Synthesis, characterization, and reactivity of iron trisamidoamine complexes that undergo both metal- and ligand-centered oxidative transformations

Article abstract of DOI:10.1021/ic702154z

Functional systems that combine redox-active metals and noninnocent ligands are no longer rare chemical oddities; they are instead emerging as significant components of catalytic and enzymatic reactions. The present work examines the synthetic and functional aspects of iron compounds ligated by a family of new trisamidoamine ligands of the type [(RNC₆H₄) ₃N]^3- (L¹). When R is the electron-rich 4-t-Bu-Ph moiety, the ligand can undergo oxidative rearrangement and store oxidizing equivalents under specific conditions. Starting ferrous complexes of the general formula [(L¹)Fe^IIsolv]^- (solv = CH₃CN, dimethylformamide) can be easily oxidized (a) by dioxygen to afford the corresponding [(L¹)Fe^IIIOH]^- complexes, featuring several cases of terminal hydroxo units, and (b) by organochlorides (R-Cl) to provide [(L¹)Fe^IIIsolv] congeners and coupled R-R products. Efforts to synthesize [(L ¹)Fe^III-O-Fe^III(L¹)]^2- by using [Cl₃Fe^III-O-Fe^IIICl₃] ^2- indicate that intrinsic Fe^IIICl units can oxidatively rearrange the ligand to afford [(L¹_re)(Cl)Fe ^II][Et₄N]₂, although the oxidizing equivalent is not retained. Compound [(L¹_re)(Cl)Fe ^II][Et₄N]₂ can be further oxidized to [(L ¹_re-2)(Cl)Fe^III][Et₄N] by CH ₂Cl₂. Finally, oxidation of [(L¹)Fe ^IIIsolv] by FeCl₃ affords [(L¹ _reH)(Cl)Fe^II(μ-Cl)₂Fe^II(Cl) (L¹_re-2H)], which features a similar ligand rearrangement that also gives rise to a diamagnetic, doubly oxidized moiety. These results underscore the complexity of chemical transformations available to systems in which both the metal and the ligand are redox-active entities.

Full text of DOI:10.1021/ic702154z

Inorg. Chem. 2008, 47, 1165−1172
Synthesis, Characterization, and Reactivity of Iron Trisamidoamine
Complexes That Undergo Both Metal- and Ligand-Centered Oxidative
Transformations
Remle C¸ elenligil-C¸
etin,‡ Patrina Paraskevopoulou,† Rupam Dinda,† Richard J. Staples,§
Ekkehard Sinn,† Nigam P. Rath,# and Pericles Stavropoulos*,†
Department of Chemistry, UniVersity of MissourisRolla, Rolla, Missouri 65409, Department of
Chemistry, Boston UniVersity, Boston, Massachusetts 02215, Department of Chemistry and
Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138, and Department of
Chemistry and Biochemistry, UniVersity of MissourisSt. Louis, St. Louis, Missouri 63121
Received November 1, 2007
Functional systems that combine redox-active metals and noninnocent ligands are no longer rare chemical oddities;
they are instead emerging as significant components of catalytic and enzymatic reactions. The present work examines
the synthetic and functional aspects of iron compounds ligated by a family of new trisamidoamine ligands of the
-
type [(RNC6H4)3N]3 (L1). When R is the electron-rich 4-t-Bu
−Ph moiety, the ligand can undergo oxidative
rearrangement and store oxidizing equivalents under specific conditions. Starting ferrous complexes of the general
formula [(L1)FeIIsolv]- (solv
) CH3CN, dimethylformamide) can be easily oxidized (a) by dioxygen to afford the
corresponding [(L1)FeIIIOH]- complexes, featuring several cases of terminal hydroxo units, and (b) by organochlorides
-
(R
−
Cl) to provide [(L1)FeIIIsolv] congeners and coupled R
−
R products. Efforts to synthesize [(L1)FeIII
−O
−
FeIII(L1)]2
by using [Cl3FeIII
−O
−
FeIIICl3]2 indicate that intrinsic FeIIICl units can oxidatively rearrange the ligand to afford
-
[(L1re)(Cl)FeII][Et4N]2, although the oxidizing equivalent is not retained. Compound [(L1re)(Cl)FeII][Et4N]2 can be further
oxidized to [(L1 2)(Cl)FeIII][Et4N] by CH2Cl2. Finally, oxidation of [(L1)FeIIIsolv] by FeCl3 affords [(L1reH)(Cl)FeII(
Cl)2FeII(Cl)(L1 2H)], which features a similar ligand rearrangement that also gives rise to a diamagnetic, doubly
µ-
re-
re-
oxidized moiety. These results underscore the complexity of chemical transformations available to systems in which
both the metal and the ligand are redox-active entities.
Introduction
witnessed a surge of well-defined enzymes (for instance,
ribonucleotide reductase,3 cytochrome c peroxidase,4 galac-
tose oxidase,5 prostaglandin H synthase,6 lipoyl synthase,7
biotin synthase,8 diol dehydrase,9 pyruvate formate lyase,10
DNA photolyase,11 lysine-2,3-aminomutase12) in which met-
Metal-bound residues and ligands not only confer neces-
sary structural elements and stereoelectronic properties that
can tune the reactivity at the metal site but also may
participate in tandem with the metal center to bring about
critical chemical and biochemical transformations. Redox-
based catalysis, in which both the metal and ligand can
possess multiple electronic states during turnover, constitutes
a most interesting case of synergistic activity, with a plethora
of rapidly accumulating examples of biological importance.1
Indeed, the growing field of radical enzymology2 has
(2) See contributions to the thematic issue: Chem. ReV. 2003, 103, 2081-
2456.
(3) Licht, S.; Gerfen, G. J.; Stubbe, J. Science 1996, 271, 477-481.
(4) Huyett, J. E.; Doan, P. E.; Gurbiel, R.; Houseman, A. L. P.; Sivaraja,
M.; Goodin, D. B.; Hoffman, B. M. J. Am. Chem. Soc. 1995, 117,
9033-9041.
(5) Whittaker, J. W. Arc. Biochem. Biophys. 2005, 433, 227-239.
(6) Dorlet, P.; Seibold, S. A.; Babcock, G. T.; Gerfen, G. J.; Smith, W.
L.; Tsai, A. L.; Un, S. Biochemistry 2002, 41, 6107-6114.
(7) Cicchillo, R. M.; Iwig, D. F.; Jones, A. D.; Nesbitt, N. M.; Baleanu-
Gogonea, C.; Souder, M. G.; Tu, L.; Booker, S. J. Biochemistry 2004,
43, 6378-6386.
* To whom correspondence should be addressed. E-mail: pericles@
mst.edu.
† University of MissourisRolla.
‡ Boston University.
(8) Berkovitch, F.; Nicolet, Y.; Wan, J. T.; Jarrett, J. T.; Drennan, C. L.
Science 2004, 303, 76-79.
(9) Magnusson, O. T.; Frey, P. A. Biochemistry 2002, 41, 1695-1702.
(10) Knappe, J.; Wagner, A. F. V. AdV. Prot. Chem. 2001, 58, 277-315.
§ Harvard University.
# University of MissourisSt. Louis.
(1) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705-762.
10.1021/ic702154z CCC: $40.75
Published on Web 01/08/2008
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 3, 2008 1165

C¸ elenligil-C¸ etin et al.
Chart 1
redox conditions, it is the long-term intention of this research
to provide conditions that would enable simultaneous support
of both high-valent metal-oxo moieties and ligand-centered
radicals in mediation of oxygenation chemistry. A summary
of the crystallographic data collected in this work is presented
in Table 1.
Results and Discussion
alloradicals, composed of redox-active metals and radical-
bearing residues (Tyr•, Cys•, Gly•, Trp•, and TrpH•+),
operate flawlessly toward storing and transferring redox
equivalents, as well as executing mechanistically diverse
chemistry (for instance, hydrogen-atom abstraction, proton
transfer, single-electron transfer) that is essential for catalytic
turnover.1,2
Ligand Synthesis and Characterization. Ligand L1H3
has been synthesized according to the general three-step
methodology depicted in Scheme 1. The tripodal 2,2′,2′′-
trinitrotriphenylamine is first synthesized by prolonged
heating of an 1:3 mixture of 2-nitroaniline and 2-fluoroni-
trobenzene in dimethyl sulfoxide (DMSO), following modi-
fication of a previously published procedure.20 The product
is obtained in 50% yield along with the bipodal 2,2′-
dinitrodiphenylamine, which is removed via washings with
hot acetone. A previously reported methodology20 gave large
amounts of the bipodal byproduct, whereas attempted Ull-
mann-type coupling, by heating a neat solution of 2-fluo-
ronitrobenzene over copper powder at 180 °C, resulted in
only miniscule amounts of the desired product.
The tripodal trinitroamine is then reduced to the corre-
sponding 2,2′,2′′-triaminotriphenylamine by means of pres-
surized dihydrogen over palladium/carbon in ethanol at
60 °C. The product can be separated from palladium by
dissolving it in hot acetone under an inert atmosphere. Small
amounts of nitro precursors can be removed by washing the
product with hot hexane. The product can be further purified
by column chromatography on silica gel (3:1 petroleum ether/
ethyl acetate). The structure of this new precursor compound,
recrystallized from benzene, is shown in Figure S1 in the
Supporting Information.
The synthesis concludes with the Pd2(dba)3/BINAP-
catalyzed coupling of 2,2′,2′′-triaminotriphenylamine with
4-tert-butylbromobenzene, following standard Hartwig-
Buchwald methodology21 for the arylation of amines. The
product is obtained as an off-white solid after purification
by column chromatography on silica gel (hexane/ethyl
acetate). Recrystallization from hexane affords a good-quality
crystalline material suitable for X-ray analysis.
The structure of L1H3 (Figure S2 in the Supporting
Information) reveals a preorganized cavity (pseudo-C3-
symmetric) that is suitable for tetradentate metalation.
Benchmark metrical parameters, which will be used to
evaluate ligand rearrangements noted in subsequent sections,
include the average C-C distance of the phenyl rings in the
triphenylamine core (1.387 Å), with values ranging between
1.371(5) to 1.407(5) Å, and the average C-N(secondary)
bond length at 1.402(4) Å.
The present study explores one member of a family of
tripodal trisamidoamine ligands of the type [(RNC6H4)3N]3-
(Chart 1, left), recently synthesized by a generic methodol-
ogy. Under certain oxidative conditions, iron complexes of
this ligand, where R is an electron-rich aryl group (R ) 4-t-
Bu-Ph in this study), can generate ligand-centered radicals,
which, in turn, are responsible for rearranging the ligand and
storing oxidation equivalents in certain cases. Unlike the
exhaustively explored [(RNCH2CH2)3N]3- (TREN) systems
(Chart 1, right),13 this noninnocent ligand system also features
a more rigid backbone than TREN does and a lack of
R-hydrogen atoms with respect to nitrogen residues (â vs
the metal), which can be vulnerable in oxidative environ-
ments. These stability rules have been highlighted in Collins’
development of oxidatively stable tetraamido macrocyclic
ligands.14 Typical â-hydride eliminations from the backbone
of the TREN ligand have also been reported,15 leading to
the loss of an arm via C-N(amine) bond cleavage. TREN-
type systems have been otherwise superbly exploited in
molybdenum/tungsten-based dinitrogen reductive chemistry16
and in some early-transition-metal organometallic chemis-
try.17 More closely to the theme of this work, TREN-type
ligands supported an FeIVCN moiety18 and gave rise to some
unique terminal FeIIIO and FeII/IIIOH units.19
While the short-term goal of the present work is to explore
the fundamental chemistry of the new family of ligands under
(11) Cheek, J.; Broderick, J. B. J. Am. Chem. Soc. 2002, 124, 2860-2861.
(12) Banerjee, R. Chem. ReV. 2003, 103, 2083-2094.
(13) (a) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16. (b) Verkade, J.
G. Acc. Chem. Res. 1993, 26, 483-489.
(14) (a) Collins, T. J. Acc. Chem. Res. 1994, 27, 279-285. (b) Bartos, M.
J.; Gordon-Wylie, S. W.; Fox, B. G.; Wright, L. J.; Weintraub, S. T.;
Kauffmann, K. E.; Mu¨nck, E.; Kostka, K. L.; Uffelman, E. S.; Rickard,
C. E. F.; Noon, K. R.; Collins, T. J. Coord. Chem. ReV. 1998, 174,
361-390.
(15) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1996, 118, 3643-3655.
(16) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76-78. (b)
Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252-
6253. (c) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3850-
3860. (d) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3861-
3878.
(17) (a) Schrock, R. R.; Adamchuk, J.; Ruhland, K.; Lopez, L. P. H.
Organometallics 2003, 22, 5079-5091. (b) Kim, Y.; Verkade, J. G.
Organometallics 2002, 21, 2395-2399.
Synthesis and Characterization of Starting Iron(II)
Complexes. Entry into the chemistry of the metalated ligand
L1H3 is best accomplished by first synthesizing ferrous
precursor species. The ligand is deprotonated with 3 equiv
(18) Cummins, C. C.; Schrock, R. R. Inorg. Chem. 1994, 33, 395-396.
(19) (a) Gupta, R.; Borovik, A. S. J. Am. Chem. Soc. 2003, 125, 13234-
13242. (b) MacBeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang,
C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289,
938-941.
(20) Gorvin, J. H. J. Chem. Soc., Perkin Trans. 1 1988, 6, 1331-1335.
(21) (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580. (b) Wolfe,
J. P.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1144-1157.
1166 Inorganic Chemistry, Vol. 47, No. 3, 2008

Iron Trisamidoamine Complexes
Scheme 1
Table 1. Summary of the Crystallographic Data for 2,2′,2′′-Triaminotriphenylamine (LH3), L1H3, and 1-9
LH3
L1H3
1
2
3
formula
Mr
cryst syst
space group
a (Å)
C21H21N4
329.42
monoclinic
P21/c
7.9816(4)
14.0735(7)
15.3216(8)
90
97.4100(10)
90
C48H54N4
686.95
monoclinic
P21/n
16.452(2)
13.4243(19)
19.063(3)
90
107.673(3)
90
4011.5(10)
4
C80H83FeN8P
1243.36
monoclinic
P21/n
10.6554(17)
23.765(4)
28.125(6)
90
98.872(8)
90
7037(2)
4
C76H80FeN5O1.5
1174.27
monoclinic
P21/n
10.579(2)
24.040(5)
28.361(6)
90
98.580(6)
90
7132(3)
4
P
C114H158Fe2K2N14O14
2138.44
cubic
Pa3h
22.3171(6)
22.3171(6)
22.3171(6)
90
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
1706.69(15)
4
1.282
11115.1(5)
4
1.278
D
calcd (g/cm3)
1.137
1.174
1.094
T (K)
173(2)
213(2)
293(2)
293(2)
100(2)
λ (Å)
0.710 73
0.078
0.0400
0.710 73
0.066
0.0738
0.2050
0.710 73
0.286
0.0824
0.2135
0710 73
0.279
0.0792
0.1918
0.710 73
0.404
0.1253
0.3323
µ (mm-1
)
R1a (4σ data)
wR2b (4σ data)
0.0864
4
5
6
7
8
9
formula
Mr
cryst syst
space group
a (Å)
C57H79FeN4O8
1004.09
orthorhombic
Pbca
22.520(10)
22.520(13)
22.520(13)
90
C51H58FeN5O
812.87
rhombohedral
R3h
14.741(15)
14.741(15)
36.61(5)
90
90
120
6890(14)
6
1.175
C50H54FeN5
780.83
rhombohedral
R3h
14.710(3)
14.710(3)
36.630(9)
90
90
120
6865(3)
6
1.133
C68H101ClFeN5O
1108.35
monoclinic
P21/n
10.804(6)
31.363(18)
20.186(8)
90
94.522(13)
90
6819(6)
4
C60H81ClFeN5O
979.60
orthorhombic
P212121
10.483(4)
17.586(6)
29.606(10)
90
90
90
5458(3)
4
1.192
C99H111Cl4Fe2N8
1666.46
triclinic
P1h
10.4415(14)
14.8145(18)
16.309(2)
100.006(3)
94.584(3)
109.878(3)
2309.9(5)
1
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
11422(11)
8
1.168
D
calcd (g/cm3)
1.045
1.198
T (K)
293(2)
0.710 73
0.318
0.0985
0.2720
293(2)
0.710 73
0.369
0.0862
0.2780
296(2)
0.710 73
0.367
0.1068
0.2358
293(2)
0.710 73
0.300
0.0813
0.2053
100(2)
0.710 73
0.369
0.0575
0.0801
293(2)
0.710 73
0.479
0.1159
0.1808
λ (Å)
µ (mm-1
)
R1a (4σ data)
wR2b (4σ data)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
of KH in tetrahydrofuran (THF) to afford the air-sensitive
compound K3L1, which precipitates from saturated THF
solutions as a light-yellow solid. The reaction of K3L1
(generated in situ in THF) with 1 equiv of anhydrous FeCl2,
added as a solid to the THF solution of K3L1 under strictly
anaerobic conditions, affords green-brown solutions of
[(L1)FeIITHF]-, whose characterization and reactivity will
be discussed elsewhere. These solutions are oxidatively
unstable even under the most stringent conditions. A more
manageable iron(II) precursor can be achieved by changing
the solvent to CH3CN and by using a bulky countercation
such as Ph4P+. The addition of [Ph4P]Cl to the deep-green-
brown solution of [(L1)FeIITHF]- in CH3CN generates an
orange-yellow solution, which upon standing affords orange
crystals of [(L1)FeIINCCH3][Ph4P] (1). The structure of 1
(Figure 1) reveals a distorted trigonal-bipyramidal geometry
around the ferrous site, with metrical parameters [average
Fe-Namido 2.027 Å; Fe-Namine 2.247(5) Å; Fe-NCH CN 2.126-
3
(6) Å] that are typical of iron(II) complexes. The average
Namido-Fe-Namine bond angle (79°) signifies the most
important deviation from ideal trigonal-bipyramidal geom-
etry, arising because of the constraints imposed by the five-
membered metallocycles. The iron atom is found at an
approximate distance of 0.375 Å off the plane formed by
the three amido nitrogen atoms, in the direction of the fifth
coordination site occupied by acetonitrile. The Fe(1)-N(5)-
C(49) angle at 153.4(6)° indicates that the coordination of
acetonitrile deviates from the usually linear η1-CH3CN
coordination mode.22 Space-filling models indicate strain
constraints imposed by the interaction of acetonitrile, en-
Inorganic Chemistry, Vol. 47, No. 3, 2008 1167

C¸ elenligil-C¸ etin et al.
Figure 1. Solid-state structure of 1 showing 50% probability ellipsoids
and the atom-labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-N(4) 2.012(4), Fe(1)-N(3) 2.024(4), Fe(1)-N(2) 2.045(4),
Fe(1)-N(5) 2.126(6), Fe(1)-N(1) 2.247(5), N(5)-C(49) 1.125(8); N(4)-
Fe(1)-N(3) 119.92(17), N(4)-Fe(1)-N(2) 118.79(16), N(3)-Fe(1)-N(2)
111.20(17), N(4)-Fe(1)-N(5) 102.97(19), N(3)-Fe(1)-N(5) 101.79(19),
N(2)-Fe(1)-N(5) 96.99(18), N(4)-Fe(1)-N(1) 78.94(17), N(3)-Fe(1)-
N(1) 79.25(16), N(2)-Fe(1)-N(1) 79.82(16), N(5)-Fe(1)-N(1) 176.79-
(16), C(49)-N(5)-Fe(1) 153.4(6).
Figure 2. Solid-state structure of 2 showing 50% probability ellipsoids
and the atom-labeling scheme. The PPh4 cation is omitted for clarity.
+
Selected interatomic distances (Å) and angles (deg): Fe(1)-O(1) 1.920-
(4), Fe(1)-N(2) 1.986(5), Fe(1)-N(4) 1.995(5), Fe(1)-N(3) 2.003(5), Fe-
(1)-N(1) 2.297(5), O(1)-H(1A) 0.8200; O(1)-Fe(1)-N(2) 107.0(2),
O(1)-Fe(1)-N(4) 99.5(2), N(2)-Fe(1)-N(4) 118.1(2), O(1)-Fe(1)-N(3)
103.2(2), N(2)-Fe(1)-N(3) 108.0(2), N(4)-Fe(1)-N(3) 118.9(2), O(1)-
Fe(1)-N(1) 174.20(18), N(2)-Fe(1)-N(1) 78.1(2), N(4)-Fe(1)-N(1)
75.31(19), N(3)-Fe(1)-N(1) 77.51(19), Fe(1)-O(1)-H(1A) 109.5.
capsulated in the cavity of the phenyl arms, along with the
bulky tert-butyl substituents. It is also noted that the acidic
protons of CH3CN make close contact with the electron-
rich, N(2)-atom-appended aryl arm of the ligand (3.104 Å
from the center of the phenyl ring), suggesting the possibility
of a πaryl-HCH CN interaction.
3
Synthesis and Characterization of Ferric Species via
Oxidation of Iron(II) Precursors by Dioxygen. (a) Reac-
tions in CH3CN. Green solutions of [(L1)FeIINCCH3]- in
CH3CN can be oxidized with dioxygen to afford dark-blue
solutions, from which a crystalline material that is not
suitable for X-ray analysis can be obtained. Cation exchange
with [PPh4]Cl, dissolved in blue acetonitrile solutions of this
crystalline material, affords [(L1)FeIIIOH][PPh4]‚CH3CN‚
0.5Et2O (2) upon diffusion of diethyl ether.
The structure of 2 (Figure 2) reveals a terminally coor-
dinating oxygen atom that is assigned to a hydroxide moiety.
This is supported by the Fe-OH bond distance at 1.920(4)
Å and further established by means of an IR-active ν(16O-
H) band at 3630 cm-1. The average Fe-Namido bond distance
at 1.995(5) Å and the significant displacement of the iron
atom from the plane of the three amido nitrogen atoms (0.449
Å) are consistent with the presence of a ferric center.
(b) Reactions in Dimethylformamide (DMF). Yellow-
green solutions generated from the reaction of K3L1 and
FeCl2inDMF,presumablycontainingtheanion[(L1)FeIIDMF]-,
can be oxidized by dioxygen to afford [(L1)FeIIIOH]-
[K(DMF)3]‚3H2O (3) as dark-blue crystals. The compound
has been characterized by X-ray analysis (Figure S3 in the
Supporting Information), albeit on poor-quality crystals due
to persistent twinning problems. The solid-state structure
reveals a terminal hydroxo moiety (Fe-OH ) 1.856(11) Å;
ν(16O-H) ) 3615 cm-1), whereas the 3-fold Fe-Namido bond
distance 1.952(6) Å is typical of a ferric site. The potassium
Figure 3. Solid-state structure of 4 showing 50% probability ellipsoids
and the atom labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-O(1) 1.863(5), Fe(1)-N(3) 1.963(6), Fe(1)-N(2) 1.964(6),
Fe(1)-N(4) 1.966(6), Fe(1)-N(1) 2.346(5), N(1)-C(13) 1.440(8), N(1)-
C(1) 1.445(8), N(1)-C(7) 1.447(8), N(2)-C(2) 1.379(8), N(3)-C(8) 1.375-
(8), N(4)-C(14) 1.386(8); O(1)-Fe(1)-N(3) 102.1(2), O(1)-Fe(1)-N(2)
102.3(2), N(3)-Fe(1)-N(2) 115.3(2), O(1)-Fe(1)-N(4) 102.3(2), N(3)-
Fe(1)-N(4) 115.7(2), N(2)-Fe(1)-N(4) 115.8(2), N(3)-Fe(1)-N(1) 77.9-
(2), N(2)-Fe(1)-N(1) 77.89(19), N(4)-Fe(1)-N(1) 77.5(2), O(1)-Fe(1)-
N(1) 179.7(2).
cation, which is separated from the hydroxo moiety by 5.753
Å, is supported by a matrix of six DMF molecules.
Compound 3 readily absorbs water molecules in the lattice,
and, indeed, if the reaction for its preparation is performed
with wet dioxygen and/or in wet DMF, then the product
obtained is instead crystalline [(L1)FeIIIOH](H3O)‚4H2O (4).
An alternative formulation of the same compound is [(L1)FeIII-
OH2]‚5H2O, but the former assignment is supported by an
Fe-OH bond distance of 1.863(5) Å in 4 (Figure 3) and a
ν(16O-H) vibration at 3616 cm-1, which are very similar to
those in 3 and inconsistent with the presence of a coordinated
water molecule. The average Fe-Namido and apical Fe-Namine
(22) (a) Barrera, J.; Sabat, M.; Harman, W. D. Organometallics 1993, 12,
4381-4390. (b) Chetcuti, P. A.; Knobler, C. B.; Hawthorne, M. F.
Organometallics 1988, 7, 650-660.
1168 Inorganic Chemistry, Vol. 47, No. 3, 2008

Iron Trisamidoamine Complexes
bond distances of 1.964(6) and 2.346(5) Å, respectively, are
also very similar to those encountered in 3 and are within
the range expected for a ferric site. Potassium ion is not
detected by atomic absorption and X-ray analysis; that leaves
the hydronium ion as the only reasonable option in the
charge-balancing role. Hence, one of the lattice water
molecules has been modeled as a hydronium ion involved
in hydrogen-atom bonding with solvated water molecules.
In summary, the reaction of [(L1)FeIIsolv]- (solv ) CH3-
CN, DMF) with dioxygen leads to the formation of
[(L1)FeIIIOH]- ions in all cases examined. Reactions with
18O2 indicate that the hydroxo oxygen atom is derived from
dioxygen, but the source of the hydrogen atom, whether due
to water traces or hydrogen-atom abstraction, is currently
unknown.
Synthesis and Characterization of Ferric Species via
Oxidation of Iron(II) Precursors by Dichloromethane and
Other Chloroorganics. Solutions of [(L1)FeIIsolv]- (solv )
DMF, CH3CN) can be easily oxidized by a series of R-Cl
(in decreasing order of reactivity, R ) benzyl, allyl > t-Bu,
i-Pr . n-Bu, Ph) according to the equation
Figure 4. Solid-state structure of 5 showing 50% probability ellipsoids
and the atom-labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-N(2) 1.946(3), Fe(1)-O(1D) 1.980(4), Fe(1)-N(1) 2.230-
(4), N(1)-C(1) 1.464(3); N(2A)-Fe(1)-N(2) 116.64(4), N(2)-Fe(1)-
O(1D) 100.70(7), O(1D)-Fe(1)-N(1) 180.0, N(2)-Fe(1)-N(1) 79.30(7).
2[(L1)FeIIsolv](K) + 2R-Cl f
2[(L1)FeIIIsolv] + R-R + 2KCl (1)
Details about the scope and mechanism of these reductive
dechlorination reactions23 will be presented elsewhere. In the
current context, the reaction can be exploited for its synthetic
utility because it affords access to the solvated adducts
[(L1)FeIIIsolv] [solv ) DMF (5), CH3CN (6)]. Dichlo-
romethane is the most practical reagent; a few drops, or
diffusion,ofCH2Cl2 inyellow-greensolutionsof[(L1)FeIIsolv]-
in the corresponding solvent readily generates dark-blue
solutions of [(L1)FeIIIsolv]. Two representative examples
[solv ) DMF (5), CH3CN (6)] have been structurally
characterized.
Figure 5. Solid-state structure of 6 showing 50% probability ellipsoids
and the atom-labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-N(2) 1.964(6), Fe(1)-N(3) 2.053(17), Fe(1)-N(1) 2.272-
(9), N(3)-C(17) 1.25(3); N(2)-Fe(1)-N(2A) 115.55(13), N(2)-Fe(1)-
N(3) 102.36(19), N(2)-Fe(1)-N(1) 77.64(19), N(3)-Fe(1)-N(1) 180.000-
(1), C(17)-N(3)-Fe(1) 180.000(2), N(3)-C(17)-C(18) 180.000(3).
Compound 5 (Figure 4) adopts a C3-symmetric geometry,
featuring a unique Fe(1)-Namido bond distance of 1.946(3)
Å, which is shorter than that found in iron(II) complex 1
and consistent with those observed for iron(III) compounds
2-4. The central iron atom Fe(1) lies 0.361 Å above the
equatorial plane of the three amido nitrogen atoms and away
from the apical N(1) nitrogen atom.
indicate that steric interactions with the t-Bu groups are
reduced in 6.
From acetonitrile solutions, the corresponding compound
6 (Figure 5) is obtained, which also features a similar C3-
symmetric configuration. As expected for the ferric state,
the average Fe-Namido bond distance of 1.964(6) Å in 6 is
shorter than that in 1 [2.027(4) Å], as is the Fe-NCCH3
bond distance in 6 [2.053(17) Å] by comparison to that in 1
[2.126(6) Å]. Accordingly, the C-N bond distance of the
coordinated CH3CN in 6 [1.25(3) Å] is longer than the
corresponding C-N bond distance in 1 [1.125(8) Å]. In
contrast to the bent Fe-N-CCH3 moiety in 1, the corre-
sponding angle in 6 is linear because space-filling models
Electrochemistry. Cyclic and linear-sweep voltammetry
experiments on [(L1)FeIIsolv]- (solv ) DMF, DMSO) under
anaerobic conditions are instructive with respect to the
oxidative transformations of the iron(II) precursors. In DMF,
a semireversible FeII/FeIII couple is observed at very acces-
sible potentials (E1/2 ) -1.298 V vs Fc+/Fc, ∆E ) 71 mV,
and ip,a/ip,c ) 1.16; Figure 6), followed by a two-electron
anodic peak at Ea ) -0.038 V that is tentatively assigned
to simultaneous oxidation of the metal and ligand ([(L1)-
FeII] f [(L1)•FeIII]). Similar behavior is observed in DMSO
(E1/2 ) -1.302 V vs Fc+/Fc, ∆E ) 81 mV, and ip,a/ip,c
)
1.23; anodic peak at Ea ) -0.063 V). Importantly, the
corresponding anodic wave in THF is significantly more
accessible (Ea ) -0.382 V) and as will be detailed elsewhere,
(23) (a) Costentin, C.; Robert, M.; Save´ant, J.-M. J. Am. Chem. Soc. 2004,
126, 16834-16840. (b) Costentin, C.; Robert, M.; Save´ant, J.-M. J.
Am. Chem. Soc. 2003, 125, 10729-10739.
Inorganic Chemistry, Vol. 47, No. 3, 2008 1169

C¸ elenligil-C¸ etin et al.
Figure 7. Solid-state structure of 7 showing 50% probability ellipsoids
and the atom-labeling scheme ([Et4N] cations have been omitted for clarity).
Selected interatomic distances (Å) and angles (deg): Fe(1)-N(4) 2.019-
(10), Fe(1)-N(2) 2.090(10), Fe(1)-N(1) 2.106(9), Fe(1)-Cl(1) 2.357(4),
Fe(1)-N(3) 2.679(10), N(1)-C(1) 1.316(14), N(1)-C(19) 1.384(16), N(2)-
C(7) 1.368(15), N(2)-C(2) 1.417(14), N(3)-C(29) 1.427(14), N(3)-C(8)
1.431(15), N(3)-C(13) 1.444(14), N(4)-C(39) 1.416(14), N(4)-C(14)
1.427(14); N(4)-Fe(1)-N(2) 116.9(4), N(4)-Fe(1)-N(1) 114.1(4), N(2)-
Fe(1)-N(1) 81.3(4), N(4)-Fe(1)-Cl(1) 120.1(3), N(2)-Fe(1)-Cl(1) 111.3-
(3), N(1)-Fe(1)-Cl(1) 106.1(3), N(4)-Fe(1)-N(3) 72.2(3), N(2)-Fe(1)-
N(3) 69.0(4), N(1)-Fe(1)-N(3) 148.2(3), Cl(1)-Fe(1)-N(3) 95.5(2).
Figure 6. Cyclic voltammetry of [(L1)FeIIDMF]- (3 mM) in DMF/0.1 M
TBAPF6 with a gold disk electrode (1.6 mm in diameter). Scan rate ) 0.1
V/s.
the reaction of [(L1)FeIITHF]- with dioxygen in THF leads
to both metal- and ligand-centered oxidation. However,
ligand-based oxidations are achievable by other strong
oxidants, as indicated below.
Incidentally, ligand L1H3 and the precursor 2,2′,2′′-
triaminotriphenylamine, which are stable toward oxygen,
exhibit an anodic wave at Ea ) 0.325 and 0.295 V,
respectively, in DMSO, whereas the deprotonated K3L1,
which is exceedingly air-sensitive, has the corresponding
wave at -0.654 V in DMSO. It is worthwhile mentioning
in this context that the oxidation of K3L1 with dioxygen
(toward so far uncharacterized products) is also associated
with a g ) 2.0 electron paramagnetic resonance (EPR) signal
during the early stages of the reaction, which decays over
time. Similar behavior is also observed in the oxidation of
[(L1)ZnIINCCH3]-, indicating that the ligand does not neces-
sarily require a redox-active metal to undergo oxidation by
dioxygen.
Scheme 2
stabilized as a ligand-centered radical within the resulting
mononuclear iron(II) compound, and its fate is not known
at the present time. The iron(II) state of 7 is supported by
the 57Fe Mo¨ssbauer effect of 7 at 78 K, which is consistent
with the presence of a high-spin ferrous site (δ ) 0.99 mm/
s, ∆EQ ) 2.84 mm/s, and Γ ) 0.44 mm/s), and by the
metrical parameters noted below.
Other Oxidizing Agents. (a) In Situ FeIIICl Moieties.
In an effort to synthesize [(L1)FeIII-O-FeIII(L1)]2-, [Cl3-
FeIII-O-FeIIICl3](NEt4)224 was added to 2 equiv of K3L1 in
THF to afford dark-green-black solutions. Rather than
obtaining the intended compound, crystals of [(L1 )FeIICl]-
re
As with complexes [(L1)FeIIsolv]- (solv ) DMF, CH3-
CN), compound 7 can also be oxidized by dichloromethane
to afford the corresponding blue-green ferric complex
(NEt4)2 (7) can be isolated from diethyl ether extracts of the
reaction mixture upon the addition of hexane. Most impor-
tantly, ligand L1 has been rearranged in 7, due to formal
cleavage of a Namine-C bond and the formation of a new
Namido-C bond (Scheme 2). It has been established that this
rearrangement is triggered by an initial one-electron oxidation
of the ligand at the Namido site, leading to an aminyl radical,
which is then involved in an 1,4-(N-to-N)-phenyl migration,
as indicated in Scheme 2. However, the initial oxidizing
equivalent is not retained in 7; hence, L1re remains a trianion.
A second point of importance is that the all-ferric oxidation
state of the starting material has been reduced to the ferrous
site of compound 7. Taken together, these observations
suggest that FeIIICl moieties act as internal oxidants of the
coordinated ligand, although the oxidizing equivalent is not
[(L1 )FeIIICl](NEt4) (8), by allowing CH2Cl2 to diffuse into
re
ethereal solutions of 7.
The structures of 7 (Figure 7) and 8 (Figure 8) are similar
and exhibit distorted tetrahedral geometries comprised of the
three Namido atoms and the coordinated chloride. The amino
nitrogen atom is also found at contact distances of 2.679-
(10) and 2.501(4) Å from the iron sites of 7 and 8,
respectively, and at a position that caps the facial coordina-
tion of the chloride atom and two of the Namido atoms. The
average Fe-Namido bond distance in 7 [2.072(10) Å] is
significantly longer than that documented in 8 [1.972(5) Å]
and comparable to the average Fe-Namido bond distance
observed for ferrous species 1. Similarly, the longer Fe(1)-
Cl(1) bond distance [2.357(4) Å] in 7 versus that of 8 [2.301-
(24) Armstrong, W. H.; Lippard, S. J. Inorg. Chem. 1985, 24, 981-982.
1170 Inorganic Chemistry, Vol. 47, No. 3, 2008

Iron Trisamidoamine Complexes
established structural feature of many ferrous chloro-contain-
ing compounds.25 Close inspection of the coordinated ligands
indicates that each ligand is not only rearranged as demon-
strated previously but also doubly oxidized within the
vulnerable o-phenylenediamine moiety to afford a diamag-
netic o-diiminobisquinone fragment (o-dibq0; see below).
Furthermore, only two amido (in effect, imino) residues are
coordinated, whereas the third one is found to be at a
nonbonding distance, presumably present as a protonated
moiety. The doubly oxidized and singly protonated ligand
(L1re-2H) in 9 is thus a monoanion. Although the reaction is
certainly complicated, the reduction of the ferric sites and
the oxidation of the ligand can be formally explained by
means of a one-electron oxidation of 5 by FeCl3 [thus
reducing the latter to iron(II)], which, in combination with
a second electron abstracted from the ligand by the iron(III)
site of 5 (or a derivative thereof), results in a two-electron
oxidative rearrangement of the ligand and reduction of the
ferric site of 5. This two-electron process is facilitated by
the presence of chloride ligands in abundance, thus stabilizing
the [FeII(Cl)(µ-Cl)]2 core.
Compound 9 possesses a crystallographic 2-fold axis
penetrating the two bridging chloride atoms. The geometry
around each five-coordinate iron atom is distorted trigonal
bipyramidal. The equatorial coordination plane is defined
by an imino nitrogen atom, the terminal chloride atom, and
one bridging chloride atom (short Fe-Clb bond), whereas
the axial coordination is occupied by the second imino
nitrogen atom and the second bridging chloride atom (long
Fe-Clb bond). Thus, each asymmetrically bridging chloride
atom lies on the equatorial plane with respect to the proximal
iron site and on the axis of the distal iron coordination. The
other two nitrogen atoms reside beyond the coordination
sphere of the iron center [Fe-N(3) ) 3.981 Å; Fe-N(4) )
3.861 Å]. The N(3) atom is one of the amido nitrogen atoms
of the original tripodal ligand (C-N bond distances are
consistent with single-bond character) and is expected to be
protonated. The N(4) atom is a bona fide three-coordinate
amine residue. The Fe-Clt bond distance [2.230(3) Å], the
short and long Fe-Clb bond distances [2.367(3) and 2.515-
(3) Å], the Clb-Fe-Clb bond angle [84.80(11)°], and the
intermetallic Fe-Fe distance (3.607 Å) are all typical of the
[FeII(Cl)(µ-Cl)]2 core unit, as established by similar ferrous
chloro complexes carrying neutral bidentate ligands (for
instance, [(tmen)(Cl)FeII(µ-Cl)2FeII(Cl)(tmen)], where tmen
) tetramethylethylenediamine).25b
Figure 8. Solid-state structure of 8 showing 50% probability ellipsoids
and the atom-labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-N(3) 1.949(5), Fe(1)-N(1) 1.971(5), Fe(1)-N(4) 1.996(4),
Fe(1)-Cl(1) 2.301(2), Fe(1)-N(2) 2.501(4), N(1)-C(1) 1.385(7), N(1)-
C(19) 1.434(7), N(2)-C(29) 1.437(7), N(2)-C(2) 1.463(8), N(2)-C(7)
1.466(8), N(3)-C(13) 1.383(6), N(3)-C(8) 1.402(7), N(4)-C(39) 1.407-
(6), N(4)-C(14) 1.413(6); N(3)-Fe(1)-N(1) 115.7(2), N(3)-Fe(1)-N(4)
81.59(18), N(1)-Fe(1)-N(4) 117.9(2), N(3)-Fe(1)-Cl(1) 123.38(18),
N(1)-Fe(1)-Cl(1) 112.20(16), N(4)-Fe(1)-Cl(1) 101.30(17), N(3)-Fe-
(1)-N(2) 74.71(17), N(1)-Fe(1)-N(2) 74.36(19), N(4)-Fe(1)-N(2)
156.29(18), Cl(1)-Fe(1)-N(2) 91.29(14).
Figure 9. Solid-state structure of 9 showing 50% probability ellipsoids
and the atom-labeling scheme. Selected interatomic distances (Å) and angles
(deg): Fe(1)-N(1) 2.019(8), Fe(1)-N(2) 2.048(9), Fe(1)-Cl(2) 2.230(3),
Fe(1)-Cl(1) 2.367(3), Fe(1)-Cl(1A) 2.515(3), Cl(1)-Fe(1A) 2.515(3),
N(1)-C(1) 1.332(12), N(1)-C(45) 1.423(12), N(2)-C(2) 1.315(11), N(2)-
C(3) 1.435(12), N(3)-C(18) 1.381(12), N(3)-C(13) 1.402(12), N(4)-C(24)
1.421(12), N(4)-C(40) 1.436(11), N(4)-C(23) 1.439(12); N(1)-Fe(1)-
N(2) 78.8(3), N(1)-Fe(1)-Cl(2) 124.4(2), N(2)-Fe(1)-Cl(2) 102.0(2),
N(1)-Fe(1)-Cl(1) 107.4(2), N(2)-Fe(1)-Cl(1) 89.2(2), Cl(2)-Fe(1)-Cl-
(1) 128.08(13), N(1)-Fe(1)-Cl(1A) 192.6(3), N(2)-Fe(1)-Cl(1A) 167.6-
(3), Cl(2)-Fe(1)-Cl(1A) 90.25(13), Cl(1)-Fe(1)-Cl(1A) 84.80(11), Fe-
(1)-Cl(1)-Fe(1A) 95.20(11).
Figure 10 depicts in greater detail the [FeII(Cl)(µ-Cl)]2 core
and the coordinated o-dibq0 moiety. Although the quality of
the structural data is not optimal, metrical parameters strongly
suggest that the two C-N bond distances represent double
bonds [N(1)-C(1) 1.315(11) Å; N(2)-C(2) 1.332(12) Å]
and the phenylene ring is characterized by the usual pattern
of four long [C(1)-C(2) 1.464(14) Å; C(2)-C(10) 1.448-
(2) Å] is consistent with the lower oxidation state of iron in
compound 7.
(b) Oxidation with FeCl3. The oxidizing role of FeIIICl
moieties was further examined by dissolving equivalent
amounts of 5 and anhydrous FeCl3 in THF to obtain brown
solutions of [(L1re-2H)(Cl)FeII(µ-Cl)2FeII(Cl)(L1re-2H)] (9) as
diamond-shaped crystals after extraction and crystallization
from hexane.
(25) (a) Raper, E. S.; Miller, A.; Glowiak, T.; Kubiak, M. Trans. Met. Chem.
1989, 14, 319-320. (b) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.
Inorg. Chim. Acta 1994, 224, 5-9. (c) Mills, D. K.; Hsiao, Y. M.;
Farmer, P. J.; Atnip, E. V.; Reibenspies, J. H.; Darensbourg, M. Y. J.
Am. Chem. Soc. 1991, 113, 1421-1423.
The structure of 9 (Figure 9) reveals the presence of a
centrosymmetric [FeII(Cl)(µ-Cl)]2 core, which is a well-
Inorganic Chemistry, Vol. 47, No. 3, 2008 1171

C¸ elenligil-C¸ etin et al.
the synthetically useful [(L1)FeIIIsolv] species. The latter can
be oxidized by the strong oxidant FeCl3 to effect complete
reduction of all iron to iron(II) (captured in a [FeII(Cl)(µ-
Cl)]2 core) along with oxidative rearrangement of the ligand
featuring doubly oxidized o-diimidophenylene rings.
(4) Efforts to synthesize [(L1)Fe-O-Fe(L1)]2- via the [Cl3-
Fe-O-FeCl3]2- precursor indicate that intrinsic FeIIICl
moieties can perform ligand oxidation, resulting in a species
([(L1 )(Cl)FeII]2-) in which all iron has been reduced and
re
the ligand has been oxidatively rearranged, although the
oxidation equivalent has not been retained. This compound
can be easily oxidized to [(L1 )(Cl)FeIII]- by dichlo-
re
romethane.
Future studies will be directed toward exploring whether
these and related ligand systems can store oxidizing equiva-
lents above the L•FeIII level in accordance with chemistry
encountered with P450 oxygenases,29 peroxidases,30 and
related synthetic analogues.31
Figure 10. Close-up picture of the o-dibq0 moiety in 9.
(13) Å; C(11)-C(48) 1.412(14) Å; C(12)-C(1) 1.434(14)
Å] and two short [C(10-C(11) 1.336(13) Å; C(12)-C(48)
1.361(13) Å] C-C bond distances. Similar metrical param-
eters are found in many other o-dibq0-containing compounds,
([FeII(dibq)3]2-,26 [FeII(CN)4(dibq)3]2-,27 and [OsII(dibq)3]2-.28
The presence of two diamagnetic o-dibq0 moieties and a
diferrous site is also consistent with the EPR-silent (X-band)
character of 9.
Acknowledgment. We thank Dr. Charles Barnes for
assistance with X-ray analysis results. We are indebted to
Dr. Nicholas Leventis for insightful comments. We gratefully
acknowledge generous financial support (to P.S.) by the NSF
(Grant CHE-0412959) and, in part, by the NIH/NIEHS
(Grant 5 P42 ES007381). We also acknowledge NSF funding
(Grant CHE-0420497) for the purchase of the diffractometer
at the University of MissourisSt. Louis.
Conclusion
The following is a summary of the principal findings of
this investigation:
Note Added in Proof: While this work was under review,
a high-yield synthesis of the precursor 2,2′,2′′-triaminotriph-
enylamine appeared in the literature.32 This precursor has
been previously reported in the Ph.D. thesis of Dr. R.
C¸ elenligil-C¸ etin.33
(1) A new family of tripodal trisamidoamine ligands has
been introduced that is capable of providing significant
electron density and steric protection to a coordinated metal
center but could also store oxidizing equivalents under
specific conditions.
(2) A series of electron-rich [(L1)FeIIsolv]- compounds
(solv ) DMF, CH3CN) have been isolated or generated in
situ, which are exceedingly air-sensitive and can be trans-
formed to a series of [(L1)FeIIIOH]- compounds featuring
terminal hydroxo groups.
(3) The [(L1)FeIIsolv]- series of compounds can reduc-
tively dechlorinate organochlorides (R-Cl reactivity order
R ) benzyl, allyl > t-Bu, i-Pr . n-Bu, Ph) and generate
Supporting Information Available: Listings of experimental
procedures, solid-state structures of 2,2′,2′′-triaminotriphenylamine
(Figure S1), ligand L1H3 (Figure S2), and compound 3 (Figure S3),
X-ray crystallographic tables for 2,2′,2′′-triaminotriphenylamine,
L1H3, and compounds 1-9 (Tables S1-S66), and X-ray crystal-
lographic data for the same compounds in CIF format. The material
is available free of charge via the Internet at http://pubs.acs.org.
IC702154Z
(29) Groves, J. T. J. Inorg. Biochem. 2006, 100, 434-447.
(30) Derat, E.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 13940-13949.
(31) Balch, A. L.; Chan, Y.-W.; Cheng, R.-J.; La Mar, G. N.; Latos-
Grazynski, L.; Renner, M. W. J. Am. Chem. Soc. 1984, 106, 7779-
7785.
(32) Jones, M. B.; MacBeth, C. E. Inorg. Chem. 2007, 46, 8117-8119.
(33) C¸ elenligil-C¸ etin, R. Synthesis, Characterization, and Reactivity of Iron
Complexes with N-Donor Ligands in Relation to Oxygenation of
Hydrocarbons. Ph.D. Thesis, Boston University, Boston, MA, 2004.
(26) Peng, S.-M.; Chen, C.-T.; Liaw, D.-S.; Chen, C.-I. Inorg. Chim. Acta
1985, 101, L31-L33.
(27) Christoph, G. G.; Goedken, V. L. J. Am. Chem. Soc. 1973, 95, 3869-
3874.
(28) Ghosh, A. K.; Peng, S.-M.; Paul, R. L.; Ward, M. D.; Goswami, S. J.
Chem. Soc., Dalton Trans. 2001, 336-340.
1172 Inorganic Chemistry, Vol. 47, No. 3, 2008