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In an attempt to understand the aqueous interactions of Cr(III) with the low-molecular-mass physiological ligand citric acid, the pH-specific synthesis in the binary Cr(III)−citrate system was explored, leading to the complex (NH 4 ) 4 [Cr(C 6 H 4 O 7 )(C 6 H 5 O 7 )]‚3H 2 O (1). 1 crystallizes in the monoclinic space group I2/a, with a ) 19.260(10) Å, b ) 10.006(6) Å, c ) 23.400(10) Å, β ) 100.73(2)°, V ) 4431(4) Å 3 , and Z ) 8. 1 was characterized by elemental analysis and spectroscopic, structural, thermal, and magnetic susceptibility studies. Detailed aqueous speciation studies in the Cr(III)−citrate system suggest the presence of a number of species, among which is the mononuclear [Cr(C 6 H 4 O 7 )(C 6 H 5 O 7 )] 4complex, optimally present around pH ≈ 5.5. The structure of 1 reveals a mononuclear octahedral complex of Cr(III) with two citrate ligands bound to it. The two citrate ligands have different deprotonation states, thus signifying the importance of the mixed deprotonation state in the coordination sphere of the Cr(III) species in aqueous speciation. The latter reveals the distribution of numerous species, including 1, for which the collective structural, spectroscopic, and magnetic data point out its physicochemical profile in the solid state and in solution. The importance of the synthetic efforts linked to 1 and the potential ramifications of Cr(III) reactivity toward both low-and high-molecular-mass biotargets are discussed in light of (a) the quest for well-characterized soluble Cr(III) species that could be detected and identified in biologically relevant fluids, (b) ongoing efforts to delineate the aqueous speciation of the Cr(III)−citrate system and its link to biotoxic Cr(III) manifestations, and (c) the synthetic utility of convenient Cr(III) precursors in the synthesis of advanced materials.

The facile copper-catalyzed synthesis of polysubstituted pyrroles from aldehydes, amines, and β-nitroalkenes is reported. Remarkably, the use of α-methyl-substituted aldehydes provides efficient access to a series of tetra- and pentasubstituted pyrroles via an overwhelming 1,2-phenyl/alkyl migration. The present methodology is also accessible to non α-substituted aldehydes, yielding the corresponding trisubstituted pyrroles. On the contrary, the use of ketones, in place of aldehydes, does not promote the organic transformation, signifying the necessity of α-substituted aldehydes. The reaction proceeds under mild catalytic conditions with low catalyst loading (0.3-1 mol %), a broad scope, very good functional-group tolerance, and high yields and can be easily scaled up to more than 3 mmol of product, thus highlighting a useful synthetic application of the present catalytic protocol. Based on formal kinetic studies, a possible radical pathway is proposed that involves the formation of an allylic nitrogen radical intermediate, which in turn reacts with the nitroalkene to yield the desired pyrrole framework via a radical 1,2-phenyl or alkyl migration.