materials Communication The Formation of a Unique 2D Isonicotinate Polymer Driven by Cu(II) Aerobic Oxidation Francisco Sánchez-Férez 1 , Teresa Calvet 2 , Mercè Font-Bardia 3 and Josefina Pons 1, * 1 Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; francisco.sanchez.ferez@uab.cat 2 Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain 3 Unitat de Difracció de Raig-X, Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), Universitat de Barcelona, Solé i Sabarís, 1-3, 08028 Barcelona, Spain * Correspondence: josefina.pons@uab.es; Tel.: +34-935-812-895 Abstract: The isolation and structural characterization of a unique Cu(II) isonicotinate (ina) material with 4-acetylpyridine (4-acpy) is provided. The formation of [Cu(ina)2 (4-acpy)]n (1) is triggered by the Cu(II) aerobic oxidation of 4-acpy using O2 . This gradual formation of ina led to its restrained incorporation and hindered the full displacement of 4-acpy. As a result, 1 is the first example of a 2D layer assembled by an ina ligand capped by a monodentate pyridine ligand. The Cu(II)-mediated aerobic oxidation with O2 was previously demonstrated for aryl methyl ketones, but we extend the applicability of this methodology to heteroaromatic rings, which has not been tested so far. The formation of ina has been identified by 1 H NMR, thus demonstrating the feasible but strained formation of ina from 4-acpy in the mild conditions from which 1 was obtained. Keywords: Cu(II) polymers; 2D materials; aerobic oxidation; isonicotinic acid; 4-acetylpyridine 1. Introduction Citation: Sánchez-Férez, F.; Calvet, T.; The discovery of graphene foregrounded the outbreak of 2D materials, which was Font-Bardia, M.; Pons, J. The Formation of a Unique 2D triggered by their fascinating electrochemical, mechanical, and optical properties. Their Isonicotinate Polymer Driven by better performance over 0D and 1D materials fostered the rapid development of 2D-based Cu(II) Aerobic Oxidation. Materials technologies for electrochemical devices, renewable energy storage, and production or 2023, 16, 3724. https://doi.org/ for catalysis. Especially in these fields, the aim is, per se, to encounter clean, renewable, 10.3390/ma16103724 and inexpensive sources, so Cu(II)-based materials were expected to be one of the most promising candidates since Cu(II) combines high natural abundance with low toxicity [1]. Academic Editor: Anastasios Within this frame, our group has previously reported the synthesis and characteri- J. Tasiopoulos zation of Cu(II) complexes with carboxylic acids and pyridine derivatives [2–4]. During Received: 30 March 2023 the assays to further extend this research with 4-acetylpyridine (4-acpy), we performed its Revised: 5 May 2023 reaction with Cu(NO3 )2 ·3H2 O in acetonitrile (ACN) as a solvent. From this reaction, the Accepted: 12 May 2023 serendipitous formation of single crystals, which were isolated and characterized, revealed Published: 14 May 2023 the formation of an isonicotinate (ina) ligand, which further coordinated to the Cu(II) center and fostered the formation of complex [Cu(ina)2 (4-acpy)]n (1). Therefore, in this contribution, we present the serendipitous formation of a Cu(II) isonicotinate (ina) material starting from Cu(NO3 )2 ·H2 O and 4-acpy. Copyright: © 2023 by the authors. Interestingly, the elucidation of its crystal structure revealed the arrangement of Licensee MDPI, Basel, Switzerland. a 2D isonicotinate material. Indeed, isonicotinic acid (ina) has been vastly employed This article is an open access article as an archetypal linker for the construction of extended networks. In particular, Cu(II) distributed under the terms and isonicotinates have presented from 0D to mostly 3D structures, benefiting from the great conditions of the Creative Commons Attribution (CC BY) license (https:// variety of coordination modes of ina ranging from monodentate to bridging. Within this creativecommons.org/licenses/by/ plethora of available structures, Cu(II) ions mainly present square pyramidal geometry 4.0/). and lead to the assembly of highly stable 3D nets [5] featured by [Cu(ina)2 ]n [6]. This Materials 2023, 16, 3724. https://doi.org/10.3390/ma16103724 https://www.mdpi.com/journal/materials Materials 2023, 16, 3724 2 of 12 tendency of forming 3D nets is only broken either by introducing polydentate chelate or bridging ligands that are able to compete with the coordination of ina, which lowers the dimensionality to 1D [7] or 2D materials [8,9], or the isolated example of a 0D structure by combining ina with a chelate linker, coordinated chloride ions, and bulky counterions [10]. Therefore, the dimensionality of Cu isonicotinates is biased towards 3D nets, and no previous examples have been reported with monodentate pyridine derivatives, which are probably triggered by its lower coordination ability. Additionally, hydrothermal conditions promoted the formation of mixed valent Cu(I)-Cu(II) isonicotinates rather than guiding a tunable dimensionality [11]. Therefore, the slow formation of ina, in combination with the excess 4-acpy, provided access to a network that has not been achieved by direct self-assembly [6]. Since the formation of ina seemed to be triggered by Cu(II) with molecular oxygen as the oxidizing agent, we tried to demonstrate the feasible formation of ina from 4-acpy in the mentioned conditions. Within the palette of available first-row transition metals, Cu(II) was already known to promote C-N, -S, and -O bond formation, avoiding air moisture sensitivity and providing elevated functional group tolerance [12]. However, despite the promising advantages, the early stages of this research with limited examples hitherto found triggered the use of Pd(II) with the consequent overshadowing of Cu(II). Conversely, during the last few decades, the course of Cu(II) research was shifted by the buoyant results that emerged. To date, Cu(II) catalytic systems have shown the ability to oxidize a plethora of organic substrates using molecular oxygen. Among the major findings of special mention are the benzylic oxidation to alcohols [13], or mixtures of alcohols and ketones [14]; the oxidation of alkanes to alcohols and ketones [15]; the oxidation of alcohols to aldehydes [16,17], or to mixtures of aldehydes and ketones [18], the oxidation of primary and secondary alcohols to aldehydes and ketones [19,20]; the oxidation of phenols to ortho-quinones, biphenols, or benzoxepines [21], and the oxidation of amines to imines and nitriles [22], not to mention Baeyer–Villiger oxidation [23]. Indeed, the catalytic oxidation of aryl methyl ketones to mixtures of aldehydes and carboxylic acids using molecular oxygen as an oxidant was tested several years ago [24], but recently, some methodologies for the isolation of aldehydes [25], esters [26], and carboxylic acids [27,28] have been published. Thus, we present the serendipitous formation of 1 promoted by the slow catalytic oxidation, using molecular oxygen, of the acetyl group of 4-acpy to the carboxylate function- ality in ina. The single-crystal X-ray diffraction of 1 displayed a 2D coordination polymer assembled by ina and 4-acpy ligands, which was subsequently characterized by EA and FTIR-ATR spectroscopy. Additionally, we demonstrated the feasibility of the formation of ina from 4-acpy, and we tried to obtain some insights into the effect of changing the synthetic conditions (changing the Cu(II) salt, the solvent, and the O2 pressure) of the resulting product. These experiments were traced by 1 H NMR spectroscopy. 2. Experimental Section 2.1. Materials and General Methods Copper(II) nitrate trihydrate (Cu(NO3)2 ·3H2O), copper(II) acetate hydrate (Cu(OAc)2 ·H2O), and 4-acetylpyridine (4-acpy) as reagents and acetonitrile (ACN) and N,N-dimethylformamide (DMF) as solvents were purchased from Sigma-Aldrich. In addition, deuterated ACN (ACN-d3 ) and DMF (DMF-d7 ) were used for the 1 H NMR experiments. Reagents and solvents were used as received without further purification. Elemental analysis (C, H, and N) was measured in a Euro Vector 3100 apparatus. FTIR-ATR spectrum was measured in a diamond window in the range from 4000 to 500 cm−1 in a Tensor 27 (Bruker) spectrometer equipped with an attenuated total reflectance (ATR) accessory (model MKII Golden Gate). 1 H NMR spectra were acquired using a Bruker Ascend 400 MHz spectrometer either in ACN-d3 or DMF-d7 at room temperature. All the chemical shifts (δ) are given in ppm. Materials 2023, 16, 3724 3 of 12 2.1.1. Synthesis of [Cu(ina)2 (4-acpy]n (1) To an ACN solution (6 mL) of Cu(NO3 )2 ·3H2 O (12.0 mg, 0.050 mmol), liquid 4-acpy (23 µL, 0.317 mmol) was added. The reaction was stirred under reflux for 16 h and then was transferred to a vial and left to slowly evaporate for 1 month. After this period, suitable green crystals of 1 were grown. 1. Elemental Analysis calcd(%) for C19 H15 CuN3 O5 (428.88 g/mol): C 53.21; H 3.53; N 9.80; found: C 52.42; H 3.29; N 9.52. FTIR-ATR (wavenumber, cm−1 ): 3072(w), 2908(w), 2795(w), 1695(m), 1631(m), 1608(w), 1590(s), 1500(m), 1489(m), 1436(s), 1384(s), 1360(s), 1323(m), 1255(s), 1239(s), 1216(m), 1170(m), 1112(m), 1074(m), 1033(s), 964(w), 933(m), 919(s), 883(m), 840(w), 831(w), 819(m), 804(s), 774(s), 767(s), 722(m), 682(m), 668(s), 590(s), 554(m). 2.1.2. X-ray Crystallographic Data A green prism-like specimen of 1 was used for the crystallographic data collection. The X-ray intensity data were measured on a D8 Venture system equipped with a multilayer monochromator and a Mo microfocus. The frames were integrated with the Bruker SAINT software package, using a narrow-frame algorithm. The integration of the data with a 0.70 Å resolution gave an average redundancy of 7.101, a completeness of 99.7%, and an Rsig of 4.05%. From this integration, 3135 (82.33%) independent reflections were greater than 2σ(|F|2 ). The structure was solved and refined using the Bruker SHELXTL Software Package (version-2018/3) [29]. The final cell constants and volume are based upon the refinement of the XYZ-centroids of reflections above 20 σ(I). Data were corrected for absorption effects using the multi-scan method (SADABS). Crystal data and relevant details of structure refinement are reported in Table 1. The entire X-ray data of 1 can be found via the CCDC in .cif format, using the code 2252448. The X-ray structure was worked with Mercury 4.3.1 software, and molecular graphics were rendered using the POV-Ray image package [30]. Color codes used for the molecular graphics are orange roughy (Cu), red (O), light blue (N), grey (C) and white (H). Table 1. X-ray crystallographic data of complex 1. Sample 1 Empirical Formula C19 H15 CuN3 O5 Formula weight 428.88 T (K) 254 (2) Wavelength (Å) 0.71073 System, space group Orthorhombic, Pbca Unit cell dimensions a (Å) 12.6673 (11) b (Å) 11.7649 (9) c (Å) 24.118 (2) α (◦ ) 90 β (◦ ) 90 γ (◦ ) 90 V (Å3 ) 3594.4 (5) Z 8 Dcalc (mg/m3 ) 1.585 µ (mm−1 ) 1.253 F (000) 1752 Crystal size (mm−3 ) 0.180 × 0.173 × 0.076 −16 ≤ h ≤ 16 hkl ranges −14 ≤ k ≤ 14 −30 ≤ l ≤ 30 Materials 2023, 16, x FOR PEER REVIEW 4 of 13 Crystal size (mm−3) 0.180 × 0.173 × 0.076 Materials 2023, 16, 3724 −16 ≤ h ≤ 16 4 of 12 hkl ranges −14 ≤ k ≤ 14 −30 ≤ l ≤ 30 Table 1. Cont. θ range (°) 2.332 to 26.747 Reflections collected/unique/[Rint] 27,042/3808/0.0405 Sample 1 Completeness to θ (%) 99.6 θ range (◦ ) 2.332 to 26.747 Absorption correction Semi-empirical from equivalents Reflections collected/unique/[Rint ] 27,042/3808/0.0405 Refinement Completeness to θmethod (%) Full-matrix99.6 least-squares on |F|2 Data/restraints/parameters Absorption correction Semi-empirical3808/1/229 from equivalents Refinement methodon F2 Goodness-of-fit 1.034 on |F|2 Full-matrix least-squares Data/restraints/parameters 3808/1/229 R1 = 0.0525, Final R indices Goodness-of-fit F2 on[I>2σ(I)] 1.034 wR2 = 0.1293 R1 = 0.0525, Final R indices [I>2σ(I)] R indices (all data) wR2R=1 0.1293 = 0.0662, R1 wR 2 = 0.1397 = 0.0662, R indices (all data) Extinction coefficient wR2 = 0.1397 n/a Extinction coefficient n/a Largest diff-peak and hole−(e. 3 Å ) −3 1.037 and −0.914 Largest diff-peak and hole (e. Å ) 1.037 and −0.914 The geometry evaluation of the Cu(II) center in the complex has been performed us- The geometry ing version evaluation 2.1 of SHAPE of the Cu(II) [31] software, center which in the on is based complex the lowhas been performed meas- continuous-shape using version 2.1 ofvalue ure (CShM) SHAPE [31] Itsoftware, S [32]. which is based is a generalizable on thedescriptor structural low continuous-shape measure used to quantitatively (CShM) evaluatevalue S [32]. distortion It is aofgeneralizable in terms symmetry and structural distance descriptor usedgeometry. from any ideal to quantitatively The cor- evaluate distortion in terms of symmetry and distance from any ideal geometry. responding atomic coordinates have been directly extracted from .cif data and the S values The corresponding atomic coordinates have been directly extracted from have been computed for any potential geometric accommodation within the correspond-.cif data and the Sing values have been computed for any potential geometric accommodation coordination number five: vOC-5 = vacant octahedron; TBPY-5 = trigonal bipyramid; within the corresponding coordination number five: vOC-5 = vacant SPY-5 = square pyramid; JTBPY-5 = Johnson trigonal bipyramid. octahedron; TBPY-5 = trigonal bipyramid; SPY-5 = square pyramid; JTBPY-5 = Johnson trigonal bipyramid. 3. Results and Discussion 3. Results and Discussion 3.1. Synthesis 3.1. Synthesis and and General General Characterization Characterization Thecomplex complex [Cu(ina) 2(4-acpy)]n was synthesized by mixing in ACN the The [Cu(ina) 2(4-acpy)]n was synthesized by mixing in ACN the Cu(NO3)2 ·3H2O Cu(NO and the3)4-acpy 2·3H2O and the 4-acpy ligand in a 1:6 molar ratio, stirring under reflux, and then ligand in a 1:6 molar ratio, stirring under reflux, and then leaving it to leaving it to stand for a month stand for a month (Scheme (Scheme 1). During 1).period, this Duringsuitable this period, greensuitable crystalsgreen crystals of 1 were of 1 grown. were grown. The product was characterized by elemental analysis (EA), FTIR-ATR The product was characterized by elemental analysis (EA), FTIR-ATR spectroscopy, and spec- troscopy, andX-ray single-crystal single-crystal X-ray diffraction. diffraction. Scheme 1. Scheme 1. Ouline Ouline of of the the synthetic synthetic conditions conditions to to the the formation formation of of 1. 1. The EA The EA ofof 11 agrees agrees with with the theproposed proposedformula. formula. The The displayed displayed signals signals within within thethe FTIR-ATR spectrum are FTIR-ATR are attributable attributabletotovibrations vibrations from from both ligands, both ina ina ligands, andand 4-acpy (S.I: 4-acpy Figure (S.I: S1). S1). Figure The most The characteristic vibrations most characteristic from 4-acpy vibrations from are found 4-acpy at found are 2908 and at 2795 2908 cm and−1, assigned 2795 − 1 cm to, [ν(C-H)] assignedal,toand at 1695alcm [ν(C-H)] , andfrom −1 at 1695 − 1 the [ν(C=O)] cm from of the theacetyl [ν(C=O)]group. of Vibrations the acetyl over 3000 group. cm−1 belong Vibrations overto3000 cm−1ar belong [ν(C-H)] from theto aromatic rings [ν(C-H)]ar fromof the both ligands,rings aromatic whereas sig- of both ligands, whereas signals from al can[ν(C=C/C=N)] −1 . nals from [ν(C=C/C=N)] be found betweenal can1631 be found between and 1436 cm−1.1631 Theand 1436 cmand formation The formation and further coordination of the ina ligand cause the raising of vibrations from the carboxylate functionality. Bands at 1590 and 1384 cm−1 have been attributed to [ν(COO)]as and [ν(COO)]s , respectively. Indeed, the coordination mode of the car- boxylate in the Cu(II) complex can be inferred from the difference between these bands Materials 2023, 16, 3724 5 of 12 (referred to as the ∆ value) [33,34]. In 1, the ∆ of 206 cm−1 suggests a monodentate coor- dination mode. Additional bands from the aromatic rings attributed to [δ(C=C/C=N)] between 1360 and 1239 cm−1 , [δ(C-H)]ip at 1033 cm−1 , and [δ(C-H)]oop at 774 cm−1 have also been identified [35]. 3.2. Crystal Structure of [Cu(ina)2 (4-acpy)]n (1) [Cu(ina)2 (4-acpy)]n (1) crystallizes in the orthorhombic Pbca space group and con- tains Cu(II) centers bearing a [CuO2 N3 ] core with a distorted square pyramidal geometry (S = 0.942 for SPY, 4.456 for TBPY, 1.006 for vOC, and 7.824 for JTBPY) [31,36]. These units are composed of four bridging (µ2 :η1 :η1 -) ina ligands and a monodentate (µ1 :η1 -) 4-acpy lig- and (Figure 1a), which are orthogonally connected by ina ligands, being 4-acpy sequenced in pairs by pointing upwards and downwards along each chain (Figure 1b). This linkage re- sults in the formation of 2D layers along the (030) plane holding an sql topology (Figure 1c). Selected Materials 2023, 16, x FOR PEER REVIEW bond lengths and angles are provided in Table 2. The equatorial 6 of 13 Cu-O and Cu-N bond lengths from isonicotinate ligand in 1 are comparable to those extracted from [Cu(ina)2 ]n (Cu-O, 1.962(3)–2.306(2) Å and Cu-N, 2.010(5)-2.026(2) Å) [6]. Instead, the apical site is(Å) occupied of Cg(J) onby 4-acpy plane I (equalwith whenanα = elongated 0); e slippage =Cu-N bond horizontal length of displacement 2.351(3) or slippage Å, which is prob- between Cg(I) and Cg(J) (equal for both centroids when α = 0); Cg(1) = N2-C8-C9-C10-C11-C12; Cg(2) = N3- ably driven by a marked Jahn–Teller effect [37]. This length is surprisingly C14-C15-C16-C17-C18; Cg(3) = N1-C2-C3-C4-C5-C6.; f distance between H atom and ring centroid J long compared to the(Å); Cu(II) complexes g perpendicular with distance of H4-acpy reported to ring plane to date J; h distance of X to(Cu-N 2.016–2.198(2) ring centroid J (Å); i angle of Å) the [38–43]. X-H bond with the Cg(J) plane. Figure 1. Representation of (a) the molecular structure of complex 1 with atom labeling, (b) the 1. Representation Figurepairing of (a) of 4-acpy within the the plane molecular displaying structure an upwards of complex or downwards 1 with and disposition, atom (c) labeling, the (b) the pairing sqlof 4-acpy within the plane displaying an upwards or downwards disposition, and (c) the sql topology. topology. Materials 2023, 16, 3724 6 of 12 Table 2. Bond lengths, angles, and intermolecular interactions in complex 1. Bond lengths Cu(1)-O(1) 1.949(3) Cu(1)-N(1)#2 2.038(3) Cu(1)-O(3) 1.957(2) Cu(1)-N(3) 2.351(3) Cu(1)-N(2)#1 2.036(3) Bond Angles O(1)-Cu(1)-O(3) 179.76(12) N(2)#1-Cu(1)-N(1)#2 167.80(13) O(1)-Cu(1)-N(2)#1 88.25(12) O(1)-Cu(1)-N(3) 90.49(12) O(3)-Cu(1)-N(2)#1 91.92(11) O(3)-Cu(1)-N(3) 89.32(11) O(1)-Cu(1)-N(1)#2 89.66(12) N(2)#1-Cu(1)-N(3) 99.33(12) O(3)-Cu(1)-N(1)#2 90.21(12) N(1)#2-Cu(1)-N(3) 92.71(13) π···π interactions a b β, γ c Slippage e Cg(I)···Cg(J) Cg···Cg A Cg(I)_Perp, Cg(J)_Perp d Cg(1)···Cg(2) 3.868(2) 6.7(2) 18.3, 24.6 3.5170(18), 3.6727(17) 1.213 C-H···π interactions C-H···Cg(J) H···Cg(J) f H-Perp g Γc X···Cg(J) h X-H, π i C18-H18···Cg(1) 2.98 2.87 15.70 3.849(5) 74 C15-H15···Cg(3) 2.96 2.70 23.86 3.777(5) 68 C19-H19B···Cg(3) 2.92 2.87 10.47 3.702(7) 59 a #1 x + 1/2, −y + 3/2, −z + 1; #2 x − 1/2, y, −z + 1/2; Cg···Cg = distance between ring centroids given in Å; a Cg···Cg = distance between ring centroids (Å); b α= dihedral angle between planes I and J (◦ ); c offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ), and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e slippage = horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0); Cg(1) = N2-C8-C9-C10-C11-C12; Cg(2) = N3-C14-C15-C16-C17-C18; Cg(3) = N1-C2-C3-C4-C5-C6.; f distance between H atom and ring centroid J (Å); g perpendicular distance of H to ring plane J; h distance of X to ring centroid J (Å); i angle of the X-H bond with the Cg(J) plane. These layers are assembled by π···π and C-H···O and C-H···π interactions resulting in a 3D supramolecular net. The aromatic ring between ina and 4-acpy ligands are stacked at 3.868(2) Å supported by C-H···O interactions between the uncoordinated O carboxylate and the ortho-H of a vicinal ina ligand (Figure 2a). Furthermore, each 4-acpy is embedded into a pocket of ina ligands and displays three C-H···π interactions (Figure 2b). 3.3. Catalytic Conversion As previously mentioned, the aerobic oxidation of aromatic methyl ketones to car- boxylic acids using Cu(II) as the catalyst has been previously reported, but no examples have been found using the analogous heteroaromatic molecules. After the obtention of complex 1, we tried to follow the conversion from 4-acpy to ina by 1 H NMR spectroscopy. To this aim, we tested the oxidation of 4-acpy to ina, modifying the precursor, the solvent, and the O2 pressure. These results have been summarized in Table 3. As a general procedure adapted from [27], 0.100 mmol of the Cu(II) salt (24.16 mg of Cu(NO3 )2 ·3H2 O or 19.96 mg of Cu(OAc)2 ·H2 O) and 0.6 mmol of 4-acpy (68 µL) were placed in a vial and dissolved in either 2 mL of ACN or DMF, and the resulting dark blue solution was degassed. Then, the vials were filled with 2.1 bars of O2 pressure, sealed, and put in the furnace at 120 ◦ C for 18 h. Then, the reaction crude was dried and dissolved in DMF-d7 or ACN-d3 for the 1 H NMR experiments. Materials Materials 2023, 16,2023, 372416, x FOR PEER REVIEW 7 of 13 7 of 12 Materials 2023, 16, x FOR PEER REVIEW 10 of 13 Figure 3. 1H NMR spectra in ACN-d3 of the catalytic assays: (a) using Cu(NO3)2·3H2O in ACN with- out O2 pressure for 18h at 120 °C; (b) using Cu(OAc)2·H2O in ACN at 2.1 bars of O2 pressure for 18 h at 120 °C; (c) using Cu(NO3)2·3H2O in ACN at 2.1 bars of O2 pressure for 18 h at 120 °C. Peaks Figure 2. 2. Representation of Representation ofthe theintermolecular intermolecular (a) π···π (a) πand ···πC-H···O and or (b) C-H···π ···O interactions C-H···πare pre- Figure belonging to 4-fopy sent in complex 1. are identified Dashed with blue lines green-filled stand for C-H··· Ostars, thoseC-H interactions, belongingor (b) to 4-acpy whereas dashed interactions green linesidentified present with in complextriangles, yellow-filled refer 1. Dashedand blue linesbelonging those to C-H···π and π···π interactions. stand for toC-Hina···are O interactions, identified with whereas dashed light blue green lines asterisks. refer to C-H···π and π···π interactions. Table3.3.3.Catalytic Summary of the results and conditions employed for the catalytic conversion of 4-acpy to Conversion Table 3. Summary of the results and conditions employed for the catalytic conversion of 4-acpy to ina and/or 4-fopy. As 4-fopy. Outline previously of the conversion mentioned, the aerobic with the tested oxidation conditions. of aromatic methyl ketones to car- ina and/or Outline of the conversion with the tested conditions. boxylic acids using Cu(II) as the catalyst has been previously reported, but no examples have been found using the analogous heteroaromatic molecules. After the obtention of complex 1, we tried to follow the conversion from 4-acpy to ina by 1H NMR spectroscopy. To this aim, we tested the oxidation of 4-acpy to ina, modifying the precursor, the solvent, and the O2 pressure. These results have been summarized in Table 3. As a general procedure adapted from [27], 0.100 mmol of the Cu(II) salt (24.16 mg of Cu(NO3)2·3H2O or 19.96 mg of Cu(OAc)2·H2O) and 0.6 mmol of 4-acpy (68 µL) were placed in a vial and dissolved in either 2 mL of ACN or DMF, and the resulting dark blue solution was degassed. Then, the vials were filled with 2.1 bars of O2 pressure, sealed, and put in the furnace at 120 °C for 18 h. Then, the reaction crude was dried and dissolved in DMF- d7 or ACN-d3 for the 1H NMR experiments. First, aiming to demonstrate the feasibility of the catalytic conversion from 4-acpy to ina under similar experimental conditions from which 1 was isolated, the reaction be- Precursor Quantity (mmol) Solvent O Pressure (bar) Product Precursor Quantitytween (mmol) Cu(NO3)2Solvent ·3H2O and 4-acpy O2 Pressure was carried out in2 ACN under autogenous (bar) Product pressure at 120 °C for 18 h. Reactions DMF were DMF atm using a closed atm conducted vessel based on n.r.previous exam-n.r. ACN rate to achieve fasteratm ples, evincing a boost in the reaction conversion [27]. 4-fopy + ina Cu(NO3 )2 ·3H2 O 24.16 mg (0.1) ACN atm 4-fopy + ina Cu(NO3)2·3H2O 24.16 mg (0.1) The 1H NMR in ACN-d3 of DMF the resulting products revealed 2.1 the recovery of 4-acpyn.r. and DMF ACN 2.1 the mixtures of the corresponding aldehyde (4-formylpyridine, 4-fopy) and2.1 n.r. ina ina, therefore verifying that theACN Cu(II) catalytic DMF 2.1 of 4-acpy to ina2.1 oxidation ina 3a). Then, is attainable (Figure n.r. Cu(OAc)2 ·H2 O 19.96 mg (0.1) to further extendDMFthese results, we scrutinized 2.1 different synthetic conditions n.r. to 4-fopy evaluate ACN 2.1 + ina Cu(OAc)2·H2O 19.96 mgthis(0.1) process. To this end, two different 2.1 solvents were employed: ACN4-fopy and DMF. It should ACN + ina be mentioned Experimental that reactions conditions: performed 2 mL of the in DMF corresponding resulted solvent in◦the at 120 recovery C for of 4-acpy 18 h using 68 µL ofwithout 4-acpy. n.r. = no Experimental conditions: 2 mL of the corresponding solvent at 120 °C for 18 h using 68 µL of 4-acpy. reaction; ACN of any trace = acetonitrile; 4-fopy norDMF = N,N-dimethylformamide. ina, regardless of the Cu(II) precursor or the addition of 2.1 bars of n.r. = no reaction; ACN = acetonitrile; DMF = N,N-dimethylformamide. O2 pressure (S.I: Figure S2). In addition, the use of Cu(OAc)2·H2O in ACN leads to the First, aiming to demonstrate the feasibility of the catalytic conversion from 4-acpy to 4. inaConclusions under similar experimental conditions from which 1 was isolated, the reaction between ◦C Cu(NO 2 ·3H2 O and We3 )reported, for4-acpy was the first carried time, the out in ACNofunder assembly autogenous a Cu(II) pressure isonicotinate at 120 bearing a 2D for 18 h.structure layered Reactions were conducted holding a monodentate using a closedderivative, pyridine vessel based on previous the 4-acpy ligand.examples, The for- evincing a boost in the reaction rate to achieve faster conversion [27]. mation of ina has been traced back, resulting in the identification of the Cu(II) catalytic The 1 of oxidation H NMR 4-acpyinusing ACN-d 3 of the resulting dioxygen products as the oxidant. revealed Thus, the recovery we extended of 4-acpy the covering ofand the the mixtures ofoxidation Cu(II)-driven the corresponding of methyl aldehyde ketones to (4-formylpyridine, a heteroaromatic4-fopy) andsuggesting molecule, ina, therefore an verifying that analogous the Cu(II) pathway tocatalytic the one oxidation followedof 4-acpy by itsto ina arylis counterparts. attainable (Figure The3a). Then, use of Cu(NO3)2·3H2O without O2 pressure or Cu(OAc)2·H2O at 2.1 bars of O2 provided a less efficient conversion, leaving unreacted 4-acpy, which probably drove the arrangement of 1. Similarly to the analogous reactions with aryl methyl ketones, the best results were gar- nered using ACN under an O2 pressure of 2.1 bars at 120 °C for 18h, from which ina was Materials 2023, 16, 3724 8 of 12 to further extend these results, we scrutinized different synthetic conditions to evaluate this process. To this end, two different solvents were employed: ACN and DMF. It should be mentioned that reactions performed in DMF resulted in the recovery of 4-acpy without any trace of 4-fopy nor ina, regardless of the Cu(II) precursor or the Materials 2023, 16, x FOR PEER REVIEW 9 of addition 13 of 2.1 bars of O2 pressure (S.I: Figure S2). In addition, the use of Cu(OAc)2 ·H2 O in ACN leads to the recovery of 4-acpy and mixtures of 4-fopy and ina (Figure 3b), yielding a similar result to Scheme 2. Mechanistic pathway for the conversion of 4-acpy into ina or 4-fopy. The orange square that of Cu(NO highlights 3 )2 ·3Hidentified: the two products 2 O without O4-fopy. ina and 2 pressure. Adapted from [28]. Figure 3. 1 H NMR spectra in ACN-d3 of the catalytic assays: (a) using Cu(NO3 )2 ·3H2 O in ACN without O2 pressure for 18 h at 120 ◦ C; (b) using Cu(OAc)2 ·H2 O in ACN at 2.1 bars of O2 pressure for 18 h at 120 ◦ C; (c) using Cu(NO3 )2 ·3H2 O in ACN at 2.1 bars of O2 pressure for 18 h at 120 ◦ C. Peaks belonging to 4-fopy are identified with green-filled stars, those belonging to 4-acpy are identified with yellow-filled triangles, and those belonging to ina are identified with light blue asterisks. tified as [Cu(ina)2(H2O]n (S.I: Figure S3). This agrees with the two requirements needed to achieve the formation of 1: the gradual formation of the ina ligand and a sufficient amount of 4-acpy that remains unreacted. Thus, it is reasonable to suggest that the heteroaromatic analogue follows the same catalytic pathway [28], and the presence of 4-fopy is due to the incomplete conversion of 4-acpy into ina (Scheme 2). Materials 2023, 16, 3724 Therefore, Cu(NO3)2·3H2O can oxidize 4-acpy to ina even without O2 pressure, 9 ofde- 12 spite having the worst performance. Among the palette of oxidative reactions from methyl ketones with Cu(II), α-oxygenation to carboxylic acids and C-C bond cleavage to alde- hydes can occur in Interestingly, employing Cu(NO3 )2 ·3H similar conditions. The2 Ooutcome and 4-acpyis mainly in ACN biased by of at 2.1 bars theOstrong 2 pressurede- and heating pendence ofup 120 ◦ C for thetocatalytic 18h were found performance on theto be the Cu(II) best experimental counterion. conditions In this case, since and as previ- only ina has been ous catalytic studiesidentified and nonoticed, have already traces of additional the products use of Cu(OAc) 2·H2are O aspresent (Figure the catalyst led3c). to This small is quantities in agreement of the with the previous results corresponding aldehyde with aromatic [25]. methyl Similarly, ketones [27]. the absence of O2Itpressure should be mentioned with Cu(NO3)that,2·3H2O from alsothis reaction, yielded a blueofpowder mixtures was ina, 4-fopy and isolated. whichAfter the the reflects filtration need forof the reaction crude and washing O2 to boost the transformation into ina. with 10 mL of DMF, the remaining solid was identified as [Cu(ina) 2 (H2 O]be It should n (S.I: noted Figure thatS3). the This agreescatalytic decreased with theactivity two requirements from Cu(OAc) needed2·H2to O achieve with re- the spectformation to Cu(NO of3)21:·3Hthe 2Ogradual formation has already of the ina been reported andligand can beand a sufficient attributed to theamount decreased of 4-acpy that remains availability of Cu(II)unreacted. Thus, itcoordinated ions that remain is reasonable to suggest to the acetate that ions.the heteroaromatic Similarly, the con- analogue version offollows 4-acpythe cansame catalyticby be hindered pathway [28], and the the coordination presence to the Cu(II) of 4-fopy ions is due to in contrast to aryl the incomplete methyl ketones. conversion of 4-acpy into ina (Scheme 2). O2 O O O OH H2O N N H2O O2 O O OH OH CH3 N H N 1,2-hydride shift O O O OH H N N OH O H2O H C-C Cleavage O2 N CO2 + H2O Scheme 2. Mechanistic pathway for the conversion of 4-acpy into ina or 4-fopy. The orange square highlights the two products identified: ina and 4-fopy. Adapted from [28]. Therefore, Cu(NO3 )2 ·3H2 O can oxidize 4-acpy to ina even without O2 pressure, de- spite having the worst performance. Among the palette of oxidative reactions from methyl ketones with Cu(II), α-oxygenation to carboxylic acids and C-C bond cleavage to aldehydes can occur in similar conditions. The outcome is mainly biased by the strong dependence of the catalytic performance on the Cu(II) counterion. In this case, and as previous cat- alytic studies have already noticed, the use of Cu(OAc)2 ·H2 O as the catalyst led to small quantities of the corresponding aldehyde [25]. Similarly, the absence of O2 pressure with Cu(NO3 )2 ·3H2 O also yielded mixtures of 4-fopy and ina, which reflects the need for O2 to boost the transformation into ina. It should be noted that the decreased catalytic activity from Cu(OAc)2 ·H2 O with re- spect to Cu(NO3 )2 ·3H2 O has already been reported and can be attributed to the decreased availability of Cu(II) ions that remain coordinated to the acetate ions. Similarly, the con- Materials 2023, 16, 3724 10 of 12 version of 4-acpy can be hindered by the coordination to the Cu(II) ions in contrast to aryl methyl ketones. 4. Conclusions We reported, for the first time, the assembly of a Cu(II) isonicotinate bearing a 2D layered structure holding a monodentate pyridine derivative, the 4-acpy ligand. The formation of ina has been traced back, resulting in the identification of the Cu(II) catalytic oxidation of 4-acpy using dioxygen as the oxidant. Thus, we extended the covering of the Cu(II)-driven oxidation of methyl ketones to a heteroaromatic molecule, suggesting an analogous pathway to the one followed by its aryl counterparts. The use of Cu(NO3 )2 ·3H2 O without O2 pressure or Cu(OAc)2 ·H2 O at 2.1 bars of O2 provided a less efficient conversion, leaving unreacted 4-acpy, which probably drove the arrangement of 1. Similarly to the analogous reactions with aryl methyl ketones, the best results were garnered using ACN under an O2 pressure of 2.1 bars at 120 ◦ C for 18 h, from which ina was isolated. This improved the conversion promoted by the formation of [Cu(ina)2 (H2 O]n that rapidly precipitated. Therefore, the remarkable difference in the conversion after changing the Cu(II) precursor demonstrates the significant effect of the counterion. Furthermore, the assays in DMF did not bring any evidence of 4-fopy nor ina ligand formation regardless of the employed Cu(II) salt. Therefore, it seemed that the catalytic performance of Cu(II) in α-oxygenation as well as in the C-C bond cleavage of heteroaromatic methyl ketones behaves similarly to their aryl analogues. Then, the formation of mixtures with 4-fopy when modifying the synthetic conditions is also provided probably as a result of incomplete conversion. It, therefore, appears that this progressive formation of ina, combined with the excess of 4-acpy, enabled its gradual nucleation and growth, granting the formation of suitable crystals for the X-ray diffraction of compound 1. Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/ma16103724/s1. Figure S1: FTIR-ATR spectrum of compound 1. Figure S2: 1 H NMR spectra in DMF-d6 of the catalytic assays using (a) Cu(NO3 )2 ·2H2 O in DMF at 2.1 bars of O2 pressure for 18 h at 120 ◦ C and (b) Cu(OAc)2 ·H2 O in DMF at 2.1 bars of O2 pressure for 18 h at 120 ◦ C. Figure S3: FTIR-ATR spectrum of compound [Cu(ina)2 (H2 O)]n . Author Contributions: Conceptualization, J.P.; data curation, F.S.-F. and M.F.-B.; formal analysis, F.S.-F. and M.F.-B.; funding acquisition, J.P.; investigation, F.S.-F.; methodology, F.S.-F.; project ad- ministration, J.P.; resources, J.P. and T.C.; software, F.S.-F.; supervision, J.P.; validation, J.P. and T.C.; visualization, F.S.-F.; writing—original draft preparation, F.S.-F.; writing—review and editing, J.P. and T.C. All authors have read and agreed to the published version of the manuscript. Funding: J.P. acknowledges financial support from the CB615921 project, the CB616406 project from “Fundació La Caixa”, and the 2021SGR00262 project from the Generalitat de Catalunya. F.S.-F. acknowledges the PIF predoctoral fellowship from the Universitat Autònoma de Barcelona. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All the data from this research has been included in this manuscript and supporting information. Conflicts of Interest: The authors declare no conflict of interest. References 1. Ren, X.; Wang, H.; Chen, J.; Xu, W.; He, Q.; Wang, H.; Zhan, F.; Chen, S.; Chen, L. Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective. Small 2023, 19, 2204121. [CrossRef] [PubMed] 2. Soldevila-Sanmartín, J.; Ayllón, J.A.; Calvet, T.; Font-Bardia, M.; Pons, J. Mononuclear and Binuclear Copper(II) Bis(1,3- Benzodioxole-5-Carboxylate) Adducts with Bulky Pyridines. 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