Incorporation of μ3-CO3 into an MnIII/MnIV Mn12 cluster: {[(cyclam)MnIV(μ-O)2MnIII(H2O)(μ-OH)]6(μ3-CO3)2}Cl8·24H2O

The centrosymmetric title cluster, hexaaquadi-μ3-carbonato-hexacyclamhexa-μ2-hydroxido-dodeca-μ2-oxido-hexamanganese(IV)hexamanganese(III) octachloride tetracosahydrate, [Mn12(CO3)2O12(OH)6(C10H24N4)6(H2O)6]Cl8·24H2O, has two μ3-CO3 groups that not only bridge octahedrally coordinated MnIII ions but also act as acceptors to two different kinds of hydrogen bonds. The carbonate anion is planar within experimental error and has an average C—O distance of 1.294 (4) Å. The crystal packing is stabilized by O—H⋯Cl, O—H⋯O, N—H⋯Cl and N—H⋯O hydrogen bonds. Two of the four independent chloride ions are disordered over five positions, and eight of the 12 independent water molecules are disordered over 21 positions.


Table 2
Hydrogen-bond geometry (Å , ).  (Chakov et al., 2005), but it has little in common with the title compound. Many Mn 12 clusters are known, some of which have mixed valent Mn III /Mn IV ions (Sun et al., 1998;Kuroda-Sowa et al., 2001;Bian et al., 2004). The most famous is Mn 12 O 12 (MeCO 2 ) 16 (H 2 O) 4 (Lis, 1980;Aubin, et al., 1996), which opened up the field of single molecule magnets. These structures are unlike that of the title compound. They contain an internal cubane of four Mn IV ions with µ 3 -O bridges and eight outer Mn III ions. The production of the title compound was unexpected. We presume that prolonged stirring of a manganese(II) chloride solution in an open vessel in the presence of base (cyclam, (1,4,8,11-tetraazacyclotetradecane)) yielded a basic Mn III /Mn IV oxide which took up CO 2 from the air and formed the triply bridged carbonate species of the title compound.
The cluster of the title compound has a center of symmetry. It has the overall formula {[(cyclam)Mn IV (µ-O) 2 Mn III (H 2 O)(µ-OH)] 6 (µ 3 -CO 3 )} 2 } 8+ with charge balanced by chloride ions. There are 24 molecules of non-coordinated water in the model, many of which are disordered. The Mn 12 cluster is shown in the Scheme.
For simplicity, only one half of the cluster is depicted in Fig. 1  the µ-oxo bridges; the average Mn IV -N(trans) distance is 2.097 (5)Å as compared to 2.040 (4)Å for Mn IV -N(cis). Fig. 3 depicts the entire Mn 12 cluster with cyclam CH 2 groups omitted for clarity. In sum, both Mn III and Mn IV have coordination number six and a pseudo-octahedral geometry. The inversion-related halves of the cluster are connected via the µ-(OH) groups. A diverse set of Mn-O bonds is exhibited in the structure, involving oxo, hydroxo, and aqua ligation to Mn III and Mn IV ions as well as intramolecular hydrogen bonding. The chloride counterions are primarily nestled in cyclam cavities, hydrogen bonded to N-H donor groups of the cylam ligands as well as to non-coordinated water molecules.

Experimental
To a mixture of MnCl 2 .4H 2 O (136 mg, 687 mmol), cyclam (1,4,8,11-tetraazacyclotetradecane) (144 mg, 722 mmol), and sodium tetraphenylborate (289 mg, 844 mmol) in a 200 ml round bottom flask was added 150 ml of acetonitrile. The reaction was continuously stirred for 5 days over which time it turned from pale yellow to dark brown to dark olive green and a solid material was formed. The solid was filtered and redissolved in a 1:2 mixture of H 2 O:acetonitrile and placed in upcapped 5 mm diameter tubes in the refrigerator. After 2 weeks, black plates formed. The crystal selected for data collection was cut from a large plate.

Refinement
Hydrogen atoms on water O and aza-N atoms were located in a difference map and subsequently refined with U iso = 1.2U eq (N or O) and distance restraints of 0.84 (1) Å for O-H, 0.93 Å for N-H and H···H of 1.32 (3) Å for water. The C-H geometry was determined by idealized geometry and a C-H distance of 0.99 Å. The C-H and N-H H atoms were refined as riding on the parent atoms. There are seven different positions for the four chloride ions in the asymmetric unit. Of these, Cl1 and Cl2 are included at full occupancy while Cl3, Cl4, and Cl6 are at half occupancy and Cl5A/Cl5B respresent a split position of occupancy 0.40/0.10 occupancy. These disordered chlorides were selected based on longer hydrogen bonding distances and reasonable distribution within the structure. Four hydrate water O atoms, O16, O17, O18, and O19, were in sites of full occupancy and were refined with anisotropic thermal parameters. The remainder were refined with isotropic thermal parameters and fixed occupancies that were determined by an ad hoc method. Most of the hydrogen atoms were not reliably located for the hydrate molecules and none were included in the structure factor calculation. The final difference map contains a number of peaks in the region of the chloride ions and solvate water molecules that are possibly additional minor water sites or part of disordered chloride sites. Fig. 1. A drawing of the asymmetric unit of the title compound. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms bonded to carbon, chloride counterions, and hydrate molecules have been omitted for clarity. One of the oxygen atoms (O13') is shown at its symmetry position, ' = 1 -x, 1 -y, 1 -z, in order to show the complete carbonate anion. Fig. 2. A view of one-half of the cluster normal to the triply bridging carbonate group. A portion of the hydrogen bonding is also depicted. Symmetry code: ' = 1 -x, 1 -y, 1 -z.