Heterocycles bidentate

In contrast with the Schiff base salen, salicylaldehyde oxime (79) (salox) complexes of Co have received comparatively little attention, but a series of bis-bidentate divalent complexes of the form iraiis-Co(sa 1 ox)2( D M SO)2 have been reported.343 The heterocyclic bidentate oxime violurate (lH,3H-pyrimidine-2,4,5,6-tetrone 5-oximate, Hvi) (80) and its /V-methyl (mvi) and /V,/V -dimethyl (dmvi) derivatives form high-spin divalent [Co(vi)]+ and Co(vi)2 complexes, whereas [Co(vi)3] is low spin.344 The mixed-ligand Co(dmvi)2(phen) complex is also low spin. The crystal structure of m-Co(pxo)2Br2 (pxo = 2-acetylpyridine-l-oxide oxime) is isostructural with the Ni11 relative.345 The dichloro complex also adopts a cis configuration. The tridentate dioximes 2,6-diformyl-4-methylphenol dioxime and 2,6-diacetyl-4-methylphenol dioxime (Hdampo) form binuclear complexes of the type (81a) and (81b) respectively.346 Cobalt oxide nanoparticles were prepared by... 

The tetrameric Zr complexes [Zr(0H)2(H20)2L]4X8 (L = bipyridyl, phenanthroline, various Schiff-bases X = Cl, NCS) were the presumed products from the reaction of ZrOCl2 with various heterocyclic bidentate ligands.336 The interaction of H20 with Zr0(C104)2 or Zr0(N03)2 was probed by NMR spectroscopy.337,338 In the latter case, the cation [Zr4(0H)8(H20)i6]8+ (80) was proposed as the product.338 The related trimer [Zr02Ci2H8(/u2-0H)]3(//3-0)Li5(THF)g(H20)5 (81) was isolated from the hydrolysis product of an organometallic precursor. X-ray structural data confirmed the planarity of the six-membered Zr3(/r2-OH)3 core.339 In related work, protometric studies of Zr hydroxide complexes have probed thermodynamic stability,340 while FT IR and theoretical investigations have addressed the details of laser-ablated group IV metal atoms that... 

Complexes with chiral heterocycles possessing P-containing substituents as P-mono- andP,N-bidentate ligands and their use in homogeneous asymmetric catalysis 98KK883. 

Chiral P-heterocycles as P-mono- and P,N-bidentate ligands in the synthesis of coordination compounds and homogeneous asymmetric catalysis 98KK883. 

The 2 1 reaction of 9-BBN with a series of dicarboxylic acids, namely oxalic acid, malonic acid, 2,2-dimethylmalonic acid, and succinic acid, in dimeth-oxyethane gives in some cases dimeric and in other cases macrocyclic (acyloxy)diorganoboranes. This has been proved by IR spectroscopy (all C = O groups are bidentate), B-NMR 5 = 10 ppm) and X-ray crystallography [47]. With oxalic acid two structures are possible (IV and V), of which the first with a five-membered boron heterocycle instead of a four-membered one is the more probable formulation (Fig. 13). 

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

The synthesis of the polycyclic 5-5-6-5 derivative 81 was realized by nucleophilic substitution of the 5,6-dichloro[ 1,2,5]oxadiazolo[5,4-7]pyrazine 79 with 5-aminotetrazole 80 (Scheme 17). This conversion took place at room temperature and the product 81 was isolated in moderate 36% yield. Many other heterocyclizations with N,N, N,0-, /V,.Y-bidentate nucleophiles gave the corresponding reaction in up to 93% yield <1997CHE1352>. 

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