Synthesis and characterization of pd complexes of a carbazolyl/bis(Imine) NNN pincer ligand

Article abstract of DOI:10.1021/ic200026p

A serendipitously discovered construction of a carbazole nucleus by lithiation of N-methylated bis(4-methyl- 2,6-dibromophenyl)amine is described. It was used to synthesize an NNN pincer-type ligand that combines a central carbazole site (N-methylated in the precursor ligand) with two flanking aldimine donors bearing mesityl substituents. The installation of this ligand on Pd was accomplished via an N-Me cleaving reaction with (COD)PdCl₂ producing MeCl and (NNN)PdCl (where NNN is an anionic carbazolyl/bis(imine) pincer ligand). Several (NNN)PdX complexes were characterized spectroscopically. (NNN)PdOTf (^-OTf = triflate or ^-O₃SCF ₃) readily reacted with stoichiometric amounts of water in benzene or dichloromethane to give a cationic water adduct [(NNN)Pd(OH2)]OTf. An X-ray diffraction study on a single crystal of (NNN)PdCl revealed an almost perfectly square planar environment about Pd and an almost perfectly planar carbazole/ bis(imine) conjugated system. Cyclic voltammetry of (NNN)PdCl showed quasi-reversible oxidation at E1/2 = 0.72 V vs Fc/Fc⁺ which is most likely ligand-based.

Full text of DOI:10.1021/ic200026p

ARTICLE
pubs.acs.org/IC
Synthesis and Characterization of Pd Complexes of a
Carbazolyl/Bis(Imine) NNN Pincer Ligand
Aaron M. Hollas, Weixing Gu, Nattamai Bhuvanesh, and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S Supporting Information
b
ABSTRACT: A serendipitously discovered construction of a
carbazole nucleus by lithiation of N-methylated bis(4-methyl-
2,6-dibromophenyl)amine is described. It was used to synthe-
size an NNN pincer-type ligand that combines a central
carbazole site (N-methylated in the precursor ligand) with
two flanking aldimine donors bearing mesityl substituents.
The installation of this ligand on Pd was accomplished via an
N-Me cleaving reaction with (COD)PdCl2 producing MeCl
and (NNN)PdCl (where NNN is an anionic carbazolyl/bis(imine) pincer ligand). Several (NNN)PdX complexes were
characterized spectroscopically. (NNN)PdOTf (-OTf = triflate or -O3SCF3) readily reacted with stoichiometric amounts of
water in benzene or dichloromethane to give a cationic water adduct [(NNN)Pd(OH2)]OTf. An X-ray diffraction study on a single
crystal of (NNN)PdCl revealed an almost perfectly square planar environment about Pd and an almost perfectly planar carbazole/
bis(imine) conjugated system. Cyclic voltammetry of (NNN)PdCl showed quasi-reversible oxidation at E1/2 = 0.72 V vs Fc/Fcþ
which is most likely ligand-based.
’ INTRODUCTION
fused six-membered metallacycles upon metalation of the
pincer (in contrast to five-membered metallacycles with PNP)
would jeopardize the preference of the pincer to bind to the
metal in a meridional fashion. Although related diarylamido/
bis(oxazoline) complexes of Fe, Co, and Ni possess a meridional
NNNA ligand,11 our concern was reinforced by the recent
report of Gardinier et al. on the complexes of the diarylamido/
bis(imidazole) NNNB ligands (Chart 1).12 (NNNB)Re(CO)3
displayed facial geometry, in contrast to the meridionality of
(PNP)Re(CO)3.8 Other anionic pincer ligands with a central
diarylamido donor and flanking neutral N-donor arms include
(NNNC)MCl complexes from the Petersgroup and (NNND)NiX
complexes by Hu et al (Chart 1).13-15
Pincer ligands can be generally defined as tridentate,
meridionally chelating ligands.1 They have become popular as
tools in transition metal chemistry for their ability to form
structurally predictable and highly robust complexes. Our group
has been particularly interested in the PNP pincer ligands2
combining the central diarylamido donor and flanking phosphine
arms (Chart 1).3-5 These PNP ligands have supported, as
spectator ligands, a number of unusual and reactive structural
fragments at a broad variety of metal centers and maintained
integrity while rather remarkable reactions took place at the
metal centers with external substrates.6 There are, nonetheless,
limits to their stability which have so far been breached by
either oxidation at the phosphorus by atom transfer7 or one-
electron oxidation of the ligand π-system.8,9 In this context,
we desired to adapt the PNP ligand design to be more resistant
to both types of oxidation, while maintaining the pincer
structure and the advantageous semirigidity of the diarylamido
framework.
To this end, we turned our attention to an NNN ligand
(Chart 1) incorporating a central carbazole module instead of a
diphenylamine and aldimines (-CH=NR) instead of phos-
phines (-PR2). The sp2-N donor should be largely immune to
oxidation at the donor N atom. The carbazole system serves
two purposes. First, it should be more difficult to oxidize. The
ionization potential of carbazole has been reported to be about
0.4 V higher than that of diphenylamine.10 Second, the extra
C-C bond in carbazole should serve to “pull back” the imine
donors and partly compensate for the extra atoms in the
metallacycles. We were concerned that the formation of two
The carbazolyl/bis(aldimine) NNN ligands we had in
mind were first reported by Gibson et al. who examined
their Mn, Fe, Co, as well as Rh complexes.16 A Pd complex of
a
closely related carbazolyl/bis(pyridine) ligand NNNE
(Chart 1) was reported by Thummel et al.17 Carbazolyl/
bis(oxazoline) ligands were also reported by Nakada et al.18
Very recently, Barbe, Kadish, and co-workers reported Cu
complexes of a variation of the NNN ligand (denoted in Chart 1
as NNNF)19 which was synthesized via Gibson’s methods.16
In the present work we report a serendipitously found and
superior synthetic route to the formation of the carbazole module
in the carbazolyl/bis(imino) ligand, its new (NNN)PdX
complexes, and their characterization, including by cyclic
voltammetry.
Received: January 5, 2011
Published: March 11, 2011
r
2011 American Chemical Society
3673
dx.doi.org/10.1021/ic200026p Inorg. Chem. 2011, 50, 3673–3679
|

Inorganic Chemistry
ARTICLE
Chart 1. Representations of the Various NNN and PNP
Pincer Complexes
Scheme 2
Scheme 1
exact lithiation step does the elimination of LiBr and concomi-
tant C-C bond formation take place. Direct nucleophilic sub-
stitution on simple aryl bromides by an aryllithium typically
does not proceed. Moreover, dilithiation of (4-Me-2-Br-
C6H3)2NMe proceeds cleanly and without carbazole
formation,20,21 so it is not an intrinsic reaction for any ortho-
brominated diarylamine. We hypothesize that the particular
substitution pattern favors a conformation for the critical partially
lithiated intermediate that results in a low barrier for the C-C
bond formation. At this point, we lack sufficient data to ascertain
the particular intimate mechanism by which the C-C bond is
formed.22
The groups of Thummel and Gibson have reported several
methods for the synthesis of a carbazole-based ligand with imino
functionalities that rely on selective substitution of carbazole that
requires multiple steps.16,17 Our procedure is more direct, high-
yielding, and requiring fewer synthetic steps. Thummel reported
the formation of the carbazole ring directly from bis(p-
tolyl)amine,17 but by using stoichiometric Pd(OAc)2 and only
in a modest yield. The drawback of our synthesis is in that it
requires an N-Me group. Although we have not tested it here, it
is likely that another, more easily removed N-protecting group
can suffice. Furthermore, our synthesis may very well be compa-
tible with a variety of substituents in the position para to the
carbazole N, something not easily achieved by syntheses starting
with the parent carbazole itself.
’ RESULTS AND DISCUSSION
Synthesis of the NNN Ligand. Bromination of 1 with excess
Br2 resulted in tetrabromination selectively in ortho-positions
and produced 2 in high yield (Scheme 1). 2 is a habitual side
product in the dibromination of 1 with Br2. N-methylation of 2 to
give 3 was accomplished by deprotonation of 2 with LiN-
(SiMe3)2 and treatment with methyl tosylate in refluxing tetra-
hydrofuran (THF). The rather harsh conditions are presumably
a reflection of the diminished nucleophilicity of the amido site in
deprotonated 2 because of the four ortho-bromine substituents,
both on steric and electronic grounds. Treatment of 3 with 3
n
equiv of BuLi in ether for 1 h, followed by addition of excess
dimethylformamide (DMF), resulted in the formation of 4,
isolated in 69% yield in 90-95% purity. This purity was sufficient
for the use in the subsequent step. Condensation of 4 with 2,4,6-
trimethylaniline to give 5 in 81% isolated yield was accomplished
by refluxing the mixture for 9 d in chloroform in the presence of
acetic acid.
Synthesis of the Pd Complexes. We have previously re-
ported that PdCl2 reacts with N-methylated PNP ligands by
cleavage of the N-Me bond and the formation of (PNP)PdCl
and MeCl.23 We hoped that this strategy would also work with 5,
and it turned out to be the case, although the N-Me cleavage
with 5 is much slower. Heating a mixture of 5 and (COD)PdCl2
in toluene at 112 °C for 19 h resulted in the formation of
(NNN)PdCl, isolated as red crystals in 71% yield (Scheme 2).
(NNN)PdH, (NNN)PdI, (NNN)PdOTf, and (NNN)PdOH
were prepared by methods closely analogous to those used
by us in the past for the synthesis of the corresponding
(PNP)Pd complexes5,21,24 and isolated in good to excellent
yields (Scheme 2).
The key step in our synthesis is the formation of the carbazole
system upon lithiation of 3. N-methylation is necessary for the
carbazole-making step. Attempts at producing the carbazole
n
system by lithiation of 2 with varying amounts of BuLi led
to complex mixtures only. The key C-C bond is ostensibly
formed by the intramolecular, interannular elimination of
LiBr from the partially lithiated 3. It is not clear to us at which
3674
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679

Inorganic Chemistry
ARTICLE
Figure 2. Cyclic voltammogram of (NNN)PdCl in CH2Cl2 using 0.1 M
[Bu4N][PF6] as the supporting electrolyte and scan rate of 200 mV/s.
surprising to us, given that (PNP)PdOTf shows no evidence of
triflate displacement by water whatsoever. It is possible that the
steric bulk of the NNN ligand is more imposing and favors
expulsion of the bulkier triflate. It is also possible that the facile
dissociation of triflate is responsible for the observed conver-
sion of (NNN)PdOTf into (NNN)PdCl in dichloromethane
solutions.
X-ray Structure of (NNN)PdCl. An X-ray diffraction study
on a single crystal of (NNN)PdCl allowed us to establish its
structure in the solid state (Figure 1). The environment about Pd
is approximately square-planar. The angular deviations from
the idealized square planar geometry in (NNN)PdCl are minor
and are smaller than in the analogous (PNP)PdCl complex.5
The N-Pd-N “pincer bite” angle involving the trans-imine
nitrogens in (NNN)PdCl is 176.77(17)°, whereas the P-Pd-P
angle in (PNP)PdCl is 163.54(2)°. The Pd-Cl distance in
(NNN)PdCl of 2.3081(15) Å is similar to that in (PNP)PdCl
(2.3157(7) Å)5 and is shorter (by ca. 0.06-0.12 Å) than the
Pd-Cl distances in various (PCP)PdCl complexes where Cl
is trans to a more strongly trans-influencing aryl donor.27-29
The Pd-Namido distance in (NNN)PdCl of 1.989(5) Å falls
in between the analogous Pd-N distance in (PNP)PdCl
(2.026(2) Å)5 and the Pd-N distance in (NNNC)PdCl of
1.962(2) Å by Peters et al.13 The Pd-Nimine distances in
(NNN)PdClareabout0.07ÅlongerthanthePd-Namido distances.
The diiminocarbazole framework in (NNN)PdCl is remarkably
planar (Figure 1), reminiscent of the (NNNC)MCl structures
(M = Ni, Pd, Pt).13 Not surprisingly, the carbazole units are
π-stacked in the crystal. The stacking is pairwise, with the two
participating molecules being related by a crystallographic center
of symmetry. The closest C-C contacts (ca. 3.5 Å) are between
the carbons of the carbazole system that link the two six-
carbon rings.
Figure 1. ORTEP drawings30 (50% probability ellipsoids) of
(NNN)PdCl showing selected atom labeling. The top view is approxi-
mately perpendicular to the plane of the carbazole backbone; the bottom
view is approximately in that plane. Hydrogen atoms are omitted for
clarity. Selected bond distances (Å) and angles (deg) follow: Pd1-N1,
1.989(5); Pd1-N2, 2.060(5); Pd1-N3, 2.055(5); Pd1-Cl1,
2.3081(15); N1-Pd1-N2, 89.36(19); N1-Pd1-N3, 90.52(19);
N2-Pd1-N3, 176.77(17); N1-Pd1-Cl1, 174.05(13); N2-Pd1-
Cl1, 90.99(13); N3-Pd1-Cl1, 89.46(13).
The NMR spectra of the (NNN)PdX complexes displayed the
same C2v symmetry on the NMR time scale. The NNN pincer
ligands are devoid of 19F or 31P reporter NMR nuclei, but the
pattern of singlets in the 1H NMR serves as an appropriate NMR
diagnostic tool. The C2v symmetry in (NNN)PdX resulted in
three CH3 resonances (in a 6H:6H:12H ratio), three singlets
from aromatic hydrogens (in a 2H:2H:4H ratio), and a 2H
singlet arising from the imine CH. C2v symmetry was also
observed in the 13C{1H} NMR spectra.
(NNN)PdOTf displayed rather unusual reactivity with water
(Scheme 2). We found that exposure of solutions of the deep-red
(NNN)PdOTf in benzene or dichloromethane to even near-
stoichiometric amounts of water resulted in the displacement
of the triflate ligand by a water molecule and formation of
the orange [(NNN)Pd(OH2)]OTf. The latter is poorly soluble
in benzene and can be conveniently isolated by filtration.
(NNN)PdOTf could be regenerated by treatment of solutions
of [(NNN)Pd(OH2)]OTf with molecular sieves or by applica-
tion of vacuum.25 [(NNN)Pd(OH2)]OTf displayed C2v symme-
try in its NMR spectra at room temperature (RT), including a
distinct resonance for the coordinated water (two equivalent
hydrogens), which appeared at δ 3.27 ppm in CD2Cl2 for the
apparently pure material but whose chemical shift and breadth of
the peak depended on the amount of extra water present in the
system. There exist other reports of displacement of triflate
coordinated to a late transition metal by water in solvents of
low polarity.26 Still, this event in the present system was rather
Electrochemical Studies of (NNN)PdCl. Cyclic voltammetry
was performed to probe the redox activity of (NNN)PdCl. We
observed quasi-reversible oxidation of (NNN)PdCl at E1/2
=
0.72 V (vs Fc/Fcþ) when carried out in CH2Cl2 solvent with
[Bu4N][PF6] as the supporting electrolyte (Figure 2). Very
recently, Barbe et al. reported19 reversible oxidation at E1/2
=
0.71 V (vs Fc/Fcþ)31 of a CuII complex (NNNF)CuCl of a very
similar carbazolyl/bis(imino) ligand. The similarity of this redox
potential to that observed by us here for (NNN)PdCl strongly
suggests that these redox events are ligand-based. Carbazole itself
is electrochemically active32 and was reported19 to undergo
irreversibleoxidationat0.81 V vsFc/Fcþ undersimilarconditions.
As expected, the NNN ligand is much harder to oxidize than
the PNP ligand. For example, (PNP)NiCl was reported7 to
undergo reversible oxidation at E1/2 = -0.06 V and the more
3675
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679

Inorganic Chemistry
ARTICLE
electron-rich (PNP)Mn(CO)3 and (PNP)Re(CO)3 were re-
ported to undergo reversible oxidation at E1/2 = -0.34 V and
-0.25 V, respectively (all vs Fc/Fcþ).8 Peters’ [(PNP0)Cu]2
underwent two reversible oxidations at -0.49 and 0.30 V vs
Fc/Fcþ; these redox events in the dimeric amido-bridged system
were judged to have “strong ligand contribution”.33 Gardinier’s
(NNNB)Re(CO)3 underwent a quasi-reversible oxidation
with E1/2 = 0.00 V vs Fc/Fcþ.12 All in all, it is clear that the
NNN ligand allows access to ligand-based redox chemistry in a
completely different potential range.
habit. The crystal mounted on a nylon loop was then placed in a cold
nitrogen stream (Oxford) maintained at 110 K. A BRUKER GADDS
X-ray (three-circle) diffractometer was employed for crystal screening,
unit cell determination, and data collection. The goniometer was
controlled using the FRAMBO software, v.4.1.05.36 The sample was
optically centered with the aid of a video camera such that no translations
were observed as the crystal was rotated through all positions. The
detector was set at 5.0 cm from the crystal sample. The X-ray radiation
employed was generated from a Cu sealed X-ray tube (KR = 1.5418 Å
with a potential of 40 kV and a current of 40 mA) fitted with a graphite
monochromator in the parallel mode (175 mm collimator with 0.5 mm
pinholes). A total of 180 data frames were taken at widths of 0.5° with
an exposure time of 15 s. These reflections were used to determine the
unit cell using Cell_Now.37 A suitable cell was found and refined by
nonlinear least-squares and Bravais lattice procedures. The unit cell was
verified by examination of the h k l overlays on several frames of data.
No supercell or erroneous reflections were observed. After careful
examination of the unit cell, a standard data collection procedure was
initiated using ω and j scans.
Integrated intensity information for each reflection was obtained by
reduction of the data frames with SAINTplus.38 The integration method
employed a three-dimensional profiling algorithm and all data were
corrected for Lorentz and polarization factors, as well as for crystal decay
effects. Finally the data was merged and scaled to produce a suitable data
set. SADABS39 was employed to correct the data for absorption effects.
Systematic reflection conditions and statistical tests using XPREP did not
indicate any suitable space group. The structure could be, however, solved
in the triclinic P1 space group using XS40 (incorporated in SHELXTL);
and the additional symmetry (space group P21/c) was determined using
PLATON.41 All non-hydrogen atoms were refined with anisotropic
thermal parameters. The Hydrogen atoms were placed in idealized
positions [C-H = 0.96 Å, Uiso(H) = 1.2Uiso(C)] and refined using
riding model. The structure was refined (weighted least-squares refine-
ment on F2) to convergence.36
’ CONCLUSION
In summary, we are reporting on an unexpected synthesis of a
carbazole nucleus from a brominated diarylamine that lends itself
well to the preparation of carbazolyl/bis(aldimine) NNN pincer
ligands. Although these ligands were only obtained with the
N-methylated central N function of the carbazole, the NNN
pincer complexes of divalent Pd are easily accessible through
thermal N-Me cleavage in a reaction of 5 with a PdCl2 source.
Subsequent transformations allowed characterization of a series
of (NNN)PdX compounds. The triflate ligand in (NNN)PdOTf
is remarkably easily displaced by water, even in solvents of low
polarity (benzene, dichloromethane). (NNN)PdCl was charac-
terized by X-ray crystallography which revealed an almost
perfectly square-planar coordination environment about Pd
and the almost perfectly planar (NNN)Pd fragment. Cyclic
voltammetry study of (NNN)PdCl revealed a quasi-reversible
oxidation at E1/2 = 0.72 V vs Fc/Fcþ that most likely takes place
at the ligand π-system.
’ EXPERIMENTAL SECTION
General Considerations. All air or water sensitive manipulations
were performed under argon with standard Schlenk line techniques or in
a glovebox. Solvents were dried with and distilled from an appropriate
drying agent (NaK/Ph2CO or CaH2). NMR spectra were recorded on a
Mercury 300 MHz, Inova 300 MHz or an Inova 500 MHz spectrometer.
FT-IR spectra were collected using a Bruker ALPHA-P FT-IR Spectro-
meter with a diamond ATR. Elemental analyses were performed by
CALI Laboratories, Parsippany, NJ. Hexanes, dichloromethane, chloro-
form, pentane, and DMF were purchased from BDH, THF and NaBH4
from Mallinckrodt Chemicals, diethyl ether from J. T. Baker, hexam-
ethyldisilazane (HMDS) from Gelest, methyl tosylate, methyl iodide,
2,4,6-trimethylaniline, nBuLi and MeLi from Sigma-Aldrich, C6D6 from
Cambridge Isotope, and Br2 from Alfa Aesar. Bis(p-tolyl)amine (1) was
prepared by Buchwald-Hartwig amination from p-bromotoluene and
p-toluidine34 and (COD)PdCl2 also as described in the literature.35
Electrochemical studies were carried out using a CH Instruments
Model 700D Series Electrochemical Analyzer/Workstation in conjunc-
tion with a three-electrode cell. The working electrode was a CHI 104
glassy carbon disk (3.0 mm diameter), and the auxiliary electrode a
platinum wire. The reference electrode was a Ag/AgCl electrode sepa-
rated from the test solution by a fine porosity frit and 1 M KCl solution.
CVs were conducted in solutions of CH2Cl2 with 0.1 M NBu4PF6 as
supporting electrolyte at scan rates of 200 mV/s. The concentration of
analyte was 2.5 ꢀ 10-3 M. CVs were referenced to the Fe(η5-C5H5)2/
Fe(η5-C5H5)2þ redox couple.
Bis-(2,6-dibromo-4-methyphenyl)amine (2). Di-p-tolylamine
(1) (16.8 g, 0.085 mol) was dissolved in CH2Cl2 (100 mL) and stirred in
an ice bath. Bromine (20 mL, 0.39 mol) was added dropwise over 40 min.
The icebathwas removed, and tubing was attached totheflask, exhausting
into a solution of NaOH. A precipitate formed within 1 h in the reaction
flask, and the reactionwas stirredovernight. Following removalofvolatiles
under vacuum, the crude residue was redissolved in 850 mL of CH2Cl2,
placed in an ice bath, and bromine (2.5 mL, 0.05 mol) was added. The ice
bath was removed after addition, and the volatiles were removed from the
reaction mixture in vacuo after 4 h. Pentane (500 mL) was added to the
solid, then cyclohexene (10 mL) to quench the remaining bromine. The
volatiles were then removed under vacuum, the residue dissolved in a
CH2Cl2/ether solution. Sodium acetate (4.9 g, 0.06 mol) was added, and
the mixture was stirred for 30 min. The solids were filtered off, and the
filtrate was concentrated and crystallized at-32 °C to yield white crystals.
Yield: 35.8 g (82%) from three crops. 1H NMR (C6D6): δ 7.03 (s, 4H,
Ar-H), 5.83 (s, 1H, N-H), 1.70 (s, 6H, Ar-CH3). 13C{1H} NMR
(C6D6): δ 136.7 (CAr), 134.6 (CAr), 133.0 (CAr), 117.9(CAr), 19.5 (Ar-
CH3). Mp = 166 °C.
Bis-(2,6-dibromo-4-methylphenyl)methylamine (3). Bis-(2,6-
dibromo-4-methylphenyl)amine (2) (10 g, 19.5 mmol) was dissolved
in THF (75 mL). In a separate flask, hexamethyldisilazane (5.8 mL,
n
27.7 mmol) and BuLi (16.2 mL, 23.5 mmol) were combined in THF
(35 mL) and stirred for 15 min. This solution was then transferred to the
reaction mixture, and it was stirred for 30 min. All volatiles were removed in
vacuo, and the solid was redissolved in THF (75 mL). Methyl tosylate
(5.3 mL, 35.1 mmol) was added, and the reaction was refluxed for 24 h.
X-ray Crystallography: Data Collection and Reduction,
Structure Solution, and Refinement. A Leica MZ 75 microscope
was used to identify a suitable red multifaceted crystal with very well-
defined faces with dimensions (max, intermediate, and min; in mm)
0.08 ꢀ 0.05 ꢀ 0.03 from a representative sample of crystals of the same
n
Then a solution resulting from mixing BuLi (0.8 mL, 1.2 mmol) and
hexamethyldisilazane (0.29 mL, 1.4 mmol) in THF was added to the
reaction, stirred for 5 min, and then the volatiles were removed in vacuo.
3676
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679

Inorganic Chemistry
ARTICLE
The residual solid was redissolved in 90 mL of THF and refluxed overnight.
The reaction mixture was dried in vacuo, and the residual solid was washed
with 9:1 water/ethanol mixture and filtered. After drying, the solid was
dissolved in CH2Cl2, filtered, concentrated, and crystallized at -32 °C to
13C{1H} NMR (C6D6): δ 162.2 (CdN), 157.2 (CAr), 140.9 (CAr),
134.1 (CAr), 131.7 (CAr), 128.7 (CAr), 128.0 (CAr), 127.6 (CAr), 126.1
(CAr), 125.7 (CAr), 117.3 (CAr), 21.3 (Ar-CH3), 20.5 (Ar-CH3), 18.8
(Ar-CH3).
1
yield off-white crystals. Yield: 8.52 g (83%). H NMR (C6D6): δ 7.14
(NNN)PdI. (NNN)PdH (225 mg, 0.38 mmol) was dissolved in
toluene. CH3I (22.5 μL, 0.36 mmol) was added and after 20 min the
volatiles were removed in vacuo. The crude product was crystallized at -
32 °C from a pentane layered solution of toluene to yield a deep-purple
microcrystalline solid. Yield: 210 mg (77%). 1H NMR (C6D6): δ 8.00
(s, 2H, Ar-H), 7.44 (s, 2H, N=CH), 6.91 (s, 2H, Ar-H), 6.79 (s, 4H,
Ar-H), 2.41 (s, 12H, Ar-CH3), 2.39 (s, 6H, Ar-CH3), 2.14 (s, 6H,
Ar-CH3). 13C{1H} NMR (CDCl3): δ 163.8 (CdN), 157.6 (CAr),
137.4 (CAr), 135.7 (CAr), 132.0 (CAr), 130.2 (CAr), 128.4 (CAr), 128.2
(CAr), 127.7 (CAr), 124.7 (CAr), 116.6 (CAr), 21.3 (Ar-CH3), 20.9
(Ar-CH3), 20.0 (Ar-CH3). Elem. An. Found (Calculated) for
C34H34N3IPd: 56.84 (56.88), 4.73 (4.77), 5.90 (5.85).
(s, 4H, Ar-H), 3.55 (s, 3H, N-CH3), 1.68 (s, 6H, Ar-CH3). 13C{1H}
NMR (C6D6): δ 142.0 (CAr), 135.6 (CAr), 135.0 (CAr), 122.1 (CAr), 43.2
(N-CH3), 19.2 (Ar-CH3). Mp = 167 °C.
1,8-Diformyl-3,6,9-trimethyl-carbazole (4). Compound 3
n
(21.9 g, 41.7 mmol) was dissolved in ether (1300 mL). 2.5 M BuLi
(50 mL, 125 mmol) was added to the solution and stirred for 1.75 h, at
which point the reaction volume was reduced to 1000 mL in vacuo. DMF
(25.0 mL, 255 mmol) was added, and the reaction stirred overnight. The
reaction volume was reduced to 300, and 100 mL of a 2% HCl solution
was added. The remaining ether was removed in vacuo, and the yellow
solid was filtered off. The solids were washed with water, and the aqueous
wash was extracted with 100 mL of CH2Cl2 and combined with the solids
in a CH2Cl2 solution. This solution was dried over MgSO4 and filtered
through Celite and silica. Yellow solids were obtained from precipitation
from CH2Cl2/pentane at -32 °C. Yield: 7.6 g (69%). Product so
obtained was of about 94% purity by NMR. 1H NMR (C6D6): δ 10.09
(s, 2H, -CHO), 7.63 (s, 2H, Ar-H), 7.56 (s, 2H, Ar-H), 3.60 (s, 3H,
N-CH3), 2.24 (s, 6H, Ar-CH3). 13C{1H} NMR (CDCl3): δ 189.9
(CdO), 140.8 (CAr), 132.8 (CAr), 129.6 (CAr), 126.4 (CAr), 125.4(CAr),
122.0 (CAr), 41.8 (N-CH3), 20.9 (Ar-CH3).
1,8-Bis[(2,4,6-trimethylphenylimino)methyl]-3,6,9-trimethyl-
carbazole (5). Compound 4 (7.6 g, 28.7 mmol) was dissolved in
CHCl3 (240 mL). 2,4,6-Trimethylaniline (12 mL, 85.4 mmol) and
acetic acid (1 mL) were added to the solution. Molecular sieves were
added, and the reaction was refluxed for 9 d. The solution was filtered
hot through Celite and silica, and the volatiles were removed in vacuo.
The crude product was dissolved in hot THF and gradually cooled to -
32 °C. Yellow powder precipitated; it was washed with pentane and
dried. Yield: 11.6 g (81%). 1H NMR (C6D6): δ 8.61 (s, 2H, N=CH),
8.39 (s, 2H, Ar-H), 7.78 (s, 2H, Ar-H), 6.90 (s, 4H, Ar-H), 3.14
(s, 3H, N-CH3), 2.41 (s, 6H, Ar-CH3), 2.22 (s, 12H, Ar-CH3), 2.22
(s, 6H, Ar-CH3). 13C{1H} NMR (CDCl3): δ 160.4 (CdN), 149.3
(CAr), 141.4 (CAr), 133.1 (CAr), 129.9 (CAr), 128.9 (CAr), 128.4 (CAr),
127.2 (CAr), 125.7 (CAr), 123.4 (CAr), 120.7 (CAr), 41.4 (N-CH3),
21.2 (Ar-CH3), 20.9 (Ar-CH3), 18.6 (Ar-CH3). Mp = 197 °C.
(NNN)PdCl. Compound 5 (290 mg, 0.58 mmol) and (COD)PdCl2
(170 mg, 0.58 mmol) were combined in a PTFE-valved flask with
toluene (20 mL). The mixture was degassed and then heated at 112 °C
for 19 h. The solution was filtered, and the product was obtained by
crystallization from toluene/pentane at -32 °C to yield red crystals.
Yield: 260 mg (71%). Crystals suitable for X-ray diffraction were grown
from a solution in toluene/pentane at -32 °C. 1H NMR (C6D6): δ 8.02
(s, 2H, Ar-H), 7.70 (s, 2H, N=CH), 6.97 (s, 2H, Ar- H), 6.79 (s, 4H,
Ar-H), 2.40 (s, 6H, Ar-CH3), 2.35 (s, 12H, Ar-CH3), 2.14 (s, 6H,
Ar-CH3). 13C{1H} NMR (CDCl3): δ 162.1 (CdN), 151.8 (CAr),
138.0 (CAr), 135.3 (CAr), 131.9 (CAr), 130.3 (CAr), 128.3 (CAr), 128.1
(CAr), 127.8 (CAr), 125.3 (CAr), 116.7 (CAr), 21.4 (Ar-CH3), 20.9
(Ar-CH3), 19.1 (Ar-CH3). M/Z (MþLiþ: 631.1818), (Mþ, -Cl-:
590.167). Elem. An. Found (Calculated) for C34H34N3ClPd: 65.20
(65.18), 5.53 (5.47), 6.75 (6.71).
(NNN)PdOTf. (NNN)PdH (1.13 g, 1.91 mmol) was dissolved in
toluene (25 mL). Methyl triflate (216 μL, 1.91 mmol) was added
dropwise, and the solution soon bubbled and turned deep red. The
volatiles were removed in vacuo after 15 min, and the red solid was
recrystallized multiple times at -32 °C from toluene/pentane. Yield:
1
1.39 g (98%). H NMR (C6D6): δ 7.91 (s, 2H, Ar-H), 7.36 (s, 2H,
N=CH), 6.89 (s, 2H, Ar-H), 6.88 (s, 4H, Ar-H), 2.53 (s, 12H, Ar-
CH3), 2.37 (s, 6H, Ar-CH3), 2.19 (s, 6H, Ar-CH3). 13C{1H} NMR
(CD2Cl2, 125.6 MHz): δ 164.3 (CdN), 148.4 (CAr), 137.1 (CAr), 136.7
(CAr), 132.5 (CAr), 130.0 (CAr), 129.3 (CAr), 129.2 (CAr), 128.9 (CAr),
125.7 (CAr), 117.0 (CAr), 21.3 (Ar-CH3), 21.0 (Ar-CH3), 19.7 (Ar-
CH3). NB: (NNN)PdOTf is slowly converted to (NNN)PdCl in dichlor-
omethane. 19F NMR (C6D6): δ -77.7. Elem. An. Found (Calculated)
for C35H34N3O3F3SPd: 56.71 (56.80), 4.58 (4.63), 5.73 (5.68).
(NNN)PdOH. (NNN)PdOTf (402 mg, 0.543 mmol) was dissolved
in wet THF (20 mL). Finely ground KOH (280 mg, 4.99 mmol) was
added, and the deep red solution turned orange within minutes. The
volatiles were removed after 1 h. The crude product was redissolved in
toluene and filtered through Celite. Pure product was obtained as an
orange powder by layering the solution with pentane and cooling to -
32 °C. Yield: 208 mg (63%). 1H NMR (C6D6): δ 8.08 (s, 2H, Ar-H),
7.82 (s, 2H, N=CH), 7.09 (s, 2H, Ar-H), 6.74 (s, 4H, Ar-H), 2.44 (s,
6H, Ar-CH3), 2.32 (s, 12H, Ar-CH3), 2.11 (s, 6H, Ar-CH3), -2.61
(br, 1H, -OH). 13C{1H} NMR (C6D6): δ 161.5 (CdN), 147.8 (CAr),
139.7 (CAr), 134.75 (CAr), 131.7 (CAr), 130.6 (CAr), 128.5 (CAr), 127.4
(CAr), 126.2 (CAr), 125.8 (CAr), 117.2 (CAr), 21.0 (Ar-CH3), 20.6
(Ar-CH3), 18.7 (Ar-CH3). Elem. An. Found (Calculated) for
C34H35N3OPd: 67.07 (67.16), 5.67 (5.80), 6.83 (6.91).
[(NNN)Pd(OH2)]OTf. To a solution of (NNN)PdOTf (31 mg, 42
μmol) in C6D6 was added H2O (1.5 μL, 83.3 μmol). The deep red
solution soon turned orange, and copious amounts of orange red
precipitate formed. The precipitates was washed with 2 mL of pentane
3 times, and then dried under vacuum for 15 min. Yield: 24 mg (76%).
1H NMR (CD2Cl2, 499.4 MHz): δ 8.41 (s, 2H, Ar-H), 8.18 (s, 2H,
N=CH), 7.72 (s, 2H, Ar-H), 6.99 (s, 4H, Ar-H), 3.27 (s, 2H, H2O),
2.68 (s, 6H, Ar-CH3), 2.30 (s, 12H, Ar-CH3), 2.26 (s, 6H, Ar-CH3).
13C{1H} NMR (CD2Cl2, 125.6 MHz): δ 162.7 (CdN), 145.2 (CAr),
138.3 (CAr), 137.2 (CAr), 132.8 (CAr), 131.3 (CAr), 130.4 (CAr), 129.8
(CAr), 129.6 (CAr), 125.9 (CAr), 117.9 (CAr), 21.3 (Ar-CH3), 21.0
(Ar-CH3), 19.7 (Ar-CH3). 19F NMR (CD2Cl2, 282.2 MHz): δ -
79.8. IR (cm-1): ν = 3494 (OH). It was found that after heating at 80 °C
overnight under vacuum about 40% of [(NNN)PdOH2]þ OTf - was
converted to (NNN)PdOTf.
(NNN)PdH. (NNN)PdCl (126 mg, 0.200 mmol) was dissolved in
THF (15 mL) and ethanol (2 mL). NaBH4 (20 mg, 0.53 mmol) was
added, and the solution quickly turned brown/yellow. After 30 min
volatiles were removed in vacuo, and the crude residue was redissolved in
toluene and filtered through Celite and silica. Toluene was removed in
vacuo, and the yellow solid was washed with cold pentane. Yield: 106 mg
(89%). 1H NMR (C6D6): δ 8.20 (s, 2H, Ar-H), 7.86 (s, 2H, N=CH),
7.19 (s, 2H, Ar-H), 6.69 (s, 4H, Ar-H), 2.50 (s, 6H, Ar-CH3), 2.33
(s, 12H, Ar-CH3), 2.11(s, 6H, Ar-CH3), -15.73 (s, 1H, Pd-H).
Regeneraton of (NNN)PdOTf. To a suspension of [(NNN)
Pd(OH2)]OTf (20 mg, 26 μmol) in C6D6 was added dry 4A molecular
sieves (0.5 g, 8-12 mesh). The mixture was allowed to stay at room
temperature for 20 h. 19F and 1H NMR spectra showed that >95% of
the starting [(NNN)Pd(OH2)]OTf was converted to (NNN)PdOTf.
3677
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679

Inorganic Chemistry
ARTICLE
’ ASSOCIATED CONTENT
(c) Lu, S.-F.; Du, D.-M.; Zhang, S.-W.; Xu, J. Tetrahedron: Asymmetry
2004, 15, 3433–3441.
(12) Wanniarachchi, S.; Liddle, B. J.; Toussaint, J.; Lindeman, S. V.;
Bennett, B.; Gardinier, J. R. Dalton Trans. 2010, 39, 3167–3169.
(13) Peters, J. C.; Harkins, S. B.; Brown, S. D.; Day, M. W. Inorg.
Chem. 2001, 40, 5083.
S
Supporting Information. Crystallographic details in the
b
form of a CIF file and a pictorial 1H NMR spectrum of [(NNN)
Pd(OH2)]OTf. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) (a) Breitenfeld, J.; Vechorkin, O.; Corminboeuf, C.; Scopelliti,
R.; Hu, X. Organometallics 2010, 29, 3686. (b) Vechorkin, O.; Csok, Z.;
Scopelliti, R.; Hu, X. Chem.—Eur. J. 2009, 15, 3889–3899.
(15) Diarylamido/bis(imino) ligands have also been reported: (a)
Black, D. St. C.; Rothnie, N. E. Tetrahedron Lett. 1978, 2835–6. (b)
Black, D. St. C.; Rothnie, N. E. Aust. J. Chem. 1983, 36, 2395–406.
(16) (a) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S. K.;
White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015. (b)
Gibson, V. C.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Dalton
Trans. 2003, 2718. (c) Britovsek, G. J. P.; Gibson, V. C.; Hoarau, O. D.;
Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Inorg. Chem. 2003,
42, 3454.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ozerov@chem.tamu.edu.
’ ACKNOWLEDGMENT
This research was supported by the NSF through the “Power-
ing the Planet” Center for Chemical Innovation (Grant CHE-
0802907), as well as the Welch Foundation (Grant A-1717), and
the Dreyfus Foundation (Camille Dreyfus Teacher-Scholar
Award to O.V.O). We thank Dr. David E. Herbert for assistance
with electrochemistry experiments and helpful discussions.
(17) Mudadu, M. S.; Singh, A. N.; Thummel, R. P. J. Org. Chem.
2008, 73, 6513–6520.
(18) Inoue, M.; Nakada, M. Heterocycles 2007, 72, 133.
(19) Barbe, J. M.; Habermeyer, B.; Khoury, T.; Gros, C. P.; Richard,
P.; Chen, P.; Kadish, K. M. Inorg. Chem. 2010, 49, 8929–8940.
(20) Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman, B. M. J. Am.
Chem. Soc. 2004, 126, 4792.
’ REFERENCES
(21) Gilman, H.; Zuech, E. A. J. Org. Chem. 1959, 24, 1394.
(22) For an overview of aromatic nucleophilic substitution, see:
March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Struc-
ture; Smith, M. B., March, J., Eds.; John Wiley and Sons: New York,
2001; Chapter 13, pp 850-893.
(1) (a) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003,
103, 1759–92. (b) The Chemistry of Pincer Compounds; Morales-Morales,
D.; Jensen, C. M., Eds.; Elsevier: Amsterdam, The Netherlands, 2007.
(2) Our diarylamido-based PNP pincer ligands were inspired by the
disilylamido-based PNP ligands of Fryzuk that have been more recently
employed by Caulton: (a) Fryzuk, M. D. Can. J. Chem. 1992, 70, 2839.
(b) Fryzuk, M. D.; Berg, D. J.; Haddad, T. S. Coord. Chem. Rev. 1990,
99, 137. (c) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. Rev. 1989,
95, 1. (d) Fan, H.; Fullmer, B. C.; Pink, M.; Caulton, K. G. Angew. Chem.,
Int. Ed. 2008, 47, 9112. (e) Ozerov, O. V.; Watson, L. A.; Pink, M.;
Caulton, K. G. J. Am. Chem. Soc. 2007, 129, 6003.
(3) (a) Liang, L.-C. Coord. Chem. Rev. 2006, 250, 1152. (b) Ozerov,
O. V. In The Chemistry of Pincer Compounds; Morales-Morales, D.
Jensen, C., Eds.; Elsevier: Amsterdam, The Netherlands, 2007;
pp 287-309. (c) Mindiola, D. J. Acc. Chem. Res. 2006, 39, 813.
(4) (a) Liang, L.-C.; Lin, J.-M.; Hung, C.-H. Organometallics 2003,
22, 3007. (b) Winter, A. M.; Eichele, K.; Mack, H.-G.; Potuznik, S.;
Mayer, H. A.; Kaska, W. C. J. Organomet. Chem. 2003, 682, 149.
(5) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004,
23, 326.
(23) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778.
(24) Fafard, C. M.; Ozerov, O. V. Inorg. Chim. Acta 2007, 360, 286.
(25) Because of the sensitivity of [(NNN)Pd(OH2)]OTf to vacuum,
it was only characterized in solution by NMR spectroscopic techniques.
(26) (a) Hughes, R. P.; Sweetser, J. T.; Tawa, M. D.; Williamson, A.;
Incarvito, C. D.; Rhatigan, B.; Rheingold, A. L.; Rossi, G. Organometallics
2001, 20, 3800–3810. (b) Baar, C. R.; Carbray, L. P.; Jennings, M. C.;
Puddephatt, R. J. Organometallics 2000, 19, 2482–2497. (c) Svetlanova-
Larsen, A.; Hubbard, J. L. Inorg. Chem. 1996, 35, 3073–6.
(27) Morales-Morales, D.; Grause, C.; Kasaoka, K.; Redon, R.;
Cramer, R. E.; Jensen, C. M. Inorg. Chim. Acta 2000, 300-302, 958.
(28) Cross, R. J.; Kennedy, A. R.; Muir, K. W. J. Organomet. Chem.
1995, 487, 227.
(29) Ozerov, O. V.; Guo, C.; Foxman, B. M. J. Organomet. Chem.
2006, 691, 4802.
(6) (a) Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 11161.
(b) Whited, M. T.; Grubbs, R. H. Acc. Chem. Res. 2009, 42, 1607. (c)
Cantat, T.; Graves, C. R.; Scott, B. L.; Kiplinger, J. L. Angew. Chem., Int.
Ed. 2009, 48, 3681. (d) Bailey, B. C.; Fan, H.; Huffman, J. C.; Baik, M.-H.;
Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 8781. (e) Fafard, C. M.;
Adhikari, D.; Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem.
Soc. 2007, 129, 10318. (f) Weng, W.; Guo, C.; elenligil-C-etin, R.;
Foxman, B. M.; Ozerov, O. V. Chem. Commun. 2006, 197.
(30) ORTEP plots were created using Ortep-3 for Windows:
Farugia, L. J. Appl. Crystallogr. 1997, 30, 565.
(31) We use the potential of 0.48 V for the Fc/Fcþ couple vs SCE
(reported in Connelly, N.G.; Geiger, W. E. Chem. Rev. 1996, 96, 877, in
0.1 M [Bu4N][ClO4]) to convert the potential values reported vs SCE
to the scale referenced to Fc/Fcþ for consistency of comparison.
(32) (a) Ambrose, J. F.; Carpenter, L. L.; Nelson, R. F. J. Electrochem.
Soc. 1975, 27, 876. (b) Ambrose, J. F.; Nelson, R. F. J. Electrochem. Soc.
1968, 115, 1159.
(7) Adhikari, D.; Basuli, F.; Fan, H.; Huffman, J. C.; Pink, M.;
Mindiola, D. J. Inorg. Chem. 2008, 47, 4439.
(33) Harkins, S. B.; Mankad, N. P.; Miller, A. J. M.; Szilagyi, R. K.;
Peters, J. C. J. Am. Chem. Soc. 2008, 130, 3478; PNP0 = (4-tBu-2-iBu2P-
C6H3)2N.
(8) Radosevich, A. T.; Melnick, J. G.; Stoian, S. A.; Bacciu, D.; Chen,
C.-H.; Foxman, B. M.; Ozerov, O. V.; Nocera, D. G. Inorg. Chem. 2009,
48, 9214.
(34) (a) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999,
576, 125. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(35) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346.
(36) FRAMBO, Program for Data Collection on Area Detectors, v.
4.1.05; BRUKER-Nonius Inc.: Madison, WI.
(37) Sheldrick, G. M. Cell_Now, Program for Obtaining Unit Cell
Constants from Single Crystal Data, version 2008/1; University of
G€ottingen: G€ottingen, Germany, 2008.
(9) Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.;
Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676.
(10) Based on ionization potential data tabulated by the NIST
Chemistry WebBook (http://webbook.nist.gov/chemistry/) for carba-
zole (several values around 7.6 eV reported) and diphenylamine (several
values around 7.2 eV).
(11) (a) Kieltsch, I.; Dubinina, G. G.; Hamacher, C.; Kaiser, A.;
Torres-Nieto, J.; Hutchison, J. M.; Klein, A.; Budnikova, Y.; Vicic, D. A.
Organometallics 2010, 29, 1451–1456. (b) Inagaki, T.; Phong, L. T.;
Furuta, A.; Ito, J.; Nishiyama, H. Chem.—Eur. J. 2010, 16, 3090–3096.
(38) SAINT, Program for Data Integration from Area Detector Frames,
Version 7; Bruker-Nonius Inc.: Madison, WI.
3678
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679

Inorganic Chemistry
ARTICLE
(39) Sheldrick, G. M. SADABS, Program for Absorption Correction for
Data from Area Detector Frames, version 2008/1; University of
G€ottingen: G€ottingen, Germany, 2008.
(40) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(41) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2006. Spek, A. L. Acta
Crystallogr. 1990, A46, C34.
3679
dx.doi.org/10.1021/ic200026p |Inorg. Chem. 2011, 50, 3673–3679