Stereochemically rigid mono and bis pyridazine complexes of [RuII(hedta)]− (hedta3− = N-hydroxyethylethylenediaminetriacetate)

ELSEVIER

inorganica Chimica Acta 279 (1998) 85-94

.....

StereochemicaUy rigid m o n o and bis pyridazine c o m p l e x e s of [ Ru u (hedta) ] - ( hedta 3 - = N-hydroxyethylethylenediaminetdacetate

)

Ya Chen, Rex E. Shepherd * Department of Chemistt3', Universityof Pittsburgh, Pittsburgh, PA 15260, USA Received 14 August 1997:revi~d I0 November 1997:accepted 27 December 1997

Abstract Pyridazine (pyd) complexes of [ Run(hedta) ( pyd),, ! • ( n = I or 2. hedta ~ = N-hydroxyethylethylenediaminetriacetate) have been studied by 'H NMR and electrochemical methods. Substitution of pyd on [Ru(hedta)(H20) ] - has a second-order rate constant of 32 M - *s at 22°C. The Ru u/iv couples appear at O. 16 V for the mono and 0.49 V for the his complexes versus NHE. in the his complex, one carboxylato donor is displaced forming an RunN4 in-plane set with two of the N donors as pyd ligands, bound via N-I attachments. 'H NMR assignments confirmed by t H J H COSY spectral data for the N- I bound pyd ligands are in ppm: ( n = I ) H-3, 9.07: H-4, 7.64; H-5, 7.55; H-6, 9.56; (n ffi2 ) H-3, 9.13; H-4, 7.66: H-5, 7.47: H-6, 8.72. Addition of the second pyd donor occurs ~ 3400 times faster than for pyrimidine, pyrazine or pyridines indicative of an acceleration via an internal base-assisted dissociation of the in-plane carboxylate of the hedta~~ chelate using the available N-2 base position of the coordinated pyd of the mono complex. The v/2 (N,N)-coordinated intermediate is rapidly displaced by a second pyd ligand. Neither the mono (n ~ I) or bis (n = 2) complex is in exchange with the pyd free ligand pool in contrast to [ Run(hedta) (pyre)., I (pym ~ pyrimidine) in which the second pyre base rapidly exchanges with free pyre. Also, whereas the second pyre site also exhibits a 1,3ometallottopic shift faster than the NMR time scale, neither [ Rut~(hedta) (pyd),,] (n ~ I or 2) exhibit evidence for a 1,2oshift up to f~0°C. Therefore the banier to a 1,2-shift fi)r the [Rutt(hedta) ] complexes of pyd is much greater than for other low-spin d" complexes including W(0), Pt(IV) and Re(l) pyd complexes, or for Ru(ll) with other ~acid ligands present as in the I RuU(porphinato)(CO) I system, an outcome predicted by former studies of S. Alvarez et al., J. Am. Chem. Soc. 109 (1987) 531¢~and by K,R, Di:~ou, Inor~ O tern. 16 (1977) 2¢181. Protonation of the mono complex (pK,,,. 3.42) occurs with a a immediate shift of the Ru"/m E,, v:tlue l~r IRu(hedta) ( pydH ) I to + O.08V versus NHE ( pH ~ I.t72) for the cis°equatorial and trans~eqt, atorial isomers. The cisopolar fi)rn~ shills more slowly to one of the other isomer l~rms in t}0 rain. The [Ru"(hedta)(pydH) ] complex has 'H NMR shifts at (ppm) He3.9.14', He4 and H-5, 7.74 (overlapl~d); He6 at 9.53 and 9.41 fi)r the two inoplane stereoisomers. In acidic conditions, the I Run(hedta)(pydH) ] complex redistributes slowly to Ibrm the bis and [Ru(hedta)( H20)] complexes. At pD ~ i.0 the additional ptotonation of the inoplant, carboxylate of [Ru(hedta)(pydH)] opens a pethway ,hat promotes rapid 1,2omvtallotropic shifts, probably via a 'ping ~mg' motion that makes and breaks the Ru n bond to N°I and N-2 rapi,'i~' 'he tH NMR timescale. The absence of fluxionality for the [Ru(hedta)(pyd)] .... and [Ru(hedta) ( pyd ) .~] ~ complexes compared to flr~ti..... '~~, in [ Ru( hedta ) ( pyre ) 2] ~ is attributed to the greater ~r basicity ofpyd compared to pyre and the greater backdonation from Ru(ll) to pyd than to pym. Both features strengthen the Ru(II)~(N-I ) bonding which creates a high barrier to I,2-metallotropic shifts for the pyd complexes. Protonation weakens the Run-pydH' it-bonding and lowers the barrier to 1,2-shifts. © 1998 Elsevier Science S.A. All rights reserved.

geywords: Rutheniumcomplexes: Pyridazine complexes

1. Introduction Pyrimidines have rc.cently been shown to make complexes with [ RuU(hedta) ] - (hedta ~~ = N.hydroxyethylethylenediaminetriacetate) through the N-I lone pair, followed by a linkage migration to ,q2(1,2), ~q2(1,6) and ~12(5,6) positions

ill: * Corresponding author. Tel.: + 1-412-624 8200: fax: + 1.412-6248552. 0020-16931981519.00 © 1998Elsevier Science S.A. All rights reserved. PI! S0020- 1693 ( 98 ) 00043°7

(hedln)#u"---N~ !

tH ~ " N ,

K Chert, R.E. Shepherd I lnorganica Ckimica Actra279 f 1998) 85-94

86

Pyflmidine nucleobases also adopt the q2( 5.6)-coordination mode [ 2], illustrating a possible alternative metallation mute for DNA by strongly ~r-donating metal centers. Fundamental studies of the coordination ofdiazines are of interest to elucidate further aspects of these 'n-type complexes. Pyridaziue (pyd) is the d i a n e most closely matching the or~ i t y and resonance energy of pyrimidine and hence the next most ~ ~date to form ,n--type ~12 complexes. Values of the pK,, a measure of o-donor strength, and resoPenCeenergy related to ~-acceptor power have been reported as indicated for their structures [ 3 !:

,N~N, pK, (1~') RIB

'N~"I 1.31 1 4 ~

2.33 I0 to 12 kadAml

Pyridazine is a somewhat better ~r-acceptor ligand than pyrimidine due to the more direct electron withdrawing influence of the second nitrogen ortho to the coordination site of the metal at N-I [4~7]. The possibility of finding new 'e-type ~2 complexes of [Ru"(hedta)] = with pyridazine prompted our interest in studying i~s complexes. However. there is also significant interest in what has been termed 'latent fluxionality', or even isolation of species which correspond to the intermediates of such 1.2-metallotropic shifts: I

that for d~'octahedral complexes there should be an increasing barrier to fluxionality in the order Cr° < Mn I < Fe" (or Mo°, W° dppe > phen > bpy :~ tmen [ 22 ] and are ~ I.? kcal mol- ~ tower than for [Re(CO)3Cl(pyd)2| + [21,22]. in the case ofRu(ll) complexes, pyridazine ligands exhibit coordination fluxionality near 25°C with 1,2-shifts tbr the Ru(CO) ( tetra-/,-( isopropylphenyl ) porphinato) ( 4.5-dimethylpyridazine) complex. The xl-~(N,N) intermediate (2) has been suggested, being formed with a low barrier of 12 to ! 5 kcal real " * [ 23 ].

(2)--

tlq - - -

Methods of preparation of pyrida~ine complexes have I~en reviewed 18,91. There are numerous recent studies of the antifenom~Mt|¢ interactions provided through bridging pyfldazine assemblies where other ligation ~mpletes the coordination sphere of the metal centers, M and M'.

In a related pyrimidine syslem, we have recently reported that the hisosubstituted complex of I Ru"(hedta) I of slruco lure (3) exhibits one stereochemically rigid pyrimidine and one fiuxion.l pyrimidine with 1.3ocoordinatio. shifts 12¢~I:

0)-

(i)-

n

(a- 1 to3)

Examples of the,~ studies for M or M ' ~ Ca( II)-bridged l l0~141 or M"~,Mn ", Co", Ni", Co*', Fe" II5,161 are representative, Although the normal monodentate No I coordination ofpyd dominates studies in the earlier coordination chemistry of pyridazine I8,91 recent attention has been given to the 1,2~ m e t a l l ~ i c shifts or pyd coordinated to the d ~ and d~ fr~o merits for ~"(PRO~CI I 171, W(CO)~ I 181, pttV(CH~)~X ( X - , ~ l i d e ) [19,201, Re*(CO)~X 121,221 and Re"(porphinato) 123 l, The transition states for the 1,2+flux~ i t y of pyridaeine complexes is viewed as weakened o~¢mating. 20 e ~- count, intermediates [18,241 for ML~(pyd) d~ ca:~s, A molecular orbital treatment suggests

In a preliminary report of early work on the mane and his pyd complexes o1" I Ru(hedta) I ~. we postulated an asymmetric intermediate (4) was importaat in the substitution chemistry ofpyd with I eu"(hedta) I .....127 I. With this background we ,~t out to explore w~ther i Ru(.ed|a) (pyd): I " has one or more lluxional pyridazines as its I Ru(hedta)(pyre) ~l - analogue, or whether the relatively low steric factors and harder ligand environment of hedta ~~~(compared to the porphinato-carbonyl case) might favor stere(x:hemically-

rigidcomplexes.

E Chen. R,E. Shepherdilnorgunicu Clmnica Arm 279 ¢1998) 85-W

4) =

)

The unusual 'q-~-coordination mode of pyrimidines for IRuU(hedta)]- [I.2,27.28] is further established by the absence of such species in the pyridazine complexes of this report.

2. Experimental Pyridazine was obtained from Aldrich. K l Ru( hedta)CI I was prepared in a modification of our earlier reported procedure for Nat Ru(hedta) (H.~O) I .41t20 by using KOH in place of NaOH upon work up 1291, or via the van EIdik procedure 1301. For NMR work there is an advantage in using the K I Ru( hedta)CI ] starting material in order to lower the amount of HOD present in the linal NMR samples. Preparation of At-purged samples containing I Run(hedta) (DaO) I ~ over Zn / Hg with the appropriate ligands have been reported elsewhere I 1,2,26]. The electrochemical studies of Ru n complexes at a glassy carbon working electrode and a saturated sodium chloride calomel reference fidlowed procedures using 40 mV s ~sweep rates and a 50 mV stepping voltage for differential pulse voltammetry or 50 mV s * sweep rates h)l' cyclic voltallm~elry as in previous work ll,2,261. The electrochemical experiments were calibrated using the El~a values Ior I Ru(NH~)~ICI~ (0.0t~ V versus NHE) and IRu(bpy),~l(CIO,~),~ (I.29 V versus NHE) as reference standards 131 I. Samples were run in 0.10 M NaCI electrolyte at ~ 2.5 × I0 ~~ M. The solvent was purged with Ar prior to transfer of a suitable sample of [ Ru(hedta) (H~O) ] ~ or the mono or his pyridazine complex by syringe techniques from a more concentrated stock solution prepared under Ar from known weights of KI Ru(hedta)Cl] and pyridazine in H~O over Zn/Hg. pH adjustments within the electrochemica, cell were made by syringe injection of small amount~ of 1.0 M HCI or NaOH while the cell contents were vigorously stirred via a rice-sized stirring bar, driven by an external magnetic stirrer. The pH was monitored via a ~fini-combination pH probe placed in a septum and mounted in a port at the top el'the electrochemical cell. The pH probe and meter combination were calibrated using commercial buffers. 'H NMR spectra were obtained at 25°C using Biuker AF300 or AMS00 NMR spectrometers averaging ~ 256 scans. IH NMR data were referenced to DSS (0.()0 ppm) weighed amounts of K[ Ra(hcdta)Ci] and pyridazine were placed in 5.0 ml round-bottom flasks such that the iinal

87

desired RuU:ligand ratios of 2.00:1,00, !.00:1.00, 0.80:1,00 or 1,00:2,00 were achieved, Small ZnlHg chips were added along with 3.00 ml of D20 and enough standard DCi to bring the pD value to 3.00. The sample was septum-sealed and purged with a stream of Ar from a needle attached to an Ar line. Exit of gas was through a second needle connected with surgical teflon tubing which passed the gas through a septumsealed NMR tube with a final needle exit. Samples were transferred to purged NMR tubes at convenient sampling times of 20.0 rain, 50.0 rain, 5.0 h and up to 14 days after mixing to access the reaction progress. It is known that action of dilute DCI solutions over Zn/Hg during the reduction steps raises the final pD value to near 6.0. For samples under acidified conditions, injection of known volumes of purged 1.0 M DC! were made via the septum of a capped NMR tube in order to bring the pD value to 1.0. The temperatures of the sample in the 'H NMR probe was controlled by software associated with the Bruker NMR spectrometers, In the case of low-temperature studies samples were prepared in CD~OD (Aldrich) instead of D20 or in 50:50 (vol.:vol.) CD~OD/ D:O to prevent freezing of the solvent phase.

3. Results and discussion 3. i. /H NMR of lRuthedta)tpyd)] ~ and IRtt(hedta)(pyd):]- in neutral solution

Ruthenium(Ii) polyaminopolycarboxylates are known to add two N.heterocyclic ligands in stepwise substitution reac* tion~ as in Eqs. ( I ) and (2) fi)r lRun(hedta)(11~O)J '-. L ~ pyridines, pyrazines or pyrimidines 132,201. /,i

IRulhedta)(H:O)l

+l,~lRu(hedta)LI /,

+1=1:O

(I~

;

/t ¸ :~

I Ru( hedta)L I + L {~ I Ru( hedta)L~ I

(2)

In prior reports of I Ru(edta) I'~....and I Ru(hedta) I come plexes, the rate el" the second substitution ol'L has been much slower than the first substitution step. allowing separate study of mono and his species. The first step involves displacement of H:O whereas the second requires the displacement of a coordinated carboxylato donor. The resultant bis complex has the two Noheterocyclic donors in the Run( N~ ) plane with remaining axially-coordinated carboxylato donors as shown for pyrimidine in ( 3 ). Step I is rapid with second-order rate constants of ~ 30 M = ' s ' I i ] whereas step 2 is limited by the dissociation ot' the in-plane carboxylate donor of the mono complex. This rate has a value of ~ 1.57 x 10 ~,s s ' at 25°C 126,33]. Hence, when I Ru n(hedta) (H20) ] ° is combined with L at I:l stoichiometries, the pyrimidines yield only the mono complex, in the case of pyridazine the second sub5tio tutio;; reaction proceeds more rapidly which creates a competition for the formation of mono and his pyddazine complexes even with I Ru(hedta)(H:O) ] in excess if mix-

E Che~ R.E. Shepherd/ Inorganica Ckimica Acta 279 (1998) 85-94

and are only modestly differentiated for the [ (CO)5W(pyd) case. These were assigned as 7.35 ppm for H-5 and 7.5 ! ppm for H-4 [ 18]. We have assigned the shifts for the mono and bis pyridazine protons as shown below: m

9.$6 H 7S$ 11

I

I ~sT~7.47 9.13

helml~c~o

9.19 ........ m

....... u

....... m

...... ~ h ....

rrpz

Fig, I, 'H NMR of pyridazine pr~ons of [Ru(hedta)(pyd)] ° ([Ru(hed~)(D~O) h s 2,61 x 10 =~ M. Ipydl,~ 1.31 x 10 ~= M (1.00:0,~) 5,0 h after mixifl~, Dol~ (@) i~icate [Ru(hed~)(pyd).,I ~ re~Mnc~; pO-~ 6.0:D~O; 2S~. it- 0,07),

ing problems provide m o m e n t , locally.high [pyd]. This is shown in Fig. I at the [ Ru" (hedta) (H=O) ], ~ :[ pyd ] ratio of 1.00:0.S0 with [Ru(ll)],,~-2,61X 10 =2 M, There are four distinct resonance lines for the mono product and four lesser abundant lines for the his product.Pyridazine ~ a free ligemd in D~O exhibits 'H NMR resonances at 9.19 ppm (p~udo doublet) for ~ equiva~t H-3 and Ho6 prmons and %83 ppm (p~udo triplet) for ~ equivalent H~4 and Ho5 protons, The~ *H NMR shifts may be u~d to help assign the ~ i n a t e d ligand features,The assignment of the laser resona~s ( Fig, I ) to the his species is suppofl~ by the loss of MI of the mono r ~ c e s and the growth of the laser ~ ) o m 3 ~ at [RuN(hedta)], =:|pyd] of 1.00:~2.00 as shown in Fi B, 2 which was taken at !:3.56, This situation is chosen to provide 2 equiv, of pyd for~)ordination, but lear. i~ excess li~andto show therelativeshiftsof thecoordinated ligands versus the un~mplexed ligand, Spectra were recorded at~r 30 min and followed for up to 13 days. The re~s ~ constant for all ~ t r a after 30 min, show.. ieq[ no ~ change in substitution, indicating equilibria I 2 are e s m b l i ~ in less than 30 min. The assignments for the coordinated lig~d protons were made on the basis of comparison with the observations for known Re(!), Pt( IV ) [19,20], and W(0) [18] pyrida~ine complexes For [ (CO)~W(pyd) ] in CDCI~ the H~3 ~ experiences only a dight dowofleld shift c o m ~ to the free ligand whereas the H~6 ~ is strongly perturbed by the nearby W(O) coordinated at N-I; the H~ proton is s h i ~ downfield by ~0,$2 ~ ~ coordination [ 18 ], Similar large downfield shiP--observed forH.6of lPt(CH~)~Cl(pyd) ] ( -0.93 ppm) and lRe(CO)~Cl(pyd)~l ( -0,67 ppm) upon coordi~ of pyd 119.201. Assignments for H-4 .,rodH-5 pyd Wormere~remore difficult. These appear as overlapped, equivalent ~ for the I~(IV) and Re(li) pyd complexes [ 19,201

,3 z~*'H 9,19 The assignment for the H-3 at 9.07 prm for mono and at 9.13 ppm for bis complexes is based on the lesser effect of the metal center on the chemical shift of this position as seen for the W(O) system, The assignment of the more perturbed re~nahce to the H-6 proton is assisted by the knowledge that ~ protons on heterocyclic rings for the fully-coordinated hedta~~ ligand induce downfield shills for [Ru'(hedta) ] -, bUl that neutral and cationic centers promote upfleld shifts 1341, In the his complex the combined, effective charge for donor,~ ~md R,(II) at the Rg(II) cemcr is zero. whereas I'or the mono specie~ the I~al charge is ~ I. Hence, the uplleld shift at 8.74 ppm for H o(~of the bis complex and the downfleld shin for H-6 in the mono complex is consistent with literature precedents 1341. These ~signmenL,; were confirmed by the 'H..-~HC O S Y spectrum (Fig.3). The 11-3 proton is torte. latedonly with the resonance at 7.66 ppm which forcesthe assignment of thisfeatureto the H.4 resonance. Hence, the 7.47 ppm feature ari~s due to H-5. The features shown by the 'hedta' region for the bis complex can be assigned as follows (ppm): CH=CH=(ea ): 3.06, 3.18: CH~CH,OH: 4.05, pendant carboxylate AB ,set 3.84; coordinated carboxylate AB .set 3.86: ~cond coordinated carboxylate AB set 3.52, CH~CH~OH: 3,18. The presence of the sharp, separate resonance lines of the free ligand, added as reference markers for the bis complex, also ~ow that ligand exchange between bound and free pyd is ab~nt at 25~. Since *H NMR 1,2-shifts have been de~ected for pyd as a ligand in the Pt(IV), Re(I), W(0) and RuU(porphinato) complexes 118-231 as the temperature increases, the spectrum for [Ru"(hedta)(pyd)21 ~ was examined at 5.0°C increments from 25.0 to 60.0°C. No exchange broadening was observed for the free ligand/complex resonances even at 60.0°C. No large shift or approach to a coalescence spec-

Y. Chert, R,E Shepherd~ inorganica Chimica Arm 279 (1998) 85-94

89

I

-~l:=~I

-"-

IU

I

" "~

9.e

w .... l

8,0

Fig. 2. *H NMR o f I Ru( hedta)(pyd).,I • ( I Ru(hedta) (D.,O) l, free pyd resonances: pD ~ 6.0; D~O, 25°C,/z - 0.07 ).

l

'/.0

--i

--

I

6.0

--I

, S.e

'

¢

"~

4.8

i

"i"

$.e

~ 2.61 x I0 * ~ M, I pydla ~ 9.29 x 10- " M (i.00:3.56), 5.0 h after mixing. Arrows point to

trum was observed. Only a 0.07 ppm downfield shift in the 55 to 60°C spectra in the H-6 proton was detected. This implies a much higher free energy of activation for 1,2-shifts of the coordinated pyd in the [ Ru'(hedta) (pyd)., I - complex than for similar low-spin d o complexes studied perviously [ 17-23 I. Hence, the barrier is much greater than the 12 to 15 kcal tool- * barrier *o the 1,2-shifl observed for I Ru"(porphinato)(CO) ] 123], or the 20 to 25 kcal mol ~ * barriers reported for Pt(IV) and Re(I) 119=221. Since the small shift of the H-6 proton occurring at 60.0°C was far from coalescence behavior, a much higher temperature would be necessary to promote 1,2-fluxionality in the I Ru(hedta)° (pyd).~l ~ case. Up to 60.0°C the 'hedta' regime it~dicated two coordinated carboxylates and one pendant carboxylate of the trans.O carboxylate coordination geometry as shown for complexes in : 3). in the pyrimidine case, but evidence of the onset of carboxylate interchange between bound and the pendant carboxylate commenced at 60.0°(2. Therefore, above 60.0°C any 1,2-shift of the pyd ligands would be complicated in its interpretation by the remaining coordination provided by the bedta "~~ chelate. Therefore we did not examine data above 60.0°C due to the carboxylate donor exchange complication.

3.2. IH NMR of lRuthedta)(pym) ~ at pD ~ 2.0 The IH NMR spectrum for [Ru(hedta) (pyd) ] - changes as a function of sample pD. if a sample is prepared such that the amount of the bis complex is kept very low as in Fig. 4(a) and the sample is then acidified with DCI to a pD of 2.0, the new spectrum Fig. 4(b) is obtained. It is noted that the pyd H-4 and H-5 pair moves slightly downfield and merges at

7.74 ppm while H-3 moves slightly from 9.07 to '~. 14 ppm. The effect on H-6 is more pronounced. The H-6 feature at 9.55 ppm in the mono complex is split into two features at 9.53 and 9.41 ppm. The areas for these two features sum to equal the resonance attributed to H-3. We attribute these changes to protonation at N-2 as shown by equilibrium (3),

+1~0 ÷ D

(3) The fact that H-6 is split into two components is explained on the basis of two main stereochemical isomers of [ Ru (hedta) ( pyd ) I ...... It has been shown previously that there are three possible isomers of [ Ru (hedta) L I - species [ 35 I. Of these, the c/s-polar is very sterically hindered. Often only the c/s-equatorial and trans.equatorial forms are abundant. Strong ~-acceptor ligands such as CO force only one isomer, the cis.equatorial, to dominate. Here, the data supports both cis.equatorial and trans-equatorial derivatives in the ratio of i.00 to 0.46. The difference in the H-6 position for the cisequatorial and trans-equatorial isomers for the protonated pydH + ligand most probably originates in the nearby pendant alcohol functionality of hedta 3- which is available for Hbonding in the cis-equatorial isomer. The alcohol function-

Y. Chen, R.E. Shepherd / Inorganica Chimica Acta 279 (1998) 85-94

]

0

: i i [ I ;

o

1000 2000 3000 I~

u

dul

Fi~, 3~ 'H~'fl COSY spectrum of [Ru(hedta)(pyd);] ° in D20: 25°C; [ Rut tl ) I , , . " 2.61 x I0 ° ~ M; ~ conditions as Fig. 2: spectrum taken at t

3~0day~,

ality cannot reach the pydH* ligand in the trans-equatorial isomer, but readily achieves H-bonded contact distance for the c/,v-equatorial isomer as clearly shown by models of each isomer, This makes both isomers of more similar nature for the neutral pyd ligand. Thus, these complexes appear to be magnetically equivalent for the mono [ Ru(hedta) (pyd) ] complex, both isomers exhibiting the same 'H NMRchemical

H-3 features would coalesce. This is not observed, and the mono species must be stereochemically rigid. When the bis complex is treated with DCI to pD---2.0, there is very little shift in any of the ring protons. This leads to the conclusions that when backdonafion is shared between Ru(ll) and two pyd rings, there is insufficient enrichment in basicity of the bis complex to allow for protonation at pD-~ 2.0. Electrochemical evidence presented in the next section support the conclusions drawn there by iH NMR data. An estimate of the pKa of the mono complex is ~ 3.42, based on complete conversion to the protonated mono complex at pH 1.92. Hence, the pKa must he between 3.5 and 4.0. Generally when (NH~) sRu 2+ or (NH3) sos "~+ coordinates to pyrimidine or pyridazine, there is an increase in basicity of the ring by about 1.0 log unit [4,6]. Applying this estimate to [ Ru(hedta) (pydH) ] yields a value of 3.33 which is close to our estimated value. Therefore at a pD of 2.0 we are certain that all of the species are converted to the protonated form. In our previous preliminary report on the pyridazine complexes [ 27 ], we obtained data between pD = 0 and pD-= !.0. Under the conditions the cis in-plane carboxylate donor is lost due to protonation. Thus the species at pD ~ 1.0 exists protonated as in (5):

TO

}5 "0)

shifts.

An important conclusion may be drawn from the main° rained differentiation of Ho3 and Ho6 Wotons in both isomers. If the g.(II) center were to migrate rapidly ~ t w ~ n No I and N~2 wtth the ~Imultaneous shift of the proton, the Ho6and

{a)

IMPM

I ~ ~l, ~H NMR ~t" ~ P y d

| Ru(~)

au

~,s

u

"e,,o

I ~ (a) pD ,~ 6,0 ~t ~ , 0 rain re,actium time, ( b ]) pD ~, Z,0 at I,O h alker acidification; I Ro( il ) h,.~~ 2.61 x 10 ": M

i'. Chert, lt.E. Shepherd# imirlclni('a ('hinli~-a Aria 279 ¢1998i 85-94

Under these more acidic conditions, the H-4 and H-5 i~sonances coalesce, The H-3 and H-6 proton resonances are very broad, This can be explained by 1,2-metallotropic shifts either at the original pydH + coordination position or by oscillation ofpyd between the original site and the position opened cis by the loss of the protonated carboxylate donor, This could occur by an intermediate similar to (4) but with the fully removed and protonated carboxylato arm. Reprotonation of the nitrogen either in (5) or by oscillating via the protonated form of (4) accounts for the broadening of H-3 and H-6 resonances and the equivalence of the H-4 and H-5 protons at pD_< I.O. Therefore in the absence of sufficient [H.~O+ ] to remove the cis carboxylato donor, the coordinated pydH + ligand is stereochemically rigid as shown by the 4-line per isomer spectrum shown in Fig. 4(b) at pD --- 2.0, These new observations at pD=2.0 revise slightly our interpretation made previously for conditions ofpD < 1.0 wherein the effect of the loss of the cis carboxylate had not been recognized [271. 3.3, Electrochemisto, The formations of IRu (hedia) (pyd) I " and IRu ( hedta)(pyd)~l- were also detected by following the differential pulse voltammognims us a function of time. Fig. 5 presents the time evolution data at [ Ru(hedta) (H~O) 1~= 2,38 x IO '=~ M at a ratio of [Ru(ll)I,,:1 pyd]~ of 1.00:2.00. Under these conditions the substitution is second-order and the formation of the his s~cics is much slower. The spectrum at zero lime is that of IRu(hedta) (H,,O) ] ill pH ~ 5.20 which shows a Ru 'Mu wave at -0.06 V and Ru mt'v processes at 1,00 and I, 14 V, described previously 136l, Within 2.0 inin (Fig,. 5) the niono complex is fully I'ormed with loss of the I Rulhedta)(H,,O)l DPP t'eatai~, and exhibits its Ru ' ' m wave at 0,16 V, comparable to I Ru(hedla) (pyridine) I ..... ill O. 13 V 12al and [ Ru(hedta) (pyrimidine) I - at 0. l0 V I I I, As shown by DPP spectra at 30,0 niin and 18.0 h ( and many

j/

l

//7

I,-A l

t

J, !

/

I

r

ell

0.41 0,1i

el0

I,i0

Ill

1,40

I

i i

i/

i-,./ ! A'" ~ss

,

..........

I

I

*

I

i

I

I

t

I •

Ilt

I

I

i

L

|__A__J

i

t

•

I

lit

I

J ~ _ _ |

Ill

_ _ _ _

__

lali

II v~. N l i I Fig. 6. pH-dependenl changes in the DPl) specirum of IRu( hedia )( pyre ) I • (ai pH=4.20: (bl p H = l . 9 2 al 3 rain. 30 rain. 90 rain. 18 h IRu(llilj=2.38x10 ~Mlpydlj~lRu(il)l,.

not presented) the conversion of the mono to bis complex is complele in 18 h. The bis [Ru(hedta)(pyd)2J = complex exhibits its Ru u~lli wave at 0.49 V versus NHE at near neutral pH, conditions corresponding to the neutral ligand coordination ob~erved by IH NMR. Upon acidification with HCI, added via a syringe to the electrochemical cell, the Ru u/m wave shifts from +0.16 V versus NHE (pH =5.20) to +0.08 V (pH ~ 1.92) (Fig. 6). This complements the observalioi~ ~:f the 'H NMR si:~¢l:lrunl of |he IRu(hedta) ( pydH ) !. Prolonalion of the N-2 nitrogen causes a ne~a:ivi~,shill in potential of 8()inV. This is opposite the shill usually observed when ~°accepl(w ligands such a~ pyrazine arc coordinated to Rulll) 14,61. Normally, the

protonation ot' il 'i"roacceptor Ill, and ellhilliees the 'ffoaceepior power, stabilizhlg l~u(II). This normally pronlote,~ a more positive Ru tt'ltt couple, In the case ot' pyriditi~ine., the No2 nitroten is close to the l u ( i l ) Celiit?r. I)rotonlltion ,~f thi~ position due to ~he enhanced basicity of the coordinated pyd Iigand occurs at the expense of increased l u ' H ~ electro° static repulsion, a destabilizing influence. The magnitude of the shift on the E~j,~ value shows that this effect costs 1.84 kcal tool ~ ~on the reduction of the Ru(Ill) complex in acidic solution in the half-cell represented by Eq. ( 4):

O,,,ll)lw"-

,l_J I , .l_i I__LI I I .l_Ll_LJ_l._____ .0,30 ~

91

(illtt/)illtl I"--1~1~ + I" i t ~ +

ill el. ]~ill Fig. 5. DPP spectra for IRu(hedta)(H20)] reacting wilh pyridazine in 0.10 M NaCI, 22°C. p H = 5 . 2 0 [ R u ( t l ) l l = 2 . 3 8 x l O ' M I p y d i l = 4.'/6 × I0 ~ M under N2 at 0.2, 65.0, 1080 rain, respectively.

(4)

9~

K Chen. R.E. SKepherd/ Inorganica ChimicaActa 279 (1998) 85-94

By contrast the bis complex, which shows its Ru"/m wave at +0.49 V versus NHE at pH-5.20, exhibits almost no change in its Ruu/m waves at pH---1.92. This half-cell appears at + 0.54 V at pH-- 1.92, a relatively small change which suggests only a small population of protonated species at equili~um with the deprotonated [Ru(hedta)(pyd)2] - . Alternately the small 0.05 V shift is from the influence of profanation of the pendant carboxylate group, forming [R u n ( ~ ) (pyd)2 ], which presents only a minor electrostatic advantage for reduction relative to [Ru(hedta)(pyd)2] -. An estimate of the pK, of a pendant carboxylate of [Ru(hedta)(pyd)z] - would be ~ 3.2, the valne reported for the pendam emboxylate of [Ru(Hedta)(HzO) | - [37]. This is in concert with the very small changes observed upon acidificationof [Ru(hedta) (pyd)2] ~ in the SH NMR specera. This observation supports the conclusion that there is more ~.backdonation from Ru(H) into one pyd ring of the mono complex d~'~ for the shared influence on each ring of

the bis species. Hence the pg, for the bis complex is below 2.0. given ~ pK, of the mono derivative is ~ 3,42. and the his species shows only minor shifts even at pH-21.92 by either JH NMR or electrochemical methods, The existence of structural isomers [ 35] for [Ru(bedta)(pydH)] is also confl~ed by the electrochemical data. When the sample at pH ~ 7.0 is shifted to 1.92by the addition of HCI, one type isomer protonated immediately, forming a species which ~m~butes to the 0.4)8 V wave for [ Ru (hedta)(pydH)] and the loss of the original 0,17 V wave of the [Ru(hedta)(pyd) ] = complex, A laser stmies which also eventually contributes to the [ Ru(hed~) ( pyd ) ] ~-wave, but it requires 90 mtn ~fore it eonve~ts to the same species with the wave at 0,08 V with the c~plete loss of the 0,17 V (Flg. f)).Since proton tra, sfer reactions are diffusion con° trolled, the inhibition to protonation must originate in a stpaCo rural diiTe~nc~ for the species which protonate immediately and those that do not, The cis~equatorial aridtr~t,v.equ~torial immet~ have coordinated p y ~ i n e s that are a~cessible to the solvent at N~2, These should be protonated at the diffusion rate. The ct$.polar isomers are hindered with the N.2 nitrogen protected by the glycinato in@lane arms 135}. Under acidification, the cis polar form may be resistant to protonation until it conveys to either c/,v-equatorial or ~mn~equatorial i~mers, Therefore, the small amount ofcis.polar lags behind in the protonation shift from 0.17 to 0.08 V in the DPP spectrum. Additionally, the electrochemical data shows a slower shift under acidic conditions that results in the growth of a wave at 0.53 V, indicative of the formation of the his complex at the expense of pmtonated mono species. These chanses were also d e , t e d in the *H NMR spectra of |Ru(bedia)(pydH) ] at pD-2.0 which is described in the next ~tton,

3.4. pH-i~IJ~Mde~redi~rribazioa of t ~ / e x e s The effect ofpH on the long term stability ofthe complexes was s~udied for the mono complex by tH NMR spectroscopy

[Ru(hedtaXp~]" + H30÷~=,m [ I t ~ ~ l "-"~ I~III* + [Re(hedaXtl20)l"

~ ~ I meH ÷+ad3

[Ru(Im~~l ptu(he~~)!

~

H3o ÷+me

+ pyd ~

ptu(hm~~!

+Hd3~

~30÷+ Ptu0mhXp~)zl"

Scheme I,

at pD= 2.0. The changes shown initially by Fig. 4(b) for protonation at N-2 is followed over a period of 18 h to produce the his corer ~, ~ ~ ~ i~:~x,:~ t~ ~ . (5). 2[ Ru(hedta)(pydH) | + 2H:O~ [RuU(hedta) (H:O) ]+ [Ru"(hedta) (pyd)2] - + 2H30 +

(5)

The driving force for this process appears to be the high stabilityof the bis complex in response to a high value for

the second substitution rate constant k2 which raises the magnitude of K2. The rate of formation of the bis complex will be accelerated by the attack of free pyd on the [Ru(hedta) (pydH) I complex as in Scheme I. Whereas the mono pyd complex itself is stable over the same time period with respect to forming the bis complex, in acidic solution the dissociation of pydH* will be enhanced relative to the loss of a neutral pyd ligand from [ Ru(hedta) (pyd) ] ~. As explained earlier for the response of the Ru wm couple to protonation at N-2 or pyd, an additional repulsion of the protonated ltgand should weaken the Rutt-N.I bonding. Upon dissociation of pydH *, the equilibrium amount of pyd can either add to the pool of [Ru(hedta) (H~O) ] ~, leadln8 to ~ v e ~ l of the process, or the free pyd can attack the pool of mono complexes, leading to the more stable bis product. With prolonged exposure to H~O +, further oxidation of the [Ru(hedta)(pyd).~] ~ species is evident by a loss of tH NMR resonance intensity. A new broad singlet appears at 6.54 ppm. In order to assess if the singlet originated from some form of fluxionally-bound pyridazine species, a low temperature spectrum was obtained in 50:50 CD3OD/DaO at - 37°C. No resolution of lines as for a slowing ofa fluxional process was observed in the singlet feature. We are drawn to the conclusion that the singlet originates via an oxidation of Re(ii) to Ru(i!1), and that the singlet originates from nearly equivalent H-5 and H-4 protons in N~t bound I Rum( hedta)(pyd) I species. The absence of easily detectable H-6 and H~3 r~.~nances for the ~me species is a result of param~netic ~ifts and broadening of the nearer ring protons.

3.5. i'~hanceme, t of k~ by ct~rdinated pyridazine The amount of bis complex which forms in competition with production of the mono pyd complex remains remarkably constant whether [Ru(hedta)(H~O)l- is in large excess with IRu(hedta)i-:lpydl of 2.00:1.00 or much

E Chen, R.E. Shepherdllnorganica Chimica Acta 279 ¢I ~ ) 85-94

lilaOmlagihO)l'÷~ ~

~ i "

93

weakened bonding to the cis position leads to a dissociative intermediate that can be scavenged a second ligand that it may encounter by diffusion. This is the structure described for species (4) presented in Section I.

Scheme 2.

4. Condusions reduced at a ratio of 1.00:1.00. If [Ru(hedta)(H20) I i-: [ pyd] of 2.00:i.00 the percentage of mono:bis is 72.0:28.0%. At !.00" 1.00 ratio on mixing the mono:bis product ratio was found to be 68.2:31.8%. These analyses are based upon integrations of IH NMR resonance lines for the mono and bis species. Since the amount of his product is relatively insensitive to the time-dependent, decreasing pool of pyd scavenger ligand, the seque:nce shown in Scheme 2 is suggested to account for the kinetic split in forming ~ 30% his product. Here, IRu(hedta)(pyd)]* represents a structurally-activated intermediate which precedes the bis species. From rates with pyridine and pyrimidine ks -- 30 M- I s- *. If we solve for the value of k2 required to provide ~ 30% his product at I:1 and 2.61 × l 0 - 2 M, an approximate value for k2 of 0.513 s- ~ is obtained. This is a factor of 3400 higher than the steps leading to bis species for [Ru(bedta)(pyre)2]- and [Ru(hedta)(pz),.I ~ determined in former studies. The first-order rate constant, independent of the external pool of entering ligand has been attributed to ratelimiting dissociation of the in-plane carboxylate donor of [Ru(hedta) ] ~ to allow formation of the his species by rapid scavenging of the dissociated intermediate by the available ligand pool, forming | Ru(hedta)L~ ] ° with L ~ pyrimidine, pyridine or pyrazine. Since pyridazine represents a special case, the explanation must be involved with the structure of pyridazine. Since its initial substitution rate for kl appears to be normal, estimated as 32 M ~ ms ~ t from electrochemical data with complete complexation in 120 s at 2.6 x 10 ~ :~M under second-order conditions, the enhancement tbr pyd must reside in the second step, itself. This step is independent of the concentration of the external entering group except at very low free iigand values as shown in the electrochemical experiments. At higher concentrations as studied by t H NMR, the second addition appears nearly independent of [ pyd],. This points to the 3400-fold acceleration of pyd substitution in step 2 originating from a structural difference in the [ Ru(hedta) (pyd) I - complex that is not available when pyridine, pyrazine or pyrimidine replaces pyridazine. The logical difference between pyridazine and the other diazines or pyridine in the coordinated species is that pyridazine has the N-2 nitrogen with its lone pair available as a nucleophile toward this cis position of the coordinated in-plane carboxylate to assist the displacement of the carboxylato donor. The others (pyridine, pyrimidine, pyrazine) must wait for the unassisted dissociation of the in-plane carboxylate to open the cis position tbr the second ligand to add. In the case of pyridazine, the dissociation of the carboxylato donor can be compensated by donation from the N-2 lone pair of the coordinated pyridazine. This accelerates the rate at which the

The evidence presented in this report shows that pyridazine forms both mono and bis complexes with [RuU(hedta) ] which have large kinetic barriers to 1,2-fluxionality, much greater than the 25 kcal mol- I barriers observed for representative Pt(IV) and Re(i) d6 low-spin complexes. Thus the molecular orbital predictions of Alvarez et al. [ 25 ] are confirmed: (i) that the barrier for Ru(ll) pyd complexes is greater than for Re(1) and (ii) when competitor iigands are ~r-acceptors, as in the case of [Run(porphinato)(pyd) (CO)] complexes, the barrier is lowered for 1,2-metailotropic shifts of coordinated pyd. The protonated pydH + ligand is destabilizing in its coordination to Ru(II) due to an electrostatic repulsion component that leads to an 80 mV negative shift on the Ru n/m El ;: value and a pathway for dissociation that slowly converts [Ru(hedta)(pydH)] into the bis [Ru(hedta)(pyd)2]complex plus [Ru(hedta)(H20)]-. This influence is in opposition to the enhancement in ~r-acceptor attraction that occurs/'or the protonated ligand. Such protonation effects normally cause positive El/: shifts when the site of protonation is more remote as in the case of pyrazine, but ligand protonation lowers the or.donating ability of LH ~ leading to weakened Ru(li )~LH* bonding. The bis complex [Ru (hedta) ( pyd ): I = exhibits little evidence for 1,2-fluxionality and no exchange with the external pool of free pyd ligand up to 60.0°C. By contrast its pyrimidine analogue has ligand exchange and 1.3.fluxionality for the ligand closest to the pendant carboxylato donor. Thus bonding between [ Run(hedta) ] ° to pyd is stronger than for pyre. This is in harmony with the fact that pyd is a stronger ore donor and a better ,rr-acceptor than pyre [ 3~7 l. The weaker bonding to pyrimidine allows for fluxionality and exchange. it has been described elsewhere 127] that the barrier for migration of a N-heterocyclic ligand from coordination at a nitrogen lone pair to a ,it.cloud ~q" site is a function of the strength of the tr-bonding plus the ,rr,acceptor ability of the iigand toward Ru(ll). Since pyridazine is a better'rr-acceptor than pyre, it will have a higher kinetic barrier to achieve ~: coordination on the ,n'-cloud than for pyrimidine. Consistent with this conclusion, the migration process is complete in 14 days at 25°C |br [Ru(hedta)(pym)] ~:, but there is no IH NMR evidence after 13 days for any form except N-I-coordinated pyd in either [Ru(hedta)(pyd)]~ or [ Ru(hedta) (pyd) 2] -. This is consistent with a larger kinetic barrier for N-1 to ,q2 migration for the pyd complex, just as there is a higher kinetic barrier to 1,2-fluxional migration. Lastly, the combined evidence of IH NMR and electrochemical techniques has ruled out any large concentration of

94

E Chen, R.E. Shepherd / Inorganica Chimica Acta 279 (1998) 85-94

a bidontate.coordinated pyri~ine species, even though it contributes in a kinetic pathway to form the bis complexes. Referenees [ i ] Y. Chtm, F.-T. Lin, R.E. Shepherd. lnorg. Chem, 36 (1997) 818. 121 (a) S. Zhang, L.A. Hell. R.E. Shepherd. Inorg. Chem. 29 (1990) 1012: (b) ILE. Shepherd. S. Zlumg. F.-T. Lin. R.A. Kones, Inorg. Chem. 31 (1992) 1457; (c) R,E. Shepherd. S. Zhang, lnorg. Chim, 191 (1992) 271. 13] (a) DJ. Brown. in: A.R. Katdnsky. C. Rees (Eds,). Comprehensive Helerocyclic Chemistry. I/ol. 3. Pergamon, New York. 1984. p. 59; (b) A.G, LenheN. R.N, Castle. in: R,N. Castle ( ~ . ) . Pyridazin~. The Chemimy of Hetemcyclic Compounds. Wiley. New York. 1973; (c) G,G. BaHin, The Pyrazines. The Chemistry of Hetcrocyclic Compounds, Wiley, New York. 1982, p, 7; (d) J. Tjebm~, Acta Chem. ~and, 16 (1962) 916: re) J.P, Cox. Tetrahedron 19 (1963) 1175. 141J. Sen, H. Taehe. Acta Chem. Seand. Set. A 33 (1979) 125. I$1R.A, Lay. R,H, Magnuson. J. Sen, H. Taube. J, Am. Chem. Soc. 104 (1982) 7~8. 161 P. Ford. D.F.P. Rudd. R.G. Gaunder. H, Taube. J. Am. Chem. Soc, 90 (1968) i 187, 17i R.B, Wiherg. T.P. Lewis. J. Am, Chem. Soc, 92 (1970) 7154. 181 P.J,Steel.Coord. C~m. Rev, 106 (1990) 22% [91 E.C. Constable.PJ. Steel.Coord, Chcm Roy. 9~ (1989) 205. [ 101 (a) T. Otieno, SJ, Rettis. R.C. Thompson. J. Trotter, lnor8, Chem. 34 (1995) 1718; (b) S,S. Tand~, L.K, Thompson. M.E, Manual, J.N, Ikid~on. Int~rl~.Chem~ 33 (19~) 5555; (c) L,K, Thompson, S.S, Tandon, M,E. Manual, lnor8, Chem, 34 (1995) 2356, [ I I ] M,J, Ilesley, P. Hubbersley, C.E, Russell, P H. Walton, J. Chain, St)c, Dalton Tr~m~. (1994) 2483. 1121C.L. Sheppard, S.S. Temdon. L.K. Thomp~n. J.M Bridm~n. D.O. Miller, M. Hand~, F. Lloret lflorl]. Chim. Acta ~ 0 (1996) 227. 1131 M, M~kttw,t, M, Munaktt|tt.T, Kuroda~aw~, Y. No~k~, J. Chain, So~,, Dalton Tr~n~, (1994) 603, 1141 (a) S, Brooker. R.K, Kelly, B, Monh~rakl, K,S, Mufl?, J,Chem, S~,, ~h~m, Commufl, (1996) 2~?t)~(hi L, Caflucci,G, Ciani,M Merit, A, Sire.i,J,Chem~ See,.O~lton Tran~ (199~) 2397, t l~l (it) N, M ~ t ~ h t , P, Cldtmi, L, Carluc,¢l, G. Ciani, O, Natant, N, Sth)ni, J, Chem S~., O~llofl Tr~fl~, (1',)94) 3(XN: (h) J, Ci,.~oi,, T~ M~itl~lu, C. Gur~n, I, Jil~ru, A, Mes~a, Ray, Roum, Chim, 41 (1~)6) S67,

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