PH-Switchability and Second-Order Nonlinear Optical Properties of Monocyclopentadienylruthenium(II)/iron(II) Tetrazoles/Tetrazolates: Synthesis, Characterization, and Time-Dependent Density Functional Theory Calculations

Article abstract of DOI:10.1021/acs.inorgchem.7b00138

Tetrazole/tetrazolate monocyclopentadienyliron(II) and ruthenium(II) compounds of general formulas [(η⁵-C₅H₅)M(dppe)(N₄(H)CC₆H₄NO₂)][PF₆]/[(η⁵-C₅H₅)M(dppe)(N₄CC₆H₄NO₂)] were investigated for their pH-switching second-order nonlinear optical (SONLO) properties. Compounds [(η⁵-C₅H₅)M(dppe)(N₄CC₆H₄NO₂)] (M = Fe, Ru) and compound [(η⁵-C₅H₅)Ru(dppe)(N₄(H)CC₆H₄NO₂)][PF₆] were fully characterized by (¹H-, ¹³C-, ³¹P-) NMR, cyclic voltammetry, and elemental analysis, and compounds [(η⁵-C₅H₅)Fe(dppe)(N₄CC₆H₄NO₂)] and [(η⁵-C₅H₅)Ru(dppe)(N₄(H)CC₆H₄NO₂)][PF₆] were further characterized by single-crystal X-ray diffraction; the synthesis of [(η⁵-C₅H₅)Fe(dppe)(N₄(H)CC₆H₄NO₂)][PF₆] was unsuccessful. Time-dependent density functional theory calculations were performed using PBE0 and CAM-B3LYP functionals to evaluate the first hyperpolarizability (β_tot) of the tetrazole/tetrazolate complexes and for a detailed analysis of the experimental data. Both functionals predict (i) high first hyperpolarizabilities for the tetrazolate complexes [(η⁵-C₅H₅)M(dppe)(N₄CC₆H₄NO₂)], with β_tot[Ru] ≈ 1.2β_tot[Fe], and (ii) a 3-fold reduction in β_tot[Ru] upon protonation, in complex [(η⁵-C₅H₅)Ru(dppe)(N₄(H)CC₆H₄NO₂)]⁺, forecasting [(η⁵-C₅H₅)Ru(dppe)(N₄CC₆H₄NO₂)]/[(η⁵-C₅H₅)Ru(dppe)(N₄(H)CC₆H₄NO₂)]⁺ complexes as on/off, pH-switchable SONLO forms.

Full text of DOI:10.1021/acs.inorgchem.7b00138

Article
pubs.acs.org/IC
pH-Switchability and Second-Order Nonlinear Optical Properties of
Monocyclopentadienylruthenium(II)/iron(II) Tetrazoles/Tetrazolates:
Synthesis, Characterization, and Time-Dependent Density Functional
Theory Calculations
Pedro R. Florindo,*,†,∥ Paulo J. Costa,*,‡,§ M. F. M. Piedade,†,‡ and M. Paula Robalo†,⊥
†Centro de Química Estrutural, Instituto Superior Tec
́
nico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
∥iMed.ULisboa, Faculdade de Farmac
́
ia da Universidade de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal
‡
§Centro de Química e Bioquímica, DQB, and Departamento de Química e Bioquímica, Faculdade de Cien
̂
cias da Universidade de
Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
⊥
́
Area Departamental de Engenharia Química, ISEL - Instituto Superior de Engenharia de Lisboa, Instituto Politec
́
nico de Lisboa, Rua
Conselheiro Emídio Navarro, 1, 1959-007 Lisboa, Portugal
S
* Supporting Information
A B S T R A C T : T e t r a z o l e / t e t r a z o l a t e
monocyclopentadienyliron(II) and ruthenium(II) compounds
of general formulas [(η5-C5H5)M(dppe)(N4(H)CC6H4NO2)]-
[PF6]/[(η5-C5H5)M(dppe)(N4CC6H4NO2)] were investigated
for their pH-switching second-order nonlinear optical
(SONLO) properties. Compounds [(η5-C5H5)M(dppe)-
(N4CC6H4NO2)] (M = Fe, Ru) and compound [(η5-
C5H5)Ru(dppe)(N4(H)CC6H4NO2)][PF6] were fully charac-
terized by (1H-, 13C-, 31P-) NMR, cyclic voltammetry, and
elemental analysis, and compounds [(η5-C5H5)Fe(dppe)-
(N4CC6H4NO2)] and [(η5-C5H5)Ru(dppe)(N4(H)-
CC6H4NO2)][PF6] were further characterized by single-crystal X-ray diffraction; the synthesis of [(η5-C5H5)Fe(dppe)(N4(H)-
CC6H4NO2)][PF6] was unsuccessful. Time-dependent density functional theory calculations were performed using PBE0 and
CAM-B3LYP functionals to evaluate the first hyperpolarizability (βtot) of the tetrazole/tetrazolate complexes and for a detailed
analysis of the experimental data. Both functionals predict (i) high first hyperpolarizabilities for the tetrazolate complexes [(η5-
C5H5)M(dppe)(N4CC6H4NO2)], with βtot[Ru] ≈ 1.2βtot[Fe], and (ii) a 3-fold reduction in βtot[Ru] upon protonation, in
complex [(η5-C5H5)Ru(dppe)(N4(H)CC6H4NO2)]+, forecasting [(η5-C5H5)Ru(dppe)(N4CC6H4NO2)]/[(η5-C5H5)Ru(dppe)-
(N4(H)CC6H4NO2)]+ complexes as on/off, pH-switchable SONLO forms.
(iii) structural or chemical modification of the bridging group,
breaking the conjugation between D and A.2a Thus, SONLO
switches can be achieved through an external stimulus such as a
pH variation, a redox process, or by interaction with
electromagnetic radiation.
In this trend, coordination complexes offer significant
advantages over organic NLO chromophores due to NLO
active charge-transfer transitions between the metal and the
ligands, and the opportunity for fine-tuning their electronic
properties by modification of the metal center, its oxidation
state and coordination sphere.3 Group 8 organometallic
complexes were first highlighted in the NLO context by
Green et al., which unveiled good second harmonic generation
(SHG) efficiencies for ferrocenyl derivatives.4 Although
interesting results have been achieved with ferrocene systems,5
INTRODUCTION
■
Compounds with enhanced second-order nonlinear optical
(SONLO) properties have been extensively investigated over
the last quarter-century due to the large panel of possible
technological applications, such as building blocks for optical
communications, optical data processing and storage, or
electro-optical devices.1 Among those, there has been a
growing interest in molecular species with switchable
SONLO properties, since the possibility to modulate the
NLO behavior of molecular materials using an external stimulus
increases their potential for application in optoelectronic and
photonic technologies.2 The quadratic hyperpolarizability (β)
of chromophores can be manipulated by reversibly modifying
specific parts of the active molecules. Since most SONLO
chromophores are of the donor(D)-π system-acceptor(A) type,
alterations are categorized in three types, (i) reducing the
donor ability of D by oxidation or protonation, (ii) reducing
the acceptor behavior of A by reduction or deprotonation, or,
Received: January 24, 2017
© XXXX American Chemical Society
A
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group 8 half-sandwich complexes have later boosted the
SONLO response of organometallic complexes due to the
possibility of d(metal)-π(chromophore) orbital conjugation.
Systematic studies performed on iron and ruthenium
complexes of general formulas [(η5-C5H5)M(PP) (chromo-
phore)] (PP = mono- or bidentate phosphanes), with different
types of chromophores (e.g., phenyl, thienyl) bound to the
metal center through nitrile or acetylide linkages revealed [(η5-
C5H5)M(PP)]+ organometallic moieties as efficient donors for
SONLO purposes, consistently leading to higher β values than
the best organic donor groups (e.g., NR2).6
[(η5-C5H5)Ru(dppe)(4-N4(H)C−C6H4−NO2)][PF6] by sin-
gle-crystal X-ray diffraction. TD-DFT calculations were
performed to evaluate the SONLO response of the tetrazole/
tetrazolate complexes, and the theoretical results are discussed
together with experimental data in view of their SONLO
potential as molecular switches.
RESULTS AND DISCUSSION
■
Synthetic Studies. Scheme 1 presents the synthetic route
for the target organometallic compounds. The ligand 5-(4-
Considerable efforts have been made in designing and
preparing metallic complexes with effective pH-, redox-, or
photoswitchable SONLO responses, in solution and in the solid
state,2,7 and the fragments [(η5-C5Me5)Fe(dppe)]+ and [(η5-
C5H5)Ru(dppe)]+ were previously studied for their redox-
switchable properties. In this family the monometallic complex
[(η5-C5Me5)Fe(dppe)(4-CC−C6H4−NO2)] revealed a re-
markable switchability,8 and more recently, [(η5-C5H5)M-
(dppe)(L)]+ complexes (M = Fe, Ru; L= 5-(3-(thiophen-2-
yl)benzo[c]thiophen-1-yl)-thiophene-2-carbonitrile) were in-
vestigated by time-dependent density functional theory (TD-
DFT) calculations, with calculated βtot values for one-electron
oxidized species increasing up to 8.3 times (M = Ru) in relation
to the nonoxidized complexes.9
Scheme 1. Synthesis of the Tetrazole Ligand T-H and the
a
Organometallic Complexes RuT+, FeT0, and RuT0
Remarkably, ruthenium10 and iron10d,11 compounds of
general formulas [(η5-C5H5)M(PP)(L)][X] (PP = mono- or
bidentate phosphanes; L = N-donor ligand; X = counterion)
have also been recognized also as good anticancer agents. Some
of us have developed tetrazole-carbohydrate ligands and the
respective ruthenium glycoconjugates,10c given the interest in 5-
substituted-1H-tetrazoles (RCN4H) as metabolism-resistant
isosteric replacements for carboxylic acids (RCO2H).12
Owing to their multiple N-donor atoms and various
coordination modes, tetrazoles and derivatives have also the
potential as functional ligands in coordination chemistry and
crystal engineering,13 and in this context, zinc(II)-tetrazole
metal−organic frameworks previously exhibited SHG up to 5
times the urea standard.14
a
(a) HONH2·HCl, pyridine, r.t.; Ac2O. NaN3, NH4Cl, DMF, 90 °C.
(b) TlPF6 (or NH4PF6), CH2Cl2, r.t. (c) TlOEt, THF, r.t.
nitrophenyl)-1H-tetrazole (T-H, Scheme 1a) was obtained in
good yield from 4-nitrobenzaldehyde by generation of the
oxime with hydroxylamine hydrochloride, in situ dehydration
with acetic anhydride, and final (3 + 2) azide cycloaddition of
the resulting nitrile.10c
With T-H in hand, we planned the synthesis of both
tetrazolate neutral complexes FeT0/RuT0 and tetrazole PF6
−
salts FeT+/RuT+ (Scheme 1b,c). Neutral tetrazolate complexes
FeT0/RuT0 were synthesized by in situ generation of
thallium(I) tetrazolate with thallium(I) ethoxide and subse-
quent reaction with the parent neutral complexes [CpM-
(dppe)X] (Cp ≡ (η5-C5H5); M = Fe, X = I; M = Ru, X = Cl),
and isolated in good yields of 78 and 89%, respectively
(Scheme 1c).
Given the above referred studies on the SONLO and
anticancer properties of [(η5-C5H5)M(PP)(L)] complexes (M
= Fe, Ru), and the interesting electronic and structural
properties of tetrazoles,15 we ended up wondering, (i) how
The synthesis of complexes FeT+/RuT+ was first attempted
by halide abstraction of the parent neutral complexes
[CpM(dppe)X] (M = Fe, X = I; M = Ru, X = Cl) with
thallium(I) hexafluorophosphate in the presence of a slight
excess of T-H (Scheme 1b). While RuT+ was obtained in good
yield following this synthetic methodology, we were not able to
isolate its iron analogue FeT+. Aware of a possible thallium(I)-
induced oxidation of the electron-rich iron(II) center, we
attempted the synthesis of FeT+ using ammonium hexafluor-
ophosphate, but unsuccessfully, while RuT+ was readily
obtained in these conditions; although with a lower yield
(79% vs 87%), this methodology avoids the manipulation of
dangerous thallium salts.
−
efficient would the tetrazolate heteroring (RCN4 ) be as metal-
chromophore linker in neutral complexes of the type [(η5-
C5H5)M(PP)(N4C-chromophore-A)], and moreover, (ii)
whether the tetrazolate/tetrazole complex pair would have
the right properties for a SONLO pH-switch, due to
conjugation loss in the protonated tetrazole complexes.
Ruthenium-alkynyl complexes of general formulas trans-[Ru-
(dppe)2Cl(CC−C6H4-R)] and the corresponding vinylidene
forms trans-[Ru(dppe)2Cl(CCHC6H4-R)]PF6, were studied
in this context,16 and variations up to 5-fold in β and β0 were
observed for alkynyl/vinylidene complex pairs.
In this context, we planned the development of new
tetrazolate complexes of general formula [(η5-C5H5)M(dppe)-
(4-N4C−C6H4−NO2)] (M = Fe, Ru; dppe = 1,2-bis-
(diphenylphosphino)ethane), conjugating the strong [(η5-
C5H5)M(dppe)]+ donor and −NO2 acceptor moieties, and of
the respective tetrazole complexes as hexafluorophosphate salts.
Here we describe their synthesis, spectroscopic (1H-, 13C-, 31P
NMR), electrochemical and structural characterization of
compounds [(η5-C5H5)Fe(dppe)(4-N4C−C6H4−NO2)] and
Interestingly, both iron15 and ruthenium17 monocyclopenta-
dienyl complexes are efficient catalysts for the (3 + 2)
cycloaddition of azide to electro-deficient nitriles in mild
conditions, affording the corresponding organometallic tetra-
zolates. On the basis of these procedures, the tetrazolate
complexes FeT0 and RuT0 were also obtained by a one-pot
reaction of the corresponding parent neutral complexes with
thallium hexafluorophosphate, sodium azide, and 4-nitro-
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benzonitrile, in dichloromethane at room temperature (Scheme
2), with yields of 83 and 85%, respectively.
low-field shift of 0.13 ppm (RuT0 to RuT+), in agreement with
the presence of tetrazole and tetrazolate complexes, respec-
tively, and evidencing hyperconjugation loss in RuT+ upon N4
protonation (Figure S2).
Scheme 2. One-Pot Catalytic Synthesis of MT0 Complexes
The protic switchability of RuT+/RuT0 complexes was
studied in dry dichloromethane, one of the solvents commonly
used in hyper-Rayleigh scattering experiments (HRS). RuT0
was first quantitatively obtained from RuT+, by reaction with an
equivalent amount of triethylamine (Et3N) and silica filtration
(Scheme 4). Reversibly, when RuT0 was reacted with an
equivalent amount of trifluoromethanesulfonic acid (triflic acid,
F3CSO3H), RuT+ was readily obtained in 89% yield, as the
corresponding triflate salt (Figure S3).
New compounds T-H, FeT0, RuT0, and RuT+ were fully
We studied also the RuT+/RuT0 conversion in “wet”
conditions (Scheme 4). First, a dichloromethane solution of
RuT+ was vigorously stirred with an equivalent volume of
water, in the presence of a catalytic amount of tetrabutylam-
monium bromide as phase transfer catalyst. After 15 min, RuT0
was recovered from the crude product by extraction with dry
ethyl ether (Et2O) with 20% yield, and unreacted RuT+
(insoluble in Et2O) was recovered by extraction of the crude
product with dry dichloromethane with 75% yield, demonstrat-
ing some stability of RuT+ in “wet” dichloromethane. When the
experiment was repeated in basic conditions, using an
equivalent volume of NaOH 35% solution and in the presence
of the same phase transfer catalyst, RuT0 was efficiently
obtained with 78% yield (Scheme 4). The reverse conversion of
RuT0 in RuT+ in acid phase transfer conditions was
unsuccessful using 50% fluoroboric acid solution (HBF4),
with RuT0 being quantitatively recovered even after reaction
times up to 2 h; nonetheless, when RuT+ was stirred with 50%
HBF4 solution for 15 min, no formation of RuT0 was noticed,
which indicates that tetrazole deprotonation of RuT+ is
inhibited in these acidic conditions (Scheme 4). Since no
signs of decomposition were found during the referred
experiments, these demonstrate also the stability of the Ru−
N bonds toward hydrolysis.
1
characterized by H, 13C, and 31P NMR spectroscopies, and
their formulations and purity were confirmed by elemental
1
analysis. Even though it was acquired in dry DMSO-d6, the H
NMR spectrum of T-H revealed only two doublets at 8.30 and
8.44 ppm (JHH = 8.8 Hz), attributed to phenyl protons
(Scheme 3), and no signal of the tetrazole proton was observed,
due to rapid exchange between 1H and 2H tautomers which
exist as a near 1:1 ratio;12 addition of D2O led to the disclosure
of a low-field shifted water residual signal, consistent with
media acidity.
Compounds FeT0 and RuT0 revealed similar NMR spectra,
1
as expected given their structural analogy, with H and 13C
cyclopentadienyl chemical shifts and dppe 31P resonances
similar to the ones found for analogous [CpM(dppe)(4-C
C−C6H4−NO2)] neutral compounds.6f,g
Tetrazolate Ct 13C NMR chemical shifts (Scheme 3) of 161.8
and 160.9 ppm for FeT0 and RuT0, respectively, confirm the
expected M-N2 coordination of the tetrazolate ring.18 In FeT0,
Ct is shielded by 2−3 ppm when compared to compounds
[CpFe(CO)(L)(N4C−C6H4−CN)] (L = CO, PPh3, P-
(OCH3)3, δ(C5) = 164.2−165.1 ppm),15 despite the superior
electron-withdrawing ability of the −NO2 group over −CN,
evidencing the donor ability of the [CpFe(dppe)]+ organo-
metallic moiety. Also, comparison of the 1H NMR spectra of T-
H with FeT0 and RuT0 shows that the phenyl protons are
significantly shielded upon coordination, with Δ(H2,6Ph)
downfield shifts of ∼0.8 and 0.9 ppm (superimposed) and
Δ(H3,5Ph) shifts of 0.37 and 0.41 ppm, respectively for RuT0
and FeT0. These results agree with increased electronic density
on the phenyl ring upon coordination, due to π-backdonation
from the electron-rich metal centers extended throughout the
hyperconjugated chromophore.
Given the unsuccessful synthesis of FeT+ (described above),
1
we studied the evolution of the H and 31P NMR spectra of
FeT0 in DMSO-d6 over time in the presence and absence of
trifluoroacetic acid, TFA (Figure S4). After 1 h of incubation
with TFA (Figure S4-B), free T-H was already perceptible by
1H NMR, indicating that protonation leads to decoordination
of the tetrazole ligand, evidencing FeT+ instability. After 24 h,
the T-H/FeT0 proportion is ∼5/4 (Figure S4-C), and the
formation of free cyclopentadiene (C5H6, δH = 1.97, 6.40, and
6.49 ppm) and of free dppe (δH = 7.23−7.26 ppm; δP = −14.0
ppm) are noticed (Figure S4-C). After 3 days of acidic
incubation, FeT0 was completely decomposed, and only T-H,
1H NMR spectrum of RuT+ in DMSO-d6 revealed
differences when compared with the RuT0 spectrum, namely,
a broad resonance at δ = 5.93 ppm attributed to the tetrazole
proton 1H coupled with residual water. Addition of D2O led to
the upfield shift of the water coupled 1H signal, while the
remaining signals are coincident with the RuT0 spectrum, as
1
cyclopentadiene and free dppe could be identified by H and
31P NMR in the now colorless solution (Figure S4-D). In the
absence of TFA, FeT0 was stable over the same period, with no
signs of decomposition. Overall, these experiments suggest that
the formation of FeT+ leads to T-H decoordination and
1
confirmed by H NMR after addition of the neutral analogue
(Figure S1). In dry CD2Cl2, the 1H NMR spectra of RuT+ and
RuT0 presented significant differences, namely, a δ(H3,5Ph)
Scheme 3. Numbering Scheme for the Ligand T-H (left), RuT+ (Middle), and MT0 Complexes (Right)
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a
Scheme 4. Evaluation of the RuT+/RuT0 Protic Switch in Dichloromethane in Dry (Top) and Wet Conditions (Bottom)
a
In wet conditions, tetrabutylammonium bromide was used as phase transfer catalyst.
Figure 1. Molecular diagrams of compounds FeT0 (left) and RuT+ (right), with numbering scheme (C1 = Ct); thermal ellipsoids are drawn at a 50%
probability. Hydrogen atoms have been omitted for clarity, except for H4 in RuT+.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds FeT0 and RuT+
bond lengths (Å)
FeT0
RuT+
bond angles (deg)
FeT0
RuT+
a
a
M−Cp
M−P1
M−P2
1.7070(6)
2.205(1)
2.203(1)
1.964(3)
1.340(4)
1.334(5)
1.338(5)
1.342(5)
1.344(5)
1.865 (15)
2.301(1)
2.297(1)
2.081(4)
1.345(7)
1.305(7)
1.346(7)
1.335(8)
1.326(6)
N2−M−Cp
P1−M−Cp
P2−M−Cp
123.95(11)
127.21(4)
126.26(4)
87.07(5)
90.4(1)
123.1 (3)
131.2(4)
127.4 (4)
82.85(5)
89.4(1)
a
a
M−N2
N1−N2
N2−N3
N3−N4
P1−M−P2
P1−M−N2
P2−M−N2
N1−N2−N3
N2−N3−N4
90.2(1)
89.3(1)
111.0(3)
107.8(3)
105.8(3)
111.7(3)
103.7(3)
122.4(2)
126.4(3)
112.1(4)
104.4(4)
110.1(5)
107.6(5)
105.8(4)
125.3(3)
122.4(3)
b
N4−C1
b
b
C1 −N1
N3−N4−C1
b
N4−C1 −N1
C1 −N1−N2
M−N2−N1
M−N2−N3
b
a
b
Centroid. C1 = Ct.
subsequent decomposition of the [CpFe(dppe)]+ organo-
metallic moiety, despite successfully modeling by density
functional theory (DFT) computational methods (see below).
Structural Studies. Compounds FeT0 and RuT+ were
further characterized by single crystal X-ray diffraction studies.
Suitable crystals of RuT+ were obtained by slow diffusion of
Et2O in a DCM solution of the compound, and of n-hexane in
DCM for FeT0. Compound FeT0 crystallizes in the
orthorhombic crystal system, Pbcn space group (centrosym-
metric), while compound RuT+ crystallizes in the monoclinic
crystal system, space group Cc (non-centrosymmetric).
Molecular diagrams of FeT0 and RuT+ are presented in Figure
1, and selected bond lengths and angles of FeT0 and RuT+ are
presented in Table 1.
D
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Compounds FeT0 and RuT+ present the usual “three-legged
piano stool” ligand distribution around the metal centers with
bond distances and angles within the range found for
[CpM(dppe)(L)] analogues (M = Fe, Ru).10,11,15,19−23 The
Ru−N length is shorter than the respective lengths in similar
[CpRu(dppe)(1-BuIm)]+ analogues (1-BuIm = 1-butyl-imida-
zole, 2.131(2) to 2.141(3)).19 Also, the Fe−N bond is shorter
in FeT0 than in an analogue tetrazolate compound
(2.007(7)).15 The different electronic environment within the
tetrazolate/tetrazole rings in compounds FeT0/RuT+ is
reflected in the respective conformations of the heterorings.
The main differences are found in the N3−N4−C1 angle,
larger in RuT+ by ∼5°, and in the N4−C1−N1, which is ∼5°
smaller. Overall, the structural features fully align with examples
of tetrazolate/tetrazole ruthenium(II) and iron(II) analogue
complexes found in the literature.15,20−23 Significantly, RuT+ is
one of the few examples of an X-ray structure of a Ru-tetrazole
complex, following the structures of [Ru(tpy) (bpy)L]+ (tpy =
2,2′:6′,2″-terpyridine, bpy = 2,2′-bipyridyl, L = 2-(1H-tetrazol-
5-yl)thiophene)21 and [Ru(tpy) (bpy)(L)]+ (L = 4-(1H-
tetrazol-5-yl)benzonitrile).23
macroscopic cancel of the SONLO response of the individual
molecules, compound RuT+ is expected to have a nonzero
SHG response in the solid state.6o
UV−vis Spectroscopic Studies. The optical absorption
spectra of complexes FeT0, RuT0, RuT+, and ligand T-H,
recorded in 10−3 to 10−5 mol dm−3 dimethylformamide (DMF)
solutions, are presented in Figure 2. Additionally, for the
Contrary to what would be expected, based on spectroscopic
evidence for interannular conjugation (vide supra) and the
DFT-optimized structures of the complexes (see below), in
complex RuT+ a quasi-planarity is verified between the phenyl
and the tetrazole rings, with a torsion angle τ of 4.5°, while in
FeT0 these rings present a significant deviation from planarity
(τ = 31.9°). This apparent disagreement was easily rationalized
for both compounds by close inspection of the supramolecular
interactions in solid state packing. The large deviation from
planarity in compound FeT0 (see Figure S5) can be attributed
to a series of very weak intermolecular interactions of the type
C−H···O between nitro group oxygens and Ph hydrogens of
neighbor molecules, both from dppe (C212−H212···O2 {2.611
Å}, C213−H213···O2 {2.597 Å}, C213−H213···O1 {2.514 Å})
and the tetrazolate ligand (C4−H4···O2 {2.628 Å}). In
compound RuT+, the planar interannular conformation
adopted by the tetrazole ligand in this compound (see Figure
S6) is mostly explained by the stronger C−H···O intermo-
lecular interactions established between nitro group oxygens
and Cp and dppe ligands of consecutive cationic complexes
(C24−H24···O2 {2.414 Å}, C113−H113···O1 {2.571 Å}),
with some “reinforcement” from short contacts of the type N−
H···F and C−H···F between tetrazole ligand hydrogens and
fluorine atoms of neighbor anions (N4−H4···F3 {1.878 Å},
N4−H4···F1 {2.640 Å}, C3−H3···F1 {2.532 Å}, C7−H7···F2
{2.496 Å}).
Figure 2. UV−vis spectra of compounds FeT0, RuT0, RuT+, and T-H,
recorded in DMF.
organometallic complexes, spectra were acquired in solvents
with different polarities, i.e., toluene (RuT+ is insoluble),
dichloromethane (DCM), tetrahydrofuran (THF), and dimeth-
yl sulfoxide (DMSO) to unveil the solvatochromic behavior of
the UV−vis absorption bands. These results are summarized in
Table 2.
Table 2. UV-Vis Spectral Data of FeT0, RuT0, RuT+, and T-H
solvent
toluene
DCM
THF
DMF
DMSO
compound
wavelength (nm) {ε × 10−3 (M−1 cm−1)}
T-H
349 {15.0}
FeT0
458 {2.9} 479 {3.5} 477 {3.3} 495 {2.3} 502 {3.2}
309 {11.6} 313 {15.6} 314 {14.8} 315 {11.4} 314 {14.8}
392 {6.3} 396 {7.0} 401 {7.0} 404 {4.7} 409 {5.0}
RuT0
RuT+
300 {11.6} 305 {11.7} 304 {14.7} 306 {12.4} 306 {13.8}
b
a
380
305
397 {6.6} 408 {5.1} 412 {5.0}
302 {16.6} 302 {15.0} 305 {15.0}
b
a
b
Insoluble. Shoulder.
The UV−vis spectrum of T-H in DMF presents only an
intense π−π* absorption band at 349 nm. Spectra of the
organometallic compounds FeT0, RuT0, and RuT+ also present
an intense band in the UV region (302−315 nm) attributed to
the same π−π* ligand absorption, which suffers a significant
blue-shift upon coordination (Figure 2, Table 2). The
complexes show also a less intense absorption band in the
visible region, at 495 nm for FeT0, 404 and 408 nm for RuT0
and RuT+, respectively, which are red-shifted with increasing
solvent polarity, up to 44 nm for FeT0 and 17 nm for RuT0
(toluene−DMSO, Table 2), consistent with metal-to-ligand
charge transfer (MLCT) bands. This assignment is supported
also by their low energy, low intensity, their absence in the
spectra of the T-H ligand and parent complexes, and by time-
dependent DFT calculations, TD-DFT (see below). These
band red-shifts are characteristic of electronic transitions with
an increase of the dipole moment upon photoexcitation. The
As referred above, compound RuT+ crystallizes in the
monoclinic crystal system, in a non-centrosymmetric space
group (Cc), which can be a key feature for SHG response in the
solid state.1 Analysis of the crystal packing of RuT+ shows that
despite the dipole cancel along the b axis, the monocationic
complex molecules are aligned along the (a, −c) direction
(Figure S7).
Even more important than the perfect alignment of the
molecular dipoles is the angle between the molecular charge
transfer axis, normally along the donor−acceptor axis, and the
polar crystal axis.1a The optimum value of this angle depends
on the crystallographic system, in order to allow quadratic
phase-matched interactions.6d This angle in complex RuT+ is
86.8°, which deviates significantly from the optimum value of
54.74° for the monoclinic system.1a Therefore, and contrary to
FeT0 in which the centrosymmetric crystallization leads to the
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lower energy of the FeT0 MLCT band can be rationalized
based on the lower oxidation potential of the iron center in
relation to ruthenium (see below). The lower intensity of the
FeT0 MLCT spectral band might be explained by its decreased
proximity of the π−π* band when compared to the ruthenium
counterparts, and its larger-base Gaussian shape; this shape may
also indicate that more than one electronic transition
contributes to the MLCT band (see Computational Studies
below).
bands. These data indicate that the photoactive species
originated by RuT+ is different in DMSO and DCM solutions.
In DCM, the protonated tetrazole form is stable even in the
presence of water, as demonstrated by solution studies (vide
supra), and hence the different optical behavior in this solvent
in relation to the tetrazolate RuT0. Comparison of the spectra
in this solvent shows also that tetrazole-tetrazolate deprotona-
tion leads to a significant intensity increase and a bathochromic
shift of the MLCT band, with both band features anticipating
improved second order NLO properties for RuT0 over the
protonated analogue, as initially expected. In DMSO, the
deprotonated tetrazolate form (RuT0) is easily formed from
Compounds RuT0 and RuT+ present quite similar UV−vis
spectra in THF, DMF, and DMSO, but not in DCM, as
depicted in Figure 3 for DMSO and DCM, and in Figure 2 for
1
RuT+ in the presence of water, as evidenced by H NMR
experiments in DMSO-d6. Given that, despite preparation from
dry solvents, UV−vis spectra were acquired in atmospheric
conditions and in (new) nonflamed glassware; we are here led
to conclude that RuT+ is switched to RuT0 due to water
absorbed by the hydrophilic solvents during the experiments.
According to the two-level model (TLM),24 the second-
order nonlinearity can be described as
Δμ f
eg
Ee3g
eg
β ∝
(1)
where Δμeg is the difference between the dipole moments of
the ground (g) and the excited state (e), feg is the oscillator
strength and Eeg is the transition energy. This model assumes
that only one excited state is coupled strongly enough to the
ground state by the applied electric field for the contribution to
β and that only one tensor component dominates the NLO
response (i.e., a unidirectional charge-transfer transition) and
has been used to compute β0 values for analogue acetylide
Figure 3. UV−vis spectra of compounds RuT0 and RuT+ recorded in
DCM and DMSO.
DMF. In DCM, complex RuT+ displays both π−π* and MLCT
bands as shoulders, while RuT0 displays Gaussian-shaped
Table 3. Summary of Electrochemical Data for T-H and Complexes FeT0, RuT0, and RuT+
a
compound
Epa (V)
Epc (V)
E1/2 (V)
Acetonitrile
Epa − Epc (mV)
Ic/Ia
HOMO−LUMO (V)
c
T-H
−0.70
−0.89
−1.21
0.22
b
−1.12
0.30
−1.17
0.26
90
80
1
FeT0
RuT0
RuT+
1
1.47
1.89
−0.88
−1.17
0.62
b
−1.07
0.70
−1.12
0.66
100
80
1
1
−0.98
−1.19
1.10
b
−1.07
1.21
−1.13
120
1
1.03
0.75
0.73
b
−1.03
−1.13
−1.18
Dichloromethane
0.32
100
1
d
FeT0
0.37
0.27
−0.92
−1.23
0.69
100
1
1.60
2.07
−1.03
0.77
200
80
RuT0
RuT+
0.73
1
−0.98
1.18
−1.30
1.03
150
0.6
0.70
−0.80
−1.26
−0.99
a
b
c
d
−
Epa(M2+/M3+) − Epc(NO2/NO2 ). Ia/Ic. E1/2 (Fc/Fc+) = 0.40 V, ΔE = 80−100 mV. E1/2 (Fc/Fc+) = 0.46 V, ΔE = 100−110 mV.
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Figure 4. Cyclic voltammograms of FeT0 and RuT0 in dichlorometane (left), and of RuT0, FeT0 (only Fe(II)/Fe(III) process) and T-H in
acetonitrile (right) (v = 200 mV·s−1). A full voltammogram of FeT0 in acetonitrile is presented in Figure S8.
compounds.6f,g,n Although computational results showed that
more than one excited state contributed to the MLCT bands of
MT0 and RuT+ complexes (see below), it allows a first intuitive
prediction of the first hyperpolarizabilities of these complexes
in relation to acetylide analogues. On the basis of this simple
model, one can anticipate (i) lower first hyperpolarizabilities for
MT0 in relation to the corresponding acetylide analogues, due
to the lower intensity of the MLCT bands of MT0 complexes
and (ii) lower first hyperpolarizability for RuT+ in relation to
RuT0, due to lower intensity of the MLCT band of RuT+.
Despite its simplicity and limitations,25 these trends fully align
with our first hyperpolarizabilities results, obtained by DFT
calculations (see below).
Electrochemical Studies. The redox behavior of the
organometallic complexes FeT0, RuT0, and RuT+ was
investigated in DCM and acetonitrile (ACN) at room
temperature by cyclic voltammetry (CV), using a Pt wire as
working electrode and a Ag wire as pseudoreference electrode;
T-H was only characterized in ACN due to its insolubility in
DCM. All potentials are collected in Table 3.
reductive process (Epc = −0.70 V), probably a tetrazole related
reduction, is absent upon coordination to [CpM(dppe)]+ metal
centers, while the second (Epc = −0.89 V, low intensity)
displays reduced intensities and is slightly shifted toward lower
potentials, and although mostly unchanged in potential, the
−
NO2/NO2 redox processes display a more irreversible
character in complexes MT0 (M = Fe and Ru) than in T-H.
These differences agree with NMR evidence for d(metal)-
π(chromophore) hyperconjugation and the increased chromo-
phore electronic density upon coordination.
The electrochemical behavior of MT0 compounds is quite
similar to the one revealed by [CpM(dppe)(4-CC−C6H4−
NO2)] analogues, which unveiled slightly lower M(II)/M(III)
redox potentials in DCM (E1/2 of 0.29 and 0.67 V for M = Fe
and Ru, respectively).6g Nonetheless, while [CpFe(dppe)(4-
CC−C6H4−NO2)] unveiled an irreversible redox process
(Ic/Ia = 0.6), compound FeT0 displays reversible Fe(II)/Fe(III)
redox waves, with Ic/Ia ratios of 1 (Figure 4) in both solvents
tested.
As demonstrated below in Computational Studies, for
complexes MT0, the highest occupied molecular orbital
(HOMO) is essentially located in the metal fragment, with
some contribution of the Cp ligand and tetrazolate ring, while
the lowest unoccupied molecular orbital (LUMO) is mostly a
Ph-NO2 orbital. It is thus fair to assume that the M(II)/M(III)
The electrochemical behavior of T-H is characterized by two
irreversible processes, at Epc = −0.70 V and Epc = −0.89 V (low
intensity), showing no anodic counterparts even when potential
was reversed at −1.00 V. It displays also a quasi-reversible
−
process at E1/2 = −1.17 V attributed to the NO2/NO2 redox
−
process.
oxidation and the NO2/NO2 reduction potential can be
The electrochemical behavior of FeT0 in DCM is
characterized by a reversible redox process at E1/2 = 0.32 V,
attributed to the Fe(II)/Fe(III) redox pair. At negative
potentials, it displays a cathodic process at Epc = −0.92 V
(very low intensity) and an irreversible process at Epc = −1.23
V (Epa = −1.03 V), attributed to the NO2/NO2− redox process.
In ACN, FeT0 presented the same general electrochemical
behavior, with a reversible redox process at E1/2 = 0.26 V,
attributed to the Fe(II)/Fe(III) redox pair, a small cathodic
process at Epc = −0.88 V, and a quasi-reversible process at E1/2
= −1.12 V attributed to the NO2/NO2− pair. Compound RuT0
evidenced the same general electrochemical behavior as FeT0 in
DCM and ACN, except for the higher potentials verified for
Ru(II)/Ru(III) redox processes when compared with Fe(II)/
Fe(III) redox processes (Figure 4).
related, respectively, to the relative magnitude of HOMO and
LUMO energies. Therefore, we calculated the HOMO−
LUMO gaps for compounds MT0, expressed as the potential
−
differences between M(II)/M(III) and NO2/NO2 redox
processes.6j,n The results (Table 3) show that the HOMO−
LUMO gaps depend significantly on the metal fragment, with
smaller gaps for FeT0 than for RuT0. This effect is mainly due
to the relative stabilization of the HOMO orbital in RuT0, since
LUMO orbitals are mostly unaffected by the change of metal
fragment and solvent, and agrees with the better donor
character of the iron(II) fragment. For both compounds, the
calculated HOMO−LUMO gaps are higher in DCM than in
ACN, in agreement with the bathochromic shifts of the MLCT
bands with increasing solvent polarity in the UV−vis spectra.
Compound RuT+ exhibited a quite interesting electro-
chemical behavior. At negative potentials in DCM, it presents
two ligand-based reductive processes being the one at −1.26 V
Comparison of the electrochemical behavior of T-H, FeT0,
and RuT0 in ACN (Table 3, Figure 4) shows that the first T-H
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−
attributed to the NO2/NO2 pair. At positive potentials, a
ruthenium centered process (oxidation) at 1.18 V with a Ic/Ia
ratio of 0.6 suggests some instability of the oxidized ruthenium
species at the electrode surface. Scan rate studies showed that it
became more reversible when the scan direction is immediately
reverted after the oxidation potential. This behavior can be
associated with a RuII/RuIII oxidation, followed by fast
decomposition. The higher potential for the RuII/RuIII
oxidation process agrees with the lower electronic density in
the cationic ruthenium(II) center in relation to RuT0.
Nevertheless, when the initial scan is directed toward the
RuT+ (vide supra). The presence of the N−H proton appears
to induce the distortion to avoid an interaction with the ortho
hydrogen of the phenyl ring. Torsion angles different from zero
are common for uncoordinated 5-phenyl-1H-tetrazole deriva-
tives, e.g., free 5-(4-aminophenyl)-1H-tetrazole (12.6°)31 and
for coordinated tetrazoles, [TiCl4(5-phenyltetrazole)2] (24.10°,
53.8°).32 Indeed, for the related [Ru(tpy)(bpy)(L)]+ (L = 4-
(1H-tetrazol-5-yl)benzonitrile) complex, a τ torsion angle of
27.2° is found in the X-ray structure,23 whereas for L = 2-(1H-
tetrazol-5-yl)thiophene, the torsion angle is close to 0° (3.9°).21
As discussed above, packing forces are strong enough to
overcome the steric repulsions between the ortho hydrogen and
the N−H proton, allowing planarity, whereas our calculations
are performed in a solvent continuum media, and a twisted
structure is invariably obtained for all MT+ complexes. A similar
question was discussed for 4,4′-bipyridine bridging ligands in
ruthenium complexes.33
−
negative potentials range and reaches the NO2/NO2 redox
process, a new oxidation wave at Epa = 0.78 V emerges and is
probably related to the cathodic wave at 0.70 V, already present
in the first scan (Figure S9). Moreover, the comparison
between this new redox process with the one obtained for the
RuT0 complex seems to indicate that the loss of the tetrazole
proton is possible during the electrochemical time scale, being
The highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) of complexes
FeT0, FeT+, RuT0, and RuT+, calculated at the PBE0 level of
theory, are depicted in Figure 5; the correspondent orbitals
calculated using CAM-B3LYP are very similar (Figure S10).
−
dependent on the NO2/NO2 process. In ACN the electro-
chemical behavior of RuT+ shows higher complexity with the
presence of three oxidation processes (Epa = 0.75 V; 1.03 and
1.21 V), the last one being attributed to the Ru(II)/Ru(III) pair
and two cathodic processes (Epc = 1.10 and 0.73 V). In
accordance with the behavior found in DCM, when the initial
scanning direction to negative potentials was reversed after the
NO2 reduction, an increase in the anodic process at Epa = 0.75
V was observed.
Computational Studies. Given the experimental con-
firmation of the interesting electronic properties preconized for
complexes FeT0, RuT0, and RuT+ and the pH switchability of
the RuT0/RuT+, DFT calculations26 were employed to provide
further insights into the experimental data and to calculate the
theoretical SONLO response (βtot) of these organometallic
complexes. Indeed, DFT calculations have been successfully
employed in the study of these properties of analogue acetylide
and nitrile organometallic complexes.9,27,28
Using the experimental X-ray data as a starting point, the
geometries of complexes FeT0, RuT0, and RuT+ were
optimized at the DFT level of theory using the popular
PBE1PBE functional, also known as PBE0,29 and the hybrid
exchange−correlation functional using the Coulomb-attenuat-
ing method (CAM-B3LYP)30 which predicts more reliable,
optical and hyperpolarizability data when compared with
experimental results.9 Since the UV−vis and NMR data showed
the stability of RuT+ in dichloromethane, all calculations used a
polarizable continuum model to account this solvent (see
Methods for further details). Although compound FeT+ is
experimentally unstable, it was successfully optimized, and
structural details are summarized in Table S1, which presents a
selection of distances and angles for the DFT-optimized
structures compared with X-ray diffraction data, when available.
Both functionals give excellent results in the prediction of the
coordination sphere of the iron and ruthenium complexes.
Nonetheless, CAM-B3LYP systematically yields longer M−L
bond lengths when compared with PBE0 and the X-ray
structures. This effect of stretched bond lengths for single
bonds was previously observed for similar systems in gas-phase
calculations.9 The calculated bond angles fall in the
experimental range, whereas the remaining calculated bond
lengths for the tetrazole/tetrazolate are equivalent for both
functionals. All the optimized structures of MT0 complexes
present a torsion angle τ near 0°, whereas RuT+ presents a τ
angle >24°, contrasting with the X-ray structures of FeT0 and
Figure 5. HOMOs (left) and LUMOs (right) of complexes FeT0,
FeT+, RuT0, and RuT+.
The HOMO of complexes MT0 have essentially metal
character with a considerable participation of the tetrazole
moiety, defining a M−N π* bond. A small contribution of the
Cp ligand is also present. Upon protonation, the π system of
the tetrazole ligand no longer contributes to the HOMO,
essentially being a Cp-M based orbital. On the other hand, the
LUMOs of MT0 and MT+ complexes are essentially located in
the acceptor Ph-NO2 moiety.
Using the optimized structures, we performed TD-DFT
calculations for complexes FeT0, RuT0, and RuT+ in dichloro-
methane, and the relevant calculated excitations are listed in
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Table 4. Relevant TD-DFT Excitation Energies (λ), Oscillator Strengths (f) and Compositions, for Complexes FeT0, RuT0, and
a
RuT+ Calculated at the PBE0 and CAM-B3LYP Level of Theory
PBE0
CAM-B3LYP
complex
λ (nm)
f
composition
λ (nm)
f
composition
λexp (nm)
FeT0
441
309
0.1804
0.4421
H → L (97%)
H-3 → L (91%)
H-18 → L (5%)
436
307
307
0.1597
H → L (97%)
479
313
0.2601
0.2047
H-3 → L (51%)
H → L+2 (41%)
H-3 → L (44%)
H → L+2 (45%)
H → L (99%)
RuT0
RuT+
464
310
0.1742
0.5076
H → L (99%)
H-3 → L (87%)
H-4 → L (5%)
H → L (99%)
H → L+1 (62%)
462
307
0.1541
0.428
396
305
H-3 → L (80%)
H-4 → L (7%)
H → L (99%)
474
323
0.0219
0.154
471
326
0.0178
0.1557
380
305
H → L+1 (63%)
H-2 → L+4 (8%)
H→ L+6 (8%)
a
The values are compared with experimental data (λexp).
Figure 6. Electron density difference maps (EDDM) in complexes FeT0, RuT0, and RuT+ for excitations presented in Table 4, calculated at the
PBE0 (left) and CAM-B3LYP levels of theory (right). Green and brown correspond to a decrease and increase of electron density, respectively.
Table 4. To aid the visualization and the character assignment
of the listed excitations, the electron density difference maps
(EDDM), which represent the changes in electron density that
occur for a given electronic transition, are presented in Figure
6.
calculated TD-DFT results also show the existence of two main
excitations, and the agreement between the experimental and
calculated spectra is satisfactory (see Figure 7). As for FeT0, the
calculated low-energy excitation at 464 nm (PBE0) or 462 nm
(CAM-B3LYP) is assigned to a MLCT corresponding to an
excitation from the metal-tetrazolate group (with some Cp
participation) to PhNO2; contrary to the iron analogue, this
band is significantly red-shifted when compared with the
experimental value (396 nm), which is opposite to the trend
found experimentally results, λ(FeT0) > λ(RuT0). The
agreement between calculated and experimental results is
much better for the high-energy absorption (experimental, 305
nm) calculated at 310 nm (PBE0) or 307 nm (CAM-B3LYP).
As for the iron analogue, this excitation is assigned as ILCT and
as for the other complexes, this result is independent of the
functional.
For complex FeT0, the low-lying absorption observed at 479
nm in dichloromethane is blue-shifted in the TD-DFT spectra
with a calculated excitation at 441 nm (PBE0) or 436 nm
(CAM-B3LYP). This excitation is assigned to a MLCT since it
corresponds to a transition from an iron centered orbital with
some tetrazole contribution, to the π system of the Ph-NO2
moiety, as depicted in Figure 6. The experimental higher-
energy absorption observed at 313 nm is calculated at 309 or
307 nm for PBE0 and CAM-B3LYP, respectively. Although
there is some metal contribution, this transition corresponds
mainly to an intraligand charge transfer (ILCT) as it transfers
electron density from the tetrazole ring to the nitrobenzene
group. Despite the blue shift observed for the low-energy band,
the TD-DFT calculations are in very good agreement with the
experimental results. This agreement is clearly observed in
Figure 7 were a superposition of the experimental spectra with
the calculated excitations is shown.
For compound RuT+, there is a low-energy excitation with
moderate oscillator strength, calculated at 474 nm (PBE0) or
471 nm (CAM-B3LYP) which should account for the observed
shoulder at 380 nm and is assigned to a ruthenium-to-
nitrophenyl MLCT. This MLCT is slightly different since the
tetrazole ring is absent from the transition whereas in RuT0, a
metal-tetrazole to nitrophenyl charge transfer is observed. At
higher energies, a stronger excitation was calculated at 323 nm
Complex RuT0 shows two maxima in the experimental UV−
vis spectra in dichloromethane at 396 and 305 nm. The
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Figure 7. TD-DFT excitations (blue, PBE0 and green, CAM-B3LYP) plotted against the UV−visible absorption spectra (red) in dichloromethane
for complexes FeT0, RuT0, and RuT+.
(PBE0) or 326 nm (CAM-B3LYP) but contrarily to RuT0, this
excitation does not correspond to an ILCT, being better
described as a MLCT from the metal to the 5-(4-nitrophenyl)-
1H-tetrazole ligand as seen in the EDDMs depicted in Figure 6,
accounting for the shoulder observed at 305 nm.
CAM-B3LYP level of theory with a similar basis set.9 The
estimated βtot values are approximately 1.2-fold larger for RuT0
when compared with FeT0 for both functionals, in agreement
with the trend found experimentally6f,g and by our calculations
for the acetylide analogs.
Our calculations further confirmed that there are relevant
MLCT-attributed electronic transitions, especially for com-
plexes FeT0 and RuT0, which are a main contributor for
SONLO properties (see below). To predict their SONLO
properties, the static first hyperpolarizabilities of the complexes,
βtot, were calculated using both functionals and the results are
presented in Table 5 (further details in Methods). For
As noticed before, the βtot values for acetylide complexes are
higher than the values calculated for MT0 complexes, by 2.7-
fold and 2.9-fold for iron complexes and by 2.4-fold and 2.6-
fold for ruthenium analogues, respectively, for PBE0 and CAM-
B3LYP functionals. The relative magnitudes of the Fe/Ru
MLCT oscillator strength, calculated for the acetylide
complexes, are also in good agreement with the ones obtained
experimentally.6f,g
Compound RuT+ revealed βtot values of 53.64 × 10−30 esu
(PBE0) and 49.49 × 10−30 esu (CAM-B3LYP). For both
functionals the values are 2.9-fold lower than the ones
calculated for the tetrazolate analogue, due to conjugation
loss upon protonation of the tetrazolate ring, reflected by the
low oscillator strengths calculated for the MLCT band (f =
0.0219 or 0.0178, for PBE0 or CAM-B3LYP, respectively)
when compared with RuT0, and in agreement with the
experimental results. Concerning the switchability of the
RuT+/0 complexes, the results confirm the potential of
tetrazole/tetrazolate forms for use as pH SONLO switches,
with a 2.9-fold increase in βtot upon tetrazole deprotonation.
Compounds [CpM(dppe)(4-CC−C6H4−NO2)] (M = Fe,
Ru) have shown intense low-lying MLCT absorptions at ∼500
nm (Fe) and ∼450 nm (Ru) and revealed HRS-determined
static first hyperpolarizabilities (β0) of 64 × 10−30 esu and 161
× 10−30 esu, respectively, when determined in THF;6g in
chloroform, the iron complex had previously revealed a β0 of 92
× 10−30 esu.6f These experimental results provide a basis for an
evaluation of our DFT and TD-DFT calculations. Both
functionals largely overestimate the experimentally determined
β0, yielding βtot values of 359.32 × 10−30 and 376.20 × 10−30
Table 5. Calculated Static First Hyperpolarizabilities βtot for
FeT0, RuT0, RuT+, and Acetylide Analogue Compounds
βTot (10−30 esu)
complex
PBE0
CAM-B3LYP
FeT0
RuT0
RuT+
132.35
155.96
53.64
119.91
143.67
49.49
CpFe(dppe)(4-CC−C6H4−NO2)
359.32
376.20
345.43
367.77
CpRu(dppe)(4-CC−C6H4−NO2)
comparison and experimental reference, the putative complexes
[CpM(dppe)(4-CC−C6H4−NO2)] (M = Fe, Ru) were
studied along with the tetrazolate/tetrazole analogues.
The calculated static first hyperpolarizabilities (βtot) for
compounds FeT0 and RuT0 were 132.35 × 10−30 and 155.96 ×
10−30 esu (PBE0) and 119.91 × 10−30 and 143.67 × 10−30 esu
(CAM-B3LYP), respectively (Table 5). These values are lower
than the ones calculated here for the acetylide analogue
complexes, and larger than the ones calculated for the
[MCp(H2PCH2CH2PH2)L]+ (L = 5-(3-(thiophen-2-yl)benzo-
[c]thiophen-1-yl)-thiophene-2-carbonitrile) systems at the
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Inorganic Chemistry
Article
esu (PBE0) and 345.43 × 10−30 and 367.77 × 10−30 esu (CAM-
B3LYP) for M = Fe and Ru, respectively. The CAM-B3LYP
functional produces systematically lower values. Although the
trend for a larger response for [CpRu(dppe)(4-CC−C6H4−
NO2)] over the iron analogue is maintained, the experimentally
determined values evidence a 2.5-fold β0 value for the
ruthenium compound (THF), significantly larger than the
calculated ratio. Nonetheless, the qualitative trends are
generally correct, and therefore, DFT and TD-DFT calculations
provide a reliable first evaluation of SONLO structure−activity
relationships and a valuable insight into the electro-optical
processes accounting for differences in βtot verified for
complexes with different metal linkers, [M-N4(H)C-R]+ vs
[M-N4C-R] vs [M-CC-R], within the compounds here
studied.
techniques (COSY, HSQC). Electronic spectra were recorded at room
temperature on an Agilent Technologies Cary 60 UV−vis
spectrophotometer in the range 200−900 nm. Combustion analyses
were performed at Laborator
́
́
io de Analises do Instituto Superior
Tec
́
nico, using a Fisons Instruments EA1108 system, and data
acquisition, integration, and handling were performed using the
software package Eager-200 (Carlo Erba Instruments), confirming
≥95% purity for all compounds.
Synthesis of 5-(4-Nitrophenyl)-1H-tetrazole. Hydroxylamine
hydrochloride (1.04 g, 15.0 mmol) was added to a stirred pyridine
solution (4 mL) of 4-nitrobenzaldehyde (1.51 g, 10.0 mmol). After 30
min, acetic anhydride (3 mL) was added, and the mixture was stirred
at r.t. for a further 1 h. After pumping to dryness, the crude was
dissolved in AcOEt (30 mL), washed with water (3 × 10 mL), rinsed
with brine (10 mL), and dried over MgSO4, and the solvent removed
under reduced pressure. The crude product was then dissolved in
DMF (10 mL), ammonium chloride (0.64 g, 12 mmol) and sodium
azide (0.71 g, 11.0 mmol) were added, and the mixture was heated to
90 °C for 2 h. After being pumped to dryness, the crude was dissolved
in AcOEt (30 mL), washed with water (3 × 10 mL), rinsed with brine
(10 mL), and dried over MgSO4, and the solvent was removed under
reduced pressure. The crude product was purified by column
chromatography (AcOEt/n-hexane 1:1 to AcOEt), affording pure 5-
CONCLUSIONS
■
We here first surveyed the second-order nonlinear optical
properties (SONLO) of tetrazolate complexes of general
formulas [CpM(dppe)(N4CC6H4NO2)] (M = Fe, Ru), and
the tetrazole/tetrazolate complex pairs [CpM(dppe)(N4(H)-
CC6H4NO2)]+/[CpM(dppe)(N4CC6H4NO2)] as pH-switch-
able SONLO on/off forms, respectively. Our experimental and
theoretical studies perspective the tetrazolate heteroring as a
promising metal linker for the development of push−pull
organometallic complexes with large first hyperpolarizabilities,
β. These studies demonstrate also that the pair [CpRu(dppe)-
(N4(H)CC6H4NO2)]+/[CpRu(dppe)(N4CC6H4NO2)] can be
reversible and efficiently interconverted in solution by pH
variation, predicting an approximate 3-fold increase in βtot upon
tetrazole deprotonation. The acidic “switch-off” of compound
[CpFe(dppe)(N4CC6H4NO2)] was not possible, due to
instability of the iron tetrazole complex [CpFe(dppe)(N4(H)-
CC6H4NO2)]+.
1
(4-nitrophenyl)-1H-tetrazole as a white crystalline solid; η = 82%. H
NMR (DMSO−d6, 400 MHz): 8.30 (d, 2H, J = 8.8, H2,6Ph), 8.44 (d,
2H, J = 8.8, H3,5Ph); 13C NMR (DMSO-d6, 100 MHz): 124.6
(C3,5Ph), 128.2 (C2,6Ph), 130.6 (C1Ph), 148.7 (C4Ph), 155.4 (Ct).
Anal. Calcd for C7H5N5O2·0.04C4H8O2: C, 44.17; H, 2.75; N, 35.97.
Found: C, 44.47; H, 2.60; N, 36.17.
Synthesis of Complexes MT0 (M = Fe, Ru). To Schlenk tubes
charged with THF solutions (20 mL) of 5-(4-nitrophenyl)-1H-
tetrazole (105 mg, 0.55 mmol) was added thallium ethoxide (39 μL,
0.55 mmol). After being stirred for 30 min at r.t., complexes
[CpM(dppe)X] were added (0.50 mmol), and the mixtures were
stirred at r.t. for further 4 h. The solutions were double filtered to
eliminate thallium chloride and pumped to dryness, and the crude
compounds were washed with n-hexane and recrystallized by slow
diffusion of n-hexane in dichloromethane solutions, affording
crystalline products.
Given that the SONLO structure−activity relationships
depicted throughout the last 20 years for nitrile compounds
of general formulas [CpM(PP)(NC-R)][X] (M = Fe, Ru;
PP = mono/bidentate phosphine; R = phenyl-, thienyl-
derivative chromophore; X = counterion), and that the
respective tetrazolate neutral analogues can be directly obtained
from those by self-catalyzed azide [3 + 2] cycloadditions, this
work establishes a straight one-step route for target neutral
tetrazolate complexes with improved, pH switchable SONLO
properties. Although revealing lower βtot values, our studies
evidenced a remarkable stability of RuT0 (and RuT+) when
compared with acetylide analogues (known for some instability,
e.g., vide ref 6n), thus evidencing organoruthenium(II)
tetrazolate complexes as stable, efficient building blocks for
the construction of SONLO materials.
1
Compound FeT0: purple; η = 78%; H NMR (CDCl3, 400 MHz):
2.72−2.86 (m, 2H, −(CH2)2−, dppe) 3.09−3.23 (m, 2H, −(CH2)2−,
dppe), 4.34 (s, 5H, η5-C5H5), 7.12 (s (br), 6H, Ph, dppe), 7.28 (s (br),
4H, Ph, dppe), 7.37−7.44 (comp., 12H, H2,6Ph + Ph, dppe), 7.97 (d,
2H, J = 8.4, H3,5Ph). 13C NMR (CDCl3, 100 MHz): 30.1 (t, JCP = 20.8,
−(CH2)2−, dppe), 79.5 (η5-C5H5), 123.6 (C3,5Ph), 126.1 (C2,6Ph),
128.1 (t, JCP = 4.5, Cmeta, dppe), 128.6 (t, JCP = 4.2, Cmeta, dppe), 129.3,
129.7 (CparaDppe, C1Ph), 131.2 (t, JCP = 4.2, Cortho, dppe), 132.6 (t,
JCP = 4.6, Cortho, dppe), 134.5 (t, JCP = 20.0, Cipso, dppe), 136.4 (Cipso
,
dppe), 141.9 (t, JCP = 20.1, Cipso, dppe), 146.9 (C4Ph), 161.8 (Ct). 31
P
1
NMR (CDCl3, 162 MHz): 106.9 (dppe). H NMR (DMSO-d6, 400
MHz): 2.94 (s (br), 2H, −(CH2)2−, dppe) 3.11 (s (br), 2H,
−(CH2)2−, dppe), 4.27 (s, 5H, η5-C5H5), 7.13 (s (br), 6H, Ph, dppe),
7.30 (s (br), 4H, Ph, dppe), 7.42 (comp. (br), 8H, Ph, dppe + H2,6Ph),
7.53 (s (br), 4H, Ph, dppe), 8.03 (s (br), 2H, H3,5Ph). 31P NMR
(DMSO-d6, 162 MHz): 105.9 (dppe). Anal. Calcd for
C38H33N5O2P2Fe: C, 64.33; H, 4.69; N, 9.87. Found: C, 63.94; H,
4.69; N, 10.04.
METHODS
■
Compound RuT0: orange; η = 89%; 1H NMR (CDCl3, 400 MHz):
2.61−2.75 (m, 2H, −(CH2)2−, dppe) 3.11−3.27 (m, 2H, −(CH2)2−,
dppe), 4.73 (s, 5H, η5-C5H5), 7.09 (s (br), 6H, Ph, dppe), 7.23 (s (br),
4H, Ph, dppe), 7.34 (s (br), 6H, Ph, dppe), 7.42 (s (br), 4H, Ph,
General Procedures. All experiments were carried out under inert
atmosphere (N2) using standard Schlenk techniques. Commercial
reagents were bought from Sigma-Aldrich and used without further
purification; solvents were dried using standard methods.34 Starting
materials were prepared following the methods described in the
literature for the synthesis of [CpRu(dppe)Cl]35 and [CpFe(dppe)-
I].6d 1H, 13C, and 31P NMR spectra were recorded on Bruker Avance
II 400 spectrometer at probe temperature. 1H and 13C NMR chemical
shifts are reported in parts per million (ppm) downfield from the
residual solvent peak; 31P NMR spectra are reported in ppm downfield
from internal hexafluorophosphate anion standard (−144.2 ppm).
dppe), 7.46 (d, 2H, J = 8.5, H2,6Ph), 8.00 (d, 2H, J = 8.4, H3,5Ph). 13
C
NMR (CDCl3, 100 MHz): 29.0 (t, JCP = 22.5, −(CH2)2−, dppe), 82.3
(η5-C5H5), 123.6 (C3,5Ph), 126.3 (C2,6Ph), 128.0 (t, JCP = 4.8, Cmeta
dppe), 128.5 (t, JCP = 4.6, Cmeta, dppe), 129.4, 129.7 (Cpara, dppe,
C1Ph), 130.8 (t, JCP = 4.9, Cortho, dppe), 132.7 (t, JCP = 5.3, Cortho
,
,
dppe), 133.6 (t, JCP = 23.0, Cipso, dppe), 136.7 (Cipso, dppe), 141.7 (t,
JCP = 20.5, Cipso, dppe), 146.9 (C4Ph), 160.9 (Ct). 31P NMR (CDCl3,
162 MHz): 86.1 (dppe). 1H NMR (DMSO-d6, 400 MHz): 2.77−2.81
(m, 2H, −(CH2)2−, dppe) 3.16−3.23 (m, 2H, −(CH2)2−, dppe), 4.69
1
Coupling constants are reported in Hz. Assignments of H and 13C
NMR spectra were confirmed with the aid of 1H-13C two-dimensional
K
DOI: 10.1021/acs.inorgchem.7b00138
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(s, 5H, η5-C5H5), 7.12 (s (br), 6H, Ph, dppe), 7.23−7.27 (m, 4H, Ph,
dppe), 7.37−7.41 (m, 6H, Ph, dppe), 7.48−7.50 (comp., 6H, H2,6Ph +
Ph, dppe), 8.07 (d, 2H, J = 8.1, H3,5Ph). 31P NMR (DMSO-d6, 162
sodium hydroxide solution. No conversions were verified when RuT0
(2 h) or RuT+ (15 min) were stirred with fluoroboric acid solutions,
with quantitative recovery of the starting products.
1
MHz): 85.1 (dppe). H NMR (CD2Cl2, 400 MHz): 2.72 (s (br), 2H,
Electrochemical Studies. Electrochemical experiments were
performed using an EG&G Princeton Applied Research model 273A
potentiostat/galvanostat and monitored using Electrochemistry
PowerSuite v2.51 software (Princeton Applied Research). Cyclic
voltammograms were obtained in 0.2 M/0.1 M solutions of
[NBu4][PF6] in dried DCM/CH3CN, respectively. Experiments
were carried out using a three-electrode configuration cell, with a
platinum-disk work electrode (1.0 or 0.7 mm diameter) probed by a
Luggin capillary connected to a silver-wire pseudoreference electrode;
a Pt wire auxiliary electrode was employed. The redox potentials were
measured in the presence of ferrocene (internal standard) and
normally quoted relative to SCE using the ferrocenium/ferrocene
redox couple (E1/2 = 0.46 and 0.40 V versus SCE for DCM or
CH3CN, respectively).
X-ray Crystallographic Procedures. Crystals were mounted with
protective oil on a cryo-loop and X-ray single diffraction was
conducted on Bruker D8 and X8 Apex II diffractometers equipped
with MoKα X-ray sources and graphite monochromators. The X-ray
generators were operated at 50 kV and 30 mA, and the X-ray data
collection were monitored using the APEX2 program.36 Psi-scan
absorption correction was applied using SAINT and SADABS
programs.37 Structures were solved by direct methods with the
program SHELXS38 and SIR2004,39 and refined by full-matrix least-
squares on F2 with SHELXL,38 all included in the package of programs
WINGX-V2014.1.40 Non-hydrogen atoms were refined with aniso-
tropic thermal parameters, with H atoms placed in idealized positions
and allowed to refine riding on the parent C atom. PLATON41 was
used to calculate bond distances and angles as well as intermolecular
interactions. Graphical representations (ESI) were prepared using
Mercury 3.5.1.42 Further crystallographic information for all
determined structures are given in Table S2. CCDC 1526279 and
1526280 contain the supplementary crystallographic data for FeT0 and
RuT+, respectively; these data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif.
−(CH2)2−, dppe) 3.21 (s (br), 2H, −(CH2)2−, dppe), 4.73 (s, 5H, η5-
C5H5), 7.12 (s (br), 6H, Ph, dppe), 7.24 (s (br), 4H, Ph, dppe), 7.35
(s (br), 6H, Ph, dppe), 7.46 (s (br), 4H, Ph, dppe), 7.55 (s (br), 2H,
H2,6Ph), 8.02 (s (br), 2H, H3,5Ph). 31P NMR (CD2Cl2, 162 MHz):
86.0 (dppe). Anal. Calcd for C38H33N5O2P2Ru·(0.4CH2Cl2,
0.3C6H14): C, 59.27; H, 4.70; N, 8.59. Found: C, 58.89, H, 4.87; N,
8.45.
One-Pot Catalytic Synthesis of Complexes MT0 (M = Fe, Ru).
To Schlenk tubes charged with [CpM(dppe)X] (0.10 mmol), 4-
nitrobenzonitrile (148 mg, 0.10 mmol), thallium hexafluorophosphate
(35 mg, 0.10 mmol) and sodium azide (65 mg, 0.10 mmol) was added
dichloromethane (5 mL). After stirring overnight, at r.t., the solutions
were double filtered and pumped to dryness, and the crude
compounds were washed with n-hexane and recrystallized by slow
diffusion of n-hexane in dichloromethane solutions, affording
crystalline products; η = 83% (M = Fe), η = 85% (M = Ru).
Synthesis of RuT+. To a Schlenk tube charged with [CpRu-
(dppe)Cl] (300 mg, 0.50 mmol), thallium hexafluorophosphate (175
mg, 0.50 mmol) and 5-(4-nitrophenyl)-1H-tetrazole (96 mg, 0.55
mmol), was added dichloromethane (20 mL) and the mixture was
stirred overnight at room temperature, under inert atmosphere. The
solution was double filtered to eliminate thallium chloride and pumped
to dryness, and the crude was washed with n-hexane and recrystallized
by slow diffusion of ethyl ether in dichloromethane solution of the
compound; η = 87%. Using ammonium hexafluorophosphate (82 mg,
0.50 mmol) instead of thallium hexafluorophosphate, RuT+ was
obtained in 79% yield.
Bright yellow; 1H NMR (DMSO-d6, 400 MHz): 2.72−2.93 (m, 2H,
−(CH2)2−, dppe) 3.10−3.21 (m, 2H, −(CH2)2−, dppe), 4.71 (s, 5H,
η5-C5H5), 5.93 (s (br), 1H, H4Tet) 7.11 (s (br), 6H, Ph, dppe), 7.24−
7.28 (m, 4H, Ph, dppe), 7.38−7.42 (m, 6H, Ph, dppe), 7.49 (comp.,
6H, H2,6Ph + Ph, dppe), 8.10 (d, 2H, J = 8.7, H3,5Ph). 13C NMR
(DMSO-d6, 100 MHz): 28.0 (t, JCP = 22.6, −(CH2)2−, dppe), 82.3
(η5-C5H5), 124.3 (C3,5Ph), 126.4 (C2,6Ph), 128.2 (t, JCP = 4.7, Cmeta
,
dppe), 129.0 (t, JCP = 4.5, Cmeta, dppe), 129.7, 129.8 (Cpara, dppe,
C1Ph), 130.9 (t, JCP= 4.8, Cortho, dppe), 132.9 (t, JCP = 5.0, Cortho
,
Computational Details. All DFT calculations were performed
with Gaussian0943 using the PBE1PBE functional, also known as
PBE029 and the hybrid exchange−correlation functional using the
Coulomb-attenuating method (CAM-B3LYP).30 The triple-ζ basis set
with one polarization function LANL2TZ(f) (ruthenium and iron)
and LANL08(d) (phosphorus) were used with the associated effective
core potential (ECP)44 downloaded from the EMSL Basis Set
Library.45 For the other elements, the standard 6-311G** basis set was
employed. Geometry optimizations were performed in dichloro-
methane, without symmetry constraints, using a polarizable continuum
model described with the integral equation formalism variant
(IEFPCM).46
dppe), 134.2 (t, JCP = 22.5, Cipso, dppe), 135.6 (Cipso, dppe), 141.3 (t,
JCP = 21.1, Cipso, dppe), 147.1 (C4Ph), 159.5 (C1). 31P NMR (CDCl3,
−
1
162 MHz): −144.2 (qt, JPF = 712.8, PF6 ), 84.9 (dppe). H NMR
(CD2Cl2, 400 MHz): 2.73 (s (br), 2H, −(CH2)2−, dppe) 3.05 (s (br),
2H, −(CH2)2−, dppe), 4.82 (s, 5H, η5-C5H5), 7.18 (s (br), 6H, Ph,
dppe), 7.29 (s (br), 4H, Ph, dppe), 7.43−7.49 (comp., 12H, H2,6Ph +
Ph, dppe), 8.15 (s (br), 2H, H3,5Ph). 31P NMR (CD2Cl2, 162 MHz):
−
−144.2 (qt, JPF = 712.2, PF6 ), 83.8 (dppe). Anal. Calcd for
C38H34F6N5O2P3Ru: C, 50.67; H, 3.80; N, 7.78. Found: C, 50.65, H,
3.90; N, 7.45%.
pH Switch Studies for RuT+/RuT0. All pH switch experiments
were made using 0.1 mmol of RuT+ (90 mg) or RuT0 (75 mg)
dissolved in 10 mL of dichloromethane. The experiments in dry
conditions were performed in flamed Schlenk tubes and under inert
atmosphere. RuT0 was obtained in 97% yield (73 mg) after silica
filtration, by addition of triethylamine (14 μL, 0.10 mmol) to RuT+.
RuT+ was obtained (as triflate salt) in 89% yield (80 mg) by addition
of triflic acid (9 μL, 0.10 mmol) to RuT0; after washing of the crude
product with ethyl ether (3 × 10 mL). The experiments in wet
conditions were performed in atmospheric conditions, and tetrabuty-
lammonium bromide (catalytic, 2 mg) was dissolved in dichloro-
methane solutions of the compounds. Water, or aqueous solutions of
sodium hydroxide (35 wt %) or of fluoroboric acid (50 wt %), were
used. After intense stirring for 15 min the phases were separated, the
organic phase was dried with anhydrous magnesium sulfate, filtered,
and pumped to dryness. RuT0 was recovered by crude extraction with
ethyl ether (3 × 10 mL) and (posteriorly) RuT+ was recovered by
crude extraction with dichloromethane (2 × 5 mL). RuT0 was
obtained in 20% yield (15 mg) from RuT+ after stirring with water (67
mg of RuT+ recovered) and in 78% yield from RuT+ after stirring with
The static first hyperpolarizabilities βtot were calculated using the
finite field (FF) method by double numerical differentiation of the
energy using the following equation.
β
=
β2 + βy2 + β2
x z
tot
(2)
calculating the individual static components
1
3
β = β +
(β + β + β )
ijj jij jji
∑
i
iii
i≠j
(3)
The same procedure was employed for the calculation of NLO
properties of η5-monocyclopentadienylmetal complexes.9 TD-DFT
calculations were also performed using the same functionals and basis
sets. The scripts to plot the electron density difference maps
(EDDMs) were retrieved from the GaussSum package.47
L
DOI: 10.1021/acs.inorgchem.7b00138
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00138.
■
S
NMR spectra of MT0 and RuT+. Supramolecular
analysis, and further crystallographic data for FeT0 and
RuT+. Cyclic voltammograms of FeT0 and RuT+.
HOMO and LUMO of complexes MT0 and MT+, and
further computational data (PDF)
Accession Codes
CCDC 1526279−1526280 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*(P.R.F.) E-mail: pedro.florindo@tecnico.ulisboa.pt.
*(P.J.C.) E-mail: pjcosta@ciencias.ulisboa.pt.
ORCID
Pedro R. Florindo: 0000-0002-8582-7036
Notes
The authors declare no competing financial interest.
■
́
Garcia-Amoros, J.; Cusido, J.; Raymo, F. M.; Castet, F.; Rodriguez, V.;
Champagne, B. Oxazines: a new class of second-order nonlinear
optical switches. J. Am. Chem. Soc. 2016, 138, 5052−5062.
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Ledoux-Rak, I.; Zyss, J. Large, concentration-dependent enhancement
of the quadratic hyperpolarizability of [Zn(CH3CO2)2(L)2] in CHCl3
on substitution of acetate by triflate. Angew. Chem., Int. Ed. 2003, 42,
456−459. (b) Maury, O.; Le Bozec, H. Molecular engineering of
octupolar NLO molecules and materials based on bipyridyl metal
complexes. Acc. Chem. Res. 2005, 38, 691−704. (c) Cariati, E.; Pizzotti,
M.; Roberto, D.; Tessore, F.; Ugo, R. Coordination and organo-
metallic compounds and inorganic−organic hybrid crystalline
materials for second-order non-linear optics. Coord. Chem. Rev.
2006, 250, 1210−1233. (d) Coe, B. J. Switchable nonlinear optical
metallochromophores with pyridinium electron acceptor groups. Acc.
Chem. Res. 2006, 39, 383−393. (e) Morrall, J. P.; Dalton, G. T.;
Humphrey, M. G.; Samoc, M. Organotransition metal complexes for
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ACKNOWLEDGMENTS
This research was financially supported by Fundaca
■
̧
o para a
̃
Cien
̂
cia e a Tecnologia (FCT) through annual funds UID/
QUI/00100/2013 and UID/MULTI/00612/2013. P.J.C.
acknowledges FCT, Fundo Social Europeu, and Programa
Operacional Potencial Humano for the Investigador FCT
contract and project (IF/00069/2014). We thank Tiago J. L.
Silva for fruitful discussions on the calculations of the
̂
hyperpolarizabilities, and Leonor Corte-Real for the kind
loaning of [(η5-C5H5)Ru(PPh3)2Cl].
(4) Green, M. L. H.; Marder, S. R.; Thompson, M. E.; Bandy, J. A.;
Bloor, D.; Kolinsky, P. V.; Jones, R. J. Synthesis and structure of (cis)-
[1-ferrocenyl-2-(4-nitrophenyl)ethylene], an organotransition metal
compound with a large second-order optical nonlinearity. Nature
1987, 330, 360−362.
DEDICATION
Dedicated to Prof. Maria Helena Garcia on her retirement.
■
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