The visible-light-driven transfer hydrogenation of nicotinamide cofactors with a robust ruthenium complex photocatalyst

Article abstract of DOI:10.1039/d0gc00331j

The highly efficient regeneration of nicotinamide cofactors has been successfully achieved with a quantum yield (Φ) of 7.9 × 10^-3via photocatalytic transfer hydrogenation in the presence of the ruthenium complex Ru(tpy)(biq)Cl₂ (where tpy = 2,2′:6′,2′′-terpyridine and biq = 2,2′-bisquinoline). The photocatalytic system is not only highly efficient but also tolerant to amino acid residues. The combination of this photocatalyst with glutamate dehydrogenase enabled the controllable and efficient synthesis of l-glutamate to be realized. A mechanism involving light-induced ligand exchange, decarboxylation and hydride transfer has been proposed. Kinetic isotope experiments revealed that the decarboxylation of [Ru(tpy)(biq)HCOO]⁺ to [Ru(tpy)(biq)H]⁺ was the rate-determining step with a small apparent activation energy of 3.2 ± 0.4 kcal mol^-1. The hydricity of [Ru(tpy)(biq)H]⁺ was estimated, via reaction equilibrium, to be 40 ± 3 kcal mol^-1

Full text of DOI:10.1039/d0gc00331j

View Article Online
View Journal
Green
Chemistry
Cutting-edge research for a greener sustainable future
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Dong, J. Tang,
L. Zhao, L. Deng and M. Xian, Green Chem., 2020, DOI: 10.1039/D0GC00331J.
Volume 20
This is an Accepted Manuscript, which has been through the
Number
May 2018
Pages 1919-2160
9
7
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Cutting-edge research for a greener sustainable future
rsc.li/greenchem
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1463-9262
PAPER
Paul T. Anastas et al.
The Green ChemisTREE: 20 years after taking root with the 12
principles
rsc.li/greenchem

Please do not adjust margins
Page 1 of 9
Green Chemistry
View Article Online
DOI: 10.1039/D0GC00331J
ARTICLE
Visible-light-driven Transfer Hydrogenation of Nicotinamide
Cofactors with a Robust Ruthenium Complex Photocatalyst
Wenjin Dong, a,b Jie Tang,a, c Lijun Zhao,a Li Deng*a and Mo Xian*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Highly efficient regeneration of nicotinamide cofactors has been successfully achieved with a quantum yield (Φ) of 7.9 ×
10-3 via a photocatalytic transfer hydrogenation in the presence of a ruthenium complex Ru(tpy)(biq)Cl2 (where tpy =
2,2’:6’,2’’-terpyridine and biq = 2,2’-bisquinoline). The photocatalytic system is not only highly efficient but also tolerant to
amino acid residues. Combining this photocatalyst and glutamate dehydrogenase, controllable and efficient synthesis of L-
glutamate has been realized. A mechanism involving light-induced ligand exchange, decarboxylation and hydride transfer
has been proposed. Kinetic isotope experiments revealed that the decarboxylation of [Ru(tpy)(biq)HCOO]+ to
[Ru(tpy)(biq)H]+ was the rate-determining step with a small apparent activation energy of 3.2±0.4 kcal mol-1. The hydricity
of [Ru(tpy)(biq)H]+ was estimated as 40±3 kcal mol-1 through a reaction equilibrium.
with a 100 % optical purity. To extend scope of photosensitizer
for artificial photosynthesis, organic dyes were screened
extensively for this transformation by Park et al.9 Eosin Y, rose
Introduction
Nicotinamide cofactors (i.e. NAD(P)+ and NAD(P)H) serve the
most biological transformation involving enzymatic redox
reactions.1-3 Efficient reductive regeneration of the expensive
cofactors is not only significant for economical application of
biological processes but also for novel artificial
bengal and phloxine B exhibited high activity with TOFs from
1178 to 1537 h-1. Besides homogeneous photosensitizers,
12
heterogeneous ones like carbon nitride,4d,10 CdSe,11 TiO2 and
COF13 were also successfully applied for this conversion.
Concerning
electron
mediators,
widely
used
4
biotransformations (e.g. artificial photosynthesis). Over past
[RhCp*(bpy)H2O]2+ (M) enables the dual electron transfer from
reduced photosensitizers to NAD(P)+ in the form of hydride,
affording 1,4-NAD(P)H selectively. Besides photochemical
regeneration, M can also catalyze transfer hydrogenation of
NAD(P)+ with formate under mild conditions.14
decades, enormous efforts have been devoted to the
regeneration of cofactors in chemical,5 electrical,6 biological7
and photochemical4a,b ways. Among them, photochemical
regeneration of cofactors is of obvious advantages. Apart from
the utilization of solar energy to mimic the photosynthesis
system I, light is able to make the regeneration of cofactors
controllable, which is of great potential to regulate the
biosynthesis by varying the ratio of reduced and oxidized
In spite of the success in photosensitizer development, two
bottleneck problems for NAD(P)H regeneration need to be
addressed.2,3 In term of sacrificial reductant, amines
forms.8 Typically
a photocatalytic regeneration system
Previous works:
Photoredox catalysis
hv
contains a photosensitizer to extract electrons from sacrificial
reductant and an electron mediator to perform the dual
electron transfer to NAD(P)+. In a pioneering work by Itamar et
al.,4a,b Ru(bpy)3Cl2 (where bpy = 2,2’-bipyridine) was employed
as photosensitizer with methyl viologen (MV2+) as mediator. As
a result, NADPH was generated with a TON of 13 after 40 h
based on Ru(bpy)3Cl2 and enzymatic reduction of butanone to
chiral 2-butanol with this photocatalytic system was achieved
Excited
Reduced
Photosensitizer
NAD(P)+
NAD(P)H
[Rh(Cp*)(bpy)H]+
Amines
Photosensitizer
[Rh(Cp*)(bpy)H2O]+
Reduced
Photosensitizer
Imines
Stepwise
Conventional transfer hydrogenation
HCOO-
[RhCp*(bpy)H2O]2+
NAD(P)+
NAD(P)H
TOF 80 h-1
Intolerant to amino acid residues
=
-
HCO3
[RhCp*(bpy)H]+
This work:
Photocatalytic transfer hydrogenation
a. CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R.
China.
Advantages:
hv
HCOO-
[Ru(tpy)(biq)(OH)]+
[Ru(tpy)(biq)H]+
NAD(P)+
NAD(P)H
Green and renewable reductant
Tolerant to amino acid residues
High efficiency (TOF up to 190 h-1
Photocontrolable
b. College of Chemical Engineering, Qingdao University of Science & Technology,
Qingdao 266042, P.R. China.
-
HCO3
)
c. University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Bifunctional photocatalyst
Scheme 1. Visible-light-driven transfer hydrogenation of
NAD(P)+.
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 9
ARTICLE
Journal Name
(e.g. TEOA, NEt3) were used giving toxic imine and aldehyde as experiments which are sensitive to air were carried out in
View Article Online
DOI: 10.1039/D0GC00331J
by-products. An inexpensive and renewable alternative is sealed tubes. A NMR tube adapter was used to apply vacuum
formate which releases only CO2 (bicarbonate) as by-product. using standard Schlenk techniques. Deuterium NMR solvents
Although it is employed in transfer hydrogenation of NAD(P)+, were degassed by freeze-pump-thaw cycling for three times.
there is so far no report on photocatalytic regeneration of Electronic absorption spectra were record on a Varian Carry 50
NAD(P)H using formate. On the other hand, the efficiency of spectrometer. HRMS measurements were performed on a
mediator M is much lower than that of photosensitizers, which mxXis II time of flight mass spectrometer (Bruker Daltonics
results in the usage of high concentration of M. For instance, Corporation). X-ray Diffraction data were collected on Smart
the concentration of M is 50-fold higher than that of dyes (250 1000 diffractometer (Bruker, Karlsruhe, Germany) with Mo K-
uM vs 5 uM).9 Moreover, deactivation of M occurs in the alpha X-ray source (λ = 0.71073 Å) at 150 K. The SUPERFLIP72
presence of amino acid residues due to the coordination on program was employed for dealing with the collected data and
thiol
and
amino
groups.15
To
overcome
this, refinement of data was achieved through the SHELX-97 and
comparmentalization strategy has been applied to avoid the OLEX2 programs. Analysis of L-glutarate was performed on a
contact of metal complex with coordinative residues in Shimadzu Prominence 20 HPLC system, equipped with a C18
enzyme. Ward et al. incorporated a half-sandwich Ir complex column and a UV-Vis detector
with biotin tag into streptavidin, creating an artificial For a typical NADH regeneration procedure, 2 mL aqueous
metalloenzyme and efficiently compartmentalizing the NADH solution containing 1 mM NAD+, 10 uM photocatalyst 7 and 0.5
dependent downstream enzyme and the Ir complex.16 For the M HCOONa was added to a Schlenk tube equipped with a
same purpose, half-sandwich Rh complex has been rubber stopper and a sampling syringe, then the tube was
coordinated to a light-harvesting polymeric material for purged with argon and placed in front of green LEDs (40 w, 525
photocatalytic reduction of NAD+.17 With the above two nm). The reaction temperature was kept lower than 40 °C
concerns on reductant and mediator in mind, we endeavour to through cooling with a fan. After a certain time irradiation, the
develop an efficient and amino acid residue tolerant reaction was sampled and NADH concentration was analyzed
photocatalytic transfer hydrogenation protocol for NAD(P)H with a Varian Carry 50 spectrometer at 340 nm.
regeneration. It is noteworthy that Ishitani et al. developed Synthesis and characterization
bifunctional photocatalyst Ru(tpy)(bpy)(DMF)(PF6)2 which
Ru(tpy)Cl3 was synthesized according to
a
reported
enabled the reduction of 1-benzylnicotinamidium cation (BNA+)
to the 1,4-dihyro form (BNAH).18 Under visible light, ligand
procedure.26 To 500 mL ethanol solution of terpyridine (1.17 g,
5 mmol) RuCl3 3H2O (1.31 g, 5 mmol) was added and the
solution was refluxed for 3 h with stirring. After cooling down
to room temperature, brown precipitate was obtained with a
near quantitative yield. For complex 1, 2, 3, 4 and 6, 221 mg
(0.5 mmol) Ru(tpy)Cl3 was added to 50 mL water : ethanol (1:3)
solution containing 0.5 mmol bipyridyl ligand, 20 mg LiCl and
100 uL NEt3. After 20 h reflux with stirring, solvent was
reduced to ca. 5 mL with a rotatory evaporator. The
concentrated solution was kept in a refrigerator overnight,
affording crystalline product. The product was washed
carefully with 2 mL chilling ethanol and 10 mL ethyl ether and
then dried in vacuum.
exchange
switched
[Ru(tpy)(bpy)DMF]2+
to
[Ru(tpy)(bpy)NEt3]2+ which proceeded intramolecular redox
reaction giving [Ru(tpy)(bpy)H]+ intermediate. Fast hydride
transfer from this complex to BNA+ affords BNAH selectively. In
this way, [Ru(tpy)(bpy)DMF]2+ functions as both
photosensitizer and dual electron (hydride) transfer mediator,
in other words the oxidation of NEt3 and reduction of BNA+
occur on the same Ru center. Moreover, irradiation of Ru
complexes is able to dissociate thioether and pyridyl ligands
which are strongly coordinating and often cause deactivation
of metal catalysts.19 Thus, it is reasonable to expect Ru
terpiridyl complexes to be tolerant towards amino acid
residues in a photocatalytic reaction system. Inspired by these
characters of Ru terpiridyl complexes, we employed them as
photocatalysts to survey visible-light-driven transfer
hydrogenation of cofactors using formate in this study.
For complex 1, 1H NMR (400 MHz, Methanol-d4) δ 10.22 (d, J =
4.9 Hz, 1H), 8.80 (d, J = 8.2 Hz, 1H), 8.67 (d, J = 8.1 Hz, 2H), 8.55
(d, J = 8.0 Hz, 3H), 8.51 (d, J = 8.1 Hz, 1H), 8.34 (t, J = 7.9 Hz,
1H), 8.18 (t, J = 8.1 Hz, 1H), 8.02 (t, J = 7.4 Hz,1H), 7.95 (t, J =
7.9 Hz, 2H), 7.76 (t, J = 8.1 Hz, 2H), 7.73 – 7.70 (m, 4H), 7.39 (d,
J = 5.2 Hz, 1H), 7.35 (t, J = 7.4 Hz, 2H), 7.06 (t, J = 7.3 Hz, 1H).
13C NMR (151 MHz, Methanol-d4) δ 158.90 , 158.89 , 158.12 ,
156.34 , 152.24 , 151.93 , 151.48 , 137.02 , 136.62 , 135.51 ,
134.12 , 127.14 , 126.68 , 126.09 , 123.47 , 123.42 , 123.22 ,
122.38. HRMS (ESI-TOF)m/z: [1-Cl]+ Calcd for C25H19ClN5Ru
526.0367; found 526.0389. Electronic absorption spectra (H2O):
λMLCT = 473 nm, ε473 nm = 5368 M-1cm-1, ε456 nm = 5368 M-1cm-1.
For complex 2, 1H NMR (400 MHz, MeOD) δ 10.44 (dd, J = 5.2,
1.3 Hz, 1H), 8.91 (dd, J = 8.2, 1.3 Hz, 1H), 8.71 (d, J = 8.1 Hz,
2H), 8.56 (d, J = 7.9 Hz, 2H), 8.39 (dd, J = 8.6, 5.7 Hz, 2H), 8.33
(dd, J = 8.2, 1.1 Hz, 1H), 8.22 (t, J = 8.1 Hz, 1H), 8.17 (d, J = 8.9
Hz, 1H), 7.91 (td, J = 7.9, 1.5 Hz, 2H), 7.76 (dd, J = 5.4, 1.1 Hz,
Experimental
Materials and general methods
All substrates and reagents were used as received from
commercial suppliers unless otherwise stated. For
monochromatic light source, Kessil PR-160-456 LEDs (456nm,
Max power = 40 w, FWHM ≈ 20 nm) were used for blue light.
Green LEDs (525 nm, Max power = 100 w, FWHM = 15~20 nm)
and red LEDs (625 nm, Max power = 100 w, FWHM = 15~20
nm) were provided by Perfectlight company. NMR spectra
were recorded on a Bruker 400 or 600 spectrometer. NMR
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
1H), 7.59-7.52 (m, 2H), 7.41 (dd, J = 8.2, 5.4 Hz, 1H), 7.22 (ddd, 1H), 8.26 (d, J = 8.7 Hz, 1H), 8.20 (t, J = 8.0 Hz, 1VHie)w, A8r.t0icl2e-O7n.l7in4e
J = 7.5, 5.6, 1.3 Hz, 2H). 13C NMR (101 MHz, MeOD) δ 158.92 , (m, 7H), 7.46 (t, J = 7.6 Hz, 1H), 7.33 (t, J D=O6I.:510H.1z0,329H/D)0, G7C.20103(3t,1JJ
158.38 , 152.96 , 152.20, 152.12, 149.07 , 147.26 , 136.96 , = 7.8 Hz, 1H). 13C NMR (101 MHz, Methanol-d4) δ 159.30 ,
135.57 , 134.44 , 134.16 , 130.87 , 130.42 , 127.90 , 127.34 , 158.97 , 158.56 , 152.48 , 151.85 , 151.78 , 151.11 , 138.36 ,
127.05 , 125.67 , 124.77 , 123.39 , 122.35. HRMS (ESI-TOF)m/z: 137.45 , 136.33 , 135.43 , 130.66 , 130.38 , 129.36 , 129.05 ,
[2-Cl]+ Calcd for C27H19ClN5Ru+ 550.0367; found 550.0373.
128.56 , 128.25 , 126.97.
For complex 3, 1H NMR (400 MHz, MeOD) δ 10.01 (d, J = 5.8 HRMS (ESI-TOF)m/z: [7-Cl]+ Calcd for C33H23ClN5Ru+
Hz, 1H), 8.65 (d, J = 8.2 Hz, 3H), 8.53 (d, J = 8.0 Hz, 2H), 8.38 (s, 626.0608; found 626.0642. Electronic absorption spectra (H2O):
1H), 8.14 (t, J = 8.1 Hz, 1H), 7.93 (dd, J = 11.4, 4.2 Hz, 2H), 7.87 λMLCT = 548 nm, ε548 nm = 4723 M-1cm-1, ε525 nm = 3761 M-1cm-1
(d, J = 5.5 Hz, 1H), 7.73 (d, J = 5.3 Hz, 2H), 7.39-7.30 (m, 2H), Ru(tpy)(biq)(Cl)PF6 was prepared by precipitating 7 in ethanol.
7.14 (d, J = 5.9 Hz, 1H), 6.89 (d, J = 5.5 Hz, 1H), 2.82 (s, 3H), To 20 mL ethanol solution of 660 mg (1 mmol) 7 add 2 mL
2.39 (s, 3H). 13C NMR (101 MHz, MeOD): δ 158.97 , 158.36, saturated NH4PF6 solution, giving ca. 500 mg product. Single
158.28, 155.86 , 151.83 , 151.44 , 150.48 , 149.07 , 148.27 , crystals of Ru(tpy)(biq)(Cl)PF6 suitable for XDR analysis were
136.77 , 133.62 , 127.54 , 127.05,126.98 (d), 124.00, 123.92, grown by vapour diffusion of benzene into a dichloromethane
123.31 , 122.21 , 19.99 , 19.42. HRMS (ESI-TOF)m/z: [3-Cl]+ solution at room temperature. The molecular structure (CCDC
Calcd for C27H23ClN5Ru+ 554.0680; found 554.0684.
number: 1920316) was shown in Figure S58. 1H NMR (400
For complex 4, 1H NMR (400 MHz, MeOD) δ 9.94 (d, J = 6.5 Hz, MHz, DMSO-d6) δ 10.10 (d, J = 4.8 Hz, 1H), 8.91 (d, J = 8.2 Hz,
1H), 8.63 (d, J = 8.1 Hz, 2H), 8.52 (d, J = 8.1 Hz, 2H), 8.38 (d, J = 1H), 8.82 (d, J = 8.1 Hz, 2H), 8.69 (d, J = 8.0 Hz, 2H), 8.64 (d, J =
2.6 Hz, 1H), 8.15-8.06 (m, 2H), 7.93 (td, J = 7.9, 1.3 Hz, 2H), 8.0 Hz, 1H), 8.36 (t, J = 8.6 Hz, 1H), 8.22 (t, J = 8.1 Hz, 1H), 8.07
7.81 (d, J = 5.4 Hz, 2H), 7.65 (dd, J = 6.5, 2.6 Hz, 1H), 7.41-7.33 (t, J = 6.1 Hz, 1H), 7.99 (t, J = 8.5 Hz, 1H), 7.78 (t, J = 7.8 Hz, 1H),
(m, 2H), 7.02 (d, J = 6.6 Hz, 1H), 6.66 (dd, J = 6.6, 2.7 Hz, 1H), 7.62 (d, J = 4.9 Hz, 2H), 7.38 (t, J = 7.1 Hz, 1H), 7.32 (d, J = 5.1
4.24 (s, 3H), 3.88 (s, 3H). 13C NMR (101 MHz, MeOD): δ 167.11, Hz, 1H), 7.08 (t, J = 7.2 Hz, 1H). 13C NMR (101 MHz, DMSO-d6) δ
166.19 , 159.46, 159.16 , 158.85 , 157.26, 152.90 , 151.89 , 160.92 , 158.09 , 157.69 , 151.75 , 150.46 , 149.94 , 137.59 ,
151.68 , 136.54 , 133.09 , 127.02 , 123.16 , 122.08 , 113.09 , 137.02 , 135.61 , 134.88 , 129.97, 129.54 , 128.75 , 128.37,
112.70 , 110.06, 109.98, 55.87 , 55.61. HRMS (ESI-TOF)m/z: [4- 128.06 , 127.81 , 127.31 , 126.67 , 123.32 , 122.52 , 122.29 ,
Cl]+ Calcd for C27H23ClN5O2Ru+ 586.0578; found 586.0583.
For complex 6, 1H NMR (400 MHz, Deuterium Oxide) δ 8.46 (d, To
120.41.
synthesize
Ru(tpy)(biq)(MeCN)(PF6)2,
100
mg
J = 8.1 Hz, 2H), 8.42 (d, J = 8.0 Hz, 1H), 8.35 (d, J = 8.0 Hz, 2H), Ru(tpy)(biq)(Cl)PF6 and 100 mg AgPF6 were dissolved in 10 mL
8.12 (t, J =8.0 Hz, 1H), 8.09 (t, J =8.0 Hz, 2H), 7.99 (d, J = 5.2 Hz, acetonitrile. The solution was heated up to 85 °C for 3 h. After
2H), 7.93 (t, J = 7.9 Hz, 2H), 7.75 (s, 1H), 7.45 (t, J = 7.9 Hz, 1H), removal of AgCl, acetonitrile was removed affording purple
7.340 (t, J = 7.0 Hz, 2H), 6.75 (d, J = 7.5 Hz, 1H), 2.84 (s, 3H), solid. The solid was washed with ethanol to remove excess
1.50 (s,3H). 13C NMR (101 MHz, Deuterium Oxide) δ 166.38 , amount of AgPF6, giving pure product. Single crystals of
165.08 , 159.98 , 159.80 , 159.59 , 153.63 , 138.16 , 137.44 , Ru(tpy)(biq)(MeCN)(PF6)2 suitable for XDR analysis were grown
136.27 , 136.05 , 123.70 , 123.04 , 121.26 , 121.00, 24.68 , by vapour diffusion of dichloromethane into a acetonitrile
23.30. HRMS (ESI-TOF)m/z: [6-Cl]+ Calcd for C27H23ClN5Ru+ solution at room temperature. The molecular structure (CCDC
1
554.0680; found 554.0683.
number: 1920254) was shown in Figure S59. H NMR (400
To prepare complex 5 and 7, 221 mg (0.5 mmol) Ru(tpy)Cl3 MHz, Acetonitrile-d3) δ 8.96 (d, J = 8.8 Hz, 1H), 8.90 (dd, J = 9.0,
and 0.5 mmol bisquinoline or 4,4’-dinitrilbipyridine were 1.7 Hz, 1H), 8.79 (d, J = 8.3 Hz, 1H), 8.63 (dd, J = 9.0, 1.7 Hz,
added to 5 mL glycol and heated up to 180 °C for 1 h. Then the 1H), 8.59 (dd, J = 8.1, 1.7 Hz, 2H), 8.41 (m, 4H), 8.11-7.92 (m,
reaction mixture was diluted with 100 mL EtOH and filtered to 4H), 7.87 (d, J = 8.2 Hz, 1H), 7.79 (d, J = 5.6 Hz, 2H), 7.53 (t, J =
remove insoluble solid. The filtrate was concentrated to ca. 1 7.6 Hz, 1H), 7.37 (t, J = 6.7 Hz, 2H), 7.30 (t, J = 8.0 Hz, 1H), 6.79
mL under vacuum and 200 mL ethyl ether was used to wash (d, J = 8.9 Hz, 1H), 2.16 (d, J = 1.7 Hz, 3H). 13C NMR (101 MHz,
the residue affording violet precipitate. The resulting violet Acetonitrile-d3) δ 160.44 , 159.80 , 159.35 , 158.90 , 158.50 ,
solid product was further washed with ethyl ether to minimize 158.05 , 153.51 , 150.75 , 139.24 , 138.82 , 138.77 , 137.56 ,
the residue glycol content.
132.13 , 131.69 , 129.68 , 129.63 , 129.52 , 129.49 , 128.79 ,
For complex 5, 1H NMR (400 MHz, Methanol-d4) δ 10.50 (dd, J 128.14 , 128.04 , 124.52 , 123.95 , 123.35 , 121.04 , 120.65 ,
= 1.9, 0.7 Hz, 1H), 9.03 (dd, J = 8.5, 0.8 Hz, 1H), 8.81 – 8.63 (m, 117.31. HRMS (ESI-TOF)m/z: [Ru(tpy)(biq)(MeCN)PF6]+, Calcd
4H), 8.57 (d, J = 8.0 Hz, 3H), 8.27 (t, J = 8.1 Hz, 1H), 8.13 (dd, J = for C35H26F6N6PRu+ 777.0899; found 777.716.
8.5, 1.8 Hz, 1H), 8.05 – 7.88 (m, 3H), 7.71 (d, J = 5.5 Hz, 2H), To synthesize Ru(tpy)(biq)(HCOO)PF6, to 1mL Ethanol solution
7.40 – 7.28 (m, 2H). 13C NMR (101 MHz, Methanol-d4) δ 158.62 of 30 mg Ru(tpy)(biq)(MeCN) (PF6)2 or Ru(tpy)(biq)(Cl)PF6 was
, 158.50 , 157.57 , 154.89 , 154.68 , 152.46 , 138.83 , 138.02 , added 1 mL 5 M HCOONa solution and the resulting mixture
137.60 , 135.45 , 127.24 , 124.97 , 124.67 , 123.76 , 122.80 , was stirred under irradiation of 40 w green LEDs for 1 h.
115.06 , 113.86 , 112.85. HRMS (ESI-TOF)m/z: [5-Cl]+ Calcd for Addition of drops of KPF6 solution afford black crystal solid.
C27H17ClN7Ru+ 576.0272; found 576.0274.
For complex 7, H NMR (400 MHz, Methanol-d4) δ 8.99 (d, J = spectrum were too weak to identify. To verify the product was
The solid was poorly soluble in MeOH-d4, so signals in 13C NMR
1
8.8 Hz, 1H), 8.94 (d, J = 8.8 Hz, 1H), 8.68 (d, J = 8.8 Hz, 1H), 8.65
a
formate complex, DCOONa was used to prepare
(d, J = 8.1 Hz, 2H), 8.49 (d, J = 8.0 Hz, 2H), 8.32 (d, J = 7.7 Hz, Ru(tpy)(biq)(DCOO)PF6 . As shown in Figure S26 and S27, the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 9
ARTICLE
Journal Name
1H NMR spectra were identical except for the disappearance of Hz, 3H), 8.45 (d, J = 7.9 Hz, 4H), 8.35 (m, J = 16.2V,ie8w.2ArtHiczle,O5nHlin)e,
DOI: 10.1039/D0GC00331J
the signal of formate moiety at 7.37 ppm. Single crystals of 8.09 – 7.95 (m, 3H), 7.88 – 7.79 (m, 1H), 7.71 (d, J = 5.5 Hz, 1H),
Ru(tpy)(biq)(HCOO)PF6 suitable for XDR analysis were grown by 7.37 (t, J = 7.0 Hz, 1H), 7.30 (d, J = 7.0 Hz, 1H), 7.12 (t, J = 7.4
vapour diffusion of n-hexane into a methanol solution at room Hz, 1H), 2.10 (s, 3H). 13C NMR (101 MHz, Acetonitrile-d3) δ
temperature. The molecular structure (CCDC number: 1942074) 158.33 , 157.59 , 155.91 , 153.12 , 152.47 , 151.18 , 138.55 ,
1
was shown in Figure S60. For Ru(tpy)(biq)(HCOO)PF6, H NMR 137.66 , 137.38 , 136.79 , 127.84 , 127.50 , 126.68 , 124.29 ,
(400 MHz, Methanol-d4) δ 9.05 (d, J = 8.9 Hz, 1H), 8.95 (d, J = 124.15 , 123.53 , 123.47 , 117.31.
8.8 Hz, 1H), 8.85 (d, J = 8.8 Hz, 1H), 8.75 (s, 0H), 8.68 (d, J = 8.1 Rh(Cp*)(bpy)Cl2 was prepared according to the following
Hz, 2H), 8.49 (d, J = 8.0 Hz, 2H), 8.35 (d, J = 6.9 Hz, 1H), 8.31 – procedure: 31 mg (0.2 mmol) 2,2’-bipyridine was added to a
8.21 (m, 2H), 7.96 (td, J = 7.9, 1.4 Hz, 3H), 7.92 – 7.79 (m, 7H), suspension
of
61.8
mg
(0.1
mmol)
7.47 (t, J = 7.5 Hz, 1H), 7.37 (s, 1H), 7.32 (t, J = 7.2 Hz, 1H), 7.25 dichloro(pentamethylcyclopentadienyl)rhodium(III) Dimer in 4
(t, J = 7.9 Hz, 1H), 6.94 (d, J = 8.9 Hz, 1H). HRMS (ESI-TOF)m/z: mL methanol. After 3 h, Rh(Cp*)(bpy)Cl2 was precipitated as
[Ru(tpy)(biq)HCOO]+ Calcd for C34H24N5O2Ru+ 636.0968; yellow solid upon addition of cold diethyl ether into reaction
found 636.0981. Electronic absorption spectra (H2O): λMLCT
555 nm, ε555 nm = 5739 M-1cm-1.
=
solution, washed with diethyl ether and dried in vaccum. The
product was dissolved in water giving hydrolyzed form of to
For Ru(tpy)(biq)(DCOO)PF6 1H NMR (400 MHz, Methanol-d4) δ [Rh(Cp*)(bpy)(H2O)]2+ (M). 1H NMR (400 MHz, Chloroform-d) δ
9.05 (d, J = 8.9 Hz, 1H), 8.95 (d, J = 8.8 Hz, 1H), 8.85 (d, J = 8.7 9.00 (d, J = 8.1 Hz, 2H), 8.86 (dd, J = 5.6, 1.4 Hz, 2H), 8.26 (td, J
Hz, 1H), 8.74 (d, J = 8.8 Hz, 1H), 8.68 (d, J = 8.1 Hz, 2H), 8.49 (d, = 7.9, 1.5 Hz, 2H), 7.82 (t, 6.2 Hz, 2H), 1.75 (s, 15H). 13C NMR
J = 8.0 Hz, 2H), 8.35 (d, J = 6.8 Hz, 1H), 8.27 (dt, J = 8.1, 3.7 Hz, (101 MHz, Chloroform-d) δ 154.60 , 151.14 , 140.84 , 128.42 ,
2H), 7.96 (t, J = 7.8 Hz, 1H), 7.92 – 7.78 (m, 5H), 7.47 (t, J = 7.5 125.53 , 97.19 , 97.11 , 9.20 .
Hz, 1H), 7.32 (t, J = 7.2 Hz, 1H), 7.25 (t, J = 7.2 Hz, 1H), 6.94 (d, J Benzyl nicotinamide bromide ((BNA)Br )was prepared by
= 8.9 Hz, 1H).
refluxing 2.13 g (17.5 mmol) nicotinamide and 2.8 mL (ca. 23
For [Ru(tpy)(biq)OH]+ (7 in D2O) 1H NMR (400 MHz, Deuterium mmol) benzyl bromide in 50 mL acetone. After 12 h reaction,
Oxide) δ 8.80 (d, J = 8.8 Hz, 1H), 8.73 (d, J = 8.9 Hz, 1H), 8.54 (d, the precipitate was filtered and washed with acetone affording
J = 8.6 Hz, 1H), 8.47 (d, J = 8.2 Hz, 2H), 8.36 (d, J = 8.9 Hz, 1H), 3.1 g product with a quantitative yield. To a saturated (BNA)Br
8.27 (m, 3H), 8.17 (t, J = 8.1 Hz, 1H), 7.98 (d, J = 8.8 Hz, 1H), aqueous solution, saturated KPF6 solution was added affording
7.91 – 7.71 (m, 4H), 7.59 (d, J = 5.1 Hz, 2H), 7.53 (d, J = 7.7 Hz, white pericipitate (BNA)PF6 which was carefully washed with
1H), 7.25 (t, J = 7.5 Hz, 1H), 7.11 (m, 3H), 6.65 (d, J = 8.9 Hz, water, dried and stored under vacuum. 1H NMR (400 MHz,
1H). HRMS (ESI-TOF)m/z:
C33H24N5ORu+ 608.1019; found 608.1059.
[Ru(tpy)(biq)OH]+ Calcd for Deuterium Oxide) δ 9.27 (s, 1H), 9.00 (d, J = 5.1 Hz, 1H), 8.82
(d, J = 8.2 Hz, 1H), 8.12 (t, J = 7.1 Hz, 1H), 7.41 (s, 5H), 5.82 (s,
Ru(tpy)(biq)(H)PF6 was prepared by the reduction of 2H). 13C NMR (101 MHz, Deuterium Oxide) δ 165.58 , 146.50 ,
Ru(tpy)(biq)(Cl)PF6 with NaBH4. A solution of 100 mg 144.30 , 144.23 , 133.98 , 132.20 , 130.14 , 129.65 , 129.27 ,
Ru(tpy)(biq)(Cl)PF6 in 30 mL water and ethanol (1 : 2) was 128.60 , 65.10.
degassed with argon. Then ca. 100 mg NaBH4 was added in To prepare benzyl-1,4-dihyronicotinmide (BNAH), 1.74 g (10
portions within 1 h. After removal of ethanol, black precipitate mmol) Na2S2O4 was added in portions to 10 mL water solution
was afforded. The hydride complex was washed with water of 0.59 g (2 mmol) (BNA)Br and 0.84 g (10 mmol) NaHCO3. The
and dried under vacuum. 1H NMR (400 MHz, Acetonitrile-d3) δ solution was stirred at room temperature for 3 h, affording
1
9.80 (d, J = 8.7 Hz, 1H), 8.87 (d, J = 8.8 Hz, 1H), 8.72 (d, J = 8.8 BNAH as yellow precipitate. H NMR (400 MHz, Chloroform-d)
Hz, 1H), 8.68 (d, J = 8.9 Hz, 1H), 8.46 (d, J = 8.1 Hz, 2H), 8.35 (d, δ 7.38 – 7.27 (m, 3H), 7.24 (d, J = 6.8 Hz, 2H), 7.16 (d, J = 1.6
J = 8.8 Hz, 1H), 8.30 – 8.21 (m, 3H), 7.99 (t, J = 8.1 Hz, 1H), 7.87 Hz, 1H), 5.74 (dd, J = 8.1, 1.7 Hz, 1H), 5.24 (s, 2H), 4.75 (dt, J =
– 7.70 (m, 5H), 7.58 (d, J = 5.3 Hz, 2H), 7.45 (t, J = 7.5 Hz, 1H), 8.0, 3.4 Hz, 1H), 4.29 (s, 2H), 3.18 (s, 2H). 13C NMR (101 MHz,
7.17 (t, J = 8.6 Hz, 1H), 7.05 (t, J = 6.6 Hz, 2H), 6.74 (d, J = 8.8 Chloroform-d) δ 170.10 , 140.05 , 128.99 , 128.83 , 127.80 ,
Hz, 1H), -14.71 (s, 1H). HRMS (ESI-TOF)m/z: [Ru(tpy)(biq)H]+ 127.18 , 103.20 , 77.02 , 57.42 , 22.88.
Calcd for C33H24N5Ru+ 592.1070; found 592.1086.
Ru(tpy)(bpy)(H)PF6 and Ru(tpy)(bpy)(MeCN)PF6
synthesized according to reported procedure.27
1,3-dimethylpyridinium iodide ((DMPY)I ) was prepared
were through a room temperature reaction of 4.65 g (50 mmol) 3-
methylpyridine with 14.2 g (100 mmol) methyl iodide in 20 mL
For Ru(tpy)(bpy)(H)PF6 1H NMR (400 MHz, DMSO-d6) δ 9.70 (d, benzene. The resulting precipitate was washed with benzene
J = 5.3 Hz, 1H), 8.87 (d, J = 8.2 Hz, 1H), 8.69-8.61 (m, 5H), 8.20 and
recrystallized
in
acetone.
To
obtain
1,3-
(t, J = 8.4 Hz, 1H), 7.96 (t, J = 8.0 Hz, 1H), 7.89 (t, J = 7.8 Hz, 2H), dimethylpyridinium hexafluorophosphate ((DMPY)PF6 ),
7.86-7.81 (m, 2H), 7.78 (d, J = 5.6 Hz, 2H), 7.31 (t, J = 7.0 Hz, substitution of iodide with hexafluorophosphate was
2H), 7.13 (t, 6.0 Hz, 1H), 7.05 (d, J = 4.9 Hz, 1H), -14.63 (s, 1H). performed in 5 mL aqueous solution of 235 mg (DMPY)I with
1
HRMS (ESI-TOF)m/z: [Ru(tpy)(bpy)HCOO]+ Calcd for drops saturated KPF6 solution. H NMR (400 MHz, DMSO-d6) δ
C26H20N5O2Ru+ 536.0655; found 536.0672. [Note: 9.13 (s, 1H), 9.03 (d, J = 6.0 Hz, 1H), 8.47 (d, J = 8.0 Hz, 1H),
[Ru(tpy)(bpy)H]+ is very sensitive to CO2, giving 8.08 (dd, J = 7.9, 6.2 Hz, 1H), 7.53 (d, J = 8.0 Hz, 2H), 7.45 (d, J =
[Ru(tpy)(bpy)HCOO]+ peak in HRMS.]
6.9 Hz, 3H), 5.79 (s, 2H). 13C NMR (101 MHz, DMSO-d6) δ
For Ru(tpy)(bpy)(MeCN)PF6 1H NMR (400 MHz, Acetonitrile-d3) 146.79 , 144.72 , 142.54 , 139.79 , 134.76 , 129.83 , 129.69 ,
δ 9.62 (d, J = 6.9 Hz, 1H), 8.65 (d, J = 8.2 Hz, 2H), 8.58 (d, J = 8.1 129.15 , 128.26 , 63.78 , 18.40.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
b)
a) 2000
L-glutamate synthesis
VAiedwditAiorntiocfle10OmMnline
20
15
10
5
Dark
Light Dark
Light
DOI: 10.1039/D0GC00331J
substrate
4 mL 0.1M phosphate buffer (pH = 7.4) solution containing 1
mM NAD+, 50 uM photocatalyst 7,
1500
1000
500
0
2 U glutamate
Light Dark Light Dark Light
dehydrogenase (GDH), 0.5 M HCOONa, 0.05 M (NH4)2SO4 and
10 mM α-ketoglutarate was added to a Schlenk tube equipped
with a rubber stopper and a sampling syringe and then the
tube was purged with argon and placed in front of green LEDs
(100 w, 525 nm). The reaction temperature was kept lower
than 40 °C through cooling with a fan. After 1.5 h irradiation,
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
20
40
60
80
100
120
Time (h)
Time (min)
another 10 mM α-ketoglutarate was added and irradiated for Figure 2. a) Time course of NADH regeneration in the presence
1 h. After a certain time irradiation, the reaction mixture was of 10 uM 7 with or without irradiation (100 w, 525 nm); b)
sampled and derivated with phenyl isothiocyanate as the Synthesis of L-glutamate using 50 uM 7 and 2 U GDH (10 mM
following procedure: To each 200 uL diluted sample was added α-ketoglutarate, 0.5 M HCOONa, 0.1 M (NH4)2SO4, in 0.1M
100uL 2.5 wt% phenyl isothiocyanate acetonitrile solution and phosphate buffer, 100 w green LEDs).
100 uL 1 M NEt3 acetonitrile solution. The resulting mixture
was kept at room temperature for 1 h, then 600 uL hexane
Substitution of bipyridine ligand at 4 and 4’-position with
was used to extract unreacted phenyl isothiocyanate. 200 uL
electron donating or withdrawing groups did not improve the
of bottom layer (acetonitrile) was added to 800 uL water and
photocatalytic activities of 4 and 5. 6, 6’-dimethyl bipyridyl Ru
filtered for HPLC analysis. Analysis of derivated L-glutamate
complex 6 gave an improved TOF of 18 h-1. This may attribute
samples was done on a Shimadzu Prominence 20 HPLC system,
to the high ligand substitution rate induced by the large steric
equipped with a C18 column and a UV-Vis detector.
hindrance of 6,6’-dimethyl bipyridine ligand.19a To our delight,
the bisquinoline containing complex 7 efficiently catalyzed the
reduction of NAD+ with a TOF of 78 h-1 under irradiation of
green light (40 w LEDs, 525 nm). This performance is
Results and discussion
comparable with M catalyzed transfer hydrogenation of NAD+
(TOF = 80 h-1). Moreover, bisquinoline ligand enhances the
electron delocalization and causes a red shift of metal-to-
ligand charge transfer (MLCT) absorption band (e.g. λMLCT of 1 =
473 nm, λMLCT of 7 = 548 nm, Figure S55-56). Under red light
(40 w, 625 nm) illumination, 7 can also catalyze the reduction
of NAD+ with a TOF of 39 h-1. The quantum yields (Φ = mol of
product / mol of absorbed photon) of NADH in the presence of
1 and 7 were measured through the ferrioxalate (for 456 nm
LEDs) and Reineckate’s salt actinometry (for 525 nm LEDs),
respectively.20 In the presence 7, the quantum yield was one
order higher than that with 1 (7.9×10-3 vs 3.6×10-4, see
supplementary information, page 5). Unlike 1, 7 could not
catalyze the reduction of NAD+ using TEOA as reductant, which
implied the rapid ligand exchange property instead of
photoredox property of 7 was the key for this conversion.
To confirm this NADH regeneration was a light-driven process,
control experiments under alternate dark and light conditions
were carried out. As a result, neglectable NADH was formed
without illumination (raise temperature to 45 °C, no NADH was
detected) whereas the concentration of NADH increased
linearly with a TOF of 159 h-1 under irradiation of a 100 w
green LEDs. After 100 min irradiation, 93 mol% NAD+ was
converted to NADH with a concentration of 1.86 mM (Figure
2a). Moreover, compared with 250 uM M as electron mediator,
only 10 uM 7 employed in this photocatalytic system.
Catalysts screening
Initially, we screened Ru terpyridyl complexes 1~7 under visible
light irradiation (40 w, 456 or 525 nm). As shown in Figure 1,
complex 1 catalyzed the reduction of NAD+ in a sodium
formate aqueous solution with a TOF of 7 h-1, which was much
higher than that using 15 % w/v TEOA (TOF = 2 h-1) or NEt3
.
18a
Similar results were obtained in the presence of 2 and 3.
100
With 1mM GSH
Without GSH
80
Cl
Cl
Cl
Cl
N
N
N
N
Ru
N
N
N
Ru
N
N
R
N
60
40
20
0
R
2
1
4
3
: R= H; : R= Me;
: R= OMe; : R= CN
5
Cl
Cl
Cl
Cl
N
N
N
N
Ru
N
Ru
N
N
N
N
N
6
7
1
2
3
4
5
6
7
M
Catalysts
Figure 1. Screening photocatalysts (10 uM) for transfer
hydrogenation of NAD+ (1mM) in the presence of HCOONa (0.5
M). For photocatalysts 1-7, 40 w LEDs (456 nm for 1-6 and 525
nm for 7) were used with a fan at room temperature; for M,
reactions were carried out at 40 °C. Black and gray bars
represent the results with and without 1 mM glutathione
(GSH).
Besides NADH, NADPH and BNAH were also regenerated with
high TOFs of 190 and 90 h-1, respectively. To prove the
regenerated NADH with 7 is enzymatically useful, glutamate
dehydrogenase (GDH) was employed with the light-driven
NADH regeneration system to synthesize L-glutamate from α-
ketoglutarate (see supplementary information, page 7). As
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 9
ARTICLE
Journal Name
100
200
180
160
140
120
100
a)
b)
shown in Figure 2b, the yield of L-glutamate could be
controlled by irradiation. A TOF of 161 h-1 was achieved after
0.5 h irradiation, indicating no significant deactivation of
photocatalyst occurred in the presence of GDH. After 1.5 h
irradiation, 10 mM α-ketoglutarate was completely converted
to L-glutamate and then another 10 mM α-ketoglutarate was
added and converted within 1 h. TONs based on 7 and NAD+
were 380 and 19, respectively. In earlier works4a, 7a, more
attention was paid to the regeneration of NAD(P)+. TONs of
NAD(P)+ over 40 were achieved, however enzyme or protein
View Article Online
DOI: 10.1039/D0GC00331J
40 w LEDs (520 nm)
1 mM NAD+
10 uM Ru(tpy)(biq)Cl2
90
80
70
60
50
40
30
y = a + b*x
a = 35.20 ± 0.97
b = 0.0735 ± 0.002
R2 = 0.9972
100 w LEDs (520 nm)
0.5 M HCOONa
10uM Ru(tpy)(biq)Cl2
0
200
400
600
800
1000
0
500
1000
1500
2000
+
Conc. NAD (uM)
Conc. HCOONa (mM)
Figure 4. a) TOF of NADH as a function of HCOONa
concentration; b) Plot of TOF of NADH vs concentration of
NAD+.
should be used to regenerate NAD(P)+. In recent works10, 11
,
efforts were devoted to the discovery of novel
photosensitizers but TONs of NAD(P)+ was low. In this work,
moderate TON of NAD(P)+ was achieved using a bifunctional
photocatalyst which play roles of both photosensitizer and
mediator.
295.5508; found 295.5493). Similarly, [Ru(tpy)(biq)Cl]+
generated [Ru(tpy)(biq)MeCN]2+ in MeCN. The two cation
structures were well defined by SC-XRD (Figure S58 and S59).
Compared to the spontaneous substitution of chloride with
solvents at ground state, fast replacement of solvents by
formate requires irradiation to generated excited states.
Electronic absorption spectra of 7 in sodium formate solution
after irradiation showed a shift of MLCT band from 553 nm to
559 nm and a rise of absorption at 625 nm, indicative of the
photosubstitution occurred at the Ru center (Figure 3a). 7a
was observed in the irradiated solution by HRMS ((ESI-TOF)
m/z: [Ru(tpy)(biq)HCOO]+ Calcd for C34H24N5O2Ru+ 636.0993;
found 636.0968). Irradiation of [Ru(tpy)(biq)MeCN]2+ also
afforded 7a whose molecular structure was characterized by
1H NMR and SC-XRD (Figure S26, S27 and S60). The rate
constant (k) for the photosubstitution of 7 in 0.5 M sodium
formate solution was determined as 0.034 s-1, corresponding
to a quantum yield of 1.2×10-2 (Figure 3b).
Tolerance to amino acid residues and light-induced ligand
exchange
With complex 7 in hand, we tested its tolerance towards
amino acid residues. 7 kept at least 86 % of catalytic activity in
the presence of bovine serum albumin, alcohol dehydrogenase,
glutathione and mercaptoethanol (Figure S1). In stark contrast,
M lost most activity (TOF decreased from 80 to 3 h-1) in the
presence of glutathione (Figure 1).
It is reasonable to attribute the good tolerance and high
efficiency of 7 to the dissociative character of the triplet metal
centered (3MC) excited state which is generated from MLCT
excited states.19 To this end, we tested the formation of
[Ru(tpy)(biq)HCOO]+ (7a) from 7 through ligand exchange with
formate anion. In water, 7 readily released chloride ligand
forming [Ru(tpy)(biq)OH]+ and [Ru(tpy)(biq)]2+ cations which
was confirmed by 1H NMR (Figure S28)19c and HRMS ((ESI-
TOF)m/z: [Ru(tpy)(biq)OH]+ Calcd for C33H24N5ORu+
608.1059; found 608.1019; [Ru(tpy)(biq)]2+ Calcd for
C33H23N5Ru2+
Reaction kinetics and mechanism
The rate constant of formate ligand exchange linearly
increased with the concentration of sodium formate,
suggesting that the photo-induced ligand exchange followed a
first-order reaction kinetic (Figure S2).
The kinetics of photocatalytic reduction of NAD+ were
investigated at different concentration of sodium formate and
NAD+. As shown in Figure 4a and 4b, TOFs of NADH had a
linear relationship with formate concentration and they
appeared irrelevant to the concentration of NAD+. The above
observations indicate that the photoreduction of NAD+ is
according with pseudo first-order reaction and the final
hydride transfer to NAD+ is a fast step, which is consistent with
the light-driven ligand exchange of complex 7 and the reported
fast hydride transfer from a terpyridiyl ruthenium hydride to
BNA+.18 To verify the transfer hydrogenation occurs via a Ru-
hydride species [Ru(tpy)(biq)H]+ (7b), reaction of 10 mM BNA+
a)
b)
0.7
0.22
0.20
0.18
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ExpDecay1 Model:
y = y0 + A1*exp[-(x-x0)/t1]
0.16
0.14
0.12
0.10
0.08
t1 = 29.34 ±1.29 s
k = 0.034 ± 0.002 s-1
R2 = 0.9971
0
50
100
150
200
250
300
Time (s)
1
and 10 mM 7b in MeCN-d3 was measure by H NMR spectra.
Within 5 min, 7b was completely consumed by BNA+, giving
BNAH as product (Figure S41). This result also suggested the
hydride transfer to NAD+ was a fast step.
350
400
450
500
550
600
650
700
750
800
Primary kinetic isotope effect (KIE = kH / kD = 2.5) was
measured in parallel experiments using HCOONa (TOF = 159 h-
1) and DCOONa (TOF = 64 h-1, the same conditions as Figure 2a
except for using DCOONa), respectively. Thus the rate-
determining step of this photoreaction should involve ether C-
Wavelength (nm)
Figure 3. a) Electronic absorption spectra of 7 in 0.5 M
HCOONa aqueous solution under green light (40 w) for 5 min
at 25 °C; b) Time course of the change in absorption of 7 at 625
nm.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
O
O
O
H
N
D
H
H
H
H bond cleavage (decarboxylation) or Ru-H bond cleavage
(hydride transfer).21 With the regard to the fact that NAD+
didn’t affect the reaction rate, the concerted decarboxylation
and hydride transfer to NAD+ could be excluded. It is
reasonable to conclude that the decarboxylation of 7a is the
rate-determining step since hydride transfer is much faster.
Based on the above observations, a plausible mechanism was
given in Scheme 2. Using Ru(tpy)(bpy)(MeCN)(PF6)2,
DCOONa (98% D atom)
H2
View Article Online
DOI: 10.1039/D0GC00331J
NH2
NH2
NH2
O
85 %
+
15 %
7, Green LEDs
N
N
Main product
O
O
H
H
H
N
NH2
NH2
Dark condition
N
+
7a
Ru(tpy)(bpy)(HCOO)PF6
and
Ru(tpy)(bpy)(H)PF6
as
photocatalysts, the regeneration of NADH was successfully
achieved, consistent with the proposed catalytic cycle.
Another strong evidence supporting this mechanism was the
reaction of BNA+ with DCOONa (Scheme 3). According to NMR
results (Figure S41), deuterated 1,4-BNAH was obtained as
main product. This indicated that formate acted as hydride
donor instead of electron donor (unlike widely used amines), 7
was converted to a hydride form 7b and finally BNA+ received
the hydride from 7b. It should be noted that the reaction
involving 7 and formate was different from that using 1 and
amines18 regarding the way to generate the key intermediate
Ruthenium hydrides.
With the rate-determining step in mind, we wondered
whether the transformation of 7a to 7b was a light-driven
process like the formation of cobalt hydride complex through
β-hydride elimination22 or a thermal reaction as the generation
of [RhCp*(bpy)H]+. With this regard, decarboxylation of
Ru(tpy)(biq)(HCOO)PF6 complex was conducted in sealed NMR
tubes under both light and dark conditions. However, no
hydride signal at high field was observed in 1H NMR. The
failure in observation of 7b was probably caused by the
undetectable low concentration of hydride species formed
Detected by NMR
Scheme 3. Light-driven transfer hydrogenation of BNA+ using
DCOONa with green LEDs and reduction of BNA+ using 7a in
the dark.
employed as hydride acceptor to drive the equilibrium to the
7b side. As a result, BNAH was afforded under dark conditions,
suggesting that the rate-determining decarboxylation of 7a
was a thermal reaction (Figure S41). Furthermore, apparent
activation energy of this step was estimated as only 3.2±0.4
kcal mol-1 via photocatalytic experiments carried out at
different temperature (Figure S3).
Thermodynamic hydricity of 7b
Thermodynamic hydricity is a crucial parameter to evaluate
the hydride donating ability of metal and organic hydride
donors.23 Hydricity of [Ru(tpy)(bpy)H]+ (1b) was carefully
measured in MeCN and water by Cruetz et al.24 after the
report of the photocatalytic reduction mediated by it.18 To our
best knowledge, the hydricity of 7b was still unknown since it’s
structure was not reported before. To understand the
thermodynamics of this visible-light-driven transfer
hydrogenation, hydricity of 7b was measured in MeCN. Thanks
to the efforts devoted to the measurement of hydricities of
metal hydrides and organic hydride donors, we could estimate
the value through a thermodynamic equilibrium. As shown in
through
a thermal dynamic equilibrium with a small
equilibrium constant. To verify the hypothesis, BNA+ was
Characterized by NMR and HRMS
figure S42-43,
a mixture of Ru(tpy)(bpy)(H)PF6 and
Ru(tpy)(biq)(MeCN)(PF6)2 generated 7b after 21 h at 298 K,
*
OH
OH
N
N
N
N
Ru
N
according to the appearance of a new peak at -14.71ppm and
N
Ru
N
N
N
hv
N
1
Fast
the decrease of the peak at -14.69 ppm in H NMR. However,
7
the
other
mixture
of
Ru(tpy)(biq)(H)PF6
and
Ru(tpy)(bpy)(MeCN)PF6 did not afforded 1b (Figure S44).
Although equilibrium of the two mixtures was not reached due
to the decomposition of hydride species, these observations
revealed that 7b owned lower hydride donating ability than
that of 1b (i.e. hydricity7b > hydricity1b). Thermodynamic
equilibrium was successfully achieved using Ru(tpy)(biq)(H)PF6
and 1,3-dimethylpyridinium hexafluorophosphate (DMPY) in
MeCN-d3, forming 1,4-dihydrodimethylpyridine (1,4-DHDMPY)
as well as 1,6-dihydrodimethylpyridine (1,6-DHDMPY) as minor
product. After 48 h at 298 K, equilibrium constant of 4.3 was
estimated by integration of the 1H NMR spectra, which
remained unchanged for the following 5 days (Figure S45-50)
and corresponded to a Gibbs free energy change of -0.9 kcal
hv
ISC
3MC state
1Ground state
1 MLCT
3MLCT
HCOO
HCOO
n+
S
OC(O)H
Ru
N
hv
N
N
N
Ru
N
N
N
N
N
N
Characterized
by SC-XRD
7a
S = solvent or unoccupied
O
Rate determining
Fast
KIE = kH / kD = 2.5
NH2
H
N
N
O
N
R
Ru
N
N
NH2
mol-1. The reported hydricity of 1,4-DHDMPY is 41±3 kcal mol-
N
N
R
1 24a
.
[Note: the value was also reported as 50.3 kcal mol-1
elsewhere.25] The hydricity of 7b in MeCN was calculated as
40±3 kcal mol-1 by summation the Gibbs free energy change
and the hydricity of 1,4-DHDMPY according to Scheme 4. The
value is 1 kcal mol-1 higher than that of 1b (39±3 kcal mol-1),
which is in good agreement of the early observation of hydride
7b
Characterized by NMR and HRMS
Scheme 2. A plausible mechanism of visible-light-driven
transfer hydrogenation of NAD(P)+.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 9
ARTICLE
Journal Name
n
H
the powerful protocol, reduction of more unsaturated
substrates using NAD(P)H dependent enzymes are underway
S
View Article Online
DOI: 10.1039/D0GC00331J
N
N
N
N
K = 4.3
Ru
N
N
Ru
N
N
N
+
+
N
+
N
N
N
in our lab.
7b
S = solvent or unoccupied
G = -RTlnK = -0.9 kcal mol-1
1,4-DHDMPY 1,6-DHDMPY
Δ
Conflicts of interest
There are no conflicts to declare.
H
OC(O)H
N
N
N
N
Ru
N
k1
Ru
N
N
N
N
N
CO2
+
k2
Dark condition
Acknowledgements
7a
7b
G = 11.5 kcal mol-1
Δ
This work was supported by the start-up fund of QIBEBT
(Y6710719ML) and Taishan Scholars Construction Projects of
Shandong (ts201712076). The authors are grateful to Prof. Dr.
Xiaowu Wang and Miss Ying Yang for helpful discussion and
support for NMR measurement.
K = k1 / k2 = e-(Δ G / RT) = 3.4 x 10-9
k2 >> k1
Scheme 4. Reaction thermodynamics of the decarboxylation of
7a to 7b.
transfer from 1b to [Ru(tpy)(biq)MeCN]2+. With the hypothesis
that relative differences in hydricity between 1b and 7b are
identical in water and MeCN (Δhydricity7b-1b,water = Δhydricity7b-
1b,MeCN), the hydricity of 7b in water could be estimated as 23.6
kcal mol-1 (hydricity7b,water = hydricity1b,water + Δhydricity7b-
Notes and references
1
2
3
4
H. Wu, C. Y. Tian, X. K. Song, C. Liu, D. Yang and Z. Y. Jiang,
Green Chem., 2013, 15, 1773-1789.
T. Quinto, V. Kohler and T. R. Ward, Top. Catal., 2014, 57,
321-331.
A. McSkimming and S. B. Colbran, Chem. Soc. Rev., 2013, 42,
5439-5488.
a) D. Mandler and I. Willner, J. Am. Chem. Soc., 1984, 106,
5352-5353; b) D. Mandler and I. Willner, J. Chem. Soc., Perk.
T. 2, 1986, 805-811; c) S. K. Kuk, R. K. Singh, D. H. Nam, R.
Singh, J. K. Lee and C. B. Park, Angew. Chem. Int. Ed., 2017,
56, 3827-3832; d) J. Kim, S. H. Lee, F. Tieves, D. S. Choi, F.
Hollmann, C. E. Paul and C. B. Park, Angew. Chem. Int. Ed.
2018, 57, 13825-13828.
a) Y. Maenaka, T. Suenobu and S. Fukuzumi, J. Am. Chem.
Soc., 2012, 134, 367-374; b) Y. Maenaka, T. Suenobu and S.
Fukuzumi, J. Am. Chem. Soc., 2012, 134, 9417-9427; c) J.
Collman, P. Wagenknecht, R. Hembre, N. Lewis, J. Am. Chem.
Soc., 1990, 112, 1294-1295; d) P. Wagenknecht, J. Penney, R.
Hembre, Organometallics 2003, 22, 1180-1182.
A. Damian, K. Maloo and S. Omanovic, Chem. Biochem. Eng.
Q., 2007, 21, 21-32.
= 22.6 + 1 kcal mol-1). The thermodynamic equilibrium
1b,MeCN
constant for reversible decarboxylation of[Ru(tpy)(bpy)HCOO]+
(1a) to 1b in water was reported as 5.9×10-10, corresponding to
a Gibbs free energy change (ΔG1a-1b,water = hydricity1a,water
–
hydricity1b,water) of 12.5 kcal mol-1. 24b With another hypothesis
that the hydricities of ruthenium formates 1a and 7a are close
(hydricity1a,water = hydricity7a,water ≈ hydricityformate,water), Gibbs
free energy change of 11.5 kcal mol-1 is obtained for the
decarboxylation of 7a (ΔG7a-7b,water
= hydricity7a,water –
hydricity7b,water = ΔG1a-1b,water –Δhydricity1b-7a,MeCN = 12.5 – 1 kcal
mol-1), corresponding to a equilibrium constant of 3.4×10-9.
The relatively higher equilibrium constant (compared to that
of 1a to 1b) favours the formation of 7b, therefore enhances
the efficiency of the regeneration of nicotinamide cofactors.
5
Conclusions
6
7
In summary, we developed a visible-light-driven transfer
hydrogenation protocol for nicotinamide cofactors
regeneration. In the presence of low concentration of
photocatalyst 7, efficient reduction of NAD+ was achieved with
a TOF of 159 h-1 and a quantum yield of 7.9 × 10-3. Compared
to the conventional transfer hydrogenation of NAD+ using M,
the light-driven protocol enables the reduction of cofactors
more efficient, controllable and tolerant to amino acid
residues, especially to thiol groups. In principle, these
advantages render the combination of photocatalytic
regeneration of nicotinamide cofactors with enzyme or whole
a) O. Abril and G. M. Whitesides, J. Am. Chem. Soc., 1982,
104, 1552-1554; b) S. Broussy, R. W. Cheloha and D. B.
Berkowitz, Org. Lett. , 2009, 11, 305-308.
8
9
a) G. Knor, Chem-Eur. J. , 2009, 15, 568-578; b) J. R. Revollo,
A. A. Grimm and S. Imai, J. Bio. Chem., 2004, 279, 50754-
50763.
a) S. H. Lee, D. H. Nam, J. H. Kim, J.-O. Baeg and C. B. Park,
ChemBioChem, 2009, 10, 1621-1624; b) S. H. Lee, D. H. Nam
and C. B. Park, Adv. Syn. & Catal., 2009, 351, 2589-2594.
10 a) J. Liu and M. Antonietti, Energ. Environ. Sci., 2013, 6, 1486-
1493; b) Y. Wu, J. Ward-Bond, D. Li, S. Zhang, J. Shi, Z. Jiang,
ACS Catal. 2018, 8, 5664-5674.
cell compatible. Controllable and efficient synthesis of L- 11 D. H. Nam, S. H. Lee and C. B. Park, Small, 2010, 6, 922-926.
12 Z. Y. Jiang, C. Q. Lu and H. Wu, Ind. Eng. Chem. Res., 2005,
44, 4165-4170.
glutamate has been realized in the presence of 7 and GDH.
Note that the reaction mechanism is different from the
photoredox regeneration of BNA+ using 1 and amines. Kinetic
study showed the light-driven ligand exchange afforded 7a
13 Y. Zhao, H. Liu, C. Wu, Z. Zhang, Q. Pan, F. Hu, R. Wang, P. Li,
X. Huang and Z. Li, Angew. Chem. Int. Ed., 2019, 58, 5376-
5381.
with a quantum yield of 1.2×10-2. Kinetic isotope experiments 14 R. Ruppert, S. Herrmann and E. Steckhan, J. Chem. Soc.,
Chem. Commun., 1988, 1150-1151.
revealed that the formation of 7b through the decarboxylation
of 7a was the rate-determining step with a small apparent
activation energy of 3.2±0.4 kcal mol-1. The hydricity of 40±3
kcal mol-1 for 7b and thermodynamic equilibrium constant of
3.4×10-9 for decarboxylation of 7a to 7b were estimated. With
15 a) J. Lutz, F. Hollmann, T. V. Ho, A. Schnyder, R. H. Fish and A.
Schmid, J. Organomet. Chem., 2004, 689, 4783-4790; b) F.
Hildebrand and S. Lutz, Chem-Eur. J. , 2009, 15, 4998-5001.
16 V. Kohler, Y. M. Wilson, M. Durrenberger, D. Ghislieri, E.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
Churakova, T. Quinto, L. Knorr, D. Haussinger, F. Hollmann,
N. J. Turner and T. R. Ward, Nature Chem., 2013, 5, 93-99.
17 K. T. Oppelt, J. Gasiorowski, D. A. M. Egbe, J. P. Kollender, M.
Himmelsbach, A. W. Hassel, N. S. Sariciftci and G. n. Knör, J.
Am. Chem. Soc., 2014, 136, 12721-12729.
View Article Online
DOI: 10.1039/D0GC00331J
18 a) Y. Matsubara, K. Koga, A. Kobayashi, H. Konno, K.
Sakamoto, T. Morimoto and O. Ishitani, J. Am. Chem. Soc.,
2010, 132, 10547-10552; b) O. Ishitani, N. Inoue, K. Koike
and T. Ibusuki, J. Chem. Soc., Chem. Commun., 1994, 367-368;
c) Y. Matsubara, H. Konno, A. Kobayashi and O. Ishitani,
Inorg. Chem., 2009, 48, 10138-10145; d) Y. Matsubara, T.
Kosaka, K. Koga, A. Nagasawa, A. Kobayashi, H. Konno, C.
Creutz, K. Sakamoto and O. Ishitani, Organometallics, 2013,
32, 6162-6165; e) K. Koga, Y. Matsubara, T. Kosaka, K. Koike,
T. Morimoto and O. Ishitani, Organometallics, 2015, 34,
5530-5539.
19 a) L. N. Lameijer, D. Ernst, S. L. Hopkins, M. S. Meijer, S. H. C.
Askes, S. E. Le Devedec and S. Bonnet, Angew. Chem. Int. Ed.,
2017, 56, 11549-11553; b) A. Bahreman, B. Limburg, M. A.
Siegler, R. Koning, A. J. Koster and S. Bonnet, Chem-Eur. J.,
2012, 18, 10271-10280; c) A. Bahreman, B. Limburg, M. A.
Siegler, E. Bouwman and S. Bonnet, Inorg. Chem., 2013, 52,
9456-9469.
20 H. Kuhn, S. Braslavsky and R. Schmidt, Pure and Appl. Chem.,
2004, 76, 2105-2146.
21 E. M. Simmons and J. F. Hartwig, Angew. Chem. Int. Ed.,
2012, 51, 3066-3072.
22 X. Sun, J. T. Chen and T. Ritter, Nature Chem., 2018, 10,
1229-1233.
23 E. S. Wiedner, M. B. Chambers, C. L. Pitman, R. M. Bullock, A.
J. M. Miller and A. M. Appel, Chem. Rev., 2016, 116, 8655-
8692.
24 a) Y. Matsubara, E. Fujita, M. D. Doherty, J. T. Muckerman
and C. Creutz, J. Am. Chem. Soc., 2012, 134, 15743-15757; b)
C. Creutz and M. H. Chou, J. Am. Chem. Soc., 2007, 129,
10108-10109; c) C. Creutz and M. H. Chou, J. Am. Chem. Soc.,
2009, 131, 2794-2795.
25 S. Ilic, U. P. Kadel, Y. Basdogan, J. A. Keith and K. D. Glusac, J.
Am. Chem. Soc., 2018, 140, 4569-4579.
26 P. Adcock, F. Keene, R. Smythe, M. Snow, Inorg. Chem., 1984,
23, 2336-2343.
27 a) H. Konno, A. Kobayashi, K. Sakamoto, F. Fagalde, N. Katz,
H. Saitoh, O. Shitani, Inorg. Chim. Acta 2000, 299, 155-163;
b) C. Hecker, P. Fanwick, D. Mcmillin, Inorg. Chem., 1991, 30,
659-666.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins