The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition metal complex [Fe(II)(L(•))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(•) coordinate equatorially while the oxygen containing the radical from L(•) coordinates axially forming a linear O(•)··Fe(II)··O(•) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.

The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.

The elaboration of readily available nitrogen heterocycles to complex targets is integral to medicinal and natural products chemistry, as such new strategies to derivatize heterocycles retain relevance. As part of ongoing studies on alkaloid synthesis, [1] we noted a lack of direct enantioselective methods for the assembly of C3-chiral carbazolones (i.e., 1), motifs common in medicinal [2] and natural products chemistry (Scheme 1). [3] This is a striking observation, as a host of methods for the stereoselective installation of functionality a to the carbonyl functionality of the parent carbazolone should potentially be viable. Furthermore, our examination of the literature reveals only one, ultimately unsuccessful, attempt to demonstrate this strategy. [4] Motivated by the notion that direct access to chiral carbazolones, such as 1, should provide a valuable chiral synthon useful in target directed synthesis, [5] we commenced studies on this topic. Herein, we report the realization of this strategy with a four-step synthesis of a diverse range of enantioenriched carbazolones and indolones (i.e., 1) by exploiting the enantioselective Pd-catalyzed decarboxylative allylation of 2 (Scheme 2). [6] Preliminary studies on the utility of carbazolones 1 have been undertaken, resulting in the completion of a formal synthesis of (+)-kopsihainanine A (3). [7] Although Pd-catalyzed decarboxylative allylation [8,9] of carbazolones are yet to be reported, [10,11] the transformations of related vinylogous esters and thioesters are known. [12] In these studies, the enhanced enantioselectivity of the thioester relative to the ester is attributed to the modest p-donation of the sulfur atom. We postulated that carbazolones should behave as vinylogous amides, [13,14] thus appropriate indole protection should allow attenuation of the p-donation of the nitrogen atom and induce high enantioselectivity. Supporting the viability of such a strategy is a recent report by Stoltz and co-workers, who demonstrate improved enantioselectivity with the allylation of lactams that bear electron-withdrawing N-protection. [15] Hence, our studies commenced with an investigation of the role of N-protection on enantioselectivity.When N-methyl-protected carbazolone 4 a was subjected to conditions developed by Stoltz and co-workers, [16] namely the PHOX ligand (L1) [17] and [Pd 2 (dba) 3 ] at room temperature, no reaction occurred. However, heating to 80 8C provided allyl carbazolone 1 a in 69 % yield and 66 % ee (Table 1, entry 1). Trosts ligand L3 was unsuitable for this reaction at any temperature (Table 1, entry 2). Changing from N-methyl to N-benzyl protection was tolerated, although carbazolone 1 b formed with decreased enantioselectivity Scheme 1. C3-chiral carbazolones 1 and representative medicinal and natural products.Scheme 2. Enantioselective Pd-catalyzed decarboxylative allylation described herein.

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The synthesis, structures, magnetism and ab initio calculations are presented for three new triangular Ln3 single molecule toroidal species, Ln = Tb, Dy and Ho, and for a new hexagonal Dy6 wheel compound.