Coordination of osmium

Organometallic chemistry of pyrrole is characterized by a delicate balance of the ti N)- and -coordination modes. Azacymantrene is an illustration of the considerable nucleophilicity of the heteroatom. However, azaferrocene can be alkylated at C2 and C3 sites. Ruthenium and osmium, rhodium, and iridium chemistry revealed the bridging function of pyrroles, including zwitterionic and pyrrolyne complex formation. The ti (CC) coordination of osmium(2- -) allows versatile derivatizations of the heteroring. 

Osmium forms a number of compounds in the +4, +5 and +6 states. OsAr4 (Ar = Ph, o-tolyl, cyclohexyl) has tetrahedral coordination of osmium (e.g. Os—C 1.994 A in Os(QH5)4). The tetraphenyl and tetra(cyclohexyl) compounds are air sensitive, but the o-tolyl is very inert (stable to O2, CO, NO) presumably owing to the orf/io-methyls shielding the osmium the latter can be oxidized to paramagnetic Os(o-tolyl)4 (with little change in the Os-C bond length). Osmium(IV) tetraaryls undergo a reductive elimination in the presence of PR3 (R, e.g. Me, OMe, OEt) by an associative mechanism [198]. 

The same group also described a highly stereoselective synthesis of pyrrolidines utilizing a Os04-catalyzed oxidative cyclization strategy. Preferential mode of cyclization is most likely due to the robust intramolecular coordination of osmium with nitrogen and oxygen atoms of the iV-tosylated aminoalcohol (eq 54). ... 

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

Coordination Compounds. Osmium in oxidation states from +2 to +8 forms a wide range of complexes with nitrogen ligands. Amine... 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

The halides in mer-OsCl3(PR3)3 can be replaced by a thiolate ligand to give paramagnetic Os(SC6F5)3(PR3)2 an agostic Os—F—C interaction is believed to complete the coordination sphere of osmium [156]. 

Imamura (11,20,21) synthesized several similar perpendicular dimers exploiting axial coordination of the 4-pyridyl free-base porphyrin to Ru(II)CO (3) and Os(II)CO (4) porphyrins (Fig. 1). The pyridine-ruthenium and pyridine-osmium interactions are much stronger than the pyridine-zinc interaction, and the complexes are perfectly stable in solution and can be isolated by precipitation. One of the ruthenium dimers was characterized by FAB-MS (11). Complexation is accompanied by characteristic changes in JH NMR chemical shift, indicating... 

To substitute the strongly bound axial CO ligand of the ruthenium or osmium center, it is necessary to employ more drastic conditions than simple stirring at room temperature. Imamura (11,20) used photolysis to synthesize porphyrin trimers on the basis of simultaneous coordination of two 4-pyridyl porphyrins to the same ruthenium porphyrin (12, Fig. 3). Some interesting photophysical behavior was observed for these systems. The trimers have an extra UV-Vis absorption band at about 450 nm which is ascribed to metal-ligand charge transfer (MLCT), a d7r(Ru(II))-7r (OEP) transition. This band shows a batho-chromic shift in more polar solvents, and decreased in intensity when... 

FIGURE 12.5 Structure of the osmium redox polymer, OsPVI, formed by coordination of an [Os(2,2 -bipyridine)2Cl]+ complex to polyvinylimidazole in a usually 1 9 ratio. 

The chemistry of osmium has been reviewed in COMC (1982) and COMC (1995)339 370 as well as in Comprehensive Coordination Chemistry II.340 A recent review covers the applications of diosmium tetracarboxylates.341... 

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