Azolyl complexes

Almost aU of the pyrrole-, indole-, and carbazole-containing systems afforded stable Fe(ll) and Co(II) complexes. The only exceptions are hgands 44 and 47 which contain only aryl substituents. This indicates that two pairs of o-phenyl groups on the N-pyrrolyl substituents impose too much of a steric hindrance and/ or are too electron-withdrawing for the corresponding [N,N,N] metal complex to form. The two Fe(lII) precatalysts 54c, 55c were synthesized to evaluate the dependence of the polymerization performance on the oxidation state of the metal center. In total, twenty-one different N-azolyl complexes were prepared aiming at a thorough structure-activity evaluation (vide infra). 

Anion displacement of halides with azolyl anions is one common route to azolyl complexes. One example of such a synthesis is shown in Equation 4.21. In other cases, azolyl complexes have been prepared by proton transfer between the free azole and a metal alkox-ide or hydroxide. An example involving the synthesis of palladium-azolyl complexes is shown in Equation 4.22. In some rare cases, reactions of pyrrole and d early metal alkyls also lead to the formation of a metal-nitrogen bond via o-bond metathesis, as shown in Equation 4.23. Finally, several late-transition-metal-azolyl complexes possessing accompanying hydride Hgands have been prepared by N-H activation of pyrrole and other azoles. 

The lone pair in the T] -pyrrolyI ligands is the most basic site in V azolyl complexes, and is the site of protonation and electrophilic attack. Novel planar chiral catalysts have been prepared, based on the azaferrocene unit, and these complexes have been used for asymmetric nucleophilic catalysis. One example of such a complex is shown on the left in Figure 4.5. Some linked versions of azaferrocenes possessing C synunetry create an unconventional chiral environment. The ligand on the right in Figure 4.5 has been used for several asymmetric copper-catalyzed transformations. ... 

Thiazyl monomer is a radical with one unpaired electron. It exhibits an IR band at 1209 cm-1. The experimental dipole moment is 1.83 0.03 D in the opposite direction to that in NO (p = 0.16 D). Much less is known about selenazyl monomer, SeN, but it has been characterized by infrared spectroscopy.36 The structure of a transition-metal complex [OsTp(NSe)Cl2] (Tp=hydrotris(l-pyr-azolyl)borate) has been determined.39... 

Dynamic Jahn-Teller Character of Manganese(IH) Spin-Crossover Complex [Mn(taa)] (H3taa = tris(l-(2-azolyl)-2-azabuten-4-yl)amine)... 

Similar TRIR experiments were performed with Tp Rh(CO)2 and Bp Rh(CO)2 [Tp = hydridotris(3,5-dimethylpjn azolyl)borate Bp = dihydridobis(3,5-dimethyIpyrazolyl)borate] in liquid Xe solution at 223 K 48). From the positions of the observed transient infrared bands, it was found that the Tp species forms two xenon complexes upon photolysis, (7j -Tp )Rh(CO)Xe and (>j -Tp )Rh(CO)Xe, and that Bp Rh(CO)Xe is produced from Bp Rh(CO)2. 

In their initial attempts to prepare classical chelate complexes containing TmMe, Hill and co-workers155 have encountered an unprecedented class of reaction for tris(azolyl)borate, namely the intramolecular activation of the bridgehead B—H bond which provides the first example of a metallaboratrane. The reaction of TmMe with [Ru(R)ClCOPPh3] (CH=CHCPh2OH) likely yield... 

The methyne proton of tris(pyrazolyl)methane is sufficiently acidic to be removed by raBuLi, and the resulting reactive intermediate readily reacts with electrophiles. Klaui prepared two new anionic tripodal nitrogen water-soluble ligands (the lithium salts of tris(pyrazolyl)- and tris(3-tert-butylpyrazol)-methanesulfonic, Tpms and Tmps B" respectively) by addition of lithiated tris(pyr-azolyl)methane to a sulfur trioxide-trimethylamine complex (Scheme 25).146... 

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. 

N-Azolyl-Containing Tridentate Ligands and their Fe/Co Complexes... 

Vhf of 1-8 Hz between the 3-CF3 fluorines and the hydrogens of the platinum-bound methyl groups has been observed by Fekl et al in the spectrum of the novel platinum(IV) complex, Tp(CF3)2PtMe3, where Tp(CF3)2 denotes hydridotris(3,5-bis(trifluoromethyl)pyrazolyl)borate. This is the first example of a formal Vf h(-c) coupling in complexes involving hydridotris(pyr-azolyl)borate ligands... 

TpRu complexes as recoverable Lewis acid catalysts for enantioselective solvent-free cycloaddition reactions (Tp = hy dr otr is (pyr azolyl) borate)... 

SCHEME 34. Carbene complexes via azolyl lithium reagents... 

Mechanistic Studies. - The mechanism of the reaction of tetra-zole-activated phosphoramidites with alcohols has been studied. A series of diethyl azolyl phosphoramidites (85) was prepared from diethyl phosphorochloridite and fully characterized, and the same compounds shown to be formed from the phosphoramidite (86) and azole. The degree of formation of (85) from (86) increases with the acidity of the azole, and the proposed mechanism is a fast protonation of (86), followed by a slow, reversible formation of (85) and a fast reaction of (85) with alcohols. Another study was concerned with the influence of amine hydrochlorides on the rate of methanolysis of the phosphoramidites (87) or (88), or tris(diethylamino)phosphine.The chloride content was measured to be 10-20 mM in doubly distilled samples which explains that "uncatalysed alcoholysis is possible. Intensive purification, including treatment with butyllithium and distillation from sodium, brought the chloride content down to 0.1-1 mM. The methanolysis reaction, in methanol as the solvent, was found to be first-order in catalyst concentration. An aJb initio calculation on N- and P-protonated aminophosphine (89) gave similar proton affinities for N and P this contrasts with earlier MNDO calculations which had ff-protonated species as the most stable. The M-protonated compound had an electronic structure reminiscent of a phosphenium ion-ammonia complex. 

Given these correlations, it seems that it should be possible to predictably turn on or off the stereochemical activity of the lead lone pair. To test this idea, Reger and co-workers prepared a series of lead complexes of polypyr-azolyl borate and polypyrazolyl methane ligands in which the number of donor atoms and the size of the ligand was systematically varied (177, 180, 183, 208). For a constant coordination number (6), only the bulkier hgand is holodirected (Fig. 12). This observation is consistent with the idea that the natural (i.e., energetically favored) state for Pb(ll) compounds is one in which the 6s and 6p orbitals are hybridized and the lone pair is stereochemically active, but that sufficient steric repulsion can overcome this effect and make the holodirected structure more stable. 

---