Organo-palladium(II) complexes bearing unsymmetrical N,N,N-pincer ligands: synthesis, structures and oxidatively induced coupling reactions.

The 2-(2′-aniline)-6-imine-pyridines, 2-(C6H4-2′-NH2)-6-(CMe=NAr)C5H3N (Ar = 4-i-PrC6H4 (HL1a), 2,6-i-Pr2C6H3 (HL1b)), have been synthesised via sequential Stille cross-coupling, deprotection and condensation steps from 6-tributylstannyl-2-(2-methyl-1,3-dioxolan-2-yl)pyridine and 2-bromonitrobenzene. The palladium(II) acetate N,N,N-pincer complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)), can be prepared by reacting HL1 with Pd(OAc)2 or, in the case of 1a, more conveniently by the template reaction of ketone 2-(C6H4-2′-NH2)-6-(CMe=O)C5H3N, Pd(OAc)2 and 4-isopropylaniline; ready conversion of 1 to their chloride analogues, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), has been demonstrated. The phenyl-containing complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdPh] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)), can be obtained by treating HL1 with (PPh3)2PdPh(Br) in the presence of NaH or with regard to 3a, by the salt elimination reaction of 2a with phenyllithium.

O,N,N-pincer ligand effects on oxidatively induced carbon-chlorine coupling reactions at palladium.

The syntheses of two families of sterically tuneable O,N,N pro-ligands are reported, namely the 2-(phenyl-2'-ol)-6-imine-pyridines, 2-(C6H4-2'-OH),6-(CMe=NAr)C5H3N [Ar = 4-i-PrC6H4 (HL1(a)), 2,6-i-Pr2C6H3 (HL1(b))] and the 2-(phenyl-2'-ol)-6-(amino-prop-2-yl)pyridines, 2-(C6H4-2'-OH),6-(CMe2NHAr)C5H3N [Ar = 4-i-PrC6H4 (HL2(a)), 2,6-i-Pr2C6H3 (HL2(b))], using straightforward synthetic approaches and in reasonable overall yields. Interaction of HL1(a/c) and HL2(a/b) with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C6H4-2'-O)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)) and [{2-(C6H4-2'-O)-6-(CMe2NHAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), which can be readily converted to their chloride derivatives, [{2-(C6H4-2'-O)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)) and [{2-(C6H4-2'-O)-6-(CMe2NHAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (4a), 2,6-i-Pr2C6H3 (4b)), respectively, on reaction with an aqueous sodium chloride solution. Treating each of 3a, 3b, 4a and 4b with two equivalents of di-p-tolyliodonium triflate at 100 °C in a toluene/acetonitrile mixture affords varying amounts of 4-chlorotoluene along with the 4-iodotoluene by-product with the conversions highly dependent on the steric and backbone properties of the pincer complex employed (viz.

From discrete monomeric complexes to hydrogen-bonded dimeric assemblies based on sterically encumbered square planar palladium(II) ONN-pincers.

The 2-(3-biphenyl-2-ol)-6-iminepyridines, 2-(3-C12H8-2-OH)-6-(CH=NAr)C5H3N (Ar = 2,6-i-Pr2C6H3 (L1a-H), 2,4,6-Me3C6H2 (L1b-H)), have been prepared in high yield via sequential Suzuki coupling, deprotection and condensation reactions from 2-methoxybiphenyl-3-ylboronic acid and 2-bromo-6-formylpyridine. Treatment of L1-H with Pd(OAc)2 or (MeCN)2PdCl2 results in deprotonation of L1-H to afford the discrete square planar ONN-chelates, [{2-(3-C12H8-2-O)-6-(CHNAr)C5H3N}Pd(OAc)] (Ar = 2,6-i-Pr2C6H3 (1a), 2,4,6-Me3C6H2 (1b)) and [{2-(3-C12H8-2-O)-6-(CH=NAr)C5H3N}PdCl] (Ar = 2,6-i-Pr2C6H3 (2a), 2,4,6-Me3C6H2 (2b)), in good yield, respectively; conversion of 1 to 2 using aqueous sodium chloride has been demonstrated. Selective reduction of the imino unit in L1-H with LiAlH4 proceeds smoothly to yield the 2-(3-biphenyl-2-ol)-6-(methylamine)pyridines, 2-(3-C12H8-2-OH)-6-(CH2-NHAr)C5H3N (Ar = 2,6-i-Pr2C6H3 (L2a-H), 2,4,6-Me3C6H2 (L2b-H)), which on reaction with Pd(OAc)2 give [{2-(3-C12H8-2-O)-6-(CH2-NHAr)C5H3N}Pd(OAc)] (Ar = 2,6-i-Pr2C6H3 (3a), 2,4,6-Me3C6H2 (3b)).