Sulfonamido tripods: Tuning redox potentials via ligand modifications

Article abstract of DOI:10.1016/j.poly.2014.09.014

A series of Fe^II-OH₂ complexes were synthesized with ligands based on the tetradentate sulfonamido tripod N,N′,N″-[2,2′,2″-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfonamido). These complexes differ by the substituent on the aryl rings and were fully characterized, including their molecular structures via X-ray diffraction methods. All the complexes were five-coordinate with trigonal bipyramidal geometry. A linear correlation was observed between the electronic effects of each ligand, given by the Hammett constants of the para-substituents, and the potential of the Fe^II/Fe^III redox couple, which were determined using cyclic voltammetry. It was found that the range of redox potentials for the complexes spanned approximately 160 mV.

Full text of DOI:10.1016/j.poly.2014.09.014

Polyhedron 85 (2015) 777–782
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Sulfonamido tripods: Tuning redox potentials via ligand modifications
⇑
Nathanael Lau, Joseph W. Ziller, A.S. Borovik
Department of Chemistry, University of California – Irvine, 1102 Natural Sciences II, Irvine, CA 92697-2025, United States
a r t i c l e i n f o
a b s t r a c t
A series of FeII–OH2 complexes were synthesized with ligands based on the tetradentate sulfonamido tri-
pod N,N0,N00-[2,20,200-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfonamido). These complexes differ by the
substituent on the aryl rings and were fully characterized, including their molecular structures via X-ray
diffraction methods. All the complexes were five-coordinate with trigonal bipyramidal geometry. A linear
correlation was observed between the electronic effects of each ligand, given by the Hammett constants
of the para-substituents, and the potential of the FeII/FeIII redox couple, which were determined using
cyclic voltammetry. It was found that the range of redox potentials for the complexes spanned approx-
imately 160 mV.
Article history:
Received 22 July 2014
Accepted 12 September 2014
Available online 23 September 2014
Keywords:
Ligand design
Iron complexes
Hydrogen bonds
Coordination complexes
Iron-aquo complexes
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
sphere can alter the redox potential of transition metal complexes
[14–17]. We have been investigating the effects on adjustments in
The structure and function of metal complexes can be under-
stood within the context of the relationships between the primary
and secondary coordination spheres [1–3]. These relationships are
best illustrated in metalloproteins, in which the protein-induced
microenvironments at the active site influence physical properties
and reactivity. For instance, the electrochemical studies on a
variety of metalloproteins showed that control of redox potentials
can occur with modulations of either the primary or secondary
coordination spheres. In series of studies, Lu showed that changes
in the primary and secondary coordination spheres of a single Cu
center had a large impact on the CuI/CuII redox process in blue cop-
per proteins [4–6]. Relatively small changes in redox potentials
resulted when a tyrosine residue near the FeIII center in rubredoxin
was modified with nonnative tyrosine residues with differing para-
substituents [7]. Analysis of electrochemical measurements did
however demonstrate strong correlation between the reduction
both coordination spheres in synthetic systems through the rational
design of multidentate ligands. Most of our work has examined the
effects of the secondary coordination sphere, whereby control is
achieved through intramolecular H-bonds [3,18,19]. More
recently we have been exploring the chemistry associated with
the tetradentate sulfonamide-based tripodal ligand N,N0,N00-(nitrilo-
tris(ethane-2,1-diyl))tris(2,4,6-trimethyl-benzenesulfonamido)
([MST]3ꢀ). Mountford first introduced these types of tripodal
ligands [20,21] and we have shown that [MST]3ꢀ can form hetero-
and homobimetallic complexes containing intramolecular H-bonds
[19,22–24]. In this report, we describe the preparation and
chemistry for a series of monomeric FeII–OH2 complexes (Fig. 1,
Scheme 1) with different sulfonamido tripodal ligands [RST]3ꢀ
,
(N,N0,N’’-[2,20,2’’-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfon-
amido) R = OMe, Me, H, Cl, CF3). The ligands differ by the para-
substituent of the aryl group on the sulfonamido arms and allowed
us to examine how these modifications correlated with individual
properties of the FeII–OH2 complexes. Our findings illustrated that
the electrochemical properties of the complexes can be predictably
tuned through modulation of the ligand but other physical proper-
ties are not significantly affected by these modifications.
potential of the FeIII center and the Hammett constants (
rp) of
the para-substituent of the nonnative tyrosine-based residues. Lar-
ger changes were reported in a series of mutated Fe superoxide
dismutases, in which a single point mutation caused a change of
greater than 500 mV for the FeII/FeIII redox couple [8].
Numerous synthetic systems have also shown that varying the
structural features on ligands can control redox processes [9–12].
The recent work of Costas illustrates how physical properties of Fe
complexes can be varied through modification of ring substituents
of a coordinated pyridine ligand [13]. Moreover, intramolecular
hydrogen bonds (H-bonds) within the secondary coordination
2. Experimental
2.1. General methods
All reagents were purchased from commercial sources and used
as received unless otherwise noted. Solvents were sparged with
argon and dried over columns containing Q-5 and molecular
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.poly.2014.09.014
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

778
N. Lau et al. / Polyhedron 85 (2015) 777–782
R
ceased. Fe(OAc)2 (68 mg, 0.39 mmol) and NMe4OAc (52 mg,
H
N
0.39 mmol), were added to the cloudy white reaction, and the solu-
tion was stirred. After 3 h, 5 mL of Et2O was added to the yellow
solution to aid the precipitation of NaOAc. The reaction mixture
was filtered through a medium porosity glass-fritted funnel to
remove the insoluble species and the filtrate was dried under vac-
uum. The resulting pale yellow solid was redissolved in 5 mL of
acetonitrile (MeCN), stirred for 20 min, and filtered using a fine
porosity glass-fritted funnel. The filtrate was concentrated under
vacuum to ca. 1 mL and treated with Et2O (10 mL) then pentane
(40 mL) to precipitate a pale yellow solid. The pale yellow solid
was collected on a medium porosity glass-fritted funnel and dried
under vacuum to give 182 mg (91%) of product. FTIR (KBr disc,
cmꢀ1, selected bands): 3413, 3046, 2964, 2902, 2861, 1608, 1490,
1403, 1326, 1263, 1134, 1062, 976, 821, 710, 622, 605. MS (ESꢀ,
N
S
3
O
O
H3RST
Fig. 1. H3RST compounds where R = ꢀCF3 ([H3F3ST]), ꢀCl ([H3CST]), ꢀH ([H3PST]),
ꢀCH3 ([H3TST]), ꢀOCH3 ([H3MOST]).
sieves. The syntheses of the ligands were carried out in air and the
preparations of metal complexes were conducted in a Vacuum
Atmospheres, Co. drybox under nitrogen or argon atmosphere.
Sodium hydride (NaH) as a 60% dispersion in mineral oil was fil-
tered with a medium porosity glass-fritted funnel and washed 5
times each with pentane and Et2O. Solid NaH was dried under vac-
uum and stored under inert atmosphere. Fe(OAc)2 was prepared by
literature procedures [25]. Water was degassed by five freeze–
pump–thaw cycles and stored under inert atmosphere. The tripodal
ligands H3MOST, H3TST, and H3PST were synthesized following
m/z): Exact mass calcd for
823.1. This salt, presumably [NMe4][FeIIF3ST] (103 mg, 0.12 mmol)
in 5 mL of CH2Cl2, was treated with H2O (8 L, 0.46 mmol) in one
C27H24N4O6S3F9Fe: 823.0, Found:
l
portion via a syringe and the mixture was stirred. After 15 min,
volatiles were removed under reduced pressure and the solid res-
idue was redissolved in CH2Cl2 (6 mL), filtered through a medium
porosity glass-fritted funnel, and layered under pentane. White
needle crystals were collected via filtration and dried under vac-
uum, to give 98 mg (94%) of crystalline product. Elemental Anal.
Calc. for [NMe4][FeIIF3ST(OH2)] C31H38N5O7S3F9Fe: C, 40.66; H,
4.18; N, 7.65. Found: C, 40.74; H, 4.06; N, 7.32%. FTIR (KBr disc,
cm1, selected bands): 3340, 3044, 2964, 2899, 2860, 1608, 1490,
1403, 1327, 1261, 1136, 1062, 977, 825, 710, 622, 605. (Nujol,
literature procedures with modifications
information for details [26–28].
– see Supporting
2.2. Preparation of ligands
2.2.1. N,N0,N00-(nitrilotris(ethane-2,1-diyl))tris(4-(trifluoromethyl)-
benzene-sulfonamide) (H3F3ST)
To a 250 mL round bottom flask containing tris(2-amino-
ethyl)amine (tren) (0.25 g, 1.7 mmol) and NaOH (0.33 g, 8.2 mmol)
in water (20 mL), 4-(trifluoromethyl)benzenesulfonyl chloride
(1.3 g, 5.4 mmol) dissolved in Et2O (10 mL) was added dropwise
at room temperature with vigorous stirring. After allowing the
reaction mixture to stir for 2 days, the Et2O was removed under
reduced pressure and the resulting white precipitate was collected
on a medium porosity glass-fritted funnel and washed with water.
The resulting white power was dried under reduced pressure over-
night, then dissolved in dichloromethane (CH2Cl2) (100 mL) and
dried with anhydrous sodium sulfate. After filtering off the insolu-
ble species, the solvent was removed and the residue was dried
under vacuum at 45 °C for 5 h. The residue was brought into the
dry box, redissolved in CH2Cl2 (10 mL), and precipitated with pen-
cmꢀ1): 3295 (OH). leff
[FeCp2]0/+
(l
B): 4.7(2). E1/2 (MeCN): ꢀ0.284 V versus
.
2.3.2. [NMe4][FeIICST(OH2)]
This salt was prepared in an analogous manner to [NMe4]
[FeIIF3ST(OH2)] using H3CST (180 mg, 0.27 mmol), NaH (20 mg,
0.81 mmol), Fe(OAc)2 (47 mg, 0.27 mmol), NMe4OAc (38 mg,
0.28 mmol) to isolate a pale yellow powder, which presumably
was the [NMe4][FeIICST] salt (141 mg, 66%). FTIR (KBr disc, cmꢀ1
,
selected bands): 3341, 3036, 2960, 2898, 2856, 1636, 1581, 1477,
1392, 1251, 1141, 1084, 975, 823, 751, 630, 593. MS (ESꢀ, m/z):
Exact mass calcd for C24H24N4O6S3Cl3Fe: 720.9, Found: 721.0. The
isolated [NMe4][FeIICST] (33 mg, 0.04 mmol) was treated with
tane to give 0.81 g (62%) of an ivory powder. FTIR (KBr disc, cmꢀ1
,
H2O (3 l
L, 0.17 mmol) in an analogous manner to [NMe4][FeIIF3
selected bands): 3353, 3303, 3262, 3106, 2965, 2824, 1610, 1405,
1323, 1165, 1106, 1062, 958, 846, 713, 602, 554. 1H NMR
(500 MHz, CDCl3, ppm): 2.61 (t, 2H), 3.06 (t, 2H), 5.91 (br s, 1H
NH), 7.79 (d, 2H), 8.08 (d, 2H). 13C NMR (500 MHz, CDCl3, ppm):
41.5, 54.8, 126.8, 128.0, 129.3, 143.6. HRMS (ES+, m/z): Exact mass
calcd for C27H27N4O6S3F9: 771.10, Found: 771.04.
ST(OH2)] to give 28 mg (83%) of crystalline product. Elemental
Anal. Calc. for [NMe4][FeIICST(OH2)] C28H38N5O7S3Cl3Fe: C, 41.26;
H, 4.70; N, 8.59. Found: C, 41.09; H, 4.57; N, 8.22%. FTIR (KBr disc,
cmꢀ1, selected bands): 3313, 3036, 2964, 2899, 2855, 1580, 1476,
1391, 1247, 1138, 1083, 975, 821, 751, 629, 593. (Nujol, cmꢀ1):
3303 (OH). leff (CDCl3,
lB): 4.8(3). E1/2 (MeCN): ꢀ0.338 V versus
2.2.2. N,N0,N00-(nitrilotris(ethane-2,1-diyl))tris(4-chloro-
[FeCp2]0/+
.
benzenesulfonamide) (H3CST)
2.3.3. [NMe4][FeIIPST(OH2)]
Prepared in an analogous manner to H3F3ST using tren (0.26 g,
1.8 mmol), NaOH (0.34 g, 8.4 mmol), 4-chlorobenzenesulfonyl
chloride (1.2 g, 5.6 mmol) to give 0.66 g (55%) product. FTIR (KBr
disc, cmꢀ1, selected bands): 3291, 3093, 2957, 2850, 1587, 1477,
1396, 1327, 1162, 1094, 955, 825, 753, 618, 567. 1H NMR
(500 MHz, CDCl3, ppm): 2.55 (t, 2H), 2.98 (t, 2H), 5.91 (br s, 1H
NH), 7.49 (d, 2H), 7.86 (d, 2H). 13C NMR (500 MHz, CDCl3, ppm):
41.4, 54.7, 129.0, 129.9, 138.5, 139.6. HRMS (ES+, m/z): Exact mass
calcd for C24H27N4O6S3Cl3: 669.02, Found: 669.01.
This salt was prepared in an analogous manner to [NMe4]
[FeIIF3ST(OH2)] using H3PST (300 mg, 0.53 mmol), NaH (38 mg,
1.6 mmol), Fe(OAc)2 (92 mg, 0.53 mmol), NMe4OAc (71 mg,
0.53 mmol) to isolate a pale yellow powder, which presumably
was the [NMe4][FeIIPST] salt (340 mg, 93%). FTIR (KBr disc, cmꢀ1
,
selected bands): 3297, 3035, 2955, 2897, 2853, 1489, 1445, 1246,
1132, 974, 821, 761, 715, 694, 608, 586. MS (ESꢀ, m/z): Exact mass
calcd for
C24H27N4O6S3Fe: 619.0, Found: 619.1. The isolated
[NMe4][FeIIPST] salt (26 mg, 0.04 mmol) was treated with H2O
2.3. Preparation of the complexes
(3
l
L, 0.14 mmol) in an analogous manner to [NMe4][FeII
F3ST(OH2)] to give 27 mg (85%) of crystalline product. Elemental
Anal. Calc. for [NMe4][FeIIPST(OH2)] C28H41N5O7S3Fe: C, 47.25; H,
5.81; N, 9.84. Found: C, 47.19; H, 5.53; N, 9.63%. FTIR (KBr disc,
cmꢀ1, selected bands): 3313, 3034, 2957, 2897, 2854, 1489, 1445,
1247, 1132, 975, 821, 761, 714, 694, 608, 586. (Nujol, cmꢀ1):
2.3.1. [NMe4][FeIIF3ST(OH2)]
A solution of H3F3ST (300 mg, 0.39 mmol) dissolved in 6 mL of
anhydrous dimethylacetamide (DMA) was treated with solid NaH
(28 mg, 1.2 mmol). The mixture was stirred until gas evolution

N. Lau et al. / Polyhedron 85 (2015) 777–782
779
NMe4
O
O
O
H
R
S
N
O
R
H
O
S
O
S
N
O
H
N
a-d
-3 NaOAc
FeII
N
R
N
S
3
R
O
O
N
[NMe4][FeIIRST(H2O)]
Scheme 1. Preparation of [NMe4][FeIIRST(OH2)] complexes. Conditions: (a) 3NaH, DMA, N2, rt; (b) Fe(OAc)2, DMA, N2, rt; (c) NMe4OAc, DMA, N2, rt; (d) 4H2O, CH2Cl2, N2, rt.
R = ꢀCF3, ꢀCl, ꢀH, ꢀCH3, ꢀOCH3.
3290 (OH). leff (CDCl3,
[FeCp2]0/+
l
B): 4.9(9). E1/2 (MeCN): ꢀ0.394 V versus
cobaltocenium/cobaltocene couple ([CoCp2]0/+) was used as an
internal reference then scaled against the ferrocene/ferrocenium
couple ([FeCp2]+/0) [30]. Tetrabutylammonium hexafluorophos-
phate (TBAP) was use as the supporting electrolyte at a concentra-
tion of 0.1 M. Electrochemical values are reported as the average of
three independent measurements.
.
2.3.4. [NMe4][FeIITST(OH2)]
This salt was prepared in an analogous manner to [NMe4]
[FeIIF3ST(OH2)] using H3TST (300 mg, 0.49 mmol), NaH (30 mg,
1.2 mmol), Fe(OAc)2 (71 mg, 0.41 mmol), NMe4OAc (55 mg,
0.41 mmol) to isolate a pale yellow powder which presumably
2.5. Crystallography
was the [NMe4][FeIITST] salt (310 mg, 86%). FTIR (KBr disc, cmꢀ1
,
selected bands): 3264, 3037, 2958, 2896, 2850, 1599, 1492, 1325,
1246, 1138, 975, 818, 664, 600, 555. MS (ESꢀ, m/z): Exact mass
calcd for C27H33N4O6S3Fe: 661.1, Found: 661.2. The isolated solid
[NMe4][FeIITST] salt (56 mg, 0.08 mmol) was treated with H2O
A Bruker SMART APEX II diffractometer and the APEX2 program
package was used to determine the unit-cell parameters and for
data collection. Crystallographic details are summarized in the
Supporting information, and in Table S1.
(5 l
L, 0.30 mmol) in an analogous manner to [NMe4][FeIIF3ST
(OH2)] to give 42 mg (74%) of crystalline product. Elemental Anal.
Calc. for [NMe4][FeIITST(OH2)] C31H47N5O7S3Fe: C, 49.40; H, 6.28;
3. Results and discussion
N, 9.29. Found: C, 49.21; H, 6.04; N, 9.02%. FTIR (KBr disc, cmꢀ1
,
3.1. Preparation and properties of the [NMe4][FeIIRST(OH2)]
selected bands): 3281, 3036, 2957, 2896, 2852, 1599, 1491, 1244,
1138, 975, 817, 664, 599, 554. (Nujol, cmꢀ1): 3249 (OH). leff
B): 4.8(1). E1/2 (MeCN): ꢀ0.419 V versus [FeCp2]0/+
.
The five salts were prepared using the synthetic route described
in Scheme 1. The starting H3RST compounds were deprotonated
with 3 equivalents of NaH in DMA. Metallation with Fe(OAc)2 and
metathesis with NMe4OAc resulted in the formation of FeII com-
plexes as the [NMe4]+ salt. Adding Et2O facilitated precipitation of
NaOAc from DMA that was removed from the reaction mixture via
filtration. The Fe salts were isolated and their properties are consis-
tent with a formulation of [NMe4][FeIIRST]. However, we were never
able to obtain acceptable elemental analysis for any of these salts,
possibly because a small amount of the complexes were five-coordi-
nate, which could arise from weakly interacting solvent molecules.
To prepare the FeII–OH2 complexes, CH2Cl2 solutions of each
[FeIIRST]ꢀ complex were treated with four equivalents of water
and the mixtures were allowed to stir for 5 min. Volatiles were
removed under reduced pressure, and the resulting salts were
recrystallized from solutions in CH2Cl2 that were layered under pen-
tane to afford crystalline products in yields ranging from 74% to 96%.
The [NMe4][FeIIRST(OH2)] salts were characterized by EPR and
FTIR spectroscopies, solution effective magnetic moment measure-
ments, and elemental analysis. The perpendicular-mode EPR spec-
tra measured at 77 K did not contain any features, which is
consistent with each complex having a high spin FeII center. This
premise is supported by the room temperature solution effective
magnetic moment of the complexes determined using the Evans’
method. The effective magnetic moments for the complexes were
statistically the same and matched closely with the spin-only mag-
(CDCl3,
l
2.3.5. [NMe4][FeIIMOST(OH2)]
This salt was prepared in an analogous manner to [NMe4]
[FeIIF3ST(OH2)] using H3MOST (200 mg, 0.30 mmol), NaH (22 mg,
0.91 mmol), Fe(OAc)2 (53 mg, 0.30 mmol), NMe4OAc (41 mg,
0.30 mmol) to isolate a pale yellow powder ([NMe4][FeIIMOST])
in 76% yield. FTIR (KBr disc, cmꢀ1, selected bands): 3264, 3036,
2958, 2898, 2847, 1597, 1496, 1304, 1250, 1134, 1013, 975, 818,
667, 601, 564. MS (ESꢀ, m/z): Exact mass calcd for C27H33N4O9S3Fe:
709.1, Found: 709.1. The isolated solid [NMe4][FeIIMOST] (62 mg,
0.08 mmol) was treated with H2O (6 lL, 0.31 mmol) in an analo-
gous manner to [NMe4][FeIIF3ST(OH2)] to give 61 mg (96%) of crys-
talline product. Elemental Anal. Calc. for [NMe4][FeIIMOST(OH2)]
C
31H47N5O10S3Fe: C, 46.44; H, 5.91; N, 8.73. Found: C, 46.34; H,
5.62; N, 8.66%. FTIR (KBr disc, cmꢀ1, selected bands): 3302, 3039,
2961, 2897, 2843, 1597, 1496, 1306, 1254, 1134, 1025, 972, 832,
667, 603, 564. (Nujol, cmꢀ1): 3247 (OH). leff (CDCl3,
lB): 4.9(3).
E1/2 (MeCN): ꢀ0.441 V versus [FeCp2]0/+
.
2.4. Physical methods
Elemental analyses were performed on a Perkin–Elmer 2400
CHNS analyzer. 1H NMR and 13C NMR were recorded on a Bruker
DRX500 spectrometer. Fourier transform infrared (FTIR) spectra
were collected on a Varian 800 Scimitar Series FTIR spectrometer.
High-resolution mass spectra were collected using Waters Micro-
mass LCT Premier Mass Spectrometer. Perpendicular-mode X-band
electron paramagnetic resonance (EPR) spectra were collected
using a Bruker EMX spectrometer at 77 K using liquid nitrogen.
Solution effective magnetic moments were measured by the Evans’
method on a Bruker DRX500 spectrometer using flame sealed
standard cores of 1:1 CHCl3:CDCl3 [29]. Cyclic voltammetric
experiments were conducted using a CH1600C electrochemical
analyzer. A 2.0 mm glassy carbon electrode was used as the work-
ing electrode at scan velocities 0.1 V sꢀ1 unless otherwise noted. A
netic moment value of 4.9 lB. These values can be attributed to an
S = 2 spin ground state from the five-coordinate FeII complexes. FTIR
spectra of the complexes recorded as a Nujol mull revealed the pres-
ence of peaks that ranged between 3300 and 3250 cmꢀ1, which is
consistent with O–H vibrations from a coordinated water molecule.
3.2. Solid-state molecular structures of [FeIIRST(OH2)]ꢀ complexes
The molecular structures of the [NMe4][FeIIRST(OH2)] salts were
characterized by X-ray diffraction methods. Selected bond lengths

780
N. Lau et al. / Polyhedron 85 (2015) 777–782
Table 1
2.782 Å. Both these distances are indicative of H-bonds formed
Selected metrical parameters for the [FeIIRST(OH2)]ꢀ complexes.
between the aqua ligand and oxygen atoms of the sulfonamido
groups from two different arms of the [RST]3ꢀ ligands. These two
intramolecular H-bonds undoubtedly assist in positioning the aqua
ligand within the complexes [33]. The other metrical parameters
are consistent with values found in similar FeII complexes [34].
[FeIIRST(OH2)]ꢀ R =
*
*
ꢀCF3
ꢀCl
ꢀH
ꢀCH3
ꢀOCH3
Bond distances (Å)
Fe1–O1
Fe1–N1
Fe1–N2
Fe1–N3
2.120(3)
2.208(3)
2.010(3)
2.071(3)
2.082(3)
2.672(3)
2.782(3)
3.005(3)
2.096(2)
2.219(2)
2.112(2)
2.094(2)
2.070(2)
2.708(2)
2.773(2)
3.087(2)
2.092(2)
2.139(1)
2.214(1)
2.107(1)
2.098(1)
2.065(1)
2.697(1)
2.676(1)
3.089(1)
2.090(1)
2.099(2)
2.227(2)
2.086(2)
2.108(2)
2.069(2)
2.763(2)
2.701(2)
3.090(2)
2.088(2)
2.164(4)
2.226(4)
2.071(4)
2.091(4)
2.041(4)
2.673(4)
2.707(4)
3.089(4)
2.068(4)
3.3. Effects of the substituents on the [RST]3ꢀ ligands: vibrational
properties
Fe1–N4
O1ꢂ ꢂ ꢂO2
O1ꢂ ꢂ ꢂO4
O1ꢂ ꢂ ꢂO6
The para-substituents on aryl groups of the [RST]3ꢀ ligands var-
ied in their electronic effects, which often causes predictable
changes in the physical and chemical properties within com-
pounds. However, we did not observe any correlations between
the substituents and either the vibrational properties of the coordi-
nated aqua ligands or the structural parameters of the complexes.
Average Fe–Neq 2.084(3)
Bond angle (°)
N1–Fe1–O1
N1–Fe1–N2
N1–Fe1–N3
N1–Fe1–N4
N2–Fe1–N3
N2–Fe1–N4
N3–Fe1–N4
174.91(1) 169.74 (6) 170.91(5) 170.25(7) 173.19(1)
80.25(1)
79.75(1)
80.98(1)
80.11 (6)
78.93 (6)
80.13(6)
79.08(5)
80.76(5)
80.58(5)
79.17(7)
80.20(7)
79.85(7)
80.50(1)
79.63(1)
80.85(1)
The energies of the m(OH) bands would be expected to trend based
on the electronic effects that the substituents have on the electron-
withdrawing ability of the tripodal ligands. As the electron density
at the metal center decreases because of the changes in the ligand
field provided by the different [RST]3ꢀ ligands, the acidity of the
aqua ligand should also increase, which would be reflected in an
115.48(1) 115.57(7) 114.60(5) 115.98(7) 112.21(2)
120.95(1) 120.18(7) 116.96(5) 115.70(7) 116.40(2)
115.24(1) 114.93(7) 119.85(5) 119.02(8) 123.04(2)
0.859 0.826 0.851 0.854 0.836
s
value
*
Indicates average values, because of two crystallographically distinct but
chemically equivalent complexes in the asymmetric unit cell of these structures.
decrease in the energy of the m(OH) band. Yet this analysis is com-
plicated by the presence of intramolecular H-bonds involving the
coordinated aqua ligand. The electronic effects of the substituents
on the [RST]3ꢀ also affects the ability of these ligands to act as
H-bond acceptors, with the more electron-withdrawing ligands
(e.g., R = ꢀCF3) being the poorest at accepting a H-bond. Therefore,
the effects of increased acidity of the Fe–OH unit versus the
decrease in H-bond accepting within the series of [FeIIRST(OH2)]ꢀ
complexes could be nearly offsetting and result in the observed
lack of a correlative property.
and bond angles are given in Table 1 and representative thermal
ellipsoid diagrams of [FeIITST(OH2)]ꢀ and [FeIIPST(OH2)]ꢀ are
shown in Fig. 2. All of the salts crystallized with their anionic
complexes as monomers, with one distinct metal complex per
asymmetric unit cell for three of the five salts: [NMe4]
[FeIICST(OH2)], [NMe4][FeIIPST(OH2)], and [NMe4][FeIITST(OH2)].
Two crystallographically distinct but chemically equivalent
complexes were contained per asymmetric unit cell in [NMe4]
[FeIIF3ST(OH2)] and [NMe4][FeIIMOST(OH2)].
The [FeIIRST(OH2)]ꢀ complexes had similar N4O primary coordi-
nation spheres around the FeII center, whereby the trigonal plane is
defined by the three deprotonated nitrogen atoms (N2, N3, N4)
from the sulfonamido groups. One apical position (N1) is occupied
by the amine nitrogen atom of the [RST]3ꢀ ligands and the other
site is taken by oxygen atom (O1) from the external aqua ligand.
The FeII center is displaced from the trigonal plane in each complex
(ꢁ0.35 Å) toward the O1. Each complex shows a modest distortion
from trigonal bipyramidal (tbp) coordination geometry, as gauged
3.4. Effects of the substituents on the [RST]3ꢀ ligands: structural
properties
There are no statistically significant trends between the
electronic properties of the para-substituent on the [RST]3ꢀ ligand
and the structural parameters of the [FeIIRST(OH2)]ꢀ complexes.
For example, the Fe1–O1 bond distances should change in
accordance with the effects of the substituent on the [RST]3ꢀ
ligand, with shortest bond distance predicted to be found in
[FeIIF3ST(OH2)]ꢀ. However, [FeIITST(OH2)]ꢀ and [FeIICST(OH2)]ꢀ
had statistically equivalent Fe1–O1 bond lengths and were the
shortest values in the series. In addition, the average Fe1–Neq bond
distances in [FeIITST(OH2)]ꢀ, [FeIIPST(OH2)]ꢀ, [FeIICST(OH2)]ꢀ,
[FeIIF3ST(OH2)]ꢀ are statistically the same.
using the structural parameter
s: the values for the complexes
range from 0.826 to 0.859 [31]. The distortions from idealized
tbp could be caused, in part, from a Jahn–Teller effect that should
be present in high-spin d6 metal complex having local C3 symme-
try [32]. This effect should be small because it arises from the dxz
and dyz orbitals that are formally non-bonding. A larger contributor
to the distortion is the contraction of the N1–Fe1–O1 angle, which
in each complex is less than 175°. Note that the ‘‘bend’’ in the angle
is such that O1 is positioned towards two of the sulfonamido arms
containing O2 and O4, with O1ꢂ ꢂ ꢂO2 distances ranging from 2.672
to 2.763 Å and the O1ꢂ ꢂ ꢂO4 distances ranging from 2.676 to
3.5. Effects of the substituents on the [RST]3ꢀ ligands: redox properties
The electrochemical properties of the [FeIIRST(OH2)]ꢀ com-
plexes were explored using cyclic voltammetry (Fig. 3). A quasi-
reversible one-electron process was observed for each complex,
Fig. 2. Thermal ellipsoid diagram depicting the molecular structure of (A) [FeIIPST(OH2)]ꢀ and (B) [FeIITST(OH2)]ꢀ. Ellipsoids are drawn at the 50% probability level, and only
the aqua H atoms are shown for clarity.

N. Lau et al. / Polyhedron 85 (2015) 777–782
781
decrease in the rate of electron transfer between the complex with
[F3ST]3ꢀ and [MOST]3ꢀ at room temperature. Moreover, the slope of
the linear trend line is 4.5 kcal molꢀ1 per Hammett unit. This is
greater than the slope of the linear trend line of 0.7 kcal molꢀ1 per
Hammett unit observed in a study on synthetic rubredoxins that
examined only secondary coordination sphere effects [7], but less
than the slope of the linear trend line of 9.9 kcal molꢀ1 per Hammett
unit observed in a study on a series of FeII bis-terpyridyl complexes
that examined primary coordination sphere effects [11].
4. Summary and conclusions
The present study describes the synthesis and characterization
of a series of FeII–OH2 complexes with sulfonamido tripodal
ligands. These ligands were designed to vary the properties at
the iron center via modifying the para-substituent of the aryl group
on the ligand sulfonamido arms. The [FeIIRST(OH2)]ꢀ complexes
have many similar properties despite the modulation of the
ligands. Structurally, all of the complexes in this series have nearly
the same distorted tbp geometry with small metrical differences in
their primary coordination spheres. The complexes were deter-
mined to have high spin S = 2 FeII centers, as each of their effective
magnetic moments agreed closely with the spin-only magnetic
moment value of 4.9
the [FeIIRST(OH2)]ꢀ complexes, specifically the energies of the
bands associated with the (OH), did not show a correlation with
lB. Moreover, the vibrational properties of
m
Fig. 3. Cyclic voltammogram of (A) [NMe4][FeIIF3ST(OH2)], (B) [NMe4]
[FeIICST(OH2)], (C) [NMe4][FeIIPST(OH2)], (D) [NMe4][FeIITST(OH2)], (E) [NMe4]
[FeIIMOST(OH2)] measured in MeCN (0.1 M TBAP). All voltammograms were
collected at 100 mV sꢀ1 and internally referenced against [CoCp2]0/+, then scaled
the substituent effects on the [RST]3ꢀ ligands. We attribute the lack
of correlative relationships to the presence of intramolecular
H-bonds within each complex. Comparing the H-bond donating
properties of the Fe–OH2 unit with the H-bond accepting ability
of the [RST]3ꢀ ligands throughout the series of complexes led us
to suggest that the H-bonds mitigate the influence that the substit-
uents may have on these properties of the complexes. Neverthe-
less, we observed a small, but significant trend in the redox
potentials within the series of [FeIIRST(OH2)]ꢀ complexes. A linear
trend between the redox potentials of the FeII/FeIII couples and the
rp for the ligand para-substituent was demonstrated, illustrating
that the electrochemical properties of the complexes can be tuned
by modifying the [RST]3ꢀ ligands.
against [FeCp2]+/0
.
which was assigned as the FeII/FeIII couple. In contrast to the
discussion above, the redox potentials for this couple was depen-
dent on the electronic effects of the ligand, as shown by a linear
Hammett plot of the E1/2 of the FeII/FeIII couple versus the rp of
the ligand para-substituents (Fig. 4) [35]. The high r2 value of
0.99 for the linear best-fit demonstrates a high correlation between
the oxidation potentials and the electronic properties the different
[RST]3ꢀ ligands. Complexes with ligands that contained para-
substituents with greater electron-withdrawing ability shifted
the FeII/FeIII couple to more positive potentials, which is consistent
with these ligands stabilizing the FeII–OH2 complexes [36].
Acknowledgment
The authors thank the National Institutes of Health, USA
(GM050781).
The potential difference between the complex with the most
electron-withdrawing ligand, [F3ST]3ꢀ, and the most electron-
donating ligand, [MOST]3ꢀ, was 157 mV or a free energy difference
of 3.62 kcal molꢀ1. This corresponds to an approximately 450-fold
Appendix A. Supplementary data
CCDC 982221, 982222, 982223, 982224 and 982225 contain the
supplementary crystallographic data for [NMe4][FeIIF3ST(OH2)],
[NMe4][FeIICST(OH2)], [NMe4][FeIIPST(OH2)], [NMe4][FeIITST(OH2)]
and [NMe4][FeIIMOST(OH2)] respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2014.09.014.
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