Complexes of Pd(II), 6-C6H6Ru(II), and 5-CpRh(III) with Chalcogenated Schiff Bases of Anthracene-9-carbaldehyde and Base-Free Catalytic Transfer Hydrogenation of Aldehydes/Ketones and N-Alkylation of Amines

Article

pubs.acs.org/Organometallics

Cite This: Organometallics XXXX, XXX, XXX −XXX

Complexes of Pd(II), η⁶‑C₆H₆Ru(II), and η⁵‑Cp*Rh(III) with

Chalcogenated Schiﬀ Bases of Anthracene-9-carbaldehyde and

Base-Free Catalytic Transfer Hydrogenation of Aldehydes/Ketones

and N‑Alkylation of Amines

Pooja Dubey, Sonu Gupta, and Ajai K. Singh*

Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi 110016, India

S

* Supporting Information

ABSTRACT: The condensation of 2-(phenylsulfanyl)ethylamine and 2-

(phenylselenyl)ethylamine with anthracene-9-carbaldehyde resulted in

Schiﬀ bases [PhS(CH₂)₂CN-9-C₁₄H₉](L1) and [PhSe(CH₂)₂CN-9-

C₁₄H₉] (L2), respectively. Na₂[PdCl₄] treatment of L1/L2 in acetone−

water mixture for 3 h at room temperature gave palladacycle [PdCl(C⁻, N,

S/Se)] (1/2; L1/L2−H = (C⁻, N, S)/(C⁻, N, Se)). The reaction of [(η⁶-

C₆H₆)RuCl(μ-Cl)]₂with L1/L2 in methanol for 8 h at room temperature

(followed by addition of NH₄PF₆) aﬀorded half-sandwich complex [(η⁶-

C₆H₆)Ru(L)Cl][PF₆], 3/4: (L = L1/L2 ≡ (N, E) ligand). The reaction of

[(η⁵-Cp*)RhCl(μ-Cl)]₂with L1 /L2 in the presence of CH₃COONa at 50

°C (followed by treatment with NH₄PF₆) resulted in [(η⁵-Cp*)Rh(L-

H)][PF₆], 5/6: (L = L1/L2). On carrying out the reaction of [(η⁵-

Cp*)RhCl(μ-Cl)]₂with these ligands at room temperature and in the

absence of CH₃COONa, complex [(η⁵-Cp*)Rh(L)Cl][PF₆], 7/8 (L = L1/

1

L2 ≡ (N, E) ligand), was formed. Complexes 1−8 were authenticated with H, ¹³C{¹H}, and ⁷⁷Se{¹H} NMR spectroscopy,

high-resolution mass spectrometry, elemental analyses, and single-crystal X-ray diﬀraction. The moisture- and air-insensitive

complexes of Pd(II) (1, 2), Ru(II) (3, 4) and Rh(III) (5−8) were thermally stable. Palladium and rhodium (under base-free

condition) species eﬃciently catalyzed transfer hydrogenation (propan-2-ol as H-source). At room temperature conversion was

90% in TH catalyzed with 0.2 mol % of 2. N-Alkylation of aniline with benzyl alcohol under base-free condition was promoted

by 3−8. The 7 was most eﬃcient for the two base-free catalytic reactions. For TH optimum loading of 1−2 and 5−8 as catalyst

is 0.05−0.2 and 0.2−0.5 mol % respectively. The optimum temperatures are 80 and 100 °C for TH and N-alkylation,

respectively. The optimum loading of 3−8 for N-alkylation is 0.5 mol %. Mercury poisoning test supported homogeneous

pathway for the two catalytic reactions. The rhodacycles probably gave real catalytic species by losing a Cp* group.

exploring for diverse applications.¹²The organochalcogen

ligands are known to form more air stable and moisture

INTRODUCTION

■

Anthracene derivatives are reported important for designing

insensitive transition metal complexes compared to their

phosphorescent, electroluminescent and semiconducting ma-

phosphine analogs.¹³As a catalyst, such complexes are

terials.^1,2Anthracene-based ligands are versatile.³⁻⁵They are

eﬃcient.¹³There is combination of anthracene and chalcogen

reported³for synthetic modeling of mono-[Fe] hydrogenase

donor site in Schiﬀ bases [PhS(CH₂)₂CN-9-C₁₄H₉](L1)

and discrimination of biologically important species. The

synthesis, structure, and properties of metal complexes of a

and [PhSe(CH₂)₂CN-9-C₁₄H₉] (L2). Consequently, they

variety of anthracene based ligands have been reported.⁶The

are worth exploring. Of their complexes with Pd(II), Rh(III),

ortho-metalation of benzene⁷is well-known. However, such

reports on naphthalene⁸are few, and those on anthracene⁹are

scanty. Moreover, the metal complexes of anthracene-based

ligands are rarely evaluated as a catalyst.¹⁰The architecture of

ligand and its arrangement around the metal center play an

important role in catalytic eﬃciency.¹¹Thus, use of anthracene

backbone in place of benzene frame in ligands can be

gratifying. Consequently, transition metal complexes of

anthracene based ligands are expected to be promising as

homogeneous catalysts for several organic reactions including

C−H activation and therefore are envisioned as worth

and Ru(II) reported herein for the ﬁrst time, 1−2 and 5−8

show good performance as catalysts for transfer hydrogenation

(base-free in case of Rh). The Ru(II) and Rh(III) complexes

(3−8) were eﬃcient catalysts for N-alkylation reaction of

amines with benzyl alcohol under base-free condition.

The palladacycles including those having metal−chalcogen

bond are well-known for Suzuki−Miyaura,^14,15Sonogashira,¹⁵

and Heck coupling¹⁶and allylation of alkenes.^15a,16However,

Received: December 15, 2018

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Scheme 1. Synthesis of 1−8

transfer hydrogenation (TH) by palladacycles having a metal−

chalcogen bond is not to our knowledge. TH based on a

hydrogen source like 2-propanol, glycerol, cyclopentanol, or

formic acid for reduction and a catalyst¹⁷has drawn attention

due to its simplicity. Moreover, these low-cost hydrogen

donors are environmentally friendly and have the ability to

dissolve many organic compounds. The important cations used

to design catalysts for TH¹⁷are Ru(II), Rh(III), Ir(III), and

Ir(I). TH by species having common transition metals, viz., Fe,

Mn, Co, and Ni is also reported.¹⁸Some of the species based

on ruthenium are very eﬃcient,¹⁹as within few minutes very

good conversion is achieved in the presence of base with 0.03

to 0.005 mol % loading of the catalyst. However, none of them

is base-free. The single Pd(II) catalyst reported so far for TH²⁰

was designed by incorporating air- and moisture-sensitive

phosphine-based pincer complex of Pd into a MOF. Its

optimum loading was reported as 5 mol % Pd. The TH of

carbonyl compounds catalyzed with Pd(II) complexes of

anthracene-based chalcogen ligands was rewarding, as

optimum loading reduced to 0.05−0.2 mol %. In TH,

undesirable decarbonylation and aldol condensation of base-

sensitive ketones and aldehydes, mediated by metal and alkali,

respectively, are matter of concern.²¹Base-free TH thus has

wider scope and is less corrosive, important characteristics for

use on industrial scale.²²The catalysts carrying out TH

eﬃciently under base-free mild conditions are not common.

Cp*Rh−guanidinato complexes are reported to catalyze base-

free TH.²³The activation of TH (H source: 2-propanol) by

complexes of Cp*Rh with anthracene-based chalcogen ligands

reported herein is base-free and 0.2−0.5 mol % loading of the

complexes as a catalyst is enough to give good to excellent

yield.

base is generally required for such N-alkylation. Some Ir

species^37a,bare known to be eﬃcient for the N-alkylation but

are expensive. Some catalysts based on Ru and Ir require

38

forcing conditions.³⁷However, Cp*IrCl₂and Ru₃(CO)₁₂(2

39b

mol %) with bulky phosphines^39aand CpRuCl(PPh₃)₂are

examples of catalysts for which working conditions used are

mild. Some ruthenium species are known to catalyze base-free

N-alkylation⁴⁰of amines with benzyl alcohols. For one such

species, [Ru(η⁶-C₆H₆)Cl₂]₂reported by Williams et al.,^40athe

optimum loading is 2.5 mol %, and the presence of an excess of

bulky phosphine ligand is essential. The only Rh species

known to catalyze base-free N-alkylation of amines is

[RhH(PPh₃)₄].⁴¹Half-sandwich complexes of Ru(II) and

Rh(III) with anthracene-based chalcogenated ligands (L1 and

L2) reported herein, catalyze N-alkylation reaction of amines

with benzyl alcohols under base-free conditions. The catalytic

activities of Ir(III) complexes of L1 and L2 have been reported

by our group recently.^13bThe reaction time with Rh complexes

as catalysts is half that of of their Ir analogues.

RESULTS AND DISCUSSION

■

The syntheses of Pd(II), Ru(II), and Rh(III) complexes (1−4,

7−8) carried out at room temperature are summarized in

Scheme 1. L1 and L2 were prepared by a reported method.^15b

Their palladation with Na₂PdCl₄in acetone and water mixture

(5:2) has resulted in 1 and 2, respectively, where L1/L2

behaves as a (C⁻, N, E) pincer ligand. Complexes 3−4 and 7−

8 were formed by chloro bridge cleavage of [(η⁶-C₆H₆)RuCl-

(μ-Cl)]₂and [(η⁵-Cp*)RhCl(μ-Cl)]₂respectively, followed by

reaction with L1 or L2 at room temperature, facilitated by

anion exchange with NH₄PF₆. 5 and 6 resulted in the reaction

of [(η⁵-Cp*)RhCl(μ-Cl)]₂with L1 and L2, respectively,

carried out in the presence of CH₃COONa at 50 °C.

The catalysts based on Au,²⁴Ag,²⁵Pd,²⁶Rh,²⁷Os,²⁸Cu,²⁹

Ni,³⁰Co,³¹Fe,³²Mn,³³Re,³⁴Ru,³⁵and Ir³⁶have been

reported for N-alkylation of amines with benzyl alcohols. The

At room temperature (as well as on heating up to 70 °C) but

in the absence of sodium acetate, the reaction of [(η⁵-

B

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Figure 1. (a) ORTEP of 1. Bond lengths (Å): Pd(1)−S(1) 2.4255(18), Pd(1)−Cl(1) 2.3364(16), Pd(2)−C(2) 1.998(6), Pd(1)−N(1) 1.982(5).

Bond angles (deg): S(1)−Pd(1)−Cl(1) 88.9(2), Cl(1)−Pd(1)−N(1) 174.57(16), Cl(1)−Pd(1)−C(2) 95.52(18), C(2)−Pd(1)−S(1)

174.27(18), C(2)−Pd(1)−N(1) 88.9(2), N(1)−Pd(1)−S(1) 85.37(15). (b) ORTEP of 2. Bond lengths (Å): Pd(1)−Se(1) 2.5231(9),

Pd(1)−Cl(1) 2.3402(18), Pd(1)−C(2) 1.992(7), Pd(1)−N(1) 1.981(6). Bond angles (deg): Se(1)−Pd(1)−Cl(1) 90.08(5), Cl(1)−Pd(1)−N(1)

175.58(18), Cl(1)−Pd(1)−C(2) 95.3(2), C(2)−Pd(1)−Se(1) 174.1(2), C(2)−Pd(1)−N(1) 88.7(3), N(1)−Pd(1)−Se(1) 85.81(17).

Figure 2. (a) ORTEP of 3. Bond lengths (Å): Ru(1)−S(1) 2.367(3); Ru(1)−Cl(1) 2.386(3); Ru(1)−C(24) 2.217(15); Ru(1)−N(1) 2.134(10).

̅

Bond angles (deg): N(1)−Ru(1)−S(1) 83.6(3); N(1)−Ru(1)−Cl(1) 86.1(3); S(1)−Ru(1)−Cl(1) 78.81(11). (b) ORTEP of 4. PF₆anion has

been omitted for clarity. Bond lengths (Å): Ru(1)−Se(1) 2.4911(10); Ru(1)−N(1) 2.091(6); Ru(1)−Cl(1) 2.397(2); Ru(1)−C(24) 2.159(11).

Bond angles (deg): N(1)−Ru(1)−Se(1) 81.82(16); N(1)−Ru(1)−Cl(1) 85.31(16); Se(1)−Ru(1)−Cl(1) 83.84(6).

Cp*)RhCl(μ-Cl)]₂with L1 and L2 has given 7 and 8,

respectively.

azomethine nitrogen with Pd/Ru/Rh. The ¹³C{¹H} NMR

spectra of complexes 1 and 2 have new quaternary carbon

signals at 144.4 and 144.8 ppm respectively (see Figures S10

Palladium complexes 1 and 2 have good solubility in DMSO

and DMF but moderate solubility in CHCl₃, CH₂Cl₂, and

CH₃CN. The solubilities of 3−8 are good in CH₃CN, but they

are sparingly soluble in CH₂Cl₂, CHCl₃, and CH₃OH.

Complexes 1−8 could be stored for 6 months without

noticeable decomposition, as revealed by their ¹H NMR

spectra.

1

and S12) implying the palladation of L1 and L2. In H NMR

spectra of 1−8, each proton of two CH₂groups becomes

diasterotopic, which results in multiplets of diﬀerent pattern,

appearing from δ 2.29 to 4.93 (Figures S9, S11, S14, S16, S19,

S21, S24, and S26) supporting M−N and M−S/Se bond (M =

Pd, Ru, or Rh) formation which prevents the free rotation in

−CH₂−CH₂−. The total number of protons in the spectra of

1, 2, 5, and 6 is lowered by one relative to those of L1 and L2

respectively, due to formation of M−C bond via C−H

activation, i.e., metallacycle. In ¹³C{¹H} NMR spectra of 1 and

2, >CHN− signals were shielded relative to those of free L1

and L2 by 3.52 and 6.15 ppm, respectively. In contrast, the

>CHN− signals in ¹³C{¹H} NMR spectra of 3−7, were

deshielded (∼2−16.0 ppm) relative to those of free L1 or L2

due to coordination of this group with Ru/Rh(III). In both ¹H

and ¹³C{¹H} NMR spectra of 5−8, the signals of η⁵-

pentamethylcyclopentadienyl group (singlet in ¹H NMR)

were shielded (maximum shift ∼0.3 and 2.1 ppm, respectively)

with respect to that of [(η⁵-Cp*)RhCl(μ-Cl)]₂. This may be

due to substitution of Cl with less electronegative S and Se.

1

Spectral Characterization. The H, ¹³C{¹H}, and ⁷⁷Se-

{¹H}NMR and mass spectra [see Figures S9 −S26] of 1−8 are

in agreement with their molecular structures depicted in

Scheme 1. In the case of 8, ¹³C{¹H} and ⁷⁷Se{¹H}NMR

spectra could not be recorded due its inadequate solubility in

all available solvents. The ⁷⁷Se{¹H} NMR spectra of 2, 4, and

6, have depicted one signal in each case at 321.4, 424.1, and

379.8 ppm respectively (see Figures S13, S18, and S23) and

were deshielded (56.4, 159.1, and 114.8 ppm respectively)

compared to the signal of free L2 in its ⁷⁷Se{¹H} NMR

spectrum, indicating the coordination of selenium with the

1

metal center. The signal of >CHN− in H NMR spectra of

1−8 was noted as deshielded up to 0.88 ppm in comparison to

those of free ligands,^13bsupporting the coordination of

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Figure 3. (a) ORTEP of 5. Selected bond lengths (Å): Rh(1)−S(1), 2.347(16), Rh(1)−N(1), 2.039(5), Rh(1)−C(12), 2.030(6), Rh−C

2.158(6)−2.243(6). Selected bond angles (deg): C(12)Rh(1)S(1) 98.47(15), N(1)Rh(1)S(1) 82.15(14), C(12)Rh(1)N(1)

85.4(3). (b) ORTEP of 6. Selected bond lengths (Å): Rh(1)−Se(1), 2.4559(7), Rh(1)−N(1), 2.041(4), Rh(1)−C(12), 2.030(5), Rh−C 2.174−

2.554. Selected bond angles (deg): C(12)Rh(1)Se(1) 98.51(13), N(1)Ir(1)Se(1) 82.63(12), C(12)Rh(1)N(1) 86.15(18).

Figure 4. (a) ORTEP of 7. Selected bond lengths (Å): Rh(1)−S(1), 2.3761(9), Rh(1)−N(1), 2.100(2), Rh(1)−Cl(1), 2.4026(9), Rh−C

2.153(3)−2.183(3). Selected bond angles (deg): Cl(1)Rh(1)S(1) 93.52(3), N(1)Rh(1)S(1) 82.88(8), Cl(1)Rh(1)N(1) 84.00(8).

(b) ORTEP of 8. Selected bond lengths (Å): Rh(1)−Se(1), 2.476(15), Rh(1)−N(1), 2.068(9), Rh(1)−Cl(1), 2.393(3), Rh−C 2.107(10)−

2.186(11). Selected bond angles (deg): Cl(1)Rh(1)Se(1) 94.11(9), N(1)Rh(1)Se(1) 82.9(3), Cl(1)Rh(1)N(1) 86.4(2).

1

The signals of η⁶-benzene in both H and ¹³C{¹H} NMR

spectra appear deshielded with respect to that of [Ru(η⁶-

benzene)Cl₂]₂.

monoanionic tridentate (S/Se, N, C⁻) bonding mode in 1 and

2, i.e., six- and ﬁve-membered chelate rings are formed by their

coordination with Pd, which shows distorted square planar

geometry. The selected bond lengths of each complex are

given below its molecular structure (Figures 1−4). In

complexes 1 and 2, bond lengths (Å) of Pd−C (1.998(6)

and 1.992(7)), Pd−N (1.982(5) and 1.981(6)), and Pd−Cl

(2.3364(16) and 2.3402(18)) are consistent with the values

reported for palladacycles of chalcogenated Schiﬀ bases,^13a14a

their reduced forms, tridentate selenated^14band indole-based

Schiﬀ bases,¹⁶and trinuclear Pd complexes of chalcogenated

NHCs.^15bThe Pd−S bond length in 1, 2.4255(18) Å, is

somewhat longer than the reported value of 2.3179(19) Å^15b

for trinuclear Pd complex of sulfated NHC and is consistent

with the value of 2.428(2) Å^14areported for a palladacycle of a

sulfated Schiﬀ base. The Pd−Se bond length in 2, 2.5231(9) Å,

is somewhat higher than the value reported (2.370(1) Å),^13a

for a complex of tridentate chalcogenated Schiﬀ base and close

to value of 2.528(11) Å^14breported for a complex of reduced

chalcogenated Schiﬀ base.

The high-resolution mass spectra (HR-MS) of complexes

1−8 were recorded. The cationic fragments of [M − Cl]⁺were

noted in the HR-MS spectra of 1 and 2 (Figures S1and S2).

However, in the HR-MS spectra of 3−8, the cationic fragments

of [M − PF₆]⁺were noticed (Figures S3 −S8). These

observations authenticate these complexes.

Crystal Structures. Single crystals of 1−8 were grown by

slow evaporation of their solutions. 1 and 2 were dissolved in

CHCl₃, and 3−8 were dissolved in diethyl ether−acetonitrile

(1:3; v/v) mixture. The crystallographic data and reﬁnement

parameters are given in Tables S1 −S3. The molecular

structures (ellipsoids at 30% probability) of complexes 1−8

determined with single crystal X−ray diﬀraction are shown in

Figures 1−4.

Complexes 1 and 2 crystallized with one molecule of

chloroform, whereas in the case of 3 and 4, two molecules of

complex crystallized with two molecules of hexaﬂuorophos-

phate anion in their crystal lattice. The crystal of 4 also has one

molecule of chloroform. The CHCl₃, (in case of 1 and 2) and

In the crystals of complexes 1 and 2, intermolecular C−H···

Cl noncovalent interactions exist as shown in Figures S27 and

S28. They result in a three-dimensional framework. In the

−

PF₆anion have been omitted for clarity. L1 and L2 exhibit

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crystal of 2, intermolecular Se···H−C7 interactions have been

noted (Figure S28). The C−H···Cl distances are given in

Table S6.

Scheme 2. Transfer Hydrogenation of Carbonyl

Compounds

Ligands L1 and L2 coordinate with Ru in a neutral bidentate

(S/Se, N) mode forming a ﬁve-membered chelate ring in

complexes 3 and 4, as corroborated by their single crystal

structures. The geometry of ruthenium is pseudo-octahedral

half-sandwich “piano-stool” type. The chlorine and η⁶-C₆H₆

group complete the coordination sphere of ruthenium.

The length of the Ru−S bond in 3, 2.367(3) Å, is shorter

than the value of 2.4079(6) Å^13dreported for ruthenium

complex of (phenylthio)methyl-2-pyridine. The Ru−N bond

lengths of 3 and 4 are 2.153(7) and 2.097(6) Å, respectively,

consistent with the values [2.075(3)−2.104(6) Å] reported for

several other species.^13d,eThe Ru−Se bond length in 4 is

2.4901(11) Å, consistent with the values 2.4859(9)−2.497(5)

Å reported^13cfor Ru(II) complex of a 1,2,3-triazole based

ligand. In the crystals of 3 and 4, C−H···F secondary

1, entry 3). At room temperature, 90% conversion occurred on

carrying out TH with 0.2 mol % of 2 as a catalyst for 12 h

(Table 1, entry 1), whereas with 0.1 mol % of 2, conversion

was reduced to 45% (Table 1, entry 2). The conversion in TH

of 4-anisaldehyde to the desired product was reduced on

lowering the amount of KOH (Table 1, entry 4). In the

absence of either 2 or KOH, there was no TH reaction (Table

1, entries 5 and 12). The combinations glycerol + KOH, 2-

propanol + HCOONa, and HCOOH + HCOONa gave

moderate conversions only (Table 1, entries 6, 8, and 9),

whereas the combination of glycerol with HCOONa did not

work at all (Table 1, entry 7). When TH reaction was

performed using ethanol as a substitute of 2-propanol, keeping

other conditions at optimum, the conversion was low (Table 1,

entry 10). The conversion of model substrate did not increase

when the loading of the catalyst was made 0.2 mol % (Table 1,

entry 11). However, on decreasing the loading of the catalyst

to 0.05 mol %, conversion diminished (Table 1, entry 13). TH

attempted with Na₂PdCl₄under conditions optimized for 1

and 2 gave poor conversion to the desired product (Table 1,

entry 14), indicating the role of ligand in the catalytic activity.

The optimum loading of 1 and 2 as a catalyst, TH at 80 °C was

0.1 mol % (Table 2). The overall optimized conditions can be

summarized as KOH (40 mol %), 2 (0.05−0.2 mol %), and 2-

propanol (5 mL) (Table 2, entry 1). Complex 1 was somewhat

less eﬃcient than 2. The scope of optimized protocols as

summarized in Scheme 2 has been explored, and results are

shown in Table 2.

The presence of electron-withdrawing or -donating groups

on benzaldehyde aﬀects its TH. The former has gaven better

yield/conversion. The conversions of benzaldehyde with

deactivating groups (Table 2, entry 2−5) into desired product

were up to 97% when loading of 1 or 2 as a catalyst was 0.05

mol %. In the presence of activating groups on benzaldehyde

(Table 2, entry 6 and 7), conversion to desired product was up

to 90% only, even with 0.1 mol % loading of the catalyst. 1 and

2 were somewhat less eﬃcient for ketones than aldehydes.

Aromatic (Table 2, entry 11−13) and aliphatic (Table 2, entry

14) ketones were reduced to corresponding alcohol in good

yield by using 0.2 mol % loading of 1 or 2. Thus, 1 and 2 can

be labeled as more eﬃcient catalysts (its required loading is

low and reaction time short) than the entity designed by

incorporation into MOF of a pincer complex of Pd which

required loading as a catalyst of 5 mol %.²⁰The TH reaction

was subjected to mercury and triphenylphosphine poisoning

tests.⁴⁵TH of 4-anisaldehyde was catalyzed with 2 in the

presence of excess Hg(0)/PPh₃under optimum conditions.

The reaction was unaﬀected and progressed to completion.

Thus, complexes 1 and 2 behave as homogeneous catalysts.

The time proﬁle of transfer hydrogenation under optimum

conditions given in Figure S35 has revealed linear increase in

the conversion up to 3 h without induction period.

−

interactions due to PF₆anion (Figures S29 and S30) have

been noted. The C−H···F distances are given in Table S6.

In complexes 5 and 6, six- and ﬁve-membered chelate rings

(one each) are formed due to coordination of L1 and L2 with

rhodium in (C⁻, S/Se, N) form, whereas only one ﬁve-

membered chelate ring is formed in 7 and 8 due to their

coordination with Rh via S/Se and N. In all four complexes 5−

8, there is a pseudo-octahedral half-sandwich “piano stool”

type disposition of donor atoms around Rh. The centroid of

η⁵-Cp* ring occupies the center of three octahedral sites

making a triangle. The, S/Se, N, and C⁻or chlorine complete

the coordination sphere. The Rh−S bond lengths in 5 and 7

[2.349(3) and 2.3761(9) Å, respectively] are somewhat

shorter than values reported for complexes [η⁵-Cp*RhCl-

(1,1′-(1,2-ethanediyl)bis(3-methylimidazole-2-thione)]Cl

[2.3967(11) Å],^42a[η⁵-Cp*RhCl{2-(phenylthiomethyl)pyri-

dine}]PF₆[2.383(2) Å],^42band [η⁵-Cp*RhCl{η²-S,P-Ph₂P-

(S)NHPPh₂}]BF₄[2.404(3) Å].^42cThe Rh−C bond lengths

of 5 and 6 [2.030(6) and 2.030(5) Å, respectively] are

consistent with the values reported for rhodacycles of N-

benzylidenemethylamine and 2-phenylpyridine [2.0285(13)

and 2.0361(13) Å, respectively]^42dand longer than the value

reported for Rh−(NHC) complex [2.013(3) Å].^42eThe Rh−

Se bond lengths of complexes 6 and 8 [2.4559(7) and

2.4732(6) Å, respectively] are somewhat shorter than the

values reported for half-sandwich complexes of Rh(III) with 2-

(phenylselenomethyl)pyridine [2.487(1) Å]^42fand [η⁵-

Cp*RhCl{η²-(SePPh₂)₂N}]BF₄[2.5266(8) Å]^42gbut some-

what longer than the values reported for a half-sandwich

complex of Rh(III) with 1,2-dicarba-closo-dodecaborane-1,2-

dichalcogenolato ligand [2.3833(5)−2.4706(6) Å].^42hThe

involvement of PF₆⁻in C−H···F secondary interactions in 5−8

(Figures S31 −S34) results in chains in the crystal structures.

The C−H···F distances are given in Table S7.

Applications of Complexes 1−8 in Catalysis. Catalytic

Transfer Hydrogenation (TH). The catalysis of TH of various

aldehydes/ketones using 2-PrOH as a hydrogen donor with

complexes 1 and 2 (0.05−0.2 mol %) was explored (Scheme

2). The KOH was the best base for this purpose,⁴³as the

catalytic TH is sensitive to the nature of base, e.g., performance

in the presence of K₂CO₃, Cs₂CO₃, KHMDS, t-BuOK, or

HCOONa^17,44is not the same.

The reaction conditions for TH were optimized using 4-

anisaldehyde as a model substrate and taking 2 as a catalyst.

The optimum reaction temperature emerged as 80 °C (Table

Base-Free Catalytic Transfer Hydrogenation. For catalysis

of TH of aldehydes/ketones to the corresponding alcohols by

E

DOI: 10.1021/acs.organomet.8b00908

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

a

Table 1. Optimization of Reaction Conditions for Transfer Hydrogenation Using 2

b

no.

base

KOH

none

KOH

HCOONa

KOH

solvent (mL)

temperature (°C)

time (h)

catalyst (mol %)

yield (%)/TOF (h⁻¹

)

1.

2

2-PrOH

glycerol

2-PrOH

HCOOH

C₂H₅OH

2-PrOH

RT

80

12

5

0.2

0.1

0.2

none

0.05

90 (37.5)

45 (37.5)

91 (182)

20 (40)

nd

3

4

c

5

6

40 (80)

<5

d

7

d

8

e

60 (120)

51 (102)

55 (110)

91 (91)

nd

9

10

11

12

13

KOH

38 (152)

8 (6)

f

14

5

0.1

a

b

c

d

Reaction conditions: 4-anisaldehyde, 1 mmol; 2-propanol, 5 mL; KOH, 0.2 mmol. ¹H NMR % yield. Using 0.1 mmol of KOH. HCOONa, 1

e

f

mmol. At a ratio of 1:3 (HCOONa/HCOOH). Na₂PdCl₄as a catalyst. nd; not detected.

Table 2. Transfer Hydrogenation of Aldehyde/Ketone

Catalyzed with 1 and 2

Table 3. Optimization of Conditions for Catalysis of TH

a

with Complexes 5−8

conversion

(%)/isolated yield

catalyst

time

(h)

no.

substrate

benzaldehyde

4-nitrobenzaldehyde

4-bromobenzaldehyde

4-chlorobenzaldehyde

4-ﬂuorobenzaldehyde

4-anisaldehyde

4-methylbenzaldehyde

2-bromobenzaldehyde

3-anisaldehyde

(mol %)

1

2

b

solvent/H-

yield %

1

2

3

4

5

6

7

8

9

0.05

0.1

0.05

0.1

2

5

95/85

92/80

93/82

90/81

89/79

89/77

87/78

89/76

85/74

88/79

97/89

96/87

94/85

91/80

93/85

91/83

90/81

92/80

90/78

91/76

no.

complex

mol % Rh source (5 mL) t (h) (TOF (h⁻¹))

1

2

3

4

5

6

7

8

9

10

11

7

5

6

8

7

0.5

0.25

0.5

1

2-PrOH

glycerol

2

88 (79)

15 (30)

<5

nd

4 (4)

79 (79)

76 (76)

83 (83)

88 (44)

88 (17.6)

12 (1)

t-butyl alcohol

2-PrOH

c

10 2-

pyridinecarboxaldehyde

0.2

11 acetophenone

0.2

12

70/61

65/58

61/50

60/53

75/63

66/54

69/56

64/53

0.5

10

24

12 propiophenone

13 4-methylacetophenone

14 octan-2-one

[Cp*RhCl₂]₂

a

Reaction conditions: acetophenone, 1 mmol, temp., 80 °C.

b

c

1

Monitored with H NMR; nd; not detected. RT.

a

Reaction conditions: aldehyde/ketone, 1 mmol; KOH, 40 mol %; 2-

propanol, 5 mL; bath temperature, 80 °C.

(Table 3, entry 3 and 4). On using 6 as a catalyst under

optimum conditions, the conversion observed was lower

(76%) compared to that achieved with 5, 7, and 8 (Table 3,

entry 7). Overall order of catalytic eﬃciency for the base-free

TH is 7 > 8 > 5 > 6 (Table 3, entries 5−8). On increasing the

reaction time and amount of 7 as a catalyst beyond the

optimized value, conversion to the reduced product was not

improved (Table 3, entries 9). TH attempted with 0.5 mol %

[Cp*RhCl₂]₂at 80 °C for 24 h has given only 12% conversion

to the desired product (Table 3, entry 11).

The scope of TH catalyzed with 5−8 was studied using

several aromatic aldehydes/ketones and aliphatic ketones. The

optimum reaction conditions were used in each case. The

results are summarized in Table 4. The catalytic TH of

benzaldehyde using any one of complexes 5−8 (0.3 mol %) in

the presence of 2-propanol under optimum conditions has

resulted in good conversion (Table 4, entry 1). TH of aromatic

complexes 5−8, base was not essential. Acetophenone was

chosen as a model substrate to optimize conditions of the

catalysis. Complexes 5−8 catalyzed its reduction (in aerobic

base-free condition) with 2-PrOH to 1-phenylethanol, at 80

°C. The detailed results are shown in Table 3.

The conversion reached maximum in 2 h with 0.5 mol % 7

(Table 3, entry 1). 2-PrOH was the best solvent cum H-source

for TH as reported earlier¹⁷(Table 3, entry 1). In the absence

of 5−8, no conversion was noticed. On lowering the amount of

7 below optimum value, conversion became poor (Table 3,

entry 2). On using 0.5 mol % 7 as a catalyst at room

temperature, only 4% conversion to the desired product was

observed after 2 h (Table 3, entry 5). When glycerol or t-butyl

alcohol was used in place of 2-propanol for TH (under

optimized other conditions) no conversion was detected

F

DOI: 10.1021/acs.organomet.8b00908

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Article

a

Table 4. Transfer Hydrogenation of Aldehyde/Ketone Using 5−8

b

conversion (%)/isolated yield

no.

substrate

benzaldehyde

4-nitrobenzaldehyde

4-bromobenzaldehyde

4-chlorobenzaldehyde

4-anisaldehyde

4-methylbenzaldehyde

2-pyridinecarboxaldehyde

acetophenone

4-methylacetophenone

4-chloroacetophenone

propiophenone

catalyst (mol %)

5

6

7

8

1

2

3

4

5

6

7

8

0.3

0.2

0.3

0.5

81/71

85/76

84/72

82/70

79/70

76/69

72/66

79/68

75/64

80/72

79/70

74/65

68/60

78/70

79/71

78/69

74/65

72/63

70/61

74/63

76/67

72/64

78/69

76/66

73/61

63/55

94/87

97/85

94/83

92/81

90/80

89/78

88/75

88/79

83/74

94/85

88/79

84/75

74/65

91/80

92/82

88/79

84/75

81/72

78/67

80/71

83/70

78/67

88/79

81/70

78/66

69/58

9

10

11

12

13

cyclopentanone

octan-2-one

a

b

1

Reaction conditions: aldehyde/ketone, 1 mmol; 2-propanol, 5 mL; bath temperature, 80 °C. Monitored with H NMR/isolated yield.

aldehydes having electron-rich (entries 5−6) as well as

electron-poor (entries 2−4) substituents has been catalyzed

with 5−8. The reactivity varied with the substituent on

benzene ring of carbonyl compound. The presence of electron-

withdrawing groups like NO₂on benzaldehyde has resulted in

high conversion in a short time, even with 0.2 mol % loading of

any one of complexes 5−8 as a catalyst. In the presence of

electron-donating groups such as OCH₃and CH₃on

benzaldehyde, 0.3 mol % loading of a complex from 5−8 as

a catalyst was essential for comparable conversion. Complexes

5−8 as catalysts were somewhat less eﬃcient for ketones

compared to aldehydes. Both aromatic (Table 4, entry 8−10)

and aliphatic (Table 4, entry 11−13) ketones were reduced to

corresponding alcohols in good yield by using 0.5 mol %

loading of any complex from 5−8 as a catalyst. In the presence

of electron-rich substituent on an aromatic ketone (Table 4,

entry 9), conversion to the reduced product was up to 83%

after 2 h when loading of any one of complexes 5−8 as a

catalyst was 0.5 mol %. For TH of aliphatic ketones complexes

5−8 were also eﬃcient as catalysts as the yield given was good

(Table 4, entries 11−13). The earlier reported η⁵-Cp*Rh(III)

guanidianto complexes are less eﬃcient than the present ones

as their reported loading for catalysis is high even for reaction

time of 4 h.²³The comparison of the performance of

complexes 5−8 as catalysts for TH with their iridium

analogues has revealed that the time taken is reasonably low

with 5−8. 5 and 6 were less eﬃcient as catalysts than were 7

and 8. The mercury and PPh₃poisoning tests⁴⁵made on TH

catalyzed with 7 were negative as expected for homogeneous

catalysis.

The time proﬁles of TH catalyzed under optimum

conditions with complexes 5 and 7, shown in Figure S36,

have revealed that there is an induction period of 0.5 h. The

conversion was increased almost linearly up to 2 h and reached

maximum in the same time for both 5 and 7. The induction

period of ∼0.5 h has indicated that the 5 or 7 is not catalyzing

TH directly and that formation of real catalytic species takes

some time. This is consistent with the proposed catalytic

species of [Cp*RhL]²⁺and Rh(L-H)]²⁺for complexes 5 and 7,

respectively.

Base-Free Catalytic N-Alkylation of Aniline with Benzyl

Alcohol. N-Alkylation of aniline with benzyl alcohol (Scheme

3) was base-free when complexes 3−8 were employed as

catalysts.

Scheme 3. N-Alkylation of Aniline with Benzyl Alcohol

This N-alkylation is environmentally friendly as water is the

byproduct. Optimization studies have shown that toluene (0.3

mL) is the best solvent for 1 mmol of the substrate under N₂in

the presence of any one of the complexes 3−8 (optimum

loading: 0.5 mol %). Other optimum conditions are temper-

ature of 100 °C and times of 6 h for 3−4 and 4 h for 5−8. The

overall results of optimization are summarized in Table 5.

The inﬂuence of diﬀerent solvents on the formation of N-

benzylaniline is given in Table 5 (entries 2−7). The catalytic

eﬃciency of complex 3 was lower than that of 4. Only 10%

product was formed by using complex 7 as a catalyst under

solvent-free condition (Table 5, entry 1). No product was

obtained by using complex 4 as a catalyst under a similar

condition (Table 5, entry 1). On using 0.25 mol % complex 4

or 7 as a catalyst, the conversion was poor (Table 5, entry 8).

No signiﬁcant diﬀerence in conversion was observed when

catalytic reaction was carried out by using 1 mol % of complex

4 or 7 (Table 5, entry 9). On lowering the temperature to 50

°C (Table 5, entry 10) the conversion was reduced to 24 and

10% when the reaction was catalyzed with complexes 4 and 7,

respectively. At room temperature no conversion was detected

(Table 5, entry 11). In the absence of catalyst (4 or 7), N-

alkylation of aniline with benzyl alcohol was not observed

(Table 5, entry 12). Under optimum conditions, and in the

presence of [(η⁵-Cp*)RhCl(μ-Cl)]₂and [Ru(η⁶-benzene)Cl₂],

conversion was 12 and 6%, respectively, even on using 1 mol %

loading catalyst and carrying out the reaction for 48 h (Table

5, entry 13).

The scope of N-alkylation of aniline and its derivatives with

aromatic benzyl alcohols was explored (Table 6). The alcohols

with electron-donating and -withdrawing substituents at para

position reacted with anilines forming N-alkylated aniline

derivatives in high yield (Table 6, entries 1−5). The presence

of electron-donating (OCH₃and CH₃) or electron-with-

drawing groups (Br and Cl) marginally aﬀects the reaction

rate. However, the presence of former has given somewhat

better yield. Complex 4 with a selenated ligand was more

eﬃcient than 3, an analogue of sulfated ligand L1, as observed

G

DOI: 10.1021/acs.organomet.8b00908

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Table 5. Optimization of Reaction Conditions for Base-Free N-Alkylation of Aniline with Benzyl Alcohol Catalyzed by 4 and

a

7

b

yield (%)/TOF (h⁻¹

)

no.

solvent

catalyst

mol % Rh/Ru

T/°C

Rh

Ru

1

2

3

4

5

6

7

8

none

THF

1,4-dioxane

ﬂuorobenzene

toluene

1,2-dichlorobenzene

dimethoxyethane

toluene

7/4

none

0.5/0.5

0.25/0.25

1/1

100

50

10 (5)

12 (6)

23 (11.5)

22 (11)

98 (49)

50 (25)

21 (10.5)

30 (30)

96 (24)

24 (12)

nd

8 (2.6)

20 (6.6)

15 (5)

96 (32)

38 (12.6)

15 (5)

22 (14.6)

94 (15.6)

10 (3.3)

nd

9

10

11

12

0.5/0.5

none

RT

100

nd

12 (0.25)

nd

6 (0.12)

c

13

toluene

[Cp*RhCl₂]₂/[Ru(η⁶-C₆H₆)Cl₂]₂

1

a

b

Reaction conditions: PhNH₂(1 mmol); PhCH₂OH (1 mmol); toluene (0.3 mL); time, 4 h for 7 and 6 h for 4; N₂atmosphere. Monitored with

c

¹H NMR. Time, 48 h; nd: not detected.

a

Table 6. N-Alkylation of Aromatic Benzyl Alcohol and Aniline Catalyzed with 3−8

b

conversion (%)/isolated yield

entry no.

substrate (1)

benzyl alcohol

4-methoxybenzyl alcohol

4-methylbenzyl alcohol

4-bromobenzyl alcohol

4-chlorobenzyl alcohol

benzyl alcohol

substrate (2)

aniline

3

4

5

6

7

8

1

2

3

4

5

6

7

8

9

10

93/83

90/82

91/79

89/76

81/73

89/80

87/76

80/71

75/66

71/60

96/89

93/83

92/80

90/79

87/77

92/81

89/80

85/73

79/68

76/63

88/77

90/81

89/78

83/74

80/71

90/79

88/76

90/80

90/73

76/67

83/74

84/71

80/72

76/68

75/63

84/75

82/71

86/77

88/79

78/67

98/90

98/89

95/82

92/79

89/81

94/82

93/79

96/84

97/86

89/76

92/80

94/83

92/81

89/77

85/73

90/79

91/76

94/85

95/84

80/71

aniline

4-methoxyaniline

4-methylaniline

4-bromoaniline

4-chloroaniline

4-ﬂuoroaniline

a

Reaction conditions: RNH₂(1 mmol), RCH₂OH (1 mmol), toluene (0.3 mL), Ru catalyst 0.5 mol %; time, 6 h for 3−4; catalyst, Rh catalyst, 0.5

b

1

mol %; time, 4 h for 5−8; nitrogen atmosphere. Monitored with H NMR/isolated yield.

earlier.^43b−dThe optimum loading of complexes 3 and 4 for

catalysis of base-free N-alkylation is lower than the values

reported for Ru-NHC complexes^40band pyridine-based Ru

complexes.^40cGrigg et al. have reported N-methylation of

pyrrolidine with methanol catalyzed with in situ generated 1

mol % of complex [RhH(PPh₃)₄].⁴¹Overall order of catalytic

eﬃciency of the present complexes for base-free N-alkylation is

7 > 8 > 4 > 3 > 5 > 6. The eﬃciency of complexes 5−8 as

catalysts for N-alkylation is comparable to those of Ir

complexes of anthracene based chalcogen ligands,^13bbut for

complexes 3 and 4 it is quite lower than those of their Ir

analogs.^13b

The time proﬁle of N-alkylation of aniline with benzyl

alcohol with ruthenium complexes under optimum conditions

shown in Figure S37 has revealed that slow initial conversion

accelerates after 1 h. In 6 h, yield reaches maximum, and there

is an induction period of ∼1 h. The time proﬁle of N-alkylation

catalyzed by Rh complexes under optimum conditions (Figure

S38) shows induction period of 1 h. These results suggest that

neither Ru nor Rh complexes are as such catalytic species.

The time proﬁles of N-alkylation of aniline with benzyl

alcohol catalyzed with 5 and 7 under optimum conditions

shown in Figure S38 indicate that the conversion is initially

slow but accelerates after 1 h. The yield has reached maximum

in 4 h with the induction period of ∼1 h. This is consistent

with the generation of a real catalytic species diﬀerent from 5

or 7 as proposed. The N-alkylation of aniline with benzyl

alcohol was catalyzed separately with 4 and 7 (0.5 mol %) in

the presence of Hg ([Ru/Rh]/Hg; 1:500)^13eor 5.0 mol % of

PPh₃under optimum conditions. The inhibition of conversion

was not noticed. Thus, the present catalysis of N-alkylation

appears to be homogeneous.

Tentative Mechanisms. Transfer Hydrogenation Cata-

lyzed by 1 and 2. The tentative mechanistic pathway for

catalytic TH with 1 and 2 is shown in Scheme 4. 1 and 2 are

precatalysts and are ﬁrst converted to the isopropanolate, a key

species of catalytic cycle (Scheme 4) with help of base KOH,

essential for catalysis.

Thereafter β-elimination of acetone aﬀords the hydride

1

species. The emergence of acetone signal in the H NMR

spectrum of the reaction mixture supports this step. However,

H

DOI: 10.1021/acs.organomet.8b00908

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molecule with the Rh center resulting in intermediate A

(Scheme 5), and proton transfer from the alcohol to the Rh

center results in intermediate B (Scheme 5). The partially

decoordinated nitrogen of ligand probably acts as an internal

base. The β-hydride elimination gives a hydride complex with

concomitant acetone loss (Scheme 5). The coordination of

carbonyl group to intermediate B (metal hydride species)

results in intermediate C. The proton transfer from metal to

carbonyl group in C gives intermediate D which reacts with 2-

propanol to give alcohol as a product and regenerate the real

catalytically active alkoxo moiety. The formation of a metal

hydride complex as a key intermediate (Scheme 5), is

supported by a new signal grown with time at δ −10.23 ppm

Scheme 4. Tentative Mechanism for TH with Catalyzed

with 1

1

in H NMR spectrum of the reaction mixture having 7 as a

catalyst (Figure S40). Intermediate B (Scheme 5) is supported

by ESI-MS (Figure S41) of complex 7 catalyzed TH reaction

mixture. The spectrum of the mixture recorded after 30 min

progress of the reaction has peak at m/z 580.1686 which may

be ascribed to B [C₃₃H₃₅RhNS]⁺.

The tentative mechanism of catalytic TH with complexes 5

and 6 is shown in Scheme 6, using 5 as a representative

catalyst. It is expected to be diﬀerent from that of 7/8.

this hydride intermediate species could not be isolated. 1,2-

Insertion of the aldehyde/ketone,⁴⁶followed by transfer of

alcoholic proton of isopropyl alcohol to alkoxy group attached

to Pd, aﬀords C−OH bond, resulting alcohol as a product. The

1

TH reaction catalyzed with 1 was monitored with H NMR

spectroscopy during its progress. A new signal at δ ≈ −7.36

ppm (Figure S39) observed in the spectrum supports the

formation of a metal−hydride species⁴⁷proposed in the

mechanism.

Scheme 6. Tentative Mechanism for Base-Free TH

Catalyzed with 5

Transfer Hydrogenation Catalyzed by Complexes 5−8. A

tentative mechanism (Scheme 5; complex 7 is used as

representative catalyst) in which the partial decoordination

of azomethine nitrogen/cleavage of M-N bond, HCl loss and

isopropanolate⁴⁸formation is proposed as a ﬁrst step for TH

catalyzed with complexes 7 and 8 in the absence of base.

The decoordination, probably an eﬀect of bulky anthracenyl

group, in part facilitates the interaction of a 2-propanol

Scheme 5. Tentative Mechanism for Base-Free TH

Catalyzed with 7

In the ﬁrst step, the addition of alcohol to catalyst complex 5

̅

proceeds with the loss of [Cp*] resulting in intermediate E.

̅

The [Cp*] is cleaved without change in oxidation state as

reported earlier,⁴⁹even for TH catalyzed with Cp*Rh based

complex.^13fThe liberated [Cp*] probably acts as an in situ

̅

generated base and promotes the base-free TH.^49,50Thereafter,

F is formed by β-hydrogen elimination from alkoxo moiety E,

with the loss of acetone molecule. Further steps in TH

catalyzed with complex 5/6 are similar to those of 7/8. The

ESI-MS of reaction mixture of TH catalyzed with complex 5

I

DOI: 10.1021/acs.organomet.8b00908

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Article

was recorded (Figure S42). There is peak at m/z = 137.0421

which may be ascribed to [(Cp*H) + H]⁺, formed by cleaved

[Cp*]⁻. The appearance in the ESI-MS of the reaction mixture

(Figure S42) recorded after 1 h a peak at m/z 502.3760

appears to be due to [E]⁺= [C₂₆H₂₅RhNOS]⁺.

Scheme 8. Tentative Mechanism for Base-Free N-Alkylation

Catalyzed with 7

The singlet at 1.40 and 1.46 ppm (due to Cp*Rh) observed

1

in H NMR spectra (see Figure S43) of catalytic reaction

mixtures having complex 5 and 6, respectively, disappeared

completely after 1 h, supporting the proposed Cp* free species

as a real catalyst. However, signals due to ligand of complex 5

were intact in ¹H NMR spectrum of reaction mixture recorded

after 1 h (see Figure S45).

N-Alkylation of Aniline with Benzyl Alcohol Catalyzed by

3−8. The tentative mechanism of base-free N-alkylation

catalyzed with complexes 3 and 4 is depicted in Scheme 7

Scheme 7. Tentative Mechanism for Base-Free N-Alkylation

Catalyzed with 4

Scheme 9. Tentative Mechanism for Base-Free N-alkylation

with Catalyzed with 5

using 3 as a representative species. The bulky anthracenyl

group partly decoordinates nitrogen of azomethine or cleaves

M−N bond, which facilitates release of HCl, resulting in the

formation of I. A β-elimination from the isopropoxide group

and the elimination of acetone from I generate hydride J,

which gives K by its addition to CN bond. Thereafter, K

reacts with alcohol to give N-alkylated product and regenerates

catalytically active alkoxo moiety I.

The ¹H NMR spectrum of reaction mixture (Catalyst:

complex 4) exhibits a new signal at δ −15.31 ppm (Figure

S44), which indicates the formation of ruthenium hydride

species J as an intermediate. On monitoring after 1 h, the

reaction mixture of 4-methoxybenzyl alcohol, aniline and 4

with ESI-MS (Figure S45), a peak at m/z = 742.1030

assignable to [{I} + (2H₂O)]⁺= [{C₃₇H₃₄NO₂RuSe} +

(2H₂O)]⁺was noticed, supporting the mechanism of Scheme

7.

The tentative mechanistic pathways of base-free N-alkylation

catalyzed with complexes 5−8 are shown in Schemes 8 and 9.

For complexes 7 and 8, the pathway is similar to that of

complex 3/4. The ¹H NMR spectrum (Figure S46) of reaction

mixture (Catalyst: complex 7) recorded with the progress of

reaction shows new signal at δ −9.33 ppm due to formation of

rhodium-hydrido species (M).

On monitoring reaction mixture of aniline, 4-methoxyben-

zylalcohol and complex 7 with ESI-MS a peak (Figure S47)

corresponding to [M]⁺= [(η⁵-Cp*)Rh(C₂₃H₂₀SN)]⁺(m/z

508.1424) was noticed after 1 h, supporting the mechanism of

Scheme 8. The mechanism of catalytic base-free N-alkylation

with complexes 5 and 6 is expected to be diﬀerent from that of

̅

complex 7 or 8 (Scheme 9). With the loss of [Cp*] complex

5/6, is converted into rhodium-alkoxo moiety (intermediate

̅

P). The [Cp*] acts as an base generated in situ and promotes

N-alkylation free from external base.^49,50It may take proton

J

DOI: 10.1021/acs.organomet.8b00908

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SHELXTL-2014/7 program.⁵²Hydrogen atoms were included in

idealized positions with isotropic thermal parameters set at 1.2 times

that of the carbon atom to which they were attached. High-resolution

mass spectral measurements were performed with a Bruker Micro

TOF-Q II instrument based on electron spray ionization (10 eV, 180

°C source temperature, sodium formate as a reference), taking sample

in CH₃CN. Fourier transform-infrared spectra in KBr pellets were

recorded on a Nicolet, Protege 460 FT-IR spectrometer. Commercial

nitrogen gas was used after passing it successively through traps

containing solutions of alkaline anthraquinone, sodium dithionite,

alkaline pyrogallol, concentrated H₂SO₄and KOH pellets. Nitrogen

atmosphere was created with Schlenk techniques. Yield refers to

isolated yield of the compound which has purity of ≥95% [established

from alcohol resulting alkoxide, another base. Thereafter, Q is

formed by β-hydrogen elimination from alkoxo moiety P.

Condensation of amine with aldehyde (or ketone) results

imine. The addition of Rh−H to CN bond results in S^51,36h

which reacts with alcohol to give N-alkylated product and

regenerates catalytically active alkoxo moiety P.^51,38a

On monitoring reaction mixture of aniline, 4-methoxyben-

zylalcohol and complex, 5 when reaction progressed for 2 h

with ESI-MS, intermediate species P [C₃₁H₂₇RhNO₂S] and Q

[C₂₃H₁₉RhNS], were identiﬁed by the appearance of peaks at

m/z 603.3896 [{P + (Na)}⁺] and 445.0936 [{Q + (H)}⁺]

(Figure S48) respectively. The peak at m/z = 137.0421

observed in ESI-MS of the reaction mixture of catalytic N-

1

by H NMR]. All reactions were carried out in glassware dried in an

alkylation with complex 5, is assignable to [{(Cp*) H⁺} + H⁺].

̅

oven, under ambient conditions.

The singlet at 1.40 ppm (due to Cp*Rh) observed in ¹H NMR

spectrum of catalytic reaction mixture of N-alkylation with 5,

disappeared after 1 h (see Figure S49), supporting the Cp*

free species as a real catalyst. However, signals due to ligand in

¹H NMR of the N-alkylation reaction mixture were observed

intact after 1h

Chemical and Reagents. Diphenyldiselenide, sodium borohy-

dride, sodium tetrachloropalladate, and NH₄PF₆procured from

Sigma-Aldrich (USA) were used as received. Thiophenol, 9-

antharacenecarboxaldehyde aldehydes, ketones, and alcohols were

procured locally. The reported methods were used to synthesize

[{(η⁶-C₆H₆)RuCl(μ-Cl)}₂],⁵³[{(η⁵-Cp*)RhCl(μ-Cl)}₂]⁵⁴and L1/

L2.^13bAll the solvents were dried and distilled by standard procedures

before use.⁵⁵Other chemicals and reagents used were available

commercially within the country.

Syntheses of Complexes [Pd(L−H)Cl] (1/2; L = L1/L2).

Na₂[PdCl₄] (0.147 g, 0.5 mmol) was dissolved in 2 mL of water, and

the solution of ligand L1 (0.170 g, 0.5 mmol)/L2(0.170 g, 0.5 mmol)

made in 5 mL of acetone was added to it with vigorous stirring. The

mixture was further stirred for 2 h. The orange red solution was

extracted with chloroform. The chloroform layer was washed with

water, dried with anhydrous Na₂SO₄, and evaporated to dryness under

vacuum to obtain 1 and 2 as an yellow/orange red colored powder.

The single crystal of complex 1 and 2 was grown from CHCl₃

mixtures over a week.

CONCLUSION

■

The palladacycle [Pd(L1/L2−H)Cl] (1/2), ruthenium

complexes [(η⁶-C₆H₆)Ru(L1/L2)Cl][PF₆] (3/4), rhodacycles

[(η⁵-Cp*)Rh(L1/L2−H)(PF₆)] (5/6), and half-sandwich

complexes [(η⁵-Cp*)Rh(L1/L2)(Cl)(PF₆)] (7/8) of bulky

ligands L1/L2 having anthracenyl group were synthesized,

characterized with multinuclei NMR, HR-MS, and single-

crystal X-ray diﬀraction, and explored for their catalytic

applications (TH and N-alkylation). Complexes 3−8 are

only precatalysts as supported by time proﬁles of catalytic

reactions. In catalysis with rhodadacycles the real catalytic

species probably result by the loss of [Cp*]⁻. The TH

catalyzed by rhodium complexes 5−8 and N-alkylation of

anilines with benzyl alcohols catalyzed by complexes 3−8 do

not require base. The optimum loading of complexes 5−8

required to catalyze base-free transfer hydrogenation of various

aldehydes/ketones with 2-propanol (H source) under ambient

conditions is 0.2−0.5 mol %. The yield in base-free N-

alkylation of anilines with aromatic benzyl alcohols in the inert

atmosphere catalyzed with 0.5 mol % complexes 5−8 is good.

Complex 7 shows maximum catalytic activity for both the

reactions. The poisoning experiments for transfer hydro-

genation and N-alkylation of anilines with benzyl alcohols

show that the two catalytic processes are predominantly

homogeneous. The alkoxide and M−H bond formation is

proposed in the tentative mechanism of the two catalytic

1. Yield: (0.207 g, 86%; mp 156 °C). Anal. Found: C, 57.21; H,

3.74; N, 2.83%. Calcd for C₂₃H₁₈ClNPdS: C, 57.27; H, 3.76; N,

1

2.90%. H NMR (300 MHz, DMSO-d₆, 25 °C, TMS): δ (ppm) =

3.32 (bs, 2 H, SCH₂mixed with dmso solvent), 4.43 (s, 2 H, NCH₂),

7.39−7.41 (t, 1H, Ar−H), 7.47−7.56 (m, 3H, Ar−H), 7.60−7.62 (m,

1H, Ar−H), 7.65−7.77 (d, 1H, Ar−H), 8.17−8.23 (t, 3H, Ar−H)

8.30 (s, 1H, CHCl₃), 8.48−8.50 (d, 1H, Ar−H), 8.77−8.81 (d, 1H,

Ar−H), 8.97 (s, 1 H, Ar−H), 9.75(s, 1H, CHN). ¹³C{¹H} NMR

(75 MHz, DMSO-d₆, 25 °C, TMS) δ (ppm) = 35.8 (SCH₂), 66.7

(NCH₂), 79.6 (CHCl₃in DMSO-d₆) 123.6, 125.3, 125.5, 125.8,

127.0, 127.4, 128.9, 129.4, 129.8, 130.0, 130.5, 131.2, 132.2, 134.0,

137.2, 139.7, (Ar−C), 144.4 (C10, Ar−C), 157.9 (NCH). HR-MS

(CH₃CN) [C₂₃H₁₈NPdS]⁺(m/z) = 446.0192; calcd value for

[C₂₃H₁₈NPdS]⁺= 446.0197 (ppm error δ: −1.3). IR (KBr, cm⁻¹):

2968 [m; ν_{C−H (aliphatic})] 3058 (m; ν_{C−Haromatic}), 1670 (m;

ν_{CNaromatic}), 1423 (m; ν_{CCaromatic}), 755 (m; ν_{C−Haromatic}). Single

crystals of 1 having one molecule of chloroform were used to record

NMR.

1

reactions on the basis of H NMR spectra and ESI-MS of the

2. Yield: (0.232 g, 88%; mp 162 °C). Anal. Found: C, 52.17; H,

reaction mixture recorded when reaction progressed for 1−2 h.

3.40; N, 2.63%. Calcd for C₂₃H₁₈ClNPdSe: C, 52.20; H, 3.43; N,

1

2.65%. H NMR (300 MHz, DMSO-d₆, 25 °C, TMS): δ (ppm) =

EXPERIMENTAL SECTION

3.27 (s, 2 H, SeCH₂), 3.43 (s, 1 H, NCH₂mixed with dmso solvent),

4.59 (bs, 1H, NCH₂), 7.35−7.40 (t, 1H, Ar−H), 7.44−7.53 (m, 3H,

Ar−H), 7.58−7.63 (m, 1H, Ar−H), 7.72−7.77 (m, 1H, Ar−H),

7.93−7.96 (d, 1H, Ar−H), 8.18−8.21 (m, 3H, Ar−H) 8.31 (s, 1H,

CHCl₃;), 8.39−8.41 (d, 1H, Ar−H), 8.73−8.83 (d, 1H, Ar−H), 8.93

(s, 1H, Ar−H), 9.66 (s, 1H, CHN). ¹³C{¹H} NMR (75 MHz,

DMSO-d₆, 25 °C, TMS) δ (ppm) = 29.0 (SeCH₂), 68.1 (NCH₂),

79.6 (CHCl₃in DMSO-d6) 123.7, 125.3, 125.8, 127.2, 127.3, 128.9,

129.5, 129.8, 130.2, 130.5, 131.1, 133.5, 133.9, 137.0, 139.3, (Ar−C),

144.8 (C10, Ar−C), 158.4 (NCH). ⁷⁷Se{¹H}NMR (57 MHz,

DMSO-d₆, 25 °C, Me₂Se) δ (ppm): 321.40. HR-MS (CH₃CN)

[C₂₃H₁₈NPdSe]⁺(m/z) = 493.9615; calcd value for [C₂₃H₁₈NPdSe]⁺

= 493.9647 (ppm error δ: 3.6). IR (KBr, cm⁻¹): 2951 [m;

ν_{C−H (aliphatic})] 3045 (m; ν_{C−Haromatic}), 1635 (m; ν_{CNaromatic}), 1475

■

Physical Measurement. A Bruker Spectrospin DPX-300 NMR

1

spectrometer was used to record H, ¹³C{¹H}, and ⁷⁷Se{¹H} NMR

spectra at 300.13, 75.47, and 57.24 MHz, respectively. The chemical

shifts are reported in ppm relative to internal standards. The C, H,

and N analyses were carried out with a PerkinElmer 2400 Series II C,

H, and N analyzer. Single-crystal diﬀraction data of 1, 2, 3, and 4 were

collected on Bruker AXS SMART Apex CCD diﬀractometer at

298(2) K using Mo Kα radiations (λ = 0.71073 Å) radiations. Frames

were collected by ω, φ, and 2θ-rotations with full-quadrant data

collection strategy (four domains each with 600 frames) at 10s per

frame with SMART. All data were processed using the programs

SAINT routine in APEX3. The structures were solved by direct

methods and reﬁned by the full-matrix least-squares on F²using the

K

DOI: 10.1021/acs.organomet.8b00908

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(m; ν_{CCaromatic}), 748 (m; ν_{C−Haromatic}). Single crystals of 2 having one

molecule of chloroform were used to record NMR.

ppm). IR (KBr, cm⁻¹): 2918 [m; ν_{C−H (aliphatic})] 3065 (m;

ν_{C−Haromatic}), 1617 (m; ν_{CNaromatic}), 1470 (m; ν_{CCaromatic}).

Syntheses of Complex [(η⁶-C₆H₆)Ru(L)Cl][PF₆] (3/4; L = L1/

L2). To a solution of L1 (0.091 g, 0.2 mmol)/L2 (0.082 g, 0.2 mmol)

made in CH₃OH (5 mL) was added a solution of [{(η⁶-

C₆H₆)RuCl(μ-Cl)}₂] (0.050 g, 0.1 mmol) in CH₃OH (5 mL). The

mixture was stirred for 8 h at room temperature. The resulting orange

solution was ﬁltered, and the volume of the ﬁltrate was reduced (∼7

mL) with a rotary evaporator. It was mixed with solid NH₄PF₆(0.032

g, 0.2 mmol), and the orange microcrystalline solid resulting

instantaneously was ﬁltered, washed with 5 mL of ice-cold CH₃OH,

and dried in vacuo. Single crystals of each of the two complexes (3/4)

were obtained from a mixture (1/4) of diethyl ether and CH₃CN.

3. Yield: (0.126 g, 90%; mp 220 °C). Anal. Found: C, 46.66; H,

3.57; N, 1.96%. Calcd for C₂₉H₂₅ClF₆NPRuS: C, 46.68; H, 3.59; N,

6. Yield: 0.130 g, 85% Anal. Calcd for C₃₃H₃₃F₆RhNPSe: C, 51.44;

H, 4.32; N, 1.82. Found: C, 51.43; H, 4.31; N, 1.81. Mp: 190.0 °C. ¹H

NMR (CD₃CN, 25 °C vs TMS) δ (ppm) 1.46 {(s, 15H, Me(Cp)},

2.57−2.66 (m, 1H, CH₂−N), 3.88−3.96 (m, 2H, CH₂−Se), 4.89−

4.93 (m, 1H, CH₂−N), 6.88−6.90 (m, 2H), 6.98−7.00 (m, 4H),

7.23−7.21 (m, 1H), 7.58−7.65 (m, 2H), 7.72−7.77 (m, 1H), 8.14−

8.17 (m, 1H), 8.46−8.49 (m, 1H), 8.76 (s, 1H), 9.43 (s, 1H).

¹³C{¹H} NMR (CD₃CN, 25 °C vs TMS): 7.7 {C of Me(Cp*)}, 34.0

(C of CH₂−Se), 66.1 (C of CH₂−N), 99.5 (C of Cp*), 122.7, 123.8,

124.7, 125.0, 125.7, 126.4,128.4, 128.8, 129.3, 129.5, 130.3, 130.5,

131.0, 131.5, 132.0, 133.2, 136.6, 140.8, 164.3 (C of CHN).

⁷⁷Se{¹H} NMR (CD₃CN, 25 °C, Me₂Se) δ (ppm): 379.8. HR-MS

(CH₃CN) [C₃₃H₃₃RhNSe]⁺(m/z) = 626.0822; calulated value for

[C₃₃H₃₃ClRhNSe]⁺= 626.0830 (δ: −1.2 ppm). IR (KBr, cm⁻¹): 2916

[m; ν_{C−H (aliphatic})] 3042 (m; ν_{C−Haromatic}), 1609 (m; ν_{CNaromatic}),

1446 (m; ν_{CCaromatic}).

1

2.00%. H NMR (300 MHz, CD₃CN, 25 °C) δ (ppm) = 2.44−2.57

(m, 1H, SCH₂), 2.92−2.99 (m, 1H, SCH₂), 3.78−3.89 (m, 2H,

NCH₂), 5.89 (s, 6 H, η⁶-C₆H₆), 7.46−7.48 (m, 1H, Ar−H), 7.62−

7.73 (m, 5H, Ar−H), 7.83−7.98 (m, 3H, Ar−H), 8.04−8.07 (d, 1H,

Ar−H), 8.19−8.26 (m, 3H, Ar−H), 8.82 (s, 1H, Ar−H), 10.25 (s,

1H, CHN). ¹³C{¹H} NMR (75 MHz, CD₃CN, 25 °C, TMS) δ

(ppm) = 37.4 (SCH₂), 60.4 (NCH₂), 87.7 (η⁶-C₆H₆) 123.7, 124.5,

126.1, 126.3, 127.9, 128.5, 129.1, 129.4, 130.2, 130.3, 130.9, 131.2

(Ar−C), 180.2 (NCH). HR-MS (CH₃CN) [C₂₉H₂₅ClNRuS]⁺(m/

z) = 556.0452; calulated value for [C₂₉H₂₅ClNRuS]⁺= 556.0438 (δ:

−2.5 ppm). IR (KBr, cm⁻¹): 2958 [m; ν_{C−H (aliphatic})] 3064 (m;

ν_{C−Haromatic}), 1646 (m; ν_{CNaromatic}), 1433 (m; ν_{CCaromatic}), 764 (m;

ν_{C−Haromatic}).

Syntheses of [Rh{(η⁵-Cp*}Cl(L)]PF₆(7/8; L = L1/L2). The L1

(0.068 g, 0.2 mmol)/L2 (0.077 g, 0.2 mmol) and [(η⁵-

C₅(CH₃)₅RhCl(μ-Cl)]₂(0.080 g, 0.1 mmol) were dissolved in

CH₃OH (10 mL). The mixture was stirred for 8 h at room

temperature. The resulting yellow solution was ﬁltered, and the

volume of the ﬁltrate reduced to ∼5 mL with a rotary evaporator. It

was mixed with solid NH₄PF₆(0.032 g, 0.2 mmol), and the resulting

yellow microcrystalline solid was ﬁltered, washed with 5 mL of ice-

cold CH₃OH, and dried in vacuo. Single crystals of 7/8 were grown

by slow evaporation of their concentrated solutions made in an

acetonitrile−diethyl ether mixture (1:4 v/v).

4. Yield: (0.224 g, 88% ; mp 200 °C). Anal. Found: C, 46.56; H,

7. Yield: 0.129 g, 85% Anal. Calcd for C₃₃H₃₄ClF₆RhNPS: C,

52.15; H, 4.51; N, 1.84. Found: C, 52.13; H, 4.50; N, 1.82. Mp: 185.0

3.34; N, 1.85%. Calcd for C₂₉H₂₅ClF₆NPRuSe: C, 46.57; H, 3.37; N,

1

1.87%. H NMR (300 MHz, CD₃CN, 25 °C, TMS): δ (ppm) =

1

°C. H NMR (CD₃CN, 25 °C vs TMS) δ (ppm) 1.48 {(s, 15H,

2.29−2.40 (m, 1 H, SeCH₂), 2.88−2.94 (m, 1H, SeCH₂), 3.71−3.77

(m, 1H, NCH₂), 4.22−4.27 (m, 1H, NCH₂), 5.93 (s, 6H, η⁶-C₆H₆),

7.60−7.67 (m, 5H, Ar−H), 7.70−7.72 (m, 1H, Ar−H), 7.82−7.85

(m, 3H, Ar−H), 8.02−8.05 (d, 1H, Ar−H), 8.19−8.25 (m, 3H, Ar−

H), 8.81 (s, 1 H), 10.24 (s, 1H, CHN). ¹³C{¹H} NMR (75 MHz,

CD₃CN, 25 °C, TMS) δ (ppm) = 29.8 (SCH₂), 61.1 (NCH₂), 86.6

(η⁶-C₆H₆) 123.3, 124.1, 125.8, 125.9, 127.0, 127.6, 128.1, 128.8,

129.0, 130.1, 130.4, 130.7 (Ar−C), 180.2 (NCH). ⁷⁷Se {¹H} NMR

(57 MHz, CD₃CN, 25 °C, Me₂Se) δ (ppm) = 424.15. HR-MS

(CH₃CN) [C₂₉H₂₅ClNRuSe]⁺(m/z) = 603.9836; calulated value for

[C₂₉H₂₅ClNRuSe]⁺= 603.9886 (δ: −8.4 ppm). IR (KBr, cm⁻¹): 2919

[m; ν_{C−H (aliphatic})] 3048 (m; ν_{C−Haromatic}), 1648 (m; ν_{CNaromatic}),

1421 (m; ν_{CCaromatic}), 766 (m; ν_{C−Haromatic}).

Me(Cp)}, 2.72−2.76 (bs, 1H, CH₂), 3.09−3.25 (bs, 1H, CH₂), 3.55.

(bs, 1H, CH₂), 3.82−3.86 (bs, 1H, CH₂), 7.52−7.67 (m, 7H), 7.70−

7.67 (m, 2H), 7.73−7.78 (m, 1H), 8.17−8.25 (m, 2H), 8.39−8.41

(m, 1H), 8.97 (s, 1H), 9.76 (s, 1H). ¹³C{¹H}NMR (CD₃CN, 25 °C

vs TMS): 8.6 {C of Me(Cp*)}, 38.2 (C of CH₂−Se), 56.9 (C of

CH₂−N), 99.7 (C of Cp*), 122.5, 123.5, 124.0, 126.0, 126.3, 127.3,

127.8, 128.5, 129.1, 129.6, 130.9, 130.9, 131.1, 177.9 (C of CHN).

HR-MS (CH₃CN) [C₃₃H₃₄ClRhNS]⁺(m/z)= 614.1179; calulated

value for [C₃₃H₃₄ClRhNS]⁺= 614.1150 (δ: −4.8 ppm). IR (KBr,

cm⁻¹): 2850 [m; ν_{C−H (aliphatic})] 3050 (m; ν_{C−Haromatic}), 1606 (m;

ν_{CNaromatic}), 1420 (m; ν_{CCaromatic}).

8. Yield: 0.145 g, 90%. Anal. Calcd for C₃₃H₃₄ClF₆RhNPSe: C,

49.12; H, 4.25; N, 1.74. Found: C, 49.13; H, 4.26; N, 1.75. Mp: 190

Syntheses of Rh{(η⁵-Cp*}(L-H)]PF₆(5/6; L = L1/L2). To a

solution of L1 (0.068 g, 0.2 mmol)/L2 (0.077 g, 0.2 mmol) and

CH₃COONa (0.016 g, 0.2 mmol) made in CH₃OH (5 mL) was

added a solution of [(η⁵-Cp*RhCl(μ-Cl)]₂(0.080 g, 0.1 mmol)

prepared in CH₃OH (5 mL). The mixture was stirred for 8 h at 50

°C. The resulting yellow solution was ﬁltered, and the ﬁltrate reduced

in volume (∼5 mL) with a rotary evaporator. It was mixed with solid

NH₄PF₆(0.032 g, 0.2 mmol), and the resulting yellow microcrystal-

line solid was ﬁltered, washed with 5 mL of ice-cold CH₃OH, and

dried in vacuo. For single crystals of 5/6, their concentrated solutions

made in an acetonitrile−diethyl ether mixture (1:4 v/v) were

evaporated slowly.

1

°C. H NMR (CD₃CN, 25 °C vs TMS) δ (ppm) 1.68 {(s, 15H,

Me(Cp)}, 2.58−2.66 (m, 1H, CH₂), 3.37−3.66 (m, 1H, CH₂), 3.93−

4.09 (m, 1H, CH₂), 4.31−4.35 (m, 1H, CH₂), 7.48 (s, 1H), 7.59−

7.66 (m, 6H), 7.74−7.83 (m, 2H), 7.89−7.92 (m, 1H), 8.21−8.23

(m, 2H), 8.33−8.45 (m, 1H), 8.81 (s, 1H), 9.48 (s, 1H). HR-MS

(CH₃CN) [C₃₃H₃₄ClRhNSe]⁺(m/z) = 662.0610; calulated value for

[C₃₃H₃₄ClRhNSe]⁺= 662.0594 (δ: −2.3 ppm). IR (KBr, cm⁻¹): 2885

[m; ν_{C−H (aliphatic})] 3042 (m; ν_{C−Haromatic}), 1607 (m; ν_{CNaromatic}),

1416 (m; ν_{CCaromatic}).

Procedure for Catalysis of Transfer Hydrogenation with 1

and 2. Aldehyde/ketone (1 mmol), KOH (0.2 mmol) and catalyst 1/

2 (0.05−0.2 mol %) were reﬂuxed in 5 mL of 2-propanol for 2/5/12

h. After cooling the mixture to 25 °C, the 2-propanol was removed

with a rotary evaporator, and the formed product was extracted with

50 mL of ethyl acetate. The solvent from the extract was evaporated

oﬀ, resulting in a residue which was puriﬁed by column

chromatography on silica gel using a mixture of hexane and ethyl

5. Yield: 0.130 g, 90%. Anal. Calcd for C₃₃H₃₃F₆RhNPS: C, 54.78;

H, 4.60; N, 1.94. Found: C, 54.76; H, 4.58; N, 1.92. Mp: 180.0 °C. ¹H

NMR (CD₃CN, 25 °C vs TMS) δ (ppm) 1.40 {(s, 15H, Me(Cp)},

2.77−2.85 (m, 1H, CH₂−S), 373−3.79 (m, 1H, CH₂−S), 4.03−4.05

(m, 1H, CH₂−N), 4.63−4.69 (m, 1H, CH₂−N), 6.81 (s, 1H), 6.95−

7.00 (m, 1H), 7.12−7.21 (m, 4H), 7.31−7.36 (m, 1H), 7.61−7.65

(m, 2H), 7.67−7.78 (m, 1H), 8.15−8.18 (m, 1H), 8.44−8.45 (m,

1H), 8.78 (s, 1H), 9.39 (s, 1H). ¹³C{¹H} NMR (CD₃CN, 25 °C vs

TMS): 7.9 {C of Me(Cp*)}, 39.7 (C of CH₂−S), 65.3 (C of CH₂−

N), 100.3 (C of Cp*) 122.8, 124.1, 125.8, 126.5, 126.7, 128.5, 128.9,

129.0, 129.5, 129.6, 130.5, 130.6, 132.1, 133.1, 136.5, 140.2, 140.2,

164.1 (C of CHN). HR-MS (CH₃CN) [C₃₃H₃₃RhNS]⁺(m/z) =

578.1378; calculated value for [C₃₃H₃₃RhNS]⁺= 578.1383 (δ: −0.5

1

acetate (20:1) as eluent. The H NMR of the puriﬁed product were

recorded and matched with literature values.

Procedure for Catalysis of Transfer Hydrogenation with 5−

8. Aldehyde/ketone (1 mmol) and catalyst 5−8 (0.2−0.5 mol %)

were reﬂuxed in 5 mL of 2-propanol for 2 h. After cooling the mixture

to 25 °C, the 2-propanol was removed with a rotary evaporator. The

hydrogenated product was extracted with 50 mL of ethyl acetate. The

solvent of the extract was evaporated oﬀ, resulting in a residue which

L

DOI: 10.1021/acs.organomet.8b00908

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

was puriﬁed by column chromatography on silica gel using a mixture

of hexane and ethyl acetate (20:1) as an eluent. The H NMR of the

Design of Organic Semiconductors for Field-Effect Transistors. J. Am.

Chem. Soc. 2013, 135, 6724−6746.

1

puriﬁed product were recorded and matched with literature values.

Procedure for Catalytic of N-Alkylation. Complex 3/4 (0.5

mol %) or 5−8 (0.5 mol %) taken in 0.3 mL of toluene was mixed

with the alcohol (1.2 mmol) and aniline (1 mmol). The mixture was

reﬂuxed for 6 (3/4) or 4 h (5−8). Thereafter solvent was evaporated

oﬀ using a rotary evaporator. The residue was extracted with 20 mL of

ethyl acetate. The organic layer was washed with water (3 × 50 mL)

and dried over anhydrous Na₂SO₄. The solvent from the extract was

evaporated oﬀ, and the residue was puriﬁed by column chromatog-

raphy on silica gel using mixture of hexane and ethyl acetate (10:1) as

eluent. The ¹H NMR of the puriﬁed product were recorded and

matched with literature values.

Hg Poisoning Test. In an excess of Hg (Hg: Pd/Ru/Rh: 400:1)

taken in a reaction ﬂask. The TH and N-alkylation reactions were

carried out optimum conditions. After standardized work up,

conversion was unaﬀected.

PPh₃Poisoning Test. The PPh₃(5 mol %) was taken in the

reaction ﬂask. The TH or N-alkylation was carried in this ﬂask out

under optimum conditions. After standardized work up conversion

remained nearly unchnaged.

(2) Dong, H.; Zhu, H.; Meng, Q.; Gong, X.; Hu, W. Organic

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ASSOCIATED CONTENT

* Supporting Information

■

S

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.organo-

met.8b00908.

Structural reﬁnement parameters, selected bond lengths

and bond angles of complexes 1−8, distances [Å] of

inter- and intramolecular interactions, ¹H and ¹³C NMR

spectra, mass spectra (PDF)

Accession Codes

CCDC 1865930−1865937 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

*E-mail: aksingh@chemistry.iitd.ac.in.

■

ORCID

(7) (a) O’Reilly, M. E.; Veige, A. S. Trianionic Pincer and Pincer-

Type Metal Complexes and Catalysts. Chem. Soc. Rev. 2014, 43,

6325−6369. (b) Morales-Morales, D.; Redon, R.; Yung, C.; Jensen, C.

M. High Yield Olefination of A Wide Scope of Aryl Chlorides

Catalyzed by the Phosphinito Palladium PCP Pincer Complex:

[PdCl{C₆H₃(OPPri₂)2−2,6}]. Chem. Commun. 2000, 0, 1619−1620.

(c) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.;

Ferguson, G.; Puddephatt, R. J. Competition between Intramolecular

Oxidative Addition and Ortho Metalation in Organoplatinum(II)

Compounds: Activation of Aryl-Halogen Bonds. Organometallics

1991, 10, 2672−2679. (d) Yang, X. Y.; Tay, W. S.; Li, Y.; Pullarkat,

S. A.; Leung, P. H. Versatile Syntheses of Optically Pure PCE Pincer

Ligands: Facile Modifications of the Pendant Arms and Ligand

Backbones. Organometallics 2015, 34, 1582−1588. (e) Baar, C. R.;

Hill, G. S.; Vittal, J. J.; Puddephatt, R. J. Stereoselectivity in

Organometallic Reactions: Intramolecular Oxidative Addition of

Aryl−Halogen Bonds to Platinum(II). Organometallics 1998, 17,

32−40.

Ajai K. Singh: 0000-0003-1650-6316

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

We thank the Department of Atomic Energy (BRNS), India

and Nanomission, Department of Science and Technology,

■

́

India, for the ﬁnancial support. P.D. and S.G. thank the

University Grants Commission (UGC), India, for their awards

of Senior Research Fellowships.

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DOI: 10.1021/acs.organomet.8b00908

Organometallics XXXX, XXX, XXX−XXX

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Complexes of Pd(II), 6-C6H6Ru(II), and 5-CpRh(III) with Chalcogenated Schiff Bases of Anthracene-9-carbaldehyde and Base-Free Catalytic Transfer Hydrogenation of Aldehydes/Ketones and N-Alkylation of Amines

DOI: 10.1021/acs.organomet.8b00908

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Article abstract of DOI:10.1021/acs.organomet.8b00908

Full text of DOI:10.1021/acs.organomet.8b00908

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