Structural and 1H NMR spectroscopic characterization of bis(N-isopropylsalicylaldiminato)iron(II)

Article abstract of DOI:10.1016/S0277-5387(02)00837-9

Coordinatively unsaturated iron(II) species form reactive organometallic and coordination complexes and are integral to the reactivity of non-heme iron proteins and their synthetic analogues. Iron(II) Schiff base complexes have proven to be easily prepared and useful starting materials for such compounds. Here we report a detailed preparative procedure and the solid-state structure of the iron(II) complex of N-isopropylsalicylaldimine (L^iprH) based on the compounds first synthesized by Larkworthy (J. Chem. Soc., A (1968) 1048). The title compound is prepared by adding 2 equiv. of salicylaldehyde (sa1H) to Fe(O₂CCH₃)₂ in KOH-CH₃OH to produce a precursor formulated as Fe(sal)₂ which is reacted subsequently with isopropylamine in THF to form Fe(L^ipr)₂ in 34% isolated yield. A single crystal X-ray crystallographic study of Fe(L^ipr)₂ reveals a mononuclear complex with two bidentate salicylaldiminate ligands bound to an iron(II) atom in a tetrahedral coordination geometry. The ¹H NMR spectrum of Fe(L^ipr)₂ in benzene-d₆ exhibits six paramagnetically shifted ligand resonances ranging from +195 to -31 ppm that are consistent with a mononuclear high spin (S = 2) iron(II) complex in solution. Upon exposure to air, Fe(L^ipr)₂ forms the oxo-bridged dinuclear iron(III) complex [(L^ipr)₂Fe]₂O as shown by ¹H NMR spectroscopy.

Full text of DOI:10.1016/S0277-5387(02)00837-9

Polyhedron 21 (2002) 705ꢀ713
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1
Structural and H NMR spectroscopic characterization of
bis(N-isopropylsalicylaldiminato)iron(II)
Michele A. Torzilli, Shalton Colquhoun, Jin Kim, Robert H. Beer *
Department of Chemistry, Fordham University, Bronx, NY 10458, USA
Received 23 August 2001; accepted 3 December 2001
Abstract
Coordinatively unsaturated iron(II) species form reactive organometallic and coordination complexes and are integral to the
reactivity of non-heme iron proteins and their synthetic analogues. Iron(II) Schiff base complexes have proven to be easily prepared
and useful starting materials for such compounds. Here we report a detailed preparative procedure and the solid-state structure of
the iron(II) complex of N-isopropylsalicylaldimine (Li prH) based on the compounds first synthesized by Larkworthy (J. Chem. Soc.,
A (1968) 1048). The title compound is prepared by adding 2 equiv. of salicylaldehyde (salH) to Fe(O2CCH3)2 in KOHꢀ
produce a precursor formulated as Fe(sal)2 which is reacted subsequently with isopropylamine in THF to form Fe(Lipr)2 in 34%
isolated yield. A single crystal X-ray crystallographic study of Fe(Li pr
reveals a mononuclear complex with two bidentate
/CH3OH to
)
2
salicylaldiminate ligands bound to an iron(II) atom in a tetrahedral coordination geometry. The 1H NMR spectrum of Fe(Li pr)2 in
benzene-d6 exhibits six paramagnetically shifted ligand resonances ranging from ꢁ195 to ꢂ31 ppm that are consistent with a
2) iron(II) complex in solution. Upon exposure to air, Fe(Li pr
forms the oxo-bridged dinuclear
/
/
mononuclear high spin (Sꢃ
/
)
2
iron(III) complex [(Li pr)2Fe]2O as shown by H NMR spectroscopy. # 2002 Elsevier Science Ltd. All rights reserved.
1
Keywords: Iron(II); Zinc(II); Schiff base; X-ray crystal structures; 1H NMR spectroscopy; Dioxygen reactivity
1. Introduction
sulfido [10], peroxo [11] and imido [12] bonds into iron
salicylaldimine compounds. Similarly, widespread re-
cent interest in iron(II) species that possess low coordi-
nation numbers has led to unusual coordination or
The salicylaldimine ligands shown in Scheme 1 are
easy to prepare and synthetically versatile and, accord-
ingly, have been popular ligands for the synthesis of
organometallic compounds [13ꢀ25]. These compounds
/
transition metal complexes [1ꢀ4]. Among metal salicy-
/
frequently exhibit desirable reactions with small mole-
cules, e.g. the activation of dioxygen by non-heme
laldimine compounds, the iron complexes have been
prepared extensively to investigate various aspects of
their magnetic and spectroscopic properties and wide-
ranging bioinorganic, organometallic, and catalytic
chemistry [2,5]. A common approach to prepare these
compounds takes advantage of the reactivity of an
iron(II) salicylaldimine complex, notably Fe(salen), in
concert with the complex being coordinatively unsatu-
rated. By virtue of its ability to be reduced or oxidized as
well as to expand its coordination number, Fe(salen) has
been employed historically to introduce iron nitrosyl [6],
alkyl [7], halide [7], quinone [8], and bridging oxo [9],
iron(II) proteins and their model complexes [26ꢀ28].
/
Iron complexes of the LRH and salen ligands shown
in Scheme 1 figured early in studies of the magnetic,
electronic and Mossbauer properties of iron(II) [5,6,29ꢀ
/
¨
31]. The bis(salicylaldiminato)iron(II) complexes, for-
mulated as Fe(LR)2 (Rꢃ
/
alkyl or aryl), were first
prepared reliably by Larkworthy [6,29]. The compounds
were determined to be high spin (Sꢃ2), but their
/
structure was not confirmed. In fact, a four coordinate
iron(II) complex of salicylaldimine or a related Schiff
base ligand with a N2O2 donor set has yet to be
characterized by single crystal X-ray crystallography
or solution NMR spectroscopy. Herein we describe
explicit details of the synthesis of the iron(II) complex of
N-isopropylsalicylaldimine (LiprH) based on a modifi-
* Corresponding author. Tel.: ꢁ1-718-817-4430; fax: ꢁ1-718-817-
4432.
E-mail address: beer@fordham.edu (R.H. Beer).
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 8 3 7 - 9

706
M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ
/
713
Table 1
Crystal data, data collection, and solution and refinement for Fe(Li pr
)
2
Crystal data
Empirical formula
Crystal habit, color
Crystal size (mm)
Crystal system
C20H24FeN2O2
plate, orange
0.35ꢄ0.30ꢄ0.05
orthorhombic
Pbca
Space group
˚
Scheme 1.
a (A)
˚
15.0650(6)
13.2631(6)
19.3374(9)
3863.8(3)
8
b (A)
˚
cation of the general procedure by Larkworthy [29] and
show that this complex is a soluble discrete mononuclear
complex in the solid state and solution. We also
c (A)
3
V (A )
˚
Z
Formula weight
demonstrate that Fe(Lipr
) reacts readily with air to
2
380.26
1.307
Dcalc (Mg mꢂ3
Absorption coefficient 0.795
)
form the corresponding oxo-bridged dinuclear iron(III)
complex indicating that it is a useful material for
synthetic strategies involving low coordinate iron(II)
salicylaldimine compounds.
(mmꢂ1
F(000)
)
1600
Data collection
˚
Wavelength (A)
Temperature (K)
0.71073
173(2)
u Range for data col- 2.11ꢀ25.04
lection (8)
/
2. Experimental
Index ranges
Reflections collected
Independent reflec-
tions
ꢂ175h 517, ꢂ155k 511, ꢂ225l 522
17 845
2.1. Materials and methods
3395 (Rintꢃ0.0763)
All reagents were used as received from commercial
suppliers. Benzene and tetrahydrofuran were distilled
under pre-purified dinitrogen from sodium benzophe-
none ketyl and stored under dinitrogen. Methanol was
dried and distilled over activated molecular sieves under
dinitrogen and stored under dinitrogen. The preparation
and manipulation of bis(N-isopropylsalicylaldimina-
to)iron(II) was carried out under argon in a glove box
and on a vacuum line using standard inert atmosphere
methods [32]. Care must be taken to ensure all equip-
ment is fully dried and solvents are de-gassed to
successfully prepare bis(N-isopropylsalicylaldimina-
to)iron(II).
Elemental analysis of bis(N-isopropylsalicylaldimina-
to)iron(II) was performed by Desert Analytics (Tuscon,
AZ). Elemental analysis of N-isopropylsalicyaldimine
and bis(N-isopropylsalicylaldiminato)zinc(II) were car-
ried out by QTI (Whitehouse, NJ). Infrared spectra were
recorded on a Mattson Satellite FTIR. The NMR
Solution and refinement
Solution
direct methods
Refinement method
Weighting scheme
full-matrix least-squares on F2
wꢃ[s2(Fo2)ꢁ(AP)2ꢁ(BP)]ꢂ1, where
Pꢃ(Fo2ꢁ2Fc2)/3, Aꢃ0.0001, and
Bꢃ5.2692
Absorption correction semi-empirical
Max/min transmission 0.58808 and 0.53736
Data/restraints/para-
meters
Final R indices
3395/0/254
R1ꢃ0.0645, wR2ꢃ0.0854
R1ꢃ0.0875, wR2ꢃ0.0914
(I ꢀ2s(I)]
R indices (all data)
Goodness-of-fit on F2 1.248
Largest difference
peak and hole (e A
0.229 and ꢂ0.269
ꢂ3
˚
)
ment of bis(N-isopropyisalicyl-aldiminato)iron(II) sum-
marized in Table 1 were carried out by routine methods
using the SHELXTL suite of programs [33]. The ORTEP
diagram and selected bond distances and angles are
given in Fig. 1 and Table 2, respectively. Full crystal-
lographic details are available as supplementary mate-
rial.
A crystal of bis(N-isopropylsalicylaldiminato)iron(II)
was attached to a glass fiber and mounted on the
Siemens SMART CCD platform diffractometer system
for a data collection at 173(2) K. An initial set of cell
constants was calculated from reflections harvested
from three sets of 20 frames. These initial sets of frames
were oriented such that orthogonal wedges of reciprocal
space were surveyed. This produced orientation matrices
determined from 45 reflections. Final cell constants were
calculated from a set of 6495 reflections from the actual
spectroscopic data were obtained on
a
Bruker
AVANCE 300 spectrometer at 23 8C and are reported
as positive shifts upfield of tetramethylsilane. The
numbering scheme for the aromatic ring protons for
the salicylaldimine anion moiety is given in Fig. 2.
2.2. X-ray crystallography of bis(N-
isopropylsalicylaldiminato)iron(II)
Single crystals suitable for X-ray crystallographic
studies were obtained by dissolving bis(N-isopropylsa-
licylaldiminato)iron(II) in tetrahydrofuran under argon
in a Schlenk tube and cooling to ꢂ
several days. The data collection, solution and refine-
/
20 8C in a freezer for

M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ
/
713
707
cular hydrogen bond was observed between H(15) and
˚
O(2) equal to 2.484 A.
2.3. Preparation of compounds
2.3.1. N-iso-propylsalicyaldimine (LiprH)
This compound was synthesized by a generally
accepted method for the preparation of salicylaldimine
ligands [2]. A solution of 16.69 g (0.137 mol) of
salicylaldehyde in 150 ml of benzene was added to a
250 ml round bottom flask fitted with a DeanꢀStark
/
trap and condenser. To this solution, 12 ml (8.3 g, 0.14
mol) of isopropylamine was added slowly with stirring
and then refluxed for 1 h. After cooling, the reaction
mixture was dried over anhydrous magnesium sulfate
and gravity filtered. The filtrate was rotoevaporated and
Fig. 1. Molecular structure of Fe(Li pr
level thermal ellipsoids.
)
2
shown with 50% probability
Table 2
dried under vacuum to yield 16.8 g (75%) of a yellowꢀ
/
i pr
˚
Selected bond distances (A) and angles (8) for Fe(L
)
2
brown oil that could be used without further purifica-
tion. Anal. Calc. for C10H13NO: C, 73.59; H, 8.03; N,
8.58. Found: C, 73.39; H, 7.84; N, 8.44%. FTIR (neat):
3060, 2969, 2869, 1679, 1631(s), 1582, 1499, 1460, 1420,
1385, 1322, 1279, 1210, 1150, 1032, 977, 951, 905, 846,
Coordination sphere
Fe(1)ꢁO(2)
Fe(1)ꢁO(1)
O(1)ꢁFe(1)ꢁN(1)
O(2)ꢁFe(1)ꢁN(2)
O(2)ꢁFe(1)ꢁO(1)
N(1)ꢁFe(1)ꢁN(2)
1.912(2)
1.916(2)
93.4 (1)
93.0(1)
Fe(1)ꢁN(1)
Fe(1)ꢁN(2)
2.040(3)
2.053(3)
114.3(1)
113.3(1)
82.7
O(1)ꢁFe(1)ꢁN(2)
O(2)ꢁFe(1)ꢁN(1)
1
755, 738, 632 cmꢂ1. H NMR (CDCl3) d 13.72 (1H, s,
a
125.8(1)
119.5(1)
f
b
OH), 8.35 (1H, s, Nꢀ
/
CH), 7.26 (2H, m, H3, H4), 6.95
¯
u
6.1, 6.9
(1H, d, H6), 6.86 (1H, t, H5), 3.52 (1H, septet,
NCH(CH3)2, 1.29 (6H, d, NCH(CH3)2) ppm.
Ligand geometry
Ligand 1
Ligand 2
O(2)ꢁC(11)
¯
¯
O(1)ꢁC(1)
1.317(4)
128.5(2)
1.293(4)
1.486(4)
122.0(2)
116.4(3)
121.5(2)
1.444(5)
1.526(5)
1.522(5)
1.396(6)
120.0(4)
1.312(4)
129.4(2)
1.296(4)
1.483(4)
122.1(2)
117.1(3)
120.8(2)
1.442(5)
1.520(5)
1.526(5)
1.398(5)
120.0(4)
C(1)ꢁO(1)ꢁFe(1)
N(1)ꢁC(7)
N(1)ꢁC(8)
C(7)ꢁN(1)ꢁFe(1)
C(7)ꢁN(1)ꢁC(8)
C(8)ꢁN(1)ꢁFe(1)
C(6)ꢁC(7)
C(8)ꢁC(9)
C(8)ꢁC(10)
CꢁC(ring)avg
CꢁCꢁC(ring)avg
C(11)ꢁO(2)ꢁFe(1)
N(2)ꢁC(17)
2.3.2. Bis(N-isopropylsalicylaldiminato)iron(II)
Fe(Lipr
)
2
N(2)ꢁC(18)
This synthetic procedure is a modification of the
preparation by Larkworthy [29]. A solution of 4.21 g
(34.5 mmol) of salicylaldehyde (salH) dissolved in 20 ml
of a 0.14 M KOH methanol solution was added to 3.00
g (17.2 mmol) of Fe(O2CCH3)2 (Aldrich) in 100 ml of
methanol with stirring. A dark red solid formed. The
solvent was decanted after 10 min. The solid product
was dried to yield 2.99 g of a red-purple powder of the
iron(II) salicylaldehyde compound ‘Fe(sal)2’. To 0.474 g
(1.78 mmol) of Fe(sal)2 dissolved in 50 ml of tetrahy-
drofuran, 0.378 g (6.40 mmol) of isopropylamine was
added under argon. The opaque solution became dark
orange and was allowed to stir for 3 h. The solvent was
evaporated under vacuum and the crude product was
dissolved in 300 ml of pentane. A small amount of
product crystallized immediately as orange flakes along
with a substantial amount of a red precipitate, a
presumed hydrolysis product. The solution, which is
extremely sensitive to moisture and dioxygen, was
C(17)ꢁN(2)ꢁFe(1)
C(17)ꢁN(2)ꢁC(18)
C(18)ꢁN(2)ꢁFe(1)
C(16)ꢁC(17)
C(18)ꢁC(19)
C(18)ꢁC(20)
CꢁC(ring)avg
CꢁCꢁC(ring)avg
a
The dihedral angle between the mean planes formed by the atoms
O(1)ꢁFe(1)ꢁN(1) and O(2)ꢁFe(1)ꢁN(2).
The two dihedral angles between the mean plane formed by carbon
b
atoms of the aromatic ring of ligand 1 (C1ꢁC6) and O(1)ꢁFe(1)ꢁN(1)
and the carbon atoms of ligand 2 (C11ꢁC16) and the mean plane
formed by O(2)ꢁFe(1)ꢁN(2).
data collection. A semi-empirical absorption correction
was carried out using the SHELXTL XPREP absorption
routine. The space group Pbca was determined based on
systematic absences and intensity statistics. A successful
direct-methods solution was calculated which provided
most non-hydrogen atoms from the E-map. Several full-
matrix least-squares/difference Fourier cycles were per-
formed which located the remainder of the non-hydro-
gen atoms. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen
atoms were placed in ideal positions and refined as
riding atoms with individual (or group if appropriate)
isotropic displacement parameters. A weak intermole-
filtered and cooled in a freezer (ꢂ20 8C) for 2 days.
/
The crystalline product was filtered and dried under
vacuum to yield 0.23g of orange flakes (34%). The solid
product is comparatively stable when exposed briefly to
air. Anal. Calc. for C20H24N2O2Fe: C, 63.17; H, 6.36; N,
7.37. Found: C, 62.97; H, 6.24; N, 7.24%. FTIR (KBr):
3447 (br), 3050, 2964, 2924, 2889, 1617, 1599 (s), 1537,
1467, 1442, 1402, 1367, 1331, 1315, 1195, 1143, 1122,

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M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ713
/
1031, 976, 919, 855, 786, 763, 756, 740, 612, 578, 493
cmꢂ1. 1H NMR (C6D6, 300 MHz, 23 8C) d 195.4 (1H,
We find, as previously determined, that the reaction of
the primary amine with the pre-formed iron salicylalde-
hyde complex forms the desired pure product in an
acceptable yield. Two equiv. of salicylaldehyde (salH)
are deprotonated in a methanol solution of potassium
hydroxide and added to 1 equiv. of iron(II) acetate as
shown in Scheme 2, Eq. (1). A precipitate forms,
presumed to be bis(salicyaldehydato)iron(II) ‘Fe(sal)2’,
which is dried under vacuum to yield a purple solid.
The 1H NMR spectrum of Fe(sal)2 in CD3CN
exhibits four broad markedly shifted resonances for
s, NCH(CH3)2), 67.7 (1H, s, H4), 62.0 (1H, s, H6), ꢂ
/
¯
15.9 (1H, s, H5), ꢂ
/
21.8 (1H, s, H3), ꢂ30.5 (6H, s,
/
NCH(CH3)2) ppm.
¯
2.3.3. Bis(N-isopropylsalicylaldiminato)zinc(II)
Zn(Lipr
)
2
To a rapidly stirring suspension of 4.17 g (25.6 mmol)
of N-isopropylsalicylaldimine in 15 ml of deionized
water, 1.74 g of ZnCl2 (12.8 mmol) was added slowly. A
10 ml solution of 1.02 g (24.3 mmol) of NaOH dissolved
in deionized water was then added slowly to the slightly
yellow solution to produce a white solid. The hetero-
geneous reaction mixture was stirred for an additional
10 min and filtered. The precipitate was washed with
the ligand aromatic ring protons ranging from ꢁ
16 ppm. This shift range is expected since iron(II)
complexes with phenolate oxygen donors ordinarily
display large upfield and downfield shifts [25,35ꢀ37].
/
45 to
ꢂ
/
/
The shifts are a consequence of the alternating sign of
delocalized unpaired electron spin density from partially
filled iron(II) d p symmetry orbitals into ligand p
deionized water (3ꢄ
/
5 ml) followed by pentane (3ꢄ5
/
ml) to remove any unreacted ligand. The product was
dried under vacuum to yield 4.12 g of a white powder
orbitals [38ꢀ41]. As a result, alternating upfield and
/
1
(83%). The product can be recrystallized from acetoneꢀ
/
downfield H NMR shifts are observed for one proton
attached to a ring carbon atom to the next. On this basis
and by analogy to the assignments of other iron(II)
phenolate complexes, the 1H NMR resonances of
water. Anal. Calc. for C20H24N2O2Zn: C, 61.62; H, 6.21;
N, 7.19. Found: C, 61.95; H, 6.10; N, 7.00%. FTIR
(KBr): 3051, 2966, 2926, 2901, 1616 (s), 1537, 1466,
1445, 1408, 1333, 1316, 1191, 1145, 1123, 1032, 999, 959,
918, 857, 759, 603, 505, 449 cmꢂ1. 1H NMR (CDCl3) d
Fe(sal)2 can be assigned to 6H(ꢁ
/
44.8), 4H(ꢁ35.3),
/
5H(ꢂ10.4) and 3H(ꢂ16.0) in accord with the ring
/
/
8.35 (1H, s, Nꢀ
/
CH), 7.29 (1H, t, H4), 7.09 (1H, d, H3),
¯
proton numbering scheme for salicylaldimine given in
Fig. 2.
Early magnetic studies of Fe(sal)2 suggest it is a high
6.85 (1H, d, H6), 6.59 (1H, t, H5), 3.63 (1H, m,
NCH(CH3)2), 1.33 (3H, d, NCH(CH3)2); 1.21 (3H, d,
¯
¯
NCH(CH3)2) [note: the NCH(CH3)2 methyl groups are
¯
spin iron(II) (Sꢃ2) complex [29], otherwise very little is
/
¯
diastereotopic] ppm.
known about this compound. The most closely related
crystallographically characterized salicylaldehyde metal
complex is the dinuclear manganese(II) compound,
Mn2(sal)4(CH3OH)2 [42], in which phenolate oxygen
atoms from two of the salicylaldehyde anions bridge the
3. Results and discussion
1
3.1. Synthesis
manganese centers. Our H NMR studies suggest that
‘Fe(sal)2’ is mononuclear in solution (though perhaps
solvated) since only one set of ring proton resonances is
observed for what we presume to be terminally coordi-
nated salicylaldehydato ligands.
A general method for the reliable synthesis of
bis(salicylaldiminato)iron(II) complexes has been de-
scribed by Larkworthy [6,29]; these compounds had
been prepared prior to this study, but because of their
acute air sensitivity the complexes frequently yielded
impure oxidized products. As indicated by Larkworthy
[29], the optimal procedure for the preparation of
bis(salicylaldiminato)iron(II) complexes was the forma-
tion of a precursor from salicylaldehyde and iron(II)
acetate in the presence of a strong base followed by the
addition of a primary amine in alcohol solvents [34]. No
preparative details, e.g. amounts of material, yields, etc.,
however, are given in Larkworthy’s or subsequent
The condensation of excess isopropylamine with the
coordinated aldehyde moiety in the complex ‘Fe(sal)2’ in
tetrahydrofuran produced a dark orange solution from
which orange microcrystalline Fe(Lipr
) could be iso-
2
lated in 34% yield (Eq. (2)). While solutions of Fe(Lipr
)
2
are very susceptible to decomposition in air, the solid
compound could be handled briefly in air. The com-
papers [6,12,29ꢀ31]. We have re-examined this synthetic
/
method to render a more detailed experimental proce-
dure for the preparation of the N-isopropylsalicylaldi-
mine derivative, Fe(Lipr)2.
The reaction of the N-isopropylsalicylaldimine with
iron(II) acetate in tetrahydrofuran failed to produce
1
pure product as indicated by H NMR spectroscopy.
Scheme 2.

M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ
/
713
709
Fig. 2. The 1H NMR spectrum of Fe(Li pr
) in benzene-d6 at 23 8C. Chemical shift (d) in ppm downfield of tetramethylsilane. The schematic
2
indicates the proton assignments (inset). The symbol (ꢄ) indicates solvent or residual solvent impurity.
pound is very soluble in non-polar solvents. The
solubility behavior is in marked contrast to its closest
analogue, Fe(salen), which requires donor solvents to
dissolve due to its assumed polymeric structure [35,43].
Though no four coordinate iron(II) salicylaldimine
complex has been structurally characterized previously,
the structure of Fe(Lipr)2 and the Zn analog [46,47] are
most comparable while those of the other first row d
block analogs Co, Ni, and Cu deviate somewhat more
significantly [48]. No simple trend is obvious and their
differences are likely due to the both the nature of the
For comparative purposes the Zn(Lipr
) analog, a
2
discrete mononuclear tetrahedral complex [45ꢀ47], was
/
also prepared. This straightforward synthesis employed
ZnCl2, N-isopropylsalicylaldimine and sodium hydrox-
ide, a general preparative method for zinc(II) N-
alkylsalicylaldimines that generally gives good yields in
metalꢀ
/
ligand bonding, the electronic structure of the
metal atoms and the structural requirements of the
˚
ligand. The average Znꢁ
/
O (1.913 A) and Feꢁ
/
O (1.914
our hands [44]. Both compounds, M(Lirp)2 (Mꢃ
Fe, Zn)
/
˚
A) bond lengths are indistinguishable compared with the
˚
˚
˚
have very similar solubility characteristics and solid-
state infrared spectra, indicative of an analogous
structure. The most intense infrared stretch, assigned
Coꢁ
bond distances. The Znꢁ
only slightly shorter than the corresponding Feꢁ
/
O (1.905 A), Niꢁ
/
O (1.896 A) and Cuꢁ
/
O (1.805 A)
˚
/
N (2.009 A) bond distance is
/N
˚
˚
typically to the imine Cꢀ
/
N bond, is found at 1599 (Fe)
and 1616 (Zn) cmꢂ1. The value for the energy of the
stretch in the iron complex is in excellent agreement with
(2.047 A) distance while those of Co (1.992 A), Ni
˚
(1.970 A), and Cu (1.979 A) are considerably shorter.
˚
The average ligand bite angles (OꢁMꢁN) for Fe (93.28),
/
/
the Cꢀ
hedral iron(II) complex of the nitrogen analogue t-
BuNꢀ
C(H)C6H4NHꢂ of the salicylaldiminate ligand
[18].
/
N stretch (1603 cmꢂ1) reported for the tetra-
Co (96.48), Ni (94.48), Cu (94.98) and Zn (96.78) are
largely invariant due to the constraints of the ligand.
/
Likewise, the corresponding interligand angles (Nꢁ
/
Mꢁ
/
Oavg, NꢁMꢁN, OꢁMꢁO) of the Fe complex (113.88,
/
/
/
/
119.58, 125.88), Co (112.38, 122.68, 118.58), Ni (112.48,
120.98, 125.18), and Zn complex (112.68, 117.28, 122.98)
diverge only slightly, while the Cu angles (100.38, 137.98,
137.28) are markedly different because of its very large
distortion from tetrahedral geometry. Overall tetrahe-
dral geometry as determined by the dihedral angle, f,
between the planes formed by the metal atom and
3.2. Structure determination by X-ray crystallography
The structure of Fe(Lipr)2 shown in Fig. 1 displays a
four coordinate iron atom coordinated to two bidentate
N-isopropylsalicylaldimine anions. The bond distances
and angles provided in Table 2 reveal relatively short
˚
corresponding ligand donor atoms, e.g. N(1)ꢁ
/
Fe(1)ꢁ
/
Feꢁ
/
O (1.912(2), 1.916(2) A) and Feꢁ
/
N (2.040(3),
˚
O(1), as well as a measure of the deviation of these
atoms from the plane formed by ligand aromatic ring
2.053(3) A) bond distances and a distorted tetrahedral
coordination geometry most likely due to the acute Oꢁ
FeꢁN bite angles (93.4(1)8, 93.0(1)8). Metrical para-
/
(dihedral angle u), are most similar for the Fe (fꢃ
82.78, uavg 6.58) and Zn (fꢃ84.28, uavg 3.98) com-
plexes, whereas Co, Ni (fꢃ948), and Cu (fꢃ608) are,
again, less comparable.
/
/
ꢃ
/
/
ꢃ
/
meters of the ligand, also given in Table 2, indicate that
the ligand is coordinated to iron in its characteristic
phenolate-imine form [2]. Overall ligand geometry is
very similar to generally accepted norms for salicylaldi-
mine ligands in this coordination mode [1,2].
/
/
The most closely related structurally characterized
tetrahedral iron(II) complex is that of the salicylaldi-

710
M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ713
/
1
3.3. Characterization by H NMR spectroscopy
mine nitrogen analogue, t-BuNꢀ
/
C(H)C6H4NHꢂ [18].
˚
The average iron imine nitrogen distance of 2.061 A in
this complex is nearly equivalent to the corresponding
distance in Fe(Lipr)2. The remaining structural para-
meters of its coordination sphere, while not as similar to
Fe(Lipr)2 as Zn(Lipr)2, are characteristic of a bidentate
monoanionic ligand bound to iron(II) in a tetrahedral
coordination geometry. Other similar iron nitrogen
imine distances include those of tetrahedral amidinato
1
The H NMR spectrum of Fe(Lipr)2 shown in Fig. 2
exhibits broad ligand resonances that experience large
upfield and downfield shifts ranging from ꢁ
/
195 to ꢂ31
/
ppm resulting from the paramagnetism of the complex
[29]. The observed peaks and their assignment are given
in Table 3. The shifts and linewidths of the peaks at
67.7, 62.0, ꢂ
those observed previously for high spin iron(II) com-
plexes [Fe(salen)X]ꢂ (Xꢃ
anion) [35] and are assigned
/
15.9 and ꢂ
/
21.8 ppm in Fig. 2 are similar to
˚
FeCl2 adducts (2.100 A) [24] and the tropocoronand N4
˚
macrocycle (2.020 A) [16].
/
An obvious comparison, that of Feꢁ
/
O bond distances
accordingly to the aromatic ring protons H4, H6, H5,
and H3. The observation of one set of ligand resonances
of low coordinate mononuclear complexes (ranging
from three to five) with terminal phenolate ligands is
more problematic. The number of these complexes is
for the ring protons in Fe(Lipr
) suggests that, like
2
Fe(sal)2, there is a single static ligand environment
consistent with a mononuclear complex (however, we
cannot exclude that a rapidly averaged environment is
present in either complex).
limited and typically have shorter Feꢁ
/
O bond distances
than observed in Fe(Lipr)2, for example, the three
coordinate homoleptic phenoxide [Fe{O(2,4,6-t-
˚
BuC6H2)}2]2 (1.822 A) [25], four coordinate trispyrazo-
lylborate phenoxide HB{(3,5-ipr)pz}3Fe(OC6F5) (1.876
˚
A) [49], and a five coordinate iron calixarene adduct of
The large peak observed at ꢂ30.5 ppm in Fig. 2 is
/
assigned to the isopropyl methyl protons based on its 6:1
relative intensity compared with each of the aromatic
ring protons. The methyl protons, which are diaster-
˚
THF (1.895 A) [17].
1
One can examine the iron ligand bond distances of the
four coordinate Fe(Lirp
eotopic in our H NMR spectrum of Zn(Lipr)2 [45,53]
)
complex reported here and
(see Scheme 3), are either not resolved in the spectrum of
Fe(Lipr)2 or its chirality is lost due ligand exchange or
inversion through a square planar intermediate [54]. The
anomalously large linewidth (700 Hz) may indicate the
former explanation.
2
corresponding examples of five and six coordinate
neutral high spin iron(II) salicylaldimine complexes,
i.e. ‘salen type’ ligands with additional donors, N,N?-
(3,3?-dipropylmethylamine)bis-(salicylideneimine)
(a
pentadentate ligand that incorporates one amine donor)
[50] and 1,10-bis(5-nitrosalicylaldehyde)-1,4,7,10-tetraa-
zadecane (a hexadentate ligand with two supplementary
amines) [51]. The average iron donor bond distances
The remaining resonance at 195.4 ppm is assigned
tentatively to the tertiary hydrogen of the isopropyl
group. Hydrogen atoms attached to carbon atoms
adjacent to tertiary amines bound to iron(II) have
been observed to experience shifts greater than 100
(Feꢁ
/
N, Feꢁ
/
O) in this series from coordination number
six to four are (2.127, 2.093), (2.045, 2.041), (1.953,
˚
1.914) A, respectively. Neglecting the differences be-
ppm, i.e. in [FeM(BPMP)(L)2]ꢁ (Mꢃ
BPMPꢃ2,6-bis[[bis(2-pyridylmethylamino]methyl]-4-
methylphenol anion; Lꢃ
O2CRꢂ or O2P(OR)2ꢂ) [37]
and (TMEDA)Fe(CH2Ph)2 (TMEDAꢃN,N,N?,N?-tet-
ramethyl-ethylenediamine) [23]. The imine hydrogen,
N, resonance has not been reported previously in
/
Fe, Zn, Ga,
/
tween the ligands, the change in iron(II)-donor bond
distance follows the predictable trend of decreasing
bond length with decreasing coordination number.
Early studies of bis(salicylaldiminato)iron(II) or re-
/
/
HCꢀ
/
¯
lated complexes, which included electronic and Moss-
¨
1H NMR studies of iron(II) Schiff base compounds
[35,36], so its absence is predictable. Since the proton of
interest is two bonds away from the iron atom in the
delocalized, conjugated imine moiety of the salicylaldi-
minate ligand, a large proportion of electron spin
density would presumably broaden this resonance
beyond detection.
bauer spectroscopic measurements as well as magnetic
measurements, suggested that the compound was either
polymeric or monomeric with a coordination number
ranging from four to six [6,29ꢀ31,52]. These conclusions
/
are reasonable given the tendency for coordinatively
unsaturated Schiff base complexes to add donors as well
as undergo bridging with the oxygen atom donor
portion of the ligand. For example, Fe(salen) is pre-
sumably a polymer in the solid state, but readily
coordinates to good donor solvents to form soluble
species or exogenous anionic ligands to form five
coordinate mononuclear species in solution [35]. Like-
wise, a more soluble Fe(salen) analogue, Fe(acen), is
dinuclear in the solid state and mononuclear in benzene
[43]. Here, in contrast, we have shown that Fe(Lipr)2 is
mononuclear and tetrahedral in the solid state.
The observation that the linewidths (in Hz) of the ring
protons H3(254), H6(200), H4(125) and H5(55) for
Fe(Lipr)2 become progressively smaller the farther the
˚
˚
proton is from the iron center, i.e. H3(4.5 A), H6(5.3 A),
˚
˚
H4(6.3 A) and H5(6.6 A), is consistent with their
assignment. The nature of the shifts of the isopropyl
protons is more difficult to discern. The fact that the
proton attached to the tertiary carbon apparently shifts
upfield while the methyl group attached to the same
carbon shifts downfield implies a contact spin delocali-

M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ
/
713
711
Table 3
Tabulated H NMR spectral data of Fe(Li pr
1
)
2
a
b
Chemical shift (dobs, ppm)
c
Linewidth (Hz)
d
Isotropic shift (d, ppm)
Assignment
Integration (number of H’s)
H3
H4
ꢂ21.8
67.7
254
125
55
1
1
1
1
1
6
ꢂ28.9
60.4
H5
H6
ꢂ15.9
62.0
ꢂ22.5
55.2
200
400
700
e
CH(CH3)2
¯
195.4
ꢂ30.5
191.8
ꢂ31.8
CH(CH3)2
¯
Concentrated sample in C6D6 at 23 8C at 300 MHz.
Ligand schematic is shown in Fig. 2.
a
b
Chemical shift positive upfield of tetramethylsilane.
Measured at half-height.
c
d
(Dv/v0)iso ꢃdFeꢂdZn
Tentative assignment.
.
e
air
2Fe(Lipr)2 0 [Fe(Lipr)2]2O
(3)
C6D6
obtained from [Fe(Lipr)2]2O prepared independently by
the literature method [57]. The small exclusively upfield
shifts are characteristic of oxo-bridged diiron(III) com-
plexes with strong antiferromagnetic coupling (J ꢀ
/
100
5/2) iron(III) centers
cmꢂ1) between high-spin (Sꢃ
/
Scheme 3.
mediated by an oxo bridge [38]. Since high spin iron(III)
has a symmetrical electronic environment (6A1g), the
shifts are primarily contact in origin and, therefore,
proportional to the magnetic susceptibility of the
zation mechanism similar to the shifts observed for
methylene protons adjacent to a coordinated tertiary
amine in [Fe2(BPMP)(O2P(OPh)2)2]ꢁ [37]. Since tetra-
hedral iron(II) has an anisotropic ground state (5E), it
generally experiences a significant dipolar contribution
to the isotropic shift which depends on the relative
complex [39ꢀ
/
41]. The relatively small magnetic moment
1.9 BM) of [Fe(LR)2]2O complexes [55] results in a
(ꢁ
/
marked attenuation of their isotropic shift compared
with Fe(Lipr)2. With the exception of a small amount of
ligand, due possibly to ligand dissociation or proto-
nolysis, the reaction did not result in an observable
precipitate or any other apparent iron species as
orientation and distance of proton to the iron atom [39ꢀ
/
41]. The isopropyl group protons in Fe(Lipr)2 are likely
to be very susceptible to dipolar interactions since they
can experience close contacts with the iron atom (as
1
monitored by H NMR.
˚
close as 2.8 A estimated from the X-ray structure and
model building) as the isopropyl group rotates freely.
Oxo-bridged diiron(III) species have been observed
previously from the reaction of air or dioxygen with
mononuclear iron(II) porphyrin [58,59], salen [9], or
acen [43] complexes. In contrast to Fe(salen), the
reaction with Fe(Lipr)2 can be carried out in non-polar,
non-coordinating solvents in which both Fe(Lipr)2 and
[Fe(Lipr)2]2O are soluble and where Fe(Lipr)2 is, plau-
sibly, a discrete mononuclear species. As emphasized
previously for Fe(acen) [43], these attributes are essential
for studying the reactions of low coordinate iron(II)
compounds. A case in point is the reaction of iron(II)
porphyrins and dioxygen in toluene to detect the
diiron(III) peroxo intermediate by 1H NMR at low
temperature [59].
3.4. Reactivity
When a solution of Fe(Lipr)2 in benzene-d6 is exposed
briefly to air (B
1 min), the 1H NMR resonances due to
/
the iron(II) complex disappear and less shifted reso-
nances ranging from 28.8 to 1.2 ppm are observed in the
spectrum accompanied by peaks due to a small amount
of free ligand (B10% by integration). The five broad
/
resonances are consistent with the formation of the oxo-
bridged dinuclear iron(III) species [Fe(Lipr)2]2O from
Fe(Lipr)2 and air (Eq. (3)). The observed 1H NMR shifts
found at 28.8 (very broad), 13.1, 10.1, 2.3 (overlapping),
and 1.2 ppm in an approximate 1:1:1:6:1 ratio can be
assigned to the (CH3)2CH, 4H, 6H, 5H and (CH3)2CH
In addition, tetradentate ligands like acen, salen or
porphyrins enforce typically a square planar geometry
about the iron center. While this allows for an often
critical available coordination site, it may be of interest
¯
¯
(overlapping) and 3H protons, respectively, based on
comparison with published spectra of [Fe(LR)2]2O [55]
and Fe(salen)2O [38,56] and their integration. A near
identical spectrum was
to probe iron(II) reactions with ligands such as Lipr
H
that allow access to coordination geometries other than
those based on square planar, e.g. as observed in the

712
M.A. Torzilli et al. / Polyhedron 21 (2002) 705ꢀ713
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4. Supporting information
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Full crystallographic information for Fe(Lipr
)
is
2
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Acknowledgements
[24] F.A. Cotton, L.M. Daniels, D.J. Maloney, J.H. Matonic, C.A.
Murillo, Polyhedron 5 (1994) 815.
The authors gratefully acknowledge financial support
from Fordham University, the Camille and Henry
Dreyfus Foundation Award of Research Corporation
Award #H50558 (to R.H.B. and S.C.), and a NSF
Division of Undergraduate Education Grant DUE
#9650684 for the NMR spectrometer at Frodham.
The authors also thank Dr. Jennifer L. Kisko for
preliminary experiments and Dr. Victor G. Young, Jr.
at the X-ray Crystallographic Laboratory at the Uni-
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Fe(Lipr)2.
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