Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 2

Very few clusters containing a Ru3Ir or Os3Ir mixed metal frame work have been reported in the literature.. In Cluster IIIa, structurally characterized in the solid state two of the bri

Chapter 2 Synthesis of Cp- and Cp*- containing ruthenium-iridium

and osmium- iridium mixed metal clusters

2.1 Introduction

Although the chemistry of mixed metal platinum group metals have been extensively

studied, a greater proportion of these studies have involved clusters of Ru, Pd and Pt

mixed metal framework Tetrahedral clusters containing Rh, Os, Ir or Ru mixed metal

framework have been less extensively studied Very few clusters containing a Ru3Ir or

Os3Ir mixed metal frame work have been reported in the literature Recently

Suss-Fink and coworkers have reported a very high yield synthesis of the carbonyl cluster

anions [Ru3Ir(CO)13]- and [Os3Ir(CO)13]- by a redox condensation reaction [1, 2] The

reaction sequences are shown in Schemes 2.1 and 2.2, respectively

Scheme 2.1

-+H

-+H 2 -CO -

+ Na[Ru 3 H(CO) 11 ] [Ir(CO)Cl(PPh 3 ) 2 ]

II III

IV

V

Scheme 2.3

The low temperature 1H NMR of cluster III suggested the presence of two isomers

IIIa and IIIb in solution In Cluster IIIa, (structurally characterized in the solid state)

two of the bridging hydrides were found to be equivalent but different from the third,

whereas in cluster IIIb, all three hydrides were found to be nonequivalent The

structures of the two isomers are shown Figure 2.1

IIIa IIIb

Figure 2.1 Two different isomeric forms of Ru3Ir(CO)11PPh3 in solution

Among the very few literature reports available on tetranuclear clusters containing the

Ru3Ir and Os3Ir mixed metal framework, only a handful are on clusters containing a

Cp or Cp* ligand Table 2.1 summarizes the known tetranuclear Ru3Ir and Ru3Rh, as well as Os3Ir and Os3Rh clusters possessing either a Cp or Cp* ligand, along with the isolated yields and 1H NMR chemical shifts

Table 2.1 Known Cp and Cp* containing Ru3Ir and Ru3Rh tetranuclear clusters

1 H NMR

δ ppm Cluster and yield reported Shape & total electron count

(Two isomers reported) 15%

Open butterfly

or spike triangle

64

1.94 -17.95 -18.64 [5]

Rh B Ru(CO) 3

H H

Ru Ru(CO) 3 H

Os Os

Rh

Rh OC

Rh CO

Os OC

Cl

25%

Tetrahedral

60 1.57 -14.06 [10]

Tetranuclear clusters containing Os3Ir and Ru3Ir mixed metal framework possessing a

Cp or Cp* ligand seems to be unexplored The next section of the thesis describes the attempts made to synthesize Ru3Ir amd Os3Ir clusters containing a Cp or Cp* ligand

2.2 Reactions of cyclopentadienyl iridium dicarbonyls with

Ru3(CO)12

Attempts were made to synthesize Cp*IrRu3 clusters by reacting various Cp*Iridium complexes with triruthenium clusters The reaction of Ru3(CO)12 with acetonitrile in the presence of TMNO led to the formation of the bis acetonitrile derivative of ruthenium, Ru3(CO)10(CH3CN)2., which was subsequently reacted with Cp*Ir(CO)2 at ambient temperature and also under photolytic conditions IR monitoring of the reactions did not suggest the formation of any new product Decomposition of

Ru3(CO)10(CH3CN)2 was observed during the reaction and the IR spectrum showed strong peaks due to unreacted Cp*Ir(CO)2

Ionic coupling between Cp*Ir(CH3COCH3)3]2+ or [Cp*Ir(CH3CN)3]2+ with [Ru3(CO)11]2- afforded an unidentified brown solid An attempt at chromatographic separation of the product mixture yielded several bands in low yields which have not been identified Attempted purification by solvent extraction also afforded a mixture The reaction of Cp*Rh(CO)2 with Ru3(CO)12 in the presence of hydrogen has been reported by Knobler and coworkers to yield Cp*RhRu3(μ-H)2(CO)10 [6] Attempts to synthesize the iridium analogue by reacting Cp*Ir(CO)2 and CpIr(CO)2 with

Ru3(CO)12 in the presence of molecular hydrogen afforded new ruthenium-iridium mixed metal clusters in moderate yields The synthetic procedures, yields and characterization of the new clusters will be discussed in the following sections

2.2.1 Reaction of Cp*Ir(CO) 2 with Ru 3 (CO) 12 and H 2

3a, and Cp*IrRu

The mixed–metal clusters Cp*IrRu3(μ-H)2(CO)10, 3(μ-H)4(CO)9, 4a,

are formed in 40% and 13% yields, respectively, when H2 is bubbled through solutions of Ru (CO)3 12 and Cp*Ir(CO) at 70-90 ºC (Scheme 2.4) 2

Ir

Ru

H H

Ru 3 (CO) 12 + Cp*Ir(CO) 2 H 2 1 atm

The molecular structure of 3a is shown in Figure.2.2 In the solid state structure, one

of the two hydrides is found bridging an edge of the Ru3 triangle and the second hydride is found bridging a ruthenium-iridium edge The molecule is asymmetric and the two hydrides are inequivalent However, the ambient temperature proton NMR spectrum shows a single sharp hydride signal at δ -18.21 ppm A VT 1H NMR experiment showed broadening of the hydride resonance on cooling, but no decoalescence was observed even down to 190 K

Figure 2.2 ORTEP diagram of Cp*IrRu3(μ-H)2(CO)10, 3a Thermal ellipsoids are

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

The cluster 4a is stable in air for a short period of time Slow decomposition to an

insoluble black solid was observed after a few hours This cluster was previously

[(NPPh3)2][Ru3(CO)9(B2H )] by Galsworthy et.al [5]; it was characterized by 5 1H NMR, IR and MS However, the solid state structure of the compound was not

reported We have determined the molecular structure of 4a by an X-ray crystallographic analysis The ORTEP plot of 4a is shown in Figure 2.3

Figure 2.3 ORTEP diagram of Cp*IrRu3(μ-H)4(CO)9, 4a Thermal ellipsoids are

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

The solution IR spectrum recorded in hexane shows bands only in the terminal carbonyl region, consistent with the solid-state structure At room temperature, the proton NMR spectrum in deuterated toluene showed a singlet at δ 1.79 ppm which could be assigned to the Cp* ligand and another sharp singlet at δ -18.59 ppm assignable to rapidly exchanging hydrides; on cooling the solution, the hydride signal broadens, but no decoalescence was observed down to 200 K A FAB-MS spectrum

of the crystals showed a very strong molecular peak at m/z = 887

2.2.2 Reaction of CpIr(CO) 2 with Ru 3 (CO) 12 and H 2

The analogous reaction of Ru (CO)3 12 with CpIr(CO)2 in the presence of hydrogen

afforded only 3b in 46% yield; the tetrahydrido cluster, CpIrRu (μ-H) (CO)3 4 9,

analogous to 4a was not formed even after refluxing the reaction mixture for 10 h

Replacing the Cp* ring with a Cp ring could have possibly influenced the reactivity Cp*Ir(CO)2 being a relatively stronger nucleophile than CpIr(CO)2 could have

probably favored further reactivity to afford 4a

The solid state structure of 3b is similar to that of cluster 3a except for the position of

the bridging hydrides (Figure 2.4)

Figure 2.4 ORTEP diagram of CpIrRu3(μ-H)2(CO)10, 3b Thermal ellipsoids are

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

The related Cp- containing clusters, CpRhRu (H)3 2(CO) and CpIrOs (H) (CO)10 3 2 10, had

similar disposition of hydrides [6, 8] The IR spectral profile of 3b in hexane was

similar to those of the above related clusters The IR spectrum in hexane solution exhibited a peak at 1820 cm-1, showing that the bridging carbonyl persisted in solution The 1H NMR spectrum of 3b recorded at room temperature showed a singlet

at δ 4.84 ppm due to Cp protons and a singlet at δ -17.84 ppm due to the bridging hydrides It was not possible to observe the low temperature limiting NMR spectrum

even at 200 K as in the case with 3a and 4a A FAB-MS spectrum showed a very

strong molecular ion peak at m/z = 842.4 The compound was highly unstable and decomposed to a black insoluble compound even in the solid state after a few hours

Reactions of cluster 3a with 1 atmosphere hydrogen for 6 h yielded cluster 4a in

about 60-70% yields However, quantitative conversion was not observed even after

prolonged heating The reverse reaction of 4a under 1 atmosphere CO resulted in

cluster fragmentation with the formation of Ru (CO)3 12 and Cp*Ir(CO)2 indicating that the metal-metal bonds in these clusters are not very strong (Scheme 2.5) This type of break down of cluster skeleton was previously reported in Cp*RhRu (μ-H) (CO)3 2 10

+ CO

H

Scheme 2.5

2.3 Reactions of cyclopentadienyl iridium dicarbonyl with triosmium clusters

Triosmium based mixed metal clusters have been previously synthesized from the formally unsaturated hydrido triosmium cluster Os3(μ-H)2(CO)10, by procedures that exploit its Lewis acid character [11-16] In the preparation of the mixed metal cluster, FeOs3(μ-H)2(CO)13, it has been shown that Os3(μ-H)2(CO)10 can also function both as

a Lewis acid and a Lewis base (Scheme 2.6) [17, 18]

Os 3 ( μ-H) 2 (CO) 10 + [Fe(CO) 4 ] 2- Os 3 ( μ-H) 2 Fe(CO) 13 (1)

Os 3 ( μ-H) 2 (CO) 10 + [Fe 2 (CO) 9 ] Os 3 ( μ-H) 2 Fe(CO) 13 (2)

2.3.1 Reaction of Cp*Ir(CO) 2 with Os 3 ( μ-H) 2 (CO) 10

The reaction of Os3(μ-H)2(CO)10, 5, with Cp*Ir(CO) , 2a, in toluene at2 120 ºC afforded the mixed-metal cluster, Cp*IrOs (μ-H) (CO)3 2 10, 3c, as a red, air-stable solid

in 71% yield, and Cp*IrOs4(μ-H)2(CO)13, 7, in trace amounts (Scheme 2.7)

Scheme 2.7

The infrared spectrum of cluster 3c exhibited a pattern similar to those of the related

clusters CpMOs (μ-H) (CO) (M = Co, Rh, Ir)3 2 10 (Figure 2.5 and Table 2.2) [6, 8, 9, 19] However, the terminal and bridging carbonyl stretching frequencies are much

lower in 3c, and may be attributed to the comparatively higher basicityof the Cp* ring The stretching frequency values were comparable to the values of the closely related cluster, Cp*RhOs3(μ-H) (CO)2 10 [9] The FAB-MS spectrum showed a very strong molecular ion peak at m/z = 1180.8 and fragment clusters of peaks corresponding to successive loss of up to 10 carbonyls Crystals of suitable quality were grown from hexane at -30 ºC to further confirm the structure by X-ray analysis

The ORTEP plot of 3c is shown in Figure 2.6

Figure 2.5 IR spectrum of 3c recorded in hexane

Table 2.2 νCO stretching frequencies of known CpMOs3 clusters

Figure 2.6 ORTEP diagram of Cp*IrOs3(μ-H)2(CO)10, 3c Thermal ellipsoids are

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

The proton NMR spectrum of 3c at room temperature consists of a single sharp peak

at δ1.72 ppm which could be assigned to the Cp* group and two broad peaks in the hydride region indicating their non-equivalence and also their fluxional nature On lowering the temperature to 233 K the two broad signals sharpen and appear at δ -20.66 and -17.65 ppm for Ha and Hb (Figure 2.7) These assignments were made based on related compounds where it has been observed that osmium-osmium bridging hydrides cis to a carbonyl bridge experience chemical shifts at higher field than -20 ppm whereas osmium-osmium bridging hydrides not cis to a bridging carbonyl have chemical shifts at lower field than -20 ppm [14, 16, 20] Presumably the exchange mechanism is the same as that for the RhRu3 analogues [21]

Figure 2.7 Tentative of 1H NMR assignments for the bridging hydrides in 3c

The pink band which was isolated from the column with hexane and dichloromethane

in a very low yield was identified as Cp*IrOs4(μ-H)2(CO)13, 7 The IR spectrum of 7

recorded in dcm solution showed the presence of terminal carbonyls X-ray diffraction-quality crystals were obtained by slow diffusion of hexane into a

dichloromethane solution at -30 ºC The ORTEP plot of 7 showing the atomic

labeling scheme, and selected bond parameters, is shown in Figure 2.8

The structure is similar to that of CpRhOs4(μ-H)2(CO)13 reported by Shore and

coworkers [22] The metal core of cluster 7 consists of a tetrahedral Os4 metal framework, edge bridged by a Cp*IrCO fragment The cluster contains a total of 74 valence electrons which is consistent to that expected for an edge bridged tetrahedron according to EAN rule Three carbonyl ligands are attached to each osmium atom The iridium atom is attached to a Cp* ring and a carbonyl All the carbonyls are terminal in nature Bridging hydrides were found to span the Os(2)-Os(4) and Os(2A)

- Os(4) edges which at 2.9459(3) Å are the longest Os-Os distances in this structure The osmiums which are bridged by iridium have the shortest Os-Os distance [Os(2)-Os(2A) = 2.7764(4) Å]

The proton NMR spectrum is consistent with the molecular structure obtained in the solid state At 300 K (CDCl3), the singlet at δ 1.85 ppm could be assigned to the Cp* protons and the sharp singlet at δ -19.74 ppm to the 2 equivalent bridging hydrides

Figure 2.8 ORTEP plot of Cp*IrOs4(μ-H)2(CO)13, 7 and selected bond parameters

Thermal ellipsoids are drawn at 50% probability level Organic hydrogens are omitted for clarity Os(1)-Os(2A) = 2.7764(4) Å; Os(2)-Os(3) = 2.8261(3) Å; Os(2)-Os(4) = 2.9459(3) Å; Os(3)-Os(4) = 2.7848(4) Å; Ir(1)-Os(2) = 2.8109(3) Å; Ir(1)-Os(2)-Os(4) = 97.098(9)º; Ir(1)-C(11)-O(11) = 170.3(8)º; Ir(1)-Os(2)-Os(3) = 120.835(8)º; Os(2)-Ir(1)-Os(2A) = 59.19(1)º; Os(2A)-Os(3)-Os(2) = 58.842(10)º

(μ-H) (CO) is often present as an impurity in Os

(μ-H)2(CO)10 and hence could be the precursor for 7 The reaction of Os4(μ-H) (CO)4 12

with Cp*Ir(CO)2 under similar conditions, however, afforded 7 only in trace amounts thus ruling out this possibility The reaction of cluster 3c with hydrogen (100 psi) in a

stainless steel autoclave at 120 ºC for 24 h afforded Cp*IrOs3(μ-H) (CO)4 9, 4bas a dark orange, air-stable solid in 90% yield after chromatographic separation (Scheme

2.8) This thus indicated that the formation of 7 did not proceed via 3c either

+ CO

4b 3c

Scheme 2.8

The IR spectral profile of 4b was similar to Cp*RhOs3(μ-H)4(CO)9 [9] The molecular structure was further confirmed by X-ray crystallographic analysis; the ORTEP plot

of 4b is shown in Figure 2.9

Figure 2.9 ORTEP diagram of Cp*IrOs3(μ-H)4(CO)9, 4b Thermal ellipsoids are

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

The proton NMR spectrum of 4b recorded at room temperature shows a singlet at δ

-19.27 ppm, suggesting the fluxional nature of the hydrides (Figure 2.10) On cooling the sample to 195 K a total of five signals could be seen, of which two singlets of

equal intensity at δ -17.64 and δ -20.22 ppm could be assigned to structure (II) The singlets at δ -18.36 and δ -20.44 and δ -20.92 ppm of relative intensities 1:2:1 could

be assigned to the structure (I), which is that observed in the solid state In solution, structure (II) appears to be the predominant isomer (Figure 2.11) The two isomers are present in a 1.0:3.7 ratio in solution The chemical shift assignments were made based

on observations by earlier workers that the chemical shift for a metal-hydride bridging

an Os-Os edge lies at a higher field than that bridging an Ir-Os edge [23, 24]

In the 1H EXSY spectrum of 4b recorded in d8-toluene (τm = 0.1 sec) at 200 K, exchange cross peaks were seen between all the five resonances indicative of hydride exchange that results in isomerisation between the two isomers as well as hydride exchange within the same isomer (Figure 2.12) Since the system seemed too complicated to derive thermodynamic and kinetic parameters, we have attempted to propose a set of exchange pathways to account for the observed cross peaks in the 1H EXSY spectrum

A plausible set of exchanges which involve either single hydride migration, or two hydride migrations (either simultaneous or step-wise) that can account for the observed exchange cross peaks is depicted in Scheme 2.9 The cross peak between Hb and He can only be accounted for by a more complex exchange mechanism, and the weaker intensity compared to the others is consistent with that

Figure 2.10 1H VT NMR of 4b recorded in d8- toluene

Ir Ha

Hb Hd