Imidazoline ligands

Phosphino-imidazoline ligands of this type were originally synthesized by Bu-sacca and coworkers and used in an enantioselective Heck reaction [28]. 

Imidazolines have been shown to have biological utility as antidepressants, and imidazoline ligands are known for a number of receptors widely distributed in both the peripheral and central nervous system.39 The imidazoline moiety has also been extensively studied as an amide bond... 

Bastero, A. Claver, C. Ruiz, A. CastUlon, S. Daura, E. Bo, C. Zangrando, E. Insights into CO/styrene copolymerization by nsing Pd catalysts containing modular pyridine-imidazoline ligands. Chem. Eur. J. 2004,10, 3747-3760. 

However, the thione form of mereaptoazole ligands is retained in the metal eomplexes of l-methyl-l,3-imidazolin-2-thione 370 (90POL981) and 371 [92ICA75 93JOM(453)47], l-phenyltetrazol-5-thione 372 [96JOM (513)63] and l,3-thiazolidin-2-thione 373 (89ICA265). 

Along these lines, imidazoline [68, 69] rather than oxazoline coordination sites better fitted into a modular concept in order to understand how the electronic properties should ideally be, since electron density can be readily adjusted by the choice of the N substituent [70]. The Cp ligand C5R5 in 47 permits steric and electronic tuning. With the electron-withdrawing effect of five Ph groups R, the... 

Ligand abbreviations bi = 2,2 -bi-2-imidazoline bt = 2,2 -bi-2-thiazoline bpy = 2,2 -bipyridine phen = 1,10-phenanthroline phy = l,10-phenanthroline-2-carbaldehyde phenylhydrazone bpp = 2,6-bis(pyrazol-3-yl)pyridine paptH = 2-(2-pyridylamino)-4-(2-pyridyl)thiazole 2-pic = 2-picolylamine L = macrocyclic ligand derived from condensation of 2,6-diacetylpyridine with 3,6-dioxaoctane-1,8-diamine Hjthpu = pyruvic acid thiosemicarbazone Hjthpx = pyridoxal thiosemicarbazone salen = dianion of W,iV -ethylenebis(salicylideneimine) H2fsa2en = dianion of fV,JV -ethylenebis(3-carboxysalicylaldimine). 

For the complex of the related ligand 2-(pyridin-2-yl)imidazoline 21 (Dq(Ni2+) 1140 cm-1) spin crossover is also observed [34]. Two forms of [Fe 213](C104)2 were isolated - one low spin and the other high spin but undergoing an abrupt and time-dependent transition to low spin from about 120 K. This system is of obvious interest and warrants further study. 

In contrast to 6-substituted-bipyridine systems such as 49, attachment of a five-membered heterocycle to the 2-position of 1,10-phenanthroline, which is a structural modification similar to the replacement of one of the terminal rings of terpyridine, does generally bring the ligand field into the crossover region and spin transitions have been observed for such systems when the heterocycle is thiazole 51 [74], imidazoline [75], triazole [76], pyrazole [77] and oxadiazole [78]. 

The phosphino-imidazoline catalyst 28 (Fig. 29.15) is a close analogue of the original PHOX ligand. 

Holt A, Wieland B, Baker GB. 2004. Allosteric modulation of semicarbazide-sensitive amine oxidase activities in vitro by imidazoline receptor ligands. Br J Pharmacol 143 495. 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

The fact that NHCs form stable compounds with beryllium, one of the hardest Lewis acids known and without p-electrons to back donate, shows the nu-cleophilicity of these ligands. Reaction of l,3-dimethylimidazolin-2-ylidene with polymeric BeCl2 results in the formation of the neutral 2 1 adduct 23 or the cationic 3 1 adduct 24. The first NHC-alkaline earth metal complex to be isolated was the 1 1 adduct 25 with MgEt2- Whereas l,3-dimesitylimidazolin-2-ylidene results in the formation of a dimeric compound, the application of sterically more demanding l,3-(l-adamantyl)imidazolin-2-ylidene gives a monomeric adduct. ... 

In certain cases it is also possible to replace anionic ligands by neutral NHC ligands. Nickelocene and chromocene are capable of coordinating one imidazolin-2-ylidene ligand by shifting one of the cyclopentadienyl anions from to... 

In cases where the free NHC cannot be synthesized the complex formation has to be accomplished in situ from a ligand precursor, e.g., the imidazolium salt in the case of imidazolin-2-ylidenes. By this method, it is often possible to prepare complexes which do not have the maximum number of NHC ligands attached to the metal center. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Some bidentate bis(imidazolin-2-ylidene)s 18 have been used for the preparation of complexes with chelating dicarbene ligands [75-80]. The synthesis, properties, and coordination chemistry of tripodal tris(imidazolin-2-ylidene) ligands like 19... 

For example, complex 37 with an imidazolin-2-ylidene and a methyl ligand in cis-position to each other decomposes to yield the 1,2,3-trimethylimidazolium salt 38, Pd°, and cod (Fig. 13) [124], Additional examples for the reductive elimination of 2-alkyl and 2-aryl substituted azohum salts from palladium or nickel NHC complexes have been reported [125, 126]. Today, reductive elimination reactions have been established as one important reaction pathway for the deactivation of catalytically active metal NHC complexes [126, 127]. 

Additional factors which lead to an increased stability of the carbene complexes towards reductive ehmination are the type of NHC ligand and the NA -substitution pattern. The stability of NHC complexes depends strongly on the electronic situation at the carbene center. The oxidative addition of p-tolyl chloride to linear Pd° complexes bearing two unsaturated imidazolin-2-ylidenes (type 5, Fig. 6) or two saturated imidazolidin-2-ylidenes (t3q>e 7, Fig. 6) proceeds readily. The Pd complex with the imidazolin-2-yhdene ligands is stable, while the one with the imida-zolidin-2-ylidene ligands reductively ehminates the C2-arylated imidazolidinium salt [134]. 

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