Octupolar organometallic Pt(II) NCN-pincer complexes; Synthesis, electronic, photophysical, and NLO properties

Article abstract of DOI:10.1016/j.jorganchem.2017.12.028

A series of organometallic octupolar 1,3,5-substituted-2,4,6-styryl-benzene complexes was synthesized by post-modification of parent [PtCl(NCN-CHO-4)], i.e. 1,3,5-tri-R-2,4,6-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene (NCN = [C₆H₂(CH₂NMe₂)₂-2,6]^– in which R = OMe, H, Br (1–3). Their synthesis involved a triple Horner-Wadsworth-Emmons reaction of [PtCl(NCN-CHO-4)] with the appropriate tris[(diethoxyphosphoryl)]methyl]benzene derivative. The ¹⁹⁵Pt{¹H} NMR chemical shift reflects the electronic properties of the π-system to which it is connected. The UV/Vis bands of the octupolar platinum complexes are only slightly red-shifted (by 5–12 nm) with respect to those of corresponding stilbenoid Pt-Cl pincer compounds (i.e., the separate branches), suggesting that there is only a limited electronic interaction between these branches. The fluorescence Stokes shift, quantum yields and lifetimes of 1–3 also are of the same order of magnitude as those of stilbenoid Pt-Cl pincer compounds, indicating that a dipolar excited state is formed, which is localized on one of the three branches. The hyper-Rayleigh scattering technique revealed hyperpolarizabilites β_HRS of 430, 870 and 183 × 10^?30 esu for 1, 2 and 3, respectively, which are among the highest for transition metal complexes. The highest value was found for the compound lacking a donor or acceptor group at the central core, lending support to the idea that dispersion overwhelms charge transfer in determining the magnitude of the first hyperpolarizability in octupolar compounds.

Full text of DOI:10.1016/j.jorganchem.2017.12.028

Accepted Manuscript
Octupolar organometallic Pt(II) NCN-pincer complexes; Synthesis, electronic,
photophysical, and NLO properties
Guido D. Batema, Cornelis A. van Walree, Gerard P.M. van Klink, Celso de Mello
Donegá, Andries Meijerink, Javier Perez-Moreno, Koen Clays, Gerard van Koten
PII:
S0022-328X(17)30722-2
10.1016/j.jorganchem.2017.12.028
JOM 20223
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 October 2017
Revised Date: 22 December 2017
Accepted Date: 23 December 2017
Please cite this article as: G.D. Batema, C.A. van Walree, G.P.M. van Klink, C. de Mello Donegá,
A. Meijerink, J. Perez-Moreno, K. Clays, G. van Koten, Octupolar organometallic Pt(II) NCN-pincer
complexes; Synthesis, electronic, photophysical, and NLO properties, Journal of Organometallic
Chemistry (2018), doi: 10.1016/j.jorganchem.2017.12.028.
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ACCEPTED MANUSCRIPT
Octupolar Organometallic Pt(II) NCN-Pincer complexes; Synthesis, electronic,
photophysical, and NLO properties#
Guido D. Batema,[a] Cornelis A. van Walree,[b,c] Gerard P.M. van Klink,[a] Celso
de Mello Donegá,[d] Andries Meijerink,[d] Javier Perez-Moreno,[e,f] Koen Clays,[e]
and Gerard van Koten*[a]
[a] Dr. G. D. Batema, Dr. G. P. M. van Klink, Prof. Dr. G. van Koten
Organic Chemistry and Catalysis, Faculty of Science, Utrecht University, Universiteitsweg 99, 3584
CH Utrecht (The Netherlands)
Fax: (+31)30-2523615
E-mail: g.vankoten@uu.nl
[b] Dr. C. A. van Walree, Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular
Research, Department of Chemistry, Faculty of Science, Padualaan 8, 3584 CH Utrecht (The
Netherlands)
[c] School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide 5001
(Australia)
[d] Dr. C. de Mello Donegá, Prof. Dr. A. Meijerink
Condensed Matter and Interfaces, Faculty of Science, Utrecht University, Princetonplein 5, 3584 CC
Utrecht (The Netherlands)
[e] Dr. J. Perez-Moreno, Prof. Dr. K. Clays
Department of Chemistry, University of Leuven, Celestijnenlaan 200D, 3001 Leuven (Belgium)
[f] Current address: Department of Physics, Skidmore College, 815 North Broadway, Saratoga Springs,
NY 12866 (USA)
Keywords: organometallic stilbenoid pincers, post-modification of organometallics, N-ligands,
conjugation, luminescence, 195Pt NMR, nonlinear optics
#
This paper is dedicated to Professor Irina P. Beletskaya in recognition of her great and innovative
research in the field of metal mediated and catalyzed organic synthesis; thank you for your friendship
and many inspiring discussions. Happy birthday!
1

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Abstract:
A series of organometallic octupolar 1,3,5-substituted-2,4,6-styryl-benzene complexes
was synthesized by post-modification of parent [PtCl(NCN-CHO-4)], i.e. 1,3,5-tri-R-
2,4,6-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene
(NCN
=
[C6H2(CH2NMe2)2-2,6]– in which R = OMe, H, Br (1–3). Their synthesis involved a
triple Horner-Wadsworth-Emmons reaction of [PtCl(NCN-CHO-4)] with the
appropriate tris[(diethoxyphosphoryl)]methyl]benzene derivative. The 195Pt{1H}
NMR chemical shift reflects the electronic properties of the π-system to which it is
connected. The UV/Vis bands of the octupolar platinum complexes are only slightly
red-shifted (by 5 – 12 nm) with respect to those of corresponding stilbenoid Pt-Cl
pincer compounds (i.e., the separate branches), suggesting that there is only a limited
electronic interaction between these branches. The fluorescence Stokes shift, quantum
yields and lifetimes of 1-3 also are of the same order of magnitude as those of
stilbenoid Pt-Cl pincer compounds, indicating that a dipolar excited state is formed,
which is localized on one of the three branches. The hyper-Rayleigh scattering
technique revealed hyperpolarizabilites βHRS of 430, 870 and 183 × 10–30 esu for 1, 2
and 3, respectively, which are among the highest for transition metal complexes. The
highest value was found for the compound lacking a donor or acceptor group at the
central core, lending support to the idea that dispersion overwhelms charge transfer in
determining the magnitude of the first hyperpolarizability in octupolar compounds.
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Introduction
The development of new optical materials containing organometallic groupings
attracts a great deal of attention in materials research. Materials with notable non-
linear optical properties1-6 or light-emitting diode properties6-8 have been discovered,
comprising molecules in which one or several organometallic fragments are
connected to a conjugated system. The observed optical and photophysical properties
of these materials often rely also on the presence of electron donor and/or acceptor
groups. Depending on the nature of these latter groupings and the conjugated system,
the electronic properties and/or the nonlinear optical (NLO) properties can be fine-
tuned.9-10
Guerchais et al. reported an example of electronic fine tuning for cyclometalated PtII
complexes, containing a 4-styryl-2-phenylpyridine ligand, with different substituents
on the styryl group.8 In a similar study we reported on 4,4'-disubstituted stilbenoid
NCN-pincer PtII complexes [PtCl(NCN-R-4)] (NCN-R-4 = [C6H2(CH2NMe2)2-2,6-R-
4]– in which R = C2H2C6H4-R'-4', with R' = NMe2, OMe, SiMe3 H, I, CN, NO2), see
Figure 1.11 For these systems a linear correlation was found between the Hammett
substituent constant σp of R', and the
δ
195Pt NMR chemical shifts of the metal center,
pointing to a substantial electronic interaction of the para positioned substituents and
the NCN-PtII fragment. Photoinduced charge separation, in which the Pt functionality
functions as an electron donor, was shown to occur. The nature of the donor-acceptor
interaction, i.e. in terms of separate inductive and mesomeric contributions, between
the para-substituent and the NCN-PtII fragment could be further studied in a series of
organometallic 4,4’-substituted benzylidene aniline complexes 4-ClPt-3,5-
(CH2NMe2)2C6H2CH=NC6H4R’-4’, with R’ = NMe2, Me, H, Cl, CN, see Figure 1.12
Figure 1. Stilbenes (X=CH) and benzylideneanilines (X=N) used for studying donor-
acceptor communication between para-R substituents and the Pt-Cl fragment.11,12
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1
Linear correlations were found between the azomethine H, the 195Pt NMR and
13
various C NMR chemical shifts, and the substituent parameters σF and σR of R’ at
the aniline site. The dual substituent parameter treatment of the azomethine 13C NMR
shift gave important insight into the unique behaviour of the Pt-pincer group as a
substituent. Inductively, it is a very strong electron-withdrawing group, whereas
mesomerically it behaves like a very strong electron donating group.12
Within non-linear optics research, octupolar compounds represent an interesting class
of materials.13-15 In general, compounds consisting of a multi-branched conjugated
backbone with three-fold (approximate D3h) symmetry have been subject of
investigation.16 These compounds consist of a central core with either electron
donating or accepting characteristics to which three π-conjugated branches are
connected, which often also contain electron accepting or donating substituents. The
relation between the NLO-characteristics and the charge transfer pattern is, however,
not immediately obvious.17 Organometallic and coordination compounds have also
been explored as octupolar NLO materials.18-21
Building on our success with the 4,4'-disubstituted stilbenoid and benzylideneaniline
bridged pincer Pt-complexes, we were interested as to whether octupolar NLO-active
compounds could be developed from these pincer compounds. Hence, in this study we
report the synthesis and optoelectronic properties of donor/acceptor substituted
octupolar stilbenoid NCN-pincer platinum complexes 1,3,5-tri-R-2,4,6-tris[(4-
(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene (R = OMe, H, Br) (1–3)
(Figure 2).
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Figure 2. Donor-acceptor star shaped stilbenoid pincers
These molecules contain a central aromatic ring, to which three styryl NCN-pincer
platinum fragments are connected on the meta positions. The central aromatic ring
will be used to tune the electronic properties by the introduction of substituents R.
The complexes are fully characterized using NMR, IR, high resolution mass
spectroscopy, and with elemental analysis. Furthermore, the electronic, physical, and
optical properties of these compounds are studied using NMR, UV/Vis, emission
spectroscopy and hyper-Rayleigh scattering (HRS).
Results and discussion
Synthesis. In our (post-modification) strategy to synthesize the organometallic
octupolar molecules 1–3, we used the Horner-Wadsworth-Emmons protocol (see
Scheme 2).22 In this reaction a benzylphosphonate ester is coupled with an aromatic
aldehyde, in the presence of a strong base (tBuOK). After hydrolysis, the two
fragments are connected via a carbon-carbon double bond, with a predominant trans
configuration. The Horner-Wadsworth-Emmons protocol was used in the previously
reported synthesis of the 4,4'-substituted stilbenoid NCN pincer Pt(II) complexes, and
showed to be compatible for the organometallic pincer platinum fragment.11 To afford
a series of electronically different octupolar complexes, first the diethyl
benzylphosphonate ester precursors 9a–c had to be synthesized. This was done
according to reported procedures (Scheme 1). Thus, benzyl bromide 8a (R = OMe)
was synthesized via bromomethylation of the aromatic ring of the commercially
available 1,3,5-trimethoxybenzene 4 using formaldehyde and a mixture of HBr/acetic
acid (reaction i).13,23,24
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Scheme 1. Synthetic route towards diethyl benzylphosphonate esters 9a–c.
Reagents and conditions: i: (CHO)n, 33% HBr/AcOH, 80°C; ii: a) MeOH, H2SO4,
reflux; b) LiAlH4, THF, reflux; iii: 33% HBr/AcOH, rt; iv: Br2, FeCl3 (cat.), C2H4Cl2,
reflux; v: NBS, BPO, CCl4, reflux; vi: P(OEt)3, 110 °C.
Benzyl bromide 8b (R = H) was obtained via a multistep reaction procedure starting
with 1,3,5-benzenetricarboxylic acid 5 which was converted into tris-benzylalcohol 6
(ii), which was subsequently converted into 8b with a mixture of HBr/acetic acid
(iii).22,25 Tribromomesitylene 7 was obtained via an aromatic bromination of
mesitylene with bromine (iv).26 In the next step (v) 7 was converted into benzyl
bromide 8c by a radical bromination of the mesityl groups using N-
bromosuccinimide.27 Finally the benzyl bromides 8a–c were converted into the
benzylphosphonate esters 9a–c, according a Michaelis-Arbusov reaction with
triethylphosphite (vide infra).13,22, 28
As described above, in order to synthesize the organometallic octupoles, metallo-
aldehyde [PtCl(NCN-CHO-4)] (10) was coupled with the corresponding
benzylphosphonate esters 9a–c. After hydrolysis of the reaction mixture, the pure
trans isomers of the octupoles 1–3 were isolated in good yields (75–92%), noting that
this reaction involves three coupling reactions on one central molecule (Scheme 2).
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Scheme 2.
1
13
The air stable solids 1–3 were fully characterized by H, C{1H}, 195Pt{1H} NMR
spectroscopy, elemental analysis, and high-resolution mass spectrometry.
1
13
NMR spectroscopy. Solutions of 1–3 in CD2Cl2 were studied by H, C{1H}, and
195Pt{1H} NMR spectroscopy. In the 1H NMR spectra resonances for the (CH3)2N and
the ArCH2N protons of the CH2NMe2 substituents were observed at
δ = 3.06 ppm and
at = 4.06 ppm, respectively, accompanied with characteristic satellites resulting
δ
3
from platinum coupling J(H, Pt). Only in 1 and 2 this coupling on the CH2 protons
could be resolved (3J(H, Pt) = 37–38 Hz). The other platinum couplings with the
protons of the CH2 and CH3 groups in these compounds appeared as a broad signal at
the base of the singlet peaks. Furthermore the characteristic resonances of the olefinic
protons were observed at
16 Hz) indicative of a trans configuration. For 1 (R = OMe) the resonances of these
protons were found at much lower field ( Htrans = 7.47/7.18 ppm), compared with the
signals of 3 (R = Br), which were found at higher field ( Htrans = 6.94/6.68 ppm),
δ
= 6.3–7.5 ppm with large coupling constants (3J(H, H) =
δ
δ
showing that these proton chemical shifts are affected by the nature of the substituent.
Also, in the 13C{1H}, and the 195Pt{1H} NMR spectra of 1–3 the net electronic effect
exerted by the substituents R on the conjugated system is directly reflected in the
positions of the Cipso (to platinum) carbon atom (δ 13Cipso = 146 to 148 ppm), and the
platinum NMR chemical shifts of the metal center (δ 195Pt = –3149 to –3158 ppm)
(Table 1). Going through from 1, which contains three donating OMe groups, towards
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3, containing the, primarily, electron withdrawing Br groups, both resonances shift to
lower field. This effect points to an electronic interaction between the central aromatic
ring and the end bound organometallic fragment; i.e., electron density is shifted from
the metal to the central core. For 4,4’-disubstituted stilbenoid Pt-pincers, 195Pt
chemical shifts of –3164 and –3158 ppm is found for the 4-MeO and H substituents,
respectively.11 The effect of the substituent on the 195Pt chemical shift in the octupolar
compounds is thus about equally large as in the stilbene-Pt pincers.
13
Table 1. Selected C{1H} and 195Pt{1H} NMR data of 1,3,5-tri-R-2,4,6-tris[(4-
(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene 1–3.[a]
Compound
δ 13Cipso{1H}
δ 195Pt{1H}[b]
1 (R = OMe)
146.8
2 (R = H)
147.3
3 (R = Br)
148.1
–3158
–3154
–3149
[a] 0.024 M solutions in CD2Cl2;
δ in ppm, [b] Na2PtCl6 as the external reference
UV/Vis spectroscopy. The complexes 1–3 are freely soluble in dichloromethane,
whereas they show a limited solubility in less polar solvents like diethyl ether (Table
2). As a result of the restricted solubility the absorption coefficient ε of the complexes
was only determined in dichloromethane. The absorption spectra for 1–3 (Figure 3,
Table 2) show a weak band between 251–253 nm originating from the excitation of
the singlet ground state (S0) to a higher excited state (e.g. S0→S2).9,11,23 A more
intense lower energy band (S0-S1), is observed in the range of 343–364 nm. The
position of the absorption band is found at the lowest wavelength for 3, followed by 1
and 2. Given that the Pt-Cl moiety behaves as an electron donor,[5] there is no clear
relation with the donor-acceptor character of R. In that case the longest wavelength
would be observed for the electron acceptor Br, followed by H and OMe. For 1 and 2
in all solvents, the absorption maxima are only slightly red-shifted (by 5 – 12 nm) in
comparison to the transitions in stilbenoid Pt-Cl pincer compounds with the same R-
group (i.e., the separate branches).[5] This suggests that there is only a limited
electronic interaction between these branches. The absorption spectra of 1–3 show no
distinct solvent polarity dependence (Table 2), indicating that the (vertical) electronic
excitation is not accompanied by a substantial change in dipole moment. Similar
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behavior was observed for three-fold symmetric diphenylamino-substituted
triphenylbenzene.30
Figure 3. Normalized UV/Vis (left) and emission spectra (right) of 1–3 in dichloromethane.
Table 2. UV/Vis and fluorescence data of 1–3.[a]
Solvent
kr (·107 s–1)[c] knr (·109 s–1)[d]
λmax (nm)
λfl (nm)[b]
φfl
1 (R = OMe)
0.010
τfl (ns)
Acetonitrile
353
251 [22.7]
359 [90.2]
361
433
425
0.17
0.17
5.9
4.7
5.8
5.8
Dichloromethane
0.008
Tetrahydrofuran
Ethyl acetate
422
420
0.006
0.003
0.14
0.15
4.3
2.0
7.1
6.6
357
Diethyl ether[e]
358
2 (R = H)
0.020
Acetonitrile
356
253 [27.1]
362 [89.5]
364
444
428
0.19
0.16
10.5
9.4
5.2
6.2
Dichloromethane
0.015
Tetrahydrofuran
Ethyl acetate
423
420
0.010
0.006
0.15
0.15
6.7
4.0
6.6
6.6
346
Diethyl ether[f]
3 (R = Br)
0.0012
Acetonitrile
343
467
454
0.20
0.15
0.6
5.0
6.6
Dichloromethane
253 [29.1]
348 [65.3]
0.015
10.0
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Tetrahydrofuran
Ethyl acetate
354
348
465
467
0.0005
0.0012
0.17
0.17
0.3
0.7
5.9
5.9
Diethyl ether[f]
[a] Lower energy absorption wavelength (nm) at room temperature in air-saturated solutions (c ≈ ·10–5 M),
absorption coefficient between brackets (
in ·103 –1 cm–1); [b] Excitation wavelength 370 nm, argon flushed
ε
ε
M
solutions at room temperature, c ≈ 10-6 M; [c] kr = Φfl / τfl; [d] knr = (1 – Φfl )/ τfl); [e] restricted solubility; [f]
insoluble.
As reflected by their absorption coefficient of
ε M
≈ 65–90 ·103 –1 cm–1 the absorption
bands of 1-3 show intense absorption bands. These values are about two (for 3) to
three times (for 1 and 2) larger than the value obtained for linear dipolar 4,4'-
substituted stilbenoid pincer platinum complexes,11 which is in agreement with the
factor found between the similar organic 4,4'-substituted stilbene and octupolar
compounds (factor 2.7).31
Emission spectroscopy. The compounds are luminescent, and were therefore studied
for their photophysical properties by determination of the fluorescence maxima (λfl),
quantum yields (Φfl) and excited state lifetimes (τfl). From these data the radiative (kr)
and non-radiative (knr) decay constants were calculated.
Emission spectra and excited state lifetimes were recorded in solvents of different
polarity (Table 2). The emission spectra of 1–3 show a broad non-structured emission
band in the range of λfl = 420–467 nm, and examples of the emission spectra in
dichloromethane are depicted in Figure 3. For compounds 1 and 2 a small
bathochromic shift of the emission maximum with an increase in solvent polarity is
observed. The occurrence of this positive solvatochromism reflects the formation of a
polar or charge separated excited state. For 3 this behavior was not observed. The
solvatochromism and Stokes’ shifts are, however, not as large as observed for other
threefold symmetric molecules like diphenylamino-substituted triphenylbenzene30 and
1,3,5-methoxy-2,4,6-trisstyrylbenzene derivatives,13 confirming that only a limited
extent of charge separation takes place. In view of the weaker donor and acceptor
used in the present study this does not come as a surprise.
The complexes are only weakly luminescent, shown by the low quantum yields (Φfl =
0.0005–0.02), with the lowest values found for 3. Although one would expect that this
may be caused by the heavy-atom effect (which favors intersystem crossing), the kr
and knr data indicate that it is rather the radiative part of the decay that is slower for 3
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(with the exception of the data in dichloromethane). Alternatively, the very low Φfl
for 3 may be caused by a fraction of non-emitting complexes that decay too fast to
contribute to the measured τfl. The observation that radiative rate constants is of the
order of 107 s–1 suggests that fluorescence is responsible for the radiative decay to the
ground state (not phosphorescence). The relaxation process is, however, dominated by
the non-radiative decay process which is two orders of magnitude larger than the
radiative decay. Presumably the presence of the NCN-PtCl fragment favors non-
radiative decay, which is likely to be caused by the heavy atom effect.
The fluorescence Stokes’ shift, quantum yields and lifetimes of 1-3 are of the same
order of magnitude as those of stilbenoid Pt pincer compounds,11 and hence their
photophysics are roughly similar. In common with the amino substituted
triphenylbenzenes, it is thus likely that a dipolar excited state is formed, which is
localized on one of the three branches. In the excited state, energy transfer between
the three branches may occur.32
NLO-properties. A general drawback of the application of linear dipolar molecules
in electro-optic devices is that they must be aligned parallel to the applied electric
field to exhibit second-harmonic generation (SHG). Several techniques have been
developed to align these molecules in a parallel fashion, opposing the tendency of
dipoles to orient in an antiparallel pattern.33 The problem can be overcome with the
development of octupolar complexes, which have no net dipole moment. Therefore,
octupolar materials containing NLO properties are very interesting for the application
in electro-optic devices.31 In this study the second-order hyperpolarizability
β was
determined for octupoles 1–3 using the hyper-Rayleigh scattering (HRS)
technique,34,35 in dichloromethane solution. The obtained data are tabulated in Table
3. This table shows the measured values βHRS, the only non-zero hyperpolarizability
tensor βxxx (〈βxxx〉2 = 〈(21/8) βHRS〉2, assuming D3h symmetry) and the static (frequency
independent) values βHRS(0). The latter ones were obtained by
(ωm2 ax – 4ω2 )(ωm2 ax – ω2 )
β
HRS (0) = βHRS (ω)
ωm4 ax
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where βHRS(0) is the hyperpolarizability at the used laser frequency ω, and ωmax is the
frequency at the absorption maximum of the NLO-active compound.36
Table 3. HRS results of R-trisstilbene pincers. Values in 10–30 esu
complex
βHRS
βxxx
βHRS(0)
1 (R = OMe)
430(±45)
700(±70)
66(±7)
2 (R = H)
3 (R = Br)
870(±105) 1410(±170)
183(±19) 300(±30)
125(±15)
36(±4)
[a] solution in CH2Cl2, laser 800 nm
The largest
β values are found for 2 (R = H). The βHRS and βxxx values are as high as
870 and 1410×10–30 esu, respectively. However, one should not forget that resonance
enhancement occurs for 1-3, as their optical transitions are relatively close to the used
laser wavelength of 800 nm. Correction with the oversimplified two-level model36
yields the numbers 36-125 × 10–30 esu. Because of the closeness of the second-
harmonic wavelength (400 nm) to the resonance wavelength of the lowest-energy
absorption (approximately 360 nm in CH2Cl2), the resulting static
underestimated. Nevertheless, the resulting values still are of the same order of
magnitude as the highest ’s of other transition metal containing NLO-complexes, viz.
metal acetylide and vinylidene complexes (
(0) values up to 406 × 10–30 esu,18 and
alkynyl ruthenium dendrimers (
(0) values up to 220 × 10–30 esu).20 They compare
β(0) values are
β
β
β
favourably to subphthalocyanines, for which βHRS (0) values of 10 - 86 × 10–30 esu)
were reported. 37 Recently, a very large value of 1700 ×10–30 esu (static value 1300
×10–30 esu was reported for an octupolar bis(porphyrinato)terbium(III) complex.21
It is of interest to also compare the data for 1-3 with those of the stilbenoid pincer
platinum complexes.6 For these compounds
β(0) values were found to range from 46
(R = SiMe3) to 216 × 10–30 esu (R= I; this value was much larger than for the other
substituents, which might be related to the large polarizability of the iodine
substituent). The optical nonlinearities for the trisstilbene analogues are thus of
comparable magnitude, the latter compounds having the advantage of a vanishing
static dipole moment.
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In comparison to 1 and 3, the
β values for compound 2 are noticeably high. Thus, the
highest value is found for the compound lacking a donor or acceptor group at the
central core, which supports the idea that the charge transfer characteristics are not
decisive in the behavior of C3 symmetric compounds.17 The subtle difference in the
absorption characteristics is responsible (Table 2 and Figure 1, notice the strong red-
shifted shoulder for 2), as it has been noticed that "dispersion overwhelms charge
transfer in determining the magnitude of the first hyperpolarizability" in octupoles.17
A more quantitative analysis of this effect would require the detailed description of
the absorption bands based on band fitting and the application of the Thomas-Kuhn
Sum rules, together with more experimental data of the strong dispersion of the first
hyperpolarizability close to the resonance wavelength.17 This would allow a much
finer dispersion analysis than the crude two level model.
Conclusions
The electronic properties of the octupolar platinum complexes do not differ very
much from those of the separate branches bearing the same substituents (i.e., the
corresponding stilbenoid Pt-Cl pincer compounds). The effect of the substituent on
the 195Pt NMR chemical shift in the octupolar compounds is about as large as in the
stilbene-Pt pincers, while UV/Vis bands of the octupolar compounds are only slightly
red-shifted. This indicates that there is only a limited electronic interaction between
the individual branches of 1-3. Furthermore, the fluorescence properties (Stokes shift,
quantum yields and lifetimes) of 1-3 also are similar to those of the stilbenoid Pt
pincer compounds. This can be explained by formation of a dipolar excited state,
which is localized on one of the three branches. However, energy transfer between the
three branches may occur.
Hyperpolarizabilities
β of the octupolar compounds are however higher than those of
Pt-stilbene compounds. In this respect, the octupolar nature of the electronic system
really makes a difference; the reported values are among the highest for
organometallic compounds. It is furthermore of interest that the highest
hyperpolarizability was found for the compound lacking a donor or acceptor group at
the central core, which supports the idea that dispersion rather than charge transfer is
important in the NLO-behaviour of C3 symmetric compounds.
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All in all, it can be concluded that the introduction of the NCN-pincer platinum
fragment as a substituent led to the development of new organometallic octupolar
systems with attractive NLO properties.
EXPERIMENTAL
General: All reactions involving air- or moisture-sensitive reagents were performed
by standard Schlenk techniques unless stated otherwise. Toluene, pentane, THF and
Et2O were distilled from Na/benzophenone, CH2Cl2 was distilled from CaH2 and
triethylamine was distilled from KOH prior to use. The platinum precursor
[PtCl(NCN-CHO-4)], and the para-substituted diethyl benzylphosphonates 8 were
prepared according to published procedures (see text).11,38 All other reagents were
1
13
obtained commercially and used without further purification. H and C{1H} NMR
spectra were recorded at 25 °C on Bruker AC 300 NMR or Varian Inova 300
spectrometers (operating at 300 and 75 MHz, respectively), chemical shifts are
depicted in ppm and referenced to residual solvents resonances. 195Pt{1H} NMR
spectra were recorded on a Varian Inova 300 MHz NMR spectrometer (operating at
64.4 MHz) and referenced to external Na2PtCl6 (1 M in D2O, δ = 0 ppm).39
Elemental analyses were performed by Kolbe, Mikroanalytisches Laboratorium
(Mülheim, Germany). ES-MS spectra were obtained from the Biomolecular Mass
Spectrometry Department of the Utrecht University. Infra-red spectra were recorded
with a Perkin Elmer Spectrum one FT-IR spectrometer. UV spectra were collected on
Cary 1 or Cary 5 spectrophotometers in spectrophotometric grade solvents.
Fluorescence emission spectra were obtained on a Spex Fluorolog spectrofluorimeter,
equipped with a Spex 1680 double excitation monochromator, a Spex 1681 emission
monochromator and a Spex 1911F detector. Fluorescence spectra were corrected for
the detector spectral response with the aid of a correction file provided by the
manufacturer. Fluorescence quantum yields were determined relative to 9,10-
diphenylanthracene (Φfl = 0.90, excitation wavelength 370 nm).40 Solvents used for
fluorescence measurements were of spectrophotometric grade (Acros).
Lifetime measurements were performed by using a Pico Quant PDL 800-B laser as
the excitation source (λexc= 406 nm, 55 ps pulse width, 2.5- 40 MHz repetition rate).
The luminescence was collected through a focusing lens, filtered through a crossed
polarizer and a combination of suitable optical cut-off filters, dispersed by a 0.1 m
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ACCEPTED MANUSCRIPT
monochromator (1350 lines/mm grating, blazed at 500 nm) and detected by a fast
Hamamatsu photo-multiplier tube (PMT) (H5738P-01). The PMT signal was
amplified by an inverting preamplifier (PAM-102-T – PicoQuant) and used as the
start input for a Time Harp 200 multi-channel computer card, which was
synchronized with the laser pulse via the stop input. The decay curves were obtained
by time correlated single photon counting (TCSPC) via time-to-amplitude conversion
(TAC). The ratio of stop to start pulses was kept low (below 0.04) to assure good
statistics. The instrument response function (IRF) was measured by using a diluted
suspension of silica particles (LUDOX) as the scattering medium and the same
experimental conditions used for the fluorescence decay measurements. The raw data
was deconvoluted from the IRF and analyzed by a fluorescence decay analysis
software (Fluofit 3.3). The instrumental resolution was 30 ps (10% of the IRF
FWHM).
Accurate hyperpolarizabilities
β were determined by hyper-Rayleigh scattering at 800
nm as described elsewhere.41 Individual samples in dichloromethane were measured
at five different concentrations and different modulation frequencies, in order to
exclude contributions of multiphoton fluorescence from the nonlinear scattering
signal.
General procedure octupolar pincer compounds 1–3:
In a dry Schlenk tube, [PtCl(NCN-CHO-4)] (10) (3 equiv.) and the appropriate
benzylphosphonate ester 9a–c (1 equiv.) were dissolved in dry degassed THF (10–15
mL). While stirring, t-BuOK (6 equiv.) was added to the reaction mixture, under a
nitrogen outflow, which directly caused a strong color change from light yellow to
orange of the reaction mixture. After stirring at RT (3h), the mixture was quenched at
0 °C by the subsequent addition of ice and an aqueous NaCl solution (to prevent
halogen abstraction on the Pt center). The formed precipitate was isolated by filtration
through a glass filter. The residue was dissolved in dichloromethane and dried using
MgSO4, filtered, and evaporated to leave the crude product. The crude product was
purified by precipitation out of a small amount of dichloromethane by the addition of
pentane, and isolated after centrifugation.
1,3,5-Tri-OMe-2,4,6-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]-
benzene (1): 1,3,5-Trimethoxy-2,4,6-tris[(diethoxyphosphoryl)methyl]benzene 9a
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(92 mg, 0.15 mmol), 10 (200 mg, 0.44 mmol). Product 1 was isolated as a light-
yellow powder (196 mg, 0.13 mmol, 87%). 1H NMR (300 MHz, CD2Cl2):
δ = 7.47 (d,
3
3J(H,H) = 16.7 Hz, 3H; trans CH=CH), 7.18 (d, J(H,H) = 16.5 Hz, 3H; trans
3
CH=CH), 7.05 (s, 6H; ArH), 4.06 (s, J(H,Pt) = 38.2 Hz, 12H; NCH2), 3.70 (s, 9H,
OCH3), 3.06 (s, 3J(H,Pt) = broad, 36H; NCH3); 13C{1H} NMR (75 MHz, CD2Cl2):
156.6, 146.8 (Cipso), 144.4, 134.7, 134.5, 122.4, 117.8, 117.0, 78.0 (NCH2), 60.3
(OMe), 54.6 (NCH3); 195Pt{1H} NMR (64 MHz, CD2Cl2):
= –3158 ; IR (ATR):
δ =
δ
=
3464, 2981, 2918, 1622, 1585, 1450, 1398, 1334, 1300, 1270, 1235, 1189, 1141,
1098, 1004, 970, 945, 879, 835, 706, 668 cm–1; MS (ES+; CH2Cl2) m/z : 1504.40
[M+H]+; elemental analysis calcd (%) for C51H69Cl3N6O3Pt3 (1505.72): C 40.68, H
4.62, N 5.58; found: C 40.76, H 4.65, N 5.43.
1,3,5-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene (2):
1,3,5-
Tris[(diethoxyphosphoryl)methyl]benzene 9b (66 mg, 0.12 mmol), 10 (200 mg, 0.44
mmol). Product 2 was isolated as an off-white powder (162 mg, 0.11 mmol, 92%). 1H
3
NMR (300 MHz, CD2Cl2):
δ
= 7.51 (s, 3H; ArH), 7.13 (d, J(H,H) = 16.7 Hz, 3H;
3
trans CH=CH), 7.07 (d, J(H,H) = 16.7 Hz, 3H; trans CH=CH), 7.04 (s, 6H; ArH),
3
3
4.06 (s, J(H,Pt) = 37.3 Hz, 12H; NCH2), 3.06 (s, J(H,Pt) = broad, 36H; NCH3);
13C{1H} NMR (75 MHz, CD2Cl2):
= 147.3 (Cipso), 144.4, 139.1, 133.0, 130.8,
125.6, 123.0, 118.0, 77.9 (NCH2), 54.6 (NCH3); 195Pt{1H} NMR (64 MHz, CD2Cl2):
= –3154; IR (ATR): = 3456, 3012, 2916, 1625, 1579, 1450, 1399, 1337, 1270,
δ
δ
1230, 1179, 1140, 1085, 1014, 958, 870, 839, 708, 689 cm–1; MS (ES+; CH2Cl2) m/z :
1415.37 [M+H]+; elemental analysis calcd (%) for C48H63Cl3N6Pt3 (1415.65): C
40.72, H 4.49, N 5.94; found: C 40.88, H 4.43, N 5.79.
1,3,5-Tri-Br-2,4,6-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene
(3): 1,3,5-Tribromo-2,4,6-tris[(diethoxyphosphoryl)methyl]benzene 9c (90 mg, 0.12
mmol), 10 (166 mg, 0.37 mmol). Product 3 was isolated as a light-yellow powder 145
mg (0.09 mmol, 75%). 1H NMR (300 MHz, CD2Cl2):
δ
= 7.04 (s, 6H; ArH), 6.94 (d,
3
3J(H,H) = 16.5 Hz, 3H; trans CH=CH), 6.68 (d, J(H,H) = 16.5 Hz, 3H; trans
3
3
CH=CH), 4.06 (s, J(H,Pt) = broad, 12H; NCH2), 3.06 (s, J(H,Pt) = broad, 36H;
13
NCH3); C{1H} NMR (75 MHz, CD2Cl2):
δ = 148.1 (Cipso), 144.5, 140.3, 138.2,
131.9, 125.9, 123.6, 118.1, 77.9 (CH2N), 54.6 (NCH3); 195Pt{1H} NMR (64 MHz,
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CD2Cl2):
δ
= –3149; IR (ATR): = 3463, 2980, 2914, 1626, 1585, 1449, 1399, 1339,
1295, 1270, 1230, 1086, 1014, 959, 875, 838, 668 cm–1; MS (ES+; CH2Cl2) m/z :
1653.08 [M+H]+; elemental analysis calcd (%) for C48H60Br3Cl3N6Pt3 (1652.34): C
34.89, H 3.66, N 5.09; found: C 34.75, H 3.76, N 4.96.
Acknowledgment
Erwin van der Geer is acknowledged for supplying of 1,3,5-tribromomesitylene (7).
This work was partially supported (GDB) by the Council for Chemical Sciences of
the Netherlands Organization for Scientific Research (NWO/CW).
Supplementary data
NMR data of compounds 1 – 3.
References
[1] E. Cariati, M. Pizzotti, D. Roberto, F. Tessore and R. Ugo, Coord. Chem. Rev.
250 (2006) 1210-1233.
[2] Molecular Organometallics for Optics, H. L. Le Bozec and V. Guerchais, (Eds.),
Top. Organometal. Chem. (2010) 28.
[3] O. Maury and H. L. Bozec, Metal-Based Quadratic Nonlinear Optical Materials,
in Molecular Materials, D. W. Bruce, D. O’Hare and R. I. Walton (Eds.), Wiley
(2010) Chapter 1.
[4] J. P. Morrall, G. T. Dalton, M. G. Humphrey and M. Samoc, Organotransition
Metal Complexes for Non-Linear Optics, in Adv. Organometal. Chem. 55 (2008) 61.
[5] V. Mamane, I. Ledoux-Rak, S. Deveau, J. Zyss and O. Riant, Synthesis (Stuttgart)
(2003) 455-467.
[6] G. D. Batema, M. Lutz, A. L. Spek, C. A. van Walree, C. De Mello Donegá, A.
Meijerink, R.W. A. Havenith, J. Pérez-Moreno, K. Clays, M. Büchel, A. van Dijken,
D. L. Bryce, G. P. M. van Klink and G. van Koten, G., Organometallics 27 (2008)
1690–1701.
[7] R. C. Evans, P. Douglas and C. J. Winscom, Coord. Chem. Rev. 250 (2006) 2093-
2126.
[8] B. Yin, F. Niemeyer, J. A. G. Williams, J. Jiang, A. Boucekkine, L. Toupet, H. Le
Bozec and V. Guerchais, Inorg. Chem. 45 (2006) 8584-8596.
[9] H. Meier, Angew. Chem., Int. Ed. Engl. 44 (2005) 2482-2506.
[10] J. M. Cole, Philosophical Transactions of the Royal Society of London Series a-
Mathematical Physical and Engineering Sciences, 361 (2003) 2751-2770.
17

ACCEPTED MANUSCRIPT
[11] G. D. Batema, K. T. L. van de Westelaken, J. Guerra, M. Lutz, A. L. Spek, C. A.
van Walree, C. de Mello Donegá, A. Meijerink, G. P. M. van Klink and G. van Koten,
Eur. J. Inorg. Chem. (2007) 1422-1435.
[12] G. D. Batema, M. Lutz, A. L. Spek, C. A. van Walree, G. P. M. van Klink and G.
van Koten, Dalton Trans. 43 (2014) 12200–12209.
[13] B. R. Cho, K. Chajara, H. J. Oh, K. H. Son and S. J. Jeon, Org. Lett. 4 (2002)
1703-1706.
[14] S. A. Soomro, A. Schulz and H. Meier, Tetrahedron 62 (2006) 8089-8094.
[15] H. Meier, E. Karpuk and H. C. Holst, Eur. J. Org. Chem. (2006) 2609-2617.
[16] F. Castet, M. Blanchard-Desce, F. Adamietz, Y. M. Poronik, D. T. Gryko and V.
Rodriguez, ChemPhysChem. 15 (2014) 2575-2581.
[17] S. van Cleuvenbergen, I. Asselberghs, E. M. García-Frutos, B. Gómez-Lor, K.
Clays and J. Pérez-Moreno, J. Phys. Chem. C 116 (2012) 12312–12321.
[18] C. E. Powell and M. G. Humphrey, Coord. Chem. Rev. 248 (2004) 725-756.
[19] I. Jung, T. W. Lee, S. O. Kang and J. J. Ko, Synthesis (Stuttgart) (2005) 986-992.
[20] M. P. Cifuentes, C. E. Powell, J. P. Morrall, A. M. McDonagh, N. T. Lucas, M.
G. Humphrey, M. Samoc, S. Houbrechts, I. Asselberghs, K. Clays, A. Persoons and
T. Isoshima, J. Am. Chem. Soc. 128 (2006) 10819-10832.
[21] J. Sun and Y. Han, RSC Advances 7 (2017) 22855-22859.
[22] E. Diez-Barra, J. C. Garcia-Martinez, S. Merino, R. del Rey, J. Rodriguez-Lopez,
P. Sanchez-Verdu and J. Tejeda, J. Org. Chem. 66 (2001) 5664-5670.
[23] A. Vacca, C. Nativi, M. Cacciarini, R. Pergoli and S. Roelens, J. Am. Chem. Soc.
126 (2004) 16456-16465.
[24] A. W. van der Made and R. H. van der Made, J. Org. Chem. 58 (1993) 1262-
1263.
[25] P. R. Ashton, D. W. Anderson, C. L. Brown, A. N. Shipway, J. F. Stoddart and
M. S. Tolley, Chem. Eur. J. 4 (1998) 781-795.
[26] L. Engman and J. S. E. Hellberg, J. Organomet. Chem. 296 (1985) 357-366.
[27] S. M. Ma and B. K. Ni, J. Org. Chem. 67 (2002) 8280-8283.
[28] G. Mehta and P. Sarma, Tetrahedron Lett. 43 (2002) 9343-9346.
[29] H. Meier, Angew. Chem., Int. Ed. Engl. 31 (1992) 1399-1420.
[30] W. Verbouwe, L. Viaene, M. van der Auweraer, F. C. De Schryver, H.
Masuhara, R. Pansu and J. Faure, J Phys Chem A. 101 (1997) 8157-8165.
[31] S. H. Lee, J. R. Park, M. Y. Jeong, H. M. Kim, S. J. Li, J. Song, S. Ham, S. J.
Jeon and B. R. Cho, ChemPhysChem 7 (2006) 206-212.
[32] W. Verbouwe, M. van der Auweraer, F. C. de Schryver, P. J. Warman, J. Am.
Chem. Soc. 120 (1998) 1319-1324.
[33] B. R. Cho, S. J. Lee, S. H. Lee, K. H. Son, Y. H. Kim, J. Y. Doo, G. J. Lee, T. I.
Kang, Y. K. Lee, M. H. Cho and S. J. Jeon, Chem. Mater. 13 (2001) 1438-1440.
[34] K. Clays and A. Persoons, Phys. Rev. Lett. 66 (1991) 2980-2983.
[35] K. Clays and A. Persoons, Rev. Sci. Instrum. 63 (1992) 3285-3289.
[36] J. L. Oudar and D.S.Chemla, J. Chem. Phys. 66 (1977) 2664-2668.
[37] C. G. Claessens, D. González-Rodríguez, M. Rodriguez-Morgade, A. Medina
and T. Torres, Chem. Rev. 114 (2014) 2192-2277.
[38] G. Rodríguez, M. Albrecht, J. Schoenmaker, A. Ford, M. Lutz, A.L. Spek and G.
van Koten, J. Amer. Chem. Soc. 124 (2002) 5127-5138.
[39] P. S. Pregosin, Coord. Chem. Rev. 44 (1982) 247-291.
[40] D. F. Eaton, Pure Appl. Chem. 60 (1988) 1107-1114.
[41] G. Olbrechts, R. Strobbe, K. Clays, A. Persoons, Rev. Sci. Instrum. 69 (1998)
2233-2241.
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Highlights
-Efficient, single step post-modification of parent [PtCl(NCN-CHO-4)] with several 1,3,5-
tris[(diethoxyphosphoryl)]methyl]benzene derivatives leads to the formation of a series of
organometallic octupolar title compounds.
- There is only limited electronic interaction between the individual branches of 1-3 pointing
to the formation of a dipolar excited state, which is localized on one of the three branches.
-Hyperpolarizabilities
β of the octupolar compounds are higher than those of the parent Pt-
stilbene compounds, i.e. the octupolar nature of the electronic system really makes a
difference
providing β-values that are among the highest for organometallic compounds.