Molybdopterin is an essential cofactor for all forms of life. The cofactor is composed of a pterin moiety appended to a dithiolene-functionalized pyran ring, and through the dithiolene moiety it binds metal ions. Different synthetic strategies for dithiolene-functionalized pyran precursors that have been designed and synthesized are discussed. These precursors also harbor 1,2-diketone or osone functionality that has been condensed with 1,2-diaminobenzene or other heterocycles resulting in several quinoxaline or pterin derivatives. Use of additives improves the regioselectivity of the complexes. The molecules have been characterized by (1)H and (13)C NMR and IR spectroscopies, as well as by mass spectrometry. In addition, several compounds have been crystallographically characterized. The geometries of the synthesized molecules are more planar than the geometry of the cofactor found in proteins.
Neutral complexes of zinc with N,N′-diisopropylpiperazine-2,3-dithione (iPr2Dt0) and N,N′-dimethylpiperazine-2,3-dithione (Me2Dt0) with chloride or maleonitriledithiolate (mnt2−) as coligands have been synthesized and characterized. The molecular structures of these zinc complexes have been determined using single crystal X-ray diffractometry. Complexes recrystallize in monoclinic P type systems with zinc adopting a distorted tetrahedral geometry. Two zinc complexes with mixed-valent dithiolene ligands exhibit ligand-to-ligand charge transfer bands. Optimized geometries, molecular vibrations and electronic structures of charge-transfer complexes were calculated using density functional theory (B3LYP/6-311G+(d,p) level). Redox orbitals are shown to be almost exclusively ligand in nature, with a HOMO based heavily on the electron-rich maleonitriledithiolate ligand, and a LUMO comprised mostly of the electron-deficient dithione ligand. Charge transfer is thus believed to proceed from dithiolate HOMO to dithione LUMO, showing ligand-to-ligand redox interplay across a d10 metal.
A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X 3 (Ln = La 3+ , Ce 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Lu 3+ ; X = NO 3 − , Cl − , OTf −) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO 3 − , T = 267−292 °C; monodentate Cl − , T = 238-262 °C; noncoordinating OTf − , T = 170-183 °C). 13 C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C A , exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C B , displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO 3 − < Cl − < OTf −. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX 3 show a systematic increase in the lanthanide-2 coordination number (CN La-mc = 2 (NO 3 −), 4 (Cl −), 5 (H 2 O, model for OTf −)) and a decrease in the mean Ln−N bond length (La−N average = 2.91 Å (NO 3 −), 2.78 Å (Cl −), 2.68 Å (H 2 O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.
The interligand communication between non-innocent dithiolene ligands of different oxidation states has been described in a Mo system. The fully reduced ene-dithiolate (Dt2−) acts as a donor moiety to the oxidized dithione (Dt0) in an LL′CT process.
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