N. Dehury, et al.
Inorganica Chimica Acta 511 (2020) 119796
Table 2
Comparison of reactivity towards the tandem α/β-alkylation/transfer hydro-
a.
genation reaction of acetophenone and 1-phenylethanol using complexes.
Entry
Catalyst (mol %)
Yields %b
C
D
E
Fig. 2. ORTEP diagram of complex 5. Ellipsoids are drawn at 20% probability
level. Hydrogen atoms and counter ions are eliminated for clarity.
1
2
3
1 (0.2)
2 (0.2)
3 (0.1)
3 (0.1)
4 (0.1)
4 (0.1)
5 (0.1)
5 (0.1)
–
43
–
36
56
13
–
11
–
–
–
51
–
37
–
–
–
12
23
–
71
95
83
61
–
c
temperature. The complex 5 was crystallized out in orthorhombic
crystal system with space group Pbca. Important crystallographic
parameters and selected bond lengths and bond angles are presented in
Tables TS1 and TS2 (SI). From the crystal structure it is observed that
both iridium and ruthenium centers display expected piano-stool geo-
metry. The iridium center is coordinated through phenanthroline end
whereas the ruthenium center is bonded with the pyridine nitrogen of
pyrazine end. It is observed that the pyridine rings are twisted out of
plane of ligand L during coordination with ruthenium center. The Ru-C,
and Ir-C distances are varying in the range of 2.163 Å – 2. 224 Å and
4
5
6
c
7
8
–
c
9
3 + 4 (0.0.5 + 0.05)
–
–
–
–
1
1
1
0
1
2
[cp*IrCl(µ-Cl)]
[(p-cym)RuCl(µ-Cl)]
[cp*IrCl(µ-Cl)]
[(p-cym)RuCl(µ-Cl)]
2
(0.1)
–
–
39
2
(0.1)
(0.05 + 0.05)
–
–
d
2
+
2
e
13
5 (0.1)
5 (0.01)
–
–
97
91
–
–
14f
2
.128 Å – 2.180 Å, respectively, which are well agreement with the
a
Reaction conditions: Acetophenone (0.5 mmol), base (0.5 mmol), PhCH
2
OH
reported complexes. The Ir-Cl (2.400 Å), Ir-N (2.126 Å −2.093 Å), Ru-
Cl (2.396 Å) and Ru-N (2.144 Å −2.147 Å) bond distances are also
matching well with the reported complexes. The observed N1-Ir1-N2
b
(1 mL), 100 °C, reaction time = 16 h. Isolated yields after column chroma-
c
tography. 1-phenylethanol (A') was taken instead of acetophenone (A).
d
Reaction carried out in presence of ligand L (10 times of metal precursors).
(
77.1˚) and N5-Ru1-N6 (85.8˚) bite angles are also similar to the pre-
e
f
Reaction temperature 130 °C at 8 h. Catalysts loading 0.01 mol% at 130 °C.
viously reported complexes. The Ir-C(Cp*), Ru-C(p-cym) and Ru-Ir dis-
tances are found to be 1.793 Å, 1.685 Å and 9.901 Å, respectively.
mechanism) and transfer hydrogenation reactions using our synthe-
sized complexes. To test the catalytic behavior of the complexes, the α-
alkylation reaction of acetophenone (A) in presence of excess benzyl
alcohol (B) was conducted. It was observed that both mono- and di-
nuclear ruthenium complexes (2 and 4) yielded only α-alkylated pro-
duct (C) (Table 2, entry 2 and 5). On the other hand, mononuclear
iridium complex 1 resulted only hydrogenated product E (Table 2,
entry 1), whereas diiridium 3 yielded hydrogenated product E along
with a small amount α-alkylated product D under similar reaction
conditions (Table 2, entry 3).[3] These results suggest that (although
low yields) ruthenium and iridium centers conduct two mechanistically
distinct reactions, independently.
The complex 5 possesses both ruthenium and iridium centers in the
same molecular framework. The same α-alkylation reaction was con-
ducted using heterodinuclear 5. It was observed that the heterodi-
nuclear 5 selectively yielded α-alkylated product (D) (the hydrogenated
product of C) (entry 7, Table 2). Thus, incorporation of second metal
ruthenium in the same molecular framework significantly improve
yield and selectivity of specific product (95% D) (via tandem α-alky-
lation/transfer hydrogenation process) which is remarkable (entry 7,
Table 2). Interestingly, an equimolar mixture of dimeric 3 and 4 dis-
played much lower yield (61%) (entry 9, Table 2) suggesting the im-
portance of different metal present into a single molecular framework.
As expected, the reaction did not proceed at all in presence of dimeric
2.3. Evaluation of catalytic performances of polypyridyl-pyrazine based
complexes
Both ruthenium and iridium complexes are known to exhibit cata-
lytic activity towards various organic transformations [72,73]. To ex-
plore the catalytic performance of the complexes first we studied ac-
ceptorless dehydrogenation using benzyl alcohol (B). The reaction was
carried out at 100 °C in toluene using KOH as base. It was observed that
homodinuclear iridium complex (3) displayed good reactivity (entry 3,
Table 1), whereas, the dinuclear ruthenium complex (4) and mono-
nuclear 1 and 2 performed poorly (entry 1, 2 and Table 1). Interest-
ingly, the heterodimetallic 5 (with one iridium center) displayed re-
activity similar to 3 (having two iridium centers). This suggests the
importance of second hetero-metal in the framework and both metal
centers are acting as catalyst. Similar reactivity was also observed while
1-phenyl ethanol(A) was used (entry 6, Table 1).
We next studied tandem α-alkylation (through borrowing hydrogen
Table 1
Acceptorless alcohol dehydrogenation of primary and secondary alcohol.a.
precursors [cp*IrCl(µ-Cl)]
Table 2), however, 40% formation of C was registered when an equi-
molar mixture of both precursors ([cp*IrCl(µ-Cl)] and [(p-cym)RuCl(µ-
Cl)] ) were used (entry 12, Table 2) in presence 10 mol % of L. This
again emphasis the importance of well-define ligand framework for
better catalytic activity. While raising reaction temperature to 130 °C
an improved yield of formation of D was observed (entry 13, Table 2).
Similarly, a generous yield of D also registered at reduced amount of
catalyst loading (0.01 mol %) at higher temperature (entry 14, Table 2).
Next, we conducted β-alkylation of 1-phenylethanol (A′) in excess
benzyl alcohol (B). It was observed that β-alkylation of 1-phenylethanol
2 2
and [(p-cym)RuCl(µ-Cl)] (entry 10 and 11,
Entry
Catalyst(mol%)
R
Yield (%)b
2
2
1
2
3
4
5
6
1 (0.2)
2 (0.2)
3 (0.1)
4 (0.1)
5 (0.1)
5 (0.1)
H
H
H
H
H
CH
39
33
83
31
82
76
3
a
Reaction conditions: Alcohol (0.5 mmol), KOH (0.5 mmol), toluene (1 mL),
00 °C, reaction time = 12 h. bIsolated yields after column chromatography.
1
3
Dehury, Niranjan
