Thiophosphoryl-, thiophosphoryloxy-, and thiophosphorylamino-benzene derivatives as novel classes of hybrid pincer ligands

Article abstract of DOI:10.1080/10426507.2010.507726

The synthetic approaches to novel families of SCE (E = S',N,O) hybrid pincertype ligands bearing thophosphoryl, thiophosphoryloxy, and thiophosphorylamino groups in various combinations with thiophosphoryl-, thiocarbamoyl-, and imine- (including that of benzothiazole ring) donating functions have been developed. All of the ligands readily undergo direct cyclometallation (metal = Pd(II), Pt(II)) to afford 5,5- or 5,6-membered pincer complexes. Palladium complexes displayed from high to excellent catalytic performance in the Suzuki cross-coupling reaction of aryl bromides and phenylboronic acid and the higher asymmetry for a complex served as a factor of its higher catalytic activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426507.2010.507726

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Phosphorus, Sulfur, and Silicon and the
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Thiophosphoryl-, Thiophosphoryloxy-,
and Thiophosphorylamino-Benzene
Derivatives as Novel Classes of Hybrid
Pincer Ligands
Vladimir A. Kozlov a , Diana V. Aleksanyan a , Andrei A. Vasil'ev b &
Irina L. Odinets a
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, Moscow, Russia
b N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of
Sciences, Moscow, Russia
Version of record first published: 25 Apr 2011
To cite this article: Vladimir A. Kozlov, Diana V. Aleksanyan, Andrei A. Vasil'ev & Irina L. Odinets
(2011): Thiophosphoryl-, Thiophosphoryloxy-, and Thiophosphorylamino-Benzene Derivatives as Novel
Classes of Hybrid Pincer Ligands, Phosphorus, Sulfur, and Silicon and the Related Elements, 186:4,
626-637
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Phosphorus, Sulfur, and Silicon, 186:626–637, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.507726
THIOPHOSPHORYL-, THIOPHOSPHORYLOXY-,
AND THIOPHOSPHORYLAMINO-BENZENE DERIVATIVES
AS NOVEL CLASSES OF HYBRID PINCER LIGANDS
Vladimir A. Kozlov,1 Diana V. Aleksanyan,1
Andrei A. Vasil’ev,2 and Irina L. Odinets1
1A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy
of Sciences, Moscow, Russia
2N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
Moscow, Russia
Abstract The synthetic approaches to novel families of SCE (E = Sꢁ,N,O) hybrid pincer-
type ligands bearing thophosphoryl, thiophosphoryloxy, and thiophosphorylamino groups in
various combinations with thiophosphoryl-, thiocarbamoyl-, and imine- (including that of
benzothiazole ring) donating functions have been developed. All of the ligands readily un-
dergo direct cyclometallation (metal = Pd(II), Pt(II)) to afford 5,5- or 5,6-membered pincer
complexes. Palladium complexes displayed from high to excellent catalytic performance in
the Suzuki cross-coupling reaction of aryl bromides and phenylboronic acid and the higher
asymmetry for a complex served as a factor of its higher catalytic activity.
Keywords Catalysis; palladium; pincer complexes; pincer ligands; platinum; Suzuki cross-
сoupling; thiophosphoryl compounds
INTRODUCTION
Pincer complexes containing terdentate anionic six-electron donor ligands of the
YEY type (Y presents NR2, SR, PR2, OPR2; E is C or N) are of unfailing interest due to the
potential for structural modifications with multiple choices of donor atoms, allowing fine
tuning of the reactivity of the metal center. Successful introduction of pincer complexes into
homogeneous catalysis as well as their application in organic synthesis or as sensors and
switches provided additional impact to this diverse and rapidly developing area and led to
the creation and detailed investigation of derivatives bearing a wide range of coordinating
groups including optically active ones, fixed on a number of central cores.1 It should be
mentioned that among pincer complexes, independently of the nature of coordination sites,
the most common are symmetrical derivatives having two identical donor groups Y and
two equivalent five-membered metallacycles. Desymmetrization of a pincer-type structure
could improve the chemical and physical properties of the corresponding complexes. Thus,
Received 1 June 2010; accepted 7 July 2010.
The authors are grateful to the Russian Basic Research Foundation (grant 08-03-00508) for financial support.
Address correspondence to I. L.Odinets, A. N. Nesmeyanov Institute of Organoelement Compounds RAS,
Vavilova str. 28, Moscow 119991, Russia. E-mail: odinets@ineos.ac.ru
626

HYBRID PINCER LIGANDS
627
nonsymmetrical (or so-called hybrid) YEYꢁ-pincer complexes can combine the properties
of a few symmetrical systems while exhibiting unique reactivity and nontypical features
compared with their symmetric analogues. Routes to desymmetrization comprise changes
in donor moieties (introducing of different substituents at the same donor atoms or using
different donor groups), variation of bridges (changing of their length and nature) the central
core (shifting of the central aromatic core to give a CYYꢁ system, application of different
aromatic rings and moieties bearing the C(sp2) vinyl carbon atom). Such alterations allow
the additional possibility of controlling the steric and electronic properties of the pincer
system in total and hence catalytic activity or photoluminescence properties compared to
their symmetric analogues.2,3
Furthermore, among pincer ligands, highly attractive are those bearing a sulfur atom as
pendant arms. They include thioether, thioamide, and phosphine-sulfide derivatives and may
form both anionic S-E-S− or dianionic S-E-S2− ligands. Though the most studied among
these species are metallacycles formed by the ligands bearing bis-thioether functions,
the complexes with coordination via thioamide and phosphine sulfide moieties4 with a
benzene, pyridine, pyrrole, or indene core have been in focus only over the last decade but
already have been found to possess high catalytic activity in cross-coupling reactions5 and
photoluminescent properties6 (Scheme 1).
X
PhCH2HN
NHCH2Ph
RHN
NR
R1R2N
NR1R2
N
N
S
S
M
S
M
Cl
S
S
M
S
Cl
Cl
I
II
III
X = CH, CAlk, N
M = Ni, Pd, Pt
M = Pd, Pt
M = Pd, Pt
Ph
Ph
X
(EtO)2P
P(OEt)2
N
Ph2P
S
P
R
PPh2
S
Ph2P
PPh2
S
S
Pd
M
S
M
S
Cl
L
Cl
IV
V
VI
X = CH, CAlk, N
M = Pd, Pt
R = H, Me, Bu, Cl, Br, OH, OMe
M = Li, Ni, Pd, Pt, Ir, Mo, Rh,
Mn, Re, Au, Ce, Nd
Scheme 1 Representative complexes of thione sulfur–based pincer ligands with (hetero)aromatic core.
Nevertheless, before the beginning of our investigations the known representatives
of hybrid systems bearing a thione sulfur atom were limited to neutral and cationic tin
complexes formed by a SCO-pincer ligand containing the soft sulfur atom of the phosphine
sulfide group along with the hard oxygen from P O coordinating moiety (Scheme 2).7
In view of the aforesaid, it seems reasonable to develop the synthetic strategies to
other types of hybrid SCE pincer ligands and investigate their ability to undergo cyclo-
metallation as well as catalytic and luminescent properties. Focusing on this problem, we

628
V. A. KOZLOV ET AL.
But
But
+
But
_
PF6
OPri
O
–
Ph3C+PF6
Ph4PBr
P(OPr–i
)
Ph2P
S
Ph2P
S
P(OPr–i
O
2
Ph2P
S
P
)
2
O
Sn
O
Sn
Ph
Sn
Ph
Ph
Ph
Ph
Ph
Ph
Scheme 2 First example of tin SCO-pincer complexes.
have developed a few series of hybrid pincer-type ligands bearing thiophosphoryl, thiophos-
phoryloxy, and thiophosphorylamino groups in various combinations with thiophosphoryl-,
thiocarbamoyl-, and imine- (including that of benzothiazole ring) donating functions and
obtained the corresponding platinum and palladium pincer complexes. This minireview
discusses the synthetic strategies used for the design of these ligands, features of their
complexation with noble metals, as well as catalytic performance in the Suzuki reaction.
The results have been partially published whereas some are communicated preliminary in
this article. All experimental details, including synthetic procedures, spectroscopic char-
acterization of the ligands and their pincer complexes, may be found in the corresponding
cited full papers.
RESULTS AND DISCUSSION
The first elaborated SCE (E = Sꢁ) system8 was designed as the formal com-
bination of two symmetric SCS analogues, namely, isophthalic acid thioamides I
and (bisthiophosphoryl)benzene IV. The multistep synthesis of the ligands, namely,
3-diphenylthiophosphorylbenzoic acid thioamides, started from commercially available 3-
bromotoluene and was completed with thionation of the intermediate 3-diphenylphosphoryl
benzamides using the Lawesson reagent (Scheme 3). The latter proceeded as a step-by-step
process and provided the mixture of the corresponding thioamides 2a,b and 3a,b
bearing diphenylphosphoryl and diphenylthiophosphoryl groups, respectively, which were
separated by chromatography.
In contrast to both symmetric prototypes I, IV, their hybrid analogues 2a,b readily
underwent direct cyclopalladation at the C-2 position of the central benzene ring reacting
with PdCl2(PhCN)2 in CH2Cl2 under ambient conditions to yield the corresponding pincer-
type complexes 4a,b with κ3-SCSꢁ coordination (Scheme 4). It should be noted that the
related O,S-thioamides 2a,b did not react with PdCl2(PhCN)2, suggesting that double
S,S-coordination was a crucial factor for cyclopalladation.8
Furthermore, we exploited the combination of phosphine sulfide and thiophospho-
ryloxy moieties as coordinating sites in the hybrid SCSꢁ-pincer systems. The desired li-
gands 6a–d were obtained via thiophosphorylation of 3-diphenylthiophosphorylphenol 5
as a single key precursor. Similar to compounds 2, the direct cyclopalladation of the lig-
ands 6a–d easily proceeded at the C-2 position of the central benzene ring in a reaction
with bis(benzonitrile)palladium dichloride and afforded the rare example of pincer com-
plexes 7a–d, containing fused 5- and 6-membered metallacycles with κ3-SCSꢁ-coordination
(Scheme 5).9
Unlike the 5,5-membered palladium complexes I-V having either two C S or two
P S groups or the hybrid complexes 4a,b bearing one P S and one C S group bound

HYBRID PINCER LIGANDS
629
1. Mg
2. Ph2PCl
3. KMnO4
1. SOCl2
2. RNH2
Ph2P
O
CNHR
O
Ph2P
O
COH
O
Br
CH3
1a: R=Ph, 81%
1b: R=CH2Ph, 91%
RL
+
Ph2P
CNHR
Ph2P
O
CNHR
∆, xylene
S
S
S
3a: R=Ph, 42%
3b: R=CH2Ph, 30%
2a: R=Ph, 22%
2b: R=CH2Ph, 60%
S
S
MeO
P
P
S
OMe
RL=
S
Scheme 3 Synthesis of 3-diphenylthiophosphorylbenzoic acid thioamides.
PdCl2(PhCN)2
Ph2P
S
CNHR
S
Ph2P
CNHR
S
CH2Cl2, rt, 2d
Pd
Cl
S
3a,b
4a: R=Ph, 72%
4b: R=CH2Ph, 89%
Scheme 4 Synthesis of 5,5-membered hybrid κ3-SCSꢁ palladium pincer complexes.
1. Mg/THF
2. Ph2PCl
3. [S]
R1R2P(S)Cl
4.Py HCl
Base
O
P
Ph2P
Ph2P
OH
R1
R2
Br
OMe
S
S
5
S
6a–d, 56–93 %
(PhCN)2PdCl2
C6H6, ∆, 2h
7a: R1=R2= NMe2, 68–71%
7b: R1=R2= NEt2, 88%
or
Ph2P
S
O
P
CH2Cl2, rt, 5d,
7c: R1=R2= Ph, 65%
R1
R2
7d: R1=Me, R2=OBu, 19%
Pd
Cl
S
Scheme 5 Synthesis of 1-thiophosphoryloxy-3-thiophosphorylbenzenes and 5,6-membered palladacycles.

630
V. A. KOZLOV ET AL.
Figure 1 General view of hybrid 5,5-membered palladacycle 4b (left) and 5,6-membered pincer Pd(II) complex
7c (right) in representation of atoms via thermal ellipsoids (p = 50%).
directly to the central phenyl ring, the P S bonds in complexes 7 significantly deviate from
the plane of the central core. The latter is reflected in significant puckering of both 5- and 6-
membered metallacycles and further distortion of the palladium configuration. Figure 1
demonstrates the general structure of representative 5,5-membered and 5,6-membered
palladacycles.
In continuation of this study, we combined thioamide functionality with thiophos-
phoryloxy (or thiophosphorylamino) coordinating moieties in the third type of hybrid
SCSꢁ-pincer systems, which should also provide distorted 5,6-membered pincer com-
plexes. A convenient synthetic approach for the desired SCSꢁ-pincer ligands was based
on the thiophosphorylation of 3-hydroxy- or 3-aminobenzoic acid (thio)anilides as suitable
precursors. In the case of thiocarbamoyl-thiophosphoryloxy-based pincer ligand 10, the in-
termediate thioamido-phenol 9 was prepared by demethylation of 3-methoxybenzoic acid
thioamide 8, readily available via the reaction of the Grignard reagent of 3-bromoanisole
with phenylisothiocyanate (Scheme 6A). The related ligand with an NH-bridge was synthe-
sized by thiophosphorylation of 3-aminobenzamide 11 followed by transformation of the
carbonyl group in the compound 12 into thiocarbonyl one by treatment with the Lawesson
reagent (Scheme 6B).
C(S)NHPh
C(S)NHPh
Py.HCl
OMe
Br
C(S)NHPh
A
1. Mg
2. PhNCS
Ph2P(S)Cl
Et3N
S
OPPh2
OMe
OH
8
9
10
C(S)NHPh
B
COOH
C(O)NHPh
1. SOCl2, DMF
2. PhNH2
C(O)NHPh
RL
3. SnCl2/HCl
Ph2P(S)Cl
Et3N
S
S
NHPPh2
NO2
NH2
NHPPh2
11
12
13
Scheme 6 Synthesis of (A) thiocarbamoyl-thiophosphoryloxy- and (B) thiocarbamoyl-thiophosphorylamino-
based pincer ligands 10 and 13.

HYBRID PINCER LIGANDS
631
The reaction of both SCSꢁ ligands 10,13 with (PhCN)2PdCl2 also proceeded readily
as the C2-cyclometallation of the central phenyl ring to afford the corresponding pincer
palladium (II) complexes 14a,15a with 5- and 6-membered fused metallacycles in good to
high yields (79–95%, Scheme 7). Furthermore, applying more severe conditions (refluxing
in acetonitrile or benzonitrile) we succeeded to perform direct cycloplatination of these
ligands (Scheme 7).10 Note that the related OCS ligand 12 bearing carbamoyl moiety did
not undergo such direct cyclopalladation.
(PhCN)2MCl2
PhHN
PhHN
X
P
X
P
Ph
Ph
Ph
Ph
S
M
S
S
S
Cl
X=O (10), NH (13)
X=O : M=Pd (14a), Pt (14b)
X=NH: M=Pd (15a), Pt (15b)
Scheme 7 Cyclometallation of thiophosphoryl-thiocarbamoyl pincer ligands 10,13.
This successful study prompted us to develop a pincer system having fused 5- and
6-membered palladacycles that would contain a thiophosphoryloxy group along with a
donor moiety of completely different nature. The imine functionality was a donating group
of choice because azomethines represent one of the most popular types of ligands in
organometallic and coordination chemistry.11 The desired SCN pincer-type ligands 16a–c
were synthesized, starting from commercially available 3-hydroxybenzaldehyde, by two
reciprocal routes differing in the sequence of thiophosphorylation and condensation with
amino component depending on the nature of the latter (Scheme 8).11 The typical cyclopal-
ladation reaction with PdCl2(PhCN)2 proceeded readily in benzene or benzene–methanol
solutions to afford the corresponding hybrid SCN-pincer complexes 17a–c with 5- and 6-
membered fused metallacycles in moderate to good yields. Performing the same reaction of
t
ligands 16a,b (R = Bu, Ph) in dichloromethane followed by treatment with alcohol (either
methanol or ethanol) resulted in unexpected formation of the related OCN palladacycles
18a,b as minor products, along with the above SCN complexes 17a,b.
Figure 2 illustrates the structure of SCN pincer complex 17b and its NCO coun-
terpartner 18b (separated by chromatography) according to the X-ray diffraction analysis
data. In general, the principal bond lengths in these complexes and the conformation of 5-
and 6-membered palladacycles along with distortions of the coordinated palladium poly-
hedron are almost the same and independent from the coordination pattern. In other words,
according to the X-ray data, the complex 18b is almost isostructural to the related 17b.
Complexes 18a,b represent the products of formal oxidation of the P S group
in the starting ligand, which was suggested to proceed in the metal ion coordination
sphere. However, the detailed mechanism of such an oxidation is still unclear. It should be
emphasized that the complexes 18a,b present only the second example of the noble metal
pincer compounds bearing a coordinated phosphoryl group. The first one, an unsymmetrical
platinum NCO-pincer complex, resulting from insertion of the terminal oxo-ligand into

632
V. A. KOZLOV ET AL.
O
N
Pd
Cl
PPh2
S
R
17a–c
[O]
iii
PdCl2(PhCN)2
O
i or ii
iv
HO
CHO
Ph
P
N
Ph
R
S
O
17a,b
+
16a–c
R = tBu (a), Ph (b), OMe (c)
N
Pd
PPh2
O
R
Cl
18a,b
Reaction conditions. i: Ph2P(S)Cl, TEBA, 10%aq.NaOH/C6H6 then RNH2/MgSO4 (R=Ph,tBu)
.
ii: MeONH2 HCl/Na2CO3 then Ph2P(S)Cl, TEBA, 10%aq.NaOH/C6H6
iii: C6H6 or C6H6/MeOH; iv: CH2Cl2/MeOH
Scheme 8 Influence of the reaction conditions on the result of cyclopalladation of meta-(diphenylthio-
phosphoryloxy)benzaldimines.
the Pt P bond of the Pt(IV) PCN-pincer complex containing the phosphine moiety, was
recently reported by Milstein’s group.12
Finally, we designed SCN ligands bearing thiophosphoryl, thiophosphoryloxy, or
thiophosphorylamino pendant arms in combination with a heterocyclic imine function of
benzothiazole ring. The latter may be either directly bonded with the central phenyl core
or via an oxygen linker. Such variations of the SCN pincer ligand system allowed the
creation of 5,5-membered or 5,6-membered fused metallacycles. Moreover, adjusting the
linker nature, we could provide the corresponding 6-membered cycle with coordination
of a metal both via the S-atom of P S group or via the N-atom of the thiazole ring.
The first ligand 19 of this series was obtained by the known oxidative cyclization13 of
secondary N-arylthioamides 2a or 3a with K3[Fe(CN)6] similar to the transformation of
1,3-bis(secondary thioamide)benzene. Naturally, starting from compound 2a an additional
thionation step was required (Scheme 9). Both routes afforded the final compound in ca.
60% yield.
Figure 2 General view of hybrid 5,6-membered SCN 17b (left) and OCN 18b (right) palladacycles.

HYBRID PINCER LIGANDS
633
1. K3[Fe(CN)6]
NaOH
2. RL
K3[Fe(CN)6]
NaOH
Ph2P
S
Ph2P
O
CNHPh
S
Ph2P
CNHPh
S
N
S
S
19
2a
3a
Scheme 9 Synthesis of 2-[3-(diphenylthiophosphoryl)phenyl]-1,3-benzothiazole 19.
It seems reasonable to choose (thio)phosphorylation of meta-benzothiazole-
substituted phenol or aniline as a suitable precursor for design of SCN ligands bearing
thiophosphoryloxy or thiophosphorylamino groups, similar to the above-described
synthesis of thiophosphoryl-thiocarbamoyl pincer ligands 10,13. Hence, the oxidative
cyclization of 3-methoxybenzothioamide 20 with K3[Fe(CN)6] followed by demethylation
afforded benzothiazole substituted phenol 21, and one-pot condensation of 3-nitrobenzoic
acid with aminothiophenol followed by reduction of nitro group with tin(II) chloride
provided the corresponding heterocyclic amine 24. Further (thio)phosphorylations under
appropriate conditions depending on the substrate nature gave the final ligands 22a,b in
good yields (Scheme 10).
1. K3[Fe(CN)6]
Ph2P(S)Cl
NaOH
Et3N, C6H6, 2 h, 60°C
2.Py.HCl
NHPh
S
C
S
MeO
HO
N
20
21
S
1. SOCl2
H2N
X
N
Ph2P
2.
S
HS
1. Ph2PCl
2. S8
22a: X=O, 82%
22b: X=NH, 53%
3. SnCl2
S
O2N
COOH
H2N
23
N
24
Scheme 10 Synthesis of thiophosphoryloxy- and thiophosphorylamino-based SCN pincer ligands 22a,b-bearing
benzothiazole moiety.
Finally, to introduce an oxygen bridge between the benzothiazole heterocycle and
phenyl core (as in the ligand 25), the reaction of sodium salt of thiophosphorylated phenol
5 with chlorobenzothiazole was used (Scheme 11).
The reaction of ligands 19, 22a,b, and 25 with (PhCN)2MCl2 (M = Pd, Pt) as a metal
source in CH2Cl2 under ambient conditions leads to the µ-Cl bridged complexes 26a–d
with coordinated P S group, which immediately precipitated from the above solutions.
Meanwhile, performing the same reaction at heating in benzonitrile solutions (for more
reactive ligand 22b in acetonitrile) afforded the formation of the corresponding 5,5- and

634
V. A. KOZLOV ET AL.
S
O
Cl
N
Ph2P
S
N
S
Ph2P
S
OH
5
25, 65%
Scheme 11 Synthesis of 2-[3-(diphenylthiophosphoryl)phenoxy]-1,3-benzothiazole 25.
5,6-membered SCN pincer complexes in ca. 60–89% yield depending on the nature of a
ligand (Scheme 12).
Ph Ph
Cl
S
P
(PhCN)2PdCl2
Y
X
P
Z
Pd
Ph
Ph
CH2Cl2, rt
5 min
S
Cl
2
S
N
26a–d, 95–98%
19, 22a,b, 25
(PhCN)2PdCl2, ∆, 2–3 h
PhCN or CH3CN
S
S
Ph2P
S
O
Ph2P
X
Pd
N
N
Pd
Cl
Pd
S
Ph2P
N
S
S
Cl
Cl
27
29
28a: X=O
28b: X=NH
Scheme 12 Synthesis of of SCN pincer complexes 27–29-bearing benzothiazole moieties.
A noteworthy observation is that µ-Cl bridged complexes 26a–d transform into the
corresponding pincer products 27–29 under the conditions of solid-phase synthesis, namely,
upon heating at 200◦C for ca. 15 min, presenting an alternative access for the synthesis of
pincer complexes (Scheme 13).
Both 5,5- and 5,6-membered palladium pincer complexes under investigation were
tested as (pre)catalysts for the Suzuki-Miyaura cross-coupling of aryl bromides and phenyl-
boronic acid, which is one of the most popular and thoroughly investigated palladium
catalyzed reactions. The main challenges in this cross-coupling pertain to the use of less
reactive chloro- and bromoarenes bearing electron-donating groups and in these cases
only a few pincer complexes displayed catalytic activity. The best results for coupling of
chloroarenes were obtained with PCP pincer complexes with diaminophosphonite phos-
phorus atoms or with palladacycles modified by carbenes. That was related to the extra

HYBRID PINCER LIGANDS
635
Ph Ph
200 °C, 15 min
Y
X
P
Cl
S
P
Ph
Ph
Z
Pd
Cl
S
Pd
S
N
Cl
2
27, 28a,b, 29
26a–d, 95–98%
60–71%
Scheme 13 Solid-phase synthesis of SCN pincer complexes 27–29-bearing benzothiazole moieties.
stability provided by these ligands toward the low-ligated catalytically active Pd(0) species
involved in a catalytic cycle.1a
This study allowed the direct estimation of the influence of the desymmetrization
nature on the catalytic properties. To draw the structure–activity relationship correctly, all
experiments were performed under the same conditions: heating in dimethylformamide
(DMF) at 120◦C using K3PO4 as a base for 5 h, similar to the procedure suggested
for testing of the best catalyst in this reaction among SCS pincer palladium complexes,
that is the complex formed by 1,3-bis[(tert-butylthio)methyl]benzene.14 The experiments
were primarily performed in the presence of Bu4NBr as an additive, a salt known to
stabilize palladium nanoparticles.15 In all cases the normal dependence on the electronic
properties of the para-substituent X (X = C(O)CH3, F, OCH3, CH3, Et2N) at the aryl
bromide was observed and the complete conversions were reached for activated sub-
strate, that is, 4-bromoacetophenone, using very low catalyst loading (up to 0.01 mol%).
Figure 3 outlines the results obtained for the representative reaction of less active
4-bromoanizole.
Figure 3 Catalytic activity of SCSꢁ and SCN pincer palladium complexes for the coupling of 4-bromoanizole
and phenylboronic acid.

636
V. A. KOZLOV ET AL.
For this substrate, 5,5-membered SCSꢁ complexes 4a,b did not provide the quan-
titative conversion even used in 3 mol%. Their bisphosphorus 5,6-membered SCSꢁ ana-
logues 7a–d possess the catalytic activity at the same level used in 1 mol%. Among these
compounds the complex 7b bearing diethylamino groups at the phosphorus atom of the
6-membered metallacycle was the most active. However, its activity decreased up to 68
and 7% as the catalyst loading was reduced to 0.1 and 0.01 mol%, respectively. The corre-
sponding monopalladacycle species formed after the catalytic cycle also possessed catalytic
activity in the above reaction but were slightly less active. Furthermore, their 5,6-membered
SCSꢁ analogues with the thiocarbamoyl group instead of phosphine sulfide (compounds
14a,15a), also decomposing over the catalytic process, as well as SCN complexes 17a–c
displayed the same level of activity used both in 0.1 and 0.01 mol%. Moreover, in some
cases the activity at lower catalyst loading of 0.01 mol% was even higher than that when
0.1 mol% of a catalyst was used. Nevertheless, in these cases the yield drastically decreased
when the catalyst loading was decreased up to 0.001 mol%. Interestingly, in the case of
catalysts 4a,b, 7a–d, and 17a–c the yields decreased by about 15% if the reaction was
performed in the absence of Bu4NBr, whereas this salt slightly inhibited the reaction with
participation of 14a,15a. For example, at the catalyst loading of 0.1 mol% the conversion
increased from 77% up to 99% for 14a and from 89% up to 99% for 15a, respectively. The
same tendency was observed for SCN palladium complexes 27–29 with a benzothiazole
ring. In the absence of Bu4NBr they provided a conversion greater than 90% when used
in 0.01 mol% only. However, a further decreased of catalyst loading up to 0.001 mol% re-
sulted in a conversion of a few percentages for this particular substrate. Furthermore, these
palladium complexes 27–29 manifested catalytic activity for the Suzuki cross-coupling
reaction of 4-chloroacetophenone with phenylboronic acid and complex 28b provided 93%
conversion used in the amount 0.1 mol%.
Monitoring of the reaction mixtures by 31P spectra revealed that the complexes 4a,b
remained unchanged after the cross-coupling reaction, excluding the possible changing
of the Cl anion for the bromine one in the presence of Bu4NBr being the component of
reaction mixture. On the other hand, other pincer palladium complexes, which are stable in
DMF solution under ambient conditions for at least 40 days, underwent decomposition of
the initial palladacycles after catalytic reaction, proceeding in different ways depending on
the nature of the substituents in the ligand. Therefore, they apparently serve as a reservoir,
providing the catalytic cycle with highly catalytically active Pd0 particles and at high level
of catalyst loading these particles may stick together while the activity passes the maximum
at 0.1–0.01 mol% loading and later on dramatically decreases.
In general, SCSꢁ and SCN complexes derived from thiophosphoryl compounds pro-
vided the highest catalytic activity in the Suzuki-Miyaura cross-coupling among all tested
SCS pincer complexes. The higher catalytic efficiency of 5,6-membered SCSꢁ and SCN
complexes may be attributed to their hemilabile properties, because they contain rather
weakly bonded thiophosphoryloxy or thiophosphorylamino groups that may facilitate sub-
strate interaction with the metal center. Regardless of whether hybrid complexes are real
catalysts or precatalysts, the general tendency is absolutely clear: the higher asymmetry for
a pincer palladium complex with at least one P S coordinating group may serve as a factor
of its higher catalytic activity in the Suzuki-Miyaura cross-coupling.
CONCLUSION
To summarize the results presented, we developed synthetic approaches to a few
novel families of organothiophosphorus pincer ligands that undergo direct cyclometallation

HYBRID PINCER LIGANDS
637
to afford hybrid SCSꢁ and NCS palladium pincer complexes having fused 5,5- or 5,6-
membered metallacycles. In the course of the study the unexpected oxidation of the thio-
phosphoryl group in the palladium ion coordination sphere providing the corresponding
NCO pincer complexes with coordination via the oxygen atom of the phosphoryl group
as well as unusual solid-phase transformation of µ-Cl-bridged complexes to the pincer
ones were revealed. In view of the easy synthetic routes, easy handling, and high catalytic
activity of the complexes obtained, further development of this area seems to be of interest.
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