Synthesis,characterization and magnetostructural correlation studies on three binuclear copper complexes of pyrimidine derived Schiff base ligands

Three binuclear Cu(II) complexes of two pyrimidine derived Schiff base ligands, 2-S-methyl-6-methyl-4-formyl pyrimidine-N(4)-ethyl thiosemicarbazone (HL₁) and salicyl hydrazone of 2-hydrazino-4,6-dimethylpyrimidine (HL₂), have been prepared. HL₁ produces a bis(μ-thiolato) Cu(II) complex co-crystallizing with its mononuclear analog, [Cu₂(L₁)₂(NO₃)₂][Cu(L₁)(NO₃)] (1). On the other hand HL₂ shows versatility by producing two different classes of binuclear Cu(II) complexes, a bis(μ-phenoxo) complex [Cu₂(L₂)₂(NO₃)₂] (2) and another a (μ-4,4′-bipyridyl) complex, [Cu₂(L₂)₂(μ-4,4′-bipyridyl)(NO₃)₂] (3) under suitable conditions. All the three complexes show distorted square pyramidal geometry around each Cu atom but to a varied extent. Magnetic behavior of complex 1 shows that it is strongly ferromagnetic in nature whereas compounds 2 and 3 are weakly antiferromagnetic in nature. A magnetostructural correlation study combined with molecular modelling on complexes 1 and 2 has thrown light on the difference on magnetic interaction between the Cu atoms in these two complexes. Various factors that may be responsible for such differences are also explored. A novel and potentially useful pH dependant conversion of 3 to 2 has also been noticed.     

Convenient method for the functionalization of the 4- and 6-positions of the androgen skeleton

The preparation and reactivity of steroidal vinyldiazo compounds is reported, providing a convenient, substituent tolerant, chemo- and stereoselective entry into 4- and 6-substituted androgen analogues from a common precursor. Under dirhodium catalysis, O-H insertion occurs at the carbenoid site, leading to 4-substituted steroids, but under silver catalysis, O-H insertion occurs at the vinylogous position, leading to 6-substituted steroids.     

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature M?ssbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.     

A causal framework for surrogate endpoints with semi-competing risks data

In this note, we address the problem of surrogacy using a causal modelling framework that differs substantially from the potential outcomes model that pervades the biostatistical literature. The framework comes from econometrics and conceptualizes direct effects of the surrogate endpoint on the true endpoint. While this framework can incorporate the so-called semi-competing risks data structure, we also derive a fundamental non-identifiability result. Relationships to existing causal modelling frameworks are also discussed.