Cationic NCN Palladium(II) - ACS Publications - American Chemical

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Cationic NCN Palladium(II) Pincer Complexes of 5-tert-Butyl-1,3bis(N‑substituted benzimidazol-2′-yl)benzenes: Synthesis, Structure, and Pd···Pd Metallophilic Interaction Varsha Rani,† Harkesh B. Singh,*,† and Ray J. Butcher‡ †

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India Department of Chemistry, Howard University, 525 College Street NW, Washington, D.C. 20059, United States

‡

S Supporting Information *

ABSTRACT: The NCN palladium(II) pincer complex [Benzoyl(N∧C∧N)PdBr] (16) was synthesized by the oxidative addition of Benzoyl(N∧C∧N)Br to Pd(dba)2 in 85% yield [(N∧C∧N) = 5-tert-butyl-1,3-bis(N-substituted benzimidazol-2′yl)phenyl)]. Then treatment of complex 16 with KI yielded the iodopalladium complex [Benzoyl(N∧C∧N)PdI] (17) in 92% yield. Furthermore, a series of cationic palladium(II) complexes, including [ B e n z o y l (N∧C∧N)Pd(MeCN)] + [BF 4 ] − (18), [Benzoyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (19), and [Benzoyl(N∧C∧N)Pd(OTf)] (20), were prepared in 68−79% yields by the reaction of the neutral palladium(II) complex (16) with AgBF4, AgSbF6, and AgOTf, respectively. Similarly, previously synthesized Tosyl(N∧C∧N)PdBr [5-tert-butyl-1,3-bis(N-tosylbenzimidazol-2′-yl)phenyl]palladium bromide (5b) was treated with AgSO3CF3 and AgSbF6 to afford cationic palladium(II) complexes [Tosyl(N∧C∧N)Pd(OTf)] (21) and [Tosyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (22) in 41 and 61% yields, respectively. 5-tert-Butyl-1,3-bis[{(N-tosylbenzimidazol-2′-yl)phenyl}palladium(II)] triflate (21) exhibited an unsupported metallophillic Pd···Pd interaction [3.166(8) Å] that is corroborated by X-ray crystallographic studies. Compared to other cationic palladium complexes, complex 21 was found to be less stable. In Atoms in Molecule (AIM) analysis, the bond critical point (ρ) between Pd and Pd atoms is 0.000865 au, supporting the presence of metallophillic interaction in complex 21. The bond strength of the Pd···Pd bond was also measured by density functional theory calculations that indicated that the calculated bond order was approximately one-fourth of the normal covalent Pd−Pd bond (natural atomic orbital bond order method). All eight complexes, two neutral and six cationic, were characterized by common spectroscopic techniques, and six complexes were corroborated by X-ray diffraction studies.

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1)], cationic gold(III) [3 (Chart 1)],5 ruthenium(II) [4 (Chart 1)],6 and iridium(III)7 complexes with bis-benzimidazole ligands have been extensively studied. These complexes show interesting photoluminescence properties. Very recently, our group has synthesized the first examples of neutral mononuclear square planar palladium(II) complexes [5 (Chart 1)]8 that were used for C−H functionalization of benzyl nitrile with N-tosylaldimines to form β-aminonitriles. Notably, complexes with different N substitutions other than tosyl were poorly soluble in common organic solvents. Therefore, to overcome the problem of solubility, we envisaged certain modifications of the ligand by replacing the tosyl group with a more polar “benzoyl” moiety on the nitrogen atom. The resonating structures of ligand 6 are depicted in Chart 2 (6a and 6b), clearly indicating the more polar character of Benzoyl (N∧C∧N)Br [(N∧C∧N) = N-substituted 5-tert-butyl1,3-bis(benzimidazol-2′-yl)-2-bromobenzene] compared to that

INTRODUCTION The pincer complexes of palladium have attracted a significant amount of attention from organometallic chemists because of their applications in organic transformations. These include C− C cross coupling reactions, Michael additions, aldol condensation reactions, and allylation of aldehydes and imines.1 The unique design, high thermal stability, and availability of only one coordination site make pincer complexes ideal catalysts. In recent years, NCN palladium(II) pincer complexes have been extensively studied. The most common types of NCN pincer ligands used to support Pd(II) complexes are bisamines (NR2), bis-oxazolines (Phebox), and bis-imidazolines (Phebim). Among several designs of the NCN pincer ligand system, bis-benzimidazole ligand scaffolds have not been frequently investigated with respect to the formation of mononuclear palladium(II) complexes because of the formation of poorly soluble palladacycles.2 Although a trinuclear palladacycle [1 (Chart 1)] with a hydrophobic cavity with bisbenzimidazole ligand scaffolds has been reported,3 other neutral and cationic pincer complexes such as platinum(II)4 [2 (Chart © XXXX American Chemical Society

Received: August 14, 2017

A

DOI: 10.1021/acs.organomet.7b00620 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Examples of Metal Complexes with Bis-Benzimidazole Ligands

Chart 2. Resonance Structures of 5-tert-Butyl-1,3-bis(N-benzoylbenzimidazol-2′-yl)-2-bromobenzene

[{(2,6-(Me2NCH2)2C6H3)Pd}2(μ-Cl)]+[X]−, where X− = [BF4]−, [B{C6H4(SiMe3)-4}4]−, or [B{C6H4(SiMe2CH2CH2C6F13)-4}4]− [8 (Chart 3)].10 The same group has reported cationic palladium(II) complexes with diolefin-substituted pyridines at the ancillary sites [9 (Chart 3)], and their utility has been investigated for olefin metathesis.11 Similar para-functionalized cationic palladium(II) complexes, para-substituted [{2,6-(Me 2NCH2) 2C6H2}Pd(OH2)]+[BF4]−, have been used for Michael reaction between methyl vinyl ketone and ethyl α-cyanoacetate.12 A unique type of chiral cationic NCN palladium pincer complex with κ5N,C,N,O,O coordination around the palladium center is reported [10 (Chart 3)] with RNRNSCSC diastereospecificity containing diphenylhydroxymethyl pyrrolidinyl moieties as chiral auxiliaries, and these complexes were further used as catalysts for the aldol condensation reaction between α-methyl isocyanoacetate and aromatic aldehydes to yield cis-oxazolines with 42% ee.13 Arai et al. have followed a new methodology, the ligand introduction route, and demonstrated the synthesis of chiral cationic palladium(II) complex [(NCN)Pd(OTf)] [11 (Chart 3)], which catalyzes the asymmetric conjugate addition of malononitriles with nitroalkenes to form the addition product with ≤93% enantioselectivity.14 Liu et al. have reported the cationic dinuclear palladium(II) pincer complex, [2,4(Me2NCH2)2C6H2-1-{Pd(H2O)(Py)}-5-{Pd(OTf)(Py)}]+[OTf]− [12 (Chart 3)], which has been used for the methanolysis of the pesticide fenitrothion.15 Another example of cationic palladium(II) NCN pincer complex (S,S)-[2,6-

of the [Tosyl(N∧C∧N)Br] derivative. Despite the more polar character of the ligand, Benzoyl(N∧C∧N)Br is much softer with a chemical hardness (η) of 1.934 eV compared to the Tosyl (N∧C∧N)Br derivative (η = 2.321 eV) (vide inf ra). The reaction of Benzoyl(N∧C∧N)Br with Pd(dba)2 led to the formation of the first fairly soluble neutral palladium(II) complex that is stable in the solution state in contrast to complex 5b.8 The chemical reactivity of [Benzoyl(N∧C∧N)PdBr] (η = 1.459 eV) is much higher than that previously reported for the [Tosyl(N∧C∧N)PdBr] (η = 1.639 eV) palladium(II) complex (vide inf ra). Therefore, we decided to investigate the chemical reactivity of this precursor, [Benzoyl(N∧C∧N)PdBr], toward silver salts to synthesize cationic palladium(II) pincer complexes with a bis-benzimidazole scaffold. It is worth noting that compared to studies of the ubiquitous neutral palladium pincer complexes, the investigation of cationic palladium(II) complexes is limited. van Koten and co-workers have reported the NCN palladium(II) complex [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Pd(H2O)]+[BF4]− [7 (Chart 3)] that has been generated in situ for the preparation of stable palladium(II) phenoxide complexes, i.e., [{2,6-(Me2NCH2)2C6H3}PdOPh], [{2,6(Me 2 NCH 2 ) 2 C 6 H 3 }Pd(OPh)·(HOPh)], and [{2,6(Me2NCH2)2C6H3}Pd(OC6H4OH)], to study the effect of hydrogen bonding in metal phenoxide.9 However, the molecular structure for the complex was reported 12 years after the first report by the same group along with new cationic complexes, dinuclear chloride bridging palladium(II) complexes B


Article

Organometallics Chart 3. Examples of Cationic Palladium(II) Pincer Complexes

Scheme 1. Syntheses of Palladium(II) Complexes of 5-tert-Butyl-1,3-bis(N-benzoylbenzimidazol-2′-yl)-2-bromobenzene (16− 23)a

a

Conditions: (i) Pd(dba)2, toluene, room temperature (rt); (ii) KI, DCM/MeOH, rt; (iii) AgSO3CF3, DCM/CH3CN, rt; (iv) AgSO3CF3, tetrahydrofuran, rt; (v) AgBF4, DCM/CH3CN, rt; (vi) AgSbF6, DCM/CH3CN, rt; (vii) AgSbF6, tetrahydrofuran. bFrom an earlier work.

alkylation.16 Takenaka and Uozumi have reported the novel chiral NCN palladium(II) pincer complexes [14 (Chart 3)] that catalyze the asymmetric Michael addition of isopropyl 2-

bis(4-isopropyl-1,4-dimethyl-4,5-dihydro-1H-imidazol-5-on-2yl)phenyl]palladium(II) tetrafluoroborate-aqua [13 (Chart 3)] has been reported and has been used for Friedel−Craft C


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Organometallics

the aromatic region from δ 9.34 ppm (C−H···Br−Pd) in the upfield direction and additional resonance signals at δ 2.66 and 2.07 ppm in the aliphatic region corresponding to methyl protons of the coordinating solvent molecule (CH3CN) in complexes 18 and 19, respectively. The 11B NMR spectrum of complex 18 showed one signal at δ −0.5 ppm. The signal in the 19 F NMR spectrum for complex 18 appeared at δ −152.5 ppm. The 19F NMR spectrum for complex 19 shows several resonance signals with 14 lines from δ −97.8 to −142.5 ppm due to the presence of the SbF6 counteranion. This can be explained in terms of coupling between 19F and 123Sb and the 121 Sb nucleide with nuclear spin 5/2Sb and 7/2Sb, respectively. The 19F NMR spectrum exhibited a single resonance signal at δ −77.3 ppm for the triflate anion in complex 20. In mass spectra (positive ion mode), peaks at m/z 679.1331, 679.1379, and 679.1397 corresponding to [C38H29N4Pd]+ confirm the presence of the same [R-Pd]+ core cation in complexes 18− 20. Additionally, in negative ion mode, the mass spectra peaks at m/z 235.1791 and 149.1074 confirm the presence of counteranions [SbF6]− and [CF3SO3]− in complexes 19 and 20, respectively. Complexes (18 and 19) exhibited an absorption band at 2291 and 2286 cm−1 for ν(CN) (Table 1). The structures of cationic palladium(II) complexes 18−20 were corroborated by X-ray diffraction studies (vide inf ra). The methodology for the formation of cationic palladium(II) complexes was further extended to previously reported complex 5b.8 Similarly, the reaction of complex 5b with silver salts (AgSO3CF3 and AgSbF6) in a tetrahydrofuran (THF)/CH3CN solvent at room temperature afforded complexes 21 and 22. Complex 21 shows novel unsupported intermolecular Pd··· Pd metallophilic interaction, which in contrast to complex 5b in which no Pd···Pd interaction was observed (vide inf ra). Interestingly, the crystals of both 5b and 21 were yellow in color. The ultraviolet−visible (UV−vis) spectra for both complexes have been recorded and compared. The spectra are quite similar, and no additional peaks in the spectra of 21 were observed (Figures S53 and S54). It was noticed that complexes 21 and 22 were not stable for a longer period of time in the solution state or in the solid state. However, the freshly prepared sample was stable enough to record the data immediately. The light yellow complex 21 turned blackish under ambient conditions after 1−2 days, and elemental analysis of complex 21 could not be performed after multiple attempts. The synthesis of the corresponding palladium cations was confirmed by spectroscopic evidence. The 1H NMR spectrum for complex 21 consists of five resonance signals in the aromatic region, one singlet at δ 8.58 ppm (two protons of the central aromatic ring), three multiplets at δ 8.19, 7.47, and 7.27 ppm, and one doublet at δ 7.73 ppm with a 3J coupling constant of 8.5 Hz, and two resonance signals in the aliphatic region, one singlet at δ 2.37 ppm for methyl protons and δ 1.45 ppm for the tert-butyl group. The 1H NMR spectrum of complex 22 is quite similar to that of complex 21. The 13C NMR spectrum of complex 22 could not be recorded because of the transformation of complex 22 to deprotected palladacycle 23 in the solution state in dimethyl sulfoxide-d6 (DMSOd6). This observation is similar to our earlier report of neutral palladium complex 5b8 and has been confirmed by NMR spectroscopy and mass spectrometry. In the 1H NMR spectrum, a new signal at δ 14.16 ppm for the protic N−H proton and a peak at m/z 471.0804 corresponding to C24H21N4Pd [M − Ts − Br]+ in the mass spectrum confirm the formation of protic palladacycle 23. However, complex 23

cyanopropionate to ethyl vinyl ketone to yield isopropyl 2cyano-2-methyl-5-oxoheptanoate with a high enantioselectivity of ≤83%.17 Soro et al. have reported dinuclear cationic palladium(II) pincer complexes with a 1,3-bis(2-pyridyl)benzene-based ligand; [Pd2(N∧C∧N)2(μ-OAc)]2+[Hg2Cl6]− is characterized by molecular structure, whereas mononuclear [Pd(N∧C∧N)(OH2)]+[BF4]− and dinuclear [Pd2(N∧C∧N)2(μ-X)]+[BAr′4]− [15 (Chart 3)] were characterized in the solution state.18 A Cambridge Crystallographic Database (version 1.19) survey and literature overview reveal that only nine solid state structures have been reported with NCN cationic palladium(II) pincer complexes. Moreover, the basic scaffold of the ligand is almost similar, i.e., a [2,6-bis(dimethylaminomethyl)phenyl]palladium backbone with some modifications. However, the syntheses of cationic cyclopalladated derivatives with a bis-benzimidazole-based ligand have not been explored. Herein, we report the first examples of cationic palladium(II) complexes with a bis-benzimidazolebased ligand that show interesting intermolecular unsupported metallophilic Pd···Pd (d8−d8) interaction (vide inf ra).

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RESULTS AND DISCUSSION 5-tert-Butyl-1,3-bis(N-benzoylbenzimidazol-2′-yl)-2-bromobenzene (6) was synthesized by the reaction of 5-tert-butyl-1,3bis(benzimidazol-2′-yl)-2-bromobenzene19 with benzoyl chloride in the presence of KOH. Compound 6 was reacted with Pd(dba)2 to give a yellowish gray palladium(II) complex [Benzoyl(N∧C∧N)PdBr] (16) via oxidative addition (Scheme 1). The reaction of complex 16 with an excess of KI in a CH3OH/ DCM solvent [1/3 (v/v)] at room temperature led to the formation of a yellow iodopalladium(II) complex, [Benzoyl(N∧C∧N)PdI] (17), via the nucleophilic substitution pathway. Both neutral palladacycles (16 and 17) are air stable and fairly soluble in chlorinated solvents like DCM and CHCl3 in contrast to complex 5.8 In the 1H NMR spectrum, the signal at δ 9.34 ppm (C−H···Br−Pd, complex 16) is shifted to δ 9.55 ppm (C−H···I−Pd, complex 17), which confirms the substitution of the bromide anion with iodide. The IR spectra for complexes 16 and 17 exhibited minor shifts in the absorption band for ν(CN) (Table 1). The structures of both complexes were corroborated by X-ray diffraction studies (vide inf ra). The synthesis of the cationic palladium(II) complexes (18− 20) was performed in one step by the reaction of complex 16 with silver salts AgBF4, AgSO3CF3, and AgSbF6 in a CH2Cl2/ CH3CN solvent [2/1 (v/v)]. The 1H NMR spectra for complexes 18−20 are very much similar to those of neutral palladium complex 16, except for the shifting of one signal in Table 1. Absorption Bands for ν(CO), ν(CN), and ν(CN) for Compounds 6 and 16−22 compound

ν(CO) (cm−1)

ν(CN) (cm−1)

ν(CN) (cm−1)

6 16 17 18 19 20 21 22

1720 1717 1727, 1717 1716 1716 1715 − −

1598 1597 1596 1596 1597 1596 1550 1551

− − − 2291 2286 − − 2271 D


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Figure 1. Molecular structures of palladium(II) complexes 16 (left) and 17 (right). Hydrogen atoms and the solvent molecule (THF) in the case of 16 have been omitted for the sake of clarity.

Table 2. Selected Bond Lengths (angstroms) and Bond Angles (degrees) for Pd(II) Complexes 16−21 Pd−C Pd−N1 Pd−N2 Pd−X C−Pd−N1 C−Pd−N2 N1−Pd−X N2−Pd−X C−Pd−X N1−Pd−N2 a

16

17

18

19

20

21

1.937(3) 2.052(2) 2.068(2) 2.534(4) 79.07(11) 79.75(11) 100.81(6) 100.32(7) 177.57(8) 158.82(9)

1.921(5) 2.055(5) 2.058(5) 2.685(5) 79.2(2) 79.1(2) 100.65(13) 100.93(13) 172.87(16) 158.29(18)

1.933(5) 2.050(5) 2.052(5) 2.112(5) 79.4(2) 79.5(2) 100.40(18) 100.73(18) 178.38(19) 158.74(18)

1.931(3) 2.050(6) 2.060(2) 2.126(3) 78.99(11) 79.83(11) 101.21(10) 99.97(10) 179.80(12) 158.82(10)

1.914(15) 2.050(13) 2.068(13) 2.173(13) 79.58(6) 79.61(6) 96.71(5) 104.11(5) 176.09(6) 159.19(5)

1.914(5) 2.027(4) 2.026(4) 2.175(3)a 79.80(19) 80.07(19) 99.9(7)a 100.1(7) 175.4(4) 159.86(16)

Average bond parameters for complex 21.

Figure 2. Molecular structures of complexes 18 (left) and 19 (right). Hydrogen atoms and the solvent molecule (THF) in 18 have been omitted for the sake of clarity.

the slow evaporation of a CDCl3 solution at room temperature. The coordination of palladium atom with the carbon atom of the central aryl ring, two nitrogen atoms of both benzimidazolyl units, and the Br or I atom at the ancillary site manifested the formation of a coplanar heptacyclic robust framework. The palladium center exhibits typical distorted square planar geometry with trans N−Pd−N angles of 158.29(18)° and 158.82(9)° and trans C−Pd−X (Br or I) angles of 177.57(8)° and 172.87(16)° for complexes 16 and 17, respectively.

was not isolated in the solid form. All the data for complex 23 were reported “as present in the solution form”.

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CRYSTAL STRUCTURES Molecular Structures of Complexes 16 and 17. The molecular structures of complexes 16 and 17 are depicted in Figure 1. The prism-shaped yellow single crystals of complex 16 were obtained via the vapor diffusion method from THF and diethyl ether, and for complex 17, the crystals were obtained by E


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Organometallics Complex 17 has shown a significant deviation of its trans C− Pd−I angle from that reported for the NCN pincer complex [2,6-{2-XylNC(H)C6H4}2C6H3PdI]20 [179.24(5)°] and unsymmetrical PCN pincer complex [(P t‑Bu CN Me )PdI] 21 [175.64(13)°]. Selected bond parameters for complexes 16 and 17 are listed in Table 2. The C−Pd bond for complex 16 [1.937(3) Å] is slightly longer than that of complex 17 [1.921(5) Å], similar to that of the neutral NCN palladium complex [Tosyl(NCN)PdBr)] (5b) [1.928(7) Å],8 and shorter than that of symmetrical PCP palladium complex [2,6(R2PCH2)2C6H3PdCl] [2.028(3) Å] (R = C6F5).22 The Pd− Br bond in [Benzoyl(NCN)PdBr] (16) [2.534(4) Å] is shorter than the Pd−I bond [2.685(5) Å] in complex 17 [Benzoyl(NCN)PdI] as expected and comparable to the Pd−I bond in the iodopalladium complex [2,6-{2-XylNC(H)C6H4}2C6H3PdI] [2.709(2) Å]20 and that in an unsymmetrical pincer complex [(Pt‑BuCNMe)PdI] [2.693(5) Å].21 It has been observed that two benzoyl fragments are syn to each other with respect to the central aromatic ring in complex 16 with an O1− C18−C18A−O1A dihedral angle of 1.720° (Figure 1). Molecular Structures of Cationic Palladium Complexes 18−20. The molecular structures of cationic palladium complexes 18 and 19 are depicted in Figure 2. The needleshaped colorless crystals of complex 18 and block-shaped yellow crystals of complex 19 were grown from THF and diethyl ether by the vapor diffusion method at room temperature. The yellow crystals of complex 20 were grown from THF by slow evaporation at room temperature (Figure 3). Complexes 18 and 19 crystallize in space group P21/n in the

palladium(II) phenoxide complexes [2,6-(Me2NCH2)2C6H3)PdOPh]9 [1.910(2) Å] and [2,6-(Me2NCH2)2(C6H3)Pd(OPh)·(HOPh)]9 [1.901(5) Å] and the dinuclear cationic palladium(II) pincer complex with a 1,3-bis(2-pyridyl)benzenebased ligand, [Pd2(NCN)2(μ-OAc)]2+[Hg2Cl6]− [1.908(5) Å].18 The Pd−C bond is comparable with those of chiral cationic NCN palladium(II) complexes 10a [1.916(4) Å] and 10b [1.918(3) Å] and longer than that of dinuclear cationic NCN palladium(II) complex 12 [2,4-(Me2NCH2)2C6H2-1{Pd(H2O)(Py)}-5-{Pd(OTf)(Py)}]+[OTf]− [1.942(8) Å].15 It has also been noticed that the Pd−C bonds in NCN pincer palladium(II) complexes are shorter than those of the unsymmetrical pyrazole-based PCN pincer complexes [(PCNMe)Pd(OTf)] [1.958(18) Å] and [(PCNH)Pd(OTf)] [1.959(2) Å].21 The average Pd···N bond length for complexes 18−20 is 2.050 Å, which is much shorter than the sum of van der Waals radii of palladium and nitrogen, 3.18 Å,23 and close to the sum of covalent radii, 2.06 Å,24 which led to the conclusion that the Pd−N bond can be considered as a single bond. The Pd−N (ancillary site) bond lengths for complexes 18 and 19 are 2.112(5) and 2.126(3) Å, respectively, values greater than that of cationic PCP palladium pincer complex [2,6-(R2PCH2)C6H3Pd(CH3CN)]+[BF4]− [2.081(4) Å].22 Similarly, the Pd−O (triflate) bond length in complex 20 is 2.173(13) Å, which is comparable with the Pd−O bond distance in unsymmetrical PCN palladium pincer complex [(PCNH)Pd(OTf)] [2.175(17) Å].21 The trans C−Pd−O (176.09°) angle in complex 20 is slightly different from the trans C−Pd−N angle [178.38(19)°] of complex 18 and an ideal angle of 180°. The crystal packing diagram of complexes 18−21 suggested that mononuclear units are arranged in a head-to-tail fashion and assembled with each other by π−π stacking interactions (Figures S57−S60). The centroid−centroid distance of complex 19 (3.504 Å) is much shorter than those of complexes 18 (∼6.718 Å) and 20 (∼7.300 Å). It is observed that the solvent molecule (THF) is sandwiched between two layers of benzimidazole rings in complex 20 (Figure S59). There is no Pd···Pd interaction found in complexes 18 (Pd···Pd, 7.932 Å) and 20 (Pd···Pd, 7.826 Å). However, in the case of complex 19, the Pd···Pd distance is 4.713 Å. Molecular Structure of Complex 21. The prismaticshaped yellow crystals of complex 21 were obtained from THF and diethyl ether through the vapor diffusion method (Figure 4). Although initial attempts to crystallize complex 21 were unsuccessful, after crystals were grown, they decomposed as the complex is not stable in a solution state for 1−2 days. Fortuitously, when the crystals were not harvested from the mother liquor, some suitable crystals could be manually picked up and mounted quickly at a lower temperature (150 K) for Xray diffraction studies. The crystals were stable enough to collect the data at a low temperature. The geometry around the palladium center is distorted square pyramidal (Figure 5). The palladium center is coordinated with the C atom of the central aromatic ring, two nitrogen atoms of benzimidazole rings, and the O atom of the triflate anion, and the fifth coordination site is coordinated with the Pd atom of another mononuclear unit (Figure 5). Selected bond parameters for complex 21 are listed in Table 2. Palladium is more strongly coordinated with the N atoms than in complexes 16−20, which is evident from the shorter Pd···N bond in complex 21 [2.027(4) Å] compared to those of complexes 16−20 [2.052(2)−2.068(13) Å].

Figure 3. Molecular structure of complex 20. Hydrogen atoms and the solvent molecule (THF) have been omitted for the sake of clarity.

monoclinic crystal system, and complex 20 crystallizes in space group P1̅ in the triclinic crystal system. The palladium center has a typical square planar geometry similar to that of neutral palladium complexes (16 and 17). It is coordinated with the C atom of the central aromatic ring and two N atoms of each benzimidazolyl unit, and the fourth coordination site is occupied by the N atom of the solvent molecule (CH3CN) in complexes 18 and 19 and the O atom of the triflate anion occupying the ancillary site in complexes 20 to generate a heptacyclic robust framework. The Pd−C bond lengths of complexes 18 and 19 are 1.933(5) and 1.931(3) Å, respectively, and are shorter than that of complex 20 [1.914(15) Å]. However, the Pd−C bond is shorter than those of neutral F


Organometallics

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DENSITY FUNCTIONAL THEORY (DFT) CALCULATIONS DFT calculations were performed on all the synthesized compounds (5b and 16−21) to develop an in-depth understanding of the structure and bonding of the palladium pincer complexes (Figure 6).27 The bond parameters are in good

Figure 4. Molecular structure of complex 21. Hydrogen atoms have been omitted for the sake of clarity.

Figure 6. Frontier molecular orbitals of complexes 5b (left) and 16 (right) and their HOMO−LUMO energy gaps.

agreement between optimized structure and those obtained from X-ray crystallographic data, thereby confirming the accuracy of the optimized geometries (Table S2). As discussed above, and by following the literature,28 the chemical reactivity of complexes 5b and 16 is compared on the basis of chemical hardness (η) as postulated by Parr and Pearson (Table 3). They have mentioned the relation between chemical hardness (η) and the energy gap (ΔE) between the frontier molecular orbitals [η = (ELUMO − EHOMO)/2].

Figure 5. Dimeric form of complex 21. Hydrogen atoms have been omitted for the sake of clarity.

Table 3. Chemical Hardness (η) Calculated for Complexes 5b and 16 Metallophilic Pd···Pd Interaction in Complex 21. The crystal packing diagram suggested that the molecules are in a parallel arrangement with each other in a head-to-tail fashion. A strong metallophilic Pd···Pd interaction, a π−π stacking interaction (3.532 Å), and hydrogen bonding forces help to adhere the mononuclear units together to form a supramolecular network. The existence of (d8−d8) metallophilic interactions (Pd···Pd) is in contrast to the reported homologues cationic Pt(II) complexes (2)4f and cationic Au(III) complex5 (3), in which no PtII···PtII or AuIII···AuIII metallophilic interactions were evident in crystal structures. The PdII···PdII center distance is 3.166(8) Å, which is comparable with those of the reported Pd···Pd metallophilic interactions in monomeric ferrocene bis-imidazoline bispalladacycle (FBIP-HOAcOTf) [3.160(1) Å]25 and the Pd··· Pd bond distance of 3.069(1) Å reported in the literature for dinuclear halide-bridged palladium(II) complex [Pd2(NCN)2(μ-OAc)][BAr′4] [Ar′ = 3,5-(CF3)2-C6H3]18 and shorter than the Pd···Pd contacts in Pd(II) complex [Pd(CNC6H4-p-F)2Cl2] [3.366(9) Å].26 The Pd···Pd interaction distance [3.166(8) Å] is shorter than the sum of van der Waals radii for Pd−Pd bonds (3.26 Å)23 and longer than the sum of the covalent radii (2.62 Å).24

chemical hardness (η)

5b

16

1.639

1.459

The natural bond orbital (NBO) analysis as summarized confirms that natural charge on the Pd atom is +0.454 to +0.508 in the cationic palladium(II) complexes (18−21); however, the charge on the Pd atom in neutral complexes (16 and 17) is in the range of +0.293 to +0.330 (Table S3). It has been observed that the charge on C, N, Br, I, and O is negative. The NCN pincer was considered as the three interacting partners in molecular orbital composition analysis. The contribution is listed as coming from palladium, ancillary ligand X (X = Br for 16; X = I for 17; X = CH3CN for 18 and 19; X = CF3SO3− for 20 and 21), and the rest of the ligand is listed as NCN. The plots of frontier molecular orbitals for complexes 16−21 reveal that the benzimidazole fragment (99%) makes the major contribution to the LUMO in neutral palladium(II) complexes (16 and 17) as well as in cationic palladium(II) complexes (18−20) (Tables S5−S9). However, for neural palladium(II) complexes, the HOMO is primarily localized on halogen atoms (74% from the pz orbital of bromine for 16 and 84% from the pz orbital of iodine for 17), and for cationic palladium(II) complexes, NCN (82−90%) G


Article

Organometallics

Figure 7. Characteristic MOs showing bonding and antibonding character in 21.

makes the primary contribution to the HOMO with the mere involvement of Pd (dyz orbital, 10−17%). The analysis of MOs for complex 21 in the dimeric form supported the intermolecular Pd···Pd interactions. The overlapping of dz2 orbitals on the palladium centers in HOMO-19 illustrated the strong bonding interaction, whereas an antibonding interaction was observed in HOMO-4 (Figure 7). These observations are similar to those observed for the monomeric ferrocene bis-imidazoline bis-palladacycle (FBIPHOAcOTf).25 However, in contrast to the bimetallic catalyst system in which intramolecular Pd···Pd supported interaction existed due to the ferrocene bis-imidazoline backbone, herein complex 21 exhibits unsupported intermolecular Pd···Pd interactions. The plots of HOMO-19 and HOMO-4 molecular orbitals for the dimeric form of 21 reveal that the palladium atoms (60%) make the major contribution to HOMO-19 with the moderate involvement of benzimidazole fragments (23%) and triflate anions (17%), whereas in the case of HOMO-4, the contributions of Pd atoms (47%) and benzimidazole fragments (51%) are almost equal, with an only 2% contribution from triflate anions. Atoms in Molecule Analysis. The metallophillic interaction of the Pd···Pd bond in complex 21 was further investigated by Atoms in Molecules (AIM) analysis29 of the geometries obtained from the crystal structure by using AIM 2000.30 The positive value of the bond critical point between Pd and Pd atoms (0.000865 au) signifies the presence of metallophilic interaction in complex 21 (Figure 8 and Table 4).

Figure 8. AIM picture of complex 21 showing the bond critical point between Pd and Pd atoms. To measure the strength of this metallophilic Pd···Pd interaction by DFT calculation, the bond order of the Pd···Pd bond was calculated by three methods; Wiberg bond indices, NAO (natural atomic orbital) bond order, and MO (molecular orbital) bond order. The calculated bond orders (Wiberg indices, 0.1051; NAO, 0.2469; MO, 0.3399) are listed in Table 4. The positive value of the bond order determined by NAO and MO methods indicates the existence of a net bonding interaction in the Pd···Pd bond. In the case of the NAO method, the Pd···Pd bond strength is found to be approximately one-fourth of that of a normal covalent bond, and by the MO bond order method, the bond strength is found to be one-third of that of a normal covalent bond.

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CONCLUSIONS A new series of neutral and cationic palladium(II) complexes (18−22) has been developed. Interestingly, complex 21 shows unsupported intermolecular metallophillic Pd···Pd interactions [3.166(8) Å], which is corroborated by X-ray crystallographic studies, and the distance is close to that of dinuclear halidebridged palladium(II) complex [Pd2(NCN)2(μ-OAc)][BAr′4] [Ar′ = 3,5-(CF3)2-C6H3]18 [3.069(1) Å]. DFT calculations predict that the metallophilic interaction arises by the strong overlap of dz2 orbitals of the palladium atoms. The bond critical point (ρ) between the Pd and Pd atoms (0.000865 au) indicates the presence of a metallophilic interaction in complex 21. The bond order was measured, which indicates that the

calculated bond order of the Pd···Pd interaction is 0.1051 (by the Wiberg index method), 0.2469 (NAO method), and 0.3399 (MO bond order method). The replacement of the tosyl group (complex 5b) with a more polar benzoyl group (complex 16) proved to be a successful strategy for tuning the solubility.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations were performed under a N2 atmosphere using standard Schlenk techniques. Solvents H


Article

Organometallics Table 4. Calculated Bond Orders of Pd···Pd Interactions in Complex 21

crystals for X-ray diffraction analysis were grown from slow diffusion of diethyl ether into a THF solution: yield 86% (0.398 g); 1H NMR (400 MHz, CDCl3) δ 9.34 (d, J = 8.0 Hz, 2H), 7.95 (d, J = 7.6 Hz, 4H), 7.77 (t, J = 7.6 Hz, 2H), 7.58 (t, J = 7.6 Hz, 4H), 7.49 (t, J = 7.6 Hz, 2H), 7.31−7.27 (m, 2H), 7.16 (m, 2H), 7.05 (s, 2H), 0.98 (s, 9H); 13 C NMR (100 MHz, CDCl3) δ 167.2, 158.3, 147.6, 140.9, 135.9, 132.9, 132.6, 131.7, 131.2, 129.7, 125.9, 125.3, 123.9, 121.6, 112.3 (aromatic carbon), 34.8, 30.9; HR-MS m/z calcd for C38H29N4O2Pd 679.1334, found 679.1326 [M − Br]+; FT-IR (KBr) 3070(w), 2955(m), 1717(s, CO), 1597(m, CN), 1481(w), 1451(s), 1398(m), 1375(m), 1292(s), 1228(m), 1179(m), 1109(w), 1014(w), 924(m), 876(w), 794(m), 757(m), 699(w) cm−1. Anal. Calcd for C38H29N4O2PdBr (760.0000): C, 60.05; H, 3.85; N, 7.37. Found: C, 59.61; H, 3.84; N, 6.99. Synthesis of Palladium(II) Complex [Benzoyl(N∧C∧N)PdI] (17). To a solution of KI (0.088 g, 0.526 mmol) in 5 mL of dry MeOH was added drop by drop Benzoyl(N∧C∧N)PdBr (16) (0.040 g, 0.0526 mmol) dissolved in 15 mL of DCM. The reaction mixture was stirred at room temperatutre for 4 h under an inert atmosphere and filtered through Celite. The solvent was evaporated in vacuo and washed with hexane to afford the pure product as a yellow solid: yield 92% (0.039 g); 1H NMR (500 MHz, CDCl3) δ 9.55 (d, J = 8.5 Hz, 2H), 7.94 (d, J = 7.5 Hz, 4H), 7.74 (t, J = 7.5 Hz, 2H), 7.57 (t, J = 7.5 Hz, 4H), 7.46 (t, J = 7.5 Hz, 2H), 7.28−7.24 (m, 2H), 7.17 (d, J = 8.5 Hz, 2H), 7.03 (s, 2H), 0.92 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 167.4, 158.9, 147.9, 141.3, 136.1, 133.1, 132.4, 131.8, 131.4, 129.9, 125.7, 125.5, 124.0, 123.2, 112.4 (aromatic carbon), 34.9, 31.1 (aliphatic carbon); HR-MS m/z calcd for C38H29N4O2Pd 679.1334, found 679.1328 [M − I]+; FT-IR (KBr) 3061(w), 2953(m), 2861(w), 1727(s, CO), 1717(s, CO), 1596(m, CN), 1503(w), 1480(w), 1450(m), 1399(m), 1375(m), 1291(s), 1263(m), 1227(m), 1178(w), 1153(w), 973(w), 922(w), 876(w), 793(w), 744(s), 698(w) cm−1. Anal. Calcd for C38H29N4O2PdI (807.0005): C, 56.56; H, 3.62; N, 6.94. Found: C, 56.25; H, 3.28; N, 6.55. General Procedure for the Formation of Cationic Palladium(II) Complexes (18−20). To a solution of Benzoyl(N∧C∧N)PdBr (16) (0.050 g, 0.0658 mmol) in a mixture of CH2Cl2 and CH3CN [15 mL, 2:1 (v/v)] were added silver salts (AgBF4, AgSbF6, and AgSO3CF3) (0.132 mmol) at room temperature (protected from light). The resulting suspension was stirred for 4 h at room tempearture under an inert atmosphere. A black precipitate appeared. The reaction mixture was filtered through Celite, and the filterate was evaporated in vacuo, affording pure 18 and 19 as white solids and 20 as a yellow solid. [Benzoyl(N∧C∧N)Pd(MeCN)]+[BF4]− (18). yield 68% (0.036 g); 1H NMR (500 MHz, CDCl3) δ 7.89 (d, J = 7.5 Hz, 4H), 7.81 (t, J = 7.5

(acetone, acetonitrile, dichloromethane, dimethylformamide, and toluene) were dried following standard methods. The starting materials and solvents (THF and DMSO) were purchased from commercial sources. 1H (400 and 500 MHz) and 13C (100 and 125 MHz) NMR spectra were recorded on Bruker AV 400 MHz and Bruker AV 500 MHz spectrometers at 25 °C. The chemical shifts were measured in parts per million relative to TMS for 1H and 13C NMR as an internal standard. Elemental analysis was performed on a Carlo Erba model 1106 elemental analyzer. Mass spectral studies were performed using a Q-tof micro (YA-105) mass spectrometer. IR spectra were recorded in the range of 400−4000 cm−1 by using KBr pellets for solid samples on the FT-IR spectrometer (PerkinElmer). UV−vis spectra were recorded by using a Shimadzu-UV-3600 UV−vis spectrometer and a Varian Cary UV0904M030 spectrometer. Synthesis of 5-tert-Butyl-1,3-bis(N-benzoylbenzimidazolyl)2-bromobenzene (6). To a solution of 5-tert-butyl-1,3-bis(benzimidazolyl)-2-bromobenzene8 (1.0 g, 2.245 mmol) in 50 mL of dry acetone was added KOH (0.516 g, 9.196 mmol). The reaction mixture was stirred at room temperature for 30 min followed by the addition of benzoyl chloride (0.803 g, 5.712 mmol). The reaction mixture was refluxed for 2 h and filtered through Whatman filter paper. The filtrate was diluted with ethyl acetate. The combined organic layer was washed with brine and dried over Na2SO4. The solvent was evaporated in vacuo. The residue was purified by column chromatography on silica gel using petroleum ether (60−80 °C) and ethyl acetate (2−10%) as the eluent to give a colorless pure product: yield 23% (0.337 g); 1H NMR (500 MHz, CDCl3) δ 7.90 (d, J = 7.95 Hz, 2H), 7.65 (d, 4H, J = 7.55 Hz), 7.54−7.50 (m, 4H), 7.39− 7.25 (m, 10H), 1.28 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 167.9, 151.9, 150.6, 142.5, 134.0, 133.9, 133.3, 132.8, 131.1, 130.5, 128.7, 125.3, 124.7, 120.7, 120.5, 113.9 (aromatic carbon), 34.9, 31.1 (aliphatic carbon); HR-MS m/z calcd for C38H30N4O2Br 653.1547, found 653.1536 [M + H]+; FT-IR (KBr) 3085(w), 2951(m), 2865(w), 1720(s, CO), 1598(m, CN), 1498(w), 1451(m), 1393(w), 1371(s), 1278(s), 1227(m), 1179(w), 929(m), 749(m), 695(m), 646(w) cm−1. Anal. Calcd for C38H29N4O2Br (653.5800): C, 69.83; H, 4.47; N, 8.57. Found: C, 69.52; H, 4.55; N, 9.22. Synthesis of Palladium(II) Complex [Benzoyl(N∧C∧N)PdBr] (16). To a solution of 5-tert-butyl-1,3-bis(N-benzoylbenzimidazolyl)2-bromobenzene (6) (0.400 g, 0.612 mmol) in dry toluene was added palladium(0) precursor Pd(dba)2 (0.387 g, 0.673 mmol). The reaction mixture was stirred at room temperature for 12 h and then filtered through Whatman filter paper. The residue was washed with toluene and later with hexane three or four times to afford a yellowish gray solid that is fairly soluble in CHCl3, CH2Cl2, and DMSO. The suitable I


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Organometallics

[Tosyl(N∧C∧N)Pd]+[SbF6]− (22). yield 61% (0.036 g); 1H NMR (500 MHz, DMSO-d6) δ 8.31 (s, 2H), 8.22 (m, 2H), 8.06 (m, 2H), 7.91 (d, J = 7.5 Hz, 4H), 7.61 (m, 4H), 7.51 (d, J = 8.0 Hz, 4H), 2.35 (s, 6H), 1.30 (s, 9H); 19F NMR {121Sb, 123Sb, coupled, 470 MHz) δ −109.1, −111.6, −113.3, −116.2, −117.5, −118.4, −120.7, −121.7, −122.9, −125.9, −127.5, −130.0; 13C NMR [could not be recorded before it decomposed to a cationic protic palladacycle in DMSO-d6 (23)]; LR-MS (ESI-MS) m/z calcd for [C38H33N4O4S2Pd] 779.0978, found 779.1369 [M − SbF6]; LR-MS (ESI-MS, negative mode) m/z calcd for [SbF6] 235, found 235.1813; FT-IR (KBr) 2959(m), 2926(m), 2855(w), 2271(w), 1732(m), 1650(w), 1595(m), 1551(w), 1479(w), 1450(m), 1393(m), 1341(m), 1286(w), 1270(w), 1195(m), 1181(m), 1089(m), 1060(w), 933(w), 813(m), 760(m), 662(s), 571(s), 542(m) cm−1. Anal. Calcd for C40H36N5O4F6S2PdSb (1057.0500): C, 45.45; H, 3.43; N, 6.63; S, 6.07. Found: C, 45.55; H, 3.23; N, 5.41; S, 5.76. [H(N∧C∧N)Pd]+[SbF6]− (23). 1H NMR (500 MHz, DMSO-d6) δ 14.16 (s, 2H), 7.86 (s, 4H), 7.74 (d, J = 8.0 Hz, 2H), 7.51 (d, J = 8.0 Hz, 4H), 7.41 (m, 4H), 7.13 (d, J = 8.0 Hz, 4H), 2.28 (s, 6H, free methyl group), 1.39 (s, 9H); 13C NMR (125 MHz, DMSO-d6) δ 158.3, 149.0, 145.3, 139.9, 137.9, 132.9, 132.2, 128.1, 125.5, 124.1, 123.8, 121.2, 116.3, 113.0 (aromatic carbon), 35.0, 31.2, 20.8; 19F NMR (470 MHz) δ −107.5, −109.1, −111.7, −113.2, −116.0, −117.4, −118.3, −120.6, −121.6, −122.9, −125.8, −127.4, −129.9; HR-MS m/ z calcd for [C24H21N4Pd] 471.0805, found 471.0804 [M − Ts − SbF6]+.

Hz, 2H), 7.74 (broad, 2H), 7.61−7.54 (m, 6H), 7.27 (m, 2H), 7.05 (s, 2H), 6.94 (d, J = 8.0 Hz, 2H), 2.66 (s, 3H), 0.98 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 166.7, 157.8, 139.0, 136.6, 132.7, 132.6, 131.4, 131.2, 130.1, 127.2, 126.1, 124.7, 117.4, 113.5 (aromatic carbon), 35.2, 31.2, 3.3 (aliphatic carbon); 11B NMR (160 MHz, CDCl3) δ 0.46; 19F NMR (decoupled) (470 MHz, CDCl3) δ −152.5; HR-MS m/z calcd for C38H29N4O2Pd 679.1334, found 679.1331 [M − CH3CN − BF4]+; FT-IR (KBr) 3068(w), 2960(m), 2925(m), 2291(w, CN), 1716(s, CO), 1596(m, CN), 1480(w), 1451(s), 1396(m), 1375(m), 1289(s), 1228(m), 1180(w), 1083(s), 1063(s), 1037(s), 927(m), 874(w), 795(w), 758(m), 747(m), 701(w), 646(w) cm−1. Anal. Calcd for C40H32BN5O2F4Pd (807.9526): C, 59.46; H, 3.99; N, 8.67. Found: C, 59.15; H, 4.11; N, 8.04. [Benzoyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (19). yield 78% (0.049 g); 1H NMR (500 MHz, DMSO) δ 8.05 (d, J = 7.5 Hz, 4H), 7.88 (t, J = 7.5 Hz, 2H), 7.81 (d, J = 8.0 Hz, 2H), 7.67 (t, J = 7.5 Hz, 4H), 7.58 (t, J = 7.5 Hz, 2H), 7.46 (t, J = 7.5 Hz, 2H), 7.27 (d, J = 8.0 Hz, 2H), 6.80 (s, 2H), 2.05 (s, 3H), 0.78 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 166.5, 163.5, 156.9, 147.2, 138.6, 136.4, 132.6, 132.1, 131.3, 131.2, 129.9, 125.9, 125.7, 124.3, 118.1, 117.1, 113.8 (aromatic carbon), 34.3, 30.5, 1.2 (aliphatic carbon); 19F{121Sb,123Sb, coupled} NMR (470 MHz, DMSO) δ −97.9, −109.1, −111.6, −113.2, −116.1, −117.4, −118.4, −120.6, −121.6, −122.9, −125.8, −127.4, −129.9, −134.5, −139.6, −141.5, −148.3; LR-MS (ESI-MS) m/z calcd for C38H29N4O2Pd 679.1334, found 679.1379 [M − CH3CN − SbF6]+; LR-MS (ESI-MS, negative mode) m/z calcd for SbF6 235, found 235.1791; FT-IR (KBr) 3070(w), 2963(m), 2286(w, CN), 1716(s, CO), 1597(m, CN), 1482(w), 1451(s), 1398(m), 1376(m), 1290(m), 1272(s), 1228(m), 1181(m), 1111(w), 1014(w), 973(w), 926(m), 795(m), 758(s), 746(m), 702(w), 658(s) cm−1. Anal. Calcd for C40H32N5O2F6SbPd (956.8994): C, 50.21; H, 3.37; N, 7.32. Found: C, 49.71; H, 3.69; N, 7.76. [Benzoyl(N∧C∧N)Pd(OTf)] (20). yield 75% (0.041 g); 1H NMR (500 MHz, CDCl3) δ 8.34 (d, J = 7.5 Hz, 2H), 7.94 (d, J = 7.5 Hz, 4H), 7.79 (t, J = 7.5 Hz, 2H), 7.60 (t, J = 8.0 Hz, 4H), 7.41 (t, J = 6.0 Hz, 2H), 7.24 (m, 2H), 7.06 (s, 2H), 7.04 (d, J = 8.0 Hz, 2H), 0.97 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 167.1, 157.1, 148.6, 139.4, 136.2, 132.9, 132.8, 131.6, 131.4, 129.9, 126.3, 125.8, 124.4, 119.5, 112.8 (aromatic carbon) 35.1, 31.2 (aliphatic carbon); 19F NMR (470 MHz, CDCl 3 ) δ −77.3; LR-MS (ESI-MS) m/z calcd for [C38H29N4O2Pd] 679.1334, found 679.1397 [M − CF3SO3]+; LRMS (ESI-MS, negative mode) m/z calcd for CF3SO3 149, found 149.1074; FT-IR (KBr) 3070(w), 2962(m), 2928(w), 2864(w), 1715(s, CO), 1596(m, CN), 1546(w), 1482(w), 1451(m), 1399(m), 1376(m), 1271(s), 1229(m), 1158(m), 1033(m), 927(w), 873(w), 795(w), 758(m), 640(m) cm −1 . Anal. Calcd for C39H29N4O5F3SPd (829.1592): C, 56.49; H, 3.53; N, 6.76; S, 3.87. Found: C, 56.07; H, 3.80; N, 6.83; S, 2.30. General Procedure for the Synthesis of Palladium(II) Cations (21 and 22). To a solution of Tosyl(N∧C∧N)PdBr complex 5b (0.050 g, 0.058 mmol) in dry THF (10 mL) and CH3CN (2 mL) were added silver salts AgSO3CF3 and AgSbF6 (0.1163 mmol) at room temperature. The resulting suspension was stirred for 12 h at room tempearture under an inert atmosphere (protected from light). The reaction mixture was filtered through Celite, and the filterate was evaporated in vacuo to dryness, to yield pure products 21 and 22 as yellow solids. [Tosyl(N∧C∧N)Pd(OTf)] (21). yield 41% (0.022 g); 1H NMR (500 MHz, CDCl3) δ 8.58 (s, 2H), 8.20−8.16 (m, 4H), 7.73 (d, J = 8.5 Hz, 4H), 7.48 (m, 4H), 7.29 (m, 2H), 2.37 (s, 6H), 1.46 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 163.7, 157.2, 149.2, 147.4, 138.8, 134.6, 133.2, 132.5, 130.8, 127.3, 127.1, 128.1, 126.8, 119.5, 114.7 (aromatic carbon), 36.1, 31.7, 21.9; 19F NMR (470 MHz) δ −77.3; LR-MS (ESIMS) m/z calcd for [C38H33N4O4S2Pd] 779.0978, found 779.1067 [M − CF3SO3]+; LR-MS (ESI-MS, negative mode) m/z calcd for [CF3SO3] 149, found 149.1215; FT-IR (KBr) 2955(m), 2925(m), 2855(m), 1726(w), 1631(m), 1595(m), 1550(w, CN), 1478(w), 1451(m), 1392(m), 1339(w), 1271(s), 1195(m), 1180(m), 1089(m), 1033(m), 932(w), 898(w), 813(w), 760(m), 744(w), 702(w), 685(w), 663(m), 640(w), 572(s), 542(m) cm−1.

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X-RAY CRYSTALLOGRAPHY STUDY The diffraction measurements for complexes 16−21 were performed on a Rigaku Saturn 724 diffractometer and an Oxford Diffraction Gemini diffractometer using graphite monochromated Mo Kα radiation (λ = 0.7107 Å). The structures were determined by a routine heavy atom technique using SHELXS 9731 and Fourier methods and refined by fullmatrix least squares with the anisotropic non-hydrogen atoms and hydrogen atoms with a fixed isotropic thermal parameter of 0.07 Å2 using SHELXL 97.32 The hydrogen atoms were partially located from difference electron density maps, and the rest were fixed by predetermined position. Scattering factors were from a common source.33 X-ray structural parameters are listed in Table S4 for complexes 16−21. CCDC 1546714 (16), 1546715 (17), 1546716 (18), 1546717 (19), 1546718 (20), and 1546719 (21) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/structures-beta/.

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COMPUTATIONAL DETAILS All the theoretical calculations were executed using the Gaussian 0934 software package. Geometrical optimization, generation of the wave function for AIM analysis, and NBO analyses of the optimized geometry were performed at the B3PW9135 level of theory by using the Lanl2dz ecp36 basis set for Pd, Br, and I atoms. For the remaining atoms, the 6-31G* basis set was used.37 AIM analysis was performed using AIM 2000.30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00620. 1 H, 13C, 19F, and 11B NMR spectra, mass spectra, IR spectra, elemental analysis report for new complexes, J


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Organometallics

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crystal packing diagrams, crystallographic data for complexes, and computational details (PDF) Cartesian coordinates for the optimized structures (XYZ) Accession Codes

CCDC 1546714−1546719 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Harkesh B. Singh: 0000-0002-0403-0149 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.B.S. is grateful to the Department of Science and Technology, New Delhi, for J. C. Bose Fellowship 15DSTFLS002. V.R. gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, and the Industrial Research and Consultancy Centre (IRCC) for financial assistance.

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REFERENCES

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