Hydride-bridged NiRh complexes with tunable N3S2 dithiolato ligands and their utilization as catalysts for hydrogenation of aldehydes and CO2 in aqueous media

Article abstract of DOI:10.1021/om300350u

New N₃S₂ dithiolato ligands, 1,4-bis(2-mercaptoethyl) -7-R-1,4,7-triazacyclononane (RTACN-S₂H₂; R = Ts, ⁱPr) were synthesized and reacted with Ni^II ion to give mononuclear complexes [Ni(RTACN-S₂)] (R = Ts (1a), ⁱPr (1b)). Complexes 1a and 1b were further transformed by treatment with Rh ^III species into a series of Ni^IIRh^III heterodimetallic compounds, [Ni(RTACN-S₂)RhCp*X]X′ (R = Ts, X = Cl, X′ = Cl, OTf ([2a]X′); R = Ts, X = X′ = NO ₃ ([3a]NO₃); R = ⁱPr, X = Cl, X′ = Cl, NO₃, PF₆ ([2b]X′)). Complexes [3a]NO₃ and [2b]NO₃ were readily reacted with H₂ (0.1 MPa) in water at room temperature to afford hydride-bridged Ni^IIRh^III dinuclear complexes, [(RTACN-S₂)Ni(μ-H)RhCp*]NO₃ (R = Ts ([4a]NO₃), ⁱPr ([4b]NO₃)), which were successfully characterized by X-ray crystallography. Upon the heterolytic activation of H₂, the Ni-Rh interatomic distances were dramatically decreased from 3.2130(6)-3.262(2) A ([2a]OTf, [2b]NO₃, [3a]NO₃) to 2.6921(6)-2.7228(4) A ([4a]NO₃, [4b]NO₃), resulting in semibridging Ni(μ-H)Rh cores. In addition, the stability of the hydride of [4a]⁺ and [4b]⁺ were interestingly tuned by varying the R group of the TACN ligand through trans influence at the Ni^II center (Ni-NR = 2.188(3) A ([4a] ⁺), 2.056(2) A ([4b]⁺). In fact, [4a]⁺ was quite stable and capable of reducing benzaldehyde in water, although [4b]⁺ quickly decomposed under similar conditions. Catalytic hydrogenation of aldehydes and CO₂ in water has been established by using [3a]NO₃ and [2a]Cl as precursors of an active species, [4a]⁺, and found to be interestingly contrasted to the inactive precursor [2b]NO₃ with R = ⁱPr. These results indicated that the properties and reactivity of the Ni(μ-H)Rh complexes can be controlled by changing the substituents of the N₃S₂ supporting ligands.

Full text of DOI:10.1021/om300350u

Article
pubs.acs.org/Organometallics
Hydride-Bridged NiRh Complexes with Tunable N3S2 Dithiolato
Ligands and Their Utilization as Catalysts for Hydrogenation of
Aldehydes and CO2 in Aqueous Media
Bunsho Kure,* Ayami Taniguchi, Takayuki Nakajima, and Tomoaki Tanase*
Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-nishi-machi, Nara, 630-8506, Japan
S
* Supporting Information
ABSTRACT: New N3S2 dithiolato ligands, 1,4-bis(2-mercap-
toethyl)-7-R-1,4,7-triazacyclononane (RTACN-S2H2; R = Ts,
iPr) were synthesized and reacted with NiII ion to give
i
mononuclear complexes [Ni(RTACN-S2)] (R = Ts (1a), Pr
(1b)). Complexes 1a and 1b were further transformed by
treatment with RhIII species into a series of NiIIRhIII
heterodimetallic compounds, [Ni(RTACN-S2)RhCp*X]X′ (R
= Ts, X = Cl, X′ = Cl, OTf ([2a]X′); R = Ts, X = X′ = NO3
i
([3a]NO3); R = Pr, X = Cl, X′ = Cl, NO3, PF6 ([2b]X′)).
Complexes [3a]NO3 and [2b]NO3 were readily reacted with
H2 (0.1 MPa) in water at room temperature to afford hydride-bridged NiIIRhIII dinuclear complexes, [(RTACN-S2)Ni(μ-
H)RhCp*]NO3 (R = Ts ([4a]NO3), iPr ([4b]NO3)), which were successfully characterized by X-ray crystallography. Upon the
heterolytic activation of H2, the Ni−Rh interatomic distances were dramatically decreased from 3.2130(6)−3.262(2) Å
([2a]OTf, [2b]NO3, [3a]NO3) to 2.6921(6)−2.7228(4) Å ([4a]NO3, [4b]NO3), resulting in semibridging Ni(μ-H)Rh cores. In
addition, the stability of the hydride of [4a]+ and [4b]+ were interestingly tuned by varying the R group of the TACN ligand
through trans influence at the NiII center (Ni−NR = 2.188(3) Å ([4a]+), 2.056(2) Å ([4b]+). In fact, [4a]+ was quite stable and
capable of reducing benzaldehyde in water, although [4b]+ quickly decomposed under similar conditions. Catalytic
hydrogenation of aldehydes and CO2 in water has been established by using [3a]NO3 and [2a]Cl as precursors of an active
species, [4a]+, and found to be interestingly contrasted to the inactive precursor [2b]NO3 with R = iPr. These results indicated
that the properties and reactivity of the Ni(μ-H)Rh complexes can be controlled by changing the substituents of the N3S2
supporting ligands.
characterized, and thus, their detailed roles in the catalytic
cycles were mostly hypothesized.
INTRODUCTION
■
Development of new water-soluble organometallic catalysts has
attracted rapidly growing attention because water is a naturally
abundant and green (nontoxic, nonflammable, and non-
hazardous) solvent, having its use to allow simple purification
of organic products and pH-dependent selectivity.1 In this
regard, catalytic hydrogenations and hydrogen-transfer reac-
tions of unsaturated compounds in water are highly desired
industrial processes rather than establishing the applied
principle of reduction by using stoichiometric hazardous
hydride reagents, such as NaBH4.2 To date, several transition-
metal complexes have been known to catalyze such reactions in
water or aqueous biphasic media.3 However, active transition-
metal hydride complexes have rarely been isolated and fully
characterized because, in its general sense, these were very
In nature, [NiFe] hydrogenases catalyze heterolytic activa-
tion of H2 into a proton and hydride along with the reduction
of substrates, such as methylene blue and NAD+, under
ambient conditions in water.7,8 The active sites of [NiFe]
hydrogenases in the Ni−C and Ni−R active states are believed
to be consisting of Ni and Fe dimetal centers bridged by two
thiolato and one hydride ligand (Chart 1b).8 Although many
structural and functional models for [NiFe] hydrogenases have
been prepared,9 only one substantial example has been reported
by Kure and Ogo et al. on hydrogenation of small molecules
catalyzed by a bisthiolate-bridged dinuclear complex under
ambient conditions in water,3g wherein hydrogenation of
aldehydes was catalyzed by the dinuclear NiRu complex in
water, and the active hydride complex [Ni(OH)(μ-SR)2(μ-
H)Ru(η6-C6Me6)] ((HSR)2 = N,N′-dimethyl-N,N′-bis(2-mer-
captoethyl)-1,3-propanediamine, Chart 1c) was isolated to
reveal that the hydroxo group on the Ni center trans to the μ-H
unstable in water.4,5 For example, Steckhan, Fish, Suss-Fink,
̈
and their co-workers independently reported water-soluble
transfer hydrogenation catalysts that were converted in situ to
the active hydride species, such as [RhCp*(H)(bpy)]+ (Chart
1a, Cp* = η5-pentamethylcyclopentadienyl, bpy = 2,2′-
bipyridine) and [RhCp*(H)(phen)]+ (phen = 1,10-phenan-
throline),6 although their definitive structures have not been
Received: April 27, 2012
Published: June 26, 2012
© 2012 American Chemical Society
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Article
Chart 1
Scheme 1
atom was crucial for its catalytic activity. When the OH ligand
was protonated at low pH, the corresponding NiRu hydride
lost its reducing ability.10 Although the detailed structure of the
active NiRu complex is not yet determined, these results
suggested that the tuning of the NiII electronic state is very
important to developing water-soluble [NiFe] hydrogenase-
mimicking catalysts.
In the present study, we have developed new water-soluble
bis(μ-thiolato)(μ-hydrido) NiIIRhIII dinuclear complexes,
[(RTACN-S2)Ni(μ-H)RhCp*]NO3 (RTACN-S2H2 = 1,4-bis-
(2-mercaptoethyl)-7-R-1,4,7-triazacyclononane; R = p-toluene-
sulfonyl (Ts) ([4a]NO3) and isopropyl (iPr) ([4b]NO3))
(Scheme 1), which were inspired from the active site of [NiFe]
hydrogenases to tune the electronic state of the NiII center by
varying the substituent group of the pendant amino group. By
introducing an electron-withdrawing Ts group onto the amino
nitrogen trans to the hydride, the hydride-bridged NiIIRhIII
complex ([4a]NO3) was significantly stabilized in water, and
consequently, we found out its efficiency to promote
hydrogenation of aldehydes and CO2 in aqueous media. We
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Article
100%) (z = 1, [iPrTACN-S2H2 + H]+ (292.18)). IR (KBr): ν 2960,
2925, 2802, 2545 (-SH), 1459, 1380, 1358, 1310, 1116, 1099 cm−1.
[Ni(TsTACN-S2)] (1a). A solution of NiCl2·6H2O (755 mg, 3.18
mmol) in methanol (60 mL) was slowly added to a solution of
TsTACN-S2H2 (1.42 g, 3.53 mmol) in chloroform (40 mL) under N2.
The mixture was stirred at room temperature for 5 h to yield a brown
solution, which was evaporated to dryness. The crude product was
purified by chromatography on an alumina column (Wako, alumina,
activated particle size = 40−150 μm) with chloroform and methanol
(95:5) as the eluent. The first purple band was collected, and the
solvent was removed by rotary evaporation. The resultant purple oil
was dissolved in acetonitrile (20 mL) and was stored for 12 h at room
temperature to afford a purple solid, which was filtered and washed
with acetonitrile and diethyl ether. Yield: 588 mg, 40% (vs
wish to report herein the synthesis and characterization of
hydride-bridged NiIIRhIII complexes by designing the new N3S2
dithiolato ligands and their catalytic activities for hydrogenation
of the carbonyl compounds in water.
EXPERIMENTAL SECTION
■
Materials. All preparative procedures were carried out under a
nitrogen atmosphere in a glovebox or using standard Schlenk
techniques. All chemicals (highest purity available) were purchased
from Wako Pure Chemical Industries, Ltd. Reagent grade solvents
were dried by the standard procedures and were freshly distilled prior
to its use. CO2 (>99.99%) and H2 (>99.999%) gases were purchased
from Sumitomo Seika Co., Ltd. Compounds, 1-(p-toluenesulfonyl)-
1,4,7-triazacyclononane,11a,b 1-isopropyl-1,4,7-triazacyclononane,11c
[RhCp*Cl2]2,11d and RhCp*(NO3)211e were prepared by the methods
described in the literature. The pH of the aqueous solution was
adjusted by using 0.1 M HNO3/H2O (pH 1−3), 25 mM CH3COOH/
CH3COONa (pH 4−6), 25 mM Na2HPO4/KH2PO4 (pH 7−8), and
0.1 M NaOH/H2O (pH 9−12) solutions.
1
NiCl2·6H2O). H NMR (300 MHz, CDCl3): δ 2.28−2.46 (m, 4H,
−CH2−), 2.48 (s, 3H, CH3(Ts)), 2.70−2.86 (m, 6H, −CH2−), 3.09−
3.19 (m, 2H, −CH2−), 3.46−3.57 (m, 2H, −CH2−), 3.67−3.77 (m,
2H, −CH2−), 4.13−4.20 (m, 2H, −CH2−), 5.90−6.05 (m, 2H,
3
3
−CH2−), 7.39 (d, JHH = 8.1 Hz, 2H, ArH(Ts)), 7.75 (d, JHH = 8.3
Hz, 2H, ArH(Ts)). 13C{1H} NMR (75 MHz, CDCl3): δ 21.7
(CH3(Ts)), 28.4 (CH2−CH2−SH), 56.2 (−CH2−(TACN)), 60.2
(−CH2−(TACN)), 61.3 (−CH2−(TACN)), 66.6 (CH2−CH2−S),
127.1 (−CH−(Ts)), 130.3 (−CH−(Ts)), 135.1 (C−SO2−(Ts)),
144.6 (C−CH3(Ts)). ESI-MS (in MeOH): m/z 481.88 (I = 100%) (z
= 1, [(1a + Na)]+ (482.05)). IR (KBr): ν 2942, 2886, 2840, 1596,
1449, 1435, 1333, 1318, 1158, 1089, 1001, 957, 922, 849, 813, 713,
693, 643, 545 cm−1. Anal. Calcd for C17H27N3NiO2S3: C, 44.36; H,
5.91; N, 9.13. Found: C, 44.46; H, 5.69; N, 9.07. Purple-green crystals
of 1a used for an X-ray analysis were obtained from a DMF/diethyl
ether solution.
1
Measurements. H NMR spectra were recorded on a Bruker 300
1
MHz spectrometer (AV-300N). The H NMR spectra in CDCl3 or
CD3CN were reported in parts per million versus tetramethylsilane
(TMS) as an external reference, and those in D2O or H2O were
performed by dissolving samples in D2O or H2O in an NMR tube
(diameter = 5.0 mm) with a sealed capillary tube (diameter = 1.5 mm)
containing 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt
(TSP) in D2O (10 mM) as an internal reference. IR spectra of solid
compounds, as KBr disks, were recorded on a JASCO FT/IR-410
spectrophotometer at ambient temperature. ESI-TOF mass spectra
were recorded on a JEOL JMS-T100LC in a positive detection mode
in the range of m/z 100−2000, equipped with an ion spray interface.
The sprayer was held at a potential of +2.0 kV, and the compressed N2
was employed to assist liquid nebulization. The orifice potential was
maintained at +20 V. The pH of the solution was determined by a pH
meter (model: TOA WM-22EP) equipped with a glass electrode
(model: TOA GST-2729C), and the pD values were corrected by
adding 0.4 to the observed values.12
[Ni(iPrTACN-S2)] (1b). A solution of NiCl2·6H2O (570 mg, 2.40
mmol) in methanol (30 mL) was slowly added to a solution of
iPrTACN-S2H2 (969 mg, 2.40 mmol) in chloroform (10 mL) under
N2. The mixture was stirred at room temperature for 5 h to yield a
brown solution, which was evaporated to dryness. The crude product
was purified by chromatography on an alumina column (Wako,
alumina, activated particle size = 40−150 μm) with chloroform and
methanol (95:5) as the eluent. The first green band was collected, and
the solvent was removed by rotary evaporation. The green residue was
washed with toluene and diethyl ether and recrystallized from
CH2Cl2/diethyl ether to give a green powder of 1b. Yield: 170 mg,
20% (vs NiCl2·6H2O). ESI-MS (in MeOH): m/z 346.98 (I = 100%)
(z = 1, [1b]+ (347.10)). IR (KBr): ν 2917, 1635, 1458, 1261, 1085,
1,4-Bis(2-mercaptoethyl)-7-(p-toluenesulfonyl)-1,4,7-triaza-
cyclononane (TsTACN-S2H2). To a solution of 1-(p-toluenesulfon-
yl)-1,4,7-triazacyclononane (TsTACN, 500 mg, 1.76 mmol) in dry
chloroform (20 mL) was added ethylene sulfide (0.7 mL, 11.8 mmol)
at room temperature, and the mixture was stirred at 50 °C for 24 h.
The excess ethylene sulfide and solvent were removed under reduced
pressure to give the product TsTACN-S2H2, which was used for
preparation of 1a without further purification. Yield: 673 mg, 95% (vs
810 cm− 1
. Anal. Calcd for C 1 6 . 5 H3 5 Cl3 N3 NiO0 . 5 S2
(1b·1.5CH2Cl2·0.5Et2O): C, 38.66; H, 6.88; N, 8.20. Found: C,
38.56; H, 6.56; N, 8.58. Green crystals of 1b for an X-ray analysis were
obtained from a dichloromethane/diethyl ether solution.
1
TsTACN). H NMR (300 MHz, CDCl3): δ 2.42 (s, 3H, CH3(Ts),
[Ni(TsTACN-S2)RhCp*Cl]X′ ([2a]X′: X′ = Cl and OTf). Complex
1a (206 mg, 0.448 mmol) was reacted with [RhCp*Cl2]2 (138 mg,
0.224 mmol) in dichloromethane (100 mL) at room temperature for 5
h to provide a red solution, from which a red powder of [2a]Cl was
obtained by removing the solvent. Yield: 341 mg, 99%. 1H NMR (300
MHz, CDCl3): δ 1.66 (s, 15H, CH3(Cp*)), 2.48 (s, 3H, CH3(Ts)),
2.54−2.62 (m, 4H, −CH2−), 2.64−2.82 (m, 8H, −CH2−), 2.95−2.98
3
(m, 4H, −CH2−), 3.26−3.29 (m, 4H, −CH2−), 7.30 (d, JHH = 8.3
3
Hz, 2H, ArH(Ts)), 7.66 (d, JHH = 8.1 Hz, 2H, ArH(Ts)). 13C{1H}
NMR (75 MHz, CDCl3): δ 21.5 (CH3(Ts)), 23.1 (CH2−CH2−SH),
51.3 (−CH2−(TACN)), 55.8 (CH2−CH2−SH), 56.2 (−CH2−
(TACN)), 61.0 (−CH2−(TACN)), 127.3 (−CH−(Ts)), 129.8
(−CH−(Ts)), 136.0 (C−SO2−(Ts)), 143.3 (C−CH3(Ts)). ESI-MS
(in MeOH): m/z 404.15 (I = 100%) (z = 1, [TsTACN-S2H2 + H]+
(404.15)). IR (KBr): ν 2925, 2850, 2574 (SH), 1598, 1449, 1335,
1158, 916, 816, 713, 694, 642, 549 cm−1.
1,4-Bis(2-mercaptoethyl)-7-isopropyl-1,4,7-triazacyclono-
nane (iPrTACN-S2H2). To a solution of 1-isopropyl-1,4,7-triazacyclo-
nonane (iPrTACN, 222 mg, 1.30 mmol) in dry benzene (10 mL) was
added ethylene sulfide (9 mL, 151 mmol) at room temperature, and
the mixture was stirred at 50 °C for 24 h. The excess ethylene sulfide
and solvent were removed under reduced pressure to give the product
iPrTACN-S2H2, which was used for the preparation of 1b and [2b]Cl
3
2.00−2.06, 2.54−2.74, 3.09−4.15 (m, 20H), 7.41 (d, JHH = 8.1 Hz,
3
2H, ArH(Ts)), 7.84 (d, JHH = 8.3 Hz, 2H, ArH(Ts)). ESI-MS (in
MeOH): m/z 731.82 (I = 100%) (z = 1, [2a]+ (732.05)). IR (KBr): ν
2914, 1625, 1491, 1455, 1340, 1162, 1087, 926, 752, 712, 694, 572,
548 cm−1. Anal. Calcd for C27.5H43Cl3N3NiO2RhS3 ([2a]-
Cl·0.5CH2Cl2): C, 40.69; H, 5.34; N, 5.18. Found: C, 40.48; H,
5.26; N, 5.31. Addition of NH4OTf (200 mg, 1.20 mmol) to an
aqueous solution (20 mL) of [2a]Cl (350 mg, 0.455 mmol) gave a red
precipitate of [2a]OTf, which was filtered, washed with H2O and
1
diethyl ether, and dried in vacuo. Yield: 360 mg, 91%. H NMR (300
MHz, CDCl3): δ 1.67 (s, 15H, CH3(Cp*)), 2.50 (s, 3H, −CH3(Ts)),
3
i
1
2.01−2.06, 2.45−3.16, 3.77−4.05 (m, 20H), 7.44 (d, JHH = 8.1 Hz,
without further purification. Yield: 334 mg, 88% (vs PrTACN). H
3
3
NMR (300 MHz, CDCl3): δ 0.97 (d, JHH = 6.5 Hz, 6H, CH3(iPr)),
2H, ArH(Ts)), 7.85 (d, JHH = 8.2 Hz, 2H, ArH(Ts)). ESI-MS (in
2.56−2.90 (m, 21H). 13C{1H} NMR (75 MHz, CDCl3): δ 18.3
(−CH(CH3)2), 23.4 (CH2−CH2−SH), 52.4 (−CH2−(TACN)), 54.8
(−CH(CH3)2), 55.9 (−CH2−(TACN)), 56.4 (−CH2−(TACN)),
61.4 (CH2−CH2−SH). ESI-MS (in CHCl3): m/z 291.99 (I =
MeOH): m/z 731.90 (I = 100%) (z = 1, [2a]+ (732.05)). IR (KBr): ν
2919, 1628, 1457, 1278, 1160, 1031, 694, 639 cm−1. Anal. Calcd for
C28H42ClF3N3NiO5RhS4: C, 38.09; H, 4.79; N, 4.76. Found: C, 37.71;
H, 4.73; N, 4.71. Red crystals of [2a]OTf·Et2O suitable for X-ray
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crystallography were obtained from a CH2Cl2 solution of [2a]OTf
layered with diethyl ether at 4 °C.
impurity of 3.5 equiv of HCOONa, which was confirmed by 1H NMR
spectroscopy with TSP as an internal standard.
[Ni(iPrTACN-S2)RhCp*Cl]X′ ([2b]X′: X′ = Cl, NO3, and PF6). A
methanolic solution (40 mL) of NiCl2·6H2O (184 mg, 0.773 mmol)
[(iPrTACN-S2)Ni(μ-H)RhCp*]NO3 ([4b]NO3). By a procedure
similar to Method A for [4a]NO3, a dark red powder of [4b]NO3
was obtained from [2b](NO3) (21 mg, 30.7 μmol). Yield: 2 mg, 10%.
ESI-MS (in H2O/MeOH = 9:1): m/z 585.98 (I = 100%) (z = 1, [4b]+
(586.13)). IR (KBr): ν 2952, 2912, 2852, 1937 (M−H), 1634, 1493,
i
was slowly added to a solution of PrTACN-S2H2 (0.773 mmol) in
CHCl3 (4 mL). After stirring for 1 h, a solution of [RhCp*Cl2]2 (240
mg, 0.388 mmol) in CHCl3 (40 mL) was added to the mixture, which
was stirred for 4 h. The solvent was removed to dryness, and the
resulting dark brown oil was extracted with CH3CN (100 mL). The
orange extract was filtered and evaporated to dryness. The crude
product was purified by chromatography on an alumina column
(Wako, alumina, activated particle size = 40−150 μm) and eluted with
chloroform and methanol (95:5). The first orange band was collected,
and the solvent was removed by rotary evaporation. The resultant red
oil was recrystallized from CH2Cl2/diethyl ether to give a dark red
1456, 1385, 1290, 1082, 759 cm−1
. Anal. Calcd for
C23H44Cl0.5N4Na0.5NiO3.5RhS2 ([4b]NO3·0.5NaCl·0.5H2O): C,
40.18; H, 6.45; N, 8.15. Found: C, 39.83; H, 6.14; N, 8.25. The
contamination of NaCl in the sample was assumed to be derived from
the phosphate buffer solution (NaH2PO4/Na2HPO4) and [2b](NO3).
Dark red crystals of [4b]NO3·1.5H2O suitable for X-ray crystallog-
raphy were obtained from an aqueous solution of [4b]NO3 at room
temperature.
Typical Procedure for Hydrogenations of Aldehydes and
CO2 in Aqueous Solutions Catalyzed by Dinuclear NiRh
Complexes. Benzaldehyde (250 μmol) and [3a]NO3 (5 μmol)
were dissolved in an aqueous phosphate buffer solution (2 mL, pH 7)
under H2 (0.1 MPa), and the mixture was vigorously stirred at 60 °C.
After 1 h, the reaction solution was quickly cooled with an ice bath and
was analyzed by 1H NMR spectroscopy to determine the initial
turnover frequencies (TOFs). The yields were similarly determined
from the same reactions after 12 h. Complex [3a]NO3 (5 μmol) was
dissolved in an aqueous phosphate buffer solution (5 mL, pH 7) in a
pressure vessel, and the solution was charged with CO2 (1 MPa) and
H2 (1 MPa). After stirring for 20 h at 20 °C, the pressure was released
and the solution was quickly cooled with an ice bath. The yield of
HCOONa was determined by 1H NMR measurements with TSP as an
internal standard. The HCOO− signal was observed at 8.41 ppm at pH
7. Blank experiments showed that no hydrogenation occurred in the
absence of [3a]NO3.
i
powder of [2b]Cl. Yield: 140 mg, 28% (vs PrTACN-S2H2). ESI-MS
(in MeOH): m/z 619.95 (I = 100%) (z = 1, [2b]+ (620.09)). IR
(KBr): ν 2912, 2852, 1652, 1495, 1448, 1377, 1288, 1271, 1084, 1062,
1029, 1010, 977, 755, 720 cm−1. Anal. Calcd for C23.5H43Cl3N3NiRhS2
([2b]Cl·0.5CH2Cl2): C, 40.34; H, 6.19; N, 6.01. Found: C, 40.18; H,
6.30; N, 6.25. A solution of [2b]Cl (210 mg, 0.320 mmol) in 50 mL of
−
H2O was loaded onto a Dowex 1-X8 column (NO3 form, 100−200
mesh) and eluted with water. The eluate was evaporated to give a red
powder of [2b]NO3, which was dried in vacuo. Yield: 216 mg, 99%.
ESI-MS (in MeOH): m/z 619.94 (I = 100%) (z = 1, [2b]+ (620.09)).
−
IR (KBr): ν 2970, 2916, 2852, 1663, 1384 (NO3 ), 1360, 1084, 1021,
830 cm−1. Complex [2b]NO3 is very hygroscopic, preventing
satisfactory elemental analysis results. Addition of NH4PF6 (50 mg,
0.307 mmol) to an aqueous solution (10 mL) of [2b]Cl (18 mg, 27.4
μmol) gave a red powder. The crude product was collected by
filtration, washed with water and diethyl ether, and recrystallized from
CH2Cl2/diethyl ether to give [2b]PF6 as a dark red powder. Yield: 12
mg, 57%. 1H NMR (300 MHz, CD3CN): δ 1.56 (s, CH3(Cp*)) ppm.
ESI-MS (in MeOH): m/z 619.93 (I = 100%) (z = 1, [2b]+ (620.09)).
IR (KBr): ν 2973, 2918, 2851, 1633, 1498, 1456, 1399, 1385, 1300,
839, 558 cm−1. Anal. Calcd for C23.5H43Cl2F6N3NiPRhS2 ([2b]-
PF6·0.5CH2Cl2): C, 34.88; H, 5.36; N, 5.19. Found: C, 34.96; H, 5.13;
N, 5.29. Red crystals of [2b]PF6 suitable for X-ray crystallography
were obtained from a mixed solvent of acetonitrile/diethyl ether.
[Ni(TsTACN-S2)RhCp*NO3]NO3 ([3a]NO3). Reaction of 1a (129
mg, 0.280 mmol) with [RhCp*(NO3)2] (101 mg, 0.279 mmol) in
methanol (40 mL) at room temperature for 5 h provided a red
solution. After filtration, the solvent was removed under reduced
pressure to yield a red powder of [3a]NO3. Yield: 230 mg, 99% (vs
1a). 1H NMR (300 MHz, D2O): δ 1.70 (s, 15H, CH3(Cp*)), 2.51 (s,
3H, −CH3(Ts)), 2.43−2.49, 2.61−2.88, 2.90−3.03, 3.36−3.80, 4.16−
X-ray Crystallographic Analyses. Crystallographic and exper-
imental data for 1a, 1b, [2a]OTf·Et2O, [2b]PF6, [3a]NO3, [4a]-
NO3·1.5H2O, and [4b]NO3·1.5H2O are listed in Tables S1−S3 (see
the Supporting Information). Measurements were carried out on a
Rigaku AFC8R/Mercury CCD diffractometer equipped with graphite-
monochromated Mo Kα radiation (0.71075 Å) using a rotating-anode
X-ray generator (50 kV, 170 mA) and a Rigaku VariMax Mo/Saturn
CCD diffractometer equipped with graphite-monochromated Mo Kα
radiation using a rotating-anode X-ray generator RA-Micro7 (50 kV,
24 mA). A total of 2160 oscillation images, covering a whole sphere of
6° < 2θ < 55°, were corrected by the ω-scan method with Δω = 0.25°.
The crystal-to-detector (70 × 70 mm) distance was set at 60 mm. The
data were processed using the Crystal Clear 1.3.5 program (Rigaku/
MSC)13 and corrected for Lorentz-polarization and absorption
effects.14 The structures of complexes were solved by direct methods
with SHELX-9715 ([2b]PF6), SIR-9216 (1a, [4a]NO3) and SIR-9716
(1b, [2a]OTf, [3a]NO3, [4b]NO3) and were refined on F2 with full-
matrix least-squares techniques with SHELXL-9715 using the Crystal
Structure 4.0 package.17 All non-hydrogen atoms (except the
disordered nitrate anion of [4b]NO3) were refined with anisotropic
thermal parameters, and the C−H hydrogen atoms were calculated at
ideal positions and refined with riding models. The hydride H atoms
of [4a]NO3 (H1, H2) and [4b]NO3 (H1) were determined from
difference Fourier maps and were refined isotropically. In the structure
of [3a]NO3, the coordinated nitrate anion was disordered and refined
with two site pivoting models (N41O32O4O5 and N42O31O4O6),
each having 0.5 occupancy with the O4 atom on the pivot axis with 1.0
4.35 (m, 20H), 7.61 (d, 3JHH = 8.3 Hz, 2H, ArH(Ts)), 7.98 (d, 3JHH
=
8.3 Hz, 2H, ArH(Ts)). ESI-MS (in H2O/MeOH = 9:1): m/z 758.94
(I = 100%) (z = 1, [3a]+ (759.07)). IR (KBr): ν 2915, 2855, 1458,
1384, 1352, 1325, 1274, 1164, 713, 693, 548 cm−1. Anal. Calcd for
C27H42N5NiO8RhS3: C, 39.43; H, 5.15; N, 8.52. Found: C, 39.04; H,
4.93; N, 8.38. Red crystals of [3a]NO3 suitable for X-ray
crystallography were obtained from a MeOH/diethyl ether solution.
[(TsTACN-S2)Ni(μ-H)RhCp*]NO3 ([4a]NO3). Method A: Complex
[3a]NO3 (38 mg, 46 μmol) was dissolved in a phosphate buffer
solution (pH 7, 25 mM, 2 mL), and H2 gas (0.1 MPa) was bubbled
through the solution at room temperature. The reaction mixture was
stirred for 5 h to afford a very air-sensitive dark red precipitate of
[4a]NO3, which was filtered off. Yield: 24 mg, 67%. Brown crystals of
[4a]NO3·1.5H2O suitable for X-ray crystallography were obtained
from a mixed solvent of MeOH/diethyl ether at 4 °C. Method B:
Complex [3a]NO3 (21 mg, 26 μmol) was treated with HCOONa (46
mg, 0.676 mmol) in H2O (1 mL) at pH 7 at room temperature for 5 h
to yield a dark red powder of [4a]NO3, which was filtered and dried in
vacuo. Yield: 9 mg, 48% (vs [3a]NO3). ESI-MS (in MeOH): m/z
697.99 (I = 100%) (z = 1, [4a]+ (698.09)). IR (KBr): ν 2912, 1923
(M−H), 1635, 1597, 1456, 1384, 1362, 1167, 1083, 691 cm−1. Anal.
Calcd for C30.5H46.5N4Na3.5NiO12RhS3 ([4a]NO3·3.5HCOONa): C,
36.65; H, 4.69; N, 5.61. Found: C, 36.67; H, 4.81; N, 5.94. The sample
for elemental analysis was prepared by Method B and contained an
−
occupancy. Both of the disordered NO3 fragments were constrained
to be planar and were refined anisotropically. In the structure of
−
[4b]NO3, the counteranion of NO3 was disordered at a special
position (N4O1O2O3) and around another special position
(N5O4O5O6) with each being 0.5 occupancy. The former nitrate
was refined anisotropically, and the latter was constrained as a planar
rigid model with isotropic thermal parameters. All calculations were
carried out on a Pentium PC with the Crystal Structure 4.0 package.17
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1
equatorial ones. The H NMR spectrum of 1a showed sharp
signals, and in contrast, that of 1b exhibited only broad
featureless signals, which is consistent with a high-spin Ni(II)
state.
RESULTS AND DISCUSSION
■
N3S2 Dithiolato Ligands RTACN-S2H2 (R = Ts, iPr). New
ligands, 1,4-bis(2-mercaptoethyl)-7-R-1,4,7-triazacyclononane
i
(RTACN-S2H2; R = Ts, Pr), were designed to prepare a NiII
Dinuclear NiRh Complexes [Ni(TsTACN-S2)RhCp*X]X′
(X = Cl, X′ = Cl, OTf ([2a]X′); X = X′ = NO3 ([3a]NO3)) and
[Ni(iPrTACN-S2)RhCp*Cl]X′ (X′ = Cl, NO3, PF6 ([2b]X′)).
The dithiolato-bridged NiIIRhIII dinuclear complex [Ni-
(TsTACN-S2)RhCp*Cl]Cl ([2a]Cl) was prepared by reacting
1a with 0.5 equiv of [RhCp*Cl2]2 in dichloromethane and was
further converted by treatment with NH4OTf in water to a red
powder of [2a]OTf quantitatively. When 1a was reacted with
an equimolar amount of [RhCp*(NO3)2] in methanol,
[Ni(TsTACN-S2)RhCp*(NO3)]NO3 ([3a]NO3) was obtained
in high yield. Complexes [2a]X′ (X′ = Cl, OTf) and [3a]NO3
were characterized by 1H NMR, ESI-MS (Figure S2 for
[2a]OTf and Figure S4 for [3a]NO3 in the Supporting
Information), IR, and elemental analysis. The IR spectrum of
[3a]NO3 (Figure S7, Supporting Information) showed peaks at
1384 cm−1 as well as at 1458 and 1274 cm−1. The former peak
center having three Ni−N and two Ni−S bonds with a
potential one vacant coordination site. In addition to the ability
of dithiolato units to bridge an additional metal, the vacant
coordination site should be readily occupied by exogenous
substrates, and furthermore, the electronic state of the Ni
center can be controlled by changing the substituent R on the
TACN ligand. In pioneering papers, the syntheses of TACN-
based ligands with one, two, and three thiol pendants and the
corresponding mononuclear and homometallic dinuclear
complexes have been reported.18 In this work, we have
constructed NiIIRhIII heterodinuclear centers in stepwise
i
fashion by using RTACN-S2H2 ligands (R = Ts, Pr), which
were synthesized from the reactions of RTACN with ethylene
sulfide in high yields (Scheme 1).
Mononuclear Ni Complexes [Ni(RTACN-S2)] (R = Ts
(1a), iPr (1b)). The mononuclear Ni complexes 1a and 1b were
prepared from the reactions of RTACN-S2H2 (R = Ts (1a),
iPr(1b)) with NiCl2·6H2O in mixed solvents of CHCl3/
methanol. The Ni1 atom of 1a has a square-planar coordination
geometry with bonding parameters that are usual for the
previously reported NiIIN2(amino)S2(thiolato) complexes
(Figure 1a).19 The distances between Ni1 and the nitrogen
−
is attributable to a free NO3 anion, and the latter two are
−
assignable to a coordinating NO3 ligand in a monodentate
fashion.21 Complexes [2a]Cl and [3a]NO3 have high solubility
in H2O ([2a]Cl, 30 mg/mL; [3a]NO3, 21 mg/mL at 20 °C)
and were stable in water in a range of pH 1−11, which were
1
confirmed by H NMR spectra.
ORTEP drawings for the complex cations of [2a]OTf and
[3a]NO3 are shown in Figure 2 and Figure S1 (Supporting
Figure 1. (a) ORTEP drawing of 1a. Hydrogen atoms are omitted for
clarity. Selected distances (Å) and angles (deg): Ni1−S1 =
2.1632(14), Ni1−S2 = 2.1698(10), Ni1−N1 = 1.965(3), Ni−N2 =
1.973(4), Ni1−N3 = 3.423(4), S1−Ni1−S2 = 93.54(5), S1−Ni1−N1
= 90.38(11), S1−Ni1−N2 = 178.09(7), S2−Ni1−N1 = 170.11(9),
S2−Ni1−N2 = 88.35(8), N1−Ni1−N2 = 87.70(13). (b) ORTEP
drawing of 1b. Selected distances (Å) and angles (deg): Ni1−S1 =
2.318(5), Ni1−S2 = 2.418(5), Ni1−N1 = 2.224(9), Ni−N2 =
2.133(8), Ni1−N3 = 2.207(8), S1−Ni1−S2 = 97.11(13), S1−Ni1−
N1 = 88.2(3), S1−Ni1−N2 = 141.3(3), S1−Ni1−N3 = 131.1(3), S2−
Ni1−N1 = 170.1(2), S2−Ni1−N2 = 86.9(3), N1−Ni1−N2 = 83.7(3).
Figure 2. ORTEP drawing of [2a]OTf. The counteranion (OTf) and
hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg): Ni1−Rh1 = 3.207(2), Ni1−S1 = 2.173(3), Ni1−S2 =
2.164(3), Ni1−N1 = 1.959(7), Ni1−N2 = 1.977(6), Ni1−N3 =
2.614(7), Rh1−S1 = 2.414(3), Rh1−S2 = 2.395(3), Rh1−Cl1 =
2.412(3), Ni1−S1−Rh1 = 89.26(8), Ni1−S2−Rh1 = 88.54(8).
Information). The complex cation of [2a]OTf is composed of
NiIIRhIII heterometallic centers bridged by dithiolato units of
the TACN ligand, without metal−metal interaction (Ni1−Rh1
= 3.207(2) Å). The Ni1 atom retains a square-planar geometry
as in 1a; the interatomic distance of Ni1−N3 (2.614((7) Å) is
appreciably shorter than that of 1a (3.423(4) Å) with lone-pair
electrons on the N3 atom directed to the Ni1 center. The Rh1
atom is ligated by η5-Cp* and a terminal Cl− ion. The structure
of [3a]+ is essentially similar to that of [2a]+ (Ni1−Rh1 =
3.2128(6) Å, Ni1−N3 = 2.601(4) Å), except that the Cl− anion
coordinating to Rh1 is replaced by a nitrate anion with a
monodentate fashion.
atoms of the TACN ligand are not equivalent, wherein the
distance of Ni1−N3 (3.423(4) Å) is quite longer than those of
Ni1−N1 and Ni−N2 (1.965(3) and 1.973(4) Å), suggesting no
interaction between the Ni1 and N3 atoms since the basicity of
the N3 amino group is considerably reduced, owing to the
electron-withdrawing Ts group. In contrast to the traditional
NiIIN2S2 complexes and 1a, the Ni1 atom of 1b has a distorted
trigonal-bipyramidal geometry (τ value = 0.48)20 with a normal
coordination of the N3 atom possessing an electron-donating
iPr group (Figure 1b). The S2 and N1 atoms occupy the axial
positions, whereas the S1, N2, and N3 atoms are in the
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By a method similar to that of [2a]Cl, reaction of 1b with 0.5
equiv of [RhCp*Cl2]2 in a mixed solvent of chloroform/
methanol was carried out, but any multinuclear complexes were
[4a]NO3 was also synthesized from reaction of [3a]NO3
with HCOONa in H2O at room temperature through β-
hydrogen elimination with evolution of CO2. Complex
[2b]NO3 reacted with H2 under a similar condition for
[3a]NO3 to afford [(iPrTACN-S2)Ni(μ-H)RhCp*]NO3
([4b]NO3), but its isolated yield was very low. Kinetic analyses
by monitoring the decrease of the absorption band at 305 nm
for [3a]NO3 and 254 nm for [2b]NO3 revealed that the
reactions with H2 (0.1 MPa) at pH 7.0 at 20 °C proceeded with
pseudo-first-order kinetics over four half-lives, as shown in
Figures S11 and S12 (Supporting Information). The rate of the
reaction of [3a]NO3 with H2 (kobs = 1.9 × 10−2 s−1) is
significantly faster than that of [2b]NO3 (kobs = 4.5 × 10−3 s−1)
under the same conditions.
i
not isolated in pure form. Alternatively, reaction of PrTACN-
S2H2 with 1 equiv of NiCl2·6H2O, followed by addition of 0.5
equiv of [RhCp*Cl2]2 in one pot, afforded a dinuclear NiRh
complex [2b]Cl that was purified by chromatography on an
alumina column (Scheme 1). Complex [2b]Cl has high
solubility in H2O (55 mg/mL at 20 °C) and is stable in
water in a range of pH 2−11 confirmed by ESI-MS spectra. To
the solution of [2b]Cl in H2O was added NH4PF6 to provide a
red powder of [2b]PF6. Treatment of [2b]Cl with anion-
exchange resin in water also afforded [2b]NO3 in good yield.
The 1H NMR spectra of complexes [2b]X′ (X′ = Cl, NO3, PF6)
in CDCl3 show only broad resonances, owing to para-
magnetism of the NiII metallo fragment of 1b. The ESI mass
spectra of [2a]PF6 is shown in Figure S3 in the Supporting
Information. An ORTEP diagram for the complex cation of
[2b]PF6 is shown in Figure 3. The structure consists of a
The ESI mass spectrum of [4a]NO3 in MeOH (Figure S5a−
c, Supporting Information) showed a prominent signal at m/z
697.99 with a characteristic distribution of isotopomers that
matched well the calculated isotopic distribution for [4a]+. To
confirm the origin of the hydride, [3a]NO3 was reacted with D2
in H2O (pH 7) for 5 h at room temperature, and the reaction
product was analyzed by ESI-MS (Figure S5d−f, Supporting
Information) to show that the signal at m/z 697.99 for [4a]+
was shifted to 698.96, corresponding to [(TsTACN-S2)Ni(μ-
D)RhCp*]+ ([4a-d]+). Similarly, in the ESI mass spectrum of
[4b]NO3 in MeOH (Figure S6a−c, Supporting Information),
the signal at m/z 585.98 was shifted to m/z 587.05 by using D2
instead of H2 (Figure S6d−f, Supporting Information). These
mass spectral results clearly demonstrated that the hydride
complexes [4a]NO3 and [4b]NO3 were generated through
heterolytic cleavage of H2 (eq 1). The IR spectrum of [4a]NO3
(Figure S8, Supporting Information) exhibited a peak at 1923
cm−1, which falls between the values for terminal (2250−1700
cm−1) and bridging (1700−1200 cm−1) metal−hydride units
and might be assigned to the asymmetrical semibridging
hydride of ν(Ni−H−Rh).21 The peak was definitely shifted to
1374 cm−1 in [4a-d]NO3, and the energy difference (549 cm−1)
was consistent with replacement of the hydride H by the D
atom. In the IR spectrum of [4b]NO3 (Figure S9, Supporting
Information), a peak at 1937 cm−1 was assigned to ν(Ni−H−
Rh) and indicated that the semibridging character of the
hydride ligand of [4b]NO3 may be reduced in comparison with
that in [4a]NO3, presumably due to stronger trans influence of
Figure 3. ORTEP drawing of [2b]PF6. The counteranion (PF6) and
hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg): Ni1−Rh1 = 3.262(2), Ni1−S1 = 2.323(2), Ni1−S2 =
2.315(2), Ni1−N1 = 2.097(6), Ni1−N2 = 2.057(6), Ni1−N3 =
2.101(6), Rh1−S1 = 2.424(3), Rh1−S2 = 2.400(3), Rh1−Cl1 =
2.410(2), Ni1−S1−Rh1 = 86.79(7), Ni1−S2−Rh1 = 87.57(6).
NiIIRhIII dinuclear core (Ni1−Rh1 = 3.262(2) Å) essentially
similar to that of [2a]OTf. Unlike complex [2a]OTf, the Ni1
atom adopts a distorted square-pyramidal geometry (τ value =
0.20),20 where the Ni1−N3 length (2.101(6) Å) is significantly
shorter than that of 2.614(7) Å in [2a]OTf due to the strong σ-
i
the amino group connected to the Pr substituent.
i
The detailed structure of [4a]NO3 was determined by an X-
donating property of the N3 atom with an Pr group. These
ray analysis. The asymmetric unit involves two independent,
results clearly demonstrated that the steric and electronic
structures of the NiII center of the dinuclear scaffold
[Ni(RTACN-S2)RhCp*Cl]+ can be varied depending on the
substituent group on the N3 amino nitrogen.
chemically identical cations of [4a]+ and two NO3 anions,
−
together with three water molecules, and an ORTEP drawing
for one of the cations is illustrated in Figure 4 (the diagram of
the other cation is shown in Figure S10, Supporting
Information). The complex contains NiIIRhIII dimetal centers
bridged by two thiolato units and a hydride ligand. The μ-H
atom was determined by difference Fourier syntheses to reveal
a semibridging mode mainly interacting with the Rh center.
The Ni atom adopts a considerably distorted octahedral
geometry with the N3S2 pentadentate ligand and the hydride
anion. The Ni−N3/N7 distance for [4a]NO3 (2.188(3),
2.192(3) Å), is fairly shorter than those for [2a]OTf
(2.614(7) Å) and [3a]NO3 (2.601(4) Å). Furthermore, the
dihedral angles defined by Ni(μ-S)2 and Rh(μ-S)2 for [4a]NO3
(76.09°, 75.96°) are significantly smaller than those for
[2a]OTf (134.455°) and [3a]NO3 (135.371°), and then the
Ni···Rh distance for [4a]NO3 (2.6921(6), 2.6906(6) Å) is
concomitantly shorter than those for [2a]OTf (3.207(2) Å)
Hydride-Bridged NiIIRhIII Dinuclear Complexes
i
[(RTACN-S2)Ni(μ-H)RhCp*]NO3 (R = Ts ([4a]NO3), Pr
([4b]NO3)). Complex [3a]NO3 readily reacted with H2 (0.1
MPa) in H2O (pH 7) at room temperature to give a hydride-
bridged NiIIRhIII complex [(TsTACN-S2)Ni(μ-H)RhCp*]NO3
([4a]NO3) in good yield (eq 1). The hydride complex
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Stability of the isolated hydride complexes was examined.
Both complexes [4a]NO3 and [4b]NO3 did not undergo an H/
D exchange reaction under mild conditions in protic media. In
particular, complex [4a]NO3 was quite stable in water at pH
7−11 under N2, and below ca. pH 6 (adjusted by the aqueous
solution of HNO3), complex [4a]NO3 slowly decomposed to
[3a]NO3 with evolution of H2. Formation of [3a]NO3 was
monitored by ESI-MS and UV−vis spectra. The first-order rate
constant (kobs) of the decomposition of [4a]NO3 (under N2,
pH 4, 20 °C) was 2.7 × 10−3 s−1 (Figure S13, Supporting
Information). In contrast, the decomposition of [4b]NO3
(under N2, pH 4, 20 °C) was too fast to determine the rate
constant accurately. These indicate that [4a]NO3 is much more
stable in H2O than [4b]NO3 because the hydride of [4a]NO3 is
assumed to be effectively stabilized by nesting between two
metal centers.
Figure 4. ORTEP drawing for one of the complex cations of [4a]NO3.
Selected distances (Å) and angles (deg): Ni1−Rh1 = 2.6921(6), Ni1−
H1 = 2.18(5), Ni1−S1 = 2.3087(11), Ni1−S2 = 2.3312(12), Ni1−N1
= 2.068(4), Ni1−N2 = 2.057(4), Ni1−N3 = 2.188(3), Rh1−S1 =
2.3986(11), Rh1−S2 = 2.3856(10), Rh1−H1 = 1.66(5), Ni1−S1−Rh1
= 69.74(3), Ni1−S2−Rh1 = 69.59(4).
Hydrogenation of Aldehydes Catalyzed by Dinuclear
NiRh Complexes. Before we tried the hydrogenation of
aldehydes into the corresponding alcohols catalyzed by the
dinuclear NiRh complexes, some stoichiometric reactions were
investigated. In the absence of H2, benzaldehyde reacted with
equimolar [4a]NO3 in water (1 h, pH 7, 60 °C) to be reduced
into benzyl alcohol in 8% yield. The low yield may be attributed
to the instability of the hydride species in the reaction mixture.
In contrast to [4a]NO3, complex [4b]NO3 did not reduce
benzaldehyde at all under the same conditions because of the
rapid decomposition of [4b]NO3 in H2O, which was confirmed
by ESI-MS. Contrary to the air-sensitive hydride complexes,
under catalytic conditions (H2 = 0.1 MPa, pH 7, 60 °C), the
air-stable precursor [3a]NO3 has proven to catalyze the
hydrogenation of benzaldehyde into benzyl alcohol in a yield
of 92% after 12 h. As shown in Table 1, the catalytic activities
for hydrogenation of benzaldehyde are dependent on the pH of
the reaction solutions (entries 1−4). The initial TOFs
(turnover frequencies after 1 h) of the hydrogenation of
benzaldehyde showed a maximum at pH 7. This pH
dependence might be related to both the stability of the
hydride complex [4a]NO3 and the activation process of
and [3a]NO3 (3.2130(6) Å). These structural features of
[4a]NO3 demonstrated that the flexible dithiolato bridges allow
such an interesting structural change triggered by incorporating
a hydride ligand that is in concert with coordination of the NTs
pendant amino group. A similar tendency has been observed in
the Ni(μ-H)Ru complex [NiII(H2O)(μ-RS)2(μ-H)RuII(η6-
C6Me6)]NO3 ((RSH)2 = N,N′-dimethyl-N,N′-bis-
(mercaptoethyl)-1,3-propanediamine).9d
An ORTEP drawing of [4b]NO3 is shown in Figure 5.
Although the overall structure of the complex cation of
Table 1. Hydrogenation of Benzaldehyde in Aqueous
Solutions Catalyzed by Dinuclear NiRh Complexes and
a
Mononuclear Rh Complexes
Figure 5. ORTEP drawing for the complex cation of [4b]NO3.
Selected distances (Å) and angles (deg): Ni1−Rh1 = 2.7228(4), Ni1−
H1 = 2.03(5), Ni1−S1 = 2.3151(8), Ni1−S2 = 2.3395(8), Ni1−N1 =
2.069(2), Ni1−N2 = 2.107(2), Ni1−N3 = 2.056(2), Rh1−S1 =
2.3926(7), Rh1−S2 = 2.3568(7), Rh1−H1 = 1.50(4), Ni1−S1−Rh1 =
70.65(2), Ni1−S2−Rh1 = 70.87(2).
yield
d
b
c
entry
catalyst
[3a]NO3
pH T (°C) TOF TON
(%)
1
2
3
6
60
60
60
60
20
40
60
60
60
60
2
8
16
32
46
43
2
32
63
92
85
4
[3a]NO3
3
[3a]NO3
7
22
18
0
4
[3a]NO3
8
5
[3a]NO3
7
[4b]NO3 is essentially identical to that of [4a]NO3, the Ni−N3
bond length of [4b]NO3 (2.056(2) Å) is notably shorter than
the corresponding values of [4a]NO3 (2.188(3), 2.192(3) Å),
and the Ni···Rh interatomic distance (2.7228(4) Å) is
appreciably elongated from those of [4a]NO3 (av 2.6914 Å).
These differences should result from the stronger σ-donating
ability of the NiPr amino group, which might cause a greater
trans influence onto the bridging hydride. The less bridging
character of the hydride in [4b]NO3 is consistent with the IR
6
[3a]NO3
7
10
0
31
2
61
4
7
[2b]NO3
7
8
[2b]NO3
10
7
0
2
3
9
[Cp*Rh(bpy)Cl]Cl
Cp*Rh(NO3)2
2
15
21
29
51
10
7
6
a
Benzaldehyde was treated with H2 in the presence of the catalyst in
aqueous buffer solutions as described in the Experimental Section for
12 h. Conditions: catalyst = 5 μmol (2.5 mM), substrate = 250 μmol
(125 mM), H2 = 0.1 MPa, buffer solution = 2 mL. TOF: initial
b
1
spectrum. The H NMR spectra of [4a]NO3 and [4b]NO3
turnover frequency ([(mol of product)/(mol of catalyst)]/h),
c
1
showed only broad featureless signals, indicating that the
complexes are paramagnetic, owing to the octahedral NiII
center.
determined by H NMR analysis after 1 h. TON: turnover number
((mol of product)/(mol of catalyst)), determined by 1H NMR analysis
d
after 12 h. Determined after 12 h.
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Article
aldehydes by protons.3j,5b,22 The rates of hydrogenation of
benzaldehyde are also dependent on the temperature of the
solutions (entries 3, 5, and 6), indicating that the higher the
temperature, the faster the rate of the hydrogenation in the
range of 20−60 °C. Complex [2b]NO3 was almost inactive,
and the mononuclear Rh complexes, such as [RhCp*Cl(bpy)]
Cl and [RhCp*(NO3)2], showed only lower catalytic activities
than that of [3a]NO3, probably because the corresponding
hydride species were unstable in H2O or they did not activate
H2 in the conditions (entries 7−10). Hydrogenation of a
variety of aldehydes was promoted by [3a]NO3 at pH 7 at 60
°C (Table 2). Both aromatic (entries 1−4) and aliphatic
Scheme 2
Table 2. Hydrogenation of Aldehydes in Aqueous Solutions
a b c d
, , ,
Catalyzed by [3a]NO3
Table 3. Hydrogenation of CO2 or Bicarbonate in Aqueous
Solutions Catalyzed by Dinuclear NiRh Complexes and
a
Mononuclear Rh Complexes
b
entry
catalyst
[3a]NO3
pH
TON
1
2
4
7
10
16
34
38
23
0
[3a]NO3
3
[3a]NO3
11
11
11
4
4
[2a]Cl
5
[4a]NO3
6
[2b]NO3
7
[2b]NO3
7
0
8
[2b]NO3
11
11
11
11
1
a
Conditions: catalyst = 5 μmol (2.5 mM), substrate = 250 μmol (125
9
[2b]Cl
1
b
mM), H2 = 0.1 MPa, T = 60 °C, buffer solution = 2 mL, pH 7. TOF:
initial turnover frequency ([(mol of product)/(mol of catalyst)]/h),
10
11
[Cp*Rh(bpy)Cl]Cl
Cp*Rh(NO3)2
1
0
c
1
determined by H NMR analysis after 1 h. TON: turnover number
a
Conditions: catalyst = 5 μmol (1 mM), H2 = 1 MPa, CO2 = 1 MPa, T
((mol of product)/(mol of catalyst)), determined by 1H NMR analysis
b
= 20 °C, t = 20 h, buffer solution = 5 mL. TON: turnover number
((mol of product)/(mol of catalyst)), determined by 1H NMR analysis
of the reaction mixture samples.
d
after 12 h. Determined after 12 h.
aldehydes (entry 5) are converted to the corresponding
alcohols. Hydrogenation of benzaldehyde (entry 1) proceeded
much more efficiently than that of benzaldehyde derivatives
with electron-donating groups, such as p-tolualdehyde,
salicylaldehyde, and p-anisaldehyde (entries 2−4). The catalytic
cycle in H2O at pH 7 may involve formation of the hydride-
bridged complex [4a]+ from heterolytic activation of H2 by the
NiIIRhIII species [3a]+ and reduction of the aldehydes by [4a]+
to restore the starting compound (Scheme 2).
Hydrogenation of CO2 with Dinuclear NiRh Com-
plexes. We have further investigated the hydrogenation of
CO2 into formate catalyzed by the dinuclear NiRh complex-
es.4a,d,23 The NiRh dinuclear complexes [2a]Cl, [3a]NO3, and
[2b]NO3, together with the mononuclear Rh complexes, were
examined under 2 MPa pressure of H2 and CO2 (1:1) in H2O
at 20 °C, and the product HCOO− was quantified by 1H NMR
(Table 3). Complex [3a]NO3 effectively catalyzed the
hydrogenation of CO2 in H2O under both acidic and basic
conditions (entries 1−3). The catalytic activity of [3a]NO3
(entry 3) was comparable to that of [2a]Cl (entry 4),
suggesting that the hydride complex [4a]+ is an active species.
Complex [4a]NO3 was also a fair catalyst for the CO2
hydrogenation with a lower TON by 33% than the
corresponding starting complex [3a]NO3 (entry 5). Contrasted
to the NiRh complexes with the TsTACN-S2 ligand, complex
i
[2b]NO3 with the PrTACN-S2 ligand (entries 6−8) and the
mononuclear Rh complexes (entries 10 and 11) showed almost
no activity. Although Himeda et al. have reported that
[RhCp*Cl(bpy)]Cl catalyzed the hydrogenation of CO2 in
H2O under higher temperature and pressure (80 °C, 4 MPa
(CO2/H2 = 1:1)),23g it showed only slight activity under the
present conditions (20 °C, 2 MPa (CO2/H2 = 1:1)) because
the postulated terminal hydride species [RhCp*(H)(bpy)]+
should be quite unstable in water. The catalytic cycle of
hydrogenation of CO2 with [2a]Cl or [3a]NO3 was estimated
as follows.4d,23h The dinuclear NiRh complexes reacted with H2
in H2O to afford the hydride complex [4a]+, which may be
much more stable in water than [4b]+ and the mononuclear
species [RhCp*(H)(bpy)]+, and consequently, the hydride
[4a]+ was able to react with CO2 to afford formate. In the
reaction solution of [2a]Cl with 2 MPa of H2 and CO2 (1:1),
only [2a]+ and [4a]+ were observed by ESI-MS and no other
NiRh dimetallic species was detected, implying that reaction of
[4a]+ and CO2 is the rate-determining step, and a reason for
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Organometallics
Article
the low activity of [2b]+ could be attributable to the instability
of the corresponding hydride [4b]+ in water.
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CONCLUSIONS
■
The hydride-bridged NiIIRhIII complexes, [(RTACN-S2)Ni(μ-
H)RhCp*]+ (R = Ts ([4a]+), Pr ([4b]+)), were successfully
i
synthesized by utilizing the new N3S2 dithiolato ligands,
through heterolytic activation of H2 by [Ni(RTACN-S2)-
i
RhCp*X]+ (R = Ts, X = NO3 ([3a]+); R = Pr, X = Cl
([2b]+)). The stability of the hydride in [4a]+ and [4b]+ could
be tuned by varying the R group of the TACN ligand through
trans influence across the NiII center. In fact, complex [4a]+
with an electron-withdrawing Ts group was quite stable and
capable of reducing benzaldehyde in water. Contrastingly,
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complex [4b]+ with an electron-donating Pr group, quickly
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[3a]+ and [2a]+ as precursors of an active species, [4a]+, and is
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AUTHOR INFORMATION
Corresponding Author
*E-mail: kure@cc.nara-wu.ac.jp (B.K.), tanase@cc.nara-wu.ac.jp
(T.T.). Tel: +81 742 20 3394. Fax: +81 742 20 3847.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research and that on Priority Area 2107 (nos. 22108521,
24108727) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and by Nara Women’s
University Grants for Research Projects. B.K. thanks Profs. S.
Ogo (Kyushu University) and S. Fukuzumi (Osaka University)
for valuable discussions.
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