High-Spin Molecules: Hexanuclear [Mn6O4Cl4(Me2dbm)6] (Me2dbmH = 4,4‘-Dimethyldibenzoylmethane) with a Near Tetrahedral [Mn6O4Cl4]6+ Core and a S = 12 Ground State

An interesting sub-area of transition-metal cluster chemistry is the small but growing family of molecules that, in their ground states, have unusually large numbers of unpaired electrons. Molecular clusters with spin (S) values as high as double figures have been discovered, with the highest value to date being S ≈ 16/2 for one of the clusters in a sample of cocrystallized Fe17 and Fe19 species,2 but examples with S g 8 are nevertheless very rare.3,4 The study of such molecules has shown that the high spin value is a result of the presence of (at least some) ferromagnetic exchange interactions between the metal ions and/ or spin frustration effects arising from the presence in certain Mx topologies of competing exchange interactions which prevent (frustrate) the preferred spin alignments that would otherwise normally yield low-spin species. The study of high-spin molecules has taken on additional importance in recent years as it has been realized that a fairly large S value is a necessary (but not sufficient) property for molecules to exhibit the new magnetic phenomenon of singlemolecule magnetism, i.e., the ability of material composed of discrete, (magnetically) noninteracting molecules to be magnetized by an external magnetic field below a critical blocking temperature (TB). The importance of a high S value in such nanoscale magnets has stimulated a search for new examples of species with this property. We herein report the preparation of a new hexanuclear MnIII cluster with an aesthetically pleasing [Mn6O4Cl4] core and show that it possesses a S ) 12 ground state, one of the highest yet identified. Treatment of [Mn4O2(O2CMe)6(py)2(Me2dbm)2] (1) (Me2dbmH ) 4,4′-dimethyldibenzoylmethane), prepared as for the dbm analogue,7 in CH2Cl2 with 6 equiv of Me3SiCl and 2 equiv of Me2dbmH gave a brown solution from which was isolated crude [MnCl(Me2dbm)2] (2) on addition of Et2O. Redissolution in MeCN:CH2Cl2 (1:1) followed by standing and slight concentration at room temperature over ∼3 weeks gave black crystals of [Mn6O4Cl4(Me2dbm)6]‚3CH2Cl2 (3‚3CH2Cl2), together with some white powder. The latter was removed by filtration and washing of the black crystals with EtOH. Complex 3 can also be obtained by dissolving purified 2 in MeCN:CH2Cl2 (1:1), and subsequent treatment as above, suggesting the formation of 3 to involve a slow hydrolysis of the mononuclear species.8 Nonoptimized yields up to 14% have been obtained to date.8 The structure9 of 3 (Figure 1) consists of a (Mn)6 octahedron with four nonadjacent faces bridged by the μ3-O ions and the other four faces by μ3-Cl ions. Six-coordinate, approximately octahedral geometry at each metal is completed by a chelating Me2dbm group. The cluster has virtual Td symmetry. As expected for high-spin, octahedral MnIII, there is a Jahn-Teller (JT) distortion, taking the form of an axial elongation of the two trans Mn-Cl bonds, making them unusually long (2.618(3)2.692(3) A). In contrast, the Mn-O2(1.876(4)-1.899(5) A) and Mn-O(Me2dbm) (1.903(5)-1.925(5) A) bond lengths are as expected. As a result of (i) the long Mn-Clversus short Mn-O2bonds and (ii) the near trigonal planar geometry at the latter (sum-of-angles (soa) ≈ 349°) compared with marked trigonal pyramidal geometry at the former (soa ≈ 223°), the [Mn6O4Cl4] core is a near tetrahedron with a Clat each vertex, a Mn at the midpoint of each edge, and a O2bridging each face. Although many [M6(μ3-X)8] face-capped metal octahedra are known,10 only a relative few contain two types of X group, e.g., the [Ti6O6Cl2], [Ti6Te6O2], and [Re6YxZ8-x] (x ) 5, Y ) S or Se, Z ) Cl; x ) 6, Y ) S, Z ) Cl)13 cores. Only the [Ti6(μ3O)4(μ3-Cl)4] core of [(C5H5Me)6Ti6O4Cl4] contains, like 3, four O2and 4 Clbridges, but the structure does not approximate to a tetrahedron. [Mn6X8] species have been unknown to date, although a [Mn6(μ3-O)4(μ3-Cl)4] unit as found in 3 but at the 2MnII, 4MnIII level is also a recognizable fragment within the higher nuclearity cluster [Mn10O4Cl12(biphen)4] (biphen ) 2,2′biphenoxide).14