Tetranuclear Copper Complexes with Bulky Aminoalcohol Ligands as Catalysts for Oxidative Phenoxazinone Synthase-like Coupling of Aminophenol: A Combined Experimental and Theoretical Study

: The new copper(II) complexes [Cu 4 (pa) 4 (Bae) 4 ] · H 2 O ( 1 ) and [Cu 4 (eba) 4 (Buae) 4 ] · H 2 O ( 2 ) (Hpa = propionic acid, HBae = 2-benzylaminoethanol, Heba = 2-ethylbutyric acid and HBuae = 2-butylaminoethanol) were synthesizsed by the interaction of a copper salt with a methanol solution of the respective ligands. The single-crystal X-ray diffraction analysis reveals that both compounds have a {Cu 4 ( µ 3 -O) 4 } cubane-like core. Both compounds show pronounced phenoxazinone synthase-like activity towards the aerobic oxidation of o -aminophenol to phenoxazinone chromophore, with the maximum initial rates W 0 up to 3.5 × 10 − 7 M s − 1 , and exhibit complex non-linear W 0 vs. [catalyst] 0 dependences. DFT//CCSD theoretical calculations (B3LYP/ma-def2-TZVP//DLPNO-CCSD(T)/ma-def2-TZVPP) were employed to investigate the most challenging steps of catalyst-free and copper-catalysed o -aminophenol oxidation (formation of o -aminophenoxyl radical). QTAIM analysis was used to study the key intermediates and weak interactions. Geometries and energies of intermediates and transition states were benchmarked against a series of popular DFT functionals. The results of the calculations demonstrate that a Cu II –OO • copper-superoxo model catalyst decreases the calculated activation barrier from 28.7 to 19.9 kcal mol − 1 for the catalyst-free and copper-catalysed abstraction of the H atom from the hydroxyl group of o -aminophenol, respectively. Finally, both complexes 1 and 2 were studied as catalysts in the amidation of cyclohexane with benzamide to give N-cyclohexyl benzamide and N-methyl benzamide employing di- tert -butyl peroxide (DTBP) as the oxidant, with a conversion of 16%, and in the oxidation of cyclohexane to cyclohexanol with aq. H 2 O 2 , with a conversion of 12%.


Introduction
The synthesis and investigation of homo-and heterometallic polynuclear complexes is an important field of contemporary chemistry. The presence of several metal centres at close proximity facilitates red-ox and spin coupling interactions that have a marked effect on catalytic [1][2][3][4][5] and magnetic properties [6][7][8]. Natural enzymes often contain polynuclear active centres, e.g., in methane monooxygenases (iron and copper dimers) [9], catechol oxidase (copper dimer) [10], photosystem II (Mn 4 Ca) [11] and many others, for which the mechanistic investigations unambiguously indicate the participation of two or more metal centres in catalytic intermediates [12,13]. Thus, the synthesis of polynuclear complexes containing proper surroundings of metal centres can often be considered as a biomimetic approach towards novel catalytic systems.
Our synthetic strategy towards polynuclear coordination compounds is based on spontaneous self-assembly, where one of more metals is typically introduced in a zerovalent form [14][15][16]. Aerobic oxidation of this powdered metal induces deprotonation of the hydroxyl groups of alcohol ligands, forcing their bridging coordination mode [14]. Another advantage of this technique is the absence (or reduced amount) of anions in the reaction mixture. Metal salts also can be used as starting materials in the spontaneous self-assembly approach. The presence of bulky aliphatic substituents in flexible chelating ligands is a second important feature of our strategy. Polynuclear complexes with a Schiff base and saturated N,O donor ligands could have pronounced catalytic activities [3,17]. However, their solubility in organic solvents is often limited, forming obstacles to the application of the respective complexes in homogeneous catalysis. The introduction of bulky hydrophobic groups not only sterically isolates and stabilises polynuclear species [18][19][20], but also makes them soluble in organic non-polar solvents, typically not adequate for complexes of this type [21,22]. Using this strategy, we described various homoand heterometallic polynuclear complexes active in the sp 3 C-H hydroxylation [23][24][25][26] or amidation processes [21,22,24], and in the phenoxazinone synthase-like aerobic oxidation of aminophenols [27].
Oxidative coupling of the substituted o-aminophenol (OAPH) to phenoxazinone chromophore (PXCR) is a natural process, occurring during the biosynthesis of Actinomycin D [28][29][30], an antitumor drug that has application in medicine. This biological process is catalysed by the phenoxazinone synthase (PHS), which contains a polynuclear copper active centre [31,32]. The construction of efficient artificial catalytic systems [10], including those based on multicopper cores [33,34], became an important research direction. Recently, we described mono-and tetranuclear copper complexes with carboxylate and aminoalcohol ligands which showed unusual behaviour in the oxidation of OAPH [27]. In continuation of this research line, we now report the synthesis, characterisation and catalytic activity study of the two novel tetranuclear complexes [Cu 4 (pa) 4 (Bae) 4 ]·H 2 O (1) and [Cu 4 (eba) 4 (Buae) 4 ]·H 2 O (2) (Hpa = propionic acid, HBae = 2-benzylaminoethanol, Heba = 2-ethylbutyric acid and HBuae = 2-butylaminoethanol, Scheme 1). Both complexes act as efficient catalysts for the aerobic oxidation of OAPH to PXCR under ambient conditions. To gain further insights into the catalytic mechanisms, we conducted a theoretical DFT//ab initio study of the principal steps of the metal-free aerobic oxidation of OAPH, as well as copper-catalysed reactions. Moreover, both complexes were studied as catalysts towards oxidative amidation and hydroxylation of cyclohexane.

Synthesis and Spectroscopic Analysis
Complexes 1 and 2 were synthesised using the self-assembly reaction of Cu(NO 3 ) 2 (1) or Cu(BF 4 ) 2 (2) with a methanol solution of aminoalcohol (2-benzylaminoethanol for 1 or 2butylaminoethanol for 2) and carboxylic acid (propionic for 1 or 2-ethylbutyric acid for 2) in the presence of triethylamine, using the molar synthetic ratio of Cu:aminoalcohol:carboxylic acid = 1:3:1. The reactions were initiated and brought to completion by heating (50-60 • C) and stirring in open air. Dark-blue solutions were obtained at the end of both reactions. Dark-blue microcrystals of 1 suitable for the X-ray crystallographic study were formed after two months from the resulting solution without any additional procedures, while the crystals of 2 grew within two weeks.
The IR spectra of 1 and 2 show complex patterns in the 400-2000 cm −1 region ( Figures S3 and S4) and confirm the presence of both carboxylate and aminoalcoholate ligands. The weak broad bands in the 3400-3500 cm −1 region are attributed to the ν(OH) frequencies of solvated water molecules. The very strong bands in the 1550-1600 cm −1 and 1390-1410 cm −1 regions are assigned to the antisymmetric and symmetric COO − stretching frequencies of the carboxylate ligands, respectively.

Crystal Structures
The single-crystal X-ray analysis reveals that the tetranuclear complexes [Cu 4 (pa) 4 (Bae) 4 ]·H 2 O (1) and [Cu 4 (eba) 4 (Buae) 4 ]·H 2 O (2) have similar structural configurations based on the cubane-like molecular core {Cu 4 (µ 3 -O) 4 }, where the copper atoms are linked together by µ 3 -O bridges from aminoalcoholate ligands ( Figure 1). The 2-benzylaminoethanolate (1) and 2-butylaminoethanolate (2) ligands are deprotonated forms of the corresponding alcohols and reveal bidentate bridging-chelate coordination, while deprotonated propionic (1) and 2-ethylbutyric (2) acids show a monodentate coordination mode. Both complexes contain four crystallographically independent copper(II) atoms, each of which is penta-coordinated and has distorted square-pyramidal geometry, with an O 4 N donor set formed by donor atoms from carboxylate and aminoalcoholate ligands. The equatorial Cu-O(N) bond lengths in 1 and 2 vary from 1.905(5) to 2.0359(11) Å, and the apical Cu-O bond distances are in the range 2.3386(8)-2.5784(9) Å (Table S2)  The observed H-bonding network results in a close packing of the neighbouring tetranuclear molecules. The intermetallic Cu···Cu non-bonded distances within the chains are in the range of 5.727-6.658 and 5.962-6.672 Å for 1 and 2, respectively. The bulky substituents of the organic ligands present in both crystal structures are arranged in a way that prevents the formation of the supramolecular assemblies of higher than 1D dimensionality.  The addition of catalytic amounts of the catalyst to an OAPH solution in methanol affords a rapid growth of the absorption at 435 nm expected for PXCR [28]. The first test using 0.4 mM of 1 and 10 mM of OAPH revealed the initial reaction rate W 0 = 7.0 × 10 −8 M s −1 . The respective UV spectra of the catalytic solutions are shown in Figure 3. The accumulations of PXCR are of linear character in the studied range. The blank test in the absence of a catalyst and using [OAPH] 0 = 10 mM shows two orders of lower activity, with W 0 = 6 × 10 −10 M s −1 . This rate agrees with that previously reported (9 × 10 −10 M s −1 ) under similar conditions [27].
Complex 2 reveals an activity comparable to that of 1, as evidenced by the initial rate of 7.5 × 10 −8 M s −1 for [2] 0 = 0.5 mM. The W 0 vs. [cat] 0 dependence is shown in Figure 4. In general, both complexes show similar behaviour, where two distinct stages can be elucidated: an initial linear stage for [Cu] total < 1.5 mM and an exponentially increasing stage for [Cu] total > 1.5 M (hereafter the concentration of catalysts are given in terms of mols of all copper present in solution). In an attempt to interpret such a dependence, we applied the model described by us earlier [27], which presumes the existence of equilibrium between tetranuclear (Cu 4 ) and dinuclear (Cu 2 ) species, where the catalytic activity of Cu 2 is negligible compared to that of Cu 4   (k e f f 4 K)  One can see that the whole data cannot be fitted as a single line, rather showing two linearised regions with the intersection points of [Cu] 0 = 1.2 and 1.6 mM for 1 and 2, respectively. Moreover, the region with higher reaction rates reveals negative slopes of the lines for both 1 and 2, suggesting that the condition k eff4 >> k eff2 is not satisfied and the model is not applicable in the present case. Another factor could be the interaction of the copper complexes with OAP, which could act as a chelating ligand and quench the activity of the complexes when the ratio [OAP] 0 /[cat] 0 is high enough (low catalyst loading).
The reaction rates exhibited by catalysts 1 and 2 (W 0 up to 3.5 × 10 −7 M s −1 , corresponding to the TOF = 1 × 10 −4 s −1 ) can be considered as high, compared to the typical range of rates from 1 × 10 −8 to 1 × 10 −6 M s −1 [10]. The prominent activity of both complexes can be associated to their polynuclear nature, where the close proximity of metal centres promotes interaction between the catalytically active intermediates. A further increase in rates could be achieved through the design of compounds of higher nuclearity towards possible higher access of the polynuclear cage surface.
The oxidation of an equimolar mixture of o-aminophenol and 2-amino-p-cresol (10 mM each) catalysed by 1 (0.4 mM) resulted in both normal and methyl-substituted phenoxazinone chromophores in a 1:1 ratio (by GC analysis) after 60 min (Scheme 3). Such behaviour is expected for 2-amino-p-cresol, for which the formation of a phenoxazinone chromophore is not possible due to the presence of a methyl group. The evolution of the visible absorption with time is depicted in Figure S6. Using the extinction coefficient of the non-substituted chromophore (2.4 × 10 4 M −1 cm −1 ), the reaction rate of 1 × 10 −7 M s −1 can be deducted. This value is slightly higher than that obtained for the oxidation of 10 mM of OAP only ( Figure 3). This increase can be explained by the twice higher concentration of a substrate. In addition, the difference between extinction coefficients of the normal and methyl-substituted phenoxazinone chromophores was neglected. The TD-DFT analysis of the chromophores was performed at the PBE0/ma-def2-TZVPP level for the B3LYP/ma-def2-TZVP optimised geometries in methanol. The calculated UV/Vis spectra are depicted in Figure S7. The first excited state S1 involves mostly HOMO − 1 → LUMO and HOMO → LUMO transitions, all of the π → π* type, and correspond to 454.7 and 454.4 nm for normal and methyl-substituted chromophores, respectively ( Figure S8, Listings S1 and S2). These values are rather close to the one observed experimentally (435 nm). In contrast, the wavelengths of the S0 → S2 transitions are considerably different: 375.9 and 387.7 nm for normal and methyl-substituted chromophores, respectively. This effect can be explained by the contribution of the molecular orbitals of a methyl group to the S2 excited state ( Figure S8, Listing S2). Therefore, even the presence of a small substituent may substantially change the absorption wavelengths and strengths.
The oxidative formation of o-aminophenoxyl radical (OAP•) and o-iminobenzoquinone (BQMI) is known to be the rate-limiting step during the aerobic OAPH oxidation to phenaxozinone chromophore (Scheme 4) [10]. Although transition metal catalysts (e.g., those of copper) can serve as terminal oxidants in all oxidation steps [27,34,35], the principal role of a catalyst is commonly considered in the formation of the BQMI intermediate in the presence of O 2 [10]. Remarkably, despite many efforts that have been made to investigate this reaction mechanism experimentally, only limited data are available in terms of the theoretical description of possible reaction pathways of BQMI formation [36]. While the enzymatic reaction starts with the two-electron oxidation of OAPH [29], artificial systems, as well as catalyst-free oxidations, are known to be sensitive to the radical traps or even initiated by stable radicals, such as 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) [37,38]. Therefore, the transformation of OAPH to BQMI may proceed through the OAP• radical. In our theoretical calculations, we considered the H atom abstraction from the hydroxyl group as a first step in the aerobic oxidation of OAPH. The interaction of OAPH with 3 O 2 (reactant 3 R 1 ) affords two stable intermediates, where the O 2 molecule is bonded either to hydroxyl group or amino group ( Figures 6 and 7, Scheme 5). The close examination of non-covalent interaction reveals that both oxygen atoms of O 2 form weak attractive interactions with OAPH ( Figure 6), namely N-H···O and C-O···O (for 3 I 1a ), and C-H···O and O-H···O (for 3 I 1b ).
The electron densities at the respective bond critical points for 3 I 1a are nearly equal for N-H···O and C-O···O contacts (ρ(r BCP ) = 3.7 × 10 −3 a.u.), which correspond to the binding energy of −0.08 kcal mol −1 per contact, according to the model developed by Emamian and Lu [40]. For the 3 I 1b configuration ( Figures 6 and 7), the O-H···O and C-H···O contacts appear to be different in binding energies, as evidenced by the electron densities of 4.0 × 10 −3 and 1.7 × 10 −3 a.u., respectively. For these densities, the model [40] predicts the binding energies of −0.16 and 0.36 kcal mol −1 , respectively. From the DFT//CCSD calculations, we found the enthalpies of OAPH + 3 O 2 interactions to be −0.68 and −0.28 kcal mol −1 (for 3 I 1a and 3 I 1b , respectively), thus showing that the abovementioned model can correctly predict the interaction magnitude but it is less relevant in certain cases [41], such as very weak interactions. It must be noted that the binding energy is based on the electronic energy only (with or without basis set counterpoise correction) [42], which does not include the zero point and vibrational energies.    [43,44]. However, this pathway appears to be less favourable due to the necessity of the presence of two reactive OAP• radicals, while the W 0 vs. [OAPH] 0 dependence of a catalyst-free oxidation does not show acceleration with the [OAPH] 0 growth ( Figure S5) [45]. Furthermore, the overall Gibbs energy changes for both 3 R 1 → 1 P 1a and 3 R 1 → 1 P 1c are less favourable for the latter process.
According to the above data, the OAPH → OAP• reaction in methanol has the lowest barrier of 28.7 kcal mol −1 for 3 TS 1b in methanol solution, which is slightly lower than the barrier between 3 R 1 and 3 TS 1a of 30.2 kcal mol −1 (Scheme 5). These barriers are not too high to impede the catalyst-free reaction completely, but account for its low rate, which agrees with the experimentally determined W 0 = 6 × 10 −10 M s −1 for [OAPH] 0 = 10 mM at ambient conditions. It should be noted that the barriers obtained from the pure DFT calculations were found to be different and often lower than for the DFT//CCSD ones. For instance, the Gibbs energy of the transition state 3 TS 1a undergoes an increase from 19.8 to 30.2 kcal mol −1 after applying the CCSD correction (Scheme S1). The same tendency is found with the range-separated ωB97X-D4 [46,47] or double-hybrid B2PLYP [48] functionals applied in a certain case (Table S3). It was also found that the chosen DFT//CCSD methods afford the barrier 3 I 1a → 3 TS 1a very close (0.9 kcal mol −1 difference) to that obtained from heavier and computationally more expensive calculations with denser integration grids, SMD solvation model at all stages and tighter SCF convergence criteria (Table S3).
Obviously, the reaction proceeds much faster in the case the catalyst decreases the activation energy. Since theoretical calculations of polynuclear complexes are computationally demanding, we considered a putative model of mononuclear copper species with aminoalcohol and carboxylate ligands, where OAPH and/or O 2 could coordinate to the copper centre. OAPH is a "classical" N,O donor ligand and known to form coordination compounds with various metals. Ghosh et al. suggested that the coordination of OAPH through the deprotonated oxido-group to a dinuclear zinc complex is most favourable [36]. However, a search via the Cambridge Structural Database (CCDC Inc., Cambridge, England; ver. 5.43; June 2022) [49] reveals that such a coordination is less frequent for coordination compounds: 3, 3 and 19 hits were found for O-, N-and O,N donor chelate bonding, respectively, for pure OAPH, and 4, 10 and 42 hits, respectively, including substituted OAPH ligands.
Firstly, we modelled the N donor coordination of the OAP molecule in the [Cu II (HMeEa) (OAc)(MeOH)(OAPH)] + ···O 2 complex (where HMeEa = N-methylethanolamine and HOAc = acetic acid; these ligands are related to those in complexes 1 and 2 but require fewer computational costs due to a smaller number of atoms and the absence of a flexible n-butyl group) that results in a stable intermediate 4 I 1d (Figures S9 and S10). However, the relaxed scan ( Figure S9) along the H···O distance (where O belongs to O 2 molecule) revealed that abstraction of the H atom from the hydroxyl group leads to the decoordination of the OAP• because its amino group becomes strictly planar and, in this way, sterically not able to coordinate a metal centre. Since this decoordination occurs at (or immediately after) the H transfer transition stage 4 TS 1d ( Figure S9), the activation energy for such a reaction is expected to be equal or higher than that for the metal-free 3 I 1b → 3 I 2b process. The DFT calculations resulted in a barrier of 30.1 kcal mol −1 (Scheme 6), which is comparable to that for the metal-free reaction (Scheme 5). In the next approximation, the bidentate coordination of OAPH to the [Cu II (HMeEa) (OAc)] + complex was considered, where the H atom in the HMeEa ligand is from the deprotonated hydroxyl group of OAPH ( Figure S11). However, while the formation of the O-centred radical upon oxidation by Cu(II) is known to be possible [50], an attempt to model such a reaction did not afford a stable Cu(I)···OAP• intermediate (optimisation of the reaction product results in the reactant geometry, i.e., the OAP• radical readily couples with the Cu(I) centre).
In the next approach, we modelled putative copper-superperoxo species. Their formation is expected from the reaction M n−1 + 3 O 2 → M n -OO•, which constitutes a typical part of the metal-mediated aerobic oxidations [3,[51][52][53]. The decrease in the absorption bands at  The energy of the barrier 3 TS 1e of the H transfer reaction and formation of the OAP• radical is 19.9 kcal mol −1 (Scheme 6). This value is substantially lower than the catalyst-free 3 R 1 → 3 TS 1b barriers of 28.7 and 30.2 kcal mol −1 (Scheme 5). The analysis of the spin density discloses that most of the spin population (1.76) is localised on the copper and oxygen atoms of the Cu-OO• superoxo fragment (Table S4), where the d yz and d z2 orbitals of the copper atom contribute (8.5, 7.4 and 7.4% for d yz and 3.4, 4.8 and 3.7% for d z2 , for 3 I 1d , 3 TS 1d and 3 I 2d , respectively) to one of the spin-up molecular orbitals. During the reaction, the spin density is transferred to the OAPH molecule (Figure 9), eventually leading to the H abstraction and release of the OAP• radical. The catalyst-free process exhibits a similar change of the spin density during the H transfer reaction (Figure 10, Table S5).  The lower energy barrier for the copper-catalysed reaction can be explained by two reasons. First, the non-covalent interactions between the OAPH and copper catalyst (Figure 8) stabilise the resulting assembly, in this way increasing the probability of successful H atom transfer from the hydroxyl group to O 2 . Next, Löwdin atomic charges (Table S3) indicate a significant increase in electronegativity of the oxygen atom of O 2 abstracting hydrogen from the hydroxyl group, while the respective Löwdin spin population is twice higher for the metal-free starting configuration (Table S3). It is known that the coordination of O 2 to a metal centre increases its nucleophilicity [54], in this way facilitating attacks on the positively charged hydrogen atom.
The Gibbs energies of the 3 TS 1a and 3 I 2a states relative to the 3 I 1a one are listed in Table S6. As can be seen, pure GGA and meta-GGA functionals predict the ∆G energies of 3 TS 1a and 3 I 2a to be significantly lower than hybrid functionals. For the difference between intermediate structures 3 I 2a and 3 I 1a , whose geometries are less variable than that of the transition state, the lowest ∆G of 8.7 kcal mol −1 was obtained by the BP86 functional, while the highest one of 18.5 kcal mol −1 by the M06-2X functional. After the correction of electronic energies by single-point DLPNO-CCSD(T) calculations, all the final ∆G values become very close, ranging from 19.8 (M06-L) to 22.3 kcal mol −1 (TPSSh). The smallest difference between pure DFT and CCSD-corrected energies is exhibited by the M06-2X functional, which shows 90% of the final DLPNO-CCSD(T) energy (Table S6).
The Gibbs energies of the 3 TS 1a transition state exhibit larger deviations. The lowest barrier of 6.8 kcal mol −1 was predicted by the BP86 functional, while the highest barrier of 30.0 kcal mol −1 by the ωB97X-D4 one. After applying the single-point DLPNO-CCSD(T) correction, the discrepancy becomes lower, but still not negligible (Table S6): the group of GGA and meta-GGA functionals shows Gibbs energies from 21.5 (M06-L) to 22.7 (TPSS) kcal mol −1 , while the hybrid functionals reveal higher energies from 24.5 (B3LYP) to 34.2 (ωB97X-D4) kcal mol −1 . The higher ∆G( 3 TS 1a ) − ∆G( 3 I 1a ) barriers observed for the case of ωB97X-D4 and M06-2X functionals can be associated to the respective geometries of the 3 TS 1a transition states, which are slightly different from those obtained for other methods ( Figure S15). However, all attempts to obtain the B3LYP-like geometry of 3 TS 1a using the ωB97X-D4 and M06-2X functionals were unsuccessful.
The above results follow the known tendency of hybrid functionals to better estimate the electronic energy due to the inclusion of the Hartree-Fock exchange [67,68]. In the present case, the ∆G( 3 I 2a ) − ∆G( 3 I 1a ) Gibbs energies (after the DLPNO-CCSD(T) correction) were varied in the 2.5 kcal mol −1 range, showing that all the functionals produce reliable geometries of the intermediates. A much larger spread in the case of ∆G( 3 TS 1a ) − ∆G( 3 I 1a ) energies (12.7 kcal mol −1 after the DLPNO-CCSD(T) correction) indicates that the proper choice of a functional is crucial for the analysis of transition state geometries. The rangeseparated hybrid GGA ωB97X-D4 and hybrid meta-GGA M06-2X functionals are known to have high accuracy for weak non-covalent interactions [68][69][70]. Since the Gibbs energies produced by these two functionals are close to those obtained after the single-point DLPNO-CCSD(T) correction (Table S6), one may consider ωB97X-D4 and M06-2X functionals as a good choice for accurate energies. However, although the B3LYP functional underestimates the Gibbs energy barrier, the correction of an electronic energy by the coupled cluster method affords ∆G close to those exhibited by other hybrid functionals (Table S6). Considering that B3LYP performs faster than ωB97X-D4 and significantly faster than M06-2X functionals ( Figure S16), the B3LYP functional, as a well-recognised calculation method, is an appropriate choice for the geometry optimisations of the transition states in the present case. The correction of the single-point energy obtained from higher-level calculations is necessary for the B3LYP calculations. These results completely agree with those suggested earlier for the B3LYP functional [71].
One should emphasise that the difference in the 3 TS 1a geometries ( Figure S15) and energies (Table S6) is stipulated by the great flexibility of the 3 TS 1a structure, where the dioxygen molecule may occupy various positions. The reoptimisation of the more "rigid" transition state 3 TS 1e by the M06-2X functional revealed the geometry to be very similar to that found using the B3LYP method ( Figure S17).

Amidation and Hydroxylation of Cyclohexane
Considering the prominent activity of 1 and 2 in the OAPH oxidation reaction, with the possible participation of radical intermediates, we studied the catalytic activity of both complexes in cyclohexane (CyH) functionalisation reactions, namely, peroxidative radical amidation and hydroxylation (Scheme 7). The reaction of 0.3 M of benzamide with 10 eq of CyH and 2 eq. of t BuOO t Bu in chlorobenzene in the presence of 1 mol% of the catalyst at 90 • C resulted in 16% conversion of benzamide after 24 h. The main products of the amidation reaction are N-cyclohexyl benzamide (Cy-N-benzamide) and N-methyl benzamide, with nearly equal amounts ( Figure S18). The formation of the methyl derivative can be understood if one considers the large contribution of a β-scission [72] of the t BuO• radical, which becomes favourable at the reaction temperature (90 • C) and, possibly, in the presence of 1 or 2: where both t BuO• and •CH 3 radicals react with a copper-benzamide intermediate [22,24,73]: The DFT//DLPNO-CCSD(T) calculations indicate that the metal-free β-scission reaction has a barrier of 12.9 kcal mol −1 and ∆G = −16.5 kcal mol −1 at 90 • C (383.15 K), in this way being a thermodynamically favourable process ( Figure S19). The barrier increases to 13.1 kcal mol −1 and ∆G to −11.5 kcal mol −1 at the standard conditions (298.15 K, 25 • C). Both t BuO• and •CH 3 radicals readily abstract the H atom from CyH, with the subsequent trapping of the Cy• radical by the Cu(II)-benzamide intermediate, according to the expected reaction mechanism [73]. In a similar way, trapping the methyl radical by the Cu(II)-benzamide intermediate leads to N-methyl benzamide. The conversion level exhibited by 1 and 2 is comparable to that observed (20%) for the dinuclear copper complexes with N-butyldiethanolamine and N-tert-butyldiethanolamine [22], but is lower than the conversion found (55%) for piperazine-based Schiff-base copper complexes [22]. However, in the latter cases, the amounts of N-methyl benzamide were significantly lower, indicating that both 1 and 2 are efficient scavengers for methyl radicals.
It is known that mono-and polynuclear complexes of copper are efficient catalysts for the oxidation of alkanes with peroxides [3]. Since complexes 1 and 2 contain N,O donor ligands that typically promote such a catalytic activity, we studied the catalytic properties of both compounds in the oxidation of cyclohexane with aqueous H 2 O 2 in the presence of nitric acid promoter in acetonitrile. The reaction was initiated by adding five equivalents of H 2 O 2 to CyH (0.2 M) in the presence of 2.5 mol% of nitric acid promoter and 0.1 mol% of the catalyst 1 or 2 (0.4 mol% if calculated per total copper amount). The presence of small amounts of the acidic promoter typically has a promoting effect on the catalytic activity due to facilitation, e.g., of the proton transfer steps. Both complexes show similar initial reaction rates: W 0 = 7.8 × 10 −7 and 1.1 × 10 −6 M s −1 for 1 and 2, respectively ( Figure S20). The yield of products was 12% after 24 h, with an almost complete selectivity towards cyclohexanol, most likely due to the quantitative reduction in cyclohexyl hydroperoxide (main reaction product) to cyclohexanol by PPh 3 (see the Experimental section). These catalytic parameters are comparable to those exhibited by some systems reported earlier, e.g., by the heterometallic polynuclear Cu/Fe complex with N-tert-butyldiethanolamine (W 0 = 3.9 × 10 −6 M s −1 for 1.4 mol% of HNO 3 promoter), suggesting that both complexes 1 and 2 are promising catalysts for this type of reaction. It should be noted that activation of a strong sp 3 C-H bond of cyclohexane is a challenging process, where the yields demonstrated by most of the batch homo-or heterogeneous catalytic system do not exceed 35% [3].
The oxidation of cyclooctane under similar conditions resulted in a W 0 of 1.2 × 10 −6 and 8.7 × 10 −7 M s −1 for 1 and 2, respectively ( Figure S21), with an almost complete selectivity towards cyclooctanol. These reaction rates are close to those found for the oxidation of cyclohexane.
The high prevalence of alcohols in the course of the oxidation of cyclohexane and cyclooctane indicates the participation of the alkyl hydroperoxides (Scheme 7). Cyclohexyl hydroperoxide (CyOOH) was directly detected by the GC-MS technique ( Figure S22), according to the analytical protocol earlier reported by some of us [21,24,[74][75][76][77][78]. The chromatogram recorded before the addition of PPh 3 to the sample revealed that CyOOH is the main reaction product ( Figure S22), since only traces of cyclohexanol and cyclohexanone were detected in this case. A similar test using cyclooctane as a substrate revealed the presence of several strong peaks on the chromatogram, completely disappearing after the addition of PPh 3 to the sample ( Figure S23). The exact EI-MS spectrum of cyclooctyl hydroperoxide was not reported earlier. However, since both cyclohexyl and cycloheptyl hydroperoxides undergo elimination of H 2 O and O 2 H fragments upon ionisation, forming distinct M-18 and M-33 m/z peaks [79], one can expect similar behaviour for cyclooctyl hydroperoxide. The search towards the respective signals ( Figure S24) revealed that the mass spectra of peaks I and II ( Figure S25) can be interpreted as cyclooctyl hydroperoxide. A much stronger intensity of the 111 m/z peak than 126 m/z in the mass spectrum of II ( Figure S25), expected for cycloalkyl hydroperoxides [79], allows its assignment to cyclooctyl hydroperoxide. The oxidation of cyclohexane and cyclooctane catalysed by complex 2 revealed the same chromatographic patterns of the products.
The oxidation of methylcyclohexane under similar conditions revealed the 1 • :2 • :3 • bond selectivities of 1:2:7 and 1:7:18 for 1 and 2, respectively ( Figure S26). Both selectivities are in the range expected for catalytic systems abstracting H atoms through the attack of a hydroxyl radical [80]. Considering this, as well as the presence of alkyl hydroperoxides as the main reaction products, the principal reaction pathway can be assigned to the broad class of mechanisms where the role of the catalyst concerns the generation of hydroxyl radicals upon reaction with H 2 O 2 [3]:

Conclusions
We have synthesised the novel complexes [Cu 4 (pa) 4

(Bae) 4 ]·H 2 O (1) and [Cu 4 (eba) 4 (Buae) 4 ]·H 2 O (2)
, starting from copper salts and flexible aminoalcohol and carboxylic acids as pro-ligands, following the spontaneous self-assembly strategy. Single-crystal X-ray analyses revealed tetranuclear cores in both 1 and 2. The Cu 4 O 4 cores are surrounded by the bulky aliphatic and aromatic groups of the ligands, making the complexes soluble in aprotic hydrophobic solvents . Complexes 1 and 2 show a notable catalytic activity in the oxidative amidation of cyclohexane with benzamide, with the conversion of benzamide at 16%. The main reaction products are N-cyclohexyl and N-methyl benzamide. The formation of methylated benzamide can be understood in terms of β-scission of t BuOO t Bu with the generation of a free methyl radical, which is a thermodynamically favourable process, according to the DFT//DLPNO-CCSD(T) calculations. Both 1 and 2 are active catalysts towards the oxidation of cyclohexane with aqueous H 2 O 2 , showing reaction rates up to 1.1 × 10 −6 M s −1 and yields of cyclohexanol of 12%.
The catalytic features of 1 and 2 were investigated in the aerobic oxidation of o-aminophenol (OAPH) to phenoxazinone chromophore (PXCR). Both complexes reveal a high catalytic activity, with a maximum initial reaction rate of 3.5 × 10 −7 M s −1 . The dependence of the initial reaction rate on the catalyst concentration was studied, showing that, for both catalysts 1 and 2, it is linear up to 0.25 mM, beyond which a pronounced non-linear growth of the reaction rate is observed. A possible explanation could be the equilibria between the tetranuclear and smaller species in solution, where all species are catalytically active.
By using the combined DFT//DLPNO-CCSD(T) theoretical calculations, we also investigated the catalyst-free and copper-mediated formation of OAP• radicals and o-iminobenzoquinone (BQMI) upon the reaction of OAPH with O 2 , which is known to be the rate-limiting stage towards the formation of the phenoxazinone chromophore. Our results indicate that the highest barrier for the catalyst-free process is 28.7 kcal mol −1 (according to DFT//DLPNO-CCSD(T)) and 19.8 kcal mol −1 (according to DFT). Within the models studied, the lowest barrier for the copper-catalysed process was found to be 19.9 kcal mol −1 (DFT//DLPNO-CCSD(T)) for the configuration where OAPH attacks a Cu II -OO• peroxyl species. The copper catalytic effect was associated not only to the coordination of OAPH, but also to the stabilisation of the starting intermediate through broad, non-covalent interactions between OAPH and the copper-peroxyl catalyst.
We expect that the experimental and theoretical results of this study should help in the design of novel biomimetic catalytic systems and contribute to a better understanding of the mechanisms of OAPH oxidation, a reaction of biological relevance. Complexes of copper are versatile catalysts that promote a wide range of organic transformations [81][82][83][84][85]. Considering that complexes 1 and 2 were found to be active catalysts in three different types of reactions, one may expect that they may have a pronounced activity in other reactions, such as aerobic oxidations of sp 3 C-H bonds, cross-coupling or click chemistry, which deserve further investigation.

General
All chemicals were of reagent grade and used as received. All experiments were carried out in air. Elemental analyses for C, H and N were carried out by the Microanalytical Service of the Instituto Superior Técnico. Infrared spectra (4000-400 cm −1 ) were recorded using an Agilent Cary 630 ATFT-IR (Agilent Technologies Inc., Santa Clara, CA, USA) spectrometer. UV/Vis spectra were recorded using a Perkin-Elmer Lambda 35 (PerkinElmer Inc., Waltham, MA, USA) spectrometer in the 330-600 nm spectral range.

Crystallography
Details of the data collection and processing, structure solution and refinement are summarised in Table S1. The single-crystal X-ray data were acquired on a Bruker D8 Quest (for 1) or AXS KAPPA APEX II (for 2) diffractometer (Bruker AXS Inc., Madison, WI, USA). Cell parameters were retrieved and refined using the Bruker SAINT programme. SADABS was used for correction of absorption [86]. Both structures were solved by direct methods by means of SHELXT-2014/5 (for 1) or Olex.Solve 1.5 (for 2) and refined against F 2 using the SHELX-2018/3 (for 1) or 2019/2 (for 2) programme [87] (Table S1). The Olex2 1.5 (OlexSys Ltd., Durham, England) programme was used during the structures' refinement [88]. The hydrogen atoms of water molecules and N-H groups in 1 were localised and refined freely. The coordinates of the hydrogen atoms of the water molecule O13 in 2 were refined freely, while U iso was fixed to 1.5U eq of the O13 atom. The remaining H atoms in 1 and 2 were placed at calculated positions and refined using the riding model with U iso = 1.5U eq (for methyl groups) or U iso = 1.2U eq (for other groups). The ethyl groups C9-C12 of 2-ethylbutyric acid in 2 were found to be disordered over two sites with occupancies of 0.316 (19) and 0.684 (19). The geometries of the bulky n-butyl (C3-C6 and C15-C18) and ethyl (C8-C12, C20-C24, C32-C36 and C44-C48) groups were not satisfactory due to the large thermal motion. The respective C-C distances and C-C-C angles were restrained to ideal values obtained from the Cambridge Structural Database (CCDC Inc., Cambridge, England) [49] statistics. Anisotropic refinement of the atoms C3-C6 and C8-C12 was not successful, thus the isotropic refinement was used instead. The above issues originate from the weak high-angle diffraction data for 2, for which I/σ(I) is below 3 for 2θ > 43 • (Figures S1 and S2).
Crystallographic data for the structures can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, quoting the deposition numbers CCDC 2201716 (1) and 2201715 (2).

Phenoxazinone Synthase Activity Study
The phenoxazinone synthase activity of 1 and 2 was monitored spectroscopically by recording the UV/Vis spectra in the 330-600 nm region (unless stated otherwise) at certain periods of time. Typically, 5 mL of a methanol solution of the complex of desired concentration (calculated as a full concentration of copper cations) was added to a 10 mL flask containing 5 mL of a methanol solution of o-aminophenol (OAPH). After 10 s of vigorous stirring a portion of the resulting mixture was transferred into a quartz cuvette (10 mm; Labbox (Labbox Labware S.L., Barcelona, Spain) MISQ-010-002) of the spectrophotometer, without further stirring. The extinction coefficient of 2.4 × 10 4 M −1 cm −1 [27,34] was used to convert absorption to molar concentration.

Catalytic Amidation of Cyclohexane
The reactions were typically carried out under N 2 atmosphere in a thermostated Schlenk tube under vigorous stirring. Firstly, 6 mg (5 µmol) of the catalyst and 60.6 mg (0.5 mmol) benzamide were introduced into the tube in the solid form. Then, chlorobenzene (1 mL) and cyclohexane (0.54 mL, 5 mmol) were added in this order. The oxidant (184 µL, 1 mmol) t BuOO t Bu was then added at room temperature and the mixture was immediately frozen with liquid nitrogen; the gas atmosphere was pumped and filled with N 2 a few times to remove air. The frozen mixture was left to warm up under vacuum (to degasify) until it became liquid, and the above procedure was repeated. Finally, the Schlenk tube was filled with N 2 and the reaction mixture was heated at 90 • C for 24 h, with the possibility for gas to escape to compensate for the excessive pressure. The reaction mixtures were cooled to room temperature, 10 mL of CH 3 CN and 1 mL of the solution of nitromethane (GC internal standard; 1 mL of CH 3 NO 2 mixed with 9 mL of CH 3 CN) were added and the resulting mixture was directly analysed by GC/GC-MS techniques. the combination of a hydrogen peroxide with organic compounds at elevated temperatures may be explosive!). The total reaction volume was 5 mL. Aliquots (ca. 0.5 mL) of the reaction mixture were transferred, upon cooling, into a vial containing an excess (ca. 150 mg) of solid Ph 3 P (according to the method developed by Shul'pin [89]) and then analysed directly by gas chromatography GC/GC-MS techniques.

Theoretical Calculations
The ORCA 5.0.3 (Max-Planck-Institut für Kohlenforschung, Mülheim a. d. Ruhr, Germany) software package was used for all calculations [90][91][92]. Unless stated otherwise, the calculations were performed using B3LYP functional [59,93] with the minimally augmented ma-def2-SVP and ma-def2-TZVP basis sets [94] for non-metal and metal atoms, respectively (preliminary optimisation of intermediates and transition state geometries) or the ma-def2-TZVP basis set for all atoms (final calculations). Dispersion correction was introduced through the D4 keyword (Grimme's atom-pairwise approach) [95]. The CCSD calculations were performed through the DLPNO-CCSD(T) scheme [96] using the ma-def2-TZVPP basis set [94] and TightPNO keyword. The iterative full LMP2 was disabled for the close-shell DLPNO-CCSD(T) calculations (keyword "UseFullLMP2Guess False"). The AutoAux keyword [97] was used to generate auxiliary basis sets in all cases. SCF convergence criteria corresponding to those invoked by the TightSCF keyword were automatically applied during geometry optimisations. Unless stated otherwise, the final enthalpies and Gibbs energies were obtained from the DFT calculations of geometry and vibrational energies where electronic energies were obtained from the CCSD calculations. Transition states were found using either nudged elastic band (NEB) [98] or relaxed scan methods, and refined at the DFT level with DLPNO-CCSD(T) correction for electronic energy. The correctness of the transition state optimisation was confirmed by the presence of a single imaginary frequency. The minimum energy crossing points (MECPs) [99] for intersection of triplet and singlet surfaces were located using the SurfCrossOpt keyword. The solvent (methanol) effects were accounted for by means of the conductor-like polarisable continuum model (C-PCM) [100] for the DFT geometry optimisation and frequency calculations, and the solvation model based on density (SMD) [101] for the CCSD single-point calculations used to extract the exact electronic energy. The correction ∆G term of 1.89 kcal mol −1 was added to the final Gibbs energies of single molecules to convert 1 atm to 1 M standard states [101]. The timedependent DFT calculations (TD-DFT) were performed at the PBE0/ma-def2-TZVPP level using the Tamm-Dancoff approximation (TDA) [102]. The PBE0 hybrid functional is known to reproduce singlet excited states of organic molecules with sufficient accuracy [103,104]. The visualisation of spin density and reduced density gradient 3D isosurfaces was made using the Avogadro 1.2 (University of Pittsburgh, Pittsburgh, PA, USA) programme [105]. Analysis of bond critical points and non-covalent interactions indexes [39] was done using the Multiwfn 3.8 (University of Science and Technology, Beijing, People's Republic of China) programme [106]. Cartesian coordinates of the DFT optimised structures are given in the Listings S3-S7.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12111408/s1, Figure S1: Reflection intensity statistics (I/σ(I) vs. 2θ) for 1. Figure S2: Reflection intensity statistics (I/σ(I) vs. 2θ) for 1. Figure S3: IR spectrum of 1. Figure S4: IR spectrum of 2. Figure S5: Left: dependence of the initial reaction rate W 0 on the OAPH concentration in the catalyst-free aerobic oxidation of OAPH in methanol at room temperature. Right: change of the UV/Vis spectra for [OAPH] 0 = 7.4 × 10 −4 M. The inset shows increase in the band at 435 nm. Figure S6: Increase in the band at 435 nm, attributable to the phenoxazinone chromophore (PCXR), over time in the aerobic oxidation of the mixture of o-aminophenol (OAPH, 10 mM) and 2-amino-p-cresol (10 mM) catalysed by 1 (0.4 mM) in methanol. Figure S7: Fragment of the TD-DFT calculated absorption spectra of the normal and methyl-substituted phenoxazinone chromophores. The absorptions corresponding to the first two strongest excitation states S1 and S2 are shown. Figure S8: Frontier molecular orbitals of the normal and methyl-substituted phenoxazinone chromophores (PBE0/ma-def2-TZVPP TD-DFT calculation using B3LYP/ma-def2-TZVP geometry in methanol; only singlet excitations were considered). Figure S9: Relaxed energy scan along the transition-state coordinate for the H atom abstraction from OAPH catalysed by the [Cu II (HMeEa)(OAc) (MeOH)] + complex. The final point at the 1.031 Å corresponds to the optimised geometry of the reaction product 4 I 2d , while the respective separation in the reactant 4 I 1d configuration constitutes 3.046 Å. The figure shows the transition state 4 TS 1d . Figure S10: Isosurfaces of the spin density (0.01 a.u. level) for the optimised intermediates 4 I 1d and 4 I 2d and transition state 4 TS 1d . Figure S11: DFT-optimised structure of the [Cu II (HMeEa)(OAc)(OAP)] complex, where OAP ligand is in the deprotonated state. Figure S12: DFT-optimised structure of the 3 I ea intermediate. Figure S13: DFT-optimised structure of the 3 I 1e intermediate with partial numbering scheme. Figure S14: DFT-optimised structure of the 3 I 1b intermediate with partial numbering scheme. Figure S15: Molecular geometries and selected interatomic distances (Å) for the 3 TS 1a transition state optimised using different functionals. In all cases, the new transitions states were found through the complete NEB-TS search between 3 I 1a and 3 I 2a intermediates using the respective functionals. Figure S16: Calculation timings for single-point and vibrational frequency calculations of the 3 I 1a intermediate using different DFT functionals. All geometries were preliminary optimised using the respective functionals. Figure S17: Molecular geometries and selected interatomic distances (Å) for the 3 TS 1e transition state optimised using B3LYP and M06-2X functionals. Figure S18 Table S1: Crystal data and structure refinement for 1 and 2. Table S2: Selected geometrical parameters (distances/Å and angles/ • ) for 1 and 2. Table S3: Comparison of the barriers between 3 TS 1a and 3 I 1a states calculated at different theory levels. Table S4: Löwdin spin population and atomic charges for selected atoms in 3 I 1e . Table S5: Löwdin spin population and atomic charges for selected atoms in 3 I 1b . Table S6: Gibbs energies of 3 TS 1a and 3 I 2a states relative to 3 I 1a obtained using indicated DFT functionals before or after correction of electronic energies at the DLPNO-CCSD(T) level. Listing S1: Fragment of the ORCA output for the TD-DFT calculation of the phenoxazinone chromophore. Listing S2: Fragment of the ORCA output for the TD-DFT calculation of the methyl-substituted phenoxazinone chromophore. Listing S3: Cartesian coordinates of the intermediates and transition states optimised at the B3LYP-D4/ma-def2-TZVP level in methanol (C-PCM model). Listing S4: Cartesian coordinates of the 3 I 1a intermediate optimised using different functionals, ma-def2-TZVP basis set and tight integration grids in methanol (C-PCM model). Listing S5: Cartesian coordinates of the 3 TS 1a transition state optimised using different functionals, ma-def2-TZVP basis set and tight integration grids in methanol (C-PCM model). Listing S6: Cartesian coordinates of the 3 I 2a intermediate optimised using different functionals, ma-def2-TZVP basis set and tight integration grids in methanol (C-PCM model). Listing S7: Cartesian coordinates of the 3 TS 1e transition state optimised using M06-2X functional, ma-def2-TZVP basis set and tight integration grids in methanol (C-PCM model).  Data Availability Statement: Crystallographic data for the structures can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, quoting the deposition numbers CCDC 2201716 (1) and 2201715 (2).

Conflicts of Interest:
The authors declare no conflict of interest.