Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)]

Polyhedron 26 (2007) 5075–5087 www.elsevier.com/locate/poly

Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)] Jeanet Conradie, Gert J. Lamprecht, Andreas Roodt, Jannie C. Swarts

*

Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa Received 16 May 2007; accepted 10 July 2007 Available online 28 August 2007

Abstract The kinetics of oxidative addition of CH3I to [Rh(FcCOCHCOCF3)(CO)(PPh3)], where Fc = ferrocenyl and (FcCOCHCOCF3) = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, 1H and 31P NMR techniques. Three definite sets of reactions involving isomers of at least two distinctly different classes of RhIII-alkyl and two different classes of RhIII-acyl species were observed. Rate constants for this reaction in CHCl3 at 25 C, applicable to the reaction sequence below, were determined as k1 = 0.00611(1) dm3 mol1 s1, k1 = 0.0005(1) s1, k3 = 0.00017(2) s1 and k4 = 0.0000044(1) s1 while k3  k3 and k4  k4 but both 6¼0. The indeterminable equilibrium K2 was fast enough to be maintained during RhI depletion in the first set of reactions and during the RhIIIalkyl2 formation in the second set of reactions. From a 1H and 31P NMR study in CDCl3, Kc1 was found to be 0.68, Kc2 = 2.57, Kc3 = 1.00, Kc4 = 4.56 and Kc5 = 1.65.

First set of reactions

Second set of reactions

Third set of reactions

The above reaction sequence represents a completely general reaction sequence for the oxidative addition of iodomethane to any bdiketonato complex of the type [Rh(b-diketonato)(CO)(PPh3)], although the equilibrium K2 may not necessarily always be fast. A temperature dependence study in chloroform led to the activation enthalpies, DH#, and activation entropies, DS#, for both the first and second sets of reactions. A solvent dependence study utilizing five different solvents showed the rate of the first set of reactions are directly proportional to the dielectric constant of the solvent. The molecular formula of all the RhIII-alkyl and RhIII-acyl species are [Rh(FcCOCHCOCF3)(CH3)(CO)(PPh3)(I)] and [Rh(FcCOCHCOCF3)(COCH3)(PPh3)(I)], respectively (Fc = ferrocenyl), but the geometries of the two RhIII isomers and the initial RhI species all differ due to different coordination spheres. A crystal structure determination (Z = 4, monoclinic, P21/c) of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)], an isomer of the RhIII-alkyl2 species, is also reported.  2007 Elsevier Ltd. All rights reserved. Keywords: Rhodium complexes; Ferrocene; b-Diketonates; Kinetics; Oxidative addition

1. Introduction *

Corresponding author. Tel.: +27 0 51 4012781; fax: +27 0 51 4446384. E-mail address: [email protected] (J.C. Swarts).

0277-5387/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.07.004

Platinum group transition metal complexes are well known for their medical applications [1] and are often

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J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

employed as either homogeneous [2] or heterogeneous [3] catalysts in a variety of chemical reactions. Rhodiumbased homogeneous catalysts in particular may be used inter alia in carbon–carbon bond formation [4], cyclopropanation [5], decarbonylation of aldehydes [6], hydroboration of olefins [7], hydroformylation of olefins [8], hydrogenation [9], hydrosilation [10] and carbon monoxide insertion reactions [11–13]. Oxidative addition of iodomethane to square planer rhodium(I) or iridium(I) complexes and subsequent CO insertion reactions is important in the Monsanto and Cativa industrial processes [12,14] for the production of acetic acid from methanol. The rate-determining step in the rhodium-based Monsanto process is the oxidative addition of methyl iodide to [Rh(CO)2I2]. In contrast, during cis-[Ir(CO)2I2]-catalyzed methyl carbonylation in the Cativa process, CO insertion in [CH3Ir(CO)2I3] to give [(CH3CO)Ir(CO)2I3] is ratedetermining. It is known [15–17] that rhodium(I) complexes of the type [Rh(L,L 0 )(CO)(PPh3)], L,L 0 = mono charged bidentate ligands, can also facilitate the formation of an acetyl species following oxidative addition of methyl iodide. Depending on what L,L 0 is, a variety of reaction mechanisms for this reaction has been reported. These mechanisms are summarized in Eq. (1) (L,L 0 ) = methyl(2methyl-amino-1-cyclopentene-1-dithiocarboxylato, [17]), (2) (L,L 0 thioacetylacetonato [15d]), (3) (L,L 0 = inter alia 1,1,1-trifluoro-2,4-pentanedionato [15a]) and (4) (L,L 0 = inter alia 2,4-pentanedionato [15b]) below. The notation RhIII-alkyl1 and RhIII-acyl1 in the four equations below indicates the first formed alkylated, [RhIII(R)], or acylated rhodium(III) species, [RhIII(COR)]. When the last number in the notation changes to ‘‘2’’, such as in RhIIIalkyl2 in Eqs. (3) and (4), it shows that after the first alkylated or acylated species has formed, it converted to a second, different but more stable, alkylated or acylated geometrical isomer. k 1 ;K

k2

RhI þ CH3 I  RhIII -alkyl1 ! RhIII -acyl1 k 1 k1

k2

RhI þ CH3 I ! RhIII -alkyl1 ! RhIII -acyl1

ð1Þ ð2Þ

k 1 ;K

RhI þ CH3 I  RhIII -alkyl1 k 1 k2

k3

7! RhIII -acyl1 ! RhIII -alkyl2

ð3Þ

k 1 ;K

RhI þ CH3 I  RhIII -alkyl1 k 1 k2

k3

k 2

k 3

 RhIII -acyl1  RhIII -alkyl2

ð4Þ

In, for example, Eq. (4), [Rh(H3CCOCHCOCH3)(CO)(PPh3)(CH3)(I)] would be a typical formula describing the two different RhIII-alkyl1 or RhIII-alkyl2 geometrical isomers. The first example of a single crystal structure determination of a RhIII-alkyl species of this type is described later in this publication and Fig. 8 shows clearly the coordination modes. For the RhIII-acyl1 isomer of Eq. (4), the formula [Rh(H3CCOCHCOCH3)(COCH3)(PPh3)(I)] is

representative. Structurally it differs from the alkyl species in that the CO ligand inserted between the Rh center and the alkyl, here a methyl ligand, to generate an acyl ligand. We were interested to establish whether a ferrocenyl (Fc) fragment as part of the b-diketonato ligand in complexes of the type [Rh(b-diketonato)(CO)(PPh3)] may enhance acyl formation during oxidative addition. Utilizing L,L 0 = (FcCOCHCOCF3) = fctfa, we show with this communication that incorporation of the ferrocenyl group in the b-diketonato ligand significantly enhances the rate of acyl formation as well as the ensuing carbonyl insertion and deinsertion to form new RhIII-alkyl and RhIII-acyl species during the oxidative addition of CH3I to [Rh(fctfa)(CO)(PPh3)]. We especially demonstrate how a variety of different spectroscopic techniques (UV/Vis, IR, 1H and 31 P NMR) may be used to obtain different information packages and a better insight into the overall process that takes place during this general type of reaction. Especially satisfying was the realization that each of the previous proposed mechanisms (Eqs. (1)–(4)) are only special cases of the more general mechanism given in the abstract for the reaction between [Rh(fctfa)(CO)(PPh3)] and CH3I. 2. Experimental 2.1. Materials and apparatus NMR measurements, at 298 K unless otherwise stated, were recorded on a Bruker Advance DPX 300 NMR spectrometer [1H (300.130 MHz), and 31P (121.495 MHz)]. Chemical shifts are reported as d values relative to SiMe4 at 0 ppm for the 1H spectra or relative to 85% H3PO4 at 0 ppm for the 31P spectra. Infrared spectra were recorded on a Hitachi 270-50 infrared spectrometer either in a KBr matrix or in chloroform or acetone solutions. UV measurements were recorded on a GBC-916 UV/Vis spectrometer. Liquid reactants and solvents were distilled prior to use, water was double distilled. DMF was dried according to published methods [18]. Solutions of [Rh(fctfa)(CO)(PPh3)] in acetone, methanol, ethyl acetate, benzene and chloroform [19] were tested for stability by means of 1H NMR and overlay IR and UV spectra for at least 24 h. Synthesis of [Rh(fctfa)(CO)(PPh3)(CH3(I)]: Methyl iodide (1.9 g, 13.4 mmol) was added to [Rh(fctfa)(CO)(PPh3)] [20] (75 mg, 0.1 mmol) dissolved in hexane (70 ml) at 30 C. N2 was purged through the reaction vessel before it was sealed and left at room temperature in the dark. After 5 days crystals suitable for X-ray structure determination were obtained from the reaction mixture, 90% yield, m.p.: 235 C; C, 47.5; H 3.2, C34H28F3FeO3RhPI requires C, 47.6; H, 3.3%). IR (KBr), mmax/cm1: 2056 (C@O); 1H NMR(CDCl3): isomer A: 1.78 (3H, d d, CH3), 4.30 (5H, s, C5H5 and 1H, m, C5H4), 4.40 (1H, m, C5H4), 4.48 (1H, m, C5H4), 4.77 (1H, m, C5H4), 5.31 (1H, s, CH) and 7.25–7.50 (15H, m, aromatic); isomer B: 1.78 (3H, d d, CH3), 4.24 (5H, s, C5H5 and 1H, m, C5H4), 4.38 (1H, m, C5H4), 4.60 (1H, m, C5H4), 4.80

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

(1H, m, C5H4), 5.45 (1H, s, CH) and 7.25 – 7.50 (15H, m, aromatic). 2.2. Crystallography Crystal density of [Rh(fctfa)(CO)(PPh3)CH3(I)] was determined by flotation in sodium iodide solution. A crystal of dimensions 0.075 · 0.150 · 0.650 mm was used for data collection on a Syntex P-1 diffractometer with Nb filtered utilizing Mo Ka radiation. The x/2h-scan technique was used with variable scan width Dx = (0.43 + 0.34 tanh), scan speed 5.49 min1 in x and a maximum scan time of 60 s per reflection. Unit cell parameters were determined from least-squares refinement of 25 reflections with 15 < h < 20 (total measuring range 3 < h < 25). Empirical absorption corrections [21] were applied with minimum and maximum correction factors of 0.4529 and 0.7413, respectively. The mean intensity of three standard reflections, measured over 7200 s of X-ray exposure time, varied from the initial value by less than 1%. All possible reflec˚ 1 in the range 12 < h < tions with (sin h)/k < 0.59 A 12.0 < k < 18.0 < l < 24 were measured, giving 4420 unique reflections of which 4414 were considered observed [I > 2.0r(I)]. The structure was solved by the heavy-atom method using SHELXS86 [22] and was subjected to anisotropic full-matrix least-squares refinement on F (a total of 380 parameters refined) using SHELXS93 [23]. The H-atom positions were calculated riding on the adjacent C atoms ˚ , and were refined with an overall assuming C–H = 0.930 A temperature factor. Neutral-atom scattering factors were taken from Cromer and Mann [24] and anomalous-dispersion corrections for rhodium from International Tables for X-ray Crystallography [25]. Final R = 0.0588 and wR = 0.1692 (unit weights), (Dq)max = 1.841, (Dq)min = 1.087 ˚ 3 and (D/r)max = 0.745. The high value of the maxieA mum residual electron density (Dq)max of 1.841 is due to irrelevant peaks near the heavy Rh-atom. 2.3. Kinetic measurements The kinetic rate constants were determined utilizing UV/ Vis (by monitoring the change in absorbance at 375, 400, 530 and 580 nm), IR (by monitoring formation and disappearance of the carbonyl peaks at 2082, 2064, 1990, 1729 and 1714 cm1), 1H and 31P NMR spectroscopy (by monitoring the change in integration units of the various signals with time as specified in Table 2 and Fig. 6). A linear relationship between UV absorbance, A, and concentration, C, confirmed the validity of the Beer Lambert law (A = eCl with l = path length = 1 cm) for the [Rh(fctfa)(CO)(PPh3)] complex. All kinetic measurements were monitored under pseudo first-order conditions with CH3I concentrations 7–10 000 times the concentration of the [Rh(fctfa)(CO)(PPh3)] complex. At least five different concentrations within this range were utilized. The concentration of [Rh(fctfa)(CO)(PPh3)] was 60.0004 mol dm3 for UV/Vis measurements and 60.01 mol dm3 for IR and NMR mea-

5077

surements. The activation parameters DH#, DS# and DG# for the reaction in chloroform and acetone were obtained from kinetic runs between 15.0 and 35.0 C. At least three temperatures, held constant within 0.1 C, were employed to establish the temperature dependence for each substitution reaction. 2.4. Calculations Pseudo first-order rate constants, k1obs, for reaction set 1 were calculated by fitting kinetic data to the first-order equation [26] ½At ¼ ½A0 expðk 1obs tÞ

ð5Þ

with [A]t and [A]0 the absorbance of the indicated species at time t and 0 (UV/Vis or IR experiments), or integral values for the specified peaks on NMR spectra, and were converted to second order rate constants, k1, by determining the slope of the linear plots of k1obs against the concentration of the incoming CH3I ligand with k obs ¼ k 1 þ k 1 ½CH3 I

ð6Þ

Here k1 is the first-order rate constant for the reverse step [26] in the general mechanism k1

A þ CH3 I  B k 1

with A = [Rh(fctfa)(CO)(PPh3)] and B = {[Rh(fctfa)(CH3)(CO)(PPh3)(I)]  [Rh(fctfa)(COCH3)(PPh3)(I)]}, or more conveniently expressed as [RhIII-alkyl1  RhIII-acyl1]. First-order rate constants for reaction set 2, k3, and reaction set 3, k4, were also determined from Eq. (5) by replacing k1obs with k3 or k4. Kinetic data for the consecutive reaction treatment [26] of IR data obtained in acetone k3 k1 and applicable to the general reaction A ! B ! C were fitted to Eq. (5) when monitoring the disappearance of A, Eq. (7) for monitoring the appearance and disappearance of B and Eq. (8) for monitoring the appearance of C, here RhIIIalkyl2. A and B were defined above. k 1obs ½A0 ½expðk 1obs tÞ  expðk 3 tÞ ð7Þ k 3  k 1obs   1 ½Ct ¼ ½A0 1  ½k 3 expðk 1obs tÞ  k 1obs expðk 3 tÞ k 3  k 1obs

½Bt ¼

ð8Þ

Activation parameters [26] were obtained from a fit on Eq. (9) with ki (i = 1, 3) the kinetic rate constants for reaction steps 1 and 3. ln

ki DH # DS # kB ¼ þ þ ln T RT R h

ð9Þ

All kinetic mathematical fits were done utilizing the fitting program MINSQ [27]. The error of all the data is presented according to crystallographic conventions. For example k1obs = 0.0446(1) s1 implies k1obs = 0.0446 ± 0.0001 s1.

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

3.1. The UV/Vis study, solvent and temperature dependence

5

3.2

3.3

3.4 3

-1

3.5

-1

2

25 °C

1

15 °C

-10.4

35 °C

-10.9

15

-11.4

25 °C 3.2

3.3

3.4

3.5

-1

3 -1 10 T /K

15 °C

10 5 0

k1

0

k 1

0.5

1

1.5

2

-3

2

rel. abs.

1 st

3

580 nm

2 nd reaction

0.2

2 t/hours

4

reaction 375 nm 400 nm 530 nm

20

t/hours

40

20

30

Fig. 3. Relationship between the second-order rate constant, k1, in units of dm3 mol1 s1, and the dielectric constant of the solvent during the UV monitoring of the first reaction set of the oxidative addition of CH3I to [Rh(fctfa)(CO)(PPh3)] at 25 C.

530 nm 580 nm

0

10

1

0

1

0.002

dielectric constant

0

3

acetone

0

t/hours

rd

0.004

0

400 nm

0.4

2

methanol

ethylacetate

0 0

chloroform

0.006

benzene

2 nd 530 nm

Fig. 2. CH3I concentration dependence for the UV/Vis monitored oxidative addition of CH3I to [Rh(fctfa)(CO)(PPh3)] in chloroform at 530 nm for the first reaction step, bottom, and for the second reaction step, top. Insets: The linear dependence between ln(k/T) and 1/T allowed the determination of activation parameters.

375 nm

1 st

375 nm

1

rel. abs.

4

[CH3I]/mol dm

k1

However, when the reaction was studied in methanol, acetone, ethyl acetate and benzene, k1 was found to be so small as to be undetected. The temperature dependence in chloroform for the first and second stages of the oxidative addition reaction is also demonstrated in Fig. 2. The increase in the rate of the first step of oxidative addition depends linearly on the dielectric constant, DE [28], of each solvent, Fig. 3. Chloroform does not fit the trend suggested by the other solvents because it follows a different mechanism (k1 is measurable). Rate constants, results of the solvent dependence study and activation parameters which were obtained from the temperature dependence study utilizing Eq. (9) are summarized in Table 1.

relative absorbance

-17

3

20

A þBC

0

-15

4

10 T /K

103kobs/s-1

As illustrated in Fig. 1, three general reaction steps for the oxidative addition reaction between CH3I and [Rh(fctfa)(CO)(PPh3)] could be identified by UV/Vis spectrophotometry. Due to the larger change in absorbance for the second and third reaction step at 375 nm, more accurate k3 and k4 rate constants were obtainable at this wavelength. For reaction step 1, data obtained at 530 nm gave more accurate k1 and k1 rate constants. The first reaction showed a first-order dependence on CH3I, while the second and third (not illustrated) reactions are independent (i.e. zero order) of CH3I concentration, Fig. 2. It is clear that the relationship between k1obs and [CH3I] in chloroform does not pass through the origin. This implies [26] that reaction step one is an equilibrium process of the form

35 °C -13

lnk3/T

6

104k3/s-1

3. Results and discussion

lnk1/T

5078

60

Fig. 1. Relative absorbance vs. time data at 25 C for the UV/Vis monitored oxidative addition of CH3I to [Rh(fctfa)(CO)(PPh3)] in chloroform at the indicated wavelengths for the first reaction step (insert top left), the second reaction step (insert top right) and the third reaction step (main graph, starting at approximately 5 h), [Rh(fctfa)(CO)(PPh3)] = 0.00015 mol dm3, [CH3I] = 0.1416 mol dm3.

Of special interest is the observation that DS# values for all the reaction steps that are independent of [CH3I] (the k1 and k3 steps), approaches zero within experimental error, whereas DS# for the second order reaction step related to k1 are large negative. This is consistent with an interpretation that the transition state for the forward step 1 reaction occurs via an associative mechanism where the coordination number changes from 4 to 6. For the other reaction steps, the coordination number changes from 6 to 5 implying the transition state undergoes much less dramatic coordination changes. This results in DS# approaching zero.

Table 1 Kinetic rate constants for the oxidative addition reaction of CH3I to [Rh(fctfa)(CO)(PPh3)] as obtained by various spectroscopic methods at 25 C by considering each reaction step in isolation during data work-up with Eq. (5), as well as by a consecutive reaction treatment of data with Eqs. (7) and (8) Solvent (DE),a (k/nm)

Reaction sequence 1bDH# = 29(3), DS# = 188(9) k1 (dm3 mol1 s1)

Reaction sequence 1bDH# = 81(6), DS# = 40(20) k1 (s1)

Reaction sequence 2b,c DH# = 93(1), DS# = 5(3) k3 (s1)

UV/Vis

CHCl3 (4.8) k = 580 k = 530 k = 400 k = 375 C6H6 (2.27) CH3COOEt (6.02) (H3C)2CO (20.7) CH3OH (32)

0.0060(1) 0.0061(1)

0.0005(1) 0.0005(1)

0.00016(2) 0.00017(2) 0.00021(2) 0.00020(1)

0.000141(2) 0.000716(4) 0.00370(4)d 0.00658(3)

0 0 0 0

0.0061(2) 0.0041(1) 0.0046(4) 0.008(2)

0.0006(2) 0 0 0

IRc

1

H NMR P NMR

31

CHCl3 (H3C)2CO, (H3C)2CO, (H3C)2CO, (H3C)2CO,

Eq. Eq. Eq. Eq.

CHCl3 (4.8) CHCl3 (4.8)

(5)e (7)e (8)e (7)e

0.006(1) 0.005(1)

RS 3b,c106 k4 (s1)

4.4(1) 4.3(1) 3.7(1)

0.00017(1) 0.00020(4) 0.00010(1) 0.00018(1) 0.00016(1)

4.7(4) 2.6(5)

0.00011(1) 0.00012(2)

5(2)

5.5(3)

Activation parameters in CHCl3 are summarized in the table heading. RS is the reaction sequence and Et is the ethyl. a DE is the dielectric constant at 25 C, except for CHCl3 where DE is at 20 C. The sited wave lengths were used to monitor the reaction by UV/Vis in CHCl3. b Units of DH# is kJ mol1, that of DS# is J mol1 K1. For the extremely slow reaction sequence 3 (t1/2 < 35 h), no activation parameters were determined. Also, k3 and k4 6¼ 0 but very small. c Cited rate constants are the average from several reactions with different CH3I concentrations. d In acetone, DH# = 50(4) kJ mol1, DS# = 124(4) J mol1 K1. e IR data for each reaction set fitted to the indicated kinetic model Eqs. (5), (7) or (8), conditions for the consecutive reaction treatment of IR data were [CH3I] = 0.2604 mol dm3, [Rh(fctfa)(CO)(PPh3)] = 0.002 mol dm3, T = 25 C.

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

Method

5079

5080

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

Results of the UV/Vis section only of this study are interpreted to imply that the first reaction step represents an oxidative addition process whereby the RhI nucleus converts to a RhIII nucleus. The product of this oxidative addition step may be either an alkyl or acyl species, [Rh(fctfa)(CH3)(CO)(PPh3)(I)] or [Rh(fctfa)(COCH3)(PPh3)(I)] (or both), but the UV/Vis technique cannot distinguish between these possibilities. Since the second and third reaction steps are independent of [CH3I], one may deduce that these two reactions either involve slow solvent exchange with any of the ligands of the newly formed RhIII complex or it may be isomerization from an alkyl to an acyl product (or vice versa). To obtain more insight on the nature of the RhIIIproducts, an infrared study of the reaction was conducted as this technique is ideal to distinguish between metal–CO bonds that vibrates at 1900–2100 cm1, and metal– COCH3 bonds that vibrates at 1600–1800 cm1.

The half-life of the first reaction step under pseudo firstorder conditions with [CH3I] = 1.673 mol dm3 and at 25 C, Fig. 4, is 63 s. Since it is highly unlikely that the RhIII-acyl1 species can be formed by fast isomerization of an undetected RhIII-alkyl species with total different ligand coordination pattern than the detected RhIII-alkyl1 species at exactly the same rate as the detected RhIII-alkyl1 species, this result is considered to imply that an equilibrium exist between the RhIII-alkyl1 and RhIII-acyl1 species that is fast enough to be maintained on the time scale of [Rh(fctfa)(CO)(PPh3)] disappearance. The same conclusion could be drawn using 1H NMR data and will be discussed under the NMR heading. This first set of reactions may, therefore, be presented by reaction sequence (10) where ki represents forward rate constants and ki reverse rate constants, i = 1 or 2.

3.2. Infrared study

ð10Þ

3.2.1. Chloroform as solvent Three reaction steps could also be identified by IR spectroscopy, Fig. 4. In the first reaction step, the rate of disappearance of the RhI species [RhI(fctfa)(CO)(PPh3)] in chloroform at 25 C, as monitored by the disappearance of the CO peak at 1990 cm1 corresponds exactly with the rate of formation of both a RhIII-alkyl1 species, [RhIII(fctfa)(CH3)(CO)(PPh3)(I)], monitored by the appearance of the CO-peak at 2082 cm1, and a RhIII-acyl1 species, [RhIII(fctfa)(COCH3)(PPh3)(I)], monitored at 1729 cm1.

The k1 step in Eq. (10) was only detected in CHCl3 solutions. The equilibrium constant between the RhIIIalkyl1 and RhIII-acyl1 species is defined as K2 = k2/k2. Pseudo first-order rate constants k1obs for reaction 10 utilizing nine different CH3I concentrations between 0.12 and 1.70 mol dm3 were determined. A graph (not shown) of k1obs versus [CH3I] in chloroform solution, was linear with y-intercept 0.0006(2) s1. This satisfies Eq. (6) with Rh(III)alkyl2

2nd reaction

3rd reaction

1

Rh(III)alkyl2

st

1 reaction

Rh(III)acyl2

relative absorbance

1.0 Rh(III)alkyl1

Rh(I)

Rh(III)alkyl1

0

1714

2064

Rh(III)acyl1

0.4 0 2082 2064

1729 Rh(III)acyl1

CH3I

0 2082

1990

1729 -1

wave number/cm

Fig. 4. The reaction between CH3I (1.673 mol dm3) and [Rh(fctfa)(CO)(PPh3)] (0.0100 mol dm3) in chloroform at 25 C as monitored by IR spectroscopy. The first set of reactions (bottom spectrum, spectrum times are t = 0 (. . . . . ., not shown for RhI due to high amplitude), 74, 149, 224 and 299 s) shows the disappearance of Rh(I) at 1990 cm1 (k1obs = 0.011 s1) and the simultaneous appearance at the same rate of a RhIII-alkyl1, at 2082 cm1, and a RhIII-acyl1 species at 1729 cm1. The middle insert illustrates the simultaneous disappearance of RhIII-alkyl1 (t = 554, 734, 1094, 1919, 2819, 3719, 4905, 7605, 10 305 and 20 205 s) and RhIII-acyl1 (t = 554, 1919, 3719, 7605 and 20 205 s) and the formation of a RhIII-alkyl2 species at 2064 cm1 (k3 = 0.00017(3) s1 for all three cases) during the second set of reactions. The third set of reactions (top right inset) is illustrated by the disappearance of the RhIII-alkyl2 species (spectra were recorded at t = 40 408, 121 558, 248 726 and 335 308 s) and the simultaneous formation of a new RhIII-acyl2 species at 1714 cm1 (k4 = 4.3(2) · 106 s1).

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

k1 the second order forward rate constant and k1 the firstorder reverse rate constant for reaction (10). Individual values for k1 and k1 are tabulated in Table 1. The mutual consistency of k1 and k1 values as determined by the UV/Vis and IR methods are obvious. A second, much slower (t1/2  1.1 h  4000 s) set of reactions that are CH3I independent is illustrated in the middle insert in Fig. 4, where the RhIII-acyl1 species at 1729 cm1 and the RhIII-alkyl1 species at 2082 cm1 both disappear at the same rate (k2 = 0.00017(1) s1) as the formation of a new RhIII-alkyl2 geometrical isomer (k3 = 0.00017(3) s1) at 2064 cm1. Again, this is regarded as additional proof that the equilibrium between the RhIIIacyl1 species and the RhIII-alkyl1 species is fast. The [CH3I]-independent second reaction step is 0.011/ 0.00017  65 times slower (from rate constants from Fig. 4) than the [CH3I]-dependent first reaction at [CH3I] = 1.673 mol dm3. Kinetic data of the second reaction is consistent with the equilibrium k3

ð11Þ

AB k 3

where A = [RhIII-alkyl1RhIII-acyl1] and B = RhIIIalkyl2. As discussed later, k3 n k3. The structure of the RhIII-alkyl2 geometrical isomer having the molecular formula [RhIII(fctfa)(CH3)(CO)(PPh3)(I)] has also been solved and is reported later in this publication. The second set of reactions was followed by a very slow (t1/2  40 h) [CH3I]-independent third reaction and corresponds to the slow first-order disappearance of the RhIIIalkyl2 species (k4 = 4.9(5) · 106 s1) at 2064 cm1 and the appearance, at the same rate, of a new RhIII-acyl2 geometrical isomer at 1714 cm1 (k4 = 4.4(2) · 106 s1, Fig. 4, top right inset). The long half-life of the third reaction implied that it could not be followed with great accuracy, as solvent evaporation became difficult to control. The kinetic data of the third reaction is consistent with the general reaction sequence k4

RhIII -acyl2 ! RhIII -alkyl2

ð12Þ

Utilizing the combined information obtained by the UV/Vis and IR study only, the overall reaction sequence for this oxidative addition reaction may therefore be represented as

ð13Þ

5081

where the superscripts ‘‘1’’ and ‘‘2’’ refer to RhIII-alkyl1, RhIII-acyl1, and RhIII-alkyl2 or RhIII-acyl2, respectively. As was the case with the first set of reactions, k3 and k4 obtained by the IR method are consistent with the results from the UV/Vis method. 3.2.2. Consecutive reaction treatment in acetone as solvent In chloroform, the kobs versus [CH3I] graph for the first step did not pass through the origin for both the UV/Vis and IR detection methods, and this is indicative of a noticeable equilibrium in the first reaction step of the total reaction. However, IR results in acetone indicate, in agreement with the UV/Vis results in acetone (Table 1), that this equilibrium was shifted so far to the right that the k1 step is not observable. The apparent absence of a k1 step in acetone allowed us to treat results from an IR study in acetone utilizing consecutive reaction kinetic models. The IR observed reaction sequence of this reaction in acetone in the range 1960–2100 cm1 was basically the same as observed in chloroform. However, the formation of the RhIII-acyl complexes could not be measured due to the strong IR absorbance of the solvent, acetone, in the 1650–1800 cm1 region. The RhI peak was monitored at 1987 cm1 and the RhIII-alkyl1 and RhIII-alkyl2 species at 2082 and 2059 cm1, respectively. By treating data for every reaction sequence in isolation utilizing Eq. (5), as was done in chloroform, rate constants was determined for all the reaction steps and corresponded to those of the UV/Vis study. Rate constants are listed in Table 1. Because the equilibrium K2 was found to be fast enough to be maintained during Rh(I) depletion, and since CH3I was in large excess over Rh(I), it was possible to determine the rate constants k1, k3 and k4 for the studied reaction in acetone by using the consecutive reaction treatment on k3 k1 k4 the general reaction sequence, A ! B ! C ! D (A, B and C was defined in Sections 3.1 and 3.2.1, D = RhIII-acyl2). By ignoring the very slow k4 step (i.e. D) in the reaction sequence and upon performing a least-squares fit of the available IR absorbance vs. time data of the RhIII-alkyl1 peak for the reaction in acetone to Eq. (7), see graph shown in Fig. 5, top, the rate constants were determined as k1obs = 0.0012(1) s1 (k1 = 0.0046 dm3 mol1 s1), and k3 = 0.00010(1) s1. The value of k1obs corresponds very close with those obtained from data treatment utilizing Eq. (5) (values summarized in Table 1). The value of k3 was slightly (37%) smaller than the value of 0.00016(2) s1 which was determined by treating reaction 2 in isolation. In addition, fitting of the available absorbance/time data of the RhIII-alkyl2 peak to Eq. (8), see Fig. 5, bottom, resulted in k1obs = 0.0022(6) s1 (k1  0.0084 dm3 mol1 s1), while k3 = 0.00018(1) s1. This value of k3 corresponds well to the value obtained by treating data sets as separate, isolated quantities for each reaction according to Eq. (5). Utilizing Eq. (8), k1 was 83% (1.83 times) larger than the value obtained from Eq. (5) but it is still kinetically equivalent. This drift in k1 and k3 was accentuated because of the difficulty of obtaining large

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J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

All things considered, the calculated rate constants obtained by the consecutive reaction treatment (Eqs. (7) and (8)) and as individual data sets utilizing Eq. (5) are regarded as mutually consistent and confirm the proposed mechanism. It is, however, important to note that consistency between the consecutive reaction treatment of data and treatment of kinetic data of each reaction step in isolation would not have been observed if reaction step 1 was not much faster than reaction step 2 and if reaction step 3 was not much slower than reaction step 2. If the rates did not differ as much as demonstrated by the half lives cited in Section 3.2.1, reaction steps 1, 2 and 3 would be overlapping so much that accurate rate constants would not be obtainable by regarding each reaction step in isolation. When the consecutive reaction treatment of data was applied to reactions performed in chloroform (data for Section 3.2.1), the mathematical models (Eqs. (7) and (8)) were not found to be appropriate as data could not be satisfactorily fitted to it. The breakdown of the model is attributed to the detected k1 step in the mechanism

2.0

relative absorbance

1.5

1.0

rel. abs.

0.4

0.5

k1

0.2

k3

AB!C k 1

The mathematical treatment of such monophasic reversible reactions is quite complex [29].

0.0 0

20

40

time/min

3.3. 1H and

31

P NMR study

0.0 0

100

200

300

400

500

time/min Fig. 5. Relative IR absorbance vs. time data of the reaction between CH3I (0.2604 mol dm3) and [Rh(fctfa)(CO)(PPh3)] (0.002 mol dm3) in acetone at 25 C. Top: Kinetic data were fitted to the consecutive reaction model Eq. (7), relative absorbancies of the RhIII-alkyl1 species are illustrated. Bottom: A data fit of RhIII-alkyl2 IR absorbances to Eq. (8) is shown. Inset: An enlargement of the initial stages of RhIII-alkyl2 formation. The slow initial formation of the RhIII-alkyl2 species in the region t < 10 min is apparent.

amounts of data points for relatively fast reactions quickly enough with overlay spectra utilizing an infrared spectrophotometer. It led to too little available data to fit complicated equations. k1 k3 k4 By ignoring the fast k1 step (i.e. A) in A ! B ! C ! D and by applying the consecutive reaction kinetic model, Eq. (7), to intermediate C for the compound series B, C and D, values of k3 and k4 were found to 1.6(1) · 104 and 5.5(3) · 106 s1. The value for k3 corresponded well to the previously determined values. The value for k4 is larger by a factor 2 than the value obtained from the treatment of data in isolation, which gave k4 = 2.6(5) · 106 s1. This twofold difference between the k4 values are again not regarded as significant. Eq. (8) could not be fitted to formation data of D, from the reaction in acetone as solvent, due to interference of the acetone carbonyl peak in the region of RhIII-acyl2 detection.

The reaction between CH3I and [Rh(fctfa)(CO)(PPh3)] was also monitored by 1H and 31P NMR in order to obtain additional insight into this reaction. By carefully comparing the positions and integrals of the different signals, the spectral parameters of different isomers could be identified, Table 2, see also Ref. [19]. The same reaction sequence as observed by IR and UV/Vis, was observed utilizing 1H NMR, Fig. 6 and 31P NMR, Fig. 7. The new feature introduced by the NMR study is the existence of more than one geometrical isomer for each intermediate. The two main isomers of each intermediate will be referred to as A and B, e.g. RhIII-alkyl1A and RhIII-alkyl1B, the choice of the labels are arbitrary and have no significance. The A and B isomers of each species exist in a fast equilibrium with each other, because the observed rate constant for the disappearance or formation of an A and a B isomer of the same species was found to be within experimental error the same and also because the ratio Rh(I)A/Rh(I)B = 1.5 (obtained by integral evaluation) is not the same as the ratio RhIIIalkyl1A/RhIII-alkyl1B = 0.39 or the ratio RhIII-acyl1A/ RhIII-acyl1B = 1.0 (Table 2). If the reaction was (RhIA + CH3I  {RhIII-alkyl1A  RhIII-acyl1A}) for the A isomers and separately (RhIB + CH3I  {RhIII-alkyl1B  RhIII-acyl1B}) for the B isomers, and if no equilibrium between isomers A and B existed, the ratio A:B of these isomers would have been the same throughout the entire reaction sequence involving steps 1, 2 and 3. Rate constants obtained from 1H NMR are summarized in Table 1. It can be seen good agreement exist between the

e

e

4.47 4.76

e

4.31 4.39

e

c

d

a

b

1.65 RhIII-acyl 2A RhIII-acyl 2B

Kc = [Rh-species B]/[Rh-species A] for the equilibrium Rh-species A  Rh-species B. In the text the CH group of fctfa is referred to as the methine proton. Fc = ferrocenyl. Peaks of the two isomers overlapped. Peaks could not uniquely be identified due to excessive overlapping.

2.91 3.01

1.78 1.77 19 81 115.2 116.5 27.09 29.33 4.56 RhIII-alkyl 2A RhIII-alkyl 2B

1.00

5083

kinetic rate constants obtained by utilizing UV/Vis, IR, 1H NMR and 31P NMR spectroscopic methods. The structure of one of the isomers of the [RhI(fctfa)(CO)(PPh3)] is reported elsewhere [20], while the structure of the [RhIII(fctfa)(CH3)(CO)(PPh3)(I)]2B species is reported in this communication. The structure of the other RhIII acyl compounds is as yet unknown and the indicated formulas relate to molecular stoichiometry only and not to a specific geometry. The 1H and 31P NMR spectrum of the crystalline isolated RhIII-alkyl2B isomer were identical to the spectrum of isomer RhIII-alkyl2B as observed during the monitoring of the second set of reactions during the oxidative addition reaction of [Rh(fctfa)(CO)(PPh3)] with CH3I in CDCl3 solutions. An important observation was that upon dissolving the solid crystalline RhIII-alkyl2B isomer in CDCl3, isomerization to the RhIII-alkyl2A isomer sets in. After 4 days, traces of Rh(I), RhIII-alkyl1 and RhIII-acyl1 were also observed on 1H and 31P NMR. This means that Eq. (13) also has a k3 and k4 step, but these rate constants are so small that the k3 and k4 steps are simply not kinetically determinable. Taking into account there exist two main isomers of each reactant and reaction product, the complete reaction sequence for the oxidative addition of CH3I to [Rh(fctfa)(CO)(PPh3)] is therefore

4.23 4.31 5.45 5.30

e

e

18 82

e

e

e

e

125 124 155 155 33.23 32.80 38.38 37.57 2.57

Rh -alkyl 1A RhIII-alkyl 1B RhIII-acyl 1A RhIII-acyl 1B

d

176.4 176.4 48.04 48.04 0.68 Rh A RhIB

III

1

P NMR

31

d I

28 72 50 50

1.47 1.42 3.05 2.98

4.29 4.34 4.17 3.94 6.055 6.055 6.08 6.08 28 72 50 50

4.73

4.27 4.83

2H 2H

3.94 4.20

4.09 4.50

5H

6.045 6.045 H NMR

60 40

d–1H (Fc)c (ppm) d–1H (CH3) (ppm) d–1H (CH)b (ppm) % of each isomer by J(31P–103Rh) (Hz) 1

d–31P (ppm) Kac Compound

Table 2 The equilibrium constant Kc and 1H and 31P NMR spectral parameters of the different isomeric forms of [Rh(fctfa)(CO)(PPh3)] and the RhIII complexes formed during the oxidative addition reaction of CH3I to [Rh(fctfa)(CO)(PPh3)]

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

First set of reactions

ð14Þ Second set of reactions

Third set of reactions

The above mechanism is a completely general mechanism. As such it caters for all previously encountered special cases sighted in Eqs. (1)–(4). Each one of these four equations can be generated by careful manipulation of the size of the individual rate constants. For example, Eq. (14) will simplify to Eq. (3) provided k2 = k3 = k4 = k4 = 0 s1. The equilibrium constants for the equilibrium between the isomers, defined as Kci = [RhB-species]/ [RhA-species], i = 1, 2, . . . 5, were determined from the 1H NMR integration data of suitable identifiable and well resolved peaks at 25 C in CDCl3 and are documented in Table 2.

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

First set of reactions

PPM

6.05

3.00

1.50

1.45

3.05

3.00

PPM

alkyl2B

acyl2B

acyl2A

acyl1A

alkyl1B

alkyl1A

PPM

PPM

3.05

PPM

2.95

2.90

1.80

1.75

PPM

PPM

5.45

5.40

5.35

5.30

3.05

PPM

3.00

1.80

PPM

alkyl1B

alkyl1A

alkyl2B

acyl1B

alkyl2B

alkyl2A

acyl1A

Second set of reactions

alkyl1A and 1B

acyl1A and 1B

6.05

acyl1B

acyl1A

RhIB

RhIII-acyl1A and 1B

6.10

Third set of reactions

RhIA

RhIII-alkyl1A and 1B

acyl1B

5084

PPM

1.75

1.45

Fig. 6. Fragments of the 1H NMR spectra in CDCl3 illustrating reaction sequences during the oxidative addition and the ensuing carbonyl insertion and deinsertion reactions of 0.3674 mol dm3 CH3I to 0.0183 mol dm3 [Rh(fctfa)(CO)(PPh3)] in CDCl3 at T = 25 C. Top left: Selected spectra for the first set of reactions illustrating changes in the signal of the methine proton of the b-diketonato ligand of the indicated isomers in the region 6.04–6.08 ppm, the growing signals of the CH3-group of indicated RhIII-acyl isomers at 3.05 and 2.98 ppm and of RhIII-alkyl isomers at 1.47 and 1.42 ppm. Bottom: Signal intensity changes of the second reaction sequence showing the methine proton of the b-diketonato ligand of the indicated isomers at 5.30–6.08 ppm, the CH3 signal disappearance for the RhIII-acyl isomers at 3.05 and 2.98 ppm and RhIII-alkyl isomers at 1.47 and 1.42 ppm, and the increase in signal intensity of the CH3-group of the overlapping alkyl 2 isomers at 1.78 ppm. Top right: The third set of reactions is illustrated by changes in signal intensity of the CH3-group of the RhIII-alkyl2 isomers at 1.78 ppm, and of the RhIII-acyl2A and 2B species at 3.01 and 2.92 ppm. The multiplicity of the signal of the CH3 group of the RhIII-alkyl1 and 2 isomers is due to coupling with Rh (spin 1/2) and P (spin 1/2). Supplementary data shows further spectra highlighting NMR changes.

traces of overlapping Rh A and Rh B

Rh -acyl1A Rh -acyl1B

Rh -alkyl1B Rh -alkyl1A

Overlapping Rh A and Rh B

Rh -alkyl2B

Rh -alkyl2B

Rh -alkyl2A

PPM

50

40

50

30

40

20

30

PPM

20

50

40

30

20

Fig. 7. Selected 31P NMR spectra illustrating doublet 31P peaks of the indicated reactants and products during the three sets of reactions for the oxidative addition of 0.500 mol dm3 CH3I to 0.021 mol dm3 [Rh(fctfa)(PPh3)(CO)] in CDCl3 at 25 C. Bottom left: at time t = 0 (reagent). Middle top: t = 1070 s (towards the end of the first set of reactions). Bottom right: t = 69520 s (near the end of the second set of reactions).

3.4. The crystal structure data of [Rh(fctfa)(CO)(PPh3)(CH3)(I)] The numbering system of the atoms in [Rh(fctfa)(CO)(PPh3)(CH3)(I)] is shown in Fig. 8 while the crystal data are summarized in Table 3. Selected bond lengths and bond angles are given in Table 4. An obvious feature of [Rh(fctfa)(CO)(PPh3)(CH3)(I)] and similar known RhIII-alkyl complexes in which the fctfa ligand is replaced

with other bidentate ligands [15,30] is that the iodo ligand, I, is bonded in the axial position of the octahedron, above or below the equatorial plane of the octahedron formed by O(1), O(2), C(5)O and C(6)H3. The Rh–I bond axis deviated 3.6(1) towards O(2) from the expected 90 and the Rh–P bond deviates by more than 2 away from O(1) and O(2). This resulted in a 2.26(5) deviation from linearity for the I–Rh–P bond axis. A similar deviation from linearity (3.4) for the I–Rh–P bond axis was found for [Rh(cupf)(CO)(PPh3)(CH3)(I)] [15]. The RhIII–I bond length of the ˚ , is shorter (0.1 A ˚ ) than Rh–I present study, 2.716(1) A bonds in complexes having the CH3 group trans to the iodine atom [30] and this is the result of the smaller trans influence of PPh3 compared to that of an alkyl group. The various ligand–rhodium–ligand bond angles in the equatorial plane formed by O(1), O(2), C(6) and C(5), viz. C(5)–Rh–C(6) = 88.4(4), C(5)–Rh–O(2) = 97.3(3), O(2)–Rh–O(1) = 88.0(2) and O(1)–Rh–C(6) = 86.1(3), deviate 2–7 from 90. The Rh–C(5)–O(3) bond angle, 175.7(8), is not completely linear, with the C(5)–O(3) bond ˚ , significantly shorter than the C(5)–O(3) length 1.12(1) A ˚ in [RhI(fctfa)(CO)(PPh3)] [20]. bond distance of 1.147(5) A In accordance with the p bonding concept [31] the ˚ is larger than the Rh–C(5) bond length of 1.830(8) A ˚ , in [RhI(fctfa)corresponding bond length, 1.801(5) A (CO)(PPh3)] because decreasing electron density on the rhodium center in changing the oxidation state from I to III, imply less electron donating capability from the metal

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

5085

Table 4 ˚ ) and angles () for [Rh(fctfa)(CO)(PPh3)Selected bond lengths (A (CH3)(I)]

Fig. 8. A perspective view of [Rh(fctfa)(CO)(PPh3)(CH3)(I)] showing atom labelling. Thermal ellipsoids were drawn at a 30% probability level. The view from above down the ferrocenyl axis (top left) shows the cyclopentadienyl ring deviates 12.7 from a fully eclipsed conformation. Right: The cyclopentadienyl rings are planar and almost parallel, they deviate 3.1(5) from co-planarity with the plane through the Rh chelate.

d-orbital into the p* orbital of the carbonyl ligand. This leads to a longer (weaker) Rh–C bond length and a stron˚ shorter than ger C–O bond. The Rh–O(1) bond is 0.085 A the Rh–O(2) bond which indicates the structure is not symmetric around the b-diketonato ligand. However, both Rh– O(1) and Rh–O(2) bonds in [RhIII(fctfa)(CO)(PPh3)(CH3)(I)] are longer than the corresponding bonds in ˚ and 2.070(3) A ˚. [RhI(fctfa)(CO)(PPh3)] [20], 2.048(3) A The Rh–O(1) bond in both the Rh(I) and Rh(III) complexes is trans to the CO group, but Rh–O(2) is trans to the PPh3 group in [RhI(fctfa)(CO)(PPh3)] and trans to the CH3 group in [RhIII(fctfa)(CO)(PPh3)(CH3)(I)]. The C(3)–O(2) and C(1)–O(1) bonds in the b-diketonato skeleton of the [Rh(b-diketonato)(CO)(PPh3)(CH3)(I)] complex ˚ and 1.265(8) A ˚ , respectively, and are similar, 1.271(9) A both bonds are smaller than the corresponding bonds in free Hfctfa [32]. The change in oxidation state from Rh(I) to Rh(III) did not influence the C(3)–O(2) and C(1)–O(1) bond lengths. The C(1)–C(2) and C(2)–C(3) bonds of the present Rh(III) complex are similar, contrary to what was found for both [RhI(fctfa)(CO)(PPh3)] (corre˚ ), or the free sponding bonds were 1.413(5) and 1.368(6) A Hfctfa ligand where asymmetric enolization was observed ˚ , C(2)–C(3) = 1.345(6) A ˚ ). As was (C(1)–C(2) = 1.432(6) A the case for [Rh(fctfa)(CO)(PPh3)], deviations up to 10 from the expected 120 for sp2 hybridization for the C

Atoms

Bond

Atoms

Bond

Rh–C(5) Rh–O(1) Rh–C(6) Rh–O(2) Rh–P Rh–I P–C(51) O(3)–C(5) O(2)–C(3) F(2)–C(4)

1.830(8) 2.075(5) 2.076(8) 2.160(5) 2.320(2) 2.716(1) 1.822(7) 1.12(1) 1.271(9) 1.40(3)

F(1)–C(4) C(1)–C(2) C(2)–C(3) C(3)–C(4) C(1)–C(11) C(11)–C(12) C(11)–C(15) C(12)–C(13) C(21)–C(22) C(41)–C(42)

1.39(3) 1.40(1) 1.38(1) 1.51(1) 1.47(1) 1.41(1) 1.45(1) 1.40(1) 1.39(2) 1.40(1)

Angles

Atoms

Angles

C(5)–Rh–O(1) O(1)–Rh–O(2) O(2)–Rh–P O(1)–Rh–I O(1)–C(1)–C(11) O(2)–C(3)–C(4) C(34)–C(33)–C(32)

173.8(3) 88.0(2) 92.9(2) 89.7(1) 114.2(7) 111.9(7) 119.6(9)

O(1)–C(1)–C(2) O(2)–C(3)–C(2) C(2)–C(3)–C(4) C(3)–C(2)–C(1) Rh–C(5)–O(3) P–Rh–I

126.5(6) 129.3(7) 118.9(7) 125.3(7) 175.7(8) 177.74(5)

The standard deviation of the last decimal is given in brackets.

atoms of the b-diketonato skeleton were found, Table 4. The largest deviations were at the C atoms bound to O(1) and O(2). In the case of the free Hfctfa [32] the corresponding bond angles were all within experimental error 120. The cyclopentadienyl rings are within experimental error, planar and almost parallel [dihedral angle 1.4(6)] while the average angle of deviation from a fully eclipsed configuration is 12.7, Fig. 8. The cyclopentadienyl ring which is bonded to the b-diketonato chelate ring at C(1) deviates 3.1(5) from co-planarity with the plane defined by atoms O(1), O(2), C(1), C(2), C(3) and Rh, i.e. the plane through the chelate ring. ˚ , in the phenyl The average C–C bond distance, 1.38(1) A rings is within experimental error near the normal value ˚ ) for the aromatic C–C bond [33]. The bond angles (1.394 A around P differ no more than 7 from 10928 0 , the angle for ˚, a regular tetrahedron. The mean P–C distance is 1.82(1) A and almost the same [20] as in the parent rhodium(I) com˚ . The plex [RhI(fctfa)(CO)(PPh3)], where it was 1.824 A III Rh –P bond distance for the present complex of

Table 3 Crystal data and structure refinement for [Rh(fctfa)(CO)(PPh3)(CH3)(I)] Empirical formula Formula weight Crystal system Space group

C34H28F3FeO3RhPI 858.2219 Monoclinic P21/c

Z, density (calculated) (g/cm3) Temperature (K) ˚) Wavelength (A Absorption coefficient (mm1)

4, 1.716 293(2) 0.71073 1.961

Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) Volume (A

10.769(2) 15.153(3) 20.413(5) 94.31(2) 3221(1)

Reflections collected Unique reflections Refinement method Data [I > 2r(I)]; parameters Final R indices [I > 2r(I)]

4717 4420 [Rint = 0.0588] Full-matrix least-squares on F2 4414; 429 R = 0.0588, Rw = 0.1692

5086

J. Conradie et al. / Polyhedron 26 (2007) 5075–5087

˚ is, however, much longer than the Rh–P bond 2.320(2) A ˚ , indicating distance for [RhI(fctfa)(CO)(PPh3)], 2.232(1) A a change in oxidation state from Rh(I) to Rh(III) results in a weaker Rh–P bond. The RhIII–P bond length for the [Rh(fctfa)(CO)(PPh3)(CH3)(I)] complex is, however, in the same order as was found for a variety of other RhIIIalkyl complexes including [Rh(cupf)(CO)(PPh3)(CH3)(I)] ˚ , and [Rh(ox)(CO)(PPh3)(CH3)I] [30b], [15c], 2.327(4) A ˚ 2.321(2) A, (average of two molecules found in one unit cell, Hcupf = N-hydroxy-N-nitroso-benzeneamine). The trifluoromethyl group in [Rh(fctfa)(CO)(PPh3)(CH3)(I)] is disordered, similar to what was found for the free b-diketone Hfctfa [32]. An intermolecular interaction was observed between F and either a hydrogen on a ferrocenyl group, or a hydrogen on a phenyl ring from another ˚, molecule (indicated by H 0 ): F(3)–H 0 (53) = 2.661 A 0 0 ˚ ˚ F(3)–H (12) = 2.669 A, F(12)–H (53) = 2.652 A, F(12)– ˚ , F(13)–H 0 (36) = 2.606 A ˚ . These intermoH 0 (13) = 2.667 A lecular interactions link one molecule to another in the unit cell and it is partially the cause of the CF3 disorder. However, the CF3 disorder is also the result of intramolecular interaction that prevents rotation of the CF3 group. Two preferred CF3 orientations are observed. The first involves a strong intramolecular interaction between F(2) and H(2) (the hydrogen bonded to C(2)) and a weak interaction between both F(1) and F(3) with O(2). The contact dis˚ , F(1)–O(2) = 2.702 A ˚ and tances are F(2)–H(2) = 2.362 A ˚ , respectively. The other CF3 orientaF(3)–O(2) = 2.718 A tion, involving F(12), F(11) and F(13), is the result of a strong interaction between F(12) and O(2) and a weak interaction of both F(11) and F(13) with H(2). The intramolecular contact distances are F(12)–O(2) = ˚ , F(11)–H(2) = 2.727 A ˚ and F(13)–H(2) = 2.624 A ˚, 2.485 A respectively.

posed mechanisms for a variety of such reactions are special cases of the general mechanism that emanated from this study. The structure of the intermediate RhIII-alkyl2B geometric isomer, [Rh(fctfa)(CO)(PPh3)(CH3)(I)], was solved. It was asymmetric in nature and the phosphine and iodo ligands were found to be the axial ligands. Acknowledgements Financial assistance by the South African NRF under Grant 2054243 and the Central Research Fund of the University of the Free State is gratefully acknowledged. Appendix A. Supplementary material CCDC 646438 contains the supplementary crystallographic data for [Rh(fctfa)(CO)(PPh3)(CH3)(I)]. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2007.07.004. References [1] [2] [3] [4] [5]

[6] [7]

4. Conclusion [8]

A three-step reaction sequence was found for the oxidative addition of iodomethane to [Rh(fctfa)(CO)(PPh3)], the final product of the reaction was found to be a RhIII-acyl species. UV/Vis studies alone could only provide reaction rates and the amount of reaction steps. Utilizing IR spectroscopic techniques, alkyl and acyl RhIII species could be identified, but only NMR spectroscopy revealed that each reaction product exists in solution as two main isomers in a fast equilibrium with each other. This study is the first to observe a fast equilibrium between RhIII-alkyl and RhIII-acyl intermediates (equilibrium K2) and it was the first to determine the actual rate of conversion from a RhIII-alkyl2 species to a RhIII-acyl2 species. The incorporation of a ferrocenyl group in the b-diketonato ligand slightly accelerated the rate of CH3I oxidative addition when compared with the rate of oxidative addition to either the corresponding CH3COCHCOCF3 [15a] or PhCOCHCOCF3 complex [34]. A completely general mechanism for the oxidative addition of CH3I to [Rh(b-diketonato)(CO)(PPh3)] could be derived and all previously pro-

[9]

[10] [11] [12] [13] [14]

[15]

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