Stereochemically-rigid and fluxional pyrimidines within the bis-[Ru(hedta)(pym)2]− complex

ELSEVIER

Inorganica Chimica Acta 268 (1998) 287-295

Stereochemically-rigid and fluxional pyrimidines within the bis-[Ru(hedta) (pym)2] - complex Ya Chen, Fu-Tyan Lin, Rex E. Shepherd * Department of Chemistry Universi~ of Pittsburgh, Pittsburgh, PA 15260, USA Received 31 October 1996; accepted 26 May 1997

Abstract The formation of the bis-pyrimidine complex [Run(hedta)(pym)2] occurs by sequential additions of pym to [Run(hedta)(H20)] (hedta 3- = N-fiydroxyethylethylenediaminetriacetate;pyre = pyrimidine). The second step is rate limited by the dissociation of an in-plane carboxylato donor of the hedta3 ligand; k = 1.57 × 10 4 s t at 22°C. Et/2 values for the Ru u/In waves in 0.10 M NaCI, T= 22°C are 0.18 V for [Run(hedta)(pym)] - and 0.54 V for [Run(fiedta)(pym)2] . The latter is similar to the bidentate 2,2'-bipyridine complex, [ Ru (hedta) (bpy) ] - , with E~/2 = 0.48 V, illustrative of the influence of two-coordinated N-heterocyclic ~'-acceptor ligands. ~H NMR spectra of the [RuU(hedta) (pyre)2] - complex reveal a differentiation in the coordination behavior of the two bound pyrimidines. One pyrimidine is stereochemically-rigid, exhibiting resonances at 9.25 (H2), 8.82 (H6), 8.68 (H4) and 7.45 (H5) ppm, respectively. The second pyrimidine is fluxional with rapid migration of the Ru u center between N- 1 and N-3 sites. This motion interchanges the H4 and H6 protons which resonate at 8.82 ppm; the H2 resonance appears at 9.15 ppm and H5 at 7.61 ppm. The weaker bonding exhibited by the fluxional pyrimidine also allows for rapid exchange of the fluxional pyrimidine with any free pyrimidine in the solution. The difference in the two pyrimidines is consistent with assisted displacement of the fluxional pyrimidine, promoted by associative attack by the pendant carboxylate arm which is displaced in forming the bis complex. The stereochemically-rigidpyrimidine is nearer the -CHzCHeOH (alcohol) arm of hedta 3- which does not associate strongly and provide an assisted ligand exchange path. Acidification of the [Run(hedta) (pym)2] complex to pD-= 1.0 results in formation of rf(1,2)-[Ru(hedta) (pyre) ] as the major product in 1.0 Da, rather than the previously established (Y. Chen, F.-T. Lin and R.E. Shepherd, Inorg. Chem., 36 (1997) 818) 2:43:22:33 distribution for N - l : r f ( 1,2):rf(5,6):,/2(1,6) isomers which is generated spontaneously from the initially N-l-bound [Ru(hedta) (pyre) ] 1:1 complex after 14 days. © 1998 Elsevier Science S.A. Keywords: Fluxionality; Ruthenium complexes; Pyrimidine complexes

1. Introduction There are only a few cases in which metal complexes of N-heterocycles have r f coordination rather than being bound at the nitrogen b a s e ' s lone electron pair. These include the (NH3)5Os n complexes of 2,6-1utidine [ 1 ] and pyrrole [2]. Until recently, the only reports of Ru n complexes having ,/2 coordination with N-heterocycles were those of kinetic transients which appear during the flash photolysis of [ ( N H 3 ) s R u ( p y ) ] 2+ [ 3 ] , and during the electrochemical reduction of amido-coordinated [ (NH3) 5Ru m(isonicotinamide) ] 2 + [ 4 ]. Our research group has studied several types of Run-polyaminopolycarboxylate ( R u t t ( p a c ) ) complexes of N-heterocycles including pyridines, pyrazines, pyridazines and nucleobases pyrimidines. [ Run(hedta) ] - has shown the capacity to make an expanding list of r f structural attach* Corresponding author. Tel.: + 1-412-624 8200; fax: + 1-412-624 8552. 0020-1693 / 98 / $19.00 © 1998 Elsevier Science S.A. All rights reserved PIISO020-1693 ( 9 7 ) 0 5 7 5 9 - 9

ments including the C - 5 - C - 6 r f - b o u n d uracils and uridines (1) [5,6], cytosine and cytidine (2) [5], three types of 772coordinated pyrimidines ( 3 ) - ( 5 ) [7,8], r/e-pyrazinium species (6) and (7) [9,10], rf-6-azauridine ( 8 ) [11], and o ~n2 ~-J~..va, L ~ ~,R~L o~[ ~ " N""If" ot]'N / ~{. . ~ ~ ~) RulIL ~ + X~/

~z)

RunL

(3)

RIIIIL

(4)

(5)

RuUL

H•RutIL N~"N

\\

//

tfT~'~ " (6)

RsalL

(7)

(8)

(9)

C (10)

288

Y. Chert et al. / Inorganica Chimica Acta 268 (1998) 287-295

r/Z-aminopyrimidines (9) and (10) [ 12]. One of the characteristic properties of the r/2-bound pyrimidines is the electrochemical wave for the Ru Ium redox couple that appears at 0.50 V versus NHE for r/2 species 3-5 [7,8]. The E~/2 value for the wave near 0.50 V is very close to the El/2 values of the R u II/III w a v e for bis-N-heterocyclic substituted Ru"(pac) complexes. For example, Diamantis and Dubrawski reported the R u ll/II! w a v e for [Ru"(edta) (py)2] 2- as 0.51 V and the 2,2'-bipyridine complex [RuH(edta) (bpy) ]2- as 0.57 V versus NHE [ 13,14]. We wished to rule out any possibility that bis species were present in the products formed from the 1:1 complex with time. We provide evidence in this report that the species responsible for the 0.50 V wave with pyrimidine r/2-bound to [RutI(hedta)] cannot be simply a bis complex. The evidence is provided by preparing the bis species which forms spontaneously when [Ru(hedta) (H20) ] combines with pyrimidine or other N-heterocyclic bases at the ratio of 1:> 2. The formation of bis complexes with Ru~(pac) s are known under these conditions from the studies of others [ 13-15]. The ]H and ~3C NMR spectra do not coincide for the bis species with the ones reported on rf(1,2), r/2(5,6) or r/2(1,6) in our prior work [7,8]. In the progress of our study it was observed that one pyre ligand is stereochemically-rigid whereas the second pym of [Ru(hedta)(pym)2] - is fluxional and rapidly exchanges.

2. Experimental Pyrimidine was obtained from Aldrich. Na[Ru(hedta)(OH2)] .4H20 was obtained from stores generated for former studies [ 15-18]. Preparation of Ar-purged samples containing [ R u " ( h e d t a ) ( D 2 0 ) ] - over Zn/Hg with the appropriate ligands have been reported elsewhere [5-12]. Similarly the electrochemical studies of Ru" complexes at a glassy carbon working electrode and a saturated sodium chloride calomel using 40 mV s t sweep rates and a stepping voltage of 50 mV with differential pulse voltammetry or 50 mV s ~in the cyclic voltammetry mode have been described elsewhere [5-12] following known calibration methods [161. ~H and ~3C NMR spectra for this study were recorded at 25°C using Bruker AF300 or AM500 NMR spectrometers averaging > 256 scans. JH NMR data were referenced to DSS (0.00 ppm) and t3C NMR spectra to 1,4-dioxane (69.1 ppm). HH COSY spectra were obtained at 500 MHz with the Bruker AM500 spectrometer with 200 scans. Methods in all regards were similar to those reported previously [7-11 ] except that [RuH(hedta) (D20) ] - : [pyrimidine] ratios were set at 1: _>3 or 1:2. After mixing the reactants in small round bottom flasks which were septum-sealed and Ar purged, samples were transferred to Ar-purged NMR tubes and spectra were taken over a period of 30.0 min through 7.0 h initially, and then over the next 5 days. The samples were prepared in slightly acidic D20 such that contact with Zn/Hg would give a final pD value near 6.0. In the case of acidified samples for

JH NMR, DC1 was injected into the NMR tube via a capping septum such that upon dilution the final pD value is ~ 1.0.

3. Results and discussion 3.1. Electrochemical studies The reaction of pyrimidine (pym) with [Ru"(hedta)( H 2 0 ) ] - occurs in two steps. When [Ru(hedta)( H 2 0 ) ] - = 2 . 6 1 X 1 0 -3 M is combined with [ p y m ] = 1.57X 10 -2 M (1:6) the known wave for [Run(hedta) (H20) ] - at 0.04 V versus NHE is already converted to a new wave at 0.18 V within 5 min (Fig. 1). Repetitive scans at 60.0 and 120.0 min show the decrease in the initial [Ru(hedta) ( p y r e ) ] - complex with the growth of the bis complex with a wave at 0.54 V versus NHE. A similar wave appears at 0.48 V when [ R u ( h e d t a ) ( H 2 0 ) ] - reacts with 2,2'-bipyridine (Fig. 2) at 1:1 with [Ru[I]tot=2.61X 10 3 M (not shown). Therefore the formation of the bis[Ru(hedta) (pym)2] - occurs with a shift in the wave of the 1:1 adduct from 0.18 to 0.54 V for the bis product. The direction of the shift and the magnitude are comparable, but not exactly the same as for the N- 1 to ,/2 migration of the 1: 1 complex which, themselves, have their Ru "/m waves at 0.50 V [7,8]. A plot of the current amplitudes for the bis addition of - I n ( i s - i T ) where i~ is the final DPP current amplitude for the bis complex and iT are the amplitudes at various reaction times gave a very good straight line (Fig. 3) with a slope of 1.57 X 1 0 - 4 S- 1 as a measure of the rate for the second addition of pym at 22°C, limited by the rupture of the in-plane RuI1-carboxylate bond in the intermediate [Ru(hedta)(pym) ]*. This is in excellent agreement of experiment using 2-methylpyrazine to scavenge [ Ru(hedta)( H 2 0 ) ] - ill forming the bis complex [Ru(hedta) (2-CH3pz)2] -

T

/ 111tlv,i,lw,l,,, o.o 1.o 0.5O

v Fig. 1. DPP study of the formation of b i s - [ R u ( h e d t a ) ( p y m ) 2 ] - at 22°C. [ R u i t ] , o t = 2 . 6 1 X 1 0 3 M [ p y m ] ~ = l . 5 7 × 1 0 -2 M in 0.10 M NaCI. Decreasing wave at 0.18 V shows scans at 5.0, 50.0 and 120.0 min, respectively; ascending waves at 0.54 and 1.10 V are the same times. Voltage axis is corrected to NHE scale; current indicator is 20 ~ V / d i v .

Y. Chen et al. /lnorganica Chimica Acta 268 (1998) 287-295

289

[Ru(hedtaXH20)]- + pyre

k=,,M-',-' >[Ru(hcdta)(pym)]- + H20

[Ru(hedtaXpym)]-

k=,.,,,,0-',-' >[gu(hedtaXpym)],

pym +[Ru(hedta)(pym)]* '~a )[Ru(hedtaXpym)2 ]Scheme 1.

t

t

~

,11

i

~

0.O

I

I

I

I

f

I

I

Fig. 4(b) after 19 h. The initial 40 min spectrum clearly exhibits features of the 1:1 complex with pyrimidine ligand resonances at 9.52 (H2), 9.18 (H6), 8.74 (H4) and 7.58 (H5) ppm, respectively, as characterized for the 1:1 N- 1bound complex [7,8]. Additional features are present with resonances at 8.68 and 7.45 ppm, as well as free ligand 9.15 (H2), 8.81 (H4, 6) and 7.59 (H5) ppm, respectively. As the formation of the bis complex reaches completion, the resonances of the 1:1 complex completely vanish (Fig. 4 ( b ) ). There are two sets of resonances identified as a broadened one at 9.25 ppm connected to the 8.68 and 7.45 ppm set. The second set at 9.15, 8.82 and 7.61 ppm are very similar to the free ligand values. However, these cannot be due to excess free ligand since there is no longer any of the 1:1 complex and the time dependent subsequent changes for a 1: 1 system [7,8] are not observed in this case. None of the features exhibited in spectrum 4(b) match those of the N-l, 772(1,2), 772(5,6) or r f ( 1 , 6 ) complexes of 1:1 stoichiometry. The areas of the 7.45 and 7.61 ppm H5 protons are equal as would be required for two distinguishable coordinated pyrimidine ligands. When the 1H N M R experiment was repeated at 1:3 and 1:3.5 ratios the product signals were the same as those of the 1:2 ratio after 14 h, however the 9.15, 8.62 and 7.61 ppm set grew in amplitude proportional to the amount of excess ligand, whereas the features at 9.25, 8.68 and 7.45 ppm were not changed by the presence of excess ligand (Fig. 4 ( c ) ) .

II

1.0

0.50

V Fig. 2. DPP spectrum of [ R u ( h e d t a ) ( b p y ) l at 22°C [ R u n ] t o t = 2.61 × 10 -3 M in 0.10 M NaCI [bpy] = 1.31 × 10 -2 M (1:0.50). The wave at - 0 . 0 8 V represents unreacted [Rul[(hedta)Cl] - . Voltage axis is corrected to NHE scale; current indicator is 20 IxA/div. Spectrum after 4.0 h.

where a rate of 1.16× 10 -4 s -~ was measured at 22°C. It is known from prior studies that the first substitution of pym to [Ru(hedta) (H20) ] - occurs in a second-order fashion with a rate constant of 31 M -~ s - ] [7,8]. The two steps are summarized in Scheme 1.

3.2. IH NMR studies Further information on the formation and the nature of the bis complex was obtained by 1H N M R methods. The formation of the bis complex was examined at a 1:2 stoichiometry pD ~ 6.0. The ]H N M R spectra of the species obtained are presented in Fig. 4(a) and 40 min reaction time and

-3.4

-3.8

7

-4.2

-4.6

i

i

40

t

I

80

t

t

I

120

time in minutes Fig. 3. Rate of [ Ru (hedta) ( p y m ) 2] - formation monitored by differential pulse voltammetry as a function of time. [ Ru ( hedta ) ( p y m ) ] i = 2.61 X 10 3 M; [pym]~ = 1.31 × l0 -2 M,/d,= 0.10 NaC1, T = 22.0°C.

290

E Chen et al. / lnorganica Chimica Acta 268 (1998) 287-295

(a)

. . . .

(b)

i . . . .

10.0

[

9.0

.

.

[

8.0

"1

I . . . . . . . . .

"/.0

I'"

(c)

"

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1

9.0

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""'"

I

'

7.0

1

10.0

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PPM PPM PPM Fig. 4. ~H NMR spectra of the [Ru(hedta)(H20)] - / p y m system at 1:2 and 1:3; [Ru(hedta) (H20) ], - = 2.61 × 10 3 M; [ p y m ] ~ = 2 × or 3 × as indicated. (a) 1:2 after 40 min; arrows show resonances of the 1:1 complex; (b) 1:2 after 19 h; (c) 1:3 pD ---6.0, 7 h (not shown: several other ratios of [complex] :[ pym] with the same behavior as C).

Adjustment of the pD to 2.3 also caused only a minor downfield shift of the set involving the free ligand, but did not change the 9.25, 8.68 and 7.45 ppm resonances. Integrations of the 1:2 spectrum indicated a hidden resonance, connected to the 9.25, 8.68 and 7.45 set, which is buried underneath the (H4, H6) resonance of the second coordinated pyrimidine and/or the free ligand H4, H6 pair at 1:3 or higher stoichiometries. The presence of the buried resonance was confirmed by HH COSY spectra obtained at 500 MHz. The cross-correlation data obtained after 40 scans (Fig. 5 ( a ) ) show the connectivities of the 9.25, 8.68 and 7.45 ppm resonances (see the three arrows near the top of Fig. 5 (a)). The presence of a buried resonance is indicated by the echos of low intensity between the C4 and C6 protons shown by two arrows at the bottom of Fig. 5(a). The coupling constant between proton H5 (7.45 ppm resonance) and H6 (buried 8.82 ppm resonance) is very small as evidenced by an expanded scale plot (Fig. 5(b) ). All four connectivities for the static pyrimidine ring are detected, however, this amplification also enhances other through-space couplings. At both data resolutions the connectivities of the three major resonances of the second pyrimidine are clearly observed. A search for interactions between the coordinated pyrimidines was made using HH NOESY methods (Section 5). No strong connections between the two types of coordinated pyrimidines were observed. Only a very weak interaction between H2 of the rigid pyrimidine yielding four resonances and H2 of the fluxional pyrimidine yielding three resonances was detected, as might be anticipated for cis bound ligands. The above observations imply that one of the coordinated pyrimidines exchanges rapidly with the free ligand pool, whereas one pyrimidine does not. Both pyrimidines are coordinated as shown by the shift in the E l l 2 values to 0.54 V in the DPP data. The pyrimidine which does not exchange has four 1H NMR features: the broad 9.25 ppm resonance (H2) the 7.45 ppm feature (H5), the buried feature at 8.82 ppm (H6) and the detectable 8.68 ppm signal of H4. The other

pyrimidine is fluxional as evidenced by the equivalency of its H4 and H6 protons in the 1:2 complex (Fig. 4 ( b ) ) . Migration of the [ R u " ( h e d t a ) ( p y m ) ] - unit from N-1 to N-3 of the second pyrimidine signal averages the H4 and H6 pair. Therefore, the strength of coordination to either N-center in the second pym is less than in the strongly bound N-1 attachment of stereochemically-rigid pyrimidine. The weakened bonding of the second pyrimidine also makes these Ru"-pym bonds subjectable to substitution by external nucleophiles, including excess pym. Hence, the rapid exchange of free ligand for these positions leads to a weighted-average chemical shift for the bound/freely-exchanging pyrimidine. Also, the amplitude of the exchangeable pyrimidine increases and shifts slightly as the concentration or pD value of the free ligand pool is adjusted. The chemical shifts of the free and second-bound pyrimidines are quite similar, such that the chemical shifts are nearly those of the free ligand. It is known that Ru~I-polyaminopolycarboxylates which form bis complexes with N-heterocyclic ligands (L) do so by the displacement of an in-plane carboxylate [ 13-15 ] : O

O

+ L

~.-

~C~-~"~a"£

In the case of edta 4- complexes, the bis adducts L and L' are stereochemically equivalent, but for hedta 3- complexes one of the N-heterocycles is near the freed carboxylate arm and the other near the pendant -CH2CH~OH arm. This leads to a differentiation in structure and kinetic behavior for pyre (denoted by L and L' in the above drawing). It has been shown by others that for the [Rum(edta) (H20) ] - and [Rum(hedta) (H~O) 1 complexes the presence of the pendant carboxylate of the former pro-

Y. Chen et al. / lnorganica Chimica Acta 268 (1998) 287-295

(a)

~

L

....

i,

7,0

75

1

• 80

!o 85

Q 90

t 90

PPM 85

80

75

70

PPM

L

(b)

291

idine can be compensated by the closer approach of ionpaired carboxylate. This would catalyze the dissociation of the weaker-coordinated pyrimidine by partial bonding of the carboxylate in the transition state for the dissociation of the labelized pyrimidine. Since the law of microscopic reversibility must hold, the entry of ligands at the second binding site, L', must occur with only partial coordination of the carboxylate arm. That this process is largely a dissociative process, with a rate constant of 1.57 × 10 - 4 s - I, is indicated by the agreement in rates for various scavenger ligands (pym, 2-CH3pz, pyd) regardless of the external concentration of the scavenger from five-fold to 55-fold as seen previously for 2-CH3pz. Therefore, if the entering ligand is another pyre from the external free ligand pool, there will be a pathway which randomizes the identity of the free ligand with the fluxionally-coordinated pym ligand, but not the stereochemically-rigid pyrimidine. The observations imply thatt the pyrimidine nearer the -CH2CHzOH arm does not benefit from a similar compensation for bond-breaking. Therefore, this pyrimidine remains strongly coordinated and does not exchange with the external ligand pool. Its 1H NMR shifts remain independent of the external [pym]. 3.3. L¢C NMR studies

70

L

~8

. m. ,m

B.o

-"

;8 ~

85

maB"

g. 90

--F

J

'PM 90

85

80 PPM

75

Confirmation of the conclusions drawn from the ~H NMR and electrochemical methods is given by the ~3C NMR spectrum of [ Ru (hedta) (pyre) 2 ] -- obtained at 1:3 stoichiometry [Ru(hedta) (H20) ] i-: [pym] l shown in Fig. 6. Data collection commenced 7 h after mixing to assure that the formation of the bis species was complete according to IH NMR and DPP results. Two coordinated carboxylates are evidenced by n3C NMR resonances at 188.2 and 187.8 ppm along with the displaced carboxylate moiety at 177.4 ppm. The pyrimidine

70

Fig. 5. (a) 500 MHz HH COSY spectrum of the [Ru(hedta)(pym)2]after 7 days, 22°C, pD -= 6.0. Arrows at the top show connectivities of H2, H4 and H5 of the static pyrimidine; arrows at the bottom reveal connectivities of weaker coupling between H4 and the buried H6 proton. (b) HH COSY spectrum of the same sample obtained at expanded scale sensitivity: four arrows at top reveal H2, H6, H4 and H5 connectivities; arrows at bottom duplicate the H4-H6 coupling seen more clearly in Fig. 5(a).

motes more rapid substitution reactions with external nucleophiles [18-22]. The case for the Ru I[ analogues has not been as well documented [ 23 ]. However, the evidence supplied herein shows that one pyrimidine suffers rapid displacement by external pym in [ R u ( h e d t a ) ( p y m ) 2 ] - , and the other coordinated pym remains rigidly bound on the ~H NMR time scale and exhibits four different JH resonances, one for each ring proton. If the already weakened Ru-pym bond (as shown by the fluxionality of the second pym ligand) is near an ion-paired arm having the carboxylate functionality attracted to Ru H, any further bond-weakening to this pyrim-

j

",

200

180

/

j

160

140

120

100

80

60

Fig. 6. ~3C NMR spectrum of the [Ru(hedta) (pym)2] complex with one additional equivalent of free pyrimidine; pD ---6.0, 7 to 19 h for data collection: arrows point to resonances for the stereochemically-rigid pyre; dots mark the exchanging pyre set.

292

Y. Chen et al. / Inorganica Chimica Acta 268 (1998) 287-295

Table 1 13C NMR shifts for [Ru(hedta) (pym)2] - ~

3.4. Long-time reaction

Carbon

Free ligand b

Rigid pym

Exchanging pym

C2 C4 C5 C6

159.8 159.7 124.9 159.7

163.8 ( - 4 . 0 ) 157.3 (+0.4) 124.8 (+0.1) 157.6 (+0.1)

159.9 ( - 0 . 1 ) 159.9 ( - 0 . 2 ) 125.1 ( - 0 . 2 ) 159.9 ( - 0 . 2 )

Even in the presence of excess free ligand to suppress the dissociation of the bis complex, there is a very slow conversion to the "02 species. The rate at 1:1 stoichiometry requires 14 days for completion. In the presence of excess ligand at 1:3.5 initial ratio (or 1 bis: 1.5 excess pym) required much longer to generate the same detectable amount of the 772 species as for the 1:1 complex. In fact when the b i s - [ R u n ( h e d t a ) ( p y m ) 2 ] - complex is present with a significant excess of free ligand (1:6) to suppress the dissociation of pym, the slow evolution of r/2 species occurred within the same sample at a rate of about 11.5 times

2 1 N~N

6t.

3

, 5

chemical shifts in ppm; A6 values are in parenthesis, a positive A6 is an upfield shift, a negative A 6 is a downfield shift.

[Ru(hedta) ( p y r e ) 2 ] - ~ [ R u ( h e d t a ) ( p y m ) ] - + pyre

b Taken from Ref. [8al.

slower than for the formation of the 772mixture via rearrangement of the 1 : 1 complex. Even at 5.0 days after mixing, only 20% of "02 species have been produced; 80% of the bis complex, with its signature resonances at 7.45 and 8.68 ppm, is still present. At 1:1, the sample would be ~ 80% converted to r f forms after 5.0 days [7,8]. The reaction sequence is summarized in Scheme 2. When a sample of the [ R u ( h e d t a ) ( p y m ) 2 ] - complex with excess ligand is acidified to pD 1.0, there was no ~H N M R shift in the stereochemically-rigid coordinated pym of b i s - [ R u ( h e d t a ) ( p y m ) 2 ] - . However, the free pyrimidine, exchanging with the second site, is protonated and shifts downfield for all of its features. The solution at pD ~ 1.0 was examined again after 24 h. This spectrum is shown in Fig. 7(a). A small amount of the bis complex remained. However, new features appear throughout the 7.3 to 10.4 ppm region. When the new features were checked against the

carbon resonances are observed as two sets indicative of the strongly coordinated (marked by arrows) and fluxionalexchanging (dots) pym ligands in Fig. 6 and as identified in Table 1. Neither of the data sets for the stereochemically-rigid pym or exchanging pym ligand match shifts of the "02(1,2), v12(5,6) or "02(1,6) 1:1 complexes reported elsewhere [8]. The lifetime of the N-1 1:1 complex is too short relative to the data collection time of the ~3C NMR method to allow ruling out an N- 1 1:1 complex on the grounds of chemical shift data for the ring carbons of pym, but the absence of any of its conversion products (the "02 complexes) identified previously [8] rules out its original presence after 7 h when the 13C data collection was commenced.

o o

~~o

9.52

-~'/I

N~N

II

7.58

o

N/-~N

t . / ,~. v 9 . 2 5 o..~/~ 0I \ N A N HO ~ ~,~8.6S 8.g2 7.45

(mmo)

l

k

(Ta-)

o

I (k/ll.5);slower O

o

O"--""~,N~O OH2

.j

w

o

o

n20,2)

n2(5,6) Scheme 2.

.o--,

[.) o

~20,6)

Y. Chen et al. / Inorganica Chimica Acta 268 (1998) 287-295

(a)

t

T lO.O

293

tI

9.o

'

U

'

4A

6C

(b)

4C

1

~A

~fl.g

48

C "*"~6~,J4

5B

l'

•

~l

.

.

.

.

i ' '

. i

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•

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~.

.

.

9.o

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.

i

'do"

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"

PPM Fig. 7. Products of the acidified [Ru(hedta)(pym)2] - complex detected by SH NMR. (a) [Ru(hedta)(pym)2]i=2.61 × 10 -3 M, pD --- 1.0, 24 h after pD adjustment of 6.0 to 1.0; ( ~ ) starting bis complex; ( 1") growth of the ,/2(1,2)-[Ru(hedta) (pym) ] - complex; ( 0 ) trace amount of the r/2(5,6) complex; major resonances are for freed pymH ÷. (b) Spectrum of'q2 species derived fromthe 1:1 [Run(hedta) (pym) ] - complex: A = 7/2( 1,2),B = r/2(5,6),C = rf(1,6) complexes; the numbers indicate H2, H4, H5 or H6 of the bound pyrimidine in the respective r f isomer (assignments per prior work [ 7,8 ] ).

various known r/2 and N-I complexes of pym with [Run(hedta) ] - (Fig. 7(b) ) the striking result, presented in Scheme 3, was observed. Namely, the acidified sample of the bis complex converts in acidic solution to the r/2(1,2) complex. A trace of r/2(5,6) may also form at a four to five-fold slower rate than by the pathway in Scheme 3. Since no shift in the ~H resonance of the rigid pym occurred initially upon acidification of the bis complex, we conclude that the pKa of the protonated form of the bis complex is less than or equal to 0.0. This indicates that any electron density donated by Ru ~I to the pyrimidine ring does not enhance the o- basicity of the lone pair at N-3 which is meta to the site of coordination and at a node in the electronic wave pattern of six-membered rings. The effect of the positive repulsive charge is manifest in the observation of a pKa of ~ 0.0 which is less than the 1.3 value of free pyrimidine. Similar effects were discussed previously in the classic paper of Taube concerning the pKa of [ (NH3) sRu (pymH) ] 3+ being lowered from the free ligand value [24].

If N-3 protonation weakens o- bonding for the N-1 nitrogen, this would facilitate the dissociation of pymH ÷ from the more weakly coordinated, second pym site. The Ru" center may require some compensation for Run-N bonding losses by the nearby pyrimidine base. This is shown by the bidentate intermediate in Scheme 3. An analogue of this intermediate has been characterized in former work with pyridazine (pyd) which makes a stable bidentate [ R u ( h e d t a ) ( p y d ) ] - complex [7,25]. However, the donation from pyrimidine with a very strained meta-bridged unit compared to the cis arrangement for both nitrogens with pyridazine, lends itself to the aquation of the bidentate pyrimidine, which converts to an adduct. Particularly, by means of controls brought about by orienting the Run-pyrimidine bonding in the bidentate structure, water adds to give a more stable bond than provided by the bidentate intermediate. The same pyrimidine is thereby placed in position to be captured predominantly in the rf(1,2) form. Since the r/2(1,2), */2(5,6) and 772(1,6) are not in rapid equilibrium, as shown by their co-existence

294

E 0

Chen et al. / lnorganica Chimica Acta 268 (1998) 287-295 0

O_%N~/O

NAN

~O

~.

,\

~-~ +

.-'N / i \ v

o

o

pKa ~ 0 (no shifts at pH 1.2)

bis

o

~e

.oi/-<(° 0 1 la.y 100%conv~mxl

pD-

o

-o~~~Ru7°H2 O

Scheme 3.

when made by separate pathways from the l:l [ Ru H(hedta) (pym) ] complex under neutral pH conditions [8], there is no rapid shift of the 72(1,2) complex to other T~2 positions about the ring. Therefore, a nearly quantitative synthetic route to the ~a(1,2) isomer of [Ru(hedta)(pym) ] - has been found via H3O+ attack on [ Ru(hedta) (pyre) 2] - .

4. Conclusions The species of [ R u ( h e d t a ) ( p y m ) ] - formed at l:l stoichiometry and identified previously a s ']72 complexes [7,8] cannot be simply bis adducts of [RuH(hedta) ] -. The electrochemical evidence shows that bis addition occurs without producing the wave at 0.50 V of the 1:1 T] 2 [ 7 , 8 ] . The rate of the second addition of pym ( 1.57 × 10 -4 s - ' ) nearly matches the measured rates of bis addition for 2-methylpyrazine ( 1.16 × 10 4 s -- 1 ) and bidentate coordination of pyridazine ( 1.35 × 10 - 4 S -~ ) [25,26]. Both processes involving bis-2-CH3pz or bidentate pyd coordination require displacement of the same in-plane carboxylate of [Run(hedta) ] - , as does bis-pym substitution. The displaced carboxylate has been detected by the 'sC NMR data. The chemical behavior of the bis complexes are very different from the 1:1 7"]2 complexes of [Ru(bedta) (pym) ] - . The 772 1:1 complex is stereochemically rigid, allowing 'H

and '3C NMR characterization of four isomers: the initial N-1 form and 72(1,2), "q2(5,6) and ~72(1,6) which at equilibrium are distributed 2:43:22:33 [ 8]. The bis system exhibits a 'H NMR spectrum requiring one stereochemically rigid and one fluxional pyrimidine. The fluxional ligand also exchanges rapidly with the external free pyre ligand pool. Under acidified conditions the bis complex dissociates the exchanging pyre easily. This presents a pathway which affords only the ~/2(1,2) isomer which cannot be prepared for kinetic reasons as a separate isomer from the 1:1 complex. All of these factors clearly disprove the possibility that the reported 7/2 complexes were misassigned bis[Ru(hedta)(pym)2] complexes. This offers further confirmation of the existence for the novel ~72 1:1 [Ru(hedta) (pyre) ] - complexes. The unusual nature of the [Ru(hedta) (pym)2] - complex deserves comment. TheRu H center enters into different chemical bonding with two equivalent pyrimidine ligands on the basis of the proximity of other nearby potential ligand donors. A poorly coordinating carboxylate donor is displaced by an entering N-heterocyclic donor, but since the carboxylate unit is tethered to remain near the reactive center, its charge and basicity alter the Ru H bonding to its new substrate. This illustrates how metals bound by protein surfaces, with other poised ligand donors set at proper distances, can alter the ligand exchange and reactivity of a metal to be different toward ligands even identically bound to the same metal center. The presence of other similar donors such as free carboxylates in solution cannot induce the same effects, as the metal center-to-ionic attractions are too transitory. But within carefully built structures, even 'non-coordinated' groups near the metal ion site may alter the reaction rates of truly coordinated ligand groups. Thus even simple multidentate ligated metals carry properties related to enzymes and biocatalytic centers.

5. Supplementary material Supplementary material includes a 500 MHz HH NOESY spectrum of ring-ring interactions in [Ru(hedta) (pyre)2] and can be obtained from the authors upon request.

Acknowledgements We gratefully acknowledge prior support of the National Science Foundation, grant CHE 8417751, the Research Corporation, and the Petroleum Research Fund, all which contributed to the laboratory support for projects related to the identification of the T]2 coordination mode of pyrimidines on [ RulI(hedta) ] . References [ 1] R. Cardone and H. Taube, J. Am. Chem. Soc., 109 (1987) 8101. [2] R. Cardone, W.D. Harman and H. Taube, J. Am. Chem. So=., 111 (1989) 5969. [3] V.A. Durante and P.C. Ford, Inorg. Chem., 18 (1979) 588.

Y. Chen et al. / lnorganica Chimica Acta 268 (1998) 287-295

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