Aluminum-stabilized low-spin iron(II) hydrido complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane

Article abstract of DOI:10.1021/ic500195q

We investigated herein the reactions of (Me₃tacn)FeCl _n (1a: n = 3, 1b: n = 2) with common aluminum hydride reagents and a bulky dihydridoaluminate {Li(ether)₂}{Al(OC₆H ₃-2,6-^tBu₂)}(μ-H)₂, which yielded the diamagnetic hydrido complexes 2-4 containing Fe(II) and Al(III). In particular, the use of divalent 1b afforded excellent isolated yields. The structures of 2-4 were determined using spectroscopic and crystallographic analyses. The crystal structures showed distorted octahedral Fe centers and fairly short Fe-Al distances [2.19-2.24 A]. The structures of cation moiety 2 and neutral complex 4 were further probed using DFT calculations, which indicated a stable low-spin Fe(II) state and strongly electron-donating nature of the (Me₃tacn)FeH₃ fragment toward the Al(III) center.

Full text of DOI:10.1021/ic500195q

Article
pubs.acs.org/IC
Aluminum-Stabilized Low-Spin Iron(II) Hydrido Complexes of 1,4,7-
Trimethyl-1,4,7-triazacyclononane
Masataka Oishi,† Togo Endo,† Masato Oshima,‡ and Hiroharu Suzuki*,†
†Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552,
Japan
‡School of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi-shi, Kanagawa 243-0297, Japan
S
* Supporting Information
ABSTRACT: We investigated herein the reactions of
(Me3tacn)FeCln (1a: n = 3, 1b: n = 2) with common
aluminum hydride reagents and a bulky dihydridoaluminate
{Li(ether)2}{Al(OC6H3-2,6-tBu2)}(μ-H)2, which yielded the
diamagnetic hydrido complexes 2−4 containing Fe(II) and
Al(III). In particular, the use of divalent 1b afforded excellent
isolated yields. The structures of 2−4 were determined using
spectroscopic and crystallographic analyses. The crystal
structures showed distorted octahedral Fe centers and fairly
short Fe−Al distances [2.19−2.24 Å]. The structures of cation
moiety 2 and neutral complex 4 were further probed using DFT calculations, which indicated a stable low-spin Fe(II) state and
strongly electron-donating nature of the (Me3tacn)FeH3 fragment toward the Al(III) center.
INTRODUCTION
RESULTS AND DISCUSSION
1. Reaction of (Me3tacn)FeCl3 (1a) and LiAlH4.
■
■
Dinuclear transition metal motifs are found as the active centers
of a number of metalloenzymes,1 wherein the transition metal
centers are typically stabilized by the amino acid residues of
polypeptides and siderophores. Successful models of hemeer-
ythrin as an oxygen-storage metalloprotein have been described
as O-bridged diiron frameworks supported by the facial
nitrogen-based ligands 1,4,7-triazacyclononanes (tacn)2 and
hydrotris(pyrazolyl)borates (Tp).3
Primarily, we reinvestigated the reaction of (Me3tacn)FeCl3
(1a) with LiAlH4 using Wieghardt’s procedure (Scheme 1).4
Scheme 1
Despite these pioneering studies on the di-iron oxo
complexes, little known are the corresponding hydrido forms
having these ligand systems. Hitherto, the syntheses of the
related dinuclear hydrido clusters of group 8 and 9 metals by
Wieghardt,4 our group,5 Paneque, and Ruiz,6 respectively, have
involved [(Me3tacn)2Rh2(H)2(μ-H)2]2+, [(Me3tacn)2M2(μ-
H)3]+ (M = Fe, Ru), and [(TpMe )2Ir2(H)2(μ-H)(μ-L)] (L =
2
After counteranion exchange with NaBPh4, a THF-soluble
fraction was separated from the reaction mixture, and two
signals in the hydrido region of the H NMR spectrum (THF-
Cl, SC4H3). We became intrigued with their reactivity toward
unactivated substrates, compared to other ligand systems of Cp,
CO, and triphos. Additionally, the formation of heterobinuclear
complexes implies anisotropic reactivity.7 For instance, a recent
study from our laboratory has shown promising reactivity of the
diruthenium hydride containing a (Me3tacn)Ru unit,
{(Me3tacn)Ru}(Cp*Ru)(μ-H)3, with CO2 (1 atm), leading to
the formation of the bis(μ-formato) complex, whereas
(Cp*Ru)2(μ-H)4 exhibited no reactivity even at a much higher
pressure of CO2.7b The development of useful starting
(hydrido) complexes is crucial before the initiation of
systematic reactivity studies. We report herein the reactions
between Me3tacn-ligated iron(II) and -(III) chlorides and
several hydride reagents and the structures of the iron(II)
hydrido complexes.
1
d8, rt) were observed at δ −24.1 and −25.7 ppm with an
integration ratio of 6:1 (56% and 9% NMR yields, respectively).
The chemical shift of the former major resonance was close to
that reported previously.4 The major component (2-BPh4) was
isolated as purple platelets by recrystallization from THF. The
reddish-purple platelet crystals of the minor product 2′-BPh4
were eventually obtained from a THF/toluene mixture. The
preliminary X-ray analysis of the sample suggested that 2′-BPh4
might be a monocation having two Fe−H3−Al fragments,8
which was analogous to the structure of 3 (shown later). The
Received: January 27, 2014
© XXXX American Chemical Society
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1
Figure 1. H NMR spectrum of 2-BPh4 (THF-d8). Asterisks denote solvent.
structure of 2-BPh4 was established using NMR and single-
crystal X-ray analyses. As reported previously, this material was
highly air- and moisture-sensitive and immediately decolorized
on exposure to air and moisture.4 Once purified, 2-BPh4
became decreasingly soluble in THF to a great extent. The
1H NMR spectrum of the pure, diamagnetic 2-BPh4 clearly
showed resonances of Me3tacn and counteranion moieties as
well as a single hydride resonance (Figure 1). The spectrum of
2-BPh4, however, showed two (Me3tacn)Fe(H)3 fragments
over one counteranion based on the integration ratio of the
tacn ligand, hydride, and borate.
metal-centered core, Fe−Al−Fe [Fe1−Al1−Fe1# = 180°]. In
the solid-state structure, each of the two Me3tacn−Fe fragments
of 2-BPh4 adopted δδδ and λλλ tacn configuration, respectively.
The Fe−Al distances [2.1918(5) and 2.1879(4) Å] were
significantly shorter than the distances of known Fe−Al bonds.9
Fischer and co-workers reported FeAl4 and FeAl5 hydrido
clusters, in which unsupported Fe−Al bonds [2.21−2.26 Å]
were shorter than the hydride-bridged Fe−Al bonds [2.32−
2.44 Å].9b However, in the crystal structure of 2-BPh4 three
bridging hydride ligands were located between Fe and Al atoms
in the difference Fourier map, and the hydride-bridged Fe−Al
distance of 2-BPh4 was shorter than those in the above
examples as well as unsupported Fe−Al-bonded complexes.10
In addition, an 27Al{1H} NMR spectrum of 2-BPh4 (THF-d8,
60 °C) demonstrated the presence of the aluminum atom,
whose resonance was observed at δ 97.9 ppm with a relatively
smaller half-height width (w1/2) of 79 Hz. The resultant 27Al
NMR parameters were reasonable for the highly symmetric
aluminum hydrides.11
Moreover, the X-ray analysis of 2-BPh4 unequivocally
demonstrated a structure with two iron fragments and one
1
borate, in accordance with the H NMR data. As shown in
Figure 2, the crystal structure of 2-BPh4 has a linear three-
In order to obtain insight into the Fe−H3−Al structure, we
performed DFT calculation of 2 at the B3LYP level of theory.
The optimized structure of cation 2 (singlet state) and selected
bond distances of the singlet and triplet states are shown in
Figure 3 and Table 1, respectively. The geometrical parameters
of the singlet state agreed with those obtained from the X-ray
analysis, in particular, in terms of Fe−Al and Fe−N distances
[2.222 and 2.059 Å, respectively]. However, the corresponding
triplet state indicated a relatively low symmetrical structure of
the two (Me3tacn)Fe fragments. Subsequently, the calculation
indicated that the triplet state of 2 was much higher in energy
compared to the corresponding singlet state; that is, the free
energy difference between the two states was estimated to be
17.9 kcal mol−1 (Table S1).
Figure 2. Crystal structure of [{(Me3tacn)Fe(μ-H)3}2Al]BPh4 (2-
BPh4) (cation structure of one of the two independent molecules).
Hydrogen atoms in Me3tacn ligands are omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond
distances (Å) and angles (deg): Fe1−Al1 2.1918(5), Fe1−N1
2.043(3), Fe1−N2 2.040(3), Fe1−N3 2.031(3); Fe1−Al1−Fe1
180.0; Fe2−Al2 2.1879(4), Fe2−N4 2.040(3), Fe2−N5 2.035(3),
Fe2−N6 2.035(3); Fe2−Al2−Fe2 180.0.
Although the molecular orbitals of singlet-state 2 were
somewhat complicated, six occupied high-lying HOMO to
HOMO−5 and six unoccupied molecular orbitals LUMO+2,
+3, +7, +8, +10, and +11 were found to have major
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lying HOMO−11, −12, −14, and −15 (Figure 4), whereas
occupied molecular orbitals were not clearly found, implying
the presence of either Al−H or Al−Fe bonding interaction. As
listed in Table 2, the NBO analysis indicated the presence of
Table 2. NBO Analysis of Optimized 2
natural charge
bond
Wiberg index
Fe (av)
Al
+0.0419
+1.0801
−0.4740
−0.2210
+0.5811
Fe−H
Fe−N
Fe−Al
Al−H
0.4726−0.4738
0.2518−0.2526
0.2764
N (av)
μ-H (av)
Me3tacn
0.3662−0.3676
Figure 3. DFT-optimized structures of 2 (singlet state).
positively charged Al (+1.08) and negatively charged N
(−0.47) and H (−0.22) centers with a higher covalent bond
order for Fe−H (0.47) than that for Fe−Al (0.28). The
relatively small covalent bond indexes for Fe−H and Fe−N
could be ascribed to contributions from Fe−H antibonding
orbital interactions found in HOMO−35, −16, −10, and −9
and from Fe−N antibonding orbital interactions found in
HOMO−13, −10, −9 to −7, and −6 (Figures S1, S2).
Although the covalent bond index for each Al−H (0.37) is
small, the sum of the bond indexes for the three Al−H (total
1.10) may indicate the presence of sufficient bonding
interaction between the Al and the almost electrically neutral
(Me3tacn)FeH3 moiety (total charge −0.040). Previous reports
on crystallographically characterized transition metal−alumi-
num complexes with triply bridging hydrides are limited to Re−
Al, W−Al, and Sc−Al hydrides.12 The formal shortness ratios
(FSRs)13 for the metal−aluminum distance in these complexes
are in a range of 0.99−1.03. In contrast, 2-BPh4 has a smaller
Table 1. Selected Bond Distances (Å) from DFT-Optimized
and Experimentally Determined Structures of 2
DFT
X-ray
2-iBu2AlCl2
bond
singlet state triplet state
2-BPh4
Fe1−Al3
2.2220
2.2233
2.1918(5)
2.1879(4)
2.1906(3)
Fe2−Al3
Fe1−N(av)
2.2220
2.0590
2.3178
2.0610
2.038
2.038
2.042
Fe2−N(av)
Fe1−H(av)
2.0590
1.6478
2.1423
1.6454
1.44
1.56
1.55
Fe2−H(av)
1.6577
1.7487
contributions from Fe-based d orbitals (Figure S1-1). Clear N−
Fe−H bonding orbital interactions were found in fairly low-
Figure 4. Molecular orbitals (HOMO−11, −12, −14, and −15) in 2 including N−Fe−H bonding interactions. Each orbital is drawn by a 0.04
contour value.
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FSR (0.91). This may be due to the presence of Fe and
Me3tacn as the electron-rich late metal and ligand. Therefore,
we concluded that the experimentally observed short Fe−Al
distance resulted from the strong donating property of the
polarized (Me3tacn)FeH3 fragments toward the Al(III) center.
Such a Fe2AlH6 core structure with any ligand systems has not
been reported hitherto; rather experimental and theoretical
studies on the structures of Fe2BH6, [{(R3P)3Fe}2B(μ-H)6]+ (R
= Et, R3 = triphos), have been reported.14 The bonding nature
of D3d-symmetric Fe2BH6 was interpreted as having direct Fe−
B bonds and a hypervalent boron center.
Scheme 3
2. Reactions of (Me3tacn)FeCl2 (1b) and Other Hydride
Reagents. The structure and iron formal oxidation state of 2-
BPh4 led us to examine the reactions of the corresponding
Fe(II) chloride as a starting complex with other known hydride
reagents. (Me3tacn)FeCl2 (1b), reported by Rauchfuss and his
co-worker, is composed of a dinuclear cation and mononuclear
anion [{Me3tacn)Fe}2(μ-Cl)3][(Me3tacn)FeCl3] (described as
“(Me3tacn)FeCl2” for simplicity).15 1b was suspended in THF
and treated with 5 equiv of DIBAL-H at 25 °C to yield a purple
solution (Scheme 2). After workup, a viscous solid was
THF but insoluble in pentane, toluene, and ether. It was readily
isolated as red-purple platelets after filtration and recrystalliza-
tion from a THF/ether mixture (76% isolated yield based on
1b). 1H NMR and COSY spectra of 3 (THF-d8) exhibited not
only the characteristic resonances of Me3tacn ligand protons (δ
2.95 and 2.93−3.06 ppm for Me and CH2CH2, respectively)
and hydride (δ −25.6 ppm) but also proton signals of the 2-
methoxyethoxy group in the typical magnetic field (δ 3.41 and
3.43−3.92 ppm for Me and CH2CH2, respectively). T1 min for
the hydride signal was measured [130(1) ms at 235 K]. The
value indicates the absence of the nonclassical hydride nature of
3.16
Scheme 2. Reaction of 1b with Either DIBAL-H or LiAlH4
The X-ray analysis of 3 was performed, and the molecular
structure is presented in Figure 5. The solid-state structure of 3
1
obtained. The H NMR spectrum in C6D6 showed that the
product, having a characteristic hydride resonance at δ −24.0
ppm, resembled 2-BPh4 except for the resonances of the
counteranion moiety. The recrystallization of the product from
a THF/pentane mixture at ambient temperature yielded purple
platelets, which allowed us to carry out single-crystal X-ray
studies. The results of both X-ray and NMR studies were
consistent with the formulation of 2-iBu2AlCl2 (see CIF file).
The S6-symmetric cation was essentially the same as that in 2-
BPh4. Cations 2 were reproducibly formed from 1b with either
DIBAL-H or LiAlH4 with over 70% isolated yields, which were
much higher and more selective than those from the trivalent
iron chloride 1a (Scheme 2).
A reaction of 1a with DIBAL-H was also performed, yielding
Figure 5. Crystal structure of 3 (cation moiety only). Hydride ligands
could not be located. All other hydrogen atoms are omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level. Selected
bond distances (Å) and angles (deg): Fe1−Al1 2.2450(17), Fe1−N1
2.064(5), Fe1−N2 2.072(5), Fe1−N3 2.044(5), Al1−O1 1.860(4),
Al1−O3 1.891(4), Al1−O7 1.780(4); O1−Al1−O3 79.05(15), O3−
Al1−O7 92.25(18), O1−Al1−O7 97.47(18).
2-iBu2AlCl2 along with a minor product, which showed a
1
hydride resonance at −25.1 ppm in the H NMR spectrum
(hydride integration ratio: 2-iBu2AlCl2/the minor product =
3.2:1). The structure of the minor product was tentatively
assigned to 2′-iBu2AlCl2.
We also reacted 1b with borohydride reagents such as LiBH4,
LiEt3BH, and NaBH4 under similar conditions; however, all
attempts resulted in the formation of paramagnetic black solids.
These materials were insoluble in THF, CH3CN, and CH2Cl2
and reacted with protic solvents. This led to the evolution of
gas; therefore, we did not characterize them any further. Next,
we turned our attention to the use of another aluminum-based
hydride reagent, NaAlH2(OCH2CH2OMe)2 (Red-Al). When 5
equiv of Red-Al was employed for 1b in THF, the reaction
proceeded at 25 °C and reached completion in 24 h, affording
the diamagnetic purple product 3 (Scheme 3). 3 was soluble in
showed two Fe−Al units linked by alkoxyl ligands on the Al
atoms and one Na atom trapped by six alkoxide and ethereal
oxygen atoms. The Fe−Al distance [Fe1−Al1 = 2.2450(17) Å]
was comparable to that of 2. The incorporation of the Na and
Cl atoms into the molecule apparently decreased the solubility
of the complex in organic solvents and resulted in a syn
configuration of the Fe1−Al1−Al2−Fe2 core. We attempted to
liberate the Na cation in the structure of 3 by addition of excess
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crown ethers (15-crown-5 and 18-crown-6). However, NaCl
elimination did not occur even at elevated temperatures.
Lithium and sodium aluminum dihydrides bearing bulky
aryloxides were reported by Noth and co-workers.17 We
̈
envisaged that the lithium hydridoaluminate {Li(OEt2)2}{Al-
(ArO)2}(μ-H)2 (Ar = C6H3-2,6-tBu2) would serve as a hydride
reagent to afford a neutral Fe−H3−Al complex, into which a
lithium salt (byproduct) would no longer be incorporated
owing to the presence of the sterically congested Al(OAr)2
moiety. The reaction of 1b with 5 equiv of {Li(OEt2)2}{Al-
(ArO)2}(μ-H)2 in THF led to the formation of Fe−Al hydride
4 (Scheme 4). 4 was readily isolated by the filtration of
Scheme 4
insoluble byproducts followed by the evaporation of THF
solvent and successive removal of the remaining aluminum
hydride reagent by simply rinsing the residue with ether and a
small amount of toluene. This was because 4 was moderately
soluble in THF but scarcely soluble in ether and toluene. The
structure of 4 was established using spectroscopy and
Figure 6. Crystal structure of 4. Hydrogen atoms in Me3tacn, alkoxide
ligands, and a crystallization THF solvent are omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level. Selected
bond distances (Å) and angles (deg): Fe1−Al1 2.2137(4), Fe1−N1
2.0622(12), Fe1−N2 2.0610(12), Fe1−N3 2.0439(12), Al1−O1
1.7444(10), Al1−O2 1.7817(10); N1−Fe1−N2 84.17(5), N2−Fe1−
N3 84.48(5), N1−Fe1−N3 84.59(5), O1−Al1−O2 96.05(5).
1
crystallography. The H NMR spectrum recorded at 25 °C
displayed a characteristic resonance at δ −25.20 ppm for the
three hydrides, which were observed to be equivalent at the
temperature. Consequently, the signals of two aryloxide ligands
on aluminum as well as methyl proton signals of Me3tacn on
iron were also observed to be equivalent on the NMR time
scale. The single crystals of 4, suitable for X-ray analysis, were
grown by the slow diffusion of ether into the THF solution.
The crystal structure of 4 is shown in Figure 6. The structure of
the distorted octahedral iron hydride moiety and intermetal
distance [Fe1−Al1 = 2.2137(4) Å] in 4 were essentially the
same as those in the complexes 2 and 3. One exception was the
pentacoordinated Al center, in which the four atoms Fe1, Al1,
O1, and O2 share the same plane: summation of aluminum
angles = 360.0° [O1−Al1−O2 = 96.05(5)°, O1−Al1−Fe1 =
134.42(4)°, O2−Al1−Fe1 = 129.54(4)°].
3. Electronic Spectra of Fe(II) Hydrides. To verify the
preference of the low-spin Fe(II) center, UV−vis spectra of
diamagnetic 2−4 were recorded (Figure 7). The detected
absorption bands in the spectra of 2−4 are listed in Table 3 for
comparison with those of related low-spin Fe(II) complexes. In
contrast to nearly colorless Fe(II) complexes 1b and
[{(Me3tacn)Fe}2(μ-Cl)3]BPh4,14,18d 2-BPh4 and 2-iBu2AlCl2,
having common cationic structure, yielded almost identical
absorptions in both the UV and visible-light regions. The
spectra include four bands, two of which (2-BPh4: 543 and 311
nm; 2-iBu2AlCl2: 537 and 328 nm) were assignable to typical
spin-allowed d−d transitions of low-spin Fe(II) complexes.14
As mentioned above in the DFT results, owing to the large
energy difference between the two spin states, there was no
contribution from the high-spin Fe(II) electronic configuration
to the spectra. These characteristic bands for low-spin Fe(II)
states were also detected in the spectra of 3 and 4; however, the
two extra bands corresponding to ∼455 and ∼390 nm detected
in the spectra of 2 were not clearly observed for 3 and 4.
Interestingly, the two common absorption bands of 2−4 (522−
543 and 310−360 nm) were significantly blue-shifted and had
much higher molar extinction coefficients than those of
previously reported low-spin Fe(II) (Table 3).
The DFT calculation of the neutral complex 4 (singlet state)
was also carried out. As summarized in Table S5, the DFT-
optimized structure of 4 is in good agreement with the crystal
structure in terms of Fe−Al, Fe−N, and Al−O distances and
O−Al−O angle (within 3% error). The molecular orbital
diagrams of 4, presented in Figure S2-1 and S2-2, are more
comprehensive than those of 2. The Fe-based nonbonding
orbitals were clearly found in the three molecular orbitals
HOMO, HOMO−1, and HOMO−2. Fe−N and Fe−H
bonding interactions were mainly observed in the low-lying
HOMO−12 and −13, whereas the LUMO+5, +6, and +7
displayed antibonding orbital interactions of Fe−N, Fe−H, and
Fe−Al (Figure S2-1). The NBO analysis of 4, summarized in
Table S6, showed nearly identical bonding interactions in the
(Me3tacn)FeH3Al moiety with those in cation 2 (natural charge
in 4: Fe −0.0421, Al +1.7106, Nav −0.4857, and Me3tacn
+0.5131; Wiberg index in 4: Fe−Al 0.2164, Fe−H 0.4916−
0.5950, Al−H 0.2214−0.3005).
CONCLUSIONS
■
The synthesis of Al-capped (Me3tacn)FeH3 complexes 2−4
was accomplished through the reaction of 1 with aluminum-
based hydride reagents under mild conditions, irrespective of
the valency of the starting iron chlorides. On the other hand,
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Figure 7. UV−vis spectra of 2−4 (0.24−1.70 mM in THF at 25 °C).
i
substituents (iPr and Bu) in the tacn ligand, dramatically
altered the spin states.19 We conclude that the low-spin Fe(II)
state in 2−4 was governed by the presence of the hydride
ligands as σ-donors. Moreover, at the same time, the Al
fragment played an important role in stabilizing the polarized
[(Me3tacn)Fe(μ-H)3] fragment. So far, several bimetallic
hydrides of Fe(II) and main group metals (e.g., B, Si, Sn)
with other facial ligand systems, e.g., triphos, have been
synthesized and structurally characterized.20 The synthetic
application of the Fe−H3−Al species to other heterobimetallic
hydrides is currently under investigation in our laboratory.
Table 3. Comparison of Electronic Transitions of Fe(II)
Complexes
complex
solvent
H2O
λ, nm (ε, L mol−1 cm−1
601 (6), 387 (17), 288 (560)
)
ref
18b
[Fe(tacn)2]Br2
[Fe(Me3tacn)
MeCN 562 (23), 388 (43)
18d
18d
18e
18f
(NCMe)3](OTf)2
[Fe(Me3tacn)
(NCMe)3](BPh4)2
MeCN 577 (40), 384 (58)
[Fe(trans-diammac)]
D2O
D2O
584 (100), 405 (140)
590 (14), 394 (15)
a
(PF6)2
c
[Fe{(NH2)2sar}]
b
(OTf)2
2-BPh4
THF
THF
THF
THF
543 (452), 457 (412), 388
this
EXPERIMENTAL SECTION
d
■
(295), 311 (1180)
work
2-iBu2AlCl2
537 (800), 454 (907), 391
this
work
General Procedures. All manipulations for air- and moisture-
sensitive compounds were carried out under an argon atmosphere
using standard Schlenk techniques or in a glovebox filled with argon
(H2O < 1 ppm, O2 < 1 ppm). Dehydrated solvents (toluene, pentane,
ether, tetrahydrofuran, acetonitrile, methylene chloride, and methanol)
were purchased from Kanto Chemical Co. Ltd. Deuterated solvents
(benzene-d6, tetrahydrofuran-d8, and toluene-d8) were purchased from
Cambridge Isotope Laboratories, Inc. and Sigma-Aldrich Co. (Non)-
deuterated hydrocarbon and ethereal solvents were distilled from Na−
K alloy and stored under an argon atmosphere. Other reagents and
organic and inorganic chemicals were used as received.
d
(990), 328 (1810)
d
3
4
535 (716), 360 (878)
this
work
d
522 (208), 310 (827) ,
e
this
work
282 (4461)
a
diammac = exo-6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazatetrade-
b
cane. (NH2)2sar = 1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-
c
d
e
eicosane. pH = 8.5. Observed as a shoulder. Assigned to an
absorption of aryloxy group by comparison with the spectrum of
{Li(OEt2)2}{Al(ArO)2}(μ-H)2 (Ar = 2,6-di-tert-butylphenyl).
Instrumentation. 1H and 13C NMR spectra were recorded on
Varian INOVA 400 and Varian 400-MR Fourier transform
spectrometers. 1H chemical shifts were referenced to the residual
proton peaks of benzene-d6 at δ 7.15 ppm or tetrahydrofuran-d8 at δ
3.58 ppm vs tetramethylsilane at δ 0.00 ppm. NMR yields were
estimated by integration changes based on integrations of 2,2,4,4-
tetramethylpentane as an internal standard. The central peak of a
triplet for benzene-d6 at δ 128.0 ppm or the central peak of a quintet
for tetrahydrofuran-d8 at δ 67.57 ppm vs tetramethylsilane δ 0.00 ppm
was used as the 13C NMR internal reference. The peak for Al(NO3)3
(D2O) was used as an 27Al NMR external reference. UV−vis spectra
were recorded on a Shimadzu UV-2550 spectrophotometer. Infrared
spectra were recorded on a JASCO FT-IR 4200 type A
spectrophotometer. Elemental analyses (C, H, and N) were recorded
on a PerkinElmer 2400II. Single crystals suitable for X-ray analysis
were coated with Paratone-N in a glovebox. Crystals of proper size
were picked by using a nylon CryoLoop and quickly transferred in a
low-temperature N2 stream to the goniometer head. The diffraction
commonly used borohydrides, such as NaBH4 and LiEt3BH,
yielded only insoluble unidentified materials. The X-ray analysis
of 2−4 clearly revealed significantly short Fe−Al distances (ca.
2.2 Å). The trihydride-bridged Fe−Al structures were probed
using DFT calculations for 2 and 4, which suggested a stable
low-spin Fe(II) state and the presence of a strongly electron-
donating property of the polarized [(Me3tacn)Fe(μ-H)3] and
the Lewis acidic Al(III) center. The UV−vis spectra of 2−4
included the characteristic absorption bands for low-spin Fe(II)
species with a significant blue shift of the LF transitions. These
spectroscopic features agreed with the diamagnetic nature, as
revealed from the NMR and DFT analysis. Hagen reported the
temperature-dependent spin-crossover behavior of [(Me3tacn)-
FeL3]2+ (L = acetonitrile and trifluoromethanesulfonate), where
the subtle structural change, i.e., the type of L and N-
F
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Table 4. Crystallographic Data of 2-BPh4, 2-iBu2AlCl2, 3, and 4
complex
2-BPh4
2-iBu2AlCl2
3
4
empirical formula
C42H68AlBFe2N6
800.47
C26H66Al2Cl2Fe2N6
699.41
C30H76Al2ClFe2N6NaO8
873.07
C41H74AlFeN3O3
739.86
mol wt (g mol−1
cryst habit
cryst syst
)
platelet
platelet
platelet
block
monoclinic
P21/a (#14)
red
monoclinic
C2/c (#15)
red
monoclinic
P21/n (#14)
red
monoclinic
P21/n (#14)
red
space group
cryst color
cryst size (mm)
a (Å)
0.15 × 0.07 × 0.05
16.0332(8)
15.8365(8)
17.3741(9)
0.54 × 0.13 × 0.05
12.3874(7)
11.9798(7)
25.1828(15)
0.39 × 0.37 × 0.10
15.5885(5)
17.5040(5)
16.9997(5)
0.51 × 0.28 × 0.23
10.1258(2)
16.6997(4)
24.9425(6)
b (Å)
c (Å)
α (deg)
β (deg)
100.921(2)
92.973(2)
97.035(3)
90.111(1)
γ (deg)
V (Å3)
4331.6(4)
4
3732.1(4)
4
4603.6(2)
4
4217.7(2)
4
Z value
measurement temp (°C)
−150
1.237
−150
1.245
−150
1.261
−150
1.165
D
calc(g cm−3
)
μ(Mo Kα) (mm−1
)
0.725
0.992
0.781
0.417
2θmax (deg)
55
51
51
55
reflns collected
indept reflns
35 022
15 313
74 772
68 315
9849 (Rint = 0.0918)
6177
3606 (Rint = 0.0411)
3062
14417 (Rint = 0.0795)
10 526
9645 (Rint = 0.0324)
8491
reflns obsd (>2σ)
abs corr type
abs transmn
empirical
0.6907 (min.)
1.0000 (max.)
0.0509
empirical
0.4513 (min.)
1.0000 (max.)
0.0356
empirical
0.5516 (min.)
1.0000 (max.)
0.0822
numerical
0.8258 (min.)
1.0000 (max.)
0.0344
R1 (I > 2σ(I))
R1 (all data)
wR2 (all data)
0.1019
0.0403
0.1120
0.0401
0.1206
0.0865
0.2477
0.0935
data/restraints/params
goodness of fit on F2
largest diff peak
9849/0/492
1.058
3429/0/193
1.057
14 417/0/462
1.064
9645/0/672
1.039
0.688
0.849
1.92
0.761
and hole (e Å−3
)
−0.842
−0.318
−1.90
−0.628
data were collected on a R-AXIS RAPID diffractometer equipped with
graphite-monochromated Mo Kα radiation (λ = 0.71069 Å).
Structures were solved by heavy-atom Patterson or direct methods
and expanded using Fourier techniques. The non-hydrogen atoms
were refined by full-matrix least-squares refinement in F2 using the
SHELXS-97 program.21 The twin refinement of complex 3 was
performed with SHELXS using the data in HKLF5 format. Hydrogen
atoms were located by difference Fourier maps and refined
isotropically. Crystallographic data of 2-BPh4, 2-iBu2AlCl2, 3, and 4
are summarized in Table 4.
Synthesis of 2-iBu2AlCl2. To a suspension of 1b (0.0906 mmol,
81 mg) in THF (5 mL) was added a solution of DIBAL-H in toluene
(0.9 mL, 1.5 M) at 25 °C under an argon atmosphere. After the
mixture was stirred at this temperature for 12 h, the resulting purple
suspension was filtered through glass frits, and the solvent was
removed from the filtrates under reduced pressure. The residue was
washed with pentane and ether to yield 2-iBu2AlCl2 (69 mg, 73%).
Single crystals of 3 were grown from THF/pentane at ambient
1
temperature. H NMR (benzene-d6, rt): δ 2.73 (18H, s, NMe), 2.70
(2H, m, AlCH2CH), 2.41 (24H, m, NCH2), 1.57 (12H, d, J = 6.6 Hz,
CH3), 0.85 (4H, d, J = 6.6 Hz, AlCH2), −24.0 (6H, s, μ-H) ppm.
COSY (benzene-d6, rt): 2.70/1.57, 0.85. 13C{1H} NMR (benzene-d6,
rt): δ 59.7 (NCH2), 59.3 (NCH3), 57.7 (AlCCH), 29.3 (AlCCCH3),
27.4 (AlCH2) ppm. HMQC (13C/1H, benzene-d6, rt): δ 59.7/2.41;
59.3/2.73; 57.7/2.70; 29.3/1.57; 27.4/0.85. HMBC (13C/1H, benzene-
d6, rt): δ 57.7/1.57, 0.85; 29.3/2.70; 27.4/2.70. IR (KBr, cm−1): ν
2852 (m), 1655 (s), 1578 (s), 1450 (s), 1052 (m), 1010 (s), 785 (s),
745 (s), 687 (s), 670 (s). Anal. Calcd for C26H66Al2Cl2Fe2N6: C,
44.65; H, 9.51; N, 12.02. Found: C, 44.91; H, 9.91; N, 11.45.
Synthesis of 2-BPh4. To a suspension of 1b (0.106 mmol, 95 mg)
in THF (6 mL) was added LiAlH4 (2.03 mmol, 77 mg) at 25 °C under
an argon atmosphere. After the mixture was stirred at this temperature
for 1.5 h while the reaction system was closed, NaBPh4 (0.16 mmol, 55
mg) was subsequently added under argon purge. After stirring for 24
h, the resulting purple suspension was filtered through glass frits and
the insoluble salts were washed with THF (0.5 mL × 2). Solvent was
removed from the combined filtrates under reduced pressure, and then
the residue was washed with ether to afford essentially pure 2-BPh4
(102 mg, 80%). Storage of a concentrated THF solution of 2-BPh4 at
Synthesis of 3. To a suspension of 1b (0.0906 mmol, 81 mg) in
THF (5 mL) was added a solution of NaAlH2(OCH2CH2OMe)2 in
toluene (3.6 M, 0.38 mL) at 25 °C under an argon atmosphere while
the mixture immediately turned purple. After the mixture was stirred at
this temperature for 12 h, the resulting purple suspension was filtered
through glass frits, and the solvent was removed from the filtrates
under reduced pressure. The residue was washed with toluene and
ether to afford 3 (91 mg, 76%). Single crystals of 3 were grown from
1
−30 °C afforded diffraction quality crystals. H NMR (THF-d8, rt): δ
7.26 (8H, m, Ph-H, ortho), 6.84 (8H, t, J = 6.8 Hz, Ph-H, meta), 6.69
(4H, t, J = 6.8 Hz, Ph-H, para), 2.95 (18H, s, NMe), 2.87 (24H, m,
NCH2), −24.1 (6H, s, μ-H) ppm. 13C{1H} NMR (THF-d8, rt): δ
164.7 (q, JBC = 49.1 Hz, Ph-ipso), 136.7 (Ph-ortho), 125.2 (Ph-meta),
121.4 (Ph-para), 60.2 (NCH2), 59.3 (NCH3) ppm. 27Al{1H} NMR
(THF-d8, 60 °C): δ 97.9 ppm (w1/2 = 79 Hz). IR (KBr, cm−1): ν 2897
(m), 1655 (s), 1578 (s), 1457 (s), 1289 (m), 1078 (m), 1011 (s), 732
(s), 706 (s), 612 (s). Anal. Calcd for C42H68AlBFe2N6: C, 62.55; H,
8.50; N, 10.42. Found: C, 62.36; H, 9.00; N, 10.47.
1
THF/ether at ambient temperature. H NMR (THF-d8, rt): δ 3.92
(4H, t, J = 5.6 Hz, OCH2), 3.79 (4H, t, J = 5.6 Hz, OCH2), 3.67 (4H,
t, J = 5.6 Hz, OCH2), 3.43 (4H, t, J = 5.6 Hz, OCH2), 3.41 (6H, s,
G
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Inorganic Chemistry
Article
OCH3), 3.33 (6H, s, OCH3), 3.06 (12H, m, NCH2), 2.95 (18H, s,
NMe), 2.93 (12H, m, NCH2), −25.6 (6H, s, μ-H) ppm. COSY (THF-
d6, rt): 3.92/3.67; 3.79/3.43; 3.06/2.93. 13C{1H} NMR (THF-d8, rt):
δ 78.1 (OCH2), 74.4 (OCH2), 61.0 (OCH2), 60.3 (NCH2), 60.0
(OCH2), 59.6 (NCH3), 58.7 (OCH3), 58.3 (OCH3) ppm. HMQC
(13C/1H, THF-d8, rt): δ 78.1/3.43; 74.4/3.67; 61.0/3.79; 60.3/3.06,
2.93; 60.0/3.92; 59.6/2.95; 58.7/3.33; 58.3/3.41. HMBC (13C/1H,
THF-d8, rt): δ 78.1/3.79; 74.4/3.92; 61.0/3.43; 60.3/3.06, 2.93; 60.0/
3.67. IR (KBr, cm−1): ν 2851 (s), 1655 (s), 1648 (s), 1638 (s), 1458
(s), 1119 (s), 1086 (s), 1014 (s), 835 (s), 763 (s), 603 (m). Anal.
Calcd for C30H76Al2ClFe2N6NaO8: C, 41.27; H, 8.77; N, 9.63. Found:
C, 41.70; H, 9.81; N, 9.64.
(2) Wieghardt, K.; Pohl, K.; Gebert, W. Angew. Chem., Int. Ed. Engl.
1983, 22, 727.
(3) Armstrong, W. H.; Spool, A.; Papaefthymiou, G. C.; Frankel, R.
B.; Lippard, S. J. J. Am. Chem. Soc. 1984, 106, 3653.
(4) Hanke, D.; Wieghardt, K.; Nuber, B.; Lu, R.-S.; McMullan, R. K.;
Koetzle, T. F.; Bau, R. Inorg. Chem. 1993, 32, 4300.
(5) Namura, K.; Kakuta, S.; Suzuki, H. Organometallics 2010, 29,
4305.
(6) Paneque, M.; Poveda, M. L.; Salazar, V.; Taboada, S.; Carmona,
E.; Gutier
139.
́
rez-Puebla, E.; Monge, A.; Ruiz, C. Organometallics 1999, 18,
(7) (a) Shima, T.; Namura, K.; Kameo, H.; Kakuta, S.; Suzuki, H.
Organometallics 2010, 29, 337. (b) Namura, K.; Ohashi, M.; Suzuki, H.
Organometallics 2012, 31, 5979.
(8) We assign the cation structure of 2′-BPh4 to [{(Me3tacn)Fe}(μ-
H)3Al(OBu)2]2M(solvent)n (M = Li or Na). The X-ray structure was
not completely solved because several carbon atoms of BuO groups on
the aluminum atoms, which should result from ring-opening of THF,
could hardly be located from the X-ray diffraction data.
(9) (a) Weiss, J.; Stetzkamp, D.; Nuber, B.; Fischer, R. A.; Boehme,
C.; Frenking, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 70. (b) Steinke,
T.; Cokoja, M.; Gemel, C.; Kempter, A.; Krapp, A.; Frenking, G.;
Zenneck, U.; Fischer, R. A. Angew. Chem., Int. Ed. 2005, 44, 2943.
(c) Buchin, B.; Gemel, C.; Kempter, A.; Cadenbach, T.; Fischer, R. A.
Inorg. Chim. Acta 2006, 359, 4833.
Synthesis of 4. In the glovebox, to a suspension of 1b (0.101
mmol, 90 mg) in THF (7 mL) was added {Li(OEt2)2}{Al(OC6H3-
2,6-tBu2)2}(μ-H)2 (1.56 mmol, 930 mg) at room temperature. The
mixture was stirred for 15 h while the system was closed. Then,
insoluble byproduct was filtered and solvent was evaporated from the
filtrate. Washing the residue with ether and a small amount of toluene
followed by dryness in vacuo gave 4 as a red-purple solid (190 mg,
94%). Single crystals were obtained from a THF solution by slow
1
diffusion of ether. H NMR (THF-d8, rt): δ 6.99 (4H, d, J = 7.6 Hz,
Ar−H, meta), 6.42 (2H, t, J = 7.6 Hz, Ar−H, para), 2.69−2.59 (12H,
t
m, NCH2), 2.37 (9H, s, NMe), 1.59 (36H, s, Bu), −25.20 (3H, s, μ-
H) ppm. COSY (THF-d8, rt): 6.99/6.42. 13C{1H} NMR (THF-d8, rt):
δ 160.2 (Ar, ipso), 139.9 (Ar, ortho), 124.9 (Ar, meta), 116.2 (Ar,
para), 60.1 (NCH2), 58.7 (NCH3), 36.3 (CMe3), 33.3 (CMe3) ppm.
HMQC (13C/1H, THF-d8, rt): δ 124.9/6.99; 116.2/6.42; 60.1/2.69−
2.59; 58.7/2.37; 33.3/1.59. HMBC (13C/1H, THF-d8, rt): δ 160.2/
6.99; 139.9/6.42; 36.3/6.99, 1.59. IR (KBr, cm−1): ν 2950 (s), 1719
(m), 1638 (m), 1408 (s), 1245 (s), 1013 (s), 895 (s), 866 (s), 750 (s),
677 (s). Anal. Calcd for C41H74AlFeN3O3: C, 66.56; H, 10.08; N, 5.68.
Found: C, 66.70; H, 10.65; N, 5.77.
(10) (a) Fischer, R. A.; Priermeier, T. Organometallics 1994, 13, 4306.
(b) Braunschweig, H.; Muller, J.; Ganter, B. Inorg. Chem. 1996, 35,
̈
7443. (c) Anand, B. N.; Krossing, I.; Noth, H. Inorg. Chem. 1997, 36,
̈
1979. (d) Jones, C.; Aldridge, S.; Gans-Eichler, T.; Stasch, A. Dalton
Trans. 2006, 1, 5357. (e) Riddlestone, I. M.; Edmonds, S.; Kaufman, P.
A.; Urbano, J.; Bates, J. I.; Kelly, M. J.; Thompson, A. L.; Taylor, R.;
Aldridge, S. J. Am. Chem. Soc. 2012, 134, 2551. (f) Burlitch, J. M.;
Leonowicz, M. E.; Petersen, R. B.; Hughes, R. E. Inorg. Chem. 1979,
18, 1097.
Calculation Methods. All geometry optimizations, vibrational
frequencies, and energy calculations were carried out using the density
functional theory (DFT)22 at the B3LYP level23,24 with a mixed basis
set in the Gaussian03 program.25 The mixed basis set contained 3-
21G26 for the carbons and hydrogens of Me3tacn and aryloxide ligands
and the oxygens of aryloxide ligands, 6-31G for nitrogens of Me3tacn
and the hydrogens of the bridging ligands, and 6-31+G(d,p)27−31 for
irons and aluminum. Wiberg bond indexes32,33 were calculated using
natural bond orbital (NBO) analysis.34
(11) Akitt, J. W. In Aluminum, Gallium, Indium, Thallium; Mason, J.,
Ed.; Multinuclear NMR; Plenum Press: New York, 1987; Chapter 9.
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G. J. Am. Chem. Soc. 1984, 106, 8128. (b) Barron, A. R.; Willkinson,
G.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1987,
837. (c) Conroy, K. D.; Piers, W. E.; Parvez, M. Organometallics 2009,
28, 6228.
(13) (a) Cotton, F. A.; Murillo, L. A.; Walton, R. A. Multiple Bonds
Between Metal Atoms, 3rd ed.; Springer: New York, 2005. (b) Pauling,
L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press:
Ithaca, 1960.
(14) (a) Hillier, A. C.; Jacobsen, H.; Gusev, D.; Schmalle, H. W.;
Berke, H. Inorg. Chem. 2001, 40, 6334. (b) Guilera, G.; McGrady, G.
S.; Steed, J. W.; Kaltsoyannis, N. New J. Chem. 2004, 28, 444.
(15) Moreland, A. C.; Rauchfuss, T. B. Inorg. Chem. 2000, 39, 3029.
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data for 2−4 in cif format. Cartesian
coordinates of 2 and 4 from the computation. Diagrams of
molecular orbitals of the singlet states of 2 and 4. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: hiroharu@o.cc.titech.ac.jp.
Notes
■
(17) Noth, H.; Schlegel, A.; Knizek, J.; Krossing, I.; Ponikwar, W.;
̈
Seifert, T. Chem.Eur. J. 1998, 4, 2191.
(18) (a) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.;
Elsevier: Amsterdam, 1984. (b) Wieghardt, K.; Schmidt, W.;
The authors declare no competing financial interest.
Herrmann, W.; Kuppers, H.-J. Inorg. Chem. 1983, 22, 2953. (c) Bossek,
̈
ACKNOWLEDGMENTS
U.; Nuhlen, D.; Bill, E.; Glaser, T.; Krebs, C.; Weyhermuller, T.;
̈
̈
■
Wieghardt, K.; Lengen, M.; Trautwein, A. X. Inorg. Chem. 1997, 36,
2834. (d) Blakesley, D. W.; Payne, S. C.; Hagen, K. S. Inorg. Chem.
The Japan Society for the Promotion of Science (a Grant-in-
Aid for Scientific Research (S), grant no. 18105002) is
gratefully acknowledged for funding this research. We also
thank Dr. Hiroyasu Sato from Rigaku Corp. for assistance with
X-ray structure determination.
2000, 39, 1979. (e) Bolrzel, H.; Comba, P.; Pritzkow, H.; Sickmuller,
̈
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A. F. Inorg. Chem. 1998, 37, 3853. (f) Martin, L. L.; Martin, R. L.;
Sargeson, A. M. Polyhedron 1994, 13, 1969.
(19) Diebold, A.; Elbouadili, A.; Hagen, K. S. Inorg. Chem. 2000, 39,
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