Pyrrol ligand

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

The attraction of the rhenium system is that deprotection of the pyrrole can readily be achieved by treatment with acid and DMSO, which replaces the kinetically labile i7- -pyrrole ligand on the metal [87JOM(326)C17 90H(31)383]. However, the relatively high cost of rhenium will almost certainly preclude its synthetic use as a pyrrole protecting group, especially when compared with the alternative methods discussed above. 

Pyrrole ligands can form both Ln-N or-bonds and tjs-n-Ln bonds. complexes with sterically less crowded pyrrole ligands [195]. The introduction of sterically demanding groups in a-position as in 2,5-di-fert-butylpyrrole led to a shielding of the nitrogen and subsequent -coordination to the lanthanide center [196]. Additionally, rj1-coordination to a sodium atom is observed in the obtained ate complex. 

Table 3. Selectivity factor SF-/x2 of calix[4]pyrrole ligands for fluoride anion relative to other anions (X2 = Cl-, Br , I- and H2PO4) in acetonitrile (MeCN), di-chloromethane (DCM), A/,A/-dimethylformamide (DMF), dimethyl sulphoxide (DMSO) and propylene carbonate (PC)...

Ihe interaction of polynuclear metal complexes with pyrrole-type molecules has also attracted some attention, and it is interesting in connection with possible interactions with surfaces, where vicinal metal atoms may simultaneously interact with the pyrrole molecule. Cluster compounds containing intact coordinated pyrrole ligands are not known, since this reaction invariably involve N-H and/or C-H bond activation to yield derivatives most frequently containing bridging pyrrolyl ligands, as exemplified in Eig. 6.2. 

Among the few known heteroruthenocenes are cationic thiophene- and bis(thiophene)ruthenium, electronic analogs to arene ruthenium sandwich complexes (Section 5), and derivatives featuring a tetramethylpyrrole (tp) ligand, Cp Ru(tp) (91) and Ru(tp)2, prepared by an adaptation of the Zn reduction method to the chloro complex [Cp RuCl2]2 (92) (equation 21). The methyl groups are necessary to prevent a-complexation of the pyrrole ligand however, under suitable conditions ([Cp Ru(acac)]2/Zn +/pyrrole in ether), the preparation of at least Cp Ru (pyrrole) (93) has been equally successful (equation 22). 

An interesting C-metalation of a coordinated pyrrol ligand in a rhenium complex has been described [19a]. Methylated aromatic hydrocarbons have been activated by (ZrCU) [19b], A novel mode of electrophilic activation of an aliphatic C-H bond, assisted by porphyrin complexes ofZr(IV) and achieved by the use of hydrides of lithium, sodium or potassium, has been reported [19c]. It has been shown that the ligands 2,3,5,6-tetraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular activation by tantalum alkylidene groups at rates 20 and 100 times slower than that of the simple 2,6-diphenylphenoxide ligand [19d]. 

Condensation of a tris-pyrrole ligand syntone 75 with its diformylated derivative 76 by Scheme 2.101 has been used in [113] for the syn-... 

Secondaiy alcohols have been shown to be competent reaction partners for the preparation of tertiaiy amines, but typically require higher temperatures. Beller and co-workers reported a method for the synthesis of cx-branched tertiary amines via N-all lation of secondary amines with secondaiy alcohols using catalytic Ru3(CO)i2 with JV-phenyl-2-(dicyclo-he)ylphosphanyl)pyrrole ligand (Scheme 12.6). The scope is limited to the adulation of cyclic amines as acyclic amines undergo trans-alkylation, likely due to competitive amine dehydrogenation. 

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