Osmium aromatic amine complexes

A recent development31 is the preparation of metal polymer complexes directly on the electrode via the electrochemically induced polymerization of the metal complex. Ruthenium(II) and osmium(II) complexes with ligands containing aromatic amines, e.g. 3- or 4-aminopyridine or 5-amino-1,10-phenanthroline, are electrochemically polymerized to yield a film of the metal polymer on the electrode surface. The polymerization involves free radicals, which are formed via the initial oxidation of the metal complex to a radical cation and subsequent reaction of the radical cation with a base to yield the free radical. 

We consider here the remarkably small number of complexes of osmium with primary and aromatic amines RNH2 all these involve phosphines as coligands. For complexes of pyridine and its analogues see p. 533 complexes of polydentate amines are considered in the sections which follow this one (p. 534). 

Aromatic amine complexes65 of osmium, and also complexes of N macrocycles, are generally similar to those of Ru, but [Os(bipy)3]2+ and [Os(terpy)2]2+ are more labile and reactive than their Ru analogues. As for Ru, some Os11 complexes are luminescent and potentially of use in photochemical molecular devices and for detection of DNA.66... 

The II state, d6. An enormous number of Ru and Os complexes with CO, PR3 and similar 7t-acid ligands are known. For other ligands, the main chemistry is that of chloro, ammonia and other amine ligands, and again I large numbers of complexes exist. The aquo ion, [Ru(H20)6]2 + has been prepared, but it is readily oxidized to [Ru(H20)6]3+. For osmium the best characterized complexes are those with aromatic amines. 

As noted in Chapters 2 and 11, a series of -q -arene complexes of osmium have been prepared, and the reactivity of these species has been studied extensively by Harman. The reactions of iq -arene complexes of Os(II) illustrate how strong backbonding can cause the uncoordinated portion of an aromatic system to be more susceptible to electrophilic attack than the corresponding free arene. ° Osmium(II) pentamine complexes of phenols, anilines, acetanilides, and anisoles react with electrophiles at the uncoordinated portion of the ring. For example, the simple phenol complex in Equation 12.78 reacts with Michael acceptors at the 4-position of the coordinated phenol in the presence of a mild tertiary amine base. This reactivity and selectivity for reaction at the 4-position is greater than the reactivity of free phenol. The reactions of electrophiles with aniline derivatives occur in a similar fashion and lead to products from alkylation of the aromatic ring predominantaly at the 4-position (Equation 12.79). Related reactions occur with complexes of electron-rich five-membered pyrrole and furan heterocycles. Examples of electrophilic attack on -q -pyrrole complexes of Os(II) are shown in Equation 12.80. ... 

Covalent adducts and/or noncovalent complexes of DNA with some chemicals, including carcinogens or cytostatics, produce specific electrochemical signals. Appearance of specific peaks was observed on DNA interactions with mitomycin C, osmium tetroxide complexes, DM, aromatic amines, and a variety of other DNA binders (Sect. 12.7). 

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],... 

Os forms many complexes with nitnte. oxalate, carbon monoxide, amines, and thio ureas. The latter arc important analytically Osmium forms the interesting aromatic sandwich" compound, osmocene. A metallocene is described under Ruthenium. See also Chemical Elements and Platinum and Platinum Group. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

The q -phenol complex undergoes conjugate addition at C4 with a variety of Michael acceptors (Fig. 7), including those with p substituents [44]. In most cases, the addition reaction is accomplished with an amine base as catalyst (see 25). Less reactive electrophiles, such as methyl acrylate or acrylonitrile, require a Lewis acid co-catalyst (e.g., 24). An example of the versatility of this reaction is shown in Fig. 7, where the aromatic steroid p-estradiol (26) is complexed (27) and subsequently alkylated exclusively at CIO (i.e.,para) at -40 °C. Since the osmium preferentially binds the a face of the steroid in 27, conjugate addition occurs from the p face, yielding the stereochemistry found in testosterone [26]. The overall yield of this transformation after decomplexation of the dienone product 29 is 69%. 

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