Highly active iridium(III)-NHC system for the catalytic B-N bond activation and subsequent solvolysis of ammonia-borane

Article abstract of DOI:10.1021/om2007437

A series of coordinatively unsaturated NHC-Ir(III) complexes are examined in terms of their catalytic activity toward the solvolysis of ammonia-borane (AB). Utilization of [Ir(I^tBu′)₂Cl] (2) and [Ir(I^tBu′)₂][PF₆] (3) at 50 ppm catalyst loading led to reactions evolving approximately 3 equiv of H₂ within 10 min at 40 °C. The activation parameters of the reaction were measured to be ΔH^? = 11.1 kcal/mol and ΔS^? = 30.9 eu. Mechanistic investigations revealed an equilibrium between [Ir(H)(NH₃)(I^tBu)(I^tBu′)][PF ₆] (4) + H₂ and [Ir(H)₂(I^tBu) ₂][PF₆] (6) + NH₃. Spectroscopic data suggest that complexes 4 and 6 are the resting state of the active catalyst. X-ray structures of complex 4 and [Ir(NH₃)(I^tBu′) ₂][PF₆] (5) are given.

Full text of DOI:10.1021/om2007437

ARTICLE
pubs.acs.org/Organometallics
Highly Active Iridium(III)ꢀNHC System for the Catalytic BꢀN Bond
Activation and Subsequent Solvolysis of AmmoniaꢀBorane
George C. Fortman, Alexandra M. Z. Slawin, and Steven P. Nolan*
EaStCHEM School of Chemistry, University of St. Andrews, St Andrews, KY16 9ST, U.K.
S Supporting Information
b
ABSTRACT: A series of coordinatively unsaturated NHCꢀ
Ir(III) complexes are examined in terms of their catalytic activity
toward the solvolysis of ammoniaꢀborane (AB). Utilization of
t
0
Ir(I Bu ) Cl] (2) and [Ir(I Bu ) ][PF ] (3) at 50 ppm catalyst
2 6
t
0
[
2
loading led to reactions evolving approximately 3 equiv of H2
within 10 min at 40 °C. The activation parameters of the
q
q
reaction were measured to be ΔH = 11.1 kcal/mol and ΔS = 30.9 eu. Mechanistic investigations revealed an equilibrium between
t
t
0
t
[
6
Ir(H)(NH )(I Bu)(I Bu )][PF ] (4) + H and [Ir(H) (I Bu) ][PF ] (6) + NH . Spectroscopic data suggest that complexes 4 and
3
6
2
2
2
6
3
t
0
2
are the resting state of the active catalyst. X-ray structures of complex 4 and [Ir(NH )(I Bu ) ][PF ] (5) are given.
3 6
t
’
INTRODUCTION
The ongoing search for alternative fuel supplies has, within the
transformed into [Ir(I Bu) (H) ][PF ] when placed under an
2 2 6
atmosphere of hydrogen. Only recently have we turned our
attention to catalytic applications of these unusual complexes.
1
4
past decade, focused attention on the hydrogen-dense ammoniaꢀ
borane (AB). Containing more hydrogen per weight then even
liquid hydrogen, ammoniaꢀborane provides a stable source
of transportable hydrogen for subsequent consumption to
produce energy. Catalytic solvolysis as shown in eq 1 is one
method to liberate the hydrogen stored in the hydrogen-rich
ammoniaꢀborane. Several recent reviews highlight advances in
Development of the bis-ortho-metalated NHCꢀIr complexes
shown in Figure 1 seemed a promising candidate for catalytic
solvolysis of AB. Herein, we report details of these catalytic
studies that showed the cationic iridium species 3 to be extremely
active in the catalytic solvolysis of AB. Additionally, mechanistic
aspects of the reaction have been investigated.
1
ꢀ6
the catalytic solvolysis of AB.
Acid-catalyzed solvolysis is the
7
’ RESULTS AND DISCUSSION
oldest known method to produce hydrogen from AB. Hetero-
geneous catalysts have shown high activity in the solvolysis.
Optimization of reaction conditions found the reaction to be
8
9
Platinum (20% on graphite) and rhodium colloids have both
been shown to be highly active in the solvolysis of AB. Amoroso
and Abdur-Rashid have recently disclosed a highly active Ir(III)
pincer-type complex which is capable of rapidly releasing near-
quantitative amounts of hydrogen at within 10 min at 40 °C with
most efficient in a 1:1 mixture of THF and H O. A mixture of
2
isopropyl alcohol and H O (1:1) resulted in similar reaction
2
times (see Figure 2). Of the two iridium complexes tested, the
cationic complex 3 was found to display higher activity. While 2
ꢀ1
was found to give an admirable TOF of 111 500 h , 3 was
1
0
ꢀ1
loadings of 90 ppm.
considerabley more active with a TOF of 173 000 h at 24 °C
15
and 500 ppm catalyst loadings (see Figure 3). Further reduc-
tion of catalyst loading of 3 to 50 ppm was possible and operated
best at the slightly elevated temperature of 40 °C, as shown in
eq 2. Figure 4 shows a plot of the pressure change as a result of
hydrogen evolution from the isothermal reaction.
½
cat:ꢁ
NH3BH3
f
nH2 v þ products
ð1Þ
ROH or H2O
In 2005 our group disclosed a series of ortho-metalated Ir and
1
1
Rh complexes. The complexes are prepared (Figure 1) by a
simple substitution reaction of [MCl(coe) ] (M = Ir, Rh; coe =
2
2
½Irꢁ 50 ppm
t
t
NH BH3
3H v þ products
ð2Þ
3
f
2
cyclooctene) with 4 equiv of I Bu (I Bu = 1,3-bis(tert-butyl)-2H-
imidazol-2-ylidene). Complex 1 ([IrHCl(I Bu )(I Bu)]) is formed
within 20 h and exhibits an ortho metalation of one I Bu ligand.
Ortho metalation of the second I Bu is much slower and occurs
t
0
t
H2O=THF ð1 : 1Þ
0°C
4
t
t
Although the byproduct of the reaction could not be unam-
biguously identified using analytical methods, single crystals suitable
for diffraction revealed the production of the ammoniumꢀborate
salt [NH ][B O H ] 2H O as shown in Figure 5. The ammo-
over a period of 5 days; it is aided by repeated removal of liberated
hydrogen using vacuum. The cationic complex is formed quickly
when reacted with AgPF . Complexes 2 and 3 are remarkable, as
6
4
5
10
4
3
2
the ortho metalation of the two NHCs occurs in a mutually cis
nia borate salt is most likely in equilibrium with other
t
arrangement. Also, the I Bu ligand intramolecular CꢀH activa-
1
2,13
tion is reversible.
Under pressures of H , 2 can be converted
Received: August 9, 2011
2
back to 1. Additionally, complex 3 has been reported to be
Published: September 22, 2011
r 2011 American Chemical Society
5487
dx.doi.org/10.1021/om2007437
|Organometallics 2011, 30, 5487–5492

Organometallics
ARTICLE
t
0
t
t
0
2
] (2),
Figure 1. Synthesis of [IrHCl(I Bu )(I Bu)] (1), [IrCl(I Bu )
t
0
2
and [Ir(I Bu ) ][PF ] (3).
t
0
2
][PF
6
Figure 3. Comparison of cationic [Ir(I Bu )
6
] (blue [) and the
t
0
neutral [IrCl(I Bu ) ] (green 2) in the solvolysis of NH BH . Reaction
conditions: 0.450 mmol of NH BH ; 500 ppm of [Ir]; 2 mL of THF/
3 3
2
3
3
3
3
2
H O (1/1); 24 °C.
i
Figure 2. Comparison of the use of PrOH/H
2
O (green 2) vs THF/
t
0
H O (blue [) for the solvolysis of NH3 BH using [Ir(I Bu ) ][PF ].
Reaction conditions: 0.450 mmol of NH BH ; 50 ppm of [Ir]; 2 mL of
3 3
2
3
3
3
2
6
2
Figure 4. Catalytic solvolysis of AB in THF/H O (1/1) with 50 ppm of
t
solvent; 40 °C.
0
2
][PF
[
Ir(I Bu )
6
] at 40 °C. The pressure change equates to approxi-
2
mately 3 mol of H released/mol of AB.
polyborate salts, as aqueous solutions of alkali borates are
1
7 11
1
known to readily take part in these equilibria.
B{ H} NMR
steric bulk of the substrate, but approximately 3 mols of H were
2
of the borate displayed a chemical shift of 19.9 ppm, consistent
evolved per mole of Me HNBH . Solvolysis of Me NHBH
2
3
2
3
with a trigonal BO , and a shift at 1.3 ppm is in line with a
3
required 40 min to release 3 mol H per mol of substrate.
ꢀ
17
2
tetrahedral BO4 boron. Formation of the polyborate salt in
conjunction with the pungent smell of ammonia of the com-
pleted reaction mixture strongly suggests that the reaction may
Kinetic studies of the catalytic solvolysis were carried out
between the temperatures of 24 and 60 °C. The reaction was
found to be first order overall in ammoniaꢀborane (see the
Supporting Information, Figures S1 and S2). An Eyring plot
(Figure 7) of the rate constants for the reaction revealed
similar enthalpies and entropies of activation for the catalytic
18
proceed as shown in eq 3. Interestingly, recent industrial
interest has been noted for [B O ][NH ] 2H O as a material
5
10
4
3
2
19
for nonlinear fiber optics.
q
q
3500 ppm
solvolysis of 2 (ΔH = +11.1 kcal/mol, ΔS = +44.6 cal/
(
q q
molK)) and3(ΔH = +10.0 kcal/mol ΔS =+41.7cal/(molK)).
NH3BH3 þ 2H2O
f
3H2
THF=H2O ð1:1Þ
These activation parameters suggest a similar rate-limiting
q
1
4
transition state for both complexes. The ꢀΔS term implies a
þ = ½NH4ꢁ½B5O10H4ꢁ þ = NH3
ð3Þ
5
5
more highly ordered transition state. Most likely this is derived
from the initial IrꢀAB interaction. The hydrolysis of the
liberated borane should be relatively barrierless, and as such
the activation enthalpies determined here are suspected
to mainly result from the cleavage of the BꢀN bond of the
ammoniaꢀborane.
If the reaction proceeds as shown in eq 3, this would denote
that half of the hydrogen produced is derived from the water
in the reaction media. To test this theory we chose to use
Me HNBH as a source of hydrogen. As shown in Figure 6,
2
3
the reactionproceeded muchslower, presumably due to the increased
5
488
dx.doi.org/10.1021/om2007437 |Organometallics 2011, 30, 5487–5492

Organometallics
ARTICLE
2 3
Figure 6. Pressure change as the result of the solvolysis of Me NHBH
t
0
using [Ir(I Bu ) ][PF ]. Reaction conditions: 0.450 mmol of Me NHBH ;
2
6
2
3
Figure 5. ORTEP representation of the major product [B O H ]-
5
10 4
t
0
2 6
][PF ]. Two mol-
50 ppm of [Ir]; 2 mL of THF/H
NHBH only possess 2 mol of H
2
. This confirms that H O is also a source of hydrogen in the reaction.
2
2
O (1/1); 40 °C. Even though Me -
[
NH
ecules of H
ellipsoids are drawn at the 50% probability level. X-ray data are
4
] from AB solvolysis promoted by [Ir(I Bu )
3
2
, the reaction still produces 3 mol of
2
O within the unit cell have been omitted for clarity. Thermal
H
2
16
consistent with those originally reported by Loboda et al.
Mechanistic studies were conducted by examining a series of
stoichiometric reactions (Figure 8). The reaction of 1 equiv of
AB with complex 3 in a D O/d -THF mixture (1/1) (eq 4)
2
8
resulted in the formation of a mixture of [Ir(H)(NH )-
3
t
t
0
I Bu)(I Bu )][PF ] (4) and [Ir(H) (I Bu) ][PF ] (6) as shown
6 2 2 6
1
t
(
in eq 4. Product identification was performed by H NMR (see
the Supporting Information, Figure S3). Addition of an extra 2
equiv of AB led to a slight decrease in intensity of the shifts
assigned to complex 4. These findings prompted us to investigate
the reaction of NH and H with complex 3.
3
2
Figure 7. Eyring plot of the catalytic solvolysis of ammoniaꢀborane
t
0
2
]
in THF/H
2
O
(1:1) with [IrCl(I Bu )
(black squares) and
t
0
[
Ir(I Bu ) ][PF ] (red triangles) releasing approximately 3 equiv of H .
2
6
2
Reactions involving equimolar amounts of 3 and methanol
solutions of ammonia resulted in the nearly quantitative
t
0
production of [Ir(NH )(I Bu ) ][PF ] (5), as shown in eq 5.
3
2
6
t
In this complex both Bu groups remain ortho metalated. To
confirm that the ortho demetalation observed in 4 was the
result of the presence of H in the reaction mixture, complex 5
Additionally a reversible equilibrium between complexes 4
1
2
was placed under 1.0 atm of H , as shown in eq 6. Somewhat
and 6 was found to exist, as shown in eq 7. Results from H NMR
experiments are shown in Figure 9 and in Figure S5 in the
Supporting Information. Spectrum A of Figure 9 shows complex
2
surprisingly, this reaction produced the dihydride [Ir(H) -
2
t
(
I Bu) ][PF ] with loss of the bound NH (see the Supporting
2
6
3
Information, Figure S4). It appears as though ortho demetala-
tion may help facilitate the loss of the weakly bound ammonia
ligand.
6 (formed in situ from 3 and 0.5 bar of H ). In spectrum B, the
2
addition of 2.65 equiv of NH led to a mixture of complexes 4
3
(88%) and 6 (12%). Subsequent addition of 0.5 atm of H to the
2
5
489
dx.doi.org/10.1021/om2007437 |Organometallics 2011, 30, 5487–5492

Organometallics
ARTICLE
t
0
2
Figure 8. Reaction of [Ir(I Bu ) ][PF ] (3; spectrum A) with 1.0 equiv of NH BH (spectrum B) and an additional 2.0 equiv (3 equiv total) of
6
3
3
3
t
3 6
t
0
NH
3
BH
3
(spectrum C). Reaction of 3 with 1.0 equiv of NH
] (6). In spectrum C, a total of 3.0 equiv of NH
evolution was observed. The NMR spectra show a decrease in intensity of the shifts assigned to 6 on going from spectrum B to spectrum C. This is
3
BH
3
shows the formation of [Ir(H)(NH
)(I Bu)(I Bu )][PF ] (4) and
3
3
t
[Ir(H)
2
(I Bu)
2
][PF
6
3
BH has been added to the reaction mixture. Upon mixing, further gas
3
3
attributed to the increase in the concentration of free NH being greater than the concentration of dissolved H in solution. This shifts the equilibrium
3
2
formed between species 4 and 6 toward a greater concentration of 4.
Complexes 4 and 6 were both tested in the catalytic solvolysis
of ammoniaꢀborane. Both exhibited equal activity to 3 (see the
Supporting Information, Figure S6). Most likely, these complexes
act as the resting state of the active catalyst, as stoichometric
reactions between 3 and AB in D O/d -THF resulted in a mixture
2
8
1
of both complexes, as determined by H NMR (see Figure 8).
Single crystals of complexes 4 and 5 suitable for X-ray analysis
were grown from slow vapor diffusion of pentane into a saturated
solution of THF. ORTEP representations of these complexes are
20
shown in Figure 10. Relatively few crystal structures of mono-
III
21
meric Ir ꢀNH complexes are known. The IrꢀN(1) bond lengths
3
confirmed the ligation of NH (as opposed to NH through
3
2
Figure 9. NMR spectra displaying the equilibrium formed between
t
t
NꢀH bond activation by the metal center). The IrꢀN bond
lengths were determined to be 2.260(7) and 2.233(7) Å for com-
plexes 4 and 5, respectively. The formation of an ammonia adduct is
2 2 6 3 2 6
[Ir(H) (I Bu) ][PF ] (6) and [Ir(H)(NH )(I Bu) ][PF ] (4). The
shifts of the complexes shown are the carbene backbone and hydride
peaks. The asterisk denotes the residual protio impurities in the solvent.
See text for details.
1
1
not surprising, as complex 3 was shown to readily bind CO.
A mechanism consistent with the aforementioned experimen-
tal results is depicted in Scheme 1. The coordinatively unsatu-
rated cationic complex is suspected of forming a loosely bonded
complex with AB. Aldridge et al. have recently reported the similar
reaction mixture resulted in a decrease of NMR shifts attributed
to species 4 (74%) and an increase in intensity of shifts of
complex 6 (26%) (Figure 9, spectrum C). Removal of H from
2
f
0
complex [Ir(IMes) (H) (μ-H) B(H)NMe ][BAr ] (IMes = N,N -
the NMR tube converts the remaining 6 into 4, as shown in
spectrum D of Figure 9.
2
2
2
3
4
2
2
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The next step
involves cleavage of the BꢀN bond of AB and subsequent hydro-
lysisof borane. In the presence of an aqueous medium, the borane
should readily hydrolyze to form a number of different borate
salts with the concomitant release of H . Reaction of the IrꢀNH
2
3
complex with H serves to promote the reverse ortho metalation
2
process, forming complex 5.
5
490
dx.doi.org/10.1021/om2007437 |Organometallics 2011, 30, 5487–5492

Organometallics
ARTICLE
combustion, then the H O used in the overall reaction of
2
hydrolysis and combustion would then, formally be used in a
catalytic fashion. The resulting energy would therefore be produced
from the recycling of water.
’
CONCLUSIONS
In conclusion, we have demonstrated the use of unusual,
formally 14e, bis-ortho-metalated IrꢀNHC complexes as highly
active catalysts in the solvolysis of ammoniaꢀborane. The reaction
is proposed to proceed through Ir-mediated cleavage of the BꢀN
bond followed by hydrolysis of the liberated borane by the reaction
solvent. Two cationic IrꢀNH complexes have been isolated and
3
are proposed to be resting states of the active catalyst. Further
t
0
development of the ortho-metalated complexes [Ir(I Bu ) Cl]
2
t
0
2
and [Ir(I Bu ) ][PF ] as catalysts is currently underway in our
6
laboratories.
t
t
0
Figure 10. ORTEP representations of [Ir(H)(NH )(I Bu)(I Bu )]-
3
t
0
’
ASSOCIATED CONTENT
[PF
6
] (4) and [Ir(NH
3
)(I Bu )
2
][PF
6
] (5). All hydrogens except
hydride and the counterion PF6 are omitted for clarity. Ir(1)ꢀ
ꢀ
S
Supporting Information. Text giving the experimental
N(1) = 2.260(7) Å (4) and 2.233(7) Å (5).
b
section and figures, tables, and CIF files giving NMR spectra for
high-pressure experiments, data for kinetic experiments, and
crystallographic data for [B O H ][NH ], 4, and 5. This material
Scheme 1. Plausible Mechanism for the Catalytic Solvolysis
of AmmoniaꢀBorane Promoted by [Ir(I Bu ) ][PF ] (3)
5
10
4
4
t
0
2
is available free of charge via the Internet at http://pubs.acs.org.
6
’
AUTHOR INFORMATION
Corresponding Author
*
E-mail: snolan@st-andrews.ac.uk.
’
ACKNOWLEDGMENT
The ERC (Advanced Investigator Award FUNCAT) is grate-
fully acknowledged for support. S.P.N. is a Royal Society-
Wolfson Research Merit Award holder.
’
REFERENCES
(1) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010,
110, 4079–4124.
(
(
2) Smythe, N. C.; Gordon, J. C. Eur. J. Inorg. Chem. 2010, 509–521.
3) Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010,
4
9, 7170–7179.
4) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem.
Soc. Rev. 2009, 38, 279–293.
5) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007,
613–2626.
(
(
2
(
(
6) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116–8118.
7) (a) Kelly, H. C.; Marchelli, F. R.; Giutso, M. B. Inorg. Chem.
1964, 3, 431–437. (b) Ryschkewitsch, G. E. J. Am. Chem. Soc. 1960,
82, 3290–3294. (c) Kelly, H. C.; Marriott, V. B. Inorg. Chem. 1979,
18, 2875–2878. (d) D’Ulivo, A.; Onor, M.; Pitzalis, E. Anal. Chem. 2004,
76, 6342–6352.
(8) Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190–194.
If a significant amount of hydrogen is present, complex 5
would be converted to the dihydride complex 6. These com-
plexes would serve as the resting state of the active catalyst.
Dissociation of the weak ammonia ligand from complex 5
followed by ligation of AB completes the catalytic cycle. Likewise,
complexation of AB to 6 could result in a species similar to those
(9) Clark, T. J.; Whittell, G. R.; Manners, I. Inorg. Chem. 2007,
6, 7522–2527.
4
(10) Graham, T. W.; Tsang, C.-W.; Chen, X.; Guo, R.; Jia, W.; Lu, S.-
M.; Sui-Seng, C.; Ewart, C. B.; Lough, A.; Amoroso, D.; Abdur-Rashid,
K. Angew. Chem. Int., Ed. 2010, 49, 8708–8711.
(
11) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.;
Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516–3526.
12) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan,
22
disclosed by the Aldridge group.
(
An interesting aspect of the reaction is that 3 mol of H is
2
S. P. Angew. Chem., Int. Ed. 2005, 44, 2512–2515.
(13) Cavallo, L.; Nolan, S. P.; Jacobsen, H. Can. J. Chem. 2009,
87, 1362–1368.
produced from 1 mol of AB. Half of the evolved hydrogen is
obtained from H O. If the ultimate fate of the released H is
2
2
5
491
dx.doi.org/10.1021/om2007437 |Organometallics 2011, 30, 5487–5492

Organometallics
ARTICLE
(
14) Fortman, G. C.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Chem.
Commun. 2011, 9723–9725.
15) Reaction conditions: 0.4504 mmol of AB, 500 ppm of catalyst,
carried out at 24 °C in 2.0 mL of THF/H O (1/1). The TOF defined the
(
2
rate of AB consumption as measured by H
2
from t = 0 s to the time that
1
mol of AB = /
more details.
2
(initial mol of AB). See the Supporting Information for
(
16) Loboda, N. V.; Antipin, M. Y.; Akomov, V. M.; Struchkov, Y. T.;
Skvortsov, V. G.; Petrova, O. V.; Molodkin, A. K. Zh. Neorg. Khim. 1993,
8, 1960–1962.
3
(17) Salentine, C. G. Inorg. Chem. 1983, 22, 3920–3924.
(18) Although ammonia is a gas and may affect the pressure
measurement used to monitor the reaction kinetics, the high solubility
of ammonia in water would easily dissolve a large majority of the
produced ammonia. As such, the added gaseous volume of NH was
3
deemed to be negligible in comparison to hydrogen production.
(
(
19) Jia, D.; Zhao, Y.; Pan, S. Chinese Patent CN 101824651, 2010.
20) CCDC-826131 (borate), CCDC-826132 (4), and CCDC-
826133 (5) contain supplementary crystallographic data for this
contribution. These data can be obtained free of charge from The
Cambridge Cryatallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(21) (a) Deeming, A. J.; Doherty, S.; Marshall, J. E.; Powell, J. L.;
Senior, A. M. Dalton Trans. 1993, 1093–1100. (b) Xu, W.; Lough, A. J.;
Morris, R. H. Can. J. Chem. 1997, 75, 475–482. (c) Rais, D.; Bergman,
R. G. Chem. Eur. J. 2004, 10, 3970–3978. (d) Hughes, R. P.; Smith, J. M.;
Incarvito, C. D.; Lam, K.-C.; Rhatigan, B.; Rhein-gold, A. L. Organome-
tallics 2004, 23, 3362–3362. (e) Vicente, J.; Chicote, M. T.; Vicente-
Hern ꢀa ndez, I.; Bautista, D. Inorg. Chem. 2008, 47, 9592–9605. (f)
Heiden, Z. M.; Gorecki, B. J.; Rauchfuss, T. B. Organometallics 2008,
27, 1542–1549. (g) Blacker, A. J.; Brown, S.; Clique, B.; Gourlay, B.;
Headley, C. E.; Ingham, S.; Ritson, D.; Screen, T.; Stirling, M. J.; Taylor,
D.; Thompson, G. Org. Process Res. Dev. 2009, 13, 1370–1378. (h)
Salomon, M. A.; Jungton, A.-K.; Braun, T. Dalton Trans. 2009, 7669–
7677.
(
22) Tang, C. Y.; Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc.
2
010, 132, 10578–10591.
5
492
dx.doi.org/10.1021/om2007437 |Organometallics 2011, 30, 5487–5492