Transition metal complexes of pyrazine-based bis-terdentate diamide ligands

Abstract

The four symmetrical, closely related pyrazine-based bis-terdentate diamide ligands N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H2 LM1), N,N'-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-dicarboxamide (H2 LM2), N,N'-bis(2-pyridylmethyl)-pyrazine-2,5-dicarboxamide (H2 LS1) and N,N'-bis[2-(2-pyridyl)ethyl]pyrazine-2,5-dicarboxamide (H2 LS2) have been synthesised and characterised. Their coordination chemistry towards first row transition metal ion has been investigated.

Several dinuclear copper(II) complexes of (HLM1)-, (H2 LM2)-, (LS1) 2-- and (LS2)2- have been obtained, of which six were structurally characterised, namely [CuII2(HL M1)(MeCN)4] (BF4 )3 · MeCN, [CUII2(LS1)(H20)2(MeCN)2] (BF4 )2 · H2O, [CuII2(LS1)(H20)4](SiF6) · 0.5 H2O, {[CuII2(LS1)(H20)2(SiF6)] · 4 H2O}∞,[CuII2(LS2)(H20)2(MeCN)2](BF4)2 and [CuII2(LS2)(H20)4(BF4)2] · 2 H2O. Curiously, only one complex of (LS2)2- contained six-coordinate copper(II) ions, whereas all other structures contained five-coordinate copper(II) ions. The preparation of the dinuclear copper(II) complexes {[CuII2(LS1)(H20)4(SiF6) · 0.5 H2O and {[CuII2(LS1)(H2O)2(SiF6)] · 4 H2O}∞, was carried out in water, employing Cu(BF4)2, which resulted in the formation of SiF62- ions, by hydrolysis of the BF4- anions and attack of the resulting HF onto the glassware.

Under identical reaction conditions, two structurally very different complexes have been obtained with the homologous ligands H2LM1 and H2LM2, namely the dimeric dinuclear copper(II) complex [CuII2(H2LM1)2(MeCN)2](BF4)4 of a zwitterionic form of H2LM1 and the square-like tetranuclear copper(II) complex [CuII4(H2LM2)2(HLM2)2](BF4)6 · 3 MeCN · 0.5 H2O incorporating (HLM2 )- and a zwitterionic form of H2LM2.

The conversion of the dimeric dinuclear copper(II) complex of H2LM1 into a [2 x 2] grid-type structure of (HLM1)- in solution with base was monitored by UV/VIS spectroscopy. The [2 x 2] grid-type complex [CuII4(HLM1)4](BF4)4 · 3.5 MeCN of (HLM1)- has been structurally characterised. The related square-like tetranuclear copper(II) complex of (HLM2)- [CuII4(HLM2)4](BF4)4 has also been obtained and structurally characterised by X-ray crystallography.

Furthermore, the nickel(II) [2 x 2] grid [NiII4(HLM2)4](BF4)4 · 10 MeCN and the cobalt(III) [2 x 2] grid [CoIII4(LS2)4](BF4)4 · 12.75 MeCN have been obtained and structurally characterised. A nickel(II) [2 x 2] grid-type complex of (HLM1)- was also obtained, but not structurally characterised.

Two mononuclear cobalt(III) complexes of zwitterionc forms of H2LM1 and H2LM2 have been obtained. The H2LM2 containing complex [CoIII(H2LM2)2](BF4)3 · 2 EtOH has been structurally characterised.

Azido bridged copper(II) complexes of (HLM1)- and (HLM2)- have been prepared. The dimeric tetranuclear μ(1,1)-azido bridged complex [CuII4(HLM1)2(μ1,1)- N3) 2(N3)3(MeCN)2](BF4)2 · MeCN of (HLM1)- and the μ(1,3)-azido bridged polymeric chain compound l[CuII2(HLM2)(μ(1,3)-N3)2](BF4) · MeCN}∞ of (HLM2)- have been structurally characterised.

IR, UV /VIS spectroscopic, cyclic voltammetric and magnetic studies have been carried out on several of the complexes mentioned above. The results are presented in this thesis.