One-pot and step-by-step N-assisted CPh-H activation in 2-(4-bromophenyl)imidazol[1,2-a]pyridine: Synthesis of a new C,N-cyclometalated compound [{Pt(CN)(μ-CI)}2] as precursor of luminescent platinum(II) compounds

Article abstract of DOI:10.1021/om901032v

The activation of a. C_ph-H bond in the phenyl ring of 2-(4-bromophenyl)imidazol[1,2-a]pyridine (HCN)by[{Pt(n³-C ₄H₇)(μ-Cl)}₂](n³-C ₄H₇ = n³-2-methylallyl) renders the new cyclometalated complex [{Pt(CN)(μ-Cl)}₂] (2) with high yield and selectivity. Complex 2 can be achieved directly in a one-pot reaction or step by step through the intermediate [Pt(n³-C₄H ₇)Cl(HCN-kN)] (1). Compound 1 could be isolated and fully characterized. The X-ray structure shows the coordination of HC N through only the N and the existence of a weak Pt...-H-C hydrogen bridging bond (Pt...H1 =2.78 A, Pt...Cl = 3.365(3) A, Pt-H1-C1. = 120.9°). Hence, the formation of this intermediate could be considered the first step in the cyclometalation process. The mononuclear complexes [PtCl(CN)L] (L = tht (3), PPh₃ (4), CN-Xy1 (5), CN-^tBu (6)) were obtained by cleavage of the bridging system in [{Pt(CN)(μ-Cl)}₂] (2) by the neutral ligands, L. The resulting geometry (trans C, Cl) is that, expected, from the electronic preferences, taking into account the degree of transphobia (T) of pairs of trans ligands, T[C(CN)/L(C1)] < T[C(CN)/L(S, P, C)]. Complexes [PtCl(C^circ;N)L] (L = CN-XyI (5), CN- ^tBu (6)) containing two strong-field ligands, a C_cN σbonded and an isocyanide ligand, are luminescent. TD-DFT calculations were performed for the singlet ground state, So, as well as for the first triplet excited state of 6 in both the gas phase and solution. Calculations indicate that the lowest-lying absorption involves mainly ¹IL (CN) mixed with, a small contribution of ¹MLCT/1L'LCT (L = CAN; L' = Cl) transitions. Complex 5 exhibits "luminescent thermochromism" in the solid state; at. 77 K it shows a green phosphorescence band assigned to ³IL transitions located on the CN group of monomer species, while at 298 K an orange-red emission is observed, being tentatively assigned, to excited states of emissive aggregates (³MMLCT/³ π-π). However complex 6 shows phosphorescence only at 77 K both, in solution and in the solid state with the emissions arising from ³IL and ³L'MLCT excited states of monomer species.

Full text of DOI:10.1021/om901032v

1396 Organometallics 2010, 29, 1396–1405
DOI: 10.1021/om901032v
One-Pot and Step-by-Step N-Assisted CPh-H Activation in
2-(4-Bromophenyl)imidazol[1,2-a]pyridine: Synthesis of a
New C,N-Cyclometalated Compound [{Pt(C∧N)(μ-Cl)}2] as Precursor of
Luminescent Platinum(II) Compounds
,†
,‡
†
†
Juan Fornies,* Violeta Sicilia,* Carmen Larraz, Jose A. Camerano, Antonio Martı
´
n,†
ꢀ
ꢀ
Jose M. Casas, and Athanassios C. Tsipis*
†
,§,^
ꢀ
†
´
ꢀ
Departamento de Quımica Inorganica, Instituto de Ciencia de Materiales de Aragon, Facultad de Ciencias,
ꢀ
Universidad de Zaragoza-CSIC, Plaza. S. Francisco s/n, 50009, Zaragoza, Spain, ‡Departamento de
´
ꢀ
ꢀ
´
Quımica Inorganica, Instituto de Ciencia de Materiales de Aragon, Escuela Universitaria de Ingenierıa
Teꢀcnica Industrial, Universidad de Zaragoza-CSIC, Campus Universitario Rio Ebro, Edificio Torres
Quevedo, 50018, Zaragoza, Spain, and §Laboratory of Inorganic and General Chemistry, Department of
Chemistry, University of Ioannina, 451 10 Ioannina, Greece
Received November 30, 2009
The activation of a CPh-H bond in the phenyl ring of 2-(4-bromophenyl)imidazol[1,2-a]pyridine
(HC∧N) by[{Pt(η3-C4H7)(μ-Cl)}2] (η3-C4H7 =η3-2-methylallyl) renders the new cyclometalated complex
[{Pt(C∧N)(μ-Cl)}2] (2) with high yield and selectivity. Complex 2 can be achieved directly in a one-pot
reaction or step by step through the intermediate [Pt(η3-C4H7)Cl(HC∧N-κN)] (1). Compound 1 could be
isolated and fully characterized. The X-ray structure shows the coordination of HC∧N through only the N
and the existence of a weak Pt H-C hydrogen bridging bond (Pt H1=2.78 A, Pt C1 = 3.365(3)
3 3 3 3 3 3 3 3 3
A, Pt-H1-C1=120.9ꢀ). Hence, the formation of this intermediate could be considered the first step in the
cyclometalation process. The mononuclear complexes [PtCl(C∧N)L] (L=tht (3), PPh3 (4), CN-Xyl (5),
CN-tBu (6)) were obtained by cleavage of the bridging system in [{Pt(C∧N)(μ-Cl)}2] (2) by the neutral
ligands, L. The resulting geometry (trans C, Cl) is that expected from the electronic preferences, taking into
account the degree of transphobia (T) ofpairsoftrans ligands, T[C(C∧N)/L(Cl)] < T[C(C∧N)/L(S, P, C)].
Complexes [PtCl(C∧N)L] (L = CN-Xyl (5), CN-tBu (6)) containing two strong-field ligands, a CC∧N σ-
bonded and an isocyanide ligand, are luminescent. TD-DFT calculations were performed for the singlet
ground state, S0, as well as for the first triplet excited state of 6 in both the gas phase and solution.
1
Calculations indicate that the lowest-lying∧absorption involves mainly IL (C∧N) mixed with a small
1
contribution of MLCT/1L0LCT (L = C N; L0 = Cl) transitions. Complex 5 exhibits “luminescent
3
thermochromism” in the solid state; at 77 K it shows a green phosphorescence band assigned to IL
transitions located on the C∧N group of monomer species, while at 298 K an orange-red emission is
observed, being tentatively assigned to excited states of emissive aggregates (3MMLCT/3π-π*). However
complex 6 shows phosphorescence only at 77 K both in solution and in the solid state with the emissions
arising from 3IL and 3L0MLCT excited states of monomer species.
Introduction
this kind of compound has been mainly due to their applica-
tion as reagents for highly regioselective organic synthesis or
as catalysts for cross-coupling, alkane dehydrogenation, or
Diels-Alder reactions, among others.1,4,5 More recently,
the increasing attention paid to transition metal cyclometa-
lated compounds is due to their attractive photochemical
and photophysical properties6-8 and their potential use as
molecular devices.
Transition metal cyclometalated compounds containing
five-membered metallacycles can be selectively synthesized
as compared to other sized ring compounds. They are very
stable organometallic compounds due to the chelating effect
and have been known for a long time.1-3 The great interest in
*Corresponding authors. E-mail: juan.fornies@unizar.es. Fax:
(þ34)976-761187.
^Corresponding author regarding the TD-DFT calculations. E-mail:
attsipis@cc.uoi.gr.
(4) Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jerpersen, K.; Gold-
man, A. S. J. Am. Chem. Soc. 2004, 126, 13044.
(1) Omae, I. Coord. Chem. Rev. 2004, 248, 995.
(2) Mohr, F.; Priver, S. H.; Bhargava, S. K.; Bennett, M. A. Coord.
Chem. Rev. 2006, 250, 1851.
(3) Crabtree, R. H.; Mingos, D. M. P. Comprehensive Organometallic
Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford,
2007.
(5) Omae, I. J. Organomet. Chem. 2007, 692, 2608.
ꢀ
(6) Lowry, M. S.; Bernhard, S. Chem.-Eur. J. 2006, 12, 7970.
(7) Termine, R.; Talarico, M.; Aiello, I.; Dattilo, D.; Pucci, D.;
Ghedini, M.; Golemme, A. Opto-Electron. Rev. 2005, 13, 287.
(8) Santoro, A.; Whitwood, A. C.; Williams, J. A. G.; Kozhevnikov,
V. N.; Bruce, D. W. Chem. Mater. 2009, 21, 3871.
r
pubs.acs.org/Organometallics
Published on Web 02/24/2010
2010 American Chemical Society

Article
In the transition metal complexes of C,N-cyclometalated
Organometallics, Vol. 29, No. 6, 2010 1397
with phosphine,26 alkylphosphine,26 alkynyl,27 cyanide,28
and isocyanide29 as ancillary ligands. In addition, Pt(II)
complexes with sterically undemanding ligands are essenti-
ally flat, and therefore ground- and excited-state interactions
become feasible depending on the concentration and proxi-
mity of the molecules. This fact leads to marked red-shifted
phosphorescence transitions with respect to those in the
monomer species. These transitions are assigned to me-
tal-metal-to-ligand charge transfer (MMLCT) or excimeric
ligand-to-ligand charge transfer and are highly dependent on
the extent of Pt Pt and π π interactions.9,10,30-32
ligands the presence of π-conjugated fragments around the
metal ion gives rise to ligand-centered (IL, π-π*, or n-π*)
and metal-to-ligand charge transfer (MLCT) excited states,
the lowest unoccupied molecular orbital (LUMO, π*) being
located mainly on the imine fragment, which lies at lower
energies than the d-d states. Moreover, the strong σ-donor
CC∧N atom affords the metal ion a very strong ligand field,
raising the energy of the nonradiative metal-centered (d-d)
excited states to relatively inaccessible energies. Addition-
ally, the introduction of strong-field ancillary ligands in the
complex increases the energy gap, ΔE, between the lowest-
lying excited state (IL/ MLCT) and higher-lying d-d state,
so the nonradiative decay is inhibited. Thus, many isolated
C,N-cyclometalated Pt(II) complexes emit from 3IL and/or
3MLCT states and are luminescent in solution under ambi-
ent conditions.9-12 Some of them have been successfully
applied in the manufacture of photosensors or phosphores-
cent organic light-emitting devices (PhOLEDs).9,11-21
Mononuclear heteroleptic complexes with a single cyclo-
metalating ligand (C∧N) allow the fine-tuning of photophy-
sical properties via variation of the electronic nature
(donating or withdrawing character) of the cyclometalating
or even the ancillary ligands.22 The electronic properties of
the ligands affect the electron density at the metal center and
consequently force the metal-to-ligand charge transfer
(MLCT) transitions to be mixed with the lowest-energy
transitions, thus altering the relative energy, radiative color,
and lifetime of the excited state.23-25 This is the case, for
example, in heteroleptic benzoquinolinate Pt(II) complexes
3 3 3
3 3 3
Following on with our interest in the chemistry and
photophysical properties of cyclometalated Pt(II) com-
pounds,28,29,33,34 our aim was to prepare new luminescent
compounds via variation of the cyclometalating ligand.
Along these lines, we describe the cyclometalation of 2-(4-
bromophenyl)imidazol[1,2-a]pyridine (HC∧N) by [{Pt(η3-
C4H7)(μ-Cl)}2] (η3-C4H7 = η3-2-methylallyl) as a one-pot
or step-by-step reaction, through the intermediate [Pt(η3-
C4H7)Cl(HC∧N-κN)], to yield the di-μ-chloro cyclometa-
lated compound [{Pt(C∧N)(μ-Cl)}2]. This dinuclear com-
pound was used as starting material for the synthesis of
the mononuclear complexes [PtCl(C∧N)L] (L = tht, PPh3,
CNXyl, CNBut), which incorporate monodentate ligands
with different electron-withdrawing/donating properties, in
order to tune the emission properties. As expected, only
complexes 5 and 6, containing the strong-field isocyanide
ligand, are luminescent. Their photophysical properties were
thoroughly investigated. To gain a better understanding of
the nature of the absorption and emission bands, time-
dependent density functional theory (TD-DFT) calculations
were performed for the singlet ground state, S0, as well as for
the first triplet excited state of 6 in both the gas phase and
solution.
(9) Williams, J. A. G.; Develay, S.; Rochester, D. L.; Murphy, L.
Coord. Chem. Rev. 2008, 252, 2596.
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platinum(II) materials for OLED applications. In Highly Efficient
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Weinheim, 2007.
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Results and Discussion
Reaction of [{Pt(η3-C4H7)(μ-Cl)}2] with 2-(4-Bromophe-
nyl)imidazol[1,2-a]pyridine (HC∧N). Synthesis of [Pt(η3-
C4H7)Cl(HC∧N)] (1) and [{Pt(C∧N)(μ-Cl)}2] (2) Compounds.
When a solution of the dichloro-bridged complex [{Pt(η3-
C4H7)(μ-Cl)}2] (η3-C4H7 = η3-2-methylallyl) and an equi-
molar amount of 2-(4-bromophenyl)imidazol[1,2-a]pyridine
(HC∧N) was refluxed in acetone (Scheme 1a and Experi-
mental Section for details), compound [Pt(η3-C4H7)Cl-
(HC∧N)] (1) precipitated as a white solid. However, by
refluxing a solution containing equimolar amounts of the
allyl complex [{Pt(η3-C4H7)(μ-Cl)}2] and HC∧N in 2-meth-
oxyethanol (Scheme 1b and Experimental Section for
details), the precipitated solid corresponded to complex
[{Pt(C∧N)(μ-Cl)}2] (2). Complex 2 can be also obtained by
refluxing a suspension of compound 1 in 2-methoxyethanol
(Scheme 1c). Compound 1 was isolated as a pure, air-stable
solid and fully characterized (see Experimental Section). A
(23) You, Y.; Seo, J.; Kim, S. H.; Kim, K. S.; Ahn, T. K.; Kim, D.;
Park, S. Y. Inorg. Chem. 2008, 47, 1476.
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Pt-Cl absorption (285 cm-1) is consistent with the presence
Chem. C 2007, 111, 4052.
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1398 Organometallics, Vol. 29, No. 6, 2010
Fornies et al.
Table 1. Selected Structural Data for 1
Bond Lengths [A]
Pt-C(14)
2.099 (4)
Pt-C(16)
2.097(4)
2.368(7)
1.424(4)
1.499(5)
1.372 (4)
1.371 (4)
1.384 (4)
Pt-C(15)
2.097 (4)
2.092 (3)
1.427(5)
1.472 (4)
1.382 (4)
1.338 (4)
1.380 (4)
Pt-Cl
Pt-N(1)
C(15)-C(16)
C(5)-C(17)
C(7)-C(8)
N(2)-C(8)
N(2)-C(13)
C(14)-C(15)
C(6)-C(7)
N(1)-C(7)
N(1)-C(13)
N(2)-C(9)
Bond Angles [deg]
N(1)-Pt-Cl
90.34(7)
N(1)-Pt-C(14)
99.94(11)
C(16)-Pt-C(14)
69.54(13)
C(16)-Pt-Cl
99.65(9)
C(14)-C(15)-C(16) C(14)-C(15)-C(17)
114.1(3) 122.3(3)
C(16)-C(15)-C(17)
122.2(3)
Figure 1. Molecular structure of complex 1.
A] is in the upper range of those observed in Pt(II) complexes
with the same kind of ligands,41,42 in agreement with the high
trans influence of the η3-2-methylallyl group.
Scheme 1
The 2-(4-bromophenyl)imidazol[1,2-a]pyridine ligand is
not planar; the phenyl ring forms an angle of 32.5(1)ꢀ with
the best plane defined for the imidazo[1,2-a]pyridine frag-
ment. Both parts of the ligand are almost perpendicular to
the platinum coordination plane, the interplanar angles
being 72.0(1)ꢀ and 69.6(1)ꢀ for the phenyl and imidazol-
[1,2-a]pyridine fragments, respectively. As can be seen, the
phenyl ring is turned in such a way that the C1-H1 vector
points toward the platinum atom. After locating the hydro-
gen atoms geometrically we found that the Pt H1 distance
3 3 3
was 2.78 A and the Pt-H1-C1 angle was 120.9ꢀ. These
parameters, together with the observed Pt C1 separation
[3.365(3) A], are in the range observed for reported hydrogen
3 3 3
bridging M H-C bonds43,44 and differ from observed
3 3 3
values for agostic systems.45 Taking into account the pre-
sence, in the molecule, of two hydrogen bond acceptor atoms
(Pt, Cl) and the nonhindered rotation of the phenyl ring
of a Pt-Cl terminal bond in trans position to a ligand with a
large trans influence.35 The presence of the allyl and the
HC∧N ligands was evident from the peak corresponding to
[Pt(η3-C4H7)(HC∧N)]þ observed in its MALDI (þ) mass
spectrum and corroborated by the X-ray study on a single
crystal of it (Figure 1, Table 1). As can be seen, 1 is a
mononuclear complex in which the platinum(II) center ex-
hibits a highly distorted square-planar environment mainly
due to the geometry and bonding mode of the η3-allyl group.
The three Pt-Callyl distances are roughly equal and shorter
than those found in other η3-allyl platinum(II) and
palladium(II) complexes containing ligands with a greater
trans influence than Cl and N.36-38 The dihedral angle
between the best plane defined for the 2-methylallyl moiety
and the platinum coordination plane (Pt, Cl, N1, C14, C16)
is 73.6(1)ꢀ. The Pt-Cl distance [2.368(7) A] fits the Pt-Cl
bond length [2.367(7) A] found in the relevant complex [(η3-
C3H5)Pt(P(tBu)3)Cl].39,40 The Pt-N bond length [2.092(25)
around the C6-C7 bond, the existence of the Pt H1
3 3 3
interaction could be considered as the step prior to the
C1-H1 activation process, to give the cyclometalated com-
plex, 2.
In solution, all the NMR spectra display the expected
signals for 1 in accordance with its X-ray structure. The H1
and H12 NMR signals (see Experimental Section and
Scheme 2) appear downfield shifted (Δδ: 0.3 ppm H1, 0.67
ppm, H12) with respect to those in the free ligand, which is a
characteristic feature of D-H Pt hydrogen interaction
3 3 3
involving the filled dz2 orbital of the Pt(II) center.43,46 As a
consequence of the nonhindered rotation around the C6-C7
and Pt-N bonds, the protons H1 and H2 are chemically
equivalent to H5 and H4 (appearing as an AA0XX0 system)
and so are C1 and C2 to C5 and C4. The methyl allyl
group (η3-C4H7) gives five 1H NMR signals, unambiguously
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R.; Facchin, G.; Zanotto, L.; Ganis, P.; Valle, G. Organometallics 1996,
15, 3250.
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1997, 1559.
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Article
Organometallics, Vol. 29, No. 6, 2010 1399
Scheme 2. Numerical Scheme for NMR Purposes
Scheme 3
use of [{Pt(η3-C4H7)(μ-Cl)}2] gave 2 as a pure solid in high
yield, and no traces of oxidative addition products were
observed. Hence, although the allyl complex [{Pt(η3-C4H7)-
(μ-Cl)}2] is not as easy to prepare as other precursors
for cyclometalation processes (e.g., K2PtCl4,51,52 [PtCl2-
(NCPh)2],50 [PtCl2(dmso)2],53,54 [PtCl2(SEt2)2]55), it has pro-
ven to be a very convenient starting material56 as in this case,
given the selectivity and yield of the described reaction. Cyclo-
metalation was generally considered to proceed by a two-step
reaction mechanism (Scheme 3).1 The intermediate coordina-
tion compounds have been detected57,58 and isolated59 in the
reactions of [Pt2Me2(μ-SMe2)2] with imines containing two
nitrogen atoms, which coordinate to the metal center as chelate
ligands. With imines containing only one single donor nitrogen
such intermediates could never been isolated and could be
detected only when the metalation step was hindered by bulky
groups.59-61 Hence, cycloplatination intermediates containing
the ligand coordinated through the N in a monodentate fashion
have hardly ever been isolated.62-65 In some cases, no interac-
assigned on the basis of a 1H-13C HSQC experiment, at 3.52
(1H syn), 3.0 (1H syn), 2.06 (1H anti), 1.98 (3H) and 1.65 ppm
(1H anti), in agreement with the lack of symmetry in complex
1. The high values of JPt-H are similar to those found in other
η3-allyl derivatives,39,40 including the starting complex
[{Pt(η3-C4H7)(μ-Cl)}2].47 The presence of two resonances
corresponding to C10 and C30 of the η3-C4H7 group with
similar chemical shifts and Pt-C coupling constants agrees
with the lack of symmetry in complex 1 and also with the
symmetrically π-bonded 2-methylallyl ligand. The JPt-C
values are all smaller than those found in complex [{Pt(η3-
C4H7)(μ-Cl)2}],48 as expected for the stronger trans influence
of a N with respect to a Cl ligand.
As is usual in di-μ-chloro cyclometalated complexes, com-
pound [{Pt(C∧N)(μ-Cl)}2] (2) is barely soluble in most
common solvents, which precludes obtaining good 13C or
195Pt{1H} NMR spectra. Its 1H NMR was recorded in
DMSO-d6 as the solvent and provides direct evidence of
the purity of the prepared sample, the elimination of the allyl
group, and the metalation of the 2-(4-bromophenyl)imi-
dazol[1,2-a]pyridine ligand (HC∧N) through one ortho-C
of the phenyl ring. Hence, in complex 2 H1 disappears and
the remaining phenyl protons are all inequivalent to each
other, giving three signals with their corresponding multi-
plicity proving the metalation of the HC∧N group. In the
solid state, the IR spectrum of 2 shows two ν(Pt-Cl) absorp-
tions at 320 and 261 cm-1. These values are significantly
higher than those described in the literature for the dinuclear
complexes [{Pt(P∧C)(μ-Cl)}2] (P∧C = -CH2C6H4PtBu(o-
tolyl), -CH2C6H4(PtBu)2,49 -CH2C6H4P(o-tolyl)250), in
agreement with the weaker trans influence of nitrogen com-
pared to phosphorus. Thus, compound 2 is a five-membered
cyclometalated compound resulting from the selective intra-
molecular activation of a CPh-H bond of 2-(4-bromo-
phenyl)imidazol[1,2-a]pyridine (HC∧N) by the allyl complex
[{Pt(η3-C4H7)(μ-Cl)}2] followed by 2-methylpropene elimi-
nation. Other attempts to cyclometalate this HC∧N ligand
using K2PtCl4 and [PtCl2(NCPh)2] as starting materials in
refluxing 2-methoxyethanol failed; in all cases mixtures of 2
and other unidentified compounds precipitated in the reac-
tion system and pure 2 could not be obtained. However the
tion between the later activated C H bond and the metal
3 3 3
atom is observed;64,65 in others, such as the N-bound ferroce-
nylamine Pt(II) complexes, trans-[PtCl2(DMSO)[(η5-C5H4-
CH2NMe2)Fe(η5-C5H5)62 and [{trans-PtCl2(DMSO)}2{μ-
Fe(η5-C5H4CH2NMe2)2)}],63 a Pt H-C interaction can be
3 3 3
adduced, although it is very weak since the Pt H distances
3 3 3
are quite long, 3.163 or 3.38 A.62 In our case, the use of [{Pt(η3-
C4H7)(μ-Cl)}2] as precursor has enabled us to isolate the
cycloplatination intermediate [Pt(η3-C4H7)Cl(HC∧N-κN)]
(1), containing the HC∧N-κN ligand coordinated in a mono-
dentate fashion and showing a clear Pt H-C interaction.
3 3 3
(51) Yeo, W.-C.; Vittal, J. J.; White, A. J. P.; Williams, D. J.; Leung,
P.-H. Organometallics 2001, 20, 2167.
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Gong, J.-F.; Song, M.-P. Organometallics 2009, 28, 3369.
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264.
(54) Crespo, M.; Anderson, C. M.; Tanski, J. M. Can. J. Chem. 2009,
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(47) 1H NMR (CD2Cl2, 400.13 MHz, 298 K, δ): 3.58 (s, 2JPt-H = 40
Hz, 2Hsyn), 2.11 (s, 3JPt-H = 85.6 Hz, 3H, Me), 2.04 (s, 2JPt-H = 85.6
Hz, 2Hanti).
nez, M.; Sales, J.; Solans, X.; Font-Bardıa, M.
´ ´
(48) 13C{1H} NMR (CD2Cl2, 100.63 MHz, 298 K, δ): 110.33 (s, JPt-C
= 108 Hz, C-2), 45.22 (s, JPt-C = 281 Hz, C-1, C-3), 22.62 (s, JPt-C = 55
Hz, C-4).
(49) Cheney, A. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1972,
754.
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(50) Fornies, J.; Martı
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n, A.; Navarro, R.; Sicilia, V.; Villarroya, P.
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Organometallics 1996, 15, 1826.
Organometallics 2002, 21, 1633.

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Fornies et al.
1400 Organometallics, Vol. 29, No. 6, 2010
with the cis arrangement of the Pt-C and Pt-P bonds in the
SP-4-4 isomer and can be seen in Figure 2. The observed
values of 3JPt-H2 are in all cases very similar to those found in
the literature for related compounds 74,75 The 31P{1H} NMR
spectrum of 4 shows a singlet with the expected platinum
satellites; the Pt-P coupling constant value (4408 Hz) is in
agreement with a trans arrangement of triphenylphosphine
and the Pt-N bond.66,75,76 The 13C{1H} and 195Pt{1H}
NMR spectra could be recorded for complexes 3 (L = tht)
and 6 (L = CN-tBu) but not for 4 (L = PPh3) and 5 (L =
CN-Xyl) because of their low solubility in most common
solvents. In each case their 195Pt NMR spectra show a sing-
let; the δ(195Pt) for 6 (-3807 ppm) is upfield shifted with
respect to that in 3 (-3646 ppm), as corresponds to the higher
electron density of the metal center, in agreement with better
σ-donor properties of the isocyanide (CtN-tBu) and its
weak electron-withdrawing character. In both 3 and 6, the
δ(195Pt) values are similar to those observed in the literature
for related platinum (II) complexes59,62,77 and are downfield
shifted with respect to that of the unmetalated complex, 1,
which indicates a smaller electronic density on the platinum
center in the metalated complexes.78 This fact contrasts with
the observed upfield shift of the 195Pt signal upon metalation
when another starting material, such as trans-PtCl2(dmso)2,
is used in the process.62 The Pt-C coupling constant values
for C13, C7, and C6 in complexes 3 (L = tht) and 6 (L =
CNtBu) are much higher than in complex 1, containing the
unmetalated HC∧N-κN ligand; for the metalated carbon
atom (C1) and the adjacent one (C2) the values are similar
to those observed in other C,N-cyclometalated Pt(II) com-
plexes.79
The molecular structure of compound 4 was determined by
X-ray diffraction studies (Figure 2, Table 2). As can be seen,
the platinum(II) center exhibits a distorted square-planar
environment due to the small bite angle of the cyclometalated
ligand [80.8(1)ꢀ]. This angle and the Pt-CC∧N and Pt-NC∧N
bond lengths are similar to those observed in other five-
membered metallacycles of Pt(II).28,29,53,73,80,81 A chloride
and a triphenylphosphine ligand complete the coordination
sphere of platinum(II). The Pt-Cl and Pt-P distances are in
the range of Pt-Cl52,53,73,81,82 and Pt-P53,73,81-83 bonds trans
to σ-carbon and nitrogen, respectively. As a consequence of
the metalation, the two fragments of the metalated ligand,
4-bromophenyl and imidazol[1,2-a]pyridine, are coplanar to
each other [the interplanar angle is 4.9(1)ꢀ] and also to the
platinum coordination plane, the interplanar angles being
2.1(1)ꢀ and 3.1(1)ꢀ, respectively. One of the phenyl rings
Figure 2. Molecular structure of complex 4.
Reactivity of [{Pt(C∧N)(μ-Cl)}2] (2) toward Monodentate
S, P, and C Donor Ligands. The dinuclear complex [{Pt-
(C∧N)(μ-Cl)}2] (2) reacts with several neutral monodentate
S, P, and C donor ligands, such as tht, PPh3, CN-Xyl, and
CN-tBu in a 1:2 molar ratio (Scheme 1d and Experimental
Section) to give the mononuclear complexes [PtCl(C∧N)L]
(L = tht (3), PPh3 (4), CN-Xyl (5), CN-tBu (6)), which were
obtained as pure and air-stable solids in moderate to good
yields and fully characterized by the usual means. All the
spectroscopic data discussed below, relative to compounds
3-6, indicate that the isomer SP-4-4 (trans C, Cl) is the only
one present in each case, as is usual for this kind of com-
pound.66 Each compound, 3-6, shows one νPt-Cl absorption
(278 cm-1 3, 283 cm-1 4, 285 cm-1 5, 284 cm-1 6) at similar
frequencies to another, which are consistent with a terminal
Pt-Cl bond trans to C.35,62,67 Complexes 5 and 6 addition-
ally show one absorption assignable to νCtNR (2179 cm-1 5,
2194 cm-1 6), at similar frequencies to those observed in
other cyclometalated complexes with terminal CtN-Xyl
and CtN-tBu29,68,69 and, as expected, at a higher frequency
than in the corresponding free ligands (CtN-Xyl: 2131
cm-1, CtN-tBu: 2125 cm-1).
1
Well-resolved H NMR spectra of 3-6 show the signals
due to the C∧N group and the corresponding co-ligand, L,
unambiguously assigned on the basis of 1H-1H COSY
experiments (see Experimental Section and Scheme 2). The
most significant change with respect to the starting complex,
2, is the significant upfield shift of the H2 signal in complex 4
(L = PPh3), which has been ascribed to the anisotropic shiel-
ding effect of the aromatic ring current of a phenyl group in
PPh3 on the H2 pointing toward it.70-73 This is consistent
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Dalton Trans. 2007, 2030.
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ssero, M. Eur. J. Inorg. Chem. 2004, 4484.
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(82) Martın, R.; Crespo, M.; Font-Bardia, M.; Calvet, T. Organome-
(71) Craig, C. A.; Garces, F. O.; Watts, R. J.; Palmans, R.; Frank, A.
J. Coord. Chem. Rev. 1990, 97, 193.
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Inorg. Chem. 1995, 34, 2334.
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ssero, M.; Manassero, C.; Minghetti, G. Organometallics 2009, 28, 2150.
tallics 2009, 28, 587.
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Organometallics 2001, 20, 4418.

Article
Organometallics, Vol. 29, No. 6, 2010 1401
Table 2. Selected Structural Data for 4
Bond Lengths [A]
Pt-C(1)
2.030(3)
2.096(3)
1.452 (5)
1.382 (4)
1.352 (4)
1.367 (4)
Pt-P
2.230 (9)
2.385 (4)
1.361 (5)
1.383 (4)
1.391 (4)
Pt-N(1)
Pt-Cl(1)
C(7)-C(8)
N(2)-C(8)
N(2)-C(13)
C(6)-C(7)
N(1)-C(7)
N(1)-C(13)
N(2)-C(9)
Bond Angles [deg]
C(1)-Pt-N(1)
80.86(12)
C(1)-Pt-P(1)
95.66(10)
N(1)-Pt-Cl(1)
92.60(8)
P-Pt-Cl(1)
90.89(3)
C(13)-N(1)-Pt
139.6(2)
C(7)-N(1)-Pt
112.9(2)
C(2)-C(1)-Pt
130.0(2)
C(6)-C(1)-Pt
114.3(2)
[C14-C19] in triphenylphosphine is oriented toward the
C2-H2 bond, and the angle between this line and the
perpendicular to that plane is 41.97ꢀ. Crystals of complex 6
were also obtained, but unfortunately the poor quality of the
X-ray diffraction data obtained precludes complete structural
determination. Nevertheless, atom connectivity can be estab-
lished without error, showing a structure very similar to that
of 4 with CN-tBu instead of PPh3.
To summarize, all the spectroscopic and structural data
discussed above indicate that in the cleavage of the bridging
system in complex [{Pt(C∧N)(μ-Cl)}2] (2) by the neutral
ligands tht, PPh3, CN-Xyl, and CN-tBu only one (SP-4-4)
of the two possible isomers (Scheme 1) is formed selectively.
This result is compatible with the expected degree of trans-
phobia (T)33,84,85 of pairs of trans ligands, T[C(C∧N)/L(Cl)]
< T[C(C∧N)/L(S, P, C)], taking into account that the greater
the trans influence86,87 of two ligands, the greater the trans-
phobia. Given that in complexes 3-6 there is no steric
hindrance between pairs of cis ligands, the resulting geo-
metry (trans C, Cl) is that expected from the electronic
preferences.
Figure 3. Normalized UV-vis spectra of complex 6 (5 ꢀ 10-5
M) in different solvents.
cm-1, which we have tentatively assigned to metal-perturbed
ligand-centered transitions (1LC π-π*) within the C∧N
ligand.27,29,68,88-90 In contrast to most C∧N-cyclometalated
Pt(II) complexes, in these cases no absorptions in the
370-450 nm range with extinctions between 2000 and 6000
M-1 cm-1 assigned as metal-to-ligand charge transfer
(MLCT) transitions29,90-92 were observed. To shed some
light on this, theoretical calculations using the time-depen-
dent density functional theory (TD-DFT) were carried out
for 6 at the SAOP/TZP level of theory. The optimized S0
structure and geometrical parameters (Figure S2a and Table
S2) agree well with the experimental values. All the relevant
data about the molecular orbitals (MOs) involved in the
excited states and the calculated electronic excitations ap-
pear in Figure S3 and Tables S3-S6. In toluene solution the
HOMO (Figure 4 and Table S4) is mostly constructed from
orbitals located on the phenyl ring of the C∧N (78%) with
small contribution of the chloride ligand (4%) and the Pt
center (3%), while the LUMO is well located on the pyridinic
and imidazolyl rings of the C∧N ligand (87%), with a very
low contribution of both Pt and CNtBu. The contribution of
Pt and Cl orbitals increases on going from the HOMO to
HOMO-1 and HOMO-2. Similar relative compositions of
the frontier orbitals have been found in imine-29,34,88 or
pyrazol-based93 cyclometalated platinum(II) complexes.
Figure 4 shows the calculated excited states in toluene
solution (bars) that fit well, within the accuracy of the
method, with the experimental absorptions in the region of
λ > 290 nm. Calculations indicate that the major contribu-
tion to the lowest-lying absorption (λ = 356 nm) involves the
HOMO f LUMO (81%) and HOMO-1 f LUMO (9.4%)
transitions, indicating a remarkable intraligand (1IL, C∧N)
character mixed with small MLCT/L0LCT, since this band
Preliminary studies on compounds 1-6 indicate that only
the mononuclear complexes [PtCl(C∧N)L] (L = CN-Xyl (5),
CN-tBu (6)) are luminescent. Both contain a σ Pt-CC∧N
bond and a strong-field ligand, L, such as isocyanide. Hence
absorption and emission spectra studies were carried out
only on them, in both the solid state and solution.
Absorption Spectra and Theoretical Calculations. Consid-
ering the low solubility of 5 in common organic solvents and
for comparative purposes, we obtained the absorption spec-
tra of 5, 6, and the unmetalated ligand HC∧N in different
solvents at low concentration (5 ꢀ 10-5 M) (Table S1, Figure
S1 for 5 and Figure 3 for 6).
In the HC∧N ligand, as in compounds 5 and 6, all the
absorptions appeared at λ < 360 nm with ε g 1 ꢀ 104 M-1
also involves transitions dPt f π*C∧N and π*Cl f π*C∧N
.
´
(84) Vicente, J.; Arcas, J. A.; Martınez-Viviente, E.; Jones, P. G.
Organometallics 2002, 21, 4454.
(85) Vicente, J.; Arcas, A.; Galvez-Lopez, M. D.; Jones, P. G.
Organometallics 2006, 25, 4247.
(86) Pregosin, P. S. Transition Metal Nuclear Magnetic Resonance;
Elsevier: New York, 1991.
Calculations also show a lower IL and larger MLCT/L0LCT
character of the absorption bands located at higher energies
(350 nm > λ > 290 nm). The same assignments could be
made for the absorptions observed for complex 5, since they
ꢀ
ꢀ
(87) Cotton, F. A. Chemistry of Precious Metals; Blackie Academic &
Professional, Chapman & Hall: London, 1997.
(88) Shao, P.; Li, Y.; Azenkeng, A.; Hoffmann, M. R.; Sun, W. Inorg.
(91) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba,
I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(92) Schneider, J.; Du, P.; Wang, X.; Brennessel, W. W.; Eisenberg,
R. Inorg. Chem. 2009, 48, 1498.
Chem. 2009, 48, 2407.
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Chem. 2009, 694, 737.
(90) Schneider, J.; Du, P.; Jarosz, P.; Lazarides, T.; Wang, X.;
(93) Develay, S.; Blackburn, O.; Thompson, A. L.; Williams, J. A. G.
Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2009, 48, 4306.
Inorg. Chem. 2008, 47, 11129.

ꢀ
Fornies et al.
1402 Organometallics, Vol. 29, No. 6, 2010
as frequently occurs in square-planar Pt(II) complexes,
although the existence of excimeric-like emissions cannot be
excluded.20,28,29,93-96 Thus, this emission is tentatively as-
signed to 3MMLCT/3π-π* transitions.
The emission intensity of the HE band for complexes 5 and
6 decreases at high concentrations,88 as illustrated in
Figure S9 for complex 6. Investigation of the dependence
of the emission lifetime (τobs) on the concentration for 5 and 6
measured at 376 nm (Figures S10, S11) shows values of τ0
and self-quenching constant (KQ) in line with those reported
for related Pt(II) complexes.34,93,96
Both complexes 5 and 6 become more emissive upon
cooling. The emission spectra of 6 at low temperature
(77 K) show the same profile in diluted (5 ꢀ 10-5 M) or
concentrated (10-3 M) solution in CH2Cl2 (Figure S6). They
exhibit a broad phosphorescent band at low energy (LE)
(λmax = 580 nm) in addition to a low-intensity band at 450
nm, which can be selectively obtained upon excitation at 300
nm a concentrated solution of 6 (10-3 M) in 2-Me-thf at 77 K
(Figure 7). The peak maxima (456, 486, and 522 nm),
vibronic progressions (ca. 1400 cm-1, typical of breathing
modes on aromatic rings), and long lifetime point to an
emission arising mainly from a 3IL excited state with a
3
slight, if any, MLCT character,91,94,97 as in the analogous
complexes [Pt(bzq)Cl(CNR)] (R = tBu, Xyl).34 The excita-
tion spectra in 2-Me-thf monitoring the LE band (λmax
=
Figure 4. (a) Frontier orbitals plots for complex 6 in toluene
obtained by DFT. (b) Calculated absorption spectrum (bars)
and experimental UV-vis spectrum of 6 in toluene (5 ꢀ 10-5 M).
580 nm) at 10-3 M resemble the lower-energy absorption
observed in the UV-visible spectrum of 6. Aiming to know
the origin of these emission bands theoretical calculations
were carried out on complex 6. The optimized structural
parameters of 6 in the S0 and T1 states computed at the BP86/
TZP level are given in Figure S2. Structural changes occur-
ring upon the S0 f T1 excitation are small (0.1-0.25 A for
the bond lengths and 0.2-1.3ꢀ for the bond angles).
appearatsimilarwavenumbersintoluene. The chargetransfer
(CT) nature of these bands is consistent with their blue-shift in
the simulated absorption spectrum of 6 in a more polar
solvent, such as acetonitrile, and causes the modest negative
solvatochromism29,88 observed experimentally for 5 and 6
(Table S1, Figure S1 for 5 and Figure 3 for 6). For both
complexes 5 and 6 the lowest-lying absorption band follows
Beer’s law in a wide concentration range (352 nm, DMF, 4 ꢀ
10-5 to 2 ꢀ 10-3 M, 5; 352 nm, CH2Cl2, 2 ꢀ 10-5 to 1 ꢀ 10-2
M, 6), suggesting that no remarkable aggregation occurs
within this range (see Figures S4 and S5).29,34 The UV-vis
spectra of compounds 5 and 6 both in solution and in the solid
state (Figure S6) barely show any influence at all of the
isocyanide substituent (xylil or tert-butyl) in the lower-energy
bands, in agreement with the noncontribution of MOs located
on these co-ligands on the HOMOs of these complexes.
Emission Spectroscopy and Theoretical Calculations. So-
lution and Glass State. Complexes [PtCl(C∧N)(CN-Xyl)] (5)
and [Pt(C∧N)(Cl)(CN-tBu)] (6) are slightly emissive in solution
at both room (298 K) and low (77 K) temperature. The results
are summarized in Table 3. In dilute solution (5 ꢀ 10-5 M)
both complexes show (Figure 5 for 5 and Figures 6 and S7 for
6) only one high-energy (HE) emission band (λmax = 367 nm,
Natural bond orbital (NBO) analysis has been employed
to estimate the changes of the natural atomic charges upon
the S0 f T1 excitation of 6; the values are shown in
Figure S12a. In general, it was found that charge is primarily
transferred from the chloride ligand and the platinum central
atom toward the C∧N ligand and, to a lesser extent, toward
the -CtN- group of the CN-tBu ligand.
The simulated emission spectra of 6 by using time-dependent
density functional theory (TD-DFT) calculations at the SAOP/
TZP level are depicted schematically in Figure S13. The calcu-
lated emission maximum, λmax, at 575 nm is in excellent
agreement with the phosphorescence emission maximum ob-
served experimentally at 77 K (580 nm CH2Cl2, 587 nm 2-Me-
thf). This band could be attributed to a SOMO-1 r SOMO
electronic transition. The calculated band maximum around
475 nm, found to be in close agreement with those observed ex-
perimentally at λ= 450 or 456 nm in a rigid matrix of CH2Cl2 or
2-Me-thf, respectively, originates primarily from a SOMO-1r
LUMOþ1 electronic transition. Moreover, the emission bands
with maxima around 330 and 260 nm mainly arise from
SOMO-2 r LUMO and SOMO-2 r LUMOþ5 transitions,
λ
exc = 300 nm 5; λmax = 376 nm, λexc = 340 nm 6) that can be
reasonably attributed to IL emissive states from the C∧N
ligand considering the emission spectrum of the unmetalated
ligand (HC∧N) in fluid CH2Cl2 solution (Figure S8). In a more
concentrated solution (10-3 M) the emission of 5 changes with
the excitation wavelength. The excitation band corresponding
to the unstructured low-energy (LE) emission appears greatly
red-shifted with respect to that for the HE emission of the
monomers. This fact, together with the color of the emission
and its appearance at high concentration, suggests that it is
presumably due to emissive ground-state aggregates generated
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Chem., Int. Ed. 2008, 47, 4542.
by weak Pt Pt and/or π-π interactions of aromatic groups,
3 3 3

Article
Organometallics, Vol. 29, No. 6, 2010 1403
Table 3. Emission Data for Complexes 5 and 6
media em [nm]
compound
color
λ
τ [μs]
5
light gray
solid (298)
solid (77)
575, 620max, 676 (λex 400)
481, 522max, 564 (λex 350)
337sh, 367max (λex 300)
0.18 (620)
341 (40%), 63.8 (60%) (522)
0.1 (367)
24.8 (95%), 138.7 (5%) (467);
1.4 (557); 4.2 (633)
92 ns (380); 0.9 (639)
2.7 (97%), 20.9 (3%) (603)
198.7 (26%), 30.7 (74%) (521);
225 (18%), 15.2 (82%) (561)
0.1 (376)
92.1 (46%), 348 (54%) (450); 181
(18%), 24.9 (82%) (580)
0.1 (376)
304 (59%), 38.2 (41%) (450);
32.3 (84%), 275 (16%) (580)
0.1 (375)
289 (75%), 37.8 (25%) (578)
0.1 (380)
403.2 (456); 161.5 (23%),
414.5 (77%) (486);
270 (35%), 35.7 (65%) (587)
CH2Cl2 5 ꢀ 10-5 M (298)
CH2Cl2 5 ꢀ 10-5 M (77)
CH2Cl2 10-3 M (298)
CH2Cl2 10-3 M (77)
solid (77)
467, 557, 640 (λex 400), 633 (λex 480)
380max, 399, 456sh (λex 330); 639 (λex 420)
603max (λex 450)
6
pale yellow
484, 521max, 563 (λex 350);
495sh, 529sh, 561max (λex 400)
360sh, 376max, 392, 418sh (λex 340)
450, 457sh, 580max (λex 350)
CH2Cl2 5 ꢀ 10-5 M (298)
CH2Cl2 5 ꢀ 10-5 M (77)
CH2Cl2 10-3 M (298)
CH2Cl2 10-3 M (77)
376 (λex 320)
450, 459sh, 580max (λex 350)
2-metilthf 5 ꢀ 10-5M (298)
2-metilthf 5 ꢀ 10-5M (77)
2-metilthf 10-3M (298)
2-metilthf 10-3M (77)
335sh, 358sh, 375max, 393sh (λex 300)
543sh, 578max, 612sh (λex 330)
380max, 394 (λex 330)
456, 486max, 522 (λex 300);
552sh, 587max, 617 (λex 360)
Figure 5. Normalized excitation and emission spectra of 5 in
CH2Cl2 at 298 and 77 K. Inset: Unnormalized emissions in the
same conditions.
Figure 7. Normalized excitation and emission spectra of 5 in the
solid state.
3
emission at 580 nm seems to be a due to a L0MLCT excited
state. However, taking into account that with the inclusion of
solvent molecules in the calculations, the Cl orbital contribution
to the HOMO diminishes with respect to the gas phase (the
HOMO is mostly constructed from orbitals located on the
phenyl ring of the C∧N with marginal contribution of the
chloride ligand and the Pt center), it seems reasonable to
3
consider this low-energy emission as a mixed IL/3L0MLCT
transition.
Solid State. In the solid state, complex 5 exhibits“lumines-
cent thermochromism” (Figure 8) presumably related to the
existence or absence of Pt Pt and/or π-π interactions.80
3 3 3
Thus at 298 K complex 5 exhibits an intense orange-red
luminescence band centered at 620 nm, with a short lifetime,
which changes to a green phosphorescence band upon cool-
ing. The differences in their excitation spectra indicate that
both emissions are due to two different excited states.80 The
excitation profile of the HE band (observable at 77 K)
resembles the UV-vis spectrum of 5. This fact, the vibronic
Figure 6. Normalized excitation and emission spectra of 6 in
2-Me-thf.
respectively. In view of the MOs participating in the electronic
transitions (Figure S12) it can be concluded that the T1 to S0 de-
excitation in the gas phase corresponds to a ligand (C∧N) to
metal (Pt) to ligand (Cl) charge transfer transition. Hence, the
3
spacing (1540, 1230 cm-1), and lifetime are typical of IL
localized on the C∧N ligand with low, if any, contributions of

ꢀ
Fornies et al.
1404 Organometallics, Vol. 29, No. 6, 2010
hydrogen bridging bond (dPt H1 = 2.78 A, dPt-C1 =
3 3 3
3.365(3) A, angle Pt-H1-C1 = 120.9ꢀ) in agreement with
the observed H NMR data. Taking into account the pre-
1
sence in the molecule of two hydrogen bond acceptor atoms
(Pt, Cl) and the nonhindered rotation of the phenyl ring
around the C6-C7 bond, the existence of the Pt H1 in-
3 3 3
teraction could presumably be considered as the prior step
for the C1-H1 activation process. The cleavage of the bridg-
ing system in [{Pt(C∧N)(μ-Cl)}2] (2) by the neutral ligands L
rendered the mononuclear complexes [PtCl(C∧N)L] (L =
tht (3), PPh3 (4), CN-Xyl (5), CN-tBu (6)) with the geometry
(trans C, Cl) expected from the electronic preferences, taking
into account the degree of transphobia (T) of pairs of trans
ligands, T[C(C∧N)/L(Cl)] < T[C(C∧N)/L(S, P, C)].
Only complexes [PtCl(C∧N)(CNR)] (R = Xyl (5), tBu (6))
are luminescent, both of which contain two strong-field
ligands, a σ Pt-CC∧N bond and a strong-field isocyanide
ligand. The change of the isocyanide substituent (xylyl or
tert-butyl) hardly causes any effect on the absorption bands
of compounds 5 and 6 both in solution and in the solid state,
in agreement with the noncontribution of MOs located on
these co-ligands on the HOMOs of these complexes. Com-
plex 6 shows phosphorescence at 77 K both in solution and in
the solid state with the emissions arising from 3IL and
3L0MLCT excited states of monomer species. Complex 5
exhibits “luminescent thermochromism” in the solid state; at
77 K it shows a green phosphorescence band assigned to 3IL
transitions located on the C∧N group of monomer species,
while at 298 K an orange-red emission is observed, being
tentatively assigned to excited states of emissive aggregates
Figure 8. Normalized excitation and emission spectra of 6 in the
solid state at 77 K.
3MLCT transitions in the monomer species.34,91,94,97 This
assignment is well supported in view of the emission spec-
trum of the unmetalated (HC∧N) ligand in the solid state at
77 K (Figure S14). However, the energy of the orange-red
emission, the lifetime, and the excitation profile monitoring
at 620 nm are similar to those of the LE emission observed in
fluid CH2Cl2 at 10-3 M, which has been tentatively assigned
to excited states of aggregates generated by Pt Pt and/or
3 3 3
π-π interactions (3MMLCT/3π-π*).20,28,29,93-95 However,
in the solid state, complex 6 is only luminescent at 77 K
(Figure 8). It is probable that the high energy of the excited
state brings it sufficiently close to higher-lying metal-cen-
tered states, which provide it with a thermally activated
nonradiative decay pathway.93 As can be seen, the emission
profile changes with the excitation wavenumber. The vibro-
nic band observed upon exciting at 350 nm seems to be due to
two phosphorescent emissions, a green high-energy (HE)
band with λmax = 521 nm and a minor contribution of a low-
energy (LE) band centered at 561 nm that becomes the more
important one upon exciting at 400 nm. Both emissions
resemble those of 6 in glassy 2-Me-thf (10-3 M). The green
emission has been assigned mainly to 3IL excited states and
(3MMLCT/3π-π*) generated by Pt Pt and/or π-π inter-
3 3 3
actions in view of the emission and excitation energy max-
ima. Thus, the “luminescent thermochromism” shown by 5 in
the solid state seems to be related to the existence or absence
of these kinds of interactions among monomer species.
With the aim of obtaining luminescent compounds con-
taining 2-(4-bromophenyl)imidazol[1,2-a]pyridinate-kC,N,
the synthesis and characterization of new heteroleptic com-
plexes containing different ancillary ligands are in progress.
Experimental Section
All the information about materials, instrumentation meth-
ods used for characterization and photophysical studies, com-
putational details concerning TD-DFT calculations, and the X-
3
the LE one to mixed IL/3L0MLCT transitions, all corre-
sponding to monomer species. The excitation profile of the
LE band shows a peak maxima ca. 3000 cm-1 red-shifted
with respect to the excitation maxima of this band in glassy
ray structure analysis of 1 and 4 CHCl3 together with the full IR
3
and NMR data corresponding to HC∧N and complexes 1-6 is
given in the Supporting Information.
3
2-Me-thf. Therefore the contribution of MMLCT/3π-π*
Synthesis of [Pt(η3-C4H7)Cl(HC∧N-KN)] (1). An orange solu-
tion of [{Pt(η3-C4H7) (μ-Cl)}2] (0.286 g, 0.5 mmol) in acetone
(20 mL) containing 2-(4-bromophenyl)imidazol[1,2-a]pyridine
(0.273 g, 1 mmol) was refluxed for 1 h. Within this process time,
1 precipitated as a white solid, which was filtered and washed
with Et2O. Yield: 0.27 g, 95%. Anal. Calcd for 1, C17H16Br-
ClN2Pt (558.76): C, 36.54; H, 2.89; N, 5.01. Found: C, 36.37; H,
2.93; N, 4.96. MS (MALDIþ): m/z 523 [Pt(HC∧N)(η3-C4H7)]þ.
Synthesis of [{Pt(C∧N)(μ-Cl)}2] (2): Method A. An orange
solution of [{Pt(η3-C4H7)(μ-Cl)}2] (0.571 g, 1 mmol) in 2-meth-
oxyethanol (20 mL) containing 2-(4-bromophenyl)imidazol-
[1,2-a]pyridine (HC∧N, 0.546 g, 2 mmol) was refluxed for 2 h.
The dark brown solid that precipitated was filtered and washed
with MeOH and Et2O, 2. Yield: 0.74 g, 74%. Method B. Awhitesus-
pension of [Pt(η3-C4H7)Cl(HC∧N-κN)] (1) (0.447 g, 0.80 mmol)
in 2-methoxyethanol (20 mL) was refluxed for 2 h, and the result-
ing solid was filtered and washed with Et2O (20 mL) to give
a dark brown solid, 2. Yield: 0.32 g, 82%. Anal. Calcd for 2,
excited states to the low-energy part of this band cannot be
excluded.
Conclusions
Complex [{Pt(η3-C4H7)(μ-Cl)}2] has been proved to be a
very convenient starting material for cyclometalation of
2-(4-bromophenyl)imidazol[1,2-a]pyridine (HC∧N) in view
of the selectivity and yield of the process to give [{Pt(C∧N)(μ-
Cl)}2] (2). Compound 2 was obtained from [{Pt(η3-C4H7)(μ-
Cl)}2] and HC∧N in a one-pot reaction in refluxing 2-
methoxyethanol or step-by-step through the intermediate
[Pt(η3-C4H7)Cl(HC∧N-κN)] (1). Complex 1 precipitates in
the reaction mixture if [{Pt(η3-C4H7)(μ-Cl)}2] and HC∧N are
reacted in refluxing acetone. The X-ray structural data in
complex 1 indicate the existence of a weak Pt H-C
3 3 3

Article
Organometallics, Vol. 29, No. 6, 2010 1405
C26H16Br2Cl2N4Pt2 (1005.32): C, 31.06; H, 1.60; N, 5.57. Found: C,
30.95; H, 1.77; N, 5.78.
Acknowledgment. This work was supported by the
Spanish MICINN (Project CTQ2008-06669-C02-01) and
Synthesis of [PtCl(C∧N)(tht)] (3). tht (53.58 μL, 0.6067 mmol)
was added to a stirred suspension of 2 (0.30 g, 0.3038 mmol) in
CH2Cl2 (15 mL). The reaction mixture was refluxed for 5 h,
cooled to room temperature, and filtered through Celite. The
resulting solution was evaporated to dryness and the residue
washed with Et2O (2 ꢀ 5 mL), yielding 3 as a gray solid. Yield:
0.15 g, 42%. Anal. Calcd for BrC17ClH16N2PtS: C, 34.61; H,
2.73; N, 4.75; S, 5.43. Found: C, 35.04; H, 2.59; N, 4.72; S, 5.42.
MS (MALDIþ): m/z 555 [Pt(C∧N)(tht)]þ.
ꢀ
the Gobierno de Aragon (Grupo de Excelencia: Quımica
´
Inorganica y de los Compuestos Organometalicos).
ꢀ
ꢀ
Supporting Information Available: Absorption data (Table
S1). Normalized UV-vis spectra of complex 5 in different
solvents and HC∧N and 6 in toluene (Figure S1). Selected
structural parameters of the ground state, S0, and first triplet
excited state, T1, calculated at the BP86/TZP level (Figure S2)
of complex 6. Cartesian coordinates of the stationary points on
the PES of complex 6 in the S0 ground state (Table S2). 3D
contour plots of all MOs involved in the electronic transitions
in the simulated absorption spectra of complex 6 (Figure S3).
Molecular orbital energies, compositions, and assignment
calculated for the ground state, S0, of 6 at the SAOP/TZP level
(Tables S3, S4, and S5). Principal singlet-singlet optical
transitions (f > 0.03) calculated for the absorptions of complex
6 (Table S6). Normalized UV-vis absorption spectra at several
concentrations at 298 K of 6 in CH2Cl2 (Figure S4) and 5 in
DMF (Figure S5). Diffuse reflectance UV-vis spectra in the
solid state of 5, 6, and HC∧N at 298 K (Figure S6). Emission
spectra of 6 (Figure S7) and HC∧N (Figure S8). Emission
spectra of complex 6 in deoxygenated CH2Cl2 solutions at
room temperature in different concentrations (Figure S9). Plot
of the measurement luminescence decay constants versus con-
centration for 6 (Figure S10) and 5 (Figure S11). Natural
atomic charges for the S0 and T1 states of 6, spin density
isosurface (0.002 au), and 3D contour plots of the relevant
MOs calculated at the BP86/TZP level (Figure S12). Calcu-
lated emission spectra of 6 (Figure S13). Emission spectra of
HC∧N in the solid state (Figure S14). General procedures and
materials, computational details, X-ray structure analysis of 1
Synthesis of [PtCl(C∧N)(PPh3)] (4). PPh3 (111.58 mg, 0.4254
mmol) was added to a stirred suspension of 2 (210.00 mg, 0.2127
mmol) in CH2Cl2 (30 mL), and the reaction mixture was
refluxed for 3 h and then cooled to room temperature. The
resulting precipitate was filtered off and washed with CH2Cl2
(2 ꢀ 5 mL) and Et2O (2 ꢀ 5 mL) to afford 4 as a gray solid. Yield:
0.30 g, 92%. Anal. Calcd for BrC31ClH23N2PPt: C, 48.68; H,
3.03; N, 3.66. Found: C, 48.27; H, 2.86; N, 3.44. MS
(MALDIþ): m/z 728 [Pt(C∧N)(PPh3)]þ, 502 [Pt(C∧N)]þ.
Synthesis of [PtCl(C∧N)(CN-Xyl)] (5). CN-Xyl (0.1020 g,
0.7775 mmol) was added to a stirred suspension of 2 (0.3838 g,
0.3887 mmol) in CH2Cl2 (20 mL) at room temperature. After 18
h the precipitate was filtered off and washed with CH2Cl2 (2 ꢀ 5
mL) and Et2O (2 ꢀ 5 mL) to afford 5 as a light gray solid. Yield:
0.29 g, 60%. Anal. Calcd for BrC22ClH17N3Pt: C, 41.69; H,
2.70; N, 4.42. Found: C, 41.47; H, 2.63; N, 4.67. MS
(MALDIþ): m/z 598 [Pt(C∧N)(CN-Xyl)]þ.
Synthesis of [PtCl(C∧N)(CN-tBu)] (6). CN-tBu (77.13 μL,
0.6828 mmol) was added to a stirred suspension of 2 (0.34 g,
0.3414 mmol) in CHCl3 (30 mL). The reaction mixture was
refluxed for 10.5 h, cooled to room temperature, and filtered
through Celite. The resulting solution was evaporated to dry-
ness and the residue washed with Et2O (3 ꢀ 5 mL), yielding 6 as a
pale yellow solid. Yield: 0.29 g, 73%. Anal. Calcd for
BrC18ClH17N3Pt: C, 36.91; H, 2.93; N, 7.17. Found: C, 36.78;
H, 3.41; N, 7.02. MS (MALDIþ): m/z 550 [Pt(C∧N)(CN-tBu)]þ,
494 [Pt(C∧N)(CN)]þ.
and 4 CHCl3 (including Table S7), full IR and NMR data for
3
HC∧N and for 1-6. Crystallographic data in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.