Osmium complexes chelating ligands

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Osmium (IV) does not react with ethylene under pressure (97), or with cyclo-octa-1,5-diene (45), but the osmium(II) complex [Os2Cl3(PEtPh2)6]Cl gives the stable, 6-coordinated complex [OsCUfPEtPl MCsHu)] on treatment with cyclo-octa-1,5-diene, which acts as a chelating ligand in the complex (40a). 

The inert hydroxo-bridged species were also a product of (very fast) hydrolysis of p-cymene osmium complexes with glycinate, L-alaninate, a-aminobutyrate and p-alaninate. However, complexes with picolinate as the chelating ligand, [Os(r 6-/> cym)Cl(pic)] 8 and [Os(r 6-biph)Cl(pic)] 9, with pyridine as /V-donor and carboxy-late as O-donor, hydrolyzed with half-lives of 0.20 and 0.52 h (298 K), and aqua adduct pKa values (pk L value for solutions in D20) of 6.67 and 6.33, respectively. Complexes 8 and 9 were cytotoxic towards A2780 human ovarian cancer cells, with IC50 values of 8 and 4.2 pM, respectively [64],... 

Novel picolinate derivatives have been synthesised that show hydrolysis rates intermediate between the slow values for the complexes with /V,/V-chelating ligands, and the rapidly-hydrolysing complexes with 0,0- or N,0- amino acidlike chelating ligands. For these organometallic osmium complexes, the hydrolysis rates fall into the range of the active ruthenium-arene relatives (Fig.l) [66]. 

Peacock AFA, Parsons S, Sadler PJ (2007) Tuning the hydrolytic aqueous chemistry of osmium arene complexes with N,O-chelating ligands to achieve cancer cell cytotoxicity. J Am Chem Soc 129 3348-3357... 

One osmium complex of the phosphorus-arsenic chelate As—Pf—As (bis(2-diphenyl-arsinoethyI)phenylphosphine, (Ph2AsCH2CH2)2PPh) has been made from 0s04 and the ligand in HC1 it is the orange Os(As—Pf—As)C13.457... 

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

In the iron, ruthenium, osmium, cobalt, and rhodium complexes the xanthato ligands are isobidentate chelating. Selected examples are zra s-Ru(S2COEt)2(P-Me2Ph)2,265 cis- and zra s-Os(S2COMe)2(PPh3)2,266 Co(S2 COMe)3.267... 

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