The Degree of D. Phil DEOFJJBER 1968
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'Transition Metal Complexes of Quadridentate ilitrogen Ligands' A thesis submitted for the Degree of D. Phil in the University of York P. A. TASKER DEOFJJBER1968 ABSTRACT B The properties of vitamin 12 and various 'model' compounds are reviewed, and the criteria for ligands which might act as substitutes for the corrin s'ystem are discussed in chapters I and 2. A series of possible 'model' ligands was prepared by the condensatiori of some diamines with o-aminoaryl carbonyl compounds in a 1: 2 molar ratio. The properties of these free ligands were examined, and the effect of variation of structural features in the ligands on the geometry, conjugation and crystal field splitting when complexedis discussed. Nickel(II) complexes of dianionic forms of these ligands were prepared and studied. All the ligands, gave planar nickel(II) complexesp even in cases where a tetrahedral arrangement of the donor atoms seemed to give a much less strained form of the ligand. In these complexes the nickel(II) ion showed. no tendency to increase its coordination number, and this behaviour contrasted with that of analogous 'N ligands derived from complexes of 202t salicyl- aldehyde. It is suggested that this difference results from electronic, rather than steric. effects. The cobalt(II) complexes were also judged to be planar. They showed less tendency to be oxygen- than 'N ated or oxidized similar complexes of 202' ligands. Some evidence is presented for reduction to to B a species analogous vitamin 12s which contains cobalt with a formal oxidation state of one, but no to B be alkyls analogous vitamin 12 coenzyme could isolated. The synthesis of macrocyclic ligands is discussed in chapter 6. A series of new macrocyclic complexes was prepared by the reaction of 4.7-diaza- 293: 8p9-diberizodecane-1,10-dione with some diamines in the presence of metal ions. Some aspects of the mechanisms of these reactions are discussed in chapter 7. Only for the reaction of 192-diamino- ethane with the, dialdehyde was a 'template' or organ- izAtional effect of the metal ion suggested. The experimental section is divided into (chapter two parts. The first io) gives general experimental detailst and includes measurement of magnetic susceptibility in solution by Evans' NMR method, and a manometric study of gas absorption.. The second part describes preparative methods and reaction details (chapter ii). ii ACKNQVLEDGEMENTS. I should like to thank Dr. M. Green for his friendly supervision and encouragement received during the course of this research. I should also like to express my gratitude to all the members of the department v1ho have given assistance, in particular to Dr. J. Smith and R. Bratchley for helpful discussionsp and to Mrs. M. Sutherland for running so many IM spectra for me. Finally I am indebted to my wife for her constant encouragement and help in the preparation of this thesis, and to my father who arranged for many of the diagrams and spectra to be reduced in size. C OITTETITS page Acknowledgments Contents Chapter 1. Vitamin Bjý and 'model' compounds Chapter 2. Choice of 'model' ligand systems 21 Chapter 3- Acyclic 'modelt ligand systems 30 Chapter 4- Nickel(II) acyclic . complexes of quadridentate nitrogen ligands 59 Chapter 5. Cobalt complexes of the acyclic 'model' ligands, 106 Chapter 6. The synthesis of macrocyclic ligands 141 Chapter 7. Some new quadridentate macrocyclic ligands 1-53 Chapter 8. Nickel(II) and copper(II) complexes of some new macrocyclic ligands 188 Chapter 9. Cobalt complexes of some new macro- cyclic ligands 209 Chapter 10. pleneral experimental details 226 Chapter 11. Preparative details 265 Pi ONH2 2 H3 CH2 Z'- I' 3"ýrb 0' ý122 /\ CONFý 0N 'CH- (. Hj 1. N., ýR 99 N H3 CONH2 23CH CH Fý , 2 0 co )NH2 NH CH, C%l 2 %C ol 1] 00 (B) Vitamin B12 cocnzyme R= iL * Vitamin B R= 011- 12 11.13. The 13 its structure of vitamin 12 and coenzyme. Derivatives differing by virtue of the nature of 'R' are called cobalamins, e. g. cyanocobalanin is vitamin B, 2; The tetrapyrrolic-rinG system will be referred o as the 'corrin' system., 1 Chapter 1. Vitamin B12 and 'Model' Compounds. 1.1 Vitamin B B 12 and vitamin 12 coenzyme. 1.1.1 Ligand replacement in vitamin B12 derivatives. 1.1.2 Electronic spectra of vitamin B12 derivatives. 1.1.3' Reduction of vitamin B12 derivatives. 1.2 'Model' systems for vitamin B12* Reduction of the 'model' compounds. 'Model' B 1.2,2 compounds of vitamin 12 coenzyme. 1.2.3 Ligand replacement in 'model' compounds. L. 1- Other systems stabilizing low valency states and alkyls of cobalt. 1.4 References. 1.1 Vitamin B B12 12 and vitamin coenzyme. been The chemistry of vitamin B12 has studied 192,394 extensively and reviews deal with references up to 1967. The structure of vitamin B12 [1.19PJ was derived mainly from X-ray crystallographic data by Hodgkin and co-workers596. Vitamin B12 coenzyme Cl. ltB] is a more functional form, but was isol'ated798 twelve years after the vitamin, because it is highly sensitive to light and cyanide ions, and moderately sensitive to acid. Consequently any of these agents in all the 2 earlier isolation procedures would have destroyed it to give other forms of vitamin B12* It is the first known naturally-occurring organo-metallic compound., For the inorganic chemist the main interest in vitamin B12 lies in its unusual properties in comparison with other cobalt complexes. The many reactions of the peripheral groups in the molecule a:re of importance in studies of the biological properties of the vitamin, and were also important in establishing the complete structure of the molecule, but will not be dealt with here. 1.1.1 Ligand Replacement in vitamin B12 derivatives. Both the cyanide or 5'-deoxyadenosyl group Min [1.11) and the benziminazole moiety may be displaced from the coordination sphere of the cobalt atom by other ligandsq9tlO but the corrin ring system has not been removed without the use of reagents which 11 cause its destruction. Certain cobalt-free corrinoids have been isolated from photosynthetic bacteria, but 12,13 have yet to be clearly characterized. The rigid equatorial ligand system of vitamin B12 makes it an ideal system for studies of re placement , In B reactions. such studies14,15,16917918 vitamin 12 and 3 its derivatives are represented by the idealized [1.21 11-33 structures and - In this way electronic transmission through the cobalt atom into cis-and trans effects /, Co LýLZ [1.2] [1 -31 Z( Co [l -41 [1.21 Vitamin B12 and coenzyme 11-31 Vitamin B derivatives 12: without the nucleotide, (1.41 Cobalt corrinoid Cis-effects are illustrated by A dependence of the- properties of the equatorial ring system on the nature of the axial ligands. For example it has been suggested. that the main effect of an axial ligand on the electronic spectrum of the equatorial ring system (cis-effect) is related to the position of the ligand in the nephelauxetic 14 series. The N. M. R. spectrum of the equatorial ring system (in particular the C-10 hydrogen atom, see [1.11 11 is also strongly dependent on the-axia ligand (R)15,16, 4 but no systematic variation with the nature of R has been recorded, and small changes in the ring shape might 15 account for the effects. A trans, -effect is manifest by a dependence of the properties of one axial ligand (e. g. R) on the nature of the other (i. e. B). For example, the equilibrium (equation [1.11 ) favours the coordination of the benziminazole moiety for the less polarizable axial 17 ligand R. In other words, increasing the polarizability of the group R decreases the stability of the cobalt- benziminazole bond relative to that of the cobalt-water bond. Similarly, by studying the cyanide stretching 18 frequency in complexes ( 11-33 B= CN-) it has been shown that the strength of the cobalt cyanide bond decreases with polarizability of the group R. R R I H20 7 ý-o7 [equation 1.1] ý, Ll---BZ H20o bz When the primary amide groups (see [1.11 )'Of dicyanocobalamin were converted to-the corresponding ethyl ester groups, an ether soluble cobalt (III) complex 11-51 5 19 was obtained. This was treated with an excess of methyl magnesium iodide and then hydrolysed, when a methyl cobalamin C1.61 was obtained in which the ethyl ester groups had been converted into the corresponding tertiary alcohol groups*19 This is one of the two important routes to derivatives B of vitamin 12 containing alkyl-cobalt bonds. CN CH3 I 1. Ether 1 CL CH3Mg -Z, co 1 2 H20 CN H20 P-51 P. 63 1.1.2. Electronic spectra of vitamin B. 2 derivatives. The spectrum of vitamin B12 is qualitatively similar to the spectra of the metalloporphyrins, apparently having a Soret (- 7S) ban&,, (361 MYL) and distinct oc and P bands (550 and 520 M)ýt )30 At f irst sight this is rather surprising since it was thought that tetra-pyrrole systems in which the conjugation around the macrocycle is interrupted, e. g. the porphyrinogens F6 Cb bl 0 Co -x 1 Ei5! e 02u im t-- CY vi > Cd r-I 4-ý V r-4 A: ýNvavossy 6 21 and corrins, would not show a Soret band. A "conjugated pathway maintained by the cobalt atom" has 21 been proposed to explain this apparent anomaly. However, even the cobalt-free corrinoid compoundS13 contain bands which correspond closely to all those in the vitamin B12 spectrum above 300 MV4 and, hence, it does not seem necessary to postulate a completely cyclic conjugated system to explain the transitions for the B responsible spectra of vitamin 12 derivatives.