Iron(II) complexes with tetradentate bis(aminophenolate) ligands: Synthesis and characterization, solution behavior, and reactivity with O2

Article abstract of DOI:10.1021/ic1016998

Tetradentate bis(aminophenolate) ligands H₂salan^X and H₂bapen^X (where X refers to the para-phenolate substituent = H, Me, F, Cl) react with [Fe{N(SiMe₃)₂} ₂] to form iron(II) com

Full text of DOI:10.1021/ic1016998

11106 Inorg. Chem. 2010, 49, 11106–11117
DOI: 10.1021/ic1016998
Iron(II) Complexes with Tetradentate Bis(aminophenolate) Ligands: Synthesis and
Characterization, Solution Behavior, and Reactivity with O2
Christopher J. Whiteoak, Rafael Torres Martin de Rosales, Andrew J. P. White, and George J. P. Britovsek*
Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AY, United Kingdom
Received August 20, 2010
Tetradentate bis(aminophenolate) ligands H2salanX and H2bapenX (where X refers to the para-phenolate substi-
tuent = H, Me, F, Cl) react with [Fe{N(SiMe3)2}2] to form iron(II) complexes, which in the presence of suitable donor
ligands L (L = pyridine or THF) can be isolated as the complexes [Fe(salanX)(L)2] and [Fe(bapenX)(L)2]. In the
absence of donor ligands, either mononuclear complexes, for example, [Fe(salantBu,tBu)], or dinuclear complexes of
the type [Fe(salanX)]2 are obtained. The dynamic coordination behavior in solution of the complexes [Fe(salanF)(L)2]
1
and [Fe(bapenF)(L)2] has been investigated by VT H and 19F NMR spectroscopy, which has revealed equilibria
between isomers with different ligand coordination topologies cis-R, cis-β and trans. Exposure of the iron(II) salanX
complexes to O2 results in the formation of oxo-bridged iron(III) complexes of the type [{Fe(salanX)}2(μ-O)] or
[{Fe(salanX)(L)}2(μ-O)]. The lack of catalytic activity of the iron(II) salan and bapen complexes in the oxidation of
cyclohexane with H2O2 as the oxidant is attributed to the rapid formation of stable and catalytically inactive oxo-bridged
iron(III) complexes.
Introduction
alkanes with dioxygen5,8-10 and for the oxidation of organic
sulfides with strong oxidants such as PhIO.11,12
Salen ligands are an important class of ligands due their
versatility in terms of steric and electronic modifications, and
many salen metal complexes have found important applica-
tions in homogeneous catalysis.1 Well known examples are
the manganese salen complexes used as alkene epoxidation
catalysts2 and the chromium and aluminum salen complexes
applied in the ring-opening polymerization of epoxides with
CO2.3,4 Iron-based salen complexes have been used in oxida-
tion catalysis, for example, as mimics for the active site in
oxygenases,5-7 as catalysts for the oxidation of alkenes and
The related ligand class of salan ligands, also referred to as
reduced salen or tetrahydrosalen ligands, has received com-
paratively less attention, probably because their synthesis is
less straightforward. However, potential benefits, such as
increased ligand flexibility and stronger nitrogen donors make
them attractive ligand targets.13 Some early studies on the syn-
thesis and the characterization of iron complexes containing
salan ligands were reported in the early 1980s by Borer and
co-workers.14,15 A dinuclear hydroxo-bridged iron(III) com-
plex of class A [Fe(salan)(μ-OH)]2 was obtained by reacting
the ligand with iron(II) salts in an aqueous environment (see
Figure 1). The oxidation of Fe(II) to Fe(III) was probably
caused by dioxygen from the air.14 Similar dinuclear iron(III)
methoxy-bridged complexes were later reported by others.16-18
A different iron(III) salan complex was reported starting
*Corresponding author. E-mail: g.britovsek@imperial.ac.uk. Telephone:
þ44-(0)20-75945863.
(1) Cozzi, P. G. Chem. Soc. Rev. 2004, 33, 410–421.
(2) McGarrigle, E. M.; Gilheany, D. G. Chem. Rev. 2005, 105, 1563–1602.
(3) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388–2410.
(4) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618–
6639.
€
from FeCl3 and H2salan. On the basis of Mossbauer data, a
(5) Fujii, H.; Funahashi, Y. Angew. Chem. Int. Ed.. 2002, 41, 3638–3641.
(6) Kurahashi, T.; Oda, K.; Sugimoto, M.; Ogura, T.; Fujii, H. Inorg. Chem.
2006, 45, 7709–7721.
dinuclear phenoxide-bridged structure [Fe(μ-O-salan)Cl]2
(7) Snyder, B. S.; Patterson, G. S.; Abrahamson, A. J.; Holm, R. H.
J. Am. Chem. Soc. 1989, 111, 5214–5223.
(13) Matsumoto, K.; Saito, B.; Katsuki, T. Chem. Commun. 2007, 3619–
3627.
€
(8) Bottcher, A.; Birnbaum, E. R.; Day, M. W.; Gray, H. B.; Grinstaff,
(14) Borer, L.; Thalken, L.; Ceccarelli, C.; Glick, M.; Zhang, J. H.;
Reiff, W. M. Inorg. Chem. 1983, 22, 1719–1724.
(15) Borer, L.; Thalken, L.; Zhang, J. H.; Reiff, W. M. Inorg. Chem. 1983,
22, 3174–3176.
M. W.; Labinger, J. A. J. Mol. Catal. A: Chem. 1997, 117, 229–242.
€
(9) Bottcher, A.; Grinstaff, M. W.; Labinger, J. A.; Gray, H. B. J. Mol.
Catal. A: Chem. 1996, 113, 191–200.
(10) Tabushi, I.; Nakajima, T.; Seto, K. Tetrahedron Lett. 1980, 21, 2565–
€
€
(16) Baran, P.; Bottcher, A.; Elias, H.; Haase, W.; Huber, M.; Fuess, H.;
2568.
Paulus, H. Z. Naturforsch. 1992, 47B, 1681–1686.
(17) Atwood, D. A.; Jegier, J. A.; Lindholm, N. F.; Martin, K. J.;
Rutherford, D. J. Coord. Chem. 1996, 38, 305–318.
(11) Bryliakov, K. P.; Talsi, E. P. Angew. Chem., Int. Ed. 2004, 43, 5228–
5230.
(12) Sivasubramanian, V. K.; Ganesan, M.; Rajagopal, S.; Ramaraj, R.
J. Org. Chem. 2002, 67, 1506–1514.
(18) Viswanathan, R.; Palaniandavar, M.; Prabakaran, P.; Muthiah,
P. T. Inorg. Chem. 1998, 37, 3881.
r
pubs.acs.org/IC
Published on Web 11/09/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11107
Figure 1. Examples of previously reported iron(III) salan complexes.
was proposed for this complex in the solid state.15 More
recently, the use of bulky substituents in the ortho position of
the phenoxy moieties has allowed the isolation of several
mononuclear complexes [Fe(salan)Cl] of classB.19-22 A third
class C comprises oxo-bridged dinuclear iron(III) salan com-
plexes.23 Oxo-bridged dinuclear iron(III) complexes
[LFe-O-FeL] are plentiful and have long been known,24
and their involvement in many nonheme enzymes and proteins
is well established.25
All previously reported iron salan complexes have in com-
mon that the iron center is formally in the þ3 oxidation state.
To the best of our knowledge, no reports on iron(II) com-
plexes containing salan ligands have been published so far. In
contrast, iron(II) salen complexes have been known for some
time.26 It should be noted that iron(II) salen complexes
[Fe(salen)], which are typically brown in color, are extremely
air sensitive, as reported by Larkworthy and Calderazzo
already in the late 1960s and more recently also by Do and
Lippard.27-29 In several reports where iron(II) salen com-
plexes were believed to be obtained as orange/red, blue, or
purple materials,30-33 oxidation of iron(II) to iron(III) has
probably taken place, most likely to give dinuclear oxo-
bridged iron(III) complexes.
Figure 2. Salan and bapen ligands.
complexes containing the methylated bis(aminophenol)-
ethylenediamine (bapen) ligands (Figure 2). Bapen ligands
are [2,2,2]-type ligands which, upon coordination to a metal
center, will form three five-membered chelate rings. This may
provide increasedstability of themetal complex, comparedto
one five-membered and two six-membered chelate rings formed
with salan-type ligands. Surprisingly, although these bapen
compounds have been known for more than 50 years,34 they
have hitherto seen very little application as ligands in metal
coordination chemistry and catalysis. A few molybdenum and
tungsten dioxo bapen complexes and their oxo transfer ability
have been reported,35,36 and very recently, Okuda and co-
workers have reported the application of Group 4 metal com-
plexes containing bapen ligands in olefin polymerization catal-
ysis.37 Some early and late transition-metal complexes using
nonmethylated derivatives have also been reported.38,39 The
structural aspects of these iron(II) salan and bapen com-
plexes in the solid state and in solution have been analyzed
by X-ray crystallography and variable-temperature (VT)
paramagnetic NMR spectroscopy. The studies have high-
lighted the importance of careful solution analysis of
paramagnetic iron(II) complexes, which can have highly
dynamic coordination behavior in solution. Complexes of this
type are increasingly investigated as bioinorganic model sys-
tems and as nonheme iron-based oxidation catalysts but are
often characterized by solid-state methods, such as X-ray
Here we present the synthesis and the characterization of
the first iron(II) and ruthenium(II) salan complexes. A series
of salan ligands and their corresponding iron(II) complexes
has been prepared containing different substituents in the
ortho- and para-position of the phenoxy donors (see Figure 2).
Furthermore, we have prepared a series of novel iron(II)
(19) Hasan, K.; Fowler, C.; Kwong, P.; Crane, A. K.; Collins, J. L.;
€
Kozak, C. M. Dalton Trans. 2008, 2991–2998.
diffraction and Mossbauer spectroscopy. As part of our
ꢀ ꢁ
(20) Mialane, P.; Anxolabehere-Mallart, E.; Blondin, G.; Nivorojkine, A.;
attempts to develop robust homogeneous oxidation catalysts,
the reactivity of these complexes toward dioxygen as well as
their catalytic potential in oxidation reactions have been
investigated.
Guilhem, J.; Tchertanova, L.; Cesario, M.; Ravi, N.; Bominaar, E. L.; Girerd,
€
J.-J.; Munck, E. Inorg. Chim. Acta 1997, 263, 367–378.
(21) Strautmann, J. B. H.; DeBeer George, S.; Bothe, E.; Bill, E.;
€
€
Weyhermuller, T.; Stammler, A.; Bogge, H.; Glaser, T. Inorg. Chem. 2008,
47, 6804–6824.
(22) Egami, H.; Katsuki, T. J. Am. Chem. Soc. 2007, 129, 8940–8941.
(23) Glaser, T.; Pawelke, R. H.; Heidemeier, M. Z. Anorg. Allg. Chem.
2003, 629, 2274–2281.
Experimental Section
All syntheses of complexes were carried out using standard
vacuum line, Schlenk, or cannular techniques and stored in a
€
(24) Pfeiffer, P.; Breith, E.; Lubbe, E.; Tsumaki, T. Liebigs Ann. Chem.
1933, 503, 84.
(25) Lippard, S. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 344.
(26) Calvin, M.; Barkelew, C. H. J. Am. Chem. Soc. 1946, 68, 2267–2273.
(27) Calderazzo, F.; Floriani, C.; Henzi, R.; L’Eplattenier, F. J. Chem.
Soc. 1969, A, 1378–1386.
(28) Do, L. H.; Lippard, S. J. Inorg. Chem. 2009, 48, 10708–10719.
(29) Earnshaw, A.; King, E. A.; Larkworthy, L. F. J. Chem. Soc. 1968,
A, 1048–1052.
(34) Freedman, H. H.; Frost, A. E. J. Org. Chem. 1958, 23, 1292–1298.
(35) Hinshaw, C. J.; Peng, G.; Singh, R.; Spence, J. T.; Enemark, J. H.;
Bruck, M.; Kristofzski, J.; Merbs, S. L.; Ortega, R.; Wexler, P. A. Inorg.
Chem. 1989, 28, 4483–4491.
(36) Wong, Y.-L.; Ma, J.-F.; Law, W.-F.; Yan, Y.; Wong, W.-T.;
Zhang, Z.-Y.; Mak, T. C. W.; Ng, D. K. P. Eur. J. Inorg. Chem. 1999,
313–321.
(30) Belokon, Y. N.; Fuentes, J.; North, M.; Steed, J. W. Tetrahedron
2004, 60, 3191–3204.
(31) Hamaker, C. G.; Mirafzal, G. A.; Woo, L. K. Organometallics 2001,
20, 5171–5176.
(32) Idage, B. B.; Idage, S. B.; Kasegaonkar, A. S.; Jadhav, R. V. Mater.
Sci. Eng., B 2010, 168, 193–198.
(33) Roy, P.; Dhara, K.; Chakraborty, J.; Nethaji, M.; Banerjee, P. Indian
J. Chem. 2007, 46A, 1947–1950.
(37) Meppelder, G.-J. M.; Fan, H.-T.; Spaniol, T. P.; Okuda, J. Organo-
metallics 2009, 28, 5159–5165.
(38) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem.
Soc. 2008, 130, 2728–2729.
€
(39) Sik Min, K.; Weyhermuller, T.; Bothe, E.; Wieghardt, K. Inorg.
Chem. 2004, 43, 2922–2931.

11108 Inorganic Chemistry, Vol. 49, No. 23, 2010
Whiteoak et al.
nitrogen-filled glovebox. NMR spectra were collected on a
Bruker AV-400 or a DRX-400 spectrometer. Chemical
then dissolved in toluene (10 mL) and to this solution was added
pyridine (5 mL). The solution was stirred for 30 min, and the
volatiles were removed. The residual brown solid was washed
with pentane (2 ꢀ 10 mL) to yield a red powder (481 mg, 73%).
1H NMR (400 MHz, d8-toluene, 333 K): δ 162.3, 97.4, 87.9, 82.1,
48.4, 28.4, 25.6, 10.4, 9.0, -7.0, -10.2. MS (LSIMS): m/z = 382,
[M - 2Py]þ. Elemental analysis for C30H36FeN4O2 (fw 540.5):
C, 66.67; H, 6.71; N, 10.37%. Found C, 66.74; H, 6.81;
N, 10.48%. μeff (BM) = 5.17.
1
shifts for H NMR are referenced to the residual protio
impurity, and 19F chemical shifts are reported relative
to CFCl3. Elemental analysis was performed by the
Science Technical Support Unit at London Metropolitan
University.
Reagents. The 2-amino-4-fluorophenol was synthesized by
nitration of 4-fluorophenol and subsequent reduction of the nitro
group.40 [Fe{N(SiMe3)2}2] was prepared according to a pub-
lished procedure.41 The ligands H2salanMe, H2salanH, H2salanF,
H2salanCl, and H2salantBu,tBu were prepared according to pre-
viously reported methods.42,43 The synthesis for the ligands
H2bapenH, H2bapenF, and H2bapenCl are analogous to the
synthesis of the methyl derivative H2bapenMe given below and
are provided in the Supporting Information. Similarly, the
synthesis for the complexes [Fe(salanH)(Py)2], [Fe(salanF)-
(Py)2], [Fe(salanCl)(Py)2], [Fe(bapenMe)(Py)2], [Fe(bapenH)-
(Py)2], [Fe(bapenF)(Py)2], and [Fe(bapenCl)(Py)2] is analogous
to the synthesis of [Fe(salanMe)(Py)2] described below and
has been included in the Supporting Information. All other
reagents are commercially available and were used without
further purification.
[Fe(salantBu,tBu)]. To a solution of H2salantBu,tBu (613 mg,
1.17 mmol) in toluene (20 mL) was slowly added [Fe{N-
(SiMe3)2}2] (440 mg, 1.17 mmol) in toluene (10 mL) at -78 °C.
With stirring the solution was allowed to slowly warm to room
temperature to yield a faintly colored solution. After stirring
at room temperature for 1 h, the solvent was removed, and
the residue was dried under vacuum overnight to yield a brown
powder (641 mg, 95%). 1H NMR (400 MHz, d6-benzene, 298 K):
δ 100.4, 39.3, 37.56, 21.6, 15.8, 5.7, 1.7. MS (LSIMS):
m/z = 578, [M]þ. Elemental analysis for C34H54FeN2O2 (fw
578.7): C, 70.57; H, 9.41; N, 4.84%. Found C, 70.48; H, 9.33; N,
4.77%.
[Fe(salantBu,tBu)(THF)2]. The ligand H2salantBu,tBu (503 mg,
0.96 mmol) and KH (77 mg, 2 equiv) were mixed in degassed
THF and left stirring overnight. The mixture was filtered, and
the filtrate was added to FeCl2 (121 mg, 1 equiv) in THF. The
solution changed color from yellow to eventually brown. After
stirring for 1 day, the solution was filtered, and the solvent
evaporated under vacuum to leave a brown solid. 1H NMR
(400 MHz, CD3CN, 298 K): 92.7, 75.8, 40.5, 36.29, 35.5, 33.8,
5.98, -3.87. μeff (BM) = 5.12. Elemental analysis for C42H70Fe-
N2O4 (fw 722.9): C, 69.79; H, 9.76; N, 3.88%. Found C, 69.89;
H, 9.67; N, 3.97%.
N,N0-Dimethyl-N,N0-bis(5-methyl-2-hydroxyphenyl)-1,2-dia-
minoethane (H2bapenMe). To a suspension of 2-amino-4-methyl-
phenol (10.0 g, 81.2 mmol) in water (20 mL) was added 1,2-
dibromoethane (7.6 g, 40.6 mmol). The mixture was refluxed
for 6 h, and the resulting solid was filtered and recrystallized
from ethanol to yield H4bapenMe as a brown powder (4.3 g,
39%). 1H NMR (400 MHz, d6-DMSO, 298 K): δ 8.93
3
(s, 2H, NH), 6.53 (d, 2H, JHH = 7.7, ArH), 6.37 (d, 2H,
3
4
4JHH = 1.7, ArH), 6.21 (dd, 2H, JHH = 7.7, JHH = 1.7,
ArH), 4.72 (s, 2H, ArOH), 3.25 (s, 4H, N(CH2CH2)N), 2.12 (s,
2H, ArCH3).
[Ru(salantBu,tBu)(CH3CN)2]. To [RuCl2(p-cymene)]2 (306 mg,
0.50 mmol) in acetonitrile (10 mL) was added the sodium salt of
To a solution of H4bapenMe (1.0 g, 3.67 mmol) in tetrahy-
drofuran (40 mL) at 0 °C was added n-butyllithium (2.5 M in
hexanes; 8.47 mL, 14.69 mmol). The reaction was stirred for 1
h before being allowed to slowly warm to room temperature.
Iodomethane (457 μL, 7.34 mmol) was added slowly, and the
solution stirred overnight. The volatiles were removed under
reduced pressure, and water (20 mL) was added. The mixture
was extracted with dichloromethane (3 ꢀ 50 mL), and the extracts
were combined and dried over sodium sulfate. The solvent was
removed, and the residual solid was washed with methanol
the ligand, Na2salantBu,tBu 2THF (713 mg, 1.00 mmol) in
3
acetonitrile (5 mL). The solution was stirred overnight, and
the yellow precipitate was collected. Toluene (10 mL) was added
to the precipitate, and the mixture was filtered. The filtrate was
then treated with pentane until a yellow precipitate formed,
which was isolated and precipitated from toluene/pentane twice
to yield a yellow powder (273 mg, 39%). 1H NMR (400 MHz, d8-
4
toluene, 298 K): 7.31 (d, 2H, JHH = 2.6, ArH), 6.93 (d, 2H,
2
4JHH = 2.6, ArH), 3.41 (d, 2H, JHH = 11.8, ArCH2N), 3.03
(d, 2H, JHH = 11.8, ArCH2N), 2.88 (d, 2H, JHH = 8.6,
2
2
1
2
to yield a white powder (705 mg, 64%). H NMR (400 MHz,
(N(CH2CH2)N)), 2.81 (s, 6H, NCH3), 2.14 (d, 2H, JHH
=
8.6, (N(CH2CH2)N)), 1.74 (s, 18H, ArtBu), 1.47 (s, 18H, ArtBu),
1.27 (s, 6H, MeCN). 13C NMR (100 MHz, d8-toluene, 298 K):
172.42, 138.29, 131.74, 125.74, 122.13, 118.89 (aromatic C),
35.53, 34.10 (quaternary C) 64.46, 60.82 (CH2), 49.56, 32.48,
30.56 (CH3). MS (LSIMS): m/z = 706, [M]þ, 663, [M - MeCN]þ,
624, [M - 2MeCN]þ. Elemental analysis for C38H60N4O2Ru
(fw 706.0): C, 64.65; H, 8.57; N, 7.94%. Found C, 64.49; H, 8.56;
N, 7.97%.
d6-DMSO, 298 K): δ 8.99 (br s, 2H, ArOH), 6.78 (s, 2H, ArH),
6.68-6.57 (m, 4H, ArH), 3.02 (s, 4H, N(CH2CH2)N), 2.66
(s, 6H, NCH3), 2.17 (s, 6H, ArCH3). 13C NMR (400 MHz,
d6-DMSO, 298 K): δ 148.84, 140.01, 127.98, 123.88, 120.96,
115.80 (aromatic C), 54.52 (CH2), 20.94 (CH3). MS (ESI):
m/z = 301, [M þ H]þ. Elemental analysis for C18H24N2O2
(fw 300.4): C, 71.97; H, 8.05; N, 9.33%. Found C, 72.09; H, 7.94;
N, 9.25%.
[Fe(salanMe)(Py)2]. To a solution of H2salanMe (400 mg,
1.22 mmol) in toluene (20 mL) was slowly added [Fe{N-
(SiMe3)2}2] (459 mg, 1.22 mmol) in toluene (10 mL) at -78 °C.
With stirring, the solution was allowed to slowly warm to room
temperature to yield a faintly colored solution. After stirring at
room temperature for 1 h, pentane (40 mL) was added, and the
white precipitate was filtered and washed further three times
with pentane (3 ꢀ 10 mL) to yield a white powder. The solid was
X-ray Crystallography. Table 1 provides a summary of the
crystallographic data for compounds [Fe(salanCl)(Py)2], [Fe-
(salanH)]2, [{Fe(salanCl)}2(μ-O)], [Fe(bapenCl)(Py)2], [{Fe(salanH)-
(Py)}2(μ-O)] and [{(salanCl)Fe(μ-O,O0-salanCl)Fe}2(μ-O)].
CCDC 789629, 789630, 789631, 789633, 789634 and 789796,
respectively.
Results and Discussion
(40) Joshi, A. V.; Baidoosi, M.; Mukhopadhyay, S.; Sasson, Y. Org. Process
Res. Dev. 2003, 7, 95.
(41) Andersen, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W. P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782.
(42) Whiteoak, C. J.; Britovsek, G. J. P.; Gibson, V. C.; White, A. J. P.
Dalton Trans. 2009, 2337–2344.
Synthesis of Salan and Bapen Ligands. The synthesis
and characterization of the ligands H2salanX used in
this study have been reported previously.42 The bis-
(aminophenol)ethylenediamine (H2bapenX) ligands were
prepared in a two-step procedure, as shown in eq 1. The
precursors H4bapenX, prepared from the appropriate
(43) Tshuva, E. Y.; Gendeziuk, N.; Kol, M. Tetrahedron Lett. 2001,
42, 6405–6407.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11109
Table 1. Crystallographic Data for Compounds [Fe(salanCl)(Py)2], [Fe(salanH)]2, [{Fe(salanCl)}2(μ -O)], [Fe(bapenCl)(Py)2], [{Fe(salanH)(Py)}2(μ -O)], and [{(salanCl)-
Fe(μ-O,O0-salanCl)Fe}2(μ-O)]
data
[Fe(salanCl)(Py)2]
[Fe(salanH)]2
[{Fe(salanCl)}2(μ -O)]
chemical formula
solvent
fw
C
28H30Cl2FeN4O2
3C5H5N
818.61
C36H44Fe2N4O4
CH3CN
749.51
C36H40Cl4Fe2N4O5
;
862.22
T (°C)
space group
a (A)
b (A)
c (A)
-100
-100
-100
Pccn (no. 56)
11.7059(2)
20.5872(4)
17.0688(3)
;
P21/n (no. 14)
11.7368(2)
17.9168(2)
18.1254(3)
;
P1 (no. 2)
9.5427(8)
9.5864(5)
11.8820(9)
103.865(6)
93.513(7)
115.405(7)
936.41(14)
1b
R (deg)
β (°)
;
;
107.1500(17)
;
3642.04(10)
4
1.367
0.71073
0.843
0.0343
γ (°)
V (A3)
Z
4113.44(13)
4a
1.322
F
calcd (g cm-3
λ (A)
)
1.529
0.71073
0.542
0.0555
1.54184
9.230
0.0530
μ (mm-1
R1(obs)c
)
wR2(all)d
0.1826
0.1005
0.1401
data
[Fe(bapenCl)(Py)2]
[{Fe(salanH)(Py)}2(μ-O)]
[{(salanCl)Fe(μ-O,O0-salanCl)Fe}2(μ-O)]
chemical formula
solvent
fw
T (°C)
space group
a (A)
C26H26Cl2FeN4O2
2C5H5N
711.46
C46H54Fe2N6O5
2C5H5N
1040.85
-100
Pbcn (no. 60)
18.6514(4)
12.1264(3)
23.1478(4)
;
C72H80Cl8Fe4N8O9
3C6H6
1942.76
-100
P21/c (no.14)
29.257(4)
12.9382(9)
27.262(4)
;
117.308(17)
;
9170(2)
4
1.407
0.71073
0.913
0.0637
-100
P1 (no. 2)
10.8416(2)
12.3542(4)
14.3132(4)
90.566(2)
110.118(2)
107.172(2)
1706.57(9)
2
b (A)
c (A)
R (°)
β (°)
;
;
γ (°)
V (A3)
Z
5235.44(19)
4a
1.321
F
calcd (g cm-3
λ (A)
)
1.385
0.71073
0.640
0.0473
0.1554
1.54184
4.888
0.0324
μ (mm-1
R1
wR2
)
c
d
0.0972
0.1430
a The molecule has crystallographic C2 symmetry. b The molecule has crystallographic Ci symmetry. c R1 = σ ||Fo| - |Fc||/σ |Fo|. For observed data,
|Fo| > 4σ(|Fo|). d wR2 = {σ[w(Fo2 - Fc2)2]/σ[w(Fo2)2]}1/2; w-1 = σ2(Fo2) þ (aP)2 þ bP.
aminophenol and dibromoethane,34,44,45 were methylated
investigated. Inspired by a recent report on the synthesis
of iron(II) phenoxyimine complexes,46 the iron(II) pre-
cursor complex [Fe{N(SiMe3)2}2] was used. The green
liquid complex [Fe{N(SiMe3)2}2] was reacted with H2sal-
anH in toluene at room temperature for 1 h, resulting in a
color change from green to colorless. The addition of pentane
resulted in the precipitation of a white solid, which is poorly
soluble in noncoordinating solvents. Addition of an ex-
cess of pyridine to a toluene suspension of this white solid
resulted in a red-brown solution. The product was pre-
cipitated from the toluene/pyridine solution with pentane
to yield a brown complex which was characterized as
[Fe(salanH)(Py)2]. This synthetic methodology, depicted
in Scheme 1, enabled the synthesis of a series of iron(II)
salan complexes [Fe(salanx)(Py)2], where X = H, Me, F,
and Cl, which were all fully characterized by elemental
analysis, 1H NMR, and MS and, in the case of [Fe(salanCl)-
(Py)2], by X-ray crystallography. All complexes are red-
brown in color, and magnetic susceptibility measure-
ments (Evans’ NMR method) have shown that they are
high spin (S = 2) at room temperature. The formation
to give the H2bapenX ligands.36
Synthesis of Iron(II) Salan Complexes. Initial attempts
to prepare iron(II) complexes of the type [Fe(salanX)] were
carried out by salt metathesis reactions starting from
FeBr2. For example, the reaction of H2salanCl with two
equivalents of KH in THF yields the potassium salt K2sal-
anCl, which upon reaction with iron(II) bromide in THF
yields a brown product. Analysis of this product suggested
the formulation of this complex as [Fe(salanCl)(THF)2]. In
order to avoid the use of donor solvents, such as THF, an
alternative synthesis toward [Fe(salanX)] complexes was
(46) Houghton, J.; Simonovic, S.; Whitwood, A. C.; Douthwaite, R. E.;
Carabineiro, S. A.; Yuan, J.-C.; Marques, M. M.; Gomez, P. T. J. Organomet.
Chem. 2008, 693, 717–724.
(44) Vaughan, O. J.; Gibson, J. F. Polyhedron 1990, 9, 1593–1601.
€
(45) Ochoa, M. E.; Rojas-Lima, S.; Hopfl, H.; Rodrıguez, P.; Castillo, D.;
´
ꢀ
Farfan, N.; Santillan, R. Tetrahedron 2001, 57, 55–64.

11110 Inorganic Chemistry, Vol. 49, No. 23, 2010
Whiteoak et al.
Scheme 1
of six-coordinate complexes [Fe(salanx)(Py)2] is different
compared to iron(II) salen complexes, which form
five-coordinate complexes [Fe(salen)(Py)] with only one
coordinated pyridine ligand.27,47,48
ligands have been reported.49-51 [Fe(salanH)]2 is the first
example of a dinuclear phenoxide-bridged iron(II) salan
complex. A related complex is a phenoxide-bridged Fe(II)
complex featuring two bidentate aminomethylphenolate
ligands reported by van Koten and co-workers.52 Based
on the results obtained so far, we postulate that the initial
reaction between [Fe{N(SiMe3)2}2] and H2salanX in to-
luene results in the formation of a tetrahedral complex of
the type [Fe(salanX)] or possibly an octahedral complex
containing weakly coordinating NH(SiMe3)2 ligands,
[Fe(salanX){NH(SiMe3)2}2], which are soluble in toluene.
Addition of pentane results in the precipitation of the white
dinuclear phenoxide-bridged complexes [Fe(salanX)]2. These
white complexes are identical to the white complexes that
precipitate from a solution of [Fe(salanX)(Py)2] in toluene
upon the addition of acetonitrile (Scheme 1).
In order to prepare mononuclear iron(II) salan com-
plexes [Fe(salan)], the sterically encumbered ortho- and
para-tert-butyl substituted salan ligand H2salantBu,tBu
was employed (Scheme 2). This ligand was first used by
Kol and co-workers for Group 4 metal complexes.43 The
additional steric bulk in the ortho positions prevents the
formation of phenoxide-bridged dinuclear complexes.
The reaction of H2salantBu,tBu with [Fe{N(SiMe3)2}2] in
toluene resulted in a brown complex, which according to
mass spectrometry and CHN analysis corresponds to
[Fe(salantBu,tBu)]. Unlike the dinuclear white complexes
[Fe(salanX)]2, this complex is highly soluble, even in non-
polar solvents, such as pentane. The structure of this iron(II)
The chloro- and methyl-substituted complexes [Fe-
(salanCl)(Py)2] and [Fe(salanMe)(Py)2] are soluble in acet-
onitrile. In contrast, the complexes [Fe(salanH)(Py)2] and
[Fe(salanF)(Py)2], when dissolved in acetonitrile at room
temperature, form a white precipitate within minutes. This
white product can be isolated but is extremely air sen-
sitive and very insoluble; only dissolving in strongly coordi-
nating solvents, such as pyridine. When a solution of
complex [Fe(salanH)(Py)2] in toluene was layered with
acetonitrile, white crystals formed afterseveral days, which
were analyzed by X-ray crystallography as a dinuclear
phenoxide-bridged complex [Fe(salanH)]2 (Scheme 1). It
appears that decoordination of the pyridine ligands takes
place in acetonitrile, and the formation of a dinuclear
phenoxide-bridged complex [Fe(salanH)]2 occurs, rather
than the formation of a bis(acetonitrile) complex [Fe-
(salanH)(CH3CN)2], analogous to the ruthenium(II) com-
plex [Ru(salantBu,tBu)(CH3CN)2] which we have prepared
separately (vide infra).
Dinuclear phenoxide-bridged iron complexes are not
uncommon, and several complexes containing salen-type
(47) Niswander, R. H.; Martell, A. E. Inorg. Chem. 1978, 17, 2341–2344.
(48) Wiznycia, A. V.; Desper, J.; Levy, C. J. Inorg. Chem. 2006, 45, 10034–
10036.
(49) Scheringer, C.; Hinkler, K.; von Stackelberg, M. Z. Anorg. Allg. Chem.
1960, 306, 35–38.
(50) Gerloch, M.; Mabbs, F. E. J. Chem. Soc. 1967, A, 1900–1908.
(51) Smith, A. L.; Day, C. S.; Que, L., Jr.; Zhou, Y.; Bierbach, U. Inorg.
Chim. Acta 2007, 360, 2824–2828.
(52) Brandts, J. A. M.; Janssen, M. D.; Hogerheide, M. P.; Boersma, J.;
Spek, A. L.; van Koten, G. Inorg. Chim. Acta 1999, 291, 326–332.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11111
Scheme 2
complex is presumed to be tetrahedral, similar to the
tetrahedral iron(II) complexes with bidentate salicylaldimine
ligands46,53 and the analogous tetrahedral zinc(II) complex
[Zn(salantBu,tBu)], which was recently reported.54 The reac-
tion of H2salantBu,tBu with two equivalents of KH results
in the dipotassium salt K2salantBu,tBu, which can subse-
quently be reacted with FeCl2 in THF to give the complex
[Fe(salantBu,tBu)(THF)2] (Scheme 2).
Extensions to Fe(III) and Ru(II) have also been inves-
tigated. The reaction of K2salantBu,tBu with FeCl3 in THF
results in the formation of the dark-purple complex [Fe-
(salantBu,tBu)Cl] (see Supporting Information, Figure S9),
which was also reported by Kozak and co-workers.19 The
synthesis of a ruthenium(II) salan complex was carried out
by reacting Na2salantBu,tBu with [RuCl2(p-cymene)]2 in
acetonitrile. This resulted in the isolation of the first
ruthenium(II) salan complex [Ru(salantBu,tBu)(MeCN)2],
which is a diamagnetic yellow solid and was characterized
by NMR, MS, and CHN analysis (Scheme 2).
or a phenoxide-bridged dinuclear complex [Fe(bapenX)]2,
in analogy to the [Fe(salanX)]2 complexes. Subsequent
addition of pyridine results in the formation of a series of
iron(II) complexes [Fe(bapenX)(Py)2], where X = H, Me,
F, and Cl (eq 2). These complexes have been fully char-
acterized by 1H NMR, MS, elemental analysis, and
magnetic susceptibility measurements and, in the case of
[Fe(bapenCl)(Py)2], by X-ray crystallography. All com-
plexes have magnetic moments at room temperature
typical for high-spin iron(II) complexes.
Synthesis of Iron bapen Complexes. Iron(II) complexes
containing the tetradentate dianionic bapenX ligands can
be prepared in a similar fashion as the salanX complexes.
The combination of [Fe{N(SiMe3)2}2] and H2bapenX in
toluene, followed by the addition of pentane, results in a
white precipitate. This intermediate is presumed to be
either a monomeric four-coordinate complex [Fe(bapenX)]
Oxidation Reactions and Reactivity Toward Dioxygen.
The iron(II) salanX and bapenX complexes prepared in
this study were applied as catalysts for the oxidation of
cyclohexane with H2O2 under our standard conditions
(see Supporting Information). No oxidation products
(cyclohexanol or cyclohexanone) were obtained under
these conditions. During the course of these studies, it
was noticed that the iron(II) salan and bapen complexes
are very reactive toward dioxygen. Initially, it was sus-
pected that the salan ligands could be susceptible to
(53) Torzilli, M. A.; Colquhoun, S.; Kim, J.; Beer, R. H. Polyhedron 2002,
21, 705–713.
(54) Dong, Q.; Ma, X.; Guo, J.; Wei, X.; Zhou, M.; Liu, D. Inorg. Chem.
Commun. 2008, 11, 608–611.

11112 Inorganic Chemistry, Vol. 49, No. 23, 2010
Whiteoak et al.
oxidative degradation upon exposure of the complexes to
an oxidant, whereby the C-N bonds are oxidized
(dehydrogenated) to CdN bonds, essentially converting
the salan ligand into a salen ligand as has been seen
previously.17,55 However, when a solution of the complex
[Fe(salanCl)(THF)2] is exposed to air, an oxidation reac-
tion takes place to give the oxo-bridged dinuclear Fe(III)
complex [{Fe(salanCl)}2(μ-O)], which was characterized
by X-ray crystallography (eq 3). Oxo-bridged dinuclear
iron(III) complexes of the type [LFe-O-FeL] are very
common, but this is only the second example featuring
salan ligands.23 Interestingly, on one occasion during the
preparation of [{Fe(salanCl)}2(μ-O)], crystals were ob-
tained from a benzene solution after exposure to air, which
were shown by X-ray analysis to be a tetranuclear mixed
iron(II)/iron(III) complex [{(salanCl)Fe(μ-O,O0-salanCl)-
Fe}2(μ-O)] (see eq 3). This tetranuclear structure, although of
insufficient quality to be analyzed and reported in detail
(see Supporting Information), clearly shows a central core
of the oxo-bridged dinuclear complex [{FeIII(salanCl)}2-
(μ-O)] with two [FeII(salanCl)] units attached by two
bridging phenolate moieties. This tetranuclear complex
can be viewed as a halfway intermediate in the oxidation
pathway starting from the dinuclear iron(II) salan com-
plex toward the oxidized dinuclear iron(III) product
[{Fe(III)(salanCl)}2O] (eq 3).
to air to give the oxo-bridged iron(III) complex [{Fe-
(salanH)(Py)}2(μ-O)] (see eq 5).
In our previous studies on iron(II) complexes with
neutral tetradentate ligands,56 we have shown that the
coordination geometry in solution of these complexes can
be highly dynamic, resulting in temperature-dependent
equilibria between species with different coordination
geometries. Crystallization of the complexes and the deter-
mination of the structure in the solid state provides
important information for their characterization but
gives no information about their behavior in solution.
Additional solution studies by variable temperature NMR
are essential to fully understand the dynamic behavior of
these complexes. In the next sections, we will first discuss
the solid-state structures before discussing their solution
behavior.
Solid-State Structures
The solid-state structure of [Fe(salanH)]2 shows the di-
nuclear complex with the two [Fe(salenH)] units bridged by
one phenoxide donor atom from each ligand (see Figure 3
and Table 2). The geometry at each iron center is slightly
distorted square-based pyramidal [τ = 0.10 at Fe(1) and 0.13
at Fe(2)] with O(18) and O(38) occupying the apical sites at
Fe(1) and Fe(2), respectively. The central Fe2O2 ring adopts a
rhomboid geometry; the ring is flat (the four atoms are
coplanar to better than 0.01 A), the Fe-O bonds are
essentially the same (lying in the narrow range 2.0662(10)
to 2.0726(10) A), and the opposite angles are likewise the
same (those at Fe(1) and Fe(2) are 73.58(4) and 73.60(4)°
respectively, while those at O(1) and O(21) are 106.30(4) and
106.51(4)° respectively).
Linear tetradentate ligands can adopt three coordination
topologies in octahedral metal complexes: cis-R, cis-β, and
trans.57 The two internal amine donors become chiral upon
coordination to themetal center andcan therefore have either
the same or opposite configuration. Assuming that the config-
uration of these two donors remains unchanged, the number
of possible topological isomers increases to five pairs of
enantiomers: cis-R (R*,R*), cis-β (R*,R*) or cis-β (R*,S*),
and trans (R*,R*) or trans (R*,S*), whereby R* and S*
denote the relative configuration at the internal donor
atoms.56 From molecular models it is immediately evident
that the cis-R (R*,S*) topology is not possible due to severe
bite-angle strain within the complex. Whichever topology a
linear tetradentate ligand will adopt preferentially in a metal
complex is not easily predicted and depends on a number of
Exposure of the complexes [Fe(salantBu,tBu)(THF)2]
and [Fe(salantBu,tBu)] to air resulted in both cases in the
formation of the known oxo-bridged dinuclear Fe(III)
complex [{Fe(salantBu,tBu)}2(μ-O)], which was character-
ized by X-ray crystallography (see eq 4 and Supporting
Information, Figure S5). This oxo-bridged complex was
previously reported by Glaser and co-workers, prepared
via a different route from the Fe(III) precursor Fe(NO3)3,
H2salantBu,tBu and NEt3.23
The complex [Fe(salanH)(Py)2], containing the stronger
coordinating pyridine ligands, also oxidizes upon exposure
€
€
(55) Bottcher, A.; Elias, H.; Jager, E.-G.; Langfelderova, H.; Mazur, M.;
€
(56) England, J.; Davis, C. R.; Banaru, M.; White, A. J. P.; Britovsek,
G. J. P. Adv. Synth. Catal. 2008, 350, 883–897.
(57) Gavrilova, A. L.; Bosnich, B. Chem. Rev. 2004, 104, 349–383.
Muller, L.; Paulus, H.; Pelikan, P.; Rudolph, M.; Valko, M. Inorg. Chem.
1993, 32, 4131–4138.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11113
Figure 3. The molecular structure of complex [Fe(salanH)]2.
Table 2. Selected Bond Lengths (A) and Angles (°) for [Fe(salanH)]2
complexes containing salan ligands.60,61 Therefore, in the case
of the octahedral [Fe(salanX)(Py)2] complexes, the trans (R*,
R*) topology was expected. Surprisingly, in the solid-state
structure of [Fe(salanCl)(Py)2], the tetradentate ligand adopts
the cis-R (R*,R*) coordination mode, with the two pyridine
ligands being coordinated cis to each other (Figure 4 and
Table 3). As will be seen in the next section, this is not the only
topology observed in solution at room temperature, but it
appears to be the one that crystallizes preferentially.
The order of stability for tetradentate [2,2,2]-type ligands
such as the bapen ligands used here, is as follows: cis-R
(R*,R*) > cis-β (R*,R*) > cis-β (R*,S*) > trans (R*,R*) .
trans (R*,S*).57 The cis-R (R*,R*) topology is indeed the only
coordination mode observed for bapen ligands so far.35-37 In
the case of complex [Fe(bapenCl)(Py)2], the molecular structure
in the solid state shows the expected cis-R (R*,R*) topology
(Figure 5 and Table 4). However, as will be seen in the next
section, the cis-R(R*,R*) topology is not the only one present in
solution, especially at elevated temperatures.
The molecular structure of the iron(III) oxo-bridged com-
plex [{Fe(salanCl)}2(μ-O)] is shown in Figure 6. The structure
of [{Fe(salanCl)}2(μ-O)] was found to be severely disordered
with two orientations being identified for every atom except
for the iron and the bridging oxygen atoms (see Figure S4,
Supporting Information). The disorder is likely to be due to
the presence of a mixture of the trans (R*,S*) and the trans
(R*,R*) topologies. Such mixtures of isomers in the solid
state were also seen in the structure of [Fe(salanMe,tBu)Cl].20
As a consequence of the disorder, little can be said about this
structure of [{Fe(salanCl)}2(μ-O)], other than that the com-
plex is the oxo-bridged dinuclear complex shown in Figure 6,
with a trans-coordination mode for the salanCl ligands, and
that the coordination geometry at the iron center is distorted
square-based pyramidal (τ = 0.29), with the bridging oxygen
in the apical position and the two salenCl ligands in an anti
Fe(1)-O(1)
Fe(1)-N(11)
Fe(1)-O(21)
Fe(2)-O(21)
Fe(2)-N(31)
2.0693(10) Fe(1)-N(8)
2.2172(12) Fe(1)-O(18)
2.0702(10) Fe(2)-O(1)
2.0662(10) Fe(2)-N(28)
2.2343(13) Fe(2)-O(38)
2.2369(12)
1.9153(11)
2.0726(10)
2.2264(12)
1.9257(11)
O(1)-Fe(1)-N(8)
86.31(4)
O(1)-Fe(1)-N(11) 148.76(5)
O(1)-Fe(1)-O(18) 118.52(5)
N(8)-Fe(1)-N(11) 80.17(4)
N(8)-Fe(1)-O(21) 142.53(4)
N(11)-Fe(1)-O(21) 100.86(4)
O(1)-Fe(1)-O(21)
73.58(4)
N(8)-Fe(1)-O(18) 108.66(5)
N(11)-Fe(1)-O(18) 92.55(5)
O(18)-Fe(1)-O(21) 108.70(5)
O(1)-Fe(2)-N(28) 141.80(4)
O(1)-Fe(2)-O(38) 107.21(5)
O(21)-Fe(2)-N(31) 149.64(4)
N(28)-Fe(2)-N(31) 80.12(4)
N(31)-Fe(2)-O(38) 92.78(4)
Fe(1)-O(21)-Fe(2) 106.51(4)
O(1)-Fe(2)-O(21)
73.60(4)
O(1)-Fe(2)-N(31) 100.72(4)
O(21)-Fe(2)-N(28) 86.59(4)
O(21)-Fe(2)-O(38) 117.50(4)
N(28)-Fe(2)-O(38) 110.90(5)
Fe(1)-O(1)-Fe(2) 106.30(4)
factors, including the number of atoms between the donors,
the solvent, and the temperature. In addition, if complexes are
labile, equilibria can exist between different topological iso-
mers in solution, which will be concentration and tempera-
ture dependent. The topology of the isomer observed by X-ray
crystallography in the solid state is only partly representative
of what is present in solution. This was previously shown to
be the case for other high-spin iron(II) complexes containing
tetradentate bis(pyridylmethyl)diamine and bis(quinolyl)-
diamine ligands.56,58,59
The order of stability for tetradentate [3,2,3]-type ligands,
such as the salan ligands used here (the numbers refer to the
number of bridging atoms between the donor atoms), is
generally as follows: trans (R*,R*) > cis-β (R*,R*) > cis-β
(R*,S*) > trans (R*,S*) . cis-R (R*,R*).57 The trans (R*,R*)
topology is observed in several five- and six-coordinate Fe(III)
ꢀ
(58) Mas-Balleste, R.; Costas, M.; van den Berg, T.; Que, L., Jr. Chem.;
Eur. J. 2006, 12, 7489–7500.
(59) England, J.; Britovsek, G. J. P.; Rabadia, N.; White, A. J. P. Inorg.
Chem. 2007, 46, 3752–3767.
(60) Lanznaster, M.; Neves, A.; Bortoluzzi, A. J.; Assumpcao, A. M. C.;
Vencato, I.; Machado, S. P.; Drechsel, S. M. Inorg. Chem. 2006, 45, 1005–1011.
(61) Setyawati, I. A.; Rettig, S. J.; Orvig, C. Can. J. Chem. 1999, 77, 2033–
2038.

11114 Inorganic Chemistry, Vol. 49, No. 23, 2010
Whiteoak et al.
Table 4. Selected Bond Lengths (A) and Angles (°) for [Fe(bapenCl)(Py)2]
Fe-O(1)
Fe-N(10)
Fe-N(21)
2.0189(12) Fe-N(7)
2.3183(14) Fe-O(16)
2.1874(14) Fe-N(31)
2.2798(14)
2.0147(11)
2.2086(15)
O(1)-Fe-N(7)
O(1)-Fe-O(16)
O(1)-Fe-N(31)
N(7)-Fe-O(16)
N(7)-Fe-N(31)
N(10)-Fe-N(21)
O(16)-Fe-N(21)
N(21)-Fe-N(31)
79.30(5)
170.24(5)
97.75(5)
93.12(5)
97.65(5)
95.90(5)
94.59(5)
88.11(5)
O(1)-Fe-N(10)
93.69(5)
92.40(5)
80.04(5)
170.43(5)
78.83(5)
O(1)-Fe-N(21)
N(7)-Fe-N(10)
N(7)-Fe-N(21)
N(10)-Fe-O(16)
N(10)-Fe-N(31) 167.72(5)
O(16)-Fe-N(31) 89.30(5)
Figure 4. The molecular structure of the C2-symmetric complex
[Fe(salanCl)(Py)2].
Table 3. Selected Bond Lengths (A) and Angles (°) for [Fe(salanCl)(Py)2]
Fe-O(1)
Fe-N(11)
2.0027(16) Fe-N(8)
2.2460(18)
2.2859(18)
Figure 6. The molecular structure of the Ci-symmetric complex
[{Fe(salanCl)}2(μ-O)].
O(1)-Fe-N(8)
85.24(6)
175.46(9)
91.05(7)
78.46(9)
O(1)-Fe-N(11)
O(1)-Fe-N(8A)
N(8)-Fe-N(11)
85.73(7)
98.30(7)
168.98(7)
O(1)-Fe-O(1A)
O(1)-Fe-N(11A)
N(8)-Fe-N(8A)
N(8)-Fe-N(11A) 96.62(7)
N(11)-Fe-N(11A) 89.86(10)
Figure 7. The molecular structure of complex [{(salanCl)Fe(μ-O,O0-
salanCl)Fe}2(μ-O)].
(salanH)(Py)2] and dioxygen, revealed the formation of a
C2-symmetric oxo-bridged dinuclear complex (Figure 8 and
Table 5). The C2 axis passes through the bridging oxygen
atom and bisects the Fe-O-FeA angle. The geometry at the
iron center is distorted octahedral with cis angles in the range
77.49(6)-99.66(6)° and with the salenH ligand coordinated in
a cis-R (R*,R*) fashion. The angle at the bridging oxygen
atom is 175.68(12)°.
Figure 5. The molecular structure of complex [Fe(bapenCl)(Py)2].
conformation with respect to each other. The Fe-O distance
is 1.7793(8) A, and the Fe-O-FeA angle is 180°.
The single crystal X-ray diffraction data for [{(salanCl)-
Fe(μ-O,O0-salanCl)Fe}2(μ-O)] are of insufficient quality to
justify a detailed analysis of the structure (see Supporting
Information). However, the structural motif as shown in
Figure 7 is believed to be correct and has been included in this
report because it provides an interesting insight into the
stepwise nature of the oxidation of dinuclear iron(II) com-
plexes to oxo-bridged dinuclear iron(III) complexes.
NMR Spectroscopy
All iron(II) complexes reported here containing salan and
bapen ligands are high-spin complexes at room temperature,
giving rise to paramagnetic NMR spectra. The combination
of the weak-field amine and phenoxide donors and the
X-ray analysis of crystals of the product [{Fe(salanH)-
(Py)}2(μ-O)], obtained from the reaction between [Fe-

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11115
Figure 9. VT 19F NMR spectra of complex [Fe(salanF)(Py)2] in
d8-toluene.
previously in several octahedral iron(III) salan com-
plexes, where the formation of the trans (R*,R*) topology
is not possible either through the formation of dinuclear
complexesofthetype[LFe(III)-(μ-OR)2-LFe(III)]14,16-18 or
by using an additional bidentate ligand.20 At 293 K, a
third species is observed at -63 ppm, which is assigned as
the cis-R (R*,R*) isomer. Heating the toluene solution to
353 K results in a single broad peak, which most likely
represents an average of the three geometries, due to a rapid
interconversion between the three topologies, as illustrated
in Scheme 3. Alternatively, the formation of a stable five-
coordinate complex [Fe(salanF)(Py)] at higher temperatures
cannot be excluded at this stage, although the chemical
shift for this complex would be expected to be different.
The mechanism for the interconversion between the
different isomers is believed to involve the dissociation of
one of the pyridine ligands. This possible dissociation of a
pyridine ligand was investigated by measuring the VT 19F
NMR spectra of complex [Fe(salanF)(Py)2] in d5-pyridine
over the temperature range from 243 to 343 K. Only one
single peak is observed which shifts from -57 ppm at
243 K to -79 ppm at 343 K due to Curie behavior
(see Supporting Information, Figure S12). These chemi-
cal shift values are essentially the same as those for the
major species observed in d8-toluene (see Figure 9), and
the complex present in d5-pyridine is therefore most likely
the isomer with the C2-symmetric trans (R*,R*) topology.
The presence of a large excess of pyridine suppresses the
formation of other isomers, which strongly suggests that
the interconversion between the different isomers observed
in toluene involves the dissociation of at least one of the
pyridine ligands.
Figure 8. The molecular structure of the C2-symmetric complex
[{Fe(salanH)(Py)}2(μ-O)].
Table 5. Selected Bond Lengths (A) and Angles (°) for [{Fe(salanH)(Py)}2(μ-O)]
Fe-O
Fe-N(8)
Fe-O(18)
1.7840(3)
2.3981(16) Fe-N(11)
1.9643(13) Fe-N(21)
Fe-O(1)
1.9560(13)
2.2259(15)
2.2255(16)
O-Fe-O(1)
92.89(6)
95.04(6)
94.94(6)
95.26(6)
91.15(6)
86.84(6)
O-Fe-N(8)
169.77(7)
O-Fe-N(11)
O-Fe-N(21)
O(1)-Fe-N(11)
O(1)-Fe-N(21)
N(8)-Fe-O(18)
O-Fe-O(18)
99.66(6)
81.00(5)
167.32(6)
77.49(6)
93.37(6)
O(1)-Fe-N(8)
O(1)-Fe-O(18)
N(8)-Fe-N(11)
N(8)-Fe-N(21)
N(11)-Fe-O(18) 85.54(6)
O(18)-Fe-N(21) 85.95(6)
N(11)-Fe-N(21) 167.84(6)
Fe-O-FeA 175.68(12)
two stronger field pyridine ligands does not provide a strong
enough ligand field to generate low-spin complexes or spin-
crossover behavior. In this study, VT 1H and 19F NMR spec-
troscopy has been used to investigate the geometries of the
iron(II) salan and bapen complexes in solution.
Fe(II) Complexes Containing Salan Ligands. The VT
19F NMR spectra for complex [Fe(salanF)(Py)2], recorded
in d8-toluene, show a major signal at 273 K at -64 ppm
and a minor signal at -50 ppm ((Figure 9). The major
signal shifts to a lower field at lower temperatures due to
Curie behavior. This signal is tentatively assigned to the
isomer with the C2-symmetric trans (R*,R*) topology,
where the pyridine ligands are in trans position to each
other. Although the solid-state structure of the analogous
chloro-substituted complex [Fe(salanCl)(Py)2] showed the
cis-R (R*,R*) topology, this assignment is preferred for
several reasons. For [3,2,3]-type tetradentate ligands, the
most stable coordination mode is generally the trans (R*,R*)
topology,57 and this is also the topology observed in the
solid-state structure of a related Fe(III) salan complex.61
Furthermore, in the case of the less-constrained complex
reported by van Koten and co-workers with two biden-
tate aminomethylphenoxide ligands, the pyridine ligands
are trans to each other.52 The minor peak observed at -50
ppm, which turns into two signals at lower temperatures,
can be assigned to the unsymmetrical cis-β (R*,R*) iso-
mer. The cis-β (R*,R*) coordination mode has been seen
Assuming that the solution behavior for [Fe(salanF)-
(Py)2] is representative for the other complexes of the type
[Fe(salanX)(Py)2], these complexes will all exist as mix-
tures of the three isomers trans (R*,R*), cis-β (R*,R*),
and cis-R (R*,R*) in solution. In the case of complex
[Fe(salanCl)(Py)2], the cis-R (R*,R*) isomer crystallized
preferentially to the other isomers under the conditions
used, and this was the structure determined in the solid
state (Figure 4 ). It is interesting to note that neither the
trans (R*,S*) nor the cis-β (R*,S*) topology appears to be

11116 Inorganic Chemistry, Vol. 49, No. 23, 2010
Whiteoak et al.
Scheme 3
Figure 10. VT 1H NMR spectra of complex [Fe(salenF)(Py)2] in
d8-toluene.
Figure 11. VT 19F NMR spectra of complex [Fe(bapenF)(Py)2] in
d8-toluene.
present. Interconversion between (R*,R*) and (R*,S*)
isomers would require an inversion of the configuration
at the amine donors, which does not appear to occur for
these salan complexes.
All possible topologies and their interconversions are
illustrated in Scheme 4.
The VT 19F NMR spectra of complex [Fe(bapenF)-
(Py)2] have also been measured in d5-pyridine over the
temperature range from 243 to 343 K. A single peak is
observed which shifts from -10 ppm at 243 K to -47 ppm
at 343 K due to Curie behavior (see Supporting Informa-
tion, Figure S13). These chemical shift values are essen-
tially the same as those for the major species observed in
d8-toluene (see Figure 11). The only complex observed in
d5-pyridine is therefore the isomer with cis-R (R*,R*)
topology. The presence of a large excess of pyridine pre-
vents the formation of other isomers, and we conclude
that for both the iron(II) salan and bapen complexes, the
interconversion between different isomers in toluene
occurs via a dissociative mechanism involving the dis-
sociation of one of the pyridine ligands. The presence of both
(R*,R*) and (R*,S*) isomers in the case of this bapen
complex suggests that an inversion of configuration at
the nitrogen donor centers can occur. It is possible that
the weaker basicity of anilines versus amines renders inver-
sion of configuration more facile for the aniline nitrogen
donors in the bapen ligands compared to the amine nitrogen
donors in the salan ligands.
The 1H NMR spectrum of [Fe(salanF)(Py)2] in
d8-toluene at room temperature is complicated due to
existence of several isomers (see Figure 10). However, at
213 K, the spectrum becomes remarkably simple, show-
ing ca. eight signals, as expected for the C2-symmetric trans
(R*,R*) isomer. Also at higher temperature (333 K), the
spectrum appears to simplify due to a fast interconversion
between the three different isomers.
Fe(II) Complexes Containing Bapen Ligands. In the
VT 19F NMR spectra of complex [Fe(bapenF)(Py)2] in
d8-toluene, only one signal is observed below 273 K,
which is tentatively assigned to the cis-R (R*,R*) geometry
(see Figure 11). This is the geometry seen in the solid-state
structure of the analogous chloro-substituted complex
[Fe(bapenCl)(Py)2] (see Figure 5), and it is generally the
most stable geometry for [2,2,2]-type ligands, where the
order of stability is: cis-R (R*,R*) > cis-β (R*,R*) > cis-β
(R*,S*) > trans (R*,R*) . trans (R*,S*).57 Above 273 K,
other isomers appear, initially the two unsymmetrical cis-β
(R*,R*) and cis-β (R*,S*) isomers (giving rise to two
signals each) and further increase of the temperature
reveals another isomer (at ca. -36 ppm at 313 K), most
likely the C2-symmetric trans (R*,R*) isomer. The
Cs-symmetric trans (R*,S*) topology is not observed,
as this is the least stable topology for [2,2,2]-type ligands.
VT-1H NMR spectra in d8-toluene below 273 K show
approximately eight signals, which indicates the presence
of a single isomer, most likely with cis-R (R*,R*) topology
(Figure 12). Some of the signals are rather broad, and full

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11117
Scheme 4
structures obtained from X-ray crystallographic analysis of
crystals grown from solutions of these complexes only
represent one isomer of the mixture present in solution and
not necessarily the major isomer. The iron(II) salan and bapen
complexes have shown no activity as catalysts for the oxida-
tion of cyclohexane with H2O2 as the oxidant under the
conditions used here. This lack of catalytic activity is believed
to be due to the rapid formation of oxo-bridged iron(III)
complexes, which are catalytically inactive. The reaction of
iron(II) salan complexes with O2 has resulted in the isolation
of several oxo-bridged Fe(III) complexes, such as [{Fe-
(salanCl)}2(μ-O)] and [{Fe(salanH)(Py)}2(μ-O)]. We believe
that these oxo-bridged iron(III) complexes are common
catalyst degradation products in many homogeneous iron-
catalyzed oxidation reactions. The prevention of their for-
mation or their regeneration will be investigated in our future
research toward highly active and robust alkane oxidation
catalysts.
Figure 12. VT 1H NMR spectra of complex [Fe(bapenF)(Py)2] in
d8-toluene.
Acknowledgment. We are grateful to EPSRC and to
Unilever for funding and to Johnson Matthey for a
generous loan of RuCl3. We thank Peter Haycock and
Richard Sheppard for their assistance with NMR mea-
surements.
assignment of the spectrum was not attempted. At higher
temperatures, multiple peaks are observed indicating the
presence of other isomers.
Conclusion
Supporting Information Available: Crystallographic data and
selected bond lengths and angles for complexes [{Fe(salantBu,tBu)}-
2(μ-O)] and [Fe(salantBu,tBu)Cl]. Synthetic procedures and analy-
tical data for ligands and metal complexes and VT NMR spectra in
d5-pyridine. This material is available free of charge via the Internet
at http://pubs.acs.org.
In conclusion, we have shown in this study the first
syntheses of iron(II) salan and bapen complexes, which can
be prepared starting from H2salan and H2bapen with [Fe{N-
(SiMe3)2}2]. Dinuclear complexes of the form [Fe(salanX)]2]
are observed in the solid state and in the presence of suitable
donors L, such as pyridine or THF, complexes of the type
[Fe(salanX)(L)2] and [Fe(bapenX)(L)2] can be isolated. The
complexes show a temperature-dependent dynamic behavior
in solution, whereby interconversion between the various
ligand topologies cis-R, cis-β, and trans occurs. The solid-state
Note Added after ASAP Publication. This paper was
published on the Web on November 9, 2010, with a
missing caption for Figure 12. The corrected version was
reposted on November 11, 2010.