Synthesis and Crystal Structure of Alkynylplatinum ( IV ) Complex Containing the Terpyridine Ligand

Reaction of square planar [PtII(C≡CPh)(tpy)]+ (tpy = 2,2:6,2-terpyridine) with bromine at low temperature provides a general route for the synthesis of octahedral alkynyl(terpyridine)platinum(IV) complex. In this first example of alkynyl(terpyridine)platinum(IV) complex, the alkynyl group is situated in trans position relative to the central nitrogen atom of the terpyridine ligand, and the two bromido ligands are situated in trans positions; an X-ray structural analysis has been completed for trans(Br)[PtIVBr 2 (C≡CPh)(tpy)].


Introduction
Platinum(II) terpyridyl complexes with alkynyl ligands have attracted great interest in recent years due to their unique photophysics [1,2] and their potential applications as photocatalysts for hydrogen evolution [3,4] and molecular frameworks for light-to-chemical energy conversion [5].On the other hand, there is a limited range of reported alkynylplatinum(IV) complexes [6][7][8]: there is no report on alkynylplatinum(IV) complex with terpyridine ligands.Although general oxidizing agents such as halogens and hydrogen peroxide are utilized for oxidation reactions of Pt II to Pt IV centers, halogens are particularly the most useful reagents [9,10].However, only one example for oxidation of alkynylplatinum(II) by halogens has been known in [Pt II (Me 2 bpy)(C≡C-4-tol) 2 ] by iodine to form [Pt IV I 2 (Me 2 bpy)(C≡C-4-tol) 2 ] [6], due to instability of the Pt II -alkynyl bond to halogencontaining oxidants [11].Therefore, other procedures using 4-nitrophenyl azide and alkynyliodine(III) reagents as oxidants have been explored for oxidation of Pt II to Pt IV [7,8,12].In order to generalize about routes to alkynylplatinum(IV) compounds by halogen-oxidation reaction without Pt-alkynyl bond breaking, we have optimized reaction conditions (the kind of oxidants and temperature) using alkynylplatinum(II) containing the terpyridine ligand as a prototype complex.

Materials and Methods
All chemicals employed here were used without further purification unless otherwise stated.All solvents purchased for organic synthesis were anhydrous and used without further purification.A Br 2 -CHCl 3 solution was prepared by addition of 12 drops of Br 2 to 10 mL of CHCl 3 [10].A Cl 2 -CHCl 3 solution was prepared by a saturated Cl 2 gas through 10 mL of CHCl 3 for approximately 15 sec [9].The platinum(II) precursor ([Pt II (C≡CPh)(tpy)]PF 6 ) was prepared in accordance with the published method [2].
as the solvent.The instrument was operated at positive ion mode using m/z range of 100-1000.IR spectra were obtained using the KBr pellet method with a JASCO FT-IR 4100 spectrometer.
2.1.X-Ray Crystal Structure Determination.Single crystals of 1Br were obtained from a solution of the complex in DMF/diethyl ether.A brown crystal of 1Br with dimensions 0.15 × 0.15 × 0.03 mm was mounted on a glass fiber.All measurements were performed on a Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo K radiation ( = 0.71075 Å).All calculations were conducted using the CrystalStructure program package [13] except for refinement, which was performed using SHELXL97 [14].
The structure was solved by direct methods using SIR92 program [15].A numerical absorption correction (ABSCOR) [16] was applied to data.Aromatic hydrogen atoms were fixed at C-H lengths of 0.95 Å refined as riding, with  iso (H) = 1.2 eq (C).Both the highest residual electron peak and the deepest hole are located within 1 Å from atom Pt1.The crystallographic data are summarized in Table 1 and the geometrical parameters are summarized in Table 2.
The crystal data for 1Br has been deposited with Cambridge Crystallographic Data Centre as supplementary publication number CCDC-1014039.

Synthesis of trans(Br)-[PtBr 2 (C≡CPh)(tpy)]
Br.The synthetic route for the formation of platinum(IV) terpyridyl complexes is summarized in Scheme 1. Addition of Br 2 -CHCl 3 solution to [Pt II (C≡CPh)(tpy)] + in acetonitrile at −20 ∘ C gave [Pt IV Br 2 (C≡CPh)(tpy)] + (1 + ) as light brown solid in 73% yield.When this reaction was performed at room temperature, 1 + was produced but a small quantity of the alkynyl dissociated species was detected.In the same condition, however, addition of Cl 2 -CHCl 3 solution gave some impurities in addition to [Pt IV Cl 3 (tpy)] + (m/z = 534, major product) and [Pt IV Cl 2 (C≡CPh)(tpy)] + (m/z = 600, minor product) (Scheme 1).These results indicate that the Pt-C bond in [Pt(C≡CPh)(tpy)] + is influenced by the strength of oxidants and reaction temperatures: milder conditions (Br 2 as an oxidant and low temperature) are required for bond retention of the Pt-alkynyl moiety.

Characterization of trans(Br)-[PtBr 2 (C≡CPh)(tpy)]
+ .The identities of the complex 1 + have been confirmed by IR spectroscopy, 1 H NMR spectroscopy, and ESImass spectroscopy.The molecular structure of 1Br has also been determined.To the best of our knowledge, this study demonstrates the first crystal structure of an alkynylplatinum(IV) with the terpyridine ligand.The IR spectrum of 1Br exhibited ](C≡C) absorption at 2165 cm −1 , which is 40 cm −1 higher than that of [Pt II (C≡CPh)(tpy)] + [17].The 1 H NMR spectra of 1 + showed resonances at more downfield regions compared to the corresponding Pt II complex [17].For example, the doublet signals of 6,6  -positions in terpyridine ligand of 1 + appeared at 9.29 ppm, whereas those of [Pt II (C≡CPh)(tpy)] + appeared at 9.10 ppm, indicating a lowering of the electron density on a Pt center.
The molecular structure of 1Br is shown in Figure 1 with atom-labeling scheme.The complex consists of a distorted octahedral geometry around the Pt IV center with three nitrogen atoms of the terpyridine ligand, two bromido ligands in trans positions, and an alkynyl group in trans position relative to the central nitrogen atom (1  -position) of the terpyridine ligand.The terpyridine ligand is coordinated in a planar tridentate fashion with the central nitrogen atom closest to the Pt IV atom.As shown in Table 2(a), the bond lengths of Pt-N are comparable to the precursor platinum(II) terpyridine complex [2].Additionally, the Pt1-C1-C2 and C1-C2-C3 fragments are nearly linear with bond angles of 175.3(12) ∘ and 174.4(14) ∘ , respectively.However, both the Pt-C and the C≡C bond lengths (2.054(10) and 1.059(17) Å, resp.) are different from those of the corresponding Pt II complex (1.98(1) and 1.19(1) Å, resp.)[2].In the crystal, the cation of 1Br features intermolecular - stacking between the phenyl group of the alkynyl ligand and the terpyridine ligand, as represented by the shortest contact of 3.256(16) Å for C5⋅ ⋅ ⋅ C23 (Table 2(b)).The phenyl ring of the alkynyl ligand is almost parallel to the plane of Pt-terpyridine with a dihedral angle of 5.438 ∘ , indicating the formation of a face-to-face - interaction (Figure 2).In addition, there are a number of intermolecular weak C-H⋅ ⋅ ⋅ Br interactions in the crystal as illustrated in Figure 2.These interactions help to stabilize the structure.

Conclusions
In summary, we have prepared an alkynylplatinum(IV) complex by simple oxidation of the corresponding alkynylplatinum(II) one.It is anticipated that the successful route described here provides a useful methodology to obtain a variety of platinum(IV) complexes with a wide range of alkynyl ligands.

Figure 2 :
Figure 2: Packing diagram.The green lines denote intermolecular hydrogen bonds.