Nayanmoni Gogoi

Origin of the Magnetic Anisotropy in Heptacoordinate Ni-II and Co-II Complexes

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2u2009H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

Heptacoordinated nickel(II) as an Ising-type anisotropic building unit: illustration with a pentanuclear [(NiL)3{W(CN)8}2] complex.

Heptacoordinated nickel(II) complexes characterized by substantial Ising-type single-ion anisotropy have been involved in the construction of two pentanuclear [Ni3W2] compounds by association with [W(CN)8](3-). For one of them, slow relaxation of magnetization was observed to occur concomitantly with antiferromagnetic ordering.

Geometry-Mediated Enhancement of Single-Ion Anisotropy: A Route to Single-Molecule Magnets with a High Blocking Temperature†

Not just any old iron ion: A linear, two-coordinate ionic Fe(I) complex with a S=3/2 ground state has a large energy barrier for magnetization reversal, Ueff =226u2005cm(-1) , and undergoes slow magnetic relaxation in the absence of an applied magnetic field. The preparation of complexes with these properties is a step towards the eventual practical application of single-molecule magnets.

Assembling discrete D4R zeolite SBUs through noncovalent interactions. 3.(1) Mediation by butanols and 1,2-bis(dimethylamino)ethane

The use of tetrameric zinc phosphate [Zn(dipp)(CH(3)OH)](4) (1; dipp = diisopropylphenylphosphate dianion) as a suitable building block for realizing new noncovalently linked extended structures, via facile replacement of coordinated methanol molecules by other alcohols, is reported herein. Compounds [Zn(dipp)(sec-butanol)](4) x 4 H(2)O (2) and [Zn(dipp)(tert-butanol)](4) x 4 H(2)O (3) have been synthesized by the addition of sec- or tert-butanol to 1 at room temperature. The reaction of zinc acetate or zinc sulfate with dippH(2) in the presence of N,N,N,N-tetramethylethylenediamine (tmeda) under similar reaction conditions results in the formation of [H(2)tmeda][Zn(4)(dipp)(4)(MeOH)(2)(OAc)(2)] x (CH(3)OH) (4) or [H(2)tmeda][Zn(3)(dipp)(3)(dippH)(2)(CH(3)OH)] x (CH(3)OH)(3) (5), respectively. Analytically pure compounds 2-5 have been isolated in the form of single crystals directly from the respective reaction mixtures in very good yields and characterized with the aid of analytical and spectroscopic studies. Single-crystal X-ray diffraction studies reveal that compounds 2 and 3 are neutral tetranuclear zinc phosphates. Compound 4 is also a tetrameric phosphate but is anionic. The core structures of compounds 2-4 resemble the double-4-ring secondary building unit (D4R SBU) in zeolites. Compound 5 is a trinuclear ionic zinc phosphate built from three fused S4R SBUs. Compounds 4 and 5 represent the first examples of discrete anionic zinc organophosphates. The presence of coordinated sec- or tert-butanol molecules and a planar water tetramer cluster in 2 and 3, and the H(2)tmeda cations and methanol solvents in 4 and 5, leads to the formation of zigzag chainlike supramolecular assemblies in the solid state.

Ab Initio Chemical Synthesis of Designer Metal Phosphate Frameworks at Ambient Conditions

Stepwise hierarchical and rational synthesis of porous zinc phosphate frameworks by predictable and directed assembly of easily isolable tetrameric zinc phosphate [Zn(dipp)(solv)]4 (dippH2 = diisopropylphenyldihydrogen phosphate; solv = CH3OH or dimethyl sulfoxide) with D4R (double-4-ring) topology has been achieved. The preformed and highly robust D4R secondary building unit can be coordinatively interconnected through a varied choice of bipyridine-based ditopic spacers L1-L7 to isolate eight functional zinc phosphate frameworks, [Zn4(dipp)4(L1)1.5(DMSO)]·4H2O (2), [Zn4(dipp)4(L2)1.5(CH3OH)] (3), [Zn4(dipp)4(L1)2] (4), [Zn4(dipp)4(L3)2] (5), [Zn4(dipp)4(L4)2] (6), [Zn4(dipp)4(L5)2] (7), [Zn4(dipp)4(L6)2] (8), and [Zn4(dipp)4(L7)2] (9), in good yield. The preparative procedures are simple and do not require high pressure or temperature. Surface area measurements of these framework solids show that the guest accessibility of the frameworks can be tuned by suitable modification of bipyridine spacers.

Auto-reusable receptors for selective colorimetric recognition of fluoride

Two new aliphatic oximes featuring additional pyridyl functionality are reported as highly selective colorimetric receptors for fluoride ion recognition. H-bonding mediated binding of fluoride ion to both the receptors are reasonably labile and the fluoride free receptor species are automatically regenerated after a short interval. This allows reuse of both the receptors for colorimetric fluoride recognition upto three cycles without any need for reactivation.

Influence of aromatic nitro-substituents on auto-reusability of oxime-based fluoride receptors

AbstractThe interaction of fluoride ion with three oxime based receptors containing additional aromatic nitro-substituents was studied by using colorimetric and NMR spectroscopy. Colorimetric responses of the receptors towards fluoride ion were found to be highly dependent on the position of the nitro substituent in the aromatic backbone. While the ortho and para nitro-substituted receptors showed intense colour change and associated colorimetric response upon addition of fluoride, the meta derivative did not provide any visible deviation in colour as well as colorimetric behaviour. Further, due to the presence of H-bond acceptor motif, the receptor–fluoride complexes formed by both ortho and para nitro-substituted derivatives are reasonably labile and dissociate to automatically result in the free receptor species within a short time span. This allowed reuse of the receptor solution for selective and successive recognition of fluoride by colorimetric technique without any external trigger.Graphical AbstractThree new oximes with additional aromatic nitro-substituents are prepared and their interaction of fluoride ion was studied. The position of nitro-substituents profoundly influences the ability to reuse the receptors for colorimetric recognition of fluoride. Based on spectroscopic studies, a possible mechanism for auto-regeneration of the fluoride free receptors is detailed.

Surprises from an old reaction: a new 1-D heterometallic tris(pyridine-2-aldoximato)nickel(II)–sodium complex

The reaction of [Ni(pao)2Cl2] (paoxa0=xa0pyridine-2-aldoxime) with pyridine (py) in the presence of NaOH produced a 1-D polymeric heterometallic species, {[Ni2Na3(pao)6(OH) (H2O)](C5H5N)(H2O)2}n (1), along with a known bis(pyridine) complex, [Ni(pao)2(py)2] (2). Complex 1 represents a hitherto unknown example of a 1-D polymeric heterometallic structure composed of tris(pyridine-2-aldoximato)nickel(II) metalloligands. Further, an expeditious route to prepare the useful building block, [Ni(pao)2(py)2] (2), in good yield is reported.Graphical Abstract