Inorg. Chem. 1989, 28, 3225-3228
3225
Contribution from the Departments of Chemistry, Kent State University, Kent, Ohio 44242, and Indiana-Purdue University a t Fort Wayne, Fort Wayne, Indiana 46805
Electron Transfer. 99. Reductions of Oxyhalogens with Dimeric Molybdenum(V)' D. E. Linn,Jr.,**2aS. K. Ghosh,2band E. S . Gould*v2b Received December 28, 1988
The bridged dimer of molybdenum(V) M 0 ~ 0 ~ ~ + ((structure aq) I) is readily oxidized to Mo(V1) by a number of oxyhalogen species in aqueous acidic media. Reactions of BrOC, HBr02, HCI02, and HOC1 with excess reductant yield the corresponding halide ions, and reduction of C l o y yields CI-, even when the oxidant is in large excess. The I(VI1) acid H5106 is reduced very rapidly to IO), which undergoes further slow reduction, yielding 12. Reductions of the anions CIOF and BrOC are independent of [H'] in the range 0.05-1.0 M H+, but reductions of the acids HBr02, HC102, and HSI06are found to be inverse-first order in [H']. Acidity patterns are consistent with reaction through a (Mov)2-anion complex, and the approaches to kinetic saturation in the CIOC and BrOC reductions allow estimates of the stability constants of such complexes in these instances. Hypochlorous acid appears to react via a (MO~)~-CIOH adduct (K = 800 M-I), which undergoes deprotonation (pK = 1) prior to electron transfer. Reductions of 10,- at concentrations > Br0< > 109- > CIOJ corresponds to that reported for le oxidations of the Co(l1) center in cob(1I)alamin. This question remains unresolved.
The recent surge of interest in the redox reactions of molybdenum has been sparked by an increased awareness of the richness of t h e solution chemistry of t h a t element3 and by its recognized occurrence in an array of redox enzymes, most notably t h e nitrogenases, nitrate reductases, and xanthine dehydrogenase^.^^,^ As with its congener, chromium, all oxidation states between 2+ and 6+ are prominently represented; a n d further variety arises from t h e ease with which each of these states forms dimeric and polymeric species. Our initial contribution to this area dealt with the reduction of very low concentrations of Mo(V1) with vitamin B12s(cob(I)alamin),5 but interpretation was complicated by t h e intrusion of autocatalytic behavior and uncertainty as to the identity of the reduced molybdenum species. In t h e present study, which treats the oxidation of dimeric molybdenum(V) with anionic oxyhalogen species, there is no ambiguity concerning oxidation states. W e have, however, encountered some unexpected variations in kinetic patterns, with t h e reduction of iodate appearing t o be unusually complex. Experimental Section Materials. Sodium chlorate (Baker and Adamson), sodium chlorite6*'
Joint sponsorshipof this work by the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. (2) (a) Indiana-Purdue University at Fort Wayne. (b) Kent State University. (3) Reviews:' (a) Stiefel, E.I. Prog. Inorg. Chem. 1977,22, 1. (b) Spivack, B.; Dori, Z . Coord. Chem. Rev. 1975,99, 17. (c) Richens, D. T.; Helm, L.; Pittet, P.-A.; Merbach, A. E. Inorg. Chim. Acta 1987,132,85. (d) Haight, G. P.; Johnson, M. D.; Rahmoeller, K. M. Isr. J. Chem. 1985, (1)
25, 177. (4) See, for example: (a) Ochiai, E.-I. Bioinorganic Chemistry; Allyn and Bacon: Boston, MA, 1977; Chapter 11. (b) Rajagopalan, K. V.; Kramer, S.; Gardlek, S. Polyhedron 1986.5, 573. (c) George, G. N.; Hawkes, T. R.; Jones, G. D.; Bray, R. C. Polyhedron 1986,5,587. (d) Kellev. B. C. Polvhedron 1986. 5 . 597. ( e ) Wilson. G. L.: Konv. M.: Tiekhk, E. R. T:; Pilbrow, J. R.;'Spencd, J. T.; Wedd, A: G. i . ' A m : Chem. SOC.1988, 110, 6923. (5) Pillai, G. C.; Bose, R. N.; Gould, E. S. Inorg. Chem. 1987, 26, 3120. (6) The purity of this sample of NaC102, as estimated by the method of Kolthoff and co-workers,' was 78.01, in agreement with the value of 80% given on the label. Attempted recrystallization did not improve its quality. Since the impurities were taken to be. NaCl (which does not
react with dimeric MoV)and NaC103 (which reacts much more slowly than NaCIO,), this oxidant was used as received and a correction was made for the estimated purity. (7) (a) Kolthoff, I. M.; Sandell, E. B.;Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Collier-Macmillan: Toronto, 1969; p 852. (b) Balasubramanian, P. N.; Gould, E. S. Inorg. Chem. 1984, 23, 3689.
0020-1669/89/1328-3225$01 S O / O
(Aldrich), sodium br~mite**~ (Aldrich), sodium hypochlorite1° (Alfa), and sodium periodate (Alfa) were used as received. Sodium iodate (Baker and Adamson) was recrystallized three times before use in rate experiments to ensure that the unexpected kinetic peculiarities observed with this oxidant (vide infra) were not artifacts attributable to impurities. Solutions of hypoiodous acid were prepared in situ as described by Chia." Sodium perchlorate solutions (for use as the supporting electrolyte in kinetic experiments) were prepared by careful neutralization of NaHCOJ and were then purged with N 2 to remove dissolved C02. All solutions were prepared in distilled water that had been previously boiled for at least 1 h and then sparged with N 2 for 4 h. The preparation of dimeric molybdenum(V) solutions, Moz0:+(aq),12 was carried out by aquation of (NH4)2MoC15.13A deaerated column (12 X 1 cm) of Dowex W-X4 resin was washed with 12 M HC104 to remove ultraviolet-absorbing impurities and subjected to the Cr(I1)scrubbing procedure of Bowen14to remove traces of 02. Typically, 0.13 g (0.40 mmol) of (NH4),MoCIS in 100 mL of 0.05 M HC104 was adsorbed onto the cation-exchange resin and washed with 60 mL of 0.2 M HCIO4 to remove chloride, after which the orange-brown Mo2042+(aq) was displaced with 25 mL of 2.0 M NaC104 under N2. Solutions of Mo(V) were standarized at 295 nm ( ~ 2 9 5= 3550 M-I cm-I).l5 The preparations thus obtained (0.01-0.02 M in dimer) were stable for several weeks in the absence of 02.Gradual oxidation yielded the characteristic blue isopolymolybdate, which eventually precipitated. Stoichiometric Studies. The stoichiometries of the five most rapid reactions were determined, taking Mo202+in excess, by adding a measured deficiency of oxidant to Mo204'+, waiting 30-80 min for completion of the reaction, and then measuring the decrease in absorbance at 295 nm (for conversion of MoVto Mo"). These changes were compared to those observed when the Mo(V) reductant was treated with excess
(8) Analysis of this reagentg indicated 0.4% NaOH and 1.7% NaBr03 but
no detectable NaBr. Since BrO,' reacts much more slowly than Br02-, it did not complicate our kinetic experiments with the latter, which were carried out with excess oxidant. (9) Balasubramanian, P. N.; Pillai, G. C.; Carlson, R. R.; Linn, D. E.,Jr.; Gould, E. S. Inorg. Chem. 1988, 27, 780. (10) The NaClO solution, analyzed by using a reported procedure (Furman, N. H. Standard Methods of Chemical Analysis, 6th ed.; Van Nastrand: Princeton, NJ, 1963; Vol. 1, part A, pp 264, 341) was found to be 1.21 M.
(11) Chia, Y.-T.; Connick, R. E. J. Am. Chem. SOC.1959, 81, 1280. (12) (a) Sasaki, Y.; Sykes, A. G. J. Chem. SOC.,Dalton Trans. 1974, 1468. (b) Sasaki, Y.; Taylor, R. S.; Sykes, A. G. J. Chem. Soc.,Dalton Trans. 1975, 396. (c) Generation of Mo204*+from (pyH),MoCl5 yielded
solutions exhibiting strong pyridinium-derivedabsorbance in the ultraviolet. Such absorbance complicated kinetic studies with this reductant. (13) Brauer, G. Handbook of Preparative Inorganic Chemistry, 2nd ed.; Academic Press: New York, 1963; p 1413. (14) Bowen, A. R.; Taube, H. Inorg. Chem. 1974, 13, 2245. (15) (a) Richens, D. T.; Sykes, A. G. Inorg. Synth. 1985,23,137. (b) Ardon, M.; Pernick, A. Inorg. Chem. 1973, 12, 2484.
0 1989 American Chemical Society
3226 Inorganic Chemistry, Vol. 28, No. 16, 1989
Linn et al.
Table I. Stoichiometries of the Reactions of Dimeric Molybdenum(V) with Oxyhalogen Species'
oxidant BrOF HBr02 CIO,-b HClO2 HOC1 HS106
[H+I, M 0.14 0.14 0.14 0.14 0.14 0.14 0.10 0.10 0.14 0.14 0.14 0.10
0.10 0.10 0.10 0.10 0.10
1O4[(Mov)2I M 5.50 5.50 5.50 5.74 5.74 5.74 18.7 18.7 5.74 5.74 5.74 5.00 5.00 2.40 2.40 4.00 4.00 I
104[0x], M 1.50 0.759 0.459 2.87 1.44 0.848 1000 1000 2.42 1.23 0.747 3.63 1.21 0.578 0.168 60 60
lO4[(MoV)zl1 M 4.34 2.38 1.41 6.00 3.18 1.86
104A[Ox],
M
6.25 6.45 4.68 2.18 1.37 3.60 1.25 0.565 0.165 1.53 1.56
A[(MOV)21/ A[Oxl 2.90 3.13 3.07 2.08 2.20 2.20 2.99 2.89 1.93 1.77 1.84 0.99 1.03 0.978 0.979 2.61 2.56
'Reactions with ( M O ~in) excess ~ were followed at 295 nm (see text). 6Reaction mixtures with CI03- in excess were analyzed for C1- titrimetrically (see text). -A[C103-] was taken as A[CI-]. CReactionmixtures with IO< in excess were analyzed for I2 spectrophotometrically. -A[IO
