RESUMEN
The synthesis of the novel macrocyclic octadentate amino-thiophenolate ligand H(2)L2 (3,7,11,19,23,27-hexaaza-33,34-dithiol-15,31-di(tert-butyl)-tricyclo[27,3,1(13.17)]-tetratriaconta-1(32),13,15,17(34),29,30-hexane) and its ability to support binuclear nickel(II) complexes with dithiolato-bridged square-pyramidal Ni(II) ions are reported. H(2)L2 is obtained as the hexahydrobromide salt from a Schiff-base condensation reaction between 1,2-bis(4-tert-butyl-2,6-diformylphenylthio)ethane and bis(3-aminopropyl)amine followed by two successive reductions with NaBH(4) and Na/NH(3). The ligand forms a green, paramagnetic, binuclear nickel(II) complex dication [Ni(II)(2)L2](2+), which can be isolated as a ClO(4)(-) (4) or BPh(4)(-) salt (5). The binuclear nickel(II) complex contains a central N(3)Ni(µ-S)(2)NiN(3) core with two square-pyramidal coordinated Ni(II) ions. The [Ni(2)L2](2+) dication does not bind further coligands, in striking contrast to the behaviour of the parent [Ni(2)L1](2+) dication supported by the smaller (L1)(2-) macrocycle (containing diethylenetriamine in place of the dipropylenetriamine units) which readily binds a variety of other coligands (L') to form bisoctahedral [Ni(2)L1(L')](+) structures. The unusual behaviour of 4 relates to two different N configurations which leads to a steric shielding of the third bridging position by the CH(2)-groups of the dipropylenetriamine chains. An analysis of the temperature-dependent magnetic susceptibility data of 5 reveals the presence of a weak antiferromagnetic exchange interaction between the spins of the nickel(II) ions with a value for the magnetic exchange coupling constant J of -23.5 cm(-1) (H = -2JS(1)S(2)). These results are further substantiated by DFT calculations.
RESUMEN
The dinickel(II) complex [Ni2L(ClO4)]ClO4 (1), where L(2-) represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [nBu4N]H2PO2 to form the hypophosphito-bridged complex [Ni2L(µ-O2PH2)](+), which can be isolated as an air-stable perchlorate [Ni2L(µ-O2PH2)]ClO4 (2) or tetraphenylborate [Ni2L(µ-O2PH2)]BPh4 (3) salt. 3·MeCN crystallizes in the triclinic space group P1Ì . The bisoctahedral [Ni2L(µ-O2PH2)](+) cation has a N3Ni(µ1,3-O2PH2)(µ-S)2NiN3 core structure with the hypophosphito ligand attached to the two Ni(II) ions in a µ1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni2L](2+) dication for more basic oxoanions. Treatment of [Ni2L(µ-O2PH2)]ClO4 with H2O2 or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence of ferromagnetic exchange interactions between the Ni(ii) (S = 1) ions with a value for the magnetic exchange coupling constant J of +22 cm(-1) (H = -2JS1S2). These results are additionally supported by DFT (density functional theory) calculations.
RESUMEN
We hereby report the synthesis, crystal structures and magnetic characterizations of three novel Cr(III) complexes: a mononuclear Cr(III)(HL((1)))3 species along with two dinuclear µ-methoxo Cr(III)2(µ-OMe)2(HL)4 (H2L((1)): 2-salicyloylhydrazono-1,3-dithiolane; H2L((2)): 2-salicyloylhydrazono-1,3-dithiane). EPR studies indicate that both dinuclear complexes (2a and 2b) are stable in solution (CHCl3). Both Cr(III) dinuclear complexes exhibit a strong antiferromagnetic coupling. DFT calculations performed for complex 2a are in agreement with a strong antiferromagnetic exchange interaction in these new dinuclear Cr(III) complexes.
RESUMEN
The first combined high pressure single-crystal X-ray diffraction and high pressure magnetism study of two polymetallic clusters is presented in an attempt to correlate the observed changes in structure with changes in magnetic response without the need for changes in external ligation. At 1.5 GPa the structure of [Mn(6)O(2)(Et-sao)(6)(O(2)CPh(Me)(2))(2)(EtOH)(6)] (1; Et-saoH(2) = 2-hydroxyphenylpropanone)--a single molecule magnet (SMM) with an effective anisotropy barrier of approximately 86 K--and of [Mn(6)O(2)(Et-sao)(6)(O(2)C-naphth)(2)(EtOH)(4)(H(2)O)(2)] 2 both undergo significant structural distortions of their metallic skeletons, which has a direct effect upon the observed magnetic response. The application of hydrostatic pressure on the two compounds (up to 1.5 GPa) flattens the Mn-N-O-Mn torsion angles weakening the magnetic exchange between the metal centres. In both compounds one interaction switches from ferro- to antiferromagnetic, with the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with respect to the plane of the three metal centres. High pressure dc chi(M)T plots display a gradual decrease in the low temperature peak height and slope, simulations showing a decrease in |J| with increasing pressure with a second antiferromagnetic J value required to simulate the data. The "ground states" change from S = 12 to S = 11 for 1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in |D|, while out-of-phase (chi(M)(//)) ac data show a large decrease in the effective energy barrier for magnetisation reversal.
RESUMEN
The dinuclear chromium complexes [Cr(2)(N,O)(3)Cl(3)] (6) (N,O = 4,4-dimethyl-2-oxazolylmethanolate), [Cr(2)(N,O(Me2))(2)(EtOH)(2)Cl(4)] (7) and [Cr(2)(N,O(Me2))(2)(H(2)O)(2)Cl(4)] (8) (N,O(Me2) = 4,4-dimethyl-2-oxazolyldimethylmethanolate) have been prepared and characterized, including by single-crystal X-ray diffraction. Complex 6 is unsymmetrical, with two chloride ligands terminally bound to one Cr atom, whereas 7 and 8 (in 8 x C(4)H(8)O) which contain two molecules of coordinated ethanol or water, respectively, are centrosymmetric. These chromium complexes are paramagnetic, and the magnetic properties of 6 and 7 in the solid state correspond to antiferromagnetic behaviour, which was confirmed by DFT calculations of their electronic structures. Complexes 6-8 were evaluated in the catalytic oligomerization and/or polymerization of ethylene with different aluminium-based cocatalysts, and MMAO proved to be the most effective one. In the presence of MMAO, the influence of different reaction parameters, such as the Al/Cr molar ratio, reaction temperature and ethylene pressure, was investigated. Complex 7 showed the highest activity for ethylene polymerization at both 1 atm and 10 atm of ethylene pressure, up to 620 000 g mol(-1)(Cr) h(-1) in the latter case.
RESUMEN
This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn6 complexes and suggests that large magnetic anisotropy is not favoured by a high spin state of the ground state.