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Reaction of the 9,9-dimethylxanthene-bis(imine)-bis(catechol) ligand XbicH 4 with half an equivalent of Zr(acac) 4 affords the neutral tetracatecholate complex (XbicH 2 ) 2 Zr, containing four iminium ions hydrogen bonded to the catecholates. The heteroleptic bis(catecholate)-tetraphenylporphyrin complex (TPP)Zr(XbicH 2 ) is formed from reaction of (TPP)Zr(OAc) 2 with XbicH 4 in the presence of base. Both compounds adopt an eight-coordinate square antiprismatic geometry around the zirconium center. NMR spectra of (TPP)Zr(XbicH 2 ) show that it is fluxional at room temperature, with homoleptic (XbicH 2 ) 2 Zr showing fluxionality at higher temperatures. Calculations and kinetic isotope effect measurements suggest that the motions involve dissociation of a single catecholate oxygen and subsequent twisting of the seven-coordinate species. The compounds show reversible one-electron oxidations of each of the bound catecholates to bound semiquinones. 

2-(Arylamino)-4,6-di- tert -butylphenols containing 4-substituted phenyl groups ( R apH 2 ) react with oxobis(ethylene glycolato)osmium( vi ) in acetone to give square pyramidal bis(amidophenoxide)oxoosmium( vi ) complexes. A mono-amidophenoxide complex is observed as an intermediate in these reactions. Reactions in dichloromethane yield the diolate ( H ap) 2 Os(OCH 2 CH 2 O). Both the glycolate and oxo complex are converted to the corresponding cis -dichloride complex on treatment with chlorotrimethylsilane. The novel bis(aminophenol) ligand EganH 4 , containing an ethylene glycol dianthranilate bridge, forms the chelated bis(amidophenoxide) complex (Egan)OsO, where the two nitrogen atoms of the tetradentate ligand bind in the trans positions of the square pyramid. Structural and spectroscopic features of the complexes are described well by an osmium( vi )-amidophenoxide formulation, with the amount of π donation from ligand to metal increasing markedly as the co-ligands change from oxo to diolate to dichloride. In the oxo-bis(amidophenoxides), the symmetry of the ligand π orbitals results in only one effective π donor interaction, splitting the energy of the two osmium-oxo π* orbitals and rendering the osmium-oxo bonding appreciably anisotropic. 

The bis(aminophenol) 2,2′-biphenylbis(3,5-di- tert -butyl-2-hydroxyphenylamine) (ClipH 4 ) forms trans -(Clip)Os(py) 2 upon aerobic reaction of the ligand with {( p -cymene)OsCl 2 } 2 in the presence of pyridine and triethylamine. A more oxidized species, cis -β-(Clip)Os(OCH 2 CH 2 O), is formed from reaction of the ligand with the osmium( vi ) complex OsO(OCH 2 CH 2 O) 2 , and reacts with Me 3 SiCl to give the chloro complex cis -β-(Clip)OsCl 2 . Octahedral osmium and ruthenium tris-iminoxolene complexes are formed from the chelating ligand tris(2-(3′,5′-di- tert -butyl-2′-hydroxyphenyl)amino-4-methylphenyl)amine (MeClampH 6 ) on aerobic reaction with divalent metal precursors. The complexes’ structural and electronic features are well described using a simple bonding model that emphasizes the covalency of the π bonding between the metal and iminoxolene ligands rather than attempting to dissect the parts into discrete oxidation states. Emphasizing the continuity of bonding between disparate complexes, the structural data from a variety of Os and Ru complexes show good correlations to π bond order, and the response of the intraligand bond distances to the bond order can be analyzed to illuminate the polarity of the bonding between metal and the redox-active orbital on the iminoxolenes. The osmium compounds’ π bonding orbitals are about 40% metal-centered and 60% ligand-centered, with the ruthenium compounds’ orbitals about 65% metal-centered and 35% ligand-centered. 

Schiff base condensation of 4,5-diamino-9,9-dimethylxanthene with 4,6-di- tert -butylcatechol-3-carboxaldehyde affords the bis(catecholimine) ligand XbicH 4 , which can bind metals in both a square bis(catecholate) upper pocket and a pentagonal N 2 O 3 lower pocket. Metalation with PhSiCl 3 results in [(XbicH 2 )SiPh][HCl 2 ], where the silicon adopts a five-coordinate, square pyramidal geometry in the upper pocket and the lower pocket binds to two protons on the imine nitrogens. Deprotonation of the imines with LiO t Bu, NaN[SiMe 3 ] 2 , or AgOAc results in binding of the univalent metal ion in the lower pocket, where it adopts an unusual pentagonal monopyramidal geometry in the solid state. The complexes show irreversible electrochemistry, with oxidations taking place at relatively high potentials. 

The tris(aminophenol) ligand tris(4-methyl-2-(3′,5′-di- tert -butyl-2′-hydroxyphenylamino)phenyl)amine, MeClampH 6 , reacts with Ti(O i Pr) 4 to give, after exposure to air, the dark purple, neutral, diamagnetic complex (MeClamp)Ti. The compound is six-coordinate, with an uncoordinated central nitrogen (Ti–N = 2.8274(12) Å), and contains titanium( iv ) and a doubly oxidized ligand, formally a bis(iminosemiquinone)-mono(amidophenoxide). The compound is unsymmetrical in the solid state, though the three ligands are equivalent on the NMR timescale in solution. Ab initio calculations indicate that the ground state is a multiconfigurational singlet, with a low-lying multiconfigurational triplet state. Variable-temperature NMR measurements are consistent with a singlet–triplet gap of 1200 ± 70 cm −1 , in good agreement with calculations. The distortion from threefold symmetry allows a low-lying, partially populated ligand-centered π nonbonding orbital to mix with largely occupied metal–ligand π bonding orbitals. The energetic accessibility of this distortion is inversely related to the strength of the metal–ligand π bonding interaction. 

Tris(2-(arylamido)-4,6-di- tert -butylphenoxo)molybdenum( vi ) complexes ( R ap) 3 Mo can be prepared either from (cycloheptatriene)Mo(CO) 3 and the N -aryliminoquinone, or from MoO 2 (acac) 2 and the aminophenol. In contrast to all other reported unconstrained transition metal tris(amidophenoxide) complexes, the molybdenum complexes show a facial geometry in the solid state. In solution, the fac isomer predominates, though a small amount of mer isomer is detectable at room temperature. At elevated temperature the two species interconvert through Rây-Dutt trigonal twists, which are faster than Bailar twists in this system, presumably because of steric effects of the N -aryl groups. Substituents on the N -aryl ring shift the fac / mer equilibrium of the complex, with more electron-withdrawing substituents generally increasing the proportion of the mer isomer. The preference for fac over mer geometry is thus suggested to be due to enhanced π bonding in the fac isomer. In contrast to analogous catecholate complexes, the tris(amidophenoxide) complexes are not Lewis acidic and are inert to nucleophilic oxidants such as amine- N -oxides.