Oxygen activation by copper camphor complexes

Abstract

Cleavage of the carboximide CN bond promoted by reaction of camphor carboximide hydrochlorides (OC₁₀H₁₅CONHCH₂COOLi(H₂O)₂·2HCl (2) or OC₁₀H₁₅CONHCH(CH₂Ph)COOLi(H₂O)·HCl (3)) with CuCl₂ leads to the corresponding amino acid complexes ([Cu(H₂NCH₂COO)₂] (I) and [Cu{H₂NCH(CH₂Ph)COO}₂]·(H₂O) (II)), and the camphor carboxylate residue (OC₁₀H₁₅COO⁻) which is catalytically oxidized to camphorquinone by oxygen from air. The process is mediated by coordination of the camphor carboxylate to copper. The reaction of the camphor carboximide hydrochloride (OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)·HCl) (4) with CuCl₂ follows a different trend. In this case the CN camphor carboximide bond remains intact and the complexes [CuCl{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (III) and [{CuCl₂}₂{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (IV) form. However, a redox process also occurs, since formation of III requires Cu(II) → Cu(I) reduction as confirmed by X-ray photoelectron spectroscopy and cyclic voltammetry. A related reduction process was identified in the formation of [CuCl(OC₁₀H₁₅COOLi)] (V) from CuCl₂ upon reaction with lithium camphor carboxylate (OC₁₀H₁₅COOLi) under nitrogen. The above results show that electron transfer is highly facilitated in the copper camphor carboximide/carboxylate system. Such an ability was used to activate oxygen from air and promote the oxidation of ethylacetoacetate to pyruvate using V as the catalyst.

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Introduction

Copper compounds play a role in areas so diverse as catalysis,^1–8 biological applications,^9–15 food and materials related processing,^16,17 waste treatment¹⁸ or agriculture (e.g. CuSO₄ in vineyards). Such a wide scope of applications impelled us to synthesize and study the properties of copper camphor complexes, with the focus on Cu(I) complexes.^19–21 Cu(I) to Cu(II) oxidation by air is a common drawback when doing Cu(I) chemistry. However, in reactions of CuCl with some camphor derivatives the process was found to enable oxidation of organic substrates. Such findings challenged further studies in particular because enzymes such as tyrosinases, laccases or catechol oxidases activate oxygen from air forming peroxo or oxo Cu(II) intermediates in processes that range from oxygen release as H₂O²² to oxidative oxygen transfer to organic substrates.²³ The activation of molecular oxygen by copper compounds in biological processes was recently reviewed by Solomon et al.²⁴ The challenge for non-enzymatic systems to enable activation of oxygen from air is to find ligands that are coordinative and electronically versatile to stabilize the metal site in different oxidation states to allow coordination and transfer of oxygen to organic substrates without decomposition of the catalyst.

From previous studies we knew that camphor derived ligands are able to stabilize transition metals (Pd, Pt, Zn)^25–28 towards electron reorganization in CC triple bonds, thus a step forward was to design camphor based ligands to obtain copper complexes that promote Cu(I) → Cu(II) → Cu(I) electron transfer at potentials suitable for oxygen activation. Herein the results show that the camphor carboximide and carboxylate copper systems fulfill the above requirements and catalytically enable oxygen activation and transfer to substrates.

Results and discussion

The synthesis of the lithium camphor carboximides hydrochlorides OC₁₀H₁₅CONHCHRCO₂Li(H₂O·HCl)_x (R = H, x = 2; (2); R = CH₂Ph, x = 1 (3)) and OC₁₀H₁₅CONH(CH₂)₂CO₂Li(H₂O·HCl) (4) is based on ketone and amine condensation upon reaction of lithium camphor carboxylate (1) with α-amino acids (H₂NCH(R)COOH)(R = H or CH₂Ph) or β-alanine (eqn (1)). In contrast, no condensation of amino acids (glycine, L-phenylalanine or β-alanine) with camphor carboxylic acid (OC₁₀H₁₅COOH, 5) was observed, neither did condensation of amino acids with camphor or camphorquinone, in agreement with high relevance of the lithium ion for the processes.

The new compounds (2, 3 and 4) were formulated based on the elemental analysis, FTIR and NMR (¹H, ¹³C, DEPT, 2D). Characteristic camphor (ν_(CO)ca. 1730 cm⁻¹) carboxylate bands (and ν_(COO)ca. 1600 cm⁻¹) were observed by FTIR. Data from NMR confirms the condensation of the camphor carboxylate (OC₁₀H₁₅COOLi, 1) with the α-amino acids (glycine R = H, L-phenylalanine R = CH₂Ph) or β-alanine (H₂NCH₂CH₂COOH) (see the Experimental section).

The structural characteristics of the camphor carboximides (2–4) favor coordination to transition metals through one, two or more atoms enabling different hapticities which render them high attractive ligands to stabilize transition metals in different oxidation states.

Reactions of lithium 3-camphor carboximides with CuCl₂

The lithium camphor carboximides (OC₁₀H₁₅CONHCH(R)CO₂Li·HCl) (2 and 3) react with CuCl₂ (in EtOH, under air) affording the amino acid complexes [Cu(NH₂CHRCOO)₂]·xH₂O (R = H, I; R = CH₂Ph, II). In the process, cleavage of the carboximide CN bond and lithium camphor carboxylate (1) release occur, concomitant to the formation of the amino acid complexes (eqn (2)).

A first observation is that the reaction proceeds in the absence of an external base (e.g. NaOH) due to the basic character of 1, in contrast with the typical syntheses of Cu(II) amino acid complexes that require the presence of a strong base (e.g. NaOH). A second and more relevant aspect is that the camphor carboxylate (OC₁₀H₁₅COO⁻) is oxidized to camphorquinone (6, OC₁₀H₁₄O) (eqn (3)) without the presence of an oxidizer other than oxygen from air.

The amount of camphorquinone increases with increase of the excess of the camphor carboximide (2, 3) or camphor carboxylate suggesting a catalytic process (Scheme 1).

Scheme 1 Proposed outline for formation of V and products of catalytic activation of oxygen.

The analytical and spectroscopic data obtained for complexes I and II as well as for camphorquinone agree well with the former published data,^29–32 thus corroborating their formulation.

In contrast with 2 and 3, lithium camphor carboximide hydrochloride 4 reacts with CuCl₂ affording [CuCl{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (III) and [{CuCl₂}₂{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (IV) that keep intact the camphor carboximide ligand. Formation of III (low yield) requires Cu(II) → Cu(I) reduction, a process which, in the absence of a reducing agent, was unexpected. Oxidation of the chloride co-ligand (Cl⁻/Cl₂) (as found in other copper systems³³) or amino acid oxidation (Strecker's type) could be the source electrons.

The Cu(I) character of III enables acquisition of the ¹H NMR spectrum (in the usual region, see the Experimental section) that was further confirmed by XPS and cyclic voltammetry (see below).

In IV, the metal site behaves as Cu(II). However, exposure to air yields a gummy solid with a strong smell, from which the Cu(I) complex [CuCl(OC₁₀H₁₅COOLi)]·Et₂O (V·Et₂O) was obtained. The fate of the amino acid residue (not found in the isolated products) conceivably are the volatile smelly products (aldehyde, ammonia) formed through a Strecker's type amino acid degradation process.^34–36 In this case, oxidation of the amino acid, chloride or both provide the electrons for Cu(II) → Cu(I) reduction. This process was not further studied. Compound V·Et₂O is not stable in air for long periods. The related compound [CuCl(OC₁₀H₁₅COOLi)]·THF (V·THF) that differs from (V·Et₂O) in the solvent, is straightforwardly obtained upon reduction of CuCl₂ by the lithium camphor carboxylate (1) in THF. Under nitrogen, V·Et₂O remains stable for long periods. In air CuCl₂ reacts with 1 to form the Cu(II) compound [CuCl₂(OC₁₀H₁₅COOLi)]·2H₂O (VI). Differences between V (pale green) and VI (blue) include color and spectroscopic properties displayed by IR [V, 1732 (ν_CO), 1623 (ν_COO) cm⁻¹; VI, 1712 (ν_CO), 1587 (ν_COO) cm⁻¹], EPR (VI typical spectra, V EPR silent) and NMR (V typical spectra, VI no spectra).

Reactions of lithium 3-camphor carboximides with CuCl

To check whether the camphor carboximide Cu(I) complexes could be obtained directly from CuCl, the reactions with camphor carboximides (3 and 4) were undertaken in THF under nitrogen. From 3, [CuCl{OC₁₀H₁₅CONHCH(CH₂Ph)COOLi}] (VII) was obtained; this is formally similar to III obtained from the reaction of 4 with CuCl₂ (see above). From the reaction of CuCl with 4, the copper amino acid complex [{CuCl}₂(H₂NCH₂CH₂COOH)] (VIII) was obtained that requires breaking of the CN carboximide bond, such as in the cases of complexes I and II obtained from CuCl₂. Complex VIII differs from I and II in what concerns the number of copper atoms per ligand which is two and one, respectively. The analytical composition of VIII suggests a zig-zag 1D-coordination polymer structure such as [{CuCl}₂(YNC₁₀H₁₄O)] (Y = NMe₂, NH₂, NH₂C₆H₄) obtained from the reaction of camphor hydrazones with CuCl.³⁷

Exposure of VIII to air (solution in DMSO) afforded blue paramagnetic crystals that were structurally characterized as trans-[Cu(H₂NCH₂CH₂COO)₂(H₂O)₂]·4H₂O (IX, Fig. 1) by single crystal X-ray diffraction (XRD). It is worthy to note that VIII and IV (see above) that have in common the metal to ligand ratio (2 : 1), rearrange to mononuclear species.

Fig. 1 ORTEP drawing for IX, showing labeling scheme. Ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity. Cu(1)–O(6) 1.9831(6); Cu(1)–N(1) 2.0010(6); N(1)–C(2) 1.480(1); C(2)–C(3) 1.517(2); C(3)–C(4) 1.514(1); C(4)–O(5) 1.256(1); C(4)–O(6) 1.272(1); O(6)–Cu(1)–N(1) 93.35(3); C(2)–N(1)–Cu(1) 116.21(5).

A structural comparison between IX and the former reported L-alanine Cu(II) complex³⁸ shows that an extended hydrogen bonding exists in IX (not observed before) which is responsible for a supramolecular arrangement formed by dyads of water molecules linking four molecules of the complex (Fig. 2, top). Crystal packing is displayed in Fig. 2 (bottom).

Fig. 2 Crystal packing showing water dyads in the lattice of IX (bottom); supramolecular arrangement, showing hydrogen bonding (top).

The camphor carboxylate (1) reacts with CuCl affording [{CuCl(OC₁₀H₁₅COOH)}] (X), which differs from V in the carboxylic residue (–COOX: X = H (X), X = Li (V). The higher acidity of CuCl solutions conceivably is responsible for Li/H exchange and isolation of X. Besides their apparently similar formula, the compounds have considerably distinct solubility (lower for V) and spectroscopic properties, namely, the IR stretching frequencies (ν_COO: 1623 cm⁻¹ (V); 1580 cm⁻¹ (X)) as well as the ¹³C NMR resonances (C=O: 215.0 ppm (V); 204.4 ppm (X), see the Experimental section for details).

XPS

X-ray photoelectron spectroscopy (XPS) was used to ascertain the composition of selected compounds (II, III and VI) focusing on unraveling the oxidation state of the metal atom. The task may not be easy for complexes with transition metal sites other than copper, as the photoelectron binding energy is not enough to identify a specific oxidation state.³⁹ However for copper the task becomes simplified since Cu(II) displays a Cu 2p region where a multiplet structure (due to spin–spin coupling) exists which is absent in the Cu(I) compounds. Fig. 3 displays the XPS Cu 2p region for the complexes under study. It is clear from this that the oxidation state of copper is one (+1) in III as attested by the absence of the multiplet structure. A peak width increase and appearance of the characteristic Cu(II) multiplet was observed upon exposure of III to air (Fig. 3, insert), in agreement with the Cu(I) → Cu(II) oxidation.

Fig. 3 XPS Cu 2p (from bottom to top) for samples: II, VI, III (as prepared). Arrows indicate the Cu 2p_3/2 doublet component assigned to the different features. Insert: XPS Cu 2p of III (upon exposure to air).

The data obtained from complexes II and VI (undoubtedly Cu(II)) surprisingly display a combination of Cu(I) and Cu(II) oxidation states, attested by the large increase of the photoelectron peak width. The Cu(I) signals were attributed to Cu(II) → Cu(I) reduction by X-ray photoelectron radiation during data acquisition, a fact was also observed (the Cu(I) component increases) during acquisition of data from a sample of III intentionally oxidized. This aspect was not further explored, although a rather similar reduction process, from V(IV) to V(III), was recently reported by using intense synchrotron radiation during characterization of V^IVO(dipicolinato)₂–lysozyme adducts.⁴⁰

EPR

Since no structural characterization could be made by single crystal XRD for most of the above mentioned complexes, EPR was used to get information on the structural arrangement and geometry of the selected Cu(II)-complexes (II and VI). Data measured in ethanol solutions confirmed the Cu(II) character of II and VI (Table 1) and structural information was also obtained from the calculated g_z/A_z ratio, which is considerably low for II (128.3 cm) in agreement with the amino acid acting as a N,O ligand bound to a square-planar Cu(II) metal center and considerably high for VI (184.1 cm) as expected for an O-ligand coordinated to a Cu(II) tetrahedral metal site.⁴¹

Table 1 Spin Hamiltonian parameters^a

Complex	g x , gy	A x , Ay × 10^4 cm^−1	g z	A z × 10^4 cm^−1	g z /Az cm
a EPR spectra (measured at 77 K) were obtained from solutions of the complexes prepared under air at room temperature. b In ethanol. c In DMSO. d In ethanol/ethyleneglycol (1/1).
II	2.041, 2.060	20.3, 5.4	2.264	176.5	128.3
VI	2.076	5.7	2.375	129.0	184.1
III ox
(Major)	—	—	2.315	162.0	143.0
(Minor)			2.270	130.0	174.6
V ox
(Major)	—	—	2.311	161.8	142.8
(Minor)			2.370	130.0	182.3

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Complexes III and V are diamagnetic, thus EPR silent. Nevertheless, there was curiosity to know how solutions of III and V would behave upon being exposed to air, due to their ability to activate oxygen from air. EPR spectra were then measured from solutions of III and V prepared under air. The first observation was that, as expected, the solutions did not remain EPR silent and at least two Cu(II) species formed in solution. A second observation is that the pattern of the hyperfine coupling constant A_z in the two products of oxidation of III and V does not differ significantly. The more abundant species exhibit A_z values (161.8 to 162.0 × 10⁻⁴ cm⁻¹) considerably higher than the less abundant species (A_z ≈ 130 × 10⁻⁴ cm⁻¹) in both solutions (Table 1).

Unfortunately, the signals are too broad to allow unambiguous determination of the g_x, and g_y values. The relatively high g_z/A_z ratios (>140 cm, Table 1) in conjunction with their representation in the Peisach–Blumberg A_z/g_z correlation plots³⁷ indicate a tetrahedral distortion in the Cu(II) metal centers both in the major and the minor oxidized species. In addition, the high g_z/A_z ratio (>170 cm) calculated for the major component of the oxidized solutions of III and V supports Cu(II) coordination of O-containing ligands to significantly distorted tetragonal metal centers. Data from EPR for the major component of the oxidized solutions of III and V does not differ appreciably from that obtained for VI suggesting similar compounds.

Cyclic voltammetry

The redox properties of Cu(I) (III, V) and Cu(II) (I, II, VI) complexes were studied by cyclic voltammetry under an inert atmosphere. Representative cyclic voltammograms are displayed in Fig. 4. For comparative purposes the behavior of CuCl₂ was also studied. The cyclic voltammograms of III (E^ox_1/2 = 0.40 V, Fig. 4a) and V (E^ox_1/2 = 0.48 V) display quasi-reversible anodic waves consistent with Cu(I) → Cu(II) oxidation processes, followed by irreversible waves (E^ox_p = 1.48 V, III; E^ox_p = 1.60 V, V) at higher potentials in the range of potential of the second oxidation wave displayed by CuCl₂ (Cu(II) → Cu(III), E^ox_p = 1.60 V).

Fig. 4 Cyclic voltammograms of complexes III (a) and VI (b) obtained under nitrogen in CH₃CN.

No anodic processes in the range of Cu(I) → Cu(II) oxidation were detected for complexes I, II, VI which display irreversible cathodic waves (E^red_p = −0.60 V, I; E^red_p = −0.57 V, II and E^red_p = −1.43 V, VI) consistent with Cu(II) → Cu(I) reduction processes. The wide dispersion of potentials (Δ = 860 mV) is consistent with different geometries and binding atoms. The low reduction potential measured for VI (Fig. 4b) corroborates the tetrahedral distorted geometry of the copper center as ascertained by EPR and the considerably higher potential confirms the square planar geometry for complex II. In Cu(II) complexes the redox potentials may be used as a sensitive indication for geometry⁴² and structural characteristics (e.g. N or O coordinating atoms).^43,44

It is worth highlighting that the potential of the reduction Cu(II) → Cu(I) in CuCl₂ is just slightly higher than that of the Cu(I) → Cu(II) oxidation in complexes III or V, pointing to facile electron transfer between them.

Catalysis – oxidation of 1,3-dicarbonyls

The oxidation of ethylacetoacetate was used as a model to extend to substrates other than camphor carboxylate, the ability of [CuCl(OC₁₀H₁₅COOLi)] (V) to catalyze the oxidation of 1,3-dicarbonyls using oxygen from air. The rationale behind this study is that the camphor carboxylate or camphor carboximide compounds used as ligands throughout this work are formally 1,3-dicarbonyls which are easily enolizable under the appropriate conditions and prone to oxidation at the α-carbon^45–49 forming α-dicarbonyls or α-hydroxycarbonyls.⁵⁰ Formation of such types of products from ethylacetoacetate by reaction with V, in the absence of oxidizing agents other than air, was used to corroborate the catalytic activity of V towards oxidation processes using molecular oxygen as the oxygen source. In the case of ethylacetoacetate the expected products are pyruvate (CH₃COCOO⁻ formed by α-oxygenation) or hydroxyacetone (CH₃COCH₂OH, which could be further oxidized to pyruvate). In fact, upon addition of ethylacetoacetate to a solution of V it changed from pale green to orange-brown and back to green upon quenching with acetic acid. This color alteration suggests copper oxidation state exchange in solution. The evolution of gas upon addition of acetic acid to quench the reaction mixture is attributed to CO₂ formed by decarboxylation of acetoacetate. The outcome of the reaction was analyzed by ¹H NMR, ¹³C NMR and HSQC. By ¹H NMR a signal at 2.18 ppm (not observed in the reagents) evidences the formation of a new compound, that was assigned to pyruvate (CH₃COCOO⁻, ca. 55% yield) according to integration of the methyl groups. The presence of hydroxyacetone (CH₃COCH₂OH) or methylglyoxal (CH₃COCHO) was excluded, since no correlation signals were observed in the HSQC spectrum attributable to CH₂ or CH groups respectively.

Formation of pyruvate from ethylacetoacetate thus corroborates molecular oxygen activation catalyzed by V as found in the case of oxidation of camphor carboxylate to camphorquinone (Scheme 1).

Oxidases, such as tyrosinases activate molecular oxygen towards phenol to quinone oxidation^1,2 through coordination to copper, such coincidence is a challenge to pursue the study of copper camphor carboxylate/carboximide systems.

Conclusions

A variety of complexes were obtained from the reaction of lithium camphor carboximide hydrochlorides (2, 3 and 4) with Cu(II) and Cu(I) chlorides. In reactions with CuCl₂, two distinct situations were found. In one, the CN carboximide bond (–C(O)NHCHRCOOLi; 2, 3) breaks and copper amino acid complexes [Cu(NH₂CHRCOO)₂] (I, II) and camphorquinone are obtained. In the other the camphor carboximide ligand (–C(O)NHCHRCOOLi; 4) remains intact and the Cu(I) complex [CuCl{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (III) unexpectedly forms, in addition to the dinuclear complex [{CuCl₂}₂{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (IV) where no reduction of the metal is evident. However, upon exposure of IV to air bad smelling vapors evolve (aldehyde, NH₃, other) due to oxidation of the amino acid ligand (Strecker type) with concomitant formation of [CuCl(OC₁₀H₁₅COOLi)] (V) where the metal has a Cu(I) character.

The Cu(II) → Cu(I) reduction and the camphorquinone formation by oxidation of camphor carboxylate are processes that require metal mediated electron transfer which are believed to be facilitated by the close potentials measured for the oxidation of camphor carboximide (III, E^ox_1/2 = 0.40 V) or camphor carboxylate complexes (V, E^ox_1/2 = 0.48 V) and reduction of CuCl₂ (E^red_1/2 = 0.50 V).

In camphorquinone formation, oxygen from air is activated by the copper camphor carboxylate/carboximide system as corroborated by formation of pyruvate from ethylacetoacetate catalyzed by [CuCl(OC₁₀H₁₅COOLi)] (V) in the absence of oxidizers other than oxygen from air.

From reactions of CuCl with lithium camphor carboxylate or lithium camphor carboximides, Cu(I) or Cu(II) complexes were obtained, although activation of oxygen from air was inhibited, since the reactions were carried out under an inert atmosphere to avoid copper oxide formation.

Experimental section

Complexes were prepared under a nitrogen atmosphere using vacuum/Schlenk techniques. In some cases, solutions obtained upon separation of the complexes by filtration were exposed to air. Anhydrous CuCl₂ was purchased from Merck. CuCl was synthesized from CuCl₂ by reduction with sodium sulphite.⁵¹ Compound 1 was prepared from camphor (Aldrich) following the procedure in ref. 52. The solvents (p.a. grade) were from Sigma-Aldrich or Panreac (EtOH). EtOH was placed over molecular sieves. IR spectra were obtained from KBr pellets or Nujol mulls using a JASCO FT/IR 4100 spectrometer. NMR spectra (¹H, ¹³C, DEPT, HSQC and HMBC) were obtained from D₂O or DMSO-d₆ solutions using Bruker Avance II⁺ (300 or 400 MHz) spectrometers. NMR chemical shifts are referred to TMS (δ = 0 ppm). ESI mass spectra were obtained using a 500-MS Ion Trap (Varian Inc., Palo Alto, CA, USA) mass spectrometer operating in the positive ion mode. The XPS data was obtained using a Kratos XSAM800 equipment and the electron paramagnetic resonance (EPR) spectra were obtained from frozen samples at 77 K using a DPPH radical as a reference and a Bruker ESP 300E X-band spectrometer. The measured 1^st derivative X-band EPR was simulated with the EPR simulation software developed by Rockenbauer and Korecz.⁵³ Cyclic voltammograms (CV) were obtained from solutions in CH₃CN of Bu₄NBF₄ (0.1 M) using a three compartment cell equipped with a glassy carbon working electrode interfaced with a VoltaLab PST050 equipment. The potentials were measured at 200 mV s⁻¹ in volts (±10 mV) versus SCE being used as internal reference, [(Fe(η⁵-C₅H₅)₂)] (E_1/2^red = 0.382 V (CH₃CN)).

Synthesis of amino acid camphor derivatives

Lithium 2-((1,4)-4,7,7-trimethyl-3-oxobicyclo[2.2.1]heptane-2-carboxamido)acetate hydrochloride (2). Lithium (1,4)-4,7,7-trimethyl-3-oxobicyclo[2.2.1]heptane-2-carboxylate (1, 0.32 g, 1.6 mmol) was dissolved in water (30 mL) and the pH adjusted to 6–7 with HCl (10% solution) before addition of glycine (0.030 g, 0.40 mmol). The mixture was stirred for 24 h and taken to dryness forming a white precipitate that was filtered, washed with ethanol (2 × 10 mL), Et₂O (2 × 3 mL) and dried under vacuum. Yield: 83%. Elemental analysis (%) for C₁₃H₁₈NO₄Li(H₂O·HCl)₂, Found: C 42.6, N 4.0, H 6.6; Calc. C 42.4, N 3.8, H 6.6. IR (cm⁻¹): 1733 (ν_CO), 1637, 1581 (ν_COO). ¹H NMR (D₂O, δ ppm): 3.48 (s, 2H), 3.29 (dd, J_HH¹ = 1.6 Hz, J_HH² = 4.6 Hz, H3), 2.40 (t, J_HH = 4.4 Hz, H4), 2.06–1.73 (m, 2H), 1.48–1.34 (m, 2H), 1.01 (s, 3H), 0.89 (s, 3H), 0.80 (s, 3H). ¹³C NMR (D₂O, δ ppm): 221.5 (C2), 176.8 (C11), 174.3 (C13), 58.9 (C3), 58.3 (C1), 46.8 (C4), 45.1 (C7), 41.1 (C12), 28.9 (C6), 22.4 (C5), 18.6, 18.0 (C9 + 10), 8.7 (C8). ⁷Li NMR (D₂O, δ ppm): −0.85.

Lithium 3-phenyl-2-((1,4)-4,7,7-trimethyl-3-oxobicyclo[2.2.1]heptane-2-carboxamido) propanoate hydrochloride (3). Compound 1 (0.081 g, 0.40 mmol) was dissolved in distilled water (20 mL) and the pH adjusted to ca. 6–7 using a solution of HCl (10%), then L-phenylalanine (0.073 g, 0.44 mmol) was added and the mixture stirred for 24 h. The solvent was fully evaporated to afford a white precipitate that was filtered off the solution and redissolved in EtOH (10 mL). Upon slow evaporation the white precipitate formed was filtered, washed with Et₂O (2 × 3 mL) and dried. Yield: 40%. IR (cm⁻¹): 1733 (ν_CO), 1600, 1582 (ν_COO). Elemental analysis (%) for C₂₀H₂₄O₄NLi(H₂O·HCl), Found: C 59.3, N 3.2, H 6.4; Calc. C 59.5, N 3.5, H 6.7. ¹H NMR (DMSO-d₆, δ ppm): 7.30–7.16 (m, 5H), 3.40, 3.38 (2d, J = 4.4 Hz, H12), 3.15, 3.12 (2d, J = 4 Hz, 1H), 3.01 (d, J_HH = 4.4 Hz, H3), 2.85, 2.82 (2d, J = 4 Hz, 1H), 2.19 (t, J_HH = 3.7 Hz, H4), 1.90–1.25 (m, 4H), 0.91 (s, 3H), 0.76 (s, 3H), 0.73 (s, 3H). ¹³C NMR (DMSO-d₆, δ ppm): 216.6 (C2), 171.4 (C11), 168.5 (C12), 138.4, 129.4, 128.2, 128.2 (C_arom), 57.8 (C1), 59.1 (C3), 46.3 (C4), 44.8 (C7), 55.8 (C12), 37.6 (C14), 29.3 (C6), 22.4 (C5), 19.3, 18.8, 9.72 (C8). ⁷Li (DMSO-d₆, δ ppm): −0.74.

Lithium 3-((1,4)-4,7,7-trimethyl-3-oxobicyclo[2.2.1]heptane-2-carboxamido)propanoate hydrochloride (4). Compound 1 (0.40 g, 2.0 mmol) was dissolved in distilled water (40 mL) and the pH adjusted to ca. 6–7 using a solution of HCl (10%). Then, β-alanine (0.070 g, 0.79 mmol) was added and the mixture stirred for 24 h. Solvent evaporation yielded a white precipitate that was dissolved in EtOH (6 mL) and the solution was allowed to evaporate slowly under air. White microcrystals formed that were filtered, washed with Et₂O (2 × 3 mL) and dried. Yield: 73%. IR (cm⁻¹): 1733 (ν_CO), 1594, 1582 (ν_COO). Elemental analysis (%) for C₁₄H₂₀O₄NLi·H₂O·HCl¼EtOH, Found: C 49.1, N 3.6, H 6.8; Calc.: C 49.5, N 4.0, H 6.7. ¹H NMR (D₂O, δ ppm): 3.28 (2d, J_HH = 4.8 Hz, H3), 3.16 (t, J_HH = 6.7 Hz, 2H), 2.54 (t, J_HH = 6.7 Hz, 2H), 2.39 (t, J_HH = 4.5 Hz, H4), 2.07–1.67 (m, 2H), 1.47–1.32 (m, 2H), 0.99 (s, 3H), 0.87 (s, 3H), 0.82 (s, 3H). ¹³C NMR (D₂O, δ ppm): 221.5 (C2), 178.4, 176.8 (C11), 58.9 (C3), 58.8 (C1), 46.7 (C4), 45.1 (C7), 36.5, 33.8 (C12), 28.9 (C6), 22.3 (C5), 18.6, 18.0, 8.7 (C8).

Synthesis of copper complexes

[Cu(H₂NCH₂COO)₂] (I). CuCl₂ (0.023 g, 0.17 mmol) and 2 (0.18 g, 0.70 mmol) were mixed and stirred under vacuum for ca. 1 h. Then, nitrogen was fluxed in the Schlenk and EtOH (8 mL) was added. The mixture was stirred overnight to form a blue precipitate that was filtered off the solution and dried. Yield: 70%. IR (cm⁻¹): 3264, 3171 (ν_NH), 1608, 1589 (ν_COO). Elemental analysis (%) for CuC₄H₈N₂O₄·3/4H₂O·1/4EtOH, Found: C 22.7, N 12.0, H 4.6; Calc.: C 22.8, N 11.8, H 4.7.

From the solution upon exposure to air yellow crystals precipitated that were identified as camphorquinone comparing the IR and NMR spectra with the published data.⁵⁴ (Yield, ca. 15%, based on 2.)

Alternative procedure: Glycine (0.10 g, 1.3 mmol) and Cu(CH₃COO)₂ (0.12 g, 0.63 mmol) were dissolved in EtOH (10 mL) and stirred overnight. A blue precipitate formed that was filtered, washed with ethanol and dried under vacuum. Yield: 66%.

[Cu{H₂NCH(CH₂Ph)COO}₂]·(H₂O) (II). CuCl₂ (0.12 g, 0.87 mmol) and compound 3 (0.70 g, 1.74 mmol) were mixed and stirred under vacuum for ½ h in a Schlenk. Then, nitrogen was introduced in the Schlenk and EtOH (3 mL) was added. The mixture was stirred for 1 h. A blue precipitate formed that was filtered and washed with cold water (open air). Yield: 64%. IR (cm⁻¹): 3333, 3249 (ν_NH), 1621 (ν_COO). Elemental analysis (%) for CuC₁₈H₂₀N₂O₄·H₂O, Found: C 52.7, N 6.6, H 5.2; Calc.: C 52.7, N 6.8, H 5.4. From the solution, by slow evaporation under air, yellow crystals of camphorquinone precipitated (yield, ca. 5%, based on 3).

Alternative procedure: Phenylalanine (0.28 g, 1.7 mmol) and CuCl₂ (0.10 g, 0.77 mmol) were dissolved in deoxygenated water (2 mL) and stirred for 1½ h. The blue precipitate was filtered, washed with cold water and dried under vacuum. Yield: 32%.

[CuCl{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (III). CuCl₂ (0.035 g, 0.26 mmol) and compound 4 (0.19 g, 0.56 mmol) were stirred under vacuum for ½ h in a Schlenk. Then nitrogen was fluxed in the Schlenk, EtOH (2 mL) added and the mixture stirred overnight under nitrogen. The precipitate was filtered off the solution, washed with Et₂O and dried. Yield: ca.10%; IR (cm⁻¹): 1731 (ν_CO), 1610 (ν_COO, ν_CN). Elemental analysis (%) for CuClC₁₄H₂₀NO₄·Et₂O, Found: C 49.1, N 3.2, H 7.1; Calc.: C 49.2, N 3.2, H 6.8. ¹H NMR (MeOH-d₄, δ ppm): 2.85–2.75 (mbr, 2H), 2.41–2.31 (mbr, 2H), 1.05 (s, 3H), 0.90 (s, 3H), 0.85 (s, 3H).

By taking the solution to dryness a precipitate was obtained that was washed with EtOH (2 × 3 mL) and dried to afford [{CuCl₂}₂{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (IV) as a green-blue powder. Yield: 46%; IR (nujol) (cm⁻¹): 1733 (ν_CO), 1627, 1597 (ν_COO). Elemental analysis (%) for Cu₂Cl₄C₁₄H₂₀NO₄Li·H₂O·½EtOH, Found: C 30.5, N 2.0, H 4.7; Calc.: C 30.9, N 2.4, H 4.3. Exposure of IV to air, undergoes formation of a smelly gummy solid that was pumped dry (under vacuum) and washed with Et₂O, affording the blue precipitate [CuCl(OC₁₀H₁₅COOLi)]·Et₂O (V·Et₂O). (IR (cm⁻¹): 1733 (ν_CO), 1626 (ν_COO) and elemental analysis (%) for CuClC₁₁H₁₅O₃Li·Et₂O, Found: C 48.8, H 6.5; Calc.: C.48.9, H 6.8.

[CuCl(OC₁₀H₁₅COOLi)]·THF (V·THF). CuCl₂ (0.059 g, 0.44 mmol) and 1 (0.36 g, 1.8 mmol) were stirred together under vacuum for ca. 1 h. Then, nitrogen was introduced into the Schlenk and THF (10 mL) was added and the reaction carried on overnight. The precipitate was filtered (mostly unreacted 1) and the light green solution taken to dryness affording a pale green precipitate. Yield: 58%. IR (cm⁻¹): 1732 (ν_CO), 1623 (ν_COO). Elemental analysis (%) for CuClC₁₁H₁₅O₃Li·1.1THF, Found: C 48.9, H 6.5; Calc. C 48.6, H 6.3. ¹H NMR (MeOH-d₄, δ ppm): 3.74 (m, 4H, THF), 2.78 (sbr, H3), 2.4 (sbr, H4), 2.18–1.4 (m, 4H), 1.89 (m, 4H, THF), 1.02 (s, 3H), 0.88 (s, 3H), 0.84 (s, 3H). ¹³C NMR (MeOH-d₄, δ ppm): 215.0 (C2), 68.85 (THF), 60.4 (C1) 54.6 (C3), 48.4 (C4), 46.4 (C7), 30.5 (C6), 26.47 (THF), 23.8 (C5), 19.9 (C9/10), 19.3 (C9/10), 9.96 (C8). ⁷Li NMR (MeOH-d₄, δ ppm): −0.54. From the residual solution, abundant yellow crystals of camphorquinone precipitated (Yield, ca. 50%).

A progressive disappearance, during spectra acquisition (MeOH-d₄) of the hydrogen atom at position 3, due to deuteration and the broadening of atom C11 (see eqn (3) for labeling) precluding detection by ¹³C NMR (as in V) is a characteristic behavior of 1.⁴⁰ Nevertheless, no doubts exist concerning C-11 in the molecule, since the carboxylate stretch (ν_COO, 1623 cm⁻¹) is observed by FTIR.

[CuCl₂(OC₁₀H₁₅COOLi)]·2H₂O (VI). Compound 1 (0.24 g, 1.2 mmol) was dissolved in H₂O (5 mL) and the solution added to CuCl₂ (0.12 g, 0.90 mmol). Upon overnight stirring a light blue precipitate formed that was filtered off the solution, washed with cold water and dried under vacuum. Yield 53%. IR (cm⁻¹): 3438, 3411, 3359 (ν_OH), 1712 (ν_CO), 1587 (ν_COO). Elemental analysis (%) for CuCl₂C₁₁H₁₉O₅Li, Found: C 35.9, H 4.9; Calc. C 35.4, H 5.1.

[CuCl{OC₁₀H₁₅CONHCH(CH₂Ph)COOH}] (VII). THF (5 mL) was added to CuCl (0.024 g, 0.24 mmol) and 3 (0.220 g, 0.52 mmol) and the mixture stirred overnight under N₂. A white suspension formed that was filtered and dried under vacuum. Yield 75%. IR (cm⁻¹): 3459 (ν_OH), 3433, 3220 (ν_NH), 1733 (ν_CO), 1624, 1580 (ν_COO). Elemental analysis (%) for CuClC₂₀H₂₄LiNO₄·2.5H₂O, Found: C 48.3, N 3.0, H 6.0; Calc.: C 48.6, N 2.8, H 5.9. ¹H NMR (MeOH-d₄, δ ppm): 7.34 (s, 5H), 4.6 (sl, 1H), 3.20 (s, H3), 2.36 (d, J = 4 Hz, H4), 1.93–1.45 (m, 4H), 1.01 (s, 3H), 0.87 (s, 3H), 0.84 (s, 3H). ¹³C NMR (MeOH-d₄, δ ppm): 218.9 (C2), 176.4 (C11), 130.45, 129.99, 128.36, 59.6 (C1), 48.3 (C4), 46.4 (C7), 30.5 (C6), 23.7 (C5), 19.9, 19.3 (C9/10), 10.0 (C8). C3, not observed due to deuteration and broadening of the signal, during data acquisition.

[{CuCl}₂(H₂NCH₂CH₂COOLi)] (VIII). CuCl (0.050 g, 0.50 mmol) and compound 4 (0.075 g, 0.275 mmol) were maintained under vacuum for ¼ h, before flux in N₂ and added THF (10 mL). The mixture was stirred at 40 °C for 24 h. The light blue precipitate was filtered and dried under vacuum. Yield 45%. IR (cm⁻¹): 3206 (ν_NH), 1587 (ν_CO). Elemental analysis (%) for Cu₂Cl₂C₃H₆NO₂Li·H₂O·0.15THF, Found: C 13.7, N 4.8, H 3.1; Calc.: C 13.4, N 4.4, H 2.9. ¹H NMR (DMSO-d₆, δ ppm): 3.15 (sbr, 2H), 1.87 (sbr, 2H).

Blue crystals were obtained from DMSO solution under air that were filtrated and dried. Yield 50%. ESI-MS (MeOH/DMSO): m/z 308, [(IX-4H₂O)+ H + MeOH]⁺. IR (cm⁻¹): 3403 (ν_OH), 3308 (ν_NH), 1589, 1555 (ν_CO). Structural analysis by single crystal X-diffraction confirmed trans-[Cu(H₂NCH₂CH₂COO)₂(H₂O)₂]·4H₂O (IX) was obtained.

[{CuCl(OC₁₀H₁₅COOH)}] (X). THF (5 mL) was added to a mixture of CuCl (0.079 g, 0.80 mmol) and 1 (0.34 g, 1.7 mmol) and the mixture was stirred overnight. A green precipitate formed that was filtered off the solution and dried. Yield: 97%; IR (cm⁻¹): 1733 (ν_CO), 1580 (ν_COO). Elemental analysis (%) for CuClC₁₁H₁₆O₃, Found: C 44.2, H 5.1; Calc.: C 43.8, H 5.3. ¹H NMR (MeOH-d₄, δ ppm): 3.74 (m, H3), 2.36 (sl, H4), 2.15–1.35 (m, 4H), 1.01 (s, 3H), 0.88 (s, 3H), 0.84 (s, 3H). ¹³C NMR (MeOH-d₄, δ ppm): 204.4 (C2), 68.8 (C3), 60.0 (C1), 48.5 (C4), 43.5 (C7), 30.6 (C6), 23.9 (C5), 20.0 (C9/10), 19.4 (C9/10), 10.0 (C8). Traces of the endo isomer exist. Such as in V, C11 was not observed.

Catalysis

The typical procedure was: ethyl acetoacetate (1 mmol) was diluted with a mixture (1 : 1) of ethanol/water (4 mL) and triethylamine (2 mmol) was added to favour the enolization. Then the catalyst was added (1%, V) and the mixture stirred at 40 °C in a vented screw-cap glass vessel, under air for 24 h. Then, the reaction was quenched with 2 mmol of acetic acid and the mixture stirred for an additional 24 h. The reaction products of were analyzed by NMR (¹H, ¹³C, HSQC), using toluene as the internal standard.

XPS analysis

Samples were irradiated with the unmonochromatic Al Kα radiation. Experimental conditions and details about data treatment were the same as described elsewhere.³³ Sensitivity factors used here were: C 1s: 0.25; O 1s: 0.66; N 1s: 0.42; Cu 2p3/2: 4.2; Cl 2p: 0.61.

Single-crystal X-ray diffraction analysis

Single crystal XRD analysis on [Cu(H₂NCH₂CH₂COO)₂(H₂O)₂]·4H₂O (IX) was performed at 150(2) K on a Bruker AXS-KAPPA APEX II area detector apparatus using graphite-monochromated Mo Kα (λ = 0.71073 Å) and were corrected for Lorentz, polarization and empirically for absorption effects. The structure was solved by direct methods using SHELX97⁵⁵ and refined by full matrix least squares against F² using SHELX97 all included in the suit of programs WinGX v1.70.01 for Windows.⁵⁶ Non-hydrogen atoms were refined anisotropically and H atoms were inserted in idealized positions and allowed refine riding on the parent carbon atom. Crystal data and refinement parameters are summarized in Table 2. Illustrations of the molecular structures were made with ORTEP3.⁵⁷

Table 2 Crystal data obtained for [Cu(H₂NCH₂CH₂COO)₂(H₂O)₂]·4H₂O (IX)

Empirical formula	C6H24N2O10Cu
Formula weight	347.81
Crystal system	Monoclinic
Space group	P21/c
Unit cell dimensions
a/Å	5.5238(2)
b/Å	7.5320(3)
c/Å	17.7276(7)
α/°	90
β/°	90.499(2)
γ/°	90
Volume (Å^−3)	737.53(5)
Z, Dcal (g cm^−3)	2, 1.566
Absorption coefficient (mm^−1)	1.526
F(000)	366
Crystal size (mm^3)	0.2 × 0.3 × 0.3
θ range for data collection (°)	2.94 to 38.62
Index ranges	—
Reflections collected/unique	9 ≤ h ≤ 9, −13 ≤ k ≤ 13, −30 ≤ l ≤ 1
Data/restraints/parameters	9
Final R (observed)	16 343/4168 [R(int) = 0.0407]
	4168/0/136
	R 1 = 0.025 wR2 = 0.065

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CCDC 1030294 contains the supplementary crystallographic data for IX.

Acknowledgements

Financial support by FCT-Fundação para a Ciência e Tecnologia (UID/QUI/00100/2013, RECI/QEQ-MED/0330/2012 and RECI/QEQ-QIN/0189/2012) and the NMR and MS Networks (IST-UTL Nodes) for facilities.

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Footnote

† CCDC 1030294. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00064e

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