New binucleating ligands to support dizirconium organometallics.

A new class of bis(amidinato) ligands, which feature dibenzofuran or 9,9-dimethylxanthene spacers, are used to prepare structurally characterized dizirconium complexes.

N-donor effects on carboxylate binding in mononuclear iron(II) complexes of a sterically hindered benzoate ligand.

Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes2ArCO2-, a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes2ArCO2)2Fe(base)2 (base = 1-methylimidazole (MeIm), pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)2 = N,N,N',N'-tetramethylethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal X-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of alpha-substitution of the N-donors. Increasing alpha-substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from eta 1 to eta 2. We surmise that this change is due to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for inducing carboxylate shifts could play a critical role in biological systems.

Ligand macrocycle structural effects on copper-dioxygen reactivity.

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric mu-eta 2:eta 2-peroxo- and bis(mu-oxo)dicopper complexes, a comparative study was undertaken of the O2 reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R3-1,4,7-triazacyclodecane (R3TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD). While the 3-coordinate complex [(iPr3TACDD)Cu]SbF6 was unreactive with O2, oxygenation of [(R3TACD)Cu(CH3CN)]X (R = Me or Bn; X = ClO4- or SbF6-) at -80 degrees C yielded bis(mu-oxo) species [(R3TACD)2Cu2(mu O)2]X2 as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN), which yielded interconverting mixtures of peroxo and bis(mu-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161), low-temperature oxygenation of [(iPr3TACD)Cu(CH3CN)]SbF6 in a variety of solvents cleanly yielded a mu-eta 2:eta 2-peroxo product, with no trace of the bis(mu-oxo) isomer. The peroxo complex was characterized by UV-vis and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder problems). Intramolecular attack at the alpha C-H bonds of the substituents was indicated as the primary decomposition pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products, which included bis(mu-hydroxo)- and mu-carbonato-dicopper complexes that were characterized by X-ray diffraction. A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(mu-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.