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

Abstract

The 2-(3-biphenyl-2-ol)-6-iminepyridines, 2-(3-C₁₂H₈-2-OH)-6-(CHNAr)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-(CHNAr)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)). Depending on the temperature of the reaction, the oxidised aldimine products 1a or 1b can also be observed as a competitive side-product during the formation of 3a or 3b. Similarly, ketimine-containing, [{2-(3-C₁₂H₈-2-O)-6-(CMeN(2,6-i-Pr₂C₆H₃))C₅H₃N}Pd(OAc)] (5), can be detected during the preparation of chiral [{2-(3-C₁₂H₈-2-O)-6-(CMeH-NH(2,6-i-Pr₂C₆H₃))C₅H₃N}Pd(OAc)] (4) from 2-(3-C₁₂H₈-2-OH)-6-(CH₂-NH(2,6-i-Pr₂C₆H₃))C₅H₃N (L3-H) and Pd(OAc)₂. Complexes 3a, 3b and 4 all exist as dimeric species in the solid state in which two anti-aligned square planar monomers are held together by two intermolecular NH_amine⋯O_phenolate interactions resulting in palladium–palladium separations of between 3.141–3.284 Å. The structurally related chloride-containing dimeric assemblies, [{2-(3-C₁₂H₈-2-O)-6-(CH₂-NHAr)C₅H₃N)}PdCl] (Ar = 2,6-i-Pr₂C₆H₃ (6a), 2,4,6-Me₃C₆H₂ (6b)), can also be isolated on treatment of 3 with aqueous sodium chloride or by reaction of L3-H with (MeCN)₂PdCl₂. Single crystal X-ray diffraction studies have been performed on L1a-H, L3-H, 1a, 1b, 2a, 2b, 3a, 3b, 4, 6a and 6b.