Transition metals, binding ligands

F. 9a-g. Types of binding interactions that can be exploited during templating a n-n interaction b hydrophobic or van der Waals interaction c covalent bonds d (transition) metal-ligand binding e hydrogen bonding f crown ether -ion interaction g ionic interaction... 

Two key issues are essential for the success of the catalytic HAT (CHAT) reaction. First, the two catalytic systems must be compatible. Because early and late transition metals usually bind ligands with opposing donor properties, they should remain mutually unaffected. Also, the Wilkinson catalyst should be stable under the slightly acidic conditions of the titanocene regeneration system (Zn, 2,4,6-collidine hydrochloride). Since it has been reported that Rh(PPh3)3Cl is even stable towards BF3 Et20, this was expected to be the case [44]. 

These assays are based upon the use of a europium or terbium chelate (a transition metal-ligand complex displaying long-lived fluorescent properties) and labeled anti-phosphopeptide or anti-phos-photyrosine antibodies that can bind to phosphorylated peptides. The antibodies are usually labeled... 

Chapters 4-6 have shown that a host of different ligands will bind to metals and that these ligands may be classified as X- or L-type. In fact, transition metals will bind to virtually any other element in the periodic table and to almost all organic molecules. Ligands may either be directly involved in the catalytic process or indirectly affect catalysis by exerting steric and/or electronic effects on the complex. 

Some C-H bonds of transition metal ligands are found to have an agostic interaction of type 12, where a C-H bond is oriented in a manner to intramolecularly bind to the metal. 

Most transition metal ions bind six ligand atoms. An important tetradentate ligand is adenosine triphosphate (ATP), which binds to divalent metal ions (such as Mg ", Mn ", Co ", and Ni " ) through four of their six coordination positions (Figure 13-2). The fifth and sixth positions are occupied by water molecules. The biologically active form of ATP is generally the Mg " complex. 

In contrast, the thermodynamic template effect involves a particular template species (usually a transition-metal ion) binding to a ligand that is complementary to itself, within an equilibrating mixture of products that are formed without the involvement of the template. The binding of the template thermodynamically stabilises the most complementary product (usually a macrocyclic compound). An excellent example is the preparation of phthalocyanine (2.8). Treatment of 1,2-dicyanobenzene with either boron trichloride or uranyl chloride results in two different-sized macrocycles (2.6 and 2.7, respectively) (Scheme 2.4). Macrocycles 2.6 and 2.7 are themselves only stable when the template is still present. On the removal of the template, the normal phthalocyanine (2.8) is formed, which is highly stable and forms many coordination complexes with a range of transition-metal ions. This is also a very attractive synthetic procedure for the preparation of unsymmetrical phthalocyanines. 

Here we discuss some representative examples in the modeling of transition metal adatoms on pristine graphene. Depending on the system, pristine graphene can be considered as a substrate or ligand for transition metals. The binding of defective graphene to transition metals is of a different nature and the topic is discussed in Section 11.3.3. 

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