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Article

New Ni(II)-Ni(II) Dinuclear Complex, a Resting State of the (α-diimine)NiBr2/AlMe3 Catalyst System for Ethylene Polymerization

by
Igor E. Soshnikov
1,*,
Nina V. Semikolenova
1,
Anna A. Bryliakova
1,2,
Artem A. Antonov
1,
Konstantin P. Bryliakov
1 and
Evgenii P. Talsi
1,*
1
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, 630090 Novosibirsk, Russia
2
Department of Natural Sciences, Novosibirsk State University, Pirogova 1, 630090 Novosibirsk, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(2), 333; https://doi.org/10.3390/catal13020333
Submission received: 19 December 2022 / Revised: 28 January 2023 / Accepted: 31 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Mechanism/Kinetic Modeling Study of Catalytic Reactions)
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Abstract

:
A novel room-temperature stable diamagnetic nickel complex 2 was detected upon activation of Brookhart-type ethylene polymerization pre-catalyst LNiBr2 (1, L = 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) with AlMe3. Using in situ 1H, 2H, and 13C NMR spectroscopy, as well as DFT calculations, this species has been identified as an antiferromagnetically coupled homodinuclear complex [LNiII(μ-Me)(μ-CH2)NiIIL]+Br−. Its behavior in the reaction solution is characteristic of the resting state of nickel catalyzed ethylene polymerization.

Graphical Abstract

1. Introduction

During the first two decades of the 21st century, Ni(II) α-diimines have remained the subject of extensive studies due to their unique ability to catalyze the formation of branched polyethylene from ethylene as the sole feedstock, as well as copolymerization of ethylene with polar monomers [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. Despite a great number of publications devoted to their mechanistic investigations [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56], some crucial details of the catalytic process remained unclear. For example, some data on the nature of the active species formed upon activation of α-diimine Ni(II) complexes with MAO and MMAO have been obtained. Namely, cationic Ni(II)-alkyl species—direct precursors of the ethylene polymerization active sites [29,30]—were detected in situ by NMR in the real catalyst systems LNiBr2/MAO and LNiBr2/MMAO [46].
In contrast, the nature of the Ni(II) complexes formed upon activation of LNiBr2 with AlMe3 have not yet been reported. Meanwhile, there have been several examples of using trimethylaluminum alone as an effective co-catalyst for Ni(II) α-diimines [25,34,51,54,55,56,57].
Recently, we reported the detection and characterization of a paramagnetic monovalent nickel complex [LNiI(μ-Me)2AlMe2] formed upon activation of complex 1 (Figure 1) with AlMe3 [51,53,56]. Herein, we report formation of novel diamagnetic nickel(II) species in this system in the presence of a small excess of AlMe3 (Al/Ni = 10). The new species have been characterized in detail by in situ 1H, 2H, and 13C NMR spectroscopy, and their assignment has been corroborated by quantum chemical (DFT) calculations.

2. Results and Discussion

2.1. The Nickel Center Spin State

It is well-known that a Ni(II) center (3d8 configuration) can exist in two spin states: paramagnetic high-spin (S = 1) and diamagnetic low-spin (S = 0). As a rule, four-coordinated Ni(II) complexes with square–planar geometry are diamagnetic, whereas distorted tetrahedral counterparts are paramagnetic (see, for example, Refs. [58,59,60,61]). The low-spin Ni(II) species display well-resolved 1H NMR resonances in the range typical for diamagnetic species. On the contrary, the high-spin Ni(II) complexes display broad and paramagnetically shifted 1H NMR peaks [62]. The paramagnetic shifts are temperature-dependent, obeying the Curie’s law (inverse proportionality with temperature). In addition, the high-spin Ni(II) species are EPR-silent in the X-band due to high zero-field splitting (D >> hν) [63].
Complexes of Ni(I) (3d9 configuration) are paramagnetic (S = 1/2) and usually display well-resolved X-band EPR resonances at g > ge due to the negative spin-orbit coupling constant. Typically, for such species, only very broad 1H NMR resonances can be detected.
Diamagnetism of nickel(I) species can also be the case due to antiferromagnetic coupling of two paramagnetic centers (S1 = S2 = 1/2; SΣ = S1 + S2 = 0 ground state); some diamagnetic dinuclear NiI-NiI complexes have been reported [64]. Besides, diamagnetic state can be realized for Ni(I) complexes with α-diimine anion-radical (L(∙−)) as a ligand. In this case, antiferromagnetic coupling between the Ni(I) center (S = 1/2) and anion-radical (L(∙−)) (S = 1/2) results in zero overall spin. In the latter two cases, the temperature dependence of the paramagnetic shifts of the 1H NMR resonances does not obey Curie’s law; these shifts increase with temperature in line with the growing population of the paramagnetic excited state (S = 1).
Complex 2, reported herein, can be obtained via the reaction of 1 with a relatively small excess of AlMe3 (10 equiv.) in toluene at room temperature. Note that 2 displays good thermal stability at room temperature and rapidly decomposes only at T > 40 °C. Nevertheless, our numerous attempts to isolate 2 from the solution for its X-ray structural characterization have remained unsuccessful. Fortunately, 2 had rather informative 1,2H and 13C NMR spectra.
The 1H NMR spectra of 2 recorded at different temperatures are shown in Figure 2. The resonances of 2 are spread over a range of 8 … −1.5 ppm, which is typical for diamagnetic species, their chemical shifts demonstrating very weak temperature dependence. It is worth noting that some resonances of 2 in the range of 6.4 … 6.8 and 1.6 … 2.3 ppm, marked by asterisks, coalesce at 40 °C. The reason for such behavior will be discussed below.

2.2. The 1H NMR Resonances of the α-Diimine Ligand of 2

The 1H NMR spectrum of the sample 1/AlMe3 (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM) recorded at 0 °C displays narrow well-resolved resonances of the α-diimine protons of the ligand of 2 (Figure 3a).
A singlet at δ 2.27 corresponds to para-Me-substituents at the aromatic rings of 2. Indeed, the 1H NMR spectrum of the sample 1′/AlMe3 (α-diimine ligand of 1′ contains no para-methyl substituents, Figure 1) displays no such resonance (Figure 3b). Two singlets at δ 0.98 and δ 1.14 are assigned to the methyl protons of the CH3-C=N moieties. To corroborate this assignment, the 2H NMR spectrum of the sample 1D/AlMe3 was recorded (toluene, [Al]/[Ni] = 10/1, CNi = 5 mM, 1D—complex 1, selectively enriched (ca. 86%) by 2H isotope in the Me-C=N moiety). The corresponding 2H resonances of the CD3-C=N groups are observed at δ 0.90 and δ 1.05, respectively (Figure 3c), witnessing a small isotopic shift compared to the 1H peaks of the non-labeled prototype.
The resonances in the range 6.5−6.8 ppm belong to the aromatic protons of the α-diimine ligand, whereas four broadened singlets at 2.21, 2.14, 1.83, and 1.82 ppm can be assigned to the o-Me protons (Figure 3a). The integral intensities of these resonances are in agreement with our assignment (Table S1, SM).
Besides the resonances of the α-diimine ligand, the 1H NMR spectrum of 1/AlMe3 (Figure 3a and Figure 4a) displays three additional signals at δ 5.44 (br d, 2JHH = 3.5 Hz), δ 3.72 (br. d, 2JHH = 3.5 Hz), and δ −1.07 (s). The last one is characteristic of the protons of Me-group, directly bonded to the Ni(II) center (see, for example, Ref. [28]). This assignment was confirmed by 13C and 1H NMR (Figure 4 and Figure 5) of the sample 1/AlMe3* containing 13C-enriched (ca. 54 at.%) trimethylaluminum and 1H-13C hxdeptbiqf NMR of 1/AlMe3 one (Figure S2). The 1H NMR spectrum of 1/AlMe3* displayed a doublet at δ −1.07 with 1JCH = 125.2 Hz (Figure 4a,b), with the corresponding singlet at in dept135 13C NMR being found at δ −25.2 (Figure 5). The corresponding cross-peak in 1H-13C NMR was also observed (Figure S2 in SM). The integral intensity of 1H NMR resonance at δ −1.07 indicates that the molecule of 2 contains two α-diimine ligands per Ni-Me group.
Each of the remaining two resonances at δ 5.44 (br. d, 2JHH = 3.5 Hz, 1H) and δ 3.72 (br. d, 2JHH = 3.5 Hz, 1H) has an integral intensity corresponding to one hydrogen atom (Figure 4a). The 1H-1H COSY NMR spectrum of 2 displays a couple of cross-peaks (Figure S1 in SM), corresponding to spin-spin coupling between these protons.
Using 13C-enriched trimethylaluminum instead of Al(12CH3)3 leads to the appearance of two additional doublets at δ 5.44 and δ 3.72 in 1H NMR due to one-bond 13C-1H spin-spin coupling 1JCH = 132.6 Hz and 1JCH = 142.8 Hz, respectively (Figure 4a,b). Note that broadened doublets δ 5.44 (1JHH = 3.5 Hz) and δ 3.72 (1JHH = 3.5 Hz) are also observed due to the presence of Al(12CH3)3 in partially labeled sample.
The resonance of the CH2-group with an integral intensity equal to those of Ni-CH3 was detected in the 13C{1H} dept135 NMR spectrum of the sample 1/AlMe3* at δ 111.1 (Figure 5). The cross peaks in 2D 1H,13C NMR clearly indicated the correlation of 13C resonance at δ 111.1 and two 1H resonances at δ 5.44 and δ 3.72, respectively (Figure S2 in SM).
Only the initial α-diimine ligand containing no 13C-lables was recovered after 1/AlMe3* sample hydrolysis with H2O, followed by extraction with Et2O, and drying. Therefore, the resonances at δ 5.44 and δ 3.72 in the 1H NMR spectrum and at δ 111.1 in the 13C NMR one undoubtedly belong to the CH2-group bound to the Ni center (or centers) rather than to the α-diimine ligand. The 2JHH coupling constant of 3.5 Hz is lower than expected for heminal protons at sp3 hybrid carbon atoms, but reasonably higher than those for heminal protons at sp2 carbon atoms [65].
Further, the 13C NMR chemical shift of the CH2 group (δ 111.1) is not characteristic of nickel(II)-bonded sp3-carbons of Ni-CH2-R moiety (such carbons display 13C NMR resonances typically in the range of 10 … 20 ppm; see ref. [38,59]). Unfortunately, 13C NMR data on the species containing Ni=CH2 moieties are lacking, which complicates speculating on the characteristic values of 13C chemical shifts of the sp2 carbons directly bonded to the Ni center. However, a number of nickel(II) complexes of chelating N-heterocyclic carbenes are well-known (see Refs. [66,67], and references therein). For these compounds, 13C NMR chemical shifts of the sp2 carbons directly bonded to the Ni center fall within the range from 180…200 ppm. Therefore, the value of δ 111.1 might be tentatively attributed to the carbon of Ni-CH2 moiety with the hybridization intermediate between sp3 and sp2.
It is reasonable to conclude that 2 is a dinuclear Ni(II)-Ni(II) complex rather than mononuclear one. Indeed, in the case of mononuclear Ni(II) species, six-coordinated Ni(II) center supported by two bidentate α-diimine ligands, one methyl, and one CH2-group does not have square-planar geometry and thus would not display the 1H NMR spectrum characteristic of diamagnetic species. We assume that complex 2 may be represented as alternative resonance structures differing in the mode of Br ligation (outer-sphere or inner-sphere, Figure 6). To discriminate between these alternatives, quantum-chemical calculations have been performed (see below).

2.3. DFT Calculations

To discriminate between the two possible structures of 2 (Figure 6), we have calculated the geometries and free energies of the bridged homodinuclear complex at the UB3LYP-D3/def2-TZVPP level of theory, which was previously shown to satisfactorily predict the gaps between different spin states of first-row transition metal complexes [68,69,70]. Crucially, starting from the draft structure [L(Me)NiII(μ-CH2)NiII(Br)L], optimization led to the doubly bridged ion pairs of the type [LNiII(μ-Me)(μ-CH2)NiIIL]+Br−, thus favoring the assignment of 2 to the ion-pair structure (Figure 6). Singlet (S = 0) appeared to be 11.8 kcal/mol lower in energy than its triplet (S = 1) counterpart (for the energies, molecular models and Cartesian coordinates see SM), which is consistent with the diamagnetic nature of species 2 as evident from NMR data.

2.4. Temperature Behavior of the Ortho-Methyl and Meta-H Resonances

The molecule of 2 is not symmetric, so magnetic non-equivalence of the ligand protons is expected. Hindered rotation of the mesityl moieties around the C-N bound leads to a number of resonances of non-equivalent o-methyl and m-H resonances in the 1H NMR at low temperatures (namely at −20 and 0 °C; Figure 2c and Figure 3a). Sample heating partially unfreezes rotation around the N-C bond, leading to slow or moderate exchange at 25 °C; it causes dramatic broadening of the resonances mentioned above. At T = 40 °C, the m-H resonances collapse, giving rise to a narrow singlet indicative of fast exchange. Further temperature increase leads to decomposition of 2, which prevents detection of narrow resonances of the o-Me groups.

2.5. The Role of 2 in Catalytic Process

It is interesting to establish the role of 2 in ethylene polymerization. The real catalytic experiments in autoclave usually require using high excess of co-catalyst (Al/Ni = 500 or more), which in additions acts as a scavenger. As an example, the results of the ethylene polymerization over 1/AlMe3 (Al/Ni = 500) catalyst system are summarized in Table 1 [51].
Unfortunately, under these conditions the Ni(II) species are not stable, undergoing rapid reduction into the corresponding Ni(I) counterparts (neutral heterobinuclear complexes of [LNiI(μ-Me)2AlMe2], [51]) predominating in the resulting reaction solution. That is why we have performed the following NMR-tube catalytic experiment instead. First, species 2 was generated in the 1/AlMe3 sample (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM). Then, ethylene was bubbled through the sample during 1 min at 0 °C, and NMR and EPR spectra were recorded right after. This was repeated five times.
According to the 1H NMR and EPR data, after each bubbling ethylene was completely polymerized, the concentration of 2 decreased, and EPR-resonances of Ni(I) species grew up (Figures S3 and S4 in SM). After the 3rd cycle, NMR resonances of 2 disappeared completely, but the sample kept its ethylene polymerization activity. This is not surprising, given the reported ability of monovalent nickel species ([LNiI(μ-Me)2AlMe2]) to give rise to polymerization-active species in the presence of excess of ethylene [51].
Altogether, 2 can be assigned to a resting state of the true catalytically active species, yet currently, it is not clear whether 2 converts into the active species directly or through monomeric nickel(I) species of the type [LNiI(μ-Me)2AlMe2]. Possible transformations of the nickel species in the catalyst system 1/AlMe3 are presented in Scheme 1.

3. Materials and Methods

3.1. General Procedures

Toluene-d8 and 1.2-difluorobenzene (Aldrich, St. Louis, MO, USA) were dried over molecular sieves (4 Å), degassed in a vacuum, and stored under dry argon. Methylene chloride, CDCl3, and CD2Cl2 (Aldrich) were dried over P2O5, distilled, and stored under dry argon. Hexane was stored over molecular sieves (4 Å), purified by refluxing over sodium-benzophenone and distilled in dry argon. AlMe3 (neat), 1,2-butanedione, 2,4,6-trimethylaniline, 2,6-dimethylaniline, NiBr2(DME), D2O (99 at.% of D), and CH3OD (99 at.% of D) were purchased from Aldrich and used without preliminary purification. Al(13CH3)3 was prepared from 13C-labelled CH3I (Aldrich) by sequential treatments with Al metal at 80 °C and Na metal in C12H26 at 100 °C and isolated by vacuum distillation [71]. All operations with oxygen-sensitive and moisture-sensitive compounds were carried out under dry argon atmosphere by using standard Schlenk techniques (synthesis of nickel complexes) or glovebox (preparing the samples for NMR and EPR experiments).

3.2. NMR and EPR Spectra Registration

1H, 2H and 13C NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer (Rheinstetten, Germany) in 5 mm (o.d.) NMR tubes at 400.130, 61.422, and 100.613 MHz, respectively. Typical operating conditions for 1H NMR measurements were as follows: spectral width 4–5 kHz for diamagnetic and 40 kHz for paramagnetic compounds; spectrum accumulation frequency 0.25–0.4 Hz; 32–64 transients, 90° pulse at 12.0 μs. Operating conditions for 2H NMR experiment: spectral width 1 kHz; spectrum accumulation frequency 0.5 Hz; 8000 transients, 90° pulse at 6.6 μs. Operating conditions for 13C NMR experiments: spectral width 20 kHz; spectrum accumulation frequency 0.2–0.01 Hz; 32–2000 transients, 45° pulse at 3.8 μs. Multiplicities and JCH coupling constants were determined from gated decoupled spectra. 1H-1H and 13C-1H correlations were established by using standard Bruker cosydfqf and hxdeptbiqf pulse programs, respectively. For calculations of 1H chemical shifts (ppm), the resonances of the residual solvent protons (δ 2.09 for CHD2C6D5, δ 7.26 for CHCl3, and δ 5.27 for CHDCl2) were used. 2H chemical shifts were referred to the signal of CDH2C6H5 in toluene (δ 2.10) whereas 13C ones were referred to the solvent resonances (δ 77.16 for CDCl3 and δ 20.4 for CD3-group of toluene-d8). The sample temperature measurement uncertainty and temperature reproducibility were less than ±1 °C.
EPR spectra were recorded on a CMS 8400 EPR spectrometer (Adani, Minsk, Belarus) at 9.44 GHz, modulation frequency 100 kHz, modulation amplitude 0.5 mT. Frozen solution EPR measurements were conducted in a quartz finger Dewar filled with liquid nitrogen (−196 °C). Toluene solution of TEMPO (2 mM) was used as an external standard for g-values calculations.
Samples 1/AlMe3 (1′/AlMe3, or 1D/AlMe3) were prepared in the argon-filled glovebox by simple reagents mixing at ambient temperatures. Septum stoppers and Parafilm “M” were used to avoid oxygen/moisture transfer into the NMR tube.

3.3. Synthesis of the Complexes 1, 1′, and 1D

In general, complexes 1, 1′, and 1D were synthesized according to published procedures (see, for example, ref. [72]) with small deviations.

3.3.1. Synthesis of 1,4-Bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene (L1)

To a stirred solution of 2,3-butanedione (100 mg, 1.16 mmol) in methanol (2 mL), a solution of 2,4,6-trimethylaniline (170 mg, 1.26 mmol) in methanol (3 mL) was added. After the addition of a droplet of glacial acetic acid, reaction mixture was refluxed overnight. After reaction mixture cooling to room temperature the solvent was removed in a vacuum to give a yellow powder of crude ligand L1. Further recrystallization from ethanol afforded bright yellow crystals of pure L1 (242 mg, 65% yield).
1H NMR (24 °C, CDCl3): δ 6.91 (s, 4H, Ar-Hm), 2.31 (s, 6H, Ar-CH3 p), 2.06 (s, 6H, N=C-CH3), 2.02 (s, 12H, Ar-CH3 o). 13C NMR (24 °C, CDCl3): δ 168.50 (2C, N=C-CH3), 145.99 (2C, Ar-Cip), 132.55 (2C, Ar-Cp), 128.71 (4C, Ar-Cm, 1JCH = 154.6 Hz), 124.68 (4C, Ar-Co), 20.86 (2C, Ar-CH3 p, 1JCH = 126.3 Hz), 17.86 (4C, Ar-CH3 o, 1JCH = 126.7 Hz), 15.93 (2C, N=C-CH3, 1JCH = 128.6 Hz). Anal. Calcd. for C22H28N2: C, 82.45; H, 8.81; N, 8.74. Found: C, 82.54; H, 8.96; N, 8.75.

3.3.2. Synthesis of 1,4-Bis-2,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene (L1′)

L1′ was obtained according to the procedure described above (2,6-dimethylaniline was used instead of mesitylamine) as bright yellow crystals in 61% yield.
1H NMR (24 °C, CDCl3): δ 7.08 (d, 4H, Ar-Hm, 3JHH = 7.7 Hz), 6.95 (t, 2H, Ar-Hp, 3JHH = 7.7 Hz), 2.06 (s, 6H, N=C-CH3), 2.05 (s, 12H, Ar-CH3 o). 13C NMR (24 °C, CDCl3): δ 168.25 (2C, N=C-CH3), 148.43 (2C, Ar-Cip), 128.08 (4C, Ar-Cm, 1JCH = 158.7 Hz), 124.83 (4C, Ar-Co,), 123.42 (2C, Ar-Cp, 1JCH = 160.3 Hz), 17.93 (4C, Ar-CH3 o, 1JCH = 126.8 Hz), 15.99 (2C, N=C-CH3, 1JCH = 128.9 Hz). Anal. Calcd. for C20H24N2: C, 82.15; H, 8.27; N, 9.58. Found: C, 82.21; H, 8.54; N, 9.60.

3.3.3. Synthesis of 2H-Enriched 2,3-butanedione

2H-enriched 2,3-butanedione was prepared by H/D exchange between 2,3-butanedione and D2O according to a published procedure [73]. To a stirred solution of 2,3-butanedione (2.00 g, 23.5 mmol) in D2O (99% of D, 10 mL, 495 mmol), concentrated hydrochloric acid (0.8 mL, 10 mmol) was added dropwise. The reaction mixture was allowed to stir for 10 days at ambient temperature. After that, acid was neutralized by addition of CaCO3 powder (2 g, 20 mmol), and the product was extracted by CH2Cl2 (3 × 15 mL). Combined organic phase was dried over Na2SO4 and filtered. Pure 2H-enriched 2,3-butanedione (84 at. % of 2H) was isolated by distillation (1.45 g, 19.3 mmol, 82% yield). Deuterium abundance in the product was determined by 1H NMR. 1H NMR (25 °C, CDCl3): δ 2.33 (s, CH3), 2.31 (t, CH2D, 2JHD = 2.2 Hz), 2.29 (quint, CHD2, 2JHD = 2.2 Hz).

3.3.4. Synthesis of 2H-Enriched 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene (L1D)

According to the procedure described for L1, the desired ligand L1D was obtained as bright yellow crystals in 65% yield. To avoid isotopic exchange between 2H-enriched 2,3-butanedione and OH-groups of methanol during synthesis, CH3OD was used as solvent instead. Deuterium abundance in the N=C-Me moiety of the resulting ligand was 84 at. %.
1H NMR (25 °C, CDCl3): δ 6.91 (s, 4H, Ar-Hm), 2.31 (s, 6H, Ar-CH3 p), 2.06–2.03 (m, 0.9H, N=C-CH3 + N=C-CH2D + N=C-CHD2), 2.02 (s, 12H, Ar-CH3 o).

3.3.5. Synthesis of 1,4-Bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene nickel(II) dibromide (complex 1)

All manipulations were performed under dry argon atmosphere using the standard Schlenk technique. Only solvents containing no water and oxygen traces were used.
A Schlenk flack was charged by a freshly prepared solution of L1 (100 mg, 0.31 mmol) in CH2Cl2 (10 mL). A preliminary weighted (in glovebox) amount of NiBr2(DME) (100 mg, 0.32 mmol) was added rapidly and reaction mixture was allowed to stir overnight at ambient temperature. After that insoluble compounds were filtered off and filtrate was dried in vacuum. The resulting dark-red powder was recrystallized from CH2Cl2/hexane to give red crystals of pure 1 (119 mg, 0.22 mmol, 70% yield).
1H NMR (25 °C, CD2Cl2): δ 34.8 (br, 6H, Ar-CH3 p, Δν1/2 = 33 Hz), 27.9 (br, 12H, Ar-CH3 o, Δν1/2 = 100 Hz), 24.1 (br, 4H, Ar-Hm, Δν1/2 = 25 Hz), −21.2 (br, 6H, N=C-CH3, Δν1/2 = 48 Hz). Anal. Calcd. for C22H28N2Br2Ni: C, 49.03; H, 5.24; N, 5.20; Br, 29.65; Ni, 10.89. Found: C, 50.35; H, 6.15; N, 5.26.

3.3.6. Synthesis of 1,4-Bis-2,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene nickel(II) dibromide (complex 1′)

Complex 1′ (dark red crystals) was prepared in a similar manner by using the corresponding α-diimine ligand L1′ in 78% yield.
1H NMR (24 °C, CDCl3): δ 28.2 (br, 12H, Ar-CH3 o, Δν1/2 = 134 Hz), 24.1 (br, 4H, Ar-Hm, Δν1/2 = 61 Hz), −17.6 (br, 2H, Ar-Hp, Δν1/2 = 74 Hz), −22.7 (br, 6H, N=C-CH3, Δν1/2 = 66 Hz). Anal. Calcd. for C20H24N2Br2Ni: C, 47.02; H, 4.73; N, 5.48; Br, 31.28; Ni, 11.49. Found: C, 47.48; H, 5.18; N, 5.46.

3.3.7. Synthesis of 2H-Enriched 1,4-bis-2,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene nickel(II) dibromide (complex 1D)

Dark red crystals of 1D were obtained in a similar manner by using the corresponding 2H-enriched α-diimine ligand L1D in 72% yield.
1H NMR (25 °C, CD2Cl2): δ 34.9 (br, 6H, Ar-CH3 p, Δν1/2 = 35 Hz), 27.9 (br, 12H, Ar-CH3 o, Δν1/2 = 105 Hz), 24.1 (br, 4H, Ar-Hm, Δν1/2 = 30 Hz), −21.4 (br, N=C-CH3, Δν1/2 = 50 Hz), −22.2 (br, N=C-CH2D, Δν1/2 = 55 Hz), −22.0 (br, N=C-CHD2, Δν1/2 = 50 Hz). Overall integral intensity of N=C-Me moiety corresponds to 0.9 H.

3.4. DFT Calculations: Computational Details

Geometry optimization and frequency analysis of 2 with different multiplicities (S = 0, S = 1; for the (S = 2) spin isomer, the calculation did not converge) were carried with the UB3PLYP density functional theory scheme (DFT) with D3 corrections [74] using GAUSSIAN 16 program suite [75], with the def2-TZVPP [76] basis set for the Ni and Br atom and 6-311G(d) [77] basis set for other atoms. Solvation effects (with toluene) were incorporated using polarized continuum model (PCM) method [78,79] as implemented in GAUSSIAN 16. The stationary points were ascertained by vibrational frequency analysis with no imaginary frequencies. The Gibbs energies reported in this paper were sum of electronic and thermal free energies. DFT optimized Cartesian coordinates and potential energies (Ha) for the calculated structures are presented in the SM.

4. Conclusions

A novel diamagnetic species 2 was detected upon activation of Brookhart-type Ni(II) α-diimine ethylene polymerization pre-catalyst 1 (LNiBr2, L = 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) with AlMe3 co-catalyst. At [Al]:[Ni] ratio of 10:1, species 2 is room-temperature stable and accounts for the major part of nickel. 1H, 2H, and 13C NMR spectroscopic characterization of 2 has witnessed its dinuclear nature, incorporating two antiferromagnetically coupled Ni(II) centers, to give an overall S = 0 spin state. DFT calculations allow to reasonably discriminate between the two possible structures, [L(Me)NiII(μ-CH2)NiII(Br)L] and [LNiII(μ-Me)(μ-CH2)NiIIL]+Br−, favoring the latter, doubly bridged outer-sphere ion pair structure. Complex 2 is sufficiently stable in the reaction solution only at low Al/Ni molar ratios, while in conditions approaching those of practical polymerization ([Al]:[Ni] = 500), quantitative reduction of Ni(II) to the monovalent state takes place. Most plausibly, 2 can be assigned to a resting state of the true Ni(II) active sites of ethylene polymerization.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13020333/s1, Figure S1: 1H-1H cosydfph NMR spectrum of the sample 1/AlMe3; Figure S2: 1H-13C hxdeptbiqf NMR spectrum of the sample 1/AlMe3; Figure S3: 1H NMR spectra of the catalyst system 1/AlMe3/C2H4; Figure S4: EPR spectra of the catalyst system 1/AlMe3/C2H4; Table S1: The integral intensities of 1H NMR resonances of 2; Cartesian coordinates and calculated energies.

Author Contributions

Conceptualization, I.E.S., K.P.B. and E.P.T.; Synthesis of Ni(II) complexes and samples preparing, A.A.A. and N.V.S.; NMR spectroscopic studies, I.E.S.; DFT calculations, A.A.B.; writing—original draft preparation, I.E.S.; writing—review and editing, I.E.S., K.P.B. and E.P.T.; project administration, K.P.B. and E.P.T.; funding acquisition, I.E.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation, grant number # 22-23-00048.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank D.E. Babushkin for the synthesis of Al(13CH3)3. NMR experiments were conducted using the equipment of the Department of Mechanisms of Catalytic Reactions, Boreskov Institute of Catalysis. AAB acknowledges the access to the supercomputer facilities of the Computing Centre of Novosibirsk State University.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00333 g001 550
Figure 1. Complexes 1 and 1′ studied herein.
Figure 1. Complexes 1 and 1′ studied herein.
Catalysts 13 00333 g001
Catalysts 13 00333 g002 550
Figure 2. The 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM) of the sample 1/AlMe3, recorded at different temperatures: 40 °C (a), 25 °C (b), and −20 °C (c). Asterisks mark the resonances that coalesce at 40 °C. Other resonances of 2 are highlighted.
Figure 2. The 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM) of the sample 1/AlMe3, recorded at different temperatures: 40 °C (a), 25 °C (b), and −20 °C (c). Asterisks mark the resonances that coalesce at 40 °C. Other resonances of 2 are highlighted.
Catalysts 13 00333 g002
Catalysts 13 00333 g003 550
Figure 3. The 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the samples 1/AlMe3 (a) and 1′/AlMe3 (b). The 2H NMR spectrum of the sample 1D/AlMe3 (toluene, [Al]/[Ni] = 10/1, CNi = 5 mM) recorded at 0 °C (c).
Figure 3. The 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the samples 1/AlMe3 (a) and 1′/AlMe3 (b). The 2H NMR spectrum of the sample 1D/AlMe3 (toluene, [Al]/[Ni] = 10/1, CNi = 5 mM) recorded at 0 °C (c).
Catalysts 13 00333 g003
Catalysts 13 00333 g004 550
Figure 4. Expanded regions (6.0…3.0 and 0.5…−1.5 ppm) of the 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the samples 1/AlMe3 (a) and 1/AlMe3* (b). AlMe3* is a trimethylaluminum enriched by 13C isotope (ca. 54 at.%).
Figure 4. Expanded regions (6.0…3.0 and 0.5…−1.5 ppm) of the 1H NMR spectra (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the samples 1/AlMe3 (a) and 1/AlMe3* (b). AlMe3* is a trimethylaluminum enriched by 13C isotope (ca. 54 at.%).
Catalysts 13 00333 g004
Catalysts 13 00333 g005 550
Figure 5. dept135 13C NMR spectrum (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the sample 1/AlMe3*. AlMe3* is a trimethylaluminum enriched by 13C isotope (ca. 54 at.%). Doubled asterisks mark solvent (toluene-d8) resonances.
Figure 5. dept135 13C NMR spectrum (toluene-d8, [Al]/[Ni] = 10/1, CNi = 5 mM, 0 °C) of the sample 1/AlMe3*. AlMe3* is a trimethylaluminum enriched by 13C isotope (ca. 54 at.%). Doubled asterisks mark solvent (toluene-d8) resonances.
Catalysts 13 00333 g005
Catalysts 13 00333 g006 550
Figure 6. Proposed resonance structures for 2.
Figure 6. Proposed resonance structures for 2.
Catalysts 13 00333 g006
Catalysts 13 00333 sch001 550
Scheme 1. Possible transformations of the nickel species in the catalyst system 1/AlMe3.
Scheme 1. Possible transformations of the nickel species in the catalyst system 1/AlMe3.
Catalysts 13 00333 sch001
Table
Table 1. Data of the ethylene polymerization over 1/AlMe3 catalyst system a.
Table 1. Data of the ethylene polymerization over 1/AlMe3 catalyst system a.
System[Al]/[Ni]Activity, kg (PE)·molNi−1·h−1Mw, g/molMn, g/molMw/MnBranches/1000 C
1/AlMe350027,00078,00039,0002.036 ± 2
a Polymerization conditions: toluene (100 mL); time 60 min; T = 50 °C; P(C2H4) = 5 bar; 2.0 μmol of pre-catalyst 1.
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MDPI and ACS Style

Soshnikov, I.E.; Semikolenova, N.V.; Bryliakova, A.A.; Antonov, A.A.; Bryliakov, K.P.; Talsi, E.P. New Ni(II)-Ni(II) Dinuclear Complex, a Resting State of the (α-diimine)NiBr2/AlMe3 Catalyst System for Ethylene Polymerization. Catalysts 2023, 13, 333. https://doi.org/10.3390/catal13020333

AMA Style

Soshnikov IE, Semikolenova NV, Bryliakova AA, Antonov AA, Bryliakov KP, Talsi EP. New Ni(II)-Ni(II) Dinuclear Complex, a Resting State of the (α-diimine)NiBr2/AlMe3 Catalyst System for Ethylene Polymerization. Catalysts. 2023; 13(2):333. https://doi.org/10.3390/catal13020333

Chicago/Turabian Style

Soshnikov, Igor E., Nina V. Semikolenova, Anna A. Bryliakova, Artem A. Antonov, Konstantin P. Bryliakov, and Evgenii P. Talsi. 2023. "New Ni(II)-Ni(II) Dinuclear Complex, a Resting State of the (α-diimine)NiBr2/AlMe3 Catalyst System for Ethylene Polymerization" Catalysts 13, no. 2: 333. https://doi.org/10.3390/catal13020333

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