Synthetic investigations into discrete polynuclear cage formation and their subsequent incorporation into extended network materials

Abstract

This thesis details the synthesis and structural characterisation of thirty novel complexes via the utilisation of a variety of ligands (seven in total) including hydroxamic acids, Schiff base ligands and their hybrid analogues. In addition, a number of co-ligands have also been used in conjunction with one of the primary seven ligands. In Chapter 2 we describe a family of planar pentanuclear Cu(II) 12-MC-4 metallacrowns constructed using the hydroxamic acid ligands 2-(dimethylamino)phenylhydroxamic acid (L1H2) or 2-(amino)phenylhydroxamic acid (L2H2). This family comprises four discrete complexes of formulae [Cu(II)5(L1)4(MeOH)4](ClO4)2 (1), [Cu(II)5(L1)4(pyr)2](ClO4)2.pyr (2), [Cu(II)5(L1)4(pyr)6](ClO4)2 (3), and [Cu(II)5(L2)4(MeOH)4](ClO4)2.H2O (7), whereby the terminal methanol ligands in 1 and 7 have been exchanged in a controlled manner with N- donor pyridine ligands to give complexes 2 and 3. The introduction of ditopic connector ligands such as 4,4'-bipyridine (4,4'-bipy), 4,4'-azopyridine (4,4'-azp), and pyrazine (pz) at the axial Cu(II) coordination sites within our discrete [Cu5] metallacrown units (1-3), results in the pre-meditated formation of the extended networks: {[Cu(II)5(L1)4(4,4'-bipy)3](ClO4)2.H2O}n (4), {[Cu(II)5(L1)4(4,4'-azp)2(MeOH)2](ClO4)2}n (5) and {[Cu(II)5(L2)4(pz)2(MeOH)](ClO4)2.3MeOH}n (6). Electrospray mass spectrometry and UV-vis analysis indicate the solution stability of the {Cu5(Lx)4}2+ (x = 1, 2) cores. Magnetic susceptibility studies carried out on 1, 4 and 6 establish strong antiferromagnetic exchange interactions between the Cu(II) ions, resulting in isolated S = 1/2 ground spin values in all cases. Chapter 3 presents the synthesis, structural and magnetic characterisation of a family of Ni(II) cages also constructed via the hydroxamate building blocks L1H2 or L2H2. This family comprises two pentanuclear 12-MCNi(II)-4 metallacrowns [Ni(II)5(L1)4(MeOH)4](ClO4)2.2MeOH (8) and [Ni(II)5(L1)4(pyr)5](ClO4)2.1H2O (9). Both complexes share analogous near-planar {Ni(II)5(L1)4}2+ cores; however they differ in the number and nature of ligands positioned at the axial Ni(II) sites. The addition of pyridine ligands in 9 converts previous square planar Ni(II) centres to square-based pyramidal/octahedral geometries, thus deliberately introducing extra paramagnetic centres and allowing us to magnetically 'switch on' diamagnetic square planar Ni(II) centres within our analogous [Ni5] metallacrowns. Subtle alterations to the reaction scheme for complexes 8 and 9 results in a change in topology as well as an increase in nuclearity via the formation of the hepta- and nonanuclear complexes [Ni(II)7(L1H)8(L1)2(H2O)6](SO4).15H2O (10), [Ni(II)9(µ-H2O)2(L2)6(L2H)4(H2O)2](SO4).29H2O (11) and [Ni(II)9(µ-H2O)2(L2)6(L2H)4(H2O)2](ClO4)2.2MeOH.18H2O (12). DFT calculations were performed on 8 and 9 to ascertain the ground spin configurations (s = 0 vs. s = 1) of all Ni(II) centres, yielding three and four paramagnetic (s = 1) Ni(II) centres in 8 and 9 respectively. Complementary DFT analysis and dc magnetic susceptibility measurements demonstrate dominant antiferromagnetic exchange pathways in 8 and 9. Magnetic susceptibility measurements carried out on 11 and 12 also indicate dominant antiferromagnetic exchange interactions, while analysis of complex 10 suggest competing ferro- and antiferromagnetic exchange pathways. Chapter 4 details the in-situ ligand formation and subsequent Cu(II) ligation of the polydentate ligands o-[(E)-(2-hydroxy-3-methoxyphenyl)methylideneamino]benzohydroxamic acid (L3H3), [[2-[(E)-(2-hydroxy-3-methoxy-phenyl)methyleneamino]benzoyl]amino]ethanimidate (L4H2) and o-[(E)-(o-hydroxyphenyl)methylideneamino]benzohydroxamic acid (L5H3), formed via the Schiff base condensation of 2-(amino)phenylhydroxamic acid and either 2-hydroxy-3-methoxybenzaldehyde (to give L3H3 and L4H2) or 2-hydroxybenzaldehyde (to give L5H3). The result is the synthesis of a family of discrete Cu(II) polynuclear cages of formulae: [Cu(II)10(L3)4(L2)2(H2O)2](ClO4)4.5MeOH.H2O (13), [Cu(II)14(L3)8(MeOH)3(H2O)5](NO3)4.2MeOH.3H2O (15), [Cu(II)14(L5)8(MeOH)6(NO3)4(H2O)2].6MeOH.10H2O (16) and [Cu(II)30(O)1(OH)4(OMe)2(L3)16(MeOH)4(H2O)2](ClO4)4.2MeOH.27H2O (17). Each member comprises a topology derived from off-set stacked near planar layers of polynuclear subunits connected through long Cu(II)-O contacts. The exact topology observed is dependent on the specific reaction conditions and methodologies employed. Furthermore, through simple modifications to the reaction scheme for 13 (namely the addition of acetonitrile), the topologies previously observed in our Cu(II) cage family (13 and 15-17) were completely transformed upon the construction of the Cu(II) 1D coordination polymer {[Cu(II)(L4)].H2O}n (14) (where L42¯ = [[2-[(E)-(2-hydroxy-3-methoxy-phenyl)methyleneamino]benzoyl]amino]ethanimidate). Chapter 5 investigates the coordination chemistry of Ln(III) metal ions with the ligand 2,6-dimethoxyphenol (L6H). L6H is specifically selected to facilitate the formation of two oxophilic compartments, making it an ideal ligand for the strategic construction of [Ln(III)2] dimers. We were proved correct and present here the microwave assisted synthesis of the dimeric series: [Ln(III)2(L6)2(ROH)x(H2O)y(NO3)4].zEtOH; where Ln = La, R = Et, x = 4, y = 0, z = 0 (18); Ln = Ce, R = Et, x = 4, y = 0, z = 0 (19); Ln = Gd, x = 0, y = 2, z = 2 (20); Ln = Gd, R = Me, x = 2, y = 0, z = 0 (21); Ln = Tb, R = Et, x = 2, y = 0, z = 1 (22); Ln = Tb, R = Me, x = 2, y = 0, z = 0 (23); Ln = Dy, x = 0, y = 2, z = 2 (24). Simple solvent selection allowed us to control the number of {Ln(III)2} units observed in the asymmetric unit (i.e. 1 versus 2). Complementary dc magnetic susceptibility measurements and DFT analysis reveal the presence of weak antiferromagnetic exchange in all paramagnetic family members. DFT calculations were also performed towards elucidating the magnetic exchange mechanisms observed in our complexes. In Chapter 6 we report the coordination chemistry of the Schiff base ligand 1-[(methylimino)methyl]-2-naphthol (L7H), as well as continuing our investigations of the ligand 2,6-dimethoxyphenol (L6H). In the first section of Chapter 6, we present the synthesis and structural analysis of a Mn(III) hydrogen bonded chain [Mn(III)(L7)2(N3)MeOH] (25) and two Mn(III) 1D coordination polymers [Mn(III)(L7)2(Cl)]n (26) and [Mn(III)(L7)2(N3)]n (27) along with a dinuclear Cu(II) metal complex of formula [Cu(II)2(L7)] (28). Coordination polymers 25-27 are the first Mn(III) based chains to be synthesised using the L7H ligand, as well as adding to a family of analogous [Mn(III)(L)2(X)]n (where L = 2-iminomethyl-6-methoxyphenol or 1-[(methylimino)methyl]-2-naphthol and X = Cl¯, Br¯, OAc¯, N3¯) chains, previously synthesised by the Jones group. The introduction of a linear ditopic secondary building unit (SBU) in the form of 4,4'-bipyridine to the reaction scheme for 28 resulted in the formation of a hydrogen bonded 2D extended network of formula [Cu(II)2(NO3)2(L7)2(MeOH)2(4,4'-bipy)]n (29). In the last section of this chapter, we describe the synthesis of the tetranuclear Co(II) cubic complex [Co(II)4(OMe)4(L6)4(MeOH)4] (30). Magnetic susceptibility measurements performed on 30 display weak ferromagnetic intra-molecular interactions between Co(II) metal centres and are suggestive of an effective S' = 2 ground state.