Some aspects of the formation and structural features of low nuclearity heterometallic carboxylates

Examples of trinuclear molecular systems with a linear metal core

The simplest case of the formation of a trinuclear heterometallic complex with a linear metal core can be observed with isomorphic substitution of part of the metal atoms in a homonuclear analog. Formally, this process can be attributed to the reactions of transmetallation of the central ion in the starting compound, although the reaction is complicated. Such an example can be demonstrated in the “virtual” conversion of the cobalt(II) homonuclear complex [Co₃(naph)₆(lut)₂] (1) into the heterometallic complex [Co₂Mg(naph)₆(lut)₂] (naph is the anion of 2-naphthoic acid; lut is lutidine) (2) (Fig. 1).

Fig. 1:

Molecular structures of compounds 1 (a) and 2 (b) (solvate molecules, CH₃ groups of lutidine molecules and H atoms are omitted).

In homonuclear complex 1, the central and peripheral Co(II) atoms are nonequivalent and have a different coordination environment. The peripheral cobalt atoms are in a distorted trigonal-bipyramidal environment (CoO₄N), while the environment of the central atom corresponds to a distorted octahedron (CoO₆). As a result, 1 can be considered as the “prototype” of the heterometallic compound in which another divalent metal ion occupies the central position. To replace the ions of a d-element, a s-metal can be selected, which, according to Pearson’s classification, has a large affinity for oxygen atoms in the coordination sphere [53], [54] and coordination numbers close to 3d-elements. The indicated requirements are met by the Mg²⁺ ion. This assumption was confirmed by the reaction which afforded magnesium-containing complex 2 in a quantitative yield according to Equation (1) [55]:

For energy assessment of pathways of the formation of complex {Co–Mg–Co}, a model reaction between cobalt carboxylate and magnesium trifluoromethanesulfonate was studied by quantum-chemical calculations, which, as shown earlier, is capable of producing the desired trinuclear Co₂Mg compound (2). Note that this scheme involves the use of the starting complex [Mg(MeCN)₄(CF₃SO₃)₂] with easily replaceable acetonitrile molecules. The lability of these coordinated solvent molecules is often used in coordination chemistry when generating active fragments with s- and d-element ions in reaction medium.

The relative energies of the compounds on the left and right sides of Equation (2) were estimated. The calculations were carried out by the density functional theory method using the B3LYP/Def2-TZVP approximation, as well as its variations, including dispersion interactions (CAM-B3LYP/Def2-TZVP and CAM-B3LYP/Def2-TZVP+D3BJ). As follows from the results of calculations of complexes with naphthoate ligands shown in Fig. 2, regardless of the approximation used, the substitution of the central cobalt ion by magnesium ion leads to a decrease in the energy of the system under consideration by 6.8–10.5 kcal/mol. Calculations for compounds with bridging pivalate groups led to a similar result; the energy gain was 8.1–9.5 kcal/mol, depending on the approximation used. Therefore, one of the reasons contributing to the formation of this type of heterometallic trinuclear systems is the thermodynamic stability of complexes including Co²⁺ and Mg²⁺ ions.

Fig. 2:

Optimized geometries of complexes 1 and 2 calculated in the CAM-B3LYP/Def2-TZVP approximation with inclusion of dispersion correction D3BJ. Relative energies of homo- and heterometallic systems are given at the bottom right.

The process of replacing the central Co(II) atom in 1 with Mg(II) atom can formally be considered as a transmetallation reaction, as a result of which heterometallic molecule 2 is formed, which is similar in structure to 1, but with completely different magnetic characteristics. However, other starting compounds and reactions can also be used to prepare heterometallic compounds similar to 2. For example, it is known that the reaction of the binuclear cobalt(II) complex with 2,4-lutidine, [Co₂(O₂CBu^t)₄(2,4-lut)₂], with magnesium pivalate (obtained in situ) leads to the formation of compound [Co₂Mg(O₂CBu^t)₆(2,4-lut)₂] (3, Fig. 3a), the structure of which is similar to 2. It can be assumed that in this case the fragment of magnesium pivalate {Mg(O₂CBu^t)₂} is introduced into the binuclear fragment {Co(μ-O₂CBu^t)₄Co} of the starting cobalt complex [56]. Although the mechanism of this kind of “introduction” has not yet been studied in detail, the fact of the appearance of the magnesium ion in the central position, apparently, involves a rather complex multi-stage transformation of the coordination environment of this ion.

Fig. 3:

Molecular structures of compounds 3 (a) and 4 (b) (solvate molecules and H atoms are omitted).

The action of 2,2′-dipyridyl (bpy) on compound 3 leads to the substitution of the monodentate N-donor ligand with the chelating ligand to form [Co₂Mg(O₂CBu^t)₆(bpy)₂] (4, Fig. 3b) [57]; although in this case, the peripheral Co(II) atoms change their coordination environment (CN=4 in 3, CN=5 in 4), which also affects the magnetic characteristics of the molecules. In this case, the trinuclear metal carboxylate core of the heterometallic molecule is quite stable and remains unchanged even with a multiple excess of the chelating ligand bpy. Such resistance to the action of a chelating ligand is very unusual, since in the case of homometallic carboxylate complexes of Co²⁺ and other metal ions M²⁺ (M²⁺=Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺) monomers [M(O₂CBu^t)₂(bpy)₂] are observed to form [2], [58], [59], [60], [61], [62]. Moreover, the action of magnesium pivalate on [M(O₂CBu^t)₂(bpy)₂] (M=Co(II), Ni(II)) leads to the quantitative formation of the heteronuclear compound [M₂Mg(O₂CBu^t)₆(bpy)₂] (M=Ni(II), Co(II)) [57], [63] and the transition of the excess bpy molecule into the solution in a free state, which, presumably, confirms the thermodynamic stability of the heterometallic structure.

In the structures of the considered trinuclear heterometallic molecules (for example, 2–4), one can formally distinguish two anionic metal fragments {(L)M(O₂CR)₃}⁻, which are associated with the central metal atom (in this case, Mg(II)) (Fig. 4) and perform the function of a scorpionate anionic metal-containing ligand in a stable heterometallic structure.

Fig. 4:

Structures of {(L)M(O₂CR)₃} fragments in compounds 2, 3 (a) and 4 (b) (Bu^t groups and H atoms are omitted) and their schematic representation (c).

Such molecules are stable, because each metal-containing anionic ligand {(L)M(O₂CR)₃}⁻ provides at least three oxygen atoms from its carboxylate groups to coordinate to the central oxophilic metal center. In the case of a divalent central metal ion, for example, the Mg²⁺ ion, this leads to the formation of neutral molecules, but what will happen if monovalent ions of alkali metals are used, for example Li⁺?

Heterometallic carboxylate architectures containing 3d-metal and lithium ions

To complicate the situation, we consider the replacement of the central position of the type 1 compound with an atom of an oxophilic monovalent metal to form the corresponding heterometallic structures. The formation of these compounds can also be expressed as a substitution of the central divalent Co²⁺ ion in the type 1 compound or as the introduction of singly charged ions into binuclear homometallic complexes. In the case of the participation of Li⁺ ions in the formation of the heterometallic framework and two anionic fragments {(L)M(O₂CR)₃}⁻ (M=Co(II), Ni(II), Cu(II), Zn(II)), the formation of molecular complexes [M₂Li₂(O₂CR)₆(L)₂] (where L is the N-donor ligand) is observed (Fig. 5). This means that one divalent ion is replaced by two monovalent Li⁺ ions, although in this situation, the geometric parameters of these ions should be taken into account.

Fig. 5:

Molecular structures of compounds 5 (a) и 6 (b) (solvate molecules and H atoms are omitted) and schematic representation of molecules [Li₂M₂(O₂CR)₆(L)₂] (c).

The {M₂Li₂} family of molecular compounds can be supplemented with complexes containing bridging L, pyrazine and pyrimidine, which bind the {Co₂Li₂(O₂CR)₆} fragments into 1D and 2D coordination polymers (Fig. 6) [64].

Fig. 6:

Binding the {Co₂Li₂(O₂CR)₆} fragments by pyrimidine molecules into 1D (a) and 2D (b) coordination polymers (solvate molecules and H atoms are omitted).

The situation becomes complicated if we use the tridentate pincer ligand 2,2′;6′,2″-terpyridine as L. In this case, the geometrical features of terpy, which has all three N donor centers in the same plane, can be a formal obstacle to the formation of {M₂Li₂} complexes. Taking into account the limited coordination capabilities of the 3d-elements (for example, for Co(II) with CN=4–6), problems arise with the arrangement of carboxylate bridges in the remaining space between M²⁺ and Li⁺ ions using the above anionic fragment {M(O₂CR)₃L}⁻. Taking the complex [Co₂Li₂(O₂CBu^t)₆(terpy)₂] (7) as an example, it was shown that in this case an unusual rearrangement of bridging carboxylate groups is observed while maintaining the composition of [M₂Li₂(O₂CR)₆(L)₂]. Heterometallic ions in 7 are bound with only two carboxylate bridges, and the other two carboxylic anions participate only in the binding of Li⁺ ions to each other (Fig. 7) [65]. As a result, the binding scheme looks like the interaction of two neutral metal-containing ligands {(terpy)Co(O₂CR)₂} with the binuclear fragment {Li(μ-O₂CR)₂Li}, which leads to quite ordinary coordination numbers of cobalt(II) (CN=5) and lithium(I) (CN=4) atoms.

Fig. 7:

Molecular structure of compound 7 (solvate molecules and H atoms are omitted).

A similar binding of Li⁺ and Cu²⁺ ions is observed in complex {[Li(Hcpdc)]₄[Cu(O₂CBu^t)₂(terpy)]₄·H₂O}_n (8) (Hcpdc⁻ is the anion of cyclopropane-1,1-dicarboxylic acid), in which the coordination environment of copper(II) is also partially hindered by terpy chelate. In this case, the binuclear fragment {LiCu(O₂CBu^t)₂(terpy)₂} is part of the composition of the polymeric chain formed by binding of Li⁺ ions to Hcpdc⁻ anions (Fig. 8) [66].

Fig. 8:

Fragment of polymeric chain of compound 8 (solvate molecules and H atoms are omitted).

To obtain complexes with lithium(I) and vanadium(IV) atoms, compounds of vanadyl (V(IV)=O)²⁺ (the formal analogue of M²⁺) and lithium carboxylates were used. Molecular complexes [Li₂(VO)₂(O₂CR)₆(bpy)₂] (9: R=Bu^t, 10: R=CF₃) were synthesized and studied, which, unlike [M₂Li₂(O₂CR)₆(bpy)₂] (M=Co(II), Ni(II), Cu(II)), have a slightly different structure of the {(VO)₂Li₂(O₂CR)₆} fragment, taking into account the specific structure of vanadyl. It was found that the vanadyl oxygen atom takes part in the formation of the coordination environment of the Li ion, and the 12-membered macrocycle {-VO-Li-OCO-VO-Li-OCO-} is formed rather than four-membered Li₂O₂ cycle (as it was found in complexes with Co²⁺, Ni²⁺, and Cu²⁺ ions) (Fig. 9) [67]. As a result of such binding, both metal atoms, Li(I) and V(IV), are in a coordination environment typical in coordination compounds with CN=4 and 6, respectively.

Fig. 9:

Molecular structures of compounds 9 (a) and 10 (b) (solvate molecules and H atoms are omitted).

Heterometallic architectures containing 3d-metal and lanthanide ions

Since lanthanide ions are highly oxophilic and in most cases present in coordination compounds in the form of trivalent metal centers, the formation of triatomic heterometallic molecules similar to 3d-Mg and 3d-Li requires an additional singly charged anion to comply with the principle of electroneutrality. As a result, the formation of molecular complexes [M₂Ln(O₂CR)₆X(L)₂] is observed (Fig. 10), where X is the anion (NO₃⁻ or RCO₂⁻), which compensates for the charge of the Ln³⁺ ion and ensures the electroneutrality of the heteronuclear compound with this composition.

Fig. 10:

Molecular structure of compound 11 (a) (solvate molecules and H atoms are omitted) and schematic representation of molecules [M₂Ln(O₂CR)₆X(L)₂] (b).

In recent years, a large series of heterometallic complexes with different Ln:M ratios in the metal core were obtained (M=Co(II), Ni(II), Cu(II), Zn(II)) [18], [68], [69], [70], [71], [72]. The Zn–Ln complexes were studied in most detail, since they are of great practical interest from the point of view of obtaining luminescent materials [11], [12], [13], [73]. For example, for a series of compounds containing Zn²⁺ and Ln³⁺ ions, depending on the substituent on the carboxylate group, the nature of the N-donor ligand and the presence of the NO₃⁻ anion in the reaction mixture, bi-, tri-, and tetranuclear complexes ZnLn, Zn₂Ln, and Zn₂Ln₂, respectively, were synthesized (Fig. 11) [11], [12], [13], [18], [77], [78], [79], [80] (Fig. 11). It can be assumed that the reasons for such a variety of complexes are the large coordination numbers of lanthanide ions, the possibility of forming polyhedra with different ligand environments (for example, LnO₈ and LnO₉), and some changes in the ionic radius in the La–Lu series as a result of lanthanide contraction. Using trimethylacetate Zn–Ln complexes with lanthanum(III) and europium(III) as an example, the dependence of the structure of the resulting compound on the radius of the Ln³⁺ ion was shown, and under the same conditions, tetranuclear and binuclear complexes were isolated, respectively (Fig. 11a,b) [74].

Fig. 11:

Molecular structures of compounds 12 (a), 13 (b), 14 (d) and 15 (c) (CH₃ groups in 15, solvate molecules and H atoms in 12–15 are omitted).

It is worth noting that, as in the case of structures having the {M₂Mg} or {M₂Li₂} core, in the listed lanthanide-containing complexes, the anionic fragment {(L)Zn(O₂CR)₃}⁻ plays the role of a scorpionate metal-containing ligand. However, here the ratio between this fragment and the lanthanide ion can be different depending on the structure and ligand environment of the lanthanide-containing fragment. Thus, in binuclear complex [(bpy)ZnEu(μ-O₂CBu^t)₃(O₂CBu^t)₂] (12), the ratio {(L)Zn(O₂CR)₃}⁻:Ln equal to 1:1 is observed similar to the situation in tetranuclear molecules [{(L)ZnLn(μ-O₂CR)₅}₂] (13: L=phen, R=Bu^t, Ln=La) (Fig. 11b). However, in the tetranuclear structure, the zinc-containing scorpionate metal ligands {(L)Zn(O₂CR)₃} are coordinated to the lanthanide metal centers of the binuclear tetracarboxylate fragment, which is often found in the series of known binuclear lanthanide carboxylates [81], [82], [83], [84]. In some cases, when the tetranuclear structure is formed, the nitrate groups present in the initial lanthanide compound are retained and heterometallic molecules of similar structure [{(L)ZnLn(μ-O₂CR)₄(NO₃)}₂] (14: L=bpy, phen, O₂CR=1-naphtoate, Ln=Eu) are formed (Fig. 11d).

In the formation of trinuclear complexes with Eu³⁺, Gd³⁺, and Tb³⁺ ions, two binuclear fragments {(L)M(O₂CR)₃}⁻ (M=Zn(II), L is pyrazino[2,3-f][1], [10]phenanthroline, pyzphen, O₂CR is O₂CBu^t) are transformed into a more complex metal-containing ligand {(pyzphen)₂Zn₂(O₂CBu^t)₄OH}, which chelates the Ln³⁺ ion in a pentadentate mode (Fig. 11c) [13]. These compounds can be obtained both by self-assembly from the reaction mixture containing Zn(O₂CBu^t)₂, pyzphen, and Ln(NO₃)₃·xH₂O in MeCN and when pyzphen acting on the linear complex [Zn₂Ln(O₂CBu^t)₆(NO₃)(MeCN)₂] [76]. Unlike bpy in the linear complex [Zn₂Ln(O₂CBu^t)₆(NO₃)(bpy)₂] [76], pyridyl moieties in pyzphen cannot rotate around the shared C‒C bond, which apparently makes steric hindrances in the environment of Zn²⁺ ions. This is confirmed by the difference in angles in the ZnO₃ fragment: the solid angles O‒Zn‒O for [Zn₂Tb(OH)(O₂CBu^t)₄(NO₃)₂(pyzphen)₂] (312.2° and 315.8°) are smaller than the corresponding angle in [Zn₂Eu(O₂CBu^t)₆(NO₃)(bpy)₂] (347.4°).

Attempts to introduce the tridentate chelating ligand 2,6-bis(2-pyridyl)-4-(4-pyridyl)pyridine (pyterpy) (terpy derivative) into the coordination sphere of the cobalt atom as L in the trinuclear fragment {Co₂Ln(NO₃)(O₂CBu^t)₆} resulted in the degradation of the {(L)M(O₂CR)₃} fragment observed in the above architectures; however, they made it possible to find the conditions for the formation of the heterometallic pentanuclear complex [{Co₂Dy(NO₃)(O₂CBu^t)₆}{Co(pyterpy)(O₂CBu^t)(H₂O)}₂](NO₃)₂·2MeCN·3H₂O (16), in which the mononuclear cations {Co(O₂CBu^t)(pyterpy)(H₂O)}⁺ are coordinated to the cobalt atoms of the central core {Co₂Dy(NO₃)(O₂CBu^t)₆} by the free nitrogen atom of the 4-pyridyl moiety (Fig. 12) [71].

Fig. 12:

Structure of the cationic fragment [{Co(pyterpy)(O₂CBu^t)(H₂O)}₂{Co₂Dy(NO₃)(O₂CBu^t)₆}]²⁺ in compound 16 (H atoms are omitted).

The effect of the type of coordination polyhedron on the ability of a particular metal to form heterometallic complexes manifested itself in a very special way in the case of complexes with Cu²⁺ ions. The reaction of the known dimer [Cu₂(O₂CBu^t)₄(HO₂CBu^t)₂] [85] and Eu(NO₃)₃ in the presence of NBu₄OH led to the formation of the trinuclear compound (NBu₄)[Cu₂Eu(O₂CBu^t)₈] (17, Fig. 13), in which each copper atom is surrounded by a square-planar environment made of four O atoms of four bridging carboxylate groups that bind the peripheral metal ions to the central europium atom [86]. The peculiarity of the electronic structure of copper(II) and the ability to form a square planar environment complements the variety of M–Ln carboxylate complexes with this example.

Fig. 13:

Fragment of crystal packing of compound 17 (solvate molecules and H atoms are omitted).

As noted above, some homometallic polynuclear complexes of transition metals can be considered as unique “prototypes” of heterometallic compounds. Such compounds, the metal atoms in which are in nonequivalent positions, can show the direction of the search for new types of heterometallic compounds. In the synthesis of the copper(II) complex with 1,1-cyclohexanediacetic acid dianions (chda²⁻) and bpy molecules starting from [Cu₂(O₂CBu^t)₄(HO₂CBu^t)₂], the ternary complex [Cu₃(chda)₂(O₂CBu^t)₂(bpy)₂] (18) was obtained (Fig. 14a), which combines the anions of two carboxylic acids [87]. The introduction of [Ln₂(O₂CBu^t)₆(HO₂CBu^t)₆] (Ln=Sm, Gd) into the reaction mixture led to the formation of the tetranuclear heterometallic complexes [Ln₂Cu₂(chda)₂(O₂CBu^t)₆(bpy)₂] (Ln=Sm (19), Gd) (Fig. 14b). Although the structure of the tetranuclear heterometallic Cu–Ln complex differs markedly from the structure of the trinuclear copper(II) complex, the structural motifs of the homometallic compound are essentially retained. The peripheral metal fragment bonded to the central copper atom in complex 18 is formed by the Bu^tCO₂⁻ bridging carboxylate group and two oxygen atoms of the carboxylate groups of both chda²⁻ dianions, while for [Ln₂Cu₂(chda)₂(O₂CBu^t)₆(bpy)₂], it is formed by the bridging carboxylate group Bu^tCO₂⁻, the O atom of one of the carboxylate groups of the chda²⁻ anion, and the O atom of the chelate-bridging Bu^tCO₂⁻ anion. The structures of trinuclear fragments, {Cu₃} in 18 and {CuLnLn} in 19, were found to be similar, which generally allows one to use the proposed scheme for bonding certain metal-containing blocks when forming heteronuclear architectures. In addition, we believe that the modeling of possible transformations of homonuclear molecular systems into heteronuclear ones is promising from the point of searching for a homometallic “prototype” that is similar in structure and electronic properties and its further transformations.

Fig. 14:

Molecular structures of compounds 18 (a) and 19 (b) (solvate molecules and H atoms are omitted).

Structural features of cadmium(II) heterometallic complexes

Our analysis of the data on the synthesis and structures of the considered heterometallic complexes makes it possible to use a block combination coupling scheme in such systems in which the anionic scorpionate metal ligand {(L)M(O₂CR)₃}⁻ with anions of 3d-elements, namely Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and VO²⁺ (only for Li–V complexes) plays a structure-forming role, binding to Li⁺, Mg²⁺ or Ln³⁺ ions. In this case, it can be noted that scorpionate fragments {(L)M(O₂CR)₃}⁻ with a monodentate N-donor as L, for example, NEt₃, pyridine, and its substituted derivatives, were observed only for Co²⁺ and Zn²⁺ ions. Apparently, this is associated with a stable tetrahedral or tetragonal-pyramidal environment of these metals in the divalent state. On the other hand, the Ni²⁺ ion in most heteronuclear carboxylates prefers octahedral environment, whereas Cu²⁺ ions form stable homometallic complexes with monodentate L, such as pyridines [88], [89], [90], [91], [92] or NEt₃ [93]. It seems that this explains the fact that heterometallic carboxylate complexes with fragments {(L)M(O₂CR)₃}⁻ (L is a monodentate N-donor ligand) have not been isolated for these metals.

In this situation, it is worth paying attention to a closely related 4d-element, cadmium(II), since its increased geometric parameters and, accordingly, a large variety of coordination environments (for example, the existence of carboxylates with CN=7 and 8 [94]) can change the binding scheme of the {(L)Cd(O₂CR)₃}⁻ fragment in heteronuclear architectures.

An attempt to obtain a heterometallic complex according to the methodology similar to 1–3 with the introduction of 1,10-phenanthroline (phen) led to the isolation of the previously known binuclear cadmium homometallic complex [(phen)₂Cd₂(O₂CBu^t)₄] [95]. However, it was found that when {Cd(O₂CBu^t)₂} was allowed to react with pivalates of alkaline-earth elements, Mg or Ca and Sr, and 2,4-lutidine (lut) in MeCN, analogues of trinuclear heterometallic 3d-metal complexes [(lut)₂Cd₂M(O₂CBu^t)₆] (20) (M=Mg(II), Ca(II), Sr(II)) could be easily prepared (Fig. 15a). The centrosymmetric linear metal core is formed by a central atom of an alkaline earth element located in a distorted octahedral environment and connected with two terminal Cd(II) atoms with one bridging and two chelate-bridging anions of pivalic acid (CN(Cd)=6, CN(Mg)=6). In this case, the Cd···M and Cd···Cd distances regularly increase with increasing radius of the cation M (Cd···M 3.45 Å (Mg)→3.63 Å (Ca)→3.75 Å (Sr)) [94].

Fig. 15:

Molecular structures of compounds 20 (a, solvate molecules and H atoms are omitted) and 21 (b, solvate molecules and H atoms of organic ligands are omitted).

Substitution reactions between 2,4-lutidine molecules and 1,10-phenanthroline made it possible to isolate complexes retaining the metal core, namely trinuclear heterometallic complexes [(H₂O)(phen)₂Cd₂Mg(O₂CBu^t)₆] (21), [(phen)₂Cd₂Ca(O₂CBu^t)₆] (22), and [(phen)₂Cd₂Sr(O₂CBu^t)₆]·2MeCN (23), which cannot be obtained by direct reaction of the starting components. In this case, a new type of heterometallic complex was identified (21), in which the central magnesium atom is bonded to each terminal Cd(II) atom only by pairs of carboxylate groups from two fragments {(phen)Cd(O₂CBu^t)₃}⁻ (Fig. 15b), while the third carboxylate anions of these fragments form hydrogen bonds with the protons of the water molecule bound to the Mg(II) atom. As a result, the coordination environment of cadmium atoms is retained, while that of the magnesium atom changes (CN=5) from an octahedral to a distorted square pyramidal. In the case of Cd–Ca and Cd–Sr compounds 22 and 23, the metal cores [(L)₂Cd₂M(O₂CBu^t)₆] are similar to those of the starting compounds with 2,4-lutidine (Fig. 16).

Fig. 16:

Molecular structures of compounds 22 (a) and 23 (b) (solvate molecules and H atoms are omitted).

Concerning heterometallic molecules with the metal core {Cd₂Ln}, where Ln³⁺ ions have a large radius, one can notice that they are similar to the mentioned trinuclear heterometallic systems containing the scorpionate fragment {(L)M(O₂CR)₃}, where M is a 3d-element. For example, the structures of the synthesized trinuclear complexes [EuCd₂(O₂CR)₆(NO₃)(bpy)₂] (R=C₆F₅) (24) (Fig. 17) and [LnCd₂(O₂CR)₆(NO₃)(phen)₂] (Ln=Tb(III), Eu(III), O₂CR is the anion of 3,5-di-tert-butylbenzoic acid) [96] are similar to those of compounds [(L)₂M₂Ln(NO₃)(O₂CR)₆] [76], [86], [97], [98], [99], [100], [101], [102], [103], [104]. However, the electronic characteristics of the central lanthanide ion affect the magnetic properties of the heterometallic molecules of the isolated complexes, and the shape of the metal core can change; in particular, the angle Cd–Ln–Cd in the complex molecule varies from 145° to 175°.

Fig. 17:

Molecular structure of compound 24 (solvate molecules, H and F atoms are omitted).

Substitution of carboxylate anions in heterometallic structures

An important property illustrating the high stability of heterometallic complexes is their ability to retain a heterometallic ensemble of metal atoms during the substitution of carboxylate anions. In this way, for example, some poorly soluble 1- and 2-naphthoate complexes were synthesized. In certain cases, when replacing carboxylate anions in heterometallic trinuclear complexes, rearrangement of the metal core can be observed. Thus, when the pivalate complex [Zn₂Ln(NO₃)(O₂CBu^t)₆(MeCN)₂] reacts with 1-naphthylacetic acid (Hnaphac) salts in the presence of a number of N-donor heterocyclic ligands (L), tetranuclear complexes of the composition [(L)₂Zn₂Ln₂(NO₃)₄(naphac)₆] (25) and [(L)₂Zn₂Ln₂(NO₃)₂(naphac)₈] (L=4,4′-dimethyl-2,2′-dipyridyl, 4,4′-di-tert-butyl-2,2′-dipyridyl) are formed [12] (see Scheme 1, Fig. 18).

Scheme 1:

Synthesis of compounds [L₂Zn₂Ln₂(NO₃)₂(naphac)₈] (L=4,4′-dimethyl-2,2′-dipyridyl, 4,4′-di-tert-butyl-2,2′-dipyridyl) from initial pivalate complex [Zn₂Ln(NO₃)(O₂CBu^t)₆(MeCN)₂].

Fig. 18:

Molecular structure of compound 25 (solvate molecules and H atoms are omitted).

One of the possible ways to synthesize new compounds based on the obtained carboxylate heterometallic complexes is the replacement of the anion of the monocarboxylic acid with the dicarboxylic acid dianion. Since such a reaction is usually accompanied by the replacement of all anions of monocarboxylic acid, this results in the formation of a polymeric structure. Substitution of carboxylate anions can be supplemented by substitution of a monodentate neutral ligand with a bridging one, for example, pyridine with 4,4′-dipyridyl. With such an integrated approach, conditions are created under which coordination polymers, including scaffolds, are formed. The described approach is widely used for the synthesis of metal organic frameworks (MOFs) from acetates or inorganic metal salts in situ. The methodology proposed by us consists in the preliminary assembly of a heterometallic metal core as part of a molecular complex with the subsequent assembly of the framework structure while maintaining or transforming the initial ensemble of metal atoms [104], [106].

Substitution of pivalate anions with terephthalate anions in the molecular complex [Zn₂Li₂(O₂CBu^t)₆(py)₂] (26) (Fig. 19a) afforded a series of framework coordination polymers [Zn₂Li₂(R-bdc)₃(4,4′-bpy)]·solv (R-bdc²⁻ is the dianion of substituted terephthalic acid; R=H, Br, NH₂, NO₂), in which the tetranuclear fragment {Zn₂Li₂(O₂C)₆N₂} is retained when substituting all ligands [107]. A new type of secondary building block {Zn₂Li₂(O₂C)₆} is able to be bound by bridging ligands at eight growth points: six carboxylate residues and two neutral bridging ligands (Fig. 19b). For the resulting range of compounds, the effect of the nature of aromatic guest molecules on luminescence and the excitation wavelength was studied.

Fig. 19:

Molecular structure of compound 26 (a, solvate molecules and H atoms are omitted), view of the secondary building block {Li₂Zn₂(O₂CR)₆} with the possible extension of the coordination structure through the bridging ligands (b), and the example of binding {Li₂Zn₂(O₂CR)₆} fragments by 4,4′-bipyridine and terephthalate anions (c).

In [108] using biphenyldicarboxylic acid (H₂bpdc) as an example, the effect of the N-donor ligand as a stabilizer of the tetranuclear fragment is shown. The reactions between [Zn₂Li₂(O₂CBu^t)₆(py)₂] (26) with H₂bpdc in DMA and N-methyl-2-pyrrolidone (NMP) led to the rearrangement of the tetranuclear fragment to form the binuclear fragment {ZnLi(OOC-)₃}, while the reaction in DMF in the presence of diazabicyclo[2.2.2]octane (dabco) afforded MOF {[Zn₂Li₂(bpdc)₃(dabco)]·9DMF·4H₂O}_n. Thus, the presence of dabco made it possible to stabilize the initial tetranuclear {Zn₂Li₂} fragment and form a framework structure, while the absence of an N-donor in the reaction mixture led to the formation of layered honeycomb polymers based on triply-bonded binuclear {ZnLi} fragments. Studies have shown that {[Zn₂Li₂(bpdc)₃(dabco)]·9DMF·4H₂O}_n demonstrates reversible crystal transitions to the amorphous phase when the guest molecule is removed or replaced. It was also found that the luminescence and quantum yield of the resulting porous polymers depend on the nature of the guest molecule.

Using reactions with the molecular complexes [M₂Li₂(O₂CBu^t)₆(py)₂] (M=Zn(II), Co(II)) as examples, the possibility of the formation of MOFs with anions of tricarboxylic acids – trimesinic (H₃btc) and 1,3,5-tricarboxylbenzoic (H₃btb) was demonstrated [109]. Two structural types of polymers were distinguished: 3D scaffolds with a chiral topology SrSi₂, in which the plane of each carboxylate group lies in the plane of the btc³⁻ benzene ring or 2D honeycomb structures in the case of the btb³⁻ linker, for which the angles between the central benzene ring and the plane of the CO₂⁻ group are close to 90°. In both cases, the binuclear triply-bounded fragments {MLi(O₂CR)₃} are the building block. Layered Zn-containing coordination polymers are capable of a guest-dependent change in luminescence. A rare example of constant porosity and selectivity of separation of CO₂/N₂ and CO₂/CH₄ mixtures with layered polymers was also demonstrated.

In addition, the reaction between [Co₂Gd(NO₃)(O₂CBu^t)₆(py)₂] and terephthalic acid (H₂bdc) in dimethylacetamide (DMA) is known, which proceeds with partial destruction of the trinuclear heterometallic metal core to the binuclear {CoGd(μ₂-RCO₂-κ¹,κ¹)₃(RCO₂-κ²)₂}, which is the secondary building block for constructing the coordination polymer [{CoGd(dma)₂}₂(bdc)₅]·4DMA (27) [110]. The three-dimensional framework 27 is porous and has two types of channels filled with coordinated and guest DMA molecules (Fig. 20).

Fig. 20:

View of the secondary building block {CoGd(μ₂-RCO₂-κ¹,κ¹)₃(RCO₂-κ²)₂} with possible extension of the coordination structure through the bridging ligands (a) and the example of binding {CoGd(O₂CR)₅} fragments by terephthalate anions (b).

As the main results of this review of heterometallic carboxylate complexes, it should be noted that due to the high stability of heterometallic fragments, their use in subsequent syntheses allows many reactions to be carried out according to the ligand substitution reaction scheme in mononuclear complexes. Thus, the ability to predict the results of synthesis is significantly increased. The preservation of metal fragments when varying carboxylate anions and neutral N- and O-donor ligands, as well as the allomerism of heterometallic complexes of various 3d-metals provide great opportunities for obtaining compounds with a given combination of properties and structural features for further synthesis.