Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

We have determined the ultra-low temperature crystal structure of the archetypal single-molecule magnet (SMM) [Mn12O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H (1) at 2 K, by using a combination of single-crystal X-ray and single-crystal neutron diffraction. This is the first structural study of any SMM in the same temperature regime where slow magnetic relaxation occurs. We reveal an additional hydrogen bonding interaction between the {Mn12} cluster and its solvent of crystallisation, which shows how the lattice solvent transmits disorder to the acetate ligands in the {Mn12} complex. Unusual quantum properties observed in 1 have long been attributed to disorder. Hence, we studied the desolvation products of 1, in order to understand precisely the influence of lattice solvent on the structure of the cluster. We present two new axially symmetric structures corresponding to different levels of desolvation of 1, [Mn12O12(O2CMe)16(H2O)4]·4H2O (2) and [Mn12O12(O2CMe)16(H2O)4] (3). In 2, removal of acetic acid of crystallisation largely resolves positional disorder in the affected acetate ligands, whereas removal of lattice water molecules further resolves the acetate ligand disorder in 3. Due to the absence of acetic acid of crystallisation, both 2 and 3 have true, unbroken S4 symmetry, showing for the first time that it is possible to prepare fully axial Mn12–acetate analogues from 1, via single-crystal to single-crystal transformations

Spin density studies on p-O2NC6F4CNSSN: A heavy p-block organic ferromagnet

A complete picture of the spin density distribution in the organic radical p-O2NC6F4CNSSN has been obtained by a combination of polarized neutron diffraction, electron paramagnetic resonance (EPR), and electron-nuclear double resonance (ENDOR) spectroscopies, and ab initio density-functional theory (DFT) calculations. Polarized neutron diffraction revealed that the spin distribution is predominantly localized on the N and S atoms (+0.25μB and +0.28μB, respectively) of the heterocyclic ring with a small negative spin density on the heterocyclic C atom (−0.06μB). These spin populations are in excellent agreement with both ab initio DFT calculations (spin populations on the C, N, and S sites of −0.07, 0.22 and 0.31, respectively) and cw-EPR studies which estimated the spin population on the N site as 0.24. The DFT calculated spin density revealed less than 1% spin delocalization onto the perfluoroaryl ring, several orders of magnitude lower than the density on the heterocyclic ring. cw-ENDOR studies at both X-band (9 GHz) and Q-band (34 GHz) frequencies probed the spin populations on the two chemically distinct F atoms. These spin populations on the F atoms ortho and meta to the dithiadiazolyl ring are of magnitude 10−3 and 10−4, respectively

Role of the hydrogen bonds in nitroanilines aggregation : charge density study of 2-methyl-5-nitroaniline.

The electron charge distribution of 2-Methyl-5-nitroaniline has been studied from high-resolution singlecrystal X-ray data at 100 K, and ab initio calculations which include X-ray structure factors computed from a superposition of ab initio molecular electron densities. Using the Hansen and Coppens' rigid pseudoatom multipolar model refinements were performed on both the experimental and the theoretical X-ray data sets from which, molecular atomic charges and dipolar moments were obtained. To understand the nature and the magnitude of the intermolecular interactions. the Atoms in Molecules theory was used to investigate the topology of the electron density of the in-crystal, both experimental interacting as well as theoretical noninteracting, and in-vacuum molecules. A meticulous analysis of the topological properties of the experimental charge density and of its Laplacian indicates, contrary to expectations, a two center character of the N-(HO)- O-. . . synthons that induce the known polar chain formation in nitroanilines and the presence of a C-methyl-H-O interaction further strengthening the chains. It also shows the attractive nature of the rather strong C-(HO)-O-. . . interactions that help the head-to-tail arrangement of the chains. They build two intermolecular six membered hydrogen bonded rings, embracing a N-(HO)-O-. . . interaction, that originate centrosymmetric dimers which impair the macroscopic second harmonic generation of the title compound. The authenticity of a previously proposed closed shell C-aryl-H(. . .)pi interaction between adjacent chains has been confirmed. The latter has not been observed in m-nitroaniline, 2-methyl-4-aniline or other related compounds with chains built from similar N-(HO)-O-. . . synthons and assembled head-to-head. Crystallization causes a molecular electric dipolar moment higher than that of the free molecule, the latter being coincident with the experimental value in solution, and with the one calculated from the refinement of the theoretical X-ray data. It also induces changes in the charge density distribution and its topology, and an enhancement of the intramolecular conjugation that can be related to a molecular aggregation mechanism ruled by the N- (HO)-O-. . . synthon. These findings strongly point to the existence of cooperative effects

A new precatalyst for the Suzuki reaction - a pyridyl-bridged dinuclear palladium complex as a source of mono-ligated palladium(0)

A dinuclear pyridyl-bridged palladium complex, trans-(P,N)-[PdBr(μ- C5H4N-C2,N)(PPh3)]2 1, was obtained from material isolated from the Suzuki cross-coupling reaction of 2-bromopyridine with 2,4-difluorophenylboronic acid in the presence of catalytic (PPh3)4Pd. Complex 1 is an effective precatalyst for the Suzuki cross-coupling reactions of a variety organoboronic acids and aryl bromides, and represents a useful source of mono-ligated palladium(0), "(Ph3P)Pd(0)"

Structural origin of the gradual spin transition in a mononuclear iron(II) complex

Variable temperature single crystal X-ray diffraction and SQUID magnetometry experiments have revealed a gradual spin transition in [FeII(L)](ClO4)2 (where L=1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane), centred around room temperature. The gradual nature of the spin transition has been attributed to the lack of significant intermolecular interactions between iron centres and the propensity of the counter ions to accommodate the internal strain in the crystal caused by spin crossover