Kinetics of substitution of ferrocenyl-containing β-diketonato ligands by phenanthroline from β-diketonato-1,5-cyclooctadienerhodium(I) complexes

www.elsevier.com/locate/ica Inorganica Chimica Acta 331 (2002) 188– 193

Kinetics of substitution of ferrocenyl-containing b-diketonato ligands by phenanthroline from b-diketonato-1,5-cyclooctadienerhodium(I) complexes Theunis G. Vosloo, W.C. (Ina) du Plessis, Jannie C. Swarts *,1 Department of Chemistry, Uni6ersity of the Free State, Bloemfontein 9300, South Africa Received 9 July 2001; accepted 23 November 2001 Dedicated to Professor A.G. Sykes

Abstract Second-order rate constants, k2, for the substitution of the ferrocene-containing b-diketonato ligands FcCOCHCOR− with R =CF3 (ferrocenoyltrifluroacetonato, fctfa, pKa 6.56), CCl3 (ferrocenoyltrichloroacetonato, fctca, 7.13), CH3 (ferrocenoylacatonato, fca, 10.01), Ph (anion of benzoylferrocenoylmethane, bfcm, 10.41) and Fc (anion of diferrocenoylmethane, dfcm, 13.1) (Ph= phenyl, Fc= ferrocenyl, values in brackets are the pKa values of the free b-diketones) from the complexes [Rh(cod)(FcCOCHCOR)] with 1,10-phenanthroline (phen, cod =1,5-cyclooctadiene) at 25 °C were found to be 560 (R =CF3), 1370 (CCl3), 30 (Ph), 18 (CH3) and 7.0 dm3 mol − 1 s − 1 (Fc), respectively. The temperature dependence of each reaction was determined and the large negative values obtained for activation, DS c B − 100 J K − 1 mol − 1 for all but R =CCl3 (DS c CCl3 = − 81 J K − 1 mol − 1), suggests an associative substitution mechanism. The rate law of the reaction was found to be R ={ks + k2[phen]}[Rh(cod)(FcCOCHCOR)]. Since the solvent-associated rate constant ks : 0 for all R except Ph (ks,RPh = 0.06 s − 1) the solvent, methanol, plays a limited role in the reaction. Results are interpreted to imply that the rate-determining step during substitution is breaking of an RhO bond and not the formation of an RhN bond. The role of b-diketone pKa and group electronegativity, , of each R group on the rate of substitution are also discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferrocene; b-Diketones; Rhodium; Group electronegativities; Substitution kinetics

1. Introduction In recent years, complexes with multi-metallic centres have received an increasing amount of attention inter alia because of the development of new materials [1] for applications such as molecular wires [2], prodrugs [3], luminescence-based sensors [4] and for catalysts in synthetic organic chemistry [5]. In many of these cases the ferrocenyl fragment is employed as an electron reservoir [1]. We have recently reported the synthesis of a number of ferrocene-containing b-diketonato complexes of * Corresponding author. Tel.: +27-51-401 2781; fax: + 27-51-430 7805. E-mail address: [email protected] (J.C. Swarts). 1 Dedicated to my friend, A. Geoffrey Sykes, who launched my career in chemistry in general, and in kinetics in particular.

rhodium(I) and rhodium(III), [6,7], as part of a program to establish how the ferrocenyl fragment will influence the reactivity of the rhodium nucleus inter alia in carbonyl insertion and substitution reactions. Towards this goal, in an electrochemical study, we described how the ferrocenyl formal reduction potential in ferrocene-containing b-diketones of the type FcCOCH2COR may be utilised to determine group electronegativities, R, of a variety of R groups including those indicated in the abstract [8]. The crystal structure of the enol form of FcCOCH2COCF3, [8], and of the rhodium(I) complex [Rh(fca)(cod)] have also been reported [9] and the kinetics of conversion from the enol to keto forms, and visa versa, for the b-diketones FcCOCH2COR with R =CF3, CCl3, CH3, phenyl and ferrocenyl has been described [10]. However, the influence of the ferrocenyl group on substitution reac-

0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 8 0 5 - 2

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189

2. Experimental

2.1. Materials

Scheme 1. Substitution reactions of [M(b-diketonato)(cod)] complexes either with s-donor ligands such as 1,10-phenanthroline (phen) or p-bonding ligands such as triphenylphosphite. M =Rh or Ir, R1 and R2 = different substituents at carbon atom numbers 1 and 3 on the b-diketonato ligand; (b-dik-C2)= b-diketonato ligand s-bound to Ir(I) via the methine carbon atom (carbon atom number 2).

tions of square-planar d8 rhodium(I) complexes remains unexplored. Monometallic b-diketonato-cod complexes of rhodium(I) and iridium(I) can undergo fundamentally two types of substitution reactions. Either the s-donating b-diketonato ligand are substituted by strong s-donor ligands such as 1,10-phenatroline [11– 13], or the pbonding ligand cod is substituted by other strong pbonding ligands such as triphenylphosphite [14] according to Scheme 1. These two types of reactions have been used successfully to determine the effects of the substituents R1 and R2 on the behaviour of (R1COCHCOR2)− as a leaving ligand [11,12]. It was found that the reactivity towards substitution in complexes of the type [M(cod)(b-diketonato)] (M= Rh or Ir) increase with an increase in the electronegativity of the substituents R1 and R2 but decrease with an increasing pKa of the b-diketone irrespective of whether the b-diketone is the leaving or non-labile ligand. The mechanistic impact of this electronegativity/reaction rate relationship is that during b-diketonato substitution, the first and the rate-determining step apparently is the breaking of the RhO bond adjacent to the R-substituent with the highest group electronegativity because this is the weakest RhO bond as explained elsewhere [15]. Conversely, when the b-diketone is the substitution-inert ligand, then the Rh– ligand bond cis to the above-described RhO bond is broken in the first, rate determining, step. This is also in accordance with the relative kinetic trans effect [16]. Substitution kinetic studies on reactions involving monometallic bdiketonato complexes of rhodium(I), as shown in Scheme 1, indicated they are associative in nature [11,12] by virtue of two criteria: large negative values for the entropy of activation [17] and negative volumes of activation [18]. With this publication we report the influence of the iron-containing ferrocenyl group on the kinetics of b-diketonato substitution from the di- and tri-metallic iron–rhodium complexes [Rh(cod)(FcCOCHCOR)] with R = CF3, CCl3, CH3, Ph and Fc.

The complexes [Rh(cod)(FcCOCHCOR)] were synthesised and characterised as described before [6], and 1,10-phenanthroline was used as purchased (Fluka). Methanol (analytic grade) was freshly distilled prior to use.

2.2. Methods and equipment UV spectra of [Rh(cod)(FcCOCHCOR)] and [Rh(cod)(phen)]+ were recorded in a Pye-Unicam SP 1700 double beam spectrophotometer. The Beer–Lambert law, A= mCl with A= absorbance, m =molar extinction coefficient, C= concentration and l =path length= 1 cm, were found to be valid [6] for all complexes in the concentration range utilised for the kinetic studies. Pseudo-first-order rate constants, kobs, were determined by monitoring the disappearance of [Rh(cod)(FcCOCHCOR)] (2.91×10 − 4 mol dm − 3 methanol solutions for the fca and fctfa complexes, 1.48× 10 − 4 mol dm − 3 solutions for the bfcm and fctca complexes and 2.90× 10 − 5 mol dm − 3 solutions for the dfcm complex) at the indicated wave lengths (Table 1) at 25.0 °C with a Durram D110 stopped-flow spectrophotometer interfaced with a personal computer. Phen concentrations were between 1.5× 10 − 3 and 4× 10 − 2 mol dm − 3. At least five different concentrations within this range were utilised for each [Rh(cod)(FcCOCHCOR)] complex to obtain each pseudo-first-order rate constant. The activation parameters DS c , DH c and DG c were obtained from kinetic runs between 15 and 39 °C, at least three temperatures were employed to establish the temperature dependence for each substitution reaction. Temperatures were held constant to within 0.01 °C.

2.3. Calculations For each kinetic run, absorbance–time data was collected and the pseudo-first-order rate constants observed, kobs, were determined from the linear fit of data in Eq. (1) ln(A0/At )= kobst

(1)

where A0 indicates initial absorbance and At absorbance at time t [19]. The pseudo-first-order rate constants determined experimentally were converted to second-order rate constants, k2, by determining the slope of the linear plots of kobs against the concentration of the incoming phen ligand. A zero intercept of this graph implied that the solvent does not significantly contribute to the reaction rate and the first-order rate constant associated with a solvent path, ks :0.

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Table 1 Second-order rate constants, k2, and activation parameters for b-diketonato substitution from [Rh(cod)(FcCOCHCOR)] with 1,10-phenanthroline in methanol at 25 °C. Group electronegativities, R, pKa values of the free b-diketones and the bond distances d(RhO) adjacent to indicated R-groups are also provided. The values in brackets indicate the uncertainty in the last digit of the numbers. R

a

CF3 (fctfa) CCl3 (fctca) CH3 (fca) Phenyl (bfcm) Ferrocenyl (dfcm)

R b

pKa c

d(RhO) (A, ) d

uexp (nm) e

k2 (dm3 mol−1 s−1) f

DH c (kJ mol−1)

DS c (J K−1 mol−1)

3.01 2.76 2.34 2.31 1.87

6.90 7.13 10.01 10.41 13.1

2.066(5)25 2.054(2)26 2.048(3)9 2.050(4)25 2.039(3)9

416 495 472 477 525

560(10) 1370(10) 18(1) 30(3) 7.0(2)

25(1) 31(2) 29(4) 30(1) 19(5)

−107(1) −81(5) −120(10) −113(1) −160(20)

(1900) (2260) (2390) (2390) (1567)

The abbreviation in brackets represents the acronym of the b-diketonate FcCOCHCOR−. Ref. [8]. c Ref. [6]. d The numbers in superscripts are the references to the bond lengths. e Wavelength at which the substitution was monitored, values in brackets represent the extinction coefficients, m, in units of dm3 mol−1 cm−1, of each rhodium complex at the experimental wavelength. f ks :0 s−1 for all complexes except bfcm, where it is 0.06(1) s−1 at 25 °C. a

b

Non-zero intercepts implied that kobs =ks +k2[phen] and that the solvent does contribute to a noticeable extent towards the reaction mechanism. Activation parameters were obtained from a fit in the Eyring equation [20] (Eq. (2)) ln(k2/T)= − DH c /(RT) + DS c /R + ln{R/(Nh)}

(2)

All mathematical fits were carried out using the fitting program MINSQ [21].

3. Results and discussion The validity of the reaction between complexes of the type [Rh(cod)(FcCOCHCOR)] and phen to liberate [Rh(cod)(phen)]+ according to Scheme 1 was confirmed earlier [6]. The general rate law applicable to this substitution reaction is given [22] by Eq. (3) Rate= {ks + k2[phen]}[Rh(FcCOCHCOR)(cod)] =kobs[Rh(FcCOCHCOR)(cod)]

to the substitution reaction during bfcm displacement compared with the other b-diketonato ligand substitutions from [Rh(b-diketonato)(cod)] complexes could be found although it is worth pointing out that complexes possessing the phenyl-containing CF3COCHCOPh ligand also deviated from the other b-diketonato complexes in having non-zero ks values [16,23]. Table 1 summarises the second-order rate constants, k2, as well as the entropy of activation, DS c , and activation enthalpy, DH c , which was determined from a temperature dependence study, for all the reactions investigated. The large negative activation entropies clearly show that the substitution process proceeds via an associative mechanism and not a dissociative mechanism. When one considers the nature of the transition state during the substitution reaction, further evidence for an associative mechanism may be presented. Reaction (4) below shows the two possible transition states if the substitution process is to proceed via a dissociative mechanism. Reaction (5) shows the expected transition

(3)

with the pseudo-first-order rate constant kobs =ks + k2[phen] and k2 the second-order rate constant for the substitution process. A graph of phen concentration against kobs was linear in all cases (Fig. 1). All complexes but [Rh(FcCOCHCOPh)(cod)] showed a zero intercept. The result ks :0 is to be expected since the displacement of the bidentate chelates FcCOCHCOR− with monodentate solvent molecules, here methanol, would be much more difficult than the displacement of a monodentate ligand by solvent molecules [22]. The small non-zero intercept ks =0.06(1) for the bfcm complex at 25 °C was surprising, but it is real: It was found to be reproducible in the follow-up experiments utilising [Rh(FcCOCHCOPh)(cod)] complexes from different synthetic batches. No obvious explanation as to why the solvent, methanol, contributes more noticeable

Fig. 1. Left: graphs of pseudo-first-order rate constants, kobs, vs. [1,10-phenanthroline] at 25 °C for the complexes [Rh(FcCOCHCOCF3)(cod)], [Rh(FcCOCHCOCCl3)(cod)], [Rh(FcCOCHCOCH3)(cod)] and [Rh(FcCOCHCOFc)(cod)] passes through the origin. Right: graphs of kobs vs. [1,10-phenanthroline] at various temperatures for the complex [Rh(FcCOCHCOPh)(cod)] had intercepts of 0.021(7) at 15.35 °C, 0.06(1) at 25.0 °C and 0.10(1) s − 1 at 38.1 °C. Rhodium complex concentrations are provided in the experimental section, the solvent was methanol.

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[Rh(FcCsOCHCsOR1)(cod)] NN

v [Rh(FcCsOCHCsOR1)(cod)(NN)] v [Rh(FcCsOCHCsOR1)(cod)(NN)

“ [Rh(cod)(NN)]+ + (FcCOCHCOR1)−

Fig. 2. Plots of log(k2) vs. pKa values of the free b-diketones in complexes of the type [Rh(b-diketonato)(cod)] and, on the inset, vs. the sum of the group electronegativities (R1 + R2) of the terminal groups R1 and R2 on b-diketonato ligands (R1COCHCOR2)−. hfaa = hexafluoroacetylacetonato, tfba = benzoyltrifluoroacetonato; tfaa = trifluoroacetylacetonato; dbm = anion of dibenzoylmethane; acac= acetylacetonato; ba = benzoylacetonato. Other abbreviations are defined in the abstract. Rate constants for substitution reactions other than those studied for this publication, were taken from Ref. [11].

states in an associative mechanism. In both reactions the notation sO indicates that the rhodium nucleus is coordinated via a coordination bond to the specified O-atom of the b-diketonato ligand. The expected transition state for a dissociative mechanism may either be the cationic 12-electron [Rh(cod)]+ species or the neutral 14-electron [Rh(FcCOCHCsOR1)(cod)] species in which the b-diketonato ligand FcCOCHCOR1 reverted from being a bidentate ligand, which complexes via the two sO-atoms, to a monodentate ligand after one RhsO bond has broken (reaction (4)). In the associative mechanism, a neutral five-coordinated 18-electron species [Rh(FcCsOCHCsOR1)(cod)(NN)], in which only one of the N-atoms are coordinated to the Rh nucleus, is expected to form first (reaction (5)). This transition state may then revert to another transition state in which the phen ligand becomes bidentate with a simultaneous breaking of at least one of the RhsO bonds. While Rh(I) are known to be involved in 16and 18-electron complexes, 12- or 14-electron species are extremely unlikely [24]. It follows that the associative mechanism as shown in reaction (5) is by far the more plausible reaction pathway. [Rh(FcCsOCHCsOR1)(cod)] v [Rh(cod)]+ + (FcCOCHCOR1)− or [Rh(FcCOCHCsOR1)(cod)] NN

“ [Rh(cod)(NN)]+

(4)

(5)

Apart from the highly negative DS values and the electron counting arguments given above to prove that the substitution reaction given above proceeds via an associative mechanism, it is well known that the associative mechanisms are characterised by negative volumes of activation [17]. This criteria has been used in a high pressure study to conclusively show that the reaction between [Rh(acac)(cod)] and phen proceeds via an associative mechanism [18]. The entropy of activation of acac substitution is − 108 J K − 1 mol − 1 and is considered to be a very large negative [12]. Comparison of this value with the DS c -values obtained for all the reactions studied in this report (Table 1), by way of analogy, strongly implies that the substitution reactions of this study proceeds via an associative mechanism as indicated in Eq. (5). It is strikingly evident (Fig. 2) that the more acidic b-diketonato ligands coordinated to the [Rh(FcCOCHCOR)(cod)] complexes have a dramatic acceleration effect on the rate of substitution with phen. Of special importance, however, is the observation that the more basic b-diketonato ligands have almost no influence on the rate of substitution. This is in sharp contrast to what was reported earlier. Leipoldt and Grobler [11] studied b-diketonato substitution from monometal complexes of the type [Rh(R1COCHCOR2)(cod)]. In their study, the pKa values of the free b-diketonato ligands varied between 4.35 and 9.35. Within these pKa limits, these authors observed a linear relationship between pKa and log(k2). This observation also made them to consider a linear relationship between pKa and log(k2) as a third diagnostic tool, alongside large negative values for the entropy of activation and negative volumes of activation, to identify associative substitution mechanisms. Three of the new ligands used in this study, fca, bfcm and dfcm, have pKa values more basic (Table 1) than those in the range studied by Leipoldt et al. [12] and it allowed us to show that a characteristic property of associative substitution mechanisms does not include a linear relationship between log(k2) and pKa of the leaving b-diketonato ligand. Our results regarding the non-linear log(k2)–pKa relationship are not entirely unexpected. It was demonstrated earlier [12] in a study where phen and a variety of phen derivatives having pKa values ranging from 3.57 to 6.31 substituted the acetylacetonato ligand in [Rh(acac)(cod)], that the pKa of the incoming phenbased NN ligands do not have any influence on the rate of acac substitution. This observation is consistent with a mechanism in which the rate-limiting step in the c

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substitution of b-diketonato with phen in [Rh(b-diketonato)(cod)] is not the rate of formation of a five-coordinate 18-electron complex via the formation of a new RhN bond as explained for the first transition state described in Eq. (5), but rather the rate of RhO bond breaking with concomitant transformation of phen from a mono- to bidentate ligand, the second transition state in Eq. (5). The fact that the rate of b-diketonato substitution becomes less dependent on b-diketonato pKa at high pKa values (Fig. 2) implies that there exists a limiting rate by which RhO bonds may be broken. An exponential fit on the data shown in Fig. 2, showed that the rate of b-diketonato substitution from complexes of the type [Rh(b-diketonato)(cod)] are predicted by the following equation log(k2)=e(2.519 − 0.2145pKa)

(6)

In principle, one may expect that for complexes in which the pKa of the b-diketonato ligand is large enough, substitution will simply cease to occur. However, in practice, this would probably never occur. Eq. (6) predicts that even if the pKa of the free b-diketone approaches the very high value of 25, the substitution reaction would still occur with k2 =1.06 dm3 mol − 1 s − 1. This rate is only seven times slower than for the dfcm (pKa 13.1) substitution. The slowest b-diketonato substitution should occur in complexes where the RhO bonds are the strongest, i.e. the shortest. A survey of the available RhO bonds adjacent to a specific R1 terminal side group in complexes of the type [Rh(R1COCHCOR2)(cod)], summarised in Table 1, support this expectation. The shortest RhO bond length thus far observed [9] in b-diketonato-cod-rhodium complexes is 2.039(3) A, for the RhO bond adjacent to the ferrocenyl group in the complex [Rh(FcCOCHCOCH3)(cod)]. The corresponding bond in [Rh(FcCOCHCOCCl3)(cod)] was for all practical purposes [26] the same, 2.040(2) A, , even though the group electronegativities of the CH3 and

Fig. 3. With the obvious exception of R = CCl3, the rate of (FcCOCHCOR)− substitution from complexes of the type [Rh(FcCOCHCOR)(cod)], expressed as log(k2), increases exponentially as the RhO bond length between the b-diketonato carbonyl group adjacent to the R =Fc, Ph, CH3 and CF3 groups and the Rh nucleus increases.

CCl3 groups differ substantially (Table 1). The only available crystal structure [25] of a CF3 and Ph complex of the type investigated in this study is [Rh(PhCOCHCOCF3)(cod)], but based on the results of the RhO bond lengths adjacent to the Fc group for the CCl3 and CH3 complexes described above, the RhO bond length on the CF3 and Ph sides of the complex [Rh(PhCOCHCOCF3)(cod)] are regarded as representatives of the RhO bond lengths adjacent to CF3 [2.066(5) A, ] and Ph [2.050(4) A, ] terminal b-diketonato side groups. By comparing the rate of dfcm and fctfa substitution from [Rh(b-diketonato)(cod)] complexes, Table 1, an increase in the RhO bond length of 0.027 A, led to a eightyfold increase in the rate of b-diketonato substitution. The only exception to the near linear relationship between log(k2) and RhO bond length, Fig. 3, is the faster than expected substitution rate for the CCl3-containing complex [Rh(FcCOCHCOCCl3)(cod)]. This deviation may be the consequence of the extreme lability [6] of this ligand. The complexes [Rh(b-diketonato)(cod)] have two RhsO bonds. Rather than looking at the RhO bond lengths to decide which of these break first, it is instructive to consider the recently determined [8] group electronegativities, , of the ferrocenyl and other R groups investigated in this study. The group electronegativity of the ferrocenyl group, 1.87, is substantially lower than those of all the other R groups (Table 1). The carbonyl group on the ferrocenyl side of the b-diketonato ligand, FcCOCHCOR−, should therefore be more electron rich than the other carbonyl group, and it should therefore interact strongly with the positively charged rhodium(I) nucleus. Conversely, the carbonyl group on the other side of the bidentate b-diketonato ligand should be relatively more positive in nature than the carbonyl group on the ferrocenyl side. Hence this carbonyl group should form a weaker bond with the positively charged rhodium nucleus of [Rh(b-diketonato)(cod)]. It follows, therefore, that the RhsO bond on the side of the b-diketonato ligand, (R1COCHCOR2)−, possessing the less electronegative side-group should initially remain intact while the other RhsO bond should break first during b-diketonato substitution. Inspection of the available rate constants, bond lengths and group electronegativities summarised in Table 1, confirm this because the trend that larger k2 values can be associated with smaller pKa values, larger R values and longer RhO bond lengths is quite obvious. Since group electronegativity considerations led to the same result as the survey of the available bond lengths with regard to predicting the RhO bond that breaks first (or fastest), we conclude that the use of group electronegativities, as explained above, is a completely general method to arrange qualitatively the relative bond strengths from strong to weak. It also follows

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that the rate of b-diketonato substitution can be related to the group electronegativities, R, of the side groups R1 and R2 of b-diketonato ligands (R1COCHCOR2)−. The inset in Fig. 2 demonstrates that the rate of substitution depends on the sum of the group electronegativities, R1 + R2, according to the following expression log(k2)=e[ − 3.024 + 0.79(R1 + R2)]

(7)

Because we have previously shown that there exists a direct relationship between pKa and group electronegativities [8], it is strikingly evident that both Eqs. (6) and (7) really leads to the same conclusion: The rate of b-diketonato substitution tends to become independent of b-diketonato pKa for strongly basic b-diketones.

4. Conclusion This study demonstrated that the substitution of b-diketonato ligands in complexes of the type [Rh(bdiketonato)(cod)] with phenanthroline proceeds via an associative mechanism with rate-determining step the breaking of a RhO bond rather than the formation of a RhN bond. The relative rate of b-diketonato substitution from [Rh(R1COCHCOR2)(cod)] complexes are related to the sum of the group electronegativities of the R1 and R2 side groups and to the pKa of the free b-diketones. However, reaction rates are less sensitive to more basic b-diketonates, i.e. b-diketonates possessing high pKa values or terminal side groups R1 and R2 with small group electronegativities. By incorporating the electron-donating ferrocenyl group in the b-diketonate ligand, we were capable of showing that b-diketonato substitution from the di- and tri-metal complexes [Rh(FcCOCHCOR1)(cod)] are slowed down to the point where the rate are almost independent of b-diketonato pKa and it is the consequence of the very strong RhO bonds that the ferrocenyl group induces.

Acknowledgements The authors acknowledge the financial support from the National Research Foundation, CANSA South Africa and the Central Research Fund of the University of the Free State.

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