Synthesis of new Fe(II) and Ru(II) η5-monocyclopentadienyl compounds showing significant second order NLO properties

Article abstract of DOI:10.1016/j.jorganchem.2013.02.028

A series of new ruthenium(II) complexes of the general formula [Ru(η⁵-C₅H₅)(PP)(L)][PF₆] (PP = DPPE or 2PPh₃, L = 4-butoxybenzonitrile or N-(3-cyanophenyl) formamide) and the binuclear iron(II) comple

Full text of DOI:10.1016/j.jorganchem.2013.02.028

Accepted Manuscript
Synthesis of new Fe(II) and Ru(II) η5-monocyclopentadienyl compounds showing
significant second order NLO properties
Andreia Valente, Sophie Royer, Milan Narendra, Tiago J.L. Silva, Paulo J.G. Mendes,
M. Paula Robalo, Manuel Abreu, Jürgen Heck, M. Helena Garcia
PII:
S0022-328X(13)00157-5
10.1016/j.jorganchem.2013.02.028
JOM 17903
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 November 2012
Revised Date: 15 February 2013
Accepted Date: 21 February 2013
Please cite this article as: A. Valente, S. Royer, M. Narendra, T.J.L. Silva, P.J.G. Mendes, M.P.
Robalo, M. Abreu, J. Heck, M.H. Garcia, Synthesis of new Fe(II) and Ru(II) η5-monocyclopentadienyl
compounds showing significant second order NLO properties, Journal of Organometallic Chemistry
(2013), doi: 10.1016/j.jorganchem.2013.02.028.
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ACCEPTED MANUSCRIPT
750 nm
1500 nm

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New organometallic Ru(II) and Fe(II) compounds revealed values in the 95-618×10-
30esu range by Hyper-Rayleigh Scattering (HRS) measurements at 1500 nm.

ACCEPTED MANUSCRIPT
Synthesis of new Fe(II) and Ru(II) η5-monocyclopentadienyl compounds showing
significant second order NLO properties
Andreia Valentea, Sophie Royerb, Milan Narendraa, Tiago J. L. Silvaa,c, Paulo J. G.
Mendesc, M. Paula Robalob,d, Manuel Abreue, Jürgen Heckf, M. Helena Garciaa*
aCentro de Ciências Moleculares e Materiais, Faculdade de Ciências da Universidade de Lisboa,
Campo Grande, 1749-016 Lisboa, Portugal
bCentro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1049-
001 Lisboa, Portugal
cCentro de Química de Évora, Universidade de Évora, Rua Romão Ramalho 59, 7002-554 Évora,
Portugal
dÁrea Departamental de Engenharia Química, Instituto Superior de Engenharia de Lisboa, Av.
Conselheiro Emídio Navarro, 1, 1959-007 Lisboa, Portugal
eLaboratorio de Óptica, Lasers e Sistemas, Departamento de Física, Faculdade de Ciências da
Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
fInstitut für Anorganische und Angewandte Chemie, Universität Hamburg, Martin-Luther- King-
Platz 6, D-20146 Hamburg
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Abstract
A series of new ruthenium(II) complexes of the general formula
[Ru(η5-
C5H5)(PP)(L)][PF6] (PP = DPPE or 2PPh3, L = 4-butoxybenzonitrile or N-(3-
cyanophenyl)formamide) and the binuclear iron(II) complex [Fe(η5-C5H5)(PP)(ꢀ-
L)(PP)(η5-C5H5)Fe][PF6]2 (L = (E)-2-(3-(4-nitrophenyl)allylidene)malononitrile), that
has been also newly synthesized) have been prepared and studied to evaluate their
potential in the second harmonic generation property. All the new compounds were fully
characterized by NMR, IR and UV-Vis spectroscopies and their electrochemistry
behaviour was studied by cyclic voltammetry. Quadratic hyperpolarizabilities (β) of three
of the complexes have been determined by hyper-Rayleigh scattering (HRS)
measurements at fundamental wavelength of 1500 nm and the calculated static β0 values
are found to fall in the range 65 - 212 × 10-30 esu. Compound presenting β0 = 212 × 10-
30
esu has revealed to be 1.2 times more efficient than urea standard in the second
harmonic generation (SHG) property, measured in the solid state by Kurtz powder
technique, using a Nd:YAG laser (1064 nm).
Keywords
Cyclopentadienyl complexes; Quadratic hyperpolarizabilities; Hyper-Rayleigh scattering;
Kurtz powder technique; Second-Order Nonlinear Optical
1. Introduction
There is current research interest in the development of second-order nonlinear optical
(NLO) materials exhibiting large first hyperpolarizabilities β because of their applications
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in laser frequency conversion, optical parameter oscillators and signal communication and
the inherent employ in electrooptic devices.[1-9] The most intensively studied NLO
chromophores are based in highly polarizable conjugated backbones presenting an electron
donor and an acceptor group attached to both ends of the backbone in order to create an
asymmetric “push-pull” system. Coordination of such systems to organometallic moieties
bring additional possibilities to enhance high hyperpolarizabilities due to the occurrence of
low energy ligand to metal or metal to ligand charge transfer. In this frame, the
organometallic moiety forms an alternative type of donor or acceptor group for the
traditional push-pull system. Additional advantages of complexes are related with the
variation of coligands that can fine tune the energy of these charge transfer transitions
besides the introduction of other variables related with size, nature and redox ability of the
transition metal.
Our approach in the search of organometallic molecular materials with strong NLO
properties has been based on “MCp” piano-stool structures where the chromophores are
linked to the metal centre (FeII, RuII, NiII and CoIII ) by functional groups such as nitrile
(N≡C) or acetylide (C≡C) that allow interactions of the suitable metal d orbitals with the
two π sets of orthogonal π and π* orbitals of the functional group, leading consequently to
an extension of the π-electron system between the metal and the terminal donor/acceptor
substituting group of the ligand. Experimental values of the hyperpolarizabilities obtained
by Hyper Rayleigh Scattering measurements were corroborated by our theoretical
calculations, showing that “FeCp” and “RuCp” were the best partners for this kind of
interaction, behaving more efficiently as electron donors than the traditional donor groups
(such as alkyl substituted amine groups).[10] Within this prospective to search for a large
NLO response in molecular materials, we continue to explore the field and we report here
our recent results concerning four new compounds. As new approaches for structural
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diversity and architectural flexibility we introduced a new structure designed to reinforce
the π-backdonation effect through the pumping of electron density from two iron centres
toward the same chromophore, and we also used the less classical chromophore N-(3-
cyanophenyl)formamide.
The first hyperpolarizabilities (β) of the new compounds were measured by Hyper-
Rayleigh Scattering with a laser emitting at 1500 nm. The NLO properties of the
compound presenting the best static first hyperpolarizability (β0) value was also studied
in the solid state, by Kurtz powder technique, with a Nd:YAG laser emitting at 1064 nm.
2. Experimental
2.1. General procedures
All the experiments were carried out under dinitrogen atmosphere using standard Schlenk
techniques. All the solvents used were dried using standard methods.[11] Starting
materials [M(η5-C5H5)(LL)X] (LL
=
2PPh3 or DPPE, ethane-1,2-diylbis-
(diphenylphosphane), and X = Cl when M = Ru; LL = DPPE and X = I when M = Fe)
were prepared following the methods described in the literature.[12,13] Compound N-(3-
cyanophenyl)formamide (L2) was synthesised using an alternative method (see below) to
the one published in [14]. Compound 4-butoxybenzonitrile (L1) was purchased and used
without further purification. FT-IR spectra were recorded in a Mattson Satelite FT-IR
1
13
spectrophotometer with KBr pellets; only significant bands are cited in text. H C and
31P NMR spectra were recorded on a Bruker Avance 400 spectrometer at probe
temperature. The 1H and 13C NMR chemical shifts are reported in parts per million (ppm)
31
downfield from internal Me4Si and the P NMR spectra are reported in ppm downfield
from external standard, 85% H3PO4. Phase sensitive NOESY with gradients was
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performed with a mixing time of 1.5 s at 45 ºC. Elemental analyses were obtained at
Laboratório de Análises, Instituto Superior Técnico, using a Fisons Instruments EA 1108
system. Data acquisition, integration and handling were performed using a PC with the
software package EAGER-200 (Carlo Erba Instruments). Electronic spectra were
recorded at room temperature on a Jasco V-560 spectrometer in the range of 200-900 nm.
2.2. Organic and organometallic synthesis
Synthesis of N-(3-cyanophenyl)formamide, L2
Compound N-(3-cyanophenyl)formamide was prepared by direct reaction of 3-
cyanophenyl isocyanate with water in a yield of 95%.
IR (KBr, cm-1 ν(N-H stretch) 3367; ν(C≡N, stretch) 2242; ν(C-H, aldehyde stretch) 2800
1
and 2750; ν(C=O, stretch) 1750. H NMR [(CD3)2CO, Me4Si, δ/ppm]: 8.57 [s, 1, COH];
8.04 [s, 1, H3]; 7.74 [d, 1, H5]; 7.50 [t, 1, H6]; 7.39 [d, 1, H7]. 13C NMR [(CD3)2CO,
δ/ppm]: 153.1 (COH); 141.4 (C4); 130.9 (C6); 126.7 (C7); 123.9 (C5); 122.4 (C3); 119.3
(C1); 113.5 (C2). UV-Vis in CH2Cl2, λmax/nm (ε/M-1cm-1): 273 (227504).
Synthesis of (E)-2(3-(4-nitrophenyl)allylidene)malononitrile), L3
The compound was synthesized by reaction of (E)-3-(4-nitrophenyl)prop-2-enal (0.02
mol) with malononitrile (0.02 mol) in methanol (40 mL) in the presence of piperidine
(0.2 mL), during 2 h at room temperature. The product was filtrated, washed with water
and dried under vacuum to give an orange solid product. Yield: 57%.
IR (KBr, cm-1): ν(NO2, aromatic) 1516 + 1344; ν(C≡N, stretch) 2225; ν(C=C stretch
1
conjugated) 1613. H NMR [(CD3)2CO, Me4Si, δ/ppm]: 8.32 [d, 2, H9+H11]; 8.17 [d, 1,
H4]; 8.08 [d, 2, H8+H12]; 7.73 [d, 1, H6]; 7.53 [dd, 1, H5]. 13C NMR [(CD3)2CO, δ/ppm]:
161.1 (C4); 149.9 (C7); 148.1 (C6); 141.4 (C10); 130.7 (C8+C12); 127.0 (C5); 125.0
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(C9+C11); 114.3+112.3 (C1+C2); 85.7 (C3). UV-Vis in CH2Cl2, λmax/nm (ε/M-1cm-1): 268
(4732), 351 (17528). Anal. Calc. for C12H7N3O2: C, 64.00; H, 3.13; N, 18.66. Found: C,
64.32; H, 3.57; N, 17.08.
Preparation of ruthenium(II) complexes, [Ru(η5-C5H5)(PP)(NCR)][PF6]
Complexes [Ru(η5-C5H5)(PP)(NCR)][PF6] were prepared by halide abstraction from the
parent neutral complexes [Ru(η5-C5H5)(PP)X] (0.5 mmol) with TlPF6 (0.51 mmol) in
dichloromethane, in the presence of a slight excess (0.51 mmol) of the ligands, L1 and
L2, at reflux for 6 h under inert atmosphere. After cooling to room temperature, filtering
and removing the solvent, the complexes were washed with n-hexane (2 x 15 mL) and
recrystallized from dichloromethane / n-hexane, giving crystalline yellow products.
[Ru(η5-C5H5)(PPh3)2(L1)][PF6], 1Ru
Yellow; recrystallized from dichloromethane / n-hexane; 70 % yield; IR (KBr, cm-1):
-
1
ν(C≡N) absent, ν(PF6 ) 840 and 560. H NMR [(CD3)2CO, Me4Si, δ/ppm]: 7.46 [m, 6,
PPh3]; 7.36 [m, 14, PPh3 + H3 + H7]; 7.25 [m, 12, PPh3]; 7.08 [d, 2, H4 + H6]; 4.73 [s, 5,
Cp]; 4.11 [t, 2, H8]; 1.77 [m, 2, H9]; 1.49 [m, 2, H10]; 0.96 [t, 3, H11]. 13C NMR
[(CD3)2CO, δ/ppm]: 164.3 (C5), 136.9-136.5 (Cq, PPh3), 135.3 (C3+C7), 134.3 (Corto),
131.7 (C1), 131.0 (Cpara), 129.3 (Cmeta), 116.4 (C4 + C6), 103.4 (C2), 84.8 (Cp), 69.1 (C8),
31
-
31.7 (C9), 19.7 (C10), 14.0 (C11). P NMR [(CD3)2CO, δ/ppm]: 41.6 [PPh3]; -144.2 [PF6
]. UV-Vis in CH2Cl2, λmax/nm (ε/M-1cm-1): 301 (26035), 374 (sh). UV-Vis in acetone,
λmax/nm (ε/M-1cm-1): 367 (sh). UV-Vis in DMSO, λmax/nm (ε/M-1cm-1): 302 (19068),
370 (sh). Anal. Calc. for C52H48P3F6NORu: C, 61.78; H, 4.79; N, 1.39. Found: C, 61.4;
H, 4.70; N, 1.30.
[Ru(η5-C5H5)(DPPE)(L1)][PF6], 2Ru
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Yellow; recrystallized from dichloromethane / n-hexane; 70 % yield; IR (KBr, cm-1):
-
1
ν(C≡N) absent, ν(PF6 ) 840 and 560. H-NMR [(CD3)2CO, Me4Si, δ/ppm]: 8.03 [m, 4,
dppe]; 7.59 [m, 6, dppe]; 7.50 [m, 10, dppe]; 6.84 [d, 2, H3+H7]; 6.59 [d, 2, H4+H6]; 5.00
[s, 5, Cp]; 4.00 [t, 2, H8]; 2.80 [m, 4, dppe]; 1.71 [m, 2, H9]; 1.43 [m, 2, H10]; 0.92 [t, 3,
H11]. 13C NMR [(CD3)2CO, δ/ppm]: 164.0 (C5), 138.0 (Cq, dppe), 134.8 (C4+C6), 134.2-
129.8 (CH, dppe), 127.9 (C1), 115.9 (C3+C7), 103.0 (C2), 83.0 (Cp), 69.0 (C8), 31.7 (C9),
28.7-28.2 (-CH2-, dppe), 19.7 (C10), 14.0 (C11). 31P-NMR [(CD3)2CO, d/ppm]: 79.35
-
[dppe]; -144.23 [PF6 ]. UV-Vis in CH2Cl2, λmax/nm (ε/M-1cm-1): 287 (22941), 323 (sh).
UV-Vis in acetone, λmax/nm (ε/M-1cm-1): no bands are observed. UV-Vis in DMSO,
λmax/nm (ε/M-1cm-1): 292 (17103), 329 (sh). Anal. Calc. for C43H42P3F6NORu: C, 57.59;
H, 4.72; N, 1.56. Found: C, 56.6; H, 4.70; N, 1.50.
[Ru(η5-C5H5)(PPh3)2(L2)][PF6], 3Ru
Yellow; recrystallized from dichloromethane / n-hexane; 50 % yield; IR (KBr, cm-1):
ν(N-H, stretch) 3392; ν(C≡N, stretch) 2233; ν(C-H, aldehyde stretch) 2800 and 2750;
-
ν(C=O, stretch) 1750; ν(PF6 ) 840 and 560. 1H NMR [(CD3)2CO, Me4Si, δ/ppm]: 8.0 [s, 1
H3]; 7.7 [d, 1, H5], 7.5-7.4 [m, 7, H6+PPh3]; 7.37 [m, 12, PPh3]; 7.25 [m, 12, PPh3]; 7.07
[d, 1, H7]; 4.8 [s, 5, Cp]. 13C NMR [(CD3)2CO, δ/ppm]: 154.4 (C9); 141.4 (C4); 136.8-
136.3 (Cq, PPh3), 134.2 (Corto), 131.1 (Cpara), 131.0 (C6), 129.4 (Cmeta), 127.3 (C7), 124.3
(C5), 121.5 (C3), 112.8 (C2); 84.8 (Cp); CN is overlapped by phosphane signals. 31P-NMR
-
[(CD3)2CO, δ/ppm]: 41.40 [s, PPh3]; -144,24 [sept, PF6 ]. UV-Vis in CH2Cl2, λmax/nm
(ε/M-1cm-1): 315 (16038). UV-Vis in acetone, λmax/nm (ε/M-1cm-1): no bands are
observed. UV-Vis in DMSO, λmax/nm (ε/M-1cm-1): 317 (12910). Anal. Calc. for
C49H41P3F6N2ORu.⅓CH3(CH2)4CH3: C, 60.61; H, 4.55; N, 2.77. Found: C, 60.1; H, 4.6;
N, 2.8.
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Preparation of complex [Fe2(η5-C5H5)2(DPPE)2(L3)][2PF6], 1Fe
[Fe2(η5-C5H5)2(DPPE)2(L3)][2PF6] was prepared by halide abstraction from the parent
neutral complex [Fe(η5-C5H5)(DPPE)I] (0.5 mmol) with TlPF6 (1.01 mmol) in
dichloromethane, in the presence of a slight excess (0.51 mmol) of the ligand L3, at
room temperature for 6 h under inert atmosphere. After cooling to room temperature,
filtering and removing the solvent, the complex was washed with n-hexane (2 x 15 mL)
and recrystallized from dichloromethane / n-hexane, giving a crystalline dark blue
-
product in 50 % yield; IR (KBr, cm-1): ν(C≡N, stretch) 2177, ν(PF6 ) 840 and 560. (m);
557
(m);
1H-NMR
[(CD3)2CO,
Me4Si,
δ/ppm]:
8.31-7.29
[m,
44,
3
3\
dppe1+2+H8+H9+H11+H12]; 7.01 [d, 1, H6; J56 = 16 Hz]; 6.14 [d, 1, H4; J45 = 12 Hz],
3
5.27 [dd, 1, H5; 3J56 = 16 Hz; J45 = 12 Hz]; 4.67 [s, 5, Cp2]; 4.57 [s, 5, Cp1]; 3.09-2.60
[m, 8, dppe1+2]. 13C NMR [(CD3)2CO, δ/ppm]: 157.2 (C4), 149.9 (C), 145.9 (C6),
141.4(C), 137.2 (Cq, dppe), 134.2-130.2 (-CH, dppe1+2), 125.7 (C5), 125.1 (-CH, dppe1),
82.4 (Cp1), 82.0 (Cp2), 28.75+28.55(-CH2-, dppe1+2), CN is overlapped by phosphane
signals. 31P-NMR [(CD3)2CO, δ/ppm]: 95.96 [s, dppe]; 94.49 [s, dppe]; -144.42 [sept,
-
PF6 ]. UV-Vis in CH2Cl2, λmax/nm (ε/M-1cm-1): 359 (17566), 535 (sh), 628 (6364). UV-
Vis in acetone, λmax/nm (ε/M-1cm-1): 356 (13590), 631 (5294). UV-Vis in DMSO,
λmax/nm
(ε/M-1cm-1):
360
(14113),
646
(6506).
Anal.
Calc.
for
C74H65P6F12N3O2Fe2.CH2Cl2: C, 54.97; H, 4.12; N, 2.56. Found: C, 55.54; H, 4.15; N,
2.49.
2.3. Electrochemical experiments
The electrochemical experiments were performed on an EG&G Princeton Applied
Research Model 273A potentiostat/galvanostat and monitored with a personal computer
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loaded with Electrochemistry PowerSuite v2.51 software from Princeton Applied
Research. Cyclic voltammograms were obtained using 10-3 M solutions of compounds in
CH2Cl2 or CH3CN, and 0.1 M of [NBu4][PF6], with a three-electrode configuration. The
working electrode was a platinum-disk (1.0 mm diameter) probed by a Luggin capillary
connected to a silver-wire pseudo-reference electrode; a Pt wire auxiliary electrode was
employed. The electrochemical experiments were performed under a N2 atmosphere at
room temperature. The redox potentials of the complexes were measured in the presence
of ferrocene as the internal standard and the redox potential values are normally quoted
relative to the SCE by using the ferrocenium/ferrocene redox couple (Ep/2= 0.46 or 0.40 V
vs. SCE for CH2Cl2 or CH3CN, respectively)[15].
The supporting electrolyte was purchased from Aldrich Chemical Co., recrystallized from
ethanol, washed with diethyl ether and dried under vacuum at 110 ºC for 24 h. Reagent
grade acetonitrile and dichloromethane were dried over P2O5 and CaH2, respectively, and
distilled under nitrogen atmosphere before use.
2.4. HRS measurements
β measurements were carried out using the harmonic light scattering technique (also
named Hyper-Rayleigh) in chloroform solutions. The 10-3-10-5 M solutions of the
complexes were placed into a 4 cm long fluorimetric cell, after being carefully filtered
through a 0.2 mm filter in order to eliminate the white light noise resulting from
microburning of any the remaining dust particles by the incoming laser beam. The
measurements were performed at a fundamental wavelength of 1500 nm as described in
[16], using a Q-switched Nd:YAG laser operating in the 10 Hz repetition range. Instead
of the third harmonic (355 nm) generated from an Nd:YAG laser with a wavelength of
1064 nm, the optical parametric oscillator (OPO)[17] in use was pumped with the second
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harmonic (532 nm). The signal intensity at 824 nm and the fundamental at 532 nm were
removed from the Idler using dichroic mirrors, a green-light filter (HR 532) and a silicon
filter (HR 650–850; transparent > 1000 nm). An additional Glan-Taylor polariser ensured
the vertical polarisation of the beam into the measurement cell. The scattered second
harmonic signal is collected at 90° with respect to the direction of the incoming laser
beam. The harmonic photons were detected by a photomultiplier, sampled by a boxcar
and processed by a computer. A rotating a half-wave plate between two crossed
polarizers varied the fundamental intensity. All measurements were carried out using
Disperse Red 1 (DR1) as external standard. The reference hyperpolarisability β of DR1 in
CH2Cl2 was calculated by comparison of the slopes of the standard in CH2Cl2 and CHCl3
to obtain the ratio βsolute[18]. Using the value β(CHCl3) = 80 × 10-30 esu[19] the
hyperpolarisability of DR1 in CH2Cl2 is estimated to be 70 × 10-30 esu. The effect of the
refractive indices of the solvents was corrected using simple Lorentz local field.[20]
Assuming that the scattering contribution from the solvent is negligibly small, this
external reference method is used to calculate the ß values of chromophores according to
eq. 1:
(1)
where S is the slope of the appropriate “I2w vs. concentration” plot and ßref is the
orientational average of the first hyperpolarizability of the reference sample.
2.5. Kurtz powder SHG measurements
The efficiency of SHG was measured using the Kurtz powder method[21] using an
alteration of our preceding experimental set-up published elsewhere[22], presented
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in Figure 1. The powder sample S is located in a proper sample holder where it is it is
exposed to a high-power pulsed laser beam. The sample is on the focus of the parabolic
mirror R which collimates the rear lobule of the SHG light. The collimated beam is
focused by the lens L on the silicon photo detector SPD. The sample holder has a line
filter F of adequate wavelength for SHG in order to cut any leak due to the stray lighting
(originated by the flash lamp), from the fundamental beam or residual florescence from
the laser. Neutral density filters N, in the sample holder, allow control of the intensity of
SHG light hitting the SPD. The SPD signal is measured with a 2 GS s−1 digital
oscilloscope DO which automatically integrates the signal. This integral is proportional to
the SHG efficiency and a quantitative value is extracted by comparing it with its
corresponding value from a reference material (urea) obtained under the same
experimental conditions. The 1064 nm laser pulses are produced directly by the Nd:YAG
laser at low power (110 mJ per pulse), this laser produces 10 ns pulses with a repetition
rate of 4 Hz. Samples used in this experiment were not standardised both in the amount of
test sample and grain size. For this reason signals between individual measurements were
seen to vary in some cases by as much as ± 20%. The material to be measured was
mulled to a fine powder and compacted in a dedicated mount and then installed in the
sample holder. For a proper comparison with the reference material, the measurements
should be averaged over several laser exposure cycles. The signal amplitude from the
SPD is measured by the oscilloscope, which is triggered by the signal itself. The silicon
photo detector positioning and the neutral density filter were adjusted in order to obtain a
good signal to noise ratio and to prevent saturation of the photodiode. The oscilloscope
produces a time integral of the SPD waveform automatically, which is proportional to the
SHG efficiency. The oscilloscope also performs the average over several laser shots
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automatically. The reference sample SHG efficiency measurement is performed under the
same experimental conditions as that of the test sample.
Figure 1. Experimental set-up for SHG measurements by Kurtz powder technique
3. Results and discussion
3.1. Synthesis of the M(II) complexes
Mononuclear complexes of the general formula [Ru(η5-C5H5)(PP)(L)][PF6] with PP =
DPPE or 2PPh3, L = 4-butoxybenzonitrile or N-(3-cyanophenyl)formamide, and the
binuclear complex [Fe(η5-C5H5)(PP)(ꢀ-L)(PP)(η5-C5H5)Fe][PF6]2 with L = (E)-2-(3-(4-
nitrophenyl)allylidene)malononitrile) were prepared, as shown in
Scheme 1, by halide abstraction with TlPF6 from the parent neutral complexes [M(η5-
C5H5)(PP)X] (M = Ru(II), X = Cl; M = Fe(II), X = I), in dichloromethane, in the presence
of a slight excess of the corresponding ligand. The new bidentate ligand (E)-2-(3-(4-
nitrophenyl)allylidene)malononitrile) (L3) was synthesised by reaction of (E)-3-(4-
nitrophenyl)prop-2-enal with malononitrile in the presence of piperidine, at room
temperature.
Scheme 1. Reaction scheme for the synthesis of the complexes [Ru(η5-
C5H5)(PP)(L)][PF6] and[Fe(η5-C5H5)(PP)(ꢀ-L)(PP)(η5-C5H5)Fe][PF6]2 and the structures
of the nitrile ligands numbered for NMR proposes
The reactions were carried out at reflux with exception of 1Fe that was performed by
stirring at room temperature for 6 h, under inert atmosphere. In the case of L3 two
approaches were used in order to obtain both the mononuclear and binuclear iron
compounds. Nevertheless only the binuclear compound 1Fe was obtained as pure
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complex. The several attempts to synthesise the compound [Fe(η5-C5H5)(DPPE)(L3)]PF6
gave mixtures difficult to purify.
The new compounds were recrystallized by slow diffusion of n-hexane in acetone or
dichloromethane solutions, giving crystalline yellow (1Ru, 2Ru and 3Ru) or dark blue
products (1Fe). All the compounds are fairly stable to air and moisture in the solid state
and were obtained in good yields (50-70%). The formulation of all the new compounds is
1
13
31
supported by analytical data, FT-IR, H, C, P NMR spectroscopic data and elemental
analyses. The solid state FT-IR spectra (KBr pellets) of the complexes presented a large
number of bands which identify the presence of the various coligands. Typical bands
were used to confirm the presence of the cyclopentadienyl ligand (ca. 3100-3040 cm-1)
-
and the PF6 anion (840 and 560 cm-1) in all the studied complexes. The disappearance of
the νN≡C was observed upon coordination of L1 to the ruthenium centre. However, in the
case of the coordination of L2 and L3 ligands (the latter presenting the good NO2
acceptor group) to the ruthenium and iron centre, respectively, negative shifts were
observed (-9 cm-1 to 3Ru and -48 cm-1 to 1Fe). These negative shifts are indicative of an
enhanced π-backdonation from the metal d orbitals to the π* orbital of the N≡C group
leading to a decreased N≡C bond order. This effect has been already observed in other
ruthenium and iron related compounds.[23-25]
1H NMR chemical shifts of the cyclopentadienyl ring are displayed in the characteristic
range of monocationic ruthenium(II) and iron(II) complexes. Relatively to the ruthenium
complexes a deshielding of the Cp protons between 0.47 and 0.68 ppm in relation the
[RuCp(PPh3)2Cl] precursor, can be observed. The same effect can be seen for the
phosphane coligands.
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The shielding effect on the nitrile ligand L1 observed upon coordination to the ruthenium
organometallic moiety became more evident for those protons adjacent to the N≡C
functional group, namely H3 and H7 protons (Table 1). Comparing the analogous
complexes 1Ru (presenting PPh3 coligand) and 2Ru, where the PPh3 coligand was
replaced by a better donor coligand, DPPE, this shielding effect was even extended to the
H4 and H6 protons as well. Relatively to 3Ru, where the previous nitrile chromophore L1
was replaced by L2, while maintaining the PPh3 coligand, one can observe a shielding of
about -0.32 ppm on the H7 proton, while the remaining aromatic protons do not suffer any
change (Table 1). This shielding effect of π-backdonation was not observed in the other
aromatic ring protons possibly due to the presence of the donor amine group in the meta
position relative to the nitrile group.
The electron withdrawing effect of the NO2 group in 1Fe clearly causes an electronic
delocalization throughout the coordinated ligand. Comparison of the chemical shifts of
H4, H5 and H6 to those of the respective uncoordinated ligand L3, show that while these
protons, closer to the C≡N group, were severely shielded up to 2.27 ppm, the aromatic
protons (H8, H9, H11 and H12), nearby the NO2 group, were not significantly affected
(peak overlapping with DPPE coligand). The upfield shift observed for H4 to H6 protons
confirms the expected shielding due to an electronic flow from the metal centre towards
the NO2 electron withdrawing group, due to a π-backdonation effect involving the
coordinated nitrile group. This effect of π-backdonation was also confirmed by the N≡C
stretching frequencies on the infrared spectra of this compound, as discussed above.[23-
25]
13C NMR data for this family of compounds confirm the evidence found for proton
spectra. The Cp ring chemical shifts are in the range usually observed for Ru(II) and
Fe(II) cationic derivatives, a significant deshielding (up to ≈ 12 ppm) being observed on
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the carbon of the N≡C functional group upon coordination. All the other carbons of the
chromophore ligand were only slightly deshielded or remained almost unchanged for the
studied compounds.
31P{1H} NMR data of the ruthenium complexes showed a single sharp signal revealing
equivalence of the coordinated phosphorous atoms, with the expected deshielding upon
coordination, in accordance to the σ donor character of these ligands. For 1Fe, two sharp
signals corresponding to the DPPE coligand could be observed, consistent with the
bimetallic character of this complex.
During NMR experiments a dependence on the temperature for complex 1Fe has been
observed (Figure 2). This effect has been attributed to an E/Z isomerisation and it is of
particular interest since the hyperpolarizabilities of nonlinear optical chromophores
depend on their geometrical E/Z isomerism.[26] In addition, when the chromophore
possesses rotable single bonds in the π-conjugated bridge, the influence of the rotation of
these bonds on nonlinearity is significant and the rotational isomers (rotamers) exhibit
different hyperpolarizabilities.[27] Analysing the coupling constants we can conclude
that the predominant species at room temperature is the E-isomer (J = 12-16 Hz).
1
Increasing the temperature one can observe the emerging of new H NMR signals that
can be attributed to the Z-isomer with coupling constants between 8-12 Hz (Figure 2).
Moreover, a NOESY study allowed the observation of the spatial interactions between H4
and H6 protons (Figure 3), corroborating the hypothesis that the E-isomer is the
predominant one.
Table 1. Selected 1H NMR data for compounds [Mn(η5-C5H5)(PP)(L)][PF6]n and the free
ligands
* Peak overlapping with DPPE
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Figure 2. 1H NMR spectrum of 1Fe where the Z-isomer signals can be observed with
increasing temperature
Figure 3. NOESY NMR spectrum for 1Fe, showing spatial interaction between H4 and
H6 protons
3.2. UV-Vis studies
The optical absorption spectra of the studied complexes [Ru(η5-C5H5)(PP)(L)][PF6]
and[Fe(η5-C5H5)(PP)(ꢀ-L)(PP)(η5-C5H5)Fe][PF6]2 and all the ligands were recorded in
10-3 – 10-6 M dichloromethane, acetone and dimethyl sulfoxide solutions (Table 2) in
order to identify MLCT and π-π* absorption bands expected for these complexes.
The electronic spectra of all the compounds showed intense absorption bands in the UV
region, which can be assigned to electronic transitions occurring both in the
organometallic fragment {MCp(PP)}+ (λ ≈ 235-260 nm) and in the coordinated
chromophore (λ ≈ 260 – 450 nm) (Table 2). For compounds 3Ru and 1Fe, evidence was
found for the existence of additional charge transfer (CT) bands. In fact, the iron complex
presented one band compatible with a MLCT nature, this being confirmed by
solvatochromism studies. As can be observed in the electronic spectra the compound 1Fe
(Figure 4) presents one MLCT broad band in DMSO. Changing the solvent polarity by
replacing DMSO for dichloromethane, the enlargement of this band together with the
appearance of one shoulder at higher energy is observed. Moreover, the bathochromic
shift (ῡ= 444 cm-1) in the main transition of this band is also compatible with its charge
transfer nature. Nevertheless in the case of 3Ru only one ILCT band can be observed, in
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spite of the spectroscopic evidence (1H NMR and νNC = -9 cm-1) that could suggest π-
backdonation and therefore the possibility of existence of a MLCT band.
Table 2. Optical spectral data for complexes [Mn(η5-C5H5)(PP)(L)][PF6]n, 1Ru, 2Ru,
3Ru, 1Fe, and the free ligands L1, L2 and L3, in dichloromethane, acetone and dimethyl
sulfoxide
Figure 4. Electronic spectra of complex 1Fe and L3 in CH2Cl2 and 1Fe in DMSO
showing the bathochromic shift for 1Fe
3.3. Electrochemical studies
In order to obtain an insight on the electron richness of the organometallic fragment and
on the coordinated ligands, the electrochemical behavior of Ru(II) and Fe(II) compounds
and also the free chromophores L1, L2 and L3, were studied by cyclic voltammetry in
dichloromethane and acetonitrile, between the limits imposed by the solvents. Studies
carried out at scan rates between 50 and 1000 mVs-1 showed the same results. Table 3
summarizes the electrochemical data obtained for all the studied compounds at room
temperature and at the scan rate of 200 mVs-1.
The ligands L1 and L2 did not present any redox processes. The electrochemistry of L3
in dichloromethane showed two irreversible reductive processes at -0.69 and -0.97 V
respectively and three irreversible oxidative processes at Epa = +0.15, -0.82 and -1.15 V.
The oxidations were found not to be dependent of the reductive waves. The behaviour of
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L3 in acetonitrile is similar showing the reductive processes in the range -0.67 and -1.28
V.
The bimetallic complex 1Fe, in dichloromethane, displayed one quasi-reversible redox
process at the potential expected for the redox Fe(II)/Fe(III) pair with E1/2 = 0.94 V, as
was reported for related compounds[28,29]. Furthermore, two quasi reversible processes
were found at E1/2 ~ 0.08 V and E1/2 = -0.99 V that can be related with ligand centred
redox processes which became more reversible upon coordination to the iron(II) moieties.
In acetonitrile, the complex 1Fe showed decomposition during the electrochemical
experiment, with the formation of the [Fe(η5-C5H5)(DPPE)(NCCH3)]+ species along
time.
Complex 1Ru present, in dichloromethane, one irreversible redox process occurring at
the expected potential for Ru(II) oxidation (Epa = 1.32V) and an irreversible reductive
process at negative potentials Epc = -0.39 V probably due to the coordinated ligand, which
as free molecule did not present any redox process. The electrochemistry run in
acetonitrile only showed one irreversible redox process (Epa = 1.06 V) attributed to
Ru(II)/Ru(III).
2Ru exhibited two redox processes at positive potentials and one irreversible redutive
process at negative potentials in dichloromethane. The first positive redox process is
quasi-reversible (at E1/2 = 1.18 V) when isolated, but the second one is irreversible with
Epa = 1.54 V. The redutive process occuring at negative potential (Epc = -0.26 V) showed
to be dependent from the second oxidative process. The electrochemical behaviour of
2Ru in acetonitrile was characterized by a quasi-reversible redox process (E1/2 = 1.08 V)
attributed to the redox pair Ru(II)/Ru(III).
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The electrochemistry of 3Ru also revealed one irreversible oxidation wave at Epa = 1.35
V with a very small counterpart at Epc = 1.14 V. The reduction of the coordinated ligand
seemed to occur in a very irreversible way at Epc = -0.30V. This behaviour is very similar
in acetonitrile.
The RuII/RuIII potential is mainly affected by the phosphane coligand since the overall
behaviour for the ruthenium compounds containing PPh3 as coligand in both solvents was
characterized by one oxidation process Ru(II)/Ru(III), with Epa in the range 1.06 – 1.35 V
with a very small cathodic counterpart, while for the dppe derivative, 2Ru, the oxidation
process Ru(II)/Ru(III) became quasi-reversible. Replacing PPh3 by DPPE results in a
decrease of ca. 0.10 V on the redox potential of the Ru(II)/Ru(III) couple (1Ru vs. 2Ru),
which agrees with the relative donating ability of the two phosphanes to the metal center.
For the same phosphane, the Ru(II)/Ru(III) couple potential remains almost unchanged
(1Ru vs. 3Ru).
Table 3. Electrochemical data for the complexes [Mn(η5-C5H5)(PP)(L)][PF6]n and the
free ligands in dichloromethane and acetonitrile
Figure 5. Cyclic voltammogram of complex 1Fe, in dichloromethane solution, at 200
mV/s, showing the reversibility of the processes: - - - Fe(II)/Fe(III); …. Reduction of
coordinated L3
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3.4. Second order NLO properties
The second order nonlinear optical properties of the complexes 1Ru, 3Ru and 1Fe were
studied by hyper-Rayleigh scattering (HRS)[30] in diluted chloroform solutions (10-3-10-5
M). To avoid the effect of fluorescence due to two-photon absorption and to obtain as
little resonance enhancement as possible[31], the stimulating laser light was shifted from
the original wavelength of the used Nd:YAG laser of 1064 nm to a higher wavelength
(1500 nm). Thus, a superposition of absorptions in the UV-Vis region and the SHG signal
(750 nm) is reduced, which makes the calculated static hyperpolarisability (β0)[31,32]
more reliable.[16,33] Another important reason for using the higher wavelength incident
beam, is an attempt to discriminate between a true SHG signal and a two photons
absorption induced fluorescence (TPAF) enhanced signal.[20,34] At the 1500 nm incident
beam HRS examinations were achieved using a tuneable optical parametric oscillator
(OPO) based set-up.[16] The obtained molecular first hyperpolarizalilities β together with
the corresponding calculated β0 values are summarized in Table 4.
Table 4. Second-order NLO values for complexes 1Ru, 3Ru and 1Fe at 1500 nm
aβ0 = β[1-(2λmax/1500)2][1-(λmax/1500)2]
Analysing the obtained values, compounds 1Ru and 3Ru present β0 values quite close to
the corresponding values of β since for both compounds the maximum of absorption is
far from the second harmonic frequency (750 nm). Based on the spectroscopic evidences
1
found for compound 1Fe (∆νNC = -45 cm-1; shielding on H NMR up to ~ 2 ppm;
solvatochromism at MLCT band centred at 628 nm) it would be expected a higher β
value. However the calculation of β0 gave a much lower value than expected. This can be
explained by the strong broad optical transition in the visible range centred at 628 nm this
affecting the β values by two-photon resonant enhancement. The well-known two-level
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model (TLM) of Oudar and Chemla[31], which is very often used in the literature to
derive static values, β0, is expected to become invalid close to resonance as it ignores any
kind of line-broadening mechanisms. This model, in which it is assumed that the lowest
energy CT transition yields the dominant contribution to β, diverges whenever the (laser
or) second-harmonic wavelength approaches the transition wavelength. This can lead to a
strong overestimation of the resonance effect and hence a significant underestimation of
the static first hyperpolarizability.
Remarkably, compound 3Ru presents a value of β0 = 212 x10-30 esu which places this
complex in the range found for the best values of β0 (200 - 400 x10-30 esu) reported in the
literature[12,35-38]. The absence of any MLCT absorption band led us to postulate that
the quite significant value found for the first hyperpolarizability might be originated by
variation of the dipolar moment between the ground state and the excited state related to
the ILCT occurring at the coordinated ligand.
Motivated by the good β0 value found for 3Ru, we decide to measure the efficiency of
second harmonic generation in the solid state using the Kurtz powder method[21], in
spite of the inexistent information concerning the crystallographic space group due to
difficulties to grow adequate single crystals. These measurements were performed at the
Nd:YAG laser fundamental wavelength (1064 nm), due to the transparency of the sample
at 532 nm, the second harmonic wavelength. It was found that compound 3Ru was 1.2
times more effcient in doubling frequency than standard urea in the same experimental
conditions. It is important to note that the results obtained by this technique depend not
only on the molecular hyperpolarisability β, but also very strongly on the crystal packing
structure, grain size, phase-matching properties, etc. Thus, this result can be explained on
the assumption that compound crystallizes in a centric crystallographic group as happens
with about 70% of the organic and organometallic compounds. At the light of these
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results, it seems that compound 3Ru can be considered with interesting NLO potential,
and thus, further studies concerning several strategies envisaging acentric crystallization
are in progress.
4. Conclusion
Four new ruthenium(II) and iron(II) piano stool structured complexes were synthesised in
view to study their NLO properties. Spectroscopic data analysed on this scope suggested
interesting potentiality as frequency doublers. Quadratic hyperpolarizabilities β were
measured by Hyper Rayleigh Scattering at 1500 nm, showing values in the range 95-618
x 10-30 esu. The transparency observed in the visible region of compound 3Ru gave a
value of static hyperpolarizability of 212 × 10-30 esu, placing this compound in the best
range of values reported in the literature. Studies in the solid state to study the ability of
doubling a Nd:YAG laser beam of 1064 nm wavelength by Kurtz powder technique,
showed that 3Ru is 1.2 times more efficient than urea standard. Although a stronger
effect would be expected considering the significant value of the found static
hyperpolarizability, this result can be explained on the basis of a probable centric
crystallization. Hence, further studies concerning several strategies to guarantee acentric
crystallization for 3Ru are currently in progress.
Acknowledgments
The authors thank financial support from the Portuguese Foundation for Science and
Technology for the projects PTDC-QUI-66148-2006, PEst-OE/QUI/UI0536/2011 and
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SFRH/BPD/80459/2011 (Andreia Valente’s postdoctoral scholarship). Technical
assistance involving Kurtz powder technique is thanked to Fernando Monteiro.
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´Synthesis of new Fe(II) and Ru(II) η5-monocyclopentadienyl compounds showing
significant second order NLO properties`
Manuscript reference: JOM 17903
Editorial reference: JOM_JORGANCHEM-D-12-00701
Highlights
•
•
•
•
Four new ruthenium(II) and iron(II) piano stool structured complexes
Spectroscopic data suggest interesting potentiality as frequency doublers
Static β0 values for the new complexes fall in the range 65 - 212 × 10-30 esu
One compound is 1.2 times better than urea for 2nd harmonic generation (SHG)