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Monomeric Mo(V) and Mo(IV) species
121
Journal of the Less-Common Metals, 54 (1977) 121 - 128 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
MONOMERIC
G. P. HAIGHT, School
MO(V) AND Mo(IV)
GERALD
of Chemical
WOLTERMANN,
Sciences,
University
SPECIES*
TAIRA
of Illinois,
IMAMURA Urbana,
and PAUL
Ill. 61801
HUMMEL
(U.S.A.)
Summary A search is under way for means of producing monomeric MO(V) and Mo(IV) species, in solution or bound to surfaces or membranes, which will function as model catalysts. A cursory survey of reactions of Mo,Oi’ complexes and reduction of MOO:- shows promise of achieving such results and reveals a rich chemistry of Mo(IV) and MO(V) to be explored further. Bidentate ligands form 3:2 and 2:2 L:Mo complexes with MosOi’. The dioxo bridges may dissociate, forming MoLs monomers or LMoLMoL chelate bridged dimers. Moz04 (oxinate)apys reacts with XCHsCHsY chelates to produce products in which 2 py are replaced; 2 py and one bridging 02- are replaced and the chelate bridges at both ends; and ESR active dimers are formed. Reduction of MOO:- by S20z- in the presence of chelates produces preparations in which Mo(V1) is reduced to MO(V) and Mo(IV). Mo(IV) can be generated bound to glutathione bound to sepharose, a stable preparation which effectively catalyses the reduction of acetylene but which is not very effective with dinitrogen.
Introduction Past studies of the kinetics of molybdenum-catalysed reductions of oxyions in aqueous systems [l], molybdenum(V) reduction of nitrate [ 21 and catalysis of acetylene and nitrogen reduction by complexes of molybdenum with thiol-containing chelates [ 31 have suggested that monomeric MO(V) and Mo(IV) are active catalysts and/or reducing agents. MO(V) is deactivated by dimerisation forming the complexes
51 ?
[Me0-Mo14+
F1/-O\”
0
[Mo\O/Mo12’
*Presented at the Conference on “The Chemistry and Uses of Molybdenum”, University of Oxford, England, 31 August - 3 September, 1976, sponsored by Climax Molybdenum Co. Ltd. and the Chemical Society (Dalton Division).
122
which are diamagnetic and inert to redox reactions [4]. Mo(IV) in catalytic systems is deactivated by disproportionation when it is formed by twoelectron reduction of Mo(V1) using Zn metal [ 51, SnCl; [ 11, or a dropping mercury electrode [6]. These phenomena suggest to us that molybdenum atoms in enzymes and catalytic surfaces may be effective redox agents and/ or catalysts when they can be tied up as monomers in complexes, membranes or solids so as to take part in redox reactions without the possibility of undergoing self-destructive dimerisation or disproportionation reactions. Other studies of inorganic nitrogen fixation reactions indicate that Ti(I1) [7] and Zr(I1) [8] , which are isoelectronic with Mo(IV), are effective reducing agents for Nz. The authors have undertaken exploratory research to discover systems in which monomers of MO(V) and Mo(IV) can be stabilised and their catalytic properties tested. Reports that thiol chelates labilise the bridging bonds in Mo,Oz’ [9], that ATP is consumed in large quantities in nitrogenasecatalysed Nz fixation [lo] and that MO is bound to sulphur [ 31 and to flavins [ 111 in enzymes, have centred our exploration on preparations involving ligands containing thiol groups, polyphosphates and Shydroxyquinoline, which binds as a flavin might.
Experimental MO(V) stock solutions were prepared by reduction of NazMoO, in 3M HCl with metallic mercury [9] . Solutions were neutralised with 3M NaOH and treated with equimolar solutions of ligand. Slow hydrolytic precipitation of MO(V) occurred when ligand was not in excess. However, changes in absorbance of visible light showed distinct breaks at 60 mol.% ligand, characteristic of an MosLs complex, or at 50 mol.% in the cases of polyphosphates. The recording of EPR spectra has been described previously [9,12,13]. Preparation of new compounds reported ~-(2-aminoethanol)-~-~-dioxobis[oxo-8-hydroxyquinolinatomolybdenum(V)], Moz0,(oxinate)2(H,NCH&H,0H)
Moz04(oxinate)spyz (1.4 g) and ethanolamine (0.121 g) were mixed in absolute alcohol (200 ml). The mixture was heated to 75 “C for 10 min. The solid product was recrystallised from CHzClz. Analysis showed C, 41.7%; H, 3.43%; N, 7.24%. The calculated values for C2,,H1sMo2N30, are C, 39.7%; H, 3.17%; N, 6.94%. ~-ethylenediamine-~-/I-dioxobis[o3co-8-hydroxyquinolinatomolybdenum(V) hydrate, Mo,O,(oxinate),(H,NCH,CH,NH,)
Moz04 (oxinate)spyz (0.7 g) and 0.12 g ethylenediamine in 200 ml absolute ethanol were heated to 75 “C for 10 min. The tan coloured product was recrystallised from CH,C&Jether. Analysis showed C, 38.7%; H, 3.52%;
123
MO, 30.6%; N, 9.08%. The calculated values for C2cH2sMo2N407 38.5%; H, 3.56%; MO, 30.9%; N, 9.00%.
are C,
~-(l-mercaptopropionato-S:O)-~1-oxobis[oxo-8-hydroxyquinolinatomolybdenum(V)], MoP0,(oxinate)2 (SCH,CH,CO,)
Moa04(oxinate)zpy, (1.0 g, 10u3 mol) and 0.3 g fl-mercaptopropionic acid (10M3 mol) in 200 ml absolute ethanol were heated to 75 “C for several hours giving a green crystalline product. Analysis showed C, 39.3%; H, 2.79%; MO, 28.9%; N, 4.68%; S, 4.86%. The calculated values for CzlH,,MozO,S are C, 39.8%; H, 2.53%; MO, 30.4%; N, 4.43%; S, 5.06%. Glu tathione
bound
This was prepared Analysis for oxidation
to sepharose-48
by the method
of Cuatrecasas
[ 141.
state of bound Mo(IV)
A suspension of bound MO in dilute sulphuric acid was titrated with standard permanganate to determine the number of reducing equivalents of MO present. The solution was then reduced with Zn metal to give Mo(II1) which was titrated with permanganate to determine the number of moles of MO present. Bound glutathione reacted too slowly with [Mn04]- to interfere.
Results and discussion Mo20i+
reactions
with bidentate
ligands
The dioxo bridged MO(V) moiety 1:: /O\&, /MoI lo/
has six vacant sites for ligand binding:
I \
Simple addition of bidentate ligands might be expected to form complexes with L:Mo ratios of 3:2 if ligands were to form bridges trans to the terminal oxygens. If 0 bridges are labilised, 4:2 complexes of Mo,Oi’ could result. Metal:ligand ratios measured by Job’s method [15] and ESR studies have yielded results which are summarised in Table 1 [9, 12, 131. Six-line signals have previously been observed with cysteine [9] and are characteristic of monomeric Mo( V). The eleven-line signals previously observed with the polypeptide, glutathione [ 121 are thought to be characteristic of a triplet state produced by breaking the dioxo bridge bonds while the chelating agent continues to bind the two molybdenum atoms together. One preparation with P-mercaptopropionic acid gives six-line signals, indicating that both types of ESR-active species may be produced by this ligand. Polyphosphate complexes of MO2 0 z’ are discussed in detail elsewhere [ 41. Complexes of HzP30f; and H2P20;- with MoaOz’ give identical UV spectra characteristic of 2:2 complexes [13]. Polyphosphates of Mo,Oz’ are especially susceptible to oxidation. Exposure to air, or the presence of NO;
124 TABLE 1 Soluble complexes of MozOF with bidentate ligandsa Ligand( L)
HSCHzCHzSH HSCH2CH2C02H HSCH2CH2NH2 NH2CH2CH2C02H NHxCHMeC02H H2P30 ii il H2P20,-
Ratio L:Mob
ESR signah? Lines
Relative areas
3:2 3 :2 3 :2 3 :2 3:2 2:2 2:2
6 11 11
All equal 1:2:3:4:5:6:5:4:3:2:1 1:2:3:4:5:6:5:4:3:2:1 None None NoneC
3od
ESR-active species
Monomer Triplet dimer Triplet dimer None None None P44W’2W214-
aThe phosphate complexes were prepared at pH 4, all others at pH 7. bFrom Job’s plot. ’ Refs. 9, 12, 13. dRef. 13.
or ClO,, results in rapid oxidation to Mo(V1). The oxidation of MO(V) was first order in [MozO4 (P207)2] 6- similar to oxidation by NH30H’ [ 161 and contrary to the one-half order observed for NO, acting on Mo20z’ tartrate complexes [2] . Slow oxidation of MO(V) occurred even under argon in the absence of obvious oxidants. Only PzO$- gives ESR-active solutions. Fiveline spectra are indicative of splitting of the MO(V) monomer signal by four equivalent phosphorus atoms. Each of the five lines is split into six by g5Mo. Effects of concentration of Mo20i’ and H,P,Oq- on the magnitude of the ESR signal permitted definition of the monomer--dimer equilibrium and the structure of the monomer complex. P20+-, with basic oxygens on adjacent P atoms, is a stronger chelate than P,OT; and more effective in displacement of bridging oxides in Mo20i’.
Reactions
of Mo20i’
with “dicys”
ligands
“Dicys” ligands, prepared by reacting cysteine with alkyl dibromides, Br(CH2),Br, contain two cysteine molecules joined by (-CH,-), bridges between sulphur atoms. If n is zero (cystine), no ESR-active species is produced [12]. If n is 2 - 6, amorphous intractable solids, suggesting polymerisation, are formed. 1 - 3 dicys generally gave L:Mo ratios of 1:4. It was hoped to produce analogues of the cysteine complexes which labilise MoOzMo dioxo bridges and produce {Mo(V)}~L complexes in the triplet state observed with glutathione, but this approach has so far failed.
125
Reactions of bidentate ligands with di-p-oxo-bis[oxo-8-hydroxyquinolinatopyridinemoly bdenum( V)] Di-~-oxo-bis[oxo-8-hydroxyquinolinatopyridinemoiybdenum(V)] :
PY
PY
has been treated (in ethanol) with a series of bidentate ligands to determine the effect of replacing the two pyridine ligands with bridging chelates in the hope of learning more of the effects of bridging chelates on labilisation of the 0 bridge bonds in Mo,Oz’. Table 2 contains information on the products formed. Amino groups replace pyridine easily. Thiol ligands labilise 0 bridges, replacing one in each of two cases. Only one ESR-active species has been found and it is not yet well characterised. Bridging chelates gave interesting fragments in mass spectra. NHzCHzCHzNHz forms strong single bonds with each MO. Ionising electrons cause rupture of the C-C bond leading to CHaNHs fragments:
In contrast, -SCHsCHsObinds to each MO with both S and 0 [17]. C-S and C-O bonds are ruptured by ionising electrons causing C2H4 (mass 28) to appear in the mass spectra. The MosOi* dimer thus serves as a template for reducing a 1,2-substituted ethane to ethylene.
Labilising -0- bridges in this case produces an even stronger chelate bridge. Mos04(oxine)apys is reduced by SaOi- to (Hpy’)[Moz04(oxine)a]which gives a strong ESR signal and must contain a mixed oxidation state:
P/“\ii ,,
May
MO(V)
Mo204(oxinate)g(
mixtures
Mog03(oxinate)2(
HSCH2CH2SH
HSCH2CH2C02H
iFrom C, H, MO, N, S analyses. m/e, major fragment.
No reaction
d,l-alanine NH2CH2CH2NH2 SCH2CH2COg)
NH2CH2CH2NH2)
SCH2CH20)C
Mog03(oxinate)g(
HSCH2CH20H
30 ( CH2NH2)
(cZH4)
NH2CH&H20H)
Mo204(oxinate)2(
NH2CH2CH20H 28
Tan coloured solid; violet in solution
544 (Mo204(oxinate)g) -
Mo204(oxinate)g(lEtOH),
HOCH2CH20H
Green solid; ESR active
Tan coloured solid
both ends
Tan coloured solid; ligand bridged at
Tan coloured solid
Other properties
Mass spectrumb
Producta
Ligand
Reaction products of potential ligands with MozOd( oxinate)pyz
TABLE 2
127
Bound
ligands for Mo( V) and/or MO(W)
To prevent dimerisation (or polymerisation) of MO(V) and disproportionation of Mo(IV) it would be convenient if inert complexes with bound ligands could be prepared. MOO:- in neutral aqueous system can be reduced by &Oz-. Such a reaction in the presence of suitable ligands has long been used to prepare MO(V) complexes [18]. Monomeric SO; radicals. presumably effect one-electron reductions. However, two-electron reductions should also be possible. A number of solids prepared by reducing MOO:- in the presence of ligands such as P2074- have contained Mo(IV). When the reduction was carried out in the presence of glutathione bound to sepharose 4B, a bound Mo(IV) complex resulted. The brown-black Mo(IV) remained bound after repeated washings with water. Suspensions of the bound Mo(IV) glutathione complex catalysed the SaOz- reduction of acetylene to ethylene and ethane in a few minutes. Attempts to reduce Nz were inconclusive. This does appear to be a promising technique for generating bound molybdenum catalysts. Attempts to monomerise dimeric Mo20i’ complexes have indicated a rich chemistry, involving monomers, dimers of Mo,Oi’, MosOi’, triplets with bridging chelates and possibly tetramers as well, to be explored further.
Acknowledgments This work has been supported in part by grants from the National Science Foundation, the National Institutes of Health and the Research Corporation. The authors are grateful to Dr. John Christopher, Professor Gerhardt Schrauzer and Mr. Paul Robinson for valuable discussions and assistance.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
G. P. Haight and W. F. Sager, J. Am. Chem. Sot., 74 (1952) 2056. J. T. Spence and T. J. Huang, J. Phys. Chem., 72 (1968) 4198. G. N. Schrauzer, Adv. Chem. Ser., 100 (1972) 1. G. P. Haight and D. Boston, J. Less-Common Met., 36 (1974) 95. G. P. Haight, Anal. Chem., 25 (1953) 642. G. P. Haight, Acta Chem. Stand., 15 (1961) 2012. E. E. Van Tamelen, Adv. Chem. Ser., 100 (1972) 95. J. E. Bercaw, personal communication, 1975. T. J. Huang and G. P. Haight, J. Am. Chem. Sot., 92 (1970) 2336. A. L. Lehninger, Biochemistry, Worth, New York, 1970, p. 561. D. T. Sawyer, J. N. Gerber, L. Amos and L. J. deHayes, J. Less-Common Met., 36 (1974) 513. T. J. Huang and G. P. Haight, J. Am. Chem. Sot., 93 (1971) 611. T. Imamura, G. P. Haight and R. E. Belford, Inorg. Chem., 15 (1976) 1047. P. Cuatrecasus, J. Biol. Chem., 245 (1970) 3059. P. Job, Ann. Chim. (Paris), 9 (1928) 113.
128 16 G. P. Haight and A. C. Swift, J. Phys. Chem., 65 (1961) 1921. 17 J. I. Gelder, J. H. Enemark, G. M. Woltermann, D. A. Boston and G. P. I-bight, Chem. Sot., 95 (1975) 1616. 18 P. C. H. Mitchell, J. Chem. Sot. A, (1970) 2421.
J. Am.
Journal of the Less-Common Metals, 54 (1977) 121 - 128 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
MONOMERIC
G. P. HAIGHT, School
MO(V) AND Mo(IV)
GERALD
of Chemical
WOLTERMANN,
Sciences,
University
SPECIES*
TAIRA
of Illinois,
IMAMURA Urbana,
and PAUL
Ill. 61801
HUMMEL
(U.S.A.)
Summary A search is under way for means of producing monomeric MO(V) and Mo(IV) species, in solution or bound to surfaces or membranes, which will function as model catalysts. A cursory survey of reactions of Mo,Oi’ complexes and reduction of MOO:- shows promise of achieving such results and reveals a rich chemistry of Mo(IV) and MO(V) to be explored further. Bidentate ligands form 3:2 and 2:2 L:Mo complexes with MosOi’. The dioxo bridges may dissociate, forming MoLs monomers or LMoLMoL chelate bridged dimers. Moz04 (oxinate)apys reacts with XCHsCHsY chelates to produce products in which 2 py are replaced; 2 py and one bridging 02- are replaced and the chelate bridges at both ends; and ESR active dimers are formed. Reduction of MOO:- by S20z- in the presence of chelates produces preparations in which Mo(V1) is reduced to MO(V) and Mo(IV). Mo(IV) can be generated bound to glutathione bound to sepharose, a stable preparation which effectively catalyses the reduction of acetylene but which is not very effective with dinitrogen.
Introduction Past studies of the kinetics of molybdenum-catalysed reductions of oxyions in aqueous systems [l], molybdenum(V) reduction of nitrate [ 21 and catalysis of acetylene and nitrogen reduction by complexes of molybdenum with thiol-containing chelates [ 31 have suggested that monomeric MO(V) and Mo(IV) are active catalysts and/or reducing agents. MO(V) is deactivated by dimerisation forming the complexes
51 ?
[Me0-Mo14+
F1/-O\”
0
[Mo\O/Mo12’
*Presented at the Conference on “The Chemistry and Uses of Molybdenum”, University of Oxford, England, 31 August - 3 September, 1976, sponsored by Climax Molybdenum Co. Ltd. and the Chemical Society (Dalton Division).
122
which are diamagnetic and inert to redox reactions [4]. Mo(IV) in catalytic systems is deactivated by disproportionation when it is formed by twoelectron reduction of Mo(V1) using Zn metal [ 51, SnCl; [ 11, or a dropping mercury electrode [6]. These phenomena suggest to us that molybdenum atoms in enzymes and catalytic surfaces may be effective redox agents and/ or catalysts when they can be tied up as monomers in complexes, membranes or solids so as to take part in redox reactions without the possibility of undergoing self-destructive dimerisation or disproportionation reactions. Other studies of inorganic nitrogen fixation reactions indicate that Ti(I1) [7] and Zr(I1) [8] , which are isoelectronic with Mo(IV), are effective reducing agents for Nz. The authors have undertaken exploratory research to discover systems in which monomers of MO(V) and Mo(IV) can be stabilised and their catalytic properties tested. Reports that thiol chelates labilise the bridging bonds in Mo,Oz’ [9], that ATP is consumed in large quantities in nitrogenasecatalysed Nz fixation [lo] and that MO is bound to sulphur [ 31 and to flavins [ 111 in enzymes, have centred our exploration on preparations involving ligands containing thiol groups, polyphosphates and Shydroxyquinoline, which binds as a flavin might.
Experimental MO(V) stock solutions were prepared by reduction of NazMoO, in 3M HCl with metallic mercury [9] . Solutions were neutralised with 3M NaOH and treated with equimolar solutions of ligand. Slow hydrolytic precipitation of MO(V) occurred when ligand was not in excess. However, changes in absorbance of visible light showed distinct breaks at 60 mol.% ligand, characteristic of an MosLs complex, or at 50 mol.% in the cases of polyphosphates. The recording of EPR spectra has been described previously [9,12,13]. Preparation of new compounds reported ~-(2-aminoethanol)-~-~-dioxobis[oxo-8-hydroxyquinolinatomolybdenum(V)], Moz0,(oxinate)2(H,NCH&H,0H)
Moz04(oxinate)spyz (1.4 g) and ethanolamine (0.121 g) were mixed in absolute alcohol (200 ml). The mixture was heated to 75 “C for 10 min. The solid product was recrystallised from CHzClz. Analysis showed C, 41.7%; H, 3.43%; N, 7.24%. The calculated values for C2,,H1sMo2N30, are C, 39.7%; H, 3.17%; N, 6.94%. ~-ethylenediamine-~-/I-dioxobis[o3co-8-hydroxyquinolinatomolybdenum(V) hydrate, Mo,O,(oxinate),(H,NCH,CH,NH,)
Moz04 (oxinate)spyz (0.7 g) and 0.12 g ethylenediamine in 200 ml absolute ethanol were heated to 75 “C for 10 min. The tan coloured product was recrystallised from CH,C&Jether. Analysis showed C, 38.7%; H, 3.52%;
123
MO, 30.6%; N, 9.08%. The calculated values for C2cH2sMo2N407 38.5%; H, 3.56%; MO, 30.9%; N, 9.00%.
are C,
~-(l-mercaptopropionato-S:O)-~1-oxobis[oxo-8-hydroxyquinolinatomolybdenum(V)], MoP0,(oxinate)2 (SCH,CH,CO,)
Moa04(oxinate)zpy, (1.0 g, 10u3 mol) and 0.3 g fl-mercaptopropionic acid (10M3 mol) in 200 ml absolute ethanol were heated to 75 “C for several hours giving a green crystalline product. Analysis showed C, 39.3%; H, 2.79%; MO, 28.9%; N, 4.68%; S, 4.86%. The calculated values for CzlH,,MozO,S are C, 39.8%; H, 2.53%; MO, 30.4%; N, 4.43%; S, 5.06%. Glu tathione
bound
This was prepared Analysis for oxidation
to sepharose-48
by the method
of Cuatrecasas
[ 141.
state of bound Mo(IV)
A suspension of bound MO in dilute sulphuric acid was titrated with standard permanganate to determine the number of reducing equivalents of MO present. The solution was then reduced with Zn metal to give Mo(II1) which was titrated with permanganate to determine the number of moles of MO present. Bound glutathione reacted too slowly with [Mn04]- to interfere.
Results and discussion Mo20i+
reactions
with bidentate
ligands
The dioxo bridged MO(V) moiety 1:: /O\&, /MoI lo/
has six vacant sites for ligand binding:
I \
Simple addition of bidentate ligands might be expected to form complexes with L:Mo ratios of 3:2 if ligands were to form bridges trans to the terminal oxygens. If 0 bridges are labilised, 4:2 complexes of Mo,Oi’ could result. Metal:ligand ratios measured by Job’s method [15] and ESR studies have yielded results which are summarised in Table 1 [9, 12, 131. Six-line signals have previously been observed with cysteine [9] and are characteristic of monomeric Mo( V). The eleven-line signals previously observed with the polypeptide, glutathione [ 121 are thought to be characteristic of a triplet state produced by breaking the dioxo bridge bonds while the chelating agent continues to bind the two molybdenum atoms together. One preparation with P-mercaptopropionic acid gives six-line signals, indicating that both types of ESR-active species may be produced by this ligand. Polyphosphate complexes of MO2 0 z’ are discussed in detail elsewhere [ 41. Complexes of HzP30f; and H2P20;- with MoaOz’ give identical UV spectra characteristic of 2:2 complexes [13]. Polyphosphates of Mo,Oz’ are especially susceptible to oxidation. Exposure to air, or the presence of NO;
124 TABLE 1 Soluble complexes of MozOF with bidentate ligandsa Ligand( L)
HSCHzCHzSH HSCH2CH2C02H HSCH2CH2NH2 NH2CH2CH2C02H NHxCHMeC02H H2P30 ii il H2P20,-
Ratio L:Mob
ESR signah? Lines
Relative areas
3:2 3 :2 3 :2 3 :2 3:2 2:2 2:2
6 11 11
All equal 1:2:3:4:5:6:5:4:3:2:1 1:2:3:4:5:6:5:4:3:2:1 None None NoneC
3od
ESR-active species
Monomer Triplet dimer Triplet dimer None None None P44W’2W214-
aThe phosphate complexes were prepared at pH 4, all others at pH 7. bFrom Job’s plot. ’ Refs. 9, 12, 13. dRef. 13.
or ClO,, results in rapid oxidation to Mo(V1). The oxidation of MO(V) was first order in [MozO4 (P207)2] 6- similar to oxidation by NH30H’ [ 161 and contrary to the one-half order observed for NO, acting on Mo20z’ tartrate complexes [2] . Slow oxidation of MO(V) occurred even under argon in the absence of obvious oxidants. Only PzO$- gives ESR-active solutions. Fiveline spectra are indicative of splitting of the MO(V) monomer signal by four equivalent phosphorus atoms. Each of the five lines is split into six by g5Mo. Effects of concentration of Mo20i’ and H,P,Oq- on the magnitude of the ESR signal permitted definition of the monomer--dimer equilibrium and the structure of the monomer complex. P20+-, with basic oxygens on adjacent P atoms, is a stronger chelate than P,OT; and more effective in displacement of bridging oxides in Mo20i’.
Reactions
of Mo20i’
with “dicys”
ligands
“Dicys” ligands, prepared by reacting cysteine with alkyl dibromides, Br(CH2),Br, contain two cysteine molecules joined by (-CH,-), bridges between sulphur atoms. If n is zero (cystine), no ESR-active species is produced [12]. If n is 2 - 6, amorphous intractable solids, suggesting polymerisation, are formed. 1 - 3 dicys generally gave L:Mo ratios of 1:4. It was hoped to produce analogues of the cysteine complexes which labilise MoOzMo dioxo bridges and produce {Mo(V)}~L complexes in the triplet state observed with glutathione, but this approach has so far failed.
125
Reactions of bidentate ligands with di-p-oxo-bis[oxo-8-hydroxyquinolinatopyridinemoly bdenum( V)] Di-~-oxo-bis[oxo-8-hydroxyquinolinatopyridinemoiybdenum(V)] :
PY
PY
has been treated (in ethanol) with a series of bidentate ligands to determine the effect of replacing the two pyridine ligands with bridging chelates in the hope of learning more of the effects of bridging chelates on labilisation of the 0 bridge bonds in Mo,Oz’. Table 2 contains information on the products formed. Amino groups replace pyridine easily. Thiol ligands labilise 0 bridges, replacing one in each of two cases. Only one ESR-active species has been found and it is not yet well characterised. Bridging chelates gave interesting fragments in mass spectra. NHzCHzCHzNHz forms strong single bonds with each MO. Ionising electrons cause rupture of the C-C bond leading to CHaNHs fragments:
In contrast, -SCHsCHsObinds to each MO with both S and 0 [17]. C-S and C-O bonds are ruptured by ionising electrons causing C2H4 (mass 28) to appear in the mass spectra. The MosOi* dimer thus serves as a template for reducing a 1,2-substituted ethane to ethylene.
Labilising -0- bridges in this case produces an even stronger chelate bridge. Mos04(oxine)apys is reduced by SaOi- to (Hpy’)[Moz04(oxine)a]which gives a strong ESR signal and must contain a mixed oxidation state:
P/“\ii ,,
May
MO(V)
Mo204(oxinate)g(
mixtures
Mog03(oxinate)2(
HSCH2CH2SH
HSCH2CH2C02H
iFrom C, H, MO, N, S analyses. m/e, major fragment.
No reaction
d,l-alanine NH2CH2CH2NH2 SCH2CH2COg)
NH2CH2CH2NH2)
SCH2CH20)C
Mog03(oxinate)g(
HSCH2CH20H
30 ( CH2NH2)
(cZH4)
NH2CH&H20H)
Mo204(oxinate)2(
NH2CH2CH20H 28
Tan coloured solid; violet in solution
544 (Mo204(oxinate)g) -
Mo204(oxinate)g(lEtOH),
HOCH2CH20H
Green solid; ESR active
Tan coloured solid
both ends
Tan coloured solid; ligand bridged at
Tan coloured solid
Other properties
Mass spectrumb
Producta
Ligand
Reaction products of potential ligands with MozOd( oxinate)pyz
TABLE 2
127
Bound
ligands for Mo( V) and/or MO(W)
To prevent dimerisation (or polymerisation) of MO(V) and disproportionation of Mo(IV) it would be convenient if inert complexes with bound ligands could be prepared. MOO:- in neutral aqueous system can be reduced by &Oz-. Such a reaction in the presence of suitable ligands has long been used to prepare MO(V) complexes [18]. Monomeric SO; radicals. presumably effect one-electron reductions. However, two-electron reductions should also be possible. A number of solids prepared by reducing MOO:- in the presence of ligands such as P2074- have contained Mo(IV). When the reduction was carried out in the presence of glutathione bound to sepharose 4B, a bound Mo(IV) complex resulted. The brown-black Mo(IV) remained bound after repeated washings with water. Suspensions of the bound Mo(IV) glutathione complex catalysed the SaOz- reduction of acetylene to ethylene and ethane in a few minutes. Attempts to reduce Nz were inconclusive. This does appear to be a promising technique for generating bound molybdenum catalysts. Attempts to monomerise dimeric Mo20i’ complexes have indicated a rich chemistry, involving monomers, dimers of Mo,Oi’, MosOi’, triplets with bridging chelates and possibly tetramers as well, to be explored further.
Acknowledgments This work has been supported in part by grants from the National Science Foundation, the National Institutes of Health and the Research Corporation. The authors are grateful to Dr. John Christopher, Professor Gerhardt Schrauzer and Mr. Paul Robinson for valuable discussions and assistance.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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