Formations of mixed-valence oxovanadiumV,IV citrates and homocitrate with N-heterocycle chelated ligand.

Dimeric mixed-valence oxovanadium citrate [V 2O 3(phen) 3(Hcit)].5H 2O ( 1) (H 4cit = citric acid, phen = 1,10-phenanthroline) was isolated from a weak acidic medium. It could be converted quantitatively into a tetrameric oxovanadium citrate adduct of 1,10-phenanthroline [V 2O 3(phen) 3(Hcit) 2(phen) 3O 3V 2].12H 2O ( 2). This was supported by the trace of infrared spectra and X-ray diffraction patterns. The two compounds feature a bidentate citrate group that chelates only to one vanadium center through their negatively charged alpha-alkoxy and alpha-carboxy oxygen atoms, while the other beta-carboxy and beta-carboxylic acid groups are free to participate in strong intramolecular and intermolecular hydrogen bonding [2.45(1) in 1 and 2.487(2) A in 2], respectively. This is also the case of homocitrato vanadate(V/IV) [V 2O 3(phen) 3( R, S-H 2homocit)]Cl.6H 2O ( 3) (H 4homocit = homocitric acid), which features a binding mode similar to that found in the R-homocitrato iron molybdenum cofactor of Mo-nitrogenase. Moreover, the homocitrato vanadate(V) [VO 2(phen) 2] 2[V 2O 4( R,S-H 2homocit) 2].4H 2O.2C 2H 5OH ( 4) is isolated as a molecular precursor for the formation of mixed-valence complex 3. The V-O alpha-alkoxy and V-O alpha-carboxy bond distances of homocitrate complexes 3 and 4 are 1.858(4) and 1.968(6) av and 2.085(4) and 1.937(5) A, respectively. They are shorter than those of homocitrate to FeVco (2.15 A). The gamma-carboxy groups of coordinated homocitrato complexes 3 and 4, and the free homocitrate salt Na 3(Hhomocit).H 2O ( 5), form strong hydrogen bonds with the chloride ion and the water molecule [2.982(5) in 3, 2.562(9) in 4, and 2.763(1) A in 5], respectively.

N-heterocycle chelated oxomolybdenum(VI and V) complexes with bidentate citrate.

A 1,10-phenanthroline (phen) chelated molybdenum(VI) citrate, [(MoO2)2O(H2cit)(phen)(H2O)2] x H2O (1) (H4cit = citric acid), is isolated from the reaction of citric acid, ammonium molybdate and phen in acidic media (pH 0.5-1.0). A citrato oxomolybdenum(V) complex, [(MoO)2O(H2cit)2(bpy)2] x 4H2O (2), is synthesized by the reduction of citrato molybdate with hydrazine hydrochloride in the presence of 2,2'-bipyridine (bpy), and a monomeric molybdenum(VI) citrate [MoO2(H2cit)(bpy)] x H2O (6) is also isolated and characterized structurally. The citrate ligand in the three neutral compounds uses the alpha-alkoxy and alpha-carboxy groups to chelate as a bidentate leaving the two beta-carboxylic acid groups free, that is different from the tridentate chelated mode in the citrato molybdate(VI and V) complexes. 1 and in solution show obvious dissociation based on 13C NMR studies.

Syntheses, spectroscopies and structures of molybdenum(VI) complexes with homocitrate.

Initial investigations into the possible role of homocitric acid in iron molybdenum cofactor (FeMo-co) of nitrogenase lead us to isolate and characterize two tetrameric molybdate(VI) species. The complexes K2(NH4)2[(MoO2)4O3(R,S-Hhomocit)2].6H2O (1) and K5[(MoO2)4O3(R,S-Hhomocit)2]Cl.5H2O (2) (homocitric acid = H4homocit, C7H10O7) are prepared from the reactions of acyclic homocitric acid and molybdates, which represent the first synthetic structural examples of molybdenum homocitrate complexes. The homocitrate ligand trapped by tetranuclear molybdate coordinates to the molybdenum(VI) atom through alpha-alkoxy and alpha-, beta-carboxy groups. The physical properties, structural parameters, and their possible biological relevances are discussed.

Enantiomeric and mesomeric mandelate complexes of molybdenum -- on their stereospecific formations and absolute configurations.

The stereospecific formation and absolute configuration of R-homocitrate coordinated FeMo-co in nitrogenase was mimicked through the structural analyses of a collection of enantiomeric and mesomeric mandelato molybdenum complexes, i.e., (NH(4))(2)[Mo(Delta)O(2)(R-mand)(2)]x3H(2)O (1a), (NH(4))(2)[Mo(Lambda)O(2)(S-mand)(2)]x3H(2)O (1b), (NH(4))(4)[Mo(Delta)O(2)(RS-mand)(2)][Mo(Lambda)O(2)(RS-mand)(2)]x8H(2)O (2), (NH(4))(2)[W(Delta)O(2)(R-mand)(2)]x2H(2)O (3a), (NH(4))(2)[W(Lambda)O(2)(S-mand)(2)]x2H(2)O (3b) (H(2)mand=mandelic acid, C(8)H(8)O(3)), which have been characterized by elemental analyses, optical rotation, circular dichroism, IR, NMR spectroscopes and X-ray single crystal studies. The R and S chiral mandelic acids induce the formations of the enantiomeric pair of chiral complexes, which are supported by the characterizations of optical rotation and circular dichroism. The configuration of the resulted metal center could be assigned as Delta or Lambda. While the RS racemic reagent yields only mesomeric compound. The Delta(R,R)-complexes 1a and 3a are enantiomers of Lambda(S,S)-1b and 3b, respectively. Of the five complexes, Mo and W atoms are all hexa-coordinated by two cis-oxo groups and two bidentate mandelate ligands through the deprotonated alpha-alkoxyl and alpha-carboxyl groups, forming a stable five-membered chelated rings. The average Mo(VI)-O bond distances with alpha-alkoxyl and alpha-carboxyl are 1.944 and 2.210 A, respectively. Further comparison indicates that bonds of alpha-alkoxyl groups in the hydroxycarboxylato molybdenum complexes are much sensitive to the change in the oxidation state of molybdenum, which support the possible Mo activation model in FeMo-co through the protonation and cleavage of alpha-alkoxyl group in homocitrate ligand.

Synthesis and characterization of homochiral polymeric S-malato molybdate(VI): toward the potentially stereospecific formation and absolute configuration of iron-molybdenum cofactor in nitrogenase.

Reaction of sodium or potassium molybdate and excess malic acid in a wide range of pH values (pH 4.0-7.0) resulted in the isolation of two cis-dioxo-bis(malato)-Mo(VI) complexes, viz. Na(3)[MoO(2)H(S-mal)(2)] and K(3)[MoO(2)H(S-mal)(2)].H(2)O (H(3)mal=malic acid). The sodium complex is also characterized by an X-ray structure analysis, showing that the mononuclear Mo units are linked together via very strong symmetric CO(2)...H... O(2)C-hydrogen bond [2.432(5) A], forming a polymeric chain. The molybdenum atoms are quasi-octahedrally coordinated by two cis-oxo groups and two bidentate malate ligands via its alkoxy and alpha-carboxyl groups, while the beta-carboxylic and carboxylate groups remain uncomplexed, as the coordination of vicinal carboxylate and alkoxide of homocitrate in FeMo cofactor of nitrogenase. The absolute configuration of the metal center in this S-malato complex is assigned as Lambda and the homochirality within the chain is established as a homochiral form ...Lambda(S)-Lambda(S)-Lambda(S)-Lambda(S)... . It is proposed that the chiral configuration of the metal center in wild-type FeMo-co biosynthesis might be induced by the early coordination of the chiral R-homocitric acid, while a mixture of raceme might be obtained in the biosynthesis of NifV(-) FeMo-cofactor. The absolute configuration of wild-type FeMo-cofactor is assigned as Delta(R).