Synthesis of iridaboratranes bearing phosphine-tethered borane: Reversible CO/PR3 (R = Me, OMe, OEt) substitution reactions induced by a σ-electron-acceptor borane ligand

Article abstract of DOI:10.1021/om300216g

The iridaboratrane [{o-(Ph₂P)C₆H₄} ₃B]IrH(CO) (1-Ir), bearing phosphine-tethered borane, was synthesized via phosphine ligand exchange between the tris(triphenylphosphine) carbonyl hydride IrH(CO)(PPh₃)₃ (2-Ir) and the tris(phosphine) borane {o-(Ph₂P)C₆H₄}₃B (3). 1-Ir was fully characterized on the basis of its ¹H, ¹¹B, and ³¹P NMR spectra, X-ray diffraction analysis, and elemental analysis. Density functional theory calculations revealed the important properties of the σ-acceptor borane ligand that led to its unique electron distribution in 1-Ir. The borane ligand extracts a significant amount of electron density from the iridium center, but the iridium center maintains an electron density similar to that of the boron-free compound 2-Ir by decreasing π back-donation from Ir to CO and strengthening the donation from the phosphorus atom (or by weakening the d_metal-σ_P-R interaction). The properties of the borane ligand can promote the reversible CO/PR₃ (R = Me, OMe, OEt) substitution reaction.

Full text of DOI:10.1021/om300216g

Article
pubs.acs.org/Organometallics
Synthesis of Iridaboratranes Bearing Phosphine-Tethered Borane:
Reversible CO/PR (R = Me, OMe, OEt) Substitution Reactions
3
Induced by a σ-Electron-Acceptor Borane Ligand
Hajime Kameo, Yasuhiro Hashimoto, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan
*
S Supporting Information
ABSTRACT: The iridaboratrane [{o-(Ph P)C H } B]IrH(CO) (1-Ir), bearing
2
6
4 3
phosphine-tethered borane, was synthesized via phosphine ligand exchange between
the tris(triphenylphosphine) carbonyl hydride IrH(CO)(PPh ) (2-Ir) and the
3
3
tris(phosphine)borane {o-(Ph P)C H } B (3). 1-Ir was fully characterized on the
2
6
4 3
1
11
31
basis of its H, B, and P NMR spectra, X-ray diffraction analysis, and elemental
analysis. Density functional theory calculations revealed the important properties of
the σ-acceptor borane ligand that led to its unique electron distribution in 1-Ir. The
borane ligand extracts a significant amount of electron density from the iridium center, but the iridium center maintains an
electron density similar to that of the boron-free compound 2-Ir by decreasing π back-donation from Ir to CO and strengthening
the donation from the phosphorus atom (or by weakening the dmetal−σ* interaction). The properties of the borane ligand can
P−R
promote the reversible CO/PR (R = Me, OMe, OEt) substitution reaction.
3
INTRODUCTION
The unique properties of σ-electron-acceptor (Z-type) borane
Scheme 1. Tautomerization in the Imidazolyl-Based Borane
Systems (R = Me, i-Pr, t-Bu)
■
1
−7
ligands have been identified. One of these is the flexibility of
the metal−boron interaction in response to the Lewis basicity of
the metal center. Hence, complexes containing a Z-type ligand
are potentially allowed to access the multielectron state and are
therefore expected to promote multielectron redox reactions.
Peters and Moret recently demonstrated the usefulness of this
property and reported dinitrogen activation using a trigonal-
We focused on the triphosphine−borane (TPB) rhodium
7b
complexes [{o-(Ph P)C H } B]RhH(CO) (1-Rh) with a frame-
2 6 4 3
bipyramidal {o-(i-Pr) PC H } BFe scaffold. Another useful
2
6
4 3
work similar to that of the well-known RhH(CO)(PPh ) (2-
3
3
property is the strong trans effect induced by electron positive
Rh) and obtained important information regarding the proper-
ties of the σ-acceptor borane ligand from direct comparison of
2i,l
borane, which can promote the formation of coordinatively
unsaturated complexes that exhibit high reactivity. A few studies
of the trans effect of σ-acceptor borane in a hydrotris(2-methyl-1-
imidazolyl)borate or similar derivative system have been
conducted to date. Hill et al. reported that the σ-acceptor
borane ligand appears to exhibit a pronounced trans effect as a σ-
boryl ligand, even though the M−B bond character in the σ-boryl
ligand is very different from that in the σ-acceptor borane ligand;
the σ-boryl ligand has a conventional one-electron covalent
bond, and the σ-acceptor borane ligand exhibits a two-center−
9
these complexes (Chart 1). Additionally, density functional
Chart 1. TPB Complex 1-M (M = Rh, Ir) and Its Related
Borane-Free Complex 2-M
8
two-electron dative interaction. Additionally, Hill et al. have
synthesized the ruthenaboratrane [Ru(CO)(PPh ){B(mt) }]
3
3
(
mt = methimazolyl) and found that the phosphine ligand trans
to the borane ligand is labile and readily and reversibly replaced
2
l
by CO. However, in their imidazolyl-based borane systems, the
presence of the base-stabilized boryl tautomer may not be
theory (DFT) calculations using 1-Rh and borane-free 2-Rh
revealed some unique properties of the Z-type borane ligand.
2
i
negligible (Scheme 1). The phosphine−borane system
developed by Bourissou et al. seems to be a more appropriate
system for discussion of the properties of the σ-acceptor borane
Received: March 16, 2012
Published: May 23, 2012
5
,1f
ligand because of the absence of such a tautomer.
©
2012 American Chemical Society
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Article
Specifically, incorporation of the borane ligand does not change
the electron density of the metal center, even though the borane
extracts the electron density from the metal center. Additionally,
the carbonyl ligand trans to the borane ligand appears to accept
relatively less electron density through π back-donation. The
electronic deviation induced by the σ-borane ligand seemingly
results in the rare CO dissociation of neutral Rh(I) complexes.
As an extension of our continuing work in this field, we
attempted to synthesize the iridium complex [{o-(Ph P)-
2
C H } B]IrH(CO) (1-Ir), bearing phosphine-tethered borane
6
4 3
3, because direct comparison of the TPB complex 1-Ir and
borane-free 2-Ir can provide valuable information regarding a
Z-type ligand in the same way as for the corresponding Rh
system. Furthermore, Hill et al. recently reported that the bond
dissociation energy for Ir→BR was greater than that for Rh→
3
2m
BR in an imidazolyl-based system. Hence, the replacement of
Rh with Ir is expected to induce the prominent functions of a
Z-type ligand owing to the relatively strong Ir→B interaction.
3
Figure 1. Molecular structure of 1-Ir with thermal ellipsoids at the 40%
probability level. The hydrogen atoms and phenyl groups (except for
ipso carbon) are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Ir−CO = 1.941(14), Ir1−P1 = 2.309(2), Ir1−P2 =
2.349(3), Ir1−P3 = 2.321(3), Ir1−B1 = 2.326(11); ∑(C−B−C) 330.9.
RESULTS AND DISCUSSION
An iridium complex with carbonyl, hydride, and phosphine-
■
tethered borane ligands, [{o-(Ph P)C H } B]IrH(CO) (1-Ir),
2
6
4 3
was synthesized in a manner similar to that used to prepare 1-
complex is also supported by the ν(CO) values of the IR spectra.
9
−1
Rh. The reaction of IrH(CO)(PPh ) (2-Ir) with {o-(Ph P)-
Specifically, the Δν(CO) values for Ir complexes (1920 cm
3
3
2
(2-Ir) → 1972 cm− (1-Ir)) are smaller than those for Rh
1
C H } B (3) at 100 °C in toluene produced 1-Ir in moderate
6
4 3
complexes (1918 cm− (2-Rh) → 1994 cm (1-Rh)). The
apparent discrepancy between the strong Ir→B interaction and
the strong Ir−CO π back-donation in comparison with the
corresponding Rh complex will be discussed later. Two
phenylene groups trans to each other are found to be unequal
in the solid state and in solution. The phenylene group bearing
the C2 atom nearly lies on the plane defined by B1−Ir1−P1, and
the torsion angle of C2−B1−Ir1−P1 is 6.2°. In contrast, another
phenylene group bearing the C38 atom shows a significant
deviation over the plane defined by B1−Ir1−P3, giving a large
C38−B1−Ir1−P3 torsion angle (41.8°). Three signals were
observed in the 31P{ H} NMR spectrum, indicating that this
unsymmetrical situation was maintained even in solution.
The 31P{ H} NMR spectrum of 1-Ir recorded at 20 °C
exhibited a slightly broad single resonance (Figure 2). When the
temperature was lowered to −40 °C, the signal split into two
mutually coupled signals at 25.3 and 26.4 ppm with an intensity
ratio of 2:1. Further measurement at lower temperatures showed
that the resonance at higher magnetic field gradually broadened
again. These findings appear to indicate that the two phosphine
ligands located at trans positions to each other differ at low
temperatures, owing to the rigid framework ligated by the o-
phenylene.
1
−1
yield (eq 1).
1
1
Although we could not obtain single crystals of 1-Ir suitable for
X-ray diffraction analysis by using common organic solvents, the
addition of an equivalent amount of 18-crown-6 to a THF
solution of 1-Ir resulted in the formation of crystals sufficient for
X-ray diffraction. Figure 1 shows the molecular structure of 1-Ir.
The iridium adapts a pseudo-octahedral structure. The Ir−B
distance (2.326 Å) is comparable to the sum of the covalent radii
12
DFT calculations were carried out to shed more light on the
nature of the Ir→B interaction. The actual experimental systems
1-Ir and 2-Ir were computed using the hybrid functional
10
of iridium and boron (2.23 Å) and slightly shorter than the
Rh−B distance in 1-Rh (2.370 Å). Because the covalent radii for
10
10
13
rhodium (134 pm) and iridium (136 pm) are similar, the
iridium in 1-Ir apparently donates more electrons to the boron
than does the rhodium in 1-Rh. The geometric deviation from
planarity in boron is slightly larger in 1-Ir than in 1-Rh (1-Ir,
B3PW91. The calculated structures for 1-Ir and 2-Ir were in
excellent agreement with the experimental data (see the
Supporting Information, STable 15), and the frontier molecular
orbitals revealed three-center B−Ir−L interactions (L = CO).
2
z
∑
(C−B−C) = 330.9°; 1-Rh, ∑(C−B−C) = 332.5°), which is
These interactions involve the d orbital of Ir, the p orbital of
consistent with the relatively stronger Ir→B interaction. The
boron, and the molecular orbital of CO, with bonding iridium−
boron and antibonding iridium−carbon interactions in the
highest occupied molecular orbital (HOMO) and an antibond-
ing iridium−boron interaction in the LUMO+1 (Figure 3).
Similar three-center−four-electron interactions have been
reported in a 2-mercapto-1-R-imidazolyl-tethered (R = Me,
11
Ir−CO bond distance increases from 2-Ir (1.887 Å) to 1-Ir
1.941 Å). The same trend was observed for the Rh system.
(
However, the extent of M−CO elongation is smaller for the Ir
system than for the Rh system, suggesting that the π back-
donation from M to CO still remains to a greater extent for 1-Ir
than for 1-Rh. This relatively strong π back-donation for the Ir
2i
3b
tBu) borane system independently by Hill et al. and Parkin et al.
4
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Article
Table 1. NBO Atomic Charges (q (M = Ir, Rh), q , q , and
M
CO
P
−1
q ) and ν(CO) (cm )
B
9
9
3
1-Ir
2-Ir
1-Rh
2-Rh
qRh
−1.67
0.37
−1.67
−1.56
0.43
1.35
1.36
1.29
4.00
0.25
0.28
0.10
1994
−1.54
qB
qP
0.88
1.41
1.29
1.29
1.30
3.88
1.25
1.24
1.26
3.75
1
1
.42
.35
∑
qP
4.18
0.30
0.26
0.08
1972
Δ(∑qP)
qCO
0.18
0.18
ΔqCO
ν(CO)
1920
1919
than in Rh. The smaller value of Δq in Ir the system is
CO
consistent with the results of the CO stretching frequencies.
16
Natural localized molecular orbital (NLMO) analyses of
-Rh and 1-Ir also supported the relatively strong Ir−B inter-
action (Table 2). The metal−boron interaction reflects more on
1
Figure 2. Variable-temperature 31P{ H} NMR spectra (163 MHz,
1
toluene-d ) of 1-Ir.
8
Table 2. Hybridization of the NLMO Involving the Metal (Rh
1
7
or Ir) and Boron Atoms and NLMO/NPA Bond Orders
1
-Rh
1-Ir
NLMO d(M)→p(B)
%
%
d(M)
p(B)
76.21
19.14
58.50
30.59
donor NBO d(M)
%
%
%
s(M)
p(M)
d(M)
0.23
8.14
91.62
3.62
24.12
72.26
1
5
Figure 3. HOMO (left) and LUMO+1 (right) of 1-Ir. Hydrogen
acceptor NBO p(B)
atoms are omitted for clarity.
%
%
s(B)
p(B)
18.80
81.19
0.571
19.98
80.01
0.667
5b
as well as in a phosphine−borane system by Bourrisou et al.
M−B bond order
The contribution of the HOMO strengthens the Ir−B bond and
weakens the Ir−CO bond, which seems to be the origin of the
14
trans influence of the σ-acceptor borane ligand in 1-Ir.
the 2s orbital character of the boron in 1-Ir (19.98%) than in
1-Rh (18.80%), and the NLMO/NPA bond order between M
and B in 1-Ir (0.667) is also larger than that between Rh and B in
2-Rh (0.571).
The coordination of the borane ligand reflects the Brønsted
basicity of the complex. To estimate the Brønsted basicity of 1-Ir,
we examined the reactions of 1-Ir with several protonic acid or
hydrido reagents. However, these reactions gave several un-
characterized products, and the basicity of 1-Ir was hardly
estimated. Previously, we reported the synthesis of anionic
rhodium complexes, [{o-(Ph P)C H } BRh(CO)][K(18-
16
17
Natural population analysis data together with ν(CO) data
for 1−3 are summarized in Table 1. A comparison of 1-Ir, 2-Ir,
and 3 revealed unique features of the σ-acceptor borane ligand.
The natural charge on boron is lower in 1-Ir (0.37) than in 3
(
0.88) and in 1-Rh (0.43), indicating the presence of a relatively
strong Ir→B interaction. However, q (−1.67) is equal to that of
Ir
boron-free 2-Ir (−1.67), irrespective of the Ir→B interaction.
This singular phenomenon has been observed in the
corresponding Rh system and can be explained by natural
charges in the phosphorus atom (q ) and CO fragment (qCO).
P
2
6
4 3
The incorporation of the boron consequently results in the
crown-6)] (4) and [Rh(CO)(PPh ) ][K(18-crown-6)] (5),
which were obtained from the reaction of the corresponding
3 3
increase of qCO and q , and the Ir center seems to retain the same
P
electron density by decreasing π back-donation to CO and
strengthening electron donation from the phosphorus ligand (or
neutral complexes 1-Rh and 2-Rh with KH in the presence of
18-crown-6. In this study, we found that the reverse reactions,
9
weakening the dmetal−σ*
interaction). The amounts of
electron transfer of CO and phosphorus atoms were estimated
namely reactions of these anionic complexes with 1 equiv of
H O, resulted in the quantitative formation of the corresponding
2
P−R
on the basis of differences in the charges of the CO (ΔqCO) and
neutral complexes; there exist perfect equilibria between the
neutral complex and the corresponding anionic complex. These
equilibrium reactions enabled us to compare the Brønsted
basicities of 1-Rh and 2-Rh. When the reaction of 1-Rh with
phosphorus atoms (Δ(∑q )) between the (TPB)M (M = Rh, Ir)
P
complexes 1 and the related boron-free complexes 2. While
ΔqCO is smaller in Ir than in Rh, Δ(∑q ) is slightly larger in Ir
P
4
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Organometallics
equiv of 5 was monitored by 31P NMR spectroscopy, the
Article
1
quantitative formation of 2-Rh and 4 was observed (eq 2). The
reverse reaction, namely, the reaction of 2-Rh with 4, did not
form 1-Rh and 5. These results strongly imply that the
coordination of the borane ligand decreases the Brønsted
basicity of the complex. It is also acceptable that the reduction
of steric repulsion in 1-Rh is a driving force of the one-way
reaction.
Similar to the reaction of 1-Rh with PMe , the reactions with
3
P(OR) (R = Me, Et) gave the corresponding hydrido phosphite
3
complexes. When the reaction of 1-Rh with 3 equiv of P(OR)
3
1
31
(
R = Me, Et) was monitored by H and P NMR spectroscopy,
the reaction was complete within 2 h. In comparison, the reaction
with 3 equiv of PMe under the same conditions afforded a 40:60
3
mixture of 1-Rh and 10a-Rh and took place relatively slowly.
These results indicate that the less bulky phosphorus compounds
lead to smooth formation of the phosphine or phosphite complexes
and that steric hindrance is more important to the substitution
reactions than the basicity of a phosphorus ligand. In fact, 1-Rh did
not react with the relatively large phosphine PR (R = Et, Ph) and
3
phosphite P(OR) (R = i-Pr) under mild conditions, and elevating
3
the reaction temperature resulted in the formation of a mixture of
unidentified products. These results implied that the vacant site
generated by carbonyl dissociation is not large enough to accept a
relatively large phosphine or phosphite ligand.
The newly synthesized compounds 10b-Rh and 10c-Rh were
unambiguously characterized on the basis of elemental analysis as
well as H, B, and P NMR spectroscopic data, which were
similar to those of the trimethylphosphine analogue 10a-Rh. The
solid-state structures of 10b-Rh and 10c-Rh were confirmed by
X-ray diffraction studies (see below).
We have recently reported preparation of the cationic silyl,
germyl, and stannyl iridium(III) complexes [{(o-Ph PC H )} -
2
6
4
3
1
11
31
EIr(H)(CO)][BF ] (6, E = Si; 7, E = Ge; 8, E = Sn), whose
4
cationic parts are isoeletronic and isostructural with 1-Ir.
Complexes 6−8 were deprotonated by 3 equiv of triethylamine
(
NEt ) to afford the corresponding neutral complexes {(o-
3
18
The TPB iridium complex 1-Ir also reacted with PR (R = Me,
Ph PC H )} EIr(CO). In this study, abstraction of one
3
2
6
4
3
OMe, OEt) to afford the corresponding phosphine and
hydride from [Ir(H) (CO)(PPh ) ][BF ] (9) was also found
2
3 3
4
phosphite complexes [{o-(Ph P)C H } B]IrH(PR ) (10a-Ir, R
to occur by 3 equiv of NEt to give 2-Ir, quantitatively, while 1-Ir
2
6
4
3
3
3
=
Me; 10b-Ir, R = OMe; 10c-Ir, R = OEt) via the elimination of
did not react with 3 equiv of NEt . These results show that 1-Ir
3
the carbonyl ligand (eq 4). Complexes 10a-Ir and 10b-Ir were
is more basic than 6−9 despite the strong Ir→B interaction
Chart 2). It is also possible that the constrained tetradentate
1
11
31
fully characterized on the basis of H, B, and P NMR spectra
and elemental analysis. The reaction with 20 equiv of trimethyl
phosphite gradually proceeded even at ambient temperature, and
the reaction at 60 °C was complete within 36 h. These reactions
were reversible, and exposure of 10-Ir to CO immediately
generated 1-Ir almost quantitatively.
(
frameworks influence the Brønsted basicity of the complexes.
The electronic deviation of 1-Ir from the related boron-free
complex 2-Ir is also expected to influence the reactivity of the
iridaboratrane. Substitution of a carbonyl ligand with a
phosphine ligand is one of the fundamental reactions in
19
organometallic chemistry and seems to be an appropriate
reaction for a better understanding of a new system. We
previously reported that the CO/PMe substitution reaction of
3
1
-Rh was induced by the strong trans-labilizing effect and
9
σ-electron-acceptor properties of the borane ligand (eq 3). To
check the limitations of the CO/PR substitution reaction for
3
1
-Rh and 1-Ir, we here examined the reaction of 1-Rh with PR3
(
(
R = Et, Ph, OMe, OEt, O(i-Pr)) and those of 1-Ir with PR3
R = Me, OMe, OEt).
Chart 2. Comparison of the Brønsted Basicities
4
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Organometallics
Article
Figure 4. Molecular structures of hydrido phosphine and hydrido phosphite complexes 10b-Rh, 10c-Rh, 10a-Ir, and 10b-Ir. Hydrogen atoms (except
for hydride) and phenyl groups (except for ipso carbon) are omitted for clarity. C2 and C3 in 10b-Ir show the disorder in the crystal (C2:C2b = C3:C3b
=
55:45).
Table 3. Structural Parameters for 10a-Rh, 10b-Rh, 10c-Rh, 10a-Ir, and 10b-Ir
M = Rh
M = Ir
9
1
0a-Rh
10b-Rh
10c-Rh
10a-Ir
10b-Ir
2.338
M−B1 (Å)
(C−B−C) (deg)
2.354(4)
330.8
2.343(6)
2.351(4)
2.336(6)
∑
330.0
331.6
327.5
328.6
M−PR (Å)
2.4077(11)
2.3463(10)
2.3128(18)
2.2951(17)
2.3451(17)
2.3017(16)
2.3083(11)
2.3126(11)
2.3454(11)
2.2934(10)
2.3719(14)
2.2984(13)
2.3339(13)
2.3018(14)
2.2776(19)
2.297(2)
2.3388(19)
2.3087(19)
3
M−PPh (Å)
2
2
2
.3033(11)
.3002(11)
It should be noted that 1-Ir undergoes a CO/PR exchange
steric repulsion between the Ph P groups in the TPB ligand and
3
2
reaction, because iridium generally has a tendency to form
extremely strong Ir−CO bonds. For example, the Ir−CO bond
dissociation energy (BDE) in Ir(CO){P(i-Pr) } Cl was
PR , although the effect of π back-donation through a
3
dmetal−σ* interaction cannot be ruled out.
P−R
3
2
To gain greater insight into the mechanisms involved in the
substitution reactions of a carbonyl ligand, kinetic studies were
carried out. The reactions of 1-Rh and 1-Ir with P(OMe)3
produced the phosphite complexes 10b-Rh and 10b-Ir,
respectively, with no observed intermediate species. The
estimated to be 72 kcal/mol, which is much greater than that
20
for any previously reported M−CO BDE. Indeed, to the best of
our knowledge there are no examples of neutral monocarbonyl
iridium(I) complexes bearing triphosphine ligands that disso-
ciate the CO ligand in not only a square-planar system but also a
31
disappearance of 1-Rh and 1-Ir was monitored by P NMR
21
trigonal-bipyramidal system. Therefore, CO dissociation can
be apparently attributed to the properties of the σ-borane ligand,
although steric factors also probably contribute to the lability of
the CO ligand in 1-Ir.
spectroscopy, and first-order reactions were found to occur in
the carbonyl complexes. The rate constant of the reaction
of 1-Ir was smaller than that of 1-Rh by a factor of about
⧧
1
0
/50 000. The enthalpy of activation (1-Rh, ΔH = 17.9 ±
The crystal structures of the hydrido phosphine and hydrido
phosphite complexes 10b-Rh, 10c-Rh, 10a-Ir, and 10b-Ir are
shown in Figure 4. The molecular frameworks of these structures
are similar to those of the hydrido carbonyl complexes 1-Rh and
−1
⧧
−1
.4 kcal mol ; 1-Ir, ΔH = 25.2 ± 0.9 kcal mol ) is the
dominant factor determining the rate constant, which can
likely be attributed to the relatively strong Ir−CO bond
(
Table 4). Furthermore, the small negative value of entropy
1-Ir. The M−B distances of 10 range from 2.336 to 2.354 Å,
which are similar to those of 1 (Table 3). These findings indicate
the presence of a M→B interaction. The boron pyramidalization
observed in 10 (∑(C−B−C) = 327.5−331.6°) was greater than
that observed in 1, probably owing to enhancement of the Lewis
basicity of the metal center derived from the better σ-donating
Table 4. Enthalpy and Entropy of Activation and the Rate
Constant for the Reaction with P(OMe) at 70 °C
3
ΔH (kcal mol−1)
⧧
ΔS (eu)
⧧
kobsd at 70 °C (s−1
)
complex
a
ability of these phosphorus ligands. The M−PR (M = Rh, Ir; R =
1-Rh
17.9 ± 0.4
25.2 ± 0.9
−3.5 ± 1.6
−3.8 ± 2.6
4.92
3
−
5
1
-Ir
9.90 × 10
Me, OMe, OEt) distances are longer in phosphine complex 10a
than in phosphite complexes 10b,c. This is likely a result of the
a
Value calculated by extrapolation.
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⧧
⧧
(
1-Rh, ΔS = −3.5 ± 1.6 eu; 1-Ir, ΔS = −3.8 ± 2.6 eu) implies a
After 24 h, the mixture was cooled to room temperature, and the solvent
was removed under reduced pressure. The residue was extracted with
diethyl ether (20 mL) and the extract then filtered through a column
packed with Celite. After the volatile materials were removed, washing
the residual solid with hexane (3 mL × 2) and cold diethyl ether (2 mL× 1)
dissociative interchange mechanism. This hypothesis is
supported by the dependence of the rate constant on the
strengthening M−CO bond as well as the absence of a detected
intermediate. On the basis of these results, we conclude that the
rate-determining step involved CO dissociation and that the σ-
acceptor and trans-labilizing abilities of the σ-borane ligand
undoubtedly weakened the Ir−CO bond and led to unprece-
dented CO dissociation in 1-Ir.
1
afforded 1-Ir as a pale yellow solid (132 mg, 130 mmol, 63%). H NMR
(
1
400 MHz, toluene-d , −80 °C): δ −9.64 (dt, J
= 103.2, JP−H = 16.4,
H, Ir−H), 6.45−6.88 (m, 30H, Ar), 7.39 (m, 3H, Ar), 7.86 (m, 6H, Ar),
8
P−H
31 1
8
.28−8.51 (m, 3H, Ar). P{ H} NMR (163 MHz, C D , 20 °C): δ 25.4
6
6
11
1
(s). B{ H} NMR (128 MHz, C D , 20 °C): δ −7.2. Anal. Found for
C H P OBIr: C, 65.07; H, 4.55. Calcd: C, 65.03; H, 4.27. IR (KBr):
6
6
55
43 3
−
1
SUMMARY
1972 cm .
■
Reaction of 1-Rh with [Rh(CO)(PPh ) ][K(18-crown-6)] (5). An
3
3
We report here the synthesis of new iridaboratranes, in which the
iridium donates its electron density to the borane through an
interaction of the d orbital of the Ir center and the p orbital of
NMR sample tube equipped with a Teflon Seal-cock was filled with
-Rh (5.9 mg, 0.0064 mmol) and THF-d (0.4 mL). To the solution was
1
8
2
z
added 5 (7.8 mg, 0.0064 mmol) in THF, and then the reaction was
carried out at ambient temperature for 24 h. The quantitative formation
of 2-Rh and [{o(Ph P)C H } BRh(CO)][K(18-crown-6)] (4) was
the boron. Structural analysis and DFT calculations indicated
that the metal→boron interaction in 1-Ir is stronger than that
observed in the corresponding Rh system. The Brønsted basicity
of 1 was strongly reflected by the coordination of the borane
ligand, and 1 exhibited a Brønsted basicity lower than that for the
related borane-free complex 2 and higher than those for the
cationic complexes 6−9. Furthermore, the σ-acceptor borane of
the phosphine−borane system exhibited a strong trans-
influencing ability similar to that of an imidazolyl-based system,
as well as significant elongation of the Ir−CO bond trans to the
boron. The electronic and structural deviation induced by the
Ir→B interaction caused an unexpected reversible CO/PR3
substitution reaction of 1-Ir. Kinetic studies strongly suggested
that the CO/PR3 substitution reaction proceeded via a
dissociative interchange mechanism, and the σ-acceptor and
trans effects of the borane ligand undoubtedly contributed to
weakening of the Ir−CO bond. Neutral monocarbonyl iridium-
2
6
4 3
confirmed by 31P NMR spectroscopy.
Reaction of [Ir(H) (CO)(PPh ) ][BF ] (9) with NEt . An NMR
2
3 3
4
3
sample tube was filled with 9 (5.9 mg, 0.0054 mmol), THF-d (0.45
8
mL), and triethylphosphine oxide (2 mg) as an internal standard. To the
solution was added NEt (2.3 μL, 0.016 mmol), and then the reaction
3
was carried out at ambient temperature for 15 h. The solution gradually
turned from colorless to yellow, and the quantitative formation of 2-Ir
31
was confirmed by P NMR spectroscopy.
Preparation of RhH{P(OMe) }[B{o-C H (PPh )} ] (10b-Rh). A
3
6
4
2 3
50 mL Schlenk tube was filled with 1-Rh (137.1 mg, 0.148 mmol),
P(OMe) (50 μL, 0.423 mmol), and toluene (3 mL). The reaction
3
mixture was stirred at room temperature for 15 h, after which the solvent
was removed under reduced pressure. Washing the residual solid with
hexane (2 mL × 2) afforded 10b-Rh as a pale yellow solid (127.4 mg,
1
0
.121 mmol, 82%). H NMR (400 MHz, C D ): δ −9.63 (m, 1H, Rh−
6
6
H), 2.94 (d, J
7
7
−
= 10.8 Hz, 9H, P(OMe) ), 6.78−6.89 (m, 18H, Ar),
P−H
3
.00−7.05 (m, 6H, Ar), 7.13−7.22 (m, 6H, Ar), 7.39−7.43 (m, 9H, Ar),
(
I) complexes lead to almost no dissociation of the carbonyl
31 1
.82−7.84 (m, 3H, Ar). P{ H} NMR (163 MHz, C D , 20 °C): δ
6
6
ligand due to the strong Ir(I)−CO bond, which is known to have
an extremely high dissociation energy. In fact, the catalytic
mechanism including Ir(I)−CO dissociation is recognized as
unreasonable. The low CO dissociation property of Ir(I)
complexes appears to be one of the reasons that IrH(CO)-
PPh ) rarely exhibits high catalytic activity in comparison with
the Rh analogue RhH(CO)(PPh ) . We are currently inves-
51.2 (dd, J
= 130.0 Hz, JP−P = 19.2 Hz, PPh ), 144.3 (dq, J
=
P−Rh
P−Rh
2
147.1 Hz, J = 19.2 Hz, P(OMe)3). 11B{ H} NMR (128 MHz, C D ,
1
P−P
6
6
20 °C): δ −7.5. Anal. Found for C H O P BRh: C, 66.65; H, 4.77.
57
52
3 4
1
9
Calcd: C, 66.95; H, 5.13.
Preparation of RhH{P(OEt) }[B{o-C H (PPh )} ] (10c-Rh). A
3
6
4
2 3
5
0 mL Schlenk tube was filled with 1-Rh (92.4 mg, 0.0997 mmol),
(
3 3
P(OEt) (50 μL, 0.292 mmol), and toluene (3 mL). The reaction
3
3
3
mixture was stirred at room temperature for 15 h, after which the solvent
was removed under reduced pressure. Washing the residual solid with
hexane (2 mL × 2) afforded 10c-Rh as a pale yellow solid (86.3 mg,
tigating a new catalytic process that takes full advantage of this
feasible CO dissociation.
1
0
.0790 mmol, 79%). H NMR (400 MHz, CD Cl ): δ −9.90 (m, 1H,
2
2
EXPERIMENTAL SECTION
General Procedures. The compounds described below were
handled under a dinitrogen atmosphere, and air and water were
removed completely using Schlenk techniques. [{o-(Ph P)C H } B]-
RhH(CO) (1-Rh), RhH(CO)(PPh ) (2-Rh), IrH(CO)(PPh3)3
2-Ir), {o-(Ph P)C H } B (3), and [Ir(H) (CO)(PPh ) ][BF ] (9)
Rh−H), 0.84 (t, J
P(OCH
= 6.4 Hz, 9H, P(OCH CH ) ), 3.49 (m, 6H,
■
H−H
2 3 3
31
1
CH
2
)
3
), 6.57−7.72 (m, 42H, Ar). P{ H} NMR (163 MHz,
), 158.0
). B{ H} NMR (128
BRh: C, 67.74;
3
C D , 20 °C): δ 51.3 (dd, JP−Rh = 128.6 Hz, JP−P = 18.5 Hz, PPh
6 6 2
11 1
(dq, JP−Rh = 136.0 Hz, JP−P = 18.5 Hz, P(OEt)
MHz, C , 20 °C): δ −7.6. Anal. Found for C60
H, 5.57. Calcd: C, 67.68; H, 5.49.
Preparation of IrH(PMe )[B{o-C
Schlenk tube was filled with 1-Ir (59.1 mg, 0.0582 mmol), a 1 M toluene
solution of PMe (0.50 mL, 0.50 mmol), and toluene (4 mL). The
2
6
4 3
3
9
22
D
6
H O P
58 3 4
3
3
6
23
9
24
(
2 6 4 3 2 3 3 4
were prepared as previously described. Benzene-d , toluene-d , THF-d ,
3
H (PPh )} ] (10a-Ir). A 50 mL
6 4 2 3
6
8
8
diethyl ether, and benzene were dried over sodium benzophenone ketyl
and distilled under a dinitrogen atmosphere. Tetrahydrofuran, hexane,
and toluene were purified using a two-column solid-state purification
system. Chloroform-d, dichloromethane-d , and dichloromethane were
dried over P O and stored over 4 Å molecular sieves. The other
reagents used in this study were purchased from commercial sources and
used without further purification. The H, B, and P NMR spectra
were recorded on a JEOL JNM-AL 400 spectrometer. H NMR data
3
reaction mixture was stirred at 80 °C. After 12 h, the mixture was cooled
to room temperature, and the solvent was removed under reduced
pressure. Washing the residual solid with hexane (3 mL × 2) afforded
2
2
5
1
10a-Ir as a pale yellow solid (52.9 mg, 0.0497 mmol, 85%). H NMR
1
11
31
(400 MHz, C
9H, PMe ), 6.77−7.05 (m, 30H, Ar), 7.39−7.42 (m, 3H, Ar), 7.74−7.78
31 1
D ): δ −11.68 (br s, 1H, Ir−H), 0.90 (d, JP−H = 6.8 Hz,
6 6
1
3
11
were analyzed with reference to the residual peaks of the solvent, and B
(m, 3H, Ar), 7.89−7.92 (m, 3H, Ar), 8.27−8.33 (m, 3H, Ar). P{ H}
NMR (163 MHz, C D , 20 °C): δ −66.6 (br s, w = 48.2 Hz, PMe3),
and 31P NMR chemical shifts were referenced to external BF ·Et O and
3
2
6
6
1/2
11
1
8
5% H PO samples, respectively. IR spectra were recorded on a Perkin-
23.8 (br s, w = 40.6 Hz, PPh ). B{ H} NMR (128 MHz, C D ,
3
4
1/2 2 6 6
Elmer FTIR-Spectrum One spectrometer. Elemental analyses were
carried out using a Perkin-Elmer 2400 II elemental analyzer.
Preparation of IrH(CO)[B{o-C H (PPh )} ] (1-Ir). A 50 mL
20 °C): δ −7.4. Anal. Found for C H P BIr: C, 64.17; H, 4.87. Calcd:
5
7
52 4
C, 64.35; H, 4.93.
Preparation of IrH{P(OMe) }[B{o-C H (PPh )} ] (10b-Ir). A
6
4
2
3
3
6
4
2 3
Schlenk tube was filled with 3 (254 mg, 0.320 mmol), 2-Ir (208 mg,
06 mmol), and toluene (4 mL). The reaction mixture was stirred at 100 °C.
50 mL Schlenk tube was filled with 1-Ir (48.5 mg, 0.0477 mmol),
2
P(OMe) (50 μL, 0.423 mmol), and toluene (4 mL). The reaction
3
4
256
dx.doi.org/10.1021/om300216g | Organometallics 2012, 31, 4251−4258

Organometallics
Article
mixture was stirred at 80 °C. After 12 h, the mixture was cooled to room
temperature, and the solvent was removed under reduced pressure.
Washing the residual solid with hexane (3 mL × 2) afforded 10b-Ir as a
crystal data and results of the analyses are given in STables 3−14
(Supporting Information).
Density Functional Theory (DFT) Calculations. DFT calcu-
1
13
pale yellow solid (46.7 mg, 0.0420 mmol, 88%). H NMR (400 MHz,
lations were carried out at the B3PW91 level in conjunction with the
27
CD Cl ): δ −11.67 (dtd, J = 82.0 Hz, JP−H = 15.2 Hz, JP−H = 3.6 Hz,
Stuttgart/Dresden ECP and associated with triple-ξ SDD basis sets.
2
2
P−H
1
H, Ir−H), 4.07 (d, J
= 7.2 Hz, 9H, P(OMe) ), 6.62−6.64 (m, 3H,
For C, H, B, O, and P, 6-31G(d) basis sets were employed. All
P−H
3
Ar); 6.79−7.09 (m, 21H, Ar), 7.13−7.25 (m, 9H, Ar), 7.42−7.45 (m,
calculations were performed without symmetry constraints, utilizing the
31
1
28
6
H, Ar), 7.76−7.79 (m, 3H, Ar). P{ H} NMR (163 MHz, C D ,
Gaussian09 program. The molecular structures were drawn using the
6
6
2
9
2
0 °C): δ 23.8 (m, PPh ), 24.9 (m, PPh ), 111.4 (m, P(OMe) ).
GaussView version 4.1.2 program. Cartesian coordinates are given in
STables 16 and 17 (Supporting Information).
2
2
3
1
1
1
B{ H} NMR (128 MHz, C D , 20 °C): δ −7.2. Anal. Found for
6
6
C H O P BIr: C, 61.27; H, 5.03. Calcd: C, 61.57; H, 4.71.
57
52
3 4
Preparation of IrH{P(OEt) }[B{o-C H (PPh )} ] (10c-Ir). A 50 mL
Schlenk tube was filled with 1-Ir (43.7 mg, 0.0430 mmol), P(OEt)3
50 μL, 0.292 mmol), and toluene (4 mL). The reaction mixture was
ASSOCIATED CONTENT
Supporting Information
3
6
4
2 3
■
*
S
(
Tables of atomic coordinates and parameters, bond lengths and
angles, torsion angles, and structure refinement details, ORTEP
drawings, and CIF files for 1-Ir, 2-Ir, 10b-Rh, 10c-Rh, 10a-Ir,
and 10b-Ir and tables of Cartesian coordinates. This material is
available free of charge via the Internet at http://pubs.acs.org.
stirred at 80 °C. After 12 h, the mixture was cooled to room temperature,
and the solvent was removed under reduced pressure. Washing the
residual solid with hexane (3 mL × 2) afforded 10c-Ir as a pale yellow
1
solid (43.4 mg, 0.0376 mmol, 87%). H NMR (400 MHz, C D ):
6
6
δ −11.68 (dtd, J = 69.6 Hz, JP−H = 15.6 Hz, JP−H = 3.6 Hz, 1H, Ir−H),
P−H
0.83 (t, J
= 7.0 Hz, 9H, P(OCH CH ) ), 3.53 (qd, J
= 7.0 Hz,
H−H
2
3
3
H−H
JP−H = 6.2 Hz, 6H, P(OCH CH ) ), 6.62−7.45 (m, 30H, Ar), 7.72−7.92
AUTHOR INFORMATION
Corresponding Author
E-mail: nakazawa@sci.osaka-cu.ac.jp.
2
3 3
■
*
31
1
(
m, 9H, Ar), 8.24−8.28 (m, 3H, Ar). P{ H} NMR (163 MHz, C D ,
6
6
11 1
2
0 °C): δ 22.0 (m, PPh ), 24.7 (m, PPh ), 106.0 (m, P(OEt) ). B{ H}
2 2 3
NMR (128 MHz, C D , 20 °C): δ −7.2. Anal. Found for
C H O P BIr: C, 62.18; H, 5.01. Calcd: C, 62.45; H, 5.07.
6
6
Notes
60
58
3 4
Reaction of 10b-Ir with CO. A 50 mL Schlenk tube was filled with
0b-Ir (34.9 mg, 0.0314 mmol) and toluene (5 mL). After the tube
was evacuated at −196 °C using a high-vacuum line, 1 atm of CO
2.0 mmol) was transferred into the tube. The reaction mixture was then
stirred at 60 °C. After 3 h, the mixture was cooled to room temperature,
and the solvent was removed under reduced pressure. Washing the
residual solid with hexane (3 mL × 2) afforded 1-Ir as a pale yellow solid
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
■
(
The present research was supported by a Challenging
Exploratory Research grant (No. 23655056) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan
and a Grant-in-Aid for Young Scientists(B) (No. 23750064)
from the Japan Society of the Promotion of Science. H.K.
acknowledges financial support from a Sasakawa Scientific
Research Grant from the Japan Science Society.
(
30.1 mg, 0.0296 mmol, 94%).
Kinetic Study of the Reactions of MH(CO)[B{o-C H (PPh )} ]
6
4
2 3
(1-Rh, M = Rh; 1-Ir, M = Ir) with Trimethyl Phosphite. A Schlenk
tube was charged with 1-Rh (55.0 mg, 0.0594 mmol), toluene-d8
1.6 mL), and triphenylphosphine (15.0 mg) as an internal standard.
(
The solution was divided evenly into four NMR sample tubes. The
NMR probe was cooled at the appropriate temperature (−30, −20, −15,
and −10 °C), and 20 equiv of trimethyl phosphite (35 μL, 0.30 mmol)
was introduced into the tube at −78 °C. The sample was then shaken
and placed in the spectrometer. Data collection with an automated
acquisition program began immediately after the sample was placed in
the probe. The rate constants were calculated on the basis of the time
conversion of [1-Rh]. The obtained rate constants are given in STable 1
REFERENCES
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(
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2
(
see the Supporting Information). The temperature dependence of the
⧧
rate constants yielded the following activation parameters: ΔH
1
=
−1
⧧
−1 −1
7.9 ± 0.4 kcal mol and ΔS = −3.5 ± 1.6 cal mol K . The rate
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60.0 mg of 1-Ir (0.0591 mmol) instead of 1-Rh. The obtained rate
constants are given in STable 2 (see the Supporting Information). The
temperature dependence of the rate constants yielded the following
⧧
−1
⧧
activation parameters: ΔH = 25.2 ± 0.9 kcal mol and ΔS = −3.8 ±
−
1
−1
2
.6 cal mol K .
Structural Determination by X-ray Diffraction. Suitable single
crystals of 2-Ir, 10b-Rh, 10c-Rh, 10a-Ir, and 10b-Ir were obtained from
slow diffusion of n-hexane into CH Cl solutions of the corresponding
2
2
complexes. Single crystals of 1-Ir were also obtained as described above.
The single crystals were mounted onto a CryoLoop with Paratone oil,
and X-ray diffraction experiments were carried out using a Rigaku/MSC
Mercury CCD diffractometer with a graphite-monochromated Mo Kα
radiation source (λ = 0.710 69 Å) at 200 K. Cell refinement and data
2
5
reduction were carried out using the CrystalClear program. The
intensity data were corrected for Lorentz−-polarization effects and
empirical absorption. The structures were determined by direct
methods (SIR 97). All non-hydrogen atoms were found on the basis
of a difference Fourier synthesis and were refined anisotropically.
26
Refinement using the SHELXL-97 package was carried out by least-
2
squares methods based on F with all measured reflection data. The
4
257
dx.doi.org/10.1021/om300216g | Organometallics 2012, 31, 4251−4258

Organometallics
Article
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bonding interaction between the d torus and the molecular orbital of
z
CO, but the antibonding (σ*) interaction between the two-bladed
2
(
4) (a) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap,
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z
orbital and the lone pair on the carbon in CO appears
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(
(
(
5) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.;
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(
2
6
4 3
[
BF ] (5) considerably weakens the Ir−CO bond trans to the Si atom in
4
the solid state (Ir−CO distance 1.965 Å), the CO/P(OMe)
3
(
c) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard,
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substitution reaction of 5 does not proceed under conditions (at 60
C) similar to those of 1-Ir. Therefore, the σ-acceptor borane ligand
°
7
appears to promote the CO elimination more strongly than the silyl
group.
1
(22) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg.
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(
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(
6) (a) Rudolf, G. C.; Hamilton, A.; Orpen, A. G.; Owen, G. R. Chem.
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Butts, C. P.; Hamilton, A.; Owen, G. R. Organometallics 2009, 28, 5222.
(
(
(
d) Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Chem.
Commun. 2011, 484.
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10) Covalent radii: Rh. 134 pm; Ir, 136 pm; B, 88 pm. Atkins, P. W.;
(
(
(
(
(
3
(
Overton, T.; Rourke, J.; Weller, M.; Armstrong, F., Shriver and Atkins
Inorganic Chemistry, 4th ed.; Oxford University Press: New York, 2006;
p 24.
(
11) We could not find the structural data for 2-Ir via the Cambridge
Structural database (version 5.32); therefore, the structure was
determined by X-ray diffraction analysis (the details are given in the
Supporting Information).
(
12) Density functional theory (DFT) calculations were carried out at
1
3
2
7
the B3PW91 level in conjunction with the Stuttgart/Dresden ECP
and associated with triple-ξ basis sets for Rh and Ir. For H, C, B, O, and
P, 6-31G(d) was employed. The optimized geometries, shown in
SFigure 15, reproduced the key features of 1-Ir and 2-Ir well, with a
maximum deviation of only 0.0319 Å in the Ir−P1 distance in 2-Ir being
observed (the details are given in the Supporting Information).
(
13) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev.
B 1988, 37, 785.
(
14) Owen recently suggested two factors that contribute to strong
8
1f
trans labilization of a σ-acceptor borane in a d octahedral system. One
is the electropositive nature of the boron atom. The other dominating
factor is similar orbital energies between the square-based pyramidal and
boron-stabilized octahedral, suggesting there is no significant gain or loss
in energy for a ligand to occupy the site trans to boron. However, the Ir−
CO interaction in 1-Ir still appears to be considerably strong, and the
kinetic studies of the CO/P(OMe)3 substitution reaction do not
support the formation of an intermediate generated by a dissociation
mechanism. Therefore, there seems to be no positive information that
supports his suggestion. We will continue our research, keeping his
suggestion in mind.
4
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dx.doi.org/10.1021/om300216g | Organometallics 2012, 31, 4251−4258