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Angew Chem Int Ed Engl. Author manuscript; available in PMC 2017 July 27.
Published in final edited form as:
PMCID: PMC5531756
NIHMSID: NIHMS884215

[Ni(cod)2][Al(ORF)4], a Source for Naked Ni(I) Chemistry

Abstract

The straightforward synthesis of the cationic, purely organometallic NiI-salt [Ni(cod)2]+[Al(ORF)4] was realized through a reaction between Ni(cod)2 and Ag[Al(ORF)4]. Crystal structure, EPR, XANES and cyclic voltammetry confirmed the presence of a homoleptic NiI-olefin complex. Weak interactions between the metal centre, the ligands and the anion provide a good starting material for further cationic NiI-chemistry.

Keywords: Weakly coordinating anion, nickel(I), crystallography, DFT, olefin complexes, EPR, cyclic voltammetry

Graphical abstract

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Homoleptic Ni(I) Olefin Complex: Straightforward access to a solvent-free organometallic Ni(I)-complex was accomplished by the reaction between Ni0(cod)2 and Ag[Al(ORF)4]. Crystal structure, EPR, XANES, and cyclic voltammetry confirmed the salt as [Ni(cod)2]+[Al(ORF)4], which is a good starting material for further cationic Ni(I)-chemistry.

Nickel is traditionally used in many homogeneous catalyses,[1] e.g. in the Reppe carbonylation, cyclotetramerization of acetylene, di- or trimerization of ethylene, as well as the Shell Higher Olefin Process (SHOP), where the catalytically active species are Ni0- and NiII-compounds.[2] However, other oxidation states of nickel are known as intermediates or isolated compounds.[3] With a d9-electron configuration, mononuclear NiI is a rather uncommon oxidation state. Previously isolated NiI-compounds were typically stabilized by electron rich ligands such as phosphanes[4], amines[5], carbenes[6], β-diketiminates[4g,7], Cp (Cyclopentadienyl)[6c,8], or were incorporated in alumo-phosphates[9]. All mononuclear compounds include strongly σ-donating C-, N-, P-, S-, O-, or halogen atoms in their ligands.[4,5,6,7,8,10] An open question is, if NiI leads to better performance in catalysis than Ni0 or NiII. Towards this goal, Stephan's dinuclear NiI β-diketiminates were used by Driess and Limberg for small molecule activation.[7k,10c,10d,10e,10k,10I] In addition, NiI-catalysts were used in Kumada cross-couplings,[6g,10b] and olefin oligomerization or polymerization reactions.[10g–j] Nevertheless, knowledge regarding NiI-olefin complexes is very scarce. The first report of a NiI-olefin complex was the marginally stable (cod)NiIX (cod = 1,5-cyclooctadiene; X = Br, I),which was published in 1967 without any characterization.[11] Later Saraev et al. described a poorly characterized NiI-olefin-species as an intermediate in EPR studies starting from Ni(cod)2.[12]

In order to study an unknown homoleptic olefin coordination sphere of NiI, by analogy with the “naked” Ni0 complexes of Wilke et al.,[13] our aim was to produce a stable NiI-olefin-complex cation in combination with a weakly coordinating anion (WCA). The closest known approximation to this goal so far is Grützmacher's complex[NiI(trop2NH)(OOCCF3)] (trop2NH = bis (5H-dibenzo[a,d]cyclo-hepten-5-yl)amine[10a] and the intermediate NiI(cod)x-species of Saraev.[12] A straightforward access to the [Ni(cod)2]+-salt of the very weakly coordinating perfluoro-tert-butoxy aluminate [Al(ORF)4] (ORF = OC(CF3)3) 1 is provided by the oxidation of Ni0(cod)2 with Ag[Al(ORF)4] in CH2Cl2 at room temperature (Scheme 1, orange crystals in 61 % yield after recrystallization; calculated to be exergonic at COSMO/PBE0/def2-TZVPP, details in Fig. S 3).

equation image
(1)

Crystal structure, IR, EPR, XANES, and cyclic voltammetric (CV) measurements confirmed the existence of a homoleptic NiI-cod-complex. The powdered product is stable at room temperature and – astonishingly – did not show oxygen or air sensitivity over weeks. By contrast, in solution the salt 1 is highly sensitive towards dioxygen. Weakly coordinating solvents like CH2Cl2 or o-difluorobenzene (o-DFB) neither replace the cod-rings nor coordinate to the nickel centre. CV-measurements of 1 in o-DFB showed an electrochemically irreversible oxidation for the redox pair NiI/II at E1/2 = +0.962 V vs. Fc/Fc+ (k0 = 2.4 · 10−4 cm·s−1, Table S 8). The reduction of NiI at E1/2 = −0.7 V vs. Fc/Fc+ includes a more complicated two electron transfer, which will be analysed later. In THF solution, 1 disproportionated to black solid Ni0 and yellow dissolved [NiII(THF)6][Al(ORF)4]2. The molecular structure of 1 was determined by single crystal x-ray crystallography and is shown in Figure 1a.

Figure 1
a) Molecular structure of 1. Thermal ellipsoids are shown at the 50% probability level. b) Cationic parts of the molecular structures of [Rh(cod)2]+[Al(ORF)4] (3), [Ni(cod)2]+[Al(ORF)4] (1) and [Ag(cod)2]+[Al(ORF)4] (2) at 100 ...

The overall structure of 1 is in-between tetrahedral and square-planar. More precisely, the torsion angle θ of the planes defined by the C=C bond centroids of adjacent cod-rings is 53.1° (NiI, d9, Figure 1b). For comparison we also prepared and crystallized tetrahedral [Ag(cod)2]+[Al(ORF)4] (2, d10, θ = 88.5°, Figure 1b), as well as square planar [Rh(cod)2]+[Al(ORF)4] (3, d8, θ = 10.0°; Figure 1b). Neutral Ni(cod)2 has a torsion angle of 84.5° (d10).[14] Compared to Ni(cod)2, the Ni-C bonds in 1 are elongated by 6 to 15 pm and the C=C bonds are actually shorter in 1 by 3 to 4 pm. They are within 135 to 136 pm, similar to the undistorted C=C double bonds in free cod (dc=c = 134 pm[15]). This is probably induced by the positively charged Ni atom that allows only minimal π-back bonding. The NMR-spectroscopic analysis and quantum chemical calculations support this hypothesis: the olefinic proton signal in the 1H-spectrum (Fig. S 6) bears no paramagnetic shift, which would be present if there were a Fermi-contact-interaction with the SOMO of nickel. This suggests that the unpaired electron-spin density is mainly centred on the metal. Calculations at the PBE0/def2-TZVPP level reproduce the molecular structure well within 0.6° (torsion angle) and 4 pm (Ni-C-distances; Fig. S 1, Table S 1), as well as a Mulliken population analysis (Table S 3, PBE0/def2-TZVPP, but also B3LYP/def2-TZVPP) localizes over 90% of the cations spin density on the nickel centre (Figure 2a, inset). EPR measurements of a concentrated solution of 1 in CH2Cl2 with a non-reactive ionic liquid ([MeN(Octyl)3]+[Al(ORF)4], 0.1 M) as a glass forming additive that prohibits aggregation and an ordered orientation of the ions in frozen solution, showed the typical signal of a nickel atom with d9 configuration (Figure 2a).[12] The experimental spectrum was simulated with g-tensor principal values of gz = 2.390, gy = 2.061 and gx = 2.047 (dashed line). After magnification of parts of the experimental spectrum, an additional small signal from a second component became visible (5 % signal intensity).* Similarly to the spectrum of a frozen solution (Figure 2a), a powdered sample of 1 shows contributions from two components (Fig. S 5). Importantly, the g-tensor components of the main species in the solid state (90% of the signal) are very similar to those of the main component in the frozen solution (a comparison of all experimental g-tensors is given in Table S 7).

Figure 2
a) Continuous-wave EPR spectrum of a frozen solution of 1 in CH2Cl2 with ionic liquid additive to facilitate glass formation ([MeN(Octyl)3]+[Al(ORF)4], 0.1 M) at X-band (9.400 GHz) at 100 K (solid line). Simulated spectra using parameters given ...

X-ray absorption near-edge spectroscopic studies (XANES; Figure 2b) were performed at the Ni K-edge (on a powdered sample of 1 diluted in boron nitride) in order to directly probe the metal oxidation states in 1, and provide support for the EPR-derived NiI assignment. 1 exhibits an edge inflection energy of ca. 8341 eV, which is typical for nickel in the +1 oxidation state.[10m,16] A shoulder along the rising edge occurs at 8334.5 eV and corresponds to a 1s → 4p shakedown transition[16a], in accordance with the distorted structure in between tetrahedral and square-planar. This transition is strongest in four-coordinate square-planar Ni-complexes, but is also observed in five-coordinate square-pyramidal geometries (it is not present in either Td or Oh geometries).

It is important to note that spectra were collected at both 19 K and 298 K and they were found to be identical at both temperatures; this excluded the possibility of any temperature dependent spin- or oxidation-state isomerism in 1. The temperature dependence of the inverse magnetic susceptibility χ−1(T) of 1 in a magnetic field (Fig. S 11) follows Curie's law indicating independent spins on the Ni site. From the slope of the fit an effective magnetic moment of 1.86 ± 0.05 μB was calculated. Taken into account the average g-factor of 2.166 ± 0.112 (EPR), and assuming a spin ½ system with J = S = 0.5, led to the theoretical value of μeff = 1.876 ± 0.097 μB. The good agreement of the experimental result with theory is in line with a NiI metal atom in 1.

Salt 1 is a good starting material for other NiI-salts: In preliminary studies we substituted the cod-ligands of 1 by σ-donors like PPh3 and 1,3-bis(diphenylphosphino)propane (dppp). The formed compounds [Ni(PPh3)3]+[Al(ORF)4] / [Ni(dppp)2]+[Al(ORF)4] indicate the high potential of 1 as a precursor for further Ni(I) chemistry. Thus, we obtained a propitious NiI-salt generated in a direct oxidation route from commercially available chemicals. The stabilization by the [Al(ORF)4] WCA allows handling of 1 at room temperature and storage as air stable powder over months, as well as application in highly oxygen sensitive solutions in CH2Cl2 and 1,2-difluorobenzene. Preliminary experiments show a simple exchange of the olefin ligands to access a variety of new (possibly catalytically active) NiI-complexes, which will be investigated in an upcoming full paper.

Supplementary Material

ESI

Footnotes

**We thank Fadime Bitgül and Dr. Harald Scherer for measuring and evaluating NMR spectra, Melanie Wernet for her work on the [Ni(dppp)2][Al(ORF)4] salt, Britta Knaebel for crystallising the rhodium complex and Prof. Dr. B. Breit for supplying the rhodium source. This work was supported by the Freiburger Material for schungszentrum (FMF) and funded by the ERC project UniChem, number 291383. K.R. thanks the Cluster of Excellence “Unifying Concepts in Catalysis” (EXC 314/2), Berlin and the Heisenberg-Programm of the Deutsche Forschungsgemeinschaft for financial support. XAS data were obtained on NSLS beamline X3A (Brookhaven National Laboratory), with support from NIH Grant P30-EB-009998 and the U.S. Department of Energy. We thank Dr. Erik R. Farquhar for help with XAS data collection and Prof. Frank Breher (KIT) for recording the very first EPR spectra of 1.

Supporting Information: In the E.S.I. a detailed introduction to NiI-chemistry, the experimental section with specifics in synthesis, details of crystallographic data, cyclic voltammetric values, EPR spectra, NMR spectra, UV-Vis spectrum, IR spectra with a table of calculated and experimental vibrational frequencies, details of XANES and SQUID measurement, as well as calculation methods and other measurement details are given.

*This signal is not yet clearly identified, but presumably emerges from a geometrical isomer or side-product. It contributes to about 5% to the measured signal and was simulated with g-tensor components of gz = 2.240, gy = 2.055 and gx = 2.044.

The DFT calculated g-tensor components of the [Ni(cod)2]+ cation are clearly sensitive to the used geometry (Table S 4). While the computed gx and gy values are in reasonable fair agreement with the experimental values, the computed gz-components are underestimated. The DFT-calculated g-anisotropies for transition metal compounds are known to be underestimated, and errors of up to 50% in the g-shifts have been reported.[17] Calculated Δgz-components, shown in Table S 4, underestimate the experimental data by about 17-41% and are within the errors observed in literature. The deviations are likely resulting from an imperfect description of the metal-olefin interactions with density functional theory (and thereby the energy separation between the SOMO and the filled orbitals), which has also been noted for some other group-9 transition metal systems.[18]

Ag[Al(ORF)4] available at: www.iolitec.de

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