Re(I) carbonyl complexes containing pyridyl-imine and amine ligands: Synthesis, characterization and their catalytic olefin epoxidation activities

Article abstract of DOI:10.1016/j.jorganchem.2016.04.010

Re(I) carbonyl complexes containing pyridyl-imine or amine ligands have been synthesized and crystallographically characterized. Their catalytic activity for the cis-cyclooctene epoxidation with tert-butyl hydroperoxide (TBHP) as oxidant and the oxidation of these Re(I) complexes by TBHP have been investigated.

Full text of DOI:10.1016/j.jorganchem.2016.04.010

Accepted Manuscript
Re(I) carbonyl complexes containing pyridyl-imine and amine ligands: Synthesis,
characterization and their catalytic olefin epoxidation activities
Xiaolu Song, Min Hwee Lim, Dara Khairunnisa Binte Mohamed, See Mun Wong, Jin
Zhao, T.S. Andy Hor
PII:
S0022-328X(16)30153-X
10.1016/j.jorganchem.2016.04.010
JOM 19466
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 January 2016
Revised Date: 26 March 2016
Accepted Date: 12 April 2016
Please cite this article as: X. Song, M.H. Lim, D.K.B. Mohamed, S.M. Wong, J. Zhao, T.S.A. Hor,
Re(I) carbonyl complexes containing pyridyl-imine and amine ligands: Synthesis, characterization and
their catalytic olefin epoxidation activities, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.04.010.
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Graphical Abstract
Re(I) carbonyl complexes containing pyridyl-imine or amine ligands have been synthesized and
crystallographically characterized. Their catalytic activity for the cis-cyclooctene epoxidation
with tert-butyl hydroperoxide (TBHP) as oxidant and the oxidation of these Re(I) complexes by
TBHP have been investigated.

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Re(I) Carbonyl Complexes containing Pyridyl-Imine and Amine Ligands:
Synthesis, Characterization and their Catalytic Olefin Epoxidation Activities
Xiaolu Songa,b, Min Hwee Limb, Dara Khairunnisa Binte Mohamedb, See Mun Wongb,
Jin Zhao*a,b, T. S. Andy Hor*a,b,c
a Institute of Materials Research and Engineering, Agency for Science, Technology and Research,
2 Fusionopolis Way, Innovis, Singapore 138634, Singapore.
b Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
117543, Singapore.
c Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
Email addresses: zhaoj@imre.a-star.edu.sg; andyhor@hku.hk
ABSTRACT
Re(I) carbonyl complexes containing pyridyl-imine or amine ligands have been synthesized and
crystallographically characterized. Their catalytic activity for the cis-cyclooctene epoxidation
with tert-butyl hydroperoxide (TBHP) as oxidant and the oxidation of these Re(I) complexes by
TBHP have been investigated.
Keywords: Rhenium(I), Schiff Base, Olefin Epoxidation, Pyridyl amine, Pyridyl imine
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Introduction
Rhenium complexes in the highest oxidation state, methyltrioxorhenium(VII) (MTO) and a
number of MTO-Lewis base adducts have been proven to be efficient catalyst for olefin
epoxidation catalyst.[1] Common ligands such as Schiff base ligands have also been introduced to
the Re based catalyst systems for olefin epoxidation.[1-3,7] For example, some MTO adducts with
Schiff base ligands derived from salicylaldehyde and aniline[2a-2f], or pyridine-imine based
ligands [2g,2h], are known to be active for the epoxidation of cis-cyclooctene when H2O2 or urea
hydrogen peroxide was used as oxidant. Re(V) oxo complexes containing Schiff base ligands,[3]
such as tetradentatebis(salicylidene)imine,[3a] acetylacetone derived Schiff base ligands,[3b] have
been also used as catalyst for the cis-cyclooctene epoxidation with tert-butyl hydroperoxide
(TBHP) as oxidant. The oxidation of Re(V) to Re(VII) complexes was proposed to occur in the
course of the catalytic reaction although no structurally defined Re(VII) complexes have been
isolated.[3] Re(I) carbonyl precursors can be oxidized to Re(VII) oxo complexes through
oxidative decarbonylation. It has been reported that (η5-C5R5)Re(CO)3 (R = H or Me) reacted
with H2O2 or TBHP to form (η5-C5R5)ReO3.[4] Complex fac-[L3Re(NO)(CO)2]+ (L = 1,4,7-
triazacyclononane) could undergo an exhaustive oxidative decarbonylation in the presence of
H2O2 to afford [L3ReO3]+.[5] Therefore, it should be possible and desirable to use air-stable Re(I)
carbonyl complexes instead of air-sensitive high-valent Re(VII) complexes as pre-catalyst for
olefin epoxidation in the presence of a suitable oxidant. This strategy has been successfully used
in Mo based catalyst systems for olefin epoxidation.[1d,6] Among the Re(I) complexes, only the
Re(I) carbonyl complexes containing Schiff base derived from 2-amino-2'-hydroxy-1,1'-
binaphthyl (NOBIN) and 3,5-dichlorosalicylaldehyde have been tested as catalyst for the
epoxidation of styrene and its derivatives with TBHP as oxidant, giving the epoxide yields
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ranged from 7 to 25% after 10 hours with 20 mol% of the catalyst loading. [7] As part of our
continuous interest in using air-stable low oxidation state metal complexes as pre-catalyst for
olefin epoxidaiton,[8] we herein report the synthesis, characterization of a series of Re(I) carbonyl
complexes containing pyridyl-imine ligand and those containing pyridyl-amine ligands derived
from the pyridyl-imines, and the examination of the catalytic activity of these Re(I) complexes in
the cis-cyclooctene epoxidation, a benchmark reaction in olefin epoxidation catalysis. The
oxidation of the Re(I) complexes by TBHP has also been studied.
Experimental
General considerations
All preparations and manipulations were performed without taking vigorous precaution to
exclude air and moisture unless stated somewhere. All chemicals were used as received.
Elemental analyses for C, H, and N were performed on a Perkin-Elmer PE 2400 CHNS
elemental analyzer. 1H NMR spectra were measured at room temperature with Brucker ACF300
or Bruker DRX400 MHz FT NMR spectrometer using CDCl3 as solvent. ESI-MS was performed
on a Finnigan LCQ quadrapole ion trap mass spectrometer. Catalytic runs were monitored by GC
methods on Shimadzu GC-2010 instrument with a J&W DB-1 column. GC-MS was carried out
on Agilent 7890A Series GC System with Agilent 5975C inert MSD. All pyridyl-imine[9] and
pyridyl-amine[10] ligands (see supplementary material), and Re(CO)5Br[11] were prepared
according to the literature procedures.
Synthesis of Re(I) complexes 1-6 and 1’-5’
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General method: Ligand L1-L6 or L1’-L5’ (0.5 mmol) and Re(CO)5Br (0.203 g, 0.5 mmol) were
dissolved in methanol (10 mL) and the mixture was refluxed overnight. After removing all
solvent under reduced pressure, the residue was dissolved in CH2Cl2. The product was
precipitated upon addition of hexane. After filtration, the product was dried under reduced
pressure.
1: (Yield: 0.180g, 67%) ESI-MS (m/z): 465 ([L1Re(CO)3]+). 1H NMR: δ(ppm) = 9.06(d, 1H, py),
8.79(s, 1H, CH=N), 8.06(t, 1H, py), 7.93(d, 1H, py), 7.58(t, 1H, py), 4.35(m, 2H, NCH2), 2.66(m,
2H, CH2S), 2.42(m, 2H, CH2CH2CH2), 2.13(s, 3H, CH3). 13C NMR (100.62 MHz): δ(ppm) =
196.8, 196.2, 186.4(CO), 167.2(CH=N), 155.4, 153.8, 139.4, 128.8, 128.4 (py), 64.4 (NCH2),
31.2 (CH2S), 28.4 (CH3), 15.5 (CH2CH2CH2). IR (KBr)/cm-1: ν(CH) 3045(vw), ν(CH) 2915(w),
ν(CO) 2019(vs), ν(CO) 1899(vs), ν(CO) 1886(vs), ν(C=N) 1598(w), 1475(m), 1436(w),
1299(m), 1234(m), 1159(w), 1055(w), 786(m), 531(w). Anal.Calcd for C13H14BrN2O3SRe: C,
28.68; H, 2.59; N, 5.15; S, 5.89. Found: C, 28.55; H, 2.71; N, 5.47; S, 5.64.
2: (Yield: 0.170g, 66%) ESI-MS (m/z): 435 ([L2Re(CO)3]+).1H NMR: δ(ppm) = 9.02(d, 1H, py),
8.70(s, 1H, CH=N), 8.05(d, 1H, py), 7.90(t, 1H, py), 7.56(t, 1H, py), 4.32-4.23(m, 2H, CH2O),
3.99-3.92(t, 2H, NCH2), 3.39(s, 3H, CH3). 13C NMR (100.62 MHz): δ(ppm) = 196.9, 196.3,
186.4(CO), 167.9(CH=N), 155.5, 153.7, 139.4, 128.7, 128.5(py), 69.6(CH2O), 65.6(NCH2),
59.2(CH3). IR(KBr)/cm-1: ν(CH) 3033(vw), ν(CH) 2922(m), ν(CO) 2018(vs), ν(CO) 1920(vs),
ν(CO) 1888(vs), ν(C=N) 1598(m), 1474(m), 1301(m), 1222(m), 784(m), 758(m), 643(m),
623(m), 540(m), 520(m), 483(m). Anal.Calcd for C12H12BrN2O4Re: C, 28.02; H, 2.35; N, 5.45.
Found: C, 28.04; H, 2.66; N, 5.64.
3: (Yield: 0.200g, 70%) ESI-MS (m/z): 497 ([L3Re(CO)3]+). 1H NMR: δ(ppm) = 9.04(d, 1H, py),
8.50(s, 1H, CH=N), 8.0(t, 1H, py), 7.82(d, 1H, py), 7.52(t, 1H, py), 7.44(m, 2H, Ar), 7.06(t, 1H,
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Ar), 6.97(d, 1H, Ar), 5.37(m, 2H, NCH2), 3.82(s, 3H, CH3). 13C NMR (100.62 MHz): δ(ppm) =
197.4, 196.3, 190.3(CO), 165.3(CH=N), 155.8, 153.7, 139.1(py), 132.9, 131.4(Ar), 128.4,
128.2(py), 122.3, 121.5, 114.4, 111.3(Ar), 64.5(NCH2), 55.8(CH3). IR(KBr)/cm-1: ν(CH)
3032(vw), ν(CH) 2962(w), ν(CH) 2921(w), ν(CO) 2015(vs), ν(CO) 1908(vs), ν(CO) 1887(vs),
ν(C=N) 1599(m), 1492(m), 1251(m), 1122(m), 1029(m), 758(s). Anal.Calcd for
C17H14BrN2O4Re: C, 35.42; H, 2.45; N, 4.86. Found: C, 35.52; H, 2.69; N, 4.67.
4: (Yield: 0.201g, 73%) ESI-MS (m/z): 473 ([L4Re(CO)3]+). 1H NMR: δ(ppm) = 9.04(d, 1H, py),
8.56(s, 1H, CH=N), 8.03(d, 1H, py), 7.85(t, 1H, py), 7.57(m, 1H, py), 7.46(m, 1H, thiophene),
7.13(m, 2H, thiophene), 5.54(s, 2H, CH2). 13C NMR (100.62 MHz): δ(ppm) = 197.1, 196.1,
186.2(CO), 165.5(CH=N), 155.5, 153.8, 139.3(py), 135.3, 130.9(thiophene), 128.8(py),
128.7(thiophene), 128.5(py), 128.3(thiophene), 62.3 (CH2). IR(KBr)/cm-1: ν(CH) 3025(vw),
ν(CH) 2923(w), ν(CO) 2015(vs), ν(CO) 1900(vs), ν(CO) 1884(vs), ν(C=N) 1597(m), 1478(m),
1446(m), 1310(m), 1230(w), 1034(w), 847(w), 768(m), 722(m). Anal.Calcd for
C14H10BrN2O3SRe: C, 30.44; H, 1.82; N, 5.07; S, 5.80. Found: C, 30.03; H, 2.12; N, 4.78; S,
5.81.
5: (Yield: 0.198g, 73%) ESI-MS (m/z): 462 ([L5Re(CO)3]+). 1H NMR: δ(ppm) = 9.04(d, 1H, py),
9.0(s, 1H, CH=N), 8.04(d, 1H, py), 7.96(t, 1H, py), 7.54(t, 1H, py), 4.39-4.21(m, 2H,
CH=NCH2), 2.54(m, 2H, CH2N(CH3)2), 2.38(s, 6H, CH3), 2.26(m, 2H, CH2CH2CH2). 13C NMR
(100.62 MHz): δ(ppm) = 196.9, 196.4, 186.5(CO), 168.0(CH=N), 155.5, 153.7, 139.5, 128.8,
128.6(py), 63.5(CH=NCH2), 55.9(CH3), 53.6(CH3), 45.2(CH2N(CH3)2), 26.8(CH2CH2CH2). IR
(KBr)/cm-1: IR (KBr)/cm-1: ν(CH) 3020(vw), ν(CH) 2951(m), ν(CO) 2020(vs), ν(CO) 1909(vs),
ν(CO) 1884(vs), ν(C=N) 1598(m), 1473(m), 1383(w), 1299(m), 783(m). Anal.Calcd for
C14H17BrN3O3Re: C, 31.06; H, 3.16; N, 7.76. Found: C, 31.29; H, 3.12; N, 7.53.
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6: (Yield: 0.21 g, 81%) ESI-MS (m/z): 528 ([L6ReBr(CO)3+H]+). H NMR: δ(ppm) = 9.02 (d,
1H, py), 8.77 (s, 1H, CH=N), 8.04(t, 1H, py), 7.94(d, 1H, py), 7.55(t, 1H, py), 4.29-4.15(m, 2H,
CH=NCH2), 3.10-2.87 (m, 2H, CH2N(CH3)2), 2.37 (s, 6H, CH3). 13C NMR (100.62 MHz):
δ(ppm) = 196.8, 196.4, 186.5(CO), 167.9(CH=N), 155.5, 153.6, 139.4, 128.7(py),
62.6(CH=NCH2), 58.2(CH2N(CH3)2), 45.6(CH3). IR (KBr)/cm-1: ν(CH) 3018(vw), ν(CH)
2960(w), 2858(w), 2828(w), 2785(w), ν(CO) 2024(vs), ν(CO) 1922(vs), ν(CO) 1894(vs),
1727(w), ν(C=N) 1596(w), 1458(m), 1299(m), 1264(m), 1071(m), 936(w), 785(s), 641(m),
532(m). Anal. Calcd for C13H15BrN3O3Re: C, 29.61; H, 2.87; N, 7.97. Found: C, 29.28; H, 2.98;
N, 7.77.
1
1’: (Yield: 0.186g, 68%) ESI-MS (m/z): 467 ([L1’Re(CO)3]+). H NMR: δ(ppm) = 8.85(d, 1H,
py), 7.90(t, 1H, py), 7.46(d, 1H, py), 7.37(t, 1H, py), 4.76(m, 1H, pyCH2), 4.17(m, 1H, pyCH2),
3.88(s, 1H, NH), 3.44(m, 2H, NCH2), 2.86(m, 2H, CH2S), 2.15(s, 3H, CH3), 2.13(m, 2H,
13
CH2CH2CH2). C NMR (100.62 MHz): δ 196.8, 195.9, 192.0(CO), 158.8, 153.8, 139.5, 125.8,
122.3(py), 60.9(pyCH2), 57.4(NCH2), 31.9(CH2S), 28.0(CH3), 16.1(CH2CH2CH2). IR (KBr)/cm-
1: ν(NH) 3178(w), ν(CH) 3020(vw), ν(CH) 2916(w), ν(CO) 2019(vs), ν(CO) 1911(vs), ν(CO)
1882(vs), 1612(w), 1482(w), 1449(m), 770(m). Anal.Calcd for C13H16BrN2O3SRe: C, 28.57; H,
2. 95; N, 5.13; S, 5.87. Found: C, 28.79; H, 2.92; N, 5.04; S, 5.62.
2’: (Yield: 0.233g, 90%) ESI-MS(m/z): 437([L2’Re(CO)3]+). 1H NMR: δ(ppm) = 8.84(d, 1H, py),
7.89(t, 1H, py), 7.45(d, 1H, py), 7.36(t, 1H, py), 4.80(m, 1H, pyCH2), 4.19(s, 1H, NH), 4.15(m,
1H, pyCH2), 3.74(m, 2H, CH2O), 3.56(m, 2H, NCH2), 3.43(s, 3H, CH3). 13C NMR (100.62
MHz): δ(ppm) = 196.8, 196.0, 192.1(CO), 159.2, 153.7, 139.4, 125.6, 122.4(py), 69.7(pyCH2),
60.5(NCH2), 59.3(CH2O), 57.7(CH3), IR(KBr)/cm-1: ν(NH) 3174(s), ν(CH) 3020(vw), ν(CH)
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2933(m), ν(CH) 2862(m), ν(CO) 2022(vs), ν(CO) 1935(vs), ν(CO) 1884(vs), 1612(m), 1486(s),
1451(s), 1225(m), 1125(s), 1047(s), 946(s), 770(s), 636(m), 533(m). Anal.Calcd for
C12H14BrN2O4Re: C, 27.91; H, 2.73; N, 5.43. Found: C, 27.81; H, 2.81; N, 5.05.
3’: (Yield: 0.210g, 73%) ESI-MS(m/z): 499([L3’Re(CO)3]+).1H NMR: δ(ppm) = 8.90(d, 1H, py),
7.83(t, 1H, py), 7.41(d, 1H, py), 7.27(m, 1H, py), 7.25(m, 2H, Ar), 6.98(m, 2H, Ar), 4.93(s, 1H,
NH), 4.85(m, 1H, pyCH2), 4.38(m, 1H, pyCH2), 4.02(m, 2H, CH2Ar), 3.85(s, 3H, CH3). 13C
NMR (100.62 MHz): δ(ppm) = 196.7, 196.5, 192.5(CO), 159.6(Ar), 158.2, 153.6, 139.3(py),
131.3, 131.1(Ar), 125.5, 124.0(py), 122.4, 121.3, 111.2(Ar), 59.5(pyCH2), 59.1(CH2Ar), 55.7
(CH3). IR(KBr)/cm-1: ν(NH) 3187(w), ν(CH) 3020(vw), ν(CH) 2925(m), ν(CO) 2018(vs), ν(CO)
1921(vs), ν(CO) 1879(vs), 1608(w), 1491(m), 1451(m), 1242(m), 780(m), 748(m). Anal.Calcd
for C17H16BrN2O4Re: C, 35.3; H, 2.79; N, 4.84. Found: C, 35.36; H, 2.93; N, 5.02.
4’: (Yield: 0.176g, 64%) ESI-MS(m/z): 475([L4’Re(CO)3]+). 1H NMR(300 MHz, CDCl3): δ(ppm)
= 8.85(d, 1H, py), 7.86(t, 1H, py), 7.40(d, 1H, py), 7.36(t, 1H, py), 7.33(m, 1H, thiophene),
7.17(m, 1H, thiophene), 7.08(m, 1H, thiophene), 4.93(m, 1H, pyCH2), 4.53(m, 1H, pyCH2),
13
4.00(m, 2H, CH2thiophene), 1.70(s, 1H, NH). C NMR (100.62 MHz): δ(ppm) = 196.4, 195.9,
192.0(CO), 158.7, 153.8, 139.4(py), 137.7, 129.5(thiophene), 128.2(py) , 127.6(thiophene),
125.8(py), 122.6(thiophene), 58.9 (PyCH2), 55.5 (CH2thiophene). IR(KBr)/cm-1: ν(NH) 3176(m),
ν(CH) 3020(vw), ν(CH) 2930(w), ν(CO) 2017(vs), ν(CO) 1920(vs), ν(CO) 1881(vs), 1608(w),
1483(m), 1450(s), 1289(m), 1254(m), 775(s), 699(s), 527(m). Anal.Calcd for C14H12BrN2O3SRe:
C, 30.33; H, 2.18; N, 5.05; S, 5.78. Found: C, 30.22; H, 2.14; N, 5.05; S, 5.71.
5’: (Yield: 0.196g, 72%) ESI-MS(m/z): 464([L5’Re(CO)3]+). 1H NMR(300 MHz, CDCl3): δ(ppm)
= 8.86 (d, 1H, py), 7.96(t, 1H, py), 7.41(d, 1H, py), 7.39(t, 1H, py), 4.71(m, 1H, pyCH2), 4.27(m,
1H, pyCH2), 3.89(s, 1H, NH), 3.43(m, 2H, NHCH2), 2.96(m, 2H, CH2N(CH3)2), 2.65(s, 6H,
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13
CH3), 1.75(m, 2H, CH2CH2CH2). C NMR (100.62 MHz): δ(ppm) = 197.2, 196.3, 192.3(CO),
159.7, 153.5, 139.5, 125.5, 122.4(py), 60.4(pyCH2), 58.6(NHCH2), 58.2(CH2N(CH3)2),
45.9(CH3), 25.7(CH2CH2CH2). IR(KBr)/cm-1: ν(NH) 3067(m), ν(CH) 3020(vw), ν(CH)
2920(m), ν(CO) 2024(vs), ν(CO) 1899(vs, br), 1610(m), 1460(m), 1295(w), 1049(w), 973(w),
772(m), 536(w). Anal.Calcd for C14H19BrN3O3Re: C, 30.94; H, 3.52; N, 7.73. Found: C, 30.58;
H, 3.52; N, 7.60.
Catalytic reactions
Cis-cyclooctene (400 mg, 3.6 mmol), mesitylene (1 g, internal standard) and 5 mol% (0.18
mmol) of catalyst were added to the reaction vessel at 50 °C. The reaction was started upon the
addition of TBHP (1.31 mL, 5.0-6.0 M in n-decane). The course of the reaction was monitored
by quantitative GC analysis. Samples were taken and diluted with the respective solvents and
treated with a catalytic amount of MgSO4 and MnO2. The resulting slurry was filtered and the
filtrate was injected into a GC column. The conversion of cyclooctene and the formation of
cyclooctene oxide were calculated from calibration curves (r2 = 0.999).
Single X-ray crystallography determination
Single crystals of 6 were obtained by slow evaporation of the sample mixture in CH2Cl2/hexane
at r.t.. Single crystals of the other compounds were obtained by layering method using
CH2Cl2/hexane solvent at -20 °C. Diffraction measurements were conducted at 293(2) K on a
Bruker AXS APEX CCD diffractometer by using Mo Kα radiation (λ = 0.71073Å). The data
were corrected for Lorentz and polarization effects with the SMART suite of programs and for
absorption effects with SADABS.[12a] Structure solutions and refinements were performed by
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using the programs SHELXS-97[12b] and SHELXL-97.[12c] The structures were solved by direct
methods to locate the heavy atoms, followed by difference maps for the light non-hydrogen
atoms. Anisotropic thermal parameters were refined for the rest of the non-hydrogen atoms.
Hydrogen atoms were placed geometrically and refined isotropically.
Results and Discussion
Synthesis of the ligands and the Re complexes
Pyridyl-imine based ligands (L1-L6) (Scheme 1) were prepared according to the literature
methods through the reaction between 2-pyridinecarboxaldehyde and corresponding amine.[9]
Pyridyl-amine based ligands (L1’-L5’) were prepared by reduction of L1-L5 using NaBH4.[10] All
1
ligands were characterized by H-NMR and ESI-MS spectroscopic analysis (see supplementary
material). The positive mode of ESI-MS shows the [L+H]+ peaks for all ligands. Complexes 1-6
and 1’-5’ were obtained from the direct reaction between Re(CO)5Br and the corresponding
ligands. They were isolated as yellow or orange air-stable solids in good yields (Scheme 1).
Among them, 4’ has been reported previously, which was obtained from the reaction between
[NEt4]2[Re(CO)3Br3] and L4’.[10b] Two carbonyl ligands in the Re precursor are displaced by two
donors from the ligands (N donor from pyridine and N donor from imine or amine) and the
donor atom O, S or N on the pendant does not coordinate to the rhenium center for all cases. This
bidentate mode of ligands in these complexes is evident from the crystal structure determination
(see below). The 1H NMR spectra show a general downfield shift for the protons on the ligands
upon complexation. The H atoms of the methylene groups directly neighbored to the N donor
atom from imine or amine give rise to a greater downfield shift. In 1’-5’, the protons of CH2
group adjacent to the pyridine ring become diastereotopic after the coordination of the amine
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nitrogen. The positive mode of ESI-MS spectra show [M-Br]+ peak for all these complexes.
Three strong bands in the region of 2025-1880 cm-1 in the IR spectra of these complexes indicate
the presence of the fac-[Re(CO)3]+ core.
Scheme 1. Synthesis of Complexes 1-6 and 1’-5’
Molecular structure of the Re(I) complexes determined by X-ray single crystal diffraction
The molecular structures of complexes 1-6, 1’ and 2’ shown in Figures 1-4 were determined by
X-ray single-crystal diffraction. In all these complexes, the metal center adopts a distorted
octahedral geometry with a fac-[Re(CO)3]+ core moiety. The pyridyl-imine or amine ligand acts
as bidentate chelating ligand, forming a five-member ring with Re center and leaving the donor
atom in the pendant uncoordinated. The molecular structure of 4’ has been reported previously
with a pendant thiophene arm.[10b] Similar (N,N) coordination is also found in other fac-
[ReX(CO)3(L)] (X= Cl, Br, I) complexes where the potential tridentate ligands such as
2,2’:6’,2”-terpyridine,[13a] 2,6-bis[(1-phenylimino)ethyl]pyridine)[13b,13c] are in a bidentate chelate
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mode. For compounds 1-6, the N-Re-N has an angle between 74.3(2) º to 75.2(2) º, indicating a
large distortion from ideal octahedron caused by the chelate-compression of the five member
ring. The Re1-N1 (N from pyridine) and Re1-N2 (N from imine) bond distances are 2.167(3)-
2.183(6) Å and 2.153(5)-2.172(4) Å, respectively, which are slightly shorter than those found in
other (N,N) neutral bidentate Schiff base complexes with pyridyl and imine N donors, such as
Re(CO)3Cl([4-(4’-methyl)-2,2’-bipyridyl)imine]-2-hydroxybenzene) ( Re-N 2.178(6) to 2.182(7)
Å) and Re(CO)3Cl(pyca-L-Asp(OMe)-OMe) (pyca = pyridine-2-carbaldehyde imine; Asp =
Aspartic acid) (Re-N 2.170(9) to 2.187(7) Å).[14] Compounds 1’, 2’ and 4’[10b] have similar
structures to their analogues 1, 2 and 4. The difference lies with the Re1-N2 bond distance (N
from amine) (2.226(6) Å) being longer than the Re1-N2 (N from imine) bond (2.163(4) Å).
Change in the pendant functions and adjustment in the spacer between the pendant and the imine
nitrogen do not impart any significant difference on the coordination motif of the ligand and the
metal geometry.
1
1’
Figure 1. X-ray structures of 1 (C13H14BrN2O3SRe) and 1’ (C13H16BrN2O3SRe) drawn at 30%
probability thermal ellipsoids, Selected bond distances (Å) and angles (deg) for 1: Re1-C11
1.910(5), Re1-C12 1.914(6), Re1-C13 1.927(5), Re1-N1 2.170(4), Re1-N2 2.172(4), Re1-Br1
2.6302(5), N1-Re1-N2 74.5(2), N1-Re1-Br1 85.13(9), N2-Re1-Br1 84.2 (1). For 1’: Re1-C13
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1.892(6), Re1-C11 1.914(5), Re1-C12 1.921(5), Re1-N1 2.177(4), Re1-N2 2.216(4), Re1-Br1
2.6229(7), N1-Re1-N2 74.8(2), N1-Re1-Br1 85.2(1), N2-Re1-Br1 83.1(1).
2
2’
Figure 2. X-ray structures of 2 (C12H12BrN2O4Re) and 2’ (C12H14BrN2O4Re) drawn at 30%
probability thermal ellipsoids, Selected bond distances (Å) and angles (deg) for 2: Re1-C3
1.919(7), Re1-C1 1.928(7), Re1-C2 1.929(6), Re1-N2 2.153(5), Re1-N1 2.169(5), Re1-Br1
2.6267(7), N2-Re1-N1 75.2(2), N2-Re1-Br1 83.7(1), N1-Re1-Br1 83.6(1). For 2’: Re1-C12
1.909(7), Re1-C10 1.909(7), Re1-C11 1.933(7), Re1-N1 2.193(6), Re1-N2 2.238(5), Re1-Br1
2.6242(7), N1-Re1-N2 75.7(2), N1-Re1-Br1 84.8(1), N2-Re1-Br1 84.9(1).
3
4
Figure 3. X-ray structures of 3 (C17H14BrN2O4Re) and 4 (C14H10BrN2O3SRe) drawn at 30%
probability thermal ellipsoids, Selected bond distances (Å) and angles (deg) for 3: Re1-C16
1.908(4), Re1-C15 1.918(5), Re1-C17 1.922(4), Re1-N2 2.163(3), Re1-N1 2.181(3), Re1-Br1
2.6142(4), N2-Re1-N1 74.7(1), N2-Re1-Br1 82.44(8), N1-Re1-Br1 84.69(8). For 4: Re1-C14
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1.908(9), Re1-C12 1.915(8), Re1-C13 1.931(9), Re1-N2 2.163(6), Re1-N1 2.183(6), Re1-Br1
2.6126(9), N2-Re1-N1 74.3(2), N2-Re1-Br1 84.3(2), N1-Re1-Br1 84.4(2).
5
6
Figure 4. X-ray structures of 5 (C14H17BrN3O3Re) and 6 (C13H15BrN3O3Re) drawn at 30%
probability thermal ellipsoids, Selected bond distances (Å) and angles (deg) for 5: Re1-C13
1.903(5), Re1-C14 1.913(6), Re1-C12 1.923(6), Re1-N2 2.163(4), Re1-N1 2.181(4), Re1-Br1
2.6145(6), N2-Re1-N1 74.5(2), N2-Re1-Br1 85.2(1), N1-Re1-Br1 83.3(1). For 6: Re(1)–C(11)
1.912(3), Re(1)–C(12) 1.930(4), Re(1)–C(13) 1.908(4), Re(1)–N(1) 2.167(3), Re(1)–N(2)
2.167(3), Re(1)–Br(1) 2.6333(4), C(13)–Re(1)–C(11) 88.7(2), C(13)–Re(1)–N(2) 94.4(1),
C(11)–Re(1)–N(2) 172.8(1), N(1)–Re(1)–N(2) 74.82(9), C(13)–Re(1)–Br(1) 176.8(1), C(11)–
Re(1)–Br(1) 92.0(1).
Application of the Re(I) complexes in cis-cyclooctene epoxidation catalysis and the oxidation of
Re(I) complexes with TBHP
The catalytic activity of complexes 1-6 and 1’-5’ towards epoxidation of cis-cyclooctene were
examined in the presence of TBHP (5.5 M in n-decane) at 50 °C with the ratio of catalyst:
substrate: oxidant of 5:100:200 (see Figure S1 for the epoxide yields and the conversion of cis-
cyclooctene after 4, 8 and 24 h in the supplementary material). Blank runs (without catalyst) and
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the reactions using imine or amine ligand as catalyst showed no significant epoxide formation
under the applied conditions. These Re(I) carbonyl Schiff base complexes gave up to 30%
epoxide yields after 24 h. Changing the imine N donor to amine N donor makes only marginal
difference in the catalytic activity. When the catalyst loading was reduced to 1 mol%,
compounds 1-5 produced only up to 17% epoxide yields after 24 h (see Table S1 in the
supplementary material). Under similar conditions, using Re(V) oxo complexes with
tetradentatebis(salicylidene)imine ligands as catalyst, up to 66% of cyclooctene oxide yield could
be obtained at 50 °C for 10 hours.[3a] Re(V) oxo complexes with acetylacetone derived Schiff
base ligand gave up to 45% the epoxide yield after 10 min.[3b] Oxorhenium(V) complexes with
ketiminato ligands could give epoxide with up to 55% of yield after 7 h.[3c]
The epoxidation reaction using Re(CO)5Br as catalyst (5 mol%) has been also conducted under
the conditions applied in this work. It showed that the epoxide yield reached its maximum of ca.
25% after 2 h reaction, after which it declined gradually. After 8 h the yield of the epoxide
decreased to 9% and after 24 h only 1% of epoxide was detected (see Table S2 in the
supplementary material). This is attributed to the secondary reaction of ring-opening of epoxide
giving diol, as evidenced by GC-MS analysis. It is significant that diol has not been observed
over a period of 24 h when the Re(I) Schiff base complexes were used as the catalysts based on
1
both H NMR spectra and GC. However, for all cases, there were notable differences between
the conversion of cis-cyclooctene and yield of cyclooctene oxide, which is due to the formation
of other insofar unidentified by-products.
ESI-MS analysis of the reaction mixtures of the Re(I) complexes and TBHP was performed in
order to get some insight of oxidation products of Re(I) complexes under the oxidative condition
(in the presence of TBHP). Thus, complexes 1-5 and 1’-5’ (3.6 µmol) and TBHP (0.36 mmol)
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were mixed and stirred at 50 ºC. After 10 min the reaction was quenched with MnO2. For all
cases, the negative mode of ESI-MS of the reaction mixture reveals several fragments attributed
to Re species. Among them, a common fragment, [LH4Re2O9Br]- can be observed which could
be assigned to di-nuclear Re(VII) species containing oxo ligand and the corresponding pyridinyl-
imine or amine ligand L (N,N coordination mode) (see supplementary material). On the basis of
the previous study on Re(V) and Re(VII) catalyst systems,[1-3] the Re(VII) fragments could be
related to the Re(VII) intermediates which are responsible for olefin epoxidation. Figure 5 shows
a fragmentation pattern of ESI-MS (negative mode) of the reaction mixture of 1’ and TBHP. The
peak at m/z 251 corresponds to the [ReO4]-. The peak at m/z 796 is assigned to the fragment of
[L1’H4Re2O9Br]-. The peak at m/z 812 is originated from the fragment of [(L1’=O)H4Re2O9Br]-
where L1’=O presents the oxidized SMe function group, S(O)Me.
Figure 5. ESI-MS spectrum of reaction mixture of 1’ and TBHP (1:200) at 50 ºC for 10 min.
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The thioether sulfur oxidation is also evident from the molecular structure of the compound
(L1'=O)Re(CO)3Br (7) based on the X-ray diffraction analysis (Figure 6). Compound 7 was
obtained from the reaction mixture of 1’ and TBHP. The mixture of 2’, with an ether pendant,
and TBHP, resulted in the crystallization of an oxidized product [H2L2’][ReO4]2 (8) after
exposure to air for a few days. X-ray diffraction analysis of 8 reveals the oxidation and the
subsequent disintegration of the Re core to perrhenate, and the liberation of ligand L2’ in its
protonated form (Figure 6).
7
8
Figure 6. X-ray structures of 7 (C13H16BrN2O4SRe) and 8 (C9H16N2O9Re2) drawn at 30 %
probability thermal ellipsoids. Selected bond distances (Å) and angles (deg) for 7. Re1-C13
1.912(7), Re1-C12 1.928(7), Re1-C11 1.943(8), Re1-N1 2.186(6), Re1-N2 2.231(6), Re1-Br1
2.6245(8), S1-O1 1.50(1), N1-Re1-N2 74.7(2), N1-Re1-Br1 82.7(2), N2-Re1-Br1 84.1(2). For 8:
Re1-O4 1.718(9), Re1-O3 1.718(8), Re1-O2 1.718(9), Re1-O5 1.723(9), Re2-O7 1.70(1), Re2-
O9 1.72(1), Re2-O6 1.726(9), Re2-O8 1.742(9), O4-Re1-O5 111.1(4), O3-Re1-O5 108.9(4), O2-
Re1-O5 110.5(5), O7-Re2-O9 108.9(5), O7-Re2-O6 110.1(5), O9-Re2-O6 109.6(5), O7-Re2-O8
109.8(5), O9-Re2-O8 109.0(5), O6-Re2-O8 109.4(5).
Conclusions
A series of Re(I) complexes with pyridyl-imine, pyridyl-amine based ligands were synthesized
and characterized. These Re(I) carbonyl complexes generally show low to moderate catalytic
activities towards the epoxidation of cis-cyclooctene under the conditions established with other
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Mo and Re complexes. The modest activity could be due to the instability of the Re(VII) specie
formed in situ in the course of the catalytic reaction which tends to decompose into catalytically
inactive perrhenate salts as observed in this work. Current activities in our laboratory are directed
towards the use of other supporting ligands to improve the catalytic activity of Re(I) complexes
and to isolate the Re(VII) complexes containing ReO3L moiety.
Acknowledgements
We are grateful to the A*STAR (WBS No. R-143-000-364-305 and R-143-000-566-305) and the
Ministry of Education (WBS No.R-143-000-361-112) for financial support. We thank Ms. G. K.
Tan, Ms. Y. M. Hong and the late Dr. L. L. Koh for crystallographic assistance.
Supplementary Information
Synthesis and characterization data of all ligands, catalytic reaction results, ESI_MS spectra of
the reaction mixtures of Re(I) complexes with TBHP, the crystallographic data and refinement
parameters of complexes 1-8, 1’, and 2’. CCDC (1063340-1063345, 1063347-1063349 and
1063351) contains the crystallographic data for this work. 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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Highlights
ꢀ
ꢀ
ꢀ
Re(I) pyridyl-imine or amine complexes were synthesized and characterized.
Their catalytic activity towards cis-cyclooctene epoxidation was reported.
The oxidation of Re(I) complexes by tert-butyl hydroperoxide was investigated.