A spectrochemical walk: Single-site perturbation within a series of six-coordinate ferrous complexes

Article abstract of DOI:10.1021/ic025616z

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe¹¹(PY5)(X)]ⁿ⁺, X = MeOH, H₂O, MeCN, pyridine, Cl^-, OBz^-, N₃^-, MeO^-, PhO^-, and CN^-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (Is) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe¹¹(PY5)]²⁺ unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK_a of the exogenously ligated MeOH in [Fe(PY5)(MeOH)]²⁺ (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)]²⁺ to yield [Fe(PY5)(OMe)]¹⁺ rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

Full text of DOI:10.1021/ic025616z

Inorg. Chem. 2002, 41, 4642−4652
A Spectrochemical Walk: Single-Site Perturbation within a Series of
Six-Coordinate Ferrous Complexes†
Christian R. Goldsmith, Robert T. Jonas, Adam P. Cole, and T. Daniel P. Stack*
Department of Chemistry, Stanford UniVersity, California 94305
Received March 26, 2002
A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5)
was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination
II
site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe (PY5)(X)]n+, X) MeOH,
H O, MeCN, pyridine, Cl-, OBz-, N -, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are
2
3
extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate
with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic
structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities
of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding
II
affinity; the [Fe (PY5)]2+ unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-
field ligands are preferentially bound over weak-field ligands. In MeOH, the pK of the exogenously ligated MeOH
a
in [Fe(PY5)(MeOH)]2+ (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate
[Fe(PY5)(MeOH)]2+ to yield [Fe(PY5)(OMe)]1+ rather than ligate to the ferrous center. Exogenous ligation by a
strongly basic ligand, however, can be achieved in polar aprotic solvents.
Table 1. Mononuclear Non-Heme Iron Enzyme Active Site Ligands
Introduction
enzymea
geometryb
ox. stateb
coordination ligandsb
ref
Ferrous iron demonstrates a remarkable ability to accom-
modate a diverse set of coordination geometries from four-
to eight-coordinate, although the six-coordinate octahedral
geometry is usually favored.1 This binding plasticity is an
important property in the bioinorganic chemistry of Fe(II),
particularly in mononuclear non-heme oxidative enzymes.
The ligand set and geometry in non-heme enzymes are much
more varied than in heme-containing enzymes. Crystal-
lographic and spectroscopic data of non-heme iron sites
support both penta- and hexacoordination with nitrogen,
oxygen, and sulfur donors derived from amino acid residues
and exogenous ligands (Table 1).2,3 This coordinative
diversity increases the number of possible mechanisms for
substrate and O2 activation. Increased efforts to discern the
relationship between structure and function in these mono-
nuclear non-heme enzymes that activate O2 have been made
3,4-PCD trig. bipyramidal Fe(III)
BphC
SLO-1
2N(His), 2O(Tyr), 1O(OH-) 40, 41
2N(His), 1O(Glu), 2O(H2O) 42, 43
sq. pyramidal
octahedral
Fe(II)
Fe(II)
3N(His), 1O(Ile-CO2-),
44, 45
1O(Asn), 1O(H2O)
-
15-RLO sq. pyramidal
Fe(II)
Fe(II)
Fe(II)
Fe(II)
4N(His), 1O(Ile-CO2
)
46
PAH
CAS
IPNS
SOD
SOR
octahedral
octahedral
octahedral
trig. bipyramidal Fe(II)
sq. pyramidal Fe(II)
2N(His), 1O(Glu), 3O(H2O) 47
2N(His), 1O(Glu), 3O(H2O) 48
2N(His), 1O(Asp), 3O(H2O) 49
3N(His), 1O(Asp), 1O(H2O) 50
4N(His), 1S(Cys)
51
a 3,4-PCD ) protocatechuate 3,4-dioxygenase; BphC ) 2,3-dihydroxy-
biphenyl 1,2-dioxygenase; SLO-1 ) soybean lipoxygenase-1; SLO-3 )
soybean lipoxygenase-3; 15-RLO ) rabbit 15-lipoxygenase; PAH ) human
phenylalanine hydroxylase; CAS ) clavaminic acid synthase; IPNS )
microbial isopenicillin N-synthase; SOD ) superoxide dismutase; SOR )
superoxide reductase. b As determined in the crystallographic analysis.
in recent years.2-7 Similar studies on small molecule models
have also been a valuable tool in the study of mononuclear
non-heme iron enzymes.
Control of the geometric plasticity and ligand lability of
Fe(II) sites can be achieved with conformationally restricted
polydentate ligands, as found with porphyrins. These tet-
* To whom correspondence should be addressed. E-mail: stack@
stanford.edu. Phone: (650) 725-8736. Fax: (650) 725-0259.
† Dedicated to Professor K. N. Raymond on the occasion of his 60th
birthday.
(1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley-Interscience: New York, 1988.
(4) Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-805.
(5) Howard, J. D.; Rees, D. C. AdV. Protein Chem. 1991, 42, 199-280.
(6) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96,
2239-2314.
(2) Iron Metabolism. Inorganic Biochemistry and Regulatory Mechanisms,
1st ed.; Ferreira, G. C.; Moura, J. J. G.; Franco, R., Eds.; Wiley:
Weinheim, 1999.
(3) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624.
(7) Solomon, E. I. Inorg. Chem. 2001, 40, 3656-3669.
4642 Inorganic Chemistry, Vol. 41, No. 18, 2002
10.1021/ic025616z CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/30/2002

Synthesis and ReactiWity of Fe(PY5)(X) Series
Scheme 1
synthesized in a variety of solvents from an equimolar ratio
of PY5 and FeII(OTf)2. The ligand accommodates a single
metal ion in a six-coordinate octahedral conformation with
the general formula [FeII(PY5)(X)](OTf)n (n ) 1 or 2). The
sixth coordination site is readily occupied by a coordinating
solvent molecule, such as MeOH, H2O, MeCN, or pyridine,
to yield [Fe(PY5)(solvent)](OTf)2. The H2O complex is
synthesized by mixing PY5 and FeII(OTf)2 in nondried
2-propanol. The exogenous solvent ligand may be displaced
with numerous anionic ligands, including chloride, methox-
ide, benzoate, azide, and cyanide, by addition of 1 equiv of
their sodium or potassium salt (Scheme 2). The chloride
complex [Fe(PY5)(Cl)](OTf) can also be synthesized by
mixing PY5 with FeIICl2, followed by 1 equiv of Ag(OTf)
to precipitate 1 equiv of Cl- as AgCl(s).
radentate ligands generally bind iron in a square-planar
coordination, allowing variation of the axial ligand(s) to yield
five- or six-coordinate complexes. This investigation con-
cerns a pentadentate ligand with Fe(II) that imposes a square-
pyramidal coordination geometry, thereby allowing facile
variation of a single exogenous ligand. The number of
noncyclic nitrogen ligands that enforce such a square-
pyramidal conformation is limited to a few tetrapodal
pentadentate ligands8-12 and open chain penta-amines.13
In an effort to isolate structurally and functionally relevant
analogues of lipoxygenase, a mononuclear non-heme iron
enzyme, the ligand, 2,6-(bis-(bis-2-pyridyl)methoxymethane)-
pyridine (PY5, Scheme 1), was designed and synthesized.14,15
The five pyridine subunits of PY5 constitute a neutral five-
coordinate metal binding cavity; complexation of a divalent
metal results in a strongly Lewis acidic metal center.
Metal Complex Characterization. The following Fe(II)
complexes,
[Fe(PY5)(MeOH)]2+,
[Fe(PY5)(H2O)]2+,
[Fe(PY5)(MeCN)]2+, [Fe(PY5)(pyridine)]2+, [Fe(PY5)-
(OBz)]1+, [Fe(PY5)(Cl)]1+, [Fe(PY5)(N3)]1+, [Fe(PY5)-
(OMe)]1+, [Fe(PY5)(OPh)]1+, and [Fe(PY5)(CN)]1+, were
isolated and characterized as their triflate salts by 1H NMR
spectroscopy, UV-vis absorption spectroscopy, mass spec-
troscopy, cyclic voltammetry, and solution magnetic sus-
ceptibility.17
A. Solution Structures. All of the examined [FeII(PY5)-
(X)](OTf)n complexes are stable upon exposure to air except
[Fe(PY5)(OMe)](OTf) (in MeOH) and [Fe(PY5)(OPh)](OTf)
(in acetone); both complexes undergo slow decomposition
to free ligand and an insoluble iron product. Isolated solid
samples of the other complexes retain their coordination
when dried in vacuo at room temperature (RT).
Reactions of PY5 with FeII(OTf)2 (OTf- ) triflate, CF3SO3 )
-
result exclusively in the isolation of six-coordinate complexes
with the general cationic formula [FeII(PY5)(X)]n+. The
structural and spectroscopic properties of a series of Fe(II)
complexes, [FeII(PY5)(X)](OTf)n, with X ) MeOH, H2O,
-
MeCN, pyridine, OBz-, Cl-, N3 , MeO-, PhO-, and CN-,
1H NMR spectroscopic characterization of the series of
ferrous complexes at RT reveals both high-spin (hs) and low-
spin (ls) complexes within the series (Table 2). The 1H NMR
spectra of [Fe(PY5)(MeOH)]2+, [Fe(PY5)(H2O)]2+, [Fe-
(PY5)(N3)]1+, [Fe(PY5)(OBz)]1+, [Fe(PY5)(Cl)]1+, [Fe(PY5)-
(OMe)]1+, and [Fe(PY5)(OPh)]1+ exhibit features of hs
ferrous complexes, with paramagnetically shifted peaks in
the range -12 to 65 ppm. Solution magnetic susceptibilities17
are in agreement with those expected for hs complexes. For
the complexes [Fe(PY5)(MeCN)]2+, [Fe(PY5)(pyridine)]2+,
are presented. In addition, the relative binding affinities of
these exogenous ligands to [FeII(PY5)]2+ in MeOH highlight
the importance of ligand charge over other electronic bonding
features in determining the stability of a metal complex. In
all cases, PY5 maintains a square-pyramidal ligation geom-
etry with a single exogenous ligand, X, completing the metal
coordination to generate a six-coordinate ferrous complex.
An intriguing aspect of this work is that variation of X is
sufficient to change the spin-state of the metal and thus the
structural and electronic properties of the complex.
1
and [Fe(PY5)(CN)]1+, diamagnetic H NMR spectra are
observed, consistent with their assignment as ls complexes.
In solution, PY5 is presumed to ligate Fe(II) in the manner
demonstrated in Scheme 1. The highly symmetric nature of
Results
Metal Complex Syntheses. The synthesis, characteriza-
tion, and X-ray structure of 2,6-(bis-(bis-2-pyridyl)-
methoxymethane)pyridine (PY5) have been described pre-
viously.14,16 The Fe(II) complexes can be anaerobically
1
the H NMR spectra of these complexes suggests dynamic
averaging on the NMR time scale; all the equatorial pyridine
resonances (Scheme 1, Py2-5) are structurally equivalent.
Cyclic voltammetric measurements of [FeII(PY5)(X)]n+
complexes in MeOH, acetone, or MeCN (Table 2) reveal
very positive oxidation potentials for all the complexes. The
four complexes with ligated solvent (X ) H2O, MeOH,
MeCN, pyridine) show irreversible oxidation peaks above
0.9 V versus SHE while five of the complexes with anionic
axial ligands (X ) Cl-, OBz-, MeO-, PhO-, CN-) show
(8) Grohmann, A.; Knoch, F. Inorg. Chem. 1996, 35, 7932-7934.
(9) Tamagaki, S.; Kanamaru, Y.; Ueno, M.; Tagaki, W. Bull. Chem. Soc.
Jpn. 1991, 64, 165-174.
(10) Takano, S.; Yano, Y.; Tagaki, W. Chem. Lett. 1981, 1177-1180.
(11) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B. L.; Que, L.,
Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 1512-1514.
(12) Bernal, I.; Jensen, I. M.; Jensen, K. B.; McKenzie, C. J.; Toftlund,
H.; Tuchagues, J.-P. J. Chem. Soc., Dalton Trans. 1995, 3667-3675.
(13) Guajardo, R. J.; Chavez, F.; Farinas, E. T.; Mascharak, P. K. J. Am.
Chem. Soc. 1995, 117, 3883-3884.
(14) Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 8566-8567.
(15) deVries, M. E.; LaCrois, R. M.; Roelfes, G.; Kooijman, H.; Spek, A.
L.; Hage, R.; Feringa, B. L. Chem. Commun. 1997, 1549-1550.
(16) Goldsmith, C. R.; Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc.
2002, 124, 83-96.
(17) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
Inorganic Chemistry, Vol. 41, No. 18, 2002 4643

Goldsmith et al.
Scheme 2
1
Table 2. Effective Magnetic Moments, H NMR Resonance Ranges, and Redox Potentials of Ferrous Complexesa
complex
µeffb (µB)
spin-state
1H NMRd rangec (ppm)
E1/2d (V)
[Fe(PY5)(MeOH)]2+
[Fe(PY5)(H2O)]2+
[Fe(PY5)(Cl)]1+
4.7
4.9
5.1
g
4.7
4.9
4.7
0
hs
hs
hs
hs
hs
hs
hs
ls
-11.3-57.0
-11.8-58.0
-11.2-64.1
-8.2-54.4
-10.8-60.1
-5.8-56.2
-9.9-54.1
4.08-9.97
0.930 (∆E ) 0.140 V)
1.360 (∆E ) 0.180 V)e
0.990 (∆E ) 0.090 V)
1.030 (∆E ) 0.160 V)
0.760 (∆E ) 0.100 V)
1.070 (∆E ) 0.090 V)e
0.740 (irrev)
[Fe(PY5)(OBz)]1+
[Fe(PY5)(OMe)]1+
[Fe(PY5)(OPh)]1+
[Fe(PY5)(N3)]1+
[Fe(PY5)(MeCN)]2+
[Fe(PY5)(pyridine)]2+
[Fe(PY5)(CN)]1+
1.150 (∆E ) 0.090 V)f
1.180 (∆E ) 0.100 V)
0.920 (∆E ) 0.090 V)
0
0
ls
ls
4.68-13.38
4.03-10.11
a Temperature ) 292 K for solution measurements. b All measurements in acetone-d6 except for [Fe(PY5)(MeCN)]2+, which was measured in CD3CN.
c Shifts are reported in ppm from a TMS internal standard in acetone-d6. d Redox potentials vs SHE. Referenced to the Fc/Fc+ couple in MeOH (0.610 V)34
except where noted. e Measured in acetone and referenced to the Fc/Fc+ couple (0.700 V).34 f Measured in MeCN and referenced to the Fc/Fc+ couple
(0.590 V).34 g [Fe(PY5)(OBz)]1+ was not soluble enough in acetone to acquire an accurate magnetic moment.
Table 3. UV-Vis Absorption Data of Ferrous Complexes
complex
solvent
MeOH
λmaxa (ꢀb)
[Fe(PY5)(MeOH)]2+
[Fe(PY5)(H2O)]2+
[Fe(PY5)(Cl)]1+
370 (1650), 785 (10), 865 (10)
2-propanol 355 (1580), 568 (20), 775 (20)
MeOH
MeOH
acetone
MeOH
MeOH
MeCN
330(1590), 390 (1930), 860 (15)
330 (1630), 395 (1800)
355 (1650), 428 (1530)
[Fe(PY5)(OBz)]1+
[Fe(PY5)(OPh)]1+
[Fe(PY5)(N3)]1+
410 (2150), 810 (20)
[Fe(PY5)(OMe)]1+
[Fe(PY5)(MeCN)]2+
340 (1300), 440 (1700), 715 (30)
362 (6900), 384 (6900),
424 (5880), 554 (180)
396 (4700), 440 (3000), 548 (190)
372 (6700), 439 (8100)
[Fe(PY5)(pyridine)]2+ pyridine
[Fe(PY5)(CN)]1+
MeOH
a Absorption maxima reported in units of nm and measured to 1000 nm.
b Extinction coefficients reported in units of M-1 cm-1
Figure 1. UV-vis spectra for complexes (a) [Fe(PY5)(Cl)](OTf), (b) [Fe-
(PY5)(OBz)](OTf), (c) [Fe(PY5)(MeOH)](OTf)2, and (d) [Fe(PY5)(H2O)]-
(OTf)2 at RT. All spectra were taken with ca. 5.5 mM samples in MeOH
with the exception of [Fe(PY5)(H2O)](OTf)2, which was taken with a 2.5
mM sample in 2-propanol.
.
quasireversible cyclic voltammograms between 0.7 and 1.1
V versus SHE. [Fe(PY5)(N3)]1+ shows an irreversible
oxidation peak at 0.740 V versus SHE. The ls complexes
tend to exhibit oxidation potentials slightly higher than the
hs complexes.
Differentiation of hs from ls ferrous complexes at RT is
possible by simple visual inspection. Solutions of the seven
hs complexes are varied shades of yellow and light orange,
while solutions of the three ls complexes are deep orange to
red. UV-vis absorption spectroscopy data corroborate the
visual assignments (Table 3). All ten complexes show high
energy transitions between 340 and 440 nm that are assigned
as metal-to-ligand charge transfers (MLCT) between the
Fe(II) center and the pyridine subunits of PY5. The maxi-
mum extinction coefficients for the MLCT absorptions of
the hs complexes [Fe(PY5)(MeOH)]2+, [Fe(PY5)(H2O)]2+,
[Fe(PY5)(Cl)]1+, [Fe(PY5)(OMe)]1+, and [Fe(PY5)(OPh)]1+
are between 1500 and 2200 M-1 cm-1 (Figure 1), much less
intense than those of the ls complexes [Fe(PY5)(MeCN)]2+,
[Fe(PY5)(pyridine)]2+, and [Fe(PY5)(CN)]1+ with extinction
coefficients in the range 5000-8000 M-1 cm-1 (Figure 2).
The ls complexes moderately absorb near 550 nm. Higher
Figure 2. UV-vis spectra for low-spin complexes: (a) [Fe(PY5)(CN)]-
(OTf), (b) [Fe(PY5)(MeCN)](OTf)2, and (c) [Fe(PY5)(pyridine)](OTf)2 at
RT. All spectra were taken of ca. 3.0 mM samples in MeOH with the
exception of [Fe(PY5)(MeCN)](OTf)2 which was taken in MeCN.
energy features below 300 nm are observed in the absorption
spectra of all these complexes and likely result from metal-
independent πfπ* transitions within PY5. The unbound
ligand absorbs strongly (ꢀ ∼ 10 000 M-1 cm-1) at 250 nm.
4644 Inorganic Chemistry, Vol. 41, No. 18, 2002

Synthesis and ReactiWity of Fe(PY5)(X) Series
Table 4. Crystallographic Data of High-Spin Ferrous Complexes
[Fe(PY5)(H2O)](OTf)2
[Fe(PY5)(Cl)](OTf)‚(MeOH)
[Fe(PY5)(OBz)](OTf)
formula
fw (g mol-1
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C31H27N5O9F6S2Fe
847.54
triclinic
P1h (No. 2)
11.794(1)
12.291(2)
13.954(4)
80.68(2)
87.72(1)
62.49(1)
1769.1(6)
2
6.36
C62H54N10O12F6S2Fe2
1491.88
C37H30N5O7F3SFe
857.6
)
monoclinic
P21/c (No. 14)
18.396(1)
14.855(1)
23.090(1)
90.00
102.262(1)
90.00
6166.4(1)
4
7.15
1.610
0.15 × 0.40 × 0.50
10.0° < 2θ < 50.0°
11 670
orthorhombic
Pnma (No. 69)
12.620(2)
14.484(2)
20.807(1)
90.00
90.00
90.00
3803.1(8)
4
5.19
Z
µ
calcd (cm-1
)
F
calcd (g cm-3
)
1.591
1.50
crystal size (mm3)
2θ range
0.20 × 0.40 × 0.60
10.0° < 2θ < 50.0°
6529
6200 (Rint ) 0.188)
3510
0.21 × 0.15 × 0.10
3.23° < 2θ < 49.43°
17 092
3621 (Rint ) 0.084)
3354
reflns collected
unique reflns
reflns with
11 358 (Rint ) 0.030)
6702
(Fo2 > 3.00σ(Fo ))
2
no. params
reflns/params ratio
p-factor
487
7.21
0.010
0.078
0.071
865
7.75
0.006
0.051
0.057
299
11.2
0.010
0.049
0.123
Ra
Rw
a
2
2
2
2
2
a R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2, where w ) 4Fo /σ2(Fo ); σ2(Fo ) ) S2(C + R2B) + (pFo )2/(Lp)2 with S ) scan rate, C
) total integrated peak count, R ) ratio of scan time to background counting time, B ) total background count, Lp ) Lorentz polarization factor, and p )
p-factor.
Mass spectroscopy data for the complexes reveal limited
information about their structures. Most of the complexes
lose their exogenous ligand before detection, such that the
typical parent ion peak is consistent with [Fe(PY5)](OTf)1+.
For the four complexes with neutral exogenous ligands (H2O,
MeOH, MeCN, pyridine), only the spectrum for [Fe(PY5)-
(H2O)](OTf)2 shows any evidence for retention of the axial
ligand, with a peak assigned to [Fe(PY5)(H2O)](OTf)1+ at
698.0. A [Fe(PY5)(X)]1+ parent ion peak is observed in the
spectra of [Fe(PY5)(Cl)](OTf), [Fe(PY5)(OMe)](OTf), and
[Fe(PY5)(CN)](OTf), as well as a less intense [Fe(PY5)]-
(OTf)1+ peak.
Table 5. Crystallographic Data of Low-Spin Ferrous Complexes
[Fe(PY5)(pyridine)]-
[Fe(PY5)(CN)]-
(OTf)2‚(pyridine)
(OTf)‚(MeOH)
formula
fw (g mol-1
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C41H35N7O8F6S2Fe
987.73
triclinic
P1h (No. 2)
10.058(4)
12.368(2)
16.847(4)
86.32(2)
79.85(3)
89.54(3)
2058.8(9)
2
5.60
C32H29N6O6F3SFe
738.52
triclinic
)
P1h (No. 2)
10.5306(4)
12.6061(6)
12.9532(6)
76.544(4)
88.724(3)
67.513(3)
1540.9(1)
2
6.30
1.592
0.15 × 0.50 × 0.50
10.0° < 2θ < 50.0°
5676
Z
µ
calcd (cm-1
)
B. Solid State Structures. The X-ray structures of five
cationic species are reported here, all of which contain triflate
counteranions: [Fe(PY5)(H2O)]2+, [Fe(PY5)(pyridine)]2+,
F
calcd (g cm-3
)
1.593
cryst size (mm3)
2θ range
0.25 × 0.40 × 0.50
8.0° < 2θ < 46.8°
5346
5006 (Rint ) 0.083)
3037
[Fe(PY5)(OBz)]1+, [Fe(PY5)(Cl)]1+, and [Fe(PY5)(CN)]1+.
reflns collected
unique reflns
reflections with
14,16
5408 (Rint ) 0.014)
4728
Structures of [Fe(PY5)(MeOH)](OTf)2
and [Fe(PY5)-
(MeCN)](ClO4)215 have been previously reported, and their
metrical parameters will be used in comparisons. Structural
and refinement data for the four hs and two ls complexes,
as assessed by solution magnetic susceptibility at 298 K (vide
supra), are summarized in Tables 4 and 5, respectively. The
atom labeling schemes for the cations of all complexes are
congruent in order to facilitate comparisons of structural
parameters. The Fe(II) center in each complex is coordinated
in a square-pyramidal geometry by PY5, and a distorted six-
coordinate octahedral environment is achieved with binding
of an exogenous anion or solvent molecule. For the purpose
of discussion, an equatorial plane (Pleq) is defined by the
least-squares-plane of the four nitrogen atoms N2-5 in their
respective pyridine subunits Py2-5 (Scheme 1). The axial
positions are occupied by the nitrogen atom (N1) of the axial
pyridine subunit (Py1) and the heteroatom of the exogenous
ligand. The centroid of each pyridine subunit (Pyn) is denoted
(Fo2 > 3.00σ(Fo ))
no. params
reflns/params ratio
p-factor
2
586
5.18
0.010
0.061
0.067
442
10.70
0.011
0.042
0.057
Ra
Rw
a
2
a R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2, where w
2
2
2
2
) 4Fo /σ2(Fo ); σ2(Fo ) ) S2(C + R2B) + (pFo )2/(Lp)2 with S ) scan rate,
C ) total integrated peak count, R ) ratio of scan time to background
counting time, B ) total background count, Lp ) Lorentz polarization factor,
and p ) p-factor.
as cPyn. The centroids of the pyridine subunits and the
quarternary carbons that link them (Cquat) are used to describe
the angles between adjacent subunits.
The asymmetric unit of [Fe(PY5)(OBz)](OTf) contains
half of the ferrous cation [Fe(PY5)(OBz)]1+ as the complex
resides on a crystallographic mirror plane. To facilitate
comparison with the other structures, the nitrogens in the
Inorganic Chemistry, Vol. 41, No. 18, 2002 4645

Goldsmith et al.
Table 6. Selected Bond Lengths and Angles for High-Spin Complexesa
Cl,
unit A
Cl,
unit B
complexb
Fe1-N1
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N5
MeOH
H2O
OBz
2.097(3) 2.131(7) 2.185(5) 2.198(5) 2.215(5)
2.152(3) 2.149(8) 2.183(4) 2.182(5) 2.192(5)
2.203(3) 2.207(8) 2.267(3) 2.272(5) 2.263(5)
2.217(3) 2.242(7) 2.267(3) 2.252(4) 2.247(5)
2.141(4) 2.171(7) 2.183(4) 2.173(5) 2.179(5)
Fe1-X (X ) O, Cl) 2.040(3) 2.034(6) 2.012(4) 2.311(2) 2.320(2)
N2-Fe1-N3
N4-Fe1-N5
N2-Fe1-N5
N3-Fe1-N4
N2-Fe1-N4
N3-Fe1-N5
83.2(1) 80.4(3) 81.9(1) 80.5(2) 80.4(2)
82.1(1) 83.7(3) 81.9(1) 80.4(2) 80.8(2)
97.7(1) 95.8(3) 97.3(2) 95.1(2) 94.0(2)
95.7(1) 98.5(3) 95.7(2) 100.5(2) 101.4(2)
170.5(1) 168.9(3) 166.4(1) 164.6(2) 163.3(2)
171.9(1) 170.7(3) 166.4(1) 166.8(2) 167.1(2)
a Bond lengths are in angstroms, and bond angles are in degrees.
Estimated standard deviations in the least-squares figure are given in
parentheses. b Complex names are abbreviated to the exogenous axial ligand
for clarity.
Figure 3. ORTEP representation of the crystal structure of [FeII(PY5)-
(H2O)]2+. Ellipsoids drawn at the 50% probability level.
Table 7. Selected Bond Lengths and Angles for Low-Spin Complexesa
[Fe(PY5)(pyridine)](OTf)2
[Fe(PY5)(CN)](OTf)
Fe1-N1
1.987(8)
2.012(8)
1.996(8)
2.060(8)
1.999(8)
1.992(8)
Fe1-N1
1.980(2)
2.000(2)
2.068(2)
2.043(2)
2.003(2)
1.925(3)
1.159(4)
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N5
Fe1-N6
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N5
Fe1-C30
C30-N6
N2-Fe1-N3
N4-Fe1-N5
N2-Fe1-N5
N3-Fe1-N4
N2-Fe1-N4
N3-Fe1-N5
85.0(3)
84.0(3)
96.3(3)
94.6(3)
178.9(3)
175.5(3)
N2-Fe1-N3
N4-Fe1-N5
N2-Fe1-N5
N3-Fe1-N4
N2-Fe1-N4
N3-Fe1-N5
Fe1-C30-N6
82.7(1)
84.9(1)
97.5(1)
94.9(1)
177.6(1)
176.2(1)
174.1(3)
a Bond lengths are in angstroms, and bond angles are in degrees.
Estimated standard deviations in the least-squares figure are given in
parentheses.
available.18 Selected bond lengths and angles are provided
in Tables 6 (for hs complexes) and 7 (for ls complexes). As
in previously reported structures,14-16 the plane of Py1 tilts
relative to the perpendicular of Pleq, and the two methoxy-
methyl groups are positioned between Py2 and Py1 and
between Py5 and Py1. These distortions create the distinctive
binding structure of PY5. The cPy2-Cquat-cPy1 and cPy5-
Cquat-cPy1 angles widen from the 108° seen in the free ligand
to values between 118° and 123°. The bond lengths of Fe-
N2 and Fe-N5 contract relative to Fe-N3 and Fe-N4,
correlating with the opposite positioning to the tilt of Py1.
Last, the Fe(II) ion is displaced above Pleq. The ligand
distortions and coordination sphere perturbations for all
cations are summarized in Table 8. The average value of
the Fe(1)-N(1-5) bond lengths is consistent with the
solution spin-state assignments for each Fe(II) metal center.
The axial cyanide ligand in [Fe(PY5)(CN)]1+ is slightly
askew with an Fe(1)-C(30)-N(6) bond angle of 174.1°. The
Fe(1)-C(30) bond length of 1.925(3) Å is consistent with
other Fe(II)-CN complexes.19,20
Figure 4. ORTEP representation of the crystal structure of [FeII(PY5)-
(OBz)]1+. Ellipsoids drawn at the 50% probability level.
coordination sphere are appropriately designated N1-5. The
asymmetric unit of [Fe(PY5)(Cl)](OTf) contains two inde-
pendent [Fe(PY5)(Cl)]1+ cations (units A and B). The axial
N6 in the structure of unit B is congruent with N1 in the
structure of unit A. Likewise, the equatorial N7-10 in the
structure of unit B are congruent with N2-5 in the structure
of unit A. To facilitate comparison with the other structures,
unit B will be treated as an independent structure with its
coordination sphere nitrogens designated N1-5. Full crystal-
lographic reports with the original atomic labeling scheme
for each complex are provided.18
All hs cations and [Fe(PY5)(CN)]1+ share the distinctive
PY5 binding motif previously reported.14-16 The overall
coordination is conserved throughout these five new stuc-
tures. An ORTEP representation of [Fe(PY5)(H2O)]2+ is
shown in Figure 3, while an orthogonal view of [Fe(PY5)-
(OBz)]1+ is shown in Figure 4. ORTEP representations of
the [Fe(PY5)(Cl)]1+ and [Fe(PY5)(CN)]1+ structures are
(19) Hsu, H. F.; Koch, S. A.; Popescu, C. V.; Mu¨nck, E. J. Am. Chem.
Soc. 1997, 119, 8371-8372.
(20) Rauchfuss, T. B.; Contakes, S. M.; Hsu, S. C. N.; Reynolds, M. A.;
Wilson, S. R. J. Am. Chem. Soc. 2001, 123, 6933-6934.
(18) See Supporting Information.
4646 Inorganic Chemistry, Vol. 41, No. 18, 2002

Synthesis and ReactiWity of Fe(PY5)(X) Series
Table 8. Coordination Environment Structural Data for Ferrous Complexes
spin-statea N(1)b av N(2,5)b av N(3,4)b av N(1-5)b N(1) tiltc Fe displd cPy2-Cquat-cPy1
cPy5-Cquat-cPy1
e
e
complex
[Fe(PY5)(Cl)]1+, unit B
[Fe(PY5)(Cl)]1+, unit A
[Fe(PY5)(OBz)]1+
hs
hs
hs
hs
hs
ls
2.215
2.198
2.183
2.131
2.097
1.980
1.987
2.186
2.178
2.185
2.160
2.147
2.002
2.006
2.255
2.262
2.267
2.225
2.210
2.056
2.028
2.22
2.22
2.22
2.18
2.16
2.02
2.01
31.0
28.9
27.6
25.2
19.4
15.7
10.9
+0.286
+0.276
+0.264
+0.195
+0.167
+0.055
+0.049
119.5
121.0
122.9
118.9
121.4
121.6
117.0
120.8
122.1
122.9
123.3
121.6
118.0
112.6
[Fe(PY5)(H2O)]2+
[Fe(PY5)(MeOH)]2+
[Fe(PY5)(CN)]1+
[Fe(PY5)(pyridine)]2+
ls
a At RT as assessed by 1H NMR. b Bond distances from Fe reported in units of angstroms. c Tilt in degrees of the axial pyridine ligand from the perpendicular
of the least squares equatorial plane N(2-5). d Displacement of the Fe(II) ion from the equatorial plane N(2-5) in Å. e Measured in degrees, using the
centroids of Py1, Py2, and Py5. Bond angles are ∼108° in free PY5.
Figure 6. Titration of 1.6 mM [Fe(PY5)(MeOH)]2+ in MeOH with
pyridine. The solid lines represent the empirical data while the dashed lines
represent the model fit. Amount of pyridine (equiv) added in order of
increasing spectral intensity: 0, 3.3, 6.6, 10, 13, 20, 33. The end spectrum
(33 equiv pyridine added) is ∼80% converted to [Fe(PY5)(pyridine)]2+ 18
.
moieties (2.007 Å) are consistent with the solution assign-
ment of a ls Fe(II) complex.
Solution Chemistry. The axial ligands bound to [FeII-
(PY5)]2+ are sufficiently labile enough to allow study of
ligand exchange except for CN-. [Fe(PY5)(CN)]1+ remains
unchanged over the course of hours in solutions with large
excesses of pyridine, water, or Cl- as assessed by optical
spectroscopy. This lack of CN- lability is similar to that
observed for the ferrocyanide anion.1 A series of spectro-
photometric titrations in MeOH provides the relative binding
Figure 5. ORTEP representation of the crystal structure of [FeII(PY5)-
(pyridine)]2+. Ellipsoids drawn at the 50% probability level. Note the unusual
positioning of the methoxy group to the right.
The conformation of the PY5 ligand in [Fe(PY5)(pyridine)]-
(OTf)2 (Figure 5) slightly differs from that of the other
structurally characterized complexes. Py1 tilts to a lesser
degree from the perpendicular of Pleq at 10.9°, and only one
of the two methoxy groups is positioned analogously to the
other structures. The second methoxy group is positioned
upward into the cleft between Py2 and Py3 in a similar
manner as found in the structure of free PY5.16 As a
consequence, the cPy2-Cquat-cPy3 angle of 113° is most
similar to the cPy5-Cquat-cPy1 angle of 117°, as both reflect
the accommodation of the methoxy group between two
pyridine subunits. The other four cPy-Cquat-cPy angles are
significantly less than the idealized 109° angle. Correlated
with the lessened tilt of Py1, the bond lengths of Fe-N2
(2.012(8) Å) and Fe-N5 (1.999(8) Å) do not contract relative
to Fe-N3 (1.996(8) Å) and Fe-N4 (2.060(8) Å) as observed
in the other ferrous complexes. The exogenous pyridine
ligand is coordinated perpendicular to Pleq, such that the 2-
and 6-position hydrogen atoms of the ring are positioned
into the clefts of the pyridyl arms. The exogenous pyridine
Fe(1)-N(6) bond length of 1.992 Å is shorter than the
average Fe(1)-N(1-5) bond length of 2.010 Å for PY5.
Together, the average Fe-N bond lengths of the six pyridine
-
affinities of MeCN, pyridine, Cl-, OBz-, N3 , and MeOH.
Spectral deconvolution of the individual titrations over the
350-500 nm range clearly shows only two significant
species. A representative titration of [Fe(PY5)(MeOH)]2+
with pyridine to give [Fe(PY5)(pyridine)]2+ is presented in
Figure 6. The equilibrium constant for each two-species
spectrophotometric titration was determined by a component
analysis and a least-squares fit to a simple equilibrium model
(eq 1).
[FeII(PY5)(X)]n+ + Y h [FeII(PY5)(Y)]n+ + X (1)
In each case, projected spectra of the two pure species closely
matched the experimentally determined spectra.18 The bind-
ing affinities of MeCN and pyridine were measured relative
to the binding affinity of MeOH, the weakest ligand
examined, while the binding affinities of Cl-, OBz-, and
-
N3 were measured relative to the binding affinity of
pyridine. The relative equilibrium constants are summarized
in Table 9. It should be noted that the solvent composition
and the ionic strength of the solution change slightly during
Inorganic Chemistry, Vol. 41, No. 18, 2002 4647

Goldsmith et al.
[Fe(phen)3]2+ (1.22 V vs SHE).21,22 The high redox potentials
of [FeII(PY5)(X)]n+ complexes disfavor the formation of
ferric complexes with PY5. Indeed, reactions of the ligand
with Fe(III) salts in MeOH result in the isolation of [FeII-
(PY5)(X)]n+ complexes, and only one [FeIII(PY5)(X)]n+
complex has been isolated to date.16 The failure to isolate
other [FeIII(PY5)(X)]n+ complexes is likely due to spontane-
ous reduction, as has been observed for other systems,
presumably through oxidation of the solvent.23,24 The redox
potential of the ferrous complexes can be readily altered by
the exogenous ligand. As observed in Table 2, anionic ligands
tend to lower the redox potentials among the complexes
examined in MeOH. The Coulombic attraction between an
anionic ligand and a cationic metal center should better
stabilize the ferric complex formed upon oxidation, thereby
lowering the potential relative to a complex with a neutral
exogenous ligand. In the case of high-spin (hs) complexes,
the antibonding σ* electron (the eg set of d orbitals in an
octahedral complex) may facilitate the loss of an electron
relative to the low-spin (ls) complexes that lack electrons in
this higher-energy orbital. Consequently, the redox potential
is lowered with anionic and weak-field exogenous ligands.
Table 9. Relative Equilibrium Constants of Ferrous-PY5 Complexes in
MeOH
liganda
K(MeOH)b
K(pyr)b
0.0003
MeOH
MeCN
pyridine
OBz-
Cl-
1
100(50)
3000(500)
5 × 106 c
5 × 106 c
2 × 108 c
1
2000(1000)
2000(1000)
80000(10000)
-
N3
a This ligand (Y) was added as the titrant. b K(X) ) ([Fe(PY5)(Y)] ×
[X])/([Fe(PY5)(X)] × [Y]) at 298 K. c Estimate based on value of K(MeOH)
for pyridine.
each titration, and titration conditions were adjusted to
minimize these changes. Ionic strength does have an effect
on the observed equilibrium in experiments involving anionic
titrants; the observed Keq for the titration of [Fe(PY5)-
(pyridine)]2+ to [Fe(PY5)(Cl)]1+ nearly doubles when the
system contains 0.06 M tetrabutylammonium perchlorate salt.
The values presented here are only estimates for MeOH
solution. The relative binding affinity for the studied ligands
-
is MeOH < MeCN < pyridine , Cl- ∼ OBz- < N3
,
CN-.
In MeOH, strong bases can deprotonate [Fe(PY5)-
(MeOH)]2+ (pKa ) 9.1 ( 0.2, MeOH)16 to give [Fe(PY5)-
(OMe)]1+, which is identifiable by its characteristic UV-
vis spectrum (vide infra). When the base is a suitable ligand,
the acid-base chemistry competes with ligand displacement.
Phenolate (pKa(phenol) ) 14.3, MeOH), for instance, readily
deprotonates [Fe(PY5)(MeOH)]2+ in MeOH to form [Fe-
(PY5)(OMe)]1+. However, when the reaction is run in
acetone and monitored by UV-vis, another species is
formed, tentatively assigned as [Fe(PY5)(OPh)]1+. The use
of an aprotic solvent medium appears to allow stronger bases
to bind to Fe(II).
The influence of the exogenous ligand on the metal center
can also be examined with UV-vis absorption spectroscopy.
Most complexes exhibit two metal-to-ligand charge transfer
(MLCT) bands in the 300-500 nm range. The hs complexes’
MLCT absorptions are weaker in intensity than those of the
ls complexes, presumably as a result of weaker overlap
between the metal π-bonding d orbitals and the ligand π*
orbitals. Broad absorption bands with small extinction
coefficients (ꢀ ) 15-20 M-1 cm-1) are observed for most
hs complexes above 700 nm while moderately low intensity
transitions are observed for most ls complexes near 550 nm.
In [Fe(PY5)(CN)]1+, this band may be obscured by the lower-
energy MLCT band. Magnetic circular dichroism spectros-
copy indicates that the two bands above 700 nm in
[Fe(PY5)(MeOH)]2+ are spin-allowed dfd transitions,25 and
it is likely that the bands seen above 700 nm for the other
hs complexes are dfd transitions as well.
The exogenous ligand X in [FeII(PY5)(X)]n+ controls the
spin-state of the ferrous complex. The complex with the
medium-field MeOH, [Fe(PY5)(MeOH)]2+, has been shown
to be a spin-transition species that is predominantly hs at
room temperature (RT).16 Stronger-field ligands such as
MeCN, pyridine, or CN- create ls complexes at RT.
Comparing the ls [Fe(PY5)(pyridine)]2+ to the hs [Fe-
(pyridine)6]2+ suggests that the five pyridine subunits of PY5
create a stronger ligand field than five independent pyridine
ligands.26 The field strength of PY5 on a per pyridine subunit
Discussion
Control of the coordination to Fe(II) is difficult because
of the intrinsic coordinative ambivalence of the metal. Such
control can be exerted through appropriate ligand design.
To this end, the pentadentate ligand PY5 was synthesized
and complexed to iron with a number of exogenous ligands.
The apparent accessibility of the ligand periphery allows for
a wide variety of substrates to bind in the sixth coordination
site. Ten ferrous complexes [FeII(PY5)(X)]n+, with X )
-
MeOH, H2O, MeCN, pyridine, Cl-, MeO-, OBz-, N3 ,
phenolate, and CN-, were examined. The exogenous ligands
were chosen for their potential biochemical relevance, their
coverage of the spectrochemical series, and the synthetic
tractability of the resultant iron complexes.
The PY5 ligand was designed as an uncharged ligand in
order to promote the formation of Lewis acidic metal centers.
Cyclic voltammetric measurements on the ferrous complexes
clearly show that the Fe(II) oxidation state is favored by PY5.
The quasireversible redox potential of the [Fe(PY5)-
(MeCN)]2+ complex in MeCN is 1.15 V versus SHE, a value
similar to the high redox potentials measured for the
hexapyridyl complexes [Fe(bipy)3]2+ (1.21 V vs SHE) and
(21) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1982, 104, 7599-7609.
(22) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593-5603.
(23) Di Vaira, M.; Mani, F.; Stoppioni, P. J. Chem. Soc., Dalton Trans.
1997, 1375-1379.
(24) Onggo, D.; Rae, A. D.; Goodwin, H. A. Inorg. Chim. Acta 1990, 178,
151-163.
(25) Pavel, E. G.; Kitajima, N.; Solomon, E. I. J. Am. Chem. Soc. 1998,
120, 3949-3962.
(26) Doedens, R. J.; Dahl, L. F. J. Am. Chem. Soc. 1966, 88, 4847-4855.
4648 Inorganic Chemistry, Vol. 41, No. 18, 2002

Synthesis and ReactiWity of Fe(PY5)(X) Series
basis is most likely on par with 2,2′-bipyridine, a ligand that
can also form spin-transition Fe(II) complexes.27,28
of the increased ligand field stabilization energy of the ls
complexes. The stabilization energy gained from going hs
to ls is much less than that gained from binding of an anionic
ligand. Consequently, weak-field anionic exogenous ligands
are bound preferentially to strong-field neutral exogenous
ligands.
The ferrous structures of PY5 with six different mono-
dentate ligands were determined by X-ray crystallography:
[Fe(PY5)(MeOH)]2+,
[Fe(PY5)(H2O)]2+,
[Fe(PY5)-
(pyridine)]2+, [Fe(PY5)(Cl)]1+, [Fe(PY5)(OBz)]1+, and [Fe-
(PY5)(CN)]1+. In each case, PY5 enforces the square
pyramidal coordination configuration of Fe(II) with the
exogenous ligand completing a six-coordinate metal geom-
etry. Systematic structural distortions from an idealized
octahedral coordination geometry are present in these
structures. The Fe(1)-N(1) bond distance to PY5 is con-
sistently the shortest of the five metal-pyridine bonds, and
the Fe(II) is consistently displaced above the least-squares
plane of N2-5, Pleq. The most significant perturbations result
from the skewing of Py1 from the perpendicular of Pleq (Table
8). Correlated with the tilt of Py1 is the positioning of the
methoxy-methyl groups into the clefts between Py2 and Py1
and between Py5 and Py1. This positioning requires the cPy2-
Cquat-cPy1 and cPy5-Cquat-cPy1 angles to be significantly
opened relative to an idealized tetrahedral angle; the posi-
tioning of the methoxy-methyl group is noninnocent (Table
8).29 As a consequence of the tilt of Py1 and methoxy group
positioning, a consistent distortion exists among the four
equatorial Fe-N bond distances. The Fe(1)-N(2,5) bond
lengths are considerably shorter than the Fe(1)-N(3,4) bond
lengths.
The electronic properties and steric demands of the
exogenous ligand modulate the magnitude of these perturba-
tions. Strong-field ligands reduce the radius of the Fe(II) ion,
thereby allowing the Fe(II) to sink further into Pleq and closer
to Py1. Py1 tilts less as the ligand field strength increases,
leading to smaller disparity in the equatorial bond distances.
Using the average length of the five Fe-N bonds of PY5 as
a measure of the Fe(II) ionic radius (Table 8), the iron size
in these structures decreases along the series [Fe(PY5)(Cl)]1+
∼ [Fe(PY5)(OBz)]1+ > [Fe(PY5)(H2O)]2+ > [Fe(PY5)-
(MeOH)]2+ > [Fe(PY5)(pyridine)]2+ ∼ [Fe(PY5)(CN)]1+,
which is analogous to the ligand ordering in the spectro-
chemical series. These trends are also reflected in the RMS
errors determined by structural overlays of the cations’
coordination spheres.18
The formation of [Fe(PY5)(OPh)]1+ successfully occurs
in acetone but not in MeOH, where the added phenolate
instead leads to the complete formation of [Fe(PY5)-
(OMe)]1+. This result is initially surprising given the superior
acidity of PhOH (14.3)30 compared to MeOH (19.6)30 in
MeOH. Even at millimolar concentrations of added PhO-,
the equilibrium concentration of PhO- should be sufficiently
larger than the equilibrium concentration of MeO- in MeOH.
This suggests that the binding affinity of MeO- to [FeII-
(PY5)]2+ is greater than that of PhO-. Unfortunately, the
binding affinity of MeO- relative to MeOH is too large to
be determined directly by a titration of MeO- with [Fe(PY5)-
(MeOH)]2+. Addition of competitive anions to solutions of
[Fe(PY5)(OMe)]1+ results in release of the Fe(II) from PY5
and concomitant bleaching of the solution. Instead, a
thermodynamic analysis that uses the pKa of [Fe(PY5)-
(MeOH)]2+ in MeOH (9.1)16 and the pKa of MeOH estimates
the relative binding affinity of MeO- to be 3 × 1010 (eq 2).
This binding constant is 2 orders of magnitude larger than
those of the other anionic ligands with the exception of CN-
and may suggest some π-bonding in the Fe(II)-O bond
similar to that observed in [FeIII(PY5)(OMe)]2+.16 When
PhO- is added to [Fe(PY5)(MeOH)]2+ in acetone, the
competition with MeO- is eliminated, allowing PhO- to bind
to [FeII(PY5)]2+. These aprotic conditions provide a new
route toward the synthesis of sub-site-differentiated [FeII-
(PY5)(X)]n+ complexes.
[Fe(PY5)(MeOH)]2+ + MeO- h
[Fe(PY5)(OMe)]1+ + MeOH (2)
Summary
The ligand PY5 was previously designed to stabilize high-
spin Fe(II) complexes for the study of mononuclear non-
heme iron enzymes. The structural analysis of a series of
iron complexes shows the intrinsic property of PY5 to chelate
Fe(II) in a square-pyramidal coordination geometry. The
ligand’s pentadenticity allows the study of single site
reactivity. A diverse set of exogenous monodentate ligands
that spans the spectrochemical series can complete the
octahedral coordination of the metal center. Both hs and ls
Fe(II) complexes are isolated at RT, hinting at an intrinsic
“spin ambivalence” of the [FeII(PY5)(X)]n+ moiety. The
exogenous axial ligand regulates many properties of the Fe-
(II) metal center, including its spin-state and redox potential.
Ligation of PY5 generates an electron-deficient Fe(II) center
that preferentially chelates anionic ligands while also favoring
strong-field ligands. The solution chemistry with respect to
Study of the ligand exchange shows two significant trends.
First, [FeII(PY5)]2+ binds all anionic ligands examined
preferentially over neutral ligands. This is readily explained
by the Coulombic attraction between anionic ligands and the
dicationic [FeII(PY5)]2+ species. The binding affinities of
anions relative to MeOH are sufficiently large to allow nearly
stoichiometric formation of [FeII(PY5)(X)]1+ (X ) Cl-,
-
OBz-, and N3 ) in pure MeOH with ca. 1 mM concentration
of [FeII(PY5)]2+. Second, at parity of charge, the ligands that
allow formation of ls Fe(II) complexes are bound preferen-
tially to those that react to form hs Fe(II) complexes because
(27) Real, J. A.; Mun˜oz, M. C.; Faus, J.; Solans, X. Inorg. Chem. 1997,
36, 3008-3013.
(28) Roux, C.; Zarembowitch, J.; Itie´, J.-P.; Polian, A.; Verdaguer, M. Inorg.
Chem. 1996, 35, 574-580.
(30) Covington, A. K.; Dickinson, T. Physical Chemistry of Organic SolVent
Systems; Plenum Press: London, 1973; p 823.
(29) Jonas, R. T.; Stack, T. D. P. Inorg. Chem. 1998, 37, 6615-6629.
Inorganic Chemistry, Vol. 41, No. 18, 2002 4649

Goldsmith et al.
precipitation of a yellow compound in nearly quantitative yield
(0.123 g, 90%). Yellow/green crystals suitable for X-ray analysis
were obtained from an 2-propanol/diethyl ether solution of the
complex, [Fe(PY5)(H2O)](OTf)2. Absorption spectrum (2-propanol)
λmax (nm), ꢀ (M-1 cm-1): 355, 1580; 568, 20; 775, 20. Mass
spectroscopy (LSIMS+, M+): m/e 698.0 (EM ) 698.8 for [Fe-
(PY5)(H2O)](OTf)1+). Cyclic voltammetry (acetone): +1.360 V
versus SHE (∆E ) 0.180 V). Solution magnetic moment (acetone-
d6): µeff ) 4.9 µB. 1H NMR (400 MHz, acetone-d6) δ (ppm): -11.8,
14.5, 17.1, 41.9, 54.4, 58.0. Anal. Calcd for C31H27N5O9F6S2Fe:
C, 43.93; H, 3.21; N, 8.26. Found: C, 44.02; H, 3.17; N, 8.18.
[Fe(PY5)(pyridine)](OTf)2. Equimolar amounts of PY5 (0.034
g) and Fe(OTf)2 (0.025 g) were dissolved in 2.0 mL of pyridine
under N2 to give a deep red/brown solution. Addition of diethyl
ether resulted in the precipitation of a brown compound in moderate
yield (0.048 g, 75%). Deep red prismatic crystals suitable for X-ray
analysis were obtained from a pyridine/diethyl ether solution of
the complex, [Fe(PY5)(pyridine)](OTf)2‚(pyridine). Absorption
spectrum (pyridine) λmax (nm), ꢀ (M-1 cm-1): 396, 4700; 440, 3000;
548, 190. Mass spectroscopy (LSIMS+, M+): m/e 680.0 (EM )
680.8 for [Fe(PY5)](OTf)1+). Cyclic voltammetry (MeOH): +1.180
V vs SHE (∆E ) 0.100 V). Solution magnetic moment (acetone-
basic ligands shows signs of being markedly different in
aprotic versus protic media.
Experimental Section
Syntheses. All starting materials were purchased from Aldrich
and used without further purification unless noted otherwise.
FeII(OTf)2 was synthesized according to a literature method.31
Sodium hydride (60% oil dispersion) was washed with hexanes
and dried in a vacuum. All solvents and gases were of analytical
grade and were purified by literature methods.32 CH2Cl2 was
distilled from CaH2 under N2 and stored over 4 Å molecular sieves.
MeCN was distilled from CaH2 under N2. MeOH was distilled from
Mg(OMe)2 under N2 and stored in darkness over 4 Å molecular
sieves. Anhydrous diethyl ether (ether) was stored over 4 Å
molecular sieves. THF was distilled from K(s) under N2. When
air-free solutions were necessary, the solvents were degassed prior
to use. All iron complexes for crystallographic analysis were
synthesized and handled under a N2 inert atmosphere using a
MBraun Labmaster 130 glovebox or standard Schlenk-line tech-
niques. Flash column chromatography was performed using silica
gel 60, 230-400 mesh from EM Science (Gibbstown, NJ) using
standard techniques.33
1
d6): µeff ) 0 µB. H NMR (400 MHz, acetone-d6): δ (ppm) 4.68
1
Instrumentation. H NMR spectra were recorded on a Varian
(6 H, s, C-OMe), 7.39 (4 H, d of d, J ) 5.6 Hz, 3-Hpy-equatorial
(eq)), 8.03 (4 H, t, J ) 7.6 Hz, 4-Hpy-eq), 8.21 (1H, t, J ) 4.4 Hz,
4-H of exogenous pyridine (py-ex)), 8.34 (1 H, t, J ) 6.0 Hz,
4-Hpy-axial (ax)), 8.94 (2H, d, J ) 6.7 Hz, 3-Hpy-ex), 9.69 (2 H,
d, J ) 6.0 Hz, 3-Hpy-ax), 10.05 (4 H, d, J ) 7.2 Hz, 5-Hpy-eq),
10.89 (4 H, b, 6-Hpy-eq), 13.38 (2 H, b, 2-Hpy-ex). Anal. Calcd
for C36H30N6O8F6S2Fe: C, 47.59; H, 3.33; N, 9.25. Found: C,
47.82; H, 3.19; N, 9.35.
[Fe(PY5)(OBz)](OTf). Equimolar amounts of [Fe(PY5)(MeOH)]-
(OTf)2 (0.036 g) and NaOBz (0.009 g) were dissolved in 4 mL of
MeOH under N2 to give a greenish-yellow solution. Addition of
diethyl ether resulted in the precipitation of a greenish-yellow
compound in nearly quantitative yield (0.032 g, 95%). Green
prismatic crystals suitable for X-ray analysis were obtained from a
slowly cooled MeOH solution of the complex, [Fe(PY5)(OBz)]-
(OTf). Absorption spectrum (MeOH) λmax (nm), ꢀ (M-1 cm-1): 330,
1630; 395, 1800. Mass spectroscopy (MALDI, M+): m/e 681.1
(EM ) 680.8 for [Fe(PY5)](OTf)). Cyclic voltammetry (MeOH):
+1.025 V vs SHE (∆E ) 0.160 V). 1H NMR (400 MHz, acetone-
d6): δ (ppm) -8.2, 5.11, 7.1, 7.9, 10.4, 18.3, 37.4, 51.5, 54.4. Anal.
Calcd for C37H30N5O7F3SFe‚MeOH: C, 53.31; H, 4.11; N, 8.40.
Found: C, 52.88; H, 3.61; N, 8.06.
[Fe(PY5)(N3)](OTf). Nearly equimolar amounts of PY5 (0.042
g) and Fe(OTf)2 (0.034 g) were dissolved in 2.0 mL of MeOH
under N2 to give a yellow solution. The addition of NaN3 (0.0058
g) as a MeOH solution turns the solution orange. Upon standing,
crystals precipitated in moderate yield (0.040 g, 65%). Absorption
spectrum (MeOH) λmax (nm), ꢀ (M-1 cm-1): 405, 2150; 810, 20.
Mass spectroscopy (MALDI, M+): m/e 681.1 (EM ) 680.8 for
[Fe(PY5)](OTf)). Cyclic voltammetry (MeOH): +0.740 V vs SHE
(irrev). Solution magnetic moment (acetone-d6): µeff ) 4.7 µB. 1H
NMR (400 MHz, acetone-d6): δ (ppm) -9.8, 7.45, 19.35, 41.9,
51.9, 54.1. Anal. Calcd for C30H25N8O5ClF3SFe: C, 49.90; H, 3.49;
N, 15.52. Found: C, 50.03; H, 3.76; N, 15.22.
[Fe(PY5)(Cl)](OTf). Equimolar amounts of PY5 (0.050 g) and
FeCl2 (0.013 g) were dissolved in 5 mL of MeOH under N2 to
give a deep orange solution. Addition of 1 equiv of Ag(OTf) (0.027
g) resulted in a deep yellow solution and the precipitation of AgCl.
After filtration of the solution, addition of diethyl ether resulted in
the precipitation of a bright yellow compound in nearly quantitative
Gemini-400 (400 MHz) NMR spectrometer at RT, and chemical
shifts are reported in ppm downfield from an internal TMS
reference. Electronic spectra at RT were measured on either a
Polytec X-dap fiber optics UV-vis diode array spectrophotometer
or a Cary 50 Bio UV-vis spectrophotometer. Electrochemical
measurements were recorded at 100 mV/s under N2 at RT using a
Bioanalytical Systems, Inc. CV-50W voltammetric analyzer, a
platinum working electrode, a platinum wire auxiliary electrode,
0.1 M (n-Bu4N)(ClO4) supporting electrolyte, and a silver wire
reference electrode, with all potentials referenced to the ferrocenium/
ferrocene couple (in MeOH ) +0.610 V vs SHE, in acetone +0.700
V vs SHE, ∆E ) 0.079 V).34 Solution magnetic moments were
determined at RT by the Evans method.17 Mass spectroscopy data
(positive FAB, MALDI, and LSIMS) were collected by the Mass
Spectrometry Facility, Department of Pharmaceutical Chemistry,
University of California, San Francisco. The matrix for MALDI
mass spectroscopy was R-cyano-4-hydroxycinnamic acid while that
for LSIMS mass spectroscopy was either 3-nitrobenzoic acid or
thioglycerol. Elemental analyses were performed by Desert Ana-
lytics (Tucson, AZ). Samples were heated under vacuum at 398 K
for 5 h prior to elemental analysis with the exception of [Fe(PY5)-
N3](OTf), which was instead dried under vacuum for 1 h at RT
prior to analysis.
Metal Complex Syntheses. The synthesis and characterization
of 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5), [Fe(PY5)-
(MeOH)](OTf)2, [Fe(PY5)(OMe)](OTf), [Fe(PY5)(MeCN)](OTf)2,14,16
15
and [Fe(PY5)(MeCN)](ClO4)2 have been described previously.
The preparation of KOPh has been described previously.35
[Fe(PY5)(H2O)](OTf)2. Equimolar amounts of PY5 (0.077 g)
and Fe(OTf)2 (0.058 g) were dissolved in 10 mL of nondried
2-propanol under N2. Addition of diethyl ether resulted in the
(31) Haynes, J. S.; Sams, J. R.; Thompson, R. C. Can. J. Chem. 1981, 59,
669-678.
(32) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 1st ed.; Pergamon Press: New York, 1988.
(33) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-
2925.
(34) Zuman, P.; Meites, L. Electrochemical Data; John Wiley & Sons:
New York, 1974; Vol. A.
(35) Krafft, T. E.; Hejna, C. I.; Smith, J. S. Inorg. Chem. 1990, 29, 2682-
2688.
4650 Inorganic Chemistry, Vol. 41, No. 18, 2002

Synthesis and ReactiWity of Fe(PY5)(X) Series
yield (0.078 g, 95%). Yellow prismatic crystals suitable for X-ray
analysis were obtained from a MeOH/diethyl ether solution of the
complex, [Fe(PY5)(Cl)](OTf)‚(MeOH). Absorption spectrum (MeOH)
λmax (nm), ꢀ (M-1 cm-1): 330, 1590; 390, 1930; 860, 15. Mass
spectroscopy (LSIMS+, M+): m/e 567.5 (EM ) 567.3 for [Fe-
(PY5)(Cl)]1+). Cyclic voltammetry (MeOH): +0.990 V vs SHE
the observed absorbance at each wavelength and that calculated
from the model for all independent spectra.
X-ray Crystallography. ORTEP representations with a detailed
numbering scheme and complete tables of positional parameters,
bond lengths, bond angles, and anisotropic thermal factors for the
described crystal structures are located in the Supporting Informa-
tion.
(∆E ) 0.090 V). Solution magnetic moment (acetone-d6): µeff
)
5.1 µB. 1H NMR (400 MHz, acetone-d6): δ (ppm) -11.2, 7.9, 18.3,
42.6, 50.2, 64.1. Anal. Calcd for C30H25N5O5ClF3SFe: C, 50.33;
H, 3.52; N, 9.79. Found: C, 49.95; H, 3.78; N, 9.57.
General Methods. For each of the X-ray crystal structures
presented, the data set was collected at low temperature <200 K
under a N2 stream. A suitably sized crystal was mounted in paratone
oil on a glass fiber and placed in a cold stream of N2 on an Enraf-
Nonius CAD-4 or a Siemens CCD diffractometer with graphite
monochromated Mo KR radiation (λ ) 0.710 73 Å). Structural and
refinement data for the four high-spin and two low-spin ferrous
complexes are summarized in Tables 4 and 5, respectively. The
data were corrected for Lorentz and polarization effects. The
structures were solved by direct methods and expanded using
Fourier techniques. All non-hydrogen atoms were refined aniso-
tropically, unless noted. Hydrogen atoms were located by difference
Fourier maps but included at idealized position 0.95 Å from their
parent atoms for the final refinement. Isotropic thermal parameters
1.2 times the parent atoms were assumed. Unless otherwise noted,
the remaining significant peaks on the final difference Fourier maps
were located near the triflate anion(s). Neutral atom scattering
[Fe(PY5)(CN)](OTf). Addition of 1 equiv of KCN (0.005 g) to
a 10 mL MeOH solution of [Fe(PY5)(MeOH)](OTf)2 (0.053 g)
under N2 resulted in the deep red ferrous cyanide species [Fe(PY5)-
(CN)](OTf) at RT. A red crystalline solid was isolated after addition
of diethyl ether in nearly quantitative yield (0.042 g, 90%). Deep
red prismatic crystals suitable for X-ray analysis were obtained from
a MeOH/diethyl ether solution of the complex, [Fe(PY5)(CN)]-
(OTf)‚(MeOH). Absorption spectrum (MeOH) λmax (nm), ꢀ (M-1
cm-1): 372, 6700; 439, 8100. Mass spectroscopy (LSIMS+, M+):
m/e 557.1 (EM ) 557.4 for [Fe(PY5)(CN)]1+). Cyclic voltammetry
(MeOH): +0.920 V vs SHE (∆E ) 0.085 V). Solution magnetic
1
moment (acetone-d6): µeff ) 0 µB. H NMR (400 MHz, acetone-
d6): δ (ppm) 4.03 (6 H, s, C-OMe), 7.46 (4 H, d of d, J ) 5.5
Hz, 3-Hpy-equatorial (eq)), 7.93 (4 H, t, J ) 6.8 Hz, 4-Hpy-eq),
8.01 (4 H, d, J ) 4.2 Hz, 5-Hpy-eq), 8.32 (3 H, m, py-axial), 10.11
(4 H, d, J ) 5.2 Hz, 6-Hpy-eq). Anal. Calcd for C31H25N6O5F3-
SFe‚MeOH: C, 50.42; H, 3.96; N, 11.38. Found: C, 49.23; H,
3.40; N, 10.39.
factors were taken from Cromer and Waber.36 Anomalous dispersion
37
effects were included in Fcalcd
;
the values for ∆f′ and ∆f′′ were
those of Creagh and McAuley.38 The values for the mass attenuation
coefficients are those of Creagh and Hubbell.39 All calculations
were performed using the teXsan crystallographic software package
of Molecular Structure Corporation. Specific details for each of
the crystal structures are available.18
[Fe(PY5)(OPh)](OTf). Equimolar amounts of [Fe(PY5)(MeOH)]-
(OTf)2 (0.050 g) and KOPh (0.010 g) were dissolved in 10 mL of
acetone under N2 to give an orange solution. Addition of diethyl
ether resulted in the precipitation of an orange-red powder in
moderate yield (0.027 g, 60%). Absorption spectrum (MeOH) λmax
(nm), ꢀ (M-1 cm-1): 355, 1660; 430, 1530. Mass spectroscopy
(MALDI, M+): m/e 681.1 (EM ) 680.8 for [Fe(PY5)](OTf)).
Cyclic voltammetry (acetone): +1.070 V vs SHE (∆E ) 0.090
V). Solution magnetic moment (acetone-d6): µeff ) 4.9 µB. 1H NMR
(400 MHz, acetone-d6): δ (ppm) -5.8, 6.7, 19.6, 38.8, 43.1, 47.1,
51.4, 54.5, 56.2.
Acknowledgment. We are grateful to the National
Institutes of Health (Grant GM50730) and the Stanford
Graduate Fellowship Fund (C.R.G.) for financial support of
this work.
(36) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch: Birmingham, 1974; Vol. IV.
(37) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781-782.
(38) Creagh, D. C.; McAuley, W. J. In International Tables for Crystal-
lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,
1992; Vol. C.; p 219-222, Table 4.2.6.8.
Equilibrium Constant Measurements. The relative binding
affinities of various exogenous ligands were determined in MeOH
by spectrophotometric titrations. The exogenous ligand titrants were
added either as neat solutions (MeCN, pyridine) or as MeOH
solutions (Et4NCl, NaN3) of the added species. MeCN and pyridine
were added to ∼0.5 mM solutions of [Fe(PY5)(MeOH)](OTf)2. The
binding affinities of anionic ligands relative to MeOH are too large
to directly assess by optical spectroscopy. Instead, the binding
affinities of anionic ligands were measured relative to that of
pyridine. These titrations were performed in a mixed pyridine/
MeOH solution (25% pyridine by volume) to ensure >99%
conversion of [Fe(PY5)(MeOH)]2+ to [Fe(PY5)(pyridine)]2+. Solu-
tions of Et4NCl and NaN3 were titrated to ∼0.5 mM [Fe(PY5)-
(pyridine)](OTf)2. Because of solubility issues, an inverse titration
was performed to measure the relative binding affinity of OBz-,
with pyridine added to a 0.2 mM MeOH solution of [Fe(PY5)-
(OBz)](OTf). For each titration, at least five independent spectra
covering the 350-500 nm wavelength range were collected. Factor
analysis in SPECFIT showed that only two colored species were
present above the limit of detection. The titration data were
sufficiently modeled with a single equilibrium expression (eq 1).
The component spectra of the two pure colored species and the
equilibrium constant were refined to minimize the residual between
(39) Creagh, D. C.; Hubbell, J. H. In International Tables for Crystal-
lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,
1992; Vol. C.; p 200-206, Table 4.2.4.3.
(40) Orville, A. M.; Lipscomb, J. D.; Ohlendorf, D. H. Biochemistry 1997,
36, 10052-10066.
(41) Ohlendorf, D. H.; Orville, A. M.; Lipscomb, J. D. J. Mol. Biol. 1994,
244, 586-608.
(42) Senda, T.; Sugiyama, K.; Narita, H.; Yamamoto, T.; Kimbara, K.;
Fukuda, M.; Sato, M.; Yano, K.; Mitsui, Y. J. Mol. Biol. 1996, 255,
735-752.
(43) Han, S.; Eltis, L. D.; Timmis, K. N.; Muchmore, S. W.; Bolin, J. T.
Science 1995, 270, 976-980.
(44) Minor, W.; Steczko, J.; Stec, B.; Otwinowski, Z.; Bolin, J. T.; Walter,
R.; Axelrod, B. Biochemistry 1996, 35, 10687-10701.
(45) Boyington, J. C.; Gaffney, B. J.; Amzel, L. M. Science 1993, 260,
1482-1486.
(46) Gillmor, S. A.; Villasen˜or, A.; Fletterick, R.; Sigal, E.; Browner, M.
F. Nat. Struct. Biol. 1997, 4, 1003-1009.
(47) Erlandsen, H.; Fusetti, F.; Martinez, A.; Hough, E.; Flatmark, T.;
Stevens, R. C. Nat. Struct. Biol. 1997, 4, 995-1000.
(48) Zhang, Z.; Ren, J.; Stammers, D. K.; Baldwin, J. E.; Harlos, K.;
Schofield, C. J. Nat. Struct. Biol. 2000, 7, 127-133.
(49) Kreisberg-Zakarin, R.; Borovok, I.; Yanko, M.; Frolow, F.; Aha-
ronowitz, Y.; Cohen, G. Biophys. Chem. 2000, 86, 109-118.
(50) Sugio, S.; Hiraoka, B. Y.; Yamakura, F. Eur. J. Biochem. 2000, 267,
3487-3495.
(51) Yeh, A. P.; Hu, Y.; Jenney, F. E., Jr.; Adams, M. W. W.; Rees, D. C.
Biochemistry 2000, 39, 2499-2508.
Inorganic Chemistry, Vol. 41, No. 18, 2002 4651

Goldsmith et al.
Supporting Information Available: Structure reports for [Fe-
(PY5)(H2O)](OTf)2, [Fe(PY5)(OBz)](OTf), [Fe(PY5)(Cl)](OTf),
[Fe(PY5)(pyridine)](OTf)2, and [Fe(PY5)(CN)](OTf); comparison
of the anticipated [Fe(PY5)(pyridine)]2+ spectrum to an indepen-
dently obtained spectrum; structural overlay RMS errors for the
ferrous complex six-coordinate coordination cores. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC025616Z
4652 Inorganic Chemistry, Vol. 41, No. 18, 2002