Synthesis and characterization of divalent manganese, iron, and cobalt complexes in tripodal phenolate/N-heterocyclic carbene ligand environments

Article abstract of DOI:10.1021/ic4024053

Two novel tripodal ligands, (BIMPN^Mes,Ad,Me)^- and (MIMPN^Mes,Ad,Me)^2-, combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex's reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by ¹H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field ⁵⁷Fe Moessbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1 - in which a pyridine molecule is situated next to the Mn-Cl bond - and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMEN^R ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.

Full text of DOI:10.1021/ic4024053

Article
pubs.acs.org/IC
Synthesis and Characterization of Divalent Manganese, Iron, and
Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene
Ligand Environments
Martina Kaß,† Johannes Hohenberger,† Mario Adelhardt,† Eva M. Zolnhofer,† Susanne Mossin,‡
̈
Frank W. Heinemann,† Jorg Sutter,† and Karsten Meyer*,†
̈
†Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-University Erlangen-Nuremberg, Egerlandstrasse
1, 91058 Erlangen, Germany
‡Department of Chemistry, Technical University of Denmark, Kemitorvet 207, 2800 Kgs. Lyngby, Denmark
S
* Supporting Information
−
ABSTRACT: Two novel tripodal ligands, (BIMPNMes,Ad,Me
)
and (MIMPNMes,Ad,Me)2−, combining two types of donor
atoms, namely, NHC and phenolate donors, were synthesized
to complete the series of N-anchored ligands, ranging from
chelating species with tris(carbene) to tris(phenolate)
chelating arms. The complete ligand series offers a convenient
way of tuning the electronic and steric environment around
the metal center, thus, allowing for control of the complex’s
reactivity. This series of divalent complexes of Mn, Fe, and Co
1
was synthesized and characterized by H NMR, IR, and UV/
vis spectroscopy as well as by single-crystal X-ray diffraction
studies. Variable-temperature SQUID magnetization measure-
ments in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of
increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field 57Fe Mossbauer spectroscopy of the
̈
Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the
series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is
evident. Particularly, the molecular structure of 1in which a pyridine molecule is situated next to the Mn−Cl bondand those
of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the
corresponding symmetrical TIMENR ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.
in ortho and para positions, respectively, were employed to
activate CO2 and other heteroallene analogues.16
INTRODUCTION
■
Tripodal ligands provide a powerful platform for small
molecule activation chemistry.1−7 The ligand field splitting
resulting from their trigonal coordination environment is
suitable for the stabilization of highly unusual metal−ligand
multiple bonds, even for relatively electron-rich mid to late
transition metals.7,8 This is most impressively exemplified by
the synthesis of an Fe(II) imido complex,9,10 the first terminal
Co(III) imido,11 and the first Fe(IV) nitrido species by Peters
et al.12
The ligand TIMENR (R = mesityl, 2,6-xylyl) enabled the
stabilization and complete characterization of structurally
characterized terminal Fe(IV) nitrido complexes.17 A series of
TIMENR (R = 2,6-xylyl) Mn nitrides in different oxidation
states (III, IV, and V) have also recently been studied.18 In all
cases, the orientation of the imidazolydene substituents of the
coordinated ligand is perpendicular to the metal−carbene
plane, thus forming a narrow cavity that allows only axial access
for small molecules to the reactive metal center. On the other
hand, in the case of the cobalt peroxo complex of the xylyl
ligand derivative,19 two of the carbene arms are pushed apart,
giving the complex an overall distorted octahedral symmetry
that allows side access of organic substrates. Consequently, the
complex was employed in O-atom transfer chemistry to organic
substrates. In contrast, the 3-fold symmetrical iron nitrido and
cobalt imido complexes of TIMENR (R = mesityl, 2,6-xylyl) did
Our laboratory routinely employs two different N-anchored
tripodal ligand systems, the symmetrical tris(phenolate) ligand,
(
R,R′ArO)3N3− (trianion of tris(3,5-R,R′-2-hydroxyphenyl)-
methyl]amine, R, R′ = alkyl)13 and the tris(carbene) ligand
TIMENR (tris[2-(3-R-imidazol-2-ylidene)ethyl]amine, R =
alkyl, aryl).14 A derivative of the tris(phenolate) ligand system
with methyl substituents in the ortho and para positions of the
phenol was first employed by Kleij and co-workers in the
catalytic cycloaddition of CO2 to epoxides and oxiranes with
iron.15 Uranium complexes of the tris(phenolate) ligand
derivative (Ad,MeArO)3N3−, with adamantyl and methyl groups
Received: September 23, 2013
© XXXX American Chemical Society
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not undergo any atom or group transfer chemistry. This could
potentially be due to the inflexibility of the ligand. Instead, the
RN2− and N3− ligands insert into the metal−carbene bond of
the Co(III) imido and one-electron oxidized Fe(V) nitrido
intermediates, forming bis(carbene) imine species (Scheme
1).20,21
We herein report the synthesis and characterization of the
novel, hybrid ligands (BIMPNR,R′,R″)− and (MIMPNR,R′,R″)2−
and their coordination chemistry with divalent manganese, iron,
and cobalt metal ions. While mixed NHC/phenolate ligands
have been reported in the past,23 most recently by Bercaw and
co-workers,24 (BIMPNR,R′,R″)− and (MIMPNR,R′,R″)2− are the
first tripodal ligands that combine NHC and phenolate
chelating arms. Together with the TIMENR and
(R′,R″ArO)3N3− ligands, they provide a complete ligand series,
ranging from N-anchored tris(carbene) to tris(phenolate)
chelating arms.
Scheme 1. Insertion Reactions of the TIMENR (R = mesityl,
2,6-xylyl) Iron and Cobalt Imido20 and Iron Nitrido
Complexes21
RESULTS AND DISCUSSION
■
Ligand Synthesis. The tris(phenolate) ligand
((Ad,MeArO)3N)3− was synthesized, according to the literature,
in a Mannich-type procedure with 2-adamantyl-p-cresole25 and
hexamethylenetetramine with a catalytic amount of p-
toluenesulfonic acid.25 The tris(phenol) amine was deproto-
nated with NaOMe in toluene, from which the sodium salt
precipitates as a colorless solid.
The syntheses of the two new hybrid ligand systems, namely,
(BIMPNR,R′,R″)− and (MIMPNR,R′,R″)2−, are summarized in
Scheme 2. In the first step, the carbene units are attached to the
nitrogen anchor in an SN2 reaction of a halido-triethylamine
with the substituted imidazole. The phenol is chloromethylated
via a Blanc reaction, after which the amine nitrogen attacks the
benzylic position to form the ligand precursors
(H3BIMPNR,R′,R″)2+ and (H3MIMPNR,R′,R″)+, respectively.
These ligand precursors are deprotonated with KOtBu in
THF to yield the potassium salts of the free ligands in near
quantitative yield.
The molecular structures of the cation of (H3-
BIMPNMes,Ad,Me)(OTs)2 and of [K2(BIMPNMes,Ad,Me)2(C6H6)]
are shown in Figure 1; their structural features are discussed in
detail in the Supporting Information.
Coordination to First-Row Transition Metals. The
deprotonated ligands react with divalent anhydrous metal
chlorides of mid to late first-row transition metals (MnCl2,
FeCl2, and CoCl2) to give the complexes 1−12, which are
summarized in Figures 2 and 3.
Consequently, we sought to reduce the steric bulk of the
chelating ligand to enable side access of substrates to the
functional entity. Our first strategy was to remove the ortho-
CH3 groups of the mesityl/xylyl substituents on the
imidazolylidenes. However, the introduction of reactive hydro-
gens in these ortho positions leads to unexpected new reactivity,
resulting in C−H activation and tris-metalation of the ligand.22
Therefore, the design strategy was modified to exchange one or
two carbene arms with phenolates. Introduction of phenolates
still provides protection against bimolecular decomposition
reactions. Nevertheless, the sterically encumbering ortho-
adamantyl substituents on the phenolates are two atoms away
from the coordinating O atom, whereas the NHC aryl
substituents are attached to the N atom next to the carbene
carbon atom; and hence, oriented perpendicularly to the Fe-
tris(carbene) coordination plane. As a result, steric bulk of the
ortho substituents on the phenolate is further away from the
metal center and side access to the metal center ought to
increase. These new ligands (BIMPNR,R′,R″)− (anion of bis[2-
(3-R-imidazol-2-ylidene)ethyl-(3,5-R′,R″-2-hydroxyphenyl)-
methyl]amine) and (MIMPNR,R′,R″)2− (dianion of mono[2-(3-
R-imidazol-2-ylidene)ethyl-bis(3,5-R′,R″-2-hydroxyphenyl)-
methyl]amine) (Chart 1) can be regarded as hybrid or
crossover ligands of the N-anchored tris(carbene) and the
tris(phenolate) ligand systems.
Coordination of the Bis(carbene) Mono(phenolate)
Ligand (BIMPNMes,Ad,Me)− to Mn, Fe, and Co. Reaction of
−
(BIMPNMes,Ad,Me
)
with 1 equiv of anhydrous manganese
chloride in an ether/pyridine mixture (15:1) at room
temperature yields the four-coordinate Mn(II) complex
[(BIMPNMes,Ad,Me)Mn(Cl)] (1) as a white powder in 55%
yield. Salt metathesis from chloride to azide affords the
colorless complex [(BIMPNMes,Ad,Me)Mn(N3)] (2), which is
Chart 1. Series of Tripodal N-Anchored Ligands from Tris(carbene) (Left) to Tris(phenolate) (Right)
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Scheme 2. Synthetic Route for K(BIMPNR,R′,R″) and K2(MIMPNR,R′,R″)
2+
Figure 1. Molecular structures of the (H3BIMPNMes,Ad,Me
)
cation in crystals of (H3BIMPNMes,Ad,Me)(OTs)2·0.2EtOAc (left) and
[K2(BIMPNMes,Ad,Me)2(C6H6)] in crystals of [K2(BIMPNMes,Ad,Me)2(C6H6)]·3Et2O (right). (50% probability ellipsoids; the tosylate counterions,
cocrystallized solvent molecules, andwith the exception of the phenol and imidazole H atomshydrogen atoms are omitted for clarity; C, gray; H,
white; N, blue; O, red; K, light blue).
−
Figure 2. Molecular structures of complexes [(BIMPNMes,Ad,Me)M(Lax)] with M = Mn (Lax = Cl, 1; N3 , 2), M = Fe (Lax = CH3CN, 3MeCN; N3, 4),
−
and M = Co (Lax = none, 5; N3 , 6) (50% probability ellipsoids; solvent molecules, noncoordinating chlorides, and hydrogen atoms omitted for
clarity; C, gray; N, blue; O, red; Mn, magenta; Fe, dark red; Co, purple).
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Figure 3. Molecular structures of complexes [(MIMPNMes,Ad,Me)M] with M = Mn (7), Fe (8), and Co (9) and anions of PNP[((Ad,MeArO)3N)M]
with M = Mn (PNP-10), Fe (PNP-11), and Co (PNP-12) (50% probability ellipsoids; solvent molecules, PNP cations (for PNP-10−PNP-12), and
hydrogen atoms omitted for clarity; C, gray; N, blue; O, red; Mn, magenta; Fe, dark red; Co, purple).
Table 1. Selected Bond Distances [Å] and Bond Angles [deg] for [(BIMPNMes,Ad,Me)MnII(Cl)]·0.5pyridine (1·0.5py),
[(BIMPNMes,Ad,Me)FeII(NCMe)]Cl·2MeCN (3MeCN·2MeCN), [(BIMPNMes,Ad,Me)CoII]Cl·MeCN (5-Cl·MeCN),
[(MIMPNMes,Ad,Me)MnII]·1.5DME·0.5THF (7·1.5DME·0.5THF), [(MIMPNMes,Ad,Me)FeII]·1.84THF·0.46Et2O (8·1.84THF·
0.46Et2O), [(MIMPNMes,Ad,Me)CoII]·2CDCl3 (9·2CDCl3), PNP[((Ad,MeArO)3N)MnII]·2.5DME (PNP-10·2.5DME),
a
PNP[((Ad,MeArO)3N)FeII]·DME·C6H6 (PNP-11·DME·C6H6), and PNP[((Ad,MeArO)3N)CoII]·DME (PNP-12·DME)
bond/angle
M···Nanchor
M−Laxial
M−Ccarb.
M−O
NNHC1−Ccarb.
NNHC2−Ccarb.
NNHC1−Ccarb2
Ccarb2−Ccarb3
NNHC2−Ccarb3
Cph1−O
Cph1−Cph2
Cph2−Cph3
Cph3−Cph4
Cph4−Cph5
Cph5−Cph6
Cph1−Cph6
Nanchor−M−Laxial
Ccarb.−M−Laxial
O−M−Laxial
Ccarb.−M−C′carb.
1
3MeCN
5-Cl
7
8
9
PNP-10
PNP-11
PNP-12
2.696
2.422
2.229
2.001
1.361
1.363
1.383
1.342
1.390
1.319
1.416
1.388
1.390
1.398
1.390
1.433
170.25
103.72
92.45
109.03
130.70
111.60
2.461
2.229
2.111
1.920
1.356
1.363
1.386
1.348
1.387
1.339
1.419
1.398
1.399
1.385
1.395
1.413
177.76
94.52
90.45
120.82
117.61
120.66
2.141
2.220
2.166
2.083
2.169
2.112
2.068
2.024
1.875
1.357
1.359
1.382
1.346
1.387
1.346
1.413
1.396
1.393
1.384
1.394
1.418
2.130
1.930
1.350
1.360
1.388
1.325
1.392
1.334
1.430
1.397
1.390
1.390
1.392
1.416
2.063
1.916
1.360
1.370
1.378
1.347
1.386
1.343
1.420
1.397
1.393
1.375
1.401
1.411
2.009
1.882
1.349
1.360
1.397
1.325
1.396
1.335
1.426
1.397
1.393
1.397
1.382
1.434
1.977
1.952
1.919
1.342
1.42
1.39
1.40
1.39
1.40
1.42
1.333
1.422
1.384
1.388
1.393
1.396
1.428
1.338
1.421
1.399
1.389
1.397
1.399
1.433
117.40
112.72
124.65
115.7
132.1
125.2
115.1
129.5
114.0
Ccarb.−M−O
O−M−O′
128.7
109.9
120.6
118.43
106.84
132.63
117.81
116.60
120.70
111.2
117.3
111.9
NNHC1−Ccarb.−NNHC2
doop
103.3
0.362
103.2
0.113
103.62
104.4
103.3
104.5
−0.262
−0.109
−0.175
−0.238
−0.107
−0.161
−0.247
a
See Chart 2 for atom labeling. Given are the mean values for each bond or angle type; a full list of metric parameters and e.s.d.’s are listed in Tables
S7−S12 (Supporting Information).
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Table 2. Selected Bond Distances [Å] and Bond Angles [deg] for the Ligand Structures (H3BIMPNMes,Ad,Me)(OTs)2·0.2EtOAc
(A) and [K2(BIMPNMes,Ad,Me)2(C6H6) ]·3Et2O·0.5C6H6 (B and B′) and the Azide Complexes [(BIMPNMes,Ad,Me)MnII(N3)]·
3C6H6 (2·3C6H6), [(BIMPNMes,Ad,Me)FeII(N3)]·1.894Et2O·0.606DMF (4·1.984Et2O·0.606DMF), and
[(BIMPNMes,Ad,Me)CoII(N3)]·2THF·0.5C6H6 (6·2THF·0.5C6H6) and, for Comparison, TIMEN Azide Complexes
[(TIMENXyl)Mn(N3)]BPh4·MeCN (C·MeCN), [(TIMENMes)Fe(N3)]BPh4·2.5THF (D·2.5THF), and
a
[(TIMENXyl)Co(N3)]BPh4·MeCN (E·MeCN)
bond/angle
M···Nanchor
M−Laxial
M−Ccarb.
M−O
A
B
B′
2
4
6
C
D
E
2.626
2.125
2.204
2.007
1.190
1.168
1.359
1.356
1.383
1.340
1.386
1.324
1.402
1.390
1.383
1.394
1.390
1.427
168.71
102.57
91.26
130.3
177.9
2.591
2.062
2.123
1.940
1.175
1.160
1.359
1.364
1.384
1.343
1.386
1.327
1.412
1.393
1.384
1.395
1.394
1.429
170.2
101.3
92.75
134.9
177.8
2.659
2.039
2.071
1.932
1.188
1.164
1.356
1.361
1.378
1.339
1.384
1.319
1.411
1.389
1.381
1.392
1.391
1.428
168.9
103.67
89.93
128.3
177.1
3.137
2.031
2.179
3.243
1.947
2.108
3.213
1.938
2.039
Nα−Nβ
Nβ−Nγ
1.184
1.160
1.361
1.370
1.387
1.339
1.388
1.182
1.156
1.362
1.365
1.383
1.339
1.386
1.161
1.169
1.357
1.364
1.384
1.337
1.383
NNHC1−Ccarb.
NNHC2−Ccarb.
NNHC1−Ccarb2
Ccarb2−Ccarb3
NNHC2−Ccarb3
Cph1−O
Cph1−Cph2
Cph2−Cph3
Cph3−Cph4
Cph4−Cph5
Cph5−Cph6
Cph1−Cph6
Nanchor−M−Laxial
Ccarb.−M−Laxial
O−M−Laxial
M−Nα−Nβ
Nα−Nβ−Nγ
1.328
1.335
1.380
1.347
1.335
1.369
1.412
1.400
1.391
1.389
1.390
1.406
1.365
1.370
1.385
1.341
1.390
1.313
1.441
1.392
1.376
1.399
1.391
1.446
1.368
1.373
1.383
1.345
1.385
1.315
1.426
1.390
1.389
1.396
1.395
1.449
175.4
103.9
177.5
105.2
174.3
104.8
167.5
174.5
179.6
115.3
112.5
112.1
166.3
178.3
118.8
111.9
110.5
179.3
119.15
110.97
113.26
Ccarb.−M−C′carb.
Ccarb.−M−O
107.90
107.8
106.95
134.27
111.63
103.1
138.33
108.30
102.9
134.03
112.31
103.0
NNHC1−Ccarb.−NNHC2
doop
108.2
101.6
101.6
103.3
0.521
102.9
0.554
103.4
0.520
0.301
0.271
0.297
a
See Chart 2 for atom labeling. Given are the mean values for each bond or angle type, as well as average values for the two independent molecules
of the [(BIMPNMes,Ad,Me)MII(N3)] complexes; all single values and their e.s.d.’s are listed in Tables S7−S10 (Supporting Information).
CDCl3, or DMSO-d6) feature signals ranging from 55 to −10
ppm, some of which are strongly broadened and, in part,
overlapping with each other, thus, preventing unequivocal
signal assignment and integration. However, the characteristic
spectra with the two comparatively sharp peaks around 50 ppm
with relative intensities of 3:1 allow for the identification and
determination of the purity of the complex. While the solid-
state molecular structure derived by X-ray crystallography
exhibits C1 symmetry (vide infra), with two complexes related
Chart 2. Schematic Drawing of Complexes with Atom Labels
Used in Tables 1 and 2
1
by an inversion center in the unit cell, the H NMR spectra in
polar solvents suggest a dynamic behavior in solution, resulting
in Cs symmetry on the NMR time scale. The structure of the
complex in solution also appears to vary depending on the
solvent’s ability to coordinate the metal and solvate the chloride
counterion, as is indicated by the markedly different signal
easily identified by the intense and very characteristic v (N ) IR
̃
3
vibrational bands (in KBr: 2077 and 2056 cm−1). Both MnII
complexes are NMR-silent. The UV/vis-spectra of both
complexes feature bands with maxima at 257 and 309 nm of
similar intensity (ε = 11 300 and 5750 M−1 cm−1 for 1, 14 900
and 7400 M−1 cm−1 for 2), which we tentatively assign to a
π−π* transition in the phenolate and a charge-transfer (CT)
transition, respectively.26
1
distribution of the H NMR spectrum in THF-d8 compared to
MeCN-d3, CDCl3, or DMSO-d6.
Salt metathesis from chloride to azide affords
[(BIMPNMes,Ad,Me)Fe(N3)] (4), which, similar to 2, is identified
A yellow powder of [(BIMPNMes,Ad,Me)Fe]Cl (3) was
obtained in a procedure analogous to that used for 1 in 80%
by the intense and characteristic v (N ) IR vibrational bands (in
̃
3
KBr: 2086, 2054, and 2001 cm−1).27 The 1H NMR spectra of 4
in polar solvents, such as CDCl3 and MeCN-d3, are practically
1
yield. The paramagnetic H NMR spectra of 3 (in MeCN-d3,
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identical to those of 3, whereas, in THF-d8, the signals of both
complexes differ significantly, clearly indicating that the azide
and chloride counterions are coordinated only in THF
solutions.
distance of ca. 3 Å; see Figure 4). This observation
demonstrates the intended steric flexibility of the
(BIMPNR,R′,R″)− ligand to provide side access for organic
substrates to the reactive center.
The green cobalt complex [(BIMPNMes,Ad,Me)Co]Cl (5-Cl)
was obtained by stirring an excess of K(BIMPNMes,Ad,Me) and
CoCl2 in benzene for at least 24 h, during which time the crude,
green complex precipitates. Impurities of KCl can gradually be
reduced by repeatedly redissolving the crude material in MeCN
or CH2Cl2, followed by filtration through Celite and
evaporation. The difficulty in removing KCl from the bulk
material of all (BIMPNMes,Ad,Me)− chloride complexes (1, 3, and
5-Cl) suggests some interaction with the complex, e.g.,
coordination of K+ to the phenol oxygens (see below).
Figure 4. Space-filling representations of the molecular structure of 1
in crystals of [(BIMPNMes,Ad,Me)MnIICl]·0.5pyridine; side view with
and without the pyridine molecule situated between two ligand arms
(left and middle) and top view (right) down the Mn−Cl axis; C, gray;
H, white; N, blue; O, red; Cl, green; Mn, magenta.
−
−
Anion exchange of Cl− to PF6 or N3 allows for a more
straightforward workup to obtain the analytically pure
complexes [(BIMPNMes,Ad,Me)Co]PF6 (5-PF6) and
[(BIMPNMes,Ad,Me)Co(N3)] (6) that are easily identifiable by
their characteristic strong IR bands (in KBr: 843 cm−1 for PF6 ,
−
2081, 2044, and 1999 cm−1 for the coordinated azide in 6).27
When dissolved in MeCN, the blue azide complex 6 changes
its color to the same green color as the azide-free complexes
[(BIMPNMes,Ad,Me)CoII]+ dissolved in MeCN. Accordingly, the
1H NMR spectra of the CoII-complexes 5-Cl, 5-PF6, and 6 in
MeCN-d3 and CDCl3 feature the same signals (within the error
margins for highly temperature-dependent paramagnetic NMR
shifts). In THF-d8, the signal distribution of 5-PF6 remains the
same, whereas 6 gives rise to a very different spectrum with
more signals. All of these observations lead to the conclusion
thatin MeCN and chloroform solutionsthe divalent Fe
and Co complexes [(BIMPNMes,Ad,Me)M]+ (M = Fe, Co) exist
in the monocationic form with the anion not coordinated, but
Nearly colorless prismatic crystals of 3 suitable for X-ray
crystallography were obtained by slow diethyl ether diffusion
into an acetonitrile solution of the complex. The molecular and
crystal structure reveals an acetonitrile molecule bound axially
to the iron center (3MeCN). The coordination geometry at the
iron center can be described as nearly trigonal-bipyramidal,
with the iron atom positioned 0.113(1) Å above the plane
defined by the two carbene carbons and the oxygen atom and a
near linear angle between the anchoring and acetonitrile
nitrogens ((NMeCN−Fe−Nanchor) = 177.76(5)°). An Fe−Nanchor
distance of 2.462(2) Å is indicative of a weak bonding
interaction between the nitrogen anchor and the iron center.
Single crystals of 5-Cl were also obtained from an acetonitrile
solution.30 While the manganese center in 1 contains an axially
coordinated chlorido ligand and the axial position in the
corresponding iron complex 3MeCN is occupied by an
acetonitrile solvent molecule, the cobalt center in 5-Cl is
coordinated solely by the chelating (BIMPNMes,Ad,Me)− ligand in
a distorted trigonal-pyramidal fashion. Thus, the cobalt ion
prefers binding to the nitrogen anchor over the chloride
counterion or an acetonitrile solvent molecule. Consequently,
going from Mn to Fe to Co, the out-of-plane shift doop above
solvated, whereas, in THF solution, the Cl− or N3 anions
−
coordinate to the metal centers.
In addition to the absorption bands observed for the Mn(II)
complexes, the UV/vis spectra of the (BIMPNMes,Ad,Me)− cobalt
complexes exhibit metal-centered Laporte and spin-forbidden
d−d transitions of low intensity.
Tables 1 and 2 summarize bond lengths and angles of all new
complexes, along with selected TIMENR azide complexes for
comparison, as well as the out-of-plane shift doop, which is
defined as the distance of the metal center from the least-
squares plane defined by the three coordinating atoms of the
tripodal ligand, i.e., the carbene carbon and phenolate oxygen
atoms. In general, the Mn−Ccarb bond distances observed in
complexes 1, 2, and 7 (2.130(5)−2.230(3) Å) compare well
with those in the literature reported Mn−NHC complexes
(2.175(3)−2.277(4) Å).28 Similarly, the Fe−Ccarb bond lengths
seen in complexes 3MeCN, 4, and 8 (2.063(4)−2.144(3) Å) fall
within the range as those observed for literature Fe−NHC
complexes (2.072(3)−2.166(10) Å).29
Crystals of 1 suitable for X-ray crystal structure determi-
nation were obtained as colorless prisms by slow diethyl ether
diffusion into a pyridine solution of the complex. The molecular
structure of this manganese complex exhibits C1 symmetry. The
central manganese(II) ion is coordinated by the NHC
carbenes, phenolate oxygen, and the chloride in a distorted
trigonal-pyramidal geometry. The manganese is located above
the plane of the three donor atoms O1, C3, and C8 (toward
Cl1), as is indicated by the corresponding doop of 0.362(2) Å.
Notably, a non-coordinated pyridine molecule is situated
between the adamantyl and one of the mesityl substituents,
with one C−H bond pointing toward the Mn−Cl bond (at a
the plane decreases from 0.362(2) Å (1) to 0.113(1) Å (3MeCN
)
to −0.261(1) Å (5-Cl) below the plane toward the N-anchor.
Accordingly, d(M−Nanchor) decreases from 2.695(2) Å (no
interaction in 1) to 2.461(2) Å (weak interaction in 3MeCN), to
2.141(2) Å (cobalt bound to Nanchor in 5-Cl).
In the molecular structures of the azide complexes 2, 4, and
6, the metal center is coordinated in a trigonal-pyramidal
fashion, with the carbene and phenolate C and O donor atoms
−
of the (BIMPNMes,Ad,Me
)
ligand occupying the equatorial
positions while the azide resides in the axial position. For all
three azide complexes, two crystallographically independent
molecules are present within the unit cell. Table 2 summarizes
the complex metrics of one representative only; parameters for
each independent molecule are listed in Tables S7−S10
(Supporting Information).
Notably, the azide is coordinated in its preferred bent
coordination mode (e.g., in 4: Fe1−N6−N7 = 134.9(2)°) in
contrast to the azide ligands in the corresponding TIMENMes
complexes, which are forced into linear coordination by the
cylindrical cavity formed by the mesityl substituents of the
NHCs ([(TIMENMes)Fe(N3)]+: 174.5(2)°).17 Therefore, even
with the bulky adamantyl substituent on the phenol, the steric
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pressure at the metal center has already been decreased
compared to that in the TIMENR-complexes by the
substitution of one of the NHCs with a phenol arm. The
azide protrudes into the gap between the phenolate and one of
the carbene arms thus widening the corresponding angles
(CCarb.−M−O) to over 130° (see Figure 5).
atoms closer to the ligand N-anchor (doop = −0.109, −0.175,
and −0.238 Å for 7, 8, and 9).
Coordination of the Tris(phenolate) Ligand
((Ad,MeArO)3N)3− to Mn, Fe, and Co. Reaction of Na3-
((Ad,MeArO)3N) with anhydrous manganese chloride in an
acetonitrile/DME mixture (9:1) in the presence of [PNP]Cl
(PNP = PNP+ = bis(triphenylphosphine)iminium) generates
1H NMR-silent, yellow [PNP][((Ad,MeArO)3N)Mn] (PNP-10)
in 75% isolated yield.
Analogously, reaction of Na3((Ad,MeArO)3N) with 1 equiv of
anhydrous ferrous chloride in a 1:1 mixture of THF and
acetonitrile at room temperature yields the four coordinate
Fe(II) complex Na[((Ad,MeArO)3N)Fe] (Na-11) as a colorless
solid in 94% yield. Exposure to oxygen or chloroform yields the
neutral, dark brown, air-stable Fe(III) complex.31 The para-
magnetic 1H NMR spectrum (THF-d8) of Na-11 features seven
signals between 30 and 0.5 ppm, consistent with the expected
C3 symmetry on the NMR time scale. Due to paramagnetic
broadening, however, the unambiguous assignment of these
characteristic signals is not possible.
Figure 5. Space-filling representations of the molecular structure of 4
in crystals of [(BIMPNMes,Ad,Me)FeIIN3]·1.894Et2O·0.606DMF; side
view (left) and top view (right), along the Nazide−Fe−Nanchor axis; C,
gray; H, white; N, blue; O, red; Co, purple.
The addition of anhydrous CoCl2, dissolved in acetonitrile,
to Na3((Ad,MeArO)3N) at room temperature yields a dark blue
solution. After filtration, removal of the solvent, and
recrystallization from THF/hexane, dark blue crystals of
Na[((Ad,MeArO)3N)Co]·2THF (Na-12·2THF) were obtained
The azide’s linear coordination mode in the [(TIMENMes)-
M(N3)]+ complexes (M = Mn, Fe, Co) is accompanied by an
M−Nα bond that is approximately 0.1 Å shorter than the
corresponding bond in the (BIMPNR,R′,R″)− azide complex of
the same metal. Additionally, the [(TIMENMes)M(N3)]+
complexes exhibit a larger displacement of the metal ions
above the plane of the three coordinating carbenes of the
TIMENMes ligand (doop), all of which indicates increased double
bond character between the metal center and the coordinating
Nα nitrogen of the azide ligand.
1
in 68% yield. The H NMR spectrum (MeCN-d3) shows six
paramagnetic signals between 39 and −3 ppm as well as
resonances from cocrystallized THF. Considering the likely C3
symmetry in solution, the signal at 8.68 ppm, which is
considerably broader than the other five signals (220 Hz
compared to 30 Hz), may arise from an accidental degeneracy
of two resonances. In spectra recorded in THF-d8, this signal is
more resolved, and hence, two overlapping signals at 9.40 ppm
(broad, half-width 300 Hz) and 8.60 ppm (half-width 90 Hz)
are detectable. The chemical shifts of the other signals are very
similar to those of the spectrum in acetonitrile. Although an
unambiguous signal assignment of the paramagnetic spectra is
Coordination of the Mono(carbene) Bis(phenolate)
Ligand (MIMPNMes,Ad,Me 2−
to Mn, Fe, and Co. Under an
)
2−
inert gas atmosphere, the reaction of the (MIMPNMes,Ad,Me
)
1
not possible, the H NMR spectra are characteristic of the
ligand with 0.7 equiv of anhydrous manganese, ferrous, or
cobalt dichloride yields [(MIMPNMes,Ad,Me)Mn] (7) and
[(M IMPNM e s , A d , M e ) F e ] (8 ) a s c ol o rl e s s a nd
[(MIMPNMes,Ad,Me)Co] (9) as green solids in approximately
60% isolated yields.
synthesized compounds and provide useful information
regarding the compounds’ identity and purity.
Single crystals of 11 and 12 were obtained after cation
exchange from a sodium counterion to the bis(triphenyl-
phosphine)iminium (PNP+) (PNP-10, PNP-12). In all three
molecular structures, the coordination of the chelating
tris(phenolate) at the central metal is best described as
distorted trigonal-pyramidal.
1
1
While complex 7 is H NMR-silent, the H NMR spectrum
of 8 (THF-d8) features five paramagnetically broadened and
shifted signals from 40 to −7 ppm with half-widths ranging
from 200 to 340 Hz. In contrast, the spectrum of 9 (CDCl3)
features 10 signals from 45 to −5 ppm. The less detailed
spectrum of 8 prevents a definitive signal assignment but is
characteristic of the spectroscopically and analytically pure
complex.
As in the (MIMPNMes,Ad,Me 2− system, all four bond distances
)
to the metal center decrease in going from Mn (PNP-10) to
Co (PNP-12), with the divalent metal ion moving more and
more below the phenolate oxygen plane toward the anchoring
N atom. For a given metal, the M−Nanchor bond distance is
shorter, and the average MO bond length is longer in the
The molecular and crystal structures of all three
[(MIMPNMes,Ad,Me)MII] (M = Mn, Fe, Co) complexes, derived
by single-crystal X-ray crystallography, feature a metal center
tris(phenolate) system than in complexes with the (MIM-
2−
PNMes,Ad,Me
)
ligand. This trend persists even when the
coordinated in a trigonal-pyramidal fashion solely by the
structure of the (BIMPNMes,Ad,Me
)
complex 5-Cl is taken
−
2−
chelating (MIMPNMes,Ad,Me
)
ligand. The metal center of the
into account.32 For the cobalt series 5-Cl, 9, and PNP-12, the
neutral complex is located below the plane defined by the
following trend applies: the more phenolates, the longer the
carbene carbon and two phenolate oxygen atoms (negative
average M−O bond, and the shorter the M−Nanchor
.
d
oop), demonstrating that binding to the nitrogen anchor seems
Interestingly, while the phenolate ligand is known to be
redox-active, and occasionally exhibits radical character,33−35
the divalent complexes presented herein exhibit full aromaticity
of the phenolate chelating arms. None of the C−C bonds of the
phenolate ring in either the potassium salt of the free ligand
favored over binding to an axial ligand trans to the N anchor,
such as available halide anions or coordinating solvent
molecules. As the ionic radius of the divalent ion decreases
from Mn to Co, all four M−L bond distances decrease and the
out-of-plane shift of the metal ions increases, moving the metal
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(structure of K(BIMPNMes,Ad,Me), vide supra) or the 12
complexes 1 to PNP-12 differ significantly from 1.39 Å (see
Tables S11 and S12 in the Supporting Information for full lists
of relevant metric parameters). The only exceptions are the two
bonds in the phenolate ring with adjacent carbon atoms bearing
substituents (the methylene bridge to the N-anchor, the
phenolate oxygen or the adamantyl substituent). Apparently,
the steric strain introduced by these substituents lengthens
these bonds by about 0.02−0.03 Å.36
The space-filling representations of the three cobalt
complexes 5-Cl, 9, and PNP-12 (Figure 6) reveal that
Figure 6. Space-filling representations of the three new Co(II)
complexes, each with side view (left) and top view (right) along the
M−Nanchor axis; C, gray; H, white; N, blue; O, red; Co, purple.
Figure 7. Temperature-dependent SQUID magnetization data (1 T)
−
−
for complexes of (BIMPNMes,Ad,Me
)
(top), (MIMPNMes,Ad,Me
)
replacement of NHCs with phenolates reduces the steric bulk
as desired. However, in the absence of an axial ligand, the
mesityl substituents on the remaining NHC rings seem to bend
down and obstruct axial access to the metal center. The
molecular structures of the azide complexes 2, 4, and 6, as well
as [(BIMPNMes,Ad,Me)Mn(Cl)] (1) with its intercalated
pyridine, prove, however, that the new ligands are flexible
enough to enable side access for organic substrates, a feature
that is not present in complexes of the tris(carbene) TIMENR
system.
Magnetization Measurements. Variable-temperature
SQUID magnetization measurements (2−300 K) confirm
high-spin ground states for all complexes and show remarkably
similar temperature dependencies of the magnetic moment for
complexes of the same metal ion across the ligand series
(Figure 7, Table 3): At room temperature, manganese
complexes 1, 7, and PNP-10 possess magnetic moments, μeff,
between 5.8 and 6.0 μB (S = 5/2 ground state) that decrease
negligibly with decreasing temperature down to 10 K. From
this temperature on, μeff drops to values between 5.1 and 5.3 μB
at 2 K. Likewise, iron complexes 8 and Na-11 possess room-
temperature magnetic moments of 5.04 and 4.92 μB (S = 2
ground state), which decrease slightly with decreasing temper-
ature (4.89 and 4.70 μB at 15 K) and result in low-temperature
values of 3.70 and 3.59 μB at 2 K. While the μeff value of the
(middle), and ((Ad,MeArO3)N)3− (bottom). Magnetic moment (μeff)
plotted vs temperature (T). Magenta: Mn complexes 1, 7, and 10;
brown: Fe complexes 8 and Na-11; blue: Co complexes 5-PF6, 9, and
Na-12; red lines: simulation. Data were corrected for underlying
diamagnetism. Reproducibility was confirmed by measuring multiple
independently synthesized samples.
manganese and iron complexes at room temperature are found
near the spin-only values (5.92 μB for S = 5/2, 4.90 μB for S =
2), the magnetic moments of the cobalt complexes are
reproducibly around 4.3−4.4 μB and, thus, significantly larger
than the spin-only value for an S = 3/2 system (3.87 μB),
suggesting a considerable contribution from spin−orbit
coupling. The magnetic moments of 5-PF6, 9, and Na-12
decrease slightly with decreasing temperature until they start
dropping at approximately 70 K to values between 3.1 and 3.4
μB at 2 K. Simulations confirm a trend for the zero-field-
splitting parameter D that is already evident by visual inspection
of the curve progressions, namely, that |D(Mn)| < |D(Fe)| ≪
|D(Co)|. Azide complexes 2 and 6 display virtually the same μeff
at RT as their analogous complexes 1 and 5-PF6 with chloride
−
and PF6 counterions; however, the μeff remains high for a
larger temperature range, indicative of considerably smaller
zero-field splittings (see Figures S11 and S25 and Table S1,
Supporting Information).
H
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Table 3. Effective Magnetic Moments μeff at Room Temperature Derived from the Magnetization Measurements and the
Corresponding Parameters Used for Simulation of the Experimental Data
complex
μeff (RT) [μB]
g value
|D| [cm−1
]
|E/D|
[(BIMPNMes,Ad,Me)MnCl]
[(BIMPNMes,Ad,Me)Mn(N3)]
[(BIMPNMes,Ad,Me)Co]PF6
[(BIMPNMes,Ad,Me)Co(N3)]
[(MIMPNMes,Ad,Me)Mn]
[(MIMPNMes,Ad,Me)Fe]
(1)
5.82
5.78
4.28
4.42
6.00
5.04
4.40
5.96
4.92
4.42
1.970
1.958
2.202
2.266
2.030
2.045
2.234
2.008
1.982
2.302
0.426
0.365
26.837
7.086
2.041
4.359
29.720
0.093
4.552
20.395
0.000
0.019
0.000
0.305
0.025
0.333
0.000
0.002
0.333
0.000
(2)
(5-PF6)
(6)
(7)
(8)
[(MIMPNMes,Ad,Me)Co]
(9)
PNP[((Ad,MeArO)3N)Mn]
Na[((Ad,MeArO)3N)Fe]
Na(THF)2[((Ad,MeArO)3N)Co]
(PNP-10)
(Na-11)
(Na-12·2THF)
Table 4. Mossbauer Parameters for All Iron Complexes Discussed Herein
̈
complex
δ [mm·s−1
]
ΔEQ [mm·s−1
]
Γfwhm [mm·s−1
]
39
[(TIMENMes)FeII]OTf2
0.59(1)
0.68(1)
0.81(1)
1.06(1)
1.04(1)
0.687(1)
0.83(1)
1.02(1)
3.28(1)
3.09(1)
2.08(1)
2.24(1)
2.267(3)
3.24(1)
0.40(1)
0.27(1)
0.38(1)
0.37(1)
0.30(1)
0.48(1)
0.31(1)
[(BIMPNMes,Ad,Me)FeII]Cl
[(MIMPNMes,Ad,Me)FeII]
Na[((Ad,MeArO)3N)FeII]
PNP[((Ad,MeArO)3N)FeII]
[(TIMENMes)FeII(N3)]BPh4
[(BIMPNMes,Ad,Me)FeII(N3)]
(3)
(8)
(Na-11)
(PNP-11)
17
(4)
Mossbauer Spectroscopy. The zero-field 57Fe Mossbauer
TIMENMes complexes, which may be accounted for by the
reduced symmetry at the iron center. This ligand contribution
to the total EFG results in a less symmetric distribution of the
electron density in the mixed NHC/phenolate complexes (the
formal valence contribution to the EFG is the same for all high-
spin Fe(II) complexes presented herein). Accordingly, the ΔEQ
of 8 is similar compared to 3, since the ligand field is
comparable in symmetry. In contrast, the (idealized) C3
symmetrical tris(phenolate) complex 11 gives rise to an “in-
between” quadrupole splitting, independent of the counterion
of the complex. The crystal structure of PNP-11, with a PNP+
cation, reveals that the C3 symmetry is somewhat distorted by
intercalation of the cation’s phenyl substituents (in the solid
state). A similar distortion may be caused by the sodium cations
binding to the phenyl moieties in complexes Na-11. Although
single-crystals of Na-11 with a sodium cation have not been
obtained, this kind of distortion has been observed in the very
closely related cobalt complex, namely, Na[(tBu,tBuArO)3N)Co]
(see the Supporting Information for synthesis and XRD
studies), where the sodium cation is bound to two of the
three aryloxides just above the Co site. This significantly
reduces the complexes’ C3 symmetry.41 However, the same
compound, crystallized with the bulky PNP+ cation, does not
have direct interactions between the cation and the aryloxide
oxygens. The two different resultant complex symmetries are
also directly observable in frozen solution at low temperature
by X-band EPR spectroscopy; see Figure 8. The EPR spectra of
the S = 3/2 Co(II) is very different depending on the solvent: If
the compound Na[(tBu,tBuArO)3N)Co]·THF is dissolved in
polar CH3CN, the result is a well-defined axial spectrum,
showing hyperfine coupling to 59Co (I = 7/2) only in the
parallel component. If the same compound is dissolved in
nonpolar benzene, a rhombic spectrum is obtained with the
same average g-values but with pronounced splitting between
the two perpendicular components, which is due to rhombic
zero-field splitting and/or splitting in the perpendicular g-
values.42 We explain the EPR spectral change from axial to
̈
̈
spectra of all new iron complexes presented herein feature
doublets with isomer shifts, δ, in agreement with high-spin
iron(II) complexes (see Table 4).37,38 The isomer shifts δ of
the respective iron(II) complexes increase stepwise as carbenes
are substituted by phenolates in the ligand series from the
tris(carbene) all the way to the tris(phenolate) ligand
derivative. This is most evident when comparing the complexes
without additional axial ligands, [(TIMENMes)Fe]OTf2 (δ =
0.59 mm·s−1), 3, 8, and Na-11 (δ = 1.06(1) mm·s−1),39 where
each phenolate arm adds 0.1−0.2 mm·s−1 to the observed
isomer shift; the same is observed for the two azide complexes
[(TIMENMes)Fe(N3)]BPh4 and [(BIMPNMes,Ad,Me)FeII(N3)]
(4). This trend is accounted for by the different electronic
properties of the NHC and phenolate ligands. While the σ-
donating NHCs also engage in metal-to-ligand π-backbond-
ing,40 the phenolates are σ and π donors. This leads to a higher
d-electron density at the metal, which, in turn, increases the
shielding of the s orbitals vs the nuclear charge. The s orbitals
expand in volume and the electron density within the iron
nucleus is effectively lowered, which increases the isomer shifts,
δ, in 57Fe Mossbauer spectroscopy.
̈
For a variety of simple, tetrahedral bis(carbene) dihalide
Fe(II) complexes, isomer shifts and quadrupole splittings of δ =
0.70−0.81 mm·s−1 and ΔEQ = 3.67−4.03 mm·s−1 have been
reported.29i,j While these Mossbauer parameters appear to be
̈
characteristic for four-coordinate, divalent iron−carbene
species, the complexes investigated in this study show
significantly different values, ranging from δ = 0.59−1.06
mm·s−1 and ΔEQ = 1.02−3.28 mm·s−1. This is most likely due
to the considerably different coordination geometry and ligand
environment.
It is less obvious to formulate a trend for the observed
quadrupole splittings, ΔEQ, in the series of high-spin Fe
complexes measured and presented herein. In general, the
−
quadrupole splitting of the (BIMPNMes,Ad,Me
)
Fe(II) com-
plexes is considerably larger than that of the corresponding
I
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corresponding TIMENR ligands, their potential to allow side
access to the reactive center for organic substrates.
The complete ligand series offers excellent tunability of the
electronic and steric environment at the metal center of the
complexes, allowing adjustment of the complexes’ reactivity.
Additionally, the modular synthesis of the mixed ligands allows
a combination of different substituents on the NHC and
phenolate moieties. We are currently exploring the complexes’
capacity for small molecule activation. Studies of the electro-
chemical and chemical redox behavior are underway. This work
includes photolysis experiments of azide complexes 2, 4, and 6
for the synthesis of potentially catalytically active nitride
complexes.
Figure 8. Left graph (rhombic): The black trace is the experimental X-
band EPR spectrum (12 K, ν = 8.99756 GHz, power = 0.249 mW,
modulation amplitude = 0.25 mT) of Na[((tBu,tBuArO)3N)CoII]·2THF
dissolved in a pure benzene glass. The red trace is the simulation using
a rhombic S = 3/2 spin Hamiltonian with a large zero-field-splitting
parameter: D ≫ hν. The simulation parameters used were: D = −10
cm−1, E/D = −0.08, gz (=g∥) = 2.02, gy = 2.10 and gx = 2.33, A∥ = 4 ·
10−4 cm−1 (hyperfine coupling to 59Co, I = 7/2), line width = 27 mT,
but only the gz value is determined independently, since both a
nonzero rhombic zero-field-splitting parameter, E, and nonequal g⊥
values will split the perpendicular line at g = 4.3. Right graph (axial):
The black trace is the experimental X-band EPR spectrum (13 K, ν =
8.99479 GHz, power = 0.249 mW, modulation amplitude = 0.25 mT)
of exactly the same salt: Na[((tBu,tBuArO)3N)CoII]·2THF, but this time
dissolved in a pure acetonitrile glass. The red trace is the simulation
using an axial S = 3/2 spin Hamiltonian with a large zero-field-splitting
parameter: D ≫ hν. The simulation parameters used were: D = −10
cm−1, E/D = 0, gz (=g∥) = 2.02, gy = gx (=g⊥) = 2.24, A∥ = 9 · 10−4
cm−1, line width = 4 and 21 mT for parallel and perpendicular,
respectively.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic and analytical details, magnetization data and
simulation parameters, and crystallographic details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: karsten.meyer@fau.de. Fax: (+49) 9131-852-7367.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Henkel AG & Co. KGaA and the Friedrich-
Alexander-University of Erlangen-Nuremberg for financial
support of this work. Martina Kaß is grateful for a Ph.D.
̈
rhombic symmetry in a nonpolar solvent in analogy with the
cation bonding motif in the X-ray structures. The sodium ion is
unable to be solvated separately from the complex anion in
benzene solution, whereas it is possible in CH3CN solution
thus giving an idealized axial symmetry of the complex anion.
fellowship provided by the Studienstiftung des Deutschen
Volkes. We thank Dr. Carola Vogel for the synthesis and
characterization of [(TIMENMes)Fe](OTf)2.
REFERENCES
■
(1) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9−16.
(2) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76−78.
(3) Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia by
Molybdenum. In Catalysis without Precious Metals; Bullock, R. M., Ed.;
Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2010;
pp 25−50.
CONCLUSIONS
■
In summary, two novel tripodal ligands, (BIMPNMes,Ad,Me)− and
(MIMPNMes,Ad,Me)2−, combining NHC carbene and phenolate
oxygen donors, were synthesized in this study. These chelators
complete the N-anchored ligands series with donor function-
alities ranging from tris(carbene) to tris(phenolate). The
coordination chemistry of these mixed donor ligand systems
and that of the all-phenolate system (Ad,MeArO3)N3− are
reported for divalent Mn, Fe, and Co ions. The ligands and
complexes presented here were characterized by single-crystal
X-ray diffraction studies, IR, UV/vis spectroscopy, and 1H
NMR spectroscopy.
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(30) The molecular structures of 5-Cl and 5-PF6 are very similar;
therefore, only one structure is discussed here and the other is given in
the Supporting Information.
(31) Details for the synthesis, crystallography, and spectroscopy of
this complex will be reported elsewhere.
(32) Because of the additional axial ligands, the other
−
(BIMPNMes,Ad,Me
comparable.
)
complexes’ molecular structures are not directly
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(27) For the coordinated azide ligand, a total of three absorption
bands are expected in the IR vibrational spectrum, originating from the
symmetric νs(N3) and the asymmetric νas(N3) stretching vibrations at
around 1300 and 2000 cm−1, respectively. The third absorption is
assigned to the deformation vibration ν(N3) at approximately 400
cm−1. Thus, strictly speaking, for a mono-azide metal complex, only
one νas(N3) absorption band is expected in the 2000 cm−1 region. We
note, however, that the azide complexes of the bis(carbene)
mono(phenolate) ligand in complexes 2, 4, and 6 crystallize with
two independent molecules per unit cell and that the chelate allows for
a variety of M−N3 conformations. This likely results in a number of
different νas(N3) vibration frequencies and absorption bands. In
addition, because of interactions with the “matrix”, such as KBr in the
solid state or a solvent molecule in solution, the number of absorption
bands can vary. In a simple, classic case of a coordinated azide, e.g.,
TlN3, Dehnicke reports ( Dehnicke, K. Z. Anorg. Allg. Chem. 1974,
409, 311 ) two absorption bands centered at 2037 and 2000 cm−1 for
νas(N3) and two bands at 1328 and 1319 cm−1 for the νs(N3)
vibration. We note that the solution IR spectrum of 6 also shows two
vibrational bands at 2081 and 2041 cm−1 (see the Supporting
Information, Figure S34).
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(41) See the Supporting Information for the synthesis and
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crystal structures.
(42) Only simulations with nonzero E result in reasonable g-values.
To determine parameters more accurately, it would be necessary to
perform an experiment at another frequency such as Q-band.
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