Journal of Cluster Science

, Volume 21, Issue 3, pp 379–396

Routes to Higher Nuclearity Mixed-Metal Carbonyl Clusters Using the [Rh(η5-C5Me5)(NCMe)3]2+ Dication as a Building Block

  • Saifun Nahar
  • John E. Davies
  • Gregory P. Shields
  • Paul R. Raithby
Original Paper

DOI: 10.1007/s10876-010-0331-9

Cite this article as:
Nahar, S., Davies, J.E., Shields, G.P. et al. J Clust Sci (2010) 21: 379. doi:10.1007/s10876-010-0331-9

Abstract

Reaction of the [Rh(η5-C5Me5)(NCMe)3]2+ (1) dication with the hexaosmium [Os6(CO)17]2− (2) dianion leads to the initial formation of [Os6(CO)17Rh(η5-C5Me5)] (3). This cluster readily adds CO to form [Os6(CO)18Rh(η5-C5Me5)] (4) which has been characterised crystallographically. 3 also adds dihydrogen to give [Os6H2(CO)17Rh(η5-C5Me5)] (5) and undergoes a substitution reaction with PPh3 to form [Os6(CO)16(PPh3)Rh(η5-C5Me5)] (6). With the [Ru6(CO)18]2− (7) dianion, [Rh(η5-C5Me5)(NCMe)3]2+ (1) reacts to form three mixed-metal clusters [Ru5(CO)15Rh(η5-C5Me5)] (8), [Ru6(CO)18Rh(η5-C5Me5)] (9) and [Ru6(CO)18Rh25-C5Me5)2] (10). The clusters have been characterised spectroscopically and the structures of 8 and 10 have been confirmed crystallographically. The cluster 8 undergoes a substitution reaction with P(OMe)3 to form the disubstituted product [Ru5(CO)13(P(OMe)3)2Rh((η5-C5Me5)] (11) which has also been characterised crystallographically.

Keywords

Cluster carbonyl Rhodium Osmium Ruthenium Cyclopentadienyl X-ray crystal structure 

Introduction

High nuclearity transition metal clusters are known throughout the d-block of the periodic table and fall into one of two general classes, early transition metal clusters supported by π-donor ligands such as halides, oxides or sulphides and late transition metal clusters stabilised by π-acceptor ligands such as carbonyls and phosphines [1]. There are also an increasing number of clusters that link the two classes having some properties of each such as the series of rhodium-hydrido clusters supported by phosphine ligands that have characterised recently by Weller et al. [2, 3, 4]. The exact definition of what constitutes a high nuclearity cluster varies significantly. Some take the view that clusters with six or more metal atoms can be classed as high nuclearity because the majority of these contain an octahedral metal core and this represents the smallest closed polyhedron the metal–metal bonding in which cannot be represented in terms of a fully localised system [5]. Others view high nuclearity clusters as those having ten or more metal atoms where some aspects of the metallic state have been observed [6]. Yet others reserve the term “high nuclearity” for clusters with hundreds or thousands of metal atoms and fall well within the nanoparticle regime [7].

Many of the most interesting of these high nuclearity clusters are mixed-metal clusters having two or more different metallic elements in the cluster core. These mixed metal clusters may contain two different transition elements, as in the tetraanion [Pd33Ni9(CO)41(PPh3)6]4− [8], or a transition element and a main group metal such as in [Cu146Se73(PPh3)30] [9]. The interest stems from the observation that the mixed metal systems are more catalytically active than similar homo-metal clusters [10, 11, 12]. Also of significance is that in the series of mixed-metal cluster species [Ru5CoC(CO)16], [Ru5RhC(CO)16] and [Ru5PdC(CO)16] the ability to substitute a carbonyl ligand with SO2 dramatically differs depending on whether Co, Rh or Pd centres are present [13].

A number of routes that can be employed to synthesise higher nuclearity carbonyl cluster compounds in which greater or lesser control can be exerted over the size and nature of the product. The two most commonly used methods are thermolysis and pyrolysis. Thermolysis involves heating in the presence of a solvent while pyrolysis involves heating in the solid state. Both methods tend to afford a range of products each in relatively low yield. However, these methods have been used successfully to prepare the highest nuclearity osmium cluster carbonyl anions, [Os17(CO)36]2− and [Os20(CO)40]2−, so far characterised [14, 15]. The largest dianion [Os20(CO)40]2−, still only has surface atoms but a metal core size of 0.9 × 0.9 × 0.9 nm, with each Os atom being coordinated to at least one carbonyl ligand [16].

A rather more controlled method of preparing high nuclearity clusters involves the addition of a metal-containing cation to a lower nuclearity cluster anion. The most commonly used approach is to couple a pre-formed cluster anion with a monometallic cationic species. In this way the nuclearity of the cluster is increased by only one metal, but in a controlled manner. For example, the neutral tetranuclear osmium cluster [Os4H4(CO)12] may be reduced with K/Ph2CO to form the dianion [Os4H4(CO)11]2−, with the loss of CO, and is then treated with the labile metal cation [M(η6-C6H6)(MeCN)3]2+ (M = Ru, Os) to afford the pentanuclear cluster [Os4MH4(CO)116-C6H6)] [17]. The method can be extended by choosing different cation and anion charges and ratios, for example, the use of [Os3(CO)11]2− with 2 equiv. of the cation [Ru(η5-C5H5)(MeCN)3]+ gives a species where the nuclearity has been built up by two metal units affording the pentanuclear cluster [Os3Ru2(CO)115-C5H5)2] [18]. In these examples the cation and anion charges are balanced in the final product to give a neutral species with only the neutral, labile acetonitrile ligands being lost from the final product.

In this context Johnson et al. [19] have reported the synthesis of the octahedral hexanuclear carbide cluster [Ru5C(CO)14Rh(η5-C5Me5)2] from the reaction between [Ru5C(CO)14]2− and [Rh(η5-C5Me5)(NCMe)3]2+. More recently, we have described the synthesis of the penta and hexanuclear mixed metal clusters [Os4Rh(μ-H)3(MeC=NH)(CO)115-C5Me5)], [Os4Rh(μ-H)2(CO)135-C5Me5)] and [Os4Rh2(μ-H)2(CO)115-C5Me5)2] from the reaction of the dianion [Os4H4(CO)11]2− with [Rh(η5-C5Me5)(NCMe)3]2+ [20]. We now report the results of cluster build up reactions between the non-carbido hexametallic cluster dianions [Os6(CO)17]2− and [Ru6(CO)18]2− with the cation [Rh(η5-C5Me5)(NCMe)3]2+. A preliminary account of some of this work has appeared previously [21].

Results and Discussion

The reaction of the potassium salt of the [Os6(CO)17]2− (2), generated in situ by the reaction of [Os6(CO)18] with freshly prepared K/Ph2CO in tetrahydrofuran, at −78 °C [22], with an excess of [Rh(η5-C5Me5)(NCMe)3][PF6]2 (1), in dichloromethane solution, lead to the isolation of the mixed-metal cluster [Os6(CO)17Rh(η5-C5Me5)] (3) in 21% yield together with traces of a second cluster, [Os6(CO)18Rh(η5-C5Me5)] (4). Cluster 3 was characterised by IR and 1H NMR spectroscopy and by positive ion FAB mass spectrometry (Table 1). The low solubility of the cluster prevented a meaningful 13C NMR spectrum from being obtained, The IR spectrum shows carbonyl stretching frequencies consistent with the presence of both terminal and edge-bridging carbonyl ligands and the 1H NMR spectrum displays a single peak at δ 1.26 corresponding to the methyl protons of the η5-C5Me5 ligand.
Table 1

Spectroscopic data for clusters 36

 

Clusters

3

4

5

6

IR (νCO/cm−1) CH2Cl2

2086 m, 2061 s, 2027 vs, 1710 w

2082 m, 2039 vs, 2026 m, 2012 w, 1946 ww (br)

2086 m, 2057 vs, 2050 vs, 2030 m, 2010 m, 1950 vw

2081 w, 2067 m, 2037 w, 2023 vs, 1994 vw, 1973 vw, 1640 vw

Mass (+FAB) m/z190Os (calc.)

1856 (1854)

1885 (1882)

1860 (1856)

2094 (2088)

1H NMR (δ) aCD2Cl2bCDCl3

a1.26 (s, C5Me5)

a1.55 (s, C5Me5)

a2.12 (s, C5Me5) −16.6 (d, JHRh 1.6 Hz) −20.15 (s) −22.13 (s)

b7.55–7.45 (m, Ph) 1.57 (s, C5Me5)

13C{1H} NMR (δ) CD2Cl2

189.2 (s, CO) 188.6 (s, CO) 100.2 (d, JRhC 9.1 Hz) 9.7 (s, C5Me5)

9.96 (s, C5Me5)

31P{1H} NMR (δ) CDCl3

−113.4 (s) −119.9 (s)

All attempts to grow single crystals of 3 were unsuccessful, however, from the formulation the electron count is 96 electrons, hence, it is isoelectronic with the known cluster [Os7(CO)176-C6H6)] [23]. In this cluster the osmium framework can be described as a bicapped trigonal bipyramid with the arene ring η5-bound to one of the apical Os atoms as illustrated in Fig. 1. It is reasonable to assume, therefore, that in the case of 3, that the metal core structure is similar to that of [Os7(CO)176-C6H6)] [23] and that a capping reaction occurs in which the “Rh(η5-C5Me5)” fragment couples with the bicapped tetrahedral core of the [Os6(CO)17]2− dianion, to give a related structure to that of [Os7(CO)176-C6H6)] with the “Os(η6-C6H6)” being replaced by the isolobal “Rh(η5-C5Me5)” fragment. Significantly, in [Os7(CO)176-C6H6)] the “Os(η6-C6H6)” unit occupies a position in the metal framework where the Os atom is coordinated to four other Os centres. This structure, therefore, does not represent a simple coupling of the “Os(η6-C6H6)” fragment to a face of the pre-formed Os6 bicapped tetrahedron, but suggests that a metal framework rearrangement has occurred. The “Os(η6-C6H6)” unit occupies an 18e site where the donor properties of the arene balance the electron distribution within the metal framework. It is likely that a similar process occurs in the formation of 3.
Fig. 1

The molecular structure of [Os7(CO)176-C6H6)] [23]

The cluster [Os6(CO)17Rh(η5-C5Me5)] (3) reacts readily with CO, H2 and PPh3 under mild conditions (Scheme 1). When CO gas is bubbled through a dichloromethane solution of 3 at room temperature an immediate colour change from brown to deep pink occurs. The reaction is quantitative and the product [Os6(CO)18Rh(η5-C5Me5)] (4) has been characterised by spectroscopic (Table 1) and by crystallographic techniques. The positive FAB mass spectrum of 4 exhibits a molecular ion peak at 1,885 amu, which corresponds to the proposed molecular formula. The 1H NMR spectrum displays a sharp resonance at δ 1.55 which can be assigned to the methyl protons of the η5-C5Me5 group while the 13C NMR spectrum shows a singlet at δ 9.7 and a doublet at δ 100.2 [J(Rh–C) = 9.1 Hz] that corresponds to the methyl and cyclopentadienyl carbons of the η5-C5Me5 ligand, respectively. The carbonyl signals appear in the 13C NMR spectrum in the range δ 188.6–189.2 confirming that they are all terminally bound to the osmium centres.
Scheme 1

 

Crystals of [Os6(CO)18Rh(η5-C5Me5)] (4) suitable for a single crystal X-ray diffraction experiment were obtained by slow evaporation of a dichloromethane-hexane solution at −20 °C for several weeks. Cluster 4 crystallises in the monoclinic space group P21/n with one molecule of the complex and one and a half molecules of dichloromethane solvent in the asymmetric unit. The molecules in the crystal are separated by normal van der Waals distances.

The molecular structure of 4 is shown in Fig. 2 which includes the key molecular dimensions. In contrast to the structure of 3, but consistent with the higher electron count of 98e the metal core geometry in 4 consists of an octahedral arrangement of Os atoms one face of which is capped by the “Rh(η5-C5Me5)” unit. The 18 carbonyl groups are all terminal and essential linear with three carbonyls bound to each of the six Os atoms.
Fig. 2

The molecular structure of [Os6(CO)18Rh(η5-C5Me5)] (4) showing the atom numbering scheme adopted. Selected bond lengths (Å) include: Os(1)–Os(6) 2.834(2); Os(1)–Os(5) 2.849(2); Os(1)–Os(3) 2.867(2); Os(1)–Os(2) 2.876(2); Os(1)–Rh(1) 2.911(3); Os(2)–Os(6) 2.839(2); Os(2)–Os(4) 2.859(2); Os(2)–Os(3) 2.914(2); Os(3)–Os(5) 2.844(2); Os(3)–Os(4) 2.876(2); Os(4)–Os(5) 2.8699(19); Os(4)–Os(6) 2.870(2); Os(5)–Rh(1) 2.783(3); Os(5)–Os(6) 2.839(2); Os(6)–Rh(1) 2.772(3); Rh(1)–C(23) 2.20(3); Rh(1)–C(21) 2.21(3); Rh(1)–C(19) 2.22(3); Rh(1)–C(25) 2.24(3); Rh(1)–C(27) 2.26(3)

The structure of 4 is similar to that of [Os7(CO)21] with which it is isoelectronic [24]. In 4 the “Os(CO)3” cap has been replaced by the “Rh(η5-C5Me5)” cap. The Os6 octahedron is fairly regular with an average Os–Os distance of 2.861 Å. The capping Rh centre is asymmetrically bound to the Os3 face with two shorter distances with an average of 2.778 Å and a longer distance of 2.911(3) Å. A similar asymmetry of the capping group is observed in [Os7(CO)21] [24] where the comparable bond lengths are 2.806 and 2.830(6) Å.

Since the cluster 4 was obtained as the sole product of the carbonylation of 3, and was also identified as a low yield side product in the initial synthesis of 3, the thermal stability of 4 was examined. After heating 4 under reflux, in toluene, for an hour 3 was recovered in quantitative yield. This facile addition and loss of CO between 3 and 4 is accompanied by a presumed change in metal framework geometry, from the bicapped trigonal bipyramid in 3 to the capped octahedron in 4. This is consistent with the formal addition and loss of two electrons, in going from the 96e system in 3 to 98e in 4. Cluster 3 may be described as having an “electron precise” metal framework as it is made up of fused tetrahedral units with the same number of framework edges as there are electron pairs for bonding. By contrast 4 has the capped octahedral framework, and the bonding within the octahedron is best described by a delocalised model in which there is a low lying delocalised molecular orbital that can accommodate the additional electron pair compared to 3. Facile metal framework rearrangements with the addition or loss of two electrons are common with cluster carbonyl chemistry and the structures can be rationalised in terms of the Mingos Condensed Polyhedral Bonding Model [25, 26].

The cluster [Os6(CO)17Rh(η5-C5Me5)] (3) also readily adds dihydrogen when hydrogen gas is bubbled through a dichloromethane solution of 3 for 30 min, the solution changing from brown to pink. The product, obtained in 80% yield, has been characterised as [Os6H2(CO)17Rh(η5-C5Me5)] (5) by spectroscopic methods (Table 1). The positive ion FAB mass spectrum gave a molecular ion at 1,860 amu consistent with the formulation [Os6H2(CO)17Rh(η5-C5Me5)] and the microanalysis was also consistent with this formulation. The 13C NMR spectrum displays a single peak at δ 9.96 that can be attributed to the methyl carbon atoms of the η5-C5Me5 ligand but, unfortunately, the poor solubility of the cluster prevented a sufficiently high quality spectrum being obtained to identify signals for the cyclopentadienyl ring carbons or the carbonyl carbon atoms. Significantly, the 1H NMR spectrum displays two singlets at δ −22.13 and δ −20.15 and a doublet at δ −16.6 (J(H–Rh) 1.6 Hz) in the metal edge-bridging hydride region as well as a singlet attributable to the methyl protons of the η5-C5Me5 group at δ 2.12. The presence of three hydride signals in the 1H NMR spectrum is consistent with the presence of more than one isomer in solution.

Since the addition of H2 to 3 to form 5 is a direct analogy of the addition of CO to 3 to form 4 it is likely that the structure of 5 is closely related to that of 4 and has a capped octahedral metal framework. It is, therefore, possible to draw a number of structural isomers of [Os6H2(CO)17Rh(η5-C5Me5)] (5) in which the two hydride ligands span different Os–Os or Os–Rh edges. However, it is unfortunately not possible to unambiguously identify which isomers are present in solution.

The reaction of equimolar quantities of [Os6(CO)17Rh(η5-C5Me5)] (3) with PPh3 affords a new cluster derivative characterised spectroscopically as the substitution product [Os6(CO)16(PPh3)Rh(η5-C5Me5)] (6) (Table 1). The solution IR spectrum of 6 shows the presence of both terminal and edge bridging carbonyl groups and both the positive ion FAB mass spectrum, which exhibits a molecular ion at 2,094 amu, and microanalysis data are consistent with the formulation. The 1H NMR spectrum in CDCl3 shows a single peak at δ 1.57 and a multiplet centred at δ 7.5 corresponding to the methyl protons in the η5-C5Me5 ligand and the phenyl protons of the PPh3 group, respectively. The low solubility of the complex precluded a 13C NMR spectrum being obtained. The 31P NMR spectrum contains two signals at δ −113.4 and −119.9 with relative intensities of 5:1.

From the spectroscopic analysis the cluster 6 is isoelectronic with the parent cluster 3 and it is likely that the clusters are structurally closely related, with the phosphine ligand having replaced one of the carbonyl groups. However, the presence of two signals in the 31P NMR spectrum suggests that two different isomers may exist in solution, with the phosphine occupying different positions of substitution. The preferred site of substitution is likely to be on an Os atom that is coordinated to three other Os atoms in the parent cluster as this site is the least sterically crowded and, in terms of localised electron counting is a 17e site, which would benefit from the enhanced σ-electron donating properties of the phosphine compared to a carbonyl group (Fig. 3a). This is also the preferred site of phosphine substitution in [Os6(CO)18] which adopts the bicapped tetrahedral metal framework [27]. The second isomer may correspond to the structure illustrated in Fig. 3b where the phosphine is coordinated to an 18e centre where the coordinated Os atom is bonded to four other Os atoms, as has been observed for the disubstituted phosphite derivative [Os6(CO)16{P(OMe)3}2] [28].
Fig. 3

Possible isomeric structures of [Os6(CO)16(PPh3)Rh(η5-C5Me5)] (6). The carbonyl groups have been omitted for clarity

Attempts were also made to prepare [Os6(CO)18Rh(η5-C5Me5)] (4) by the direct reaction of [Rh(η5-C5Me5)(NCMe)3]2+ with the octahedral hexaosmium dianion [Os6(CO)18]2− [29] but the reaction was unsuccessful. The reactivity was dominated by the redox chemistry of the hexaosmium dianion and the only product that could be isolated was the neutral [Os6(CO)18] cluster.

In order to develop the chemistry further the reaction of the [N(PPh3)2]+ salt of the [Ru6(CO)18]2− (7) dianion [30] with [Rh(η5-C5Me5)(NCMe)3][PF6]2 was explored. When the reaction was carried out in dichloromethane, at room temperature, there was an immediate colour change of the solution to dark green and then to orange. In contrast to the reaction with [Os6(CO)18]2− three products were isolated after chromatographic separation on silica. The major orange product was identified as [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) spectroscopically and crystallographically. A preliminary report of this cluster has appeared which gives details of the crystal structure analysis and the spectroscopic data [21]. The most remarkable feature of this cluster that there has been a major framework rearrangement with the metal core opening out to give a bi-edge bridged tetrahedral framework with the “Rh(η5-C5Me5)” unit forming the apex of the central tetrahedron and two μ42-CO ligands sitting over the open framework faces as illustrated in Fig. 4. The framework can be viewed as derived from an “electron precise” 84e cluster by the breaking of two “edges” to give an “electron precise” 88e cluster which is the electron count for 8. The presence two μ42-CO ligands within the same cluster is rare but it has been observed previously, the first example being in the mixed-metal cluster [Mo2Ru5(CO)1442-CO)25-C5H5)24-S)] [31].
Fig. 4

The molecular structure of [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) [21]. Selected bond lengths (Å) from the two independent half molecules include Ru(1)–Ru(2) 2.787(1) [2.784(1)]; Ru(1)–Ru(3) 2.850(1) [2.841(1)]; Ru(2)–Ru(2A) 2.636(2) [2.639(2)]; Ru(2)–Ru(3) 2.766(1) [2.764(1)]; Ru(2)–Rh(1) 2.823(1) [2.803(1)]; Ru(3)–Ru(2A) 2.767(1) [2.764(1)]; Ru(3)–Rh(1) 2.769(2) [2.762(2)]; Ru(3)–Ru(1A) 2.850(1) [2.841(1)]; Rh(1)–Ru(2A) 2.823(1) [2.803(1)]

The two minor products, the green [Ru6(CO)18Rh(η5-C5Me5)] (9) and the brown [Ru6(CO)18Rh25-C5Me5)2] (10) were isolated in ca. 1–2 and 4–5% yields, respectively. A small amount of [Ru3(CO)12] was also isolated from the reaction. Cluster 9 could only be characterised by IR spectroscopy and FAB mass spectrometry (Table 2). The IR spectrum of 9 showed, in addition to bands due to the presence of terminal carbonyl ligands a weak broad band at 1,828 and a medium band at 1,724 cm−1 which can be assigned to bridging carbonyl ligands. The molecular ion peak in the mass spectrum corresponds to the proposed formulation. This would give an electron count of 98e and, although, it has not proved possible to obtain crystals suitable for an X-ray structure determination, it may be proposed that 9 has the same metal framework arrangements as in [Os7(CO)21] [24] or [Os6(CO)18Rh(η5-C5Me5)] (4), that of a capped octahedron.
Table 2

Spectroscopic data for clusters 9–11

 

Clusters

9

10

11

IR (νCO/cm−1) CH2Cl2

2074 s, 2042 m, 2018 vs, 1989 w,sh, 1952 vw, 1919 w,br, 1828 w, 1724 w

2060 w, 2042 m, 2022 vs, 2010 s, 1984 vw, 1961 w, 1919 vw, 1835 w, 1786 w, 1671 vw, 1603 w

2045 s, 2021 s, 2000 vs, 1977 vw, 1923 vw, 1726 vw, 1601 vw, 1549 w

Mass (+FAB) (Calc. 101Ru, 103Rh)

1349 (1346)

1590 (1586)

1358 (1355)

1H NMR CD2Cl2

2.04 (s, C5Me5) 1.86 (s, C5Me5)

1.86 (s, C5Me5) 3.7–3.9 (m, OMe)

13C{1H} NMR CD2Cl2

9.4 (s, C5Me5) 108.5 (d, JRh–C 4.7 Hz, C5Me5

The FAB mass spectrum of 10 exhibits a molecular ion peak at 1,590 amu which corresponds to the formulation [Ru6(CO)18Rh25-C5Me5)2] (10). The IR spectrum displays signals that can be attributed to terminal, edge-bridging, face-capping and μ42-CO ligands (Table 2). The 1H NMR spectrum displays two resonances at δ 1.86 and 2.04 that can be assigned to the methyl protons of the η5-C5Me5, and is consistent with there being two η5-C5Me5 ligands in different environments. Unfortunately there was insufficient yield of 10 to allow a 13C NMR spectrum to be run. It was possible to obtain suitable single crystals of 10 by recrystallisation from dichloromethane.

The molecular structure of [Ru6(CO)12(μ-CO)4(μ-CO)(μ42-CO)Rh25-C5Me5)2] (10) is illustrated in Fig. 5 which also includes some key bond parameters. The molecule crystallises in the monoclinic space group P21/n with one molecule and a CH2Cl2 solvent molecule in the asymmetric unit. The molecules are separated by normal van der Waals distances. The metal framework is best described as a Ru5Rh octahedron one edge of which is bridged by an additional Ru atom (Ru(6)) and this Ru atom is also linked to the second Rh(η5-C5Me5) fragment by a dicarbonyl bridged Ru–Rh bond. While the two C5Me5 ligands coordinate to the two Rh centres in the normal η5-fashion the carbonyls adopt all the possible bonding modes from terminal to μ42-CO. The carbonyl C(54)O(54) adopts the μ42-CO bonding mode observed in 8 with C(54) capping the Ru(1)Ru(4)Ru(5) triangular faces and the C(54)–O(54) forms a π bond to Ru(6). Carbonyl C(53)O(53) adopts a μ3-capping mode over the Ru(2)Ru(3)Ru(5) face. In addition to C(61)O(61) and C(62)O(62), which asymmetrically bridge the Ru(6)–Rh(2) bond, the carbonyls C(23)O(23) and C(33)O(33) asymmetrically bridge the Ru(1)–Ru(2) and Ru(3)–Ru(4) edges of the octahedron asymmetrically. The remaining carbonyls are essentially linear and are terminally bound, including C(90)O(90) which is bonded to the Rh(2) centre.
Fig. 5

The molecular structure of [Ru6(CO)12(μ-CO)4(μ-CO)(μ42-CO)Rh25-C5Me5)2] (10) showing the atom numbering scheme. Selected bond lengths (Å) include: Ru(1)–C(11) 1.895(15); Ru(1)–C(12) 1.872(16); Ru(1)–C(23) 2.206(14); Ru(1)–C(54) 2.338(14); Ru(1)–Ru(2) 2.7976(16); Ru(1)–Ru(6) 2.8351(16); Ru(1)–Ru(4) 2.8441(17); Ru(1)–Ru(5) 2.9019(17); Ru(1)–Rh(1) 2.9626(17); Ru(2)–C(53) 2.429(14); Ru(2)–Ru(5) 2.7660(17); Ru(2)–Ru(3) 2.9207(18); Ru(2)–Rh(1) 2.9483(19); Ru(3)–C(53) 2.380(14); Ru(3)–Ru(5) 2.7718(17); Ru(3)–Ru(4) 2.8159(16); Ru(3)–Rh(1) 2.9131(17); Ru(4)–C(33) 2.193(13); Ru(4)–C(54) 2.317(13); Ru(4)–Ru(6) 2.8430(15); Ru(4)–Ru(5) 2.8798(19); Ru(4)–Rh(1) 2.9219(17); Ru(5)–C(53) 2.092(14); Ru(6)–C(62) 1.986(15); Ru(6)–C(61) 1.991(14); Ru(6)–O(54) 2.139(9); Ru(6)–C(54) 2.405(13); Ru(6)–Rh(2) 2.7374(15); Rh(2)–C(61) 2.096(14); Rh(2)–C(62) 2.155(15)

The Ru(6)–Rh(2) distance, at 2.7374(15) Å, is similar to the equivalent bond (2.7395(8) Å) in [Ru4RhH(η5-C5Me5)(CO)13(BH2)] [32] which has a similar “spiked” triangular arrangement to that in 10. The average Ru–Rh(1) distance, at 2.937 Å, within the Ru5Rh octahedron is significantly longer and is also longer than the average Ru–Rh distance of 2.873 Å found in the carbido-centred Ru5Rh octahedron in [Ru5C(CO)14Rh(η5-C5Me5)] [19]. The Ru–Ru distances in the Ru5Rh octahedron in 10 span the range 2.766(1)–2.902(2) Å which is similar to the range of 2.815–2.957 Å found in [Ru5C(CO)14Rh(η5-C5Me5)] [19]. The average Rh(1)–C(C5Me5) distance of 2.215 Å is slightly shorter than the average Rh(2)–C(C5Me5) distance of 2.256 Å in 10.

The electron count for 10 is 114e, however, using the Mingos Condensed Polyhedral Model [25, 26] the electron count ought to be 116e [{octahedron (86) + triangle (48) + spike (34)} − {common edge (34) + common atom (18)}. The deficit of 2e may be accounted for by considering the bonding of the “Rh(η5-C5Me5)CO” unit which has an electron count of 16e and requires two more electrons to make it an 18e centre. This could be made up if each of the bridging carbonyls donates an electron to the Rh(2) centre or if the Ru(6)–Rh(2) bond has multiple bond character; notably this bond is significantly shorter than the Ru–Rh bonds within the Ru5Rh octahedron. A similar description of the bonding has been put forward for [Ru4RhH(η5-C5Me5)(CO)13(BH2)] and [Ru5H25-C5Me5)(CO)13(BH2)], the analysis being supported by molecular orbital calculations [32].

The thermal stability of the major product [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) from the reaction of [Ru6(CO)18]2− (7) dianion with [Ru(η5-C5Me5)(NCMe)3]2+ (1) was investigated as a parallel to the study of the thermal stability of [Os6(CO)18Rh(η5-C5Me5)] (4). Cluster 8 was heated under reflux, in CH2Cl2, for 2 days, and the reaction repeated using heptane and octane as solvent. However, no change was observed in the IR spectrum of the reaction mixture so this complex appears to be resistant to carbonyl loss under these conditions.

However, in parallel to [Os6(CO)17Rh(η5-C5Me5)] (3), [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) does undergo substitution reactions with P(OMe)3. The reaction of 8 with 1 or 2 equiv. of P(OMe)3, in the presence of Me3NO (which oxidises carbonyl ligands to CO2), in dichloromethane solution, affords a high yield of the disubstituted cluster [Ru5(CO)10(μ-CO)(μ42-CO)2(P(OMe)3)2Rh(η5-C5Me5)] (11). Cluster 11 have been fully characterised by IR, 1H, and 13C NMR spectroscopy and by FAB mass spectrometry. The structure of the cluster has also been determined crystallographically.

The IR spectrum of 11 shows, in addition to signals corresponding to the presence of terminal and edge bridging carbonyl ligands, low frequency signals around 1,550 cm−1 indicating that the μ42-CO ligands were still present in the cluster. The 1H NMR spectrum of the cluster showed a singlet resonance at δ 1.86 corresponding to presence of the methyl protons on the C5Me5 group and a multiplet in the range δ 3.7–3.9 corresponding to the methyl protons of the P(OMe)3 ligands. The 13C NMR spectrum displays signals at δ 9.4 corresponding to the methyl carbons of the C5Me5 ligands and a signal at δ 108.5 with J(Rh–C) = 4.7 Hz corresponding to the cyclopentadienyl carbon atoms of the C5Me5 ring. It was not possible to observe resonances for the carbonyl carbon atoms. The positive ion FAB mass spectrum displayed a molecular ion at 1,358 amu which is consistent with the proposed formulation for the cluster.

The molecular structure of [Ru5(CO)10(μ-CO)(μ42-CO)2(P(OMe)3)2Rh(η5-C5Me5)] (11) is illustrated in Fig. 6 which also contains a selection of important bond parameters. The complex crystallises in the triclinic space group P-1 with one independent molecule in the asymmetric unit. The molecules are separated by normal van der Waals distances.
Fig. 6

The molecular structure of [Ru5(CO)10(μ-CO)(μ42-CO)2(P(OMe)3)2Rh(η5-C5Me5)] (11) showing only one orientation of the disordered phosphite group. Selected bond lengths (Å) include: Rh(1)–Ru(2) 2.7547(10); Rh(1)–Ru(4) 2.8010(11); Rh(1)–Ru(5) 2.8036(10); Ru(1)–C(2) 1.890(8); Ru(1)–O(13) 2.165(4); Ru(1)–C(13) 2.262(6); Ru(1)–P(1) 2.280(2); Ru(1)–Ru(5) 2.8061(11); Ru(1)–Ru(2) 2.8603(9); Ru(2)–C(4) 1.889(8); Ru(2)–C(11) 2.155(6); Ru(2)–C(13) 2.168(6); Ru(2)–Ru(4) 2.7639(11); Ru(2)–Ru(5) 2.7684(10); Ru(2)–Ru(3) 2.8579(9); Ru(3)–C(5) 1.901(7); Ru(3)–O(11) 2.154(4); Ru(3)–C(11) 2.238(6); Ru(3)–P(2) 2.280(2); Ru(3)–Ru(4) 2.8280(9); Ru(4)–C(12) 2.090(7); Ru(4)–C(11) 2.153(7); Ru(4)–Ru(5) 2.6558(9); Ru(5)–C(12) 2.114(7); Ru(5)–C(13) 2.139(7)

The structure of 11 is closely related to that of the parent cluster 8 with two of the terminal carbonyls on the two edge bridging Ru atoms of the bi-edge bridged tetrahedral core replaced by phosphite ligands to give the cluster approximate C2 symmetry. As with 8 the shortest Ru–Ru distance in the cluster is the carbonyl bridged Ru(4)–Ru(5) edge at 2.656(1) Å. Generally, the variation in metal–metal distances in 11 and 8 follow similar trends to those in [Os6(CO)18] [33] and its phosphine and phosphite derivatives [Os6(CO)16(PR3)2] [27, 28]. The presence of the phosphite ligands in 11 causes little variation in the Ru–CO distances in the parameters associated with the μ42-CO ligand compared to those in 8.

Conclusions

The ionic coupling reaction of [Rh(η5-C5Me5)(NCMe)3]2+ with [Os6(CO)17]2− and [Ru6(CO)18]2− does not yield simple coupling products as has been observed previously with the coupling of the carbido dianion [Ru5C(CO)14]2− with [Rh(η5-C5Me5)(NCMe)3]2+ [19] or with low nuclearity osmium anions [20, 34]. In the reactions with the non-carbido dianions significant framework rearrangements occur in the reaction products. In this study we have found that the reaction product [Os6(CO)17Rh(η5-C5Me5)] (3), by analogy with the known structure of [Os7(CO)176-C6H6)] [23], adopts a metal framework in which the “Rh(η5-C5Me5)” unit occupies a site bonded to four Os atoms rather than a simple cap bonding to three Os atoms. This cluster readily adds CO and H2 to form the clusters [Os6(CO)18Rh(η5-C5Me5)] (4) and [Os6H2(CO)17Rh(η5-C5Me5)] (5) in with the addition of a two electron donor ligand results in a further metal framework rearrangement to a Rh-capped Os6-octahedron. In the reaction with [Ru6(CO)18]2− three products [Ru5(CO)15Rh(η5-C5Me5)] (8), [Ru6(CO)18Rh(η5-C5Me5)] (9) and [Ru6(CO)18Rh25-C5Me5)2] (10) with a variety of metal framework geometries not all based on an octahedron are obtained. Presumably, the absence of a carbide to stabilise the core geometry and the greater kinetic lability of ruthenium centres compared to osmium centres allow for this greater diversity in reaction chemistry. The ability of [Os6(CO)17Rh(η5-C5Me5)] (3) to readily accept a further two electron donor ligand and, in the case of the carbonylation product [Os6(CO)18Rh(η5-C5Me5)] (4), release it with a facile change in metal core geometry is another novel feature of this system.

Experimental

All the reactions were performed under an atmosphere of dry, oxygen-free nitrogen using standard Schlenk techniques. Technical grade solvents were purified by distillation over the appropriate drying agents and under an inert atmosphere prior to use. Routine separation of products was performed by thin layer chromatography (TLC), using commercially prepared glass plates, precoated to 0.25 mm thickness with Merck Kieselgel 60 F254, as supplied by Merck, or using laboratory prepared glass plates coated to 1 mm thickness with Merck Kieselgel F254. FAB Mass spectra were recorded using a Kratos model MS 902. IR spectra were recorded on a Perkin-Elmer 1710 FT-IR spectrometer, using 0.5 mm NaCl or CaF2 cells. 1H, 13C{1H} and 31P{1H} (referenced against 85% H3PO4 in H2O) NMR spectra were recorded on a Bruker WH 250 MHz or a WH 400 MHz spectrometer. The complexes [Rh(η5-C5Me5)(NCMe)3][PF6]2 [35], K2[Os6(CO)17] [22] and [(Ph3P)2N]2[Ru6(CO)18] [30] were prepared by literature procedures.

Synthesis of [Os6(CO)17Rh(η5-C5Me5)]

To a cooled THF solution (10 mL) (−78 °C) of [Os6(CO)18] (50 mg, 0.03 mmol), a solution of freshly prepared K/Ph2CO in THF was added dropwise until the IR spectrum of the solution indicated the presence of νCO frequencies that corresponded to the anionic cluster [Os6(CO)17]2− (2) (compared to the literature) were present [22]. The solvent was removed under reduced pressure and the orange residue redissolved in CH2Cl2 (20 mL) and was treated with an excess of [Rh(η5-C5Me5)(NCMe)3][PF6]2 (1) (24.3 mg, 0.045 mmol) at room temperature. The solution was stirred for a further 1 h and TLC work up using 50:50 hexane:CH2Cl2 as eluent afforded three bands. The first brown band was identified as unreacted [Os6(CO)18], the second as [Os6(CO)17Rh(η5-C5Me5)] (3) (21%) and the final deep pink band as [Os6(CO)18Rh(η5-C5Me5)] (4) (trace). Elemental analysis (%): Calc. for C27H15O17Os6Rh: C, 17.50; H 0.81; Found C, 17.41; H, 0.95.

Reaction of 3 with CO

CO gas was bubbled through a CH2Cl2 solution (20 mL) of 3 for 30 min. The colour changed from brown to deep pink, and after TLC work up with 50:50 hexane:CH2Cl2 as eluent [Os6(CO)18Rh(η5-C5Me5)] (4) was isolated in quantitative yield. Elemental analysis (%): Calc. for C28H15O18Os6Rh: C, 17.87; H, 0.80; Found C, 17.52; H, 0.81.

Decarbonylation of 4

The cluster [Os6(CO)18Rh(η5-C5Me5)] (4) (25 mg) was heated, under reflux for 1 h, in toluene and after TLC work [Os6(CO)17Rh(η5-C5Me5)] (3) was isolated in above 90% yield.

Reaction of 3 with H2

H2 gas was bubbled through a CH2Cl2 solution (20 mL) of 3 (20 mg, 0.011 mmol) for 30 min. The colour of the solution changed from brown to pink. TLC work up using 50:50 hexane:CH2Cl2 as eluent afforded one major band which was characterised as [Os6H2(CO)17Rh(η5-C5Me5)] (5) (80%). Elemental analysis (%): Calc. for C27H17O17Os6Rh: C, 17.47; H, 0.92; Found C, 17.37; H, 0.89.

Reaction of 3 with PPh3

To a solution of 3 (50 mg, 0.027 mmol) in CH2Cl2 (20 mL), 1.1 equiv. PPh3 (7.8 mg, 0.03 mmol) was added and the mixture stirred at room temperature for 2 days. TLC work up using 50:50 hexane:CH2Cl2 as eluent afforded the purple cluster [Os6(CO)16(PPh3)Rh(η5-C5Me5)] (6) (80%). Elemental analysis (%): Calc. for C44H30O16Os6PRh: C, 25.2; H, 1.44; Found C, 24.9; H, 1.5.

Reaction of [(Ph3P)2N]2[Ru6(CO)18] (7) with [Rh(η5-C5Me5)(NCMe)3][PF6]2 (1)

To a solution of the [(Ph3P)2N]+ salt of the [Ru6(CO)18]2− (7) (200 mg, 0.09 mmol) in CH2Cl2 (25 mL), [Rh(η5-C5Me5)(NCMe)3][PF6]2 (1) was added at room temperature. The solution immediately became dark green and then quickly changed to deep orange. After stirring for 1 h the solvent was removed. The solid residue was chromatographed by TLC using hexane:CH2Cl2 (50:50) as eluent. Three bands were separated on the TLC plate. The orange band was characterised as [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) (35–40%), followed by the green band as [Ru6(CO)18Rh(η5-C5Me5)] (9) (1–2%) and the brown band as [Ru6(CO)12(μ-CO)4(μ-CO)(μ42-CO)Rh25-C5Me5)2] (10) (4–5%). Elemental analysis (%) for 8: Calc. for C25H15O15RhRu5: C, 25.80; H, 1.30; Found C, 25.19; H, 1.24. Elemental analysis (%) for 10: Calc. for C39H32Cl2O18Rh2Ru6: C, 28.01; H, 1.93; Found C, 28.45; H, 2.10.

Reaction of [Ru5(CO)12(μ-CO)(μ42-CO)2Rh(η5-C5Me5)] (8) with P(OMe)3

Cluster 8 (25 mg, 0.022 mmol) was stirred in CH2Cl2 (20 mL) with 2 equiv. of P(OMe)3 (22.3 mg, 0.042 mmol) at −78 °C. A solution of Me3NO (2.2 equiv., 3.4 mg) in CH2Cl2 was added dropwise over a period of 10 min. The solution was allowed to warm to room temperature, stirring for a further 30 min. By this time the colour of the solution had changed from orange to green; the IR spectrum of the solution also changed confirming that no starting material remained. After reducing the volume, the residue was chromatographed by TLC using 2:1 hexane:CH2Cl2 as eluent. The product [Ru5(CO)10(μ-CO)(μ42-CO)2(P(OMe)3)2Rh(η5-C5Me5)] (11) was isolated as the major green band (80%). Elemental analysis (%): Calc. for C29H33O19P2RhRu5: C, 25.69; H, 2.46; Found C, 25.50; H, 2.70.

X-Ray Crystallography

Suitable single crystals of 4, 10 and 11 were obtained by recrystallisation from hexane:CH2Cl2 solution. A crystal of each cluster was mounted on a glass fibre using either epoxy resin glue (for 4) or perfluoroether oil (for 10 and 11) which freezes at reduced temperatures and holds the crystal static in the X-ray beam. The X-ray data for 4 and 10 was recorded on a Rigaku AFC7R four-circle diffractometer using ω/2θ scans and for 11 on a Rigaku AFC5R diffractometer, both instruments being equipped with an Oxford Cryosystems Cryostream crystal cooling apparatus. The data for 10 and 11 was recorded at 150 K and that for 4 at room temperature. Graphite monochromated Mo-Kα radiation, λ = 0.71073 Å, was used in all instances. Semi-empirical absorption corrections based on ψ-scans were applied. The structures were solved using direct methods (SHELXS-86) [36] and from subsequent Fourier difference syntheses and were refined using full-matrix least-squares on F2 (SHELXL-96) [37]. Structure 4 contained 1.5 molecules of dichloromethane in the asymmetric unit and 10 contained one molecule of dichloromethane in the asymmetric unit; neither exhibited disorder. Methyl hydrogen atoms were placed in idealised positions and were allowed to ride on the relevant carbon atom with the displacement parameter assigned 1.5 times the value of that of the carbon atom. The hydrogen atoms on the carbon atoms of the dichloromethane molecules were also placed in idealised positions and allowed to ride on the carbon atom, with an isotropic displacement parameter set at 1.2 times that of the carbon atom. Because of the low reflection to parameter ratio only the Os and Rh atoms (and the solvent Cl atoms) were refined with anisotropic displacement parameters for 4, while for 10 only the Rh, Ru and O atoms were refined with anisotropic displacement parameters. Also, for 4, the Os–C and C–O (terminal carbonyl groups) and the C–Cl distances in the solvent molecule were restrained to be similar using the SHELX SADI command. All non-hydrogen atoms were refined with anisotropic displacement parameters for 11. One of the phosphite groups in 11 displayed positional disorder over two sites. These disordered atoms were refined with partial occupancies that were summed to unity. Refinement continued until convergence was reached. In the final cycles of refinement a weighting scheme was introduced that afforded a flat analysis of variance. The crystal data, data collection details and refinement details are presented in Table 3.
Table 3

Crystallographic data for clusters 4, 10 and 11

 

4

10

11

Empirical formula

C29.5H18Cl3O18Os6Rh

C39H32Cl2O18Rh2Ru6

C29H33O19P2RhRu5

Formula weight

2010.9

1671.79

1355.75

Temperature (K)

293(2)

150(2)

150(2)

Crystal system

Monoclinic

Monoclinic

Triclinic

Space group

P21/n

P21/n

P-1

Unit cell dimensions

   

 a (Å)

10.352(3)

11.878(5)

12.530(3)

 b (Å)

33.910(3)

16.905(6)

16.171(3)

 c (Å)

11.963(2)

23.857(3)

10.785(3)

α (°)

90

90

97.52(2)

β (°)

112.61(2)

96.11(2)

95.74(2)

γ (°)

90

90

104.07(2)

V3)

3876.7(13)

4763(3)

2081.7(9)

Z

4

4

2

Dc (mg/m3)

3.445

2.331

2.163

μ(Mo Kα) (mm−1)

20.281

2.707

2.304

Crystal size (mm)

0.20 × 0.10 × 0.10

0.12 × 0.10 × 0.10

0.25 × 0.15 × 0.10

θ range (°)

2.52–22.44

2.56–22.51

2.63–25.01

Reflections collected

5314

6568

7688

Independent reflections

4998

6212

7324

R(int)

0.106

0.0481

0.0389

Data/restraints/parameters

4998/81/282

6212/0/400

7324/0/512

R1 (I > (I))

0.0715

0.0544

0.0366

wR2 (all data)

0.1961

0.1273

0.0857

Gof

1.036

0.994

1.060

Crystallographic data (excluding structure factors) has been deposited with the Cambridge Crystallographic data Centre as supplementary publication nos. CCDC 768923–768925. Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, e-mail: data_request@ccdc.cam.ac.uk or fax: +44 1223 336033.

Acknowledgments

We gratefully acknowledge the support of the Cambridge Commonwealth Trust and the United Kingdom Committee of Vice Chancellors and Principals (to S. N.) and the Cambridge Crystallographic Data Centre (to G. P. S.). We are also grateful to the EPSRC for a Senior Fellowship (to P. R. R.).

Copyright information

© Springer Science+Business Media, LLC 2010

Authors and Affiliations

  • Saifun Nahar
    • 1
  • John E. Davies
    • 1
  • Gregory P. Shields
    • 1
  • Paul R. Raithby
    • 1
    • 2
  1. 1.Department of ChemistryUniversity of CambridgeCambridgeUK
  2. 2.Department of ChemistryUniversity of BathBathUK

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