Diphenylphosphino- or dicyclohexylphosphino-tethered boryl pincer ligands: Syntheses of PBP Iridium(III) complexes and their conversion to iridium-ethylene complexes

Article abstract of DOI:10.1021/om9006455

Two hydroborane precursors (1b,c) for PBP pincer ligands bearing phenyl or cyclohexyl groups on phosphorus atoms were synthesized, and their complexation reactions with iridium were investigated. Reaction of hydroborane lb, bearing phenyl groups, with Ir[

Full text of DOI:10.1021/om9006455

6234 Organometallics 2009, 28, 6234–6242
DOI: 10.1021/om9006455
Diphenylphosphino- or Dicyclohexylphosphino-Tethered Boryl Pincer
Ligands: Syntheses of PBP Iridium(III) Complexes and Their Conversion
to Iridium-Ethylene Complexes
Yasutomo Segawa, Makoto Yamashita,* and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of
Tokyo 7-3-1, Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan
Received July 21, 2009
Two hydroborane precursors (1b,c) for PBP pincer ligands bearing phenyl or cyclohexyl groups on
phosphorus atoms were synthesized, and their complexation reactions with iridium were investigated.
Reaction of hydroborane 1b, bearing phenyl groups, with Ir[P(p-tol)3]2(CO)Cl afforded an 18-electron
complex, [PhPBP]Ir(H)(CO)Cl (3b), but reactions with other iridium(I) sources gave complicated
mixtures. The characteristic IR peaks in 3b compared with the previously reported tBu-PBP derivative
3a were discussed on the basis of theoretical calculations. Complexation of cyclohexyl-substituted
hydroborane 1c with [Ir(C2H4)2Cl]2 afforded a 16-electron complex, [CyPBP]Ir(H)Cl (2c). X-ray
˚
structure and computational studies on 2c revealed that the hydride ligand is close [B-H = 1.90(5) A]
to the boron atom. Reaction of 2c with LiTMP (TMP = 2,2,6,6-tetramethylpipyridide) under ethylene
atmosphere gave monovalent iridium complex [CyPBP]Ir(C2H4) (4c) by a similar procedure to the
previously reported [tBuPBP] system. Exposure of a CD2Cl2 solution of 2c to ethylene without additional
base resulted in a quantitative formation of the intermediate [CyPBP]Ir(H)(C2H4)Cl (8c), as was
characterized by NMR spectroscopy. In contrast, no spectral change was observed in the same procedure
using [tBuPBP]Ir(H)Cl (2a) probably due to a difference in steric bulk. The observation of 8c is a rare
example of a spectroscopically identified boryl(hydrido)olefin metal complex.
Introduction
In all these reactions, boryl ligands act as “reactive” ligands. It
is known that the boryl ligand has a higher σ-donor ability than
other monoanionic ligands of second-row p-block elements
such as C, N, and O.18-24 If the boryl ligand is stabilized and
acts as a “supporting” ligand, the strong electron-releasing
property of the boryl ligand would be applied to functionaliza-
tions other than borylation.
To retain highly reactive anionic ligands on metal cen-
ters, multidentate ligands are generally utilized to take
advantage of the chelate effect. Among them, PXP pincer
ligands, meridional tridentate ligands shown in Figure 1,
could provide remarkable thermodynamic stability to metal
complexes.25 Recent development of bulky and electron-rich
phosphine-based PCP pincer ligands has disclosed unique
Transition metal boryl complexes1-5 have been widely
synthesized and applied to catalytic borylation reactions.6-17
*Corresponding authors. E-mail: makotoy@chembio.t.u-tokyo.ac.
jp; nozaki@chembio.t.u-tokyo.ac.jp. Phone and Fax: þ81-3-5841-7261.
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Article
Organometallics, Vol. 28, No. 21, 2009 6235
complexes (B).37,38 Diborane(4)-bridged metallocene, which
could be considered as a bidentate boryl ligand precursor,
reacted with a platinum(0) complex via B-B oxidative
addition to afford a series of platinum(II) complexes C having
a bidentate diboryl ligand.39-41
On the other hand, boron-containing multidentate ligands
with other coordination modes have also been recently
developed. Most established examples are metal complexes,
having a metal-to-ligand M-B σ-dative bond with one-
(D),42 two- (E),43-48 or three (F)47-65 tethered coordinating
group(s). Compounds such as F, with a cage-like structure,
are generally called “metallaboratoranes”. The other exam-
ples consist of metal complexes bearing a bidentate base-
stabilized boryl ligand (G)66 and a bidentate base-stabilized
borylene ligand (H).37
Figure 1. Representative PXP pincer complexes (X = N, C, Si).
Scheme 1. Formation of Transition Metal Complexes Bearing a
Bidentate Boryl Ligand
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C-H,26-32 N-H,33,34 and O-H35 bonds.
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6236 Organometallics, Vol. 28, No. 21, 2009
Segawa et al.
Figure 2. Previously reported transition metal complexes D-H
bearing a boron-containing multidentate ligand.
Scheme 2. Syntheses of [tBuPBP]Ir Complexes
Figure 3. Crystal structure of 6b (50% thermal ellipsoids; hy-
drogen atoms except N-H were omitted for clarity). Half of the
entire structure constitutes an asymmetric unit where the num-
bers with asterisks are in the second asymmetric unit.
the literature procedure (R = Ph).68,69 Diphenyl- or dicyclo-
hexylphosphine and paraformaldehyde were heated to af-
ford oily products, which were hydroxymethylphosphines
5b,c containing small amount of starting materials and
byproducts (phosphine oxides). Condensation between 5b,
c and o-phenylenediamine took place at room temperature to
give 1,2-bis(phosphinomethylamino)benzenes 6b,c. Com-
pound 6b could be isolated in 63% yield as a colorless crystal,
while 6c was used for the next step without isolation. The
crystal structure of 6b is shown in Figure 3. Protons bonding
to nitrogen atoms were surrounded by diphenylphosphino
groups, and no hydrogen bonding was observed. Treatment
Scheme 3. Syntheses of Hydroborane 1b,c
of 6b,c with 3.3 equiv of BH3 SMe2 afforded 7b,c with
3
formation of a benzodiazabororidine ring and protection
of phosphines in one step. The deprotection of the phos-
phine-borane moiety of 7b,c using diethylamine or n-butyl-
amine proceeded without decomposition of the benzo-
diazaborole ring to give hydroboranes 1b,c (1b: 66%, 1c:
21%; from o-phenylenediamine). The characteristic proton
signals bound to the boron atom (1b: 4.39 ppm, 1c: 5.18 ppm)
were detected by 1H{11B} NMR spectroscopy. The 11B
NMR chemical shifts of 1b,c are around 25 ppm, which
indicates that there is no Lewis acid-base interaction be-
tween boron and phosphorus atoms. Hydroboranes 1b,c
were crystallized from toluene/hexane at rt. The molecular
structures of 1b,c are shown in Figures 4 and 5. Hydrogen
atoms on each boron atom were observed in the Fourier
Recently, we have communicated the synthesis of PBP
iridium complex 2a by an oxidative addition of the B-H
bond in 1a, possessing di-tert-butylphosphinomethyl groups
on nitrogen atoms. The resulting [tBuPBP]Ir(H)Cl complex
2a could be converted to [tBuPBP]Ir(H)(CO)Cl complex 3a
and [tBuPBP]Ir(C2H4) complex 4a (Scheme 2).67 Herein, we
report further systematic study on the PBP iridium com-
plexes, including the syntheses of hydroboranes with phenyl
or cyclohexyl groups on their phosphorus atoms as PBP
ligand precursors, their complexation with iridium to form
iridium(III) complexes, solid state structures and DFT stu-
dies on the resulting complexes, and their conversion to the
corresponding iridium complexes having an ethylene ligand.
˚
map, in which B-H bond lengths are around 1.1 A. As indi-
cated by NMR study, neither intra- nor intermolecular B-P
Lewis acid-base interaction was observed in these solid state
structures. No significant structural difference between each
benzodiazabororidine moiety of 1b,c has been observed.
[PhPBP]Ir(H)(CO)Cl Complex 3b. Reactions of hydro-
borane 1b bearing phenyl groups with various iridium(I)
sources were examined (Scheme 4). Attempted synthesis of
[PhPBP]Ir(H)Cl (2b) by the reactions with [Ir(cod)Cl]2
(cod = 1,5-cyclooctadiene), [Ir(coe)2Cl]2 (coe = cis-cyclooc-
tene), and [Ir(C2H4)2Cl]2 afforded complicated mixtures.
Reactivity of 1,3-(Ph2PCH2)2C6H4 (PhPCHP) similar to 1b
Results and Discussion
Preparation of PBP Ligand Precursors. The synthetic
route to hydroboranes 1b,c bearing phenyl or cyclohexyl
groups as PBP ligand precursors is shown in Scheme 3.
Compounds 5b,c and 6b,c were synthesized according to
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2001, 20, 2171–2177.

Article
Organometallics, Vol. 28, No. 21, 2009 6237
Figure 4. Crystal structure of hydroborane 1b (50% thermal
ellipsoids; hydrogen atoms except B-H were omitted for clarity).
Figure 6. Crystal structure of 3b (50% thermal ellipsoids; minor
part of disordered moiety and hydrogen atoms were omitted for
clarity). Half of the entire structure constitutes an asymmetric unit
where the numbers with asterisks are in the second asymmetric unit.
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) of 3b and 3a
Figure 5. Crystal structure of hydroborane 1c (50% thermal
ellipsoids; hydrogen atoms except B-H were omitted for clarity).
3b
3a
Ir-Cl
Ir-B
Ir-C(1)
C(1)-O
Ir-P
2.511(2)
2.010(8)
1.985(11)
1.155(11)
2.313(2)
2.5307(13)
2.010(6)
1.923(5)
1.139(6)
2.3546(13)
Scheme 4. Reaction of Hydroborane 1b with Various Iridium(I)
Sources
2.3588(13)
174.36(16)
150.44(4)
179.3(5)
B-Ir-Cl
P-Ir-P
Ir-C(1)-O
N-B-N
180
156.92(7)
177.9(8)
107.3(6)
106.3(4)
Table 2. Characteristic Peaks in the IR Spectra (cm-1, KBr) of
Complexes 3a,b and Reference Compounds 3a-opt, 3b-opt, and 8
wavenumbersa
has been found by Yao et al.,70 whereas the reaction of
PhPCHP with [Rh(cod)Cl]2 did not give [PhPCP]Rh(H)Cl,
but did afford a mixture of dinuclear complexes. This is
probably due to an inadequacy of steric bulkiness and/or
electron donation, although the coordinatively unsaturated
[tBuPCP]Rh(H)Cl complex can be obtained from the reac-
3a67
3a-opt
3b
3b-opt
1985(s)
1976(s)
1994(m)
1977(m)
2170(m)
2121(m)
2091(s)
2073(s)
a s: strong, m: medium.
tion of 1,3-(tBu2PCH2)2C6H4 (tBuPCHP) with RhCl3
axis through B-Ir-Cl exists in the molecule, and a hydride
and a carbonyl group are disordered, while the previously
reported 3a has pseudo-Cs symmetry. The hydride ligand could
not be found in the Fourier map. The chloride ligand was
located at trans position to boron in the solid state structure of
3b. Selected bond lengths and angles of 3b and previously
reported 3a are shown in Table 1. The Ir-Cl and Ir-P bond
lengths of 3b are slightly shorter than those of 3a, although
there is no significant difference in Ir-B length and the
structure of the benzodiazabororidine moiety. Bond lengths
of the carbonyl group in 3b follow the tendency observed in CO
stretching wavenumbers, where Ir-C(1) is longer and C(1)-O
is somewhat shorter compared with those in 3a.
3
H2O71 or [Rh(coe)2Cl]2.72 On the other hand, reaction with
Ir[P(p-tol)3]2(CO)Cl at 100 °C gave 18-electron [PhPBP]Ir-
(H)(CO)Cl complex 3b. A triplet hydride signal [-7.92 ppm,
t, 2JPH = 18 Hz] and a triplet carbonyl signal [177.4 ppm, t,
2JPC = 7 Hz] in the 1H and 13C NMR spectra of 3b indicated
that these two nuclei were coupled with two magnetically
equivalent phosphorus nuclei.
Recrystallization of 3b from THF/hexane gave colorless
crystals of 3b. The ORTEP drawing is shown in Figure 6. A C2
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The IR spectroscopy of 3b showed the characteristic CO
and Ir-H peaks at 1994 and 2089 cm-1, as summarized
for the previously reported compound 3a67 in Table 2. For
J. M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539–12544.

6238 Organometallics, Vol. 28, No. 21, 2009
Segawa et al.
Figure 7. Structures of 3a-opt and 3b-opt (all hydrogen atoms
except Ir-H were omitted for clarity).
Figure 8. Vibrational modes for 3a-opt (a,b) and 3b-opt (c,d),
where the red arrows show force constants.
comparison, theoretical calculation has been performed for
3a and 3b. Starting from the crystal structures of 3a and 3b,
the optimized structures 3a-opt and 3b-opt were obtained by
use of B3LYP/LanL2DZ/6-31G(d) calculations (Figure 7,
structural parameters are found in the Supporting In-
formation). The calculated structure of 3a-opt well-repro-
duced the experimentally obtained structure of 3a, while the
structure of 3b-opt has a pseudo-Cs symmetry compared with
3b, possessing a C2 axis along with B-Ir and Ir-Cl bonds.
Results from vibrational analyses for 3a-opt and 3b-opt are
also shown in Table 2.73
The νCO peak of 3b had a slightly higher wavenumber and
the νIrH vibration appeared at significantly lower wavenum-
ber compared with those in 3a (1985 and 2170 cm-1). The
higher wavenumber of νCO in 3b than that of 3a may be
explained by a difference in electron density on the metal
center. On the other hand, the calculation showed similar
νCO values for 3a-opt and 3b-opt probably due to the
structural difference between 3b and 3b-opt as described
above. It is also noteworthy that the relative intensities of
these two peaks in 3b are reversed from those in 3a. Although
the reason for the significantly lower wavenumber of νIrH in
3b compared to that of 3a is not clear so far, vibrational
analyses in 3b-opt could reproduce the experimental obser-
vation. Similarly, the reversal of intensities for two peaks in
3a-opt and 3b-opt was also indicated by calculation, with a
slightly smaller difference on νIrH peaks (48 cm-1) compared
to the experimental result (79 cm-1). Considering force
constants obtained from calculations on 3a-opt and 3b-opt
(Figure 8), lower wavenumber peaks (a,c) can be assigned as
a combination of H-Ir-C antisymmetrical stretching and
C-O stretching. Higher wavenumber peaks (b,d) corre-
spond to a combination of H-Ir-C symmetrical stretching
and C-O stretching. Thus, the reversal of wavenumbers and
peak intensities is attributable to the difference between the
contribution of C-O stretching to νIrH peaks in 3a and 3b;
namely, it may result from the difference in the donor
Figure 9. Crystal structure of 2c (50% thermal ellipsoids; hy-
drogen atoms except Ir-H were omitted for clarity).
Scheme 5. Syntheses of [CyPBP]Ir(H)Cl Complex 2c
chemical shift was shifted from -11.6 ppm (1c) to 63.4 ppm (2c)
to show a complexation of the PBP ligand with iridium. The 11B
NMR signal showed a low-field shift from 25.9 ppm (1c) to
32.5 ppm (2c), which indicates formation of a B-Ir bond. The
hydride of 2c was observed as a sharp triplet due to coupling with
t
abilities between Ph2P and Bu2P groups or the conforma-
tional difference between 3a and 3b.
two phosphorus atoms in its 1H NMR [-26.19 ppm, t, 2JPH
=
14 Hz in CD2Cl2] and was not sharpened by decoupling of the
11B nucleus. These characteristics are similar to those of 2a.67
A single crystal of 2c was subjected to X-ray analysis. An
ORTEP drawing is given in Figure 9, and selected bond
lengths and angles of 2a,c are summarized in Table 3. As
expected, [CyPBP]Ir(H)Cl complex 2c has a similar structure
to previously reported [tBuPBP]Ir(H)Cl complex 2a. The
position of the hydride ligand in 2c was indicated by a
[CyPBP]Ir(H)Cl Complex 2c. Reaction of 1c with [Ir-
(C2H4)2Cl]2 at toluene refluxing temperature forms [CyPBP]-
Ir(H)Cl complex 2c with appearance of an Ir-H vibration
[νIrH: 2361 cm-1] in the IR spectrum (Scheme 5). The31P NMR
(73) Obtained vibration numbers were scaled with a factor of 0.961
for B3LYP/LanL2DZ calculations. See: NIST Computational Chem-
istry Comparison and Benchmark Database, NIST Standard Reference
Database Number 101 Release 14, Sept 2006, Johnson, R. D., III, Ed.;
http://srdata.nist.gov/cccbdb.
˚
Fourier map to be close [B-H = 1.90(5) A] to the boron

Article
Organometallics, Vol. 28, No. 21, 2009 6239
˚
Table 3. Selected Interatomic Distances (A) and Angles (deg) of 2c
and 2c-opt
Scheme 6. Syntheses of [PBP]Ir(C2H4) Complex 4c
2c
2c-opt
Ir-Cl
Ir-B
Ir-H
B-H
Ir-P
2.3836(13)
1.979(5)
1.51(5)
1.90(5)
2.3103(13)
2.3109(13)
2.467
1.986
1.591
2.095
2.357
slightly shorter than that in 2c-opt.75 In both 2c and 2c-opt,
the hydride ligand is close to the boron atom [1.90(5) and
B-Ir-Cl
P-Ir-P
B-Ir-H
Ir-B-H
Ir-B-N
144.03(16)
157.11(4)
64.5(19)
45.6(15)
126.4(4)
126.8(4)
106.6(4)
136.9
159.6
70.7
45.8
126.6
˚
2.095 A, respectively]. Similarly distorted trigonal-bipyra-
midal structures of (hydrido)(boryl)rhodium complexes
(iPr3P)2Rh(H)(Cl)Bpin and Bcat8,14 were investigated in-
depth by X-ray, neutron diffraction, and DFT calculations,
˚
N-B-N
106.8
which showed a similar B-H separation of around 2.0 A
(neutron data).76 As was concluded for the structure of 2a,50
2c could also be best described as (boryl)(hydrido)iri-
dium(III) rather than (σ-borane)iridium(I) from a structural
point of view, in addition to the 1H{11B} NMR experiment.
[PBP]Ir(C2H4) Complex 4c. The compound 2c was con-
verted to ethylene-coordinated iridium(I) complex 4c by
reaction with LiTMP under ethylene atmosphere
(Scheme 6). A C2v symmetrical structure of complexes 4c
was manifested by 1H NMR spectroscopy, where four
cyclohexyl groups were magnetically equivalent in C6D6
solution. The ethylene signal was observed as a singlet in
1
the H (2.77 ppm) and 13C (38.1 ppm) NMR spectra. The
low-field-shifted 11B chemical shift (54.0 ppm) was in the
typical range for boryl complexes, similar to that of 4a.
In the crystal structure of 4c shown in Figure 11, two
ethylene carbons are almost coplanar to the plane defined by
PBP and the central iridium atom. Although the bond
lengths and angles around the ethylene ligand are quite
similar to those of previously reported 4a,67 a significant
difference was observed in the orientation of bulky alkyl
groups on the phosphorus atoms. As shown in Figure 12,
four tert-butyl groups of 4a formed a C2 symmetrical or-
ientation, where two faces of the square plane are equivalent.
On the other hand, a Cs symmetrical orientation was ob-
served in the structure of 4c. Thus, the central iridium atom
of 4c has a larger coordination sphere due to the smaller
steric hindrance of the cyclohexyl groups compared to tert-
butyl groups; even crystal packing may affect the orientation
of the alkyl groups.
To observe the ethylene-coordinating intermediate, reac-
tions of 2a,c with ethylene gas were monitored by NMR
spectroscopy (Scheme 7). Noteworthy, a significant differ-
ence was observed between 2a and 2c. A CD2Cl2 solution of
2a under ethylene atmosphere did not show any spectral
change except for the appearance of the dissolved ethylene
signal. On the other hand, 2c was quantitatively converted to
the corresponding ethylene-coordinated complex 8c. This
may be explained by the larger coordination site of the
[CyPBP]Ir complex to be occupied by ethylene. Dissociation
of ethylene from 8c to regenerate 2c occurred under vacuum
to prevent isolation of 8c. The ethylene-coordinating
Figure 10. Structure of 2c-opt (all hydrogen atoms except Ir-H
were omitted for clarity).
atom. In the structure of 2c, the central iridium atom formed
a distorted trigonal-bipyramidal structure, consisting of two
apical phosphine ligands and equatorial hydride, boryl, and
chloride ligands with a small angle of B-Ir-H [64.5(19)°], as
was observed in the structure of 2a [63(2)°].74 The sum of the
three angles around the boron atom, not including the
hydride, is 359.8°, indicating sp2 hybridization of the boron
center. The bent B-Ir-Cl angle in 2c is about 14° smaller
than that of 2a probably due to the difference of steric
hindrance between cyclohexyl and tert-butyl groups.
To clarify the position of the hydride ligand of 2c, DFT
calculations were performed with B3LYP/LanL2DZ/6-31G-
(d). The crystal structure of 2c was used as an initial structure
for optimization, and the resulting structure of 2c-opt is
illustrated in Figure 10. Selected interatomic distances and
angles are shown in Table 3. The Ir-H bond length in 2c is
(74) A distorted TBP structure possessing a small B-M-H angle has
been reported with a related boryl(hydrido)rhodium chloride complex
bearing two iPr3P ligands. See: (a) Lam, W. H.; Shimada, S.; Batsanov,
A. S.; Lin, Z.; Marder, T. B.; Cowan, J.; Howard, J. A. K.; Mason, S. A.;
McIntyre, G. J. Organometallics 2003, 22, 4557-4568. The reason for the
formation of a small angle in the distorted TBP geometry of d6-ML5
complexes has been attributed to the lack of pπ-dπ interaction between
two non-π-donor ligands such as hydrides and the central metal by Eisenstein
with theoretical calculations. See: (b) Rachidi, I. El-I.; Eisenstein, O.; Jean, Y.
New J. Chem. 1990, 14, 671. (c) Riehl, J. F.; Jean, Y.; Eisenstein, O.;
Pelissier, M. Organometallics 2002, 11, 729-737. So far, it is not clear why
the “π-acceptor'” boryl ligand led to a similar geometry.
(75) It is often said “H is easy to locate through quantum calculations
but hard to locate through experimental techniques”; we could believe
the location of the hydride ligand generated by calculation rather than
X-ray analysis. See: Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O.
Chem. Rev. 2000, 100, 601-636.
(76) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z. Y.; Marder, T.
B.; Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J.
Organometallics 2003, 22, 4557–4568.

6240 Organometallics, Vol. 28, No. 21, 2009
Segawa et al.
2JHH = 12 Hz) revealed a Cs symmetrical structure. The
hydride was also detected as a triplet signal at high field
(-26.32 ppm, 3JPH = 15 Hz) in the 1H NMR spectroscopy.
Coordinating ethylene signals were observed as a single peak
in each of the 1H and 13C NMR spectra (3.20 and 49.4 ppm,
respectively) distinct from a signal of dissolved free ethylene
(5.38 and 112.1 ppm) at -90 °C. Coordinating and free
ethylene signals were broadened due to rapid exchange at
20 °C. Although the relative configuration of hydride,
chloride, and ethylene was not fully confirmed, chloride
may be located at the trans position of the boryl ligand as
was characterized for the structures of [PBP]Ir(H)(CO)Cl
complexes 3a,b. A boryl(hydrido)(alkene)metal species has
been proposed as an intermediate of transition metal cata-
lyzed hydroboration of an alkene77 and was observed in a
few examples before.78-81
Conclusion
Two hydroborane precursors (1b,c) for the PBP pincer
ligand bearing phenyl or cyclohexyl groups on phosphorus
atoms were synthesized. Although all the trials for complexa-
tion of phenyl-substituted hydroborane 1b with chloro-
iridium(I) alkene complexes afforded complicated mixtures,
reaction with Ir[P(p-tol)3]2(CO)Cl gave an 18-electron com-
plex, [PhPBP]Ir(H)(CO)Cl (3b). The characteristic IR peaks in
3b compared with the previously reported tBu-PBP derivative
3a were discussed on the basis of theoretical calculations.
Introduction of the PBP ligand using cyclohexyl-substituted
1c with [Ir(C2H4)2Cl]2 formed a 16-electron complex, [Cy-
PBP]Ir(H)Cl (2c). The location of hydride atoms in 2c was
proven to be close to the boron atom, as suggested by a
computational study, although no 11B-1H coupling was ob-
served in the NMR study. Treatment of 2c with base afforded
the square-planar iridium(I) ethylene complex 4c. Ethylene-
coordinated iridium(III) complex [CyPBP]Ir(H)(C2H4)Cl (8c)
could be observed as an intermediate in the formation of 4c, in
contrast to the case of 4a, where no intermediate was detected,
probably due to a difference in steric bulk. The observation of
8c is a rare example of a spectroscopically identified boryl-
(hydrido)olefin metal complex.
Figure 11. Crystal structure of 4c (50% thermal ellipsoids; hy-
˚
drogen atoms were omitted for clarity). Selected bond lengths (A)
and angles (deg): Ir-B, 2.069(5); Ir-C(1), 2.199(5); Ir-C(2),
2.181(5); Ir-P(1), 2.2808(13); Ir-P(2), 2.2821(13); C(1)-C(2),
1.405(7); B-N(1), 1.434(7); B-N(2), 1.431(7); C(1)-Ir-C(2),
37.42(19); Ir-C(1)-C(2), 70.6(3); Ir-C(2)-C(1), 72.0(3); P(1)-
Ir-P(2), 144.86(5); B-Ir-C(1), 159.6(2); B-Ir-C(2), 158.7(2);
N(1)-B-N(2), 104.8(4).
Experimental Section
Figure 12. Simplified structures of the orientation of bulky
substituents on the phosphorus atom in 4a,c with space-filling
models.
General Procedures. All manipulations were carried out in an
argon-filled glovebox (Miwa MFG) unless otherwise noted. The
1H, 1H{11B}, 11B, 11B{1H}, 13C{1H}, and 31P{1H} NMR spectra
were recorded on 500 MHz spectrometers with residual pro-
Scheme 7. Difference in Reactivities of 2a and 2c toward Gaseous
Ethylene and One Possible Structure of 8c
1
1
tiated solvent for H and H{11B}, deuterated solvent for 13C-
{1H}, external BF3 OEt2 for 11B and 11B{1H}, and external 85%
3
H3PO4 for 31P{1H} used as reference. Elemental analyses
were performed by the Microanalytical Laboratory of the
Department of Chemistry, Faculty of Science, Graduate
School of Science, The University of Tokyo. FAB-MS
were measured on a JEOL JMS-700 mass spectrometer. Elec-
trospray ionization-time-of-flight (ESI-TOF) mass spectra
were recorded on a Micromass LCT KB 201 mass spectrometer.
complex 8c was characterized by 1H, 13C, 31P, and 11B NMR
spectroscopic data in CD2Cl2. The low-field 11B signal of
48.0 ppm and one singlet 31P signal of 38.9 ppm, which were
obviously different from those of 2c (δB: 32.5 ppm, δP:
63.4 ppm), indicated that the PBP ligand coordinated to
the iridium center in a meridional fashion, where two phos-
phorus atoms are magnetically equivalent. Geminal cou-
pling of two magnetically inequivalent methylene protons
between N and P (two doublets at 3.78 and 3.96 ppm,
(77) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957–5026.
(78) Daura-Oller, E.; Segarra, A. M.; Poblet, J. M.; Claver, C.;
Fernandez, E.; Bo, C. J. Org. Chem. 2004, 69, 2669–2680.
(79) Segarra, A. M.; Daura-Oller, E.; Claver, C.; Poblet, J. M.; Bo,
ꢀ
ꢀ
C.; Fernandez, E. Chem.;Eur. J. 2004, 10, 6456–6467.
(80) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.;
Miyaura, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263–14278.
(81) Caballero, A.; Sabo-Etienne, S. Organometallics 2007, 26, 1191–
1195.

Article
Organometallics, Vol. 28, No. 21, 2009 6241
X-ray crystallographic analyses were recorded on a Rigaku
Mercury CCD diffractometer. IR spectroscopies were measured
on a Shimadzu FTIR-8400 and are uncorrected. Melting points
were measured on a MPA100 Optimelt automated melting point
system and are uncorrected. Ether, THF, and hexane were
purified by passing through a solvent purification system
(Glass Contour). Carbon monoxide (>99.95 vol %) and ethy-
lene (>99.9 vol %) were obtained from Takachiho Chemical
Industrial Co., Ltd. LiTMP,82 Ph2PCH2OH (5b),68
[Ir(cod)Cl]2,83 [Ir(coe)2Cl]2,83 [Ir(C2H4)2Cl]2,84 and Ir[P(p-
tol)3]2(CO)Cl85 were synthesized according to literature proce-
dures.
Synthesis of 1b. A solution of 5b (4.77 g, 22.1 μmol) and o-
phenylenediamine (1.19 g, 11.0 μmol) in CH2Cl2 (50 mL) was
stirred for 1 day. The resulting solution was filtered through a
short silica column with 200 mL of THF, and then volatiles were
evaporated to afford a colorless oil of 6b. Then the oil was
BH3 SMe2 (1.0 M in CH2Cl2, 55.0 mL, 55 mmol) was added to a
3
solution of 6c in CH2Cl2 (100 mL) at 0 °C. The resulting solution
was stirred for 12 h at room temperature. After solvents were
evaporated under reduced pressure, n-BuNH2 (50 mL) was
added to the residue. The resulting n-BuNH2 solution was
stirred for 24 h at refluxing temperature. After volatiles were
evaporated under reduced pressure, the resulting suspension
was filtered through a silica gel column with toluene (300 mL).
An analytically pure sample was obtained from the recrystalli-
zation from Et2O (1.90 g, 21% calcd from o-phenylenediamine).
1H{11B} NMR (C6D6, 500 MHz) δ 1.13-1.32 (m, 20H),
2
1.52-1.83 (m, 24H), 3.91 (d, JPH = 3 Hz, 4H), 5.18 (s, 1H),
7.18 (dd, J = 6, 3 Hz, 2H), 7.45 (dd, J = 6, 3 Hz, 2H); 13C NMR
(C6D6, 125 MHz) δ 26.9 (CH2), 27.6 (d, 3JPC = 2 Hz, CH2), 27.7
(d, JPC = 4 Hz, CH2), 29.9 (d, JPC = 11 Hz, CH), 30.2 (d,
3
2
2JPC = 12 Hz, CH), 33.6 (d, 1JPC = 17 Hz, CH). 40.2 (d, 1JPC
=
19 Hz, CH2), 110.6 (d, 4JPC = 5 Hz, CH), 119.4 (CH), 138.6 (4°);
31P NMR (C6D6, 202 MHz) δ -11.6 (s); 11B NMR (C6D6, 160
MHz) δ 25.9 (br s); mp 126.8-128.2 °C. Anal. Calcd for
C32H53BN2P2: C, 71.37; H, 9.92; N, 5.20. Found: C, 71.21; H,
10.07; N, 5.12.
diluted with 100 mL of CH2Cl2. A solution of BH3 SMe2 (1 M
3
in CH2Cl2, 100 mL) was added to the solution of 6c at rt. The
resulting solution was stirred for 12 h at room temperature.
After solvents were evaporated under reduced pressure, THF
(100 mL) and Et2NH (30 mL) were added to the residue. The
resulting Et2NH solution was stirred for 12 h at refluxing
temperature. After volatiles were evaporated under reduced
pressure, the resulting suspension was filtered through a silica
gel column with toluene (300 mL). An analytically pure sample
was obtained from the recrystallization from Et2O (3.74 g, 66%
calcd from o-phenylenediamine). 1H{11B} NMR (C6D6, 500
Synthesis of 3b. To a red solution of Ir[P(p-tol)3]2(CO)Cl (380
mg, 439 μmol) in toluene (7 mL) was added 1b (226 mg, 439
μmol), and the resulting solution was stirred for 24 h at refluxing
temperature to afford a pale yellow solution. Volatiles were
removed from the solution, and the residue was reprecipitated
from THF/hexane to afford an analytically pure sample (285
mg, 84%). Recrystallization from THF/hexane gave colorless
crystals: 1H NMR (C6D6, 500 MHz) δ -7.92 (t, 2JPH = 18 Hz,
2
MHz) δ 4.19 (d, JPH = 5 Hz, 4H), 4.39 (s, 1H), 7.00-7.04
2
2
(m, 12H), 7.12 (dd, J = 6, 3 Hz, 2H), 7.22 (dd, J = 6, 3 Hz, 2H),
7.28-7.31 (m, 8H); 13C NMR (C6D6, 125 MHz) δ 45.8 (d,
1JPC = 12 Hz, CH2), 110.5 (d, 4JPC = 5 Hz, CH), 119.7 (CH),
128.8 (d, 3JPC = 6 Hz, CH), 129.0 (CH), 133.4 (2JPC = 18 Hz,
CH), 137.8 (d, 1JPC = 15 Hz, 4°), 138.1 (4°); 31P NMR (C6D6,
202 MHz) δ -21.8 (s); 11B NMR (C6D6, 160 MHz) δ 25.2 (br s);
mp 134.5-135.5 °C. Anal. Calcd for C32H29BN2P2: C, 74.72; H,
5.68; N, 5.45. Found: C, 74.51; H, 5.96; N, 5.39.
1H), 3.91 (dvt, JHH = 12 Hz, JPH = 3 Hz, 2H), 4.46 (d,
2JHH = 12 Hz, 2H), 6.86-6.97 (m, 10H), 7.02 (t, J = 7 Hz, 4H),
7.11 (dd, J = 5, 3 Hz, 2H), 7.69 (dvt, 3JHH = 7 Hz, 3JPH = 5 Hz,
4H), 8.13 (dvt, JHH = 7 Hz, JPH = 5 Hz, 4H); 13C NMR
(C6D6, 125 MHz) δ 51.5 (vt, 1JPC = 25 Hz, CH2), 110.0 (CH),
119.2 (CH), 128.8 (vt, 3JPC = 5 Hz, CH), 129.0 (vt, 3JPC = 5 Hz,
CH), 130.7 (CH), 131.2 (CH), 131.4 (vt, 2JPC = 6 Hz, CH), 134.4
(vt, 2JPC = 6 Hz, CH), 137.0 (vt, 1JPC = 24 Hz, 4°), 140.2 (vt,
3JPC = 7 Hz, 4°), 177.4 (t, 2JPC = 7 Hz, 4°); 31P NMR (C6D6,
202 MHz) δ 22.1 (s); 11B NMR (C6D6, 160 MHz) δ 43.2 (br s);
3
3
Isolation of 6b. A solution of 5b (2.26 g, 10.4 mmol) and o-
phenylenediamine (565 mg, 5.22 mmol) in CH2Cl2 (15 mL) was
stirred for 1 day. The resulting solution was filtered through a
short silica gel column with 100 mL of THF, and then volatiles
were evaporated to afford a colorless oil. Addition of Et2O (10
mL) to the oil led to the formation of a white precipitate, and the
resulting solid was filtered. The crude product was dissolved in
THF and recrystallized with THF/hexane to yield colorless
mp 166.8-169.4 °C (dec); IR (KBr) νIrH = 2089 cm-1, νCO
=
1994 cm-1. Anal. Calcd for C33H29BClIrN2OP2: C, 51.47; H,
3.80; N, 3.64. Found: C, 51.67; H, 4.00; N, 3.45.
Synthesis of 2c. To a red solution of [Ir(C2H4)Cl]2 (222 mg,
391 μmol) in toluene (15 mL) was added 1c (421 mg, 781 μmol),
and the resulting solution was stirred for 12 h at refluxing
temperature to afford a red solution. Volatiles were removed
from the solution, and the residue was reprecipitated from
CH2Cl2/hexane to afford an analytically pure sample (356 mg,
61%): 1H NMR (CD2Cl2, 400 MHz) δ -26.19 (t, 2JPH = 14 Hz,
1H), 1.21-1.55 (m, 22H), 1.71 (d, J = 3 Hz, 4H), 1.84 (br s, 8H),
1.98 (d, J = 12 Hz, 2H), 2.08 (d, J = 10 Hz, 2H), 2.23 (t, J = 12
Hz, 2H), 2.63 (t, J = 12 Hz, 2H), 3.36 (d, J = 12 Hz, 2H), 3.53 (d,
J = 12 Hz, 2H), 6.98 (dd, J = 5, 3 Hz, 2H), 7.15 (dd, J = 5, 3 Hz,
2H); 13C NMR (CD2Cl2, 100 MHz) δ 25.8 (CH2), 26.0 (CH2),
26.3 (vt, 2JPC = 6 Hz, CH2), 26.5 (vt, 2JPC = 6 Hz, CH2), 26.7
(CH2), 27.1 (CH2), 28.3 (CH2), 29.4 (CH2), 32.9 (vt, 1JPC = 12
Hz, CH), 34.1 (vt, 1JPC = 12 Hz, CH), 39.2 (vt, 1JPC = 20 Hz,
1
crystals (1.66 g, 63%): H NMR (C6D6, 500 MHz) δ 3.27 (s,
2H), 3.51 (d, 2JPH = 4 Hz, 4H), 6.68 (dd, J = 6, 3 Hz, 2H), 6.92
(dd, J = 6, 3 Hz, 2H), 7.02-7.03 (m, 12H), 7.34-7.37 (m, 8H);
1
13C NMR (C6D6, 125 MHz) δ 44.0 (d, JPC = 11 Hz, CH2),
113.2 (CH), 120.3 (CH), 128.9 (CH), 129.0 (d, 3JPC = 4 Hz, CH),
133.2 (d, 2JPC = 18 Hz, CH), 137.5 (d, 1JPC = 13 Hz, 4°), 138.1
(d, 3JPC = 7 Hz, 4°); 31P NMR (C6D6, 202 MHz) δ -17.9 (s); mp
121.1-123.0 °C. Anal. Calcd for C32H30N2P2: C, 76.18; H, 5.99;
N, 5.55. Found: C, 75.97; H, 6.13; N, 5.33.
Synthesis of 1c. A suspension of Cy2PH (6.59 g, 33.2 mmol)
and paraformaldehyde (998 mg, 33.2 mmol) was stirred for 4 h
at 130 °C to afford a colorless oil. A solution of o-phenylene-
diamine (1.80 g, 16.6 mmol) in CH2Cl2 (100 mL) was added to
the oil, and the resulting solution was stirred for 3 days. The
reaction mixture was filtered through a short silica gel column
with 300 mL of THF, and all the volatiles were removed under
reduced pressure to afford a colorless oil (6c). A solution of
CH2), 107.2 (CH), 117.2 (CH), 139.1 (vt, 3JPC = 6 Hz, 4°); 31
NMR (C6D6, 202 MHz) δ 63.4 (s); 31P NMR (CD2Cl2, 202
MHz) δ 63.3 (s); 11B NMR (C6D6, 160 MHz) δ 32.5 (br s); 11
P
B
NMR (CD2Cl2, 160 MHz) δ 32.4 (br s); mp 186.0-189.7 °C
(dec); IR (KBr) νIrH = 2361 cm-1. Anal. Calcd for C32H53-
BClIrN2P2: C, 50.16; H, 6.97; N, 3.66. Found: C, 49.90; H, 6.92;
N, 3.54.
(82) Hall, P. L.; Gilchrist, J. H.; Harrison, A. T.; Fuller, D. J.;
Collum, D. B. J. Am. Chem. Soc. 1991, 113, 9575–9585.
(83) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18–20.
(84) Onderdelinden, A. L.; Van der Ent, A. Inorg. Chim. Acta 1972, 6,
420–6.
(85) Rappoli, B. J.; Churchill, M. R.; Janik, T. S.; Rees, W. M.;
Synthesis of 4c. Gaseous ethylene (1 atm) was introduced to a
freeze-thawed solution of 2c (600 mg, 783 μmol) in THF
(10 mL). After stirring for 5 min at rt, a solution of LiTMP
(127 mg, 862 μmol in 10 mL of THF) was added, and the
resulting solution was stirred for 12 h at refluxing temperature.
After volatiles were removed, Et2O (100 mL) was added and
Atwood, J. D. J. Am. Chem. Soc. 2002, 109, 5145–5149.

6242 Organometallics, Vol. 28, No. 21, 2009
Segawa et al.
resulting suspension was filtered through a Celite pad. After
evaporation of Et2O, recrystallization of the residue from Et2O
solution gave orange microcrystals (480 mg, 81%). Single
crystals suitable for X-ray analysis were obtained from recrys-
tallization with THF/hexane: 1H NMR (C6D6, 500 MHz) δ
0.96-1.29 (m, 20H), 1.55 (d, J = 13 Hz, 4H), 1.60 (br s, 8H),
1.69 (d, J = 14 Hz, 4H), 1.94 (d, J = 8 Hz, 4H), 2.16 (t, J = 10 Hz,
4H), 2.77 (s, 4H), 3.70 (s, 4H), 7.07 (dd, J = 5, 3 Hz, 2H), 7.19 (dd,
Mercury CCD diffractometer. Graphite-monochromated Mo
˚
KR radiation (λ = 0.71070 A) was used. The structures were
solved by direct methods with (SIR-97)86 and refined by full-
matrix least-squares techniques against F2 (SHELXL-97).87 The
intensities were corrected for Lorentz and polarization effects.
The non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms were refined isotropically in the difference Fourier
maps or placed using AFIX instructions.
J = 5, 3 Hz, 2H); 13C NMR (C6D6, 125 MHz) δ 27.2 (vt, 2JPC
5 Hz, CH2), 27.3 (vt, 2JPC = 6 Hz, CH2), 29.4 (CH2), 29.5 (CH2),
36.9(vt, 1JPC = 14 Hz, CH), 38.1(CH2, ethylene), 42.9 (vt, 1JPC
=
Computational Studies. The Gaussian 03 program88 running
on a SGI Altix4700 system was used for optimization (B3LYP/
LANL2DZ for Ir, 6-31G(d) for others). Visualization of the
results was performed by use of POV-Ray for Windows v3.5
software.
=
21 Hz, CH2), 108.3 (CH), 117.8 (CH), 140.9 (vt, 3JPC = 8 Hz, 4°);
31P NMR (C6D6, 202 MHz) δ 69.8 (s); 11B NMR (C6D6,
160 MHz) δ 54.0 (br s); mp 190.0-195.0 °C (dec); HRMS-
ESITOF (m/z) [M] þ Hþ calcd for C34H57BIrN2P2, 759.3719;
found, 759.3341; [M] þ Hþ - C2H4 calcd for C32H53BIrN2P2,
731.3406; found, 731.3430. Although the observed fragment peak
of [M] þ Hþ - C2H4 satisfied the calculated mass number, the
parent ion [M] þ Hþ was slightly different from calculation
probably due to overlapping with a signal of impurity. Elemental
analysis failed due to rapid decomposition of 4c.
Acknowledgment. This work was supported by KA-
KENHI (Nos. 21245023 and 21685006) from MEXT,
Japan. M.Y. thanks the General Sekiyu Research Promo-
tion Foundation. Y.S. acknowledges a JSPS fellowship
for young scientists.
Supporting Information Available: The details of X-ray and
computational study, cif files, NMR spectroscopies, and mass
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
Observation of 8c. Gaseous ethylene (1 atm) was introduced to
a freeze-thawed CD2Cl2 (0.6 mL) solution of 2c (10.0 mg, 13.1
μmol) in a J-Young NMR tube at rt. The NMR tube was
brought into an NMR probe and was cooled to -90 °C, and
the reaction was immediately monitored by NMR spectroscopy.
Complete disappearance of 2c was observed, and formation of
(86) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
1
1
8c was confirmed by H, 13C, 31P, and 11B NMR spectra. H
NMR (CD2Cl2, 400 MHz, -90 °C) δ -26.32 (t, 2JPH = 15 Hz,
1H), 0.87-1.28 (m, 22H), 1.51-1.80 (m, 14H), 2.05 (s, 2H), 2.12
(s, 2H), 2.47 (s, 2H), 2.61 (s, 2H), 3.20 (4H, s, coordinating
ethylene), 3.78 (d, 2JHH = 12 Hz, 2H), 3.96 (d, 2JHH = 12 Hz,
2H), 5.38 (s, free ethylene), 6.78 (dd, J = 5, 3 Hz, 2H), 6.83 (dd,
J = 5, 3 Hz, 2H); 13C NMR (CD2Cl2, 100 MHz) δ 24.7 (CH2),
24.9 (CH2), 25.2 (CH2), 25.9 (CH2), 26.6 (CH2), 26.8 (CH2), 27.2
(CH2), 32.3 (vt, 1JPC = 16 Hz, CH), 35.5 (vt, 1JPC = 11 Hz, CH),
39.2 (vt, 1JPC = 8 Hz, CH2), 49.4 (CH2, coordinating ethylene),
107.0 (CH), 116.2 (CH), 122.1 (CH2, free ethylene), 137.5 (vt,
3JPC = 8 Hz, 4°); 31P NMR (CD2Cl2, 202 MHz, 20 °C) δ 38.9 (s);
11B NMR (CD2Cl2, 160 MHz, 20 °C) δ 48.0 (br s).
€
SHELXL-97; University of Gottingen:
(87) Sheldrick, G. M.
€
Gottingen, Germany, 1997.
(88) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian03W, Revision D.01; Gaussian, Inc.:
Wallingford, CT, 2004.
X-ray Crystallography. Details of the crystal data and a
summary of the intensity data collection parameters for 6b,
1b,c, 3b, 2c, and 4c are listed in the Supporting Information. In
each case, a suitable crystal was mounted with mineral oil on a
glass fiber and transferred to the goniometer of a Rigaku