Synthesis and characterization of PBP pincer iridium complexes and their application in alkane transfer dehydrogenation

Article abstract of DOI:10.1021/acs.organomet.6b00762

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis(phosphine) PBP pincer ligand. The previously reported complexes (PBP)Ir(Ph)- (Cl) (1) and (PBP)Ir(H)(Cl) (2) were converted to the new complexes (PBP)IrH4 (3) and (PBP)Ir(Ph)(H) (4). Complexes 3 and 4 serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs tertbutylethylene. The dicarbonyl complex (PBP)Ir(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4. Intermediates (PBPhP)Ir(H)(CO)2 (5) and (PBP)IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.

Full text of DOI:10.1021/acs.organomet.6b00762

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of PBP Pincer Iridium Complexes and
Their Application in Alkane Transfer Dehydrogenation
Wei-Chun Shih and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S
* Supporting Information
ABSTRACT: This work reports on the synthesis of several new
complexes of Ir supported by a diarylboryl/bis(phosphine) PBP
pincer ligand. The previously reported complexes (PBP)Ir(Ph)-
(Cl) (1) and (PBP)Ir(H)(Cl) (2) were converted to the new
complexes (PBP)IrH4 (3) and (PBP)Ir(Ph)(H) (4). Complexes
3 and 4 serve similarly as precatalysts for transfer dehydrogen-
ation of cyclooctane. The turnover numbers achieved were
relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs tert-
butylethylene. The dicarbonyl complex (PBP)Ir(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4.
Intermediates (PBPhP)Ir(H)(CO)2 (5) and (PBP)IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained
in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.
Chart 1. Previously Reported Pincer Complexes with a
Central Boryl Donor (Top) snd Synthesis of Complex 1 by
Insertion into a B−Ph Bond (Bottom)
INTRODUCTION
■
Pincer ligands are a particularly advantageous class of ancillary
ligands in transition-metal chemistry that have allowed for the
pursuit of well-defined fundamental reactivity as well as given
rise to impressive catalytic applications.1 Alkane dehydrogen-
ation and the associated catalytic transformations of hydro-
carbons are among the most impressive accomplishments for
pincers in catalysis.2−4
Although diverse designs abound, the most common
construction of a pincer ligand combines flanking phosphine
side arms with a different central donor. Among these central
donors, boryls have been among the latest to be introduced.
Boryls are particularly interesting because they are arguably the
most donating and the most trans influencing among donor
ligands that can be incorporated into polydentate ligands. The
first boryl/bis(phosphine) PBP* pincer ligand was reported by
Nozaki and Yamashita in 2009 (Chart 1).5 In the following
years, several groups accessed boryl-centered pincer ligands
where the boryl donor is incorporated into an m-carborane cage
(Chart 1).6 Yamashita later reported a study testing the utility
of PBP* complexes of Ir in alkane dehydrogenation, with
modest success.7 Yamashita’s PBP* ligand has also been used
by other groups.8
the apparent donor ability of the PBP ligand toward Ir in
comparison with other pincer ligands. Herein we report the
results of these pursuits.
In a previous publication,9 we showed how insertion of an Ir
center into a B−Ph bond of a PBPhP precursor10 provides
access to the new boryl/bis(phosphine) complex (PBP)Ir(Ph)-
(Cl) (1; Chart 1). Palladium complexes of this PBP pincer were
recently reported by Tauchert et al.11 We reported9 that
complex 1 can be converted to the hydrido/chloride 2 via
intermediacy of 2-H2 in an overall hydrogenolysis reaction
(Scheme 1). In order to evaluate the potential of the (PBP)Ir
system in alkane dehydrogenation, we sought to prepare a
(PBP)Ir polyhydride derivative. In addition, we envisaged
synthesizing a carbonyl derivative for the purpose of comparing
RESULTS AND DISCUSSION
■
Synthesis and Charaterization of PBP-Type Pincer
Iridium Complexes. Stirring 2 and sodium triethylborohy-
dride in toluene or C6D6 at room temperature followed by
placing the reaction mixture under 1 atm of H2 afforded a pale
yellow solution of (PBP)IrH4 (3), from which a 64% isolated
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: September 28, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00762
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Scheme 1. Synthesis and Reactivity of (PBP)Ir Complexes
Figure 1. POV-Ray rendition of the ORTEP drawing (50% thermal
ellipsoids) of 3 showing selected atom labeling. Hydrogen atoms are
omitted, except for the hydrogens on Ir, for clarity. Selected bond
distances (Å) and angles (deg): Ir1−B1, 2.159(3); Ir1−H1, 1.64(3);
Ir1−H2, 1.71(3); Ir1−H7, 1.57(3); Ir1−H8, 1.50(3); B1−H1,
1.48(3); B1−H2, 1.34(3); Ir1−P1, 2.2840(7); Ir1−P2, 2.2804(7);
B1−Ir1−H1, 43.3(10); B1−Ir1−H2, 38.3(10); B1−Ir1−H7,
138.6(11); B1−Ir1−H8, 139.9(10); H1−Ir1−H8, 176.0(14); H2−
Ir1−H7, 176.0(13); H1−B1−H2, 101.5(17); H7−Ir1−H8, 81.5(15).
Chart 2. Different Structural Types for Interaction of R2BH2
with a Transition Metal, As Presented by Radius and
Braunschweig (A−C), and Yamashita’s Assignment of the
Intermediate Structure D
yield was obtained upon workup (Scheme 1).12 Treatment of
solutions of 3 with 1.0 equiv of N-chlorosuccinimide (NCS) at
room temperature was found to convert 3 back to 2. The in situ
mixture contained (PBHP)Ir(H)2(Cl) (2-H2) and (PBP)Ir(H)-
(Cl) (2) in a 15:85 ratio, and the removal of volatiles at 100 °C
under vacuum fully converted the mixture into 2. Complex 3
1
displayed C2v symmetry in its H NMR spectrum, with two
different hydride resonances apparent: a broad signal at δ −6.82
ppm and a triplet at δ −13.47 ppm (t, JH−P = 12.7 Hz). We
ascribe these resonances to the pair of bridging and terminal
hydrides, respectively. A similar picture was observed in the
hydride region by Lin and Peters for (PBP*)CoH4,8a albeit
only at −90 °C and under an H2 atmosphere (PBP* =
Yamashita’s PBP ligand).
An X-ray diffraction study (Figure 1) of 3 permitted
identification of the two bridging hydride ligands H1 and H2
and the two terminal hydride ligands H7 and H8, located via
the Fourier difference map. Radius, Braunschweig, et al.
discussed the limiting structural motifs for complexes of
[R2BH2]− fragments with transition metals (Chart 2).13 They
argued that a more Lewis acidic boron center favors structural
type A (κ2-dihydroborate) over B (σ-B−H complex of a metal
hydride), whereas a more electron-rich metal would favor the
higher valence type C (boryl/dihydride). It should be noted
that type B may correspond to two degenerate forms (B and
B′), which may average out to a more symmetric structure: for
example, by NMR spectroscopy. These issues have been
explored in detail by Sabo-Etienne and co-workers in Ru
chemistry.14 In addition, A and C can be viewed as limiting
structures at either end of a possible continuum of intermediate
structures. In this vein, Yamashita et al. ascribed type D to the
structures of (PBP*)(μ-H)2RuX (where X = BH4,15 OAc16).
These compounds possessed a Ru−B bond distance com-
parable to that of unambiguous 2c-2e Ru−Bboryl bonds yet
clearly contained bridging hydrides. D can be viewed as either
incomplete insertion of the metal into the pair of B−H bonds
in type A or as a variation of C that retains B···H contacts.
The Ir···B bond distance in 3 (2.159(3) Å) is shorter than
the Ir···Bborate bond distances (2.19−2.28 Å) in the dihydrido
borate iridium complexes. It is slightly longer than the Ir−B
distances in 2-H2 (2.135−2.137 Å) possessing a σ-B−H
coordination mode.7,9 Although this distance is ca. 0.15 Å
longer than the 2c-2e Ir−B bonds in 1 and 2, it is only ca. 0.02
Å longer than the 2c-2e Ir−B distance in 6 (vide infra). The
11B{1H} NMR chemical shift observed for 3 (45.6 ppm; Table
1) is decidedly upfield of that for the unambiguous boryl
species but also clearly not upfield enough to fall in the region
of sp3-hybridized B. On the basis of these observations, it seems
reasonable to suggest that complex 3 lies somewhere along the
continuum between A and C, and probably closer to type A.
The fact that the solid-state structure is symmetric with respect
Table 1. Chemical Shifts in 11B{1H} NMR and 31P{1H}
NMR Spectra of Ir Complexes (in ppm)
1
2
2-H2
3
4
5
6
7
11B
31P
75.9
44.9
72.6
61.9
53.7
53.5
45.6
65.4
53.1
56.8
5.7
99.4
61.0
42.4
66.2
51.7
B
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to the B−Ir vector argues against B as the ground state, at least
in the solid.
Another potential transfer dehydrogenation precatalyst
(PBP)Ir(Ph)(H) (4) was also successfully synthesized (Scheme
1). Stirring 1 and sodium triethylborohydride in toluene or
C6D6 at room temperature resulted in an immediate formation
of a dark red solution. Solution multinuclear NMR analysis
demonstrated the near-quantitative formation of complex 4.
Relative to 1 (75.9 ppm) or 2 (72.6 ppm), the 11B NMR
resonance of 4 (53.1 ppm) is shifted considerably upfield, and
the hydride resonance (δ −11.51 ppm) is broad, which suggests
a degree of interaction between B and H in 4 that is not present
in 2. This interaction is likely also responsible for the thermal
stability of 4 with respect to reductive elimination of benzene,
as it lowers the energy of 4 as a benzene addition product to
(PBP)Ir. Without the auxiliary B−H interaction, elimination of
benzene from 4 would be expected to be quite facile.17
Figure 2. POV-Ray rendition of the ORTEP drawing (50% thermal
ellipsoids) of 6 showing selected atom labeling. Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles (deg): Ir1−
B1, 2.137(3); Ir1−C1, 1.935(3); Ir1−C2, 1.890(3); C1−O1,
1.144(4); C2−O2, 1.142(4); Ir1−P1, 2.3082(14); Ir1−P2,
2.2985(13); B1−Ir1−C1, 159.32(13); B1−Ir1−C2, 88.42(13); C1−
Ir1−C2, 112.26(14); P1−Ir1−P2, 139.45(3); C2−Ir1−P1,
109.97(11); C2−Ir1−P2, 104.32(12).
Exposure of a solution of 4 to 1 atm of CO led to the
formation of a pale yellow solution of (PBPhP)Ir(H)(CO)2 (5)
after a few minutes (Scheme 1). The presence of two carbonyl
ligands was evident from the two downfield 13C NMR
resonances (179.0 and 168.5 ppm) as well as two bands in
the IR spectrum (νCO 2015 and 1978 cm−1). The 11B NMR
spectrum showed one upfield resonance at 5.7 ppm, consistent
with an sp3-hybridized boron. One triplet peak at −10.66 ppm
(t, JH−P = 15.0 Hz, Ir−H) in the 1H NMR spectrum showed no
broadening from interaction with boron, consistent with a
terminal Ir hydride. These observations suggest the structure of
5 as depicted in Scheme 1, with the triarylborane moiety acting
as a Z-type ligand. Triarylboranes have been at the center of the
development of new appreciation for Z-type ligands over the
past decade or so.18−21 The formation of 5 can be viewed as
CO-induced reductive elimination of a B−C bond from 4.
Thermolysis of complex 4 at 120 °C under a CO atmosphere
led to a gradual change in the color of the solution to dark-red,
corresponding to the formation of the new complex (PBP)Ir-
(CO)2 (6). Conversion of the intermediately formed 5 into 6
requires loss of benzene. While we have not ascertained the
mechanism by which this happens, a likely scenario involves
migration of Ph to Ir with loss of CO, followed by C−H
reductive elimination of benzene and recoordination of CO.
The downfield 11B NMR chemical shift (99.4 ppm; Table 1)
confirmed the presence of an sp2-hybridized boryl central
donor in 6. Complex 6 displayed C2v symmetry in solution on
the NMR time scale at room temperature: a singlet resonance
at 61.0 ppm in the 31P{1H} NMR spectrum, one methine
resonance and two methyl resonances from the isopropyl
groups in the 1H NMR spectrum, and one CO resonance in the
31C{1H} NMR spectrum. However, the solid-state IR spectrum
revealed two CO stretching bands at 1960 and 1913 cm−1. The
presence of two coordinated carbonyls was confirmed in the
course of an X-ray diffraction study on a single crystal of 6
(Figure 2). The geometry about Ir in 6 is distorted five-
coordinate and is closer to square pyramidal than to trigonal
bipyramidal (the τ parameter22 is 0.33). The solid-state
structure is approximately Cs symmetric. The C2v symmetry
on the NMR time scale in solution suggests that rapid
interconversion between the degenerate, less symmetric ground
state forms takes place rapidly. The sum of angles about the
boron center was found to be 360°, consistent with a trigonal-
planar, sp2-hybridized boryl. However, the Ir−B distance (2.137
Å) was decidedly longer than typical PBP-type Ir−Bboryl
distances (1.97−2.05 Å).5,7,9 The origin of this relative
elongation in 6 is not clear.
Monovalent Ir complexed by PXP-type pincer ligands
typically prefers to bind only a single carbonyl ligand. Although
coordination of the second CO is possible, it is typically bound
weakly and readily lost upon workup and/or exposure to
vacuum.23,24 The fact that 6 persists as a dicarbonyl in solution
in the absence of a CO atmosphere is thus rather unusual. It is
possible that the strongly donating nature of the boryl ligand
improves back-bonding to CO ligands, resulting in stronger
bonds. On the other hand, it is also reasonable to suggest that
the putative square planar monocarbonyl complex may be
destabilized by the tenuous placement of the very strongly trans
influencing boryl trans to CO.
Complex 6 can also be synthesized by addition of 1 atm of
CO to a toluene solution of 3 at 100 °C overnight via loss of 2
equiv of H2. When 3 was exposed to an atmosphere of CO at
room temperature for 1 h, the new intermediate (PBP)Ir-
(H)2(CO) (7) was observed to constitute 52% of the mixture,
along with 22% of 3 and some other unidentifiable products
(percentages of total 31P NMR intensity given). This reaction
mixture gradually evolved into a dark red solution that
contained 43% of 6 (31P NMR evidence) after 24 h. The
reverse reaction was observed during the thermolysis of the
C6D6 solution of 6 under 1 atm of H2 at 100 °C overnight,
yielding a mixture of 7 and 6 in a ratio of 41 to 47, with the
balance being due to two minor unidentified products.
Although a pure bulk sample of 7 could not be isolated by
the reaction of 3 with CO due to the facile conversion to 6, it
was easily synthesized by comproportionation of 3 and 6 at 50
°C in toluene (Scheme 2). The signal in the 11B{1H} NMR
spectrum and a broad resonance of the bridging hydrides in the
1H NMR spectrum of 7 (42.4 and −6.26 ppm) are similar to
those of compound 3 (45.6 and −6.82 ppm). It can be
concluded that the same arguments (vide supra) regarding the
nature of the apparent dihydroborate binding to Ir are
applicable to 7 as well as 3.
Transfer Dehydrogenation of COA to COE Catalyzed
by (PBP)Ir(H)4 (3) and (PBP)Ir(Ph)(H) (4). Complexes 3 and
4 were tested as precatalysts for transfer dehydrogenation of
C
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However, they fall significantly short of the best Ir examples
Scheme 2. Comproportionation of 3 and 6 into 7
utilizing pincer ligands of the aryl/bis(phosphine) type.3,4
CONCLUSION
■
In summary, we report the preparation of new diarylboryl/
bis(phosphine) PBP complexes of Ir that can serve as
precatalysts for alkane transfer dehydrogenation. The activity
displayed by this system was found to be modest. Structural
and spectroscopic characterization of a series of PBP complexes
enabled analysis of nonclassical BH/Ir interactions.
EXPERIMENTAL SECTION
■
General Considerations. Unless specified otherwise, all manip-
ulations were performed under an Ar atmosphere using standard
Schlenk line or glovebox techniques. Screw-capped culture tubes with
PTFE-lined phenolic caps were used to perform catalytic reactions.
Toluene, pentane, and isooctane were dried and deoxygenated (by
purging) using a solvent purification system (Innovative Technology
Pure Solv MD-5 Solvent Purification System) and stored over
molecular sieves in an Ar-filled glovebox. C6D6 was dried over NaK/
Ph2CO/18-crown-6, distilled or vacuum-transferred, and stored over
molecular sieves in an Ar-filled glovebox. Hexamethyldisiloxane, COA,
1-hexene, and TBE were degassed by three freeze−pump−thaw cycles
and stored over molecular sieves in an Ar-filled glovebox. (PBP)Ir-
(Ph)(Cl) (1), (PBHP)Ir(H)2(Cl) (2-H2), and (PBP)Ir(H)(Cl) (2)
were prepared via literature procedures.9 All other chemicals were used
as received from commercial vendors. NMR spectra were recorded on
a Varian Inova 400 (1H NMR, 399.535 MHz; 11B NMR, 128.185
MHz; 31P NMR, 161.734 MHz) or a Varian Inova 500 (1H NMR,
499.703 MHz; 13C NMR, 125.697 MHz; 31P NMR, 202.265 MHz)
spectrometer. Chemical shifts are reported in δ (ppm). Coupling
cyclooctane (COA) to cyclooctene (COE) with tert-butyl-
ethylene (TBE) or 1-hexene as the hydrogen acceptor (Table
2). A significant amount of work on related PCP/POCOP-type
Table 2. TONs for Dehydrogenation of COA to COE
Catalyzed by (PBP)Ir(H)4 (3) and (PBP)Ir(Ph)(H) (4)
a
entry
cat.
R
T (°C)
time (h)
TON
1
3
3
3
3
3
3
3
3
3
3
4
4
4
CMe3
CMe3
CMe3
CMe3
CMe3
CMe3
CMe3
CMe3
nBu
100
150
200
200
200
200
200
200
150
200
150
200
200
26
26
0.5
1
0
30
2
3
32
4
34
1
constant values in the H NMR spectra should be interpreted with a
5
2
36
0.2 Hz uncertainty. For 1H and 13C NMR spectra, the residual solvent
6
4
36
peak was used as an internal reference (1H NMR, δ 7.16 for C6D6; 13
C
7
6
36
NMR, δ 128.06 for C6D6). 11B NMR spectra were referenced
externally with BF3 etherate at δ 0. 31P NMR spectra were referenced
externally with 85% phosphoric acid at δ 0. Elemental analyses were
performed by CALI Laboratories, Inc. (Parsippany, NJ).
8
26
26
26
26
15
26
43
9
78
10
11
12
13
nBu
221
CMe3
CMe3
nBu
38
47
158
1
(PBP)Ir(H)4 (3). In a 25 mL Teflon screw-capped round-bottomed
flask, NaEt3BH (500 μL, 0.50 mmol, 1.0 M in toluene) was added to a
toluene solution (2 mL) of (PBP)Ir(H)(Cl) (2; 313 mg, 0.50 mmol).
The solution was degassed via freeze−pump−thaw, and the flask was
refilled with H2 (1 atm). The reaction mixture was stirred at room
temperature for 1 h. During the stirring, the solution gradually
changed from dark red to pale yellow. The solution was filtered
through Celite, and the volatiles were removed under vacuum. The
resulting solid was washed with isooctane, yielding a pale yellow solid
(189 mg, 64%). 31P{1H} NMR (202 MHz, C6D6): δ 65.4. 11B NMR
(128 MHz, C6D6): δ 45.6 (br). 1H NMR (500 MHz, C6D6): δ 8.62 (d,
JH,H = 7.5 Hz, 2H, Ar-H), 7.43 (m, 2H, Ar-H), 7.30 (t, JH,H = 7.4 Hz,
2H, Ar-H), 7.15 (m, 2H, Ar-H), 2.18 (m, 4H, CHMe2), 1.24 (dvt, JH,H
≈ JH,P = 7.3 Hz, 12H, CHMe2), 0.98 (dvt, JH,H ≈ JH,P = 7.1 Hz, 12H,
CHMe2), −6.82 (br s, 2H, B-H2-Ir), −13.47 (t, JH,P = 12.7 Hz, 2H, Ir-
H2). 13C{1H} NMR (101 MHz, C6D6): δ 162.6 (br s, C-B), 149.3 (vt,
JP,C = 22.7 Hz, C-P), 131.5 (vt, JP,C = 7.8 Hz), 130.5 (s), 130.3 (s),
126.2 (vt, JP,C = 3.5 Hz), 26.6 (vt, JP,C = 15.7 Hz, CHMe2), 20.9 (t, JP,C
= 2.8 Hz, CHMe2), 19.0 (s, CHMe2). Anal. Calcd for C24H40BIrP2: C,
48.57; H, 6.79. Found: C, 48.65; H, 6.59. Mp: 190 °C dec.
(PBP)Ir(H)(Ph) (4). In a 50 mL Schlenk flask, NaEt3BH (1.00 mL,
1.00 mmol, 1.0 M in toluene) was added to a toluene solution (10
mL) of (PBP)Ir(Ph)(Cl) (1; 702 mg, 1.00 mmol). The reaction
mixture was stirred at room temperature for 1 h. The solution was
filtered through Celite, and the volatiles were removed under vacuum.
The resulting solid was washed with hexamethyldisiloxane, yielding an
orange solid (567 mg, 93%). 31P{1H} NMR (202 MHz, C6D6): δ 56.8.
11B NMR (128 MHz, C6D6): δ 53.1 (br). 1H NMR (500 MHz, C6D6):
δ 8.50 (d, JH,H = 7.1 Hz, 2H), 7.43 (d, JH,H = 6.7 Hz, 1H), 7.36 (m,
4H), 7.21 (m, 5H), 6.93 (t, JH,H = 7.0 Hz, 1H), 2.57 (m, 2H, CHMe2),
a
TONs were calculated on the basis of the formation of COE in H
NMR with mesitylene (3.59 mmol) as internal standard. Reaction
conditions: COA (10.0 mmol), TBE or 1-hexene (10.0 mmol), cat. 3
or 4 (0.01 mmol).
pincer complexes of Ir indicates that intermediacy of the
monovalent [(pincer)Ir] fragment is crucial.3 3 and 4 may be
able to access a corresponding transient (PBP)Ir intermediate
via reductive elimination of benzene or H2 or by hydrogenation
of the sacrificial olefin. Although there was no conversion at
100 °C (entry 1), a turnover number (TON) of 30 was
achieved at 150 °C for 26 h (entry 2). The TONs were slightly
increased to 43 upon elevation of the reaction temperature to
200 °C (entry 8), but it was clear that most of the TONs took
place early in the reaction (entries 3−7). Utilization of the
sterically less imposing 1-hexene instead of TBE as the
hydrogen acceptor enhanced the obtained TONs to 78 at
150 °C and 221 at 200 °C (entries 9 and 10).25 Using complex
4 as the precatalyst with TBE or 1-hexene as the hydrogen
acceptor gave TONs (entries 11−13) similar to those for use of
3 as the precatalyst. The activities of 3 and 4 as precatalysts for
alkane transfer dehydrogenation are modestly higher than those
of Yamashita’s Ir complexes supported by the PBP* ligand.7
D
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2.49 (m, 2H, CHMe2), 1.18 (dvt, JH,H ≈ JH,P = 7.3 Hz, 6H, CHMe2),
1.24 (dvt, JH,H ≈ JH,P = 6.9 Hz, 12H, CHMe2), 0.89 (dvt, JH,H ≈ JH,P =
7.0 Hz, 12H, CHMe2), −6.26 (br s, 2H, Ir-H). 13C{1H} NMR (126
MHz, C6D6) δ 187.0 (s, CO), 162.4 (br s, C-B), 146.9 (vt, JP,C = 25.8
Hz, C-P), 132.0 (vt, JP,C = 8.6 Hz), 130.3 (s), 130.0 (vt, JP,C = 2.3 Hz),
126.1 (vt, JP,C = 3.9 Hz), 28.1 (vt, JP,C = 15.7 Hz, CHMe2), 20.3 (vt,
JP,C = 1.8 Hz, CHMe2), 18.9 (s, CHMe2).
1.09 (dvt, JH,H ≈ JH,P = 7.3 Hz, 6H, CHMe2), 0.94 (dvt, JH,H ≈ JH,P
=
6.7 Hz, 6H, CHMe2), 0.84 (dvt, JH,H ≈ JH,P = 7.3 Hz, 6H, CHMe2),
−11.51 (br s, 1H, Ir-H). 13C{1H} NMR (101 MHz, C6D6): δ 165.0 (t,
JP,C = 8.6 Hz, Ir-C), 161.2 (br s, C-B), 145.3 (vt, JP,C = 23.5 Hz, C-P),
138.1 (s), 130.4 (m), 129.9 (s), 128.6 (s), 126.9 (vt, JP,C = 3.2 Hz),
125.0 (s), 121.5 (s), 25.5 (vt, JP,C = 13.6 Hz, CHMe2), 23.4 (vt, JP,C
=
General Procedure of Transfer Dehydrogenation of COA
using Complex 3. In a screw-capped culture tube, complex 3 (5.9
mg, 0.01 mmol) was dissolved in a solution of COA (1.35 mL, 10.0
mmol) and a hydrogen acceptor (1.29 mL for TBE or 1.25 mL for 1-
hexene, 10.0 mmol). The tube was sealed, and the reaction mixture
was heated in a preheated oil bath at a suitable temperature for the
required time (Table 2, entries 1−10). After the tube was cooled to
room temperature, mesitylene (0.50 mL, 3.59 mmol) was added as
internal standard and the TON was calculated on the basis of the
14.5 Hz, CHMe2), 20.5 (s, CHMe2), 19.0 (s, CHMe2), 18.8 (s,
CHMe2), 17.4 (s, CHMe2). Anal. Calcd for C30H42BIrP2: C, 53.97; H,
6.34. Found: C, 53.68; H, 6.46. Mp: 114 °C dec.
(PBPhP)Ir(H)(CO)2 (5). In a 25 mL Teflon screw-capped round-
bottomed flask, (PBP)Ir(H)(Ph) (4; 67 mg, 0.10 mmol) was dissolved
in C6H6 (1.0 mL). The solution was degassed twice via freeze−pump−
thaw, and the flask was refilled with CO (1 atm). After 10 min, the
solution changed from orange to pale yellow. The volatiles were
removed under vacuum, and the resulting solid was washed with cold
pentane, yielding a pale yellow solid (52 mg, 72%). The purity of
isolated samples of 5 was gauged to be >95% by NMR spectroscopy.
31P{1H} NMR (202 MHz, C6D6): δ 51.7. 11B NMR (128 MHz,
1
formation of COE in the H NMR spectrum.
General Procedure of Transfer Dehydrogenation of COA
using Complex 4. In a screw-capped culture tube, complex 4 (6.7 mg
for 4, 0.01 mmol) was dissolved in a solution of COA (1.35 mL, 10.0
mmol) and a hydrogen acceptor (1.29 mL for TBE or 1.25 mL for 1-
hexene, 10.0 mmol). The tube was sealed, and the reaction mixture
was heated in a preheated oil bath at a suitable temperature for the
required time (Table 2, entries 11−13). After the tube was cooled to
room temperature, mesitylene (0.50 mL, 3.59 mmol) was added as
internal standard and the TON was calculated on the basis of the
1
C6D6): δ 5.7. H NMR (500 MHz, C6D6): δ 7.81 (d, JH,H = 7.3 Hz,
2H), 7.20 (t, JH,H = 7.3 Hz, 2H), 7.16 (m, 2H), 7.12 (m, 2H), 7.03 (t,
JH,H = 6.7 Hz, 2H), 6.96 (t, JH,H = 7.2 Hz, 1H), 6.89 (d, JH,H = 7.2 Hz,
2H), 2.37 (m, 2H, CHMe2), 2.27 (m, 2H, CHMe2), 1.14 (dvt, JH,H
JH,P = 7.2 Hz, 6H, CHMe2), 0.99 (m, 12H, CHMe2), 0.69 (dvt, JH,H
≈
≈
JH,P = 7.3 Hz, 6H, CHMe2), −10.66 (t, JH,P = 15.0 Hz, 1H, Ir-H).
13C{1H} NMR (101 MHz, C6D6): δ 179.0 (s, CO), 174.2 (br, B-CAr),
168.5 (s, CO), 164.6 (br, B-CPh), 141.4 (vt, JP,C = 30.3 Hz, C-P), 135.7
(s), 133.8 (vt, JP,C = 11.0 Hz), 129.6 (s), 128.5 (vt, JP,C = 3.5 Hz),
126.1 (s), 124.2 (vt, JP,C = 4.2 Hz), 124.1 (s), 29.7 (vt, JP,C = 13.7 Hz,
CHMe2), 27.9 (vt, JP,C = 17.9 Hz, CHMe2), 20.2 (s, CHMe2), 20.1 (s,
CHMe2), 19.5 (s, CHMe2), 18.6 (s, CHMe2). IR (KBr), νCO 2015,
1978 cm−1.
1
formation of COE in the H NMR spectrum.
X-ray Data Collection, Solution, and Refinement for (PBP)-
Ir(H)4 (3). A colorless, multifaceted block of suitable size (0.26 × 0.09
× 0.04 mm) was selected from a representative sample of crystals of
the same habit using an optical microscope and mounted onto a nylon
loop. Low-temperature (110 K) X-ray data were obtained on a Bruker
APEXII CCD-based diffractometer (Mo sealed X-ray tube, Kα
0.71073 Å). All diffractometer manipulations, including data collection,
integration, and scaling, were carried out using the Bruker APEXII
software.26 An absorption correction was applied using SADABS.27
The space group was determined on the basis of systematic absences
and intensity statistics, and the structure was solved by direct methods
and refined by full-matrix least squares on F2. The structure was solved
in the monoclinic P21/c space group using XS28 (incorporated in
SHELXLE). All non-hydrogen atoms were refined with anisotropic
thermal parameters. All hydrogen atoms were placed in idealized
positions and refined using a riding model, with the exception of the
hydrogen bound to iridium, which was located from the difference
map. The structure was refined (weighted least-squares refinement on
F2), and the final least-squares refinement converged. No additional
symmetry was found using ADDSYM incorporated in the PLATON
program.29
X-ray Data Collection, Solution, and Refinement for (PBP)-
Ir(CO)2 (6). A red, multifaceted block of suitable size (0.20 × 0.16 ×
0.04 mm) was selected from a representative sample of crystals of the
same habit using an optical microscope and mounted onto a nylon
loop. Low-temperature (150 K) X-ray data were obtained on a Bruker
APEXII CCD-based diffractometer (Mo sealed X-ray tube, Kα =
0.71073 Å). All diffractometer manipulations, including data collection,
integration, and scaling, were carried out using the Bruker APEXII
software.26 An absorption correction was applied using SADABS.27
The space group was determined on the basis of systematic absences
and intensity statistics, and the structure was solved by direct methods
and refined by full-matrix least squares on F2. The structure was solved
in the monoclinic P21/c space group using XS28 (incorporated in
SHELXLE). All non-hydrogen atoms were refined with anisotropic
thermal parameters. All hydrogen atoms were placed in idealized
positions and refined using a riding model. The structure was refined
(weighted least-squares refinement on F2) and the final least-squares
refinement converged. No additional symmetry was found using
ADDSYM incorporated in the PLATON program.29
(PBP)Ir(CO)2 (6). Method A. In a 25 mL Teflon screw-capped
round-bottomed flask, (PBP)Ir(H)4 (3; 89 mg, 0.15 mmol) was
dissolved in toluene (2 mL). The solution was degassed twice via
freeze−pump−thaw, and the flask was refilled with CO (1 atm). The
reaction mixture was stirred at 100 °C overnight. During the heating,
the color gradually changed from pale yellow to dark red. The volatiles
were removed under vacuum, and the resulting solid was washed with
cold pentane, yielding a dark red solid (80 mg, 83%).
Method B. In a 25 mL Teflon screw-capped round-bottomed flask,
(PBP)Ir(H)(Ph) (4; 134 mg, 0.20 mmol) was dissolved in toluene (2
mL). The solution was degassed twice via freeze−pump−thaw, and
the flask was refilled with CO (1 atm). The reaction mixture was
stirred at 120 °C for 3 days. The volatiles were removed under
vacuum, and the resulting solid was washed with cold pentane, yielding
a dark red solid (98 mg, 76%). 31P{1H} NMR (202 MHz, C6D6): δ
1
61.0. 11B NMR (128 MHz, C6D6): δ 99.4 (br). H NMR (500 MHz,
C6D6): 8.09 (d, JH,H = 7.4 Hz, 2H), 7.38 (m, 2H), 7.20 (m, 2H), 7.11
(m, 2H), 2.31 (m, 4H, CHMe2), 1.09 (dvt, JH,H ≈ JH,P = 6.9 Hz, 12H,
CHMe2), 0.90 (dvt, JH,H ≈ JH,P = 6.9 Hz, 12H, CHMe2). 13C{1H}
NMR (126 MHz, C6D6): δ 185.6 (s, CO), 163.2 (br s, C-B), 152.2 (vt,
JP,C = 25.2 Hz, C-P), 130.4 (vt, JP,C = 11.1 Hz), 130.1 (s), 129.4 (s),
128.6 (s), 28.8 (vt, JP,C = 14.3 Hz, CHMe2), 19.74 (vt, JP,C = 2.4 Hz,
CHMe2), 18.43 (s, CHMe2). IR (KBr), υCO νCO 1960, 1913 cm−1.
Anal. Calcd for C26H36BIrO2P2: C, 48.38; H, 5.62. Found: C, 48.76; H,
5.68.
(PBP)Ir(H)2(CO) (7). In a 25 mL Teflon screw-capped round-
bottomed flask, (PBP)Ir(H)4 (3; 237 mg, 0.40 mmol) and
(PBP)Ir(CO)2 (6; 284 mg, 0.44 mmol) were dissolved in toluene
(10 mL). The reaction mixture was stirred at 50 °C overnight. During
the heating, the mixture gradually changed from dark red to orange.
After the mixture was cooled to room temperature, the volatiles were
removed under vacuum, and the resulting solid was redissolved in
toluene, layered with pentane, and placed in a −35 °C freezer, yielding
orange crystals (486 mg, 98%). The purity of isolated samples of 7 was
gauged to be >95% by NMR spectroscopy. 31P{1H} NMR (202 MHz,
1
C6D6): δ 66.2. 11B NMR (128 MHz, C6D6): δ 42.4 (br). H NMR
(500 MHz, C6D6): 8.38 (d, JH,H = 7.5 Hz, 2H), 7.31 (m, 2H), 7.27 (m,
2H), 7.09 (td, JH,H = 7.4, JH,H = 1.2 Hz, 2H), 2.22 (m, 4H, CHMe2),
E
DOI: 10.1021/acs.organomet.6b00762
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(11) Schuhknecht, D.; Ritter, F.; Tauchert, M. E. Chem. Commun.
2016, 52, 11823−11826.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00762.
■
S
(12) On the basis of the procedure used for the synthesis of (PCP)
IrH4: (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C.
M. Chem. Commun. 1996, 2083−2084. (b) Gupta, M.; Hagen, C.;
Kaska, W. C.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997,
119, 840−841.
NMR spectra (PDF)
crystallographic information (CIF)
(13) Arnold, N.; Mozo, S.; Paul, U.; Radius, U.; Braunschweig, H.
Organometallics 2015, 34, 5709−5715.
(14) (a) Gloaguen, Y.; Benac-Lestrille, G.; Vendier, L.; Helmstedt,
́
U.; Clot, E.; Alcaraz, G.; Sabo-Etienne, S. Organometallics 2013, 32,
4868−4877. (b) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.;
Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624−
5625. (c) Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.;
Chaudret, B.; Barthelat, J.-C.; Sabo-Etienne, S. Organometallics 2005,
24, 2935−2943.
(15) For X = BH4, the BH4 ligand is clearly type A.
(16) Miyada, T.; Huang Kwan, E.; Yamashita, M. Organometallics
2014, 33, 6760−6770.
AUTHOR INFORMATION
Corresponding Author
*E-mail for O.V.O.: ozerov@chem.tamu.edu.
ORCID
Oleg V. Ozerov: 0000-0002-5627-1120
Notes
The authors declare no competing financial interest.
■
(17) Wang, D. Y.; Choliy, Y.; Haibach, M. C.; Hartwig, J. F.; Krogh-
Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2016, 138, 149−163.
(18) (a) Hill, A. F. Organometallics 2006, 25, 4741−4743. (b) Parkin,
G. Organometallics 2006, 25, 4744−4747.
(19) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu,
K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.;
Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729−
16738.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (grant
CHE-1300299 to O.V.O.), the Welch Foundation (grant A-
1717 to O.V.O.), and the Taiwan Ministry of Education
(Graduate Fellowship for Study Abroad to W.-C.S.) for support
of this research. We are grateful to Haomiao Xie for assistance
with acquisition of IR spectra.
(20) (a) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47 (3),
859−871. (b) Bouhadir, G.; Bourissou, D. Chem. Soc. Rev. 2016, 45
(4), 1065−1079.
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DOI: 10.1021/acs.organomet.6b00762
Organometallics XXXX, XXX, XXX−XXX