Journal of Crystallography

Volume 2013 (2013), Article ID 312310, 4 pages

http://dx.doi.org/10.1155/2013/312310

Research Article

Synthesis and Crystal Structure of trans-Diiodobis(piperidine-1-carbonitrile)platinum(II)

Aleksey L. Mindich,¹ Matti Haukka,² and Nadezhda A. Bokach¹

¹Department of Chemistry, Saint Petersburg State University, Universitetsky Pr. 26, Stary Petergof 198504, Russia

²Department of Chemistry, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland

Received 24 May 2013; Accepted 3 October 2013

Academic Editors: M. Akkurt, J. Jasinski, T. Mino, P. R. Raithby, and E. Suresh

Copyright © 2013 Aleksey L. Mindich et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Table 1: Crystallographic details for trans-[PtI₂(NCNC₅H₁₀)₂].

Empirical formula
C₁₂H₂₀I₂N₄Pt

Fw
669.21

Temp. (K)
100(2)

()
0.71073

Cryst syst
Monoclinic

Space group
2₁/c

a ()
6.7497(7)

b ()
15.8212(16)

c ()
8.2773(8)

(deg)
104.110(4)

()
857.25(15)

Z
2

(Mg/m³)
2.593

(Mo K) (mm⁻¹)
11.779

Number of reflections
27391

Unique reflections
4203

GOOF (F²)
1.229

0.0257

R1a ()
0.0160

wR2b ()
0.0377

R1 = . wR2 = [/ ]1/2.

Table 2: Selected bond lengths and angles (Å, °).

Pt(1)–N(1)
1.9537(15)
N(1)–Pt(1)–I(1)#1
90.69(5)

Pt(1)–I(1)
2.6117(2)
N(1)–Pt(1)–I(1)
89.31(5)

N(1)–C(1)
1.153(2)
N(1)–C(1)–N(2)
178.1(2)

N(2)–C(1)
1.314(2)
C(1)–N(1)–Pt(1)
178.37(15)

N(2)–C(6)
1.477(2)
C(1)–N(2)–C(6)
119.89(15)

N(2)–C(2)
1.478(2)
C(1)–N(2)–C(2)
117.16(15)

C(6)–N(2)–C(2)
116.18(13)

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, NCH ₂), 1.76–1.65 (m, 8H, NCH₂CH ₂), 1.65–1.56 (m, 4H, NCH₂CH₂CH ₂). ¹³C{¹H} NMR (100.61 MHz, CDCl₃): δ 118.8 (NCN), 49.9 (NCH₂), 24.7 (NCH₂CH₂), 22.6 (NCH₂CH₂CH₂) (Scheme 1).

Scheme 1: Preparation of trans-[PtI₂(NCNC₅H₁₀)₂].

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.

Figure 1: The molecular structure of trans-[PtI₂(NCNC₅H₁₀)₂], showing the atom-labeling scheme and displacement ellipsoids drawn at the 50% probability level.

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.

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