Anion Exchange in [ni(η 5 -c 5 H 4 R)(cl)(nhc)]. Counterion Effect on the Structure and Catalytic Activity

Supporting Information VT 1 H measurements of complex 3c In order to estimate the thermodynamic parameters of complex 3c a series of variable-temperature 1 H NMR experiments was performed in the temperature range from-55 to 100 ºC (Table S1). The function δ=f(1/T) used to fit the experimental data has the following form: where δ is the observed chemical shift, δ ls is the chemical shift fitted for diamagnetic species, ΔHº and ΔSº are the enthalpy and entropy, respectively, of the transition between low-spin and high-spin states, T is the absolute temperature in Kelvin, and C is a constant related to the molar susceptibility of the high-spin species. The experimental data and the fitting curve are shown in Figure S1. Final set of parameters with asymptotic standard errors is as follows: Table S1. Set of parameters used for fitting chemical shifts in 3c t [ºC] T [K] 1/T [1/K] Chemical shift of Cp [ppm]

However, reactions of 1a or 1b with AgCF 3 CO 2 in acetonitrile or toluene afforded complexes 3a and 3b with the coordinated carboxylate (Scheme 3).Attempts to extend this methodology to other carboxylates (acetate, pivalate) were not successful.
In contrast to AgCF 3 CO 2 , reaction of 1a with AgNO 3 in acetonitrile afforded cationic complex 2j.However, when this reaction was repeated in toluene/THF, neutral complex 3c with a coordinated nitrate was isolated.Moreover, complex 2j could be also obtained by dissolving 3c in acetonitrile (Scheme 4).In contrast to 2a-2i that were stable in CDCl 3 solutions (see below), dissolving 2j in CDCl 3 resulted in a red solution giving NMR spectra corresponding to 3c.A detailed inspection of these spectra revealed also a residual broad singlet at 4.77 ppm that could be assigned to 2j.This behaviour suggests that 2j and 3c exist in equilibrium in a polar solvent, with 3c being the major species. 18aracterization NMR studies.The NMR spectra of 2a-2i were routinely recorded in CDCl 3 at ambient temperature.These spectra featured all expected resonances, i.e. that of the cyclopentadienyl, of the carbene, and of the coordinated nitrile.The Cp protons appeared as singlets from 4.67 ppm to 4.76 ppm for 2a-2c and 2e-2g.For the weaker-donating benzimidazole-based NHC ligand Bn 2 -bimy, 19 the Cp resonances were shifted significantly downfield to 5. The carbene carbon atom chemical shift varied from 159.9 ppm to 199.4 ppm, depending on the type of NHC ligand.For IMes complexes (2a, 2e-2g, 2j, and 3a), the carbene carbon atom signal appeared from 159.9 to 166.2 ppm.The 13 C NMR spectra of Bn 2 -bimy complexes 2h and 2i displayed their carbene atom signals at 174.1 ppm and 174.2 ppm, respectively.The highest chemical shift of the carbene carbon atom in the range of 195.6-199.4ppm was observed for SIMes complexes (2b, 2d and 3b).While the spectra of trifluoroacetates 3a and 3b were unexceptional, the spectra of nitrates 2j and 3c deserve a further comment.
In contrast to the other ionic complexes, NMR spectra of 2j could be recorded only in CD 3 CN since 2j appeared to easily dissociate the nitrile ligand in a polar solvent (e.g. in CDCl 3 ) to form the neutral complex 3c (see Scheme 4).Thus, NMR spectra of 2j featured all expected signals within the usual ranges, e.g. a sharp singlet of Cp protons at 4.77 ppm.
However, NMR spectra of 3c were far from routine: we first noticed rather unusual chemical shift and linewidth of the Cp signal: in CDCl 3 at ambient temperature it appeared at 3.53 ppm with ν 1/2 = 5. 6 Hz, and at 35 °C it appeared at higher field at 2.77 ppm with ν 1/2 = 13 Hz.Moreover, the parameters of the Cp signal varied considerably also with the solvent used: it appeared at 2.34 ppm (ν 1/2 = 8.9 Hz) in C 6 D 6 at ambient temperature. 20n the 13 C NMR spectrum of 3c no carbene carbon atom signal was detected; moreover, the Cp signal at 97.1 ppm was unusually broad with ν 1/2 = 6.2 Hz, while for the other complexes ν 1/2 was in the range 1.6-2.1 Hz.VT NMR studies in toluene-d 8 in the temperature range from −55 °C to 100 °C were therefore performed (Fig. 1).The Cp signal appeared as a singlet at 4.31 ppm at −55 °C and shifted to −3.01 ppm at 100 °C.This upfield shift with increasing temperature was accompanied by signal broadening from ν 1/2 = 5 Hz to ν 1/2 = 36 Hz.At the same time the imidazole singlet shifted downfield slightly from 5.81 ppm to 7.33 ppm.We explain this behaviour of 3c in terms of spin equilibrium, i.e. equilibrium between a diamagnetic singlet ground state and a paramagnetic triplet excited state. 21The absence of an observable carbene carbon atom signal might be explained by its merging with the baseline as a result of the paramagnetic broadening.
The spin equilibrium was further modelled by using a Boltzmann distribution of spins 22 (for details, see the ESI †) and the thermodynamic parameters for 3c thus obtained were as follows: ΔH°= (15.15 ± 0.43) kJ mol −1 , ΔS°= (16.7 ± 4.0) J (mol K) −1 .The value of the high-spin species to low-spin species equilibrium constant K eq of 0.02 calculated at 298.15 K shows that at this temperature there is a large excess of the diamagnetic form of 3c.This value also explains why the magnetic susceptibility measurement by Evans' method 23 that we had attempted failed to give any significant result.At the same time such a placement of the equilibrium substantiates the observed chemical shift for Cp protons in 3c.For nickelocene (two unpaired electrons, μ eff = 2.88μ B ) 24 the magnitude of paramagnetic 1 H NMR chemical shift (δ = ca.−250 ppm) 25 is considerably larger than for 3c even though both in nickelocene and in 3c the distances between the nickel atom and the Cp plane are comparable (1.8177(4) Å 26   chemical shifts in both compounds should be of a similar order of magnitude.In the case of 3c this contribution is relatively small, creating a downfield shift of only several ppm.While it is not clear why the nitrate anion modifies the electronic properties of 3c in comparison with the other studied anions, the solid state structure of 3c (see below) revealed the expected, three-coordinate geometry.
Solid-state structures.We have focused our efforts to grow X-ray quality crystals on complexes with novel structural features, mainly on those with anions that have not been reported previously for [Ni(η 5 -C 5 H 5 )(A)(NHC)] complexes.In particular, the intriguing solution properties of nitrates prompted us to study them in detail.Gratifyingly, the solid-state structures of complexes 2c, 2j, 3a, and 3c have been determined by singlecrystal X-ray diffraction (Fig. 2).Selected crystallographic data, the parameters for data collection and refinement procedures are presented in Table 1.Selected bond lengths and angles are given in Table 2, and in Tables S2 and S3 in the ESI.† Single crystal X-ray structure analysis reveals that compounds 2c, 3a and 2j crystallise in the triclinic P1 ˉ(no.2) space group whereas complex 3c is the only one to yield non-centrosymmetric crystal structure in the orthorhombic Pca2 1 (no.29) space group.While crystal structures of compounds 3a and 2j contain one molecule and a pair of cation and anion, respectively, in the asymmetric unit, there are two independent molecules or two independent pairs of cations and anions in crystal structures of complexes 3c and 2c, respectively.The sum of bond angles around Ni atoms, that is, X-Ni-C (NHC) , C (NHC) -Ni-C g and C g -Ni-X angles, where C g denotes the centre of gravity of Cp rings and X stands for N or O, amounts to 360°within 3 s.u.'s in all studied compounds which indicates planar trigonal coordination of nickel atoms (see Table S2 in the ESI †).
Fig. 2 The molecular structure of cations of complexes 2c and 2j and of neutral complexes 3a and 3c.Hydrogen atoms are omitted for clarity.Thermal ellipsoids are drawn at the 50% probability level.In the case of complexes 2c and 3c, where two independent molecules are present in the asymmetric unit, only one of them is presented.See Fig. S3 in the ESI † for the second ones of 2c and 3c.
The plane of the carboxylate group in complex 3a deviates considerably from the Ni coordination plane (Ni, C g , X, C (NHC) ) as evidenced by the value of C6-Ni1-O1-C29 torsion angle equal to −153.50(14)°.This is even more pronounced for nitrate anions in compound 3c where C6-Ni1-O1-N3 and C36-Ni2-O31-N33 torsion angles are −116.2(4)and 115.2(5)°, respectively.This twist results in the monodentate binding of carboxylate and nitrate ligands to nickel (Ni⋯O distances for unbound oxygen atoms amount to 3.2238(13) Å in 3a and 2.905(6) Å on average in 3c).As shown in Table 2, the distances from nickel to Cp carbon atoms differ significantly from each other within every complex.talytic activity Styrene polymerization.The activity of complexes 2a-3c in styrene polymerization was examined under conditions similar to those described in our previous reports. 5,8Briefly, an excess of MAO (Al : Ni = 100 : 1) was added to a toluene suspension of complex 2 or 3.After stirring for 30 min at ambient temperature, neat styrene (styrene : Ni = 1000 : 1) was added and the polymerization was run in a sealed Schlenk tube for 3 h at 50 °C.The results of styrene polymerization are summarized in Table 3.
Disappointingly, hexafluorophosphates 2a and 2b (entries 1 and 4) were one order of magnitude less active than the neutral parent complexes. 5In control experiments we established that 2a without MAO gave no polymer (entry 2).Similarly, MAO itself did not yield polystyrene (entry 3).Analogously to what was observed for the chloride series, introduction of the more bulky NHC ligand (SIPr vs. SIMes, entry 5) or a substituent on the Cp ligand (entry 6) resulted in significantly lower yields than for 2b.

−
) had no significant effect on the activity (entries 7, 8, and 11).Complex 2g with the more bulky nitrile (entry 9) provided the same efficiency as 2a.Introduction of the weaker donating benzimidazole-based NHC ligand resulted in a low yield of the polymer (entry 10).The highest activity was achieved with complexes 3a and 3b (entries 12 and 13) bearing covalently bound carboxylates; however, neutral nitrate 3c was less effective (entry 14).These findings show that strongly  a N3 denotes the nitrogen atom of the coordinated acetonitrile for 2c and 2j; O1 denotes the oxygen atom of the trifluoroacetate or nitrate anions for complexes 3a and 3c, respectively (see Fig. 2 for the atom numbering scheme).coordinating anions, i.e. chloride or trifluoroacetate, are the most efficient in this type of polymerization.
The obtained polystyrenes were examined by 13 C NMR, GPC, and MALDI-TOF MS.The 13 C NMR spectra were consistent with atactic microstructure of all polymers. 28GPC analyses showed that in most cases M n was lower than that obtained with the corresponding chlorides, while M w /M n was higher than for the chlorides.The trifluoroacetates 3a and 3b produced polystyrenes with similar M n and M w /M n to those obtained with 1a. 5 MALDI-TOF MS (see the ESI †) suggested that the polystyrene chains were terminated with CvC double bonds.
Previously, we proposed that the initial reaction of complexes 1 with MAO resulted in cationic species [Ni(η 5 -C 5 H 5 )-(NHC)] + (Scheme 5, path a). 29Consequently, the efficiency of styrene polymerization with 1/MAO depended mainly on the stabilization of these intermediate species, meaning that the strongly coordinating chloride that irreversibly reacts with MAO was the most suitable counterion. 5In this study, we anticipated that the labile nitrile ligand 12,30 in complexes 2 would be readily displaced with styrene (Scheme 5, path b) to produce the same intermediates as with 1.However, the low efficiency of complexes 2 in the styrene polymerization suggests that the nitrile binds to the Ni centre rather strongly and actually inhibits the polymerization.To further address this issue, we studied reactions of complexes 2 with styrene (Scheme 6).
However, despite our best efforts to use as many various reaction conditions as possible (type of the nitrile ligand, solvent and temperature), the exchange of the nitrile ligand  12 Surprisingly, chloride complexes and the corresponding hexafluorophosphates provided almost identical conversions.Encouraged by these results, we decided to test our new complexes 2 and 3 in the cross-coupling of 4′-bromoacetophenone with phenylboronic acid.The results are summarized in Table 4.
All studied complexes provided high yields of the expected cross-coupling product, i.e. 4-acetylbiphenyl (A), with excellent selectivity.The highest yield was achieved with cationic nitrate 2j (entry 10); however, the advantageous effect of this counterion was not confirmed with neutral nitrate 3c (entry 13).The weakly donating Bn 2 -bimy ligand was consistently less efficient (entries 8 and 9) than the other NHC ligands.With the more challenging substrate, 4′-chloroacetophenone, complex 2e was significantly less efficient than with 4′-bromoacetophenone (entry 14).The absence of a pronounced structure-activity relationship for the studied series of complexes is consistent with previous hypothesis that the Ni(II) complexes 2-3 serve as a convenient source of Ni(0) in this catalytic reaction. 9,12

Conclusions
In summary, we have shown that, depending on the anion binding properties and reaction conditions, cationic 2 or neutral complexes 3 were obtained by anion metathesis in complexes 1.In the case of nitrate, both cationic complex 2j and neutral 3c could be isolated.Complexes 2j and 3c were found to easily interconvert with each other in a solution.This facile exchange of ligands opens up prospects for further optimization of electronic and catalytic properties of these complexes, in particular discovery of systems with switchable magnetic properties and plausible applications in spintronics.

General
All manipulations (except polymer separation and purification, and work-up of the Suzuki cross-coupling reactions) were performed under an inert atmosphere of argon using Schlenk techniques.Solvents were purified with conventional methods. 32Styrene (ReagentPlus®, Aldrich) was distilled from  CaH 2 under reduced pressure and passed through a column with neutral Al 2 O 3 .Other reagents were purchased from commercial suppliers and were used without further purification.Complexes 1a-1e were prepared from nickelocene 33 or 1,1′-bis-(allyl)nickelocene 34 and the appropriate imidazolium salt according to the published method with minor modifications. 3MR spectra were recorded, unless otherwise noted, at ambient temperature on a Mercury-400BB spectrometer operating at 400 MHz for 1 H NMR, at 101 MHz for 13 C NMR, at 376 MHz for 19 F NMR, and at 162 MHz for 31 P NMR.ESI MS were measured on a Mariner spectrometer.EI MS (70 eV) were measured on a AutoSpec Premier (Waters) spectrometer.MAL-DI-TOF MS of polystyrenes were acquired with a Bruker Daltonics ultrafleXtreme™ mass spectrometer (DCTB matrix with AgCF 3 CO 2 ).The average molecular weights of PS were measured on a LabAlliance liquid chromatograph equipped with a Jordi Gel DVB Mixed Bed column (250 mm × 10 m) using CH 2 Cl 2 as the mobile phase at 30 °C and calibrated with standard PS.Conversion and selectivity of Suzuki reactions were determined on an Agilent Technologies 7820 GC System equipped with a FID detector and an Agilent 19091J-413 column.Tetradecane was used as an internal standard.

Catalytic activity
General procedure for styrene polymerization.To a suspension of [Ni(η 5 -C 5 H 5 )(IMes)(CH 3 CN)] + (ClO 4 ) − (2e) (9.3 mg, 16.4 μmol) in toluene (10.0 mL) a solution of MAO in toluene (10% wt.from Aldrich) was added (1.15 mL, Al/Ni = 100).The colour of the reaction mixture changed immediately from pale yellow to brown and white fumes appeared.After stirring for 30 min at room temperature, styrene was added (neat, 1.95 mL, 17.0 mmol).The resulting mixture was immersed in a preheated oil bath maintained at 50 °C and stirred vigorously for 3 h at this temperature.After cooling to the room temperature the reaction mixture was poured into methanol (ca.200 mL).The resulting polystyrene was isolated by filtration, washed with methanol, and dried in vacuo.Yield: 1.77 g, 31%. 13  10 Ag] + .General procedure for Suzuki cross-coupling.4′-Bromoacetophenone (55.1 mg, 0.277 mmol) and phenyl-boronic acid (44.0 mg, 0.361 mmol, 1.3 eq.) were dissolved in toluene (0.80 mL) in a Schlenk tube.Solid K 3 PO 4 (153 mg, 0.722 mmol, 2.6 eq.) and tetradecane (internal standard, 7.0 μL) were then added, followed by 2e (5.0 mg, 8.8 μmol, 3.2% mol ).The tube was immersed in a preheated oil bath maintained at 90 °C and stirred for 1 h at this temperature.After cooling to the room temperature, the reaction mixture was diluted with diethyl ether, washed with water and dried over anhydrous Na 2 SO 4 .The substrate conversion (74%) and selectivity were determined with GC.

X-ray diffraction studies
Single crystals of 2c suitable for X-ray studies were obtained from a CH 2 Cl 2 -hexanes (1 : 2) solution; single crystals of 3a and 3c were obtained from saturated toluene-hexane solutions at 4 °C; compound 2j was crystallized from a mixture of acetonitrile and diethyl ether at 4 °C.Diffraction data of suitable single crystals were measured on an Agilent κ-CCD Gemini A Ultra diffractometer with graphite-monochromated Mo-Kα radiation at 100(2) K for compounds 2c and 3a, and at 293(2) K for 2j and 3c.Cell refinement and data collection as well as data reduction and analysis were performed with the CrysAlis PRO software. 35The structures were solved by direct methods and subsequent Fourier-difference synthesis with ShelXS and refined by full-matrix least-squares against F 2 with ShelXL within the Olex2 program suite. 36,37All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were introduced at calculated positions and refined as riding atoms with isotropic displacement parameters related to that of the parent atoms.Asymmetric unit of complex 3a contained one half of a severely disordered toluene molecule which was treated with the SQUEEZE procedure implemented in PLATON. 38The structure model of compound 3c was refined as an inversion twin with the twin ratio refined to 43 : 57.Attempts to refine the crystal structure in centrosymmetric space group failed giving unphysical ADPs.Data analysis was carried out using Olex2 and PLATON.Crystal data and structure refinement parameters are given in Table 1 and CCDC 972867-972870.
22 and 5.24 ppm for 2h and 2i.An interesting feature of the proton NMR spectra of these benzimidazole-based NHC complexes was the presence of the Ph-CH 2 -signals as two doublets with chemical shifts in the range from 6.19 to 6.45 ppm ( 2 J = 13.5 Hz) which suggests their diastereotopic character.The resonances of the coordinated acetonitrile molecule were observed as singlets from 2.03 to 2.26 ppm in CDCl 3 .
0.1 Å compared to the other ones.This variation in the Ni-C Cp distances can be attributed to the trans effect of the NHC ligand which leads to the elongation of Ni-C Cp bonds trans to the carbene and, consequently, shortening of the Ni-C Cp bonds trans to the L ligand.The C-C bond lengths in cyclopentadienyl ligands vary from 1.35 to 1.45 Å which is typical for Ni complexes comprising both Cp and NHC ligands deposited in the Cambridge Structural Database (1.35-1.46Å).