Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 5

Chapter 5 Oxidative addition chemistry of Cp*IrOs3 μ-H2CO10 with Group 16 substrates 5.1 Chalcogenide transition metal carbonyl clusters The reaction of group 16 substrates with transi

Chapter 5 Oxidative addition chemistry of Cp*IrOs3( μ-H)2(CO)10 with

Group 16 substrates

5.1 Chalcogenide transition metal carbonyl clusters

The reaction of group 16 substrates with transition metal carbonyl clusters often

resulted in oxidative-addition rather than simple substitution For example, oxidative

addition of PhSeSePh to Os3(CO)10(CH3CN)2 was reported to occur with Se-Se bond

cleavage to give Os3(CO)10(SePh)2 (Scheme 5.1) [1]

Os 3 (CO) 10 (CH 3 CN) 2 PhSeSePh (CO) 4 Os Os(CO) 3

PhSe Os(CO) 3

SePh

isomerisation in refluxing hexane

Os (CO) 3

O (CO) 3

(CO) 3 Os

-CO

Scheme 5.1

Chalcogenide transition metal carbonyl clusters have been attracting attention in

recent years both in fundamental research and in technological fields [2] They are of

interest in organometallic chemistry because of the unusual coordination modes and

geometries they exhibit [3, 4] Furthermore, the presence of chalcogenide ligands

often appears to be a key factor in cluster growth reactions [5] The chalcogenide atoms also play a major role in cluster catalysis due to their potential to act as stabilizing ligands, thus preventing cluster fragmentation even under forcing conditions Some of these clusters also display unique catalytic activity For example, chalcogenide ruthenium derivatives have shown promising catalytic activity in oxygen reduction reactions in polymer electrolyte fuel cells (PEMFC) [6] The presence of selenium in these clusters is thought to be responsible for the catalytic activity

One of the effective methods for synthesizing transition metal clusters containing bridging chalcogenido ligands involves the tertiary phosphine chalcogenides R3PE (E

= S, Se, Te) This method takes advantage of the frailty of the P=E bond, which leads

to its formal oxidative addition to the cluster, resulting in the transfer of the selenium atom to low-valent metal-centres, sometimes followed by release or addition of metal fragments The product distribution in these reactions is strongly dependent on the reaction conditions and the cluster to phosphine molar ratio For example, reaction of the tetrahedral mixed-metal clusters, MCo3(μ-H)(CO)12 (M = Ru or Fe), with phosphine selenides afforded new chalcogenido-carbonyl bimetallic clusters These reactions gave two main types of products: (1) trinuclear selenido clusters of the type MCo2(μ3-Se)(CO)9-n(L)n] (n = 1, 2 with L = monodentate ligand) resulting from selenium transfer, and (2) tetranuclear clusters of the type [MCo3(μ-H)(CO)12-n(L)n] obtained by substitution of carbonyl groups by the deselenized phosphine ligand (Scheme 5.2) [7]

Co Se

H MCo 3 ( μ-H)(CO) 12 + Ph 2 RP=Se

(Ph 2 RP)

(Ph 2 RP) (Ph 2 RP)

Scheme 5.2

Reactions of homonuclear metal clusters with organosulphur substrates have also been quite extensively studied, and provide an entry into sulphur-containing clusters Ruthenium, osmium, rhodium and iridium clusters containing sulphide bridges are of special interest since they are known to be good hydrodesulfurization catalysts [8] Adams has given a detailed report on compounds containing bridging sulfid0 ligand and the transition element osmium [9] Reactions of aryl or alkyl sulfides with

M3(CO)12 (M = Ru, Os) have been reported to afford the clusters Os3SR) (μ3,η2-C6H4)(CO)9 (R = Me, iPr) and Ru3(μ-SPh)(μ -η1 η6-C6H5)(CO)8 as a result

(μ-H)(μ-of aryl-S cleavage (Scheme 5.3) [10]

Ru Ru

Scheme 5.3

The chemistry of tellurium-containing clusters can be quite different from those of sulphur and selenium analogues For instance, the difference in reactivity of the trinuclear iron clusters, Fe3(CO)9(μ3-E)2, (E = S, Se, Te), towards Lewis bases has been attributed to the larger size of the Te atom which results in a more strained Fe-Te-Fe angle in Fe3(CO)9(μ3-Te)2 than in its sulphur and selenium analogues [11]; adduct formation can release the strain by cleavage of the bond between the apical and one of the basal iron atoms (Scheme 5.4)

Te

Fe PPh 3

No reaction PPh 3

Scheme 5.4

Mixed-metal clusters containing chalcogenides are much less common Some examples are the clusters [Fe2MTe3(CO)11]2⎯ (M = Mo, W) which have been synthesized methanothermally using a mixture of Fe3(CO)12 and M(CO)6 (M = Mo, W) with Na2Te2 in a sealed tube at 80 ºC [12]

Although there are number of reports on the reaction of homonuclear carbonyl clusters with group 16 substrates, relatively little is known on the reactivity of these substrates with mixed-metal clusters To date there have been no reports on the reactivity of osmium-iridium mixed-metal clusters with chalcogenides In

continuation of our reactivity studies on 3c, we have investigated its reactivity with

chalcogenide substrates under mild conditions The results of these studies are discussed in the following sections

5.2 Reaction of Cp*IrOs3( μ-H)2(CO)10 with thiophenol

The reaction of Cp*IrOs3(μ-H)2(CO)10, 3c, with excess thiophenol under chemical

activation with TMNO at room temperature afforded the novel cluster Cp*IrOs3H)3(CO)9(μ-SPh), 22, as a bright orange crystalline product in ~87% yield (based on

(μ-consumed 3c) Diffraction-quality crystals were grown by slow diffusion of hexane

into a dichloromethane solution The ORTEP plot is shown in Figure 5.1

Figure 5.1 ORTEP diagram and selected bond parameters for 22 Thermal ellipsoids

are drawn at 50% probability level Organic hydrogens are omitted for clarity Os(2) = 2.9037(3) Å; Ir(1)-Os(3) = 2.9242(3) Å; Ir(1)-Os(4) = 2.9327(3) Å; Os(2)-Os(3) = 2.8387(3) Å; Os(2)-Os(4) = 2.8448(3) Å; Os(3)-S(5) = 2.4343(15) Å; Os(4)-S(5) = 2.4257(14) Å; S(5)-Os(3)-Os(2) = 78.00(3)º; S(5)-Os(4)-Os(2) = 78.01(3)º; Os(3)-Os(2)-Ir(1) = 61.213(7)º; Os(4)-Os(2)-Ir(1) = 61.340(7)º

Ir(1)-The molecule of 22 contains a wingtip-bridged butterfly cluster core consistent with

the total valence electron count of 62 An osmium and iridium atom occupied the hinge of the butterfly while the wingtips were occupied by two osmium atoms The wingtip osmium atoms were bridged by a benzene thiolato group Three hydrides were found bridging the three iridium-osmium edges; their presence was further confirmed by 1H NMR spectroscopy Bridging hydrides tend to elongate metal-metal bond distances and thus the presence of hydride bridges across all the three iridium-osmium bonds led to elongation of the bond distances Ir(1)-Os(2) = 2.9037(3) Å, Ir(1)-Os(3) = 2.9242(3) Å and Ir(1)-Os(4) = 2.9327(3) Å The Os(3)…Os(4) distance

at 3.6970(8) Å was clearly too large to allow any significant direct metal-metal bonding The open edge of the cluster contained the bridging benzene thiolato group which served formally as a three-electron donor The Os(3) and Os(4) atoms were asymmetrically bridged by the benzene thiolato moiety [Os(3)-S(5) = 2.4343(15) Å, Os(4)-S(5) = 2.4257(14) Å] The Os-S distances closely matched the Os-S distances

in Os3(μ-H)(μ-SMe)(μ-η2-C6H4)(CO)9 (2.418(4) and 2.433(5) Å, respectively) [13] The S(5)-Os(4)-Os(2) and Os(4)-S(5)-Os(3) bond angles were 78.01(3) and 99.05(5)º, respectively The thiolato bridge was oriented perpendicular to the triosmium plane The carbon-sulphur bond distance [C(4)-S(5)] measuring 1.790(5) Å matched with the S-C bond distance reported in Ru3(μ-SPh)(μ,η1η6-C6H5)(CO)8

[1.796(4) Å] [10] Rosenberg et.al and Lewis et al have investigated the reaction of thiophenol with the triosmium clusters viz., [Os3(CO)10 (μ-dppm)] and [Os3(CO)9(μ-dppm)(MeCN)] [14, 15] Oxidative addition of an SH bond occurred in both the cases

to give the product [Os3(CO)8(μ-S)(μ-dppm)(μ-H)] However, metal-metal bond cleavage was not observed in these reactions

The formation of 22 from 3c here involves oxidative addition across the S-H bond via

cleavage of an Os-Os bond (Scheme 5.5)

Ir

Os S

H H H

Os

Os Os

Ir

H H

PPh 3

Scheme 5.6

The compounds were partially separated by TLC on silica-gel, with hexane/ dichloromethane (4:1, v/v) as eluant The fastest moving band afforded the novel compound Cp*IrOs3(μ-Η)2(CO)9(μ3-Se), 23, which was completely characterized

spectroscopically and analytically, as well as by a single crystal X-ray structural analysis The ORTEP diagram of 23, together with selected bond parameters, is given

in Figure 5.2

Figure 5.2 ORTEP diagram and selected bond parameters for 23 Thermal ellipsoids

are drawn at 50% probability level Organic hydrogens are omitted for clarity Os(3) = 2.8163(5) Å; Ir(1)-Os(2) = 2.8927(5) Å; Os(2)-Se(5) = 2.5196(9) Å; Os(4)-Se(5) = 2.5242(9) Å; Os(2)-Os(4) = 2.8123(5) Å; Os(2)-Os(3) = 2.8477(5) Å; Os(3)-Os(4) = 2.9484(5) Å; Ir(1)-Se(5) = 2.4057(9) Å; Ir(1)-Se(5)-Os(4) = 105.81(3)º

Ir(1)-Cluster 23 consists of a butterfly Os3Ir core, with two osmium atoms forming the hinge The butterfly cluster core is the result of cleavage of an iridium-osmium bond

in the parent tetrahedral cluster 3c There is a triply bridging selenium atom along an

IrOsOs edge The selenium atom acts as a four-electron donor, and the cluster has a total valence electron count of 62, consistent with five metal-metal bonds

In butterfly clusters, the Os-Se distances to the wingtip osmium atoms are generally

longer than those to the hinge metal atoms Thus, in 23, the Os-Se distance to the

wingtip osmium is longer than to the hinge The Os-Se bond distances are in agreement to those observed in related clusters, Os3(µ3-Se)(µ-H)2(CO)8(PPh3) [2.5190(8), 2.5061(7) and 2.5065(8) Å] and Os3(µ3-Se) (CO)9(PPh3), [2.5315(7) and 2.5161(6) Å] [16]

The clusters Cp*IrOs3(μ-H)2(CO)9(PPh3), 17a, and Os3(μ-H)2(CO)7(μ3-Se)(PPh3)2,

24, moved together as a single band on the TLC plate They were crystallized and

separated by hand, the former as red crystals and the latter as orange-yellow crystals

Cluster 17a was also obtained from the reaction of 3c with PPh3 and has been described (Chapter 4)

Cluster 24 has been characterized by X-ray crystallography; the ORTEP plot is shown

in Figure 5.3 It is a triosmium cluster with a capping selenium atom It is structurally quite similar to the previously reported cluster Os3(CO)9(μ3-Se)(PPh3), except that two of the carbonyls are replaced by a phosphine ligand and two bridging hydrides [16]

Figure 5.3 ORTEP diagram and selected bond parameters for 24 Thermal ellipsoids

are drawn at 50% probability level Organic hydrogens are omitted for clarity Os(3) = 2.7940(5) Å; Os(1)-Os(2) = 2.9858(4) Å; Os(2)-Os(3) = 2.9419(4) Å; Os(2)-Se(4) = 2.5042(9) Å; Os(3)-Se(4) = 2.5020(9) Å; Os(1)-P(1) = 2.334(2) Å; Os(2)-P(2) = 2.356(2) Å; P(1)-Os(1)-Os(3) = 154.56(5)º; P(2)-Os(2)-Se(4) = 162.88(6)º

Os(1)-The two slowest moving bands afforded red crystalline samples of Cp*IrOs3H)2(μ3-Se)(CO)8(PPh3), 25, and Cp*IrOs3(μ-H)2(μ3-Se)2(CO)7(PPh3), 26,

(μ-respectively Both have been characterized spectroscopically, analytically and by single crystal X-ray analyses Their ORTEP diagrams, with selected bond parameters, are given in Figures 5.4 and 5.5, respectively

Cluster 25 is essentially a phosphine-substituted derivative of 23 The 1H NMR

spectrum of 25 showed a doublet at δ -16.87 (2JHH = 3.3 Hz) which could be assigned

to the hydride bridging an Os-Ir bond, while the doublet of doublets at δ-13.95 ppm (2JPH = 11.6 Hz) may be assigned to the hydride bridging an Os-Os bond

Figure 5.4 ORTEP diagram and selected bond parameters for 25 Thermal ellipsoids

are drawn at 50% probability level Organic hydrogens are omitted for clarity

Ir(1)-Os(4) = 2.8395(7) Å; Ir(1)-Os(2) = 2.8512(7) Å; Os(3) = 2.8135(7) Å;

Os(4) = 2.8489(7) Å; Os(3)-Os(4) = 2.9812(7) Å; Ir(1)-Se(5) = 2.4038(13) Å;

Os(2)-Se(5) = 2.5379(12) Å; Os(3)-Os(2)-Se(5) = 2.316(3) Å; Os(3)-P(6) = 2.316(3) Å

Figure 5.5 ORTEP diagram and selected bond parameters for 26 Thermal ellipsoids

are drawn at 50% probability level Organic hydrogens are omitted for clarity Os(2) = 2.9232(5) Å; Os(2)-Os(3) = 2.9154(6) Å; Os(2)-Os(4) = 3.0043(5) Å; Os(3)-Os(4) = 2.7423(5) Å; Ir(1)-Se(5) = 2.4753(10) Å; Ir(1)-Se(6) = 2.4967(10) Å; Os(2)-Se(5) = 2.553(1) Å; Os(3)-Se(5) = 2.4674(10) Å; Os(3)-Se(6) = 2.5346(10) Å; Os(4)-Se(6) = 2.5613(10) Å; Os(4)-P(7) = 2.337(2) Å; P(7)-Os(4)-Os(2) = 173.39(6)º

Ir(1)-The molecular structure of 26 consists of a spiked-triangular array of metal atoms

The iridium spike does not lie in the plane of the Os3 triangle The two selenium atoms act as four electron donors, giving the cluster a total valence electron count of

64 with four metal-metal bonds, consistent with the spiked-triangle geometry The

cluster may be regarded as derivable from 25 by the capping of the IrOs2 wing with an additional μ3-Se fragment

The Os(3)-Os(4) bond distance in 26 is extremely short measuring 2.7423(5) Å and

may be attributed to the two selenium For example, chalcogen bridged Os-Os bond distances in Os3(μ3-Se)(CO)9PPh3 [2.8528(3), 2.8308(3) and 2.7703 Å] and Os3(μ-H)2(CO)8(μ3-Se)(PPh3) [non-hydride bridged distance of 2.8054(4) Å] are shorter than the value of 2.877 Å in Os3(CO)12 [16] The cluster has both the shortest and the longest Os-Se bond distances [Os(3)-Se(5) = 2.4674(10) Å, Os(4)-Se(6) = 2.5613(10)

Å] in comparison to clusters 23-25, in which the Os-Se bond distances range

between 2.5530(10)-2.4957(9) Å

The structures 23-26 readily suggest the reactivity relationships among them and we

have made attempts to establish this inter-relationship by carrying out a series of

reactions on 17a, 23, 24 and 25 with PPh3 or Ph3PSe, both in the presence, and absence, of TMNO The results obtained from these reactions are summarized in Scheme 5.7

Os Os

Ir

Os H H

Os

Os Os

Ir

H H

Scheme 5.7

Some of the observations made are as follows:

1 Monitoring the original reaction of 3c by 1H NMR spectroscopy showed that

17a, 23-26 were formed directly from the reaction

2 Formation of 17a evidently resulted from the reaction of free PPh3 with 3c

under TMNO activation(Chapter 4)

3 Liberation of free PPh3 was observed during the reaction which was confirmed

by both 31P NMR and FAB- MS spectroscopy and also suggests that Ph3PSe mainly functioned as a selenium transfer reagent This function was again

evident when 17a in turn reacted with Ph3PSe in the presence of TMNO to

afford 25

4 We have found that 23 reacted with PPh3 even in the absence of TMNO, to

afford 24 and 25 and forms 26 when reacted with Ph3PSe under TMNO

activation; these point to 23 as the precursor to 24, 25 and 26

5 Interestingly, 25 did not give rise to 26 either with PPh3 or Ph3PSe both in the

presence or absence of TMNO It was also found that a 1:2.6:2.6 ratio of 3c:

TMNO:Ph3PSe was required to ensure completion of reaction of 3c; reducing

the ratio of TMNO or Ph3PSe resulted in recovery of large amount of

unreacted 3c This evidently points to the subsequent reactions of 23 or the formation of 3c being more rapid than the formation of 23 itself

We have thus proposed a reaction sequence as given in Scheme 5.8

The formation of 24 from 23 requires the loss of a Cp*IrCO fragment although we do

not know at this point what the ultimate fate of this fragment is An example of such a cluster fragmentation with Ph3PSe is that of the reaction of RuCo3(μ-H)(CO)12 which resulted in the loss of a Co fragment to give trinuclear clusters [7]

Thus the reaction of 3c with Ph3PSe has given rise to three different types of products:

(a) tetranuclear clusters (23 and 25) with a μ3-Se capping ligand resulting from Se

transfer (b) tetranuclear cluster (17a) obtained by simple substitution of the deselenized phosphine ligand and (c) trinuclear (24) or tetranuclear clusters (26)

formed by oxidative addition of Ph3PSe