Carboxyl-Bonded Low-Spin Iron(III). Chemistry of a Family of Coordination Type cis-FeN4O2

Article abstract of DOI:10.1021/ic950976q

Three tridentate ligands H₂PhL (Ar = Ph), H₂ToL (Ar = p-tolyl), and H₂NpL (Ar = α-naphthyl) of structural type HON=C(Ar)N=NC₆H₄CO₂H have been synthesized. These (general abbreviation H₂ArL) react with iron(DI) chloride, affording the pink Et₄N[Fe(ArL)₂] (s= 1/2 ; μ_eff=1.98-2.04μ_B) in which azo oxime chelation ensures spin-pairing while carboxylate coordination stabilizes the trivalent state. The EPR spectra of the complexes in frozen dimethylformamide-toluene glass (77 K) are rhombic with g values in the range 1.997-2.173. The X-ray structure of Et₄N[Fe(NpL)₂]·H₂O·CH ₂Cl₂ has been determined, revealing meridional binding of the two ligands affording Cis-FeN₄O₂ geometry. The effect of spin-pairing is expressed in the relatively short (～1.90 A?) Fe-O and Fe-N lengths. The water of crystallization is hydrogen-bonded to carboxyl O atoms. The iron(III)-iron(II) reduction potentials lie near ～0.1 V vs SCE. Crystal data for Et₄N[Fe(NpL)₂·H₂CH₂Cl ₂:crystal system triclinic, space group P1?, a = 11.539(9) A?, b = 13.306(13) A?, c = 14.60(2) A?, α = 89.30(9)°, β= 82.65(9)°, y = 79.90(8)°, V = 2189(4) A?₃, Z = 2, R = 6.59%, R_w = 6.23%.

Full text of DOI:10.1021/ic950976q

Inorg. Chem. 1996, 35, 1935-1939
1935
Carboxyl-Bonded Low-Spin Iron(III). Chemistry of a Family of Coordination Type
cis-FeN4O2
Soma Karmakar and Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India
ReceiVed July 29, 1995X
Three tridentate ligands H2PhL (Ar ) Ph), H2ToL (Ar ) p-tolyl), and H2NpL (Ar ) R-naphthyl) of structural
type HONdC(Ar)NdNC6H4CO2H have been synthesized. These (general abbreviation H2ArL) react with iron(III)
chloride, affording the pink Et4N[Fe(ArL)2] (s ) 1/2; µeff ) 1.98-2.04 µB) in which azo oxime chelation ensures
spin-pairing while carboxylate coordination stabilizes the trivalent state. The EPR spectra of the complexes in
frozen dimethylformamide-toluene glass (77 K) are rhombic with g values in the range 1.997-2.173. The X-ray
structure of Et4N[Fe(NpL)2]‚H2O‚CH2Cl2 has been determined, revealing meridional binding of the two ligands
affording cis-FeN4O2 geometry. The effect of spin-pairing is expressed in the relatively short (∼1.90 Å) Fe-O
and Fe-N lengths. The water of crystallization is hydrogen-bonded to carboxyl O atoms. The iron(III)-iron(II)
reduction potentials lie near -0.1 V vs SCE. Crystal data for Et4N[Fe(NpL)2]‚H2O‚CH2Cl2: crystal system triclinic,
space group P1h, a ) 11.539(9) Å, b ) 13.306(13) Å, c ) 14.60(2) Å, R ) 89.30(9)°, â ) 82.65(9)°, γ )
79.90(8)°, V ) 2189(4) Å3, Z ) 2, R ) 6.59%, Rw ) 6.23%.
Introduction
Coordination by the carboxyl group is a pervasive feature of
iron(III) chemistry and biochemistry.1-4 But for one exception,5
systems incorporating such binding are universally high-spin
(s ) 5/2 per iron(III)). Species of the low-spin (s ) 1/2) category
remain a synthetic and structural curiosity, and the prospect of
realizing them aroused our interest. The ligation of less common
3d spin states has been a general concern of this laboratory.6-8
Herein we report the design of a family of acyclic tridentate
ligands that has afforded low-spin iron(III) complexes of
coordination type cis-FeN4O2. Carboxyl binding is authenti-
cated by X-ray structure determination. Magnetic, spectral, and
redox behaviors of the complexes are scrutinized.
Results and Discussion
Ligands. The three ligands used are H2PhL, H2ToL, and
H2NpL, generally abbreviated as H2ArL, 1. These yellow solids
incorporate carboxyl, azo, and oxime functions as potential
donor sites. The starting material for synthesis of H2ArL
(Scheme 1; structures 2-4) is 2-aminobenzoic acid. The amino
group is transformed into the azo oxime function by the reaction
sequence of Scheme 1, the carboxyl group being preserved
throughout.
* Corresponding author. Telefax: +91-33-473-2805. email: icac@
iacs.ernet.in.
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
(1) Que, L. Jr.; True, A. E. Progr. Inorg. Chem. 1990, 38, 97.
(2) (a) Powell, A. K. Coord. Chem. ReV. 1994, 134, 130. (b) Ward, M.
D. Coord. Chem. ReV. 1992, 115, 1.
The carboxyl function is a weaker-field ligand than water,9
and further the spin-pairing energy of the iron(III) (3d5) is
especially high.10 To achieve low-spin configuration for
iron(III) in an environment having carboxyl coordination, it is
evidently necessary to have companion donor sites that strongly
promote spin-pairing. The selection of azo oxime as the
companion function was based on our experience with low-
spin azo oxime tris chelates of type 5. Here the iron(II) state (z
(3) Hartman, J. R.; Rardin, R. L.; Chaudhuri, P.; Pohl, K.; Wieghardt,
K.; Nuber, B.; Weiss, J.; Papaefthymiou, G. C.; Frankel, R. B.;
Lippard, S. J. J. Am. Chem. Soc. 1987, 109, 7387.
(4) (a) Turowski, P. N.; Armstrong, W. H.; Liu, S.; Brown, S. N.; Lippard,
S. J. Inorg. Chem. 1994, 33, 636. (b) Taft, K. L.; Papefthymiou, G.
C.; Lippard, S. J. Inorg. Chem. 1994, 33, 1510.
(5) Curtis, N. F.; Xin, L.; Weatherburn, D. C. Inorg. Chem. 1993, 32,
5838.
(6) Karmakar, S.; Choudhury, S. B.; Chakravorty, A. Inorg. Chem. 1994,
33, 6148.
(7) (a) Basu, P.; Pal, S.; Chakravorty, A. Inorg. Chem. 1988, 27, 1848.
(b) Chattopadhyay, S.; Basu, P.; Pal, S.; Chakravorty, A. J. Chem.
Soc., Dalton Trans. 1990, 3829.
(8) (a) Basu, P.; Choudhury, S. B.; Pal, S.; Chakravorty, A. Inorg. Chem.
1989, 28, 2680. (b) Basu, P.; Chakravorty, A. Inorg. Chem. 1992,
31, 4980. (c) Basu, P.; Chakravorty, A. J. Chem. Soc., Chem.
Commun. 1992, 809.
(9) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
New York, 1984; p 750.
(10) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry: A
ComprehensiVe Text, 4th ed.; Wiley-Interscience: New York, 1980;
p 644.
0020-1669/96/1335-1935$12.00/0 © 1996 American Chemical Society

1936 Inorganic Chemistry, Vol. 35, No. 7, 1996
Karmakar and Chakravorty
around the C-N bond away from the azo-oxime chelate ring
by 12.2° in A and 18.3° in B. The carboxyl function is nearly
coplanar with the benzene ring in A, but in B it forms a dihedral
angle of 15.4°.
The carboxyl function of chelate A alone is involved in
hydrogen-bonding with the water of crystallization. The
uncoordinated atom, O2, has an O2‚‚‚O7 contact of length
2.952(11) Å. The water molecule forms a second slightly longer
(O7‚‚‚O3′, 3.030(11) Å) hydrogen bond with the coordinated
carboxyl oxygen, O3′, of an adjacent complex ion. The
O2‚‚‚O7‚‚‚O3′ angle is 105.4°. In effect, the lattice consists of
centrosymmetric water-bridged dimers, as shown in the structure
fragment 6. The coordinates of primed and unprimed atoms in
6 are xj, 1 - y, jz and x, y, z, respectively.
) -1) is stable, but the iron(III) state (z ) 0) is not.11 We
guessed that insertion of a carboxyl substituent in the azo oxime
Ph group might afford a ligand with the desired iron(III) binding
properties.
Low-Spin Iron(III) Complexes. The reaction of anhydrous
iron(III) chloride and H2ArL in 1:2 ratio in methanol containing
Et4NCl afforded a pink salt of composition Et4N[Fe(ArL)2] in
excellent yields. Single crystals of the NpL2- complex could
however be grown only in a solvated form; Vide infra.
Selected characterization data are listed in Table 1. The 1:1
electrolytic complexes display characteristic absorption spectra
in the visible region (Table 1; Figure 1). The bands are believed
to be of π(ArL2-) f d(Fe) origin.
The four Fe-N lengths are virtually the same within
experimental error, the average value being 1.897(6) Å. On
the other hand, the Fe-O3 length, 1.919(6) Å, is ∼0.04 Å longer
than the Fe-O6 length, 1.879(6) Å. Significantly, the O3 atom
is involved in hydrogen-bonding as in 6, but O6 is not. Spin-
pairing leads to an effective decrease in metal radius.13 For
high-spin iron(III) complexes, the expected Fe-N (sp2 hybrid-
ized) and Fe-O (carboxylate) lengths are ∼2.1 and ∼2.0 Å,
respectively.14 The lengths in Fe(NpL)2- are shorter than these
values by nearly 0.2 and 0.1 Å, respectively.
The room-temperature magnetic moments of Et4N[Fe(ArL)2]
5
lie close to 2.0 µB (Table 2), corresponding to the t2 config-
uration with incomplete quenching of orbital moment. The
variable-temperature moment of Et4N[Fe(PhL)2] (Figure 2)
reveals the expected slight decrease with temperature.12
In frozen dimethylformamide-toluene (1:1) glass (77 K),
Et4N[Fe(ArL)2] displays well-resolved rhombic EPR spectra
(Table 2, Figure 2), consistent with the cis-FeN4O2 coordination
sphere revealed by structure determination (see below). This
geometry can at best have a 2-fold axis and is therefore
inherently rhombic.
The only known low-spin iron(III)-carboxylate complex is
macrocycle 7, in which the FeN4O2 coordination sphere has trans
Crystal and Molecular Structure. Only the NpL2- complex
afforded good single crystals in the form of the solvate
-
Et4N[Fe(NpL)2]‚H2O‚CH2Cl2. A view of the Fe(NpL)2 ion
is shown in Figure 3, and selected bond parameters are listed
in Table 3. The CH2Cl2 molecule and the Et4N+ cations occur
as discrete entities in the lattice without any unusual nonbonded
contacts with other atoms. The water molecule is hydrogen-
bonded to the complex anion (see below).
The two ligands span meridionally, binding the metal in the
hexadentate FeN4O2 fashion utilizing pairs of oximato N, azo
N, and carboxyl O atoms, resulting in cis configuration for the
FeN4O2 coordination sphere. Distortions from the idealized
octahedral geometry are large. The angles at the metal center
between cis-positioned donor pairs span the range 79.1(3)-
99.5(3)°, and those between trans-positioned pairs are 165.9(3)-
178.5(3)°.
The two tridentate ligands have the same gross structure but
with some subtle conformational differences. The ligands
incorporating N1 and N4 will be respectively identified as A
and B. For both, the five-membered azo-oxime chelate ring
including the pendant oximato O atom is an excellent plane
with mean deviation of ∼0.02 Å. The naphthyl plane is inclined
to it by 59.0° in A and by 51.2° in B. The six-membered chelate
rings deviate significantly from planarity (mean deviation ∼0.1
Å). The benzene ring bearing the carboxyl function is rotated
configuration.5 The average Fe-N and Fe-O lengths are
1.994(3) and 1.895(2) Å, respectively, the N atoms being sp3
hydridized unlike those in our complex (sp2). The shorter (by
-
∼0.1 Å) average Fe-N length observed in Fe(NpL)2 is
consistent with this change in nitrogen hybridization. The
-
average Fe-O distances in 7 and Fe(NpL)2 are nearly equal.
In Fe(ArL)2-, we now have the first family of low-spin iron(III)
carboxylates derived from acyclic ligands.
Metal Redox. In dimethylformamide solution (0.1 M TEAP)
the Et4N[Fe(ArL)2] complexes display a cyclic one-electron
response near -0.06 V vs SCE. Reduction potential and
coulometric data are collected in Table 1. Exhaustive reduction
at -0.3 V leads to one-electron transfer affording a green
solution which has the same voltammogram (initial scan anodic)
(11) (a) Pal, S.; Melton, T.; Mukherjee, R. N.; Chakravarty, A. R.; Tomas,
M.; Falvello, L. R.; Chakravorty, A. Inorg. Chem. 1985, 24, 1250.
(b) Pal, S.; Mukherjee, R. N.; Tomas, M.; Falvello, L. R.; Chakravorty,
A. Inorg. Chem. 1986, 25, 200. (c) Manivannan, V.; Chattopadhyay,
S.; Basu, P.; Chakravorty, A. Polyhedron 1993, 12, 2725.
(12) (a) Figgis, B. N. Trans. Faraday Soc. 1961, 57, 198. (b) Ibid. 1961,
57, 204.
(13) (a) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (b) Shannon,
R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925.
(14) Lin, W.; Welsh, W. J.; Harris, W. R. Inorg. Chem. 1994, 33, 884.

Carboxyl-Bonded Low-Spin Iron(III)
Inorganic Chemistry, Vol. 35, No. 7, 1996 1937
Table 1. Conductivity, Electrochemical,a and Electronic Spectral Data in Dimethylformamide at 298 K
E1/2,b V
compd
Λ, Ω-1 cm2 M-1
(∆Ep,c mV); nc,d
UV-vis data: λmax, nm (ꢀ, M-1 cm-1
)
Et4N[Fe(PhL)2]
Et4N[Fe(NpL)2]
Et4N[Fe(ToL)2]
74
69
70
-0.05 (70); 0.99
-0.08 (80); 1.01
-0.06 (80); 1.02
925 (920), 600f (5680), 520 (14 130)
925 (930), 600f (5900), 500 (15 560)
925 (916), 600f (5220), 500 (12 440)
a At a platinum disk electrode; supporting electrolyte tetraethylammonium perchlorate (TEAP, 0.1 M); scan rate 50 mV s-1; reference electrode
b
SCE; solute concentration ∼10-3 M. E1/2 is calculated as the average of anodic (Ep ) and cathodic (Ep ) peak potentials. c ∆Ep ) Ep - Ep . d n )
a
c
c
Q/Q′, where Q is the observed Coulomb count and Q′ is the calculated count for 1e transfer. e Constant-potential electrolysis performaed at 200 mV
below Ep for reduction and 200 mV above Ep for oxidation. f Shoulder.
c
a
Table 2. Magnetic Momentsa and EPR Datab
Scheme 1a
g values
compd
µeff, µB
g1
g2
g3
Et4N[Fe(PhL)2]
Et4N[Fe(NpL)2]
Et4N[Fe(ToL)2]
2.01
1.98
2.04
2.173
2.173
2.173
2.127
2.121
2.124
1.997
1.997
1.998
a In the solid state at 298 K. b Frozen dimethylformamide-toluene
(1:1) glass at 77 K.
a (i) Concentrated HCl, NaNO2, 0 °C. (ii) SnCl2, 0 °C. (iii) NaOAc,
HOAc, ArCHO, 25 °C. (iv) n-BuNO2, NaOMe, MeOH, reflux. (v)
Dilute NaOH, extract with Et2O, neutralize aqueous part with dilute
H2SO4, 0 °C.
Figure 2. (a) Plots of the magnetic moment µeff (µB) and the magnetic
susceptibility øM (emu/mol) against temperature T (K) for Et4N[Fe-
(PhL)2]. (b) X-band EPR spectrum of Et4N[Fe(NpL)2] in frozen 1:1
dimethylformamide-toluene solution at 77 K.
2-
Electrogenerated solutions of Fe(ArL)2 are EPR-inactive
consistent, with either low-spin (s ) 0) or high-spin (s ) 2,
EPR-silent) states. In general, ligands which promote spin-
pairing in iron(III) do so in iron(II) also.6,8,11c This happens
because the drastically reduced (between iron(III) and iron(II))
spin-pairing energy is still more than balanced by the crystal
field stabilization even though it is also sizably reduced.9,10 On
2-
this count, the Fe(RL)2 species are believed to be low-spin.
A representative electronic spectrum of the reduced complex
is shown in Figure 1. The bands at 710 and 475 nm are
tentatively assigned to d(Fe) f π*(ArL2-) excitation. Because
Figure 1. (a) Electronic spectra of Et4N[Fe(PhL)2] (s) and electro-
generated Fe(PhL)22- (- - -) in dimethylformamide. (b) Cyclic voltam-
mograms (scan rate 50 mV s-1) of 10-3 M solutions of Et4N[Fe(PhL)2]
(s) and electrogenerated Fe(PhL)22- (- - -) in dimethylformamide (0.1
M TEAP) at 298 K at platinum electrode.
of the low E1/2 value of the couple of eq 1, FeII(ArL)22- solutions
-
are spontaneously oxidized by air to FeIII(ArL)2
.
-
-
It is instructive to compare Fe(ArL)2 with the tris chelate
5. In the latter, the iron(III)-iron(II) E1/2 is only moderately
high, ∼0.3 V, but even then, the iron(III) species are unstable.11
The azo-oxime function is essentially a soft and strong-field
ligand with superior iron(II) affinity. On the other hand,
carboxyl oxygen is a hard donor, binding iron(III) well. In
Fe(ArL)2-, azo-oxime chelation ensures spin-pairing while
as the parent Fe(ArL)2 complex (initial scan cathodic). We
thus have the couple of eq 1. The two species are believed to
2-
FeIII(ArL)2- + e h FeII(ArL)2
(1)
have the same gross structure, since the redox response is nearly
reversible.

1938 Inorganic Chemistry, Vol. 35, No. 7, 1996
Karmakar and Chakravorty
netometer fitted with a Walker Scientific L75FBAL magnet and a
Model 153N dewar along with a temperature controller. A Perkin-
Elmer 240C elemental analyzer was used to collect microanalytical
data (CHN). Electrochemical measurements were performed under
nitrogen atmosphere on a PAR 370-4 electrochemistry system using
solvents and supporting electrolyte purified/prepared as before.8a,15
Solution electrical conductivities were measured with the help of a
Philips PR 9500 bridge, the solute concentration being ∼10-3 M.
Synthesis of Ligands and Complexes. The ligands were prepared
by the same general procedure outlined in Scheme 1, and complexes
were also synthesized by a general method. Details are given below
for H2NpL and its iron(III) complex. The solvents and chemicals used
for synthesis were of analytical grade.
2-(Carboxyphenyl)hydrazine Hydrochloride, 3. A solution of
sodium nitrite (2.50 g, 0.036 mol) in 5 mL water was added slowly to
a suspension of o-aminobenzoic acid, 2 (2.74 g, 0.020 mol), in 10 mL
of concentrated HCl at 0 °C, affording a clear greenish yellow solution,
which was left at 0 °C for a further period of 0.25 h. A cold solution
(0 °C) of stannous chloride (12 g, 0.053 mol) in 10 mL of concentrated
HCl was then added with vigorous stirring which was continued for
1.5 h. The white precipitate that separated from the mixture was
collected by filtration and dissolved in 20 mL of water to obtain a
clear and transparent solution, and then 150 mL of concentrated HCl
was added to it with constant stirring. The hydrazine hydrochloride,
3, started precipitating immediately, and the mixture was cooled
overnight in a refrigerator. The silky white solid was collected by
filtration and was used without further purification in the next step.
Yield: 3.0 g (79%).
Naphthaldehyde 2-(Carboxyphenyl)hydrazone, 4. A 3.0 g (0.016
mol) sample of 3 was dissolved in 15 mL of water containing sodium
acetate trihydrate (4.53 g, 0.033 mol) and glacial acetic acid (10-15
mL). The solution was filtered to remove insoluble material, if present.
To this was added slowly naphthaldehyde (2.49 g, 0.016 mol) with
vigorous stirring which was continued for 2 h. The pale yellow
hydrazone appeared immediately. It was collected by filtration washed
thoroughly with water, and then dried in vacuo over fused CaCl2 to
obtain the required hydrazone, as a pale yellow solid. Yield: 3.5 g
(76%). Anal. Calcd for C18H14N2O2: C, 74.48; H, 4.83; N, 9.65.
Found: C, 74.55; H, 4.80; N, 9.69. Mp: 208-210 °C.
Figure 3. ORTEP plot and atom-labeling scheme for Et4N[Fe(NpL)2].
All atoms are represented by their 50% thermal probability ellipsoids.
Table 3. Selected Bond Distances (Å) and Angles (deg) and Their
Estimated Standard Deviations for Et4N[Fe(NpL)2]‚H2O‚CH2Cl2
Distances
Fe-N1
Fe-N3
Fe-N4
Fe-N6
Fe-O3
Fe-O6
N1-N2
N4-N5
1.892(6)
1.907(6)
1.888(6)
1.899(7)
1.919(6)
1.879(6)
1.299(8)
1.287(9)
O2-C1
O3-C1
O5-C19
O6-C19
O1-N3
O4-N6
O2‚‚‚O7
O7‚‚‚O3′
1.232(10)
1.285(11)
1.219(10)
1.263(8)
1.249(8)
1.257(8)
2.952(11)
3.030(11)a
Angles
N1-Fe-N3
N1-Fe-N4
N1-Fe-N6
N3-Fe-N4
N3-Fe-N6
N4-Fe-N6
O3-Fe-N1
O3-Fe-N3
81.1(3)
178.5(3)
99.5(3)
98.5(3)
80.0(3)
79.1(3)
92.1(2)
170.7(2)
O3-Fe-N4
O3-Fe-N6
O3-Fe-O6
O6-Fe-N1
O6-Fe-N3
O6-Fe-N4
O6-Fe-N6
O2‚‚‚O7‚‚‚O3′
88.1(2)
95.0(3)
96.8(2)
87.9(2)
89.4(3)
93.5(2)
165.9(3)
105.4(3)a
Anal. Calcd for 4 (Ar ) Ph), C14H12N2O2: C, 70.00; H, 5.00; N,
11.67. Found: C, 70.08; H, 4.90; N, 11.62. Mp: 203-205 °C. Anal.
Calcd for 4 (Ar ) To), C15H14N2O2: C, 70.87; H, 5.51; N, 11.02.
Found: C, 70.75; H, 5.60; N, 11.15. Mp: 200-203 °C.
a Coordinates of primed atom: jx, 1 - y, jz.
carboxylate coordination stabilizes the trivalent state, also
lowering the iron(III)-iron(II) reduction potential.
((2-Carboxyphenyl)azo)-1-naphthaldoxime, 1. A 3.5 g (0.012
mol) sample of 4 was suspended in 25 mL of dry methanol, and 10
mL of freshly prepared n-BuNO2 was added to the suspension, followed
by a solution of NaOMe in methanol (prepared from 0.70 g of Na in
20 mL of hot methanol). The blood red mixture was stirred and then
heated to reflux for 1 h. It was then cooled, and an ice-cold solution
of 1.20 g of NaOH in 20 mL water was added slowly. The mixture
was allowed to stand for 12 h at room temperature and then extracted
with diethyl ether. The aqueous layer was cooled in ice and then
neutralized slowly with cold 1 N H2SO4 until it became just acidic.
During this process, the color of the solution became progressively
lighter and finally the ligand, 1, separated out as a crystalline yellow
solid which was collected by filtration. It was washed several times
with water and finally dried in vacuo over fused CaCl2. Yield: 3.2 g
(83%). Anal. Calcd for 1, C18H13N3O3: C, 67.71; H, 4.07; N, 13.16.
Found: C, 67.65; H, 4.12; N, 13.08. Mp: 150-152 °C. λmax, nm (ꢀ,
cm-1 M-1): 360 (5680), 330 (6290).
Concluding Remarks. The main findings of this work will
now be summarized. The oxime-azo-carboxylate ligand
system, H2ArL, has been designed, affording low-spin carboxyl-
bonded iron(III) salts of the type Et4N[Fe(ArL)2] in which the
metal coordination geometry is cis-FeN4O2. Upon collective
-
consideration of stabilities and reduction potentials of Fe(ArL)2
and the tris chelate 5, it is proposed that in the former azo-
oxime chelation ensures spin-pairing while carboxylate coor-
dination stabilizes the trivalent state. The Fe-N and Fe-O
-
lengths in Fe(NpL)2 are significantly shorter (by 0.1-0.2 Å)
than those in representative high-spin species.
Prior to this work, only one low-spin carboxyl-bonded
iron(III) complex of a macrocyclic ligand had been documented
(trans-FeN4O2 coordination). The present complexes are the
first species incorporating an acyclic ligand system (cis-FeN4O2
coordination). With judicious choice of coligand functions, it
should in principle be possible to tailor new ligands that could
stabilize the difficultly achievable low-spin state of iron(III) in
a carboxyl-coordinated environment.
Anal. Calcd for 1 (Ar ) Ph), C14H11N3O3: C, 62.45; H, 4.09; N,
15.61. Found: C, 62.54; H, 4.21; N, 15.52. Mp: 145-147 °C. λmax
,
nm (ꢀ, cm-1 M-1): 360 (5790), 330 (6230). Anal. Calcd for 1 (Ar )
To), C15H13N3O3: C, 63.60; H, 4.59; N, 14.84. Found: C, 63.71; H,
4.52; N, 14.95. Mp: 142-144 °C.
(5780), 330 (6360).
λ
max, nm (ꢀ, cm-1 M-1): 360
Tetraethylammonium Bis[((2-carboxylatophenyl)azo)-1-naph-
thaldoximato]ferrate(III), Et4N[Fe(NpL)2]. The complex was pre-
pared by adding a methanolic solution (10 mL) of H2NpL (0.39 g, 1.22
Experimental Section
Physical Measurements. A Hitachi 330 spectrophotometer was
used to record UV-vis spectra. EPR spectra were studied with a Varian
E-109C spectrometer fitted with a quartz dewar. Magnetic susceptibili-
ties were measured with a Model 155 PAR vibrating-sample mag-
(15) Lahiri, G. K.; Bhattacharya, S.; Ghosh, B. K.; Chakravorty, A. Inorg.
Chem. 1987, 26, 4324.

Carboxyl-Bonded Low-Spin Iron(III)
Inorganic Chemistry, Vol. 35, No. 7, 1996 1939
Table 4. Crystallographic Data for Et4N[Fe(NpL)2]‚H2O‚CH2Cl2
Table 5. Essential Atomic Coordinates (×104) and Equivalenta
Isotropic Displacement Coefficients (Å2 × 103) for
Et4N[Fe(NpL)2]‚H2O‚CH2Cl2
empirical formula C45H46Cl2N7O7Fe
Z
T, °C
λ, Å
2
22
0.710 73
1.401
5.26
0.75-0.85
6.59
6.23
1.20
fw
923.6
P1h
space group
a, Å
x
y
z
U(eq)
11.539(9)
13.306(13)
14.60(2)
89.30(9)
82.65(9)
79.90(8)
2189(4)
F
calcd, g cm-3
Fe
285(1)
-299(5)
868(5)
236(5)
236(5)
517(5)
724(5)
1920(5)
2360(5)
487(6)
-1354(5)
-2095(5)
-366(6)
1086(8)
2366(7)
2757(7)
1600(7)
95(8)
-1191(7)
-1883(8)
-1567(6)
1835(7)
2090(1)
543(4)
4459(4)
3192(4)
545(4)
3667(4)
2786(3)
1622(4)
748(4)
2416(1)
3645(4)
799(4)
40(1)
61(2)
77(3)
52(2)
58(2)
68(2)
52(2)
40(2)
43(2)
45(2)
42(2)
45(2)
46(2)
56(4)
49(3)
42(3)
48(3)
52(3)
42(3)
47(3)
43(3)
110(3)
b, Å
c, Å
µ, cm-1
transm coeff
Ra
O1
O2
O3
O4
O5
O6
N1
N2
N3
N4
N5
N6
C1
C2
C7
C8
C19
C20
C25
C26
O7
R, deg
â, deg
γ, deg
V, Å3
1554(3)
1134(3)
4653(3)
3392(3)
2036(3)
2368(4)
3111(4)
2775(4)
2441(4)
1669(4)
1184(5)
1168(5)
1542(4)
2942(5)
3998(5)
3904(5)
3348(5)
1848(5)
-565(5)
b
Rw
GOFc
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
;
2
w-1 ) σ2(|Fo|) + g|Fo| ; g ) 0.000 for Et4N[Fe(NpL)2]‚H2O‚CH2Cl2.
c The goodness of fit is defined as [w(|Fo| - |Fc|)2/(no - nv)]1/2, where
no and nv denote the numbers of data and variables, respectively.
863(4)
2531(4)
2062(4)
1231(5)
3630(6)
3112(5)
2161(5)
311(5)
3385(5)
3789(5)
3398(5)
1322(5)
5852(6)
mmol) containing Et4NCl (0.12 g, 0.650 mmol) to a solution containing
anhydrous ferric chloride (0.10 g, 0.620 mol). A pink color developed,
and it darkened quickly. The mixture was stirred at room temperature
for 1 h. The deposited dark crystalline solid was filtered off, washed
several times with aqueous methanol (1:1), and dried in vacuo over
CaCl2. Yield: 0.38 g (76%). Anal. Calcd for Et4N[Fe(NpL)2],
C44H42N7O6Fe: C, 64.43; H, 5.13; N, 11.96. Found: C, 64.50; H, 5.03;
N, 11.92.
Anal. Calcd for Et4N[Fe(PhL)2], C36H38N7O6Fe: C, 60.04; H, 5.28;
N, 13.62. Found: C, 60.10; H, 5.15; N, 13.71. Anal. Calcd for
Et4N[Fe(ToL)2], C38H42N7O6Fe: C, 61.00; H, 5.62; N, 13.11. Found:
C, 61.14; H, 5.74; N, 13.01.
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
X-ray Structure Determination. Crystals of Et4N[Fe(NpL)2]‚
H2O‚CH2Cl2 (0.38 × 0.24 × 0.40 mm3) were grown by slow diffusion
of hexane into wet dichloromethane solution followed by slow
evaporation. The unit cell parameters were determined by the least-
squares fit of 30 machine-centered reflections having 2θ values in the
range 15-25°. Data were collected at 296 K by the ω-scan method
over a 2θ range of 3-45° on a Nicolet R3m/V diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). Two
check reflections measured after every 98 reflections showed no
significant intensity reduction during 50.45 h of exposure to X-rays.
Lorentz and polarization corrections as well as a semiempirical
absorption correction16 were made. The structure was successfully
solved in space group P1h. Of the 5732 unique reflections, 3403
satisfying I > 2.5σ(I) were used for structure solution. The structure
was solved by direct methods and was refined by full-matrix least-
squares procedures, making all non-hydrogen atoms anisotropic.
Hydrogen atoms were included in calculated positions with fixed U
()0.08 Å2). The highest residual was 0.68 e Å-3. Significant crystal
data are listed in Table 4. Essential atomic coordinates and isotropic
thermal parameters are collected in Table 5. Computations were carried
out on a Micro VAX II computer using the SHELXTL-PLUS program
system,17 and crystal structure plots were drawn using ORTEP.18
Acknowledgment. Financial support received from the
Department of Science and Technology, New Delhi, is ac-
knowledged. Affiliation with the Jawaharlal Nehru Centre for
Advanced Scientific Research, Bangalore, India, is also ac-
knowledged.
Supporting Information Available: Complete atomic coordinates
(Table S1), bond distances (Table S2) and angles (Table S3), anisotropic
thermal parameters (Table S4), and hydrogen atom positional parameters
(Table S5) for Et4N[Fe(NpL)2]‚H2O‚CH2Cl2 (9 pages). Ordering
information is given on any current masthead page.
IC950976Q
(17) Sheldrick, G. M. SHELXTL-PLUS 88. In Structure Determination
Software Programs; Nicolet Instrument Corp.: 5225-2 Verona Rd,
Madison, WI 53711, 1988.
(16) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr. 1968,
A24, 351.
(18) Johnson, C. K. ORTEP. Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.