Base metal Schiff base complexes applied as catalysts for the oxidation of n-octane

Article abstract of DOI:10.1016/j.ica.2017.04.032

The catalytic oxidation of n-octane to value-added oxygenates has been studied with a series of salen-type complexes of iron. The four iron Schiff base complexes N,N′-bis(salicylidene)-L-iron(III) chloride [L?=?1,2-phenylenediamine (5); ethylenediamine (6); propylenediamine (7) and butylenediamine (8)] were synthesised by variation of the bridging diamine backbone. All the complexes are fully characterised and the crystal structure of 7 is for the first time reported as a centrosymmetric dimer observed to crystallise in the triclinic P1￣ system. The application of 5–8 in this oxidation reaction represents a rare example of the systematic use of iron-salen complexes for the liquid-phase functionalisation of C_sp³–H bonds of straight chain alkanes. All catalysts displayed activity with H₂O₂ and TBHP as oxidants that resulted in a mixture of oxygenated products dominated by the ketones (2-, 3- and 4-octanones). Observed trends and variations in catalytic activities of the complexes is related to differences in chemical/structural features and changes in reaction conditions. A detailed analysis of selectivity parameters of the complexes to the variety of oxygenates is also presented and rationalised.

Full text of DOI:10.1016/j.ica.2017.04.032

Accepted Manuscript
Research paper
Base metal Schiff base complexes applied as catalysts for the oxidation of n-
octane
Ebrahim Kadwa, Holger B. Friedrich, Muhammad D. Bala
PII:
S0020-1693(16)30913-6
http://dx.doi.org/10.1016/j.ica.2017.04.032
ICA 17537
DOI:
Reference:
Inorganica Chimica Acta
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18 November 2016
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18 April 2017
Please cite this article as: E. Kadwa, H.B. Friedrich, M.D. Bala, Base metal Schiff base complexes applied as catalysts
for the oxidation of n-octane, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.04.032
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Base metal Schiff base complexes applied as catalysts for the oxidation
of n-octane
Ebrahim Kadwa, Holger B. Friedrich and Muhammad D. Bala
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001,
Durban 4000, South Africa
Abstract
The catalytic oxidation of n-octane to value-added oxygenates has been studied with a series
of salen-type complexes of iron. The four iron Schiff base complexes N,N'-bis(salicylidene)-
L-iron(III) chloride [L = 1,2-phenylenediamine (5); ethylenediamine (6); propylenediamine
(7) and butylenediamine (8)] were synthesised by variation of the bridging diamine
backbone. All the complexes are fully characterised and the crystal structure of 7 is for the

first time reported as a centrosymmetric dimer observed to crystallise in the triclinic P
1
system. The application of 5-8 in this oxidation reaction represents a rare example of the
3
systematic use of iron-salen complexes for the liquid-phase functionalisation of Csp –H bonds
of straight chain alkanes. All catalysts displayed activity with H2O2 and TBHP as oxidants
that resulted in a mixture of oxygenated products dominated by the ketones (2-, 3- and 4-
octanones). Observed trends and variations in catalytic activities of the complexes is related
to differences in chemical/structural features and changes in reaction conditions. A detailed
analysis of selectivity parameters of the complexes to the variety of oxygenates is also
presented and rationalised.
Keywords: Paraffin functionalisation; Schiff base; Iron complexes; Regioselectivity
parameter
*Correspondence to: e-mail- bala@ukzn.ac.za; fax- +27 31 260 3091

1. Introduction
The functionalisation of linear (n-) alkanes under mild conditions is notoriously difficult and
has been the subject of much research effort [1-3]. The holy-grail of linear alkane activation
and oxidation [4] is the cytochrome P450 enzyme which contains a heme group, a complex
containing iron tetra-coordinated in the plane of a porphyrin ring via four N-donors at the
active site [5]. Salen-type ligands are often used as cytochrome P450 active site mimics as
they share certain similarities with porphyrins: N-donor groups, a formal charge of (2-), tetra-
coordination to the metal with the metal coordinated in a plane formed by the N- and O-
donor groups. Additionally, some salen-type complexes have been shown to reversibly bind
dioxygen (O2). Hence, the catalytic oxidation of many organic substrates has been
accomplished with salen-type complexes of Mn, Co, V, Cr, Ti and other metals [6-12]. The
general popularity of salen-type Schiff base ligands as “heme mimics” in oxidation catalysis
is due to their ease of synthesis, availability of starting materials at a relatively low cost, the
large range of possible variants with different substituents and the possibility of their
coordination to, and stabilisation of, most metal centres [13].
Although iron complexes of salen-type ligands are well known [14-19], there exists a general
paucity of reports on their use as alkane oxidation catalysts and to the best of our knowledge,
the nearest catalytic application to this report is the use of an FeSalenCl complex for the
oxidation of amino acids [20] cyclohexane [21], various alkenes and chalcones [7], certain
organic dyes [22, 23], sulphides and sulphoxides [9] and phenol derivatives [24]. And for the
purpose of close mimicry to heme in the design of the supporting architecture around the
ONNO donor atoms of the salen ligand, research has proven that variation in the
rigidity/flexibility and functionality of the backbone atoms has more general influence on the
behaviour of a coordinated metal than variation of the substituents on the bridging phenyl
rings [25]. Hence, in this study we have varied the backbone architecture of the four salen-
type ligands and their corresponding complexes and used iron as the basis of the catalyst
design to further benefit from advantages that include reversible redox behaviour, low cost,
earth abundance, ready availability and low toxicity [26]. Therefore, as a continuation of our
studies on paraffin activation [3, 27] we report the first application of cheap iron centred
Schiff base complexes as biomimetic catalysts for the oxidation of n-octane to value added
oxygenated products, including terminal products such as 1-octanal.

2. Results and discussion
2.1. Synthesis of complexes
Scheme 1 represents a general synthetic route to the four iron Schiff base complexes. Full
characterisation details for all the ligands (1-4) have previously been published [27]. All the
metal complexes (5-8) are paramagnetic, however, they were unambiguously characterised
by their sharp melting points, exact CHN elemental analysis, IR, HRMS and for 7 single
crystal XRD.
(R)
N
O
N
O
Fe
Cl
(R)
R = o-C6H4 (5)
(CH2)2 (6)
N
N
(CH2)3 (7)
FeCl3
OH HO
+
(R)
R = o-C6H4 (1)
(CH2)2 (2)
N
N
(CH2)3 (3)
(CH2)4 (4)
-
OH
FeCl4
O
Fe
N
O
HO
N
(R)
R = (CH2)4 (8)
Scheme 1. Route to the synthesis of iron Schiff base complexes 5-8.
2.2. IR Spectroscopy
Selected peaks from the IR spectra of ligands and complexes are presented in Table 1. Also
shown are the peak frequency shifts upon complexation.
Table 1. Comparison of infrared frequencies of selected functional groups of compounds 1-
8.
Ligand vs
imine C=N
/ cm-1
phenolic C-O
/ cm-1
phenyl C=C
/ cm-1
phenyl C-H
/ cm-1
761
complex
1613
1277
1481
1 salophen
5 FesalophenCl
1604
1315
1461
760

Shift*
-9
+38
-20
-1
1636
1618
-18
1284
1303
+19
1498
1445
-53
742
758
+6
2 salen
6 FesalenCl
Shift*
1637
1611
-26
1282
1301
+19
1499
1472
-27
753
757
+4
3 salpn
7 FesalpnCl
Shift*
1633
1661
+28
1285
1285
0
1498
1476
-22
753
760
+7
4 salbn
8 Fe(salbn)2FeCl4
Shift*
* Shift = (freq. complex - freq. ligand)
One of the most important observations from the data in Table 1 is the weakening (shift to
lower wavenumbers) of the imine C=N bond upon complexation to the metal, implying there
is loss of some double bond character as the pi electrons are delocalised in the metallocycle.
This weakening is proportional to the length of the ligand backbone (phenylene weakens the
least, followed by ethylene and then propylene) except for the butylene backbone. In the case
of the butylene backbone, IR data suggest that complexation strengthens the C=N bond. A
possible explanation for this may be ascribed to the increased degrees of rotational freedom
due to the multiple conformation possibilities for the very flexible four carbon chain of the
backbone leading to the imine-Fe-imine bonds being more linear. This high flexibility of the
backbone may be responsible for the difficulty we encountered on the attempted isolation of
quality single crystals of complex 8. Also noted from the IR data is the general strengthening
(shift to higher wavenumbers) of the phenolic C-O bond upon complexation to Fe and
subsequent formation of the O-Fe-O bonds. For the complexation of salbn (4) to yield
Fe(salbn)2FeCl4 (8), the peak at 1285 cm-1 remained unchanged (due to the uncoordinated
phenolic C-O), with a poorly resolved shoulder at circa 1300 cm-1, attributed to the
coordinated C-O functionality in the complex. In general, there is an overall shift in the peak
positions of the significant functional groups upon complexation of respective ligands to Fe.

The cyclometallated moieties that form upon complexation impart stability to the metal
complexes formed.
2.3. X-Ray Crystallography
The molecular structure of complex 7 FesalpnCl was determined by X-ray diffraction
analysis of its single crystal data, while those of 5 and 6 we isolated and determined, exactly
matched previously reported [15, 28] results and are henceforth excluded from this discussion
except where necessary for the purpose of comparison. However, the molecular structure of
complex 8 is yet to be reported by single crystal XRD due probably to the high flexibility of
the six membered butylene diamine backbone which in comparison to the other complexes
has a higher degree of freedom that often leads to disorder, distortion and poor quality of
diffraction data. An ORTEP representation of the arrangement and connectivity of atoms in
complex 7 is presented in Fig 1.
Figure 1. ORTEP diagram of 7 with thermal ellipsoids drawn at the 50% probability level.
Hydrogen atoms and a second, poorly resolved dimer are omitted for clarity. Selected bond
lengths (Å) and angles (°) Fe1-N1, 2.145 (2); Fe1-N2, 2.152 (2); Fe1-O1, 2.0241 (17); Fe1-O2, 1.9030 (17);
Fe1-Cl1, 2.3309 (11); Fe1-O1a, 2.1136 (17); N1-Fe-N2, 91.32 (8); O1-Fe-O2, 95.74 (7); N1-Fe-O1, 84.26 (7);
N2-Fe-O2, 87.89 (8); Cl-Fe-O1, 96.16 (6); Cl-Fe-O2, 94.59 (6); Cl-Fe-N1, 90.61 (6); Cl-Fe-N2, 92.44 (7).

Amongst the earliest structural data determinations of salen-type Schiff base complexes to be
published are those of 5 and 6, which were previously reported by Elmali et al. [28] and
Gerloch and Mabbs [15]. Related structures have been published for (ferrous and ferric) iron
complexes of 3, which contain axial ligands other than a chloride, often with additional
substituents on the ligand [29-32], but the crystal structure obtained for 7 is new. However,
no crystal structure of complex 8 was found from a survey of current literature. The
asymmetric unit of complex 7 crystallised as dimeric species in the centrosymmetric triclinic

P system, similar to that reported for complex 6. The Fe-O-Fe molecular dimers occurred
1
through the sharing of one oxygen atom from each ligand between two iron nuclei. The two
types of Fe-O bonds that resulted (Fe1-O1 and Fe1-O1a), are non-equivalent with the dimer-
forming bonds (Fe1-O1a) being longer than the normal phenolic (Fe1-O1) bonds due
primarily to the effects of the two connected Fe centres. Each iron centre in the dimeric
crystal of 7 is hexa-coordinated in an octahedral environment that is situated approximately
0.54 Å away from the basal plane defined by the donor ONNO atoms which, due to non-
equivalent axes, show rhombic distortions of the octahedral planes [33]. When compared
with related previously reported counterparts (5 and 6), the distortions were less pronounced
in 7 than in 6 as flexibility of the propylene backbone is greater than that of the ethylene
backbone, and the bond angle formed between the nitrogen donors and the iron centre (N-Fe-
N) increased with an increase in the carbon chain length and flexibility of the ligand
backbone.
2.4. Catalytic conversion of the substrate n-octane using peroxide based oxidants
The type, accessibility and cost of the oxidant (source of oxygen) is very important in
oxidation catalysis. Air and molecular oxygen are the preferred sources of oxygen, but for
highly unreactive substrates such as alkanes, activated oxidants, mostly peroxides, are often
employed. In this study we have employed hydrogen peroxide (H2O2) and tertiary butyl
hydroperoxide (TBHP), which are common peroxide-based oxidants, for the oxidation of the
substrate n-octane under catalytic conditions with the isolated complexes 5-8 as catalysts.
The result presented in Figure 2 indicates that a blank reaction (in the absence of any added
catalyst) with H2O2 as the oxidant displayed a very low conversion, while use of catalysts
with this oxidant led to significantly improved total conversions. The homogeneous catalysts
(5-8) were all catalytically active in the conversion of n-octane to oxygenates and complex 6
with the tightest carbon backbone architecture was the least active in this catalytic process.

An increase in the length of the alkyl backbone from an ethylene (6) to a propylene (7) group
afforded greater catalytic activity, while a longer butylene (8) group did not impart any
additional advantage to the catalytic activity, remaining at circa 10% conversion. About 4%
conversion of the substrate with the more stable TBHP oxidant occurred even in the absence
of an added catalyst. However, increased activities were observed with the use of the
catalysts. In summary, all the complexes 5-8 were active for the activation of the substrate
and a direct correlation relating catalytic activity with the variation in ligand backbone is
observed (Fig. 2). The general trend in catalytic activity could directly be linked to the
hydrophobicity of the ligand backbone. Where reactivity increased with an increase in the
alkyl chain length (for catalysts 6-8) or possession of an aromatic group (for the higher
reactivity of catalyst 5). In previous studies with monooxygenase enzymes like cytochrome
P-450 and other biomimetic catalysts for paraffin activation, it was established that the
hydrophobic regions of the catalysts enabled the ready uptake, correct orientation, and
hydroxylation of C-H bonds [34]. Other related homogeneous systems have obtained similar
conversions with H2O2 [35] and higher conversions with TBHP [3, 36].
12
10
8
6
H2O2
4
2
0
TBHP
blank
5
6
7
8
Catalyst
Figure 2. Catalytic conversion of n-octane by complexes 5-8 using H2O2 and TBHP as
oxidants.
2.5. Product selectivity and analysis of the regioselectivity parameter

Statistically, depending on the position of attack and extent of oxidation, a total of nine
oxygenated products (octanols, octanal, octanones and octanoic acid) are possible for the
functionalisation of n-octane with an atom of oxygen. These are the six products of internal
carbon activation (4-, 3-, and 2- octanols and octanones) and three of terminal carbon
activation (1-octanol, octanal and octanoic acid- the only product that requires the addition of
two oxygen atoms). A combination of these in the product stream creates a selectivity
difficulty which is to date the greatest challenge of paraffin activation. Thus the need is to
develop new catalysts capable of maintaining a lean distribution of preferably the three
products of terminal carbon activation. Hence, the concept of the regioselectivity parameter
has become useful in paraffin activation as a means to express catalyst efficiency and
selectivity or product distribution. The regioselectivity parameter for n-octane (C1:C2:C3:C4)
is the relative reactivity at carbon positions 1, 2, 3, 4 respectively along its backbone and is
used to analyse selectivity to the highlighted plethora of products possible in the oxidation
reaction data. It represents the relative reactivity of the internal methylene (CH2) hydrogen
atoms normalised to the terminal methyl (CH3) hydrogen atoms at carbon position 1.
Products were grouped together based on the position of the inserted oxygen, with
normalisation carried out by dividing the total selectivity at each carbon by the number of
hydrogens at that position (three for the terminal CH3 and two for each unique internal CH2).
As a reference point, the reactivity at the terminal carbon 1 was set to one. For this study, the
selectivity data expressed in terms of the various product groups and the selectivity parameter
expressing total reactivity at each carbon atom are presented in Table 2.
Table 2. Product distribution in the oxidation of n-octane catalysed by catalysts 5-8.
Oxidant
H2O2
TBHP
Product distribution (%)
Product distribution (%)
A
c
Regioselectivity
parameter*
Regioselectivity
parameter*
A
c
i
-one
-nol
-one
-nol
-nal
-nal
i
C1 : C2 : C3 : C4
C1 : C2 : C3 : C4
(4-;3-;2-)
(4-;3-;2-;1-)
(4-;3-;2-)
(4-;3-;2-;1-)
d
Catalyst
d
100
(29; 36; 35)
54
74
(21; 24; 28)
78
13
(3; 0; 4; 5)
12
1
0
Blank
0
0
10
10
8
0
0
3
4
0.0 : 1.2 : 1.3 : 1.0
1.0 : 1.5 : 1.3 : 1.4
1.0 : 2.2 : 1.8 : 2.0
1.0 : 1.2 : 0.9 : 1.2
3
3
2
0
1.0 : 2.5 : 2.0 : 2.0
1.0 : 3.9 : 3.0 : 3.1
1.0 : 3.3 : 2.7 : 2.5
1.0 : 3.8 : 3.7 : 2.9
36
(8; 2; 9; 16)
25
5
6
7
7
9
9
(16; 20; 18)
62
(23; 26; 29)
74
(4; 0; 5; 3)
15
(19; 23; 21)
54
(8; 1; 8; 7)
35
(21; 24; 29)
72
(4; 3; 5; 4)
18
(17; 19; 18)
(8; 0; 7; 20)
(21; 23; 29)
(3; 8; 4; 3)

53
34
76
15
1
0
8
8
4
1.0 : 1.3 : 0.9 : 1.2
0
1.0 : 3.8 : 3.0 : 2.9
(16; 18; 19)
(8; 1; 7; 18)
(22; 24; 29)
(4; 2; 4; 4)
Conditions for all entries: 5 ml MeCN, catalyst: n-octane: oxidant = 1: 50: 150 (mol/mol), 80 °C, 48h, stirred at 100 rpm. * =
Regioselectivity parameter (C1:C2:C3:C4) is the relative reactivity at carbon positions 1, 2, 3 and 4 respectively on the n-octane backbone.
Reactivity at C1 is normalised to 1.0.
Previous reports of regioselectivity on the oxidative functionalisation of n-alkanes have been
published for n-hexane, n-heptane, n-octane and n-decane where it was generally established
that differences in competitive functionalisation at the terminal and internal C-H bonds has
led to two distinct regioselectivity possibilities. On the one hand, zero terminal C-H
selectivity may lead to the observance of only products of internal C-H activation (mainly
ketones). This scenario has been observed for systems using: an iron-based metal-organic
framework (MOF) catalyst, [37] a titanium-doped silicate catalyst, [38] an osmium-based
catalyst with an iron-based promoter [39] and, a salen-type manganese complex immobilised
onto a polymer membrane support [40]. Thermal oxidation of n-octane in the liquid phase
with oxygen as the oxidant also resulted exclusively in the functionalisation of internal
carbons, except when cracking of a C-C bond resulted in two carboxylic acids [41]. On the
other hand, even when functionalisation does occur at the terminal primary C-H bonds,
reactivity at each of the possible internal C-H bonds is always higher leading to more ketones
and secondary alcohols, as observed for the oxidation of n-octane with a bimetallic MOF-5
catalyst containing iron and zinc [42] and a biphasic water/ionic liquid catalyst system [35].
We have observed reactivity at the terminal and internal C-H bonds. The C1:C2:C3:C4 ratio
is very low, most notably using catalysts 7 and 8 with H2O2 where the ratio 1:1:1:1 obtained
is the most even product distribution at carbon atoms of n-octane reported to date.
In agreement with some of the findings and established trends highlighted above, this study
also indicates that the dominant product groups identified with H2O2 and TBHP as oxidants
are the ketones comprising over 50 and 70% product selectivity respectively. Analysis of the
regioselectivity profiles for selectivity to more desired terminal products (1-octanol, octanal
and octanoic acid) indicates that H2O2 as oxidant yielded statistically more terminal C-H
activation with reasonable amounts of, especially, octanal observed with all the catalysts at
up to 10% conversion. This is an interesting observation that might serve as a technique for
the oxidation of an existing primary alcohol to the aldehyde [43]. In addition, trace amounts
of octanoic acid were observed with all the catalysts and all four alcohols were typically
produced with 1-octanol as the dominant product. Similar selectivity profiles were obtained
for 2- and 4-octanol, while the catalysts were usually least selective towards 3-octanol. This

contrasts with the selectivity to ketones where 3-octanone was the most prevalent, possibly
indicating that further oxidation from octanol to octanone is more facile at the C3 position. In
general, unlike the overall catalyst activity, the selectivity to oxygenated products was
observed to be independent of the ligand backbone.
2.6. Catalyst deactivation and nature of the active intermediate
The colour of the solution changed from dark red to yellow as the reaction proceeded. After
48 h a light orange solid precipitated out of the solution. This was washed with diethyl ether,
dried and analysed by IR (ATR). The IR spectrum of the isolated species contained OH peaks
indicating the presence of iron hydroxo species. We were unable to perform any further
analysis on the recovered species due to its absolute insolubility. UV analysis of the reaction
mixture showed that the complex changed in solution upon addition of H2O2. This change
was due to H2O2 and not as a result of n-octane (heating the complex with only n-octane
added did not alter the pattern of its UV spectrum). A UV/vis time dependent monitoring of
the reaction mixture showed that the absorbance peak at 470 nm which is due to the ligand to
metal (PhO-  Fe3+) charge transfer (LMCT) transition decreased in intensity as the reaction
progressed, which is in agreement with earlier observations [9]. Thence we propose high-
valent iron-salen oxo specie [FeIV=O(salen)]+· as the active intermediate in this study.
Formation of the active specie occurs via an oxidative reaction of the complex with H2O2 in
solution. This specie has been studied and characterised by others [44, 45] using a variety of
spectroscopic techniques and its function as a medium for the transfer of oxygen to the
substrate is widely accepted [9, 20, 23, 46]. Relating this to the observed product distribution
(vide supra) clearly indicates that the formation of octyl hydroxo compounds was the initial
step in the oxidation of n-octane to the variety of products (octanols, octanones and octanoic
acid) observed in this study.
3. Conclusions
Catalysts based on a combination of cheap ligands (Schiff bases) and environmentally
3
friendly metal (Fe) have been employed for the activation of highly unreactive Csp –H bonds
of n-octane. In comparison to related results for homogeneous paraffin oxidation, the
catalysts used in this study showed improved product distribution in favour of terminal
carbon (C1) functionalisation utilising a simple H2O2 oxidant. Thus there is a regioselectivity
advantage for the use of these catalyst systems in the direct oxidation of n-octane to
industrially useful terminal products such as 1-octanal and octanoic acid.

4. Experimental
4.1. Ligand synthesis
All the four ligands N,N'-bis(salicylidene)-1,2-phenylenediamine (1, salophen), N,N'-
bis(salicylidene)ethylenediamine (2, salen), N,N'-bis(salicylidene)propylenediamine (3,
salpn) and N,N'-bis(salicylidene)butylenediamine (4, salbn) were synthesised by established
methods we reported earlier [27].
4.2. Synthesis of iron complexes
General procedure for the synthesis of the complexes: A suspension of FeCl3 (ca. 15 mmol)
in 50 mL of ethanol was added dropwise to a hot (ca. 50 °C) solution of ligand (ca. 12 mmol)
in 50 mL of ethanol, which was kept stirring in a round bottomed flask. An immediate colour
change from orange/yellow to dark red/purple was observed. A reflux condenser was then
attached to the round bottomed flask and the reaction was maintained at reflux overnight.
After cooling to room temperature, the precipitated complex was filtered off, washed with
ice-cold ethanol and dried in air.
FesalophenCl (5): Red powder, yield = 72 %, m.p. = 347-350 °C. Selected IR peak
frequencies (cm-1): 1604 (C=N), 1315 (C=O), 1461 and 760. m/z = 370.0399, corresponding
to [5-Cl]+. Analysis found (calculated) for C20H14N2O2FeCl (%): C, 58.53 (59.22); H, 3.13
(3.48); N, 6.98 (6.91).
FesalenCl (6): Red powder, yield = 76 %, m.p. = 257-263 °C. Selected IR peak frequencies
(cm-1): 1618 (C=N), 1303 (C=O), 1445 and 758. m/z = 322.0398, corresponding to [6-Cl]+.
Analysis found (calculated) for C16H14N2O2FeCl (%): C, 53.44 (53.74); H, 3.44 (3.95); N,
8.16 (7.83).
FesalpnCl (7): Red powder, yield = 54 %, m.p. = 202-205 °C. Selected IR peak frequencies
(cm-1): 1611 (C=N), 1301 (C=O), 1472 and 757.m/z = 336.0555, corresponding to [7-Cl]+.
Analysis found (calculated) for C17H16N2O2FeCl (%): C, 54.86 (54.94); H, 4.34 (4.34); N,
6.53 (7.54).
Fe(salbn)2FeCl4 (8): Dark purple powder, yield = 69 %, m.p. = 237-238 °C. m/z = 646.2252,
corresponding to [8-FeCl4]+. Selected IR peak frequencies (cm-1): 1661 (C=N), 1285 (C=O),
1476 and 760. Analysis found (calculated) for C36H38N4O4Fe2Cl2.6H2O (%): C, 44.92
(45.40); H, 4.40 (5.29); N, 5.56 (5.88).

4.3. Catalytic testing for the oxidation of n-octane
The catalysts 5-8 were tested for activity in the oxidation of n-octane in the presence of tert-
butyl hydroperoxide (TBHP) and hydrogen peroxide (H2O2) as oxidants. Reactions were
carried out in batches in 25 mL round bottom flasks fitted with condensers and sealed at the
top to prevent loss of any volatilised n-octane. In a typical reaction, a catalyst (0.01 g) was
weighed into the flask and solvent (5 mL acetonitrile (MeCN)) was added using a pipette.
Catalyst, substrate and oxidant were added in that order in the (molar) ratio 1:50:150,
respectively. Pentanoic acid (ca. 0.008 g, weighed accurately) was used as the internal
standard. Reactions were stirred (100 rpm) at 80 °C for 48 h. Conditions were kept constant
to allow for comparison of the various catalysts and oxidants. At the end of each catalytic
run, the reaction vessel was allowed to cool to room temperature. Duplicate samples were
removed from the reaction vessel and analysed by GC using the single point internal standard
method.
4.4. X-ray structure collection, solution and refinement on complex 7
A single crystal of complex 7 with dimensions 0.23 x 0.15 x 0.08 mm-3 was selected and
glued on to the tip of a glass fibre. The crystal was then mounted in a stream of cold nitrogen
at 100(1) K and centred in the X-ray beam by using a video camera. The crystal evaluation
and data collection were performed on a Bruker APEXII diffractometer with Mo Kα (λ =
0.71073 Å) radiation and diffractometer to crystal distance of 4.00 cm. The initial cell matrix
was obtained from three series of scans at different starting angles. Each series consisted of
12 frames collected at intervals of 0.5º in a 6º range about with the exposure time of 10
seconds per frame. The reflections were successfully indexed by an automated indexing
routine built in the APEXII program suite [47]. The final cell constants were calculated from
a set of 6460 strong reflections from the actual data collection. The data were collected by
using the full sphere data collection routine to survey the reciprocal space to the extent of a
full sphere to a resolution of 0.75 Å. A total of 7866 data were harvested by collecting 2216
frames at intervals of 0.5º scans in  and φ with exposure times of 15 sec per frame. These
highly redundant datasets were corrected for Lorentz and polarization effects. The absorption
correction was based on fitting a function to the empirical transmission surface as sampled by
multiple equivalent measurements [47]. The systematic absences in the diffraction data were

1
uniquely consistent for the space group P that yielded chemically reasonable and
computationally stable results of refinement [48]. A successful solution by the direct methods

of SIR97 [49] provided all non-hydrogen atoms from the E-map. All non-hydrogen atoms
were refined with anisotropic displacement coefficients. All hydrogen atoms were included in
the structure factor calculation at idealized positions and were allowed to ride on the
neighbouring atoms with relative isotropic displacement coefficients. The final least-squares
refinement of 567 parameters against 7866 data resulted in residuals R (based on F2 for I≥2σ)
and wR (based on F2 for all data) of 0.0409 and 0.1106, respectively. The final difference
Fourier map was featureless. The molecular diagram is drawn with 50% probability
ellipsoids.
Crystallographic data for the structure of complex 7 has been deposited with the Cambridge
Crystallographic Data Centre, CCDC 1507900. The data can be obtained free of charge at
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road Cambridge CB2 1EZ, UK; Fax: +44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk).
Acknowledgements
We are grateful for financial grants from the Department of Science and Technology-
National Research Foundation (DST-NRF) Centre of Excellence in Catalysis (c* change,
grants for PAR06 and PAR08). We also thank Dr Bernard Owaga for crystal data collection
and analysis.
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O
O
3
1
2
4
O
(R)
3
1
2
4
OH
OH
N
O
N
O
n‐octane
+
Fe
Cl
3
1
2
4
TBHP or H2O2
OH
OH
R = o‐C6H4; (CH2)2; (CH2)3; (CH2)4
O
catalysts
O
OH
C8 oxygenates

Highlights
Four salen-type iron complexes were prepared, each with a different ligand diamine bridging
backbone.
Variation of the ligand backbone altered the electronics and sterics around the iron centres as
observed by the use of IR spectroscopy and X-Ray crystallography.
All the complexes were found to be catalytically active in the oxidation of n-octane with
H2O2 and TBHP as oxidants. The dominant product class was the ketones.