Protonation vanadium complexes

The tetrameric vanadium complex contains also two sodium ions. Each is coordinated by two oxygen atoms of methanol molecules and by two oxygen atoms of different catechol molecules. Because of this coordination, oxygen atoms are Hj-bridging Na and V atoms. One of the three independent catechol molecules participates in the coordination of the vanadium atoms by one O only whereas the second atom is protonated and does not participate in the metal coordination. It is apparent that the complex... 

Top row the three components of the Yandulov-Schrock cycle for catalytic dinitrogen reduction. Left the catalyst with the substrate Nj coordinated to Mo (Ar refers to the aryl substituent drawn in detail for one of the nitrogens). Centre the reductant bis(pentamethylcyclopentadienyl)chromium, CpjCr. Right the proton source 2,6-lutidinium borate. Bottom row vanadium complexes with intermediates of nitrogen reduction activated dinitrogen or diazenido(2—) (46a), imide (46b) and ammonia (46c). 

Dinuclear vanadium complexes with bridging diazenido(2—). The ligands are 2,6-bis(l-iminoethyl)pyridine (53)h h mesityl(l—) (54),h l 2-dimethylaminophenyl(l—) and pyridine (55),h l tolane, trimethylphosphine and dimethylamino-Cp(l—) (56),and benzamidinate(l —) (57).P 1 The complexes 54 and 55, containing (r-aUcyls, produce ammonia and hydrazine on protonation. 

Hydrolysis of [LVCI3] (L = 1,4,7-triazacyclononane) under anaerobic conditions yields the green binuclear cation [L2V2(/a-CH3C02)] / Binuclear vanadium complexes of triethylenetetraaminehexaacetate, which may be formulated as [V20(ttha)], react with O2 by proton-independent and assisted pathways (ko = 140 s- k, = 2.08 x 10" M- s ) to yield an initial [V(III)V(IV)Ol]... 

The complex C5H5V(CO)3CNH arises from protonation of [C5H5V-(CO)jCN] (56). This is the only vanadium isocyanide complex known. 

In most paramagnetic sandwich complexes the proton hf structure is not resolved in low temperature powder or single crystal EPR spectra. In vanadium dibenzene, a typical example of this type of compound, the poor resolution is due to the fact that the aromatic rings are rigidly frozen at T < 50 K and thus the proton hfs tensors of the benzene rings are no longer magnetically equivalent. 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

Protonation of a carbonyl oxygen rather than the metal may be encouraged in this case by the high coordination number of vanadium. This would then promote halide attack on the carbonyl carbon to yield an intermediate hydroxyhalocarbene, which reacts further to yield the indicated products. This system represents a potential photoassisted water-gas shift catalyst system since H3V(CO)3(diars) upon photolysis with a mercury vapor lamp yields H2, and in the presence of CO regenerates the starting complex HV(CO)4(diars). The feasibility of coupling these two reactions in the same reaction solution remains to be demonstrated. 

---