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George Christou

Publications and source records attributed to George Christou.

At least 19 recordsLinked to original sources
PubMed2006

[Mn7O5(OR)2(O2CPh)9(terpy)] (R = Me, CH2Ph) complexes with a fused cubane/butterfly core and an S = 6 ground-state spin.

Two new heptanuclear Mn clusters, [Mn7O5(OMe)2(O2CPh)9(terpy)] (1) and [Mn7O5(OCH2Ph)2(O2CPh)9(terpy)] (2), were prepared from the partial alcoholysis of the trinuclear complex [Mn3O(O2CPh)6(py)2(H2O)] (3) in the presence of terpy (terpy = 2,2':6',2' '-terpyridine). Complexes 1 and 2 crystallize in the triclinic P and the orthorhombic Pbca space groups, respectively. The clusters are both mixed valent, containing three Mn oxidation states: MnIV, 5MnIII, and MnII. The Mn ions are held together by nine doubly bridging benzoates, four mu3-O2- ions, one mu5-O2- ion, and either two mu-MeO- (1) or two mu-PhCH2O- (2) groups. The single terpy chelate in each complex is attached to the MnII ion. The core topology is novel and very unusual, comprising a cubane and a butterfly unit fused by sharing a MnIII and the mu5-O2- ion. Solid-state dc and ac magnetic susceptibility studies establish that complexes 1 and 2 both possess an S = 6 ground-state spin. Fits of variable-temperature and -field magnetization data gave S = 6, g = 1.88, and D = -0.21 cm-1 for 1 and S = 6, g = 1.86, and D = -0.18 cm-1 for 2. Single-crystal magnetization vs dc field scans down to 0.1 K for 2 show only very little hysteresis at 0.1 K.

Journal Article↗
PubMed2006

A family of [Mn6] complexes featuring tripodal ligands.

The synthesis and magnetic properties of four new Mn complexes containing tripodal alcohol ligands are reported: [Mn6(OAc)6(H2tea)2(tmp)2].2MeCN (1.2MeCN), [Mn6(acac)4(OAc)2(Htmp)2(H2N-ep)2] (2), [Mn6(OAc)8(tmp)2(py)4].2py (3.2py), and [Mn6(OAc)8(thme)2(py)4].2py (4.2py) [H3tea, triethanolamine; H3tmp, 1,1,1-tris(hydroxymethyl)propane; H2N-H2ep, 2-amino-2-ethyl-1,3-propanediol; H3thme, 1,1,1-tris(hydroxymethyl)ethane]. All complexes are mixed-valent with a [Mn(III)2Mn(II)4] oxidation assignment and are constructed from four edge-sharing triangles but differ slightly in that complexes 1 and 2 display a [Mn(III)2Mn(II)4(mu2-OR)6(mu3-OR)4]4+ core, while complexes 3 and 4 feature [Mn(III)2Mn(II)4(mu2-OR)2(mu3-OR)4]8+ and [Mn(III)2Mn(II)4(mu2-OR)4(mu3-OR)4]6+ cores, respectively. dc and ac magnetic susceptibility studies in the 2-300 K range for complexes 1-4 reveal the presence of dominant antiferromagnetic exchange interactions, leading to ground states of S = 0 for 1 and 2, while complexes 3 and 4 display S = 4 ground states with D = -0.44 and -0.58 cm(-1), respectively. Single-molecule magnetism behavior was confirmed for 3 and 4 by the presence of sweep-rate and temperature-dependent hysteresis loops in single-crystal M vs H studies at temperatures down to 40 mK. Theoretical density functional calculations were used to evaluate the individual pairwise exchange interactions present, confirming the diamagnetic ground states for 1 and 2 and the S = 4 ground states for 3 and 4.

Journal Article↗
PubMed2006

Making "wheels" and "cubes" from triangles.

[Mn(IV)Mn(II)3] triangular units directed by the presence of tripodal alcohols self-assemble in the presence of azide and acetate ligands to form either a [Mn24] "wheel" or a [Mn32] "cube".

Journal Article↗
PubMed2006

The properties of the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets in truly axial symmetry: [Mn12O12(O2CCH2Br)16(H2O)4].4CH2Cl2.

Detailed studies are reported of a Mn(12) single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn(12)O(12)(O(2)CCH(2)Br)(16)(H(2)O)(4)].4CH(2)Cl(2) (2.4CH(2)Cl(2) or Mn(12)-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn(12) family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1 or Mn(12)-Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal (55)Mn NMR spectra on Mn(12)-BrAc display much sharper signals than a single crystal of Mn(12)-Ac, and this allows one Mn(III) signal to show an almost baseline-resolved quintet from quadrupolar splitting ((55)Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn(12)-BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn(12)-BrAc is far superior to Mn(12)-Ac for the study of the intrinsic properties of the Mn(12) family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian.

Journal Article↗
PubMed2006

Ligand-induced distortion of a tetranuclear manganese butterfly complex.

The reaction of the pentadentate Schiff-base ligand 1,3-bis(salicylideneamino)-2-propanol (salproH3) with [Mn3O(O2CR)6(py)3] (R = Me, Et, But) gives the corresponding tetranuclear manganese product [Mn4O2O2CR)5(salpro)] (4Mn(III)). The syntheses, structure and magnetochemical characterization of these complexes are reported. The structure of the [Mn4(mu3-O)2]8+ is butterfly-like much more closed than in previous complexes with this core as a result of the alkoxide oxygen of the salpro ligand bridging the two wingtip Mn atoms. Variable-temperature, solid-state magnetic susceptibility studies reveal that these complexes possess S = 0 ground state spins. Fitting of the magnetic susceptibility data to the theoretical chiMT vs. T expression derived for a C2v symmetry complex, assuming an isotropic Heisenberg spin-Hamiltonian and using the Van Vleck equation, revealed that the various exchange parameters are all antiferromagnetic, and the core thus experiences spin frustration effects.

Crystallography, X-Ray↗
PubMed2006

Largest mixed transition metal/actinide cluster: a bimetallic Mn/Th complex with a [Mn10Th6O22(OH)2]18+ core.

A high-nuclearity mixed transition metal/actinide complex has been prepared from the reaction of a Mn(III)4 complex with Th(NO3)4 in MeCN/MeOH. The complex [Th6Mn10O22(OH)2(O2CPh)16(NO3)2-(H2O)8] is the largest such complex to date and the first Th/Mn species. It is rich in oxide groups, which stabilize all of the metals in the high Th(IV) and Mn(IV) oxidation levels. Magnetic characterization establishes that the complex has an S = 3 ground-state spin value.

Journal Article↗
PubMed2005

Single-molecule magnets: a reductive aggregation route to new types of Mn12 complexes.

Three dodecanuclear Mn clusters [Mn12O10(OMe)3(OH)(O2CC6H3F2)16(MeOH)2].8MeOH (1), [Mn12O10(OMe)4(O2CBu(t))16(MeOH)2] (2), and [Mn12O12(O2CBu(t))16(MeOH)4] (3) synthesized by reductive aggregation reactions are reported. Clusters 1 and 2 possess a central alkoxide-bridged planar Mn4 topology, whereas 3 is a new high-symmetry member of the normal Mn12 family. Complexes 1 and 2 crystallize in the monoclinic space groups C2/c and P2(1)/n, respectively. Both consist of four Mn(IV) and eight Mn(III) ions held together by 10 mu3-O2- ions, and either (i) one mu-OH- and three mu-MeO- groups for 1 or (ii) four mu-MeO- groups for 2. Complex 3 crystallizes in the orthorhombic space group Aba2 and possesses the normal Mn12 structure but with terminal MeOH molecules. The cyclic voltammogram (CV) of 1 exhibits no reversible redox processes. Variable-temperature, solid-state dc and ac magnetic susceptibility measurements on 1 and 2 reveal that they possess S = 5 and 9 ground states, respectively. In addition, ac susceptibility measurements on complex 1 in a zero dc field in the temperature range 1.8-10 K and in a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display a nonzero frequency-dependent out-of-phase (chi(M)'') signal at temperatures below 3 K, with the peak maxima lying at temperatures below 1.8 K. For complex 2, two frequency dependent chi(M)'' signals are seen, one in the higher temperature range of 3-5 K and a second at lower temperatures with its peak maxima at temperatures below 1.8 K. Single-crystal magnetization vs dc field scans down to 0.04 K for 1.8MeOH and 2 show hysteresis behavior at <1 K, confirming that both complexes are new examples of SMMs.

Journal Article↗
PubMed2005

Initial example of a triangular single-molecule magnet from ligand-induced structural distortion of a [MnIII3O]7+ complex.

The reaction of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et) with methyl 2-pyridyl ketone oxime (mpkoH) in a 1:3 molar ratio in MeOH/MeCN leads to [Mn3O(O2CR)3(mpko)3](ClO4) in 80-90% isolated yield. Ferromagnetic exchange interactions between the three MnIII ions in the nonplanar [MnIII3O]7+ triangular core lead to a spin ground state of S = 6; single-crystal studies reveal the temperature and sweep rate dependent hysteresis loops expected for a single-molecule magnet.

Journal Article↗
PubMed2005

1,1,1-Tris(hydroxymethyl)propane in manganese carboxylate chemistry: synthesis, structure and magnetic properties of a mixed-valence [MnIII4MnII4] cluster featuring the novel [MnIII4MnII4(mu3-OR)6(mu2-OR)8]6+ core.

The reaction between MnBr(2).4H(2)O with H(3)tmp (1,1,1-tris(hydroxymethyl)propane) in MeCN in the presence of Na(O(2)CCMe(3)) and NBu(4)Br produces the complex [Mn(8)(O(2)CCMe(3))(2)(tmp)(2)(Htmp)(4)Br(4)(H(2)O)(2)].2MeCN (1.2MeCN) in good yield. The centrosymmetric octanuclear molecule consists of four Mn(III) and four Mn(II) ions assembled together by fourteen alkoxo bridges to give a [Mn(III)(4)Mn(II)(4)(mu(3)-OR)(6)(mu(2)-OR)(8)](6+) rod-like core in which the metal centres are arranged in a planar zigzag fashion. Peripheral ligation is provided by a combination of bridging pivalate ions, terminal bromides and water molecules. Dc magnetic susceptibility measurements reveal the presence of dominant antiferromagnetic interactions leading to a spin ground state of S = 0. A rationalization of this result is attempted by structural comparison with previously reported tetranuclear manganese complexes containing the [Mn(III)(2)Mn(II)(2)(mu(3)-OR)(2)(mu(2)-OR)(4)] core in which the magnetic interactions are ferromagnetic.

Bromides↗