Indian Journal of Chemistry Vol. 38A, July 1999, pp.686-69I

Mechanisms of the acid catalysed bridge cleavage reactions of two trinuclear ruthenium(lII) complex ions [Ru3 (NH3)I/SCN)X2]6+, (X = CI-', Br-)

Biswanath Chakravarty*, Ruma Bhattacharya, Shambhunath Bisai & Md. Munsur Rahman Department of Chemistry, University of Kalyani, Kalyani 741 235, India

Received 22 Jalll/ary 1999; revised 13 ApriL 1999

Acid catalysed bridge cleavage reaction of two doubly bridged trinucIear complex ions [Ru)(NH) IJ(SCN)X1J"', (X= Cl-, B,) have been investigated at 59" in perchloric and nitric acid media respectively. Both the complexes show Iwo stages of reaction. Plots of k nt><

versus [acid] are linear for both the complexes in the two stages of reactions. Only the plots for first stage have intercept on rate axis. The rate constants for the chloro and the bromo complexes for acid - dependent path are as follows : tirst stage 5.15 x 10-' & 2.43 xl 0'" dm) mol- I S-I and second stage 7.35 x 10-) & 4.02 xl Q-4 dm) mol -1s-1 respectively. An associative mechanism has been invoked for the reactions in which bond formation by the incoming solvent (water) molecule with the middle ruthenium atom leads to

rupture of bridging bonds.

Sykes and other workers have initiated studies on the acid and base catalysed bridge cleavage reactions of dinuclear complexes of cobalt(III) and chromium(III)I . Studies on the acid and base catalysed bridge cleavage reactions of ruthenium(III) complexes are Iimited2

-6

.

Studies on the bridged complexes having two different me tal ions as ruthenium(II) and rhodium(lII) or iridium(III), have also been reported7

. In the present in­vestigation, we have reported bridge cleavage reactions of two trinuclear complexes of ruthenium(III) in acidic medium. The complexes have thiocyanate and chloride, and thiocyanate and bromide bridges respective ly. Mechanisms of bridge cleavage in a doubly bridged trinuclear complex might be interesting as this can indi­cate the order of cleavage of bridges in a multiply bridged complex and in thi s regard the present one would be an interesting addition.

The experimental complexes are:

Materials and Methods

Preparation of complexes

The two complexes under investigation, [Ru3(NH')13(SCN)CI2]CI6 and [Ru/NH3)13(SCN)BrJBro were prepared according to literature methodx. Their purity was checked by elemental analysis and spectral matching with authentic samples.

All the chemicals used in this study were of analar grade. Kinetic runs were followed spectrophotometri­cally using a SICO uvispec spectrophotometer (model no. 100). Reactions were initiated by adding requi site amounts of the complex to thermally pre-equilibrated solutions of respective acids, HCl0

4, HN03 or PTS and

their sodium salts (for maintaining ionic strength). Sam­ple quenching technique was adopted for absorbance measurements at suitable time interval s. The kinetics were followed initially at 488 nm and later at 400 nm (4 12 nm for Br- complex) . All the kinetic experiments were carried out under pseudo-first order conditions lI S­

ing a large excess of reagent. The complexes under in­vestigation are all air stable and no spec ial precautions were adopted for their kinetic runs.

l-

CHAKRAVARTY el at.: BRIDGE CLEAVAGE REACTIONS OF Ru(lII) COMPLEXES 687

0·55

0·50

0·45

0·40

0·35

0·30 <II u c 0·25 0 .D .... 0 UI .D

0·20 4:

0·15

0-·10

Wove length,nm

Fig. 1 -Spectral scanning of the chloride bridged complex (temp Sry) [complex] = 7.5 x 10--4 mol dm-l; [HCI04] = O.S mol dm-3

(i) 0 min , (ii ) I h , (ii i) 2 h, (iv) 2.S h, (v) 3 h and (vi) 4 h.

Results and Discussion

D issoc iat io n of th e trin uclea r sp ec ies fR u (NH ) (SCN) Cl]Jf>+ in perchloric acid medium :

3 3 /3

Absorption spectra and reaction stoichiometry .

The absorption spectra of the ch loro complex in aque­ous perchloric acid solution is given in Fig. l . Curve (i) shows two absorption maxima in the visible range at 488 nm and around 360 nm. In the presence of perchloric ac id (0.5 mol dm-3) there is a little shift in maxima posi­tion, (from 495 nm to 488 nm) most probably due to protonation of the complex species. Absorbance at 488 nm decreases continuously fo r nearly two hours wi thout any change in the spectral pattern . However, after two hours decrease in absorbance at 488 nm is very litt le, but at 400 nm absorbance increases sign ificantly. Rate constants measured at thi s wavelength at the later pe­riod (after 2 hours) are different from those obtained at

the beginning of the reaction. It is likely that the reac­tion enters into a new phase at this stage. We have meas­ured the rate constant at this wave length also to deter­mine the rate constant of the possible reaction taking place at this stage.

Dissociation of the trinuclear complex at different bridge bonds will lead to a number of pentaammine and tetraammine products. Tetraammine complexes may exist in cis-trans isomeric forms and the thiocyanato species in linkage isomeric forms. Dinuclear complex species may also form in the course of the reaction.

Near the end of the first reaction (around ninety min­utes after the start of the reaction), the reaction was ar­rested by cooling the reaction mixture to room tempera­ture ( -200

) and neutralizing the remaining acid with KHCOr After cooling to ice temperature and separat­ing deposited KCI04, the reaction mixture was loaded on to a column of cation exchange resin (15 cm length, Biorad AG 50W-X2), in the Na+ form, and eluted first with water and then with acidic sodium perchlorate so­lution (PH-3.D). Concentration of sodium perchlorate in the eluent was gradually increased from an initial 0.2 mol dm-3 to a final 3.0 mol dm-3. Aliquots of 10 cm3

each were collected and their UV-visible spectra were recorded. Four different complex species were detected in the reaction mixture. Two of them are major constitu­ents, [Ru(NH

3)sNCSP+ (Amax in the visible range at 495

nm, difference from Ru-SCN complex having Amax in the visible range peak at 505 nm 9 and trans -[Ru(NH3)/ HP )2]'+' having Amax at 332 nm. These two components separated with the eluents 1.2 and 2.6 mol dm-3 NaCI0

4. Presence of two other minor components

have a lso been de tec ted . Th ese a re trans -[Ru(NH)/HP)CIF+ and trans - [Ru(NH)4CI2]+ (eluted with 0.5 and 1.8 mol dm-3 NaCI04). These two species have been identified by matching their spectra with au­thentic samples 10. I I.

The colour of the reaction mi xture, after substantial progress of the second stage intensified (deep violet) and after 2-3 days a bl ack residue was deposited. This resi­due was insoluble in all solvents and seems to be poly­meric in nature. No attempt was made to determi ne its composition.

D issocia t ion of th e Trinuclear sp ecies [Ru./NH)jSCN)Br)f>+

Dissociation of this compound could not be investi­gated in perchloric ac id solu tion as the perchlorate salt of the compound is insoluble in water and quickly set-

688 INDIAN J CHEM. SEC A, JULY 1999

CII u C o .0 '­o VI .0 ci

370 530 570 610 Wove length, nm

Fig.2 -Spectral scanning of the chloride bridged complex in pres­ence of nitric acid (temp 59") [complex] = 7.5 x 10-4 mol dm-l; [HNO)l = 0.5 mol drn-) (i) 0 min, (ii) 15 min, (iii) 30 min , (iv) 45 min , (v) I h, (vi) 2 h, (vii) 3 h, (viii) 4 h.

tIes as an insoluble residue. So the study of the present complex was made in nitric acid_ The spectral scanning of the reaction mixture shows some initial (for ca. 30 minutes) rapid change in absorbance at 488 nm and there­after slow increase in absorbance at 412 nm Fig. 2. Af­ter 2-3 hours the colour of the solution intensified (vio­let-red) and on concentration on a rotary evaporator some sparingly soluble material deposited at the bottom of the vessel. The remaining solution on cooling in a refrigera­tor overnight after adding a little (20 percent) alcohol deposited crystals of [Ru(NH,)/HP)2P+' The sparingly soluble material was identified as trans - [Ru(NH,)4Br2] NO, by elemental analysis and also by matching its spec­tra with the authentic sample'~. The diaquo complex was identified from its elemental analysis. Trans - structure seems logical for it in parity with other products and also from the fact that all such ruthenium (III) complexes are stereoretetive"'.

As in the case of the chloro complex, during the course of the dissoc iation reaction (about one hour from the beginning) the reaction was arrested by cooling the re­action mixture to room temperature (-20°), excess acid was neutralised with sodium bicarbonate and then loaded on to a column of cation exchange resin (Bio-rad AG

50W-X2, 15 cm column) and eluted with acidified (PH - 3.0) sodium nitrate solution. Concentration of sodium nitrate in the eluent was gradually increased from 0.1 to 3.0 mol dm-', Aliquots of 10 cm3 each were collected and their UV-visible spectra were recorded. In the case of the bromo complex also two major species were identified in the solution as [Ru(NH ) NCS] (NO ), and

3 5 , •

trans - [Ru(NH')4Br2]NO,. Identification was made by matching their respective spectra with known samples 9

. '2.Trace amount of other species

[Ru(NH)4Br(HP)F+ and trans - [Ru(NH,)iHP)2P+ were also detected in the mixture, the first one by equili­brating with a 0.1 mol drn-3 sodium bromide solution and matching the spectra with that of the dibromo com­plex as before and the diaqua complex as earlier ill . These species were eluting out with 1.2, 1.6, 0.5 and 2.4 mol dm-3 sodium nitrate solutions respectively.

Kinetics of the dissociation of chloride bridged complex

Kinetics of the acid catalysed dissoc iation of the trinuclear chloride complex was followed at 59°. Rate constants were determined by Guggenheims plot at 400 nm (Table I). Plot of rate constant versus [acid] for the first stage of the reaction is linear having a slope and an intercept on rate axis. However, for the second stage of the reaction, plot of rate constant versus [H+] is li near with posi tive slope, but no intercept. All the slopes and intercepts reported in the Table I are determined by least squares method and the observed deviations are stand­ard deviation. The nature of acid dependence of the re­action gives the empirical rate equations (I, 2).

First stage,

Second stage,

... ( I )

.. . (2)

Rate constant values of these acid-dependent kll and k21 and acid-independent (kill) paths are given in Table 2.

It has been amply demonstrated that ruthenium(III) , and also ruthenium(II), indicate a more associative character for water exchange and other substitution re­actions(,' ". In aqueous solution, incoming aqua ligand may form a seven-coordinated aqua intermediate with any of the ruthenium atoms. However, it is difficult to dislodge coordinated ammonia ligand because of strong metal-nitrogen bond . Approach of the aqua ligand to first ruthenium atom would lead to the formati o n of aquapentaammine complex . However, this species could not be detected in the reaction mixture. Hence, it is very

CHAKRAVARTY e/ at.: BRIDGE CLEAVAGE REACTIONS OF Ru(lII) COMPLEXES 689

likely that the first ruthenium atom does not form aqua intermediate with the incoming water molecule. With remaining two ruthenium centres, incoming water mol­ecule may replace either of the chloride ligands, bridg­ing or terminal, or the bridging thiocyanate ion. Thiocy­anate is strongly bonded to two ruthenium centres and forms a stronger bridge than chloride ion. The bridging chloride ion is under a greater strain than the terminally coordinated chloride ion.

Approach of the incoming water molecule may be directed to any of the ruthenium atoms, but it is the mid­dle one that forms the intermediate with greater ease than others. Formation of the aqua intermediate by the mid­dle ruthenium atom will lead to chloride bridge bond breaking between the second and the third rutheniun at­oms. Nevertheless, we were unable to detect the dinuclear species with NCS- bridge. Such bridged species forms only with rutheniu m (II)~. As soon as the chloride bridge breaks down, rupture of the SCN- bridge also takes place giv in g the products. Thiocyanate ion, which is an ambidentate li gand , is linked to two adjacent ruthenium atoms through Sand N donor atoms. Ruthenium (III) is a moderately hard acid; so Ru-N bond is stronger than Ru-S bond. Dissociation of the complex takes place by the breaking of Ru-S bond. Product analysi s indicated formation of (85 ± 10)% Ru(NH),NCS2+ species. Traces of sulphur bonded species might be there but of negl igi­ble sign ificance.

Attack of RO+ (or H,O) on the central ruthenium atom .' -

takes place in the direction of one of the C2

axis which is perpendicular to the C

4 axis of the molecular ion, as thi s

direction is least blocked by the ligands.

The ac id-independent and acid-dependent paths of the complex ion may be shown as :

kif)

-Ru-NCS-Ru-CI-RlI-Ci + Hp ~

( I) m (3) s low

fast -Ru -NCS ---- Ru ---- CI - Ru - CI ~

I

(I) : (2) (3) Hp

Hp

(NH)5 RlISCNl> + Ru(NH).(HP)/+ + RlI (NHJ)}HP)Cll+ ( I ) (2) (2, 3)

+ Ru(NH J).Cl1•

(3)

In the experimental temperature and acidity range both the chloroaquo and the dichlorotetraammine complexes undergo aquation to diaquo complex III . The products obtained with the second and the third ruthenium atoms would be trans isomers only since the trinucJear com­plex has a linear structure. It has been observed that dur­ing a substitution reaction , cis-tetramine complexes of ruthenium(III) retain their configuration without under­going conversion lO

. In the present substitution reaction if any cis species is formed at any stage of the reaction, it would remain as cis throughout the course of the reac­tion . However, product diaquo species has a trans struc­ture; so only trans species is formed in the course of the reaction. In the presence of perchloric acid , it has been observed earlier that these trans products undergo dis­sociation and oxidation by the released coordinated am­monia molecules with the formation of aquonitroso com­plexes l4

• These aquonitroso complexes undergo polym­erization 14 as observed in the second stage of the reac­tion. The acid dependent path may also be shown in a s imilar way as the acid-independent path. In this case a hydronium ion (H jO+) approaches the central (second in order) ruthenium atom. The products are the same, with central ruthenium atom giving a protonated species.

The second stage of the reaction is primarily a po­lymerization process. Polymerization may take place "vith the aquonitroso complex or its aquation or dissociation products, probably with more coordinated aqua and chlo­ride ions . This type or polymerization has been observed with a number of ruthenium(III) complexes having co­ordi nated aqua and chloride ions I.'. Composition of this polymer could not be determined with certainty. 1t is dif­ficult to predict the mechanism of this polymerization reaction at this stage, but it has been observed earl ier that in the presence of perchlorate ion photochemical oxidation of aqua complexes of ruthenium(III) leads to formation of polymeric products l (, .

Dissociation o{ the bromide and thiocyanate bridged complex

The rate plot of this complex shows that during dis­sociation two consecutive reactions are taking place. The rate constants of these reactions were determined using Guggenheims procedure by measing absorption at 488 and 412 nm respectively. The rate constant values thus obtained are given in Table I. Plot of rate constant ver­sus [acid] for both the early and the later period reac­tions are linear with positive slopes but only positive intercept for the first one. The second plot has no

690 INDIAN J CHEM. SEC A, JULY 1999

Table I - Dissociation of [Ru)(NH) ,)(SCN)X2

] 6+ in acidic aqueous solution

[Complex] = 5 X 10-4 mol dm-); 1= 1.0 mol dm-); temp. 59" (Error in k values is within ± 3%)

100k, S·I

X Acid [W] mol dm-) 0.3 0.5 0.7

CI - HCIO. k I 4.86 6.0 1 7.3 1

HCIO. kl 2.35 3.80 5.00

HNO) k, 5.02 5.88 7.41

Br- HNO) k ) 2.64 3. 10 3.61

HNO) k. 1.10 1.90 2.71

Table 2 - Rate constants for ac id independent and acid dependent paths (x I (1) at 59"C

Complex

Chloro (HCIO)

Chloro (HNO)

Bromo (HNO)

Acid independent ( S- I)

(First stage)

(ki ll) 3.48 ± 0.26

(klO

) 3.52 ± 0.22

(k),) 1.90 ± 0.1 3

intercept. Considering the nature of acid-dependence, the empirical rate equations may be given as in Eqs (3 and 4) .

First stage,

Second stage,

k (lIhslll = IB,

k (nh.~J I = lO r

k)(, + k),[W]

k41

[W]

... (3)

... (4)

The rate constant values of the acid-catalysed k 11 and k41 and acid free k,o paths for this complex are also given in Table 2.

Product analysis of the reaction mixture after nearly thirty minutes indicates that both the bromide and the ch loride bridged complexes dissociate in similar fash­ion, that is , by the approach of the incoming Hp or Hp+ li gand towards the central ruthenium atom with ultimate bond cleavage between this ruthenium atom and the bridging thiocyanate ion and also by the cleavage of the bond between the third ruthenium atom and the bromide bridge in a fast reaction step. Rate constants are smaller compared to the chloro complex, may be due to higher enthalpy necessary for metal-bromide bond rupture. In halopentaammi ne comp lexes of cobalt(III), chromium(III), rhodium(III) and iridium(III) it has been

Acid dependent , k(dm)mol- 's- ')

(First stage)

(k ,,) 5.15 ± 0.23

(k,, ) 5.06 ± 0.28

(k )2 ) 2.43 ± 0.10

(Second stage)

(kll

) 7.35 ± 0.28

(k.1) 4.02 ± 0.28

observed earlier that the metal -bromide bond rupture requires a higher enthalpy than the metal-chloride bond rupture 17. Bond making by the incoming aqua ligand is more important in the case of the ruthenium(lH) com­plex than in cobalt(III) complexes. Bromide ion is com­paratively a more soft base than the chloride ion. It should lead to destabilization of the transition state in an SN2 process and consequently slower reactions with higher activation energy may be observed. It is long known that halide ions (CI-, Br-, 1- apart from F-) retain some basic­ity (Brollsted) even after coordination to a metal ion IX.

In acid solution, protonation of these halide ions pro­duce a reactive protonated species that hydrolytically cleaves more easily. Chloride ion coordinated to ruthe­nium ion retains significant basicity' Y, and as both the chl oride and the bromide ions differ little in th e ir Bronsted bas icity, protonation of bromide ion in thi s case a lso produces a spec ies that cleaves more eas il y. Aquation of the aquobromo complex formed after first stage of the reaction may be shown as follows :

CHAKRAVARTY et al.: BRIDGE CLEAVAGE REACTIONS OF Ru(III) COMPLEXES 691

k4 [Ru(NH)).(Hp)(HBr)p+ + Hp -) [Ru(NH).(HP)Y+ + HBr ... (6)

slow

This mechanism is in conformity with the empirical rate equation proposed in (4). The thiocyanato pentaammine complex formed during the reaction does not undergo aquation at this experimental condition. It was observed earlier that at 59" the pentaamminethiocyanato complex does not hydrolyse even in the presence of 1.0 mol dm-3 PTS20. Cobalt pentaammine complex also shows a similar trend where the thiocyanato complex indicates a greater inertness and under similar conditions its rate of reaction is at least 104 times less than either of the chloride or the bromide complexes21 •

Final reaction products for the chloro and the bromo complexes are different as the investigations were car­ried out for these two complexes in two different acidic medium, viz. perchloric and nitric acids . In order to com­pare the reactions of these complexes, the kinetics of the chloro-complex was also investigated in HNO, medium . Spectral scanning of this complex in 0 .5 mol dm-3 HN0

3

at room temperature (ca. 25") shows similar behavior as with the HCI04 with absorption maxima at 488 nm (first stage) and 400 nm (second stage). The rate constants for the first stage as determined by Guggenheim's proce­dure at 59" are given in Table I . Within the limits of experimental error the values are the same as obtained in HCI0

4. No attempt was made to determine the rate

constant for the second stage of the reaction . The final product in thi s case is not a polymeric one, but the diaquo complex [Ru(NH) 4(HP)2P+' as obtained with the bromo complex. So, the mechanisms of the reaction of both the complexes in nitric acid medium are similar.

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