Titanocene sulfide chemistry

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Review

Titanocene sulfide chemistry Michal Horáˇcek J. Heyrovsk´ y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejˇskova 3, 182 23 Prague 8, Czech Republic

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Titanocene hydrosulfides Ti-(SH)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1. Preparation of titanocene hydrosulfides Ti-(SH)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1.1. Reactions with H2 S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1.2. Reactions with elemental sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2. Reactivity of titanocene hydrosulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2.1. Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2.2. Deprotonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2.3. Photolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Titanocene sulfides Ti-(S)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Preparation of titanocene sulfides Ti-(S)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. Reactivity of titanocene sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2.1. Modification of sulfide moiety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2.2. Chelate transfer processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Titanocene thiolates Ti-(SR)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.1. Preparation of titanocene thiolates Ti-(SR)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.1.1. Reactions with dialkyl or diaryl sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.1.2. Metathetic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.2. Reactivity of titanocene thiolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

a r t i c l e

i n f o

Article history: Received 26 June 2015 Received in revised form 17 September 2015 Accepted 23 September 2015 Available online xxx Dedicated to Karel Mach for his extraordinary contribution to titanocene chemistry.

a b s t r a c t This review is focused on the chemistry of titanocene sulfide compounds. The linking element for all compounds discussed in this review is the presence of a bent titanocene fragment consisting of two aromatic cyclopentadienyl rings and one electron deficient central titanium atom coordinated between them. Several types of sulfur ligands bonding to titanocene fragment, such as hydrosulfide, sulfide or thiolate are described, as well as the various types of sulfur ligand functions in titanocene complexes have been observed. The titanocene sulfide chemistry is more than fifty years old, but since it is still evolving, it offers a plethora of surprises and challenges. © 2015 Elsevier B.V. All rights reserved.

1. Introduction There is a large number of compounds in literature having the simple sulfide ion S2− as a ligand comparable to O2− that are mononuclear, binuclear or polynuclear and incorporating sulfur

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bridges. These and related compounds derived from polysulfide ions (Sn2− ) have been intensively studied, partly because sulfur bridged species are known to occur in Nature. Organometallic compounds containing sulfur atoms are very important in systems exhibiting biological and/or catalytical activity. Although known for a long time, but still under intense studies are sulfide complexes of biogenic transition metals (Fe, Co, Ni) constituting a family of life-important enzymes [1–4], whereas sulfides of some other transition metals (Mo, W, Ru, Os, Rh, Ir, Re) gained importance as

http://dx.doi.org/10.1016/j.ccr.2015.09.011 0010-8545/© 2015 Elsevier B.V. All rights reserved.

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Ti

Cl Cl

+

THF, Et3N

n H2S

methyl-substituted titanocene complexes starting from the unsubstituted up to the permethylated derivative. In addition to introducing methyl groups to the cyclopentadienyl ligands, further fine tuning or changes in crucial chemical and physical properties of titanocene complexes can be achieved by introduction or transformation of functional groups on cyclopentadienyl ligands [16].

SH

Ti

- Et3NH.Cl

SH

Scheme 1.

2. Titanocene hydrosulfides Ti-(SH)n catalysts for sulfur removal from petroleum via hydrodesulfurization [5,6]. These applications stimulate the unceasing interest in investigating the whole range of either mononuclear or homo- and hetero-polynuclear transition metal complexes containing sulfur ligands as bridging moieties [7–14]. The titanocene-sulfide chemistry, in spite of its undoubted progress since the sixties of the last century, remained relatively underdeveloped, with its scope limited to some basic knowledge on simple highly substituted titanocene(TiIV and TiIII )-hydrosulfide and sulfide complexes. The titanocene thiolate chemistry has also been explored due to the ability of thiolate derivatives to undergo coordination of different transition metal fragments and forming heterometallic complexes [15]. A line (thread) connecting all the presented (or reviewed) compounds is the presence of titanocene fragment “Cp2 Ti”, which is an electronically and coordinatively unsaturated species playing an important role in catalytic reactions with different substrates. In general, the titanocene fragment is an unstable 14-electron species with d2 configuration Ti(II). One of the possibilities to increase the stability of the titanocene fragment is its substitution on the cyclopentadienyl ligand. It has been well established that the presence of methyl substituents at the cyclopentadienyl ring increases the electron density at the metal center, this effect being more pronounced with the increasing number of methyl groups. The chemical properties change rapidly in the range of

Ti

SH

- 2H2

SH

+ 2H2

2.1. Preparation of titanocene hydrosulfides Ti-(SH)n 2.1.1. Reactions with H2 S Hydrogen sulfide (H2 S) is the most versatile reagent for synthesis of hydrosulfido compounds. A number of ligands (alkyl, aryl, hydrido, halogeno, alkoxo, amido, thiolato) can be substituted by the hydrosulfido group upon their reaction with H2 S. Low-valent metal complexes undergo oxidative addition to form hydrosulfido species. The first titanocene hydrosulfide containing two unsubstituted cyclopentadienyl rings Cp2 Ti(SH)2 was prepared in 1965 by the reaction of Cp2 TiCl2 with excess of H2 S in the presence of NEt3 at room temperature (Scheme 1) [17]. This method was revised in 1980 by McCall and Shaver [18] with the aim to find titanocene precursors for synthesis of complexes Cp2 TiSx where x = 3, and 4; the compound with x = 5 being previously known [19]. The easily isolable derivative (MeCp)2 Ti(SH)2 was prepared in crystalline form by Rauchfuss from the monomethyl substituted titanocene dichloride (MeCp)2 TiCl2 using the above-mentioned method [20]. This monomethylated titanocene dihydrosulfido complex has been suggested to undergo dehydrogenation yielding the [(MeCp)2 TiS2 ]2 dimer including a 1,4-dititanacyclohexasulfane fragment (Scheme 2).

Ti

S

S

S

S

Ti

Scheme 2.

Ti

CO

+

Ti

2 H 2S

SH

+

SH

CO

H2

+

2 CO

Scheme 3.

Ti S

S

Ti Ti

Ti

S 10

Ti

CO CO

+

12 H2S

2

S

S Ti

+

7H2

+

20CO

+ 10CpH

S

Scheme 4.

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Ti

NtBu

H2S - py

Ti

py

NHtBu

H2S - NH2tBU

SH

Ti

SH

SH

Cp2TI(NtBU)py - NH2tBU - py

H2S

- NH2tBU

Ti

3

S

Ti

S

Ti

S

Scheme 5.

Ti

CH2 CH2

+ H2 S

py

Ti

S

+

Ti

py

SH SH

Scheme 6.

Ti

S

H2 - py

Ti

py

S

SH

Ti

H

H

H

PR3, H2

1/8 S8

- S=PR3 (R= Me, Ph)

S Ti S

H2

Ti

SH SH

Scheme 7.

The sterically most demanding Cp∗2 TiCl2 was shown to be inert towards H2 S in the presence of NEt3 at room temperature. However, this titanocene complex reacted with Li2 S5 to afford a titanocene-sulfur complex containing a titanacyclobutasulfine ring [21–23]. The more convenient method to prepare Cp∗2 Ti(SH)2 compound was discovered by Bottomley, who explored the stoichiometric oxidative addition reaction of titanium(II) complex Cp∗2 Ti(CO)2 with H2 S (Scheme 3) [24]. The Cp∗2 Ti(SH)2 complex was stable in solid state, however in solution it rapidly oxidized giving a yellow insoluble material. The analogic reaction between the unsubstituted Cp2 Ti(CO)2 and H2 S carried out in toluene at 80 ◦ C gave the moderately airsensitive dark green-brown complex (CpTi)5 S6 (Scheme 4) [25,26]. The reaction proceeded with a slow but still observable rate at room temperature, and the (CpTi)5 S6 complex formed was similar to the

extremely stable oxygen cluster (CpTi)6 O8 prepared analogously from Cp2 Ti(CO)2 and water [27]. In 1997 Mountford reported that the titanocene complex Cp2 Ti(Nt Bu)py reacts with excess H2 S yielding free pyridine, Cp2 Ti(SH)2 and t BuNH2 in an equimolar ratio [28]. It was proposed that the monohydrosulfido complex Cp2 Ti(NHt Bu)(SH) is the first formed intermediate, which rapidly eliminates t BuNH2 and forms a transient titanocene complex Cp2 Ti S. This intermediate dimerizes to [Cp2 TiS]2 , followed by the reaction with another molecule of H2 S giving the final product Cp2 Ti(SH)2 (Scheme 5). In 1999 the titanocene complex Cp∗2 Ti(C2 H4 ) [29] was reacted with one equivalent of H2 S in THF solution in the presence of pyridine (py). Products (low yield) of this condensation reaction were found to be Cp*2 Ti(py) S and Cp∗2 Ti(SH)2 [24] together with ethane and a small amount of ethylene (Scheme 6) [30].

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CH2 Ti

H2S

Ti

H2S

SH

Ti

- CH4

Me

Scheme 8.

SiMe3 CH2

H 2C Ti

H2S

Ti

SH

H2S

SH

- Me3SiCH=CHSiMe3

C

Ti

C SiMe3

Scheme 9.

Bergman and Andersen studied the previously mentioned reactions due to dihydrogen activation of the terminal sulfido ligands [30,31]. The complex Cp*2 Ti(py) S which was prepared in high yield by the reaction with elemental sulfur (see below), reacts immediately with dihydrogen to form the Cp∗2 Ti(SH)(H) complex while losing a pyridine. The starting sulfido compound and the product of the reaction with dihydrogen are in equilibrium, which was studied via NMR spectroscopy. The presence of ␩2 -dihydrido intermediate Cp∗2 Ti(S)(H)2 was found. As already mentioned, the Cp*2 Ti(py) S complex was prepared in high yield by the equivalent reaction with elemental sulfur; however, after its formation this compound reacts further with either another equivalent of elemental sulfur or with ethylene sulfide upon forming the disulfido complex Cp∗2 TiS2 . This disulfido complex undergoes under dihydrogen atmosphere hydrogenation of its S S bond and Cp∗2 Ti(SH)2 was obtained as the sole product after 4 days at 70 ◦ C. In the presence of phosphine, sulfur abstraction is taking place and the Cp∗2 Ti(S)(H)2 complex is formed (Scheme 7). In 2011 Mach and coworkers reported the reactions of hydrogen sulfide with tucked-in titanocene complexes and their precursors.[32] Both the titanocene complex [Ti(III)␩5 :␩1 C5 Me4 (CH2 )(Cp*)] and its precursor Cp∗2 TiMe yielded the corresponding titanocene hydrosulfide Cp∗2 TiSH (Scheme 8). Titanocene doubly tucked-in compound [Ti␩4 :␩3 -C5 Me3 (CH2 )2 (Cp*)] and its precursor [Cp*2 Ti(␩2 -Me3 SiC CSiMe3 )] are forming dihydrosulfide complex Cp∗2 Ti(SH)2 (Scheme 9). The use of the tucked-in compounds is advantageous in the sense of “optimum atom economy” as no other products are formed during stoichiometric reactions; using the other reactants methane and hydrogenated bis(trimethylsilyl)ethyne derivatives are formed. The protolytic reactions of H2 S run also smoothly with the intramolecular Ti C bonds of the ansa-titanocene compounds (Scheme 10). In 2014 the synthesis of titanocene dihydrosulfides and monohydrosulfides containing one bulkier group (CH2 Ph or t Bu) on otherwise methylated cyclopentadienyl ligands was reported [33]. The titanocene dihydrosulfide compounds (Me4 Cp(CH2 Ph))2 Ti(SH)2 and (Me4 Cp(t Bu))2 Ti(SH)2 were obtained by addition of H2 S to the corresponding doubly tucked-in titanocenes (Scheme 11). The titanocene hydrosulfide compounds (Me4 Cp(CH2 Ph))2 Ti(SH) and (Me4 Cp(t Bu))2 Ti(SH) were obtained by H2 S-induced

Si

Si HC

Ti

H2S

CH2

Ti

SH

CH2

H2C

Si

Si

Si

Si

HC HC

Ti

H2S

CH2 CH2

Ti

SH SH

Si

Si

Scheme 10.

protonolysis of ␴-Ti–C bonds in (Me4 Cp(CH2 Ph))Ti(III)(␩5 :␩1 Me4 Cp(CH2 -o-C6 H4 )) and (Me4 Cp(t Bu)Ti(III)(␩5 :␩1 -Me4 Cp(CMe2 CH2 )), respectively (Scheme 12). Addition of excess of gaseous H2 S to a toluene solution of [Me4 Cp(SiMe2 NMe2 )]2 TiCl2 afforded [Me4 Cp(SiMe2 S-␬S)]2 Ti complex as the main product. This compound is of limited stability on air, releasing sulfane under contact with air humidity (Scheme 13) [34]. 2.1.2. Reactions with elemental sulfur Titanocene-sulfide complex Cp*2 Ti(py) S was formed quantitatively in reaction of Cp*2 Ti(C2 H4 ) [29] with 1/8 S8 in toluene solution in the presence of pyridine (py) overnight (Scheme 14). Based on MS analysis, the disulfide complex Cp∗2 Ti(␩2 -S2 ) was found as an intermediate [30]. On the other hand addition of two equivalents of elemental sulfur to toluene solution of Cp∗2 Ti(C2 H4 ) leads to the formation of complex Cp∗2 Ti(␩2 -S2 ) in good yield; however, a small amount of Cp∗2 Ti(␩2 -S3 ) complex was also isolated. The separation of compounds Cp∗2 Ti(␩2 -S2 ) and Cp∗2 Ti(␩2 -S3 ) from reaction mixture proved to be difficult. Nevertheless complex Cp∗2 Ti(␩2 -S2 )

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CH

H 2C

CH2

H 2C Ti

SH

Ti

+

5

+

2 H 2S

Ti SH

CH2

H2C Ti

CH2

H2C Ti

+

2 H2S

+

SH

Ti

SH

Scheme 11.

R C

CH2 Ti

+

H2S

Ti

H2S

SH

Ti

+

R

CH2

C R = CH2Ph t

Bu

Scheme 12.

Ti

CH2 CH2

+ 1/8 S8

py

Ti

S py

Ti

S py

1/8 S8 +

or

Ti - py

S

S S

Scheme 15. Scheme 13.

Ti

CH2 CH2

+ 1/8 S8

py

Ti

S py

in the presence of pyridine afforded Cp*2 Ti(py) S. Treatment of Cp∗2 Ti(␩2 -S2 ) complex with acetylene leads to vinyldisulfide compound, titanocene dihydrosulfido complex was formed when the solution was heated to 70 ◦ C under 1 atm of H2 and treatment of benzene solution and pyridine with trimethyl- or triphenylphosphine produced Cp*2 Ti(py) S and corresponding phosphine sulfide (Scheme 16) [30]. 2.2. Reactivity of titanocene hydrosulfides

Scheme 14.

can be prepared in a higher yield by reaction of Cp*2 Ti(py) S with one equivalent of elemental sulfur or ethylene sulfide (Scheme 15). The reactivity of Cp∗2 Ti(␩2 -S2 ) complex was studied towards various species and its reaction with ethylene complex Cp∗2 Ti(C2 H4 )

2.2.1. Coordination Due to their low acidity titanocene dihydrosulfides with Cp and (MeCp) were shown to have rather unreactive S H bonds. However their sulfur atoms were able to coordinate M(CO)4 species to form Cp2 Ti(SH)2 Mo(CO)4 complexes (Scheme 17) [35]. The reactivity of the SH group is affected by its coordination to the Mo fragment; the bridging SH ligand was found to be a better nucleophile

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Ti

S

S

Ti py

py

HC

S

1/8 S8

Ti

S

S

S

PR3, py

CH H2

Ti

S

S

S

Ti Ti

py

SH SH +

S=PR3 R = Me, H Scheme 16.

Ti

SH

Mo(CO)4(diene)

SH

H S Mo(CO) 4

Ti S H

Scheme 17.

compared with the terminal SH group in the corresponding titanocene dihydrosulfide complex. The synthesis of cubane-type sulfido clusters containing ruthenium atoms was reported by Hidai in 1999. The titanocene dihydrosulfide complex reacted at room temperature with tetrameric ruthenium chloro-bridged cluster containing pentamethylcyclopentadienyl ligands [Cp*RuCl]4 afforded bis(hydrosulfido)bridged titanium-ruthenium complex in good yield. The conversion of this complex into cubane cluster was treated by triethylamine followed by elimination of one cyclopentadienyl ligand from titanium and HCl (Scheme 18) [36,37]. 2.2.2. Deprotonation The unexpected elimination of a CpH molecule from Cp2 Ti(SH)2 upon deprotonation with NaH was reported by Kubas in 1996. As the main product of this reaction an anionic dimer was obtained, which was the first organometallic titanium compound equipped with terminal sulfido ligands (Scheme 19) [38]. The mono-deprotonation of Cp2 Ti(SH)2 with butyllithium in THF afforded anion [Cp2 Ti(SH)(S)]− acting as an intermediate in the general route for preparation of sulfido-bridged tetranuclear titanium-iridium [39] or rhodium [40] complexes. The formation of the intermediate anion [Cp2 Ti(SH)(S)]− is followed by hydrogen transfer from the hydrosulfido group to one cyclopentadienyl ring under formation of dianionic complex [Cp2 Ti2 (␮-S)2 (S)2 ]2− (Scheme 20). The titanocene dihydrosulfide complex Cp2 Ti(SH)2 is also able to protonate the methoxo ligand in rhodium complexes [Rh(␮-OMe)(diolefin)]2 [40,41] yielding the heterotetranuclear complexes containing CpTi fragment and bridging sulfido ligands (Scheme 21). This family of early-late transition metals complexes exhibit a good stability for the catalytic hydroformylation of olefins [42]. Generally the reactions of titanocene dihydrosulfide are very sensitive to moisture and other oxygen ligands leading to the formation

of oxo-sulfide titanium complexes. From this reason deprotonation reactions with iridium and rhodium complexes depend on the type of oxygen ligands in reactants. Thus in the case of acetylacetonate or 8-oxyquinolinate ligands the trinuclear heterobimetallic complexes were formed (Scheme 22) [40,43,44]. All these additive-deprotonation reactions staring from titanocene dihydrosulfide complex and rhodium and iridium compounds were summarized by Oro in 2003 [45]. A new synthetic method for preparation of azadithiolato complexes was investigated by Rauchfuss in 2011. The monomethyl substituted titanocene dihydrosulfido complex was reacted with cyclic imines (CH2 NR)3 where R = Ph, Me and CH2 Ph and as a result 2-aza-1,3-dithiolato chelate titanocene complexes (MeCp)2 Ti[(SCH2 )2 NR] were obtained (Scheme 23) [46]. The titanocene hydrosulfide Cp∗2 Ti(SH) and ansa-titanocene hydrosulfide ansa-((CH2 Me2 Si)Me4 Cp)2 Ti(SH) maintain the protolytic capability of their S–H moiety as demonstrated by reactions with [Ti(III)␩5 :␩1 -C5 Me4 (CH2 )(Cp*)] and ansa[Ti(III)␩1 :␩5 :␩5 -Me4 Cp(SiMe2 CHCH2 SiMe2 )Me4 Cp] (Scheme 24). Both reactions afforded green paramagnetic sulfides (Cp∗2 Ti)2 S and ansa-((CH2 SiMe2 )Me4 Cp)2 Ti}2 S in virtually quantitative yields [32]. Paramagnetic sulfides could be also obtained by utilizing half molar equivalent of H2 S to Ti(III)␩5 :␩1 -C5 Me4 (CH2 )(Cp*) or Cp∗2 TiMe and ansa-Ti(III)␩1 :␩5 :␩5 -Me4 Cp(SiMe2 CHCH2 SiMe2 )Me4 Cp, respectively. Reactions between a titanocene hydrosulfide and a titanocene alkyl can be used for the preparation of sulfides with diverse metallocene moieties. This was exemplified by reacting ansa-((CH2 Me2 Si)Me4 Cp)2 Ti(SH) with Cp∗2 TiMe which resulted in the formation of (ansa-(CH2 SiMe2 Me4 Cp)2 TiSTiCp∗2 ) (Scheme 25). 2.2.3. Photolysis A recent study of synthesis and stability of pentamethylsubstituted titanocene hydrosulfides Cp∗2 Ti(SH)2 and Cp∗2 Ti(SH) has indicated their photosensitivity [32]. Exposure to sunlight of red solutions of Cp∗2 Ti(SH)2 and blue solutions of Cp∗2 Ti(SH) either in hexane or toluene led to changing their color to brown within several hours. Monitoring their NMR spectra has indicated the possibility of formation of some highly reactive [Cp*TiS] species in both cases. The sunlight photolysis of Cp∗2 Ti(SH)2 in toluene appeared to be surprisingly selective, affording titanium-sulfide clusters [(Cp*Ti)4 (␮3 -S)3 (␮2 -S)3 ] and [(Cp*Ti)6 (␮3 -S)8 ] in 50% and 3% yield,

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Scheme 18.

Ti

SH

NaH, THF

S

Ti

SH

S

S

2Na+.4THF + CpH +

Ti

H2

S

Scheme 19.

2-

-

Ti

BuLi

SH SH

Ti

SH S

S

Ti

- CpH

Ti

S

S

S

Scheme 20.

Scheme 21.

2 [M(acac)(diolefine)] - CpH, - Hacac Ti

SH SH

Ti

O

S S

O

M M

M = Rh, Ir

2 [M(quinol)(diolefine)]

N

- CpH, - Hquinol

Ti O

S S

M M

Scheme 22.

respectively [47]. In hexane, the system becomes heterogeneous soon because of the low solubility of [(Cp*Ti)6 (␮3 -S)8 ] as the main product. On the other hand, isolation of products in early stages of photolysis allowed to detect a trace amount of [(Cp*Ti)3 (␮3 S)(␮2 -S)3 ] whose structure is of importance for understanding of the mechanism of Cp∗2 Ti(SH)2 photodecomposition (Scheme 26).

In summary, the formation of [(Cp*Ti)4 (␮3 -S)3 (␮2 -S)3 ] cluster as the main product of photolysis Cp∗2 Ti(SH)2 fulfils the stoichiometry of equation. hv

4CP∗2 Ti(SH)2 −→4CP∗ H + 2H2 S + (Cp∗ Ti)4 S6

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R N

Ti

R

SH

N

N

R

Ti

- MeCN

SH

S

R

N

S

Scheme 23.

Scheme 24.

Scheme 25.

Cp*

Cp* S Cp* S Cp*

Ti S

Ti S

Cp*

Ti

Ti

S

S

S Ti S

Cp* Cp*

Ti S S

S S

Cp* Ti Ti Cp*

Ti Ti

S S

Ti

S

S Cp*

S Cp*

Ti

Ti Cp* S

Cp* Scheme 26.

A clue to the mechanism of photolytic decomposition of Cp∗2 Ti(SH)2 (Scheme 27) can be sought in recent results dealing with a detailed investigation of photolysis of the closely related [Cp∗2 Ti(OH)2 ], which was performed by Rosenthal [48] and Beweries [49] in an effort to find a photolytically driven decomposition of water into elements. The overwhelming formation of Cp*H and H2 S in photolysis Cp∗2 Ti(SH)2 indicates that the mechanism is to be very similar. On the other hand, studies were conducted not only on methyl substituted cyclopentadienyl ligands. The properties of permethylated metallocene compounds can further be finely tuned by substituting one methyl group on both cyclopentadienyl rings for a different substituent. In 2014 Mach reported the

photodecomposition of titanocene dihydrosulfides containing one bulkier group R on otherwise methylated cyclopentadienyl ligands compounds [(C5 Me4 R)2 Ti(SH)2 ] (R = benzyl and tert-butyl) [33]. The aim was to establish the influence of the mentioned bulky substituents on the sunlight photolysis of substituted titanocene dihydrosulfides. Surprisingly, the photolysis of substituted titanocene dihydrosulfides exhibits a striking difference in the structure of products and the efficiency of process (Scheme 28). The photodecomposition of [(C5 Me4 CH2 Ph)2 Ti(SH)2 ] proceeded analogously to that of Cp∗2 Ti(SH)2 under elimination of cyclopentadiene and hydrogen sulfide affording the cage product [(C5 Me4 CH2 Ph)Ti]4 S6 in 48% yield. In contrast, compound [(C5 Me4 t-Bu)2 Ti(SH)2 ] afforded compound [(C5 Me4 t Bu)2 TiSH] in

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Cp*

Cp* IV Cp*2Ti(SH)2

III 2 Cp*Ti(SH)2

[Cp* ]

III Cp*Ti

[ H ]

S

Cp*Ti(SH)2

SH

Ti

S

[ H ]

SH

SH

IV Ti

S

IV Ti

Cp*

SH SH HS III SH Ti Cp*

Cp*

Cp* Ti III SH HS Ti Cp* HS HS S S

Ti S

Ti

Cp*Ti(SH)2 [ H ]

S

SH HS SH

S

Ti

Ti

- 2 H 2S

Cp*

Ti S

Ti S Cp* S

S

2 H 2S

S

Cp*

Cp*

Cp*

[ H ]

Ti

Cp*

Cp*

Ti

Cp*

S

Cp*

Cp*

Ti

Ti

S

Cp*

S

S Ti

S S

Cp*

Ti Ti Cp*

S

Ti

S S

S

S Cp*

Ti

S

2

S

Cp*

Ti

Ti Cp* S

S

Ti Cp* Scheme 27.

Cp'

R

Ti S Cp' Cp'

Ti S

S Ti S

S Ti

ν hν

SH Ti

Cp'

hν ν

Ti

SH

SH

S

R R = t-Bu, CH2Ph

Cp' =

Scheme 28.

87% yield after exposing to sunlight for only one hundredth of the time for the large scale photolysis of [(C5 Me4 CH2 Ph)2 Ti(SH)2 ]. The overwhelming photodissociation of SH radical has been tentatively attributed to the steric effect of tert-butyl substituents.

Ti

Cl Cl

+ Li2S2/3S

THF

Ti

S S S

S

S 3. Titanocene sulfides Ti-(S)n Scheme 29.

3.1. Preparation of titanocene sulfides Ti-(S)n The history of titanocene-sulfide chemistry begins with the discovery of the titanacyclohexasulfane complex Cp2 TiS5 reported in 1966 by Samuel [50] and independently by Kopf [19,51]. The Xray structures of both of its crystallographic modifications revealed the chair structure of its six-membered ring TiS5 [52–54]. Later, the titanocene pentasulfide complexes with substituted cyclopentadienyl rings were reported: for methylsubstituted compound [55], for trimethylsilylsubstituted cyclopentadienyl ring [56] and for ansa-dimethylsilylbridged complexes [57]. In all cases the same cyclohexane-like chair structural configurations were observed. The replacement of one selenium atom in titanocene pentaselenide complex led to the preparation of titanocene sulfur-selenium chelate compound with similar geometric parameters [58].

There are several synthetic routes already available for the preparation of Cp2 TiS5 complex at present time [59–62]. As an example, the reaction of Cp2 TiCl2 with sulfur transfer reagent Li2 S2 /3S [63] in THF afforded the Cp2 TiS5 complex in 60% yield (Scheme 29) [18]. The chemistry of unsubstituted and monomethyl substituted titanocene pentasulfide complexes, together with their reactions with nucleophiles and organophosphines in the presence of acetylene were studied by Rauchfuss in 1987 [64–66]. The reactivity including dimerization and intermolecular rearrangement were also being reported. The structure of this polysulfido chelated complex Cp2 TiS5 together with the presence of the six-membered titanapentasulfido moiety appeared to be suitable either for the

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S S Ti

S S S

Ti

S

S S S

S S

S Ti

S S

S Scheme 30.

Ti

S

CO + CO

S

S

- 2CO

Ti S

Scheme 31.

thermally induced insertion reaction or migration of organic substrates to the sulfur atoms (Scheme 30) [66]. The cleavage of the TiS5 ring with aqueous (NH4 )2 S was used for the insertion of CR2 unit into the titana-sulfido ring through nucleophilic addition of ketones or alkylhalides to Cp2 TiSx 2− species [65]. The first introduction of the dimethylsilanedithiolate ligand to the monomethylsubstituted titanocene fragment was achieved by treating the THF solution of Me2 SiCl2 with two equivalents of Li2 S followed by reaction with (MeCp)2 TiCl2 . The (MeCp)2 TiS2 SiMe2 complex was shown to act as an efficient, nonreductive atom- or chelate-transfer agent [67]. In 1992 Steudel found that titanocene carbonyl complex Cp2 Ti(CO)2 activated the S S bond in cyclic organic disulfides and that this way should be used for synthesis of titanocene chelating ligands containing sulfur and methylene groups (Scheme 31). Various sulfur chlorides or bissulfenyl chlorides may be subsequently used for preparation of corresponding sulfur containing organic ligands [68]. The same research group has also shown the ability of titanocene carbonyl complex Cp2 Ti(CO)2 in activating sulfur monocycles (S6 , S7 and S8 ) [68]. The products of this activation depended on the number of sulfur atoms and also on reaction conditions. The reaction with S6 runs at 0 ◦ C and yields titanocene complex having eight sulfur atoms, confirmed by its reactions with sulfur chlorides SCl2 and S2 Cl2 resulting in the formation sulfur homocycles S9 and S10 , respectively. On the contrary, the analogous reaction with S8 proceeds at higher temperature due to the high activation energy of the S8 ring. This reaction afforded the previously mentioned titanocene complex Cp2 TiS5 . The presence of titanocene fragment was found to be useful for synthesis of the cyclic sulfurimides S7 NH, S8 NH, S9 NH, S11 NH and small sulfurimide heterocycles S5 NR, S6 NR, and S7 NR [69–71]. As an example, titanocene complexes with cycloheptasulfurimides ligands were prepared by the reaction of titanocene dicarbonyl Cp2 Ti(CO)2 and S7 NH and S7 NCH3 via insertion of titanocene unit into S-S bond and liberation of two molecules of CO (Scheme 32). Both reactions led to the clean formation of one product only, which has been suggested either by high-resolution HPLC, and also by 1 H NMR, which exhibited only one signal for the methyl group in the Cp2 Ti(S7 NCH3 ) complex. The crystal structures surprisingly showed that the compound with S7 NH ligand featured disulfide and pentasulfide bridges, while the S7 NCH3 analogue exhibited trisulfide and tetrasulfide ones. An explanation for the

exclusive formation of one single isomer during each reaction and also the mutual difference of the isolated products could not be equivocally ascribed either to steric or to electronic effects. One thing is certain that nitrogen atoms are void of coordination to the titanocene fragment due to near planar geometry at the nitrogen atoms and also to the partial delocalization of nitrogen “lone pair” to vacant sulfur orbitals. The above mentioned complexes were treated with SCl2 and S2 Cl2 —the corresponding amides were obtained [69]. The unsubstituted titanocene carbonyl complex was reacted with perthiophosphinic acid anhydrides R2 P2 S4 to form thiophosphoryl complexes of bis(cyclopentadienyl titanium (Scheme 33). The products of these syntheses depend on the nature of the R substituent and allowed for the preparation of two types of titana-ring species—TiS2 P2 (for anisolyl and phenetolyl) and TiS2 P (for anisolyl, tert-butyl and cyclohexenyl) [72]. In the case of anisolyl substituted perthiophosphinic acid anhydrides R2 P2 S4 , both types of products were isolated. The solid-state structural studies combined with 31 P NMR experiments indicated some interrelationship, equilibrium and interchange of the RPS fragment between both types of products Cp2 TiS4 P2 R2 and Cp2 TiS3 PR in solution. This interconversion was proposed to occur through an unsymmetrical isomer TiS2 P2 ring accompanied with a phosphorus(III) intermediate (Scheme 34). The authors concluded that steric effects of the substituents were mostly responsible for the formation of disparate titana-rings. The same complexes were prepared either by using titanocene dichloride and reduced R2 P2 S4 obtained via its treatment with LiHBEt3 or by the metathesis reaction with Li2 S3 PR [72]. Titanocene complexes containing secondary phosphine sulfide ligand Cp2 Ti(S2 PR2 ) where R = Me, Et, Ph, Cy were reported by Stephan in 1987 [73]. These complexes were prepared by several routes starting from Ti(IV) and Ti(III) precursors Cp2 TiCl2 , [Cp2 TiCl]2 , and Cp2 Ti(CO)2 ; in the latter case a photochemical activation was necessary (Scheme 35). The formation of titanocene complexes Cp2 TiSPR2 from Cp2 TiCl2 and LiSPR2 precursors was studied by EPR spectroscopy due to the observed quick decomposition of titanocene(IV) species via reduction. The titanocene(III) complexes Cp2 TiSPR2 are unstable and undergo conversion to other titanocene(III) complexes Cp2 TiS2 PR2 rapidly. The application of [Cp2 TiCl]2 together with LiSPR2 led to the synthesis of Cp2 TiS2 PR2 complexes without the formation of Cp2 TiSPR2 species, which is caused by the facile abstraction of a sulfur atom from LiSPR2 . The direct synthesis of Cp2 TiS2 PR2 complexes from [Cp2 TiCl]2 by a simple nucleophilic substitution mechanism was described using LiS2 PR2 as precursor. The novel heteroleptic complexes of unsubstituted titanocene dithiolene on a thiazole backbone were synthetized by Lorcy. Several approaches were developed in order to prepare the dithiolene ligands which were reacted with titanocene dichloride. Different substitution on the nitrogen atom in thiazole core were studied giving space for other functionalities through this atom (Scheme 36) [74].

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Ti

CO

S7NH

CO

- 2CO

S

S

H N

Ti S

S S

H N

S

S

SnX2

S

11

S

S

- Cp2TiX2

S

S

S

S (S)n

n = 1, 2 or 4 X = Cl or SCN CH3

Ti

CO

S7NCH3

CO

- 2CO

S

S

S N

Ti

CH3

S

N

S S

S

- Cp2TiX2

S

S

S

SnX2

S

S

S S (S)n

Scheme 32.

S S

P

R

S

P

S

Ti CO

Ti

S

R +

P

CO

S

S

R = 4-C6H4OCH3 = 4-C6H4OC2H5

R

P S

R S

S P

Ti

R

S

R = 4-C6H4OCH3 = tBu = 3-cyclohexenyl

Scheme 33.

S

S S

S +

P

Ti S

S RPS

P

S

R

Ti

Ti

R

S

P R

S S

S

R

S

P

P

S

P

S

P

R

Ti

R

S R

R = 4-C6H4OCH3, 4-C6H4OC2H5, tBu, 3-cyclohexenyl Scheme 34.

With the aim to prepare titanocene complexes containing a bidentate aromatic ligand Woollins reported in 2006 the activation of naphthol[1,8-cd]-1,2-dithiole with superhydride LiBEt3 H followed by reaction with one equivalent of Cp∗2 TiCl2 (Scheme 37) [75]. The similar titanocene dithiolato complexes with different naphthalene and biphenyl backbone Cp2 TiL were reported by Woollins [76]. The steric and electronic effects of substituents on the naphthalene backbone were studied in solution by temperature-dependent 1 H NMR spectroscopy, which suggested the envelope conformation of the six-membered TiS2 C3 chelate rings to undergo rapid inversion. For all synthesis titanocene dicarbonyl complex was used as a starting material. Moreover the using of naphtho-[1,8-cd]-1,2-dithiole-1,1,2-trioxide and dibenzo[ce]-1,2-dithiine-5-oxide derivatives resulted in thiolato sulfinato O-bond (Scheme 38). An unusual metallacycle 1,3,2,4-dithiaboratitanetane was reported by Okazaki in 1995. This air- and moisturestable titanocene compound was prepared by dilithiation of dimerkaptoboprane TbtB(SH)2 (Tbt = 2,4,6-tris[bis(trimethylsilyl)

methyl]phenyl) followed by transmetalation reaction of titanocene dichloride Cp2 TiCl2 (Scheme 39) [77,78]. The used dimercaptoborane TbtB(SH)2 containing the sterically demanding Tbt group was proved to be a useful reagent for the preparation of highly reactive sulfur-containing organo-boron complexes comprising of group 4, 14 and 15 elements. The monomeric aluminum dihydrosulfide species [LAl(SH)2 ] was converted into lithium salt and reacted with titanocene dichloride in order to prepare heterobimetallic sulfides containing aluminum in AlS2 Ti ring, having a similar structure as the previous complexes with boron [79]. Following the motivation for the synthesis of new stechiometric sulfides, different mixed complexes of titanocene selenide sulfides [Cp2 TiSex S5−x ] were synthetized from titanocene dichloride and lithium polyselenide and polysulfide mixtures. Upon using initial molar quantities of selenium and sulfur close to each other, mixtures of products were obtained, with the main components being [Cp2 TiSe4 S], [Cp2 TiSe3 S2 ], [Cp2 TiSSe3 S] and [Cp2 TiSe2 S3 ]. On the other hand, analogous reactions of titanocene dichloride with mixtures consisting of either molar excess of sulfur or selenium led

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Ti

Cl

LiSPR2

SPR2

Ti

S

Ti

PR2

S

hν ν SPR2H or (SPR2)2

LiSPR2 or LiS2PR2

Cl Ti

SPR2

Ti

SPR2

Cl

CO

Ti

Ti

CO

Cl

Scheme 35.

SLi

S

S Cp2TiCl2

S N

S

CsOH, H2O

S

Ti

SLi

S

S

Cp2TiCl2

N

N

R

R

S(CH2)2CN

S

S(CH2)2CN

H3 C

R = Me, Ph Cp2TiCl2

THF, Δ

S

S

S S

CH3 N S

Zn N

S

(NEt4)2

S

S

H3C Scheme 36.

SLi

S

S

SLi S

Cp*2TiCl2

2 Li(BEt3)H

Ti S

Scheme 37.

O

O

O

S

O

O S

O

O S Ti S

Ti

CO CO

O

S S

O Ti

O S S

Scheme 38.

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3.2. Reactivity of titanocene sulfides

Ti

Cl

S

TbtB(SLi)2

Ti

B

Cl

3.2.1. Modification of sulfide moiety Monomethyl substituted titanocene pentasulfide complex (MeCp)2 TiS5 undergoes desulfurization in the presence of tree equivalent of PBu3 to form unique dimeric complex [(MeCp)2 TiS2 ]2 containing the heterocyclic Ti2 S4 moiety (Scheme 41) [20]. The formation of the same dimer was obtained from (MeCp)2 TiS5 by transferring the S2 fragment to [Ir(dppe)2 ]Cl (dppe = 1,2-bis(diphenylphosphino)ethane). The crystalline [Ir (dppe)2 S2 ]Cl observed and isolated in a high yield. [(MeCp)2 TiS2 ]2 was isolated after its chromatographic workup in the form of a red crystalline material. The structure of this complex equipped with two S S bonds was confirmed by cleavage reactions with dimethyl acetylenedicarboxylate which form dithiolene complex (Scheme 42) [61]. Similarly, two equivalent of PPh3 were used in reaction with titanocene pentasulfide for preparation of diamagnetic dimeric complex containing 1,5-dititanacyclohexasulfane (Scheme 43) [84]. The solid-state structure of these compounds was proved by X-ray crystallographic analysis of the methyl-substituted analogue [(MeCp)2 TiS3 ]2 . The reactivity of this type of compounds was verified on the unsubstituted analogue (Scheme 44). Heating of [Cp2 TiS3 ]2 in benzene forms a red pentasulfide together with some insoluble polysulfide species. Reactivity towards acetylene affording the dithiolene complex similar as was observed for methyl-substituted complex. Protonolysis of [Cp2 TiS3 ]2 with anhydrous HCl provides equimolar mixture of titanocene dichloride and pentasulfide complex. However, in all cases the loss of one or more sulfur atoms was found.

Tbt

S Scheme 39.

Ti

Cl

Na2(S)3(CH3As)2

Cl

THF

Ti

S

As As

S

S

Scheme 40.

to the formation of [Cp2 TiS5 ] or [Cp2 TiSe5 ] complexes. Apart from their synthesis, the crystal structures of these compounds and their 77 Se NMR were studied [80,81]. The titanocene arsenic-sulfur complex Cp2 Ti(-S(CH3 )AsS(CH3 ) AsS-) with a similar structure (chair like conformation) as the allsulfur titanacycle Cp2 TiS5 was reported by Rheingold in 1997. This is in contrast with the all-organoarsenic complexes Cp2 Ti(RAs)3 which form only the four-membered titanacycle ring. The complex was prepared from the sodium-reduced form of the mixed trimer and tetramer of cyclo-(CH3 AsS)3,4 and titanocene dichloride in THF (Scheme 40) [82]. The present polychalcogenide titanacycle has been excellent reagents for synthesis of main-group heterocycles. The dinuclear titanocene complex with two 2 -S sulfidobridges between Cp2 Ti and CpTiCl fragments and one 2 -␩1 -S bridge connecting two Cp2 TiCl moiety were obtained from reaction of titanocene dichloride with S(SiMe3 )2 [83].

Ti

S S S

+ 6 PBu3

S

3.2.2. Chelate transfer processes The titanocene dithiolene complexes were study due to transferring their chelates to transition-metal and organic halides under mild conditions. In 1982 Rauchfuss used these ligand-exchange reactions for preparation of new complexes of Ni, Pt and Rh [20,61]. The thiophosphoryl complexes of titanocene as reagents were used

CH2Cl2

Ti

S

S

S

S

+ 6 SPBu3

Ti

S Scheme 41.

Ti

S

S

S

S

Ti

+ 2 RC

S

CH2Cl2

CR

R

Ti S

R

R = CO2CH3 Scheme 42.

2

Ti

S S S

S

+ 4 PPh3

CH2Cl2

Ti

S

S S

S S

S

Ti

+ 4 SPPh3

S

Scheme 43.

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Ti

S S S

S

S Δ, C6H6

S

Ti

S S

S S

RC

Ti

Ti

S

R = CO2CH3

S

R

S

CR

R

HCl

Ti

Cl Cl

Ti

+

S S S

S

S Scheme 44.

S S

R

P

Ti S

P

S + NiCl2(dppe)

(dppe)Ni

S

S + Cp2TiCl2 +

P S

1

/3 (RPS)3

R

R Scheme 45.

Ti

Cl

Ti

+

(RS)2

Cl

2

Ti

SR Cl

R = Me, Et, Bz, Ph Scheme 46.

for synthesis of sulfur compounds via chelate transfer processes two years later for nickel species (Scheme 45) [72]. 4. Titanocene thiolates Ti-(SR)n 4.1. Preparation of titanocene thiolates Ti-(SR)n 4.1.1. Reactions with dialkyl or diaryl sulfides The reactions of the Ti(III) complex (Cp2 TiCl)2 with dialkyl and diaryl disulfides were used for preparing titanocene monosulfides in high yield (Scheme 46) [85]. Similar oxidative addition was employed to prepare titanocene complexes containing aryloxo and alkyl/arylsulfide CpCp Ti(Oar)(SR) [86]. In 1990 Grubbs used titanocene methylidene compound Cp2 Ti CH2 .PMe3 in reactions with various sulfur-containing compounds (alkene sulfides, triphenylphosphine sulfide) including elemental sulfur to prepare titanocene ␩2 -thioformaldehyde trimethylphosphine compounds (Scheme 47) [87]. Interestingly, this complex (titanocene ␩2 -thioformaldehyde trimethylphosphine) did not undergo insertion reaction with various triple (qC C and C N) and double bonds (C C, C O, C S). On the other side, the protolysis of this compound induced by a number of proton sources (HCl, CF3 CO2 H, H2 O, CH3 OH) was successful (Scheme 48). Disproportionation of this species after hydrogenolysis resulted in the formation of Cp2 Ti(SMe)2 together

with some unidentified products. Reactions with methyl iodide produced cationic species with the iodide acting as a counterion. In addition silver salt and copper chloride was used to metathesize the iodine anion, while reactions with acetylchloride resulted in the formation of titanocene chloride. When trimethylene sulfide with a lower ring strain was used, a six-membered ring is formed and a different product was obtained (Scheme 49). The mechanism for the formation of these products was investigated using NMR spectroscopy and a biradical mechanism involving an initial electron transfer was suggested.

4.1.2. Metathetic reactions Other frequently used method for preparation of titanocene thiolates is the metathetic reaction of titanocene precursors. The most frequently used precursor is titanocene dichloride, reacted with alkali metal salt of a sulfide or with thiols in the presence of base (Et3 N). These stoichiometric reactions 1:1 led to the formation of Cp2 Ti(Cl)(SR) [88–97], Cp2 Ti(Me)(SR) [88,90,97], Cp2 Ti(NCX)(SPh) [98,99] or Cp2 TiS2 C2 (CN)2 [100,101] compounds. Using a molar ratio 1:2, bis-substituted complexes were prepared [35,102–111]. Reaction of titanocene dichloride Cp2 TiCl2 with cysteamine HSCH2 CH2 NH2 results in formation of ionic species [Cp2 Ti(Cl)SCH2 CH2 NH3 ]Cl, that undergoes further Cp cleavage to form air-sensitive polymer [CpTi(Cl)SCH2 CH2 NH]n [91]. Titanocene dichloride reacts also with alkali aminothiophenolates NaSC6 H4 NH2 -o or LiSC6 H4 NH2 -p, depending on used stoichiometry one or both Cl atoms can be substituted and mono or bis(aminothiophenolato) complexes can be prepared [90]. Similarly complexes with thiocresolate ligands Cp2 Ti(Cl)(SC6 H4 CH3 -o) and Cp2 Ti(Cl)(SC6 H4 CH3 -p) were obtained from the reactions of Cp2 TiCl2 with NaSC6 H4 CH3 -o or LiSC6 H4 CH3 -p, respectively. To form methyl analogues, the Cp2 Ti(Cl)CH3 complex can be used [88]. The substituent effects to the conformations of bent metallocenethiolate complexes and their heterobimetallic analogues were studied by Darensbourg [109].

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S

CH2

Ti

15

PMe3

+

S

Ti

+

R

PMe3

R Scheme 47.

SCH3

Ti

H2

Ti

SCH3

S

2 CF3CO2H

Ti

O2CCF3 O2CCF3

PMe3 O CH3I

CH3CCl

+

O Ti

I

-

PMe3

CH3

Cl

SCH3

Ti

S

Scheme 48.

Ti

CH2

+

S

- PMe3

S

Ti

PMe3 Scheme 49.

CH3

Ti

Cl Cl

S +

S

SCH3

X

X

Ti S

-

S Cs

+

- 2 CsCl

S

S

S 2

X = O, S, Se Scheme 50.

The mixed ligand complex of titanium containing two thioether thiolate ligands (4-methylthio-1,3-dithiole-2-one-5thiolate (dmidCH3 )) and two monomethylated cyclopentadienyl ring were prepared from the corresponding titanocene dichloride (Scheme 50) [112–114]. The synthesis, structure and properties of pentamethylsubstituted titanocene dithiolene complexes were studied by Guyon in 1995 [115]. Dimeric titanocene Ti(III) chloride reacts with NaSCH3 in THF under mild conditions forming [Cp2 Ti(SCH3 )]2 in a high yield. This titanium thiolate was used for reactions with Rh complexes and thiolate abstraction and ligand exchange have been observed (Scheme 51) [116]. Similar dimeric compound containing phenyl substituent [Cp2 Ti(SPh)]2 was observed as the product of thermally induced decomposition of Cp2 Ti(CH2 SPh)2 in toluene at 100 ◦ C during which the methylene group was liberated (Scheme 52) [117].

In 2000 Taylor reported the synthesis of titanocene dithiophenolates via reaction of two equivalents of LiSPh with titanocene dichloride having on one cyclopentadienyl ring t Bu substituent (Scheme 53). Molecular structures of titanocene compound as well as zirconium and hafnium were determined showing intriguing difference between the titanium and zirconium analogues considering the orientation of the thiophenyl groups relative to the t Bu substituent [118]. A series of titanocene compounds containing trimethylsilyl substituted cyclopentadienyl rings was prepared by Delgado in 1994 by reacting titanocene dichloride with thiols in the presence of base [119] in order to prepare heterobimetallic Ti-M (M = Pd, Pt) complexes [120]. New titanocene complexes containing diphenylthiophosphorylcyclopentadienyl and trimethylsilylcyclopentadienyl ligands were reported by the same authors in 1998 [121]. The diphenylthiophosphorylcyclopentadienyl derivatives

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Cl

Ti

Ti

S

Ti

+ 2 NaSCH3

Rh Rh

S

Ti

S

Cl

Cl

Rh

Cl Rh

S

Scheme 51.

Ph Cl

Ti

Ti

+ 2 LiCH2SPh

CH2SPh

Δ, 100 °C Ti

CH2SPh

Cl

S

Ti

S Ph

Scheme 52.

Ti

Cl

Ti

+ 2 LiSPh

SPh SPh

Cl Scheme 53.

were obtained by the reaction with S8 acting as an oxidizer (Scheme 54). The presence of the trimethylsilyl group in titanocene complexes increases the stability of these compounds in solution when these compounds are compared with their homoleptic derivatives containing only one diphenylphosphinosubstituent. The replacement of halide by alkyl- or arylthio groups on transition metals was studied using methyl- or phenylthiotrimethylsilane by Jenkins and Abel in 1968 [122]. They demonstrated the formation of titanocene thiolates Cp2 Ti(SCH3 )2 and Cp2 Ti(SPh)2 from Cp2 TiCl2 in good yield (Scheme 55).

Ph Ph P S

Ph

Ph

P Ph

P Ph Ti

The lithium silanethiolates LiSSiMe2 R undergo transmetallation reaction with titanocene dichloride and NiCl2 (dppe) complex to afford Cp2 Ti(SSiMe2 R)2 and Ni(SSiMe2 R)2 (dppe) compounds. The latter one readily reacted with CpTiCl3 to produce the Ti-Ni heterometallic compound CpTiCl(S)2 Ni(dppe) [123]. The stereochemical characterization of Cp2 Ti(SPh)2 and the geometrical effect of an unpaired electron on the titanocene fragment was reported by Dahl in 1976 [54,124]. The crystal structures of titanocene compounds with bis(ethylthiolato) ligand Cp2 Ti(SC2 H5 )2 [125] and bis(tert-butylthiolato) ligand Cp2 Ti(St Bu)2 [126] were published eight and twenty years later, respectively. The bidentate unsaturated 1,2-dithiolate ligands were reacted with one equivalent of titanocene dichloride in the presence of triethylamine to yield titanocene dithiolate complexes. The same method was used for preparation of titanocene bis(dithiolate) complexes which can be subjected to acidic degradation with HCl/CHCl3 to yield analytically pure bis(dithiolate ligand) [127–130]. The crystal structures of titanocene dithiolato complexes were also reported [131,132]. The P-H functionalized 2-(phosphanyl)ethanethiols RHPCH2 CH2 SH afforded under reaction with titanocene dichloride in the

Cl

2HSPh, 2NEt3

Ti

SPh SPh

Cl

SiMe3

SiMe3

1

/8 S8

Ti

SPh SPh

SiMe3

Scheme 54.

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Ti

Cl

SR

Ti

+ 2 RSSi(CH3)3

17

ClSi(CH3)3

+ SR

Cl Scheme 55.

Ti

Cl +

Cl

S

SH

2 NEt3

SH

- 2 NHEt3Cl

Ti S

Scheme 56.

Ti

CO

- CO

+ 2 RSSSR

Ti

R

SSR

Ti

SR

CO

SPh + PhSSPh

C

Ti C

NR'

R

NR' Scheme 57. Scheme 59.

presence of Et3 N a diastereomeric mixture of rac- and mesotitanocene dithiolates [133]. Similar titanocene complexes bearing hybrid bidentate ligands with sulfur and dimethyl-substituted phosphorus as donor atoms were prepared from titanocene dichloride and corresponding lithium salts [134]. The synthesis of different heterocycles from titanacycles via reaction with electrophilic reagents such as SCl2 , S2 Cl2 , PhPCl2 , Ph2 GeCl2 were reported by Choi. Titanocene complexes containing a titanacyclopentane ring were prepared from titanocene dichloride and variously substituted dithiolate precursors [135–137]. The reaction of ␣,␣ -dimercapto-o-xylene (H2 dmox) with unsubstituted and monomethylsubstituted titanocene dichlorides in the presence of Et3 N produced the thiolate complexes Cp2 Ti(dmox) and (MeCp)2 Ti(dmox) which were used for investigation of cytotoxycity against several human cancer cell lines (Scheme 56) [138]. The tetralithiated salt of tetrathiafulvalene tetrathiolate in THF was reacted with isopropyl substituted titanocene dichloride to afford homobimetallic compound (i PrCp)2 Ti[S2 TTFS2 ]Ti(i PrCp)2 . This building block material was studied for its electronic, magnetic and optical properties [139–141]. Similar dinuclear

chelate-bridged titanocene complexes with SiMe3 substitution on the cyclopentadienyl ring were reported for 1,2,4,5tetramercaptobenzene [142]. The “one pot” synthetic method for preparation of organometallic compounds incorporating the titanocene fragment “Cp2Ti” is the reduction of titanocene dichloride with magnesium followed by in situ treatment of the reaction mixture in the presence of some RSSR species. This method was used for the preparation of titanium thiols having the general formula Cp2 Ti(SR)2 for R = 1-C10 H7 , CH2 CHCH2 , n-C5 H11 , CH2 CO2 Me, CH2 CO2 Et, (C5 H5 )Fe(C5 H4 CH2 ) by Song in 2002 [143]. Titanocene alkynethiolato complexes containing unsubstituted and monomethylsubstituted cyclopentadienyl rings were prepared by the reaction of the corresponding titanocene dichloride with two equivalents of LiSC CR where R = Ph or t Bu. The reaction of this product with Ni(cod)2 complex was tested and a linear trimetallic complex Ti2 Ni was obtained, in which the Ni atom is connected to the two titanocene alkyne-thiolato units through interactions with thiolate sulfur bridges [144]. The first thioalkyne derivatives of titanocene with SiMe3 group on cyclopentadienyl ring were

Ti

Ti

SH

SS

R

SS

R

R = CHMe2

+ 2 R-S-imide

SH

Ti

SH SSS

R

R = p-C6H4Me, C6H5 Scheme 58.

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S Ti

CS2

C Ti

1

/2

+

S Scheme 60.

R1

O R2 R3

Ti

+

R1

SPh

R2

R4

Ti

R1

R3 R4

OH

SPh Scheme 61.

reported by Delgado in 1998 and their reactivity towards Mo, Pt and Pd metal complexes containing labile ligands was investigated [145]. Pentamethylcyclopentadienyland cyclopentadienyltitanium(IV) complexes with C8 H4 S8 ligand were prepared by reaction of titanocene dichlorides with lithium or amonium salt of S8 H4 S8 ligand and their electrical properties were studied [146]. The titanocene derivatives of ring-opened dibenzothiophene (DBT) were synthetized from in situ generated lithium salts of DBT and led to the preparation of titanocene complexes with different metallacycles [147]. The ability of the titanocene Ti(II) complex Cp2 Ti(CO)2 to activate sulphur [54], disulfides [148] and trisulfides species was demonstrated by Shaver. Cp2 Ti(CO)2 undergoes oxidative additions of trisulfides RSSSR to form Cp2 Ti(SR)(SSR) complexes, where R = CHMe2 , CMe3 , CH3 Ph, and 4-C6 H4 Me (Scheme 57) [149]. The presence of the disulfanido ligand was confirmed by the reaction with PhCH2 Br to give CpTiBr2 , PhCH2 SR, and PhCH2 SSR. These complexes desulfurized slowly in C6 D6 solution to give Cp2 Ti(SR)2 . This process accelerated in the presence of PPh3 . Analogous compounds were prepared when the Cp2 Ti(SH)2 complex was treated with two equivalents of RS-imide where imide = phthalimide and succinimide, R = CHMe, p-C6 H4 CH3 , C6 H5 (Scheme 58) [150]. The Cp2 Ti(SSR)SH complex was suggested as the most reasonable intermediate for all mentioned RS-imides. The symmetrical disulfane Cp2 Ti(SSR)2 for R = CHMe and the unsymmetrical trisulfanes Cp2 Ti(SR)(SSSR) for R = p-C6 H4 CH3 , C6 H5 were isolated as a products. Similarly, Teuben reported that titanocene iminoacyl complexes Cp2 TiC(R) NR where (R = Cl, C6 F5 , C6 H5 , o-CH3 C6 H4 and R = oCH3 C6 H4 , 2,6-(CH3 )2 C6 H3 ) are oxidized by disulfide PhSSPh to give Cp2 Ti(SPh)C(R) NR complex with ␩2 -coordinated C N group to titanium (Scheme 59) [151]. Tree years later Klei reported that alkyl titanocene derivative of Cp2 Ti(␩3 -allyl) reacted with carbon disulfide to form Cp2 Ti(␩2 -CS2 ) and Cp2 Ti(␩1 -allyl)2 via disproportionation reaction and where the presence of CS2 induced allyl coupling with Cp2 Ti(␩1 -allyl)2 to form 1,5-hexadiene (Scheme 60) [152]. Metal activated reductive head-to-head dimerization of CS2 to give C2 S4 bridged ligand was observed when the reaction of titanocene dicarbonyl with CS2 solution was taking place under N2 at room temperature. The diamagnetic product of this reaction [Cp2 Ti]2 (C2 S4 ) complex was characterized by spectroscopic, magnetic susceptibility, and electrochemical measurements [153].

4.2. Reactivity of titanocene thiolates The titanocene(II)-1-butene or titanium complex Cp2 Ti[P(OEt)3 ]2 were reacted with different substituted allylic sulfides or thioacetals in order to prepare allyltitanocene species. These allyltitanocene species prepared by desulfurizative titanation reaction undergo reactions with cyclic and acyclic ˛-chiral ketones to produce tert-homoallylic alcohols, amines, ethers, dienes or anilides (Scheme 61) [154–171]. In 1988 the metalloligand Cp2 Ti(SCH2 CH2 CH2 PPh2 )2 was used by Stephan for synthesis of heterobimetallic Ti-Ni complexes. This titanocene species has the ability to act as a tetradentate ligand bonded to a late transition metal by two sulfur and two phosphorus atoms [172]. The titanium phosphinethiolate and titanium thiolate ligands were used for complexation of other transition metals as Cu [173,174], Pt [174–177], Pd [174], Mo [178–181], W [178] The first thiolato-bridged Ru-Ti complexes containing titanocene fragments were reported in 1998 by Mitsudo. These [Cp2 Ti(SR)2 RuCl(Cp*)] complexes were prepared from titanocene dithiolates Cp2 Ti(SR)2 where R = Ph and Me, and [RuCl(Cp*)(cod)] and their cationic and hydrido complexes were obtained [182,183]. In 2000 Hidai published the synthesis and reactivity of cubane-type sulfido clusters containing titanium and ruthenium fragments with cyclopentadienyl ligands [184,185]. 5. Conclusion This review has reported on the development and current progress in the field of titanocene sulfide chemistry. A large number of the results come from the end of the last century; however, current research in this field shows new opportunities and awareness of scientific background which has evolved during the past decades proves to be of significant advantage. Titanocene sulfur compounds have been found to be active either in preparing and modifying titanium-late transition metal complexes or in the formation of titanium sulfur clusters or even were found to act as precursors in MOCVD processes. Similarly, these species have found application in organic synthesis via chelate or ligand transfers and also in modification procedures of the sulfur containing ligand(s). The large majority of titanocene compounds described in this review incorporated either completely unsubstituted or less substituted cyclopentadienyl ligand(s), and this is precisely an area which is opening new possibilities, since steric and electronic properties are effectively tuneable via substituent changes on the cyclopentadienyl rings, allowing these compounds to fit specific

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applications. The important motivation in the investigation of compounds with early transition metals containing sulfide ligands has been generally their relevance to biological systems. Accordingly, the biological activity of naturally occurring gaseous H2 S formed during the enzymatic decomposition of amino acids is intensely studied. This important signalling biological compounds have been shown to play an important role e.g., in the central nervous and/or the cardiovascular system. From this point of view transition metal complexes with sulfide ligands could be conceived as potential “in-vivo” or “in-vitro” sources of H2 S as occurring in the case of photolysis of titanocene hydrosulfide complexes. Moreover, the titanocene sulfide complexes containing hydrophilic ligands were tested for antitumor activity. Acknowledgement Financial support from the Czech Science Foundation (grant no. P207/12/2368) is gratefully acknowledged. I also wish to thank Dr. Róbert Gyepes for many helpful comments and excellent assistance. References [1] J.C. Gordon, G.J. Kubas, Organometallics 29 (2010) 4682. [2] M.Y. Darensbourg, E.J. Lyon, X. Zhao, I.P. Georgakaki, Proc. Natl. Acad. Sci. U.S.A. 100 (2003) 3683. [3] I.P. Georgakaki, L.M. Thomson, E.J. Lyon, M.B. Hall, M.Y. Darensbourg, Coord. Chem. Rev. 238 (2003) 255. [4] R.C.R. Linck, T.B. Bioorganometallics, Wiley-VCH, Weinheim, Germany, 2006. [5] R.J. Angelici, Acc. Chem. Res. 21 (1988) 387. [6] C. Bianchini, A. Meli, Acc. Chem. Res. 31 (1998) 109. [7] F. Gloaguen, T.B. Rauchfuss, Chem. Soc. Rev. 38 (2009) 100. [8] M. Yuki, Y. Miyake, Y. Nishibayashi, Organometallics 29 (2010) 5994. [9] J.J. Perez-Torrente, M.V. Jimenez, M.A.E. Hernandez-Gruel, M.J. Fabra, F.J. Lahoz, L.A. Oro, Chem. Eur. J. 15 (2009) 12212. [10] J.L. Krinsky, J. Arnold, R.G. Bergman, Organometallics 26 (2007) 897. [11] T. Amitsuka, H. Seino, M. Hidai, Y. Mizobe, Organometallics 25 (2006) 3034. [12] U. Helmstedt, P. Lonnecke, E. Hey-Hawkins, Inorg. Chem. 45 (2006) 10300. [13] S. Kuwata, M. Hidai, Coord. Chem. Rev. 213 (2001) 211. [14] M. Draganjac, T.B. Rauchfuss, Angew. Chem. Int. Ed. 24 (1985) 742. [15] D.W. Stephan, T.T. Nadasdi, Coord. Chem. Rev. 147 (1996) 147. [16] J. Pinkas, M. Lamaˇc, Coord. Chem. Rev. 296 (2015) 45. [17] H. Kopf, M. Schmidt, Angew. Chem. Int. Ed. 4 (1965) 953. [18] J.M. Mccall, A. Shaver, J. Organomet. Chem. 193 (1980) C37. [19] H. Kopf, B. Block, M. Schmidt, Chem. Ber. Recl. 101 (1968) 272. [20] C.M. Bolinger, J.E. Hoots, T.B. Rauchfuss, Organometallics 1 (1982) 223. [21] A. Shaver, J.M. Mccall, Organometallics 3 (1984) 1823. [22] P.H. Bird, J.M. Mccall, A. Shaver, U. Siriwardane, Angew. Chem. Int. Ed. 21 (1982) 384. [23] A. Shaver, J.M. Mccall, P.H. Bird, U. Siriwardane, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 47 (1991) 659. [24] F. Bottomley, D.F. Drummond, G.O. Egharevba, P.S. White, Organometallics 5 (1986) 1620. [25] F. Bottomley, G.O. Egharevba, P.S. White, J. Am. Chem. Soc. 107 (1985) 4353. [26] F. Bottomley, R.W. Day, Can. J. Chem. 70 (1992) 1250. [27] J.C. Huffman, J.G. Stone, W.C. Krusell, K.G. Caulton, J. Am. Chem. Soc. 99 (1977) 5829. [28] P. Mountford, J. Organomet. Chem. 528 (1997) 15. [29] S.A. Cohen, P.R. Auburn, J.E. Bercaw, J. Am. Chem. Soc. 105 (1983) 1136. [30] Z.K. Sweeney, J.L. Polse, R.G. Bergman, R.A. Andersen, Organometallics 18 (1999) 5502. [31] Z.K. Sweeney, J.L. Polse, R.A. Andersen, R.G. Bergman, M.G. Kubinec, J. Am. Chem. Soc. 119 (1997) 4543. [32] J. Pinkas, I. Císaˇrová, M. Horáˇcek, J. Kubiˇsta, K. Mach, Organometallics 30 (2011) 1034. [33] M. Horáˇcek, I. Císaˇrová, R. Gyepes, J. Kubiˇsta, J. Pinkas, M. Lamaˇc, K. Mach, J. Organomet. Chem. 755 (2014) 141. ˇ epniˇcka, J. Kubiˇsta, M. Horáˇcek, K. Mach, Inorg. Chem. [34] J. Pinkas, R. Gyepes, P. Stˇ Commun. 7 (2004) 1135. [35] C.J. Ruffing, T.B. Rauchfuss, Organometallics 4 (1985) 524. [36] S. Kabashima, S. Kuwata, K. Ueno, M. Shiro, M. Hidai, Angew. Chem. Int. Ed. 39 (2000) 1128. [37] S. Kabashima, S. Kuwata, M. Hidai, J. Am. Chem. Soc. 121 (1999) 7837. [38] P.J. Lundmark, G.J. Kubas, B.L. Scott, Organometallics 15 (1996) 3631. [39] M.A. Casado, M.A. Ciriano, A.J. Edwards, F.J. Lahoz, L.A. Oro, J.J. Perez-Torrente, Organometallics 18 (1999) 3025. [40] R. Atencio, M.A. Casado, M.A. Ciriano, F.J. Lahoz, J.J. PerezTorrente, A. Tiripicchio, L.A. Oro, J. Organomet. Chem. 514 (1996) 103.

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