Synthesis and characterization of cationic iron complexes supported by the neutral ligands NPi-Pr3, NArPi-Pr3, and NSt-Bu3

Article abstract of DOI:10.1139/v05-017

This paper compares the local geometries, spin states, and redox properties of a series of iron complexes supported by neutral, tetradentate NP₃ (tris(phosphine)amine) and NS₃ (tris(thioether)amine) ligands. Our consideration of an F

Full text of DOI:10.1139/v05-017

332
Synthesis and characterization of cationic iron
complexes supported by the neutral ligands
NPi-Pr , NArPi-Pr , and NSt-Bu 1
3
3
3
Cora E. MacBeth, Seth B. Harkins, and Jonas C. Peters
Abstract: This paper compares the local geometries, spin states, and redox properties of a series of iron complexes
supported by neutral, tetradentate NP3 (tris(phosphine)amine) and NS3 (tris(thioether)amine) ligands. Our consideration
of an Fe-mediated N2 fixation scheme similar to that proposed by Chatt for molybdenum motivates our interest in sys-
tems of these types. This report specifically describes the synthesis and characterization of cationic Fe(II) chloride
complexes supported by the neutral ligands NPi-Pr (NPi-Pr = [N(CH2CH2P-i-Pr2)3]), NArPi-Pr (NArPi-Pr = [N(2-
3
3
3
3
diisopropylphosphine-4-methylphenyl)3]), and NSt-Bu (NSt-Bu = [N(CH2CH2S-t-Bu)3]). The solid-state structures, elec-
3
3
trochemistry, and magnetic properties of these complexes are reported. Whereas the NPi-Pr and NArPi-Pr3 ligands pro-
3
vide pseudotetrahedral S = 2 ferrous cations [Fe(NPi-Pr3)Cl]PF6 (1[PF6]) and [Fe(NArPi-Pr3)Cl]BPh4 (2[BPh4]) featuring
a long Fe—N bond distance, the NSt-Bu ligand gives rise to a trigonal bipyramidal structure with a S = 1 ground state
3
and a much shorter Fe–N interaction. The complexes 1[BPh4] and 2[BPh4] can be reduced under CO to give rise to the
five-coordinate Fe(I) monocarbonyls [Fe(NPi-Pr3)CO]BPh4 (4[BPh4]) and [Fe(NArPi-Pr3)CO]BPh4 (5[BPh4]). The solid-
state structures and electrochemistry of 4[BPh4] and 5[BPh4] are described, as is the EPR spectrum of 4[BPh4]. The
synthesis and characterization of the hydride–dinitrogen complex [Fe(NPi-Pr3)(N2)(H)]PF6 (6[PF6]) has also been accom-
plished and its properties are also reported.
Key words: nitrogenase, iron, polydentate phosphines, thioether ligands, N2 chemistry, nitrogen, Fe(I).
Résumé : Dans ce travail, on compare les géométries locales, états de spin et propriétés redox d’une série de com-
plexes de fer supportés par les ligands neutres et tridentates NP3 (tris(phosphine)amine) et NS3 (tris(thioéther)amine).
Notre considération d’un schéma de fixation du N2 catalysé par le Fe semblable à celui proposé par Chatt pour le mo-
lybdène motive notre intérêt dans les systèmes de ce type. Dans ce travail, on décrit spécifiquement la synthèse et la
caractérisation de complexes cationiques du chlorure de Fe(II) supportés par les ligands neutres NPi-Pr (NPi-Pr
=
=
3
3
[N(CH2CH2P-i-Pr2)3]), NArPi-Pr (NArPi-Pr = [N(2-diisopropylphosphine-4-méthylphényl)3]) et NSt-Bu (NSt-Bu
3
3
3
3
[N(CH2CH2S-t-Bu)3]), y compris les structures à l’état solide, l’électrochimie et les propriétés magnétiques de ces
complexes. Alors que les ligands NPi-Pr et NArPi-Pr conduisent à des cations ferreux pseudotétraédriques S = 2
3
3
[Fe(NPi-Pr3)Cl]PF6 (1[PF6]) et [Fe(NArPi-Pr3)Cl]BPh4 (2[BPh4]) comportant une longue liaison Fe—N, le ligand NSt-Bu
conduit à la formation d’un complexe de structure bipyramidale trigonale avec un état fondamental S = 1 et une
interaction Fe–N beaucoup plus courte. Les complexes 1[BPh4] et 2[BPh4] peuvent être réduits sous l’influence du
CO pour conduire à des complexes monocarbonylés et pentacoordinés du Fe(I), [Fe(NPi-Pr3)CO]BPh4 (4[BPh4]) et
[Fe(NArPi-Pr3)CO]BPh4 5[BPh4]. On décrit aussi les structures solides et l’électrochimie des complexes 4[BPh4] et
3
5[BPh4] ainsi que le spectre RPE du composé 4[BPh4]. On a aussi réalisé la synthèse et la caractérisation du complexe
hydrure–diazote [Fe(NPi-Pr3)(N2)(H)]PF6 (6[PF6]) et on rapporte ses propriétés.
Mots clés : nitrogénase, fer, phosphines polydentates, ligands thioéther, chimie du N2, azote, Fe(I).
[Traduit par la Rédaction] MacBeth et al. 340
Introduction
FeMo-nitrogenase (FeMo-N2ase) enzyme (2), the biochemi-
cal (3), theoretical (4), and synthetic (1, 5), model communi-
ties have focused the bulk of their attention on
understanding its mode of mechanistic action. The presence
of a single molybdenum center in the FeMo-cofactor (2, 3a),
in addition to a wealth of structurally diverse functional
The site(s) of substrate binding and reactivity (e.g., N2,
HCϵCH, H+, CO) at the inorganic cofactors of nitrogenase
enzymes is a fascinating and complicated issue (1). Because
crystallographic data is presently available only for the
Received 9 September 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 23 March 2005.
C.E. MacBeth, S.B. Harkins, and J.C. Peters.2 California Institute of Technology, Division of Chemistry and Chemical
Engineering, Mail Code 127-72, 1200 E. California Blvd., Pasadena, CA 91125, USA.
1This article is part of a Special Issue dedicated to Dinitrogen Chemistry.
2Corresponding author (e-mail: jpeters@caltech.edu).
Can. J. Chem. 83: 332–340 (2005)
doi: 10.1139/V05-017
© 2005 NRC Canada

MacBeth et al.
333
Scheme 1.
model systems (6), has reinforced the suggestion that a sin-
gle molybdenum site may be responsible for mediating N2
uptake and reduction in FeMo-N2ase. Particularly attractive
is a scheme akin to that originally put forward by Chatt et al.
(7) and later modified by Pickett (8) in which a vacant MoIII
site, perhaps generated via homocitrate dechelation under
electron loading conditions, binds N2 to mediate its succes-
sive reduction to NH3 by the successive transfer of protons
and electrons. Such a scenario is schematically outlined in
the top portion of Scheme 1. Significant in this regard is the
recent work of Yandulov and Schrock (9), who have demon-
strated that such a cycle can be accomplished in a small
molecule synthetic system. They have shown that certain
triamidoamine-supported molybdenum complexes are capa-
ble of catalytically shuttling among various oxidation states
(MoIII/MoIV/MoV/MoVI) as nitrogen is taken up and reduced
to ammonia.
These successes not withstanding, it needs to be empha-
sized that iron is the only metal known to be ubiquitous to
all nitrogenase enzymes (10) (also true of hydrogenase en-
zymes), and biochemical and spectroscopic data (11) are
available to suggest that the iron sites of the FeMo-cofactor
play a critical, albeit ill-defined, role in facilitating N2 reduc-
tion, and myriad proposals have been put forth stressing an
Fe-mediated N2 fixation scheme. Problematic is that the
inorganic dinitrogen chemistry of iron is far less well-
developed than that for molybdenum, and therefore evaluat-
ing the chemical feasibility of such schemes is at present
difficult. Most of the Fe-mediated scenarios being consid-
ered in the literature invoke multiple iron centers during N2
uptake–reduction (5). This is due to the presence of multiple
iron centers in the cofactor, and perhaps more importantly,
to the implicit assumption that a single iron site cannot sup-
port the various Nx intermediates, oxidation states, and
multi-electron transformations that would need to be invoked
en route to NH3 production from N2 (bottom of Scheme 1,
steps A–G), in sharp contrast to the case of redox-rich mo-
lybdenum.
Inorganic coordination chemistry can play a key role with
respect to expanding our understanding of the N2 chemistry
available to iron, and in testing the validity of the assump-
tions mentioned above. The recent structural assignment of a
central light atom in the MoFe-cofactor core (2) suggests
that, if one or more iron atoms in the central section of the
cofactor provides an initial site for N2 uptake (3a), then
coordinatively unsaturated iron centers provide plausible
starting points for inorganic iron model chemistry (12). At
the bottom of Scheme 1 we sketch a N2 reduction scenario
that mirrors that of the molybdenum postulate, but in this
case invokes a hemi-labile role for the central light atom that
exposes a low-valent iron site for N2 uptake under electron
loading conditions. Such a scenario would set the stage for
the further reduction of N2 to ammonia, perhaps via a Chatt-
type cycle based upon iron (steps A–G). Fe–Nx model com-
plexes will help to evaluate the chemical feasibility of this
mechanistic scheme. Depending on the degree to which the
Fe–X linkage is broken during the cycle, four- and (or) five-
coordinate complexes may be considered as the most appro-
priate starting points.
In this context, our own group has recently developed
four-coordinate iron, phosphine-supported systems of the ge-
neric type L3Fe–Nx (13). These pseudotetrahedral platforms
exhibit: (i) an unprecedented degree of redox flexibility for
mononuclear iron systems; (ii) a fascinating propensity to
populate low-spin, ground-state configurations for pseudo-
tetrahedral Fe2+, Fe3+, and Fe4+ species; and (iii) a capacity
to accommodate strongly π-basic (e.g., N3–, NR2–), π-acidic
–
(e.g., N2), and intermediate type (e.g., N2R–, NR2 ) ligands
© 2005 NRC Canada

334
Can. J. Chem. Vol. 83, 2005
Scheme 2.
Scheme 3.
FeCl2
CH2Cl2, RT
NSt-Bu
3
TlPF6
- TlCl
NPi-Pr
[(NPi-Pr )FeCl][PF9]
3
3
t-Bu
t-Bu
1[PF6]
S
t-Bu
S
Sacconi's
prep
S
S = 2, colorless, 81%
N
FeCl2, NaBPh4
THF, 65 ºC
3 LiP-i-Pr2
[(NArPi-Pr )FeCl][PF9]
NArPi-Pr
3
3
- 78
C
°
Cl
NPi-Pr
2[BPh4]
3
N
3
S = 2, yellow, 97%
i-Pr2P
i-Pr2P
i-PPr2
92% isolated
yield
FeCl2
THF, RT
N
TlPF6
- TlCl
[(NSt-Bu )FeCl][PF9]
NSt-Bu
3
3
Br
3[PF6]
6 t-BuLi / Et2O
- 78
3 CIP-i-Pr2
S = 1, colorless, 62%
°C
- 78
C
°
NAri-Pr
N
3
- 3 LiBr
- 3 C4H8
- 3 C4H10
3
i-Pr2P
i-Pr2P
modification of the literature procedure for NPPh (NPPh
=
P-i-Pr2
N(CH2CH2PPh2)3) (14). Accordingly, the addition 3of 3 equiv.
of lithium diisopropylphosphide to an ethereal solution
of tris(2-chloroethyl)amine at low temperature affords
3
N
NArPi-Pr in excellent isolated yield (92%) following work-
66% isolated
yield
3
up. Preparation of NArPi-Pr requires a combination of a
suitable carbanion nucleophi3le and chlorophosphine electro-
phile. We find that the addition of chlorodiisopropylphos-
phine to an ethereal solution of [2-Li-4-Me-C6H3]3N, generated
in situ by the addition of tert-butyllithium to tris(o-bromo-p-
at a single coordination site. An obvious question that
emerges is whether five-coordinate iron scaffolds will dis-
play similar properties.
tolyl)amine, affords crystalline NArPi-Pr in good yield (66%)
3
after work-up.
In the present paper, we compare the local geometries,
spin states, and redox properties of a series of iron com-
plexes supported by neutral, tetradentate NP3 (tris(phos-
phine)amine) and NS3 (tris(thioether)amine) ligands (14).
As shown in Scheme 3, cationic Fe(II)–chloride com-
plexes of these NP3-type ligands can be prepared by reacting
the neutral phosphine ligands with FeCl2 in the presence of a
halide abstraction reagent (TlPF6 or NaBPh4) in either
CH2Cl2 or THF, providing colorless [Fe(NPi-Pr3)Cl]PF6
(1[PF6]) and yellow [Fe(NArPi-Pr3)Cl]BPh4 (2[BPh4]). For
structural and magnetic comparison, we also prepared the
colorless complex [Fe(NSt-Bu3)Cl]PF6 (3[PF6]) by the addi-
Two new NP3-type ligands are introduced, NPi-Pr (NPi-Pr
=
3
N(CH2CH2P-i-Pr2)3) and NArPi-Pr (NArPi-Pr
=
N3(2-
diisopropylphosphine-4-methylpheny3l)3) are described, and
we establish the nature of their ferrous “(NP3)FeII–X” ions.
For comparison, we also describe a structurally related
“(NSt-Bu3)FeII–X” system (NSt-Bu = (e.g., N(CH2CH2S-t-
3
tion of FeCl2 to a THF solution of NSt-Bu followed by chlo-
3
Bu)3)) (14). It is established that3the NPi-Pr and NArPi-Pr
ride abstraction with TlPF6. The isolated yield of 3[PF6] is
fairly good (62%), although not in comparison to the high
yields obtained for 1[PF6] and 2[BPh4]. Somewhat surpris-
scaffolds support unusual monovalent, trigonal bipyramida3l
3
monocarbonyl derivatives. Additionally, it is demonstrated
ing is that the iron chemistry of Sacconi’s NSt-Bu ligand has
that the NPi-Pr ligand supports N2 coordination to iron.
3
3
not been previously described, though its coordination to the
first row transition metals Co (18), Ni (19), and Cu (20) has
been known for some time. Given the sulfur-rich environ-
ment of the central iron sites of the MoFe-cofactor, and the
Results and discussion
Five-coordinate iron complexes supported by neutral,
tetradentate NP3 (15, 16) and PP3 (17) ligands are well-
known, dating back to the early pioneering efforts of
Sacconi and co-workers (14, 15). Despite the general utility
of these types of polydentate phosphines, little attention has
been paid to derivatives with sterically encumbering phos-
phine substituents. To begin to explore such systems we
have targeted the preparation of the isopropyl-substituted
possible presence of an interstitial N atom, NSt-Bu and re-
3
lated ligands provide an interesting structural starting point
for functional iron model studies.
X-ray diffraction analyses of single crystals of 1[PF6],
2[BPh4], and 3[PF6] (Fig. 1) establishes that each complex is
monomeric. NP3-supported 1[PF6] and 2[BPh4] are best de-
scribed as tetrahedral complexes and are structurally quite
similar to the ferrous cation of NPPh (15b). Accordingly,
1[PF6] features an elongated Fe-N di3stance of 2.801(3) Å
systems NPi-Pr and NArPi-Pr (Scheme 2). Isopropyl-
substituted NPi3-Pr can be prepared by a straightforward
3
3
© 2005 NRC Canada

MacBeth et al.
335
Fig. 1. Displacement ellipsoid (50%) representation of 1[PF6], 2[BPh4], and 3[PF6]. Hydrogen atoms and counteranions have been re-
moved for clarity. Selected bond lengths (Å) and angles (°) for 1[PF6]: Fe—Cl 2.262(2), Fe—P1 2.455(1), Fe—N 2.801(3), N-Fe-Cl
180.00(11), P1-Fe-P′ 112.10(2), P1-Fe-Cl 106.70(2). For 2[BPh4]: Fe—Cl 2.253(2), Fe—P1 2.467(2), Fe—P2 2.485(2), Fe—P3
2.481(2), Fe—N 2.701(2), P1-Fe-C1 103.83(6), P2-Fe-Cl 115.89(5), P3-Fe-Cl 105.64(4), Fe-N 2.701(2). For 3[PF6]: Fe—Cl 2.298(1),
Fe—S1 2.447(1), Fe—S2 2.453(1), Fe—S3 2.407(1), Fe—N 2.378(3), N-Fe-Cl 178.32(8), S1-Fe-Cl 101.38(4), S2-Fe-Cl 97.54(4),
S3-Fe-Cl 100.11(4).
(2.701(2) Å for 2[BPh4]) (21). The iron centers in 1[PF6]
and 2[BPh4] lie above the P3 plane, projected into the hemi-
sphere opposite the apical N atom. Both systems are high
spin (S = 2), expected for conventional pseudotetrahedral
iron(II) systems.
in high yield (>85%). Attempts to prepare the related iron(I)
NSt-Bu3Fe(CO)+ species by similar strategies have not been
successful.
Structurally characterized, Fe(I) monocarbonyl complexes
are rare. To our knowledge, Parkin and co-workers (22) have
reported the only crystallographically characterized example
of such a species, which is pseudotetrahedral, using an an-
ionic substituted tris(pyrazolyl)borate ligand auxiliary. Re-
cently, we have reported the structural characterization of an
iron(I) dicarbonyl complex, which is approximately square
pyramidal, supported by an anionic tris(phosphino)borate
ligand (13a). The solid-state structures of 4[BPh4] and 5[BPh4]
were obtained for comparison and are shown in Fig. 2. Each
cationic complex is rigorously trigonal bipyramidal. The
Fe—N bond distances observed for 4[BPh4] (2.150(2) Å)
and 5[BPh4] (2.164(2) Å) are dramatically contracted
(>0.6 Å) compared with their Fe(II)–chloride precursors.
Each carbonyl derivative also displays an unusually short
Fe—C bond length (1.680(4) Å for 4[BPh4], 1.709(2) Å for
5[BPh4]) because of the presence of very strong π back-
bonding from the iron(I) centers into the π* orbitals of the
coordinated CO ligand. The strength of this interaction is
also reflected by the observation of very low ν(CO) stretch-
ing frequencies (1881 cm–1 for 4[BPh4], 1880 cm–1 for
5[BPh4]; THF/KBr). For comparison, Parkin’s four-
coordinate iron(I) monocarbonyl [PhTpt-Bu]Fe(CO) exhibits
a longer Fe—C bond length of 1.789(3) Å, and a ν(CO)
value of 1907 cm–1 (22). Despite their cationic nature, the π-
backbonding ability of 4[BPh4] and 5[BPh4] is appreciably
stronger than in Parkin’s tetrahedral derivative, presumably
resulting from the presence of a fifth donor group.
It is interesting that the local geometry of sulfur-supported
3[PF6] is better described as trigonal bipyramidal. It displays
a much shorter Fe–N interaction (2.378(3) Å), which in turn
places the Fe center closer to the plane created by its three
sulfur donor atoms. The trigonal bipyramidal geometry of
3[PF6] gives rise to an average S-Fe-Cl bond angle of 99.7°.
For comparison, the average P-Fe-Cl angles of 1[PF6] and
2[BPh4] are 106.7° and 108.5°, respectively, closer to that
expected for an idealized tetrahedral geometry (109.5°). The
fact that complex 3[PF6] adopts a higher coordination num-
ber is perhaps reflective of the decreased electron-releasing
character of sulfur relative to phosphorus, resulting in a
more electron-deficient iron center that draws the additional
N-atom donor inward. The spin state of 3[PF6] (S = 1) also
reflects the change in coordination number. Two unpaired
electrons are expected for an σ-only d-orbital splitting
scheme under a trigonal bipyramidal ligand field.
Electrochemical characterization of 1[PF6] and 2[BPh4]
shows that each system supports a reversible FeII/I redox
process (–1.65 V for 1[PF6] and –1.51 V for 2[BPh4]; inter-
nally referenced to Cp2Fe in THF). Only 1[PF6] affords a
reversible FeIII/II couple (0.54 V). Cationic 2[BPh4] is irre-
versibly oxidized at above ca. 0.5 V. For comparison, we
note that NS3-supported 3[PF6] does not feature any well-
defined redox processes in the potential window scanned, al-
though problematic adsorption of the complex to the glassy
carbon electrode cannot be discounted. Electrochemical data
for 1[PF6] and 2[BPh4] suggests that Fe(I) species should be
accessible, at least for the phosphine-supported complexes.
The cyclic voltammetry of complexes 4[BPh4] and
5[BPh4] was also studied. As shown in Fig. 3, both species
exhibit two fully reversible redox processes that we assign as
FeI/0 (–1.6 V for 4[BPh4] and 5[BPh4]) and FeII/I (–0.3 V for
4[BPh4], –0.2 V for 5[BPh4]) redox events. Bianchini et al.
(23) have explored the cyclic voltammetry of a related iron
system, [(PP3)Fe(CϵCPh)]BPh4, which shows a reversible
FeII/I process but an irreversible FeI/0 event. The neutral π-
acidic CO ligand of 4+ and 5+ is better able to stabilize the
zero-valent state of iron than the anionic phenylacetylide
ligand of [(PP3)Fe(CϵCPh)]BPh4.
While the neutral iron(I) chloride complexes of NPi-Pr and
3
NArPi-Pr have proven synthetically elusive, formal replace-
ment of3the chloride by CO leads to well-defined cationic
iron(I) species. Thus, tetraphenylborate salts of 1+ and 2+
can be reduced by Na/Hg amalgam in THF under a blanket
of CO to provide deep red, crystalline material. Thorough
analysis establishes the products to be the cationic, mono-
carbonyl derivatives [Fe(NPi-Pr3)CO]BPh4 (4[BPh4]) and
[Fe(NArPi-Pr3)CO]BPh4 (5[BPh4]), each of which is isolated
Because complexes 4[BPh4] and 5[BPh4] represent rather
© 2005 NRC Canada

336
Can. J. Chem. Vol. 83, 2005
Fig. 2. Displacement ellipsoids (50%) of 4[BPh4], 5[BPh4], and 6[PF6]. Hydrogen atoms and counteranions have been removed for
clarity. Selected bond lengths (Å) and angles(°) for 4[BPh4]: Fe—C 1.680(4), Fe—N 2.150(2), Fe—P1 2.2939(8), Fe—P2 2.2379(8),
Fe—P3 2.2851(8), C-Fe-N 177.43(11). For 5[BPh4]: Fe—C 1.709(3), Fe—N 2.164(2), Fe—P1 2.3091(7), Fe—P2 2.2936(7), Fe—P3
2.2722(7), C-Fe-N 177.6(1). For 6[PF6]: Fe—N2 1.804(3), Fe—N1 2.142(3), Fe—H 1.53(4), Fe—P1 2.245(1), Fe—P2 2.250(1), Fe—P3
2.286(1), N1-Fe-N2 176.4(1).
Fig. 3. Cyclic voltammograms of 5[BPh4] and 4[BPh4] recorded
at 100 mV/s in THF vs. Ag/AgNO3.
Fig. 4. EPR spectrum of 4[BPh4] (5.4 K, ν = 9.475 GHz) re-
corded in a frozen glass of 1-methyltetrahydrofuran. (A) Experi-
mental spectrum, (B) simulated spectrum: g1 = 2.302, a1(P)
=
30 G; g2 = 2.0365, a2(P) = 15 G; g3 = 2.0075, a3(P) = 28 G.
unusual examples of mononuclear iron(I) carbonyls it was
prudent to undertake their magnetic characterization. Solu-
tion moments for 4[BPh4] and 5[BPh4] (24, d8-THF) were
measured to be 2.05 µB and 1.96 µB, respectively, implying a
doublet ground state for each species. We also collected a
glassy 1-methyltetrahydrofuran EPR spectrum of 4[BPh4] at
5.4 K (Fig. 4A). The observed signal, which is rhombic with
three components at g1 = 2.3020, g2 = 2.0365, and g3 =
2.0075, shows well-resolved superhyperfine coupling to
the phosphorus nuclei of the ligand (3 × P, Ip = 1/2). The
spectrum can be satisfactorily simulated as an S = 1/2 spin
system by inclusion of the superhyperfine interactions
(Fig. 4B). The three-component, rhombic signal observed is
qualitatively comparable to that reported by Bianchini et al.
(23) for the iron(I) derivative (PP3)Fe(CϵCPh). However,
Bianchini notes the absence of superhyperfine interactions
between the unpaired electron and the phosphorous nuclei of
PP3 as a result of a spin-spin exchange narrowing processes
occurring at sufficiently fast rates. The presence of resolv-
able superhyperfine interactions in the spectrum of 4[BPh4]
likely benefits from the acquisition of the spectrum in the
glassy phase.
diamagnetic [Fe(NPi-Pr3)(N2)(H)]PF6 (6[PF6]) in good yield.
Complex 6[PF6] has been thoroughly characterized and its
X-ray crystal structure is shown in Fig. 2. The spectroscopic
and structural data for 6[PF6] compare favorably with those
reported for [(NPPh3)Fe(H)(N2)][BPh4] (16). The ν(N2) vi-
bration is observed at only slightly lower energy for 6[PF6]
(2087 vs. 2090 cm–1 in THF/KBr), and the Fe—NN2 bond
distances are indistinguishable (1.81(1) Å). The ν(Fe-H) vi-
bration for 6[PF6], which has been confirmed by preparation
of a deuterated sample, is shifted to a much lower frequency
than for [(NPPh3)Fe(H)(N2)][BPh4] (1865 vs. 1913 cm–1),
likely reflecting the presence of an isopropyl-substituted
rather than a phenyl-substituted phosphine donor trans to the
hydride.
In conclusion, we have attempted to motivate renewed
interest in the small molecule coordination chemistry of
five-coordinate iron systems supported by ligands that allow
access to low-valent states. Understanding potential reactiv-
ity patterns for local iron sites of the cofactor of nitrogenase
provides one such impetus. In this regard, we have expanded
the versatile NP3 ligand set popularized by Sacconi and
co-workers (15) to include the bulkier, more electron-
releasing NArPi-Pr and NPi-Pr derivatives that feature
The reduction of the cationic complexes 1, 2, and 3 under
dinitrogen is a topic of ongoing study. For the present con-
tribution, we note that N2 uptake can be achieved by the
(NPi-Pr3)Fe system by slight modification of the route used
for the original preparation of [(NPPh3)Fe(H)(N2)][BPh4]
3
3
isopropyl-substituted phosphine donors. The iron coordina-
tion chemistry of these new generation polyphosphine lig-
ands is well-defined, and iron(I) carbonyl and iron(II)
(NPPh = N(CH2CH2PPh2)3) (15a, 16). Addition of a THF
3
solution of Li[BEt3H] to 1[PF6] under N2 generates yellow,
© 2005 NRC Canada

MacBeth et al.
337
dinitrogen adducts have been described. A key feature of
these tetradentate ligands concerns the flexibility of the
ligand. The apical Fe–N donor interaction reacts to the na-
ture of the ligand in the fifth binding site: it contracts for π-
acidic CO (Fe(I) relative to chloride (Fe(II)). The related but
measurements were recorded in a glovebox under a dini-
trogen atmosphere using a BAS CV 100 W (Bioanalytical
Systems Inc., West Lafayette, Indiana). A glassy carbon
working electrode, a platinum wire auxiliary electrode, and a
Ag/AgNO3 nonaqueous reference electrode were used in all
studies. All measurements were carried out in THF solutions
of electrolyte (0.32 mol/L tetra-n-butylammonium hexa-
fluorophosphate) and analyte. All electrochemical measure-
ments are referenced to Fc/Fc+ as an internal standard.
X-band EPR measurements were recorded using a Bruker
EMX spectrometer (controlled by Bruker Win EPR software
version 3.0) equipped with a rectangular cavity working in
the TE102 mode. Low-temperature measurements were con-
ducted with an Oxford continuous-flow helium cryostat. Ac-
curate frequency values were recorded from a frequency
counter on the microwave bridge. Solution spectra were ac-
quired in 1-methyltetrahydrofuran. Samples were prepared
in a glovebox under dinitrogen in a quartz EPR tube equipped
with a ground glass joint.
Crystallographic data for complexes 1–6 appear in Ta-
bles 1 and 2. X-ray diffraction studies were carried out at the
Beckman Institute Crystallographic Facility on a Bruker
Smart 1000 CCD diffractometer. Crystals were mounted on
a glass fiber with Paratone-N oil. Crystallographic data were
collected on a Bruker P4 diffractometer (0.710 73 Å MoKα)
with a CCD area detector. Data were collected using the
Bruker SMART program, collecting scans at five settings.
Data reduction was performed using Bruker SAINT v6.2.
Structure solution and structure refinement were performed
using SHELXS-97 (Sheldrick, 1990) and SHELXL-97
(Sheldrick, 1997).
less electron-releasing NSt-Bu ligand supports a well-
3
defined, cationic (NS3)FeII–Cl species whose local geometry
and donor ligands are interesting given the local geometry of
the central iron sites of the MoFe-cofactor of nitrogenase. A
difference in spin state is observed when comparing the
(NSt-Bu3)FeII–Cl complex 3[PF6] (S = 1) and its (NP3)FeII–Cl
analogues (S = 2). This difference results from a severe con-
traction of the apical Fe–N linkage to lend a trigonal
bipyramidal rather than a tetrahedral geometry for the diva-
lent iron center in 3[PF6]. Future work will examine the in-
stallation of nitrogenous donor ligands in the fifth binding
site of these scaffolds.3
Experimental section
All manipulations were carried out using standard Schlenk
or glovebox techniques under a nitrogen atmosphere. All
solvents were deoxygenated and dried by sparging with N2
gas and passage through an activated alumina column. Non-
halogenated solvents were tested with a standard purple
solution of sodium benzophenone ketyl in tetrahydrofuran to
ensure dioxygen and water removal. Deuterated solvents
were purchased from Cambridge Isotopes Laboratories, Inc.
and were degassed and stored over activated 3 Å molecular
sieves prior to use. All other reagents and solvents were pur-
chased from commercial vendors and used without further
purification unless otherwise noted. NSt-Bu (14), LiP-i-Pr2
3
Refinement of F2 against all reflections
(25), and N(CH2CH2Cl)3 (26) were prepared according to
literature procedures. CAUTION: N(CH2CH2Cl)3 (a mustard
agent) and thallium reagents are toxic and should be han-
dled with extreme care.
The weighted R factor (wR) and goodness of fit (S) are
based on F2, conventional R factors (R) are based on F, with
F set to zero for negative F2. The threshold expression of
F2 > 2σ(F2) is used only for calculating R factors(gt), etc.,
and is not relevant to the choice of reflections for refine-
ment. R factors based on F2 are statistically about twice as
large as those based on F, and R factors based on all data
will be even larger.
All esds (except the esd in the dihedral angle between two
l.s. planes) are estimated using the full covariance matrix.
The cell esds are taken into account individually in the esti-
mation of esds in distances, angles, and torsion angles; cor-
relations between esds in cell parameters are only used when
they are defined by crystal symmetry. An approximate (iso-
tropic) treatment of cell esds is used for estimating esds in-
volving l.
Elemental analyses were performed by Desert Analytics,
Tucson, Arizona. A Varian Mercury-300 spectrometer was
1
used to record H and 31P NMR spectra at room tempera-
ture. 1H and 13C NMR chemical shifts were referenced to re-
sidual solvent. 31P NMR are reported relative to an external
standard of 85% H3PO4 (0 ppm). Abbreviations for reported
signal multiplicites are as follows: singlet (s), doublet (d),
triplet (t), quartet (q), multiplet (m), broad (br). IR spectra
were recorded on a Bio-Rad Excalibur FTS 3000 spectrome-
ter controlled by Win-IR Pro software using a KBr solution
cell. MS data were obtained by injection of a hydrocarbon
solution onto a Hewlett-Packard 1100MSD mass spectrome-
ter (ES+) or an Agilent 5973 mass selective detector (EI).
UV–vis measurements were recorded on a Varian Cary 50
Bio spectrophotometer controlled by Cary WinUV Software.
All measurements were recorded using a quartz cell fitted
with a Teflon® cap.
Synthesis of NPi-Pr
3
In a 100 mL flask, tris(2-chloroethyl)amine (1.72 g,
8.41 mmol) was dissolved in THF (100 mL) and cooled to
–78 °C, while stirring, for 30 min. Diisopropyllithiophos-
phine (3.34 g, 26.9 mmol) was dissolved in THF (50 mL)
Room temperature solution magnetic susceptibilities were
determined using the method of Evans (24). Electrochemical
3 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml for information on ordering electronically).
CCDC 261768–261770 and 240830–240832 contain the crystallographic data for this manuscript. These data can be obtained, free of
charge, via ww.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

338
Can. J. Chem. Vol. 83, 2005
Table 1. Crystallographic data and detail of refinement for 1[PF6], 2[BPh4], and 3[PF6].
Crystal
[Fe(NPi-Pr3)Cl]PF6
C24H54NP4ClF6Fe
[Fe(NArPi-Pr3)Cl]BPh4
C63H80BClFeNP3
[Fe(NSt-Bu3)Cl]PF6
C18H39NPS3ClF6Fe
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
685.86
Cubic
P213
1046.30
Triclinic
P-1 (No. 2)
10.992(9)
14.940(12)
17.893(15)
83.95(3)
79.03(3)
89.11(3)
2 869(4)
2
601.95
Monoclinic
P21/n
16.7388(15)
12.0505(11)
27.017(3)
14.8923(8)
α (°)
β (°)
101.593(2)
γ (°)
V (Å3)
3 302.8(3)
4
1.379
5 338.4(8)
8
1.498
Z
Dcalcd (Mg/m3)
1.211
Radiation (MoKα)
T (K)
λ = 0.710 73 Å
100(2)
λ = 0.710 73 Å
100(2)
λ = 0.710 73 Å
100(2)
θ Range (°)
2.36–29.47
43 333
3 511
116
0.045, 0.072
1.37–34.70
70 110
21 858
1.86–31.49
52 733
15 469
Total reflections
Unique reflections
Parameters
646
605
R1, wR2 (I > 2σ(I))a
0.0452, 0.0836
0.0595, 0.0977
Goodness-of-fit on F2
2.594
1.236
.
1.460
aR1 = Σ||Fo| – |Fc|| / Σ|Fo|; wR2 = {Σ[w(Fo – Fc2)2] / Σ[w(Fo )2]}1/2
2
2
Table 2. Crystallographic data and detail of refinement for 4[BPh4], 5[BPh4], and 6[PF6].
Crystal
[Fe(NPi-Pr3)CO]BPh4
C48H74BNOP3Fe
[Fe(NArPi-Pr3)CO]BPh4·THF
C64H80BFeNOP3·C4H8O
[Fe(NPi-Pr3)N2(H)]PF6
C24H55F6FeN3P4
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
852.66
Monoclinic
P21/c
17.4370(9)
11.3322(6)
24.3169(13)
1110.96
679.44
Triclinic
P-1
Monoclinic
P 1 21/c 1 (No. 14)
11.1069(6)
22.6213(13)
23.9289(14)
9.9335(9)
11.2697(10)
14.8239(14)
99.275(2)
97.251(2)
95.764(2)
1 612.3(3)
2
α (°)
β (°)
104.5680(10)°
91.349(2)
γ (°)
V (Å3)
4 650.5(4)
4
1.218
6 010.5(6)
4
1.228
Z
Dcalcd (Mg/m3)
1.400
Radiation (MoKα, Å)
T (K)
λ = 0.710 73
100(2)
λ = 0.710 73
100(2)
λ = 0.710 73
100(2)
θ Range (°)
1.73–28.43
61 863
11 004
1.70–33.81
102 345
21 841
1.9–32.7
52 733
8 000
Total reflections
Unique reflections
Parameters
517
700
378
R1, wR2 (I > 2σ(I))a
0.0626, 0.0988
0.0678, 0.1397
0.0604, 0.1519
Goodness-of-fit on F2
2.206
1.006
1.09
2
2
aR1 = Σ||Fo| – |Fc|| / Σ|Fo|; wR2 = {Σ[w(Fo – Fc2)2] / Σ[w(Fo )2]}1/2
.
and added dropwise to the stirring, cold solution of tris(2-
chloroethyl)amine over the course of 30 min. After addition
of the diisopropyllithiophosphine was complete, the reaction
was slowly warmed to room temperature and stirred for 3 h.
All volatiles were removed in vacuo to yield a pale yellow
oil and white precipitate. The reaction mixture was then
stirred in diethylether (30 mL) for 20 min. The resulting
mixture was then filtered through a 60 mL, medium poros-
ity, scintered glass frit that was layered with 1 inch (1 inch =
25.4 mm) of Celite® and 1 inch (1 inch = 25.4 mm) of silica
gel. The white solids collected on the frit where then washed
with two 20 mL portions of petroleum ether. All volatiles
© 2005 NRC Canada

MacBeth et al.
339
were removed from the filtrate to yield NPi-Pr as a very vis-
cous, pale yellow oil (3.48 g, 92%). H NMR (300 MHz,
Synthesis of [Fe(NArPi-Pr3)Cl]BPh4 (2[BPh4])
A 50 mL reaction vessel was charged with NArPi-Pr
3
1
3
C6D6): 2.87 (dt, 5.4 Hz, 6H), 1.5–1.75 (m, 12H) 1.05 (m,
36H). 13C NMR (75 MHz, C6D6): 53.2 (d, 29.5 Hz), 24.1 (d,
14.3 Hz), 20.6 (t, 16.6 Hz), 19.4 (d, 9.7 Hz). 31P NMR
(121.4 MHz, C6D6): 0.78 (br s). FAB-MS (m/z): 450.3526
(1.0 g, 1.57 mmol), FeCl2 (0.20 g, 1.57 mmol), sodium
tetraphenylborate (0.540 g, 1.57 mmol), and THF (25 mL)
and sealed with a Teflon® stopper. The reaction was heated
at 65 °C for 12 h, cooled, and the solvent was removed in
vacuo. The resultant yellow solids were washed with petro-
leum ether (2 × 20 mL) and dried under reduced pressure.
The product was extracted into CH2Cl2, filtered through a
plug of Celite® and layered petroleum ether, which afforded
analytically pure yellow crystals upon standing (1.60 g,
97%). Crystals suitable for X-ray diffraction were obtained
by layering an CH2Cl2 solution with diethylether. µeff (ace-
tone-d6, 298 K): 5.3 µB. 1H NMR (300 MHz, CD2Cl2): 140.9
(br s), 30.2 (br s), 19.4 (br s), 15.3 (s), 15.0 (s), 9.76 (s),
7.32 (s), 7.03 (t, 7.8 Hz), 6.87 (m), 5.03 (s), –3.05 (br s),
–16.1 (br s), –20.7 (br s). Anal. calcd. for C63H80BClFeNP3:
C 72.32, H 7.71, N 1.34; found: C 72.08, H 7.53, N 1.46.
(NPi-Pr + H+).
3
Synthesis of NArPi-Pr
3
In a 100 mL flask, tris(2-bromo-p-tolyl)amine (3.0 g,
5.73 mmol) was dissolved in ether (50 mL) and cooled to
–78 °C. Solid t-BuLi (2.2 g, 34.4 mmol) was added portion-
wise over 15 min. The reaction was warmed to room tem-
perature after 30 min. Following 3 h of stirring, the reaction
mixture was yellow in color with LiBr precipitate. The reac-
tion vessel was cooled to –78 °C and chlorodiisopropyl-
phosphine (2.8 g, 18.0 mmol) was added over 5 min. The
mixture was stirred at ambient temperature for 18 h at which
time it was filtered through a plug of silica gel and the sol-
vent removed in vacuo. The resulting tacky solid was dis-
solved in CH2Cl2 (50 mL), filtered through Celite®, and
dried to a solid. Trituration of the filtrate in methanol
(150 mL) afforded an analytically pure fine white solid,
which was collected on a fritted glass funnel and dried thor-
oughly under reduced pressure (2.4 g, 66%). 1H NMR
(300 MHz, CDCl3): 7.25 (s), 7.16 (br s), 6.95 (s), 6.93 (s),
6.87 (dd, 2.0 Hz, 8.0H), 6.48 (dd, 4.0 Hz, 8.0H), 2.54 (m),
2.30 (s), 2.28 (s), 1.51 (d sept, 2.0 Hz, 7.0H), 1.21 (q,
7.5 Hz, 14.5H), 1.13 (dd, 7.0 Hz, 10.0H), 1.00 (br s), 0.91
(dd, 7.5 Hz, 10.0H), 0.825 (br s), 0.41 (7.5 Hz, 14H). 13C
NMR (126 MHz, CDCl3): 155.1 (m), 154.5 (m), 135.0 (m),
134.7 (s), 133.9 (br s), 131.9 (s), 129.9 (br s), 129.3 (s),
127.7 (s), 23.9 (m), 23.4 (m), 21.9 (m), 21.1 (s), 21.0 (s),
19.2 (m). 31P NMR (121 MHz, CDCl3): –2.87 (br m),
–11.28 (s). Anal. calcd. for C39H60NP3: C 73.67, H 9.51, N
2.20; found: C 73.57, H 9.25, N 2.05.
Synthesis of [Fe(NSt-Bu3)Cl]PF6 (3[PF6])
NSt-Bu (0.133 g, 0.364 mmol) was stirred in 5.0 mL of
3
THF and solid FeCl2 (0.0457 g, 0.364 mmol) was added. A
THF solution (4.0 mL) of TlPF6 (0.127 g, 0.364 mmol) was
added to the reaction dropwise over 20 min. A white precipi-
tate began to form immediately. The reaction mixture was
stirred for 3 h, concentrated to dryness, dissolved in CH2Cl2
(3.0 mL), and filtered through a pad of Celite®. Layering the
resulting colorless filtrate with Et2O produced colorless
1
crystals (0.136 g, 62%) of the desired product. H NMR
(300 MHz, d8 -THF): 20.5 (b), 10.9 (b), 2.6 (b). 31P NMR
(121.4 MHz, THF-d8): –143.5 (septet, 711 Hz, PF6). Anal.
calcd. for C18H39ClF6FeNP: C 35.91, H 6.16, N 2.33; found:
C 35.23, H 6.16, N 2.46.
Synthesis of [Fe(NPi-Pr3)(CO)]BPh4 (4[BPh4])
A 0.23 weight% amalgam was prepared in a 25.0 mL
Shlenk flask by dissolving solid Na (5.1 mg, 0.22 mmol)
in mercury (2.21 g). The flask was then charged with
[Fe(NPi-Pr3)Cl]BPh4 (0.190 g, 0.221 mmol) dissolved in THF
(5.0 mL) and a stir bar. The headspace of the flask was
quickly evacuated and refilled three times with 1 atm of
CO(g) (1 atm = 101.325 kPa). After the third cycle, the flask
was sealed under 1 atm of CO(g) (1 atm = 101.325 kPa) and
stirring commenced. The reaction mixture changed immedi-
ately from colorless to bright yellow. The reaction mixture
was stirred for 12 h over which the color changed again
from golden yellow to dark red. All volatiles were removed
in vacuo and the resulting red-brown solid was extracted into
THF (3.0 mL) and filtered through a pad of Celite® to re-
move insoluble material. The resulting red solution was lay-
ered with Et2O and stored at –35 °C overnight. The next day,
large red crystals of the product were isolated on a medium
porosity frit and dried. IR (THF, KBr, cm–1) ν(CO):
Synthesis of [Fe(NPi-Pr3)Cl]PF6 (1[PF6])
Solid FeCl2 (0.335 g, 2.65 mmol) was added to a stirring
solution of NPi-Pr (1.19 g, 2.56 mmol) dissolved in CH2Cl2
3
(2.0 mL). The slurry was stirred for 10 min and TlPF6 was
added dropwise as a THF (4.0 mL) solution over 15 min. A
white precipitate began to form immediately. The reaction
was stirred for 4 h and then filtered through Celite®. The pad
of Celite® was washed with two portions of CH2Cl2 (5.0 mL).
All volatiles were then removed in vacuo to yield a white
solid. The white solid was extracted into CH2Cl2 (5.0 mL)
and again filtered through a pad of Celite®. The colorless so-
lution was then layered with Et2O and stored at –35 °C for
2 days to afford colorless crystals of the desired product
1
(1.48 g, 81.3%). µeff (CD2Cl2, 298 K): 5.04 µB. H NMR
(300 MHz, CD2Cl2): 27.5 (b), 3.2 (b), –8.5 (b). 31P NMR
(121.4 MHz, CD2Cl2): –141.5 (septet, 711 Hz, PF6). Anal
calcd. for C24H54ClF6FeNP4: C 42.03, H 7.94, N 2.04;
found: C 42.13, H 8.00, N 2.05.
1
1881. µeff (THF-d8, 298 K): 2.15 µB. H NMR (THF-d8):
19.1 (b), 7.65 (b), 7.2 (b), 7.0 (b), 3.6 (b), –0.47 (b). Anal.
calcd. for C49H75BFeNOP4: C 68.94, H 8.86, N 1.64; found:
C 68.96, H 8.62, N 1.66.
Synthesis of [Fe(NPi-Pr3)Cl]BPh4 (1[BPh4])
The tetraphenylborate salt was synthesized in a similar
maner as 1, with the following exceptions: sodium tetra-
phenylborate was used in place of TlPF6, and methanol was
used in place of THF.
Synthesis of [Fe(NArPi-Pr3)(CO)]BPh4 (5[BPh4])
In a 50 mL Schlenk flask, [(NArPi-Pr3)FeCl][BPh4]
(60 mg, 0.057 mmol) was dissolved in THF (50 mL) and so-
© 2005 NRC Canada

340
Can. J. Chem. Vol. 83, 2005
4. (a) V. Vrajmasu, E. Münch, and E.L. Bominaar. Inorg. Chem.
42, 5974 (2003); (b) H. Uwe, R. Ahlrichs, and D. Coucou-
vanis. J. Am. Chem. Soc. 126, 2588 (2004).
dium metal (1.3 mg, 0.057 mmol) amalgamated in mercury
(1.84 g, 9.17 mmol) was added. The solution was sparged
with CO(g) for 10 min and then stirred vigorously for 18 h
during which time the solution became green in color. The
solvent was removed under reduced pressure, and the resul-
tant green solids were extracted with THF (5 mL) and then
filtered. The green filtrate was layered with petroleum ether
(15 mL) and the desired product precipitated as analytically
pure red crystals (49 mg, 83%). µeff (THF-d8, 298 K):
2.1 µB. IR (THF, KBr, cm–1) ν(CO): 1880. Anal. calcd. for
C63H80BFeNOP3: C 73.99, H 7.76, N 1.35; found: C 73.76,
H 7.70, N 1.47.
5. (a) D. Coucouvanis, J. Han, and N. Moon. J. Am. Chem. Soc.
124, 216 (2002); (b) J. Vela, S. Stoian, C.J. Flashenriem, E.
Münck, and P.L. Holland. J. Am. Chem. Soc. 126, 4522 (2004).
6. (a) J. Chatt, J.R. Dilworth, and R.L. Richards. 78, 589 (1978);
(b) F. Barriere. Coord. Chem. Rev. 236, 71 (2003).
7. J. Chatt, J.R. Dilworth, and R.L. Richards. Chem. Rev. 78, 589
(1978).
8. C.J. Pickett. J. Biol. Inorg. Chem. 1, 601 (1996).
9. D.V. Yandulov and R.R. Schrock. Science, 301, 76 (2003).
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Santos, D.R. Dean, L.C. Seefeldt, and B.M. Hoffman. J. Am.
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Lukat-Rodgers, K.R. Rodgers, and P.L. Holland. J. Am. Chem.
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Synthesis of [Fe(NPi-Pr3)(N2)(H)]PF6 (6[PF6])
Solid 1[PF6] (0.222 g, 0.324 mmol) was dissolved in THF
(10 mL) and cooled to –78 °C. To this solution a solution of
Li(HBEt3) (1.0 mol/L in ether) (356 µL, 0.356 mmol) was
added dropwise via syringe. The reaction mixture changed
slowly from colorless to pale yellow over 1 h. The reaction
mixture was warmed to room temperature and filtered
through a pad of Celite®. The yellow filtrate was dried in
vacuo to yield a yellow-brown solid. The solid was extracted
into THF (4.0 mL) and filtered through a pad of Celite®.
The filtrate was layered with Et2O (4.0 mL) and stored at
–35 °C for 3 days to afford light yellow crystals of the de-
sired product (0.142 g, 66%). IR (THF, KBr, cm–1): ν(N2)
2090, ν(FeH) 1865. 1H NMR (300 MHz, THF-d8): –8.77 (dt,
JPH = 74.1 Hz (cis), JPH = 38.1 Hz (trans)), 1.4–1.6 (m,
36H), 1.8 (t, 2H), 1.94 (m, 4H), 1.98 (m, 2H), 2.15–2.25 (m,
4H), 2.32 (m, 2H), 2.48 (m, 2H) 2.70 (m, 2H). 31P NMR
(121.4 MHz, THF-d8): 67.6 (d, 2P Jpp= 41.4 Hz), 62.2 (t,
1P), –141.5 (septet, 1P, JFP = 711 Hz, PF6). Anal. calcd. for
C24H55F6FeN3P4: C 42.43, H 8.16, N 6.18; found: C 42.82,
H 8.16, N 5.72.
14. R. Morassi and L. Sacconi. Inorg. Synth. 16, 174 (1976).
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and L. Sacconi. Inorg. Chem. 16, 1518 (1977).
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Inorg. Chem. 34, 1295 (1995).
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Chem. Soc. 113, 8485 (1991).
Synthesis of [Fe(NPi-Pr3)(N2)(D)]PF6
Fe(NPi-Pr3)(N2)(D)]PF6 was prepared by following the
synthetic protocol outlined for 6[PF6] and using Li(DBEt3)
in place of Li(HBEt3). The IR (THF, KBr) for this complex
exhibits a ν(FeD) = 1341 cm–1.
20. (a) J.M. Baumeister, R. Alberto, K. Ortner, and O.B. Spingler.
J. Chem. Soc. Dalton Trans. 4143 (2002); (b) E.A. Ambundo,
A.J. Grall, N. Aguera-Vega, L.T. Dressel, T.H. Cooper, M.J.
Heeg, L.A. Ochrymowyc, and D.B. Rorabacher. Inorg. Chem.
38, 4233 (1999).
Acknowledgement
We acknowledge the DOE (Department of Energy)
(PECASE), the NSF (National Science Foundation) (CHE-
0132216), and the Beckman Institute Senior Research Fel-
lows Program (CEM) for financial support. Larry Henling
and Dr. Michael Day provided crystallographic assistance.
21. Long M-N distances are often observed for [(NP3)M(II)–X]+
complexes. See for example: (a) C.A. Ghilardi, C. Mealli, S.
Midollini, and A. Orlandini. Inorg. Chem. 24, 164 (1985);
(b) M. Di Vaira and A.B. Orlandini. Inorg. Chem. 12, 1292
(1973).
22. J.L. Kisko, T. Hascall, and G. Parkin. J. Am. Chem. Soc. 120,
10 561 (1998).
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