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Synthesis and characterisation of a heterodinuclear ruthenium(II)palladium(II) complex with two different cyclometallating sites
Inorganica Chimica Acta 245 ( 1996) 69-73
Synthesis and characterisation of a heterodinuclear ruthenium( II) palladium( II) complex with two different cyclometallating sites Abdelaziz Jouaiti a, Michel Geoffroy a, Jean-Paul Collin b aDepartment of Physical Chemistry, 30 Quai Ernest Ansermet, University of Geneva, 1211 Geneva, Switzerland b Laboratoire de Chimie Organo-Minkrale, UA au CNRS 422, Facultk de Chimie, Universite Louis Pasteur, 1 rue Blake Pascal, F-67008 Strasbourg, France
Received I1 April 1995; revised 17 August 1995
Abstract A bis-cyclometallating ligand bearing two different terdentate coordination sites (N-C-N: dipyridyl-benzene; P-C-P: diphosphaalkenebenzene moieties) has been synthesised. Selective reactions of appropriate metal complex precursors afforded a heterodinuclear ruthenium( II)-palladium( II) complex characterisedby ‘H, 31P NMR spectroscopy andFAB-MS techniques. We have comparedits electrochemical and spectroscopic properties (absorption and emission) with the individual ruthenium( II) and palladium( II) subunits. Keywords:
Ruthenium complexes; Palladium complexes; Cyclometallated ligand complexes; Heterodinuclear complexes
1. Introduction
2. Experimental 2.1. Instrumentation
During recent years, the chemistry of coordination complexes containing cyclometallated terdentate ligands has received significant attention. Different sets of donor atoms (P-C-P, N-N-C, N-C-N, S-C-S etc.) have been used to prepare a wide range of metal complexes [l-14] (Ru, OS, Rh, Ni, Ta, Pd, Pt etc.). As a result of the strong metal-carbon (T bond, the ligand field and the electron density at the metal centre are increased by comparison with nitrogen or phosphorus donor atoms. Unusual reactivities [ 12,15-171 and new photophysical properties [ 10,181 have been evidenced in this type of complex. More recently, mixed-valence complexes (Run-Run’, Osn-Gs’“) based on the N-C-N [ 19,201 or N-N-C [ 211 sets lead to an exceptional electronic coupling between the two metal centres. Similarly to this bisterdentate ligand, we previously reported a dinuclear palladium complex containing two diphosphaalkene-benzene moieties [ 221. We report here the synthesis and characterisation of a new bis-cyclometallating ligand 1 incorporating a dipyridyl-benzene and a disphosphaalkene-benzene fragment. The different metal-binding sites of 1 permit the preparation of the heterodinuclear ruthenium( II)-palladium( II) complex 2 by selective reaction with the appropriate metal complex precursors ( Scheme 1) . 0020.1693/96/$15,00 0 1996 Elsevier Science S.A. All rights reserved SSDfOO20-1693(95)04799-6
3’P and ‘H NMR spectra were acquired on a Bruker WP2OOSY spectrometer. Mass spectra were obtained by using VG mass lab TRIO-2 and VG ZAB-HF mass spectrometers. Electronic absorption and emission spectra were recorded with Kontron Uvikon 860 and Shimadzu RF 450 fluorimeters. Electrochemical measurements were carried out at room temperature by using a PAR model 362 potentiostat. Cyclic voltammetries, in THF containing 0.1 M n-Bu,NPF, as supporting electrolyte, were performed using a three-electrode system with a platinum working electrode, a platinumwire counter electrode and an SCE reference electrode. 2.2, Preparations Most of the syntheses were performed under an argon atmosphere, in carefully dried glassware. All the reagents were purchased from Fluka and used as received. 4’-Tolyl2,2’:6’2”-terpyridine (tterpy) [ 231, Ru( tterpy)Cl, [ 231, (dpbH = 1,3-dipyridyl-benzene) Wtterpy) (dpb)P& [ 191, [ 2,4,6-( ter-Bu),) ] PH, [ 241 and Pd[ di( tri-t-butylphenylphospha-ethylene)]-1,3-benzene [ 1 l] were synthesised according to previously reported procedures.
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A. Joudi
ef al. /Inorganica
Chimica Acra 245 (1996) 69-73
71
(d, 2H, 4.8 Hz); 7.03 (d, 2H, 4.8 Hz); 6.91 (m, 2H); 6.64 (m, 2H); 2.54 (s, 3H); 1.78 (s, 36H); 1.42 (s, 18H). FABMS (nitrobenzyl alcohol matrix) : m/z = 1450 ( CszHssN5P,ClPdRu+ requires 1450). 1
3. Results and discussion 3. I. Synthesis of ligand 1 and complex 2 The synthetic scheme leading to 1 is summarised in Fig. 1. The key step is the cross coupling reaction between lbromo-3,5-dipyridyl-benzene (3) and the organozinc derivative of 4 bearing two protected aldehyde functions. This reaction, catalysed by the palladium tetrakis-triphenylphosphine complex, affords the intermediate 5 in moderate yield (62%). The synthesis of 3, previously reported [ 261, uses the Stille-type cross coupling [ 271. It is little improved by substitution of 2-trimethylstannyl pyridine by the pyridylzinc reagent. After the deprotection of the precursor aldehyde, the two phosphaalkene substituents may be introduced following the method of Yoshifuji et al. [ 281 and previously described in the synthesis of 2,6-diphosphaalkene-pyridine [ 291. As in this case, a mixture of two isomers (E-E and E-Z isomers) was obtained (see Fig. 1) . Therefore, the aromatic region of the ‘H NMR spectra shows a complicated pattern. On the other hand, the ratio of the isomers ( 86% for the E-E isomer) can be evaluated from the signal of the phosphorus atoms in the 31P{ ‘H) NMR spectrum. The mass spectrum of 1 showed
1+
\0
CH3 BF4
Fig. 2. Reactions used in the synthesis of the heterodinuclear complex 2.
a molecular peak at m/z = 885, and the expected fragmentations also confirm the proposed structure. The reaction (Fig. 2) of 1 with the transition metal precursors (Ru( tterpy) (acetone),‘+ and Pd( C,H,CN) &12) readily leads to the heterodinuclear complex 2 in high yield (80%). In view of the a priori numerous possibilities of reaction between the ditopic ligand 1 and the two inorganic fragments, the high preparative yield observed is indeed surprising. The palladium precursor can react with the two different sites of coordination of 1 contrary to the ruthenium one. Actually, all attempts to introduce ruthenium into the diphosphaalkene-benzene moiety were unsuccessful. This difference of reactivity of the two terdentate sites towards ruthenium complexation can be exploited to prepare selectively some heterodinuclear complexes in two successive steps. The formulation of complex 2 was established by ‘H, 31P( ‘H) NMR spectroscopy and by FAB-MS. The main peak at m/z = 1450 corresponds to the mass of the monocationic complex 2+ and has the appropriate isotopic pattern. 3.2. Electrochemical measurements
Fig. 1. Reactions used in the synthesis of the bridging ligand 1.
The electrochemical behaviour of 2 has been compared with those of the mononuclear palladium complex 7 and ruthenium complex 8 (Scheme 2). The redox potentials of the complexes 2,7 and 8 obtained by cyclic voltammetry in THF are reported in Table 1.
A. Jouairi et al. /Inorganica Chimica Acta 245 (19%) 69-73
72
of the heterodinuclear complex 2 displays three redox couples directly attributable to Ru(II) oxidation (0.57 V), diphosphaalkene-benzene moiety reduction ( - 1.22 V) and terpyridine-centred reduction ( - 1.48 V) by comparison with the values of the isolated constituents 7 and 8. The main feature of this behaviour is the small effect of each of the organometallic parts on the other. This situation contrasts strongly with other homodinuclear complexes where a large influence is observed [ 19-211. The neutral charge of the organo-palladium moiety is possibly one of the factors leading to a low interaction between the two subunits. Indeed, both electronic and electrostatic factors are much less predominant in this case. Another important parameter is the torsion angle around the interannular C-C bond. A torsion angle close to 90” leads to a negligible overlap between the r-electron clouds of the metallated rings.
BF4-
Scheme 2.
3.3. Absorption and luminescence properties
Table 1 Electrochemical
data for 7,8,2
and Ru( tterpy)2Z’
El’* (V) (vs. SCE)
Compound
R”m~II
PdL” -
’
RuL+”
- 1.23 ;: 2 Ru(tterpy),‘+
’
0.61 0.57 1.25
- 1.22
-1.47 - 1.48 - 1.24
“Ref. [ll]. bRef. [19]. ‘Ref. [23]. Table 2 UV-Vis spectral data for complexes
7.8 and 2 in CH&
Compound
Amax(nm)
7 8 2
475 (0.5). sh 350 (8.1), 318 (19.95). 269 (24.6) 511 (13.8), 375 (11.8). 288 (58.4), 245 (43.4) 512 (22.3). 316 (88.3), 286 (117.5). 242 (87.4)
(eX
10’ (1 mol-’ cm-‘))
Each electrochemical reaction is consistent with single metal-based oxidation and ligand-based reduction. The redox couples show characteristic patterns for a reversible couple ( AEp close to 60 mV, Zpa= I,,. and Ii, proportional to ui’*) . As previously reported [ 11],7 shows a reversible reduction wave at - 1.23 V. It is attributed to the addition of one electron in the rr antibonding orbital of the diphosphaalkene subunit, on the basis of the EPR spectrum of the reduced species. The redox couples of the ruthenium complex 8 indicate that the u donating properties of the deprotonated dipyridyl benzene strongly affects both the metal centre and the external terpyridine ligand. The stability of the Ru( III) state is illustrated by a drastic shift of potential (0.65 V) between 8 and the Ru(tterpy),*+ species. Otherwise, the terpyridinecentred reduction became more negative by at least 0.25 V as compared to the Ru(tterpy)22+ complex. The CV curve
In Table 2 are reported the A,, and e_ values for the dinuclear complex 2 and related monomers 7 and 8. The ruthenium complex 8 exhibits ligand-centred w-@ transitions in the UV region and broad metal-to-ligandcharge tranfer (MLCT) bands in the visible region. The palladium complex 7 shows also ligand-localised transitions at high energy. In the lower energy region (h> 330 nm) a broad structureless absorption band can be assigned to a perturbed ligand-localised transition as already described in Pd(I1) ortho-metallated complexes [ 301. In addition, a broad band at 475 nm with a low extinction coefficient was assigned to a metal-centred transition. The spectrum of the heterodinuclear complex 2 is dominated by the rr-rr* transition and MLCT bands. In fact, the spectrum of 2 coincides almost with the sum of the spectra of the component subunits 7 and 8. On the basis of the spectroscopic and electrochemical data, we can again conclude that the ruthenium and palladium parts in 2 interact at most only slightly with each other. The emission spectra of 2, 7 and 8 were recorded in degassed CH,Cl, and CH,CN solutions at room temperature. Generally, luminescence lifetimes of cyclometallated palladium complexes are very short in room-temperature fluid solution. We also did not observe any emission spectrum for complex 7. As previously reported, complex 8 displays a weak emission band centred at 785 nm (4 = 4.5 X 10e5) due to the MLCT excited state [ 3 11. This result can be interpreted as being caused by the o-donating ability of the cyclometallating ligands. The emission spectrum of the heterodinuclear complex 2 exhibits a similar band to 8 (A,, = 800 nm), but with a lower intensity (45% of the intensity of 8 in the same conditions). The decrease in intensity is slightly different in CH3CN (30%). Considering the spectroscopic and redox properties of 2 in the frame of the localised orbital approach, an energy or electron transfer reaction cannot explain this phenomenon: (i) energy tranfer is excluded by the lack of an accessible excited state of the palladium part; (ii) the electron transfer reaction towards the palladium subunit is
A. Jouaiti et al. /Inorganica Chimica Acta 245 (19%) 69-73
slightly endergonic (0.2 eV). More probably, we suppose that the palladium part acts as a tuner. Indeed, small perturbations of the energy levels of the ruthenium excited state may have important effects on the decay processes. Examples of this type of perturbation concerning Ru(II)-Pt(I1) and Ru( II)-Pd( II) complexes have been already observed [ 32,331.
Acknowledgements This work was supported by a grant from the Swiss National Research Fund (Switzerland) and by the CNRS (France). We also acknowledge Dr J.P. Sauvage for helpful discussions.
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2075.
Synthesis and characterisation of a heterodinuclear ruthenium( II) palladium( II) complex with two different cyclometallating sites Abdelaziz Jouaiti a, Michel Geoffroy a, Jean-Paul Collin b aDepartment of Physical Chemistry, 30 Quai Ernest Ansermet, University of Geneva, 1211 Geneva, Switzerland b Laboratoire de Chimie Organo-Minkrale, UA au CNRS 422, Facultk de Chimie, Universite Louis Pasteur, 1 rue Blake Pascal, F-67008 Strasbourg, France
Received I1 April 1995; revised 17 August 1995
Abstract A bis-cyclometallating ligand bearing two different terdentate coordination sites (N-C-N: dipyridyl-benzene; P-C-P: diphosphaalkenebenzene moieties) has been synthesised. Selective reactions of appropriate metal complex precursors afforded a heterodinuclear ruthenium( II)-palladium( II) complex characterisedby ‘H, 31P NMR spectroscopy andFAB-MS techniques. We have comparedits electrochemical and spectroscopic properties (absorption and emission) with the individual ruthenium( II) and palladium( II) subunits. Keywords:
Ruthenium complexes; Palladium complexes; Cyclometallated ligand complexes; Heterodinuclear complexes
1. Introduction
2. Experimental 2.1. Instrumentation
During recent years, the chemistry of coordination complexes containing cyclometallated terdentate ligands has received significant attention. Different sets of donor atoms (P-C-P, N-N-C, N-C-N, S-C-S etc.) have been used to prepare a wide range of metal complexes [l-14] (Ru, OS, Rh, Ni, Ta, Pd, Pt etc.). As a result of the strong metal-carbon (T bond, the ligand field and the electron density at the metal centre are increased by comparison with nitrogen or phosphorus donor atoms. Unusual reactivities [ 12,15-171 and new photophysical properties [ 10,181 have been evidenced in this type of complex. More recently, mixed-valence complexes (Run-Run’, Osn-Gs’“) based on the N-C-N [ 19,201 or N-N-C [ 211 sets lead to an exceptional electronic coupling between the two metal centres. Similarly to this bisterdentate ligand, we previously reported a dinuclear palladium complex containing two diphosphaalkene-benzene moieties [ 221. We report here the synthesis and characterisation of a new bis-cyclometallating ligand 1 incorporating a dipyridyl-benzene and a disphosphaalkene-benzene fragment. The different metal-binding sites of 1 permit the preparation of the heterodinuclear ruthenium( II)-palladium( II) complex 2 by selective reaction with the appropriate metal complex precursors ( Scheme 1) . 0020.1693/96/$15,00 0 1996 Elsevier Science S.A. All rights reserved SSDfOO20-1693(95)04799-6
3’P and ‘H NMR spectra were acquired on a Bruker WP2OOSY spectrometer. Mass spectra were obtained by using VG mass lab TRIO-2 and VG ZAB-HF mass spectrometers. Electronic absorption and emission spectra were recorded with Kontron Uvikon 860 and Shimadzu RF 450 fluorimeters. Electrochemical measurements were carried out at room temperature by using a PAR model 362 potentiostat. Cyclic voltammetries, in THF containing 0.1 M n-Bu,NPF, as supporting electrolyte, were performed using a three-electrode system with a platinum working electrode, a platinumwire counter electrode and an SCE reference electrode. 2.2, Preparations Most of the syntheses were performed under an argon atmosphere, in carefully dried glassware. All the reagents were purchased from Fluka and used as received. 4’-Tolyl2,2’:6’2”-terpyridine (tterpy) [ 231, Ru( tterpy)Cl, [ 231, (dpbH = 1,3-dipyridyl-benzene) Wtterpy) (dpb)P& [ 191, [ 2,4,6-( ter-Bu),) ] PH, [ 241 and Pd[ di( tri-t-butylphenylphospha-ethylene)]-1,3-benzene [ 1 l] were synthesised according to previously reported procedures.
OZ’L f (HZ ‘S) 9P’L f (HP ‘w ) 9S’L ! (HP ‘S) 99’L f (ZH L’L ‘HZ ‘1) 69-L f (ZH L’L ‘HZ ‘1) 88’L f (ZH 2’8 ‘HZ ‘P) 20’8 f (ZH 2’8 ‘HZ ‘P) 21’8 t(HZ ‘S) PE’8 f(ZH 8 ‘HZ ‘P) EP’8 t(HZ ‘S) S8’8 9 :(‘13’c13 ‘ZHDI OOP) WJN HI ‘822 9 :(POd’H/E13a3 ‘ZHl4 18) WW {Hl)dIE ‘(%08) Z =nd3o %m OZ P~PI~!~
(1uanIa w Hoag+QZH3 ‘Ia%t?~I!s) Aqdvr%oltzuIoJq3‘q E 103 ‘uo~.n?lapun ‘palrps SI?Mamlxy uog3~a~ aql (Iounu LEO’0‘8m PI) z13z(N35H93)Pd30 Uo!?PPe JaUV ‘(lm 01) ZDZH~ u! pahIoss!p WManp!saJ ayl put! palerodeaa uaql S\?M 1ua~Ios aq~ +qOZ ~03xngal le paloaq SBMuopnIos aql pw (Iouuu Lgj.0 %u SE) l pue%!I ayl pappe st2~ uo!lnIos s!ql OJ, . (p ~1) Ioulzqla u! pahIoss!p SPManpysal aql pue paleJo -dtrAasv~ luayos ayl ‘uopqy Jal3v .~!tzuf q Z 103paxnga.t SBM (Iw OI ) auola3t! y (IouIur EI I.0 ‘%uI ZZ) Pd@v pue (I0u.u.u L~0.0 %u 0Z) EI3(kl.wll)n2g 30 awxw v
. (S’pfjfj sal!nbal ‘dZNPLHo93) s88=z/w :w ‘3, ZSI-OSI=%~ .(2-g lames!) P’PPZ Pue S.192 t(F-2 James!) Z’E9Z 9 :(POdEH/E13C13 ‘zHW 18) WTN (H,}dlE ‘(%IS ‘2 SE’O) HOlEI-lava Jo amlx!uI of ~1013pazgIelsA13aJ aJaM paugqo (OhpI) z-2 put2 (ob98) g-2 slauros! OMI aql 30 amlxy au ’(1uanIa se amlx!uI JaqlQI3ZH3) Aqdw%olstuoJq:,Ia%e~I!s dq pay -pnd a.xaMsl3npord 8ugInsaJ au ‘q ZZ103arnlwadural uloo~ ltr pangs SBM amlx!uI ayl ‘(lomw 6.0) 9 apdqapIwp aql30 uoypp~ JaiJv ‘pappv ICIayssaDDnsaJaM (Iowur 8’I ) gn8u Put! (IOUUJ8’1 ‘3 LZ’O)nBZaDI!S13 ‘uaq,.L‘(Ia SZ) tIH.L u! (Iounu 8-I ‘8 S’o) awqdsoqd- [ Ihaqd-( Ihnq-1-In-9‘p‘Z) ] 30 uoyyos e 01 ‘~08~ lapun ‘papptt SBM(I0uu.u 8.1) !Tnfy
palDwxa put? pasdIoIpliq uaql St?M11 *lq%!waAopaxngal SBMamlx!uI uoyD=aJ au . (10un.a p0.c) [ sz] (p) auazuaq -Imacw-auaIddold!p-S&s-otuolq-I 30 ayleylap DU!Zaql 01 pappe alaM (Iounu 80.0 %u Z6) lsI(Iew~ um!paIIad aql pw (Iowa PO.5‘8 96.0) c aAp+ap 0uIoJq au ‘c 103 paqp3sap poqlaur aql %upn pas!SaqWh SBM punoduro:, sy._L
‘(6’OIE
sal!nbaJ .I~~NI’H~I~) ()I e = 21~4 :SN ‘&ZL = ‘d.H . (HZ ‘w> 6Z’L !(HP ‘m) E8’L !(zH 9’1 ‘HI ‘P) PZ’8 !(zH 9’1 ‘HZ ‘1) 69’8 !(HZ ‘m) OL’8 3 :(“13”a3) lDYN H[ *(%ZP) 2 E8’0
pIa!lC!p!Ios al!qM e SBpalyos! S~ZM1Dnpoldamd aqL . (Hoam -ZIaZHa :luanIa) Ia%e~I!s %u!sn liqdar%olwuo~~ uumIo3 dq paqs!IduroDDt?SAM uogwyynd *paAotuaJSAM 1uaAIosaql pue *OS%I,~I JaAo pa!lp aJaM slcwlxa 2y%!?~opatuqwoa aqL . (1~ OSI x 2 ) z~3Z~3 qw pmwa pue IYHN 30 uowos snoanbe ut! ql!M paq3uanb SBMam1xy.I uorJX?al aql ‘%~I003 lal3v ‘q OZ103 paxnI.3a.ISBM amlxy aq.I_*pappt!alaM (Iourw PC.9 ‘8 Z) aUaZUaqOUIOlq~.ll-S‘E‘ PUU I (IO”” 82.0 ‘%ZE.0) ?( c ( SH93)d)pd *amlwadural UIOOl01 ULlt?M01 PaMOIIVSBM amlx!ur aql pue pappe SBM(Iounu 6’s I ‘8 6’I ) z13uz snoop -dquv .up_u0~ 103panuyuo:, SAM %u!Ll!ls *eInuw v!h pappt! ~IMOIS SBM (aualuad u! fl p’I !Tna-130 uognIos ‘ItuoZ) gn8 -1 pw 3, 8L - 01 paloo:, uaql S~ZM 31 .passv%ap SBM uoyqos aw PUB a,01 - oi paloo3 SBMwu au . (1~ 0E ) tnu Pawi -S!p li]qSay pw (IOUIUI 8’EI ‘8 81.2) au!p!.tAdouroIq-Z ql!M pa8tzqD S~ZM ‘urnldas iaqqnl B qlj~ paddw turn ap!s a pu?? .xasuapuoDxnua1 e ‘.ralauroulIaql e ql!M pally put?Jeq %u!U!ls cyau%sur E %uluyuo~ YSEIJpauxolloq-pun01 yDau-aaql v
. (WE saqnbal ZOZN~IH*Z~)ws = z/zu :SK ‘3, LOZ= ‘d.K ’(zH I Pm S ‘L ‘HZ ‘PPP) PE’Lf (ZH 8’1 Pue S’L ‘HZ ‘Pl) 98’L ! (ZH 8 ‘HZ ‘P ‘14) L6’L f (ZH S’I ‘HI ‘1) OP.8 t(zH 8’1 ‘HZ ‘P) EP’8 t(zH S’I ‘HZ ‘P) LS’8 t(zH L’I ‘HI ‘1) 99’8 t(HZ ‘m) LL’8 !(HZ ‘S) IZ’OI 9 :(“13a3)
2IbW.I HI
. (%96 ‘8 9E.O) p!IOSal!qM a s??pauyqo SBM9 ‘(1uanIa St? HOan-z13ZH3 k’!I!S) Aqdw%owmq~ put! (1” 001 X Z) Z13ZHs ljl!M uofl~wxa lal3v .paqDeaJ SBM 8 Hd Igun pappv SBME03H~N 30 UoynIos snoanbe ua ‘UaqJ_‘q 9 ~03 amlwadwal LUOOJ it! pa.uys SBM amlxrur uo!lX?aJ au ’(p 0~) auolacw u! (I0uu.u wO’I ‘2 S.0) s 30 uoyqos fz 01 asrM
-do.tp pappt? STZM ( IUI9) I3H w p 30 UoyIos snoanbe uv
*(08~ saqnbal POZN8ZH0’3) 089 =2/m :!?I4 ‘3,261= .dm ‘(ZH S’I PU’JS’Z ‘EI ‘HZ ‘JJP) WI !(ZH S PUv ZI ‘EI ‘HZ ‘1lP) 81’2 t(zH S’P Pug ZI ‘HP ‘JP) 86’s f (ZHS’I PUE S ‘ZI ‘HP ‘PPP) WP !(HZ ‘S) 9S’S t(zH I P’JE S’P ‘S’L ‘HZ ‘PPP) 8Z’L !(zH S’I ‘HI ‘1) SS’L !(ZH L’I ‘HZ ‘P) 8L !L t(zH 8’1 Pug S’L ‘HZ ‘Pl) 6L’L t(zH I P’JE S’L ‘HZ ‘1P) 16’L f(zH L’I ‘HZ ‘P) 62’8 !(ZH L’I ‘HI ‘1) E9’8 t(zH I P’JE 8’1 ‘S’P ‘HZ ‘PPP) IL’8 9 :(z13zC13) 2wN Hr . (%Z9) punodmo:, arnd3o 2 6’0 papla!A (z13ZH3 U!
HOa&qohI ‘Ia%WJS) dqdw8oleruo~q~ ‘z13ZH3 ql!M aDIM fL-fj9(9661)g$.;pz twy
V3!‘u!‘/3 v3!uvaJoq/
70 la !l!vnof ‘V
OL
A. Joudi
ef al. /Inorganica
Chimica Acra 245 (1996) 69-73
71
(d, 2H, 4.8 Hz); 7.03 (d, 2H, 4.8 Hz); 6.91 (m, 2H); 6.64 (m, 2H); 2.54 (s, 3H); 1.78 (s, 36H); 1.42 (s, 18H). FABMS (nitrobenzyl alcohol matrix) : m/z = 1450 ( CszHssN5P,ClPdRu+ requires 1450). 1
3. Results and discussion 3. I. Synthesis of ligand 1 and complex 2 The synthetic scheme leading to 1 is summarised in Fig. 1. The key step is the cross coupling reaction between lbromo-3,5-dipyridyl-benzene (3) and the organozinc derivative of 4 bearing two protected aldehyde functions. This reaction, catalysed by the palladium tetrakis-triphenylphosphine complex, affords the intermediate 5 in moderate yield (62%). The synthesis of 3, previously reported [ 261, uses the Stille-type cross coupling [ 271. It is little improved by substitution of 2-trimethylstannyl pyridine by the pyridylzinc reagent. After the deprotection of the precursor aldehyde, the two phosphaalkene substituents may be introduced following the method of Yoshifuji et al. [ 281 and previously described in the synthesis of 2,6-diphosphaalkene-pyridine [ 291. As in this case, a mixture of two isomers (E-E and E-Z isomers) was obtained (see Fig. 1) . Therefore, the aromatic region of the ‘H NMR spectra shows a complicated pattern. On the other hand, the ratio of the isomers ( 86% for the E-E isomer) can be evaluated from the signal of the phosphorus atoms in the 31P{ ‘H) NMR spectrum. The mass spectrum of 1 showed
1+
\0
CH3 BF4
Fig. 2. Reactions used in the synthesis of the heterodinuclear complex 2.
a molecular peak at m/z = 885, and the expected fragmentations also confirm the proposed structure. The reaction (Fig. 2) of 1 with the transition metal precursors (Ru( tterpy) (acetone),‘+ and Pd( C,H,CN) &12) readily leads to the heterodinuclear complex 2 in high yield (80%). In view of the a priori numerous possibilities of reaction between the ditopic ligand 1 and the two inorganic fragments, the high preparative yield observed is indeed surprising. The palladium precursor can react with the two different sites of coordination of 1 contrary to the ruthenium one. Actually, all attempts to introduce ruthenium into the diphosphaalkene-benzene moiety were unsuccessful. This difference of reactivity of the two terdentate sites towards ruthenium complexation can be exploited to prepare selectively some heterodinuclear complexes in two successive steps. The formulation of complex 2 was established by ‘H, 31P( ‘H) NMR spectroscopy and by FAB-MS. The main peak at m/z = 1450 corresponds to the mass of the monocationic complex 2+ and has the appropriate isotopic pattern. 3.2. Electrochemical measurements
Fig. 1. Reactions used in the synthesis of the bridging ligand 1.
The electrochemical behaviour of 2 has been compared with those of the mononuclear palladium complex 7 and ruthenium complex 8 (Scheme 2). The redox potentials of the complexes 2,7 and 8 obtained by cyclic voltammetry in THF are reported in Table 1.
A. Jouairi et al. /Inorganica Chimica Acta 245 (19%) 69-73
72
of the heterodinuclear complex 2 displays three redox couples directly attributable to Ru(II) oxidation (0.57 V), diphosphaalkene-benzene moiety reduction ( - 1.22 V) and terpyridine-centred reduction ( - 1.48 V) by comparison with the values of the isolated constituents 7 and 8. The main feature of this behaviour is the small effect of each of the organometallic parts on the other. This situation contrasts strongly with other homodinuclear complexes where a large influence is observed [ 19-211. The neutral charge of the organo-palladium moiety is possibly one of the factors leading to a low interaction between the two subunits. Indeed, both electronic and electrostatic factors are much less predominant in this case. Another important parameter is the torsion angle around the interannular C-C bond. A torsion angle close to 90” leads to a negligible overlap between the r-electron clouds of the metallated rings.
BF4-
Scheme 2.
3.3. Absorption and luminescence properties
Table 1 Electrochemical
data for 7,8,2
and Ru( tterpy)2Z’
El’* (V) (vs. SCE)
Compound
R”m~II
PdL” -
’
RuL+”
- 1.23 ;: 2 Ru(tterpy),‘+
’
0.61 0.57 1.25
- 1.22
-1.47 - 1.48 - 1.24
“Ref. [ll]. bRef. [19]. ‘Ref. [23]. Table 2 UV-Vis spectral data for complexes
7.8 and 2 in CH&
Compound
Amax(nm)
7 8 2
475 (0.5). sh 350 (8.1), 318 (19.95). 269 (24.6) 511 (13.8), 375 (11.8). 288 (58.4), 245 (43.4) 512 (22.3). 316 (88.3), 286 (117.5). 242 (87.4)
(eX
10’ (1 mol-’ cm-‘))
Each electrochemical reaction is consistent with single metal-based oxidation and ligand-based reduction. The redox couples show characteristic patterns for a reversible couple ( AEp close to 60 mV, Zpa= I,,. and Ii, proportional to ui’*) . As previously reported [ 11],7 shows a reversible reduction wave at - 1.23 V. It is attributed to the addition of one electron in the rr antibonding orbital of the diphosphaalkene subunit, on the basis of the EPR spectrum of the reduced species. The redox couples of the ruthenium complex 8 indicate that the u donating properties of the deprotonated dipyridyl benzene strongly affects both the metal centre and the external terpyridine ligand. The stability of the Ru( III) state is illustrated by a drastic shift of potential (0.65 V) between 8 and the Ru(tterpy),*+ species. Otherwise, the terpyridinecentred reduction became more negative by at least 0.25 V as compared to the Ru(tterpy)22+ complex. The CV curve
In Table 2 are reported the A,, and e_ values for the dinuclear complex 2 and related monomers 7 and 8. The ruthenium complex 8 exhibits ligand-centred w-@ transitions in the UV region and broad metal-to-ligandcharge tranfer (MLCT) bands in the visible region. The palladium complex 7 shows also ligand-localised transitions at high energy. In the lower energy region (h> 330 nm) a broad structureless absorption band can be assigned to a perturbed ligand-localised transition as already described in Pd(I1) ortho-metallated complexes [ 301. In addition, a broad band at 475 nm with a low extinction coefficient was assigned to a metal-centred transition. The spectrum of the heterodinuclear complex 2 is dominated by the rr-rr* transition and MLCT bands. In fact, the spectrum of 2 coincides almost with the sum of the spectra of the component subunits 7 and 8. On the basis of the spectroscopic and electrochemical data, we can again conclude that the ruthenium and palladium parts in 2 interact at most only slightly with each other. The emission spectra of 2, 7 and 8 were recorded in degassed CH,Cl, and CH,CN solutions at room temperature. Generally, luminescence lifetimes of cyclometallated palladium complexes are very short in room-temperature fluid solution. We also did not observe any emission spectrum for complex 7. As previously reported, complex 8 displays a weak emission band centred at 785 nm (4 = 4.5 X 10e5) due to the MLCT excited state [ 3 11. This result can be interpreted as being caused by the o-donating ability of the cyclometallating ligands. The emission spectrum of the heterodinuclear complex 2 exhibits a similar band to 8 (A,, = 800 nm), but with a lower intensity (45% of the intensity of 8 in the same conditions). The decrease in intensity is slightly different in CH3CN (30%). Considering the spectroscopic and redox properties of 2 in the frame of the localised orbital approach, an energy or electron transfer reaction cannot explain this phenomenon: (i) energy tranfer is excluded by the lack of an accessible excited state of the palladium part; (ii) the electron transfer reaction towards the palladium subunit is
A. Jouaiti et al. /Inorganica Chimica Acta 245 (19%) 69-73
slightly endergonic (0.2 eV). More probably, we suppose that the palladium part acts as a tuner. Indeed, small perturbations of the energy levels of the ruthenium excited state may have important effects on the decay processes. Examples of this type of perturbation concerning Ru(II)-Pt(I1) and Ru( II)-Pd( II) complexes have been already observed [ 32,331.
Acknowledgements This work was supported by a grant from the Swiss National Research Fund (Switzerland) and by the CNRS (France). We also acknowledge Dr J.P. Sauvage for helpful discussions.
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