Configurational isomerization of dinuclear iridium and rhodium complexes with a series of NPPN ligands, 2-PyCH2(Ph)P(CH2) nP(Ph)CH2-2-Py (Py = Pyridyl, n = 2-4)

Article abstract of DOI:10.1021/om400965j

New heterodonor NPPN tetradentate ligands, 2-PyCH₂(Ph)P(CH ₂)_nP(Ph)CH₂-2-Py (meso- and rac-Lⁿ; n = 2-4, Py = pyridyl), were prepared and reacted with [CpMCl₂] ₂ (M = Ir, Rh; Cp* is pentamethylcyclopentadienyl) in the presence of NH₄BF₄ to afford a series of dinuclear complexes [(CpMCl)₂(meso-Lⁿ)](BF₄)₂ (M = Ir, n = 2 (2a), 3 (3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and [(CpMCl)₂(rac-Lⁿ)](BF₄)₂ (M = Ir, n = 2 (2b), 3 (3b), 4 (4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), which were characterized by IR, ¹H and ³¹P{¹H} NMR, and ESI mass spectroscopic techniques and X-ray crystallography. The configurations around the two metal centers were controlled by the configuration of the coordinated P atoms so as to avoid repulsive interaction between the phenyl group on P and the chloride ligand, resulting in the formation of stereospecific isomers; a meso configuration of the metal centers is induced from meso-L ⁿ (abbreviated as meso-P₂/meso-M₂), and in contrast, a rac configuration is induced from rac-Lⁿ (rac-P ₂/rac-M₂). Furthermore, inversion of metal centers for the Ir₂ complexes occurred in DMSO at higher temperatures (60-100 C), generating equilibrium mixtures of minor diastereomers (meso-P ₂/rac-M₂ or rac-P₂/meso-M₂) in low ratios together with the major isomers (meso-P₂/meso-M₂ or rac-P₂/rac-M₂). The equilibrium constants, K = [minor isomer]/[major isomer], varied appreciably depending on the lengths of the methylene chains as well as configurations of the NPPN ligands; the overall propensity for the K values was observed to be L² < L³ < L⁴ and meso-Lⁿ < rac-Lⁿ, while rac-L³, rac-L⁴, and meso-L⁴ showed almost identical equilibrium constants, presumably resulting from no steric influence between the two metal centers.

Full text of DOI:10.1021/om400965j

Article
pubs.acs.org/Organometallics
Configurational Isomerization of Dinuclear Iridium and Rhodium
Complexes with a Series of NPPN Ligands,
2‑PyCH2(Ph)P(CH2)nP(Ph)CH2‑2-Py (Py = Pyridyl, n = 2−4)
Takayuki Nakajima,* Yuki Fukushima, Minori Tsuji, Naoko Hamada, Bunsho Kure,
and Tomoaki Tanase*
Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-nishi-machi, Nara 630-8506, Japan
S
* Supporting Information
ABSTRACT: New heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P-
(CH2)nP(Ph)CH2-2-Py (meso- and rac-Ln; n = 2−4, Py = pyridyl), were
prepared and reacted with [Cp*MCl2]2 (M = Ir, Rh; Cp* is pentamethylcy-
clopentadienyl) in the presence of NH4BF4 to afford a series of dinuclear
complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3 (3a), 4 (4a); M
= Rh, n = 2 (2c), 3 (3c), 4 (4c)) and [(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n =
2 (2b), 3 (3b), 4 (4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), which were
1
characterized by IR, H and 31P{1H} NMR, and ESI mass spectroscopic
techniques and X-ray crystallography. The configurations around the two metal
centers were controlled by the configuration of the coordinated P atoms so as
to avoid repulsive interaction between the phenyl group on P and the chloride
ligand, resulting in the formation of stereospecific isomers; a meso configuration
of the metal centers is induced from meso-Ln (abbreviated as meso-P2/meso-M2),
and in contrast, a rac configuration is induced from rac-Ln (rac-P2/rac-M2). Furthermore, inversion of metal centers for the Ir2
complexes occurred in DMSO at higher temperatures (60−100 °C), generating equilibrium mixtures of minor diastereomers
(meso-P2/rac-M2 or rac-P2/meso-M2) in low ratios together with the major isomers (meso-P2/meso-M2 or rac-P2/rac-M2). The
equilibrium constants, K = [minor isomer]/[major isomer], varied appreciably depending on the lengths of the methylene chains
as well as configurations of the NPPN ligands; the overall propensity for the K values was observed to be L2 < L3 < L4 and meso-
Ln < rac-Ln, while rac-L3, rac-L4, and meso-L4 showed almost identical equilibrium constants, presumably resulting from no steric
influence between the two metal centers.
which terminal N-donor units such as pyridyl and quinoline-
methyl groups are connected to flexible diphosphine chains,6
owing to the difficulty of separating stereoisomers arising from
chirality of the P atoms, while unique reactivity may be induced
by cooperation of two flexibly linked, adjacent hemilabile metal
centers.
In the present study, we have prepared the new series of
NPPN tetradentate ligands 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-
2-Py (Ln; n = 2−4, Py = pyridyl), where two phosphorus atoms
with 2-picolyl substituent groups are connected with methylene
chains of variable length with the aim of tuning the separation
of two metal centers. With respect to the chirality of the P
atoms, the NPPN ligands exist as meso and racemic
diastereomers and both are able to capture two Cp*MCl (M
= IrIII, RhIII) fragments to afford the series of dinuclear
complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3
(3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and
[(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n = 2 (2b), 3 (3b), 4
(4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)). X-ray analyses of
INTRODUCTION
■
The design of multidentate ligands is of crucial importance to
fabricate functional metal centers and thus remains a main
subject in modern molecular sciences of inorganic and
organometallic chemistry with relevance to a wide variety of
valuable applications.1 In particular, multidentate mixed-donor
ligands have attracted considerable recent interest because they
should notably involve (1) hemilabile chelating effects that
stabilize and activate the metal center and (2) bridging effects
that connect two or more metals and induce cooperative and
synergistic functions exerted by the metals.2,3 There have been
a number of studies on mononuclear transition-metal
complexes with PN heterodonor ligands, which are attractive
as homogeneous catalysts because the hard−soft and hemilabile
characters of PN ligands permit metal coordination sites
suitable for substrate binding and activation.4 Dinucleating
ligands with PN heterodonor units are usually concerned with
PNNP tetradentate systems, in which diphenylphosphino and
diphenylphosphinomethyl groups are incorporated into rigid
N-donor scaffolds such as 2,2′-bipyridyl, 1,10-phenanthroline,
1,8-naphthylidine, pyrazole, phthalazine, and so on.5 In
contrast, examples are quite limited with NPPN ligands, in
Received: October 2, 2013
Published: November 22, 2013
© 2013 American Chemical Society
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the solid-state structures indicated that the configuration of the
two metal centers was regulated by the configuration of the
coordinated P atoms to form stereospecific isomers, in which a
meso configuration for the metal centers is derived from meso-Ln
and, in contrast, a rac configuration of the metal centers is
derived from rac-Ln. In addition, inversion of metal centers for
the Ir2 complexes was found to occur in DMSO at higher
temperatures (60−100 °C), affording equilibrium mixtures of
the major and minor isomers with the equilibrium constants
appreciably varying depending on the lengths of the methylene
chains as well as the configuration of the NPPN ligands.
Scheme 2. Preparations of 2a−d, 3a−d, and 4a−d
RESULTS AND DISCUSSION
and/or crystallization, to isolate the series of dinuclear metal
complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3
(3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and
[(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n = 2 (2b), 3 (3b), 4
(4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), even though the
isolated yields were extremely low. By using the pure isomers of
meso-L2 and rac-L4, complexes 2a,c and 4b,d were obtained in
good yields. While the six configurational isomers are
potentially generated from the chirality of the two phosphorus
atoms and the two metal centers (Scheme 3), the 31P{1H}
■
Synthesis of NPPN Tetradentate Ligands meso/rac-Ln
(n = 2−4). A new series of NPPN tetradentate ligands meso-/
rac-Ln (n = 2−4) were synthesized as shown in Scheme 1.
Scheme 1. Preparations of NPPN Tetradentate Ligands,
meso- and rac-Ln (n = 2−4)
Scheme 3. Stereoisomers of [(Cp*MCl)2(meso- or rac-Ln)]2+
(M = Ir, Rh; n = 2−4)
Deprotonation of Ph(H)P(CH2)nP(H)Ph with n-BuLi and
subsequent addition of picolyl chloride afforded 1:1 mixtures of
meso and rac isomers of the NPPN ligands Ln in moderate
yields which were monitored by 31P{1H} NMR spectroscopy (δ
−14.8, −15.4 (L2); δ −19.6, −19.7 (L3); δ −18.5, −19.7 (L4))
in CDCl3. Addition of HCl dissolved in Et2O to a solution of
meso-/rac-Ln in Et2O yielded the HCl adducts meso-/rac-Ln·
2HCl as white solids, which were used for the synthesis of
complexes without further purification.
While attempts to separate the stereoisomers of meso-/rac-Ln
and meso-/rac-Ln·2HCl by crystallization were generally
unsuccessful, meso-L2 and rac-L4 ligands were able to be
isolated on a millimole scale successfully via their phosphine−
borane adducts. Treatment of meso-/rac-L2 and meso-/rac-L4
with BH3−THF, followed by precipitation from MeOH,
afforded white crystals of meso-L2·2BH3 and rac-L4·2BH3 in
12 and 18% yields, respectively. The solid-state structures of
meso-L2·2BH3 and rac-L4·2BH3 were determined by X-ray
crystallography as shown in Figures S1 and S2 (see the
Supporting Information). The removal of BH3 from meso-L2·
2BH3 and rac-L4·2BH3 was accomplished by treatment with
diethanolamine or tetrafluoroboric acid to afford the desired
meso-L2 and rac-L4 ligands in almost quantitative yields.
Synthesis of Dinuclear Iridium and Rhodium Com-
plexes [(Cp*MCl)2(meso-Ln)](BF4)2 and [(Cp*MCl)2(rac-
Ln)](BF4)2 (M = Ir, Rh, n = 2−4). Reactions of [Cp*MCl2]2
(M = Ir (1a), Rh (1b)) with 1:1 meso and rac mixtures of the
HCl adducts of NPPN ligands (meso-/rac-Ln·2HCl, n = 2−4)
and excess NH4BF4 in the presence of NEt3 in CH2Cl2 yielded
the respective mixtures of [(Cp*MCl)2(meso-Ln)](BF4)2 and
[(Cp*MCl)2(rac-Ln)](BF4)2 in a ca. 1:1 ratio (Scheme 2),
which were separated by silica gel column chromatography
NMR spectra of the isolated complexes showed only one
singlet corresponding to the major isomers, meso-P2/meso-M2
and rac-P2/rac-M2 (Scheme 3a), which were unambiguously
determined by X-ray crystallography (Figure 1). The ORTEP
diagrams for the complex cations of 2a,b, 3a, 3d′, and 4a,b are
illustrated in Figure 1, and those of 2c and 4d are supplied in
the Supporting Information (Figures S3 and S4). The
crystallographic data and selected structural parameters are
given in Tables S2−S4 in the Supporting Information. In all of
the structures, two Cp*MCl fragments were chelated by 2-
PyCH2P moieties to form a three-legged piano-stool structure
and bridged by the central P(CH2)nP diphosphine chains
without any metal−metal interactions. The Cp*MCl(2-
pyCH2(Ph)P) structures are almost identical irrespective of
the metal species, Ir or Rh, and the length of the diphosphine
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Figure 1. ORTEP diagrams for the complex cations of (a) 2a, (b) 2b, (c) 3a, (d) 3d′, (e) 4a, and (f) 4b with the atomic numbering schemes. The
thermal ellipsoids are drawn at the 40% probability level, and the hydrogen atoms are omitted for clarity.
chains on the basis of the structural parameters given in Table
S4 (Supporting Information): M−Cl = 2.386−2.414 Å, M−P =
2.260−2.295 Å, M−N = 2.093−2.154 Å, and P−M−Cl =
85.32−93.65°. The structures of 2a (M = Ir) and 2c (M = Rh)
with meso-L2 have pseudo or crystallographic Ci symmetry to
reduce the steric repulsion between the two metal fragments of
{Cp*MCl(2-pyCH2(Ph)P)}, resulting in the metal−metal
separations of 8.336(5) Å (2a) and 8.370(4) (2c) Å (Figure
1a and Figure S3 (Supporting Information)). On the other
hand, the complex cation of 2b (M = Ir) with rac-L2 has a
pseudo C2 axis passing through the middle of the diphosphine
chain (Figure 1b), where the metal−metal distance of 6.989(3)
Å is remarkably shorter than those with meso-L2. For the
propylene-bridged complexes 3a (M = Ir) and 3d′ (M = Rh),
the former (3a) with meso-L3 has a pseudo Cs symmetry (Figure
1c). However, the latter (3d′) with rac-L3 possesses a pseudo
C2 symmetry as observed in 2b (Figure 1d); the metal−metal
separation of 3d′ (8.461(3) Å) is shorter by ca. 1.1 Å than that
of 3a (9.550(4) Å). For the butylene-bridged complexes, the
structure of 4a (M = Ir, Figure 1e) with meso-L4 has an
inversion center like that of 2c and those of 4b (M = Ir, Figure
1f) and 4d (M = Rh, Figure S4 (Supporting Information)) with
rac-L4 adopting a pseudo C2 symmetry as found in 2b and 3d.
The metal−metal separation of 10.878(5) Å (4a) is slightly
longer than those of 4b (10.612(2) Å) and 4d (10.613(3) Å).
The distances between the two metal centers (dMM) vary
depending on the length (n) of the central methylene chains of
the NPPN ligands (Ln) with the apparent propensity of
d
MM(meso-Ln) > dMM(rac-Ln) for the same number of n and
ΔdMM(L2) > ΔdMM(L3) > ΔdMM(L4) for ΔdMM = dMM(meso-Ln)
− dMM(rac-Ln), as shown in Figure S5 (Supporting
Information).
The X-ray crystal structures clearly revealed that the
configuration around the metal centers is completely regulated
by the configuration of the coordinated P atoms so as to avoid
steric repulsion between the phenyl group and the chloride
ligand, resulting in the formation of stereospecific isomers in
which the meso configuration for the metal centers is derived
from meso-Ln (abbreviated as meso-P2/meso-M2) in 2a,c, 3a, and
4a, and in contrast, a rac configuration of the metal centers is
induced from rac-Ln (rac-P2/rac-M2) in 2b, 3d′, and 4b,d.
Because no influence depending on the length of diphosphine
chains was observed, the preferential formations of the meso-
P2/meso-M2 and rac-P2/rac-M2 isomers are attributed to steric
repulsion between the chloride ligand attached to the metal ion
and the phenyl substituent on the phosphorus atom (Figure 2).
Since the preferred configuration of the {Cp*MCl(2-
pyCH2(Ph)P)} fragment is represented as RM,RP or SM,SP in
CIP notation, the absolute configurations for the two M and
two P centers are described as RM,RP,SP,SM for the stereoisomer
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rac-P2/rac-M2′ (RM,SP,SP,RM/SM,RP,RP,SM) were estimated to
be generated in very small amounts, which tentatively
corresponded to the small singlet peaks marked with asterisks
in Figure 3b,d. For the rhodium complexes 2c and 4d, heating
of their DMSO solutions at higher temperature led to reaction
mixtures containing a decomposed species and the isomers
generated by configurational inversion of the metal centers,
which prevented further detailed discussion.
The equilibrium constants of the two stereoisomers (K =
[meso-P2/rac-M2]/[meso-P2/meso-M2] or [rac-P2/meso-M2]/
[rac-P2/rac-M2]) of 2a,b, 3a,b, and 4a,b were measured at
various temperatures between 60 and 100 °C in DMSO-d6 by
integration of the 31P{1H} NMR resonances and indicated that
the central methylene chains and the configurations of the
NPPN ligands have appreciable influences on the isomerization
(Figure 4). The K values apparently increase in the order meso-
Figure 2. Repulsive interaction between the phenyl group of the P
atom and the Cl ligand. RM or SM indicates the absolute configuration
around the metal center and RP or SP that of the P atom.
of meso-P2/meso-M2, and as SM,SP,SP,SM/RM,RP,RP,RM for the
stereoisomer of rac-P2/rac-M2. These configurational structures
were retained even in the solution state at room temperature,
which was confirmed by 31P{1H} NMR spectra showing a
singlet peak at δ 17.9−20.5 ppm for the two equivalent
phosphorus atoms of the iridium complexes (2a,b, 3a,b, 4a,b),
1
and a doublet peak at δ 47.9−49.9 ppm with JRhP = 132−136
Hz for those of the rhodium complexes (2c,d, 3c,d, 4c,d).
Furthermore, the 31P{1H} NMR spectra of the reaction
mixtures of [Cp*MCl2]2 with Ln·2HCl also indicated that the
meso-P2/meso-M2 and rac-P2/rac-M2 isomers (Scheme 3a) were
formed dominantly as major products together with only small
amounts of the minor isomers, as shown in Scheme 3b,c.
Equilibrium Studies. Although the meso-M2/meso-P2 and
rac-M2/rac-P2 structures were retained in the solution state at
room temperature, inversion of the metal centers for the
iridium complexes occurred in DMSO upon increasing
temperature at 60−100 °C to afford equilibrium mixtures of
the major (meso-P2/meso-M2 or rac-P2/rac-M2) and minor
(meso-P2/rac-M2 or rac-P2/meso-M2) isomers, which were
monitored by 31P{1H} NMR spectroscopy (Figure 3). The
31P{1H} NMR spectra of meso-P2/rac-M2 and rac-P2/meso-M2
isomers showed two resonances due to the presence of two
nonequivalent phosphorus atoms. Inversion of metal centers
seems to proceed through dissociation of chloride ligands.13
The meso-P2/rac-M2 isomer (RM,RP,SP,RM/SM,RP,SP,SM) was
formed from the meso-P2/meso-M2 isomer (for example 4a in
Figure 3a,b) and the rac-P2/meso-M2 isomer (RM,SP,SP,SM/
SM,RP,RP,RM) from the rac-P2/rac-M2 isomer (for example 4b in
Figure 3c,d) via chiral inversion of one metal center. The
doubly inverted isomers of meso-P2/meso-M2′ (SM,RP,SP,RM) or
Figure 4. Plots of equilibrium constant K vs temperature in the
□
○
●
▲
reaction solutions from 2a ( ), 2b ( ), 3a (Δ), 3b ( ), 4a ( ), and
■
4b ( ).
L2 < meso-L3 < rac-L2 < rac-L3 ≈ meso-L4 ≈ rac-L4. The overall
propensity for the K values was observed to be L2 < L3 < L4 and
meso-Ln < rac-Ln, although rac-L3, rac-L4, and meso-L4 showed
almost identical equilibrium constants, presumably resulting
from no steric influence existing between the two metal centers
bridged by long methylene chains. In contrast, isomerization
from 2a,b and 3a with meso-/rac-L2 and meso-L3 involve some
steric influence between the two metal fragments, which should
Figure 3. 31P{1H} NMR spectra in DMSO-d6 of (a) 4a at room temperature, (b) 4a after heating at 100 °C, (c) 4b at room temperature, and (d) 4b
after heating at 100 °C.
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Table 1. Thermodynamic Parameters of ΔHo and ΔSo Obtained from van’t Hoff Plots for the Equilibrium Reactions from 2a−
a
4a with meso-Ln (n = 2−4) and 2b−4b with rac-Ln (n = 2−4)
2a, meso-L2
2b, rac-L2
3a, meso-L3
3b, rac-L3
4a, meso-L4
4b, rac-L4
ΔH° (kJ/mol)
ΔS° (J/(K mol))
5.6(3)
3.0(8)
0.79(5)
5.9(6)
2.1(3)
2.5(3)
2.0(2)
−8(2)
5.2(18)
−2.5(8)
−1.3(8)
−2.7(5)
a
See Figure S6 in the Supporting Information.
meso- and rac-Bis{phenyl(2-picolyl)phosphine}propane (meso/
rac-L3). A mixture of meso- and rac-L3 was obtained in 81% yield (2.37
g, 9.09 mmol) from 1,2-bis(diphenylphosphino)propane. 1H NMR
(CDCl3): δ 1.21−1.63 (m, 4H, CH2), 3.18−3.19 (m, 4H, PyCH2),
6.76−8.40 (m, 18H, Ph, Py). 31P{1H} NMR (CDCl3): δ −19.6 (s, 1P),
−19.7 (s, 1P).
meso- and rac-Bis{phenyl(2-picolyl)phosphine}butane (meso/
rac-L4). A mixture of meso- and rac-L4 was obtained in 55% yield
(1.43 g, 5.23 mmol) from 1,2-bis(diphenylphosphino)butane. 1H
NMR (CDCl3): δ 1.24−2.46 (m, 8H, CH2), 3.17−3.18 (m, 4H,
PyCH2), 6.80−8.42 (m, 26H, Ph, Py). 31P{1H} NMR (CDCl3): δ
−18.5 (s, 1P, rac), −19.7 (s, 1P, meso).
General Procedure for Synthesis of meso- and rac-NPPN·
2HCl (meso-/rac-Ln·2HCl: n = 2−4) Ligands. To a solution of meso-
and rac-NPPN (meso/rac-Ln: n = 2−4) ligands in Et2O was added 4
equiv of HCl (1.57 M in Et2O). The resulting white precipitate was
filtered, washed with Et2O, and dried under reduced pressure to give
the ligands meso- and rac-NPPN·2HCl (meso-/rac-Ln·2HCl: n = 2−4),
which were used without further purification for the synthesis of
complexes.
meso- and rac-1,2-Bis{phenyl(2-picolyl)phosphino}ethane·2HCl
(meso-/rac-L2·2HCl). A mixture of meso- and rac-L2·2HCl was
obtained in 69% yield (5.28 g, 12.3 mmol) from meso/rac-L2.
31P{1H} NMR (CD3OD): δ −9.1 (s, 1P), −9.8 (s, 1P).
meso- and rac-1,3-Bis{phenyl(2-picolyl)phosphino}propane·2HCl
(meso-/rac-L3·2HCl). A mixture of meso- and rac-L3·2HCl was
obtained in 76% yield (3.24 g, 7.32 mmol) from meso/rac-L3.
31P{1H} NMR (CD3OD): δ −12.7 (s, 1P), −12.6 (s, 1P).
meso- and rac-1,4-Bis{phenyl(2-picolyl)phosphino}butane·2HCl
(meso/rac-L4·2HCl). A mixture of meso- and rac-L4·2HCl was obtained
in 83% yield (1.31 g, 2.87 mmol) from meso/rac-L4. 31P{1H} NMR
(CD3OD): δ −11.6 (s, 1P).
meso-1,2-Bis{phenyl(2-picolyl)phosphino}ethane·2BH3
(meso-L2·2BH3). To a solution of meso-/rac-L2 (4.7 g, 11.0 mmol) in
THF (40 mL) was added BH3−THF (30 mL, 33.0 mmo) at 0 °C. The
mixture was stirred overnight at room temperature, and then degassed
H2O (30 mL) was added. The organic phase was separated, and the
aqueous phase was extracted with AcOEt. The combined organic
extracts were dried over Na2SO4, filtered, and concentrated under
reduced pressure to give an oil. By addition of dry MeOH, white
precipitates were formed, which were filtered off, washed with hexane,
and dried under vacuum to afford white powders of meso-L2·2BH3
(531 mg, 12%). 1H NMR (CDCl3): δ 0.25−1.25 (br, 6H, BH3), 1.89−
2.01 (m, 2H, CH2), 2.30−2.36 (m, 2H, CH2), 3.36(m, 4H, PyCH2),
6.87 (d, 2H, J = 8 Hz, Py), 7.06 (dd, 2H, J = 9, 5 Hz, Py), 7.36−7.53
(m, 12H, Ph, Py), 8.33 (d, 2H, J = 4 Hz, Py). 31P{1H} NMR (CDCl3):
δ 20.4 (br s, 2P).
result in suppression of inversion around the metal centers. The
van’t Hoff plots of −ln K vs 1/T provided thermodynamic
parameters of ΔH° and ΔS° (Table 1 and Figure S6). While
the absolute values of ΔS° are very small in all, ΔH° values for
the reactions from 2a (meso-L2) and 3a (meso-L3) are
remarkably large in comparison with those for other complexes,
suggesting that steric interaction between the two metal
fragments prohibits the configurational inversion to some
extent.
CONCLUSION
■
The series of dinuclear iridium and rhodium complexes
[(Cp*MCl)2(meso-Ln)]2+ and [(Cp*MCl)2(rac-Ln)]2+ (M =
Ir, Rh; n = 2−4) were synthesized by the reaction of new
tetradentate NPPN ligands meso-/rac-Ln with [Cp*MCl2]2. X-
ray analyses of their solid-state structures revealed that the
configurations of the two metal centers were determined by the
configuration of the coordinated P atoms, resulting in the
stereospecific formation of meso-P2/meso-M2 and rac-P2/rac-M2
isomers, avoiding steric repulsion between the chloride ligand
and the phenyl substituent on the phosphorus atom.
Configurational inversion of the metal centers for the iridium
dinuclear complexes was investigated at higher temperatures,
which demonstrated that the central methylene chains and the
configurations of the NPPN ligands have considerable influence
on the isomerization.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out under a
nitrogen atmosphere by using standard Schlenk techniques. Solvents
were dried by standard procedures and freshly distilled prior to use.
Other reagents were used as purchased without further purification.
The compounds Ph(H)P(CH2)nP(H)Ph (n = 2−4) were prepared by
the reported procedure.7 1H and 31P{1H} NMR spectra were recorded
on a JEOL JMN-AL-400 spectrometer at 400 and 160 MHz,
respectively. The chemical shifts were calibrated to TMS (1H) or
80% H3PO4 (31P) as an external reference. IR spectra were measured
on KBr pellets with a JASCO FT/IR-410 spectrometer. ESI-TOF-MS
spectra were obtained with a JEOL JMS-T100LC high-resolution mass
spectrometer with positive ionization mode.
General Procedure for Synthesis of meso- and rac-NPPN
(meso/rac-Ln: n = 2−4) Ligands. To a solution of the
diphenylphosphine Ph(H)P(CH2)nP(H)Ph (n = 2−4) in THF was
added dropwise 2 equiv of n-BuLi (1.57 M in hexane) at −78 °C.
Then, 2 equiv of 2-picolyl chloride in THF was added at the same
temperature. The cooling bath was removed, and the mixture was
stirred overnight at room temperature before degassed H2O was
added. The organic phase was separated, and the aqueous phase was
extracted with Et2O. The combined organic extract was dried over
Na2SO4, filtered, and concentrated under reduced pressure to a give
mixture of meso- and rac-Ln ligands (n = 2−4).
meso- and rac-Bis{phenyl(2-picolyl)phosphine}ethane (meso/
rac-L2). A mixture of meso- and rac-L2 was obtained in 58% yield
(5.21 g, 21.2 mmol) from 1,2-bis(diphenylphosphino)ethane. 1H
NMR (CDCl3): δ 1.51−1.89 (m, 4H, CH2CH2), 3.15, 3.16 (s, 4H,
PyCH2), 6.76−7.35 (m, 16H, Ph, Py), 8.37 (br s, 2H, Py). 31P{1H}
NMR (CDCl3): δ −15.4 (s, 1P, rac), −14.8 (s, 1P, meso).
rac-1,4-Bis{phenyl(2-picolyl)phosphino}butane·2BH3 (rac-L4·
2BH3). To a solution of meso-/rac-L4 (23.3 g, 51.0 mmol) in THF (40
mL) was added BH3−THF (152 mL, 152 mmol) at 0 °C. The mixture
was stirred overnight at room temperature before degassed H2O (30
mL) was added. The organic phase was separated, and the aqueous
phase was extracted with AcOEt. The combined organic extracts were
dried over Na2SO4, filtered, and concentrated under reduced pressure
to give an oil. By addition of dry MeOH, white precipitates were
formed, which were filtered off, washed with hexane, and dried under
vacuum to afford white powders of rac-L4·2BH3 (4.5 g, 18%). 1H
NMR (CDCl3): δ 0.23−1.20 (br, 6H, BH3), 1.31−1.45 (m, 2H, CH2),
1.49−1.54 (m, 2H, CH2), 1.85−1.92 (m, 4H, CH2), 3.37 (d, 4H, J =
11 Hz, PyCH2), 6.96 (d, 2H, J = 8 Hz, Py), 7.11 (t, 2H, J = 6 Hz, Py),
7.33−7.57 (m, 12H, Ph, Py), 8.33 (d, 2H, J = 4 Hz, Py). 31P{1H}
NMR (CDCl3): δ 16.8 (br d, 2P, J = 59 Hz).
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meso-1,2-Bis{phenyl(2-picolyl)phosphino}ethane (meso-L2).
To a solution of meso-L2·2BH3 (400 mg, 0.0878 mmol) was added
diethanolamine in MeOH at room temperature, and the mixture was
heated at 100 °C for 2 h. After addition of degassed H2O (20 mL), the
mixture was heated at 80 °C. At room temperature, white precipitates
were collected by filtration, washed with hexane, and dried under
mmol), and the mixture was stirred overnight at room temperature
before filtration. The filtrate was concentrated to ca. 3 mL. After
addition of Et2O (1 mL), the solution was allowed to stand in
refrigerator to afford yellow crystals of 2c. Yield: 20 mg, 25%. Anal.
Calcd for C46H56B2Cl2F8N2P2Rh2: C, 48.08; H, 4.91; N, 2.44. Found:
C, 48.05; H, 4.67; N, 2.73. IR (KBr): ν 1473 (m), 1435 (m), 1083 (s),
reduced pressure to afford white solids of meso-L2 (357 mg, 95%). H
1062 (s) cm−1
. ESI-MS (MeOH): m/z 1060.97 (z1,
1
1
NMR (CDCl3): δ 1.69−1.85 (m, 4H, CH2), 3.18 (s, 4H, PyCH2), 6.80
(d, 2H, J = 8 Hz, Py), 6.99 (dd, 2H, J = 9, 5 Hz, Py), 7.25−7.53 (m,
12H, Ph, Py), 8.40 (d, 2H, J = 4 Hz, Py). 31P{1H} NMR (CDCl3): δ
−14.8 (s, 2P).
[[(Cp*RhCl)2(meso-L2)](BF4)]+ (1061.14)). H NMR (DMSO-d6):
δ 1.31 (d, 30H, J = 3 Hz, Cp*), 1.80−1.84 (m, 2H, CH2), 2.61−2.66
(m, 2H, CH2), 4.33 (d, 4H, J = 11 Hz, PyCH2), 7.54−7.58 (m, 12H,
Ph, Py), 7.83 (d, 2H, J = 8 Hz, Py), 8.12 (t, 2H, J = 8 Hz, Py), 8.53 (d,
rac-1,4-Bis{phenyl(2-picolyl)phosphino}butane (rac-L4). To a
solution of rac-L4·2BH3 (200 mg, 0.40 mmol) in CH2Cl2 (20 mL) was
added HBF4 (1.5 mL, 11 mmol, 52.9 wt % in Et2O), and the mixture
was stirred overnight at room temperature. After addition of 1 M
NaOH (12 mL, 12 mmol), the organic phase was separated and the
aqueous phase was extracted with CH2Cl2. The combined organic
extracts were dried over Na2SO4, filtered, and concentrated under
2H, J = 6 Hz, Py). 31P{1H} NMR (DMSO-d6): δ 49.7 (dt, 2P, JP−Rh
136 Hz, JP−P′ = 25 Hz).
=
[(Cp*RhCl)2(rac-L2)](BF4)2 (2d) (Method A). Yield: 10%. IR (KBr):
ν 1472 (m), 1437 (m), 1084 (s), 1056 (s), 1034 (s) cm−1. ESI-MS
(MeOH): m/z 1061.17 (z1, [[(Cp*RhCl)2(rac-L2)](BF4)]+
(1061.14)). 31P{1H} NMR (DMSO-d6): δ 48.5 (dt, 2P, JP−Rh = 135
Hz, JP−P′ = 25 Hz). Elemental analysis was not performed because only
a small amount of the pure sample was isolated.
1
reduced pressure to give a colorless oil of rac-L4 (164 mg, 90%). H
[(Cp*IrCl)2(meso-L3)](BF4)2 (3a) (Method A). 3a was obtained in
15% yield from crystallization (CH2Cl2/Et2O) of the reaction mixture
without column chromatography. Anal. Calcd for
C47.5H59B2Cl3F8Ir2N2P2: C, 41.21; H, 4.30; N, 2.02. Found: C,
41.36; H, 4.04; N, 2.29. IR (KBr): ν 1472 (m), 1437 (m), 1082 (s),
1060 (s), 1036 (s) cm−1. ESI-MS (MeOH): m/z 1255.36 (z1,
NMR (CDCl3): δ 1.36−1.41 (m, 4H, CH2), 1.63−1.72 (m, 4H, CH2),
3.18 (s, 4H, PyCH2), 6.85 (d, 2H, J = 8 Hz, Py), 7.00 (t, 2H, J = 6 Hz,
Py), 7.21−7.58 (m, 12H, Ph, Py), 8.44 (d, 2H, J = 4 Hz, Py). 31P{1H}
NMR (CDCl3): δ −18.5 (s, 2P).
General Procedure for Synthesis of [(Cp*MCl)2(meso-Ln)]-
(BF4)2 and [(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, Rh). Method A. To a
solution of meso-/rac-Ln·2HCl (n = 2−4) in MeOH was added 4 equiv
of NEt3 at room temperature, and the solvent was removed under
reduced pressure. To the residue was added CH2Cl2 (10 mL), 2 equiv
of [Cp*MCl2]2, and 5 equiv of NH4BF4 successively, and the mixture
was stirred overnight at room temperature. After addition of degassed
H2O, the organic phase was separated and the aqueous phase was
extracted with CH2Cl2. The combined organic extracts were dried over
Na2SO4, filtered, and dried under reduced pressure. The residue was
chromatographed on silica gel (eluent CH2Cl2/ MeOH 99/1) to
separate it into [(Cp*MCl)2(meso-Ln)](BF4)2 and [(Cp*MCl)2(rac-
Ln)](BF4)2, which were further purified by crystallization from
CH2Cl2/Et2O.
1
[[(Cp*IrCl)2(meso-L3)](BF4)]+ (1255.27)). H NMR (DMSO-d6): δ
1.41 (s, 30H, Cp*), 1.74 (m, 2H, CH2), 2.42 (m, 2H, CH2), 2.81 (m,
2H, CH2), 4.11−4.30 (m, 4H, PyCH2), 7.49 (m, 6H, Ph, Py), 7.57 (br
s, 6H, Ph), 7.80 (d, 2H, J = 8 Hz, Py), 8.08 (d, 2H, J = 8 Hz, Py), 8.62
(d, 2H, J = 6 Hz, Py). 31P{1H} NMR (DMSO-d6): δ 17.9 (s, 2P).
[(Cp*IrCl)2(rac-L3)](BF4)2 (3b) (Method A). Yield: 11%. IR (KBr): ν
1471 (m), 1437 (m), 1084 (s), 1062 (s), 1031 (s) cm−1. ESI-MS
(MeOH): m/z 1255.30 (z1, [[(Cp*IrCl)2(rac-L3)](BF4)]+
1
(1255.27)). H NMR (DMSO-d6): δ 1.34 (s, 30H, Cp*), 1.78−3.21
(m, 6H, CH2), 4.03−4.23 (m, 4H, PyCH2), 6.40−8.56 (m, 18H, Ph,
Py). 31P{1H} NMR (DMSO-d6): δ 19.4 (s, 2P). Elemental analysis
was not performed because only a small amount of the pure sample
was isolated.
[(Cp*IrCl)2(meso-L2)](BF4)2 (2a) (Method A). 2a was obtained in
11% yield from crystallization (CH2Cl2/Et2O) of the reaction mixture
without column chromatography.
[(Cp*RhCl)2(meso-L3)](BF4)2 (3c) (Method A). Yield: 12%. IR
(KBr): ν 1472 (m), 1435 (m), 1084 (s), 1057 (s) cm−1. ESI−MS
(MeOH): m/z 1075.16 (z1, [[(Cp*RhCl)2(meso-L3)](BF4)]+
(1075.16)). 31P{1H} NMR (DMSO-d6): δ 49.9 (d, 2P, JP−Rh = 135
Hz). Elemental analysis was not performed because only small amount
of the pure sample was isolated.
2a (Method B). To a solution of [Cp*IrCl2]2 (44.9 mg, 0.0664
mmol) in CH2Cl2 (10 mL) were added a solution of meso-L2 (28.4 mg,
0.0664 mmol) in CH2Cl2 (10 mL) and NH4BF4 (24.4 mg, 0.233
mmol), and the mixture was stirred overnight at room temperature
before filtration. The filtrate was concentrated to ca. 3 mL. After
addition of Et2O (1 mL), the solution was allowed to stand in
refrigerator to afford yellow crystals of 2a. Yield: 31 mg, 35%. Anal.
Calcd for C46H56B2Cl2F8Ir2N2OP2: C, 41.67; H, 4.04; N, 2.44. Found:
C, 41.61; H, 4.25; N, 2.11. IR (KBr): ν 1473 (m), 1437 (m), 1084 (s),
1056 (s), 1034 (s) cm−1. ESI-MS (MeOH): m/z 1241.01 (z1,
[(Cp*RhCl)2(rac-L3)](BF4)2 (3d) (Method A). Yield: 4%. IR (KBr): ν
1472 (m), 1437 (m), 1084 (s), 1057 (s) cm−1. ESI-MS (MeOH): m/z
1133.10 (z1, [[(Cp*RhCl)2(rac-L3)](PF6)]+ (1133.12)). 31P{1H}
NMR (DMSO-d6): δ 48.5 (d, 2P, JP−Rh = 135 Hz). Elemental analysis
was not performed because only a small amount of the pure sample
was isolated. A similar procedure using NH4PF6 instead of NH4BF4
gave a small amount of the crystalline compound [(Cp*RhCl)2(rac-
L3)](PF6)2·0.5Et2O (3d′·0.5Et2O), which was characterized by X-ray
crystallography.
1
[[(Cp*IrCl)2(meso-L2)](BF4)]+ (1241.26)). H NMR (DMSO-d6): δ
1.33 (s, 30H, Cp*), 1.80−1.84 (m, 2H, CH2), 2.60−2.62 (m, 1H,
CH2), 3.58−3.62 (m, 1H, CH2), 4.33−4.41 (m, 4H, PyCH2), 6.85 (d,
2H, J = 8 Hz, Py), 7.00 (t, 2H, J = 6 Hz, Py), 7.21−7.58 (m, 12H, Ph,
Py), 8.44 (d, 2H, J = 4 Hz, Py). 31P{1H} NMR (DMSO-d6): δ 20.0 (s,
2P).
[(Cp*IrCl)2(meso-L4)](BF4)2·0.5CH2Cl2 (4a·0.5CH2Cl2) (Method A).
Yield: 4%. Anal. Calcd for C48.5H61B2Cl3F8Ir2N2P2: C, 41.66; H, 4.40;
N, 2.00. Found: C, 41.86; H, 4.01; N, 2.07. IR (KBr): ν 1473 (m),
1436 (m), 1055 (s) cm−1. ESI-MS (MeOH): m/z 591.17 (z2,
[[(Cp*IrCl)2(meso-L4)]]2+ (591.14)). 1H NMR (CD2Cl2): δ 1.31 (m,
1H, CH2), 1.42 (s, 30H, Cp*), 1.54 (m, 1H, CH2), 1.66 (m, 1H,
CH2), 2.03−2.14 (m, 2H, CH2), 2.34−2.43 (m, 2H, CH2), 3.93 (dd,
2H, J = 17, 12 Hz, PyCH2), 4.23 (dd, 2H, J = 17, 12 Hz, PyCH2),
7.41−7.46 (m, 6H, Ph, Py), 7.57−7.59 (m, 6H, Ph), 7.99−8.01 (m,
4H, Py), 8.53 (d, 2H, J = 6 Hz, Py). 31P{1H} NMR (CD2Cl2): δ 19.5
(s, 2P).
[(Cp*IrCl)2(rac-L2)](BF4)2·5CHCl3 (2b·5CHCl3) (Method A). Yield:
2%. IR (KBr): ν 1472 (m), 1437 (m), 1083 (s), 1056 (s), 1034 (s)
cm−1. ESI-MS (MeOH): m/z 1241.35 (z1, [[(Cp*IrCl)2(rac-L2)]-
1
(BF4)]+ (1241.26)). H NMR (DMSO-d6): δ 1.33 (s, 30H, Cp*),
1.80−3.21 (m, 4H, CH2), 4.32 (m, 4H, PyCH2), 7.40−8.51 (m, 28 H,
Ph, Py). 31P{1H} NMR (DMSO-d6): δ 20.5 (s, 2P). The formula was
determined by X-ray crystallography because only a small amount of
the pure sample was isolated.
[(Cp*RhCl)2(meso-L2)](BF4)2 (2c) (Method A). 2c was obtained in
11% yield from crystallization (CH2Cl2/Et2O) of the reaction mixture
without column chromatography.
[(Cp*IrCl)2(rac-L4)](BF4)2·0.75CH2Cl2 (4b·0.75CH2Cl2) (Method A).
Yield: 3%.
4b·0.75CH2Cl2 (Method B). To a solution of [Cp*IrCl2]2 (50.4 mg,
0.063 mmol) in CH2Cl2 (5 mL) were added a solution of rac-L4 (29.5
mg, 0.063 mmol) in CH2Cl2 (10 mL) and NH4BF4 (33.2 mg, 0.31
mmol), and the mixture was stirred overnight at room temperature
2c (Method B). To a solution of [Cp*RhCl2]2 (43.5 mg, 0.0704
mmol) in CH2Cl2 (5 mL) were added a solution of meso-L2 (30.3 mg,
0.0708 mmol) in CH2Cl2 (10 mL) and NH4BF4 (30.0 mg, 0.286
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Organometallics
Article
before filtration. The filtrate was concentrated to ca. 3 mL. After
addition of Et2O (2 mL), the solution was allowed to stand in a
refrigerator to afford yellow crystals of 4b. Yield: 37 mg, 75%. Anal.
Calcd for C48.75H61.5B2Cl3.5F8Ir2N2P2: C, 41.25; H, 4.37; N, 1.97.
Found: C, 41.32; H, 4.23; N, 2.07. IR (KBr): ν 1470 (m), 1437 (m),
separations of 2a−c, 3a, 3d′, and 4a,b,d, and van’t Hoff plots
for 2a,b, 3a,b, and 4a,d. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
1055 (s) cm− 1
. ESI-MS (MeOH): m/z 590.98 (z2,
■
[[(Cp*IrCl)2(meso-L4)]]2+ (591.14)). 1H NMR (CD2Cl2): δ 1.43
(d, 30H, J = 2 Hz, Cp*), 1.59 (m, 4H, CH2), 2.00−2.13 (m, 2H,
CH2), 2.27−2.38 (m, 2H, CH2), 3.93 (dd, 2H, J = 18, 11 Hz, PyCH2),
4.20 (dd, 2H, J = 17, 12 Hz, PyCH2), 7.39−7.47 (m, 6H, Ph, Py),
7.56−7.59 (m, 6H, Ph), 7.91 (d, 2H, J = 8 Hz, Py), 7.98 (t, 2H, J = 8
Hz, Py), 8.53 (d, 2H, J = 6 Hz, Py). 31P{1H} NMR (CD2Cl2): δ 18.5
(s, 2P).
*T.N.: e-mail, t.nakajima@cc.nara-wu.ac.jp; fax, +81 742-20-
3847; tel, +81 742-20-3847.
*T.T.: e-mail, tanase@cc.nara-wu.ac.jp.
Notes
The authors declare no competing financial interest.
[(Cp*RhCl)2(meso-L4)](BF4)2 (4c) (Method A). Yield: 2%. IR (KBr):
ν 1471 (m), 1437 (m), 1055 (s) cm−1. ESI-MS (MeOH): m/z
1089.17 (z1, [[(Cp*RhCl)2(meso-L4)(BF4)]]+ (1089.22)). 31P{1H}
NMR (DMSO-d6): δ 48.5 (d, 2P, JP−Rh = 134 Hz). Elemental analysis
was not performed because only a small amount of the pure sample
was isolated.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research and on Priority Area 2107 (Nos. 22108521,
24108727) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. T.N. is grateful to Tokuyama
Science Foundation, Kurata Memorial Hitachi Science and
Technology Foundation, and Nara Women’s University for a
research project grant.
[(Cp*RhCl)2(rac-L4)](BF4)2·0.75CH2Cl2 (4d·0.75CH2Cl2) (Method
A). Yield: 3%.
4d·0.75CH2Cl2 (Method B). To a solution of [Cp*RhCl2]2 (50.2
mg, 0.081 mmol) in CH2Cl2 (5 mL) were added a solution of rac-L4
(37.1 mg, 0.081 mmol) in CH2Cl2 (10 mL) and NH4BF4 (42.2 mg,
0.40 mmol), and the mixture was stirred overnight at room
temperature. The mixture was filtered and concentrated to ca. 3 mL,
and after addition of Et2O (2 mL), it was allowed to stand in a
refrigerator to afford yellow crystals of 4b. Yield: 74 mg, 77%. Anal.
Calcd for C48.75H61.5B2Cl3.5F8N2P2Rh2: C, 47.18; H, 5.00; N, 2.26.
Found: C, 47.14; H, 4.77; N, 2.31. IR (KBr): ν 1471 (m), 1436 (m),
REFERENCES
■
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(b) Broussard, M. E.; Juma, B.; Train, S. G.; Peng, W.-J.; Laneman, S.
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1054 (s) cm−1
. ESI-MS (MeOH): m/z 1089.17 (z1,
[[(Cp*RhCl)2(meso-L4)(BF4)]]+ (1089.22)). H NMR (CD2Cl2): δ
1.42 (d, 30H, J = 2 Hz, Cp*), 2.00−2.12 (m, 2H, CH2), 2.32−2.44 (m,
2H, CH2), 3.90 (dd, 2H, J = 16, 14 Hz, PyCH2), 4.12 (dd, 2H, J = 17,
13 Hz, PyCH2), 7.42−7.46 (m, 4H, Ph), 7.51 (t, 2H, J = 7 Hz, Py),
7.56−7.61 (m, 6H, Ph), 7.80 (d, 2H, J = 8 Hz, Py), 7.97 (t, 2H, J = 8
Hz, Py), 8.55 (d, 2H, J = 6 Hz, Py). 31P{1H} NMR (CD2Cl2): δ 47.9
(d, 2P, JP−Rh = 132 Hz).
1
(2) For recent examples, see: (a) Angamuthu, R.; Byers, P.; Lutz, M.;
Spek, A. L.; Bouwman, E. Science 2010, 327, 313−315. (b) Matano, Y.;
Miyajima, T.; Nakabuchi, T.; Imahori, H.; Ochi, N.; Sakaki, S. J. Am.
Chem. Soc. 2006, 128, 11760−11761. (c) Gagliardo, M.; Selander, N.;
́
Mehendale, N. C.; van Koten, G.; Gebbink, R. J. M. K.; Szabo, K. J.
Chem. Eur. J. 2008, 14, 4800. (d) Lee, C.-C.; Ke, W.-C.; Chan, K.-T.;
Lai, C.-L.; Hu, C.-H.; Lee, H. M. Chem. Eur. J. 2007, 15, 582−591.
(e) Miranda-Soto, V.; Grotjahn, D. B.; Dipasquale, A. G.; Rheingold,
A. L. J. Am. Chem. Soc. 2008, 130, 13200−13201.
X-ray Crystallography. Crystals of meso-L2·2BH3, rac-L4·2BH3,
2a−c, 3a,d′, and 4a,b,d were quickly coated with Paratone N oil and
mounted on top of a loop fiber at room temperature. The crystal and
experimental data are summarized in Tables S1−S3 (see the
Supporting Information). All data were collected at −120 °C (2a−c,
3a, 3d′, 4a) or −150 °C (meso-L2·2BH3, rac-L4·2BH3, 4b,d) on a
Rigaku AFC8/Mercury CCD diffractometer equipped with graphite-
monochromated Mo Kα radiation using a rotating-anode X-ray
generator (50 kV, 200 mA) and a Rigaku VariMax Mo/Saturn CCD
diffractometer equipped with graphite-monochromated Mo Kα
radiation using a rotating-anode X-ray generator (RA-Micro7, 50 kV,
24 mA). A total of 720−1440 oscillation images, covering a whole
sphere of 6° < 2θ < 55°, were collected by the ω-scan method. The
crystal-to-detector (70 × 70 mm) distance was set at 45 mm. The data
were processed using the Crystal Clear 1.3.5 program (Rigaku/MSC)8
and corrected for Lorentz−polarization and absorption effects.9 The
structures were solved by direct methods with SHELXS-9710 (2a,b,
4b) and SIR-9211 (meso-L2·2BH3, rac-L4·2BH3, 2c, 3a, 3d′, 4a,d) and
were refined on F2 with full-matrix least-squares techniques with
SHELXL-9710 using the Crystal Structure 4.0 package.12 All non-
hydrogen atoms were refined with anisotropic thermal parameters, and
the C−H hydrogen atoms were calculated at ideal positions and
refined with riding models. All calculations were carried out on a
Windows PC running the Crystal Structure 4.0 package.12
(3) For our recent examples, see: (a) Nakajima, T.; Sakamoto, M.;
Kurai, S.; Kure, B.; Tanase, T. Chem. Commun. 2013, 49, 5239−5338.
(b) Kure, B.; Nakajima, T.; Tanase, T. J. Organomet. Chem. 2013, 733,
28−35. (c) Goto, E.; Begum, R.; Hosokawa, A.; Yamamoto, C.; Kure,
B.; Nakajima, T.; Tanase, T. Organometallics 2012, 31, 8482−8497.
(d) Kure, B.; Taniguchi, A.; Nakajima, T.; Tanase, T. Organometallics
2012, 31, 4791−4800. (e) Nakajima, T.; Kurai, S.; Noda, S.; Zouda,
M.; Kure, B.; Tanase, T. Organometallics 2012, 31, 4283−4294 and
referenses cited therein. (f) Yoshii, A.; Takenaka, H.; Nagata, H.;
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