Synthesis, structures, and electronic spectroscopy of luminescent acetylene- and (buta-1,3-diyne)platinum complexes

Article abstract of DOI:10.1002/ejic.200600773

The electronic absorption and emission spectroscopy of a series of diphenylaceylene- and (buta-1,3-diyne)-Pt⁰ complexes (L)Pt[(1,2-η²)-R-(C≡C)_n-R] and [(dppp)Pt] ₂[μ-(1,2-η²):(3,4-η²)-R-(C≡C) ₂-R] (R = Ph or CH₃, L = dppp or (PPh₃) ₂, n = 1 or 2) was investigated. The structures of (dppp)Pt[(1,2-η²)-Ph-C≡C-Ph], (dppp)Pt[(1,2- η²)-PhC₄Ph] and [(dppp)Pt]₂[μ-(1,2- η²):(3,4-η²)-Ph-(C≡C)₂-Ph] were characterized by X-ray diffraction. The complexes all display intense absorptions that were attributed to Pt→P(dπ*) and Pt→acetylene(π_x*) transitions. Except for the CH ₃C₄CH₃ complexes, the complexes all exhibit two emissions at 380-550 nm and 500-800 nm. The higher energy emission could arise from the ³[P(dπ*)→Pt] transition, and the lower energy emission, which has a longer lifetime than the higher energy one, was attributed to the ³[acetylene(π_x*)→Pt] transition. The energy of the MLCT absorption and emission was affected by the electronic properties of the acetylenes and the ancillary phosphanes. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600773

FULL PAPER
DOI: 10.1002/ejic.200600773
Synthesis, Structures, and Electronic Spectroscopy of Luminescent Acetylene-
and (Buta-1,3-diyne)platinum Complexes
Ke Zhang,[a] Jian Hu,[a] Kwok Chu Chan,[b] Kwok Yin Wong,[b] and John H. K. Yip*[a]
Keywords: Spectroscopy / Luminescence / Platinum / Acetylene / Charge-transfer
The electronic absorption and emission spectroscopy of a
complexes, the complexes all exhibit two emissions at 380–
series of diphenylaceylene- and (buta-1,3-diyne)-Pt0 com- 550 nm and 500–800 nm. The higher energy emission could
plexes (L)Pt[(1,2-η2)-R–(CϵC)n–R] and [(dppp)Pt]2[µ-(1,2-
η2):(3,4-η2)-R–(CϵC)2–R] {R = Ph or CH3, L = dppp or
(PPh3)2, n = 1 or 2} was investigated. The structures of
arise from the 3[P(dπ*)ǞPt] transition, and the lower energy
emission, which has a longer lifetime than the higher energy
one, was attributed to the 3[acetylene(πx*)ǞPt] transition.
The energy of the MLCT absorption and emission was affec-
ted by the electronic properties of the acetylenes and the an-
cillary phosphanes.
(dppp)Pt[(1,2-η2)-Ph–CϵC–Ph],
and
(dppp)Pt[(1,2-η2)-PhC4Ph]
were
[(dppp)Pt]2[µ-(1,2-η2):(3,4-η2)-Ph–(CϵC)2–Ph]
characterized by X-ray diffraction. The complexes all display
intense absorptions that were attributed to PtǞP(dπ*) and
PtǞacetylene(πx*) transitions. Except for the CH3C4CH3
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
gand ππ* character to the MLCT excited state. In principle,
the mixing could be subdued if the metal orbitals are much
higher in energy than the ligand orbitals. This requires an
electron-rich metal center. In search of organometallics
showing low-energy MLCT transition, our attention was
therefore drawn to acetyleneplatinum(0) π complexes, which
possess an electron-rich metal center and low-lying π*-or-
bitals.
Although the π complexes have a fairly long history,[5]
most studies to date focus on the structures and reactivity
aspects of the compounds.[6] On the other hand, electronic
spectroscopy and photophysics of the complexes remain
sparsely studied.[7] The luminescent properties of a series of
acetylene-Pt0 complexes have been reported by Forniés and
co-workers.[7a] They assigned the vibronic phosphorescence
exhibited by [(PPh3)2Pt2]2(HCϵC–1,4-C6H4–CϵCH) to
3MLCT emission. The assignment was supported by ex-
tended Hückel calculations that showed that the HOMO
contains a substantial metal character and the LUMO is
mainly localized in the π*-orbital of the acetylene.
Introduction
Recent interest in linear organometallics that have a low-
lying metal-to-ligand charge-transfer (MLCT) excited state
arose from their potential as molecular devices.[1] For exam-
ple, it had been demonstrated that the intense MLCT ab-
sorption of RuII acetylides is associated with their large sec-
ond-order nonlinear optical effect.[1c] In addition, MLCT
excitation is accompanied with charge separation, which is
pertinent for the operation of photonic wires.[2]
Currently, organometallic complexes containing π-conju-
gated ligands are receiving a lot of attention mainly because
of their wire-like geometry and the ability of π-conjugated
carbon chains to mediate electron transfer.[3] Most of the
studies have focused on metal acetylides and metal polyyne
σ complexes. One way to lower the MLCT energy is to in-
crease π-conjugation of the polyyne ligand. For instance,
the complex [Re(L)(CO)3(CϵCCϵC)Re(L)(CO)3] (L =
4,4Ј-di-tert-butyl-2,2Ј-bipyridine) displayed a low-energy
MLCT transition [dπ(Re)Ǟπ*(CϵCCϵC–Re)] and
3MLCT emission.[4] However, a potential drawback of this
approach is that the gap between the metal dπ- and the
ligand π-orbitals is reduced, resulting in substantial mixing
of the orbitals, and hence introduction of significant intrali-
A better understanding of the electronic structures and
photophysics of the complexes could be achieved by ex-
panding the scope of spectroscopic study to cover molecules
that contain different acetylenes and ancillary ligands. To
this end, we have carried out a comparative study of a series
of acetylene- and (buta-1,3-diyne)platinum(0) (LPt)[(1,2-
η2)-R–(CϵC)n–R] {L = (PPh3)2 or dppp [1,3-bis(diphenyl-
phosphanyl)propane]; R = Ph or CH3; n = 1 or 2} and
[a] Department of Chemistry, National University of Singapore,
10 Kent Ridge Crescent, 119260 Singapore, Singapore
Fax: +65-7791691
E-mail: chmyiphk@nus.edu.sg
[b] Department of Applied Biology and Chemical Technology,
Hong Kong Polytechnic University,
binuclear
[(dppp)Pt]2[µ-(1,2-η2):(3,4-η2)-R–(CϵC)2–R]
Hunghom, Kowloon, Hong Kong SAR, People’s Republic of
China
complexes (Scheme 1). The complexes are different by the
substituents attached to the CϵC bond, extent of conjuga-
tion, number of metal atoms or ancillary phosphane atoms.
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
384
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Luminescent Acetylene- and (Buta-1,3-diyne)platinum Complexes
FULL PAPER
Scheme 1.
Except for complexes 1 and 3, all of the complexes are new. frequency and intensity have been observed for other re-
While binuclear metal complexes of buta-1,3-diyne in µ- ported acetylene-Pt0 complexes.[5,6] The low stretching fre-
(1,2-η2):(3,4-η2) coordination mode are known, the com- quency is due to the metal-to-ligand π-back donation.[11]
plex 5 is the first Pt2-butadiyne π complex ever charac- The IR spectra of 3, 4 and 6 show two resonance signals at
terized by X-ray crystallography.[8] The first diplatinum 1719–1749 and 2160–2196 cm–1 arising from stretching of
complex of PhC4Ph, [(PPh3)2Pt]2[µ-(1,2-η2):(3,4-η2)- the coordinated and the uncoordinated CϵC bonds. The
PhC4Ph], was synthesized by Stone but its crystal structure dinuclear complexes 5 and 7 show only a single peak at
was not available.[9]
1758 and 1705 cm–1 for ν(CϵC), respectively. This indicates
that both CϵC bonds in the complexes are equivalent.
Results and Discussion
(B) Structures
(A) Synthesis and Characterization
The complexes 2, 4, and 5 were characterized by single-
crystal X-ray diffraction analysis, while crystal structures of
1[12] and 3[6b] have been previously reported by other re-
search groups (Table 3). The selected bond lengths and
angles are listed in Table 1.
There are two literature methods for the preparation of
acetylene-Pt0 complexes. The first method[10] involves re-
duction of the Pt(PPh3)2Cl2 ion by N2H4 in the presence of
the acetylene ligand in alcohol. In this work, (PPh3)2Pt[(1,2-
η2)-PhC2Ph] (1) and (PPh3)2Pt[(1,2-η2)-PhC4Ph] (3) were
prepared by this method. However, synthesis of (dppp)-
Pt[(1,2-η2)-PhC2Ph] (2) and (dppp)Pt[(1,2-η2)-PhC4Ph] (4)
by the same methods was not successful. Instead the
(dppp)Pt[(1,2-η2)-PhC2Ph] (2)
There are two independent molecules 2a and 2b of very
stronger reductant NaBH4 is needed to reduce Pt(dppp)Cl2 similar structure in a unit cell of crystal 2·THF (Figure 1
to Pt0(dppp). This could be due to the fact that dppp is for structure of 2a).
more electron-donating than PPh3, making Pt(dppp)Cl2
Like the reported structure of 1, complex 2 displays an
more difficult to reduce than Pt(PPh3)2Cl2. Another litera- approximate C2v symmetry. The acetylene bond length (C1–
ture method[9] involves substitution of ethylene in (R3P)2- C1A) [1.301(7) Å] is significant longer than the average
Pt(C2H4) by acetylenes. This method was used to prepare CϵC bond length (ca. 1.2 Å). This indicates a weakening
(dppp)Pt[(1,2-η2)-CH3C4CH3] (6) and the dinuclear of the bond due to σ-donation from the acetylene to the
[(dppp)Pt]2[µ-(1,2-η2):(3,4-η2)-PhC4Ph] (5) and [(dppp)Pt]2- metal and π-back donation from the metal to the acetylene.
[µ-(1,2-η2):(3,4-η2)-CH3C4CH3] (7).
The Pt center adopts a distorted square-planar coordina-
The ESI-MS spectra of the complexes all show peaks tion geometry with a small dihedral angle of 2.98° between
attributable to the corresponding molecular ion M+. The the two C1–Pt1–C1A and P1–Pt1–P1A triangles. The Pt–C
IR spectra of 1 and 2 display weak bands at 1741 and and the Pt–P bond lengths are normal. Similar to 1, the
1748 cm–1, respectively, which are assigned to stretching of linear C2–C1ϵC1A–C2A backbone is distorted with the
the coordinated CϵC bonds. Absorption bands of similar C2–C1–C1A angle being 144.5(2)°. It is slightly smaller
Eur. J. Inorg. Chem. 2007, 384–393
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FULL PAPER
Table 1. Selected interatomic distances and angles in 2a, 4, and 5.
2a
4
5
Distances [Å]
Pt1–C1
Pt1–P1
C1–C1A
C1–C2
2.041(3)
2.2705(9)
1.301(7)
1.464(5)
Pt1–C2
Pt1–C1
Pt1–P1
Pt1–P2
C1–C2
C1–C5
C2–C3
C3–C4
C4–C6
2.043(4)
2.044(4)
2.2629(10)
2.2686(10)
1.299(6)
1.463(6)
1.393(6)
1.201(6)
1.433(6)
Pt1–C1
Pt1–C2
Pt1–P1
Pt1–P2
Pt2–C4
Pt2–C3
Pt2–P3
Pt2–P4
C1–C2
C1–C5
C2–C3
C3–C4
C4–C6
2.034(3)
2.051(3)
2.2478(9)
2.2581(9)
2.017(3)
2.063(3)
2.2423(8)
2.2755(9)
1.303(5)
1.451(5)
1.411(4)
1.296(5)
1.454(5)
Angles [°]
C1A–Pt1–C1
C1–Pt1–P1A
C1–Pt1–P1
P1–Pt1–P1A
C1A–C1–C2
C2–C1–Pt1
C1–C1A–Pt1
37.2(2)
C2–Pt1–C1
C2–Pt1–P1
C1–Pt1–P1
C2–Pt1–P2
C1–Pt1–P2
P1–Pt1–P2
C2–C1–C5
C2–C1–Pt1
C5–C1–Pt1
C1–C2–C3
C1–C2–Pt1
C3–C2–Pt1
C4–C3–C2
C3–C4–C6
37.05(16)
155.79(13)
118.81(11)
111.48(12)
148.40(11)
92.44(4)
137.7(4)
71.5(2)
150.1(3)
146.7(4)
71.5(2)
141.3(3)
174.7(5)
176.5(5)
C1–Pt1–C2
C1–Pt1–P1
C2–Pt1–P1
C1–Pt1–P2
C2–Pt1–P2
P1–Pt1–P2
C4–Pt2–C3
C4–Pt2–P3
C3–Pt2–P3
C4–Pt2–P4
C3–Pt2–P4
P3–Pt2–P4
C2–C1–C5
C2–C1–Pt1
C5–C1–Pt1
C1–C2–C3
C1–C2–Pt1
C3–C2–Pt1
C4–C3–C2
C4–C3–Pt2
C2–C3–Pt2
C3–C4–C6
C3–C4–Pt2
C6–C4–Pt2
37.20(13)
118.33(10)
155.45(10)
150.64(10)
113.44(10)
91.00(3)
37.02(13)
109.14(10)
145.49(10)
154.86(10)
118.57(10)
95.76(3)
138.6(3)
72.1(2)
149.3(3)
146.8(3)
70.7(2)
142.0(3)
146.4(3)
69.6(2)
150.47(10)
113.35(10)
96.02(5)
144.5(2)
144.0(3)
71.42(10)
144.0(3)
138.6(3)
73.4(2)
147.6(2)
than the corresponding angles of 140 and 139° in 1. The
phenyl rings and the coordination triangle C1–Pt1–C1A are
not coplanar, showing a dihedral angle of 26.89°.
(dppp)Pt[(1,2-η2)-PhC4Ph] (4)
The molecular structure of complex 4 (Figure 2) is sim-
ilar to that of {[(PPh3)2Pt](PhC4Ph)} (3) reported by Deem-
ing.[6b] The lengths of the coordinated CϵC bond (C1–C2)
and the uncoordinated one (C3–C4) are 1.299(6) and
1.201(6) Å.
The coordination geometry at the Pt center in 4 is also
nearly planar with the dihedral angle between triangles of
C1–Pt1–C2 and P1–Pt1–P2 being 4.31°. The two Pt–C and
Pt–P bond lengths are almost identical [Pt1–C1, 2.044(4) Å;
Pt1–C2, 2.043(4) Å; Pt1–P1, 2.2629(10) Å; Pt1–P2,
2.2686(10) Å]. While the C5–C1–C2–C3 linkage is bent
[ЄC5–C1–C2 137.7(4)°; ЄC1–C2–C3 146.7(4)°] like that in
2a, the uncoordinated triple bond remains linear [ЄC2–C3–
C4 = 174.7(5)°; ЄC3–C4–C6 176.5(5)°]. The dihedral angle
between the coordination triangle of C1–Pt1–C2 and the
adjacent phenyl ring of the acetylene is 18.36°.
Figure 1. ORTEP diagram (thermal ellipsoid = 50%) of 2a. H
atoms and solvent molecules are omitted; C3 and C3X represent
two positions of the disordered carbon atom, with 80% and 20%
occupancy, respectively.
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Luminescent Acetylene- and (Buta-1,3-diyne)platinum Complexes
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[(dppp)Pt]2[{µ-(1,2-η2):(3,4-η2)-PhC4Ph}] (5)
The molecular structure of complex 5 is shown in Fig-
ure 3. The butadiyne is coordinated to the two Pt atoms in
µ-(1.2-η2):(3,4-η2) mode. Both Pt atoms show nearly
square-planar coordination geometry. The C1–C2–C3–C4
backbone of the butadiyne shows a gauche conformation
with a torsion angle of 94.79°. Similar conformation has
been observed in the µ-(1,2-η2):(3,4-η2)-butadiyne com-
plexes (PyCl4W)2(Me3SiCϵCCϵCPh),[8d] (Cp2V)2(Me3Si–
CϵCCϵC–SiMe3),[8e]
and
[(R2PCH2CH2PR2)Ni]2-
(RЈCϵCCϵCRЈ)[8f] (R = iPr, RЈ = H or R = tBu, RЈ = Ph)
which display a torsion angle of 74–98°. The two Pt atoms
are related by a C2-rotation. The C1–Pt1–C2 and C3–Pt2–
C4 metallacycles show a gauche conformation with a dihe-
dral angle of 86.01°. The lengths of the two acetylene bonds
are 1.303(5) Å (C1–C2) and 1.296(5) Å (C3–C4), similar to
the acetylene bond lengths observed in 2a and 4.
Notably, the “inner” Pt–C bonds, Pt1–C2 [2.051(3) Å]
and Pt2–C3 [2.063(3) Å] and the “inner” Pt–P bonds, Pt1–
P2 [2.2581(9) Å] and Pt2–P4 [2.2755(9) Å], are slightly but
consistently longer than the “outer” Pt1–C1 [2.034(3) Å],
Pt2–C4 [2.017(3) Å], and Pt1–P1 [2.2478(9) Å] and Pt2–P3
[2.2423(8) Å], respectively. Such differences could be caused
by the steric repulsions between two neighboring dppp li-
gands. Similar to compounds 1–4, the carbon atoms at the
two ends of the acetylene bond are bent away from the
metal atom. The phenyl rings of the butadiyne are almost
coplanar with the PtC2 unit, showing dihedral angles of
15.14 and 16.95°.
Figure 2. ORTEP diagram (thermal ellipsoid = 50%) of 4. H atoms
and solvent molecules are omitted.
31P-NMR Spectroscopy
The 31P NMR spectra of all the complexes are consistent
with the X-ray crystal structures of the complexes. The
spectra of 1 and 2 show only two singlets at δ = 27.07 and
4.11 with Pt satellites (1JPt-P = 3448 and 3155 Hz, respec-
tively), and the spectrum of 3 shows two doublets of triplets
at δ = 26.53 and 26.19 ppm (1JPt-P = 3521 and 3456 Hz,
2JP-P = 22.9 Hz). The spectra of 4 and 6 show similar
[AB]2 splitting patterns. On the other hand, the spectrum
of 5 is poorly resolved, because of the similar chemical
shifts of different P atoms in the complexes (4.18 and
3.94 ppm, 2JP-P = 26.7 Hz). In the spectrum of 7 the signals
of the two phosphorus atoms are isochronous at δ =
1
8.54 ppm with very close JPt-P (3227 and 3258 Hz). Al-
though the X-ray crystal structures of 6 and 7 are not avail-
able, the similar NMR spectra suggest the molecular struc-
tures of 6 and 7 should resemble their PhC4Ph counter-
parts, 4 and 5, respectively.
(C) Absorption Spectroscopy
The UV/Vis absorption spectra of complexes 1, 2, and
the parent ligand diphenylacetyleme are shown in Figure 4.
The ligand displays intense (ε Ͼ 104 –1 cm–1) vibronic ab-
sorptions at 250–310 nm. Similar vibronic bands are ob-
Figure 3. (a) ORTEP diagram (thermal ellipsoid = 50%) of 5. H
atoms and solvent molecules are omitted; (b) a side view showing
the staggered conformation of 5.
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K. Zhang, J. Hu, K. C. Chan, K. Y. Wong, J. H. K. Yip
FULL PAPER
bands of
3 and 4 maximize at 370 nm (εmax =
9ϫ103 –1 cm–1) and 374 nm (εmax = 1.09ϫ104 –1 cm–1),
respectively. The low energy absorption (λmax ≈ 360 nm,
εmax = 2.31ϫ104 –1 cm–1) of 5 is more intense than that of
3 and 4.
The absorption spectra of 6, 7, and CH3C4CH3 are
shown in Figure 6. Unlike PhC2Ph and PhC4Ph, the spec-
trum of CH3C4CH3 shows little absorption at wavelengths
longer than 250 nm (ε Ͻ 200 –1 cm–1). However, the spec-
tra 6 and 7 show intense absorption bands around 250 nm
and
a less intense band at ca. 320 nm (εmax =
4.3ϫ103 –1 cm–1) and 315 nm (εmax = 1.3ϫ104 –1 cm–1),
respectively.
Figure 4. UV/Vis absorption spectra of complexes 1, 2, and ligand
PhC2Ph in CH2Cl2 at room temperature.
served in the spectrum of 1. In addition, the spectrum dis-
plays a broad and moderately intense absorption (ε = 1–
3.5ϫ103 –1 cm–1) ranging from 310–400 nm. No vibronic
structure is observed in the spectrum of 2. Instead the spec-
trum shows a broad and intense band peaked at 270 nm
(εmax = 2.34ϫ104 –1 cm–1) and a broad intense absorption
at 300–420 nm. The latter absorption consists of more
than one band, showing a peak at 327 nm (εmax
=
1.38ϫ104 –1 cm–1) and a shoulder at ca. 355 nm (ε =
9.1ϫ103 –1 cm–1).
The absorption spectrum of the ligand PhC4Ph shows a
clear vibronic structured band at 270–340 nm with a pro-
gression of 2000 cm–1 which should be associated with the
vibration of the CϵC bond in the ππ* excited state (Fig-
ure 5). The three complexes exhibit very intense absorption
around 250 nm with no distinct vibronic peak. While the
parent ligand does not absorb beyond 350 nm, the com-
plexes display broad intense absorption bands extending to
450 (3 and 4) or 500 nm (5). The low energy absorption
Figure 6. UV/Vis absorption spectra of complexes 6, 7, and
CH3C4CH3 in CH2Cl2 at room temperature.
(D) Emission Spectroscopy
Table 2 summarizes the photophysical data of the com-
plexes. The complexes are all not luminescent in degassed
solution at room temperature. On the other hand, the solids
of 1–5 are emissive at room temperature, showing two emis-
sion bands at 380–550 nm and 520–800 nm (Figure 7). The
lifetimes (τ) of the higher energy emissions are shorter than
the detection limit of our nanosecond laser flash photolysis
set-up (Ͻ0.1 µs), but the 520–800 nm emissions show long
lifetimes (τ = 1.57–3.86 µs).
Table 2. Photophysical data of the acetylene-Pt complexes.
Emission (in EtOH, frozen at 77 K) Emission (solid state at 298 K)
λmax [nm] (τ [µs])
λmax [nm] (τ [µs])
1
2
3
4
447 (Ͻ0.1), 547 (58.32)
429 (Ͻ0.1), 570 (39.97)
404 (Ͻ0.1), 589 (21.76)
475 (Ͻ0.1), 622 (10.07)
446 (Ͻ0.1), 562 (3.86)
443 (Ͻ0.1), 588 (2.19)
457 (Ͻ0.1), 597 (2.72)
459 (Ͻ0.1), 617 (1.57)
458 (Ͻ0.1), 630 (too weak to
be measured)
5
468 (Ͻ0.1), 605 (5.65)
6
7
490 (19.00)
518 (29.20)
No emission
No emission
Figure 5. UV/Vis absorption spectra of complexes 3, 4, 5, and li-
gand PhC4Ph in CH2Cl2 at room temperature.
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The lower energy emissions show vibronic shoulders. The
spacing is estimated at 1750 cm–1 in the spectrum of 1.
The emission spectra of frozen solutions of 3, 4, and 5
at 77 K (supporting materials) also show two bands at 380–
550 and 520–750 nm. Similar to the solid-state emission,
the energy of the lower energy luminescence (λmax = 590–
620 nm) is slightly lower than the corresponding emissions
of 1 and 2. The lifetime of the low energy emissions (τ =
5.65–21.76 µs) is much longer than that of the higher energy
ones (τ Ͻ 0.1 µs).
Unlike the PhC4Ph and PhC2Ph complexes, 6 and 7 are
not emissive in the solid state at room temperature. At
77 K, the solids show weak emission at ca. 500 nm with τ
of 5.10 µs (6) and 3.25 µs (7). The frozen solution spectra
display similar emissions at 490 nm (6, τ = 19.00 µs) and
518 nm (7, τ = 29.20 µs) (Figure 9). The emissions of 6 and
7 are far less intense than those of 1–5.
Figure 7. Solid-state emission spectra of complexes 1–5 at room
temperature [a: intensities of the emission maximum in the low en-
ergy region (550–700 nm) are normalized; asterisks at ca. 485 and
529 nm denote instrumental artifacts]. The excitation wavelength is
320 nm.
The energy of the long-lived emissions of PhC4Ph com-
plexes 3, 4, and 5 (λmax = 594–630 nm) is lower than
PhC2Ph complexes 1 (λmax = 562 nm) and 2 (λmax
=
588 nm). Apart from the acetylene, the emission energy is
also affected by the ancillary phosphane atoms: the PPh3-
containing complexes 1 and 3 emit at slightly higher energy
than their dppp counterparts 2 and 4.
The emission spectra of 77 K frozen EtOH solutions of
1 and 2 are shown in Figure 8. Excitation at 320 nm leads
to relatively weak emissions of short lifetimes (τ Ͻ 0.1 µs)
located at 350–500 nm. In addition, the spectra of 1 and 2
display more intense lower energy emissions of long life-
times at 547 nm (τ = 58.32 µs) and 570 nm (τ = 39.97 µs),
respectively.
Figure 9. 77 K frozen ethanol solution emission spectra of com-
plexes 6 and 7. The excitation wavelength is 320 nm.
Spectroscopic Assignments
In previous spectroscopic studies on acetylene-Pt0 com-
plexes, the low energy absorptions, which were absent in the
spectra of the free acetylenes, were assigned to the
Pt(5d)Ǟacetylene(π*) charge transfer transition.[7a,7c] This
assignment was further supported by semi-empirical calcu-
lations showing that the HOMO and LUMO are mainly
composed of the dz2-orbital and acetylene π*-orbital.[7a]
Likewise, the complexes 1–7 all exhibit intense absorption
bands that are lower in energy than the ligand absorptions.
Scheme 2 shows a qualitative molecular orbital of acetyl-
ene-Pt complexes constructed from orbital interactions be-
tween a two-coordinate phosphane-Pt0 and a bent acetyl-
ene.
Three metal orbitals dz2, dxz, and dx2–y2 are destabilized
by interacting with either the πx- or the πz-orbital of the
acetylenes. The dz2-orbital is expected to be the HOMO of
the complexes as its energy is raised by the strong antibond-
ing interactions with the πz-orbital. Similarly, the dx2–y2 or-
Figure 8. 77 K frozen ethanol solution emission spectra of com-
plexes 1 and 2. The excitation wavelength is 320 nm.
Eur. J. Inorg. Chem. 2007, 384–393
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389

K. Zhang, J. Hu, K. C. Chan, K. Y. Wong, J. H. K. Yip
FULL PAPER
The low energy absorption of the complexes at wave-
lengths longer than 320 nm should belong to the
PtǞP(dπ*) and PtǞacetylene(πx*) transitions. The order
of these MLCT transitions primarily depends on the energy
gap between the d-orbitals and πx*-orbitals. Results of
semi-empirical calculations show that the HOMO and the
LUMO of the acetylene are mainly composed of πx and πx*
of the acetylene, respectively. The HOMO–LUMO gap is
8.25, 8.38, and 10.48 eV for PhC4Ph, PhC2Ph, and
CH3C4CH3, respectively (supporting information), and is
consistent with the order of the πǞπ* absorption energy of
the acetylenes. Because of the π-conjugation, the order of
the LUMO energy is PhC4Ph (–0.65) Ͻ PhC2Ph (–0.51 eV)
Ͻ CH3C4CH3 (0.73 eV). On the other hand, the HOMOs
of PhC4Ph and PhC2Ph have almost the same energy
(–8.89 eV) and are higher energy than the HOMO of
CH3C4CH3 (–9.75 eV). Although the acetylenes in the com-
plexes are distorted from linearity, the energy order of the
HOMOs and LUMOs should be the same as that of the
free ligands. Accordingly, the PtǞacetylene(πx*) MLCT
transition energy of the complexes should follow the order
3, 4, 5 Ͻ 1, 2 Ͻ 6, 7. The same trend is observed in the
absorption of the complexes. Given their low-lying πx*-or-
bitals, the PtǞacetylene(πx*) transitions in 1, 2, 3, 4, and 5
should be lower in energy than the PtǞP(dπ*). In addition,
it is reasonable to expect the MLCT transitions of 3 and 4
to be lower in energy than those of 1 and 2. Indeed, the
Scheme 2.
bital is involved in σ-interaction with the πz-orbital but the MLCT absorption of the PhC4Ph complexes extends to
interactions should be weak because of the poor overlap. 500 nm while the PhC2Ph complexes do not absorb beyond
The dxz is in the correct symmetry to overlap with the πx- 400 nm. On the other hand, the energy of the PtǞace-
orbital. On the other hand, the dyz-orbital should be tylene(πx*) transition could be close to or even higher than
strongly stabilized via π-back bonding with the πz*-orbital. that of the PtǞP(dπ*) for the complexes 6 and 7 as the
The dyx-orbital is involved in δ-interactions with the πx*- πx*-orbitals of CH3C4CH3 are high in energy.
orbital and should be slightly lowered in energy.
The solid state and frozen solution emission spectra of
The πz*-orbital of the acetylene, strongly destabilized by 1–5 show emissions of short lifetimes at ca. 380–550 nm
the back bonding, should be higher energy than the πx*- and of longer lifetimes at ca. 500–800 nm. The long life-
orbital which is slightly destabilized by the weak δ-interac- times of the lower energy emission suggests that lumines-
tions. In this molecular orbital picture, the lowest energy cence arises from a triplet excited state. It is further sup-
allowed PtǞacetylene transition is the 1(dz2Ǟπx*) transi- ported by the large Stoke shift between the absorption and
1
1
x2–y2
tion. The (dxzǞπx*) and (d
Ǟπx*) transitions should the emission. The frozen solution spectrum of 1 shows a
be close in energy to the (dz2Ǟπx*). Because of the mixing vibronic spacing of ca. 1750 cm–1, that is close to the
1
between the metal and ligand orbitals, these transitions ground state stretching frequency of the coordinated acetyl-
1
3
have some intraligand character. Especially the (dxzǞπx*) ene. This suggests the excited state could be MLCT(πx*)
1
transition, which could mix with the (πxǞπx*) transition, arising from the PtǞacetylene(πx*) excitation. The order of
should be more intense than the other two transitions. the emission energy is 3, 4, 5 Ͻ 2 Ͻ 1 and is in agreement
Apart from the PtǞacetylene transition, a PtǞP(dπ*) with the energy of the πx*-orbitals of the complexes and
charge transfer from Pt to the dπ*-orbitals of the phos- hence the MLCT assignment. That the MLCT emission of
phane atoms is also possible. Gray showed that the complex 1 is slightly higher in energy than that of 2 could be due to
Pt0(dppp)2 exhibited an intense Pt(dx2–y2, dz2)Ǟ(dσ*, dπ*) the fact that the PPh3 in 1 is a strong electron-acceptor and
charge transfer absorption (εmax = 24,000 –1 cm–1) at weaker electron-donor than the dppp in 2.
360 nm.[13] As PhC2Ph and PhC4Ph are better π-acceptors
The short-lived 350–550 nm emission displayed by 1–5
and weaker σ-donors than dppp, it is likely that the could arise from an intraligand (acetylene) ππ* excited state
3
PtǞP(dπ*) transitions in the present complexes locate at and/or a MLCT(dπ*). PhC2Ph and PhC4Ph fluoresce at
wavelengths shorter than 360 nm in view of the position 330 and 365 nm in solution, respectively (supporting infor-
and energy of the very intense bands peaked at ca. 320 nm mation), while CH3C4CH3 is not emissive. To a different
in the spectra of 2, 6, and 7. Both position and intensity of extent, the metal–ligand interactions in the acetylene com-
the absorption bands are consistent with the PtǞP(dπ*) plexes stabilize the πx- and πz-orbitals and destabilize the
transition.
πx*- and πz*-orbitals. Accordingly, the lowest energy intrali-
390
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Luminescent Acetylene- and (Buta-1,3-diyne)platinum Complexes
FULL PAPER
the mixture gave a pale yellow suspension. The reaction mixture
was stirred overnight at room temperature. The yellow solid col-
lected by filtration was washed with excess ethanol. Analytically
pure product was obtained by recrystallization in THF/n-hexane.
Yield: 0.25 g (80%). C41H36P2Pt (785.8): calcd. C 62.7, H 4.6;
gand πxǞπx* transition, and hence the corresponding ππ*
emission should be blue shifted from that of the free acetyl-
ene. Possibly, the higher energy emissions of the complexes
3
come from a MLCT(dπ*) that arises from the PtǞP(dπ*)
transition. It should be noted that the complex Pt0(dppp)2
1
found C 62.5, H 4.5. H NMR (300 MHz, CDCl3): δ = 7.66–6.96
3
exhibits MLCT(dπ*) phosphorescence at 622 nm. As the
(m, 30 H, Ph), 2.61–2.58 (m, 4 H, -CH2-P-Pt-), 2.05–1.91 (m, 2
H, -C-CH2-C-) ppm. 31P{1H} NMR (121.5 MHz, CDCl3): δ = 4.11
[1J(PtP) = 3155 Hz] ppm. IR (KBr): ν(CϵC) = 1748 (w) cm–1. ESI-
MS (m/z, assignment): 786 [M]+. Single crystals of 2·THF for X-
Pt center in the acetylene complexes is less electron rich
than the one in Pt0(dppp)2, it is reasonable for the
3MLCT(dπ*) emission to occur at a higher energy.
Unlike the other complexes, 6 and 7 are not luminescent ray diffraction analysis were obtained by slow diffusion of n-hexane
into a concentrated THF solution of 2.
at room temperature and show a single emission at low tem-
perature. The emission energy is slightly higher than that of
1–5 by 1800–3800 cm–1. Given the fact that the πx*-orbital
of CH3C4CH3 is about ca. 1.3 eV or 10000 cm–1 higher than
those of PhC4Ph and PhC2Ph, it is unlikely the emissive
(dppp)Pt[(1,2-η2)-PhC4Ph] (4): The procedures were similar to
those for preparing 2. Yield: 70%. C43H36P2Pt (809.8): calcd. C
1
63.8, H 4.5; found C 63.9, H 4.3. H NMR (300 MHz, CDCl3): δ
= 7.74–7.01 (m, 30 H, Ph), 2.64–2.55 (m, 4 H, -CH2-P-Pt-), 2.09–
1.94 (m, 2 H, -C-CH2-C-) ppm. 31P{1H} NMR (121.5 MHz,
CDCl3): δ = 4.87 [2J(PP) = 15.3 Hz, 1J(PtP) = 3349 Hz], 3.20
3
state in 6 and 7 is MLCT(πx*). The emissive excited state
3
is therefore tentatively assigned to the MLCT(dπ*).
1
[2J(PP) = 15.3 Hz, J(PtP) = 3086 Hz] ppm. IR (KBr): ν(CϵC) =
2160 (m, uncoordinated), 1719 (m, coordinated) cm–1. ESI-MS
(m/z, assignment): 810 [M]+. Single crystals of 4 for X-ray diffrac-
tion analysis were obtained by slow diffusion of n-hexane into a
concentrated CH2Cl2 solution of 4.
Concluding Remarks
In this work, MLCT emission of a series of phosphane-
supported acetylene-Pt0 complexes in both the solid state
and frozen solution was observed. All of the complexes dis-
play intense charge transfer absorptions. Dual emissions are
observed in the spectra of 1–5. The higher energy emission
[(dppp)Pt]2[µ-(1,2-η2):(3,4-η2)-PhC4Ph] (5): 4 (0.11 g, 0.14 mmol)
and 1 mol equiv. of Pt(dppp)(CH2=CH2) (0.086 g, 0.14 mmol) were
added to diethyl ether (15 mL) and stirred at room temperature for
24 h. The pale yellow suspension was then filtered and washed with
diethyl ether. The pale yellow crude product was recrystallized from
THF/diethyl ether to give the pure product. Yield: 0.11 g (55%).
C70H62P4Pt2 (1417.3): calcd. C 59.3, H 4.4; found C 59.2, H 4.4.
1H NMR (300 MHz, C6D6): δ = 6.81–7.91 (m, 50 H, Ph), 2.24 (m,
8 H, -CH2-P-Pt-), 1.61–1.68 (m, 4 H, -C-CH2-C-) ppm. 31P{1H}
NMR (121.5 MHz, C6D6): δ = 4.18 [2J(PP) = 26.7 Hz, 1J(PtP) =
3258 Hz], 3.94 [2J(PP) = 26.7 Hz, 1J(PtP) = 3267 Hz] ppm. IR
(KBr): ν(CϵC) = 1758 (m) cm–1. ESI-MS (m/z, assignment): 1417
[M]+. Single crystals of 5 for X-ray diffraction analysis were ob-
tained by slow diffusion of diethyl ether into a concentrated THF
solution of 5.
3
could arise from intraligand ππ* and/or MLCT(dπ*) ex-
cited states. It is demonstrated that the MLCT emission en-
ergy is subject to the electron-donating ability of the phos-
phane atom and the electronic structures of the acetylene.
The lowest energy emissive excited state of 1–5 is assigned
3
to MLCT(πx*) as the πx*-orbitals of PhC4Ph and PhC2Ph
are substantially stabilized via conjugation with the phenyl
rings. On the other hand, we assign the phosphorescence of
3
6 and 7 to (dπ*ǞPt) transitions. It is believed that for the
two complexes, the dπ*-orbitals of dppp are lower in energy
(dppp)Pt[(1,2-η2)-CH3C4CH3] (6): Pt(dppp)(CH2=CH2) (0.108 g,
0.17 mmol) was treated with twofold excess of 2,4-hexadiyne
(0.027 g, 0.34 mmol) in diethyl ether at room temperature for 24 h.
The pale yellow suspension was then filtered and washed with a
small amount of diethyl ether. The product was purified by
recrystallization in THF/diethyl ether. Yield: 0.08 g (70%).
than the πx*-orbitals.
Experimental Section
General Methods: All reactions were carried out using standard
Schlenk techniques. 1,4-diphenylbuta-1,3-diyne, PPh3, dppp,
NaBH4, and N2H4·H2O purchased from Aldrich, 2,4-hexa-1,3-di-
yne purchased from TCI, and PtCl2 purchased from Oxkem were
used without further purification. Diphenylacetylene and ethylene
were obtained from Aldrich. All solvents used for syntheses and
spectroscopic measurements were purified according to literature
methods. Pt(CH3CN)2Cl2 was prepared by refluxing PtCl2 in a
large excess of CH3CN. cis-Pt(PPh3)2Cl2 and Pt(dppp)Cl2 were pre-
pared by reacting 2 equiv. of PPh3 and 1 equiv. of dppp, respec-
tively, with Pt(CH3CN)2Cl2 in CH2Cl2. (dppp)Pt[(1,2-η2)-
CH2=CH2] was prepared by reduction of Pt(dppp)Cl2 in the pres-
ence of ethylene in ethanol. 1[5a] and 3[6b] were synthesized accord-
ing to the reported methods. Their NMR spectra are identical to
the reported ones.
1
C33H32P2Pt (685.6): calcd. C 57.8, H 4.7; found C 57.5, H 4.5. H
NMR (300 MHz, C6D6): δ = 7.98–7.92, 7.71–7.65 and 7.10–6.98
3
4
(m, 20 H, Ph), 2.76 [doublet of triplets, J(PtH) = 39.7 Hz, J(PH)
= 7.6 Hz, 3 H, CH3 on the coordinated CϵC], 2.15 (m, 4 H,
-CH2-P-Pt-), 1.90 [doublet of triplets, 5J(PtH) = 18.8 Hz, 6J(PH) =
3.5 Hz, 3 H, CH3 on the uncoordinated CϵC], 1.62 (m, 2 H, -C-
CH2-C-) ppm. 31P{1H} NMR (121.5 MHz, C6D6): δ = 8.47 [2J(PP)
1
1
= 31.1 Hz, J(PtP) = 3353 Hz], 7.29 [2J(PP) = 31.1 Hz, J(PtP) =
3124 Hz] ppm. IR (KBr): ν(CϵC) = 2196 w (uncoordinated), 1749
s (coordinated) cm–1. ESI-MS (m/z, assignment): 686 [M]+.
[(dppp)Pt]2[µ-(1,2-η2):(3,4-η2)-CH3C4CH3] (7): The preparative
procedures for 7 were similar to those for preparing 5. Yield: 40%.
C60H58P4Pt2 (1293.2): calcd. C 55.7, H 4.5; found C 55.5, H 4.5.
1H NMR (300 MHz, C6D6): δ = 8.06–8.00, 7.86–7.80 and 7.10–
6.93 (m, 40 H, Ph), 2.51 (m, 6 H, CH3), 2.26 (br., 8 H, -CH2-P-
Pt-), 1.74 (br., 4 H, -C-CH2-C-) ppm. 31P{1H} NMR (121.5 MHz,
C6D6): δ = 8.54 [1J(PtP) = 3227 and 3258 Hz] ppm. IR (KBr):
ν(CϵC) = 1705 (w) cm–1. ESI-MS (m/z, assignment): 1293 [M]+.
Synthesis
(dppp)Pt[(1,2-η2)-PhC2Ph] (2): Pt(dppp)Cl2 (0.26 g, 0.39 mmol)
was added to a 30-mL ethanolic solution of diphenylacetylene
(0.085 g, 0.42 mmol). Slow addition of NaBH4 (0.1 g, 2.6 mmol) to
Eur. J. Inorg. Chem. 2007, 384–393
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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391

K. Zhang, J. Hu, K. C. Chan, K. Y. Wong, J. H. K. Yip
FULL PAPER
Table 3. Crystal data and structure refinements for compounds 2, 4, and 5.
Compound
2
4
5
Empirical formula
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system
Space group
C45H44OP2Pt
857.83
295(2)
0.71073
orthorhombic
Pnma
C43H36P2Pt
809.75
223(2)
C70H62P4Pt2
1417.26
223(2) K
0.71073 Å
monoclinic
P2(1)/n
0.71073
triclinic
¯
P1
Unit cell dimensions (Å and deg)
a = 30.0064(15)
b = 23.4345(12)
c = 10.7582(5)
α = β = γ = 90
a = 10.7412(5)
b = 12.6475(6)
c = 13.7503(6)
α = 83.3900(10)
β = 88.0370(10)
γ = 66.3590(10)
1699.68(14)
a = 10.8152(7)
b = 20.0454(12)
c = 26.8225(17)
α = γ = 90
β = 92.4740(10)
Volume [Å3]
7565.0(6)
5809.6(6)
Z
8
2
4
Calculated density [mg/m3]
Absorption coefficient [mm–1]
F(000)
1.506
3.828
3440
1.582
4.252
804
1.620
4.963
2792
Crystal size [mm3]
θ range for data collection [°]
0.38ϫ0.26ϫ0.20
1.61 to 27.50
–38 Յ h Յ 36, –30 Յ k Յ
30, –12 Յ l Յ 13
51463
0.40ϫ0.10ϫ0.08
1.77 to 25.00
–12 Յ h Յ 12, –15 Յ k Յ
15, –16 Յ l Յ 16
18338
0.50ϫ0.20ϫ0.20
1.52 to 30.01
–14 Յ h Յ 15, –28 Յ k Յ
26, –20 Յ l Յ 37
46864
Index ranges
Reflections collected
Independent reflections
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2sigma(I)]
R indices (all data)
8897 [R(int) = 0.0408]
0.5148 and 0.3241
8897/17/459
5990 [R(int) = 0.0271]
0.7272 and 0.2811
5990/0/556
16579 [R(int) = 0.0317]
0.4368 and 0.1904
16579/0/685
1.021
1.080
1.025
R1 = 0.0318, wR2 = 0.0664
R1 = 0.0461, wR2 = 0.0707
1.222 and –1.152
R1 = 0.0276, wR2 = 0.0695
R1 = 0.0287, wR2 = 0.0702
3.840 and –0.667
R1 = 0.0325, wR2 = 0.0752
R1 = 0.0440, wR2 = 0.0792
2.971 and –0.682
Largest diff. peak and hole [e Å–3]
Physical Measurements: Elemental analyses of all the compounds
prepared were carried out in the microanalysis laboratory in the
Department of Chemistry, the National University of Singapore.
1H- and 31P{1H}-NMR spectra were recorded at room temperature
with a Bruker ACF 300 spectrometer. Electrospray ionization mass
spectra (ESI-MS) were measured with a Finnigan MAT 731 LCQ
spectrometer. IR spectra (KBr) were recorded using a Bio-Rad
Win-IR spectrophotometer. UV/Vis absorption spectra were re-
corded with a Shimadzu UV-1601PC UV/Vis spectrophotometer.
Emission spectra were recorded with a SPEX-Fluorolog2 model
F111A1 fluorescence spectrofluorometer. The lifetimes were mea-
sured using the Quanta Ray DCR3 Nd:YAG laser with a pulse-
width of 8 ns and excitation wavelength of 355 nm. Sample solu-
tions for 77 K frozen glass emission spectra measurement were pre-
pared as follows: (i) a small amount of solid samples were added
to ethanol solution; (ii) the solutions were then dissolved using the
ultra-sonic bath; (iii) the solutions were filtered; (iv) the filtrates
were introduced into a quartz tube; (v) the quartz tubes were im-
mersed in liquid nitrogen in a quartz optical Dewar flask for meas-
uring.
and SHELXTL[16] for space group determination, structure solu-
tion, and least-squares refinements on |F|2. The crystals were
mounted at the end of glass fibers and used for the diffraction
experiments. Anisotropic thermal parameters were refined for the
rest of the non-hydrogen atoms. The hydrogen atoms were placed
in their ideal positions. The X-ray crystal data and parameters used
in structural refinements are summarized in Table 3.
CCDC-623835 (for 2), -623836 (for 4), and -623837 (for 5) contain
the supplementary crystallographic data. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see also the footnote on the first page of
this article): Molecular orbitals of the acetylenes, emission spectra
of diphenylacetylene and 1,4-diphenyl-buta-1,3-diyne, emission
spectra of frozen glass solutions of 3–5 at 77 K, syntheses and char-
acterizations of 1 and 3.
Acknowledgments
Molecular Orbital Calculations: The calculations were performed
using the SPARTAN semiempirical program SGI/R10K, release
5.1.3, with geometry optimization. The model RHF/PM3 was used
in the calculations.
J. H. K. Y. thanks The National University of Singapore for finan-
cial support. K. Y. W. acknowledges the support from the Hong
Kong Polytechnic University. We are grateful to Ms. Tan Geok
Kheng (NUS) for her assistance with X-ray crystal structure deter-
mination.
X-ray Crystallography: The diffraction experiments were carried
out with a Bruker AXS SMART CCD 3-circle diffractometer at T
= 223 K (except that: T = 295 K for crystal 2·THF), 2θ-ω scan
with a sealed tube at 23 °C using graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å). The software used were: SMART[14] for
collecting frames of data, indexing reflection, and determination of
lattice parameters; SAINT[14] for integration of intensity of reflec-
tions and scaling; SADABS[15] for empirical absorption correction;
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Luminescent Acetylene- and (Buta-1,3-diyne)platinum Complexes
FULL PAPER
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Received: August 19, 2006
Published Online: November 27, 2006
Eur. J. Inorg. Chem. 2007, 384–393
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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