Abstract
Treatment of dichlorobis(piperidine-1-carbonitrile)platinum(II) with potassium iodide in water/methanol mixture results in the halide ligand exchange giving novel trans-diiodobis(piperidine-1-carbonitrile)platinum(II) complex. The reaction proceeds under mild conditions (20°C, 40 min) giving trans-[PtI2(NCNC5H10)2] in 70% isolated yield. The iodide complex was characterized by IR, 1H and 13C NMR spectroscopies, and high resolution ESI-MS, and its structure was determined by a single crystal X-ray diffraction.
1. Introduction
Interest in metal complexes bearing NCR (R = Alk, Ar, NAlk2) ligands is stimulated by enhanced reactivity of nitrile functionality in such compounds toward nucleophilic addition and 1,3-dipolar cycloaddition [1, 2]. Moreover, these metal-mediated processes allow the conductance of the reactions, which are not feasible without involvement of metal centers. Our recent results demonstrate unexpectedly high activity of platinum(II)-bound push-pull nitriles NCNAlk2 toward cycloaddition of nitrones and nucleophilic additions [3]. Following this project focused on investigations of unusual reactivity of push-pull nitrile ligands, we studied novel platinum(II) complex bearing push-pull dialkylcyanamide ligands.
2. Materials and Methods
All reagents and solvents were obtained from commercial sources and used as received. Isomeric mixture of approximately 1 : 1 cis/trans-dichlorobis(piperidine-1-carbonitrile)platinum(II) was prepared in accordance with the published method [4]. Infrared spectrum was recorded on a Shimadzu FTIR 8400S instrument in KBr pellets. 1H and 13C NMR spectra were measured on a Bruker-DPX 300 and Bruker 400 MHz Avance spectrometers at ambient temperature. Electrospray ionization mass spectrum was obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source and MeOH was used as the solvent. The instrument was operated at positive ion mode using a m/z range of 50–3000.
2.1. X-Ray Crystal Structure Determination
The crystal of of trans-[PtI2(NCNC5H10)2] was immersed in cryo oil, mounted in a Nylon loop, and measured at a temperature of 100 K. The X-ray diffraction data was collected on Bruker Kappa Apex II Duo diffractometer using Mo Kα radiation (λ = 0.71073 Å). The SAINT program package [5] was used for cell refinement and data reduction. The structure was solved by direct methods using SHELXS-97 [6] program with a WinGX [7] graphical user interface. A numerical absorption correction (SADABS) [8] was applied to data. Structural refinement was carried out using SHELXL-97 [6]. Hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C–H = 0.99 and Uiso = 1.2·Ueq (parent atom). The crystallographic details are summarized in Table 1 and selected bond lengths and angles in Table 2.
2.2. Preparation of trans-[PtI2(NCNC5H10)2]
Water solution (0.5 mL) of KI (41 mg, 0.25 mmol) was added to a suspension of cis/trans-[PtCl2(NCNC5H10)2] (49 mg, 0.1 mmol) in MeOH (0.5 mL). The reaction mixture was intensively stirred for 40 min (20–25°C) and then was left to stand at −5°C for 3 h. The resulting solid was filtered off and washed with cold methanol/water mixture (two 0.5-mL portion) and dried on air to give trans-[PtI2(NCNC5H10)2] as the light-yellow solid (47 mg, 70%).
Elemental analysis, Pt: 29.27% (29.15%, calcd). High resolution ESI+-MS, m/z: 542.0331 [M–I]+ (542.0381 calcd), 691.9263 [M+Na]+ (691.9323 calcd). IR spectrum in KBr, selected bands, cm−1: 2941 s, 2864 s ν(C–H), 2293 s ν(C≡N). 1H NMR (300.13 MHz, CDCl3): δ 3.32 (t, 8H, NCH2), 1.76–1.65 (m, 8H, NCH2CH2), 1.65–1.56 (m, 4H, NCH2CH2CH2). 13C{1H} NMR (100.61 MHz, CDCl3): δ 118.8 (NCN), 49.9 (NCH2), 24.7 (NCH2CH2), 22.6 (NCH2CH2CH2) (Scheme 1).
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3. Results and Discussion
3.1. Synthesis of trans-[PtI2(NCNC5H10)2]
General route to iodide platinum(II) complexes bearing two neutral ligands is the exchange reaction between K2[PtCl4] and excess KI in water followed by reaction with neutral ligands [9–12]. In our case, this reaction was unselective and furnished the target compound in low yield with concomitant formation of the platinum black as the by-product.
Trans-diiodobis(piperidine-1-carbonitrile)platinum(II) complex (trans-[PtI2(NCNC5H10)2]) was obtained as the light-yellow solid by the reaction of dichlorobis(piperidine-1-carbonitrile)platinum(II) with potassium iodide in water/methanol mixture under mild conditions (20°C, 40 min) in 70% isolated yield. Strong trans-effect of iodide ligands determines the selective generation of the trans-isomer and allows the use of the cis/trans mixture of the starting of [PtCl2(NCNC5H10)2] complex [4]. The advantages of this method are also mild reaction conditions, facile isolation, and high yield of the reaction product.
3.2. Characterization of trans-[PtI2(NCNC5H10)2]
In ESI+ mass spectrum, only two intensive signals with characteristic isotopic pattern of [M−I]+ and [M+Na]+ were detected. In IR spectrum of the title compound, the most characteristic band is from C≡N stretching vibrations (2293 cm−1). This value is higher than that in free 1-piperidinecarbonitrile (2210 cm−1) [13] and is very close to ν(C≡N) bond in trans-[PtCl2(NCNC5H10)2] (2292 cm−1) [4]. In 1H NMR, only one set of signals corresponding to the methylene groups of the piperidine ring was observed. The most characteristic signal in 13C NMR spectrum, sensitive to structure changes, is the one from NCN fragment at 118.8 ppm.
A single-crystal X-ray diffraction study was performed for a crystal grown by the slow evaporation of CH2Cl2/nBu2O (2 : 1, v/v) solution of trans-[PtI2(NCNC5H10)2] (Figure 1). The coordination polyhedron of the compound is formed by two piperidine-1-carbonitrile ligands in the trans-orientation and two iodide ligands, resulting in a typical square planar geometry.
Bond angles around the platinum(II) center are very close to 90° (89.31(5)° and 90.69(5)°). The Pt(1)–I(1) distances (2.6117(2) Å) are typical for Pt–I bonds [14, 15]. The Pt(1)–N(1) (1.9537(15) Å) bond length is equal to such bonds in similar platinum(II) complexes bearing push-pull nitriles [4, 16]. The Pt(1)–N(1)–C(1) and N(1)–C(1)–N(2) fragments are nearly linear with bond angles of 178.37(15)° and 178.1(2)°, correspondingly. The bond distance N(1)–C(1) (1.153(2) Å) is equal, within 3σ, to the distance in similar complexes with push-pull NCNR2 [4, 16] or with conventional NCR nitriles [17, 18]. The bond C(1)–N(2) (1.314(2) Å) is shorter than a regular single C–N bond and it is close to the typical C=N bond [19]. This could be explained by a significant contribution of the structure Pt–N−=C=N+R2 with double bond C(1)=N(2) [20]. This data is in agreement with slightly nonlinear structure of the fragment Pt(1)–N(1)–C(1) and it is also supported by the previous investigations of complexes of similar structures [4, 16].
The angles around the N(2) atom are close to 120° varying from 116.18(13)° to 119.89(15)° thus pointing out the sp2-hybridization of the N(2) atom and the amide character of the NR2 group. The piperidine ring has a typical chair conformation and all bond lengths are usual for single C–C bonds (ranging from 1.517(3) Å to 1.529(3) Å) [19].
4. Conclusion
The novel complex trans-[PtI2(NCNC5H10)2] bearing the push-pull nitrile ligands was prepared in high yield and characterized by IR, 1H and 13C NMR spectroscopy, and high resolution ESI-MS. Proposed structure was confirmed by single crystal X-ray diffraction. The trans-effect of I− ligands determines the selective generation of trans-isomer.
Platinum has square planar environment with almost linear fragment Pt(1)–N(1)–C(1)–(N2). The bond length C(1)–N(1) is close to double C=N bond that is caused by significant contribution of N−=C=N+R2 structure stabilized by the platinum(II) center.
Acknowledgments
The authors would like to thank the Russian Fund for Basic Research (Grants 12-03-00076 and 12-03-33071) and Saint Petersburg State University for the research Grant 2013–2015 (12.38.781.2013). The authors also acknowledge the Centre for Magnetic Resonance of Saint Petersburg State University for performing of NMR studies. L. D. Funt is thanked for experimental assistance at an early stage of the project.