CuII bis(oxamato) end-grafted poly(amidoamine) dendrimers

Abstract

Successive treatment of the diethyl ester of 3,4-bis(oxamate)phenylene benzoic acid (3,4-bopbH₂Et₂OH; 1) with [Bu₄N]OH and MCl₂·nH₂O (M = Cu, Ni; n = 2, 6) gave, upon extraction into CH₂Cl₂, binuclear complexes [Bu₄N]₄[{M(3,4-bopbO)}₂CH₂] (M-3, M = Cu, Ni) as novel symmetric methylene diesters. In contrast, subsequent addition of [Bu₄N]OH and anhydrous NiCl₂ to 1 afforded the very hygroscopic [Bu₄N]₃[Ni(3,4-bopbO)] (4) instead. In order to verify reaction conditions to achieve the corresponding carboxamides to 1 – as a crucial step for the synthesis of higher generation dendrimers – 1 was shown to react with RNH₂ (R = Me (5a), Pr (5b)) to [RNH₃][tdqc] (R = Me (6a), Pr (6b); tdqc = 1,2,3,4-tetrahydro-2,3-dioxo-6-quinoxaline carboxylate) by rearrangement reactions. Alternatively, if 1 was converted first to 3,4-bopbH₂EtF (8) and treated next with PrNH₂, the corresponding carboxamide 3,4-bopbH₂Et₂NPrH (7b) could be obtained in high yields. The reaction of 7b with [Bu₄N]OH and MCl₂·nH₂O (M = Cu, Ni; n = 2, 6) gave conveniently mononuclear [Bu₄N]₂[M(3,4-bopbNPrH)] (M-9b, M = Cu, Ni). After thus optimizing reaction conditions, for the synthesis of higher-branched bis(oxamato) type complexes the (polyamido)amines 5c (en^IIH₂), 5d (den^IIIH₃), 5e (den^VH₅) and 5f (den^XH₁₀), possessing two, three, five or ten terminal amino groups, were reacted with appropriate equiv. of 8 to give the corresponding carboxamides en^II(3,4-bopbH₂Et₂)₂ (7c), den^III(3,4-bopbH₂Et₂)₃ (7d), den^V(3,4-bopbH₂Et₂)₅ (7e) and den^X(3,4-bopbH₂Et₂)₁₀ (7f). Compounds 7c–f were converted to the corresponding Cu^II-containing complexes 9c–f, which were treated with the corresponding equiv. of [Cu(pmdta)][BF₄]₂ to afford 10c–f. Complexes of the series 9c–f and 10c–f possess two, three, five or ten end-grafted mononuclear {Cu(3,4-bopb)}²⁻ and trinuclear {Cu₃(3,4-bopb)(pmdta)₂}²⁺ complex fragments, respectively. The identities of 7c–f were established by NMR spectroscopy and ESI-MS measurements. For 9c–f and 10c–f, ESI-MS, UV/Vis and ESR studies were applied to confirm the identities of these species. The magnetic properties of the {Cu₃(3,4-bopb)(pmdta)₂}²⁺ end-grafted poly(amidoamine) dendrimers 10c–f were studied by susceptibility measurements vs. temperature to give J values between −112 (10c) and −118 cm⁻¹ (10d), which are typically observed for discrete trinuclear Cu^II-containing bis(oxamato) type complexes.