Dual utility of a single diphosphine-ruthenium complex: A precursor for new complexes and, a pre-catalyst for transfer-hydrogenation and Oppenauer oxidation

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Dual utility of a single diphosphine–ruthenium

complex: a precursor for new complexes and,

a pre-catalyst for transfer-hydrogenation and

Oppenauer oxidation†

Cite this: RSC Adv., 2021, 11, 15617

Aparajita Mukherjee and Samaresh Bhattacharya

*

The diphosphine–ruthenium complex, [Ru(dppbz)(CO)

2

Cl

2

] (dppbz ¼ 1,2-bis(diphenylphosphino)benzene),

where the two carbonyls are mutually cis and the two chlorides are trans, has been found to serve as an

eﬃcient precursor for the synthesis of new complexes. In [Ru(dppbz)(CO)

2 2

Cl ] one of the two carbonyls

undergoes facile displacement by neutral monodentate ligands (L) to aﬀord complexes of the type

[

Ru(dppbz)(CO)(L)Cl

Ru(dppbz)(CO) Cl

2 2 2

Ru(dppbz) Cl ]. The two carbonyls and the two chlorides in [Ru(dppbz)(CO) Cl ] could be displaced

2

] (L ¼ acetonitrile, 4-picoline and dimethyl sulfoxide). Both the carbonyls in

[

2

] are displaced on reaction with another equivalent of dppbz to aﬀord

2

together by chelating mono-anionic bidentate ligands, viz. anions derived from 8-hydroxyquinoline (Hq)

and 2-picolinic acid (Hpic) via loss of a proton, to aﬀord the mixed-tris complexes [Ru(dppbz)(q) ] and

], respectively. The molecular structures of four selected complexes, viz.

Ru(dppbz)(CO)(dmso)Cl ], [Ru(dppbz) Cl ], [Ru(dppbz)(q) ] and [Ru(dppbz)(pic) ], have been determined

2

[

Ru(dppbz)(pic)

2

[

2

Received 28th February 2021

Accepted 21st April 2021

by X-ray crystallography. In dichloromethane solution, all the complexes show intense absorptions in the

visible and ultraviolet regions. Cyclic voltammetry on the complexes shows redox responses within 0.71

DOI: 10.1039/d1ra01594j

to ꢀ1.24 V vs. SCE. [Ru(dppbz)(CO)

2 2

Cl ] has been found to serve as an excellent pre-catalyst for catalytic

rsc.li/rsc-advances

transfer-hydrogenation and Oppenauer oxidation.

about. Synthesis of any new ruthenium complex is essentially

achieved via displacement of pre-coordinated ligand(s) from an

Introduction

There has been considerable current interest in the chemistry of appropriate starting ruthenium precursor. Predictable displace-

ruthenium complexes largely because of their versatile catalytic ability of ligand(s) in any ruthenium complex is thus of signi-

1

applications. Complexes of ruthenium are also nding applica- cant importance, and is a much sought aer property. In the

2

tion in biology, several ruthenium-species, such as RAPTA, present study, which has emerged from a recently reported work

NAMI-A, KP1019, etc., have already shown promise as anti-tumor of us involving a mixed-ligand ruthenium(II) diphosphine

3

agents. A third application of ruthenium complexes, viz. complex, [Ru(dppbz)(CO)

2

Cl

2

], where dppbz depicts 1,2-bis(di-

4

synthetic application, is also there, which is relatively less talked phenylphosphino)benzene (Chart 1), our aim was to explore

potential of this complex as a precursor for (i) synthesis of new

complexes, and (ii) catalysis. This [Ru(dppbz)(CO) Cl ] complex

2

Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, was found to serve as an useful precursor for the in situ genera-

Kolkata–700 032, India. E-mail: samaresh_b@yahoo.com; Fax: +91-33-24146223

Electronic supplementary information (ESI) available: Selected bond

parameters for [Ru(dppbz)(CO)(dmso)Cl ], (Table S1); energy diﬀerences

between the cis- and trans-isomers of [Ru(dppbz)(CO)(CH CN)Cl and

Ru(dppbz)(CO)(4-picoline)Cl ], (Fig. S1 and S2); DFT-optimized structures of

trans-[Ru(dppbz)(CO)(CH CN)Cl and trans-[Ru(dppbz)(CO)(4-picoline)Cl ],

Fig. S3 and S4); some computed bond parameters of the DFT-optimized

structures of [Ru(dppbz)(CO)(CH CN)Cl and [Ru(dppbz)(CO)(4-picoline)Cl ],

Table S2); selected bond parameters for [Ru(dppbz) Cl ], [Ru(dppbz)() ] and

Ru(dppbz)(pic) ], (Tables S3–S5); parameters and gures from TDDFT

2

tion of a ruthenium(0) species, viz. [Ru(dppbz)(CO) ], which

could eﬃciently catalyze C–C and C–N coupling reactions. Herein

†

2

3

2

]

[

2

3

2

]

2

(

3

2

]

2

(

2

[

2

calculations, (Tables S6–S17 and Fig. S5–S10); optimization for catalytic

transfer-hydrogenation, (Table S18); optimization for catalytic Oppenauer

oxidation, (Tables S19 and S20); crystal data (Table S21). CCDC

1

489487–1489490. For ESI and crystallographic data in CIF or other electronic

format see DOI: 10.1039/d1ra01594j

Chart 1

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we wish to describe two types of utility of the same [Ru(dppbz)(CO)(L)Cl

2

], herein we have carried out reactions of

[

Ru(dppbz)(CO) Cl ] complex, viz. synthetic utility and catalytic [Ru(dppbz)(CO) Cl ] with three selected monodentate ligands,

2

utility, which are not associated with any change of oxidation viz. dimethyl sulfoxide (dmso), acetonitrile and 4-picoline. Each

state of the metal center. There are four monodentate ligands in of these ligands is found to readily react with

this complex, two carbonyls that are mutually cis and two chlo- [Ru(dppbz)(CO)

rides that are trans. We came up with experimental methods product of type [Ru(dppbz)(CO)(L)Cl

whereby one, two or all four of these monodentate ligands can be picoline) in quantitative yields. Bulk characterization data on

predictably displaced by ligand(s) of appropriate nature leading these three complexes were found to be consistent with their

to generation of new molecules – a phenomenon demonstrating compositions. The structure of one member of this family, viz.

2

Cl

2

] and aﬀord the expected monosubstituted

] (L ¼ dmso, CH CN, 4-

2

3

synthetic utility of the parent complex.

[Ru(dppbz)(CO)(dmso)Cl ], was determined by X-ray crystal-

2

Ruthenium complexes with a pre-existing Ru–H bond or with lography. The structure is shown in Fig. 1 and selected bond

the capability of formation of such a fragment in situ, have the parameters are presented in Table S1 (ESI†). The structure

potential to serve as catalyst/pre-catalyst in transfer- conrms the loss of one CO group. The ancillary dmso ligand is

5

–11

hydrogenation of suitable substrates.

Complexes having S-bonded to the ruthenium center. The Ru–S distance is

13

Ru–Cl bond(s) are particularly useful in this respect, as they can normal, and the other bond distances around ruthenium

We have also successfully compare well with those in the parent [Ru(dppbz)(CO)

explored this possibility in the [Ru(dppbz)(CO)

7f,g,i

easily give rise to a Ru–H fragment.

2

Cl

2

]

4

2

Cl ] complex. complex. It was interesting to note that unlike the disposition

2

Herein we report our observations on exploration of dual utility of the CO and chloride ligands in the parent complex, the

of the [Ru(dppbz)(CO) Cl ] complex, with particular reference to remaining carbonyl and a chloride have undergone a positional

2

(

i) formation and characterization of the new complexes, and interchange that results in the formal trans-to-cis isomerization

ii) eﬃciency of the [Ru(dppbz)(CO) Cl ] complex in catalyzing of the two chlorides in [Ru(dppbz)(CO)(dmso)Cl ]. In order to

help rationalize this change in geometry, the structures of both

the trans and cis isomers of [Ru(dppbz)(CO)(dmso)Cl ] were

optimized by DFT. The computed energies of the optimized

2

transfer-hydrogenation reactions.

2

ꢀ1

Results and discussion

Syntheses and structures

isomers (Fig. 2) indicate that the cis isomer is 8.40 kcal mol

more stable than the trans isomer. We speculate that the initial

ligand substitution occurs stereoselectively with the displace-

ment of a carbonyl from the parent complex by dmso to furnish

As outlined in the introduction, this study was intended to

explore two types of reactivity in [Ru(dppbz)(CO) Cl ] without

bringing about any change in the oxidation state of ruthenium.

The reactivity that we describe rst has its origin in our previous

study involving the same ruthenium complex. During our

attempts to grow single crystals of [Ru(dppbz)(CO) Cl ] from its

2

the trans form of [Ru(dppbz)(CO)(dmso)Cl ] as the kinetic

product of substitution; conversion of this isomer into the

thermodynamically more stable cis isomer results via a subse-

quent sequence of events involving the mutual permutation of

the carbonyl and one chloride from the trans isomer. The

isomerization seems to be facilitated by the elevated tempera-

ture of reuxing dimethyl sulfoxide.

2

solutions in diﬀerent solvents, we noticed that whenever

a coordinating solvent was used for such crystallization, the

crystalline solid obtained back was

Ru(dppbz)(CO) Cl ] and another species that seemed to have

only one carbonyl in it, as indicated by IR. Prompted by this

indication that a carbonyl in [Ru(dppbz)(CO) Cl ] probably

undergoes facile displacement by a monodentate ligand (L)

leading to formation of new complex of type

a

mixture of

[

2

12

2

a

Fig. 2 DFT-optimized structures of the cis and trans isomers of the

2

[Ru(dppbz)(CO)(dmso)Cl ], and the energy diﬀerence (DE) between

2

Fig. 1 Crystal structure of [Ru(dppbz)(CO)(dmso)Cl ].

them.

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As crystals of the other two [Ru(dppbz)(CO)(L)Cl

RSC Advances

2

] (L ¼

CH CN and 4-picoline) complexes could not be grown, struc-

3

tures of both the trans and cis isomers of these two complexes

were examined by DFT method, and the computed energies of

the optimized isomers (Fig. S1 and S2; ESI†) indicate that the cis

isomer is thermodynamically more stable than the trans isomer

in both cases. However, in view of the relatively milder experi-

mental conditions used for the synthesis of these two

complexes,

viz.

[Ru(dppbz)(CO)(CH

3

CN)Cl

2

]

and

[

Ru(dppbz)(CO)(4-picoline)Cl ], they are believed to have the

2

trans structures, which are shown in Fig. S3 and S4 (ESI†). Some

computed bond parameters of these DFT-optimized structures

are listed in Table S2 (ESI†), which are found to compare well

with each other, and also with those in the crystal structure of

[

Ru(dppbz)(CO)(dmso)Cl

Formation of the [Ru(dppbz)(CO)(L)Cl

-picoline) complexes demonstrates the labile nature of one

2

].

2

] (L ¼ dmso, CH

3

CN,

4

carbonyl in the parent [Ru(dppbz)(CO) Cl ] complex. It is also

2

2 2

Fig. 3 Crystal structure of [Ru(dppbz) Cl ].

noteworthy in this context that use of an excess quantity of the

monodentate ligand (L) does not lead to the displacement of

Encouraged by the facile displacement of carbonyl(s) from

Ru(dppbz)(CO) Cl ] by a variety of neutral ligands, we next

investigated the lability of the coordinated chloride(s) in the same

complex. Here we selected two bidentate chelating ligands, viz.

quinolin-8-ol (Hq) and 2-picolinic acid (Hpic), both of which are

known to bind to ruthenium, and also other metal ions, as

both carbonyls from [Ru(dppbz)(CO)

of [Ru(dppbz)(CO) Cl ] with a second equivalent of dppbz was

found to successfully displace both carbonyls and furnish the

bis-dppbz complex, [Ru(dppbz) Cl ], in good yield. It is relevant

2 2

Cl ]. However, treatment

[

2

to mention here that presence of acetonitrile during the

synthesis was found to be crucial, as the bis-dppbz complex is

not formed at all when the same synthesis was attempted in

dichloromethane alone. This indicates that displacement of

one carbonyl by acetonitrile probably takes place rst, which

then allows the added dppbz to bind itself to the metal center

and eventually furnish the bis-dppbz complex. This hypothesis

is further supported by the fact that reaction of

monoanionic N,O-donors, via loss of the acidic proton, to form

14,15

ve-membered chelate rings (I and II).

The pyridine-type

nitrogen was expected to displace carbonyl and the phenolate or

carboxylate oxygen was expected to displace chloride. The reaction

of quinolin-8-ol with [Ru(dppbz)(CO)

equimolar ratio with the intention of obtaining a complex of type

Ru(dppbz)(q)(CO)Cl], but we only obtained a complex formulated

as [Ru(dppbz)(q) ] in low yield. This result indicates that,

compared to [Ru(dppb)(CO) Cl ], the initial product from the

2 2

Cl ] was rst carried out in

[

3 2

[Ru(dppbz)(CO)(CH CN)Cl ] with dppbz ligand in dichloro-

2

methane under ambient conditions readily aﬀords the same

2

bis-dppbz complex. The molecular structure of [Ru(dppbz) Cl ],

2

reaction, [Ru(dppbz)(q)(CO)Cl], is kinetically more reactive

towards the deprotonated quinolin-8-ol. And thus it rapidly

undergoes an additional substitution with the deprotonated

which was established by X-ray crystallography, is shown in

Fig. 3, and some relevant bond parameters are presented in

Table S3 (ESI†). The structure of [Ru(dppbz) Cl ] reveals the

2 2

2

quinolin-8-ol to aﬀord [Ru(dppbz)(q) ]. Realizing the requirement

presence of two chelating dppbz ligands that share a common

equatorial plane with the ruthenium center, and the two chlo-

rides are mutually trans as in the parent complex. Formation of

of two moles of Hq per mole of [Ru(dppbz)(CO) Cl ] for the

2

unstoppable production of [Ru(dppbz)(q) ], it was synthesized in

2

good yield from a reaction between these two ingredients in 2 : 1

mole ratio. Depending on the relative disposition of the two N,O-

[

Ru(dppbz) Cl ] thus exemplies stereoretentive displacement

2 2

of the two mutually cis carbonyls from [Ru(dppbz)(CO) Cl ] by

2

ligands, three geometrical isomers, viz. ct, tc and cc, may be

a neutral chelating ligand.

16

envisioned for [Ru(dppbz)(q)

2

]. In order to sort out this isomer

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2

Fig. 4 Crystal structure of [Ru(dppbz)(q) ].

2

Fig. 5 Crystal structure of [Ru(dppbz)(pic) ].

assignment problem, crystal structure of the isolated

Ru(dppbz)(q) ] complex was determined. The structure (Fig. 4

The reaction of [Ru(dppbz)(CO)

with 2-picolinic acid (Hpic) in 1 : 2 mole ratio, and the bis-

picolinate complex, [Ru(dppbz)(pic) ], was isolated in good

yield. The molecular structure of this complex was determined

by X-ray crystallography (Fig. 5 and Table S5 (ESI†)), which

shows that the picolinate ligands are chelated to the ruthenium

in the usual fashion (II). The complex has a similar stereo-

2 2

Cl ] was next carried out

[

2

17

and Table S4 (ESI†)) reveals that both the quinolin-8-olate ligands

are chelated to ruthenium in the usual fashion (I), with the

nitrogens exhibiting a cis disposition and contained in the plane

dened by the ruthenium center and the two phosphorus atoms

of the dppbz ligand. The two oxygens atoms adopt a trans orien-

tation relative to the plane containing the RuP N atoms. There-

2

chemical disposition of the two N,O-donor ligands as observed

fore the isolated [Ru(dppbz)(q)

Comparison of the stereochemistry around ruthenium in the

parent [Ru(dppbz)(CO) Cl complex and the derived

Ru(dppbz)(q) ] complex reveals that, interestingly, the anionic

2

] complex has the ct-geometry.

ꢀ

in [Ru(dppbz)(q)

2

]. The Ru–P distances (2.2553 A mean

distance) are again found to be signicantly shorter in

Ru(dppbz)(pic) ] than in the parent complex. The observed

2

]

[

2

[

2

bond parameters in the Ru(pic) fragments compare well with

halide positions and the neutral carbonyl sites in parent complex

are taken up respectively by the anionic phenolate-oxygens and

neutral pyridine-nitrogens in the derived bis-quinolinolate

complex. Bond parameters within the Ru(q) fragments are

similar to those distances and angles reported in related struc-

15

those data reported in other pic-chelated compounds.

Formation of these two [Ru(dppbz)(N–O) ] complexes (where

2

N–O ¼ quinolin-8-olate, 2-picolinate) demonstrates that all of

2 2

the monodentate ligands in [Ru(dppbz)(CO) Cl ], viz. the two

neutral carbonyls and the two anionic chlorides, are easily

displaced by appropriate chelating bidentate ligands. And in

view of the electronic nature of the displacing as well as dis-

placed donor sites, the observed displacement reactions are

stereoretentive in nature.

14

tures having the same quinolinolate auxiliary. The mean Ru–P

ꢀ

distance of 2.2489 A in [Ru(dppbz)(q)

2

] is 0.1443 A shorter than

Cl ], a feature we

the mean Ru–P distance in [Ru(dppbz)(CO)

2

attribute to the relatively poor p-acid character of the heterocyclic

platform of the quinolin-8-olate ligands compared to the two

carbonyls in the parent complex.

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2 2

Scheme 1 Formation of diﬀerent types of complexes from [Ru(dppbz)(CO) Cl ].

[

Ru(dppbz)(CO) Cl ] thus serves as a useful precursor for the ruthenium(II) complexes based on [Ru(dppbz)(CO) Cl ] may be

2

synthesis of new and unique dppbz-chelated ruthenium(II) synthesized via selective displacement of monodentate

complexes, as summarized in Scheme 1. Neutral monodentate ligand(s).

ligands (L) are found to displace only one carbonyl from the

parent complex, independent of the amount of ligand used, to

furnish complexes with the formula [Ru(dppbz)(CO)(L)Cl

Whereas, use of a rigid diphosphine ligand promotes the facile

displacement of both carbonyls to give[Ru(dppbz) Cl ]. The loss

of the mutually cis carbonyls in [Ru(dppbz)(CO) Cl ] is attrib-

2

]. Spectral properties

Magnetic susceptibility measurements conrm that all six of

the new complexes are diamagnetic, consistent with their

formulated structures and 2+ oxidation state for ruthenium

2

utable to the chelate eﬀect imposed by the dppbz ligands.

Mono-anionic chelating ligands (N–O) are found to bring about

stereoretentive displacement of the monodentate CO and

6

1

(

low-spin d , S ¼ 0). The H NMR spectra of all six complexes

show broad signals within 6.36–8.10 ppm for the dppbz ligand.

1

The H chemical shi for the methyl group(s) in [Ru(dppbz)(-

chloride ligands in [Ru(dppbz)(CO)

the type [Ru(dppbz)(N–O) ]. The reactions illustrated in Scheme

demonstrate that, by proper choice of ligands, new octahedral

2 2

Cl ] to yield complexes of

CO)(L)Cl

.16 and 2.31 ppm, respectively. In both [Ru(dppbz)(N–O)

O ¼ q, pic) complexes, appropriate signals for the N–O ligands,

2

] (L ¼ dmso, CH

3

CN, 4-picoline) is observed at 3.37,

2

] (N–

1

Table 1 Electronic spectral and cyclic voltammetric data

Complex Electronic spectral data lmax, nm (3, M cm

a

ꢀ1

b

)

Cyclic voltammetric data E/V vs. SCE

c

d

[

Ru(dppbz)(CO)(dmso)Cl

Ru(dppbz)(CO)(CH CN)Cl

Ru(dppbz)(CO)(4-picoline)Cl

Ru(dppbz) Cl

Ru(dppbz)(q)

Ru(dppbz)(pic)

2

]

340 (2418), 302 (2181), 270 (13 200)

ꢀ1.20

c

d

3

2

]

356 (800), 316 (2000), 268 (14 000)

ꢀ1.16

c

d

2

]

363 (3100), 307 (6100), 269 (15 100)

ꢀ1.17

c

e

f

d

2

]

370 (1800), 313 (1500), 276 (3900)

686 (620), 475 (5000), 366 (6300), 248 (19 000)

0.91 (99) , ꢀ1.03

e f e

f

d

2

0.99 (263) , 0.14 (83) , ꢀ1.16

e f d

c

2

]

432 (1200), 343 (8100), 259 (16 200)

0.71 (94) , ꢀ1.24

a

b

ꢀ1

c

d

e

In dichloromethane. Solvent, dichloromethane; supporting electrolyte, TBHP; scan rate, 50 mV s

.

Shoulder.

E

pc value.

E

1/2 value, where E1/

f

2

¼ 0.5(Epa + Epc). DE

p

value, where DE

p

¼ Epa ꢀ Epc

.

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2

consistent with C symmetry existing in these molecules, are Electrochemical properties

observed.

The redox properties of the mixed-ligand ruthenium dppbz

complexes have been examined in dichloromethane solution

The IR spectra of the complexes show many bands of varying

intensities within 450–2100 cm , among which several bands

ꢀ

1

(

0.1 M TBHP) by cyclic voltammetry. Voltammetric data are

presented in Table 1. The three [Ru(dppbz)(CO)(L)Cl

] (L ¼

dmso, CH CN, 4-picoline) complexes show an irreversible

ꢀ1

observed from 492–1584 cm are attributed, by comparison

with the spectrum of the parent [Ru(dppbz)(CO) Cl ] complex,

to the Ru(dppbz) unit. The [Ru(dppbz)(CO)(L)Cl

] (L ¼ dmso,

CH CN, 4-picoline) complexes show a strong n(CO) stretch

2

3

2

20

reduction near ꢀ1.2 V. In [Ru(dppbz)

2 2

Cl ], a similar reduction

3

is observed at ꢀ1.03 V, along with a reversible oxidation recor-

ded at 0.91 V that is tentatively assigned to the Ru(II)/Ru(III)

redox couple. It is interesting to note that this ruthenium-based

ꢀ

1

within 1973–1980 cm , while no such band was found in the

spectra of the [Ru(dppbz) Cl ] and [Ru(dppbz)(N–O) ] (N–O ¼ q,

2

pic) complexes consistent with the absence of a terminal CO

2 2

oxidation is not observed in [Ru(dppbz)(CO) Cl ] within the

positive limit of the voltage window, indicating that the two

carbonyls in the parent complex more eﬀectively stabilize

2

ruthenium(II) than the dppbz ligand. [Ru(dppbz)(q) ] shows

ligand. In [Ru(dppbz)(pic) ] a broad and strong band was

observed at 1703 cm , which is due to the carboxylate fragment

in the coordinated picolinates.

The mixed-ligand ruthenium dppbz complexes are readily

soluble in dichloromethane and chloroform, producing yellow

2

ꢀ

1

a reversible Ru(II)/Ru(III) oxidation at 0.14 V and an irreversible

reduction at ꢀ1.16 V. A second oxidative wave is observed at

solutions for [Ru(dppbz)(CO)(L)Cl

solution for [Ru(dppbz)(q)₂], and orange solution for

Ru(dppbz)(pic) ]. The electronic spectra of the complexes were

2 2 2

] and [Ru(dppbz) Cl ], red

0

8

.99 V, which is believed to be due to oxidation of the quinolin-

21

-olate ligand. In [Ru(dppbz)(pic) ], the Ru(II)/Ru(III) oxidation

2

[

2

is observed at 0.71 V and the reduction of dppbz at ꢀ1.24 V. The

recorded in dichloromethane solutions, and the spectral data

are given in Table 1. Each complex shows several absorptions in

the visible and ultraviolet region. To gain insight into the nature

of the observed absorptions TDDFT calculations, which include

dichloromethane as solvent, have been performed on all six

anodic shi in the Ru(II)/Ru(III) oxidation couple in

2 2

[Ru(dppbz)(pic) ] compared to [Ru(dppbz)(q) ] reects a lower

eﬃciency of the carboxylate moiety, compared to phenolate

moiety, to stabilize the ruthenium(III) oxidation product.

18

complexes using the Gaussian 09 program package. The main

calculated transitions and compositions of the molecular

Catalytic transfer-hydrogenation

orbitals associated with these transitions for all the six As indicated in the introduction, the second objective of this

complexes are presented in Tables S6–S17 (ESI†), and contour study was to explore catalytic activity of the parent

plots of the same molecular orbitals are shown in Fig. S5–S10 [Ru(dppbz)(CO) Cl ] complex. The presence of Ru–Cl bond in

2

(

ESI†). Each of the [Ru(dppbz)(CO)(L)Cl

2

] (L ¼ dmso, CH

3

CN, 4- this complex, together with the labile nature of one carbonyl as

picoline) complexes show three absorptions. The lowest energy observed during exploration of its synthetic utility, tempted us

absorption, observed within 340–363 nm, is found to result to assess its catalytic potential towards transfer-hydrogenation

from a combination of transitions involving several lled and reaction. We began our study by examining the transfer-

vacant orbitals, and based on the composition of these hydrogenation of 4-chlorobenzaldehyde to 4-chlorobenzyl

participating orbitals this absorption is attributable to alcohol. Aer extensive optimization it was found that

t

a combination of metal-to-ligand charge-transfer (MLCT), 0.02 mol% catalyst, 0.2 mol% KO Bu, 1-propanol as solvent,

ligand-to-ligand charge-transfer (LLCT), ligand-to-metal charge- a reaction temperature of 95 C, and a reaction time of 6 h

ꢁ

transfer (LMCT), and intra-ligand charge-transfer (ILCT) tran- furnished an excellent (99%) yield of the product (Table S18,

19

sitions. The second absorption observed within 302–316 nm, entry 1; ESI†). Using the optimized reaction conditions,

and the third one near 270 nm, are also found to be of quali- transfer-hydrogenation of twelve diﬀerent aldehydes has been

2 2

tatively similar nature. In the [Ru(dppbz) Cl ] complex the performed (entries 1–12, Table 2). Benzaldehyde and, para-

lowest energy absorption at 370 nm is found to result due to substituted benzaldehydes having both electron-donating and

a transition from the HOMO to LUMO+2. As the HOMO has electron-withdrawing substituent at the para-position, fur-

dominant (61%) ruthenium character and the LUMO+2 is nished the corresponding alcohols in excellent (97–99%) yields

localized almost entirely (99%) on the dppbz ligand, this (entries 1–5). However, reduction of bulkier aldehydes could be

absorption is assignable to a predominantly MLCT transition achieved with less eﬃciency as reected in lower yields of the

with minor LLCT character. The next two absorptions at 313 nm product alcohols (entries 6–8). Both the aldehyde groups in 1,3-

and 276 nm are found to be primarily due to ILCT transition, diformyl benzene are found to be reduced similarly and

with much less LLCT and LMCT character. In the smoothly (entry 9). In cinnamaldehyde the aldehyde function

[

Ru(dppbz)(q) ] complex the absorptions at 686 nm and 475 nm underwent selective reduction in presence of the olenic frag-

2

are found to be due mainly to ILCT transition with some MLCT ment (entry 10). Aldehydes having another strong donor atom,

character. The third absorption at 366 nm has major MLCT such as salicylaldehyde or pyridine-2-aldehyde, are found hard

character, while the fourth one at 248 nm has major ILCT or impossible to hydrogenate (entries 11 and 12 respectively),

character. In the [Ru(dppbz)(pic) ] complex, the band at 433 nm presumably due to catalyst inhibition caused through coordi-

2

has dominant MLCT nature with much less ILCT component. nation. Aer successful transfer-hydrogenation of aryl alde-

The next band at 343 nm also has dominant MLCT nature, while hydes, we attempted reduction of aryl ketones under the similar

22

the third band at 259 has major LLCT character.

experimental condition. Several aryl ketones were used as

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a

Table 2 Catalytic transfer-hydrogenation of aldehydes and ketones

Table 2 (Contd.)

b

Entry

1

Substrate

Yield (%) Entry

Substrate

Yield (%)

99

2

3

4

5

98

1

25

99

97

12

13

14

15

0

97

99

90

c

6

7

61

91

c

1

6

7

8

92

82

81

8

9

88

c

1

2

9

78

32

1

0

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Table 2 (Contd.)

known as Oppenauer oxidation, is also achievable using the

same catalyst precursor. Ru-catalyzed Oppenauer oxidation is

well precedent in the literature. Some of these reactions are

24

known to proceed via the intermediacy of ruthenium-hydrido

24a,c,f,j

species,

Ru(dppbz)(CO)

of [Ru(dppbz)(CO)

that is also derivable in situ from

Cl ]. We planned to explore catalytic potential

Cl ] towards Oppenauer-type oxidation of

[

2

b

Entry

Substrate

Yield (%) secondary alcohols, a green oxidation protocol that utilizes

acetone as hydrogen acceptor. We rst examined the oxidation

of cyclohexanol to cyclohexanone and, aer several trials for

optimization, we have observed that the best result is obtained

with 3 : 2 toluene-acetone as solvent, 0.1 mol% of catalyst,

t

ꢁ

KO Bu as base, 100 C temperature and 6 h reaction time (Table

S19, entry 1; ESI†). Presence of acetone in this oxidation was

found to be a must, as manifested in reaction in toluene alone

that did not yield any oxidized product (entry 12). However,

acetone alone was not found to be the best solvent either, as it

aﬀorded the product in signicantly lower yield (entry 13). And

a 3 : 2 toluene-acetone mixture was found to be the best choice.

Scope of the reaction is illustrated in Table 3. When 2-propanol

was used as the substrate, acetone was the expected product,

c

2

1

0

a

t

Reaction conditions: aldehydes and ketones (1.0 mmol), KO Bu (0.2

mol%), Ru catalyst (0.02 mol%), 1-propanol (5 mL). Yields are

determined by GCMS based on the quantity of substrate remaining

b

aer the reaction. Besides the substrate and the expected product, no and hence acetone could not be utilized as a reagent. We

c

other species was detected in any of the reactions. For ketones Ru

therefore tried oxidizing 2-propanol using 1,4-benzoquinone as

catalyst 0.2 mol% and 10 h time are required, instead of Ru catalyst

the alternative hydrogen acceptor, which furnished the ex-

0

.02 mol% and 6 h time respectively.

25

pected product in excellent yield (entry 1). Also in the oxida-

tion of 2-butanol or 3-methyl-2-butanol, 1,4-benzoquinone was

found to be a better reagent than acetone (entries 2 and 3). For

the oxidation of the other three secondary alcohols, both

acetone and 1,4-benzoquinone were found to show comparable

eﬃciency (entries 4–6).

substrate, and majority of them could be reduced to their cor-

responding alcohols with good (78–97%) yields (entries 13–19).

We have also attempted dehydrogenation of primary alco-

hols using the same [Ru(dppbz)(CO) Cl ] complex as the

2 2

2-Cyclohexen-1-one was found to undergo selective reduction of

the keto-group in presence of the olenic fragment (entry 19). 2-

catalyst-precursor. We investigated benzyl alcohol as the rst

substrate and, aer screening the experimental parameters,

found that the same experimental condition used for oxidation

of secondary alcohols was most eﬀective, which aﬀorded benzyl

benzoate as the major product along with benzaldehyde in

much lower yield (Table S20, entry1; ESI†). The scope of this

dehydrogenation reaction is presented in Table 4. Six primary

alcohols were tried as substrate, and with acetone as the

acceptor of hydrogen, the corresponding ester was obtained as

the major dehydrogenation product from each reaction. While

Hydroxyacetophenone gave the corresponding alcohol in poor

(32%) yield (entry 20), while 2-acetylpyridine did not aﬀord the

expected alcohol at all (entry 21); a manifestation of catalyst

inhibition via coordination also observed in case of aldehyde

reduction.

The observed catalysis is believed to be initiated by the

formation of a Ru–H bonded species, which is formed in situ via

interaction of the Ru–Cl moiety in the catalyst-precursor with 1-

propanol. The other steps are envisaged to be similar as

23,7f,7g,7i

described by us and others.

Facile dissociation of one

1,4-benzoquinone was utilized as hydrogen acceptor, yield of

carbonyl from the parent [Ru(dppbz)(CO) Cl ] complex in

2

the ester decreased and that of the aldehyde was found to

improve signicantly (entries 1–6). We also tried 1,5-pentane-

diol as a substrate with two alcoholic groups in a single mole-

cule, and, with acetone as the hydrogen acceptor, the

corresponding cyclic ester or lactone, viz. 1-oxacyclohexan-2-

one, was obtained in excellent yield along with the mono-

aldehyde as a minor product (entry 7). Such oxidation of

primary alcohol to ester in presence of acceptor appears to be

unprecedented. The catalytic eﬃciency of our complex towards

oxidation of secondary alcohol to ketone is better than that of

presence of monodentate neutral ligands (vide supra) seems to

be an added advantage for substrate approach to the catalyst.

The observed catalytic activity of [Ru(dppbz)(CO)

2 2

Cl ] is found

6

,7a–d,g

to be better than that of many other reported complexes,

23b–e

including our own reports.

However, catalytic eﬃciency of

[

Ru(dppbz)(CO) Cl ] is comparable to that of some reported Ru-

2

7f,i,j

7h,23a

catalysts,

and less than few others.

The observed eﬃciency in transfer-hydrogenation of alde-

hydes and ketones using [Ru(dppbz)(CO) Cl ] as the pre-

2

catalyst, reects successful intermediacy of the in situ gener-

ated ruthenium-hydrido species. This encouraged us to

examine whether a reverse reaction, viz. catalytic dehydroge-

nation of alcohols to corresponding oxidation product(s),

24b–h

24a,j

many other Ru-catalysts,

and comparable to that of few.

It is interesting to mention here that Oppenauer type oxidation

using 1,4-benzoquinone as acceptor of hydrogen appears to be

unprecedented.

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[

Ru(dppbz)(CO)(4-picoline)Cl

Ru(dppbz)(CO) Cl ] (50 mg, 0.07 mmol) in dichloromethane

2 2

The present study shows that the [Ru(dppbz)(CO) Cl ] complex (30 mL), 4-picoline (9.6 mg, 0.10 mmol) was added, and the

2

].

To

a

solution

of

Conclusions

[

2

can be utilized for two purposes, as a precursor for synthesis of solution was stirred for 4 h under ambient condition. The

new complexes and as a catalyst precursor for transfer- volatiles were removed by evaporation under reduced pressure,

hydrogenation and Oppenauer type oxidation. The coordi- and the solid mass, thus obtained, was washed thoroughly with

nated carbonyls and chlorides in this complex could be dis- hexane to get rid of any adhering 4-picoline, and dried in air.

placed under relatively mild condition, and the nature of Yield: quantitative anal. calc. for C H NOP Cl Ru: C, 60.01; H,

3

7

31

2

1

displacement can be predicted based on the nature of displac- 4.19; N, 1.89. Found: C, 60.13; H, 4.14; N, 1.91. H NMR (300

ing ligands. Moreover, [Ru(dppbz)(CO) Cl ] is found to be an MHz, CDCl ): d (ppm) ¼ 2.31 (CH ), 6.88–7.72 (28H)*, 8.88 (d,

2

3

ꢀ

1

excellent pre-catalyst for transfer-hydrogenation of aldehydes 2H, J ¼ 4.0). IR (KBr, cm ): 525, 559, 669, 694, 747, 813, 998,

and ketones, as well as Oppenauer type oxidation of alcohols. 1027, 1092, 1114, 1187, 1434, 1482, 1619, and 1980.

While secondary alcohols yield ketones as expected, primary

2 2 2 2

[Ru(dppbz) Cl ]. To a solution of [Ru(dppbz)(CO) Cl ]

alcohols are found to furnish esters, which is quite unusual. (50 mg, 0.07 mmol) in 1 : 1 dichloromethane–acetonitrile (40

Besides, the application of 1,4-benzoquinone as an alternative mL) was added 1,2-bis(diphenylphosphino)benzene (35 mg,

to acetone in Oppenauer type oxidation is another interesting 0.080 mmol), and the solution was reuxed for 4 h. Upon partial



nding of this study.

(ꢂ50%) evaporation of the solvents under ambient condition,

Ru(dppbz) Cl ] separated as a crystalline yellow solid, which

was collected by ltration, washed thoroughly with diethyl

ether, and dried in air. Yield: (57 mg) 72%. Anal. calc. for C60

[

2

Experimental

Materials

-

1

48 4 2

H P Cl Ru: C, 67.67; H, 4.51. Found: C, 67.72; H, 4.48. H NMR

ꢀ

1

(

4

9

300 MHz, CDCl ): d (ppm) ¼ 7.26–7.70 (48H)*. IR (KBr, cm ):

3

Ruthenium trichloride was purchased from Arora Matthey,

Kolkata, India. 1,2-Bis(diphenylphosphino)benzene (dppbz)

was procured from Aldrich. [{Ru(CO) Cl } ] was prepared by

following

synthesized starting from [{Ru(CO)

us. Tetrabutylammonium hexauorophosphate (TBHP) used

in the electrochemical studies was purchased from Aldrich. All

other chemicals and solvents were reagent grade commercial

materials and were used as received.

68, 492, 511, 521, 531, 551, 618, 667, 697, 727, 748, 758, 924,

98, 1027, 1092, 1109, 1156, 1187, 1251, 1431, 1451, 1482, and

2

2 n

1569.

Ru(dppbz)(q)

.15 mmol) in hot toluene (40 mL) containing triethylamine

15 mg, 0.15 mmol) was added [Ru(dppbz)(CO) Cl ] (50 mg, 0.07

2

6

a

reported method.

[Ru(dppbz)(CO)

Cl } ] as reported earlier by

2 n

2 2

Cl ] was

[

2

]. To a solution of 8-hydroxyquinoline (22 mg,

2

0

4

(

2

mmol). The solution was reuxed for 4 h, during which time the

solution color gradually changed from yellow to deep reddish-

a

Table 3 Oxidation of secondary alcohols to ketone

Synthesis of the complexes

[Ru(dppbz)(CO)(dmso)Cl

2 2 2

]. [Ru(dppbz)(CO) Cl ] (50 mg,

0.07 mmol) was dissolved in dimethyl sulfoxide (25 mL), and

the solution was heated at reux for 1 h. The solvent was then

evaporated under reduced pressure to aﬀord [Ru(dppbz)(-

CO)(dmso)Cl ] as a crystalline yellow solid, which was washed

2

with hexane to remove any adhering dimethyl sulfoxide, and

b

dried in air. Yield: Quantitative anal. calc. for C33

H

30

O

2

P

2

SCl

2

-

Entry

Reactant

Oxidant

Acetone

Yield , %

1

Ru: C, 54.69; H, 4.14. Found: C, 54.72; H, 4.09. H NMR (300

c

2

7

1

2

3

4

5

6

2-Propanol

—

95

57

95

66

97

96

91

89

98

99

MHz, CDCl

(

3

): d (ppm) ¼ 3.37 (2CH

3

); 7.20–8.10 (24H)*. IR

d

1

,4-Benzoquinone

ꢀ

1

KBr, cm ): 460, 495, 528, 545, 578, 592, 671, 697, 735, 757, 968,

c

2-Butanol

Acetone

9

1

99, 1025, 1091, 1100, 1117, 1192, 1306, 1434, 1483, 1635 and

973.

1

,4-Benzoquinone

c

3-Methyl-2-butanol

Cyclopentanol

Cyclohexanol

Diphenylmethanol

Acetone

1

,4-Benzoquinone

[

Ru(dppbz)(CO)(CH CN)Cl ].

To

a

solution

of

3

2

c

Acetone

[

Ru(dppbz)(CO) Cl ] (50 mg, 0.07 mmol) in 10 mL of dichloro-

2 2

1

,4-Benzoquinone

methane, 20 mL of acetonitrile was added, and the solution was

stirred for 4 h under ambient condition. Upon evaporation of

c

Acetone

1

,4-Benzoquinone

c

the solvents [Ru(dppbz)(CO)(CH

a crystalline yellow solid in quantitative yield. Anal. calc. for

C H NOP Cl Ru: C, 57.63; H, 3.93; N, 2.04. Found: C, 57.25; H,

3

CN)Cl

2

] was obtained as

Acetone

1,4-Benzoquinone

a

t

3

27

2

1

Reaction conditions: secondary alcohol (1.0 mmol), KO Bu

b

3

6

7

1

.95; N, 2.05. H NMR (300 MHz, CDCl ): d (ppm) ¼ 2.16 (CH ), (0.25 mol%), Ru catalyst (0.1 mol%). Yields are determined by

3

ꢀ1

GCMS based on the quantity of substrate remaining aer the

reaction. Besides the substrate and the expected product, no other

species was detected in any of the reactions. toluene (3 mL), acetone

.36–7.62 (24H)*. IR (KBr, cm ): 468, 508, 528, 551, 669, 695,

29, 746, 999, 1028, 1092, 1114, 1188, 1255, 1433, 1451, 1483,

637, and 1978.

c

d

(2 mL). toluene (5 mL), benzoquinone (1.0 mmol).

©

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orange. The solvent was evaporated, and the crude product was separated, which was extracted with acetonitrile. Evaporation of

puried by thin-layer chromatography on a silica plate. Using the acetonitrile extract aﬀorded [Ru(dppbz)(pic) ] as a crystal-

2

1

: 7 acetonitrile–benzene as the eluant a slow moving reddish- line orange solid. Yield: (42 mg) 71%. Anal. calc. for C H -

42 32

orange band separated, which was extracted with acetonitrile. N O P Ru: C, 63.71; H, 4.05; N, 3.54. Found: C, 63.67; H, 4.11; N,

2

4 2

1

Evaporation of the acetonitrile extract aﬀorded [Ru(dppbz)(q)

as a deep red solid. Yield: (46 mg) 74%. Anal. calc. for C48

N O P Ru: C, 68.95; H, 4.31; N, 3.35. Found: C, 69.34; H, 4.29; N, (KBr, cm ): 507, 530, 551, 671, 695, 761, 851, 998, 1026, 1047,

2 2 2

2

]

-

3.56. H NMR (300 MHz, CDCl

3

): d (ppm) ¼ 7.05–7.35 (6H)*,

H

36

7.58 (t, H, J ¼ 7.6), 7.62–7.79 (8H)*, 8.32 (d, H, J ¼ 7.1). IR

ꢀ

1

3

6

7

.32. H NMR (300 MHz, CDCl

3

): d (ppm) ¼ 6.51 (d, H, J ¼ 7.6), 1095, 1111, 1161, 1186, 1283, 1348, 1384, 1433, 1482, 1564,

.58 (d, H, J ¼ 7.9), 6.74 (s, H), 6.77 (s, H), 6.80 (d, H, J ¼ 3.3), 1595, 1629, and 1703.

.82 (d, H, J ¼ 3.6), 6.97 (t, H, J ¼ 7.1), 7.09 (t, H, J ¼ 7.9), 7.24–

.42 (7H)*, 7.65 (s, H), 7.67 (s, H), 8.14 (d, H, J ¼ 4.3). IR

Physical measurements

ꢀ

1

(

KBr, cm ): 490, 508, 531, 548, 595, 628, 671, 694, 740, 780, 801,

Microanalyses (C, H, N) were performed using a Heraeus Carlo

Erba 1108 elemental analyzer. IR spectra were obtained on

a Perkin Elmer Spectrum Two IR spectrometer as KBr pellets.

8

1

18, 915, 998, 1028, 1096, 1109, 1170, 1186, 1217, 1288, 1321,

366, 1383, 1432, 1456, 1483, 1496, 1563, and 1592.

[

Ru(dppbz)(pic)

.15 mmol) in hot toluene (40 mL) containing triethylamine

Cl ] (50 mg, 0.07

2

]. To a solution of 2-picolinic acid (18 mg,

Magnetic susceptibilities were measured using a Sherwood MK-

0

(

1

balance. H NMR spectra were recorded in CDCl

3

solution on

15 mg, 0.15 mmol) was added [Ru(dppbz)(CO)

2

a Bruker Avance DPX 300 NMR spectrometer using TMS as the

internal standard. Electronic spectra were recorded on a JASCO

V-630 spectrophotometer. Electrochemical measurements were

made using a CH Instruments model 600A electrochemical

analyzer. A platinum disc working electrode, a platinum wire

mmol). The solution was heated at reux for 5 h to yield an

orange solution. The solvent was evaporated and the product

was puried by thin-layer chromatography on a silica plate.

Using 1 : 5 acetonitrile–benzene as the eluant an orange band

a

Table 4 Oxidation of primary alcohols to ester

b

Entry

Reactant

Oxidant

Acetone

Yield ,% (Ester)

Yield ,% (Aldehyde)

c

1

2

3

4

5

6

Benzyl alcohol

4-Methoxy benzyl alcohol

1-Butanol

75

59

41

48

70

33

65

18

68

29

72

59

10

32

43

40

12

56

9

51

8

62

11

32

d

1

,4-Benzoquinone

c

Acetone

1

,4-Benzoquinone

c

Acetone

1

,4-Benzoquinone

c

Ethanol

Acetone

1

,4-Benzoquinone

c

1-Propanol

Acetone

1

,4-Benzoquinone

c

Isoamyl alcohol

Acetone

1,4-Benzoquinone

c

Acetone

7

d

1,4-Benzoquinone

a

t

b

Reaction conditions: secondary alcohol (1.0 mmol), KO Bu (0.25 mol%), Ru catalyst (0.1 mol%). Yields are determined by GCMS based on the

quantity of substrate remaining aer the reaction. Besides the substrate and the reported product(s), no other species was detected in any of the

c

d

reactions. Toluene (3 mL), acetone (2 mL). Toluene (5 mL), benzoquinone (1.0 mmol).

15626 | RSC Adv., 2021, 11, 15617–15631

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auxiliary electrode and an aqueous saturated calomel reference

General procedure for oxidation of primary and secondary

electrode (SCE) were used in the cyclic voltammetry experi- alcohol. In a typical run, an oven-dried 10 mL round bottom

ments. All electrochemical experiments were performed under ask was charged with the primary/secondary alcohol (1.0

t

a dinitrogen atmosphere. The reported electrochemical data mmol),

a

known mole percent of the catalyst, KO Bu

were collected at 298 K and are uncorrected for junction (0.25 mol%) and 2 mL of acetone/benzoquinone (1.0 mmol)

potentials. Geometry optimization by density functional theory dissolved in toluene. The ask was placed in a preheated oil

(DFT) method and electronic spectral analysis by time- bath at required temp. Aer the specied time, the ask was

dependent density-functional theory (TDDFT) calculation were removed from the oil bath and the resultant solution was

18

performed using the Gaussian 09 (B3LYP/GEN) package. GC- ltered through tight-packed slurry of silica (100–200 mesh) in

MS analyses were performed using a Perkin Elmer CLARUS (1 : 1) mixture of diethyl ether and hexane, and the ltrate was

680 instrument.

analyzed by GC-MS.

Crystallography

Conﬂicts of interest

Single crystals of the four complexes were obtained by: (i)

There are no conicts to declare.

[Ru(dppbz)(CO)(dmso)Cl

2

]: slow diﬀusion of hexane into

a dichloromethane solution of the complex; (ii) [Ru(dppbz) Cl ]:

slow diﬀusion of acetonitrile into a dichloromethane solution

2

Acknowledgements

of the complex; (iii) [Ru(dppbz)(q) ]: slow diﬀusion of toluene

into a dichloromethane solution of the complex; and (iv)

2

The authors thank the reviewers for their constructive

comments, which have been very helpful in preparing the

revised manuscript. Financial assistance received from the

Council of Scientic and Industrial Research (CSIR), New Delhi

2

[Ru(dppbz)(pic) ]: slow evaporation of solvent from a solution of

the complex in acetonitrile. Table S21 (ESI†) shows the X-ray

data processing and collection parameters for the four

complexes. Data on all the crystals were collected on a Bruker

SMART CCD diﬀractometer. X-ray data reduction, structure

solution, and renement were done using the SHELXS-97 and

[Sanction no. 01(2994)/19/EMR-II] is gratefully acknowledged.

The DST-FIST and DST-PURSE programs of the Department of

Chemistry, Jadavpur University, are also gratefully acknowl-

edged for providing nancial and infrastructural supports. AM

thanks CSIR for her fellowship [grant number 09/096(0962)/

28

SHELXL-97 packages. The structures were solved by the direct

methods.

2019-EMR-I]. The authors thank Prof. Saurabh Das (Department

of Chemistry, Jadavpur University) for his help in recording the

UV-vis spectra of the complexes.

Computational modeling details

All calculations were done using density functional theory (DFT)

29

with the B3LYP exchange correlation functional, as imple- References

18

mented in Gaussian 09 program. The lanl2dz basis set was

30

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elements. Vertical electronic excitations were computed based

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(

b) G. Chelucci, S. Baldino and W. Baratta, Ruthenium and

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groups or atoms to each molecular orbital.

Application as catalysts

General procedure for transfer-hydrogenation of aldehyde/

ketone. In a typical run, an oven-dried 10 mL round bottom



ask was charged with the aldehyde/ketone (1.0 mmol),

t

a known mole percent of the catalyst and KO Bu (0.2 mol%)

dissolved in 1-propanol (5 mL). The ask was tted with

a condenser, the other end of which was attached with

a mercury seal. The ask was then placed in a preheated oil bath

at the required temperature. Aer the specied time the ask

was removed from the oil bath and water (20 mL) added, fol-

lowed by extraction with ether (4 ꢃ 10 mL). The combined

organic layers were washed with water (3 ꢃ 10 mL), dried over

anhydrous Na SO , and ltered. Ether was removed under

2

electrochemical CO reduction, Chem. Rev., 2015, 115,

12936–12973; (f) Q. Zeng, F. W. Lewis, L. M. Harwood and

F. Hartl, Role of ligands in catalytic water oxidation by

mononuclear ruthenium complexes, Coord. Chem. Rev.,

2015, 304–305, 88–101; (g) L. Duan, L. Wang, F. Li, F. Li

and L. Sun, Highly eﬃcient bioinspired molecular Ru

water oxidation catalysts with negatively charged backbone

ligands, Acc. Chem. Res., 2015, 48, 2084–2096; (h)

C. Gunanathan and D. Milstein, Bond activation and

2

4

vacuum, and the residue obtained was dissolved in hexane and

analyzed by GC-MS.

©

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Dual utility of a single diphosphine-ruthenium complex: A precursor for new complexes and, a pre-catalyst for transfer-hydrogenation and Oppenauer oxidation

DOI: 10.1039/d1ra01594j

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