Azoles metal complexes

Additional sections of this chapter cover (1) tautomeric reactions of azoles and their derivatives in electronically excited states and (2) stabilization of certain tautomers in their metal complexes. 

Tautomeric rearrangements of transition-metal complexes with azole ligands are relatively scarce. The fluxional behavior of the rhodium complex 43 with a neutral 3,5-dimethylpyrazole was explained as the result of rapid processes of metallotropy and prototropy occurring simultaneously (Scheme 24) [74JOM(C)51],... 

In contrast with aminoazole ligands, hydrazones of azoles adopt the imino form in their metal complexes 360 [X = O (93ZNK863), X = S (91MC110)]. 

Pinacolone, o-(diphenylphosphino)benzoyl-coordination chemistry, 2, 401 Ping-pong reactions copper(II) complexes, 5, 717 Piperidine, /V-hydroxy-metal complexes, 2, 797 P a values azole ligands, 2, 77 Plant roots amino acids, 2, 962 carboxylic adds, 2,962 Plants... 

Arylamines are commonplace. They are part of molecules with medicinally important properties, of molecules with structurally interesting properties, of materials with important electronic properties, and of transition metal complexes with catalytic activity. An aryl-nitrogen linkage is present in nitrogen heterocydes such as indoles [1, 2] and benzopyr-azoles, conjugated polymers such as polyanilines [3-9], and readily oxidizable triarylamines used in electronic applications [10-13]. The ability of aryl halides and triflates to form arylamines allows a single group to be used as a synthetic intermediate in aromatic carbon-... 

The transmetallation of lithiated heterocycles has been described as a method to provide NHC complexes, and other Fischer-type carbene complexes [161]. The method proceeds via the alkylation of an alkylimidazole with BuLi to generate the lithiated azole, which can transmetallate to the metal complex. Further reaction with an acid or an alkylating agent would provide the desired NHC - M complex (Scheme 47). 

An interdisciplinary approach should lead to their future prospects as building blocks of a variety of chemical structures. Thus, betaines 1 can be incorporated as a subunit(s) in host molecules and could confer unusual properties to the supramolecules, either cavitates or clathrates. Their capacity for specific physical behavior should also be considered together with their use as neutral ligands (azolate ligands without counterion) in forming metal complexes. Advances in the chemistry of betaines 1, to be of any real significance, must result from coordinate efforts directed toward supramolecular chemistry, advanced organic materials, and heteroarene coordination chemistry. 

The transition-metal complexes with unsymmetrical binucleating ligands based on substituted azole moieties are of interest as subjects for mechanistic studies of oxidation. A good example is the oxadiazole-based ligand (71) that has been reported.103... 

Retrosynthetically, preparation of N-azolyl 2,6-bis(imino)pyridyl metal complexes as precatalysts for polymerization involves (i) synthesis of N-amino azoles, (ii) condensation of 2,6-diacetylpyridine with two equivalents of N-amino azole, and (iii) complexation of die appropriate transition metal halide with the terdentate ligands. The latter two reactions present no real challenge and are more or less analogous to fhe published preparation of N-aryl 2,6-bis(imino)pyridyl complexes [13], and fhe more demanding synthesis of N-amino-azoles was covered in a preceding section. 

Amino-derivatives (X NH) give rise to two types of structures in their metal complexes classical bis-chelates 200 (9I CS(D)2045) and tricyclic salen complexes 201 (97JC S(D)4539). Chelates of aldimines of 5-oxop5u-azoles (X = O) are represented by coordination compounds 202 and 203. Complexes 202 include bis-chelates of cobalt (M = Co, R = C6H4-3-CI, = Ph, R2 = H, no L) (04CJAC49), (M = Co, R = C10H7,... 

However, others have shown that oxidative additions of much less acidic substrates, such as water, ° alcohols, unactivated anilines and azole heterocycles, " occur to low-valent transition metal complexes. For example, iridium(I) complexes containing tri-ethylphosphine as ligand undergo oxidative addition of water rapidly at room temperature (Equation 7.17). Both [Ir(PEtj)J and [Ir(PEt3)jCl] also add aniline rapidly at room temperature (Equation 7.17). The latter reaction is a step in the catalytic hydroamination of norbornene. These substrates react readily with early metal complexes, but tliese reactions do not occur by oxidative addition. ... 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

Bakova R, Chergui M, Daniel C, Vlcek A Jr, Zalis S (2011) Relativistic effects in spectroscopy and photophysics of heavy-metal complexes illustrated by spin-orbit calculations of Re(imid-azole)(CO)3(phen). Coord Chem Rev 255 975-989... 

Many examples are known of complexes between metal cations and both neutral azoles and azole anions. Overlap between the cf-orbltals of the metal atom and the azole rr-orbitals is believed to increase the stability of many of these complexes. 

---