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Formation and Reactivity of [(tacn)-N-CO-ReIIIBr(CO)2]+ in Water: a Theoretical and Experimental Study

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journal contribution
posted on 01.06.2009, 00:00 by Fabio Zobi, Olivier Blacque, Gideon Steyl, Bernhard Spingler, Roger Alberto
The chemistry of [(tacn)-N-CO-ReIII(CO)2Br]X (X = Cl or Br), obtained in good yield from the reaction of fac-[(tacn)ReI(CO)3]Br (1, tacn = 1,4,7-triazacyclononane) with X2 in water, is described. The [(tacn)-N-CO-ReIII(CO)2Br]X complex (2 with X = Br; 2a with X = BrCl2), which we have previously communicated, is characterized by an unusual three-membered ring acyl amide bond. Complex 2 is stable as a solid but is reactive in aqueous solution. Under basic conditions (1 M NaOH), reductive decarbonylation was observed, and the bis-carbonyl complex [(tacn)ReI(CO)2Br] (3) was obtained in quantitative yield. The Br ligand in 3 could be replaced by CN, giving the neutral complex [(tacn)ReI(CO)2(CN)] (4). In acidic media (1 M HBr), complex 2 partially converted to the monocarbonyl μ-oxo bridged dinuclear complex {[(tacn)ReIII(CO)Br]2O}2+ (5 as [PF6] salt). Under mild oxidative conditions the trioxo [(tacn)ReVIIO3](ReO4) (6) was formed almost quantitatively, and small amounts of the uncommon ReVI complex [(tacn)ReVIO2Br](ReO4) (7) were identified. Mechanistic investigations at the density functional level of theory (DFT) showed that the elementary steps in the formation of 2 from 1 and 3 from 2 involved reactions of the complexes with hydroxide. The calculated pathway is strongly exothermic (ca. −137 kcal/mol), confirming the energetically and kinetically highly favored formation of 3. The X-ray structures of 2a and 35 are reported and discussed.