Table Of Contentstereochemistry of organometallic and inorganic compounds 2
Stereochemical and
Stereophysical Behaviour of
Macrocycles
Edited by
IVAN BERNAL
Department of Chemistry, University of Houston, Houston TX 77004, U.S.A.
Elsevier
Amsterdam — Oxford — New York — Tokyo 1987
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V
Introduction to Volume 2
Volume 1 appeared in the Spring of 1986, was twice reviewed in
journals and a number of times in oral or written private communications.
There is general agreement that the series will serve a valuable role in
surveying complex subjects for readers who need the unifying synthesis
only an expert can provide; however, as expected, there is no overall
consensus as to what should be the content of each individual volume.
Most commentators seem to favor dedication of each tome to a single
topic-- a few spoke favorably on behalf of variety. Many understand the
problem of filling an entire volume with equally relevant and timely
material and all understand the problem of gathering, at a given point in
time, a group of experts who have the time and inclination to write a
survey of their specialties, even when, admittedly, those specialties are
ripe for evaluation.
Nonetheless, an effort was made to provide a sharper focus in the
content of Volume 2 which will deal exclusively with coordination
compounds of macrocyclic ligands. The three chapters deal with the
relationship between stereochemistry and other chemical and physical
properties of metal macrocyclics. The rationale for this selection is
stated in Chapter 1 by Boeyens and Dobson: "the interest in metallic
macrocyclics, as reflected in the current literature, establishes this field
of study as one of the most prolific topics in coordination chemistry."
Their review was largely and "arbitrarily restricted to the
stereochemistry of transition-metal macromonocycles, for ring sizes not
exceeding sixteen, and excluding crown ethers."
The relationship between thermodynamics and stereochemistry of
macrocyclics and cryptâtes is explored in Chapter 2 by Buschmann,who
remarks that "only one year after Werner discovered chelate compounds
[1893, ed.] the first crown ether was synthesized by Vorländer"; yet the
rate at which synthetic work has progressed in recent years is such that
"in 1985 Izatt and co-workers tabulated the known thermodynamic data
for cation-macrocyclic interactions (ref. 19). At that time the complexa-
tion properties of almost 260 different ligands were given. Thus, the
properties of 90% of the known ligands have not yet been estimated."
Matthes and Parker review, principally, stereochemical aspects of
the macrocycles of second and third row transition elements with additio
nal comments on the stereochemistry of Cu and Ni in unusual oxidation
states and their survey, though somewhat briefer in appearance, covers, as
do all others, information published well into 1986 when the first drafts
VI
of manuscripts were due.
We hope this volume elicits interest in the topics covered among
those not familiar with the subject reviewed and serves as an update for
those active in the field. Our aim is to provide an educational vehicle for
the neophyte and an update for those actively participating in the growth
of the discipline, and your comments will inform us of the degree to
which these expectations are met.
Many thanks to those who have taken the time to privately convey,
orally or in writing, their reaction to Volume 1 and for suggestions as to
topics we should cover, and by whom. Such information is invaluable to us
and, to the extent that we can persuade suitable reviewers to contribute,
efforts will be made to cover those topics if, in the estimation of the
potential reviewer, they have not recently been treated elsewhere.
Volumes 3 to 5 are already committed and, to no small degree, will reflect
the judicious choice of people who cared to make suggestions of topical
material which interests them. Heartfelt thanks to them, also.
A letter by Professor John C. Bailar, Jr., commenting on a chapter in
Volume 1, was particularly satisfying and I share it with the chemical
community so as to preserve a bit of history of American chemistry.
Among other comments, he remarked that ( and I quote with his kind
permission):
"Jackson has done an excellent job on the inversion reactions [i.e.,
The Bailar Inversion Reaction; ed] and he is to be commended for it. He
obviously has a tremendously wide knowledge of all the reactions involved.
When Auten and I observed the reaction in 1934 I did not realize it would
precipitate such a great amount of subsequent research, and that it would
lead to new ideas about reaction mechanisms. Incidentally this was the
first piece of work which I did in coordination compounds, and my organic
chemical background, of course, played an important role in it. It seemed
quite reasonable to me that since Waiden has observed products of
different chirality by using potassium hydroxide and silver hydroxide that
we would find comparable results in the inorganic field. Auten, who was
an undergraduate student at that time, had serious doubts about it and I
think was a little loath to try the experiment. Naturally we were
delighted when we found that there is an inversion in the reaction which
we studied."
X
CONTRIBUTORS TO THIS VOLUME
J.C.A. Boeyens and S.M. Dobson
Structural Chemistry Group
Department of Chemistry
University of the Witwatersrand
1 Jan Smuts Avenue
Johannesburg
South Africa
H.-J. Buschmann
Physikalische Chemie
Universität-GH Siegen
Postfach 101240
D-5900 Siegen
Federal Republic of Germany
K.E. Matthes and D. Parker
Department of Chemistry
University of Durham
South Road
Durham DH1 3LE
United Kingdom
1
CHAPTER 1
STEREOCHEMISTRY OF METALLIC MACROCYCLICS
JAN CA. BOEYENS and SUSAN M. DOBSON
Structural Chemistry Group, Department of Chemistry,
University of the Witwatersrand,
1 Jan Smuts Avenue, Johannesburg, South Africa.
2
STEREOCHEMISTRY OF METALLIC MACROCYCLICS
1. INTRODUCTION
The interest in metallic macrocyclics, as reflected in the
current literature, establishes this field of study as one of the
most prolific topics in coordination chemistry. It deals with the
encirclement of a metal ion by a cyclic ligand, which seems to
require a minimum ring size of about nine atoms. All compounds of
interest are organic heterocycles containing at least three
heteroatoms like N, P, 0 or S. The crown ethers belong to this
class, but distinguish themselves by their marked ability to
complex alkali metal ions. All the ligand atoms in crown
complexes are oxygen by definition, and because of the
physiological importance of the alkali ions these compounds are of
special biological significance. For this reason, crown ethers
tend to be more comprehensively and regularly reviewed than the
nitrogen, phosphorus, sulphur and mixed-donor macrocyclics. The
emphasis is reversed in this review.
The special relative disposition of cation and ligand in
macrocyclics generates properties distinct from those of regular
coordination compounds. The macrocyclic effect refers to their
special stability and is but one, even though the best documented
example. No discussion of the exceptional properties of macro
cyclics can therefore be complete without taking proper account of
stereochemical factors.
1.1 HISTORICAL SURVEY
The earliest known examples of metallic macrocyclics were found
in naturally occurring biologically active substances like
hemoglobin, chlorophyll and vitamin B , containing Fe, Mg and Co
12
respectively. The unadorned macrocyclic moieties, basic to these
substances have skeletons of the type shown in Figure 1. A
synthetic isomeric variety thereof has recently been described
(1).
In this type of structure a planar configuration is dictated by
the extensive delocalization and supported by the fused five-
membered rings. Without these features, the molecular conforma
tion of macromonocycles is a more flexible function of ring
strain, degree of unsaturation and the nature of the central metal
3
ion. Stereochemical variety will be limited only by the ingenuity
of the synthetic chemist.
Figure 1. Basic skeleton of naturally occurring macrocyclics.
The first synthetic metallic macrocycle was reported by Curtis
(2) and in the intervening two-and-a-half decades, a large variety
of macrocycles have been synthesized and their complexation
chemistry has been studied extensively. By analogy with the known
natural products most of the early synthetic materials were
tetradentate nitrogen donor complexes. For a macrocycle of this
type to encircle a first-row transition metal ion, it must consist
of 13 to 16 members and the spacing between donor atoms must be
such that 5, 6 or 7 membered chelate rings are formed (3) . For a
smaller ring or a larger metal ion the macrocycle has to fold, in
order to attain maximum coordination.
An obvious method to produce macromonocycles is by cyclization
of linear polydentate ligands. Comparison of the metal complexes
of the open-chain and cyclic analogues show that the cyclization
has a dramatic effect on complex properties. This is typically
manifested as
(i) an increased kinetic inertness towards complex formation
in aqueous medium and towards decomposition (4);
(ii) increased ligand-field strength (5);
(iii) an increase by orders of magnitude in thermodynamic
stability (6);
(iv) improved stabilization of high metal ion oxidation states
(7).
These factors collectively define the well-known macrocyclic
effect (6).
4
Various aspects of macrocyclic chemistry have been reviewed in
the literature. Gokel and co-workers (8) reviewed the synthesis
of aliphatic nitrogen-donor macrocyclics. Christensen and co-
workers (9) reviewed the synthesis of multidentate macrocyclics
and their ion-binding properties. Izatt and co-workers (10)
collated and reviewed the kinetic and thermodynamic data on
cation-macrocycle interactions, and Lindoy (11) discussed the
transition metal macrocyclics.
1.2 CHEMICAL CLASSIFICATION
The chemical identity of a macromonocyclic ligand depends on
the ring size and the number, nature and location of the donor
atoms and double bonds. The IUPAC rules (12) for naming organic
compounds result in the assignment of unequivocal, but extremely
complicated names to these compounds and a more popular procedure
is to introduce some abbreviated version. Most widely accepted is
the set of rules proposed by Melson (13). In its simplest form it
consists of a number specifying the ring size, a term denoting
degree of unsaturation and the symbols of ligating atoms in
alphabetical order. The numbering of ring atoms starts with the
heteroatom of highest priority according to Chemical Abstracts
rules, i.e. 0 > S > Se > N > P > As > Sb, etc., and proceeds in
the direction that yields the lowest locants for all heteroatoms
and sites of unsaturation. The rules are illustrated by the
following examples:
4
HN7 NH
x /
— N N
[9]ane-4,7-N -1-S 11,13,13-Me -[13]-10ene-l,4,7,10-N
2 3 4
or or
[9]ane-N S Me [13]ene-N
2 3 4
5
5,7,7,12,14,14,-Me-[14]-4,11-diene-l,4,8,11-Ν
6 4
or
Me [14]4,ll-dieneN
6 4
1.3 MOLECULAR CONFORMATION
The idea of molecular shape is an integral part of chemical
thinking (14). It features at all explanatory levels, shaping
important concepts like chemical reactivity, reaction mechanism,
phase transition and reaction pathway. This is often collectively
referred to as the structure-function relationship, although
molecular structure is rarely invoked explicitly. It enters the
argument in the guise of concepts like steric congestion, ligand-
field stabilization, barriers to conversion, overcrowding, packing
considerations, cone-angles, Jahn-Teller distortion, trans
effects, steric control, vibronic coupling, transition states,
reaction intermediates, and many other terms, making brief
appearances in the literature, only to explain some poorly
characterized observations. This diversity of concepts have one
feature in common. They cannot be reduced to fundamental
electronic factors and represent some manifestation of molecular
conformation. There is nothing amiss with this, provided the
fundamental importance of molecular shape is appreciated. This
requires a primary ability to identify conformational classes and
provide a logical basis for structure-function relationships.
