Hydroxyl group effect in novel NNN type pyridine based ruthenium (II) complex for the transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.catcom.2016.07.009

The new NNN type pyridine ligands were prepared by using low cost and readily available starting materials and metalated with RuCl₂(PPh₃)₃ to obtain ruthenium(II) complexes. All structures were illuminated by NMR, HRMS, and FT-IR spectroscopy. The complexes exhibited good catalytic activity in transfer hydrogen reaction of ketones and it was found that a hydroxyl group on β-position of the pyridine ring had a dramatic effect on the catalyst efficiency.

Full text of DOI:10.1016/j.catcom.2016.07.009

Hydroxyl group effect in novel NNN type pyridine based ruthenium (II)
complex for the transfer hydrogenation of ketones
Irfan Sahin, Sedat Emir, Esin Ispir, Idris Karakaya, Selcuk Gumus, Mahmut
Ulusoy, Semistan Karabuga
PII:
DOI:
Reference:
S1566-7367(16)30231-X
doi: 10.1016/j.catcom.2016.07.009
CATCOM 4713
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 May 2016
6 July 2016
12 July 2016
Please cite this article as: Irfan Sahin, Sedat Emir, Esin Ispir, Idris Karakaya, Selcuk
Gumus, Mahmut Ulusoy, Semistan Karabuga, Hydroxyl group effect in novel NNN type
pyridine based ruthenium (II) complex for the transfer hydrogenation of ketones, Catalysis
Communications (2016), doi: 10.1016/j.catcom.2016.07.009
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

ACCEPTED MANUSCRIPT
Hydroxyl Group Effect in Novel NNN Type Pyridine Based Ruthenium (II) Complex for
the Transfer Hydrogenation of Ketones
Irfan Sahina, Sedat Emira, Esin Ispira, Idris Karakayab, Selcuk Gumusc, Mahmut Ulusoyd,
Semistan Karabugaa*
aDepartment of Chemistry, Faculty of Science and Letters, Kahramanmaras Sutcu Imam
University, 46100 Kahramanmaras/Turkey,
bDepartment of Chemistry, Faculty of Science and Letters, Osmaniye Korkut Ata University
80000 Osmaniye/Turkey,
cDeparment of Chemistry, Faculty of Science, Yuzuncu Yil University, 65080, Van/Turkey
dMersin University Engineering Faculty, Department of Metallurgical & Materials
Engineering 33343, Mersin/Turkey
Abstract
The new NNN type pyridine ligands were prepared by using low cost and readily available
starting materials and metalated with RuCl2(PPh3)3 to obtain ruthenium(II) complexes. All
structures were illuminated by NMR, HRMS, and FT-IR spectroscopy. The complexes
exhibited good catalytic activity in transfer hydrogen reaction of ketones and it was found that
a hydroxyl group on β-position of the pyridine ring had a dramatic effect on the catalyst
efficiency.
Keywords: Transfer hydrogenation, substituent effect, ruthenium, NNN ligand, imino
pyridine
Introduction
The design of efficient transition-metal catalysts plays a key role in achieving high
productivity and selectivity in many catalytic reactions. Among the various catalytic
reactions, transfer hydrogenation reactions (TH) have attracted a great deal of attention.[1]
Complexes derived from ligands containing nitrogen donor atoms have been extensively
studied and they were reported to exhibit high reactivity in TH [2-8]. Particularly, ruthenium
based complexes coordinated with NNN,[9-12] NN,[13, 14] NO,[13] CNN [15-17] type
ligands have been frequently applied as catalysts. Clearly, Noyori’s Ru-ethylenediamine
system is stated to be one of the most significant preliminary report in this area.[18, 19]
Inspired from these systems, a variety of catalysts possessing heteroatoms with ligands such
as quinazoline,[13, 20] pyridine,[21] imidazoyl,[10] etc. have been developed. Despite the
fact that quinazoline 1 based ruthenium complex gave only 400 h-1 TOF values, its pyridine
counterparts 2 were reported to have a TOF value of 2400 h-1 under similar reaction
conditions.[5, 13] Yu and coworkers reported that ruthenium complexes coordinated with
heterocyclic compounds bearing both pyridine and imidazoyl moiety 3a gave high TOF
values up to 705600 h-1 in TH reaction.[6] They also investigated the substituent effects of the
electron-donating or electron-withdrawing functional groups on the imidazoyl moiety of the
complex 3b-e in catalytic activity studies and found that the stronger electron donating groups
onto the coordinative imidazolyl arm of the NNN ligands were the higher the catalytic
activities, which is presented in the following order 3d > 3c > 3b > 3e. (Scheme 1).[22]

ACCEPTED MANUSCRIPT
Scheme 1. Some examples of NN and NNN type complex
In this regard, we reported our own efforts towards achievement of these objectives by using
ruthenium complexes coordinated with pyridinyl-based NNN ligand and investigated the
catalytic behaviors in TH reactions. Complex 6a bearing hydroxy substituent which is
expected to be suitable for derivatization and immobilization has been found to display high
catalytic activity in the TH reaction.
Results and Discussion
3-hydroxypyridine-2,6-dicarbaldehyde was prepared in a two-step reaction in accordance with
the known procedure with a minor revision.[23] Firstly, 3-hydroxy pyridine 4 was converted
to dicarbinol pyridine 6 by using excess amount of formaldehyde water solution in the
reaction in order to eliminate the monocarbinol pyridine 5 and the residue was purified
through column chromatography. Then, the dialcoholpyridine 6 was oxidized with SeO2 in
dioxane at reflux for overnight to afford pyridine dicarbaldehyde 7. The imine 8a was
synthesized via condensation of dicarbaldehyde 7 and aniline in EtOH under reflux.
Scheme 2 Reagent and Conditions: i-ii: 4 mol equiv. formaldehyde, NaOH in EtOH, at 90 °C
for 24 h, iii: SeO2, dioxane, reflux, overnight, iv: 2.2 mol equiv. aniline in EtOH at reflux for
4 h.
The complexes 9a,b were prepared with high yields via treatment of RuCl2(PPh3)3 with ligand
8a,b in degassed toluene at reflux temperature for 3 h under nitrogen (scheme 3). The
complexes 9a,b were characterized using NMR, HRMS, and FT-IR analyses. The signals for
CH protons of imine in the complex 9a were shifted upfield (at 7.76 and 7.74 ppm) with
31
respect to the ligand 8a. Besides, the signal at 36.84 ppm as a single peak in the P NMR
spectrum indicated the existence of the phosphorus moiety on the complex 9a as well. While
the complex 9a was air and moisture stable, some decomposition was observed in complex
9b.

ACCEPTED MANUSCRIPT
Scheme 3. Preparation of complexes 9a,b.
Yu and coworkers explained that the electron donating groups onto the coordinative
imidazolyl arm of the ligand promoted the catalytic activity of the Ru(II) catalysts.[22] While
obtaining a TOF value of 5820 h-1 from methyl substitued imidazoyl ring, unsubstituted ones
showed a TOF = 3880 h-1. In this respect, we assumed that the hydroxyl group on β-position
of the pyridine ring of complex 9a could have a similar accelerating effect onto the TH
reaction. Therefore, we investigated this influence of complex 9a by comparing the results in
the absence of OH group as substituent on pyridine of complex 9b. Initial tests were carried
out in TH reaction of acetophenone by using the complexes 9a,b in the presence of NaOH as
i
a base in PrOH at reflux. The complex 9a was enhanced with 69% conversion in 10 min
(TOF = 1380 h-1), but the complex 9b resulted in 59% conversion in 60 min (TOF = 196 h-1)
under the same reaction conditions (Scheme 4).
Scheme 4. The effect of substituent on the catalyst in the TH reaction
With the aim of obtaining reliable results, the full geometry optimizations for ground states of
studied complexes 9a,b were performed through the application density functional theory
(DFT) calculations. The geometry optimized structures of 9a, 9b, and 9c are illustrated in
Figure 1. One way to amplify the catalytic activity of the complex is to generate more active
hydride species, which may be achieved through base activation.[24] Introduction of a base
results in deprotonation of acidic groups, that is, OH in the present case. In order to determine
whether base activation via deprotonation has any kind of influence on the activation of
hydride in hydrogen transfer reaction, the NBO analysis has been performed for O(H)
complexes. The NBO analysis examines all possible interactions between filled (donor)

ACCEPTED MANUSCRIPT
Lewis-type NBOs and empty (acceptor) non-Lewis NBOs and estimates their energetic
importance by second-order perturbation theory.[25] Natural (localized) orbitals are used to
calculate distribution of the electron density in atoms and in bonds between atoms. The
catalytic activity of the Ru complex has been influenced by the negative charge development
on the hydrogen atom bonded to Ru. The larger negative charge absolutely on hydrogen is,
the more powerful is hydride source the complex. The electronic properties of the complex at
the metal center were modulated by deprotonation of OH group, providing a more electron
rich hydride. The results of NBO analysis reveal that the positive charge on central Ru atom
decreases together with an increase in negative charge development on hydrogen going from
pyridinyl 9b to pyridinyl oxide 9c. Therefore, 9b is expected to possess better catalytic
activity.
Figure 1. Geometry optimized structures of complexes and charges on central atoms.
Hydrogens are deleted for clarity.
Several screenings were performed in order to determine the most convenient base and
substrate-catalyst ratios. We began optimization reactions by screening the base (NaOH,
KOH, NaOiPr, KOtBu and NaOtBu) with acetophenone as a model substrate in 10 min at
reflux. As can be seen in Table 1, the best result was obtained in the presence of NaOiPr
(entry 3). (Increasing the catalyst loading up to 5/1000 ratio slightly improved the
conversion). The catalyst loading up to 5/1000 ratio to acetophenone increased the
corresponding alcohol conversion (entries 6 and 7). Finally, rising the amount of base did not
lead to any change in the previous result (entries 8 and 9).
Table 1. Optimization studies for complex 9a in TH reaction of acetophenone
entry
1
C/S
base
time (min)
10 (60)
conv. %a
69 (90)
3/1000
NaOH

ACCEPTED MANUSCRIPT
2
3
4
5
6
7
8
9
3/1000
3/1000
3/1000
3/1000
4/1000
5/1000
5/1000
5/1000
KOH
10
10
10
10
10
10
10
10
58
80
63
56
84
93
87
77
NaOiPr
NaOtBu
KOtBu
NaOiPr
NaOiPr
NaOiPrb
NaOiPrc
i
Conditions: 0.003 mmol 9a, 1 mmol acetophenone, 0.3 mL 0.1 M base, in 10 mL PrOH,
reflux, C/S: catalyst/substrate, adetermined by GC, b0.5 mL, c0.8 mL.
Optimized condition in hand (0.3 mL 0.1M NaOiPr as a base, 0.5 mmol % of catalyst 9a and
i
in 10 mL PrOH at reflux), a series of aromatic and aliphatic ketones were explored in TH
reaction and the related results are shown in Figures 2. In general, the reduction of acetyl
group bearing aromatic substrates 10-23 proceeded with high conversions ranging between
84-97%. Interestingly, the o-methoxy substitute acetophenone 14 resulted in very low
conversion (4%) in 2h. When methyl group of acetyl was displaced with isopropyl group 25,
relatively moderate conversion was obtained because of the steric hindrance. The similar
result was also observed from the reduction of the tetralone 26 as well.
Figure 2. Transfer hydrogenation of ketones catalyzed by complex 9a
In addition, we carried out the TH reaction of acetyl heteroaromatic ketones. The catalysis on
the reaction rate diminished significantly from furyl 27 to thiophenyl 28 and it was realized
that no product was observed from the reduction of acetyl pyridine 29. So, we assumed that
the presence of an extra chelating nitrogen atom could be coordinated with ruthenium and the
reaction did not progress due to any kind of an interaction between catalyst and substrate.
When a mixture of acetophenone and acetyl pyridine was used in the reaction condition, trace
amount of acetophenone was obtained (less than 1% conversion) and also 31P NMR spectrum

ACCEPTED MANUSCRIPT
of acetyl pyridine and complex 9a mixture supported this assumption as well (phosphorus
signal was shifted to downfield from 36.84 to 39.07 ppm). The reduction of phenyl 3-
pyridinyl ketone 39 displayed the similar result. Several aliphatic ketones 30-35 were also
examined in TH reaction, which gave the corresponding secondary alcohols with high
conversions. The catalyst displayed low reactivity in the reduction of sterically hindrance
pinacolone 33. In the case of the diaromatic ketones 36-38, the catalyst demonstrated
considerable activity.
Conclusion
In summary, we prepared novel NNN type pyridine based ruthenium (II) complexes and
explored their use as catalysts for TH reaction of ketones. Due to the electronic effects, a
significant difference was observed on the catalysis efficiency between the ruthenium
complexes. Besides, the complex 9a enabled potential solubility in a polar solvent by virtue of
a hydroxyl group on β-position of the pyridine ring. Extended studies on the design and
synthesis of transition metal complexes which are capable of preparing a heterogeneous
catalyst by deposition on available commercial solid material linking with hydroxyl group are
ongoing.
Acknowledgements
This presented study was supported by The Scientific and Technological Research Council of
Turkey (TUBITAK) (project number is 113Z294). We thank David N. Primer (University of
Pennsylvania) for proof reading.
References
[1] D. Wang, D. Astruc, Chem. Rev., 115 (2015) 6621-6686.
[2] R.V. Jagadeesh, G. Wienhofer, F.A. Westerhaus, A.E. Surkus, H. Junge, K. Junge, M.
Beller, Chem. Eur. J., 17 (2011) 14375-14379.
[3] M. Rueping, A.R. Antonchick, T. Theissmann, Angew. Chem. Int. Ed., 45 (2006) 3683-
3686.
[4] S. Sanz, M. Benitez, E. Peris, Organometallics, 29 (2010) 275-277.
[5] W. Baratta, E. Herdtweck, K. Siega, M. Toniutti, P. Rigo, Organometallics, 24 (2005)
1660-1669.
[6] F.L. Zeng, Z.K. Yu, Organometallics, 28 (2009) 1855-1862.
[7] Y.Y. Li, H. Zhang, J.S. Chen, X.L. Liao, Z.R. Dong, J.X. Gao, J. Mol. Catal. A: Chem.,
218 (2004) 153-156.
[8] L. Gok, H. Turkmen, Tetrahedron, 69 (2013) 10669-10674.
[9] W.W. Jin, L.D. Wang, Z.K. Yu, Organometallics, 31 (2012) 5664-5667.
[10] F.L. Zeng, Z.K. Yu, Organometallics, 27 (2008) 2898-2901.
[11] W.J. Ye, M. Zhao, W.M. Du, Q.B. Jiang, K.K. Wu, P. Wu, Z.K. Yu, Chem. Eur. J., 17
(2011) 4737-4741.
[12] O. Dayan, B. Cetinkaya, J. Mol. Catal. A: Chem., 271 (2007) 134-141.
[13] S. Karabuga, S. Bars, I. Karakaya, S. Gumus, Tetrahedron Lett., 56 (2015) 101-104.
[14] L.T. Ghoochany, S. Farsadpour, Y. Sun, W.R. Thiel, Eur. J. Inorg. Chem., (2011) 3431-
3437.
[15] W. Baratta, L. Fanfoni, S. Magnolia, K. Siega, P. Rigo, Eur. J. Inorg. Chem., (2010)
1419-1423.

ACCEPTED MANUSCRIPT
[16] W. Baratta, K. Siega, P. Rigo, Chem. Eur. J., 13 (2007) 7479-7486.
[17] W. Baratta, F. Benedetti, A. Del Zotto, L. Fanfoni, F. Felluga, S. Magnolia, E.
Putignano, P. Rigo, Organometallics, 29 (2010) 3563-3570.
[18] R. Noyori, T. Ohkuma, Angew. Chem. Int. Ed., 40 (2001) 40-73.
[19] T. Ohkuma, H. Ooka, S. Hashiguchi, T. Ikariya, R. Noyori, J. Am. Chem. Soc., 117
(1995) 2675-2676.
[20] C. Kucukturkmen, A. Agac, A. Eren, I. Karakaya, M. Aslantas, O. Celik, S. Ulukanli, S.
Karabuga, Catal. Commun., 74 (2016) 122-125.
[21] W. Baratta, P. Rigo, Eur. J. Inorg. Chem., (2008) 4041-4053.
[22] H.N. Chai, T.T. Liu, Q.F. Wang, Z.K. Yu, Organometallics, 34 (2015) 5278-5284.
[23] T.D. Lash, S.T. Chaney, Chem. Eur. J., 2 (1996) 944-948.
[24] K. Abdur-Rashid, S.E. Clapham, A. Hadzovic, J.N. Harvey, A.J. Lough, R.H. Morris, J.
Am. Chem. Soc., 124 (2002) 15104-15118.
[25] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO 3.1, Theoretical
Chemistry Institute, University of Wisconsin, Madison, WI, 1996.

ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Highlights



Easily prepared atom economic catalyst
Hydroxyl group on the complex shows a dramatic effect on the catalyst efficiency
Capable of preparing heterogeneous catalysts via immobilization on hydroxyl group