摘要:
J. CHEM. SOC. DALTON TRANS. 1994 895Electrochemical Oxidation of Diaquaruthenium(l1)Complexes of Quaterpyridines and Crystal Structureof [ RuL1( PPh,),] [CIO,]; ( L1 = 3",5,5',5"'-tetramethyl-2,2' : 6',2" : 6",2"'-quaterpyridine) tChin-Wing Chan, Ting-Fong Lai and Chi-Ming Che"Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong KongThe perchlorate salts of [RuL~(OH,),]~+ 1 and [RuL~(OH,),]~+ 2 (L2 = 2,2':6',2":6",2"'-quaterpyridine.L' = 3",5,5',5"'-tetramethyl-2,2': 6',2": 6".2"'-quaterpyridine) have been isolated. In acidic solutionsboth 1 and 2 exhibit reversible RuV'-Ru'" and Ru~~'-Ru" couples, though the RU~~-RU"' couples areonly quasireversible. The effect of pH on the €; of these couples has been investigated. At pH 1, the €"values of the [RuL~O,]~+-[RUL~(O)(OH,)]~+ and [RuL~O,]~+-[RUL~(O)(OH~)]~+ couples are 1.12 and1.05 V vs.saturated calomel electrode respectively. The crystal structure of [ RuL'( PPh,),] [CIO,],*MeCN-H,O has been determined: monoclinic space group P2,/n, a = 13.21 O(4). b = 29.71 1 (7),c =14.878(3) A, p = 98.89(2)", Z = 4. In this complex the quaterpyridine ligand adopts a non-planarconformation with a dihedral angle of 28.3" between the two central pyridyl rings. The two triphenyl-phosphine ligands are in a trans configuration [P(l)-Ru-P(2) 177.6"]. The complex [RUL~(OH,),]~+is an active catalyst for the electrochemical oxidation of propan-2-01,Ruthenium occupies its special niche in redox chemistrybecause a repertoire of oxidation states are accessible withina narrow potential range from diaquaruthenium(I1).Thecomplex rrans-[R~(tmc)O,]~ + (tmc = 1,4,8,11 -tetramethyl-1,4,8, I I-tetraazacyclotetradecane)' could be generated at 0.66 V(us. saturated calomel electrode, SCE) in pH 1 aqueous solution.Under the same conditions, incorporation of chelating pyridylgroups into the co-ordination sphere of ruthenium wouldincrease the E" of trans-dioxoruthenium(v1) to 0.89 V (us.SCE) in [RuLO,]' + [L = N,Nf-dimethyl-N,N'-bis(2-pyridyl-methyl)propylenediamine]2 and even 1.01 V in [Ru(bipy),-O2]' + (bipy = 2,2'-bi~yridine).~ These high-valent oxo-ru-thenium species have been shown to be competent oxidantsfor organic ~ x i d a t i o n . ~ , ~We have been interested in investigating the photo-physical and electrochemical properties of transition-metalcomplexes containing polypyridine ligands.In this context,the two tetradentate chelating ligands 3",5,5',5"'-tetramethyl-2,2' : 6',2" : 6",2"'-quaterpyridine (L')6 and 2,2' : 6',2": 6",2"'-quaterpyridine (L2)7 are of interest to us. Previous studiesby Lehn,' C~nstable,~ and Che' and their co-workers havedemonstrated the versatile co-ordination chemistry of L' andL2. We expect that dioxoruthenium(vr) complexes of theseligands, if generated, are strongly oxidizing and may be stablewith regard to ligand dissociation. Herein is described thesynthesis and electrochemistry of tran~-[Ru"L~(OH,),]~+and rran.~-[Ru"L'(OH,),]~+ and the X-ray crystal analysisof rran.s-[ R u" L ( PPh,),] [ClO,] , .ExperimentalMatt.riu/s.- -The quaterpyridines L' (ref.6 ) and Lz [ref. 7(a)]were prepared as described, as were [RU(OHZ),] [p-MeC6H4-S03]z9 and [Ru(PPh,),(MeCO,),].'O All chemicals were ofanalytical grade and used as received. Water for electrochemicalstudies was distilled twice from KMnO,, and acetonitrilesuccessively from KMnO, and CaH,.-f Supplrmentur.i~ duta available: see Instructions for Authors, J. Chem.Soc., Dalton Truns., 1994, Issue 1, pp. xxiii-xxviii.A' 4 "'4 4"2, 2':6', 2":6", 2"'-quaterpyridineR = H or MeZ = MeCN, H20, or PPh3Physical Measurements and Instrumentation.-Infrared spec-tra were measured on a Nicolet 20 SXC FT-IR spectrometer,UVjVIS spectra on a Milton Roy Spectronic 3000 spectro-photometer and 'H NMR spectra on a JEOL GSX 270 FT-NMR spectrometer with SiMe, as internal standard.Elementalanalyses were performed by the Shanghai Institute of OrganicChemistry, Chinese Academy of Science and the Departmentof Chemistry at National Taiwan University.Cyclic voltammograms were measured using a PrincetonApplied Research (PAR) model 273 potentiostat and recordedon a Kipp & Zonen X-Y recorder. A conventional two-compartment cell was used in all measurements. The workingedge-plane pyrolytic graphite and auxiliary platinum electrodeswere separated from the saturated calomel electrode by 896 J . CHEM. SOC. DALTON TRANS. 1994sintered-glass disc. The surface of the graphite electrode wastreated as described before." The potentials (E") of allreversible couples were reported as estimates of (Epa + Epc)/2,and the Epa values were recorded for irreversible electrodeprocesses.Crystal Structure Determination.-Crystal data. C,,H5,-Cl,N,O,P,Ru~MeCN~H,O, A4 = 1250.09, monoclinic, spacegroup P2,/n, a = 13.210(4), b = 29.711(7), c = 14.878(3) A,p = 98.89(2)", U = 5769(4) A3, T = 296 K, D, = 1.45 g cm-3,2 = 4, D, = 1.44 g crn-,, F(OO0) = 2576, p(Mo-Ku) = 4.73cm '.Crystal dimensions 0.07 x 0.21 x 0.35 mm.Data collection and processing. A dark red prism of[RuL'(PPh,),][CIO,], was obtained by slow evaporation ofan acetonitrile solution of the complex at room temperature.Diffraction data were collected on an Enraf-Nonius CAD4diffractometer (graphite-monochromated Mo-Ka radiation,h = 0.71073 A) in -20 mode.The unit-cell dimensions wereobtained from a least-squares fit of 25 reflections in the range20 < 20 -= 30". Of I 1 801 data collected (at 20,,, = 52" andrange hk ? 1), 11288 were considered unique. The data werecorrected for Lorentz and polarization effects, but not forabsorption. Three reflections were monitored every 2 h, butshowed no significant variation in intensity. Calculations werecarried out on 7627 reflections [m, F, > 30(F0)] and 698parameters ( p ) with a Micro Vax I1 computer using the Enraf-Nonius SDP programs.12 The position of the ruthenium atomwas obtained from a Patterson synthesis, and the rest of thenon-hydrogen atoms were located from subsequent Fouriermaps. Refinement was by full-matrix least squares with aweighting scheme w = 4FO2/[o2(Fo2) + (0.055F02)2].One ofthe perchlorate groups was disordered and appeared to rotateabout one of its Cl-0 bonds. The three disordered oxygenatoms were located at six idealized sites from a Fourierdifference map, each with an occupancy factor of 0.5, and theirparameters were not adjusted in subsequent refinement. Duringthe final least-squares cycles all non-hydrogen atoms except theoxygen atom in the water molecule were refined anisotropically.The final agreement factors R(Fo) = E/IFoI - ~ F c ~ ~ / Z ~ F o ~ , R' =p)]' were 0.058, 0.080, and 1.96, respectively. The hydrogenatoms in the methyl groups and water molecule were omittedwhile all other hydrogen atoms at calculated positions wereincluded in the structure-factor calculation.The final Fourierdifference map contained extrema at 1.89 and - 1.00 eTable 1 lists the atomic coordinates for the non-hydrogenatoms, Table 2 selected bond distances and angles.Additional material available from the Cambridge Crystallo-graphic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond lengths and angles.CZyf-,I - I ~ c 1 ) 2 / ~ ~ I ~ 0 1 2 1 4 and s = C W I F O I - IFc1>2/(m -Synthe~is.-[RuL~(OH,)~][ClO~]~ 1. A solution of(40.4 mg, 1.4 equivalents) in degassed 95% ethanol (100 cm3)was refluxed under a nitrogen atmosphere for 3 h. The bluishpurple solution was filtered while warm, treated with an excessof sodium perchlorate, concentrated to about 10 cm3 underreduced pressure, and kept at 4 "C for 2 h.A purple solid (23mg) was isolated in 27% yield (Found: C, 37.4; H, 3.1; N, 8.3.Calc. for C,,Hl,CI,N,Ol,Ru: C, 37.15; H, 2.80; N, 8.65%). IR(cm-'): 3439s, I143s, 1120s, 1088s, 769s, 639m and 627m.UVjVIS in 0.1 mol dm-, CF,CO,H (h,,,/nm, 10-'&/dm3 mol-'cm-'): 281 (24.3), 291 (26.6), 320 (23.7), 348 (8.4), 480 (2.8) and552 (2.8).[RuL'(OH,),][CIO~]~ 2. The above procedure was adoptedexcept that L' was used. Yield 22%. IR (cm-'): 3439s, 2980w,1 143s, 1120s, 1088s, 769s, 639m and 627m. UV/VIS in 0.1 moldm-, CF,CO,H (h,,,/nm, 10-3~/dm3 mol-' cm-'): 300 (20),330 (20), 355 (8), 460 (3) and 550 (3).[RuL'(PPh,),][ClO,], 3. A mixture of diacetatobis-(triphenylphosphine)ruthenium(rr) (222 mg, 3 x lo-, mol)[RU(OH,),]Cp-MeC,H,SO,], (50 mg, 9.1 X lop5 mol) and L2and L' (100 mg, 2.73 x lo-, mol) in degassed methanol(120 cm3) was stirred under a nitrogen atmosphere at roomtemperature for 45 h.The reddish solution was filteredpromptly, treated with an excess of lithium perchlorate, andconcentrated to about 50 cm' under reduced pressure. Themicrocrystals formed were filtered off, washed with a fewcm3 of cold methanol and dried under reduced pressure. Yield72% (Found: C, 60.3; H, 4.05; N, 4.60. Calc. for C,,H5,Cl,N,-O,P,Ru: C, 60.50; H, 4.40; N, 4.70%). 'H NMR (CD,CN):6 9.74 (s, H6,6"'), 7.60-7.31 (m, H3*4*3'94'~"95"33'''*4'"), 7.43-7.35(m, HJ, 7.15-7.09 (t, Hb), 6.67-6.60 (m, Ha), 2.49 (s, 5'- and3"-CH,), and 2.40 (s, 5- and 5"'-CH,).IR (cm-'): 1483w,1120m, 1089s, 700m, 624m and 517m. UV/VIS in MeCN(h,,,/nm, 10-,&/drn3 mol-' cm-'): 267 (76), 297 (35), 312 (35),344 (18), 358 (24), 415 (4.1) and 450 (4).[RuL~(PP~,),][CIO,]~ 4. The above procedure for pre-paration of complex 3 was adopted except that L2 was used.Yield was 33%. 'H NMR (CD,CN): 6 9.93-9.53 (m, H6,6"')) (Found: C,58.9; H, 4.1; N, 4.8. Cak. for C5,H,,C1,N,0,P,Ru: C , 59.25;H,3.90; N, 4.95%). IR (cm-'): 1476m, 1144s, 1120s, 1108s, 775m,749m, 699m, 624m and 518m. UV/VIS in MeCN (h,,,/nm,10-3&/dm-3 mol-' cm-I): 268 (33), 288 (26), 333 (12), 348 (14),41 6 (2.8), 463 (3.4) and 523 (2.0).[RuL2(MeCN),][C10,], 5. The compound L2 (52 mg,1.7 x mol) and [RU(OH,),]Cp-MeC,H,SO,], (88 mg,1.0 x lo-, mol) were refluxed in degassed acetone-methanol(1 : 1 v/v, 50 cm3) for 48 h.The solution was reduced to half itsoriginal volume under reduced pressure. A black solid wasfiltered off, redissolved in acetonitrile and treated with anexcess of lithium perchlorate. Recrystallization by ether dif-fusion afforded dark red needles (23 mg) in 20% yield (Found:C, 41.55; H, 2.90; N, 12.00. Calc. for C,,H,,C12N,0,Ru: C,41.60; H, 2.90; N, 12.15%). 'H NMR (CD,CN): F 9.40 (ddd,H6,6"'), 8.42 (dd, H3'.5',3".5'' ), 8.35-8.32 (dd, H333'"), 8.27 (td,H4g4"'), 8.15 (t, H4'"''), 7.87 (ddd, H5v5"') and 1.79 (s, CH,CN).IR (cm-'): 1 105s, 1089s, 768s and 623s. UV/VIS in MeCN(h,,,/nm, 10-3~/dm3 mol ' cm-'): 283 (42.1), 327 (15.1), 342(20.0), 376 (1.8), 420 (3.2), 476 (5.0) and 501 (4.3).and 8.17-6.55 (m, C6H5 and H3-5.3"5'."-5''.3'''-5"'Results and DiscussionProperties, Crystal and Molecular Structure.-There havebeen a number of studies on 2,2' : 6',2" : 6",2"'-quaterpyridinecomplexes.7 , 8 Although considerable strain in the planar tetra-dentate co-ordination mode has been noted in transition-metal complexes, 7 * 8 isolation of ruthenium(I1) quaterpyridinecomplexes is possible with [Ru(OH,)J2 + and [Ru(MeCO,),-(PPh,),] as starting materials.Both [RuL ' (OH,),][ClO,] , and [RuL2(OH,),] [CIO,],are less stable than expected. Attempts to grow suitable crystalsof trans-[RuL2(OH,),][C10,], for X-ray crystal analysiswere unsuccessful. Solid samples of the diaqua complexesare susceptible to air oxidation especially in the presence ofmoisture.Purple solutions of these complexes turn brownishgreen when exposed to air within a few hours. This is notunexpected since the related tr~ns-[Ru(bipy),(OH~)~]~ + complex is unstable upon prolonged storage. 'The first unequivocal evidence for the tetradentate co-ordination mode of L2 came from the crystal analysis of[CoL2(0H,)(S0,)]N0,~H20. l4 In contrast, the methyl groupsin L' should prevent it from acting as a planar tetradentateligand as in the case of [CU,L'][C~O,]~~H,O.~ Tetradentateco-ordination of L' is possible when there are strong metal-ligand interactions as in the case of [PtL']2+.8" To ourknowledge, no crystal structure has been reported onmononuclear ruthenium(r1) complexes of L' or L2.Thatdescribed here consists of, in addition to quaterpyridine, twobulky triphenylphosphine ligands in a trans configuration. Aperspective view of the complex cation [RuL1(PPh,),12 + isshown in Fig. 1. The two bipyridyl units are twisted with J . CHEM. SOC. DALTON TRANS. 1994 897Fig. 1 Perspective view of [RuL'(PPh,),][ClO,],200 300 400 500 600 700hfnmFig. 2CF,CO,H aqueous solutionThe UViVIS spectrum of [RUL~(OH,),]~+ in 0.1 mol dm-,dihedral angle of = 28", indicating the inherent instability of L'in the tetradentate co-ordination mode. On the other hand, thetwo bulky phosphine ligands only slightly deviate from a transconfiguration [P( 1)-Ru-P(2) 177.6"].The UVNIS absorption spectra of all ruthenium(I1)quaterpyridine complexes are similar.Basically, there areintense intraligand transitions around 300 nm (E > lo4 dm3mol ' cm ') and metal-to-ligand charge transfer (m.1.c.t.)transitions from 400 to 500 nm (E > lo3 dm3 mol-' cm-')(Fig. 2). The positions of the absorption bands for complexes1 and 2 are similar to those of trans-[R~(bipy),(OH,),]~ + .'Electrochemical Studies.-Electrooxidation of trans-diaqua-ruthenium(r1) complexes in aqueous media have been welld o c ~ m e n t e d . ~ . ~ In aqueous media both [RUL~(OH,),]~+ and[RuL'(OH,)J2 + display very similar cyclic voltammograms tothose of tr~ns-[Ru(tmc)O,]~ + and ~rans-[Ru(bipy),(OH,)]~ +except with a shift in El values. Fig. 3 shows the cyclicvoltammogram of complex 1 in pH 1 solution. As for theoxidation of trans-diaquaruthenium(I1) to trans-dioxoruthen-ium(vI), there are three reversible couples: I, at 0.58 V with apeak-to-peak separation (AE,) z 50 mV, I1 at 0.91 V with AEp> 100 mV, and I11 at I .12 V with AE, 30-40 mV. For complex 2in pH 1 solution, the corresponding three redox couples are at0.46,0.90 and 1.05 V.Studies on the effect of pH on E+ values revealed that20 pAI11 I1 II I I I I I 11.4 1 .o 0.6 0.2EIVvs. SCEFig. 3 Cyclic voltammogram of [RUL~(OH,),]~+ in 0.1 mol dm-3CF,CO,H aqueous solution. Insert: repeat cycling between 1 .O and1.3 V; scan rate 50 mV s-'these are all proton-coupled electron-transfer processes. ThePourbaix diagrams shown in Fig. 4(a) and 4(h) cover the pHrange from 1 to 9 for complex 1 and from 1 to 10 for 2.Sinceboth 1 and 2 are unstable in aqueous solution upon prolongedstanding, we found it very difficult to determine the n values ofthe redox couples 1-111 by coulometric experiments. However,with reference to previous studies on the well established trans-dioxoruthenium(v1) systems, these couples can be assigned asin equations (1)-(5). As expected the E" values are almostCouple Ia (pH z 1):[RU~"L(OH,),]~+ + e - [Ru"L(OH,)~]~+ (1)Couple Ib (pH 1.4-6.0):[Ru"'L(OH)(OH,)]~+ + e + H+ - [ Ru"L( OH , ) ,] + (2)Couple Ic (pH 3 6):[Ru"'L(OH),]+ + e + 2H+ - [RU"L(OH,),]~' (3)Couple I1 (pH 1-6):[Ru'vL(0)(OH,)]2~ + e + H + --+[Ru"'L(OH)(OH ,)I + (4)Couple 111 (pH 1-6):[RuV1L0,l2+ + 2e + 2H' --+ [RU'~L(O)(OH,)]~+ (5)constant from pH 1 to 1.4 for 1 and 2.At pH I .46.0, the beststraight lines for E" values have slopes of -56 and -60 mVper pH unit for 1 and 2, respectively. This accounts for a oneproton-one electron couple. At pH >6, the slopes change to- 118 mV per pH unit for 1 and 2, suggesting a two proton-one electron couple [equation (3)]. Couple I1 is assigned to theone proton-one electron oxidation of Ru"' to Ru" where thebest straight lines in the Pourbaix diagram from pH 1 to 6 haveslopes of - 59 and - 66 mV per pH unit for 1 and 2, respectively.Couple 111 is assigned to a two proton-two electron oxidationof Ru" to RuV' based on the smaller value of AE, ( z 30-40 mV)and previous studies on related systems such as trans-[R~(bipy),(OH,),]~ +. Furthermore, the peak height forcouple I11 is much larger than those of I or 11.In the Pourbai898 J. CHEM. SOC. DALTON TRANS. 1994~ ~~p~~Table 1 Fractional atomic coordinates of non-hydrogen atoms with their estimated standard deviations in parentheses for [RuL'(PPh,),][ClO,],X0.161 lO(3)0.173 0(1)0.151 5(1)0.1 14 3(3)0.294 3(3)0.252 8(3)0.052 7(3)0.022 8(4)0.007 3(5)0.093 9(5)0.187 O(5)0.198 3(4)0.297 7(4)0.390 4(5)0.471 O ( 5 )0.469 O(4)0.379 l(4)0.560 7(5)0.355 7(4)0.421 8(4)0.380 5(5)0.280 3(4)0.214 5(4)0.102 l(4)0.050 6(4)- 0.099 6(5)- 0.056 O(5)-0.106 9(4)-0.049 7(4)0.528 5(5)0.056 O(4)-0.223 l(4)- 0.038 O(5)-0.125 l(5)- 0.1 15 2(6)-0.021 3(7)0.065 2(6)0.235 6(4)0.177 O(6)0.224 O(7)0.326 3(8)0.384 8(6)* Occupancy factor = 0.5.Y0.133 33(1)0.130 28(5)0.133 13(5)0.063 8( 1)0.1864(1)0.189 O( 1)0.044 4(2)0.102 2(1)-0.001 8(2)- 0.029 2(2)-0.01 1 O(2)0.035 7(2)0.057 3(2)0.036 9(2)0.063 9(2)0.110 5(2)0.129 7(2)0.139 l(3)0.21 1 9(2)0.255 3(2)0.264 l(2)0.227 8(2)0.229 7(2)0.269 4(2)0.268 8(2)0.228 9(2)0.189 6(2)0.204 9(3)0.224 9(3)0.129 9(2)0.121 3(2)0.1 17 5(2)0.122 4(2)0.132 2(3)0.136 O(2)0.078 6(2)0.044 O(2)0.003 5(2)0.030 4(3)-0.021 O(2)0.177 2(2)- 0.003 O(3)70.294 60(3)0.134 88(9)0.455 42(9)0.283 3(3)0.321 9(3)0.309 3(3)0.280 l(3)0.257 8(4)0.248 9(4)0.268 6(5)0.295 9(4)0.303 5(4)0.331 9(4)0.368 l(4)0.397 6(5)0.387 6(4)0.342 l(4)0.21 8 3(6)0.434 9(6)0.3 17 8(4)0.300 O(4)0.293 5(5)0.297 5(5)0.300 6(4)0.291 4(3)0.294 9(4)0.290 2(4)0.280 l(4)0.273 5(4)0.277 9(5)0.276 5 ( 5 )0.051 2(4)0.075 2(4)0.009 3(5)-0.081 O ( 5 )-0.104 6(5)- 0.040 2(4)0.105 8(4)0.060 8(4)0.044 l(5)0.070 9(6)0.115 8(5)x0.339 5 ( 5 )0.241 3(5)0.238 8(7)0.332 3(6)0.378 5(6)0.335 O ( 5 )0.212 O(4)0.318 8 ( 5 )0.363 4(6)0.300 8(7)0.195 6(7)0.15 1 7(5)0.205 6(4)0.299 9(5)0.332 2(5)0.274 9(6)0.180 4(6)0.148 7(5)0.025 7(4)0.193 5(5)- 0.060 4(4)- 0.155 O(4)-0.163 5 ( 5 )- 0.079 4(5)0.016 6(5)0.685 O( 1 )0.599 5( 1)0.681 2(7)0.787 O(5)0.622 O ( 5 )0.655 4(8)0.600 6(5)0.659 60.636 50.493 20.529 90.559 60.694 80.232 4(8)0.353(1)0.293 O(9)0.056( 1)Y0.070 7(2)0.177 l(2)0.218 7(2)0.256 5(2)0.252 7(3)0.173 6(2)0.184 l(2)0.188 5(2)0.230 l(3)0.267 2(2)0.263 5(2)0.222 3(2)0.083 6(2)0.081 l(2)0.041 l(3)0.002 7(2)0.004 l(2)0.044 4(2)0.133 l(2)0.1194(2)0.1 15 9(2)0.125 4(2)0.138 5(2)0.142 7(2)0.091 91(6)0.1 12 61(6)0.054 4(3)0.097 6(3)0.088 6(3)0.128 7(3)0.078 2(2)0.101 60.151 60.120 10.148 30.095 50.127 90.139 O(4)0.077 3(6)0.109 9(5)0.362 5(6)0.211 5(3)0.133 7(4)0.090 9(4)0.088 O(4)0.055 1(5)0.025 3(5)0.027 6( 5)0.060 O ( 5 )0.506 O(4)0,524 l(4)0.547 l(5)0.549 9(5)0.534 l(5)0.511 6(4)0.5 17 9(4)0.576 7(5)0.6 17 O( 5 )0.602 1(5)0.545 O ( 5 )0.502 9(4)0.492 l(4)0.433 O(4)0.462 l(4)0.550 7(4)0.610 9(4)0.582 7(4)0.1864(1)0.703 5( 1)0.134 l(5)0.231 O(6)0.252 O(5)0.132 2(5)0.638 3(5)0.784 30.672 50.718 60.663 20.782 10.732 90.790 3(7)0.866 3(9)0.827 9(7)0.449( 1)Table 2 Selected bond lengths (A) and angles (") for [RuL'(PPh,),]-~C10412Ru-P(I) 2.407( 1) P( 1 )-Ru-P( 2) 177.60(5)Ru-P(2) 2.416(1) N( I)-Ru-N(3) I59.2(2)Ru-N(I) 2.155(5) N( l)-Ru-N(4) 123.0(2)Ru-N(2) 1.973(4) N( 2)-R U-N( 3) 80.7( 2)Ru-N(4) 2.176(4) N(3)-Ru-N(4) 77.8(2)Ru-N(3) 1.979(4) N(2)-Ru-N(4) 158.2(2)diagram, the best straight lines from pH 1 to 6 have slopes of-58 and -55 mV per pH unit for 1 and 2, respectively.Theobserved E+(RU~'-RU'~) of [RuL'O,]'+ at 1.12 V (in pH 1solution) is the highest ever reported for trans-dioxoruthe-n i u m ( v ~ ) .~ ~ ~ For complex 2 E,(Ru~'-Ru'~) is observed at 1.05 V(in pH 1 solution).It is interesting that the high-valent oxoruthenium speciesgenerated by electrochemical oxidation of complex 1 in acidicmedia are considerably more stable than that of 2. Repeatedcycling of the former between 1 .O and 1.3 V shows no significantdeterioration of couple 111 (insert of Fig. 3). However, repetitivecyclic voltammetric scans of 2 in aqueous solutions revealed afaster rate of decomposition of [RuL'O,]' + . This suggests thatthe twisted conformation of L' is also unfavourable for trans-dioxoruthenium(v1).In basic solutions, cyclic voltammetric studies of complexes1 and 2 are complicated. Couples I1 and 111 become illdefined and the complexes were found to degrade fairly rapidly.Nevertheless, the Ru"'-Ru" couple is still reversible and its E,can be measured.The planar aromaticity of co-ordinated quaterpyridine, asrevealed by crystal analysis, is an important property forfabrication of chemically modified electrodes.Complex 1 wasfound to adsorb on edge-plane pyrolytic graphite.15 Fig. 5shows the cyclic voltammogram of such an electrode in pHI .4 Na(O,CCF,)-CF,CO,H buffer after it had been cycled inz lo-' mol dm-, 1 and washed subsequently with a stream ofdistilled water. The redox couples are due to complex 1adsorbed on the surface of the electrode. With the addition of%lop4 mol dm-, propan-2-01 a large catalytic current isgenerated at about 1 V where a very small current was observedin the absence of the adsorbed ruthenium complex undersimilar conditions.In the presence of four methyl groups in the 3",5,5',5"'positions no surface adsorption of complex 2 is observed.Although structural data for 2 are not available, it can beinferred that the co-ordinated ligand L' probably has a non-planar conformation similar to that in 3.Such a conformationis expected not to favour adsorption of the ruthenium complexon the electrode surfaceJ . CHEM. SOC. DALTON TRANS. 1994I 1899, / I / i , , , l , i ' i i ' i i 1 1 I I1.21 .o0.80.60.40.208 -0.2P3, 1.20u11 .o0.80.60.40.20-0.2-0.4Ruw - Ru"FluV1- Ru"Fig.4 Pourbaix diagrams for (a) [RUL~(OH,),]~' and (6)[RuL'(OH,),]~+AcknowledgementsWe acknowledge support from The Hong Kong ResearchGrants Council and The University of Hong Kong. C.-W. C. isI I I I I I I i 11.2 0.8 0.4 0.0EN vs. SCEFig. 5 Residual current from surface-adsorbed [RUL~(OH,),]~ +on edge-plane pyrolytic graphite and catalytic current resulting fromoxidation of propan-2-01 (z mol dm-3). Measurements weretaken in a mixture of 0.04 mol dm-3 CF3C0,H and 0.06 mol dm-3Na(O,CCF,) aqueous solution with scan rate of 50 mV s-'grateful for the award of a Hung Hing-Ying Scholarship(1992-1993) administered by The University of Hong Kong.References1 C. M. Che, K. Y . Wong and C. K. Poon, Znorg. Chem., 1985,24,1797.2 C. M. Che, W.T. Tang and C. K. Li, J. Chem. Soc., Dalton Trans.,1990,3735.3 C. M. Che, K. Y. Wong, W. H. Leung and C. K. Poon, Znorg. Chem.,1986,25,345; J. C. Dobson and T. J. Meyer, Inorg. Chem., 1988,27,3283.4 C. M. Che, W. H. Leung, C. K. Li and C. K. Poon, J . Chem. Soc.,Dalton Trans., 1991,379; C. M. Che, C. Ho and T. C. Lau, J. Chem.SOC., Dalton Trans., 1991, 1259.5 W. P. Griffith, Chem. Soc. Rev., 1992,21, 179.6 J. M. Lehn, J. P. Sauvage, J. Simon, R. Ziessel, C. Piccini Leopardi,G. Germain, J.-P. Declercq and M. Van Meersche, N o w . J . Chim.,1983,7,413.7 (a) E. C. Constable, S. M. Elder, J. Healy and D. A. Tocher, J. Chem.Soc. Dalton Trans., 1990, 1669; (6) E. C . Constable, S. M. Elderand D. A. Tocher, Polyhedron, 1992, 11, 1337; (c) E. C. Constable,S. M. Elder, J. Healy and M. D. Ward, J. Am. Chem. Sue., 1990,112,4590.8 (a) C. W. Chan, C. M. Che, M. C. Cheng and Y. Wang, Znorg. Chem.,1992,31,4874; (b) C. M. Che, Y. P. Wang, K. S. Yeung, K. Y. Wongand S. M. Peng, J. Chem. Soc., Dalton Trans., 1992, 2675; ( c ) S. M.Yang, K. K. Cheung and C. M. Che, J . Chem. Soc., Dalton Trans.,1993,3515.9 P. Bernhard, H.-B. Burgi, J. Hauser, H. Lehmann and A. Ludi,Znorg. Chem., 1982,21, 3936.10 R. W. Mitchell, A. Spencer and G. Wilkinson, J. Chem. Soc.,Dalton Trans., 1973, 846.I 1 C. M. Che, K. Y. Wong and F. C. Anson, J. Electrounal. Chem.Interfacial Electrochem., 1987, 226, 21 1.12 Enraf-Nonius Structure Determination Package, Enraf-Nonius,Delft, 1985.13 B. Durham, S. R. Wilson, D. J. Hodgson and T. J. Meyer, J . Am.Chem. Soc., 1980,102,600.14 E. N. Maslen, C. L. Raston and A. H. White, J. C'hem. Soc.,Dalton Trans., 1975, 323.15 K. Y. Wong, W. 0. Lee, C. M. Che and F. C. Anson, J. Electroanal.Chem. Interfacial Electrochem., 1991,319, 207 and refs. therein.Received 29th October 1993; Paper 310648 1
ISSN:1477-9226
DOI:10.1039/DT9940000895
出版商:RSC
年代:1994
数据来源: RSC