925216-59-7Relevant academic research and scientific papers
Ultrafast and ultraslow oxygen atom transfer reactions between late metal centers
Fortner, Kevin C.,Laitar, David S.,Muldoon, John,Pu, Lihung,Braun-Sand, Sonja B.,Wiest, Olaf,Brown, Seth N.
, p. 588 - 600 (2007/10/03)
Oxotrimesityliridium(V), (mes)3Ir=O (mes = 2,4,6- trimethylphenyl), and trimesityliridium(III), (mes)3Ir, undergo extremely rapid degenerate intermetal oxygen atom transfer at room temperature. At low temperatures, the two complexes conproportionate to form (mes) 3Ir-O-Ir(mes)3, the 2,6-dimethylphenyl analogue of which has been characterized crystallographically. Variable-temperature NMR measurements of the rate of dissociation of the μ-oxo dimer combined with measurements of the conproportionation equilibrium by low-temperature optical spectroscopy indicate that oxygen atom exchange between iridium(V) and iridium(III) occurs with a rate constant, extrapolated to 20 °C, of 5 × 107 M-1 s-1. The oxotris(imido) osmium(VIII) complex (ArN)3Os=O (Ar = 2,6-diisopropylphenyl) also undergoes degenerate intermetal atom transfer to its deoxy partner, (ArN) 3Os. However, despite the fact that its metal-oxygen bond strength and reactivity toward triphenylphosphine are nearly identical to those of (mes)3Ir=O, the osmium complex (ArN)3Os=O transfers its oxygen atom 12 orders of magnitude more slowly to (ArN)3Os than (mes)3Ir=O does to (mes)3Ir (kOsOs = 1.8 × 10-5 M-1 s-1 at 20 °C). Iridium-osmium cross-exchange takes place at an intermediate rate, in quantitative agreement with a Marcus-type cross relation. The enormous difference between the iridium-iridium and osmium-osmium exchange rates can be rationalized by an analogue of the inner-sphere reorganization energy. Both Ir(III) and Ir(V) are pyramidal and can form pyramidal iridium(IV) with little energetic cost in an orbitally allowed linear approach. Conversely, pyramidalization of the planar tris(imido)osmium(VI) fragment requires placing a pair of electrons in an antibonding orbital. The unique propensity of (mes)3Ir=O to undergo intermetal oxygen atom transfer allows it to serve as an activator of dioxygen in cocatalyzed oxidations, for example, acting with osmium tetroxide to catalyze the aerobic dihydroxylation of monosubstituted olefins and selective oxidation of allyl and benzyl alcohols.
Osmium imido complexes: Synthesis, reactivity, and SCF-Xα-SW electronic structure
Schofield, Mark H.,Kee, Terence P.,Anhaus, Jens T.,Schrock, Richard R.,Johnson, Keith H.,Davis, William M.
, p. 3595 - 3604 (2008/10/08)
Osmium tetraoxide reacts with 3 equiv of 2,6-diisopropylphenyl isocyanate in refluxing heptane over a period of 20 h to afford red-brown crystalline trigonal planar Os(NAr)3 (1; Ar = 2,6-C6H3-i-Pr2) in modest (50%) yield. 1 reacts with relatively small phosphines and phosphites to afford square-planar complexes of the type trans-Os(NAr)2L2 (4; L = PMe2Ph, PMe3, P(OMe)3) in high yield and with trimethylamine oxide to give Os(NAr)3O, which in turn reacts with olefins C2H2R2 to give metallaimidazolidine complexes, Os[N(Ar)CHRCHRN(Ar)](NAr)(O). An X-ray study of the complex made from ethylene (5b; P1, a = 9.607 (2) ?, b = 22.294 (6) ?, c = 8.997 (2) ?, α = 100.08 (2)°, β = 112.44 (2)°, γ = 90.85 (2)°, V = 1746.6 (8) ?3, Z = 2, ρ(calcd) = 1.977 g cm-3, R1 = 0.034, R2 = 0.047) showed it to be a pseudotetrahedral complex in which each nitrogen atom in the ring is trigonal planar. trans-Os(NAr)2(PMe2Ph)2 (4b) reacts with Me3NO to give Os(NAr)2O2, with methyl or ethyl iodide to give complexes of the type Os(NAr)2(R)I(PMe2Ph), and with iodine to give Os(NAr)2(PMe2Ph)I2 (7). An X-ray structure of 7 (P21/n, a = 15.2139 (9) ?, b = 14.0999 (9) ?, c = 18.158 (1) ?, β = 106.795 (4)°, V = 3729.0 (8) ?3, Z = 4, ρ(calcd) = 1.354 g cm-3, R1 = 0.051, R2 = 0.057) showed it to be a trigonal bipyramid with two imido ligands and one iodide in the equatorial plane. 7 reacts with KS2CNEt2 to give Os(NAr)2(S2CNEt2)2, with 2 equiv of AgOAc to give Os(NAr)2(OAc)2(PMe2Ph) (12), and with 2 equiv of MeMgCl to give Os(NAr)2Me2(PMe2Ph). From 12 can be prepared Os(NAr)2(S-t-Bu)2 and Os(NAr)2R2 (R = CH2-t-Bu, CH2SiMe3). An SCF-Xα-SW analysis of Os(NH)3 in D3h symmetry agrees with a qualitative MO description. The HOMO (3a1′) is essentially an osmium-centered weakly σ-antibonding orbital and the LUMO (2e″) a low-lying π-antibonding level. The occupied 1a2′ orbital, which is largely nitrogen-centered (76% vs 3% osmium), prevents the species from being a true 20-electron complex. An SCF-Xα-SW analysis of Os(NH)2(PH3)2 in C2h symmetry also confirmed the qualitative MO description. The two highest occupied MO's are osmium-based (3bg (dxz) and 7ag (dz2)) with the 7ag approximately 1 eV higher in energy. The LUMO is an in-plane π* orbital (8ag). An occupied nonbonding level (6bu) localized almost entirely at nitrogen and phosphorus prevents this species from being a true 20-electron complex.
