Photochemical Synthesis and Ligand Exchange Reactions of Ru(CO)4(n2-alkene) Compounds

Abstract

Substituted mononuclear metal carbonyls play a central role in the discussion of organometallic chemistry. In the undergraduate laboratory, these compounds are most often prepared by thermal reaction of the parent binary metal carbonyl with a suitable ligand, L (1). When thermal routes fail, photochemical synthesis is a viable alternative for carbonyl ligand replacement (2), but, with a few notable exceptions (3-7), this approach can often be too technically demanding for the conventional undergraduate laboratory. Even in these instances, the use of sophisticated equipment can be required (4, 5), potentially limiting the applicability of the experiment to only the most well-equipped institutions. Furthermore, the inherent challenges of carrying out synthetic photochemistry on metal carbonyl compounds have thus far limited undergraduate laboratory investigation to the group six metals (Cr, Mo, W) (3-7).Ruthenium pentacarbonyl is thermally unstable and is difficult to handle (8), which often renders thermal reactions inconvenient for the synthesis of Ru(CO) 4 L compounds. However, advantage can be taken of the efficient photochemical metal-metal bond cleavage in Ru 3 (CO) 12 to prepare Ru(CO) 4 (η 2 -alkene) compounds; when a sufficiently labile alkene is used, the latter can serve as [Ru(CO) 4 ] transfer reagents to generate different Ru(CO) 4 L compounds (9, 10). In contrast, thermal reactions of Ru 3 (CO) 12 typically yield ligand-substituted trinuclear clusters (11).From a pedagogic perspective, the students are introduced to the notion that it is not always a metal-carbonyl bond that cleaves in photochemical reactions, but that larger metal clusters can be induced to fragment with useful results. Although Ru 3 (CO) 12 is relatively expensive, 1 the experiments that will be described can be carried out on a scale of only a few mg͞student, with the compounds characterized in situ by IR spectroscopy rather than being isolated. This exposes the students to a common microscale method used in research laboratories when working with reagents that are either very costly or are difficult to prepare in large quantities. We have found that our students appreciate being exposed to the (somewhat novel) prospect of carrying out a photochemical synthesis and being able to learn a great deal about the chemistry of a system without necessarily needing to submit an isolated compound at the conclusion of the experiment. Experimental BackgroundThe photochemical fragmentation of Ru 3 (CO) 12 can take place under a variety of conditions, ranging from filtered high-intensity light (9, 10) to sunlight or conventional fluorescent lighting (12), rendering the experiment quite flexible in terms of the sophistication of equipment required. The UV-vis spectrum of Ru 3 (CO) 12 reveals a single band at the short wavelength extreme of the visible spectrum, centered at 390 nm ( Figure 1). This band has been assigned to a σ → σ* transition in the Ru 3 framework (13), and allows one to understand why a metal-metal bond is cleaved upon photochem...

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“…The fluxionality of complex 4 is made evident by the SST experiment and by the broad signals in both the 1 H and the 31 P{ 1 H} NMR spectra which exhibit temperature dependent chemical shifts and line-shapes. Variable temperature 31 P{ 1 H} NMR spectroscopic analysis of 4 shows that the 6:1 ratio of the non-symmetric complex to the C s -symmetric P,P 0 -chelate between -80 and 50 °C remains relatively unchanged (even after replacing CD 2 Cl 2 solvent with CD 3 NO 2 ), although all signals sharpen upon cooling. Furthermore, between 50 and 80 °C, the signal belonging to the C s -symmetric isomer is coincidentally overlapping with the more downfield signal of the non-symmetrical species.…”

“…A 100 mL hexane solution of [Ru(CO) 4 (η 2 -C 2 H 4 )] was prepared photolytically with the use of [Ru 3 (CO) 12 ] (42 mg, 66 μmol) and an ethylene purge. 31 Under anhydrous conditions and Ar atmosphere, the solution was transferred, via cannula, to a prepared 250 mL Schlenk flask containing mapm (90 mg, 180 μmol). The pale-yellow slurry was stirred for 5 min then 10 mL of dichloromethane was added.…”

“…A 125 mL n-pentane solution of [Ru(CO) 4 (η 2 -C 2 H 4 )] was prepared photolytically with the use of [Ru 3 (CO) 12 ] (48 mg, 75 μmol) and an ethylene purge. 31 Meanwhile, a solution of mapm (52 mg, 104 μmol) in 5 mL of dichloromethane was prepared under Ar atmosphere. In complete darkness, the mapm solution was then transferred to the solution of [Ru(CO) 4 (η 2 -C 2 H 4 )] by cannula, and the resulting solution was heated to 50 °C with a water bath while stirring under a brisk flow of Ar to remove the solvents.…”

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“…218 After irradiation, the 1-benzyl-3-trifluoromethylpyridinium cation 365 was added to obtain the corresponding hydride reduction product 366. 220 The aim of this study was to demonstrate that under light excitation a metal−metal bond can be cleaved instead of the more classical metal−carbonyl bond breakage. The reaction could be performed under different conditions ranging from filtered high-intensity light to sunlight or conventional fluorescent lighting.…”

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