Synthesis, structures, and dearomatization by deprotonation of iron complexes featuring bipyridine-based PNN pincer ligands

Article abstract of DOI:10.1021/ic401432m

The synthesis and characterization of new iron pincer complexes bearing bipyridine-based PNN ligands is reported. Three phosphine-substituted pincer ligands, namely, the known ^tBu-PNN (6-((di-tert-butylphosphino) methyl)-2,2′-bipyridine) and the two new ⁱPr-PNN (6-((di-iso-propylphosphino)methyl)-2,2′-bipyridine) and Ph-PNN (6-((diphenylphosphino)methyl)-2,2′-bipyridine) ligands were synthesized and studied in ligation reactions with iron(II) chloride and bromide. These reactions lead to the formation of two types of complexes: mono-chelated neutral complexes of the type [(R-PNN)Fe(X)₂] and bis-chelated dicationic complexes of the type [(R-PNN)₂Fe]²⁺. The complexes [(R-PNN)Fe(X)₂] (1: R = ^tBu, X = Cl, 2: R = ^tBu, X = Br, 3: R = ⁱPr, X = Cl, and 4: R = ⁱPr, X = Br) are readily prepared from reactions of FeX₂ with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility measurements show that these complexes have a high-spin ground state (S = 2) at room temperature. Employing a 2-fold or higher excess of ⁱPr-PNN, diamagnetic hexacoordinated dicationic complexes of the type [( ⁱPr-PNN)₂Fe](X)₂ (5: X = Cl, and 6: X = Br) are formed. The reactions of Ph-PNN with FeX₂ in a 1:1 ratio lead to similar complexes of the type [(Ph-PNN)₂Fe](FeX₄) (7: X = Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer from the Fe^II centers to the neutral bipyridine unit based on the determined bond lengths. Density functional theory (DFT) calculations were performed to compare the relative energies of the mono-and bis-chelated complexes. The doubly deprotonated complexes [(R-PNN*)₂Fe] (9: R = ⁱPr, and 10: R = Ph) were synthesized by reactions of the dicationic complexes 6 and 8 with KO ^tBu. The dearomatized nature of the central pyridine of the pincer ligand was established by X-ray diffraction analysis of single crystals of 10. Reactivity studies show that 9 and 10 have a slightly different behavior in protonation reactions.

Full text of DOI:10.1021/ic401432m

Article
pubs.acs.org/IC
Synthesis, Structures, and Dearomatization by Deprotonation of Iron
Complexes Featuring Bipyridine-based PNN Pincer Ligands
Thomas Zell,† Robert Langer,† Mark A. Iron,‡ Leonid Konstantinovski,‡ Linda J. W. Shimon,‡
Yael Diskin-Posner,‡ Gregory Leitus,‡ Ekambaram Balaraman,† Yehoshoa Ben-David,†
and David Milstein*,†
†Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, 76100 Rehovot,
Israel
S
* Supporting Information
ABSTRACT: The synthesis and characterization of new iron
pincer complexes bearing bipyridine-based PNN ligands is
reported. Three phosphine-substituted pincer ligands, namely,
the known tBu-PNN (6-((di-tert-butylphosphino)methyl)-
i
2,2′-bipyridine) and the two new Pr-PNN (6-((di-iso-
propylphosphino)methyl)-2,2′-bipyridine) and Ph-PNN (6-
((diphenylphosphino)methyl)-2,2′-bipyridine) ligands were
synthesized and studied in ligation reactions with iron(II)
chloride and bromide. These reactions lead to the formation of
two types of complexes: mono-chelated neutral complexes of the type [(R-PNN)Fe(X)2] and bis-chelated dicationic complexes of
the type [(R-PNN)2Fe]2+. The complexes [(R-PNN)Fe(X)2] (1: R = tBu, X = Cl, 2: R = tBu, X = Br, 3: R = iPr, X = Cl, and 4: R
i
= Pr, X = Br) are readily prepared from reactions of FeX2 with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility
measurements show that these complexes have a high-spin ground state (S = 2) at room temperature. Employing a 2-fold or
higher excess of iPr-PNN, diamagnetic hexacoordinated dicationic complexes of the type [(iPr-PNN)2Fe](X)2 (5: X = Cl, and 6:
X = Br) are formed. The reactions of Ph-PNN with FeX2 in a 1:1 ratio lead to similar complexes of the type [(Ph-
PNN)2Fe](FeX4) (7: X = Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer
from the FeII centers to the neutral bipyridine unit based on the determined bond lengths. Density functional theory (DFT)
calculations were performed to compare the relative energies of the mono- and bis-chelated complexes. The doubly deprotonated
complexes [(R-PNN*)2Fe] (9: R = iPr, and 10: R = Ph) were synthesized by reactions of the dicationic complexes 6 and 8 with
KOtBu. The dearomatized nature of the central pyridine of the pincer ligand was established by X-ray diffraction analysis of single
crystals of 10. Reactivity studies show that 9 and 10 have a slightly different behavior in protonation reactions.
under mild conditions. Furthermore, we found that the pincer
INTRODUCTION
■
t
complex [(tBu-PNP)Fe(H)2(CO)] (C, Scheme 1, Bu-PNP =
Iron pincer complexes have attracted much interest in recent
years. Since the initial reports on the application of bis(imino)-
pyridine iron complexes as catalysts in olefin oligomerization and
polymerization reactions by Brookhart1 and Gibson2 in the late
1990s, iron pincer complexes have been extensively studied in
these reactions.3 Chirik and co-workers later showed that
bis(imino)pyridine-iron complexes are efficient catalysts for
various reactions such as hydrogenation of olefins and alkynes,4
and hydrosilylation of olefins, alkynes, aldehydes, and
ketones.4a,5
Besides the use of iron pincer complexes as homogeneous
catalysts for various types of reactions,3h this class of complexes
was used for the specific binding6 and stoichiometric activation of
substrates,7 and it has been discussed in the context of molecular
sensors and switches.8
2,6-bis(di-tert-butylphosphinomethyl)pyridine) is an efficient
catalyst for the hydrogenation of carbon dioxide to sodium
formate in aqueous sodium hydroxide solutions at remarkably
low pressures and temperatures.11 Further investigations on the
application of this complex as catalyst for the formal reverse
reaction revealed a very high activity in the decomposition of
formic acid to CO2 and H2 in the presence of trialkylamines.12
Over the past few years, we have explored the dearomatized,
bipyridine-based pincer complex [(tBu-PNN*)Ru(H)(CO)]
t
(D, Bu-PNN = 6-(di-tert-butylphosphinomethyl)-2,2′-bipyri-
dine Scheme 1, the asterisk denotes a dearomatized pincer
ligand) as a catalyst for various environmentally benign catalytic
processes. D has been used as a catalyst for the hydrogenation of
amides to the corresponding alcohols and amines,13 the
hydrogenation of urea derivatives to amines and methanol,14
the hydrogenation of organic carbonates, carbamates, and
Our group recently reported the synthesis of the PNP-based
iron pincer complex [(iPr-PNP)Fe(H)(CO)(Br)]9 (A, Scheme
i
1, Pr-PNP = 2,6-bis(di-iso-propylphosphinomethyl)pyridine)
and the related [(iPr-PNP)Fe(H)(CO)(η-BH4)]10 (B, Scheme
1), which are efficient catalysts for the hydrogenation of ketones
Received: June 6, 2013
© XXXX American Chemical Society
A
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PNN)Fe(Cl)2] (1) and [(tBu-PNN)Fe(Br)2] (2) as red and
black complexes, respectively (Scheme 3). Elemental analyses of
the compounds are in good agreement with the expected
formulation, and the electrospray ionization-mass spectrometry
(ESI-MS) spectra show characteristic ions, such as the
corresponding complex cations [(tBu-PNN)Fe(X)]+.
Scheme 1. Iron and Ruthenium Pincer Complexes A−D
The 1H NMR spectra of both compounds show nine
paramagnetically shifted resonances, which are tentatively
assigned to the seven resonances of the bipyridyl unit, one
resonance for the CH2 group, and one resonance for the methyl
t
protons of the Bu groups. This pattern indicates a plane of
symmetry through the iron pincer moiety in solution. The signals
appear in the range of −15.76 to 128.43 ppm and −13.02 to
116.50 ppm for complexes 1 and 2, respectively. In the 31P{1H}
NMR spectra, no resonances are observed for these compounds
in the range of −4000 to +4000 ppm. Magnetic measurements in
solution at room temperature (Evans’ method23) gave effective
magnetic moments of μeff = 5.0 (for complex 1) and 5.3 μB (for
complex 2), which are consistent with S = 2 spin ground states.
X-ray diffraction analyses clearly confirm the formation of the
dihalide complexes 1 and 2 (see Figure 1). The molecular
structures of both complexes in the solid state are best described
as strongly distorted trigonal bipyramidal coordination of the
iron atoms, with N1 and P occupying the apical positions. Other
structurally characterized pincer complexes of the type [(PNN)-
Fe(X)2] exhibit similar structures.24 The N1−Fe−P angles of
151.37(5)° (for 1) and 148.57(4)° (for 2) strongly deviate from
the ideal linear geometry. The CH2P arm is bent out of the plane
of the bipyridine−iron unit (N1, N2, Fe): the distances of the P
atoms to this plane are 0.6388(6) and 0.5974(4) Å for complexes
1 and 2, respectively. The Fe−P bond distance of complex 1
(2.5574(7) Å) is somewhat longer than in the case of complex 2
(2.5034(5) Å).
formates to the corresponding alcohols and amines,15 and the
hydrogenation of cyclic diesters to afford 1,2-diols.16 Further-
more, coupling of nitriles with amines to form imines,17 the
cross-esterification reaction of primary alcohols with secondary
alcohols,18 and the transformation of alcohols to carboxylic acids
and dihydrogen using water as an oxygen source are catalyzed by
D.19 Very recently the direct synthesis of pyrroles via
dehydrogenative coupling of β-aminoalcohols with secondary
alcohols,20 as well as selective deuteration reactions of alcohols
employing D2O as deuterium source21 catalyzed by D were
reported.
The extraordinary activity of the bipyridine-based ruthenium
pincer complex D prompted us to investigate the synthesis and
the properties of the corresponding iron complexes. Herein we
report on the ligation of the P-substituted bipyridine-based PNN
ligands (tBu-PNN = 6-((di-tert-butylphosphino)methyl)-2,2′-
bipyridine, iPr-PNN = 6-((di-iso-propylphosphino)methyl)-2,2′-
bipyridine, and Ph-PNN = 6-((diphenylphosphino)methyl)-
2,2′-bipyridine; Scheme 2) with iron(II) chloride and bromide.
During the finalization of our manuscript, a report on the
application of complex 1 (Scheme 3) as precatalyst for the
hydroboration of alkenes with pinacolborane was published by
Huang and co-workers.22
t
As in the cases of Bu-PNN, the complexes [(iPr-PNN)Fe-
(X)2] (X = Cl 3 and X = Br 4, Scheme 4) are obtained by the
i
reactions of Pr-PNN with FeCl2 or FeBr2 in a 1:1 ratio. The
elemental analyses of the red products are in a good agreement
with a 1:1 ratio of the ligand and the iron dihalide. Complexes 3
1
and 4 feature resonances in the paramagnetic region of the H
Scheme 2. Ligands Used in This Paper
NMR spectra. For complex 3, a solution-state magnetic
measurement at ambient temperature is in accordance with an
S = 2 spin state with effective magnetic moment of μeff = 5.3 μB.
Because of its low solubility, the magnetic susceptibility
measurement for 4 was performed in the solid state at 23 °C
using a SQUID magnetometer. The measurement, resulting in a
magnetic moment of μeff = 5.20 μB, again indicates an S = 2 spin
ground state. Characteristic complex ions containing ligand−
iron fragments (e.g., [(iPr-PNN)Fe(X)]+) were detected in the
ESI-MS spectra for both complexes.
According to the X-ray diffraction analysis, the mono-chelated
complex [(iPr-PNN)Fe(Br)2] 4 is a penta-coordinated iron
complex. It is isostructural to complex 2 and features a distorted
trigonal bipyramidal iron center (Figure 2, left). The N1−Fe−P
angle between the axial atoms is 151.72(6)°. The P-arm of the
ligand is not perfectly located in the plane through the
bipyridine−iron unit (N1, N2, Fe), and the distances of the
benzylic carbon and the phosphorus atom from this plane are
0.5277(26) and 0.2113(6) Å, respectively.
Scheme 3. Synthesis of Complexes 1 and 2
Treatment of either FeCl2 or FeBr2 with a 2-fold excess of iPr-
PNN in THF initially results in red solutions, and after one or
two days of stirring at room temperature the formation of purple
precipitates is observed. These solids are insoluble in apolar
solvents but are highly soluble in alcohols and acetonitrile. The
RESULTS AND DISCUSSION
■
The reactions of tBu-PNN with FeCl2 or FeBr2 in tetrahydrofur-
an (THF) lead to formation of the dihalide complexes [(tBu-
B
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Figure 1. ORTEP diagram of the molecular structure of [(tBu-PNN)Fe(Cl)2] (1, left) and [(tBu-PNN)Fe(Br)2] (2, right) (ellipsoids set at the 50%
probability level). The solvent molecules and hydrogen atoms are omitted for clarity. Bond lengths (Å) and angles (deg): Complex 1: Fe−N1 2.177(2),
Fe−N2 2.2019(19), Fe−P 2.5574(7), Fe−Cl1 2.3082(7), Fe−Cl2 2.3534(6), N1−Fe−N2 73.52(7), N1−Fe−P 151.37(5), N1−Fe−Cl1 95.48(6),
N1−Fe−Cl2 89.44(5), N2−Fe−P 78.18(5), N2−Fe−Cl1 142.11(5), N2−Fe−Cl2 108.81(5), P−Fe−Cl1 104.31(2), P−Fe−Cl2 103.87(2), Cl1−Fe−
Cl2 107.18(2). Complex 2: Fe−N1 2.1550(14), Fe−N2 2.2039(13), Fe−P 2.5034(5), Fe−Br1 2.4636(3), Fe−Br2 2.4873(3), N1−Fe−N2 73.97(5),
N1−Fe−P 148.57(4), N1−Fe−Br1 96.19(4), N1−Fe−Br2 94.40(4), N2−Fe−P 77.27(4), N2−Fe−Br1 113.17(4), N2−Fe−Br2 139.34(4), P−Fe−
Br1 106.669(14), P−Fe−Br2 99.236(13), Br1−Fe−Br2 106.664(11).
Scheme 4. Synthesis of Complexes 3−6 (P = PiPr2)
Figure 2. ORTEP diagram of the molecular structure of [(iPr-PNN)Fe(Br)2] (4, left) and [(iPr-PNN)2Fe](Br)2 (6, right) (ellipsoids set at the 50%
probability level). The solvent molecules, hydrogen atoms, and the bromide counterions of 6 are omitted for clarity, while the iso-propyl substituents of 6
are shown as thin lines. Bond lengths (Å) and angles (deg): Complex 4: Fe−N1 2.153(2), Fe−N2 2.2060(19), Fe−P 2.5073(7), Fe−Br1 2.4780(4),
Fe−Br2 2.5014(4), N1−Fe−N2 74.14(7), N1−Fe−P 151.72(6), N1−Fe−Br1 96.36(6), N1−Fe−Br2 91.35(5), N2−Fe−P 78.01(5), N2−Fe−Br1
137.88(5), N2−Fe−Br2 113.64(5), P−Fe−Br1 101.021(18), P−Fe−Br2 104.33(2), Br1−Fe−Br2 107.418(15). Complex 6: Fe−N1 1.995(2), Fe−N2
1.946(2), Fe−N3 1.987(2), Fe−N4 1.940(2), Fe−P1 2.2813(8), Fe−P2 2.2696(8), N1−Fe−N2 80.82(9), N1−Fe−N3 80.63(9), N1−Fe−N4
98.94(9), N1−Fe−P1 163.20(7), N1−Fe−P2 91.45(7), N2−Fe−N3 98.12(9), N2−Fe−N4 179.02(10), N2−Fe−P1 84.24(7), N2−Fe−P2 98.11(7),
N3−Fe−N4 80.90(10), N3−Fe−P1 93.98(7), N3−Fe−P2 160.54(7), N4−Fe−P1 95.87(7), N4−Fe−P2 82.84(7), P1−Fe−P2 98.26(3).
1H, 13C{1H}, and 31P{1H} NMR spectra of the resulting
X-ray diffraction analysis of 6 shows the formation of the
dicationic complex [(iPr-PNN)2Fe]2+ with the iron center
coordinated to two pincer ligands in a distorted octahedral
geometry (Scheme 4 and Figure 2, right). Similar to other
diamagnetic complexes [(iPr-PNN)2Fe](X)2 (5: X = Cl and 6:
X = Br, Scheme 4) are identical. The 31P{1H} NMR spectra have
1
one resonance at δ = 60.6 ppm. All resonances in the H and
examples in the literature,25 both Pr-PNN ligands are
i
13C{1H} NMR spectra were fully assigned by 2D NMR
experiments. These spectra show one set of signals for the
pincer ligand, containing diastereotopic isopropyl groups. ESI-
MS measurements indicate the formation of a bis-chelated
iron(II) complex, and the results of elemental analyses are in a
good agreement with a ligand:iron dihalide ratio of 2:1. In
accordance with low-spin ground states (S = 0), no paramagnetic
shifts were observed in the magnetic measurements (Evans’
method) of both compounds.
coordinated to the Fe2+ center in a meridional fashion. The
trans N−Fe−P angles of the pincer ligands, are 163.20(7) (for
N1−Fe−P1) and 160.54(7)° (for N3−Fe−P2), showing
significant distortion from the ideal octahedral geometry. The
dication [(iPr-PNN)2Fe]2+ 6, featuring an iron(II) center in a
low-spin configuration shows, as expected, significantly shorter
Fe−N and Fe−P bond distances than found in the iron(II) high-
spin complex 4. The Fe−P distances are 2.51 Å and 2.27−2.28 Å
C
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for the mono-chelated and bis-chelated complexes 4 and 6,
respectively. The Fe−N distances in cis-position to the P atom
are 2.21 Å and 1.94−1.95 Å, for 4 and 6, respectively, and the
Fe−N distances in the para-position to the P atom are 2.15 Å and
1.99−2.00 Å
The reactions of the Ph-PNN ligand with FeCl2 and FeBr2 in a
1:1 ratio lead to the formation of intensely orange colored
products [(Ph-PNN)2Fe](FeX4) (7: X = Cl, and 8: X = Br,
Scheme 5). The resulting complexes are insoluble in THF and
CH2Cl2, poorly soluble in MeCN, and highly soluble in MeOH.
The NMR spectra of 7 and 8 are identical and show resonances in
the typical diamagnetic range. The H and 13C{1H} NMR
1
Figure 3. ORTEP diagram of the molecular structure of [(Ph-
PNN)2Fe](FeBr4) (8) (ellipsoids set at the 50% probability level).
The solvent molecules, hydrogen atoms, and the FeBr42− counterion are
omitted for clarity, while the phenyl substituents are shown as thin lines.
Bond lengths (Å) and angles (deg): Fe−N1 1.981(9), Fe−N2 1.936(9),
Fe−N3 1.993(10), Fe−N4 1.952(8), Fe−P1 2.249(3), Fe−P2
2.242(3), N1−Fe−N2 82.0(4), N1−Fe−N3 86.2(4), N1−Fe−N4
98.5(4), N1−Fe−P1 165.9(3), N1−Fe−P2 91.4(3), N2−Fe−N3
93.7(4), N2−Fe−N4 175.0(4), N2−Fe−P1 84.7(3), N2−Fe−P2
99.6(3), N3−Fe−N4 81.4(4), N3−Fe−P1 89.8(3), N3−Fe−P2
166.0(3), N4−Fe−P1 94.3(3), N4−Fe−P2 85.4(3), P1−Fe−P2
95.64(12).
spectra exhibit two sets of resonances for the inequivalent phenyl
substituents, indicating the absence of a plane of symmetry
through the iron pincer moiety. The protons of the methylene
group give rise to two distinct resonances with a geminal
coupling constant of 2JHH = 17.5 Hz and a coupling constant of
2JPH = 8.1 Hz to the phosphorus atom. The phosphorus
resonance appears at δ = 54.4 ppm in the 31P{1H} NMR
spectrum. Solution-state measurements of the magnetic
susceptibility of complexes 7 and 8 at room temperature
(Evans’ method) resulted in values expected for S = 2 complexes.
These magnetic moments of μeff = 5.1 μB for complex 7 and 8,
stem from the high-spin FeX4 anions.26 The EI-MS spectra
2−
While the tert-butyl substituted ligand leads exclusively to
formation of the paramagnetic complexes 1 and 2 (type I,
Scheme 6), reactions of the phenyl substituted ligand lead
exclusively to complexes 7 and 8, which contain the diamagnetic
show signals characteristic for iron(II) fragment ions containing
one and two pincer ligands (e.g., [(Ph-PNN)2Fe]2+).
Scheme 5. Synthesis of Complexes 7 and 8 (P = PPh2)
2−
dication [(Ph-PNN)2Fe]2+ (type II, Scheme 6) with FeX4
counteranions. With the iso-propyl substituted ligand, both types
of complexes are obtained depending on the initial reaction
conditions.
Scheme 6. Mono-Chelated Iron Dihalide Complexes (I) and
a
bis-Chelated Dications (II)
X-ray diffraction analysis unequivocally confirms the for-
mation [(Ph-PNN)2Fe](FeBr4) (8, Figure 3). The dicationic 8 is
isostructural to [(iPr-PNN)2Fe]2+ (6). The iron atom is
coordinated by two pincer ligands in a distorted octahedral
geometry. The trans N−Fe−P angles of 165.9(3)° (for N1−Fe−
P1) and 166.0(3)° (for N3−Fe−P2) are slightly larger than in 6,
while the Fe−N and Fe−P bond distances are almost equal in
both dications.
Bipyridine-based ligands (bpy) are known on occasion to be
redox noninnocent and can exist in three different oxidation
states, namely, as a neutral ligand (bpy0), as a π-radical
monoanion (bpy•−), and as a diamagnetic dianion (bpy2−).27
The one- and two-electron reduction of bipyridine ligands is
accompanied by structural changes within the five-membered
metallacycle involving a distinct stepwise shortening of the C−C
bond connecting the two pyridines. Experimental bond lengths
for (bpy0), (bpy•−), and (bpy2−), are approximately 1.49, 1.43,
and 1.38 Å, respectively. The interpyridine C−C bond distances
of the complexes reported here range from 1.467(4) (for
complex 6) to 1.492(3) Å (for complex 1) and are indicative of a
neutral bipyridine unit in these complexes.
a
X = Cl or Br, and P = PR2.
To gain additional insight, we performed DFT calculations to
compare the relative energies of the reactions leading to the
complexes type I and II (Scheme 6). Geometries were optimized
at the DF-PBE/SVP level of theory, and energies were calculated,
using a double hybrid functional employing a solvation model
(MeOH) at 298 K (see Computational Details section for full
details). For the iron complexes, all possible spin states were
considered. In accordance with the magnetic measurements, the
2−
mono-chelated complexes of the type I and the FeX4 anions
were found to have quintet ground states (S = 2), while the
dicationic 18-electron complexes of the type II have singlet
ground states (S = 0).
t
For the Bu-PNN ligand, only type I complexes (i.e.,
The nature of the substituents of the phosphine moiety
determines if mono-chelated iron complexes of the type
[(PNN)Fe(X)2] or bis-chelated dications of the type
[(PNN)2Fe]2+ are obtained as products upon reaction of the
PNN ligands with FeX2 (X = Cl, Br) in a 1:1 ratio (Scheme 6).
complexes 1 and 2) were observed experimentally. This can be
explained by the steric bulk of the PtBu2 moiety, the pincer ligand
being too large to allow for two ligands to bind to the iron center.
When trying to build the geometry of isomer II for the tBu-PNN
ligand, it became readily apparent that it is not possible for two
D
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ligands to be bound to the iron center without the methyl groups
of the two tBu substituents overlapping.
observed as broad resonances at δ = 4.03 (for complex 9) and
4.43 ppm (for complex 10) in the 1H and 1H{31P} NMR spectra.
The resonances of the benzylic carbon atoms were observed as
multiplets centered at δ = 67.2 and 60.7 ppm in the 13C{1H}
NMR spectra of 9 and 10 respectively. The 31P{1H} NMR
spectra contain singlets at δ = 51.8 and 50.6 ppm for complex 9
and 10, respectively.
The reactions of the mono-chelated iPr-PNN complexes 3 and
4 with an additional ligand iPr-PNN (leading to the formation of
the bis-chelated complexes 5 and 6) are calculated to be slightly
endergonic, by 6.4 and 10.7 kcal/mol, respectively (Scheme 7).
This is in agreement with the clean formation of [(iPr-
i
1H-15N-HMQC NMR measurements were performed for
complexes 6, 8, 9, and 10 to compare the chemical shifts of the
bipyridine nitrogen atoms of these complexes (see Table 1 and
Supporting Information). The deprotonation reactions lead to
distinct shifts of the 15N resonances of the central pyridine rings
(N2) to significantly higher fields and slightly less pronounced
shifts of the nitrogen signals of the terminal pyridines (N1) to
lower fields.
PNN)Fe(X)2] from 1:1 mixtures of FeX2 and Pr-PNN.
However, the salts 5 and 6, formed in reactions with an excess
of iPr-PNN, are insoluble in THF, and the lattice energy, which is
not considered in the calculations, helps to drive the reactions
toward the products. In the case of the Ph-PNN ligand, the
experiments show that formation of the salts [(Ph-PNN)2Fe]-
(FeX4) (7 and 8) is favored over formation of the mono-chelated
complexes [(Ph-PNN)Fe(X)2]. The difference in ΔG298 was
calculated to be 3.3 kcal/mol for the chloride complex 7 and 0.0
kcal/mol for the bromide complex 8 in favor of the salts (Scheme
7), not considering the additional stabilization provided by the
lattice energy of the precipitated salts.
Table 1. 15N Chemical Shifts [ppm] of Complexes 6, 8, 9, and
10 Obtained by 1H-15N-HMQC NMR Measurements (41
MHz, 23 °C)
Scheme 7. Calculated Gibbs Energies [kcal/mol] of the
a
Reactions Leading to the Formation of 5−8
a
a
b
c
complex 6
complex 8
complex 9
complex 10
N1
N2
254.7
260.7
256.8
254.9
273.2
199.7
269.5
204.7
a
b
Spectrum measured in CD3OD. Spectrum measured in C6D6.
c
Spectrum measured in toluene-d8.
a
The structure of complex 10 was unequivocally confirmed by
X-ray diffraction analysis (Figure 4), which shows a neutral
complex with the iron center coordinated by two deprotonated
pincer ligands in a distorted octahedral fashion. A comparison of
this structure with the structure of the dication 8 reveals distinct
changes in the bond distances resulting from the deprotonation
of the benzylic arms of the ligands. While deprotonation does not
significantly affect the coordination sphere around the iron
center, differences in the bond distances are observed within the
pincer ligands. Table 2 summarizes the bond distances in the
pincer ligands of 8 and 10. The bond distances of the benzylic
carbon atom (C11) to the pyridine ipso carbon atom (C10) and
to the phosphorus atom decrease by 0.114 and 0.087 Å,
respectively. The average of the benzylic C−Cipso bond distances
is 1.504 Å (for 8) and 1.390 Å (for 10) and the average C−P
bond distance decreases from 1.839 Å (for 8) to 1.752 Å (for 10).
The shortening of these bonds is accompanied by an elongation
of the bonds to the ipso carbon atom (C10) within the pyridine
ring. The averaged C9−C10 bond distance changes from 1.386 Å
(for 8) to 1.433 Å (for 10) and the C10−N2 bond from 1.358 Å
(for 8) to 1.390 Å (for 10). This suggests a strong double bond
character between C10 and C11. These values are in the range
reported for dearomatized pyridine-based metal complexes.28 It
is noteworthy that this is, to our knowledge, the first example of a
structurally characterized metal complex featuring a deproto-
nated bipyridine-based pincer ligand and, furthermore, the first
example of a dearomatized iron pincer complex.
ΔG298,MeOH given at the SMD(MeOH)-DSD-PBEP86/TZVP//DF-
PBE/SVP/SVPfit level of theory.
The bis-chelated dicationic complexes of the type
[(PNN)2Fe]2+ readily undergo deprotonation of the benzylic
arms upon reaction with different bases, such as MeLi, KOtBu, or
LiHMDS. The doubly dearomatized complexes [(iPr-
PNN*)2Fe] (9) and [(Ph-PNN*)2Fe] (10) were synthesized
with concomitant formation of KBr and HOtBu (and FeBr2 or
K2[FeBr4] for 10) by treatment of the complexes 6 and 8 with 2
equiv of KOtBu in dry THF for 8 h (Scheme 8). These complexes
are diamagnetic and, in contrast to the starting materials, are
soluble in THF, toluene, and benzene.
Complexes 9 and 10 were characterized by multinuclear NMR
spectroscopy; all resonances in the H and 13C{1H} NMR
1
spectra were fully assigned by 2D-NMR measurements. Both
complexes have two sets of inequivalent substituents on the
phosphorus atoms in the H and 13C{1H} NMR spectra. The
1
benzylic protons integrate to 1 H per pincer ligand and are
Scheme 8. Deprotonation Reactions Resulting in the
Dearomatized Complexes 9 and 10
Metal−ligand cooperation by ligand aromatization-dearoma-
tization of transition metal complexes featuring pyridine-based
pincer ligands is of importance in bond activation reactions and
catalytic processes.29 It has been shown that dearomatized
complexes are capable of activating of H−H, C−H (sp2 and sp3),
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accompanied by the coordination of E− to the metal center.
This coordination might be, in many cases, an additional driving
force for these activation reactions. It, however, cannot take place
on coordinatively saturated complexes, unless one ligand
dissociates.
Thus, it is not surprising that the dearomatized complexes 9
and 10 do not react with H2 (under 2 atm. pressure) or with an
excess of hexylamine (∼400 equiv). However, the complexes are
readily protonated by an excess of methanol or water (Scheme
9). A comparison of both complexes in these experiments
suggests that the iso-propyl-substituted complex 9 is more basic
than the phenyl-substituted complex 10.
Figure 4. ORTEP diagram of the molecular structure of [(Ph-
PNN*)2Fe] (10) (ellipsoids set at the 50% probability level). The
solvent molecules and all hydrogen atoms, other than those on the PNN
ligand arms, are omitted for clarity, while the phenyl substituents are
shown as thin lines. Bond lengths (Å) and angles (deg): Fe−N1
1.9690(13), Fe−N2 1.9383(12), Fe−N3 1.9820(13), Fe−N4
1.9410(12), Fe−P1 2.2380(4), Fe−P2 2.2231(4), C10−C11
1.387(2), C33−C34 1.393(2), N1−Fe−N2 80.65(5), N1−Fe−N3
90.43(5), N1−Fe−N4 99.40(5), N1−Fe−P1 165.21(4), N1−Fe−P2
88.64(4), N2−Fe−N3 95.01(5), N2−Fe−N4 175.58(5), N2−Fe−P1
84.60(4), N2−Fe−P2 99.85(4), N3−Fe−N4 80.57(5), N3−Fe−P1
89.80(4), N3−Fe−P2 164.75(4), N4−Fe−P1 95.22(4), N4−Fe−P2
84.57(4), P1−Fe−P2 94.959(16).
Scheme 9. Protonation Reactions of 9 and 10
Dissolving 9 in CD3OD (∼1700 equiv) or D2O (∼4500
equiv) leads to formation of intensely red solutions and full
conversions to [(iPr-PNN)2Fe]2+ as observed by NMR spec-
troscopy. However, dissolving 10 in CD3OD (∼2050 equiv)
results in the formation of a roughly 1:1 mixture of the doubly
protonated dicationic complex [(Ph-PNN)2Fe]2+ and the singly
protonated complex [(Ph-PNN)(Ph-PNN*)Fe]+. The mono-
cationic complex [(Ph-PNN)(Ph-PNN*)Fe](PF6) (12) was
alternatively synthesized by reaction of the doubly deprotonated
complex [(Ph-PNN*)2Fe] (10) with 1 equiv of the doubly
protonated complex [(Ph-PNN)2Fe](PF6)2 (11) in THF
(Scheme 10). Complex 12 can also be obtained by
deprotonation of 11 with 1 equiv KOtBu in MeCN. The
mono-dearomatized complex 12 exhibits a characteristic AB
system in the 31P{1H} NMR spectrum with two doublets located
Table 2. Comparison of Bond Lengths [Å] between the
Aromatized Dication of 8 and the Doubly Dearomatized
Complex 10
a
[(Ph-PNN)2Fe]2+ (8)
[(Ph-PNN*)2Fe] (10)
bond
Fe−P
ligand 1
ligand 2
ligand 1
ligand 2
2.249(3)
2.242(3)
1.993(10)
1.952(8)
1.398(18)
1.35(2)
2.2380(4)
1.9690(13)
1.9383(12)
1.377(2)
1.391(2)
1.382(2)
1.391(2)
1.469(2)
1.383(2)
1.405(2)
1.366(2)
1.433(2)
1.387(2)
1.350(2)
1.3616(19)
1.357(2)
1.3895(19)
1.7508(16)
2.2231(4)
1.9820(13)
1.9410(12)
1.384(2)
Fe−N1
Fe−N2
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C7
C7−C8
C8−C9
C9−C10
C10−C11
N1−C1
N1−C5
N2−C6
N2−C10
P−C11
1.981(9)
1.936(9)
at δ = 61.5 and 43.9 ppm featuring a coupling constant of 2JPP
=
1.409(18)
1.358(19)
1.394(19)
1.397(16)
1.484(15)
1.377(16)
1.407(18)
1.380(18)
1.377(15)
1.516(15)
1.339(15)
1.351(14)
1.355(14)
1.357(14)
1.842(11)
1.386(2)
47.6 Hz. The dearomatized pincer ligand features a broad singlet
at δ = 4.03 for the benzylic hydrogen atom in the H NMR
1
1.41(2)
1.382(2)
1.393(18)
1.478(16)
1.399(15)
1.392(18)
1.390(17)
1.395(15)
1.492(15)
1.331(15)
1.374(15)
1.348(14)
1.359(13)
1.835(11)
1.392(2)
spectrum, and the aromatized ligand shows two resonances at δ =
3.74 and 4.54 ppm for the diastereotopic benzyl hydrogen atoms
with a geminal coupling of 2JHH = 18.0 Hz.
The addition of D2O (∼4650 equiv) to complex 10 results in
the formation of a suspension that consists of an orange colored
solution and a dark precipitate. Increasing the amount of D2O to
approximately 12500 equiv does not completely dissolve the
1.467(2)
1.383(2)
1.398(2)
1.372(2)
1.433(2)
1.393(2)
1.3451(19)
1.3609(19)
1.3580(19)
1.3880(19)
1.7525(15)
Scheme 10. Synthesis of Complexes 11 and 12 (P = PPh2)
a
Ligands 1 and 2 refer to the crystallographically inequivalent pincer
ligands of the bis-chelated complexes.
O−H, and N−H bonds without changing the formal oxidation
state of the metal. Dearomatized complexes can activate these E-
H bonds by cooperation between the metal and the ligand,
thereby regaining aromatization of the ligand by protonation of
the benzylic carbon atom. These reactions are usually
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precipitate. In solution, only the dication [(Ph-PNN)2Fe]2+ was
observed by NMR spectroscopy. However, carrying out the
reaction of 10 with an excess of D2O (∼2450 equiv) in THF
results in the formation of an intensely greenish-brown solution.
1H and 31P{1H} NMR measurements show the formation of
[(Ph-PNN)2Fe]2+ and [(Ph-PNN)(Ph-PNN*)Fe]+ in a mixture
of roughly 1:6.
The protonation reaction of complex 10 with CD3OD is fully
reversible. Evaporation of the solvent from the reaction of 10
with CD3OD and exposure of the residue to high vacuum leads to
full conversion to the doubly dearomatized complex 10. This
reaction is accompanied by a change in color from redish-brown
to brown. Partial deuterium incorporation into the benzylic
positions was observed by 2D NMR, and the integration of the
residual resonance of benzylic proton in the 1H NMR spectrum
gives about 0.7 H per pincer ligand. The 31P{1H} NMR spectrum
of the resulting mixture of differently deuterated complexes
shows a multiplet (see Supporting Information). The multiplet
consists of the two superimposed resonances of the non-
deuterated complex 10HH, which exhibits a singlet at δ = 50.5
ppm, and the mono-deuterated complex 10HD, which shows a
symmetrical multiplet centered at δ = 50.4 ppm resulting from
second order spin coupling of the AA′X system.
Reactions of FeX2 with Ph-PNN in a 1:1 ratio yield the bis-
chelated salts [(Ph-PNN)2Fe](FeX4) (7 and 8). The formation
of 7 and 8 is thermodynamically favored over the formation of
the mono-chelated complexes [(Ph-PNN)Fe(X)2]. The bis-
chelated dication [(Ph-PNN)2Fe]2+ in these salts is isostructural
i
with the Pr substituted dication. The bis-chelated dicationic
complexes [(iPr-PNN)2Fe]2+ and [(Ph-PNN)2Fe]2+ undergo
deprotonation of the benzylic carbon atoms in the presence of
suitable bases. The doubly deprotonated complexes [(iPr-
PNN*)2Fe] (9) and [(Ph-PNN*)2Fe] (10) were synthesized
by reactions of 6 and 8 with KOtBu. The structural analysis of 10
unequivocally confirms the resulting dearomatization of the
pincer ligands. These complexes are reversibly protonated in the
presence of an excess of water or methanol, and the reactions
performed suggest that complex 9 is more basic than complex 10.
A very recent report by Huang et al. on the application of
complex 1 as precatalyst for hydroboration reactions22 indicates
the high catalytic potential of these compounds. Studies of the
scope of complexes 1−4 in different catalytic reactions are
currently underway in our laboratories.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under a
nitrogen atmosphere in a glovebox or using standard Schlenk
techniques. All solvents were reagent grade or better. Tetrahydrofuran,
benzene, toluene, diethylether, and pentane were refluxed over sodium
and distilled under an argon atmosphere. Methylene chloride, methanol,
and acetonitrile were degassed by freeze-pump thaw cycles and stored in
the glovebox over the appropriate molecular sieves. Deuterated solvents
were sparged with argon and stored in the glovebox over the appropriate
molecular sieves. Other commercially available reagents were used as
received.
To obtain a higher percentage of deuteration, CD3OD (∼1900
equiv) was added to 10 and evaporated. Repeating this
procedure five times resulted in 85% deuteration of the complex
in the benzylic position. The 31P{1H} NMR spectrum (see
Supporting Information) of the complex mixture shows a second
order spin system of a mono-deuterated complex 10HD that is
superimposed with a broad singlet at δ = 50.4 ppm of the doubly
deuterated complex 10DD
.
As with 10, evaporation of all volatiles from the reaction of 9
with CD3OD results in a change of color from red to brown.
Extraction of the resulting brown solid with toluene-d8 gives a
brown solution and an insoluble, unidentified, green residue. The
1H and 31P{1H} NMR spectra of the solution show formation of
minor amounts 9 along with major amounts of the mono-
deuterated free ligand. The 31P{1H} NMR spectrum exhibits a
multiplet for 9, similar to 10. The integration of this multiplet
with respect to the pseudo triplet resonance of the mono-
deuterated ligand (δ = 11.2 ppm; 2JPD = 42.2 Hz) in the 31P{1H}
NMR spectrum gave a ratio 9 to the free ligand of approximately
1:7.
NMR spectra were recorded using Bruker AMX-300, AMX-400, and
AMX-500 NMR spectrometers. 1H and 13C{1H} NMR chemical shifts
are reported in ppm downfield from tetramethylsilane. 31P{1H} NMR
chemical shifts are reported in ppm downfield from H3PO4 (0.0 ppm)
and are referenced to an external 85% solution of phosphoric acid in
D2O. NMR assignments (Scheme 11) of the diamagnetic compounds
were assisted by 1H-1H-COSY, 1H-31P-HMQC, 1H-13C-HSQC, 1H-13C-
HMBC, and 13C-DEPTQ NMR spectroscopy, as required. 15N chemical
shifts were identified by 1H-15N-HMQC NMR measurements and are
reported downfield from liquid ammonia (0.0 ppm). 1H NMR spectra of
the paramagnetic compounds were recorded with a d1 time of 50 ms,
from samples of approximately 2 mg substance in 0.6 mL of solvent. The
effective magnetic moments in solution were measured by the Evans’
method23 at ambient temperature. The measurement of the magnetic
susceptibility of a sample of complex 4 (20.7 mg powder) was performed
on a Quantum design MPMS XL SQUID magnetometer, employing a
field of 3000 Oe. IR spectra were recorded on a Nicolet FT-IR
spectrophotometer. Elemental analyses and ESI-MS spectroscopy were
performed by the Department of Chemical Research Support,
Weizmann Institute of Science.
CONCLUSIONS
■
In summary, bipyridine-based PNN-type iron pincer complexes
were readily prepared from reactions of bipyridine-based PNN
ligands with FeX2 (X = Cl and Br). Three different ligands, the
t
i
known Bu-PNN ligand and the new Pr-PNN and Ph-PNN,
were studied in this work. Depending on the ratios between the
ligand and metal salt, and on the PR2 substituents, three different
types of complexes were obtained. Reactions of the most bulky
ligand tBu-PNN with FeX2 give the mono-chelated, paramagnetic,
high-spin complexes [(tBu-PNN)Fe(X)2] 1 and 2, exclusively.
The analogous complexes [(iPr-PNN)Fe(X)2] 3 and 4 are
obtained in reactions with iPr-PNN when the ligand and FeX2 are
reacted in a 1:1 ratio. Increasing this ratio to 2:1, or reacting [(iPr-
PNN)Fe(X)2] with an additional equivalent of the ligand, leads
to formation of the dicationic, bis-chelated, diamagnetic, low-spin
complexes [(iPr-PNN)2Fe](X)2 (5 and 6). DFT calculations
show that the second ligation step is slightly endergonic, but the
reactions are driven by the formation of the insoluble salts.
Synthesis of PNN Ligands. 6-Chloromethyl-2,2′-bipyridine was
prepared according to the literature.30
Scheme 11. Assignment of the Carbon and Hydrogen Atoms
of the PNN Ligands
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tBu-PNN (6-Di-tert-butylphosphinomethyl-2,2′-bipyridine). This
was prepared as reported previously.13
128.43 ppm. No resonance was detected in the 31P{1H} NMR spectrum
in a range from −4000 to 4000 ppm. Magnetic susceptibility (Evans):
μeff = 5.0 μB (1,4-dioxane in CDCl3, 23 °C).
iPr-PNN (6-Di-iso-propylphosphinomethyl-2,2′-bipyridine). A pres-
sure vessel was charged with a mixture of 6-chloromethyl-2,2′-bipyridine
(1.03 g, 5.00 mmol), di-iso-propyl phosphine (710 mg, 6.00 mmol), and
MeOH (15 mL). The reaction mixture was heated to 50 °C for 48 h with
stirring. The reaction mixture was allowed to reach room temperature,
and triethylamine (910 mg, 9.00 mmol) was added. After stirring the
reaction mixture at room temperature for 1 h, all volatiles were removed
in vacuo. The crude product was extracted from the colorless residue
with ether (2 × 10 mL). Upon evaporation of the ether in vacuo a pale
yellow liquid was obtained. The product was purified by column
chromatography over basic alumina using hexane as eluent to yield 874
mg (61%) of a pale yellow liquid. Anal. Calcd. (found) for C17H23N2P
[286.16 g·mol−1]: C 71.30 (71.52), H 8.10 (8.34). ESI-MS: 309.16 (M +
[(tBu-PNN)Fe(Br)2] (2). A solution of tBu-PNN (228 mg, 0.726 mmol)
in 3 mL of THF was added to a solution of FeBr2 (153 mg, 0.716 mmol)
in 20 mL of THF. Immediately after addition the reaction mixture
turned black. After 16 h of stirring at room temperature, the resulting
black suspension was layered with pentane (60 mL). The suspension
was filtered, the residue was washed with pentane (10 mL), and dried in
vacuo to yield 360 mg (95%) of a black powder. Single crystals of 2 were
obtained by cooling a saturated solution in acetonitrile to −20 °C. Anal.
Calcd. (found) for C19H27Br2FeN2P [530.06 g·mol−1]: C 43.05
(43.25), H 5.13 (5.25), N 5.28 (5.27). ESI-MS: (m/z, pos.): 449.04
([(tBu-PNN)FeBr]+ = M-Br− = C19H27BrFeN2P+); (m/z, neg.): 80.90
(Br−). 1H NMR (300 MHz, CD3CN, 23 °C) δ = −13.02, 7.56, 10.17,
15.75, 53.91, 55.99, 77.82, 87.44, 116.50 ppm. No resonance was
detected in the 31P{1H} NMR spectrum in a range from −4000 to 4000
ppm. Magnetic susceptibility (Evans): μeff = 5.3 μB (1,4-dioxane in
CDCl3, 23 °C).
1
Na+, C17H23N2PNa+), 287.16 (M + H+, C17H24N2P+). H NMR (400
MHz, CD2Cl2, 23 °C): δ = 1.11 (dd, 6 H, 3JPH = 13.8 Hz, 3JHH = 7.2 Hz,
A
B
iPr−CH3 ), 1.17 (dd, 6 H, 3JPH = 14.4 Hz, 3JHH = 7.2 Hz, iPr−CH3 ),
1.89 (m, 2 H, iPr−CH), 3.30 (d, 2H, 2JPH = 1.8 Hz, PCH2), 7.73 (vt, 1 H,
3JHH = 7.8 Hz H6), 7.82 (dvt, 1 H, 3JHH = 7.8 Hz, 4JHH = 1.8 Hz, H11),
8.21 (d, 1H, 3JHH = 7.8 Hz, H7), 8.46 (d, br, 1H, 3JHH = 8.1 Hz, H10),
8.68 (m, 1H, H13) ppm. 13C{1H} NMR (100 MHz, CD2Cl2, 23 °C): δ
[(iPr-PNN)Fe(Cl)2] (3). A solution of iPr-PNN (57.3 mg, 0.20 mmol)
in 3 mL of THF was added to a suspension of FeCl2 (25.2 mg, 0.20
mmol) in 7 mL of THF. Upon addition the reaction mixture turned red.
After 16 h of stirring at room temperature, the resulting red suspension
was layered with pentane (10 mL). The suspension was filtered, the
residue was washed with pentane (20 mL), and dried in vacuo to yield
73.8 mg (89%) of a red powder. Anal. Calcd. (found) for
C17H23Br2FeN2P [413.10 g·mol−1]: C 49.43 (49.72), H 5.62 (5.65),
N 6.78 (6.62). ESI-MS: (m/z, pos.): 377.08 ([(iPr-PNN)Fe(Cl)]+ =
C17H23ClFeN2P+), 170.97 ([(iPr-PNN)Fe]2+ = C17H23FeN2P2+). The
compound is paramagnetic and shows 12 resonances in the 1H NMR:
1H NMR (300 MHz, CD3CN, 23 °C) δ = −17.99, 0.74, 7.56, 8.65, 8.81,
9.01, 29.96, 51.09, 54.37, 79.88, 80.17, 117.03, 134.25 ppm. No
resonance was detected in the 31P{1H} NMR spectrum in a range from
−4000 to 4000 ppm. Magnetic susceptibility (Evans): μeff = 5.3 μB (1,4-
dioxane in CDCl3, 23 °C).
= 18.9 (d, 2JPC = 10.5 Hz, iPr-CH3 ), 19.6 (d, 2JPC = 15.0 Hz, iPr-CH3 ),
A
B
1
1
i
23.6 (d, JPC = 15.0 Hz, PCH2), 32.5 (d, br, JPH = 22.5 Hz, Pr-CH),
117.6 (s, C7), 120.8 (s, C10), 123.5 (s, C12), 123.6 (s, C5), 136.7 (s,
C11), 136.9 (s, C6), 149.1 (s, C13), 155.2 (s, C8), 156.4 (s, C9), 161.39
(d, 2JPC = 9.0 Hz, C4) ppm. 31P{1H} NMR (162 MHz, CD2Cl2, 23 °C):
δ = 15.8 (s) ppm.
Ph-PNN (6-Diphenylphosphinomethyl-2,2′-bipyridine). A
Schlenk flask equipped with a reflux condenser and a dropping funnel
was charged with a solution of diphenylphosphine in ether (30 mL). The
solution was cooled to 0 °C, and a solution of potassium tert-butoxide
(74 mg, 6.6 mmol) in THF (5 mL) was added dropwise during a period
of 10 min. The resulting brown colored mixture was stirred for 1 h at 0
°C, and a solution of 6-chloromethyl-2,2′-bipyridine (1.02 g, 5 mmol) in
ether (20 mL) was added dropwise during a period of 10 min. Upon
complete addition the reaction mixture was stirred at 0 °C for 1 h and
was allowed to reach room temperature overnight. Degassed water (20
mL) was added, and the organic phase was separated under nitrogen.
The water phase was extracted with ether (2 × 20 mL), the organic
fractions were combined, dried over Na2SO4, and filtered. The solvent
was removed in vacuo to yield a brownish solid. The crude product was
purified by column chromatography over basic alumina using a hexane/
ether mixture (10:1) as eluent to yield 1.08 g (61%) of colorless crystals.
Anal. Calcd. (found) for C17H23N2P [354.13 g·mol−1]: C 78.13
(77.95), H 5.61 (5.40). ESI-MS: 377.08 (M + Na+, C23H19N2PNa+),
355.20 (M + H+, C23H20N2P+). 1H NMR (300 MHz, CD2Cl2, 23 °C): δ
= 3.81 (s, 2H, P−CH2), 7.13 (d, 3JHH = 7.5 Hz, 1H, H7), 7.32 (m, 1H,
H14), 7.43 (m, 6H, Ph-H), 7.60 (m, 4H, Ph-H), 7.69 (vt, 1H, 3JHH = 7.8
Hz, 1H, H8), 7.80 (dvt, 1H, 3JHH = 7.8 Hz, 4JHH = 2.1 Hz, H-13′), 8.28
(d, br, 2H, JHH = 8,0 Hz, H9 + 12), 8.69 (m, 1H, H15) ppm. 13C{1H}
NMR (100 MHz, CD2Cl2, 23 °C): δ = 38.4 (d, 1JPC = 16.3 Hz, PCH2),
118.1 (s, C9), 121.0 (s, C12), 123.5 (s, C14), 123.6 (d, JPC = 8.4 Hz, C7),
128.3 (s, Ph-C), 128.4 (s, Ph-C), 128.7 (s, Ph-C), 132.9 (s, Ph-C), 133.1
(s, Ph-C), 136.6 (s, C13), 137.0 (s, C8), 138.7 (d, JPC = 15.0 Hz, Ph-
Cipso), 149.0 (s, C13), 155.5 (s, C10), 156.2 (s, C11), 157.6 (d, JPC = 7.5
Hz, C6) ppm. 31P{1H} NMR (162 MHz, CD2Cl2, 23 °C): δ = −8.3 (s)
ppm.
[(iPr-PNN)Fe(Br)2] (4). A solution of iPr-PNN (57.3 mg, 0.20 mmol)
in 3 mL of THF was added to a solution of FeBr2 (43.0 mg, 0.20 mmol)
in 7 mL of THF. Upon addition a black suspension was formed. After 16
h of stirring at room temperature, the resulting red suspension was
layered with pentane (10 mL). The suspension was filtered, the residue
was washed with pentane (20 mL), and dried in vacuo to yield 98.7 mg
(99%) of a red powder. Single crystals of 4 were obtained by slow
evaporation of the solvent from a saturated solution in methylene
chloride. Anal. Calcd. (found) for C17H23Br2FeN2P [502.00 g·mol−1]:
C 40.67 (40.69), H 4.62 (4.49), N 5.58 (5.44). ESI-MS: (m/z, pos.):
421.02 ([(iPr-PNN)Fe(Br)]+ = C17H23BrFeN2P+), 314.11 ([(iPr-
PNN)2Fe]2+ = C34H46FeN4P22+), 170.97 ([(iPr-PNN)Fe]2+
=
C17H23FeN2P2+); (m/z, neg.): 78.94 (Br−). NMR spectroscopy of the
red powder shows a paramagnetic compound (with 12 resonances in the
1
1H NMR). H NMR (400 MHz, CD2Cl2, 23 °C) δ = −17.16, 1.27,
10.32, 10.55, 10.92, 18.67, 51.71, 57.49, 81.56, 87.42, 120.57, 131.01
ppm. Magnetic susceptibility (SQUID): μeff = 5.20 μB (23 °C).
[(iPr-PNN)2Fe](Cl)2 (5). A solution of iPr-PNN (21.0 mg, 73.3 mmol)
in 1 mL of THF was added to a solution of FeBr2 (5.0 mg, 23.4 mmol) in
4 mL of THF. Upon addition a black suspension was formed. Pentane
(15 mL) was added to the purple suspension after 72 h of stirring at
room temperature. The suspension was filtered, the residue was washed
with pentane (20 mL) and dried in vacuo to yield 17.5 mg (95%) of a
Synthesis of Complexes 1−12. [(tBu-PNN)Fe(Cl)2] (1). A solution
purple powder. ESI-MS: (m/z, pos.): 314.11 ([(iPr-PNN)2Fe]2+
=
t
of Bu-PNN (160 mg, 0.51 mmol) in 3 mL of THF was added to a
C34H46FeN4P22+). 1H NMR (400 MHz, CD3OD, 23 °C): δ = 0.37 (dd,
A
suspension of FeCl2 (63.4 mg, 0.50 mmol) in 12 mL of THF. Upon
addition the reaction mixture turned red. After heating the suspension
for 5 h to 65 °C, 80 mL of hexane were added. The resulting suspension
was filtered, and the residue was dried in vacuo to yield 190 mg (86%) of
a red powder. Single crystals of 1 were obtained by slow diffusion of
pentane into a saturated solution in methylene chloride. Anal. Calcd.
(found) for C19H27Cl2FeN2P [440.06 g·mol−1]: C 51.73 (51.80), H
6.17 (6.23), N 6.35 (6.29). ESI-MS: (m/z, pos.): 405.10 ([(tBu-
PNN)FeCl]+ = M-Cl− = C19H27ClFeN2P+). 1H NMR (300 MHz,
CD3CN, 23 °C) δ = −15.76, 8.75, 14.38, 23.8, 53.56, 54.14, 76.79, 80.58,
3 H, 3JPH = 14.0 Hz, 3JHH = 7.2 Hz, iPr−CH3 ), 0.73 (dd, 3 H, 3JPH = 15.3
Hz, 3JHH = 7.3 Hz, iPr−CH3‘A), 0.82 (dd, 3 H, 3JPH = 12.1 Hz, 3JHH = 7.0
i
B
3
3
i
Hz, Pr−CH3 ), 1.45 (dd, 3 H, JPH = 10.6 Hz, JHH = 7.0 Hz, Pr−
CH3‘B), 2.09 (m, 1 H, iPr−CH), 2.45 (m, 1 H, iPr−CH′), 3.30 (s, br, 2H,
PCH2), 7.33 (vt, (dd), 1 H, 3JHH = 5.7 Hz, H12), 7.36 (d, 1 H, 3JHH = 5.1
Hz, H13), 8.00 (m, (d+t), 2 H, H5 + 11), 8.23 (vt, 1 H, 3JHH = 7.7 Hz,
H6), 8.52 (d, 1 H, 3JHH = 7.9 Hz, H10), 8.66 (d, 1 H, 3JHH = 7.8 Hz, H7)
ppm. 1H{31P} NMR (400 MHz, CD3OD, 23 °C): δ = 0.37 (d, 3 H, 3JHH
A
= 7.2 Hz, iPr−CH3 ), 0.73 (d, 3 H, 3JHH = 7.3 Hz, iPr−CH3‘A), 0.82 (d, 3
B
H, 3JHH = 7.1 Hz, iPr−CH3 ), 1.45 (d, 3 H, 3JHH = 7.0 Hz, iPr−CH3‘B),
H
dx.doi.org/10.1021/ic401432m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2.09 (sept, 2 H, 3JHH = 7.0 Hz, iPr−CH), 2.45 (sept, 2 H, 3JHH = 7.1 Hz,
iPr−CH′), 3.30 (s, br, 2H, PCH2), 7.33 (vt, (dd), 1 H, 3JHH = 5.8 Hz,
H12), 7.36 (d, 1 H, 3JHH = 5.1 Hz, H13), 8.00 (m, (d+t) 2 H, H5 + 11),
8.23 (vt, 1 H, 3JHH = 7.8 Hz, H6), 8.52 (d, 1 H, 3JHH = 7.9 Hz, H10), 8.66
(d, 1 H, 3JHH = 7.9 Hz, H7) ppm. 13C{1H} NMR (100 MHz, CD3OD,
room temperature pentane (10 mL) was added. The suspension was
filtered, and the residue was washed with pentane (20 mL) and dried in
vacuo to yield 110.1 mg (97%) of an orange colored powder. Single
crystals of 6 suitable for X-ray analysis were obtained by slow diffusion of
Et2O into a saturated solution of the complex in acetonitrile. Anal.
Calcd. (found) for C46H38Br4Fe2N4P2 [1135.80 g·mol−1]: C 48.46
(48.76), H 3.36 (3.42), N 4.91 (4.64). ESI-MS: (m/z, pos.): 845.13
([(Ph-PNN)2Fe(Br)]+ = C46H38BrFeN4P2+), 382.02 ([(Ph-
A
23 °C): δ = 19.0 (br, iPr-CH3 ), 19.5 (vt, (dd), JPH = 1.5 Hz, iPr-CH3‘A),
19.6 (vt, (dd), JPH = 4.5 Hz, iPr-CH3‘B), 19.8 (vt, (dd), JPH = 4.5 Hz, iPr-
B
CH3 ), 27.0 (vt, (dd), JPH = 9.0 Hz, iPr-CH), 31.6 (vt, (dd), JPH = 7.7 Hz,
−
PNN)2Fe]2+ = C46H38FeN4P22+); (m/z, neg.): 294.07 (FeBr3 ), 80.97
iPr-CH), 49.3 (vt, (dd), JPH = 21.4 Hz, CH2P), 124.1 (s, C6), 125.0 (s,
C10), 126.0 (vt, (dd), JPC = 4.0 Hz, C5), 128.7 (s, C12), 139.0 (s, C6),
140.0 (s, C11), 151.5 (s, C13), 159.7 (s, br, C9), 160.4 (vt, (dd), JPC = 1.7
Hz, C8), 168.0 (vt, (dd), JPC = 3.0 Hz, C4) ppm. 31P{1H} NMR (121
MHz, CD3OD, 23 °C): δ = 60.6 (s) ppm. Magnetic susceptibility
(Evans): no paramagnetic shifting of the reference could be observed
(benzene in CD3OD, 23 °C).
1
(Br−). H, 13C{1H}, and 31P{1H} NMR spectra are identical to the
chloro complex 7. 15N NMR (41 MHz, CD3OD, 23 °C): δ = 254.9
(N2), 256.7 (N1) ppm. Magnetic susceptibility (Evans): μeff = 5.1 μB
(benzene in CD3OD, 23 °C). This magnetic moment stems from the
FeBr42− anion (S = 2).26
[(iPr-PNN*)2Fe] (9). Complex 6 (32.0 mg, 40.7 μmol) and KOtBu
(11.2 mg, 0.10 mmol) were suspended in THF (10 mL), and the
reaction mixture was stirred at room temperature for 8 h. All volatiles
were removed in vacuo, and 20 mL of a 1:2 mixture of toluene and
benzene was added. The resulting suspension was filtered, and all
volatiles were removed in vacuo to obtain 23.0 mg (90%) of a dark
brown solid. 1H NMR (500 MHz, C6D6, 23 °C): δ = 0.60 (dd, 3 H, 3JPH
[(iPr-PNN)2Fe](Br)2 (6). A solution of iPr-PNN (21.0 mg, 73.3 μmol)
in 1 mL of THF was added to a solution of FeBr2 (5.0 mg, 23.4 μmol) in
4 mL of THF. Upon addition a black suspension was formed. Pentane
(15 mL) was added to the purple suspension after 72 h of stirring at
room temperature. The suspension was filtered, the residue was washed
with pentane (20 mL), and dried in vacuo to yield 17.5 mg (95%) of a
purple powder. Single crystals of 6 were obtained by slow diffusion of
diethylether into a saturated solution of the complex in acetonitrile.
Anal. Calcd. (found) for C34H46Br2FeN4P2 [788.36 g·mol−1]: C 51.80
(52.07), H 5.88 (5.75), N 7.11 (7.09). ESI-MS: (m/z, pos.): 314.11
([(iPr-PNN)2Fe]2+ = C34H46FeN4P22+); (m/z, neg.): 78.94 (Br−). 1H,
13C{1H}, and 31P{1H} NMR spectra are identical to the chloro complex
5. 15N NMR (41 MHz, CD3OD, 23 °C): δ = 254.7 (N1), 260.7 (N2)
ppm. Magnetic susceptibility (Evans): no paramagnetic shifting of the
reference could be observed (benzene in CD3OD, 23 °C).
A
B
= 12.3 Hz, 3JHH = 7.2 Hz, iPr−CH3 ), 0.90 (m, 3 H, iPr−CH3 ), 1.27 (m,
1 H, iPr−CH), 1.35 (dd, 3 H, 3JPH = 13.7 Hz, 3JHH = 7.3 Hz, iPr−CH3‘A),
1.43 (dd, 3 H, 3JPH = 10.2 Hz, 3JHH = 7.5 Hz, iPr−CH3‘B), 2.89 (m, 1 H,
iPr−CH′), 4.03 (s, br, 1 H, H3), 5.99 (vt, br, 1 H, 3JHH = 6.7 Hz, H12),
6.37 (d, 1 H, 3JHH = 6.3 Hz, H7), 6.47 (d, br, 1 H, 3JHH = 8.5 Hz, H5),
6.54 (vt, 1 H, 3JHH = 8.1 Hz, H11), 6.78 (vt, 1 H, 3JHH = 7.7 Hz, H6), 7.01
(d, br, 1 H, 3JHH = 8.0 Hz, H10), 7.81 (d, br, 1 H, 3JHH = 5.2 Hz, H13)
ppm. 1H{31P} NMR (500 MHz, C6D6, 23 °C): δ = 0.60 (d, 3 H, 3JHH
=
A
B
7.2 Hz, iPr−CH3 ), 0.90 (d, 3 H, 3JHH = 7.3 Hz, iPr−CH3 ), 1.27 (m, 1
H, iPr−CH), 1.35 (d, 3 H, 3JHH = 7.2 Hz, iPr−CH3‘A), 1.43 (d, 3 H, 3JHH
= 7.4 Hz, iPr−CH3‘B), 2.90 (sept, 1 H, 3JHH = 7.1 Hz, iPr−CH′), 4.03 (s,
[(Ph-PNN)2Fe](FeCl4) (7). Ph-PNN (72.0 mg, 0.203 mmol) was added
to a suspension of FeCl2 (25.2 mg, 0.200 mmol) in 12 mL of THF. Upon
addition the reaction mixture turned red. After 16 h of stirring at room
temperature, the volume of the resulting suspension was reduced in
vacuo to about 5 mL, and pentane (15 mL) was added. The suspension
was filtered, the residue was washed with pentane (20 mL) and dried in
vacuo to yield 88.5 mg (93%) of an orange colored powder. Anal. Calcd.
(found) for C46H38Cl4Fe2N4P2 [960.00 g·mol−1]: C 57.42 (57.23), H
3.98 (4.04), N 5.82 (5.72). ESI-MS: (m/z, pos.): 799.18 ([(Ph-
br, 1 H, H3), 5.99 (vt, br, 1 H, 3JHH = 6.9 Hz, H12), 6.37 (d, 1 H, 3JHH
=
6.5 Hz, H7), 6.47 (d, br, 1 H, 3JHH = 8.6 Hz, H5), 6.54 (vt, 1 H, 3JHH = 8.2
Hz, H11), 6.78 (vt, 1 H, 3JHH = 7.7 Hz, H6), 7.01 (d, br, 1 H, 3JHH = 7.6
Hz, H10), 7.81 (d, br, 1 H, 3JHH = 5.0 Hz, H13) ppm. 13C{1H} NMR
(125 MHz, C6D6, 23 °C): δ = 19.9 (m, br, iPr-CH3B + iPr-CH3‘B), 20.2
A
(s, br, iPr-CH3 ), 20.9 (s, br, iPr-CH3‘A), 29.1 (vt, (dd), JPH = 11.2 Hz,
iPr-CH′), 31.1 (vt, (dd), JPH = 7.2 Hz, iPr-CH′), 67.2 (m, C3), 101.2 (s,
C7), 110.9 (vt, (dd), JPH = 7.1 Hz, C5), 119.3 (s, C10), 122.7 (s, C12),
129.8 (s, C6), 132.5 (s, C11), 151.1 (s, C13), 157.7 (s, C8), 162.9 (s,
C9), 172.1 (vt, (dd), JPH = 9.7 Hz, C4) ppm. 31P{1H} NMR (202 MHz,
C6D6, 23 °C): δ = 51.8 (s) ppm. 15N NMR (41 MHz, C6D6, 23 °C): δ =
199.7 (N2), 273.2 (N1) ppm. Magnetic susceptibility (Evans): no
paramagnetic shifting of the reference could be observed (dioxane in
C6D6, 23 °C).
+
PNN)2Fe(Cl)]+ = C46H38ClFeN4P2 ), 445.11 ([(Ph-PNN)Fe(Cl)]+ =
C26H19ClFeN2P+), 382.02 ([(Ph-PNN)2Fe]2+ = C46H38FeN4P22+); (m/
−
z, neg.): 160.84 (FeCl3 ). 1H NMR (400 MHz, CD3OD, 23 °C): δ =
4.04 (dd, 1 H, 2JHH = 17.5 Hz, 2JPH = 8.1 Hz, Ph2PCHH), 4.91 (m, 1 H,
Ph2PCHH), 6.60 (m, 1 H, H15), 6.74 (m, 2 H, H3′), 6.74 (m, 1 H,
H14), 6.80 (m, 2 H, H3), 6.92 (vt, 2 H, 3JHH = 7.4 Hz, H2), 7.03 (t, 1 H,
3JHH = 7.4 Hz, H1), 7.21 (vt, 2 H, 3JHH = 7.4 Hz, H2′), 7.42 (t, 1 H, 3JHH
=
[(Ph-PNN*)2Fe] (10). Complex 8 (34.8 mg, 30.6 μmol) and KOtBu
(7.9 mg, 70.4 μmol) were suspended in THF (10 mL), and the reaction
mixture was stirred at room temperature for 8 h. All volatiles were
removed in vacuo, and 20 mL of a 1:2 mixture of toluene and benzene
was added. The resulting suspension was filtered, and all volatiles were
removed in vacuo to obtain 20.4 mg (87%) of a dark brown solid. 1H
NMR (500 MHz, C6D6, 23 °C): δ = 4.43 (s, br, 1 H, Ph2PCH), 5.80 (t, 1
H, 2JHH = 6.3 Hz, H14), 6.27 (t, 1 H, 2JHH = 7.3 Hz, H13), 6.35 (d, 1 H,
2JHH = 7.0 Hz, H9), 6.56 (d, 1 H, 2JHH = 8.0 Hz, H12), 6.60 (m, br, 3 H,
H1′+H2′), 6.77 (m, 1 H, H1), 6.84 (t, 2 H, 2JHH = 7.3 Hz, H2′), 6.93 (d,
br, 1 H, 2JHH = 8.5 Hz, H7), 7.04 (t, 1 H, 2JHH = 7.8 Hz, H8), 7.25 (br, 2
H, H3′), 7.62 (d, br, 1 H, 2JHH = 5.3 Hz, H15), 7.68 (br, 2 H, H3) ppm.
1H{31P} NMR (500 MHz, C6D6, 23 °C): δ = 4.43 (s, br, 1 H, Ph2PCH),
5.80 (t, 1 H, 2JHH = 6.3 Hz, H14), 6.27 (t, 1 H, 2JHH = 7.3 Hz, H13), 6.35
(d, 1 H, 2JHH = 6.9 Hz, H9), 6.57 (d, 1 H, 2JHH = 7.8 Hz, H12), 6.60 (m,
br, 3 H, H1′+H2′), 6.77 (m, 1 H, H1), 6.85 (t, 2 H, 2JHH = 7.3 Hz, H2′),
6.94 (d, 1 H, 2JHH = 8.5 Hz, H7), 7.05 (t, 1 H, 2JHH = 7.8 Hz, H8), 7.26
(d, 2 H, 2JHH = 7.6 Hz, H3′), 7.62 (d, br, 1 H, 2JHH = 5.3 Hz, H15), 7.65
7.4 Hz, H1′), 7.64 (vt, 1 H, 3JHH = 7.4 Hz, H13), 8.26 (d, 1 H, 3JHH = 8.1
Hz, H12), 8.29 (d, 1 H, 3JHH = 8.0 Hz, H7), 8.40 (vt, 1 H, 3JHH = 8.0 Hz,
H8), 8.66 (d, 1 H, 3JHH = 8.0 Hz, H9) ppm. 1H{31P} NMR (400 MHz,
CD3OD, 23 °C): δ = 4.02 (d, 1 H, 2JHH = 18.1 Hz, Ph2PCHH), 4.91 (d, 1
H, 2JHH = 18.2 Hz, Ph2PCHH), 6.59 (d, br, 1 H, 2JHH = 5.1 Hz, H15),
6.74 (m, 2 H, H3′), 6.74 (m, 1 H, H14), 6.78 (d, 2 H, 3JHH = 7.7 Hz, H3),
6.90 (vt, 2 H, 3JHH = 7.4 Hz, H2), 7.02 (t, 1 H, 3JHH = 7.4 Hz, H1), 7.20
(vt, 2 H, 3JHH = 7.4 Hz, H2′), 7.41 (t, 1 H, 3JHH = 7.4 Hz, H1′), 7.64 (vt, 1
H, 3JHH = 7.4 Hz, H13), 8.26 (d, 1 H, 3JHH = 8.1 Hz, H12), 8.29 (d, 1 H,
3JHH = 8.0 Hz, H7), 8.40 (vt, 1 H, 3JHH = 8.0 Hz, H8), 8.66 (d, 1 H, 3JHH
=
8.0 Hz, H9) ppm. 13C{1H} NMR (100 MHz, CD3OD, 23 °C): δ = 37.4
(m, br, CH2PPh2), 124.1 (s, C9), 124.6 (s, C12), 126.2 (m, C7), 128.3 (s,
C3′), 130.0 (m, C2′), 130.4 (m, C3), 130.5 (m, C2′) 130.8 (s, C1), 131.4
(m, C14), 131.4 (d, 2JCP= 38.5 Hz, C4′), 131.7 (d, 2JCP= 39.6 Hz, C4),
132.2 (s, C1′), 139.0 (s, C13), 139.9 (s, C8), 151.0 (br, C15), 157.6 (br,
C11), 160.2 (m, C10), 165.9 (m, C6) ppm. 31P{1H} NMR (162 MHz,
C6D6, 23 °C): δ = 54.4 (s) ppm. The resonances of H3′and H14 are
overlapping in the 1H and 1H{31P} NMR spectra. Magnetic
susceptibility (Evans): μeff = 5.1 μB (benzene in CD3OD, 23 °C).
This magnetic moment stems from the FeCl42− anion (S = 2).26
[(Ph-PNN)2Fe](FeBr4) (8). Ph-PNN (72.0 mg, 0.203 mmol) was
added to a solution of FeBr2 (43.0 mg, 0.201 mmol) in 10 mL of THF.
Upon addition the reaction mixture turned red. After 16 h of stirring at
2
(d, br, 2 H, JHH = 7.3 Hz, H3) ppm. 13C{1H} NMR (100 MHz,
CD3OD, 23 °C): δ = 60.7 (m, CHPPh2), 101.8 (s, C9), 124.6 (s, C12),
111.6 (vt, 3JCP = 8.2 Hz, C7), 119.6 (s, C12), 122.9 (s, C14), 126.1 (s,
C1′), 127.0 (vt, 3JCP = 4.2 Hz, C2′), 127.2 (vt, 3JCP = 4.6 Hz, C2), 127.3
(s, C1), 130.5 (vt, 2JCP = 4.3 Hz, C3′), 131.0 (s, C8), 131.8 (vt, 2JCP= 4.4
I
dx.doi.org/10.1021/ic401432m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Hz, C3), 132.4 (s, C13), 138.9 (m, C4′), 139.5 (m, C4), 150.8 (s, C15),
157.3 (vt, 3JCP = 2.2 Hz, C10), 161.0 (s, C11), 173.9 (vt, 2JCP = 11.7 Hz,
C6) ppm. 31P{1H} NMR (162 MHz, C6D6, 23 °C): δ = 50.6 (s) ppm.
15N NMR (41 MHz, toluene-d8, 23 °C): δ = 204.7 (N2), 269.5 (N1)
7.93 (d, br, 1 H, 3JHH= 7.9 Hz, H12), 8.01 (d, br, 1 H, 3JHH= 7.7 Hz, H7),
3
3
8.18 (t, 1 H, JHH= 7.9 Hz, H8), 8.38 (d, br, JHH= 8.0 Hz, H9) ppm.
13C{1H} NMR (125 MHz, CD3CN, 23 °C): δ = 39.0 (m, CH2PPh2),
1
59.5 (d, JCP = 67.8 Hz, CH2PPh2), 104.3 (m, C9*), 114.9 (m, C7*),
1
ppm. The resonances of H1′and H2′ are overlapping in the H and
121.3 (s, C12*),122.9 (s, C9), 123.3 (s, C12), 124.0 (m, C7), 125.6 (s,
C14*), 126.6 (s, C14), 128.2 (d, JCP = 8.7 Hz, phenyl-C), 128.4 (d, JCP =
1H{31P} NMR spectra. Magnetic susceptibility (Evans): no para-
magnetic shifting of the reference could be observed (dioxane in C6D6,
23 °C).
2.2 Hz, phenyl-C), 128.6 (d, JCP = 2.2 Hz, phenyl-C), 128.6 (d, JCP = 9.2
Hz, phenyl-C), 139.1 (d, JCP = 2.2 Hz, phenyl-C), 129.2 (m, 2x phenyl-
C), 130.0 (d, JCP = 8.5 Hz, phenyl-C), 130.3 (d, JCP = 8.5 Hz, phenyl-C),
131.2 (d, JCP = 2.1 Hz, phenyl-C), 131.6 (d, JCP = 9.0 Hz, phenyl-C),
132.2 (m, phenyl-Cipso), 132.5 (d, JCP = 9.9 Hz, phenyl-C), 133.2 (br,
C8), 134.5 (m, phenyl-Cipso), 136.3 (s, C8), 136.5 (s, C13), 136.7 (s,
C9*), 138.4 (m, 2x phenyl-Cipso), 149.9 (s, C15*), 151.7 (s, C15), 156.2
(m, C10*), 157.9 (s, C11), 160.3 (s, C11*), 160.9 (m, C10) 165.7 (m,
C6), 174.2 (m, C6*) ppm. 31P{1H} NMR (121 MHz, CD3CN, 23 °C):
[(Ph-PNN)2Fe](PF6)2 (11). Complex 8 (98.0 mg, 86.2 μmol) and
TlPF6 (136.0 mg, 0.39 mmol) were suspended in DCM (20 mL), and
the reaction mixture was stirred at room temperature for 48 h. The
insoluble precipitate was filtered off and washed with DCM (2 × 10
mL). The solutions were combined, and all volatiles were removed in
1
vacuo to obtain 52.4 mg (58%) of an orange colored solid. H NMR
(400 MHz, CD3CN, 23 °C): δ = 4.01 (dvt, 1 H, 2JHH = 18.0 Hz, JPH = 4.4
−
δ = −143.4 (sept, 1JPF = 706.4 Hz, PF6 ), 43.9 (d, 2JPP = 47.6 Hz, Ph-
2
Hz, Ph2PCHH), 4.76 (dvt, 1 H, JHH = 18.1 Hz, JPH = 7.0 Hz,
2
PNN*), 61.5 (d, JPP = 47.6 Hz, Ph-PNN) ppm. 19F{1H} NMR (377
Ph2PCHH), 6.54 (d, br, 1 H, 3JHH = 5.5 Hz, H14), 6.71 (m, 2 H, H3′),
6.71 (m, 1 H, H14), 6.77 (m, 2 H, H3), 6.93 (vt, 2 H, 3JHH = 7.4 Hz, H2),
7.07 (t, 1 H, 3JHH = 7.4 Hz, H1), 7.22 (vt, 2 H, 3JHH = 7.4 Hz, H2′), 7.45
(t, 1 H, 3JHH = 7.4 Hz, H1′), 7.61 (vt, 1 H, 3JHH = 8.2 Hz, H13), 8.08 (d, 1
H, 3JHH = 8.0 Hz, H12), 8.19 (d, 1 H, 3JHH = 7.8 Hz, H7), 8.36 (vt, 1 H,
−
MHz, CD3CN, 23 °C): δ = −73.6 (sept, 1JPF = 706.9 Hz, PF6 ) ppm.
Reactivity Studies of the Complexes 9 and 10. Reaction of
Complex 9 with D2O. Complex 9 (7.3 mg, 11.7 μmol) was dissolved in
D2O (0.6 mL, 32.7 mmol) under formation of an intense red solution.
Full conversion of 9 to [(iPr-PNN)2Fe]2+ was observed according to the
1H and 31P{1H} NMR spectra. The sample did not show any change
3
1
3JHH = 7.9 Hz, H8), 8.48 (d, 1 H, JHH = 8.0 Hz, H9) ppm. H{31P}
NMR (400 MHz, CD3CN, 23 °C): δ = 4.01 (d, 1 H, 2JHH = 18.2 Hz),
4.76 (d, 1 H, 2JHH = 18.2 Hz), 6.54 (d, br, 1 H, 3JHH = 5.3 Hz, H14), 6.71
(m, 2 H, H3′), 6.71 (m, 1 H, H14), 6.77 (m, 2 H, H3), 6.93 (vt, 2 H, 3JHH
= 7.8 Hz, H2), 7.07 (t, 1 H, 3JHH = 7.5 Hz, H1), 7.22 (vt, 2 H, 3JHH = 7.8
Hz, H2′), 7.45 (t, 1 H, 3JHH = 7.5 Hz, H1′), 7.61 (vt, 1 H, 3JHH = 7.8 Hz,
H13), 8.08 (d, 1 H, 3JHH = 8.0 Hz, H12), 8.19 (d, 1 H, 3JHH = 7.8 Hz,
H7), 8.36 (vt, 1 H, 3JHH = 7.9 Hz, H8), 8.47 (d, 1 H, 3JHH = 8.0 Hz, H9)
ppm. 31P{1H} NMR (162 MHz, CD3CN, 23 °C): δ = −143.4 (sept, 1JPF
after 18 h.
Reaction of Complex 9 with CD3OD. Complex 9 (5.3 mg, 8.5 μmol)
was dissolved in CD3OD (0.6 mL, 14.8 mmol) under formation of an
intense red solution. Full conversion of 9 to [(iPr-PNN)2Fe]2+ was
observed according to the 1H and 31P{1H} NMR spectra. The sample
did not show any change after 14 h. All volatiles were evaporated, and
the sample was exposed to high vacuum for about 6 h resulting in the
formation of a brown solid. The solid was extracted with toluene-d8 (0.6
mL) resulting in a green insoluble residue and a brown solution. 1H and
31P{1H} NMR analysis of the solution shows a mixture of complex 9
with the mono-deuterated ligand (iPr-PNNHD) in a ratio of about 1:7.
31P{1H} NMR (162 MHz, toluene-d8, 23 °C): δ = 11.2 (vt, 2JPD = 42.2
Hz, iPr-PNNHD), 51.6 (m, 9HD) ppm.
−
= 706.6 Hz, PF6 ), 54.2 (s, Ph-PNN) ppm. 19F{1H} NMR (377 MHz,
1
−
CD3CN, 23 °C): δ = −73.6 (sept, JPF = 706.9 Hz, PF6 ) ppm. The
resonances of H3′and H14 are overlapping in the 1H and 1H{31P} NMR
spectra. Magnetic susceptibility (Evans): no paramagnetic shifting of
the reference could be observed (dioxane in CD3CN, 23 °C).
[(Ph-PNN)(Ph-PNN*)Fe](PF6) (12). Route 1. Complex 10 (7.9 mg,
10.4 μmol) and complex 11 (10.7 mg, 10.2 μmol) were suspended in
THF (12 mL), and the reaction mixture was stirred for 36 h at room
temperature. All volatiles were removed in vacuo to obtain 18.6 mg
(quantitative) of a brownish green solid. Route 2: A solution of KOtBu
(10.0 μmol) in CD3CN (0.6 mL) was added to complex 11 (10.5 mg,
10.0 μmol). Full conversion to complex 12 was observed by NMR
spectroscopy. Integration of the benzylic protons as well as the
resonances in the 31P{1H} NMR spectrum indicate deuterium
incorporation in these positions. A deuteration of about 50% was
observed in the protonated as well as the deprotonated benzylic
positions after storing the sample for 9 h at room temperature. The
asterisk denotes the resonances of the dearomatized pincer ligand. The
1H and 1H{31P}NMR show overlapping resonances. The chemical shifts
and the multiplicities of bipyridine resonances that are overlapping with
other resonances were determined by 2D NMR experiments (these data
is represented in italic). 1H NMR (500 MHz, CD3CN, 23 °C): δ = 3.74
(m, 1 H, H5), 4.03 (s, br, 1 H, H5*), 4.54 (m, 1 H, H5′), 6.40 (m, 1 H,
H14*), 6.46 (m, br, 1 H, H15*), 6.67 (m, 5 H, phenyl-H + H9* (6.66
ppm, d)), 6.82 (m, 7 H, phenyl-H + H7* (6.84 ppm, d)), 6.91 (m, 4 H,
phenyl-H + H14 (6.89 ppm, t)), 6.98 (vt, 1 H, 3JHH = 7.3 Hz, phenyl-
Hpara), 7.19 (m, 7 H, phenyl-H + H13* (7.22 ppm, t) + H8* (7.14 ppm,
t)), 7.35 (vt, 1 H, phenyl-Hpara), 7.44 (d, br, 1 H, 3JHH = 8.1 Hz, H12*),
7.48 (vt, 1 H, 3JHH= 8.4 Hz, H13), 7.55 (d, br, 1 H, 3JHH= 5.5 Hz, H15),
7.94 (d, br, 1 H, 3JHH= 8.0 Hz, H12), 8.01 (d, br, 1 H, 3JHH= 7.7 Hz, H7),
8.18 (vt, 1 H, 3JHH= 7.9 Hz, H8), 8.38 (d, br, 3JHH= 7.9 Hz, H9) ppm.
1H{31P} NMR (500 MHz, CD3CN, 23 °C): δ = 3.74 (d, 1 H, 2JHH = 18.2
Hz, H5), 4.03 (s, br, 1 H, H5*), 4.56 (m, 1 H, 2JHH = 17.8 Hz, H5′), 6.40
(m, 1 H, H14*), 6.46 (m, br, 1 H, H15*), 6.67 (m, 5 H, phenyl-H + H9*
(6.66 ppm, d)), 6.82 (m, 7 H, phenyl-H + H7* (6.84 ppm, d)), 6.91 (m, 4
H, phenyl-H + H14 (6.89 ppm, t)), 6.98 (vt, 1 H, 3JHH = 7.3 Hz, phenyl-
Hpara), 7.19 (m, 7 H, phenyl-H + H13* (7.22 ppm, t) + H8* (7.14 ppm,
t)), 7.35 (vt, 1 H, phenyl-Hpara), 7.44 (d, br, 1 H, 3JHH = 8.0 Hz, H12*),
7.48 (vt, 1 H, 3JHH= 7.8 Hz, H13), 7.55 (d, br, 1 H, 3JHH= 5.6 Hz, H15),
Dissolving 9 in Hexylamine. Complex 9 (6.7 mg, 10.7 μmol) was
dissolved in HexNH2 (0.5 mL, 3.4 mmol) and C6D6 (0.1 mL) under
formation of an intense brown solution. No protonation of 9 was
observed according to the 1H and 31P{1H} NMR spectra. The sample
did not show any change after 18 h.
Exposing 9 to H2 Pressure. Complex 9 (3.0 mg, 4.8 μmol) was placed
in a J. Young NMR tube and dissolved in C6D6 (0.6 mL) under
formation of an intense brown solution. The NMR tube was pressurized
two bars of H2. No change in color was observed, and the NMR spectra
did not indicate any reaction after about 3 h.
Reaction of Complex 10 with D2O (neat). D2O (0.6 mL, 32.7 mmol)
was added to complex 10 (5.3 mg, 7.0 μmol) under formation of
suspension (orange colored solution, brown precipitate). Further
addition of D2O (1.0 mL, 54.5 mmol) and sonication in an ultrasonic
bath did not lead to the formation of a solution. Full conversion of 10 to
[(Ph-PNN)2Fe]2+ was observed in the 1H and 31P{1H} NMR spectra in
the orange colored solution. The sample did not show any change after
16 h.
Reaction of Complex 10 with D2O (in THF). Complex 10 (6.8 mg,
8.9 μmol) was dissolved in THF (0.5 mL) to give a brown solution. A
31P{1H} NMR spectrum was recorded, and D2O (0.4 mL, 21.8 mmol)
was added. Upon addition a change in color to intense green was
observed. 1H and 31P{1H} NMR measurements showed the formation
of [(Ph-PNN)(Ph-PNN*)Fe]+ (by comparison to 12) and [(Ph-
PNN)2Fe]2+ (by comparison to 11) in a ratio of 5.9 to 1.0 (according to
integration in the 31P{1H}). The sample did not show any change after
20 h.
Reaction of Complex 10 with CD3OD. Complex 10 (5.6 mg, 7.3
μmol) was dissolved in CD3OD (0.6 mL, 14.8 mmol) forming of an
1
intense redish brown solution. H and 31P{1H} NMR measurements
showed the formation of [(Ph-PNN)(Ph-PNN*)Fe]+ (by comparison
to 12) and [(Ph-PNN)2Fe]2+ (by comparison to 11) in a ratio of 1:1
(according to integration in the 31P{1H} NMR). The sample did not
J
dx.doi.org/10.1021/ic401432m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Comparison of the Spin-State Energies (ΔEe or ΔG298, kcal/mol) for Selected Cases Obtained Using DFT and
Multireference Methods
a
b
c
b
c
b
d
d
complex
geometry
S
DF-PBE
DSD /SVP
DSD /TZVP
CASSCF
NEVPT2
[FeCl4]2−
triplet (C2v)
quintet (D2)
quintet (C1)
1
2
1
2
0
1
2
27.4
0.0
43.4
0.0
41.9
0.0
57.8
0.0
54.2
0.0
27.4
0.0
43.4
0.0
41.9
0.0
60.0
0.0
57.2
0.0
complex 3
3.3
48.1
37.4
0.0
42.4
32.7
0.0
76.0
50.6
0.0
67.6
43.0
0.0
0.0
3.0
a
CASSCF and NEVPT2 results are for the geometry optimized for this spin-state; the DFT results used fully optimized geometries for each spin
b
c
d
state. Energies are ΔG298. DSD = DSD-PBEP86. Energies are ΔEe.
a
Table 4. Crystal Data and Summary of Data Collection and Refinement for 1, 2, 4, 6, 8, and 10
1
2
4
6
8
10
formula
C19H27Cl2FeN2P
C19H27Br2FeN2P + C17H23Br2FeN2P
C2H3N
C34H46FeN4P2 + 2(Br) 2(C46H38FeN4P2) +
C46H36FeN4P2 +
1.5(C6H6)
+ C2H3N
2(Br4Fe) + C2H3N
diffractometer
Bruker Apex II
red prism
Bruker Apex II
orange plate
Bruker Apex II
red chunk
Bruker Apex II
black prism
Nonius
Bruker Apex II
black prism
0.37 × 0.28 × 0.25
879.74
crystal description
crystal size, [mm3]
red plate
0.28 × 0.20 × 0.20 0.24 × 0.20 × 0.04 0.32 × 0.28 × 0.24 0.22 × 0.17 × 0.07
441.15
0.30 × 0.30 × 0.10
2321.22
FW, [g·mol−1
]
571.12
502.01
829.41
space group
P21/c
P1
̅
P21/c
P21/n
Cc
P21/c
crystal system
a, [Å]
monoclinic
8.1888(6)
14.6190(11)
17.4370(13)
triclinic
8.6815(3)
10.2730(4)
13.7241(5)
81.688(2)
87.376(2)
85.020(2)
1205.87(8)
2
monoclinic
8.7592(6)
14.3618(8)
15.2256(2)
monoclinic
10.2676(5)
17.5319(9)
21.1256(11)
monoclinic
12.674(3)
24.218(5)
16.214(3)
monoclinic
17.2483(6)
13.6776(5)
18.6190(6)
b, [Å]
c, [Å]
α, [deg]
β, [deg]
γ, [deg]
cell volume, [Å3]
Z
93.843(4)
90.642(3)
99.049(3)
108.19(3)
102.225(2)
2082.7(3)
4
1915.2(3)
4
3755.5(3)
4
4728.1(19)
2
4292.9(3)
4
ρcacld, [g·cm−3
μ, [mm−1
]
1.407
1.062
17194
4764
1.573
1.741
5.044
22009
4385
1.467
2.647
65745
9497
1.630
4.099
45070
11096
1.361
0.470
46580
12721
]
4.017
no. of reflections
19742
no. of unique
reflections
5991
2Θmax, [deg]
Rint
54.42
0.045
232(0)
56.76
0.021
260(0)
55.12
0.046
212(0)
57.20
0.042
424(0)
55.16
0.068
548(2)
58.26
0.032
584(0)
no. of parameters
(restraints)
b
final R
0.0385
0.0584
1.023
0.0232
0.0287
1.033
0.0284
0.0407
1.056
0.0368
0.0483
0.0536
0.0592
1.073
0.0362
0.0501
1.013
c
final R
GooF
1.144
a
b
c
Collected using MoKα radiation (λ = 0.71073 Å). For data with I > 2σ(I). For all data.
show any change in the product ratio after 26 h. All volatiles were
evaporated, and the sample was exposed to high vacuum for about 24 h,
resulting in the regeneration of complex 10 as a dark brown solid, which
was fully soluble in toluene. 1H NMR analysis of the residue showed that
H/D exchange took place, as a result of CD3OD addition/elimination to
the dearomatized complex 10, resulting in approximately 30%
deuteration of complex 10. 31P{1H} NMR (162 MHz, toluene-d8, 23
°C): δ = 50.4 (m, 10HD), 50.6 (s, 10HH) ppm. The 31P{1H} NMR
spectrum (see Supporting Information) exhibits two superimposed
resonances of 10HH (s) and 10HD (m). The 2D NMR spectrum shows a
broad multiplet of the deuterium atoms in the benzylic position centered
at δ = 4.40 ppm (with respect to the natural abundance deuterium
resonances of toluene-H8).
Exposing 10 to H2 Pressure. Complex 10 (4.1 mg, 5.4 μmol) was
placed in a J. Young NMR tube and dissolved in C6D6 (0.6 mL) under
formation of an intense brown solution. The NMR tube was pressurized
with two bars of H2. No change in color was observed, and the NMR
spectra did not indicate any reaction after about 3 h.
Repeated Reaction of 10 with CD3OD. Complex 10 (10.0 mg, 13.1
μmol) was dissolved in CD3OD (1.0 mL, 24.7 mmol) resulting in the
formation of an intense reddish brown solution. The reaction was stirred
at room temperature for 15 min, and all volatiles were removed in vacuo
resulting in the formation of a brown solid. This procedure was repeated
5 times. The solid was fully soluble in toluene. 1H NMR analysis of the
residue showed approximately 85% deuteration as a result of H/D
exchange of 10 with CD3OD. 31P{1H} NMR (162 MHz, toluene-d8, 23
°C): δ = 50.4 (s, br, 10DD), 50.4 (m, 10HD) ppm. The 31P{1H} NMR
spectrum (see Supporting Information) exhibits two superimposed
resonances of 10DD (s, br) and 10HD (m). The 2D NMR spectrum show
a broad resonance of the deuterium atoms in the benzylic position
centered at δ = 4.35 ppm (with respect to the natural abundance
deuterium resonances of the toluene-H8).
Dissolving 10 in Hexylamine. Complex 10 (5.4 mg, 7.1 μmol) was
dissolved in HexNH2 (0.5 mL, 3.4 mmol) and C6D6 (0.1 mL) under
formation of an intense brown solution. No protonation of 10 was
1
observed according to the H and 31P{1H} NMR spectra. The sample
did not show any change after 18 h.
K
dx.doi.org/10.1021/ic401432m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Computational Details. All DFT calculations were carried out
using Gaussian 09 Revision C.01.31 Two DFT functionals were used.
Geometries were optimized and vibrational frequencies were calculated
using the Perdew−Burke−Ernzerhof (PBE) GGA functional.32
Accurate energies were calculated using one of the latest double−
hybrid functionals by Kozuch and Martin: DSD−PBEP86.33 This
double−hybrid functional incorporates the PBE34 DFT exchange
functional with “exact” Hartree−Fock exchange, the Perdew-86 (P86)
correlation functional35 with “exact” spin−component scaled36 second-
order Møller−Plesset37 (SCS−MP2) correlation, and an empirical
dispersion correction,38 specifically Grimme’s third version of his
empirical dispersion correction (DFTD3)38a,39 with Becke−Johnson
(BJ) dampening.39,40
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the European Research Council
under the FP7 framework (ERC No. 246837) and by the Kimmel
Center for Molecular Design. T.Z. received a Postdoctoral
Fellowship from the MINERVA Foundation, and R.L. received a
postdoctoral fellowship from the Deutsche Forschungsgemein-
schaft (DFG). D.M. holds the Israel Matz Professorial Chair of
Organic Chemistry.
With these two functionals, the split-valence (double-ζ quality) and
the triple-ζ basis sets, each with added polarization functions (i.e., the
SVP and TZVP basis sets, respectively) of Ahlrichs and co-workers were
used.41 The SVP basis set was used for geometry optimizations while
energies were calculated using the TZVP basis set.
To improve the efficiency of the calculations, density-fitting basis sets
(DFBS) were employed during the calculation of the Coulomb
interaction;42 this is indicated by appending “DF-” to the functional
name. The SVPFit density fitting basis set,43 specifically designed for
SVP, was used.
DEDICATION
■
This paper is dedicated to Prof. Dr. Werner Uhl on the occasion
of his 60th birthday.
The accuracy of the DFT method was improved by adding the second
generation empirical dispersion correction recommended by Grimme.44
The older version (DFTD2)38c is available, with analytical gradients and
Hessians, in Gaussian09 and was used during geometry optimizations
and frequency calculations. As noted above, DSD−PBEP86 includes, per
definition, a DFTD3 correction with BJ−scaling in its functional form,
which was calculated using a program written by Grimme.38a
Bulk solvent effects were approximated by single point energy
calculations using a polarizable continuum model (PCM),45 specifically
the integral equation formalism model (IEF-PCM)45a,b,46 with the same
solvents as in the experiments. Specifically, Truhlar and co-workers’
Solvation Model with Dispersion (SMD) was used.47
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data for the complexes 1, 2, 4, 6, 8, and 10
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Corresponding Author
*E-mail: david.milstein@weizmann.ac.il.
L
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