Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a Group 13 Metalloligand: Remarkable Effect of an Aluminum-Metalloligand in Hydrosilylation of CO2

Article abstract of DOI:10.1021/jacs.7b02553

Efficient synthesis and catalysis of a series of palladium complexes having a group 13 metalloligand (Al, Ga, In) are reported utilizing 6,6-bis(phosphino)terpyridine as a new scaffold for Pd-E bonds (E = Al, Ga, In). Systematic investigation revealed unique characteristics of the Al-metalloligand in both structure and reactivity, which exhibited the highest catalytic activity for hydrosilylation of CO₂ ever reported (TOF = 19 300 h^-1). This study demonstrated fine-tuning of catalyst activity by the precisely designed metalloligand is a promising approach for new catalyst development in synthetic organometallic chemistry.

Full text of DOI:10.1021/jacs.7b02553

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Communication
Synthesis, Structure, and Catalysis of Palladium Complexes
Bearing a Group 13 Metalloligand: Remarkable Effect of
an Aluminum-Metalloligand in Hydrosilylation of CO2
Jun Takaya, and Nobuharu Iwasawa
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 19 Apr 2017
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Synthesis, Structure, and Catalysis of Palladium Complexes Bearing
a Group 13 Metalloligand: Remarkable Effect of an Aluminum-
Metalloligand in Hydrosilylation of CO2
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Jun Takaya* and Nobuharu Iwasawa*
Department of Chemistry, School of Science, Tokyo Institute of Technology, Oꢀokayama, Meguroꢀku, Tokyo 152ꢀ8551,
Japan
In 2011, Lu and coꢀworkers reported an efficient synꢀ
thesis of various transition metal complexes having aluꢀ
minum as a metalloligand utilizing a threeꢀfold N,Pꢀ
ABSTRACT: Efficient synthesis and catalysis of a seꢀ
ries of palladium complexes having a group 13 metalꢀ
loligand (Al, Ga, In) are reported utilizing 6,6”ꢀ
bis(phosphino)terpyridine as a new scaffold for Pd–E
bonds (E = Al, Ga, In). Systematic investigation reꢀ
vealed unique characteristics of the Alꢀmetalloligand
in both structure and reactivity, which exhibited highꢀ
6a
multidentate ligand. They also demonstrated reactivity
tuning of Ni complexes by varying group 13 metalloligꢀ8
ands (Al, Ga, In) for catalytic hydrogenation of alkenes.
However, the catalytic activity was not very high (TOF
–1
=
ca. 12 h ) compared to commonly employed transiꢀ
est catalytic activity for hydrosilylation of CO ever
2
tion metal catalysts. We envisioned that the suitable deꢀ
sign of a scaffold for M–E bonds satisfying both synꢀ
thetic ease and high reactivity is strongly required to
realize efficient catalysis by group 13 metalloligands.
Herein we report 6,6”ꢀbis(phosphino)terpyridine as an
efficient scaffold enabling a facile synthesis of a series
of palladium complexes having a group 13 metalloligꢀ
and (Al, Ga, In). A remarkable effect of an Alꢀ
metalloligand is also demonstrated in hydrosilylation of
–
1
reported (TOF = 19300 h ). This study demonstrated
fine tuning of catalyst activity by the precisely designed
metalloligand is a promising approach for new catalyst
development in synthetic organometallic chemistry.
Development of new supporting ligands for transition
metals has been an important challenge in synthetic orꢀ
ganometallic chemistry. Reactivity of metals can be fineꢀ
ly tuned by appropriate choice of elements and ligand
structure, leading to high activity and new catalysis. Reꢀ
cently, group 13 elements have emerged as a new class
of supporting ligands by utilizing rationally designed
multidentate ligands as a scaffold for a M–E bond (E =
CO , achieving the highest catalytic efficiency ever reꢀ
2
ported.
1
group 13). Preꢀintroduction of a group 13 element into
the ligand followed by complexation with a metal (M)
potentially enabled a facile synthesis of a variety of M–
E bonds. The group 13 element (E) worked as a σꢀ
acceptor ligand via dative bonding from the metal (M) to
the ligand (E), realizing reversal of the role of the metal
2
and the ligand. Strong trans influence and cooperative
3
molecular activation were also reported. However, theꢀ
se studies mostly focused on boron, and utilization of
heavier group 13 elements as a metalloligand has been
4
ꢀ7
rather limited (Figure 1). This is partly due to difficulꢀ
ty of incorporating and handling highly Lewis acidic,
reactive metals as a supporting ligand for transition metꢀ
als. In particular, the systematic synthesis and evaluation
of a series of group 13 metalloligands have remained a
formidable challenge despite the possibility of discoverꢀ
ing and understanding unique catalysis by these metalꢀ
loligands.
Figure 1. Various scaffolds for group 13 metalloligands
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–
Figure 2. ORTEP drawings of 2a and 3a at 30% probability level. [AlCl ] for 2a and hydrogen atoms are omitted for clarity.
4
We focused on 6,6''ꢀbis(diphenylphosphino)ꢀ2,2':6',2''ꢀ
terpyridine 1 as a scaffold for M–E bonds (E = Al, Ga, I
Scheme 1. Synthesis of a series of Pd complexes hav-
ing a group 13 metalloligand (E = Al, Ga, In)
9
n). We expected the terpyridineꢀbased rigid and planar
N P ꢀstructure would hold two metals tightly to stabilize
3
2
the M–E bond effectively. Terpyridine is one of the most
widely utilized polynitrogen ligand motifs, thus enabling
incorporation of a wide range of metals as a metalloligꢀ
and. In terms of reactivity, the planar, pincerꢀtype coorꢀ
dination of the obtained metalloligand to a transition
metal would allow easier access of substrates than the
Lu’s triphosphine ligand, leading to high catalytic activiꢀ
1
0
ty while keeping sufficient stability.
The 6,6''ꢀbis(diphenylphosphino)ꢀ2,2':6',2''ꢀterpyridine
was easily synthesized by S Ar reaction of lithium
1
N
diphenylphosphide to 6,6”ꢀdibromoterpyridine (See the
Supporting Information). Incorporation of aluminum
into the terpyridine moiety was achieved by treatment of
1 with an excess amount of AlCl in THF at room temꢀ
3
perature, affording a terpyridineꢀaluminum complex 2a
in 93% yield (Scheme 1). The structure of 2a was deꢀ
termined by Xꢀray analysis to be a trigonal bipyramidal,
The Pd–Al distance of 3a (2.461(1) Å) is slightly longꢀ
than
Bu)(PCy
er
that
of
)Pd(AlCl ) complex (2.3790(6) Å) having an
3
previously
reported
(It-
1
1
3
–
cationic N,N,NꢀAlCl complex having a [AlCl ] as a
unsupported dative bond from Pd to Al. This is probably
due to the rather restricted structure of the scaffold in 3a.
The Pd–E distances are shorter than the sum of the corꢀ
responding covalent radii, supporting bonding between
Pd and group 13 metals. Notable structural differences
between Al, Ga, and Inꢀmetalloligands were found in the
Pd–Cl bond length (Table 1). The Pd–Cl bond of 3a is
largely elongated to 2.5475(9) Å compared to those of
3b and 3c (2.457(1) Å for 3b, 2.426(1) Å for 3c). This is
the longest value among those of pincerꢀtype palladium
chloride complexes, suggesting the Pd–Cl bond is
highly destabilized by the Alꢀmetalloligand. Interestingꢀ
ly, the Pd–Cl bond lengths are seemingly in inverse proꢀ
portion to the radii of group 13 metals. Moreover, the P
NMR chemical shift of 3a is largely deviated from those
of 3b and 3c (δ = 42.8 as a singlet for 3a vs. 63.9 for 3b
and 68.3 for 3c), invoking unique reactivity of aluminum
2
4
counter anion (Figure 2a). The distance between two
phosphorous atoms is ca. 4.88 Å, which would be suitaꢀ
ble for complexation with various late transition metals.
The reaction of 2a with Pd (dba) proceeded smoothly at
12
2
3
room temperature to give a palladium complex 3a bearꢀ
ing an Alꢀmetalloligand in 67% yield as a pale yellow
solid. Recrystallization of 3a from DMFꢀTHF afforded a
single crystal suitable for Xꢀray analysis, and the
ORTEP diagram is depicted in Figure 2b and 2c. The
geometry around palladium is square planar and alumiꢀ
num is octahedral, forming a tetracoordinate palladiꢀ
um(II) chloride complex having a PAlPꢀpincer type ligꢀ
and. Furthermore, corresponding gallium and indium
analogues 3b and 3c were also prepared successfully
according to the similar procedure, demonstrating synꢀ
thetic utility of this method for preparation of a variety
of metalloligands and their metal complexes (Scheme 1).
Structures of 2b, 2c and 3b, 3c were also fully characterꢀ
ized by NMR and Xꢀray analyses to be almost the same
with those of 2a and 3a (see SI).
13
31
14
among the heavier group 13 metalloligands. More deꢀ
tailed investigation on the bonding and electronic propꢀ
erty of palladium complexes 3 is described in supporting
information.
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The AlꢀPd complex 3a exhibited not only unique reacꢀ
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tivity differences in Al triad, but also the highest catalytꢀ
ic activity among known catalysts for hydrosilylation of
Table 1. Structural data of E–Pd complexes 3
–3
CO . Over 90% conversion was achieved with 4.8 x 10
2
mol% of 3a (s/c = ca. 21000) at 25 ˚C under 1 atm CO2,
affording silyl formate in 92% yield (Scheme 2). The
–1
turnover frequency (TOF) reached 19300 h , which is
the highest value ever reported. Previously, Baba reportꢀ
ed a Cuꢀdiphosphine complex as one of the most active
–1
catalysts for hydrosilylation of CO (TOF = 10,300 h )
2
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although the reaction required heating at 60 ˚C to afford
a
18
E =
Al (3a)
Ga (3b)
In (3c)
3
1% of product after 6 h. Therefore, the AlꢀPd comꢀ
Pd–Cl / Å 2.5475(9)
2.457(1)
2.3956(7)
2.289(1)
2.293(1)
2.426(1)
2.4802(5)
2.321(1)
2.347(1)
plex 3a clearly achieved the most efficient hydrosilylaꢀ
tion of CO proceeding at ambient temperature under 1
2
Pd–E / Å
2.461(1)
1
atm CO
2
, demonstrating high practical utility of the Alꢀ
Pd–P / Å 2.273(1)
19
Pd catalyst for CO ꢀfixation. Although the detailed
2
2
Pd–P / Å 2.2898(8)
reaction mechanism is not clear yet, the unique electronꢀ
ic property of the Alꢀmetalloligand would play a key
role for the high catalytic activity as suggested in the
structural evaluation. Further experimental and theoretiꢀ
cal investigations on the reaction mechanism and origin
of the high reactivity of the Alꢀmetalloligand are in proꢀ
gress.
31
b
c
c
δ
P
42.8
63.9
68.3
a
The unit cell contained four independent molecules, and
bond lengths of one of them are depicted. The lengths and
angles of those four molecules are in a range of Pd–Cl =
2
.510(1)ꢀ2.54759(9) Å, Pd–Al = 2.455(1)ꢀ2.472(1) Å, Pd–
b
c
P = 2.271(1)ꢀ2.2798(8) Å. in D O. in DMSO.
2
Table 2. Evaluation of catalytic activity for hydrosilylation
After extensive screening of the reactivity of a series of
palladium complexes, the AlꢀPd complex 3a was found
to be an active catalyst for hydrosilylation of carbon
a
of CO2
dioxide. CO is an abundant, but less reactive molecule,
2
and its utilization as a C1 source has been a challenging
14
task in current chemistry. In particular, hydrosilylation
of CO is widely investigated strategies for converting
2
CO to useful compounds such as silyl formates, methꢀ
2
oxysilanes, and methane because the reaction is thermoꢀ
dynamically favorable and proceeds under neutral condiꢀ
Entry Catalyst
Time
3 h
Yield / %
b
1
2
3
4
5
6
7
8
9
3a
quant.
15
tions. Numerous catalysts including metals and orꢀ
–
3 h
n.d.
n.d.
n.d.
2%
ganocatalysts have been developed, however, there still
remain several problems in efficiency, reaction condiꢀ
3a w/o Csꢀpivalate
2a
3 h
tions, and product selectivity toward practical CO ꢀ
24 h
24 h
24 h
24 h
24 h
24 h
24 h
2
16
c
fixation. We found that in the presence of 0.1 mol% of
Pd (dba) + 1 + AlCl
3
2
2
3
a and 1.0 mol% of cesium pivalate as an additive, the
3b
3c
10%
7%
reaction of dimethylphenylsilane with atmospheric presꢀ
sure of CO in DMF proceeded smoothly at 25 ˚C, givꢀ
2
ing silyl formate in quantitative yield (Entry 1, Table
5% Pd/C
17%
4%
17
d
2
). The reaction was highly selective for formate forꢀ
Pd(dba)2
mation, and no over reduction was observed. Both 3a
and a pivalate salt were crucial for this reaction although
the role of the pivalate is not clear yet (Entries 2ꢀ4). In
situ preparation of 3a in DMF was not successful (Entry
d
10
PdCl (PhCN)2
27%
2
a
Dimethylphenylsilane (1.63 mmol), cesium pivalate (1.5
–2
–3
x 10 mmol), and catalyst (1.5 x 10 mmol) in DMF (1.0
mL) under CO (1 atm, balloon). The yield was calculated
5
). It should be noted that other Pd complexes having a
2
based on the amount of formic acid obtained after hydrolyꢀ
Gaꢀ or an Inꢀmetalloligand 3b and 3c afforded silyl forꢀ
mate in low yield even after extended reaction time (Enꢀ
tries 6 and 7), highlighting uniquely high reactivity of
the Alꢀmetalloligand among group 13 metals. Common
b
c
sis. >95% silane was consumed in 1 h by GCꢀanalysis.
–3
–3
Pd (dba) (0.7 x 10 mmol), 1 (1.6 x 10 mmol), and
2
3
–3
d
AlCl (3.0 x 10 mmol) were used. In the presence of 1
3
–3
(
1.6 x 10 mmol).
palladium complexes such as Pd/C, Pd(dba) , and
2
PdCl (PhCN) were not effective to promote the hydrosꢀ
Scheme 2. Large scale reaction catalyzed by 3a
2
2
ilylation (Entries 8ꢀ10).
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queu, K.; Bouhadir, G.; Bourissou, D. Organometallics 2013,
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32, 6780. (c) Sircoglou, M.; Mercy, M.; Saffon, N.; Coppel, Y.;
Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem. Int. Ed.
2
009, 48, 3454. (d) Derrah, E. J.; Sircoglou, M.; Mercy, M.;
Ladeira, S.; Bouhadir, G.; Miqueu, K.; Maron, L.; Bourissou, D.
Organometallics 2011, 30, 657.
(5) Halide or alkyl abstraction and coordination to Al were reported
in many cases. For selected examples, see; (a) Devillard, M.; Niꢀ
colas, E.; Appelt, C.; Backs, J.; MalletꢀLadeira, S.; Bouhadir, G.;
Slootweg, J. C.; Uhl, W.; Bourissou, D. Chem. Commun. 2014,
In conclusion, 6,6''ꢀbis(diphenylphosphino)ꢀ2,2':6',2''ꢀ
terpyridine was established as an efficient scaffold for
formation of M–E bonds, realizing a facile synthesis of a
series of Pd complexes with group 13 metalloligands.
The systematic evaluation of group 13 metals disclosed
uniquely high catalytic activity of the Alꢀmetalloligand,
achieving the highest efficiency in hydrosilylation of
5
0, 14805. (b) Boudreau, J.; Fontaine, F.ꢀG. Organometallics
2011, 30, 511. (c) Thibault, M.ꢀH.; Boudreau, J.; Mathiotte, S.;
Drouin, F.; Sigouin, O.; Michaud, A.; Fontaine, F.ꢀG. Organo-
metallics 2007, 26, 3807. (d) Fontaine, F.ꢀG.; Zargarian, D. J.
Am. Chem. Soc. 2004, 126, 8786.
0
1
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0
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5
6
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0
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2
3
4
5
6
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0
(
6) Alꢀmetalloligands: (a) Rudd, P. A.; Liu, S.; Gagliardi, L.;
Young, V. G., Jr.; Lu, C. C. J. Am. Chem. Soc. 2011, 133,
20724. (b) Rudd, P. A.; Planas, N.; Bill, E.; Gagliardi, L.; Lu, C.
C. Eur. J. Inorg. Chem. 2013, 3898. (c) Cowie, B. E.; Tsao, F.
A.; Emslie, D. J. H. Angew. Chem. Int. Ed. 2015, 54, 2165. (d)
Devillard, M.; Nicolas, E.; Ehlers, A. W.; Backs, J.; Malletꢀ
Ladeira, S.; Bouhadir, G.; Slootweg, J. C.; Uhl, W.; Bourissou,
D. Chem. Eur. J. 2015, 21, 74. (e) Devillard, M.; Declercq, R.;
Nicolas, E.; Ehlers, A. W.; Backs, J.; SaffonꢀMerceron, N.;
Bouhadir, G.; Slootweg, J. C.; Uhl, W.; Bourissou, D. J. Am.
Chem. Soc. 2016, 138, 4917.
CO . This study demonstrated high utility of the preciseꢀ
2
ly designed metalloligand for new catalyst development
in synthetic organometallic chemistry. Further mechanisꢀ
tic studies and application to other catalytic reactions are
ongoing in our group.
(
7) Formation of metal complexes featuring a Ga–Au and an In–Pd
bond were reported in ref 4c and 4d.
ASSOCIATED CONTENT
(8) Cammarota, R. C.; Lu, C. C. J. Am. Chem. Soc. 2015, 137,
2486.
Supporting Information. Preparative methods and specꢀ
tral and analytical data. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
(9) Formation of a cyclic, binuclear bis(phosphine)rhodium complex
without a metal–metal bond was briefly mentioned utilizing a reꢀ
lated bis(phosphino)terpyridine derivative. Ziessel, R., Tetrahe-
dron Lett. 1989, 30, 463.
10) The pincer type metal complexes are well recognized as reactive
catalysts in synthetic chemistry. See: (a) van der Boom, M. E.;
Milstein, D. Chem. Rev. 2003, 103, 1759. (b) Selander, N.;
Szabó. K, J. Chem. Rev. 2011, 111, 2048. (c) Pincer and Pincer-
Type Complexs; Szabó, K. J.; Wendt, O. F. Ed.; WileyꢀVHC:
Weinheim, Germany, 2014.
11) Bauer, J.; Braunschweig, H.; Damme, A.; Gruβ, K.; Radacki, K.
Chem. Commun. 2011, 47, 12783.
12) Cordero, B.; Gómez, V.; PlateroꢀPrats, A. E.; Revés, M.; Echeꢀ
verría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans.
2008, 2832.
(
AUTHOR INFORMATION
Corresponding Author
takayajun@chem.titech.ac.jp
niwasawa@chem.titech.ac.jp
(
(
ACKNOWLEDGMENT
This research was supported by JSPS KAKENHI Grant
Number JP15H05800, 15KT0059, and 26620024.
(
13) (a) Tsang, M. Y.; Viñas, C.; Teixidor, F.; Planas, J. G.; Conde,
N.; SanMartin, R.; Herrero, M. T.; Domínguez, E.; Lledós, A.;
Vidossich, P.; ChoquesilloꢀLazarte, D. Inorg. Chem. 2014, 53,
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(16) Recent examples, see; (a) Scheuermann, M. L.; Semproni, S. P.;
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kura, K.; Naijo, M.; Yamaguchi, S.; Miyaji, A.; Baba, T. Chem.
Lett. 2015, 44, 1217. (c) Motokura, K.; Naijo, M.; Yamaguchi,
S.; Miyaji, A.; Baba, T. Chem. Lett. 2015, 44, 1464. (d)
Metsänen, T. T.; Oestreich, M. Organometallics 2015, 34, 543.
(e) Chen, J.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E.
Y.ꢀX. J. Am. Chem. Soc. 2016, 138, 5321.
4
5, 5254. (b) Fontaine, F.ꢀG.; Boudreau, J.; Thibault, M.ꢀH. Eur.
J. Inorg. Chem. 2008, 5439. (c) Yamashita, M. Bull. Chem. Soc.
Jpn. 2016, 89, 269. (d) Maity, A.; Teets, T. S. Chem. Rev. 2016,
1
2
16, 8873. (e) Bouhadir, G.; Bourissou, D. Chem. Soc. Rev.
016, 45, 1065.
(
(
2) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859.
3) For selected examples, see; (a) Zhu, J.; Lin, Z.; Marder, T. B.
Inorg Chem 2005, 44, 9384. (b) Segawa, Y.; Yamashita, M.;
Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201. (c) Harman, W.
H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080. (e) Harman,
W. H.; Lin, T.ꢀP.; Peters, J. C. Angew. Chem. Int. Ed. 2014, 53,
1
1
081. (f) Lin, T.ꢀP.; Peters, J. C. J. Am. Chem. Soc. 2014, 136,
3672.
(17) DMSO and EtMe SiH were also employable for the reaction
2
+
–
(4) Formation of zwitterionic [M] [E] complexes by intramolecular
abstraction of Cl from M over the formation of M–E bonds was
instead of DMF and Me PhSiH. See SI.
2
(18) (a) Motokura, K.; Kashiwame, D.; Takahashi, N.; Miyaji, A.;
Baba, T. Chem. Eur. J. 2013, 19, 10030. (b) Motokura, K.;
Kashiwame, D.; Miyaji, A.; Baba, T. Org. Lett. 2012, 14, 2642.
(19) See SI for comparison with previously reported metal catalysts
for hydrosilylation of CO2.
i
i
reported when using (oꢀ( Pr
2 6 4 2 2 6 4 3
P)C H ) AlCl or (oꢀ( Pr P)C H ) E
(
E = Al, Ga, In) as a metalloligand precursor. See: (a) Sircoglou,
M.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Bourissou, D. Organ-
ometallics 2008, 27, 1675. (b) Sircoglou, M.; Saffon, N.; Miꢀ
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