Multinuclear Complexes of

Mechanism of Formation of Multinuclear Complexes. Probably least relevant to biology and not the focus of this chapter is the topic of formation of multinuclear complexes of nucleobases involving deprotonation of nucleobases at pH values far below their relevant pK values. Once deprotonated, nucleobases appear to be particularly prone to multiple metal binding, at least in the case of divalent metal ions. 

Multinuclear complexes of Y alkoxide have been S3uithesized with multidentate amines and they have shown high activity in ROP (Fig. 10) (63,64). Although the polymerization of e-CL can be controlled, the polydispersity is rather high (1.5 < PDI < 1.7), and parasitic aggregation and clustering can be the source of complexity. 

Aluminium compounds contained in rocks, soil and sediments are very slightly soluble, and in natural waters very low concentrations of aluminium occur (mostly <20(xg/l). Aluminium ions are easily hydrolysed even in neutral media and form insoluble multinuclear complexes of either aluminium ions or aluminium hydroxide Al(OH)3. In addition, in a soil solution at neutral pH, the aluminium concentration is low and its intake by plants is very limited. For example, soil acidification by acid rain increases the solubflity of aluminium compounds and aluminium can thus enter the hydrosphere and biosphere in higher amounts. In sHghtly acidic solutions (pH 5), the predominant cations are [Al(OH)2]" > [Al(H20)g] + > [Al(OH)] +. Under these conditions, aluminium is actually toxic to certain organisms. A higher intake of aluminium by birds leads to abnormalities of their eggs. Soluble compounds of aluminium in plants cause slower growth, because aluminium is a phytotoxic substance. 

Organometallic Compounds. Mononuclear carbon monoxide complexes of palladium are relatively uncommon because of palladium s high labihty, tendency to be reduced, and competing migratory insertion reactions in the presence of a Pd—C bond (201). A variety of multinuclear compounds... 

Linear multinuclear metal complexes are attracting attention in the context of molecular electronics due to their projected use as molecular wires. 01igo(pyridyl)amido ligands are efficient scaffolds for lining up several Ni11 ions like a string. The first structurally characterized trinickel complex of this type, [Ni3(dpa)4Cl2] (dpa = bis(2-pyridyl)amide), showed a nearly linear Ni3 unit with Ni—Ni distances of around 244 pm.209 Penta-, hepta-, and nonanuclear systems have... 

Pyridyl functionalized tris(pyrazolyl)borate ligands show some interesting properties including the formation of polynuclear zinc complexes.23,1 Some of these contain extensive H bonding and have potential as models for multinuclear zinc enzymes such as phospholipase C or PI nuclease.235 A bis-ligand complex of the hydrotris(5-methyl-3-(3-pyridyl)pyrazolyl)borate ligand (23) shows octahedral coordination of all six pyrazole nitrogen donors despite the steric bulk. 

Phosphido-bridged multinuclear complexes have been formed from zinc chloride of the form [Zn4(PPh2)4Cl4(PRR 2)2], where R and R are alkyl or phenyl groups. Related tetrameric structures can be formed with a bridging dialkyl phosphide or (trimethyl)silylphenyl phosphide.303... 

Crabtree s catalyst is an efficient catalyst precursor for the selective hydrogenation of olefin resident within nitrile butadiene rubber (NBR). Its activity is favorably comparable to those of other catalyst systems used for this process. Under the conditions studied the process is essentially first order with respect to [Ir] and hydrogen pressure, implying that the active complex is mononuclear. Nitrile reduces the catalyst activity, by coordination to the metal center. At higher reaction pressures a tendency towards zero order behavior with respect to catalyst concentration was noted. This indicated the likelihood of further complexity in the system which can lead to possible formation of a multinuclear complex that causes loss of catalyst activity. 

In contrast to the vast number of mono- and multinuclear binary carbonyl complexes of the transition metals, no isolable binary carbonyls of titanium, zirconium, or hafnium have been reported. 

The reaction of bis(silanethiolato) complexes of group 10 metals with transition metal complexes gives the corresponding multinuclear complexes accompanied by the elimination of the silyl chloride (Scheme 14).117,118... 

Monomeric arrangements are rare for alkoxide complexes of bismuth and require excess anionic ligands or bulky substituents. Otherwise the alkoxide ligands typically impose dimerization or multinuclear clustering, which are expressed in a variety of currently unusual structural arrangements (Table I). 

The substitution and protonation behavior of the rcms-dioxotetra-cyanometalate complexes of Re(V), Tc(V), W(IV), Mo(IV), and Os(VI) have been extensively investigated in the past decade and selected aspects have been reviewed (1, 2). Previous studies demonstrated the use of oxygen-17 NMR in different metal systems (3), including the complex oligomeric Mo(IV) aqueous systems (4), and therefore, since these oxocyano complexes contain an even wider range of nuclei such as 13C, 15N, 170, "Tc, and 183W, they are attractive model complexes to study by multinuclear NMR. Thus, detailed studies on the dynamics therein have been investigated in the past few years (5-8). 

It is known that the nitrosonium cation is a strong oxidant (54). In (55) it was found by multinuclear NMR ( H, 13C, 19F and 14N) that the interaction of nitrosonium tetrafluoroborate with 2,2,6,6-tetramethyl-4-R-piperidine-1 -oxyl radicals 22a-e resulted in formation of 4-R-2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoroborates (Scheme 16). Cations 23a-e could be classified as nitrosonium complexes of biradicals 24a-e. 

Surface complex-formation to bi-or multinuclear complexes or surface films, blockage of surface groups by metal cations... 

This section summarizes work carried out on polynuclear complexes containing M(bpy)2 units, an area in which there is much interest, in particular with respect to energy transfer. Dendritic systems are excluded from this review, but are covered elsewhere in CCC The complexes-as-ligands strategy is commonly exploited for the controlled construction of multinuclear complexes and examples are seen in this section. 

The TMEDA and THE complexes of the very sterically crowded methyllithium derivative tris(trimethylsilyl)methyUithium (9, Scheme 1) were studied by multinuclear solid state NMR spectroscopy in combination with solution NMR spectroscopy and X-ray crystallography to reveal the strucmre and dynamic behaviour of the complexes . In the solid state, this complex crystallizes as an ate-complex, with one lithium cation interacting with two substituted methyl anions while the other lithium cation is complexed by two TMEDA ligands. In solution, the afe-complex is partly transformed into solvated monomers or aggregates, depending on the experimental conditions. 

Titanium alkoxides are also effective and sought-after initiators for the ROP of lactides due to a low toxicity, which minimizes the problems linked to the presence of catalyst residues in commercial PLA products [18, 19]. Despite impressive advancements in the use of Lewis acidic metal initiators in the preparation of PLAs, surprisingly little attention has been paid to the group 4 metal (Ti, Zr, Hf) initiators, probably due to the highly oxophilic nature of M(1V) which has a natural tendency to form aUcoxy-bridged multinuclear complexes. Verkade and coworkers previously demonstrated a series of titanium aUcoxide complexes 118-122 (Fig. 17) that function as moderately efficient initiators in bulk homopolymeization of L-lactide and rac-lactide, some of these initiators displaying a well-controlled polymerization behavior [119]. 

With few other ligand classes do zinc and cadmium form such a structurally varied series of complexes.7676 Multinuclear complexes and intriguing geometries abound. 

Organometallic Compounds. Mononuclear carbon monoxide complexes of palladium are relatively uncommon because of palladium s high lability, tendency to be reduced, and competing migratory insertion reactions in the presence of a Pd—C bond (201). A variety of multinuclear compounds are known (202), including [Pd2Cl4(CO)2] [75991-68-3], [Pd3(P(C6H5)3)3(CO)3] [36642-60-1], and [Pd7(P(CH3)3)7(p3-CO)3(p2-CO)4] [83632-51-3]. 

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