Syntheses, crystal structures and photophysical measurements of phosphite-substituted schiff base and azobenzene liagands

Article abstract of DOI:10.1002/ejic.201000557

Organic chromophores with charge asymmetry may exhibit significant second-order nonlinear optical (NLO) properties. Metal complexes have been used as the donor, the acceptor, or as the bridge in some of these chromophores. Metal complexes may also be usef

Full text of DOI:10.1002/ejic.201000557

FULL PAPER
DOI: 10.1002/ejic.201000557
Syntheses, Crystal Structures and Photophysical Measurements of
Phosphite-Substituted Schiff Base and Azobenzene Ligands
Makeba B. Murphy-Jolly,*[a] Samuel B. Owens Jr. ,[a] Jason L. Freeman,[a]
Gary M. Gray,*[a] Christopher M. Lawson,[b] and David P. Shelton[c]
Keywords: Nonlinear optics / Schiff bases / Azo compounds / P ligands / Transition metals
Organic chromophores with charge asymmetry may exhibit
significant second-order nonlinear optical (NLO) properties.
Metal complexes have been used as the donor, the acceptor,
(3)2 (7). The X-ray crystal structures of 2 and 5 show that
coordination of the phosphite ligand to the metal atom does
not alter the conformation of the chromophore. Hyper-Ray-
or as the bridge in some of these chromophores. Metal com- leigh scattering (HRS) measurements of the compounds in
plexes may also be useful in dipolar chromophore orientation
and in the building of large molecular structures, but this ap-
proach remains largely unexplored. We herein report the
syntheses and characterization of a novel class of phosphite-
containing chromophores, O2N-1-C6H4-4-CH=N-1-C6H4-4-
1,4-dioxane at 1064 nm indicate that phosphite functionaliza-
tion causes a small decrease in the β values of the Schiff-base
chromophores {β [esu]: 47ϫ10–30 (1), 25ϫ10–30 (2), 30ϫ10–
30
(3} and no change in the β value of the azo chromophore
{β [esu]: 62ϫ10–30 (4)}. The larger β values of the cis-Mo(CO)4-
OP(OC6H4)2 (2) and O2N-1-C6H4-4-X=N-1-C6H4-4-OP- L2 complexes {β [esu]: 38ϫ10–30 (5), 41ϫ10–30 (7)} as com-
(OC10H6)2 [X = CH (3), N (4)], and their transition-metal com-
plexes, cis-Mo(CO)4(2)2 (5), PdCl2(2)2 (6), and cis-Mo(CO)4-
pared to those of the ligands (2 and 3) are consistent with the
90° orientation of two chromophores in the complexes.
A potential and complementary approach to developing
molecules with even stronger second-order NLO responses
Introduction
Second-order NLO organic and organometallic materials is to coordinate known organic chromophores such as (E)-
have been found to be useful in computers, data storage, stilbenes, diaryl Schiff bases, and azobenzene compounds
electro-optic modulation, medical imaging, and telecommu- that are known to have large β values to transition-metal
nications.[1–8] Because the success of many of these applica- centers. Orientations, which result in alignments of the di-
tions depends on the strength of the second-order NLO ma- poles in the complexes, should result in materials with even
terial, there has been a significant effort to improve the per- higher second-order NLO responses. We have initiated a
formance of second-order NLO organic and organometallic project to synthesize phosphite-modified organic chromo-
materials by designing molecules with large first hyperpo- phores that can be easily attached to metal centers and to
larizibilities (β).[9–20] Many of the most promising second- characterize the manner in which coordination to metal
order NLO materials are dipolar aromatic molecules pos- centers affects their β values. We herein report the synthesis
sessing an electron donor and acceptor group, because of novel phosphite-substituted Schiff bases and azoben-
larger second-order optical nonlinearities can arise from the zenes and their respective transition-metal complexes. Co-
intramolecular charge transfer between the opposite groups ordination of the phosphite ligands to transition-metal cen-
through their π-conjugated systems.[21–25] Organometallic ters allows the groups to be oriented in different manners
and coordination metal complexes represent a growing class relative to one another. The introduction of a binaphthyl-
of second-order NLO compounds that offer additional flex- diyl phosphite group into some of these derivatives allows
ibility and tunability.[26]
chiral materials to be prepared. The compounds have been
fully characterized by elemental analyses, multinuclear
NMR, and UV/Vis spectroscopy. Compounds 2 and 5 have
been characterized by X-ray crystallography. The hyper-
Rayleigh scattering (HRS) technique has been used to mea-
sure the β values of the ligands and complexes.
[a] Department of Chemistry, University of Alabama –
Birmingham,
1530 Third Ave S., Birmingham, Alabama 35294-1170, USA
Fax: +1-205-934-2543
E-mail: mbmjolly@uab.edu
gmgray@uab.edu
[b] Department of Physics, University of Alabama – Birmingham,
1530 Third Ave S., Birmingham, Alabama 35294-1170, USA
[c] Department of Physics and Astronomy, University of Nevada –
Las Vegas,
Results and Discussion
Ligand Syntheses
The phosphite-substituted Schiff base and azobenzene li-
gands 2, 3, and 4 were synthesized in moderate to high
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yields by using the procedure shown in Scheme 1. All of the
Of particular interest is the remarkable stability of these
1
reactions were relatively rapid. The synthesis of 2, which ligands. Ligand 2 did not show any changes in its H and
has the smaller biphenyldiyl phosphite group, reached com- 31P{1H}NMR spectra even after six months of exposure to
pletion at ambient temperature after 1 h, and the syntheses both air and water. This enhanced stability of the phos-
of 3 and 4, which have the larger binaphthyldiyl phosphite phites may be due to the D–π–A electronic system in the
groups, reached completion at room temperature after 2 h. Schiff base and azobenzene moieties of the molecule that
The ligands were purified by flash chromatography to avoid presumably reduces the availability of the lone pair of elec-
the significant product loss that occurs upon prolonged ex- trons on the phosphorus atom.
posure to silica gel during traditional column chromatog-
raphy. Ligand 2 was recrystallized by layering hexanes on a
dichloromethane solution of 2 to give X-ray quality crys-
tals. Attempts to obtain X-ray quality crystals of ligand 4
Complex Syntheses
were unsuccessful.
The cis-Mo(CO)4L2 complexes 5 and 7 were synthesized
by simultaneous additions of solutions of Mo(CO)4(nbd)
(nbd = 2,5-norbornadiene) and ligands 2 or 3, respectively,
in dichloromethane by syringe as shown in Scheme 2. Al-
though both reactions were relatively rapid, steric hindrance
due to the bulkier binaphthyl group slowed down the for-
mation of 7. The complexes were purified by fractional
recrystallization by layering hexanes on saturated dichloro-
methane solutions of the complexes. This procedure gave
X-ray quality crystals of 5, but yielded 7 as a microcrystal-
line solid. The two diastereomers of 7 show sufficiently dif-
ferent solubilities so that they could be separated in this
manner. Analytically pure samples of both complexes were
obtained in moderate to good yields by this method.
The reaction of ligand 2 with PdCl2(MeCN)2 in MeCN,
shown in Scheme 2, gave a quantitative yield of analytically
pure 6, which quickly precipitated from the solution. Com-
plex 6 was insoluble in all common organic solvents except
warm [D6]DMSO. Attempts to form dichloropalladium(II)
and dichloroplatinum(II) complexes of the more bulky bi-
naphthyl ligands 3 and 4 were unsuccessful.
NMR Characterization of Ligands and Complexes
The ligands and complexes were characterized by using
1H, 31P{1H}, and 13C{1H} NMR spectroscopy and elemen-
tal analyses. All characterization data are consistent with
the expected structures and demonstrate the high purity of
1
the ligands. The H and 13C{1H} NMR resonances were
partially assigned by using 2D heteronuclear multiple-bond
correlation (HMBC) spectroscopy, ACD/Labs NMR pre-
diction and simulation software, and the assignments for
previously synthesized molecules with similar biphenyl and
binaphthyl backbones.
X-ray Crystal Structures of 2 and 5
Crystals of ligand 2 and of its cis-Mo(CO)4 complex 5
suitable for X-ray diffraction were obtained by slow dif-
fusion of hexanes into concentrated dichloromethane solu-
tions of the compounds. The molecular structures of 2 and
5 were determined and are shown in Figures 1 and 2,
respectively. Both 2 and 5 crystallize in centrosymmetric
monoclinic space groups (P21/c for 2 and P21/n for 5).
Scheme 1. Syntheses of phosphite-substituted Schiff base and azo-
benzene ligands 2–4.
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Phosphite-Substituted Schiff Base and Azobenzene Ligands
Scheme 2. Syntheses of transition-metal complexes of phosphite-substituted Schiff base and azobenzene ligands 5–7.
Figure 1. ORTEP[39] plot of the molecular structure of compound
2. Hydrogen atoms are omitted, and the atomic displacement ellip-
soids are drawn at 25% probability.
Figure 2. ORTEP[39] plot of the molecular structure of compound
5. Hydrogen atoms are omitted, and the atomic displacement ellip-
soids are drawn at 25% probability.
The most important conformational aspect of the struc-
tures of 2 and 5 is the relative planarity of the NLO chro-
mophore in the compounds, because this could affect the
aromaticity of the chromophore. The relative planarity of
the chromophore is defined by rotations about the three 2 and 5 arise from rotations about the N1–C8 bonds
bonds as measured by the torsion angles: (1) C13–C8–N1– [20.1(14) to 39.2(14)°]. There are also smaller rotations
C1, (2) N1–C1–C2–C7, and (3) C4–C5–N2–O1. As the data about the C5–N2 bonds in 5 [12(2) and 18.9(15)°] and
in Table 1 shows, major distortions from planarity in both about the C1–C2 bond in one of the ligands in 5 [169.4(9)°].
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In contrast, the rotations about the C5–N2 and C1–C2 groups having slightly shorter Mo–C bond lengths than do
bonds in 2 are considerably smaller [C4–C5–N2–O1: the carbonyl groups trans to each other (difference ca. 2σ).
4.8(7)°; N1–C1–C2–C7: 178.4(4)°].
This is consistent with the fact that phosphites are stronger
σ-donors and weaker π-acceptors than their carbonyl coun-
terparts. However, the expected longer C–O bonds of the
ligands trans to ligand 2 were not observed, and this is likely
due to the crystal-packing forces in the solid-state crystal
structure.[29,30]
Table 1. Selected torsion angles [°] for 2 and 5 (5L/5LЈ represent
the two ligands in complex 5).
2
5L
5LЈ
C10–C11–O3–P
C11–O3–P–O4
C11–O3–P–O5
C13–C8–N1–C1
N1–C1–C2–C7
C4–C5–N2–O1
C14–C19–C20–C25
C25–O5–P–O4
C20–C25–O5–P
C14–O4–P–O5
C19–C14–O4–P
–167.4(3)
–81.2(3)
177.3(3)
–23.6(7)
178.4(4)
4.8(7)
43.4(6)
48.1(3)
–77.2(4)
43.8(3)
–74.4(4)
–140.2(7)
–78.9(7)
178.3(6)
39.2(14)
–172.7(9)
18.9(15)
43.3(13)
52.6(7)
–143.4(7)
–83.9(7)
174.7(4)
20.1(14)
–169.4(9)
12(2)
43.2(13)
49.8(6)
–75.7(9)
41.9(6)
Table 2. Selected bond angles [°] for 2 and 5 (5L/5LЈ represent the
two ligands in complex 5).
2
5L
5LЈ
5
O5–P–O3
O5–P–O4
O3–P–O4
C11–O3–P 122.6(3)
C1–N1–C8 121.0(4)
C6–C5–N2 119.5(4)
92.33(15) 91.5(3)
100.70(15) 102.0(3)
102.35(15) 103.7(3)
91.4(3)
P1Ј–Mo–P1
98.33(8)
100.7(3)
103.4(3)
127.2(6)
122.0(8)
C27–Mo–P1 87.8(3)
C26–Mo–P1 175.2(3)
C28–Mo–P1 86.5(3)
C29–Mo–P1 92.3(3)
–74.6(9)
36.6(8)
–72.1(10)
126.0(5)
121.2(9)
–73.9(10)
119.5(10) 119.1(14) C26–Mo–P1Ј 86.3(3)
C27–Mo–P1Ј 173.9(3)
C28–Mo–P1Ј 91.2(3)
C29–Mo–P1Ј 88.9(3)
Another interesting aspect of the structures of 2 and 5 is
the conformation of the seven-membered dioxaphosphepin
rings (2: P–O4–C14–C19–C20–C25–O5; 5L: P1–O4–C14–
C19–C20–C25–O5; 5LЈ: P1Ј–O4Ј–C14Ј–C19Ј–C20Ј–C25Ј–
O5Ј). This is best defined by the five torsion angles C14–
C19–C20–C25, C25–O5–P–O4, C20–C25–O5–P, C14–O4–
P–O5, and C19–C14–O4–P. The similarity of these angles
in 2, 5L, and 5LЈ suggests that coordination to the phos-
phorus atom does not significantly affect the conformation
of the dioxaphosphepin ring. This is consistent with what
has been previously observed for cis-Mo(CO)4(2,2Ј-
C12H8O2POCH3)2, which has simpler phosphepin li-
gands,[27] and indicates that the phosphepin ring has a
strongly preferred conformation that is not significantly af-
fected by hydrogen bonding or crystal-packing forces.
The orientation of the chromophore relative to the phos-
phepin ring is also important. Given the rigidity of the
phosphepin ring described in the previous paragraph, this
is determined solely by rotations about the P–O3 and O3–
C11 bonds. The rotation about the P–O3 bond as shown
by the C11–O3–P–O4 and C11–O3–P–O5 torsion angles in
Table 3. Selected bond lengths [Å] for 2 and 5 (5L/5LЈ represent
the two ligands in complex 5).
2
5L
5LЈ
5
O3–P
O4–P
O5–P
1.626(3) 1.611(6) 1.606(6)
1.625(3) 1.617(6) 1.607(6)
1.620(3) 1.606(6) 1.636(5)
P1–Mo
2.429(3)
P1Ј–Mo 2.428(2)
C26–Mo 2.018(11)
C27–Mo 1.980(11)
C28–Mo 2.028(11)
C29–Mo 2.033(10)
C26–O6 1.135(10)
C27–O7 1.150(11)
C28–O8 1.154(11)
C29–O9 1.130(10)
C11–O3 1.385(5) 1.396(10) 1.403(9)
C8–N1
C1–N1
C1–C2
C5–N2
N2–O1
N2–O2
1.432(5) 1.424(11) 1.443(10)
1.249(5) 1.258(11) 1.255(10)
1.465(6) 1.466(13) 1.471(12)
1.465(6) 1.470(13) 1.491(16)
1.211(6) 1.191(10) 1.200(17)
1.214(5) 1.222(11) 1.216(15)
Linear and Nonlinear Optical Characterization
The linear absorption of solutions of chromophores
Table 1 is nearly the same in both the free ligand 2 and in NHA and 1, ligands 2–4 and the metal complexes 5 and 7
the two ligands in 5. In contrast, the rotation about the O3– in 1,4-dioxane are summarized in Table 4. All the com-
C11 bond as shown by the C10–C11–O3–P torsion angle pounds show an absorption band between 340 and 380 nm.
changes significantly (an average of 25°) when the ligands These bands undergo a hypsochromic shift with both phos-
are coordinated to the cis-Mo(CO)4 center. This reduction phite functionalization of the precursor chromophore and
in the C10–C11–O3–P torsion angle upon coordination ap- complexation of the phosphite to the molybdenum center.
pears to minimize the interaction between the adjacent li- Shoulders observed between 230 and 280 nm are due to the
gands in the complex. In spite of the difference in the tor- overlapping πǞπ* transitions of the biaryl moieties[31] and
sion angles, the chromophores are oriented to point away the chromophore.
from the phosphepin groups in both the free ligand 2 and
in the two ligands in 5.
The β values of solutions of 1–5 and 7 in 1,4-dioxane
were measured by using the HRS technique. The HRS mea-
The final conformational aspect of interest is the coordi- surements were carried out at a laser wavelength of 1064 nm
nation environment of the molybdenum atom in 5, which is and a sample temperature of 25 °C. Calibration of the re-
a slightly distorted octahedron. The P1Ј–Mo–P1 bite angle sults of the HRS experiment requires a reference standard,
is slightly larger than 90°, which causes the C27–Mo–P1, and 4-nitroaniline was used for this purpose, because its
C28–Mo–P1, C26–Mo–P1Ј, and C29–Mo–P1Ј angles to be second-order nonlinearity is well studied and has been de-
slightly less than 90°, as shown in Table 2. The molybde- scribed previously in great detail.[32–35] Table 4 summarizes
num–carbonyl bond lengths in Table 3 show the expected the data obtained from the HRS measurements of these
trend[27,28] with the carbonyl groups trans to the phosphepin ligands.
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Phosphite-Substituted Schiff Base and Azobenzene Ligands
Table 4. Linear and nonlinear measurements of compounds (NHA, bulk of the biphenyl- and binaphthyl-derived phosphites,
1–5, and 7).
because the two groups should have similar electron-with-
drawing abilities.
[a]
Sample λmax [nm]
β/βpNA
β (ϫ10–30 esu)
IVV/IHV
It is also interesting to compare the β values for the cis-
Mo(CO)4L2 complexes 5 and 7 with the predictions of a
simple additive model for a Λ-shaped dimer composed of
1D chromophores (for which βzzz is the only nonzero tensor
component).[37] In the case that the angle between the chro-
mophores is θ = 90°, the model predicts that the nonzero
tensor components for the dimer are βzzz = βzxx, the depo-
larization ratio is IVV/IHV = 5, and βHRS for the dimer is
1.87-times βHRS for the monomer. For ligands 2 and 3 the
observed depolarization ratio 4.4Ϯ0.2 is consistent with
βzxx/βzzz = –0.06, so the assumed dominance of βzzz for the
free ligand is a good approximation, and the angle θ =
pNA
NHA
354
374
377
346
356
351
359
372
1
21.3
4.8Ϯ0.1
4.8Ϯ0.1
4.8Ϯ0.1
4.4Ϯ0.2
4.4Ϯ0.1
4.6Ϯ0.1
4.6Ϯ0.1
4.2Ϯ0.1
2.83Ϯ0.10
2.20Ϯ0.08
1.18Ϯ0.05
1.42Ϯ0.05
2.90Ϯ0.10
1.78Ϯ0.06
1.91Ϯ0.07
60Ϯ2
47Ϯ2
25Ϯ1
30Ϯ1
62Ϯ2
38Ϯ1
41Ϯ1
1
2
3
4
5
7
[a] β = 2470 au for pNA in 1,4-dioxane at 1064 nm;[38] 1 au =
3.20636ϫ10–53 C3mJ–2 = 8.6392ϫ10–33 esu = 3.6213ϫ10–42 m4 V–1.
Many nonlinear optical chromophores exhibit significant 85.8(3)° from the X-ray crystal structure of 5 is close to θ
two-photon fluorescence (2PF).[32,36] This has sometimes = 90°. Both the depolarization ratios (5: 4.6Ϯ0.1; 7:
led to overestimation of the β values obtained from HRS 4.2Ϯ0.1) and the ratios of βHRS for the dimer and mono-
measurements. Since the 2PF spectrum is typically much mer (5: 1.51Ϯ0.08; 7: 1.35Ϯ0.07) for the complexes are
broader than the HRS spectrum, spectral measurements are smaller than the values predicted for θ = 90°. Adjustment
the most reliable way to distinguish between HRS and of the angle can force agreement for either the depolariza-
2PF.[32,36] In this work, the HRS signal is measured with tion ratio (at θ ≈ 100°) or βHRS (at θ ≈ 125°), but not both
two different detection bandwidths to assess possible 2PF simultaneously. However, the dipole–induced-dipole inter-
contamination of the HRS signal. The 60 cm–1 (1.6 nm) action between the chromophores is expected to decrease
bandwidth includes the entire HRS spectrum but may also βzzz and leave βzxx unchanged. Fixing the angle at θ = 86°
include significant 2PF. The 0.3 cm–1 (0.01 nm) bandwidth and reducing the additive model βzzz for the dimer by the
effectively excludes 2PF but cannot be used alone since it factor A (5: 0.72; 7: 0.58) gives agreement with all the obser-
cuts off the wings of the HRS spectrum. For pNA, 58 % of vations. This suggests that the simple noninteracting model
the intensity of the entire HRS spectrum (measured with correctly predicts the large off-diagonal component βzxx
,
60 cm–1 bandwidth) falls within the 0.3 cm–1 bandwidth. but interaction between the chromophores reduces the diag-
The fraction will be larger for all the other chromophores onal component βzzz for the complex.
in this study since the slower reorientation of these larger
Finally, the relationship between the β and λmax values
molecules results in a narrower HRS spectrum. The of the compounds is of interest. It can be used to predict
measured fraction is in the range 74–85% for all the ligands the β values of similar compounds, and it also gives an indi-
except 4, which indicates negligible 2PF contribution to the cation of the trade-off between nonlinearity and trans-
signal. The fraction for ligand 4 is 62%, which indicates parency.[38] Transparency is highly desirable for these com-
that 2PF contributes 15%Ϯ5% of the signal measured with pounds; however, it must not compromise their nonlinear-
60 cm–1 bandwidth. The results reported for 4 are corrected ity. Figure 3 shows a logarithmic plot of β vs. λmax for all
for this 2PF contribution.
compounds with the Schiff base chromophore (1–3, 5, and
The structure–property relationships in this initial library
of compounds provide insight into the design of the next
generation of compounds with optimized second-order
nonlinear optical properties. One point of interest is the ef-
fect of phosphite substitution on β. A comparison of the β
values of the NLO chromophores with the corresponding
phosphites (1; cf. 2/3) shows that phosphite functionaliza-
tion of the second-order NLO chromophores causes a small
decrease in the β values. This is not surprising, because
phosphites are electron-withdrawing, therefore a phosphite
ester oxygen atom should be a poorer donor than a hydroxy
oxygen atom. However, when NHA is compared with 4,
there is little change in the observed β. This indicates that
the electron-withdrawing effect of the phosphite group is
counteracted by the electron-withdrawing effect of the azo
bridge. It is also interesting that the binaphthyl-derived
phosphite 3 exhibits a smaller decrease in β than does the
analogous biphenyl-derived phosphite 2. The differences in
Figure 3. Plot of logβ vs. logλmax for Schiff base compounds 1–3,
the β values are likely due to the difference in the steric 5, and 7.
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1
4.07 g (84.5%) of 1. H NMR (400 MHz, MeOD, 25 °C): δ = 8.65
(s, 1 H, CH=N), 8.29 (d, 3JH,H = 8.7 Hz, 2 H, CArH), 8.07 (d, 3JH,H
7). There is a good correlation between logβ and logλmax
,
with β ϰ λmax6.8. NHA and compound 4 are diazo deriva-
tives (different functional group) and therefore are not in-
cluded in the correlation. The slope of the line in Figure 3
(6.8) is intermediate between the slopes similarly obtained
for para-substituted benzenes (4.4) and stilbenes (8.7).[38]
3
= 9.0 Hz, 2 H, CArH), 7.26 (d, JH,H = 9.0 Hz, 2 H, CArH), 6.83
(d, 3JH,H = 8.7 Hz, 2 H, CArH) ppm. 13C NMR (400 MHz, MeOD,
25 °C): δ = 157.63 (s, CH=N), 155.17 (s, CArO), 129.17 (s, CArH),
123.88 (s, CArH), 123.01 (s, CArH), 115.91 (s, CArH) ppm.
C13H10N2O3 (242.23): calcd. C 64.46, H 4.16; found C 64.44, H
4.14.
1,1Ј-Biphenyl-2,2Ј-diyl 4-{[(4-Nitrophenyl)methylene]amino}phenyl
Phosphite, O2N-1-C6H4-4-CH=N-1-C6H4-4-OP(OC6H4)2 (2): A
solution of 1,1Ј-biphenyl-2,2Ј-diyl phosphorochloridite (1.30 g,
5.20 mmol) in THF (7 mL) was added to a stirred mixture of 4-
{[(4-nitrophenyl)methylene]amino}phenol (1.25 g, 5.20 mmol) and
NEt3 (0.73 mL, 5.20 mmol) in THF (20 mL) in a 100 mL Schlenk
tube. The mixture was stirred at room temperature for 1 h during
which time the triethylammonium chloride byproduct precipitated
from the solution. The solution was separated from the precipitate
by cannula filtration. The filtrate was then concentrated to dryness
under reduced pressure. The residue was dissolved in dichlorometh-
ane (15 mL) and quickly filtered through a ca. 2 cm thick pad of
silica gel in a 15 mL medium sintered glass funnel under nitrogen.
The filtrate was then concentrated to dryness under reduced pres-
sure to give a bright yellow solid. Recrystallization from CH2Cl2/
hexanes by using a layering technique gave 2.06 g (89.3%) of 2 as
yellow crystals. 31P{1H} NMR (400 MHz, CDCl3, 25 °C): δ =
Conclusions
We have developed a novel class of remarkably stable
phosphite ligands with second-order NLO chromophores
as substituents and their corresponding transition-metal
complexes. The ligands have been prepared with phosphite
groups. HRS measurements of the ligands in solution dem-
onstrate that the second-order NLO properties of the li-
gands are primarily a function of the chromophores, but
are also sensitive to the nature of the phosphite substituent.
This demonstrates that modification of both the chromo-
phore and the phosphite moiety can be used to improve the
first hyperpolarizability (β) of the ligand. Modification of
the phosphite substituent and the transition-metal center to
which it is attached can also be used to orient the chromo-
phores. A plot of logβ vs. logλmax shows a direct relation-
ship between the two properties and also confirms the exis-
tence of a trade-off between the transparency and the non-
linearity of the compounds. We are currently exploring the
use of phosphorus-based substituents on other known chro-
mophores with better donor abilities.
1
144.33 (s) ppm. H NMR (400 MHz, CDCl3, 25 °C): δ = 8.56 (s, 1
3
3
H, CH=N), 8.33 (d, JH,H = 8.8 Hz, 2 H, CArH), 8.07 (d, JH,H
=
3
4
8.8 Hz, 2 H, CArH), 7.51 (dd, JH.H = 7.6, JP,H = 1.7 Hz, 2 H,
C
ArH), 7.42–7.22 (m, 10 H, CArH) ppm. 13C NMR (400 MHz,
2
CDCl3, 25 °C): δ = 157.16 (s, CH=N), 151.30 (d, JP,C = 7.3 Hz,
CArO), 149.66 (s, CArNO2), 149.26 (d, 2JP,C = 5.1 Hz, CArO), 147.47
3
(s, CArN=C), 141.95 (s, CArCH=N), 131.50 (d, JP,C = 3.3 Hz,
CArCAr), 130.51 (s, CArH), 129.78 (s, CArH), 129.73 (s, CArH),
126.04 (s, CArH), 124.46 (s, CArH), 122.91 (s, CArH), 122.50 (d,
Experimental Section
3
|3JPC| = 1.3 Hz, CArH), 121.72 (d, JP,C = 7.6 Hz, CArH) ppm.
Materials and Methods: All manipulations were carried out under
dry nitrogen by using standard Schlenk-line and glove-box tech-
niques.[40] All solvents and NEt3 were distilled from the appropriate
drying agent and stored over molecular sieves (4 Å) under nitrogen
until use. Unless otherwise stated, all reagents were obtained com-
mercially and used without further purification. The 1,1Ј-biphenyl-
2,2Ј-diol, (Ϯ)-1,1Ј-binaphthyl-2,2Ј-diol, and (–)-(S)-1,1Ј-binaphth-
yl-2,2Ј-diol, purchased from Aldrich, were dried by azeotropic dis-
tillation with toluene. The 1,1Ј-biphenyl-2,2Ј-diyl phosphorochlor-
C25H17N2O5P (456.39): calcd. C 65.79, H 3.75; found C 65.62, H
3.95.
(؎)-1,1Ј-Binaphthyl-2,2Ј-diyl 4-{[(4-Nitrophenyl)methylene]amino}-
phenyl Phosphite, O2N-1-C6H4-4-CH=N-1-C6H4-4-OP(OC10H6)2
(3): A solution of (Ϯ)-1,1Ј-binaphthyl-2,2Ј-diyl phosphorochlorid-
ite (1.21 g, 3.45 mmol) in THF (5 mL) was added to a stirred mix-
ture of 4-{[(4-nitrophenyl)methylene]amino}phenol (0.83 g,
3.45 mmol) and NEt3 (0.48 mL, 3.45 mmol) in THF (10 mL) in a
50 mL Schlenk tube. The mixture was stirred at room temperature
for 2 h during which time the triethylammonium chloride byprod-
uct precipitated from the solution. The solution was separated from
the precipitate by cannula filtration. The filtrate was then concen-
trated to dryness under reduced pressure. The solid residue was re-
dissolved in 15 mL of dichloromethane and filtered through a ca.
2 cm thick pad of silica gel in a 15 mL medium sintered glass funnel
under nitrogen. Concentration to dryness under reduced pressure
gave 1.28 g (67.0%) of pure 3 as a bright yellow solid. 31P{1H}
NMR (400 MHz, CD2Cl2, 25 °C): δ = 145.71 (s) ppm. 1H NMR
(400 MHz, CD2Cl2, 25 °C): δ = 8.60 (s, 1 H, CH=N), 8.33 [d,
3J(H,H) = 9.0 Hz, 2 H, CArH], 8.10–7.97 (m, 6 H, CArH), 7.62–
7.20 (m, 12 H, CArH) ppm. 13C NMR (400 MHz, CD2Cl2, 25 °C):
idite,[41]
(Ϯ)-1,1Ј-binaphthyl-2,2Ј-diyl
phosphorochloridite,[42]
(–)-(S)-1,1Ј-binaphthyl-2,2Ј-diyl phosphorochloridite,[43] 4-hy-
droxy-4Ј-nitroazobenzene (NHA),[44] PdCl2(MeCN)2[45], and Mo-
(CO)4(nbd)[46] were prepared according to literature methods.
1
Characterization: 31P{1H}, 13C{1H} and H NMR spectra were re-
corded with a Bruker DRX400 FT-NMR spectrometer with 85%
H3PO4 and (CH3)4Si as external and internal references, respec-
tively. Chemical shifts (δ) are in parts per million (ppm). Elemental
analyses were performed by Atlantic Microlab, Inc.
4-{[(4-Nitrophenyl)methylene]amino}phenol, O2N-1-C6H4-4-CH=N-
1-C6H4-4-OH (1): The synthesis and purification of 1 was adapted
from literature methods that have been used to prepare related
compounds.[47,48]
A
solution of 4-nitrobenzaldehyde (3.00 g,
δ = 157.49 (s, CH=N), 151.16 (d, JP,C = 7.9 Hz, CArO), 149.68 (s,
2
2
20.0 mmol) in MeCN (50 mL) was added slowly to a stirred solu-
tion of 4-aminophenol (2.20 g, 20.0 mmol) in MeOH (50 mL) at
room temperature. The reaction mixture was stirred for 2 h, until
a bright orange color was observed. The bright orange solution
was then concentrated to dryness at 50 °C under aspirator vacuum,
and the solid residue was recrystallized from hot toluene to give
CArNO2), 147.85 (d, JP,C = 4.4 Hz, CArO), 147.71 (s, CArN=C),
2
147.28 (d, JP,C = 2.3 Hz, CArO), 142.05 (s, CArCH=N), 133.15 (s,
CArH), 132.87 (s, CArH), 132.18 (s, CArH), 131.72 (s, CArH), 131.07
(s, CArH), 130.39 (s, CArH), 129.75 (s, CArH), 128.87 (s, CArH),
128.79 (s, CArH), 127.17 (s, CArH), 127.10 (s, CArH), 126.90 (s,
CArH), 126.81 (s, CArH), 125.77 (s, CArH), 125.61 (s, CArH), 124.63
5268
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5263–5271

Phosphite-Substituted Schiff Base and Azobenzene Ligands
(s, CArH), 124.36 (s, CArH), 124.32 (s, CArH), 122.96 (s, CArH),
121.97 (s, CArH), 121.59 (s, CArH), 121.51 (s, CArH) ppm.
C33H21N2O5P (556.51): calcd. C 71.22, H 3.80; found C 70.93, H
4.11.
simultaneously and dropwise to a 100-mL Schlenk flask equipped
with a stirrer bar. The solution was stirred at room temperature
for 7 h. The orange-yellow solution was then concentrated under
reduced pressure, and the solid residue was triturated with hexanes.
Recrystallization by layering hexanes on a CH2Cl2 solution of the
solid residue gave 0.07 g (50%) of pure 7 as an orange-yellow
microcrystalline solid. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C):
(–)-4-Hydroxy-4Ј-nitroazophenyl (S)-1,1Ј-Binaphthyl-2,2Ј-diyl Phos-
phite, O2N-1-C6H4-4-N=N-1-C6H4-4-OP(OC10H6)2 (4): The pro-
cedure for 3 was repeated with a solution of (–)-(S)-1,1Ј-binaphth-
yl-2,2Ј-diyl phosphorochloridoite (0.69 g, 2.0 mmol) in THF
(5 mL) and a mixture of 4-hydroxy-4Ј-nitroazobenzene (NHA)
(0.48 g, 2.0 mmol) and NEt3 (0.27 mL, 2.0 mmol) in THF (10 mL)
to yield 0.54 g (50%) of pure 4 as a bright orange, microcrystalline
1
δ = 178.57 (s) ppm. H NMR (400 MHz, CD2Cl2, 25 °C): δ = 8.32
3
(d, JH,H = 8.8 Hz, 4 H, CArH), 8.24 (s, 2 H, CH=N), 8.00–7.90
3
(m, 10 H, CArH), 7.77 (d, JH,H = 8.9 Hz, 2 H, CArH), 7.58 (d,
3JH,H = 8.8 Hz, 2 H, CArH), 7.50–7.25 (m, 14 H, CArH), 7.03 (d,
4
3JH,H = 8.7 Hz, 4 H, CArH), 6.84 (dd, 3JH,H = 8.8, JH,H = 1.9 Hz,
solid. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ = 145.02 (s)
4 H, CArH) ppm. 13C NMR (400 MHz, CD2Cl2, 25 °C): δ = 157.17
(s, CH=N), 150.40 (s, CArO), 149.28 (s, CArNO2), 148.59 (s, CArO),
147.76 (s, CArN=C), 147.08 (s, CArO), 141.57 (s, CArCH=N), 132.84
(s, CArH), 132.32 (s, CArH), 131.71 (s, CArH), 131.21 (s, CArH),
130.50 (s, CArH), 129.95 (s, CArH), 129.32 (s, CArH), 129.32 (s,
CArH), 128.27 (s, CArH), 127.04 (s, CArH), 126.80 (s, CArH), 126.40
(s, CArH), 126.36 (s, CArH), 125.40 (s, CArH), 125.34 (s, CArH),
123.95 (s, CArH), 122.98 (s, CArH), 122.80 (s, CArH), 122.47 (s,
CArH), 122.17 (s, CArH), 122.06 (s, CArH), 121.01 (s, CArH) ppm.
C72.4H46.8MoN4O14P2 (7·0.5C6H12) (1354.67): calcd. C 64.19, H
3.48; found: C 64.47, H 3.52.
3
ppm. 1H NMR (400 MHz, CD2Cl2, 25 °C): δ = 8.38 (d, JH,H
=
8.9 Hz, 2 H, CArH), 8.09–7.98 (m, 8 H, CArH), 7.51–7.33 (m, 10 H,
ArH) ppm. C32.1H20.2Cl0.2N3O5P (4·0.1CH2Cl2) (565.99): calcd. C
C
68.11, H 3.60; found: C 67.94, H 3.59.
Tetracarbonyl(1,1Ј-biphenyl-2,2Ј-diyl 4-{[(4-nitrophenyl)methylene]-
amino}phenyl phosphite)molybdenum(0), cis-Mo(CO)4(2)2 (5): Solu-
tions of 2 (0.20 g, 0.45 mmol) in CH2Cl2 (5 mL) and Mo(CO)4-
(nbd) (0.07 g, 0.2 mmol) in CH2Cl2 (5 mL) were added simulta-
neously and dropwise to a 100 mL Schlenk flask equipped with a
stirrer bar. The solution was stirred at room temperature for 3 h.
The orange-yellow solution was then concentrated under reduced
pressure, and the solid residue was triturated with hexanes.
Recrystallization by layering hexanes on a CH2Cl2 solution of the
solid residue gave 0.17 g (70%) of pure 5 as orange-yellow crystals.
X-ray Data Collection and Solution: Suitable single crystals of 2
and 5 were glued on glass fibers with epoxy and aligned upon an
Enraf–Nonius CAD4 single-crystal diffractometer under aerobic
conditions. Standard peak search and automatic indexing routines
followed by least-squares fits of 25 accurately centered reflections
resulted in accurate unit cell parameters for each. The space groups
of the crystals were assigned on the basis of systematic absences
and intensity statistics. All data collections were carried out by
using the CAD4-PC software,[49] and details of the data collections
are given in Table 5. The analytical scattering factors of the com-
plex were corrected for both ∆fЈ and i∆fЈЈ components of anoma-
lous dispersion. All data were corrected for Lorentz and polariza-
tion effects. The data for compound 5 were corrected for absorp-
tion by using ψ-scans of four reflections with χ Ͼ 80°, and empiri-
cal absorption corrections were applied when necessary. All crystal-
lographic calculations were performed with the Siemens
SHELXTL-PC program package.[50] The Mo and P positions were
located by using the Patterson method with all non-hydrogen atoms
located in difference Fourier maps. Full-matrix refinements of the
positional and anisotropic thermal parameters for all non-hydrogen
atoms vs. F2 were carried out. All hydrogen atoms were placed in
calculated positions with the appropriate molecular geometry and
δ(C–H) = 0.93 Å for aromatic hydrogen atoms. The isotropic ther-
mal parameter of each hydrogen atom was fixed equal to 1.2-times
the Ueq value of the atom to which it is bound. Selected torsion
angles, bond angles, and bond lengths for 2 and 5 are given in
Tables 1, 2, and 3, respectively. The dihedral angle between the li-
gands in compound 5 was found to be 85.8(3)°. CCDC-750122 (for
2) and -750123 (for 5) contain the crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ = 174.84 (s) ppm. H
NMR (400 MHz, CD2Cl2, 25 °C): δ = 8.39 (s, 2 H, CH=N), 8.24
3
3
(d, JH,H = 8.9 Hz, 4 H, CArH), 7.99 (d, JH,H = 8.8 Hz, 4 H,
C
3
4
ArH), 7.45 (dd, JH,H = 8.0, JH,H = 1.9 Hz, 4 H, CArH), 7.32–
7.05 (m, 20 H, CArH) ppm. 13C NMR (400 MHz, CD2Cl2, 25 °C):
δ = 210.62 (m, AXXЈ, trans-CO), 205.62 (m, AX2, cis-CO), 157.58
(s, CH=N), 150.86 (s, CArO), 149.69 (s, CArNO2), 149.64 (s, CArO),
148.14 (s, CArN=C), 141.98 (s, CArCH=N), 130.64 (s, CArCAr),
130.47 (s, CArH), 129.89 (s, CArH), 129.76 (s, CArH), 126.29 (s,
CArH), 124.35 (s, CArH), 122.68 (s, CArH), 122.63 (s, CArH), 122.58
(s, CArH) ppm. C54H34MoN4O14P2 (1120.76): calcd. C 57.87, H
3.06; found C 57.46, H 3.00.
(1,1Ј-Biphenyl-2,2-diyl 4-{[(4-nitrophenyl)methylene]amino}phenyl
phosphite)dichloridopalladium(II), PdCl2(2)2 (6): Ligand 2 (0.043 g,
0.097 mmol) was added to a stirred solution of PdCl2(MeCN)2
(0.0085 g, 0.049 mmol) in CH3CN (20 mL). A pale yellow precipi-
tate was observed after stirring for 10 min. The reaction mixture
was stirred for an additional 30 min. The solid was allowed to settle
to the bottom of the flask, and the supernatant removed with a
Pasteur pipette. The residue was washed with hexanes and dried
under vacuum overnight to yield 0.051 g (Ͼ99%) of pure 6 as a
pale yellow solid. 31P{1H} NMR (400 MHz, [D6]DMSO, 25 °C): δ
= 109.34 (s) ppm. 1H NMR (400 MHz, [D6]DMSO, 25 °C): δ =
8.68 (s, 2 H, CH=N), 8.31 (d, 3JH,H = 8.6 Hz, 2 H, CArH), 8.16 (d,
3
3JH,H = 8.6 Hz, 4 H, CArH), 7.62 (d, JH,H = 7.1 Hz, CArH), 7.50–
7.05 (m, 20 H, CArH) ppm. 13C NMR (400 MHz, [D6]DMSO,
25 °C): δ = 160.20 (s, CH=N), 149.73 (s, CArO), 149.10 (s, CArNO2),
148.10 (s, CArO), 147.60 (s, CArN=C), 142.13 (s, CArCH=N), 131.52
(s, CArCAr), 130.57 (s, CArH), 129.04 (s, CArH), 128.32 (s, CArH),
124.90 (s, CArH), 123.75 (s, CArH), 122.56 (s, CArH), 122.39 (s,
CArH), 121.58 (s, CArH) ppm. C50H34Cl2N4O10P2Pd (1090.11):
calcd. C 55.09, H 3.14; found C 55.10, H 3.18.
Linear and Nonlinear Optical Characterization: The linear optical
properties of the ligands in solution were characterized by UV/
Vis spectroscopy. Electronic spectra were recorded of 1.0ϫ10–5

solutions of the ligands and complexes in 1,4-dioxane with a Varian
4-{[(4-nitrophenyl)methylene]- Cary-100 UV/Vis spectrophotometer. The experimental methods
[(–)-(S)-1,1Ј-Binaphthyl-2,2Ј-diyl
amino}phenyl phosphite]tetracarbonylmolybdenum(0), cis-Mo(CO)4- used for the HRS measurements were similar to those previously
(3)2 (7): Solutions of 3 (0.25 g, 0.45 mmol) in CH2Cl2 (5 mL) and
reported.[32,51] Linearly polarized pulses (1064 nm wavelength,
Mo(CO)4(nbd) (0.07 g, 0.2 mmol) in CH2Cl2 (5 mL) were added
35 µJ pulse energy, 100 ns pulse duration, 4 kHz repetition rate)
Eur. J. Inorg. Chem. 2010, 5263–5271
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5269

M. B. Murphy-Jolly, G. M. Gray et al.
FULL PAPER
Table 5. Crystal data and structure refinement for 2 and 5.
2
5
Empirical formula
Formula mass
Temperature [K]
Wavelength [Å]
Crystal system
Space group
C25H17N2O5P
456.38
293(2)
0.71073
monoclinic
P21/c
9.910(2)
C54H34MoN4O14P2
1120.73
293(2)
0.71073
monoclinic
P21/n
a [Å]
10.697(2)
b [Å]
c [Å]
28.093(6)
7.8199(16)
35.989(7)
13.090(3)
β [°]
101.08(3)
2136.6(7)
4
1.419
0.170
944
92.51(3)
5034.7(17)
4
1.479
0.397
Volume [Å3]
Z
Density (calculated) [Mg/m3]
Absorption coefficient µ [mm–1]
F(000)
2280
Crystal size [mm]
θ range for data collection [°]
Index ranges
0.4ϫ0.6ϫ0.8
2.09–22.48
–10 Յ h Յ 10
30 Յ k Յ 30
–8 Յ l Յ 1
0.2ϫ0.4ϫ0.8
2.22–22.47
–1 Յ h Յ 11
0 Յ k Յ 38
–14 Յ l Յ 14
7810
Reflections collected
6932
Independent reflections
Completeness to θ = 22.48°
Absorption correction
Max./min. transmission
Refinement method
2794 [R(int) = 0.0759]
100.0%
none
6559 [R(int) = 0.0593]
99.9%
empirical
0.2608/0.2362
full-matrix least squares on F2
6559/0/676
0.964
R1 = 0.0640, wR2 = 0.1530
R1 = 0. 1801, wR2 = 0.1953
0.655/–0.507
–
full-matrix least squares on F2
2794/0/298
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
1.106
R1 = 0.0658, wR2 = 0.1637
R1 = 0.1026, wR2 = 0.1809
0.489/–0.341
Largest difference peak/hole [eÅ–3]
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Received: May 19, 2010
Published Online: October 5, 2010
Eur. J. Inorg. Chem. 2010, 5263–5271
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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