Syntheses, Structural Aspects, Solution Behavior, and Catalytic Utility

Publication Date (Web): September 6, 2016 ... The reactions of six-membered cyclopalladated N,N′,N″-triarylguanidines [κ2(C,N)Pd(μ-X)]2 (3–7) ...
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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 (3,5dmpzH) in CH2Cl2 at RT for 24 h afforded a new class of cyclopalladated N,N′,N″-triarylguanidines, [κ 2 (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 1 H 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.

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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 The reactions of 1 and 2 with pyrazoles and substituted pyrazoles (PzH) in 1:1 and 2:1 (Pd:PzH) ratios in toluene © XXXX American Chemical Society

Scheme 1

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)] Received: June 3, 2016

A

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Organometallics Chart 1. Known Six-Membered Cyclopalladated N,N′,N″Triarylguanidines Bridged by Pyrazolato- and Acetatopyrazolato Ligands

respectively. The aforementioned transformations are illustrated in Scheme 2. 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 1

H 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 symmetrical 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 (SCXRD) and RT NMR (1H, 13C, and 19F) for all and VT 1H 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 sixmembered [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 sixmembered [κ2(C,N)Pd] ring adopts a pseudoboat conforma-

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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 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)

8

10

12

14

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. Cyclopalladated N,N′,N″-triarylguanidines, cis/trans[κ2(C,N)Pd(Lewis base)X], were shown to exist as either an

Figure 2. Molecular structures of 8 and 10 at the 30% probability level. Only relevant hydrogen atoms are shown for clarity.

C

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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 eightmembered [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-

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 Chart 3. Four Possible Conformers of 8−12 and 14a

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) 2.92(1) 3.30(2) 3.20(1)

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

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 onedimensional 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(κ1NPzH)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-(2pyridyl)pyrazole) does the eight-membered [Pt(κ1N-3,5dmpzH)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 usually forms a 10-membered ring, R22(10), as observed in [Pd(pzH) 4 ](OC(O)CH 2 NHC(O)Me) 2 and [Pd(3,5-

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.

a

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 αout 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 onedimensional chain-like structure mediated by a N−H···X hydrogen bond depending upon the substituents in the third 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 trifluoroacetato moiety forms an eight-membered [Pd(κ1N-PzH)2O] ring, R21(8), as illustrated in Figure 5. The voluminous C,N chelate in 10 and 12 reduces the NPzHPdNPzH bond angle and thus becomes more suitable for the formation of the R21(8) ring.

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). Solution Behavior. 1H NMR spectra of 8, 12, 13, and 14 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 presence of three species in about 1:0.57:0.25 ratio. 1H 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 measurements of 10 (CD2Cl2, 400 MHz, 10 K interval) were carried out as a representative example. The VT 1H 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 The 19F NMR spectrum of 10 measured at 298 K in CDCl3 revealed a singlet at δ −74.95, which contradicts the results of 1 H NMR studies discussed in the preceding paragraph (see also Experimental Section). Therefore, we measured 19F NMR

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 R22(10) and R12(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

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 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, which is also the likely two major species revealed in the 1H 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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Organometallics Scheme 4. Proposed Key Intermediates Formed during the Interconversion of β-out and β-in Conformers of 10

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). 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. 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

F 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 intermediates F (= I) and H (= K). The 1H NMR spectrum 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 by 1H and 13C NMR spectroscopy (see Chart 4).17 Further, these palladacycles revealed molar conductance values of 154.6 and 143.1, respectively, in MeCN, which clearly indicated a 1:1 19

Chart 4. Known Cationic Cyclopalladated N,N′,N″Triarylguanidines

G

DOI: 10.1021/acs.organomet.6b00442 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 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 more noncoordinating PF6− present in 14 than those anions 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 hydrogenbond-donating PzH in the latter. Plausible Mechanism. A plausible mechanism of formation 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. 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 4bromotoluene 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

Table 4. Screening of 8−14 and Pd(OAc)2 for Suzuki− Miyaura Coupling Reactions of PhB(OH)2 with 4Bromotoluenea

s. no.

catalyst

mol %

yield (%)b

TONc

1 2 3 4 5 6 7 8

8 9 10 11 12 13 14 Pd(OAc)2

1 1 1 1 1 1 1 1

100 90 99 0 67 0 99 83

100 90 99 0 67 0 99 83

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), DMF/H2O (2/2 mL), 20 h, and 100 °C. bThe product was purified by column chromatography on silica gel using an n-hexane/ethyl acetate (94−85/6−15, v/v %) mixture. cTurnover number (TON) = (mole of substrate) × (% yield of product)/(mole of complex).

Chart 5. Known Cyclopalladated N,N′,N″-Triarylguanidine Bridged by Pyrazolato Ligands

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-bromobenzonitrile, 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

Article

Organometallics

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 Aryl Bromides with PhB(OH)2 Catalyzed by Palladacycle 8a s. no.

Ar

mol %

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

4-MeC6H4 4-MeC6H4 4-MeC6H4 C6H5 C6H5 C6H5 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

1 0.1 0.01 1 0.1 0.01 0.01 1 0.1 0.01 0.10 0.01 1 0.01 1 1

100 90 98 98 97 98 79 100 0 0 86 94 93 84 44 53

TON 100 900 9800 98 970 9800 7900 100 0 0 860 9400 93 8400 44 53

(9943;c 9683d)

(9908d) (9336;c 9366d)

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EXPERIMENTAL SECTION

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} NMR (CDCl3, 100.5 MHz): δ 113.3, 115.3 (q, 1JCF = 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 (ArC/ArCH, and CN), 164.0 (q, 2JCF = 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 3,5dmpzH (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 (m); ν(CN) 1626 (vs). The 1H 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 3,5dmpzH located at δ 1.98, 2.10 (isomer 1) and δ 2.04, 2.26, and 2.41 (isomer 2). 1H 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-

(9803c)

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 an n-hexane/ethyl acetate (94−85/6−15, v/v %) mixture. cTON determined by 1H NMR spectroscopy. dTON determined by gas chromatography.

Chart 6. Cross-Coupling Products Discussed in Table 5

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 2bromoanisole. In these cases, even when the catalyst loading is high, the yields of the coupling products are low (Table 5, entries 15 and 16).

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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 [κ 2 (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. 1H, 13C{1H}, and 19 F NMR spectroscopic data of 8−14 revealed the presence of a I

DOI: 10.1021/acs.organomet.6b00442 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

3,5-dmpzH, isomers 1 and 3), 13.50, 13.94 (each br, 2 × 1H, NH, 3,5dmpzH, 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, 3,5dmpzH), 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(μ3,5-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 integrals of CH3 protons of the pyrazole moieties. 1H NMR (CDCl3, 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, 3,5dmpzH, 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), 117.3 (q, 1JCF = 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), 161.7 (q, OC(O)CF3, 2JCF = 34.2 Hz). The presence of isomer 2 was detectable only at δ 13.9. 19F NMR (376.2 MHz, CDCl3): δ −75.01.

dmpzH, isomer 2), 13.62 (s, 1H, NH, 3,5-dmpzH, isomer 1). The 13 C{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. 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) 1632 (vs). 1H 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. 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). The 1H 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, J

DOI: 10.1021/acs.organomet.6b00442 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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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. 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, 1 JCF = 218.1 Hz, CF3), 129.0, 129.6, 130.4, 131.0, 136.4, 136.6, 137.4, 141.5, 142.9, 145.4, 146.4, 147.2, 148.2, 162.1 (app q, 2JCF = 33.5 Hz, 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. 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, cm−1): ν(NH) 3420 (m), 3390 (m); ν(C = N) 1630 (vs); ν(PF6−) 840 (s), 556 (m). The 1H NMR spectrum of 14 indicated the presence of two isomers in about 1:0.54 ratio as estimated from the integrals of NH protons of the 3,5-dmpzH moiety. 1H NMR (CDCl3, 400 MHz): δ 1.71 (s, 2 × 3H, CH3, isomers 1 and 2), 2.00 (br, 4 × 3H, CH3, isomers 1 and 2), 2.13, 2.24 (each s, 4 × 3H, CH3, isomer 1), 2.35 (s, 2 × 3H, CH3, isomer 2), 2.50 (br, 2 × 3H, CH3, isomer 2), 5.50 (br, 2 × 1H, NH, isomer 1), 5.78 (s, 2 × 1H, CH, 3,5-dmpzH, isomer 1), 6.03 (br, 1H, NH, isomer 2), 6.04 (s, 2 × 1H, CH, 3,5-dmpz, isomer 2), 6.52−6.54 (m, 7H, isomer 1 (3H, ArH)), isomer 2 (3H (ArH) and 1H (NH)), 6.75 (d, JHH = 7.3 Hz, 2 × 1H, ArH, isomers 1 and 2), 7.06 (t, JHH = 7.3 Hz, 3H, ArH, isomer 1), 7.16−7.18 (m, 2 × 1H, ArH, isomers 1 and 2), 7.22−7.24 (m, 2 × 1H, ArH, isomers 1 and 2), 7.28−7.31 (m, 7H, ArH, isomer 1 (2H), isomer 2 (5H)), 9.78 (br, 1H, NH, 3,5-dmpzH, isomer 1), 9.92, 10.73 (each br, 2 × 1H, NH, 3,5dmpzH, isomer 2), 10.92 (br, 1H, NH, 3,5-dmpzH, isomer 1). 19F NMR (376.2 MHz, CDCl3): δ −71.40 (d, 1JPF = 715.0 Hz). 31P{1H} NMR (161.8 MHz, CDCl3): δ −143.2 (septet, 1JPF = 714.5 Hz). TOFMS ES+ m/z (%) [ion]: 965 (50) [{κ2(C,N)Pd}2(μ-3,5-dmpz)]+, 516 (15) [LH32‑tolyl+PF6− + K]+, 475 (60) [LH3+PF6−], 330 (100) [LH32‑tolyl]+. ΛM (Ω−1 cm2 mol−1) = 77.0 (10−3 M) in MeCN.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00442. 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 8, 9, and 12, 1H 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)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (N. Thirupathi): [email protected], thirupathi_ [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.A. acknowledges Council of Scientific and Industrial Research, New Delhi, for a fellowship. The authors acknowledge the (i) Department of Science and Technology, New Delhi, for a research grant, (ii) University Science Instrumentation Center, University of Delhi, for infrastructure facilities, and (iii) NMR Research Center, Indian Institute of Science, Bengaluru, for VT 1H NMR and VT 19F NMR experiments. We acknowledge Dr. Surendra Singh, Department of Chemistry, University of Delhi, for assisting us in gas chromatographic studies.

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00442 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.6b00442 Organometallics XXXX, XXX, XXX−XXX