Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a salen type ligand

Inorganica Chimica Acta 362 (2009) 1405–1411

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Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a salen type ligand Jenna Welby a, Linah N. Rusere a, Joseph M. Tanski b, Laurie A. Tyler a,* a b

Department of Chemistry and Biochemistry, Union College, Schenectady, NY 12308, United States Department of Chemistry, Vassar College, Poughkeepsie, NY 12604, United States

a r t i c l e

i n f o

Article history: Received 25 April 2008 Received in revised form 17 June 2008 Accepted 18 June 2008 Available online 25 June 2008 Keywords: Cobalt Salen Imine Phenolate Dinuclear Dimer Trinuclear

a b s t r a c t Three cobalt complexes containing the salen type ligand, bis(salicylidene)-meso-1,2-diphenylethylenediaminato (mdpSal2), are reported. The complexes differ in nuclearity and include the mononuclear, Co(mdpSal) (1), which contains a Co(II) metal center bound to one mdpSal2 ligand frame in a square planar geometry. The second complex is the dinuclear [Co(mdpSal)Cl]2 (2) in which both cobalt ions have been oxidized to the +3 oxidation state. The overall geometry of complex 2 is an edge-sharing bioctahedron with the coordination sphere around each cobalt metal center consisting of one mdpSal2 ligand and one Cl ion. The shared edge between the Co(III) ions contains two bridging phenolate groups, one from each ligand frame. Complex 3 is a linear, mixed valence, trinuclear species, [Co(mdpSal)(OAc)(lOAc)]2Co, with the oxidation states of the metal centers assigned as Co(III)–Co(II)–Co(III). The terminal Co(III) centers are equivalent with the central Co(II) lying on the inversion center of the molecule. Each cobalt ion in 3 adopts an octahedral geometry with the terminal Co(III) ions being bound to one mdpSal2 ligand each. All phenolate groups bridge to the central Co(II). The coordination sphere about each metal center in the trinuclear complex is completed by four acetate groups, two of which bind in a l-fashion bridging from the terminal Co(III) metal centers to the central Co(II). The complexes have been characterized by X-ray crystallography as well as UV–Vis and IR spectroscopy. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Organic molecules containing imine groups have been of interest to inorganic chemists for some time. These types of molecules have been shown to be good ligands for transition metals and have found utility in a broad range of applications. Undoubtedly, the Schiff base ligands receiving the most attention have been those classified as Salen type ligands. Ligands categorized in this class are tetradentate, consisting of two imine nitrogen and two phenolic oxygen donors that coordinate in the basal plane of the metal ion, and are readily prepared from the condensation of a salicylaldehyde and a diamine. Factors that have contributed to the widespread and continued interest in Salen type ligands are (a) the initial recognition that metal complexes containing these types of ligands can reversibly bind O2 [1–4], (b) the similarity between Salen type ligands and heme and hence their use in model complexes [5,6] and finally, (c) the more recent discovery of Salen complexes as efficient chiral catalyst, a field which continues to be at the forefront of research efforts [7–11].

* Corresponding author. E-mail address: [email protected] (L.A. Tyler). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.06.023

The success of Salen type ligands in coordination compounds is due in part to the ease of which a variety of substituents can be appended to the ligand periphery and the affinity of this donor set for many different metal ions [12–16]. These inherent properties have made this ligand class a unique platform to perform systematic studies on in order to achieve a desired outcome. Indeed, changing one of these two factors has great influence on the characteristics of the coordination complex. For example Jacobsen employed the same Salen type ligand system with both Cr and Mn and found the Cr analogue to be an effective catalyst for the enantioselective ring opening of epoxides while the Mn derivative catalyzes the enantioselective epoxidation of olefins (Fig. 1) [17–19]. Another characteristic of the Salen type ligands, albeit less studied, is their potential to form multinuclear complexes. This feature arises due to the susceptibility of deprotonated phenolic oxygen donors to bind more than one metal ion [20–22]. Interest in homo-dinuclear complexes continues to develop owing to their discovery in such biological systems as the di-nickel containing ureases [23] and the bifunctional enzyme ACS/CODH [24], the diiron core of the hydroxylase unit of methane monooxygenases [25] and the di-copper hemocyanins [26] to name a few. In addition, homo- and hetero-multinuclear complexes display novel magnetic, structural and redox properties which continue to make them of interest [27–30].

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(s, 2H, OH). Selected IR bands: (cm1) 1620, 1576 (m, mNC), 1493 (m), 1278 (m), 1052 (m), 758 (s), 701 (s).

N

N M O

t-Bu

t-Bu

Cl

t-Bu

O

t-Bu

Fig. 1. Salen type ligand employed by Jacobsen et al. M = Cr or Mn.

Ph

Ph

H

H N

OH

N

HO

Fig. 2. N,N0 -Bis(salicylidene)-meso-1,2-diphenylethylenediamine (mdpSalH2).

We have isolated three new cobalt containing complexes with the Salen type ligand N,N0 -bis(salicylidene)-meso-1,2-diphenylethylenediamine (mdpSalH2) (Fig. 2) which differ in nuclearity, namely the mononuclear Co(mdpSal) (1), the dinuclear [Co(mdpSal)Cl]2 (2) and the linear, mixed valence, trinuclear species [Co(mdpSal)(OAc)(l-OAc)]2Co (3). This work describes the synthesis, characterization and crystal structure of each complex, 1–3. 2. Experimental 2.1. Materials Salicylaldehyde (98%), meso-1,2-diphenylethylenediamine (98%), Co(OAc)2  4H2O and anhydrous solvents were purchased from Sigma–Aldrich Chemical Co. and used without further purification. 2.2. Physical measurements Infrared spectra were obtained with a Thermoelectron, Avatar 330 FT-IR spectrophotometer equipped with a Smart Orbit reflectance insert, diamond window. Absorption spectra were measured on a Hewlett-Packard 8453 diode array spectrophotometer. 1H spectra were recorded on a Varian 200 MHz spectrometer. 2.3. Preparation of compounds The ligand, mdpSalH2, was prepared using a method similar to those used to synthesize other Salen type ligands [31]. The starting materials (salicylaldehyde and meso-1,2-diphenylethylenediamine) were dissolved in a minimum amount of EtOH and added together in a 1:1 ratio. Upon mixing, an immediate bright yellow precipitate formed. The reaction was stirred for 2 h and the solid collected and dried under high vacuum. Yield: 85%. 1H NMR (CDCl3, 200 MHz, 25 °C, d from TMS): 4.75 (s, 2H, ethylene), 6.81 (t, 2H), 6.92 (d, 2H), 7.07 (d, 2H), 7.15 (m, 12H), 8.09 (s, 2H, imine), 13.1

2.3.1. Synthesis of Co(mdpSal) (1) A batch of 0.194 g (0.499 mmol) of mdSalH2 was suspended in 20 mL of MeOH. After 20 min, 0.124 g (0.449 mmol) of Co(OAc)2  4H2O dissolved in 10 mL of MeOH were added to it. Upon addition of the metal, the solution immediately turned redbrown and a bright orange precipitate began to form. The mixture was stirred for 3 h and the solid collected by gravity filtration and dried under high vacuum. Yield: 0.15 g (63%). Selected IR bands: (cm1) 1599, 1578 (m, mNC), 1524 (m), 1428 (m), 1146 (m), 757 (s), 701 (s). Electronic absorption spectrum in CH2Cl2: kmax (nm) (e, M1 cm1) 415 (10 248), 350 (9798), 265 (30 580), 240 (40 071). Anal. Calc. for C28H22CoN2O2: C, 70.44; H, 4.64; N, 5.87. Found: C, 70.30; H, 4.62; N, 5.87%. 2.3.2. Synthesis of [Co(mdpSal)Cl]2 (2) Complex 1 (0.125 g, 0.262 mmol) was dissolved in CH2Cl2 and refluxed. After 30 min, 1 drop of concentrated HCl was added to it. The solution immediately changed from orange to red-orange in color. Slow diffusion of diethyl ether (Et2O) into the solution resulted in the isolation of 2 as red blocks. Yield: 67 mg (50%). Selected IR bands: (cm1) 1603, 1597 (m, mNC), 1443 (m), 1323 (m), 1238 (w), 908 (w), 700 (s). Electronic absorption spectrum in CH2Cl2: kmax (nm) (e, M1 cm1) 340 (11 193), 250 (43 005). Anal. Calc. for C28H22Co2N2O2Cl2: C, 65.71; H, 4.64; N, 5.38. Found: C, 65.28; H, 4.46; N, 5.30%. 2.3.3. Synthesis of [Co(mdpSal)(OAc)(l-OAc)]2Co (3) Complex 3 was isolated as a byproduct from the reaction which resulted in the formation of complex 1. Whilst complex 1 was filtered off from the reaction mixture, slow evaporation of the methanolic mother liquor afforded isolation of complex 3 as orange microcrystalline solid. The solid was collected and dried under high vacuum for 12 h. Yield: 0.11 g (36%, based on Co). Selected IR bands: (cm1) 1622 (m), 1601, (s, mNC), 1548 (m), 1447 (m), 1293 (s), 1206 (w), 907 (m), 739 (s), 701 (s). Electronic absorption spectrum in CH2Cl2: kmax (nm) (e, M1 cm1) 400(sh) (10 991), 375 (18 412), 374 (16 402), 300(sh) (24 965), 275 (sh) (50 805), 240 (124 661). Anal. Calc. for C64H56Co3N4O12: C, 61.50; H, 4.52; N, 4.48. Found: C, 61.10; H, 4.65; N, 4.47%. 2.4. X-ray data collection and structure solution and refinement Crystals suitable for X-ray analysis were obtained using the following procedures at room temperature: Red blocks of Co(mdpSal) (1) were obtained within 24 h by diffusion of Et2O into a solution of 1 in CH2Cl2. Red blocks of the dietherate of [Co(mdpSal)Cl]2 (2  2Et2O) were obtained by slow diffusion Et2O into a solution of 2 in CH2Cl2 after 48 h. Slow diffusion of Et2O into a methanolic solution of [Co(mdpSal)(OAc)(l-OAc)]2Co yielded orange needles of 3 as a solvate (3  8CH3OH) within 4 h. X-ray diffraction data were collected on a Bruker APEX 2 CCD platform diffractometer (Mo Ka (k = 0.71073 Å)) equipped with and Oxford liquid nitrogen cold stream. Suitable crystals were mounted in a nylon loop with Paratone-N cryoprotectant oil. The structures were solved using direct methods and standard difference map techniques, and were refined by full-matrix least-squares procedures on F2 with SHELXTL (Version 6.14) [32]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon were included in calculated positions and were refined using a riding model. Hydrogen atoms on oxygen were refined semifreely using a distance restraint. Crystal data and refinement details are presented in Table 1 for complexes 1–3 while selected

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bond distances for the complexes are listed in Table 2 and bond angles are given in Table 3. The structure of 2 contains disordered diethyl ether solvate. The atoms of the disordered ether molecules were included in the refinement as a diffuse contribution to the scattering using the program SQUEEZE in the PLATON suite of programs [33]. In the case of 3, several crystals of [Co(mdpSal)(OAc)(l-OAc)]2Co were screened and found to be multiply twinned. X-ray diffraction data were collected at 125 K on a crystal that was determined with CELL_NOW to contain two twin domains, the second rotated by 180° around the 0 0 1 reciprocal axis of the first. Both domains were integrated with SAINT using the two-component orientation matrix produced by CELL_NOW. The data were absorption corrected and scaled with TWINABS (version 2008/1). Initial solutions were found and refined with merged and roughly detwinned HKLF 4 format data before final refinement against HKLF 5 format twin data. The twin ratio (BASF) refined to 0.331(2). The structure contains four molecules of methanol solvate per asymmetric unit, one of which is disordered over two sites. The methanol solvate was refined with the help of similarity restraints on displacement parameters (SIMU) and rigid bond restrains on 1–2 and 1–3 distances and anisotropic displacement parameters (DELU). 3. Results and discussion 3.1. Synthesis The synthesis of complex 1 followed closely to other Co(II)– Salen syntheses previously reported starting with Co(OAc)2  4H2O as the metal salt in MeOH [13,16]. Complex 1 is slightly soluble in MeOH with 63% of the product that immediately precipitates out of the reaction mixture upon the addition of the Co(OAc)2  4H2O to the dissolved ligand. This complex was first reported by Costa et al. who investigated how the sterics of tetradentate ligand frames affect the mechanism and rate of coordination and redox reactions of Co(II) complexes [34]. These studies did not include X-ray analysis and the structure is reported here for the first time. Interestingly, the slight solubility of mononuclear 1 in the methanolic solution led us to the isolation of the trinuclear complex 3 from

this reaction mixture as well. After removal of the solid Co(mdpSal) via filtration, slow evaporation of the homogeneous mother liquor resulted in the formation of orange microcrystalline solid within 24 h. These crystals had a tinge of green color to the eye as well as a different morphology from that of crystalline 1 (needle versus block) which led us to believe that it was a different product. This resulted in the subsequent isolation and full characterization of the linear, trinuclear complex, [Co(mdpSal)(OAc)(l-OAc)]2Co (3). Combining the yield of complex 1 with that of 3 from the reaction accounts for 99% of the Co starting material. Only two classes of linear, trinuclear cobalt complexes have been reported; those that have oxidation states assigned as Co(II)–Co(II)–Co(II) and those with a Co(III)–Co(II)–Co(III) configuration. The first class of linear, trinuclear Co(II) complexes has received considerably more attention due to the discovery that Co3(dppa)4Cl2 exists in two different forms [35,36]. In addition, the Co(II) centers in these types of complexes can exist in different spin states, which have made for interesting temperature dependent magnetic studies [37,38]. There are notably fewer examples of the second class available in the literature [39–41]. Complex 3 is a neutral species that contains four, mono-anionic acetate groups and two di-negative mdpSal2 ligands. Summing the anionic groups gives rise to a total 8 charge. The +8 charge must therefore be assigned among the three Co ions. Because the terminal Co centers are equivalent and contain Co–N bond lengths less than 1.89 Å, they are assigned as low spin, Co(III) centers. The bond lengths to the cobalt(III) centers in complex 3 are similar to other Co(III)–Nimine and Co(III)–Ophen bond lengths previously reported (see Table 2 for a list of bond lengths). The central Co–Ophen bond distances in complex 3 are longer than those observed for the terminal Co(III) centers (each longer than 2.09 Å) and by comparison of the bonding parameters of the central Co ion to similar complexes previously reported, it is assigned a +2 oxidation state. We therefore assign complex 3 to the latter class of linear, trinuclear complexes, Co(III)–Co(II)–Co(III), based on (1) charge considerations and (2) bond length analysis and comparison to other examples. The isolation of dimeric 2 from a solution of 1 was obtained by chance. Upon setting up crystallization of complex 1 in CH2Cl2, one

Table 1 Summary of crystal data and intensity collection and structure refinement parameters for complexes 1–3

Empirical formula Molecular weight Crystal color, habit Crystal size (mm) Temperature (K) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3), Z Dcalc (mg m3) Absorption coefficient (l, mm1) U Range collected (°) Completeness to U max (%) Reflections collected/unique (Rint) Data/restraints/parameters R1, wR2 (I > 2rI) R1, wR2 (all data) Goodness of fit on F2 Largest diff peak/hole (e/Å3)

[Co(mdpSal)] (1)

[Co(mdpSal)Cl]2 (2  2Et2O)

[Co(mdpSal)Co(OAc)4Co(mdpSal)] (3  8MeOH)

C28H22CoN2O2 477.41 red, block 0.23  0.17  0.14 125(2) monoclinic P21/n

C56H44Cl2Co2N4O4  2(C4 H10O) 1173.95 orange, plate 0.25  0.08  0.02 125(2) triclinic  P1

C64H56Co3N4O12  8(CH3OH) 1506.25 orange, needle 0.15  0.10  0.08 125 (2) monoclinic C2/c

12.4870(7) 11.3889(7) 16.263(1) 90 106.252(1) 90 2220.4(2), 4 1.428 0.802 1.83–26.37 99.9 23 764/4532 (0.0365) 4532/0/298 0.0306, 0.0743 0.0377, 0.0790 1.029 0.379/0.318

8.5153(7) 11.9213(9) 12.791(1) 95.201(1) 91.576(1) 94.986(1) 1287.4(2), 1 1.514 0.810 1.60–26.73 99.5 14 684/5445 (0.0390) 5445/0/307 0.0358, 0.0763 0.0536, 0.0801 1.016 0.351/0.328

28.247(6) 12.039(3) 24.538(5) 90 110.770(3) 90 7802(3), 4 1.282 0.699 1.54–24.81 93.9 13 460/6321 (0.0714) 6321/40/468 0.0789, 0.2192 0.1072, 0.2357 1.058 1.675/0.648

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batch resulted in the formation of orange plates, not red blocks as expected. Crystallographic analysis of this material confirmed it to be that of dimeric 2. These findings prompted us to investigate the reaction conditions which resulted in the formation of [Co(mdpSal)Cl]2. It has been shown that Cl radical exists in chlorinated solvents, especially in the presence UV light1 [42] and that certain Co(II) salen complexes can react with organic halides [43]. This led us to consider the likelihood of complex 1 undergoing an oxidative addition reaction with CH2Cl2 and concomitant phenolate bridge formation. To investigate this possibility we tried several different reactions employing both UV irradiation [44] as well as radical initiators (AIBN and benzoyl peroxide) however we were unable to isolate 2 from any of these attempts. We also carried out several reactions that added a slurry of NEt4Cl in CH2Cl2 to a solution of 1 in CH2Cl2 as well as a similar reaction in MeOH. These attempts were also unsuccessful, possibly due to the low solubility of either the chloride salt in CH2Cl2 or that of complex 1 in MeOH. We were able to find two similar Co(II)(salen) complexes in the literature that, when oxidized, formed dimeric species. The first report found oxidation occurred after heating a solution of the monomeric Co(II) complex in ethanol/water to 40° C for 48 h [44]. We carried out similar studies in CH2Cl2 and CHCl3 (due to very low solubility in H2O) with no apparent oxidation occurring (monitored by UV–Vis spectroscopy). The second study reports conversion of a cobalt salen monomer to a dicobalt(III) dichloride complex after a chloroform solution of it was left standing in air for several weeks [45]. Our solutions standing in CH2Cl2 and CHCl3 resulted in re-crystallization of monomeric 1 in less than 48 h. We were however able to isolate dimeric 2 by adding a drop of HCl to a boiling solution of 1 in CH2Cl2. The crystal structure of the product of this synthesis was analogous to the first report and confirmed that complex 2 is reproducibly isolated from these syntheses. We therefore conclude that our initial isolation of complex 2 was due most probably to contamination from cleaning glassware with HCl. 3.2. Spectral characterization The IR spectrum of the ligand shows stretching frequencies at 1620 cm1 and 1278 cm1 which are assigned as the mN@C and mC–O, respectively. Once reacted with Co(OAc)2  4 H2O, the mN@Cband shifts by 20 cm1 to lower energy in complexes 1–3 to 1600, 1597 and 1601 cm1, respectively [46,47]. Conversely, the mC–O is seen at 20 cm1 higher energy when compared to the ligand which indicates the phenolic oxygen groups are bonded to the metal center [48]. The overlap of the stretching frequencies of the acetate groups (bridging versus non-bridging) in complex 3 gives rise to broad bands in the 1622–1580 cm1 range. However, the frequencies at ca. 1620 and 1445 cm1 are assigned masm cooand msym coo- and are characteristic of bridging acetate groups [49]. The electronic spectra of the ligand and complexes 1–3 were recorded in CH2Cl2 and can be seen in Fig. 3. The spectrum of the ligand exhibits intense absorption bands in the high energy range (265 and 320 nm) which are identified as p ? p* transitions. The higher energy band arises from the phenyl rings while the lower energy transition is due to the azomethine chromophore [46,47]. Complexation of the ligand results in a slight blue shift of the p ? p* transition of the phenyl rings by 15–25 nm and a red shift of the p ? p* azomethine chromophore by 25 nm, which is most notable in complexes 1 and 3. In addition, complex 1 exhibits a band at 415 nm which supports the square planar geometry in solution [48].

3.3. Structure of complexes 3.3.1. Structure of Co(mdpSal) (1) The structure of 1 is shown in Fig. 4. The neutral species contains one Co(II) ion ligated to one mdpSal2 ligand. The complex

Table 2 Selected bond distances (Å) of complexes 1–3 [Co(mdpSal)] (1) Co(1)–N(1) Co(1)–O(1) O(1)–C(1)

1.858(2) 1.846(1) 1.314(2)

[Co(mdpSal)Cl]2  2Et2O (2  2C4H10O) Co(1)–N(1) 1.892(2) Co(1)–O(1) 1.931(1) O(1)–C(1) 1.361(2) Co(1)–Cl(1) 2.2382(6)

Co(1)–N(2) Co(1)–O(2) N(1)–C(7)

1.867(1) 1.841(1) 1.303(2)

Co(1)–N(2) Co(1)–O(2) N(1)–C(7) Co(1)–Co(2)

1.885(2) 1.875(1) 1.280(2) 2.9842(6)

[(Co(mdpSal)(OAc)(l-OAc))2Co]  8MeOH (3  8CH3OH) Co(1)–N(1) 1.881(7) Co(1)–N(2) Co(1)–O(1) 1.903(6) Co(1)–O(2) O(1)–C(1) 1.30(1) N(1)–C(7) Co(1)–O(3) 1.926(6) Co(1)–O(5) Co(2)–O(1) 2.141(6) Co(2)–O(2) Co(2)–O(4) 2.052(6) Co(1)–Co(2)

1.892(7) 1.912(6) 1.28(1) 1.879(6) 2.096(6) 3.090(1)

Table 3 Selected bond angles (°) of complexes 1–3 [Co(mdpSal)] (1) O(2)–Co–O(1) O(2)–Co–N(1) N(1)–C(7)–C(6)

85.91(5) 173.36(6) 125.0(2)

N(1)–Co—-N(2) O(1)–Co–N(2) C(1)–O(1)–Co

86.39(6) 169.32(6) 128.24(1)

[Co(mdpSal)Cl]2  2Et2O (2  2C4H10O) O(2)–Co–O(1) 87.08(6) O(2)–Co–N(1) 178.46(7) N(1)–C(7)–C(6) 123.8(2) Co–O(1)–Co 98.80(6)

N(1)–Co–N(2) O(1)–Co–N(2) C(1)–O(1)–Co O(1)–Co–O(1A)

85.14(7) 171.75(7) 118.7(1) 81.20(6)

[(Co(mdpSal)(OAc)(l-OAc))2Co]  8MeOH (3  8CH3OH) O(2)–Co–O(1) 82.1(2) N(1)–Co–N(2) O(2)–Co–N(1) 177.5(3) O(1)–Co–N(2) N(1)–C(7)–C(6) 125.7(9) C(1)–O(1)–Co(1) Co(1)–O(1)–Co(2) 99.5(2) O(1)–Co(1)–O(2) O(3)–Co(1)–O(5) 177.7(3) O(1)–Co(2)–O(2)A

84.9(3) 178.6(3) 123.6(6) 82.1(2) 108.7(2)

1

0.8

Absorbance

1408

0.6

0.4

0.2

0 225

275

325

375

425

475

525

Wavelength (nm) 1 The experimental apparatus used was an Atlas Suntest XLS+ Solar Simulator with 750 W intensity light.

Fig. 3. Electronic absorption spectra of complexes 1–3 in MeCl2. mdpSalH2 (— - — -) Co(mdpSal) (—), [Co(mdpSal)Cl]2 (– - – -) and [Co(mdpSal)(OAc)(l-OAc)]2Co (- - - -).

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adopts a slightly distorted square planar geometry with identical N(1)–Co–O(1) and N(2)–Co–O(2) bond angles (94.47(6)°) and nearly 10° larger than the corresponding N(1)–Co–N(2) and O(1)–Co–O(2) bond angles (86.39(6)° and 85.91(5)°, respectively). The average Co–N bond distance is 1.862(6) Å while the Co–O bond distances are slightly shorter with an average of 1.843(4) Å. Both distances are similar to other Co(II)–salen bond lengths previously reported [50].

Fig. 4. Thermal ellipsoid plot (30% probability level) of complex 1 showing the numbering scheme. H atoms have been omitted for clarity.

Fig. 5. Thermal ellipsoid plot (30% probability level) of complex 2 showing the numbering scheme. H atoms have been omitted for clarity.

3.3.2. Structure of [Co(mdpSal)Cl]2 (2) The structure of the neutral, dimeric 2 is shown in Fig. 5. Each coordination sphere for the metal ions is comprised of one mdpSal2 (N2O2) ligand frame, one Cl, and one phenolate oxygen bridge, giving rise to an overall octahedral geometry around each Co(III) center. The distance between the two equivalent Co(III) metal centers is 2.9842(6) Å. The average Co(III)–N bond distance is 1.888(5) Å, which is slightly longer (0.026 Å) than the average Co(II)–N bond distance in complex 1. Likewise, the non-bridging Co–O(2) bond length in 2 is 1.8754(1) Å, a distance that is 0.0320 Å longer than in complex 1. The Co–O(1) and Co–O(1 A) bridging bond distances in complex 2 are both longer (1.9312(1) Å and 1.9989(1) Å, respectively) than the non-bridging Co–O(2) bond length. The difference of 0.0677 Å between the two lengths is due to one ligand bound to one Co (shorter) bridging to the other (longer) Co center. All Co–O bonds in 2 are similar in length to previously reported complexes [44,45]. The Co–Cl bond length is 2.2382(6) Å and is within the range of other Co(III)–Cl bond lengths [46]. The Co–ligand bond angles for complex 2 are slightly smaller than the analogous angles found in complex 1 with the exception of the N(2)–Co–O(2) bond angle which is slightly larger. 3.3.3. Structure of [Co(mdpSal)(OAc)(l-OAc)]2Co  3CH3OH (3  8CH3OH) The complete structure of 3 is shown in Fig. 6 while the asymmetric unit of the complex is given in Fig. 7. The neutral, trinuclear species consists of three cobalt metal centers, two mdpSal2 ligand frames and four acetate ions, giving rise to an octahedral environment around each metal ion. The two terminal Co(III) ions are equivalent giving the molecule C2 symmetry. Each outer Co(III) is bound to one mdpSal2 ligand in the equatorial plane with two acetate groups occupying the axial sites. The middle Co(II) is located at the crystallographic inversion center and one acetate from each of the outer Co(III) ions coordinates in a l-fashion to it. These l-acetate ligands occupy two sites that are located cis to each other on the Co(II). The coordination sphere around the central cobalt is

Fig. 6. Thermal ellipsoid plot (30% probability level) of complex 3 showing the numbering scheme. H atoms have been omitted for clarity.

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lengths. As expected, the non-bridging Co–Ophenbond distance in complex 3 is 0.0558(14) Å shorter than the bridging Co–Ophen length [41]. Interestingly, the bridging Co(III)–Ophenbond distances in complex 2 are longer than the those found in complex 3 (1.931(1) Å versus an average 1.908(6) Å) which is likely due to the fact that in the dimer (2), the phenolate bridges to another Co(III) while in the trinuclear species (3), the bridge is to a Co(II) ion [42]. The other three types of Co–O bonds present in 3 are formed between the metal and the acetate groups. We found the bond distances to lengthen as follows: Co(II)–Ol-acetate < Co(III)– Ol-acetate < Co(III)–Oacetate, which, unlike the mdpSal2 ligand frame, exhibit shorter Co(III)–ligand bond lengths. Acknowledgements The Jerome A. Schiff Charitable Trust and Union College are thanked for their generous financial support (LAT). We also thank the National Science Foundation (NSF 0521237) for supporting the X-ray diffraction facility at Vassar College (JMT). Fig. 7. Thermal ellipsoid plot (30% probability level) of the asymmetric unit of complex 3 showing the numbering scheme. H atoms have been omitted for clarity.

completed by four bridging phenolate oxygens, two from each of the mdpSal2 ligand frames. The greatest deviations from octahedral geometry is in the center O(4)l-acetate-Co(II)–O(2A)phen bond angle which measures 162.6(2)°. The average Co(III)–N bond length is 1.887(7) Å, a value that is nearly identical to the analogous bond lengths in complex 2. The average Co(III)–Ophen bond length is 1.908(6) Å, which is shorter than the bridging Co–O bond lengths in 2. Both the Co(III)–Oacetate and the Co(III)–Ol-acetate bond lengths are shorter than the Co(II)–Ol-acetate distance (1.879(6), 1.926(6) Å and 2.052(6), respectively). The Co(III)–Co(II) bond length is 3.0902(13). All bond lengths are similar to other linear Co(III)–Co(II)–Co(III) Salen complexes previously reported [39]. Additionally, the differences in the Co(III)–O and the Co(II)–O bond distances in complex 3 suggest that the complex exits as a localized mixed valence species in the solid state [51,52]. A comparison of the bond lengths between the donor atoms of the mdpSal2 ligand frame and the cobalt metal centers in complexes 1–3 is given in Table 4 while a comparison of the Co–Oacetate bond lengths within complex 3 is given in Table 5. It can be seen that the ligand in complex 1 binds the metal in a nearly symmetric fashion with the Co–O(1) and Co–N(1) bond lengths nearly equivalent to the Co–O(2) and Co–N(2) distances (D = 0.005(1) and 0.009(1) Å, respectively). In all cases, the Co(II)–Nimine and Co(II)– Ophenbond distances are shorter than the corresponding Co(III)

Table 4 Comparison of Co-mdpSal bond lengths (Å) in complexes 1–3

Co–O(1) Co–O(2) Co–N(1) Co–N(2)

MonoNuc-Co(II) (1)

DiNuc-Co(III) (2)

TriNuc-Co(III) (3)

TriNuc-Co(II) (3)

1.846(1) 1.841(1) 1.858(1) 1.867(1)

1.931(1) 1.875(1) 1.892(2) 1.885(2)

1.903(6) 1.912(6) 1.881(7) 1.892(7)

2.141(6) 2.096(6)

Table 5 Co–OAc bond lengths (Å) in complex 3 Co(III)–OAc Co(III)–lOAc Co(II)–lOAc

1.879(6) 1.926(6) 2.052(6)

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