Spectroscopic studies of molybdenum complexes as models for nitrogenase
Because biological nitrogen fixation requires Mo, there is an interest in inorganic Mo complexes which mimic the reactions of nitrogen-fixing enzymes. Two such complexes are the dimer Mo/sub 2/O/sub 4/ (cysteine)/sub 2//sup 2 -/ and trans-Mo(N/sub 2/)/sub 2/(dppe)/sub 2/ (dppe = 1,2-bis(diphenylphosphino)ethane). The H/sup 1/ and C/sup 13/ NMR of solutions of Mo/sub 2/O/sub 4/(cys)/sub 2//sup 2 -/ are described. It is shown that in aqueous solution the cysteine ligands assume at least three distinct configurations. A step-wise dissociation of the cysteine ligand is proposed to explain the data. The Extended X-ray Absorption Fine Structure (EXAFS) of trans-Mo(N/sub 2/)/sub 2/(dppe)/sub 2/ is described and compared to the EXAFS of MoH/sub 4/(dppe)/sub 2/. The spectra are fitted to amplitude and phase parameters developed at Bell Laboratories. On the basis of this analysis, one can determine (1) that the dinitrogen complex contains nitrogen and the hydride complex does not and (2) the correct Mo-N distance. This is significant because the Mo inn both complexes is coordinated by four P atoms which dominate the EXAFS. A similar sort of interference is present in nitrogenase due to S coordination of the Mo in the enzyme. This model experiment indicates that, given adequate signal to noise ratios, the presence or absence of dinitrogen coordination to Mo in the enzyme may be determined by EXAFS using existing data analysis techniques. A new reaction between Mo/sub 2/O/sub 4/(cys)/sub 2//sup 2 -/ and acetylene is described to the extent it is presently understood. A strong EPR signal is observed, suggesting the production of stable Mo(V) monomers. EXAFS studies support this suggestion. The Mo K-edge is described. The edge data suggests Mo(VI) is also produced in the reaction. Ultraviolet spectra suggest that cysteine is released in the course of the reaction.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 6428156
- Report Number(s):
- LBL-12823; ON: DE81023019; TRN: 81-011402
- Resource Relation:
- Other Information: Thesis
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
59 BASIC BIOLOGICAL SCIENCES
ACETYLENE
CHEMICAL REACTIONS
MOLYBDENUM COMPLEXES
NMR SPECTRA
STRUCTURAL CHEMICAL ANALYSIS
X-RAY SPECTROSCOPY
CYSTEINE
ELECTRON SPIN RESONANCE
LIGANDS
NITROGENASE
ORGANIC NITROGEN COMPOUNDS
ORGANIC PHOSPHORUS COMPOUNDS
ULTRAVIOLET SPECTRA
ALKYNES
AMINO ACIDS
CARBOXYLIC ACIDS
COMPLEXES
ENZYMES
HYDROCARBONS
MAGNETIC RESONANCE
NITRO-GROUP DEHYDROGENASES
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC SULFUR COMPOUNDS
OXIDOREDUCTASES
RESONANCE
SPECTRA
SPECTROSCOPY
THIOLS
TRANSITION ELEMENT COMPLEXES
400201* - Chemical & Physicochemical Properties
400301 - Organic Chemistry- Chemical & Physicochemical Properties- (-1987)
550200 - Biochemistry