Dinuclear Oxomolybdenum(V) and Oxotungsten(V) Complexes of the Hexadentate Ligands N,N,N′,N′-Tetrakis(2-pyridylmethyl)ethylenediamine and Its Propylenediamine Analog

Abstract

Abstract Four new dinuclear oxomolybdenum(V) and oxotungsten(V) complexes, [M2O2(μ-O)2(μ-tpen-N,N′)]2+ and [M2O2(μ-O)2(μ-tppn-N,N′)]2+ (tpen = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, tppn = N,N,N′,N′-tetrakis(2-pyridylmethyl)propylenediamine ((R)- and (R,S)-forms have been used); M = Mo, W), have been prepared. X-Ray structural analysis (except for the W–tppn complex) revealed ‘basket type’ structures similar to those of the edta and pdta analogs, [M2O2(μ-O)2(μ-edta-N,N′ or μ-pdta-N,N′)]2− (edta = ethylenediamine-N,N,N′,N′ -tetraacetate(4−); pdta = propylenediamine-N,N,N′,N′-tetraacetate(4−)). The metal–metal distances of the Mo–tpen, Mo–tppn, and W–tpen complexes are 2.546(1), 2.541(2), and 2.561(1) Å, respectively. Asymmetric distortion along the Mo–Mo axis of the Mo–R-tppn complex is in the opposite direction to that of the corresponding R-pdta complex. The difference may be related to the longer Mo–N(pyridyl) distance and larger N(pyridyl)–Mo–N(pyridyl) angle of the tppn complex as well as the direction of skew conformation of five membered chelate rings M–N(pyridyl)–C–C–N(amine). Electronic absorption spectral patterns of the new complexes are generally similar to the corresponding edta complexes. Circular dichroism (CD) spectra of the R-tppn complexes in the region >300 nm show a pattern nearly enantiomeric to those of the corresponding R-pdta complexes, indicating that the asymmetric distortion is retained in water and in acetonitrile. Kinetic parameters of the inversion of the pseudo-gauche conformation of diamine moiety in the tpen complexes were evaluated from the temperature dependence of 1H NMR spectra in acetonitrile. ΔH‡’s are significantly larger and ΔS‡’s are significantly more positive than those of the edta analogs. The inversion may take place through dissociative bond cleavage of the metal–N(pyridyl) bond. The new dimolybdenum complexes show two reversible one electron reduction processes (at −1.40 and −1.86 V vs. Ag/Ag+) in acetonitrile (0.1 mol dm−3). Electronic absorption spectrum of the Mo2(IV,V) mixed valence state was obtained using spectroelectrochemical techniques. (Remark: This summary is partially truncated in the html because it exceeds the limit of the system (4000 characters per note). The complete text is given in the pdf.)