Aqua and perfluoro anion (BF₄^-, AsF₆^-) coordination isomers of manganese and rhenium carbonyls

Abstract

Pentacarbonyl- and tricarbonyl-rhenium bromide complexes react with AgAsF₆ and AgBF₄ in dichloromethane to form the respective aqua species, [Re(CO)₅(OH₂)]⁺ Y^- (where, Y = AsF₆^-, BF₄^-) and [Re(CO)(L₂)(OH₂)]⁺ BF₄^- [where L₂ = bpy, dppe, P(OPh)₃].* A variable-temperature ¹⁹F nuclear magnetic resonance study of the [Re(CO),₃(tmen)(OH₂)]⁺ BF₄^- system establishes that an equilibrium exists between the coordinated and 'free' tetrafluoroborate anion in solution (CD₂Cl₂):

Re(CO)₃(tmen)FBF₃ + H₂O ℰ [Re(CO)₃(tmen)(OH₂)]⁺ + BF₄^-

This equilibrium is forced to the right by low temperature (c. -10°C) and by removing the water with phosphorus pentoxide. Thus, under controlled reaction conditions (c. - 10°C, dry nitrogen atmosphere), AsF₆^- and BF₄^- are seen by infrared spectra to be coordinated in the complexes Re(CO)₃(bpy)FAsF₅.δ(H₂O) and Re(CO)₃(tmen)FBF₃.δ(H₂O). The species [Re(CO)₃(L₂)(OH₂)]⁺ Y^- [where (i), L₂ = 2CO, Y = AsF₆^-; (ii), L₂ = tmen, Y = AsF₆^-, BF₄^-] have been characterized by their crystal structures. Crystals of [Re(CO)₅(OH₅)]⁺ AsF₆^- are monoclinic, space group C2/c with a 27.085(5), b 7.208(1), c 13.404(3) Ǻ and β 105.50(1)°. A full-matrix least-squares refinement when using the absorption corrected data converged with R = 0.027 and R, = 0.028. The species [Re(CO)₃(tmen)(OH₂)] + AsF₆^- crystallizes in the monoclinic space group P2, with a 7.858(1), b 15,68(1), c 7.887(2) Ǻ and β 117.11(1)°. The final full-matrix refinement converged with R = 0.021 and R_w = 0.022. [Re(CO)₃(trnen)(OH₂)]⁺ BF₄^- crystallizes in themonoclinic space group C2/m with a 14.134(4), b 16.010(4), c 7.729(1) Ǻ and β 113.16(2)°. In these structures the water molecule coordinates to the rhenium(1) at an average distance of 2,246(5) Ǻ. This is significantly larger than the rhenium-fluoride bond length of 2.040(4) Ǻ.

Similar manganese aqua complexes, [Mn(CO)₃ {P(OPh)₃}₂(H₂O)]⁺ Y^- (where, Y^- = AsF₆^-, BF₄^-), also exhibit the same infrared features characteristic of coordinated water [i.e. the presence of sharp δ(H-O-H) bending absorption near 1600 cm^-1]. This feature is absent from all the species which only have lattice water and a coordinated fluoro anion.