Synthesis of a Ni Complex Chelated by a [2.2]Paracyclophane- Functionalized Diimine Ligand and Its Catalytic Activity for Oleﬁn Oligomerization

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Article Synthesis of a Ni Complex Chelated by a [2.2]Paracyclophane- Functionalized Diimine Ligand and Its Catalytic Activity for Oleﬁn Oligomerization

Daisuke Takeuchi 1,2,*, Yoshi-aki Tojo 1 and Kohtaro Osakada 1,3,*

1 Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan; [email protected] 2 Department of Frontier Materials Chemistry, Faculty of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki-shi, Aomori 036-8561, Japan 3 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan * Correspondence: [email protected] (D.T.); [email protected] or [email protected] (K.O.)

Abstract: A diimine ligand having two [2.2]paracyclophanyl substituents at the N atoms (L1) was prepared from the reaction of amino[2.2]paracyclophane with acenaphtenequinone. The ligand reacts

with NiBr2(dme) (dme: 1,2-dimethoxyethane) to form the dibromonickel complex with (R,R) and (S,S) conﬁguration, NiBr2(L1). The structure of the complex was conﬁrmed by X-ray crystallography. NiBr2(L1) catalyzes oligomerization of ethylene in the presence of methylaluminoxane (MAO) co-catalyst at 10–50 ◦C to form a mixture of 1- and 2-butenes after 3 h. The reactions for 6 h ◦ and 8 h at 25 C causes further increase of 2-butene formed via isomerization of 1-butene and formation of hexenes. Reaction of 1-hexene catalyzed by NiBr2(L1)–MAO produces 2-hexene via

Citation: Takeuchi, D.; Tojo, Y.-a.; isomerization and C12 and C18 hydrocarbons via oligomerization. Consumption of 1-hexene of the ‡ −1 Osakada, K. Synthesis of a Ni reaction obeys first-order kinetics. The kinetic parameters were obtained to be ∆G = 93.6 kJ mol , ‡ −1 ‡ −1 −1 Complex Chelated by a ∆H = 63.0 kJ mol , and ∆S = −112 J mol deg . NiBr2(L1) catalyzes co-dimerization of ethylene [2.2]Paracyclophane-Functionalized and 1-hexene to form C8 hydrocarbons with higher rate and selectivity than the tetramerization Diimine Ligand and Its Catalytic of ethylene. Activity for Olefin Oligomerization. Molecules 2021, 26, 2719. https:// Keywords: oligomerization; olefin; nickel; catalysts; N-ligand doi.org/10.3390/molecules26092719

Academic Editor: Michal Szostak 1. Introduction Received: 15 April 2021 The oligomerization of oleﬁns catalyzed by transition metal complexes has attracted Accepted: 3 May 2021 Published: 5 May 2021 attention, as shown by many review articles on this topic over the last decades [1–16] as well as recent original reports [17–19]. It is related to the industrial production of unsatu-

Publisher’s Note: MDPI stays neutral rated hydrocarbon materials. The mechanistic studies are of interest from the viewpoint with regard to jurisdictional claims in of catalytic and organometallic chemistry. Various complexes of early and late transition published maps and institutional affil- metals are employed as the catalyst for the oligomerization. Transition metal complexes iations. were reported to promote cross-dimerization of two alkynes and of alkyne with vinyl compounds to form enynes and dienes, respectively [20–24]. The cross-dimerization of two vinyl compounds has been focused on hydrovinylation of styrene and of olefins containing polar fnctional groups [25–35]. α,ω-Dienes undergo transition metal–catalyzed intramolecular hydrovinylation, which provides a convenient route to the cycloolefins [36–39]. On the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. other hand, intermolecular cross-dimerization of two hydrocarbon alkenes is rare. Hessen This article is an open access article reported that a constrained geometry complex (CGC)-type Ti complex catalyzed cross- distributed under the terms and trimerization of ethylene with 1–octene to form C12 products [40]. conditions of the Creative Commons Ni and Pd complexes with diimine ligands having bulky N-aryl substituents were Attribution (CC BY) license (https:// found to catalyze high-mass polymerization of ethylene and 1-olefins as well as copolymer- creativecommons.org/licenses/by/ ization of ethylene with acrylates [41]. The complexes with 2,6-disubstituted aryl groups 4.0/). at the coordinating nitrogens, 1a–1f, catalyze ethylene polymerization. Complexes 1g

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Ni and Pd complexes with diimine ligands having bulky N-aryl substituents were found to catalyze high-mass polymerization of ethylene and 1-olefins as well as copoly- Molecules 2021, 26, 2719 merization of ethylene with acrylates [41]. The complexes with 2,6-disubstituted2 of 13 aryl groups at the coordinating nitrogens, 1a–1f, catalyze ethylene polymerization. Complexes 1g and 1h with 4-substituted aryl groups at the nitrogen atoms catalyze oligomerization andof ethylene1h with 4-substitutedto form α-olefins aryl groups with Schultz–Flory at the nitrogen atomsdistribution catalyze [42,43]. oligomerization Subsequent of studies α ethyleneusing Ni to and form Pd -olefinscomplexes with with Schultz–Flory strically distributionbulky diimine [42, 43ligands,]. Subsequent 1i–1p, studiesas the catalysts using Ni and Pd complexes with strically bulky diimine ligands, 1i–1p, as the catalysts revealed the polymerization and co-polymerization of olefins with high productivity and revealed the polymerization and co-polymerization of olefins with high productivity and selectivityselectivity [44 [44–56].–56]. OccurrenceOccurrence of of polymerization polymerization or oligomerization or oligomerization of ethylene of ethylene depending depending on the sub- on the sub- stituentsstituents of of the the diimine diimine ligand ligand is rationalized is rationalized by the by insertion– the insertion–β-hydrogenβ-hydrogen elimination elimination mechanism,mechanism, as as shown shown in Schemein Scheme1. The 1. growingThe growing polymer polymer having having an alkyl–nickel an alkyl–nickel bond bond undergoesundergoesβ -hydrogenβ-hydrogen elimination elimination of vinylof vinyl group-terminated group-terminated oligomer oligomer to form to a hy-form a hydride(olefin)nickel(II)dride(olefin)nickel(II) species species (A). (A).

SchemeScheme 1. 1.Polymerization Polymerization vs. vs. oligomerization. oligomerization. (A,B )( denoteA) and intermediates (B) denote intermediates of the reactions. of the reactions.

IntermediateIntermediate (A ()A with) with the the ligand ligand having having 2,6-disubstituted 2,6-disubstitutedN-aryl N groups-aryl groups prefers re-prefers re- insertioninsertion of of the the vinyl vinyl group group into theinto Ni–H the bond,Ni–H andbond, resumes and resumes the polymer the growth polymer (path growth (i)). (path Ni center of intermediate (A) having the diimine ligand with 4-substituted N-aryl groups (i)). Ni center of intermediate (A) having the diimine ligand with 4-substituted N-aryl is sterically less crowded, and undergoes associative coorindation of an ethylene monomer atgroups the apical is sterically coordination less crowded, site of square-planar and underg Ni(II)oes associative center, forming coorinda intermediatetion of an (B) ethylene (pathmonomer (ii)). The at reactionthe apical is followed coordina bytion elimination site of square-planar of the oligomer havingNi(II) center, a vinyl endforming group interme- anddiate insertion (B) (path ofethylene (ii)). The into reaction the H–Ni is followed bond. Further by elimination insertion of ethyleneof the oligomer molecules having into a vinyl theend Ni–C group bond and provides insertion new of oligomer ethylene molecules. into the H–Ni In this bond. study, weFurther synthesized insertion the of Ni ethylene complexmolecules with into a diimine the Ni–C ligand bond having provides [2.2]paracyclophanyl new oligomer substituents molecules. at theIn Nthis-positions. study, we syn- Thethesized complex the is Ni expected complex to showwith newa diimine catalytic ligand properties having because [2.2]paracyclophanyl of the sterically bulky substituents Nat-cycloparaphenyl the N-positions. groups The complex of the ligand. is expected It catalyzes to oleﬁnshow oligomerization, new catalytic properties and ethylene– because of 1-hexene co-dimerization, in particular. Here, we report synthesis and structure of the new the sterically bulky N-cycloparaphenyl groups of the ligand. It catalyzes olefin oligomer- Ni-diimine complexes as well as its catalysis. ization, and ethylene–1-hexene co-dimerization, in particular. Here, we report synthesis 2.and Results structure and Discussion of the new Ni-diimine complexes as well as its catalysis. 2.1. Preparation and Structure of Ni Complexes 2. ResultsMono-substituted and Discussion [2.2]paracyclophane has a double-decker structure with a chiral center2.1. Preparation in the molecule. and Structure The transition of Ni metal Complexes complexes with the paracyclophane-containing nitrogen ligand, such as a Ti-Salen complex [57] and Au and Rh complexes with N- heterocyclicMono-substituted carbene (NHC) [2.2]paracyclophane ligands [58–60], were has employed a double-decker as the catalyst structure for stereos- with a chiral electivecenter in reactions. the molecule. We conducted The transition condensation metal ofcomplexes acenaphtenequinone with the paracyclophane-contain- with two molar equivalentsing nitrogen of amino[2.2]paracyclophaneligand, such as a Ti-Salen with complex expecting [57] formation and Au of aand diimine Rh complexes ligand hav- with N- ingheterocyclic two [2.2]paracyclophanyl carbene (NHC) substituents. ligands [58–60], The reaction were in employed reﬂuxing EtOH-AcOH as the catalyst proceeds for stereose- smoothlylective reactions. to form the We ligand, conducted according condensati to Equationon (1). of Bothacenaphtenequinone racemic and optically with active two molar amino[2.2]paracyclophanesequivalents of amino[2.2]paracyclophane were used in the ligand with synthesis. expecting formation of a diimine ligand having two [2.2]paracyclophanyl substituents. The reaction in refluxing EtOH-AcOH proceeds smoothly to form the ligand, according to Equation (1). Both racemic and optically active amino[2.2]paracyclophanes were used in the ligand synthesis.

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(1)

Figure 1a shows the 1H NMR spectra of the ligand L1, obtained from the racemic 11 FigureFigure(upper)1 a1a and shows shows optically the the HHactive NMRNMR (lower) spectraspectra amino[2.2]paracyclophanes, ofof thethe ligandligand L1L1,, obtainedobtained respectively. fromfrom thethe racemicracemic The charac- (upper)(upper)teristic andand optically opticallyaromatic activeactive hydrogen (lower)(lower) signals amino[2.2]paracyclophanes,amino[2.2]paracyclophanes, near the imine group are respectively.respectively. observed at TheThe the charac-charac- same posi- teristicteristictions. aromaticaromatic Total hydrogen hydrogenspectra of signals signalsthe ligand near near thefrom the imine theimine groupracemic group are and are observed opticallyobserved at theactive at samethe starting same positions. posi- materials Totaltions. spectra areTotal also spectra of identical. the ligand of the It from ligandsuggests the from racemic that the the racemic and ligand optically and from optically active a racemic starting active mixture materialsstarting has materials are(R,R also) or (S,S) identical. It suggests that the ligand from a racemic mixture has (R,R) or (S,S) conﬁguration. are alsoconfiguration. identical. It The suggests ligand that havi theng aligand meso structurefrom a racemic with (R mixture, S) or (S ,has R) configuration(R,R) or (S,S) is not The ligand having a meso structure with (R, S) or (S, R) conﬁguration is not contained in configuration.contained The in theligand product. having Figure a meso 1b structure shows results with ( Rof, SFAB-MAS) or (S, R) configurationmeasurement is of not L1 . The thecontained product. in Figurethe product.1b shows Figure results 1b ofsh FAB-MASows results measurement of+ FAB-MAS of measurementL1. The parent of peakL1. The at parent peak at m/z = 593 corresponds+ to [M–H] of L1. These spectroscopic data as well as m/z = 593 corresponds to [M–H] of L1. These+ spectroscopic data as well as the results parentthe peak results at m of/z elemental= 593 corresponds analysis toclea [M–H]rly indicateof L1 .the These formation spectroscopic of ligand data L1 asin wella pure as form. of elemental analysis clearly indicate the formation of ligand L1 in a pure form. Thus, the resultsThus, of condensation elemental analysis of acenaphtenequinone clearly indicate the formation with racemic of ligand amino[2.2]paracyclophane L1 in a pure form. condensation of acenaphtenequinone with racemic amino[2.2]paracyclophane forms L1 Thus,forms condensation L1 diastereoselectively. of acenaphtenequinone We usedwith racemicthe ligand amino[2.2]paracyclophane obtained from racemic diastereoselectively. We used the ligand obtained from racemic amino[2.2]paracyclophane formsamino[2.2]paracyclophane L1 diastereoselectively. for Weprepartion used ofthe the catalystsligand obtainedof this study. from racemic foramino[2.2]paracyclophane prepartion of the catalysts for of prepartion this study. of the catalysts of this study.

Figure 1. (a) 1H NMR spectra (aromatic hydrogen region) of ligand L1 obtained from racemic 1 FigureFigureamino[2.2]paracyclophane 1.1. ((aa)) 1HH NMR NMR spectra spectra (aromatic (aromatic (upper) hy anddrogen hydrogen from region) the region) optically of ligand of ligandamino[ L1 obtained2.2]paracyclophaneL1 obtained from from racemic racemic (lower). (b) amino[2.2]paracyclophaneamino[2.2]paracyclophaneFAB-MS spectrum of (upper) (upper) racemic and and L1 from obtainedfrom the the optically byoptically using amino[2.2]paracyclophane 2-nitrobamino[2.2]paracyclophaneenzyl alcohol as (lower).the (lower). matrix. (b) FAB-(b) MSFAB-MS spectrum spectrum of racemic of racemicL1 obtained L1 obtained by using by using 2-nitrobenzyl 2-nitrobenzyl alcohol alcohol as the as matrix. the matrix. The above 1H NMR spectra of L1 in Figure 1a contains the signals with a more num- 11 TheTheber abovethan that HH NMR expected NMR spectra spectra from of of theL1L1 in molecular inFigure Figure 1a stru1 containsa containscture. theIt theis signals attributed signals with with ato more the a morepresence num- of numberber thanconformational than that that expected expected isomers from from theof thethe molecular molecularcompounds stru structure. cture.in the Itsolution. Itis is attributed attributed Figure to to2a the depicts presence two ofisomersof conformationalconformationaldue to E and isomersisomers Z geometry ofof thethe compoundsaboutcompounds the C=N inin thebond,the solution.solution. while Figure FigureFigure 22b 2aa depictsshows depicts possible two two isomers isomers isomers by duedue totorotation EE andand ZZ of geometrygeometry the C-N about aboutbond thethebetween C=NC=N bond,bond,the [2.2]paracyclophanyl whilewhile FigureFigure2 2bb shows shows group possible possible and isomersthe isomers imine by by group. rotationrotationSterically ofof thethe C-N C-Ncrowded bondbond structure betweenbetween of thethe the [2.2]paracyclophanyl[2.2]paracyclophanyl molecule renders interconversion groupgroup andand thethe of imineimine the isomers group.group. diffi- Sterically crowded structure of the molecule renders interconversion of the isomers difﬁcult Stericallycult crowdedeven in the structure solution. of Figurethe molecule 2c shows renders the 1H interconversion NMR spectra at of high the isomerstemperatures. diffi- The even in the solution. Figure2c shows the 1H NMR1 spectra at high temperatures. The signals cult evensignals in the are solution. broadened Figure above 2c 90shows °C, but the doH notNMR undergo spectra coalescence, at high temperatures. which suggests The that are broadened above 90 ◦C, but do not undergo coalescence, which suggests that the signalsthe are interconversion broadened above among 90 °C, the butconformational do not undergo isomers coalescence, is slower which than the suggests NMR time that scale. interconversionthe interconversion among among the conformationalthe conformational isomers isomers is slower is slower than than the the NMR NMR time time scale. scale.

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1 FigureFigure 2. (2.a,b (a,b) Possible) Possible conformational conformational isomers isomers of L1of .L1 (c.) (Temperaturec) Temperature dependent dependent 1H 1 NMRH NMR spectra spectra FigureFigure 2. 2. (a,b(a,)b Possible) Possible conformational conformational isomers isomers of of L1L1. (.(c)c Temperature) Temperature dependent dependent 1HH NMR NMR spectra spectra of L1of L1at 25–120at 25–120 °C.◦ °C. ofof L1L1 atat 25–120 25–120 °C.C.

Ligand L1 reacts with NiBr2(dme) (dme = 1,2-dimethoxyethane) at room temperature LigandLigand L1L1 reactsreacts with with NiBr NiBr2(dme)2(dme) (dme (dme = 1,2-dimethoxyethane) = 1,2-dimethoxyethane) at atroom room temperature temperature Ligand L1 reacts with NiBr2(dme) (dme = 1,2-dimethoxyethane) at room temperature 2 to totoform formform the thethe complex complexcomplex formulated formulatedformulated as as asNiBr NiBr NiBr2(L12((L1),L1 as),), as asshown shownshown in ininEquation EquationEquation (2). (2).(2). A AdirectA directdirect reaction reactionreaction to form the complex formulated as NiBr2(L1), as shown in Equation (2). A direct reaction 2 ofof ofNiBr NiBrNiBr2 with2 withwith 2,5-dimethylaniline 2,5-dimethylaniline2,5-dimethylaniline and andand acetonaphtequinone acetonaphtequinoneacetonaphtequinone produces producesproduces Ni NiNi compex compexcompex with withwith a aa of NiBr2 with 2,5-dimethylaniline and acetonaphtequinone produces Ni compex with a ligandligandligand having havinghaving 2,5-dimethylphenyl 2,5-dimethylphenyl2,5-dimethylphenyl subs substituents substituentstituents at at theat the the imine imine imine nitrogen, nitrogen, nitrogen, NiBr NiBr NiBr2(L22(2(L2),L2 ),as), as asshown shownshown ligand having 2,5-dimethylphenyl substituents at the imine nitrogen, NiBr2(L2), as shown inin EquationEquation (3).(3). LigandLigand L2L2 alsoalso hashas 2,5-disubstituted2,5-disubstituted arylaryl groupsgroups atat thethe imineimine nitrogens,nitrogens, inin EquationEquation (3).(3). LigandLigand L2L2 alsoalso hashas 2,5-disubstituted 2,5-disubstituted arylaryl groupsgroups at at thethe imineimine nitrogens, nitrogens, similarsimilar totoL1 L1,, butbut isis stericallysterically muchmuch less less bulky bulky than thanL1 L1.. CatalyticCatalytic activityactivity of of the the complex complex is is similarsimilar to to L1 L1, ,but but is is sterically sterically much much less less bulky bulky than than L1 L1. .Catalytic Catalytic activity activity of of the the complex complex is is comparedcomparedcompared with withwith that thatthat of ofofNiBr NiBrNiBr2(L122((L1),L1 having),), havinghaving the thethe sterically stericallysterically more moremore crowded crowdedcrowded ligand. ligand.ligand. compared with that of NiBr2(L1), having the sterically more crowded ligand.

(2)(2)(2)

(3)(3)(3)

Figure 3 shows the molecular structure of NiBr2(L1) determined by X-ray crystallog- Figure 3 shows the molecular structure of NiBr2(L1) determined by X-ray crystallog- raphyFigureFigure [61]. 33 Twoshowsshows [2.2]paracyclophanyl the the molecular structure substituents ofof NiBrNiBr2( (L1areL1)) determinedorientated determined to by the X-ray opposite crystallogra-crystallog- side of raphyraphyphythe acenaphtene [ [61].61[61].]. Two TwoTwo [2.2]paracyclophanyl [2.2]paracyclophanyl[2.2]paracyclophanyl group. The Ni center substituents substituentssubstituents has the distorted are areare orientated orientatedorientated tetrahedral to toto the thethe structure, opposite oppositeopposite sidesuggesting sideside of of theof thetheacenaphteneparamagnetic acenaphtene acenaphtene group. high-spingroup. group. The The The Nicomplex Ni Ni center center center hasof has ahas thed8 th metalth distortedee distorted distorted center. tetrahedral tetrahedral Thetetrahedral [2.2]paracyclophanyl structure, structure, structure, suggesting suggesting suggesting substitu- para- 8 paramagneticparamagneticmagneticents of the high-spin ligandhigh-spin high-spin are complex complex expectedcomplex of of aofto da 8a influenced metald 8metal metal center. center. center.stability The The The of [2.2]paracyclophanyl [2.2]paracyclophanyl[2.2]paracyclophanylthe intermediates with substituents substitu- substitu- polymer entsentsofand theofof monomer thethe ligand ligandligand are ligands areare expected expectedexpected and toselectivity inﬂuencetoto influenceinfluence of stabilitythe stabilitystability reaction. of theofof thethe intermediates intermediatesintermediates with withwith polymer polymerpolymer and andandmonomer monomer monomer ligands ligands ligands and and and selectivity selectivity selectivity of of theof the the reaction. reaction. reaction. The crystal structure indicates that the ligand and Ni center forms a C2 symmetrical space around the Ni center. Ni and Pd complexes 1k, 1l, 1n in Chart1 also have coordination of the diimine ligand with C2 symmetrical structures. Polymerization of oleﬁns using these complexes as the catalyst was reported to occur stereoelectively. Investigation of a dinickel catalyst having a C2 symmetrical space around the Ni(II) center revealed relevance of the detailed coordination structure of the complex to productivity and selectivity of the catalysis [62].

Figure 3. Crystallographic structure of NiBr2(L1)·(C2H4Cl2). Selected bond distances (Å) and angles (°): Ni1–Br1 2.3375(15), Ni1–Br2 2.3392(16), Ni1–N1 2.028(5), Ni1–N2 2.035(5), Br1–Ni1–Br2

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Figure 2. (a,b) Possible conformational isomers of L1. (c) Temperature dependent 1H NMR spectra of L1 at 25–120 °C.

Ligand L1 reacts with NiBr2(dme) (dme = 1,2-dimethoxyethane) at room temperature to form the complex formulated as NiBr2(L1), as shown in Equation (2). A direct reaction of NiBr2 with 2,5-dimethylaniline and acetonaphtequinone produces Ni compex with a ligand having 2,5-dimethylphenyl substituents at the imine nitrogen, NiBr2(L2), as shown in Equation (3). Ligand L2 also has 2,5-disubstituted aryl groups at the imine nitrogens, similar to L1, but is sterically much less bulky than L1. Catalytic activity of the complex is compared with that of NiBr2(L1), having the sterically more crowded ligand.

Figure 3 shows the molecular structure of NiBr2(L1) determined by X-ray crystallography [61]. Two [2.2]paracyclophanyl substituents are orientated to the opposite side of the acenaphtene group. The Ni center has the distorted tetrahedral structure, suggesting paramagnetic high-spin complex of a d8 metal center. The [2.2]paracyclophanyl substitu- Molecules 2021, 26, 2719 5 of 13 ents of the ligand are expected to influence stability of the intermediates with polymer Molecules 2021, 26, 2719 5 of 13 and monomer ligands and selectivity of the reaction.

128.90(5), Br1–Ni1–N1 104.45(15), Br1–Ni1–N2 113.19(15), Br2–Ni1–N2 104.02(15), Br2–Ni1–N1 113.54(15), N1–Ni1–N2 83.7(2). The solvent molecule and hydrogen atoms are omitted for simplicity.

The crystal structure indicates that the ligand and Ni center forms a C2 symmetrical space around the Ni center. Ni and Pd complexes 1k, 1l, 1n in Chart 1 also have coordination of the diimine ligand with C2 symmetrical structures. Poly merization of olefins us-

Figureing these 3. Crystallographic complexes as thestructure catalyst of NiBr was2( L1report)·(C2Hed4Cl to2). occur Selected stereoelectively. bond distances (Å)Investigation and angles Figure 3. Crystallographic structure of NiBr2(L1)·(C2H4Cl2). Selected bond distances (Å) and angles ((°):◦of): aNi1–Br1Ni1–Br1 dinickel 2.3375(15),2.3375(15), catalyst Ni1–Br2 Ni1–Br2having 2.3392(16), a2.3392(16), C2 symmetrical Ni1–N1 Ni1–N1 2.028(5), space 2.028(5), around Ni1–N2 Ni1–N2 the 2.035(5), 2.035(5), Ni(II) Br1–Ni1–Br2 centerBr1–Ni1–Br2 revealed 128.90(5), rel- Br1–Ni1–N1evance of the 104.45(15), detailed Br1–Ni1–N2coordination 113.19(15), structure Br2–Ni1–N2 of the complex 104.02(15), to productivity Br2–Ni1–N1 and 113.54(15), selectiv- N1–Ni1–N2ity of the catalysis 83.7(2). The [62]. solvent molecule and hydrogen atoms are omitted for simplicity.

ChartChart 1.1.Ni Ni andand PdPd diiminediimine complexescomplexes forfor catalyticcatalytic polymerization polymerization and and oligomerization. oligomerization.

2.2.2.2. OleﬁnOlefin OligomerizationOligomerization CatalyzedCatalyzed byby NiNi ComplexesComplexes

OligomerizationOligomerization of ethylene and and 1-hexene 1-hexene was was studied studied by by using using NiBr NiBr2(L12()L1 as) the as thecat- catalystalyst and and methylaluminoxane methylaluminoxane (MAO) (MAO) as as the the co-catalyst. co-catalyst. Table Table 11 summarizes summarizes results results of of ◦ ethyleneethylene oligomerizationoligomerization catalyzedcatalyzed by NiBr2((L1L1).). The The reactions reactions at at 10 10 °CC form form mixtures mixtures of ◦ of1-butene 1-butene and and 2-butene 2-butene (entries (entries 1, 1, 2). 2). The The reactions reactions at at 25 °CC form the butenesbutenes inin largerlarger amountsamounts andand C6C6 hydrocarbonhydrocarbon products, products, as as conﬁrmed confirmed by by GPC GPC analysis analysis (entries (entries 3–6). 3–6).

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Table 1. Ethylene oligomerization catalyzed by NiBr2(L1) a Table 1. Ethylene oligomerization catalyzed by NiBr (L1) a. Conditions 2 Products/mmol Entry Catalyst C4TOF/h−1 b Temp/°CConditions Time/h 1-Butene Products/mmol 2-Butene Hexenes Entry Catalyst C4TOF/h−1 b 1 NiBr2(L1) Temp/10◦C Time/h 3 1-Butene 0.32 2-Butene 0.11 Hexenes 0.00 29 2 NiBr2(L1) 10 6 0.85 0.90 0.00 58 1 NiBr2(L1) 10 3 0.32 0.11 0.00 29 3 NiBr2(L1) 25 1 0.15 0.05 0.01 40 2 NiBr2(L1) 10 6 0.85 0.90 0.00 58 34 NiBrNiBr2(L12(L1) ) 2525 1 3 0.15 0.67 0.05 0.63 0.01 0.10 40 86 45 NiBrNiBr2(L12(L1) ) 2525 3 6 0.67 0.98 0.63 2.78 0.10 0.75 86 126 5 NiBr2(L1) 25 6 0.98 2.78 0.75 126 6 NiBr2(L1) 25 8 0.99 3.75 1.10 124 6 NiBr2(L1) 25 8 0.99 3.75 1.10 124 7 NiBr2(L2) 25 1 c – c c– c c– c c– c 7 NiBr2(L2) 25 1 – – – – 88 NiBrNiBr2(L12(L1) ) 5050 3 3 0.12 0.12 0.18 0.18 0.00 0.00 20 20 99 NiBrNiBr2(L12(L1) ) 5050 6 6 0.20 0.20 0.34 0.34 0.00 0.00 18 18 aa Conditions: [Ni] [Ni] 0.010 0.010 mmol, mmol, MAO MAO co-catalyst co-catalyst ([Al]/[Ni] ([Al]/[Ni] = 300), = 300), toluene toluene 1 mL, 1 ethylene mL, ethylene 1 atm. b 1TOF atm. [mol b − − × 1 1 c −1 −1 c (2TOFC4)][mol [mol (2 cat.]× C4)][molh . cat.]The product h . The was product polyethylene was withpolyethyleneMn = 1000 with (GPC). Mn = 1000 (GPC).

TheThe reactionreaction yieldsyields 1-1- andand 2-butenes2-butenes inin 3:13:1 molarmolar ratioratio afterafter 11 h,h, whilewhile furtherfurther reactionreaction causescauses relativerelative increase of of 2-butene 2-butene and and format formationion of of hexenes hexenes after after 3 h. 3 Figure h. Figure 4 plots4 plots time timeprofile proﬁle of the of reaction, the reaction, which which suggests suggests that init thatially initially formed formed 1-butene 1-butene is isomerized is isomerized into 2- intobutene 2-butene during during the reaction. the reaction. Turn Turnover overfrequency frequency (TOF) (TOF) for formation for formation of the of thebutenes butenes in- increasescreases for for initial initial 6 h, 6 h,and and becomes becomes constant constant afte afterr 6 h. 6 h.It suggests It suggests that that active active species species of the of thecatalysis catalysis are areincreased increased slowly slowly under under the the conditions. conditions.

FigureFigure 4.4. ReactionReaction proﬁleprofile of ethylene oligomerization catalyzed catalyzed by by NiBr NiBr2(2L1(L1)–MAO;)–MAO; (i) (i) 1-butene, 1-butene, (ii)(ii) 2-butene,2-butene, (iii)(iii) 1-hexene.1-hexene. Et Ethylene:hylene: 1 1 atm, atm, Ni: Ni: 0.010 0.010 mol, mol, [A [Al]/[Ni]l]/[Ni] = 300, = 300, toluene toluene 10 10mL, mL, 25 °C. 25 ◦ C. ProductProduct amountsamounts areare determineddetermined by by GPC GPC analysis. analysis.

MaximumMaximum TOFTOF ofof thethe reactionreaction isis calculatedcalculated from from the the total total amountamount of of 1-1- andand 2-butenes2-butenes toto bebe 124–126124–126 (h −1)1 )under under 1 1atm atm ethylene ethylene at at25 25°C ◦(entriesC (entries 5, 6). 5, Ni-diimine 6). Ni-diimine complex complex with with4-methylphenyl 4-methylphenyl substituents substituents at the atimine the nitrogen, imine nitrogen, 1g, was1g reported, was reported to catalyze to catalyzeethylene ethyleneoligomerization oligomerization to α-olefins to α up-oleﬁns to C20 up with to C20 TOF with of 53,000–57,000 TOF of 53,000–57,000 (h−1) at 35 (h −°C1) under at 35 ◦ C56 underatm of 56 ethylene atm of ethylene[42]. TOF [42 of]. the TOF reaction of the reaction catalyzed catalyzed by NiBr by2(L1 NiBr) and2(L1 averaged) and averaged carbon carbonnumber number of the ofproducts the products are smaller are smaller than 1g than, even1g, evenwhen when different different temperature temperature and andeth- ethyleneylene pressure pressure are are considered. considered. It Itis is ascribed ascribed to to severe severe steric steric hindrance hindrance of the NiNi centercenter ofof NiBrNiBr22((L1L1)) bondedbonded with with the the diimine diimine ligand ligand with with [2.2]paracyclophane [2.2]paracyclophane substituents. substituents. Reaction Reac- oftion ethylene of ethylene catalyzed catalyzed by NiBrby NiBr2(L22)(L2 under) under similar similar conditions conditions did did not not form form C4- C4- nor nor C6-C6- oligomers,oligomers, butbut producedproduced aa lowlow molecularmolecular weightweight polyethylene polyethylene as as a a wax wax solid solid ( M(Mnn= = 1000,1000, MMww/MMnn == 2.87 2.87 based based on on GPC GPC using polystyrene standards) (entry 7).7). TheThe activityactivity ofof thethe ◦ ◦ reactionreaction byby NiBrNiBr22((L1L1)) catalystcatalyst atat 5050 °CC isis muchmuch lowerlower thanthan 2525 °CC (entries(entries 8,9).8,9). TheThe catalyticcatalytic activityactivity ofof NiBrNiBr22((L1L1)) isis comparedcompared withwith thethe Ni-diimineNi-diimine complexescomplexes re-re- portedported soso far.far. TheThe NiNi complexcomplex havinghaving 4-alkylphenyl4-alkylphenyl groupsgroups atat thethe imineimine nitrogennitrogen ofof thethe diaminediamine ligandligand catalyzescatalyzes ethyleneethylene oligomerizationoligomerization withwith highhigh TOFTOF becausebecause ofof frequentfrequent ββ-- hydrogen elimination of the oligomers caused by associative exchange of the coordinated

Molecules 2021, 26, 2719 7 of 13

oligomer molecule by a new ethylene monomer [42]. The complex with 2,5-disubstituted phenyl group, NiBr2(L2), also produces the oligomer with Mn = 1000, as shown above. The complexes having bulky 2,6-disubstituted or 2,4,6-trisubstituted aryl groups at the diimine nitrogen catalyze high mass polymerization of ethylene because the associative chain transfer of the polymer molecule is inhibited strictly by the bulky aryl groups at the imine nitrogen [41]. NiBr2(L1) of this study has a more bulky ligand than the ligands of the above studies, and catalyzes dimerization and trimerization of ethylene. Reaction of 1-hexene catalyzed by NiBr2(L1)-MAO ([Al]/[Ni] = 300) causes isomerization of the substrate to 2-hexene and dimerization and trimerization of 1-hexene to form C12 and C18 products. The isomerization occurs more readily than the oligomerization under the examined conditions. Results of the reactions under different conditions are summarized in Table2. The reactions at 10 ◦C with MAO ([Al]/[Ni] = 300) and at 25 ◦C with a smaller amount of MAO ([Al]/[Ni] = 50) (entries 1–3) show lower catalytic activity than those at 25 ◦C and [Al]/[Ni] = 300 (entry 4, 5). At 35 ◦C and 50 ◦C, TOF for the oligomerization is high for the initial 0.5 h (28 and 61/h−1, respectively) and become much lower after 6 h. It indicates that the catalytic activity decreases rapidly for several hours. The addition of MAO in a larger amount ([Al]/[Ni] = 1000) does not increase the oligomer yields. The product ratios after the reaction for 24 h vary depending on the temperature (entries 2, 5, 9, 12), which is shown in Figure5. The reaction for 24 h at 50 ◦C forms the trimer as the main product (entry 12). Use of modiﬁed methylaluminoxane (MMAO) as the co-catalyst decreases the oligomer yields (entry 13). The reactions using AlMe3 and Et2AlCl co-catalysts yield 2-hexene exclusively (entries 14, 15).

a Table 2. Oligomerization of 1-hexene catalyzed by NiBr2(L1) .

Conditions Products (%) b C12,C18 Entry Co-Catalyst − Temp/◦C Time/h 2-Hexene C12 C18 TOF/h 1 1 MAO (300) 10 6 34 2.9 6.6 4.8 2 MAO (300) 10 24 79 9.5 8.6 2.3 3 MAO (50) 25 24 66 2.6 17 2.5 4 MAO (300) 25 6 56 7.3 13 10.2 5 MAO (300) 25 24 62 11 18 3.6 6 MAO (1000) 25 24 21 2.1 18 2.5 7 MAO (300) 35 0.5 33 4.6 0.0 28 8 MAO (300) 35 6 54 16 5.6 11 9 MAO (300) 35 24 45 22 28 6.3 10 MAO (300) 50 0.5 49 7.2 3.0 61 11 MAO (300) 50 6 40 11 34 23 12 MAO (300) 50 24 23 12 57 8.6 13 MMAO (300) 25 6 37 6.8 18 12 14 AlMe3 (300) 50 1 98 0.0 0.0 0.0 15 Et2AlCl (300) 50 1 95 0.0 0.0 0.0 a b Conditions: catalyst NiBr2(L1), [Ni] 0.010 mmol, [1-hexene]/[Ni] = 300, solvent toluene (1.5 mL). [Al]/[Ni] is shown in parenthesis.

Figure6 shows time-conversion (a) and first-order plots (b) of the total reaction at 10 ◦C, 25 ◦C, 35 ◦C, and 50 ◦C. The reaction obeys first-order kinetics to the concentration of 1-hexene. The kinetic parameters of the reaction were determined from Eyring plots to be ∆G‡ = 93.6 kJ mol−1, ∆H‡ = 63.0 kJ mol−1, ∆S‡ = −112 J mol−1deg−1. Isomerization of 1- hexene into 2-hexene proceeds via insertion of the olefin into a Ni–H bond and subsequent β-hydrogen elimination of the internal olefin. Formation of C12 and C18 products is induced by insertion of 1-hexene into the Ni-C bond followed by β-hydrogen elimination of the products. The above kinetics for the reaction suggests that insertion of 1-hexene into the Ni–H and Ni–C bonds is the rate-determining step of the reaction. Molecules 2021, 26, 2719 8 of 13

100

C18 80

60 C12 yield (%) yield MoleculesMolecules 20212021, 26,, 262719, 2719 40 8 of 13 8 of 13

20 2-hexene

100 10 oC 25 oC 35 oC 50 oC temperature C18 80 Figure 5. Temperature effect of oligomerization of 1-hexene. 1-Hexene: 3.0 mmol, Ni: 0.010 mol, [Al]/[Ni]60 = 300, toluene 10 mL, 24 h.C12 Conversion of 1-hexene is 83–95% for the reactions. yield (%) yield 40Figure 6 shows time-conversion (a) and first-order plots (b) of the total reaction at 10 °C, 25 °C, 35 °C, and 50 °C. The reaction obeys first-order kinetics to the concentration of 1-hexene.20 The kinetic parameters of the reaction were determined from Eyring plots to be 2-hexene ΔG‡ = 93.6 kJ mol−1, ΔH‡ = 63.0 kJ mol−1, ΔS‡ = −112 J mol−1deg−1. Isomerization of 1-hexene into 2-hexene proceeds via insertion of the olefin into a Ni–H bond and subsequent β- o o o o hydrogen10 elimination C 25 Cof the35 internal C olefin. 50 C Formation of C12 and C18 products is induced by insertion of 1-hexenetemperature into the Ni-C bond followed by β-hydrogen elimination of the FigureFigureproducts. 5. 5.Temperature Temperature The above effect kinetics effect of oligomerization forof oligomerization the reaction of 1-hexene. suggests of 1- 1-Hexene:hexene. that insertion 1-Hexene: 3.0 mmol, of 1-hexene Ni: 3.0 0.010 mmol, mol, into Ni: the 0.010 mol, [Al]/[Ni][Al]/[Ni]Ni–H and = = 300, Ni–C300, toluene toluene bonds 10 mL,is 10 the 24mL, h.rate-determining Conversion24 h. Conversi of 1-hexene onstep of of is1-hexene 83–95%the reaction. for is the83–95% reactions. for the reactions.

Figure 6 shows time-conversion (a) and first-order plots (b) of the total reaction at 10 °C, 25 °C, 35 °C, and 50 °C. The reaction obeys first-order kinetics to the concentration of 1-hexene. The kinetic parameters of the reaction were determined from Eyring plots to be ΔG‡ = 93.6 kJ mol−1, ΔH‡ = 63.0 kJ mol−1, ΔS‡ = −112 J mol−1deg−1. Isomerization of 1-hexene into 2-hexene proceeds via insertion of the olefin into a Ni–H bond and subsequent β- hydrogen elimination of the internal olefin. Formation of C12 and C18 products is induced by insertion of 1-hexene into the Ni-C bond followed by β-hydrogen elimination of the products. The above kinetics for the reaction suggests that insertion of 1-hexene into the Ni–H and Ni–C bonds is the rate-determining step of the reaction.

FigureFigure 6. 6.Oligomerization Oligomerization of 1-hexeneof 1-hexene catalyzed catalyzed by NiBr by 2NiBr(L1)–MAO.2(L1)–MAO. (a) Time (a) conversionTime conversion of 1-hexene of 1- athexene 25 ◦C. at (b )25 First-order °C. (b) First-order plots of the plots total of reaction. the total reaction.

ReactionReaction of of a mixture a mixture of ethylene of ethylene and 1-hexene and 1-hexene catalyzed byNiBrcatalyzed2(L1 )-MMAObyNiBr2( formedL1)-MMAO C8formed products C8 products in a higher in a amount higher amount than C10–C16 than C10–C16 products. products. Figure7 Figurecompares 7 compares results of results GLCof GLC measurement measurement of the of reactionthe reaction mixture mixture with wi thatth that of ethylene of ethylene oligomerization oligomerization under under similarsimilar conditions. conditions. The The products products of of the the reaction reaction of 1-hexeneof 1-hexene under under ethylene ethylene atmosphere atmosphere containcontain C8 C8 (0.92 (0.92 mmol), mmol), C10 C10 (0.24 (0.24 mmol), mmol), and and C12 C12 (0.095 (0.095 mmol), mmol), as shown as shown in Figure in Figure7a. 7a. Figure7b shows the results of the reaction of ethylene, producing C4 and C6 hydrocarbons Figure 7b shows the results of the reaction of ethylene, producing C4 and C6 hydrocar- in main. The amounts of higher hydrocarbon products, C8 (0.076 mol), C10 (0.046 mmol), bons in main. The amounts of higher hydrocarbon products, C8 (0.076 mol), C10 (0.046 and C12 (0.017 mmol), are smaller than the reaction of ethylene and 1-hexene, as shown in Figuremmol),7b. and Thus, C12 the (0.017 reaction mmol), of ethylene are smaller and 1-hexenethan the reaction forms the of hydrocarbon ethylene and via 1-hexene, cross- as dimerizationshown in Figure much 7b. more Thus, rapidly the thanreaction tetramerization of ethylene ofand ethylene 1-hexene and forms cross-trimerization the hydrocarbon via cross-dimerization much more rapidly than tetramerization of ethylene and cross-tri- (C10 hydrocarbons), and cross-tetramerization (C12 hydrocarbons). The experimental resultsmerization at present, (C10 however,hydrocarbons), are not sufﬁcientand cross-tetramerization to discuss detailed reaction(C12 hydrocarbons). pathways for theThe ex- selectiveFigureperimental 6. cross-dimerization. Oligomerization results at present, of however,1-hexene arecatalyzed not sufficient by NiBr to2 (discussL1)–MAO. detailed (a) Time reaction conversion path- of 1- hexeneways for at the 25 selective°C. (b) First-order cross-dimerization. plots of the total reaction.

Reaction of a mixture of ethylene and 1-hexene catalyzed byNiBr2(L1)-MMAO formed C8 products in a higher amount than C10–C16 products. Figure 7 compares results of GLC measurement of the reaction mixture with that of ethylene oligomerization under similar conditions. The products of the reaction of 1-hexene under ethylene atmosphere contain C8 (0.92 mmol), C10 (0.24 mmol), and C12 (0.095 mmol), as shown in Figure 7a. Figure 7b shows the results of the reaction of ethylene, producing C4 and C6 hydrocarbons in main. The amounts of higher hydrocarbon products, C8 (0.076 mol), C10 (0.046 mmol), and C12 (0.017 mmol), are smaller than the reaction of ethylene and 1-hexene, as shown in Figure 7b. Thus, the reaction of ethylene and 1-hexene forms the hydrocarbon via cross-dimerization much more rapidly than tetramerization of ethylene and cross-trimerization (C10 hydrocarbons), and cross-tetramerization (C12 hydrocarbons). The experimental results at present, however, are not sufficient to discuss detailed reaction pathways for the selective cross-dimerization.

Molecules 2021, 26, 2719 9 of 13 Molecules 2021, 26, 2719 9 of 13

Figure 7. GC elution of the product. (a) Reaction of ethylene and 1-hexene catalyzed by NiBr2(L1) Figure 7. GC elution of the product. (a) Reaction of ethylene and 1-hexene catalyzed by NiBr2(L1) −MMAO. (b) Reaction of ethylene catalyzed by NiBr2(L1) −MMAO. Ni complex 0.010 mmol, −MMAO. (b) Reaction of ethylene catalyzed by NiBr2(L1) −MMAO. Ni complex 0.010 mmol, [Al]/[Ni] = 300. Pentane 8 mL, 25 °C, 24 h. [Al]/[Ni] = 300. Pentane 8 mL, 25 ◦C, 24 h.

3.3. ConclusionsConclusions ThisThis paperpaper presentspresents diastereoselectivediastereoselective preparationpreparation ofof diminedimine ligandligand L1L1 withwith twotwo [2.2]paracyclophanyl[2.2]paracyclophanyl groups,groups, via condensation of of acenaphtenequinone acenaphtenequinone with with two two equiva- equiv- alentslents of of amino[2.2]paracyclopheylene, and and its complexation with Ni(II)Ni(II) centercenter toto formform NiBrNiBr22((L1).). X-ray crystallographic results ofof NiBrNiBr22((L1)) showedshowed thethe molecularmolecular structurestructure whosewhose paracyclophanylparacyclophanyl groups are at at the the positi positionsons close close to to the the Ni Ni center. center. The The complex, complex, in inthe the presence presence of of MAO MAO co-catalyst, co-catalyst, catalyzes catalyzes o oligomerizationligomerization of of ethylene toto formform mixturesmixtures ofof 1-1- and 2-butenes at at 10–50 10–50 °C◦C with with the the highest highest TOF TOF for for butene butene formation formation (126 (126 h−1). h −The1). Thereaction reaction of 1-hexene of 1-hexene using using the thesame same catalyst catalyst causes causes isomerization isomerization into into 2-hexene 2-hexene and and oli- oligomerizationgomerization to to C12 C12 and and C18 C18 products. products. The The total total reaction reaction obeys obeys first-order first-order kineticskinetics toto thethe amountamount ofof 1-hexene,1-hexene, suggestingsuggesting thethe rate-determiningrate-determining step step at atthe theinsertion insertionof of1-hexene 1-hexene into into Ni–HNi–H andand Ni–C Ni–C bonds. bonds. NiBr NiBr2(2L1(L1)) catalyzes catalyzes cross-dimerization cross-dimerization of of ethylene ethylene with with 1-hexene 1-hexene to formto form C8 C8 products products in thein the presence presence of MMAO,of MMA whichO, which occurs occurs more more readily readily than than tetrameriza- tetramer- tionization of ethylene of ethylene and and than than the cross-oligomerizationthe cross-oligomerization of the of two the olefins,two olefins, giving giving C10 andC10 C12and products,C12 products, under under the same the conditions.same conditions. Thus, NiBrThus,2(L1 NiBr) with2(L1 an) with extremely an extremely bulky diimine bulky liganddiimine catalyze ligand dimerizationcatalyze dimerization and trimerization and trimerization of ethylene of ratherethylene than rather formation than formation of higher oligomersof higher oligomers or high mass or high polymers. mass polymers. The unique The properties unique properties of the catalysis of the iscatalysis a selective is a formationselective formation of the cross-dimer of the cross-dimer of ethylene of and ethylene 1-hexene. and The 1-hexene. elucidation The ofelucidation the mechanism of the formechanism the selective for the co-dimerization selective co-dimerization reaction is a problemreaction is left a problem for future left research. for future research. 4. Experimental Section 4. Experimental Section 4.1. General 4.1. General All the chemicals were commercially available. MAO and MMAO were purchased fromAll Tosoh the Co.chemicals Ltd. (Tokyo, were commercially Japan) as toluene available. solutions. MAO1 Hand and MMAO13C{1H} were NMR purchased spectra 1 13 1 werefrom acquiredTosoh Co. on Ltd. a Bruker as toluene AV-400M. solutions. Thechemical H and shiftsC{ H} wereNMR referenced spectra were with acquired respect toon a Bruker AV-400M. The chemical1 shifts were referenced with respect to CHCl3 (δ 7.26), CHCl3 (δ 7.26), HDO (δ 4.79) for H, and CDCl3 (δ 77.0), DSS (sodium 3-(trimethylsilyl)-1- 1 propanesulfonate)HDO (δ 4.79) for (δH,0.0) and for CDCl13C as3 ( internalδ 77.0), DSS standards. (sodium 3-(trimethylsilyl)-1-propanesulfonate) (δ 0.0) for 13C as internal standards. 4.2. Preparation of Racemic Ligand L1 4.2. Preparation(rac)-Amino[2.2]paracylophane of Racemic Ligand L1 was prepared by the reported method [63], and modi- fication(rac of)-Amino[2.2]paracylophane the final step, Curtius rearrangement, was prepared results by the in thereported product method in overall [63], 61% and yield.mod- Aification mixture of of therac -amino[2.2]paracyclophanefinal step, Curtius rearrangement, (0.40 g, 1.8 results mmol), in acenaphtenequinonethe product in overall (0.15 61% g, 0.81yield. mmol), A mixture and aof small rac-amino[2.2]paracyclophane amount of acetic acid in EtOH (0.40 (35 g, 1.8 mL) mmol), was heated acenaphtenequinone for 35 h under reflux.(0.15 g, After 0.81 removalmmol), and of the a small solvent, amount purification of acetic by acid silica in gel EtOH column (35 mL) (hexane/CH was heated2Cl 2for, 2:1; 35 1 13 1 Rhf =under 0.3) yielded reflux. ligandAfter L1removalas an orangeof the solidsolvent, (0.26 purification g, 0.43 mmol, by 53%). silica The gelH column and C{ (hex-H} NMRane/CH spectra2Cl2, 2:1; indicated Rf = 0.3) the yielded presence ligand of conformation L1 as an orange isomer solid whose (0.26 g, structural 0.43 mmol, details 53%). were The not1H and clarified. 13C{1H} The NMR obtained spectra ligand indicated was used the presence for preparation of conformation of the complex isomer directly. whose Anal.struc- Calcd for C44H36N2: C 89.15; H 6.12; N 4.73. Found C 89.28; H 6.12, N 4.60.

Molecules 2021, 26, 2719 10 of 13

4.3. Preparation of Optically Active Ligand L1 A mixture of (R)-(–)-amino[2.2]paracyclophane (50 mg, 0.22 mmol) [64–68] and acetonaphtequinone (19 mg, 0.10 mmol) and a small amount of acetic acid in EtOH was heated for 24 h under reﬂux. Puriﬁcation by alumina column (hexane/CH2Cl2, 2:1; Rf = 0.3) yielded ligand L1 as an orange solid (34 mg, 0.56 mmol, 50%). The 1H and 13C{1H} NMR spectra are identical with the compound formed from racemic starting materials. Anal. Calcd for C44H36N2·0.3H2O: C 88.35; H 6.17; N 4.68. Found C 88.25; H 5.98, N 4.65.

4.4. Preparation of NiBr2(L1)

A mixture of NiBr2(dme) (dme: 1,2-dimethoxyethane) (120 mg, 0.39 mmol) and (rac)- L1 (240 mg, 0.41 mmol) in Et2O was stirred for 24 h at room temperature. The resulted solid was obtained by ﬁltration, washed with Et2O to yield NiBr2(L1) as an dark brown solid (280 mg, 0.34 mmol, 96%). Anal. Calcd for C44H36N2Br2Ni: C 65.14; H 4.47; N 3.45. Found C 65.39; H 4.50, N 3.29. The reaction of (R,R)-L1 with NiBr2(dme) was carried out analogously.

4.5. X-ray Crystallography of NiBr2(L1)

Single crystals of NiBr2(L1)·(C2H4Cl2) suited to X-ray diffraction study were obtained by recrystallization from 1,2-dichloroethane–Et2O, and mounted on MicroMounts (MiTe- Gen). The crystallographic data were collected on a Bruker SMART APEXII ULTRA/CCD diffractometer equipped with monochromated Mo Kα radiation (λ = 0.71073 Å). Calcula- tions were carried out using the program package Olex2 [69]. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: deposition number CCDC-2076633, which can be obtained free of charge via http://www.ccdc.cam.ac.uk/ conts/retrieving.html.

4.6. Preparation of NiBr2(L2)

A mixture of NiBr2 (200 mg, 0.90 mmol), 2,5-dimethylaniline (0.26 mL, 2.2 mmol), and acetonaphthenequinone (180 mg, 1.00 mmol) was dissolved in acetic acid (5 mL) at 80 ◦C. After heating for 1 h at the temperature, the resulted solid was collected by ﬁltration, washed with acetic acid and then Et2O, and dried in vacuo to give NiBr2(L2) as a yellow brown solid (540 mg, 0.89 mmol, 99%). Anal. Calcd for C28H24N2Br2Ni·0.5H2O: C 54.59; H 4.09; N 4.55. Found C 54.47; H 4.25, N 4.38.

4.7. Oligomerization 4.7.1. Oligomerization of Ethylene

To a 25 mL Schlenk flask containing NiBr2(L1) (0.10 mmol) under nitrogen atmosphere was added dried toluene (10 mL) and naphthalene (64 mg, internal standard). The system was degassed by two freeze-thaw cycles. The flask was connected to a balloon filled with ethylene (1 atm), and MAO solution ([Al]/[Ni] = 300) was added to the mixture through septum. The reaction was conducted in a thermostated bath. A part of the product was extracted from the system by a syringe and analyzed by 1H NMR and GLC.

4.7.2. Oligomerization of 1-Hexene

To a 25 mL Schlenk flask containing NiBr2(L1) (0.10 mmol) under nitrogen atmosphere was added dried toluene (1.5 mL) and a hexane solution of naphthalene (internal standard). The system was degassed by two freeze-thaw cycles, and the flask was filled with nitrogen. A hexane solution of MAO ([Al]/[Ni] = 300) was added through septum, and the reaction was carried out in a thermostated bath. A part of the product was extracted from the mixture, and analyzed by1H NMR and GLC.

4.7.3. Co-Dimerization of Ethylene and 1-Hexene A toluene solution of MMAO was evacuated to remove the solvent, and the remain- inig MMAO was dissolved in pentane. To a 25 mL Schlenk ﬂask containing NiBr2(L1) Molecules 2021, 26, 2719 11 of 13

(0.10 mmol) under nitrogen atmosphere was added a pentane (5 mL) solution of 1-hexene and naphthalene (internal standard). The ﬂask was connected to a balloon ﬁlled with ethylene (1 atm). The pentane solution of MMAO was added to the system via a syringe through septum. The reaction was carried out in a thermostatted bath, and a part of the product was extracted from the solution via a syringe.

Author Contributions: Conceptualization, D.T.; methodology, D.T.; investigation, D.T. and Y.-a.T.; writing—original draft preparation, K.O.; writing—review and editing, K.O.; supervision, K.O.; funding acquisition, D.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Research Grant from JSPS, grant number 22685012 and 23655098. Institutional Review Board Statement: Not Applicable. Informed Consent Statement: Not Applicable. Data Availability Statement: Data is contained within this article. Acknowledgments: The authors are grateful to ‘Dynamic Alliance for Open Innovation Bridging’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) for their support. We thank our colleagues in Center for Advanced Materials Analysis of our institute, Tokyo Institute of Technology for NMR (Yoshihisa Sei), elemental analysis (Chieko Hara), X-ray crystallography (Yoshihisa Sei, Center for Advanced Materials Analysis). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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