J . CHEM. SOC. DALTON TRANS. 1983 857Synthesis, Characterisation, and Electrochemical Studies of aDioxo-bridged Molybdenum(v) Glycinate. Generation of an 0x0-bridgedMov-Mov' SpeciesMu ktimoy C haud huryDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032, IndiaThe binuclear compound [ M 0 ~ 0 ~ ( g l y O ) ~ ( H20)2] (gly0 = NH2CH2COO-), has been prepared andcharacterized. The aqueous solution of the compound is stable for at least 24 h when the pH of thesolution is maintained above 5.5. Below pH 5.5, the compound in aqueous solution undergoesproton-assisted oxidation to a dimeric MoV-MoV1 species giving a new electronic absorption band at850 nm ( E 241 dm3 mo1-l cm-l) which is believed to be due to MoV-MoV1 intervalence chargetransfer.The kinetics of this oxidation have been studied in aqueous solution in the pH range 5.0-3.7.The oxidation step is preceded by the formation of a protonated species [ M o ~ O ~ ( ~ I ~ O ) ~ ( H ~ O ) ~ H ] + and is first-order dependent on hydrogen ion concentration. The second-order rate constant is2.52 dm3 mo1-l s-1 at 25 "C, I = 0.1 rnol dm-3. The MoV-MoV1 compound could not be isolated inthe solid state. Cyclic voltammetric and magnetic studies of the compounds are described.It is now well established that molybdenum participates in anumber of biochemical reactions associated with the func-tioning of at least six enzymes, which are distributed amongbacteria, plants, mammals etc.Id3 Although some molybden-um(v) has been detected by e.s.r.4-8 in four of the enzymes inthe presence of substrate, the oxidation state(s) of the molyb-denum atom in the native enzymes is still not clearly under-stood. At present it is generally accepted that the valency ofmolybdenum in these enzymes alternates between MoV1 andMoIV states during active electron transport. The redoxpotentials determined for xanthine oxidase 9910 and nitratereductase confirm that the metal ion readily undergoesreversible one- or two-electron reduction, and at some stage ofreduction the enzymes contain MoV. Several MoV compoundsthat mimic some of the properties of enzymes have beenrep~rted.'~*'~In this paper we report the preparation and spectral charac-teristics of di-p-0x0-bis[aqua(glycinato-0N)oxomolybdenum-Wl, [Mo*04(glY0)2(H20)21 (gly0 = NHzCHzCOO -1, to-gether with an investigation of its redox and electrochemicalbehaviour in aqueous acidic medium.ExperimentalAll reactions and physical measurements were carried out indoubly distilled water.All chemicals were analytical grade andused without further purification.Infrared spectra were recorded as KBr discs with a Beck-man IR-20A spectrometer. Electronic spectra were recordedwith a Pye Unicam model SP8-150 spectrophotometer.Proton n.m.r. spectra were obtained on a Varian model T-60spectrometer. Magnetic susceptibility was measured on aGouy balance with Co[Hg(SCN),] as calibrant or by Evansmethod.', Cyclic voltammetric measurements were madewith a PAR 174A polarographic analyser, PAR 175 universalprogrammer, and a PAR RE 0074 XY recorder.The threeelectrode measurements were carried out using a MetrohmE410 hanging mercury drop electrode as the working elec-trode, a platinum-wire auxiliary electrode, and a saturatedcalomel reference electrode (s.c.e.). For coulometry a mercurypool electrode was used. All measurements were performed insolutions which were 0.1 mol dm-3 in NaCl at 298 K under anitrogen atmosphere. The potentials reported in this paperwere uncorrected for junction potentials.Preparation of Di-p-0x0-bis[aqua(glycinato-0N)oxomolyb-denum(v)], [Mo20s(glyO)2(H20)r].-Glycine (1.5 g, 20 mmol)was dissolved in water (25 cm3) to which was added sodiummolybdate dihydrate (4.84 g, 20 mmol) dissolved in water(20 cm3). The solution was cooled to 5 "C and the pH care-fully brought to 6.0 (monitored by a pH meter) by the slowaddition of a 4 mol dm-3 sodium hydroxide solution. To thestirred solution sodium dithionite (1.7 g) dissolved in water(15 cm3) was added.The solution developed a deep orangecolour which was then filtered. Isopropyl alcohol (40 cm3) wasadded to the filtrate and on keeping the mixture in an icechest for 3 h a deep orange layer separated at the bottom.After decantation of the supernatant the orange layer wasdissolved in a minimum volume of water and was againtreated with an equal volume of Pr'OH. The orange layerobtained after cooling for 2 h, on trituration, gave a browncompound in microcrystalline form. The compound was re-crystallised twice from water-Pr'OH (1 : 1 v / ~ ) and dried invacuum to achieve satisfactory analytical results (Found : C,10.4; H, 2.85; Mo, 43.65; N, 6.20.C4H12M~2N2010 requiresC, 10.90; H, 2.75; Mo, 43.60; N, 6.35%). Yield, 0.6g.Kinetic Measurements.-Fresh solutions of [M0~0~(glyO)~-(H20I2] (ca. rnol dms) in doubly distilled water were pre-pared before kinetic runs were taken. The pH of the solutionswere adjusted between 5.0 and 3.7 with 0.2 mol dm-3 sodiumacetate-acetic acid buffer, and the ionic strength was main-tained at 0.1 mol dm-3 with sodium perchlorate. The rate ofincrease in concentration of the mixed-valent MoV-MoV1species was followed at 850 nm, where it has a strong absorp-tion band (E 241 dm3 mo1-l cm-I).Kinetic runs were takenunder pseudo-first-order conditions in a thermostatted cellcontrolled at 25 f 0.1 "C.Coufome fric Reduction u f [ Mo204(gly0)2( HzO)~]. -20 cm3of a solution of the molybdenum complex (1.25 x rnoldm-3) adjusted to pH 5.6 by acetate buffer and containing0.1 mol dm-3 of NaCl was electrolysed at a constant potentialof - 1.1 V us. s.c.e. using a mercury pool working electrodeto a constant coulomb count. The ratio of this count to thecalculated count is equal to n, the total number of electronstransferred.Results and DiscussionAn attempt by Melby l5 to prepare [M020~(glyO)~(H~O)~ J fromMoC15 resulted in a brown intractable sludge whose i.r. spec-trum even lacked ligand absorptions. Our initial attempts t8581.61.4+J.CHEM. SOC. DALTON TRANS. 1983---I .z 1.0C'0a - 0.88 0.6.- c040.2I 1 1 I , . I 1 I 1 1280 320 360 " 590 670 750 830 910Wavelength (nm)Figure 1. rime-intel-val spectra ot' 1Mot04(gly0)2( H20), J in aqueoussolution at pH 4.48. Concentrations of the solutions: ( ( I ) 6.6 x 10niol dni 3, (h) 1.32 x rnol dm 3. Initial scan (I), 15 (2), 30( 3 ) , 45 (4), 60 ( 5 ) , 75 (6), 115 (7), 175 min (8), and 24 h (9)prepare this compound using [NH4I2[MoOC'I,] were alsounsuccessful. By the present method a reasonable yield (507;)of the crude product was obtained which however substan-tially depleted during two recrystallizations. The compound ishygroscopic and is stable for several months when kept in theabsence of air. The aqueous solution of the compound whenprepared in degassed doubly distilled water is stable for atleast 24 h when the pH is maintained above 5.5.Thecompoundis diamagnetic.Itjjrured Spectra.-Of particular interest in the oxomolyb-denum(v) species are two vibrations due to terminal Mo'Ostretches in the region 900-1 OOO cm-1.16-'y The complex[M0~0~(glyO)~(H~0)~] shows two such bands at 980 and 920cm-'. In accordance with the vibrations reported 17*20*21 for aMo:~;Mo moiety, a weak band observed at 470 cm-' anda strong one at 710 cm-I are due to the symmetric and anti-symmetric vibrational modes respectively. Two mediumintensity bands appeared at 3 210 cm-' [v(NHz)] and 1 610cm-I [6(NH2)] which are indicative of amino-group co-ordin-ation.A broad band centred at 3 420cm-' isassigned to v(0H).The carboxylic group here is of unidentate nature as the separ-ation between vnsytn(C02-) (1 625 cm-') and vsym(C02-)(1 405 cm-') is 220 cm-', a value greater than that due to thefree ligand (197 cm-1).2z The results clearly show that glycinatein this complex acts as a bidentate ligand co-ordinatingthrough the amino-nitrogen and unidentate carboxylic group.Electronic Absorption Spectra.-The electronic spectrum of[M0~0~(glyO)~(H~0)~] in aqueous solution a t pH 5.5 ischaracterized by two bands at 306 nm (E 4 240 dm3 mol-' cm-')and 695 nm (E 36 dm3 mo1-I cm-I). Similar bands at ca. 300and ca. 700 nm with comparable molar extinctions are welldocumented in the literature 15~23 for MoV amino-acid com-plexes.No change in spectral characteristics of the solution atpH 5.5 is noted at least for 24 h. On longer standing in air thesolution gradually becomes colourless due to oxidation to theMoV1 state. The compound remains stable in the pH range5.5-6.8. At a pH of less than 5.5, the original orange colourof the solution gradually changes to green. The rate of colourchange is faster, the lower the pH of the solution. Figure 1represents the interval scan spectra of the compound inaqueous solution at pH 4.48. It may be noted that the initialpeak positions for the fresh solution (spectrum 1) are at 306and 695 nm. However, with the passage of time the intensityof the band at 306 nm gradually decreases with concomitantincrease in the intensity of the band at 695 nm together withthe appearance of a new band at 850 nm.After ca. 24 h, theband at 306 nm has completely vanished, and that at 850 nmhas developed into a strong band. This new band at 850 nm( E 241 dm3 m o P cm-') is of particular interest to us. We as-sume that this new low-energy band exhibited by the greensolution at higher acidity is due to MoV-MoV* intervalencecharge transfer which results as a sequel to partial aerialoxidation of the compound.Similar low-energy bands have also been observed in thevisible to near-i.r. region of the electronic spectra of a numberof mixed-valence compounds, for example pyrazine-bridgeddiruthenium-type ions 24 and biferrocene molecules.25 Ac-cording to Hush 26 these bands are due to intervalence transfertransitions where light-induced electron transfer occurs be-tween different valence state sites.In the context of molyb-denum chemistry, to our knowledge there exist no examplesof a MoV-MoV1 species in the literature. However, two dif-ferent groups of workers have investigated formal Mo'-Moocompounds. For example Stone and co-workers 27 have iso-lated a series of q7-cycloheptatrienylmolybdenum complexes,[ M O ~ ( ~ - X ) ~ ( ~ ~ - C ~ H ~ ) ~ ] (X = CI, Br, or I). These compoundsabsorb strongly in the region 950-1 380 nm and photo-electron spectroscopic studies have established that these areMoI-Mo" species with a localised trapped-valence groundstate. A similar conclusion has been reached for [(q3-C7H7)-(C0),Mo(OR),Mo(q7-C7H7)] (R = alkyl) by Zeigler and co-workers z8 from e.s.r.studies. Since in our case the isolationof the MoV-MoV1 species in the solid state was unsuccessful,the mixed valence in this compound is further examined insolution by cyclic voltammetry and n.m.r. studies (see later).Thus on the basis of elemental analyses, ix., electronicspectral features, and diamagnetic behaviour, (I) appears tobe the structure of the compound.Electrochemical Behauiour.-Figure 2 shows the cyclicvoltammogram of [Moz04(glyO)z(Hz0)z] mol dm-3) inwater with 0.1 mol dm-3 NaCl as supporting electrolyte atpH 5.6. In the potential range 0 to - 1.4 V at a scan rate of 50mV s-', a large cathodic peak at -0.90 V* and a relativelyweak anodic peak at -0.42 V have been observed.The cath-odic wave at -0.90 V appears to involve transfer of two rnolof electrons per mol of the molybdenum(v) dimer as evidentfrom the large peak current and steep initial slope of the wave.This has been confirmed from coulometric reduction of[M0~0~(glyO)~(H~0)~] at pH 5.6 at apotential of -1.1 V us.s.c.e. which shows that the number of electrons transferredper mol is 1.8. This would mean that the Mov-MoV speciesundergoes two-electron reduction to a MoIV species which is*All potentials were measured us. s.c.eJ. CHEM. SOC. DALTON TRANS. 1983 859I//I I 1 1 I I I-0.2 -0.4 -0-6 -0.8 -1.0 -1.2 -1.6N V vs s.c.e.Figure 2. Cyclic voltammogram of [M0~0~(glyO)~(H~0)~1 moldm-3) in water containing 0.1 rnol dm-3 NaCl at pH 5.6 at a scanrate of 50 mV s-'in turn oxidised to a Mo" species in the anodic sweep at -0.42V. Since the ratio of the cathodic to anodic current (Ipc/lpa) isalmost 2, we infer that the oxidation product is a monomericMoV complex.29The cyclic voltammogram of the proposed MoV-MoV'species is shown in Figure 3 (voltammogram is recorded after2 h of dissolution of [M0~0~(glyO)~(H,0),] in water at pH4.6 with 0.1 mol dm-j NaCl as the supporting electrolyte).The voltammogram consists of two cathodic peaks at -0.65and -0.74 V and an anodic peak at -0.38 V.It appears thatat the first stage of reduction, that is, at -0.65 V, the MoV-MeV' complex reduces to a MoV-MoV complex (different fromthe original MoV-MoV complex) which at -0.74 V undergoesfurther reduction to a MoIV species.This Mo" species ap-pears to be identical to that obtained by reduction of [Mo204-(gl~o)~(H~O)~] at -0.90 V because in both the cases oxidationof the MoIV to MoV species takes place at -0.40 V (the slightdifference in the potentials, -0.38 against -0.42 V, is mostlikely due to the difference in pH of the medium). The mono-meric MoV species is dimerized to the more stable [Mo204-(glyO),(H20)2]. The redox processes are schematically shownbelow in Scheme 1.0 0glyO-Mo Mo- gly0I \ A / lDimerisation Ily,O, II p H+/i02 1(pH ca 4.6) -e- l I U IH20 I OH2J -2e-Scheme 1.J I I I 1 10 -0.2 -0.4 - 0 6 -0.8 -1.0E/V vs s.c.eFigure 3. Cyclic voltammogram of [M0~0~(glyO)~(H~0)~]rnol dm-3) in water containing 0.1 mol dm-3 NaCl at pH 4.6.Voltammogram at a scan rate of 10 mV s-I taken 2 h after dissolutionof the compoundKinetics of the Proton-assisted Oxidation of [M0~0~(glyO)~-(H20)2].-The rate of proton-assisted aerial oxidation of[M0~0~(glyO)~(H~0)~] from orange to green below pH 5.5increases with decrease in pH.However, at pH <3.6, thesolution rapidly becomes blue and the spectrum lacks theband at 850 nm which is monitored for kinetic studies. Wetherefore restricted our kinetic measurements to the pHrange 5.0-3.7. The first-order plots of log (A, - A,) againsttime (t) are linear over 80% of the course of reaction, whereA, and A, are absorbances at 850 nm after 24 h and at a timet respectively. The first-order rate constant kobs.is dependenton w + ] in the pH range studied. The variation in glycinateconcentration at constant pH is found to have an insignificanteffect on the rate of oxidation.From these observations it may be concluded that in the pHrange 5.0-3.7 protonation of [M0~0~(glyO)~(H~0)~] takesplace. The protonated species is then attacked by the dissolvedoxygen in water, giving the MoV-MoV' species as shown inScheme 2 below. Considering the pseudo-first-order condition0 0H20 OH2(orange) 40 0Scheme 2.of the experiment, the observed rate constant can be expressedby kobp, = ko[H+]. Thus a plot of kobs. us. m + ] should yielda straight line from which ko can be evaluated. In the pHrange 5.0-3.7, a plot of kobs. us. [H+] gives a good linear fitof data passing through the origin.The second-order rateconstant, ko, evaluated from the slope is 2.52 dm3 mol-' s-l860 J. CHEM. SOC. DALTON TRANS. 1983The formation of intermediate MoV-MoV1 species has alsobeen proposed30B31 in studying the kinetics of oxidation of[ Mo204(ed t a)]' - (ed t a = e t h ylenediamine t et ra-ace t a t e) by[IrCl6I2-, [Fe(phen)$+ (phen = 1,lO-phenanthroline), and[(NH3)5Co(02 -)Co(NH3),I5 + . Although an intervalencecharge-transfer band for the MoV-MoV1 species is expected,no such band has ever been reported in the literature. Toour knowledge we are the first to report this band at 850 nmfor a molybdenum compound. In as much as a MoV-MoV1species is expected to have a paramagnetic MoV centre, furtherevidence for the formation of MoV-MoV1 species has beenobtained from the measurement of magnetic susceptibility ofan aqueous solution of [M0~0~(glyO)~(H~0)~] by the Evansmethod.I4 The n.m.r.spectrum is recorded 2 h after thedissolution of [ M O ~ O ~ ( ~ ~ ~ O ) ~ ( H ~ O ) ~ ] in water containing 2%t-butyl alcohol, the pH being maintained at 4.6. The value ofperf. thus obtained wasca. 0.6B.M. (ca. 0.556 x A m2).All our efforts to isolate the MoV-MoV1 species in the solidstate proved unsuccessful. It is generally believed that bio-logical molybdenum is dimeric in the native enzymes, and if itremains dimeric in the presence of substrate, the MoV e.s.r.signals observed in four of the molybdenum enzymes in thepresence of substrate must be due to a mixed oxidation state,possibly a MoV-MoV1 dime^-.^*^^ Formation of similar MoV-MoV1 species has also been proposed by Howie and Sawyer 29during the course of their electrochemical studies on someoxo- and sulphido-bridged binuclear MoV complexes.Thepresent study offers a possible pathway for the generation of adimeric MoV-MoV1 species.AcknowledgementsThanks are due to Dr. K. Nag for helpful discussions andDr. D. Dattafor carrying out the electrochemical experiments.I am indebted to Professor A. Chakravorty for providing hisElectrochemistry S ys tern.References11, 107.1 R. C. Bray and J. C. Swann, Struct. Bonding (Berlin), 1972,2 E. I. Stiefel, Prog. Inorg. Chem., 1977, 22, 1.3 E. I. Stiefel, W. E. Newton, G. D. Watt, K. L. 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