Syntheses, structural aspects, solution behavior, and catalytic utility of cyclopalladated N,N′,N″-triarylguanidines [κ2(C,N))Pd(Pyrazole)2X] (X = Br, OC(O)CF3, and PF6) in Suzuki-Miyaura coupling reactions of a

Article abstract of DOI:10.1021/acs.organomet.6b00442

The reactions of six-membered cyclopalladated N,N′,N″-triarylguanidines [κ²(C,N)Pd(μ-X)]₂ (3-7) with 2 equiv of pyrazole (pzH) and 3,5-dimethylpyrazole (3,5-dmpzH) in CH₂Cl₂ at RT for 24 h afforded a new class o

Full text of DOI:10.1021/acs.organomet.6b00442

Article
pubs.acs.org/Organometallics
Syntheses, Structural Aspects, Solution Behavior, and Catalytic Utility
of Cyclopalladated N,N′,N″‑Triarylguanidines [κ2(C,N))Pd(Pyrazole)2X]
(X = Br, OC(O)CF3, and PF6) in Suzuki−Miyaura Coupling Reactions of
Aryl Bromides
Pallavi Agarwal,† Natesan Thirupathi,*,† and Munirathinam Nethaji‡
†Department of Chemistry, University of Delhi, Delhi 110 007, India
‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bengaluru 560 012, India
S
* Supporting Information
ABSTRACT: The reactions of six-membered cyclopalladated
N,N′,N″-triarylguanidines [κ2(C,N)Pd(μ-X)]2 (3−7) with 2
equiv of pyrazole (pzH) and 3,5-dimethylpyrazole (3,5-
dmpzH) in CH2Cl2 at RT for 24 h afforded a new class of
cyclopalladated N,N′,N″-triarylguanidines, [κ2(C,N)Pd-
(PzH)2X] (Ar = 2-MeC6H4; X = Br; PzH = 3,5-dmpzH (8),
pzH (9); X = OC(O)CF3; PzH = 3,5-dmpzH (10); Ar = 2-
(MeO)C6H4; X = Br; PzH = 3,5-dmpzH (11); OC(O)CF3
(12); Ar = Ph; X = OC(O)CF3; PzH = 3,5-dmpzH (13)) in
good yields. The reaction of 8 with NH4PF6 in CH2Cl2 at RT
for 24 h afforded [κ2(C,N)Pd(3,5-dmpzH)2(PF6)] (14) in
83% yield. Complexes 8−14 were characterized by elemental
analyses, IR, NMR (1H, 13C, and 19F) spectroscopic
techniques, and conductivity measurements. Molecular structures of six-membered cyclopalladated N,N′,N″-triphenylguanidine
[κ2(C,N)Pd(μ-OC(O)CF3)]2·PhMe (7·PhMe), 8, 10, 12, and 14 were determined by single-crystal X-ray diffraction, which
revealed transoid in-in (7·PhMe), β-out (10 and 14), and α-out (12) conformations. The eight-membered “[Pd(κ1N-PzH)2X]”
rings in 8, 10, 12, and 14 that consist of a pair of N−H···X hydrogen bonds are hitherto unknown in palladacyclic chemistry. VT
1H NMR (CD2Cl2, 400 MHz) spectra of 10 revealed the presence of a mixture of three species, while VT 19F NMR (CD2Cl2,
376.2 MHz) spectra revealed the presence of a mixture of seven species at 213 K. The presence of more than one solution
species of 10 was explained by invoking the presence of a mixture of conformers and intermediates formed between a pair of
related conformers. The catalytic efficacy of 8−14 in Suzuki−Miyaura coupling reactions of various aryl bromides with
PhB(OH)2 was evaluated, and these cyclopalladated guanidines showed a wide substrate scope in the coupling reactions with low
catalyst loadings (1.00, 0.1, and 0.01 mol %) under mild reaction conditions.
Scheme 1
INTRODUCTION
■
The bridge-splitting reaction (bsr) of cyclopalladated imines,
amines, amidines, guanidines, and related compounds of the
type [κ2(C,N)Pd(μ-X)]2 (A; X = chloro, bromo, and
carboxylato) with a monofunctional Lewis base, L, such as
pyridine and phosphine, is a well-known method to generate
more soluble monomeric species, cis/trans-[κ2(C,N)Pd(L)X]
(B; see Scheme 1).1−4 The bsr of A with a bifunctional Lewis
base such as pyrazole and substituted pyrazoles (PzH) was not
studied at all in the literature since the resulting monomeric
palladacycle, C, was believed to be unstable.5 The κ1N
coordination mode of PzH when present proximal to X as is
present in C was considered as one of the rare coordination
modes,5 and under basic conditions elimination of HX from C
followed by dimerization of the resulting species to afford the
dinuclear species such as D is inevitable (see Chart 1).6
under reflux conditions afforded the corresponding dimers
[κ2(C,N)Pd(μ-Pz)]2 (D) and [{κ2(C,N)Pd}2(μ-OAc)(μ-Pz)]
The reactions of 1 and 2 with pyrazoles and substituted
pyrazoles (PzH) in 1:1 and 2:1 (Pd:PzH) ratios in toluene
Received: June 3, 2016
© XXXX American Chemical Society
A
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respectively. The aforementioned transformations are illus-
trated in Scheme 2.
Chart 1. Known Six-Membered Cyclopalladated N,N′,N″-
Triarylguanidines Bridged by Pyrazolato- and Acetato-
pyrazolato Ligands
Scheme 2
(E), respectively, in high yields (see Charts 1 and 2).8 We
wished to study the reactions of 3−7 with PzH by controlling
1H NMR spectra of 8−12 revealed the presence of more
than one solution species, and we thought this observation is
partly due to two distinct orientations of the o-substituent of
the aryl ring in the NAr moiety of the C,N chelate with
reference to the basal plane of the boat of the six-membered
[κ2(C,N)Pd] ring (see later). We believed the more sym-
metrical Ph substituent in the NPh moiety of the C,N chelate
in 13 than the aryl substituent in the NAr moiety of the C,N
chelate in 8−12 would reduce the number of solution species,
and this would enable us to better understand the solution
behavior of the latter palladacycles. In order to simplify the
spectral pattern and confirm our hypothesis mentioned above,
we also synthesized 7 in 65% yield from the cyclopalladation
reaction of N,N′,N″-triphenylguanidine, (PhNH)2CNPh,
with Pd(OC(O)CF3)2 (Pd:guanidine = 1:1) in toluene under
reflux conditions for 4 h following the literature procedure
published for related six-membered cyclopalladated N,N′,N″-
triarylguanidines, [κ2(C,N)Pd(μ-OC(O)CF3)]2.4 Further, the
reaction of 7 with 2 equiv of 3,5-dmpzH in CH2Cl2 at RT for
24 h gave 13 in 85% yield. Cyclopalladated guanidine 8 was
treated with 1 equiv of NH4PF6 in CH2Cl2 at ambient
temperature for 24 h to afford 14 in 83% yield (see Scheme 3).
Chart 2
Scheme 3
the Pd:PzH ratio to 1:1 in more polar solvents such as CH2Cl2
at ambient conditions with a view to understand how such
variation in reaction conditions influences the nature of the
products. From this endeavor, we were able to isolate 8−13 in
40−45% yields (see Chart 2). Subsequently, cyclopalladated
guanidines 8−13 were obtained from the reactions of the
corresponding dimer 3−7 with PzH in 80−85% yields by
controlling the Pd:PzH ratio to 1:2, as will be discussed here.
The solid-state structures and solution aspects of representative
new palladacycles were studied by single-crystal diffraction
1
(SCXRD) and RT NMR (1H, 13C, and 19F) for all and VT H
NMR and VT 19F NMR for 10, which revealed the presence of
three and seven species, respectively, in solution. Further, the
utility of new palladacycles as phosphine-free catalysts in
Suzuki−Miyaura coupling reactions of various aryl bromides
with PhB(OH)2 is reported.
SCXRD Studies. Molecular structures of 7·PhMe, 8, 10, 12,
and 14 have been determined by SCXRD and are depicted in
Figures 1−3. Selected bond parameters are listed in Tables 1
and 2. Palladacycle 7·PhMe is a dimer consisting of two six-
membered [κ2(C,N)Pd] units bridged by two trifluoroacetato
moieties and revealed a transoid in−in conformation, which is
typical for six-membered cyclopalladated guanidines of the type
[κ2(C,N)Pd(μ-OC(O)R)]2 (R = Me and CF3).4 The Pd atom
in 8, 10, 12, and 14 is surrounded by the imine nitrogen atom
and palladated carbon atom of the C,N chelate on one side and
the imine nitrogen atom of two pyrazoles on the other side,
thereby revealing a distorted square planar geometry. The six-
membered [κ2(C,N)Pd] ring adopts a pseudoboat conforma-
RESULTS AND DISCUSSION
■
Reactivity Studies. The separate reactions of 3 with 2
equiv of 3,5-dmpzH and pyrazole (pzH) in CH2Cl2 at RT for
24 h afforded 8 and 9 both in 80% yield, and the analogous
reaction involving 4 and 3,5-dmpzH afforded 10 in 82% yield.
The separate reactions of 5 and 6 with 3,5-dmpzH in CH2Cl2 at
RT for 24 h afforded 11 and 12 in 80% and 81% yields,
B
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Figure 1. Molecular structure of 7·PhMe at the 30% probability level.
Toluene and all hydrogen atoms except the NH hydrogen atoms are
removed for clarity.
Figure 3. Molecular structures of 12 and 14 at the 30% probability
level. Only relevant hydrogen atoms are shown for clarity.
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg) for 7·PhMe
Pd(1)−C(15)
Pd(1)−N(1)
Pd(1)−O(1)
Pd(1)−O(2)
1.960(6)
2.014(5)
2.086(4)
2.185(4)
C(15)−Pd(1)−O(1)
N(1)−Pd(1)−O(1)
C(15)−Pd(1)−O(2)
N(1)−Pd(1)−O(2)
91.0(2)
179.0(2)
173.9(2)
95.3(2)
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for 8, 10, 12, and 14
8
10
12
14
Pd(1)−C(17)
Pd(1)−N(1)
Pd(1)−N(4)
Pd(1)−N(6)
C(17)−Pd(1)−N(1)
C(17)−Pd(1)−N(4)
C(17)−Pd(1)−N(6)
N(1)−Pd(1)−N(4)
N(1)−Pd(1)−N(6)
N(4)−Pd(1)−N(6)
1.984(3)
2.023(3)
2.030(3)
2.130(3)
89.3(1)
90.7(1)
177.6(1)
178.7(1)
93.2(1)
86.9(1)
1.986(4)
2.028(3)
2.028(3)
2.145(4)
89.1(1)
91.0(2)
179.2(1)
178.2(1)
91.5(1)
88.4(1)
1.994(4)
2.040(3)
2.048(4)
2.153(4)
89.3(2)
90.8(2)
178.9(2)
178.3(2)
89.8(1)
90.1(1)
1.992(6)
2.028(5)
2.043(5)
2.152(6)
88.5(2)
91.2(2)
178.5(2)
177.8(2)
91.9(2)
88.4(2)
tion wherein the Pd and the endocyclic NH hydrogen atoms
occupy the tips of the boat, while the remaining four atoms N1,
C1, C16, and C17 occupy the basal plane of the boat.
Figure 2. Molecular structures of 8 and 10 at the 30% probability level.
Only relevant hydrogen atoms are shown for clarity.
Cyclopalladated N,N′,N″-triarylguanidines, cis/trans-
[κ2(C,N)Pd(Lewis base)X], were shown to exist as either an
C
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α-conformer or β-conformer depending upon the nature of the
ortho substituent of the aryl moiety in the NAr unit and its
upward and downward orientations.4a,b The Pd atom in 8, 10,
12, and 14 is part of both the six-membered [κ2(C,N)Pd] ring
and the eight-membered [Pd(κ1N-PzH)2X] ring, and as a
result, the tips (endocyclic NH proton and X unit) of these two
rings can orient toward and away from each other, thereby
generating “in” and “out” conformers, respectively. Thus, four
conformers, namely, α-in, α-out, β-in, and β-out are possible for
8−12 and 14 while only “in” and “out” conformers are possible
for 13, as this palladacycle lacks the ortho substituent in the Ph
ring of the NPh unit (see Chart 3). The ortho methyl
and fifth positions of PzH and the shape of the anion, X.9 In 8,
10, 12, and 14, the two pyrazole planes form a dihedral angle of
84.15(2)° (8), 83.95(2)° (10), 89.96(2)° (12), and 83.16(4)°
(14), and the torsion angles, τ, defined by the four N atoms of
the two pyrazoles involved are 16.74(4)° (8), −22.94(4)° (10),
−3.37(4)° (12), and −23.97(6)° (14). This orientation of
pyrazoles permits the formation of an eight-membered
[Pd(κ1N-PzH)2X] ring (X = Br, and O) wherein the Br or
the oxygen atom of OC(O)CF3 is simultaneously hydrogen
bonded to the NH hydrogen atom of pyrazoles. The eight-
membered [Pd(κ1N-PzH)2X] ring contains one medium
strength and one weak intramolecular N−H···X hydrogen
bond (8 and 10) or only medium-strength N−H···X hydrogen
bonds (12 and 14; see Table 3). The X atom in the eight-
a
Chart 3. Four Possible Conformers of 8−12 and 14
Table 3. Intramolecular Hydrogen Bond Parameters (Å and
deg) Present in 8, 10, 12, and 14
palladacycle
D−H···A
H−A
D−A
D−H−A
8
N5−H5···Br1
N7−H7···Br1
N5−H5···O1
N7−H7···O1
N7−H7···O4
N5−H5···O4
N5−H5···O5
C27−H27b···O5
N5−H5···F5
N5−H5···F6
N7−H7···F1
N7−H7···F6
2.4225(7)
2.6118(8)
1.93(5)
2.13(4)
2.04(5)
2.05(5)
2.54(6)
2.61(6)
2.45(2)
2.14(1)
2.50(1)
2.50(1)
3.281(4)
3.282(3)
2.771(6)
2.826(5)
2.883(6)
2.822(7)
3.324(8)
3.455(1)
3.26(2)
176.6(2)
135.6(2)
166.6(3)
137.5(3)
150.7(3)
148.6(3)
151.2(3)
146.4(4)
157.1(5)
151.5(5)
155.2(5)
138.5(5)
10
12
14
2.92(1)
3.30(2)
3.20(1)
membered [Pd(κ1N-PzH)2X] ring of 8 and 10 is accepting
bifurcated. The second oxygen atom of the trifluoroacetato
moiety in 10 is linked to an exocyclic N(H)Ar proton of the
adjacent molecule related by a 2-fold screw axis through a N−
H···O hydrogen bond. The aforementioned intermolecular N−
H···O hydrogen bond grows along the b-axis to afford a one-
dimensional chain-like structure as illustrated in Figure 4.
Interestingly, out of two oxygen atoms of the trifluoroacetato
moiety in 12, one is part of the eight-membered [Pd(κ1N-
PzH)2O] ring and accepting bifurcated as previously discussed
for 10. However, the second oxygen atom of the
trifluoroacetato moiety is acceptor bifurcating, simultaneously
involved in one intramolecular N−H···O hydrogen bond and
one intramolecular C−H···O hydrogen bond. This feature in
12 contrasts with that observed in 10, and this difference is
likely ascribed to lesser steric pressure exerted by the ortho
anisyl moiety in the NAr unit of the C,N chelate in the
former palladacycle. The formation of an eight-membered
[Pd(κ1N-PzH)2X] (X = Br and O) ring in 8, 10, and 12 is
hitherto unknown in the literature in general and in
palladacyclic chemistry in particular. Only in [κ2(N,N)(fppz)-
Pt(3,5-dmpzH)2]Cl (fppzH = 3-(trifluoromethyl)-5-(2-
pyridyl)pyrazole) does the eight-membered [Pt(κ1N-3,5-
dmpzH)2Cl] ring contain a pair of N−H···Cl hydrogen
bonds,11 analogous to that present in 8. The hydrogen bond
parameters associated with the N−H···X (X = Br, O) unit in 8,
10, and 12 closely match those parameters reported for related
complexes.10,11 The cis-Pd(κ1N-PzH)2 fragment upon N−H···
O hydrogen bond formation with the carboxylato moiety
a
α and β indicate upward and downward orientations, respectively, of
the ortho substituent of the aryl moiety in the NAr unit, while “in”
and “out” indicate inward and outward orientations of X, respectively,
with respect to the basal plane of the boat.
substituent of the aryl ring in the NAr unit of 8 revealed a
rotational disorder, and this hampered α and β nomenclature
with certainty. The perusal of molecular structures of 10 and 14
suggests a β-out conformation, while that of 12 suggests an α-
out conformation. The molecular structure of 11·PhMe was
also determined by SCXRD and revealed a greater R factor, but
the quality of X-ray diffraction data is sufficient enough to
assign an α-out conformation (see Figure S1 in the Supporting
Information, SI). The six-membered [κ2(C,N)Pd] and the
eight-membered [Pd(κ1N-PzH)2X] rings in 8, 10, 11·PhMe,
12, and 14 revealed a pseudoboat conformation. The Pd(1)−
N(6) distances in 8, 10, 12, and 14 are longer than the Pd(1)−
N(4) distance due to a greater trans influence of palladated
carbon in the C,N chelate.
It has been shown that [(η3-allyl)Pd(κ1N-PzH)2X]-type
complexes revealed either a discrete dimeric structure or a one-
dimensional chain-like structure mediated by a N−H···X
hydrogen bond depending upon the substituents in the third
2
usually forms a 10-membered ring, R2 (10), as observed in
[Pd(pzH)4](OC(O)CH2NHC(O)Me)2 and [Pd(3,5-
D
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Figure 4. Intra- and intermolecular N−H···O hydrogen bonds present in 10 illustrating the formation of a one-dimensional chain-like structure.
Intermolecular hydrogen bond parameters (Å and deg): H2−O2 = 2.262; N2−O2 = 2.958; N2−H2−O2 = 137.97.
dmpzH)4](OC(O)(CH2)3C(O)OH)2,12 but in 10 and 12, the
cis-Pd(κ1N-3,5-dmpzH)2 fragment coordinated to the trifluor-
oacetato moiety forms an eight-membered [Pd(κ1N-PzH)2O]
by an inversion/glide plane through a pair of intermolecular
C−H···F hydrogen bonds, thereby generating a sheet-like
structure (see Figure S2 in the SI).
1
1
Solution Behavior. H NMR spectra of 8, 12, 13, and 14
ring, R2 (8), as illustrated in Figure 5. The voluminous C,N
revealed the presence of two species in about 1.0:0.50, 1.0:0.04,
1.0:0.05, and 1.0:0.54 ratios, respectively, and the presence of
two solution species in the case of 8 and 12 was also revealed
from 13C{1H} NMR spectroscopy. The 1H NMR spectrum of 9
revealed the presence of a single species, while the 13C{1H}
NMR spectrum revealed the presence of two solution species.
Interestingly, the 1H NMR spectrum of 10 revealed the
chelate in 10 and 12 reduces the NPzHPdNPzH bond angle and
thus becomes more suitable for the formation of the R2 (8)
ring.
1
1
presence of three species in about 1:0.57:0.25 ratio. H NMR
and 13C{1H} NMR spectra of 11 revealed the presence of a
single species in solution.
To better understand the number and nature of solution
species of 8−14, variable-temperature (VT) 1H NMR measure-
ments of 10 (CD2Cl2, 400 MHz, 10 K interval) were carried
1
out as a representative example. The VT H NMR stack plot
for CH3 protons of 3,5-dmpzH and those CH3 protons present
in the guanidine moiety is illustrated in Figure 7. Two sharp
singlets were observed at δ 1.70 and 2.12 and several less
intense broad signals were observed at 303 K. The broad signals
became sharper and clearer upon lowering the temperature at
regular intervals. At 253 K, 12 signals can be seen at δ 1.72 (a, a,
c), 1.84 (b), 1.93 (b, b, c), 2.04 (b, b, c), 2.10 (a), 2.14 (c), 2.15
(b), 2.20 (c, c), 2.30 (a, a), 2.42 (a), 2.45 (b), and 2.50 (a, c),
which are assigned to species 1−3 in about 1:0.6:0.2 ratio, and
the signals of these species are indicated as a, b, and c,
respectively, within parentheses. The NH proton of two
separate pyrazoles revealed a pair of downfield shifted sharp
singlets for isomers 1 and 3 (δ 12.8 and 14.3) and another pair
of downfield-shifted sharp singlets for isomer 2 (δ 13.4 and
13.9) at temperatures ≤ 273 K (see Figure S3 in the SI). These
spectral features further indicate the presence of three isomers,
and the downfield position of signals of the NH proton of
pyrazoles indicates the retention of an intramolecular N−H···O
hydrogen bond in all three species in solution as well. The
sharpness of 1H NMR signals of the NH proton of 3,5-dmpzH
in 10 at temperatures ≤ 273 K is ascribed to a 14N−1H
decoupling mechanism.13
Figure 5. Types of intramolecular N−H···O hydrogen bonds present
in related palladium pyrazole complexes known in the literature (a)
and in 10 and 12 (b). In R2 (10) and R1 (8), R refers to ring, 2 and 2
in (a) and 2 and 1 in (b) refer to the number of hydrogen bond
donors/acceptors, while the number in parentheses refers to the size of
the hydrogen-bonded ring.12
2
2
In 14, the NH proton of two pyrazoles forms a pair of
intramolecular N−H···F hydrogen bonds with the fluorine
atom F6, and thus this fluorine, F6, is accepting bifurcated. The
NH proton of two pyrazoles is also involved in an
intramolecular N−H···F hydrogen bond with F1 and F5, and
thus each NH hydrogen atom is also donor bifurcating. The
fluorine atom F5 is also involved in a C−H···F hydrogen bond
with one of the CH3 protons of one of the pyrazole rings. Thus,
the fluorine atom F5 is also accepting bifurcated. The three F
atoms mentioned above constitute one face of an octahedron of
−
−
PF6 . Out of the remaining three fluorine atoms of PF6 , F3
and F4 are involved in intermolecular C−H···F and N−H···F
hydrogen bonds, respectively, with the N(H)Ar unit of the
neighboring molecule related by a 2-fold screw axis. The
aforementioned C−H···F and N−H···F hydrogen bonds grow
along the a-axis to afford a chain-like structure as shown in
Figure. 6. The chain-like structure is linked to the adjacent
identical chain-like structure constituted by molecules related
The 19F NMR spectrum of 10 measured at 298 K in CDCl3
revealed a singlet at δ −74.95, which contradicts the results of
1H NMR studies discussed in the preceding paragraph (see also
Experimental Section). Therefore, we measured 19F NMR
E
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Figure 6. Intermolecular hydrogen bond interactions in the crystal lattice of 14 forming a chain-like structure. Hydrogen bond parameters (Å and
deg): H2−F4 = 2.531; N2−F4 = 3.187; N2−H2−F4 = 133.74; H10−F3 = 2.624; F3−C10 = 3.410; F3−H10−C10 = 142.64.
Figure 7. VT 1H NMR spectra of 10 in CD2Cl2 (400 MHz). The symbols a, b, and c represent signals of CH3 protons of three species in solution.
spectra of 10 at 273, 243, and 213 K to gain better insight
concerning the number and nature of solution species. 19F
NMR spectra of 10 revealed the presence of two (δ −75.31
(br) and −74.77 (br); 273 K), three (δ −75.14 (br), − 74.94
(br), and −74.57 (br); 243 K), and seven (δ −75.74 (c), −
75.47 (d), −75.03 (a), −74.92 (e), −74.79 (b), −74.49 (f), and
−74.43 (g); 213 K) signals for the OC(O)CF3 fluorine (see
Figure 8). The presence of a mixture of two major species,
1
which is also the likely two major species revealed in the H
NMR spectrum, and five minor species as revealed by 19F NMR
spectroscopy indicates the presence of seven species in total.
It is well known that the 19F nucleus covers a wide range of
about 400 ppm and is sensitive to even subtle modifications in
the chemical environment, and thus a greater number of
solution species was detected for 10 in VT 19F NMR
spectroscopy. The seven species observed for 10 can arise
Figure 8. VT 19F NMR spectra of 10 in CD2Cl2 (376.2 MHz).
F
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Scheme 4. Proposed Key Intermediates Formed during the Interconversion of β-out and β-in Conformers of 10
19F NMR spectrum of 13 revealed two sharp singlets at δ
−75.16 and −75.25 and a broad signal at δ −75.86 in about
2:2:1 ratio, which is likely ascribed to the presence of any three
species from “in” and “out” conformers and the two
from α-in, α-out, β-in, and β-out as illustrated in Chart 3;
intermediates such as F, G, and H formed between the pair of
β-conformers, as illustrated in Scheme 4, and intermediates
such as I, J, and K formed between the pair of α-conformers, as
illustrated in Scheme S1 (see the SI).
1
intermediates F (= I) and H (= K). The H NMR spectrum
The Pd−N(PzH) bond that is trans to the Pd−C bond in the
β-out conformer of 10 could cleave in solution due to a greater
trans influence of the palladated carbon than the imine nitrogen
atom of the C,N chelate while simultaneously X forms a bond
with Pd to afford an intermediate F. The dissociated pyrazole
could be linked to the palladacycle via the intermolecular N−
H···X hydrogen bond as shown in F or in free form along with
G. The five-membered [Pd(κ1N-PzH)X] ring in G would invert
and subsequently be linked to the PzH through a N−H···X
hydrogen bond to afford the intermediate H. The rear side
attack of Pd by the imine nitrogen atom of PzH in H followed
by Pd−X bond cleavage can give rise to the β-in conformer of
10. Also, the β ⇌ α interconversion through the six-membered
[κ2(C,N)Pd] ring inversion can take place, as discussed
previously for cyclopalladated N-donor ligands cis/trans-
[κ2(C,N)Pd(Lewis base)X] in the literature.4a,b,14 Thus, 10
can also reveal the presence of α-in and α-out conformers and
intermediates I, J, and K as shown in Scheme S1 in the SI.
Thus, a maximum of eight solution species are possible for 10
assuming that intermediates G and J would be too unstable to
be detected by 19F NMR spectroscopy even at 213 K.
of 13 revealed the presence of two species in about 1:0.05 ratio
as identified from the signals of CH3 protons of 3,5-dmpzH,
and this number and ratio differ from those revealed by 19F
NMR spectroscopy. The chemical environments faced by 19F
and 1H nuclei of various conformers of 13 could be different for
each conformer in solution, and further chemical shift windows
are different for these two nuclei. The presence of intermediates
such as F, G, and H or I, J, and K is less likely for 14, as PF6− is
noncoordinating, and the presence of two solution species can
thus arise via either Pd−N(PzH) bond rotation or β ⇌ α
interconversion through the six-membered [κ2(C,N)Pd] ring
inversion.
Palladacycles 15 and 16, published earlier from our group,
were shown to exist as a single species in solution, as revealed
17
1
by H and 13C NMR spectroscopy (see Chart 4). Further,
these palladacycles revealed molar conductance values of 154.6
and 143.1, respectively, in MeCN, which clearly indicated a 1:1
Chart 4. Known Cationic Cyclopalladated N,N′,N″-
Triarylguanidines
Palladacycles 8, 9, 11, and 12 can also reveal the presence of
up to a maximum of eight solution species as analogously
discussed for 10. In contrast, 13 can reveal the presence of only
four species, namely, “in” and “out” conformers and
intermediates such as F (= I) and H (= K). The linkage of
[3,5-dmpzH2]+ to Re via the coordinated OSO3 through the
−
charge-assisted N−H···O hydrogen bond has been discussed
for [Re(OSO3)(CO)3(3,5-dmpzH)2]·[3,5-dmpzH2] in the
literature,15 and a complete dissociation of Br during the
interconversion of two conformers was ruled out for
[Ni2(PMe3)2(PzH)(μ2-η1:η3-CH2-o-C6H4)(μ-Pz)]Br.16 The
G
DOI: 10.1021/acs.organomet.6b00442
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Organometallics
Article
electrolyte nature in solution.17 Palladacycles 8−14 revealed
molar conductance values of 59.7, 51.7, 39.0, 62.6, 61.4, 53.4,
and 77.0, respectively, in MeCN, which are significantly smaller
than that anticipated for a 1:1 electrolyte but greater than that
anticipated for a nonelectrolyte.17,18 Probably, the hydrogen
bond interactions between the cation and the anion in 8−14
can be invoked for the low conductance values observed for
these palladacycles in solution. The greater conductance value
observed for 14 among 8−14 is ascribed to the presence of
Table 4. Screening of 8−14 and Pd(OAc)2 for Suzuki−
Miyaura Coupling Reactions of PhB(OH)2 with 4-
a
Bromotoluene
b
c
s. no.
catalyst
mol %
yield (%)
TON
1
2
3
4
5
6
7
8
8
9
1
1
1
1
1
1
1
1
100
90
99
0
100
90
99
0
−
more noncoordinating PF6 present in 14 than those anions
10
present in 8−13. [Au(κ1N-3,5-dmpzH)2][X]·nH2O (n = 0; X =
AuCl2, and BF4; n = 1, X = NO3) revealed lower conductance
values than anticipated for 1:1 electrolytes in acetone, and this
was ascribed to cation−anion association in solution as well.19
The difference in the number of solution species between 15
and 16 on one hand and 8−14 on the other is ascribed to the
difference in shape of the Lewis base: a rod-like MeCN in the
former two palladacycles, while a flat, less symmetric hydrogen-
bond-donating PzH in the latter.
Plausible Mechanism. A plausible mechanism of for-
mation of 8−13 has been proposed based on a point zero
charge (pzc) model that was originally proposed by Ramanan
and Whittingham to explain the formation of metal organic
frameworks (MOFs) and metal organic polymers (MOPs).20
The first step involves symmetrical cleavage of the dimer A by
PzH to form a pzc species, C, as illustrated in Scheme S2 in the
SI. The species C in the presence of another equivalent of PzH
can afford pzc species F or I depending upon the nature of the
aryl substituent in the guanidine moiety. The species F or I
subsequently rearranges to 8, 9, and 11 or 10, 12, and 13
depending upon the nature of PzH and X. Thus, the Pd:PzH
ratio in conjunction with the presence of a hydrogen bond
donor functionality in PzH is responsible for the formation of
8−13.
11
12
67
0
67
0
13
14
99
83
99
83
Pd(OAc)2
a
Reaction conditions: 4-bromotoluene (171.1 mg; 1.00 mmol),
PhB(OH)2 (158.6 mg; 1.30 mmol), K2CO3 (276.5 mg; 2.00 mmol),
b
DMF/H2O (2/2 mL), 20 h, and 100 °C. The product was purified by
column chromatography on silica gel using an n-hexane/ethyl acetate
c
(94−85/6−15, v/v %) mixture. Turnover number (TON) = (mole of
substrate) × (% yield of product)/(mole of complex).
Chart 5. Known Cyclopalladated N,N′,N″-Triarylguanidine
Bridged by Pyrazolato Ligands
Suzuki−Miyaura Coupling Reactions. The role of [C,E]
palladacycles (E = N, P, S) in catalysis gained momentum since
the discovery of [C,P] palladacycles developed by Hermann,
Beller, and co-workers.21 However, there has been a great
interest in designing phosphine-free cyclopalladated N-donor
ligands as catalysts for Suzuki−Miyaura coupling reactions in
the last two decades since catalysis can be carried out under
aerobic conditions, and moreover these [C,N] palladacycles are
less toxic than their phosphine analogues.22−29 Further, trans-
[(Me2N)2CNnBu)2Pd(OAc)2] and related complexes have
been used as phosphine-free catalysts in Suzuki−Miyaura
coupling reactions involving aryl bromides/aryl chlorides with
aryl boronic acids.30
The Suzuki−Miyaura coupling reactions involving 4-
bromotoluene and PhB(OH)2 were carried out in the presence
of 1 mol % of 8−14 as designer catalysts and Pd(OAc)2 as a
benchmark catalyst in the presence of 2 equiv of K2CO3 in a
DMF/water (1:1, v/v) mixture, and the results of our study are
listed in Table 4. As can be seen 8, 10, and 14 perform better
than 9, which in turn performs much better than 12, while 11
and 13 are inactive, indicating the role of both the substituent
in the aryl rings of the guanidine moiety and the anion, X. The
greater yield of coupling products using 8, 10, and 14 as
catalysts than using Pd(OAc)2 signifies the beneficial effect of
the coordination environment around Pd provided by ancillary
ligands in cyclopalladated guanidines. Moreover, 1 mol % of the
dinuclear palladacycle 17 was found to be inactive in the
Suzuki−Miyaura coupling reaction (see Chart 5).8 The
catalytically active nature of 8, 10, and 14 and inactive nature
of 17 in Suzuki−Miyaura coupling reactions parallel the
conclusion published by Buchwald and co-workers for the
related Pd-pyrazole complexes.31 Catalysts used in the present
investigation are stable to air/moisture even at 100 °C. We
chose 8 for Suzuki−Miyaura coupling reactions of PhB(OH)2
with various aryl bromides, although 10 and 14 are only slightly
less effective.
Suzuki−Miyaura coupling reactions of various aryl bromides
with 1.3 equiv of PhB(OH)2 were carried out in the presence of
1, 0.1, and 0.01 mol % of 8, under the conditions mentioned in
Table 4. The reactions were carried out under relatively mild
conditions, and the isolated yields of coupling products are
listed in Table 5. The structures of coupling products are
collected in Chart 6. The formation of Pd black was not
observed during the reactions, indicating the absence of
degradation of catalysts. The reactions of PhB(OH)2 with
activated aryl bromide such as 4-bromotoluene, bromobenzene,
4-bromoanisole, and 3-bromotoluene gave the respective
coupling products in good to excellent yields even in the
presence of 0.01 mol % of the catalyst (Table 5, entries 3, 6, 7,
and 14).
The Suzuki−Miyaura coupling reaction involving PhB(OH)2
with deactivated aryl bromide such as 4-bromobenzaldehyde in
the presence of 1 mol % of 8 gave the corresponding coupling
product in 100% yield (Table 5, entry 8). Other activated aryl
bromides such as 1-bromo-2-nitrobenzene, 4-bromobenzoni-
trile, and 4-bromophenol upon Suzuki−Miyaura coupling
reactions with PhB(OH)2 under the conditions indicated in
H
DOI: 10.1021/acs.organomet.6b00442
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mixture of species in solution due to conformational equilibria
via neutral species, as revealed by conductivity measurements.
Palladacycles 8−14 were screened for Suzuki−Miyaura
coupling reactions of 4-bromotoluene with PhB(OH)2, and
from the screening experiments 8, 10, and 14 were found to be
efficient catalysts. Palladacycle 8 was further utilized in Suzuki−
Miyaura coupling reactions involving PhB(OH)2 and various
aryl bromides, and the coupling products were isolated in
moderate to high yield. Catalysts prepared in the present
investigation are stable to air and moisture and thus offer the
advantage of syntheses of biaryls under aerobic conditions.
Table 5. Suzuki−Miyaura Coupling Reactions of Various
a
Aryl Bromides with PhB(OH)2 Catalyzed by Palladacycle 8
b
s. no.
Ar
mol % yield (%)
TON
1
4-MeC6H4
4-MeC6H4
4-MeC6H4
C6H5
1
100
90
98
98
97
98
79
100
0
100
900
2
0.1
0.01
1
c
d
3
9800 (9943; 9683 )
4
98
5
C6H5
0.1
0.01
0.01
1
970
d
6
C6H5
9800 (9908 )
c
d
7
4-(MeO)C6H4
4-(OHC)C6H4
4-(OHC)C6H4
4-(OHC)C6H4
2-(O2N)C6H4
4-(NC)C6H4
4-(HO)C6H4
3-MeC6H4
2-MeC6H4
2-(MeO)C6H4
7900 (9336; 9366 )
8
100
0
EXPERIMENTAL SECTION
■
9
0.1
0.01
0.10
0.01
1
Palladacycle 7. Pd(OC(O)CF3)2 (100 mg, 0.301 mmol) and
(PhNH)2CNPh (90.7 mg, 0.313 mmol) were dispersed in toluene
(30 mL) in a 50 mL RB flask, and the flask was fitted to a water
condenser. The contents in the flask were simultaneously stirred and
refluxed for 4 h under a N2 atmosphere. During the course of the
reaction, the heterogeneous mixture slowly became a clear yellow
solution. Subsequently, the reaction mixture was cooled, concentrated
under vacuum, and stored at ambient temperature for 2 days to afford
7 as a hygroscopic yellow solid in 65% (110.4 mg, 0.098 mmol; 7·
1.25PhMe) yield. Palladacycle 7 is soluble in CH2Cl2, CHCl3, ethanol,
methanol, acetone, and acetonitrile. Suitable crystals of 7·PhMe for
SCXRD were grown from a mixture of toluene and CH2Cl2 over a
period of 1 week. Mp: 150.4 °C. Anal. Calcd for Pd2C42H32N6F6O4·
1.25PhMe (MW: 1010.05 + 115.075): C, 54.10; H, 3.76; N, 7.46.
Found: C, 54.18; H, 4.04; N, 7.15. IR (KBr, cm−1): ν(NH) 3407 (br
s); ν(CN) 1594; (vs) νa(OCO) 1641 (sh); νs(OCO) 1466 (w). 1H
NMR (CDCl3, 400 MHz): δ 5.73 (s, 1H, NH), 6.42 (d, JHH = 7.3 Hz,
1H, ArH), 6.52 (s, 1H, NH), 6.54 (s, 2 H, ArH), 6.85 (d, JHH = 7.9 Hz,
2H, ArH), 7.04−7.11 (m, 5H, ArH), 7.24−7.27 (m, 2 H, ArH), 7.34
(t, JHH = 7.8 Hz, 1H, ArH), 7.52 (d, JHH = 6.7 Hz, 1H, ArH). 13C{1H}
10
11
12
13
14
15
16
0
0
86
94
93
84
44
53
860
9400
93
c
0.01
1
8400 (9803 )
44
53
1
a
Reaction conditions: aryl bromide (1.00 mmol), PhB(OH)2 (1.30
mmol), K2CO3 (2.00 mmol), DMF/H2O (2/2 mL), 20 h, and 100 °C.
b
Products were purified by column chromatography on silica gel using
c
an n-hexane/ethyl acetate (94−85/6−15, v/v %) mixture. TON
d
determined by 1H NMR spectroscopy. TON determined by gas
chromatography.
Chart 6. Cross-Coupling Products Discussed in Table 5
1
NMR (CDCl3, 100.5 MHz): δ 113.3, 115.3 (q, JCF = 291.6 Hz,
OC(O)CF3), 118.6, 122.2, 124.5, 125.3, 125.4, 126.3, 126.5, 127.2,
128.3, 129.2, 129.6, 130.4, 135.2, 135.4, 136.2, 138.0, 144.0, 144.4
2
(ArC/ArCH, and CN), 164.0 (q, JCF = 37.7 Hz, OC(O)CF3). 19F
NMR (376.2 MHz, CDCl3): δ −74.69.
Palladacycle 8. Palladacycle 3 (50 mg, 0.038 mmol) and 3,5-
dmpzH (7.5 mg, 0.078 mmol) were dispersed in CH2Cl2 (20 mL) in a
25 mL RB flask, and the contents in the flask were stirred at ambient
temperature for 24 h. During the course of the reaction, the solid
slowly dissolved to afford a clear solution. The reaction mixture was
concentrated under vacuum to afford 8 as a white solid in 80% (43.1
mg, 0.061 mmol) yield. Crystals suitable for SCXRD were grown by
slow evaporation of the reaction mixture at ambient temperature over
a period of 2 days. Mp (DSC): 209.79 °C. Anal. Calcd for
PdC32H38N7Br (MW: 707.024): C, 54.36; H, 5.42; N, 13.87. Found:
C, 54.73; H, 5.56; N, 13.89. IR (KBr, cm−1): ν(NH) 3409 (m), 3374
Table 5 gave the respective coupling products in high yields
(Table 5, entries 11−13). Palladacycle 1 catalyzes those
coupling reactions in which the ortho position of the aryl
bromides are hindered such as 2-bromotoluene and 2-
bromoanisole. In these cases, even when the catalyst loading
is high, the yields of the coupling products are low (Table 5,
entries 15 and 16).
1
(m); ν(CN) 1626 (vs). The H NMR spectrum of 8 indicated the
presence of two isomers in about 1.0:0.50 ratio as estimated from the
integrals of CH3 protons of the guanidine moiety and those of 3,5-
dmpzH located at δ 1.98, 2.10 (isomer 1) and δ 2.04, 2.26, and 2.41
1
(isomer 2). H NMR (CDCl3, 400 MHz): δ 1.69 (s, 4 × 3H, CH3,
isomers 1 and 2), 1.98 (s, 3H, CH3, isomer 1), 2.04 (s, 3H, CH3,
isomer 2), 2.10 (br, 2 × 3H, CH3, isomer 1), 2.26 (br, 3H, CH3,
isomer 2), 2.34 (s, 2 × 3H, CH3, isomers 1 and 2), 2.41 (s, 3H, CH3,
isomer 2), 2.66 (each s, 2 × 3H, CH3, isomers 1 and 2), 5.37 (s, 1H,
NH, isomer 2), 5.46 (s, 2 × 1H, CH, 3,5-dmpzH, isomer 2), 5.69 (br,
1H, NH, isomer 2), 5.73 (s, 2 × 1H, CH, 3,5-dmpzH, isomer 1), 5.90
(s, 1H, NH, isomer 1), 6.00 (br d, JHH = 7.3 Hz, 1H, ArH, isomer 1),
6.10 (br, 1H, NH, isomer 1), 6.48−6.58 (m, 2 × 3H, ArH, isomers 1
and 2), 6.71 (d, JHH = 7.3 Hz, 1H, ArH, isomer 1), 6.76 (br, 1H, ArH,
isomer 2), 6.98−7.02 (m, 2H, ArH, isomer 1), 7.08 (br, 1H, ArH,
isomer 2), 7.20 (br, 2 × 1H, ArH, isomers 1 and 2), 7.27 (br, 1H, ArH,
isomer 2), 7.28 (br, 2 × 2H, ArH, isomers 1 and 2), 7.29 (br, 2 × 1H,
ArH, isomers 1 and 2), 7.32 (br, 1H, ArH, isomer 2), 12.39 (s, 1H,
NH, 3,5-dmpzH, isomer 1), 12.61, 13.18 (each s, 2 × 1H, NH, 3,5-
CONCLUSION
■
Six-membered cyclopalladated N,N′,N″-triphenylguanidine
[κ2(C,N)Pd(μ-OC(O)CF3)]2 (7) and seven six-membered
cyclopalladated N,N′,N″-triarylguanidines [κ2(C,N)Pd-
(pyrazole)2X] (8−14) have been synthesized in good yields
and characterized by elemental analyses, IR, and multinuclear
NMR (1H, 13C, and 19F) spectroscopy. The molecular
structures of 8, 10, 12, and 14 have been determined by
SCXRD. To the best of our knowledge, the presence of Pd
simultaneously in both six- and eight-membered rings as found
in 8−14 is unprecedented in the literature. H, 13C{1H}, and
1
19F NMR spectroscopic data of 8−14 revealed the presence of a
I
DOI: 10.1021/acs.organomet.6b00442
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dmpzH, isomer 2), 13.62 (s, 1H, NH, 3,5-dmpzH, isomer 1). The
13C{1H} NMR chemical shifts of isomer 2 are assigned wherever
possible. 13C{1H} NMR (CDCl3, 100.5 MHz): δ 10.8 (CH3), 11.0
(CH3), 13.9 (br, CH3, isomer 2), 14.3 (CH3), 14.9 (CH3), 17.1 (CH3),
17.8 (CH3), 18.0 (CH3, isomer 2), 18.1 (CH3, isomer 2), 19.8 (CH3),
103.6 (br, CH, 3,5-dmpzH, isomer 2), 104.2 (br, CH, 3,5-dmpzH),
105.2 (br, CH, 3,5-dmpzH), 120.3 (ArC), 120.7 (br, ArC, isomer 2),
121.9 (ArCH), 123.6 (br, ArCH, isomer 2), 124.4 (ArCH), 124.5
(ArCH), 125.8 (ArCH), 126.1 (ArCH), 126.6 (ArCH), 127.4 (br,
ArCH, isomer 2), 127.8 (ArCH), 128.5 (ArCH), 129.0 (ArCH), 130.6
(br, ArCH, isomer 2), 131.8 (ArCH), 133.0 (br, ArC, isomer 2), 134.3
(ArC), 134.6 (ArC), 135.3 (br, ArCH), 135.6 (ArCH), 136.3 (ArC),
140.8 (br, ArC, isomer 2), 141.2 (ArC), 142.2 (ArC, isomer 2), 142.5
(ArC), 143.3 (ArC), 143.8 (br, ArC, isomer 2), 146.4 (ArC), 147.0
(ArC), 147.6 (br, ArC, isomer 2), 148.5 (CN). TOF-MS ES+ m/z
(%) [ion]: 965 [κ2{(C,N)Pd}2(μ-(3,5-dmpz))]+, 950 (8) [κ2{(C,N)-
Pd}2(μ-Br)]+, 570 (12) [κ2(C,N)Pd(3,5-dmpzK)]+, 532 (100)
[κ2(C,N)Pd(3,5-dmpz)]+, 434 (74) [κ2(C,N)Pd − H]+, 328 (80)
[LH2‑tolyl]+. ΛM (Ω−1 cm2 mol−1) = 59.7 (10−3 M) in MeCN.
3,5-dmpzH, isomers 1 and 3), 13.50, 13.94 (each br, 2 × 1H, NH, 3,5-
dmpzH, isomer 2), 14.38 (br, 2 × 1H, NH, 3,5-dmpzH, isomers 1 and
3). 19F NMR (376.2 MHz, CDCl3): δ −74.95. TOF-MS ES+ m/z (%)
[ion]: 627 (15) [κ2(C,N)Pd(3,5-dmpzH)2]+, 571 (50) [κ2(C,N)Pd +
3,5-dmpzH + K]+, 531 (100) [κ2(C,N)Pd(3,5-dmpz)]+, 475 (20)
[κ2(C,N)Pd − H + K]+, 330 (95) [LH32‑tolyl]+. ΛM (Ω−1 cm2 mol−1) =
39.0 (10−3 M) in MeCN.
Palladacycle 11. The title complex was prepared from 5 (50 mg,
0.045 mmol) and 3,5-dmpzH (8.7 mg, 0.090 mmol) in CH2Cl2 (20
mL) following the procedure previously outlined for 8. Suitable
crystals of 11·PhMe for SCXRD were grown from a CH2Cl2/toluene
mixture at ambient temperature over a period of several days. Yield:
80% (45.9 mg, 0.061 mmol, 11·0.25C7H8). Mp: 187.7 °C. Anal. Calcd
for PdC32H38N7O3Br·0.25C7H8 (MW: 755.02 + 23.035): C, 52.10; H,
5.18; N, 12.60. Found: C, 52.50; H, 5.24; N, 12.40. IR (KBr, cm−1):
ν(NH) 3391 (m), 3331 (m); ν(CN) 1622 (vs). 1H NMR (CDCl3,
400 MHz): δ 2.00, 2.26, 2.37, 2.41 (each s, 4 × 3H, CH3), 3.75, 3.77,
3.83 (each s, 3 × 3H, OCH3), 5.41, 5.73 (each s, 2 × 1H, CH, 3,5-
dmpzH), 5.78 (d, JHH = 7.3 Hz, 1H, ArH), 6.44 (d, JHH = 7.3 Hz, 1H,
ArH), 6.57−6.63 (m, 2H, ArH), 6.89 (d, JHH = 8.0 Hz, 1H, ArH), 6.93
(t, JHH = 8.1 Hz, 1H, ArH), 7.02 (t, JHH = 7.7 Hz, 1H, ArH), 7.08 (t,
JHH = 7.7 Hz, 1H, ArH), 7.27 (m, 3H, ArH), 7.85 (br, 1H, NH), 8.05
(s, 1H, NH), 12.21, 13.69 (each s, 2 × 1H, NH, 3,5-dmpzH). 13C{1H}
NMR (CDCl3, 100.5 MHz): δ 10.8 (CH3), 11.0 (CH3), 14.1 (CH3),
14.8 (CH3), 54.4 (br, OCH3), 55.4 (br, OCH3), 55.8 (OCH3), 102.9
(ArCH), 105.1 (ArCH), 106.0 (ArCH), 109.5 (ArCH), 111.2 (ArCH),
120.8 (ArCH), 121.2 (ArCH), 121.8 (ArCH), 122.1 (ArCH), 125.0
(ArCH), 125.9 (ArCH), 126.2 (ArC), 126.6 (ArC), 127.0 (ArC), 129.2
(ArCH), 131.4 (ArCH), 134.3 (ArC), 140.7 (ArC), 142.4 (ArC), 145.0
(ArC), 145.7 (ArC), 147.4 (ArC), 148.7 (ArC), 150.3 (ArC), 151.2
(CN). TOF-MS ES+ m/z (%) [ion]: 1043 (27) [{κ2(C,N)Pd}2(μ-
3,5-dmpz) − Me]+, 578 (98) [κ3(C,N,O)Pd(3,5-dmpz)]+, 521 (100)
[κ2(C,N)Pd − H+ + K]+, 481 (88) [κ2(C,N)Pd − H]+, 378 (95)
[LH32‑anisyl]+. ΛM (Ω−1 cm2 mol−1) = 62.6 (10−3 M) in MeCN.
Palladacycle 12. The title complex was prepared from 6 (57.5 mg,
0.048 mmol) and 3,5-dmpzH (9.4 mg, 0.098 mmol) in CH2Cl2 (20
mL) following the procedure outlined previously for 8. Suitable
crystals of 12 for SCXRD were grown from CH2Cl2 over a period of 1
week. Yield: 81% (62.0 mg, 0.079 mmol). Mp (DSC): 219.49 °C
(dec). Anal. Calcd for PdC34H38N7O5F3 (MW 788.14): C, 51.81; H,
4.86; N, 12.44. Found: C, 51.51; H, 5.08; N, 12.05. IR (KBr, cm−1):
ν(NH) 3390 (m), 3346 (m); ν(CN) 1628 (vs), νa(OCO) 1587 (s),
νs(OCO) 1424 (s). The 1H NMR spectrum of 12 revealed the
presence of two isomers in about 1.0:0.04 ratio as estimated from the
Palladacycle 9. The title complex was prepared from 3 (100 mg,
0.077 mmol) and pzH (10.7 mg, 0.158 mmol) in CH2Cl2 (30 mL)
following the procedure previously mentioned for 8. Yield: 80% (80.1
mg, 0.123 mmol). Mp: 185.4 °C. Anal. Calcd for PdC28H30N7Br
(MW: 650.92): C, 51.67; H, 4.64; N, 15.06. Found: C, 51.72; H, 4.69;
N, 15.04. IR (KBr, cm−1): ν(NH) 3417 (m); 3373 (m); ν(CN)
1
1632 (vs). H NMR (CDCl3, 400 MHz): δ 1.70, 2.12, 2.47 (each s, 3
× 3H, CH3), 5.86 (br, 1H, CH, pzH), 5.98, 6.18 (each br, 2 × 1H,
NH), 6.21 (br, 1H, CH, pzH), 6.52−6.56 (m, 2H, ArH), 6.75 (d, JHH
= 7.3 Hz, 1H, ArH), 6.98 (br, 3H, ArH), 7.14 (br d, JHH = 5.8 Hz, 2H,
CH, pzH), 7.20 (br t, JHH = 4.8 Hz, 2H, CH, pzH), 7.27−7.30 (m, 3H,
ArH), 7.38, 7.57 (each br, 2 × 1 H, ArH), 13.20, 14.00 (each br, 2 × 1
H, NH). 13C{1H} NMR (DMSO-d6, 100.5 MHz): δ 17.5 (CH3), 17.8
(CH3), 18.0 (CH3), 18.7 (CH3), 18.8 (br, CH3), 105.5 (br, CH, pzH),
106.9 (CH, pzH), 122.4 (ArC), 123.2 (ArCH), 125.9 (ArCH), 126.4
(br, ArCH), 127.4 (ArCH), 127.5 (ArCH), 127.8 (ArCH), 128.1
(ArCH), 128.3 (br, CH, pzH), 128.8 (ArCH or pzH), 130.8 (CH,
pzH), 131.4 (ArCH), 131.6 (ArCH), 131.8 (ArCH or pzH), 133.6
(ArC), 134.7 (ArC), 135.4 (ArC), 136.0 (ArCH), 136.7 (ArC), 139.2
(br, ArC), 141.2 (ArCH), 144.4 (ArC), 148.7 (CN). The signals of
the minor isomer were detectable only at δ 18.0 and 18.8. TOF-MS
ES+ m/z (%) [ion]: 937 (12) [{κ2(C,N)Pd}2(μ-pz)]+, 515 (35)
[κ2(C,N)Pd + Br]+, 473 (100) [κ2(C,N)Pd − H + K]+, 434 (75)
[κ2(C,N)Pd − H]+, 328 (92) [LH2‑tolyl]+. ΛM (Ω−1 cm2 mol−1) = 51.7
(10−3 M) in MeCN.
1
integrals of CH3 protons of the pyrazole moieties. H NMR (CDCl3,
Palladacycle 10. The title complex was prepared from 4 (96.6 mg,
0.087 mmol) and 3,5-dmpzH (17.1 mg, 0.178 mmol) in CH2Cl2 (30
mL) following the procedure previously mentioned for 8. Suitable
crystals for SCXRD were grown from CH2Cl2 over a period of 1 week.
Yield: 82% (105.5 mg, 0.142 mmol). Mp: 208.4 °C. Anal. Calcd for
PdC34H38N7O2F3 (MW 740.14): C, 55.17; H, 5.17; N, 13.24. Found:
C, 54.94; H, 4.98; N, 13.34. IR (KBr, cm−1): ν(NH) 3423 (m), 3246
(br); ν(CN) 1623 (m), νa(OCO) 1598 (s); νs(OCO) 1412 (m).
400 MHz): δ 1.76 (s, 3H, CH3, isomer 2), 1.92 (s, 2 × 3H, CH3,
isomers 1 and 2), 2.00 (s, 3H, CH3, isomer 1), 2.26 (s, 3H, CH3,
isomer 2), 2.32 (s, 2 × 3H, CH3, isomers 1 and 2), 2.43 (s, 3H, CH3,
isomer 1), 3.75, 3.77, 3.84 (each s, 6 × 3H, OCH3, isomers 1 and 2),
5.41, 5.73 (each s, 2 × 2H, CH, 3,5-dmpzH, isomers 1 and 2), 5.79 (d,
JHH = 7.3 Hz, 2 × 1H, ArH, isomers 1 and 2), 6.44 (d, JHH = 8.0 Hz, 2
× 1H, ArH, isomers 1 and 2), 6.58−6.65 (m, 2 × 2H, ArH, isomers 1
and 2), 6.88−6.94 (m, 2 × 3H, ArH, isomers 1 and 2), 7.00−7.10 (m,
2 × 3H, ArH, isomers 1 and 2), 7.59 (br, 2 × 1H, ArH, isomers 1 and
2), 8.05 (s, 2 × 1H, NH, isomers 1 and 2), 12.51 (s, 1H, NH, 3,5-
dmpzH, isomer 1), 12.72 (s, 1H, NH, 3,5-dmpzH, isomer 2), 14.42 (s,
2 × 1H, NH, 3,5-dmpzH, isomers 1 and 2). One NH proton of the
guanidine moiety was not observed in the 1H NMR spectrum of 12 for
both the isomers, which could be due to a rapid exchange with traces
of water present in CDCl3. 13C{1H} NMR (CDCl3, 100.5 MHz): δ
10.8 (CH3), 11.0 (CH3), 13.9 (CH3) 14.2 (CH3), 14.8 (CH3), 54.4
(br, OCH3), 55.4 (OCH3), 55.8 (OCH3), 103.0 (ArCH), 105.1
(ArCH), 106.1 (ArCH), 106.4 (ArCH), 109.6 (ArCH), 111.2 (ArCH),
1
The H NMR spectrum of 10 revealed the presence of three isomers
in about 1.0:0.57:0.25 ratio as estimated from the integrals of CH3
protons of the pyrazole moieties. 1H NMR (CDCl3, 400 MHz): δ 1.70
(s, 3 × 3H, CH3, isomer 1 (6H), isomer 3 (3H)), 1.83 (s, 3H, CH3,
isomer 2), 1.94 (s, 3 × 3H, CH3, isomer 2 (6H), isomer 3 (3H)), 2.04
(s, 2 × 3H, CH3, isomer 2), 2.10 (br, 2 × 3H, CH3, isomers 1 and 2),
2.14 (s, 3H, CH3, isomer 3), 2.19 (s, 2 × 3H, CH3, isomer 3), 2.30 (br,
3 × 3H, CH3, isomer 1 (6H), isomer 3 (3H)), 2.41 (br, 3H, CH3,
isomer 2), 2.43 (br, 3H, CH3, isomer 1), 2.50 (s, 2 × 3H, CH3, isomers
1 and 3), 5.37 (s, 2 × 1H, CH, 3,5-dmpzH, isomers 2 and 3), 5.47 (br,
2 × 1H, CH, 3,5-dmpzH, isomers 1 and 3), 5.70 (s, 2 × 1H, NH,
isomer 3), 5.73 (br, 2 × 2H, NH, isomers 1 and 2), 5.92 (s, 2 × 1H,
CH, 3,5-dmpzH, isomers 1 and 2), 6.01 (d, JHH = 8.1 Hz, 2 × 1H,
ArH, isomers 1 and 3), 6.19 (d, JHH = 7.3 Hz, 1H, ArH, isomer 2), 6.50
(m, 3 × 3H, ArH, isomers 1−3), 6.72 (d, JHH = 7.3 Hz, 2 × 1H, ArH,
isomers 1 and 3), 6.77 (br, 1H, ArH, isomer 2), 6.94−7.09 (m, 3 ×
2H, ArH, isomers 1−3), 7.20−7.23 (m, 3 × 1H, ArH, isomers 1−3),
7.28−7.30 (br, 3 × 3H, ArH, isomers 1−3), 12.82 (br, 2 × 1H, NH,
1
117.3 (q, JCF = 294.1 Hz, CF3), 120.8 (ArCH), 121.2 (ArCH), 122.0
(ArCH), 122.1 (ArCH), 125.0 (ArCH), 126.0 (ArCH), 126.3 (ArC),
126.8 (ArC), 127.1 (ArC), 129.3 (ArCH), 130.8 (ArCH), 134.5 (ArC),
141.2 (ArC), 142.8 (ArC), 144.7 (ArC), 145.1 (ArC), 145.8 (ArC),
147.6 (ArC), 148.8 (ArC), 150.3 (ArC), 151.2 (ArC), 151.3 (CN),
2
161.7 (q, OC(O)CF3, JCF = 34.2 Hz). The presence of isomer 2 was
detectable only at δ 13.9. 19F NMR (376.2 MHz, CDCl3): δ −75.01.
J
DOI: 10.1021/acs.organomet.6b00442
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
TOF-MS ES+ m/z (%) [ion]: 1084 (8) [{κ2(C,N)Pd}2(μ-3,5-dmpz) +
Na]+, 1061 (4) [{κ2(C,N)Pd}2(μ-3,5-dmpz)]+, 579 (100) [κ2(C,N)-
Pd(3,5-dmpz))]+, 482 (88) [κ2(C,N)Pd]+, 378 (98) [LH32‑anisyl]+. ΛM
(Ω−1 cm2 mol−1) = 61.4 (10−3 M) in MeCN.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00442.
Palladacycle 13. The title complex was prepared from 7 (100.0
mg, 0.098 mmol) and 3,5-dmpzH (19.3 mg, 0.201 mmol) in CH2Cl2
(30 mL) under the conditions identical to those adopted for 8. Yield:
85% (120.2 mg, 0.167 mmol, 13·0.25 C7H8). Mp: 214.1 °C. Anal.
Calcd for PdC31H32N7O2F3·0.25C7H8 (MW: 698.06 + 23.015): C,
54.55; H, 4.75; N, 13.60. Found: C, 54.76; H, 4.47; N, 13.84. IR (KBr,
cm−1): ν(NH) 3408 (m); ν(CN) 1676 (vs). The 1H NMR
spectrum of 13 revealed the presence of two isomers in about 1:0.05
ratio as estimated from the integrals of CH3 protons of the pyrazole
moiety. 1H NMR (CDCl3, 400 MHz): δ 1.87 (s, 3H, CH3, isomer 2),
2.03, 2.18, 2.23, 2.27 (each s, 4 × 3 H, CH3, isomer 1), 2.33, 2.36, 2.70
(each s, 3 × 3H, CH3, isomer 2), 5.41 (s, 1H, CH, 3,5-dmpzH, isomer
1), 5.52, 5.72 (each s, 2 × 1H, CH, 3,5-dmpzH, isomer 2), 5.76 (s, 1H,
CH, 3,5-dmpzH, isomer 1), 5.87 (s, 1H, NH, isomer 2), 6.10 (s, 1H,
NH, isomer 1), 6.23 (d, JHH = 7.6 Hz, 2 × 1H, ArH, isomers 1 and 2),
6.38 (d, JHH = 9.2 Hz, 2 × 1H, ArH, isomers 1 and 2), 6.61 (s, 2 × 1H,
NH, isomers 1 and 2), 6.66 (t, JHH = 8.0, 2 × 1H, ArH, isomers 1 and
2), 6.92−6.99 (m, 2 × 3H, ArH, isomers 1 and 2), 7.08 (d, JHH = 7.6
Hz, 2 × 3H, ArH, isomers 1 and 2), 7.14−7.16 (m, 2 × 2H, ArH,
isomers 1 and 2), 7.30 (t, JHH = 7.6 Hz, 2 × 1H, ArH, isomers 1 and
2), 7.42 (t, JHH = 7.6 Hz, 2 × 2H, ArH, isomers 1 and 2), 13.15, 14.21
(each s, 4 × 1H, NH, 3,5-dmpzH, isomers 1 and 2). 13C{1H} NMR
(CDCl3, 100.5 MHz): δ 10.9 (CH3), 11.1 (CH3), 14.1 (CH3), 15.0
(CH3), 104.1, 105.3, 114.2, 122.8, 124.3, 125.0, 125.6, 126.2, 126.4 (q,
1JCF = 218.1 Hz, CF3), 129.0, 129.6, 130.4, 131.0, 136.4, 136.6, 137.4,
General considerations, details pertinent to SCXRD
studies, crystallographic data for the palladacycles, DEPT
90 spectra of 8, 9, 11, and 12, HETCOR NMR spectra of
1
8, 9, and 12, H NMR spectra of 10 coupling products,
and GC traces for three reaction mixtures (PDF)
Crystallographic data for 7·PhMe, 8, 10, 12, and 14
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail (N. Thirupathi): tnat@chemistry.du.ac.in, thirupathi_
n@yahoo.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.A. acknowledges Council of Scientific and Industrial
Research, New Delhi, for a fellowship. The authors acknowl-
edge the (i) Department of Science and Technology, New
Delhi, for a research grant, (ii) University Science Instrumen-
tation Center, University of Delhi, for infrastructure facilities,
and (iii) NMR Research Center, Indian Institute of Science,
2
141.5, 142.9, 145.4, 146.4, 147.2, 148.2, 162.1 (app q, JCF = 33.5 Hz,
1
Bengaluru, for VT H NMR and VT 19F NMR experiments.
OC(O)CF3). 19F NMR (376.2 MHz, CDCl3): δ −75.86 (br), −75.25,
−75.16 (intensity ratio ≈ 1:2:2). TOF-MS ES+ m/z (%) [ion]: 698
(7) [M]+, 433 (100) [κ2(C,N)Pd + K]+. ΛM (Ω−1 cm2 mol−1) = 53.4
(10−3 M) in MeCN.
We acknowledge Dr. Surendra Singh, Department of
Chemistry, University of Delhi, for assisting us in gas
chromatographic studies.
Palladacycle 14. Palladacycle 8 (50 mg, 0.071 mmol) and NH4PF6
(12.1 mg, 0.074 mmol) were dispersed in CH2Cl2 (20 mL) in a 50 mL
RB flask. The contents in the flask were stirred at ambient temperature
for 24 h. During the course of the reaction, the solid slowly dissolved
and afforded a clear, colorless solution. Subsequently, the reaction
mixture was stored at ambient temperature for 2 days to afford single
crystals of 14 suitable for SCXRD. Yield: 83% (45.2 mg, 0.058 mmol).
Mp: 163.9 °C. Anal. Calcd for PdC32H38N7PF6 (MW: 772.074): C,
49.78; H, 4.96; N, 12.70. Found: C, 49.75; H, 5.10; N, 12.67. IR (KBr,
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1
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