Synthesis and Single-Crystal X-ray Investigation of 4-Azido-2-(triphenylphosphinimino)-3,5,6-trifluorobenzonitrile: A Chromogenic Nitrene Precursor for Photolabeling

Full text of DOI:10.1021/ic9510021

3716
Inorg. Chem. 1996, 35, 3716-3718
performance liquid chromatography (HPLC) with a multiwave-
length detector.
Synthesis and Single-Crystal X-ray Investigation
of 4-Azido-2-(triphenylphosphinimino)-3,5,6-
trifluorobenzonitrile: A Chromogenic Nitrene
Precursor for Photolabeling
A second goal is to introduce bifunctional character to the
photoprobe by attaching chelating agents for the incorporation
of transition metals. Ligands with phosphorus-nitrogen back-
bones are of particular interest by virtue of their chelation
potential with a variety of transition metals (e.g., Pd, Re, Rh,
etc.), the parallel radioisotopes of which are useful in radiop-
harmaceuticals.17 In continuation of our earlier studies on the
development of photolabeling agents,18-23 we report here the
synthesis and single crystal X-ray structure of 4-azido-2-
(triphenylphosphinimino)-3,5,6-trifluorobenzonitrile, 2, a chro-
mogenic nitrene precursor, and the complexation of 2 with
Pd(II). The fundamental coordination properties of the Pd(II)
complex system can be extended to the corresponding analogues
with 109Pd, a â-emitting therapeutic radionuclide for use in
radioimmunotherapy.
Raghoottama S. Pandurangi,† Kattesh V. Katti,§
Robert R. Kuntz,*,† Charles L. Barnes,† and
Wynn A. Volkert‡,§
Department of Chemistry, University of Missouri, Columbia,
Missouri 65211, H. S. Truman VA Hospital, Columbia,
Missouri 65211, and Center for Radiological Research and
MU Research Reactor, Alton Building Laboratories,
301 Business Loop 70 W, Columbia, Missouri 65302
ReceiVed August 2, 1995
Introduction
Experimental Section
Development of new photolabeling agents with improved
photochemical properties has been a long sought goal for
molecular biologists and chemists.1-9 The minimum charac-
teristics for a UV-activated photolabeling agent include high
quantum efficiencies for forming the reactive precursor and high
reactivity for insertion into unactivated bonds.10,11 Although
perfluoro azides are projected to be potential photolabeling
agents,12-14 most require photoactivation in the spectral region
where proteins and nucleic acids15 also absorb light.
One goal of our work is to develop chromogenic photore-
active nitrene precursor that can be activated by irradiation at
wavelengths beyond the protein absorption tail (i.e., λmax > 320
nm) since photolysis at these wavelengths preclude the pos-
sibility of denaturing of proteins by radiation16 and can
compensate for a lower quantum yield of the photoprobe. The
spectral separation of the photoprobe and its photolysis products
from the absorption region of the biomolecule also provides a
spectral handle for monitoring and separating the photolabeled
biomolecules from the nonlabeled products using high-
All reactions and other manipulations were carried out under an
atmosphere of dry nitrogen or under vacuum. Solvents were prepu-
rified, dried and distilled under nitrogen prior to use. 1H, 31P, and 19
F
NMR spectra were recorded on a Bruker WH-300 instrument for
samples in CDCl3 solvent. 1H NMR chemical shifts are reported in
ppm, downfield from external standard, SiMe4. The 31P NMR chemical
shifts are reported with respect to 85% H3PO4 as an external standard
and for 19F NMR, trifluorotoluene is used as an external standard.
Reagents such as PdCl2(PhCN)2 and perfluorobenzonitrile were pur-
chased from Aldrich Chemical Co. All elemental analysis were done
by Oneida Research Services, Inc., Whitesboro, NY. FTIR spectra were
taken in a Mattson Galaxy 3000 spectrophotometer instrument using a
Nujol mull.
Synthesis of 4-Azido-2-(triphenylphosphinimino)-3,5,6-trifluo-
robenzonitrile, 2. A two-necked flask, equipped with a nitrogen inlet
adapter and an addition funnel was charged with dry methylene chloride
(100 mL) and 4-azido-2,3,4,5-tetrafluorobenzonitrile, 1 (5 g, 21.3
mmol). A solution of (trimethylsilyl)triphenylphosphinimine R3PNT
(R ) Ph and T ) (CH3)3Si, 8.17 g, 23.4 mmol), prepared by refluxing
triphenylphosphine with trimethylsilyl azide,24 was slowly added
dropwise at 0 °C and stirred for an hour during which the reaction was
monitored by 31P NMR spectroscopy. The solvent was evaporated
under the atmosphere of nitrogen and the solid was redissolved in dry
acetonitrile and evaporated slowly to give yellow crystals 2 in 90%
yield. mp: 126 °C. Anal. Calcd for C25H15N5F3P: C, 63.43, H, 3.19,
N, 14.79, Found: C, 63.23, H, 3.41, N, 14.83. 1H NMR: δ 7.0-8.0
(m, 15H, aromatic protons). 31P NMR: δ 9.8(s). 19F NMR: δ -73.6
(m, 1F), -74.6 (m, 1F and -99.4 (m, 1F) IR: 2250, 2122, 1628, 1377,
1258, 1200, 1109, 1045, 719, 525 cm-1 UV: 260 nm (ꢀ )15 340 M-1
cm-1), 348 nm. (ꢀ )1632 M-1 cm-1).
* To whom correspondence to be addressed.
† University of Missouri.
‡ H. S. Truman VA Hospital.
§ Alton Building Laboratories.
(1) Brunner, J. Annu. ReV. Biochem. 1993, 62, 4893 and references therein.
(2) Chavan, A. J.; Richardson, S. K.; Kim, H., Haley, E. B.; Watt, D. S.
Bioconjugate Chem. 1993, 4, 268.
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Academic Press: New York, 1984; Chapter 9, p 433.
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Photobiol. 1984, 49, 785.
Synthesis of [4-Azido-2-triphenylphosphinimino-3,5,6-trifluo-
robenzonitrile-PdCl2], 3. A solution of the PdCl2(PhCN)2 precursor
(17) (a) Nicot, B. D.; Mathiew, R.; Montauzon, D. D.; Lavigne, G.; Majoral,
J. P. Inorg. Chem. 1994, 33, 434, (b) Katti, K. V.; Reddy, V. S.; Slingh,
P. R. Chem. Soc. ReV. 1995, 97 and references therein, (c) Mague, J.
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Moran, T; Walton, R. A. Inorg. Chem. 1989, 28, 1384.
(18) Pandurangi, R. S.; Karra, S. R.; Kuntz, R. R.; Volkert, W. A.
Bioconjugate Chem. 1995, 6, 630.
(19) Pandurangi, R. S.; Kuntz, R. R.; Volkert, W. A.; Barnes, C. L.; Katti,
K. V. J. Chem. Soc., Dalton Trans. 1995, 565.
(20) Pandurangi, R. S.; Katti, K. V.; Barnes, C. L.; Volkert, W. A.; Kuntz,
R. R. J. Chem. Soc. Chem., Commun. 1994, 1841.
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(21) Pandurangi, R. S.; Katti, K. V.; Volkert, W. A.; Kuntz, R. R.
Proceedings, International Conference on Photochemistry; Vancouver,
Canada Proceedings 1993; p 367.
(15) Marquet, J.; Rabecas, L.; Cantos, A.; Manas, M. M.; Cervera, M.;
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S0020-1669(95)01002-0 CCC: $12.00 © 1996 American Chemical Society

Notes
Inorganic Chemistry, Vol. 35, No. 12, 1996 3717
Figure 1. ORTEP plot for orientation A of 4-azido-2-(triphenylphos-
phinimino)-3,5,6-trifluorobenzonitrile, 2.
Table 1. Crystallographic Data for Compound 2
Figure 2. ORTEP plot for orientation B of 2.
formula
C25H15N5F3P
M
473.4
Table 2. Selected Bond Lengths and Angles for Compound 2
cryst size/mm 0.10 × 0.25 × 0.50 cryst syst
monoclinic
16.425(7)
14.861(9)
4477(4)
8
space group P21/c
a/Å
Lengths (Å)
b/Å
â/deg
F(000)
Dc/g cm-3
µ/cm-1
18.653(5)
100.490(10)
1936
1.405
15.1
c/Å
PA-N2A
1.566(3)
1.806(3)
1.798(3)
1.807(3)
1.121(5)
1.354(4)
1.196(5)
1.406(5)
1.130(6)
1.352(4)
1.353(5)
1.344(4)
1.425(5)
1.364(5)
1.435(5)
1.390(5)
1.376(5)
1.393(6)
1.354(6)
PB-N2B
1.578(3)
1.796(4)
1.800(4)
1.800(4)
1.166(6)
1.361(5)
1.215(5)
1.393(5)
1.144(6)
1.352(4)
1.355(4)
1.352(4)
1.404(5)
1.377(6)
1.406(6)
1.399(5)
1.372(5)
1.389(5)
1.353(6)
U/Å3
Z
PA-C8A
PB-C8B
PA-C14A
PA-C20A
N1A-C7A
N2A-C2A
N3A-N4A
N3A-C4A
N4A-N5A
F1A-C3A
F2A-C5A
F3A-C6A
C1A-C2A
C1A-C6A
C1A-C7A
C2A-C3A
C3A-C4A
C4A-C5A
C5A-C6A
PB-C14B
PB-C20B
N1B-C7B
N2B-C2B
N3B-N4B
N3B-C4B
N4B-N5B
F1B-C3B
F2B-C5B
F3B-C6B
C1B-C2B
C1B-C6B
C1B-C7B
C2B-C3B
C3B-C4B
C4B-C5B
C5B-C6B
2θmax/deg
hkl ranges
120
-18 to +18,
0-20, 0 to -16
no. of tot. data 6919
Ra
0.057
no. of unique data 6623
R′
b
0.084
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. b R′ ) [∑w(|Fo| - |Fc)2/∑|Fo| ]1/2; w-1
2
) [σ2|Fo| +0.0008(Fo)2].
in dichloromethane (0.380 g, 1 mmol) was slowly added dropwise to
a solution of 2 (0.461 g, 0.97 mmol) also taken in dichloromethane at
25 °C. The reaction mixture was stirred for 10-15 min, during which
time it was monitored by 31P NMR spectroscopy. The solvent was
partially removed under vacuum and n-hexane was added to induce
precipitation. The precipitate was washed with n-hexane (3 × 20 mL)
to remove the benzonitrile, then dried in vacuum to give a yellow
crystalline Pd complex in 75% yield. Mp: 265 °C dec. Anal. Calcd
for C25H15N5F3PPdCl2: C, 46.14, H, 2.32, N, 10.76. Found: C, 46.56,
H, 2.61, N, 10.53. 1H NMR: δ 7.0-7.8, (m, 15H, aromatic protons).
31P NMR: δ 35.6. 19F NMR: δ -59.3 (m, 1F), -65.3 (m, 1F) -86.2
Angles (deg)
N2A-PA-C8A
N2A-PA-C14A
N2A-PA-C20A
PA-N2A-C2A
114.65(16) N2B-PB-C8B
117.03(15) N2B-PB-C14B
103.90(15) N2B-PB-C20B
137.81(24) PB-N2B-C2B
114.61(17)
115.44(17)
104.41(17)
(m, 1F) IR: 2228, 2120 , 1622, 1520, 1480, 1250, 1109, 721 cm-1
.
133.0(3)
UV: 260 (ꢀ ) 14 543 M-1 cm-1), 348 (ꢀ ) 9567 M-1 cm-1).
N4A-N3A-C4A 118.7(3)
N3A-N4A-N5A 169.6(4)
N2A-C2A-C1A 118.1(3)
N2A-C2A-C3A 127.4(3)
N4B-N3B-C4B 116.5(4)
N3B-N4B-N5B 169.2(5)
N2B-C2B-C1B 118.5(3)
N2B-C2B-C3B 126.6(3)
Results and Discussion
The nucleophilic substitution of 1 by substituted phosphin-
imine resulted in the preferential ortho substitution product as
evidenced by 19F NMR which shows three multiplets for the
three fluorines instead of the two multiplet AA1XX1 pattern for
the symmetrical parent azide.14,20 The 31P NMR spectrum shows
a singlet at 9.8 δ shifted from 1.6 δ for the phosphinimine
derivative 2. This compound is an air stable solid that readily
dissolves in ethanol. The presence of a strong IR band at 2123
cm-1 indicates that the azide moiety is still present. The
interesting aspect of this nucleophilic substitution is the shifting
of λmax from 270 nm for 1 to 348 nm in 2. This permits selective
irradiation of the probe in presence of biomolecules.
attachment of the ligating system, a single-crystal X-ray stuctural
analysis of phosphinimine was carried out. The ORTEP25,26
for the two orientations is shown in Figures 1 and 2. The
compound crystallizes in monoclinic form P21/c and has two
crystallographically independent orientations in the solid state.
The salient crystallographic data are summarized in Table 1.
Selected bond lengths and bond angles are tabulated in Table
2. Typical bond lengths of the azide moiety along with bond
angles indicate that they are in the normal range reported for
the other azides.19
X-ray crystal structure of C25H15N5F3P, 2. To obtain a
conclusive proof for the nondisturbance of the azide moiety by
The presence of a nitrogen on the phosphinimine and another
on the nitrile group within the same molecule opens up a
possibility of using them for chelation to transition metals
presumably through the formation of six membered ring
(Scheme 1). For example, 2 reacted with PdCl2(PhCN)2 to
produce a new metallacyclic compound 3 containing Pd(II) in
(25) Johnson C. K ORTEP; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(26) International Tables for X-Ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol. 4.

3718 Inorganic Chemistry, Vol. 35, No. 12, 1996
Notes
Scheme 1
high yield(>75%). The chemical constitution of 3 as established
by C, H, and N analytical data indicates that the complex has
one metal center per phosphinimine unit. The additional support
was provided by 31P NMR spectrum, which shows a singlet
signal at 36.4 δ with a moderate shift of 26 δ when compared
to the ligand 2. 19F NMR of the Pd complex shows all 3 fluorine
signals being shifted downfield compared to the parent ligand.
Comparison of the ν(CtN) stretching frequency of the free
ligand with the corresponding Pd(II) complex shows a shift from
2250 cm-1 to 2228 cm-1. This observation, along with the C,
H, and N analytical data indicate the formation of the Pd
complex (Scheme 1). A strong IR band at 2125 cm-1 is in a
range similar to that for other azides reported13 and indicates
that the azide moiety has not been perturbed during chemical
manipulations.
Photolysis of the precursor azide 1 (Scheme 1) produces a
very high yield of singlet nitrene intermediates capable of
inserting into C-H bonds.20 Screening for the ability of the
ligand 2 to retain this favorable characteristic was carried out
with the pyridine trapping technique27,28 which converts singlet
nitrenes into stable pyridinium ylides. These results are shown
in Figure 3, which compares the characteristic ylide formation
from a high yield nitrene precursor 1 to the ligand 2. The
characteristic absorption of the pyridinium ylide at 394-416
nm clearly indicates that 2 also produces singlet nitrene
intermediates upon photolysis, but with a much lower quantum
yield than the parent perfluoroazide.
Figure 3. Spectral changes of 1 and 2 as a function of exposure time
to the full beam of a 200 W Hg lamp filtered to remove wavelengths
below 320 nm. The solvent was 1:1 pyridine-methylene chloride.
Key: (A) exposure time for 1 (λmax ) 265 nm, spectra below 300 nm
were masked by pyridine absorption) was (a) 0, (b) 10, (c) 20, and (d)
80 s; (B) photolysis of 2 was (a) 0, (b) 1, (c) 2, and (d) 5 min.
of 2 to form singlet nitrene intermediates upon photolysis
coupled with its ability to form a Pd(II) complex demonstrates
the bifunctional character to the new photoprobe. The high yield
for formation of the Pd(II) complex suggests that 2 should be
useful for incorporating the 109Pd radionuclide. The reaction
of 2 with 109Pd (as 109PdCl42-) forms a complex which has been
characterized by paper and thin layer chromatography by
radioanalytical imaging AMBIS.29 Although the quantum yield
for singlet nitrene formation with this compound is low, the
ability to excite the photoprobe at wavelengths which do not
perturb the biomolecule provides the flexibility of a longer
photolysis time to achieve labeling. Further investigations into
developing bifunctional photolabeling agents will focus on
increasing the singlet nitrene yield while retaining the com-
plexation ability of the ligand.
Acknowledgment. This work was supported by a grant from
the DOE, No. DE FGO2 89ER60875. Partial funding of the
X-ray diffractometer by National Science Foundation Grant No.
CHE(90-11804) is gratefully acknowledged.
Conclusion
The modification of perfluorocyanoazide through the nucleo-
philic substitution by phosphinimine resulted a considerable
bathochromic shift to 350 nm, leading to a new chromogenic
nitrene precursor 2. This permits the selective electronic
excitation of the chromophore avoiding possible photo damage
to biomolecules. The newly synthesized photoprobe crystallizes
in P21/c and shows the azide moiety to be intact. The ability
Supporting Information Available: Tables giving crystallographic
details, atomic coordinates, thermal parameters, bond angles, torsion
angles, and bond lengths for 2 (9 pages). Ordering information is given
on any current masthead page.
IC9510021
(27) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz, M. S. J.
Am. Chem. Soc. 1992, 114, 5054 and references therein.
(28) Jones, M. B.; Platz, M. S. J. Org. Chem. 1991, 56, 1694.
(29) Pandurangi, R. S.; Kuntz, R. R.; Volkert, W. A. Unpublished results.