Clusters as ligands: Synthesis, structure and coordination chemistry of ruthenium clusters derived from 4- and 5-ethynyl-2,2′-bipyridine☆
Graphical abstract
Introduction
The development of transition metal cluster chemistry owes much to the curiosity driven endeavours and synthetic prowess of the early pioneers of organometallic chemistry [1], [2], [3], [4]. The reactions of the group 8 trimetallic dodecacarbonyl clusters (M3(CO)12, M = Fe (1-Fe), Ru (1-Ru), Os (1-Os)), which are readily obtained in high yields from simple reactions and offer opportunity to explore trends down the group, have proved to be particularly useful scaffolds upon which to explore metal framework rearrangements, growth and fragmentation processes, ligand exchange and coupling reactions, cluster dynamics and so on [5].
Given the state of knowledge of the cluster reactions with an immense array of different functional groups that has been accumulated over the last 5 or so decades, it is interesting to consider cluster-ligand combinations as construction units within the ‘Tinkertoy’ approach to the design of novel molecular structures [6], [7]. This approach requires the careful selection of orthogonal reactions to sequentially introduce metal centres and ligands to a growing molecular, cluster-based scaffold (which might be described here in terms of the “clusters as ligands” concept [8]).
The Tinkertoy approach to molecular design and synthesis requires the linking of metal-complex ‘nodes’ by various bi-functional ligands. Selective synthetic design requires the use of both mono-functionalised fragments to cap or block the growing molecular structure, and bi- (or higher multi) functional fragments to serve as connectors that link the growing structure with incoming building blocks. We have recently become interested in the organometallic chemistry of ethynyl-substituted heterocycles [9], [10], including 5-ethynyl-2,2′-bipyridine [11]. The bi-functional nature of these compounds makes them ideally suited for the construction of multi-metallic systems, and 5-ethynyl-2,2′-bipyridine has found application as a linking unit within the Tinkertoy-based construction of multi-metallic frameworks based on mononuclear nodes [12], [13]. Further exploration has revealed the capacity of such ligands to promote weak electronic interactions between the tethered nodes [14].
The cluster chemistry of alkynes has been particularly well explored for the group 8 metals, with demonstrated capacity to serve as a terminal end-on, 1-e donor η1-C≡CR fragment, 3-e donor μ-C2R fragment bridging a metal–metal bond or edge, a 4-e donor face capping alkyne μ-RC2R′, 5-e donor μ-C2R fragment or even a 6-e donor C2 -fragment. The chemistry of cluster-bound alkynes, acetylides and related all-carbon fragments has been reviewed in a wide variety of contexts [15], [16], [17], [18], [19], [20]. In contrast, literature concerning the course of reactions between group 8 clusters and 2,2′-bipyridine derivatives is rather more sparse, as described below.
The initial investigations in the area of group 8 clusters with 2,2-bipyridine (bpy) were undertaken by the Lewis group, with reaction of 2,2′-bipyridine and [Ru3(CO)10(NCMe)2] affording [Ru3(CO)10(μ-N,N′-bpy)], and subsequent thermolysis resulting in ortho-metallation of the heterocycle to give [Ru3(μ-H)(μ-{κ2-N,C-η1-N′-C10H7N2})(CO)9] [21], the structure of the Os analogue having been determined [22]. From [Ru3(CO)12] and bpy, crystallographically characterised [Ru3(μ-CO)2(κ2-N′,N-bpy)(CO)8] with an Fe3(CO)12-like structure was obtained [23], [24], [25]: thermolysis afforded the ortho-metallated product [24]. Unsurprisingly given the weaker Fe–Fe bonds, trimethylamine-N-oxide promoted reactions of [Fe3(CO)12] and [Fe2(CO)9] with bpy gave mono- [Fe(CO)3(bpy)] and bi-metallic [Fe2(κ2-N′,N-bpy)(CO7)] products [25]. Similarly, whilst the higher nuclearity cluster [Fe5(μ5-C)(CO)15] fragments into [Fe(bpy)3][Fe4(μ-H)(μ4-C)(CO)12]2 on reaction with bpy [25], carbonyl substitution and ortho-metallation processes are observed in analogous reaction with higher nuclearity Ru clusters [Ru4(μ-H)4(CO)12] [26] and [Ru5(μ5-C)(CO)15] [27], although [Ru6(μ6-C)(CO)17] proved rather more prone to cluster fragmentation in the presence of bpy [27]. Interestingly, the methyl C–H bonds in 6,6′-dimethyl-2,2′-bipyridine are readily activated on reaction with [Ru3(CO)12] or [Os3(CO)10(NCMe)2] to give products containing methylene (CH2), methyne (CH) and carbyne (C) clusters [28], [29], [30].
In the spirit of the early pioneers in the field, we were therefore prompted to consider the reactions of the prototypical clusters [Ru3(CO)12] and [Ru3(CO)10(dppm)] with 4- and 5-ethynyl-2,2′-bipyridine, both to explore the competitive reactivity of the acetylene and bipyridine in the same ligand towards the metal framework, and also as a potential route to the assembly of larger structures via Tinkertoy concepts.
Section snippets
Synthesis
Survey reactions of [Ru3(CO)12] (1-Ru) with 4- and 5-ethynyl-2,2’-bipyridine (2, 3) were conducted in hexane and monitored by TLC and IR spectroscopy. In contrast to reactions between 1-Ru and Me3SiCCCCCCSiMe3 carried out in this fashion, which affords the tetrametallic cluster [Ru4(CO)12(μ-Me3SiCCC2CCSiMe3)] (35%) as the major product [31], these survey reactions gave a plethora of products, none of which could be satisfactorily isolated or characterised. Similar difficulties were encountered
Conclusion
The 4-ethynyl and 5-ethynyl-derivatives of 2,2′-bipyridine react readily with [Ru3(CO)10(dppm)] under mild thermally activated conditions to give the anticipated electron precise 48 CVE hydrido clusters, [Ru3(μ-H)(μ-C2bpy)(CO)7(dppm)] in moderate yield. The bipyridyl moiety is not hampered by coordination of the ethynyl pendant to the cluster, and remains available for further reaction, allowing use as a cluster-based ligand and compatible with the ‘Tinkertoy’ approach to the assembly of larger
General conditions
The compounds Ru3(CO)10(dppm) [39], 4-ethynyl-2,2′-bipyridine [40], [41](b), [41](a), [43], [42], [41] 5-ethynyl-2,2′-bipyridine [41], [41](b), [41](a) and PdCl2(NCPh)2 [44] were synthesised following literature procedures. All reaction solvents were used dried from an Innovative Technology solvent purification system. All reagents were used as purchased. Reactions were performed under inert nitrogen atmosphere. No special precautions were taken to exclude air or moisture during work up.
NMR
Acknowledgements
We gratefully acknowledge the Australian Research Council for funding (DP DP140100855). P.J.L. holds an ARC Future Fellowship (FT FT120100073). S.B. holds an International Postgraduate Research Scholarship, and S.B. and C.F.M. were both awarded Australian Postgraduate Award and the UWA Safety Net Top-Up Scholarships by The University of Western Australia and the Australian Government. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian
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Dedicated to memory of Professor Lord Sir Jack Lewis, FRS, a true organometallic pioneer.