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Macedonian Journal of Chemistry and Chemical Engineering. Vol. , No. , pp.
, ISSN MJCCA UDC . . Received September , Accepted October , Review
SCHIFF BASES DERIVED FROM HYDROXYARYL ALDEHYDES MOLECULAR AND
CRYSTAL STRUCTURE, TAUTOMERISM, QUINOID EFFECT, COORDINATION
COMPOUNDS
Anita Blagus, Dominik Cini, Tomislav Frii, Branko Kaitner, Vladimir Stilinovi Department of
Chemistry, J. J. Strossmayer University, Franje Kuhaa , Osijek, Croatia Laboratory of
General and Inorganic Chemistry, Department of Chemistry, Faculty of Science, University of
Zagreb, Horvatovac a, Zagreb, Croatia Present address Department of Chemistry, University
of Cambridge, Lensfield Road, Cambridge CB EW, UK
kaitnerchem.pmf.hr
During the last years a vast number of structural studies on Schiff bases derived from
hydroxyaryl aldehydes and their coordination compounds have been undertaken. In this
review we present a systematic and brief overview of the most important discoveries and
achievements accomplished in this field. The occurrence of Schiff bases and their complexes
derived from nine most commonly used hydroxyaryl aldehydes in the Cambridge Structural
Database CSD has been investigated. Ketoenol tautomerism and intramolecular hydrogen
bonding in salicylaldimines and naphthaldimines is discussed, with a comment of the most
common errors in determining the correct tautomeric form. Also, the interrelationship of
crystal packing and the substituents on the Schiff base is studied. Finally, we give a short
overview of conformational differences between free Schiff bases and Schiff bases bonded
to metal ions as well as some structural characteristics of Schiff base metal complexes. Key
words Schiff bases Schiff base complexes tautomerism quinoid effect supramolecular
interaction
, J, , . . . . , . , .
Anita Blagus, Dominik Cini, Tomislav Frii, Branko Kaitner, Vladimir Stilinovi
INTRODUCTION Nsubstituted imines, also known as Schiff bases represent one of the most
widely used families of organic compounds and their chemistry is essential material in many
organic chemistry textbooks . In general, they are easily prepared by the condensation
reaction of primary amines with carbonyl compounds. The first reports of this kind of reaction
have been published by Hugo Schiff in the s . Thereafter Schiff bases have been intensively
used as synthetic intermediates and as ligands for coordinating transition and inner transition
metal ions, and recently also for coordinating anions . During the past two decades they have
become some of the most typical ligands in the field of coordination chemistry . Particularly, a
large number of transition metal complexes of Schiff base ligands derived from the
condensation of salicylaldehyde and hydroxynaphthaldehyde with various primary amines
became the topic of contemporary research , . These Schiff base ligands may act as
bidentate N,O, tridentate N,O,O, N,O,N, N,O,S, tetradentate N,N,O,O, hexadentate
N,N,O,O,S,Sdonor ligands , etc., which can be designed to yield mononuclear or binuclear
complexes or onedimensional D, twodimensional D and threedimensional D metalorganic
frameworks . The synthesis and structural research of Schiff bases derived from the above
mentioned aldehydes and amines bearing various alkyl and aryl Nsubstituents, as well as
their metal complexes have been of interest in our research group for over two decades .
Schiff base ligands may contain a variety of substituents with different electrondonating or
electronwithdrawing groups, and therefore may have interesting chemical properties. They
have attracted particular interest due to their biological activities , e.g. acting as
radiopharmaceuticals for cancer targeting , . They have also been used as model systems
for biological macromolecules , . Besides the biological activity, solidstate thermochromism
and photochromism are an another characteristic of these compounds leading to their
application in various areas of materials science such as the control and measurement of
radiation intensity, display systems and optical memory devices . Schiff bases derived from
ohydroxyaromatic aldehydes and ketones are excellent models for the study of ketoenol
tautomerism both in solution and in the solid state . During the past decade such Schiff
bases started drawing attention due to their physical properties in the crystalline state .
These properties are greatly influenced by the topochemistry of the Schiff base molecules
which in turn is highly affected by the crystal structure. Therefore the study of crystal packing
and the intermolecular interactions in the crystal structures of various Schiff bases can lead
to valuable data for the design and synthesis of new materials Figure .
SCHIFF BASES IN THE CAMBRIDGE STRUCTURAL DATABASE CSD
Searches to determine the number of CSD entries for Schiff bases derived from hydroxyaryl
aldehydes as well as for their complexes
Fig. . Varying the type of amine aliphatic or aromatic and carbonyl aldehyde or ketone
subunits, as well as substituents on them, a great number of Schiff bases with different
supramolecular bonding capabilities can be prepared, which in turn enables great diversity of
crystal packings Maced. J. Chem. Chem. Eng. ,
Also. Chem. J. Chem. ovanillin ovan. chlorosalicylaldehyde clsal. chlorosalicylaldehyde
clsal.Schiff bases derived from hydroxyaryl aldehydes Scheme . nitrosalicylaldehyde nosal.
ethoxysalicylaldehyde etsal. For Schiff base complexes the search criteria used were based
on molecular diagrams involving any transition metal bonded to deprotonated hydroxyl
oxygen and imine nitrogen of the respective deprotonated Schiff base. Repeat
determinations of the same structure. V. The analyses of entries for Schiff bases and their
complexes are summarized in Table and Table . three filtering criteria were imposed upon
the search entries must have their D coordinates determined. Mn were performed. exhibit no
errors and should have no transition metals present. February and May . Eng. ovanillin ovan.
Zn. . salicylaldehyde sal. salicylaldehyde sal. ethoxysalicylaldehyde etsal. . polymorphs and
salts were removed from the count manually. hydroxynaphthaldehyde napht. searches to
determine the number of entries for a particular metal Cu. Also. Co. For free ligand the
search criteria used were drawings of Table Number of unique structures in CSD for the
selection of Schiff bases derived from nine different hydroxy aryl aldehydes Aldehyde
hydroxynaphthaldehyde hydroxynaphthaldehyde salicylaldehyde ovanillin vanillin
ethoxysalicylaldehyde nitrosalicylaldehyde chlorosalicylaldehyde bromosalicylaldehyde
Number of free Schiff bases the respective motifs of Schiff bases derived from nine different
aldehydes hydroxynaphthaldehyde napht. The data analysis was performed using the
program Vista version . Ni. nitrosalicylaldehyde nosal. bromosalicylaldehyde brsal were
performed using ConQuest version . Table Number of CSD hits for hydroxyl aryl Schiff base
complexes with transition metals there are no entries corresponding to complexes of Schiff
bases derived from vanillin and hydroxynaphthaldehyde Aldehyde hydroxynaphthaldehyde
salicylaldehyde ovanillin ethoxysalicylaldehyde nitrosalicylaldehyde chlorosalicylaldehyde
bromosalicylaldehyde Number of complexes Cu Ni Co V Fe Zn Mn Maced. Fe.
bromosalicylaldehyde brsal Scheme . vanillin van. respectively. with three updates
November . Molecular diagrams of hydroxynaphthaldehyde napht. vanillin van.
hydroxynaphthaldehyde napht. .
. Of these. Of these. Fig. As many as entries for salicylaldehydebased Schiff bases were
found in the CSD . Eng. J. There are substantially less structural data on the Schiff bases
derived from hydroxynaphthaldehyde with only entries found for compounds of this type. . b
coordination compounds with salicylaldimine Schiff bases. c Schiff bases derived
hydroxynaphthaldehyde and d coordination compounds with naphthaldimine Schiff bases
Maced. Chem. Branko Kaitner. have been studied more intensely than any other Schiff base
derivative Figure . as a percentage of the total number of entries obtained in this overview.
The distribution of the particular aldehyde type across the nine searched groups of Schiff
bases and complexes in the CSD is shown graphically in Figure . Chem. have been derived
by condensation with aliphatic amines and have been derived by using aromatic amines as
condensation partners. Number of CSD entries per year for a Schiff bases derived from
salicylaldehyde. . The occurrence of Schiff bases derived from different aldehydes and their
complexes in the CSD year year year year Fig. Dominik Cini. Anita Blagus. It is remarkable
that Schiff bases derived from salicylaldehyde make up of the entire sample of studied Schiff
bases and their complexes make up of the entire sample of studied Schiff base complexes.
Tomislav Frii. . Vladimir Stilinovi The search of the CSD revealed that during the last years
the Schiff bases of the salicylaldimine type. have been derived by condensation with aliphatic
amines and have been derived from aromatic amines. as well as their transition and inner
transition metal complexes.
Schiff bases derived from hydroxyaryl aldehydes MOLECULAR STRUCTURE. . The
presence of a particular tautomer in the crystal depends mostly on the parent ohydroxyaryl
aldehyde and the type of the Nsubstituent i. The electron withdrawing or donating ability of
the Nsubstituents. In the literature one can notice two main types of reported structures. The
stabilization of the ketoamino tautomer in naphthaldimines is usually connected with the
quinoid effect since the structure of this tautomer is similar to that of onaphthoquinone
Scheme . . The first group for example . this is not supported by currently available structural
data which indicates the existence of nonplanar ketoamino salicylaldimines as well as
perfectly planar enolimino naphthaldimines with Naryl substituents Figure . . Unlike
salicylaldimines. Figure a. Examples of planar and nonplanar naphthaldimine Schiff bases
with the Naryl substituent a N. However. the authors must also take molecular geometry i. .
particular care must be taken when determining the molecular structures of such compounds
in the crystalline state. There does not seem to be any common relationship between
molecular conformation and the sort of tautomer present in the crystalline state. aryl or alkyl.
The presence of a fused aromatic ring stabilizes the quinoid ring and renders the geometry of
the naphthalene moiety virtually independent on the position of the hydrogen atom along the
OHN bond. This is clearly represented by the distribution of CarO bond lengths Figure a.e.
Because of the ability of naphthaldimines to exist in both tautomeric forms. that. Figure into
account. very often this does not seem to be the case. TAUTOMERISM AND QUINOID
EFFECT Aldimine compounds derived from the aromatic aldehydes having a hydroxyl group
in the ortho position to the aldehyde group are of interest mainly due to the existence of
either O HN or OHN type of intramolecular hydrogen bond and the related tautomerism
between such enolimine and ketoamine forms. INTRAMOLECULAR HYDROGEN
BONDING. in which misinterpretation of structural data is likely. . in naphthaldimines the
ketoamino tautomer appears somewhat prevalent over the enolimino tautomer Figure b.
which makes it evident that out of entries belong to the enolimino and only to the ketoamino
tautomer.dichlorophenylhydroxynaphthalideneamine d
NpyridinylaminomethyleneHnaphthalenone Maced. It has been noted . Unfortunately. Thus.
as well as hydrogen bond donoracceptor properties. The existence of the enolimino tautomer
has been established in most crystal structures of Nsubstituted salycilaldimines listed in the
CSD. b NmethylquinonylaminomethyleneHnaphthalenone c N. J. the process of proton
transfer from hydroxyl group to the imino nitrogen atom does not alter the approximate Dh
symmetry of the naphthalene moiety .e.diisopropylphenylhydroxynaphthalideneamine . their
position and stereo chemistry. Chem. consists of reports where the hydrogen atom is simply
generated on the oxygen atom Fig. Chem. bond lengths. . . together with locating the
hydrogen atom from an electron density difference map. Eng. During previous decades it
was generally believed that the ketoamino naphthaldimines and salicylaldimines with the
Naryl substituent are planar and the enolimino tautomers are nonplanar. can stabilize one or
another tautomer in the crystal. .
and d the distribution of CarO bond lengths of Schiff bases derived from
hydroxynaphthaldehyde deposited in the CSD as enolimino tautomers bond lengths shown
left from dotted line clearly belong to CO double bonds indicating that they belong to
ketoamino rather than enolimino tautomers probably because a hydroxyaldehyde was a
starting compound instead of being located from the electron difference map. . Because of
this. . Chem. Ketoamino tautomerisation in naphthaldimines Fig. but the bond lengths were
not taken into account when assigning the bond type single or double and thus in some
cases of ketoamine tautomers instead of the quinoid form the benzenoid form was used. lead
to a structural model with a misplaced hydrogen atom and thus to a misinterpretation of the
structure as an enolimino tautomer. This can. and the struc Scheme. c scattergram of CN
and CarO bond lengths of Schiff bases derived from hydroxynaphthaldehyde. J. In the
second group for example . Tomislav Frii. Anita Blagus. . All bond lengths are given in and
angles in Maced. . the hydro gen atom was located from the difference map on the nitrogen
atom. b for ketoamino tautomer. . a The distribution of CarO bond lengths of Schiff bases
derived from salicylaldehyde. Figure b. c for a Schiff base coordinated to a metal ion.
Dominik Cini. . Eng. Chem. . b the distribution of CarO bond lengths of Schiff bases derived
from hydroxynaphthaldehyde. the nitrogen atom seems to be positively and the oxygen atom
negatively charged. Vladimir Stilinovi Fig. Branko Kaitner. Characteristic bond lengths red
and angles blue for free Schiff bases derived from ohydroxyaryl aldehydes and their
complexes represented as average values for corresponding entries in the CSD a for
enolimino tautomer. if the molecule is actually a ketoamine tautomer.
. The presence of strong hydrogen bond donors and acceptors on the molecular periphery
leads almost without exception to crosslinking of molecules via strong hydrogen bonds into
dimers. Unless additional hydrogen bond donors and acceptors are present on the subunits
themselves. On the other hand if only additional hydrogen acceptors usually O and N atoms.
Although all the Schiff Maced. NpropylaminomethyleneHnaphthalenone Figure . J. Chem. In
both cases the CO bond lengths clearly indicate that CO is in fact a double bond. which are a
wellknown class of predominantly thermochromic compounds. namely thermochromism and
photochromism . while the CN bond lengths show that CN bonds are single tures are then
reported as zwitterions instead of as ketoamine tautomers with no charge separation. while
planar molecules of this type Schiff bases exhibit thermochromism . Examples of two most
common types of errors ensuing from the failure to detect the quinoid effect in
naphthaldimines a enolimino and b zwitterion . then CHO and CHN weak hydrogen bonds
are usually the most important interactions between molecules. oligomers. it was established
that these two effects are conformationdependent. . Eng. the packing of the Schiff base will
often be governed by weak interactions dispersion forces. . . . . the central ketoamino moiety
can interact with neighbouring molecules only if one of the subunits is not sterically
demanding as is the case in some Schiff bases with aliphatic Nsubstituent .
SUPRAMOLECULAR INTERACTIONS AND CRYSTAL PACKING Crystal packing in Schiff
bases is determined by interactions between molecules which in turn are determined by
functional groups available for forming interactions with neighbouring molecules Figure . is
an example of a Schiff base where the substituent on the imino nitrogen atom is the sterically
nondemanding propyl group. which is a much more likely case. The transfer of proton from
hydroxyl oxygen atom of the parent aldehyde to the imino nitrogen atom in crystal state
causes remarkable changes in the distribution of electrons and such changes are closely
connected with physical properties in the crystalline Schiff bases. The only noteworthy
interaction . bases with aromatic Nsubstituent discussed in this review have the ability of
forming intramolecular hydrogen bond. On the basis of structural studies of salicylaldimines.
Nonplanar molecules can exhibit photochromism. chains and other supramolecular
assemblies. CH hydrogen bonding. . alone. stacking etc. Chem.Schiff bases derived from
hydroxyaryl aldehydes Fig.
Nacetylphenyl salicylideneamine Fig. Tomislav Frii. the keto oxygen atom is the hydrogen
acceptor in a CarHO interaction of . in the crystal structure of N. In the crystal structure of N.
a Molecular diagram. . between the imino methine and the oxygen atom of the central
ketoamino group. . Nhydroxyphenylsalicylideneamine . . and c view of the packing of
molecules for N. Chem. leading to the formation of chains Figure
.NbutylenebisaminomethyleneHnaphthalenone the central ketoimino groups of neighbouring
molecules are sufficiently close to one another that an intermolecular NHO hy Fig. The
described interaction can be observed by close analysis of the twodimensional fingerprint
plot derived from the Hirshfeld surface of the molecule. Vladimir Stilinovi
Fig.NhexylenebisaminomethyleneHnaphthalenone . a bis Schiff base derived from
naphthaldehyde and an aliphatic diamine. Chem. Anita Blagus. .
Nnitroxyphenylsalicylideneamine . . b twodimensional fingerprint plot. a Molecular diagram.
Branko Kaitner. J. Similarly. a Molecular diagram of a Schiff base derived from
salicylaldehyde. Dominik Cini. Ncyanophenylsalicylideneamine . and c fragment of the chain
in the crystal of NpropylaminomethyleneHnaphthalenone between molecules in the crystal
structure is a weak CHO interaction of . b fragment of the chain in the crystal.
Nmethylphenylsalicylideneamine . Eng. b twodimensional fingerprint plots and fragments of
packing for six Schiff bases with different substituent on Nsubstituted benzene ring
Nphenylsalicylideneamine .NhexylenebisaminomethyleneHnaphthalenone Maced.
b twodimensional fingerprint plot marked corresponding region for OHN hydrogen bond.
those derived from m. . connect the molecules into chains Figure . b twodimensional
fingerprint plot marked corresponding region for NHO hydrogen bond. Each amino hydrogen
atom participates in an intramolecular . it can form a hydrogen bond with the ketoamino
oxygen atom. . If an additional hydrogen donor is present. Because . Unlike Schiff bases
derived from ohydroxyaromatic aldehydes. a Molecular diagram. providing the Nsubstituent
is sufficiently small. a Molecular diagram. differ only in the spatial orientation of Nnaphthyl
group with regard to the oxynaphthaldeimine . . Both molecules show a high degree of
planarity with essentially identical interplanar twist angles between the Nsubstituents and the
rest of the molecule. a free hydrogen bond donor hydroxyl group and an acceptor imino
group are available for intermolecular hydrogen bonding. a Molecular diagram. Chem.Schiff
bases derived from hydroxyaryl aldehydes Fig. Instead. and c a fragment of the
hydrogenbonded chain in the crystal of Nmethylphenylhydroxymethoxybenzilideneamine
renders the participation of hydrogen donors and acceptors of the central ketoamino group in
intermolecular bonding less probable. namely NnaphthylaminomethyleneHnaphthalenone
and NnaphthylaminomethyleneHnaphthalenone. Fig. This bonding results in the formation of
chains in the crystal structure Figure and is reflected in a considerably higher melting point. .
The dimers in the crystal structure are further linked via weak CarHO contacts generating a
Dnetwork Figure . Eng. the packing is governed by weak interactions. Chem. This occurs in
the crystal structure of NhydroxyethylaminomethyleneHnaphthalenone where the
Nsubstituent is a primary alcohol which acts as a hydrogen donor forming a OHO
intermolecular hydrogen bond of . b fragment of the chain in the crystal. This can be
observed in the structure of Nmethylphenylhydroxymethoxybenzilideneamine where
intermolecular OHN hydrogen bonds of . with the ketoamino oxygen atom forming dimers.
Two Schiff bases. In such cases. and c view of the packing of molecules for
NhydroxyethylaminomethyleneHnaphthalenone drogen bond is formed. and a somewhat
longer intermolecular hydrogen bond . and c a fragment of the hydrogenbonded chain in the
crystal of N. Increasing the size of the Nsubstituent Maced. . and if additional hydrogen
donors and acceptors are absent.NbutylenebisaminomethyleneHnaphthalenone Fig.and
phydroxyaromatic aldehydes will not be able to form an intramolecular hydrogen bond as
described above. J.
due to different directionality Fig. Crystal structures of Schiff bases derived from aldehydes
and amines which have additional groups containing nonbonding electron pairs alkoxy. The
crystal structure comprises of CarHO hydrogenbonded dimers Figure b. In aminobenzonitrile
derivative. . Anita Blagus. nitro. The adjacent chains are further linked via weak CHp
contacts contacts and generate a herringbone motif Figure . pyridine etc. a combination of
CarHN and CarHO interactions results in the formation of molecular chains Figure b. cyano. .
all interactions between neighbouring molecules in both compounds are on the basis of the
weak intermolecular contacts. Chem. J. acyl. where methoxy oxygen and pyridine nitrogen
atoms participate as acceptors of intermolecular hydrogen bonds. which are linked via
another set of weak CHO interactions into a Dnetwork. . . . In the case of the
aminobenzonitrile derivative. The discrete molecules are stacked to give a pstacked chains.
Vladimir Stilinovi Fig. b fragments of packing in the crystal of
Npyridylmethoxysalicylideneamine Maced. which is comparable with CHN intermolecular
bonding in other Schiff bases derived from pyridylamines . which may act as hydrogen bond
acceptors are often directed by weak hydrogen bonds formed between such groups and
aromatic hydrogen donors. Branko Kaitner. b NnaphthylaminomethyleneHnaphthalenone of
the lack of hydrogenbonding functionalities on the Nsubstituents. The layers are further
interconnected by CHN interactions between imino methine group and pyridine nitrogen with
CN distances of . Dominik Cini. a Molecular diagram. Chem. Tomislav Frii.and
aminobenzonitrile demonstrating the effect of the position of the cyano group on the crystal
packing . Molecular diagram. . Eng. CHN intermolecular bonding frequently occurs in crystal
structures of Schiff bases derived from aminonitriles. An example of such intermolecular
bonding can be seen in the crystal structure of Npyridylmethoxysalicylideneamine Figure a.
twodimensional fingerprint plot marked corresponding regions red arrow for p contacts and
red circle for CHp contacts and view of the packing and pp stacking for compounds a
NnaphthylaminomethyleneHnaphthalenone. Figure depicts hydrogen bonding motifs in Schiff
bases derived from naphthaldehyde and .
Schiff bases derived from hydroxyaryl aldehydes Fig. where CarHN and CarHO interactions
leads to a Dnetwork Figure c. the hydrogen bond involving the same donor/acceptor set
results in a different supramolecular topology. Molecular diagram. and b
NcarboxyphenylaminomethyleneHnaphthalenone Maced. J. Chem. . . twodimensional
fingerprint plot marked corresponding regions red arrow for contacts and red circle for OHO
hydrogen bond and view of the packing and stacking for compounds a
NcarboxyphenylaminomethyleneHnaphthalenone . In the crystal structures of Schiff bases
with carboxyl substituents the formation of strong hydrogen bonds is expected. A different
type of supramolecular topology is observed in the crystal structure of the aminobenzonitrile
derivative. Fig. with the carboxyl group acting as the hydrogen donor and either carboxyl
group or the central ketoamino group acts as hydrogen acceptors. . Both cases are
illustrated by Schiff bases derived from hydroxynaphthaldehyde and different aminobenzoic
acids Figure . . c NcyanophenylaminomethyleneHnaphthalenone of the cyano group. Chem.
while in the structure of NcarboxyphenylaminomethyleneHnaphthalenone Figure b the
hydrogen bond acceptor is the carboxyl group which results in the formation of characteristic
carboxyl dimers. The molecules are connected into chains via CarHN interactions and the
chains are further connected by CarHO interactions into layers Figure a. In the crystal
structure of NcarboxyphenylaminomethyleneHnaphthalenone Figure a the hydrogen bond
acceptor is the ketoamino oxygen atom and hydrogen bonding leads to chains. Eng. b
NcyanophenylaminomethyleneHnaphthalenone . Molecular diagram and fragment of the
chain in the crystal for compounds a Ncyanophenylhydroxynaphthalideneamine .
the average bite distance rising from . in VIII complexes. . the so called bite distance of the
chelate ligand. Such behaviour which is to be expected since the metal ion replaces a much
smaller hydrogen. The distance increases upon the binding of the metal ion. Fig. Anita
Blagus. Consequently. can have a significant influence on the geometry of the molecule .
and b their complexes c The distribution of NO bite distance of Schiff bases derived from
hydroxyMaced. The average bite distance in complexes of CuII is . . Representation of
conformational differences between free Schiff bases and Schiff bases bonded to metal ions
The distribution of ON bite distances for free Schiff bases and their complexes is shown in
Figure . . . Dominik Cini. J. Tomislav Frii. ii changes in bond distances and angles in what
becomes a chelate ring Figure c which is associated with the redistribution of electrons
pseudoaromaticity. in FeII complexes. a The distribution of NO bite distance of Schiff bases
derived from salicylaldehide. Eng. in free Schiff bases to . In the complex chelate ring of
bisNmethylphenylhydroxynaphthalidenato Fig. in complexes. . The main structural feature
distinguishing the free Schiff base from the coordinated ligand appears in i a large difference
in the ON separation. . . and iii a significant change in the spatial orientation of the
Nsubstituent Figure. . and in hexacoordinated . in ZnII complexes and . in MnII complexes. .
Chem. The bite distance is also noticeably affected by the coordination of the metal ion the
average bite distance in tetracoordinated NiII complexes is . Vladimir Stilinovi METAL
COMPLEXES OF SCHIFF BASES Replacing the hydrogen atom from the intramolecular
hydrogen bond in the salicylaldimine and naphthaldimine Schiff bases by a metal ion. Branko
Kaitner. the bite distance of a Schiff base in a complex is very dependent on the metal atom.
Chem. . in CoII complexes .
diisopropylphenyl. and f hydroxynaphthaldehyde Maced. . a
Nmethylphenylhydroxynaphthalidenamine . The fragments are fitted by best overlap of
naphthalene atoms naphthaldehyde. .naphthalidenamine . and crystal packing . there has
been continuous interest in the coordination compounds of CuII and NiII. Chem. CarO bond
lenghts of Schiff base complexes derived from salicylaldehyde. b Nnaphthyloxy. Similar
stereochemical behaviour was observed in the analogous nickelII complexes with N. The
twisting of the otolyl moiety is affected by the molecular environment and packing in the
crystal. As noted earlier. . Chem. and Nnaphthylhydroxynaphthalidenamine . In most
complexes both electronic effect . d and their complexes e The plot of CN vs. and . Fig. as a
consequence of sterical hindrance due to the approaching of two ligands to each other in the
complex and the increase of bond angles in the chelate ring. for free and coordinated
molecule. due to the twist of the otolyl ring connected on the amino nitrogen out of the
naphthalene moiety plane with the corresponding dihedral angles ca .and naphthaldehydes.
and lengthening of the NC bond distances in comparison to the corresponding bonds in the
free Schiff base ligand. The naphthaldimine moiety in the complex exhibits the quinoidal
bond lengths assembly.Schiff bases derived from hydroxyaryl aldehydes nickelII there is a
general tendency of shortening the OC and CC. This is mostly due to the variability of the
geometric features of the coordination sphere around the metal which can range from square
planar to deformed tetrahedral geometry . The ligand itself is not planar. Eng. with Schiff
bases derived from salicyl. J. and to a somewhat lesser degree of CoII. Overlap of free Schiff
base red and a Schiff base coordinated to nickelII green. respectively Figure . have been
invoked as the driving forces responsible for the distortion.
crystal packing forces.g. c dNN / dNO correlation for cobaltII complexes data points. dNN /
dNO plots for mononuclear bisbidentate Schiff base complexes of tetracoordinated nickelII.
In the case of nickel complexes there is an extreme tendency towards transplanar
coordination with only several examples of tetrahedral complexes Figure b. Anita Blagus. J. d
dNN / dNO correlation for copperII complexes data points. Chem. for tetrahedral dNN / dNO
and for cisplanar dNN / dNO / . A simple measure of the coordination polyhedron geometry
for bisbidentate Schiff base coordination compounds is the dNN / dNO ratio i. For detailed
discussion see text. The distribution of coordination geometries in mononuclear bisbidentate
Schiff base coordination compounds in the CSD. a A diagram of the molecular fragment
used for the CSD search with the definition of parameters dNN and dNO b dNN / dNO
correlation for nickelII complexes data points. Eng. Chem. e.e. a significant role in the
coordination geometry of the ligands around the metal atom is played by the nature and
volume of the Nsubstituent and intermolecular interactions. Branko Kaitner. Dominik Cini. For
transplanar expected value of this ratio is dNN / dNO dNN / dNO for a perfect square and
dNN / dNO gt in most real complexes. i. . The transplanar nickelII complexes are usually
negligibly tet Fig. . the ratio of distances from one donor atom e. N of one ligand to the
equivalent N and to the nonequivalent O donor atom of the other ligand Figure a. . Vladimir
Stilinovi However. Maced. Tomislav Frii. cobaltII and copperII are given in Figure and show
distribution of data points into regions corresponding to above mentioned coordination
geometries.
Although this usually does not affect the properties of the bulk material since the majority of
reported structures crystallised as racemates. An example of such behaviour is bisN. while
only four structures can be described as tetrahedraly coordinated Figure . . there are cases
where the chirality of the coordination polyhedron dictates the chirality of crystal packing. It is
interesting to note that all the coordination polyhedra achievable in bisbidentate Schiff base
complexes. are chiral. The coordination to copperII is usually less rigid than that to cobalt
and nickel. where chiral tetrahedraly disordered planar coordination of the copper ions leads
to a helicity of binuclear coordination species and in turn to crystallization of enantiomeric
compounds in enantiomeric space groups P Fig. The majority of compounds exhibit a
transplanar coordination which can be somewhat distorted towards tetrahedral. Cisplanar
coordination is entirely absent as is among the nickel compounds. the data points are still
grouped into three well distin guishable regions corresponding to transplanar. Formation of
dimers can also be noticed in nickelII and cobaltII complexes but there it occurs much less
frequently. . Chem. as will be discussed below. The second most abundant group
corresponds to the cisplanar coordinated species. In spite of the smearing. J. Another feature
of copperII complexes is their tendency to form dimers . This corresponds to the rotation of
one chelate ring plane in respect to the other and indicates that such rotation is energetically
less demanding than in corresponding copper and nickel compounds. c cisplanar Fig. This is
demonstrated in the dNN / dNO plot by the smearing of the data points along the line
connecting top left and bottom right corners of the
plot.NhexamethylenedisalicylaldiminatodicopperII. b tetrahedral . Chem. and tetramers . .
Cisplanar NiII complexes occur only in polynuclear species where the Schiff base ligands
bridge between two or more metal ions as well as in complexes with tetradentate Schiff
bases usually derived from diamines such as ophenylenediamine . tetrahedral and cisplanar.
other than the perfectly planar ones. b Helical stacking of molecules along the c axis
exhibiting a symmetry Maced. Examples of limiting coordination geometries of bisbidentate
Schiff basecopperII complexes a transplanar . a Molecular diagram of bisN.Schiff bases
derived from hydroxyaryl aldehydes rahedraly deformed. but can show an increase of both
dNN and dNO compared to a perfect square due to stepwise bending of complex molecules.
Although it is commonly accepted that the stereochemistry of cobaltII is similar to that of
nickelII. Eng. there is a marked increase of the occurrence of tetrahedral bisbidentate Schiff
basecobaltII complexes in comparison to those of nickelII with tetrahedral coordination
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it is possible to control the chirality of the coordination sphere and thus the entire complex
molecule. . . Many such complexes have been found to be potent chiral catalysts in organic
synthesis . Tomislav Frii. Anita Blagus. Crystal packing of
bisethoxysalicylideneiminatonickelII a molecular diagram. . Branko Kaitner. c packing of
chains Maced. Vladimir Stilinovi and P Figure . for which a small step distance is expected
since there is Fig. so that in bisNnaphthylhydroxynaphthaldiminatonickelII the step distance
is S . steric and packing effects seem to have an important contribution to the formation of
stepped conformation. Molecular structures of bisNmethylchlorosalicylideneiminatonickelII
and bisNethylchlorosalicylideneiminatonickelII viewed along the edge of the coordination
plane. A schematic representation of a complex molecule in a stepped conformation
indicating the step distance S tors . . while the ethyl derivative adopts a stepped
conformation. . J. and electronic effects . . Chem. The methyl compound is almost planar
step distance S . Such molecules are comprised of two planar regions which lie on parallel
planes separated by a distance S referred to as step distance Figure . due to the close
contact between the methylene hydrogen atoms of one ligand and the coordinated oxygen
atom of the other Figure . Eng. This is seen in the crystal structure of
bisethoxysalicylideneiminatonickelII Figure . The step distance S is defined as the
perpendicular separation between the planes through the chelate ring and/or through the
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bisNmethylchlorosalicylideneiminato nickelII and bisNethylchlorosalicylideneiminatonickelII.
Complexes with bulkier N substituents can exhibit even larger values of step distance. The
conformation of the coordination polyhedron is also largely influenced by the crystal packing.
Fig. sometimes even to a greater extent then the Nsubstituent. . and even larger in
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base ligand itself. The methyl derivative is almost perfectly planar. A feature of the molecular
geometry of many salicylaldimine and naphthaldimine bisbidentate complexes with divalent
copper and nickel is their stepped conformation arising from a distinct bend in the otherwise
planar molecules along the line joining the two donors within each separate ligand. The
occurrence of stepped conformation has been mainly associated with steric fac Fig. while
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distance S . Chem. However. . . Large N substituents favour larger step distances.
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Acknowledgement We are indebted to the Ministry of Science. X. Am. . The McGrawHill
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Economiche. . Education and Sport of the Republic of Croatia for the longlasting financial
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