Ni(II)-Aroylhydrazone Complexes As Catalyst Precursors Towards Eﬃcient Solvent-Free Nitroaldol Condensation Reaction

catalysts

Article Ni(II)-Aroylhydrazone Complexes as Catalyst Precursors Towards Eﬃcient Solvent-Free Nitroaldol Condensation Reaction

Manas Sutradhar * , Tannistha Roy Barman, Armando J. L. Pombeiro and Luísa M.D.R.S. Martins *

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] (T.R.B.); [email protected] (A.J.L.P.) * Correspondence: [email protected] (M.S.); [email protected] (L.M.D.R.S.M.)

Received: 9 June 2019; Accepted: 17 June 2019; Published: 20 June 2019

Abstract: The aroylhydrazone Schiﬀ bases 2-hydroxy-(2-hydroxybenzylidene)benzohydrazide and (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide have been used to synthesize the bi- and tri-nuclear Ni(II) complexes [Ni (L1) (MeOH) ](1) and [Ni (HL2) (CH OH) ] (NO ) (2). 2 2 4 3 2 3 8 · 3 2 Both complexes have been characterized by elemental analysis, spectroscopic techniques [IR spectroscopy and electrospray ionization-mass spectrometry (ESI-MS)], and single-crystal X-ray crystallography. The coordination behavior of the two ligands is diﬀerent in the complexes: The ligand exhibits the keto form in 2, while coordination through enol form was found in 1. Herein, the catalytic activity of 1 and 2 has been compared with the nitroaldol condensation reaction under various conditions. Complex 2 exhibits the highest activity towards solvent-free conditions.

Keywords: Ni(II) complex; aroylhydrazone; X-ray structure; nitroaldol condensation

1. Introduction The nitroaldol (Henry) reaction, a very useful and significant reaction, plays a vital role in the synthetic organic chemistry. It is widely used to build C–C bonds by the formation of β-nitroalcohols derived from coupling of a carbonyl compound with an alkyl nitro one with α- hydrogen atoms. The reaction takes place via construction of asymmetric centers (one or two) through the new C–C junction leading to optically active products. Inorganic metal complexes or inorganic/organic bases can catalyze this reaction [1–10]. The prepared functionalized β-nitroalcohols are known as important synthetic precursors, can be converted by nucleophilic displacement into other functionalities like α-hydroxy ketones, aldehydes, carboxylic acids, azides, sulphides, etc., of pharmaceutical significance [5–8], polyfunctionalized materials [11], and are used to generate compounds of biological importance such as 1,2-diaminoalcohol, [12] aminosugars, [13] nitroketones, [14] α,β-unsaturated nitro compounds [15], and many other important bifunctional compounds. The stereoselectivity nature of the Henry reaction was identified for the first time by Shibasaki et al. [16] using several chiral metal complexes [17–29] and chiral organocatalysts [20], exhibiting the growing interest in this field. The reaction has been studied using different conditions like homogeneous [5–8], heterogeneous [21–23], ionic liquids or supercritical media [24], materials of mesoporous nanocomposite [25], etc. The design of the catalyst plays a crucial role in controlling the diastereo- and enantioselectivity of the products, leading to a serious task for researchers to search for efficient and selective catalysts from synthetic, economic, and environmental perspectives. Di and polynuclear metal complexes including coordination polymers can efficiently catalyze the nitroaldol reaction between aldehydes and nitroalkanes [26–28]. Solvent-free organic synthesis has received significant interest from the

Catalysts 2019, 9, 554; doi:10.3390/catal9060554 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 554 2 of 13 chemist worldwide due to its advantage over chemical wastes in terms of sustainability and the requirements of green chemistry [29]. The use of solvent-free condition is also applied in the nitroaldol reaction [30,31]. Some nickel complexes showed good catalytic activity towards nitroaldol reactions under various conditions such as in homogeneous reaction states [32,33], under solvent-free microwave irradiation [25], in ionic liquids [34,35], or under heterogeneous [36–38] conditions. Aroylhydrazone Schiff bases form highly stable complexes with transition metals in various oxidation states, coordination numbers, and nuclearities, and can be tuned easily by changing different carbonyl derivatives [39–46]. In this study, we describe the synthesis and characterization of one dinuclear and one trinuclear Ni(II) complex derived from two different aroylhydrazones, and their activity as catalyst precursors towards nitroaldol (Henry) reactions to achieve desired functionalized products under different homogeneous catalytic reactions conditions. Solvent-free conditions are found most efficient in our catalytic system. This is probably due to more accessibility of substrate molecules to the metal centre under solvent-free condition than the presence of solvent molecule; 94%–97% yields are observed in our case, which is relatively higher than the yield found in other di or polynuclear catalytic system [26].

2. Results and Discussion

2.1. Syntheses and Characterizations In this study, we have used two different aroylhydrazone Schiff bases, namely, 1 2-hydroxy-(2-hydroxybenzylidene)benzohydrazide (H2L ) and (2,3-dihydroxybenzylidene)-2- 2 hydroxybenzohydrazide (H3L )[47,48], to synthesize two different (one dinuclear and one trinuclear) 1 Ni(II) complexes. In the dinuclear complex [Ni2(L )2(MeOH)4](1), the ligand exhibits the enol 2 form with the loss of all (two) acidic hydrogens whereas H3L undergoes coordination with the loss of two acidic hydrogens (out of three) and one remains protonated to form the trinuclear [Ni (HL2) (CH OH) ] (NO ) (2) (Scheme1). In both complexes, the phenolate oxygen at the ortho - 3 2 3 8 · 3 2 position (from the aldehyde moiety of the ligand) forms a phenoxido bridge between two Ni(II) ions. Characterizations of 1 and 2 have been carried out by elemental analysis, spectroscopic methods (IR spectroscopy, ESI-MS), and X-ray diffraction (single crystal) techniques. Beside similar characteristic 1 stretching signals of the ligand, a band at 1609 cm− appears in the IR spectrum of 2 that corresponds to the C=O stretching frequency [47,48]. The m/z value of 2 suggests the loss of two noncoordinate nitrate ions and one acidic proton from one of the two ligands present in 2 (see Experimental). The catalytic properties of 1 and 2 were investigated towards solvent-free nitroaldol condensation reaction and their activities were compared.

2.2. General Description of the Crystal Structures Crystals of [Ni (L1) (DMF) ] [Ni (L1) (DMF) (H O) ] 2DMF (1A) suitable for X-ray diﬀractions 2 2 4 2 2 2 2 2 · were obtained upon re-crystallization of 1 from DMF-water and slow evaporation of 2 from the methanolic solution, at ambient temperature. The crystallographic data are summarized in Table1, representative molecular structures are displayed in Figures1 and2, and selected dimensions are presented in Table2. The asymmetric unit of the compound 1A comprises a half unit of two diﬀerent molecules and one solvent DMF. One half unit of one molecule contains the nickel(II) cation with one coordinated ligand, one DMF, and one water molecule. Another half consists of nickel(II) cation with one coordinated ligand and two DMF molecules. All the Ni(II) centers exhibit a distorted octahedral coordination environment. The structure of 1A contains crystallographically generated inversion centres in the i ii middle of the Ni1–Ni1 or Ni2–Ni2 bonds, therefore in the heart of the respective Ni2O2 planes. The aroylhydrazones act as dianionic and tetradentate ONOO chelating equatorial ligands, binding to one of the Ni(II) centers via the enolate oxygen, the imino nitrogen, and the deprotonated phenolate oxygen, which is connected to other metal cation. Thus, the structure displays two µ-O bridges in each molecular unit, which connect the two nickel(II) centers. In the axial positions, there are two Catalysts 2019, 9, 554 3 of 13

DMF oxygens atoms in one unit and DMF and water in the other unit. The N N bond distances of − 1.385 (3) and 1.397 (3) Å for the coordinated ligand indicate their single bond hydrazino character. The ODMF–Ni–ODMF or ODMF–Ni–Owater groups are nearly linear (170.47 (9)◦ and 172.52 (7)◦) and in - Catalyststhe Ni20192(, µ9, xO) FOR2 cores, PEER REVIEW the Ni–O–Ni angles are ca. 100◦. The Ni–Ni contact distances are about3 of 13 3.10 Å.

Scheme 1. Syntheses of 1 and 2. Scheme 1. Syntheses of 1 and 2.

2.2. General DescriptionTable of 1.theCrystal Crystal data Structures and structure reﬁnement details for complexes 1A and 2.

1 1 Crystals of [Ni2(L )2(DMF)4] [Ni2(L )2(DMF)2(H2O)2]·2DMF1A (1A) suitable for X-ray 2 diffractions were obtained upon re-crystallization of 1 from DMF-water and slow evaporation of 2 from the Empirical formula C H N NiO C H N Ni O methanolic solution, at ambient temperature. The crystallographic20 24 4 5.50data are summarized18 26 3 1.50 in10 Table 1, Formula Weight 467.14 532.48 representative molecular structuresCrystal system are displayed in Figures Triclinic 1 and 2, and selected Triclinic dimensions are presented in Table 2. Space group P¯1 P¯1 The asymmetric unit ofTemperature the compound/K 1A comprises a 296 half (2) unit of two different 296 (2) molecules and one solvent DMF. One half unita of/Å one molecule contains12.766 the nickel(II) (3) cation with 9.114 one (3) coordinated ligand, one DMF, and one waterb/Å molecule. Another half13.714 consists (4) of nickel(II) 10.821 cation (3) with one coordinated ligand and two DMFc/Å molecules. All the Ni(II)14.344 centers (4) exhibit a distorted 13.221 (4) octahedral α coordination environment. The structure/◦ of 1A contains84.87 crystallographically (1) 71.721generated (10) inversion β/◦ 63.594 (10) 83.309 (11) centres in the middle of the Ni1–Ni1i or Ni2–Ni2ii bonds, therefore in the heart of the respective Ni2O2 γ/◦ 79.445 (9) 71.328 (10) planes. The aroylhydrazones actV (Å as3 )dianionic and tetradentate2211.2 (10)ONOO chelating 1172.8 equatorial (6) ligands, binding to one of the Ni(II) centersZ via the enolate oxygen, the4 imino nitrogen, and the 2 deprotonated 3 phenolate oxygen, which isD calcconnected(g cm− )to other metal cation.1.403 Thus, the structure 1.508displays two µ-O 1 bridges in each molecularµ (Mounit, K whichα) (mm connect− ) the two nickel(II)0.92 centers. In the axial 1.27 positions, there are two DMF oxygensRﬂs. collectedatoms in/unique one unit/observed and DMF 35885and water/8156/ 6683in the other 10924 unit./4209 The/2646 N−N bond distances of 1.385 (3) and 1.397 (3)Rint Å for the coordinated ligand0.029 indicate their single 0.091 bond hydrazino Final R1 a, wR2 b (I 2σ) 0.038, 0.108 0.093, 0.224 character. The ODMF–Ni–ODMF or ODMF–Ni–O≥ water groups are nearly linear (170.47 (9)° and 172.52 (7)°) Goodness-of-ﬁt on F2 1.03 1.05 and in the Ni2(μ-O)2 cores, the Ni–O–Ni angles are ca. 100°. The Ni–Ni contact distances are about a b 2 2 2 2 4 1 2 2 2 R = Σ||Fo| |Fc||/Σ|Fo|; wR(F ) = [Σw(|Fo| |Fc| ) /Σw|Fo| ] 2 , w = 1/[ σ (Fo ) + (aP) + bP]. 3.10 Å. − − Compound 2 crystallizes in the triclinic P¯1 space group. Its unit cell contains one molecule of the compoundCompound and two2 crystallizes nitrate anions. in the Compound triclinic P ¯21 is space trinuclear group. with Its H unit3L2 coordinating cell contains oneto the molecule Ni(II) of the 2 cationscompound in the dianionic and two (HL nitrate2)2− form anions. using Compound both phenolate2 is trinuclearO atoms, the with keto H 3OL atom,coordinating and the imine to the Ni(II) 2 2 N atom.cations The in phenolate the dianionic oxygen (HL at) the− form ortho using position both of phenolatethe aldehyde Oatoms, moiety the exhibits keto Oµ-O atom, bridges and with the imine N the other metal cation, which is located at the inversion centre. The asymmetric units of 2 contains half of the molecules, i.e., one and a half nickel cations, one (HL2)2− ligand, four methanol molecules, and one nitrate anion. All the axial positions are occupied by methanol molecules. The terminal nickel cations exhibit distorted octahedral N1O5 coordination environment but the Ni(II) at center of

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 13

Ni2—O3 1.948 (6) N2—Ni1—O4 87.6 (3) O3—Ni2—O7 88.8 (3) CatalystsNi2—O32019, 9, 554iii 1.948 (5) O5—Ni1—O4 93.5 (3) O3iii—Ni2—O7 91.2 (3) 4 of 13 Ni2—O2iii 2.108 (5) O2—Ni1—O4 93.1 (2) O2iii—Ni2—O7 93.3 (2) Ni2—O2 2.108 (5) O1—Ni1—O4 86.8 (2) O2—Ni2—O7 86.7 (2) atom. The phenolate oxygen at the ortho position of the aldehyde moiety exhibits µ-O bridges with the Ni2—O7 2.143 (8) N2—Ni1—O6 91.9 (3) O3—Ni2—O7iii 91.2 (3) other metal cation,iii which is located at the inversion centre. The asymmetriciii unitsiii of 2 contains half of Ni2—O7 2.143 (8) O5—Ni1—O6 86.7 2(3)2 O3 —Ni2—O7 88.8 (3) the molecules, i.e., one and a half nickel cations, one (HL ) − ligand,iii four methanoliii molecules, and one CatalystsN1—N2 2019, 9, x FOR PEER1.387 REVIEW (8) O2—Ni1—O6 88.8 (2) O2 —Ni2—O7 86.7 (2) 5 of 13 nitrateC1—O1 anion. All the1.250 axial (9) positions O1—Ni1—O are occupied6 by91.3 methanol (3) molecules.O2—Ni2—O7 Theiii terminal93.3 nickel(2) cations exhibit distorted octahedral N1O5 coordination environment but the Ni(II)iii at center of inversion -- - - O4—Ni1—O6 O4—Ni1—O6 178.0 178.0 (3) (3) O7—Ni2—O7 O7—Ni2—O7 iii 180.0180.0 displays more-- regular octahedral- - Ni1—O2—Ni2 geometry.Ni1—O2— TheNi2 Ni–Ni125.2 125.2 contact (2) (2) distances- - are 3.764 Å, longer - - than the distances foundSymmetrySymmetry in 1A. codes: (i)(i) − −xx, ,− −yy + + 1, 1, - z- z+ +1; 1; (ii) (ii) − x− +x 1,+ 1,−y − +y 2,+ −2,z −+z 2; + (iii)2; (iii) −x +− x1, + − 1,y +− 1,y +− z1,. −z.

(a)) (b)( b) Sky-blue doted lines represent hydrogen bonds. Symmetry codes (i) −x,1 − y,1 − z (ii) 1 − x,2 − y,2 − z. Sky-blue doted lines represent hydrogen bonds. Symmetry codes (i) −x,1 − y,1 − z (ii) 1 − x,2 − y,2 − z. Figure 1. Molecular structure of compound 1A with atom numbering scheme. (a) nickel(II) coordinated Figure 1. Molecular structure of compound 1A with atom numbering scheme. (a) nickel(II) with twoFigure DMF, 1. Molecular (b) nickel(II) structure coordinated of compound with one 1A DMF with and atom one numbering water molecule. scheme. Sky-blue (a) nickel(II) doted lines coordinated with two DMF, (b) nickel(II) coordinated with one DMF and one water molecule. representcoordinated hydrogen with bonds. two DMF, Symmetry (b) nickel(II) codes (i)coordinatedx, 1 y, 1withz (ii)one 1 DMFx, 2 andy ,one 2 waterz. molecule. − − − − − −

Sky-blue doted lines represent hydrogen bonds. Symmetry code (i) 1 − x, 1 − y, −z. Figure 2. Molecular structure of compound 2 with atom numbering scheme. Sky-blue doted lines representSky-blue hydrogenFigure doted2. bonds. Molecular lines Symmetry represent structure code hydrogenof compound (i) 1 x bonds., 1 2 withy, Symmetryz atom. numbering code (i) 1scheme. − x, 1 − y, −z. − − − Figure 2. Molecular structure of compound 2 with atom numbering scheme. 2.3. Catalytic Studies 2.3. Catalytic Studies The catalytic activity of complexes 1 and 2 for the nitroaldol (or Henry) reaction was evaluated using benzaldehydes and nitroethane as models (Scheme 2, Tables 3 and 4). An excess nitroethane was used to obtain maximum conversion of benzaldehydes into products. The selectivity for the β-nitroethanol products was 100% for all the experiments (the only compounds found besides syn- and anti- β-nitroethanols were non-reacted substrates).

Catalysts 2019, 9, 554 5 of 13

Table 2. Selected bond distances (Å) and angles (◦) in complexes 1A and 2.

1A Ni1—N1 1.995 (2) N1—Ni1—O1 89.97 (8) N5—Ni2—O6 89.33 (8) Ni1—O1 2.0231 (17) N1—Ni1—O1i 170.34 (8) N5—Ni2—O6ii 169.38 (8) Ni1—O1i 2.0256 (16) O1—Ni1—O1i 80.43 (7) O6—Ni2—O6ii 80.07 (8) Ni1—O2 2.039 (2) N1—Ni1—O2 78.97 (9) N5—Ni2—O7 79.06 (8) Ni1—O4 2.128 (2) O1—Ni1—O2 168.93 (7) O6—Ni2—O7 168.18 (8) Ni1—O5 2.1753 (18) O1i—Ni1—O2 110.62 (7) O6ii—Ni2—O7 111.52 (8) Ni2—N5 2.009 (2) N1—Ni1—O4 95.68 (9) N5—Ni2—O9 92.27 (8) Ni2—O6 2.0342 (18) O1—Ni1—O4 89.38 (8) O6—Ni2—O9 93.83 (8) Ni2—O6ii 2.0488 (18) O1i—Ni1—O4 85.34 (8) O6ii—Ni2—O9 88.04 (8) Ni2—O7 2.059 (2) O2—Ni1—O4 92.13 (8) O7—Ni2—O9 84.51 (8) Ni2—O9 2.0975 (19) N1—Ni1—O5 91.79 (8) N5—Ni2—O10 92.08 (8) Ni2—O10 2.1161 (19) O1—Ni1—O5 90.17 (7) O6—Ni2—O10 94.69 (8) N1—N2 1.385 (3) O1i—Ni1—O5 87.23 (7) O6ii—Ni2—O10 89.24 (8) N5—N6 1.397 (3) O2—Ni1—O5 89.74 (8) O7—Ni2—O10 88.00 (8) O2—C8 1.276 (3) O4—Ni1—O5 172.52 (7) O9—Ni2—O10 170.47 (9) O7—C28 1.267 (3) Ni1—O1—Ni1i 99.57 (7) Ni2—O6—Ni2ii 99.93 (8) 2 Ni1—O4 2.089 (7) N2—Ni1—O5 170.5 (2) O3—Ni2—O3iii 180.0 Ni1—N2 1.991 (6) N2—Ni1—O2 90.7 (2) O3—Ni2—O2iii 99.4 (2) Ni1—O5 2.025 (6) O5—Ni1—O2 98.7 (2) O3iii—Ni2—O2iii 80.6 (2) Ni1—O2 2.031 (5) N2—Ni1—O1 78.7 (2) O3—Ni2—O2 80.6 (2) Ni1—O1 2.088 (6) O5—Ni1—O1 91.9 (2) O3iii—Ni2—O2 99.4 (2) Ni1—O6 2.092 (6) O2—Ni1—O1 169.4 (2) O2iii—Ni2—O2 180.0 Ni2—O3 1.948 (6) N2—Ni1—O4 87.6 (3) O3—Ni2—O7 88.8 (3) Ni2—O3iii 1.948 (5) O5—Ni1—O4 93.5 (3) O3iii—Ni2—O7 91.2 (3) Ni2—O2iii 2.108 (5) O2—Ni1—O4 93.1 (2) O2iii—Ni2—O7 93.3 (2) Ni2—O2 2.108 (5) O1—Ni1—O4 86.8 (2) O2—Ni2—O7 86.7 (2) Ni2—O7 2.143 (8) N2—Ni1—O6 91.9 (3) O3—Ni2—O7iii 91.2 (3) Ni2—O7iii 2.143 (8) O5—Ni1—O6 86.7 (3) O3iii—Ni2—O7iii 88.8 (3) N1—N2 1.387 (8) O2—Ni1—O6 88.8 (2) O2iii—Ni2—O7iii 86.7 (2) C1—O1 1.250 (9) O1—Ni1—O6 91.3 (3) O2—Ni2—O7iii 93.3 (2) - - O4—Ni1—O6 178.0 (3) O7—Ni2—O7iii 180.0 - - Ni1—O2—Ni2 125.2 (2) - - Symmetry codes: (i) x, y + 1, z + 1; (ii) x + 1, y + 2, z + 2; (iii) x + 1, y + 1, z. Catalysts 2019, 9, x FOR PEER REVIEW − − − − − − − − − 6 of 13

2.3. CatalyticThe catalytic Studies activity of complexes 1 and 2 for the nitroaldol (or Henry) reaction was evaluated usingThe benzaldehydes catalytic activity and ofnitroethane complexes as1 modelsand 2 for (Sch theeme nitroaldol 2, Tables (or3 and Henry) 4). An reaction excess wasnitroethane evaluated usingwas used benzaldehydes to obtain maximum and nitroethane conversion as models of benzaldehydes (Scheme2, Tablesinto products.3 and4). An excess nitroethane was used toThe obtain selectivity maximum for the conversion β-nitroethanol of benzaldehydes products was into 100% products. for all the experiments (the only compounds found besides syn- and anti- β-nitroethanols were non-reacted substrates).

Scheme 2. Scheme 2. Nitroaldol condensation condensation reaction reaction of of nitroethane nitroethane and and benzaldehydes. benzaldehydes.

Trinuclear complex 2 showed a higher activity than the dinuclear one, at the same temperature and under similar conditions (Table 3). Consequently, the reaction conditions (temperature, reaction time, amount of catalyst, and type of solvent) were optimized using a model nitroethane- benzaldehyde system with catalyst 2 (Table 3). Thus, the optimal experimental conditions found are exhibited in entry 10 of Table 3, leading to a β-nitroethanol yield of 94% (TOF = 3.9 h−1) and a good selectivity syn:anti (77:23).

Table 3. Catalytic activity of 1 and 2 in the model nitroaldol condensation reaction a of nitroethane and benzaldehyde.

Amount Yield or Time of Temp. Selectivity TOF Entry. Catalyst Solvent Conversion (h) catalyst (°C) c syn: anti /h−1 d (%) b (mol%) 1 1 24 1.0 ambient - 86.6 70:30 3.6 2 1 24 1.0 ambient H2O 80.2 70:30 3.3 3 1 24 1.0 ambient MeOH 72.2 64:36 3.0 4 1 24 1.0 ambient MeCN 56.4 58:42 2.4 5 2 24 1.0 ambient - 89.2 72:28 3.7 6 2 24 1.0 ambient H2O 82.4 70:30 3.4 7 2 24 1.0 ambient MeOH 72.6 66:34 3.0 8 2 24 1.0 ambient MeCN 58.0 60:40 2.4 9 2 24 1.0 40 - 90.8 72:28 3.8 10 2 24 1.0 60 - 94.0 77:23 3.9 11 2 24 1.0 75 - 90.6 76:24 3.8 12 Blank 24 − 60 - - - - 13 Ni(OAc)2 24 1.0 60 - 9 62:38 0.3 14 2 24 2.0 60 - 94.2 76:24 1.9 15 2 24 3.0 60 - 92.8 74:26 1.3 16 2 24 5.0 60 - 91.6 71:29 0.8 17 2 6 1.0 60 - 44.4 68:32 7.4 18 2 12 1.0 60 - 60.8 72:28 5.1 19 2 48 1.0 60 - 92.6 74:26 1.9 a Experimental conditions: Benzaldehyde (1 mmol); nitroethane (2 mmol); solvent (2 mL); b Determined using 1H NMR analysis; c Molar ratios, calculated using 1H NMR (see ESI for entry 10); d Estimated as moles of syn- and anti-β-nitroethanol/mol of 1 or 2, per hour. A blank test was performed with neat benzaldehyde and nitroethane in the absence of any metal catalyst, at 60 °C. No β -nitroalkanol was detected after 24 h of reaction time (entry 12, Table 3). The

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Table 3. Catalytic activity of 1 and 2 in the model nitroaldol condensation reaction a of nitroethane and benzaldehyde.

c 1 Time Amount of Catalyst Temp. Yield or Selectivity TOF/h− Entry. Catalyst Solvent b d (h) (mol%) (◦C) Conversion (%) syn: anti Catalysts 2019, 9, x FOR PEER REVIEW 7 of 13 1 Catalysts1 2019, 924, x FOR PEER REVIEW 1.0 ambient - 86.6 70:30 3.6 7 of 13 Catalysts 2019, 9, x FOR PEER REVIEW 7 of 13 2 Catalysts1 2019, 924, x FOR PEER REVIEW 1.0 ambient H2O 80.2 70:30 3.3 7 of 13 3 Catalystsnitroaldol1 2019 ,reaction 924, x FOR PEER also REVIEW 1.0did no t occur ambient appreciably MeOH (9% yield 72.2 of nitroethanol) 64:36 by using 3.0 Ni(OAc)7 of 132 nitroaldolCatalysts 2019 reaction, 9, x FOR PEERalso REVIEWdid no t occur appreciably (9% yield of nitroethanol) by using Ni(OAc)7 of 132 4 nitroaldolCatalystsinstead1 2019 of thereaction, 924, x catalyst FOR PEERalso precurso REVIEWdid 1.0 no rt 1occur or ambient 2 (entry appreciably 13, MeCN Table (9 %3). yield 56.4 of nitroethanol) 58:42 by using 2.4 Ni(OAc)7 of 132 5 nitroaldolinstead2 of thereaction24 catalyst also precurso did 1.0 nort 1 occur or ambient2 (entry appreciably 13, -Table (9 3).% yield 89.2 of nitroethanol) 72:28 by using 3.7 Ni(OAc)2 insteadnitroaldol of thereaction catalyst also precurso did nor t1 occuror 2 (entry appreciably 13, Table (9 3).% yield of nitroethanol) by using Ni(OAc)2 6 nitroaldol2To determine reaction24 thealso catalytic 1.0did no performancet occur ambient appreciably of H 22 Oin the(9% nitroaldol yield 82.4 of nitroethanol)condensati 70:30on byreaction, using 3.4 aNi(OAc) series of2 insteadnitroaldolTo ofdetermine thereaction catalyst thealso precursocatalytic did nor performance t1 occuror 2 (entry appreciably 13,of 2 Table in the(9 3).% nitroaldol yield of condensatinitroethanol)on reaction,by using aNi(OAc) series of2 7 insteadaromaticTo2 ofdetermine and the24 aliphaticcatalyst the precursocatalytic aldehydes 1.0 rperformance 1 orwere ambient2 (entry selected of13, MeOH 2 Tableto in screen the 3). nitroaldol as 72.6starting condensati materials 66:34on (Table reaction, 4). 3.0 Ina series general, of 8 aromaticinsteadTo2 ofdetermine and the24 aliphaticcatalyst the precurso catalyticaldehydes 1.0 rperformance 1 wereor ambient2 (entry selected of13, MeCN 2 toTable in screen the 3). nitroaldol as 58.0starting condensati materials 60:40on (Table reaction, 4). 2.4 Ina seriesgeneral, of aromaticthe reactivityTo determine and ofaliphatic the the substituted catalyticaldehydes benzaldehydesperformance were selected of is 2to lein screenss the than nitroaldol asbenzaldehyde starting condensati materials itself,on possibly(Table reaction, 4). the In a presenceseriesgeneral, of 9 aromaticthe reactivityTo2 determine and24 ofaliphatic the the substituted catalyticaldehydes 1.0 benzaldehydesperformance were 40 selected of is 2to - lein ssscreen the than nitroaldol asbenzaldehyde 90.8starting condensati materials itself, 72:28on possibly(Table reaction, 4). 3.8 the In a presence seriesgeneral, of 10 thearomaticof stericreactivity2 andhindrance.24 ofaliphatic the substituted In aldehydes the 1.0 case benzaldehydes ofwere substitute 60 selected dis -tolearomatic ssscreen than benzaldehydeasaldehydes 94.0starting materialshaving itself, 77:23 electron-withdrawingpossibly (Table 4). 3.9 the In presence general, thearomaticof steric reactivity hindrance.and ofaliphatic the substituted In aldehydesthe case benzaldehydes ofwere substitute selected dis toaromaticle ssscreen than asbenzaldehydealdehydes starting materialshaving itself, electron-withdrawing possibly(Table 4). the In presence general, 11 ofthesubstituents steric reactivity2 hindrance. 24in of parathe substituted -Inposition, the 1.0 case higher benzaldehydes of substitute yields 75 (entriesd is - aromaticless 5than and benzaldehydealdehydes 7, 90.6 Table 4) having are itself, 76:24obtained, electron-withdrawing possibly which 3.8 the presence may be 12ofsubstituentsthe steric Blankreactivity hindrance. 24in of para the substituted -position,In the case higher benzaldehydes of substitute yields60 (entriesd is - aromaticless 5than and benzaldehydealdehydes 7, -Table 4) having are itself, obtained, -electron-withdrawing possibly which -the presence may be substituentstheof stericreactivity hindrance. in of parathe substituted- position,In the− case higher benzaldehydes of substitute yields (entries dis learomaticss 5than and benzaldehyde aldehydes7, Table 4) having are itself, obtained, electron-withdrawingpossibly which the presence may be 13 ofattributed Ni(OAc)steric 2hindrance. to24 the higher In electrophilicitythe 1.0 case of substitute 60 of the substrated -aromatic compared aldehydes 9 to the having aldehydes 62:38 electron-withdrawing bearing 0.3 electron- substituentsattributedof steric hindrance. to inthe para higher -position,In electrophilicitythe case higher of substitute yields of the (entries substrated aromatic 5 andcompared aldehydes7, Table to 4)the having are aldehydes obtained, electron-withdrawing bearing which electron- may be 14 attributedsubstituentsdonating2 moieties.to 24thein para higher A-position, maxi electrophilicity 2.0mum higher yield yields 60ofof 97%the (entries substrate with - a 5syn: andcompared anti 7, 94.2 Tablediastereoselectivity to the4) are aldehydes 76:24obtained, ratiobearing which 1.9 of 80:20 electron- may was be attributeddonatingsubstituents2 moieties. to thein para higher A- position,maxi electrophilicitymum higher yield yieldsof of 97% the (entries substratewith a 5syn: andcompared anti 7, diastereoselectivityTable to the4) are aldehydes obtained, ratio bearing which of 80:20 electron- may was be 15 donatingsubstituentsattributed moieties. to 24 inthe para higher A-position, maxi electrophilicity 3.0mum higher yield yieldsof 60 of 97% the (entries substratewith - a 5syn: andcompared anti 7, 92.8 diastereoselectivityTable to 4)the are aldehydes 74:26obtained, ratio bearing which 1.3of 80:20 electron- may was be 16 attributedobserved2 forto24 the the higher aldehyde, electrophilicity 5.0 which is 60 ofpara-nitro-substituted the substrate - compared 91.6 (entry to the 7, aldehydesTable 71:29 4). Forbearing 0.8the electron-aliphatic donatingattributedobserved moieties. forto the the higher aldehyde, A maxi electrophilicitymum which yield is of para-nitro-substitutedof 97% the substratewith a syn: compared anti diastereoselectivity(entry to the 7, aldehydesTable 4). ratio Forbearing theof 80:20 electron-aliphatic was 17 observeddonatingaldehydes,2 moieties.for acetaldehyde6 the aldehyde, A maxi 1.0andmum which propionaldehyde yield is 60ofpara-nitro-substituted 97% with (entries - a syn: 8 andanti 44.4 9,diastereoselectivity(entry Table 7, 4), Table a 68:32maximum 4). ratioFor of 7.4 theof 78.2% 80:20aliphatic yield was 18 observedaldehydes,2 for acetaldehyde12 the aldehyde, 1.0and whichpropionaldehyde is 60para-nitro-substituted (entries - 8 and 60.8 9,(entry Table 7, 4), Table a maximum 72:28 4). For of 5.1the 78.2% aliphatic yield aldehydes,donatingobserved moieties.for acetaldehyde the aldehyde, A maxi andmum whichpropionaldehyde yield is ofpara-nitro-substituted 97% with (entries a syn: 8 andanti 9,diastereoselectivity(entry Table 7,4), Tablea maximum 4). ratioFor of theof 78.2% 80:20 aliphatic yield was 19 aldehydes,wasobservedwas obtainedobtained2 for acetaldehyde48 forforthe propionaldehydpropionaldehyd aldehyde, 1.0and whichpropionaldehydeee (entry(entry is 60para-nitro-substituted 9,9, TableTable (entries - 4)4) withwith 8 aanda syn:syn: 92.6 9, (entry antianti Table diastereoselectivitydiastereoselectivity 7,4), Tablea maximum 74:26 4). For of 1.9 ratiotheratio 78.2% aliphatic ofof 74:26.74:26.yield wasobservedaldehydes, obtained for acetaldehyde forthe propionaldehyd aldehyde, and whichpropionaldehydee (entry is para-nitro-substituted 9, Table (entries 4) with 8 aand syn: 9,(entry anti Table diastereoselectivity 7, 4), Table a maximum 4). For of ratiothe 78.2% aliphatic of 74:26. yield a Experimentalwasaldehydes, obtained conditions: acetaldehyde for propionaldehyd Benzaldehyde and propionaldehyde (1 mmol);e (entry nitroethane 9, Table (entries (24) mmol); with 8 a solventand syn: 9, anti (2Table mL); diastereoselectivityb 4),Determined a maximum using 1of Hratio 78.2% of 74:26. yield aldehydes,c acetaldehyde and propionaldehyde1 (entries 8 and 9, Tabled 4), a maximum of 78.2% yield NMRwas analysis; obtainedTable Molar4. Solvent-freefor ratios,propionaldehyd calculated nitroaldol usinge condensation (entryH NMR 9, (seeTable of Figure diffe 4) with S1rent for aaldehydes entry syn: 10); anti Estimatedanddiastereoselectivity nitroethane as moles catalyzed of syn ratio- by of 2 74:26. was obtainedTable 4. Solvent-free for propionaldehyd nitroaldol econdensation (entry 9, Table of diffe 4) withrent aldehydesa syn: anti and diastereoselectivity nitroethane catalyzed ratio by of 2 74:26. andwasanti -obtainedβTable-nitroethanol 4. Solvent-free for/ molpropionaldehyd of 1 or nitroaldol2, per hour.e condensation (entry 9, Table of diffe a4). withrent aldehydesa syn: anti and diastereoselectivity nitroethane catalyzed ratio byof 274:26. Table 4. Solvent-free nitroaldol condensation of diffea. rent aldehydes and nitroethane catalyzed by 2 Table 4. Solvent-free nitroaldol condensation of diffea. rent aldehydes and nitroethane catalyzed by 2 Table 4. Solvent-free nitroaldol condensation of diffea. rent aldehydesb and nitroethane catalyzedc a by− 12d Table 4.EntryEntryTableSolvent-free 4. Solvent-free nitroaldol Substrate Substrate nitroaldol condensation condensation of Yield Yield diﬀerent oror of Conversion Conversiondiffe aldehydesa. rent aldehydes and (%)(%) nitroethane b andsynsyn nitroethane:: antianti catalyzed ratioratio catalyzed byc 2TOF/hTOF/h. by−1 2 d a b c −1 d Entry Substrate Yield or Conversiona . (%) syn: anti ratio TOF/h Entry Substrate Yield or Conversion. (%) b syn: anti ratio c TOF/h−1 d Entry Substrate YieldYield or Conversion or (%) b syn: anti ratio c TOF/h−1 d EntryEntry1 Substrate Substrate Yield or Conversion94.0 syn (%): anti b synratio: anti77:23c ratioTOF / hc 1TOF/hd 3.9 −1 d Entry1 Substrate YieldConversion or Conversion94.0 (%) b (%) b syn: anti77:23 ratio c − TOF/h3.9 −1 d 1 94.0 77:23 3.9 1 94.0 77:23 3.9 1 94.0 77:23 3.9 1 94.0 77:23 3.9 1 1 94.094.0 77:2377:23 3.9 3.9

22 60.860.8 72:2872:28 2.52.5 2 60.8 72:28 2.5 2 60.8 72:28 2.5 2 60.8 72:28 2.5 2 2 60.860.8 72:2872:28 2.5 2.5 2 60.8 72:28 2.5

33 56.456.4 70:3070:30 2.32.3 3 56.4 70:30 2.3 3 56.4 70:30 2.3 3 3 56.456.4 70:3070:30 2.3 2.3 3 56.4 70:30 2.3 3 56.4 70:30 2.3

44 74.074.0 76:2476:24 3.13.1 4 74.0 76:24 3.1 4 4 74.074.0 76:2476:24 3.1 3.1 4 74.0 76:24 3.1 4 74.0 76:24 3.1

55 88.488.4 74:26 74:26 3.7 3.7 5 5 88.488.4 74:26 74:26 3.7 3.7 5 88.4 74:26 3.7 5 88.4 74:26 3.7 5 88.4 74:26 3.7

5 88.4 74:26 3.7 6 70.2 70:30 2.9 6 6 70.270.2 70:30 70:30 2.9 2.9 6 70.2 70:30 2.9 6 70.2 70:30 2.9 6 70.2 70:30 2.9 6 70.2 70:30 2.9 7 97.0 80:20 4.0 77 97.097.0 80:2080:20 4.04.0

7 97.0 80:20 4.0 7 97.0 80:20 4.0 87 CH3CHO 66.697.0 72:2880:20 2.8 4.0 8 CH3CHO 66.6 72:28 2.8 87 CH3CHO 66.697.0 72:2880:20 2.84.0 97 CH3CH2CHO 78.297.0 74:2680:20 3.2 4.0 89 CHCH3CH3CHO2CHO 66.678.2 72:2874:26 2.83.2 a 89 CHCH3CH3CHO2CHO 66.678.2 72:2874:26b 2.83.2 Experimental conditions: Catalyst3 2, 1.0 mol%, aldehyde (1 mmol), nitroethane (2 mmol), 24 h, 60 ◦C; Determined 1 a Experimental98 CHconditions:CH3CHCHO2CHO Catalyst c 2, 1.0 mol%, aldehyde78.266.6 (11 mmol), nitroethaned 74:2672:28 (2 mmol), 24 h,3.22.8 60 °C; using Ha NMRExperimental98 analysis (see CHconditions:CH Experimental);3CH3CHO2CHO Catalyst Molar 2, 1.0 ratio, mol%, calculated aldehyde78.266.6 using (1 mmol),H NMR; nitroethaneEstimated74:2672:28 as(2 moles mmol), of syn24 -h,3.22.8 60 °C; a 8 CH3CHO 66.6 72:28 2.8 and anti-βbExperimental -nitroethanolDetermined9 /mol using conditions:CH of32CH 1,H per 2NMRCHO hour. Catalyst analysis 2, 1.0 (see mol%, Experimental); aldehyde78.2 (1 c mmol),Molar ratio, nitroethane calculated74:26 (2 mmol), using 241H h, NMR;3.2 60 °C; d ab ExperimentalDetermined9 using conditions:CH3 CH1H NMR2CHO Catalyst analysis 2, 1.0 (see mol%, Experimental); aldehyde78.2 (1 c mmol),Molar ratio, nitroethane calculated74:26 (2 mmol), using 241H h, NMR;3.2 60 °C; d ba DeterminedExperimental9 using CHconditions:3 CH1H NMR2CHO Catalyst analysis 2β, 1.0 (see mol%, Experimental); aldehyde78.2 (1 c Molarmmol), ratio, nitroethane calculated74:26 (2 mmol), using 124H h,NMR;3.2 60 °C; d baEstimated as moles 1of syn- and anti-β-nitroethanol/mol of 2c , per hour. 1 d aEstimated DeterminedExperimental as molesusing conditions: ofH syn NMR- Catalystand analysis anti -2,-nitroethanol/mol 1.0 (see mol%, Experimental); aldehyde of (12 , Molarmmol),per hour. ratio, nitroethane calculated (2 mmol), using 24H h,NMR; 60 °C; Estimatedb ExperimentalDetermined as moles using conditions: of1H syn NMR- Catalystand analysis anti -2β, -nitroethanol/mol1.0 (see mol%, Experimental); aldehyde of (12 ,c mmol),Molarper hour. ratio, nitroethane calculated (2 mmol), using 241H h, NMR; 60 °C; d b Determined using 1H NMR analysisβ (see Experimental); c Molar ratio, calculated using 1H NMR; d EstimatedbThe Determined catalytic as molesusing activity of1H syn NMR -of and analysiscompound anti-β-nitroethanol/mol (see Experimental);2 was also of compared2 c, Molarper hour. ratio, in calculated the reactions using 1usingH NMR; various d TheEstimated catalytic as moles activity of syn -of and compound anti-β-nitroethanol/mol 2 was also of compared2, per hour. in the reactions using various substitutedTheEstimated catalytic aromatic as moles activity ofaldehyde syn -of and compounds anti with-β-nitroethanol/mol different 2 was nitroalkanes,also of compared 2, per hour. producing in the reactionsthe corresponding using various β - substitutedTheEstimated catalytic aromatic as moles activity ofaldehyde syn -of and compounds antiwith- -nitroethanol/mol different 2 was nitroalkanes,also of compared2, per hour. producing in the reactionsthe corresponding using various β - substitutednitroalkanolsThe catalytic aromatic (Scheme activity aldehyde3), leading of compounds towith yields different ranging2 was nitroalkanes, alsofrom compared38% to 94%producing in (Table the reactions5).the corresponding using various β - substitutednitroalkanolsThe catalytic aromatic (Scheme activity aldehyde3), leading of compounds towith yields different ranging2 was nitroalkanes, alsofrom compared38% to 94%producing in (Table the reactions5).the corresponding using various β - nitroalkanolssubstituted aromatic (Scheme 3),aldehyde leadings towith yields different ranging nitroalkanes, from 38% to 94%producing (Table 5).the corresponding β - nitroalkanolssubstituted aromatic (Scheme aldehyde3), leadings towith yields different ranging nitroalkanes, from 38% to 94%producing (Table 5).the corresponding β - nitroalkanols (Scheme 3), leading to yields ranging from 38% to 94% (Table 5). nitroalkanols (Scheme 3), leading to yields ranging from 38% to 94% (Table 5).

Catalysts 2019, 9, 554 7 of 13

The selectivity for the β-nitroethanol products was 100% for all the experiments (the only compounds found besides syn- and anti- β-nitroethanols were non-reacted substrates). Trinuclear complex 2 showed a higher activity than the dinuclear one, at the same temperature and under similar conditions (Table3). Consequently, the reaction conditions (temperature, reaction time, amount of catalyst, and type of solvent) were optimized using a model nitroethane-benzaldehyde system with catalyst 2 (Table3). Thus, the optimal experimental conditions found are exhibited in 1 entry 10 of Table3, leading to a β-nitroethanol yield of 94% (TOF = 3.9 h− ) and a good selectivity syn:anti (77:23). A blank test was performed with neat benzaldehyde and nitroethane in the absence of any metal catalyst, at 60 ◦C. No β -nitroalkanol was detected after 24 h of reaction time (entry 12, Table3). The nitroaldol reaction also did not occur appreciably (9% yield of nitroethanol) by using Ni(OAc)2 instead of the catalyst precursor 1 or 2 (entry 13, Table3). To determine the catalytic performance of 2 in the nitroaldol condensation reaction, a series of aromatic and aliphatic aldehydes were selected to screen as starting materials (Table4). In general, the reactivity of the substituted benzaldehydes is less than benzaldehyde itself, possibly the presence of steric hindrance. In the case of substituted aromatic aldehydes having electron-withdrawing substituents in para-position, higher yields (entries 5 and 7, Table4) are obtained, which may be attributed to the higher electrophilicity of the substrate compared to the aldehydes bearing electron-donating moieties. A maximum yield of 97% with a syn: anti diastereoselectivity ratio of 80:20 was observed for the aldehyde, which is para-nitro-substituted (entry 7, Table4). For the aliphatic aldehydes, acetaldehyde and propionaldehyde (entries 8 and 9, Table4), a maximum of 78.2% yield was obtained for propionaldehyde (entry 9, Table4) with a syn: anti diastereoselectivity ratio of 74:26. CatalystsThe catalytic 2019, 9, xactivity FOR PEER of REVIEW compound 2 was also compared in the reactions using various substituted8 of 13 aromatic aldehydes with diﬀerent nitroalkanes, producing the corresponding β-nitroalkanols (SchemeCatalysts 32019), leading, 9, x FOR toPEER yields REVIEW ranging from 38% to 94% (Table5). 8 of 13

Scheme 3. Nitroaldol condensation reaction of various aldehydes and different nitroalkanes. Scheme 3. Nitroaldol condensation reaction of various aldehydes and diﬀerent nitroalkanes. Scheme 3. Nitroaldol condensation reaction of various aldehydes and different nitroalkanes. Table 5. Solvent-free nitroaldol condensation of different aldehydes and nitroalkanes catalyzed by 2 a a Table 5. Solvent-free nitroaldol condensation of diﬀerenta. aldehydes and nitroalkanes catalyzed by 2 . Table 5. Solvent-free nitroaldol condensation of different aldehydes and nitroalkanes catalyzed by 2 −1 a. YieldYield or Conversion or syn: anti TOF/h−1 EntryEntry Aldehyde Aldehyde Nitroalkane Nitroalkane Yield or Conversionsyn : antisynratio: antic TOFTOF/h/h 1 d Entry Aldehyde Nitroalkane b b c d− Conversion/%b /% ratioc d Yield or Conversion syn: anti TOF/h−1 Entry1 1 Aldehyde Nitroalkanenitromethanenitromethane 94.294.2 - - 3.93.9 1 nitromethane /%b94.2 ratioc - d 3.9 22 nitroethane nitroethane 94.094.0 77:2377:23 3.93.9 13 nitromethane nitropropane 94.2 54.2 68:32 - 3.9 2.5 3 nitropropane 54.2 68:32 2.5 2 3 nitroethanenitropropane 94.054.2 77:23 68:32 3.9 2.5 4 nitromethane 56.6 - 2.3 3 4 nitropropanenitromethane 54.256.6 68:32 - 2.5 2.3 4 5 nitroethanenitromethane 56.644.4 -66:34 2.31.8 4 5 nitromethanenitroethane 56.644.4 - 66:34 2.3 1.8 55 6 nitroethanenitropropane nitroethane 44.4 44.438.4 66:34 66:3458:42 1.8 1.81.6 6 nitropropane 38.4 58:42 1.6 a a6 a Experimental conditions: Catalystnitropropane 2, (1.0 mol%), aldehyde38.4 (1 mmol), nitroalkane58:42 (2 mmol),b 241.6 h, 60 ExperimentalExperimental conditions: conditions: Catalyst Catalyst2, (1.0 mol%), 2, (1.0 aldehyde mol%), (1 aldehyde mmol), nitroalkane (1 mmol), (2 nitroalkane mmol), 24 h, 60(2◦ mmol),C; Determined 24 h, 60 1 b 1 c c 1 d 1 using°C;H b Determined NMR analysis using (see Experimental); 1H NMR analysisMolar (see ratio, Experimental); determined usingc MolarH ratio, NMR; determinedEstimated using as moles 1H ofNMR;syn- a Experimental°C; Determined conditions: using CatalystH NMR analysis2, (1.0 mol%), (see Experimental); aldehyde (1 mmol), Molar nitroalkane ratio, determined (2 mmol), using 24 h,H 60 NMR; and antid -β-nitroethanol/mol of 2, per hour. β d Estimated as moles of syn- and anti-β-nitroethanol/mol of 2, per hour. °C; bEstimated Determined as usingmoles 1 Hof NMRsyn- and analysis anti- (see-nitroethanol/mol Experimental); ofc Molar 2, per ratio, hour. determined using 1H NMR; d EstimatedIn general, as moles the yield of syn of- and β-nitroalkanol anti-β-nitroethanol/mol decreased of in 2, theper hour.order of nitromethane > nitroethane > 1- In general,In general, the the yield yield of ofβ-nitroalkanol -nitroalkanol decreased decreased in in the the orderorder ofof nitromethane >> nitroethanenitroethane > >1- nitropropane for both aldehydes,β and the same molecular size dependent behavior was found for the 1-nitropropanenitropropaneIn general, for thefor both bothyield aldehydes, aldehydes,of -nitroalkanol and and the the decreased same same molecular mo inlecular the sizeorder size dependent dependentof nitromethane behavior behavior > nitroethane was was found found > for for1- the the nitropropanealdehydes (Tablefor both 5). aldehydes, A proposed and catalytic the same cycle molecular prom otedsize dependentby Ni(II) centre behavior is presented was found in for Scheme the 4 aldehydesshowing (Table the reaction 5). A proposed pathway catalytictowards cyclethe formation promoted of by the Ni(II) β-nitroalkanols. centre is presented in Scheme 4 showing the reaction pathway towards the formation of the β-nitroalkanols.

Scheme 4. Proposed catalytic cycle for the β-nitroalkanol formation in the Ni(II) catalyzed nitroaldol Schemereaction. 4. Proposed catalytic cycle for the β-nitroalkanol formation in the Ni(II) catalyzed nitroaldol reaction. 3. Materials and Methods 3. Materials and Methods

Catalysts 2019, 9, x FOR PEER REVIEW 8 of 13

Scheme 3. Nitroaldol condensation reaction of various aldehydes and different nitroalkanes.

Table 5. Solvent-free nitroaldol condensation of different aldehydes and nitroalkanes catalyzed by 2 a.

Yield or Conversion syn: anti TOF/h−1 Entry Aldehyde Nitroalkane /%b ratioc d 1 nitromethane 94.2 - 3.9 2 nitroethane 94.0 77:23 3.9 3 nitropropane 54.2 68:32 2.5 4 nitromethane 56.6 - 2.3 5 nitroethane 44.4 66:34 1.8

6 nitropropane 38.4 58:42 1.6

a Experimental conditions: Catalyst 2, (1.0 mol%), aldehyde (1 mmol), nitroalkane (2 mmol), 24 h, 60 °C; b Determined using 1H NMR analysis (see Experimental); c Molar ratio, determined using 1H NMR; d β CatalystsEstimated2019, 9, 554 as moles of syn- and anti- -nitroethanol/mol of 2, per hour. 8 of 13 In general, the yield of β-nitroalkanol decreased in the order of nitromethane > nitroethane > 1- nitropropanealdehydes (Table for both5). A aldehydes, proposed catalyticand the same cycle mo promotedlecular size by Ni(II)dependent centre behavior is presented was found in Scheme for the4 aldehydesshowing the (Table reaction 5). A pathway proposed towards catalytic the cycle formation prom ofoted the byβ -nitroalkanols.Ni(II) centre is presented in Scheme 4 showing the reaction pathway towards the formation of the β-nitroalkanols.

Scheme 4. 4. ProposedProposed catalytic catalytic cycle cycle for the for β-nitroalkanol the β-nitroalkanol formation formation in the Ni(II) in the catalyzed Ni(II) nitroaldol catalyzed reaction.nitroaldol reaction. 3. Materials and Methods 3. Materials and Methods The syntheses for this study were performed in air and reagents and solvents (commercially available) that were used as received, without further puriﬁcation. The metal source for the synthesis of complexes was Ni(NO3)2 6H2O. Elemental analyses (C, H, and N) were carried out · 1 by the Microanalytical Service of the Instituto Superior Técnico. Infrared spectra (4000–400 cm− ) were recorded on a Bruker Vertex 70 instrument in KBr pellets (Bruker Corporation, Ettlingen, 1 1 Germany); wavenumbers are in cm− . The H NMR spectra were recorded on a Bruker Avance II + 400.13 MHz (UltraShieldTM Magnet) spectrometer at room temperature. The internal reference was tetramethylsilane. The chemical shifts are reported in ppm in the 1H NMR spectra. Mass spectra were recorded in a Varian 500-MS LC Ion Trap Mass Spectrometer (Agilent Technologies, Amstelveen, The Netherlands) equipped with an electrospray (ESI) ion source. For electrospray ionization, the drying gas and ﬂow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was carried out from m/z 100 to 1200 in methanol solution. The compounds were observed in the positive and negative mode (capillary voltage = 80–105 V).

3.1. Syntheses of the Pro-Ligand H2L 1 The aroylhydrazone pro-ligand 2-hydroxy-(2-hydroxybenzylidene)benzohydrazide (H2L ) and 2 (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide (H3L ) (Scheme1) were prepared by a reported method [47,48] upon condensation of the 2-hydroxybenzohydrazine with 2-hydroxybenzaldehyde or 2,3-dihydroxybenzaldehyde.

3.2. Synthesis of the Nickel(II) Complexes Synthesis of [Ni (L1) (MeOH) ](1) • 2 2 4 First, 0.26 g (1.01 mmol) of H L1 was dissolved in 25 mL of methanol, and Ni(NO ) 6H O (0.32 g, 2 3 2· 2 1.10 mmol) were added to it. The resultant mixture was stirred for 15 min at 50 ◦C; a dark green solution was obtained. The dark green solution was then ﬁltered, and the solvent was allowed to Catalysts 2019, 9, 554 9 of 13 evaporate slowly. After 1 d, a green microcrystalline powder was obtained, washed 3 times with cold methanol, and dried in open air. Yield: 0.293 g (78%, with respect to Ni(II)). Anal. Calcd for (1)C32H36N4Ni2O10: C, 50.97; H, 4.81; 1 N, 7.43. Found: C, 50.93; H, 4.78; N, 7.39. IR (KBr; cm− ): 3416 ν(OH), 1609 ν(C=N), 1254 ν(C–O) enolic and 1154 ν(N–N). ESI-MS(+): m/z 753 [1+H]+ (100%).

Synthesis of [Ni (L1) (DMF) ] [Ni (L1) (DMF) (H O) ] 2DMF (1A) • 2 2 4 2 2 2 2 2 · The green microcrystalline powder of 1 was dissolved in DMF and few drops of water was added. After ca. 3 d, nice green crystals were isolated from the solution. Isolated compound was washed 3 times with cold methanol and kept in open air for drying. The crystals are insoluble in common organic solvents. Anal. Calcd for (1A)C80H96N16Ni4O22: C, 51.42; H, 5.18; N, 11.99. Found: C, 51.39; H, 5.16; 1 N, 11.93. IR (KBr; cm− ): 3403 ν(OH), 1608 ν(C = N), 1258 ν(C–O) enolic and 1157 ν(N–N).

Synthesis of [Ni (HL2) (CH OH) ] (NO ) (2) • 3 2 3 8 · 3 2 2 First, 0.28 g, 1.02 mmol of H3L was dissolved in 25 mL of methanol and 0.58 g, 2.0 mmol of Ni(NO ) 6H O were added to it. The resultant mixture was stirred for 15 min at 50 C to obtain 3 2· 2 ◦ a dark green solution. A clear solution was obtained by ﬁltering the mixture and the solvent was then allowed to evaporate slowly. After 1 d, green X-ray quality single crystals were isolated. The isolated compound was then washed 3 times with cold methanol and dried in open air. Yield: 0.394 g (74%, with respect to Ni(II)). Anal. Calcd for (2)C36H52N6Ni3O20: C, 40.60; H, 4.92; 1 N, 7.89. Found: C, 40.54; H, 4.88; N, 7.85. IR (KBr; cm− ): 3386 ν(OH), 2964 ν(NH), 1603 ν(C=O), 1387 ν(NO3−) and 1068 ν( N–N). ESI-MS(-): m/z 939 [2-2(NO3)-H]− (100%).

3.3. X-ray Measurements Single crystals of complexes 1A and 2 appropriate for X-ray diffraction analysis were immersed in cryo-oil, mounted in Nylon loops, and measured at 296 K. A Bruker AXS PHOTON 100 diffractometer with graphite monochromated Mo-Kα (λ 0.71073) radiation was used to collect the intensity data. Data collections were recorded using omega scans of 0.5◦ per frame and full sphere of data were obtained. Cell parameters were retrieved using Bruker SMART [49] software and the data were refined using Bruker SAINT [49] on all the observed reflections. Absorption corrections were done using SADABS [49]. Structures were solved by direct methods by applying SIR97 [50] and refined with SHELXL2014 [51]. Calculations were carried out using WinGX v2014.1 [52]. All non-hydrogen atoms were refined anisotropically. Those H-atoms bonded to carbon were placed in the model at geometrically calculated positions and refined using a riding model. Uiso(H) were defined as 1.2Ueq of the parent carbon atoms for phenyl and methyne residues and 1.5Ueq of the parent carbon atoms for the methyl groups. Least square refinements were employed with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic for the remaining atoms.

3.4. Catalytic studies All catalytic tests were run under atmospheric ambiance under with the following conditions for each essay: 1.0–5.0 mol% (0.1–0.5 µmol) of the catalyst precursor 1 or 2 (usually 1 mol%) under solvent-free condition contained 2 mmol of nitroethane and 1 mmol of aldehyde, in that order, or added solvent (2 mL) for reaction in a particular solvent. The reaction mixture was stirred at the particular temperature for the required duration. The solvent was then evaporated, the residue was dissolved 1 in CDCl3, and analyzed by H NMR. In the case of solvent-free conditions, the reaction mixture 1 was dissolved in CDCl3 and analyzed by H NMR. A previously reported method was employed to determine the yield of the β-nitroalkanol product (relatively to the aldehyde) by 1H NMR [47,48]. To verify the adequacy of the procedure, a number of 1H NMR analyses were performed in the presence Catalysts 2019, 9, 554 10 of 13

of 1,2-dimethoxyethane as an internal standard, added to the CDCl3 solution, giving yields similar to those obtained by the above method. Moreover, the internal standard method also conﬁrmed the absence of other side products formed. The ratio between the syn and anti isomers was also determined by 1H NMR spectroscopy. The values of vicinal coupling constants (for the β-nitroalkanol products), in the 1H NMR spectra, between the α-N–C–H, and the α-O–C–H protons identify the isomers, being J = 7–9 or 3.2–4 Hz for the syn or anti isomers, respectively [53,54].

4. Conclusions Binuclear and trinuclear aroylhydrazone Ni(II) complexes in two different tautomeric forms 1 2 1 (enol and keto) [Ni2(L )2(MeOH)4](1) and [Ni3(HL )2(CH3OH)8] (NO3)2 (2), where H2L = 2 · 2-hydroxy(2-hydroxybenzylidene)benzohydrazide, and H3L = (2,3-dihydroxybenzylidene)-2- hydroxybenzohydrazide, have been synthesized and fully characterized. They act as efficient catalysts for the solvent-free Henry reaction of β-nitroalkanols formation with excellent yields and diastereoselectivity. The avoidance of a solvent, either organic or ionic liquid, in this reaction brings a number of benefits render it a safer, more economical, and environmentally benign C–C coupling process.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/6/554/s1, Figure S1: 1H NMR spectrum for the nitroalol product of nitroethane and benzaldehyde in CDCl3 (Table3, entry 10); CCDC 1914037-1914038 for 1A and 2 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif. Author Contributions: Conceptualization, M.S.; data curation, T.R.B.; formal analysis, T.R.B.; funding acquisition, A.J.L.P. and L.M.D.R.S.M.; investigation, M.S.; methodology, M.S. and T.R.B.; project administration, A.J.L.P. and L.M.D.R.S.M.; resources, A.J.L.P.; software, M.S.; supervision, M.S. and L.M.D.R.S.M.; visualization, L.M.D.R.S.M.; writing—original draft, M.S. and T.R.B.; writing—review and editing, A.J.L.P. and L.M.D.R.S.M. Funding: Authors are grateful to the Fundação para a Ciência e a Tecnologia: FCT (projects UID/QUI/00100/2019, PTDC/QEQ-ERQ/1648/2014 and PTDC/QEQ-QIN/3967/2014), Portugal, for financial support. Acknowledgments: M.S. acknowledges the FCT and IST for a working contract “DL/57/2017” (Contract no. IST-ID/102/2018). Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS measurements. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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