The first amino acid bound manganese–calcium clusters: a {[Mn^III₃Ca]₂} methylalanine complex, and a [Mn^III₆Ca] trigonal prism

Abstract

An amino acid containing octanuclear heterometallic {[Mn^III₃Ca]₂} cluster has been synthesized, alongside a structurally-related trigonal prismatic [Mn^III₆Ca]²⁺ cage.

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Perhaps one of the most stimulating events in the course of bioinorganic chemistry was the discovery of a pentanuclear heterometallic manganese–calcium cluster in the active center of Photosystem II (PSII). The Oxygen Evolving Center (OEC) of PSII is responsible for the photosynthetic process, more specifically for the oxidative splitting of water to molecular dioxygen, four protons and 4 electrons (per two water molecules), by sunlight. Molecular oxygen is consequently released into the atmosphere, while the protons produced are used to generate ATP chemical energy coins.¹ Yet, the complete nature and chemical composition of PSII has been puzzling bioinorganic chemists and crystallographers for several years, with details at the atomic level only appearing in the last two decades.² The water splitting site is positioned at the heart of the OEC and consists of a pentanuclear [Mn₄Ca] complex, whose structure has been described as a cubane-like [Mn₃CaO₄]ⁿ⁺ moiety linked to a fourth Mnⁿ⁺ atom via oxo-bridges. The structure is held in position by the carboxylate groups of the side chains of amino acids (D1-Asp170, D1-Glu333, D1-Asp342, CP43-Glu354), as well as the carboxy-terminal residue of D1-Ala344, responsible for the intra-cubane linkage of the calcium ion to a manganese ion. The role of the pentametallic cluster is to serve as an e⁻ donor during consequent cycles through a procedure that involves four oxidation steps, S_0–4 (Kok cycle). At the end of the catalytic cycle the cluster, short of 4 e⁻, “engages” two water molecules oxidizing them to O₂, protons and e⁻, in order to regenerate.³ During the last years numerous compounds have been synthesized and characterized as potential synthetic models of the active center of the enzyme. More specifically, compounds have been reported displaying structural, spectroscopic, and/or catalytic similarity to the OEC. Most of these compounds are polynuclear manganese clusters with the metal ions in various oxidation states.⁴ However, only a few examples of mixed metal manganese–calcium clusters (i.e. those containing both of the metal centers present in the active site) displaying structural similarities and/or similar metal-content to the OEC have been reported.⁵

We previously reported the first examples of polynuclear manganese clusters containing amino acid ligands, in which the manganese ions existed in moderate-high oxidation states.⁶ This motivated us to investigate whether amino acids could also be used to build heterometallic Mn/Ca clusters, and herein report the synthesis, structure and magnetic properties of the first two clusters of this new family, namely {[Mn^III₃CaO(OH)(mAla)(O₂CPh)₃(HL^a)₂(MeOH)]₂}·6.2MeCN·0.9H₂O (1·6.2MeCN·0.9H₂O) and [Mn^III₆CaO₃L^b₆(H₂O)₃](ClO₄)₂·14.1H₂O (2·14.1H₂O), using the amino acid methylalanine, mAlaH, and either the naphthalene-based triol ligand 2-(β-naphthalideneamino)-2-hydroxyethyl-1-propanol, H₃L^a, or the Schiff-base ligand created in situ upon the condensation of mAlaH and salicylaldehyde, H₂L^b, as co-ligands (Scheme S1†). To the best of our knowledge compounds 1 and 2 are the first examples of any heterometallic Mn/Ca clusters containing amino acids as ligands, with obvious relation to OEC, mimicking not only the metallic content of the active center, but also the amino acid moiety bridging the Mn and Ca ions.

Mn(ClO₄)₂·6H₂O (181 mg, 0.5 mmol), Ca(O₂CPh)₂·2H₂O (318 mg, 1 mmol), mAlaH (51 mg, 0.5 mmol) and H₃L^a (130 mg, 0.5 mmol) were stirred in MeCN/MeOH (1 : 1, 20 ml) in the presence of excess base, NEt₃, for 2 hours to form a dark brown suspension. The precipitate was removed by filtration and the dark-brown solution was layered with Et₂O (20 ml) to form dark brown crystals of {[Mn^III₃CaO(OH)(mAla)(O₂CPh)₃ (HL^a)₂(MeOH)]₂}·6.2MeCN·0.9H₂O (1· 6.2MeCN·0.9H₂O) after 4 days in ∼30% yield. For 2, Mn(ClO₄)₂·6H₂O (181 mg, 0.5 mmol), Ca(NO₃)₂·4H₂O (354 mg, 1.5 mmol), mAlaH (25 mg, 0.5 mmol), salicylaldehyde (0.0552 mL, 0.5 mmol) and NBu₄MnO₄ (MeCN solution, 0.05 M, 1 ml, 0.05 mmol) were stirred in MeCN (20 ml) in the presence of excess base NEt₃, for 30 minutes to form a dark brown solution, which was left to slowly evaporate, forming dark brown crystals of [Mn^III₆CaO₃L^b₆(H₂O)₃](ClO₄)₂·14.1H₂O (2·14.1H₂O) after ∼2 days, in ∼40% yield‡

Compound 1 crystallizes in the triclinic space group P1̄, with the asymmetric unit (ASU) comprising half the molecule. Its structure (Fig. 1) consists of two {Mn^III₃Ca^IIμ₄-O} tetrahedra linked together via two μ₃-OH ions (O1 and symmetry equivalent (s.e.)), which also bridge to Mn1 and s.e. The basal plane of the tetrahedron comprises an oxo-centered [Mn^III₃O]⁷⁺ triangle in which the O²⁻ ion (O2 and s.e.) resides 0.233 Å ‘above’ the [Mn^III₃] plane, towards the center of the cage, bonding to the Ca²⁺ ion at a distance of ∼2.63 Å. The benzoate ligands are of three types: one μ- bridges between Ca1 and Mn1′ (and s.e.), linking the two halves of the molecule; one μ- bridges between Mn2 and Mn3 (and s.e.) in the basal plane of the tetrahedron; and one is terminally bonded to Mn1 (and s.e.), the non-bonded arm H-bonding to the protonated arm of the neighbouring triol ligand (O2C⋯(H)–O15F, 2.67 Å). Each η²:η¹:μ-HL^{a 2−} ligand connects a Mn^III ion in the basal plane of the tetrahedron to the Ca^II ion in the upper vertex, with the chelating mAla ligand being bound to Mn1 (and s.e.) forming a five-membered ring through the N and one O_carboxylate atoms. The manganese ions are all in the 3+ oxidation state (BVS = 3.04, 2.94 and 2.89, for Mn1, Mn2 and Mn3, respectively) displaying Jahn–Teller elongated octahedral geometries (O1C–Mn1–O2A, 2.170–2.210 Å; O1B–Mn2–O1C, 2.138–2.498 Å; O2B–Mn3–O2A, 2.142–2.509 Å). The JT axes all lie within the [Mn^III₃] plane. The Ca^II ions are seven-coordinate adopting capped octahedral geometries (SHAPE analysis⁷), the last coordination site being occupied by a MeOH molecule. The Mn⋯Mn distances fall in the range ∼3.2–3.4 Å, while the Mn⋯Ca_tetrahedron distance are in the narrow range ∼3.4–3.5 Å. Intermolecular interactions are mediated via the π–π stacking of the naphthyl groups of the HL²⁻ and benzoate ligands (C⋯C, ∼3.4 Å).

Fig. 1 The molecular structure of 1, highlighting the chelate mode of the mAla ligands (top). The metallic core of 1, showing the connection of the two {Mn₃CaO} tetrahedral units (bottom). Color code: Mn^III = red, Ca^II = yellow, O = green, N = blue, C = grey. H-atoms and solvate molecules have been omitted for clarity.

Compound 2 crystallizes in the trigonal space group R3̄c (with two crystallographically independent cluster cations). Their metallic skeleton consists of a {Mn^III₆} trigonal prism encapsulating a nine-coordinate Ca^II ion at its center (Fig. 2). Each triangular face describes an almost ideal equilateral triangle (Mn^III⋯Mn^III ∼ 5.26–5.28 Å), the Mn ions being bridged by one syn, anti carboxylate group of a mAla Schiff-base ligand (O23, O24, O33 and O34 and s.e.), which further bridges to the central divalent calcium ion in a η²:η¹:η¹:μ₃-fashion. The two triangles are interconnected by three μ-H₂O molecules (Mn–O_W–Mn, 81.85°) and three μ₃-O⁻² ligands (O2 and s.e, Mn–O2–Mn, 114.28°) which also bind to the central Ca^II center. The approximate dimensions of the cage are 5.3 (Mn–Mn, triangular face) × 3.1 Å (Mn–Mn, upper-to-lower vertex), with the Mn–Ca distance being approximately 3.4 Å. The overall 2+ charge of the complex cation is compensated by the presence of perchlorate anions. The manganese centers are six-coordinate and in the +3 oxidation state (BVS = 2.93), adopting JT-elongated geometries, the JT axes being oriented along the O_W–Mn–O(carboxylate) vectors, aligned midway between parallel and perpendicular to the [Mn₃] plane. The central calcium ion is nine-coordinate with a tricapped trigonal prismatic {CaO₉} geometry (SHAPE analysis⁷) with Ca–O distances in the range 2.446–2.511 Å. In the extended structure the clusters are surrounded by a large number of H₂O molecules, several of which have partial occupancy, as well as several perchlorate anions. The solvate and coordinated μ-H₂O molecules, along with the perchlorate anions participate in a large number of hydrogen bonding interactions. There is also a significant number of C–H⋯O (from H₂O or ClO₄⁻ anions) interactions in the lattice that bridge the clusters together, creating a complicated 3D H-bonded framework (Fig. S1†). There are not any π–π or C–H⋯π interactions present.

Fig. 2 The molecular structure of the dication of 2 (left). The metallic core of 2, highlighting the coordination mode of the amino acid part of the Schiff-base, the bridging water molecules and the bridging μ₃-O²⁻ anions (right). Color code: Mn^III = red, Ca^II = yellow, O = green, N = blue, C = grey. H-atoms, counter ions and solvate molecules have been omitted for clarity.

The structural similarities of 1 and 2 to the actual OEC are: (i) the bridging mode of the carboxylate group of the amino acid ligand (D1-Ala344 in the OEC vs. mAla in 2 which bridges the Ca and Mn centers, (ii) the formation of a [Mn₃Ca] tetrahedron, (iii) the presence of the sp² N-atom of the mAla Schiff-base ligand in 2 mimicking the amide N-atoms present in the OEC, and (iv) the presence of six-coordinate Mn ions in all cases. In addition, the +3 oxidation state for all manganese centers may correspond to the dark stable S₁ state as has been suggested previously,⁸ although it is still not clear whether S₁ adopts a [Mn^III₄] or a [Mn^IV₂Mn^III₂] state. Furthermore, the presence of the six bridging water molecules in 2 is of great biological importance, given the presence of the water molecules near the active site of the enzyme that serve as a substrate.

Variable temperature dc magnetic susceptibility data were collected for 1 and 2 in the temperature range 5.0–300 K under an applied field of 0.1 T, and are plotted as χ_MT versus T in Fig. 3 and 4. For 1, the room temperature value of 8.35 cm³ mol⁻¹ K per {Mn₃Ca} unit is slightly lower than the theoretical value of 9.00 cm³ mol⁻¹ K expected for three non-interacting Mn^III centers (g = 2.00). Upon cooling χ_MT decreases slowly to ∼7.49 cm³ mol⁻¹ K at ∼130 K, before rapidly decreasing to a minimum value of 3.21 cm³ mol⁻¹ K at 5 K. This suggests the presence of weak antiferromagnetic exchange between the three Mn^III ions. For 2, the room temperature χ_MT value of 17.63 cm³ mol⁻¹ K is very close to the theoretical value of 18.00 cm³ mol⁻¹ K expected for six non-interacting Mn^III centers (g = 2.00). Upon cooling χ_MT remains approximately constant until ∼150 K, below which it increases slowly to reach ∼19.32 cm³ mol⁻¹ K at ∼35 K, before rapidly decreasing to a minimum value of 5.92 cm³ mol⁻¹ K at 5 K. This behavior is indicative of the presence of both ferro- and antiferromagnetic exchange interactions.

Fig. 3 χ _M T vs. T plot for complex 1 under an applied dc field of 1000 G. The solid lines represent fit of the data in the 5–300 K (see text for details). Inset: plot of reduced magnetization (M/Nμ_B) versus B for 1 in the field and temperature ranges 0–5 T and 2–6 K. The solid lines correspond to the fit of the data.

Fig. 4 χ _M T vs. T plot for complex 2 under an applied dc field of 1000 G. The solid lines represent fit of the data in the 5–300 K (see text for details). Inset: plot of reduced magnetization (M/Nμ_B) versus B for 2 in the field and temperature ranges 0–5 T and 2–6 K.

The susceptibility and magnetization data for 1 can be simultaneously fitted through the use of the program PHI⁹ employing spin-Hamiltonian (1) and the exchange coupling scheme shown in Fig. S2,† where Ŝ is a spin-operator, J the pairwise exchange coupling constant, B the applied magnetic field, and D the axial zero field splitting parameter for the Mn^III ion. The exchange coupling model assumes one exchange interaction (J₁) between Mn1–Mn2 and Mn1–Mn3 mediated by one μ₄-O²⁻ bridge and one monoatomic-bridge benzoate ligand, and a second exchange interaction (J₂) between Mn2–Mn3 mediated by a μ₄-O²⁻ bridge and a bridging η¹:η¹:μ₂ benzoate. The best fit (Fig. 3) parameters are J₁ = +0.50 cm⁻¹, J₂ = −7.00 cm⁻¹ and D_Mn = −6.56 cm⁻¹ with g fixed, g_Mn = 1.98. The fit can be improved with the inclusion of an intermolecular exchange interaction term, zJ′ = 0.12 cm⁻¹. A fit of the susceptibility data on its own using only the isotropic exchange part of spin-Hamiltonian (1) affords J₁ = +0.65 cm⁻¹, J₂ = −6.0 cm⁻¹ with g_Mn = 1.98. These J values belong to a well-defined minimum for this system in the {0.0, +1.0 cm⁻¹}(J₁)–{−5.5, −7.5 cm⁻¹}(J₂) region, as shown by the contour error plot for the data of 1 as a function of J₁ and J₂ (Fig. S3†).

The weak ferromagnetic interaction J₁ may be potentially attributed to the small Mn–O_{monoatomic benz}–Mn angles present (Mn–O–Mn: 85.22 and 87.67°, for Mn1–Mn2 and Mn1–Mn3, respectively), in addition to the in-plane arrangement of the JT axes with respect to the {Mn₂O₂} plane.¹⁰ In this simple “giant spin” model the ground-state of 1 is an S = 2 state, with the first S = 3 excited state located ∼10 cm⁻¹ above.

The size of complex 2 precludes the use of an anisotropic spin-Hamiltonian through standard computational techniques, but the susceptibility data can be fitted by adopting isotropic spin-Hamiltonian (2) which describes a 2-J model with: (1) J₁ between neighboring manganese centers located at the corners of the triangular faces, mediated by the syn, anti carboxylate group of the mAla Schiff-base ligand, and (ii) J₂ responsible for the connection of the two triangular faces, mediated by one μ-H₂O molecule and one μ₃-O²⁻ ligand (Fig. S4†).

Best fit parameters (Fig. 4) are J₁ = −0.35 cm⁻¹, J₂ = +1.94 cm⁻¹ with g fixed, g_Mn = 1.98. This suggests the ground-state is a diamagnetic S = 0 state with the first S = 1 excited state located only ∼0.35 cm⁻¹ above. Magnetization data are in agreement with this picture (Fig. 4) with M rising in a near linear fashion with B.

Conclusions

In conclusion, we have reported the synthesis and characterization of the first two examples of heterometallic Mn/Ca clusters containing an amino acid ligand, methylalanine. Both species are structurally related to the heterometallic cluster found in the active site of the OEC of PSII. Magnetic susceptibility and magnetization measurements reveal the presence of weak antiferro- and ferromagnetic exchange interactions in agreement with magneto-structural correlations developed for structurally similar Mn^III clusters. Attempts are currently underway in order to probe the possibility of isolating heterometallic Mn/Ca species upon employment of the natural amino acids Asp, Glu and/or Ala, as a means of modelling the actual content of the OEC.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under the HFRI PhD Fellowship grant (GA no. 4826). Part of this work was carried out in fulfillment of the requirements for the Master thesis of EKA according to the curriculum of the International Graduate Program in “Inorganic Biological Chemistry”, which operates at the University of Ioannina within the collaboration of the Departments of Chemistry of the Universities of Ioannina, Athens, Thessaloniki, Patras, Crete, and the Department of Chemistry of the University of Cyprus. EKB thanks the EPSRC for funding grants EP/N01331X/1 and EP/P025986/1.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1816563 and 1999601. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt01916j

‡ Crystal data for 1·6.2MeCN·0.9H₂O: C_128.4H_144.4Ca₂Mn₆N_12.2O_34.9, M = 2826.75, triclinic, space group P1̄, a = 14.303(4) Å, b = 15.673(5) Å, c = 15.979(5) Å, α = 107.04(3)°, β = 99.23(3)°, γ = 97.21(3)°, V = 3323.5(19) ų, Z = 1, T = 80 K, R₁ (I > 2σ) = 0.067 and wR₂ (all data) = 0.201 for 29 522 reflections collected, 12 519 observed reflections (I > 2σ(I)) of 18 327 (R_int = 0.039) unique reflections, GOF = 1.01. Crystal data for 2·14.1H₂O: C₆₆H_100.20CaCl₂Mn₆N₆ O_46.10, M = 2155.94, trigonal, space group R3̄c, a = 21.029(4) Å, c = 143.48(6) Å, V = 54 949(31) ų, Z = 24, T = 100 K, R₁ (I > 2σ) = 0.106 and wR₂ (all data) = 0.208 for 40 308 reflections collected, 6462 observed reflections (I > 2σ(I)) of 11 186 (R_int = 0.099) unique reflections, GOF = 1.13. Full details can be found in the CIF files with CCDC reference numbers 1816563 and 1999601, for 1 and 2, respectively.†

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