Neutral cis-Alkyl Olefin Rhodium(I) Complexes: Models of

Neutral cis-Alkyl Olefin Rhodium(I) Complexes: Models of Intermediates in Late Transition Metal Olefin Polymerization with Surprising Structure. Heiko...
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Organometallics 2010, 29, 5496–5503 DOI: 10.1021/om100413m

Neutral cis-Alkyl Olefin Rhodium(I) Complexes: Models of Intermediates in Late Transition Metal Olefin Polymerization with Surprising Structure† Heiko Urtel, Claudia Meier, Frank Rominger, and Peter Hofmann* Organisch-Chemisches Institut, Universit€ at Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Received May 3, 2010

The reaction of the γ-agostic 14 VE complex [(κ2-dtbpm)Rh(Np)] (1) with olefins opened an access to neutral, cis-alkyl olefin complexes (κ2-dtbpm)Rh(Np)(η2-olefin) (2a/b, Np = neopentyl, olefin = ethylene, methylacrylate) bearing the cis chelating bisphosphine ligand dtbpm (bis(di-tert-butylphosphino)methane, tBu2P-CH2-PtBu2). They represent the first structurally characterized examples of neutral Rh systems with cis bisphosphine ligation, isoelectronic to the presumed resting states in late transition metal (e.g., Pd, Ni) catalyzed polymerization reactions. Their solid-state molecular structures revealed an unexpected, non-square-planar coordination mode of the olefin moiety, with the center of the CdC double bond moved 34° out of the P2RhCNp plane. In solution rapid dynamic processes interconvert equivalent structures with the olefin above or below this plane. DFT and QM/ MM (ONIOM) calculations reproduce correctly the minimum geometries found in the crystal structures. They are caused by electronic effects, as shown by fragment MO (NBO analysis) and frontier MO arguments. The unusual structure motif carries over to the chloro complexes (κ2-dtbpm)Rh(Cl)(η2-olefin) (5a/b, olefin = fumaronitrile, acrylonitrile). No polymerization is observed when 2a is exposed to an excess of ethylene, indicating a rather high olefin insertion barrier for this uncharged RhI species, in agreement with DFT calculations of the transition state for ethylene insertion into the Rh-CNp bond of (κ2-dhpm)Rh(Np)(η2-C2H4) (2a*). This contrasts with the high catalytic activity of isolectronic NiII and PdII systems.

Introduction cis-Alkyl olefin complexes are generally accepted to be the resting states of catalytic olefin polymerization involving late transition metals.1 Little is known, however, about their detailed structural features2 for active polymerization catalysts, especially with phosphorus spectator ligands, although a number of cis-alkyl olefin complexes capable of olefin insertion reactions and polymer production have been reported to exist at low temperature in solution.3 Their spectroscopic characterization, however, has not allowed drawing definite conclusions about their minimum energy

geometries, and none of them could be characterized by X-ray. The cis coordination geometry in these catalysts is best enforced by using suitable bidentate chelating ligands such as diimine4 or bisphosphine5d-g systems. Here we report the syntheses and the structures of two cisalkyl olefin complexes, (κ2-dtbpm)Rh(Np)(η2-olefin), bearing a bulky, electron-rich cis-chelating bisphosphine ligand (dtbpm = bis(di-tert-butylphosphino)methane, (t Bu)2PCH2P(tBu)2; Np = neopentyl, olefin = ethylene, methylacrylate). They are neutral isoelectronic structural models for the resting state of olefin polymerization reactions efficiently catalyzed by d8-L2M(alkyl)þ cations5 (M=NiII, PdII),

† Part of the Dietmar Seyferth Festschrift. *Corresponding author. E-mail: [email protected]. (1) (a) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534. (b) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (c) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888. (2) (a) Ganis, P.; Orabona, I.; Ruffo, F.; Vitagliano, A. Organometallics 1998, 17, 2646. (b) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo, F. Organometallics 1997, 16, 5981. (c) Shiotsuki, M.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2007, 129, 4058. We are well aware that in addition to these two examples there are many other cases of structurally characterized cis-(R)(olefin) complexes especially with Pt and nitrogen chelate ligands. A CSD search (June 16, 2010) resulted in over 70 hits. Platinum systems, however, neither are active polymerization catalysts (they may dimerize olefins) nor do they show structures analogous to the ones reported here.

(3) (a) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 16. (b) Dias, E. L.; Brookhart, M.; White, P. S. Organometallics 2000, 19, 4995. (c) Zuccaccia, C.; Macchioni, A.; Orabona, I.; Ruffo, F. Organometallics 1999, 18, 4367. (d) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137. (e) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (f ) Shultz, C. S.; L. K.; Ledford, J.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6351. (g) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686. (h) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 5272. (i) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 3068. ( j) Cavallo, L.; Macchioni, A.; Zuccaccia, C.; Zuccaccia, D.; Orabona, I.; Ruffo, F. Organometallics 2004, 23, 2137, and references therein. (4) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267. (b) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1999, 18, 65.

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Scheme 1

Scheme 2

modeling especially systems with L2 = phosphine ligands5d-g (Scheme 1). For an uncharged RhI species we expected the olefin insertion barrier to be higher, and we hoped that cisalkyl olefin complexes might become sufficiently stable to be isolated.

Results and Discussion Some time ago, we prepared and fully characterized the very reactive complex [(κ2-dtbpm)Rh(Np)] (1)6 formally representing a T-shaped d8-ML3 fragment with a 14 VE count, stabilized only by a very labile Np to Rh γ-agostic interaction. Treatment of a solution of 1 in THF, toluene, or Et2O with 1 bar of ethylene at temperatures below -20 °C yields the cis-alkyl ethylene complex [(κ2-dtbpm)Rh(Np)(η2-H2CdCH2)] (2a) almost quantitatively, as determined by NMR spectroscopy (Scheme 2). The ethylene complex 2a could be isolated as a yellow solid by suspending 1 in liquid ethylene at -110 °C, followed by evaporation of the solvent at -78 °C. Further purification procedures of the product are not possible due to its facile (5) (a) We were able to isolate and characterize by X-ray analysis the THF adduct [(κ2-dtbpm)Ni(CH3)(THF)]þ[BArF]- as well as the η3benzyl complex [(κ2-dtbpm)Ni(η3-benzyl)]þ[BArF]- (see Supporting Information). Both compounds are highly active catalysts for ethylene polymerization to long-chain linear PE. The active species presumably are cations [(κ2-dtbpm)Ni(CH3)]þ and [(κ2-dtbpm)Ni(η1-benzyl)]þ, direct analogues of neutral [(κ2-dtbpm)Rh(Alkyl)]: Eisentr€ager, F. Ph. D. Thesis, Universit€at Heidelberg, Germany, 2000. (b) Schultz, M.; Eisentr€ager, F.; Rominger, F.; Hofmann, P. In preparation. (c) Kristen, M. O.; Hofmann, P.; Eisentr€ager, F. WO 0202573 A1 2002 (BASF AG, Germany). For other bulky bisphosphine complexes of Ni or Pd active in polymerization reactions see: (d) Dossett, S. J.; Gillon, A.; Orpen, A. G.; Fleming, J. S.; Pringle, P. G.; Wass, D. F.; Jones, M. D. Chem. Commun. 2001, 699. (e) Ledford, J.; Shultz, C. S.; Gates, D. P.; White, P. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 5266. (f ) Cooley, N. A.; Green, S. M.; Wass, D. F. Organometallics 2001, 20, 4771. (g) Dennett, J. N. L.; Gillon, A. L.; Heslop, K.; Hyett, D. J.; Fleming, J. S.; Lloyd-Jones, C. E.; Orpen, A. G.; Pringle, P. G.; Wass, D. F.; Scutt, J. N.; Weatherhead, R. H. Organometallics 2004, 26, 6077. (6) Urtel, H.; Meier, C.; Eisentr€ager, F.; Rominger, F.; Joschek, J. P.; Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 781.

Figure 1. Molecular structures of 2a/b (top/bottom) in the crystal: ORTEP plots, 50% probability, hydrogen atoms except on C1 and C2 omitted for clarity. Scheme 3

decomposition upon washing (even with liquid C2H4 at -110 °C) or upon attempted recrystallization. Analogously, the orange complex (κ2-dtbpm)Rh(Np)(η2-H2CdCHCO2Me) (2b) was obtained by treating solutions of 1 in Et2O with 1.5 equiv of methylacrylate at -20 °C (Scheme 2). In the solid state, the olefin complexes 2a/b decompose to unidentified products at ambient temperature within several hours. In solution they decay at temperatures above 0 °C. It is possible, however, to detect 2a/b by NMR spectroscopy at ambient temperature even after 1 h. Storing a [D8]THF solution of 2a under an ethylene atmosphere (1 to 10 bar) for 3 days at 25 °C yielded the two methallyl complexes [(κ2-dtbpm)Rh(Np)(η3-C3H4Me)], 3-syn and 3-anti, as major products (Scheme 3), in 68% and 13% yield (NMR), respectively. They were identified by comparison with authentic 3 synthesized independently7 (Supporting Information). Polymers could not be detected. We tentatively interpret the dimerization process of two C2 units to methallyl fragments (7) 3-syn and 3-anti have been identified by comparison of the NMR spectroscopic data with independently synthesized samples: Joschek, J. P. Ph.D. Thesis, Universit€at Heidelberg, Germany, 1999. Details are given in the Supporting Information.

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Table 1. Comparison of Important Bond Lengths [A˚] and Angles [deg] from the Crystal Structure Data of 2a/b and 5a/b with the Calculated Values (DFT, B3PW91) of the Corresponding Model Complexes 2a*/b* and 5a*/b* and ONIOM Calculated Results of 2a

Rh-P1 Rh-P2 Rh-C1 Rh-C2 Rh-CNp/Cl C1-C2 P1-Rh-P2 C1-Rh-C2 free olefin b

2a

2a*

2a

2b

2b*

5a

5a*

5b

5b*

X-ray

DFT

ONIOM

X-ray

DFT

X-ray

DFT

X-ray

DFT

2.378(1) 2.357(1) 2.124(2) 2.161(2) 2.123(1) 1.389(3) 73.43(1) 37.81(8) 1.339a 34.8

2.322 2.416 2.138 2.154 2.073 1.407 71.6 38.3 1.329 36.5

2.478 2.504 2.141 2.150 2.089 1.402 71.5 38.4 1.329 30.2

2.345(2) 2.428(2) 2.095(7) 2.146(6) 2.120(6) 1.376(11) 73.30(6) 37.8(3) 1.345a 33.6

2.313 2.423 2.106 2.167 2.077 1.417 72.1 38.7 1.333 41.0

2.329(1) 2.276(1) 2.102(2) 2.183(2) 2.383(1) 1.420(3) 74.08(2) 38.64(7) 1.327c 28.6

2.341 2.276 2.124 2.150 2.324 1.428 73.1 39.0 1.347 30.4

2.326(1) 2.275(1) 2.108(3) 2.185(3) 2.387(1) 1.406(5) 74.76(3) 38.17(12) 1.343a 27.7

2.334 2.268 2.134 2.168 2.334 1.412 71.6 38.3 1.338 23.5

a Obtained from microwave spectroscopy.11 b Angle between the P2Rh plane and the center of the CC double bond. c Obtained from cocrystallized fumaronitrile in the unit cell of 5a.

as involving vinylic C-H activation, because the 1H NMR spectra contain signals corresponding to free neopentane, which is the expected product of the reductive elimination step after vinylic C-H activation on Rh. The detailed mechanism leading to 3 was not studied. The molecular structures of 2a/b in the solid state could be determined by X-ray structure analysis, as it was possible to obtain single crystals from Et2O solutions at low temperature. The data revealed an unexpected coordination geometry of the olefin moiety at Rh (Figure 1, Table 1). The center of the CdC double bonds is located 34° above the P2Rh coordination plane, which also contains the neopentyl carbon C5, so that both olefinic carbon atoms lie well above this plane. Typically, square-planar, olefin-containing d8-complexes display solid-state structures with the olefin upright and the center of the double bond located at one of the four corners of a square-planar geometry, as observed in Zeise’s salt,8 although it has been noted that some of them show slight distortions, which have been interpreted as most likely due to crystal packing effects.9 To our knowledge, the severely distorted coordination mode in 2a/b is unprecedented for d8-ML3(olefin) complexes and has only been reported once, in Grotjahn’s related Ir ketene system [(κ2dtbpm)IrCl(η2-(C,C)-Ph2CdCdO)],10a where the electronic (8) (a) Zeise, W. C. Poggendorffs Ann. Phys. Chem. 1831, 21, 497. (b) Seyferth, D. Organometallics 2001, 20, 2, and references therein. (c) Love, R. A.; Koetzle, T. F.; Williams, G. J. B.; Andrews, L. C.; Bau, R. Inorg. Chem. 1975, 14, 2653. (d) Eller, P. G.; Ryan, R. R.; Schaeffer, R. O. Cryst. Struct. Commun. 1977, 6, 163. (e) Bokii, G. B.; Kukina, G. A. Zh. Strukt. Khim. (Russ.) (J. Struct. Chem.) 1965, 6, 706. (f ) Hamilton, W. C.; Klanderman, K. A.; Spratley, R. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Crystallogr. 1969, 25, S172. (g) Jarvis, J. A. J.; Kilbourn, B. T.; Owston, P. G. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1971, 27, 366. (9) (a) Ghosh, R. E.; Waddington, T. C.; Wright, C. J. J. Chem. Soc., Faraday Trans. 2 1973, 69, 275. (b) Price, D. W.; Drew, M. G. B.; Hii, K. K. M.; Brown, J. M. Chem. Eur. J. 2000, 6, 4587. (10) (a) Grotjahn, D. B.; Bikzhanova, G. A.; Collins, L. S. B.; Concolino, T.; Lam, K.-C.; Rheingold, A. L. J. Am. Chem. Soc. 2000, 122, 5222. (b) Urtel, H.; Bikzhanova, G. A.; Grothjahn, D. B.; Hofmann, P. Organometallics 2001, 20, 3938. (c) Werner, H.; Bleuel, E. Angew. Chem., Int. Ed. 2001, 40, 145. (d) Ricci, J. S.; Ibers, J. A. J. Organomet. Chem. 1971, 27, 261. (e) Borowski, A. F.; Iraqi, A.; Cupertino, D. C.; Itvine, D. J.; Cole-Hamilton, D. J. J. Chem. Soc., Dalton. Trans. 1990, 29. (f ) Burch, R. R.; Shusterman, A. J.; Muetterties, E. L.; Teller, R. G.; Williams, J. M. J. Am. Chem. Soc. 1983, 105, 3546. (g) Tau, K. D.; Meek, D. W.; Sorrell, T.; Ibers, J. A. Inorg. Chem. 1978, 17, 3454. (h) Cationic amine olefin complexes [(κ2-dtbpm)Rh(piperidine-κN)(η2-olefin)]þ[BF4]- (olefin = ethylene, acryl ester) display the same type of distorted solid-state structures: Nisar, Y. Ph.D. Thesis, Universit€at Heidelberg, Germany, 2006. Nisar, Y.; Gross, J. H.; Rominger, F.; Meier, C.; Hofmann, P. In preparation (see also Supporting Information).

reason for the distortion has been analyzed for the specific case of a ketene.10b,c There are a few structures in the literature where in a tetracoordinate, square-planar d8-system (Rh, Ir) olefinic ligands are moved somewhat out of the coordination plane. In all of them, however, the metalbound olefin ligands are part of a chelating moiety, for which undistorted geometries with the center of the CdC bond in the ligand plane are hardly accessible for steric reasons.10d-f In one (cis-P2)Rh(Cl)(η2-olefin) structure closely related to 2a with a cis-chelating 1,1,3-triphenyl-1,3-diphosphinopropane ligand containing a P-bound -(CH2)2-CHdCH2 unit, which coordinates its olefin moiety to the metal cis to Cl, a normal undistorted square-planar geometry was found.10g The Rh-C1 distances (2.1244(18)/2.095(7) A˚) in 2a/b are shorter than the Rh-C2 bonds (2.1607(18)/2.146(6) A˚). The olefinic C1-C2 bonds are elongated compared to the free olefins (by 0.05 and 0.03 A˚, respectively) (Table 1), indicative of back-donation of electron density from the metal to the olefin. This is supported by 1H and 13C{1H} NMR data in [D8]THF: the resonances of the olefinic protons of 2a are shifted upfield compared to free ethylene by about 2 ppm to δ = 3.29. Those of the carbon atoms are shifted by 72 ppm to δ = 51.8 (500 MHz, -20 °C). For 2b, comparable upfield shifts are observed. These observations suggest the possibility of a resonance description of 2a/b as metallacyclopropanes with a formal þIII metal oxidation state. Overall, a square-pyramidal rather than a square-planar coordination geometry is adopted, positioning the carbon atoms of the olefin in a pseudobasal and a pseudoapical ligand position, respectively.10h We, of course, note that the equivalence of both carbon atoms of the coordinated ethylene of 2a in solution NMR spectra is inconsistent with its solid-state structure. This discrepancy does not mean that in solution there is symmetric olefin coordination contrasting the solid-state minimum geometry. The apparent symmetry in our opinion is rather caused by rapid dynamic processes (rotation plus up-down motion of the olefin, rotation around the Rh-Np bond), interconverting the two equivalent (enantiomeric) structures of 2a, with the C2H4 unit above or below the P2RhCNp plane. These dynamic processes cannot be frozen out in solution at -80 °C and are indicative of only a rather small energy preference for the distorted minimum geometry, in agreement with DFT results (vide infra and Supporting Information). In 2b the same fluxional process is operative. To further investigate the origin of the unusual coordination geometry in 2a/b, we also synthesized olefin complexes

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Scheme 4

of 1 with a series of other olefins12 and with modified steric and electronic properties, in which the pure σ-donor ligand Np was replaced by Cl, a weaker σ-donating group with less steric crowding and with π-donating properties. Addition of 1.5 equiv of fumaronitrile to the yellow dimer [(κ2-dtbpm)RhCl]2 (4)13 in toluene or pentane (Scheme 4) resulted in the direct precipitation of the red olefin complex (κ2-dtbpm)Rh(Cl)(η2-NCHCdCHCN) (5a). With acrylonitrile, the analogous compound (κ2-dtbpm)Rh(Cl)(η2-H2CdCHCN) (5b) was isolated. As solids, 5a/b are thermally robust and air stable, but in solution, equilibrium mixtures of 5a/b with 4 and the free olefins are observed.14 The molecular structures of 5a/b in the solid state again reveal that the center of the CdC units is located above the metal coordination plane, this time somewhat less than in 2a/b, 29° and 27°, respectively (Figure 2, Table 1). In all four structures 2a/2b and 5a/5b the hydrogens at the olefin carbon atoms have been found and could be refined. Despite the uncertainty of H atom positions in X-ray structure determinations, we can consider the angle R between the CdC vector and the normals to the planes defined by each olefin carbon and its σ-bound substituents (H, COOMe, CN), taking into account the low accuracy of H positions via error estimates from standard deviations. The angles R describe the pyramidality at each olefin C atom (a trigonalplanar olefin carbon has R = 90°, a tetrahedral carbon would be described by R = 35.26°); thus values of 90° - R (0° for planar, 54.74° for tetrahedral) directly describe the bending back angle. The following R values can be derived: 2a, C1 69(2)°, C2 69(2)°; 2b, C1 73(5)°, C2 75(4)°; 5a (two independent molecules in the unit cell), C1 65(3)°, C2 75(2)° and C1 74(2)°, C2 68(2)°; 5b, C1 65(1)°, C2 73(1)°. Clearly there is pyramidalization at all olefinic C atoms, indicative of rehybridization and Rh to olefin π* back-bonding. DFT calculations for the simplified model systems (κ2-dhpm)Rh(Np)(η2-olefin) (olefin = ethylene (2a*), methylacrylate (2b*)) and (κ2dhpm)Rh(Cl)(η2-olefin) (olefin = fumaronitrile (5a*), acrylonitrile (5b*)), in which the real ligand dtbpm was substituted by the simplified model ligand dhpm (diphosphinomethane, H2PCH2PH2), reproduced the surprising geometrical features of the experimental structures of 2a/b (11) Lide, D. R. Handbook of Chemistry and Physics, 76th ed.; CRC Press Inc.: Boca Raton, FL, 1996. (12) In LT-NMR experiments in [D8]THF, formation of olefin complexes was observed with 1 and acceptor-substituted olefins (fumaronitrile, acrylonitrile), with alkyl-substituted olefins (propene, 1-pentene), and with the donor-substitued olefin ethoxy vinyl ether, but crystals could not be obtained. (13) Hofmann, P.; Meier, C.; Englert, U.; Schmidt, M. U. Chem. Ber. 1992, 125, 353. (14) Our attempts to coordinate electron-rich olefins (ethylene, ethoxy vinyl ether) to the [(κ2-dtbpm)Rh(Cl)] fragment failed. With methyl acrylate only small equilibrium amounts of an olefinic complex species could be observed at -70 °C in NMR experiments in CD2Cl2. (15) The maximum deviation from X-ray data was smaller than 0.06 A˚ for bond lengths and less than 3° for bond angles. For computational details see the Experimental Section and Supporting Information.

Figure 2. Molecular structures of 5a/b (top/bottom) in the crystal: ORTEP plots, 50% probability, hydrogen atoms except on the olefin moiety are omitted for clarity.

and 5a/b perfectly (Table 1).15 The computed out-of-plane distortion angles (line b in Table 1) for all dhpm model systems except 5b* are slightly larger than those obtained from X-ray structures of the real systems; the out-of-plane olefin distortion is slightly more pronounced, which may be simply a steric consequence of removing the four tBu groups at the P atoms. QM/MM calculations (ONIOM16) of untruncated 2a also led to a quite accurate minimum geometry compared to the crystal structure; in this case the computed out-of-plane distortion angle is smaller than in the real molecule. We refrain from an interpretation of such small differences between computed minimum energy structures of model systems and experimental structure data of 2a/b and 5a/b. The uncommon olefin coordination mode must be caused entirely by electronic and not by steric effects, because the calculations with the sterically nondemanding ligand dhpm also resulted in distorted geometries. It is interesting to note that in the solid-state structures of 2b and 5b with one π-acceptor substituent on the alkene moiety there is the same site preference for the π-acceptors -COOMe and -CN. (16) Morokuma, K. J. Phys. Chem. 1996, 100, 19357.

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Figure 3. Relevant frontier orbital interactions of a [(κ2-dhpm)Rh(R)] or [(κ2-dtbpm)Rh(R)] fragment with π and π* of ethylene in the square-planar (left) and in the observed distorted structure (right).

Both point away from their cis-neighboring ligands Np and Cl, as one would expect for steric reasons. Both, however, are located at the pseudobasal olefin carbon C2 rather than the pseudoapical one, although this position looks more crowded. As analyzed below, this coordination mode is in line with the electronic structure description (vide infra) of these systems. There is no indication that cyano-substituted olefins like acrylonitrile or fumaronitrile coordinate via their nitrogens to Rh. This is consistent with the assumption that back-bonding from the electron-rich d8 metal center to the ligands plays a crucial role. Distortions away from a square-planar d8 structure have been described for related systems with very strong doublefaced π-acceptors such as CO or NOþ.17 In those cases, the bonding situation was described in line with predictions of Hoffmann et al.18 Our DFT calculations and NBO analyses show that it is indeed the back-bonding from the d8-ML3 metal fragment d-orbitals to the single-faced olefinic π* MOs that determines the actual minimum energy structures. Metal to olefin back-bonding within the frame of the DewarChatt-Duncanson model19 and according to the rules of perturbation theory depends upon the (relative and absolute) energies of the metal d-orbitals as well as on the energetic position and shape of the olefin π*-levels. The minimum geometry adopted turns out to depend on rather subtle electronic differences.20 As sketched in Figure 3, backbonding can occur either through a metal orbital that is a linear combination of dxz/dyz (olefin CdC midpoint in plane, Figure 3 left) or, if the midpoint of the CdC double bond is moved out of the PRhP plane, through dz2 (Figure 3 right). The calculations also reveal that, contrasting the dominant role of metal to olefin back-bonding, the dative bond from the olefin π-MO to the dxy LUMO of the metal unit is rather independent of the olefin location (the olefin π-population is unchanged by the distortion; the π*-population increases). In d8-ML3(olefin) systems with strongly σ-donating ligands L (electron-rich phosphines, alkyl groups, and the (17) (a) Whittlesey, M. K.; Perutz, R. N.; Virrels, I. G.; George, M. W. Organometallics 1997, 16, 268. (b) Gottschalk-Gaudig, T.; Huffman, J. C.; Gerard, H.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 2000, 39, 3957. (c) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1996, 118, 10189. (d) Ogasawara, M.; Huang, D.; Streib, W. E.; Huffman, J. C.; Gallego-Planas, N.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 8642. (18) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. (19) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79. (b) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (20) Hoffmann, R.; Minot, C.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 2001.

Urtel et al.

like) the metal frontier MO of highest energy is dz2. Metal to olefin π* back-bonding will be most efficient and stabilizing for a dz2/π* (HOMO-LUMO) interaction, which is only possible by moving the olefin out of the square plane. The other d-type fragment MO (a linear combination of dxz/dyz), which would allow back-bonding to olefin π* in an undistorted square-planar geometry (olefin upright), lies well below dz2. Even for C2H4 as the olefin, the distorted structure is preferred if the metal fragment is [(κ2-dtbpm)Rh(Np)]. For π-acceptor-substituted olefins with lower lying π*-LUMOs the frontier MO gap is further reduced, and dz2 to π* backbonding and distortion (as far as sterics allow) are enhanced and can be seen even for d8-ML3 metal fragments like [(κ2-dtbpm)Rh(Cl)] in 5a/b, although due to Cl to metal π-donation the energy difference between the two filled d-orbitals dz2 and the dxz/dyz linear combination decreases (still with dz2 as HOMO). For electron-rich olefins their π*-LUMO is shifted to higher energies, and back-bonding becomes less differentiating between dz2 and dxz/dyz, especially if their energy separation is small, as in the [(κ2dtbpm)Rh(Cl)] fragment. Here a smaller preference for distortions or even a square-planar geometry is expected. These predictions were supported by DFT calculations on the model complexes [(κ2-dhpm)Rh(Cl)(η2-olefin)]. Whereas electron-poor olefins such as fumaronitrile and acrylonitrile resulted in distorted geometries, more electron-rich olefins (ethylene, tetramethylethylene, or methyl vinyl ether) were found to have the square-planar structure as their calculated minimum energy geometries. Using the more electron-rich metal fragment [(κ2-dhpm)Rh(alkyl)] (alkyl = Me, Np), all olefins preferred the distorted coordination mode, indicating the greater importance of back-bonding due to the higher lying, filled metal dz2 orbital. The site preference of the -COOMe and -CN groups in 2b and 5b is a consequence of the polarization of the π*-LUMO of acrylic ester and acrylonitrile by these substituents: the larger coefficient is at the β-carbon of the CdC unit. This causes a larger overlap of dz2 of Rh with the olefin π*-orbitals for the experimentally observed geometries as opposed to isomers with the substituents in a pseudoapical position. Thus the olefin orientation preference in 2b and 5b is a corollary of the electronic origin of the structure distortions in the systems described here. As already mentioned above, cis-alkyl olefin complexes are generally accepted to be the resting states of catalytic olefin polymerization involving late transition metals. Therefore the unusual geometries found for the Rh systems reported here, isoelectronic to their cationic Ni and Pd congeners, are of some mechanistic interest. We therefore calculated the transition state for ethylene insertion into the Rh-CNp bond for (κ2-dhpm)Rh(Np)(η2-C2H4) (2a*). It was found to lie 28.1 kcal mol-1 above the starting complex. Insertion resulted in the thermodynamically slightly favored primary insertion product 6* (Erel = -1.5 kcal mol-1) with just the same γ-agostic stabilization as we had seen experimentally in 1 (Scheme 5). The computed insertion barrier is consistent with the experimental fact that no polymerization products can be observed in the reaction of 2a/b with an excess of olefin. The barrier is too high to be overcome at the low temperatures at which 2a has to be handled to avoid decomposition. The calculations underline the fact that the complexes synthesized here can serve as reasonable models for cis P-ligated cis-alkyl olefin complexes in late transition metal

Article

Organometallics, Vol. 29, No. 21, 2010 Scheme 5

catalyzed polymerization reactions. They suggest that the olefin insertion step into M-alkyl bonds (more precisely, the alkyl to olefin migration) can start directly from the observed distorted olefin to metal coordination mode. This is especially interesting in the context of Ziegler’s QM/MM calculations on Brookhart’s NiII diimine catalysts, predicting that polymerization can occur directly from a similar distorted geometry.21 Although the diimine ligand set is quite different from the bisphosphine applied in our work, and because to the best of our knowledge there is no experimental structure information available for cationic cis-olefin-alkyl intermediates of Ni or Pd (contrasting the situation with Pt), our structures may represent the first experimental proof for the existence of the postulated distorted coordination geometry.22

Conclusion In summary, the first structurally characterized cis-alkyl olefin rhodium(I) complexes 2a/b bearing a cis-chelating diphosphine ligand and a nontethered olefin provide interesting experimental information related to the postulated resting state of olefin polymerization with late transition metal d8-systems with bidentate spectator ligands.

Experimental Section General Methods. All manipulations were performed under an inert atmosphere of purified argon (standard vacuum line, Braun glovebox) using standard Schlenk techniques. All solvents were purified and dried according to literature procedures and were degassed by freeze-pump-thaw cycles and stored under an atmosphere of argon. NMR spectra were recorded on either a Bruker DRX 300 or a 500 MHz spectrometer. 1H and 13 C{1H} NMR data were reported in units of δ relative to TMS referenced to the residual solvent resonance as internal reference. J(H,P) coupling constants were assigned by 1H{31P} NMR measurements. 31P{1H} NMR spectra were externally referenced to 85% H3PO4, while low-temperature measurements (21) (a) Deng, L. Q.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 6177. (b) Deng, L. Q.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 1094. (22) DFT calculations for the Ni species [(κ2-dhpm)Ni(CH3)(η2C2H4)]þ, the presumable first intermediate and a model compound for the resting state of ethylene polymerization with [(κ2-dtbpm)Ni(CH3)(THF)]þ[BArF]- (refs 5a, 5b), also resulted in a distorted geometry. (23) Gross, J. H. Rapid Commun. Mass Spectrom. 1998, 12, 1833. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.5; Gaussian, Inc.: Pittsburgh, PA, 1998.

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were calibrated at room temperature. Mass spectra were obtained on a Jeol LMS-700 instrument (CIþ or low-temperature FABþ in toluene),23 and for IR spectroscopy a Bruker Equinox 55 FT-IR spectrometer was used. Computational Details. Calculations on the model systems (κ2-dhpm)RhR(η2-olefin) 2a*/b*, 5a*/b*, TS 2a*-6*, and 6* (dhpm = bis(diphosphino)methane, H2PCH2PH2) were performed with the Gaussian 98 package.24 All geometries were fully optimized using density functional theory (DFT) with Becke’s three-parameter hybrid exchange (B3)25 and the Perdue-Wang (PW91)26 correlation functional, without applying any symmetry restrictions. Local minima were identified by the absence of any negative eigenvalues (NIMAG = 0) of the Hessian matrix in a vibrational frequency analysis; transition states. by NIMAG = 1. For Rh and P the Stuttgart-Dresden basis sets with the corresponding effective core potentials (replacing 28 core electrons for Rh and 10 for P) were applied.27,28 For P, one d polarization function with an exponent of 0.387 was added.29 The basis set for C, H, O, N, and Cl was 6-31G**.30 NBO analyses were performed for the model complexes (κ2-dhpm)Rh(Cl)(η2-H2CdCH2), (κ2-dhpm)Rh(Cl)(η2-NCCHd CHCN), and (κ2-dhpm)Rh(Me)(η2-H2CdCH2) at the B3PW91 level of theory. QM/MM calculations of the ONIOM type16 were performed for the real system 2a with the actual dtbpm ligand. In that case, the DFT method B3PW91 was applied for the metal core [(κ2dhpm)Rh(Np)(η2-C2H4)], using the same parameters as for the pure DFT calculations (vide supra). The tBu groups of the dtbpm ligand were calculated within the MM part using the Universal force field (UFF).31 Preparation of [(K2-dtbpm)Rh(Np)(η2-H2CdCH2)] (2a). About 2 mL of ethylene was condensed into an evacuated Schlenk tube filled with [(κ2-dtbpm)RhNp] (1, 50 mg, 0.10 mmol) at -196 °C. The yellow suspension was stirred for 5 min at -110 °C. The liquid ethylene was evaporated at -78 °C via an overpressure valve, resulting in a yellow powder of 2a. Washing with pentane led to decomposition, so further purification was impossible. The resulting air-, moisture-, and temperature-sensitive product was dried for 5 min under mild vacuum. Alternatively, 2a can be easily generated in situ, treating solutions (THF, toluene, Et2O) of 1 with 1 bar of ethylene at temperatures below -10 °C. IR (KBr): only decomposition products detectable, solution IR due to temperature sensitivity not possible. Mass spectra could not be obtained (methods tried: EI, CI, LT-FABþ). Single crystals were obtained by slowly concentrating solutions of 2a in Et2O at -20 °C by solvent evaporation. NMR data of 2a: 31P{1H} NMR (202 MHz, [D8]THF, -20 °C): δ -1.2 (dd, 1J(P,Rh) = 75.7 Hz, 2J(P,P) = 9.9 Hz, P trans to Np); -6.1 (dd, 1J(P,Rh) = 168.7 Hz, 2J(P,P) = 9.9 Hz, P cis to Np). 1H NMR (500 MHz, [D8]THF, -20 °C): δ 5.38 (s, xs of free C2H4); 3.29 (br s, 4 H, coord C2H4); 3.22 (t, J(H,P) = 6.4 Hz, 2 H, PCH2P); 1.43 (d, 3J(H,P) = 11.7 Hz, 18 H, tBu); (25) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3089. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (26) Perdue, J. P.; Wang, Y. Phys. Rev. B 1991, 45, 13244. (27) Andrae, D.; H€aussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (28) Bergner, A.; Dolg, M.; K€ uchle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431. (29) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (30) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 55, 2257. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (d) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (e) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Phys. Chem. 1984, 80, 3265. (31) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.

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1.34 (d, 3J(H,P) = 11.3 Hz, 18 H, tBu); 1.01 (s, 9 H, CH2C(CH3)3); 0.70 (t, J = 7.4 Hz, 2 H, RhCH2). 13C{1H} NMR (125 MHz, [D8]THF, -20 °C): δ 123.4 (s, xs of free C2H4); 51.8 (t, 1J(C,Rh) = 2J(C,P) = 13.3 Hz, coord C2H4); 38.1 (m, PCMe3 cis to Np); 37.2 (dd, J = 6.9 Hz, J = 5.9 Hz, PCMe3 trans to Np); 36.2 (s, CH2C(CH3)3); 35.7 (t, J = 3.4 Hz, CMe3 Np); 33.7 (m, RhC); 32.0 (m, PCH2P); 31.8 (d, 2J(C,P) = 6.4 Hz, PC(CH3)3 cis to Np); 30.7 (d, 2J(C,P) = 6.4 Hz, PC(CH3)3 trans to Np). Preparation of [(K2-dtbpm)Rh(Np)(η2-H2CdCHCO2Me)] (2b). [(κ2-dtbpm)RhNp] (1, 68.8 mg, 0.14 mmol) was dissolved in Et2O (8 mL) at -78 °C. Then 20 μL of methylacrylate (1.5 equiv, 18.7 mg, 0.22 mmol) was added. The red solution was stirred for 20 min. The solvent was removed in vacuo at -78 °C, resulting in a deep red oil. Addition of 5 mL of cold pentane (-78 °C) led to precipitation of the red product, which was washed with pentane (2  2 mL, -78 °C, soluble in pentane to some extent). The resulting air-, moisture-. and temperaturesensitive product was dried in vacuo. Further purification by crystallization was impossible, as only small amounts of crystals are obtainable (in Et2O). Yield: 56 mg (69%). Single crystals were obtained by slowly concentrating solutions of 2b in Et2O at -20 °C by solvent evaporation. Mp: 68 °C. IR (KBr, cm-1): 2950 (s); 2899 (s); 2868 (s); 1693 (CO, m); 1473 (m); 1393 (w); 1366 (w); 1175 (m); 1075 (m); 1019 (m); 813 (m). MS (LT-FABþ, m/z): 493 (Mþ - Np); 478 (Mþ - C2H3CO2Me). 31P{1H} NMR (202 MHz, [D8]THF, -30 °C): δ 10.4 (dd, 1J(P,Rh) = 178.0 Hz, 2 J(P,P) = 19.1 Hz, P cis to Np); 1.8 (dd, 1J(P,Rh) = 65.1 Hz, 2 J(P,P) = 19.1 Hz, P trans to Np). 1H NMR (500 MHz, [D8]THF, -30 °C): δ 4.19 (td, 3J(H,Htrans) = 3J(H,P) = 11 Hz, 3J(H,Hcis) = 3 Hz, 1 H, CHCO2Me); 3.53 (s, 3 H, OCH3); 3.34 (m, 2 H, PCHHP þ CHdCHH); 3.12 (m, 2 H, PCHHP þ CHdCHH); 2.36 (t, 2J(H,H) = 3J(H,P) = 12.6 Hz, 1 H RhCHH exo); 1.44 (d, 3J(H,P) = 11.6 Hz, 9 H, tBu); 1.40 (d, 3J(H,P) = 11.6 Hz, 18 H, tBu); 1.30 (d, 3J(H,P) = 10.9 Hz, 9 H, tBu); 1.10 (s, 9 H, CH2C(CH3)3); -0.51 (t, 2J(H,H) = 3J(H,P) = 9.9 Hz, 1 H RhCHH endo). 13C{1H} NMR (125 MHz, [D8]THF, -20 °C): δ 176.9 (s, CO2Me); 50.3 (s, OCH3); 49.9 (dd, J = 25.7 Hz, J = 8.0 Hz, CHdCH2); 48.8 (d, J = 23.7 Hz, CHd CH2); 37.6 and 38.1 (m, PCMe3 and CMe3 Np); 35.2 (d, J = 5.0 Hz, CH3 Np); 31.8 (d, 2J(C,P) = 5.8 Hz, PC(CH3)3); 33.3 (d, 2 J(C,P) = 5.4 Hz, PC(CH3)3); 31.0 (m, 2 PC(CH3)3); 30.5 (m, PCH2P), 29.4 (m, RhCH2, only detected via C-H COSY). Preparation of [(K2-dtbpm)Rh(Cl)(η2-NCHCdCHCN)] (5a). [(κ2-dtbpm)RhCl]2 (4, 100 mg, 0.11 mmol) was dissolved in toluene (10 mL). To the orange solution was cannulated a solution of fumaronitrile (26.5 mg, 0.34 mmol) in toluene (5 mL), yielding a red solution. After 10 min stirring at ambient temperature, a red solid precipitated. The red suspension was stirred for a further 50 min. The mixture was filtered, and the remaining red solid was washed with hexanes (2  4 mL). The resulting air-stable product was dried in vacuo. Dissociation to 4 and free olefin occurs in solutions of 5a at ambient temperature: 100% in THF and toluene and to a small extent in CH2Cl2 (about 8%). Crystals suitable for X-ray diffraction analysis were obtained from solutions of 5a and an additional 4 equiv of fumaronitrile in CH2Cl2 by slow solvent evaporation at ambient temperature. Yield: 115 mg (98%). Mp: 290 °C dec. Anal. Calcd for C21H40ClN2P2Rh (520.9): C, 48.42; H, 7.74; N, 5.38; Cl, 6.81; P, 11.89. Found: C, 48.69; H, 7.81; N, 5.42; Cl, 7.06; P, 11.92. 31P{1H} NMR (202 MHz, CD2Cl2, -20 °C): δ 4.6 (dd, 1 J(P,Rh) = 116.6 Hz, 2J(P,P) = 45.9 Hz, P trans Cl); -17.9 (dd, 1 J(P,Rh) = 140.2 Hz, 2J(P,P) = 45.9 Hz, P cis Cl). 1H NMR (500 MHz, CD2Cl2, -20 °C): δ 4.80 (m, 3J(H,H) = 10.4 Hz, 2 J(H,Rh) = 3.8 Hz, 1 H, CHCN above plane); 3.60 (t, J(H,P) = 8.6 Hz, 2 H, PCH2P); 3.45 (dd, 1 H, 3J(H,H) = 10.4 Hz, 3 J(H,P) = 6.4 Hz, CHCN in plane); 1.62 (d, 3J(H,P) = 14.4 Hz, 9 H, tBu); 1.52 (d, 3J(H,P) = 14.9 Hz, 9 H, tBu); 1.41 (d, 3 J(H,P) = 14.4 Hz, 9 H, tBu); 1.28 (d, 3J(H,P) = 14.2 Hz, 9 H, t Bu). 13C{1H} NMR (125 MHz, CD2Cl2, -20 °C): δ 122.0

Urtel et al. (d, 3J(C,P) = 6.6 Hz, CN); 119.5 (d, 3J(C,P) = 3.3 Hz, CN); 38.5 (d, 1J(C,P) = 14.5 Hz, PCMe3); 37.0 (m, 2  PCMe3); 36.5 (dd, 1J(C,P) = 8.9 Hz, 3J(C,P) = 2.3 Hz, PCMe3); 32.2 (dd, J = 29.6 Hz, J = 3.2 Hz, CdC in plane); 31.2 (d, 2J(C,P) = 3.3 Hz, PC(CH3)3); 30.8 (dd, 1J(C,P) = 17.4 Hz, 1J(C,P) = 12.2 Hz, PCH2P); 30.5 (dd, J = 19.1 Hz, J = 4.0 Hz, CdC above plane); 30.3 (d, 2J(C,P) = 3.9 Hz, PC(CH3)3); 29.8 (d, 2J(C,P) = 6.0 Hz, PC(CH3)3); 29.5(s, PC(CH3)3). IR (KBr, cm-1): 2998 (s); 2976 (s); 2951 (s); 2920 (s); 2900 (s); 2868 (s); 2216 (CN, m); 2197 (CN, vs); 1482 (s); 1472 (s); 1456 (m); 1439 (m); 1397 (m); 1374 (s); 1202 (w); 1170 (s); 1100 (w); 1024 (w); 1012 (w); 942 (w); 811 (w); 791 (w); 727 (m); 698 (m); 674 (w); 564 (w); 505 (w); 485 (w). MS (EI, m/z): no Mþ; 442 (Mþ - C2H2(CN)2); 385 (Mþ - tBu); 294 (Mþ - C4H9 - C4H8 - Cl); 248 (dtbpmþ - C4H8); 191 (dtbpmþ - C4H9 - C4H8); 135 (dtbpmþ - C4H9 - 2 C4H8). Preparation of [(K2-dtbpm)Rh(Cl)(η2-H2CdCHCN)] (5b). [(κ2-dtbpm)RhCl]2 (4, 130 mg, 0.15 mmol) was suspended in pentane (3 mL). Then 0.4 mL of acrylonitrile was added to the yellow suspension, yielding directly a color change to deep orange. After 2 h stirring at ambient temperature the mixture was filtered and the remaining orange solid was washed with cold pentane (2  4 mL). The resulting air-stable product was dried in vacuo. Crystals suitable for X-ray diffraction analysis were obtained from solutions of 5b in acrylonitrile at -60 °C. NMR spectra were recorded in the presence of an excess of acrylonitrile (5 equiv) to prevent dissociation. Yield: 134 mg (92%). Mp: 230 °C dec. Anal. Calcd for C20H41ClNP2Rh (495.5): C, 48.45; H, 8.33; N, 2.82; Cl, 7.15; P, 12.49. Found: C, 48.40; H, 8.27; N, 2.85; Cl, 7.05; P, 12.83. 31P{1H} NMR (202 MHz, CD2Cl2, -20 °C): δ 7.4 (dd, 1J(P,Rh) = 132.7 Hz, 2 J(P,P) = 46 Hz, P trans Cl); -17.0 (dd, 1J(P,Rh) = 141.4 Hz, 2 J(P,P) = 46 Hz, P cis Cl). 1H NMR (500 MHz, CD2Cl2, -20 °C): δ 6.2, 6.1, and 5.6 (free acrylonitrile); 4.77 (dm, 3 J(H,H) = 11.9 Hz, 2J(H,H) = 2.4 Hz, 1 H, CHHtransCHCN); 4.47 (dm, 3J(H,H) = 8.9 Hz, 2J(H,H) = 2.4 Hz, 1 H, CHHcisCHCN); 3.41 (t, 3J(H,P) = 8.4 Hz, 2 H, PCH2P); 3.14 (“q”, 3 J(H,H) = 11.9 Hz, 3J(H,H) = 8.9 Hz, 1 H, CH2CHCN); 1.55 (d, 3J(H,P) = 13.9 Hz, 9 H, tBu); 1.43 (d, 3J(H,P) = 14.3 Hz, 9 H, tBu); 1.39 (d, 3J(H,P) = 13.6 Hz, 9 H, tBu); 1.31 (d, 3 J(H,P) = 13.5 Hz, 9 H, tBu). 13C{1H} NMR (125 MHz, CD2Cl2, -20 °C): δ 137.7 (s, dCH2), free acrylonitrile); 117.0 (s, CN, free acrylonitrile); 107.1 (s, dCH(CN)); 124.2 (d,3J(C,P) = 6.3 Hz, CN); 58.5 (d, 2J(C,P) = 15.5 Hz, CH2d); 37.2 (d, 1J(C,P) = 12.2 Hz, PCMe3); 37.0 (dm, 1J(C,P) = 9.5 Hz, PCMe3); 36.9 (dd, 1J(C,P) = 5.9 Hz; J = 2 Hz, PCMe3); 36.6 (d, 1J(C,P) = 18.6 Hz, PCMe3); 35.1 (m, CH(CN)); 31.5 (dd, 1J(C,P) = 12.0 Hz, 1J(C,P) = 16.3 Hz, PCH2P); 31.1 (d, 2J(C,P) = 4.0 Hz, PC(CH3)3); 30.3 (d, 2J(C,P) = 4.6 Hz, PC(CH3)3); 30.2 (d, 2J(C,P) = 4.6 Hz, PC(CH3)3); 29.7 (d, 2 J(C,P) = 2.3 Hz, PC(CH3)3). IR (KBr, cm-1): 2950 (s, br); 2235 (shoulder, CN, m); 2190 (CN, vs); 1615 (w); 1475 (s); 1395 (m); 1370 (w); 1175 (s); 1090 (w); 1020 (w); 935 (w); 810 (m); 740 (m); 490 (w). X-ray Diffraction Analysis. The data sets for the structure analyses of 2a, 2b, 5a, and 5b were collected on a Bruker Smart CCD diffractometer with a sealed tube Mo KR radiation source (λ = 0.71073 A˚) and a graphite monochromator at 200 K. Three sets of 0.3 deg 20 s omega scans were taken, covering a whole sphere in reciprocal space. The intensities were corrected for Lorentz and polarization effects, and an empirical absorption correction was applied using SADABS32 based on the Laue symmetry of the reciprocal space. The structures were solved by direct methods and refined against F2 with a full-matrix leastsquares algorithm using the SHELXTL (version 2008/4) software package.33 Hydrogen atoms were treated using appropriate riding models, except of the olefinic hydrogen atoms, which (32) Sheldrick, G. M. SADABS; Bruker Analytical X-ray-Division, Madison, WI, 2008. (33) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.

Article were refined isotropically. CCDC 775509 (2a), 775510 (2b), 775511 (5a), and 775512 (5b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 2a: red crystal (polyhedron), dimensions 0.50  0.32  0.07 mm3, crystal system monoclinic, space group P21/c, Z = 4, a = 11.2157(1) A˚, b = 11.7153(1) A˚, c = 21.1546(1) A˚, β = 101.082(1)°, V = 2727.78(4) A˚3, F = 1.233 g/cm3, θmax = 30.49°, 33 922 reflections measured, 8305 unique (R(int) = 0.0351), 6749 observed (I >2σ(I )), μ = 0.75 mm-1, Tmin = 0.84, Tmax = 0.96, 285 parameters refined, goodness of fit 1.05 for observed reflections, final residual values R1(F ) = 0.027, wR(F2) = 0.059 for observed reflections, residual electron density -0.37 to 0.83 e A˚-3. 2b: red crystal (lamina), dimensions 0.48  0.13  0.02 mm3, crystal system monoclinic, space group P21/c, Z = 4, a = 9.0537(4) A˚, b = 17.0093(8) A˚, c = 19.0625(8) A˚, β = 94.062(1)°, V = 2928.2(2) A˚3, F = 1.281 g/cm3, θmax = 27.41°, 29 668 reflections measured, 6651 unique (R(int) = 0.1365), 3901 observed (I >2σ(I)), μ = 0.71 mm-1, Tmin = 0.45, Tmax = 0.98, 308 parameters refined, goodness of fit 1.04 for observed reflections, final residual values R1(F) = 0.071, wR(F2) = 0.168 for observed reflections, residual electron density -1.85 to 2.58 e A˚-3. 5a: red crystal (polyhedron), dimensions 0.30  0.14  0.10 mm3, crystal system monoclinic, space group Pn, Z = 4, a = 10.9515(1) A˚, b = 14.1024(2) A˚, c = 15.3310(1) A˚, β = 97.331(1)°, V = 2348.40(4) A˚3, F = 1.402 g/cm3, θmax = 27.44°, 23 754 reflections measured, 10 640 unique (R(int) = 0.0268), 9552 observed (I >2σ(I)), μ = 0.98 mm-1, Tmin = 0.80, Tmax = 0.92, 499 parameters refined, Flack absolute structure parameter -0.029(13), goodness of fit 1.07 for observed

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reflections, final residual values R1(F) = 0.023, wR(F2) = 0.050 for observed reflections, residual electron density -0.54 to 0.36 e A˚-3. 5b: red crystal (polyhedron), dimensions 0.32  0.26  0.04 mm3, crystal system monoclinic, space group P21/n, Z = 4, a = 10.8889(1) A˚, b = 15.4067(2) A˚, c = 16.1672(1) A˚, β = 95.629(1)°, V = 2699.16(5) A˚3, F = 1.378 g/cm3, θmax = 27.47°, 27 333 reflections measured, 6176 unique (R(int) = 0.0354), 5107 observed (I >2σ(I)), μ = 0.87 mm-1, Tmin = 0.85, Tmax = 0.97, 291 parameters refined, goodness of fit 1.02 for observed reflections, final residual values R1(F) = 0.022, wR(F2) = 0.048 for observed reflections, residual electron density -0.38 to 0.33 e A˚-3.

Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (Graduate College Fellowship to H.U.), BASF SE, Degussa-H€ uls AG, the European Union (PROCOPE program), and the Fonds der Chemischen Industrie. We thank Dr. J. P. Joschek for providing his data for the independently synthesized compound syn/anti 3a. Supporting Information Available: CIF files giving crystal data for 2a/b and 5a/b, and pictures and Cartesian coordinates of the calculated structures 2a (ONIOM), 2a*/b*, 5a*/b*, TS 2a*-6*, and 6*. Independent synthesis and spectra (1H, 31P) of 3-syn/anti. Energy profile (DFT, B3PW91) for the dynamic processes in model complex (κ2-dhpm)Rh(Np)(η2-C2H4) (2a*). ORTEP plots of Ni and Rh complexes of refs 5a, 10h. This material is available free of charge via the Internet at http://pubs.acs.org.