Supramolecules from cobaloximes

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Inorganica Chimica Acta 362 (2009) 682–690 www.elsevier.com/locate/ica

Review

Supramolecules from cobaloximes Renata Dreos *, Lucio Randaccio *, Patrizia Siega Dipartimento di Scienze Chimiche, Universita` di Trieste, Via L.Giorgieri 1, 34127 Trieste, Italy Received 31 October 2007; accepted 25 January 2008 Available online 1 February 2008 Dedicated to Professor Bernhard Lippert.

Abstract This short review describes the use of organocobaloximes in the self-assembly of supramolecular systems by means of functionalized aromatic boronic or diboronic acids. With the former, the cyclization is achieved by the coordination of the nitrogen donor of the boronic acid to one of the Co axial positions, with simultaneous insertion of the boron in the hydrogen bond of the other cobaloxime unit. Cyclic supramolecular species of different nuclearity, depending either on the kind of boronic acid or on the side substituents of the cobaloxime, can be obtained. The use of diboronic acids, which link two organonaquocobaloxime units, allows the formation of cyclic oligomers only in the presence of suitable guests. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Cobaloximes; Boronic acids; X-ray structures; 1H NMR characterization

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions with functionalized aromatic boronic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. General synthetic conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. X-ray structures of the products of the reactions with (dmgH)2 derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. X-ray structures of the products of the reactions with (dpgH)2 derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. X-ray structures of the products of the reactions with (mpgH)2 derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions with diboronic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

682 683 683 684 685 686 687 689

1. Introduction

*

Corresponding authors. Fax: +39 0405583903. E-mail addresses: [email protected] (R. Dreos), randaccio@univ. trieste.it (L. Randaccio). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.01.021

Supramolecular chemistry exploits non-covalent interactions to assemble large superstructures from molecular subunits in a rapid and reversible way [1]. Since these processes occur under thermodynamic control, many of these superstructures are not very stable, particularly in solution. The

R. Dreos et al. / Inorganica Chimica Acta 362 (2009) 682–690

H3C

CH3

OH = dmgH2

HO

N

683

N OH

B HO

= L1 N

OH

H3C = mpgH2 HO

N

B HO

N OH

= L2 N

OH = dpgH2 HO

N

N OH

B HO

= L3

Scheme 1.

NH2

possibility of constructing molecular architectures linked by more robust covalent bonds under thermodynamic control (dynamic covalent chemistry) [2] is, therefore, extremely appealing. A number of different covalent chemical reactions have been identified that are reversible under mild conditions [3]. Among these, the reversible reaction that leads to the formation of boronates from boronic acids and diols has been widely used in the construction of receptors and sensors for saccharides [4]. Organobis(dimethylglyoximato)Co(III) complexes [Co (CH3)(dmgH)2L] (dmgH2 = bis(dimethylglyoxime), L = neutral ligand), also known as organocobaloximes (Scheme 1), have been studied for a long time as vitamin B12 models [5]. Their derivatives with the intramolecular O  H  O bridges replaced by organoboryl groups are well known and have recently been reviewed [6]. In recent years, organocobaloximes and related derivatives with either one or both of the methyl groups of the equatorial ligand replaced by a phenyl group (mpgH2 = methylphenylglyoxime, dpgH2 = bis(diphenylglyoxime), Scheme 1) have also shown to be good building blocks for the assembly of supramolecular systems by reaction with functionalized aromatic boronic acids [7–11]. Alternatively, the use of a diboronic acid to link two organoaquacobaloxime units leads to the formation of cyclic oligomers with the coordination positions inside the cavity occupied by suitable guests [12]. This article reviews the synthesis and the structural characterization of the supramolecular species obtained by these two approaches. 2. Reactions with functionalized aromatic boronic acids 2.1. General synthetic conditions Functionalized aromatic boronic acids (4-pyridinylboronic, L1, 3-pyridinylboronic, L2, and 3-aminophenylboronic, L3, Scheme 2) direct the assembly of supramolecular species from [M(CH3)(dioximate)2L] (M = Co,

Scheme 2.

Rh; dioximate = dmgH, dpgH, mpgH; L = py, H2O) through the reaction of the B(OH)2 group with the oxime bridge of one bis(dioximate) moiety, with simultaneous coordination of the nitrogen atom to the metal center of another unit. The reactions are strongly pH dependent because all these boronic acids are zwitterionic species. The macroscopic pKa values for the first and the second deprotonations are 3.83 and 8.2, respectively, for L1 [8], 4.0 and 8.2 for L2 [13], and 4.56 and 9.14 for L3 [9]. Both 1H NMR spectra and UV–Vis studies show that in alkaline medium only the axial coordination of the nitrogen donor occurs, without the insertion of boron in O  H  O frame of the equatorial ligand. To obtain the supramolecular species, the reactions have to be carried out at neutral pH. The most spectacular evidence of the crucial role of pH arises from the reaction of [Co(CH3)(dmgH)2H2O] with L3. Indeed, when the pH of an aqueous solution containing organoaquacobaloxime and 3-aminophenylboronic acid in approximately equimolar amounts is adjusted to about 7, the supramolecular species precipitates immediately from the solution. Further acidification of the solution to below pH 4 results in the dissolution of the aggregate, which is re-formed by restoring the pH of the solution back to neutrality and is re-dissolved at alkaline pH values (above pH 9). The precipitation/dissolution cycle may be repeated at least 20 times. The neutral pH values required for the formation of the supramolecular species suggest that the reaction could involve the neutral form of the acid (Scheme 3), with loss of one water molecule; alternatively, it could involve the zwitterionic form of the acid (Scheme 4), with loss of a water molecule in a first step, deprotonation of pyridine, which in this way becomes available for coordination, and, finally, loss of a second water molecule. As the reac-

R. Dreos et al. / Inorganica Chimica Acta 362 (2009) 682–690

Ó American Chemical Society 2001

684

Ó American Chemical Society 2001

Scheme 3. Reprinted with permission from Ref. [8].

Scheme 4. Same as Scheme 3.

tion of boronic acids with the O  H  O frame of bis(dioximato) derivatives strongly resembles the interaction of boronic acids with diols [4], the lack of reactivity of the anionic form may be surprising. Indeed, it is commonly accepted that the formation of a hydroxyboronate species containing a tetrahedral boron is essential to strong binding in the boronic acid–diol complexes formed in aqueous solution [14]. This led to the conclusion that the tetrahedral boronate ion was the reactive species, even if this conclusion has been recently called into question [15]. The lack of reactivity of the anionic form of the acid in the reactions with [M(CH3)(dioximate)2L] may be rational-

ized considering that the O  H  O frame contains only one proton, and not two, as is the case for the diols. Consequently, the reaction with the anionic form of the acid should require the release of an OH group, which is clearly a disfavored process in alkaline medium. All the syntheses were carried out in protic solvents (water, methanol, or mixtures of methanol and aprotic solvent). The presence of a protic solvent is essential to the formation of the hydroxyboronate species and, at the same time, drives the equilibrium toward the formation of the insoluble supramolecular derivatives by precipitation. The supramolecular species are quite soluble in aprotic solvents. The 1H NMR spectra in CD2Cl2 or CDCl3 are entirely consistent with the solid state structures (see below). Indeed, the absence of a protic solvent ‘‘freezes” the boronic acid–boronate equilibrium, hindering dissociation or equilibration with species of different nuclearity. 2.2. X-ray structures of the products of the reactions with (dmgH)2 derivatives The reactions of [Co(CH3)(dmgH)2H2O] with L1, L2, and L3 afforded products of different nuclearity and geometry, depending on the boronic acid used as linker. Thus, the reaction of methylaquacobaloxime with 4- and 3-pyridinylboronic acid gave a dinuclear ‘‘molecular box” [7](1, Fig. 1) and a dinuclear ‘‘molecular parallelogram” [9] (2, Fig. 1), respectively, whereas that with 3-aminophenylboronic acid gave a trinuclear molecular triangle [9] (3, Fig. 1). The crystal of 1 is built up by dimeric units arranged on a crystallographic symmetry center, such that the 4-pyridinyl group of one monomeric unit axially coordinates the Co atom of the symmetry-related moiety. The Co and B atoms are at the vertices of a slightly distorted rectangle. Each pyridine faces its symmetry-related residue, with an ˚ . Since the pyridinyl groups interplanar distance of 3.3 A are also approximately perpendicular to the equatorial moiety (88.4°), compound 1 can be considered as a ‘‘rectangular” open box. A dimeric arrangement was also found in

Fig. 1. Structures of 1, 2, and 3. The hydrogen bonds connecting the oxygen atoms of B(OH) groups with the central water molecule in 3 are represented by dotted lines. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

R. Dreos et al. / Inorganica Chimica Acta 362 (2009) 682–690

2, but in this derivative the two pyridine residues, related ˚ and by a symmetry center, are coplanar within 0.2 A approximately lie in the plane of the two Co and two B atoms. The latter atoms are at the vertices of a parallelogram, whose mean plane makes an interplanar angle of 1.53° with the pyridinyl groups. The dimerization occurs in both cases by exploiting the conformational freedom (up or down) of the B bridge, which allows compensation for the different position of the pyridinyl N donor. In fact, in 1 the pyridinyl ligand is in the axial position, whereas in 2 it is in the equatorial position. Nevertheless, the Co  Co distances in 1 and 2 are very similar, being 7.592(5) and ˚ , respectively. In both 1 and 2, the OH group 7.365(3) A bound to the boronic acid has been esterified by the methanol used as a solvent. The X-ray structure of the crystals of 3 (Fig. 1) shows that the supramolecule is approximately an equilateral triangle of cobalt atoms with an approximate C3 symmetry. The single molecule is chiral, whereas both enantiomers are present in the crystal. The side of the triangle is about ˚ . Each B–OH group (not esterified, because the syn9.5 A thesis was carried out in water) makes an intramolecular H-bond with the amino group coordinated to Co, with relatively short NH  O distances in the range 2.95(1)– ˚ . The ring closure is ensured by the torsion angles 3.04(1) A around the B–C and C–NH2 bonds, which vary in the ranges 25–30° and 97–109°, respectively. The mean plane of the three C atoms of the methyl groups, each from one bis(dimethylglyoximate) moiety, is nearly parallel to that of the Co triangle (at an interplanar distance of ˚ ). On the other side, the Co plane is 1.76 A ˚ from 1.35(1) A the approximately parallel mean plane of the O atoms of the boron OH groups. Therefore, one side of 3 is hydrophilic (OH groups not esterified), while the other side is hydrophobic. A water molecule lies approximately in the middle point of the three hydroxyl O atoms and is Hbonded to the three boron OH groups at distances in the ˚. range 2.82(1)–2.85(1)A The substitution of Co by Rh in the reaction with 3-aminophenylboronic acid produced a dramatic structural change, leading to an inorganic polymer (4, Fig. 2). Indeed, the reaction of [Rh(CH3)(dmgH)2H2O] with 3-aminophenylboronic acid in water at pH 7 resulted in the displacement of H2O by 3-aminophenylboronic acid without

Fig. 2. Structure of 4. Rhodium atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen and deuterium atoms are omitted for clarity.

685

the insertion of boron in the O  H  O bridge, as suggested by an incomplete X-ray structural analysis and by the 1H NMR spectra in CD3OD. The polymeric form 4 deposited from CD3OD in the NMR tube with time. The local geometry of each unit in 4 is very similar to that of ˚ in the unit in 3, in spite of the increase of about 0.1 A the coordination distances of rhodoximes with respect to those of the analogous cobaloximes. An intramolecular H-bond between the coordinated amino group and the axially directed OCD3 group, arising from esterification of B(OH) from the solvent, is still present. Conformational changes, essentially due to significant variation in the B– C and C–NH2 torsion angles with respect to 3, lead to the chain arrangement. As both the geometry and the coordination properties of methylaquacobaloxime and methylaquarhodoxime are very similar, we believe that the determining factor is the esterification of the OH groups in 4. This hypothesis is supported by the evidence that 3 can also be recovered from methanol, provided the precipitation is immediate, but it decomposes if left in contact with this solvent for longer period. It is likely that the esterification reduces the stabilization arising from the H bonds between the water molecule and the three boron OH groups in the cyclic form 3. The steric hindrance of OCD3 could contribute to prevent cyclization in 4. It should be noted that molecular triangles with metal ions at vertex positions are still relatively rare. Most are based on square planar Pt(II) and Pd(II) complexes [16] and the closure of the triangle is made possible by rather flexible ligands. This approach often results in the formation of equilibrium mixtures, in which less strained larger species, typically squares, are also present. Recently, a solvent-induced reversible interconversion of a homochiral triangular macrocycle and a helical coordination polymer has been reported [17]. 2.3. X-ray structures of the products of the reactions with (dpgH)2 derivatives The above results suggested that slight variations in the geometry of the building blocks could strongly affect the structure of the assembled polynuclear systems. Therefore, we decided to study the effect of an increase in steric hindrance in the equatorial moiety of the metal complex, by substituting the methyl groups of the equatorial bis(dimethylglyoximato) ligand by phenyl groups (Scheme 1) [10]. The synthesis of the starting complexes ([Co(CH3)(dpgH)2L], L = py or H2O) has been previously reported [18]. The reactions of [Co(CH3)(dpgH)2L] with L1, L2, and L3 were carried out in a mixture of solvents (CH2Cl2/methanol, 1:5) because the starting complexes are poorly soluble in methanol. X-ray quality crystals of 5 and 6 (Fig. 3) were obtained from the reaction with L1 and L3, after 1 week and after 4 days, respectively. Only complicated mixtures of products were obtained from the reaction with L2, despite several attempts carried out at different pH values and in different solvents. The reaction

686

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Fig. 3. Structures of 5 and 6. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

time for the formation of 5 and 6 is substantially longer than that required for the assembly of the corresponding systems starting from [Co(CH3)(dmgH)2L]. This effect may be due to both electronic and steric factors. In fact, the electron withdrawing power of the phenyls decreases the electron density on the oxime oxygens with respect to cobaloximes and hinders the electrophilic attack of the sp2-hybridized B of the boronic acid. Furthermore, the steric hindrance of the phenyls may also make the approach of L both to the oxime bridge and to the axial coordination position more difficult. The geometry of 5 (Fig. 3) is very close to that found in 1. The most interesting structural consequence of the increase of the steric hindrance in the equatorial ligand on going from bis(dimethylglyoximato) to bis(diphenylglyoximato) is the change in the nuclearity of the supramolecular species assembled through L3 (6, Fig. 3). The molecular triangle 3 was obtained starting from bis (dimethylglyoximato) whereas the dimeric species 6 was obtained from bis(diphenylglyoximato). The dimeric unit may be considered a molecular box, similar to that of 1, but more distorted. The assembly is still possible owing to the conformational freedom of the boron bridge, which is directed toward the methyl group in 6, whereas it is directed toward the opposite side in triangle 3; however, the tri-

meric arrangement is prevented because of the steric hindrance of the phenyl groups. 2.4. X-ray structures of the products of the reactions with (mpgH)2 derivatives To the best of our knowledge, only two reports exist relative to cobalt complexes with the asymmetric methylphenylglyoximate ligand (mpgH) [19] (Scheme 1). Due to the asymmetry of the equatorial ligand, these complexes exist both as cis and trans isomers, and, when the axial ligands are different, the trans isomer is chiral. The synthesis of [Co(CH3)(mpgH)2py] [11] led to a mixture of trans and cis isomers, which were separated by fractional crystallization from CH2Cl2/CH3OH (1:1) and characterized by 1H NMR spectroscopy and X-ray structural analysis. The chiral trans isomer was isolated as a racemic mixture of the enantiomers. The reactions of the cis and trans isomers, separately, with L1, L2, and L3 were carried out in a mixture of solvents (chloroform/methanol, 1: 6), because the starting complexes are poorly soluble in methanol. X-ray quality crystals were obtained from the solutions containing the racemic mixture of the trans derivative and either 4-pyridinylboronic acid, 7, or 3-pyridinylboronic acid, 8 (Fig. 4). Small crystals, not of X-ray

Fig. 4. Structures of 7 and 8. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

R. Dreos et al. / Inorganica Chimica Acta 362 (2009) 682–690 OCH3 CH3 Co B CH3 N

CH3

H5 C 6 H5C6

H5C6 H5C6

Co

B

H3C

CH3

CH3O

CH3 Co B CH3 N H3C B H3C

N H3C

CH3

OCH3

C6H5 C6H5

CH3O

9

N

C6H5

Co CH3

C6H5

10 Scheme 5.

quality, were also obtained from the reactions of the cis isomer with 4-pyridinylboronic acid and 3-pyridinylboronic acid (9 and 10, respectively, Scheme 5) These were identified by ESI MS and 1H NMR spectroscopy in noncoordinating solvents (see below). The crystals of 7 and 8 are built up by dimeric units arranged on a crystallographic symmetry center (Fig. 4). The dimers are formed by two enantiomeric trans[Co(CH3)(mpgH)2] moieties held together by two 4- and 3-pyridinylboronic acid residues, respectively. This arrangement results in both the cases in meso species. The structure of 7 is very similar to those found in the analogous derivatives 1 and 5 containing dmgH and dpgH units, respectively. The geometry of 8 is similar to that found in the analogous bis(dimethylglyoximato) derivative 2, but the two symmetry-related 3-pyridinyl rings, which are essentially coplanar in 2, remain parallel in 8, but dis˚ . As a consequence, the ‘‘molecular paralplaced by 1.45 A lelogram” in 2 approaches a highly squeezed ‘‘molecular box” in 8. It is noticeable that the reaction of the racemic trans[Co(CH3)(mpgH)2py] with either 3- or 4-pyridinylboronic acid can produce three structurally different dimeric species (two homodimers and one heterodimer), but only the heterochiral dimer crystallized from the reaction mixture. To clarify if the high enantiospecificity of the assembly is to be attributed to a enantiomeric self-recognition in solution [20] or to a preferential crystallization [21], the reaction is monitored by 1H NMR spectroscopy in CD3OD/CDCl3 (1:1) at a pH of about 5. When an equivalent amount of 3- (or 4-) pyridinylboronic acid was added to the racemic mixture, the 1H NMR spectrum consisted of a complicated pattern of peaks, which did not simplify on standing for two weeks, attesting to the presence of several species besides 7 (or 8). Removal of the solvent under vacuum yielded a mixture of products, which were not further characterized. However, if the solvent was left to evaporate slowly from the solution, the crystals deposited on the walls of the NMR tubes consisted of pure 7 (or 8), respectively. Redissolution of 7 (or 8) in CD3OD/CDCl3 (1:1) resulted in their partial and slow decomposition. These results suggest that the meso species is sorted out from the solution in consequence of a preferential crystallization.

687

The structures of 9 and 10 were inferred from the 1H NMR spectra in CDCl3. The spectra show similar features except for the signals of the pyridinyl residue. As a consequence of the presence of a symmetry center, only one set of signals is evident. The axial methyl, the B(OCH3) and the residual oximic protons give rise to three singlets, whereas the phenyl protons appear as a multiplet. The presence of only one singlet for the equatorial methyls in both 9 and 10 and the internal equivalence of the ortho and the meta protons of pyridine in 9 are consistent with the presence of a mirror plane bisecting the O  H  O and the O  B  O bridges. The remarkable upfield shift of pyridine protons in 9 with respect to the corresponding proton of the free 4pyridinyl boronic acid (8.05 and 8.47 ppm in D2O) [8] is due to the shielding effect of the two facing pyridinyl rings. Three py protons of 10 give rise to three signals in the range 7.30–8.46 ppm (7.7–8.5 in the free 3-pyridinyl boronic acid in CD3OD). The signal of the ortho N–CH–C(CH)–B proton is noticeably shifted to lower field (9.72 ppm) with respect to the corresponding proton of the free 3-pyridinyl boronic acid (8.6 ppm in CD3OD) [10], due to the deshielding effect of the magnetic anisotropy of the parallel pyridinyl ring. In the cis-[Co(CH3)(mpgH)2] isomer, the insertion of the boronic acid may occur either in the O  H  O bridge near to the methyls or in that near the phenyl groups. The close proximity of the resonances of the equatorial methyls of 9 and 10 to the resonances of CH3C@N–OB in 7 and 8, respectively, and the presence of a cross-peak between B(OCH3) and the equatorial CH3 in the ROESY spectrum strongly support the insertion of the boron in the O  H  O bridge near to the methyls. The proposed structures for 9 and 10, very similar to those of 7 and 8, are depicted in Scheme 5. 3. Reactions with diboronic acids In another approach, diboronic acids were used as connecting elements between cobaloxime units to generate a number of oligomers by exploiting the reaction of the B(OH)2 groups with the oxime bridges. In this way, the axial position of the metal complex trans to the R group is available for the coordination of a guest molecule in the aggregate cavity. The reaction of [Co(CH3)(dmgH)2H2O] and 1,4-phenylenediboronic acid in the presence of pyrazine (pz) led to 11. The molecule of 11 may be described as an open centrosymmetric rectangular box with opposite faces made by the diborylated cobaloxime units and by two phenyl groups (Fig. 5). The box is filled by the planar pz ligand, almost ˚) parallel to the phenyl faces (at a mean distance of 3.27 A and bridging the Co atoms in one axial position, the other being occupied by the CH3 group [12]. The self-assembly process of 11 was monitored by timeresolved 1H NMR experiments (Fig. 6). After the addition of an equimolar amount of 1,4-phenylenediboronic acid to

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Fig. 5. Structure of 11. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

Fig. 7. Structure of 12. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

Ó American Chemical Society 2003

a solution of [Co(CH3)(dmgH)2H2O] in CD3OD (pD about 8), the spectra of the solution showed several singlets in the range 0.0–1.2 ppm (axial methyls) and couples of singlets in the range 1.9–2.5 ppm (equatorial methyls). The doubling of the signals number for the equatorial methyls indicates the formation of species containing one boryl bridge (Fig. 6b). The concentration of these species increases with time (Fig. 6c). The addition of pyrazine in a ratio of 0.5:1, about 1 h after the addition of the boronic acid, increased the complexity of the spectra because of the formation of species with axially coordinated pyrazine (Fig. 6d and e). One set of signals, whose integration ratio was constant with time, was immediately evident among the others. The low field shift of the axial methyl signal and the presence of two singlets for the equatorial methyls indicate that this set arises from a species with one inserted boryl bridge.

Furthermore, the high field shift of both the pyrazine and the phenyl signals suggests that in this species the aromatic rings are facing and shield each other. This spectral pattern is in agreement with a dinuclear species with two cobaloxime units bound by one diboronic acid molecule and one pyrazine. This species can be considered a ‘‘precursor” of the thermodynamically more stable 11. The concentration of the ‘‘precursor” species decreased with time, while the signals of 11, which appeared about half an hour after the addition of pyrazine, increased in intensity. After some days 11 was the prevailing species in solution (Fig. 6f and g). Addition of pyrazine before the formation of a sufficient amount of monoborylated species caused the immediate precipitation of the already known [Co(CH3)(dmgH)2]2lpz [22], identified by 1H NMR. Precipitation prevented further reactions. If pyrazine were not added to the solution of [Co(CH3)(dmgH)2H2O] and 1,4-phenylenediboronic acid, X-ray quality crystals of methylaquacobaloxime with a molecule of diboronic acid inserted in the O  H  O moiety, 12, precipitated from the solution (Fig. 7).

Fig. 6. Monitoring of the guest-induced self-organization of 11 by 1H NMR. (a) 30 mM solution of [Co(CH3)(dmgH)2H2O] in CD3OD (pD about 8); (b) immediately after the addition of a 1:1 amount of 1,4-phenylenediboronic acid; (c) after 450 ; (d) immediately after the addition of a 1:0.5 amount of pyrazine; (e) after 450 ; (f) after two days; (g) after 10 days. The signals of 11 are marked with a dagger and those of the ‘‘precursor” (see text) with an asterisk. Reprinted with permission from Ref. [12].

R. Dreos et al. / Inorganica Chimica Acta 362 (2009) 682–690

689

CH3O H3C B H3C

H3C

N B

H3C CH3O Fig. 8. ORTEP drawings for 13. Cobalt atoms are colored in dark gray, oxygen atoms in gray and boron atoms in light gray. Hydrogen atoms are omitted for clarity.

CH3 OCH3 CH3 Co B CH3 N

Co B

CH3

CH3 CH3 OCH3

14 Scheme 6.

1

H NMR spectra show that 11 is also formed the by reaction of the dimer [Co(CH3)(dmgH)2]2l-pz dissolved in CDCl3/CD3OD (50%) with 1,4-phenylenediboronic acid in a ratio of 1:1; and from a solution of 12 in CD3OD upon the addition of pyrazine in the ratio 1:0.5. In both the cases, the preliminary formation of the ‘‘precursor” species occurs. Attempts to remove the guest from 11 by protonation of pyrazine were carried out at very acidic pH values, owing to the low pKa value of pyrazine (0.6) [23]. In these conditions the removal of the guest led to the destruction of the host. The above results show that pyrazine is not included in a preformed box assembled from methylcobaloxime and diboronic acid, but rather its coordination thermodynamically drives the assembly process toward the most stable species. Interestingly, 11, once formed, does not precipitate immediately and in this case the equilibrium is attained in solution. To probe the role of the guest in the formation of the host, several ditopic ligands (ethylenediamine, piperazine, DABCO, 4-aminopyridine, azide, SCN) were used and the process monitored by 1H NMR spectroscopy. None of them proved to be effective and only ethylenediamine resulted in the formation of 13 (Fig. 8). Therefore, the action of pyrazine seems quite specific. The effectiveness of pyrazine probably arises from it having both a correct geometry and a sufficiently low pKa value to be in the basic form at the neutral pH required for the insertion of boron. Furthermore, a favorable p–p stacking interaction between the aromatic rings could stabilize 11. The relevance of the p–p interactions between host and guest for the formation of cyclic boronic esters has been outlined in a recent paper reporting the formation of 2:2 and 3:3 complexes between a racemic tetrol and 1,4-phenylenediboronic acid induced by toluene and benzene, respectively [24]. Currently, we are trying to obtain cyclic oligomers with different diboronic acids. Preliminary results indicate that the assembly of two cobaloxime units also occurs with 4,40 -biphenyldiboronic acid in the presence of 4,40 -bipyridine (14, Scheme 6) [25].

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