A large C 6D 6 solvent effect on the [formula omitted] NMR shifts of pyrazines coordinated to the M(CO) 5 series (M = W, Mo, Cr)

Abstract

The pyrazine series of complexes M(CO) 5(pzR n ) with M = W, Mo or Cr; R = CH 3, pzR n = pyrazine ( n = 0), 2-methylpyrazine ( n = 1), 2,3-dimethylpyrazine and 2,6-dimethylpyrazine ( n = 2) and 2,3,6-trimethylpyrazine ( n = 3) have been prepared by UV photolyses in acetone. 1 H NMR spectra of the M(CO) 5(pzR n ) series, the free ligands, pzR n , and their conjugate acids, pzR n H +, were obtained in CDCl 3 and C 6D 6. The pzR n ring protons experience an upfield shift in C 6D 6 relative to other solvents which do not selectively associate with the ring C–H bonds. The shift difference, δ ̂ = δ(CDCl 3−C 6D 6), is attenuated with increasing methylation (∑ n) as a function of both molecular volume and an electronic factor proportional to the Hammett constant σ p for CH 3. Thus, δ ̂ = C o + (0.56 σ p + bV) ∑ n where C o = 0.58, σ p = −0.129, bV = −0.069 for pyrazines. Coordination of the Lewis acids M(CO) 5 (M = W, Mo, Cr) enhances the shift difference δ ̂ relative to the free pyrazine ligands, and increases the molecular volume such that ( bV complex/ bV free L) = 3.6 (W), 2.9 (Mo), and 2.9 (Cr). Compared to H + with a formal +1/2 charge at each ring N, the M(CO) 5 moieties exert a fractional charge influence in the order of 0.46 (W), 0.36 (Mo), and 0.34 (Cr) in the polarization of C–H bonds adjacent to the site of protonation or metallation. This action as a withdrawing group with W(CO) 5 > Mo(CO) 5 > Cr(CO) 5 is in harmony with 13 C NMR evidence on the degree of back-donation to the CO ligands (W > Mo > Cr). 1,4-Metallotropic shifts were not observed for any of the pzR n N-bound complexes (up to ca. 340 K) before ligand dissociation prevents further study. The M(CO) 5 series M = W, Mo, Cr promote downfield shifts of α-ring protons and upfield shifts of β-ring protons, thus behaving more like anionic ML 5 units (M(CN) 5 3−, M(hedta) −; Fe II and Ru II), toward N-heterocyclic ligands. An isomer with M(CO) 5 attached to the π-cloud, in addition to the N-bound complexes, was obtained only for Mo(CO) 5(2,6-Me 2pz). Reasons for the η 2 attachment near the C–3–N–C–4 arc of Mo(CO) 5(2,6-Me 2pz), and the absence of η 2 species for the remainder of the M(CO) 5(pzR n ) series, are suggested. A binuclear complex [W(CO) 5] 2(2,3-Me 2pz) forms more readily than for the unsubstituted pyrazine ligand in spite of larger steric factors. An elevation of the basicity of the terminal N of 2,3-Me 2pz provided by the methyl groups offsets the withdrawing influence of the W(CO) 5 center, raising the nucleophilicity of the W(CO) 5(2,3-Me 2pz) monomer compared to W(CO) 5(pz).