Redox chemistry of bis(phosphaethenyl)pyridine iron complexes

Article abstract of DOI:10.1021/om201280z

Redox reactions of iron complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl] pyridine (BPEP), are reported. The Fe(II) dibromide [FeBr₂(BPEP)] (1) is readily reduced by [Cp₂Co] to afford the four-coordinate Fe(I) monobromide [FeBr(BPEP)] (2), while 2 reacts with PhCH₂Br to reproduce 1. Treatment of 1 with MesMgBr or Mes₂Mg(THF)₂ (Mes = 2,4,6-Me₃C₆H₂) results in one-electron reduction of 1, followed by transmetalation of the resulting 2 with mesitylmagnesium compounds to give the Fe(I) mesityl complex [FeMes(BPEP)] (3). The single-crystal diffraction study of 3 has revealed a distorted trigonal monopyramidal arrangement around the iron center. SQUID magnetometry has established a low-spin ground state (S = 1/2) of 3. Complex 2 reacts with Me₂Mg(THF)₂ to afford Fe(0) and Fe(II) complexes (4 and 5, respectively) coordinated with novel multidentate ligand systems containing a phosphonium ylide structure. The formation processes of 4 and 5 via an [FeMe(BPEP)] intermediate are discussed on the basis of their X-ray structures.

Full text of DOI:10.1021/om201280z

Article
pubs.acs.org/Organometallics
Redox Chemistry of Bis(phosphaethenyl)pyridine Iron Complexes
Yumiko Nakajima* and Fumiyuki Ozawa*
International Research Center of Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan
S
* Supporting Information
ABSTRACT: Redox reactions of iron complexes bearing a
PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-
(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine
(BPEP), are reported. The Fe(II) dibromide [FeBr2(BPEP)]
(1) is readily reduced by [Cp2Co] to afford the four-
coordinate Fe(I) monobromide [FeBr(BPEP)] (2), while 2
reacts with PhCH2Br to reproduce 1. Treatment of 1 with MesMgBr or Mes2Mg(THF)2 (Mes = 2,4,6-Me3C6H2) results in one-
electron reduction of 1, followed by transmetalation of the resulting 2 with mesitylmagnesium compounds to give the Fe(I)
mesityl complex [FeMes(BPEP)] (3). The single-crystal diffraction study of 3 has revealed a distorted trigonal monopyramidal
arrangement around the iron center. SQUID magnetometry has established a low-spin ground state (S = 1/2) of 3. Complex 2
reacts with Me2Mg(THF)2 to afford Fe(0) and Fe(II) complexes (4 and 5, respectively) coordinated with novel multidentate
ligand systems containing a phosphonium ylide structure. The formation processes of 4 and 5 via an [FeMe(BPEP)]
intermediate are discussed on the basis of their X-ray structures.
behavior of 2,6-bis(imino)pyridine Fe(II) complexes
INTRODUCTION
■
i
[FeX2(iPrPDI)] (X = Cl, Br; PrPDI = 2,6-(iPr2C6H3N
Phosphaalkenes with a PC bond constitute an interesting
class of supporting ligands in coordination chemistry. They
possess an extremely low-lying π* orbital and undergo effective
π-back-bonding with transition metals.1,2 We have documented
that 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phospha-
ethenyl]pyridine (BPEP) as a PNP-pincer-type phosphaalkene
ligand effectively stabilizes coordinatively unsaturated com-
plexes in low-oxidation states.3 Representative examples include
[FeBr(BPEP)] (2) with a four-coordinate Fe(I) center, which
is formed by one-electron reduction of [FeBr2(BPEP)] (1) with
KC8 (eq 1).3b While Fe(I) complexes having low-coordination
numbers have been limited,4 the four-coordinate structure of 2
has been established by experimental and theoretical studies.
The complex adopts a distorted trigonal monopyramidal con-
figuration around iron. This unique geometry enables effective
orbital interactions between Fe (dπ) and BPEP (π*) to form
highly delocalized orbitals, responsible for the stability of 2.
CMe)2C5H3N) in detail.8 The Fe(II) dihalides undergo step-
wise two-electron reduction by sodium amalgam under a nitro-
gen atmosphere to afford [Fe(N2)2(iPrPDI)] via monohalide
intermediates [FeX(iPrPDI)]. Interestingly, while the dinitro-
gen and monohalide complexes adopt the formal oxidation
states of 0 and +1, respectively, the iron centers of both com-
plexes have been proven to remain in an Fe(II) state in reality.
i
The reduction occurs on the PrPDI ligand, and the reducing
electrons are accommodated in ligand-centered π* orbitals.
On the other hand, since the π* orbital of the BPEP ligand
effectively interacts with the dπ orbital of iron to form highly
delocalized orbitals, it is expected that complexes 1 and 2
exhibit unique redox properties, significantly different from
i
those of the PrPDI complexes. Thus, we examined their reac-
tivities toward reducing agents. It has been found that
treatment of 1 with MesMgBr or Mes2Mg(THF)2 (Mes =
2,4,6-Me3C6H2) in benzene results in one-electron reduction of
1 to afford 2, followed by transmetalation of the resulting
2, giving [FeMes(BPEP)] (3) as a four-coordinate Fe(I) complex.
On the other hand, the reaction of complex 2 with Me2Mg-
(THF)2 forms unexpected phosphonium ylide complexes.
RESULTS AND DISCUSSION
■
Redox Properties of 1 and 2. First, redox properties of 1
were examined by an electrochemical method. As shown in
Figure 1, the cyclic voltammogram recorded in CH2Cl2 in
the presence of [Bu4N][BF4] at room temperature exhibited
three sets of redox waves: (i), (ii), and (iii). The potentials are
This paper reports redox reactions of 1 and 2. Redox
chemistry of iron complexes has drawn increasing attention in
connection with the activation of small molecules in enzymatic
systems.5 Such information is also important to develop iron-
catalyzed organic transformations such as cross-coupling reac-
tions.6 However, studies using well-defined complexes have
been limited.7 Recently, Chirik et al. investigated reduction
Received: December 27, 2011
Published: February 22, 2012
© 2012 American Chemical Society
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Reactions with Mesitylmagnesiums. Complex 1 was
readily reduced by Grignard reagents as well. The reaction with
2.6 equiv of MesMgBr (Mes = 2,4,6-Me3C6H2) in benzene at
room temperature afforded [FeMes(BPEP)] (3) in 88% iso-
lated yield as a brownish-green solid. The reaction proceeded
along with the formation of bimesityl (40%/1) (eq 4). In this
system, slow addition of MesMgBr over 1 h was essential to
obtain 3 in high yield. Otherwise, a mixture of 1, 2, and 3 was
formed, and further addition of MesMgBr (4 equiv) caused
decomposition of 3, providing a complex mixture of unidentified
compounds. Complex 3 is probably formed by one-electron
reduction of 1 with MesMgBr, followed by transmetalation of
the resulting 2 with another molecule of MesMgBr. Actually,
complex 2, which was independently prepared from 1 and KC8,
reacted with 1 equiv of MesMgBr to afford 3 in 55% yield.
A related reductive alkylation process has been reported for the
reactions of [FeCl2(iPrPDI)] with MeLi and Me3SiCH2Li.10
Unlike the present system, however, the occurrence of Fe(II)
alkyl intermediates [Fe(R)Cl(iPrPDI)] (R = Me, CH2SiMe3)
has been proposed.
Figure 1. Cyclic voltammogram of [FeBr2(BPEP)] (1) in CH2Cl2 (0.1
mM) at 22 °C (0.1 M [Bu4N][BF4], scan rate 100 mV/s, versus Fc/Fc+).
reported in V vs Fc/Fc+. The reversible wave (i) at +0.20 V
(E1/2) corresponds to the [FeBr2(BPEP)]/[FeBr2(BPEP)]+
cycle, while the small oxidation wave (ii) at −0.75 V (Ep)
may be formed from a decomposition product because its
intensity gradually increases with time. On the other hand, the
irreversible wave (iii) at −1.31 V (Ep) is attributed to the
conversion of 1 to 2, and its reduction potential is much less
negative than that of [FeCl2(iPrPDI)] (Ep = −1.87 V). Indeed,
complex 1 was instantly reduced by [Cp2Co] as a mild reducing
agent (E0 = −1.33 V)9 at room temperature, giving 2 in 57%
isolated yield (eq 2).
The conversion of 1 to 3 was accomplished more cleanly by
using Mes2Mg(THF)2 instead of MesMgBr (eq 5). The
reaction was completed with 1 equiv of Mes2Mg(THF)2, and
complex 3 was isolated in 69% yield. Unlike the reaction with
MesMgBr, complex 3 was formed as the sole product even if
the magnesium reagent was added in one portion to the system.
The reaction is likely to proceed via a two-step process con-
sisting of one-electron reduction and transmetalation. Indeed, a
mixture of 2 and 3 was obtained under controlled reaction
conditions using less than 0.5 equiv of Mes2Mg(THF)2.
Treatment of 2 with Mes2Mg(THF)2 (0.6 equiv) resulted in
selective formation of 3, which was isolated in 70% yield.
On the contrary, complex 2 reacted with PhCH2Br (1 equiv)
at room temperature to afford 1 (63% after isolation), together
with PhCH2CH2Ph (46%/2) (eq 3). Thus, the interconversion
between 1 and 2 was accomplished by chemical reactions. In
the reaction with PhCH2Br, no trace of toluene was detected even
in the presence of 1,4-cyclohexadiene as a good hydrogen atom
donor, indicating the occurrence of a rapid one-electron oxidation
process involving a short-lived benzyl radical. On the other hand,
complex 2 was stable toward 3-hexenyl bromide, cyclohexyl bro-
mide, and tert-butyl bromide under the same reaction conditions.
While complex 3 did not give a satisfactory elemental ana-
lysis, probably due to instability toward air and moisture, its
structure could be confirmed by X-ray diffraction analysis of a
single crystal grown from a toluene solution at −35 °C. Figure 2
shows the ORTEP diagram, together with selected bond dis-
tances and angles. Similarly to 2, complex 3 adopts a distorted
trigonal monopyramidal configuration with the basal plane
consisting of P1, P2, C8, and Fe atoms. The sum of the three
bond angles in the basal plane is 357.4°. The N−Fe−C8 angle
(135.98(12)°) is much larger than the corresponding bond
angle of 2 (i.e., N−Fe−Br = 119.19(13)°), reflecting the
bulkiness of the Mes group. The bond lengths around iron are
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This assumption is consistent with the fact that [FeMes(BPEP)]
(3) decomposes in the presence of an excess amount of MesMgBr.
1
On the other hand, the H NMR spectrum of the reaction
solution of 1 with Me2Mg(THF)2 (1 equiv) in benzene-d6
indicated the formation of two products, 4 and 5, which
exhibited characteristic signals at δ −3.16 and −5.16, res-
pectively. Although this reaction had a reproducibility problem, 4
and 5 were successfully prepared by the reaction of 2 with
Me2Mg(THF)2 (1.0 equiv) in toluene at room temperature (eq 6).
The solvent was evaporated, and the residue was extracted
with Et2O and cooled to −30 °C, giving a mixture of red and
purple crystals of 4 and 5 in 46% total yield. As we discuss
below, both complexes are likely to be formed from [FeMe-
(BPEP)] as a common intermediate.
Figure 2. Molecular structure of 3 with 50% probability ellipsoids.
Hydrogen atoms and a disordered Bu group are omitted for clarity.
t
Selected bond lengths (Å) and angles (deg): Fe−P1, 2.2603(10); Fe−
P2, 2.2552(10); Fe−N, 2.042(3); Fe−C8, 1.980(3); P1−C1,
1.735(3); P2−C2, 1.735(3); C2−C3, 1.448(4); C3−C4, 1.397(5);
C4−C5, 1.384(5); C5−C6, 1.378(5); C6−C7, 1.396(5); C7−C1,
1.440(4); P1−Fe−P2, 137.25(4); P1−Fe−C8, 111.00(10); P2−Fe−C8,
109.10(10); N−Fe−C8, 135.98(12); N−Fe−P1, 80.93(8); N−Fe−P2,
80.60(8).
in a typical range of Fe(I) complexes.4b−g The P−C bond
lengths (both 1.735(3) Å) are comparable to those of iron(I)
monobromide 2 (1.713(6) and 1.719(6) Å), but longer than
the values of the corresponding iron(II) dibromide 1
(1.682(11) and 1.712(11) Å). These structural deviations are
consistent with the occurrence of an effective π-back-donation
from the Fe(I) center to BPEP. Geometrical distortion in the
pyridine ring, which is often observed for redox-active 2,2′-
bipyridine complexes after reduction,11 was not detected. Thus,
little contribution of an anionic pyridyl form to the structure of
3 was evidenced.
Complexes 4 and 5 could not be isolated independently by
recrystallization, but their structures were successfully deter-
mined by X-ray diffraction studies. The single crystal of 4
contains two crystallographically independent molecules with
essentially the same structures in the asymmetric unit. Figure 4
1
The H NMR signals of 3 were significantly broadened,
showing its paramagnetic nature. SQUID measurements estab-
lished the μeff values ranging from 1.68 to 1.94 μB at 50−300 K,
which correspond to the S = 1/2 ground state (Figure 3). Thus,
unlike 2 in the S = 3/2 state,3b complex 3 adopts a low-spin
Figure 4. Molecular structure of 4 (molecule 1) with 50% probability
ellipsoids. One of the two independent molecules (molecule 2),
hydrogen atoms, and a solvent molecules (Et2O) are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Fe−P1,
2.1012(8); Fe−N, 1.9237(17); P1−C1, 1.725(3); P2−C2,
1.6751(19); P1−C10, 1.827(3); C2−C3, 1.432(3); C3−C4,
1.400(4); C4−C5, 1.391(4); C5−C6, 1.374(3); C6−C7, 1.4127;
C1−C7, 1.416(3); N−C3, 1.415(3); N−C7, 1.382(3); N−Fe−P1,
81.16(6); P2−C2−C9, 122.36(15); C3−C2−C9, 112.19(19); P1−
C1−C7, 110.16(18); P1−C1−C8, 127.1(2); C7−C1−C8, 122.7(2).
Figure 3. Variable-temperature SQUID magnetic data for 3.
state with one unpaired electron. It is likely that the Mes group
as a higher-field ligand than the Br ligand in 2 provides the low-
spin state. It is also possible that the relatively more planar
configuration of 3 than 2 contributes to the change in the spin
state (i.e., N−Fe−C8 = 135.98(12)° in 3 versus N−Fe−Br =
119.19(13)° in 2).
Reactions with Methylmagnesiums. The reduction of 1
also took place with PhMgBr and MeMgBr. However, while
complex 2 was generated in low yields (up to 22%), phenyl and
methyl complexes were not detected in the systems. This
is probably due to the instability of Fe(I) complexes of the
type [FeR(BPEP)] (R = Ph, Me) toward Grignard reagents.
shows the ORTEP diagram of one of the molecules. It is seen
that the BPEP ligand transforms into an η6-benzene ligand with
a PN-chelate arm, which combines with an Fe(0) center to
form the two-legged piano stool structure of 4. The Fe−N and
Fe−P1 bond lengths (1.9237(17) and 2.1012(8) Å, respec-
tively) are typical of Fe(0) complexes.8b,12 The P1−C1 bond in
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the PN-chelate preserves the phosphaalkene structure. On the
other hand, the other phosphaalkene part (P2−C2) is con-
verted to a phosphonium ylide consisting of four-coordinate
phosphorus (P2) and three-coordinate carbon (C2) atoms,
where a benzophospholane framework is constructed by intra-
molecular cyclization of the P2−Mes* moiety via P−C bond
formation. The P2−C2 bond length (1.6751(19) Å) is among
the longest values of phosphonium ylides.13 The sum of the
three angles around C2 is 358.3°. It is known that phos-
phonium ylides stabilized by a π-conjugating substituent on the
carbon have a significant contribution of the ylene form. As a
result, the carbon atom exhibits planarity and the P−C bond
length becomes longer. The structural parameters around P2−
C2 faithfully reproduce this feature.
bond remains a neutral phosphaalkene ligand. On the other
hand, the P2−C2 bond is incorporated into the five-membered
PN-chelate with λ5-phosphaethenyl-P and pyridine units, which
serve as anionic and neutral ligands, respectively. It is known
that this type of ligand system possesses a structural contribu-
tion of the PN-chelate consisting of a neutral λ3-phosphane and
an anionic 2-methylenepyridyl ligand.14,15 Thus, the structure
of 5 is described by canonical structures A and B in eq 7. In
both structures, the iron center has a formal oxidation state
of +2. The P2−C2 bond (1.766(11) Å) is longer than the P1−
C1 double bond (1.693(10) Å), but significantly shorter than
the P1−C10(Mes*) single bond (1.872(9) Å). The C2−
C3(pyridine) distance is 1.449(13) Å. This value is clearly
longer than that of a CC bond (ca. 1.34 Å) and comparable
to that of C1−C7(pyridine) (1.460(13) Å). Therefore, it is
reasonable that the contribution of A is predominant over that
of B in 5. The structure A may be regarded as a metallo-
phosphonium ylide in the ylene form.
Figure 5 shows the X-ray structure of 5 having a tetraden-
tate PNPC-chelate ligand, which may be formed by oxidative
Scheme 1 illustrates a plausible reaction process for the
formation of 4 and 5. The Fe(I) complex 2 formed by one-
electron reduction of 1 reacts with Me2Mg(THF)2 to give
[FeMe(BPEP)] (C) and MeMgBr. Migration of the methyl
ligand to one of the low-coordinate phosphorus atoms forms
D, bearing a highly coordinatively unsaturated Fe(I) center,
which subsequently undergoes oxidative addition of the termi-
Figure 5. Molecular structure of 5 with 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Fe−P1, 2.312(3); Fe−P2, 2.205(3); Fe−N, 2.032(8);
Fe−C8, 2.051(10); P1−C1, 1.693(10); P1−C10, 1.872(9); P2−C2,
1.766(11); C2−C3, 1.449(13); C3−C4 1.455(13); C4−C5,
1.386(13); C5−C6, 1.408(13); C6−C7, 1.378(13); C7−C1,
1.460(13); N−C3, 1.396(11); N−C7, 1.449(11); P1−Fe−P2,
165.58(12); P1−Fe−C8, 109.2(3); P2−Fe−C8, 83.1(3); N−Fe−C8,
163.0(3); N−Fe−P1, 82.0(2); N−Fe−P2, 84.3(2); C7−C1−C9,
116.2(8); C7−C1−P1, 118.0(7); C9−C1−P1, 125.8(7); C3−C2−
C11, 121.1(9); C3−C2−P2, 113.8(7); C11−C2−P2, 125.1(7).
t
nal C−H bond of the Bu group to give Fe(III) hydride E.
Homolysis of the Fe−H bond in E followed by P−C reductive
elimination in F forms 4, accompanied by π-coordination of the
phenyl group. Finally, P−C oxidative addition in 4 affords 5. In
this stage, MeMgBr generated in the system possibly induces
the Fe−H bond cleavage. Finally, P−C oxidative addition in 4
affords 5.
CONCLUSION
It has been found that the Fe(II) complex [FeBr2(BPEP)] (1)
supported by BPEP as a tridentate phosphaalkene ligand readily
■
addition of the P2−C(Ar) bond of the phosphonium ylide in 4,
along with dissociation of the η6-benzene ligand. The P1−C1
Scheme 1. Plausible Reaction Process for the Formation of 4 and 5 from 2 and Me2Mg(THF)2
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Table 1. Crystal Data and Details of the Crystal Structure Determination for 3, 4, and 5
3
4
5
empirical formula
fw
C64H82FeNP2
983.10
C56H73FeNP2·C4H10O
970.59
C56H73FeNP2
877.94
cryst syst
space group
a (Å)
monoclinic
P21/n
triclinic
monoclinic
C2/c
P1
̅
18.960(4)
10.792(2)
27.945(6)
16.2808(2)
17.5874(2)
22.8589(4)
96.1553(6)
106.0147(7)
114.7727(10)
5523.06(13)
4
41.171(11)
9.514(3)
b (Å)
c (Å)
27.100(15)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
99.235(4)
107.470(15)
5644(2)
4
10126(7)
8
dcalc (g cm−3
)
1.157
1.145
1.152
103
T (K)
103
173
μ(Mo Kα) (mm−1
F(000)
)
0.363
0.370
0.397
3776
2116
2056
cryst size
0.16 × 0.15 × 0.09
3.06-31.38
52 106
0.03 × 0.02 × 0.01
1.32−32.40
63 205
0.15 × 0.05 × 0.03
2.13−25.00
32 464
θ range (deg)
no. of reflns collected
no. of unique reflns (Rint)
no. of reflns with I > 2σ(I)
no. of params (restraints)
GOF on F2
16797 (0.0871)
9539
33934 (0.0398)
15 466
8782 (0.1890)
4208
611 (0)
1143 (12)
0.652
541 (0)
1.077
1.073
R1, wR2 (I > 2σ(I))
0.0781, 0.1824
0.1488, 0.2337
1.051, −0.589
0.0578, 0.1552
0.0980, 0.1689
2.491, −0.646
0.1195, 0.2530
0.2254, 0.3277
1.402, −0.565
R1, wR2 (all data)
largest peak and hole (e Å−3
)
University. ESI-mass spectra were recorded on a Bruker micrOTOF I
spectrometer for 3 and on a Bruker solariX Hybrid Qq-FTMS system
for 4 and 5. Solid-state magnetic moments were recorded using a
Quantum Design SQUID magnetometer. Magnetization versus
temperature data were recorded in a magnetic field of 10 000 G,
using crystalline samples sealed in quartz tubes. An impurity correction
was made by fitting experimental data to the formula χimpurity = C/T.
Cyclic voltammetry was performed with a BAS ALS600B. The
data were collected in CH2Cl2 (1 mM of 1) in the presence of
[Bu4N][BF4] electrolyte (0.1 M), using a glassy carbon working
electrode, platinum wire as the counter electrode, and silver electrode
as the reference electrode in a drybox equipped with electrochemical
outlets, at a scan rate of 100 mV/s at 295 K. GLC analysis was
performed on a Shimadzu GC-14B instrument (FID; CBP-1, 25 m ×
0.25 mm).
undergoes one-electron reduction by electrochemical and
chemical means to afford [FeBr(BPEP)] (2), having a distorted
trigonal monopyramidal Fe(I) center. While complex 2 is
isolated by using [Cp2Co] as a mild reducing agent, the use
of Mes2Mg(THF)2 leads to further conversion of 2 into
[FeMes(BPEP)] (3) via transmetalation. The unique ligand
properties of BPEP, which undergoes effective dπ−pπ
interactions with an Fe(I) center, will be responsible for the
easy reduction of the Fe(II) complex 1 to the Fe(I) complexes
2 and 3. The single-crystal diffraction study has revealed the
distorted trigonal monopyramidal structure of 3, which is in
sharp contrast to the planar structure of related [FeMe-
(iPrPDI)].10 We have demonstrated for 2 using DFT calcula-
tions3b that the structural distortion to trigonal monopyramidal
geometry enables effective dπ−pπ interactions between Fe and
BPEP. The sequence of one-electron reduction and trans-
metalation is operative for Me2Mg(THF)2 as well. In this case,
however, the resulting [FeMe(BPEP)] undergoes intramolec-
ular migration of the methyl ligand to a low-coordinate phos-
phorus atom, leading to the formation of 4 and 5, bearing novel
ligand systems containing a phosphonium ylide structure.
Reaction of 1 with [Cp2Co]. Complex 1 (48.3 mg, 0.0471 mmol)
was dissolved in C6H6 (0.4 mL), and to this was added a C6H6
solution (0.1 mL) of [Cp2Co] (8.90 mg, 0.0471 mmol) at room
temperature. Immediately, the solution was filtered through a Celite
pad, and the solvent was removed under vacuum. The residue was
washed with hexane and dried under vacuum to give 2 as a green solid
1
(26.5 mg, 0.0269 mmol, 57%). Formation of 2 was confirmed by H
NMR spectroscopy.
Reaction of 2 with PhCH2Br. To a C6H6 solution (0.4 mL) of 2
(15.5 mg, 0.0164 mmol) was added PhCH2Br (2.0 μL, 0.0168 mmol)
at room temperature. Immediately, the green color of the solution
slightly darkened. GLC analysis revealed the formation of biphenyl
(0.00748 mmol, 46%/2). After the solvent was removed under
vacuum, the residue was washed with hexane repeatedly and dried
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under a nitrogen atmosphere using Schlenk techniques or a glovebox.
Solvents (toluene, benzene, benzene-d6, Et2O) were dried over sodium
benzophenone ketyl and distilled. MesMgBr was purchased from a
commercial source (Sigma-Aldrich) and used without purification.
1
under vacuum to afford 1 (10.6 mg, 0.0104 mmol, 63%). The H
NMR spectrum in C6D6 revealed selective formation of 1.
Mes2Mg(THF)2,16 Me2Mg(THF)2,16 and KC8 were prepared
17
Reaction of 2 with PhCH2Br in the Presence of 1,4-Cyclo-
hexadiene. Similarly, the reaction of 2 (7.48 mg, 0.00792 mmol) with
PhCH2Br (1 μL, 0.0084 mmol) in C6D6 (0.4 mL) was examined in the
presence of 1,4-cyclohexadiene (1 μL, 0.011 mmol). GLC analysis
revealed the formation of biphenyl (0.0033 mmol, 83%). No trace
according to the literature procedure.
1H NMR spectra were recorded on a Bruker AVANCE 400 spec-
trometer (400.13 MHz). Chemical shifts are reported in δ (ppm),
referenced to the residual solvent signal as an internal standard. Ele-
mental analysis was performed by ICR Analytical Laboratory, Kyoto
2013
dx.doi.org/10.1021/om201280z | Organometallics 2012, 31, 2009−2015

Organometallics
Article
amount of toluene was detected. Complex 1 was isolated in 73% yield
(5.93 mg, 0.00579 mmol).
cell; one Et2O molecule was disordered with an occupancy ratio of
87:13. Anisotropic refinement was applied to all non-hydrogen atoms
except for disordered groups. Hydrogen atoms were placed at
calculated positions. The crystallographic data and the summary of
solution and refinement are listed in Table 1.
Reaction of 1 with MesMgBr. Complex 1 (15.6 mg, 0.0152 mmol)
was dissolved in C6H6 (2 mL), and a THF solution of MesMgBr
(0.2 M, 200 μL, 0.040 mmol) was slowly added over 1 h at room
temperature. The solvent was removed under vacuum, and the residue
was washed with hexane (0.5 mL × 3), extracted with C6H6, and
filtered through a Celite pad. The filtrate was concentrated to dryness
under vacuum to afford 3 as a dark brown solid (13.1 mg, 0.0130
mmol, 88%). The hexane washings were concentrated under vacuum,
ASSOCIATED CONTENT
■
S
* Supporting Information
Cyclic voltammogram of [FeCl2(iPrPDI)], ESI-mass spectrum
of 3, and crystallographic data (CIF files). This material is
available free of charge via the Internet at http://pubs.acs.org.
1
dissolved in C6D6 (0.4 mL), and analyzed by H NMR spectroscopy
using CH2Cl2 (2 μL, 0.031 mmol) as an internal standard, showing the
formation of bimesityl (0.0061 mmol, 80%). Single crystals of 3 for
X-ray diffraction analysis were grown by slow evaporation of a toluene
solution at room temperature. 1H NMR (C6D6, 20 °C): δ −4.89 (brs),
−2.38 (brs), −1.29 (brs), 0.52 (s), 4.75 (s), 5.86 (brs), 8.37 (brs),
21.13 (brs). ESI-MS (m/z): 982.5 (M+). This complex did not give a
satisfactory elemental analysis.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nakajima@scl.kyoto-u.ac.jp; ozawa@scl.kyoto-u.ac.jp.
■
Notes
Reaction of 2 with MesMgBr. To a C6H6 solution (2 mL) of
2 (12.2 mg, 0.0129 mmol) was added a THF solution of MesMgBr
(0.06 M, 200 μL, 0.012 mmol) over 1 h at room temperature. The
solution was stirred for 1 h and filtered through a Celite pad. The solvent
was evaporated, and the residue was washed with hexane (0.5 mL) and
dried under vacuum to give 3 (7.06 mg, 0.0718 mmol, 55%).
Reaction of 1 with Mes2Mg(THF)2. To a C6H6 solution (1 mL)
of 1 (30.6 mg, 0.0299 mmol) was added a C6H6 solution (0.5 mL) of
Mes2Mg(THF)2 (12.0 mg, 0.0295 mmol). The mixture was stirred at
room temperature for 1 h and filtered through a Celite pad. Removal
of the solvent under vacuum afforded 3 as a dark brown solid (20.3 mg,
0.0206 mmol, 69%).
Reaction of 2 with Mes2Mg(THF)2. Complex 2 (26.8 mg, 0.0284
mmol) was dissolved in C6H6 (1 mL), and a C6H6 solution of
Mes2Mg(THF) (6.92 mg, 0.0170 mmol) was added. The mixture was
stirred at room temperature for 30 min and filtered through a Celite
pad. The solvent was evaporated, and the residue was washed with
hexane (0.5 mL) and dried under vacuum to give 3 (19.4 mg, 0.0198
mmol, 70%).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research from MEXT Japan and the JST PRESTO program.
We are grateful to Dr. Sasamori and Dr. Mizuhata (ICR, Kyoto
Univ.) for crystallographic assistance and to Prof. Ono (ICR,
Kyoto Univ.) for SQUID measurement.
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