A family of polynuclear cobalt complexes upon employment of an indeno-quinoxaline based oxime ligand†

23068 | RSC Adv., 2014, 4, 23068–230 A family of polynuclear cobalt complexes upon employment of an indeno-quinoxaline based oxime ligand† Angelos B. Canaj, Lydia E. Nodaraki, Katarzyna Ślepokura, Milosz Siczek, Demetrios I. Tzimopoulos, Tadeusz Lis and Constantinos J. Milios* The reaction of Co(OAc)2$4H2O with LH (LH 1⁄4 11H-indeno[1,2-b]quinoxalin-11-one oxime) in MeOH in the presence of NEt3 forms the complex [Co III 2Co O(OAc)3L3]$0.5MeOH$0.2H2O (1$0.5MeOH$0.2H2O), while repeating the reaction under solvothermal conditions yielded the heptanuclear cluster [Co7L9 (OH)2(OAc)2.7(MeO)0.3(H2O)]$4.6MeOH$3.3H2O (2$4.6MeOH$3.3H2O). Changing the starting metal salt to Co(ClO4)2$6H2O and upon the reaction with LH in the presence of NEt3 under high temperature and pressure, we managed to isolate the decanuclear cluster [Co10L14(OH)3.6(MeO)0.4](ClO4)2$8.5MeOH$5.75H2O (3$8.5MeOH$5.75H2O), while under normal bench conditions and upon employment of pivalates in the reaction mixture complex [Co4L4(piv)4(MeOH)2]$MeOH$H2O (4$MeOH$H2O) was formed. Furthermore, the reaction of Co(ClO4)2$6H2O with LH and aibH (2-amino-isobutyric acid) in the presence of NEt3 in MeOH gave the mononuclear complex [CoL(aib)2]$3H2O (5$3H2O), while upon increasing the metal–ligand ratio cluster [Co2Co L4(aib)2(OH)2]$7.9MeOH (6$7.9MeOH) was isolated. Finally, repeating the reaction that yielded the mononuclear complex 5$3H2O under solvothermal conditions, gave the octanuclear cluster [Co8L10(aib)2(MeO)2](ClO4)2$6.8MeOH$7H2O (7$6.8MeOH$7H2O). Variable temperature dc magnetic susceptibility studies for complexes 2, 3, 4 and 7, reveal that all clusters display dominant antiferromagnetic interactions leading to small or diamagnetic ground-states, S.


Introduction
In the last few years metal-oxime coordination chemistry has proven to be a fruitful source for the synthesis and characterization of numerous metallic complexes. 1Although initially triggered by the employment of salicyl-and pyridyl-based oximes in manganese cluster chemistry, 2 it later expanded to other 3d, 4f and 3d-4f clusters as well, while a palette of various oxime-based ligands is nowadays utilized for the synthesis of such species. 3In addition, many of these clusters have been found to display interesting magnetic properties, such as single molecule magnetism behaviour (SMM), i.e. they can retain their magnetization once magnetized at very low temperatures in the absence of an external magnetic eld.
We recently reported the use of a new indeno-quinoxaline based oxime ligand, LH (Scheme 1), in Ni(II) chemistry, which led to the synthesis of ve nickel complexes with nuclearities ranging from 3 up to 8. 4 We have now expanded our studies in Co(II/III) chemistry, and herein we report the use of this Scheme 1 The structure of LH (top) and its coordination modes in 1-7 (bottom).

X-ray crystallography
Diffraction data for complexes 1, 3, 4 and 6 at low temperatures were collected on an Xcalibur R diffractometer with CCD Ruby and for 2, 5 and 7 on a KM4 diffractometer with a CCD Sapphire camera and Mo Ka radiation (l ¼ 0.71073 Å).The crystal for 3 was slowly cooled from 230 K to 80 K, at which data collection was carried out.All structures were solved by direct methods and rened by full-matrix least-squares techniques on F 2 (SHELXL-97). 6Data collection parameters and structure solution and renement details are listed in Table S1.†

Syntheses
The reaction between Co(OAc) 2 $4H 2 O and LH in the presence of base (NEt 3 ) in MeOH yields the trinuclear cluster [Co III 2 Co II O(OAc) 3 L 3 ]$0.5MeOH$0.2H 2 O (1$0.5MeOH$0.2H 2 O) in moderate yield.Given the employment of excess base in the reaction, it is reasonable that all of the LH ligands found in 1 were in their deprotonated monoanionic form, L À .Furthermore, the acetates present in the reaction can also act as a base, thus aiding the deprotonation of the ligand.Two of the three metallic centers were oxidized in the +3 oxidation state, presumably upon oxidation from the atmospheric dioxygen, a commonly observed phenomenon in cobalt chemistry.In order to investigate the effect of high pressure/temperature on the identity of the product, we repeated the same reaction under solvothermal conditions, and we obtained the heptanuclear cluster [Co II 7 L 9 (OH) 2 (OAc) 2.7 (MeO) 0.3 (H 2 O)]$4.6MeOH$3.6H 2 O (2$4.6MeOH$3.6H 2 O) according to eqn (1), proving the effect of the solvothermal conditions on increasing the nuclearity of the product: Co II 7 L 9 ðOHÞ 2 ðOAcÞ 2:7 ðMeOÞ 0:3 ðH 2 OÞ Furthermore, the oxidation of the metallic centers is now suppressed, since in the trinuclear cluster the ratio of Co III : Co II was 2 : 1, while in the heptanuclear cluster all cobalt centers are maintained in the 2+ oxidation state, due to the "reducing" environment present within the autoclaves under high temperature/pressure.
While the reaction forming 3 under normal laboratory conditions did not lead to the formation of any crystalline product, upon addition of pivalates in the reaction mixture we were able to isolate the tetranuclear cluster [Co II 4 L 4 (piv) 4 (MeOH) 2 ]$MeOH$H 2 O (4$MeOH$H 2 O), according to eqn (3): With the identity of cluster 4 established, we checked whether addition of other carboxylates would lead to analogous tetranuclear clusters.Therefore, we employed the articial amino acid 2-amino-isobutyric acid, aibH, and we managed to characterize the mononuclear complex [Co III L(aib) 2 ]$3H 2 O (5$3H 2 O), whose identity was not surprising given the chelate coordination mode of the amino acid ligand (vide infra).In order to increase the nuclearity of the cluster, we increased the metal-ligands ratio as a means of "forcing" the cluster to propagate into bigger species due to the lack of sufficient number of chelates.Indeed, upon repeating the same reaction with twice the initial metal salt concentration, we isolated the trinuclear complex [Co III 2 Co II L 4 (aib) 2 (OH) 2 ]$7.9MeOH (6$7.9MeOH) in moderate yields.Furthermore, repeating the reaction that yielded the mononuclear complex 5$3H 2 O under solvothermal conditions, gave the octanuclear cluster [Co II 8-L 10 (aib) 2 (MeO) 2 ] (ClO 4 ) 2 $6.8MeOH$7H 2 O (7$6.8MeOH$7H 2 O) according to eqn (4): Finally, even though the formation of the complexes containing trivalent Co III ions, clusters 1, 5 and 6, cannot be "strictly" given by chemical equations since we are not absolutely sure of the oxidizing-reduced species, their formation can be possibly described by the following eqn ( 5)- (7), assuming that he most likely oxidizing agent responsible for the Co(II) to Co(III) transformation is O 2 :

Description of structures
The molecular structures of complexes 1-7 are presented in Fig. 1-7, while selected interatomic distances and angles are listed in Tables S2-8.† Cluster 1 crystallizes in the trigonal R 3 space group.Its structure (Fig. 1) describes an oxo-centered equilateral triangular unit, which consists of three diatomic (-N-O-) oximate bridges, from three deprotonated L À ligands, and three h 1 :h 1 :m acetate ligands.All L À ligands adopt coordination mode B (Scheme 1), while each Co center is in a cis-O 4 N 2 coordination sphere.The formula of the molecule necessitates a [Co III 2 Co II ] oxidation states' distribution, but since the molecule lies on a special position (on a three-fold axis), and is disordered around it, we cannot assign the oxidation states of the metallic atoms.Similar examples of disordered mixed oxidation states have been previously reported in the literature. 8omplex 2 crystallizes in the triclinic P 1 space group.Its core (Fig. 2) describes an almost planar heptametallic disc.The metallic disk is held together via nine deprotonated monoanionic L À ligands, three acetate and two methoxide ligands.Cluster 3 also crystallizes in the triclinic P 1 space group; its metallic core consists of ten edge-sharing [Co 3 ] triangles, forming a slightly bent [Co 10 ] disc (Fig. 3) with dimensions of $10 Â 6 Å.The ten metallic centers are held in position by fourteen monoanionic L À ligands which adopt six different coordination modes: (i) ve are found in mode B, (ii) three in mode F in which the O oximate forms a six-member chelate ring with the N aromatic and further bridges to two cobalt centers, (iii) two in mode D, (iv) one in mode G, (v) one in mode C, according to which the O oximate forms a six-member ring with the N aromatic and further bridges to one metallic center, and (vi) two in mode A. In addition, the coordination environment of the metallic centers is completed by the presence of three m 3 -OH À and one m-OH À .Co5, Co8 and Co7 are ve-coordinate adopting distorted tetragonal pyramidal for the former two centers and highly distorted trigonal bipyramidal geometry for the remaining center, with s values of 0.26, 0.30 and 0.62 respectively, while all remaining centers are six-coordinate adopting octahedral geometry. 9 The mononuclear complex 5 crystallizes in the monoclinic P2 1 /c space group.Its structure (Fig. 5) consists of a central Co 3+ ion coordinated by one chelate L À ligand via its oximate and one  aromatic N atoms (mode E), and two aib À ligands in a chelate mode, thus forming three ve-member chelate rings.The metal ion is found in a cis-O 2 N 4 sphere, while BVS calculations yielded a value of 3.11 for the central metal atom.
Complex 6 crystallizes in the monoclinic space group P2 1 /n.The structure consists of a trimetallic unit with a "V-shape" arrangement of the three metallic centers (Fig. 6), which are held together by two deprotonated ligands, L À , found in mode B, and two m-OH À groups.Each terminal metallic atom is further coordinated to a deprotonated chelate L À ligand (mode E) and to one chelate aib À ligand.The formula of the complex necessitates a mixed-valent Co III 2 Co II oxidation state, and according to BVS calculations the two terminal cobalt atoms are in the 3+ oxidation state (BVS values of 3.15 and 3.22 for Co1 and Co3, respectively), while the central one in the 2+ (BVS value: 2.01).All metallic centers are six-coordinate adopting      Finally, complex 7 crystallizes in the triclinic P 1 space group in special positioncenter of symmetry.Its metallic core (Fig. 7) consists of a planar octametallic disk, in which the eight metallic centers are held in position by ten deprotonated, L À , ligands adopting four different coordination modes; (i) two in mode A, (ii) four in mode B, (iii) two in a monoatomic bridge fashion via the O oximate , mode H, and (iv) two in mode F. In addition, two deprotonated aib À ligands are present in an h 2 : h 1 : h 1 : m 3 fashion, while two m 3 -OCH 3 À groups complete the coordination environment of the octametallic disc.All cobalt atoms are six-coordinate, besides Co3 (and its symmetry related Co3 0 ) which is ve coordinate adopting tetragonal pyramidal (s z 0).Finally, all cobalt centers are in the 2+ oxidation state, as evidenced by BVS calculations giving values in the 1.75-2.01range.
For complex 2 ([Co 7 ]) the room temperature c M T value of 15.39 cm 3 K mol À1 corresponds to seven non-interacting Co II ions with S ¼ 3/2 and g ¼ 2.17.Upon cooling the c M T value decreases to reach a minimum value of 1.70 cm 3 K mol À1 at 5 K, suggesting the presence of dominant antiferromagnetic interactions.For cluster 4([Co 4 ]) the room temperature c M T value of 4.64 cm 3 K mol À1 is very close to the spin-only value of 4.411 cm 3 K mol À1 corresponding to two non-interacting Co II ions.Upon cooling, this value remains constant until $90 K, before it decreases to reach the minimum value of 0.62 cm 3 K mol À1 at 5 K, with this behavior suggesting the presence of dominant, albeit weak, antiferromagnetic interactions.A similar behavior is encountered for cluster 7 ([Co 8 ]); the room temperature c M T value of 17.61 cm 3 K mol À1 is very close to the spin-only value of 17.65 cm 3 K mol À1 corresponding to eight non-interacting Co II ions.Upon cooling, this value remains constant until $120 K, below which it decreases to reach the minimum value of 1.43 cm 3 K mol À1 at 5 K.
On the other hand, cluster 3 ([Co 10 ]) displays different behavior than the three complexes already mentioned; the room temperature c M T value of 30.51 cm 3 K mol À1 is larger than the expected value for ten non-interacting S ¼ 3/2 ions of 18.7 cm 3 K mol À1 (for g ¼ 2), suggesting the presence of signicant orbital contribution, as has been previously reported for other [Co II 10 ] clusters. 12Upon cooling the c M T value decreases rapidly to reach the minimum value of 3.12 cm 3 K mol À1 , suggesting the presence of dominant and strong antiferromagnetic interactions.
The presence of the antiferromagnetic interactions in all complexes was further established by Curie-Weiss analysis of the high temperature (50-300 K) magnetic susceptibility data (Fig. 9), yielding negative q values for all of them.
Complex 2 joins a small family of twenty six structurally characterized heptanuclear clusters (Table 1), in various oxidation states, while complexes 7 and 3 join a family of structurally characterized octanuclear and decanuclear cobalt clusters, respectively, in which the vast majority consists exclusively of divalent metal atoms (Tables 2 and 3).
Complexes 2, 3, 4 and 7 were also studied by alternating current ac magnetic susceptibility studies as a means of investigating potential single molecule magnetism behavior, but no out-of-phase signals were observed, thus ruling out this possibility.

Conclusions
In conclusion, we have reported the syntheses, structures and magnetic properties of seven new Co complexes stabilized by an indeno-quinoxaline based oxime ligand, LH.Following our initial results on nickel cluster chemistry, we have now found that this ligand can lead to the synthesis of cobalt clusters, as well.More importantly, the ligand adopts eight different coordination modes leading to clusters with various nuclearities.
Studies are underway for the employment of the ligand in 3d-4f and 4f chemistry, and the ndings will be reported in the future.
The nine L À ligands found in the structure, adopt four different coordination modes: (i) four are found in mode B (Scheme 1), forming a chelate ring via the oximate and one aromatic N atoms and bridging to a further Co center via the O oximate atom, (ii) two form a chelate ring via the oximate and one aromatic N atoms and bridging to two further Co centers via the m-O oximate atom (mode A), (iii) two are found in a rather unusual mode, bridging three metals via the monoatomic m-O oximate bridge and the N oximate atom (mode D), and (iv) one in mode C (Scheme 1) via the O oximate and N aromatic atoms.Two acetate ligands adopt the usual h 1 : h 1 : m mode, while the third one is found terminally bound.Finally, the two hydroxides are found in a m 3 fashion, and in addition a H 2 O molecule bridges Co2 and Co7 as a monoatomic bridge.All cobalt centers are found in the 2+ oxidation state, as evidenced by their bond distances, as well as BVS calculations (BVS values ranging from 1.76-2.03),and are found in an octahedral geometry adopting cis-O 4 N 2 (Co1, Co2, Co4), cis-O 2 N 4 (Co6, Co3), O 5 N (Co5) and O 6 (Co7) coordination environment.
Furthermore, the coordination spheres of the metallic centers are O 6 (Co1), O 5 N (Co2, Co9), cis-O 2 N 4 (Co3, Co4, Co6) and fac-O 3 N 3 (Co10) sphere.Finally, all metallic centers are in the 2+ oxidation state, as derived from BVS calculations yielding values in the 1.77-2.04range.Complex 4 (Fig. 4) crystallizes in the monoclinic space group C2/c in special position of C 2 symmetry; its metallic core describes a tetrahedron of four Co 2+ ions linked together by (i) four fully deprotonated L À ligands in mode A, forming a distorted [Co 4 (NO) 4 ] 4+ cube comprising alternate single (O) and double (N-O) atom edges, and (ii) two h 1 : h 1 : m pivalate ligands.Finally, two terminal pivalate ligands and two methanol molecules complete the coordination environment of the metal centers.The dimensions of the metallic tetrahedron are in the range 3.09-4.43Å with the shortest distance between Co1 and Co2, and the longest between Co1 and Co1 0 .All metals are found in the 2+ oxidation state (BVS values: 2.16 for Co1 and 2.17 for Co2, Fig. 4), and are six-coordinate.

Fig. 1
Fig. 1 Molecular structure of complex 1. Color code: Co ¼ pink, O ¼ green, N ¼ blue, C ¼ grey.Minor components of acetate and H atoms are omitted for clarity.

Fig. 2 Fig. 3
Fig. 2 Molecular structure of cluster 2. Color code: same as in Fig. 1.Disorder and H atoms as well as solvent molecules not shown for clarity.

Fig. 6
Fig. 6 Molecular structure of complex 6.Color code: same as in Fig. 1.Disorder and H atoms not shown for clarity.

Fig. 5
Fig. 5 Molecular structure of complex 5. Color code: same as in Fig. 1.Disorder and H atoms as well as solvent molecules not shown for clarity.
octahedral geometry, with Co1 and Co3 in a mer-O 3 N 3 coordination sphere, and Co2 in a cis-O 4 N 2 arrangement.