Carborane Complexes of Ruthenium(III): Studies on Thermal Reaction Chemistry and the Catalyst Design for Atom Transfer Radical Polymerization of Methyl Methacrylate

Abstract

The heating of the 18-electron complex [3,3-(dppb)-3-H-3-Cl-closo-3,1,2-RuC(2)B(9)H(11)] (3) in benzene at 80 °C in the presence of a small amount of CCl(4) as initiator afforded paramagnetic 17-electron species [3,3-(dppb)-3-Cl-closo-3,1,2-RuC(2)B(9)H(11)] (4) along with minor amounts of two P-phenylene ortho-cycloboronated derivatives [3-Cl-3,3,8-{Ph(2)P(CH(2))(4)PPh-μ-(C(6)H(4)-ortho)}-closo-3,1,2-RuC(2)B(9)H(10)] (5) and [3,7-Cl(2)-3,3,8-{Ph(2)P(CH(2))(4)PPh-μ-(C(6)H(4)-ortho)}-closo-3,1,2-RuC(2)B(9)H(10)] (6) in total yield of ca. 80%. The heating of either 3 or 4 in toluene at 95 °C in the absence of CCl(4) led to the selective formation of 5, which was isolated in 64% and 46% yield, respectively. Thermolysis of 3 at higher temperatures (boiling toluene, 110 °C) gives novel paramagnetic species [3-Cl-3,3,7,8-{Ph(2)P(CH(2))(4)P-μ-(C(6)H(4)-ortho)(2)}-closo-3,1,2-RuC(2)B(9)H(9)] (7) featuring bis(ortho-cycloboronation) of both P-phenyl groups at the same phosphorus atom of the ruthenium-bound dppb ligand. All new paramagnetic complexes 4-7, as well as starting diamagnetic species 3, were characterized by single-crystal X-ray diffraction and, in addition, by EPR spectroscopic studies of odd-electron complexes. Ruthenacarboranes 3-5 and 7 all display high efficiency as catalysts for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Complex 5 gave the best catalyst performance in terms of polydispersity; the PDI (M(w)/M(n)) of the polymer samples is as low as 1.15.

Mononuclear closo-ruthenacarborane complexes containing a rare eight-membered metal-diphosphine ring

“…1). These spectra prove to be quite similar to those observed earlier for a series of paramagnetic Ru(III)-carborane complexes with dppb ligand at the metal vertex, such as [3,3-(dppb)-3-Cl-closo-3,1,2-RuC 2 B 9 H 11 ] and its ortho-phenylenecycloboronated derivatives [11] and are typical of low-spin 3d 5 ruthenium complexes found in the literature [7,16,17]. …”

“…The chemical reactivity of diamagnetic complex 3 towards terminal hydride elimination reaction proved to be very similar to that observed earlier for the closely related complex 4 [11]. Both complexes 3 and 4 remain intact after thermal treatment in benzene in anaerobic conditions for more than 8 h. However, when heated in benzene or toluene in the presence of CCl 4 , each 3 and 4 gave paramagnetic species [3,3-{k 2 -{Ph 2 P(CH 2 ) 5 PPh 2 }-3-Cl-closo-3,1,2-RuC 2 B 9 H 11 ] (5) and [3,3-{k 2 -{Ph 2 P(CH 2 ) 4 PPh 2 }-3-Cl-closo-3,1,2-RuC 2 B 9 H 11 ] (6) [11,13], respectively.…”

“…This is, presumably, due to a relatively fast rearrangement of a short-lived chelate exo-nido intermediate into its closo isomer 3 occurring in solution. As shown earlier [11], complex 1 displays similar reactivity towards 1,4-bis(diphenylphosphino) butane (dppb), where [3,3-{k 2 -{Ph 2 P(CH 2 ) 4 PPh 2 }}-3-Cl-3-H-closo-3,1,2-RuC 2 B 9 H 11 ] (4) has been obtained as the only product. However in the latter case it was possible to isolate and then examined a chelate exo-nido intermediate species by 1 H and 31 P { 1 H} NMR spectroscopy if the reaction was terminated immediately after the consumption of the starting reagent 1 [6].…”

“…110 C) leading to the loss of dihydrogen and subsequent, finally, ortho-cycloboronation to form 7 can be explained by thermal activation of 5 which might be a consequence of proximity of Ph-ortho-CH and B(8)-H hydrogen atoms. Similar thermal orthocycloboronation reaction is observed for complex 6 [11] as well as for many other metallaborane compounds [14]. On the other hand, taking into account that the formation of bis(ortho-cycloboronated) complex 8 from 7 occurring in toluene at 110 C but only in the presence of CCl 4 as a source of radicals, we can safely assume, that the latter reaction might has the radical mechanism.…”

“…Both complexes 3 and 4 remain intact after thermal treatment in benzene in anaerobic conditions for more than 8 h. However, when heated in benzene or toluene in the presence of CCl 4 , each 3 and 4 gave paramagnetic species [3,3-{k 2 -{Ph 2 P(CH 2 ) 5 PPh 2 }-3-Cl-closo-3,1,2-RuC 2 B 9 H 11 ] (5) and [3,3-{k 2 -{Ph 2 P(CH 2 ) 4 PPh 2 }-3-Cl-closo-3,1,2-RuC 2 B 9 H 11 ] (6) [11,13], respectively. The crude product 5 contained a very small amount of new paramagnetic species (7), which always appeared during preparation of 5 under this radical reaction conditions (Scheme 2).…”