Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 5. Experimental and Theoretical Study of the Rotation Conformational Preferences of Mo₂Cl₄(R-py)₄ (R-py = Substituted Pyridine) Molecules

Abstract

The relative stabilities of rotation conformational isomers of Mo2Cl4(R-py)4 molecules in solution have been studied. R-py represents a substituted pyridine, 4-tert-butylpyridine, 4-picoline, or 3,5-lutidine, and the MoCl2(R-py)2 portions of each molecule have the trans configuration. In previously reported crystal structures, these molecules have been found with several different conformations: D 2 h , D 2 d (both eclipsed), and D 2 (partly staggered). From the visible spectra of these solids, it is found that the D 2 h and D 2 d conformations give rise to a δ→δ* band at ∼570 nm, while a D 2 conformation with a twist angle of ∼20° from D 2 h has a band at ∼645 nm. For solutions in CH2Cl2 we find both bands, but the intensity ratio is strongly dependent on temperature. In addition the intensity ratio varies over a range of 100 in THF/CH2Cl2 mixtures. Calculations by the DFT method provide an estimate of the energy variation as a function of internal rotation angle from the D 2 h through D 2 to D 2 d conformations. The major conclusions are: (1) the D 2 h conformation, although frequently found in crystals, is not stable in solution or the gas phase; (2) the relative stabilities of the D 2 d and D 2 conformations vary greatly with solvent and temperature; (3) absorption bands at ∼25 000 cm-1, 20,000 cm-1, and 15,000−18,000 cm-1 can be assigned to δ→d x 2 - y 2 , π→δ*, and δ→δ* transitions, respectively.

Alkylpyridine Complexes of Tungsten(II) and Chromium(II). First Rotational Isomers of W2X4L4Molecules withD2handD2Symmetries

“…The lack of a crystalline compound in which there is a D 2 d molecule is unfortunate. It would be tempting to assign the two bands observed in solution to D 2 d (680 nm) and D 2 (720 nm) isomers as in the analogous molybdenum case, but we cannot confirm either of these assignments by citing data on crystals. One might suppose (and this receives support from preliminary DFT calculations) that the δ → δ* separations in the D 2 h and D 2 d conformers should be about the same.…”

“…The coordination of the central Cr(1) atom, with inclusion of the weak contacts to Cl (2), can be considered as tetragonally elongated octahedral, while the terminal Cr(2) centers adopt angularly distorted square-pyramidal geometry. Although the structure of 3 is unique, some of its features are well-known.…”

“…Three rotamers have been recognized in the solid state: two eclipsed forms with D 2 h and D 2 d virtual symmetries and a range of partially staggered ones, D 2 (Chart ). The spectroscopic study of these molecules has shown that only two of them, D 2 d and D 2 , are present in solution and that their relative stabilities vary greatly with solvent and temperature. The major conclusion from the research is that the solvation and packing effects profoundly influence the dependence of energy on the angle of internal rotation.…”

12 Chromium, molybdenum and tungsten

“…95 The relative stabilities of the rotation conformational isomers of [Mo 2 Cl 4 (R-py) 4 ] (R-py 4-Bu t -py, 4-picoline, 3,5-lutidine) have been studied spectroscopically for samples in solution, and compared to earlier observations made for samples in the gas phase and in crystals. 96 Syntheses and magnetic and spectral properties have been reported for the molybdenum(II) complexes [Mo(CN) 3 L 2 (NO)] for L nicotinimide, nicotinic acid hydrazole, isonicotinic acid hydrazide, 2-benzoylpyridine, nicotine and 2-(hydroxymethyl)pyridine. In all cases, the heterocyclic ligand behaves as a monodentate ligand co-ordinating to the metal through the ring nitrogen atom.…”

A Bis(diazadiene) Adduct of MoCl2: Mononuclear, Octahedral, Undistorted and Diamagnetic

“…This problem is commonplace in organometallic chemistry, for instance for M-olefin, M-diene, M-carbene complexes, and so forth. [18] The title compound may be described by the two limiting forms as a bis(diazadiene) Mo [19Ϫ24] but also nitriles, [25] pyridines, [26,27] bipyridines and other aromatic amines. [28] This observation may once again be rationalized by considering the strong π-accepting properties of the dad ligand.…”