A large C6D6 solvent effect on the H-1 NMR shifts of pyrazines coordinatedto the M(CO)(5) series (M = W, Mo, Cr)

The pyrazine series of complexes M(CO)(5)(pzR(n)) with M = W, Mo or Cr; R = CH3, pzR(n) = pyrazine (n = 0), 2-methylpyrazine (n = 1), 2,3-dimethylpyra zine and 2,6-dimethylpyrazine (n = 2) and 2,3,6-trimethylpyrazine (n = 3) h ave been prepared by W photolyses in acetone. H-1 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 CDCl3 and C6D6. The pzR(n) ring protons experienc e an upfield shift in C6D6 relative to other solvents which do not selectiv ely associate with the ring C-H bonds. The shift difference, <(delta)over c ap> = delta(CDCl3-C6D6), is attenuated with increasing methylation (Cn) as a function of both molecular volume and an electronic factor proportional t o the Hammett constant sigma(p) for CH3. Thus, <(delta)over cap> = C-o + (0 .56 sigma(p) + bV) Sigma n where C-o = 0.58,sigma(p) = -0.129, bV = -0.069 for pyrazines. Coordination of the Lewis acids M(CO)(5) (M = W, Mo, Cr) enh ances the shift difference <(delta)over cap> a relative to the free pyrazin e ligands, and increases the molecular volume such that (bV(complex)/bV(fre e 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 inf luence in the other of 0.46 (W), 0.36 (Mo), and 0.34 (Cr) in the polarizati on of C-H bonds adjacent to the site of protonation or metallation. This ac tion as a withdrawing group with W(CO)(5) > Mo(CO)(5) > Cr(CO)(5) is in har mony with C-13 NMR evidence on the degree of back-donation to the CO ligand s (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 preve nts further study. The M(CO)(5) series M = W, Mo, Cr promote downfield shif ts of alpha-ring protons and upfield shifts of beta-ring protons, thus beha ving more like anionic ML5 units (M(CN)(5)(3-), M(hedta)(-); Fe-II and Ru-I I), toward N-heterocyclic ligands. An isomer with M(CO)(5) attached to the pi-cloud, in addition to the N-bound complexes, was obtained only for Mo(CO )(5)(2,6-Me(2)pz). Reasons for the eta(2) attachment near the C-3-N-C-4 are of Mo(CO)(5)(2,6-Me(2)pz), and the absence of eta(2) species for the remai nder of the M(CO)(5)(pzR(n)) series, are suggested. A binuclear complex [W( CO)(5)](2)(2,3-Me(2)pz) forms more readily than for the unsubstituted pyraz ine ligand in spite of larger steric factors. An elevation of the basicity of the terminal N of 2,3-Me2pz provided by the methyl groups offsets the wi thdrawing influence of the W(CO)(5) center, raising the nucleophilicity of the W(CO)(5)(2,3-Me(2)pz) monomer compared to W(CO)(5)(pz). (C) 1999 Elsevi er Science S.A. All rights reserved.