Iminoxolene Complexes of Molybdenum, Vanadium, Chromium, and Osmium

Tris(2-(arylamido)-4,6-di-tert-butylphenoxo)molybdenum(VI) complexes (^Rap)₃Mo can be prepared either from (cycloheptatriene)Mo(CO)₃ and the N-aryliminoquinone, or from MoO₂(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.

The tris(aminophenol) ligand MeClampH₆ (tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-4-methylphenyl)amine) reacts with Cr(THF)₃Cl₃ to make the octahedral (MeClamp)Cr complex. X-ray crystallography shows that (MeClamp)Cr is octahedral, and has little interaction between the metal and the central triarylamine nitrogen (Cr–NAr₃ = 3.32 Å). Through the use of variable-temperature NMR the singlet-triplet energy gaps for (MeClamp)Cr and Cr(PhNC₆H₂^tBu₂O)₃, its meridonal tris(chelate) equivalent, were determined to be 4.28 ± 0.10 kcal mol^-1 and 3.26 ± 0.04 kcal mol^-1 respectively. Neutral (MeClamp)Cr can be oxidized by one electron which gives an EPR signal diagnostic of a ligand centered radical. (MeClamp)V is prepared by reacting MeClampH₆ and vanadium(V) oxytriisopropoxide. The neutral complex is seven coordinate in the solid state (V– NAr₃ = 2.38 Å) but the optical spectrum gives evidence to suggest it is six-coordinate while in solution. Neutral (MeClamp)V can be reduced or oxidized by one electron with redox titrations showing isosbestic points indicative of a clean conversion from one complex to the other. The electronic structure of both (MeClamp)Cr and (MeClamp)V is best described using a covalent model in which the metal center is supported by iminoxolene ligands with substantial π donation.

Oxobis(iminoxolene)osmium(VI) compounds (^Rap)₂OsO (^Rap = 2-(4-RC₆H₄N)-4,6-^tBu₂C₆H₂O) are deoxygenated by phosphines and phosphites to give five-coordinate (^Rap)₂Os(PR′₃) or six-coordinate (^Rap)₂Os(PR′₃)₂ complexes. Analysis of the intraligand distances indicates that this net two-electron reduction is accompanied by the apparent oxidation of the iminoxolene ligands. This change is due to the increased ability of the iminoxolene ligands to engage in π donation to the reduced form of the osmium complex. Kinetic studies indicate that the rate-determining step is the attack of the phosphorus(III) reagent on the five-coordinate oxo complexes. While varying the substituents of the aryl groups on the iminoxolene ligands or on the triarylphosphines has little effect on the rate of oxygen atom transfer, the best correlation of the data is shown between the rates of oxygen atom transfer and the HOMO−LUMO gap of the oxo complexes. This correlation suggests that the osmium oxo group shows a balance between both electrophilic and nucleophilic character in the oxygen atom transfer reactions with phosphorus(III) reagents.

(^Hap)₂Os(L) complexes react with substrates such as styrene oxide. When L = PPh₃, the reaction proceeds rapidly at room temperature to form products where the epoxide has opened and formed a new C-N bond to the iminoxolene ligand. The complexes are capable of catalytic reduction of aryl- and vinyl-substituted epoxides to the corresponding alkenes using triphenylphosphine at 50 °C. When cis-stilbene oxide is used, isomerization of the epoxide is observed in addition to formation of stilbene. Changing the aryl substituents on the phosphines has small effects on rates and product distributions, while changing to PCy₃ or IMes has dramatic effects on the relative amounts of deoxygenation vs. isomerization observed. Epoxide isomerization as well as iminoxolene ligand modification suggests a radical mechanism where the ligand-modified products are the resting state of the active catalyst.