Cyclometalations on the imidazo[1,2-a][1,8]naphthyridine framework

Article abstract of DOI:10.1021/om4004658

Cyclometalation on the substituted imidazo[1,2-a][1,8]naphthyridine platform involves either the C₃-aryl or C₄′-aryl ortho carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in compariso

Full text of DOI:10.1021/om4004658

Article
pubs.acs.org/Organometallics
Cyclometalations on the Imidazo[1,2‑a][1,8]naphthyridine
Framework
Prosenjit Daw,† Tapas Ghatak,† Henri Doucet,‡ and Jitendra K. Bera*,†
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
‡Institut Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
Beaulieu, 35042 Rennes, France
́
de Rennes, “Catalyse et Organometalliques”, Campus de
S
* Supporting Information
ABSTRACT: Cyclometalation on the substituted imidazo[1,2-a][1,8]naphthyridine platform involves either the C3-aryl or C4′-
aryl ortho carbon and the imidazo nitrogen N3′. The higher donor strength of the imidazo nitrogen in comparison to that of the
naphthyridine nitrogen aids regioselective orthometalation at the C3/C4′-aryl ring with Cp*IrIII (Cp* = η5-
pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C3-aryl and imidazo C5′-H, creating
six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl2 into
the Ir−Cl bond. Pd(OAc)2 afforded an acetate-bridged dinuclear ortho-metalated product involving the C3-aryl unit. Metalation
at the imidazo carbon (C5′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor
Pd2(dba)3 (dba = dibenzylideneacetone). Regioselective C−H/Br activation on a rigid and planar imidazonaphthyridine platform
is described in this work.
Scheme 2. Imidazo[1,2-a][1,8]naphthyridine and Atom-
Numbering Scheme
INTRODUCTION
■
The utility of 1,8-naphthyridine (NP) ligands for a wide range
of applications, including axial C−H bond activation1 and C−C
bond formation on metal−metal-bonded complexes,2 bimet-
allic3 and bifunctional catalysis,4 and construction of self-
assembled inorganic architectures,5 has been demonstrated in
recent years. Functionalization of imidazole with different
naphthyridines provided ligands that afford discrete dinuclear
complexes, tetranuclear metallamacrocycles, and polymeric
chain compounds with the RhI(COD) unit.6 Naphthyridine-
functionalized N-heterocyclic carbenes (NHCs) exhibit topo-
logical flexibility and form complexes with a variety of metal
components (Scheme 1).7
We sought to develop ligand systems based on fused
imidazo[1,2-a][1,8]naphthyridine, illustrated in Scheme 2. The
purpose was to employ it as a cyclometalating ligand and also to
access a precursor for an abnormal NHC ligand upon
quaternization of imidazolyl nitrogen.8 Initial use of Cp*IrIII
led to orthometalation involving the phenyl substituent at C4′
and the imidazo nitrogen N3′. Relocating the phenyl
substituent from the C4′ to the C3 position afforded a six-
membered cyclometalated ring system. A second cyclo-
metalation at C5′ could only be achieved upon carrying out
the reaction for a longer duration. Although facile cyclo-
metalation occurs with Pd(OAc)2 involving N3′ and the phenyl
unit at C3, activation of C5′−H could not be accomplished
under normal cyclometalating conditions. Oxidative cleavage of
C5′−Br with zero-valent Pd followed by N3′ coordination gave
a cyclic cyclometalated Pd6 framework. This report describes an
interesting account of cyclometalation reactions on the
imidazo[1,2-a][1,8]naphthyridine platform.
Scheme 1. Multifaceted Naphthyridine-Functionalized N-
Heterocyclic Carbenes
Received: May 24, 2013
Published: July 15, 2013
© 2013 American Chemical Society
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simulated mass and isotope distribution patterns (Figure S14,
Supporting Information).
The molecular structure of 1, as shown in Figure 1, confirms
that the ligand is ortho-metalated through C11 of the phenyl
RESULTS AND DISCUSSION
■
Cyclometalation with Cp*IrIII. Imidazo[1,2-a][1,8]-
naphthyridines L1H and L2H2 are synthesized by condensation
of the appropriate 2-aminonaphthyridine derivatives with α-
halogenocarbonyl compounds (Scheme 3).9 L3Br is synthesized
by the treatment of L2H2 with liquid bromine in acetic acid at
room temperature (Scheme 4).
Scheme 3. Synthesis of L1H and L2H2
Scheme 4. Synthesis of L3Br
Figure 1. Molecular structure (50% probability thermal ellipsoids) of 1
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−C11
= 2.066(6), Ir1−N1 = 2.085(6), Ir1−Cl1 = 2.410(2), Ir1−C3 =
2.143(7), Ir1−C2 = 2.173(7), Ir1−C4 = 2.186(7), Ir1−C5 = 2.196(8),
Ir1−C1 = 2.236(8); C11−Ir1−N1 = 76.9(3), C11−Ir1−C3 = 99.3(3),
C11−Ir1−C4 = 102.3(3), N1−Ir1−C2 = 98.1(3).
Typical cyclometalation reactions were carried out in
dichloroethane (DCE) with excess NaOAc under reflux
conditions. Treatment of 0.5 equiv of [IrCp*(μ-Cl)Cl]2 with
5,7-dimethyl-4′-phenylimidazo[1,2-a][1,8]naphthyridine (L1H)
in the presence of excess NaOAc in dichloroethane at reflux for
4 h afforded ortho C−H activation of C4′ phenyl, resulting in
the five-membered cyclometalated complex [Cp*Ir(Cl)L1] (1)
(Scheme 5). The 1H NMR spectrum of 1 reveals cyclo-
metalation with disappearance of the ortho proton of the phenyl
ring. The 13C NMR signal corresponding to the cyclometalated
carbon appears downfield at δ 159.2 ppm in comparison to the
free ligand. The ESI-MS of compound 1 exhibits a signal at m/z
600 (100%) which is assigned to [1 − Cl]+ on the basis of
fragment and Cp*Ir is additionally anchored to the imidazo
nitrogen N1. A chloride is coordinated (Ir1−Cl1 = 2.410(2) Å)
to complete the piano-stool geometry around the metal. The
Ir1−C11 and Ir1−N1 distances are 2.066(6) and 2.085(6) Å,
respectively, comparable to those in reported IrIII C∧N-
cyclometalated compounds.10 The imidazonaphthyridine and
phenyl rings adopt a planar configuration, reflecting orthome-
talation. Differences in distances between iridium and carbon
atoms of the Cp* ligand are noted. The Ir1 exhibits a longer
distance with C1 (2.236(8) Å) situated trans to the
cyclometalated carbon C11 (∠C11−Ir1−C1 = 163°) in
comparison to the other iridium−carbon distances
(2.196(8)−2.143(7) Å) with Cp*.
Scheme 5. Synthesis of 1−5
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It was argued at this point that the ortho methyl group of the
naphthyridine unit does not allow close approach of the bulky
Cp*Ir unit to the naphthyridine nitrogen N8. To test this
hypothesis, we proposed a ligand that is devoid of o-methyl
substituents on the naphthyridine unit. However, unsubstituted
imidazonaphthyridine could not be synthesized due to lack of
an easy synthetic protocol for 2-aminonaphthyridine. Instead,
we synthesized 3-phenyl-2-aminonaphthyridine by Friedlander
condensation of 2-aminonicotinaldehyde with benzyl cyanide.11
Condensation of 3-phenyl-2-aminonaphthyridine with α-
chloroacetaldehyde afforded 3-phenylimidazo[1,2-a][1,8]-
naphthyridine (L2H2).
absence of one of the imidazo proton signals (δ 8.47 in 2). The
other imidazo proton signal is found at δ 7.70 ppm. The Cp*
protons appear as two singlets at δ 1.84 and 1.40 ppm. The
cyclometalated carbons exhibit resonances at δ 154 and 156
ppm in the 13C NMR spectrum. The ESI-MS of compound 3
exhibits a signal at m/z 934 which is assigned to [3 − Cl]+.
The double cyclometalations on a single ligand framework
consisting of five- and six-membered rings have been confirmed
by X-ray crystallography (Figure 3). The second cyclo-
The reaction of L2H2 with [IrCp*(μ-Cl)Cl]2 following a
protocol identical with that followed for 1 led to the C−H
cleaved product [IrClCp*(L2H)] (2). The C3 phenyl is ortho-
metalated with Cp*Ir which is further anchored to the imidazo
1
nitrogen, creating a six-membered metalacycle. The H NMR
spectrum shows multiple resonances corresponding to aromatic
protons in the range δ 7.06−8.60 ppm. Two imidazo protons
appear at δ 8.47 and 7.71 ppm, and Cp* protons appear as a
singlet at δ 1.42 ppm. The cyclometalated carbon exhibits a
resonance at 154 ppm in the 13C NMR spectrum. The ESI-MS
of compound 2 exhibits a signal at m/z 572 which is assigned to
[2 − Cl]+.
Figure 3. Molecular structure (50% probability thermal ellipsoids) of 3
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−C35
= 2.019(13), Ir1−C3 = 2.085(12), Ir1−C2 = 2.097(11), Ir1−C4 =
2.155(11), Ir1−N3 = 2.155(11), Ir1−C1 = 2.173(10), Ir1−C5 =
2.208(11), Ir1−Cl1 = 2.398(4), Ir2−C21 = 2.049(13), Ir2−N1 =
2.067(12), Ir2−C12 = 2.127(16), Ir2−C11 = 2.159(14), Ir2−C15 =
2.169(13), Ir2−C13 = 2.211(15), Ir2−C14 = 2.239(14), Ir2−Cl2 =
2.416(3); C35−Ir1−N3 = 78.5(5), C35−Ir1−Cl1 = 89.0(3), C21−
Ir2−N1 = 87.6(5), C21−Ir2−C11 = 95.3(5).
The molecular structure of 2 is shown in Figure 2, and
important metrical parameters are provided in the correspond-
metalation occurs through the imidazo carbon C35 and
naphthyridine nitrogen N3, creating a five-membered ring.
The Ir−C(cyclometalated) distances are 2.049(13) Å (Ir2−
C21) and 2.019(13) Å (Ir1−C35); Ir−N distances are
2.067(12) Å (Ir2−N1) and 2.155(11) Å (Ir1−N3). The
chelate angles (∠C−Ir−N) for the five- and six-membered
rings are 78.5(5) and 87.6(5)°, respectively. Two Cp* rings are
positioned syn to each other with respect to the ligand plane.
The Ir−C(Cp*) distances are slightly shorter (2.085(12)−
2.208(11) Å) when the metal is part of a five-membered
metallacycle in comparison to a six-membered-ring system
(2.239(14)−2.416(3) Å).
Figure 2. Molecular structure (50% probability thermal ellipsoids) of 2
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−N1
= 2.054(5), Ir1−C11 = 2.054(6), Ir1−Cl1 = 2.417(1), Ir1−C5 =
2.126(6), Ir1−C1 = 2.182(6), Ir1−C2 = 2.173(6), Ir1−C4 = 2.235(6),
Ir1−C3 = 2.263(6); N1−Ir1−C11 = 86.4(2), C11−Ir1−C5 =
105.9(2), N1−Ir1−C5 = 105.7(2), N1−Ir1−C1 = 143.2(2), C11−
Ir1−C1 = 93.2(2), N1−Ir1−C2 = 156.8(2).
We recently initiated a program to prepare mixed-metal Ir−
Sn compounds based on cyclometalated organometallic
constructs.12 Insertion of SnCl2 into the Ir−Cl bond in 1
proceeds efficiently, forming the mixed Ir−Sn compound
[Cp*Ir(SnCl3)(L1)] (4) in high yield (93%) (Scheme 5). The
1H and 13C NMR spectra of 4 exhibit signals very similar to
those of 1, indicating structural correspondence between them.
A similar reaction with 3 afforded the heterobimetallic
cyclometalated compound 5, where the double-cyclometalated
ligand framework is unchanged and two SnCl2 units readily
insert into both metal−chloride bonds.
The X-ray structure of 4 confirms the insertion of SnCl2 into
the Ir−Cl bond (Figure 4). The overall molecular structure is
similar to that of 1, except that the chloride is replaced by a
SnCl3 unit. The Sn atom displays a distorted-tetrahedral
environment consisting of three chlorides and one iridium. The
Ir−Sn−Cl angles are larger (114.3(5)−123.1(1)°) than the
ing figure caption. The structural features of 2 are similar to
those of compound 1 except that the C3 phenyl is
cyclometalated instead of the C4′ phenyl. The Ir1−C11
(cyclometalated carbon) and Ir1−N1 distances of the chelating
ligand are 2.054(6) and 2.054(5) Å, respectively. Other bond
parameters are comparable to those of compound 1.
A prolonged reaction (12 h) of L2H2 with an equivalent
amount of [IrCp*(μ-Cl)Cl]2 in the presence of excess NaOAc
in refluxing DCE afforded the double-cyclometalated product
[Ir2Cl2Cp2*(L2)] (3). Both imidazo C5′−H and C3-phenyl
ortho C−H are metalated. The same compound could be
obtained by treating 2 with another 1 equiv of the metal
1
precursor in the presence of NaOAc. The H NMR spectrum
of 3 exhibits signals very similar to those of 2 with additional
cyclometalation through the imidazo carbon, indicated by the
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bulky SnCl3 units is possibly responsible for an arrangement
that keeps them away from each other in 5.
Cyclopalladation. Treatment of L2H2 with Pd(OAc)2 in
acetonitrile in the presence of excess Na2CO3 resulted in
1
cyclometalation of the ligand. The H NMR spectrum of the
product Pd2(OAc)2(L2H)2 (6) reveals cyclometalation by the
absence of one of the ortho protons of the phenyl ring (Scheme
6). The methyl protons of the acetate display a sharp singlet at
Scheme 6. Synthesis of 6 and 7
Figure 4. Molecular structure (50% probability thermal ellipsoids) of 4
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−C11
= 2.076(5), Ir1−N1 = 2.082(4), Ir1−Sn1 = 2.522(1), Sn1−Cl3 =
2.378(1), Sn1−Cl2 = 2.374(1), Sn1−Cl1 = 2.374(1), Ir1−C5 =
2.171(5), Ir1−C1 = 2.195(5), Ir1−C2 = 2.205(5), Ir1−C4 = 2.224(4),
Ir1−C3 = 2.251(5); C11−Ir1−N1 = 77.5(1), C11−Ir1−Sn1 =
85.3(1), N1−Ir1−Sn1 = 90.9(1), Cl2−Sn1−Cl1 = 98.1(1), Cl2−
Sn1−Cl3 = 100.5(1), Cl1−Sn1−Cl3 = 96.9(1), Cl2−Sn1−Ir1 =
114.4(1), Cl1−Sn1−Ir1 = 119.4(1).
δ 2.47 ppm. The 13C NMR signal corresponding to the
cyclometalated carbon atom appears downfield at δ 180.2 ppm.
The ESI-MS of compound 6 exhibits a signal at m/z 391
(100%) which is assigned to [M − (CH3COO) + (CH3CN)]+
on the basis of simulated mass and isotope distribution
patterns.
The molecular structure of 6, as shown in Figure 6, confirms
two cyclometalated palladium units that are held in proximity
by bridging acetates. Only half of the molecule is located in the
asymmetric unit related to the other half by a C2 axis that
bisects the Pd···Pd vector. The ligand is cyclometalated through
Cl−Sn1−C1 angles (96.9(1)−100.5(1)°). The Ir1−Sn1
distance is 2.522(1) Å, whereas the three Sn−Cl distances
are comparable (2.374(1), 2.374(1), and 2.378(1) Å). The
SnCl3 unit is a weak σ donor but exhibits a large trans effect due
to its ability to form a strong dπ−dπ bond to the metal. This is
reflected in longer Ir−C(Cp*) distances (2.251(5)−2.171(5)
Å) in comparison to those in 1 (2.196(8)−2.143(7) Å).
The Ir−Sn distances in 5 are 2.549(1) and 2.553(1) Å.
Interestingly, two Cp* rings are positioned diagonal with
respect to the ligand plane (Figure 5). This is in contrast to the
syn orientation of Cp* units in 3. Steric congestion involving
Figure 5. Molecular structure (40% probability thermal ellipsoids) of 5
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−C35
= 2.068(5), Ir1−Sn1 = 2.553(1), Ir1−C4 = 2.190(5), Ir1−C1 =
2.210(5), Ir1−C2 = 2.241(5), Ir1−C3 = 2.232(5), Ir1−N3 = 2.135(4),
Ir1−C5 = 2.161(5), Sn1−Cl2 = 2.368(1), Sn1−Cl3 = 2.367(1), Sn1−
Cl1 = 2.374(1), Ir2−N1 = 2.059(4), Ir2−C21 = 2.087(5), Ir2−C15 =
2.189(5), Ir2−C14 = 2.226(5), Ir2−C13 = 2.217(5), Ir2−C12 =
2.246(5), Ir2− C11 = 2.241(5), Ir2−Sn2 = 2.549(1), Sn2−Cl4 =
2.385(1), Sn2−Cl6 = 2.392(1), Sn2−Cl5 = 2.403(1); C35−Ir1−N3 =
79.56(18), C35−Ir1−Sn1 = 85.10(14), N3−Ir1−Sn1 = 89.16(12),
N1−Ir2−C21 = 87.42(18), N1−Ir2−Sn2 = 88.19(12), C21−Ir2−Sn2
= 89.59(14).
Figure 6. Molecular structure (50% probability thermal ellipsoids) of 6
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Pd1−N1
= 1.982(3), Pd1−C11 = 1.992(4), Pd1−O1 = 2.066(3), Pd1*−O2 =
2.144(3), O1−C1 = 1.264(5), O2− C1 = 1.257(5), C1−C2 =
1.508(6), Pd1−Pd1* = 2.861(1), N1−Pd1−C11 = 91.09(14); C11−
Pd1−O1 = 92.80(14), N1−Pd1−O2 = 91.03(12), N1−Pd1−O1 =
175.21(12). Symmetry code: −x, y, 1.5 − z.
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the ortho carbon C11 of L2H and the imidazole nitrogen N1,
creating a six-membered ring. The Pd1−C11 and Pd1−N1
distances are 1.992(4) and 1.982(3) Å, respectively, comparable
with those in previously reported palladium(II) cyclometalated
compounds.13 The Pd···Pd distance of 2.861(1) Å is consistent
with an expected bond order of zero,13a and two {Pd(C∧N)}2
units are spanned by two bridging acetates with Pd−O
distances of 2.066(3) and 2.144(3) Å.
The acetate-assisted cyclometalation proceeds through initial
coordination of Pd(OAc)2 to the imidazo nitrogen (N3′)
followed by the interaction of the ortho C−H bond with the
metal. Finally, the hydrogen is transferred to the metal-bound
acetate via a highly ordered six-membered transition state
(Scheme 7). This proposal is in accordance with the concerted
Scheme 7. Acetate-Assisted Palladation via a Six-Membered
Transition State
Figure 7. Molecular structure (50% probability thermal ellipsoids) of 7
with important atoms labeled. Hydrogen atoms are omitted for the
sake of clarity. Selected bond distances (Å) and angles (deg): Pd1−
C31 = 1.999(10), Pd1−Br1 = 2.5023(13), Pd1−N3 = 2.016(7), Pd1−
N4 = 2.076(7); C31−Pd1−N4 = 82.2(3), C31−Pd1−Br1 = 175.1(2),
C31−Pd1−N3 = 91.7(3), N3−Pd1−N4 = 173.1(3), N3−Pd1−Br1 =
92.9(2), N4−Pd1−Br1 = 93.1(2). Symmetry code: −x, −y, 2 − z.
metalation−deprotonation (CMD) mechanism as proposed by
Ryabov, Fagnou, and others.14 Since the imidazo nitrogen is
more basic than the naphthyridine nitrogen, this allows
orthometalation of the C3-aryl ring over the C5′ hydrogen.
Regioselective cyclometalation at C5′ could not be achieved by
this method, even after employing 1 equiv more of Pd(OAc)2
followed by prolonged heating.
distance is 2.502(2) Å. The imidazo nitrogen binds to the Pd
center with a Pd1−N3 distance of 2.016(7) Å. Six Pd centers
adopt a pseudo-chair conformation with the closest and farthest
Pd···Pd distances being 6.106 and 10.296 Å, respectively
(Figure S18, Supporting Information). Two neighboring
imidazonaphthyridine planes are nearly perpendicular (inter-
planar angle 77.1°) in the hexanuclear metallamacrocycle.
Oxidative addition is an alternate pathway to access
cyclometalated compounds.15 Oxidative addition of the C−H
bond to electron-rich, low-valent complexes of late transition
metals (Re, Fe, Ru, Os, Ir, Pt) has afforded a plethora of
metalated compounds.16 Nolan and Crabtree independently
proposed oxidative addition of the imidazolium C−H to Pd(0)
for the synthesis of PdII−NHC compounds.17 Cavell followed a
similar procedure to isolate the metal−NHC−hydride complex
[Pt(H)(dmiy)(PR3)2]BF4 (dmiy = 1,3-dimethylimidazolin-2-
ylidene and R = phenyl, cyclohexyl).18 Albrecht also applied
C−I oxidative addition in halide-substituted imidazolium salts
with low-valent Pd precursors to obtain Pd−NHC complexes.19
To accomplish selective C5′ metalation, we employed 5′-
bromo-3-phenylimidazo[1,2-a][1,8]naphthyridine (L3Br). Oxi-
dative addition of L3Br to Pd2(dba)3 (dba = dibenzylidenea-
cetone) affords the cyclometalated palladacycle {Pd(L3)Br}6
(Scheme 6). The 13C NMR signal corresponding to the
cyclometalated carbon appears downfield at δ 153.1 ppm. The
ESI-MS exhibits a signal at m/z 392 which is assigned to
[Pd(L3)(CH3CN)]+.
The X-ray structure of 7 reveals a hexanuclear cluster with a
circular topology (Figure 7).20 The asymmetric unit is
comprised of a {Pd(L3)Br}3 unit related to the other half by
a center of inversion. The geometries around the Pd atoms are
very similar, and we describe one {Pd(L3)Br} unit here in
detail. The {Pd(L3)Br} unit chelates through the imidazo
carbon (C31) and naphthyridine nitrogen (N4). The imidazo
nitrogen N6 connects to the neighboring Pd. The square-planar
geometry around Pd is completed by a bromide that arises from
the oxidative cleavage of the C−Br bond. The Pd1−
C31(cyclometalated) distance is 1.999(10) Å, the Pd1−
N4(naphthyridine) distance is 2.076(7) Å, and the Pd1−Br1
CONCLUDING REMARKS
■
Owing to the superior donor ability of imidazo nitrogen,
regioselective ortho C−H bond activation occurs at the C3/C4′-
phenyl ring, affording cyclometalated Cp*Ir(C∧N)Cl com-
pounds. Double cyclometalations at C3-aryl and C5′-H are
achieved upon prolonged heating, creating five- and six-
membered metallacycles on a single platform. Insertion of
SnCl2 into the Ir−Cl bond afforded easy access to the
corresponding mixed-metal Ir/Sn compounds. Similar to the
case for Cp*IrIII, Pd(OAc)2 caused regioselective orthometa-
lation involving the C3-aryl unit; however, C5′-H could not be
activated under similar conditions. Oxidative addition of C5′-Br
to Pd2(dba)3 allows regioselective metalation involving C5′ and
the naphthyridine nitrogen N8. Efforts are underway to access a
metal-aNHC complex via N3′ quaternization of the cyclo-
̂
metalated (C5′N) complex, a process commonly known as
post-carbene generation.21
EXPERIMENTAL SECTION
■
General Procedures. All reactions with metal complexes were
carried out under an atmosphere of purified nitrogen using standard
Schlenk-vessel and vacuum-line techniques. Glasswares were flame-
dried under vacuum prior to use. Solvents were dried by conventional
1
methods prior to use. H and 13C NMR spectra were obtained on
JEOL JNM-LA 500 MHz spectrometers. 1H NMR chemical shifts
were referenced to the residual hydrogen signal of the deuterated
solvents. Elemental analyses were performed on a Thermoquest
EA1110 CHNS/O analyzer. ESI-MS was recorded on a Waters-
Micromass Quattro Micro triple-quadrupole mass spectrometer.
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Melting points were measured in open capillaries on a JSGW melting
point apparatus and are uncorrected.
Materials. Solvents were dried by conventional methods, distilled
(d, J = 4.6 Hz, 1H, NP), 8.47 (d, J = 1.7 Hz, 1H, Im), 8.18 (dd, J =
7.85 Hz, J = 1.15 Hz, 1H, NP), 7.95 (s, 1H, NP), 7.94 (d, J = 8 Hz,
1H, Ph), 7.89 (d, J = 7.75 Hz, 1H, Ph), 7.71 (d, J = 1.8 Hz, 1H, Im),
7.51 (dd, J = 8.15 Hz, J = 4.25 Hz, 1H, NP), 7.06 (t, J = 7.17 Hz, 1H,
Ph), 7.06 (t, J = 5.9 Hz, 1H, Ph), 1.42 (s, 15H,CpMe5). 13C NMR (125
MHz, CDCl3, 292 K): 154.3 (Ir−Ccyclometalated), 152.1 (NCNNP), 147.7
(NCCNP), 143.4 (NCCNP), 137.1 (NCCNP), 134.1 (CCCNP), 132.1
(CCCNP), 130.7 (CCCNP), 128.7 (NCCPh), 124.3 (CCCPh), 122.9
(CCCPh), 122.1 (CCCPh), 118.8(NCCIm), 113.3 (NCCIm), 88.0
(CpMe5), 8.8 (CpMe5). ESI-MS: m/z 572 [M − Cl]+, where M =
Ir(L2H)Cp*Cl. Anal. Calcd for C26H25N3ClIr: C, 51.38; H, 4.15; N,
6.91. Found: C, 51.70; H, 4.25; N, 6.72.
Synthesis of 3. A mixture of [Cp*IrCl2]2 (40 mg, 0.05 mmol),
NaOAc (25 mg, 0.31 mmol), and L2H2 (14 mg, 0.05 mmol) was
reacted following a procedure similar to the synthesis of 1 for a period
of 12 h. Compound 3 was isolated as a red crystalline solid. Crystals
suitable for X-ray study were grown by slow vapor diffusion of diethyl
ether into a saturated dichloromethane solution of the compound.
Yield: 45 mg (91%). Mp: >250 °C. 1H NMR (500 MHz, CDCl3, 292
K): δ 8.72 (d, J = 5.5 Hz, 1H, NP), 8.34 (dd, J = 5.5 Hz, J = 1.25 Hz,
1H, NP), 8.02 (dd, J = 8.07 Hz, J = 1.25 Hz, 1H, Ph), 8.00 (dd, J =
7.35 Hz, J = 1.25 Hz, 1H, Ph), 7.70 (s, 1H, Im), 7.40 (dd, J = 8.1 Hz, J
= 4.6 Hz, 1H, NP), 7.40 (s, 1H, NP), 7.04 − 6.97 (m, 2H, Ph), 1.81 (s,
15H, CpMe5), 1.40(s, 15H, CpMe5). 13C NMR (125 MHz, CDCl3,
292 K): 155.9 (Ir−-Ccyclometalated), 154.2 (Ir−Ccyclometalated),146.1
(NCNNP), 144.1 (NCCNP), 137.2 (NCCNP), 134.4 (NCCNP), 132.4
(CCCNP), 129.7 (CCCNP), 128.8 (CCCNP), 122.2 (CCCNP), 126.2
(CCCPh), 124.4 (CCCPh), 122.8 (CCCPh), 121.2(CCCPh), 119.9-
(CCCPh), 114.5(NCCIm), 88.9 (CpMe5), 87.9 (CpMe5) 9.6 (CpMe5),
8.9 (CpMe5). ESI-MS: m/z 934 [M − Cl]+, where M = Ir2(L2)-
Cp2*Cl2. Anal. Calcd for C36H39N3Cl2Ir2: C, 44.57; H, 4.05; N, 4.33.
Found: C, 44.37; H, 4.07; N, 4.27.
under nitrogen, and deoxygenated prior to use. IrCl3·xH2O,
Pd(CH3COO)2, and PdCl2 were purchased from Arora Matthey
22
Kolkata (India). The compounds [IrCp*(μ-Cl)Cl]2
and
[Pd2(dba)3·CHCl3]23 were prepared according to the literature
procedures. 2,4-Dimethyl-7-amino[1,8]naphthyridine, 3-phenyl-2-
amino[1,8]naphthyridine, and 6,8-dimethyl-2′-phenylimidazo[1,2-a]-
[1,8]naphthyridine (L1H) were synthesized as reported earlier.9
Synthesis of L2H2. 3-Phenyl-2-amino[1,8]naphthyridine (165 mg,
0.74 mmol) was refluxed in ethanol/water (10/1) with a small excess
of α-chloroacetaldehyde (0.2 mL, 1.10 mmol) for 4 h in the presence
of an equivalent amount of Na2CO3 (988 mg, 1.10 mmol). After it was
cooled, the solution was evaporated to dryness and the residue was
chromatographed on silica and eluted with methanol/dichloromethane
to give the pure product. Yield: 169 mg (92%). Mp: 105−108 °C. 1H
NMR (500 MHz, CDCl3, 292 K): δ 8.72 (d, J = 4.85 Hz,1H, NP), 8.5
(s, 1H, Im), 8.20 (d, J = 7.45 Hz, 1H, NP), 7.98 (d, J = 9.15 Hz, 2H,
Ph. NP), 7.77 (s, 1H, Im), 7.60 (s, 1H, NP), 7.58−7.46 (m, 4H, Ph).
13C NMR (125 MHz, CDCl3, 294 K): 148.6 (CCNNP), 142.8
(NCNNP), 137.1 (CCCNP), 135.3 (NCCNP), 131.5 (CCCNP), 130.8
(CCCNP), 129.2 (CCCPh), 129.2 (CCCPh), 129.1 (NCCPh), 129.
(CCCPh), 129.0 (CCCPh), 128.8 (CCCPh), 123.48 (CCCNP), 121.6
(CCCNP), 118.7 (NCCIm), 112.60 (NCCIm).
Synthesis of L3Br. A 500 mg amount (2.03 mmol) of L2H2 was
dissolved in glacial acetic acid (10 mL), and 0.2 mL of bromine was
added. The mixture was stirred at room temperature for 15 h. The
precipitate was filtered and redissolved in water. The suspension was
made basic with sodium carbonate and extracted with dichloro-
methane. The organic layer was dried to give the pure product. Yield:
1
556 mg (86%). Mp: 204−206 °C. H NMR (500 MHz, CDCl3, 292
K): δ 8.87 (d, J = 4.3 Hz,1H, NP), 8.32 (d, J = 8.3 Hz, 1H, NP), 7.84
(s, 1H, Im), 7.82−7.78 (m, 3H, NP, Ph), 7.65 (dd, 1H, NP), 7.54−
7.46 (m, 3H, Ph). 13C NMR (125 MHz, CDCl3, 294 K): 148.6
(CCNNP), 143.7 (NCNNP), 142.1 (CCCNP), 138.2 (NCCNP), 137.6
(CCCNP), 133.1 (CCCNP), 130.7 (CCCPh), 130.0 (CCCPh), 129.4
(NCCPh), 129.3 (CCCPh), 129.2 (CCCPh), 129.1 (CCCPh), 127.6
(CCCNP), 123.8 (CCCNP), 122.8 (NCCIm), 119.5 (NCCIm).
Conversion of 2 to 3. Compound 2 (40 mg, 0.065 mmol) was
taken up in a 10 mL DCE solution, [Cp*IrCl2]2 (26 mg, 0.033 mmol)
and NaOAc (32 mg, 0.39 mmol) were added, and the mixture was
refluxed for 12 h; after that a procedure similar to the synthesis of 1
was followed. Yield: 61 mg (90%).
Synthesis of 4. A THF solution (10 mL) of anhydrous SnCl2 (15
mg, 0.082 mmol) was added dropwise to a dichloromethane solution
(10 mL) of 1 (51 mg, 0.08 mmol), whereupon the initial yellow
solution turned bright orange. The resultant solution was concentrated
under vacuum, and 15 mL of petroleum ether was added with stirring
to induce precipitation. Repeated washing followed by prolonged
drying under vacuum provided an orange crystalline solid. Crystals
suitable for X-ray study were grown by layering petroleum ether over a
dichloromethane solution of the compound. Yield: 62 mg (93%). Mp:
>250 °C. 1H NMR (500 MHz, DMSO-d6, 292 K): δ 9.09 (s, 1H, NP),
8.20 (d, J = 9.15 Hz, 1H, NP), 8.0 (dd, J = 6.85 Hz, J = 2.45 Hz, 1H,
Ph), 7.62 (dd, J = 6.42 Hz, J = 1.85 Hz, 1H, Ph), 7.54 (s, 1H, Im), 7.44
(d, J = 10.1 Hz, 1H, NP), 7.19−7.15 (m, 2H, Ph), 2.72 (s, 3H,
CH3NP), 2.70 (s, 3H, CH3NP), 1.71 (s, 15H, CpMe5). 13C NMR (125
MHz, DMSO-d6, 294 K): 159.9 (Ir−Ccyclometalated), 155.6 (NCNNP),
147.3 (CCCNP), 142.7 (NCCNP), 138.9 (CCCNP), 138.1 (CCCNP),
129.5 (CCCNP), 128.3 (CCCNP), 126.8 (NCCNP), 124.2 (CCCPh),
124.1 (CCCPh), 123.4 (CCCPh), 122.2 (CCCPh), 122.1 (CCCPh),
112.0 (NCCIm), 105.1 (NCCIm), 92.0 (CpMe5), 24.8 (CH3),
18.5(CH3), 9.9 (CpMe5). ESI-MS: m/z 600 [Ir(L1)Cp*]+. Anal.
Calcd for C28H29N3Cl3SnIr: C, 40.72; H, 3.54; N, 5.09. Found: C,
40.31; H, 3.52; N, 5.11.
Synthesis of 5. A THF solution (10 mL) of anhydrous SnCl2 (30
mg, 0.164 mmol) was added dropwise to a dichloromethane solution
(10 mL) of 3 (78 mg, 0.08 mmol) following a procedure similar to the
synthesis of 4. Compound 5 was isolated as a red crystalline solid.
Yield: 100 mg (93%). Mp: >250 °C dec. 1H NMR (500 MHz, DMSO-
d6, 292 K): δ 8.68(d, J = 5.1 Hz, 1H, NP), 8.55 (s, 1H, Im), 8.41 (d, J
= 7.75 Hz, 1H, Ph), 8.31 (d, J = 8.3 Hz, 1H, NP), 7.83 (d, J = 7.75 Hz,
1H, Ph), 7.70−7.65 (m, 1H, NP), 7.26 (t, J = 7.6 Hz, 1H, Ph), 7.15 (J
= 8.3 Hz, 1H, NP), 7.13 (t, 1H, Ph), 2.00 (s, 15H, CpMe5), 1.52 (s,
15H, CpMe5).
Synthesis of 1. [Cp*IrCl2]2 (44 mg, 0.06 mmol), NaOAc (28 mg,
0.34 mmol), and L1H (33 mg, 0.12 mmol) were dissolved in
dichloroethane (10 mL), and the mixture was refluxed for 4 h under
nitrogen. The resulting orange-red suspension was filtered through a
small bed of Celite. The filtrate was then concentrated under reduced
pressure, followed by the addition of 15 mL of diethyl ether with
stirring to induce precipitation. Repeated washings followed by
prolonged drying under vacuum provided the cyclometalated
compound 1 as a red solid. Crystals suitable for X-ray study were
grown by slow vapor diffusion of diethyl ether into a saturated
dichloromethane solution of the compound. Yield: 66 mg (86%). Mp:
1
>250 °C. H NMR (500 MHz, CDCl3, 292 K): δ 8.54 (s, 1H, NP),
7.82 (d, J = 7.75 Hz, 1H, Ph), 7.69 (d, J = 8.05 Hz, J = 10 Hz, 1H,
NP), 7.63 (d, J = 7.15 Hz, 1H, Ph), 7.57 (d, J = 9.75 Hz, 1H, NP), 7.19
(s, 1H, Im), 7.11 (t, J = 7.3 Hz 1H, Ph), 7.0 (t, J = 7.42 Hz, 1H, Ph),
2.73 (s, 3H, CH3NP), 2.67 (s, 3H, CH3), 1.73 (s, 15H, CpMe5). 13C
NMR (125 MHz, CDCl3, 292 K): 159.2 (Ir−Ccyclometalated), 154.3
(NCNNP), 146.6 (CCCNP), 143.2 (NCCNP), 139.0 (CCCNP), 136.2
(CCCNP), 129.7 (CCCNP), 128.7 (CCCNP), 126.8 (NCCNP), 122.8
(CCCPh), 122.6 (CCCPh), 122.2 (CCCPh), 122.1 (CCCPh), 121.5
(CCCPh), 113.3 (NCCIm), 104.1 (NCCIm), 87.5 (CpMe5), 24.8(CH3),
18.7(CH3), 9.47 (CpMe5). ESI-MS: m/z 600 [M − Cl]+, where M =
Ir(L1)Cp*Cl. Anal. Calcd for C28H29N3ClIr: C, 52.89; H, 4.60; N,
6.61. Found: C, 52.47; H, 4.56; N, 6.45.
Synthesis of 2. A mixture of [Cp*IrCl2]2 (40 mg, 0.05 mmol),
NaOAc (25 mg, 0.31 mmol), and L2H2 (27 mg, 0.11 mmol) were
reacted by following a procedure similar to the synthesis of 1.
Compound 2 was isolated as a red crystalline solid. Crystals suitable
for X-ray study were grown by slow vapor diffusion of diethyl ether
into a saturated dichloromethane solution of the compound. Yield: 60
mg (91%). Mp: >250 °C. 1H NMR (500 MHz, CDCl3, 292 K): δ 8.60
4311
dx.doi.org/10.1021/om4004658 | Organometallics 2013, 32, 4306−4313

Organometallics
Article
Synthesis of 6. Pd(CH3COO)2 (30 mg, 0.134 mmol), Na2CO3
(16 mg, 0.136 mmol), and L2H2 (33 mg, 0.13 mmol) were dissolved in
acetronitrile (10 mL), and the mixture was refluxed for 6 h under
nitrogen. The resulting yellow suspension was filtered through a small
bed of Celite. The filtrate was concentrated under reduced pressure,
followed by the addition of 15 mL of diethyl ether with stirring to
induce precipitation. Repeated washing (3 × 10 mL) with diethyl ether
followed by prolonged drying under vacuum provided the cyclo-
metalated compound 6 as a yellow crystalline solid. Crystals suitable
for X-ray study were grown by layering diethyl ether over a saturated
dichloromethane solution of the compound. Yield: 61 mg (86%). Mp:
ASSOCIATED CONTENT
* Supporting Information
Figures, a table, and CIF files giving crystallographic data, NMR
spectra, and ESI-MS data of compounds and the pseudo-chair
conformation of 7. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*J.K.B.: e-mail, jbera@iitk.ac.in; fax, +91-512-2597436; tel, +
91-512-2597336.
■
1
>250 °C. H NMR (500 MHz, DMSO-d6, 292 K): δ 8.79 (d, J = 4.2
Hz, 1H, NP), 8.07 (d, J = 7.6 Hz, 1H, NP), 7.89 (d, J = 1.8 Hz, 1H,
Im), 7.69 (dd, J = 7.7 Hz, J = 4.6 Hz, 1H, NP), 7.30 (d, J = 1.95 Hz,
1H, Im), 7.24 (d, J = 8.25 Hz, 1H, Ph), 7.06 (d, J = 1.85 Hz, 1H, Im),
7.03 (d, J = 7.65 Hz, 1H, Ph), 6.78 (t, J = 7.4 Hz, 1H, Ph), 6.69 (t, J =
7.4 Hz, 1H, Ph), 3.15 (s, 3H, CH3COO). 13C NMR (125 MHz,
CDCl3, 294 K): 180.2 (Pd−Ccyclometalated), 149.3 (NCNNP), 138.0
(CCCNP), 134.7 (NCCNP), 130.3 (CCCNP), 130.2 (CCCNP), 128.4
(CCCNP), 126.3 (CCCNP), 126.2 (NCCNP), 123.9 (CCCPh), 123.7
(CCCPh), 122.6 (CCCPh), 122.5 (CCCPh), 122.5 (CCCPh), 119.8
(NCCIm), 112.6 (NCCIm), 119.8 (CH3COO), 24.76 (CH3COO). ESI-
MS: m/z 391 [Pd(L2H)(CH3CN)]+. Anal. Calcd for C34H26N6O4Pd2:
C, 52.81; H, 3.20; N, 10.27. Found: C, 52.63; H, 3.43; N, 10.22.
Synthesis of 7. Pd2(dba)3·CHCl3 (30 mg, 0.134 mmol) and L3Br
(33 mg, 0.136 mmol) were dissolved in dichloromethane (10 mL),
and the mixture was stirred for 4 h under nitrogen. The resulting
yellow suspension was filtered through a small bed of Celite. The
filtrate was concentrated under reduced pressure, followed by the
addition of 15 mL of diethyl ether with stirring to induce precipitation.
Repeated washing followed by prolonged drying under vacuum
provided cyclometalated compound 7 as a yellow solid. Crystals
suitable for X-ray study were grown by layering diethyl ether over a
dichloromethane solution of the compound. Yield: 50 mg (86%). Mp:
115−117 °C. 1H NMR (500 MHz, DMSO-d6, 292 K): δ 8.75 (d, 1H,
NP), 8.49 (d, 1H, J = 7.75, NP), 8.01 (d, J = 7.35 Hz, 1H, NP), 7.94
(q, 1H, NP), 7.78−7.74 (m, 1H, Ph), 7.72 (s, 1H, Im), 7.67 (dd, J =
7.45 Hz, 1H, Ph), 7.50 (t, J = 8.25 Hz, 1H, Ph), 7.46−7.42 (m, 2H,
Ph). 13C NMR (125 MHz, DMSO-d6, 292 K): 153.1 (Pd−
Ccyclometalated), 147.9 (NCNNP), 143.3 (CCCNP), 138.3 (NCCNP),
135.4 (CCCNP), 129.9 (CCCPh), 129.8 (CCCPh), 129.4(CCCPh),
129.7 (CCCPh), 128.8 (CCCPh), 128.7 (CCCPh), 126.1 (CCCNP),
124.2 (CCCNP), 122.5 (CCCNP), 119.4 (CCCNP), 115.7 (NCCIm).
ESI-MS: m/z 392 [Pd(L3)(CH3CN)]+. Anal. Calcd for
C16N3H10BrPd: C, 44.76; H, 2.34; N, 9.79. Found: C, 44.36; H,
2.41; N, 9.67.
X-ray Data Collection and Refinement. Single-crystal X-ray
structural studies were performed on a CCD Bruker SMART APEX
diffractometer equipped with an Oxford Instruments low-temperature
attachment. Data were collected at 100(2) K using graphite-
monochromated Mo Kα radiation (λα = 0.71073 Å). The frames
were indexed, integrated, and scaled using the SMART and SAINT
software packages,24 and the data were corrected for absorption using
the SADABS program.25 The structures were solved and refined using
the SHELX suite of programs,26 while additional crystallographic
calculations were performed for compound 1, 2, 4, and 7 by the
“SQUEEZE” option in PLATON.27 The crystallographic figures have
been generated using Diamond 3 software28 (50% probability thermal
ellipsoids). The hydrogen atoms were included in geometrically
calculated positions in the final stages of the refinement and were
refined according to the “riding model”. Hydrogen atoms of the water
molecule were not included in the structure of 7. All non-hydrogen
atoms, except C1, C4, and C5 in compound 3, were refined with
anisotropic thermal parameters. Anisotropic treatment of these three
atoms resulted in nonpositive-definite displacement tensors and were
therefore subjected to isotropic refinement. CCDC files 938956−
938962 contain supplementary crystallographic data for compounds
1−7. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This paper is dedicated to Prof. Akhil R. Chakravarty on the
occasion of his 60th birthday. This work was financially
supported by the Department of Science and Technology
(DST) of India and the Indo-French Centre for the Promotion
of Advanced Research (IFCPAR). J.K.B. thanks the DST for a
Swarnajayanti fellowship. T.G. thanks the UGC of India, and
P.D. thanks the CSIR of India for an SPM fellowship.
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