A silver-free route has been employed for the synthesis of a number of Pd and Pt complexes supported by an NCN "pincer" ligand (NCN ) [2,6-(Me 2 NCH 2 ) 2 C 6 H 3 ]-) via halide abstraction. This was achieved by the use of o-, m-, and p-hydroxypyridines or o-and p-mercaptopyridines in basic ethanol solutions. The acidic OH or SH protons are removed to generate the formally anionic ligands. X-ray crystal structure determination of these complexes shows that the bonding of the substituted pyridine ligands occurs exclusively through the pyridine N for hydroxypyridines and exclusively through the thiol S in mercaptopyridines. In the hydroxypyridine case, the ortho and para isomers tautomerize to generate pyridone structures with a covalent M−N bond, whereas the m-hydroxypyridine complex, which is unable to tautomerize, is found to still be bound through the pyridine N but in a zwitterionic structure. The mercaptopyridine complexes are the first reported examples of pincer-ligated Pd and Pt thiolate species. The pyridine ligands can be quantitatively exchanged for chloride by reaction with excess NH 4 Cl, a potentially useful chemical switch for ligation/decomplexation of this ligand type.
Dedicated to Prof. Dr. Richard Schrock on the occasion of his 60 th birthday and in recognition of his highly appreciated contribution to chemistry in general and his pioneering studies on ofefin metathesis in particular.Abstract: A general method to prepare macroheterocycles is reported in which a rigid symmetric platinated tris-pincer tricationic complex (3) is used as a template. The system uses bisolefin functionalized pyridine ligands (1, 2) which selectively bind to the three platinum centers of the template resulting in pre-organization of the olefinic tails. By subjecting these pre-organized complexes (4) to olefin metathesis reaction conditions, the olefinic tails were interconnected affording several macroheterocycles of different compositions and sizes. Various pyridines with different substitution patterns (2,6-and 3,5-disubstituted) and different olefinic tails (aliphatic, polyether, and styryl) were synthesized and used in the cyclization reactions, showing the diversity of this method. It was found that only the use of 2,6-disubstituted pyridines resulted in the formation of the desired macrocycles while 3,5-difunctionalized pyridines gave only pyridinophane formation. Furthermore, the length of the olefinic tails needs to be at least eleven atoms (C or O) in order to suppress oligomerization in favor of macrocycle formation. Yields of up to 70% of the macrocycles were obtained using the first-generation [Cl 2 (Cy 3 P) 2 Ru ¼ CHPh] or second-generation [Cl 2 (Cy 3 P)(IMes)Ru ¼ CHPh] Grubbs catalysts. Reactions of the platinated pincer pyridines with Schrock Mo-based metathesis catalysts results in exclusive olefin isomerization and no productive metathesis. The generated macrocycles (hosts) could effectively be separated from the template by addition of aqueous sodium chloride and reattached to the (recycled) template (guest). The X-ray structure determination of host-guest complex 10 is the definite proof for this reattachment and the perfect cyclic structure of the macrocycle.
The reaction of [PdCl{C6H3(CH2NMe2)2-2,6}] ([PdCl(NCN)]) with either the sodium or silver
salts of the borate anions [B{C6H4(SiMe3)-4}4]- and [B(C6H4{SiMe2(CH2CH2C6F13)}-4)4]-
under various reaction conditions resulted in the exclusive formation of the respective
dinuclear [{Pd(NCN)}2(μ-Cl)][B{C6H4(SiMe2R)-4}4] salts 4a (R = Me) and 4b (R = CH2CH2C6F13). When using BF4
-, both dinuclear [{Pd(NCN)}2(μ-Cl)]BF4 (4c) and mononuclear
[Pd(NCN)(OH2)]BF4 (5c) could be obtained. The molecular structure of 5c showed the
presence of hydrogen-bridging interactions between coordinated H2O molecules and the
fluorine atoms of BF4
-. Attempts to isolate complexes of the type [Pd(NCN)(OH2)][B{C6H4(SiMe2R)-4}4] were unsuccessful. This has been ascribed to a difference in stabilization of
the respective mono- and dinuclear cations by the borate anions BF4
- and [B{C6H4(SiMe2R)-4}4]-, respectively. The conclusions are supported by DFT calculations.
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