3- Aminopyridine, metal 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. 

Metal-catalysed hydrolysis of / -nitrophenyl picolinate at pH 7.5 was in the order Cu(II) > Ni(II) > Zn(II) > Co(II) > La(III). The probable mechanism is via attack by external HO- on the metal-ion complex (80).80 High catalytic activity in the hydrolysis at pH 7 of p-nitrophenyl picolinate, but not / -nitrophenyl acetate, was displayed by the metal complexes M(2-aminopyridine)2(OAc)2 (M = Zn, Ni), showing that they were good models for hydrolytic metalloenzymes.81... 

Figure 5.12. Cyclic voltammograms at 50 mV on CoTPP bonded GC in N2 and C02 atmospheres. Bridging compound is 4-aminopyridine. The solution was pH 6.8 phosphate buffer solution. Reprinted from Figure 5 H. Tanaka, A. Aramata, Aminopyridyl cation radical method for bridging between metal complex and glassy carbon Cobalt(II) tetraphenylporphyrin bonded on glassy carbon for enhancement of CO2 electroreduction. Journal of Electroanalytical Chemistry, 437 (1997) 29-35. Copyright 1997, with permission of Elsevier.

Aga et showed that CO2 can be catalytically electroreduced to form CO as the main product, on N4 cobalt complexes chemically bonded to GCE through -CONH-pyridine, with the N of the pyridine forming a coordinate bond with the Co center of the N4 complexes. Tanaka et al. employed aminopyridine as a bridge between the metal complex and the GCE, and the potential for the reduction of CO2 of -1.1 V was obtained . TheN4 complexes used as catalysts were CoNPc, CoTMPP, CoPc, vitamin B12, CoTPP, and CoDO (Figure 7.4a) and the modified electrode may be denoted as GC-py-Co. CoDO was however not able to reduce CO2. The mechanism for the reduction of CO2 by the rest of the N4 complexes was proposed as follows (Equation 7.11-7.14) ... 

Schemes 58-62. A new non-rigid phosphine ligand was synthesized and reacted with Fe(CO)5 to form the mononuclear iron complex (Equation (81)). Phosphino-oxazoline ligands were used as assembling ligands for hetero-metallic complexes, where the phosphorus atom binds to iron and the nitrogen atoms act as donor atoms to copper, cobalt, or palladium (Scheme 58). The copper complex catalyzes cyclopropanation and Diels-Alder reactions. When 2-(A -diphenylphosphinomethyl-A -cyclohexyl)aminopyridine (NNP) reacts with Fe(CO)5 in ethanol, /ra .r-(OC)3Fe(NNP)2 is formed (Scheme 59). This monometallic complex can then be reacted with a copper salt in CH2GI2 to form a complex having an Fe-Cu dative bond. The complex was demonstrated to be an efficient catalyst for the cyclopropanation of styrene by ethyl diazoacetate and for the Diels-Alder reaction of cyclopentadiene and methacrolein. No other heterometallic complexes have been shown to have such reactivity. Previously known...

Pyridine oxides form metal complexes , and 2-aminopyridine 1-oxide gives chelates with metals. ... 

Alkali metal derivatives of 2-(trimethylsilyl)aminopyridines can be further derivatized by insertion of 1,3-dicyclohexylcarbodiimide. Functionalized guani-dinates are formed in this reaction via a 1,3-silyl shift. Scheme 170 illustrates the reaction sequence as well as the preparation of an aluminum complex of the modified ligand, which exhibits pseudo jS-diketiminate binding of the metal center, thus exemplifying the coordinative versatility of this new multi-N-donor system. ... 

To see whether water could be activated and added to Jt-systems other than alkynes, the metal-catalyzed hydrolysis of nitriles was studied [20]. For this purpose novel homodimeric and heterodimeric bis(acetylacetonato)ruthenium(ii) complexes bearing the 6-diphenylphosphino-N-pivaloyl-2-aminopyridine (10a) and 3-diphenylphosphinoisoquinolone (Ila) ligands were prepared. The molecular structures of these precatalyst were studied in solution and also in the solid state and revealed some unusual hydrogen-bonding patterns, in particular for the heterodimeric system in which the acetylacetonato ligand is involved (Scheme 2.7). 

Figure 16 MM model of the trisaminophosphorus derivative of 2-aminopyridine (left) and its complex with a metallic atom (right)...

A number of tethered osmium boryl complexes have been developed recently in which one of the boryl substituents features an additional pendant donor which coordinates to the metal centre as a tether . For systems resulting from reactions with bifunctional donors such as 2-aminopyridine two possibilities exist, depending on which donor atom interacts with the boron centre and which with the metal. Thus, tethered boryl complexes result from coordination of the anionic donor at boron, with the neutral donor tether coordinated at osmium the reverse coordination possibility leads to the formation of intramolecular base-stabilized borylene complexes. 

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