Organoplatinum(ii) complexes containing chelating or bridging bis(N-heterocyclic carbene) ligands: Formation of a platinum(ii) carbonate complex by aerial CO2 fixation

Article abstract of DOI:10.1039/c1dt10736d

The preparation of two new bis(N-heterocyclic carbene) platinum(ii) complexes, in which NHC rings are joined by a CH₂ linker group, is described. While, the chelate complex [PtMe₂(bis-NHC1)], 1, was formed with large tert-butyl wingtips, the iso-propyl N-substituent analogue favors formation of the cluster complex [Pt₂Me₄(μ- SMe₂)(μ-bis-NHC2)]₂(μ-Ag₂Br₂), 2, in which two binuclear platinum(ii) complexes are linked together by an Ag₂Br₂ unit. The chelating platinum complex 1 undergoes aerial CO₂ fixation and forms platinum(ii) carbonate complex [Pt(CO₃)(bis-NHC1)], 3.

Full text of DOI:10.1039/c1dt10736d

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An international journal of inorganic chemistry
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Volume 40 | Number 37 | 7 October 2011 | Pages 9329–9620
ISSN 1477-9226
COVER ARTICLE
Jamali et al.
Organoplatinum(II) complexes
containing chelating or bridging
bis(N-heterocyclic carbene) ligands
1477-9226(2011)40:37;1-O

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Organoplatinum(II) complexes containing chelating or bridging
bis(N-heterocyclic carbene) ligands: formation of a platinum(II) carbonate
complex by aerial CO2 fixation†‡
Sirous Jamali,*a Dalibor Milic´,b Reza Kia,c Zahra Mazloomia and Hallimeh Abdolahia
Received 21st April 2011, Accepted 17th May 2011
DOI: 10.1039/c1dt10736d
The preparation of two new bis(N-heterocyclic carbene)
platinum(II) complexes, in which NHC rings are joined
by a CH2 linker group, is described. While, the chelate
complex [PtMe2(bis-NHC1)], 1, was formed with large tert-
butyl wingtips, the iso-propyl N-substituent analogue favors
formation of the cluster complex [Pt2Me4(l-SMe2)(l-bis-
NHC2)]2(l-Ag2Br2), 2, in which two binuclear platinum(II)
complexes are linked together by an Ag2Br2 unit. The chelat-
ing platinum complex 1 undergoes aerial CO2 fixation and
forms platinum(II) carbonate complex [Pt(CO3)(bis-NHC1)],
3.
that allow the preparation of organometallic compounds with a
variety of geometries and chemical reactivities.4 An interesting
type of bis(NHC) ligands comprises pairs of NHC rings that are
joined by a -(CH2)n- linker group. Several studies have been re-
cently published in order to rationalize the preference for chelating
versus bridging coordination mode of bis-NHC ligands. It seems
that the main factors that determine the coordination mode of
the bis-NHC ligand are the length of the linker between the two
azole rings, N-substituents and the nature of the counterion. The
effect of all these parameters on the coordination mode of the
bis-NHC ligands, as observed in a family of complexes of type
[Rh(bis-NHC)(COD)]+, was described previously.5
Carbon dioxide enters the air by burning fossil fuels such as oil,
natural gas and coal. The rate of annual global emission of CO2
into the air is roughly proportional to the global rate of energy con-
sumption. Greenhouse gases such as carbon dioxide trap radiation
upwelling from the earth’s surface and the expected result is global
warming and an unprecedented rate of climate change.6 Thus, the
development of methods for reduction of the atmospheric concen-
tration of CO2 would be highly desirable. Among these methods,
the chemical fixation of atmospheric CO2 is especially attractive
and transition metal complexes are well established as successful
compounds for CO2 fixation.7 Here we report the synthesis and
structural characterization of complexes [PtMe2(bis-NHC1)], 1,
and [Pt2Me4(m-SMe2)(m-bis-NHC2)]2(m-Ag2Br2), 2, in which bis-
NHC1, 1,1¢-methylene-3,3¢-bis[(N-(tert-butyl)imidazol-2-ylidene]
and bis-NHC2, 1,1¢-methylene-3,3¢-bis[(N-(iso-propyl)imidazol-
2-ylidene], act as chelating and bridging ligands, respectively.
Also, it is demonstrated that the fixation reaction of CO2 from
air to 1 at room temperature gives the corresponding platinum
carbonate complex [Pt(CO3)(bis-NHC1)], 3. Treatment of silver
carbene complex, [Ag2(bis-NHC1)Br2], in situ generated from the
corresponding bis(imidazolium) dibromide salt and Ag2O,5,8 with
1/2 equiv. of [Pt2Me4(m-SMe2)2] in CH2Cl2 at room temperature al-
lowed the isolation of bis(carbene)platinum complex [Pt(Me)2(bis-
NHC)], 1, in moderate yields (Scheme 1). Complex 1 is a white
solid that is stable in acetone solution for several hours.
Carbene ligands, mainly the N-heterocyclic carbenes (NHCs), are
regarded as important ligands that have attracted the attention of
numerous chemists due to their applications in organometallic
chemistry and homogeneous catalysis.1 There has also been
significant interest in the synthesis of new carbene ligands, their
metal complexes and exploiting the remarkable structural and
bonding properties of these compounds.2 In this area, a large
number of carbene ligand frameworks with tunable electronic and
steric parameters have been prepared in the past decade.3 Poly(N-
heterocyclic carbene) ligands are an interesting subclass of NHCs
aChemistry Department, Sharif University of Technology, PO Box 11155-
3516, Tehran, Iran. E-mail: sjamali@sharif.ir
bLaboratory of General and Inorganic Chemistry, Department of Chemistry,
Faculty of Science, University of Zagreb, Horvatovac 102a, 10000, Zagreb,
Croatia
cDepartment of Chemistry, Science and Research Branch, Islamic Azad
University, Tehran, Iran
† Electronic supplementary information (ESI) available. CCDC reference
numbers 797046, 797047 and 818324. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt10736d
‡ Crystal data for 1: C17H30N4Pt, orthorhombic, Pna21, a = 11.5536(2)
3
˚
˚
˚
˚
A, b = 18.5617(3) A, c = 8.6213(2) A, V = 1848.88(6) A , T = 150(1) K,
Z = 4, F(000) = 952, R1 = 0.0218, wR2 = 0.0419, GOF = 1.055, collected
reflections = 27179, independent reflections = 5842 (Rint = 0.0332).
¯
Crystal data for 2: C47H94Ag2Br2N8O3Pt4S2, triclinic, P1, a = 9.8360(3)
◦
◦
˚
˚
˚
A, b = 12.2936(4) A, c = 13.7271(4) A, a = 74.311(3) , b = 85.741(2) ,
◦
3
˚
g = 79.721(3) , V = 1571.80(8) A , T = 150(1) K, Z = 1, F(000) = 964,
The 13C NMR spectrum of 1 displays the characteristic reso-
nance for the carbene carbon atoms connected to the platinum
R1 = 0.0279, wR2 = 0.0486, GOF = 0.871, collected reflections = 12100,
independent reflections = 7113 (Rint = 0.0335).
˚
Crystal data for 3: C16H30N4O6Pt, monoclinic, P21/c, a = 8.8501(2) A, b =
1
center at d 185.8 ppm with JPtC = 778.2 Hz.9 This coupling
◦
3
˚
˚
˚
17.2896(4) A, c = 13.1079(2) A, b = 94.015(2) V = 2000.78(7) A , T =
120(2) K, Z = 4, F(000) = 1120, R1 = 0.0386, wR2 = 0.0698, GOF = 1.004,
collected reflections = 13622, independent reflections = 6565 (Rint = 0.0332).
1
constant is in the range of previously reported JPtC observed
for monodentate-NHC methylplatinum complexes.9 The two
9362 | Dalton Trans., 2011, 40, 9362–9365
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The Royal Society of Chemistry 2011
©

Fig. 2 The molecular structure of complex 1, showing 40% probability
displacement ellipsoids and the atomic numbering. The H atoms are shown
as sphere of arbitrary radius. (except for the tert-butyl H atoms, which were
omitted for clarity).
Scheme 1
dimethylplatinum complex 1 to air for 24 h led to isolation of the
platinum carbonate complex [Pt(CO3)(bis-NHC1)], 3, as a white
solid in 65% yield (Scheme 1). It seems that the carbonate ligand
connected to platinum atom originates from hydration of aerial
CO2 to give H2CO3, which in turn undergoes deprotonation in
reaction with 1 and affords the stable bis-NHC platinum carbonate
complex 3. The presence of the carbonate ligand was confirmed
by the 13C NMR spectrum of complex 3, which displayed the
carbonate carbon atom as a singlet at d 168 ppm. This value
of chemical shift falls in the same range (165–170 ppm) as
those previously reported for platinum carbonate complexes.12
The carbene carbon atoms appeared as a singlet at d 140 ppm
equivalent carbon atoms of the methyl ligands appeared as a
singlet signal at d -7.7 ppm with JPtC = 606.1 Hz. The presence
1
of two equivalent Pt–Me ligands in complex 1 is supported by
a singlet resonance in the 1H NMR spectrum(acetone-d6) at
d 0.23 ppm, which is flanked by 195Pt satellite signals (2JPtH
=
67.5 Hz), due to the Me protons. The protons Ha and Hb of the
methylene linker appear as an AB pattern at d 5.3 and 6.1 ppm
with 2JHH = 12.2 Hz respectively indicating that the six-membered
chelate ring of bis-NHC ligand is not planar and adopts a boat
conformation (Scheme 1). The Hb resonance in the 1H NMR
spectrum of 1 is shifted downfield by 0.8 ppm versus Ha and shows
a coupling with the platinum center (JPtH = 19.5 Hz) (Fig. 1a).
These NMR data suggest a weak electrostatic interaction between
the methylene linker proton Hb and the platinum metal center
and therefore not an agostic interaction, which is characterized by
an upfield 1H NMR chemical shift.10 Crystals of complex 1 were
obtained by the slow diffusion of ether into an acetone solution
of 1 under a CO2-free atmosphere in a refrigerator for a week.
A view of the molecular structure of 1 is depicted in Fig. 2.
As anticipated by NMR analysis, the bis-NHC ligand adopts
a bidentate coordination mode through the C(carbene) atoms,
forming a six-membered chelate ring with the bite angle 83.0(1)◦.
1
with a considerably higher JPtC value close to 1318 Hz because
of the lower trans influence of O atoms of carbonate ligand as
compared with that of C atoms of the methyl ligands in complex
1
1. The H NMR spectrum of complex 3 in DMSO-d6 displayed
2
an AB pattern at d 5.95 and 5.87 with JHH = 12.6 Hz for the
methylene linker protons (Fig. 1b). Furthermore, no methyl proton
resonances associated with the methyl ligands could be observed in
the 1H NMR spectrum of complex 3. The IR spectrum of complex
3 displays a strong band at 1600 cm-1, indicating that the carbonate
ligand is bonded to the platinum atom as a bidentate ligand.13
Complete structural characterization of 3 as a carbonate complex
was achieved by X-ray crystallographic analysis. The crystals of
3 suitable for X-ray crystallographic analyses were obtained by
slow evaporation of an acetone solution of complex 1. As shown
in Fig. 3a, complex 3 adopts a slightly distorted square planar
˚
The Pt–C(carbene) bond distances of C(21)–Pt(1) = 2.039(3) A and
˚
C(22)–Pt(1) = 2.049(3) A lie in the range of previously reported
Pt(II)–C(NHC) bond lengths9,11 and the NHC ring planes are tilted
by an average of 56.6◦ from the coordination plane.
˚
geometry (the Pt atom displaced -0.09(1) A from the coordination
plane) and three water molecules in the asymmetric unit are bound
to 3 via hydrogen bonds to the oxygen atoms of carbonate ligand.
˚
The Pt–C(carbene) bond lengths 1.959(5) and 1.969(4) A are in
the expected range of Pt(II)–C(NHCs) bonds.
The bite angle of the bis-NHC ligand [C(22)–Pt(1)–C(21)] is
84.0(2)◦ and the dihedral angle between the coordination plane
of the complex and the NHC ring plane [48.1(2)◦] is twisted
significantly from the favored orthogonal arrangement. This might
be caused by a steric clash between the tert-butyl groups of
the carbene ligand. The yaw distortion, q for two azole rings
in complex 3 (with an average of 7.5◦) is significantly smaller
than that observed in 1 (12.5◦). Further examination of the
crystal structure of 3 also reveals that the molecules of complex
3 are connected into infinite 1-D extended chains along the
[100] direction through O(carbonate) ◊ ◊ ◊ H–O–H ◊ ◊ ◊ O(H)–H.O
(carbonate) H-bonds (Fig. 3b).
Fig. 1 The CH2 linker region 1H NMR spectra of (a) complex 1 and (b)
complex 3.
The hydrogen atom H102 of C10 (the methylene linker carbon
˚
atom) is at distance 2.91 A from the platinum atom with the C–
H–Pt angle of 89.4◦. This confirms the presence of an electrostatic
interaction between H102 proton and platinum center, as observed
1
in the H NMR spectrum of 1 and discussed above. The most
interesting feature of complex 1 is its capability to fix aerial CO2
at room temperature. Exposure of an acetone solution of the
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9362–9365 | 9363
©

Fig. 4 The molecular structure of complex 2, showing displacement
ellipsoids at the 30% probability level. H-atoms were removed for clarity.
The dashed lines show the close contacts between the designated atoms.
Label “A” denotes atoms centrosymmetrically related to those in the other
half of a molecule.
˚
C(13) = 2.057(6) A, indicating the carbene ligand exerts higher
trans influence than the dimethylsulfide ligand. The 195Pt NMR
spectrum of 2 gave a broad resonance at d -2420 ppm with no
detectable coupling 1JAgPt. The 1H NMR spectrum of complex 2 in
acetone-d6 solvent, shows two equal intensity signals at d 0.34 and
0.25 ppm with 2JPtH = 91 and 66 Hz due to the Me protons trans to
dimethylsulfide and carbene ligands, respectively. The presence of
a bridging dimethylsulfide ligand was shown by the appearance of
the two equal intensity MeS resonances at d 2.8 and 2.2 ppm in the
1H NMR spectrum as two 1 : 8 : 18 : 8 : 1 quintets due to coupling to
two adjacent 195Pt atoms with 3JPtH = 19.5 and 18.3 Hz, respectively.
The 13C NMR spectrum of 2 showed a singlet resonance at d
183.5 ppm due to the C atoms of the carbene ligand with no
detectable 1JPtC coupling. Finally, there were two methylplatinum
resonances in the 13C NMR spectrum at d -11.9 and -7.9 ppm;
the coupling 1JPtC = 736 Hz for the methyl groups trans to sulfur
Fig. 3 (a) The molecular structure of complex 3, showing 40% probability
displacement ellipsoids and the atomic numbering. The H atoms are
shown as sphere of arbitrary radius. (b) A part of the crystal packing
of complex 3 showing a 1-D infinite chain along the [100] direction
through intermolecular hydrogen bonding. Only the H atoms involved
in the H-bonding are shown.
As observed in the present and previously published works,5
the use of large N-substituent groups and short linker CH2
group favors the chelating mode of bis(NHC) ligands. However,
the reaction of 1/2 equiv. [Pt2Me4(m-SMe2)2] with a mixture of
methylene bis(N-(iso-propyl)imdazolium bromide) and Ag2O in
CH2Cl2 gave the hexanuclear cluster complex [Pt2Me4(m-SMe2)(m-
bis-NHC2)]2(m-Ag2Br2), 2 in 70% yield, in which bis(NHC2) acts
as a bridging ligand (Scheme 1). Crystals of the acetone solvated of
2 were obtained by the slow diffusion of n-pentane into an acetone
solution of complex 2 in a refrigerator. The complex crystallizes
1
was greater than the value JPtC = 582 Hz for the methyl group
trans to C atoms of carbene ligand. Electrospray ionization (ESI)
mass spectrum (positive ions) of complex 2 shows a base peak
at m/z 1022 (Fig. 5) assignable to the species [Pt2Me4(bis-NHC)2-
Ag]+ formed by the displacement of a labile dimethyl sulfide ligand
with a bis-NHC2 ligand and cleavage of Ag–Br bonds. Examples
of formation of bis(carbene)silver complexes (C–Ag–C), in the gas
phase from silver NHCs with solid-state motifs of C–Ag–X and
C–Ag–X2 have been reported previously.14
¯
in a triclinic P1space group. The structure of 2 in the solid state
(Fig. 4) indicates two binuclear platinum units [Pt2Me4(m-SMe2)(m-
bis-NHC2)] that are linked together by an Ag2Br2 entity to form
centrosymmetric cluster complex 2.
The complex contains two different types of Pt–Ag bonds, short
and long. The bond distance for the short bonds, Pt(1)–Ag(1) or
˚
Pt(1A)–Ag(1A) = 2.8377(6) A, is near to the sum of the metallic
˚
radii of Pt and Ag (2.83 A), indicating the formation of a Pt–
Ag dative bond; note that each bond is also supported by two
secondary weak interactions of the Ag atom with the C atoms
of the methyl ligands [C(12) or C(14)] that are coordinated to
the Pt atom (dashed bonds in Fig. 4) with Ag–C distances of
˚
˚
Ag(1)–C(12) = 3.000(7) A and Ag(1)–C(14) = 3.075(7) A. The
˚
long bonds, Pt(2)–Ag(1) or Pt(2A)–Ag(1A) = 2.8982(5) A, are
shorter than the sum of the van der Waals radii of Pt and Ag (3.45
˚
A), demonstrating the presence of Pt–Ag bonding interactions.
The Pt–C bond distances of methyl ligands trans to C(carbene)
˚
˚
atom, Pt(1)–C(12) = 2.087(5) A and Pt(2)–C(14) = 2.097(5) A are
longer than the Pt–C bond distances of the methyl ligands trans
Fig. 5 Expanded ESI mass spectrum of complex 2 Inset: Simulated
isotope pattern for the [Pt2Me4(bis-NHC)2-Ag]+ cation.
˚
to the dimethylsulfide ligand, Pt(1)–C(11) = 2.045(6) A and Pt(2)–
9364 | Dalton Trans., 2011, 40, 9362–9365
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L. Y. Helgard, G. Raubenheimer and M. Albrecht, Chem. Rev., 2009,
109, 3445–3478.
Conclusions
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In summary, we have prepared two new bis(N-heterocyclic
carbene) platinum complexes in which the bis(N-heterocyclic
carbene) acts as chelating or bridging ligand and forms mono-
or multinuclear complexes 1 and 2. The chelating complex
1 demonstrated the ability to fix aerial CO2. Current efforts
are focused on the synthesis, structure and reactivity of new
bis(N-heterocyclic carbene) platinum complexes with different N-
substituents and longer aliphatic linker chains.
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The financial support of Sharif University of Technology
Research Council and the Iran National Science Fundation (Grant
No. 89000977) gratefully acknowledged. SJ thanks Professor
William Henderson for many good suggestions and helpful dis-
cussions on ESI-Mass analysis. We thank the Ministry of Science,
Education and Sports of the Republic of Croatia (Grant No. 119-
ˇ
1193079-1084) and Professor Dubravka Matkovic´-Calogovic´ for
the X-ray diffraction facility.
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Dalton Trans., 2011, 40, 9362–9365 | 9365
©