An introduction to the work in the thesis is given.
A survey of the literature describes general methods of
synthesis of transition metal macrocycles and porphyrins and
their relevance to substrate binding and activation. The
aims of this study are introduced.
The synthesis and characterization of four neutral 14-
membered tetraaza 2 ,6 -pyridyl macrocycles L 1- ! 1* with varying
carbon and nitrogen alkylation is described. Extensive 1H
and 1 3C n.m.r. studies in combination with a single X-ray
crystal structure of the free ligand I^.E^O were undertaken.
The free ligand was found to adopt a puckered arrangement
in order to minimise nitrogen donor lone pair interactions.
The insertion of the platinum metals Ru(II), Rh(III),
Ir(III), Pd(II), and Pt(II) into the tetraaza macrocyclic
ligands L¹ - L⁴ is described. All new compounds were fully
characterized by n.m.r., infrared and mass spectral techniques and by the X-ray crystal structures of
cis [RuL³ (CO) Cl] BPh₄, trans [RhL² Cl₂„ ] PF₆, cis[IrL⁴(H)Cl]PF₆, and [PdL¹](BPh₄)₂. The Rh(III) and Pd(II) structures both
show the macrocyclic ligands bound to four equatorial sites
around each metal centre. In contrast the Ir(III) and Ru(II)
complexes display folded ligand structures with one nitrogen
donor, N(7), bent away to occupy an apical position to
give a-is coordination of the remaining two ligands. 1H
and ¹³C n.m.r. studies indicated that in most cases several
ligand isomers were present in solution.
The X-ray crystal structure of the square planar
[Pd (TMC) ] cation shows the macrocycle to adopt the R.S.R.S.
The complex shows a fully reversible one electron
reduction process in acetonitrile at E,= -1.45V vs. A g / A g+,
ΔEp=65mV at Pt electrodes. Controlled potential electrolysis
of [Pd(TMC)]²+ at -1.50V in acetonitrile afforded a reduction
product which was assigned as a d⁹, Pd(I) complex [Pd(TMC)]+,
(g₁₁ = 2.302, g₁=2.076) with coupling to ¹⁰⁵Pd (22.2%, I = 5/2),
A₁₁= 53 , A₁=40G. From an investigation of TMC and six other
tetraaza macrocyclic complexes, the Pd(II)/(I) redox couple
was found to occur at potentials on average 300mV more cathodic
than the corresponding Ni(II)/(I) couples. No Pd(II)/(III)
redox process was observed within the anodic range of the
acetonitrile solvent; this contrasts with the corresponding
nickel macrocyclic complexes which all show Ni(II)/(III)
redox couples in the range +0.68—»+1.33V.
A comparison of the redox chemistry of the square planar
nickel diiminopyridyl macrocyclic complexes [Ni(n₃X)]²⁺ was
investigated. (X = n,p where n₄, n₃p are respective tetraaza
and triazaphosphorus donor sets) . Both complexes show two
reversible one electron reductions. Characterization of
the first reduction product by e.s.r. spectroscopy shows
the reduction of [Ni(n₄)]²⁺ to be ligand based, whereas for
[Ni (n₃p)]²⁺the reduction was found to be metal based. The
binding of CO, phosphines and related ligands to the mono
valent complex is described. The single crystal X-ray
structure of [Ni(n₃p)] (PF₆)₂ was determined confirming the
square planar nickel geometry.
A structural investigation of the monohalo derivatives
[Ni(n₃X)Cl]+, (X = p,s, where n^s is a triazathia donor set)
was u n d e r t a k e n . [Ni (n₃p)Cl]+ shows a 5 coordinate square
based pyramidal geometry around Ni(II) analogous to the
previously reported [Ni(n4)Br]+ complex. In contrast a
dimeric structure with dichloro bridges between octahedral
nickel(II) centres is found for the sulphur analogue
Parallel studies were instigated into the isoelectronic
reactions of [NiL]+ with dioxygen and [NiL]²+ with superoxide
ion in acetonitrile at 230K (L = TMC, C-rac HMC).The product
of the reactions was a reactive paramagnetic pale green
solution with an anisotropic e.s.r. signal, g₁=2.195, g₁₁=2.056.
⁶¹Ni labelling experiments confirmed the predominantly metal
based radical nature of the species of proposed stoichiometry
[NiL(O₂)]+ . A respective one or two electron transfer from
nickel to superoxide or dioxygen is proposed to form a reactive, formal nickel(III) - peroxo complex. A rhombic e.s.r.
signal from a paramagnetic green solution for L=TMC is also
assigned as a nickel (III) - peroxo species.