Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 4

Chapter 4 Subtitution chemistry of Cp*IrOs3μ-H2CO10 4.1 Carbonyl substitution in metal carbonyl clusters The ligand substitution chemistry of homometallic clusters have been very well

Chapter 4

Subtitution chemistry of Cp*IrOs3(μ-H)2(CO)10

4.1 Carbonyl substitution in metal carbonyl clusters

The ligand substitution chemistry of homometallic clusters have been very well

documented but ligand substitution in heterometallic clusters has been the subject of

relatively few reports The varying constituent metals of the heterometallic cluster

core afford the possibility of not only metalloselectivity upon ligand substitution but

also site selectivity due to a decrease in molecular symmetry To understand the

substitution reactions in heterometallic clusters, it will be useful to have some

knowledge on the substitution reactions of homometallic clusters

4.1.1 Carbonyl substitution in trinuclear metal carbonyl clusters

There are two types of CO ligands in M3(CO)12 (M= Ru, Os) Those in the same plane

as the M3 triangle are referred to as equatorial and those that are perpendicular to the

plane are axial (Figure 4.1)

a - axial

e - equatorial

M M

M

a a

a

e

e a e a e

a

M M

M

a a

a e

e a e a e a

Figure 4.1 Substitution positions in trinuclear clusters

When CO is substituted by a phosphine on a cluster, the coordination site adopted is restricted by the steric and electronic requirements of the comparatively bulky phosphine ligand In trinuclear clusters, the first phosphine is generally found to replace an equatorial ligand In Os3(CO)12, this preference for equatorial substitution has been accounted for in steric terms, since simple calculations on Os3(CO)12-xLxsystems have shown that the equatorial sites in (approximately) anticuboctahedral structures of ligands are less sterically hindered than axial [1-7]

Pomeroy and coworkers have given a detailed discussion on the influence of phosphine substitution on Os3(CO)11PR3 structures [8] There has also been earlier studies by Bruce and coworkers whodetermined the structures of 12 different mono- and disubstituted phosphine derivatives of Ru3(CO)12 and Os3(CO)12 to investigate the steric and electronic influences of phosphine substitution in these clusters [9, 10] The following conclusions were obtained from these studies:

1 The group 15 ligand prefers equatorial coordination site in monosubstituted derivatives In the disubstituted derivatives, the two ligands take up positions that are as far apart as possible; each occupying an equatorial site on adjacent metal atoms

2 Due to steric interactions between the group 15 ligand and the CO group cis to

it on the adjacent metal atom, the M-M bond cis to the phosphine ligand is the

longest of the three M-M separations; for the disubstituted derivatives, no such pronounced lengthening of this M-M bond has been observed

3 Introduction of two group 15 ligands into the M3(CO)12 cluster results in a twisting of the ML4 groups about the M-M axis, distorting the original D3h symmetry of the parent cluster to D3 symmetry

4 In the monosubstituted derivatives, the M-P bond length increases with

increasing cone-angle while in the disubstituted derivatives, the M-P bond lengths are almost the same as those in the corresponding monosubstituted derivatives

5 Axial M-CO bonds are longer than equatorial M-CO bonds

4.1.2 Carbonyl substitution in heteronuclear tetrahedral clusters

Heteronuclear tetrahedral clusters are of interest as species in which to scrutinize metalloselectivity and site selectivity Basically there are three positions available for substitution in tetrahedral M’M3 clusters, namely axial, equatorial and apical (Figure 4.2)

M'

M

eq eq eq

eq ax

ax ax

eq

ap ap

ax

ax ax

eq

ap ap ap

Figure 4.2 Substitution sites in heterometallic tetrahedral clusters

Studies on metalloselectivity of these clusters have revealed that the selectivity could

be influenced by factors such as the nature of the metals and the nature of both the existing and incoming ligands For example, the clusters MCo3(μ-H)(CO)12 (M= Fe, Ru) reacted with secondary and tertiary phosphines to afford monosubstituted derivatives in which the phosphine ligand was always bonded to Co, while Ru3Rh(μ-H)(CO)12 reacted with phosphines to produce monosubstituted derivatives in which

the PR3 ligand was attached to Rh [11-14] In the disubstituted product, RuCo3(CO)10(μ-H)(PMe2Ph)2, the second PMe2Ph was substituted at Ru, whereas in the case of RuCo3(μ-H)(CO)10(PPh3)2, the phosphines were bonded to one cobalt atom each [15] Pakkanen and coworkers have recently reported the structures of two isomers of [Ru3Ir(μ-H)3(CO)11(PPh3)] (Figure 4.3) [16] In one of the isomers, the phosphine ligand was coordinated to an axial position in the Ru3 basal triangle and in the other isomer the phosphine was found coordinated apically to the iridium atom

Ir

Ru H H

Figure 4.3 Isomeric structures of [Ru3Ir(μ-H)3(CO)11(PPh3)]

Table 4.1 summarizes the preferred site of substitution of phosphines for various tetrahedral mixed-metal clusters reported in the literature It can be noted that axial or apical substitution was always observed for the first phosphine ligand, while the second substitution could be equatorial or axial For example, the first substitution by PPh3 in FeRu3(μ-H)2(CO)13 occurred at an axial position on one of the basal ruthenium atoms

For tetrahedral mixed metal clusters possessing a Cp or Cp* ligand, the substitution almost always occurred at the basal metal triangle For example, the reaction of CpRu3Rh(H)2(CO)10 with phosphines afforded mono- and disubstituted phosphine derivatives where the substitution occurred at the ruthenium triangle This could be attributed to the fact that in these clusters, the bulky Cp ligand was attached to the unique heterometallic vertex Proton NMR studies of the derivatives have revealed at

least three isomers for the monosubstituted derivatives and two isomers for the

disubstituted derivatives in solution (Figure 4.4)

Table 4.1 Preferred phosphine substitution sites for tetrahedral mixed-metal clusters

Phosphine derivatives Cluster

IrRu3(μ-H)3(CO)12

apical (Ir) axial (Ru)

- [20]

CpRhRu3(μ-H)2(CO)10

axial (Ru) [major]

equatorial (Ru) [minor]

axial, equatorial (Ru, Ru) [21]

Cp*RhRu3(μ-H)2(CO)10 equatorial (Ru) [major] axial, equatorial (Ru, Ru) [21]

Rh

Ru L

L

L=PPh3

(a) axial

(b) equatorial cis to bridging CO

(c) equatorial trans to bridging CO

(d) axial,equatorial cis to bridging CO

(e) axial, equatorial trans to bridging CO

In contrast, the most stable isomer of the monosubstituted phosphine derivative in the Cp* analogue did not involve phosphine coordination at Ru(1); substitution occurred either at Ru(2) or Ru(3)

The reaction of the mixed metal cluster CpNiOs3(μ-H)3(CO)9 with phosphines in the presence of TMNO afforded disubstituted derivatives in which the phosphines were reported to be bound axially to two osmium atoms Isomers were not observed in solution

In contrast to phosphines, the substitution chemistry with isocyanides is relatively unexplored Isocyanide ligands generally show a propensity for axial substitution in derivatives of M3(CO)12 (M=Ru, Os) In complexes of the type Os3(μ-H)(H)(CO)10(CNR) (R = Me, Ph), only axial coordination was observed However, for R = tBu, equatorial substitution occurred, as has been confirmed by X-ray crystallographic analysis of Os3(μ-H)(H)(CO)9(CNBut) [23, 24] Also, in

Os3(CO)11(CNBut), there is NMR evidence for equatorial/axial isomerisation of the

t

BuNC ligand [25] In the related complex, Ru3(CO)11(CNBut), both axial and equatorial forms existed in solution However, in the solid state only the axial isomer was observed [26] Therefore it appears that for CNR ligands, the substituting ligand can occupy an equatorial or axial position depending on the steric requirements of the

R group Although electronic factors favour axial coordination of isocyanide groups, steric constraints may result in equatorial substitution In addition to the normal η1terminal bonding mode exhibited, the isocyanides also showed some propensity for

μn,η2 C-N bridged bonding modes (Table 4.2)

Table 4.2 Bridged bonding modes in isocyanides

Pt(μ-H)2(CO)10(PCy3)(CNCy), which has a butterfly structure with the CyNC bonded to

an osmium center On the other hand, reaction at 273 K produced three isomers which were in dynamic equilibrium All the three isomers exhibited butterfly geometry for the metal core, with the CyNC ligand bonded to an osmium center Refluxing a hexane solution of Os3Pt(μ-H)2(CO)10(PCy3)(CNCy) resulted in facile decarbonylation to afford a tetrahedral 58 electron unsaturated cluster Os3Pt(μ-H)2(CO)9(PCy3)(CNCy) in which the CyNC ligand was bonded to the platinum atom There was thus a transfer of the CyNC ligand from Os to Pt in the decarbonylation of the butterfly adduct as it closed to form the tetrahedral unsaturated cluster (Scheme 4.1)

H

Pt H PCy 3

Os

Pt H

Reaction of CpWIr3(CO)11 with stoichiometric amounts of isocyanides was reported

to afford the clusters [CpWIr3(CO)11-n(CNR)n] in 47-63% yields (Scheme 4.2) [32] The solid state structure of CpWIr3(CO)9(CNXy) [Xy = C6H3Me2-2,6] showed that both the isocyanides were bonded to the same iridium atom

4.2 Substitution type reactions of Cp*IrOs3( μ-H)2(CO)10, 3c

It is evident from the foregoing that clusters containing mixed metals are of interest

as sites to probe metallo selectivity and site selectivity Although there has been some reports on the synthesis of Cp- and Cp*- containing osmium-iridium clusters, there have been no reports on the reactivity of these clusters The following sections present

our studies on the substitution reaction of 3cwithvarious group 15 donor substrates like phosphines, phosphites, isocyanides and pyridine

4.2.1 Reaction of Cp*IrOs 3 ( μ-H) 2 (CO) 10 , 3c, with triphenylphosphine

Cluster 3c was found to undergo facile substitution in the presence of trimethylamine N-oxide Reaction of cluster 3c with PPh3 in dichloromethane at ambient temperature

in the presence of TMNO (dropwise addition via a dropping funnel) led to gradual deepening of the original orange-red solution to deep red over a period of 2 h Chromatographic separation of the reaction mixture on silica-gel TLC plates afforded Cp*IrOs3(μ-Η)2(CO)9(PPh3), 17a, and Cp*IrOs3(μ-Η)2(CO)8(PPh3)2, 18a,

respectively Both clusters have been completely characterized, including by single

crystal X-ray crystallographic studies The ORTEP plot of 17a is shown in Figure 4.5 The solid state structure of 17a revealed that the tetrahedral core of the parent cluster

was retained; the bridging carbonyl and the two bridging hydrides in the parent cluster were also intact and one of the axial carbonyls attached to the osmium triangle was substituted by a phosphine ligand The 1H NMR spectrum taken at 300 K in d8 toluene consisted of four resonances; one doublet at δ -19.94 ppm (2JP-H = 9.0 Hz) and three broad singlets On lowering the temperature to 233 K, the 1H NMR spectrum consisted of well-resolved resonances at δ -16.65d (2JPH = 9.1 Hz), -17.44s, -19.67d (2JPH = 10.7 Hz) and -20.01d (2JPH = 9.1 Hz) ppm (Figure 4.6)

Figure 4.5 ORTEP diagram of 17a Thermal ellipsoids are drawn at 50% probability

level Organic hydrogens are omitted for clarity

Two singlets at δ 2.14 and 2.10 ppm were also observed, assignable to two groups of Cp* methyl protons Integration of the 1H resonances supported the existence of two isomers in the ratio 1:0.14 (at 233 K) in solution A 31P selective decoupling

experiment performed on 17a by irradiating at the phosphorous resonances confirmed

that the splitting of the hydrides was indeed due to coupling with phosphorous atoms (Figure 4.7) The two doublets observed at δ -16.65 and -19.67 ppm which are of

equal intensity, and the Cp* signal at δ 2.14 ppm, could be assigned to the major isomer The magnitude of the coupling constants indicated that the phosphine was in a relative cis-position to the hydrides This suggested that the major isomer was that obtained from the X-ray structural study The singlet resonance at δ -17.44 ppm and a well-resolved doublet at δ -20.01 ppm, which are of equal intensity, and the Cp* signal at δ 2.07 ppm, could be assigned to the minor isomer The coupling constants further suggested that one of the hydride resonances was coupled to a phosphorous (2JP-H = 9.1 Hz) whereas the other hydride resonance was not

-20.4 -20.0 -19.6 -19.2 -18.8 -18.4 -18.0 -17.6 -17.2 -16.8 -16.4 -16.0

-19.2 -18.8 -18.4 -18.0 -17.6 -17.2 -16.8 -16.4 -16.0 -15.6 -15.2

-14.8

(ppm)

Before decoupling irradiation at -39.8 PPM irradiation at -23.67 PPM

Figure 4.7 P resonance coupled and decoupled H NMR spectra of 31 1 17a recorded in

d -toluene 8

It has been reported by Churchill et al and Gladfelter et al that osmium-osmium bridging hydrides cis to a carbonyl bridge have 1H NMR chemical shifts at higher field than -20 ppm and osmium-osmium bridging hydrides not cis to a bridging carbonyl have 1H NMR chemical shifts at lower field than -20 ppm [33, 34] It appears that there is a preference for axial substitution by phosphines in clusters of this general structure, even in minor isomers [21, 22, 35] If the further assumption is made that the relative arrangements of the bridging carbonyl and hydrides remain as

in all the solid-state structures obtained thus far, this arrangement being preserved also for similar derivatives of the clusters CpRhRu3(μ-H)2(CO)10 and Cp*RhRu3(μ-H)2(CO)10, then the minor isomer would correspond to substitution at one of the axial positions of either Os(3) or Os(4) (Figure 4.8) [21, 35] A 31P{1H} NMR spectrum taken at 300 K in CDCl3 showed two singlets, at δ 0.15 and δ 16.26 ppm in the ratio 1:0.14 which could be assigned to the phosphines of the major and minor isomers, respectively

Figure 4.8 Tentative structures of the minor isomer of 17a

The IR spectra of 17a recorded in dichloromethane and as a KBr pellet showed two

broad peaks in the bridging carbonyl region; the latter spectrum is shown in Figure 4.9 This suggested that the isomers existed in solution as well as in the solid state and that both the isomers had bridging carbonyls

Figure 4.9 IR spectrum of 17a recorded as a KBr pellet

The 13C NMR spectrum of 17a recorded in d8-toluene at 300 K showed the presence

of two singlets at δ 97.97 and δ 97.50 ppm, in the ratio 1:0.14, which could be assigned to the Cp* ring carbons of the major and minor isomers, respectively

Singlets at δ 10.93 and 10.77 ppm, could be assigned to the Cp* methyl carbon signals of the major and minor isomers, respectively No signals due to carbonyls were observed and this can be attributed to the low intensities

A 31P NMR spin-saturation transfer experiment on 17a at 300 K indicated that the two

isomers were undergoing chemical exchange with each other as well (see Appendix) The 1H EXSY spectrum of 17a recorded at 273 K (τm = 0.5 s) indicated that at that temperature, there was only mutual chemical exchange of the hydride resonances of the major isomer, i.e., a fluxional process (Figure 4.10) In the 1H EXSY spectrum recorded at 300 K, exchange cross peaks between all the hydrides were observed, i.e., there is an additional isomerisation process (Figure 4.11) These suggest that the simple fluxional exchange within the major isomer is more facile than the isomerisation process The former can be understood in terms of the rocking motion which has been described for the RhRu3 analogues [21, 35]; this corresponds to an incomplete merry-go-round involving the bridging carbonyl and the terminal carbonyls B, F, A’ and D’ which effectively moves the bridging carbonyl from the Ir(1)-Os(3) edge to the Ir(1)-Os(4) edge For the isomerisation process, a hydride migration will also be required (Scheme 4.3)

H A H C Intramolecular exchange crosspeak

Figure 4.10 1H EXSY spectrum of 17a recorded at 273 K (τm = 0.5 s)

Intermolecular exchange crosspeaks

Figure 4.11 1H EXSY spectrum of 17a recorded at 300 K (τm = 0.5 s)

A E F

B C'

A' D'

Scheme 4.3

The 1H NMR spectrum of 18a consisted of well-resolved resonances The singlet

resonance at δ 2.08 ppm could be assigned to the Cp* protons The doublet of doublets at δ -18.18 ppm could be assigned to the bridging hydride cis to the bridging carbonyl and the doublet at δ -15.71 ppm could be assigned to the hydride trans to the bridging carbonyl The 31P{1H} NMR spectrum showed two singlet resonances at δ -11.02 and 14.94 ppm due to the two different phosphorous atoms Both 1H and

31P{1H} NMR spectra did not suggest the presence of isomers in solution

The IR spectrum of 18a suggested the presence of a bridging carbonyl (Figure 4.12)

Substitution of electron donating phosphines for the carbonyl ligands had resulted in lowering of the νCO in 17a compared to 3c and further still in 18a The ORTEP diagram of 18a is shown in Figure 4.13 The molecular structure of 18a consisted of a

tetrahedral framework of osmium-iridium atoms Two phosphine ligands, P(5) and P(6) were coordinated to two adjacent osmium atoms in an axial and equatorial fashion, respectively

Figure 4.12 IR spectrum of 18a recorded in dichloromethane

Figure 4.13 ORTEP diagram of 18a Thermal ellipsoids are drawn at 50% probability

level Organic hydrogens are omitted for clarity

4.2.2 Reaction of 3c with P(OMe) 3

Reaction of 3c with P(OMe)3 in the presence of TMNO yielded the monosubstituted derivative, Cp*IrOs3(μ-Η)2(CO)9P(OMe)3, 17b, and the disubstituted derivative

Cp*IrOs3(μ-Η)2(CO)8[P(OMe)3]2, 18b, respectively The solution IR spectral pattern

of 17b was similar to that of 17a, suggesting axial coordination of the P(OMe)3 ligand, which was further confirmed by a single crystal X-ray crystallographic study

(Figure 4.14) The overall structure of 17b was similar to 17a with the P(OMe)3ligand axially coordinated to Os(2) of the osmium triangle No isomers were detected

in the 1H and the 31P{1H} NMR spectra

Figure 4.14 ORTEP diagram of 17b Thermal ellipsoids are drawn at 50% probability

level Organic hydrogens are omitted for clarity

The IR spectral profile of 18b was similar to that of 17b, suggesting a similar

substitution pattern The structure was further confirmed by a single crystal X-ray

crystallographic study The ORTEP plot of 18b is shown in Figure 4.15

Figure 4.15 ORTEP diagram of 18b Thermal ellipsoids are drawn at 50% probability

level Organic hydrogens are omitted for clarity

4.3 Solid state structures of mono and disubstituted PR3 [R = Ph or

OMe] derivatives of 3c

The general structural features of the mono- and disubstituted PR3 derivatives of 3c

were similar Selected bond lengths and bond angles are listed in Table 4.3 together with the common cluster numbering scheme

Table 4.3 Selected bond lengths (Å) and bond angles (º) for PR3 derivatives of 3c

Ir

Os(2) H H

Os Os