Triphenylphosphine-cyclometallated iridium(III) pyrimidine complexes: Synthesis, crystal structures and application in α-alkylation of ketones with alcohols

Article abstract of DOI:10.1016/j.jorganchem.2011.11.033

Two triphenylphosphine (PPh₃)-cyclometallated iridium(III) pyrimidine complexes Ir(NDMP)₂PPh₃Cl 1 and Ir(NDMP)(PPh₃)₂Cl₂ 2 (NDMP = 2-(2-naphthyl)-4,6- dimethyl-pyrimidine) were synthesized and characterized by NMR, IR, ESI-MS, and elemental analysis. Additionally, their detailed structures have been determined by X-ray single-crystal diffraction. The complex 1 was a PPh₃-biscyclometalated Ir(III) complex, while 2 was a PPh ₃-monocyclometalated Ir(III) complex. These complexes were found to be efficient catalysts for α-alkylation of ketones with alcohols.

Full text of DOI:10.1016/j.jorganchem.2011.11.033

Journal of Organometallic Chemistry 700 (2012) 214e218
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Triphenylphosphine-cyclometallated iridium(III) pyrimidine complexes:
Synthesis, crystal structures and application in
alcohols
a-alkylation of ketones with
,
b
a
b
b
b
a
Chen Xu a , Xin-Ming Dong , Zhi-Qiang Wang , Xin-Qi Hao , Zhen Li , Lu-Meng Duan , Bao-Ming Ji ,
*
Mao-Ping Song b
,
*
a College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, China
b Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two triphenylphosphine (PPh3)-cyclometallated iridium(III) pyrimidine complexes Ir(NDMP)2PPh3Cl 1
and Ir(NDMP)(PPh3)2Cl2 2 (NDMP ¼ 2-(2-naphthyl)-4,6- dimethyl-pyrimidine) were synthesized and
characterized by NMR, IR, ESI-MS, and elemental analysis. Additionally, their detailed structures have
been determined by X-ray single-crystal diffraction. The complex 1 was a PPh3-biscyclometalated Ir(III)
complex, while 2 was a PPh3-monocyclometalated Ir(III) complex. These complexes were found to be
Received 1 October 2011
Received in revised form
20 November 2011
Accepted 25 November 2011
efficient catalysts for a-alkylation of ketones with alcohols.
Keywords:
Ó 2011 Elsevier B.V. All rights reserved.
Triphenylphosphine-cyclometallated Ir(III)
complex
Crystal structure
a
-Alkylation
Ketone
Alcohol
1. Introduction
Cyclometalated Ir(III) complexes have attracted considerable
attention in molecular materials [19,20] and organic synthesis
[13,14]. For example, bis(imino)aryl Ir(III) pincer complexes were
used as the catalysts for the transfer hydrogenation of acetophenone
and benzophenone [21]. To the best of our knowledge, there are no
The
a-alkylation of ketones with electrophiles such as alkyl
halides is the conventional method to form CeC bonds [1]. In recent
years, alcohols have emerged as alternative alkylating agents of
carbonyl compounds by transition-metal catalysts [2e5]. This
strategy can improve both the atom efficiency and the regiose-
lectivity of the process producing only water as a side product.
Several groups have reported the use of Ru [6e8], Pd [9e11] and Ni
[12] catalysts for the
addition, Ir complexes serve as efficient cataltsts for hydrogen
transfer from alcohols to aldehydes [13,14], and they have been
utilized in a-alkylation of ketones. Among them, [Ir(cod)Cl]2 has
reports concerning cyclometalated Ir(III) complexes-catalyzed
a-
alkylation of ketones. In recent years, part of our research effort has
focused on the synthesis and application of cyclometalated
complexes [22e24]. In a preliminary communication [25], we
synthesized a new naphthalenylpyrimidine NDMP and its Ir(III)
complexes. In view of these findings and our continuous interest in
the synthesis and applications of other cyclometallated NDMP Ir(III)
complexes, we prepared two PPh3-cyclometallated NDMP Ir(III)
complexes Ir(NDMP)2PPh3Cl 1 and Ir(NDMP)(PPh3)2Cl2 2 and
a-alkylation of ketones with alcohols. In
been the most commonly employed Ir catalyst for this reaction
[15e17]. For instance, the above reaction was successfully accom-
plished by Ishii et al. with [Ir(cod)Cl]2/PPh3 catalyst [15]. However,
these reactions require highly catalytic loading and excessive
examined their catalytic activity in the a-alkylation of ketones with
alcohols. The results are presented below.
ligand. Until now, other Ir(III) complexes-catalyzed a-alkylation of
ketones have been less reported [18]. In this regard, convenience,
2. Results and discussion
along with efficacy, is of significance.
2.1. Synthesis and structures of complexes 1e2
* Corresponding authors. Tel./fax: þ86 379 65515113.
E-mail addresses: xubohan@163.com (C. Xu), maopingsong@zzu.edu.cn
(M.-P. Song).
The synthetic routes to complexes 1e2 are depicted in Scheme 1.1
was prepared by reaction of Ir(III) Cl-bridged dimer [Ir(NDMP)2Cl]2
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.11.033

C. Xu et al. / Journal of Organometallic Chemistry 700 (2012) 214e218
215
N
N
N
N
Cl
Cl
N
N
PPh3
Cl
Ir
Ir
Ir
PPh3
2
2
2
(a)
1
N
N
PPh3
N
N
Cl
IrCl3·3H2O
PPh3
Ir
Cl
PPh3
(b)
2
Scheme 1. Synthesis of 1e2.
[25] with PPh3 according to the literature procedure [26]. 2 was also
accessed by cyclometalation reaction of NDMP with IrCl3 and PPh3
[27]. The two complexes are air and moisture stable both in the solid
state and in solution. They were fully characterized by elemental
analysis, IR, 1H NMR, 13C NMR and ESI-MS. In order to further
investigate the structures of these complexes, their detailed struc-
tures have been determined by X-ray single-crystal diffraction. Fig. 1
and Fig. 2 show that 1$CH3CH2OH-2 are PPh3-biscyclometalated and
PPh3-monocyclometalated Ir(III) complexes, respectively. The Ir
atom in 1$CH3CH2OH is located at a distorted octahedral environ-
ment composed of C2N2PCl donors. The two NDMP ligands are cis-
arranged with the two C donors in cis-position and the two N
donors in trans-orientation. However, 2 contains only one NDMP
ligand, the two phosphines occupy the axial site. Both IreN and IreC
bond lengths of 1$CH3CH2OH are similar to those of 2. IreP bond
length of 1$CH3CH2OH are similar to those of the related complexes
[IrCl(arylpyridine)2(PPh3)] [28e30], while it is longer than those of 2
and the related PPh3-monocyclometalated Ir(III) complexes [27,31].
In the crystal of 1$CH3CH2OH, chlorine atom forms OeH/Cl
Fig. 2. Molecular structure of complex 2. Displacement parameters are drawn at the
50% level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (ꢀ):
Ir(1)eC(3) 2.029(3), Ir(1)eN(1) 2.127(3), Ir(1)eP(1) 2.3927(9), Ir(1)eP(2) 2.3672(9),
Ir(1)eCl(1) 2.3755(8), Ir(1)eCl(2) 2.5045(9), and P(1)eIr(1)eP(2) 176.46(3), Cl(1)e
Ir(1)eN(1) 172.30(7), Cl(2)eIr(1)eC(3) 175.77(9), N(1)eIr(1)eC(3) 80.95(11), P(1)e
Ir(1)eN(1) 91.25(7), P(2)e Ir(1)eN(1) 91.31(7).
hydrogenbond(the distanceis2.457 Å)with theadjacent OeH group
of ethanol. In the crystal of 2 there exist two types of intermolecular
CeH/Cl hydrogen bonds (the distances are 2.845 Å and 2.937 Å)
(Fig. 3).
2.2. PPh3-cyclometalated Ir(III) complexes-catalyzed
ketones with alcohols
a-alkylation of
Ishii reported direct a-alkylation of ketones and acetates with
primary alcohols catalyzed by an [Ir(cod)Cl]2/PPh3/base system
[15,17]. The reaction is thought to proceed via the following three
key steps: oxidation of the primary alcohol, base-catalyzed aldol
condensation and selective hydrogenation of
a,b-unsaturated
ketones and esters. Based on these results and in connection with
our experience in the synthesis and application of cyclometalated
Ir(III) complexes [22,25], we were interested to see whether the
obtained PPh3-cyclometalated Ir(III) complexes would be efficient
catalysts for the a-alkylation of ketones with alcohols. Initially, the
use of complex 2 as catalyst for the oxidation of benzyl alcohol was
examined. Using 2 mol% of 2 in the presence of 0.5 equivalent of
NaOH as base in dioxane at 110 ꢀC provided benzaldehyde in an
excellent yield (96%).
In the following experiments, the a-alkylation of acetophenone
with benzyl alcohol was carried out with various bases and solvents
in the presence of 2 mol% of 2. The results from this study are
summarized in Table 1. After screening a variety of bases (e.g.,
NaOH, KOH, Na2CO3, K2CO3 and t-BuOK) (entries 1e5), NaOH and
KOH were found to give the good results (80 and 78% yield),
Na2CO3, and K2CO3 displayed moderate efficiency, producing the
products in 58 and 60% yield, respectively. Among the tested
solvents, dioxane was much better than other solvents such as
toluene and THF (entries 6e7). Furthermore, the activities of
several related catalytic systems in the same model reaction were
investigated. The Ir(III) Cl-bridged dimer [Ir(NDMP)2Cl]2 generated
the product only in 42% yield under the same conditions (entry 8).
Fig. 1. Molecular structure of complex 1$CH3CH2OH. Displacement parameters are
drawn at the 50% level. Non-hydrogen bonding H atoms are omitted for clarity.
Selected bond lengths (Å) and angles (ꢀ): Ir(1)eC(1) 2.019(8), Ir(1)eC(17) 1.986(9),
Ir(1)eN(2) 2.106(7), Ir(1)eN(3) 2.108(7), Ir(1)eCl(1) 2.529(2), Ir(1)eP(1) 2.483(2), and
C(17)eIr(1)eCl(1) 174.6(2), C(1)e Ir(1)eP(1) 169.4(2), N(2)eIr(1)eN(3) 168.9(2),
C(17)eIr(1)eN(3) 80.4(3), N(3)eIr(1)eP(1) 87.86(18), C(1)eIr(1)eN(3) 89.9(3).

216
C. Xu et al. / Journal of Organometallic Chemistry 700 (2012) 214e218
Fig. 3. Partial packing view of complex 2 showing the CeH/Cl hydrogen bonds. Non-hydrogen bonding H atoms are omitted for clarity.
Increasing catalyst loading to 2 mol% (Ir 4 mol%) gave 70% yield
(entry 9). The yield was greatly improved by the addition of PPh3
suggesting that PPh3 participated in the catalytic cycles (entry 10).
Additionally, 1 also showed comparable activity producing the
product in 75% yield (entry 11). However, the activities of 1 and
[Ir(NDMP)2Cl]2/PPh3 system were still lower than that of 2.
entry 12), while the reaction with RuCl2(DMSO)4 gave the expected
product with only a very low yield (17%) under the same reaction
conditions [8].
3. Conclusions
Under the optimized reaction conditions (2, NaOH, dioxane),
various ketones were subjected to react with primary alcohols in
order to investigate the reaction scope and several representative
results are summarized in Table 2. The reaction also gave good
results using acetophenone and 4-chlorophenylmethanol (entry 1).
For 3-phenylpropanol and 1-butanol, the yields decreased in 62
and 53% yields (entries 2e3). The alkylations of 4-
chlorophenylethanone and p-tolylethanone were not significant
affected by the position and electronic nature of the substituent on
the aromatic ring of ketones (entries 4e8). In the case of dialkyl
ketone, the product yields were lower than those of the case of
alkyl aryl ketones, the alkylation took place with complete regio-
selectivity at less-hindered position over methyl (entries 9e11).
Finally, it was noteworthy that the reaction of acetylferrocene with
benzyl alcohol also gave the expected product in a good yield (83%,
In summary, we have prepared and characterized two PPh3-
cyclometallated iridium(III) pyrimidine complexes 1e2. Their
detailed structures have been determined by X-ray single-crystal
diffraction. These complexes were found to be efficient catalysts
for a-alkylation of ketones with alcohols.
4. Experimental section
4.1. General procedures
Solvents were dried and freshly distilled prior to use. All other
chemicals were commercially available expect for ligand NDMP and
its cyclometalated Ir(III) Cl-bridged dimer [Ir(NDMP)2Cl]2 which
were prepared according to the published procedure [25]. Melting
points were measured using a WC-1 microscopic apparatus and
were uncorrected. IR spectra were collected on a Bruker VECTOR22
spectrophotometer in KBr pellets. NMR spectra were recorded on
a Bruker DPX-400 spectrometer in CDCl3 with TMS as an internal
standard. Mass spectra were measured on an LC-MSD-Trap-XCT
instrument. Elemental analyses were determined with a Carlo
Erba 1160 Elemental Analyzer.
Table 1
Influence of base, solvent and catalyst on the
a
-alkylation of acetophenone with
benzyl alcohol.a
O
O
Cat
+
Base, Solvent
HO
4.2. Synthesis of Ir(NDMP)2PPh3Cl 1
Entry
Catalyst (mol%)
Base
Solvent
Yield (%)b
A
solution of cyclometalated Ir(III) Cl-bridged dimer
1
2
3
4
5
6
7
8
2 (2)
2 (2)
2 (2)
2 (2)
2 (2)
2 (2)
2 (2)
NaOH
KOH
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Toluene
THF
Dioxane
Dioxane
Dioxane
Dioxane
80
78
58
60
46
47
20
42
70
72
75
[Ir(NDMP)2Cl]2 (0.1 mmol) and PPh3 (0.22 mmol) in dichloro-
methane (15 mL) was stirred at room temperature for 2 h. The
product 1 was separated by passing through a short silica gel
column with CH2Cl2/ethyl acetate (3:1) as eluent. Yellow solid,
yield: 82%, m.p.>300 ꢀC. IR (KBr, cmꢁ1): 2922, 2360, 1616, 1593,
1532, 1465, 1435, 1372, 1186, 1142, 1017, 959, 877, 794, 739, 721. The
Na2CO3
K2CO3
t-BuOK
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
[Ir(NDMP)2Cl]2 (1)
[Ir(NDMP)2Cl]2 (2)
[Ir(NDMP)2Cl]2/PPh3 (1/2)
1 (2)
1H NMR (400 MHz, CDCl3):
d 8.65 (s, 2H, AreH), 7.96 (d, 2H, AreH),
9
10
11
7.78 (m, 5H, AreH), 7.70 (s, 2H, AreH), 7.52 (t, 2H, AreH), 7.42 (t, 2H,
AreH), 7.22 (m, 8H, AreH), 7.09 (s, 2H, AreH), 6.65 (s, 2H, AreH),
6.56 (s, 2H, AreH), 2.68 (s, 6H, eCH3), 2.40 (s, 6H, eCH3). 13C NMR
(100 MHz, CDCl3): 136.4, 135.8, 135.5, 134.8, 134.7, 134.6, 130.4,
130.2, 129.3, 128.9, 127.4, 127.3, 127.2, 127.1, 126.2, 126.0, 119.7, 29.4,
a
Reaction conditions: acetophenone (1.0 mmol), benzyl alcohol (2.0 mmol), base
(1.0 mmol), solvent (3 mL), 110 ꢀC, 12 h.
b
Isolated yields (average of two experiments).

C. Xu et al. / Journal of Organometallic Chemistry 700 (2012) 214e218
217
Table 2
-Alkylation of ketones with alcohols catalysed by complex 2.a
a
Entry
Ketone
Alcohol
Product
Yield (%)b
O
O
O
O
1
Cl
81
62
Cl
OH
OH
2
O
O
nC3H7
nC3H7
3
4
53
82
OH
O
O
OH
O
O
O
5
6
Cl
85
66
Cl
OH
OH
O
O
O
O
7
8
76
78
Cl
Cl
Cl
Cl
OH
O
Cl
Cl
OH
O
O
O
9
72
75
45
OH
O
10
11c
Cl
Cl
OH
O
O
OH
O
O
12d
83
Fe
Fe
OH
a
Reaction conditions: ketones (1.0 mmol), alcohols (2.0 mmol), 2 (2 mol%), NaOH (1.0 mmol), dioxane (3 mL), 110 ꢀC, 12 h.
Isolated yields (average of two experiments).
Alcohol (4.0 mmol) was used.
b
c
d
KOH (1.0 mmol) was used.
24.2. MS-ESIþ: m/z 921.3 [M ꢁ Cl]þ. Anal. Calcd for C50H41ClIrN4P: C,
AreH), 7.09 (m, 6H, AreH), 6.95 (m, 14H, AreH), 2.41 (s, 3H, eCH3),
2.24 (s, 3H, eCH3). 13C NMR (100 MHz, CDCl3): 137.3, 135.9, 133.7,
132.5, 132.3, 132.2, 132.1, 130.6, 130.3, 129.4, 128.7, 128.6, 127.9,
62.78; H, 4.32; N, 5.86. Found: C, 62.97; H, 4.12; N, 5.99.
127.1, 125.9, 123.3, 118.8, 29.8, 24.7. MS-ESIþ: m/z 950.2 [M ꢁ 2Cl]2þ
.
4.3. Synthesis of Ir(NDMP)(PPh3)2Cl2
2
Anal. Calcd for C52H43Cl2IrN2P2: C, 61.17; H, 4.25; N, 2.74. Found: C,
61.36; H, 4.01; N, 2.97.
Iridium trichloride (0.5 mmol), ligand NDMP (1.5 mmol),
PPh3(1.5 mmol), 2-ethoxyethanol (15 mL), and water (5 mL) were
added into a three-neck flask. The mixture was refluxed under
a nitrogen atmosphere for 12 h and then cooled to room temper-
ature. A yellow precipitate was filtered and washed with water and
ethanol several times. The resulted solid was purified by passing
through a short silica gel column with CH2Cl2/ethyl acetate (5:1) as
eluent to give yellow solid 2. Yield: 70%, m.p.: 240e241 ꢀC. IR (KBr,
cmꢁ1): 3048, 2071, 1599, 1532, 1480, 1432, 1338, 1186, 1155, 1089,
4.4. General procedure for a-alkylation of ketones with alcohols
To a solution of Ir(III) complex (0.02 mmol) and base (1.0 mmol)
in solution (3 mL) was added the corresponding ketone (1.0 mmol)
and alcohol (2.0 mmol) under nitrogen. The reaction mixture was
then placed in an oil bath and heated at 110 ꢀC for 12 h, cooled and
quenched with water. The organic layer was separated and the
aqueous layer was extracted with ethyl acetate, then the combined
organic layers were washed with water, dried over MgSO4, filtered,
1027, 999, 905, 827, 797, 745, 692. 1H NMR (400 MHz, CDCl3):
(s, 1H, AreH), 7.70 (d, 1H, AreH), 7.42(m, 12H, AreH), 7.19 (t, 3H,
d 8.28

218
C. Xu et al. / Journal of Organometallic Chemistry 700 (2012) 214e218
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 20902043) and the Natural Science Foundation of Henan
Province and Henan Education Department, China (No.
102300410220 and 2009B150019) for financial support of this work.
Appendix A. Supplementary material
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CCDC 844745e844746 contain the supplementary crystallo-
graphic data for 1$CH3CH2OH-2, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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