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Mononuclear oxovanadium complexes oftris(2-pyridylmethyl)amine†

Yasunobu Tajika, Kiyoshi Tsuge and Yoichi Sasaki*Division of Chemistry, Graduate School of Science, Hokkaido University,Sapporo, 060-0810, Japan

Received 20th September 2004, Accepted 24th February 2005First published as an Advance Article on the web 11th March 2005

Mononuclear oxovanadium(IV) and dioxovanadium(V) complexes of tris(2-pyridylmethyl)amine (tpa) have beenprepared for the first time. Crystal structure determinations of three oxovanadium(IV) complexes, [VO(SO4)(tpa)],[VOCl(tpa)]PF6, or [VOBr(tpa)]PF6, and a dioxovanadium(V) complex [V(O)2(tpa)]PF6 disclosed that the tertiarynitrogen of the tpa ligand always occupies the trans-to-oxo site. The structures of an oxo-peroxo complex[VO(O2)(tpa)]Cl that was prepared previously and of a l-oxo vanadium(III) complex [{VCl(tpa)}2(l-O)](PF6)2 havealso been determined. The tertiary nitrogen is located at a trans site to the peroxo and chloride ligands, respectively.The total sums of the four V–N bond lengths from the tpa ligand are remarkably similar among the six complexes,indicating that the vanadium oxidation states become less influential in tpa bonding due primarily to thecoordination of electron-donating oxo ligand(s). Absorption spectra of [VOCl(tpa)]+ in acetonitrile showed asignificant change upon addition of p-toluenesulfonic acid and HClO4, but not on addition of benzoic acid.Protonation at the oxo ligand by the former two acids is suggested. Cyclic voltammetric studies in acetonitrile verifiedthe proton-coupled redox behavior of the VIII/VIV process involving the oxo ligand for the first time. From thedependence of the added p-toluenesulfonic acid to the CV, redox potentials for the following species have beenestimated: [VIVOCl(tpa)]+/[VIIIOCl(tpa)] (E1/2 = −1.59 V vs. Fc+/Fc), [VIV(OH)Cl(tpa)]2+/[VIII(OH)Cl(tpa)]+ (Epc =−1.34 V), [VIV(OH2)Cl(tpa)]3+/[VIII(OH2)Cl(tpa)]2+ (Epa = −0.49 V), and [VIVCl2(tpa)]2+/[VIIICl2(tpa)]+ (E1/2 =−0.89 V). The reduction of [VV(O)2(tpa)]+ in 0.05 M [(n-Bu)4N]PF6 acetonitrile showed a major irreversible reductionwave V(V)/(IV) at −1.48 V. The metal reduction potentials of the oxovanadium(IV) and dioxovanadium(V) species arevery close, reinforcing the significant influence of the oxo ligand(s).

IntroductionTris(2-pyridylmethyl)amine (tpa) is a typical tripodal tetraden-tate ligand that forms five-membered chelate rings with ametal ion, and thus gives stable complexes with many metalions.1 The tpa and analogous tetradentate ligands furnishvarious octahedral metal complexes leaving two cis sites fortwo monodentate ligands or a didentate chelate.2–8 Edge-shared dioctahedral complexes are also known.3–5,9,10 A series ofisostructural stable tpa complexes are known; examples beingcis-dichloro-tpa compexes [MCl2(tpa)]+ (M = Re(III), Ru(III),and Os(III))6 and di-l-oxo dimeric complexes [M2(l-O)2(tpa)2]4+

(M = Mn(IV),10 Re(IV),7 Os(IV)11). Such isostructural complexesare useful in the study of the influence of d electron number andof size of metal ions on the detailed steric structures and otherproperties. Another interesting approach to the chemistry of tpacomplexes is the preparation of a series of stable complexes indifferent oxidation states with a single metal element, providinga useful redox series. Such an example is rare, however.

Vanadium takes various oxidation states with differing num-bers of oxo ligands; of the most frequently found structures,none in V(III), one in VIVO, and two in cis-VVO2 (or three inmer-VVO3).12 We thought that tpa may provide a series of stablevanadium complexes in different oxidation states, where twomutually cis positions could be occupied by different numbersof oxo and aqua ligands. Such a series of complexes wouldform an interesting redox series coupled with the change inthe degree of protonation at the ligand H2O/OH−/O2−. A

† Electronic supplementary information (ESI) available: Scan ratedependence of current peaks in cyclic voltammograms of [VOCl(tpa)]+/[VOCl(tpa)] (Fig. S1), figures of simulated cyclic voltammograms(Figs. S2, S3) and Scheme S1 describing the included electrochemicaland chemical reactions for the simulations. See http://www.rsc.org/suppdata/dt/b4/b414532a/

literature survey revealed, however, that vanadium complexesof tpa and its analogues are dominated by dinuclear species.Previously known tpa complexes of vanadium are limited tofour complexes; namely three oxo-bridged dimeric complexes,[{VIIIBr(tpa)}2(l-O)]Br2,13 [{VIV(O)(tpa)}2(l-O)](ClO4)2,14 and[{VIV,V(O)(tpa)}2(l-O)](ClO4)3,15 and an oxo-peroxo complex[VV(O)(O2)(tpa)]ClO4,16 among which X-ray structures of thethree dimeric complexes have been determined. It appears thattpa complexes of vanadium have a strong tendency to formoxo-bridged dimeric complexes in various oxidation states. Nosimple monomeric complexes of general formula [VIIIX2(tpa)]n+or [VIVOX(tpa)]n+ (n = 1, X = anionic monodentate ligand; n =2, X = H2O), as well as [VV(O)2(tpa)]+, have been preparedpreviously.

Our initial attempts to prepare a series of monomeric oxo-aqua vanadium(III) and (IV) complexes (X = H2O) with tpahad been seriously hampered by the facile formation of oxo-bridged dinuclear species. Nevertheless, we could prepare severalmononuclear tpa complexes of vanadium(IV) for the first timeby using methanol as a solvent and with the incorporation ofmonodentate anionic ligand(s). A dioxo vanadium(V) complexwas also prepared successfully. X-Ray structural determinationshave been made for all these complexes and also the previ-ously mentioned oxo-peroxo complex and a newly prepareddinuclear complex [{VCl(tpa)2}(l-O)]2+. This paper describes thepreparation, structures and redox properties of these vanadiumcomplexes of tpa.

ExperimentalMaterials

Tris(2-pyridylmethyl)amine (tpa) was prepared by the standardmethod reported in the literature.17 Acetonitrile for electro-chemical measurements was dried over calcium hydride andD

OI:

10.1

039/

b41

4532

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1 4 3 8 D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7 T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5

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was distilled under an argon atmosphere. Tetrabutylammoniumhexafluorophosphate ([(n-Bu)4N]PF6) was recrystallized twicefrom ethanol. All other commercially available reagents wereused as purchased.

Preparations of the complexes

Oxo-sulfato-tris(2-pyridylmethyl)amine-vanadium(IV). Theligand tpa (200 mg; 0.69 mmol) was dissolved in 200 cm3 ofmethanol. To the solution was added VO(SO4)·3H2O (149.5 mg.0.69 mmol). While stirring for 5 h the mixture became atransparent violet solution. After evaporation, the concentratedsolution was layered with diethyl ether. As diethyl ether diffusedinto the solution, violet crystals deposited. Yield, 268 mg (75.1%)(Found: C, 45.59; H, 4.59; N, 10.96; S, 6.18. C20H26N4O7SV([VO(SO4)(tpa)]·2CH3OH) requires C, 46.42; H, 5.06; N, 10.83;S, 6.2%). Beer’s law was not maintained suggesting partialdissociation of coordinated sulfate in methanol. The complexis insoluble in acetonitrile.

Oxo-chloro-tris(2-pyridylmethyl)amine-vanadium(IV) chloride.To a methanol solution containing BaCl2·2H2O (169 mg;0.69 mmol in 50 cm3) was added VO(SO4)·3H2O (150 mg;0.69 mmol), and the mixture was stirred until the blue oxo-vanadium(IV) sulfate was dissolved completely. The mixture wascentrifuged to remove BaSO4. The green solution was slowlyadded to a methanol solution of tpa (200 mg (0.69 mmol) in50 cm3). The color of the solution changed to reddish violetand finally to blue. The solution was evaporated to half volumeand then layered with diethyl ether as in the case of the sulfatocomplex. Blue needles were filtered off and dried under vacuum.Yield, 236 mg (70.9%) (Found: C, 44.55; H, 4.88; N, 11.84:Cl, 14.72. C18H24N4O4Cl2V ([VOCl(tpa)]Cl·3H2O) requires C,44.83; H, 5.02; N, 11.62; Cl, 14.70%) UV-vis (nm (e/M−1 cm−1))in methanol: 725 (45), 574(26); in CH3CN; 731(45), 574 (25).

Oxo-chloro-tris(2-pyridylmethyl)amine-vanadium(IV) hexaflu-orophosphate. To a methanol solution of [VOCl(tpa)]Cl wasadded 5 equivalents of NH4PF6. The blue precipitate wasrecrystallized from CH3CN/Et2O to give blue crystals of thePF6

− salt (yield, 52%) (Found: C, 40.94; H, 3.70; N, 11.24.C19H19.5N4.5OF6PClV ([VOCl(tpa)]PF6·0.5CH3CN) requires C,40.88; H, 3.52; N, 11.29%).

Oxo-bromo-tris(2-pyridylmethyl)amine-vanadium(IV) bromide.This complex was prepared similarly to [VOCl(tpa)]Cl exceptthat BaBr2·2H2O was used in place of BaCl2·2H2O (yield, 66.6%)(Found: C, 38.83; H, 3.86; N, 10.24; Br, 28.89. C18H22N4O3VBr2

([VOBr(tpa)]Br·2H2O) requires C, 39.09; H, 4.01; N, 10.13; Br,28.89%) UV-vis (nm (e/M−1 cm−1)) in CH3CN: 731(48), 582 (29).In CH3OH, Beer’s law was not maintained suggesting partialdissociation of coordinated bromide in methanol.

Oxo-bromo-tris(2-pyridylmethyl)amine-vanadium(IV) hexaflu-orophosphate. The PF6

− salt was obtained by dissolving thebromide salt in methanol and adding 5 equivalents of NH4PF6.Two types of crystal were deposited. The blue crystals ofthe PF6

− salt were mechanically separated from the bluishgreen crystals. The latter crystals were found to be thoseof {VIV,V(O)(tpa)}2(l-O)](PF6)3 by X-ray structural analysis.Yield, 32.8% (Found: C, 37.13; H, 3.16; N, 9.75; Br, 13.68.C18H18N4OF6PVBr ([VOBr(tpa)]PF6) requires C, 37.14; H, 3.12;N, 9.62; Br, 13.73%).

Dioxo-tris(2-pyridylmethyl)amine-vanadium(V) hexafluoro-phosphate. The ligand tpa (300 mg; 1.03 mmol) was dissolved inmethanol (5 cm3) and added to an aqueous solution of Na[V(O)3](126 mg; 1.03 mmol in 20 cm3) and the mixture was stirred for20 h. On addition of NaPF6 (800 mg, 4.76 mmol) to the reactionmixture, a pale yellow solid was precipitated (slight reduction ofthe volume by rotary evaporation was often required to causeprecipitation). The pale yellow solid was filtered off and washedwith water, and dried over diphosphorus pentaoxide. Yield,

458 mg (96.7%). This solid was recrystallized from methanolsolution by diffusion of diethyl ether (Found: C, 41.64: H, 3.64;N, 10.64. C18H18N4O2F6PV ([V(O)2(tpa)]PF6) requires C, 41.72;H, 3.50; N, 10.81%). 1H NMR (270 MHz, CD3CN, TMS)/ppm:d –CH2–; 4.647 (2H, d), 5.354 (2H, d), 4.466 (2H, s), pyridylrings; 7.613 (2H, d), 7.991 (2H, dt); 7.426 (2H, t), 8.445 (2H,d), 7.087 (1H, d); 7.725 (1H, dt); 7.402 (1H, t), 9.695 (1H, d).UV-vis: no absorption peak in the visible region.

Oxo-peroxo-tris(2-pyridylmethyl)amine-vanadium(V) hexaflu-orophosphate. This complex cation was prepared previously bythe reaction of H2O2, V2O5 and tpa·3HClO4.16 The cation wasprepared by a different method in this study. The vanadium(IV)complexes, [VO(SO4)(tpa)], [VOCl(tpa)]Cl, or [VOBr(tpa)]Br,were dissolved in a mixed solvent of methanol and diethyl etherand sealed in air. The air inside of the sample tube was renewedseveral times. After several days, the solution turned red in colorand red crystals deposited on diffusion of diethyl ether. Depend-ing on the initial complexes, HSO4

− (yield, 78.3%), Cl− (yield,88.0%), or Br− salts (yield, 73.6%), were obtained (For Cl− salt,Found: C, 47.01; H, 4.72; N, 12.33; Cl, 7.78. C18.5H23N4O5ClV([VO(O2)(tpa)]Cl·0.5CH3OH·1.5H2O) requires C, 47.5; H, 4.96;N, 11.98; Cl, 7.58%). UV-vis (nm(e/M−1 cm−1)) in CH3CN: 447(287).

On dissolving one of these salts in acetonitrile and addingNH4PF6, crystals of the the PF6

− salt were obtained (yield,82.8%) (Found: C, 40.55; H, 3.44; N, 10.28. C18H18N4O3F6PV([VO(O2)(tpa)]PF6) requires C, 40.47; H, 3.40; N, 10.49%). 1HNMR (270 MHz, CD3CN, TMS)/ppm: d –CH2–; 5.013 (2H, d),5.626 (2H, d), 4.577 (2H, s), pyridyl rings; 7.813 (2H, d), 8.149(2H, dt); 7.685 (2H, t), 9.455 (2H, d), 6.895 (1H, d); 7.452 (1H,dt); 7.098 (1H, t), 7.666 (1H, d).

l-Oxo-bis(chloro-tris(2-pyridylmethyl)amine-vanadium(III))hexafluorophosphate. The preparation was carried out ina Schlenk tube using methanol which was well dried overmolecular sieves in an attempt to obtain a mononuclearvanadium(III) complex of tpa. All the methanol solutionsdescribed below were deoxygenated in advance under a streamof argon gas. The ligand tpa (76.6 mg; 0.26 mmol) wasdissolved in methanol (50 cm3), and the solution was added to amethanol solution (50 cm3) of vanadium(III) chloride (41.5 mg;0.26 mmol). To the red-brown reaction mixture was addeda methanol solution of excess NH4PF6. Violet air-sensitiveneedle crystals were obtained (yield 1.3%). The product turnedout to be a l-oxo vanadium(III) dimer and the mononuclearvanadium(III) complex was not obtained (Found: C, 40.25; H,3.41; N, 10.39. C36H36N8OF12P2Cl2V2 ([{VCl(tpa)}2(l-O)](PF6)2)requires C, 40.81; H, 3.42; N, 10.58%) UV-vis (nm) in CH3CN:533.

Physical measurements

UV-visible absorption spectra were recorded on a Hitachi U3000spectrophotometer. Infrared absorption spectra were obtainedusing a Hitachi 270-30 spectrophotometer. 1H NMR spectrawere measured with a JEOL JNM-EX270 spectrometer. Cyclicvoltammograms were measured with a HOKUTO HZ-3000cyclic voltammetric analyzer at a scan rate of 100 mV s−1. Theworking and counter electrodes were a glassy-carbon disk and aplatinum wire, respectively. The sample solutions (ca. 0.001 M)in 0.05–0.08 M electrolyte–acetonitrile (0.1 M electrolyte in thecase of [VO(O2)(tpa)]PF6) were deoxygenated with a stream ofargon gas. The reference electrode was Ag/AgCl. The potentialswere recorded against internal Fc+/Fc. The cyclic voltammetrywas simulated using the DIGISIM program (version 2.1)distributed by the Bioanalytical Systems Corporation.31

Crystallographic studies

A single crystal of each of the six complexes [VO(SO4)(tpa)]·2CH3OH, [VOCl(tpa)]PF6·CH3CN, [VOBr(tpa)]PF6·CH3CN,

D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7 1 4 3 9

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[V(O)2(tpa)]PF6, [VO(O2)(tpa)]Cl·0.5CH3OH·1.5H2O, and[{VCl(tpa)}2(l-O)](PF6)2, was mounted on a glass fiber.Measurements were made on a Mercury CCD area detectorcoupled with a Rigaku AFC-8S diffractometer with graphite-monochromated Mo-Ka radiation. Final cell parameters wereobtained from a least-squares analysis of reflections withI > 10r(I). Space group determinations were made on thebasis of systematic absences, a statistical analysis of intensitydistribution, and the successful solution and refinement of thestructures. Data were collected at a temperature of −120 ◦C toa maximum 2h value of 55◦.

Data were processed using Crystal Clear.18a An empirical ab-sorption correction resulted in acceptable transmission factors.The data were corrected for Lorentz and polarization factors.All the calculations were carried out on a Silicon Graphics O2computer system using TEXSAN.18b The structures were solvedby direct methods and expanded using Fourier and differenceFourier techniques. Details of crystal parameters and structurerefinements are given in Table 1. Selected bond lengths andangles are shown in Tables 2 and 3. ORTEP18c was used to plotthe molecular structures.

CCDC reference numbers 250590-250595.See http://www.rsc.org/suppdata/dt/b4/b414532a/ for cry-

stallographic data in CIF or other electronic format.

Results and discussionPreparations of the mononuclear vanadium complexes oftris(2-pyridylmethyl)amine

In previous studies, vanadium tpa complexes were dominatedby oxo-bridged dinuclear complexes. The oxo-bridged divana-dium complexes have been characterized in three differentoxidation states; [{VIIIBr(tpa)}2(l-O)]Br2,13 [{VIV(O)(tpa)}2(l-O)](ClO4)2,14 and [{VIV,V(O)(tpa)}2(l-O)](ClO4)3.15 The vana-dium(III) dimer was prepared in ethanol. Water contaminationof the solvent appeared to be the source of the oxide bridge. Themixed solvent CH3CN–H2O was used for the preparation ofthe vanadium(IV) dimer, from which the vanadium(IV,V) mixedvalence species was derived. It appears that the use of water as asolvent tends to give l-oxo dimers. In the present study, by usingmethanol, monomeric tpa vanadium(IV) complexes have beenprepared for the first time. The dioxo complex [VV(O)2(tpa)]+

was obtained from a water-containing medium. In the caseof vanadium(III), however, our effort to obtain monomericcomplexes only gave l-oxo dimers even if the solvent wascarefully dehydrated.

The peroxo complex, [VO(O2)(tpa)]+, was obtained unex-pectedly from the monomeric vanadium(IV) complexes whenthe solution was left in contact with diethyl ether. Oxidationof the vanadium(IV) complexes to peroxo complexes was alsoobserved in the solution containing tetrahydrofuran (THF). Theoxidation did not take place when diethyl ether (or THF) wasnot present. Formation of the peroxo complex proceeds probablythrough diethyl ether (or THF) peroxide formed in the solvent.The reaction proceeded similarly in the dark, indicating that thereaction is not significantly promoted by light, if at all.

Crystal structures of the vanadium complexes oftris(2-pyridylmethyl)amine

X-Ray structural analyses have been carried out for six vana-dium tpa complexes. Table 1 shows the crystal data of thefive mononuclear and one l-oxo dinuclear complexes. ORTEPdrawings of the neutral sulfato complex, [VO(SO4)(tpa)], and thefour complex cations, [VOCl(tpa)]+, [VOBr(tpa)]+, [V(O)2(tpa)]+,and [VO(O2)(tpa)]+, are shown in Fig. 1. An ORTEP drawing ofthe dimeric cation, [{VCl(tpa)}2(l-O)]2+, is also given in Fig. 1.All the complexes have fully coordinated tpa ligands leavingtwo mutually cis positions available for anionic ligands. Selected T

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1 4 4 0 D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7

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Table 2 Selected bond lengths (A) of [VO(SO4)(tpa)], [VOX(tpa)]PF6 (X = Cl−, Br−), [VV(O)2(tpa)]PF6, [VVO(O2)(tpa)]Cl, and [{VIIICl(tpa)}2(l-O)](PF6)2

a

[VIVO(SO4)(tpa)] [VIVOCl(tpa)]PF6 [VIVOBr(tpa)]PF6 [VV(O)2(tpa)]PF6 [VVO(O2)(tpa)]Clb [{VIIICl(tpa)}2(l-O)](PF6)2

A Oterminal Oterminal Oterminal Oterminal Operoxo, Operoxo ClB Osulfato Cl Br Oterminal Oterminal Obridge

V–A 1.596(1) 1.597(2) 1.597(2) 1.630(1) 1.8595(30) 2.3570(9)1.863(3)

V–B 1.953(1) 2.3264(9) 2.4920(8) 1.629(1) 1.609(2) 1.7960(3)V–N1 2.282(1) 2.276(2) 2.279(2) 2.264(2) 2.203(3) 2.146(2)V–N2 2.114(1) 2.115(2) 2.112(2) 2.099(2) 2.1495(30) 2.105(2)V–N3 2.136(1) 2.127(2) 2.132(3) 2.311(2) 2.241(3) 2.211(2)V–N4 2.112(1) 2.107(2) 2.107(2) 2.093(2) 2.1565(30) 2.104(2)Operoxo–Operoxo 1.434(4)total sumc 8.644 8.625 8.630 8.767 8.750 8.566

a A: the ligand in trans position to N1(amino), B: the ligand in trans position to N3(pyridyl). b Average values of two independent cations in the unitcell. c Sum of four V–N bond lengths. The total sum of other V–tpa dimers: [{VIV(O)(tpa)}2(l-O)](ClO4)2,14 8.683 A; [{VIV,V(O)(tpa)}2(l-O)](ClO4)3,15

8.573 A; [{VIIIBr(tpa)}2(l-O)]Br2,13 8.583 A.

bond distances and angles of all the complexes are listed inTables 2 and 3.

The present study provides a unique opportunity to com-pare the structural characteristics of the oxovanadium(IV),dioxovanadium(V) and oxo-peroxovanadium(V) complexes con-taining identical ligand environments. Comparison may befurther extended to oxo-bridged dimers such as the newlyprepared [{VIIICl(tpa)}2(l-O)](PF6)2 and previously reported[{VIIIBr(tpa)}2(l-O)]Br2,13 [{VIV(O)(tpa)}2(l-O)](ClO4)2,14 and[{VIV,V(O)(tpa)}2(l-O)](ClO4)3.15

All three oxovanadium(IV) complexes have a distorted octahe-dral structure with the tertiary nitrogen of the tpa ligand at thetrans position to the oxo donor. The vanadium atom is shiftedfrom the equatorial plane towards the oxo ligand, as found inother oxovanadium(IV) complexes.19 The V–O(oxo) distances areca. 1.60 A which is similar to those observed in other oxovana-dium(IV) complexes.19 The geometrical characteristics of the tpaligand around the vanadium ion are remarkably similar amongthe three vanadium(IV) complexes. Thus, although the four V–N bond-distances in one complex are significantly different(2.11–2.28 A), the difference in each V–N distance among thecomplexes is rather small (within 0.01 A). The equatorial N–V–N angles in adjacent nitrogen donor atoms of tpa span the rangefrom 75 to 90◦, but each angle is again very similar (within 3.3◦)among the three complexes. The V–N(amino) distance is longerthan those of the three V–N(py) bonds which are all cis to theoxo ligand. It is noted that the V–N(py) bond (N3) that is transto the anionic monodentate ligand is ca. 0.02 A longer than theother two mutually trans V–N(py) bonds. Also the O(oxo)–V–N3(py) angle (91.3–92.9◦) is smaller than the other two O(oxo)–V–N(py) angles (98.2–109.5◦). It appears that the V–N(3) bondmay be elongated by the moderately stronger trans influence ofthe monodentate anionic ligand than that of pyridyl nitrogen.Longer distance weakens the repulsion between N3(py) and theoxo ligand to allow for smaller O–V–N angles.

The VIVO complexes may be structurally related to thedioxovanadium(V) complexes by replacing the monodentateanionic ligand by an oxide ion. Due to the strong trans influenceof the oxo ligand, the V–N3 distance (2.311(1) A) in the V(V)complex is significantly longer than those of the VIVO complexes(2.127–2.136 A). The other V–N distances are slightly shorter(0.013–0.019 A) than the corresponding ones in the VIVOcomplexes. The N–V–N bond angles of the dioxo complex aresimilar to the corresponding ones in the VIVO complexes exceptfor those involving N3 nitrogen. The angles involving N3 are

generally much smaller than those of the corresponding ones inthe VIVO complexes, as the longer V–N3 distance decreases therepulsion with the adjacent donor atoms. The coordination areaof tpa around the vanadium(V) ion is thus somewhat narrowerin angle than that in the VIVO complexes.

In the oxo-peroxo complex [VVO(O2)(tpa)]+, in trans-positionto the amino nitrogen of tpa is peroxo rather than the oxoligand. The V–N1 distance is shorter than those in the VIVOand VV(O)2 complexes, indicating that the trans influence ofthe peroxo ligand is rather modest as compared with oxo. Theother two V–N(py) distances are longer than the correspondingdistances of the dioxo complex and interestingly even than thoseof the VIVO complexes in which the oxidation state is lowerthan the peroxo complex. The tertiary nitrogen donor at thetrans-to-peroxo site appears to be a common feature for theoxo-peroxo vanadium(V) complexes with tetradentate chelat-ing ligands. Thus in the oxo-peroxo vanadium(V) complexesof N-(carbamoylmethyl)iminodiacetate,16,20 N-(2-hydroxyethyl)-iminodacetate,20,21, and N-(2-pyridylmethyl)iminodiacetate,22

the trans site to the peroxo ligand is coordinated by the amino-nitrogen.

In the case of the l-oxo divanadium(III) complex,[{VCl(tpa)}2(l-O)]2+, the trans position to the oxide bridge isoccupied by the pyridyl nitrogen rather than the tertiary one.The complex cation has a crystallographic inversion center atthe bridging oxo ligand. The geometrical structure is identicalto that of the previously reported Br analogue.13 The V–N(py)distance trans to the bridge is only moderately longer than theother two V–N(py) bonds and the trans influence of the bridgingoxide is not significant.

It can be seen from Table 2 that the difference in the V–Ndistances among all the complexes is rather small for those withoxidation states ranging from III to V, except for the distancesunder the trans influence of the oxo ligand. These observationsseem to indicate that the monodentate and bridging-oxo ligandseffectively compensate for the influence of the different oxidationstates on the V–N bonds of the tpa ligand. This conclusion maybe supported by the remarkable similarity of the total sum ofthe four V–N bonds as listed in Table 2. Although the precisemeaning of the sum is somewhat ambiguous, it is surprising thatthe total sum23 deviates by only 1.2% (0.1 A) of their averagevalue (8.664 A). We have examined the structural data of aseries of oxovanadium(IV), dioxovanadium(V) and oxo-peroxo-vanadium(V) complexes of bis(2,2′-bipyridine),24 bis(1,10-phenanthroline),24c,24d,25 hydrotris(3,5-diisopropylpyrazole),26

D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7 1 4 4 1

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Tab

le3

Sele

cted

bond

angl

es(◦ )

of[V

O(S

O4)(

tpa)

],[V

OX

(tpa

)]P

F6

(X=

Cl−

,Br−

),[V

V(O

) 2(t

pa)]

PF

6,[

VVO

(O2)(

tpa)

]Cl,

and

[{V

III C

l(tp

a)} 2

(l-O

)](P

F6) 2

a

[VIV

O(S

O4)(

tpa)

][V

IVO

Cl(

tpa)

]PF

6[V

IVO

Br(

tpa)

]PF

6[V

V(O

) 2(t

pa)]

PF

6[{

VII

I Cl(

tpa)} 2

(l-O

)](P

F6) 2

[VVO

(O2)(

tpa)

]Clb

AO

term

inal

Ote

rmin

alO

term

inal

Ote

rmin

alC

lO

2:O

pero

xoO

3:O

pero

xo

BO

sulf

ato

Cl

Br

Ote

rmin

alO

brid

geO

1:O

term

inal

A–V

–B10

5.61

(6)

100.

96(8

)98

.56(

8)10

7.49

(7)

100.

48(2

)O

2–V

–O1

106.

5(1)

O3–

V–O

110

6.3(

1)A

–V–N

116

7.24

(6)

168.

05(9

)16

8.3(

1)16

1.70

(6)

167.

39(5

)O

2–V

–N1

152.

4(1)

O3–

V–N

115

2.5(

1)A

–V–N

210

9.53

(6)

105.

56(1

0)10

5.9(

1)10

3.14

(7)

101.

25(5

)O

2–V

–N2

81.1

(1)

O3–

V–N

212

6.0(

1)A

–V–N

392

.88(

6)91

.27(

10)

91.7

(1)

89.4

9(6)

89.9

6(5)

O2–

V–N

388

.15(

10)

O3–

V–N

388

.55(

10)

A–V

–N4

98.1

5(6)

102.

46(1

0)10

2.6(

1)10

0.23

(7)

97.7

2(5)

O2–

V–N

412

5.85

(10)

O3–

V–N

481

.05(

10)

B–V

–N1

85.0

5(5)

90.8

4(6)

92.8

7(6)

90.5

2(6)

91.8

5(5)

88.3

(1)

O2–

V–O

345

.3(1

)B

–V–N

291

.80(

5)90

.29(

6)91

.73(

7)96

.19(

7)89

.95(

5)93

.1(1

)B

–V–N

316

1.51

(5)

167.

74(6

)16

9.61

(7)

162.

66(6

)16

8.73

(5)

164.

0(1)

B–V

–N4

87.7

0(5)

90.2

1(6)

89.8

5(7)

94.6

3(6)

91.8

9(5)

93.0

5(10

)N

1–V

–N2

76.5

0(5)

75.9

8(8)

76.0

9(8)

77.3

8(6)

81.2

5(7)

75.0

(1)

N1–

V–N

376

.55(

5)76

.98(

8)77

.02(

9)72

.76(

5)77

.96(

7)75

.65(

10)

N1–

V–N

474

.90(

5)75

.36(

8)74

.79(

8)74

.48(

6)79

.07(

7)74

.8(1

)N

2–V

–N3

82.1

9(5)

85.5

0(8)

83.6

2(9)

76.0

4(6)

83.8

0(7)

82.5

(1)

N2–

V–N

415

1.34

(5)

151.

35(8

)15

0.88

(9)

149.

87(6

)16

0.28

(7)

148.

9(1)

N3–

V–N

489

.31(

5)88

.03(

9)89

.74(

9)85

.47(

6)90

.90(

7)83

.2(1

)

aA

:the

ligan

din

tran

spo

siti

onto

N1(

amin

o),B

:the

ligan

din

tran

spo

siti

onto

N3(

pyri

dyl)

.bA

vera

geva

lues

oftw

oin

depe

nden

tca

tion

sin

the

unit

cell.

and N-(2-hydroxyethyl)iminodiacetate,20,21,27 and found ana-logous structural similarity to that found for the tpa ligand.

The tpa complexes of an oxometal center are still very rare, andthe only other monooxometal tpa complex so far reported is thatof ReVO, [ReVO(OCH3)(tpa)]2+.28 The trans position to the oxoligand is again occupied by an amine nitrogen. The remarkablyshort trans-to-oxo Re–N distance (1.86 A) is noted. On thecontrary, the equatorial Re–N(pyridyl) distances are in the range2.12–2.14 A which is similar to the corresponding V–N(pyridyl)distances of the oxovanadium(IV) complexes. Detailed com-parison of the bond angles of the oxovanadium(IV) complexesand [ReVO(OCH3)(tpa)]2+ indicates that the short Re–N(amine)distance is not the consequence of the geometrical requirementof the coordination of tpa, but may reflect the higher oxidationstate of Re(V). As a consequence of the short distance, tpa takesa highly stressed structure in [ReVO(OCH3)(tpa)]2+. In fact, oneof the pyridylmethyl arms is dissociated in acetonitrile solutionas shown in the 1H NMR spectrum.28a With the cooperationof a didentate chelating ligand, tpa easily gives oxorhenium(V)complexes with one dangling pyridylmethyl arm.28 Examplesare [ReVO(OCH2CH2O)(tpa)]+ and [ReVO(catecholate)(tpa)]+.Attempts to prepare similar oxovanadium(IV) complexes witha dangling pyridylmethyl arm with the aid of chelating lig-ands such as oxalate and ethyleneglycolate were unsuccessful.It is also interesting to note that trioxorhenium(VII)28a andtrioxomolybdenum(VI)29 with a tridentate tpa are known, buta dioxo complex analogous to [VV(O)2(tpa)]+ has not beenprepared so far for ReVII and MoVI.

Electronic absorption spectra of mononuclear vanadiumcomplexes of tris(2-pyridylmethyl)amine

Numerical data of the visible absorption spectra of the newcomplexes have been given in the Experimental section. Amongthe three VIVO complexes, bromo and sulfato complexes inmethanol appear to undergo partial dissociation of the coor-dinated anionic ligands as verified by their failure to obey Beer’slaw. Absorption spectra of the Br− as well as the Cl− complexeswere obtained in acetonitrile. Weak visible absorption peaks areassigned to d–d transitions. The visible absorption peak of theoxo-peroxo complex is assigned to LMCT transition as foundfor other oxo-peroxo vanadium(V) complexes.

The spectra of the VIVO complex [VIVOCl(tpa)]+ has beenmeasured in the presence of various acids in order to findinformation on the possible protonation at the oxo ligand. Fig. 2shows the spectral change in acetonitrile in the presence of p-toluenesulfonic acid. Chloride ions are added as [(n-Bu)4N]Cl(0.05 M) to avoid the possible dissociation of the coordinatedCl− from [VIVOCl(tpa)]+. The change is accompanied by theobservation of isosbestic points by the addition of up to ca.14 fold excess of the acid. Further amounts of acid causedeviation from the isosbestic points. Thus as a quantitativeanalysis was not possible the final spectrum and equilibriumconstant associated with the spectral change were not obtained.It is possible that ligand dissociation could take place in thepresence of an excessive amount of the acid. A similar spectralchange was observed on the addition of HClO4, but no spectralchange occurred in the case of added benzoic acid. The spectralchange was highly reversible as the original spectrum was fullyrecovered on the addition of triethylamine (equimolar amount tothe acid). When [(n-Bu)4N]ClO4 was added instead of HClO4, nospectral change was observed. The simple interpretation of thespectral change is protonation of the oxo ligand. It was suspectedinitially that the change might be due to some other reactions,however, the oxo ligand in the VIVO complexes were expected tobe highly acidic. With the aid of electrochemical measurements(vide infra), it was concluded that the spectral change is mostlikely due to the protonation at the oxo ligand. Thus while p-toluenesulfonic acid and HClO4 can donate a proton to the oxoligand, the weaker benzoic acid is unable to protonate it. The

1 4 4 2 D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7

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Fig. 1 ORTEP plots of the structures of (a) [VO(SO4)(tpa)], (b) [VOCl(tpa)]+, (c) [VOBr(tpa)]+, (d) [V(O)2(tpa)]+, (e) [VO(O2)(tpa)]+, and(f) [{VCl(tpa)}2(l-O)]2+ with 50% probability ellipsoids. In (f), the atoms with ‘ correspond to the atoms at equivalent positions (1 − x, −y, −z).

observed spectral change is rather small and the extent of theprotonation is not large at this concentration range of the acid.

Redox properties of mononuclear vanadium complexes oftris(2-pyridylmethyl)amine

Cyclic voltammograms (CV) of the three VIVO-tpa complexeswere measured mainly with a scan rate of 100 mV s−1. Thevoltammograms in methanol were complicated due to possibledissociation of the ligands and dimerization to form oxo-bridgedspecies by reacting with the small amount of water in thesolvent (see Fig. 3(a) for the chloro complex). The two halogenocomplexes are soluble in acetonitrile. The Cl− complex showeda quasi-reversible redox wave, but the Br− complex only gaveirreversible waves. Fig. 3 shows the CV of the chloro complex[VIVOCl(tpa)]+. The CV of the CH3CN solution in the presence

of 0.05 M [(n-Bu)4N]PF6 (Fig. 3(b)) showed a quasi-reversibleone-electron reduction wave at −1.59 V (DE1/2 = 0.08 V). Whilethe oxidation wave of Cl− was observed at ca. +1.14 V forthe solution of [VIVOCl(tpa)]+ in methanol, no oxidation waveassociated with free Cl− was observed in acetonitrile, indicatingthat the solvent CH3CN does not replace the coordinated Cl−

of [VIVOCl(tpa)]+.30 The chloride salt [(n-Bu)4N]Cl was usedinstead of [(n-Bu)4N]PF6 in order to suppress any possibleCl− dissocation, but no appreciable change was observed inthe CV (Fig. 3(c)). The quasi-reversible wave is thus assignedto the [VIVOCl(tpa)]+/[VIIIOCl(tpa)] redox process. The quasi-reversible nature of the wave was confirmed by the ratio ofanodic and cathodic currents, scan-rate dependence (see Fig.S1†), as well as DE1/2. An irreversible oxidation wave assignableto the [VVOCl(tpa)]2+/[VIVOCl(tpa)]+ process was observed atca. +1.37 V.

D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7 1 4 4 3

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Fig. 2 The change in the absorption spectrum of [VOCl(tpa)]+ inacetonitrile in the presence of 0, 4, 8, 12, 14 (solid line), and 30 (dashedline) equivalents of p-toluenesulfonic acid.

Fig. 3 Cyclic voltammograms of 0.001 M [VIVOCl(tpa)]PF6 in(a) 0.05 M [(n-Bu)4N]PF6/methanol containing 10% v/v acetoni-trile (acetonitrile was added to dissolve the complex), (b) 0.05 M[(n-Bu)4N]PF6/acetonitrile, and (c) 0.08 M [(n-Bu)4N]Cl/acetonitrilewith glassy carbon working electrode at a scan rate of 100 mV s−1.

On addition of p-toluenesulfonic acid to the 0.05 M [(n-Bu)4N]PF6 acetonitrile solution of [VIVOCl(tpa)]+, a new re-duction wave appeared at around −1.3 V at the expense ofthe original reduction wave. As Fig. 4((a) and (b)) shows,the original wave decreased to nearly half current intensityon addition of an equimolar amount of the acid, and hadcompletely disappeared by the time 2 equivalents of the acidhad been added. A new irreversible oxidation wave was observedat −0.49 V. The original CV was fully recovered by adding anequivalent amount (to the acid) of triethylamine. Judging fromthe amount of acid required for the complete disappearanceof the original wave, the final reduction product should bethe V(III) aqua species [VIII(OH2)Cl(tpa)]2+. It is reasonableto consider, however, that the reduction takes place at thehydroxo species, [VIV(OH)Cl(tpa)]2+, since further protonationto form [VIV(OH2)Cl(tpa)]3+ would be highly unfavorable asthe V(IV) hydroxo species is expected to be highly acidic. The

Fig. 4 Cyclic voltammograms of 0.001 M [VIVOCl(tpa)]PF6 in 0.05 M[(n-Bu)4N]PF6/acetonitrile in the presence of (a) 0.001 M and (b)0.002 M p-toluenesulfonic acid, 0.005 M [(n-Bu)4N]Cl and 0.05 M[(n-Bu)4N]PF6/actonitrile in the presence of (c) 0.001 M and (d) 0.002 Mp-toluenesulfonic acid, and in 0.08 M [(n-Bu)4N]Cl/acetonitrile in thepresence of (e) 0.001 M and (f) 0.002 M p-toluenesulfonic acid, withglassy carbon working electrode at a scan rate of 100 mV s−1.

reduced V(III)-hydroxo complex would be sufficiently basicto form [VIII(OH2)Cl(tpa)]2+, if protons were available. Thusthe proton-coupled reduction processes are summarized as ineqns. (1) to (3).

[VIVOCl(tpa)]+ + H+ → [VIV(OH)Cl(tpa)]2+ (1)

[VIV(OH)Cl(tpa)]2+ + e− →[VIII(OH)Cl(tpa)]+ (2)

[VIII(OH)Cl(tpa)]+ + H+ →[VIII(OH2)Cl(tpa)]2+ (3)

The reduction wave at around −1.3 V must correspond toprocess (2).

The weak oxidation wave at −0.49 V would correspond tothe oxidation of the aqua species, [VIII(OH2)Cl(tpa)]2+. Overallreactions are summarized in Scheme 1.

The current intensity of the proton-coupled reduction waveobserved at around −1.3 V is higher than the original redoxwave at −1.59 V. The intensity of the −1.3 V wave decreasedon repeated scans. Also the wave potential tends to shift in anegative direction with an increase in the amount of added acid.The current intensity further increased on addition of more than2 equivalents of the acid. These observations are reasonablyexplained by considering the overlap of the H+ reduction wave(2H+ + 2e− → H2) to the vanadium reduction wave. The H+

reduction wave could be observed in this potential region.The proton-coupled redox reactions of [VIVOCl(tpa)]+ were

also studied in the presence of different amounts of [(n-Bu)4N]Cl(Fig. 4(c–f)). In the presence of 0.08 M [(n-Bu)4N]Cl, a newquasi-reversible oxidation wave was observed at −0.89 V. Twooxidation waves at −0.89 and −0.49 V were simultaneouslyobserved in the presence of 0.005 M [(n-Bu)4N]Cl and 0.05 M [(n-Bu)4N]PF6. The reasonable interpretation of these observationsis to consider the formation of dichloro species, [VIIICl2(tpa)]+, byreaction of the electrochemically generated [VIII(OH2)Cl(tpa)]2+

with excess Cl−. The wave observed at −0.89 V is assigned to the[VIVCl2(tpa)]2+/[VIIICl2(tpa)]+ process. When the concentrationof [(n-Bu)4N]Cl is 0.005 M, two species [VIII(OH2)Cl(tpa)]2+ and[VIIICl2(tpa)]+ are in equilibrium resulting in two oxidation waves(eqn. (4)).

[VIII(OH2)Cl(tpa)]2+ + Cl− → [VIIICl2(tpa)]+ + H2O (4)

On repeated scans of the CV for the solution containing0.08 M [(n-Bu)4N]Cl, a new reduction wave was clearly observedat around −0.93 V as a counterpart of the oxidation wave at−0.86 V. These waves were more clearly observed at higherscan rates, where the successive reaction is suppressed. Fig. 5shows the 5th cycle of the CV (scan rate, 1000 mV s−1) ofthe complex in 0.08 M [(n-Bu)4N]Cl solution. The coupledwave (Epc = −0.93 V; Epa = −0.84 V) is thus assigned to

1 4 4 4 D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7

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Scheme 1 Overall reaction scheme for the proton-coupled redox reaction of [VIVOCl(tpa)]+ based on experimental results (the charge of each speciesis omitted for clarification).

Fig. 5 Fifth cycle of the cyclic voltammogram of 0.001 M[VIVOCl(tpa)]PF6 in 0.08 M [(n-Bu)4N]Cl/acetonitrile (a) in the ab-sence of p-toluenesulfonic acid and (b) in the presence of 0.002 Mp-toluenesulfonic acid with glassy carbon working electrode at a scanrate of 1000 mV s−1.

[VIVCl2(tpa)]2+/[VIIICl2(tpa)]+. The oxidation wave at ca. −1.4 Vmay be the overlap of the reduction wave of [VIV(OH)Cl(tpa)]2+

and a small contribution from the reduction wave of H+.Although the wave of the counterpart of the reduction ataround −1.2 V was not clearly seen, the re-oxidation wave ofthe hydroxo species [VIII(OH)Cl(tpa)]2+ was actually observedas a broad feature at around −1.2 V. The CV curve shown inFig. 5 is successfully reproduced by the simulation.31 The redoxreactions involved in the presence of excess Cl− and H+ arealso summarized in Scheme 1 except for the H+ reduction. Theredox potential and the relevant rate constants to reproduce theobserved CV curve are given in Scheme S1.† The simulated CVis also available as ESI† (Figs. S2 and S3). The relevant redoxpotentials are summarized in Table 4.

The CV of [VO(SO4)(tpa)] in methanol was measured in thepresence of MgSO4 in an attempt to suppress the dissociationof SO4

2−. Several irreversible reduction waves were still observedat −1.04, −1.45, and −1.70 V, as well as oxidation waves at−0.13, +0.31, and +0.79 V. The bromo complex [VOBr(tpa)]+

in 0.05 M [(n-Bu)4N]Br CH3CN gave two irreversible reduction

Table 4 Redox potentials of the complexes which appeared in cyclicvoltammograms of [VOCl(tpa)]+

[VIVOCl(tpa)]+ E1/2/V (vs. Fc+/Fc) −1.59[VIV(OH)Cl(tpa)]2+ Epc/V (vs. Fc+/Fc) −1.34[VIII(OH2)Cl(tpa)]3+ Epa/V (vs. Fc+/Fc) −0.49[VIVCl2(tpa)]2+ E1/2/V (vs. Fc+/Fc) −0.89

waves at −1.55 and −1.89 V and no oxidation waves in thepotential region up to +0.03 V (Fig. 6(a)). On addition of p-toluenesulfonic acid new waves appeared at −1.36 and −1.44 Vat the expense of the waves at −1.55 and −1.89 V, respectively,the change being complete upon addition of 2 equivalents ofthe acid. A possible interpretation is that the bromo complexis partially solvated to give [VO(CH3CN)(tpa)]2+, and both[VOBr(tpa)]+ and [VO(CH3CN)(tpa)]2+ undergo proton-assistedreduction as observed for the chloro complex.

Fig. 6 Cyclic voltammograms of (a) [VIVO(tpa)Br]Br, (b) [VV(O)2-(tpa)]PF6, and (c) [VVO(O2)(tpa)]PF6. *Reduction of remaining O2.

The VV(O)2 complex, [V(O)2(tpa)]+, in 0.05 M [(n-Bu)4N]PF6

acetonitrile shows a major irreversible reduction wave at −1.48 V(Fig. 6(b)). The wave was assigned to the reduction of the central

D a l t o n T r a n s . , 2 0 0 5 , 1 4 3 8 – 1 4 4 7 1 4 4 5

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metal ion from V(V) to V(IV) as the reduction of the ligand is notexpected to occur in this potential range. Additional irreversiblereduction waves were observed at more negative potentials,which have not been assigned precisely. No oxidation waves wereobserved in the range from −1.43 V to +1.57 V. The oxo-peroxocomplex, [V(O)(O2)(tpa)]+, in 0.05 M [(n-Bu)4N]PF6 acetonitrilealso showed an irreversible reduction wave at −1.43 V which maybe assigned to the V(V)/V(IV) process (Fig. 6(c)). In contrastto the dioxo complex, the peroxo complex showed reversibleoxidation waves at +1.37 V which may be assigned to theperoxo/superoxo redox reaction.32

It is interesting that the redox potentials of the metal centersof the series of vanadium-tpa complexes is similar betweenthe V(IV/III) process of the oxovanadium(IV) complexes andthe V(V/IV) of the dioxovanadium(V) complex (Fig. 6). Itappears that the vanadium(V) oxidation state is stabilized by thecoordination of an additional oxo ligand, and the metal redoxpotentials of the two different redox processes coincide with eachother. It is interesting to refer to the fact that the V-ligand dis-tances (except for that trans to V–O) are very similar between theoxovanadium(IV) and dioxovanadium(V) complexes (vide supra).

ConclusionA series of mononuclear complexes of vanadium in tetra- andpenta-valent oxidation states have been prepared by using atripodal tetradentate ligand tris(2-pyridylmethyl)amine (tpa).With these new complexes, [VIVOX(tpa)]n+ (n = 0, X = SO4

2−;n = 1, X = Cl−, Br−), [VV(O)2(tpa)]+ and [VVO(O2)(tpa)]+,systematic studies on the structural and redox properties ofvanadium complexes in these oxidation states have been carriedout. In previous studies, tpa tends to give oxo-bridged dinuclearvanadium(III) and (IV) complexes in water-containing solventmedia. The new mononulcear oxovanadium(IV) complexes havebeen prepared by using methanol as a reaction medium.Even with carefully dehydrated methanol, the mononuclearvanadium(III) complex was not obtained, however.

The X-ray structural determinations have revealed that thecoordination geometries of tpa including coordination bondlengths and angles are very similar among the complexes regard-less of oxidation state and, in the case of the VIV complexes, ofthe type of anionic ligands. It appears that the coordinationof the strongly negative oxo ligands significantly modifies theinfluence of the oxidation state on the metal–ligand bondlengths. The remarkable similarity of the total sum of the four V–N distances regarding tpa strongly supports this view.

The most significant result in this study is the verification ofthe proton-coupled redox behavior involving the protonationof the oxo ligand in the VIV/VIII process. Protonation to theoxo ligand in [VIVOCl(tpa)]+ has been demonstrated by theelectronic absorption change on addition of strong acids suchas p-toluenesulfonic acid and HClO4. With this information, thechange in the CV of [VIVOCl(tpa)]+ in the presence of addedp-toluenesulfonic acid (0.05 M [(n-Bu)4N]PF6) was successfullyinterpreted. Without added acid, a quasi-reversible wave wasobserved at E1/2 = −1.59 V vs. Fc+/Fc. In the presence of 2equivalents of acid, the reduction of [VIV(OH)Cl(tpa)]2+ was ob-served at ca. −1.34 V, and the reoxidation of doubly protonated[VIII(OH2)Cl(tpa)]2+ at −0.49 V. In the presence of 0.08 M [(n-Bu)4N]Cl, the dichloro complex, [VIVCl2(tpa)]2+, is formed whichis reduced at E1/2 = −0.89 V. These results provide quatitativeinformation on proton assisted redox behavior involving theprotonation at the oxo ligand in the VIVO complexes.

AcknowledgementsThis work was supported by Grant-in-Aid for Scientific Re-search on Priority Areas (No. 16033201, “Reaction Control ofDynamic Complexes”) from Ministry of Education, Culture,Sports, Science and Technology, Japan.

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