Encapsulated Metal Ions: Mononuclear and Binuclear Complexes of Schiff-Base Macrocycles and Cryptands

Abstract

Schiff-base condensation of 2,6-dlacetylpyridine with diamines on Pb(II) afforded complexes of binucleating [2+2] macrocyclic ligands P (1) and MC (2). The complexes were isolated as the dilead macrocyclic tetrathiocyanate. Interest derives from the unusual binding mode of thiocyanate as a single atom 'N-only' bridge between the pair of lead ions bound by the macrocycle. An X-ray crystallographic structure determination, carried out by Dr. V .McKee, on the dilead MC complex demonstrates the existence of a single thiocyanate 'N-only' bridge between the lead ions. The N-only bridging mode of thiocyanate is retained in derivatives containing two or three triflate anions. Transmetallation of the P complex, Pb2P(NCS)4 with transition metal triflates gave homobinuclear products with M = Cu(II), Co(II) and heterobinuclear Pb/M products with M = Mn(II), Fe(II), Ni(II), whereas transmetallation of the dilead MC complex afforded homobinuclear Cu(II) and Ni(II) complexes and heteroblnuclear complexes with M = Mn(II) and Fe(II). Infrared spectra suggest that the 'N-only' bridging mode of thiocyanate exists between the metal ions in the heterobinuclear MnPbMC and FePbMC complexes and X-ray structural confirmation has been obtained for the manganese/lead MC complex. There was evidence of the existence of heterobinuclear FeCuP and FeCuMC complexes which were formed by metal exchange of the lead ion, in the respective FePb macrocyclic complex, for Cu(I). The cryptand GT, (3) was farmed by Schiff-base condensation of the tripod amine N(CH2CH2NH2)3 with glyoxal, using group (11) metal ions as template. A wide range of metal ion GT cryptates were obtained by transmetallation, including transition metal ion, lanthanide ion and group(1) metal ion cryptates. An X-ray crystal structure of the cobalt(II) GT cryptate has been obtained. Generally the cryptates are mononuclear with respect to the metal ion, an exception being the binuclear copper (I) cryptate. Transmetallation with copper (ID produces either the mononuclear or the dinuclear cryptate depending on conditions employed. The binuclear copper(II) cryptate has a particularly unusual 7-line esr spectrum which suggests formation of a mixed valence Cu(II)/Cu(I) species. Cyclic voltammetry has shown strong stabilisation of the Cu(I) state, with E½ for the reduction lying in the range found for blue copper proteins. Synthesis of transition metal ion complexes by transmetallation of disilver or dilead 3Bp (4), or metal ion insertion into the 3Bp ligand was investigated. Dicobalt(II) and dicopper (I) cryptates of 3Bp were obtained. The corresponding reduced metal-free cryptand, R3Bp (5), gave an interesting series of binuclear complexes of Ni(II), Cu(I), Cu(II) and Co(II). The dicopper(II) and dicobalt(II) cryptates are of particular interest due to the presence of a hydroxo bridge between the metal ions. Attempted insertion of Mn(II) into R3Bp failed but gave the tetraprotonated R3Bp triflate whose structure was crystallographically determined. Attempts to insert metal ions into cryptand 3Bm (6) failed. However insertion of Ni(II) gave ring opening and a pendant arm macrocyclic complex was obtained. The reduced metal-free ligand, R3Bm (7) produced a wider range of complexes, including the dinickel(II), dicopper(II) and dicobalt(II) cryptates. Transition metal complexes throughout, have been characterised using infrared, electronic, and e.s.r. spectroscopy. Magnetic susceptibility measurements and n.m.r. were employed where appropriate. The work described in Chapters 2, 3, 4 and 5 is preceeded by a review of the relevant literature, in Chapter 1.