Synthesis, Structure, and Solution Dynamic Behavior of Nickel

Article pubs.acs.org/Organometallics

Synthesis, Structure, and Solution Dynamic Behavior of Nickel Complexes Bearing a 1,3-Diallyl-Substituted NHC Ligand Agata Włodarska, Andrzej Kozioł, Maciej Dranka, Adam Gryff-Keller, Przemysław Szczeciński, Jakub Jurkowski, and Antoni Pietrzykowski* Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland S Supporting Information *

ABSTRACT: Reaction of 1,3-diallylimidazolium bromide with 1,1′-dimethylnickelocene (NiCp′ 2) afforded [NiCp′(all2NHC)]+Br− (1a) (all2-NHC = 1,3-diallylimidazol-2-ylidene) and NiCp′(all2-NHC)Br (1b), while the reaction of 1,3diallylimidazolium chloride with nickelocene gave NiCp(all2NHC)Cl (2) and [NiCp(all2-NHC)2]+Cl− (3). Molecular structures of all four complexes have been established by single-crystal X-ray diffraction. 1H NMR spectra of 1b indicated hindered rotation about the Ni−carbene bond at room temperature, which resulted in the nonequivalence of N−CH2 protons. For compounds 2 and 3 the spectra were dynamic at room temperature and the nonequivalence of N−CH2 protons appeared at low temperature. These observations were confirmed by DFT calculations. ince the first stable, crystalline carbene was isolated by Arduengo in 1991, the chemistry of this ligand has evolved considerably and N-heterocyclic carbenes have become an important class of ligands in organometallic chemistry.1,2 Of the many transition metal NHC complexes, those containing nickel have been rather neglected, even though it appears that they might exhibit interesting properties and enhanced catalytic activity.3 In particular, ionic nickel NHC complexes have been relatively unexplored.4 Our interest has focused on hemilabile complexes because of their unique role in catalysis.5 Hahn and co-workers reported that the reaction of nickelocene with 1,3-diallylimidazolium bromide in THF gave a neutral complex, NiCp(all2-NHC)Br.6 The product was then transformed, in the reaction with AgBF4 in dichloromethane, to the ionic complex containing one allylic double bond coordinated to the nickel atom. By contrast, when we attempted to synthesize an ionic form of NiCp(all2-NHC) Br in THF, only the neutral complex was formed, even at ambient temperature.7 However, the reaction of equimolar amounts of the more electron rich 1,1′-dimethylnickelocene with 1,3-diallylimidazolium bromide in THF at room temperature led to the formation of two forms of 1: ionic [NiCp′(all2NHC)]+Br− (1a) and neutral NiCp′(all2-NHC)Br (1b) (Scheme 1). Differences in the solubility of the two complexes allowed facile separation. The ionic form 1a is soluble in acetonitrile, while the neutral form 1b is soluble in toluene. The complexes were crystallized from their respective acetonitrile and toluene solutions, and subsequently their solid-state structures were established by single-crystal X-ray diffraction. The molecular

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© XXXX American Chemical Society

Scheme 1. Formation of 1a and 1b in the Reaction of 1,1′Dimethylnickelocene with 1,3-Diallylimidazolium Bromide

structures of 1a and 1b are presented in Figures 1 and 2. Their crystal data, structure refinement parameters, and selected

Figure 1. Ortep plot of 1a with atom-numbering scheme. Received: September 12, 2014

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the same reason all protons and carbons of cyclopentadienyl ligands are equivalent. There are only a few examples of cyclopentadienylnickelNHC complexes bearing two NHC ligands.4a,e,g Herein, we report the synthesis of a biscarbene complex in a direct reaction of a carbene salt and nickelocene (Scheme 2). The reaction of nickelocene with 1,3-diallylimidazolium chloride afforded red neutral complex 2 (37% yield) and green ionic 3 (51% yield). Scheme 2. Formation of 2 and 3 in the Reaction of Nickelocene with 1,3-Diallylimidazolium Chloride

Figure 2. Ortep plot of 1b with atom-numbering scheme.

bonds and angles are given in the Supporting Information. Complex 1a crystallizes as a monoclinic crystal system with space group P21 (Z = 4). The structure determination revealed an ion-separated structure, with discrete ions: a NiCp′(all2NHC)]+ cation and nonsolvated bromide anion Br−. There are two almost identical crystallographically independent cations in the asymmetric unit. The bond lengths and bond angles in both molecules are similar. In view of this, the following discussion is limited to just one of them. An overlay of 1a molecules showing differences in their molecular geometry and conformation along with the comparison of bond distances is given in the Supporting Information. The independent cations feature two different allyl groups. One allyl substituent of the NHC ligand has a η2-coordinated CC double bond, while the second allyl group is dangling. According to the NMR data, the connectivity in the cations is preserved in solution. The geometry around the nickel center can be described as trigonal planar by taking the Cp′cent, the NHC carbene atom, and the center of the coordinated CC bond as vertices of the trigonal plane (the sum of bond angles around the Ni atom = 359.5(5)°; the distance of the Ni atom from the plane is 0.0748(6) Å). The Ni−Ccarbene and Ni−Cp′cent distances measure 1.895(5) and 1.733(1) Å, respectively, and are similar to those reported for [NiCp(all2-NHC)]+[BF4]−.6 The distances between the nickel atom and the carbon atoms of the allylic double bond measuring 2.031(5) and 2.036(5) Å are similar to those described for olefin double bonds, intramolecularly coordinated to nickel(II).6,8 Complex 1a is a good model for comparison of coordinated (C5−C6 = 1.399(7) Å) and noncoordinated (C8− C9 = 1.311(7) Å) olefin double-bond lengths. Complex 1b crystallizes as a monoclinic crystal system with space group P21/c (Z = 8). The geometry of this complex is trigonal planar at the nickel center considering Cp′cent, the carbene carbon, and the bromine atoms (the sum of bond angles = 359.8(3)°). The Ni−Ccarbene and Ni−Cp′cent distances measure 1.886(2) and 1.764(3) Å, respectively, and are similar to corresponding distances in 2 and those reported for other [NiCpX(NHC)] complexes.3c,d,f,7 1 H NMR spectra of 1a and 1b reveal nonequivalence of N− CH2 protons of allyl substituents, which indicates that rotations about the Ni−Ccarbene bond are hindered at ambient temperature. This phenomenon was also reported by Hahn and coworkers.6 The rapid rotation of the methylcyclopentadienyl ligand and the symmetry plane of the rest of the molecule result in equivalence of the appropriate 1H and 13C nuclei of Cp′. For

Compound 2 crystallizes in space group P21/n (Z = 4). The molecular structure is presented in Figure 3; the crystal data,

Figure 3. Ortep plot of 2 with atom-numbering scheme.

structure refinement parameters, and selected bonds and angles are given in the Supporting Information. The molecular structure of 2 does not differ significantly from complex 1b with regard to the coordination mode and geometry. The geometry of the complex is trigonal planar at the nickel center considering Cp′cent, the carbene carbon, and the chlorine atoms (the sum of bond angles = 359.9(3)°). The Ni−Ccarbene and Ni−Cp′cent distances measure 1.876(2) and 1.770(1) Å, respectively, and are similar to corresponding distances in 1b and those described in the literature for other [NiCpX(NHC)] complexes.3c,e,g,7 Contrary to the ionic complex 1a, which was soluble in THF, complex 3 precipitated from THF solution during the reaction. Therefore, it can easily be isolated at high yield. Complex 3 crystallizes from acetonitrile solution in monoclinic space group P21/n (Z = 4). The molecular structure of 3 is shown in Figure 4. Crystal data, structure refinement parameters, and selected bonds and angles are given in the Supporting Information. Complex 3 is a biscarbene complex comprising a chloride anion and a cationic nickel complex with η5-Cp and two NHC ligands. There are no close cation−anion contacts, and the geometry at the metal center is trigonal planar; the sum of the bond angles is 360.0(3)°. However, the C(1)−Ni(1)−C(10) angle of 96.1(1)° is significantly smaller than the idealized 120° B

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smaller coupling constants of the CH2 protons with the olefinic one are invisible. The described details of the NMR spectra of the complexes NiCp(all 2 -NHC)Br, 2, and 3, are the consequences of their conformational behavior in the applied measurement conditions. The DFT calculations, performed for model compounds in which allyl substituents have been replaced by methyl ones, support the rationalization of these observations. Indeed, these calculations show that there are two equivalent minimum energy conformations with the imidazoyl ligands roughly perpendicular to the plane defined by three nickel bonds. Fast exchange between these conformations explains the equivalence of allylic substituents and imidazoyl ring protons. An obvious explanation of the apparent isochronism of the CH2 protons of 3, which is observed at room temperature, is the fast (on the NMR time scale) rotation of carbenoid ligands about their Ni−C bonds. Indeed, the sufficient lowering of the measurement temperature and slowing of the internal rotation result in the expected changes of the spectral pattern involved. It is worth emphasizing that DFT calculations on a model compound are in good agreement with the geometry of 3 observed in the crystal structure (see the Supporting Information, Figures S5 and S6). One may ask why at room temperature the rotation about the Ni−C bond is fast in complexes 3 and 2, while in NiCp(all2-NHC)Br it is slow. Additional DFT calculations of the energies for conformations that have to be passed through during the rotation of the imidazoyl rings provide some explanation for those observations. In particular, it is to be expected that, in the cases of NiCp(all2-NHC)Br and complex 2, the state with the imidazoyl ring coplanar with the plane occupied by Ni−ligand bonds responds to the maximum of the energy curve describing the internal rotation involved. Similarly, the rotation of the imidazoyl rings in 3 has to go through the state in which one of the rings is coplanar with the plane of the Ni−ligand bonds. We have found that the appropriate energy differences (activation barriers) are 52.0, 43.9, and 29.9 kJ/mol for the methyl analogues of NiCp(all2-NHC)Br, 2, and 3, respectively. Although the energetic barriers to the imidazoyl ring rotation in the allylic complexes can differ somewhat from the above values, nevertheless the obtained results follow the order of the rotation rates observed by NMR for the studied complexes.

Figure 4. Ortep plot of 3 with atom-numbering scheme.

angles of a trigonal geometry; the observed value is typical for nickel Cp complexes. Corresponding angles C(1)−Ni(1)− Br(1) in 1b and C(1)−Ni(1)−Cl(1) in 2 measure 97.1(1)° and 95.1(1)°, respectively, and are similar to those in 3. The five-membered carbene rings are nearly perpendicular to each other, with an angle of 87.6(1)°, and their planes are oblique at dihedral angles of 74.4(1)° and 66.9(1)° with respect to the Cpcent−Ni(1)−C(1)−C(10) plane such that the observed geometry is approximately C2. Twisting of the rings occurs most probably to minimize steric hindrance of allyl substituents in the solid state and can support internal rotation of the Cp ring. It is worth noting that the cyclopentadienyl group in 3 exhibits crystallographic disorder. Structural distortions of the Cp rings are observed when the molecule possesses C2 or lower rotational symmetry due to broken degeneracy in the doubly degenerate e orbitals.9 This could lower the energy barrier to internal rotation of the ligand rings. A sample of 3, the NMR spectrum of which has been measured, was contaminated with 1,3-diallylimidazolium chloride. Diverse attempts to remove this contamination failed. Fortunately, the signals of 3 and 1,3-diallylimidazolium chloride were sufficiently separated to be distiguishable. The unambiguous assignment of the signal sets coming from both compounds was made on the basis of changes of relative intensities in the spectrum recorded for the sample to which a portion of the 1,3-diallylimidazolium chloride was added. Some observations concerning the proton NMR spectra of complexes NiCp(all2-NHC)Br, 2, and 3, are worth commenting upon in more detail, since this should shed some light on the properties of the still somewhat mysterious bonds between metal and a carbene ligand. In all cases the imidazole ring protons of these compounds give singlet signals in the spectra, and only one set of vinylic proton signals is observed. On the other hand, the form of the signals of N−CH2 protons is significantly different for these three compounds at room temperature. These protons give two doublets of doublets (AB part of the ABX spin system) for NiCp(all2-NHC)Br, an extremely broad signal, the maximum of which cannot be established precisely, for 2 and a broadened doublet (dynamically broadened A part of A2X spin system) for 3. However, for complexes 2 and 3 the nonequivalence of N− CH2 protons does appear at low temperature. The discussed signal converts into two doublets of doublets for 2 and two broadened, AB-like doublets for 3 (see the Supporting Information). In the case of complex 3, at the applied temperature, the spectrum is still dynamic, and therefore the

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CONCLUSIONS Covalent and ionic nickel complexes bearing one or two 1,3diallyl-substituted NHC ligands can be directly obtained in reactions of nickelocene or 1,1′-dimethylnickelocene with (bis)allylimidazolium halides. The change of the cyclopentadienyl groups in nickelocene for the more electron rich methylcyclopentadienyl ones facilitates the formation of ionic complexes, and the change of the bromide anion for chloride in the imidazolium salt allows the formation of the ionic cyclopentadienylnickel complex bearing two 1,3-diallyl-substituted NHC ligands. 1 H NMR spectra reveal interesting dynamic behavior of the studied compounds in solution. Rotations about the Ni− Ccarbene bond in the complexes NiCp(all2-NHC)Br and NiCp′(all2-NHC)Br (1b) are hindered at ambient temperature, while in NiCp(all2-NHC)Cl (2) and also in [NiCp(all2NHC)2]+Cl− (3), with two bulky NHC ligands, the rotation is fast. DFT calculation results confirmed this quite unexpected order of the rotation rates in the studied complexes. C

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suitable for single-crystal X-ray diffraction were grown. Yield: 0.228 g, 31%. [NiCp′(all2-NHC)]+Br− (1a). 1H and 13C NMR data and spectra are given in the Supporting Information. Anal. Calcd for C15H19BrN2Ni: C, 49.24; H, 5.23; N, 7.66. Found: C, 49.66; H, 5.40; N, 7.98. ESIHRMS: calcd for [NiCp′(all2-NHC)]+ = C15H19N258Ni 285.0902; found 285.0900. NiCp′(all2-NHC)Br (1b). 1H and 13C NMR data and spectra are given in the Supporting Information. Anal. Calcd for C15H19BrN2Ni: C, 49.24; H, 5.23; N, 7.66. Found: C, 49.53; H, 5.46; N, 7.38. EI-MS (70 eV) m/z (relative intensity) (58Ni, 79Br): 364 (M+, 46%), 285 ([M − Br]+, 48%), 216 ([Cp′NiBr]+, 88%),136 ([Cp′Br]+, 97%). EIHRMS: calcd for C15H1979BrN279Br58Ni 364.0085; found 364.0076. Synthesis of NiCp(all2-NHC)Cl (2) and [NiCp(all2-NHC)2]+Cl− (3). A solution of nickelocene (0.412 g, 2.18 mmol) in THF (30 mL) was added to 1,3-diallylimidazolium chloride (0.603 g, 3.27 mmol). The mixture was stirred for 24 h at ambient temperature. The solution slowly turned color from dark green to dark red. After decanting the red solution, a dark green solid remained as a residue. The solvent was evaporated from the red solution, and the remaining solid was washed with hexane (3 × 20 mL) in order to remove unreacted nickelocene. The resulting residue was extracted with toluene (3 × 10 mL), and the solution was reduced in volume to about 5 mL. After standing overnight at 4 °C red crystals of 2, suitable for X-ray measurements, were obtained. Yield: 0.247 g, 37%. The green solid was washed with THF (3 × 20 mL), dissolved in acetonitrile (20 mL), and filtered. After concentrating to about 5 mL and keeping overnight at 4 °C green crystals of 3, suitable for single-crystal X-ray diffraction, were grown. Unfortunately, complex 3 could not be cleanly separated from the imidazolium salt. We have made a lot of effort to isolate pure complex 3, trying various methods. The main problem is that the imidazolium salt is an oil, soluble in the same solvents as complex 3. Crystal picking allowed us to determine the crystal structure, but the crystals were still covered with the oil, and washing them in order to prepare a pure solution for NMR measurements was unsuccessful. Yield (impure): 0.506 g, 51%. NiCp(all2-NHC)Cl (2). 1H and 13C NMR data and spectra are given in the Supporting Information. Anal. Calcd for C14H17ClN2Ni: C, 54.69; H, 5.57; N, 9.11. Found: C, 55.02; H, 5.72; N, 9.53. EI HRMS: calcd for C14H1735ClN258Ni 306.0432; found 306.0427. [NiCp(all2-NHC)2]+Cl− (3). 1H and 13C NMR data and spectra are given in the Supporting Information. ESI-HRMS: calcd for [NiCp(all2NHC)2]+ = C23H29N458Ni 419.1746; found 419.1746.

EXPERIMENTAL SECTION

General Information. All manipulations were carried out using standard Schlenk techniques under an atmosphere of purified argon. All solvents were purified and dried by standard procedures and distilled under argon. Toluene, THF, cyclohexene, acetonitrile, and dichloromethane were dried over potassium, KOH, CaH2, and P2O5, respectively, and distilled under argon before use. Nickelocene,10 1,1′dimethylnickelocene,11 1,3-diallylimidazolium bromide,12 and 1allylimidazole 13 were synthesized according to the literature procedures. All other chemicals were purchased from commercial suppliers and used without further purification. NMR spectra were recorded at ambient temperature on a Mercury-400BB spectrometer operating at 400 MHz for 1H NMR and at 101 MHz for 13C NMR and Varian NMR System 500 MHz. EI (70 eV) mass spectra were recorded on an AMD-604 spectrometer. ESI mass spectra were recorded on a Waters Maldi SYNAPT G2-S HDMS. The molecular geometry optimizations without constraints and with the constant value of one dihedral angle were performed for isolated molecules by the DFT method using the hybrid B3LYP14 functional and 6-31+G* basis set. Next, for the found geometries the energies were calculated using the 6-311++G(2d,p) basis. All calculations were performed using the Gaussian 03 program.15 Crystal Structure Determination. The selected single crystal was mounted in inert oil and transferred to the cold gas stream of the diffractometer. Diffraction data were measured at 120.0(2) K with graphite-monochromated Mo Kα radiation (λ = 0.710 73) on an Oxford Diffraction κ-CCD Gemini A Ultra diffractometer. Cell refinement and data collection as well as data reduction and analysis were performed with the CrysalisPro software.16 The structure was solved by direct methods using the SHELXS-97 structure solution program and refined by full-matrix least-squares against F2 with SHELXL-201417 and OLEX218 programs. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms. Structure 1a was refined as a two-component inversion twin with a Flack parameter equal to 0.450(11). Thermal ellipsoids for compounds 1−3 (Figures 1−4) are drawn at the 50% probability level. Hydrogen atoms were omitted for clarity. Crystallographic data for compounds 1−3 are given in the Supporting Information.These data have also been deposited with the Cambridge Crystallographic Data Centre, Nos. CCDC 1020123− 1020126. They can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Synthesis of 1,3-Diallylimidazolium Chloride. Allyl chloride (9.2 mL, 108 mmol) was added to 1-allylimidazole (11.664 g, 108 mmol) in acetonitrile (50 mL). The solution was refluxed for 5 h. The solvent was evaporated under reduced pressure. The resulting dark brown oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield: 65% (12.951 g). 1H NMR (400 MHz, CDCl3): δ = 9.50 (s, 1H, NCHN), 7.01 (s, 2H, NCHCHN), 5.21 (m, 2H, −CH), 4.63 (d, 2H, CHHtrans, 3JH,H = 16.8 Hz), 4.54 (d, 2H, CHHcis, 3JH,H = 8.4 Hz), 4.24 (d, 4H, NCH2−, 3JH,H = 4.8 Hz). 13C (100 MHz, CDCl3): δ = 134.8 (NCN), 128.6 (NCCN), 120.8 (−CH), 120.2 (CH2) 49.9 (NCH2−). ESI-MS (m/z): 149.11 [M − Cl]+. ESI-HRMS: calcd for [C9H13N2]+ 149.1077; found 149.1085. Synthesis of [NiCp′(all2-NHC)]+Br− (1a) and NiCp′(all2-NHC) Br (1b). A solution of 1,1′-dimethylnickelocene (0.437 g, 2.01 mmol) in THF (30 mL) was added to 1,3-diallylimidazolium bromide (0.465 g, 2.03 mmol). The mixture was stirred for 6 days at room temperature. The color of the solution changed slowly from dark green to dark brown. The solvent was removed under reduced pressure, and the residue was washed with hexane (3 × 20 mL). The resulting brown solid was extracted with toluene (3 × 10 mL), and the solution was concentrated to 5 mL. After standing at 4 °C crystals of 1b suitable for single-crystal X-ray diffraction were grown. Yield: 0.264 g, 36%. The remaining residue was dissolved in acetonitrile (20 mL) and filtered. After concentrating and standing at 4 °C crystals of 1a

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ASSOCIATED CONTENT

S Supporting Information *

CIF file, figures of asymmetric units, tables of bond lengths and angles for compounds 1−3, NMR data and spectra of 1−3 and NiCp(all2-NHC)Br. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A. Pietrzykowski). Tel: +48 22 2347116. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Science Centre (grant no. N N205 012234). REFERENCES

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