A hydrogen-bond facilitated cycle for oxygen reduction by an acid- and base-compatible iron platform

Article abstract of DOI:10.1021/ic9006668

We report a hydrogen-bond functionalized N4Py ligand platform (N,N-bis(2-R-6-pyridylmethyl)-N-bis(2-pyridyl)methylamine; R = neopentyl-NH, N4Py^2NpNH, 9; R = phenyl-NH, N4Py^2PhNH, 10) and the ability of its iron(II)triflate [N4Py^2RFe^II(OTf)][OTf] complexes (R = NpNH, 11; R = PhNH, 12) to activate and reduce dioxygen in a synthetic cycle by coupled proton and electron transfer. A pair of iron(III)-hydroxlde [N4Py^2RFe^III(OH)][OTf]₂ complexes (R = NpNH, 13; R = PhNH, 14) are isolated and structurally and spectroscopically characterized after exposure of the iron(II)-triflate precursors to 1 atm of O₂ at ambient temperature. The stability of this system to acids and bases allows regeneration of the labile iron(II)-triflate starting materials by sequential electron and proton transfer with cobaltocene and triflic acid, respectively, or through direct proton-coupled reduction with ascorbic acid. In the stepwise process, reduction of the iron(III)-hydroxide complexes with cobaltocene gives structurally homologous iron(II)-hydroxlde [N4Py^2RFe^II(OH)][OTf] congeners (R = NpNH, 15; R = PhNH, 16) that can be prepared independently from 11 and 12 with 20% aq. NaOH. Additions of triflic acid to complexes 15 and 16 furnish the starting compounds 11 and 12, respectively, to complete the synthetic cycle. The combined data establish a synthetic cycle for O₂ reduction by an iron platform that manages proton and electron transfer through its first and second coordination spheres.

Full text of DOI:10.1021/ic9006668

10024 Inorg. Chem. 2009, 48, 10024–10035
DOI: 10.1021/ic9006668
A Hydrogen-Bond Facilitated Cycle for Oxygen Reduction by an Acid- and
Base-Compatible Iron Platform
Han Sen Soo,†, Alexis C. Komor,† Anthony T. Iavarone,§ and Christopher J. Chang*,†,‡,
†Department of Chemistry, §QB3/Chemistry Mass Spectrometry Facility, and the ‡Howard Hughes Medical
Institute, University of California, Berkeley, California 94720, and Chemical Sciences Division, Lawrence
Berkeley National Laboratory, Berkeley, California 94720
Received April 5, 2009
We report a hydrogen-bond functionalized N4Py ligand platform (N,N-bis(2-R-6-pyridylmethyl)-N-bis(2-pyri-
dyl)methylamine; R = neopentyl-NH, N4Py2NpNH, 9; R = phenyl-NH, N4Py2PhNH, 10) and the ability of its iron(II)-
triflate [N4Py2RFeII(OTf)][OTf] complexes (R = NpNH, 11; R = PhNH, 12) to activate and reduce dioxygen in a
synthetic cycle by coupled proton and electron transfer. A pair of iron(III)-hydroxide [N4Py2RFeIII(OH)][OTf]2
complexes (R = NpNH, 13; R = PhNH, 14) are isolated and structurally and spectroscopically characterized after exposure
of the iron(II)-triflate precursors to 1 atm of O2 at ambient temperature. The stability of this system to acids and bases allows
regeneration of the labile iron(II)-triflate starting materials by sequential electron and proton transfer with cobaltocene and
triflic acid, respectively, or through direct proton-coupled reduction with ascorbic acid. In the stepwise process, reduction of
the iron(III)-hydroxide complexes with cobaltocene gives structurally homologous iron(II)-hydroxide [N4Py2RFeII(OH)][OTf]
congeners (R = NpNH, 15; R = PhNH, 16) that can be prepared independently from 11 and 12 with 20% aq. NaOH.
Additions of triflic acid to complexes 15 and 16 furnish the starting compounds 11 and 12, respectively, to complete the
synthetic cycle. The combined data establish a synthetic cycle for O2 reduction by an iron platform that manages proton and
electron transfer through its first and second coordination spheres.
Introduction
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r
pubs.acs.org/IC
Published on Web 09/25/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10025
iron oxo, hydroxo, and aquo species that ligate oxygen in a
formally reduced O2- state.1,2,8,10-30
anionic iron(V)-oxo complexes by first-sphere appro-
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via oxygen or water using ethereal/alcohol solvents or other
additives.21,23,26 In addition, Collman and Chidsey have
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models that faithfully reduce O2 directly to H2O.13,14,85
We have initiated a program aimed at influencing inor-
ganic small-molecule activation chemistry by the addition of
second-sphere functionalities, focusing on iron and other
earth-abundant metals.86,87 In this report, we present a new
N4Py platform that incorporates hydrogen-bonding and the
ability of its iron(II) complexes to activate and reduce O2 in a
synthetic cycle by dual proton and electron transfer. This
system is distinguished from other hydrogen-bonded cavities
by its stability to strong acids and bases, allowing synthetic
isolation and structural characterization of homologous
iron(III)- and iron(II)-hydroxide intermediates and their
Elucidating structure-activity relationships between me-
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10026 Inorganic Chemistry, Vol. 48, No. 21, 2009
Soo et al.
conversion back to labile iron(II) precursors by controlled
electronand protontransfer. Mass spectrometry andinfrared
absorption measurements with various isotope labels are
consistent with inner-sphere O2 activation and reduction by
first- and second-sphere participation. The collective data
establish a synthetic O2 reduction cycle mediated by syn-
ergistic first- and second-sphere coordination chemistry.
yields as yellow crystalline materials (Scheme 2). Both
congeners were identified and characterized by X-ray
structural analyses (vide infra), H NMR spectroscopy,
1
and elemental analyses. The addition of second-sphere
hydrogen-bond groups markedly affects the structural
and magnetic properties of the iron-N4Py platform. The
complexes 11 and 12 each possesses an inner-sphere
triflate anion as an axial ligand (vide infra), whereas the
parent iron(II)-triflate complex [N4PyFeII(CH3-
CN)][OTf]2 that lacks hydrogen-bond pendants binds
an acetonitrile ligand at its apical sixth coordination
Results and Discussion
Design, Synthesis, and Oxygen Reactivity of Hydrogen-
Bonded Iron(II) N4Py2R Complexes. Our initial approach
to testing second-sphere effects on metal ion reactivity
relies on modulating established metal sites by the addi-
tion of secondary hydrogen bonding. Inspired in the
context of iron-oxygen chemistry by cationic iron(IV)-
oxo species with polyamine/pyridine ligands produced by
Wieghardt,50,51 Que,2,26,52-76,79,81-84 Nam,21,23,74,77-79
and others,58,62,67-71,80-84 as well as anionic hydrogen-
bonded iron(III)-oxo and iron(II)/(III)-hydroxide
complexes stabilized by Borovik’s trianionic amide
cavities,24,27,31,38,39,41-44,49 we sought to create hybrid
ligand systems that combine a neutral primary ligand
sphere and a well-defined secondary hydrogen-bond ar-
chitecture. We reasoned that pyridine-rich inner-sphere
metal platforms with pendant amines would provide the
acid and base stability necessary to facilitate O2 activa-
tion, reduction, cleavage, and cycling by proton and
electron additions.
To this end, we prepared two new N4Py-type ligand
frameworks bearing second-sphere neopentyl amine or
aniline groups as outlined in Scheme 1 (N4Py2R; R =
NpNH, 9; PhNH, 10). The pentadentate N4Py platform
supports six-coordinate metal complexes with one ex-
changeable apical site, and the neopentyl amine and
aniline hydrogen-bond pendants provide steric bulk as
well as a comparison between aliphatic and aromatic
amines. Related tripodal systems with secondary amides
and amines have been reported to exhibit enhanced
hydrolytic capabilities.9,33-35,37,46 Using N-bromosucci-
nimide (NBS), the appropriate 6-substituted-2-pico-
lines82,88 (1 and 3) were subjected to free radical bromina-
tion conditions to give statistical mixtures of products,
from which the monobrominated products82 (4 and 5)
were isolated by silica column chromatography. The
N4Py2R derivatives (6 and 7) are assembled at room
temperature by combining di-2-pyridylmethanamine89,90
with 4 and 5, respectively, in the presence of cesium carbo-
nate and sodium iodide. After acid hydrolysis of 6 to afford
the primary amine 8, reductive amination with pivaldehyde
and sodium triacetoxyborohydride provides the ligand 9 as
a white foamy material after aqueous workup. Acid depro-
tection of 7 to remove the tert-butyloxycarbonyl (Boc)
group produces the ligand 10 as a white foamy material.
Metalations of 9 and 10 with Fe(OTf)2(CH3CN)2 in
ethereal or nitrile solvents at room temperature proceed
smoothly to give the corresponding [N4Py2R FeII(OTf)]-
[OTf] complexes (R=NpNH, 11; R=PhNH, 12) in good
1
site.58,62,67-71,77,84 Moreover, H NMR spectra indicate
that the second-sphere functionalized iron(II)-triflates
are paramagnetic, as opposed to the diamagnetic
[N4PyFeII(CH3CN)][OTf]2.58,62,67-71,77,84 The effective
magnetic moment values determined by the solution
Evans’ method for complexes 11 (μeff = 5.1) and 12
(μeff = 4.7) are indicative of high-spin iron(II) (S = 2)
ground states, compared to the low-spin iron(II) (S = 0)
ground state for the parent N4Py iron(II)-triflate complex.
The structural and electronic differences induced by
second-sphere hydrogen bonding have patent conse-
quences for O2 reactivity. Whereas the parent
(N4Py)FeII system is indefinitely stable to air,62,67-71,84
exposure of 1 atm of O2 to orange acetonitrile solutions of
complexes 11 and 12 leads to clean formation of isolable
dark blue [N4Py2RFeIII(OH)][OTf]2 counterparts (R =
NpNH, 13; R = PhNH, 14), as determined by X-ray
structural (vide infra) and elemental analyses (Scheme 2).
The monomeric iron(III)-hydroxide complexes can also
be prepared using PhIO as the oxidant. Solution magnetic
susceptibility measurements by the Evans’ method are
consistent with high-spin iron(III) (S = 5/2) products,
and complex 13 was additionally identified by electro-
spray ionization (ESI) mass spectrometry. Because of the
sterically encumbering and stabilizing hydrogen-bond
pendants, we do not observe the FeIII2(μ-O) sinks that form
during side reactions with the parent N4Py system.69
The second-sphere hydrogen-bond facilitated activa-
tion and cleavage of O2 by this 4-fold symmetric, cationic
iron(II) platform is complementary to Borovik’s 3-fold
symmetric, anionic iron(II) systems,24,27,31,38-45,47-49,91
but a distinguishing feature of the cationic iron-N4Py2R
complexes reported here is their enhanced stability to
both acids and bases. Because the generation of mono-
meric N4Py2R iron(III)-hydroxide species from O2 repre-
sents a formal reduction of substrate to the O2- oxidation
state, we sought to complete the synthetic cycle at the
metal site by addition of electron and proton equivalents
to regenerate the iron(II) starting material and release
H2O. Along these lines, cyclic voltammograms of 13 and
14 show reversible iron(III)/iron(II) reduction waves at 0
mV (13) and -210 mV (14) versus Fcþ/Fc (Figure 1).
Treatment of the dark blue complexes 13 and 14 with
cobaltocene generates their red iron(II)-hydroxide
[N4Py2RFeII(OH)][OTf] counterparts (R = NpNH, 15;
R = PhNH, 16) as determined by X-ray structural (vide
infra) and elemental analyses (Scheme 2). Both complexes
1
were further characterized by H NMR and IR spectro-
scopy, and the μeff values (μeff = 4.7, 15; μeff = 4.6, 16)
(88) Kadish, K. M.; Nguyen, M.; Van Caemelbecke, E.; Bear, J. L. Inorg.
Chem. 2006, 45, 5996–6003.
(89) Chang, J.; Plummer, S.; Berman, E. S. F.; Striplin, D.; Blauch, D.
Inorg. Chem. 2004, 43, 1735–1742.
(90) Renz, M.; Hemmert, C.; Meunier, B. Eur. J. Org. Chem. 1998, 1271–
1273.
(91) Shook, R. L.; Gunderson, W. A.; Greaves, J.; Ziller, J. W.; Hendrich,
M. P.; Borovik, A. S. J. Am. Chem. Soc. 2008, 130, 8888–8889.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10027
Scheme 1. Synthetic Scheme for Ligands 9 and 10 (Np = neopentyl; Ph = phenyl)
Scheme 2. Preparation and Reactivity of 11 and 12
determined by the Evans’ method in solution suggest high-
spin iron(II) (S = 2) ground states. The monomeric N4Py2R
iron(II)-hydroxide compounds can also be prepared inde-
pendently by the reaction of 11 and 12 with 20% aq. NaOH,
showing the stability of this system to bases. Furthermore,
protonation of 15 and 16 with triflic acid regenerates the
starting complexes 11 and 12, respectively, as confirmed by
1HNMRandUV-visible spectroscopy, establishing the acid
stability of these hydrogen-bond cavities.24,27,31,38-45,47-49,91
This stepwise addition of electron and proton equivalents
closes the synthetic cycle for the full reduction of O2 to the
O
iron(II)-triflate starting materials (Scheme 2). Alterna-
tively, a direct and mild proton-coupled electron transfer
reduction of 13 and 14 to 11 and 12, respectively, can also
be achieved by ascorbic acid (Scheme 2).92
2- oxidation state with concomitant release of the labile
(92) Warren, J. J.; Mayer, J. M. J. Am. Chem. Soc. 2008, 130, 7546–7547.

10028 Inorganic Chemistry, Vol. 48, No. 21, 2009
Soo et al.
Figure 2. UV-visible absorption changes during the reaction of 11 and
O2 in CH3CN at room temperature to generate 13.
The molecular structures of 11- 14 and 16 are shown in
Figures 3 - 5 and Scheme 2.
Single crystals of 11 and 12 were obtained at room
temperature as yellow blocks from 1,2-dimethoxyethane
(DME) solutions layered beneath pentane. The com-
plexes are isostructural but not isomorphous (Figure 3).
Both 11 and 12 each possesses a triflate anion at the apical
sixth site, as opposed to the parent [N4PyFeII(CH3-
CN)][OTf]2 complex that possesses an axial coordinated
acetonitrile ligand.58,62,67-71,77,84 For both complexes 11
and 12, the N7 amine and the O1 triflate atoms are closer
than the van der Waal’s radii for usual N-O distances
and suggest the ability of these hydrogen-bonding cavities
to stabilize electronegative, anionic ligands. The iron-
ligand bonds for the second-sphere N4Py2R iron(II) com-
plexes are also longer on average than those of the parent
N4Py system, as anticipated for high spin S = 2 species
compared to low spin S=0 complexes.58,62,67-71,77,84
Single crystals of the isostructural 13 and 14 (Figure 4)
were grown at -35 °C as dark blue blades from solutions
of tetrahydrofuran (THF) or DME layered below solu-
tions of diethyl ether (Et2O) and pentane. These iron(III)
complexes possess shorter metal-ligand bond distances
than their corresponding iron(II)-triflate complexes be-
cause of higher effective nuclear charges for the former,
but the observed differences are not large since oxidation
from high-spin d6 iron(II) to high-spin d5 iron(III) in this
ligand field removes an electron from a largely non-
bonding dxy orbital (Table 2). The N4Py2R Fe-OH
Figure 1. (a) Cyclic voltammogram for a 1 mM solution of 13 in
CH3CN, with the data collected using a glassy carbon electrode at a scan
rate of 100 mV s-1; the reversible reductive wave is at 0 V relative to
ferrocene. (b) Cyclic voltammogram for a 1 mMsolution of14in CH3CN,
with the data collected using a glassy carbon electrode at a scan rate of
100 mV s-1; the reversible reductive wave is at -210 mV relative to
ferrocene.
Control experiments were performed to rule out ad-
ventitious outer-sphere oxidation and aquation pathways
and support an inner-sphere pathway. Cyclic voltammo-
grams of the iron(II)-triflate complexes 11 and 12 show no
oxidative waves within the acetonitrile solvent window
(up to þ1500 mV versus Fcþ/Fc, ferrocene, Supporting
Information, Figure S1), which precludes outer-sphere
oxidation. Indeed, exposing iron(II)-hydroxide com-
plexes 15 and 16 to 1 atm of O2 does not produce the
corresponding oxidized iron(III)-hydroxide congeners 13
and 14; in particular, compound 16 is indefinitely
unreactive toward O2. In addition, mass spectrometry
studies using 18O-labeled dioxygen show 18O enrichment
in the iron(III)-hydroxide product, consistent with inner-
sphere chemistry (vide infra). Finally, to rule out adven-
titious free radical chain autoxidation pathways, reac-
tions were carried out with 20 mM of 11 under 1.0 atm O2
in the presence of 1 equiv of the radical chain quencher
butylated hydroxytoluene (BHT). Rather than terminat-
ing or inhibiting the reaction, the observed initial rate for
O2 reduction was comparable with or without the BHT
additive, suggesting that the activation of O2 had not been
quenched. Reactions showed clean isosbestic conversions
of 11 to 13 at room temperature (Figure 2), with no
observable intermediates.
˚
˚
distances of 1.837 A (R = NpNH) and 1.835 A (R =
PhNH) are short but comparable to previously structu-
rally characterized iron(III)-oxo and iron(III)-hydroxide
units in synthetic24,27,31,41,44,46,86,93,94 and biological sys-
tems.95 Table 3 provides a comparison of Fe-O bond
lengths for selected systems. Owing to hydrogen-bonding
interactions, the observed O1-N5/N7 distances of 2.8 to
˚
2.9 A are shorter than typical van der Waal’s distances
˚
(ca. 3.4 A).
Finally, the reduced iron(II)-hydroxide complex 16
(Figure 5) was crystallized from THF/pentane mixtures.
As anticipated, the inner-sphere metal-ligand bonds for
Solid State Structures of Complexes 11- 14 and 16.
Single-crystal X-ray analysis of compounds 11-14 and 16
provide solid-state structural snapshots of the hydrogen-
bond facilitated cycle for oxygen reduction (Scheme 2).
Crystallographic data are given in Table 1, and selected
bond distances and angles are summarized in Table 2.
(93) Ogo, S.; Wada, S.; Watanabe, Y.; Iwase, M.; Wada, A.; Harata, M.;
Jitsukawa, K.; Masuda, H.; Einaga, H. Angew. Chem., Int. Ed. 1998, 37,
2102–2104.
(94) Yeh, C. Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2001, 123,
1513–1514.
(95) Green, M. T. J. Am. Chem. Soc. 2006, 128, 1902–1906.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10029
Table 1. Crystallographic Data for Complexes 11-14 and 16
11
13
12
14
16
chemical formula
formula weight
T (K)
λ (A)
crystal system
C35H43F6FeN7O6S2
891.73
126
C43H60F6FeN7O9S2
1052.95
165
0.71073
monoclinic
P21/n (No. 14)
9.9988(9)
11.1928(10)
44.421(4)
90
93.670(2)
90
4961.2(8)
4
1.410
C39H36F6FeN7O7S2
948.72
155
0.71073
monoclinic
P21/c (No. 14)
10.2218(5)
33.5162(17)
12.3587(6)
90
104.229(1)
90
4104.1(4)
4
1.535
C41H42F6FeN7O9S2
1010.79
160
0.71073
triclinic
P1(No. 2)
12.0572(15)
12.9077(16)
17.156(2)
69.948(2)
87.976(2)
65.822(2)
2258.8(5)
2
1.486
0.513
1042
14627
9030 (0.017)
0.83
C148H128F12Fe4N29O16S4
3148.47
124
0.71073
triclinic
P1(No. 2)
12.2707(13)
15.1201(16)
20.548(2)
90.005(2)
101.953(2)
109.126(2)
3514.3(6)
1
1.489
0.557
1624
22767
14048 (0.036)
0.89
˚
0.71073
triclinic
P1(No. 2)
9.8538(18)
11.491(2)
18.506(3)
77.179(2)
77.311(2)
80.914(2)
1980.2(6)
2
1.496
0.568
924
10073
6477 (0.013)
0.89
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
F
calcd (g/cm3)
μ(mm-1
F(000)
reflctns collected
)
0.470
2204
27666
10062 (0.033)
0.88
0.555
1948
23326
8381 (0.028)
0.88
ind reflctns (Rint
Tmin/Tmax
)
data/restr/params
R, Rall, wR2
GOF
5205/0/514
0.065, 0.080, 0.20
1.06
7566/0/657
0.084, 0.011, 0.22
1.08
7034/92/609
0.052, 0.063, 0.13
1.08
6951/45/628
0.056, 0.073, 0.17
1.07
9861/4/978
0.064, 0.096, 0.16
1.02
max shift/error
0.01
2.95
0.01
0.01
0.17
16OH with 16OCH3, with almost 50% exchange within
45 s. An alternative pathway could involve exchange of
H218O with intermediates from autoxidation of 11, but is
less likely since 11 does not show readily accessible outer-
sphere oxidations in acetonitrile (Supporting Informa-
tion, Figure S1).
IR absorption measurements on iron(II)-hydroxide
species generated from O2 cleavage/reduction or OH-
reaction pathways with deuterated ligands suggest that
the second-sphere aniline hydrogen-bond pendants par-
ticipate in acid-base reactivity. We performed measure-
ments on these reduced iron(II) species as no distinct
ν(O-H) stretches are observed for the corresponding
iron(III)-hydroxide complexes; the ν(O-H) signature
is presumably masked by strong intramolecular hydro-
gen bonding (Supporting Information, Figures S3, S4,
and S9).31,39,42-44
˚
Table 2. Selected Bond Lengths (A) and Angles for Complexes 11-14 and 16
11
13
12
14
16a
Fe1-O1
Fe1-N1
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N6
2.084(3)
2.180(3)
2.264(4)
2.171(3)
2.212(3)
2.243(3)
1.837(3)
2.183(3)
2.160(4)
2.164(4)
2.119(4)
2.111(4)
2.865
2.073(2) 1.835(2) 1.915(3)
2.179(2) 2.172(2) 2.236(3)
2.242(2) 2.138(2) 2.199(3)
2.187(2) 2.151(2) 2.204(4)
2.219(2) 2.132(2) 2.255(3)
2.227(2) 2.148(3) 2.221(3)
O1-(H5)-N5 3.352
O1-(H7)-N7 3.101
3.356
2.992
2.867
2.796
2.782
2.786
2.857
O1-Fe1-N1 168.96(13) 172.27(13) 165.79(9) 176.54(9) 177.05(13)
a Data taken from only one of two distinct, but similar molecules in
the asymmetric unit.
this iron(II)-hydroxide were longer than in its iron(III)
analogue (Table 2). The observed Fe-OH distances of
˚
˚
1.915 A and 1.934 A for the two molecules in the asym-
metric unit are within the normal range of previously
identified iron(II)-hydroxide species,24,95 and the Fe-O
Using a partially deuterated ligand 10-ND (25% deut-
erated at the aniline N-H and 85% deuterated at the
methine hydrogen as determined by H NMR spectro-
˚
bond elongation of 0.1 A going from Fe(III) to Fe(II) is
1
comparable to Borovik’s tris-ureayl iron(III)/iron(II)-hy-
droxidepairaswellasenzymaticsystems(Table3).24,27,31,95
Taken together, the structural characterization of discrete,
isolable iron(II)-triflate, iron(III)-hydroxide, and iron(II)-
hydroxide units from reactions with O2, reductants, bases,
and/or acids builds a foundation for the O2 reduction cycle
mediated by the N4Py2R system.
Isotope Labeling Experiments. Mass spectrometry
experiments with 18O-labeled dioxygen suggest inner-
sphere activation and cleavage of O2 by the hydrogen-
bonded complexes 11 and 12. A mass spectrum of synthe-
tically prepared 13 from the reaction of 11 with 18O2
shows about 30% incorporation of 18O into the bound
hydroxide group (Figure 6). We speculate that the re-
duced 18O content in the final product was a result of
exchange during the synthetic recrystallization workup
and mass spectral analysis. As illustrated in Supporting
Information, Figure S2, trace amounts of methanol in the
mass spectrometer that had been flushed thoroughly with
dry CH3CN prior to use show rapid exchange of the labile
scopy), we prepared the corresponding iron(II)-triflate
(12-ND) for IR studies. We observe ν(N-D) stretches at
2510 and 2425 cm-1, corresponding to ν(N-H) stretches
at 3395 cm-1 and 3290 cm-1, respectively (Supporting
Information, Figure S5a); quantitative information
about the amount of deuterium incorporation is not
available owing to the paramagnetic nature of 12-ND.
The baseline protonated iron(II)-hydroxide complex 16
exhibits distinct ν(O-H) and ν(N-H) stretches at 3670
and 3185 cm-1, respectively (Supporting Information,
Figure S5b). The fully deuterated iron(II)-deuteroxide
complex [N4Py2PhNDFeII(OD)][OTf] (16-OD) prepared
by treatment of 12 with 30% NaOD in D2O displays a
strong ν(O-D) stretch at 2705 cm-1 with ν(N-D)
stretches at 2280 cm-1 and 2215 cm-1 (Supporting Infor-
mation, Figure S5c).
The reaction of 12-ND bearing a partially labeled N-D
ligand with O2 in CH3CN followed by cobaltocene re-
duction results in a final FeII-OH/OD species with a

10030 Inorganic Chemistry, Vol. 48, No. 21, 2009
Soo et al.
Figure 3. Solid-statestructuresof(a) complex11and (b) complex12. The triflatecounterions and most ofthe hydrogens have beenomitted for clarity. The
H5 and H7 hydrogens on the amines have been included to highlight the hydrogen bonding interactions. The Fe, S, F, O, N, and C are shown in green,
yellow, pink, red, blue, and gray, respectively, and all ellipsoids are shown at the 50% probability level.
Figure 4. Solid-state structures of (a) complex 13 and (b) complex 14. The triflate counterions, solvent molecules, and most of the hydrogens have been
omitted for clarity. The H1a of the hydroxide and the H5 and H7 of the amines have been included to highlight the hydrogen bonding interactions. The Fe,
O, N, and C are shown in green, red, blue, and gray, respectively, and all ellipsoids are shown at the 50% probability level.
diminished ν(N-D) stretch at 2305 cm-1 and growth of a
minor feature at 2700 cm-1 attributable to a ν(O-D)
stretch (Supporting Information, Figure S5d), which
suggests an internal HAT between the N-D group of
the ligand and a higher-valent iron transient. Compared
to the IR spectrum in Supporting Information, Figure
S5c, which shows global deuterium incorporation into
multiple parts of the ligand and therefore fewer absorp-
tions in the 2700-3200 cm-1 region, the ν(O-D) feature
at 2700 cm-1 in Supporting Information, Figure S5d is
predominantly masked by the more intense N-H and
C-H stretches in this region because the aniline deuter-
ium label in 12-ND can only be conserved or lost during
workup.
Concluding Remarks
In summary, we have presented a hydrogen-bond functio-
nalized N4Py platform that facilitates O2 reduction in a
discrete synthetic cycle by O-O activation, cleavage, and
coupled electron and proton transfer. The second-sphere
pendants not only turn on iron-O2 reactivity but funnel their
chemistry to structurally and spectroscopically characterized
Figure 5. Solid-state structure of one of the two molecules of 16 in the
asymmetric unit. The triflate counterions, solvent molecules, and most of
the hydrogens have been omitted for clarity. The H1a of the hydroxide
and the H5 and H7 of the amines have been included to highlight the
hydrogen bonding interactions. The Fe, O, N, and C are shown in green,
red, blue, and gray, respectively, and all ellipsoids are shown at the 50%
probability level.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10031
˚
Table 3. Comparison of Fe-O Bond Lengths (A) for 13, 14, and 16 Relative to
inner-sphere O2 binding tothe labile, high-spiniron(II) center
followed by O-O activation, cleavage, and hydrogen atom
transfer (HAT) steps to afford monomeric iron(III)-hydro-
xide products that can be cycled back to the starting iron(II)-
triflate complexes by electron and proton transfer.92,96
Whereas outer-sphere oxidation of complexes 15 and 16 with
AgOTf was facile (Scheme 2), the corresponding reaction
with O2 as the oxidant did not produce the counterparts 13
and 14, since displacement of the strongly hydrogen-bonded
OH- ligand is more difficult compared to the OTf- anion (cf.
Figures 3 and 5, Table 2). Moreover, cyclic voltammetry
measurements on both 11 and 12 showed no reversible or
irreversible oxidative waves in the electrochemical window of
CH3CN (Supporting Information, Figure S1), precluding
outer-sphere oxidations of both iron(II)-triflates by O2.
However, autoxidative reaction trajectories are difficult to
rule out in these complex O2 activation pathways; with clean
isosbestic conversions of iron(II)-triflate to iron(III)-hydro-
xide units with no observable intermediates as monitored by
UV-visible absorption changes (Figure 2), alternative ap-
proaches would be necessary to further elaborate the intri-
cacies of this reaction pathway. We are currently pursuing
further studies to understand the present O2 reduction system
as well as expanding this second-sphere approach to other
small-molecule activation reactions.
Selected Published Molecules
˚
Oxidation State
Fe-O (A)
13a
III
III
III
III
III
III
III
III
III
II
1.837(3)
1.835(2)
1.813(3)
1.926(2)
1.867(2)
1.876(2)
1.868(3)
1.877(3)
1.8861(11)
1.915(3)
1.934(3)
2.048(2)
14a
[FeH3buea(O)]2-b
[FeH3buea(OH)]-b
[tpaMesFe(OH)]-c
[Fe(tnpa)(PhCOO)(OH)]þd
[(HPX-CO2H)Fe(OH)]e
[Fe0iPr(OH)]-f
[FeH1iPr(OH)]-f
16a
[FeH3buea(OH)]2-b
II
a This work. b H3buea = tris[(N0-tert-butylureayl)-N-ethylene]amine.24,27,31
c tpaMes = tris-mesityl tris(pyrrolylmethyl)amine.86 d tnpa = tris(6-neo-
pentylamino-2-pyridylmethyl)amine).93 e HPX-CO2H = hanging por-
phyrin xanthene.94 f 0iPr = tris[N-isopropylcarbamoylmethyl]aminato;
H1iPr=[(N0-tert-butylureayl)-N-ethyl]-bis(N00-isopropylcarbamoylmethyl)-
aminato.44
Experimental Section
Synthetic Materials and Methods. Silica gel P60 (SiliCycle)
and activated basic aluminum oxide (Brockmann I, 150 mesh,
˚
58 A, Sigma-Aldrich) were used for column chromatography.
Analytical thin layer chromatography was performed using
Silicycle 60 F254 silica gel (precoated sheets, 0.25 mm thick)
and EMD Chemicals Inc. 60 F254 basic aluminum oxide
(precoated sheets, 0.25 mm thick). Unless otherwise stated, all
air-and moisture-sensitive manipulations were performed either
in a Vacuum Atmospheres Company glovebox under an atmo-
sphere of purified nitrogen or using standard Schlenk techni-
ques. Enriched 18O2 (97.9% enriched, >99.8% purity) was
purchased from Cambridge Isotope Laboratories and used as
received. Acetonitrile, dichloromethane (CH2Cl2), Et2O, DME,
pentane, THF, and toluene were passed through a Vacuum
Atmospheres Solvent Purifier system prior to use. Acetonitrile-
d3 (Cambridge Isotope Laboratories) was refluxed over calcium
˚
hydride and distilled under dinitrogen. Celite and 4 A molecular
Figure 6. (a) Electrospray mass spectrum of 13-18OH, generated by
reacting 11 with 18O2, with insets showing details for the isotopic distri-
sieves were dried in vacuo at 160 °C. NMR chemical shifts
are reported in parts per million with respect to residual protic
solvent peaks; coupling constants are reported in hertz. The
known compounds 2-pivaloylamido-6-methylpyridine (1),82
2-pivaloylamido-6-(bromomethyl)pyridine (4),82 and di-2-pyri-
dylmethanamine90 were prepared by slight modification of
previously reported procedures.
butions of the [N4Py2NpNHFeIII(OH)]2þ and [N4Py2NpNH
-
FeIII(OH)][OTf]þ ions (denoted as “2þ ion” and “1þ ion”, respectively).
Comparison between isotopic distributions measured for samples gen-
erated by reaction with 18O2 (b, d) and with non-enriched O2 (c, e). The
asterisks denote the isotopic peaks at m/z = 306.15 (2þ ion) and 761.25
(1þ ion), which were measured at 38%, 13%, 40%, and 15% abundance
in (b), (c), (d), and (e), respectively, relative to the most abundant
isotopic peak in the distribution.
Instrumentation. NMR spectra were recorded using Bruker
spectrometers operating at 300 or 400 MHz at room tempera-
ture as specified. UV-visible absorption spectra and kinetic
runs were recorded using Varian Cary 50 UV-visible spectro-
photometers, with solution samples housed in Schlenk-modified
cuvettes. Cyclic voltammograms (CVs) were obtained with a
BASi EC Epsilon electrochemical analyzer. Mass spectrometry
and elemental analyses were performed at the QB3/Chemistry
Mass Spectrometry and Microanalytical facilities, respectively,
at the University of California, Berkeley. Experimental details
for the mass spectrometry measurements of 13 and 13-18OH are
included in a following section.
monomeric iron(III)-hydroxide products. The acid- and
base-stability of the N4Py2RFe system allows regeneration
of the starting iron(II)-triflate complexes by electron and
proton transfer to release water and complete the synthetic
cycle. X-ray crystallographic characterization of the homo-
logous iron(II)-triflate, iron(III)-hydroxide, and iron(II)-hy-
droxide N4Py2R complexes provide structural snapshots of
the overall process. Mass spectrometry and infrared absorp-
tion data with various isotope labels suggest that the hydro-
gen-bond facilitated O2 reduction cycle is complex. A
possible scenario is that the reaction proceeds through
(96) Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363–390.

10032 Inorganic Chemistry, Vol. 48, No. 21, 2009
Soo et al.
6-Methyl-N-phenylpyridin-2-amine (2).88 Freshly distilled
aniline (5.56 mL, 61.3 mmol) and 2-bromo-6-methyl-pyridine
(3.00 g, 17.4 mmol) were mixed together in a heavy-walled
reaction flask. The resulting yellow solution was heated to
170 °C for 2 days and then cooled to room temperature. The
resulting purplish-black solution was mixed with 20% aqueous
NaOH (30 mL) and subsequently extracted with CH2Cl2 (3 ꢀ
30 mL). The purple organic layer was dried over anhydrous
Na2SO4, and the solvent was removed to yield a purple oil. After
purification by column chromatography (silica, gradient from
CH2Cl2 to 50% CH2Cl2/50% EtOAc), 3.24 g (quantitative
yield) of the product were obtained as yellow oil. 1H NMR
(400 MHz, CDCl3): δ = 2.45 (s, 3 H), 6.50 (s, 1 H), 6.62 (d, J =
10.0 Hz, 1 H), 6.74 (d, J=11.2Hz,1H),7.05(m,1H),7.31(m,5H).
tert-Butyl 6-methylpyridin-2-yl(phenyl)carbamate (3). Com-
pound 2 (3.21 g, 17.4 mmol) and di-tert-butylcarbonate (Boc2O)
(9.52 g, 43.6 mmol) were added to a 100 mL round-bottom flask.
DMAP (2.13 g, 17.4 mmol) and NEt3 (11.7 mL, 87.2 mmol) were
dissolved in CH2Cl2 (15 mL) and added to the reaction, and the
resulting solution was refluxed for 2 days and concentrated to a
brown solid after TLC analysis (silica, 4:1 hexanes:EtOAc)
confirmed complete consumption of starting material. After
purification by column chromatography (silica, gradient from
5% EtOAc/95% hexanes to 15% EtOAc/85% hexanes), 4.61 g
(d, J = 8.0 Hz, 2 H), 8.58 (d, J = 4.8 Hz, 2 H). 13C NMR (400
MHz, CDCl3): δ = 56.9, 71.7, 106.5, 114.0, 119.9, 122.0, 122.3,
123.9, 129.1, 136.2, 138.0, 140.8, 149.2, 155.2, 158.7, 160.3.
HRFABMS (Mþ) m/z calcd for C35H32N7 550.2719, found
550.2710.
2-Pivaloylamido-6-(bromomethyl)pyridine (4).82 A 250 mL
2-neck round-bottom flask was charged with 182 (13.38 g, 69.6
mmol), NBS (18.59 g, 104 mmol), and AIBN (1.14 g, 6.96
mmol). CCl4 (75 mL) was added and the resulting mixture was
sparged with N2 for 15 min. The mixture was refluxed for 21 h,
filtered, and rinsed with CH2Cl2 (30 mL). After purification by
column chromatography (silica, gradient from 10% EtOAc/
90% hexanes to 30% EtOAc/70% hexanes), 8.52 g (45%) of 4
were obtained as colorless solid. The identity of 4 was estab-
1
lished by matching the H NMR spectrum with that reported
previously.82 1H NMR (400 MHz, CDCl3): δ = 1.34 (s, 9 H),
4.43 (s, 2 H), 7.15 (dd, J = 0.8, 7.6 Hz, 1 H), 7.69 (t, J = 7.6 Hz,
1 H), 8.00 (bs, 1 H), 8.19 (dd, J = 0.8, 7.6 Hz, 1 H).
N,N0-(6,60-(Dipyridin-2-ylmethylazanediyl)bis(methylene)bis-
(pyridine-6,2-diyl))bis-(tert-butylamide) (6). To a 100 mL round-
bottom flask was added 4 (4.90 g, 18.1 mmol), di-2-pyridyl-
methanamine90 (1.67 g, 9.03 mmol), Cs2CO3 (11.77 g, 36.1
mmol), NaI (2.71 g, 18.1 mmol), and dry CH3CN (10 mL).
The resulting brown solution was stirred for 48 h, filtered, rinsed
with EtOAc, and dried by rotary evaporation. After purification
by column chromatography (basic alumina, gradient from 50%
EtOAc/50% hexanes to 70% EtOAc/30% hexanes), 2.96 g
(58%) of 6 were obtained as white powdery material. 1H
NMR (400 MHz, CDCl3): δ = 1.33 (s, 18 H), 3.88 (s, 4 H),
5.36 (s, 1 H), 7.10-7.20 (m, 2 H), 7.29 (d, J =7.6 Hz, 2 H),
7.60-7.70 (m, 6 H), 7.96 (bs, 2 H), 8.07 (d, J = 8.0 Hz, 2 H), 8.58
(m, 2 H). HRFABMS (Mþ) m/z calcd for C33H40N7O2
566.3243, found 566.3258.
1
(93%) of 3 were obtained as a colorless solid. H NMR (400
MHz, CDCl3): δ = 1.47 (s, 9 H), 2.49 (s, 3 H), 7.00 (d, J = 7.6
Hz, 1 H) 7.18 (m, 2 H), 7.25 (m, 2 H), 7.34 (m, 2 H), 7.58 (t, J=7.8
Hz, 1 H). HRFABMS (Mþ) m/z calcd for C17H21N2O2
285.1603, found 285.1599.
tert-Butyl 6-(bromomethyl)pyridin-2-yl(phenyl)carbamate (5).
A 250 mL 2-neck round-bottom flask was charged with 3 (4.61 g,
16.2 mmol), N-bromosuccinimide (NBS) (3.47 g, 19.5 mmol),
and azobis(isobutyronitrile) (AIBN) (0.267 g, 1.63 mmol). CCl4
(35 mL) was added, and the resulting mixture was sparged with
N2 for 15 min. The mixture was refluxed for 48 h under a
nitrogen atmosphere, filtered, and rinsed with CH2Cl2 (15 mL).
After purification by column chromatography (silica, 10%
EtOAc/90% hexanes), 3.07 g (52%) of 5 were obtained as
colorless crystals. 1H NMR (400 MHz, CDCl3): δ = 1.46 (s, 9
H), 4.42 (s, 2 H), 7.24 (m, 4 H), 7.34 (m, 3 H), 7.69 (t, J=10 Hz, 1
H). HRFABMS (Mþ) m/z calcd for C17H20N2O2Br1 363.0708,
found 363.0710.
6-((((6-Aminopyridin-2-yl)methyl)(dipyridin-23-ylmethyl)amino)-
methyl)pyridin-2-amine (8). Compound 6 (1.88 g, 3.32 mmol) was
dissolved in 3 M aqueous HCl (20 mL) and refluxed for 24 h.
After cooling to room temperature, the pH was adjusted to >12
with 20% aqueous NaOH (50 mL), and the aqueous phase was
extracted with CH2Cl2 (3 ꢀ 40 mL). The combined organic
phases were dried over anhydrous Na2SO4 and evaporated
1
down to yield 1.11 g (79%) of 8 as white foamy material. H
NMR (400 MHz, CDCl3): δ = 3.78 (s, 4 H), 4.39 (bs, 4 H), 5.42
(s, 1 H), 6.34 (d, J = 8.0 Hz, 2 H), 7.01 (d, J = 7.2 Hz, 2 H), 7.14
(t, J = 6.0 Hz, 2 H), 7.39 (t, J = 7.6 Hz, 2 H), 7.69 (m, 4 H), 8.56
(d, J = 4.8 Hz, 2 H). HRFABMS (Mþ) m/z calcd for C23H24N7
398.2093, found 398.2105.
tert-Butyl 6,60-(dipyridin-2-ylmethylazanediyl)bis(methylene)-
bis(pyridine-6,2-diyl)- bis(phenylcarbamate) (7). To a 100 mL
round-bottom flask was added 5 (3.07 g, 8.46 mmol), di-
2-pyridylmethanamine90 (0.681 g, 3.68 mmol), Cs2CO3 (7.18 g,
22.1 mmol), NaI (1.65 g, 11.1 mmol), and dry CH3CN (12 mL).
The resulting tan mixture was stirred for 24 h, filtered, rinsed
with EtOAc, and the solvent was removed by rotary evapora-
tion. After purification by column chromatography (basic
alumina, gradient from 50% EtOAc/50% hexanes to 75%
EtOAc/25% hexanes) 2.72 g (98%) of 7 were isolated as a brown
6-(((Dipyridin-2-ylmethyl)((6-(neopentylamino)pyridin-2-yl)-
methyl)amino)methyl)-N-neopentylpyridin-2-amine (9). Com-
pound 8 (0.607 g, 1.59 mmol) and trimethylacetaldehyde (0.70
mL, 6.4 mmol) were dissolved in 1,2-dichloroethane (25 mL)
and stirred for 30 min at room temperature. Sodium triacetox-
yborohydride (2.02 g, 9.53 mmol) was added, and the reaction
mixture was stirred for a further 24 h. The reaction was
quenched with 20% aqueous NaOH (20 mL) and the aqueous
phase was extracted with CH2Cl2 (2 ꢀ 30 mL). The combined
organic phases were dried over anhydrous Na2SO4 and evapo-
rated down to give 9 as white foamy material in quantitative
yield. 1H NMR (400 MHz, CDCl3): δ = 0.98 (s, 18 H), 3.04 (d,
J = 6.4 Hz, 4 H), 3.75 (s, 4 H), 4.51 (t, J = 6.2 Hz, 2 H), 5.40 (s,
1 H), 6.24 (d, J = 8.4 Hz, 2 H), 6.97 (d, J = 7.2 Hz, 2 H), 7.14 (t,
J = 6.0 Hz, 2 H), 7.38 (t, J = 8.0 Hz, 2 H), 7.67 (t, J = 7.4 Hz, 2
H), 7.75 (d, J = 7.6 Hz, 2 H), 8.56 (d, J=4.0 Hz, 2 H). 13C NMR
(400 MHz, CDCl3): δ = 27.5, 32.0, 54.0, 56.7, 71.0, 103.7, 111.1,
121.8, 123.9, 136.1, 137.8, 149.2, 158.3, 159.0, 160.3.
HRFABMS (Mþ) m/z calcd for C33H44N7 538.3658, found
538.3656.
1
oil. H NMR (400 MHz, CDCl3): δ = 1.44 (s, 18 H), 3.60 (s,
4 H), 5.04 (s, 1 H), 7.09 (d, J = 9.6 Hz, 4 H), 7.37 (m, 12 H), 7.52
(m, 6 H), 8.49 (d, J = 4.0 Hz, 2 H). ESI (Mþ) m/z calcd for
C45H48N7O4 750.9, found 750.3.
6-(((Dipyridin-2-ylmethyl)((6-(phenylamino)pyridin-2-yl)methyl)-
amino)methyl)-N-phenylpyridin-2-amine (10). A 100 mL round-
bottom flask was charged with 7 (2.12 g, 3.63 mmol) at 0 °C,
after which 3 M aqueous HCl (12 mL) was added. The resulting
yellow solution was warmed to room temperature and stirred
for 12 h. Saturated Na2CO3 (15 mL) was then added to give a
colorless and foamy mixture. This mixture was extracted with
CH2Cl2 (3 ꢀ 30 mL), dried over anhydrous Na2SO4, and the
solvent was removed by rotary evaporation to yield 1.96 g (98%)
of 10 as a colorless foamy product. 1H NMR (400 MHz,
CDCl3): δ = 3.87 (s, 4 H), 5.46 (s, 1 H), 6.53 (bs, 2 H), 6.73
(d, J = 8.4 Hz, 2 H), 7.03 (t, J = 6.8 Hz, 2H), 7.09 (d, J = 7.6 Hz,
2H), 7.15 (t, J = 6.0 Hz, 2 H), 7.36 (m, 10 H), 7.67 (m, 2 H), 7.77
Complex 11. Ligand 9 (0.570 g, 1.06 mmol) was dissolved in
DME (7 mL) and added to Fe(OTf)2(CH3CN)2 (0.462 g, 1.06
mmol) to produce an orange solution. After 2 h, the solution was

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10033
dried in vacuo, and the orange residue was crystallized from
5 mL of DME layered beneath 10 mL of pentane. In two crops,
11 was collected as 0.600 g (63%) of yellow crystals suitable for
9.93. UV-vis: λmax = 230 nm, ε = 6300 M-1 cm-1; λmax =270
nm, ε = 7000 M-1 cm-1; λmax=615 nm, ε = 1100 M-1 cm-1
.
(b) With O2 as the oxidant: 12 (0.042 g, 0.046 mmol) dissolved
in CD3CN (1 mL) was concentrated in a 25 mL Schlenk tube
before excess O2 (1 atm) was introduced at room temperature.
After 2 days, the deep blue solution was dried in vacuo.
Repeated recrystallization from THF (1 mL) layered below
Et2O (3 mL) gave 14 as 0.031 g (73%) of blue crystals. The 1H
NMR spectrum and unit cell volume for the isolated compound
matched data from material obtained by method (a).
(c) From 16 with AgOTf as the oxidant: A solution of AgOTf
(0.0037 g, 0.014 mmol) dissolved in THF (2 mL) was added to 16
(0.011 g, 0.014 mmol) at room temperature. The reaction
mixture turned purple before becoming a dark blue mixture
with gray precipitate within 5 min. After 1 h, the mixture was
filtered through Celite, dried in vacuo, and the residue recrys-
tallized from THF (1 mL) and Et2O (3 mL) to afford 0.0051 g
(48%) of dark blue, microcrystalline 14. The 1H NMR spectrum
and unit cell volume for the isolated compound matched the
data from material obtained by method (a).
1
X-ray diffraction. H NMR (400 MHz, CD3CN): δ = -3.30,
-1.59, -0.02, 28.65, 51.53. Evans’ Method μeff = 5.1 μB. Anal.
Calcd for C35H43N7O6F6S2Fe: C, 47.14; H, 4.86; N, 11.00.
Found: C, 47.04; H, 4.98; N, 10.94.
Complex 13. Complex 11 (0.198 g, 0.22 mmol) dissolved in
CH3CN (10 mL) was degassed by 3 freeze-pump-thaw cycles
before excess O2 (1 atm) was introduced at room temperature.
After 16 h, the dark green solution was dried in vacuo. Recrys-
tallization from THF (2 mL) layered below Et2O (2 mL)/
pentane (4 mL) gave 13 as 0.080 g (40%) of blue crystals,
suitable for X-ray diffraction. Subsequent preparations on
1
smaller scales provided up to 92% yields of the product. H
NMR (400 MHz, CD3CN): δ = -4.94, 2.68, 17.39, 37.20, 88.40,
134, 159. 19F NMR (400 MHz, CD3CN): δ = -78.05. Evans’
Method μeff = 5.6 μB. Anal. Calcd for C43H60N7O9F6S2Fe: C,
49.05; H, 5.74; N, 9.31. Found: C, 48.97; H, 5.61; N, 9.48.
UV-vis: λmax = 240 nm, ε = 2200 M-1 cm-1; λmax = 315 nm,
ε=1100 M-1 cm-1; λmax=620 nm, ε = 770 M-1 cm-1
.
Complex 16. (a) From 12 and NaOH: A solution of 20%
aqueous NaOH (0.02 mL, 0.14 mmol) was added to 12 (0.045 g,
0.058 mmol) dissolved in THF (2 mL) under N2. The solution
gradually changed color from yellow to deep red over 1 h, after
which it was dried in vacuo. The residue was extracted with
CH2Cl2 and filtered to remove NaOTf and excess NaOH. After
recrystallization from THF (1 mL) layered below pentane (3
mL), 0.032 g (84%) of the red 16, suitable for X-ray diffraction,
Complex 15. A solution of 20% aqueous NaOH (0.03 mL,
0.21 mmol) was added to 11 (0.053 g, 0.059 mmol) dissolved in
THF (2 mL) under N2. The solution gradually changed color
from yellow to deep red over 1 h, after which it was dried in
vacuo. The residue was extracted with CH2Cl2 and filtered to
remove NaOTf and excess NaOH. After recrystallization from
DME (2 mL) layered below pentane (6 mL), 0.028 g (63%) of the
red 15 were isolated. 1H NMR (400 MHz, CD3CN): δ = 1.77,
2.31, 21.49, 29.99, 38.56, 45.91, 51.48, 60.98, 92.70, 104.38,
135.05. Evans’ Method μeff =4.7 μB. Anal. Calcd for C34H44-
N7O4F3SFe: C, 53.76; H, 5.84; N, 12.91. Found: C, 53.93; H,
5.98; N, 12.79.
1
were isolated. H NMR (400 MHz, CD3CN): δ = 2.11, 7.38,
7.55, 7.72, 8.36, 21.82, 29.97, 40.01, 44.64, 52.06, 59.93, 96.94,
111.41, 138.12. 19F NMR (400 MHz, CD3CN): δ = -78.09.
Evans’ Method μeff = 4.6 μB. Anal. Calcd for C36H32N7O4-
F3SFe: C, 56.04; H, 4.18; N, 12.71. Found: C, 55.87; H, 4.12;
N, 12.63.
Regeneration of 11 from 13 and Ascorbic Acid. Complex 13
(0.364 g, 0.040 mmol) dissolved in CH3CN (2 mL) was added to
ascorbic acid (0.0035 g, 0.020 mmol) at room temperature. The
dark blue solution turned green before turning yellow over 1 h as
the ascorbic acid dissolved. After 2 h, the yellow solution was
dried in vacuo. The 1H NMR spectrum matched data obtained
for 11, synthesized independently as described above. After
extracting the residue with trifluorotoluene and recrystallization
from pentane, 0.0150 g (42%) of 11 was isolated. Because of
possible coordination of ascorbic acid byproduct, 11 could not
be isolated in higher yields after further recrystallization.
Complex 12. In a glovebox, 10 (0.500 g, 0.910 mmol) was
dissolved in DME (4 mL) and added to Fe(OTf)2(CH3CN)2
(0.397 g, 0.910 mmol) to yield an orange solution. After 1 h, the
solution was dried in vacuo, and the orange residue was crystal-
lized from DME (2 mL) layered beneath pentane (6 mL). The
mother liquor was decanted to yield 0.703 g (78%) of 12 as
yellow crystals suitable for X-ray diffraction. 1H NMR (400
MHz, CD3CN): δ = 0.52, 4.29, 4.72, 4.96, 5.65, 16.70, 17.19,
(b) From 14 and CoCp2: CoCp2 (0.0039 g, 0.021 mmol)
dissolved in THF (1 mL) was added to 14 (0.019 g, 0.021 mmol).
The dark blue solution turned red within 1 min and was dried
in vacuo after 15 min. The residue was recrystallized from THF
(1 mL) layered below pentane (3 mL) to afford 0.013 g (76%) of
red crystals, while the poorly soluble yellow CoCp2(OTf) was
filtered off. The 1H NMR spectrum for the isolated material was
identical to data obtained using compound synthesized by
method (a).
Complex 16-OD. A solution of 30% NaOD in D2O (0.03 mL,
0.3 mmol) was added to 12 (0.035 g, 0.039 mmol) dissolved in
THF (2 mL) under N2. The solution gradually changed color
from yellow to deep red over 1 h, after which it was dried in
vacuo. The residue was extracted with CH2Cl2 and filtered to
remove NaOTf and excess NaOD. After recrystallization from
CH2Cl2 (1 mL) layered below pentane (3 mL), 0.017 g (56%) of
16-OD were isolated as a red microcrystalline material. This
product was used for IR studies.
1
26.52, 32.38, 43.98, 50.00, 52.03, 62.64, 63.53. H NMR (400
Complex 14-OD. A solution of AgOTf (0.0041 g, 0.016 mmol)
dissolved in THF (3 mL) was added to 16-OD (0.012 g, 0.015
mmol) at room temperature. The reaction mixture turned purple
before becoming a dark blue mixture with gray precipitate
within 5 min. After 1 h, the mixture was filtered through Celite,
dried in vacuo, and the residue was recrystallized from THF
(1 mL) and Et2O (3 mL) to afford 0.013 g (90%) of dark blue,
microcrystalline 14-OD. This material was used for IR studies.
Regeneration of 12 from 16 and Triflic Acid (HOTf). A stock
solution of 21 mM HOTf in DME (1.2 mL, 0.024 mmol) was
added to 16 (0.019 g, 0.024 mmol) dissolved in DME (2 mL) at
room temperature. The red solution turned yellow on complete
addition of the acid. After 10 min, the yellow solution was dried
in vacuo. Recrystallization from DME (1 mL) layered below
pentane (3 mL) gave 12 as 0.180 g (81%) of a yellow crystalline
product. The 1H NMR and UV-visible absoprtion spectra
matched that of 12 synthesized by an independent route.
MHz, CD2Cl2): δ = -60.35, -9.46, -0.90, 0.76, 3.94, 4.68,
5.09, 8.44, 10.90, 16.81, 17.75, 28.00, 32.63, 49.92, 53.68, 62.32,
62.81, 63.86, 69.99, 137.49, 148.78, 150.32. Evans’ Method
(CD3CN) μeff = 4.7 μB. Anal. Calcd for C37H31N7O6F6S2Fe:
C, 49.18; H, 3.46; N, 10.85. Found: C, 49.42; H, 3.40; N, 10.87.
Complex 14. (a) With PhIO as the oxidant: 12 (0.054 g, 0.060
mmol) was dissolved in CH3CN (5 mL) and added to PhIO
(0.025 g, 0.114 mmol). The solution rapidly changed color from
yellow to blue. The solution stirred for 1.5 h, filtered, and
dried in vacuo. The blue residue was crystallized out of DME
(2 mL) layered beneath Et2O (2 mL)/pentane (4 mL). The
mother liquor was decanted to yield 0.053 g (quantitative) of
blue 14•DME crystals suitable for X-ray diffraction. 1H NMR
(400 MHz, CD3CN): δ = -5.62, 4.70, 8.85, 91.84, 137, 155.
Evans’ Method μeff = 5.0 μB. Anal. Calcd for C41H42N7O9-
F6S2Fe: C, 48.72; H, 4.19; N, 9.70. Found: C, 48.38; H, 4.16; N,

10034 Inorganic Chemistry, Vol. 48, No. 21, 2009
Soo et al.
6-(((Dipyridin-2-ylmethyl)((6-(phenylamino-D)pyridin-2-yl)-
methyl)amino)methyl)-N-phenylpyridin-2-amine-D (10-ND). A
solution of 4 M DCl in D2O (2.5 mL) was added to 7 (0.676 g,
0.902 mmol) at room temperature. The resulting yellow solution
was stirred at room temperature for 1 h. A solution of 30%
NaOD in D2O (5 mL) was then added to give a white precipitate.
This mixture was extracted with CDCl3 (3 ꢀ 2 mL), dried over
anhydrous Na2SO4, and evaporated to yield 0.378 g (76%) of a
Under these conditions, a mass resolving power97 of 1.0 ꢀ 104
(measured at m/z = 759) was achieved, which was more than
sufficient to resolve the isotopic distributions of the singly and
multiply charged ions measured in this study. This enables an
ion’s mass and charge to be determined independently (i.e., the
charge is determined from the reciprocal of the spacing between
adjacent isotope peaks in the m/z spectrum).97 Mass spectra
were accumulated over a period of 2 min. Mass calibration was
performed immediately prior to measurements, using solutions
of sodium formate. Mass spectra were processed using Mass-
Lynx software (version 4.1, Waters).
1
colorless foamy product. H NMR indicated that the product
was only about 25% deuterated at the aniline positions. Inter-
estingly, the methine hydrogen appeared to have been substi-
tuted with up to 85% deuterium.
Infrared (IR) Spectroscopic Experiments. IR samples were
prepared in a glovebox filled with N2. Typically, about 5 mg of
the sample was dissolved in CH2Cl2 and transferred into a
solution sample holder that was equipped with KBr windows.
Between 4 and 32 scans were collected, and both the ambient
background and the CH2Cl2 spectra were subtracted from the
acquired data. The samples were prepared as described in
the synthetic section above unless otherwise specified. The
spectra shown only include the relevant region for OH/D and
NH/D stretches from 1500 to 4000 cm-1. Both OH to OD
shifts (ν(OH)/(ν(OD) = 1.36; calcd. = 1.37) and NH to ND
shifts are observed (ν(NH)/(ν(ND) = 1.43; calcd.=1.37). These
data clearly demonstrate that the hydrogens in the second-
sphere cavity can be exchanged both intra- and intermolecu-
larly.
Complex 12-ND. Ligand 10-ND (0.058 g, 0.106 mmol) was
dissolved in DME (5 mL) and added to Fe(OTf)2(CH3CN)2
(0.046 g, 0.106 mmol) to yield an orange solution. After 1 h, the
solution was dried in vacuo, and the orange residue was crystal-
lized from DME (2 mL) layered beneath pentane (6 mL). The
mother liquor was decanted to yield 0.066 g (69%) of 12-ND as
yellow crystals.
Electrochemical Experiments. Cyclic voltammetry measure-
ments were carried out in a glovebox under an atmosphere of N2
with 1 mM of 11-14 and 100 mM of nBu4NPF6 in CH3CN. The
working electrode was a glassy carbon electrode, while the
reference and auxiliary electrodes were both platinum wires.
Scans were measured at 100 mV s-1 and only changed to 50 and
400 mV s-1 if the redox waves appeared quasi-reversible. For 11
and 12, measurements were first performed in the absence of
ferrocene, before ferrocene was added as the internal standard
(Supporting Information, Figure S1). For 13 and 14, measure-
ments were first performed in the absence of cobaltocene, before
cobaltocene was added as the internal standard. Cobaltocene
was used instead of ferrocene because the reversible redox wave
for both 13 and 14 had similar potentials as ferrocene. The
potentials relative to ferrocene are 0 and -210 mV for 13 and 14,
respectively.
General Methods for X-ray Crystallography. Crystals were
mounted on Kapton loops in Paratone-N hydrocarbon oil. All
data collections were performed on Bruker (formerly Siemens)
APEX and SMART 1000 diffractometers/CCD area detector
with graphite monochromated Mo KR radiation and equipped
with low temperature apparatus. Data integration was per-
formed using the program SAINT. Data was analyzed for
agreement and possible absorption using XPREP. An empirical
absorption correction based on comparison of redundant and
equivalent reflections was applied using SADABS. The struc-
tures were solved within the WinGX package98 by direct meth-
ods (SHELXS-97 and SHELXS-86) and expanded using
Fourier techniques (SHELXL-97). All non-hydrogen atoms
were refined anisotropically except when specified below,
whereas hydrogen atoms were included in calculated positions,
but were not refined. For 13, 14, and 16, the hydrogens for the
hydroxide groups were first located in the Fourier difference
maps, before being constrained as O-H groups and were not
refined as well. In the case of chiral space groups, the correct
enantiomer was determined by comparison of calculated and
observed Friedel pairs.
Complex 11 crystallized with an unbound molecule of triflate.
The triflate anion was reasonably modeled and refined aniso-
tropically. Compound 13 crystallized with two unbound mole-
cules of triflate and two THF molecules. One of the triflate
anions was modeled adequately, whereas the second triflate
anion was disordered and refined isotropically with no re-
straints. One of the THF molecules was reasonably modeled,
whereas the second molecule of THF was refined anisotropically
over two positions. Complex 12 crystallized with an unbound
molecule of triflate and a well-behaved molecule of DME. The
triflate anion was disordered and modeled by refining aniso-
tropically over two positions with restraints for the bond
distances and angles. Complex 14 crystallized with two unbound
molecules of triflate and a relatively well-behaved molecule of
DME. One of the triflate anions was modeled adequately,
whereas the second triflate anion was disordered and refined
isotropically over two positions with restraints for the bond
distances and angles. Complex 16 crystallized with two parent
molecules in the unit cell, their associated unbound triflate
molecules and a disordered molecule of CH3CN. One triflate
Mass Spectrometry Experiments. Mass spectra were acquired
in the positive ion mode using a quadrupole time-of-flight
(Q-Tof) mass spectrometer equipped with a Z-spray electro-
spray ionization (ESI) source (Q-Tof Premier, Waters, Milford,
MA). Compounds 13 and the 18O-enrichcd 13-18OH were
dissolved in acetonitrile on site, withdrawn into a 250 μL Gas-
tight syringe (Hamilton, Reno, NV), and infused into the ESI
probe at a flow rate of 5 μL/min using a syringe pump. Swift and
precise action during data collection was necessary to prevent
degradation of 13-18OH because the OH- ligand was found to
be labile and readily exchanged with trace amounts of solvents
such as methanol in the instrument. The electrospray was
emitted from a stainless steel capillary with an inner diameter
of 127 μm. The voltages applied to the ion source optics were
adjusted for optimum desolvation and transmission of the
complexes of interest, in the following order: (1) sampling cone,
(2) extraction cone, and (3) ion guide. The ESI capillary voltage
was subsequently adjusted to maintain ion counts below the
dead-time threshold (<0.1 ions per push) to prevent spectral
distortion effects due to detector saturation. The ion source
parameters used for the measurements reported here were as
follows: ESI capillary voltage 1.0 kV, nebulizing gas flow rate
800 L/h, sampling cone voltage 10 V, extraction cone voltage 1
V, ion guide voltage 0.1 V, source block temperature 80 °C, and
nebulizing gas temperature 70 °C. Dry nitrogen (boil-off from
liquid nitrogen, Praxair, Danbury, CT) was used as the nebuliz-
ing gas. No cone gas was used. The pressures in the different
stages of the instrument were as follows: first pumping stage 1.2
mbar, ion transfer stage 4 ꢀ 10-4 mbar, quadrupole analyzer 2 ꢀ
10-5 mbar, argon-filled cell 8 ꢀ 10-3 mbar, and Tof analyzer
7 ꢀ 10-7 mbar. The Tof analyzer was operated in “V” mode.
(97) Marshall, A. G.; Hendrickson, C. L. Annu. Rev. Anal. Chem. 2008, 1,
579–599.
(98) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10035
anion was adequately modeled, whereas the second triflate
anion was disordered and refined with restraints for the bond
angles and distances. The CH3CN was disordered with N15
occupying the inversion center and was refined isotropically
without hydrogen atoms. Because of the disordered position of
C73 in CH3CN, the maximum shift/error was 0.17 after 8 least-
squares refinement cycles. All the salient crystallographic data
are summarized in Tables 1 and 2.
LBNL/DOE Helios SERC (51HE112B) for funding this work.
H.S.S. thanks Dr. Frederick Hollander and Dr. Allen Oliver
for advice on X-ray crystallography, as well as Prof. Robert
Bergman and Mr. Shaun Wong for insightful discussions and
suggestions. The mass spectrometer used in this study was
acquired with support from the National Institutes of Health
(1S10RR02239301).
Supporting Information Available: Experimental details of the
IR spectroscopic measurements, cyclic voltammograms, and
mass spectrometric studies. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the University of California,
Berkeley, the Dreyfus, Beckman, Packard, and Sloan Founda-
tions, the LBNL Chemical Sciences Division (403801), and the