Structures, electrochemical and spectroscopic properties of ternary ruthenium(II)-polypyridyl complexes with additional carboxylate, biguanide or sulfonamide donors

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Pergamon PII] S9166!4276"87#99034!3

Polyhedron Vol[ 06\ No[ 19\ pp[ 2430Ð2449\ 0887 Þ 0887 Elsevier Science Ltd All rights reserved[ Printed in Great Britain 9166Ð4276:87 ,08[99¦9[99

Structures\ electrochemical and spectroscopic properties of ternary ruthenium"II#!polypyridyl complexes with additional carboxylate\ biguanide or sulfonamide donors Samantha M[ Couchman\ Jose M[ Dominguez!Vera\ John C[ Je}ery\ Colin A[ McKee\ Simon Nevitt\ Matthias Pohlman\ Claire M[ White and Michael D[ Ward School of Chemistry\ University of Bristol\ Cantock|s Close^ Bristol\ BS7 0TS\ U[K[ "Received 05 February 0887^ accepted 06 April 0887# Abstract*The following complexes have been prepared] ðRu"bipy#1"pic#ŁðPF5Ł "bipy1\1?!bipyridine^ Hpic picolinic acid#^ ðRu"terpy#"dipic#Ł "terpy1\1?]5?\1ý!terpyridine^ H1dipicdipicolinic acid#^ ðRu"bipy#1 "Hbig#ŁðPF5Ł1 "Hbigbiguanide#^ and ðRu"bipy#1"apps#ŁðPF5Ł ðwhere Happs is the p!tolylsulfonamide of 1! "1!aminophenyl#!pyridineŁ[ The latter three have been characterised by X!ray crystallography and are all mononuclear pseudo!octachedral complexes[ Electrochemical studies reveal the relationship between the poten! tial of the metal!based Ru"II#:Ru"III# couple\ with the potential values being clearly related to the s!donor or p!acceptor capabilities of the ligands[ The energies of the lowest!energy m[l[c[t[ maxima in the electronic spectra also correlate with ligand properties[ ðRu"bipy#1"Hbig#ŁðPF5Ł1\ which has a dissociable proton on the biguanide ligand\ undergoes deprotonation in strongly basic conditions "pKa01[229[1#\ which results in a red!shift of the m[l[c[t[ transition consistent with weakening of the ligand _eld due to the increased p!donor capability of the deprotonated ligand[ Þ 0887 Elsevier Science Ltd[ All rights reserved Keywords] ruthenium complexes^ electrochemistry^ crystal structures[ ———————————————————————————————————————————————

INTRODUCTION We ð0Ð2Ł and others ð3\ 4Ł have been interested in studying the properties of ruthenium complexes with a variety of ligand donor sets[ One impetus for this is the particularly clear correlations which can be observed between electrochemical properties and ligand donor set ð0\ 3Ł[ Another is the widespread use of redox!active ruthenium fragments as components of polynuclear complexes linked by conjugated bridg! ing ligands\ in which inter!valence charge transfer is studied as a model for electron transport in molecular wires ð5Ł[ The kinetic stability of ruthenium in several di}erent oxidation states "particularly ¦1 and ¦2#\ the often reversible nature of the ¦1:¦2 couple\ and the relative ease with which mixed!ligand complexes

 Author to whom correspondence should be addressed[

can be prepared by controllable stepwise methods\ all make ruthenium complexes particularly attractive targets of study[ We describe here the syntheses and the electro! chemical and electronic spectroscopic properties of four mononuclear complexes of ruthenium"II# with mixed donor sets[ These complexes are ðRu"bipy#1 "pic#ŁðPF5Ł "bipy1\1?!bipyridine^ Hpicpyridine!1! carboxylic acid\ also known as picolinic acid#^ ðRu "terpy#"dipic#Ł "terpy1\1?]5?\1ý!terpyridine^ H1dipic pyridine!1\5!dicarboxylic acid\ also known as dipi! colinic acid#^ ðRu"bipy#1"Hbig#ŁðPF5Ł1 "Hbig biguanide#^ and ðRu"bipy#1"apps#ŁðPF5Ł ðwhere Happs is the p!tolylsulfonamide of 1!"1!aminophenyl# pyridineŁ[ The ligands are shown below^ the latter three complexes have been crystallographically characterised[ The complexes ðRu"bipy#1"pic#ŁðPF5Ł ð6\ 7Ł and ðRu"terpy#"dipic#Ł ð8Ł were both reported very recently while this work was in progress but have not been directly compared[

2430

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S[M[ Couchman et al[

EXPERIMENTAL General details ðRu"bipy#1Cl1Ł = 1H1O ð09Ł\ ðRu"terpy#Cl2Ł ð00Ł\ biguanide ð01Ł\ Happs ð02Ł and ðRu"bipy#1"pic#ŁðPF5Ł ð6\ 7Ł were prepared according to the published pro! cedures[ Dipicolinic acid was obtained from Aldrich and used as received[ Instrumentation used for routine spectroscopic and electrochemical analyses has been described in earlier papers ð0\ 1Ł[ Preparations ðRu"terpy#"dipic#Ł[ A mixture of dipicolinic acid "04[1 mg\ 9[98 mmol# and Na1CO2 "19 mg\ 9[07 mmol# was dissolved in water and heated to re~ux[ ðRu "terpy#Cl2Ł "39 mg\ 9[98 mmol# was then added and the mixture heated to re~ux for a further 4 hours\ during which time a purple solid precipitated and the super! natant solution became green[ The solid was _ltered o}\ washed with water\ and air!dried to give ðRu"terpy# "dipic#Ł in 54) yield[ X!ray quality crystals were grown by di}usion of diethyl ether vapour into a saturated solution of the complex in dmf[ Found] C\ 38[9^ H\ 2[4^ N\ 09[3)[ Required for C11H03N3O3Ru = H1O] C\ 38[2^ H\ 2[3^ N\ 09[4)[ The complex is not su.ciently volatile to give a good mass spectrum[ ðRu"bipy#1"Hbi`#ŁðPF5Ł1[ A mixture of ðRu "bipy#1Cl1Ł = 1H1O "9[029 g\ 9[14 mmol# and Hbig "9[918 g\ 9[18 mml# in dry ethanol "04 cm2# was heated to re~ux under N1 for 3 hours to give a purple solution[ After cooling\ aqueous KPF5 was added and the mix! ture was concentrated in vacuo until a purple pre! cipitate appeared[ This was collected by _ltration and dried to give ðRu"bipy#1"Hbig#ŁðPF5Ł1 in 79) yield[ Recystallisation from MeCN:ether a}orded X!ray

quality crystals[ Found] C\ 22[7^ H\ 2[9^ N\ 05[4)[ Required for C11H12F01N8P1Ru = MeCN] C\ 23[0^ H\ 2[0^ N\ 05[5)[ Electrospray MS] m:z 402 ð49)\ "Ru"bipy#1"big##¦Ł[ 0H NMR "299 MHz\ CD2CN#] d 7[81 "1 H\ d\ J 4[4#\ 7[32 "1 H\ d\ J 6[8#\ 7[23 "1 H\ d\ J 6[8#\ 7[97 "1 H\ td\ J 6[8\ 0[4#\ 6[72 "1 H\ td\ J 6[8\ 0[4#\ 6[55 "3 H\ m#\ 6[06 "1 H\ ddd\ J 6[4\ 4[6\ 0[2 Hz#\ 4[53 "ca[ 0 H\ br s\ NH#\ 4[91 "ca[ 1 H\ br s\ NH#[ ðRu"bipy#1"apps#ŁðPF5Ł[ A solution of ðRu"bipy#1 "H1O#1Ł1¦ "9[05 mmol# was generated by reaction of ðRu"bipy#1Cl1Ł = 1H1O "73 mg\ 9[05 mmol# with AgNO2 "1 equiv[# in EtOH:H1O "0]0\ 29 cm2# at re~ux for 0 hour\ followed by removal of the AgCl by _ltration[ To this was added Happs "49 mg\ 9[05 mmol# and the mixture was re~uxed with stirring for a further 0 h[ Addition of aqueous KPF5 precipitated a solid which was collected by _ltration and dried[ The crude pro! duct was puri_ed by chromatography on silica with MeCN:H1O:saturated aqueous KNO2 "03]1]0# as eluent[ The main deep red:purple band was collected\ the solution concentrated in vacuo\ and the product precipitated by addition of aqueous KPF5[ Filtration and drying in vacuo a}orded pure ðRu"bipy#1"apps#Ł ðPF5Ł in 69) yield[ Recrystallisation from CH1CI1: hexane a}orded X!ray quality crystals[ Found] C\ 38[9^ H\ 2[3^ N\ 7[2)[ Required for C27H20F5N5O1PRuS = CH1Cl1] C\ 37[4^ H\ 2[3^ N\ 7[6)[ FAB MS] m:z 626 ð79)\ "Ru"bipy#1"apps##¦Ł[ X!ray crystallo`raphy Suitable crystals were quickly transferred from the mother liquor to a stream of cold N1 at −099>C on a Siemens SMART di}ractometer _tted with a CCD! type area detector[ In all cases data were collected at −099>C to a 1u limit of 44> using graphite!mono! chromatised MoÐKa radiation[ A detailed experi!

Structures\ electrochemical and spectroscopic properties of ternary ruthenium"II#!polypyridyl complexes 2432 mental description of the methods used for data col! lection and integration using the SMART system has been published ð03Ł[ Table 0 contains a summary of the crystal parameters\ data collection and re_nement[ Empirical absorption corrections were applied to the datasets using SADABS ð04Ł[ The structures were solved by conventional heavy!atom or direct methods and re_ned by the full!matrix least!squares method on all F1 data using the SHELXTL 4[92 package on a Silicon Graphics Indy computer ð05Ł[ Non!hydrogen atoms were re_ned with anisotropic thermal par! ameters^ hydrogen atoms were included in calculated positions and re_ned with isotropic thermal par! ameters riding on those of the parent atom[ The struc! tural determinations of ðRu"bipy#1"Hbig#ŁðPF5Ł1 = MeCN and ðRu"bipy#1"apps#ŁðPF5Ł = CH1Cl1 were straightforward and presented no particular prob! lems^ the only point to note is that in ðRu"bipy#1 "Hbig#ŁðPF5Ł1 = MeCN the asymmetric unit contains two crystallographically independent molecules of the complex and two molecules of MeCN[ Crystals of ðRu"terpy#"dipic#Ł = H1O = Et1O were very thin plates "those used had a thickness of 9[91 mm# and therefore di}racted rather weakly[ Only data to a 1u limit of 49> was used in the _nal re_nement\ as no signi_cant di}racted intensity was observed between 49> and 44>\ and inclusion of the higher!angle data only increased the R indices with no concomitant improvement in the precision of the structure[ The

molecule lies on a C1 axis which passes through C"13#\ N"10#\ Ru"0#\ N"20# and C"23#^ there are therefore 05 complete molecules in the Fddd unit cell which has 21 general positions[ Each asymmetric unit contained one half of the complex molecule\ one half of a water molecule "the oxygen atom also being on a special position#\ and a badly disordered solvent molecule around the intersection of two C1 axes[ This could only be modelled by putting carbon atoms at the sites of maximum electron density and allowing the site occupancies to re_ne[ The result was approximately 1[4 atoms\ which we assume corresponds to half a disordered ether molecule in the asymmetric unit per half complex molecule and half solvent molecule\ i[e[ ðRu"terpy#"dipic#Ł = H1O = Et1O[ Hydrogen atoms were not included in the re_nement for the disordered solvent molecule[

RESULTS AND DISCUSSION Synthesis of and crystal structure of ðRu"terpy#"dipic#Ł Preparation and puri_cation of this complex was straightforward and it was isolated cleanly in good yield[ It is only very sparingly soluble in dmf and dmso\ and insoluble in other solvents\ so a good mass spectrum could not be obtained[ Attempts to obtain NMR spectra were likewise unsuccessful\ due to a

Table 0[ Crystallographic data for the new complexes Compound

ðRu"terpy#"dipic#Ł = H1O = Et1O C15H15N3O5Ru 480[47 Orthorhombic\ Fddd 7[635"1# 08[786"3# 42[803"01# 89 89 89 8271"3# 05 0[564 9[610 3721 9[49×9[49×9[91 07649\ 1965\ 9[9709

ðRu"bipy#1"Hbig#ŁðPF5Ł1 = MeCN C13H15F01N09P1Ru 734[45 Triclinic\ P 0¹ 00[3289"02# 03[437"1# 08[601"1# 79[587"8# 65[690"6# 77[063"7# 2049[2"5# 3 0[672 9[696 0577 9[3×9[2×9[0 21247\ 03085\ 9[9167

ðRu"bipy#1"apps#ŁðPF5Ł = CH1Cl1 C28H22Cl1F5N5O1PRuS 855[60 Monoclinic\ P10:c 7[7704"8# 08[892"1# 11[746"4# 89 83[16"1# 89 3918[2"09# 3 0[483 9[574 0841 9[4×9[1×9[0 14498\ 8113\ 9[9335

Formula M System\ space group  a:A b:A ý c:A ý a:> b:> g:> U:A ý2 Z Dc` cm−2 m:mm−0 F"999# Crystal size:mm Re~ections collected] total\ independent\ Rint 1u limits for data\> Data\ restraints\ parameters Final R0\ wR1a\b Weighting factors "a\b#b  −2 Largest peak\ hole :e A

2Ð49 1960\ 9\ 073 9[9437\ 9[0364 9[9775\ 1[9073 ¦0[461\ −0[070

2Ð44 03085\ 9\ 774 9[9231\ 9[9896 9[9386\ 9 ¦0[006\ −9[576

2Ð44 8113\ 9\ 423 9[9396\ 9[0990 9[9348\ 9[8365 ¦9[699\ −9[752

a Structure was re_ned on F 19 using all data^ the value of R0 is given for comparison with older re_nements based on Fo with a typical threshold of F3s"F#[ b wR1ðSðw"F 19−F 1c #1Ł:Sw"F 19#1Ł0:1 where w−0ðs1"F 19#¦"aP#1¦bPŁ and Pðmax"F 19\9#¦1F 1c Ł:2[

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S[M[ Couchman et al[

combination of poor solubility and the ease with which it oxidises in solution to the paramagnetic Ru"III# state "see below#[ However the elemental analysis and crystal structure "below# provide unam! biguous characterisation[ ðRu"terpy#"dipic#Ł "Fig[ 0\ Table 1# has the expected pseudo!octahedral structure\ with a trans!N3O1 donor set[ The terpy ligand shows the usual pattern of RuÐ N bond lengths\ with a short bond to the central  Ł and longer ones to atom ðRu"0#ÐN"10#\ 0[853"4# A the terminal two atoms ðRu"0#ÐN"00#:N"00A#\  Ł\ which arise from the steric limitations of 1[959"4# A the terpyridyl ligand ð06Ł[ The dipicolinate ligand behaves similarly for the same steric reasons\ with the two RuÐO bonds being signi_cantly longer than the four RuÐN bonds\ and the localisation of single and double CÐO bond character being apparent in the two carboxylate fragments ð6\ 7Ł[ The presence of two anionic s!donor ligands on the same axis trans to one another has not resulted in any signi_cant lengthening of the RuÐO bonds compared to those complexes with N4O donor sets where a carboxylate donor is trans to a pyridyl ring\ as in ðRu"bipy#1"pic#ŁðPF5Ł ð7Ł and ðRu"bpy!CO1#1Ł "where bpy!CO1H is 1\1?!bipyridine! 5!carboxylic acid# ð6Ł[ The non!coordinated oxygen atom O"25# is hydrogen!bonded to the lattice water molecule\ with the non!bonded O"25# = = = O"water#  \ typical of OÐH = = = O hydro! separation being 1[80 A gen!bonding[

Synthesis and "Hbi`#ŁðPF5Ł1

crystal

structure

of

ðRu"bipy#1

Although complexes of _rst!row transition metal ions with biguanide have been known for a long time ð07Ð13Ł\ there are no reports of any complexes with ruthenium[ As a bidentate ligand with two unsatu! rated "formally sp1!hybridised# N!donors there is an obvious similarity with other diimines such as 1\1?! bipyridine "bipy#[ A notable property of biguanide is that it can coordinate in either the neutral form "Hbig# or in the deprotonated form ðbigŁ− with a delocalised structure reminiscent of the acetylacetonate anion ð07Ł\ and crystal structures of both types of complex are known[ Reaction of ðRu"bipy#1Cl1Ł with Hbig in re~uxing ethanol\ followed by precipitation of the complex as its hexa~uorophosphate salt and recrystallisation\ a}orded in high yield a material whose elemental analysis was consistent with the formulation ðRu "bipy#1"Hbig#ŁðPF5Ł1[ This implies that the biguanide ligand is neutral\ rather than coordinating in the anionic deprotonated form which would have given ðRu"bipy#1"big#ŁðPF5Ł with only one hexa~uoro! phosphate anion and a substantially di}erent elemen! tal analysis[ The ES mass spectrum con_rmed the presence of the expected complex cation\ and the structure was _nally con_rmed by X!ray crystallo! graphy "Figure 1\ Table 1#[

Fig[ 0[ Crystal structure of ðRu"terpy#"dipic#Ł[

Structures\ electrochemical and spectroscopic properties of ternary ruthenium"II#!polypyridyl complexes 2434 Table 1[ Selected bond lengths "A ý # and angles "># for the three crystal structures ðRu"terpy#"dipic#Ł = H1O = 9[4Et1O

ðRu"bipy#1"Hbig#ðPF5Ł1 = MeCN

ðRu"bipy#1"apps#ðPF5Ł = CH1Cl1

RuÐN"10# RuÐN"20# RuÐN"00A# RuÐN"00# RuÐO"26A# RuÐO"26#

RuÐN"000# RuÐN"030# RuÐN"010# RuÐN"020# RuÐN"045# RuÐN"040#

RuÐN"10# RuÐN"20# RuÐN"00# RuÐN"30# RuÐN"40# RuÐN"59#

0[853"4# 0[868"5# 1[959"4# 1[959"4# 1[000"2# 1[000"2#

N"10#ÐRuÐN"20# N"10#ÐRuÐN"00A# N"20#ÐRuÐN"00A# N"10#ÐRuÐN"00# N"20#ÐRuÐN"00# N"00A#ÐRuÐN"00# N"10#ÐRuÐO"26A# N"20#ÐRuÐO"26A# N"00A#ÐRuÐO"26A# N"00#ÐRuÐO"26A# N"10#ÐRuÐO"26# N"20#ÐRuÐO"26# N"00A#ÐRuÐO"26# N"00#ÐRuÐO"26# O"26A#ÐRuÐO"26#

079[9 68[50"01# 099[28"01# 68[59"01# 099[39"01# 048[1"1# 090[18"8# 67[60"8# 77[6"1# 84[3"1# 090[18"8# 67[60"8# 84[3"1# 77[6"1# 046[3"1#

1[935"1# 1[935"1# 1[949"1# 1[952"1# 1[956"1# 1[969"1#

N"000#ÐRuÐN"030# N"000#ÐRuÐN"010# N"030#ÐRuÐN"010# N"000#ÐRuÐN"020# N"030#ÐRuÐN"020# N"010#ÐRuÐN"020# N"000#ÐRuÐN"045# N"030#ÐRuÐN"045# N"010#ÐRuÐN"045# N"020#ÐRuÐN"045# N"000#ÐRuÐN"040# N"030#ÐRuÐN"040# N"010#ÐRuÐN"040# N"020#ÐRuÐN"040# N"045#ÐRuÐN"040#

Fig[ 1[ Crystal structure of the cation of ðRu"bipy#1 "Hbig#ŁðPF5Ł1[

The asymmetric unit contains two independent ðRu"bipy#1"Hbig#ŁðPF5Ł1 = MeCN groups[ Figure 1 shows one of the independent complex cations^ the structure of the alternate one is very similar[ It has the expected octahedral structure with unremarkable bond lengths and angles in the coordination sphere[ There is no sign of any signi_cant trans e}ect\ as the RuÐN"pyridyl# bonds trans to the biguanide donor

89[51"7# 67[83"8# 85[47"8# 85[16"7# 67[73"8# 062[30"8# 062[45"8# 81[37"7# 84[98"7# 78[76"7# 80[39"7# 063[98"8# 78[15"8# 84[31"8# 75[96"8#

N"10#ÐRuÐN"20# N"10#ÐRuÐN"00# N"20#ÐRuÐN"00# N"10#ÐRuÐN"30# N"20#ÐRuÐN"30# N"00#ÐRuÐN"30# N"10#ÐRuÐN"40# N"20#ÐRuÐN"40# N"00#ÐRuÐN"40# N"30#ÐRuÐN"40# N"10#ÐRuÐN"59# N"20#ÐRuÐN"59# N"00#ÐRuÐN"59# N"30#ÐRuÐN"59# N"40#ÐRuÐN"59#

1[947"2# 1[959"1# 1[950"2# 1[952"1# 1[988"2# 1[030"2# 71[33"09# 67[75"09# 099[09"09# 83[52"09# 67[46"09# 062[37"09# 099[15"09# 064[91"09# 73[47"09# 85[86"09# 061[38"8# 82[85"8# 84[26"09# 80[98"8# 72[72"09#

atoms are very similar to the other two[ The RuÐ N"biguanide# distances are consistent with the N atoms being sp1!hybridised[ Two geometric features of the coordinated biguanide ligands are worth a com! ment[ Firstly\ the angle at the central N atom\ C"044#Ð N"043#ÐC"041#\ is 018[9"1#> ð018[0"2#> at N"143# in the independent complex dicationŁ[ This is consistent with the ligand being in the neutral Hbig form\ with steric repulsion between the two bulky arms attached to the central N atom resulting in the CÐNÐC angles being considerably {{opened out||^ ð19\ 10Ł when the ligand is deprotonated\ the increased delocalisation through! out the chelate ring results in these CÐNÐC angles being much closer to the ideal 019> ð19\ 11\ 13Ł[ Secondly\ the Ru atom is not quite in the same plane as the "near!planar# biguanide ligand^ the angle between the mean plane of the _ve ligand atoms in the chelate ring ðN"040#\ C"041#\ N"043#\ C"044#\ N"045#Ł and the RuN1 coordination plane ðRu"0#\ N"040#\ N"045#Ł is 5[8> in the cation depicted "8[1> for the independent complex dication#[ This is again a common\ but not consistent\ feature of biguanide complexes ð19\ 10Ł[ The solution 0H NMR spectrum showed the pres! ence of the expected 7 aromatic proton environments "relative intensity 1 H each# for the bipyridyl ligands\ which are equivalent but within which the two pyridyl rings are inequivalent[ Two broad singlets at 4[91 and 4[54 ppm we assign to NH protons\ but the integrals are rather low\ presumably due to exchange with pro!

2435

S[M[ Couchman et al[

Fig[ 2[ Crystal structure of the cation of ðRu"bipy#1"apps#ŁðPF5Ł[

tons in the moisture that is present in the solvent[ We would expect three di}erent NH proton environments on the basis of the crystal structure\ so we assume that the most acidic proton "the central NH# is in fast exchange with the H1O protons and the signal for this is therefore invisible[ Synthesis and "apps#ŁðPF5Ł[

crystal

structure

of

ðRu"bipy#1

We recently reported the synthesis of Happs\ a bide! natate chelating ligand with one pyridyl and one sul! fonamide donor\ and the crystal structures of some of its complexes ð02Ł[ The new complex ðRu"bipy#1"apps#Ł ðPF5Ł\ with _ve pyridyl ligands and one anionic sul! fonamide ligand\ was simply prepared by reaction of Happs with the labile complex ðRu"bipy#1"H1O#Ł1¦^ we found that this gave a cleaner product than that obtained from the reaction of Happs directly with ðRu"bipy#1Cl1Ł[ The elemental analysis and FAB mass spectrum con_rmed the formulation of the complex[ The 0H NMR spectrum in MeCN gave the correct ratio of integrals for the aromatic and aliphatic "methyl# protons\ but the 17 inequivalent aromatic protons occurred in an overlapping mass between 5[4 amd 7[4 ppm which could not be individually assigned[ The crystal structure "Figure 2\ Table 1# shows that the usual octahedral structure has been adopted

despite the bulky and sterically hindering nature of the p!tolyl!sulfonamide fragment[ There is an interaction between O"0# of the sulfonamide group and one of the  between O"0# pyridyl rings\ with a distance of 2[00 A and the mean plane of this pyridyl ring[ This appears to lengthen the RuÐN bonds to the apps ligand\ as RuÐN"40# is the longest of the RuÐN"pyridyl# bonds  \ and the RuÐN"sulfonamide# distance at 1[988"2# A  ð1[030"2# AŁ is even longer[ This contrasts with the four!coordinate\ relatively unhindered\ complexes of the same ligand with Cu"II#\ Co"II# and Pd"II# in which the metal!N"sulfonamide# bonds were shorter than\ or about the same length as\ the metal!N "pyridyl# bonds ð02Ł[

Electrochemical properties of ðRu"bipy#1"pic#ŁðPF5Ł and ðRu"terpy#"dipic#Ł Cyclic voltammetry in dmf showed that ðRu"bipy#1 "pic#ŁðPF5Ł has a reversible one!electron wave at ¦9[39 V vs[ the ferrocene:ferrocenium couple "Fc:Fc¦#\ corresponding to a metal!centred Ru"II#: Ru"III# couple\ and two reversible one!electron waves at −0[76 and −1[01 V vs[ Fc:Fc¦\ corresponding to ligand!centred processes "electrochemical data are collected in Table 2#[ By {{reversible|| is meant that the waves were symmetric "equal cathodic and anodic peak currents#\ with peak!peak separations in the

Structures\ electrochemical and spectroscopic properties of ternary ruthenium"II#!polypyridyl complexes 2436 Table 2[ Electronic spectral and electrochemical data for the complexes[ The maxima of the lowest!energy m[l[c[t[ transitions are in bold type

Complex

lmax "nm# ð09−2e "dm2 mol−0 cm−0#Ł

Cyclic voltammetric data ðE0:1 V vs[ Fc:Fc¦Ł

ðRu"bipy#1"pic#ŁðPF5Ł ðRu"terpy#"dipic#Ł ðRu"bipy#1"Hbig#ŁðPF5Ł1 ðRu"bipy#1"apps#ŁðPF5Ł

384 "03#\ 344 "sh#\ 258 "02#\ 184 "67#a 559 "sh#\ 484 "sh#\ 421 "7[2#\ 282 "09[5#\ 212 "16#\ 168 "13#a 493 "3[8#\ 259 "5[9#\ 239 "5[9#\ 181 "39#\ 133 "14#b 459 "sh#\ 383 "2[4#\ 222 "5[5#\ 186 "17#a

¦9[39 ¦9[01 ¦9[10c ¦9[19

a

−0[76\ −1[01a −1[90a −0[80c

In dmf^ b in H1O^ c in MeCN[

region of 59Ð099 mV\ at a range of scan rates[ These results agree well with recently reported values of ¦9[77\ −0[32 and −0[57 V vs[ SCE in acetonitrile ð6Ł[ The Ru"II#:Ru"III# couple of ðRu"bipy#2ŁðPF5Ł1 under the same conditions is ¦9[78 V vs[ Fc:Fc¦\ so replacement of a neutral pyridyl ligand by an anionic carboxylate therefore stabilises the Ru"III# state by 389 mV[ This may be compared with the 759 mV stabilisation of the Ru"III# state which occurs on replacing a pyridyl residue of ðRu"bipy#2Ł1¦ with a phenolate group in ðRu"bipy#1"no#ŁðPF5Ł ðHno is 1!"1! hydroxyphenyl#pyridineŁ\ which also has a N4O donor set containing one anionic oxygen donor ð0Ł[ The com! parison of ðRu"bipy#1"pic#Ł¦ with ðRu"bipy#1"no#Ł¦ allows us to cancel out the electrostatic part of the stabilisation of the Ru"III# state which arises just as a consequence of reducing the overall charge on the complex from ¦1 to ¦0\ which will be common to both complexes[ The fact that a phenolate donor results in a cathodic shift of the Ru"II#:Ru"III# couple which is 269 mV larger than that caused by the car! boxylate donor is an indication of the di}erent s! donor:p!acceptor properties of the two groups[ Clearly carboxylate is a poorer net electron donor than phenolate\ because of the partial delocalisation of the negative charge within the carboxylate group onto the non!coordinated oxygen atom[ The poten! tials of the two one!electron bipy!based reductions may be compared with ðRu"bipy#2Ł1¦\ whose _rst two reductions occur at −0[61 and −0[81 V vs[ Fc:Fc¦[ The more negative potentials required to reduce the bipy ligands of ðRu"bipy#1"pic#Ł¦ re~ect the smaller positive charge on the complex[ Cyclic voltammetry of ðRu"terpy#"dipic#Ł in dmf revealed reversible one!electron waves centred at ¦9[01 V and −1[90 V vs[ Fc:Fc¦[ The _rst of these corresponds to the Ru"II#:Ru"III# couple\ which has undergone an additional cathodic shift of 179 mV compared to ðRu"bipy#1"pic#ŁðPF5Ł "which in turn was cathodically shifted by 389 mV from ðRu"bipy#2Ł1¦#[ The couple at −1[90 V vs[ Fc:Fc¦ we assign to reduction of the terpy ligand^ a reduction of the

doubly negatively!charged dipicolinate ligand was not observed[ The relationship between Ru"II#:Ru"III# redox potential and donor set in the series ðRu"bipy#2Ł1¦\ ðRu"bipy#1"pic#Ł¦ and ðRu"terpy#"dipic#Ł "with N5\ N4O and trans!N3O1 donor sets respectively# is clearly not linear] the second substitution of pyridyl by car! boxylate has a much smaller e}ect than the _rst sub! stitution[ In ðRu"terpy#"dipic#Ł the two carboxylates are trans to one another\ and each will repel the other electrostatically\ reducing their ability to donate elec! tron density to the metal centre compared to the car! boxylate donor of ðRu"bipy#1"pic#Ł¦ which is trans to an electron!withdrawing pyridyl group "although this is not really detectable in the crystal structures\ as described above#[ Further support for this comes from the observation that in ðRu"bpy!CO1#1Ł\ with a cis! N3O1 donor set\ the shift in the potential of the Ru"II#: Ru"III# couple is almost exactly double that found for ðRu"bipy#1"pic#ŁðPF5Ł\ since in this case both car! boxylate donors are trans to pyridyl groups ð6Ł[

Electrochemical properties of ðRu"bipy#1"hbi`#ŁðPF5Ł and ðRu"bipy#1"apps#ŁðPF5Ł Cyclic voltammetry of ðRu"bipy#1"Hbig#ŁðPF5Ł1 in MeCN using a glassy carbon working electrode gave a single reversible wave at ¦9[10 V vs[ Fc:Fc¦[ We ascribe this to the Ru"II#:Ru"III# couple[ Replacing one bipy ligand by Hbig therefore results in stabil! isation of the Ru"III# state by 579 mV\ indicating that the metal centre of ðRu"bipy#1"Hbig#ŁðPF5Ł1 is much more electron rich than that of ðRu"bipy#2Ł1¦ because of the poorer p!acceptor capability of Hbig compared to the highly delocalised bipy[ At high negative poten! tials only poorly!de_ned irreversible processes could be observed which were not examined further[ Use of a Pt working electrode instead of glassy carbon resulted in a very asymmetric wave for the Ru"II#: Ru"III# couple because of absorption of the oxidised species onto the electrode surface[ Cyclic voltammetry of ðRu"bipy#1"apps#ŁðPF5Ł at a

2437

S[M[ Couchman et al[

Fig[ 3[ Electronic spectra of ðRu"bipy#1"Hbig#Ł1¦ in water at pH 6 "solid line#\ and ðRu"bipy#1"big#Ł¦ in water at pH 02 "dotted line#[

Pt!bead electrode in MeCN gave reversible waves cen! tred at ¦9[19 V and −0[80 V vs[ Fc:Fc¦\ assignable to Ru"II#:Ru"III# and bipy!centred couples respectively[ The stabilisation of the Ru"III# state by 589 mV com! pared to ðRu"bipy#2Ł1¦ arises because apps is both a better s!donor than bipy because of its negative charge\ and a poorer electron!acceptor because it con! tains only one pyridyl ring[ Interestingly\ the Ru"II#: Ru"III# couple of ðRu"bipy#1"apps#ŁðPF5Ł is 199 mV more cathodic than that of ðRu"bipy#1"pic#ŁðPF5Ł\ indi! cating that the anionic sulfonamide group is a better net electron donor to ruthenium than the anionic car! boxylate group\ possibly because nitrogen is less elec! tronegative than oxygen and therefore more polarisable[

Electronic spectra of the complexes and pH titration of ðRu"bipy#1"Hbi`#ŁðPF5Ł1 The electronic spectra of the complexes are sum! marised in Table 2[ In all cases the lowest!energy feature\ in the visible region\ may be ascribed to the Ruðd"p#Ł:L"p# m[l[c[t[ process\ where L  bipy or terpy as appropriate[ The red!shift of these compared to the same transition in ðRu"bipy#2ŁðPF5Ł1 "ca[ 349 nm# ð14Ł arises because of the weakening of the ligand _eld by replacement of a strong!_eld pyridyl donors by weaker!_eld carboxylate\ sulfonamide or biguanide ligands\ which raises the d"p# levels[ The incremental weakening of the ligand _eld on addition of one and then two carboxylates to the donor set is apparent in the shift of the m[l[c[t[ maximum from 349 to 384 and

then 423 nm in the series ðRu"bipy#2ŁðPF5Ł1\ ðRu "bipy#1"pic#ŁðPF5Ł and ðRu"terpy#"dipic#Ł[ All of the complexes have one or two near!UV transitions in the 299Ð399 nm range\ and the usual intense ligand! centred transitions in the higher!energy UV region[ A pH titration of the electronic spectrum of ðRu "bipy#1"Hbig#ŁðPF5Ł1 was e}ected by dissolving it in aqueous 9[0 M KCI acidi_ed to pH 0\ and slowly rais! ing the pH to 02 by addition of very small portions of solid KOH[ In the pH range 0 to 00 the spectrum was invariant\ but in the pH range 00Ð02 the spectrum changes signi_cantly\ which we ascribe to depro! tonation of the central NH group of the coordinated Hbig ligand to give ðRuII"bipy#1"big#Ł¦\ since ðRu "bipy#2Ł1¦ undergoes no such pH!induced spectral change[ The pair of transitions at 239 and 259 nm become red!shifted to 237 and 273 nm\ and the lowest! energy MLCT band at 493 nm is likewise red!shifted to 422 nm "Figure 3#[ A graph of lmax"MLCT# vs[ pH "Figure 4# indicates that the pK value is 01[2 "29[1#\ and the same pK value could be obtained from plot! ting other spectral parameters "e[`[ absorbance at 399 nm# as a function of pH[ The red!shift of the m[l[c[t[ transitions indicates that on deprotonation the metal d"p# orbitals are raised\ i[e[ ðbigŁ− is a weaker! _eld ligand than Hbig^ that the bipy!based orbitals are not signi_cantly perturbed is shown by the fact that the two intense UV transitions are hardly a}ected by deprotonation of the complex[ Deprotonation of Hbig to ðbigŁ− results in formation of a negatively! charged\ delocalised {{acac!like|| ligand ð08Ł which is expected to be a better p!donor than neutral Hbig\ consistent with the above observation[

Structures\ electrochemical and spectroscopic properties of ternary ruthenium"II#!polypyridyl complexes 2438

Fig[ 4[ Wavelength of the m[l[c[t[ maximum of ðRu"bipy#1"Hbig#Ł1¦ as a function of pH[

Acknowled`ements*We thank the EPSRC for _nancial sup! port[

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