Which is the Stronger Nucleophile, Platinum or Nitrogen in Rollover

College of Sciences, Shiraz University, Shiraz 71467-13565, Iran. Inorg. Chem. , 2017, 56 (23), pp 14706–14713. DOI: 10.1021/acs.inorgchem.7b026...
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Which is the Stronger Nucleophile, Platinum or Nitrogen in Rollover Cycloplatinated(II) Complexes? Fatemeh Niroomand Hosseini,*,† S. Masoud Nabavizadeh,‡,§ and Mahdi M. Abu-Omar‡ †

Department of Chemistry, Shiraz Branch, Islamic Azad University, Shiraz 71993−37635, Iran Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States § Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565, Iran ‡

S Supporting Information *

ABSTRACT: The rollover cyclometalated platinum(II) complexes [PtMe(2,X′-bpy-H)(PPh3)], (X = 2, 1a; X = 3, 1b; and X = 4, 1c) containing two potential nucleophilic centers have been investigated to elucidate which center is the stronger nucleophile toward methyl iodide. On the basis of DFT calculations, complexes 1b and 1c are predicted reacting with MeI through the free nitrogen donor to form N-methylated platinum(II) complexes, while complex 1a reacts through oxidative addition on platinum to give a platinum(IV) complex, which is in agreement with experimental findings. The reasons for this difference in selectivity for complexes 1a−1c are discussed based on the energy barrier needed for N-methylation versus oxidative addition reactions.

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(II) complex such as [PtMe(Ĉ N)PPh3] (Ĉ N = 2-phenylpyridyinate or benzo[h]quinolate) gives the corresponding dimethylplatinum(IV) complex.19 The rollover complexes are a new class of cyclometalated complexes formed with bidentate ligands such as 2,2′bipyridine. These rollover cycloplatinated complexes contain a free nitrogen donor atom, which can serve as a site for protonation20,21 and can influence catalytic processes.22,23 The Pt center of these rollover complexes can also undergo oxidative addition reactions. As an example, the rollover complex [PtMe(2,2′-bpy-H)(PPh3)]24,25 reacts with MeI to give the corresponding cyclometalated platinum(IV) complex, [PtIMe2(2,2′-bpy-H)(PPh3)], exclusively, despite N-methylation of the free N atom of bpy-H ligand can act as a competing reaction. Another example in which Pt(II) center in platinum complex is more nucleophilic than a free nitrogen donor is the reaction of complex [PtMe2(6-dppd)] (6-dppd = 4-di-2pyridyl-5,6,7,8,9,10-hexahydrocycloocta[d]- pyridazine) with mercury(II) salts HgX2 (X = Cl, Br, OAc). In this case complex [PtMe2(6-dppd)] reacts with HgX2 to give the platinum(IV) complex [PtX(HgX)Me2(6-dppd)] instead of

INTRODUCTION Oxidative addition of alkyl halides in low valent transition-metal complexes1,2 and N-alkylation reactions of free nitrogen are among the key industrial or organic transformations for the production of a variety of added-value chemicals.3,4 Among the commonly used alkyl halides is methyl iodide (MeI) such as in Monsanto’s acetic acid production.5 Selective N-methylation of amines is used for the preparation of pharmaceuticals and fine chemicals.6 The traditional procedure of methylation of amines is based on alkylating reagents such as methyl iodide and dimethyl sulfate.7 Beydoun et al. reported ruthenium-catalyzed direct methylation of primary and secondary aromatic amines using carbon dioxide and molecular hydrogen.8 Pyridine methylation is a commonly used method for modifying bioactive compounds in medicinal chemistry.9 The connection of a methyl group to a drug candidate can improve its biological activity.10 On the other hand, oxidative addition reactions in catalytic processes have been well established.11,12 Among transition metal complexes, cycloplatinated(II) complexes having Ĉ N ligands are highly reactive toward oxidative addition reactions, which typically proceed by an SN2 mechanism as has been confirmed experimentally1,2,13−16 and computationally.17,18 For example, MeI oxidative addition to a cyclometalated platinum© XXXX American Chemical Society

Received: October 17, 2017

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DOI: 10.1021/acs.inorgchem.7b02678 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Oxidative Addition and N-Methylation Competition Reactions

Scheme 2. Suggested Mechanisms for N-Methylation and Oxidative Addition of Complexes 1a−1c

methylation of the rollover ligand, we embarked on a detailed mechanistic investigation of MeI addition reactions with cycloplatinated complexes to examine and understand site selectivity.

formation of a bimetallic complex through coordination of mercury(II) salt to free pyridyl and pyridazine nitrogen donor.26 Puddephatt and co-workers have also reported other examples of MeI oxidative addition to dimethyl platinum(II) complexes having N-donor ligands with free donor atoms.27 Considering the importance of rollover cyclometalated complexes, there is a need to understand why the Pt center of rollover cyclometalated complexes can be a stronger nucleophile toward MeI than the free nitrogen donor of the rollover ligand. To understand factors governing these two competing reactions, oxidative addition on Pt versus N-

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EXPERIMENTAL SECTION

1

H and 31P NMR spectra were recorded on a Varian 400 or 600 MHz spectrometer, and referenced to CDCl3 (7.26 ppm) as an internal reference. All the chemical shifts and coupling constants are in ppm and Hz, respectively. The compounds 1-methyl-4-(2′-pyridyl)B

DOI: 10.1021/acs.inorgchem.7b02678 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Computed Activation Parameters and Reaction Thermodynamics for Oxidative Addition and N-Methylation Reactions Shown in Scheme 1 complex

ΔS‡ (cal K−1 mol−1)

ΔH‡ (kcal mol−1)

ΔG‡ (kcal mol−1)

ΔE‡ (kcal mol−1)

ΔSo (cal K−1 mol−1)

ΔHo (kcal mol−1)

ΔGo (kcal mol−1)

ΔE (kcal mol−1)

Oxidative Addition Reaction solvent 1a 1b 1c solvent 1a 1b 1c solvent 1a 1b 1c

= CH2Cl2 −37.8 −35.7 −36.2 = CHCl3 −37.2 (−27.3)a −37.9 −37.7 = CH3CN −37.4 −36.5 −37.3

solvent = CH2Cl2 1a −35.2 1b −32.3 1c −33.8 solvent = CHCl3 1a −43.1 1b −41.4 1c −34.3 solvent = CH3CN 1a −34.8 1b −32.2 1c −33.9 a

11.7 12.0 11.7

23.0 22.6 22.6

12.3 12.6 12.3

−44.0 −43.8 −42.2

−1.5 −1.4 −1.5

11.6 11.7 11.1

−0.9 −0.7 −0.9

12.3 (13.2)a 12.6 12.3

23.4 (21.3)a 23.9 23.6

12.9 13.2 12.9

−51.3 −42.4 −44.4

−1.9 −1.2 −1.2

13.4 11.4 12.0

−1.3 −0.6 −0.6

11.2 11.5 11.4

22.4 22.4 22.5

11.8 −44.2 12.1 −43.9 12.0 −43.6 N-Methylation Reaction

−1.8 −1.5 −1.6

11.4 11.5 11.4

−1.2 −0.9 −1.0

15.2 8.8 8.1

25.7 18.4 18.2

15.8 9.4 8.7

−15.7 −16.2 −11.4

−3.1 −16.5 −17.3

1.6 −11.7 −13.9

−3.1 −16.5 −17.3

15.3 8.8 8.6

28.2 21.1 18.9

15.9 9.3 9.2

−15.2 −11.3 −12.3

6.0 −7.0 −8.6

10.6 −3.6 −4.9

6.0 −7.0 −8.6

14.7 8.4 7.7

25.1 18.0 17.9

15.2 9.0 8.4

−15.1 −18.7 −10.7

−10.7 −23.9 −24.5

−6.2 −18.4 −21.3

−10.7 −23.9 −24.5

Values in parentheses are experimental values from ref 24.

pyridinum iodide (abbreviated as Me-2,4′-bpy-I),28 and [Pt2Me4(μSMe2)2]29 were prepared as reported. Synthesis of [PtMe(Me-2,4′-bpy-H)(PPh3)]I, P3. Pure 1-methyl4-(2′-pyridyl)pyridinum iodide (104 mg, 0.35 mmol) was added to a solution of [Pt2Me4(μ-SMe2)2] (100 mg, 0.18 mmol) in acetone (20 mL) at room temperature under Ar. The solution immediately turned red. The solution was stirred for 24 h. Then to the in situ solution of [PtMe(Me-2,4′-bpy-H)(SMe2)]I was added PPh3 (91.6 mg, 0.35 mmol) and the solution was further stirred for 24 h. The solvent was removed under reduced pressure. The residue was triturated with ether (5 × 3 mL), and the product as a red solid was dried under vacuum. Yield: 84%. Very small amount of PtMe2(PPh3)2 ( CH2Cl2 (E= −3.1 kcal mol−1, ε = 8.9) > CHCl3 (E = +6.0 kcal mol−1, ε = 4.8). Charge Effect. Analysis of the population changes obtained from the natural bond orbital (NBO) revealed interesting points (Table 2). During the oxidative addition reaction of 1a with MeI, the electron population of Pt decreases (the positive charge on platinum increases from 0.058 in 1a to 0.135 in 2a), which supports the oxidation of Pt center on going from 1a to

Figure 4. Comparative energy barriers for the rate-determining step of the studied reactions in CH2Cl2.

oxidative addition or N-methylation by MeI. Though the reaction of complex 1a with MeI proceeds through oxidative addition to give Pt(IV), the analogous complexes 1b and 1c are predicted to attend in N-methylation reaction versus oxidative addition. The energy barriers (ΔE‡) for N-methylation of complexes 1a, 1b and 1c are 15.8, 9.4, and 8.7 kcal mol−1, respectively, in CH2Cl2. This large difference between 1a and 1b−1c is attributed to the position of the nitrogen atom in the bpy-H ligand. In complex 1a, steric hindrance prohibits Nmethylation, whereas the accessibility of the free N atom in complexes 1b and 1c favors N-methylation. It should be noted that the energy barriers for oxidative additions with all complexes 1a, 1b, and 1c (12.3, 12.6, and 12.3 kcal mol−1, F

DOI: 10.1021/acs.inorgchem.7b02678 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Dihedral angle of two rings of Ĉ N ligand in complexes P1 and P3.

tributions as a scientist and for being a true RENAISSANCE man.

respectively) are comparable, showing that the Pt centers in all complexes have similar nucleophilicity. Products of oxidative addition 2a−2c have the same energy due to lacking of steric hindrance; however, among the products of N-methylation reaction, complex P1 is unstable compared to P2 and P3 because of steric effect between incoming Me ligand and ortho ring substituent. This steric is shown in Figure 5 when the optimized structures of P1 and P3 are compared in which the dihedral angle of C55−C41−C43− C46 in P3 is almost zero, whereas the corresponding dihedral angle of N58−C41−C42−C47 in P1 is about 26.5°.

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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.inorgchem.7b02678. DFT data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or fniroomand55@yahoo. com. ORCID

Fatemeh Niroomand Hosseini: 0000-0002-5856-8104 S. Masoud Nabavizadeh: 0000-0003-3976-7869 Mahdi M. Abu-Omar: 0000-0002-4412-1985 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.N.H. acknowledges the support of the Islamic Azad University, Shiraz Branch, for her sabbatical leave at University of California, Santa Barbara. We acknowledge support from the Center for Scientific Computing from the CNSI, MRL: an NSF MRSEC (DMR-1121053) and NSF CNS-0960316, and support from the Department of Chemistry and Biochemistry at UCSB.

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DEDICATION Dedicated to the memory of Prof. Mehdi Rashidi, the father of Organometallic Chemistry in Iran, for his outstanding conG

DOI: 10.1021/acs.inorgchem.7b02678 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02678 Inorg. Chem. XXXX, XXX, XXX−XXX