DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2347–2350 2347 Reversible proton-coupled ReVII–ReVI and ReVI–ReV couples and crystal structure of [ReVO2(OH2)(Me3tacn)]BPh4 (Me3tacn = 1,4,7- trimethyl-1,4,7-triazacyclononane) Chi-Ming Che,*,a Jack Y. K. Cheng,a Kung-Kai Cheung a and Kwok-Yin Wong b a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong The electrochemistry of [ReVIIO3(Me3tacn)]PF6 (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) in aqueous solution has been studied.At pH 1 it shows two quasi-reversible couples I at 20.14 and II at 20.36 V vs. saturated calomel electrode. Constant-potential coulometry at 20.50 V shows that the total number of electrons transferred for these two couples is two. The Pourbaix diagram over the range pH 0.9–12.2 shows that E2� 1 of couple I shifts cathodically by 60 mV per pH unit.For couple II there are two straight-line segments with slopes of 2118 mV (0.9 < pH < 4.1) and 260 mV (pH > 4.1) per pH unit. The complex [ReVO2(OH2)(Me3tacn)]BPh4 was prepared and structurally characterized by X-ray crystal analysis: monoclinic, space group P21/n (no. 14), a = 10.387(9), b = 21.176(4), c = 15.452(2) Å, b = 91.38(63)8, Z = 4. The Re]OH2 distance is 2.10(2) Å and the Re]] O distances are 1.78(1) and 1.82(1) Å. The two oxo groups are cis to each other with an angle of 106.7(5)8.Proton-coupled electron-transfer reactions constitute an important area in the oxidation chemistry of high-valent oxometal complexes. Extensive studies on the electrochemistry of some d1–d4 oxo-metal complexes in aqueous solution have been reported. With a pyrolytic edge-plane graphite electrode, it has been possible to observe the reversible M]] O æÆ M]OH couple in aqueous solution.1 The E8 values of these couples provide useful information in understanding the reactivities of these oxometal complexes.Surprisingly, there are only few reports on the proton-coupled electron-transfer reactions of d0 oxometal complexes 2 which exhibit promising oxidation chemistry. 3 It would be interesting to determine the E8 of ReVII]] O complexes, which provides a quantitative measure of the oxidizing strength of this class of complexes. Our previous work revealed that macrocyclic amines are good ligand systems for investigating the redox chemistry of high-valent oxometal complexes in aqueous solution.4 Herein is described the electrochemistry of [ReVIIO3(Me3tacn)]PF6 (Me3tacn = 1,4,7-trimethyl- 1,4,7-triazacyclononane) and the crystal structure of [ReVO2(OH2)(Me3tacn)]BPh4. The latter is one of the few examples of a six-co-ordinate d2 cis-dioxometal complexes to be structurally characterized.Experimental Materials All preparations were performed using standard Schlenk techniques. The compound Re2O7 was obtained from Strem; 1,4,7- trimethyl-1,4,7-triazacyclononane 5 and the complex [ReVIIO3- (Me3tacn)][ReO4] 6 were prepared by published procedures.Physical measurements Cyclic voltammetry was performed with a Princeton Applied Research model 273 A potentiostat. Rotating-disc voltammetry was performed with a Pine Instrument model AFMRX rotator. The working electrode used was edge-plane pyrolytic graphite (Union Carbide). The E2� 1 values were taken as the average of the anodic and cathodic peak potentials.All potentials are quoted with reference to the saturated calomel electrode (SCE). Constant-potential electrolysis was performed in a threecompartment cell under a nitrogen atmosphere with a carbon cloth obtained from Sigri Carbon as working electrode. The UV/VIS absorption spectra were recorded on a Milton Roy 3000 spectrophotometer. X-Ray crystallography A pale blue crystal of complex 2 of dimensions 0.20 × 0.05 × 0.35 mm was used for data collection at 25 8C on a Rigaku AFC7R diffractometer with graphite-monochromatized Mo-Ka radiation (l = 0.710 73 Å) using w–2q scans with w-scan angle (0.73 1 0.35 tan q)8 at a scan speed of 16.08 min21 [up to four scans for reflection I < 15s(I )].Intensity data (in the range of 2qmax = 508; h 0–11, k 0–23, l 217 to 17; three standard reflections measured after every 300, 3.67% decay), were corrected for decay and for Lorentz-polarization effects and empirical absorption corrections were applied based on the y scan of four strong reflections (minimum and maximum transmission factors 0.391 and 1.000).Upon averaging the 6401 reflections, 6054 of which were uniquely measured, 3225 with I > 3s(I) were considered observed and used in the structural analysis. The space group was determined from systematic absences and the structure solved by direct methods (SIR 927) and Fourier-difference syntheses and refined by full-matrix least squares using the software package TEXSAN8 on a Silicon Graphics Indy computer.All non-H atoms except the B atom were refined anisotropically. Hydrogen atoms at calculated positions with thermal parameters equal to 1.3 times that of the attached C atoms were not refined. The two H atoms of the H2O molecule were not included. Convergence for 366 variable parameters by least-squares refinement on F with w = 4Fo 2/ s2(Fo 2), where s2(Fo 2) = [s2(I) 1 (0.026 Fo 2)2] for 3225 reflections with I > 3s(I), was reached at R = 0.053 and R9 = 0.068.The final Fourier-difference map was featureless. The ORTEP9 drawing of the complex cation shows thermal ellipsoids at the 50% probability level. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 186/513.2348 J.Chem. Soc., Dalton Trans., 1997, Pages 2347–2350 Syntheses [ReVIIO3(Me3tacn)]X 1 (X = PF6 2 or BPh4 2). The complex [ReVIIO3(Me3tacn)][ReO4] (0.1 g) was dissolved in distilled water (30 cm3). The solution was filtered if necessary and addition of an excess of NH4PF6 or NaBPh4 caused immediate precipitation of [ReVIIO3(Me3tacn)]PF6 or [ReVIIO3(Me3tacn)]BPh4. The product was filtered off and washed with water and diethyl ether.Yield 80%. 1H NMR (CD3CN) of [ReVIIO3(Me3tacn)]- BPh4: d 7.29–7.20 (m, 8 H, Haryl), 7.05–6.95 (m, 8 H, Haryl), 6.87– 6.82 (m, 4 H, Haryl), 3.54–3.36 (m, 6 H, CH2), 3.30 (s, 9 H, Me) and 3.3–3.14 (m, 6 H, CH2). [ReVO2(OH2)(Me3tacn)]BPh4 2. A mixture of [ReVIIO3- (Me3tacn)]BPh4 (0.1 g) and an excess of zinc dust in aqueous MeOH (30 cm3, 95%) was refluxed for 4 d. The reaction mixture was filtered through Celite and the filtrate evaporated to dryness.The pale green residue was extracted with acetonitrile. Diffusion of diethyl ether into the acetonitrile extract gave pale green crystals; yield 5%. 1H NMR (CD3CN): d 7.29–7.20 (m, 8 H, Haryl), 7.05–6.95 (m, 8 H, Haryl), 6.87–6.82 (m, 4 H, Haryl), 3.88–3.52 (m, 2 H, CH2), 3.45–3.32 (m, 2 H, CH2), 3 41 (s, 3 H, Me), 3.16 (s, 6 H, Me), 3.14–3.04 (m, 3 H, CH2) and 2.83–2.75 (m, 5 H, CH2) [Found (Calc.): C, 54.1 (54.55); H, 6.20 (5.92); N, 5.40 (5.78)%]. Results and Discussion d0 Oxometal complexes have long been known to be effective oxidative catalysts and one of the recent examples is methyltrioxorhenium( VII).3 However, no electrochemical study has been carried out on the ReVII]] O æÆ ReV]OH couple.The only analogous example is [MoO3(Me3tacn)],2 which shows a quasireversible two-electron four-proton transfer couple at E8 = 20.075 V vs. the normal hydrogen electrode in 0.1 mol dm23 MeSO3H, equation (1). [MoVIO3(Me3tacn)] 1 2e2 1 4H1 [MoIVO(OH2)2(Me3tacn)]21 (1) In this work, the cyclic voltammograms of [ReVIIO3(Me3- tacn)]PF6 recorded at edge-plane pyrolytic graphite at various pH are shown in Fig. 1. Two quasi-reversible couples I and II are observed. At pH 1.0 the E2� 1 values of I and II are 20.14 and 20.36 V vs. SCE respectively. Constant-potential couloat 20.50 V indicated that the total number of electrons transferred for these two couples is two. As shown in Fig. 2, the plateau currents for couples I and II from a rotating-disc voltammetric experiment are similar in magnitude; hence, it is reasonable to assign them to the ReVII]ReVI and ReVI]ReV couples respectively. The reversibility of these two couples and particularly that of II depends on pH.At pH < 7 the peak-topeak separations for couples I and II are much larger than the 60 mV value expected for a reversible one-electron couple. For couple II and at low pH the reoxidation wave is usually broad. The two couples become more reversible at higher pH.With reference to previous electrochemical studies on Ru]] O æÆ Ru]OH2 couples,10 the above finding is ascribed to the kinetic barrier associated with protonation/deprotonation electron-transfer reactions. The cyclic voltammetry data are listed in Table 1 and the Pourbaix diagram over the range pH 0.9–12.2 is shown in Fig. 3. At pH 0.9–12.2 the E2� 1 of couple I shifts cathodically by 60 mV per pH unit. For couple II two straight-line segments with slopes of 2118 (0.9 < pH < 4.1) and 260 mV per pH unit (pH > 4.1) are found.On the basis of these results, the electrode reactions are assigned as in equations (2)–(6). For couple II, the electrochemical data could not differentiate [ReVO(OH2)(OH)(Me3tacn)]21 from [ReV- (OH)3(Me3tacn)]21 or [ReVO2(OH2)(Me3tacn)]1 from [ReVO- (OH)2(Me3tacn)]1. Attempts to isolate the reduced species of 1 by constant-potential electrolysis were unsuccessful but chem- Couple I [ReVIIO3(Me3tacn)]1 1 H1 1 e2 [ReVIO2(OH)(Me3tacn)]1 (2) Couple II 0.9 < pH < 4.1 [ReVIO2(OH)(Me3tacn)]1 1 2H1 1 e2 [ReVO(OH2)(OH)(Me3tacn)]21 (3) or [ReVIO2(OH)(Me3tacn)]1 1 2H1 1 e2 [ReV(OH)3(Me3tacn)]21 (4) pH > 4.1 [ReVIO2(OH)(Me3tacn)]1 1 H1 1 e2 [ReVO2(OH2)(Me3tacn)]1 (5) or [ReVIO2(OH)(Me3tacn)]1 1 H1 1 e2 [ReVO(OH)2(Me3tacn)]1 (6) ical reduction of 1 by zinc powder in aqueous methanol (95%) afforded [ReVO2(OH2)(Me3tacn)]BPh4 2 which was isolated in low yield.Fig. 1 Cyclic voltammogram of [ReVIIO3(Me3tacn)]1 at various pH; scan rate 100 mV s21 Fig. 2 Rotating-disc voltammetric studies of [ReVIIO3(tmtacn)]1 at pH 5.9; scan rate 5 mV s21, disc rotation rate 400 revolutions min21J. Chem. Soc., Dalton Trans., 1997, Pages 2347–2350 2349 Fig. 4 shows the perspective view of [ReVO2(OH2)(Me3tacn)]1, crystallographic details are given in Table 2 and selected bond lengths and angles in Table 3. The co-ordination geometry of the rhenium atom is a highly distorted octahedron comprised of three nitrogen atoms from the macrocyclic tmtacn and three oxygen atoms from one aqua and two cis oxides.The Re]O(1) and Re]O(2) distances of 1.78(1) and 1.82(1) Å are slightly longer than the average bond distance of 1.761 Å for transdioxorhenium( V) complexes.11 Interestingly, these distances are even 0.06 Å longer than those in cis-[ReVO2(bipy)(py)2]1 (bipy = 2,29-bipyridine, py = pyridine) average Re]O 1.74 Å).12 This may be correlated with the O]] Re]] O angle, which has a smaller value in 2 [106.7(5)8] than in cis-[ReVO2(bipy)(py)2]1 [121.4(4)8].Indeed, the O]] Re]] O angle of 2 is comparable to the related values of 112.0(4) and 105.9(2)8 in cis-[RuVIO2L]21 (L = Table 1 Values of E2� 1 (V vs. SCE) for redox couples of [ReVIIO3- (Me3tacn)]1 at different pH values pH Couple I Couple II 0.9 20.14 20.36 1.8 20.20 20.51 2.6 20.25 20.54 3.6 20.30 20.68 4.7 20.36 20.76 5.9 20.44 20.86 7.0 20.52 20.91 8.0 20.58 20.99 9.2 20.63 21.03 9.9 20.67 21.07 11.4 20.74 21.12 12.2 20.80 21.19 Table 2 Crystal data for complex 2 Empirical formula M C33H43BN3O3Re 726.74 Crystal colour, habit Blue, plate Crystal dimensions/mm 0.20 × 0.05 × 0.35 Crystal system Monoclinic Space group P21/n (no. 14) a/Å 10.387(9) b/Å 21.176(4) c/Å 15.452(2) b/8 91.38(3) U/Å3 3397(2) Z 4 Dc/g cm23 1.421 F(000) 1464 m(Mo-Ka)/cm21 36.12 No. reflections measured 6401 No. unique reflections (Rint) 6054 (0.034) Function minimized Sw(|Fo| 2 |Fo|)2 No.observations [I > 3.00s(I)] 3225 No. variables 366 R, R9 0.053, 0.068 Goodness of fit 2.23 Maximum shift/error in final cycle 0.01 Maximum, minimum peaks in final difference map/e Å23 1.34, 21.01 Table 3 Selected bond lengths (Å) and angles (8) for complex 2 Re]O(1) 1.78(1) Re]O(2) 1.82(1) Re]O(3) 2.10(2) Re]N(1) 2.199(8) Re]N(2) 2.26(1) Re]N(3) 2.26(1) O(1)]Re]O(2) 106.7(5) O(1)]Re]O(3) 101.2(5) O(1)]Re]N(1) 87.5(4) O(1)]Re]N(2) 162.1(4) O(1)]Re]N(3) 89.6(5) O(2)]Re]O(3) 99.4(5) O(2)]Re]N(1) 90.2(5) O(2)]Re]N(2) 84.9(4) O(2)]Re]N(3) 160.0(5) O(3)]Re]N(1) 164.5(5) O(3)]Re]N(2) 90.1(5) O(3)]Re]N(3) 88.4(5) N(1)]Re]N(2) 78.7(4) N(1)]Re]N(3) 78.7(4) N(2)]Re]N(3) 76.6(4) N,N,N9,N9-3,6-hexamethyl-3,6-diazaoctane-1,8-diamine) 13 and [MoVIO2(OMe)(Me3tacn)]1 respectively.14 Interestingly, the Mo]] O of 1.786 Å in [MoVIO2(OMe)(Me3tacn)]1 is also larger than that in other oxomolybdenum complexes.The Re]O(3) distance in 2 of 2.10(2) Å is indicative of a single bond and as expected longer than that of the Re]O (alkoxide) distance of 1.94 Å in [ReVO(O2C2H4)(Me3tacn)]1.15 On comparing with the complexes [ReVOCl2(tu)2(H2O)]1 and [ReVOCl3(tu)(H2O)] (tu = thiourea) for which the Re]OH2 distances are 2.231(10) and 2.291(20) Å respectively,16,17 it is reasonable to assign O(3) to a co-ordinated aqua group.On the basis of the 18-electron rule, the Re]] O bonds in 2 are double in character and exert a trans effect on the Re]N(2) and Re]N(3) bonds which are 0.06 Å longer than Re]N(1), which is trans to O(3).Such a trans effect of the ReV]] O bonds is less important than that in the [ReVO- (O2C2H4)(Me3tacn)]1 complex. In the latter case, the O]ReV bond is considered as triple in character [D(Re]Ntrans to oxo) 2 (Re]Ncis to oxo) = 0.09 Å]. The UV/VIS spectrum of complex 2 is shown in Fig. 5. It Fig. 3 Pourbaix diagram of [ReVIIO3(Me3tacn)]1 over the range pH 0.9–12.2 Fig. 4 Perspective view of [ReVO2(OH2)(Me3tacn)]12350 J.Chem. Soc., Dalton Trans., 1997, Pages 2347–2350 shows similar low-energy d–d transition(s) to those of [ReO(O2C2H4)(tacn)]1 and [ReO(OMe)2(tacn)]1 (tacn = 1,4,7- triazacyclononane).15 Based on these findings and the result from electrochemical studies, the electrode reactions of 1 can be assigned as (2) and (3) at 0.9 < pH < 4.1, and (2) and (5) at pH > 4.1. The break point of the E2� 1 versus pH plot for couple II occurs at pH 4.1, which is assigned to the pKa value of [ReVO(OH)(OH2)(Me3tacn)]21. There are relative few electrochemical studies on the d0 M]] O æÆ d2 M]OH2 couple.Compared to the E8 of [MoO3(Me3tacn)] (20.075 V vs. normal hydrogen electrode) at the same pH, 1 is less oxidizing. Acknowledgements We acknowledge support from the University of Hong Kong, the Hong Kong Polytechnic University, the Croucher Foundation and the Hong Kong Research Grants Council. Fig. 5 Room-temperature absorption spectrum of complex 2 measured in acetonitrile solution (7.8 × 1024 mol dm23) References 1 C.M. Che and V. W. W. Yam, Adv. Inorg. Chem., 1992, 39, 233; Adv. Transition Met. Coord. Chem., 1996, 1, 209. 2 W. Herrmann and K. Wieghardt, Polyhedron, 1986, 5, 513. 3 W. A. Herrmann, W. Wagner, U. N. Flessner, U. Volkhardt and H. Komber, Angew. Chem., Int. Ed. Engl., 1991, 30, 1636; W. A. Herrmann, R. W. Fischer and D. W. Marz, Angew. Chem., Int. Ed. Engl., 1991, 30, 1638; W.A. Herrmann and M. Wang, Angew. Chem., Int. Ed. Engl., 1991, 30, 1641; Z. Zhu and J. H. Espenson, J. Org. 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Johnson, ORTEP II, Report ORNL-5318, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. 10 C. M. Che, K. Y. Wong and F. C. Anson, J. Electroanal. Chem. Interfacial Electrochem., 1987, 226, 211. 11 J. M. Mayer, Inorg. Chem., 1988, 27, 3899. 12 R. L. Blackbourn, L. M. Jones, M. S. Ram, M. Sabat and J. T. Hupp, Inorg. Chem., 1990, 29, 1791. 13 C. K. Li, C. M. Che, W. F. Tong, W. T. Tang, K. Y. Wong and T. F. Lai, J. Chem. Soc., Dalton Trans., 1992, 2109. 14 K. S. Bürger, G. Haselhorst, S. Stötzel, T. Weyhermüller, K. Wieghardt and B. Nuber, J. Chem. Soc., Dalton Trans., 1993, 1987. 15 G. Böhm, K. Wieghardt, B. Nuber and J. Weiss, Inorg. Chem., 1991, 30, 3464. 16 T. Lis, Acta Crystallogr., Sect. B, 1976, 32, 2707. 17 T. Lis, Acta Crystallogr., Sect. B, 1977, 33, 944. Received 24th February 1997; Paper 7/01300K