Complexes Containing a Seven-Membered Metallacycle - American

Article pubs.acs.org/Organometallics

A Novel Series of CoIII(salen-type) Complexes Containing a SevenMembered Metallacycle: Synthesis, Structural Characterization and Factors Affecting the Metallacyclization Rate Patrizia Siega,† Renata Dreos,*,† Giovanna Brancatelli,† Ennio Zangrando,† Claudio Tavagnacco,† Višnja Vrdoljak,‡ and Tomica Hrenar‡ †

Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a,10000 Zagreb, Croatia

‡

S Supporting Information *

ABSTRACT: A series of electronically tuned trans-[CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2,2′-ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis β organometallic derivative containing a sevenmembered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the −CN− groups of the chelate. The X-ray structures of cyclized complexes, [CoIII(chelCH2)(py)(H2O)]+, show a dependence of the conformation of the seven-membered metallacycle on the different positions of substituents in the chelate. In fact, in the complex having methyls on the −CN− groups, the conformation is characterized by having the methylene carbon atom significantly displaced (ca. 1.26 Å) from the aromatic ring plane, whereas in the complex lacking methyl groups in those positions, the atoms of the Ph−O−CH2 fragment are coplanar. The standard Gibbs energies obtained by quantum chemical calculation reveal that the different conformations observed in the solid state are mainly the result of the energetically unfavorable setup of the methyls on the −CN− groups and of the energetically favorable displacement of the CH2 group out of plane of the aromatic ring. 1H NMR data suggest that the different conformations of the metallacycle are, at least partially, retained in solution.

■

The interest in cis β metal Schiff base complexes arises from the consideration that they proved to be efficient asymmetric catalysts, being chiral and having two labile mutually cis coordination sites.1d,2 Furthermore, as trans and cis β configurations of metallosalen complexes are interchangeable under the appropriate reaction conditions, the possibility that a trans metallosalen complex reacts in the cis β configuration should be considered.1c,d Generally, the cis β configuration is induced either by a bidentate ligand,3 which occupies two cis sites in the coordination sphere, or by the bonding nature of the two unidentate ligands, which strongly demand a cis coordination.4 A lengthening of the polymethylenic chain bridging the two imine nitrogen atoms5 and an increase of the size of the central metal atom6 may also favor a strained nonplanar configuration. Little information is available about the effect of the presence of substituents on the chelate on the conformation of metal complexes. However, it was observed in a series of titanium− Schiff base complexes that substantial steric effects within the

INTRODUCTION

The salen ligand is a Schiff base obtained by reaction of 2 equiv of salicylaldehyde with 1 equiv of ethylenediamine, but the more general term salen-type ligand currently refers to [N2O2] tetradentate ligands prepared by condensation of variously substituted salicylaldehydes with a diamine. salen-type ligands are able to stabilize a large number of metals in different oxidation states.1 The tetradentate salen-type ligands can adopt three configurations in the formation of octahedral complexes: trans, cis α, and cis β (Chart 1). Most of the complexes adopt a trans configuration with two unidentate ligands occupying the apical positions. There are relatively few examples in which the quadridentate ligand assumes a cis configuration. Chart 1

Received: October 25, 2013 Published: February 6, 2014 © 2014 American Chemical Society

909

dx.doi.org/10.1021/om401038v | Organometallics 2014, 33, 909−920

Organometallics

Article

both in solution and in the solid state, a number of complexes containing either two or four methyl substituents in different positions in the equatorial ligand (Chart 2) together with their corresponding cyclized products, [CoIII(chelCH2)(py)(H2O)]ClO4. Herein we describe the effect of the substituents on the cyclization rates of the organometallic [CoIII(chel)(CH2Cl)(L)] (L = H2O, py) complexes and the electrochemical properties of the strictly related [CoII(chel)] and [CoIII(chel)(py)2]ClO4 derivatives. Furthermore, we discuss the metallacycle conformational change induced by the methyl substituents in the [CoIII(chelCH2)(py)(H2O)]ClO4 products.

ligand are required to achieve a folding of the ligand so that it takes up a cis geometry.7 This result indicates that a metallosalen complex can adopt a cis β configuration due to the steric repulsion caused by introducing substituents on the salen ligand. We have reported that trans-[CoIII(4,4′,7,7′tmsalen)(CH2Cl)(S)] (4,4′,7,7′tmsalen = 4,4′,7,7′-tetramethylsalen, S = solvent; Chart 2) affords the cis β organometallic derivative Chart 2

■

RESULTS Syntheses. The chelates (chel) were obtained by reaction of the appropriately substituted acetophenone or benzaldehyde with ethylenediamine in methanol, according to the procedure previously described for 4,4′,7,7′tmsalen (Scheme S1a, Supporting Information).14 The corresponding CoII complexes were prepared by reaction of the ligands with Co(CH3COO)2· 4H2O in alkaline methanol under nitrogen (Scheme S1b, Supporting Information), whereas the [CoIII(chel)(py)2]ClO4 complexes were synthesized by carrying out the same reaction in the presence of pyridine and with continuous stirring in air (Scheme S1c, Supporting Information).14 The trans dimeric [CoIII(chel)(CH2Cl)]2 species were obtained by reduction of the corresponding [CoIII(chel)(py)2]ClO4,with NaBH4/PdCl2 in alkaline methanolic solution followed by the oxidative addition of CH2ClI (Scheme S1d, Supporting Information). The products recovered from the reaction mixture immediately after the completion of the reaction are dimers in the solid state (the X-ray structure of [CoIII(4,4′dmsalen)(CH2Cl)]2 (1) is reported below) and monomers in solution, presumably with a solvent molecule in the sixth coordination position.8,14 The cyclized cis β derivatives [Co(chelCH2)(L)(S)]+ of the complexes containing a methyl group at positions 7,7′, i.e. [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(3,3′,7,7′tmsalenCH2)(py)(H2O)]ClO4, were obtained in the same way, by allowing the reaction mixture to stand for 5 days after the alkylation reaction (Scheme S2a, Supporting Information). This synthetic procedure, when applied to complexes lacking the methyls at positions 7,7′, led to mixtures of products which could not be separated in spite of several attempts. Therefore, [CoIII(3,3′dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 were prepared by starting from the corresponding [CoIII(chel)(CH2Cl)]2 dissolved in methanol in the presence of an excess of pyridine (Scheme S2b, Supporting Information). The reaction mixture was heated and the progress of the reaction monitored by TLC. The solution was evaporated to dryness in vacuo, and the products were separated by flash chromatography on silica gel using 3/1 CHCl3/C2H5OH as eluent. The mixture contained the desired [CoIII(chelCH2)(py)(H2O)]Cl together with minor amounts of unreacted [CoIII(chel)(CH2Cl)]2 and a third green species identified as the pyridinium ylide [CoIII(chel)(CH2py)(Cl)], whose synthesis and characterization will be described elsewhere. The corresponding [CoIII(3,3′dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 complexes were obtained by treatment of the chloride derivatives with an excess of aqueous NaClO4 in methanolic solution. 1 H NMR Characterization. The assignments of the NMR spectra of ligands and complexes were facilitated by a

[CoIII(4,4′,7,7′-tmsalenCH2)(S)2]ClO4 through an intramolecular cyclization reaction (Scheme 1).8 In this case, the Scheme 1

conformational change of the ligand is driven by the chemical reaction, which probably occurs through the internal nucleophilic attack of a chelate oxygen on the axial chloroalkyl group. Few examples of intramolecular reactions of XCH2 axial groups (X = Cl, Br, I) with imino-oxime or amino-oxime equatorial ligands have been described.9−11 In all cases, the generation of an equatorial negatively charged nitrogen is required and the reaction occurs only in strongly basic medium. In contrast, the cyclization of [CoIII(4,4′,7,7′tmsalen)(CH2Cl)(S)] occurs also in neutral medium, likely due to the presence of negatively charged oxygens in the equatorial ligand.8,12 The cyclized product consists of a racemic mixture of Δ and Λ enantiomers (Scheme 1; the numbering scheme of the [CoIII(chelCH2)(py)(H2O)]+ cations refers to the Δ enantiomer throughout the text). Kinetic data obtained for the cyclization reaction of [CoIII(4,4′,7,7′tmsalen)(CH2Cl)(S)] in the presence of L = 4-X-pyridine (X = CN, H, NH2), whose steric effect is assumed to be similar, show that the cyclization rate constants for [CoIII(4,4′,7,7′tmsalen)(CH2Cl)(L)] increase with the pKa of L.13 Therefore, in this case the accelerating effect of the axial ligand is mainly inductive in nature. In order to gain a deeper insight into the factors that influence the propensity of the trans-[CoIII(chel)(CH2Cl)(L)] complexes to undergo the intramolecular cyclization in methanolic solution, we have synthesized and characterized, 910


Organometallics

Article

Table 1. 1H NMR Data for the Complexes [CoIII(chel)(CH2Cl)(S)] in CD3OD complex [CoIII(3,3′dmsalen) (CH2Cl)(S)] [CoIII(4,4′dmsalen) (CH2Cl)(S)] [CoIII(7,7′dmsalen) (CH2Cl)(S)]a [CoIII(3,3′,7,7′tmsalen) (CH2Cl)(S)] a

CH2CH2

phenyl protons

CH2Cl

C7-H

2.22 (s, 6H)

CH3-Ph

3.76 (s, 4H)

5.34 (s, 2H)

7.92 (s, 2H)

2.18 (s, 6H)

3.79 (s, 4H)

6.36 (d, 2H, C4-H), 6.88 (s, 2H, C2-H), 7.07 (d, 2H, C5-H) 6.97 (m, 6H)

5.35 (s, 2H)

7.94 (s, 2H)

2.56 (s, 6H)

3.87 (m, 4H)

6.52 (m, 2H), 7.08 (m, 4H), 7.55 (d, 2H)

5.28 (s, 2H)

2.52 (s, 6H)

3.83 (m, 4H)

6.36 (d, 2H, C4-H), 6.90 (s, 2H, C2-H), 7.42 (d, 2H, C5-H)

5.26 (s, 2H)

2.20 (s, 6H)

CH3-CN

From ref 21.

salicylaldiminate fragment forming dihedral angles of 25.10(13) and 2.9(2)° with the N2O2 coordination donor set. It is worth noting that the bridging oxygen O1 causes an elongation of the equatorial Co−O1 bond length (1.925(3) Å) with respect to the Co−O2 length (1.890(3) Å). However, the bond lengths do not show any anomalies and agree with those measured in similar derivatives. The axial Co−C and Co−O1′ distances are 2.043(4) and 2.189(3) Å, the latter in agreement with the values found in [Co(4,4′,7,7′tmsalen)(CH3)]214 and [Co(4,4′,7,7′tmsalen)(CF3CH2)]214 of 2.209(2) and 2.199(2) Å, respectively, due to the trans influence of the methylene group. The C(17)H2−Cl bond is oriented above the five-membered chelating ring in the direction of the N−Co−N bond angle bisector. The structural determination of complexes [ Co I I I (4,4′dmsalenCH 2 )(py)(H 2 O)]ClO 4 (2) and [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 (3) confirms the cyclization reaction with the tetradentate salen ligand coordinating the metal in a cis fashion. In both cationic species the cobalt atom exhibits a slightly distorted octahedral geometry, completing the coordination sphere through an aqua and a pyridine molecule. The resulting complexes (depicted in Figure 2) are chiral, in spite of the fact that they crystallize as a racemic mixture. The coordination bond distances Co−N(py), Co−O, and Co−C (Table 3) of complexes [Co I I I (4,4′dmsalenCH 2 )(py)(H 2 O)] + and [CoIII(7,7′dmsalenCH2)(py)(H2O)]+ are closely comparable within their esd’s. On the other hand, the Co−N(salen) distances in the latter evidence a slight difference (of 1.900(4) and 1.942(4) Å), as previously observed in the β cis derivatives [CoIII(4,4′,7,7′tmsalenCH2)(py)(H2O)] and [CoIII(4,4′,7,7′tmsalenCH2)(N-methylimidazole)2],8 but in 2 the values are analogous to those measured in the trans salen derivative [CoIII(4,4′dmsalen)(CH2Cl)]2. The Co−OH2 bond lengths, 2.108(3) and 2.162(3) Å in 2 and 3, respectively, are affected by the trans influence of the CH2 group, although they are significantly shorter than the value measured in [Co(4,4′,7,7′tmsalenCH2)(H2O)(py)]+ of 2.2130(9) Å.8 Since the aqua ligands in 2 and 3 are involved in H bonds of comparable geometry with the oxygen atom O1 of a symmetry-related complex and with a perchlorate oxygen, the differences may be ascribed to an electronic effect of the number and position of methyl groups on the salen ligand. A feature worth noting in the two complexes is the different conformation assumed by the seven-membered metallacycle ring upon coordination (Figure 3). In fact, the C11−C16− O2−C17 torsion angle of the chelating arm, −3.4(6) and 90.1(6)° in 2 and 3, respectively, indicates conformational diastereomer complexes (neglecting the methyl groups on the chelate). The conformation detected in 3 is same as that found

comparison with the corresponding data previously obtained for the fully characterized analogues.8,14 The following discussion will be restricted to the most interesting features of the spectra of organometallic derivatives. The spectra of the [CoIII(chel)(CH2Cl)(S)] complexes (Table 1) display a relatively low number of signals, owing to the symmetry of the molecule, and are very similar to each other. The singlet arising from the axial CH2Cl group (Figure S1, Supporting Information) is observed at 5.34 ppm for [CoIII(3,3′dmsalen)(CH2Cl)(S)], 5.35 ppm for [CoIII(4,4′dmsalen)(CH2Cl)(S)], 5.28 ppm for [Co III (7,7′dmsalen)(CH 2 Cl)(S)], and 5.26 ppm for [CoIII(3,3,′,7,7′tmsalen)(CH2Cl)(S)], suggesting that the methyls at positions 7,7′ cause an upfield shift of these signals by increasing the charge density on the axial group. The CH2CH2 protons resonate in the range 3.8−3.9 ppm (Table 1). In particular, they appear as a singlet for [CoIII(3,3′dmsalen)(CH2Cl)(S)] and [CoIII(4,4′dmsalen)(CH2Cl)(S)] and as a multiplet for [Co I I I (7,7′dmsalen)(CH 2 Cl)(S)] and [CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)] (Figure S1). The 1H NMR spectra of the cis-β-[CoIII(chelCH2)(py)(S)]+ complexes (Table 2 and Figure S2 (Supporting Information)) show a larger number of signals owing to the lack of elements of symmetry. The two methyls at positions 3,3′ or 4,4′ (Scheme 1) resonate as singlets in the range 2.1−2.3 ppm, while those at 7,7′ appear as singlets in the range 2.5−2.7 ppm. The ethylene bridge gives rise to three multiplets which integrate for 1, 2, and 1 H, respectively, at 3.6, 3.9, and 5.0 ppm for [CoIII(3,3′dmsalenCH2)(py)(S)]+ and [CoIII(4,4′dmsalenCH2)(py)(S)]+, and at 3.6, 4.2, and 4.7 ppm for [Co(7,7′dmsalenCH2)(py)(S)] + and [Co(3,3′,7,7′ tmsalenCH2)(py)(S)] + (Table 2). The phenyl protons resonate in the range 6.4−7.7 ppm, while the protons at 7,7′ give rise to two singlets at 7.9 and 8.8 ppm. Remarkably, the two diastereotopic protons of CH2, i.e. those bonded to the organometallic C, display a different chemical shift in the complexes having methyls at positions 7,7′ with respect to the complexes lacking them. Thus, two singlets are observed at 5.3 and 6.8 ppm for [CoIII(3,3′dmsalenCH2)(py)(S)]+ and [CoIII(4,4′dmsalenCH2)(py)(S)]+, and at 6.2 and 6.9 ppm for [Co(7,7′dmsalenCH2)(py)(S)]+ and [Co(3,3′,7,7′tmsalenCH2)(py)(S)]+. X-ray Structures. The molecular structure of [CoIII(4,4′dmsalen)(CH2Cl)]2 (1) is depicted in Figure 1, and coordination bond distances are reported in Table 3. The crystal contains neutral dimeric units arranged about a crystallographic inversion center where the tetradendate salen adopts a planar trans geometry. The oxygen atom of one Cosalen unit completes the coordination sphere of the metal ion of the other unit and vice versa. The salen ligand assumes an umbrella-shaped conformation with the mean plane of each 911


complex

a

2.13 (s, 3H,b), 2.17 (s, 3H,b)

2.10 (s, 3H, ), 2.28 (s, 3H,b) 2.13 (s, 3H,c), 2.21 (s, 3H,c)

b

CH3-Ph

CH2CH2 3.64 (m, 1H), 3.94 (m, 2H), 4.97 (m, 1H), 3.66 (m, 1H), 3.98 (m, 2H), 5.00 (m, 1H) 3.63 (m, 1H), 4.24 (m, 2H), 4.76 (m, 1H) 3.61 (m, 1H), 4.20 (m, 2H), 4.72 (m, 1H)

CH3-CN

2.55 (s, 3H,d), 2.68 (s, 3H,d) 2.53 (s, 3H,d), 2.65 (s, 3H,d)

axial CH2

6.40 (s, 1H), 6.47 (d, 1H), 6.96 (s, 1H), 7.08−7.18 (m, 5.32 (s, 1H), 3H) 6.84 (s, 1H) 6.48 (d, 1H), 6.91 (m, 1H), 7.04−7.17 (m, 4H) 5.36 (s, 1H), 6.82 (s, 1H) 6.61 (m, 1H), 6.81 (m, 2H), 6.98−7.14 (m, 4H), 7.70 6.27 (s, 1H), (d, 1H), 6.94 (s, 1H), 6.45 (d, 1H), 6.59 (d, 1H), 6.64 (s, 1H), 6.84 (s, 1H), 6.25 (s, 1H), 6.94 (d, 1H), 7.58 (m, 1H) 6.91 (s, 1H)

phenyl protons

See Scheme 1 for the numbering scheme. bCH3-C3 or CH3-C3′. cCH3-C4 or CH3-C4′. dCH3-C7 or CH3-C7′.

[Co (3,3′dmsalenCH2) (py)(H2O)]+ [CoIII(4,4′dmsalenCH2) (py)(H2O)]+ [CoIII(7,7′dmsalenCH2) (py)(H2O)]+ [Co(3,3′,7,7′tmsalenCH2) (py)(H2O)]+

III

Table 2. 1H NMR Data (CD3OD) for [CoIII(chelCH2)(py)(H2O)]+ a H-CN 7.88 (s, 1H), 8.82 (s, 1H) 7.90 (s, 1H), 8.84 (s, 1H)

py 7.08−7.18 (m, 2H, meta), 7.59 (m, 1H, para), 8.47 (m, 2H, ortho) 7.04−7.17 (m, 2H, meta), 7.59 (m, 1H, para), 8.46 (m, 2H, ortho) 7.16 (m, 2H, meta), 7.63 (m, 1H, para), 8.22 (m, 2H, ortho) 7.16 (m, 2H, meta), 7.62 (m, 1H para), 8.21 (m, 2H ortho)

Organometallics Article

Figure 1. Molecular structure of the centrosymmetric neutral complex 1.

Table 3. Coordination Bond Distances (Å) for Complexes 1−3 complex 1

Co−N(1) Co−N(2) Co−O(1) Co−O(2) Co−C(17) Co−O(1)′a

a

912

complexes 2 and 3

1.880(3) 1.882(4) 1.925(3) 1.890(3) 2.043(4) 2.189(3) Co−N(1) Co−N(2) Co−N(3) Co−O(1) Co−C(17) Co−O(1w) 2 3

1.891(3) 1.891(3) 1.982(3) 1.897(3) 1.964(4) 2.108(3) 1.900(4) 1.942(4) 1.982(4) 1.886(3) 1.960(5) 2.162(3)

Atom O(1)′ in 1 at −x + 1, −y, −z + 1.

Figure 2. Molecular structures of the complex cations of 2 and 3.

Figure 3. View of complexes 2 and 3 showing the different conformations of the metallacycle and the position of the coordinating methylene group with respect to the mean plane through the aromatic ring.

in the [CoIII(4,4′,7,7′tmsalenCH2)(py)(H2O)]+ derivative,8 where the cited torsion angle is 87.8°.


Organometallics

Article

In [CoIII(4,4′,7,7′tmsalenCH2)(py)(H2O)]+ we observed8 that the delocalization in the methoxy moiety is strongly reduced with respect to the salicylaldiminate group, mainly evidenced by the differences detected in the NC−C groups. Here these differences in bond lengths are not meaningful, the values being comparable within their esd’s. However, in complexes 2 and 3 significant variations are detected in bond angles about the phenolate carbon atom C(16) in addition to a widening of the C−O−C angle (from 112.2(4) to 121.5(3)°) and a small elongation of the C(Ph)−O bond length observed in the 7,7′ molecular cation (Table 4). In both cases, the pyridine ring forms a π-stacking interaction with the salen cyclized ring (centroid to centroid distance of ca. 3.64 Å).

where the axial CH2 group is placed out of plane of the aromatic ring. These complexes were built by starting from the geometry of 3. In the former set, where the axial CH2 group is in plane with the aromatic ring, changes of one methyl group position from 4′ to 7′ (4,7′ derivative) or of another methyl from 4 to 7 (7,4′ derivative) are energetically unfavorable (11.50 and 16.61 kJ mol−1, respectively) (first row of Table 5). The change of placement for both methyl groups (7,7′ derivative) gives the even higher value of 22.66 kJ mol−1. However, if the change of methyl group position from 4 to 7 (7,4′ derivative) is accompanied by the displacement of the CH2 group out of plane, a stabilization of about 8 kJ mol−1 occurs (second and third row of Table 5) in comparison to the complex with the axial CH2 group placed in plane with the aromatic ring. Similar stabilization could be found for the 7,7′ derivative. Thus, the difference in standard Gibbs energies is the result of the energetically unfavorable steric hindrance with methyl groups in positions 7 and 7′ that causes the rotation of both aromatic rings followed by the energetically favorable displacement of the axial CH2 group out of plane and toward the cobalt. Electrochemical Studies. Cyclic voltammetry was carried out on the [CoII(chel)] and [CoIII(chel)(py)2]ClO4 complexes, whose redox processes are not complicated by the coupled cleavage of the Co−C bond, which occurs for the corresponding organometallic complexes.15 The CV measurements were performed in DMSO + 0.1 M TEAP at 25.0 °C on a glassy-carbon (GC) electrode; the CV signals are very similar for all of the examined compounds and change only in peak potential values (Epc and Epa). The cyclic voltammograms of the [CoII(chel)] complexes in the range from +0.5 to −2.0 V vs SCE at 50 mV/s scan rate show two characteristic anodic/cathodic signal couples. The more negative of these is attributed to the CoII/CoI process (Figure 4), while the more positive couple is assigned to the CoIII/CoII process. In any case, the Epa − Epc value is in agreement with a quasi-reversible monoelectronic redox process.16 The species CoI, CoII, and CoIII, are all stable on the CV time scale under these experimental conditions, as was proved by the fact that the peak currents ipa and ipc are practically the same for all four peaks. For [CoIII(chel)(py)2]ClO4 complexes, a scan in negative direction from +0.5 to −2.0 V shows the presence of some partially overlapped cathodic peaks between −0.1 and −0.5 V, followed by a higher cathodic peak at more negative potentials. The overlapped peaks at more positive potentials are attributed to the CoIII/CoII reduction process, the [CoIII(chel)(py)2]+ complex being in equilibrium with other species in which either one or both of the py ligands are replaced by the solvent. In fact, for all of the complexes, the more positive of these overlapped reduction peaks occurs at the same value observed in the CV of the corresponding authentic [CoII(chel)] species

Table 4. Geometrical Distortions (Distances in Å and Angles in deg) Observed in the Ph−O−CH2 Fragment of 2 and 3 upon the out-of-Plane Displacement of CH2 in the Latter C(16)−O(2) C(10)−C(11) C(17)−O(2)−C(16) O(2)−C(16)−C(11) O(2)−C(16)−C(15) C(11)−C(16)−C(15) C(17)−O(2)−C(16)−C(11) N(2)−C(10)−C(11)−C(16)

2, R = H

3, R = Me

1.369(5) 1.455(6) 112.2(4) 128.0(4) 113.2(4) 118.8(4) −3.4(6) −34.2(7)

1.392(6) 1.492(7) 121.5(5) 119.3(5) 119.8(5) 120.8(5) 90.1(6) 49.3(8)

Quantum Chemical Calculations. Calculations of standard Gibbs energies reveal that complex 2 is 16.9 kJ mol−1 higher in energy than complex 3 (Table 5). Apart from placement of the methyl groups, the most notable feature in these two complexes is the different position of the axial CH2 group relative to the plane of the aromatic ring (Figure 3). In 2, the carbon atom of the methylene group is coplanar with the aromatic ring, whereas in 3 it is significantly displaced from it (Figure 3). This conformational change appears to be induced by the presence of the methyl groups in positions 7,7′. In order to maintain the coordination to cobalt, the CH2 group is shifted out of plane of the aromatic ring involved in the metallacycle. This conformation is accompanied by the phenyl ring rotation of ca. 15° (Figure 3). To investigate the reasons that cause the difference in standard Gibbs energies, we studied the geometries of six additional complexes with different positions of methyl groups and the axial CH2 group placed in two limiting conformations: out of plane and in plane (Figure 3 and Table 5). Starting from the geometry of complex 2, where the axial CH2 group is in plane with the aromatic ring, the corresponding derivatives with chel = 7,4′dmsalen, 4,7′dmsalen, 7,7′dmsalen were built and optimized. Additionally, geometry optimizations were carried out for 4,4′dmsalen, 7,4′dmsalen, and 4,7′dmsalen derivatives

Table 5. Standard Gibbs Energies (ΔrG°, kJ mol−1) at 298.15 K and 1 atm for CoIII(dmsalen) complexes Calculated for the inPlane (ip) and the out-of-Plane (oop) Positions of the Axial CH2 Group Relative to the Plane of the Aromatic Ringa ΔrG°4,4′ ip oop ΔrGθoop − ΔrGθip

b

0.00 9.57 9.57

ΔrG°7,4′

ΔrG°4,7′

ΔrG°4,7′ − ΔrG°7,4′

ΔrG°7,7′

ΔrG°7,7′ − ΔrG°7,4′

ΔrG°7,7′ − ΔrG°4,7′

11.50 21.42 9.92

16.61 8.33 −8.28

5.11 −13.09

22.66 16.90b −5.76

11.16 −4.52

6.05 8.57

a

M06/def2-TZVP level of theory, solvent effects incorporated using the SMD method. The positions of the methyls in the chelate are indicated by subscripts. bGeometry optimization starting from the crystal structure. 913


Organometallics

Article

[CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)] (Figure 5), with absorbance vs time traces showing a good first-order exponential

Figure 4. Cyclic voltammetry of [CoIII(3,3′,7,7′tmsalen)(py)2]ClO4 (black line) and of [CoII(3,3′,7,7′tmsalen)] (red line). Experiments were performed in DMSO + 0.1 M TEAP as supporting electrolyte on a glassy-carbon electrode at a 50 mV/s scan rate at 25 °C. The concentration of the complexes was about 1 mM.

Figure 5. Spectral changes with time of an approximately 2 × 10−4 M solution of [CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)] in 60% methanol− 40% water (v/v, I = 0.1 M NaClO4) at 25 °C. Spectra were recorded at time intervals of 30 min.

(Figure 4). The well-defined peaks at more negative potentials are attributed to the CoII/CoI process. The presence of electron-donating methyl groups in the equatorial chelate shifts the redox potentials of both the CoIII/ CoII and CoII/CoI electron transfer processes toward more negative values. The effect is clearly evident in the CoII/CoI processes of both [CoIII(chel)(py) 2]+ and [CoII(chel)] compounds and in the CoIII/CoII reduction signals of [CoII(chel)]. A similar trend can be detected for the CoIII/ CoII reduction peaks of [CoIII(chel)(py)2]+, although these signals are quite broad (see above). A comparison of the E1/2 values (Table 6) with that obtained for the CoII/CoI reduction of unsubstituted [CoII(salen)] (−1.218 V)15 shows that the insertion of methyls in positions 3,3′ or 4,4′ shifts the potentials of 0.045 and 0.022 V, respectively, toward more negative values. Substitution at positions 7,7′ has a stronger effect (0.120 V). For the tetrasubstituted [CoII(3,3′,7,7′tmsalen)] complex (Table 6) the shift of the redox potentials toward more negative values is further enhanced. The already reported E1/2 value for the CoIII/CoII redox process of [CoII(4,4′,7,7′tmsalen)] (−0.083 V)15 does not agree with this trend: the result was confirmed by a repeated experiment and could not be rationalized. Kinetics and Mechanism. When time-resolved UV−vis spectra of the [CoIII(chel)(CH2Cl)(S)] complexes were recorded in 60% methanol−40% water (v/v, I = 0.1 M NaClO4) in the temperature range 20−40 °C, a well-defined process was observed for [CoIII(7,7′dmsalen)(CH2Cl)(S)] and

behavior (Figure S3, Supporting Information). For [CoIII(3,3′dmsalen)(CH2Cl)(S)] and [CoIII(4,4′dmsalen)(CH2Cl)S], a further slow process becomes evident before the conclusion of the cyclization reaction at elevated temperatures (Figure S4, Supporting Information). It had been previously observed that the product of cyclization of [CoIII(4,4′,7,7′tmsalen)(CH2Cl)(S)], which presumably contains two solvent molecules as unidentate ligands, is not stable in methanolic solution and further reactions occur after the completion of cyclization.8 The second process is attributed to the decomposition of the cyclized species in the absence of a nitrogen base. The present results suggest that the cyclization/decomposition processes tend to overlap for complexes lacking the methyls in positions 7,7′. Despite this complication, it was possible to perform a kinetic study of the first process by carrying out the reactions at wavelengths for which the changes in absorbance related to the second reaction were very small. Time-resolved UV−vis spectra of the [CoIII(chel)(CH2Cl)(S)] complexes under the same experimental conditions but in the presence of an excess of py show two processes for all of the complexes. The processes are well separated for [CoIII(7,7′dmsalen)(CH2Cl)(S)] and [CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)] at any concentration and temperature but again tend to overlap for [CoIII(3,3′dmsalen)(CH2Cl)(S)] and [CoIII(4,4′dmsalen)(CH2Cl)(S)] at high pyridine concentrations and elevated temperatures (Figure S5, Supporting

Table 6. Redox Potentials (V vs SCE) in DMSO + TEAP 0.1 M Solution for [CoII(chel)] on a GC Electrode at 25 °C

a

complex

Epc(II/I)

Epa(I/II)

E1/2a

Epc(III/II)

Epa(II/III)

E1/2a

[CoII(3,3′dmsalen)] [CoII(4,4′dmsalen)] [CoII(7,7′dmsalen)] [CoII(3,3′7,7′tmsalen)]

−1.312 −1.270 −1.386 −1.525

−1.213 −1.210 −1.290 −1.379

−1.263 −1.240 −1.338 −1.452

−0.162 −0.140 −0.180 −0.228

−0.031 −0.045 −0.030 −0.060

−0.097 −0.093 −0.105 −0.144

E1/2 = (Epc + Epa)/2. 914


Organometallics

Article

Information). The problem was circumvented as described above. As previously found for [CoIII(4,4,′7,7′tmsalen)(CH2Cl)(S)],13 the plots of kobs vs [L] show a nonzero intercept and a significant curvature at high pyridine concentrations (Figure 6). Table 7 gives kinetic rate constants and activation parameters for the cyclization reaction.

Scheme 2

This scheme involves the substitution of the solvent by py in a fast pre-equilibrium step followed by the cyclization of both [Co(chel)(CH2Cl)(S)] and [Co(chel)(CH2Cl)(py)] at different rates. The corresponding rate law is given in eq 1, where K kobs =

k1 + k 2K[py] 1 + K[py]

(1)

represents the equilibrium constant for the substitution of the solvent by py in the axial position and k1 and k2 are the cyclization rate constants of [Co(chel)(CH2Cl)(S)] and [Co(chel)(CH2Cl)(py)], respectively. The value of k1 has been independently determined by carrying out the cyclization reaction in the absence of pyridine (Table 7). Preliminary estimates of k2 and K have been obtained from the linear plots

Figure 6. Plots of the pseudo-first-order rate constant, kobs, versus [py] for the cyclization of [CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)] in 60% methanol−40% water (v/v, I = 0.1 M NaClO4) at various temperatures. The solid lines show the fit based on eq 1 of the text with the parameters reported in Table 7.

The results have been interpreted as indicated in Scheme 2.

Table 7. Kinetic Rate Constants and Activation Parameters for the Cyclization Reaction in 60% Methanol−40% Water (v/v, I = 0.1 M NaClO4) T, K

105k1, s−1

104k2, s−1

[Co (3,3′dmsalen)(CH2Cl)(S)]

293.1 298.1 303.1 308.1 313.1

[CoIII(4,4′dmsalen)(CH2Cl)(S)]

293.1 298.1 303.1 308.1 313.1

[CoIII(7,7′dmsalen)(CH2Cl)(S)]

293.1 298.1 303.1 308.1 313.1

[CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)]

293.1 298.1 303.1 308.1 313.1

1.73 ± 0.01 3.78 ± 0.02 7.30 ± 0.01 12.5 ± 0.1 20.7 ± 0.2 ΔH1* = 92 ± 4 kJ mol−1 ΔS1* = −23 ± 13 J mol−1K−1 3.59 ± 0.01 6.96 ± 0.01 12.1 ± 0.1 20.4 ± 0.2 41.8 ± 0.4 ΔH1* = 67 ± 7 kJ mol−1 ΔS1* = −99 ± 22 J mol−1K−1 4.77 ± 0.01 8.39 ± 0.01 a 15.1 ± 0.1 27.9 ± 0.1 45.6 ± 0.1 ΔH1* = 85 ± 1 kJ mol−1 ΔS1* = −39 ± 5 J mol−1K−1 6.86 ± 0.01 16.4 ± 0.1 22.3 ± 0.1 50.4 ± 0.1 63.8 ± 0.1 ΔH1* = 83 ± 9 kJ mol−1 ΔS1* = −41 ± 29 J mol−1K−1

1.7 ± 0.1 3.3 ± 0.4 6.0 ± 0.2 10.33 ± 0.5 16.7 ± 0.7 ΔH2* = 85 ± 2 kJ mol−1 ΔS2* = −27 ± 7 J mol−1 K−1 2.7 ± 0.4 4.3 ± 0.1 8.4 ± 0.8 14.5 ± 1 24.1 ± 2 ΔH2* = 64 ± 5 kJ mol−1 ΔS2* = −96 ± 17 J mol−1 K−1 2.5 ± 0.2 5.6 ± 0.8 a 8.2 ± 0.5 15.2 ± 0.2 27.8 ± 0.8 ΔH2* = 86 ± 4 kJ mol−1 ΔS2* = −21 ± 14 J mol−1 K−1 4.4 ± 0.3 8.1 ± 0.1 13.5 ± 1.2 27.5 ± 0.7 36 ± 2 ΔH2* = 78 ± 6 kJ mol−1 ΔS2* = −42 ± 21 J mol−1 K−1

complex III

915

K, M−1 85 33 32 26 27

± ± ± ± ±

4 3 1 1 1

35 31.5 22 22 13

± ± ± ± ±

4 0.8 3 1 2

62 25 35 26.7 14.0

± ± ± ± ±

4 5a 2 0.2 0.4

46 59.2 38 12.9 27

± ± ± ± ±

2 0.7 3 0.4 1


Organometallics

Article

of 1/(kobs − k1) versus 1/[py]. The final values were derived from a nonlinear fit of kobs versus [py], and the activation parameters calculated according to the Eyring equation (eq 2) are summarized in Table 7. ln

ki ΔHi* ΔSi* κ = ln − + T h RT R

circumvent the problem, owing to the scarce solubility of the starting products at low temperatures and the fast formation of ylides at high temperatures (see above). In the proposed reaction mechanism, the cyclization is a formally unimolecular reaction,19 which should have ΔS* near zero or positive, if the activation process involves bond breaking. The considerably negative values of the activation entropy may stem from the loss of conformational freedom on going from the ground state to an almost cyclic transition state. A second contribution may arise from the change in solvation, as the cyclization involves the formation of ions from a neutral complex. If the activated complex may be described almost as an ion pair or a polar complex approaching an ion pair, a large negative value of the activation entropy due to the freezing of the solvent around the incipient ions should be expected. The X-ray structures of the cyclized complexes indicate that the conformation of the seven-membered metallacycle shows a dependence on the positions of the methyl substituents in the chelate. In fact, the conformation assumed by the Ph−O− CH2−Co fragment within the seven-membered metallacycle ring in complexes having methyl substituents on the −CN− groups is different from that of the complexes lacking methyls at these positions. In the [CoIII(4,4′dmsalenCH2)(py)(H2O)]+ complex, the carbon atom of the methylene group lies in the plane of the aromatic ring, whereas in the [CoIII(7,7′dmsalenCH2)(py)(H2O)]+ complex this carbon atom is significantly displaced from the phenol ring plane, as previously found in the [CoIII(4,4′,7,7′dmsalenCH2)(py)(H2O)]+ complex.8 Quantum chemical calculations reveal that the standard Gibbs energy for the optimized structure of the 7,7′-methylsubstituted derivative is 16.9 kJ mol−1 greater than that of the 4,4′ species (Table 5). This appears to agree with the largely preferred coplanar conformation (also in the presence of an ortho substituent) of methoxyphenyl groups observed in crystal structure data.20 In addition, the ΔG° values of variously methyl substituted salen species indicate that the difference in conformation is a balanced effect between the energetically unfavorable state represented by methyl groups in positions 7 and 7′ and the energetically favorable out-of-plane conformational displacement of the CH2 group from the aromatic ring. The computations corroborate the conformational features resulting from the X-ray structural analyses, indicating that the conformation of 3 is likely induced by steric hindrance of the methyl groups in positions 7,7′. In order to allow the coordination to the metal, the out-of-plane coordinating methylene in 3 implicates significant geometrical distortions in the geometry of the Ph−O−CH2−Co fragment in comparison to complex 2. It is worth noting that the 1H NMR spectra of the complexes lacking methyls on the −CN− groups show a considerable upfield shift of one of the two diastereotopic protons bonded to the axial methylene group. This effect can be ascribed to the different conformations of this group in 2 and 3, which cause different orientations of the protons in the magnetic anisotropy cones of aromatic rings of the complexes. Thus, these data suggest that the different conformations of the metallacycle are, at least partially, retained in solution by 2 and 3, and presumably a similar behavior can be ascribed to [CoIII(3,3′dmsalenCH2)(py)(H2O)]+ and [CoIII(3,3′,7,7′tmsalenCH2)(py)(H2O)]+, whose X-ray structures are not available.

(2)

■

DISCUSSION The trans-[CoIII(chel)(CH2Cl)]2 complexes, which are dimers in the solid state and presumably monomers in solution with a solvent molecule in the sixth coordination position,8,14 form cis β organometallic derivatives containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl ligand by the salen phenolate oxygen. The syntheses of the complexes [Co III(7,7′dmsalenCH 2 )(py)(H 2O)]ClO 4 and [CoIII(3,3′,7,7′tmsalenCH2)(py)(H2O)]ClO4 were carried out by the procedure already described for [CoIII(4,4′,7,7′tmsalenCH2)(py)(H2O)]ClO4.8 This method was revealed to be useless for complexes whose chelates lack methyls in positions 7,7′, leading to intractable mixtures of products. Therefore, [CoIII(3,3′dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 were synthesized from the corresponding trans chloromethyl derivatives in the presence of a large excess of pyridine, which was demonstrated to accelerate the cyclization rate.13 Under these experimental conditions, the formation of the rather uncommon nonstabilized pyridinium ylide species [CoIII(chel)(CH2py)(Cl)]17 is observed in addition to the desired products. The proposed metallacyclization mechanism involves the internal nucleophilic attack of the phenolate oxygen on the axial chloromethyl group with the detachment of a chloride ion (Scheme 2). From the data of Table 7 it appears that the cyclization rate of [Co(chel)(CH2Cl)(py)] is about 1 order of magnitude greater than that of [Co(chel)(CH2Cl)(S)]. It has been previously shown for [CoIII(4,4′,7,7′tmsalen)(CH2Cl)(S)] that the k2 values increase remarkably with the pKa of the nitrogen base.13 The data of Table 7 show that the cyclization rate depends on the number of the methyl substituents in the chelate: the values of k1 and k2 for 3,3′,7,7′tmsalen and 4,4′,7,7′tmsalen13 derivatives are almost double those of the dmsalen complexes. This result is in agreement with the electrochemical data that evidence a general shift of the E1/2 potentials toward more negative values with an increase in the number of methyl sustituents. The cyclization rate is also affected by the substituent position: in the dmsalen derivatives it increases in the order 3,3′ < 4,4′ < 7,7′. The reactivity order does not match exactly the trend of redox potentials, but both the electrochemical and the kinetic data evidence a remarkable influence of methyls at the 7,7′ positions. The above results suggest that the substituent effect on the intramolecular cyclization rate is inductive in origin and that the rate enhancement is due to the increased electron density on the equatorial chelate, which makes the oxygen atoms more nucleophilic. The activation entropies do not show a clear trend with the position and the number of methyl substituents (Table 7); the lack of correlation may arise from the inaccuracy of obtaining ΔS* by a long extrapolation of Eyring plots to 1/T = 0.18 Unfortunately, we could not use a wider temperature range to 916


Organometallics

Article

Table 8. Elemental Analyses and Synthetic Yields for Chelates and [CoII(chel)] and [CoIII(chel)(py)2]ClO4 Complexes calcd (found) (%)

■

compound

formula

C

H

N

yield (%)

3,3′dmsalen 4,4′dmsalen 3,3′,7,7′tmsalen [CoII(3,3′dmsalen)] [CoII(4,4′dmsalen)] [CoII(7,7′dmsalen)] [CoII(3,3′,7,7′tmsalen)] [CoIII(3,3′dmsalen)(py)2]ClO4] [CoIII(4,4′dmsalen)(py)2]ClO4] [CoIII(3,3′,7,7′tmsalen)(py)2]ClO4]

C18H20N2O2 C18H20N2O2 C20H24N2O2 C18H18CoN2O2 C18H18CoN2O2 C18H18CoN2O2 C20H22CoN2O2 C28H28CoN4O6Cl C28H28CoN4O6Cl C30H32CoN4O6Cl

72.9 (72.8) 72.9 (72.3) 74.0 (73.4) 61.2 (60.6) 61.2 (60.5) 61.2 (59.8) 63.0 (61.4) 55.1(54.5) 55.1(54.5) 56.4(56.0)

6.80 (7.15) 6.80 (6.75) 7.46 (8.08) 5.13 (5.53) 5.13 (5.86) 5.13 (6.27) 5.81 (7.43) 4.62(4.92) 4.62(4.71) 5.05(5.25)

9.45 (9.59) 9.45 (9.60) 8.59 (8.73) 7.93 (7.80) 7.93 (7.88) 7.93 (7.77) 7.35 (7.18) 9.17(9.05) 9.17(8.95) 8.77(8.56)

89.1 92.5 90.2 76.7 67.1 73.2 78.2 98.0 97.1 97.3

mmol) dissolved in methanol (200 mL). The reaction mixture was warmed to 50 °C for 2 h and then allowed to stand overnight. The yellow solid was collected by filtration and dried. General Procedure for Synthesis of [CoII(chel)] Complexes. A suspension of 2 mmol of the ligand in 120 mL of CH3OH was deaerated with nitrogen several times. A slight excess of Co(CH3COO)2·4H2O (0.5 g, 2.0 mmol) and one pellet of NaOH were added under a nitrogen atmosphere, and the stirring was continued for 1 h at 35 °C. The orange solid was collected by filtration under nitrogen, washed with methanol, and dried. General Procedure for Synthesis of [CoIII(chel)(py)2]ClO4 Complexes. To a solution of 0.5 mmol of the ligand suspended in methanol (200 mL) were added a slight excess of Co(CH3COO)2· 4H2O (1.4 g, 5.6 mmol) and then 10 mL of pyridine. Air was bubbled in the solution with stirring for 3 h. An aqueous solution of NaClO4 was added and the brown product collected by filtration and washed with water. Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. General Procedure for Synthesis of [CoIII(chel)(CH2Cl)]2 Complexes. As the complexes are light sensitive, all manipulations were performed in the dark. A methanol solution of [CoIII(chel)(py)2]ClO4 (1.4 mmol in 80 mL of CH3OH) containing one pellet of NaOH was thoroughly dearated by several vacuum/nitrogen cycles. A solution of NaBH4 (0.060 g, 1.59 mmol) in 1 mL of water was added to the suspension, followed by addition of a few drops of an aqueous 10% PdCl2 solution. A very fast reduction occurred, evidenced by the instantaneous change of color from light brown to red. A slight excess of CH2ClI (0.25 mL, 3.43 mmol) was then added, and the stirring under nitrogen was continued for 1.30 h. After filtration, 100 mL of water was added to the solution drop by drop and the precipitate was collected by filtration, washed with water, and dried in air. X-rayquality crystals of [CoIII(4,4′dmsalen)(CH2Cl)]2 (1) were obtained by slow evaporation from a saturated methanolic solution at 4 °C. [CoIII(7,7′-dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(3,3′,7,7′tmsalenCH2)(py)(H2O)]ClO4. A methanol solution of the appropriate [CoIII(chel)(py)2]ClO4 (0.82 mmol in 80 mL of CH3OH) containing one pellet of NaOH was thoroughly deaerated by several vacuum/ nitrogen cycles. A solution of NaBH4 (0.060 g, 1.59 mmol) in 1 mL of water was added to the suspension, followed by addition of a few drops of an aqueous 10% PdCl2 solution. A very fast reduction occurred, evidenced by the instantaneous change of color from light brown to red. A slight excess of CH2ClI (0.25 mL, 3.43 mmol) was then added, and the stirring under nitrogen was continued for 1.30 h. The solution was poured in a flask and allowed to stand for 5 days in

EXPERIMENTAL SECTION

General Methods. The synthesis and characterization of the Co complexes with chel = 4,4′,7,7′tmsalen have already been reported by

Table 9. Yields and ESI-MS Data (MeOH) for [CoIII(chel)(CH2Cl)]2 and [CoIII(chelCH2)(py)(H2O)]+ Complexes ESI-MS for [CoIII(chel)(CH2Cl)]+ (m/z) complex III

[Co (3,3′dmsalen) (CH2Cl)]2 [CoIII(4,4′dmsalen) (CH2Cl)]2 [CoIII(3,3′,7,7′tmsalen) (CH2Cl)] 2

yield (%)

calcd

found +

27.0

402.8

53.9

402.8

402.0 (+H , 3%), 425.0 (+Na+, 8%) 402.1 (+H+, 17%)

74.2

430.8

453.0 (+Na+, 6%) ESI-MS for [CoIII(chelCH2) (py)(H2O)]+ (m/z)

complex III

[Co (3,3′dmsalenCH2)(py) (H2O)]Cl [CoIII(3,3′dmsalenCH2)(py) (H2O)]ClO4 [CoIII(4,4′dmsalenCH2)(py) (H2O)]Cl [CoIII(4,4′dmsalenCH2)(py) (H2O)]ClO4 [CoIII(3,3′,7,7′tmsalenCH2)(py) (H2O)]ClO4

yield (%)

calcd

found

35.4

464.1

367.1 (−py, 100%)

73.6

464.1

367.1 (−py, 100%)

51.5

464.1

367.1 (−py, 100%)

52.7

464.1

367.1 (−py, 100%)

62.0

492.2

395.1 (−py, 100%)

us,8 as well as [CoIII(7,7′dmsalen)(py)2]ClO4 and [CoIII(7,7′dmsalen)(CH2Cl)]2.21 All other reagents were analytical grade and were used without further purification. UV−vis spectra were monitored with an UVIKON 941 PLUS spectrophotometer. 1D and 2D NMR spectra were recorded on a JEOL EX-400 instrument (1H at 400 MHz and 13C at 100.4 MHz) with the solvent as internal reference. Electrospray mass spectra were recorded in positive mode by using an API 1 mass spectrometer (Perkin-Elmer). General Procedure for Synthesis of Ligands. A solution of ethylenediamine (0.6 g, 10 mmol) in methanol (50 mL) was mixed with the appropriately substituted acetophenone or benzaldehyde (20

Table 10. 1H NMR Data (CDCl3) for Chelates chel

CH3-Ph

3,3′dmsalen 4,4′dmsalen 3,3′,7,7′tmsalen

2.31 (s, 6H) 2.25 (s, 6H) 2.29 (s, 6H)

CH3-CN

CH2CH2

phenyl protons

C7-H

OH

6.66 (d, 2H), 6.74 (s, 2H), 7.10 (d, 2H) 6.84 (d, 2H), 7.00 (s, 2H), 7.09 (d, 2H) 6.58 (d, 2H), 6.71 (s, 2H), 7.38 (d, 6H)

8.30 8.29

2.34 (s, 6H)

3.89 (s, 4H) 3.92 (s, 4H) 3.94 (s, 4H)

13.21 (bs, 2H) 12.97 (s, 2H) 15.96 (s, 2H)

917


For all of the complexes, a second set of signals with lower intensity (integration ratio 0.1:1) is observed and is attributed to a complex in which one py is substituted by a solvent molecule.

2.19 (s, 6H)

2.82 (s, 6H)

4.12 (s, 4H)

6.53 (m, 2H, C3-H or C4-H), 7.19 (overlapped m, 4H, C3-H or C4-H; C2-H or C5H), 7.58 (d, 2H, C2-H or C5-H) 6.36 (d, 2H, C4-H), 7.03 (s, 2H, C2-H), 7.39 (d, 2H, C3-H) 2.86 (s, 6H)

4.16 (s, 4H)

7.05 (s, 2H, C5-H), 7.11 (s, 4H, C2-H and C3-H) 4.18 (s, 4H) 2.16 (s, 6H)

[Co (3,3′dmsalen) (py)2]ClO4 [CoIII(4,4′dmsalen) (py)2]ClO4 [CoIII(7,7′dmsalen) (py)2]ClO4 [CoIII(3,3′,7,7′tmsalen) (py)2]ClO4

the dark. The suspension was then filtered, 15 mL of water was added, and the solvent was removed under reduced pressure to give a red solid, which was collected by filtration and dried on P2O5. X-rayquality crystals of [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 (3) were obtained by slow evaporation of the solvent. [Co I I I (4,4′-dmsalenCH 2 )(py)(H 2 O)]Cl and [Co I I I (3,3′dmsalenCH2)(py)(H2O)]Cl. A 0.0947 g amount (0.118 mmol) of the corresponding [CoIII(chel)(CH2Cl)]2 was dissolved in 100 mL of methanol, and 5 mL of pyridine was added. The solution was warmed to 40 °C in the dark, and the progress of the reaction was monitored by TLC. After ca. 2 h the heating was stopped and the solvent was evaporated in vacuo. The mixture was separated by flash chromatography on a silica gel column using 3/1 CHCl3/C2H5OH as eluent. The first brown and the third green fractions were discarded. The red central fraction afforded the desired product after evaporation of the solvent in vacuo. [Co III (4,4′-dmsalenCH 2 )(py)(H 2 O)]ClO 4 and [Co III (3,3′dmsalenCH2)(py)(H2O)]ClO4. A solution of NaClO4·H2O (2.00 g, 1.42 × 10−2 mol) in 5 mL of MeOH was added to 90 mg (2.2 × 10−4 mol) of the corresponding [CoIII(chelCH2)(py)(H2O)]Cl dissolved in about 15 mL of MeOH. After addition of 3 mL of H2O the solution was set aside for partial evaporation and the precipitate was collected by filtration. X-ray-quality crystals of [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 (2) were obtained by slow evaporation of the solvent. Characterization of Compounds. Characterization data (elemental analysis, ESI MS mass spectra, and 1H NMR spectra) are collected in Tables 1, 2, and 8−11. Crystallographic Measurements. Data for complexes [CoIII(4,4′dmsalen)(CH2Cl)]2 (1) and [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 (3) were collected at room temperature on a Bruker rotating anode diffractometer equipped with a Kappa-CCD detector and graphite-monochromated Cu Kα radiation (λ = 1.54178 Å). Intensity data for [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 (2) were collected at the X-ray diffraction beamline (λ = 1.0000 Å, 100 K) of the Elettra synchrotron (Trieste, Italy). Data reductions were made by using the programs Denzo and Scalepack.22 All of the structures were solved by direct methods23 and refined with full-matrix least squares based on F2 with all observed reflections using the SHELXL program on F2 with all observed reflections.23 Hydrogen atoms were positioned geometrically and allowed to ride on their respective parent atom, with the exception of water molecules in [CoIII(4,4′dmsalenCH2)(py)(H2O)]ClO4 and [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 located on the difference Fourier map with O−H distances restrained at 0.85 Å. [CoIII(7,7′dmsalenCH2)(py)(H2O)]ClO4 revealed the presence of two chloroform molecules, one of which was refined with an occupancy of 0.5. The calculations were made using the WinGX System, vesion 1.80.05.24 Crystal data and details of refinements are given in Table S1 (Supporting Information), whereas selected bond distances and angles are given in Table S2 (Supporting Information). Hydrogen bond parameters are given in Table S3 (Supporting Information). Computational Methods. All quantum chemical calculations were performed using the Gaussian 09 program package (Table S4, Supporting Information).25 Geometry optimizations for ground states were performed using the hybrid functional M0626 and def2TZVP27−29 basis set.30,31 For all optimized structures harmonic frequencies were calculated to ensure that obtained geometries correspond to the local minimum on the potential energy surface. Solvation effects were incorporated in the calculations using the reformulation of polarizable continuum model (PCM)32,33 known as the integral equation formalism (IEFPCM) of Tomasi and coworkers34−38 with radii and nonelectrostatic terms for Truhlar and coworkers’ SMD solvation model.39 The standard Gibbs energies were calculated at T = 298.15 K and p = 1 atm. Cyclic Voltammetry Experiments. The electrochemical measurements were carried out in freshly distilled DMSO (Fluka) that was kept under molecular sieves in dark bottles. The supporting electrolyte was tetraethylammonium perchlorate (TEAP, 0.1 M; Fluka), which was used after recrystallization from hot water and vacuum drying at 30 °C. Voltammetric experiments were performed using an Amel 552

a

(m, 2H para),

8.27 (s, 2H) (m, 2H para),

(m, 2H para),

8.26 (s, 2H) (m, 2H para),

Article

7.31 (m, 4H, meta), 7.81 8.14 (m, 4H, ortho) 7.30 (m, 4H, meta), 7.81 8.15 (m, 4H, ortho) 7.26 (m, 4H, meta), 7.78 8.18 (m, 4H, ortho) 7.26 (m, 4H, meta), 7.77 8.15 (m, 4H, ortho) 6.45 (d, 2H, C4-H), 7.05 (s, 2H, C2-H), 7.16 (d, 2H, C3-H) 4.16 (s, 4H) 2.24 (s, 6H)

CH2CH2 complex

CH3-Ph

CH3-CN III

Table 11. 1H NMR Data for [CoIII(chel)(py)2]ClO4 Complexes in CD3ODa

phenyl protons

py

C7-H

Organometallics

918


Organometallics

Article

(b) Carroll, K. M.; Schwartz, J.; Ho, D. M. Inorg. Chem. 1994, 33, 2707−2708. (c) Calligaris, M.; Manzini, G.; Nardin, G.; Randaccio, L. J. Chem. Soc., Dalton Trans. 1972, 543−547. (d) Kushi, Y.; Tada, T.; Fujii, Y.; Yoneda, H. Bull. Chem. Soc. Jpn. 1982, 55, 1834−1839. (4) (a) Yamanouchi, K.; Yamada, S. Inorg. Chim. Acta 1974, 9, 161− 164. (b) Cilla, A. C.; Goghi, L.; Manfredotti, A. G.; Guastini, G. Cryst. Struct. Commun. 1974, 3, 551−554. (c) Thornback, J. R.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1978, 110−115. (d) Motevalli, M.; Parkin, B.; Ramnauth, R.; Sullivan, A. Dalton Trans. 2000, 2661−2662. (e) Xu, Z.-J.; Fang, R.; Zhao, C.; Huang, J.-S.; Li, G.-Y.; Zhu, N.; Che, M.-C. J. Am. Chem. Soc. 2009, 121, 4405−4417. (5) (a) Tisato, F.; Refosco, F.; Mazzi, U.; Bandoli, G.; Dolmella, A.; Nicolini, M. Inorg. Chim. Acta 1991, 189, 97−103. (b) Gatehouse, B. M.; Reichert, B. E.; West, B. Acta Crystallogr., Sect. B 1976, 32, 30−34. (c) Chattopadhyay, S.; Drew, M. G. B.; Ghosh, A. Eur. J. Inorg. Chem. 2008, 1693−1701. (6) (a) Belokon, Y. N.; Caveda-Cepas, S.; Green, B.; Ikonnikov, N. S.; Khrustalev, V. N.; Larichev, V. S.; Moscalenko, M. A.; North, M.; Orizu, C.; Tararov, V. I.; Tasinazzo, M.; Timofeeva, G. I.; Yashkina, L. V. J. Am. Chem. Soc. 1999, 121, 3968−3973. (b) Belokon, Y. N.; Blacker, A. J.; Carta, P.; Clutterbuck, L. A.; North, M. Tetrahedron 2004, 60, 10433−10447. (7) Corden, J. P.; Errington, W.; Moore, P.; Wallbridge, M. G. H. Chem. Commun. 1999, 323−324. (8) Dreos, R.; Nardin, G.; Randaccio, L.; Siega, P.; Tauzher, G.; Vrdoljak, V. Inorg. Chem. 2003, 42, 6805−6811. (9) (a) Polson, S. M.; Hansen, L.; Marzilli, L. G. J. Am. Chem. Soc. 1996, 118, 4804−4808. (b) Marzilli, L. G.; Polson, S. M.; Hansen, L.; Moore, S. J.; Marzilli, P. Inorg. Chem. 1997, 36, 3854−3860. (10) (a) Dreos, R.; Felluga, A.; Nardin, G.; Randaccio, L.; Siega, P.; Tauzher, G. Inorg. Chem. 2001, 40, 5541−5546. (b) Dreos, R.; Felluga, A.; Nardin, G.; Randaccio, L.; Tauzher, G. Organometallics 2003, 22, 2486−2491. (11) Dreos, R.; Randaccio, L.; Siega, P.; Vrdoljak, V. Croat. Chem. Acta 2009, 82, 455−461. (12) Dreos, R.; Mechi, L.; Randaccio, L.; Siega, P.; Zangrando, E.; Ben Hassen, R. J. Organomet. Chem. 2006, 691, 3305−3309. (13) Dreos, R.; Siega, P. Organometallics 2006, 25, 5180−5183. (14) Dreos, R.; Nardin, G.; Randaccio, L.; Siega, P.; Tauzher, G.; Vrdoljak, V. Inorg. Chim. Acta 2003, 349, 239−248. (15) Siega, P.; Vrdoljak, V.; Tavagnacco, C.; Dreos, R. Inorg. Chim. Acta 2012, 387, 93−99 and references therein. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (17) (a) Saito, T.; Urabe, H.; Sasaki, Y. Transition Met. Chem. 1980, 5, 35−9. (b) Saito, T. Bull. Chem. Soc. Jpn. 1978, 51, 169−73. (18) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition Metal Complexes; VCH: Weinheim, Germany, 1991; p 89. (19) de La Mare, P. B. D.; Swedlun, B. E. In The Chemistry of the Carbon-Halogen Bond; Patai, S., Ed.; Wiley: Bristol, U.K., 1973; Chapter 7. (20) Hummel, W.; Huml, K.; Burgi, H.-B. Helv. Chim. Acta 1988, 71, 1291−1302. (21) Mechi, L.; Siega, P.; Dreos, R.; Zangrando, E.; Randaccio, L. Eur. J. Inorg. Chem. 2009, 2629−2638. (22) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data Collected in Oscillation Mode. In Methods in Enzymology; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276, pp 307−326. (23) Sheldrick, G. M. SHELX97 Programs for Crystal Structure Analysis (Release 97-2); University of Göttingen: Göttingen, Germany, 1998. (24) Farrugia, J. J. Appl. Crystallogr. 1999, 32, 837−838. (25) Frisch, M. J. G.; Trucks, W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;

potentiostat/galvanostat connected with an Amel 568 function generator and equipped with a three-electrode thermostated jacked cell at 25 ± 0.1 °C, under a stream of purified N2 to avoid O2 reduction signals. Many electrodes as Au, Pt, and glassy-carbon (GC) discs were tried as working electrodes, but a 2 mm GC disc (EG&G) was finally chosen because it showed the best signal/noise ratio. Prior to each experiment the electrode was micropolished, using successively 1 and 0.05 μm γ-alumina (Buehler) on a polishing cloth, and was then sonicated three times for 5 min in DMSO. Finally, an electrochemical cleaning was performed, which consisted of repeated potential scans in DMSO between +0.7 and −2.0 V at a 50 mV/s scan rate. All of the potentials were measured and quoted with respect to a Hg|Hg2Cl2, NaCl saturated electrode (SCE) contained in a glass tube filled with the supporting electrolyte solution, separated from the solution by a Vycor frit and located close to the tip of the working electrode to minimize the ohmic drop. The counter electrode was a Pt ring or rod directly dipped in the solution. The complex concentration was about 1 mM. Kinetic Studies. The cyclization kinetics were followed spectrophotometrically in the mixed solvent 60% methanol−40% H2O (v/ v, I = 0.1 M NaClO4). Generally, (2−4) × 10−4 M solutions of complex were used. As the complexes did not dissolve completely, the suspension was filtered before the addition of pyridine. The ligand concentration was in large excess (pseudo-first-order conditions). Observed rate constants were obtained from the linear plots of ln(A − A∞), where A is the absorbance at the time t and A∞ is the final absorbance, versus time. Kinetic data were analyzed with ORIGIN, version 6.

■

ASSOCIATED CONTENT

S Supporting Information *

CIF files giving X-ray data for compounds 1−3, Schemes S1 and S2, Figures S1−5, giving 1H NMR spectra in CD3OD of the complexes [CoIII(chel)(CH2Cl)(S)], 1H NMR spectra in CD3OD of complexes [CoIII(chelCH2)(py)(S)]+, a plot of firstorder kinetic data for the cyclization reaction of [CoIII(3,3′,7,7′tmsalen)(CH2Cl)(S)], a plot of absorbance vs time for the cyclization reaction of [CoIII(4,4′dmsalen)(CH2Cl)(S)], and a plot of absorbance vs time for the cyclization reaction of [CoIII(4,4′dmsalen)(CH2Cl)(S)] in the presence of 0.25 M py, and Tables S1−S4, giving crystallographic data and details of refinement for complexes 1−3, complete bond distances and angles, H bond parameters for complexes 2 and 3, and optimized geometries and electronic energies (Eel) for CoIII(dmsalen) complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for R.D.: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) For selected reviews on this subject see: (a) Canali, L.; Sherrington, D. C. Chem. Soc. Rev. 1999, 28, 85−93. (b) Cozzi, P. G. Chem. Soc. Rev. 2004, 33, 410−421. (c) Katsuki, T. Coord. Chem. Rev. 1995, 140, 189−214. (d) Katsuki, T. Chem. Soc. Rev. 2004, 33, 437− 444. (e) Wezenberg, S. J.; Kleij, A. W. Angew. Chem., Int. Ed. 2008, 47, 2354−2364. (f) Kleij, A. W. Chem. Eur. J. 2008, 14, 10520−10529. (g) Kleij, A. W. Eur. J. Inorg. Chem. 2009, 193−205. (h) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421−431. (2) Matsumoto, K.; Saito, B.; Katsuki, T. Chem. Commun. 2007, 3619−3627. (3) (a) Cyriac, A.; Jeon, J. Y.; Varghese, J. K.; Park, J. H.; Choi, S. Y.; Chung, Y. K.; Lee, B. Y. Dalton Trans. 2012, 41, 1444−1447. 919


Organometallics

Article

Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (26) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (27) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (28) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (29) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (30) Feller, D. J. Comput. Chem. 1996, 17, 1571−1586. (31) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045−1052. (32) Miertuš, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117− 129. (33) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027−2094. (34) Cancès, E.; Mennucci, B. J. Math. Chem. 1998, 23, 309−326. (35) Cancès, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032−3041. (36) Mennucci, B.; Cancès, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506−10517. (37) Mennucci, B.; Cammi, R.; Tomasi, J. J. Chem. Phys. 1998, 109, 2798−2807. (38) Scalmani, G.; Frisch, M. J. J. Chem. Phys. 2010, 132, 114110. (39) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378−6396.

920


Complexes Containing a Seven-Membered Metallacycle - American

Recommend Documents