Functionalized bipyridyl rhodium complex capable of electrode attachment for regeneration of NADH

Article abstract of DOI:10.1016/j.poly.2013.04.011

A Rh(III) complex having a functionalized bpy-OH ligand that is potentially linkable to electrode surfaces was synthesized and fully characterized. The hydrido complex, which could be generated either electrochemically by cathodic reduction of the [η⁵-Cp^*Rh(bpy-OH)Cl]Cl complex at -771 mV (versus Ag/AgCl) or chemically with formate, transformed NAD ⁺ efficiently into NADH with a TOF = 710 at 60 C.

Full text of DOI:10.1016/j.poly.2013.04.011

Polyhedron 57 (2013) 52–56
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Functionalized bipyridyl rhodium complex capable of electrode attachment
for regeneration of NADH
⇑
Dharmalingam Sivanesan, Sungho Yoon
Department of Chemistry, College of Natural Sciences, Kookmin University, 861-1 Jeoungnung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A Rh(III) complex having a functionalized bpy-OH ligand that is potentially linkable to electrode surfaces
was synthesized and fully characterized. The hydrido complex, which could be generated either electro-
Received 15 February 2013
Accepted 4 April 2013
Available online 10 April 2013
chemically by cathodic reduction of the [
g
5-Cp⁄Rh(bpy-OH)Cl]Cl complex at ꢀ771 mV (versus Ag/AgCl)
or chemically with formate, transformed NAD+ efficiently into NADH with a TOF = 710 at 60 °C.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
NADH regeneration
Indirect electrochemical method
Chemical method
Crystal structure
1. Introduction
The biotransformation of organic substrates by various
enzymes, including D-lactate dehydrogenase (LDH) [10] and horse
The regeneration of co-enzyme 1,4-NADH, the reduced form of
nicotinamide adenine dinucleotide (NAD+), is of significant impor-
tance in the highly chemoselective, regioselective, and stereo spe-
cific biotransformations of organic substrates by various enzymes
[1–5]. The conversion of NAD+ to NADH by direct and indirect elec-
trochemical methods has been extensively studied in order to in-
crease the rate of regeneration while maintaining the necessary
1,4-dihydro regioselectivity [6–8]. The direct electrochemical
reduction of NAD+ requires high over potentials, leading to the
generation of enzymatically inactive NAD-dimers through a one-
electron transfer reaction. Meanwhile, one indirect method was
found successful through use of a second enzyme, such as ferre-
doxin–NAD+-reductase, which could accept two electrons and then
transfer one electron pair collectively to NAD+. Steckhan and co-
workers introduced the cationic (bipyridine)rhodium complex
liver alcohol dehydrogenase (HLADH) [2,11], has been performed
successfully in combination with the electrochemical regeneration
of NADH utilizing Rh metal complexes as mediators. However, it
was recently reported that spatial separation between the enzymes
and metal complex mediators should be implemented to ensure
the long-term stability of the electro-enzymatic biotransformation
[12]. Unfortunately, limited effort was made to covalently immobi-
lize the mediators to generate reagent-free bioelectrocatalytic
devices, partially because of the absence of available active medi-
ators with linker units. Herein, we report the synthesis of a
bis(hydroxymethyl)-bipyridine-containing rhodium complex and
its reactivity in the regeneration of NADH by chemical and electro-
chemical methods. Our strategy was to devise a new effective
mediator bearing functionality through which it could be linked
to the surface of electrodes.
[g
5-Cp⁄Rh(bpy)Cl]+ as an elaborated redox mediator, enabling
the one-step transfer of two electrons, to efficiently catalyze the
regeneration of NADH by NAD+, in order to avoid using a second
enzyme [9]. Inspired by its magnificent reactivity, the detailed
mechanism of this electrocatalytic process has been delivered,
drawing the conclusion that metal complexes with bpy ligands
with unsubstituted or electron-releasing groups in the 4-position
of each pyridine show the highest activities toward the regenera-
tion of the co-enzyme [9].
2. Experimental
2.1. General procedure and physical measurements
All chemicals purchased were of analytical grade and used with-
out further purification unless mentioned otherwise. All the sol-
vents were distilled prior to the use; CH3CN and dichloromethane
were distilled over CaH2; THF and diethyl ether were distilled over
a mixture of sodium and benzophenone under nitrogen atmo-
sphere. The rhodium dimer {g l-Cl)Cl}2 was prepared
5-Cp⁄Rh(
⇑
using the reported procedure [13]. FT-IR spectra were recorded
using the KBr disk method with a Varian 640-IR FT-IR spectrometer.
Corresponding author.
E-mail address: yoona@kookmin.ac.kr (S. Yoon).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.011

D. Sivanesan, S. Yoon / Polyhedron 57 (2013) 52–56
53
Table 1
Crystal data and structure refinement for 1.
Empirical formula
Formula weight
T (K)
C22H27Cl2N2O2Rh
525.27
200(2)
k (Å)
0.71073
Crystal system
Space group
Unit cell dimensions
a (Å)
triclinic
P1
ꢀ
8.1023(18)
b (Å)
11.252(2)
c (Å)
13.153(3)
a
(°)
103.005(5)
b (°)
93.296(5)
109.067(5)
1093.0(4)
Scheme 1. Synthesis of bpy-OH and 1. Reagents: (a) SOCl2, Reflux (b) EtOH, Reflux
(c) NaBH4, MeOH (d){ -Cl)Cl}2, MeOH.
5-Cp⁄Rh(
c
(°)
g
l
V (Å3)
Z
2
Dcalc (Mg/m3)
1.596
1.047
UV–Vis spectra were recorded on an OPTIZEN 2120UVspectropho-
tometer. 1H NMR and 13C NMR were recorded on Varian 500 MHz
and Varian 125 MHz instruments, respectively. Mass data were
recorded on an Agilent 6130 Single Quadrupole LC/MS.
Absorption coefficient (mmꢀ1
Refinement method
Goodness-of-fit (GOF) on F2
Absorption correction
)
full-matrix least-squares on F2
1.064
multi-scan
Final R indices [I > 2
r
(I)]
R1 = 0.0630, wR2 = 0.1118
R1 = 0.1072, wR2 = 0.1502
0.769 and ꢀ1.431
R indices (all data)
2.2. Diethyl 2,20-bipyridyl-4,40-dicarboxylate
Largest difference in peak and hole (e Åꢀ3
)
The schematic representation of the total reaction route is de-
picted in Scheme 1. 2,20-bipyridyl-4,40-dicarboxylic acid (0.200 g)
and SOCl2 (6.00 mL) were refluxed under N2 atmosphere for 12 h
at 90 °C. The excess SOCl2 was removed from the reaction mixture
by distillation and the residue was dried under reduced pressure to
afford a yellow solid. Yield: 0.215 g (93%). The yellow solid, 2,20-
bipyridyl-4,40-dicarbonyl dichloride (0.220 g, 0.803 mmol) was dis-
solved in ethanol (15 mL) at 0 °C and triethylamine (0.223 mL,
1.60 mmol) was added. The temperature was slowly increased to
room temperature and the reaction was refluxed for 5 h. Ethanol
was removed under reduced pressure and the residue was diluted
with dichloromethane and deionized water. The separated organic
layer was dried over Na2SO4 and concentrated on a rotavapor to af-
ford a white solid. Yield: 0.220 g (90%). 1H NMR (CDCl3) d 8.95
(s, 2H), 8.65 (d, 2H), 4.45 (q, 2H), 1.45 (t, 3H).
Yield (0.061 g, 92%). 1H NMR (CD3OD) d 8.91 (d, 2H), 8.50 (s, 2H),
7.85 (d, 2H), 4.90 (s, 4H), 1.72 (s, 15H); 13C NMR (CD3OD) d
157.24, 154.51, 151.71, 125.40, 120.59, 97.53, 61.70, 7.72; FT-IR
(KBr, cmꢀ1) 3522 (m), 3274 (s), 3063 (s), 3063 (s), 2893 (w),
2835 (w), 2629 (w), 1619 (s), 1561(s), 1488 (s), 1413 (s), 1292(s),
1239 (s), 1160 (w), 1070 (s), 1022 (m), 896 (m), 826 (w), 605
(w); LC/MS (ESI): m/z 489.10 [g
5-Cp⁄ Rh(bpy-OH)Cl]+.
2.5. X-ray single crystal structural analysis
A single crystal was mounted at room temperature on the tips
of quartz fibers, coated with Paratone-N oil, and cooled under a
stream of cold nitrogen. Intensity data were collected on a Bruker
CCD area diffractometer running the SMART software package, with
Mo Ka radiation (k = 0.71073 Å). The structure was solved by direct
2.3. 4,40-bis(hydroxymethyl)-2,20-bipyridine (bpy-OH)
methods and refined on F2 using the SHELXTL software package [14].
Multi-scan absorption correction was applied with SADABS [15], part
of the SHELXTL program package, and the structure was checked for
higher symmetry by the program PLATON [16]. The pentamethylcy-
clopentadienyl ligand (Cp⁄) in complex 1 was disordered in two
positions with equal occupancy. All non-hydrogen atoms were re-
fined anisotropically. In general, hydrogen atoms were assigned
idealized positions and given thermal parameters equivalent to
1.2 times the thermal parameter of the carbon atom to which they
were attached. Data collection and experimental details for the
complex are summarized in Table 1.
To
a
solution of diethyl 2,20-bipyridyl-4,40-dicarboxylate
(0.200 g, 0.666 mmol) in ethanol (20 mL), NaBH4 (0.580 g,
16.0 mmol) was added. This mixture was refluxed at 80 °C for
10 h. The progress of the reaction was monitored by thin layer
chromatography. After completion, the reaction was diluted with
saturated NH4Cl (25 mL) and extracted twice with ethyl acetate
(60 mL). The combined ethyl acetate layers were dried over Na2SO4
and concentrated under reduced pressure to yield the desired
product as a white solid. Yield: 0.120 g (83%). 1H NMR (DMSO-
d6) d 7.80(d, 2H), 7.42(s, 2H), 6.60(d, 2H), 3.95(s, 4H); 13C NMR
(DMSO-d6) d 154.9, 147.95, 120.45, 177.97, 61.37; FT-IR (KBr,
cmꢀ1)3369 (s), 3194 (m), 2877 (m), 2819 (m), 2735 (m), 2634
(w), 1598(s), 1556 (s), 1456 (s), 1381 (s), 1308 (m), 1234 (m),
1207 (w), 1049 (s), 996 (s), 906 (m), 821 (m), 758 (m), 652 (w),
605 (w).
2.6. Electrochemistry
Cyclic voltammetry studies were carried out using a ZAHNER-
elektrik IM6 potentiostat and a WonA Tech potentiostat. The cell
was a standard three-electrode system with a platinum wire as a
counter electrode, a glassy carbon or a Pt electrode as a working
electrode, and a Ag/AgCl or a Ag/AgNO3 electrode as a reference
electrode. Tetrabutylammonium hexafluorophosphate [(Bu4N)PF6],
0.1 M in CH3CN or Tris/HCl (0.1 M, pH 7.50) was used as an
electrolyte.
2.4. [g
5-Cp⁄Rh(bpy-OH)Cl]Cl (1)
To a methanol(2 mL) solution of 4,40-bis(hydroxymethyl)-2,20-
bipyridine (0.027 g, 0.129 mmol), { -Cl)Cl}2 (0.040 g,
5-Cp⁄Rh(
g
l
0.064 mmol) was added under N2 atmosphere. The color of the
reaction mixture slowly turned to orange yellow from reddish
brown. After 3 h stirring at room temperature, the volume was re-
duced to 1 mL; upon addition of diethyl ether (10 mL) precipitate
was formed. Suitable single crystals for X-ray diffraction analysis
were obtained from MeOH and diethyl ether by vapor diffusion.
2.7. Regeneration of NADH from NAD+
The catalytic conversion of NAD+ [8 mM] to NADH by 1 [8
was carried out at various temperatures in phosphate buffer (5 mL,
lM]

54
D. Sivanesan, S. Yoon / Polyhedron 57 (2013) 52–56
Table 2
Chemical method regeneration of NADH by 1.
Entry
Catalyst
ns/nc
T (°C)
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
1
1
1
1
1000
1000
1000
1000
1000
100
100
100
100
100
25
40
60
100
60
25
40
60
80
100
60
60
200
710
310
[g
5-Cp⁄Rh(bpy)Cl]+Cl
875 [17]
80
1
1
1
1
1
135
193
189
36
9
10
11
[
g
5-Cp⁄Rh(bpy)Cl]+Cl
100
96
ns – conc of [NAD+], nc – conc of 1, 0.350 M of [HCOONa], Phosphate buffer (pH
7.25), TOF was calculated based on the UV–Vis spectrum of NADH.
pH 7.25) with sodium formate as a reducing agent. The regenera-
tion of NADH was confirmed by UV absorption at 340 nm and its
corresponding emission peak at 469 nm. Turnover frequencies
(TOF) were calculated after 30 min of catalytic conversion of
NAD+ to NADH (Table 2) [9,17].
Fig. 1. ORTEP drawing of the catalyst 1 partial labeling (without a counter anion
Clꢀ), showing the 50% probability of thermal ellipsoids.
3. Results and discussion
3.1. General consideration and synthesis of a new mediator
Table 3
Bond lengths (Å) and Bond angles (°) for 1.
In the design of the new mediator for the regeneration of NADH,
improvement was required in the catalytic activity, and a possible
anchoring group was desired to covalently link the mediator to the
surface of the electrodes. To increase the rate of regeneration, elec-
tron-releasing groups such as –NH2, –CH3, –CH2OH, –CH2SH, –CH2
NH2, or –OCH3 should be incorporated into the bipyridine ligands,
because Rh catalysts with bpy ligands bearing electron-releasing
groups in the 4-position of each pyridine showed the highest activ-
ities towards the regeneration of NADH [9]. Recently, it was re-
Bond lengths (Å)
Rh(1)–N(1)
Rh(1)–N(2)
Rh(1)–Cl(1)
Rh(1)–C(13A)
2.107(5)
2.104(5)
2.363(3)
2.120(2)
Rh(1)–C(14A)
Rh(1)–C(15A)
Rh(1)–C(16A)
Rh(1)–C(17A)
2.20(2)
2.21(2)
2.20(2)
2.16(3)
Bond angles (°)
N(1)–Rh(1)–N(2)
N(1)–Rh(1)–Cl(1)
N(2)–Rh(1)–Cl(1)
77.3(2)
88.3(3)
87.3(3)
N(2)–Rh(1)–C(14A)
N(2)–Rh(1)–C(13A)
N(1)–Rh(1)–C(15A)
146.4(9)
108.3(9)
103.0(8)
ported that derivatization of [g
5-Cp⁄Rh(bpy)Cl]+ species with
thiol or amino groups led to suppressed catalytic reduction of
NAD+ [18]. Thus, functionalized mediators bearing –CH2OH groups,
which could be used as linkers to graft or tether the Rh mediator on
the electrode surfaces, were considered to be suitable synthetic
targets.
cation was bound to the
bpy-OH, and one chloride anion, generating the mono cationic
5-Cp⁄Rh(bpy-OH)Cl]+ species. One additional Clꢀ anion was
g
5-Cp⁄, the two nitrogen atoms of the
[g
located in the crystal lattice, balancing the charge state. As we
acknowledged, compound 1 is first Rh–Cl complex with a bpy
motif ligand with the crystal structure; structurally characterized
The dihydroxy methylated ligand, 4,40-bis(hydroxymethyl)-
2,20-bipyridine (bpy-OH), was synthesized in good yield from
2,20-bipyridine-4,40-bisdicarboxylic acid by esterification using
SOCl2 and ethanol [19], followed by the reduction of the resulting
ester with excess NaBH4 (Scheme 1). The trivalent Rh complex,
all (g
5-Cp⁄)Rh compounds with bpy motif ligand contains a H2O
ligand instead of Clꢀ, due to lability of the Clꢀ anion ligand. The
distance between the Rh and bound Clꢀ anion ligand was
2.363(3) Å in 1, indicating that chloride ion may easily dissociate
from the metal centre to form complexes with Lewis bases such
[g
5-Cp⁄Rh(bpy-OH)Cl]Cl (1), was synthesized in excellent yield
by treating two equivalents of bpy-OH with {g l-Cl)Cl}2.
5-Cp⁄Rh(
as H2O generating [g
5-Cp⁄Rh(bpy-OH)(H2O)]2+ species.
The electronic spectrum of the resulting highly water-soluble
mediator1 showed three peaks at 230 nm (4.37 ꢁ 104 cmꢀ1 Mꢀ1),
The distances between the Rh(III) cation and the nitrogen atoms
304 nm (1.75 ꢁ 104 cmꢀ1 Mꢀ1), and 313 nm (1.76 ꢁ 104 cmꢀ1 Mꢀ1
)
of the bpy-OH ligand were 2.107(5) and 2.104(5) Å, comparable to
the ones of previously reported
2.093(4) and 2.115(4) Å [21]. The Rh–C distance of Rh-(
[g
5-Cp⁄Rh(bpy)(H2O)]OTf2,
(Fig. S1). In the 1H NMR spectrum of complex 1, the peaks of the
aromatic protons in the pyridine ligand were shifted downfield
compared to those of bipy-OH. In addition, the peak for the meth-
ylene protons of compound 1 was also shifted, from 3.95 to
4.90 ppm (Fig. S2) [20]. These results indicate the shift of electron
density toward the central Rh(III) metal cation from the pyridine
ligand upon complexation.
g
5-Cp⁄)
in 1 ranged from 2.115(17) to 2.363(3) Å. The angle between
N(1)–Rh(1)–N(2) was 77.3(2)°, compared to the N(1)–Rh(1)–N(2)
angle of 77.50(14)° for the water-coordinated complex [g
5-Cp⁄
Rh(bpy)(H2O)]OTf2. These results indicate that the substitution of
the Clꢀ ligand with a H2O ligand may not alter the structural
properties of remain unit of compound 1.
3.2. Description of the crystal structure of compound 1
3.2.1. Electrochemical studies
Yellow block-shaped crystals of compound 1 were obtained
from the methanol/diethyl ether diffusion system over 2 days.
The structure of 1 is shown in Fig. 1 and selected bond distances
and bond angles are given in Table 3. The eight-coordinate Rh(III)
Deleterious dimeric NADH inactivation by direct electron trans-
fer occurs at ꢀ942 mV versus Ag/AgCl through the first reduction
of NAD+ [22]. To selectively reduce NAD+ to active monomeric
NADH by an indirect electrochemical method, the catalyst or

D. Sivanesan, S. Yoon / Polyhedron 57 (2013) 52–56
55
Fig. 2. Cyclic voltammogram of 1 [2 mM], without NAD+ (plain line), 1 with (2 mM)
NAD+ (dot line) and 1 [2 mM] in CH3CN (dash and dot line). Inset: Plot of cathodic
currents vs. {scan rate (mV/s)}1/2:[square (j) – 1, circle (s) – 1 with NAD+].
mediator should have a reduction potential that is more anodic
than ꢀ942 mV versus Ag/AgCl. To test the possibility of compound
1 as such mediator, the electrochemical behavior was studied by
cyclic voltammetry at a stationary platinum disc in either CH3CN
solution or aqueous solution (0.1 M Tris/HCl (pH 7.50)). In CH3CN,
compound 1 produced a reduction peak (Ipc) at ꢀ793 mV versus
Ag/AgCl attributed to the Rh(III)/Rh(I) couple (DEp = 250 mV and
Scheme 2. Proposed mechanism for the catalytic reduction of NAD+.
E1/2 = ꢀ670 mV versus Ag/AgCl) in Fig. 2 (dash and dot line). The
[g g
5-Cp⁄Rh(III)(bpy-OH)Cl]+ species might be reduced to [ 5-Cp⁄
Rh(I)(bpy-OH)Cl]ꢀ species (Fig. 2); in the reverse scan, the reduced
one was oxidized from Rh(I) to Rh(III) at ꢀ543 mV versus Ag/AgCl.
The redox responses of compound 1 in aqueous solution are
also shown in Fig. 2 (plain line). Because the compound 1 is readily
converted to water-coordinated species in aqueous medium, redox
Based on the previously reported studies of electro-enzymatic
transformations, NADH regenerating mediators that can be at-
tached to the electrode may be very important for ensuring the
long-term stability of the system. These studies on the electro-
chemical properties and structural aspects of compound 1 with
the potential anchoring groups may reveal its potential as such a
catalyst.
active species may be [g
5-Cp⁄Rh(bpy-OH)(H2O)]2+ instead of 1. An
irreversible reduction peak appeared at ꢀ771 mV versus Ag/AgCl
at a scan rate of 75mV sꢀ1. The irreversibility observed for the
oxidative response may originate from a short-lived reduced state
g g
5-Cp⁄Rh(bpy-OH)(H2O)]2+, generating [ 5-Cp⁄Rh(bpy-OH)H]+
4. Regeneration of NADH by a chemical method
of [
species. The Rh(I) species, generated by reduction of previously
reported complexes with the [
5-Cp⁄(N-donor ligand)Rh(H2O)]+
motif, are well known to take up a proton and transform the corre-
sponding rhodium hydride species having
5-Cp⁄(N-donor
Chemically regenerated NADH was combined with HLADH from
horse liver for the transfer hydrogenation of 4-phenylbutan-2-one
in enantiomeric excesses up to 96% [2,17]. Steckhan and co-work-
g
[
g
ers reported the regeneration of NADH by [g
5-Cp⁄Rh(bpy)Cl)]+
ligand)RhH]+ formula [23]. Steckhan and co-workers reported the
5-Cp⁄Rh(bpy)(H2O)]2+ at ꢀ0.760 V versus
using sodium formate as a hydrogen source in aqueous phosphate
reduction potential of [
g
buffer medium. To compare the hydroxyl group-containing cata-
Ag/AgCl. Because the bpy-OH ligand has electron-releasing
–CH2OH groups at the 4- and 40-positions of the bipyridine, the
rhodium complex 1 may exhibit a reduction peak at the more
lyst 1 with frequently used rhodium complex, [g
5-Cp⁄Rh(bpy)Cl)]+,
the catalytic conversion was carried out in phosphate buffer with
sodium formate. To obtain the maximum TOF, the catalyst/sub-
strate ratio and temperature were varied (Table 1). The catalyst 1
negative potential than the [g
5-Cp⁄Rh(bpy)Cl]+ species [9].
showed similar catalytic activity to [
g
5-Cp⁄Rh(bpy)Cl)]+. Addition-
as for
3.2.2. Electrochemical regeneration of NADH
ally, similar trend was observed for catalyst 1
a
The enormous number of applications of indirect electrochem-
ical NADH regeneration in organic synthesis drove us to study the
[g
5-Cp⁄Rh(bpy)Cl)]+ at temperatures from 25 to 60 °C with differ-
ent substrates and catalyst ratios. The NAD+ to 1 ratio of 1000:1 at
60 °C, the TOF was 710 hꢀ1. The corresponding value reported for
indirect electrochemical reduction of NAD+ by compound
1
[24,25]. The electrochemical study of equimolar concentrations of
NAD+ and compound 1 showed a reduction peak at ꢀ814 mV
(Fig. 2. (dot line)). The reduction current that was produced from
this system was much higher than that from compound 1 alone.
This clearly indicated the combined reduction of Rh(III)/Rh(I) and
the fast reduction of NAD+ to NADH by Rh(I) species regenerating
Rh(III) species; The plot of current versus (scan rate)1/2 for
compound 1 with NAD+ revealed a slope twice that observed in
the absence of NAD+ (Fig. 2. inset).
[g
5-Cp⁄Rh(bpy)Cl)]+ was 875 hꢀ1. The unsubstituted phenanthro-
line(phen) complex, [g
5-Cp⁄Rh(phen)Cl]Cl, was reported to have
maximum TOF 2000 hꢀ1 [17]. However, the substituted (phen)–
Rh complexes were less active than the unsubstituted (phen)–Rh
complexes [17,26].
For the catalytic conversion of NAD+ to NADH, the mechanism
was proposed similar to that proposed by Fish and co-workers
(Scheme 2) [21]. Catalyst 1 reacts with formate anion in aqueous
solution to give the intermediate [g
5-Cp⁄Rh(bpy-OH)H]+ [1a],

56
D. Sivanesan, S. Yoon / Polyhedron 57 (2013) 52–56
which forms a kinetically favorable six-membered ring with NAD+.
This provides regioselective hydride transfer to give NADH. The
hydroxyl methyl groups of bpy-OH in 1 donate electrons towards
the Rh metal centre which enhances the intermediate hydrido-
Rh(I) formation and it converts the NAD+ to active NADH.
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5. Conclusions
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In summary, we have synthesized complex 1 with free hydroxyl
methyl functional groups and fully characterized its structure. The
Rh(III) catalyst 1 was reduced to Rh(I) species by the chemical and
electrochemical reduction methods, and the generated hydrido-
Rh(I) species transform NAD+ to active NADH with the maximum
TOF (710 hꢀ1). In future, the free hydroxyl groups in 1 can be
modified with suitable anchoring groups and the complex will be
attached on the surface of the electrode to regenerate the
co-enzyme.
(b) E. Steckhan, S. Herrmann, R. Ruppert, E. Dietz, M. Frede, E. Spika,
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Acknowledgements
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We acknowledged the financial support by grants from Korea
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CCDC 900961 contains the supplementary crystallographic data
for [g
5-Cp⁄Rh(bpy-OH)Cl]Cl (1). These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
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