A triad of homologous, air-stable compounds containing short, quadruple bonds between metal atoms of Group 6

Abstract

Each of the metals chromium, molybdenum, and tungsten forms a compound of molecular formula M2(mhp)4, where mhp represents the anion of 2-hydroxy-6-methylpyridine, Hmhp (6-methylpyridone). These compounds, which are yellow, orange, and red, respectively, are all stable toward dry air as solids and can be easily and quickly prepared in large quantities from readily available starting materials. Cr2(mhp)4 is stable toward even humid air and appears to be more chemically and physically stable than any other chromium(II) compound. The Cr and Mo compounds were prepared by reactions of Cr2(02CCH3)4 or Cr2(02CCH3)4(H20)2 and Mo2(02CCH3)4, respectively, with the sodium salt of the ligand; W2(mhp)4 was obtained by reaction of W(CO)8 with Hmhp in refluxing diglyme. All three compounds are volatile and give definitive mass spectra. The Raman spectra have M-M stretching frequencies (±3 cm-1) at 556, 425, and 295 cm-1, for the Cr, Mo, and W compounds, respectively. All three give crystals of composition M2(mhp)4(CH2Cl2) which are isomorphous; each structure has been determined by x-ray crystallography, revealing the M-M distances to be Cr-Cr, 1.889 (1) Á; Mo-Mo, 2.065 (1) Á; W-W, 2.161 (1) A. The compounds form monoclinic crystals (space group P2\/n with Z = 4) with unit cell dimensions at 22 ± 2 °C as follows, where the values for the Cr, Mo, and W compounds are given in that order for each: a (A) = 12.792 (2), 12.959 (1), 12.924 (1); b (A) = 16.985 (3), 16.948 (2), 16.948 (2); c (A) = 12.904 (2), 13.064 (2), 13.010 (2); ß (deg), 104.42 (1), 104.90 (1), 104.78 (1).

Tetrakis[μ-(6-methyl-2-pyridinolato)]dirhodium

“…Similar shortenings of M-M bonds in [M2(mhp) 4] and similar complexes compared with carboxylato-bridged species have already been noted for Cr, Mo and W (Cotton, Koch & Millar, 1977;Cotton, Fanwick, Niswander & Sekutowski, 1978, and references therein), and can be attributed to the electronic structure of the four mhp ligands, which reduce the net positive charge on the metal atoms, encouraging M-M bonding, while simultaneously sterically blocking the axial positions for possible ligands.…”

“…]According to the sign convention of Klyne & Prelog (1960), these and all other torsion angles in the following discussion are negative.] Corresponding values for the l M2(mhpL] structures already published (Cotton, Fanwick, Niswander & Sekutowski, 1978) are: Cr 1.7, 2.2, 1.8, 2.0 (mean 1.9°); Mo 1.3, 1.4, 1.5, 1.1 (mean 1.3°); W 1.2, 0.3, 1.8, 2.1 (mean 1.4 °) (e.s.d. 's for these torsion angles are not available, but are probably greater than for the Rh complex by factors of approximately 1-2, 2-3 and 4-5 for M = Cr, Mo and W respectively, in view of the relative e.s.d.…”

“…1) has no crystallographically imposed symmetry, but the geometry is close to a,2m (D2a), Table 2. Other [ME(mhp) 4] molecules (mhp represents the anion of 6-methyl-2-pyridinol, Hmhp) possess the same molecular symmetry; the complexes of Cr, Mo and W (Cotton, Fanwick, Niswander & Sekutowski, 1978) and of Ru (Berry, Garner, Hillier, MacDowell & Clegg, 1980b) are all isostructural, crystallizing with one un- (Cotton, DeBoer, LaPrade, Pipal & Ucko, 1971). In the carbonatobridged complexes Csa[Rh2(CO3)4(H20)21.6H20 and Cs4Na2IRhz(CO3)4CI21.8H20 the Rh-Rh bonds are shorter, 2.378 (I) and 2.380 (2) A respectively (Cotton & Felthouse, 1980); the shortening relative to IRh2(OAc)41.2H20 is not great, but is probably real.…”

Tungsten Compounds

“…Specifi cally, W 2 4+ tetracarboxylates W 2 (O 2 CR) 4 and salts of the [W 2 Cl 8 ] 4-anion have not shown the synthetic utility of the Mo 2 4+ analogs. Therefore, progress in the synthesis, structural characterization, and reactivity studies of W 2 4+ compounds has relied on new developments in synthetic methodologies and new ligand types.…”

“…1, [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] In Section 5.2, the fi rst attempts to synthesize the tetracarboxylates of the type W 2 (O 2 CR) 4 , culminating in their successful isola tion and characterization in the early 1980's, are described. Subsequent sections focus on the comparatively small but growing number of other W 2 4+ and MoW 4+ quadruply bonded complexes and paddlewheel compounds with either W 2 5+ or W 2 6+ cores.…”