Stable organoplatinum complexes as intermediates and models in hydrocarbon functionalization

Journal of Organometallic Chemistry xxx (2015) 1e13

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Stable organoplatinum complexes as intermediates and models in hydrocarbon functionalization Elisey S. Rudakov a, **, Georgiy B. Shul'pin b, * a

L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, Ulitsa R. Luxemburg, dom 70, Donetsk 83114, Ukraine b Semenov Institute of Chemical Physics, Russian Academy of Sciences, Ulitsa Kosygina, dom 4, Moscow 119991, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 December 2014 Received in revised form 23 January 2015 Accepted 24 January 2015 Available online xxx

This review narrates a story about the early stages of developing the alkane-platinum and aryl-platinum chemistry. Characterization and isolation of intermediate s-organyl complexes is in the focus of the paper. The first works in this area have been carried out in Chernogolovka (Shilov), Donetsk (Rudakov) and Moscow (Shul'pin). The review concerns also a progress in this area. A comparison of alkane reactions with Pt(II) on the one hand and Pd(II) as well as other CeH-activating reagents on the other hand shows that Shilov's system is “a black swan” in this field and further detailed studies of its intimate mechanism seem to be required. At the same time, in alkane activation a striking similarity can be seen between behavior of Pt(II) complexes and hydroxyl radicals. An aqua-platinum(II) complex can be compared with a wasp which pricks CeH bonds using a sting (coordinated hydroxyl radical). © 2015 Elsevier B.V. All rights reserved.

This historical review is dedicated to the Memory of Professor A.E. Shilov. Keywords: Functionalization of CH bonds Alkanes Arenes Catalysis Kinetics Shilov chemistry

Introduction It is commonly accepted that saturated hydrocarbons (alkanes) are the most inert organic compounds and very “difficult” substrates for metal complex catalysis [1]. Only at temperatures higher than 300e500
C and in the presence of heterogeneous catalysts alkanes can be easily transformed into valuable products in reactions of complete and partial oxidation, dehydrogenation, aromatization, creaking, isomerization, conversion with water vapor. These reactions constitute the basis of contemporary industrial usage of hydrocarbons of oil and natural gas. Nevertheless, under relatively mild conditions (temperature lower than 150
C, in the absence of radiation) saturated hydrocarbons, and especially methane and ethane, are usually very unreactive. Development of new methods for alkane transformations under mild conditions is very important because low-temperature processes are less

* Corresponding author. Tel.: þ7 495 9397317; fax: þ7 495 6512191. ** Corresponding author. E-mail addresses: [email protected] (E.S. Rudakov), [email protected], gbsh@ mail.ru (G.B. Shul'pin).

energy-demanding and more selective ones. Many practically important low-temperature reactions of alkanes are thermodynamically favorable. It is worthy that Nature solved this problem and transforms hydrocarbons including methane in living cells at ambient temperature and pressure. Enzymes containing certain metal ions (mainly iron-based methanemonooxygenase, MMO, and cytochrome P450) catalyze conversions of alkanes and other CeH compounds. In the end of 1960s Aleksandr Shilov with his coworkers at the scientific center Chernogolovka (Moscow region, 59 km far from the Kremlin; see Fig. 1) inspired by the idea of his teacher Nobel laureate Nikolay Semenov (“to imitate Nature in chemical flasks”) decided to explore the possibility to model enzymes using metal complexes. The Chernogolovka group (Shilov, Shteinman, Gol'dshleger and newcomers Lavrushko, Shestakov, Moravskii, Geletii and others) discovered and then carefully studied the H/D exchange in alkanes, RH, under the action of a simple platinum(II) salt, Na2PtCl4 in solution of D-containing water or acetic acid (1969): Na2 PtCl4

RH þ D2 O











!RD þ HOD

(1)

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Fig. 1. Surrounded by fantastic forests scientific town Chernogolovka in Moscow region.

In 1972, the same team described the oxidation of alkanes with H2PtCl6 in the presence of a catalytic amount of Na2PtCl4. In this case alkane was transformed into the corresponding alcohol and alkyl chloride: Na2 PtCl4

RH þ H2 PtCl6 þ H2 O











!RCl þ ROH þ H2 PtCl4 (2) It is astonishing that simple platinum salts in water or aqueous acetic acid under relatively mild conditions could carry out such challenging reactions. Indeed, these reactions (“Shilov system”, “Shilov chemistry” or “the Shilov reactions”; see an early review [2]) occurred in aqueous solution which is typical for processes in biological systems. However, temperature of both reactions was higher (around 100
C) than ambient, the most efficient solvent is aggressive (acetic acid) and it is hard to say that this is a typical model of natural enzymes. Nevertheless, in discussions of the recent years Shilov began to draw a parallel between the alkaneplatinum chemistry and methane oxidation in Nature under anaerobic conditions. On the other hand, the Shilov reactions presented the first examples of “genuine, organometallic” (that is occurring with the formation of carbon-metal bonds) activation of CeH bonds in alkanes by metal complexes. Many publications, including books and reviews, have been devoted to the description of peculiarities [3], mechanisms [4] and quantum chemical calculations [5] of these reactions. Careful studies showed that certain difference exists between behavior of Shilov reactions and alkane oxidation in other oxidizing systems. This is why we can compare Shilov chemistry with “a black swan” (see below). Shilov system was similar to the system proposed by Garnett and Hodges [6a,b] for the H/D exchange in aromatic compounds. A few papers on alkane activation with such type systems have been published by Hodges, Webster and Wells [6c]. In 1972, the new group of researchers headed by Rudakov (Tretyakov, Zamaschikov, Litvinenko, Rudakova, Lobachev, Zimzeva, Lutsyk, Volkova, and later newcomers Mitchenko and Popov) in Donetsk (see Fig. 2) began studies of alkane reaction with Pt(II) and other reagents. The most important achievements of this group were new details of the alkane activation mechanism, discovery of the convenient method

for preparation of stable alkylplatinum complexes and a study of their decomposition to afford alkanes and alkyl derivatives (the reaction which is reversed one relative to the Shilov reaction). The investigations of alkane activation by platinum complexes in comparison with various reactions of alkanes with other reagents were carried out. Prof. Shilov was an initiator of creating a special group on hydrocarbon functionalization under the guidance of Shul'pin (Nikitaev, Kitaigorodskiy, Nizova, Serdobov) in Moscow (see Fig. 3). Dr.
ge
e of the unfortunate John Stille, Jane Zeile Krevor (USA), a prote spent some time in the Shul'pin group studying reactions of hydrocarbons with platinum complexes. The most impressive results have been obtained on the synthesis of s-aryl-platinum(IV) complexes by direct reaction between arenes and H2PtCl6. Contacts between scientists of the three groups were very close and many common papers have been published. The first in the world Conference on new methods for hydrocarbon functionalization was held in Donetsk in 1982 (Fig. 4). This historical review is devoted mainly to works made by Donetsk and Moscow groups and emphasizes the studies of organylplatinum complexes which are intermediate compounds in hydrocarbon functionalization. Formation of s-organyl platinum complexes Kinetics and mechanism of alkane functionalization by platinum salts (Shilov reaction) Groups of Shilov and Rudakov concentrated their efforts on kinetic studies of H/D exchange in alkanes in protonic media. In early publications, Shilov and Shteinman wrote that both H/D exchange with the participation of Pt(II) and the alkane (RH) oxidation by Pt(IV) catalyzed with Pt(II), i.e. reactions (1) and (2), involve intermediate formation of Pt‒R [2]. It is noteworthy that at that time the generation of organometallic s-alkylplatinum derivatives in protonic and even acidic media at 100
C seemed impossible to many scientists. Nevertheless, in the groups of Chernogolovka and Donetsk this hypothesis became a postulate. All proposed mechanisms included a stage of the alkylplatinum formation. Let us consider briefly these mechanisms.

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Fig. 2. L.M. Litvinenko Institute of Physical Organic and Coal Chemistry in Donetsk.

Multiple exchange, ethyl-ethylene mechanism Multiple H/D exchange has been found in the ethane-platinum system [7а,b]. For example, in the case of ethane, similar to the alkane exchange in heterogeneous reaction with D2 on films of

transition metals, this process shown in eq. (3) occurs with the intermediate formation of s-ethyl and p-ethylene Pt(II) complexes. If [Dþ] >> [Hþ] each cycle IeIIeI or IIeIIIeII leads to the introduction of one deuterium atom into ethane molecule. When the rates

Fig. 3. The old building (built in 1755) of N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, in Moscow.

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Fig. 4. Participants of the first in the world Symposium “New ways for transformations of saturated hydrocarbons”. This All-Union conference was held in Donetsk (USSR) in 1982. Co-organizers and presidents A.E. Shilov and E.S. Rudakov are in the center of first row.

of decay in routes 1 and 2 are comparable during the time of one cycle IeIIeI a few internal cycles IIeIIIeII are realized, and a few D atoms are incorporated into the alkane in the one act of coordination in complex II. An average number of D atoms entered into the alkane molecule during one act of interaction between RH and PtII is characterized by the value M ¼ Sidi/Sdi, where i is the number of D atoms in the alkane. For the simple process M ¼ 1, whereas for the multiple exchange we have M > 1.

(3)

depending on the reaction conditions. In the solution of 50% acetic acid monotonous decreasing distribution of deuterated ethanes di was observed (d1 > d2 > d3 > d4 > d5 > d6). For the solution in pure water a stepwise increasing distribution was found and all ethanes are divided into groups d1, d2, d3 and d4, d5, d6. In each group the increasing profile of the deuterium content (d1 < d2 < d3 and d4 < d5 < d6) was noticed. The first troika of peaks d1ed3 in the MS spectra corresponds to the exchange in one of methyl groups, the second troika d4ed6 reflects the exchange in the second methyl. An enhanced difference between d3 and d4 is due to the somersault of the ethane fragment. Alkane-alkyl mechanism This mechanism described by Eq. (5) [7d,e] can also explain the di-distribution mode but this model is better because reflects the common mechanism of both H/D exchange and oxidation. Step 3 is the somersault of the ethane fragment.

Alkyl-carbene mechanism. Descending and ascending distributions Because a character and rates of H/D exchange for the cases of ethane and methane were similar despite methane cannot form the alkene complex of type III the alternative alkyl-carbene mechanism (4) was accepted for methane, ethane and other alkanes [7b,c].

(5) Table 1 demonstrates that model (5) is more precisely describes the mode of the second troika in di-distribution. For the alkanealkyl model (5) the values [(d1/d2) þ 1]/[(d2/d3) þ 3] ¼ 0.5 and k3/ k‒1 ¼ 0.2 ± 0.05 are constant in the broad range of conditions when the k1/k‒1 is in the interval 0.1 ÷ 2.4.

(4) II

As carbon atoms in the complex Pt eCH2eCH3 are not equivalent, H-atoms in a-position should enter into the exchange first whereas the exchange in b-position is possible only after a somersault of the ethane fragment. It has been shown in the experiments that, indeed, two different types of the exchange occur

Kinetics of the alkane activation. Comparison of rates of H/D exchange and oxidation A flood of investigations of mild alkane functionalization inspired by Shilov's discovery almost did not touch mechanisms of the initial CeH bond cleavage, and this was due to the difficulty in the study of activation kinetics. In 1973, Rudakov and coworkers

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E.S. Rudakov, G.B. Shul'pin / Journal of Organometallic Chemistry xxx (2015) 1e13 Table 1 The stepwise increasing distribution of di-ethanes in experiments [7c] and in accordance with calculations [7d].

Experiment Calculations (alkyl-carbene mechanism) Calculations (alkane-alkyl model)

d1

d2

d3

d4

d5

d6

17.1 17.2 17.1

26.5 23.2 23.1

38.9 45.2 43.8

3.3 6.0 4.7

5.3 3.7 5.2

8.9 4.5 6.2

discovered the alkane oxidation in sulfuric acid by palladium(II) ions [7e] as analogs of Pt(II), and later by HgII, PtIII, sulfuric acid itself and other reagents (Rg) [7feh]. They began an accurate study of activation rate constants measuring consumption of the alkane trace amounts including the Shilov system. Two original methods have been developed, which allow us to study directly the kinetics of alkane activation measuring consumption of low alkane concentrations [7gei]:

ðd½RH=dtÞ ¼ k½RH½Rg at ½RH < < ½Rg

5

Comparison of alkane reactions with Pt(II), Pd(II), and Pt(III). Shilov's system as a “black swan” A detailed study of CeH bond activation demonstrated that Shilov reaction astoundingly differs from the alkane reaction with palladium(II) ions. The latter process occurs only in strongly acidic media (>70% H2SO4). This oxidative dehydrogenation transforms quantitatively cyclohexane into benzene [7e] and cyclopentane into tropilium cation [7e,8h]. The electrophilic species PdII(HSO4)þ is inserted into the CeH bond. A stepwise dehydrogenation of cyclohexane with the consecutive generation of cyclohexene, cyclohexadiene and benzene (8) requires the participation of intermediate complexes Pdn (n ¼ 0e2) which protect olefins against the side reactions of protonation, oligomerization and isomerization giving benzene in almost quantitative yield (“palladium protection of the mechanism”).

(6)

The first (kinetically distributive) method is based on the measurement of the effective rate constants kl ¼ dln[RH]g/dt for the reaction in a closed shaken reactor containing gas phase (g) and solution(s). The true (liquid-phase) rate constant can be calculated from Rudakov's equation k ¼ kl(1 þ al), where a ¼ ([RH]g/ [RH]s)[RH]/0, is the thermodynamic coefficient g/s distribution of RH, and l ¼ Vg/Vs is the ratio of volumes in the reactor. The second (syringe-reactor) method permits the study of the activation kinetics without gas phase, k ¼ kl at l ¼ 0. This method allowed us to strictly examine reactions with platinum chlorides. A kinetic study of the alkane oxidation in aqueous solutions of PtIIePtIV at 98
C demonstrated that the reaction is of the first order relative the alkane and Pt(II) and its rate does not depend on the presence of Pt(IV), see Eq. (6), Rg ¼ PtII [7feh]. Selectivity of the reaction, the role of sterical hindrance and the influence of halogenide ligands have been studied [7h,8aec]. A combined investigation was carried out [8def] of rate constants kexch and kox of the H/D exchange and oxidation, respectively, under conditions when only species PtIICl‒3 are active. When concentration [Pt(IV)] rises and [Dþ] decreases the value kexch for exchange decreases and the constant for oxidation kox grows. However, the sum of constant for both processes is maintained constant and equal to the activation rate constant kexch þ kox ¼ kact. This testifies that both processes occur with the participation of common intermediates. It was found that the rate constant for interaction of RPtII with PtIV is 104 times higher than the constant for reaction of RPtII with Dþ. A qualitative model which takes into account these data has been proposed [7d,h,8g]. This mechanism (7) inсludes as a first step the equilibrium pre-activation of PtII complex which is the transformation of the complex leading to the generation of coordination vacancy. A very active (PtII)* complex containing a coordination vacancy forms then an adduct with the alkane RH. In the consequent step, all accessible CeH bonds of RH are splitted with almost equal rate.

(7)

(8) In the comparable systems the activation kinetics follow Eq. (6) but dependences of k on the reaction conditions in the systems PtII/water and PdII/H2SO4 substantially differ. The following peculiarities of the reaction with PtII have been identified: (i) values k do not depend on the presence of PtIV and concentration of H2SO4 in the region 0.01e2 M; (ii) the complex PtIICln(H2O)4‒n which contains both halogenide and aqua ligands is approximately equally active in the reaction with isobutane or cyclohexane; (iii) aqua-complexes which do not contain chloride ligands (n ¼ 0) and chloride complexes which do not contain aqua ligands (n ¼ 4) are inactive; (iv) bromides are two times more active in comparison with chlorides; (v) the activity of chlorides and bromides is the highest one at n ¼ 3 or n ¼ 2; (vi) an unusual bond selectivity has been discovered [7b,8a]: 1
:2
:3
z 1:1:0. In contrast, in the PdII/H2SO4 system it has been found that the activation rate exponentially enhances with growth of [H2SO4], the presence of Cl‒ or Br‒ ions inhibits the reaction and the normal selectivity 1
<< 2
<< 3
typical for electrophilic and oxidative II reagents (HgII/H2SO4, NOþ 2 /H2SO4 or CrO3/H2SO4) [7g,h]. Thus, Pt in the alkane activation is neither typical electrophile nor oxidant, and we can say that platinum is like “a black swan” (an equivalent of Russian “white crow”) among other known oxidizing systems (ordinary white swans). Certainly, anomalous Shilov system is by no means “a black sheep”. This is a pearl of homogeneous catalysis. The Shilov system can be also compared with the PtIII/H2SO4 combination where the anion [Pt2(SO4)4(H2O)2]2- with the structure of the dimer [H2OePtIIIePtIIIeOH2]2e, in which ions PtIII are additionally bound each to other by four bridged sulfate ligands eOeSO2eOe [8i]. Like the PdSO4/H2SO4 system, a derivative of PtIII in the solution of H2SO4 oxidizes cyclohexane to evolve SO2, but benzene is not formed. The reaction is first order in both RH and PtIII. It is noteworthy that selectivity parameters (1
<<2
<< 3
) differ substantially from that measured for Pt(II) [8j,k].

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An enigma of Shilov chemistry: Similarity between alkane reactions with Pt(II) and with hydroxyl radicals. A hypothetical wasp's sting mechanism Interestingly, in contrast to the case of palladium, a striking similarity of kinetic parameters for Shilov reaction and that for the reaction of alkanes with hydroxyl radicals in water has been detected [7g,8a]. Both reactions exhibit low bond selectivity 1
:2
z 1:1. Values of kinetic isotope effect (KIE, cyclo-C6H12/cycloC6D12) are equal to ~1.5 for Pt(II) at 98
C and 1.3 for HO radicals at 50
C. To explain low selectivity and KIE parameters of the reaction with hydroxyl radicals a model has been proposed which considers the formation of a cage in the stage of initial interaction of reactants [7g,8l]:

1

k*

* < RH/ OH > ! R þ H2 O RHþ OH ) 

1



2



(9)

Here * are the oriented complexes in the water cage. It has been assumed that rate constants k* are identical for the reaction in water and gas and parameters k1 and k‒1 are equal for RH and RD. The relation between KIEs in the gas phase (cg) and water (cw) was found: cw ¼ [(ycg
þ 1)/(y þ 1)]  cg, where kg and kw are rate constants in gas and water, y ≡ (k‒1/kH*) ¼ (cw e 1)/ (cgecw), is the return parameter equal y ¼ (cw e 1)/(cg ‒ cw). For the gas-phase reaction k1 >> k2 and the cage effect is absent. Recently, unusual reversed temperature dependence (kH/kD) ¼ (AH/ AD)exp[(ED e EH)/RT] was measured [8m] which supports the model described above. In the gas-phase reaction KIE decreases with temperature growth and value ED e EH > 0. In water (Fenton system) the KIE parameter grows with enhancing the temperature and ED e EH < 0. The same remarkable anomaly ED e EH < 0 was recently found [8m] for Shilov system. This dependence testifies similarity of both systems: PtII/H2OeRH/RD and OH/H2OeRH/RD). The only exception is sterical hindrance [8nep] in Shilov system leading to the reversed selectivity sequence 1
:2
:3
z 1:1:0 [1c,7h,8a]. It could be assumed that an active form of the aquachloride Cl2(H2O)2PtII is an adduct of the hydride PtIII derivative with hydroxyl radical in the form of unseparated radical pair Cl2(H2O) H(PṫIII) OH. This coordinatively unsaturated complex has a vacancy to add an R fragment from the reacting alkane. Coordinated hydroxyl radical can abstract hydrogen atom from RH. A collapse of radical pair (PṫIII) and R will afford an alkylhydride derivative of platinum(IV) postulated for one stage of the catalytic cycle [1a,c,2]. Thus, considering such hypothesis we can say that one of the main peculiarities of Shilov chemistry is a crypto-radical character of its intimate mechanism. It is possible to compare the platinum complex Cl2PtII(OH2)2 with a bee, which “pricks” (activates) alkanes using its sting (coordinated hydroxyl radical). A collation of the platinum catalyst with a wasp would be even more proper because this insect (unlike a “stoichiometric” bee) is able to bite “substrates” many times being in our analogy “a catalyst”. It should be mentioned that some time ago Bach and Dmitrenko [8q] proposed a mechanism of alkane oxidation catalyzed by cytochrome P450. It involves the rearrangement (somersault motion) of the metal hydroperoxide to its inverted isomeric form with a hydroxyl radical which is hydrogen bonded to the metal oxide: MOeOH / M]O/HO. The coordinated hydroxyl radical abstracts a hydrogen atom from the CeH bond of alkane. Evidently, this mechanism has certain similarities with the hypothetical platinum “wasp's sting mechanism” described above. The mystery of Shilov reaction requires, certainly, a further careful study of the mechanism which is still far from its clear understanding.

Unstable Pt(IV) complexes generated in the “alkane/Pt(IV) chloride” system In addition to traditional kinetic methods which have been employed in studies of the mechanisms of Shilov chemistry an alternatives approaches were used for the investigation of the H/D exchange in alkanes and their oxidation. These approaches consist of the direct detection of intermediate alkylplatinum complexes or the synthesis of such complexes by non-direct methods from compounds other than alkanes. In an early study, Shilov and his co-workers [9a] showed that the reaction mixture of methane and H2PtCl6 þ PtCl2 in aqueous trifluoroacetic acid contained a 4 methylplatinum complex. This compound reacted with hydrazine to produce methane and under the action of DCl in D2O afforded CH3D. Upon heating in water the complex gave the products typical for the methane oxidation by platinum, namely CH3Cl and CH3OH. The Moscow group found that the reaction of hexachloroplatinic acid with n-hexane in the dark gave p-hex-1-ene platinum(II) complex in low yield which is apparently formed from s-hexyl platinum(IV) intermediate [9b]:

CH3 ðCH2 Þ4 CH3 þ H2 PtCl6 /CH3 ðCH2 Þ4 CH2PtCl2 5   /CH3 ðCH2 Þ3 CH¼CH2 PtCl 3

(10)

This reaction can be also induced by light (l > 300 nm) [9cee] or

g-quanta [9f].

Reactions of platinum derivatives with organometallic compounds to afford alkylplatinum complexes Alkylmercury compounds react with simple platinum salts to afford alkylplatinum complexes which have not been isolated. However peculiarities of the H/D exchange which accompanies this transalkyation testify that alkylplatinum derivatives are formed in the process [10a]. Deuterated alkanes detected in these experiments are generated as a result of a hydrolytic cleavage of alkylplatinum complexes. Tetramethyltin reacts with Pt(IV) or Pt(II) salts to produce relatively stable methylplatinum complexes [10b]: 2 ðCH3 Þ4 Sn þ PtCl2 6 /CH3PtCl5 þ ðCH3 Þ3 SnCl

(11)

2 0 ðCH3 Þ4 Sn þ 2PtCl2 4 þ H2 O/CH3PtCl4 ðH2 OÞ þ Pt

þ ðCH3 Þ3 SnCl þ 3Cl

(12)

Under light irradiation the s-ethyl complex is converted into the p-ethylene complex of Pt(II) [10b]: 2 ðC2 H5 Þ2 SnðCH3 Þ2 þ PtCl2 6 /CH3PtCl5 þ CH3 CH2  PtCl2 5 /ðp­CH2¼CH2 ÞPtCl3

(13)

Preparation of alkylplatinum complexes by the interaction of platinum(II) chlorides with alkyl iodides. The “reverse” Shilov reactions During long time the attempts of the three teams to isolate and directly investigate complexes intermediate in the alkane activation were unsuccessful. Rudakov, Zamaschikov and coworkers discovered that platinum(II) chlorides in solution of

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aqueous acetic acid reduce alkyl iodides to the corresponding alkanes, catalyzing replacement of iodide anion by chloride anion [11a,b]:

Table 2 Rate constants ki (M1 s1) of reactions between alkyl iodides and Pt(II) in aqueous solution at 80
C. Complex

MeIa

EtI

i-BuI

PtCl2‒ 4 PtCl3(H2O)‒ PtCl2(H2O)2 PtBr2‒ 4

0.14 ± 0.01 0.19 ± 0.02 0.05 ± 0.05 0.95

0.03

0.010 ± 0.001 0.047 ± 0.003 0.28 ± 0.01

a b

(14) Reaction (14) opened a new field of studies of alkane and alkyl halogenide activation with the participation of d8 complexes PtII and PdII (in some cases the behavior of these ions was similar) [11b]. Detected, carefully characterized and even isolated alkylplatinum complexes allowed the chemists to support mechanisms deduced on the basis of the kinetic studies. Reactions of ethyl- and isobutyl iodide with Pt(II) are the first order in platinum(II) and strongly retarded when chloride or bromide ions are added. The rate of interaction between methyl iodide and Pt(II) only slightly depends on halogenide ion concentration. The reaction order in Pt(II) at low concentration of chloride anion is between first and second ones, and this order is the first at high concentration of platinum(II). The kinetic equation for the reaction of i-BuI with PtII is shown below:

k¼

4 h i X d½RI ¼ ki PtII Xi ðH2 OÞ4i ; ½RIdt

(15)

i¼0

where k is the total first order constant (measured for RI consumption at high excess of PtII), ki are the constants of routes for individual complexes PtIIXi(H2O)4i. Table 2 summarizes calculated values ki for the studied systems. The kinetic equation for the reaction of methyl iodide with Pt(II) includes two additional terms of ‒ the second order k2,4[PtCl2(H2O)2][PtCl2 4 ] and k3,4[PtCl3(H2O) ] [PtCl2 4 ]. The reaction of alkyl iodides RI with Pt(II) chlorides in aqueous solution at room temperature affords stable in these solution organometallic compounds RPtIVCl4(H2O)‒ [11]: H2 O

PtII Cl2
!RPtIV Cl4 ðH2 OÞ þ I 4 þ RI





þ

2  C2 H5 PtIV Cl2
!C2 H5 PtII Cl3



!C2 Di H6i þ PtII Cl3 5


2I

iD

0.14b

Constants k2,4 ¼ 0.066 ± 0.06 and k3,4 ¼ 0.03 ± 0.01. Estimated at high excess of halogenide ions.

Iodide anions produced in reaction (16) are captured by an excess of platinum(II) compounds to produce scarcely soluble iodide complex. Reactions of stable alkyl Pt(IV) complexes prepared using alkyl iodides Stable in aqueous solution organometallic compounds RPtIVCl4(H2O)‒ obtained in reaction (16) of alkyl iodides RI with Pt(II) chlorides can be involved into various consequent reactions [11ceh]. Total scheme of the transformations with the participation of the methyl complex MePtIV is depicted by Eq. (18). Here Red ¼ I‒ or SnCl2, Ox ¼ Pt(IV) or I2. If NaI is added to the solution of MePtIV in water methyl iodide is liberated immediately (route 1). A small amount of methane is also formed via routes 5 and 6. Methane accumulation is continuing further due to the catalytic reduction of methyl iodide. Addition of NaCl, NaBr and NaSCN leads to slow decomposition of MePtIV to afford methyl chloride, methyl bromide and methyl thiocyanate via routes 2, 3 and 4, respectively. Under the action of NaOH the hydroxo complex MePtIVCl4(OH)2‒ is formed (route 8). The interaction of MePtIV with reducing agents (SnCl2, NaBH4, as well as I‒ in acidic media) leads to the liberation of methane via routes 5 and 6. It should be noted that in the absence of a reducing agent the protolysis to afford methane (route 7) is impossible.

(16)

Here R ¼ CH3, C2H5, CH2COCH3, CH2COOH, C6H5 and others. Some of these complexes have been isolated in the form of scarcely soluble cesium salts Cs[RPtCl5] [11cef]. The alkyl complexes RPtIV are stable in the presence of acids, do not enter into H/D exchange with solvent, and under the action of reducing agents (for example, I‒) they are transformed into alkyl complexes of Pt(II). The latter compounds produce alkanes in fast protolysis, and in the Dþ ‒ D2O afford a set of deuteroalkanes similar to the products of the multiple exchange in alkanes catalyzed by Pt(II) salts: 

7

(17)

A combination of reactions (16) and (17) gives us a reversed process of the alkane oxidation on platinum complexes. Taking these complexes into consideration allowed us to more deeply understand the mechanism of the Shilov reaction. Indeed, the methyl complex CH3PtII generated in reaction (16) turned out [11a] to be identical to the intermediate which is formed in the reaction of methane with platinum chloride in water at 120
C.

(18) IV

The kinetic equation for the complex MePt decomposition (accumulation of MeCl) when an excess of Cl‒ is used at 308e333 K is the following:

    d½MeCl k1 MePtIV Cl   ¼ dt 1 þ K Cl

(19)

This dependence can be explained either in the frames of intra sphere decay of CH3PtIVCl2 5 : K

II 2 MePtIV Cl4 ðH2 OÞ þ Cl %MePtIV Cl2 5 þ H2 O/MeCl þ Pt Cl4

(20)

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E.S. Rudakov, G.B. Shul'pin / Journal of Organometallic Chemistry xxx (2015) 1e13

or in terms of a SN2 mechanism which includes the attack of a nucleophilic chloride anion at the carbon atom in CH3PtIVCl4(H2O)‒: þCl



þCl ;K



2 





IV
! MeCl þ PtII Cl2






MePtIV Cl5 4 ) MePt Cl4 ðH2 OÞ  Cl

k1

(21) The two mechanisms cannot be kinetically distinguished. Nevertheless, scheme (21) seems to be more preferable. The s-ethyl complex decomposes in a few routes [11e] as shown in Eq. (22).

If concentrations of the intermediates are stationary, scheme (25) leads to the Eq. (26) which describes experimental results at [PtIV] ¼ (0 ÷ 0.20) M and [Hþ] ¼ (0.015 ÷ 0.75) M.

io.n n  h ½1 þ ðk4 =k3 Þ kCH3 CH3 ¼ðk1 =k3 Þ 1 þ k4 PtIV io h  ½1 þ ðk3 =k2 Þ PtIV

(26)

The calculations gave the following values of constants for stages presented in scheme (25): k1 ¼ (1.9 ± 0.3)  107 s1, k4/k‒3 ¼ 2.0 ± 0.3, k2/k3 ¼ 0.10 ± 0.04. Complex Cs2PtIV(CH3)2Cl4 (103 M) in solution DCl (3 M)/D2O at 368 K decomposes [11k] to produce deuterated ethanes C2H6iDi in the ratio 1:0.85:2.0:2.2:0.2:0.3:0.3 for i ¼ 0‒6, respectively.

Further progress in alkylplatinum chemistry

(22) The main products of reaction (22) are ethyl alcohol and ethyl chloride (yields are 75 and 17%, respectively). The ethyl complex of platinum(IV) can be introduced into all reactions typical for the MePtIV shown in Eq. (18). Thus, addition of iodide ions leads to the very fast formation of ethyl iodide. When either NaCl is added or of starting K2PtCl4 is used in higher concentration, the yield and rate of ethyl chloride evolution grow. Treating the solution by reducing agents (SnCl2 or NaBH4) gives ethane. A reaction of alkyl iodide reduction to produce alkanes is catalyzed by PtII containing iodide ions in aqueous solutions [11c,d]. In reactions of platinum(IV) alkyl complexes iodide ions play dual role as shown in Eq. (18). As nucleophile iodide ion leads to the regeneration of starting alkyl iodide, and as a reducing agent it affords alkylplatinum(II) which in acidic media is rapidly protolyzed to give the corresponding alkane. This phenomenon has been used for the catalytic reduction of alkyl iodides and alcohols (via the equilibrium ROH þ Hþ þ I‒ ¼ RI þ H2O) to alkanes. The reaction in the cases of methanol, primary alcohols and alkyl iodides can be carried out in almost quantitative yield. Iodide ions are reducing agent and platinum(II) is a catalyst: PtII

RI þ Hþ þ I



!RH þ I2

(23)

The mechanism includes a few stages: I2

Hþ ;I

IV 2





! RI þ PtII I2





RPtI2
!RH þ PtII I2 4 %RPt I5 3






4 þI2

(24) Decomposition of the dimethylplatinum(IV) complex in accordance with Eq. (25) to afford ethane [11iek] includes the intermediate formation of ethylplatinum(IV) hydride. Stage ‒2 corresponds to the crucial step in the ethane activation by Pt(II).

In subsequent years, several groups of scientists have been engaged in research of alkylplatinum complexes. In many cases such reactions are models of certain stages of Shilov chemistry. We will only very briefly mention here selected recent publications. Labinger and Bercaw [12], Vedernikov [13], Goldberg [14], Sanford [15] and their co-workers studied such complexes and reactions by chemical and kinetic methods. Siegbahn and Crabtree [16], Cundari and Bercaw [17] carried out the quantum chemical studies of stages of Shilov reaction. Works by Olah [18a], Labinger and Bercaw [18b] and Periana (Periana system [18c]) have been directed at approaches to the practical use of the Shilov system. Numerous publications were devoted to alkylplatinum(II) complexes and oxidative addition of haloalkanes to platinum(II) derivatives [19]. Some very recent works of these authors are published in this memorial issue of the Journal of Organometallic Chemistry.

The reaction of H2PtCl6 with aromatic compounds to afford stable s-aryl Pt(IV) complexes Synthesis and transformations of stable s-aryl Pt(IV) complexes by direct reaction with arenes Attempts of the Moscow team to isolate s-aryl derivatives of platinum(IV) crowned with impressive success. Shul'pin [20] has discovered that heating a solution of H2PtCl6 and an aromatic compound ArH in the CF3COOHeH2O mixture or in CH3COOH leads to the formation of fairly stable s-aryl complexes of platinum(IV) in yields up to 95%, which can be isolated in the form of anionic adducts with ammonia after chromatography on silica gel containing a small amount of adsorbed ammonia (this silica gel can be easily prepared by exposure of silica gel to ammonia vapor in a desiccator). For example, the reaction with naphthalene depicted in Eq. (27).

(25)

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9

(27)

The same s-aryl complexes of platinum(IV) can be prepared by the reaction of H2PtCl6 with mercury, tin, lead or boron aryls [21]. The platinum fragment can attack only b-position of naphthalene for steric reason. The process of formation of the s-tolyl complex of platinum(IV) is accompanied by its pem isomerization [20a,22]. On prolonged heating, the complexes decompose [20a,b,23] to produce mainly biaryls and platinum(II) derivatives. Thus, this reaction is a model of oxidative coupling of arenes under the action of palladium(II) compounds [24]. The aryl Pt(IV) complexes arylate olefins [25]. Complexes can be obtained in situ from aryl derivatives of non-transition elements [21c,25]. Arylation of olefins with arenes in the presence of platinum(IV) compounds is a model of the Fujiwara reaction which uses palladium(II) [26a], arylation with aryl derivatives of non-transition metals models various wellknown coupling reactions catalyzed by palladium(II) [26b]. The substituent does not enter the o-position of monosubstituted benzenes for steric reasons. The kinetic isotope effect of the reaction is small (~3 for benzene and ~2.3 for toluene). In the case of toluene the activation energies of the formation and pem isomerization are 25 kcal mol1. The method of competing reactions showed [20a,22a,27a] that the relative rates of reactions (given in parenthesis) with participation of the arenes PhX decrease in the following sequence of substituent X: OH > OCH3 > CH3 > C2H5 > OC6H5 > CH(CH3)2 > H > C6H5 > F > COCH3 > COOH > Cl > NO2. Table 3 summarizes the relative initial reaction rates for the reaction with monosubstituted benzenes. The logarithms of these values are correlated with the Hammett constant s and the Brown constant sþ (with the parameters r ¼ 3.0 and rþ ¼ 1.5; see

Fig. 5). The correlation is better if the s or sþ p values for compounds Nos. 1e9 and the sm (¼ sþ m) constants for compounds Nos. 10e13 are used. Thus, the reaction may be regarded as electrophilic substitution in aromatic compounds. The following simplified kinetic scheme for the reaction of arene RH has been proposed [22] on the basis of experimental data:

k28

 






! PtCl2






PtCl5 þ Cl 6 k28

(28)

k29


! PtCl5 ðH2 OÞ












PtCl5 þ H2 O k29

(29)

Table 3 Relative initial rates of the reaction of H2PtCl6 with arenes (PhR) in solution.a Substrate no.

Substituent, R

Rate Vtherm 0

Rate Vphoto 0

1 1a 2 3 4 5 6 7 8 9 10 11 12 13

OH OCH2CH3 OCH3 CH3 CH2CH3 OPh CH(CH3)2 H Ph F COCH3 COOH Cl NO2

16.0

8.0 4.1 4.0 1.0

8.5 3.0 2.7 2.0 1.9 1.0 0.9 0.3 0.1 0.09 0.08 0.04

1.9

a Adapted from Refs. [20a,22a,27a]. Conditions. In the thermal reaction in CF3COOH/H2O rates were measured after 15 min; 90C; Vtherm ¼ W0(PhR)/W0(PhH). 0 Photochemical reaction was carried out in CH3COOH at 15
C; and Vphoto ¼ W0(PhR)/ 0 W0(PhCH3).

Fig. 5. Plots of the logarithm of V0 for the thermal reaction between H2PtCl6 and monosubstituted arenes (26) versus constants s (line A) and sþ (line B). Lines B and C: the analogous correlations with sp and sþ p for the photochemical reaction. For compound numbers, see Table 3. Adapted from Refs. 20a,22a,27a.

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E.S. Rudakov, G.B. Shul'pin / Journal of Organometallic Chemistry xxx (2015) 1e13 k30








! PtCl
ðp­RHÞPtCl5 5 þ RH







(30)

k30

k31 2 ðp­RHÞPtCl
!s­RPtCl5 5





þ Hþ

(31)

The kinetic analysis of Schemes (28)e(31) in quasi-equilibrium approximation led to the following expression for the initial rate of complex formation:

W0 ¼

i h d s­RPtCl2 5 dt

¼ keff

½PtðIVÞ0 ½RH   K33 þ Cl

(33)

K33 ¼

PtCl5 ðH2 OÞ Cl i h PtCl2 6



¼

k28 k29 ½H O k28 k29 2

(34)

Photoinduced synthesis of stable s-aryl Pt(IV) complexes by direct reaction with arenes Continuing detailed investigations of the platinum‒arene chemistry, Shul'pin and co-workers discovered that the reaction between H2PtCl6 and arenes (ArH) can be induced not only by heating [20a,22a] but also by light [9c,22a,28a] or g-irradiation [28b]. In contrast to the thermal reaction, the photoinduced process results in the formation of p-isomers of toluene and anisole complexes and no p-m-isomerization was observed. The correlation [22a] of the relative reaction rates with s constants is shown in Fig. 5C and D. The kinetics of the photoinduced process was studied for anisole, toluene and naphthalene. On the basis of these data the following kinetic scheme has been proposed [22a]. Like in the case of the thermal Shul'pin reaction, at least two starting platinum(IV)  complexes can be considered: PtCl2 6 and PtCl5(H2O) . Light irradiation accelerates the interconversion between the two complexes shown by Eq. (33). The first stage of the photochemical variant of reaction (27) is photo-excitation of the starting species:

h i hv 2 * PtCl2
! PtCl6 6







h i* h i  k39 PtIV Cl5 ðH2 OÞ þArH




! ½ArHþ PtIII Cl4 ðH2 OÞ þCl B

D

h i* h i  k39 PtIV Cl5 ðH2 OÞ þArH




! ½ArHþ PtIII Cl2 þH2 O 5 B

(41)

C

The ioneradical pairs can decompose to produce the initial compounds:

(42)

C

Addition of Pt(II) derivative in the form of salt Na2PtCl4 leads to the acceleration of the interaction between H2PtCl6 and arene [22a]. Heating a solution of an aromatic compound (benzene or toluene) with Na2PtCl4 in aqueous trifluoroacetic acid affords a saryl complex of platinum(II) which is much less stable in comparison with a corresponding complex of platinum(IV). This complex was identified after the oxidation with H2PtCl6 (at room temperature) to produce the platinum(IV) derivative [27b].

k35

(39)

C

A

h i k  42  ½ArHþ PtIII Cl2




!ðArHÞþPtIV Cl5 5

is equal to

i

h i* h i  k39 PtIV Cl2 þArH




! ½ArHþ PtIII Cl2 þCl 6 5

(40)

K 33

h

(38)

In a productive route, these species interact with an arene molecule ArH to form the ioneradical pair C or D:

(32)

where [Pt(IV)]0 is total initial concentration of platinum(IV), keff ¼ k28k30k31/k‒28k‒30 and the equilibrium constant K33 for the equilibrium  





! PtCl2 6 þ H2 O





PtCl5 ðH2 OÞ þ Cl

h i* k 37 PtCl5 ðH2 OÞ




!PtCl5 ðH2 OÞ

(35)

h i k  42 ½ArHþ PtIII Cl4 ðH2 OÞ




!ðArHÞþPtIV Cl4 ðH2 OÞ

(43)

D

In the productive route, the ioneradical pairs collapse to generate the Wheland complexes E and F:

h i k    44 2 ½ArHþ PtIII Cl2




! Arþ H PtIV Cl5 5

(44)

E

C

h i k    44 ½ArHþ PtIII Cl4 ðH2 OÞ




! Arþ H PtIV Cl4 ðH2 OÞ D

(45)

F

Under the action of a base, the Wheland complexes give the saryl complexes of platinum(IV):

  k46 2 Arþ H PtIV Cl2
! s­ArPtIV Cl5 þHþ 5





(46)

G

E

  k46 Arþ H PtIV Cl4 ðH2 OÞ




!s­ArPtIV Cl4 ðH2 OÞ þHþ F

(47)

H

Kinetic analysis this scheme in steady-state approximation led to the following equation for the initial rate of the reaction: photo

W0

  d s­RPtIV ¼ k42 ð½C þ ½DÞ dt  h i h i  k39 k44 ½ArH0 k35 PtCl2 þ k36 PtCl5 ðH2 OÞ 6 0  0  ¼ ðk42 þ k44 Þ k37 þ k39 ½ArH0 ¼

(48) Assuming k1 z k2 and k5 >> k6, we come to the simplified equation:

A hv

h



PtCl5 ðH2 OÞ




! PtCl5 ðH2 OÞ k36

i*

photo

(36)

B

The excited species A and B can be deactivated:

h i* k 37 2 PtCl2




!PtCl6 6

(37)

W0

¼

k k k ½ArH0 1 4 6  ½PtðIVÞ0 k5 k3 þ k4 ½ArH0

(49)

It is noteworthy that the irradiation of a solution of (nBu4N)2PtCl6 and an arene (anisole, phenetole, diphenyloxide and benzene) in CH2Cl2 at 15
C gave complexes NH4[s-Ar‒PtCl4NH3] after addition of NH4OH or the chromatography on silica gel containing 1e2 mg of NH3 per 1 g of SiO2 (it can be easily prepared by

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exposure of silica gel to ammonia vapor in desiccator) [28c]. The yield of complexes was 10e87%.

Recent works on s-aryl Pt(IV) complexes Mitchenko, Beletskaya and co-workers [29] demonstrated that mechanochemical activation of the salt K2PtCl6 in the presence of vapor of benzene or toluene leads to the generation of s-aryl complexes of platinum(IV) obtained in accord with reaction (27) and described in previous sections. These complexes have been characterized by NMR and IR spectra but were not isolated. Sanford [30a] carried out the thermal reaction (27) with 1,2difluorobenzene and using modified Shul'pin's procedure (a chromatography on ammonia-free silica gel) obtained the complex [s(F2C6H4)PtIVCl4(OH2)‒(H3O)þ]. Platinum fragment does not enter into o-positions. This complex is a likely intermediate in Na2PtCl4catalyzed CeH arylation reaction with diaryliodonium salts. Indeed, the reaction of this complex with anisole gave p-platinated anisole which in the course of prolong heating was transformed into the corresponding substituted diphenyl. Arylplatinum(IV) complexes can be considered as models of intermediates in CeH bond activation at palladium(IV) centers [30]. Matsumoto [31a] described the reaction of the pivalamidatebridged platinum(III) dinuclear complex [Pt2(NH3)4(tBuCONH)2(OH2)2]4þ with phenol in water to afford 2hydroxyphenyl platinum dinuclear complex [Pt2(NH3)4(tBuCONH)2(C6H4(OH))]3þ. Interestingly, this interaction occurred via ortho CeH bond activation in the axial position and gave a derivative containing a substituent in ortho position to the bulky high-valent platinum fragment. The authors note that “the steric hindrance is not so critical in the present reaction with phenol compared to Shul'pin's reaction”. Possibly this can be explained by existence of a hydrogen bonding between the phenolic oxygen and the NH3 ligand. Litvinenko and Zamaschikov [31b] reported on the coupling of aromatic hydrocarbons with p-iodonitrobenzene under phase transfer conditions using 50% aqueous KOH and 18-crown-6. The reaction is catalyzed by the phenanthroline complex Pt(phen)Cl2. The proposed mechanism includes the following stages. The first step (49), leading to the activation of ArH, involves the oxidative addition of the CeH bond to Pt(II) to afford an arylhydride complex of platinum(IV):

ArH þ PtII %ArPtIVH

(50)

Step (51) features the loss of a proton in a reductive elimination by the action of base to give an aryl complex of platinum(II):

ArPtIVH þ HO /ArPtII þH2 O

(51)

This species is more nucleophilic than the starting complex and is capable of the oxidative addition of the aryl iodide, giving a diaryl derivative of platinum(IV) in step (52):

ArPtII þ Ar0 I/ArPtIVAr0

(52)

Final step (53) features formation of a diphenyl and regeneration of platinum(II):

ArPtIVAr0 /ArAr0 þ PtII

(53)

The reaction of complex [PtMe2(NN)] and B(C6H5)3/H2O in CF3CH2OH with arenes gives compound [PtAr{HOB(C6F5)3}(NN)] where NN is the bis(pyridyl) ligand [31c]. Gunnoe, Cundari and coworkers described Pt(II)-catalyzed reactions of hydrophenylation of olefins [32]. In recent years, publications on Pt(II) and Pt(IV) aryl

11

complexes [33] were mainly devoted to the synthesis of these compound not by direct method described above but via alternative routes, for example, by oxidation of Pt(II) complexes [33d,e] or oxidative addition [33f,g]. Finally, complexes of other platinum metals have been used as catalysts, for example, recently Jones, Goldberg and co-workers described catalytic arene exchange with rhodium and iridium complexes [34a], Oro and co-workers found that hydride-rhodium(III)-N-heterocyclic carbene complexes are very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions [34b], a paper by Larrosa and co-workers was devoted to ruthenium-catalyzed regioselective functionalization of meta-C(Ar)-H bonds [34c]. Acknowledgments The authors thank the Russian Foundation for Basic Research (grant 12-03-00084-a) and the “Science without Borders Program, BrazileRussia”, CAPES (grant A017-2013) for support. References [1] (a) A.E. Shilov, G.B. Shul'pin, Chem. Rev. 97 (1997) 2879e2932; (b) A. Sen, Acc. Chem. Res. 31 (1998) 550e557; (c) A.E. Shilov, G.B. Shul'pin, Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer Academic Publishers, Dordrecht/Boston/London, 2000; (d) A.S. Goldman, K.I. Goldberg (Eds.), Activation and Functionalization of CeH Bonds, Wiley, Washington, D.C., 2004; (e) H. Schwarz, Angew. Chem. Int. Ed. 50 (2011) 10096e10115; (f) T. Newhouse, P.S. Baran, Angew. Chem. Int. Ed. 50 (2011) 3362e3374; (g) M. Sun, J. Zhang, P. Putaj, V. Caps, F. Lefebvre, J. Pelletier, J.-M. Basset, Chem. Rev. 114 (2014) 981e1019; (h) M.C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R.M. Navarro, J.L.G. Fierro, Catal. Today 171 (2011) 15e23;
rez (Ed.), Alkane CeH Activation by Single-site Metal Catalysis, (i) P.J. Pe Springer, Heidelberg, 2012; (j) V.N. Cavaliere, D.J. Mindiola, Chem. Sci. 3 (2012) 3356e3365;
rez, Chem. Soc. Rev. 42 (2013) 8809e8820; (k) A. Caballero, P.J. Pe (l) F.J. Fernandez-Alvares, M. Iglesias, L.A. Oro, V. Passarelli, in: second ed., in: J. Reedijk, K. Poeppelmeier (Eds.), Comprehensive Inorganic Chemistry II, vol. 8, Elsevier, 2013, pp. 399e432 (Chapter 8.09); (m) A. Sivaramakrishna, P. Suman, E.V. Goud, S. Janardan, C. Sravani, T. Sandeep, K. Vijayakrishna, H.S. Clayton, J. Coord. Chem. 66 (2013) 2091e2109; (n) G.B. Shul'pin, Dalton Trans. 42 (2013) 12794e12818; (o) R.H. Crabtree, J. Organomet. Chem. 751 (2014) 174e180; (p) E.G. Chepaikin, J. Mol. Catal. A Chem. 385 (2014) 160e174. [2] A.E. Shilov, A.A. Shteinman, Kinet. Catal. 14 (1973) 149e163. [3] (a) N. Basickes, A. Sen, Polyhedron 14 (1995) 197e202; (b) M. Lin, C. Shen, E.A. Garcia-Zayas, A. Sen, J. Am. Chem. Soc. 123 (2001) 1000e1001; (c) R.H. Crabtree, J. Chem. Soc. Dalton Trans. 17 (2001) 2437e2450; (d) U. Fekl, K.I. Goldberg, Adv. Inorg. Chem. 54 (2003) 259e320; (e) J.A. Labinger, J. Mol. Catal. A Chem. 220 (2004) 27e35; (f) R.H. Crabtree, J. Organomet. Chem. 689 (2004) 4083e4091; (g) B.L. Conley, W.J. Tenn III, K.J.H. Young, S.K. Ganesh, S.K. Meier, V.R. Ziatdinov, O. Mironov, J. Oxgaard, J. Gonzales, W.A. Goddard III, R.A. Periana, J. Mol. Catal. A Chem. 251 (2006) 8e23; (h) B.G. Hashiguchi, S.M. Bischof, M.M. Konnick, R.A. Periana, Acc. Chem. Res. 45 (2012) 885e898. [4] (a) A.A. Fokin, P.R. Schreiner, Chem. Rev. 102 (2002) 1551e1593; (b) M. Lersch, M. Tilset, Chem. Rev. 105 (2005) 2471e2526; (c) M. Hirano, S. Tatesawa, M. Yabukami, Y. Ishihara, Y. Hara, N. Komine, S. Komiya, Organometallics 30 (2011) 5110e5122; (d) J.A. Labinger, J.E. Bercaw, Top. Organomet. Chem. 35 (2011) 29e60; (e) O. Mironov, S.M. Bischof, M.M. Konnick, B.G. Hashiguchi, V.R. Ziatdinov, W.A. Goddard III, M. Ahlquist, R.A. Periana, J. Am. Chem. Soc. 135 (2013) 14644e14658. [5] (a) D. Balcells, E. Clot, O. Eisenstein, Chem. Rev. 110 (2010) 749e823; (b) D.H. Ess, T.B. Gunnoe, T.R. Cundari, W.A. Goddard III, R.A. Periana, Organometallics 29 (2010) 6801e6815; (c) D. Balcells, O. Eisenstein, in: second ed., in: J. Reedijk, K. Poeppelmeier (Eds.), Comprehensive Inorganic Chemistry II, vol. 9, Elsevier, 2013, pp. 695e726 (Chapter 9.26). [6] (a) J.L. Garnett, R.J. Hodges, J. Am. Chem. Soc. 89 (1967) 4546e4547; (b) J.L. Garnett, R.J. Hodges, J. Phys. Chem. 72 (1968) 1673e1682; (c) R.J. Hodges, D.E. Webster, P.B. Wells, J. Chem. Soc. (A) (1971) 3230e3238; (1972) 2571e2576.

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