Biradical o-iminobenzosemiquinonato(1−) complexes of nickel(ii): catalytic activity in three-component coupling of aldehydes, amines and alkynes

The six-coordinated bis-o-iminosemiquinone complex, NiL2BIS, in which LBIS is the o-iminosemiquinone 1-electron oxidized form of the tridentate o-aminophenol benzoxazole-based ligand H2LBAP, was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiN4O2 coordination sphere in which Ni(ii) has been surrounded by two tridentate LBIS ligands. This compound exhibits (SNi = 1) with both spin and orbital contribution to the magnetic moment and antiferromagnetic coupling between two electrons on two LBIS ligands which results in a triplet spin ground state (S = 1). The electronic transitions and the electrochemical behavior of this open-shell molecule are presented here, based on experimental observations and theoretical calculations. The electrochemical behavior of NiL2BIS was investigated by cyclic voltammetry and indicates ligand-centered redox processes. Three-component coupling of aldehydes, amines and alkynes (A3-coupling) was studied in the presence of the NiL2BIS complex, and the previously reported four-coordinated bis-o-iminosemiquinone NiL2NIS. Furthermore, among these two o-iminobenzosemiquinonato(1−) complexes of Ni(ii) (NiL2NIS and NiL2BIS), NiL2NIS was found to be an efficient catalyst in A3-coupling at 85 °C under solvent-free conditions and can be recovered and reused for several cycles with a small decrease in activity.


Introduction
Ligands have the ability of reverse accepting and donating electrons and, therefore, modied redox transformations can be assumed as "electron reservoir" ligands. This property allows electron reservoir ligands to store redox equivalents and consequently, has generated important interest in their multielectron reactivity features. 2 One of the classes of ligands which has electron reservoir ability, is redox-active ligands with different oxidation states. When some of these ligands are coordinated to some metal centers and produce complexes, both the metal and the ligand lack dened oxidation states. It means that the mentioned ligands bound to the metal ion could have a different oxidation state and signicantly inuence the oxidation state of the metal center. These ligands are known as (redox) non-innocent ligands. The interesting part in complexes of non-innocent ligands is that, the frontier orbitals of transition-metal and ligand are close in energy which leads to powerful mixing between these orbitals and it is difficult to assign the oxidation states to metal and ligand components alone.
The existence of redox-active ligands with different oxidation states and the cooperation of these ligands with the metal ion center causes the tuning of oxidation states of the central metal which is the key requirement to reach both developed catalytic activity and improved applicability of the overall complex in catalytic and enzymatic reactions. 3 Among the numerous kinds of non-innocent ligands, oamidophenolates due to their archetypal coordination abilities and spectroelectrochemical properties, have attracted signicant attention. This non-innocent ligand can exist in the completely reduced closed-shell aromatic mono-or dianions of o-aminophenolate, or organic, open-shell radical (S rad ¼ 1/2) of one-electron oxidized o-iminobenzosemiquinone or the closedshell neutral fully oxidized o-iminobenzoquinone (Scheme 1). 4 Therefore, designing and synthesis of the o-aminophenol ligand complexes as unique examples of non-innocent ligands, have been studied considerably and up to now, a number of examples of transition metal complexes (Cu, Pd, Ni, Ir, Ru, Os, Mo, V, Fe, Co, Mn) with two o-iminobenzoquinone ligands have a Department of Chemistry, College of Sciences, Shiraz University, 71454, Shiraz, Iran been synthesized and characterized due to their structure properties and magnetism. 5 We present here the synthesis and characterization of nickel(II) complex, NiL 2 BIS , which combines two o-aminophenol benzoxazole-based ligands that acquire a non-innocent character (Scheme 2). Ni II central atom in this complex and in some similar reported complexes of CuL BIS X, X ¼ (Cl, OAC), 6a X¼(Br À , I À , N 3 À , NO 3 À ) 6b and Cu(NNO ISQ ) 6c is supported by deprotonated (o-iminosemiquinone) form of H 2 L BAP . Reaction of Ni(OAC) 2 .4H 2 O with o-aminophenol H 2 L BAP in acetonitrile and in the presence of NEt 3 leads to the formation of the desired complex.
As a part of our ongoing effort we try to investigate the catalytic activity of NiL 2 BIS complex and the previously reported four-coordinated bis-o-iminosemiquinone of NiL 2 NIS 1 (in which L NIS is the bidentate o-iminosemiquinone 1-electron oxidized by 2-amino-4,6-di-tert-butyl-phenol ligand of L NAP ), (Scheme 3), and the comparison between these two Ni(II) iminosemiquinone complexes in synthesis of propargylamines from three-component coupling of aldehydes, amines and alkynes, A 3coupling reactions (Scheme 4). Propargylamines are useful building blocks for the synthesis of numerous nitrogen-containing heterocyclic compounds, and also important intermediates for preparation of natural complex products and active bio-molecules. 7 Typically, propargylamines are synthesized by the nucleophilic addition of a metal alkynylide to C]N electrophiles which oen needs stoichiometric value of extremely active organometallic reagents like Grignard reagents, 8 organolithium 9 and organozinc reagents. 10 Therefore, it is less attractive in terms of low endurance of functional groups, operational complexity and harsh reaction conditions. For the last decade, transition metal catalyzed coupling of aldehyde, alkyne, and amine that is usually referred as A 3coupling, has received greater attention due to its atom economy, step efficiency, and high chemical selectivity. 11 This reaction was recommended to be carried out via the addition of in situ generated metal-alkynylide to iminium ion, that is formed in situ, via a reaction between amine and aldehyde and water molecule is the only side product.
Transition metal complexes, particularly coinage metal complexes (Ag, Cu and Au), and also In, Zn, Ni, Fe, Ir, Co, Mn, Bi, Hg and Cd have been established as the catalysts for this reaction, among which, there is an increasing interest for Ni catalysts due to their abundance and low costs. In this regard we decided to study the catalytic activity of two Ni(II) complexes for the mentioned reaction and compare their efficiency in A 3coupling based on our observations.

Results and discussion
Synthesis and general characterization of NiL 2

BIS
The o-aminophenol H 2 L BAP , was synthesized and puried according to the literature by the reaction of 2-amino benzyl amine and 3,5-di-tert-butyl-quinone (DTBQ) in 2 : 1 molar ratio. 6a The complex NiL 2 BIS was synthesized in good yield, by stirring Ni II (OAc) 2 $4H 2 O, H 2 L BAP and Et 3 N in CH 3 CN at room temperature. Slow evaporation from 1 : 2 MeOH/CH 2 Cl 2 afforded NiL 2 BIS narrow cubic crystals.
The identication of the complex was conrmed by elemental analysis, IR and single-crystal X-ray structural analysis, temperature-dependent magnetic studies and cyclic voltammetry studies.
In the IR spectrum of the NiL 2 BIS complex, the sharp and strong n O-H and n N-H stretch band of the ligand disappears, which conrms the L BIS ligation to the Ni(II) center. All the vibrations of the ligand,n ¼ 1164 cm À1 (C-N stretching),n ¼ 1614 cm À1 (C]C stretching)n ¼ 1470 (C]N stretching) and the tert-butyl groups bands atn ¼ 2962 cm À1 exist in the IR spectrum of the complex, that conrms the presence of the ligand in the structure of complex (Fig. S1 †).

Crystal structure of NiL 2 BIS complex
In Matte crimson crystals of NiL 2 BIS complex, the asymmetric unit of the reported structure consists of the single molecule of Ni II L 2 BIS (Fig. 1). The voids are found in the crystal lattice (solvent accessible voids, 18.4% of the unit cell) with no interpretable electron density ( Fig. 2) (see Methods). The diffraction experiments and the structure renement are summarized in Table 1. The selected bond lengths and angles are given in Table  2. The complete crystallographic data, bond lengths and angles and torsion angles are given in Table S1, S2 and S3 respectively. † The complex has two L BIS ligands coordinated to the central Ni(II) in the tridentate manner. Each ligand forms the Scheme 4 A 3 -coupling or the aldehyde-alkyne-amine reaction which produces propargyl-amine. coordination bonds via the phenolate O1, imine N1 and benzoxazole N2, with O1 and N2 positioned trans in the coordination sphere. Therefore, in the reported complex the octahedral coordination sphere NiN 4 O 2 is found. Due to the relative rigidity of the L BIS molecules, two ligands form the structures approximately perpendicular to each other. The shortest coordination bonds are formed by imine N1 and N41 atoms of both ligands (  Table 2). These values indicate signicant deformation of the coordination octahedron, that can be attributed to the rigidity of the tridentate L BIS ligand. The valence geometry of both L BIS ligands is almost identical. In both phenolic rings, bond lengths C2-C3 and C4-C5, and their equivalents range from C4-C5 1.368(5) to C44-C45 1.375(4)Å, indicating the localization of double bonds in these positions. These bonds are signicantly shorter than other C-C bonds in the phenolic rings, which range from 1.427(5) to 1.466(4)Å and reveal signicant contributions of single bonds   Table 2). The O1-C1 and O41-C41 bonds lengths of 1.286(3) and 1.283(3)Å, respectively, reveal their double rather than single bond character. The C6-N1 and C46-N41 bonds are 1.355(4) and 1.363(3)Å, respectively, and are shorter by 10s than N1-C7 and N41-C47. Such distribution of the double bonds in the o-aminophenole fragment corresponds to o-iminosemiquinoate form of both L BIS ligands. That form seems to be consistent with both the bond distribution and the neutral charge of the complex with Ni II center. The tridentate coordination of L BIS causes the conformational adjustments resulting in the lack of planarity of the ligand. The additional factor affecting the conformation is the presence of bulky tBu substituents at the benzoxazole moieties. Their spatial arrangement in the complex molecule results in the intramolecular interactions of their methyl groups C34 to C69 and C31 to C74 (Fig. 1). The observed twist of both ligands can be quantied with the dihedral angles between best planes of the phenolic and benzoxazole moieties, being 58.82(12) and 46.53(12) for ligand 1 and 2, respectively. The dihedral angles between the central phenyl ring and phenolic and benzoxazole rings are 51.66(17), 34.89 (15) and 49.01 (14), 29.19(13) for O1-O2 and O41-O42 ligands, respectively.
The intramolecular p/p interactions are detected. The benzoxazole ve-membered heterocyclic moieties form the gravity centers Cg/Cg 3.8891(16)Å interaction. That results in the interactions of two benzoxazole moieties with the distance Cg/Cg of 3.8770(14)Å, with the dihedral angle between their planes being 39. 21(10) .
The intramolecular hydrogen bonds are shown in Table 3.

Magnetic susceptibility measurements
Variable-temperature magnetic susceptibility measurement for crystalline samples of the ligand H 2 L BAP and NiL 2 BIS complex was carried out with an applied magnetic eld of 1000 Oe in the temperature range 1.8-300 K (Fig. 4). The effective magnetic moment values of NiL 2 BIS change from 3.7 BM (at 1 K) to 3.88 BM (at 125 K) and 3.78 (at 300 K). These values differ from that of the spin-only moment, which amounts to 2.83 BM. This difference between the measured and calculated values results from spin-orbital coupling and displays positive and commonly large deviations from the spinonly contribution of 2.83 BM. The reported compound exhibits (S Ni ¼ 1) because of the Ni II center and antiferromagnetic coupling of both tridentate L BIS ligand radicals coordinated to Ni(II) ion whose spin alignment seems to be [([)-([[)-(Y)] (Scheme 5). It indicates that the nickel complex exists in an 102.55 (8) C54-N42-Ni1 131.18 (17) a Symmetry transformations used to generate equivalent atoms: #1 À x, Ày, Àz. octahedral triplet ground state. The ground state conguration of Ni(II) ion in a regular octahedral eld is 3 A 2g (t 6 2g e 2 g ) and it will be paramagnetic with two unpaired electrons. 12

Electrochemistry
The cyclic voltammetry (CV) electrochemical behavior of complex Ni II L 2 BIS has been recorded in CH 2 Cl 2 solution containing 0.10 M NBu 4 ClO 4 as supporting electrolyte at a glassy carbon as working electrode, and an Ag/AgNO 3 reference electrode at room temperature.
The electrochemical behavior of the complex Ni II L 2 BIS is similar to the previously studied o-iminobenzoquinone based complexes of Ni(II), 1 owing to the presence of three one-electron ligand-centered redox transitions on the voltammogram ( Fig. 5 and Table 4). The ligand-centered voltammograms are observed in the positive potential range showing radical-ligand based iminobenzosemiquinone/iminobenzoquinone (Ni II L 2 BIS /Ni II L-BIS L BIQ and Ni II L BIS L BIQ /Ni II L 2 BIQ ) redox couples and voltammograms observed in the negative potential range corresponding to iminobenzosemiquinone/amidophenoxide redox couples (Ni II L 2 BIS /Ni II L BIS L BAP ) (Scheme 6).    a The potential reported here is the average of anodic and cathodic peak potentials for a reversible process or the peak potential for an irreversible process.

Electronic spectroscopy
The UV-Vis/NIR spectra for the studied compound in CH 2 Cl 2 solution were recorded in the range of 280-780 nm. The electronic spectrum of the ligand H 2 L BAP is shown in Fig. S2 † and the electronic spectrum of the complex Ni II L 2 BIS is shown in Fig. 6. Ni II L 2 BIS exhibits four absorption bands in the visible, near infrared and near ultraviolet regions (Fig. 6). A ligand p / p* charge transfer was found for the complex in the ultraviolet eld of the electronic spectra (313 nm). The electronic absorption band in region 395 nm is consistent with iminosemiquinone (p)-to-Ni-(dp*), ligand-to-metal charge-transfer (LMCT) transition. The broad electronic absorption band in the region around 493 nm corresponded to metal-to-ligand (MLCT) charge transfer. Also, the extinction coefficient of the ligand and the Nicomplex have been extracted by the plot of the absorption maxima at a selected wavelength versus varied concentrations reply to Beer-Lambert law. We did the uorescence experiment for the H 2 L BAP ligand and Ni II L 2 BIS complex. The ligand was emissive and the complex was non-emissive. The emission spectrum of the ligand is given in Fig. S2. †

Computational details
Description of the Ni II L 2 BIS structure Optimized geometries were conrmed to be minima by the frequency analysis. The DFT calculation shows that triplet Ni II octahedral complex, Ni II L 2 BIS , has the lowest energy. The singlet electronic structure is 33.4 kcal mol À1 higher in energy with respect to the triplet solution.
The optimized structure of Ni II octahedral complex is shown in Fig. 7 and in Table S4. † Some of the selected bond lengths are included. The root-mean-square deviation (RMSD) for the bond lengths from the crystal structure are in the order of 0.02Å. Therefore, the optimized geometry of the complex is in good agreement with the experimental structure from X-ray crystallography. The C-O, C-N and aryl C-C bond distances of the redox-active fragments of both ligands are similar to the iminosemiquinone (L BIS ) 1À oxidation state reported by Brown. 13 The predicted spin density for the Ni II L 2 BIS complex (Fig. 8) also shows the delocalization of a electron density over one ligand and b electron density over another ligand in agreement with the iminosemiquinone (L BIS ) 1À oxidation state for both ligands.
Scheme 6 Schematic representation of NiL 2 BIS complex oxidation state variation. Fig. 6 Electronic spectra of 0.02 mM CH 2 Cl 2 solutions of NiL 2 BIS . Fig. 7 The optimized structure of the Ni II L 2 BIS complex. For clarity, hydrogen atoms are omitted.

Theoretical analysis of the UV-Vis transitions for Ni II L 2 BIS
A detailed theoretical study of the UV-Vis electronic absorption spectrum of Ni II L 2 BIS complex was carried out using the TD-DFT approach to gain insight into the spectral features of the complex. In order to get an adequate comparison between the theoretical and experimental spectra, we performed selfconsistent reaction eld (SCRF) calculations in the presence of a solvent using the Polarizable Continuum Model (PCM). 14 The solvent chosen was that used for the experimental spectrum, that is dichloromethane (CH 2 Cl 2 ), with a dielectric constant 3 ¼ 8.94. The wavelengths, excitation energies, and oscillator strengths of the absorption bands calculated with the TD-DFT methodology are displayed in Table 5. For each transition, molecular orbitals with major contribution are also mentioned in Table 5. The nature of most of the TD-DFT transitions involve the excited states, that are not dominated by one singly excited conguration. For analyzing the nature of such electronic transitions, a Natural Transition Orbital (NTO) analysis 15,16 has been performed as implemented in Gaussian 16 and the dominant NTO orbital pairs were calculated. The most important advantage of this procedure is that an electronic transition is readily attributed, for the majority of cases, to (at most) two NTO pairs (one occupied and one virtual) that qualitatively depict the electron density reorganization due to the excitation, thus facilitating the assignment of the transition nature. Table 6 gives the NTO pairs describing the transitions for the Ni II L 2 BIS complex. The two predicted lower energy bands are assigned as ligand-to-ligand charge transfer (LLCT) transitions (T14 and T34 states). These transitions can be identied as LLCT within the aromatic system of two ligands (p / p* transition). The highest-energy band (T61 state) has mixed character and the electron density migrates form metal-toligand (d / p* transition) or ligand to ligand (p / p* transition) groups (mixed MLCT/LLCT state).
Catalytic activity of Ni II L 2 BIS and Ni II L 2 NIS in A 3 -coupling reactions General reaction of catalytic activity of Ni II L 2 BIS and Ni II L 2 NIS in A 3 -coupling reactions is shown in Scheme 7. This reaction was optimized for the various parameters such as temperature, solvent and catalyst loading. The reaction temperature was initially optimized by performing the model reaction of benzaldehyde, pyrrolidine and phenylacetylene under solvent-free conditions at different temperatures (Table 7 entries [1][2][3][4][5]. The model reaction was screened at RT in the presence of 2 mol% NiL 2 NIS catalyst, but a good yield was not obtained during 5 h. As the reaction temperature increased, the reaction time decreased, and the best result was obtained at 85 C. With increasing temperature up to the optimized temperature (85 C), a decrease in the yield of desired product was observed, that is related to the evaporation of the volatile precursors from the reaction media in solvent free conditions and also increasing by-products of the reaction.
To optimize the catalyst load, the model reaction was performed in the presence of various amounts of the catalyst 1-3 mol%. According to the results, 2 mol% NiL 2 NIS catalyst shows the best efficiency (Table 7, entries 4, 6-8). The effect of various solvents was also monitored by performing the model reaction in the presence of 2 mol% NiL 2 NIS catalyst (Table 7, entries 9-11).
Then we tried to investigate the A 3 -coupling reaction for the other Ni II complex, NiL 2 BIS . It is noteworthy to mention that, this reaction did not happen in any measurable amount (Table  7, entries 12-16), which is due to the lack of free space in the coordination sphere of the six-coordinated NiL 2 BIS complex.     It other words, in the solution medium this complex keeps its stability and departure of ligands does not occur.
Also, a catalyst recycle experiment (Table 8) was done. The catalyst was recovered by centrifugation and the reaction was carried out for three cycles with a slight decrease in activity.
Encouraged by the optimization results, we turned our attention to various aldehydes and amines. Interestingly, various aldehydes reacted effectively with pyrrolidine, morpholine and piperidine.
As exemplied in Table 9, this protocol is rather general for a wide variety of electron-rich as well as electron-decient aromatic aldehydes and also various secondary amines. Results show the reaction was performed faster for the aldehydes bearing the electron-withdrawing group such as -NO 2 . In addition, pyrrolidine has the highest activity among the used amines (Table 9, entries e and g).
Then we tried to compare the activity of our catalyst, NiL 2 NIS , and other catalysts which had been studied for A 3 -coupling in the literature (Table 10). 17a-g To the best of our knowledge, there is only one more report of the Ni-catalyzed three-component coupling of aldehyde, alkyne and amine (Table 10, entry 1). Also, the mechanism involved in our Ni complex-catalyzed A 3 -coupling reaction can be different from the reaction mechanism which is dominant in other complex-catalyzed crosscoupling reactions due to the ability of NiL 2 NIS to present radical species. Both Ni 0 and Ni II species are used as Ni sources in Nicatalyzed cross-couplings and Ni 0 sources are generally considered as the catalytically active ones. While the easiest way is to use Ni 0 reagents like Ni-(COD) 2 and Ni(PPh 3 ) 4 , 18 these nickel sources are difficult to handle because of the high air sensitivity and thermal instability. Alternatively, Ni II complexes are more convenient as pre-catalysts in terms of their availability and easy handling. Nevertheless, these Ni II catalysts should be activated in situ with some additives 19 such as base, I 2 or PPh 3 or used as bimetallic systems of Ni(II)/M(Zn(0), Mn(0), Cu(I), Ag(I). In some cases, Ni complex itself can act as the active catalyst (Table 10, entry 1) 19a by cleavage of one of the oxo bridges of the ligand and generating an initial nickel(II) acetylide intermediate in A 3 -coupling reactions. Also, our Ni complex NiL 2 NIS can easily undergo a switch among the different oxidation states of the ligand easily. This event supports the non-innocent behavior of o-aminophenol ligand that can act as an "electron reservoir" and accept and donate electrons in a reverse catalytic cycle, result in a high reactivity. This behavior Scheme 7 General reaction for testing the catalytic activity of Ni II L 2 BIS and Ni II L 2 NIS in A 3 -coupling reactions.   propargylamine product E and regenerates the nickel catalyst A for the subsequent reactions.

Material and method
All reagents were acquired via commercial sources unless stated otherwise. 3,5-DTBQ (3,5-di-tert-butylcyclohexa-3,5-diene-1,2dione) was synthesized from the procedure reported in literature. 20 The ligand H 2 L BAP was synthesized following the reported procedure. 6a Elemental analyses (C, H, and N) were done by the Elementar, Vario EL III. Fourier transform infrared spectroscopy with KBr pellets was performed on a FT IR Bruker Vector 22 instrument. NMR spectra were performed at 400 MHz on a Bruker DRX spectrometer in CDCl 3 solution. UV-Vis absorbance spectra was recorded by using a CARY 100 Bio spectrophotometer. Cyclic voltammetry (CV) was carried out on a PAR-263A potentiometer. The cell was prepared with an Ag wire reference electrode, a glassy carbon working electrode, and a Pt counter electrode with 0.1 M NBu 4 ClO 4 (TBAP) solutions in CH 2 Cl 2 . Ferrocene was used as an internal standard. The magnetic measurements were achieved with the use of a Quantum Design SQUID magnetometer MPMS-XL between 1.8 and 290 K with a dc applied eld of 1000 Oe. Measurement was done on polycrystalline sample of 35 mg for NiL 2 BIS . Sample for X-band measurement was placed in 4 mm outer-diameter sample tubes with sample volumes of $300 mL.
All catalytic reactions were monitored by TLC (thin layer chromatography) and all yields refer to isolated products. 1 H NMR spectra were recorded in CDCl 3 on a Bruker DRX-400 AVANCE (400 MHz for 1 H and 100 MHz for 13 C) spectrometer.

X-ray crystallography
The X-ray data were collected with the Oxford Sapphire CCD diffractometer, at 293(2) K using MoKa radiation l ¼ 0.71073Å. The structure was solved by direct methods with SHELXT and rened with the full-matrix least-squares method on F 2 with the use of SHELX2017 program package. 21 The analytical absorption correction was applied by CrysAlis 171.38.43 package of programs. 22 The large voids are found in the structure, covering 18.4% of the unit cell volume, but no signicant density peaks have been located there. For the solvent accessible volume 1344Å 3 , 24 electrons were found. Therefore, to account for the disordered solvent contribution to the structure factors, the SQUEEZE (Version 160617) program was used. 23 The hydrogen atom positions were determined from the difference maps, and all hydrogen atoms were constrained during renement. Four tBu groups (C24, C28, C32 and C68) revealed the rotational disorder. The attempt to dene the discrete odel forthat disorder has given no improvement in the model quality. Therefore, the SIMU constraints for the disordered tBu groups and ISOR constraint the C25 methyl group were used in the nal renement. A summary of the crystal data and renement details for compound 1 are given in Table 1. The structural data have been deposited at the Cambridge Crystallographic Data Center: (CCDC no. 2035886).

Computational details
Geometry optimization was performed on the Ni complex using the hybrid B3LYP method, with LANL2DZ basis set for nickel and 6-31G* basis set for all other atoms, using the program Gaussian 16. 24 General procedure for the synthesis of propargylamine derivatives In a typical reaction benzaldehyde (1 mmol), pyrrolidine (1.1 mmol), phenylacetylene (1.2 mmol) and catalyst, Ni II L 2 NIS , (2 mol%, 19 mg) were added into a 5 mL round bottom ask and were stirred at 85 C. The progress of the reaction was monitored by TLC. Aer completion of the reaction, n-hexane (2 mL) was added to the reaction mixture and the solution was centrifuged at 3000 rpm for 2 min. Finally, the excess of solvent was removed under reduced pressure to give the corresponding product. Further purication was achieved by thin layer chromatography on silica gel using n-hexane/ethyl acetate. Then, the recovered catalyst was reused in recycle experiment of A 3 coupling reaction of benzaldehyde, pyrrolidine and phenylacetylene. Physical and spectroscopic data for selected compounds are given in Fig. S3 to S16 †).

Conclusion
Complex Ni II L 2 BIS was synthesized and characterized in the current work. A combination of experimental and theoretical studies is used to investigate the electronic properties of this nickel complex. The synthesis of Ni II L 2 BIS neutral complex was achieved by ligation of the o-iminosemiquinone 1-electron oxidized form of the tridentate o-aminophenol benzoxazolebased ligand H 2 L BAP and characterized by X-ray crystallography. The bond lengths of the L BIS ligand indicate that is found in the semiquinone form. This observation is also supported by the electronic conguration as determined by density functional theory calculations. Then the catalytic activity of this complex, NiL 2 BIS , and the previously reported complex, NiL 2 NIS1 in three-component coupling of aldehydes, amines and alkynes (A 3 -coupling) was investigated and the four-coordinated NiL 2 NIS complex was found to be significantly more efficient catalyst due to the non-innocent o-aminophenol ligand that acts as an "electron reservoir" and can accept and donate electrons in the C-H activation stage of catalytic cycle resulting a high reactivity in A 3 -coupling reaction. This procedure is also environmentally friendly and is done under solvent-free conditions as it does not require any organic solvents. Good yields and mild reaction conditions are other remarkable advantages of this process. The catalyst can be readily recovered and reused for three cycles with a slight decrease in activity.

Conflicts of interest
There are no conicts to declare.