Cyclometalated platinum(II) complexes bearing o-phenylenediamine derivatives: Synthesis and electrochemical behavior

Journal of Organometallic Chemistry 696 (2011) 1232e1235

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Cyclometalated platinum(II) complexes bearing o-phenylenediamine derivatives: Synthesis and electrochemical behavior Take-aki Koizumi*, Kazunori Fukuju Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 September 2010 Received in revised form 29 November 2010 Accepted 6 December 2010

New cyclometalated Pt(II) complexes bearing 1,2-phenylenediamine (pda) derivatives were synthesized and their chemical and electrochemical properties were investigated. Two Pt complexes, [Pt(bzqn) (pda)] (PF6) (bzqn ¼ benzo[h]quinoline, [1](PF6): pda ¼ 1,2-phenylenediamine, [2](PF6): pda ¼ 4,5-dimethyl1,2-phenylenediamine), were synthesized by the reaction of (Bu4N)[PtCl2(bzqn)] with corresponding pda derivatives. The Ptepda complexes were converted to Ptebda (bda ¼ 1,2-benzenediamide) complexes by treatment of 2 mol equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and they showed two-step reversible redox couples in cyclic voltammetry. The cyclometalated ligand gave a strong electronic effect to the bda ligand to take place the negative shift of the bda-based redox potentials. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Platinum complex Cyclometalated compounds Non-innocent ligand Electrochemistry

1. Introduction Non-innocent ligands, such as dioxolenes, dithiolenes, and benzoquinone diimines, coordinated transition metal complexes have received wide interests for their characteristic electrochemical behavior [1]. 1,2-Phenylenediamine (pda) is coordinatable to metals by a chelate form. The di-deprotonated pda ligand, benzenediamide (bda), undergoes reversible two-step redox reactions as shown in Scheme 1. acid-base reaction NH2 NH2

-2H+ +2H

1,2-phenylenediamine (pda)

+

1st oxidation NH-

-e -

-

+e-

NH

benzenediamide (bda)

2nd oxidation NH _ NH semiquinone diimine (sqdi)

NH

-e +e-

NH benzoquinone diimine (bqdi)

Scheme 1. Redox behavior of di-deprotonated 1,2-phenylenediamine.

The bda ligand can regulate the electron density of the centered metal in the complex. Although electrochemistry of Ru complexes containing a bda-derived ligand has been studied in detail [2], researches for other transition metal complexes have been limited. In Pt complexes, several Pt complexes having two semiquinonediimine (sqdi) ligands have been synthesized and their electrochemical properties have been investigated [3e8], however, there have been * Corresponding author. Tel.: þ81 45 924 5222; fax: þ81 45 924 5976. E-mail address: [email protected] (T.-a. Koizumi). 0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2010.12.003

no reports about electrochemical behavior of one di-deprotonated pda ligand-coordinated Pt complexes [9]. Previously, we reported synthesis and electrochemical properties of a Pt complex having bdaderived ligand, [Pt(PhePh0 ePh(NH)2ePh0 ePh) (cod)] (cod ¼ 1,5cyclooctadiene) [10]. The complex showed two-step reversible redox waves based on the Ptebqdi/Ptesqdi and Ptesqdi/Ptebda at E1/2 ¼ 0.260 V and þ0.341 V (vs. Fcþ/Fc), respectively. If this unique redox process can be set to a more negative side, multi-step oxidation process-including transition metal complexes in a low potential can be constructed. Cyclometalated ligands have a strong electrondonating ability to the metal center; therefore, introduction of a cyclometalated ligand to transition metal complexes leads the redox potential(s) of the complex to lower. It is considered that introduction of a cyclometalated ligand to Ptebda complexes causes the lower shift of the characteristic two-step redox potentials based on the bda ligand by the interligand electronic interaction. In this paper, we report synthesis and characterization of Ptepda complexes bearing a cyclometalated ligand. The Ptepda complexes were converted to corresponding Ptebda complexes by the treatment of a base. Electrochemical properties of the formed Ptebda complexes are also described.

2. Results and discussion 2.1. Preparation and characterization of Ptepda complexes Ptepda complexes [1](PF6) and [2](PF6) were prepared as follows. (Bu4N)[PtCl2(bzqn)] (bzqn ¼ benzo[h]quinoline) [11]

T.-a. Koizumi, K. Fukuju / Journal of Organometallic Chemistry 696 (2011) 1232e1235

1233

(1)

reacted with 1,2-phenylenediamine (pda) in 2-methoxyethanol at 70  C to give the pda-ligated Pt complex, [1](PF6), in 72% yield. [2] (PF6) was obtained similar way using 4,5-dimethyl-1,2-phenylenediamine instead of 1,2-phenylenediamine in 90% yield (eq. (1)). ESI-MS spectra of [1]þ and [2]þ showed the parent peak at m/z ¼ 481 and 509, respectively, indicating that the pda derivatives were coordinated to Pt by a pda fashion. Fig. 1 depicts the molecular structure of [1](PF6) determined by X-ray crystallography, and selected bond lengths and angles are summarized in Table 1. The bond lengths of Pt and two N atoms in the pda ligand are 2.059(5) and 2.123(3) Å, which are longer than those of Ptebda complex (1.974(2) and 1.9652(19) Å) [10] and [Pt (sqdi)2] complexes (1.954(9)e1.984(7) Å) [3c,3f,6,7b], and similar to those of [Pt(pda)2]Cl2 (2.0402(17) Å) [7a]. Besides, N1eC1 and N2eC2 bond lengths are 1.451(5) and 1.466(8) Å, respectively, which are longer than those of Ptesqdi complexes (1.331(9)e1.368 (7) Å) [3c,3f,4,7b], and similar to that of [Pt(pda)2]Cl2 (1.450(2) Å) [7a], indicating that the ligand in [1](PF6) coordinates to Pt as a pda fashion. The Pt1eN2 bond which is situated at trans position of C17 is longer than Pt1eN1 one due to the large trans influence of the C ligand in the unsymmetrical bzqn ligand. In 1H NMR spectra of [1](PF6) and [2](PF6), both complexes displayed 8 resonances for the bzqn ligand in the aromatic region. For [1](PF6), signals of the aromatic protons in pda ligand appeared at around d 7.4 as an AA0 BB0 pattern, and eNH2 protons are observed at d 8.56 and 7.78 as broadened signals. The aromatic protons of pda ligand in [2](PF6) showed two singlets at d 7.21 and 7.20, and methyl protons in the ligand are observed at d 2.30 as a slightly broadened peak due to the unsymmetrical structure of [2] (PF6). The broadened peaks observed at d 8.40 and 7.61 were assigned to eNH2 protons. The signals of pda ligand in [2](PF6) were shifted to a higher magnetic field than those of [1](PF6) due to the introduction of electron-donating methyl groups in the ligand. 2.2. Electrochemical properties Cyclic voltammograms (CV) of [1](PF6) and [2](PF6) were measured in an N,N-dimethylformamide (DMF) solution containing

0.10 M Bu4NPF6 as a supporting electrolyte at a glassy carbon working electrode and an AgNO3/Ag reference electrode. Fig. 2 shows CV curves of [1](PF6) and [2](PF6). As depicted in Fig. 2(a), [1](PF6) shows an irreversible oxidation peak at Epa ¼ þ0.964 V (vs. Fcþ/Fc) based on the oxidation of the coordinating pda ligand and its reduction peak at Epc ¼ 0.495 V (vs. Fcþ/Fc), and two irreversible reduction peaks at 2.146 and 2.427 V (vs. Fcþ/Fc) which are assigned to the reduction of pda and bzqn ligands, respectively. The corresponding oxidation wave for the reduction is observed at about 0.5 V as broadened wave. The CV of [2](PF6) displays similar chart to [1](PF6), but each peaks are slightly lower shifted than those in [1](PF6) (Fig. 2(b)). When two equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to the solution, drastic change of the CV curve was observed. Fig. 3 displays CV curves of 2 mol equiv of DBU-added [1] (PF6) and [2](PF6). As shown in Fig. 3(a), the oxidation and reduction peaks based on the pda ligand (cf. Fig. 2(a)) disappeared completely, and reversible two-step redox couples newly appeared at E1/2 ¼ 1.001 and 0.516 V (vs. Fcþ/Fc). Appearance of the twostep redox waves indicates that the pda ligand in the Pt complex received di-deprotonation by the treatment of DBU to convert the corresponding bda ligand. There have been no evidence for generation of Pt(I) or Pt(III) species in the redox process, and the central Pt ion is thought to be divalent in all anionic, neutral, and cationic forms. The formed Ptebda complex may show the redox behavior as shown in Scheme 2. As shown in Fig. 3(b), the CV curve of [2](PF6) was changed similarly to [1](PF6) after addition of 2 mol equiv of DBU, and twostep reversible redox waves appeared at E1/2 ¼ 1.070 and 0.664 V (vs. Fcþ/Fc). Their E1/2 potentials are lower shifted by 69 and 148 mV, respectively, than those of [1](PF6). These negative shifts are leaded to the introduction of electron-donating methyl groups to the bda ligand.

3. Conclusion We have synthesized two novel cyclometalated Pt(II) complexes having a pda-derived ligand, [1](PF6) and [2](PF6), and characterized their chemical and electrochemical properties. Ptepda complexes were converted to corresponding Ptebda complexes easily by the treatment of a base. Introducing of the cyclometalated ligand to the

Table 1 Selected bond lengths (Å) and angles ( ) of [1](PF6).

Fig. 1. ORTEP drawing of [1]þ with the 50% ellipsoidal level. PF 6 and hydrogen atoms are omitted for clarity.

Pt1eN1 Pt1eN3 N1eC1 C1eC2 C3eC4 C5eC6 N1ePt1eN2 N1ePt1eC17 N2ePt1eC17 Pt1eN1eC1

2.059(5) 2.023(5) 1.451(5) 1.380(7) 1.383(9) 1.388(6) 82.25(18) 96.6(2) 178.6(2) 111.6(3)

Pt1eN2 Pt1eC17 N2eC2 C2eC3 C4eC5 C1eC6 N1ePt1eN3 N2ePt1eN3 N3ePt1eC17 Pt1eN2eC2

2.123(3) 2.009(4) 1.466(8) 1.380(6) 1.382(8) 1.402(9) 178.33(13) 99.19(18) 81.9(2) 110.0(3)

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Ptebda unit caused the lower shift of the bda ligand-based redox couples by 800e900 mV than the cod-ligated Ptebda complex [10]. 4. Experimental 4.1. General, measurement, and materials 1 H NMR spectra were recorded on a Bruker Biospin Avance III spectrometer. Electrochemical measurements were performed with an ALS/CHI 1200A electrochemical analyzer. A conventional three-electrode configuration was used, with glassy carbon working (BAS electrode) and platinum wire auxiliary electrode (Tokuriki, special order) and a 0.01 M AgNO3/Ag reference (BAS RE5). Cyclic voltammograms were recorded at a scan rate of 100 mV s1. Fcþ/Fc ¼ þ81 mV vs. 0.01 M AgNO3/Ag, Fcþ/Fc ¼ þ425 mV vs. SCE. (Bu4N)[PtCl2(bzqn)] [11] was prepared according to the literature methods.

4.2. Synthesis of [1](PF6) To a 2-methoxyethanol (35 mL) solution of (Bu4N)[PtCl2(bzqn)] (232 mg, 0.33 mmol) added 1,2-phenylenediamine (55 mg, 0.50 mmol) and stirred at 70  C for 2 h. The yellowegreen precipitate was collected by filtration, then dissolved in a small amount of DMSO and poured into an aqueous solution of NH4PF6. The resultant solid was collected by filtration, washed by CH2Cl2, and dried in vacuo (gray solid, 153 mg, 0.24 mmol, 72%). Anal. Calcd for C19H16F6N3PPt: C, 36.43; H, 2.57; N, 6.71. Found: C,36.31; H, 2.60; N, 6.71. 1H NMR (400 MHz, DMSO-d6): d 9.06 (d, 1H, J ¼ 5.4 Hz, bzqn), 8.79 (d, 1H, J ¼ 8.1 Hz, bzqn), 8.56 (br s, 2H, pda-NH2), 7.96 (d, 1H, J ¼ 8.8 Hz, bzqn), 7.87 (t, 1H, J ¼ 8.0, 5.4 Hz, bzqn), 7.86 (d, 1H, J ¼ 8.8 Hz, bzqn), 7.78 (br s, 2H, pda-NH2), 7.78 (d, 1H, J ¼ 7.3 Hz, bzqn), 7.72 (d, 1H,

Fig. 3. Cyclic voltammograms of 2 mol equiv of DBU-added (a) [1](PF6) and (b) [2](PF6) in DMF.

J ¼ 6.5 Hz, bzqn), 7.62 (t,1H, J ¼ 7.6, 7.4 Hz, bzqn), 7.48 (m, 2H, pda-H), 7.38 (m, 2H, pda-H). ESI-MS: m/z ¼ 481 [M]þ. 4.3. Synthesis of [2](PF6) To a 2-methoxyethanol (25 mL) solution of (Bu4N)[PtCl2(bzqn)] (154 mg, 0.22 mmol) added 4,5-dimethyl-1,2-phenylenediamine (35 mg, 0.25 mmol) and stirred at 70  C for 2 h. The reddish orange solution was concentrated ca. 1 mL, and poured into an aqueous solution of NH4PF6; yellow ocher precipitate was produced. Resulting solid was collected by filtration, washed with H2O, diethyl ether, and CH2Cl2 successively, and dried in vacuo. (reddish brown solid, 131 mg, 0.20 mmol, 90%). Anal. Calcd for C21H20F6N3PPt: C, 38.54; H, 3.08; N, 6.42. Found C, 38.33; H, 2.97; N, 6.28. 1H NMR (400 MHz, DMSO-d6): d 9.04 (d, 1H, J ¼ 5.4 Hz, bzqn), 8.78 (d, 1H, J ¼ 8.1 Hz, bzqn), 8.40 (br s, 2H, pda-NH2), 7.96 (d, 1H, J ¼ 8.8 Hz, bzqn), 7.87 (t, 1H, J ¼ 8.0, 5.5 Hz, bzqn), 7.86 (d, 1H, J ¼ 8.8 Hz, bzqn), 7.77 (d, 1H, J ¼ 7.8 Hz, bzqn), 7.70 (d, 1H, J ¼ 6.7 Hz, bzqn), 7.62 (t, 1H, J ¼ 7.8, 7.2 Hz, bzqn), 7.61 (br s, 2H, pda-NH2), 7.21 (s, 2H, pda-H), 2.30 (s, 6H, pda-CH3). ESI-MS: m/z ¼ 509 [M]þ.

Fig. 2. Cyclic voltammograms of (a) [1](PF6) and (b) [2](PF6) in DMF.

Scheme 2.

T.-a. Koizumi, K. Fukuju / Journal of Organometallic Chemistry 696 (2011) 1232e1235

4.4. Crystal structure determination Crystals of [1](PF6) for X-ray analysis were obtained from a CH2Cl2-acetone solution. Suitable crystals were mounted on glass fibers. Data collection for [1](PF6) was performed at 160  C on a Rigaku/MSC Saturn CCD diffractometer with graphite monochromated Mo-Ka radiation (l ¼ 0.71070 Å). The structures were solved by using the Crystal Structure software package [12]. Atom scattering factors were obtained from the literature [13]. Refinements were performed anisotropically for all non-hydrogen atoms by the full-matrix least-square method. Hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters. The residual electron densities were of no chemical significance. Crystal data: C19H16F6N3PPt, Mr ¼ 626.41, triclinic, Space group P-1 (No.2), a ¼ 8.200(7), b ¼ 10.558(9), c ¼ 12.193(9) Å, a ¼ 68.48(3), b ¼ 70.83 (3), g ¼ 78.62(4) , V ¼ 923.9(13) Å3, Z ¼ 2, Dcalc ¼ 2.251 g cm3, m ¼ 77.206 cm1, 3918 unique reflns used in calculations. R1(R) ¼ 0.0266 (0.0335), Rw ¼ 0.0294.

[2]

[3]

Acknowledgements We gratefully acknowledge Prof. Koji Tanaka of Institute for Molecular Science for helpful discussions. This work was financially supported by Grants-in-Aid for Scientific Research for Young Chemists (No. 20750045) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

[4] [5] [6] [7] [8]

Appendix A. Supplementary material [9]

CCDC No. 793963 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data center via www.ccdc.ac.uk/datarequest/cif. References

[10]

[11] [12]

[1] (a) E. Uhlig, Pure Appl. Chem. 60 (1988) 1235; (b) M. Fourmigué, Coord. Chem. Rev. 178e180 (1998) 823; (c) A. Vl cek Jr., Coord. Chem. Rev. 230 (2002) 225;

[13]

1235

(d) K.P. Butin, E.K. Beloglazkina, N.V. Zyk, Russ. Chem. Rev. 74 (2005) 531; (e) C. Mealli, A. Ienco, A.D. Phillips, A. Galindo, Eur. J. Inorg. Chem. (2007) 2556. (a) P. Belser, A. von Zelewsky, M. Zehnder, Inorg. Chem. 20 (1981) 3098; (b) H. Masui, A.B.P. Lever, P.R. Auburn, Inorg. Chem. 30 (1991) 2402; (c) H. Masui, A.B.P. Lever, E.S. Dodsworth, Inorg. Chem. 32 (1993) 258; (d) R.A. Metcalfe, A.B.P. Lever, Inorg. Chem. 36 (1997) 4762; (e) S.I. Gorelsky, A.B.P. Lever, J. Organomet. Chem. 635 (2001) 187; (f) F.N. Rein, R.C. Rocha, H.E. Toma, Electrochem. Commun. 4 (2002) 436; (g) A. Anillo, S. Garcia-Granda, R. Obeso-Rosete, A. Galindo, A. Ienco, C. Mealli, Inorg. Chim. Acta 350 (2003) 557; (h) B. Milliken, L. Borer, J. Russell, M. Bilich, M.M. Olmstead, Inorg. Chim. Acta 348 (2003) 212; (i) T. Koizumi, B.J. Choi, T. Yamamoto, Inorg. Chim. Acta 361 (2008) 2131. (a) S.E. Nefedov, A.A. Sidorov, A.V. Reshetnikov, M.O. Ponina, I.L. Eremenko, Russ. Chem. Bull. 47 (1998) 729; (b) I.L. Eremenko, S.E. Nefedov, A.A. Sidorov, M.O. Ponina, P.V. Danilov, T.A. Stromnova, I.P. Stolarov, S.B. Katser, S.T. Orlova, M.N. Vargaftik, I.I. Moiseev, Yu.A. Ustynyuk, J. Organomet. Chem. 551 (1998) 171; (c) A.V. Reshetnikov, A.A. Sidorov, S.S. Talismanov, G.G. Aleksandrov, Y.A. Ustynyuk, S.E. Nefedov, I.L. Eremenko, I.I. Moiseev, Russ. Chem. Bull. 49 (2000) 1771; (d) I.G. Fomina, A.A. Sidorov, G.G. Aleksandrov, S.E. Nefedov, I.L. Eremenko, I.I. Moiseev, J. Organomet. Chem. 636 (2001) 157; (e) A.V. Reshetnikov, M.O. Talismanova, A.A. Sidorov, S.E. Nefedov, I.L. Eremenko, I.I. Moiseev, Russ. Chem. Bull. Int. Ed. 50 (2001) 142; (f) I.G. Fomina, S.S. Talismanov, A.A. Sidorov, Yu.A. Ustynyuk, S.E. Nefedov, I.L. Eremenko, I.I. Moiseev, Russ. Chem. Bull. Int. Ed. 50 (2001) 515; (g) M.O. Talismanova, I.G. Fomina, A.A. Sidorov, G.G. Aleksandrov, N.T. Berberova, A.O. Okhlobystin, E.V. Shinkar, I.F. Golovaneva, I.L. Eremenko, I.I. Moiseev, Russ. Chem. Bull. Int. Ed. 52 (2003) 2701. D. Herebian, E. Bothe, F. Neese, T. Weyhermüller, K. Wieghardt, J. Am. Chem. Soc. 125 (2003) 9116. K. Takeda, I. Shirotani, K. Yakushi, Synth. Met. 133-134 (2003) 415. T. Kubo, M. Sakamoto, H. Kitagawa, K. Nakasuji, Synth. Met. 153 (2005) 465. (a) Y. Konno, N. Matsushita, Bull. Chem. Soc. Jpn. 79 (2006) 1046; (b) Y. Konno, N. Matsushita, Bull. Chem. Soc. Jpn. 79 (2006) 1237. W. Wenseleers, E. Goovaerts, A.S. Dhindsa, A.E. Underhill, Chem. Phys. Lett. 254 (1996) 410. One bda type ligand-coordinated Pt complexes have been reported: (a) S. Muto, K. Tasaka, Y. Kamiya, Bull. Chem. Soc. Jpn. 50 (1977) 2493; (b) J.M. Clemente, C.Y. Wong, P. Bhattacharyya, A.M.Z. Slawin, D.J. Williams, J.D. Woollins, Polyhedron 13 (1994) 261. (a) T. Koizumi, T. Yamamoto, Abstracts for the 57th Symposium on Coordination Chemistry of Japan (2006) 143; (b) T. Koizumi, T. Teratani, T. Yamamoto, Inorg. Chem. Commun. in press. S.-W. Lai, M.C.W. Chan, K.-K. Cheung, S.-M. Peng, C.-M. Che, Organometallics 18 (1999) 3991. Crystal Structure: Crystal Analysis Package, Rigaku and Rigaku/MSC, 2000e2006. International Tables for X-ray Crystallography, vol. 4, Kynoch, Birmingham, England, 1974.