Seven coordinate complexes

Reaction of the pentadentate bispidine-based systems with H2O2 in methanol at low-temperature yields a purple solution of the low-spin iron(III) end-on hydroperoxo complexes that of the isomer with an in-plane coordination of the hydroperoxide is, as expected, considerably more stable than that with an axially coordinated substrate. Deprotonation to the end-on high-spin peroxo complex (seven-coordinate) is reversible for the more stable isomer, with a p T value of... 

Complete replacement of chloride to give complexes based upon Mo(NO)X3 (X = CN, N3, SR or S2Q R2) has also been observed. For example, the compound [Mo(CO)3L] (L = l,4,7-triazacyclononane) reacts with nitric add to yield [Mo(CO)2(NO)L] which contains Mo , but inclusion of KCN in the reaction mixture causes formation of [Mo(CN)3(NO)L]. Probably all the nitrogens of the cyclononane are coordinated, making this complex seven-coordinate. Reaction of molybdate [MoOJ " at pH 8 in the presence of KCN and L (L = bipy or phen) with hydroxylamine can give rise to [Mo(CN)3(NO)L]-H20, though the precise products are a function of reaction conditions. In an unusual reaction [Mo(CO)3(terpy)] and MeSiNs in nitromethane produce yellow [Mo(NO)(N3)3(terpy)]. This has a pentagonal bipyramidal structure with NO and a bent azide in axial positions. Mo— N is 1.777(4) and N—O 1.196(5) A, and Z.M0NO is practically linear [176.2(4) ]. ... 

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

Seven-coordinate pentagonal-bipyramidal In has been found in the chloroindium complex of 1,4,7-triazacyclononanetriacetic acid... 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

A mononuclear diastereopure high-spin Fe alkylperoxo complex with a pen-tadentate N,N,N,0,0-ligand 33 (Scheme 17) was reported by Klein Gebbink and coworkers [109, 110]. The complex is characterized by unusual seven-coordinate geometry. However, in the oxidation of ethylbenzene the iron complex with 33 and TBHP yielded with large excess of substrate only low TON s (4) and low ee (6.5%) of 1-phenylethanol. 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

Co2(CO)8 with HC1 gas in the presence of 15-crown-5 and water in toluene. This led to a complex species which included seven-coordinate pentagonal bipyramidal Co11 centers (98) which included the metal ion in the macrocyclic polyether.422... 

Iridium(V) complexes, although not common, take a variety of forms, including polynuclear and multidentate structures as well as five-, six- and seven-coordinate species, some of which are stable and have been characterized by X-ray crystallography. 

Acetone has been structurally characterized as a ligand in a zinc crown ether (15-crown-5) complex, where the zinc is seven-coordinate bound to all five ring ether donors, a chloride, and the acetone molecule.351... 

An N5O2 donor open chain precursor for a macrocycle forms a seven-coordinate zinc complex.899... 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

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