C-S bond activation and partial hydrogenation of thiophene by a dinuclear trihydride platinum complex

FULL PAPER

DOI: 10.1002/ejic.200700932

C–S Bond Activation and Partial Hydrogenation of Thiophene by a Dinuclear

Trihydride Platinum Complex

Ainara Nova,^[a]Fernando Novio,^[a]Pilar González-Duarte,*^[a]Agustí Lledós,*^[a]and

Rubén Mas-Ballesté^[a]

Keywords: C–S activation / Desulfurization / Thiophene / Hydride ligands / Platinum / Reaction mechanisms

[(dppp)₂Pt₂H₃]ClO₄[I, dppp = Ph₂P(CH₂)₃PPh₂] was found to

activate the C–S bond and to cause partial hydrogenation of

thiophene. Thus, the reaction of I with neat thiophene (T)

at reflux temperature yields [(dppp)Pt(SC₄H₄-C,S)] (II) and

[(dppp)₂Pt₂(µ-SC₄H₅-C,S)]ClO₄(III) at a 2:3 molar ratio. The

same reaction in toluene or benzene solvent affords the same

complexes II and III but in a 1:9 molar ratio. The concomitant

formation of II and III was interpreted on the basis of theoret-

ical calculations (DFT), which have provided a detailed in-

sight into the reaction mechanism. Thus, the presence of T

nated hydride to the thiophene ligand in [(dppp)PtH(C₄H₄S-

κS)] and the subsequent assistance of the {(dppp)Pt} frag-

ment formed from [(dppp)PtH₂] by hydrogen elimination. As

a result of the involvement of [(dppp)PtH₂] in the formation

of both II and III, the two reaction pathways are competitive.

The reactivity of II and III with various sources of hydride

ligands and different protonic acids has also been examined.

Thus, the reaction of II with HBF₄leads to the thiolate-

bridged dinuclear complex [(dppp)₂Pt₂(µ-SC₄H₅)₂](BF₄)₂

(IV). In the case of III the addition of HBF₄leads to [(dppp)₂-

Pt₂(µ-SC₄H₆)](BF₄)₂(V), which transforms into III by the ad-

dition of a base. The X-ray structural characterization of the

unprecedented complex V is reported here.

causes

I to dissociate into [(dppp)PtH₂] and [(dppp)-

PtH(C₄H₄S-κS)], this process being a common step for the

two ensuing reaction pathways. After dissociation, the acti-

vation of the thiophene C–S bond to yield II involves partici-

pation of [(dppp)PtH₂] exclusively. However, the reaction

leading to III requires the internal migration of the coordi- Germany, 2007)

ies dealing with several aspects of heterogeneous HDS,^[19–23]

Introduction

have already appeared. Remarkably, despite the large

number of studies, the mechanism for the heterogeneous

HDS of thiophenic molecules is not completely known and

current technology has been unsuccessful in treating these

compounds to achieve the sulfur levels required by new

legislation.

The reaction of thiophenes with homogeneous transition

metals is considered a good strategy for modeling hetero-

geneous reactions that occur during the hydrodesulfuriza-

tion (HDS) of petroleum.^[1–6]This is a step in industrial oil

refining in which sulfur is removed from crude oil by being

chemically converted to H₂S and hydrocarbon products.^[7,8]

Conventionally, the HDS process involves a reaction with

hydrogen gas (up to 200 atm pressure) at temperatures of

300–450 °C over an alumina supported cobalt (or nickel)

molybdenum catalyst. The evident importance of HDS to-

gether with environmental regulations that dictate increas-

ingly smaller amounts of sulfur in gasoline and gasoil

fuels^[9,10]account for the current interest in developing

more efficient technologies. Several reviews of the results

obtained in the search for new heterogeneous cata-

lysts,^[11–15]in homogeneous modeling studies with transi-

tion-metal complexes^{[1,2,4,16–18]}and in computational stud-

The reactions of thiophenes with soluble metal com-

plexes have provided mechanistic knowledge of the HDS

process.^{[1,5,24–26]}Significantly, many metals (Ru, Os, Ir, Re,

Pt, Pd) have been shown to be more active as HDS pro-

moters than the molybdenum–cobalt (or nickel) mixtures

usually employed in industrial HDS.^{[10,12–14,27–30]}Thus, the

reaction of thiophene (T) with mono- or dinuclear transi-

tion-metal complexes has usually led to the cleavage of a

single thiophene C–S bond and thus to the formation of a

thiametalacycle as the product^[4,31–41](Scheme 1, right-hand

side). Concerning these reactions, the most thorough mech-

anistic study involves a mononuclear rhodium species.^[34]

Although no intermediates were detected in this reaction,

the use of selectively deuterated reagents and theoretical

calculations made it possible to propose the corresponding

mechanism.^[42–44]In terms of mono- and dinuclear transi-

tion-metal complexes, the only example of a complete de-

sulfurization of thiophene to butadiene was achieved by

[a] Departament de Química, Universitat Autònoma de Barcelona,

08193 Bellaterra, Barcelona, Spain

Fax: +34-935813101

E-mail: Pilar.Gonzalez.Duarte@uab.es

agusti@klingon.uab.es

Supporting information for this article is available on the

WWW under http://ww w .eurjic.org or from the author.

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Scheme 1. Reported products obtained in the reaction of T with [MH_x] (x = 0–3) transition-metal complexes.

means of a dinuclear polyhydride iridium complex in the eral hours are required for optimum rate and yield. While

presence of a hydrogen acceptor compound.^[45]Several in- under these experimental conditions we always observed

termediate species identified by NMR showed how the acti- the concomitant formation of two platinum complexes,

vation of the second C–S bond is subsequent to the hydro- their relative amounts were dependent on the presence or

genation of the thiophenic chain. Within this context, it is absence of benzene or toluene solvent. The isolation pro-

worth noting that in the examples where the first C–S bond cedure and complete characterization of the complexes ob-

activation is accompanied by hydrogenation (Scheme 1, left- tained are given in the Experimental Section.

hand side),^[32,46,47]the complete desulfurization has usually

required further treatment with H₂. However, the same re-

The Reaction of [(dppp)₂Pt₂H₃]ClO₄(I) with Thiophene

action with the thiametalacycle products is unreported.

Monitoring by ³¹P NMR spectroscopy (Figure 1) indi-

cated concomitant formation of a mononuclear and a dinu-

clear platinum species (Scheme 2). However, neither the

NMR nor the ESI MS data gave evidence of intermediate

species being formed. Similar observations were obtained

in the monitoring of the reaction of I (0.15 mmol) with

thiophene (15 mL) in toluene or benzene solvent (15–

20 mL) at reflux temperature. The only difference in these

reactions was the relative molar ratio of the two new plati-

num complexes formed (2:3 in neat thiophene and 1:9 in

In this paper, a dinuclear polyhydride platinum complex

was investigated for its ability to react with T. An approach

followed in earlier studies of platinum-mediated desulfur-

ization of thiophene, benzothiophene, and dibenzothio-

phene involved their reaction with [(PEt₃)₃Pt] and subse-

quent treatment with various hydride reagents.^[35]While the

completely desulfurized hydrocarbons were obtained from

the three heterocycles, the amount of desulfurization in the

case of T was about 4%. Overall, the notion that two metal

centers might be required for cleavage of both thiophenic

C–S bonds,^[16]the high HDS activity shown by platinum

metals,^[48,49]and the fact that a polyhydride complex al-

ready contains the hydrogen needed for thiophene hydro-

genolysis led us to explore the reaction of [(dppp)₂Pt₂H₃]-

ClO₄[dppp = 1,3-bis(diphenylphosphanyl)propane] with T

and the mechanism of this reaction. Within the context of

its possible relevance in the modeling of HDS studies, we

first investigated the reactivity of [(dppp)₂Pt₂H₃]ClO₄to-

ward sulfur-containing species such as Na₂S or NaSH. The

set of reactions obtained provided solid evidence that be-

hind the apparent simplicity of platinum complexes con-

taining Pt–H, Pt–SH, or Pt–S fragments, there is an out-

standing chemistry.^[50]

1

toluene or benzene). According to the ³¹P and H NMR

spectroscopic data recorded during the reaction, both II

and III not only form simultaneously but also maintain a

constant molar ratio until the end of the reaction.

The ³¹P NMR features of the reaction mixture that were

attributable to a mononuclear platinum complex, literature

data,^[35,36]and the peak at m/z = 692.2 in the ESI MS spec-

trum were consistent with the formation of an unsymmetri-

cal species by insertion of the {(dppp)Pt} fragment into one

thiophenic C–S bond. The formulation of this complex as

[(dppp)Pt(SC₄H₄-C,S)] (II) was fully confirmed by ³¹P,

1

¹⁹⁵Pt{¹H} and bidimensional H and ¹³C spectra (NOESY,

COSY, HMQC C-H, and HSQC-edit C-H). ¹H and ¹³C

NMR spectroscopic data were consistent with those re-

ported for the related [(PEt₃)₂Pt(SC₄H₄-C,S)] complex.^[35]

The remaining signals in the ³¹P NMR spectrum of the

reaction mixture, together with ¹H NMR and ESI MS data

obtained during the monitoring of the reaction of I with T

Results and Discussion

Experimental Results

The reaction between [(dppp)₂Pt₂H₃]ClO₄(I) and T was gave evidence that III was probably a dinuclear platinum

1

monitored by H and ³¹P NMR and mass measurements complex. Full characterization of this complex after its iso-

until no further progress was observed. Consistent with lit- lation by column chromatography from crude mixture after

erature data,^[35,36,45]high temperature, significant excess of 20 h of reaction allowed us to formulate it as [(dppp)₂Pt₂(µ-

thiophene with respect to the platinum complex, and sev- SC₄H₅-C,S)]ClO₄(III).

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Eur. J. Inorg. Chem. 2007, 5707–5719

C–S Bond Activation and Partial Hydrogenation of Thiophene

Figure 1. Monitoring by ³¹P{¹H} NMR in Cl₃CD of the reaction of [(dppp)₂Pt₂H₃]ClO₄(I) with neat thiophene evidences formation of

[(dppp)Pt(SC₄H₄-C,S)] (II) and [(dppp)₂Pt₂(µ-SC₄H₅-C,S)]ClO₄(III) after 6 h, and total consumption of I after 20 h.

resonating at δ = 5.43 or 5.61 ppm (corresponding to the

–CH=CH– fragment) with respect to those of the aliphatic

CH₂groups in the phosphane ligands, in agreement with

the dinuclear nature of III. The absence of hydride ligands

is fully confirmed by ¹H and HMBC Pt–H NMR, and

agrees with infrared data. Finally, the ESI-MS spectrum

shows a major peak at m/z = 1299.2401 [M⁺] that fits well

to the [(dppp)₂Pt₂(SC₄H₅)]⁺cation. Elemental analyses and

IR data are consistent with III being an ionic molecule with

–

ClO₄as counterion.

Additional Reactions

In order to further elucidate the mechanism leading to

II and III, and to further analyze the reactivity of these

Scheme 2. The mono- and dinuclear platinum complexes obtained

in the reaction of I with thiophene.

complexes, we carried out complementary reactions. These

1

were monitored for several hours by ³¹P and H NMR as

Thus, the ³¹P{¹H} NMR spectrum of III is second-order, earlier described for the reaction of I with T. In some reac-

consisting of two broad signals (Figure 1) that fit well with tions the deuterated analog of I, [(dppp)₂Pt₂D₃]ClO₄(I-d₃),

2

the simulation of this spectrum including long-range P–Pt was employed, and in these cases H NMR spectra were

couplings (see electronic supporting information). Concern- also recorded. The synthesis and NMR features of I-d₃have

ing the ¹⁹⁵Pt{¹H} NMR spectrum, it displays the same already been reported.^[50]On the whole the NMR features

pattern as II with broader signals. These features corrobo- allowed identification of the compounds formed and

rate the dinuclear nature of III and the symmetrical behav- eventually led to the synthesis and characterization of new

ior of the bridging ligand. The SC₄H₅ligand in III was fully complexes, as depicted in Scheme 3.

1

characterized by H NMR, with additional NOESY and

The reactions involving I-d₃gave evidence that complex

COSY experiments, as well as by HMBC P–H, ¹³C NMR I cleaves in the presence of thiophene. Thus, the heating at

(DEPT), and HSQC-edit C–H data. The ¹³C chemical shifts reflux temperature of a 1:1 mixture of I and I-d₃in the

at δ = 126.1 and 137.1 ppm, which are in the proper range presence of thiophene showed that after 6 h all isotopomers

for olefins, gave evidence of one double bond. This fact, [(dppp)₂Pt₂H_(3–x)D_x]⁺(x = 0–3) were present in comparable

in addition to the symmetry of the complex, even at low amounts. However, when the same process was carried out

temperatures, allows an η³coordination for this bridge to in the absence of thiophene, only negligible amounts of

be discarded and makes the formulation {S–CH₂– mixed species were present after 12 h of heating. This indi-

CH=CH–CH} the most reasonable alternative.^[51]The stoi- cated that T, and not the presence of solvent, causes the

chiometric ratio of dppp to SC₄H₅ligands in III is 2:1, as cleavage of I, and that the breaking of the dinuclear plati-

deduced from the integration of the signals of the proton num complex I is a reversible process, both issues being

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A. Nova, F. Novio, P. González-Duarte, A. Lledós, R. Mas-Ballesté

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Scheme 3. Overall reactivity of complexes I–V: (a) No interconversion between II and III is observed; (b) II with HBF₄affords IV and,

similarly, III gives V; (c) V goes back to III by addition of a base (B = MeO^–, Et₃N); (d) complexes II–V react with HCl, and also with

NaBH₄, but not with I.

consistent with literature data.^[32]In addition to the above SC₄H₆)](BF₄)₂(V), whose formation involves the insertion

reactions, the use of I-d₃provided information on the of a proton in the thiophenic chain of III without cleavage

source and location of the inserted hydrogen atom into the of the dinuclear species. As shown in Scheme 3, this reac-

thiophenic chain of III. This was obtained by the reaction tion is easily reversed by addition of a base (NaMeO,

of I-d₃and T in toluene at reflux temperature, which af- NEt₃). Characterization of V by solution NMR measure-

1

2

forded II and III-d₁. Characterization of III-d₁by H, H, ments at room temperature evidenced the symmetrical be-

and ¹³C DEPT-135 NMR spectroscopic data allows to es- havior of the bridging ligand with respect to the two plati-

tablish that deuterium is bound to the carbon atom next to num centers. It also allows to establish that the proton in-

the sulfur atom.

sertion in the bridging S–CH₂–CH=CH–CH ligand in III

Another set of experiments, aimed at examining whether causes an electronic rearrangement leading to a S–CH₂–

II and III could convert into each other even though their CH₂–CH=CH species in V. As a result, one of the two σ

concentrations increased concomitantly during the reaction Pt–C5 bonds in III evolved into a π interaction with the

of I with T, both in the presence and absence of toluene or C4=C5 double bond in V, consistently with ¹H and ¹³C

benzene solvent, was indicative of parallel reaction path- NMR spectroscopic data. However, the symmetry observed

ways. Within this context, II and III were independently in the latter complex for the phosphorus nuclei in the

heated in toluene solvent at reflux temperature for several ³¹P{¹H} NMR spectrum indicated a fluxional behavior for

hours, during which no changes were observed. The same the CH=CH ligand moiety, which was consistent with low-

procedure with a mixture of I (as a possible platinum and temperature ³¹P{¹H} NMR spectroscopic data. These

hydride source) and II at various molar ratios in toluene showed that the two signals corresponding to the phospho-

did not afford III, thus reinforcing that II and III are inde- rus nuclei, particularly that at δ = 0.1 ppm, widen with

pendently formed.

decreasing temperature without reaching complete deco-

While the reaction of I with T involves the activation of alescence. According to the structure determined by single-

one C–S bond of thiophene, the conditions for the cleavage crystal X-ray diffraction (see below), the features above are

of the second S–C bond in II and III were also examined. consistent with a fluxional behavior for the bridging ligand,

To this end, complexes II and III were made to react with which involves the σ Pt–C5 bond and the C4=C5 double

protonic acids (HBF₄, HCl) and hydride sources (NaBH₄, bond to exchange sites between the two platinum centers.

I). These reactions are summarized in Scheme 3 and the

corresponding conditions given in the Exp. Section.

The Reaction of II–V with HCl

The Reaction of II and III with HBF₄

The reaction of II–V with a slight excess of HCl always

The reaction of II with HBF₄yielded the di-µ-thiolato led to the separation of solid [(dppp)PtCl₂] in high yield

dinuclear complex [(dppp)₂Pt₂(µ-SC₄H₅)₂](BF₄)₂(IV) in (about 75%). The ease of formation of this complex in re-

high yield. Identification of this complex was achieved on lated systems is well known.^[52]In the case of III–V, various

the basis of NMR spectroscopic data, which were fully con- thiols were also detected in the reaction mixture by CG-

sistent with those reported for the [(PEt₃)₄Pt₂(µ-SC₄H₅)₂]- MS, as depicted in Scheme 3. Thiophene was also identified

(BF₄)₂analog, also obtained in a parallel reaction.^[52]

in the reaction of IV, and it was the only additional product

Remarkably, replacement of II by III in the above reac- in the case of II, both results being consistent with reported

tion led to the unprecedented complex [(dppp)₂Pt₂(µ- data.^[52]

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C–S Bond Activation and Partial Hydrogenation of Thiophene

Overall, the reaction of II–V with HBF₄or HCl does Table 1. Selected distances [Å] for the cation of complex V.

not involve activation of the second thiophenic C–S bond.

Instead, these reactions allow the formation of thiols and,

in the case of HCl, the recovery of platinum as [(dppp)-

PtCl₂].

Distances^[a]

Pt1–S1A

Pt2–S1A

Pt1–C5A

Pt2–C5A

Pt1–C4A

S1A–C2A

C2A–C3A

C3A–C4A

C4A–C5A

Pt1–P1

2.339(4)

2.357(4)

2.255(12)

2.119(12)

2.376(15)

1.789(11)

1.569(14)

1.616(14)

1.131(14)

2.273(2)

2.284(2)

3.2272(6)

Pt2–S1B

Pt1–S1B

Pt2–C5B

Pt1–C5B

Pt2–C4B

S1B–C2B

C2B–C3B

C3B–C4B

C4B–C5B

Pt2–P3

2.343(6)

2.391(6)

2.30(2)

2.23(2)

2.38(2)

1.786(13)

1.572(15)

1.618(15)

1.131(15)

2.261(2)

2.281(2)

The Reaction of II–V with I and NaBH₄

Complexes II–V did not react with I in toluene at reflux

temperature but started degrading after 12 h of heating, as

evidenced by the darkening of the reaction mixture, which

was attributed to the formation of Pt⁰. The same features

were observed in the attempts to treat II–V with NaBH₄at

room temperature. However, with NaBH₄the darkening of

the solution was accompanied by the formation of either T

(II) or H₂S (III–V), the former detected by GC-MS, and

the latter by formation of silver sulfide. Significantly, no

other sulfur-containing species were identified in these reac-

tions.

On the basis of the reactions above, it becomes apparent

that the activation of the second thiophenic S–C bond by

hydride sources is only achieved in compounds III–V. The

reluctance of this bond to be activated in II is probably

due to the greater aromatic character of the corresponding

thiametalacycle. In fact, the cleavage of the C–S bond in II

requires previous treatment with protons in the presence

of noncoordinating counterions (HBF₄), which leads to IV.

Overall, the activation of the C–S bond in II–V by NaBH₄

involves the total decomposition of these complexes.

Pt1–P2

Pt1–Pt2

Pt2–P4

[a] The A/B labels refer to the two disordered SC₄H₆fragments.

Table 2. Selected angles [°] for the cation of complex V.

Angles^[a]

Pt1–Pt2–S1A

Pt2–Pt1–S1A

Pt1–Pt2–C5A

Pt2–Pt1–C5A

Pt1–S1A–Pt2

Pt1–C5A–Pt2

S1A–Pt1–C5A

S1A–Pt1–C4A

S1A–Pt2–C5A

46.36(10)

46.83(11)

44.1(3)

Pt2–Pt1–S1B

Pt1–Pt2–S1B

Pt2–Pt1–C5B

Pt1–Pt2–C5B

Pt2–S1B–Pt1

Pt2–C5B–Pt1

S1B–Pt2–C5B

S1B–Pt2–C4B

S1B–Pt1–C5B

47.64(16)

46.41(15)

45.4(6)

43.8(6)

85.9(2)

90.8(7)

80.4(5)

77.4(4)

82.8(5)

40.8(3)

86.81(13)

95.0(5)

80.5(3)

79.1(3)

82.9(3)

S1A–C2A–C3A 113.4(8)

C2A–C3A–C4A 115.2(10)

C3A–C4A–C5A 137.3(12)

S1B–C2B–C3B 110.4(10)

C2B–C3B–C4B 116.0(12)

C3B–C4B–C5B 141.2(16)

Pt1–S1A–C2A

Pt2–S1A–C2A

Pt1–C5A–C4A

102.9(5)

92.7(6)

81.9(9)

Pt2–S1B–C2B

Pt1–S1B–C2B

Pt2–C5B–C4B

Pt1–C5B–C4B

P3–Pt2–S1B

P2–Pt1–S1B

P4–Pt2–C5B

P1–Pt1–C5B

P4–Pt2–S1B

P1–Pt1–S1B

P3–Pt2–C5B

P2–Pt1–C5B

C5B–Pt2–C4B

P3–Pt2–P4

104.8(7)

90.7(10)

80.0(16)

114.5(14)

94.64(16)

94.29(16)

92.4(6)

X-ray Structure of the [(dppp)Pt(µ-SC₄H₆)Pt(dppp)]²⁺

Cation

Pt2–C5A–C4A 115.0(10)

P1–Pt1–S1A

P4–Pt2–S1A

P2–Pt1–C5A

P3–Pt2–C5A

P1–Pt1–C5A

P2–Pt1–S1A

P3–Pt2–S1A

P4–Pt2–C5A

C5A–Pt1–C4A

P1–Pt1–P2

94.37(11)

94.34(11)

93.8(3)

The structure showed evidence of a twinned crystal hav-

ing two components related by a twofold rotation along the

reciprocal axis c*. The asymmetric unit of the unit cell con-

tains two cations [(dppp)Pt(µ-SC₄H₆)Pt(dppp)]²⁺together

90.1(3)

90.7(5)

164.4(3)

173.08(11)

172.45(11)

171.4(4)

28.1(4)

92.13(8)

126.72(6)

129.58(6)

171.63(17)

173.54(16)

171.9(5)

174.6(7)

27.9(4)

92.99(8)

128.55(6)

129.30(6)

–

with three BF₄and one ClO₄anions, three acetone and

one water molecules.^[53]As the bonding parameters of the

two cations are comparable, the geometry and selected

bond lengths and angles given in the text correspond to one

of them (Figure 2, Tables 1 and 2).

P1–Pt1–Pt2

P2–Pt1–Pt2

P3–Pt2–Pt1

P4–Pt2–Pt1

[a] The A/B labels refer to the two disordered SC₄H₆fragments.

While the solution of the structure of the cation showed

evidence of disorder of the bridging SC₄H₆ligand, on the

basis of the NMR spectroscopic data already described it

was modeled as S–CH₂–CH₂–CH=CH. On the other hand,

crystallographic resolution of a related cobalt complex^[41]

allowed us to consider that there were two bridging ligands

with opposite orientation, in which the sulfur and C=C

double bond exchange sites and the double bond changes

the platinum atom, to which it is η²-bound (effectively a C₂

rotation of the SC₄H₆group perpendicular to Pt–Pt axis).

Moreover, there was no evidence for disordered atoms cor-

responding to {(dppp)Pt} moieties, suggesting that these

Figure 2. Structure of the cation in complex V with 50% prob-

ability displacement ellipsoids showing the two opposite orienta-

tions (a/b) of the thiolate bridge. Anions, hydrogen atoms, and

phenyl rings are omitted for clarity.

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A. Nova, F. Novio, P. González-Duarte, A. Lledós, R. Mas-Ballesté

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atoms lie in virtually the same locations in the disordered clusion of solvent is mandatory to obtain a reliable energy

partners. profile. Thus, the relative energies of intermediates and re-

The bond lengths and angles observed for the SC₄H₆li- action barriers calculated in benzene are given in the

gand are fully consistent with the sequence S–CH₂–CH₂– schemes. The resulting ∆G values in benzene and details of

CH=CH, whose atoms are puckered in such a fashion so geometries of the transition states are given in the electronic

that sulfur, C2, C3, and C4 are nearly planar, but with C5 supporting information.

lying above this plane. While one of the platinum atoms

(Pt1) is bound to the SC₄H₆bridge through sulfur and the

Fragmentation of the Dinuclear [(dhpp)₂Pt₂H₃]⁺Cation

(1)

C4=C5 double bond, the other platinum (Pt2) binds to the

same sulfur but only to C5 through a σ bond. As a result

of the different coordinative behavior of the bridging li-

gand, two different square-planar environments are ob-

served for the platinum centers. The platinum–platinum dis-

tance of 3.2272(6) Å is consistent with the only reported

related species, where a weak metal–metal interaction is

proposed.^[54]Concerning the terminal dppp ligands, they

adopt a distorted chair conformation that compares very

well with that usually found in dinuclear complexes enclos-

ing different bridging ligands between two {(dppp)Pt^II}

fragments.^[55]This is the first structurally characterized ex-

ample of a dinuclear platinum compound resulting from the

S–C activation of thiophene.

All attempts to make 1 react with T have led to the cleav-

age of the dinuclear platinum complex with the concomi-

tant coordination of T to one of the platinum centers. In

this way two mononuclear Pt^IIcomplexes are formed:

[(dhpp)PtH₂] (2a) and either [(dhpp)PtH(C₄H₄S-κS)]⁺(2b)

or [(dhpp)PtH(η²-C₄H₄S)]⁺(3b), depending on whether the

coordination mode of thiophene to platinum is through sul-

fur or a double bond, respectively. As shown in Scheme 4,

both types of coordination involve similar energy barriers:

16.0 kcal/mol (TS1) to give 2a and 2b; and 17.4 kcal/mol

(TS2) to give 2a and 3b. Also, both pathways are clearly

endothermic, the products being 21 kcal/mol above reac-

tants.

The energy for the cleavage of 1 was also calculated in

the absence of T in order to quantify the thiophene assist-

ance to the fragmentation process. The energy cost associ-

ated to the formation of [(dhpp)PtH]⁺and [(dhpp)PtH₂]

(2a) from [(dhpp)Pt₂H₃]⁺in benzene is 39.5 kcal/mol, thus,

considerably higher than in the presence of thiophene

(21.7 kcal/mol).

Computational Study of the Reaction Mechanism of I

with T

Computational chemistry has proven to be a useful tool

in mechanistic studies of homogeneous catalytic pro-

cesses.^[56,57]However, theoretical analyses of HDS mecha-

nisms using organometallic reagents are still scarce,^[43,44]

and in general have focused on investigating the nature and

energetics of the interactions between T and single-metal

Formation of [(dhpp)Pt(C₄H₄S-C,S)] (6a)

Both 2a and 2b species are candidates to afford the

organometallic complexes.^[58–64]In this section we present a {(dhpp)Pt⁰} fragment, which is the precursor of 6a (II).

complete computational study of the reaction mechanism However, theoretical calculations show that formation of

of I with T using DFT methods. From this study and the this fragment by reductive elimination of H₂from 2a is

experimental results presented above we propose a plausible much more easily achieved than the elimination of a S-pro-

reaction pathway for the formation of complexes II and III. tonated thiophene from 2b. The calculated energy for the

All complexes have been modeled using H₂P(CH₂)₂PH₂latter reaction is 69.5 kcal/mol, too high to be feasible. Con-

(dhpp) instead of the actual dppp ligand. The calculated sequently, only the reaction from 2a to yield 6a has been

intermediates are numbered from 1 to 10 and the transition taken into account. Concerning energy values, as 2a and 2b

states are labeled as TS1, TS2, and so forth. The van der can exist as separate entities in solution, they are taken as

Waals complexes are indicated by 2a···2b while 2a+2b indi- the zero of energy in the following discussions.

cates the sum of the energies of the individual species.

The set of reactions that account for the evolution from

Given the cationic nature of some intermediates, the in- 2a to 6a (Scheme 5) is initiated by the concomitant re-

Scheme 4. Reaction pathway proposed for the fragmentation of I (1). Relative energies of intermediates and energy barriers (in brackets)

are included. All values calculated in benzene solvent [kcal/mol].

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C–S Bond Activation and Partial Hydrogenation of Thiophene

duction of platinum and elimination of H₂(2a) to give S insertion), TS4 lies 34.2 kcal/mol above 2a. However, the

[(dhpp)Pt] (3a), which is 24.7 kcal/mol less stable than 2a. insertion product [(dhpp)Pt(C₄H₄S-C,S)] (6a, Scheme 5) is

This is consistent with already reported experimental^[65–67]remarkably stable, as shown by the exothermicity of the in-

and theoretical^[68,69]studies. The energy barrier for the H₂sertion process (from 3a to 6a).

elimination is given by the energy difference between 3a+H₂

and 2a. However, as this energy increases monotonously

Transfer of Hydride from the Metal to Thiophene

along the H₂elimination process, the TS cannot be found.

This dissociative reaction is entropically favored and, conse-

quently, considering ∆G_benzenevalues, 3a+H₂lie only

14.5 kcal/mol above 2a.

The cationic complex 2b formed in the first step of the

reaction (Scheme 4) follows a different route than 2a. First,

the apparent rotation of thiophene from κS to η²-C,C coor-

dination mode gives way to complex [(dhpp)PtH(η²-

C₄H₄S)]⁺(3b), with an energy barrier of 5.0 kcal/mol (TS5)

(Scheme 6). Complex 3b can also be formed by direct reac-

tion of 1 with T through TS2 (Scheme 4).

Once 3b is formed, the hydride migration to the double

bond in C2 or C3 becomes feasible. The migration to C2

gives an agostic intermediate 4b with an energy barrier of

12.6 kcal/mol (TS6). In going from 4b to 5b the agostic in-

teraction is lost and the thiophene ring rotates in order to

adopt η³coordination. The energy cost of this process is

very low, only 2.6 kcal/mol (TS7). Finally, from 5b a subse-

quent rotation of the thiophene ring with an energy barrier

of 5.3 kcal/mol (TS8) gives the most stable conformation of

this system (6b), 8.8 kcal/mol more stable than 2b. In 6b the

platinum is bonded to C5 and to the sulfur, and the π sys-

tem has been rearranged.

From 3b the hydride migration to C3 proceeds in a sim-

ilar way, with an energy barrier of 11.9 kcal/mol. After the

first transition state (TS10) an agostic intermediate 8b is

formed. The rotation of the thiophenic group in this case

gives compound 9b, where Pt is bonded to the C2 and sul-

fur atoms. This product is 5.3 kcal/mol more stable than 2b.

Considering the energy barriers for the hydride transfer

Scheme 5. Reaction pathway proposed for the formation of II (6a).

Relative energies of intermediates and energy barriers (in brackets)

are included. All values calculated in benzene solvent [kcal/mol].

The next step is the coordination of thiophene to 3a, and for the thiophene ring rotation, all species displayed in

which, analogously to the [(dhpp)PtH]⁺fragment Scheme 6 within the dashed-line square are in equilibrium

(Scheme 4), can occur through either the sulfur atom or the with each other, although it is displaced towards formation

double bond, thus forming [(dhpp)Pt(C₄H₄S-κS)] (5a) or of 6b. From this complex, the C–S bond activation of thio-

[(dhpp)Pt(η²-C₄H₄S)] (4a), respectively. The conversion of phene to give complex 7b is accessible with an energy bar-

4a to 5a is possible through an energy barrier of 12.4 kcal/ rier of 21.0 kcal/mol. The C–S bond activation from the less

mol (TS3). In addition, despite 4a being more stable than stable complex 9b was also considered. This activation not

5a (see Scheme 5), the oxidative addition of platinum to the only has a higher energy barrier (25.3 kcal/mol), but also it

C–S bond requires that T is bound through sulfur. The en- is a more endothermic reaction. Thus, while 7b is 6.3 kcal/

ergy barrier for the oxidative addition process is 12.8 kcal/ mol above 2b, 10b is 15.5 kcal/mol, so the activated product

mol (TS4). Considering the two steps (H₂elimination + C– 7b is clearly favored with respect to 10b.

Scheme 6. Reaction pathway proposed for hydride insertion in 2b. Relative energies of intermediates and energy barriers (in brackets) are

included. All values calculated in benzene solvent [kcal/mol].

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FULL PAPER

Scheme 7. Reaction pathways proposed for the formation of 6a (II) and 10 (III).

Figure 3. Energy profile of the reaction leading to complexes 6a (II) and 10 (III), showing the pathway: (a) from the dissociation of 1

(2a+2b) either to product 6a or to intermediate 7b, by means of plain or discontinuous lines, respectively, and (b) from 3a+7b to 10 by

dotted lines.

Formation of the [(dhpp)Pt(µ-SC₄H₅-C,S)Pt(dhpp)]⁺

Cation (10)

(more favored than to C3) leads to 6b. This intermediate is

able to activate the C–S bond to give the very unstable 7b

complex. Reaction of the latter species with 3a gives the

dinuclear platinum complex 10 without energy barrier (Fig-

ure 3). The energy of this complex is 29.5 kcal/mol below

that of 2a+2b and, despite the unfavorable entropic contri-

bution of the association process, the 2a+2b Ǟ 10 reaction

Up to now the successive pathways leading to complexes

6a and 7b have been considered separately (Schemes 4, 5,

and 6). In this subsection they will be assembled to enable

the formation of 10, model of the experimental complex III.

The overall mechanism is summarized in Scheme 7, where

the complexes described in the experimental section are

highlighted within dashed-line squares. The energy profile

for the sequence of reactions that take place after fragmen-

tation of I to finally afford 6a and 10 is depicted in Figure 3.

As shown in Scheme 7, fragmentation of the dinuclear

platinum trihydride leads to 2a and 2b. At this point, these

is also favored on Gibbs energy grounds with ∆G_benzene

=

–20.8 kcal/mol.

Calculations versus Experiments

All previous considerations account for the overall

complexes react through different pathways. On the one mechanism from 1 (I) to either 6a (II) or 10 (III). The next

hand, complex 2a undergoes H₂elimination to give 3a, step is to analyze their consistency with the experimental

which subsequently reacts with thiophene to yield 6a (II). results. Thus, concerning the reaction of I with I-d₃, the

On the other, the hydride migration to C2 in complex 2b formation of a mixture of isotopomers I-d_x(x = 0–3) was

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C–S Bond Activation and Partial Hydrogenation of Thiophene

experimentally observed only in the presence of thiophene forded the mononuclear [(dppp)Pt(SC₄H₄-C,S)] (II) metal-

at reflux temperature. This is consistent with the equilib- acycle that involves activation of the thiophenic C–S bond

rium between 1+T and 2a+2b depicted in Scheme 4. This exclusively, and the dinuclear [(dppp)₂Pt₂(µ-SC₄H₅-C,S)

reaction has an energy cost of 21.7 kcal/mol while the frag- ClO₄(III) complex, where activation of this bond is con-

mentation of I without the presence of thiophene requires comitant with the partial hydrogenation of thiophene

39.5 kcal/mol. On the other hand, replacement of I by I-d₃(Scheme 2).

in the synthesis of III showed that the hydrogen atom in-

Additional experiments and computational results have

serted into the thiophenic ring comes from I and adds to evidenced the role of T as promoter of the cleavage of I into

the C2 atom. This result fits well with the mechanism the [(dppp)PtH₂] and [(dppp)PtH(C₄H₄S-κS)]⁺fragments

showed in Scheme 7, in which the formation of 10 (III) in- in the first step of the reaction. Also, experimental observa-

volves an initial insertion of the hydride into the C2 posi- tions showing the noninterconversion equilibrium between

tion of the coordinated thiophene in 2b.

II/III indicate that two concurrent mechanisms are at work.

The general mechanism proposed can also account for This is fully consistent with the theoretical calculations,

the noninterconversion between II and III even in the pres- which allow us to propose two different reaction pathways

ence of external hydride ligands. Comparison of II and III that originate after the dissociation of I. While formation

indicates that this transformation would imply the hydride of II involves exclusive participation of the former frag-

insertion into the thiophenic ring of II and the subsequent ment, both are required to take part in the pathway leading

coordination of an additional {(dppp)Pt} fragment. On the to III. In fact, formation of this complex implies the co-

basis of Scheme 7, two steps leading to III deserve special ordination of {(dppp)Pt} to [(dppp)Pt(µ-SC₄H₅-C,S)]⁺

consideration. First, the intramolecular hydride migration (Scheme 7). The former species, which originates from

in species 2b, which entails a low energy barrier, and sec- [(dppp)PtH₂], is responsible for the stabilization of the lat-

ond, the stabilization of the intermediate 7b by means of its ter, which arises from the hydride migration into the thioph-

coordination to a {(dppp)Pt} fragment. Remarkably, none enic ring in [(dppp)PtH(C₄H₄S-κS)]⁺and subsequent C–S

of these two features can be accomplished in the experimen- bond activation. Remarkably, complex III is the first exam-

tal system. On the one hand, II does not enclose platinum- ple of a platinum complex involving C–S bond activation

bound hydride ligands, thus hampering an intramolecular and hydrogenation of thiophene.

hydride migration. On the other, formation of III would

The reactions of platinum complexes II–V with protonic

require the assistance of the {(dppp)Pt} fragment, which acids and hydride sources have shown the ability of these

only forms from the thiophene-promoted dissociation of I. reagents to give total desulfurization (Scheme 3). Thus, the

Overall, theoretical results account for the fact that II does reaction of II and III with protonic acids affords either par-

not evolve into III, even in the presence of I or NaBH₄.

tial hydrogenation of the thiophenic chain (with HBF₄) or

Another experimental feature of the reaction of I with T formation of thiols (with HCl). On the other hand, the reac-

refers to the different relative amounts of the reaction prod- tion with NaBH₄entails the formation of H₂S only from

ucts, II and III, depending on the reaction conditions. Thus, complexes where the thiophenic chain has been partially

the reaction affords II (6a) and III (10) in a 2:3 or 1:9 molar hydrogenated (III–V). This result is consistent with the as-

ratio using neat thiophene or organic solvent (toluene or sumption that metal hydride complexes are keystones in the

benzene), respectively. The energy profile depicted in Fig- desulfurization of thiophene.

ure 3 shows that the highest energies involved in the forma-

Overall, the combination of experimental and theoretical

tion of 6a and 10 (TS4: 34.2 kcal/mol and 3a+7b: 31.0 kcal/ studies of the reaction of [(dppp)₂Pt₂H₃]ClO₄with thio-

mol, respectively) are very similar. Consequently, small phene has provided new insights into the mechanism of the

changes in the reaction conditions can affect these energies, homogeneous HDS. Notwithstanding this progress,

modifying the molar ratio of products. In addition, the con- designing systems able to perform the reaction of com-

centration of thiophene also influences the II to III molar pletely and efficiently desulfurizing thiophene and in softer

ratio because the two parallel pathways leading to 6a and conditions still appears to be the main goal. The impor-

10 involve a different number of thiophene molecules. The tance of this reaction makes any effort in this direction

minor relative amount obtained for II in benzene or toluene worthwhile.

can be explained considering that two molecules of T are

required to form this complex, one to dissociate the starting

complex 1 and the other to react with 3a (see Scheme 7).

Overall, the formation of II in toluene or benzene will be

disfavored versus III, whose formation requires only one

molecule of T.

Experimental Section

Materials and Methods: All reactions were carried out under pure

dinitrogen, and conventionally dried and degassed solvents were

used throughout. These were Purex Analytical Grade from SDS.

Thiophene (Ն99%) was purchased from Aldrich. It was purified

following a reported procedure.^[70]Commercial 0.5  NaMeO in

methanol and NEt₃(99%, d = 0.726 g/cm³) were used without fur-

Conclusions

ther purification. The synthesis and spectroscopic characterization

C–S bond activation of thiophene (T) by the dinuclear

of [(dppp)₂Pt₂H₃]ClO₄(I) and [(dppp)₂Pt₂D₃]ClO₄(I-d₃) as well as

trihydride platinum complex [(dppp)₂Pt₂H₃]ClO₄(I) af- the X-ray structure of I have already been reported.^[50]Elemental

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FULL PAPER

analyses were performed on a Carlo–Erba CHNS EA-1108 ana-

lyzer. IR spectra were recorded with a Perkin–Elmer FT-2000 spec-

trophotometer using KBr pellets. ¹H, ¹H{³¹P}, ¹³C DEPT, ³¹P,

toluene and the solution chromatographed on a neutral alumina

column. Elution with a hexane/benzene mixture at 20% ratio

showed separation of two bands, yellow and red-brown, which

³¹P{¹H} ³¹P–¹H HMBC, ¹⁹⁵Pt, and ¹⁹⁵Pt{¹H} NMR and ¹H–¹H respectively contained complexes II and III. Extraction of the for-

NOESY, ¹H–¹H COSY, ¹³C–¹H HMQC, and ¹³C–¹H HSQC-edit

spectra were performed unless otherwise indicated from samples

mer band followed by removal of the solvent under vacuum pro-

duced a residue that was dissolved in acetone and precipitated with

diethyl ether. The yellow solid thus formed was collected by fil-

in solution at room temperature with a Bruker Avance DRX-360

1

spectrometer operating at 360.13 MHz for H, 90.56 MHz for ¹³C, tration, washed with diethyl ether, and vacuum dried. Yield 52 mg,

145.79 MHz for ³¹P, and 77.42 MHz for ¹⁹⁵Pt. ¹H and ¹³C chemical

shifts are relative to SiMe₄, ³¹P to external 85% H₃PO₄, and ¹⁹⁵Pt

to external 1  Na₂[PtCl₆] in D₂O. The ³¹P{¹H} NMR spectrum

of III was simulated on a Pentium^®-200 computer using the gNMR

V4.0.1 program.^{[71] 2}H NMR experiments were performed from

samples in CDCl₃/CHCl₃mixture solutions at room temperature

with a Bruker Avance-500 operating at 76.75 MHz. ²H NMR

chemical shifts are relative to CDCl₃. ¹⁹F NMR experiments for

IV and V were performed from samples in acetone solutions at

25%. H NMR (360 MHz, CDCl₃, 25 °C): δ = 6.75 (t, 1 H, CH),

6.92 (1 H, CH), 7.18 (1 H, CH), 7.34 (1 H, CH) ppm. ¹³C DEPT

NMR (91 MHz, CDCl₃, 25 °C): δ = 119.1 (CH), 120.8 (CH), 124.4

(CH), 130.2 (CH) ppm. ³¹P{¹H} NMR (146 MHz, CDCl₃, 25 °C):

1

2

1

δ = 6.56 (d, J_Pt,P= 3064.4, J_P,P= 33.0 Hz), –1.10 (d, J_Pt,P

=

1601.8, ²J_P,P= 33.0 Hz) ppm. ¹⁹⁵Pt NMR (77 MHz, CDCl₃, 25 °C):

1

δ = –4525 (dd, J_P,Pt= 3066 and 1605 Hz) ppm. ESI-MS: m/z =

692.2 [M + 1]⁺. C₃₁H₃₀P₂PtS (691.2): calcd. C 53.83, H 4.37, S

4.64; found C 53.95, H 4.29, S 4.75. Attempts to obtain crystals

room temperature with

a Bruker DPX-250 operating at suitable for X-ray diffraction were unsuccessful.

235.33 MHz. ¹⁹F NMR chemical shifts are relative to CFCl₃. H

Synthesis of [(dppp)₂Pt₂(µ-SC₄H₅-C,S)]ClO₄(III): The same reac-

tion procedure as for II but in the presence of toluene or benzene

led essentially to III. Thus, I (200 mg, 0.15 mmol) and neat thio-

phene (15 mL) were made to react in degassed toluene or benzene

(20 mL). Column chromatography under the same experimental

conditions as above allowed elution of the second red-brown band

with benzene. Removal of the solvent under vacuum produced a

residue that was dissolved in acetone and precipitated with diethyl

ether. The precipitated yellow-brown solid was collected by fil-

tration, washed with diethyl ether, and vacuum dried. Yield 83 mg,

43%. ¹H NMR (360 MHz, [D₆]acetone, 25 °C): δ = 1.62 (br., 2 H_a),

and C labels for complexes III–V are shown in Figure 4.

3

2.77 (br., H_d), 5.36 (dt, J_Hb,Ha= 3.6, J_Hb,Hc= 9.4 Hz, H_b), 5.46

3

(dt, J_Hc,Hb= 9.4, J_Hc,Hd= 8.2 Hz, H_c) ppm. ¹³C DEPT NMR

(91 MHz, [D₆]acetone, 25 °C): δ = 33.0 (C2), 42.7 (C5), 126.1 (C3),

137.1 (C4) ppm. ³¹P{¹H} NMR (146 MHz, [D₆]acetone, 25 °C): δ

1

= 3.02 (d, J_Pt,P= 3869.6 Hz), 1.96 (¹J_Pt,P= 1857.0 Hz) ppm. On

the basis of the simulation of the ³¹P{¹H} NMR spectrum includ-

3

ing long-range P–P and P–Pt couplings: J_PЈ,PЈ= 22.0, J_PЈ,PЈЈ

=

2

4.7, and J_PЈ,PЈЈ= –22.3 Hz, where PЈ and PЈЈ are nonequivalent

2

phosphorus nuclei, and J_P,Pt= 56 Hz (δ_P= 1.96 ppm) and 37 Hz

(δ_P= 3.02 ppm). ¹⁹⁵Pt{¹H} NMR (77 MHz, [D₆]acetone, 25 °C): δ

1

= –4670 (dd, J_P,Pt= 3872 and 1867 Hz) ppm. ESI-MS: m/z =

1299.2401 [M – ClO₄]⁺. C₅₈H₅₇ClO₄P₄Pt₂S (1399.64): calcd. C

49.77, H 4.10, S 2.29; found C 49.02, H 3.82, S 2.11. IR (cm^–1): ν

˜

Figure 4. H and C labels for complexes III–V.

= 1094 i, br. (ClO₄). Attempts to obtain crystals suitable for X-ray

diffraction were unsuccessful.

The ESI-MS measurements were performed on a VG Quattro

Micromass Instrument under already described experimental con-

ditions.^[50]The carrier was a 1:1 mixture of acetonitrile and water

containing formic acid (1–10%). The exact mass for III was deter-

mined on a micrOTOF-Q instrument using direct injection from

HPLC 1200RR and Apollo II ESI ionization source. GC-MS spec-

tra were recorded using a Hewlett–Packard 6890 instrument and a

Hewlett–Packard 5973 electronic impact mass detector.

Replacement of I by I-d₃in the above reaction procedure allowed

unequivocal identification of [(dppp)₂Pt₂(µ-SC₄H₄D-C,S)]⁺(III-d₁)

on the basis of NMR spectroscopic data. ¹H NMR (360 MHz, [D₆]-

acetone, 25 °C): δ = 1.63 (br., H_a), 2.77 (br., H_d), 5.36 (br., H_b),

2

5.46 (br., H_c) ppm. H NMR (77 MHz, CDCl₃/CHCl₃, 25 °C): δ =

1.58 (br., Da) ppm. ¹³C-DEPT NMR (91 MHz, [D₆]acetone,

25 °C): δ = 32.6 (C2), 42.7 (C5), 126.1 (C3), 137.1 (C4) ppm. NMR

features corresponding to the dppp terminal ligands were coinci-

dent with those of III.

Caution: Although no problems were encountered in this work, all

perchlorate compounds are potentially explosive, and should be han-

dled in small quantities and with great care!

Synthesis of [(dppp)₂Pt₂(µ-SC₄H₅)₂](BF₄)₂(IV): Aqueous HBF₄

(70 µL of 4  solution, 0.28 mmol) was added to a solution of II

Synthesis of [(dppp)Pt(SC₄H₄-C,S)] (II): The white-beige suspen- (50 mg, 0.07 mmol) in CH₃CN (10 mL). The mixture was stirred

sion formed by addition of I (200 mg, 0.15 mmol) to degassed neat

thiophene (15 mL) was heated at reflux temperature (100 °C) for

24 h, by which time it had turned into a red-brown solution con-

taining a minor amount of a black solid residue. This was separated

by filtration through Celite and the remaining solution evaporated

under vacuum to yield a brown residue. This solid was dissolved in

(15 min) and then evaporated to 5 mL under vacuum. Addition of

diethyl ether gave a white precipitate, which was dissolved in ethyl

acetate and purified on a neutral alumina column using the same

solvent as eluent. Evaporation of the solvent afforded a white solid.

1

Yield 47 mg, 84%. H NMR (360 MHz, CD₃CN, 25 °C): δ = 4.75

3

2

3

(d, J_Ha,Hc= 10, J_Ha,HbϽ 1 Hz, H_a), 4.90 (d, J_Hb,Hc= 16.5 Hz,

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C–S Bond Activation and Partial Hydrogenation of Thiophene

H_b), 5.34 (dt, ³J_Hc,Ha= ³J_Hc,Hd= 10, ³J_Hc,Hb= 16 Hz, H_c), 5.45 (d,

lyzed by CG-MS. The solution was shown to contain thiophene in

the case of II, but no thiols and/or organic volatiles were detected.

3

³J_He,Hd= 10 Hz, H_e), 5.74 (t, J_Hd,He= ³J_Hd,Hc= 10 Hz, H_d) ppm.

¹³C DEPT NMR (91 MHz, CD₃CN, 25 °C): δ = 136.6 (C4), 127.4

(C3), 120.8 (C2), 117.4 (C5) ppm. ³¹P{¹H} NMR (146 MHz,

CD₃CN, 25 °C): δ = –2.00 (¹J_Pt,P= 2897.4 Hz) ppm. ¹⁹F NMR

(235 MHz, [D₆]acetone, 25 °C): δ = –152.2 ppm. ¹⁹⁵Pt{¹H} NMR

(77 MHz, CD₃CN, –20 °C): δ = –4415.8 ppm. C₆₂H₆₂B₂F₈P₄Pt₂S₂

(1558.94): calcd. C 47.77, H 4.01, S 4.11; found C 47.96, H 4.16, S

4.27. ESI MS (m/z): 693.5 (25%) corresponding to [(dppp)₂-

Pt₂(SC₄H₅)₂]²⁺, and 1387.0 (100%) corresponding to [(dppp)₂-

X-ray Crystallographic Characterization: A summary of crystal

data, data collection, and refinement parameters for the structural

analysis of complex [(dppp)Pt(µ-SC₄H₆)Pt(dppp)]₂(BF₄)_2.95

-

(ClO₄)_1.05is given in Table 3. Measurements of diffraction intensity

data were collected on a Bruker SMART CCD-1000 area-detector

diffractometer with graphite-monochromated Mo-K_αradiation (λ

= 0.71073 Å). During the initial unit cell determination it was rec-

ognized that the crystal was twinned. Several crystals were assayed

but all possessed the same diffraction pattern, so a small but dif-

fracting crystal was chosen. The collected reflections were pro-

cessed with the program CELLNOW,^[75]which established two twin

components with a ratio of 0.72/0.28. In order to process the

twinned data, the absorption correction was applied using the

multiscan TWINABS^[75]program. Cell parameters were obtained

from least-squares fit on the observed setting angles of all signifi-

cant intensity reflections. A Patterson map solution of the structure

to locate platinum atoms, followed by expansion of the structure

with the program DIRDIF,^[76]revealed all non-hydrogen atoms.

The structure was resolved by direct methods and refined by full-

matrix least-squares based on F², with the aid of SHELX-97^[77]

software. The thiolate ligand was modeled as two disordered

SCH₂CH₂CH=CH moieties with opposite orientations, finding a

refined population ratio of 63/37 in one dinuclear complex and 69/

31 in the other. All non-hydrogen atoms were anisotropically re-

fined. All hydrogen atoms were included at geometrically calculated

positions with thermal parameters derived from the parent atoms.

Molecular graphics are represented by ORTEP-3 for Windows.

Pt (SC H ) ]⁺. IR: ν = 1051 i, br. (BF ).

˜

2

4

5 2

4

Synthesis of [(dppp)₂Pt₂(µ-SC₄H₆)](BF₄)₂(V): Aqueous HBF₄

(50 µL of 4  solution, 0.20 mmol) was added to a solution of III

(75 mg, 0.05 mmol) in degassed acetonitrile (10 mL) and the mix-

ture stirred for 30 min. Then it was filtered through Celite, concen-

trated to 1–2 mL, and precipitated with addition of diethyl ether

to yield a yellow solid. The solid was collected by filtration, washed

1

with diethyl ether, and vacuum dried. Yield 65 mg, 82%. H NMR

(360 MHz, [D₆]acetone, 25 °C): δ = 2.67 (br., 2H_b), 2.78 (br., 2H_a),

3

5.31 (dt, J_Hc,Hb= 3.2, J_Hc,Hd= 9.2 Hz, H_c), 5.90 (d, J_Hd,Hc

=

9.2 Hz, H_d) ppm. ¹³C DEPT NMR (91 MHz, [D₆]acetone, 25 °C): δ

= 26.8 (C2), 33.3 (C3), 119.7 (C5), 129.7 (C4) ppm. ³¹P{¹H} NMR

1

(146 MHz, [D₆]acetone, 25 °C): δ = 0.10 (d, J_Pt,P= 2612.4 Hz),

–0.92 (¹J_Pt,P= 2965.3 Hz) ppm. ¹⁹⁵Pt{¹H} NMR (77 MHz, [D₆]-

1

acetone, 25 °C): δ = –4675 (dd, J_P,Pt= 2612 and 2966 Hz) ppm.

¹⁹F NMR (235 MHz, [D₆]acetone, 25 °C): δ = –152.2 ppm. ESI-

MS: m/z = 650.3 (15%) and 1300.3 (100%) corresponding to

[(dppp)₂Pt₂(SC₄H₆)]²⁺and [(dppp)₂Pt₂(SC₄H₆)]⁺, respectively.

C₅₈H₅₈B₂F₈P₄Pt₂S (1474.80): calcd. C 47.23, H 3.96, S 2.17; found

C 47.14, H 4.01, S 2.12. IR: ν = 1051 i, br. (BF ).

˜

4

Table 3. Crystallographic data for [(dppp)Pt(µ-SC₄H₆)Pt(dppp)]₂-

The same reaction procedure but using only 1.2 equiv. of HBF₄

allowed separation of a yellow solid, whose acetone solution af-

forded adequate crystals for X-ray diffraction.

(BF₄)_2.95(ClO₄)_1.05

.

Empirical formula

2(C₅₈H₅₆P₄Pt₂S)·2.95(BF₄)·

1.05(ClO₄)·3(C₃H₆O)·(H₂O)

Reaction of V with Bases (NaOMe, Et₃N): A methanol solution of

NaOMe (100 µL, 0.05 mmol) or Et₃N (17.5 µL, 0.125 mmol) was

added to a suspension of V (75 mg, 0.05 mmol) in CH₃CN

(10 mL). In both cases the resulting solution was stirred at room

temperature for 30 min, after which time it was concentrated to

dryness. Then, the solid was redissolved in benzene, filtered and

precipitated by addition of diethyl ether. ¹H and ³¹P NMR spectro-

scopic data of this solid in acetone solution evidenced the complete

transformation of V into III.

M

3153.06

monoclinic

Cc

Crystal system

Space group

a [Å]

b [Å]

c [Å]

α [°]

43.8827(12)

14.1377(4)

22.2630(6)

90

β [°]

116.223(1)

90

12390.5(6)

100.0(1)

4

1.690

4.735

γ [°]

V [Å³]

Reaction of II–V with HCl: An excess of HCl was added to separate

stirred CH₂Cl₂(10 mL) solutions containing II, III, IV, or V

(25 mg) in a 3–4:1 molar ratio at room temperature. The initial

yellow-brown solution turned pale yellow and a white solid precipi-

tated. After 30 min of stirring, the mixture was filtered and the

soluble part analyzed by CG-MS. The solid was unequivocally

identified as [(dppp)PtCl₂] (65–80% yield) by NMR spec-

troscopy.^[72,73]The solution was shown to be composed mainly of

thiophene in the case of II, thiapentadiene or an isomer thereof

(C₄H₆S) and thiophene^[52]in a 4:1 molar ratio for IV, and a mixture

T [K]

Z

D_calcd[g/cm³]

µ [mm^–1]

Reflections collected/

unique reflections/R_int

Parameters/restraints

Goodness-of-fit on F²

Twin law

Twin domain populations

Absolute structure parameter

R₁, wR₂[I Ͼ 2σ(I)]

R₁, wR₂(all data)

Largest diff. peak, hole [e/Å³]

222291/45071/0.0826

1289/68

0.926

twofold axis along c*

0.720(4)/0.280(4)

0.000(4)

0.0437, 0.0970

0.0776, 0.1056

1.228, –1.130

[74]

of HS–CH₂–CH=CH–CH₃and HS–CH₂–CH₂–CH=CH₂,^[47]in

4:1 and 1:1 molar ratios, for III and V, respectively.

Reaction of II–V with NaBH₄: Solid NaBH₄was made to react with

separate stirred THF/EtOH_abssolutions (10 mL) containing II, III,

IV, or V (25 mg) in a 4:1 molar ratio at room temperature under a

closed atmosphere. In the reactions involving III–V, formation of

a black precipitate of Ag₂S on a filter paper soaked with silver

nitrate gave evidence of formation of H₂S. In all cases, after 20 h

of stirring the initial yellow-brown solution became a dark-brown

suspension. The black solid was separated and the solution ana-

CCDC-651579 {compound [(dppp)Pt(µ-SC₄H₆)Pt(dppp)]₂(BF₄)_2.95

-

(ClO₄)_1.05} contains the supplementary crystallographic data for

this paper. These data can be obtained free of charge from The

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/

data_request/cif.

Eur. J. Inorg. Chem. 2007, 5707–5719

www.eurjic.org

5717

A. Nova, F. Novio, P. González-Duarte, A. Lledós, R. Mas-Ballesté

FULL PAPER

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Published Online: November 2, 2007

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5719

Article Doi

C-S bond activation and partial hydrogenation of thiophene by a dinuclear trihydride platinum complex

DOI: 10.1002/ejic.200700932

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Article abstract of DOI:10.1002/ejic.200700932

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