Synthesis and reactivity of [NEt4]2[ReBr3(CO)3]. Formation and structural characterization of the clusters [NEt4][Re3(µ3-OH)(µ-OH)3(CO)9] and [NEt4][Re2(µ-OH)3(CO)6] by alkaline titration

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Abstract
The dianionic rhenium(I) complex [ReBr3(CO)3]2– has been synthesized and characterized. This complex is an important starting material for compounds containing the fac-Re(CO)3 moiety since the three bromide ligands are very weakly bound. Particularly in co-ordinating solvents, the bromides are substituted by solvent molecules thus generating the strong 12e– Lewis acid [Re(H2O)3(CO)3]+2. This formula was confirmed, particularly in water, by IR spectroscopic methods. Complex 2 is stable in aqueous solution even when exposed to air for weeks. It has been titrated by NaOH solutions. The change in pH was detected potentiometrically and found to occur very slowly, indicating a reaction other than protonation deprotonation. Depending on the rate of titration and the total concentration of OH–, the two rhenium(I) hydroxo complexes [NEt4][Re3(µ3-OH)(µ-OH)3(CO)9] and [NEt4][Re2(µ-OH)3(CO)6] were isolated in good yield and structurally characterized. The former crystallizes in the orthorhombic space group Pnma with a= 10.752(7), b= 13.783(8) and c= 18.254(12)Å. The skeletal framework is that of a cube lacking the Re(CO)3 unit in one corner. The latter crystallizes in the orthorhombic space group P2mm with Z= 1, a= 6.551(2), b= 7.413(2) and c= 12.084(2)Å. The two Re(CO)3 moieties are bridged by three OH– ligands which create a mirror plane. A second mirror plane is perpendicular to it along the Re-Re axis.

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