Low-spin iron(III) complexes with N,S coordination: Syntheses, structures, and properties of bis(N-2-mercaptophenyl-2′-pyridylmethyleniminato)iron(III) tetraphenylborate and bis(N-2-mercapto-2-methylpropyl-2′-pyridylmethyleniminato)iron(III) tetraphenylborate

Article abstract of DOI:10.1016/s0020-1693(98)00354-5

The chemistry of the iron complexes of two Schiff-base ligands namely, N-2-mercaptophenyl-2′-pyridylmethylenimine (PyASH, 1) and N-2-mercapto-2-methylpropyl-2′-pyridylmethylenimine (PyMSH, 2) has been explored in a systematic manner. Use of DMF as solvent allows one to isolate the Fe(II) complexes in pure forms. The Fe(III) complexes [Fe(PyAS)₂]X (X = BF₄, BPh₄ (3)) and [Fe(PyMS)₂]BPh₄ (4) have been synthesized via oxidation of the corresponding Fe(II) complexes with ferrocenium salts. In 3 and 4, the deprotonated ligands are coordinated in mer fashion with two thiolato S donors cis to each other. The deprotonated PyAS^- ligand frames in 3 are essentially planar due to extensive electron delocalization. The Fe(III) centers of these two complexes remain low-spin in the temperature range 8-300 K and exhibit rhombic EPR signals with small (g_max - g_min) values. Results of this and other previously reported works suggest that tridentate Schiff-base ligands with N₂S donor set (L) readily afford low-spin Fe(III) centers in [Fe^III(L)₂]⁺ complexes.

Full text of DOI:10.1016/s0020-1693(98)00354-5

Inorganica Chimica Acta 285 (1999) 269±276
Low-spin iron(III) complexes with N,S coordination: syntheses,
structures, and properties of bis(N-2-mercaptophenyl-20-pyridylmethyl-
eniminato)iron(III) tetraphenylborate and bis(N-2-mercapto-2-methyl-
propyl-20-pyridylmethyleniminato)iron(III) tetraphenylborate
Juan C. Noverona, Ruth Herradoraa, Marilyn M. Olmsteadb, Pradip K. Mascharaka,*
aDepartment of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, USA
bDepartment of Chemistry, University of California, Davis, CA 95616, USA
Received 21 May 1998; accepted 31 July 1998
Abstract
The chemistry of the iron complexes of two Schiff-base ligands namely, N-2-mercaptophenyl-20-pyridylmethylenimine (PyASH, 1) and
N-2-mercapto-2-methylpropyl-20-pyridylmethylenimine (PyMSH, 2) has been explored in a systematic manner. Use of DMF as solvent
allows one to isolate the Fe(II) complexes in pure forms. The Fe(III) complexes [Fe(PyAS)2]X (X ˆ BF4, BPh4 (3)) and [Fe(PyMS)2]BPh4
(4) have been synthesized via oxidation of the corresponding Fe(II) complexes with ferrocenium salts. In 3 and 4, the deprotonated ligands
are coordinated in mer fashion with two thiolato S donors cis to each other. The deprotonated PyAS ligand frames in 3 are essentially
planar due to extensive electron delocalization. The Fe(III) centers of these two complexes remain low-spin in the temperature range 8±
300 K and exhibit rhombic EPR signals with small (gmax gmin) values. Results of this and other previously reported works suggest that
‡
tridentate Schiff-base ligands with N2S donor set (L) readily afford low-spin Fe(III) centers in [FeIII(L)2] complexes. # 1999 Elsevier
Science S.A. All rights reserved.
Keywords: Crystal structures; Iron complexes; Schiff base complexes
1. Introduction
synthetic procedures that afford these species in pure form
and in high yields. To date, most of the low-spin Fe(III)
complexes of Schiff bases have been synthesized via con-
densation of the amine and mercaptoaldehyde (or ketone) on
Fe3‡ template [8,12,13] since addition of preformed ligands
to Fe(III) salts often results in reduction and formation of the
Fe(II) complexes and/or S-bridged polymeric species. In
general, syntheses of Fe(III) complexes with Fe(III)±S(thio-
lato) bonds are beset with problems of oligomerization and/
or autoredox reactions generating Fe(II) and RSSR [14,15].
Current methods of synthesizing stable Fe(III)±S(thiolato)
bonds include (a) use of sterically hindered thiols and aprotic
solvents like DMF [16±19], (b) in situ formation of multi-
dentate ligands with thiolato S donors in the presence of
Fe(III) salts [8,12,13], and (c) air oxidation of suitable Fe(II)
complexes [7,20]. In this paper, we report two low-spin
Fe(III) complexes with N4S2 coordination derived from
the two Schiff base ligands PyASH (1) and PyMSH (2)
where H is the dissociable thiol H. The two complexes
[Fe(PyAS)2](BPh4) (3) and [Fe(PyMS)2](BPh4) (4) have
The discovery of a low-spin (S ˆ 12) non-heme [FeIIIN2S3]
center in the enzyme nitrile hydratase [1±6] has raised
interest in mononuclear model Fe(III) complexes with
N,S coordination. Several groups have structurally charac-
terized Fe(III) complexes with NxSy coordination and com-
pared their spectroscopic properties with the biological iron
site [7±11]. In such modeling work, it has become apparent
that complexes derived from S-containing Schiff bases
(chelating ligands with imine nitrogen atoms and thiolato
sulfur atoms as donor atoms) comprise low-spin Fe(III)
centers [8,11]. The existing literature on Schiff base ligands
include few more examples of low-spin Fe(III) complexes
with NxSy coordination [12,13]. The fact that the redox and
reactivity parameters of such complexes provide insight into
the function of biological non-heme iron sites warrants new
*Corresponding author. Tel.: +1-408-459-4251; fax: +1-408-459-2935;
e-mail: pradip@hydrogen.ucsc.edu
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00354-5

270
J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
been synthesized via oxidation of the corresponding Fe(II)
(3.1 mmol) of iron(II) acetate in 10 ml of DMF. A green
color developed almost immediately. The mixture was
stirred for 24 h when a green precipitate was formed. The
volume of the reaction mixture was reduced to 10 ml and the
solid was isolated by ®ltration. Yield: 1.27 g (84%).
Selected IR bands (KBr pellet, cm 1): 3048 (m, br),
1591 (m, ꢁCN), 1575 (s), 1485 (s), 1461 (s), 1445 (s),
1284 (m), 1257 (m), 1150 (m), 1066 (s), 841(m), 758 (s),
738 (s), 656 (m). Electronic absorption spectrum in DMF
[ꢂmax, nm (ꢃ, M 1 cm 1)]: 805 (9 500), 660 (6 900), 450
(16 500).
‡
‡
complexes with (Fc )(BF4 ) (Fc ˆ ferrocenium) in DMF.
The success of this facile and clean oxidative procedure
indicates that the method could be used for other Fe(III)
complexes with one or more Fe(III)±S(thiolato) bonds. Both
3 and 4 are low-spin in the temperature range 4±300 K.
2.4. [Fe(PyAS)2]BF4
‡
A solution of 0.72 g (2.63 mmol) of (Fc )(BF4 ) in 5 ml
2. Experimental
of DMF was slowly added to a solution of 1.27 g
(2.63 mmol) of [Fe(PyAS)2] in 10 ml of DMF and the
mixture was stirred continuously. An immediate change
in color from green to red occurred and the mixture became
homogeneous within a minute. After 30 min, 15 ml of
acetonitrile was added to this solution followed by slow
addition of 15 ml of dry diethyl ether. The deep red reaction
mixture was then ®ltered and the ®ltrate was stored ®rst at
48C for 2 h and then at 208C for 12 h. The microcrystalline
solid which separated during this period was ®ltered and
dried under vacuum. Yield: 920 mg (62%). Selected IR
bands (KBr pellet, cm 1): 3060 (m, br), 1600 (m, ꢁCN),
1580 (m), 1475 (s), 1302 (w), 1260 (m), 1185 (m), 1065 (vs,
2.1. Preparation of compounds
‡
Ferrocenium tetra¯uoroborate (Fc )(BF4 ), sodium tet-
raphenylborate, 2-pyridinecarboxaldehyde, and 1-amino-2-
methyl-2-propanethiol hydrochloride were procured from
Aldrich and were used without further puri®cation. Iron-
(II)acetate was purchased from Sigma. The ligand 2-(2-
pyridyl)-benzothiazoline (PyASH in the thiazoline form)
was synthesized by the published procedure [21]. All
manipulations were performed under an atmosphere of pure
and dry dinitrogen. All the iron complexes afforded satis-
factory elemental analysis.
ꢁBF ), 768 (s), 742 (m), 514 (m). Electronic absorption
1
spectrum in DMF [ꢂmax, nm (ꢃ, M
4
cm 1)]: 565
(6 500), 430 (8 300). Value of ꢄeff (298 K, polycrystalline):
2.2. 2-(2-Pyridyl)-4-dimethylthiazolidine
1.95mB.
A batch of 2.78 g (19.6 mmol) of 1-amino-2-methyl-2-
propanethiol hydrochloride was added to a solution of
0.45 g (19.6 mmol) of elemental sodium in 25 ml of
degassed methanol. Next, 1.81 ml (19.6 mmol) of 2-pyri-
dinecarboxaldehyde was added and the mixture was heated
to re¯ux for 6 h. The solvent was then removed by rotary
distillation and the remaining oil was redissolved in 40 ml of
chloroform. The solution was ®ltered to remove NaCl and
washed twice with aqueous solution of NaHCO3 and once
with saturated NaCl solution in water. Then the chloroform
solution was dried over anhydrous MgSO4 and evaporated
in vacuo. The deep brown oil thus obtained was distilled
under reduced pressure. Yield: 3.2 g. (78%). 1H NMR
(CDCl3, 500 MHz), ꢀ from TMS: 1.49 (s, CH3), 1.55 (s,
CH3), 2.98 (d, J ˆ 12 Hz, CH), 3.22 (d, J ˆ 12 Hz, CH),
3.9 (NH, br), 5.84 (s, CH), 7.18 (t), 7.23 (d), 7.63 (t), 8.59;
13C NMR (CDCl3, 125.58 MHz), ꢀ from TMS: 28.38, 31.60,
59.50, 66.69, 73.80, 121.89, 122.93, 136.67, 149.77, 158.50.
The complex [Fe(PyAS)2]BPh4 (3) was isolated by slow
diffusion of diethyl ether into a solution of [Fe(PyAS)2]BF4
and NaBPh4 (5 equiv.) in 1:1 DMF:acetonitrile. This highly
crystalline compound was used in the X-ray diffraction
work.
2.5. [Fe(PyMS)2]
To a slurry of 0.632 g (3.63 mmol) of iron(II) acetate in
10 ml of DMF was added a solution of 1.48 g (7.63 mmol)
of 2-(2-pyridyl)-4-dimethylthiazolidine in 15 ml DMF and
the mixture was stirred continuously. A green color devel-
oped within 10 min. The mixture was stirred for 24 h. The
volume of the deep green solution was then reduced to 5 ml,
and 10 ml of acetonitrile was added to it. The green pre-
cipitate thus obtained was ®ltered, washed with 10 ml of
acetonitrile and dried in vacuo. Yield: 1.13 g (72%).
Selected IR bands (KBr pellet, cm 1): 2950 (m, br),
1619 (m, ꢁCN), 1585 (m), 1550 (s), 1470 (m), 1440 (vs),
1370 (m), 1298 (m), 776 (s), 670 (s). Electronic absorption
2.3. [Fe(PyAS)2]
1
A solution of 1.352 g (6.3 mmol) of 2-(2-pyridyl)-ben-
zothiazoline in 10 ml DMF was added to a slurry of 0.548 g
spectrum in DMF [ꢂmax, nm (ꢃ, M cm 1)]: 665 (1700),
580 (1200), 430 (1050).

J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
271
Table 1
2.6. [Fe(PyMS)2]BPh4ÁMeOH (4)
Summary of crystal data and intensity collection and structure refinement
parameters for [Fe(PyAS)2](BPh4) (3) and [Fe(PyMS)2](BPh4)ÁMeOH (4)
‡
A solution of 0.74 g (2.71 mmol) of (Fc )(BF4 ) in 5 ml
of DMF was added to a solution of 1.20 g (2.71 mmol) of
[Fe(PyMS)2] in 10 ml of DMF. The green color immediately
changed to red and the reaction mixture became homoge-
neous within 1 min. After 30 min of stirring, the volume of
the reaction mixture was reduced to 5 ml, and 15 ml of
methanol was added to it. Dropwise addition of a solution of
1.39 g (4 mmol) of sodium tetraphenylborate in 7 ml of
methanol to this red solution resulted in precipitation of the
desired complex as a ®ne powder. The precipitate was
collected by ®ltration and dried in vacuo. Another batch
of crystalline product was obtained from the ®ltrate upon
storage at 208C for 12 h. Total yield: 1.34 g (70%).
Selected IR bands (KBr pellet, cm 1): 3050 (s), 1605 (m,
3
4
Formula
C
48H38N4S2BFe
C45H50N4OS2BFe
793.67
Molecular weight
Crystal color, habit
T (K)
801.60
black, parallelepiped
130(2)
triclinic
dark red, plate
123(2)
Crystal system
monoclinic
P21/c
ꢀ
P1
Space group
Ê
a (A)
Ê
b (A)
8.864(2)
14.633(3)
16.943(5)
112.81(2)
95.56(2)
99.60(2)
1965.6(8)
2
14.269(3)
16.687(2)
18.189(3)
90
Ê
c (A)
ꢅ (8)
ꢆ (8)
ꢇ (8)
108.616(13)
90
Ê 3
V (A )
4104.5(12)
4
Z
3
dcalc (g cm
Absorption coefficient
)
1.354
0.531
1.284
4.196
ꢁ
CN), 1578 (m), 1477 (s), 1426 (m), 1294 (m), 1132 (m), 734
(s), 706 (s), 613 (m). Electronic absorption spectrum in
DMF [ꢂmax, nm (ꢃ, M cm )]: 850 (1050), 500 (5500).
Value of ꢄeff (298 K, polycrystalline): 2.20mB.
1
ꢄ (mm
)
1
1
GOF a on F2
R1b (%)
1.012
5.17
10.41
1.036
7.23
15.01
wR2c (%)
P
2.7. X-ray data collection and structure solution and
refinement
2
2
1=2
a GOF ˆ ‰ fwꢀFo
Fc † g=ꢀM N†Š
(M ˆ No. of reflections,
2
N ˆ No. of parameters refined).
P
P
b R1 ˆ jjFoj jFcjj= jFoj:
P
P
2
2 1=2
2
c wR2 ˆ ‰ fwꢀFo
Fc † g= wꢀFo † Š
2
2
Black parallelopipeds of 3 were grown by slow diffusion
of diethyl ether into a solution of the complex in 1:1
DMF:CH3CN. Dark red plates of 4 were obtained by slow
cooling of a solution of the complex in 3:1 MeOH:DMF.
The diffraction data were collected at 130 K. For 3, Mo Ka
radiation (graphite monochromator) was employed while
Cu Ka radiation (nickel ®lter) of a rotating anode was used
in collecting data for 4. The structure of 3 was solved by the
Patterson method. Direct methods were used to solve the
structure of 4. Both structures were re®ned by full-matrix
least-squares method (SHELXTL 5, Sheldrick, 1994). Hydro-
gen atoms bonded to carbon and nitrogen atoms were added
geometrically and re®ned with a riding model. An absorp-
tion correction (XABS2) was applied to both data sets [22].
In the ®nal cycles of re®nement, all non-hydrogen atoms
were re®ned with anisotropic thermal parameters. Machine
parameters, crystal data, and data collection parameters are
summarized in Table 1. Selected bond distances and angles
are listed in Table 2. The two sets of crystallographic data
have been submitted as supplementary material (see Sec-
tion 4).
Matthey magnetic susceptibility balance. Electrochemical
measurements were performed with standard Princeton
Applied Research instrumentation and a Beckman Pt inlay
electrode. Potentials were measured at 258C versus an
aqueous saturated calomel electrode (SCE) as reference.
3. Results and discussion
As expected, the two ligands PyASH (1) and PyMSH (2)
are isolated in the thiazoline (1a) and thiazolidine (2a)
forms, respectively. The Schiff base forms of these ligands
are generated only when metal ions and bases are added to
them in polar solvents [21,23±25].
2.8. Other physical measurements
Infrared spectra were obtained with a Perkin-Elmer 1600
FTIR spectrophotometer. Absorption spectra were mea-
sured on a Perkin-Elmer Lambda 9 spectrophotometer.
1H and 13C NMR spectra were recorded on a Varian
500 MHz Unity Plus instrument. EPR spectra were mon-
itored at X-band frequencies by using a Bruker ESP-300
spectrometer. Room temperature magnetic susceptibility
measurements on solid samples were made with a Johnson
The ligand 1 has been employed previously in the synth-
eses of complexes of various M2‡ ions [23,26±30]. How-
ever, no Fe(III) complex of either ligand has been reported
so far. In the present work, the Fe(II) complexes of both
ligands have been synthesized by reacting Fe(II) acetate

272
J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
Table 2
Table 2 (Continued )
Ê
Selected bond distances (A) and angles (8)
C(3)±N(1)±C(2)
C(13)±N(3)±C(12)
C(12)±N(3)±Fe
C(2)±C(1)±C(9)
N(1)±C(3)±C(4)
121.0(6)
120.1(6)
119.8(5)
110.2(6)
117.3(7)
C(8)±N(2)±C(4)
C(13)±N(3)±Fe
C(9)±C(1)±C(10)
C(2)±C(1)±S(1)
N(2)±C(4)±C(3)
118.8(6)
119.3(5)
110.5(6)
105.6(5)
111.9(7)
Complex 3
Fe±N(1)
1.924(3)
1.997(3)
2.2228(11)
1.750(3)
1.295(4)
1.332(4)
1.297(4)
1.395(5)
1.378(5)
1.446(5)
1.642(5)
1.636(5)
1.396(5)
1.395(4)
Fe±N(2)
2.015(3)
1.935(3)
2.2109(12)
1.751(4)
1.427(4)
1.341(4)
1.417(4)
1.390(5)
1.394(5)
1.446(5)
1.645(5)
1.644(5)
1.401(4)
1.388(5)
Fe±N(3)
Fe±N(4)
Fe±S(1)
Fe±S(2)
C(20)±C(11)±C(12) 112.7(6)
N(4)±C(14)±C(13) 113.2(6)
C(23)±C(22)±C(21) 123.1(7)
C(20)±C(11)±C(19) 109.6(6)
C(39)±B±C(21) 113.1(6)
C(27)±C(28)±C(29) 122.5(7)
S(1)±C(1)
N(1)±C(7)
N(2)±C(12)
N(4)±C(18)
C(1)±C(2)
C(14)±C(15)
C(7)±C(8)
B±C(25)
S(2)±C(24)
N(1)±C(6)
N(3)±C(13)
N(4)±C(19)
C(8)±C(9)
C(19)±C(20)
C(17)±C(18)
B±C(31)
with the ligands in anhydrous DMF. The acetate ions
promote deprotonation of the Schiff base forms of the
ligands in such reactions. Both [Fe(PyAS)2] and
[Fe(PyMS)2] have been isolated as analytically pure green
solids which have been characterized by spectroscopic
techniques. Interestingly, when methanol is used as the
solvent, addition of Fe(II) acetate to either ligand does
not afford the green complex; a black insoluble solid is
obtained in the case of PyASH while a blue solution is
produced in reactions with PyMSH. In a previous account
Garg and Tandon [26], reported the formation of an Fe(II)
complex of PyASH in methanol which has the composition
[Fe(PyAS)2(H2O)2]. The authors suggested that the ligands
in this complex are bidentate with the pyridine groups not
coordinating. We have found that use of anhydrous DMF
does not give rise to any complication and affords the bis
complexes [Fe(PyAS)2] and [Fe(PyMS)2] as dark green
solids. It is important to note that when isolated green
[Fe(PyMS)2] is redissolved in methanol or ethanol, one
obtains a blue solution1. Collectively, these ®ndings suggest
that both ligands possibly fail to coordinate in tridentate
fashion in alcohols.
B±C(37)
B±C(43)
C(25)±C(26)
C(37)±C(42)
C(31)±C(32)
C(47)±C(48)
N(1)±Fe±N(2)
N(1)±Fe±N(4)
N(4)±Fe±N(2)
N(1)±Fe±S(2)
N(1)±Fe±S(1)
N(4)±Fe±S(2)
N(2)±Fe±S(1)
C(1)±S(1)±Fe
C(7)±N(1)±C(6)
C(6)±N(1)±Fe
C(13)±N(3)±Fe
C(18)±N(4)±Fe
C(2)±C(1)±C(6)
C(1)±C(6)±N(1)
C(23)±C(24)±S(2)
C(37)±B±C(25)
C(26)±C(25)±B
81.01(11)
178.84(11)
100.07(11)
92.58(8)
88.08(8)
87.78(9)
167.36(8)
97.10(12)
123.7(3)
119.4(2)
127.5(2)
116.5(2)
118.6(3)
115.1(3)
121.9(3)
112.9(3)
123.8(3)
N(1)±Fe±N(3)
N(3)±Fe±N(2)
N(4)±Fe±N(3)
N(2)±Fe±S(2)
N(3)±Fe±S(1)
N(4)±Fe±S(1)
S(2)±Fe±S(1)
98.41(11)
84.21(11)
81.32(11)
92.66(8)
91.13(8)
90.79(8)
94.20(4)
97.20(12)
116.9(2)
118.8(3)
112.5(2)
119.2(2)
121.2(3)
115.8(3)
119.5(3)
102.1(2)
120.8(3)
C(24)±S(2)±Fe
C(7)±N(1)±Fe
C(12)±N(2)±C(8)
C(17)±N(3)±Fe
C(19)±N(4)±Fe
C(2)±C(1)±S(1)
N(1)±C(7)±C(8)
C(19)±C(24)±S(2)
C(37)±B±C(43)
C(30)±C(25)±B
C(27)±C(26)±C(25) 122.9(3)
C(48)±C(43)±B
C(34)±C(35)±C(36) 120.5(3)
C(48)±C(47)±C(46) 120.1(3)
122.8(3)
The integrity of the Fe(II) complexes in aprotic solvents
like DMF (and acetonitrile) is further exempli®ed by the fact
that both [Fe(PyAS)2] and [Fe(PyMS)2] can be oxidized to
the corresponding Fe(III) species by ferrocenium salts in
anhydrous DMF or acetonitrile. The tetraphenylborate salt
of both Fe(III) complexes can be isolated as highly crystal-
Complex 4
Fe±N(1)
1.928(6)
1.908(6)
2.225(2)
1.859(7)
1.454(9)
1.375(9)
1.482(9)
1.365(9)
1.504(10)
1.528(10)
1.535(10)
1.532(9)
1.634(10)
1.397(9)
1.388(10)
Fe±N(2)
2.054(6)
2.030(6)
2.211(2)
1.861(7)
1.289(9)
1.340(8)
1.274(8)
1.339(9)
1.504(10)
1.367(10)
1.525(10)
1.373(10)
1.628(10)
1.394(11)
1.414(10)
Fe±N(3)
Fe±N(4)
Fe±S(1)
Fe±S(2)
S(1)±C(1)
N(1)±C(2)
N(2)±C(4)
N(3)±C(12)
N(4)±C(14)
C(1)±C(2)
C(1)±C(10)
C(11)±C(19)
C(11)±C(12)
B±C(21)
S(2)±C(11)
N(1)±C(3)
N(2)±C(8)
N(3)±C(13)
N(4)±C(18)
C(1)±C(9)
C(4)±C(5)
C(11)±C(20)
C(17)±C(18)
B±C(39)
‡
line solid although [Fe(PyAS)2] has been isolated with
other counter ions (ClO4 and BF4 ) as well. The oxidation
reaction is clean and can be followed by absorption spectro-
scopy (vide infra). Since ferrocene, the other product of the
reaction, remains soluble in DMF and acetonitrile medium,
the isolation of the Fe(III) complexes is quite straightfor-
ward. This method of synthesizing [Fe(PyAS)2](BPh4) and
[Fe(PyMS)2](BPh4) is by far the most convenient one; use
of air as the oxidant leads to partial decomposition of the
complexes (as evident by the absorption spectra) and reac-
tions of Fe(III) salts and ligands invariably lead to in situ
reduction and formation of the Fe(II) complexes. Oxidation
of [Fe(PyAS)2] and [Fe(PyMS)2] to the corresponding
Fe(III) complexes could also be achieved by oxidation with
(NEt4)3[Fe(CN)6] [31]. However, when one attempts to
isolate 3 or 4 from such oxidation reaction, the desired
C(21)±C(22)
C(40)±C(41)
C(30)±C(31)
O(1)±C(45)
N(1)±Fe±N(2)
N(3)±Fe±N(1)
N(3)±Fe±N(2)
N(1)±Fe±S(1)
N(1)±Fe±S(2)
N(2)±Fe±S(2)
N(2)±Fe±S(1)
S(2)±Fe±S(1)
C(11)±S(2)±Fe
79.4(2)
178.4(3)
102.1(2)
85.5(2)
N(1)±Fe±N(4)
N(3)±Fe±N(4)
N(4)±Fe±N(2)
N(3)±Fe±S(1)
N(3)±Fe±S(2)
N(4)±Fe±S(2)
N(4)±Fe±S(1)
C(1)±S(1)±Fe
C(3)±N(1)±Fe
99.5(2)
80.0(2)
85.0(2)
92.9(2)
86.4(2)
163.3(2)
90.1(2)
99.7(2)
117.8(5)
94.3(2)
88.5(2)
163.1(2)
100.22(8)
102.0(2)
1[Fe(PyAS)2] is insoluble in alcohols.

J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
273
Fig. 2. Computer-generated thermal ellipsoid (probability level 50%) plot
‡
Fig. 1. Computer-generated thermal ellipsoid (probability level 50%) plot
‡
of [Fe(PyMS)2] (cation of 4) with the atom-labeling scheme. Hydrogen
atoms are omitted for the sake of clarity.
of [Fe(PyAS)2] (cation of 3) with the atom-labeling scheme. Hydrogen
atoms are omitted for the sake of clarity.
‡
Fe(III) complex is always contaminated with (NEt4)2-
[Fe(CN)6].
3.3. Oxidation of the Fe(II) complexes by (Fc )(BF4
)
Oxidation of [Fe(PyAS)2] and [Fe(PyMS)2] by ferroce-
nium tetra¯uoroborate proceeds smoothly in DMF solution.
3.1. Structure of [Fe(PyAS)2]BPh4 (3)
‡
As shown in Fig. 3, addition of (Fc )(BF4 ) to a solution of
[Fe(PyAS)2] in DMF ([Fe(PyAS)2] is partially soluble in
DMF) brings about a sharp color change of green to red as a
The structure of the cation of 3 is shown in Fig. 1 and
selected bond distances and angles are listed in Table 2. The
coordination geometry around iron is distorted octahedral
and the two deprotonated PyAS ligands are bound in a mer
fashion. Both PyAS frames are essentially planar due to
extensive delocalization (Fig. 1). The two imino nitrogen
atoms are trans to each other while the two thiolato sulfurs
occupy cis positions. The average Fe(III)±Sthio (thio ˆ
‡
result of conversion of [Fe(PyAS)2] to [Fe(PyAS)2] . Also,
as the oxidation reaction proceeds, more [Fe(PyAS)2] dis-
solves and the reaction goes toward completion quite
rapidly. Presence of two isosbestic points in the region
650±450 nm indicates the presence of two species,
‡
[Fe(PyAS)2] and [Fe(PyAS)2] , in solution. Isolation of
the Fe(III) complexes 3 and 4 in high yields from such
reaction mixtures suggests that the oxidation reactions are
virtually stoichiometric.
When the oxidation is carried out with oxygen instead of
ferrocenium salts, the green solutions ®rst turn red and then
slowly to deep brown. The absorption spectra indicate that
the Fe(III) complexes formed initially, decompose in such
reaction mixtures. No Fe(III) complex has been isolated
from these reactions.
Ê
thiolate) distance is 2.217(1) A and is quite typical of
Ê
Fe(III)±Sthio distances (2.21±2.31 A) observed in complexes
of multidentate ligands with one or more thiolato S donor
centers [7,11,13,20]. The Fe(III)±Nimino distances of 3
Ê
(1.924 (3) and 1.935 (3) A) are practically identical to those
of bis(2-aminoethylthiosalicylideneiminato)iron(III) chlor-
ide [13] and the Fe(III)±Npy (py ˆ pyridine) distances are
well within the range of Fe(III)±Npy distances noted for
other low-spin iron(III) complexes [32,33]. Since all the
chelate rings in 3 are ®ve-membered, signi®cant deviations
from ideal octahedral geometry are noted in the bond angles
like S(1)±Fe±N(2) (167.36(8)8) and N(1)±Fe±N(2)
(81.01(11)8).
3.4. Properties
Both [Fe(PyAS)2] and [Fe(PyMS)2] are green diamag-
netic microcrystalline solids which are moderately sensitive
to oxygen. Solutions of these Fe(II) complexes in (CD3)2SO
exhibit well-resolved NMR spectra, a fact that con®rms the
low-spin con®guration of iron in them. In DMF,
[Fe(PyAS)2] exhibits strong charge transfer bands in the
region 700±800 nm (Fig. 3). These absorptions appear in a
considerably low energy region due to extensive electronic
conjugation in the PyAS ligand frame. [Fe(PyMS)2], an
analogous complex with less electronic conjugation, exhi-
bits such bands at 550±700 nm.
3.2. Structure of [Fe(PyMS)2]BPh4ÁMeOH (4)
The structure of the cation of 4 (Fig. 2) is similar to the
structure of the cation of 3 (Fig. 1) and the metric para-
meters are very comparable (Table 2). Here also the geo-
metry around iron is distorted octahedral and the two
PyMS ligands are bonded to the Fe(III) center in a mer
fashion. However, the presence of the ±CH2± link between
the imine group and the thiolato sulfur in the PyMS ligand
removes the planarity that one observes in the coordinated
PyAS ligand frame.
In the solid state, the two Fe(III) complexes 3 and 4 are
inde®nitely stable in dry dinitrogen atmosphere. When kept
in air, they slowly decompose to brown powders of unknown

274
J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
‡
‡
Fig. 3. Changes in electronic absorption spectra during oxidation of [Fe(PyAS)2] to [Fe(PyAS)2] by ferrocenium tetrafluoroborate in DMF. Arrows indicate
the direction of growth and decay of the bands of [Fe(PyAS)2] upon oxidation to [Fe(PyAS)2] by ferrocenium ion.
compositions. In solutions, the complexes are sensitive to
both oxygen and moisture.
Kovacs and coworkers have reported two Fe(III) com-
plexes of Schiff base ligands, [Fe(AMIT)2]Cl and
[Fe(ADIT)2]Cl,2 which are low-spin at ambient temperature
[8]. The Fe(III) complex of the Schiff-base 2-ami-
noethylthiosalicylideneimine (5) is also low-spin at room
temperature [13]. In contrast, the Fe(III) complexes of two
macrocyclic N3S3 ligands reported by Wieghardt and cow-
orkers, exist in an electronic spin equilibrium between the
high-spin (S ˆ 52)
and low-spin (S ˆ 12) state [7]. At ambient temperature,
polycrystalline 3 and 4 exhibit effective magnetic moments
of 1.98mB and 2.20mB, respectively. These magnetic
moment values con®rm that both Fe(III) complexes remain
low-spin (S ˆ 12) at room temperature. The low-spin con-
®guration of the Fe(III) centers in 3 and 4 is further
con®rmed by their EPR spectra at low temperatures. The
EPR spectra of 3 and 4 at 100 K are shown in Fig. 4 while
the EPR spectrum of 3 at 8 K is displayed in Fig. 5. These
spectra demonstrate that the Fe(III) centers remain low-spin
(S ˆ 12) at temperatures as low as 8 K. The g values of 3
(2.17, 2.11, and 2.01) and 4 (2.13, 2.09, 2.03) are very
similar to those of [Fe(AMIT)2]Cl and [Fe(ADIT)2]Cl. The
complexes 3, 4, [Fe(AMIT)2]Cl, and [Fe(ADIT)2]Cl all
‡
‡
exhibit (gmax gmin) values in the range (0.10±0.21) and
all four complexes have (a) one axial thiolato S atom in the
®rst coordination sphere and (b) the two thiolato S atoms in
the cis con®guration. The low-pH form of nitrile hydratase
exhibits a (gmax gmin) value of 0.21 [6]. In contrast, low-
Fig. 4. X-band EPR spectra of [Fe(PyAS)2] (top) and [Fe(PyMS)2]
(bottom) in DMF glass at 100 K. Spectrometer settings: microwave
frequency, 9.43 GHz; microwave power, 13 mW; modulation frequency,
100 kHz, modulation amplitude, 2 G.
spin Fe(III) centers in NxOy coordination sphere display
(gmax gmin) values in the range of 0.5±1.4 [6,34]. Small
spread of g values therefore appears to be a characteristic
2AMIT ˆ 5-amino-2-methyl-3-azapent-2-ene-1-thiolate; ADIT ˆ 6-
amino-2,3-dimethyl-4-azahex-3-ene-2-thiolate.

J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
275
of 4 is more negative ( 0.51 V versus SCE) and the redox
process is quasireversible. In the case of homoleptic thio-
lates of the type [Fe(SR)4] and [Fe(SAr)4] , the reduction
potentials of alkyl thiolates are more negative than those of
the arene thiolates [19]. This trend suggests that the more
basic alkyl thiolates provide additional stability to Fe(III)
centers. The E1/2 values of 3 and 4 support this conclusion.
Interestingly, complex 4 also exhibits a quasireversible
voltammogram with E1/2 at ‡0.47 V (versus SCE). This
redox process is most possibly associated with ligand
oxidation. Similar ligand-based oxidation has been
observed in electrochemical studies on Fe(III) complexes
of N3S3 macrocyclic ligands [7].
‡
Fig. 5. X-band EPR spectrum of [Fe(PyAS)2] in DMF glass at 8 K.
In summary, the Fe(III) complexes of two Schiff-base
ligands namely, [Fe(PyAS)2]X (X ˆ BF4, BPh4) and
[Fe(PyMS)2] BPh4 have been synthesized via oxidation
of the corresponding Fe(II) complexes with ferrocenium
salts. The Fe(III) centers of these two complexes remain
low-spin in the temperature range 8±300 K. Results of this
and other previously reported works suggest that tridentate
Schiff-base ligands with N2S donor set (L) give rise to low-
Spectrometer settings: microwave frequency, 9.43 GHz; microwave
power, 13 mW; modulation frequency, 100 kHz, modulation amplitude,
2 G.
of low-spin Fe(III) centers with ligated thiolato S donor
atoms.
Six-coordinate Fe(III) Schiff-base complexes with NxOy
‡
‡
‡
‡
donor sets like [Fe(N2O)2] , [Fe(N2O2)L2] , [Fe(N3O2)L]
spin Fe(III) centers in [FeIII(L)2] complexes.
‡
and [Fe(N4O2)] are either high-spin (S ˆ 52) or exhibit spin
equilibria [35,36]. Even the hexadentate Schiff-base ligands
with N4O2 donor set do not afford low-spin (S ˆ 12) Fe(III)
complexes [37]. Thus, the O-analogs of tridentate Schiff-
base ligands like AMITH or 5 give rise to Fe(III) complexes
4. Supplementary material
Tables of crystal data and intensity collection and re®ne-
ment parameters, positional coordinates, complete tables of
bond distances and angles, H atom coordinates, and aniso-
tropic and isotropic thermal parameters for 3 and 4 (24
pages) are available from the authors upon request.
‡
of the type [Fe(N2O)2] which are either high-spin or
exhibit spin equilibria. In contrast, Schiff-base ligands with
N2S donor set like 1, 2, AMITH, ADITH, and 5 all afford
Fe(III) bis complexes that are low-spin at all temperatures.
The change in spin con®guration due to replacement of O
with S in these ligands indicates stronger interaction
between thiolato sulfur and Fe(III) center.
Acknowledgements
The complex 3 exhibits a reversible one-electron redox
process with half-wave potentials (E1/2) of 0.13 V (versus
SCE) in DMF (Fig. 6). As expected, the same voltammo-
gram is obtained with [Fe(PyAS)2] when the potential scan
is initiated from the negative (reducing) side. The E1/2 value
This work was supported by a grant from the Energy
Institute of the University of California. JCN received
partial support from the NIH-MBRS Grant GM08132 and
was a recipient of a GAANN fellowship from the US
Department of Education.
References
[1] W. Huang, J. Jia, J. Cummings, M. Nelson, G. Schneider, Y.
Lindqvist, Structure 5 (1997) 691.
[2] M.J. Nelson, H. Jin, I.M. Turner Jr., G. Grove, R.C. Scarrow, B.A.
Brennan, L. Que Jr., J. Am. Chem. Soc. 113 (1991) 7072.
[3] R.C. Scarrow, B.A. Brennan, J.G. Cummings, H. Jin, D.J. Duong,
J.T. Kindt, M.J. Nelson, Biochemistry 35 (1996) 100078.
[4] H. Jin, I.M. Turner Jr., M.J. Nelson, R.J. Gurbiel, P.E. Doan, B.M.
Hoffman, J. Am. Chem. Soc. 115 (1993) 5290.
[5] B.A. Brennan, J.G. Cummings, D.B. Chase, I.M. Turner Jr., M.J.
Nelson, Biochemistry 35 (1996) 10068.
[6] Y. Sugiura, J. Kuwahara, T. Nagasawa, H. Yamada, J. Am. Chem.
Soc. 109 (1987) 5848.
Fig. 6. Cyclic voltammogram of [Fe(PyAS)2]BPh4 (3) in DMF (0.1 M
Et4NClO4, Pt electrode, 100 mV/s scan speed). Potentials are quoted vs.
aqueous SCE.
[7] T. Beissel, K.S. Burger, G. Voigt, K. Wieghardt, C. Butzlaff, A.X.
Trautwein, Inorg. Chem. 32 (1993) 124.

276
J.C. Noveron et al. / Inorganica Chimica Acta 285 (1999) 269±276
[8] S.C. Shoner, D. Barnhart, J.A. Kovacs, Inorg. Chem. 34 (1995) 4517.
[21] L.F. Lindoy, S.E. Livingstone, Inorg. Chim. Acta 1 (1967) 365.
[22] S.R. Parkin, B. Moezzi, H. Hope, J. Appl. Crystallogr. 28 (1995) 53.
[23] L.F. Lindoy, S.E. Livingstone, Inorg. Chem. 7 (1968) 1149.
[24] H. Jadamus, Q. Fernando, H. Freiser, J. Am. Chem. Soc. 86 (1964)
3056.
[9] A.-S. Chauvin, D.E. Over, J.-C. Chottard, Inorg. Chim. Acta 59
(1995) 388.
[10] A.L. Nivorozhkin, A.I. Uraev, G.I. Bondarenko, A.S. Antsyshkina,
V.P. Kurbatov, A.D. Garnovskii, C.I. Turta, N.D. Brashoveanu,
Chem. Commun. (1997) 1711.
[25] H. Jadamus, Q. Fernando, H. Freiser, Inorg. Chem. 3 (1964) 928.
[26] A. Garg, J.P. Tandon, Transition Met. Chem. 12 (1987) 42.
[27] F. Capitan, F. Salinas, L.F. Capitan-Vallvey, Bull. Soc. Chim. 5
(1979) 185.
[11] J.C. Noveron, M.M. Olmstead, P.K. Mascharak, Inorg. Chem. 37
(1998) 1138.
[12] P.J. Marini, K.S. Murray, B.O. West, J. Chem. Soc., Dalton Trans.
(1983) 143.
[28] A.P. Koley, R. Nirmala, L.S. Prasad, S. Ghosh, P.T. Manoharan,
Inorg. Chem. 31 (1992) 1764.
[13] G.D. Fallon, B.M. Gatehouse, J. Chem. Soc., Dalton Trans. (1975)
1344.
[29] A.C. Braithwaite, C.E.F. Rickard, T. Neil Waters, Transition Met.
Chem. 1 (1975) 5.
[14] R.H. Holm, J.A. Ibers, in: W. Lovenberg (Ed.), Iron±Sulfur Proteins,
Academic Press, New York, 1977, vol. III, Ch. 7.
[15] E. Deutsch, M.J. Root, D.L. Nosco, Adv. Inorg. Bioinorg. Mech. 1
(1982) 269.
[30] W.E. Hill, N. Atabay, C.A. McAuliffe, F.P. McCullough, S.M.
Razzoki, Inorg. Chim. Acta 35 (1979) 35.
[31] P. Mascharak, Inorg. Chem. 25 (1986) 245.
[16] M. Millar, J.F. Lee, S.A. Koch, R. Fikar, Inorg. Chem. 21 (1982)
4105.
[32] X. Tao, D.W. Stephan, P.K. Mascharak, Inorg. Chem. 26 (1987) 754.
[33] W.R. Scheidt, Y.J. Lee, D.K. Geiger, K. Taylor, K. Hatano, J. Am.
Chem. Soc. 104 (1982) 3367.
[17] M. Millar, S.A. Koch, R. Fikar, Inorg. Chim. Acta 88 (1984) L15.
[18] S.A. Koch, L.E. Maelia, M. Millar, J. Am. Chem. Soc. 105 (1983)
5944.
[34] Y. Sugiura, J. Am. Chem. Soc. 102 (1980) 5208.
[35] J.-P. Costes, F. Dahan, J.-P. Laurent, Inorg. Chem. 29 (1990) 2448.
[36] K. Ramesh, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1992)
83.
[19] L.E. Maelia, M. Millar, S.A. Koch, Inorg. Chem. 31 (1992) 4594.
[20] N. Govindaswamy, D.A. Quarless Jr., S.A. Koch, J. Am. Chem. Soc.
117 (1995) 8468.
[37] M. Tweedle, L.J. Wilson, J. Am. Chem. Soc. 98 (1976) 4824.