Bromonium Ion Addition to Triruthenium and Triosmium Dodecacarbonyl:  The Markedly Different Structures of the [M₃(CO)₁₂(Br)]⁺ (M = Ru, Os) Ions

Abstract

Recently, Brown and co-workers reported the use of the bromonium ion of adamantylideneadamantane, as the triflate salt ([AdAdBr](OTf)), to transfer Br + to acceptor olefins. 1 On the other hand, work from this laboratory has shown that reaction of iodine with Os 3 (CO) 12 proceeds via the [Os 3 (CO) 12 -(I)] + cation, and it was suggested that bromination of Os 3 (CO) 12 proceeded through a similar intermediate. 2 Here we report the use of the bromonium ion reagent to prepare the [M 3 (CO) 12 -(Br)] + (M ) Ru, Os) ions and the determination of their crystal structures. These ions are the first halonium ions of metal carbonyl cluster compounds to be structurally characterized. The two cations have markedly different structures: the ruthenium derivative can be considered as an organometallic analogue of the bromonium ion of ethylene, [C 2 H 4 Br] + , whereas the osmium cation can be thought of as the corresponding ion of cyclopropane, [C 3 H 6 Br] + . Furthermore, evidence is presented that the form of [Os 3 (CO) 12 (Br)] + characterized here is not an intermediate in the bromination of Os 3 (CO) 12 .Addition of [AdAdBr]{B[C 6 H 3 (CF 3 ) 2 ] 4 } 3 to M 3 (CO) 12 (M ) Ru, Os) in CH 2 Cl 2 at or below room temperature readily afforded [M 3 (CO) 12 (Br)]{B[C 6 H 3 (CF 3 ) 2 ] 4 }, which on addition of hexane formed air-stable, red-orange (M ) Ru) or pale yellow (M ) Os) crystals. 5 The structure of the ruthenium cation (1) as found in the X-ray structure determination of [Ru 3 (CO) 12 -(Br)]{B[C 6 H 3 (CF 3 ) 2 -3,5] 4 } 6 is shown in Figure 1. It consists of a bent chain of three ruthenium atoms (Ru(1)Ru(2)Ru(3) ) 157.19(4)°), one Ru-Ru bond of which is bridged by the

Formation of [Os3(CO)10(EPh3)2(μ-Br)]+ (E = P, As) by Bromination: First Direct Evidence for the Bromonium Mechanism in Cluster Chemistry

“…The X-ray crystal structure of the cluster cation in 2a (Figure ) shows that the osmium−osmium edge carrying the two group 15 substituents has been cleaved (Os(1)···Os(2) distances of 4.038(5) and 4.027(7) Å for 2a and 2b , respectively) to form an open bromonium-bridged compound. Their structures are similar to that of the earlier reported [Os 3 (CO) 12 (μ-Br)] + , and they may thus be viewed as group 15 substituted derivatives of the latter cluster cation …”

“…Essentially, the two major possibilities are via a radical mechanism or a bromonium intermediate . It has recently been shown that the reaction of Br + with the parent carbonyl cluster Os 3 (CO) 12 gave the bromonium cluster [Os 3 (CO) 12 (μ-Br)] + , and it was also demonstrated that this cluster was not the intermediate involved in the direct bromination reaction . In contrast, we have recently reported that the bromination of the substituted clusters Os 3 (CO) 11 (EPh 3 ) (E = P, Sb) gave two isomeric products, (Br) 2 Os(CO) 3 Os(CO) 4 Os(CO) 4 (EPh 3 ) and (Br)(OC) 4 OsOs(CO) 4 Os(CO) 3 (Br)(EPh 3 ) and suggested that the intermediate was a bromonium cluster with a terminal Br atom …”

Stable Bromiranium Ions with Weakly‐Coordinating Counterions as Efficient Electrophilic Brominating Agents

“…Instead, Brown prepared the triflate salt 2-OTf,b ut also the triflate interacts with the bromiranium ion. [17] For its preparation we used as alt metathesis reactionw ith sodium tetrakis[3,5-bis(trifluoromethyl)-phenyl]b orate (NaBArF).T he stoichiometric addition of NaBArF to 2-Br 3 in dichloromethanea tr oom temperature formed 2-BArF (Scheme 2). Furthermore,t he triflate is still nucleophilic and 2-OTf reacts with cyclohexenet og ive 2bromocyclohexyl triflate.…”

“…To obtain a bromiranium salt with a weakly‐coordinating anion we chose to prepare the salt 2‐BArF . This salt has been used previously as a halogenating agent for an osmium carbonyl complex, but no procedure for its preparation or data on its reactivity have been reported . For its preparation we used a salt metathesis reaction with sodium tetrakis[3,5‐bis(trifluoromethyl)‐phenyl] borate (NaBArF).…”

“…[10] To obtain ab romiranium salt with aw eakly-coordinating anion we chose to preparet he salt 2-BArF.T his salt has been used previouslya sah alogenating agent for an osmium carbonyl complex, but no procedure for its preparation or data on its reactivity have been reported. [17] For its preparation we used as alt metathesis reactionw ith sodium tetrakis[3,5-bis(trifluoromethyl)-phenyl]b orate (NaBArF).T he stoichiometric addition of NaBArF to 2-Br 3 in dichloromethanea tr oom temperature formed 2-BArF (Scheme 2). The reaction was allowed to stir overnight and sodium bromide and Br 2 as by-products were removed by filtration and evaporation, respectively.T he crude product could be purifiedb yc rystallization from dichloromethane/hexanea tÀ20 8C.…”