William E. Newton

Syntheses and characterization of ammonium and tetraalkylammonium thiomolybdates and thiotungstates

Abstract Improved methods for the preparation of [NH 4 ] 2 [MO 2 S 2 ] and [NH 4 ] 2 [MS 4 ] are summarized and new syntheses of [NH 4 ] 2 [MOS 3 ] are reported (M = Mo, W). The facile conversion of the [NH 4 ] + salts to [Et 4 N] + salts via reaction with aqueous [Et 4 N]OH is also described. Infrared and electronic spectral data for the thiometallates are summarized as an aid to future characterization.

Synthesis and infrared spectra of 16O- and 18O-substituted oxomolybdenum complexes

Abstract A series of complexes of stoichiometry Mo 18 O 2 (LL) 2 , Mo 18 O(LL) 2 , Mo 2 18 O 3 (LL) 4 and Mo 2 18 O 4 (LL) 2 (LL = S 2 CNR 2 , S 2 P(OR) 2 , S 2 PR 2 ; not all compounds were formed with each ligand) was produced by: (a) the controlled hydrolysis with H 2 18 O of diazene adducts, Mo(LL) 2 (RN 2 R) 2 ( cis Mo 18 O 2 (LL) 2 and Mo 2 18 O 4 (LL) 4 ); (b) reduction of Mo 18 O 2 (LL) 2 with tertiary phosphine (Mo 18 O(LL) 2 ) and (c) combination of these two products (Mo 2 18 O 3 (LL) 4 ). Reaction of Mo 2 18 O 4 (LL) 2 with H 2 S gave Mo 2 18 O 3 S(LL) 2 . By comparison of the spectra of these products with their 16 O analogues (prepared similarly or by literature methods), definitive unambiguous assignments of both terminal and bridging molybdenum-oxygen stretching vibrations were made. Many incorrect assignments and disagreements in the literature were thus resolved. These assignments may be important in the identification of the various molybdenum cores in a variety of molybdenum-containing species.

Binding and activation of enzymic substrates by metal complexes : iii. reactions of mo(co)2 [s2cn(c2h5)2]2

Abstract The complex Mo(CO) 2 L 2 [L = S 2 CNEt 2 ] reacts with acetylenes to yield both Mo(CO)(RC 2 R)L 2 and Mo(RC 2 R) 2 L 2 , with diazenes giving Mo(RN 2 R)L 2 and Mo(RC 2 R) 2 L 2 , with diazenes giving Mo(RN 2 R)L 2 and Mo(RN 2 R) 2 L 2 , and with CO and PPh 3 to form Mo(CO) 3 L 2 and Mo(CO) 2 (PPh 3 )L 2 .

Preparation of some nitrile-ruthenium(II) complexes and their reactions with carbon monoxide

Abstract A new series of complexes of the type dichlorotetra(nitrile)ruthenium(II) [(RCN) 4 RuCl 2 ; R = CH 3 , C 2 H 5 , n-C 3 H 7 , i-C 3 H 7 , C 6 H 5 , C 6 H 5 CH 2 ] was prepared by the catalyzed reduction of “RuCl 3 , 3H 2 O” by hydrogen gas over Adams catalyst in solution in the appropriate nitrile. (RCN) 4 RuBr 2 (R = CH 3 , C 2 H 5 ) were also prepared by both this direct method and by metathesis of the dichloride with lithium bromide in methanol. (CH 3 CN) 4 RuBrCl was prepared by a similar short-term metathesis. Far i.r. spectroscopy indicated a trans configuration for all the complexes. Reaction with carbon monoxide in boiling methanol produced (RCN) 3 (CO)RuCl 2 , while in boiling acetone (RCN) 2 (CO) 2 RuCl 2 resulted.

Studies on the reactivity of oxodihalobis(diethyldithiocarbamato) molybdenum(VI) and tungsten(VI) complexes

Abstract The complexes OMoX 2 L 2 (L = S 2 )CNEt 2 ) 2 react with Na 2 S 2 O 4 to yield OMoL 2 (X = Cl), with PhNCO to yield (PhN)MoX 2 L 2 (X = Cl), with PPh 2 Et to give MoX 2 (PPh 2 Et)L 2 (X = Cl, Br), and with OMol 2 to produce OMoXL 2 (X = Cl, Br). The epr spectra of OMoXL 2 display halogen superhyperfine splitting [A( 35,37 Cl) = ∼3 gauss; A(bb]79,81Br) = 12.3 gauss). Reaction of WCl 6 with NaL in MeOH yields the new complex OWCl 2 L 2 which either does not react at all or gives no characterizable products with the above reagents. The results demonstrate the lack of atom transfer ability of tungsten(VI) relative to its molybdenum analog.

Thermochemical studies of molybdenum dithio-carbamate complexes as models for molybdoenzymes☆

Abstract Calorimetric measures for interconverting the oxidation states (IV), (V) and (VI) in dithiocarbamato complexes of molybdenum with accompanying oxo group transfer were carried out in 1,2-dichloroethane at 25 °C. Supplementary ΔH data for known substrates of molybdoenzymes were also measured. Combinations of these data yielded thermochemical results for hypothetical reactions between biological substrates and the oxidation states (IV), (V) and (VI) of molybdenum in these dithiocarbamato model compounds. Bond energies for terminal and bridging molybdenum-oxygen bonds in these complexes were calculated to be +96 and +101 kcal mol −1 , respectively.

Chemical Properties of the Fe-Mo Cofactor from Nitrogenase

The existence of a molybdenum cofactor common to molybdenumcontaining enzymes was first postulated by Pateman et al.1 who discovered a common genetic determinant for nitrate reductase and xanthine dehydrogenase in Aspergillus nidulans. The cofactor con cept was further developed by Nason and co-workers who studied a mutant of Neurospora crassa (nit-1), which was blocked in the terminal portion of its nitrate reductase electron transport chain (where molybdenum is located), and which also lacked xanthine dehydrogenase activity.2,3 A key discovery in the field was that crude extracts prepared from the nit-1 mutant could be restored to activity by treatment with an acid hydrolysis product obtainable from any molybdenum-containing enzyme,4,5 including the molybdenumiron protein of nitrogenase.6 Additional studies showed that although molybdate could enhance the activity produced by the nit-1 extract plus the acid-treated products of the molybdoenzymes, neither molybdate nor any of the simple molybdenum complexes tested could activate the nit-1 extracts alone. Furthermore, when 99Mo04 2− was used, it became incorporated into the newly activated nitrate reductase, but only in the presence of the acid-treated products of the molybdoenzymes. These data led to the hypothesis of a labile molybdenum complex being one of the products of all acid-treated molybdoenzymes.7

HD Formation by Nitrogenase: A Probe for N2 Reduction Intermediates

Nitrogenase is a complex of two separately purifiable proteins, the molybdenum-iron protein [MoFe] and the iron protein [Fe].1,2 In addition to catalyzing the reduction of N2 to ammonia, nitro genase has ATP-hydrolyzing activity,3 ATP-dependent H2-evolution activity,4 and supports a reaction between D2 and protons (from H2O) to form HD.5 In the absence of other substrates, all the reductant consumed is used to reduce protons to H2. When N2 is added as a substrate, an apparent maximum of 75% of the electrons reduce N2 while the remainder still reduce protons.6

EPR ligand superhyperfine interactions in mononuclear Mo(V) complexes: Implications for Mo enzymes

Abstract Reaction of Mo2O4(S2CNEt2)2 or Mo2O4(S2PPr2i)2 with thiol-containing ligands leads to the preparation of mononuclear Mo(V) complexes which display EPR absorption. For example, Mo2O4(S2CNEt2)2 reacts with o-NH2·C6H4·SH to yield Mo(S2CNEt2)(S·C6H4·NH)2. In a like manner Mo(S2CNEt2)(NR·C6H4·S)2 (R = D, Me), Mo(S2CNEt2)(S·C6H4· S)2 and Mo(S2PPr2i)(NH·C6H4· S)2 were prepared. Mo(S2CNEt2)(NH·C6H4· S)2 displays an EPR spectrum with superhyperfine splitting from 14N (2.4 G) and 1H (7.4 G). EPR spectra of the N-methyl and N-deutero complexes confirm these assignments. The anion [Mo(NH·C6H4·S)3]− generated in situ from Mo(NH·C6H4·S)3 shows A( 1 H ) 6.4 and A( 14 N ) 2.1 G. Splitting from 31P is seen in the dithiophosphinato complexes Mo(S2PPr2i)(NH·C6H4·S)2 and MoO(S2PPr2i)(O·C6H4·S). Variable temperature studies are required for the maximum resolution of ligand superhyperfine structure. The ligand splittings are qualitatively correlated with the symmetry of the orbital bearing the unpaired electron. These results show that substantial 1H superhyperfine splitting is possible for protons on coordinated ligands and establish the feasibility of a similar assignment for proton splittings in Mo enzymes.

Sulfide and Other Sulfur Containing Ligands in Metalloproteins and Enzymes

Abstract Sulfur is an essential constituent of living matter. It occurs as an integral component of the metal-containing prosthetic groups of many metalloproteins in either an organic form, as the amino acids cysteine and methionine, or as the inorganic entity, sulfide. Examples of sulfur-ligated prosthetic groups occur in proteins concerned with storage and transport, electron-transfer proteins and enzymes of the oxido-reductase type. Metalloproteins containing cysteine or methionine as their only sulfur-based donors are most often found with copper and iron, with well-documented examples occuring for zinc, manganese, molybdenum and tungsten also. The occurence of sulfide (S 2- ) as a ligand in proteins and enzymes is known for only iron and molybdenum where, with just one known exception in xanthine oxidase/dehydrogenase, it acts as a bridging ligand between two metals.