Synthesis, crystal structure, and spectroscopic properties of [Mo2(O2CMe)2(MeCN)6][BF4]2, [Rh2(O2CMe)2(MeCN)6][BF4]2, and [Rh2(O2CMe)2(MeCN)4(py)2][BF4]2

Abstract

Treatment of [M₂(O₂CMe)₄](M = Mo or Rh) with [OMe₃][BF₄](ca. 1 : 4 mol ratio) in MeCN produces the corresponding [M₂(O₂CMe)₂(MeCN)₆][BF₄]₂ compound; dissolution of the rhodium derivative in pyridine (py) produces [Rh₂(O₂CMe)₂(MeCN)₄(py)₂][BF₄]₂. The structure of each of these compounds has been confirmed by X-ray crystallography: [Mo₂(O₂CMe)₂(MeCN)₆][BF₄]₂ crystallises in the monoclinic space group P2₁/m, with a= 7.160(1), b= 10.642(1), c= 19.402(2)Å, β= 99.47(1)°, and Z= 2; [Rh₂(O₂CMe)₂(MeCN)₆][BF₄]₂ crystallises in the monoclinic space group C2/c, with a= 15.749(2), b= 18.417(3), c= 13.965(2)Å, β= 91.34(2)°, and Z= 4; [Rh₂(O₂CMe)₂(MeCN)₄(py)₂][BF₄]₂ crystallises in the monoclinic space group C2/m, with a= 19.448(3), b= 10.476(1), c= 18.990(3)Å, β= 113.25(1)°, and Z= 4. The final R factors (and observed reflections) for the refined structures were 0.053 (2 774), 0.040 (2 936), and 0.059 (2 391), respectively. In each compound, the cation involves a cis arrangement of the two µ-acetato-groups, four equatorial N-bonded MeCN molecules, and two axial ligands (MeCN or pyridine) co-ordinated to the dimetal(II) centre. For each of the cations, the bond lengths are normal for such species and comparisons within and between values for the [M₂(O₂CR)₂(MeCN)₆]²⁺(M = Mo or Rh) provide a clear indication of the static trans effect of a Mo[graphic omitted]Mo quadruple bond and a Rh–Rh single bond, together with a clear documentation of the structural similarities and differences for analogous Mo₂⁴⁺ and Rh₂⁴⁺ complexes: Mo–Mo 2.136(1), Mo–O_eq 2.084(13), Mo–N_eq 2.148(4), Mo–N_ax 2.759(19); Rh–Rh 2.534(1), Rh–O_eq 2.015(4), Rh–N_eq 1.983(4), and Rh–N_ax 2.232(4)Å. The dimensions of the Rh₂O₄N₆ cores of [Rh₂(O₂CMe)₂(MeCN)₄L₂]²⁺(L = MeCN or py) are virtually identical; the latter complex involves an orthogonal arrangement of the two pyridine rings. Proton and ¹³C n.m.r. studies have shown that, at ambient temperatures, both the axial and equatorial MeCN molecules of [Mo₂(O₂CMe)₂(MeCN)₆]²⁺ undergo rapid exchange with CD₃CN, whereas for the corresponding rhodium complex only the axial molecules undergo such exchange. U.v.-visible and i.r. data are also reported.