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May 12, 2010 - (TS, Scheme 1).12,15,16 At the TS, the equatorial plane is occu- pied by the ..... (O-O0)(CO)2 complexes in the Cambridge Structural Da...
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Organometallics 2010, 29, 2446–2458 DOI: 10.1021/om1000138

Substitution and Isomerization of Asymmetric β-Diketonato Rhodium(I) Complexes: A Crystallographic and Computational Study Kathrin H. Hopmann,† Nomampondomise F. Stuurman,‡ Alfred Muller,§ and Jeanet Conradie*,†,‡ †

Center for Theoretical and Computational Chemistry and Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway, ‡Department of Chemistry, University of the Free State, 9300 Bloemfontein, Republic of South Africa, and §Department of Chemistry, University of Johannesburg (APK Campus), P.O. Box 524, Aucklandpark, Johannesburg 2006, Republic of South Africa Received January 5, 2010

Ligand substitution of PPh3 for CO in the asymmetric square-planar β-diketonato complex [Rh(PhCOCHCOCH2CH3)(CO)2] leads to formation of the monosubstituted [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] product. Two geometrical product isomers are possible, which unexpectedly crystallize in the same crystal lattice. DFT calculations of the substitution reaction support a twostep mechanism, involving temporary dissociation of a β-diketonato ligand and formation of a square-planar intermediate. Analysis of the mechanisms involved in product isomerization suggests several noncatalyzed and solvent-assisted interconversion pathways.

Introduction Square-planar rhodium(I) complexes of the type [Rh(βdiketonato)(CO)2] and their substituted derivatives [Rh(βdiketonato)(L)(L0 )] (where L, L0 = CO, olefin, phosphine- or phosphite-based ligands) are used as catalyst precursors in various reactions, in particular hydroformylation of olefins1-7 but also CO2 hydrogenation8 and asymmetric allylic alkylation.9 The first [Rh(β-diketonato)(CO)2] complexes were reported by Bonati and Wilkinson.10 Substitution reactions of these complexes with olefins results in replacement of both CO ligands, while reactions with triphenylphosphine (PPh3, Ph = C6H5) or triphenylarsine (AsPh3) generally result in replacement of only one of the CO units.10 The monosubstitution reaction with PPh3 has been employed to study the relative thermodynamic trans influence of the bonding atoms in β-diketonate and similar bidentate ligands, because it was assumed that the reaction with PPh3 would result in substitution of the CO group *To whom correspondence should be addressed. Tel: þ27-514012194. Fax: þ27-51-4446384. E-mail: [email protected]. (1) Mieczynska, E.; Trzeciak, A. M.; Zi olkowski, J. J. J. Mol. Catal. 1993, 80, 189. (2) Trzeciak, A. M.; Mieczynska, M.; Zi olkowski, J. J. Top. Catal. 2000, 11/12, 461. (3) Jin, H.; Subramaniam, B.; Ghosh, A.; Tunge, J. AIChE J. 2006, 52, 2575. (4) van Rooy, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J.; Veldman, N.; Spek, A. L. Organometallics 1996, 15, 835. (5) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.; Horiuchi, T.; Takaya, H. J. Am. Chem. Soc. 1997, 119, 4413. (6) Kainz, S.; Leitner, W. Catal. Lett. 1998, 55, 223. (7) Franci o, F.; Leitner, W. Chem. Commun. 1999, 1663. (8) Angermund, K.; Baumann, W.; Dinjus, E.; Fornika, R.; Gorls, H.; Kessler, M.; Kruger, C.; Leitner, W.; Lutz, F. Chem.—Eur. J. 1997, 3, 755. (9) Hayashi, T.; Okada, A.; Suzuka, T.; Kawatsura, M. Org. Lett. 2003, 5, 1713. (10) Bonati, F.; Wilkinson, G. J. Chem. Soc. 1964, 3156. pubs.acs.org/Organometallics

Published on Web 05/12/2010

trans to the donor atom with the largest trans influence.11-14 X-ray crystal structures of the monophosphine product complexes [Rh(RCOCHCOR0 )(CO)(PPh3)] typically display only one of two possible geometric isomers,12,14-16 which was taken as an indication that only one isomer was formed in the substitution reaction. As both donor atoms of β-diketonate are oxygens, their relative trans influence should be governed by the adjacent substituents, R and R0 , on the ligand backbone. The crystal structures of the complexes [Rh(C4H3SCOCHCOCF3)(CO)(PPh3)]14 and [Rh(CH3COCHCOCF3)(CO)(P(p-ClPh)3)]16 exhibit CO substitution trans to the oxygen adjacent to the more electron-donating group (C4H3S and CH3, respectively), indicating that the oxygen atom nearest a strongly electron-attracting group, such as CF3, has the smallest trans influence.17 This is in agreement with the Grinberg polarization theory18 and the σ-trans effect,19 as the oxygen nearest the CF3 group will be a weaker σ-donor as a result of the electron attraction by CF3. The apparent group electronegativities, χR, in Gordy scale (in parentheses) of the substituents are R = C4H3S (2.10)20 ∼ CH3 (2.34)21 < CF3 (3.01).22 The CO substitution (11) Graham, D. E.; Lamprecht, G. J.; Potgieter, I. M.; Roodt, A.; Leipoldt, J. G. Transition Met. Chem. 1991, 16, 193. (12) Leipoldt, J. G.; Basson, S. S.; Nel, J. T. Inorg. Chim. Acta 1983, 74, 85. (13) Leipoldt, J. G.; Grobler, E. C. Inorg. Chim. Acta 1982, 60, 141. (14) Leipoldt, J. G.; Bok, L. D. C.; van Vollenhoven, J. S.; Pieterse, A. I. J. Inorg. Nucl. Chem. 1978, 40, 61. (15) Leipoldt, J. G.; Basson, S. S.; Potgieter, J. H. Inorg. Chim. Acta 1986, 117, L3. (16) Steynberg, E. C.; Lamprecht, G. J.; Leipoldt, J. G. Inorg. Chim. Acta 1987, 133, 33. (17) Leipoldt, J. G.; Bok, L. D. C.; Basson, S. S.; Gerber, T. I. A. Inorg. Chim. Acta 1979, 34, L293. (18) Grinberg, A. A. Acta Physiochim., USSR 1935, 3, 573. (19) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W.A. Benjamin Inc.: New York, 1965; pp 31 ff. (20) Conradie, M. M.; Muller, A. J.; Conradie, J. S. Afr. J. Chem. 2008, 61, 13. r 2010 American Chemical Society

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Scheme 1. Proposed Effects of trans Influence and Steric Factors on CO Substitution in [Rh(β-diketonato)(CO)2] Complexes (Based on Suggestions by Leipoldt and Co-workers)12,15,16a

a

Red dashed lines indicate bonds that are broken/formed. The product isomers can interconvert and are in equilibrium.23,26,27.

selectivity might also be affected by steric interactions, which could explain why the solid-state structures of related [Rh(RCOCHCOCF3)(CO)(PPh3)] complexes (where R = CH2CH316 (χR = 2.31),23 CH(CH3)212 (χR = 2.29),24 C(CH3)315 (χR = 2.27),24 or Fc25 (χR = 1.87)22) exhibit substitution trans to the oxygen adjacent to CF3, although this is expected to have the smaller trans influence. Leipoldt and co-workers have attempted to explain these findings on the basis of a one-step substitution reaction, involving a trigonal-bipyramidal (TBP) transition state (TS, Scheme 1).12,15,16 At the TS, the equatorial plane is occupied by the entering group (PX3), the leaving ligand (CO0 ), and the β-diketonato oxygen (O0 ) with the stronger trans influence (Scheme 1, TStrans). If the substituent (R0 ) close to the oxygen with stronger trans influence is bulky, it might preferably be positioned in an axial position (TSsteric, Scheme 1),16 which leads to replacement of the opposite CO ligand and formation of a different product isomer. Trzeciak and Zi olkowski have pointed out that the presence of a single [Rh(RCOCHCOR0 )(CO)(PPh3)] product isomer in the solid state is a consequence of the crystallization conditions.26,27 They showed that both product isomers of [Rh(RCOCHCOCF3)(CO)(PPh3)] (where R = CH3, Ph, C4H3S) exist in solution.26 NMR studies on [Rh(RCOCHCOPh)(CO)(PPh3)] (where R = CH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3)23,28 and [Rh(RCOCHCOFc)(CO)(PPh3)] (where R = CH3, Ph, CF3)27 also confirmed the presence of both isomers in solution. The isomers are able to interconvert, with a resulting equilibrium constant (Kc) (21) Kagarise, R. E. J. Am. Chem. Soc. 1955, 77, 1377. (22) Du Plessis, W. C.; Vosloo, T. G.; Swarts, J. C. J. Chem. Soc., Dalton Trans. 1998, 2507. (23) Stuurman, N. F.; Conradie, J. J. Organomet. Chem. 2009, 694, 259. (24) Klaas, P. Synthesis, Electrochemical, Kinetic and Thermodynamic Properties of New Ferrocene-containing Betadiketonato Rhodium(I) Complexes with Biomedical Applications. M.Sc.Thesis, University of the Free State, R.S.A., 2002. (25) Lamprecht, G. J.; Swarts, J. C.; Conradie, J.; Leipoldt, J. G. Acta Crystallogr. 1993, 49, 82. (26) Trzeciak, A. M.; Zi o1kowski, J. J. Inorg. Chim. Acta 1985, 96, 15. (27) Conradie, J.; Lamprecht, G. J.; Otto, S.; Swarts, J. C. Inorg. Chim. Acta 2002, 328, 191. (28) Purcell, W.; Basson, S. S.; Leipoldt, J. G.; Roodt, A.; Preston, H. Inorg. Chim. Acta 1995, 234, 153. (29) Conradie, M. M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 208. (30) Conradie, M. M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 2285.

that depends inter alia on solvent polarity and temperature effects.23,26,27,29,30 Crystallization conditions appear to favor one isomer over the other, but which isomer crystallizes does not necessarily reflect the isomer ratio in solution.27 Only in a single instance has it been possible to crystallize both geometrical isomers of an asymmetric β-diketonato Rh(I) complex, [Rh(PhCOCHCOCH3)(CO)(PPh3)], in the same crystal lattice.28 The β-diketonato substituents of [Rh(PhCOCHCOCH3)(CO)(PPh3)] have similar Gordy group electronegativities, χPh = 2.2122 and χCH3 = 2.34,21 which might have contributed to the equimolar isomer ratio observed in the solid state. Recent density functional theory (DFT) studies of the PPh3 substitution for CO in the symmetric [Rh(acac)(CO)2] (acac = (CH3COCHCOCH3)-) complex have shown that CO substitution does not proceed through a single step via an associative interchange mechanism as proposed by Leipoldt and co-workers (Scheme 1),16 but that the reaction instead occurs through two interchange steps, involving temporary loss of an Oacac ligand in the square-planar (SQP) intermediate.31 We were interested to determine if a similar twostep substitution mechanism operates in asymmetric β-diketonato complexes and if both possible product isomers are primary kinetic products, or if one isomer is formed from the other. Here we present a combined experimental and computational study of the substitution reaction of one CO group in the asymmetric β-diketonato complex [Rh(PhCOCHCOCH2CH3)(CO)2] with PPh3. The β-diketonato substituents Ph and CH2CH3 were chosen on the basis of similar steric and electronic properties (χPh = 2.2122 ≈ χCH2CH3 = 2.3123). We present crystallographic characterizations of the dicarbonyl reactant and the carbonyl triphenyl-phosphine products, followed by a theoretical analysis of the substitution reaction and possible isomerization pathways leading to isomer interconversion during and after product formation.

Results and Discussion Synthesis and Crystallography. Nucleophilic attack of PPh3 on the dicarbonyl complex [Rh(PhCOCHCOCH2CH3)(CO)2] leads to substitution of one CO group. Two different isomers of the [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] product are (31) Hopmann, K. H.; Conradie, J. Organometallics 2009, 28, 3710.

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Scheme 2. Substitution of PPh3 for CO in [Rh(PhCOCHCOCH2CH3)(CO)2] Gives an Isomeric Mixture of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] Complexes, Isomer A (SP-4-2), and Isomer B (SP-4-3)

Table 1. Selected Bond Lengths (A˚) and Angles (deg) for [Rh(PhCOCHCOCH2CH3)(CO)2] and the Two Structural Isomers A (SP-4-2) and B (SP-4-3) of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] [Rh(PhCOCHCOCH2CH3)(CO)2 Rh-C1 Rh-C2 Rh-O3 Rh-O4 O1-C1 O2-C2 C1-Rh-C2 C2-Rh-O4 C1-Rh-O3 O3-Rh-O4 C5-C6-C7 O1-C1-Rh

1.849(2) 1.852(2) 2.0288(13) 2.0285(14) 1.135(2) 1.137(2) 89.14(9) 89.87(7) 90.53(7) 90.36(6) 126.34(19) 179.4(2)

isomer A Rh2-C42 Rh2-P2 Rh2-O5 Rh2-O4 C35-O4 C33-O5 C35-C34 C33-C34 O5-Rh2-O4 O6-C42-Rh2 C42-Rh2-O5 C42-Rh2-O4 O5-Rh2-P2 P2-Rh2-C42 C35-C34-C33

isomer B 1.788(5) 2.2387(9) 2.032(3) 2.067(3) 1.285(4) 1.282(5) 1.376(6) 1.380(6) 87.84(11) 179.1(5) 178.59(17) 91.68(14) 92.99(8) 87.47(12) 127.2(4)

Rh1-C12 Rh1-P1 Rh1-O2 Rh1-O1 C5-O2 C3-O1 C5-C4 C3-C4 O1-Rh1-O2 O3-C12-Rh1 C12-Rh1-O2 C12-Rh1-O1 O2-Rh1-P1 P1-Rh1-C12 C5-C4-C3

1.797(4) 2.2376(9) 2.032(2) 2.060(3) 1.289(4) 1.263(5) 1.382(6) 1.390(6) 89.39(10) 177.7(5) 179.08(17) 90.84(15) 91.62(7) 88.15(14) 127.0(4)

Figure 1. Molecular diagrams of (I) [Rh(PhCOCHCOCH2CH3)(CO)2] with 30% probability displacement ellipsoids (data collection at 100 K) and the two geometrical isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)], (II) isomer A (SP-4-2), and (III) isomer B (SP-4-3), with 30% probability displacement ellipsoids (data collection at 293 K). Hydrogen atoms were omitted for clarity. The ethyl carbons are disordered, with 0.74:0.26 for C1A and C1B and 0.66:0.34 for C31A and C31B.

possible, which here are labeled isomer A (IUPAC SP-4-232) and isomer B (IUPAC SP-4-332 Scheme 2). Both isomers might be formed directly from the substitution reaction or through a subsequent isomerization reaction. A priori it is not clear if the substitution reaction favors any of the two isomers, nor is it possible to predict which isomer is more likely to crystallize. The substitution reaction as shown in Scheme 2 was performed in hot n-hexane as solvent. Crystals suitable for single-crystal X-ray studies of the parent compound [Rh(PhCOCHCOCH2CH3)(CO)2] and the product [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] were obtained after recrystallization (32) http://www.iupac.org/publications/books/rbook/Red_Book_2005. pdf, p 180. SQP complexes are designated SP-4, followed by a single digit referring to the priority number of the ligating atom trans to the ligating atom of priority number 1.

from acetone. The crystal data and details of data collection and refinement are given in the Supporting Information (Table S1), whereas selected bond lengths and angles are listed in Table 1. Solid-State Structure of [Rh(PhCOCHCOCH2CH3)(CO)2]. The molecular diagram of [Rh(PhCOCHCOCH2CH3)(CO)2] is presented in Figure 1I. The dicarbonyl complex packs in the P21/n space group with Z = 4, resulting in molecules lying on general positions in the unit cell. The rhodium atom has a nearly perfect square-planar coordination sphere (Rh atom displaced 0.0395(9) A˚ from the plane formed by four coordinating atoms, rms deviation of fitted atoms = 0.0047). The bond angles in the rhodium polyhedron are also within experimental error of the expected 90° for dsp2 hybridization; however, deviations of up to 6° were found in the sp2-hybridized

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C atoms of the β-diketonato skeleton, possibly due to strain from chelation with the metal. The Ph and CH2CH3 substituents on the β-diketonato ligand are twisted out of the plane formed by the acetylacetone backbone (dihedral of 50.92(13)° and 23.62(11)°, respectively). Weak intermolecular CH 3 3 3 O/Rh interactions (between C11-H11 3 3 3 O4i and C13-H13 3 3 3 Rhii)33 are observed with the Ph substituent on the β-diketonato ligand, possibly resulting in the preferred orientation. The conformation of CH2CH3 is mainly due to the packing arrangements of neighboring molecules. The Ph and CH2CH3 substituents in [Rh(PhCOCHCOCH2CH3)(CO)2] have similar electron-donating properties, which is confirmed by similar Rh-O bond lengths (both Rh-O3 and Rh-O4 are 2.029 A˚). Also the Rh-C1 and Rh-C2 bonds, 1.849 and 1.852 A˚, respectively, did not show a clear difference due to the trans influence of O4 and O3, respectively. The thermodynamic trans influence is a ground-state phenomenon leading to a weakening of the metal-ligand bond in the trans position.34 On grounds of the similar Rh-CO bond lengths, it thus is not possible to forecast which CO group preferably will be replaced in the substitution reaction (Scheme 2). In general, the Rh-C (mean distance 1.834 A˚, with a spread of 0.125 A˚) and Rh-O (mean distance 2.044 A˚, with a spread of 0.146 A˚) bond distances are fairly sensitive to changes on the β-diketonato backbone.35 Solid-State Structure of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]. NMR spectra in CDCl3 of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] show the existence of both geometrical product isomers in solution in a ratio of 1.00 (isomer A):0.70 (isomer B).23 Solution mixtures of product isomers have been observed for other [Rh(RCOCHCOR0 )(CO)(PPh3)] complexes also,26,27,29,30 but typically only a single product isomer is observed in the solid state.12,15,16,26,36 Factors such as temperature and solvent polarities are likely to play a role in determining which isomer in the mixture crystallizes. The substitution product [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] crystallized in the triclinic system in the space group P1, requiring two independent molecules (Z = 4) in the unit cell. The two molecules turned out to be both isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)], namely, isomer A, with PPh3 trans to the oxygen next to the Ph group (SP-4-2, Figure 1II), and isomer B, with PPh3 trans to the oxygen next to the CH2CH3 group (SP-4-3, Figure 1III). Simultaneous crystallization of both product isomers of an asymmetric β-diketonato Rh(I) complex in the same crystal lattice has been reported only once previously.28 The differences in Rh-O, Rh-P, and Rh-C bond lengths (Table 1) for the two isomers are within experimental error. The ligand-Rh-ligand bond angles of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] deviate by (2° from the expected 90° for a dsp2 hybridization, while deviations of up to 7° were found in the sp2-hybridized C atoms of the β-diketonato skeleton. The average C-C bond lengths of the Ph rings of PPh3 (1.389 A˚) and the β-diketonato substituent (1.385 A˚) in [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] are close to the (33) Symmetry codes are i = 1/2þx, 3/2-y, -1/2-z and ii = 1/2-x, 1/2þy, 1/2-z. (34) Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic Systems, 3rd ed.; Oxford University Press: New York, 2007. (35) Averages taken from 31 observations from square-planar Rh(O-O0 )(CO)2 complexes in the Cambridge Structural Database (CSD), Version 5.30, August 2008 update. (36) Roodt, A.; Steyn, G. J. J. Recent Res. Dev. Inorg. Chem. 2000, 2, 1.

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Figure 2. Overlay of the Rh(CCOCHCOC)(CO)(PPh3) moieties of the two isomers of (A) [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] and (B) [Rh(PhCOCHCOCH3)(CO)(PPh3)].28

expected value for an aromatic C-C bond (1.394 A˚).37 In both isomers, the Rh-O bond trans to PPh3 (2.067(3) and 2.060(3) A˚) is longer than the Rh-O bond trans to CO (2.032(3) and 2.032(2) A˚), indicating that the trans influence of PPh3 is larger than that of CO in this type of complexes.38 All the bond angles of the Ph rings are equal to 120° within the experimental error, e.g., C16-C15-C14 = 120.5(5)° and C44-C45-C46 = 120.8(4)°. The phosphorus atom displays a distorted tetrahedral geometry and is surrounded by the rhodium atom and three carbon atoms of the Ph rings bonded to it. The bond angles around P differ 11° from 109°, which is the angle for a regular tetrahedron. The O-C-Rh angle is close to linear (179.1(5)°), with a C-O bond length of 1.164(6) A˚ for isomer A (177.7(5)° and 1.133(5) A˚ for isomer B). The Rh-P bond length, 2.2387(9) A˚ (isomer A) and 2.2376(9) A˚ (isomer B), is comparable to related [Rh(βdiketonato)(CO)(PPh3)] complexes with β-diketonato = (CH3COCHCOCH3)- (2.244(2) A˚),38 (PhCOCHCOCH3)(2.248(3) and 2.249(3) A˚ for the two isomers),28 and (PhCOCHCOPh)- (2.237(7) A˚).39 The plane of the Ph ring on the β-diketonato ligand makes an angle of 22.44(12)° with the O-C-C-C-O plane of the β-diketonato skeleton of [Rh(PhCOCHCOCH2CH3)(CO)2] and 20.38(26)° and 28.33(18)° for the two isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)], respectively. These angles are similar to the angle between the plane of the Ph ring and the plane of the β-diketonato ligand in [Rh(PhCOCHCOCH3)(CO)(PPh3)] (21.23° and 27.80° for the two isomers)38 and in [Rh(PhCOCHCOPh)(CO)(PPh3)] (10.32° and 6.61° for the two Ph groups of the one molecule and 11.39° and 7.82° for the two Ph groups of the other molecule in the same unit cell).39 Steric Parameters. It is interesting that the PPh3 ligands of the two isomers of [Rh(β-diketonato)(CO)(PPh3)] adopt similar spatial orientations, despite different weak interactions to the Ph substituents (C16-H16 3 3 3 Rh1i, C16-H16 3 3 3 O2i, and C58-H58 3 3 3 O3ii).40 The Rh(CCOCHCOC)(CO)(PPh3) moieties of the two isomers can be superimposed, showing remarkable similarities (see Figure 2A for graphical representation). As a result, the steric parameters of these two ligands are similar (cone angles = 156.7° and 158.5°;41 solid angles = 26.41% and 26.36%42 for P1 and P2, respectively).43 A possible explanation (37) Sutin, L. E. Tables of Interatomic Distances and Configuration in Molecules and Ions, Supplement 1956-1959,; The Chemical Society: London, p S16s. (38) Leipoldt, J. G.; Basson, S. S.; Bok, L. D. C.; Gerber, T. I. A. Inorg. Chim. Acta 1978, 26, L35. (39) Lamprecht, D.; Lamprecht, G. J.; Botha, J. M.; Umakoshi, K.; Sasaki, Y. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1997, 53, 1403. (40) Symmetry codes are i = -1þx, y, z and ii = 1-x, 1-y, 1-z. (41) For calculation of cone angles: Taverner, B. C. Steric v1.12B, available at http://www.gh.wits.ac.za.

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Figure 3. Solid angle projections on isomer B of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]. (A) Presentation of the 8.5% overlap of the solid angle projection of CO (blue) and PPh3 (green). (B) Solid angle projection of the (PhCOCHCOCH2CH3)- ligand. (C) Solid angle projection of the Ph and CH2CH3 substituents on the (PhCOCHCOCH2CH3)- ligand.

is that due to the 90° angle between the CO and PPh3 ligands, the carbonyl ligand is wedging itself between the phenyl substituents of the PPh3 ligand, thereby forcing the PPh3 into a specific conformation. The overlap from the solid angle calculations42 for the CO and PPh3 ligands displayed in Figure 3A is ca. 8.5%. A different conformation of the PPh3 ligand would generally result in more overlap and is more unlikely. Incidentally this conformational effect is also observed in the previously reported case, where both isomers crystallized in the same lattice ([Rh(PhCOCHCOCH3)(CO)(PPh3)], Figure 2B).28 Calculation of the solid angle42 of the (PhCOCHCOCH2CH3)- ligand also yields information regarding the steric size of the Ph and CH2CH3 substituents. A solid angle projection42 of the ligand shows two comparable lobes for the substituents of the β-diketonato ligand (Figure 3B). Separation of these (Figure 3C) shows that they shield 12.48% and 12.43% of the sphere, indicating similar steric properties. This observation may also contribute to the stability of both isomers that are found in the solid state. DFT Studies. The presence of both isomers A and B of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] in solution and in the solid-state X-ray crystal structure can be due to formation of both isomers through the substitution reaction or, instead, through formation of only one isomer, followed by conversion into the other isomer (Scheme 2). We have used density functional theory to analyze the substitution pathways leading to formation of the two isomers as well as possible noncatalyzed and solvent-assisted mechanisms involved in product isomerization. Substitution Reaction. The optimized reactant complex comprising PPh3 and [Rh(PhCOCHCOCH2CH3)(CO)2] is shown in Figure 4. The SQP rhodium complex exhibits symmetric Rh-ligand bonds (1.88 A˚ for both Rh-CO bonds and 2.07 A˚ for both Rh-Oβ-diketonato bonds), indicating that the β-diketonato oxygens have similar trans influence. This supports the conclusions made based on the experimental structure. Comparison of [Rh(PhCOCHCOCH2CH3)(CO)2] to the X-ray crystal structure (Table 1) reveals that the rhodium-ligand bond distances are overestimated by 0.03 to 0.04 A˚, which is not considered to be critical.44 The optimized carbon-carbon and carbon-oxygen bonds are (42) Solid angle calculations: Guzei, I. A., Wendt, M. Program Solid-G; UW-Madison, WI, 2004. (43) Calculations were preformed with the Rh-P distance fixed to 2.28 A˚ to eliminate deviations incorporated by differences in bond distances. (44) Hehre, W. J. A. Guide to Molecular Mechanisms and Quantum Chemical Calculations; Wavefunction Inc.: Irvine, CA, 2003; pp 153, 181.

Figure 4. DFT-optimized reactant geometry comprising PPh3 and [Rh(PhCOCHCOCH2CH3)(CO)2]. Distances are in angstroms. The inset shows angles and overall conformation of the rhodium complex.

within 0.01 A˚ of the experimental structure. These results show that the employed computational method provides adequate geometric structures. Two aspects of the PPh3 geometry and mode of attack should be considered here. First, PPh3 can exist in two different helicities, which have the Ph substituents oriented in a clockwise (P) or anticlockwise (M) fashion.45,46 In solution, inversion of the helicity occurs readily, and the possible chirality introduced by the PPh3 moiety is normally disregarded.45 All calculations here were performed with PPh3 in M helicity. Second, attack of PPh3 on [Rh(PhCOCHCOCH2CH3)(CO)2] can occur from two different faces, leading to TBP transition states of chirality A and C, respectively (Scheme 3).47 Each pathway can give rise to both product isomers A and B (depending on which CO ligand dissociates); that is, there are four possible reaction pathways. The TS structures of chirality C are energetically slightly preferred and the A_(C) pathway is presented here (figures and energies for the B_(C), A_(A), and B_(A) pathways and all coordinates are given in the Supporting Information).  Haberhauer, G.; Hyla-Kryspin, I.; Grimme, S. Chem. (45) Pinter, A; Commun. 2007, 3711. (46) Costello, J. F.; Davies, S. G. J. Chem. Soc., Perkin Trans. 1998, 2, 1683. (47) http://www.iupac.org/publications/books/rbook/Red_Book_2005. pdf, p 187. The structure is oriented so that the viewer looks down the TBP axis, with the axial atom with higher priority closer to the viewer. Using this orientation, if the sequence of the atoms in the trigonal plane is in a clockwise fashion, the chirality symbol C is assigned, and if it is anticlockwise, the symbol A is assigned.

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Scheme 3. Attack of PPh3 Can Occur from Two Different Faces of [Rh(PhCOCHCOCH2CH3)(CO)2], Leading to TBP Complexes with Chirality (A) or (C) (IUPAC nomenclature47)a

a

Each path can give rise to either isomer A or B, depending on the CO that is substituted (dashed lines).

Scheme 4. CO Substitution in β-Diketonato Rh(I) Complexes Having a Bulky β-Diketonato Substituent R0 : (A) Leipoldt and Co-workers Proposed a One-Step Substitution Process with the Bulky Substituent Placed Axially;12,15,16 (B) Optimized Angles of the Two-Step Substitution Process Computed in This Study Indicate Preferred Equatorial Placement of a Bulky Group R0 at TS2

Kinetic Pathway for the Formation of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]. Isomer A (SP-4-2) of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] is formed if the CO group trans to the Oβ-diketonato adjacent to the Ph substituent is displaced. The reaction (A_(C) path, Scheme 3) proceeds through two pseudo-trigonal-bipyramidal transition states and involves formation of a SQP intermediate (Scheme 4B). The first TS comprises (i) the nucleophilic attack of PPh3 on Rh, (ii) cleavage of a Oβ-diketonato bond, and (iii) rotation of one CO ligand (Figure 5A). At the TS, the phosphorus atom is positioned 2.64 A˚ from rhodium, whereas the scissile RhOβ-diketonato bond is 2.29 A˚. The displacement vector of the imaginary frequency shows that this TS also comprises a rotation of the equatorial CO group around the TBP axis (arrow in Figure 5A). The movement converts the TBP TS geometry into a SQP intermediate. A TBP intermediate could not be located. Dissociation of a Rh-Oβ-diketonato bond instead of a Rh-CO bond can be rationalized with the relative trans influence of the ligands. For [Rh(acac)(CO)2], the trans (48) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335.

influence series is PPh3 > CO > Oacac.38,48 The series is likely the same for the asymmetric β-diketonato studied here, given the similar apparent group electronegativities of the substituents (χCH3 = 2.34,21 χCH2CH3 = 2.31,23 χPh = 2.2122). Thus, nucleophilic attack by PPh3 will lead to cleavage of the Rh-Oβ-diketonato bond instead of the Rh-CO bond. The optimized intermediate (Figure 5B) exhibits a fourcoordinated SQP geometry, with the displaced β-diketonato oxygen positioned 2.54 A˚ from the rhodium center. The Rh-P bond has shortened to 2.44 A˚, and the PPh3 group is now in an optimal position to weaken the Rh-CO bond trans to it. The second TS (Figure 5C) involves (i) the reformation of the Rh-Oβ-diketonato bond, (ii) cleavage of the Rh-CO bond trans to PPh3, and (iii) formation of a new SQP geometry. At the TBP transition state, the attacking oxygen atom is positioned 2.32 A˚ from rhodium, whereas the scissile Rh-CO bond is elongated to 2.09 A˚. Following CO dissociation, the TBP geometry collapses to the SQP product isomer A (Figure 5D). Comparison of the optimized product to the X-ray structure (Table 1) shows a slight overestimation of Rh-ligand bonds of 0.03 to 0.07 A˚, similar to the results observed for the dicarbonyl reactant structure.

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Figure 5. DFT-optimized geometries for formation of isomer A (SP-4-2) of [Rh(PhCOCHCOCH2CH3)(CO) (PPh3)] (A_(C) path, Scheme 3). (A) TS1_A, (B) Inter_A, (C) TS2_A, (D) Prod_A (isomer A, Scheme 2). Table 2. Computed Relative Energies for the Formation of Isomer A (Isomer B in Parentheses) of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] from [Rh(PhCOCHCOCH2CH3)(CO)2] and PPh3a electronic energy/kJ mol-1 ΔH298K/kJ mol-1 ΔS298K/J K-1 mol-1 ΔG298K/kJ mol-1 ΔG342K/kJ mol-1 reactant TS1_A (TS1_B) Inter_A (Inter_B) TS2_A (TS2_B) Prod_A (Prod_B) Prod_A, separated (Prod_B, separated) a

0.0 (0.0) 30.3 (33.9) 24.1 (29.7) 50.8 (50.3) 15.0 (17.6) 19.7 (16.8)

0.0 (0.0) 27.1 (30.8) 23.4 (29.3) 46.5 (45.7) 14.0 (16.5) 14.9 (12.1)

0.0 (0.0) -89.7 (-95.7) -63.9 (-62.5) -63.7 (-77.9) -10.3 (-9.1) 55.9 (57.8)

0.0 (0.0) 53.9 (59.3) 42.4 (48.0) 65.5 (68.9) 17.1 (19.2) -1.8 (-5.1)

0.0 (0.0) 57.8 (63.5) 45.2 (50.7) 68.3 (72.4) 17.5 (19.6) -4.2 (-7.6)

Results for the substitution reaction via path (C) in Scheme 3 are presented here. See Table S2 for results via path (A).

Formation of isomer B (SP-4-3) of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] occurs analogously to formation of isomer A, but results in displacement of the CO group trans to the Oβ-diketonato adjacent to CH2CH3 (see Supporting Information, Figure S1 for the (B_(C) path and Figure S3 for the (B_(A) path, Scheme 3). The optimized distances for the forming and breaking bonds are very similar to the pathways leading to formation of isomer A (Figure 5 for the A_(C) path and Figure S2 for the A_(A) path). Thermochemical Analysis of Substitution Reaction. Thermochemical quantities (ΔS, ΔH, ΔG, 298 K) for the A_(C) and B_(C) routes (Scheme 3) to formation of isomers A and B are given in Table 2. The computed free energies of activation at 298 K are 53.9 and 59.3 kJ mol-1 for TS1_A and TS1_B, respectively. The large negative entropies show that both TSs are very ordered.49 The intermediates have relative energies of 42.4 and 48.0 kJ mol-1 (ΔG298K), respectively, indicating that they are unstable despite the SQP coordination around the rhodium atom. The second reaction step, (49) Note that entropy calculations (and hence Gibbs free energies) are sensitive to low-frequency modes; that is, the computed ΔS and ΔG values should be considered estimations only.

dissociation of CO, is rate limiting. The overall activation barrier is 65.5 and 68.9 kJ mol-1 (ΔGq,298K) for formation of isomer A and B, respectively. The difference in barrier is so small that it cannot be considered significant. The reaction free energies for formation of both product isomers are small positive (17.1 and 19.2 kJ mol-1, respectively). However, the reaction is driven to completion by evaporation of CO. Calculation of the separated [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] and CO units indicates a free energy of -1.8 and -5.1 kJ mol-1 for formation of product isomer A and B, respectively (298 K, Table 2). The reaction energies at the experimental23 conditions (boiling point of n-hexane ∼69 °C or 342 K) are -4.2 and -7.6 kJ mol-1 for isomer A and B, respectively (342 K, Table 3).50 The ΔG298K values of the free products indicate that isomer B is 3.3 kJ mol-1 more stable (50) The lower energy of the separated fragments is due to an increase in entropy, as indicated above; these values are considered estimations only. Also note that the reported energies correspond to gas phase values and do not include solvent effects. This is considered a reasonable approximation here, because the substitution reaction is performed in very low polarity media (the dielectric constant of n-hexane is 1.89 at 20 °C) and polarizing effects of the surroundings should thus be small.

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Table 3. Computed Activation Energies at 298 K for Formation of Isomer A (Isomer B in Parentheses) of [Rh(PhCOCHCOCH2CH3)(CO)(PH3)] from [Rh(PhCOCHCOCH2CH3)(CO)2] with PH3 as Nucleophile

TS1 TS2

ΔHq/kJ mol-1

ΔSq/J K-1 mol-1

ΔGq/kJ mol-1

40.3 (40.6) 66.9 (66.9)

-76.9 (-76.3) -65.1 (-60.6)

63.3 (63.3) 86.3 (84.9)

than isomer A (Table 2). NMR studies in CDCl3 instead report an excess of isomer A (1.00 A:0.70 B).23 Single-point calculations on the optimized products in CHCl3 revert the isomer order, with isomer A being 2.4 kJ mol-1 more stable. Thus, the equilibrium between isomers could be affected by the polarity of the surroundings. It should be noted though that the energy differences are small and fall within the expected error margin of DFT calculations. The computed values of this study can also be compared to available experimental data of similar substitution reactions. The measured reaction enthalpy for formation of [Rh(acac)(CO)(PPh3)] from [Rh(acac)(CO)2] of 20.1 kJ mol-1 in CH2Cl2 at 30.0 °C51 compares well with the computed values of 12.0 to 20.3 kJ mol-1 (Table 2 and S2). The entropy and enthalpy of activation have been measured for the reaction of [Rh(acac)(CO)2] with the nucleophiles P(OPh)3 and P(NC4H4)3 in toluene at 25 °C.52 Activation entropies of respectively -113 and -125 J K-1 mol-1 were obtained, which are of similar order as the activation entropies observed here (-89.7 to -102.0 J K-1 mol-1, Table 2 and S2). The enthalpy of activation was reported to be 8 kJ mol-1 for both reactions, which is smaller than the activation enthalpies computed here, but might be due to differences in the nucleophiles. Steric Effects. Leipoldt and co-workers have proposed that steric effects at the TBP transition state affect the selectivity of CO substitution, leading to formation of a different product isomer than expected on electronic grounds (Scheme 1).12,15,16 Several points can be made on the basis of the optimized geometries. The transition states for the first reaction step (TS1) exhibit relatively small equatorial angles between incoming PPh3 and leaving Oβ-diketonato ligand (88.8° to 91.2° or ∼90°, Scheme 4), which might lead to steric interactions if entering or leaving groups are bulky.53 Leipoldt and co-workers have suggested that the bulkier β-diketonato substituent preferably will be placed axially to reduce steric interactions with the incoming ligand.12,15,16 However, the optimized geometries of TS1 show that the angle between PPh3 and the axially placed ligands is of similar magnitude (90.8° to 91.5° or ∼91°, Scheme 4), leading to comparable axial steric interactions. The optimized geometries for TS1_A and TS1_B show steric interactions between the Ph groups of the attacking PPh3 and both the equatorially and the axially placed β-diketonato ligand (Supporting Information, Figure S4). Thus, at TS1 there is no obvious preference for an axial or equatorial placement of a bulky β-diketonato substituent. At TS2, the PPh3 group is positioned trans to the leaving ligand, which results in an equatorial angle between PPh3 and the attacking β-diketonato oxygen of ∼135° (Scheme 4). The angles between PPh3 and the axial β-diketonato ligands (51) Serron, S.; Huang, J.; Nolan, S. P. Organometallics 1998, 17, 534–539. (52) Simanko, W.; Mereiter, K.; Schmid, R.; Kirchner, K.; Trzeciak, A. M.; Ziolkowski, J. J. Organomet. Chem. 2000, 602, 59–64. (53) Lin, Z.; Hall, M. B. Inorg. Chem. 1991, 30, 646.

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at TS2 are similar to TS1 (87.7° to 89.2° or ∼88°, Scheme 4). Thus, at TS2, the steric interactions between PPh3 and the axial β-diketonato ligand (also referred to as cis ligand) are larger than the interactions between PPh3 and the equatorial β-diketonato substituents. Steric effects at TS2 (which is rate-limiting) should therefore favor placement of bulky groups in an equatorial position (see Supporting Information, Figure S5). This is in line with Gray and Langford’s observation that sterically demanding substituents on trans ligands have less effect on reaction rates of SQP substitution reactions than bulky substituents on cis ligands.19,54 Hence, for [Rh(RCOCHCOR0 )(CO)2] complexes, where R and R0 differ substantially in size, CO substitution should be promoted trans to the bulkier β-diketonato substituent (Scheme 4), opposite the substitution model proposed by the Leipoldt model.12,15,16 Electronic Effects. In order to eliminate steric effects due to the Ph groups of PPh3, the substitution reactions employing phosphine (PH3) as nucleophile are also computed (optimized coordinates and energies are given in the Supporting Information, Table S3). This should make it possible to isolate electronic effects originating from the β-diketonate substituents. The same substitution pathway involving two transition states and a square-planar intermediate (Scheme 4B) is obtained. Activation parameters obtained for the formation of isomer A or B were very similar (Table 3). For example, at the first TS, involving nucleophilic attack of PH3, the free energy of activation is the same for both product isomers (63.3 kJ mol-1). The second TS, involving dissociation of CO, has a free energy barrier of 86.3 and 84.9 kJ mol-1 for isomer A and B, respectively. These results illustrate that the electronic effects of the two β-diketonate substituents on the reaction barriers are virtually identical, as could be expected from the almost identical group electronegativities of Ph22 and CH2CH3.23 Isomerization Pathways. The presented DFT study of the substitution of PPh3 for CO in the asymmetric square-planar β-diketonato complex [Rh(PhCOCHCOCH2CH3)(CO)2], with β-diketonato substituents Ph and CH2CH3, reveals that formation of both product isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] is energetically feasible. The presence of both product isomers in the reaction mixture is thus likely to be due to formation of both isomers in the substitution reaction. However, as pointed out above, the product isomers also have the possibility to isomerize; that is, formation of one product complex might be followed by isomerization to the other isomer. NMR studies on [Rh(CF3COCHCOFc)(CO)(PPh3)] (Fc = ferrocenyl) have shown that if pure crystals of one product isomer are dissolved, the second isomer is rapidly formed.27 Product isomerization occurs in solvents with very different polarity, and coordination propensity (e.g., C6D6, CDCl3, CD3CN), resulting in different isomer ratios.27,29 Consequently, a central part of our theoretical investigation of β-diketonate Rh(I) complexes is to establish possible isomerization pathways of the SQP compounds. Different noncatalyzed and solvent-assisted pathways can be envisioned, depending on if they occur in the low-polarity environment of the substitution reaction (i.e., n-hexane27) or in the presence of polar/coordinating solvent (such as CH3CN or CHCl3, which typically are employed in NMR studies27). Here we have considered three noncatalyzed (54) Basolo, F.; Chatt, J.; Gray, H. B.; Pearson, R. G.; Shaw, B. L. J. Chem. Soc. 1961, 2207.

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Scheme 5. Possible Isomerization Pathways of SQP β-Diketonate Complexes: (I) Isomerization of SQP Intermediate Formed during the Substitution Reaction, (II) Pseudorotation of Product Complex through Quasi-tetrahedral (THD) TS, (III) Temporary Ligand Dissociation and Rearrangement of the Three-Coordinated Intermediate, (IV) Coordination of L (PPh3/solvent molecule) and Consecutive Berry Pseudorotations of TBP Intermediate, (V) Coordination of L and Isomerization of SQP Intermediate through an Interchange Mechanisma

a

R0 = Ph and R = CH2CH3 in this study.

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Figure 6. Isomerization of SQP intermediates (Scheme 5I): (A) Inter_A_(C), (B) TSi_int, (C) Inter_B_(A).

isomerization mechanisms: (I) isomerization of the SQP intermediate formed during the substitution reaction (Scheme 5I), (II) isomerization of the product through a pseudorotation mechanism (Scheme 5II),55-58 and (III) product isomerization through a dissociative pathway (Scheme 5III).57 Additionally, two solvent-assisted (or PPh3-assisted) pathways can occur in the presence of coordinating solvent (or excess PPh3), (IV) a Berry pseudorotation mechanism involving a TBP complex (Scheme 5IV)59-61 or (V) an interchange mechanism involving a SQP intermediate (Scheme 5V). Isomerization in a Low-Polarity Environment. Mechanism I (Scheme 5I) operates prior to product formation and involves isomerization of the SQP intermediate formed during the substitution reaction (Scheme 4B, Figure 5B). We propose that this intermediate is able to isomerize by interchanging the two β-diketonato oxygens (Scheme 5I). The TS for conversion of Inter_A (where the Oβ-diketonato next to the Ph group is dissociated, Figure 6A) to the opposite SQP intermediate (where the Oβ-diketonato next to the ethyl group is dissociated, Figure 6C) has a TBP structure (Figure 6B). The equatorial plane of the TS is occupied by the CO and the Oβ-diketonato groups. The computed interconversion barrier is ΔGq,298K =11.6 kJ mol-1 relative to Inter_A (ΔHq,298K = 0.8 kJ mol-1, ΔSq,298K = -36.2 J K-1 mol-1) and ΔGq,298K = 54.0 kJ mol-1 (ΔHq,298K = 24.2 kJ mol-1, ΔSq,298K = -100.1 J K-1 mol-1) relative to the reactant (Figure 4). Given that the barrier for intermediate isomerization is 10 to 15 kJ mol-1 lower than TS2 (Table 2), it is likely that a given SQP intermediate isomerizes before it proceeds through TS2 to liberate CO, thus giving rise to both product isomers. Mechanism II involves isomerization of the product complex through a pseudotetrahedral TS (Scheme 5II). Such a mechanism has been analyzed for SQP palladium ([PhPd(PMe3)2OAc], [PhPd(PMe3)2I], [(CH2dCH)Pd(PH3)2Br])55,56 and platinum ([Pt(Cl)(SnCl3)(PH3)2])58 complexes. Barriers of ΔGq,298K = 75 to 85 kJ mol-1 (BP86, ΔHq,298K = 77 to 85 kJ mol-1)55 and 85 kJ mol-1 (B3LYP)56 were obtained for the Pd complexes and ΔGq,298K = 109 kJ mol-1 (MP2, ΔHq,298K = 112 kJ mol-1, ΔSq,298K = 8 J K-1 mol-1) for cis-trans (55) Goossen, L.; Koley, D.; Hermann, H. L.; Thiel, W. Organometallics 2005, 24, 2398. (56) Braga, A. A. C.; Ujaque, G.; Maseras, F. Organometallics 2006, 25, 3647. (57) Andersson, G. K.; Cross, R. J. Chem. Soc. Rev. 1980, 9, 185. (58) Rocha, W. R; de Almeida, W. B. J. Braz. Chem. Soc. 2000, 11, 112. (59) Berry, R. S. J. Chem. Phys. 1960, 32, 933. (60) Casado, A. L.; Espinet, P. Organometallics 1998, 17, 954. (61) Sakaki, S., Mizoe, N., Musashi, Y., Sugimoto, M. J. Mol. Struct. (THEOCHEM) 1999, 461-462, 533.

Figure 7. Pseudotetrahedral TS (TSi_prod) for isomerization of product isomers (Scheme 5II).

isomerization of the Pt complex.58 We have optimized the pseudorotation TS leading to interconversion between the product isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] (Figure 7). The TS can be viewed as a rotation of the CO group around the P-Rh bond to yield the opposite isomer. The barrier relative to product isomer A is ΔGq,298K = 111.7 kJ mol-1 (ΔHq,298K = 110.8 kJ mol-1, ΔSq,298K = -2.8 J K-1 mol-1), which is similar to the barrier for isomerization of [Pt(Cl)(SnCl3)(PH3)2].58 Mechanism III results in product isomerization through dissociation of a ligand (Scheme 5III). Three different ligands might be lost from a product complex: CO, PPh3, or a β-diketonato oxygen. The computed cost for dissociating PPh3 is ΔGr298K = 110.1 kJ mol-1 (ΔHr298K = 164.2 kJ mol-1, ΔSr298K = 172.9 J K-1 mol-1), making formation of an [Rh(RCOCHCOR0 )(CO)] intermediate unlikely but possible. Attempts to optimize a TS for rearrangement of the three-coordinated intermediate failed; however, it is clear that the barrier would be higher than 110.1 kJ mol-1. Dissociation of CO from the product complex has a significantly higher cost of ΔGr298K =147.8 kJ mol-1 (ΔHr298K= 195.4 kJ mol-1, ΔSr298K = 159.8 J K-1 mol-1) and is therefore ruled out here. Temporary loss of a β-diketonato ligand should be more feasible than loss of either CO or PPh3, in particular as the dissociated ligand stays in close vicinity, whereas the dissociated CO or PPh3 are likely to diffuse away from the complex. However, attempts to optimize a three-coordinated intermediate with one unbound β-diketonato oxygen failed. Mechanisms IV and V involve coordination of a fifth ligand to the product complex. If the resulting compound is trigonal bipyramidal, it could isomerize through two consecutive Berry pseudorotations (Scheme 5IV). During the substitution reaction in n-hexane, a possible fifth ligand is

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Figure 8. PPh3-assisted isomerization of product isomer A into isomer B (Scheme 5V): (A) Nucleophilic attack of PPh3 on isomer A (TS1_PPh3_A), (B) SQP diphosphine intermediate, (C) TS for isomerization (TSi_PPh3), (D) isomerized SQP diphosphine intermediate, (E) TS for PPh3 release (TS2_PPh3_B).

PPh3. Studies on PH3-catalyzed cis-trans isomerization of [PtH(SiH3)(PH3)2] through a Berry pseudorotation report a barrier of 113 kJ mol-1 (MP2, ΔEq).61 We attempted to test such a mechanism, but optimization of a diphosphine TBP intermediate failed. Instead, a SQP diphosphine compound is formed, in which the Oβ-diketonato trans to PPh3 has dissociated. The result is not surprising, as the substitution reaction also proceeds through a SQP intermediate rather than a TBP geometry. The SQP intermediate cannot isomerize through a Berry pseudorotation, but we propose that isomerization can occur through an interchange mechanism analogous to mechanism I; see mechanism V, Scheme 5. The optimized geometries for the PPh3-assisted isomerization according to mechanism V are shown in Figure 8, with the relative energies and thermodynamic parameters given in Table S4 (Supporting Information). Attack of PPh3 on [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] leads to a TBP TS, with one β-diketonato oxygen and the two PPh3 ligands in the equatorial plane (Figure 8A). The computed barrier is ΔGq,298K = 61.3 kJ mol-1. In the formed SQP intermediate, the β-diketonato oxygen has dissociated (Figure 8B). Nucleophilic attack of the free oxygen leads to a new TBP TS, where the two β-diketonato oxygens and CO occupy the equatorial plane (Figure 8C). The barrier for interconversion is ΔGq,298K = 50.0 kJ mol-1. Dissociation of PPh3 from the isomerized intermediate has a barrier of ΔGq,298K = 62.2 kJ mol-1 (Figure 8E). The results indicate that PPh3-mediated isomerization could be a feasible route to product isomerization. Our results are also in agreement with the experimental observation of Trzeciak and Zi olkowski that the presence of free phosphine molecules causes fast exchange between free and rhodium-coordinated phosphine.26 Isomerization in a Polar Environment. We have performed single-point calculations in acetonitrile (ε = 36.64) to estimate the effect of polar solvents on the energetics of mechanisms II,

III, and V (Scheme 5). The barrier for pseudorotation (mechanism II) is almost unaffected by inclusion of solvent effects, resulting in a slight reduction to ΔGq,298K = 109.7 kJ mol-1. The dissociative mechanism (mechanism III) shows significant solvent effects, resulting in a reduction in the cost of PPh3 dissociation to 62.4 kJ mol-1 (ΔGr298K). In coordinating solvent, however, the empty coordination site might be occupied, preventing isomerization and recoordination of PPh3. The PPh3-assisted isomerization (Scheme 5, mechanism V) is likely to occur during the substitution reaction in n-hexane, but could also take place in a polar environment when the formed product complex is redissolved. If the solvent replaces a PPh3 ligand from one complex, the liberated PPh3 could catalyze isomerization of other complexes. Such a mechanism has been suggested for isomerization of [PdX2L2] complexes.57 Calculations of the solvent effect on the PPh3assisted isomerization according to mechanism V increase the rate-limiting step to 80.4 kJ mol-1, which still can be considered feasible. In coordinating solvent, a solvent molecule could also catalyze isomerization (mechanism V, Scheme 5). We have tested this with CH3CN as coordinating solvent. The reaction occurs analogously to the PPh3-assisted pathway through formation of a SQP intermediate (optimized geometries and energies are given in the Supporting Information, Figure S6 and Table S5). The overall Gibbs free energy barrier for CH3CN-mediated product isomerization is 80.6 kJ mol-1 (including solvent correction), which is similar to the PPh3mediated pathways. However, the energies for mechanism V might vary depending on size and nucleophilicity of the coordinating solvent as well as the polarity of the environment. Summary of Overall Substitution and Isomerization Mechanism. The DFT results show that formation of both isomers A and B of [Rh(RCOCHCOR0 )(CO)(PPh3)] occurs through an interchange mechanism (Scheme 6), analogous to

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Scheme 6. Overall Mechanism for Formation and Isomerization of [Rh(RCOCHCOR0 )(CO)(PPh3)] Complexesa

a

L = free PPh3 or a solvent molecule. R0 = Ph and R = CH2CH3 in this study.

the mechanism computed for CO substitution in the symmetric [Rh(acac)(CO)2] complex.31 Reversible attack of PPh3 leads to dissociation of a β-diketonato oxygen (TS1_A or TS1_B), resulting in formation of a SQP intermediate. The two possible SQP intermediates are able to isomerize through a trigonal-bipyramidal TS (TSi_int), which has a barrier comparable to TS1 when R0 = Ph and R = CH2CH3. Dissociation of CO through either TS2_A or TS2_B results in formation of the SQP product isomers, isomer A and isomer B. This step is rate-limiting and irreversible, as CO is removed from the reaction mixture. The product isomers are able to interconvert, either through a slow noncatalyzed pseudorotation pathway (TSi_prod) or through a fast PPh3assisted mechanism (TSi_sol). In a coordinating solvent (such as acetonitrile or chloroform), isomerization might also occur through a solvent-assisted pathway (TSi_sol).

Conclusions Substitution of PPh3 for CO in the SQP β-diketonato complex [Rh(PhCOCHCOCH2CH3)(CO)2] leads to the crystallization of two structural isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] in the same crystal lattice. The two β-diketonato side groups Ph and CH2CH3 exhibit similar electronic properties, as reflected by their near equal group electronegativities as well as the similar Rh-CCO bond lengths (thermodynamic trans influence) in the reactant [Rh(PhCOCHCOCH2CH3)(CO)2]. The steric size of the two β-diketonato side groups Ph and CH2CH3 is also similar, as determined by the contribution of each group to the solid angle of the (PhCOCHCOCH2CH3)- ligand. A DFT analysis of the substitution reaction shows that nucleophilic attack of PPh3 on [Rh(PhCOCHCOCH2CH3)(CO)2] results in formation of a SQP intermediate, where one Rh-Oβ-diketonato bond is broken. Different intermediates are formed, depending on which Oβ-diketonato dissociates. The intermediates have the possibility to isomerize, followed by irreversible CO release to form the product isomers A and B. Isomerization of the product can occur through different noncatalyzed and solvent-assisted pathways. Especially a novel PPh3-mediated isomerization mechanism involving a

SQP diphosphine intermediate appears as a feasible route. The theoretical study further shows that the β-diketonato ligand does not act as a trans directing ligand, but functions as a leaving group at the first TS and as an entering group at the second TS. The substitution selectivity thus cannot be related directly to the relative trans influence of the β-diketonato oxygens. Steric interactions in the axial position of the TBP transition states appear to be more pronounced than for the equatorial position; that is, placement of bulky groups should be favored equatorially, not axially as was proposed by Leipoldt.12,15,16

Experimental Details Synthesis. [Rh(PhCOCHCOCH2CH3)(CO)2] and [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] were synthesized as described earlier.23 Crystals suitable for X-ray diffraction were obtained after recrystallization from acetone. X-ray Structure Determination. Crystals were mounted on glass fibers. The X-ray intensity data were measured on a Bruker X8 Apex II 4K CCD diffractometer area detector system equipped with a graphite monochromator and Mo KR finefocus sealed tube (λ = 0.71073 A˚) operated at 1.5 kW power (50 kV, 30 mA). The detector was placed at a distance of 3.75 cm from the crystal. Crystal temperature during the data collection for [Rh(PhCOCHCOCH2CH3)(CO)2] was kept constant at 100(2) K using an Oxford 700 series cryostream cooler. Collection for [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] was done at room temperature (293 K). The initial unit cell and data collection were achieved with Apex2 software62 utilizing COSMO63 for optimum collection of more than a hemisphere of reciprocal space. A total of 1127 (2473) ([Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] parameters given in parentheses) frames were collected with a scan width ω in j and an exposure time of 10 (20) s frame-1. The frames were integrated using a narrowframe integration algorithm and reduced with the Bruker SAINT-Plus64 and XPREP64 software packages, respectively. The integration of the data yielded a total of 14 494 (64 974) reflections to a maximum θ angle of 28.30° (28.37°), of which (62) Apex2 (Version 1.0-27); Bruker AXS Inc.: Madison, WI, 2005. (63) COSMO, Version 1.48; Bruker AXS Inc.: Madison, WI, 2003. (64) SAINT-Plus, Version 7.12 (including XPREP); Bruker AXS Inc.: Madison, WI, 2004.

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3154 (13 157) were independent with a Rint = 0.0342 (0.0408). Analysis of the data showed no significant decay during the data collection. Data were corrected for absorption effects using the multiscan technique SADABS65 with minimum and maximum transmission coefficients of 0.7293 and 0.8550 (0.8488 and 0.9641), respectively. Structures were solved by the direct methods package SIR9766 and refined using the WinGX software package67 incorporating SHELXL.68 The final anisotropic full-matrix least-squares refinement on F2 with 164 (651) variables converged at R1 = 0.0219 (0.0436) for the observed data and wR2 = 0.0543 (0.1055) for all data. The GOF was 1.105 (1.060). The largest peak on the final difference electron density synthesis was 0.53 e A˚-3 at 0.88 A˚ from Rh and the deepest hole -0.60 e A˚-3 at 0.91 A˚ from Rh (1.88 e A˚-3 at 0.86 A˚ from Rh2 and the deepest hole -0.79 e A˚-3 at 0.60 A˚ from Rh2). The aromatic, methylene, and methyl H atoms were placed in geometrically idealized positions (C-H = 0.93-0.98 A˚) and constrained to ride on their parent atoms with Uiso(H) = 1.2Ueq(C) for aromatic and methylene and Uiso(H) = 1.5Ueq(C) for methyl. The methyl H’s were located from a Fourier difference map and refined as a rigid rotor. Non-hydrogen atoms were refined with anisotropic displacement parameters. Atomic scattering factors were taken from the International Tables for Crystallography Volume C.69 The molecular plot was drawn using the DIAMOND program70 with a 30% thermal envelope probability for non-hydrogen atoms. Hydrogen atoms were drawn as arbitrarily sized spheres with a radius of 0.135 A˚. The refinement of the two [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] isomers showed large thermal vibrations in the CH2CH3 chain. This was treated by disordered refinement techniques to obtain more satisfactory refinement parameters. The refinement was kept stable with additional geometric and anisotropic restraints. Site occupancies for the terminal carbons in the ethyl groups of both isomers were left to refine freely but restricted to add up to one for each methyl, resulting in 0.737:0.263 for C1A and C1B and 0.664:0.336 for C31A and C31B (Figure 1). Computational Details. All calculations were performed with the hybrid DFT functional B3LYP71,72 as implemented in the Gaussian 03 program package.73 Geometries were optimized in the gas phase with the triple-ζ basis set 6-311G(d,p) on all atoms except (65) SADABS, Version 2004/1; Bruker AXS Inc.: Madison, WI, 1998. (66) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (67) Farrugia, L. J. WinGX Version 1.70.01. J. Appl. Crystallogr. 1999, 32, 837. (68) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refinement; University of G€ottingen: Germany, 1997. (69) International Tables for Crystallography; Kluwer Academic Publishers: Dordrecht, The Netherlands, Vol. C, p 1002. (70) Brandenburg, K.; Putz, H., DIAMOND, Release 3.1a; Crystal Impact GbR: Bonn, Germany, 2005. (71) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (72) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

Hopmann et al. rhodium, where we used LANL2DZ (corresponding to the Los Alamos Effective Core Potential plus DZ74). Additional calculations employing an added f-polarization function on rhodium (coefficient = 0.4 (LANL2DZþf040) or 1.35 (LANL2DZþf135))75,76 yielded virtually identical geometries and similar energies (see Tables S6 and S7, Supporting Information). Frequency calculations were performed on all optimized structures at the same level of theory as geometry optimizations. Minima exhibited only positive frequencies, while all transition states exhibited one negative frequency. Thermochemical data and temperature corrections were computed at 298.15 K for all optimized geometries. Additional thermochemistry calculations at 342 K were performed with the freqchk functionality implemented in Gaussian 03. Single-point calculations in CH3CN solvent (dielectric constant = 36.64) and CHCl3 (dielectric constant = 4.9) were performed with the solvent model IEFPCM.

Acknowledgment. This work was supported by the Research Council of Norway and the National Research Foundation of the Republic of South Africa (Grant Unique Number 65507). Supporting Information Available: Crystallographic data in CIF format, optimized Cartesian coordinates, computed thermochemical data for selected systems, and additional figures. This material is available free of charge via the Internet at http://pubs. acs.org. (73) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (74) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H.F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284–298. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (75) Ehlers, A. W.; B€ ohme, M.; Dapprich, S.; Gobbi, A.; H€ ollwarth, A.; Jonas, V.; K€ ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (76) Nsouli, N. H.; Mouawad, I.; Hasanayn, F. Organometallics 2008, 27, 2004.