy and 4-Me2Npy) with corresponding 2,20-bipyridine
ligands L–L (L–L ¼ bipy, 4-Mebipy, 4-MeObipy and 4-
Me2Nbipy)
are soluble in nitrobenzene, chloroform. They have been char-
acterized by 1H NMR spectroscopy, elemental analyses, mass
spectroscopy and single-crystal X-ray diffraction analysis. Full
synthetic and analytical data of the newly synthesized bipyr-
idine complexes are given in the experimental section. 1H NMR
spectroscopic data of the mononuclear pyridine complexes and
[PtMe3(bipy)I] are also given in the experimental section.
The room temperature 1H NMR spectra of all the synthesized
bipyridine complexes exhibit sharp, well resolved signals. The
1H NMR spectra of the bipyridine complexes in CDCl3 and in
nitrobenzene-d5 at room temperature are given in the ESI.† The
platinum-methyl region in all the complexes exhibit two signals,
each with two satellites due to 195Pt–H scalar coupling, in an
intensity ratio 2 : 1, reveals the presence of two non-equivalent
An equimolar mixture of mononuclear pyridine complexes
[PtMe3L2I] and corresponding 2,20-bipyridine ligands were dis-
solved in CDCl3 and in nitrobenzene-d5 in vials and the progress
of the reactions were monitored by sequential 1H NMR
measurements at room temperature at different time interval.
Result and discussion
The mononuclear pyridine complexes [PtMe3L2I] (L ¼ py, 4-
Mepy, 4-MeOpy, 4-Me2Npy) were synthesized by the reaction of
trimethylplatinum(IV) iodide with pyridine ligands, following
literature methods.32–34 On the other hand, the pale-yellow to
yellow crystalline mononuclear trimethylplatinum(IV) bipyr-
idine complexes, [PtMe3(L–L)I] (L–L ¼ bipy, 4-Mebipy, 4-MeO-
bipy, 4-Me2Nbipy) were obtained in 84–87% yield through
treatment of trimethylplatinum iodide with 2,20-bipyridine
ligands in equimolar ratio (Scheme 2). Among these complexes,
[PtMe3(bipy)I] was synthesized according to
a literature
Scheme 2 Syntheses of mononuclear trimethylplatinum(IV) iodide
method.34 The synthesized pyridine and bipyridine complexes complexes of bipyridines.
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methyl groups in the complexes: equatorial methyl (trans to mononuclear trimethylplatinum(IV)–terpyridine complexes
bipyridines) and axial methyl (trans to iodide ligand) environ- where the metal–nitrogen bond distance to the central pyridine
2
ments. The JPt–H scalar coupling constants observed for the ring is longer than the corresponding distance to the outer
methyl groups trans to iodide ligands were higher than for the ring.41,42 This shows that the Pt–N bond distances in the bipyr-
methyl groups trans to bipyridines, which indicates that the idine complexes are not inuenced signicantly by the elec-
bipyridines exert stronger trans inuence than iodide. For the tronic effect of the bipyridine substituent as observed in the
bipy complex, the aromatic region comprised of four signals case of pyridine substituent.32,33 However, the Pt–N bond
while it is three for the 4,40-disubstituted 2,20-bipyridine distances in the bipyridine complexes are marginally shorter
complexes. The highest frequency signal in each of the than the Pt–N bond distances in the corresponding pyridine
complexes displayed measurable 195Pt coupling by virtue of its complexes,32,33 reecting the better p acceptor character of the
proximity to the metal-coordinated nitrogen. Similar to the bipyridines compared to the pyridines. The Pt–I bond distance
bipyridine complexes, the platinum-methyl region in the in all the structures is almost the same. All other key bond
mononuclear pyridine complexes also exhibit well-resolved, distances and angles (see Table S1 in the ESI†) are very much
sharp signals (with satellites) for the two different types of comparable to the value for the previously reported mono-
methyl groups (trans to pyridine ligands and trans to iodide nuclear trimethylplatinum(IV)–bipyridine complexes.22,25
ligands) in an intensity ratio 2 : 1.32 ESI-MS analysis of synthe-
sized bipyridine complexes showed that the most abundant m/z
value corresponds to the loss of iodide, giving [PtMe3(L–L)]+ (L–
L ¼ bipyridines). In addition, the isotope patterns observed for
each of the complexes were consistent with those calculated for
the formulated species.
Reaction of mononuclear pyridine complexes [PtMe3L2I] (L ¼
py, 4-Mepy, 4-MeOpy, 4-Me2Npy) with corresponding 2,20-
bipyridine ligands L–L (L–L ¼ bipy, 4-Mebipy, 4-OMebipy and
4-Me2Nbipy): ligand-exchange study
Reaction of mononuclear pyridine complexes [PtMe3L2I] (L ¼
py, 4-Mepy, 4-MeOpy, 4-Me2Npy) with the corresponding 2,20-
bipyridines L–L (L–L ¼ bipy, 4-Mebipy, 4-MeObipy and 4-Me2-
Nbipy) in both the solvents (CDCl3 and nitrobenzene-d5) leads
X-ray crystallographic characterization of the complexes,
[PtMe3(L–L)I] (L–L ¼ 4-Mebipy, 4-MeObipy and 4-Me2Nbipy)
Single crystals of [PtMe3(4-MeObipy)I] were obtained by slow to the substitution of pyridines by the corresponding bipyridine
diffusion of n-hexane into its chloroform solution, while single ligands, thereby resulting in the formation of more stable
crystals of [PtMe3(4-Mebipy)I] and [PtMe3(4-Me2Nbipy)I] were chelate bipyridine complexes. The progress of each of the four
separated from the solution when 1 : 1 mixture of trimethyl- ligand substitution reactions was monitored by sequential NMR
platinum(IV) iodide and corresponding bipyridine ligands in spectroscopic measurements until the population of the pyri-
benzene were le undisturbed for few days. Complexes dine and bipyridine complexes changes extremely slowly;
[PtMe3(4-Mebipy)I] and [PtMe3(4-MeObipy)I] were crystallize in however, true thermodynamic equilibrium is most likely not
monoclinic crystal system, whereas [PtMe3(4-Me2Nbipy)I] crys- achieved within a reasonable time period in these reaction
tallizes in orthorhombic crystal system. Selected bond distances mixtures due to the inertness of the platinum(IV) complexes.
and angles of the complexes are given in Table S1 in the ESI.† The 1H NMR spectra of an equimolar mixture of pyridine
The crystal structure of [PtMe3(4-Mebipy)I] contains molecule of complexes and the corresponding 2,20-bipyridine ligands,
the crystallization solvent (benzene) in the crystal lattice. The measured at room temperature, exhibit well resolved signals for
crystal structure of all the complexes consists of a discrete the three trimethylplatinum(IV) complexes (pyridine complex
monomeric unit in which the platinum atom is hexa- [PtMe3L2I] (I), two chelate bipyridine complexes [PtMe3(L–L)I]
coordinated by the three methyl groups in a facial arrange- (II) and [PtMe3(L–L)L]I (III)), free pyridine ligand L, and
ment, a 2,20-bipyridine ligand in a bidentate fashion and an unreacted 2,20-bipyridine ligand L–L (see Scheme 3). The 1H
iodine atom (see Fig. 1). The platinum(IV) metal center in all the NMR measurements of 1 : 1 reaction mixtures for the ligand-
complexes exhibits distorted octahedral geometry with intra- exchange reactions show that two chelate bipyridine
ligand bite angle (N1–Pt1–N2) in the range 75.9(4)–76.42(13)ꢁ. complexes II and III are formed in solution (see Fig. 2); however,
The structure of [PtMe3(4-MeObipy)I] was found to be closely reaction of complex [PtMe3(4-Me2Npy)2I] with the bipyridine
related to the previously reported structure [PtMe3 ligand 4-Me2Nbipy results in the formation of exclusively one
(bipy)(OAc)]$(H2O) in which the primary coordination sphere of chelate complex, [PtMe3(4-Me2Nbipy)(4-Me2Npy)]I (III) (see
the platinum atom was also built up by the three methyl groups Scheme 4). A very small traces of [PtMe3(4-Me2Npy)3]I is also
in facial binding fashion, a bipyridine ligand and a monodentate formed in solution. As expected, 1H NMR spectra for each of the
acetate ligand.25 The N1–Pt1–N2 bite angle in the complexes is three trimethylplatinum(IV) complexes exhibit two platinum-
close to the value found in [PtMe3(bipy)(OAc)]$(H2O) (76.7ꢁ),25 methyl resonances with satellites, due to 195Pt–H scalar
[PtMe3(bipy)I] (76.5ꢁ),23 [PtMe3(bipy)Br] (76.6ꢁ),24 [PtMe3 coupling, with an intensity ratio of 2 : 1 (see Fig. 2 and ESI†).
(bipy)(pydz)][BF4] (75.3ꢁ),22 [PtMe3(terpy)I] (76.1ꢁ).41
The individual components in the solution were identied by
The Pt–N bond distances in all the structures do not differ comparison with reference data for the pyridine and bipyridine
1
signicantly (2.12–2.16 A). Similar feature of almost equal Pt–N complexes and free ligands.32–34 The H NMR spectrum of a 1 : 1
˚
bond distances also observed in previously reported bipyridine mixture of [PtMe3(4-MeOpy)2I] and (4-MeObipy) at room
complexes23–26 and is in contrast to the situation found for the temperature in nitrobenzene-d5, measured aer 7 days, is
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Fig. 1 ORTEP plots of crystal structures of [PtMe3(4-Mebipy)I] (a), [PtMe3(4-MeObipy)I] (b) and [PtMe3(Me2N-bipy)I] (c) showing the atom
labelling scheme. In case of [PtMe3(4-Mebipy)I], solvated benzene molecule omitted for clarity.
1
shown in Fig. 2, while 1H NMR spectra of 1 : 1 mixture of methyl resonance shied to higher frequency in the H NMR
[PtMe3(4-Mepy)2I] and 4-Mebipy at different time interval in spectrum. The difference in chemical shis for the two
1
nitrobenzene-d5 are plotted in Fig. 3. The H NMR spectra of platinum-methyl resonances is larger in the bipyridine
other pyridine complexes and corresponding bipyridine complexes than in the pyridine complexes. Moreover, chemical
ligands, measured at different time interval at room tempera- shi difference for the two platinum-methyl resonances is
ture, in CDCl3 and in nitrobenzene-d5 are illustrated in ESI.† greater in the bipyridine complexes II ([PtMe3(L–L)I] (L–L ¼ 2,20-
1
The H NMR spectroscopic data for the ligand-exchange reac- bipyridines)) than in bipyridine–pyridine complexes III
tions in CDCl3 and in nitrobenzene-d5 are also reported in Table ([PtMe3(L–L)L]I where L–L ¼ 2,20-bipyridines, L ¼ correspond-
2
S2 in ESI.†
ing pyridines) complexes. The JPt–H scalar coupling constants
An analysis of Table S2 in the ESI† shows that on changing trans to bipyridines are lower than to the corresponding pyri-
the solvent from CDCl3 to nitrobenzene-d5, the platinum- dines in III, which indicates that the trans inuence of the 2,20-
Scheme 3 The substitution of pyridine ligands by the corresponding 2,20-bipyridines in trimethylplatinum(IV) iodide complexes.
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Fig. 2 The platinum-methyl region in the 1H NMR spectrum (measured after 7 days) of 1 : 1 mixture of [PtMe3(4-MeOpy)2I] and 4-MeObipy in
nitrobenzene-d5 at room temperature, showing presence of three platinum(IV) complexes [PtMe3(4-MeOpy)2I] (I), [PtMe3(4-MeObipy)I] (II) and
[PtMe3(4-MeObipy)(4-MeOpy)]I (III) in solution. Inset showing the methoxy region for the spectrum. The signal at the left correspond to the
methoxy group of 4-MeObipy in complex III, while the signal at the utmost right correspond to the methoxy group of 4-MeOpy in III.
bipyridines is stronger than the corresponding pyridines (trans four ligand-exchange reactions, which reveals that the bipyr-
inuence: bipy > py, 4-Mebipy > 4-Mepy, 4-MeObipy > 4-MeOpy, idine complexes are thermodynamically more stable than the
4-Me2Nbipy > 4-Me2Npy).
corresponding pyridine complexes. Although bipyridine
The population ratios of the bipyridine complexes to the complexes are more stable compared to the corresponding
pyridine complexes and the relative population of different pyridine complexes as indicated from the population ratio of
bipyridine complexes for the reaction of 1 : 1 mixture of the bipyridine complexes to the corresponding pyridine
mononuclear pyridine complexes and corresponding 2,20- complexes, a close analysis of Table 2 reveals that the speciation
bipyridine ligands in two different solvents (CDCl3 and nitro- of the different platinum(IV) complexes in solution was found to
benzene-d5) at different time intervals are tabulated in Table 2. be dependent on the nature of the substituent as well as on the
The compositions of the platinum(IV) complexes at different type of solvent used for carrying out the reaction. As the electron
time intervals for all the four ligand-exchange reactions are also donating property of the substituent increases from H to NMe2,
given in Tables S3–S10 in the ESI.† Inspection of the data in the population ratio of bipyridine–pyridine complex III to
Table 2 immediately shows that the solution speciation in both bipyridine complex II also increases in both the solvents. This
the solvents strongly favours the formation of chelate bipyridine can be rationalized in terms of the stability of complex III in
complexes over the non-chelate pyridine complexes for all the solution. As the electron donating ability of the substituent
Scheme 4 The substitution of 4-Me2Npy by 4-Me2Nbipy in trimethylplatinum(IV) iodide complex.
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Fig. 3 The platinum-methyl region in the 1H NMR spectrum of 1 : 1 mixture of [PtMe3(4-Mepy)2I] and 4-Mebipy in nitrobenzene-d5 at 300 K at
different time interval, showing presence of three platinum(IV) complexes I, II and III in solution. For labeling of methyl groups, see Scheme 3.
increases, the electron density in both the pyridine and bipyr- (nitrobenzene-d5) than in apolar solvent (CDCl3). The strength
idine ring nitrogen also increases which in turn causes an of the chelate effect (determined from the population ratio of
increase in the extent of Pt–N interaction in solution. This leads the chelate bipyridine complexes (II + III) to the non-chelate
to an increase in the stability of complex III in solution and in corresponding pyridine complexes I aer keeping the reaction
the case of –NMe2 substituent, the stability of complex III is so mixtures for 2 weeks at room temperature) for the different
high (due to extremely strong Pt–N interaction) that exclusively substituents in both the solvents is in the order: NMe2 > H > Me
[PtMe3(4-Me2Nbipy)(4-Me2Npy)]I is formed in solution. > OMe. To explain the above order of chelation among the
Furthermore, we have also found that for a particular substit- substituents (H, Me, and OMe), the stability of the reactant
uent, the population ratio of III to II is higher in nitrobenzene- complexes in solution can also be taken into consideration. As
d5 than in CDCl3. This is supported by the fact that the complex the electron donating ability of the substituent increases from H
III being ionic in nature is more stable in polar solvent to OMe, the stability of the reactant non-chelate pyridine
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Table 2 Population of different Pt(IV) complexes in 1 : 1 mixture of [PtMe3L2I] (L ¼ pyridines) and corresponding 2,20-bipyridines (L–L) at different
time interval at 300 K in CDCl3 and in nitrobenzene-d5
In CDCl3
In nitrobenzene-d5
Substitution of
Bipyridine complexes
(II + III)/pyridine
complex (I)
Bipyridine
complex III/bipyridine
complex II
Bipyridine complexes
(II + III)/pyridine
complex (I)
pyridine ligands by
Reaction
time
Bipyridine complex
III/bipyridine complex II
2,20-bipyridines
py by bipy
Aer 1 d
Aer 3 d
Aer 7 d
Aer 14 d
Aer 1 d
Aer 3 d
Aer 7 d
Aer 14 d
Aer 1 d
Aer 3 d
Aer 7 d
Aer 14 d
Aer 1 d
Aer 3 d
Aer 7 d
Aer 14 d
1.65
4.05
10.63
15.67
1.12
2.66
9.20
12.16
0.88
2.40
7.55
8.52
1.86
7.06
10.36
19.00
0.08
0.11
0.17
0.19
0.30
0.43
0.60
0.65
1.00
1.40
1.68
1.98
1.17
3.06
8.26
13.28
0.96
1.90
3.37
4.00
0.62
1.32
2.60
3.29
3.85
9.31
12.51
15.67
0.50
0.56
0.56
0.55
0.85
0.98
1.11
1.10
1.82
1.87
2.19
2.24
4-Mepy by 4-Mebipy
4-MeOpy by 4-MeObipy
4-Me2Npy by 4-Me2Nbipy
Only III presenta
Only III presenta
Only III presenta
Only III presenta
Only III presenta
Only III presenta
Only III presenta
Only III presenta
a
In the case of the substitution of 4-Me2Npy by 4-Me2Nbipy, only the bipyridine complex [PtMe3(Me2N-bipy)(4-Me2Npy)]I is present in solution.
complexes also increases due to the increasing Pt–N interaction much on the nature of the substituent as well as on the solvent.
in the reactant complexes, thereby leading to a decrease in the Increasing electron donating nature of the substituent leads to
strength of the chelate effect. Also, the strength of chelation for an increase in the affinity for the formation of bipyridine–
a particular substituent is greater in CDCl3 than in nitroben- pyridine complexes III in solution which is further enhanced in
zene-d5. This is most likely due to the lower solvent stabilization polar solvent due to better solvent stabilization.
of the reactant pyridine complexes in CDCl3 than in nitroben-
zene-d5. Investigation of the kinetics of the reactions (see Tables
S3–S10 in the ESI†) also indicates that all the four ligand-
Conclusion
Reaction of trimethylplatinum(IV) iodide with 2,20-bipyridine
ligands (bipy, 4-Mebipy, 4-MeObipy and 4-Me2Nbipy) results in
the formation of trimethylplatinum(IV)–bipyridine complexes.
The complexes were characterized by 1H NMR, elemental
analysis, mass spectroscopy and X-ray single crystal diffraction
analysis. Also, the mononuclear trimethylplatinum(IV) iodide
complexes of pyridines, [PtMe3L2I] (L ¼ py, 4-Mepy, 4-MeOpy, 4-
Me2Npy) undergo reaction with the corresponding 2,20-bipyr-
idines (bipy, 4-Mebipy, 4-MeObipy, 4-Me2Nbipy) to form the
chelate bipyridine complexes, [PtMe3(L–L)I] and [PtMe3(L–L)L]I
(L–L ¼ 2,20-bipyridines, L ¼ corresponding pyridines). Each of
the four substitution reactions strongly favours the formation of
chelate complexes. The formation of chelate bipyridine
complexes in solution also found to be dependent on the elec-
tronic effect of the substituents. Increasing electron donating
ability of the substituent leads to an increase in the stability of
the [PtMe3(L–L)L]I in solution. The tendency for the chelation is
very high for strong electron donating substituents such as
NMe2; however, for H and moderately electron donating
substituents such as methyl and methoxy, increasing electron
donating ability decreases the strength of the chelate effect. The
nature of solvent also played a signicant role in the solution
speciation in the substitution reaction. As the polarity of the
solvent increases, the stability of the bipyridine–pyridine
exchange reactions are not proceeding to the same extent and
advancement of the reactions depends on the substituent as
well as on the nature of solvent. Compositions of the plati-
num(IV) complexes at different time intervals (see Tables S3–S10
in ESI†) show that for a strongly electron donating substituent
such as –NMe2, the progress of the ligand-exchange reaction is
much faster compared to substituents like H, Me and OMe
groups. Among H, Me and OMe groups, the progress of the
ligand-exchange reactions follows the order: H > Me > OMe. The
observed differences in the advancement of the reactions can
again be explained on the basis of the stability of the reactant
and product platinum(IV) complexes associated with Pt–N
interactions. The advancement of the reaction for a particular
substituent is found to be faster in CDCl3 than in nitrobenzene-
d5. The sequential 1H NMR measurements of the reaction
mixtures also gives an idea regarding the mechanism of the
above ligand substitution reactions. The composition of the
platinum complexes reveals that both the bipyridine complexes
II and III are forming at the same time, i.e., substitution of both
the pyridines by the bipyridine ligand leading to the formation
of bipyridine complex II as well as substitution of one iodide
and one pyridine by the bipyridine ligands (for formation of
bipyridine–pyridine complex III) found to take place at the same
time; however, formation of complexes II and III depends very
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complex [PtMe3(L–L)L]I also increases; however, the strength of 19 A. V. Pawlikowski, A. D. Getty and K. I. Goldberg, J. Am.
chelation is greater in CDCl3 than in nitrobenzene-d5. Analysis Chem. Soc., 2007, 129, 10382–10393.
of the kinetics of the ligand-exchange reactions shows that both 20 A. T. Luedtke and K. I. Goldberg, Inorg. Chem., 2007, 46,
the nature of the substituent and solvent played an important 8496–8498.
role in the advancement of the reactions. Investigation of the 21 U. Fekl, W. Kaminsky and K. I. Goldberg, J. Am. Chem. Soc.,
kinetics of the reactions also reveals that both the bipyridine 2001, 123, 6423–6424.
complex [PtMe3(L–L)I] (formed through substitution of both the 22 E. W. Abel, P. J. Heard, K. G. Orrell, M. B. Hursthouse and
pyridine ligands by bipyridine ligand) and bipyridine–pyridine K. M. A. Malik, J. Chem. Soc., Dalton Trans., 1995, 3165–3171.
complex [PtMe3(L–L)I] (results via exchange of iodide and one 23 S. M. Nabavizadeh, S. Habibzadeh, M. Rashidi and
pyridine ligand by bipyridine ligand) are formed at the same
time.
R. J. Puddephatt, Organometallics, 2010, 29, 6359–6368.
24 B. Z. Momeni, M. Rashidi, M. M. Jafari, B. O. Patrick and
A. S. Abd-El-Aziz, J. Organomet. Chem., 2012, 700, 83–92.
25 C. Vetter, C. Wagner and D. Steinborn, Acta Crystallogr., Sect.
E: Struct. Rep. Online, 2010, 66, m286.
Acknowledgements
We thank Dr Michael Seran for the help in crystallographic
studies.
26 C. Vetter, C. Bruhn and D. Steinborn, Acta Crystallogr., Sect.
E: Struct. Rep. Online, 2010, 66, m941–m942.
27 D. Steinborn and H. Junicke, Chem. Rev., 2000, 100, 4283–
4317.
28 C. Vetter, C. Wagner, J. Schmidt and D. Steinborn, Inorg.
Chim. Acta, 2006, 359, 4326–4334.
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