www.elsevier.com/locate/ica

Inorganica Chimica Acta 360 (2007) 3414–3423

Electrochemical oxidation of Mo(CO)4(LL)and Mo(CO)3(LL)(CH3CN): Generation, infrared characterization,and reactivity of [Mo(CO)4(LL)]+ and [Mo(CO)3(LL)(CH3CN)]+

(LL = 2,2 0-bipyridine, 1,10-phenanthroline and related ligands)

Ryan Johnson, Humair Madhani, John P. Bullock *

Division of Natural Sciences & Mathematics, Bennington College, Bennington, VT 05201, United States

Received 30 March 2007; accepted 18 April 2007Available online 4 May 2007

Abstract

Mo(CO)4(LL) complexes, where LL = polypyridyl ligands such as 2,2 0-bipyridine and 1,10-phenanthroline, undergo quasi-reversible,one-electron oxidations in methylene chloride yielding the corresponding radical cations, [Mo(CO)4(LL)]+. These electrogenerated spe-cies undergo rapid ligand substitution in the presence of acetonitrile, yielding [Mo(CO)3(LL)(CH3CN)]+; rate constants for these sub-stitutions were measured using chronocoulometry and were found to be influenced by the steric and electronic properties of thepolypyridyl ligands. [Mo(CO)3(LL)(CH3CN)]+ radical cations, which could also be generated by reversible oxidation ofMo(CO)3(LL)(CH3CN) in acetonitrile, can be irreversibly oxidized yielding [Mo(CO)3(LL)(CH3CN)2]2+ after coordination by an addi-tional acetonitrile. Infrared spectroelectrochemical experiments indicate the radical cations undergo ligand-induced net disproportiona-tions that follow first-order kinetics in acetonitrile, ultimately yielding the corresponding Mo(CO)4(LL) and [Mo(CO)2(LL)(CH3CN)3]2+

species. Rate constants for the net disproportionation of [Mo(CO)3(LL)(CH3CN)]+ and the carbonyl substitution reaction of[Mo(CO)3(LL)(CH3CN)2]2+ were measured. Thin-layer bulk oxidation studies also provided infrared characterization data of[Mo(CO)4(ncp)]+ (ncp = neocuproine), [Mo(CO)3(LL)(CH3CN)]+, [Mo(CO)3(LL)(CH3CN)2]2+ and [Mo(CO)2(LL)(CH3CN)3]2+

complexes.� 2007 Elsevier B.V. All rights reserved.

Keywords: Molybdenum carbonyl compounds; Polypyridyl compounds; Cyclic voltammetry; Infrared spectroelectrochemistry

1. Introduction

Since the syntheses of Group 6 tetracarbonyl bipyridylcomplexes, M(CO)4(bpy), were initially reported in 1935[1], these and related a-diimine complexes have been thesubject of intense study due to their interesting spectro-scopic, photochemical and electrochemical properties [2].Much of the electrochemical work has focused on the reduc-

0020-1693/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.04.029

* Corresponding author. Tel.: +1 802 440 4472; fax: +1 802 440 4461.E-mail address: jbullock@bennington.edu (J.P. Bullock).

tion of these complexes, the initial processes of which areligand-based, one-electron transfers of varying reversibility;in some cases the radical anions have been characterized byinfrared [3–5], visible [6], and EPR spectroscopy [4,7]. Elec-trochemical oxidations of these complexes are metal-cen-tered, one-electron processes, the chemical reversibility ofwhich is highly dependent on the metal [8–14]. For example,W(CO)4(tmp), where tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline, is reported to undergo an irreversibleoxidation on the cyclic voltammetry time-scale. The chro-mium analog, Cr(CO)4(tmp), in contrast, exhibits a chemi-cally reversible oxidation, the product of which has been

1 Electrochemical Simulation Program, version 2.4, downloaded fromhttp://lem.ch.unito.it/chemistry/esp_manual.html.

R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423 3415

characterized spectroscopically [14]. Reversible or quasi-reversible behavior has also been observed for other chro-mium tetracarbonyl derivatives of 1,10-phenanthroline(phen), several bipyrazine and bipyrimidine derivatives[10], and the related diazadienes [15].

Despite the above-mentioned observations that haveappeared in the literature, detailed studies of the electro-chemical oxidation of M(CO)4(a-diimine) complexes, par-ticularly those of molybdenum and tungsten, have beenlacking, with little published beyond the relevant oxidationpotentials and qualitative descriptions of cyclic voltamme-try responses. One notable exception, however, was a thor-ough study of Mo(CO)4(bpy) in a number of solvents [13].It was shown that the chemical reversibility of the oxida-tion is dependent on the coordinating ability of the solvent.Thus, whereas the oxidation is completely irreversible incoordinating solvents, such as acetonitrile and dimethyl-sulfoxide, it can exhibit substantial reversibility innon-coordinating solvents under some conditions. The irre-versibility of the oxidation in coordinating solvents wasshown to be due to a rapid solvent substitution undergoneby the 17-electron electrogenerated species (Eq. (1)). Simi-lar behavior is also seen among the structurally analogousmolybdenum tetracarbonyl diazadiene complexes [15].Such rapid ligand substitutions are not unusual and havebeen noted for other 17-electron species [16]. The enhancedkinetic stability of the chromium(I) radicals mentionedabove has been attributed to steric effects. Specifically,attack of the solvent on these electrogenerated complexesoccurs via associative pathways [17]; the smaller size ofchromium makes the formation of a sterically congestedtransition state less energetically favorable than for thelarger molybdenum [14,18].

½MoðCOÞ4ðLLÞ�þ þ CH3CN

!k1 ½MoðCOÞ3ðLLÞðCH3CNÞ�þ þ CO ð1Þ

This paper examines the electrochemical oxidation of a ser-ies of Mo(CO)4(LL) complexes, where LL is bpy, phen,several of its methyl-substituted derivatives, and 2,2 0-biquinoline (biq), in acetonitrile and methylene chloride.We have found that the initial anodic processes show con-siderably greater reversibility in the latter solvent than haspreviously been reported; in fact we have obtained infraredspectral data on [Mo(CO)4(ncp)]+ (ncp = neocuproine, or2,9-dimethyl-1,10-phenanthroline), to our knowledge thefirst such radical cation of a molybdenum tetracarbonyla-diimine to be so characterized. Since the electrogener-ated, 17-electron species are susceptible to rapid ligandsubstitution, we have also studied the electrochemical oxi-dation of the acetonitrile substitution products, Mo-(CO)3(LL)(CH3CN), to gain insight concerning the fate ofthe oxidation products of the corresponding tetracarbonylcomplexes. We found the generation of [Mo(CO)3(LL)-(CH3CN)]+, either by direct oxidation of the tricarbonyl,or by rapid ligand substitution of the tetracarbonyl radical,is followed by a series of reactions, including dispropor-

tionation and further ligand substitutions, that we haveelucidated and kinetically characterized.

2. Experimental

2.1. Materials

Solvents (Pharmco, HPLC grade) were dried over acti-vated 4 A molecular sieves prior to use. Mo(CO)4(LL)complexes were synthesized by direct reaction betweenMo(CO)6 (Strem) and the polypyridyl ligand according toliterature methods [19]. Ligands 2,2 0-bipyridine (Alfa-Aesar), 1,10-phenanthroline (Aldrich), 4,7-dimethyl-1,10-phenanthroline, dmp (GFS), 3,4,7,8-tetramethyl-1,10-phenanthroline (GFS), neocuproine (Aldrich), and 2,2 0-biquinoline (Aldrich), were used as received. Mo(CO)3-(LL)(CH3CN) complexes were prepared by refluxing thecorresponding tetracarbonyl in acetonitrile for 2–3 h undernitrogen; during this time the solutions deepened in colorconsiderably. Infrared spectroscopy was used to monitorreaction progress. Several attempts to isolate the productsof these reactions by solvent removal via rotary evapora-tion or under vacuum led to decomposition. Therefore,the tricarbonyl complexes were not isolated after prepara-tion, but were delivered to the electrochemical cell viagastight syringe. Electrochemical experiments were per-formed in 0.1 M solutions of tetrabutylammonium hexaflu-orophosphate, TBAH (Sigma), which was used as received.Electrolyte solutions were dried over activated 4 A molec-ular sieves prior to use.

2.2. Equipment

Electrochemical experiments were performed with aBioanalytical Systems CV-50W Voltammetric Analyzerat room temperature using a glassy carbon working elec-trode and a Ag/AgCl reference (E1/2 value of the ferro-cene/ferrocenium couple in acetonitrile was 435 mV witha peak separation of 95 mV at 250 mV/s). Solutions werepurged with nitrogen presaturated with dried solvent; allcyclic voltammetry experiments were also performed inthe presence of activated molecular sieves to minimizethe concentration of adventitious water. Digital Simula-tions of cyclic voltammetric responses were performedusing the Electrochemical Simulation Package version2.4 by Carlo Nervi.1 Spectroelectrochemical experimentswere performed at room temperature, unless otherwisespecified, using a cell equipped with a gold mesh workingelectrode, a silver wire pseudo-reference, and CaF2 win-dows [20]. Infrared spectra were collected using a NicolletAvatar 370 FTIR.

Fig. 1. Cyclic voltammograms of Mo(CO)4(bpy) in CH2Cl2/TBAH in thepresence of (a) 0, (b) 0.4, (c) 1.6, and (d) 25 equiv. of acetonitrile.

3416 R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423

3. Results and discussion

3.1. Electrochemistry of Mo(CO)4(LL) complexes

Our observations concerning the major features of theelectrochemical oxidation of Mo(CO)4(LL) complexes inacetonitrile are consistent with those of previous studies.Briefly, these complexes exhibit irreversible one-electronoxidations coupled to the reversible reduction of[Mo(CO)3(LL)(CH3CN)]+. The oxidation peak potentialsof the parent tetracarbonyls and the E1/2 values of theresulting tricarbonyl substitution products are presentedin Table 1. A second oxidation peak is seen for these com-pounds in acetonitrile; owing to the speed of the carbonylsubstitution on the cyclic voltammetry time-scale, however,this peak is assigned to the oxidation of [Mo(CO)3(LL)-(CH3CN)]+, the products of which are discussed at greaterlength later in this paper.

We observed substantially greater reversibility for theoxidation of Mo(CO)4(LL) in methylene chloride than pre-vious studies would have suggested. For example, Hanzlıkand co-workers reported that oxidation of Mo(CO)4(bpy)in the non-coordinating electrolyte dichloroethane/TBAF(TBAF = tetrabutylammonium tetrafluoroborate) at aplatinum electrode displayed reversible behavior only atlow temperature (�10 �C); at ambient temperature nocathodic return attributable to the electrogenerated specieswas observed [13]. In contrast, the cyclic voltammogram ofMo(CO)4(bpy) in CH2Cl2/TBAH that we observed(Fig. 1a) exhibits a quasi-reversible oxidation (E1/2 =692 mV; DE = 147 mV) with a current ratio, ic/ia, of 0.73at 250 mV/s; this increases to 0.91 at 1000 mV/s. Thebehavior illustrated by Fig. 1a was typical for that seenfor the complexes examined in this study (Table 1). Notethat there is a significant electronic effect of the ligand onthe oxidation potentials, with phenanthroline ligands thatare more highly substituted with electron-releasing methyl

Table 1Cyclic voltammetric characterization of Mo(CO)4(LL) complexesa

LL In CH2Cl2/TBAH

Mo0/+

E1/2 DE ic/ia Additional reducti

bpy 692 147 0.73 183 (0.003)�299 (0.034)

phen 682 101 0.66 170 (0.020)�285 (0.012)�467 (0.011)

dmp 635 107 0.66 �363 (0.029)

tmp 628 120 0.79 �395 (0.010)�571 (0.007)

ncp 659 124 0.88 179 (0.011)

biqb 706 89 0.65 240 (0.084)

a Scan rates 250 mV/s.b The biq complex had very limited solubility in acetonitrile. See also Ref. [

substituents showing the lowest oxidation potentials, pro-vided that steric interactions with the metal are similar.This reflects the greater stability of the cation due to theenhanced basicity of the nitrogen donor ligands. Thus theoxidation potentials follow the order: phen > dmp > tmp,with tmp being 54 mV more negative than phen. A similarshift in oxidation potential was observed between

In CH3CN/TBAH

Mo+/2+ Mo0/+

on peaks (ic/ia) Ea Ea E1/2 (coupled peaks)

1490 648 267

1490 639 237

1451 602 224

1440 581 208

1550 651 264

1590 671 317

11].

Fig. 2. Qr/Qf chronocoulometric data (E1 = 400 mV, E2 = 900 mV) of thefirst oxidation peak of Mo(CO)4(bpy) in the presence of 3.2 equiv. ofacetonitrile. The molybdenum concentrations employed were 0.40 mM(+), 0.56 mM (s), 0.92 mM (·), 1.45 mM (n), 2.20 mM (h), and2.95 mM (e). The solid curve represents the theoretical curve for asecond-order EC mechanism.

Table 2Rate constants for reactions of [Mo(CO)4(LL)]+ and[Mo(CO)3(LL)(CH3CN)]+a

LL k1 (M�1 s�1) k5, observed (M�1) k7, observed (M�1)

bpy 1900 0.068 0.057phen 2500 0.041 0.025

R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423 3417

Cr(CO)4(tmp) and Cr(CO)4(phen) [14]. Furthermore, whilethe oxidation peaks of Mo(CO)4(LL) are irreversible inacetonitrile, and therefore provide no thermodynamicinsight, they follow the same trend as that seen in methy-lene chloride.

In addition to the reduction peak due to the[Mo(CO)4(LL)]+ radicals, we observed several other smal-ler coupled reductions in CH2Cl2, the peak potentials andcurrent ratios (with respect to the corresponding bulk ano-dic process) of which are included in Table 1. The presenceof these peaks indicates that these radicals follow severaldecomposition pathways which may include attack byadventitious water, substitution of CO by solvent or elec-trolyte anions, or disproportionation. These complexesalso undergo a second one-electron oxidation in methylenechloride (not shown in Fig. 1) that are irreversible for allcomplexes examined2; the cathodic return scans for severalof these complexes showed signs of product deposition onthe electrode.

Rate constants for the substitution of carbon monoxideby acetonitrile (Eq. (1)) were measured using chronocoul-ometry. The results obtained for Mo(CO)4(bpy) are shownin Fig. 2; these data were acquired using Mo(CO)4(bpy)solutions in methylene chloride with concentrationsbetween 0.40 and 2.95 mM in the presence of acetonitrileat a constant CH3CN:Mo ratio of 3.2:1. The charge ratio,Qr/Qf, for the bulk oxidation process (E1 = 400 mV,E2 = 900 mV) is plotted against the term logs [Mo] k,where k is the rate constant for the second-order reaction,and s is the step-time of the experiment. The data were fitto the theoretical response curve (solid line) for a second-order EC mechanism [21] to determine the rate constant,

2 The electrochemical characterization of Mo(CO)4(biq) presented inthis study differs markedly from an earlier paper which reported tworeversible oxidations in CH2Cl2/TBAH at 420 and 780 mV versus SCE.See Ref. [11].

which in this case is estimated at 1900 M�1 s�1 (± 10%).Additional experiments were performed in which the aceto-nitrile concentration was varied while keeping the molyb-denum concentration constant; these gave resultsconsistent with results obtained by the above method.

The rate constants for the carbonyl substitution reac-tions of the other complexes studied are summarized inTable 2 under the heading k1. There are two clear trendsin these results. First, for remotely substituted phenanthro-line ligands (tmp,dmp), the rate of ligand substitutiondecreases with greater numbers of methyl substituents.Thus, complexes with relatively lower oxidation potentialsalso undergo slower substitution; this reflects the loweraffinity for the nucleophile for more electron-rich metalcenters. Second, sterically crowding ligands (ncp, biq), i.e.,those having substituents directed toward the equatorialcoordination plane, such as the methyl groups in the twoand nine positions in ncp, show markedly lower rate con-stants. This reflects the greater energetic barrier to formingan associative transition state with a sterically crowdedmetal center. This effect is fairly large, with ncp having arate constant less than 5% that of phenanthroline and lessthan 10% that of dmp. It is interesting to note that whilethe steric crowding of ncp slows the substitution in the rad-ical cations, the same ligand accelerates the rate of substi-tution in the neutral, 18-electron compound by about afactor of three compared to phen [22]. Since the substitu-tion mechanism of the neutral complexes involves bothassociative and dissociative pathways [23], presumablythe more sterically crowding ncp ligand enhances the rateof the dissociative pathway. The steric crowding aroundthe metal center of Mo(CO)4(ncp) may also be responsiblefor the fact that its oxidation in CH2Cl2/TBAH shows thegreatest reversibility of any of the tetracarbonyls examinedin this study under the similar conditions.

Given the kinetic stability of [Mo(CO)4(ncp)]+ weattempted to characterize this complex via infrared spec-troelectrochemistry. Fig. 3 shows the spectral changes inthe carbonyl stretching region that accompanied the oxida-tion of Mo(CO)4(ncp) at 900 mV versus the silver pseudo-reference at 5 �C. The peaks due to the starting materialdecrease in intensity as a new set of peaks due to[Mo(CO)4(ncp)]+ grow in isosbestically until the electroly-sis is about 70% complete (75 s elapsed time). Isosbesticbehavior is lost after this point, but to better illustrate

dmp 1600 0.029 0.021tmp 600 0.021 0.020ncp 120 0.010 0.007biq 700 0.018 0.011

a Rate constants are estimated to be correct to within 10%.

Fig. 3. Infrared spectroelectrochemical results for the oxidation ofMo(CO)4(ncp) in CH2Cl2/TBAH. Isosbestic behavior is lost afterapproximately 70% of the starting compound is converted to the radicalcation (75 s elapsed time); the heavy trace shows the spectrum of[Mo(CO)4(ncp)]+ as calculated from data obtained while isosbesticbehavior was maintained.

3418 R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423

the spectrum of the radical cation, the final spectrum of thethin-layer oxidation if isosbestic behavior had held was cal-culated and is shown in Fig. 3 (heavy gray trace).

It is interesting to compare this spectrum to thatobtained for [Cr(CO)4(tmp)]+ at �80 �C in propionitrile[14]. The spectrum of that complex has a sharp peak at2097 cm�1 and a broad, featureless peak centered at about1986 cm�1 (width at half-height = 55 cm�1). The latter fea-ture was assigned, with the aid of DFT calculations, byFarrell and co-workers, to the collapse of the three lowenergy carbonyl stretching modes (A1

1, B1, B2) into a singlebroad absorption due to equalization of the axial andequatorial carbonyl ligands upon oxidation. We observesimilar spectral features for [Mo(CO)4(ncp)]+. Specifically,the spectrum features a sharp peak at 2095 cm�1 (assignedto the symmetric A2

1 stretching mode) and a broad(�55 cm�1 width at half-height) lower energy stretch cen-tered at 1985 cm�1. However, the broad peak of the molyb-denum radical does show three distinct features: a shoulderat 2019, a larger peak at 2005, and a broad shoulder at1970 cm�1. The infrared spectra of [Mo(CO)3(LL)-(CH3CN)]+ complexes (vide infra) show a similar broaden-ing of the lower frequency absorption. However, of thosecompounds, only the ncp complex showed any distinct sep-aration of the two E bands. This raises the possibility thatthe distinct shoulders seen in the low energy carbonylstretch for [Mo(CO)4(ncp)]+, but which are absent in[Cr(CO)4(tmp)]+, could be a result of the steric crowdingassociated with that particular ligand, i.e., crowding ofthe equatorial plane could prevent as complete an equaliza-tion of the axial and equatorial carbonyl ligands in[Mo(CO)4(ncp)]+ as occurs in [Cr(CO)4(tmp)]+.

Cyclic voltammetry of Mo(CO)4(LL) complexes inCH2Cl2/TBAH in the presence of low levels of acetonitrileprovided additional insights into the reactivity of the elec-trogenerated radical cations. Figs. 1b–d show the results

obtained for Mo(CO)4(bpy) in the presence of 0.4, 1.6and 25 equiv. of acetonitrile per molybdenum complexat a scan rate of 250 mV/s. As indicated in the figure,subequivalent levels of acetonitrile result in marked dimi-nution of the reduction peak current due to [Mo(CO)4-(bpy)]+ as well as a lack of current enhancement of the bulkoxidation. Two coupled reduction peaks, at +160 and�280 mV, emerge as the acetonitrile concentrationincreases; these are assigned to the reduction of the car-bonyl substitution product, [Mo(CO)3(bpy)(ACN)]+, anda molybdenum(II) species, [Mo(CO)3(bpy)(ACN)2]2+, thegeneration of which is discussed below. The peak currentdue to the molybdenum(I) substitution product increaseswith the level of acetonitrile until reaching a plateau ataround 25 equiv. of acetonitrile per molybdenum. In con-trast, the peak current due to the molybdenum(II) complexincreases only until reaching a maximum current at about6 equiv. of acetonitrile per molybdenum, at which point itbegins to slowly decrease. These observations are consis-tent with a disproportionation in which [Mo(CO)4(bpy)]+

oxidizes [Mo(CO)3(bpy)(CH3CN)]+ (Eq. (2)) to yield a16-electron molybdenum(II) species which then rapidlycoordinates an additional molecule of acetonitrile to yieldan 18-electron, seven-coordinate complex (Eq. (3)) thatcan be formally viewed as an oxidative-addition productof Mo(CO)3(bpy)(CH3CN). Thus, the molybdenum(II)reduction peak is most pronounced under conditions wheneach of the molybdenum(I) radical cations are present inappreciable quantities. At low levels of acetonitrile thereis an insufficient level of the coordinating ligand to generatea large amount of [Mo(CO)3(bpy)(CH3CN)]+ via reaction1; the extent of reaction 2 and the production of theMo2+ species is accordingly limited as well. At high levelsof acetonitrile reaction 2 is instead limited by the availabil-ity of [Mo(CO)4(bpy)]+, most of which is rapidly convertedto the acetonitrile substitution product. Infrared evidencefor the production of [Mo(CO)3(bpy)(CH3CN)2]2+ is pre-sented later in this paper.

½MoðCOÞ4ðbpyÞ�þ þ ½MoðCOÞ3ðbpyÞðCH3CNÞ�þ

!MoðCOÞ4ðbpyÞ þ ½MoðCOÞ3ðbpyÞðCH3CNÞ�2þ ð2Þ

½MoðCOÞ3ðbpyÞðCH3CNÞ�2þ þ CH3CN

! ½MoðCOÞ3ðbpyÞðCH3CNÞ2�2þ ð3Þ

We employed the above reaction sequence as the basis fordigital simulation of the cyclic voltammetry response forthe oxidation of Mo(CO)4(bpy) in the presence of differentlevels of acetonitrile. Based on the assumption that theMo2+ tricarbonyl oxidative-addition product undergoesan irreversible two-electron reduction, as is common forsuch species [24], we estimate (to within approximately25%) the rate constant of reaction 2 to be 700 M�1 s�1.The simulated responses gave good agreement with the ob-served data with respect to the lack of measurable currentenhancement of the bulk oxidation peak, a feature thatcould potentially arise from such a regenerative mechanism,

Fig. 4. Infrared spectroelectrochemical results for the oxidation ofMo(CO)4(bpy) in CH3CN/TBAH. The final trace shows a single carbonylcontaining species assigned as [Mo(CO)2(bpy)(CH3CN)3]2+. Two inter-mediates are also observed, the largest peaks of which are indicated byarrows pointed in opposite directions, and are assigned as [Mo(CO)3(b-py)(CH3CN)]+ and [Mo(CO)3(bpy)(CH3CN)2]2+.

R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423 3419

as well as the magnitude of the coupled reduction peaksover the range of acetonitrile concentrations examined.

In addition to the infrared spectroelectrochemical exper-iments of Mo(CO)4(ncp) in methylene chloride describedearlier, we also performed a series of analogous experi-ments in acetonitrile. Given the cyclic voltammetry resultsdiscussed above for Mo(CO)4(LL) complexes, we expectedthat bulk oxidation of these compounds would readilyyield [Mo(CO)3(LL)(CH3CN)]+ radical cations. Fig. 4shows the infrared spectral changes observed upon oxida-tion of Mo(CO)4(bpy) in CH3CN/TBAH at 800 mV.Exhaustive oxidation at this potential ultimately yielded asingle product with peaks at 1990 and 1930 cm�1, buttwo distinct intermediate species were also observed.Thin-layer oxidation of Mo(CO)3(bpy)(CH3CN) producedthe same intermediates, which we assign as [Mo(CO)3-(bpy)(CH3CN)]+ and [Mo(CO)3(bpy)(CH3CN)2]2+ (vide

infra). The pattern and position of the carbonyl peaks ofthe final product are consistent with a molybdenum(II)dicarbonyl; the peak intensity ratio (symmetric/antisym-

Table 3Carbonyl stretching frequencies of Mo(CO)4(LL) and related complexes in CH

Compound LL

bpy phen dmp

Mo(CO)4(LL) 2016, 1904, 2015, 1904, 2014, 11876, 1831 1877, 1831 1874, 1

Mo(CO)3(LL)(ACN) 1907, 1788 1908, 1789 1906, 1Mo(CO)3(LL)(ACN)+ 2026, 1905 (broad) 2026, 1906 (broad) 2024, 1MoðCOÞ3ðLLÞðACNÞ22þ 2093, 2024, 1990 2092, 2024, 1990 2091, 2MoðCOÞ3ðLLÞðACNÞ32þb 1995, 1930 (1.59) 1996, 1932 (1.71) 1992, 1

a Remaining peaks obscured by other absorbances from other species.b The symmetric to antisymmetric peak ratios are given in parentheses for th

metric = 1.5) indicates that the carbonyl bond angle isabout 76� [25]. We therefore assign these peaks to theseven-coordinate, [Mo(CO)2(bpy)(ACN)3]2+. The otherMo(CO)4(LL) complexes studied show similar behavior;carbonyl stretching frequencies of the observed[Mo(CO)2(LL)(CH3CN)3]2+ complexes, along with the rel-ative intensities of the symmetric to antisymmetric peaks,are summarized in Table 3. Thus, spectroscopic evidenceindicates that oxidation of Mo(CO)4(LL) complexes yieldsproducts resulting from multiple carbonyl substitutionreactions with the solvent and a net two-electron oxidation,despite the fact that the cyclic voltammetry data indicatethat the initial oxidation involves only one electron. Underthe assumption that [Mo(CO)3(LL)(CH3CN)]+ is indeedrapidly formed after an initial one-electron oxidation ofthe tetracarbonyl starting material, we decided to examinethe electrochemical oxidation of the Mo(CO)3(LL)-(CH3CN) complexes directly to further elucidate the mech-anism by which the molybdenum(II) oxidation productsare generated.

3.2. Electrochemistry of Mo(CO)3(LL)(CH3CN)complexes

Mo(CO)4(LL) complexes readily undergo thermal sub-stitution in acetonitrile [13]. In this work, we refluxed thetetracarbonyl starting compounds under nitrogen to yieldthe corresponding acetonitrile substitution products in amatter of hours. The reactions were accompanied by dis-tinct red-shifts in the visible absorbance and deepening ofthe solution colors. Infrared spectra of the resulting com-pounds are consistent with fac-Mo(CO)3(LL)(CH3CN).The substitution reactions proceeded cleanly, as evidencedby the lack of any carbonyl compounds detectable by infra-red spectroscopy, or redox-active species detectable in thesolvent–electrolyte window, other than the acetonitrile sub-stitution product. Samples used in this study were removedfrom the refluxed solution without isolation.

Oxidation of all Mo(CO)3(LL)(CH3CN) complexes inacetonitrile show substantial reversibility; current ratiosand E1/2 values for the complexes examined are summa-rized in Table 4. These oxidation potentials show similartrends to those already discussed for the Mo(CO)4(LL)

3CN/TBAH

tmp ncp biq

901, 2013, 1900, 2017, 1905, 2018, 1910,830 1874, 1829 1874, 1825 1878, 1828785 1905, 1784 1907, 1786 1907, 1794905 (broad) 2023, 1900 (broad) 2025, 1922, 1876 2026, 1908 (broad)020, 1992 2089, 2018, 1991 2089, 2019, 1990 2088a

927 (1.80) 1991, 1925 (1.74) 1993, 1930 (1.67) 1999, 1939 (1.42)

ese complexes.

Table 4Cyclic voltammetric characterization of Mo(CO)3(LL)(CH3CN) com-plexes in CH3CN/TBAHa

LL Mo0/+ Mo+/2+

E1/2 DE ic/ia Ea Coupled reduction peaks

bpy 267 69 0.82 1110 �280, �617phen 253 74 0.90 1130 �328, �624dmp 224 82 0.88 1127 �456, �714tmp 208 64 0.88 1093 �471, �735ncp 266 80 0.96 1256 �350, �704biq 332 86 0.87 1255 �362, �625

a Scan rates 250 mV/s.

3420 R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423

compounds. The tricarbonyl complexes also exhibit a sec-ond, irreversible, one-electron oxidation. We attribute thelack of a return peak for this oxidation to the rapid coor-dination of acetonitrile to the electrogenerated 16-electronspecies, [Mo(CO)3(LL)(CH3CN)]2+ (Eq. (3)). There are,however, two cathodic peaks coupled to the second oxida-tion. In the case of Mo(CO)3(bpy)(CH3CN) they are seenat �280 and �617 mV. The former is assigned to the reduc-tion of [Mo(CO)3(LL)(CH3CN)2]2+, as already discussed,while the latter is attributed to the molybdenum(II) dicar-bonyl, [Mo(CO)2(LL)(CH3CN)3]2+, observed during thethin-layer bulk oxidations of Mo(CO)4(LL).

Given the substantial chemical reversibility of theMo(CO)3(LL)(CH3CN) oxidations, we attempted to spec-troscopically characterize the radical cations using infraredspectroelectrochemistry. The infrared spectral changes inthe carbonyl stretching region observed during a thin-layerbulk oxidation of Mo(CO)3(ncp)(CH3CN) at 500 mV areshown in Fig. 5. This series of spectra show a nearly isos-bestic conversion of the neutral starting material to[Mo(CO)3(ncp)(CH3CN)]+. The peaks due to the parentspecies give way to three new peaks: a sharp band at2025 cm�1, which we assign to the symmetric A1 stretch,

Fig. 5. Infrared spectral changes observed upon oxidation ofMo(CO)3(ncp)(CH3CN) in CH3CN/TBAH. The peaks that grow induring the electrolysis are assigned to [Mo(CO)3(ncp)(CH3CN)]+.

and two broader peaks at 1922 and 1876 cm�1, assignedto the E modes, which are split in the radical cation intotwo discernable peaks. The assignment of these new peaksto the radical cation is supported by the similarity of thecarbonyl stretching frequencies of this compound to similar17-electron species [26], such as [W(CO)3(CH3CN)3]+

(mCO = 2018, 1900 cm�1) [27]. In addition, the magnitudeof the shift in the carbonyl stretching frequencies and thesplitting of the E-band upon oxidation are similar to thespectral changes seen upon the one-electron oxidation of(g6-mesitylene)Cr(CO)3 in methylene chloride [20].

Experiments with other Mo(CO)3(LL)(CH3CN) com-plexes failed to yield the corresponding radical cations ascleanly as the ncp compound. Rather, the infrared spectraafter completion of these electrolyses typically had a totalof six carbonyl peaks, four of which exactly match thoseof the corresponding Mo(CO)4(LL) compounds. In thecase of Mo(CO)3(bpy)(CH3CN), the remaining two peaks(at 1990 and 1930 cm�1) are identical to those resultingfrom bulk electrolysis of Mo(CO)4(bpy) in CH3CN at800 mV and are attributed to [Mo(CO)2(bpy)(CH3CN)3]2+.In addition to these species, the same intermediatesobserved during the thin-layer bulk oxidation ofMo(CO)4(bpy), namely [Mo(CO)3(bpy)(CH3CN)]+ and[Mo(CO)3(bpy)(CH3CN)2]2+, were seen during the oxida-tion of the tricarbonyl. Similar results were found usingcomplexes of the other ligands used in this study; the infra-red data for the various starting materials and oxidationproducts are summarized in Table 3. The basis for theinfrared assignments of the intermediate complexes is pro-vided below.

Infrared spectra of the intermediate species observed inthe electrolyses of Mo(CO)4(LL) and Mo(CO)3(LL)-(CH3CN) were obtained by subtracting the absorbancesof known species from spectra in which the peaks due tothe intermediates were relatively strong. In the case ofMo(CO)3(bpy)(CH3CN), the spectrum of one intermediatehas a sharp peak at 2026 cm�1 and a very broad peak cen-tered at 1900 cm�1 (width at half-height is �75 cm�1). Onthe basis of the similarity of this peak pattern to that of[Mo(CO)3(ncp)(CH3CN)]+, we assign this to [Mo(CO)3(b-py)(CH3CN)]+, the initially formed electrogenerated prod-uct. The second intermediate has three carbonyl peaks(mCO = 2093, 2024, 1990 cm�1), the general peak intensitypattern of which is very similar to that of seven-coordi-nate molybdenum(II) tricarbonyls previously reported[28]. We therefore assign these peaks to [Mo(CO)3(bpy)-(CH3CN)2]2+. The stretching frequencies observed for thiscompound are similar to those of [W(CO)3(CH3CN)4]2+

(mCO = 2087, 2010 cm�1), the product of the two-electronoxidation of W(CO)3(CH3CN)3 in acetonitrile [27].

The generation of a molybdenum(II) intermediate isconsistent with the known propensity of molybdenum(I)species to disproportionate [24]. Specifically, dispropor-tionation of the electrogenerated 17-electron species (Eq.(4)) followed by rapid association of a solvent moleculeto the resulting 16-electron molybdenum(II) product

R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423 3421

(shown earlier in Eq. (3)) would yield the observed seven-coordinate molybdenum(II) intermediate. This species thenundergoes a subsequent substitution reaction in which acarbonyl is replaced by another acetonitrile, thereby gener-ating the final thin-layer bulk oxidation product (Eq. (5)).It should be noted that the fate of the molybdenum(II) dis-proportionation product in this scheme differs from an ear-lier proposal [13]. Based on several features of the cyclicvoltammogram of Mo(CO)4(bpy) in CH3CN, including apronounced ‘‘curve-crossing’’ of the anodic and cathodictraces, Hanzlık and co-workers concluded that[Mo(CO)3(bpy)(CH3CN)]+ disproportionates as indicatedby Eq. (4) but that the resulting molybdenum(II) product,[Mo(CO)3(LL)(CH3CN)]2+, oxidizes Mo(CO)4(bpy) fromthe bulk solution to yield two 17-electron species,[Mo(CO)4(bpy)]+ and [Mo(CO)3(bpy)(CH3CN)]+.

2½MoðCOÞ3ðbpyÞðCH3CNÞ�þ

!MoðCOÞ3ðbpyÞðCH3CNÞþ ½MoðCOÞ3ðbpyÞðCH3CNÞ�2þ ð4Þ

½MoðCOÞ3ðbpyÞðCH3CNÞ�2þ þ CH3CN

!k5 ½MoðCOÞ2ðbpyÞðCH3CNÞ3�2þ þ CO ð5Þ

Additional thin-layer bulk electrolyses of Mo(CO)3(LL)-(CH3CN) confirm that the corresponding radical cationsindeed disproportionate. Again, using the bpy complex asa representative example, a solution of Mo(CO)3(bpy)-(CH3CN) was electrolyzed at 500 mV for roughly 40 s,the time required for the radical cation to reach its maxi-mum concentration under the experimental conditionsemployed; the electrolysis was stopped and spectralchanges that accompanied the reactions of the intermedi-ates were then monitored. Peaks due to Mo0 and Mo2+

products were seen to grow as those due to the radical cat-ion decayed. As mentioned earlier, however, the molybde-num (0) product observed was not the tricarbonyl startingmaterial, but Mo(CO)4(bpy). The formation of this speciescan be explained by the rapid ligand substitution of theMo0 disproportionation product, Mo(CO)3(bpy)(CH3CN),with free CO (Eq. (6)), present in solution as a result of theloss of CO from [Mo(CO)3(bpy)(CH3CN)2]2+. Such a reac-tion would be consistent with the high affinity for COexhibited by low valent metal species.

MoðCOÞ3ðbpyÞðCH3CNÞ þ CO

!MoðCOÞ4ðbpyÞ þ CH3CN ð6Þ

Kinetic analyses of the radical decay provided additionalinsight into the mechanism of the disproportionation.The rate laws for the reaction of [Mo(CO)3(LL)(CH3CN)]+

complexes were obtained by measuring the decay of theabsorbance due to the A1 symmetric stretch. Surprisingly,second-order plots of the data were curved, but first-orderplots were linear. This is consistent with a ligand-inducedprocess under pseudo first-order conditions, wherein anacetonitrile coordinates to the electrogenerated molybde-

num(I) to form a 19-electron species in the rate-determin-ing step (Eq. (7)); this could then act as a one-electronreductant towards [Mo(CO)3(bpy)(CH3CN)]+ in a fast-step (Eq. (8)).

½MoðCOÞ3ðbpyÞðCH3CNÞ�þ

þ CH3CN!slow

k7

½MoðCOÞ3ðbpyÞðCH3CNÞ2�þ ð7Þ

½MoðCOÞ3ðbpyÞðCH3CNÞ2�þ þ ½MoðCOÞ3ðbpyÞ

� ðCH3CNÞ�þ !fast½MoðCOÞ3ðbpyÞðCH3CNÞ2�2þ

þMoðCOÞ3ðbpyÞðCH3CNÞ ð8Þ

This reaction sequence is similar to the proposed mechanismfor the oxidation of (g6-mesitylene)W(CO)3 in acetontrile[27]. In that system, the initially formed [(g6-mesityl-ene)W(CO)3]+ is rapidly coordinated by a solvent moleculeto form the seven-coordinate, 19-electron intermediatewhich is then oxidized at the electrode; this occurs fast en-ough on the cyclic voltammetry time-scale that the parentcompound appears to undergo an irreversible two-electronoxidation. In the case of [Mo(CO)3(LL)-(CH3CN)]+, thesolvent-coordination step is much slower and oxidation atthe electrode is a one-electron process. Rate laws for the de-cay of all [Mo(CO)3(LL)(CH3CN)]+ radicals examined inthis study have the same form; observed rate constants forthe process are given in Table 2 under k7. The influence ofthe electron-donating ability of the phenanthroline ligandsis less pronounced on these rate constants than on thosefor the ligand substitution reaction of [Mo(CO)4(LL)]+,k1. The effect of sterics is substantial, however, with therate of the net disproportionation for the sterically con-gested ncp complex being a third that of the isomeric dmpcomplex, and nearly an order of magnitude less than thatof bpy.

The loss of carbonyl from [Mo(CO)3(LL)(CH3CN)2]2+

was examined using cyclic voltammetry and infraredspectrolelectrochemistry. As mentioned above, [Mo(CO)3-(LL)(CH3CN)2]2+ can be generated by the secondoxidation of Mo(CO)3(LL)(CH3CN) in acetonitrile. Theirreversibility of these oxidations and the observed coupledreduction peaks are entirely consistent with the series ofreactions outlined above for these species. Thin-layer bulkoxidations of Mo(CO)3(LL)(CH3CN) at potentials morepositive than the second oxidation peak of the startingmaterial were performed to rapidly generate the desired[Mo(CO)3(LL)(CH3CN)2]2+ complex and to minimize pro-duction of the radical cation, the decomposition of whichwould complicate the kinetic analyses. For each complexstudied, decay of the high frequency vibration of the tricar-bonyl molybdenum(II) complex was found to follow first-order kinetics, the observed rate constants for which arepresented in Table 2 under k5. Once again, the stericallydemanding ncp and biq ligands show the smallest rate con-stants. This is consistent with an associative substitutionmechanism under pseudo first-order conditions. In addi-tion, the electron-releasing substituents on phenanthroline

Scheme 1. Electrochemical oxidation of Mo(CO)4(LL) and Mo(CO)3(LL)(CH3CN) and coupled chemical reactions.

3422 R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423

have a stabilizing effect on the tricarbonyl intermediate anddecrease the rate of carbonyl substitution.

The kinetic information described above correspondswell with a cyclic voltammetry scan rate study of the reduc-tion peaks coupled to the second oxidation of Mo(CO)3-(LL)(CH3CN). As Eq. (5) would predict, the reductionpeak due to the molybdenum(II) tricarbonyl is larger thanthat of the dicarbonyl substitution product at fast scanrates, but gets relatively smaller at lower scan rates. Forexample, at 1000 mV/s, the peak due to the dicarbonyl isa small shoulder on the larger tricarbonyl reduction peak;at 50 mV/s, the tricarbonyl peak is virtually absent, leavingthe dicarbonyl reduction as the only clearly discernablepeak directly coupled to the second oxidation process. Dig-ital simulations of the scan rate study were performed usingthe rate constants obtained from the infrared data and gavegood agreement with the observed relative peak heights ofthe coupled reductions.

4. Conclusion

The reaction pathways undergone by the one-electronoxidation products of Mo(CO)4(LL) and Mo(CO)3(LL)-(CH3CN) in the presence of acetonitrile are summarizedin Scheme 1. [Mo(CO)3(LL)(CH3CN)]+, generated eitherby oxidation of the neutral parent or via ligand substitutionof [Mo(CO)4(LL)]+, is susceptible to disproportionation ordirect oxidation, yielding [Mo(CO)3(LL)-(CH3CN)2]2+

after coordination by the solvent. The latter then undergoessubstitution of another carbonyl ligand by the solvent. Thenature of the pathway is independent of the nature of thepolypyridyl ligand but the specific rate constants are influ-enced by their steric and electronic characteristics. In the

absence of coordinating ligands, however, [Mo(CO)4(LL)]+

radical cations are long-lived enough to give quasi-revers-ible cyclic voltammetric behavior.

We are currently investigating the reaction pathwaysundergone by analogous tungsten complexes; preliminarydata indicate that, as expected, oxidation of W(CO)4(LL)complexes is less reversible in non-coordinating solventsthan their molybdenum analogs, [14] but undergo parallelligand substitution chemistry, the products of which showgreater rates of disproportionation. These studies will bethe subject of a forthcoming report.

Acknowledgements

This work was supported by a grant from VermontEPSCoR, administered by the National Science Founda-tion. Funds to purchase the electrochemical workstationand the FTIR spectrometer were generously provided byKate Merck (Bennington College’, 46) and Al Merck.

References

[1] W. Hieber, F. Muhlbauer, Z. Anorg. Chem. 221 (1935) 337.[2] A. Vlcek Jr., Coord. Chem. Rev. 230 (2002) 225.[3] R.E. Dessy, L. Wieczorek, J. Am. Chem. Soc. 91 (1969) 4963.[4] S. Zalis, C. Daniel, A. Vlcek Jr., J. Chem. Soc. Dalton Trans. (1999)

3081.[5] I.R. Farrell, F. Hartl, S. Zalis, T. Mahabiersing, A. Vlcek Jr., J.

Chem. Soc. Dalton Trans. (2000) 3081.[6] (a) Y. Kaizu, H. Kobayashi, Bull. Chem. Soc. Jpn. 45 (1972) 470;

(b) D. Miholova, B. Gas, S. Zalis, J. Klima, A.A. Vlcek, J.Organomet. Chem. 330 (1987) 75;(c) B.C. Noble, R.D. Peacock, Spectrochim. Acta A 46 (1990) 407.

[7] W. Kaim, S. Kohlmann, Inorg. Chem. 26 (1987) 68 (and referencescited therein).

R. Johnson et al. / Inorganica Chimica Acta 360 (2007) 3414–3423 3423

[8] M.K. Lloyd, J.A. McCleverty, D.G. Orchard, J.A. Connor, M.B.Hall, I.H. Hillier, E.M. Jones, G.K. McEwen, J. Chem. Soc., DaltonTrans. (1973) 1743.

[9] R.J. Crutchley, A.B.P. Lever, Inorg. Chem. 21 (1982) 2276.[10] S. Ernst, W. Kaim, J. Am. Chem. Soc. 108 (1986) 3578.[11] A.O. Youssef, M.M.H. Khalil, R.M. Ramadan, A.A. Soliman,

Transition Met. Chem. 28 (2003) 331.[12] E. Ioachim, G.S. Hanan, Can. J. Chem. 83 (2005) 1114.[13] J. Hanzlık, L. Pospısil, A.A. Vlcek, M. Krejcık, J. Electroanal Chem.

331 (1992) 831.[14] I.R. Farrell, F. Hartl, S. Zalis, M. Wanner, W. Kaim, A. Vlcek Jr.,

Inorg. Chim. Acta 318 (2001) 143.[15] H. tom Dieck, E. Kuhl, Z. Naturforsch, B: Anorg. Chem., Org.

Chem. B 37 (1982) 324.[16] J.K. Kochi, Organometallic Mechanisms and Catalysis, Academic

Press, New York, 1978.

[17] D.R. Tyler, Prog. Inorg. Chem. 36 (1988) 125.[18] A.M. Bond, R. Colton, K. McGregor, Inorg. Chem. 25 (1986) 2378.[19] M.H.B. Stiddart, J. Chem. Soc. (1962) 4712.[20] J.P. Bullock, D.C. Boyd, K.R. Mann, Inorg. Chem. 26 (1987) 3084.[21] M.K. Hanafey, R.L. Scott, T.H. Ridgway, C.N. Reilley, Anal. Chem.

50 (1978) 116.[22] S. Shrestha, J.P. Bullock, unpublished results.[23] J.R. Graham, R.J. Angelici, Inorg. Chem. 6 (1967) 992.[24] A.M. Bond, R. Colton, Coord. Chem. Rev. 166 (1997) 161.[25] P.S. Braterman, Metal Carbonyl Spectra, Academic Press, New

York, 1975.[26] M.C. Baird, Chem. Rev. 88 (1988) 1217.[27] Y. Zhang, D.K. Gosser, P.H. Rieger, D.A. Sweigart, J. Am. Chem.

Soc. 113 (1991) 4062.[28] A.M. Bond, R. Colton, J.J. Jackowski, Inorg. Chem. 17 (1978)

105.