Ru 3 ( 6-NHC ) ( CO ) 10 ] : synthesis , characterisation and reactivity of rare 46-electron tri-ruthenium clusters †

[Ru3(CO)12] reacts at room temperature with N-alkyl substituted 6-membered ring N-heterocyclic carbenes (6-NHC) to form [Ru3(6-NHC)(CO)10] (6-NHC = 6Pr 1, 6-Et 2 and 6-Me 4), rare examples of coordinatively unsaturated (46-electron) ruthenium clusters. Complexes 1, 2 and 4 have been structurally characterised, along with the tetranuclear ruthenium cluster [Ru4(6-Et)2(CO)11] 3 that is formed along with 2. The degradation of the 6-Pr derivative 1 by pyrimidinium salt elimination helped to explain the poor activity of the complex in the catalytic acylation of pyridine.


Introduction
Despite the unquestionable impact that N-heterocyclic carbene (NHC) ligands have had on organometallic chemistry over the last 25 years, their application to low-valent metal cluster chemistry has received only limited attention. 1 However, on the basis of what has been observed, it is clear that combining NHCs with metal clusters frequently leads to very interesting observations. Thus, there are examples in which cluster structure is retained upon reaction with one type of NHC substitution pattern, 2 but cleaved upon only relatively small changes to either the substituents or reaction stoichiometry. 3,4 In very recent cases, clusters with unusually high NHC content have been identified as catalyst deactivation products. 5 Arguably of most interest have been the observations of atypical NHC binding modes, 6 the detection of unprecedented reaction intermediates 7 and the formation of multiply activated carbene ligands. 2b,8 The group 8 tricarbonyl precursors [M 3 (CO) 12 ] (M = Fe, 9 Ru,1b,2,3,6,7 Os) 1b,2c,7,10,11 have proven to be the most fertile area of cluster chemistry for reaction with NHCs, largely due to their ease of accessibility and the well-known differences in properties that are seen upon descending the group. 12 For example, osmium exhibits a willingness to adopt 46-electron counts in some Os 3 clusters (e.g. [Os 3 (CO) 10 (μ-H) 2 ]), whereas ruthenium shows a greater tendency to maintain coordinative saturation (i.e. 48 electrons) meaning that electron-deficient Ru 3 systems are not very common. 13 Indeed, [Ru 3 (CO) 10 (μ-H) 2 ] has only been generated photchemically 14 and its chemistry explored only to a very limited extent. 15 We 16 (and others) 17 have shown that NHCs with a ring size of >5 can be used to stabilise low coordination numbers in a wide range of mononuclear transition metal complexes, but as far as we are aware, reactions between this class of so-called 'ring-expanded carbenes' 18 and transition metal clusters have not been described. 19 Herein, we report that N-alkylated, 6-membered ring NHCs (denoted as 6-NHC) react with [Ru 3 (CO) 12 ] at room temperature to afford novel 46-electron Ru 3 clusters of general formula [Ru 3 (6-NHC)(CO) 10 ]. Their structures, together with studies of reactivity alongside other Ru 3 clusters in catalytic C-H bond functionalisation, are described.
The solution IR spectrum of 2 was essentially identical to that of 1. However, in contrast to the sharp, well-resolved room temperature 1 H NMR spectrum of 1, the spectrum of 2 comprised of three broad resonances at δ 3.43, 3.27 and 2.07, together with a sharp triplet at δ 1.31. The two higher frequency broad signals resolved into three sharper multiplets (relative integrals of 2 : 2 : 4) for the eight NCH 2 protons upon cooling to 235 K.
The N-Me substituted ligand 6-Me behaved similarly to 6-i Pr in yielding only the [Ru 3 (6-NHC)(CO) 10 ] product, [Ru 3 (6-Me)(CO) 10 ] 4 (Scheme 1). The carbonyl absorption bands in the IR spectrum of 4 partially merged to give a total of nine bands compared to the eleven bands seen for both 1 and 2. In the proton NMR spectrum, both the N-Me singlet and NCH 2 CH 2 quintet were sharp, while the NCH 2 triplet was noticeably broader, suggestive of fluxionality (cf. 2).
The molecular structure of 4 ( Fig. 1) also closely resembles that of 1 and 2 with the three Ru-Ru distances following the same trend (Ru(1)-Ru (2)

Stoichiometric and catalytic reactions of 1 involving CO
The comparatively poor yields of 2 and 4, as well as the need to manually separate 2 from 3, led us to use 1 for investigations into the reactivity of the [Ru 3 (6-NHC)(CO) 10 ] complexes. Given the coordinative unsaturation, we were surprised to find that there was no reaction of [Ru 3 (6-i Pr)(CO) 10 ] with CO (1 atm in THF-d 8 ), even upon heating to 80°C. However, exposure of 1 to 1 atm 13 CO led to the appearance of a 13 C enhanced carbonyl signal at δ 200 in the 13 C{ 1 H} NMR spectrum at room temperature ( Fig. S11 †), implying that although [Ru 3 (6-i Pr)(CO) 10 ] will not add CO, it can undergo facile CO exchange. 24 The stability of 1 to CO led us to test it as a precursor in the catalytic acylation of pyridine (Table 1). Moore and co-workers reported in 1992 25 that the insertion of CO and a terminal alkene into the ortho C-H position of pyridine was catalysed by [Ru 3 (CO) 12 ] at high pressure (10 atm CO) under forcing conditions (150°C, 16 h) to give predominantly linear acylation products. In our hands, we were unable to achieve the 65% yield with 1-hexene described by Moore using [Ru 3 (CO) 12 ], achieving instead a more modest 31% average yield. 26 Disappointingly, 1 exhibited lower activity than [Ru 3 (CO) 12 ], as did the coordinatively saturated, abnormally bound 5-membered ring NHC clusters, [Ru 3 (ab-I t Bu)(CO) 11 ] 6a and [Ru 3 (ab-IAd)(CO) 11 ] (IAd = 1,3-bis(adamantyl)imidazol-2ylidene). 6b

Degradation of 1 through loss of [6-i PrH] + and 6-i Pr
In an effort to rationalise the poor catalytic activity, 1 was heated in the presence of 4 equiv. pyridine in an NMR tube scale reaction. Warming to 85°C brought about loss of the 1 H NMR resonances of 1 and appearance of signals due to the pyridinium cation [6-i PrH] + . 27 The presence of low frequency proton signals at δ −15.5 and δ −19.2 suggested that this was partnered with anionic ruthenium carbonyl hydride species and, indeed, an X-ray study of a small number of orange-yellow crystals isolated from the reaction yielded a structure of [6-i PrH] 2 [Ru 4 (CO) 12 H 2 ] (Fig. S14 †). 28 The di-potassium salt of [Ru 4 (CO) 12 H 2 ] 2− has a hydride chemical shift of δ −19.3. 29 A likely pathway to [6-i PrH] + formation involves C-H activation of pyridine by 1, 23a,30 followed by reductive elimination from a resulting {(6-i Pr)RuH} moiety. Support for reductive elimination from such a species came upon reacting 1 with H 2 at 60°C (THF-d 8 ), which again generated pyrimidinium proton NMR signals, together with hydride signals at δ −12.1 and −12.5, in <1 h.
Further evidence for the low stability of the [Ru 3 (6-NHC) (CO) 10 ] complexes comes from the reaction of 1 with phosphines, where we found that 1 reacted with 1-3 equiv. PPh 3 at 60°C with complete loss of the starting material and the formation of multiple phosphorus containing species. Efforts to characterise the product mixture led to isolation of just the known phosphine carbonyl cluster, [Ru 3 (PPh 3 ) 3 (CO) 9 ] (Fig. S18 †), 31 indicating that 1 also appears to be susceptible to loss of free carbene under quite mild conditions.

Conclusions
The synthesis and structural characterisation of rare examples of 46-electron tri-ruthenium clusters has been achieved upon reacting [Ru 3 (CO) 12 ] with N-alkyl substituted, 6-membered ring N-heterocyclic carbenes under very mild conditions. Reactivity studies of the [Ru 3 (6-NHC)(CO) 10 ] complexes carried out using the 6-i Pr derivative 1 indicated that loss of the carbene ligand took place upon addition of PPh 3 , mild heating under H 2 or in the presence of pyridine, thereby limiting the potential of these complexes in catalytic applications.
The formation of very different products in the reaction of [Ru 3 (CO) 12 ] with 6-NHCs to those formed with 5-membered ring analogues is notable, 1b suggesting that (i) investigations with 6-and/or 7-membered ring NHCs bearing, for example, N-aryl substituents, and/ or (ii) the use of other group 8 carbonyl clusters as precursors, is worthy of investigation.

Catalysis
Solid samples of Ru 3 precursors (0.0265 mmol), together with 2,4,6-(MeO) 3 C 6 H 3 (0.0265 mmol) as an internal standard, were weighed into a Parr autoclave inside a glovebox. A solution of 1-hexene (2 mmol) in pyridine (7.5 mL, dried over activated 3 Å molecular sieves) was added by cannula, and the autoclave assembly put together under a flow of argon. After purging twice with CO, the autoclave was pressurised to 10 atm and heated at 150°C for 16 h. After cooling and depressurising, a small amount of the red-orange solution was diluted with CDCl 3 and analysed by 1 H NMR spectroscopy. Product resonances were assigned by comparison to the literature. 25

Conflicts of interest
There are no conflicts of interest to declare.