Petrosamine Revisited. Experimental and Computational Investigation of Solvatochromism, Tautomerism and Free Energy Landscapes of a Pyridoacridinium Quaternary Salt

Petrosamine (1)—a colored pyridoacridine alkaloid from the Belizean sponge, Petrosia sp., that is also a potent inhibitor of acetylcholine esterase (AChE)—was investigated by spectroscopic and computational methods. Analysis of the petrosamine-free energy landscapes, pKa and tautomerism, revealed an accurate electronic depiction of the molecular structure of 1 as the di-keto form, with a net charge of q = +1, rather than a dication (q = +2) under ambient conditions of isolation-purification. The pronounced solvatochromism (UV-vis) reported for 1, and related analogs were investigated in detail and is best explained by charge delocalization and stabilization of the ground state (HOMO) of 1 rather than an equilibrium of competing tautomers. Refinement of the molecular structure 1 by QM methods complements published computational docking studies to define the contact points in the enzyme active site that may improve the design of new AChE inhibitors based on the pyridoacridine alkaloid molecular skeleton

Isolation of bastadin-6-O-sulfate and expedient purifications of bastadins-4, -5 and -6 from extracts of Ianthella basta

Bastadin-6-34-O-sulfate ester (8) was isolated from methanol extracts of Ianthella basta. The structure of 8 was characterized by analysis of MS and NMR data, and conversion through acid hydrolysis, to the parent compound, bastadin-6, which was identical by HPLC, MS and NMR with an authentic sample. An improved procedure for procurement of pure samples of bastadins-4 (4), -5 (5) and -6 (6) is described

What Controls the Magnetic Exchange and Anisotropy in a Family of Tetranuclear {Mn₂^IIMn₂^III} Single-Molecule Magnets?

Twelve heterovalent, tetranuclear manganese­(II/III) planar diamond or “butterfly” complexes, <b>1</b>–<b>12</b>, have been synthesized and structurally characterized, and their magnetic properties have been probed using experimental and theoretical techniques. The 12 structures are divided into two distinct “classes”. Compounds <b>1</b>–<b>8</b> place the Mn­(III), <i>S</i> = 2, ions in the body positions of the butterfly metallic core, while the Mn­(II), <i>S</i> = 5/2, ions occupy the outer wing sites and are described as “Class 1”. Compounds <b>9</b>–<b>12</b> display the reverse arrangement of ions and are described as “Class 2”. Direct current susceptibility measurements for <b>1</b>–<b>12</b> reveal ground spin states ranging from <i>S</i> = 1 to <i>S</i> = 9, with each complex displaying unique magnetic exchange parameters (<i>J</i>). Alternating current susceptibility measurements found that that slow magnetic relaxation is observed for all complexes, except for <b>10</b> and <b>12</b>, and display differing anisotropy barriers to magnetization reversal. First, we determined the magnitude of the magnetic exchange parameters for all complexes. Three exchange coupling constants (<i>J</i>_bb, <i>J</i>_wb, and <i>J</i>_ww) were determined by DFT methods which are found to be in good agreement with the experimental fits. It was found that the orientation of the Jahn–Teller axes and the Mn–Mn distances play a pivotal role in determining the sign and strength of the <i>J</i>_bb parameter. Extensive magneto-structural correlations have been developed for the two classes of {Mn^II₂Mn^III₂} butterfly complexes by varying the Mn_b–O distance, Mn_w–O distance, Mn_b–O–Mn_b angle (α), Mn_b–O–Mn_b–O dihedral angle (γ), and out-of-plane shift of the Mn_w atoms (β). For the magnetic anisotropy the DFT calculations yielded larger negative <i>D</i> value for complexes <b>2</b>, <b>3</b>, <b>4</b>, and <b>6</b> compared to the other complexes. This is found to be correlated to the electron-donating/withdrawing substituents attached to the ligand moiety and suggests a possible way to fine tune the magnetic anisotropy in polynuclear Mn ion complexes

What Controls the Magnetic Exchange and Anisotropy in a Family of Tetranuclear {Mn₂^IIMn₂^III} Single-Molecule Magnets?

Twelve heterovalent, tetranuclear manganese­(II/III) planar diamond or “butterfly” complexes, <b>1</b>–<b>12</b>, have been synthesized and structurally characterized, and their magnetic properties have been probed using experimental and theoretical techniques. The 12 structures are divided into two distinct “classes”. Compounds <b>1</b>–<b>8</b> place the Mn­(III), <i>S</i> = 2, ions in the body positions of the butterfly metallic core, while the Mn­(II), <i>S</i> = 5/2, ions occupy the outer wing sites and are described as “Class 1”. Compounds <b>9</b>–<b>12</b> display the reverse arrangement of ions and are described as “Class 2”. Direct current susceptibility measurements for <b>1</b>–<b>12</b> reveal ground spin states ranging from <i>S</i> = 1 to <i>S</i> = 9, with each complex displaying unique magnetic exchange parameters (<i>J</i>). Alternating current susceptibility measurements found that that slow magnetic relaxation is observed for all complexes, except for <b>10</b> and <b>12</b>, and display differing anisotropy barriers to magnetization reversal. First, we determined the magnitude of the magnetic exchange parameters for all complexes. Three exchange coupling constants (<i>J</i>_bb, <i>J</i>_wb, and <i>J</i>_ww) were determined by DFT methods which are found to be in good agreement with the experimental fits. It was found that the orientation of the Jahn–Teller axes and the Mn–Mn distances play a pivotal role in determining the sign and strength of the <i>J</i>_bb parameter. Extensive magneto-structural correlations have been developed for the two classes of {Mn^II₂Mn^III₂} butterfly complexes by varying the Mn_b–O distance, Mn_w–O distance, Mn_b–O–Mn_b angle (α), Mn_b–O–Mn_b–O dihedral angle (γ), and out-of-plane shift of the Mn_w atoms (β). For the magnetic anisotropy the DFT calculations yielded larger negative <i>D</i> value for complexes <b>2</b>, <b>3</b>, <b>4</b>, and <b>6</b> compared to the other complexes. This is found to be correlated to the electron-donating/withdrawing substituents attached to the ligand moiety and suggests a possible way to fine tune the magnetic anisotropy in polynuclear Mn ion complexes

Anion Dependent Redox Changes in Iron Bis-terdentate Nitroxide {NNO} Chelates

The reaction of [Fe II(BF4)2]·6H2O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L•), produces the mononuclear transition metal complex [FeII(L•)2](BF4)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 Fe2+ 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 Fe2+ 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 [FeIII(L-)2](BPh4)·0.5H2O (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

CCDC 925602: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 925600: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures